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الموضوع: Protection System in Power System

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    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
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    Protection System in Power System

    Protection System in Power System- مواضيع هامة منقولة ...

    This portion of our website covers almost everything related to protection system in power system including standard lead and device numbers, mode of connections at terminal strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power system protection relays and schemes including special power system protection schemes like differential relays, restricted earth fault protection, directional relays and distance relays etc. The details of transformer protection, generator protection, transmission line protection and protection of capacitor banks are also given. It covers almost everything about protection of power system.
    The switchgear testing, instrument transformers like current transformer testing voltage or potential transformer testing and associated protection relay are explained in detail.The close and trip, indication and alarm circuits different of circuit breakers are also included and explain.
    ******ive of Power System Protection

    The ******ive of power system protection is to isolate a faulty section of electrical power system from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system. For satisfying this quick disconnection the protection relays should have following functional requirements.
    Protection System in Power System

    Let’s have a discussion on basic concept of protection system in power system and coordination of protection relays.In the picture the basic connection of protection relay has been shown. It is quite simple. The secondary of current transformer is connected to the current coil of relay. And secondary of voltage transformer is connected to the voltage coil of the relay. Whenever any fault occurs in the feeder circuit, proportionate secondary current of the CT will flow through the current coil of the relay due to which mmf of that coil is increased. This increased mmf is sufficient to mechanically close the normally open contact of the relay. This relay contact actually closes and completes the DC trip coil circuit and hence the trip coil is energized. The mmf of the trip coil initiates the mechanical movement of the tripping mechanism of the circuit breaker and ultimately the circuit breaker is tripped to isolate the fault.Functional Requirements of Protection Relay

    Reliability

    The most important requisite of protective relay is reliability. They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.Selectivity

    The relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.
    Sensitivity

    The relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.
    Speed

    The protective relays must operate at the required speed. There must be a correct coordination provided in various power system protection relays in such a way that for fault at one portion of the system should not disturb other healthy portion. Fault current may flow through a part of healthy portion since they are electrically connected but relays associated with that healthy portion should not be operated faster than the relays of faulty portion otherwise undesired interruption of healthy system may occur. Again if relay associated with faulty portion is not operated in proper time due to any defect in it or other reason, then only the next relay associated with the healthy portion of the system must be operated to isolate the fault. Hence it should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation.
    Important Elements for Power System Protection

    Switchgear

    Consists of mainly bulk oil circuit breaker, minimum oil circuit breaker, SF6 circuit breaker, air blast circuit breaker and vacuum circuit breaker etc. Different operating mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are employed in the circuit breaker. Circuit breaker is the main part of protection system in power system and it automatically isolate the faulty portion of the system by opening its contacts.
    Protective Gear

    Consists of mainly power system protection relays like current relays, voltage relays, impedance relays, power relays, frequency relays, etc. based on operating parameter, definite time relays, inverse time relays, stepped relays etc. as per operating characteristic, logic wise such as differential relays, over fluxing relays etc. During fault the protection relay gives trip signal to the associated circuit breaker for opening its contacts.
    Station Battery

    All the circuit breakers of electrical power system are DC (Direct Current) operated. Because DC power can be stored in battery and if situation comes when total failure of incoming power occurs, still the circuit breakers can be operated for restoring the situation by the power of storage station battery. Hence, the battery is another essential item of the power system. Some time it is referred as the heart of the electrical substation. An electrical substation battery or simply a station battery containing a number of cells accumulate energy during the period of availability of AC supply and discharge at the time when relays operate so that relevant circuit breaker is tripped at the time failure of incoming AC power.
    Trip Circuit Supervision

    There are different contacts connected in series along a trip circuit of an electrical circuit breaker. There must be some situation when the circuit breaker should not trip even a faulty current passes through its power contacts. Such situations are low gas pressure in SF6 circuit breaker, low air pressure in pneumatic operated circuit breaker etc. In this situation the trip coil of the CBmust not be energized to trip the CB. So there must be NO contacts associated with gas pressure and air pressure relays, connected in series with breaker trip coil. Another scheme of trip coil is that it should not be re energized once the circuit breaker is opened. That is done by providing one NO contact of breaker auxiliary switch in series with trip coil. In addition to that the trip circuit of a CB has to pass through considerable numbers of intermediate terminal contacts in relay, control panel and circuit breaker kiosk. So if any of the intermediate contacts is detached, the circuit breaker fails to trip. Not only that, if dc supply to the trip circuit fails, the CB will not trip.To overcome this abnormal situation, trip circuit supervision becomes very necessary. The figure below shows the simplest form of trip circuit healthy scheme. Here one series combination of one lamp, one push bottom and one resistor is connected across the protective relay contact as shown. In healthy situation all the contacts except protective relay contact are in close position. Now if push bottom (PB) is pressed, the trip circuit supervision network is completed and lamp glows indicating that the breaker is ready for tripping.[IMG]https://www.electrical4u.com/wp-*******/uploads/2013/04/trip-circuit-supervision-1.png[/IMG]The above scheme is for supervision while circuit breaker is closed. This scheme is called post close supervision.There is another supervision scheme which is called pre and post close supervision. This trip circuit supervision scheme is also quite simple. The only difference is that here in this scheme, one NC contact of same auxiliary switch is connected across the auxiliary NO contact of the trip circuit. The auxiliary NO contact is closed when CB is closed and auxiliary NC contact is closed when CB is open and vice versa. Hence, as shown in the figure below when the circuit breaker is closed the trip circuit supervision network is completed via auxiliary NO contact but when the circuit breaker is open the same supervision network is completed via NC contact. The resistor is used series with the lamp for preventing unwanted tripping of circuit breaker due to internal short circuit caused by failure of the lamp.[IMG]https://www.electrical4u.com/wp-*******/uploads/2013/04/trip-circuit-supervision-2.png[/IMG]So far whatever we have discussed it is only for local controlled installation but for a distance control installation, relay system is necessary. The figure below shows the trip circuit supervision scheme wherever a remote signal is required.[IMG]https://www.electrical4u.com/wp-*******/uploads/2013/04/trip-circuit-supervision-3.png[/IMG]When trip circuit is healthy and circuit breaker is closed, relay A is energized which closes the NO contact A1 and hence relay C is energized. Energized relay C keeps NC contact in open position. Now if the circuit breaker is open, relay B is energized which closes No contact B1 hence relay C is energized. As C is energized, it keeps the NC contact C1 in open position. While CB is closed, if there is any discontinuity in the trip circuit relay A is de-energized which opens contact A1 and consequently relay C is de-energized and which make the NC contact C1 in close position and hence alarm circuit is actuated. Trip circuit supervision is experienced by relay B with the circuit breaker is open in a similar manner as relay A with the circuit breaker is closed. Relays A and C are time-delayed by copper slugs to prevent spurious alarms during tripping or closing operations. The resistors are mounted separately from the relays and their values are chosen such that if any one component is inadvertently short-circuited, a tripping operation will not take place.
    The alarm circuit supply should be separated from main trip supply so that the alarm can be actuated even the trip supply failed.
    Differential Protection of Generator or Alternator

    ny internal fault inside the stator winding is cleared by mainly differential protection scheme of the generator or alternator.
    The differential protection is provided in the generator by using longitudinal differential relay.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/generator-differential-protection1.gif[/IMG]Generally instantaneous attracted armature type relays are used for this purpose because all they have high speed operation and also they are free from being affected by any AC transient of the power circuit.There are two sets of current transformers one CT is connected to the line side of the generator and other is connected to the neutral side of the generator in each phase. It is needless to say that the characteristics of all current transformers installed against each phase must be matched. If there is any major mismatched in the current transformer’s characteristics of both sides of the generator, there may be high chance of malfunctioning of differential relay during the fault external to the stator winding and also may be during normal operating conditions of the generator.
    To ensure that the relay does not operate for the faults external to the operated zone of the protection scheme, a stabilizing resistor is fitted in series with the relay operating oil. It also ensures that if one set of CT has been saturated, there will be no possibility of malfunctioning of the differential relay.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/generator-differential-protection-1.gif[/IMG]It is always preferable to use dedicated current transformers for differential protection purpose because common current transformers may cause unequal secondary loading for other functionalities imposed on them. It is also always preferable to use all current transformers for differential protection of generators or alternators should be of same characteristics. But practically there may be some difference in characteristics of the current transformers installed at line side to those installed in neutral side of the generator. These mismatches cause spill current to flow through the relay operating coil. To avoid the effect of spill current, percentage biasing is introduced in differential relay.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/differentially-protection-generator-2.gif[/IMG]The percentage biased differential relay comprises two restraint coils and one operating coil per phase. In the relay, the torque produced by operating coil tends to close the relay contacts for instantaneous tripping of circuit breakers but at the same time the torque produced by the restraint coils prevents to close the relay contacts as restraint coils torque is directed opposite of the operating coil torque. Hence during through fault the differential relay would not be operated because the setting of the relay is increased by restraint coils and also it prevents malfunctioning of relay due to spill current. But during internal fault in the winding of the stator, the torque produced by restraint coils is ineffective and the relay closes its contact when setting current flows through the operating coil.
    Differential current pickup setting/bias setting of the relay is adopted based on the maximum percentage of allowable mismatch adding some safety margin.
    The spill current level for the relay is to just operate it; is experienced as a percentage of the through fault current causing it. This percentage is defined as bias setting of the relay.



    Annunciation System Alarm Annunciator




    Now days, a major portion of electrical and electronics industry have switched upon industrial automation system. In electrical system, all time attention is given to the processes and plants are very much essential, irrespective of the voltage levels. In electrical and electronics systems, the word Annunciator literally means the device which announces the mishaps or unusual activities, coming from the system or process associated with it.What is Alarm Annunciator?

    It is basically an audio visual warning system, which highlights the fault or mishap which is going on, or even before it happens. This is very necessary for safety concern also, and sometimes the warning comes before improper procedure which warns the operator to avoid unwanted accident etc. This is the basic concept of Alarm Annunciator, and the alarm annunciation system. Let us look at the operation of a typical alarm annunciator device.Operation of Alarm Annunciator

    Alarm Annunciation System

    In order to understand the fundamental operation and connections of alarm Annunciator, we have to understand the basic concept of alarming system in process monitoring. Suppose, an electromagnetic coil is energized by power supply and acts as an electromagnet for certain application. Now, because of over voltage a portion of the coil has been burnt. As a consequence, the entire process associated with it gets hampered. So, when finding the very cause of this mishap, you have to check each and every part of the system in order to find and recognize the actual fault. Now think you have 50 such coils, which you have to monitor. In this case finding the actual faulty coil becomes very difficult and time consuming too.But if you connect a bulb in series with the power supply of each coil, it glows if and only if the coil is energized and healthy. In this way, for 50 such electromagnetic coils you need to use 50 bulbs each of them connected in series with each of the individual coil through which you can monitor the processes by viewing the glow status of those bulbs. This is the basic and easiest model of process monitoring.
    Alarm Annunciator is a centralized model, which gives audio visual signals for the faulty processes. Latest models of annunciators are based upon microprocessor or microcontroller circuitry, which ensures the maximum reliability as well as enhanced wide ranges of features and functionalities.
    Connection of Alarm Annunciator

    There are two types of connections for each annunciation system; they are input fault contacts and output relay changeover contacts. Input fault contacts are simple connection normally open (or NC Selectable) with respect to a common C contact. Usually these input fault contacts are potential free contacts. The logic is, if any fault contacts and the common contact C becomes short circuited by any means, the respective fascia or fault window will start blinking, and the output relay contact will changeover instantly.Suppose, you are using 8 windows annunciation system, which means you are monitoring 8 operations at a time, by the annunciation system. Let us think your fault 1 (F1) is assigned as over voltage alarm of motor 1 and your fault 2 (F2) is assigned as overheating of a motor 2 armature. You will connect an over voltage relay with motor 1 and a PTC thermistor relay with Motor 2, and the respective outputs (Normally open output, changes to close when faulty) of those relays will be connected across F1 (fault input) and C (common), and F2 (fault input) and C (common) of the annunciator system. Therefore, if the voltage of motor 1, is increased beyond the predefined safe level, the over voltage relay will operate and will make a closed loop between F1 and Common. So, the F1 window will start blinking which indicates the motor 1 is getting over voltage. At the same time, the annunciator relay will changeover, and if you connect a hooter previously with its output contacts, then the hooter will start alarming.Similarly, if the armature temperature of motor 2 is increased beyond the predefined safe level, then the PTC thermistor relay will changeover and will make a loop path between F2 and Common C of annunciation system. So, the F2 window will start blinking which indicates the motor 2 is getting over heated. At the same time, the annunciator relay will changeover, and the hooter connected with its contacts, it will start alarming. Basically, the annunciator output relay changeover is common, irrespective of any faults. A single hooter is used for all fault windows. An auxiliary AC/DC supply is necessary to operate the annunciator and in modern annunciators, there is also a window and connection provided for monitoring its own auxiliary supply.
    Modern Alarm Annunciators consist of a power supply unit SMPS, a programming unit CPU and other connections including fault contacts and facial display units. The blinking windows are generally acrylics, which are enlightened by LED with very low power consumption. Typically, annunciation effectively starts from 4 faults that is 4 windows, if the number of faults to be monitored is more than 64, it is preferable to install the programming unit CPU, power supply unit PSU and the display facial unit individually, which ensures the maximum accuracy and effectiveness.

    Rotor Earth Fault Protection of Alternator or Generator

    The rotor of an alternator is wound by field winding. Any single earth fault occurring on the field winding or in the exciter circuit is not a big problem for the machine. But if more than one earth fault occur, there may be a chance of short circuiting between the faulty points on the winding. This short circuited portion of the winding may cause unbalance magnetic field and subsequently mechanical damage may occur in the bearing of the machine due to unbalanced rotation. Hence it is always essential to detect the earth fault occurred on the rotor field winding circuit and to rectify it for normal operation of the machine. There are various methods available for detecting rotor earth fault of alternator or generator. But basic principle of all the methods is same and that is closing a relay circuit through the earth fault path. There are mainly three types of rotor earth fault protection scheme used for this purpose.
    1. Potentiometer method
    2. AC injection method
    3. DC injection method

    Let us discuss the methods one by one.Potentiometer Method of Rotor Earth Fault Protection in Alternator

    The scheme is very simple. Here, one resistor of suitable value is connected across the field winding as well as across exciter. The resistor is centrally tapped and connected to the ground via a voltage sensitive relay. As it is seen in the figure below, any earth fault in the field winding as well as exciter circuit closes the relay circuit through earthed path. At the same time the voltageappears across the relay due to potentiometer action of the resistor.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/potentiometer-method.gif[/IMG]This simple method of rotor earth fault protection of alternator has a big disadvantage. This arrangement can only sense the earth fault occurred in the any point except the center of the field winding. From the circuit it is also clear that in the case of earth fault on the center of the field circuit will not cause any voltage to be appeared across the relay. That means simple potentiometer methods of rotor earth fault protection, is blind to the faults at the center of the field winding. This difficulty can be minimized by using another tap on the resistor somewhere else from the center of the resistor via a push button. If this push button is pressed, the center tap is shift and the voltage will appear across the relay even in the event of central arc fault occurs on the field winding.AC Injection Method of Rotor Earth Fault Protection in Alternator

    Here, one voltage sensitive relay is connected at any point of the field and exciter circuit. Other terminal of the voltage sensitive relay is connected to the ground by a capacitor and secondary of one auxiliary transformer as shown in the figure below.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/ac-injection-method.gif[/IMG]Here, if any earth fault occurs in the field winding or in the exciter circuit, the relay circuit gets closed via earthed path and hence secondary voltage of the auxiliary transformer will appear across the voltage sensitive relay and the relay will be operated. The main disadvantage of this system is, there would always be a chance of leakage current through the capacitors to the exciter and field circuit. This may cause unbalancing in magnetic field and hence mechanical stresses in the machine bearings.Another disadvantage of this scheme is that as there is different source of voltage for operation of the relay, thus the protection of rotor is inactive when there is a failure of supply in the AC circuit of the scheme.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/ac-injection-method-2.gif[/IMG]
    DC Injection Method of Rotor Earth Fault Protection in Alternator

    The drawback of leakage current of AC injection method can be eliminated in DC Injection Method. Here, one terminal of DC voltage sensitive relay is connected with positive terminal of the exciter and another terminal of the relay is connected with the negative terminal of an external DC source. The external DC source is obtained by an auxiliary transformer with bridge rectifier. Here the positive terminal of bridge rectifier is grounded.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/alternator-rotor-protection.gif[/IMG]
    It is also seen from the figure below that at the event of any field earth fault or exciter earth fault, the positive potential of the external DC source will appear to the terminal of the relay which was connected to the positive terminal of the exciter. In this way the rectifier output voltage appears across the voltage relay and hence it is operated.

    Loss of Field or Excitation Protection of Alternator or Generator

    Loss of field or excitation can be caused in the generator due to excitation failure. In larger sized generator, energy for excitation is often taken from a separate auxiliary source or from a separately driven DC generator. The failure of auxiliary supply or failure of driving motor can also cause the loss of excitation in a generator. Failure of excitation that is failure of field system in the generator makes the generator run at a speed above the synchronous speed.
    In that situation the generator or alternator becomes an induction generator which draws magnetizing current from the system. Although this situation does not create any problem in the system immediately but over loading of the stator and overheating of the rotor due to continuous operation of the machine in this mode may create problems in the system in long-run. Therefore special care should be taken for rectifying the field or excitation system of the generator immediately after failure of that system. The generator should be isolated from rest of the system till the field system is properly restored.There are mainly two schemes available for protection against loss of field or excitation of a generator. In 1st scheme, we use an undercurrent relay connected in shunt with main field winding circuit. This relay will operate if the excitation current comes below its predetermined value. If the relay is to operate for complete loss of field along, it must have a setting lies well below the minimum excitation current value which can be 8 % of the rated full load current. Again when loss of field occurs due to failure of exciter but not due to problem in the field circuit (field circuit remains intact) there will be an induced current at slip frequency in the field circuit. This situation makes the relay to pick up and drop off as per slip frequency of the induced current in the field. This problem can be overcome in the following manner.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/loss-of-field-protection.gif[/IMG]In this case a setting of 5 % of normal of full load current is recommended. There is a normally closed contact attached with the undercurrent relay. This normally closed contact remains open as the relay coil is energized by shunted excitation current during normal operation of the excitation system. As soon as there is any failure of excitation system, the relay coil becomes de-energized and the normally closed contact closes the supply across the coil of timing relay T1.As the relay coil is energized, the normally open contact of this relay T 1 is closed. This contact closes the supply across another timing relay T 2 with an adjustable pickup time delay of 2 to 10 seconds. Relay T 1 is time delayed on drop off to stabilize scheme again slip frequency effect. Relay T 2 closes its contacts after the prescribed time delay to either shut down the set or initiate an alarm. It is time delayed on pickup to prevent spurious operation of the scheme during an external fault.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/loss-of-field-protection-2.gif[/IMG][IMG]https://electrical4u.com/electrical/wp-*******/uploads/loss-of-field-protection-3.gif[/IMG]For larger generator or alternator, we use a more sophisticated scheme for that purpose. For larger machines, it is recommended to trip the machine after a certain prescribed delay in presence of swing condition resulting from loss of field. In addition to that there must be subsequent load shedding to maintain stability of the system. In this scheme of protection, an automatic imposition of load shedding to the system is also inherently required if the field is not restored within the described time delay. The scheme comprises an offset mho relay, and an instantaneous under voltage relay. As we have said earlier that it is not always required to isolate the generator immediately in the event of loss of field, unless there is a significant disturb in system stability.We know that system voltage is the main indication of system stability. Therefore the offset mho relay is arranged to shut the machine down instantaneously when operation of generator is accompanied by a system voltage collapse. The drop in system voltage is detected by an under voltage relay which is set to approximately 70 % of normal rated system voltage. The offset mho relay is arranged to initiate load shedding to the system up to a safe value and then to initiate a master tripping relay after a predetermined time.

    Overvoltage Protection

    There are always a chance of suffering an electrical power system from abnormal over voltages. These abnormal over voltages may be caused due to various reason such as, sudden interruption of heavy load, lightening impulses, switching impulses etc. These over voltage stresses may damage insulation of various equipments and insulators of the power system. Although, all the over voltage stresses are not strong enough to damage insulation of system, but still these over voltages also to be avoided to ensure the smooth operation of electrical power system.
    These all types of destructive and non destructive abnormal over voltages are eliminated from the system by means of overvoltage protection.Voltage Surge

    The over voltage stresses applied upon the power system, are generally transient in nature. Transient voltage or voltage surge is defined as sudden sizing of voltage to a high peak in very short duration.The voltage surges are transient in nature, that means they exist for very short duration. The main cause of these voltage surges in power system are due to lightning impulses and switching impulses of the system. But over voltage in the power system may also be caused by, insulation failure, arcing ground and resonance etc.
    The voltage surges appear in the electrical power system due to switching surge, insulation failure, arcing ground and resonance are not very large in magnitude. These over voltages hardly cross the twice of the normal voltage level. Generally, proper insulation to the different equipment of power system is sufficient to prevent any damage due to these over voltages. But over voltages occur in the power system due to lightning is very high. If over voltage protection is not provided to the power system, there may be high chance of severe damage. Hence all over voltage protection devices used in power system mainly due to lightning surges.
    Let us discuss different causes of over voltages one by one.
    Switching Impulse or Switching Surge

    When a no load transmission line is suddenly switched on, the voltage on the line becomes twice of normal system voltage. This voltage is transient in nature. When a loaded line is suddenly switched off or interrupted, voltage across the line also becomes high enough current chopping in the system mainly during opening operation of air blast circuit breaker, causes over voltage in the system. During insulation failure, a live conductor is suddenly earthed. This may also caused sudden over voltage in the system. If emf wave produced by alternator is distorted, the trouble of resonance may occur due to 5th or higher harmonics. Actually for frequencies of 5th or higher harmonics, a critical situation in the system so appears, that inductive reactance of the system becomes just equal to capacitive reactance of the system. As these both reactance cancel each other the system becomes purely resistive. This phenomenon is called resonance and at resonance the system voltage may be increased enough.
    But all these above mentioned reasons create over voltages in the system which are not very high in magnitude.
    But over voltage surges appear in the system due to lightning impulses are very high in amplitude and highly destructive. The affect of lightning impulse hence must be avoided for over voltage protection of power system.Methods of Protection Against Lightning

    These are mainly three main methods generally used for protection against lightning. They are
    1. Earthing screen.
    2. Overhead earth wire.
    3. Lighning arrester or surge dividers.

    Earthing Screen

    Earthing screen is generally used over electrical substation. In this arrangement a net of GI wire is mounted over the sub-station. The GI wires, used for earthing screen are properly grounded through different sub-station structures. This network of grounded GI wire over electrical sub-station, provides very low resistance path to the ground for lightning strokes.This method of high voltage protection is very simple and economic but the main drawback is, it can not protect the system from travelling wave which may reach to the sub-station via different feeders.
    Overhead Earth Wire

    This method of over voltage protection is similar as earthing screen. The only difference is, an earthing screen is placed over an electrical sub-station, whereas, overhead earth wire is placed over electrical transmission network. One or two stranded GI wires of suitable cross-section are placed over the transmission conductors. These GI wires are properly grounded at each transmission tower. These overhead ground wires or earth wire divert all the lightning strokes to the ground instead of allowing them to strike directly on the transmission conductors.Lightning Arrester

    The previously discussed two methods, i.e. earthing screen and over-head earth wire are very suitable for protecting an electrical power system from directed lightning strokes but system from directed lightning strokes but these methods can not provide any protection against high voltage travelling wave which may propagate through the line to the equipment of the sub-station.
    The lightning arrester is a devices which provides very low impedance path to the ground for high voltage travelling waves.
    The concept of a lightning arrester is very simple. This device behaves like a nonlinear electrical resistance. The resistance decreases as voltage increases and vice-versa, after a certain level of voltage.
    The functions of a lightning arrester or surge dividers can be listed as below.
    1. Under normal voltage level, these devices withstand easily the system voltage as electrical insulator and provide no conducting path to the system current.
    2. On occurrence of voltage surge in the system, these devices provide very low impedance path for the excess charge of the surge to the ground.
    3. After conducting the charges of surge, to the ground, the voltage becomes to its normal level. Then lightning arrester regains its insulation properly and prevents regains its insulation property and prevents further conduction of current, to the ground.

    There are different types of lightning arresters used in power system, such as rod gap arrester, horn gap arrester, multi-gap arrester, expulsion type LA, value type LA. In addition to these the most commonly used lightning arrester for over voltage protection now-a-days gapless ZnO lightning arrester is also used.
    Motor Protection Circuit Breaker or MPCB

    Page De******ion: Learn what a motor protection circuit breaker is, motor protection circuit breaker working principle, chart, sizing (selection guide) and etc.
    Motor protection circuit breakers are a specialized type of electrical protection device that is designed specifically for electric motors, like their name implies. Electric motors have plenty of applications and are used to drive mechanical devices of all types, so it is very important to protect them adequately with MPCBs. The following are just a few examples of devices driven by electric motors in commercial and industrial buildings:
    • Rooftop air conditioners, chillers, compressors, heat pumps and cooling towers.
    • Extraction and injection fans, as well as air handling units.
    • Water pumping systems.
    • Elevators and other hoisting devices.
    • Industrial conveyor belts and other machinery used in manufacturing processes.

    In all of these industrial and commercial applications of electric motors, the MPCB has the key role of providing electrical protection.[IMG]https://www.electrical4u.com/e4u-*******/images/power-system/motor-protection-circuit-br.jpg[/IMG]
    What is a Motor Protection Circuit Breaker and what are its Functions?

    A motor protection circuit breaker, or MPCB, is a specialized electromechanical device that can be used with motor circuits of both 60 Hz and 50 Hz. It has several functions that allow it to provide a safe electrical supply for motors:
    • Protection against electrical faults such as short circuits, line-to-ground faults and line-to-line faults. The MPCB can interrupt any electrical fault that is below its breaking capacity.
    • Motor overload protection, when a motor draws electric current above its nameplate value for an extended period of time. Overload protection is normally adjustable in MPCBs.
    • Protection against phase unbalances and phase loss. Both conditions can severely damage a three-phase motor, so the MPCB will disconnect the motor in either case as soon as the fault is detected.
    • Thermal delay to prevent the motor from being turned back on immediately after an overload, giving the motor time to cool down. An overheated motor can be permanently damaged if it is turned back on.
    • Motor Circuit Switching – MPCBs are normally equipped with buttons or dials for this purpose.
    • Fault Signaling – Most models of motor protection circuit breakers have a LED display that is turned on whenever the MPCB has tripped. This is a visual indication for nearby personnel that a fault has occurred and the electric motor must not be connected again until the fault is addressed.
    • Automatic Reconnection – Some MPCB models allow a cool down time to be input in case there is an overload, after which the motor will restart automatically.
      Electric motors are expensive equipment, so the role of the motor protection circuit breaker is very important. If a motor is not protected correctly, it may be necessary to carry out costly repair works or even replace the equipment completely. An electric motor that is adequately protected with an MPCB will have a much longer service life.

    Motor Protection Circuit Breaker Working Principle

    The motor protection circuit breaker can be considered a subtype of thermal magnetic circuit breaker, but with additional functions that are specially designed to protect electric motors. The basic working principle is similar to all other circuit breakers.
    • Thermal protection is used to guard the electric motor against overload. It is based on an expanding and contracting contact that disconnects the motor if excessive current is detected. It is very important to know that thermal protection has a delayed response, to allow the high inrush currents when a motor starts. However, if the motor is unable to start for some reason, thermal protection will trip in response to the extended inrush current.
    • Magnetic protection is used when there is a short circuit, line fault, or other high current electric fault. Unlike thermal protection, magnetic protection is instantaneous; to immediately disconnect the dangerous fault currents.
    • The main difference between the MPCB and other circuit breakers is that the MPCB can provide protection against phase unbalance and phase loss. Three-phase circuit motors require three live conductors with balanced voltages in order to operate effectively. An unbalance of more than 2% will be detrimental to the motor’s service life. If one of the phase voltages is suddenly lost, the effect is even more damaging because the motor will keep on running with only two phases. The motor protection circuit breaker is capable of detecting these conditions by measuring the differences among phase voltages, and disconnects the motor immediately when they occur. It is important to note that phase current unbalance is normal in three-phase systems that power separate single-phase loads, but is unacceptable when the three-phase circuit powers an electric motor.
    • MPCBs are also equipped with a manual interruption mechanism, allowing disconnection of electric motors for replacement or maintenance.
    • Motor protection circuit breakers are available in a wide variety of current ratings, and one of their best features is that many models allow the current rating to be adjusted. This means that the same MCPB can be configured to protect motors of different capacities.

    Asynchronous Motor Protection

    Most motors used in the industry are a asynchronous motors, also known as squirrel-cage induction motors. These motors use three-phase power to create a rotating magnetic field, which in turn magnetizes the rotor an creates rotational movement. When designing the electrical protection for an asynchronous motor and selecting motor protection circuit breakers, there are some very important factors to consider that aren’t present when protecting other types of electrical circuits.
    • Asynchronous motors draw a very high inrush current during startup, because they must establish a rotating magnetic field. This current can reach values of 500% to 800% of the rated value for a few fractions of a second. For this reason, the MPCB magnetic protection trips at values greater than 10 times the rated current, unlike some types of miniature circuit breakerswhich trip at values as low as 3 times rated current. In these cases, using a circuit breaker other than an MPCB will not even allow starting the motor before the magnetic protection trips. In order to reduce the inrush current, a very common practice is to complement the motor protection circuit breaker with a reduced voltage motor starter.
    • Asynchronous motors require the three phase conductors to have a balanced voltage in order to operate properly. If the phase conductors have an unbalance greater than 2%, the motor will suffer damage over time and will have a reduced service life. The electric motor will also tend to overheat, causing additional energy expenses as waste heat. For this reason, a motor circuit breaker must be able to detect phase imbalance and disconnect the motor accordingly.
    • If one of the phases is disconnected completely, the motor will keep operating but the current in the remaining two phases will rise above the rated value due to the electrical unbalance, and will probably burn the motor’s windings. For this reason, motor protectors must trip immediately as soon as phase unbalance or phase loss is detected. This is normally achieved by measuring the differences in current among the phase conductors. If one of the phase currents rises or drops considerably compared with the other two, it is indicative of unbalance. Likewise, if one of the phase currents drops to zero while the other two remain, a phase loss has occurred.

    Then, what kinds of breakers can be used for the protection of asynchronous motors? Manufacturers generally offer three different motor protection circuit breakers, available for a wide range of voltages and currents, in order to meet most asynchronous motor protection needs.It is very common to complement motor protection circuit breakers with a contactor to allow automatic control of motor startup and disconnection. The system might also include and under-voltage protection device, which disconnects the motor in case the system voltage drops considerably below the rated value.
    Motor Protection Circuit Breaker Sizing (Selection Guide)

    The two main factors that determine the adequate motor protection circuit breaker size are the nameplate voltage and nameplate current, of the motor itself.
    • The MPCB voltage rating must match the nameplate voltage of the motor. Normally, motor protection circuit breakers can be used in a wide variety of voltage ratings such as 230 V, 380 V, 415 V, 440 V, 500 V, and 660 V AC.
    • Once the voltage is known, it is necessary to check the nameplate current of the electric motor. It is important to note that the actual operating current may be lower than nameplate current, especially if the motor isn’t fully loaded. However, the MPCB must always be selected according to nameplate current value in order to allow the inrush current when a motor starts. For example, a motor with a nameplate current of 20 amperes might draw a much lower current during part-load operation, but the MPCB must be selected according to the rated value of 20 amperes, or it might trip if the motor is used at full load.
    • Motor protection circuit breakers can then be calibrated to the exact current value that is adequate for the electric motor being protected. They typically have an adjustment range. For example, a MPCB rated at 32 amperes might be usable for motors with rated currents as low as 22 amperes. This is very useful if an electric motor is replaced with a more efficient model that requires a lower current, since it will not be necessary to replace the motor breaker.
    • Even if a motor protection circuit breaker is sized correctly according to the electric motor being protected, it is also important to use adequate wiring. In order to provide adequate protection, the wire must be able to conduct the rated current safely. An undersized wire will overheat, the insulation will melt, and electric faults may occur even with a breaker installed.

    Motor Protection Circuit Breaker Specification Chart

    MPCB manufacturers typically provide charts where the technical specifications of the circuit breaker are presented, in order to simplify the selection process. The following chart, provided as an example, is for the motor circuit breaker SGV2-ME model manufactured by CGSL. The current values at which the thermal and magnetic protections operate are displayed in the thermal release and magnetic release columns. Before installing a MPCB, it is very important to verify that voltage and current ratings are compatible with the motor being protected.[IMG]https://www.electrical4u.com/e4u-*******/images/power-system/table.png[/IMG]Conclusions of Motor Protection Circuit Breaker

    Motor protection circuit breakers have a very important role in electrical safety, since the motors they protect have a wide variety of applications in commercial buildings and industry.
    Asynchronous motors, the most common type of electric motor in industrial and commercial settings, has special protection requirements that can only be met by a motor protection circuit breaker. It is also possible to complement MPCB with other protection or automation devices such as under-voltage protection, timers, and reduced voltage motor starters.
    Adequate selection of the MPCB is key in order to provide reliable motor protection. An undersized MPCB will not even allow the motor to start, while an oversized MPCB might be unable to detect over-current conditions for the electric motor being protected.






























    لاإله إلا الله محمد رسول الله

  2. #2
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
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    رد: Protection System in Power System

    Stator Earth Fault Protection of Alternator

    This is to be noted that, the star point or neutral point of stator winding of an alternator is grounded through an impedance to limit the ground fault current. Reduced ground fault current causes less damage to the stator core and winding during ground or earth fault. If the ground impedance is made quite high, the ground fault current may become even less than normal rated current of the generator. If so, the sensitivity of phase relays becomes low, even they may fail to trip during fault. For example, a current lower than rated current makes it difficult to operate differential relays for ground fault. In that case, a sensitive ground/earth fault relay is used in addition to the differential protection of alternator. What type of relaying arrangement will be engaged in stator earth fault protection of alternator depends upon the methods of stator neutral earthing. In the case of resistance neutral earthing the neutral point of stator winding is connected to the ground through a resistor.Here, one current transformer is connected across the neutral and earth connection of the alternator. Now one protective relay is connected across the current transformer secondary. The alternator can feed the power system in two ways, either it is directly connected to the substation bus bar or it is connected to substation via one star delta transformer. If the generator is connected directly to the substation bus bars, the relay connected across the CT secondary, would be an inverse time relay because here, relay coordination is required with other fault relays in the system. But when the stator of the alternator is connected to the primary of a star Delta transformer, the fault is restricted in between stator winding and transformer primary winding, therefore no coordination or discrimination is required with other earth fault relays of the system. That is why; in this case instantaneous armature attracted type relay is preferable to be connected across the CT secondary.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/stator-earth-fault-protection.gif[/IMG]It is should be noted that, 100 % of the stator winding cannot be protected in resistance neutral earthing system. How much percentage of stator winding would be protected against earth fault, depends upon the value of earthing resistance and the setting of relay.
    The resistance grounding of stator winding can also be made by using a distribution transformer instead of connecting a resistor directly to the neutral path of the winding. Here, primary of a distribution transformer is connected across earth and neutral point of the stator winding.
    Secondary of the transformer is loaded by a suitable resistor and one over voltage relay is also connected across the secondary of the transformer. The maximum allowable earth fault current is determined by the size of the transformer and the value of loading register R. This resistance is connected with the secondary, reflects to the primary of the transformer by the square of the turns ratio, thereby adding resistance to the neutral to ground path of the stator winding.


    Inter Turn Fault Protection of Stator Winding of Generator

    Inter turn stator winding fault can easily be detected by stator differential protection or stator earth fault protection. Hence, it is not very essential to provide special protection scheme for inter turn faults occurred in stator winding. This type of faults is generated if the insulation between conductors (with different potential) in the same slot is punctured. This type of fault rapidly changes to earth fault.
    The high voltage generator contains a large number of conductors per slot in the stator winding hence, in these cases the additional inter turn fault protection of the stator winding may be essential. Moreover in modern practice, inter turn protection is becoming essential for all large generating units.Several methods can be adopted for providing inter turn protection to the stator winding of generator. Cross differential methods is most common among them. In this scheme the winding for each phase is divided into two parallel paths.
    Each of the paths is fitted with identical current transformer. The secondary of these current transformers are connected in cross. The current transformer secondary's are cross connected because currents at the primary of both CTs are entering unlike the case of differential protection of transformer where current entering from one side and leaving to other side of the transformer. The differential relay along with series stabilizing resistor are connected across the CT secondary loop as shown in the figure. If any inter turn fault occurs in any path of the stator winding, there will be an unbalanced in the CT secondary circuits thereby actuates 87 differential relay. Cross differential protection scheme should be applied in each of the phases individually as shown.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/stator-inter-turn-protection.gif[/IMG]An alternative scheme of inter turn fault protection of stator winding of generator is also used. This scheme provides complete protection against internal faults of all synchronous machines irrespective of the type of the winding employed or the kind of methods for connection. An internal fault in the stator winding generates second harmonic current, included in the field winding and exciter circuits of the generator. This current can be applied to a sensitive polarized relay via a CT and filter circuit.
    The scheme operation is controlled by a direction of negative phase sequence relay, in order to prevent operation during external unbalanced faults or asymmetrical load conditions. Should there be any asymmetry external to the generator unit zone, the negative phase sequence relay prevents a complete shutdown, only allowing the main circuit breaker to be tripped, to prevent the rotor damage due to the over rating effects of second harmonic currents.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/stator-inter-turn-protection-3.gif[/IMG]Small and Large Motor Protection Scheme

    The electric motor is most essential drive in modern era of industrialization. From fractional hp AC motor used for different home appliances to giant synchronous motor and induction motor of up to 10,000 hp are used for different industrial applications. It should be protected against different electrical and mechanical faults for serving their purposes smoothly. The motor characteristics must be very carefully considered in selecting the right motor protection scheme.
    The abnormalities in motor or motor faults may appear due to mainly two reasons -
    1. Conditions imposed by the external power supply network,
    2. Internal faults, either in the motor or in the driven plant.

    Unbalanced supply voltages, under-voltage, reversed phase sequence and loss of synchronism (in the case of synchronous motor) come under former category. The later category includes bearing failures, stator winding faults, motor earth faults and overload etc.
    The degree of motor protection system depends on the costs and applications of the electrical motor.
    Small Motor Protection Scheme

    Generally motors up to 30 hp are considered in small category. The small motor protection in this case is arranged by HRC fuse, bi****llic relay and under voltage relay - all assembled into the motor contactor - starter itself. Most common cause of motor burn outs on LV fuse protected system is due to single phasing. This single phasing may remain undetected even if the motors are protected by conventional bi****llic relay. It can not be detected by a set of voltage relays connected across the lines. Since, even when one phase is dead, the motor maintains substantial back emf on its faulty phase terminal and hence voltageacross the voltage relay is prevented from dropping - off.
    The difficulties of detecting single phasing can be overcome by employing a set of three current operated relays as shown in the small motor protection circuit given below.
    The current operated relays are very simple instantaneous relays. There are mainly two parts in this relay one is a current coil and other is one or more normally open contacts (NO Contacts). The NO contacts are operated by the mmf of the current coil. This relay is connected in series with each phase of the supply and backup by HRC fuse. When the electrical motor starts and runs then the supply current passes through the current coil of the protective relay. The mmf of the current coil makes the NO contacts closed. If suddenly a single phasing occurs the corresponding current through the current coil will fall and the contacts of the corresponding relay will become to its normal open position. The NO contacts of the all three relays are connected in series to hold - in the motor contractor. So if any one relay contact opens, results to release of motor contractor and hence motor will stop running.
    Large Motor Protection

    Large motor especially induction motors require protection against-

    1. Motor bearing failure,
    2. Motor over heating,
    3. Motor winding failure,
    4. Reverse motor rotation.

    Motor Bearing Failure

    Ball and roller bearings are used for the motor up to 500 hp and beyond this size sleeve bearings are used. Failure of ball or roller bearing usually causes the motor to a standstill very quickly. Due to sudden mechanical jamming in motor bearing, the input current of the motor becomes very high. Current operated protection, attached to the input of the motor can not serve satisfactorily. Since this motor protection system has to be set to override the high motor starting current. The difficulty can be overcome by providing thermal over load relay. As the starting current of the motor is high but exists only during starting so for that current there will be no over heating effect. But over current due to mechanical jamming exists for longer time hence there will be an over heating effect. So stalling motor protection can be offered by the thermal overload relay. Stalling protection can also be provided by separate definite time over current relay which is operated only after a certain predefined time if over current persists beyond that period. In the case of sleeve bearing, a temperature sensing device embedded in the bearing itself. This scheme of motor protection is more reliable and sensitive to motor bearing failure since the thermal withstand limit of the motor is quite higher than that of bearing. If we allow the bearing over heating and wait for motor thermal relay to trip, the bearing may be permanently damaged. The temperature sensing device embedded in the bearing stops the motor if the bearing temperature rises beyond its predefined limit.
    Motor Over Heating

    The main reason of motor over heating that means over heating of motor winding is due to either of mechanical over loading, reduced supply voltage, unbalanced supply voltage and single phasing. The over heating may cause deterioration of insulation life of motor hence it must be avoided by providing proper motor protection scheme. To avoid over heating, the motor should be isolated in 40 to 50 minutes even in the event of small overloads of the order of 10 %. The protective relay should take into account the detrimental heating effects on the motor rotor due to negative sequence currents in the stator arising out of unbalance in supply voltage. The motor should also be protected by instantaneous motor protection relay against single phasing such as a stall on loss of one phase when running at full load or attempting to start with only two of three phases alive.
    Motor Winding Failure

    The motor protection relay should should have instantaneous trip elements to detect motor winding failure such as phase to phase and phase to earth faults. Preferably phase to phase fault unit should be energized from positive phase sequence component of the motor current and another instantaneous unit connected in the residual circuit of the current transformers be used for earth faults protection.
    Reverse Motor Rotation

    Specially in the case of conveyor belt, the reverse motor rotation must be avoided. The reverse rotation during starting can be caused due to inadvertent reversing of supply phases. A comprehensive motor protection relay with an instantaneous negative sequence unit will satisfy this requirement. If such relay has not been provided, a watt-meter type relay can be employed.
    NB: However, we have to provide some additional motor protection system for synchronous motor which is discussed in details in synchronous motor protection topic.




    Motor Thermal Overload Protection

    For understanding motor thermal overload protection in induction motor we can discuss the operating principle of three phase induction motor. There is one cylindrical stator and a three phase winding is symmetrically distributed in the inner periphery of the stator. Due to such symmetrical distribution, when three phase power supply is applied to the stator winding, a rotating magnetic field is produced. This field rotates at synchronous speed. The rotor is created in induction motor mainly by numbers solid copper bars which are shorted at both ends in such a manner that they form a cylinder cage like structure. This is why this motor is also referred as squirrel cage induction motor. Anyway let's come to the basic point of three phase induction motor - which will help us to understand clearly about motor thermal overload protection.
    As the rotating magnetic flux cuts each of the bar conductor of rotor, there will be an induced circulating current flowing through the bar conductors. At starting the rotor is stand still and stator field is rotating at synchronous speed, the relative motion between rotating field and rotor is maximum.Hence, the rate of cuts of flux with rotor bars is maximum, the induced current is maximum at this condition. But as the cause of induced current is, this relative speed, the rotor will try to reduce this relative speed and hence it will start rotating in the direction of rotating magnetic field to catch the synchronous speed. As soon as the rotor will come to the synchronous speed this relative speed between rotor and rotating magnetic field becomes zero, hence there will not be any further flux cutting and consequently there will not be any induced current in the rotor bars. As the induced current becomes zero, there will not be any further need of maintaining zero relative speed between rotor and rotating magnetic field hence rotor speed falls. As soon as the rotor speed falls the relative speed between rotor and rotating magnetic field again acquires a non zero value which again causes induced current in the rotor bars then rotor will again try to achieve the synchronous speed and this will continue till the motor is switch on. Due to this phenomenon the rotor will never achieve the synchronous speed as well as it will never stop running during normal operation. The difference between the synchronous speed with rotor speed in respect of synchronous speed, is termed as slip of induction motor.The slip in a normally running induction motor typically varies from 1 % to 3 % depending upon the loading condition of the motor. Now we will try to draw speed current characteristics of induction motor – let’s have an example of large boiler fan.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/2013/04/motor-starting-current.gif[/IMG]In the characteristic Y axis is taken as time in second, X axis is taken as % of stator current. When rotor is stand still that is at starting condition, the slip is maximum hence the induced current in the rotor is maximum and due to transformation action, stator will also draw a heavy current from the supply and it would be around 600 % of the rated full load stator current. As the rotor is being accelerated the slip is reduced, consequently the rotor current hence stator current falls to around 500 % of the full load rated current within 12 seconds when the rotor speed attains 80% of synchronous speed. After that the stator current falls rapidly to the rated value as the rotor reaches its normal speed.Now we will discuss about thermal over loading of electrical motor or over heating problem of electric motor and the necessity of motor thermal overload protection.
    Whenever we think about the overheating of a motor, the first thing strikes in our mind is over loading. Due to mechanical over loading of the motor draws higher current from the supply which leads to excessive over heating of the motor. The motor can also be excessively over heated if the rotor is mechanically locked i.e. becomes stationary by any external mechanical force. In this situation the motor will draw excessively high current from the supply which also leads to thermal over loading of electrical motor or excessive over heating problem. Another cause of overheating is low supply voltage. As the power id drawn by the motor from the supply depends upon the loading condition of the motor, for lower supply voltage, motor will draw higher current from mains to maintain required torque. Single phasing also causes thermal over loading of motor. When one phase of the supply is out of service, the remaining two phases draw higher current to maintain required load torque and this leads to overheating of the motor. Unbalance condition between three phases of supply also causes over heating of the motor winding, as because unbalance system results to negative sequence current in the stator winding. Again, due to sudden loss and reestablish of supply voltagemay cause excessive heating of the motor. Since due to sudden loss of supply voltage, the motor is de-accelerated and due to sudden reestablishment of voltage the motor is accelerated to achieve its rated speed and hence for that motor draws higher current form the supply.
    As the thermal over loading or over heating of the motor may lead to insulation failure and damage of winding, hence for proper motor thermal overload protection, the motor should be protected against the following conditions

    1. Mechanical over loading,
    2. Stalling of motor shaft,
    3. Low supply voltage,
    4. Single phasing of supply mains,
    5. Unbalancing of supply mains,
    6. Sudden Loss and rebuilding of supply voltage.

    The most basic protection scheme of the motor is thermal over load protection which primarily covers the protection of all the above mentioned condition. To understand the basic principle of thermal over load protection let’s examine the schematic diagram of basic motor control scheme.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/2013/04/motor-protection3.gif[/IMG]In the figure above, when START push is closed, the starter coil is energized through the transformer. As the starter coil is energized, normally open (NO) contacts 5 are closed hence motor gets supply voltage at its terminal and it starts rotating. This start coil also closes contact 4 which makes the starter coil energized even the START push button contact is released from its close position. To stop the motor there are several normally closed (NC) contacts in series with the starter coil as shown in the figure. One of them is STOP push button contact. If the STOP push button is pressed, this button contact opens and breaks the continuity of the starter coil circuit consequently makes the starter coil de-energized. Hence the contact 5 and 4 come back to their normally open position. Then, in absence of voltage at motor terminals it will ultimately stop running. Similarly any of the other NC contacts (1, 2 & 3) connected in series with starter coil if open; it will also stop the motor. These NC contacts are electrically coupled with various protection relays to stop operation of the motor in different abnormal conditions.
    Let’s look at the thermal over load relay and its function in motor thermal overload protection.
    The secondary of the CTs in series with motor supply circuit, are connected with a bi****llic strip of the thermal over load relay (49). As shown in the figure below, when current through the secondary of any of the CTs, crosses it’s predetermined values for a predetermined time, the bi-****llic strip is over heated and it deforms which ultimately causes to operate the relay 49. As soon as the relay 49 is operated, the NC contacts 1 and 2 are opened which de-energizes the starter coil and hence stop the motor.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/2013/04/motor-protection-3.gif[/IMG]Another thing we have to remember during providing motor thermal overload protection. Actually every motor does have some predetermined overload tolerance value. That means every motor may run beyond its rated load for a specific allowable period depending on its loading condition. How long a motor can run safely for a particular load is specified by the manufacturer. The relation between different loads on motor and corresponding allowable periods for running the same in safe condition is referred as thermal limit curve of the motor. Let’s look at the curve of a particular motor, given below.[IMG]https://www.electrical4u.com/wp-*******/uploads/2013/04/thermal-limit-curve.png[/IMG]Here Y axis or vertical axis represents the allowable time in seconds and X axis or horizontal axis represents percentage of overload. Here it is clear from the curve that, motor can run safely without any damage due to overheating for prolonged period at 100 % of the rated load. It can run safely 1000 seconds at 200 % of normal rated load. It can run safely 100 seconds at 300 % of normal rated load. It can run safely 15 seconds at 600 % of normal rated load. The upper portion of the curve represents the normal running condition of the rotor and the lower most portion represents the mechanical locked condition of the rotor.
    Now the operating time Vs actuating current curve of the chosen thermal over load relay should be situated below the thermal limit curve of the motor for satisfactory and safe operation. Let’s have a discussion on more details-[IMG]https://www.electrical4u.com/wp-*******/uploads/2013/04/thermal-overload-relay-curv.jpg[/IMG]Remember the characteristics of starting current of the motor – During start up of the induction motor, the stator current goes beyond 600 % of normal rated current but it stays up to 10 to 12 seconds after that stator current suddenly falls to normal rated value. So if the thermal overload relay is operated before that 10 to 12 seconds for the current 600 % of normal rated then the motor cannot be started. Hence, it can be concluded that the operating time Vs actuating current curve of the chosen thermal over load relay should be situated below the thermal limit curve of the motor but above the starting current characteristics curve of the motor. Probable position of the thermal current relay characteristics is bounded by these two said curves as shown in the graph by highlighted area.Another thing has to be remembered during choosing of thermal overload relay. This relay is not an instantaneous relay. It has a minimum delay in operation as the bi****llic strip required a minimum time to be heated up and deformed for maximum value of operating current. From the graph it is found that the thermal relay will be operated after 25 to 30 seconds if either the rotor is suddenly mechanically blocked or motor is fail to start. At this situation the motor will draw a huge current from the supply. If the motor is not isolated sooner, severer damage may occur.[IMG]https://electrical4u.com/electrical/wp-*******/uploads/2013/04/motor-protection-4.gif[/IMG]This problem is overcome by providing time over current relay with high pickup. The time current characteristics of these over current relays are so chosen that for lower value of over load, the relay will not operate since thermal overload relay will be actuated before it. But for higher value of overload and for blocked rotor condition time over load relay will be operated instead of thermal relay because former will actuate much before the latter.
    Hence both the bi****llic over load relay and time over current relay are provided for complete motor thermal overload protection.
    There is one main disadvantage of bi****llic thermal over load relay, as the rate of heating and cooling of bi-****l is affected by ambient temperature, the performance of the relay may differ for different ambient temperatures. This problem can be overcome by using RTD or resistance temperature detector. The bigger and more sophisticated motors are protected against thermal over load more accurately by using RTD. In stator slots, RTDs are placed along with stator winding. Resistance of the RTD changes with changing temperature and this changed resistive value is sensed by a Wheatstone bridge circuit.
    This motor thermal overload protection scheme is very simple. RTD of stator is used as one arm of balanced Wheatstone bridge. The amount of current through the relay 49 depends upon the degree of unbalancing of the bridge. As the temperature of the stator winding is increased, the electrical resistance of the detector increases which disturbs the balanced condition of the bridge. As a result current start flowing through the relay 49 and the relay will be actuated after a predetermined value of this unbalanced current and ultimately starter contact will open to stop the supply to the motor.




    لاإله إلا الله محمد رسول الله

  3. #3
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
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    مكة المكرمة
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    1,401

    رد: Protection System in Power System

    Electrical Fault Calculation | Positive Negative Zero Sequence Impedance

    Before applying proper electrical protection system, it is necessary to have through knowledge of the conditions of electrical power system during faults. The knowledge of electrical fault condition is required to deploy proper different protective relays in different locations of electrical power system.
    Information regarding values of maximum and minimum fault currents, voltages under those faults in magnitude and phase relation with respect to the currents at different parts of power system, to be gathered for proper application of protection relaysystem in those different parts of the electrical power system. Collecting the information from different parameters of the system is generally known as electrical fault calculation .
    Fault calculation broadly means calculation of fault current in any electrical power system. There are mainly three steps for calculating faults in a system.
    1. Choice of impedance rotations.
    2. Reduction of complicated electrical power system network to single equivalent impedance.
    3. Electrical fault currents and voltages calculation by using symmetrical component theory.

    Impedance Notation of Electrical Power System

    If we look at any electrical power system, we will find, these are several voltage levels. For example, suppose a typical power system where electrical power is generated at 6.6 kV then that 132 kV power is transmitted to terminal substation where it is stepped down to 33 kV and 11 kV levels and this 11 kV level may further step down to 0.4 kv. Hence from this example it is clear that a same power system network may have different voltage levels. So calculation of fault at any location of the said system becomes much difficult and complicated it try to calculate impedance of different parts of the system according to their voltage level. This difficulty can be avoided if we calculate impedance of different part of the system in reference to a single base value. This technique is called impedance notation of power system. In other wards, before electrical fault calculation, the system parameters, must be referred to base quantities and represented as uniform system of impedance in either ohmic, percentage, or per unit values.
    Electrical power and voltage are generally taken as base quantities. In three phase system, three phase power in MVA or KVA is taken as base power and line to line voltage in KV is taken as base voltage. The base impedance of the system can be calculated from these base power and base voltage, as follows,
    Per unit is an impedance value of any system is nothing but the radio of actual impedance of the system to the base impedance value.
    Percentage impedance value can be calculated by multiplying 100 with per unit value.
    Again it is sometimes required to convert per unit values referred to new base values for simplifying different electrical fault calculations. In that case,
    The choice of impedance notation depends upon the complicity of the system. Generally base voltage of a system is so chosen that it requires minimum number of transfers.
    Suppose, one system as a large number of 132 KV over head lines, few numbers of 33 KV lines and very few number of 11 KV lines. The base voltage of the system can be chosen either as 132 KV or 33 KV or 11 KV, but here the best base voltages 132 KV, because it requires minimum number of transfer during fault calculation.
    Network Reduction

    After choosing the correct impedance notation, the next step is to reduce network to a single impedance. For this first we have to convert the impedance of all generators, lines, cables, transformer to a common base value. Then we prepare a schematic diagram of electrical power system showing the impedance referred to same base value of all those generators, lines, cables and transformers. The network then reduced to a common equivalent single impedance by using star/delta transformations. Separate impedance diagrams should be prepared for positive, negative and zero sequence networks.
    There phase faults are unique since they are balanced i.e. symmetrical in three phase, and can be calculated from the single phase positive sequence impedance diagram. Therefore three phase fault current is obtained by,
    Where, I f is the total three phase fault current, v is the phase to neutral voltage z 1 is the total positive sequence impedance of the system; assuming that in the calculation, impedance are represented in ohms on a voltage base.
    Symmetrical Component Analysis

    The above fault calculation is made on assumption of three phase balanced system. The calculation is made for one phase only as the current and voltage conditions are same in all three phases. When actual faults occur in electrical power system, such as phase to earth fault, phase to phase fault and double phase to earth fault, the system becomes unbalanced means, the conditions of voltages and currents in all phases are no longer symmetrical. Such faults are solved by symmetrical component analysis. Generally three phase vector diagram may be replaced by three sets of balanced vectors. One has opposite or negative phase rotation, second has positive phase rotation and last one is co-phasal. That means these vectors sets are described as negative, positive and zero sequence, respectively.
    The equation between phase and sequence quantities are,
    Therefore,
    Where all quantities are referred to the reference phase r. Similarly a set of equations can be written for sequence currents also. From, voltage and current equations, one can easily determine the sequence impedance of the system. The development ofsymmetrical component analysis depends upon the fact that in balanced system of impedance, sequence currents can give rise only to voltage drops of the same sequence. Once the sequence networks are available, these can be converted to single equivalent impedance. Let us consider Z1, Z2 and Z0 are the impedance of the system to the flow of positive, negative and zero sequence current respectively. For earth fault
    Phase to phase faults
    Double phase to earth faults
    Three phase faults
    If fault current in any particular branch of the network is required, the same can be calculated after combining the sequence components flowing in that branch. This involves the distribution of sequence components currents as determined by solving the above equations, in their respective network according to their relative impedance. Voltages it any point of the network can also be determine once the sequence component currents and sequence impedance of each branch are known.
    Sequence Impedance

    Positive Sequence Impedance

    The impedance offered by the system to the flow of positive sequence current is called positive sequence impedance.
    Negative Sequence Impedance

    The impedance offered by the system to the flow of negative sequence current is called negative sequence impedance.
    Zero Sequence Impedance

    The impedance offered by the system to the flow of zero sequence current is known as zero sequence impedance. In previous fault calculation, Z1, Z2 and Z0 are positive, negative and zero sequence impedance respectively. The sequence impedancevaries with the type of power system components under consideration:-
    1. In static and balanced power system components like transformer and lines, the sequence impedance offered by the system are the same for positive and negative sequence currents. In other words, the positive sequence impedance and negative sequence impedance are same for transformers and power lines.
    2. But in case of rotating machines the positive and negative sequence impedance are different.
    3. The assignment of zero sequence impedance values is a more complex one. This is because the three zero sequence current at any point in a electrical power system, being in phase, do not sum to zero but must return through the neutral and /or earth. In three phase transformer and machine fluxes due to zero sequence components do not sum to zero in the yoke or field system. The impedance very widely depending upon the physical arrangement of the magnetic circuits and winding.
      1. The reactance of transmission lines of zero sequence currents can be about 3 to 5 times the positive sequence current, the lighter value being for lines without earth wires. This is because the spacing between the go and return(i.e. neutral and/or earth) is so much greater than for positive and negative sequence currents which return (balance) within the three phase conductor groups.
      2. The zero sequence reactance of a machine is compounded of leakage and winding reactance, and a small component due to winding balance (depends on winding tritch).
      3. The zero sequence reactance of transformers depends both on winding connections and upon construction of core.

    4. External and Internal Faults in Transformer

    5. It is essential to protect high capacity transformers against external and internal electrical faults.External Faults in Power Transformer

      External Short Circuit of Power Transformer

      The short - circuit may occur in two or three phases of electrical power system. The level of fault current is always high enough. It depends upon the voltage which has been short-circuited and upon the impedance of the circuit up to the fault point. The copper loss of the fault feeding transformer is abruptly increased. This increasing copper loss causes internal heating in the transformer. Large fault current also produces severe mechanical stresses in the transformer. The maximum mechanical stresses occur during first cycle of symmetrical fault current.High Voltage Disturbance in Power Transformer

      High voltage disturbance in power transformer are of two kinds,
      1. Transient Surge Voltage
      2. Power Frequency Over Voltage

      Transient Surge Voltage

      High voltage and high frequency surge may arise in the power system due to any of the following causes,
      • Arcing ground if neutral point is isolated.
      • Switching operation of different electrical equipment.
      • Atmospheric Lightening Impulse.

      Whatever may be the causes of surge voltage, it is after all a traveling wave having high and steep wave form and also having high frequency. This wave travels in the electrical power system network, upon reaching in the power transformer, it causes breakdown of the insulation between turns adjacent to line terminal, which may create short circuit between turns.Power Frequency Over Voltage

      There may be always a chance of system over voltage due to sudden disconnection of large load. Although the amplitude of this voltage is higher than its normal level but frequency is same as it was in normal condition. Over voltage in the system causes an increase in stress on the insulation of transformer. As we know that, voltage , increased voltage causes proportionate increase in the working flux. This therefore causes, increased in iron loss and proportionately large increase in magnetizing current. The increase flux is diverted from the transformer core to other steel structural parts of the transformer. Core bolts which normally carry little flux, may be subjected to a large component of flux diverted from saturated region of the core alongside. Under such condition, the bolt may be rapidly heated up and destroys their own insulation as well as winding insulation.Under Frequency Effect in Power Transformer

      As, voltage as the number of turns in the winding is fixed.
      Therefore,From, this equation it is clear that if frequency reduces in a system, the flux in the core increases, the effect are more or less similar to that of the over voltage.Internal Faults in Power Transformer

      The principle faults which occurs inside a power transformer are categorized as,
      1. Insulation breakdown between winding and earth
      2. Insulation breakdown in between different phases
      3. Insulation breakdown in between adjacent turns i.e. inter - turn fault
      4. Transformer core fault

      Internal Earth Faults in Power Transformer

      Internal Earth Faults in a Star Connected Winding with Neutral Point Earthed through an Impedance

      In this case the fault current is dependent on the value of earthing impedance and is also proportional to the distance of the fault point from neutral point as the voltage at the point depends upon, the number of winding turns come across neutral and fault point. If the distance between fault point and neutral point is more, the number of turns under this distance is also more, hence voltage across the neutral point and fault point is high which causes higher fault current. So, in few words it can be said that, the value of fault current depends on the value of earthing impedance as well as the distance between the faulty point and neutral point. The fault current also depends up on leakage reactance of the portion of the winding across the fault point and neutral. But compared to the earthing impedance,it is very low and it is obviously ignored as it comes in series with comparatively much higher earthing impedance.Internal Earth Faults in a Star Connected Winding with Neutral Point Solidly Earthed

      In this case, earthing impedance is ideally zero. The fault current is dependent up on leakage reactance of the portion of winding comes across faulty point and neutral point of transformer. The fault current is also dependent on the distance between neutral point and fault point in the transformer. As said in previous case the voltage across these two points depends upon the number of winding turn comes across faulty point and neutral point. So in star connected winding with neutral point solidly earthed, the fault current depends upon two main factors, first the leakage reactance of the winding comes across faulty point and neutral point and secondly the distance between faulty point and neutral point. But the leakage reactance of the winding varies in complex manner with position of the fault in the winding. It is seen that the reactance decreases very rapidly for fault point approaching the neutral and hence the fault current is highest for the fault near the neutral end. So at this point, the voltage available for fault current is low and at the same time the reactance opposes the fault current is also low, hence the value of fault current is high enough. Again at fault point away from the neutral point, the voltage available for fault current is high but at the same time reactance offered by the winding portion between fault point and neutral point is high. It can be noticed that the fault current stays a very high level throughout the winding. In other word, the fault current maintain a very high magnitude irrelevant to the position of the fault on winding.Internal Phase to Phase Faults in Power Transformer

      Phase to phase fault in the transformer are rare. If such a fault does occur, it will give rise to substantial current to operate instantaneous over current relay on the primary side as well as the differential relay.Inter Turns Fault in Power Transformer

      Power Transformer connected with electrical extra high voltage transmission system, is very likely to be subjected to high magnitude, steep fronted and high frequency impulse voltage due to lightening surge on the transmission line. The voltage stresses between winding turns become so large, it can not sustain the stress and causing insulation failure between inter - turns in some points. Also LV winding is stressed because of the transferred surge voltage. Very large number of Power Transformer failure arises from fault between turns. Inter turn fault may also be occurred due to mechanical forces between turns originated by external short circuit.Core Fault in Power Transformer

      In any portion of the core lamination is damaged, or lamination of the core is bridged by any conducting material that causes sufficient eddy current to flow, hence, this part of the core becomes over heated. Sometimes, insulation of bolts (Used for tightening the core lamination together) fails which also permits sufficient eddy current to flow through the bolt and causing over heating. This insulation failure in lamination and core bolts causes severe local heating. Although these local heating, causes additional core loss but can not create any noticeable change in input and output current in the transformer, hence these faults cannot be detected by normal electrical protection scheme. This is desirable to detect the local over heating condition of the transformer core before any major fault occurs. Excessive over heating leads to breakdown of transformer insulating oil with evolution of gases. These gases are accumulated in Buchholz relay and actuating Buchholz Alarm.
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