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الموضوع: DISTRIBUTION SYSTEM VOLTAGE PROFILE CONTROL AND APPLICATIONS

  1. #1
    Junior Engineer
    تاريخ التسجيل
    May 2006
    الدولة
    Giza - Egypt
    المشاركات
    12

    DISTRIBUTION SYSTEM VOLTAGE PROFILE CONTROL AND APPLICATIONS

    DISTRIBUTION SYSTEM VOLTAGE PROFILE CONTROL AND APPLICATIONS



    1. The Function Of The Power System

    The primary function of the power system is to satisfy load and energy requirements for customers with a reasonable assurance of continuity and quality of supply. In addition, customers expect to receive electricity at the lower possible cost. The electricity sector in all countries is interested in improving system quality not only to achieve legal obligations and standard regulation but also to increase the revenue. Improving system quality leads to increasing electricity sales, which represent a profit to the utility, and contract damage. The appliances and equipments will operate more efficiency during its lifetime, and the damage of the customer due to supply interruption would also decrease which is a return to the customer. System quality includes several items such as; voltage profile, reliability, harmonics, line outage, losses, and stability. The two main items identifying the quality of service; the voltage profile and the continuity of service “Reliability” are only our interest throughout the present project.


    2. Voltage Drop Problem

    The general problem, which can arise from poor voltage regulation (due to voltage drop), is a direct loss of revenue. Proper system voltage control will improve a company’s revenue, this consideration is vital. During the low voltage conditions, many types of equipment will draw more than rated current without any increase in the power consumption, which means more power losses for the utility and low efficiency of customer equipment and a decrease in life time.

    The voltage at consumer’s terminals must not vary by more than 6% of the declared supply voltage for that system. This legal obligation is described in the electricity supply regulations. Moreover, the IEE regulation recommend that the maximum voltage drop in an installation must not exceed 2.5% of the declared supply voltage when the circuit carries full load current.

    In the following sections, the voltage drop is identified and is analysed to show clearly its effect on both customer and utility. The net saving as a result of improving voltage profile is also given. Besides, the proposed techniques for such improvement are also outlined.


    3. The Impacts Of The Voltage Drop On Both Customer And Utility

    Distribution lines have series impedance (resistance R and reactance X) on most distribution lines, the capacitive reactance effect is small and can be considered negligible. The resistance and inductive reactance are actually distributed along the time, but we have shown them lumped into one point for simplicity. When no load is applied to this feeder, the voltage that would be received by the customer Vr, would be equal to the voltage at the sending end Vs. If a load is applied to this line, the voltage at the receiving end would be:
    Vr = Vs – IrR – IxX
    Vr – Vs = - IrR – IxX = - V.D
    Where: Ir The power component of the current. [ Ir = I cos φ ]
    Ix The reactive component of the current. [ Ix = I sin φ ]

    If the current has a lagging power factor, the value (Vr – Vs) is negative, thus we have a low voltage condition. If the current has a leading power factor, the value (Vr – Vs) is positive, thus we have a high voltage condition. Depending upon the specific load on that line, there could be a low voltage condition or a high voltage condition. In order to highlight the economical revenue gained by the utility when improving the voltage profile, consider the following case:
    For a radial feeder feeding a distribution area having X Kw peak load. Consider only Z division of this load as a voltage sensitive load (lighting load). It is known that, the lighting load power is related to the voltage violations as Pα V1.6. Then, the annual saving to the utility as a result of regulating the voltage due to the considered division is given by the following equation:

    Annual Egyption Pounds = X * Y * 8760 * (HRS/Year) * (EPS/Kwh) *
    (Z /total Kwh) * ([Vn/Vo]1.6- 1)
    Where:
    X : Annual feeder peak. Vn : Regulated voltage.
    Y : Annual load factor. Vo : Unregulated voltage.
    Z : Voltage sensitive load Kwh.
    EPS : Cost of Kwh in L.E.

    This equation makes the increased revenue of proper voltage evident. Since the regulated voltage Vn, would normally be held higher than unregulated voltage Vo, then the Annual Egyption Pounds will have to be increased. Actually, Voltage Regulation is a good business.

    4. How To Improve Voltage Profile?

    To keep distribution circuit voltage within permissible limits, means must be provided to control the voltage i.e., to increase the circuit voltage when it is low and to reduce it when it is too high. There are numerous ways to improve the distribution system’s overall voltage regulation such as;

    1. Application of voltage regulation required in the distribution substations.
    2. Application of capacitors in the distribution substation.
    3. Balancing of the loads on the primary feeders.
    3. Increasing of feeder conductor size.
    4. Transferring of loads to new feeders.
    5. Installing of new substations and primary feeders.
    6. Application of voltage regulators out on the primary feeders.
    7. Application of shunt capacitors on the primary feeders.

    It was found that, the voltage regulator is the best alternative to improve the voltage profile. We will investigate its theory, connection, and operation in the following section.


    5. Voltage Regulator

    5.1. Theory

    The following illustrates the theory of operation, connection and installation of the voltage regulators in a feeder. Voltage-regulating apparatus are designed to maintain automatically a predetermined level of voltage that would other wise vary with the load. As the load increases, the regulating apparatus boosts the voltage at the substation to compensate for the increase voltage drop in the distribution feeder. In cases where customers are located at a long distances from the substation or where voltage along the primary circuit excessive, additional regulators or capacitors located at selected points on the feeder, provide supplementary regulation. Many utilities have experienced, that the most economical way of regulating the voltage within the required limits is to be apply both step voltage regulators and shunt capacitors. Capacitors are installed out on the feeder and on the substation bus in adequate quantities to accomplish the economic voltage control on distribution feeders. power factor. Many of these installations have sophisticated controls designed to perform automatic switching of course a fixed is not a voltage regulator and can not be directly compared to regulators. But in some cases automatically switched capacitors can replace conventional step-type voltage regulators.


    5. 2. Step Voltage Regulator Operation

    The step of voltage regulator is basically an auto-transformer. The auto-transformer similar to a convential two-winding transformer except that both windings are connected electrically as will as magnetically and the secondary windings is a part of primary windings. The basic auto-transformer is shown in Fig.(1).


    Figure (1) Basic auto-transformer.

    We can obtain the step regulator from auto-transformer by tapping the series winding into eight equal parts. Since the total voltage in this series winding is 10% of the input, each part is 1.25%. If this tapped winding were connected to add to the input, the output could be raised 8 small steps of 1.25% each. However, the trouble now is that, an interruption results during each range from neap to the next. To eliminate this interruption between tap changes, we can incorporate two fingers fastened together and far enough a part so that both fingers can never be out of contact at the same time. The step regulator is shown in Fig.(2).


    Figure (2) The step regulator.


    5. 3. Sequence Of Regulator Operation

    The sequence of regulator operation is illustrated as follows:
    # In the first position, both fingers are on the neutral contact and are at the same
    potential as the neutral contact. The center tap is at the same potential as both
    fingers.
    # On contact 1 and the trailing finger is on contact 0. There is a 1.25% potential
    difference between the two moving fingers.
    # The next position of the moving fingers-both or contact 1-puts the center tap at
    the same potential as both moving fingers.

    To reach the other operating positions, there is a movement of the fingers and own the series winding. Alternately, putting them on the same contact position and then in the bridging position. This sequence of operation is shown in Fig.(3).


    Figure (3) The sequence of regulator operation.

    5. 4. Regulator Reverse Switch

    In the later technique, we can use a switch to reverse the polarity of the series winding. Therefore, the regulator can be lower, as will as rise the o/p voltage. Figure(4) illustrating this technique.

    Figure (4) Regulator Reverse Switch

    5. 5. Static Control Operation

    The step regulator is basically an auto-transformer, capable of raising or lowering the voltage in 32.5/8 percent steps. A voltage regulator is also an automatic device; therefore, there must be some means of controlling or directing its operation. This function is accomplished by the control unit, or by the brains of the regulator. It is important to understand the operation and the function of the regulator controls.
    The introduction of the integrated circuit (I.C.) board was an improvement of the static control. The circuit board was completely redesigned to take advantage of the significant advances in electronic component technology with particular emphasis on reliability in field service. The I.C. board is smaller, light, more rugged, provides easier trouble shooting and maintenance, and is directly interchangeable with the older static control board . Also, the static control was a significant advancement over the electro-mechanical control in voltage-regulator control.

    Figure(5) : The main elements of the voltage regulator control circuit.

    The introduction of the integrated circuit (I.C.) board was an improvement of the static control. The circuit board was completely redesigned to take advantage of the significant advances in electronic component technology with particular emphasis on reliability in field service. The I.C. board is smaller, light, more rugged, provides easier trouble shooting and maintenance, and is directly interchangeable with the older static control board . Also, the static control was a significant advancement over the electro-mechanical control in voltage-regulator control. Although, discussion of control theory can be come quite involved, proper adjustment of control settings is very simple because all calibrated control knobs are located on the front panel.

    The major elements of the control circuit in their operating sequence are; potential transformer, line-drop compensator, voltage sensor, and timer. These are the components, which will determine when the tap-changing motor will operate the tap changer to raise or lower the voltage on the secondary of the auto-transformer. The potential transformer, tap-changing motor, and tap changer are physically located inside the voltage-regulator tank under oil. Figure (5) represents these elements.

    5. 5. 1. Potential Transformer

    The potential transformer steps down the load-side line voltage (for example, 7200 volts) to a usable, accurate voltage for the control circuitry, (for example, 120 volts) and provides the voltage for the tap-changing motor. A variation in load-side line voltage produces a variation at the secondary of the potential transformer, which feeds the control circuitry.

    5. 5. 2. Line-Drop Compensator LDC

    The line-drop compensator is a modeling device, which modifies the secondary voltage of the potential transformer before it goes to the voltage sensor. The function of the line-drop compensator is to provide a predetermined voltage rise at the regulator, which minimizes the voltage, swings along the line due to changes in load current. The LDC has adjustable resistance and reactance elements in the control circuit, which make it possible to simulate actual line impedance. A combination of these adjustments provides a range of 0 to  24 volts.
    Sample calculations, which illustrate how these equations, are applied for various circuit configurations and regulator connections are shown. The line resistance, reactance and current-transformer primary rating may be obtained as follows :

    In the regulator circuit, Rset , Xset representing the line impedance R, X are given as:

    ICT* x resistance of the line
    Rset = -----------------------------------
    Potential transformer ratio

    ICT * x reactance of the line
    Xset = -----------------------------------
    Potential transformer ratio

    Where:
    ICT Current transformer primary current rating.



    5. 5. 3. Voltage Sensor

    The voltage sensor compares the input voltage to a pre-selecting called “voltage level” and tolerance called “bandwidth” as shown in the Fig.(6). If the voltage input to the sensor is out of the bandwidth, the control will be operate after the time delay has been exceeded. Thus, the voltage output of the regulator will be either raised or lowered until the voltage feed back into the sensor is again within the bandwidth.

    The voltage level may be adjusted between 105 and 135 volts according to the feeder voltage profile and the regulator capability. The factor setting for the voltage level is 120 volts. The bandwidth is calibrated for 1.5, 2, 2.5, 3 volts.

    Figure(6) : Voltage level and bandwidth

    5. 5. 4. Time-Delay Circuit

    The time delay circuit permits the regulator to ignore brief, self–correcting voltage variations. If a regulator tried to correct every minor fluctuation in voltage on line, it would make unnecessary operations. The time delay enables the regulator to correct only those voltage variations, which exist for longer than pre-set time. Also, it delays only the first tap change if more than one switch operation is required to correct the voltage on the line. The regulator incorporates the integrating type of timer. Integrating means that the device will add the total amount of time that the voltage is outside of the pre-selected bandwidth. When the voltage is out-side more than in-side the bandwidth by an amount greater than the pre-set time-delay setting the time-delay circuit energizes the appropriate relay. As shown in Fig.(7), for time delay setting of 30 sec., the regulator will operate at time of
    t = 35 – 5 = 30 sec.
    The time delay may be adjusted from 10 to 90 sec., and the time is set for 30 sec. at the factory.

    Figure (7)
    5. 5. 5. Tap Changer
    After the voltage has been out of the bandwidth for the pre-set time, a signal is sent to the tap-changing motor to operate the tap changer to choose the suitable number of steps.

    6. Selecting The Proper Regulator And Regulator Connection

    The KVA rating of a single-phase feeder voltage regulator is the product of the rated load current in amperes, and the rated range of regulation in Kilovolts. The rated range of regulation is the range that the regulator will raise or lower its rated voltage. The rated voltage of a feeder voltage regulator is the voltage from which the performance characteristics are based.

    To choose the proper regulator, the following data has to be supplied:
    The type of circuit (single-phase or three-phase), the voltage rating of the circuit, the rating of the circuit, and the amount of voltage correction that will be required.

    6. 1. Voltage Regulator Rating

    Firstly, calculate the setting of the line drop compensator (LDC) [Rset, X set], which sense the voltage drop along the line.

    CTP
    Rset = --------- * Reff
    PTN

    Where:
    CTP Rating of the current transformer is primary.
    PTN Potential transformers turns’ ratio.
    Reff The effective resistance of a feeder conductor from regulator station
    to regulation point.
    CTP
    Xset = --------- * Xeff
    PTN

    Where:
    Xeff Effective reactance of a feeder conductor from regulator station to
    regulation point.



    6. 2. Regulator Connections


    Figure(8) shows the connection of one regulator to a single-phase circuit. Three-phase four-wire circuit with three single-phase regulators connection is given in Fig.(9 a,b).

    Figure (8) One regulator, single-phase circuit



    (a)

    (b)
    Figure(9) Three-phase four wire with three single-phase regulators

    Three-phase three-wire circuit with two single-phase regulator connections is shown in Fig.(10 a,b)


    (a)

    (b)

    Figure(10) Three-phase three wire with single-phase regulator

    Three-phase, three-wire circuit with three-single-phase regulator connection is shown in Fig.(11 a,b).

    (a)


    (b)
    Figure(11) Three-phase three wire circuit with three single-phase regulators
    7. Installation On An Energized Line

    Bypass Switch B, allows to put in or take out the regulator of service.
    Important: Set the regulator on neutral position, that is illustrated in 4 steps;

    First B, C, and A are open. As shown in the Fig.(12)

    Figure (12) First step of voltage-regulator installation

    Second B is closed A, C are open, as shown in the Fig.(13). The line is energized, and the regulator is out of service.

    Figure (13) Second step of voltage-regulator installation.

    Third A is closed, and then C is opened turning on the control switch as shown in the Fig.(14).


    Figure (14) Thired step of voltage-regulator installation.

    Fourth You must put the regulator in the neutral position to prevent short-circuiting of the exciting winding of the regulator, which cause high short circuit current, which may damage the regulator. C is closed, and B is opened as shown in Fig.(15).

    Figure(15) Fourth step of voltage-regulator installation.

    8. Voltage Profile Control of Distribution Feeders
    (Using Voltage Regulators)

    The voltage drop problem in distribution feeders can be solved using step type voltage regulator. Data processed in this section are for real radial feeders in Egyptian Distribution Networks. Data of the feeder is fed in a special format to the load flow program for radial feeders, the program output lists feeder data . The load flow results are also given including active and reactive power for all feeder sections, the power loss and the voltage at all load points and feeder connection points.. From the voltage profile of each feeder, we notice that the voltage drop will exceed the permisable standard limits, so, feeder voltage needs to be regulated..

    8. 1. Voltage Drop Problem and Solution

    The problem of voltage drop is direct loss of revenue. During the low voltage conditions many types of loads such as motors will draw higher current than its rated one without any increase in the power consumption. This higher current will lead to greater kilo-watt loss in the way between source and load such as lines and distribution transformers. The voltage drop problem will take another shape with other kinds of loads like incandescent lamp, which will take less than its rated watt under low voltage conditions, this cause the problem of drop in utility income.
    Therefore, the effect of voltage drop in distribution systems leads to excessive power losses due to high currents drawn in addition to drops in consumption and energy sales as a result of decreased voltage levels.
    Many alternatives are offered to improve voltage profile. The voltage regulator is selected due to its advantages over the other techniques. Therefor, the voltage regulator is highly recommended to improve the voltage profile.
    Load flow program for radial feeders is utilized to obtain the voltage profile of the distribution feeder. The suitable location and size of the voltage regulator are, then, identified.



    8. 2. The Load Flow Program for Radial Feeders

    Load flow program for radial feeders is a program used to process feeder data to determine the voltage drop at any point on the feeder and hence plotting the voltage profile of the feeder.
    The input data to the program are; the number of nodes of the feeder, number of sections of the feeder, substation name, feeder name, the input voltage of the feeder, the total current drawn from the feeder, power factor, and loss factor of the feeder. Also the transformer ratings and loading percentages at every point of the feeder must be defined to the program. Feeder sections, the cross section area in mm2, length in m, and number of parallel circuits and type of section (over head transmission line or under ground cable) should be defined as input data. All those input data of the feeder are collected with some constants in an input file and they are written in a special format.
    After operating the load flow program on the input data file, the program gives an output file. The output file contains data about every load point on the feeder. These data are the transformer rating in KVA, load active, reactive and apparent power, the percentage of loading on the transformer, the voltage value in KV, and the percentage of voltage regulation (percentage of voltage drop) at all feeder points. The output file gives data about every section of the feeder. These data are:
    The points which the section is between them, the conductor code of the section, the length of the section in m, the current passes through this section in Amp., the active power consumed at the section, loading percentage of the section, active and reactive power losses in the section. At the end of the file, it gives a statistical results for the total active power loss in KW in the feeder, total reactive power loss in KVAR in the feeder, percentage of total active power loss, percentage of total reactive power loss, average energy of feeder in Kwh, energy loss in Kwh, and percentage of energy loss of the feeder.

    Since the voltage drops at every load point on the feeder are given, the voltage profile of the feeder can be drawn. This enables correct choice of the regulator rating and its suitable site.

    8. 3. Case 1

    Feeder -1 is one of 66/11 Kv substation supplying small cities and rural areas in Lower Egypt. Besides, the single line diagram of the feeder is shown in Fig.(1).

    Feeder data are prepared in the suitable form to be fed to the load flow program of radial feeders. Two cases were considered for the supply voltage value and the two cases were analysed as follows.






    Figure(1) Single-line diagram of Feeder - 1


    • In the first case, we assumed the input voltage of the feeder is 11 KV.
    Figure(2) express the voltage profile of Feeder - 1 at 11 KV substation
    voltage for both light and full load conditions.

    • In the second case, we assumed the input voltage of the feeder is 10.5 KV.
    Figure(3) shows the voltage profile of Feeder - 1 with 10.5 KV input voltage
    and for both light and full load conditions.


    a. Voltage Profile of Feeder - 1


    This case is represented in Fig.(2). As shown in the figure, under full load condition the load point at the end of the feeder suffers from severe voltage drop, it is nearly 9% drop in voltage. This value is too large and exceeds the permissible limits ( 5%), so we recommend to install a voltage regulator on the feeder to decrease the voltage drop at the regulated point (R.P.), which is the load point at the end of the feeder.





















    Figure(2) Voltage profile of Feeder – 1 at substation
    voltage 11 KV for both light and full load

























    Figure(3) Voltage profile of Feeder – 1 at substation voltage 10.5 KV
    for both full and light load


    b. Voltage Regulator Location

    Many loads are affected by the voltage flactuation ΔV ( the difference between the actual terminal voltage and the rated voltage). We choose the voltage of the regulated point (R.P.) and the voltage regulator site such that the voltage after the voltage regulator is in the permissible band ± 5% and ΔV+, ΔV- are nearly equal to cancel each other. We choose the voltage of the regulated point as 97.5%, as result of this choice the voltage regulator location is before load point number 5 directly, in other words it is at 2570 m far from the substation. The voltage regulator must be installed near a load point, so we choose load point number 5 because at this point ΔV± are minimum. Figure(4) shows the voltage profile of Abo-Mashour feeder at 11 KV input voltage and full load condition with and without installing the voltage regulator.

    From Fig.(4) we can notice that, the voltage regulator will elevate the voltage by +6.5 %. If we look at Fig.(5) which shows the voltage profile of the same feeder under the same input voltage 11KV but at light load, we find that the voltage regulator will mentain the regulated point at voltage of 97.5%, so it will increase the voltage by less than 1% . From the obvious we can say that, the voltage regulator will increase the voltage at the worth case by 6.5%, so we can use a voltage regulator with minimum rating and overloading it.




















    Figure(4) Voltage profile of Feeder - 1 in P.U values at 11 KV input voltage,
    at full load condition , with and without installing the voltage regulator.




















    Figure(5) Voltage profile of Feeder - 1 in P.U values at 11 KV input voltage,
    under light load condition before and after installing the voltage regulator.


    c. Voltage Regulator rating calculation

    By applying the method mentioned in chapter one for determining the voltage regulator size on our case to get the voltage regulator rating, the result will be as follows :

    Reff = 1.27625 Ω Xeff = 0.61812 Ω

    CTp = 250 PTN = 36.5

    IL = 423.8 A θ = 25.8315o
    CPT
    Sfeeder 3Φ = 4662 KVA Rset = -------- * Reff = 5.02463 V
    PTN
    CPT
    Xset = --------- * Xeff = 2.43354 V
    PTN

    Sfeeder.3Φ
    SV.Reg.1Φ = ------------- * %Regulation
    3

    If we make the maximum percentage value regulated by the voltage regulator, SV.Reg.1Φ = (4662/3)*0.10 = 155.4 KVA, is ±7% we can use another voltage regulator with less rating and overloaded by 120 %, then the new one will be 129.5 KVA, but the nearest rating from standard voltage regulators is 138 KVA, so we approximate the rating from 129.5 to 138 KVA. The standard voltage regulators, regulate the voltage by ±10% in 32 steps bucking or boosting conditions, but in our overloaded case which its regulation percentage is 7% the number of steps will be 12 steps for boosting condition.

    We can tabulate the results of case one to facilitate resulted data manipulation as expressed from Table(1).

    Table(1) The results of case one of feeder-1

    Voltage regulator site Just beside load point No. 5
    Voltage regulator rating 138 KVA
    Rset 5.02463 V
    Xset 2.43354 V
    No. of steps bucking or boosting 12 steps
    Voltage variation before installing the voltage regulator 0.9045 – 1 P.U
    Voltage variation after installing the voltage regulator 0.964 – 1.02963 P.U
    8. 4. Case 2
    a. Voltage Profile of Feeder - 1

    In this case, the input voltage coming from the substation to feeder - 1 is not regulated and assumed to be 10.5KV. Fig.(3) illustrates the voltage profile of feeder -1 at substation voltage of 10.5KV for both full load and light load conditions. It is recommended to install a step-type voltage regulator on the feeder just before load point number 3. The voltage regulator will be at a distance of 670 m far from the substation.

    b. Voltage Regulator Location

    The site of the voltage regulator is chosen considering the full load condition and consequently, the light load condition will be satisfied. Fig.(6) illustrates the voltage profile of the feeder in P.U values at 10.5KV input substation voltage and under full load condition with and without voltage regulator installation, we can notice that the presence of the voltage regulator on the feeder improves the voltage profile of the feeder. The standard voltage drop limitations (±5%) are, then, met. The voltage regulator increases the the value of the voltage at load point number 3 by approximately 10%. So, the voltage regulator percentage loading is 100%. Under light load condition, the recommended feeder voltage profile is also satisfied.


















    Figure(6) Voltage profile of Feeder – 1 in P.U values at 10.5 KV,
    under full load condition, with and without voltage regulator installation.























    Figure(7) Voltage profile of Feeder – 1 in P.U values at 10.5 KV,
    light load condition, with and without voltage regulator installation.


    c. Voltage Regulator Rating Calculation

    We should calculate the apparent power of the voltage regulator and Xset & Rset that should be adjusted from the control panel of the voltage regulator. Utilizing program output results and manufacture data of the step regulator, the regulator rating can be estimated as follows :.

    Reff = 1.92394 Ω Xeff = 0.93181 Ω
    CTp = 250 PTN = 36.5
    IL = 423.8 A θ = 25.8315o

    Sfeeder 3Φ = 7603 KVA

    CPT
    Rset = -------- * Reff = 5.02463 V
    PTN

    CPT
    Xset = --------- * Xeff = 2.43354 V
    PTN

    Sfeeder.3Φ
    SV.Reg.1Φ = ------------- * %Regulation = 253.43333 KVA
    3


    The nearest rating from General Electric typical equipment’s is 276KVA, so we will choose it. We can tabulate the results of case two to facilitate resulted data manipulation as expressed from Table(2).

    Table(2) The results of case two of Abo-Maashour feeder.

    Voltage regulator site Just beside load point no. 3
    Voltage regulator rating 276 KVA
    Rset 5.02463 V
    Xset 2.43354 V
    No. of steps bucking or boosting 10 steps
    Voltage variation before installing the voltage regulator 0.864 – 0.9545 P.U
    Voltage variation after installing the voltage regulator 0.946 – 1.0134 P.U

  2. #2
    V.I.P Member الصورة الرمزية dyiaeldeen
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    رد: DISTRIBUTION SYSTEM VOLTAGE PROFILE CONTROL AND APPLICATIONS

    شكراً لك اخي الكريم

  3. #3
    نائب رئيس مجلس الإدارة الصورة الرمزية ali yaghi
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    Post رد: DISTRIBUTION SYSTEM VOLTAGE PROFILE CONTROL AND APPLICATIONS

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    آخر مشاركة: 10-10-09, 01:14 AM
  2. DISTRIBUTION SYSTEM VOLTAGE REGULATION AND CONTROL
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