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  1. #1
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
    الدولة
    مكة المكرمة
    المشاركات
    1,111

    Rotor Earth Fault Protection of Alternator or Generator

    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.


    لاإله إلا الله محمد رسول الله

  2. #2
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
    الدولة
    مكة المكرمة
    المشاركات
    1,111

    رد: Rotor Earth Fault Protection of Alternator or Generator

    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.
    لاإله إلا الله محمد رسول الله

  3. #3
    Junior Engineer
    تاريخ التسجيل
    Oct 2015
    الدولة
    السعودية
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    99

    رد: Rotor Earth Fault Protection of Alternator or Generator

    ياريت لو تكون المواضيع بصيغة ملف وورد او pdf افض بكثير
    من النسخ واللصق من جوجل

  4. #4
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
    الدولة
    مكة المكرمة
    المشاركات
    1,111

    رد: Rotor Earth Fault Protection of Alternator or Generator

    أكثر من ألف مشاركة لي هي ملفات لم تتوفر إلا طريقة القص واللصق ، ومن يحتاج منها معلومة - بإذن الله سينتفع منها - أرجو منك التلطف بالعبارة ، وانتقاء الكلمات
    لاإله إلا الله محمد رسول الله

  5. #5
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
    الدولة
    مكة المكرمة
    المشاركات
    1,111

    رد: Rotor Earth Fault Protection of Alternator or Generator

    الأخ الفاضل أبا الوليد : أرجو من حضرتك الاطلاع على رد مشرف قسم المحولات على مشاركة مماثلة والمقارنة مع الشكر
    http://www.sayedsaad.com/montada/showthread.php?t=81613
    لاإله إلا الله محمد رسول الله

  6. #6
    Junior Engineer
    تاريخ التسجيل
    Oct 2015
    الدولة
    السعودية
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    99

    رد: Rotor Earth Fault Protection of Alternator or Generator

    الاخ الفاضل اسامه
    اولا اشكرك على مشاركاتك في المنتدى
    وثانيا انا لم اقصد شيء من كلامي وانما الذي قصدته
    انك اخذت معلومتك نسخ من هذا الموقع
    https://www.electrical4u.com/rotor-earth
    فقلت لك يا حبذا انها كانت بصيغةpdf. لغاية الترتيب فقط لاغير
    فلا ارى يا اخي الكريم اي كلام جارح بحقك وبحق جميع الزملاء
    فنحن هنا جميعا نكمل بعضنا البعض تحت هذا الصرح الكبير من العلم
    والشكر الخاص ايضا لصاحب الفضل المهندس سيد سعد المحترم
    وجزانا واياكم الجنه ان شاء الله
    واعذرني مره اخرى اذا اخطأت بحقك وسامحني

  7. #7
    Senior Engineer الصورة الرمزية zackariya
    تاريخ التسجيل
    Apr 2010
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    algeria
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    392

    رد: Rotor Earth Fault Protection of Alternator or Generator

    شكرا لك سيدي على هذا المجهود و اسمح لي بسؤال : رأيت في مولدة 400 فولت باستطاعة 1MVA في Exciter عند مخرج diode القطب + الموجب لصق في rotor, هل هذا طبيعي أو هناك مواصفات حول الموضوع, و شكرا .

  8. #8
    Consultant Engineer الصورة الرمزية أسامة حسن البلخي
    تاريخ التسجيل
    Sep 2014
    الدولة
    مكة المكرمة
    المشاركات
    1,111

    رد: Rotor Earth Fault Protection of Alternator or Generator

    الأخ المحترم زكريا وظائف جسور الديود هي التقويم من متناوب إلى مستمر ، لذا فهي موجودة فقط عند الحاجة إلى التحويل من المتناوب إلى المستمر حيث الاستخدامات الخاصة ... فقط
    لاإله إلا الله محمد رسول الله

  9. #9
    Senior Engineer الصورة الرمزية zackariya
    تاريخ التسجيل
    Apr 2010
    الدولة
    algeria
    المشاركات
    392

    رد: Rotor Earth Fault Protection of Alternator or Generator

    شكرا لك أخي اسامة و لكن السؤال هل القطب الموجب بعد التقويم يوصل ب rotor mas, أول مرة رأيته في هذه المولدة بعد القيام بأعمال صيانة فيها , و شكرا لك

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المواضيع المتشابهه

  1. stator and rotor earth fault protection
    بواسطة المهندس علي البدراني في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 2
    آخر مشاركة: 23-12-11, 11:46 AM
  2. Generator 100% Earth Fault Protection
    بواسطة المنذر عبد الله في المنتدى محاضرات و دروس فى هندسة الوقاية الكهربائية
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    آخر مشاركة: 04-09-11, 10:02 PM
  3. Generator Stator Earth Fault Relay(64G) - 27TN.
    بواسطة khaled hamza في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 2
    آخر مشاركة: 24-06-11, 04:45 PM
  4. rotor earth fault causes
    بواسطة زكريا محمد جعفر في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 0
    آخر مشاركة: 20-05-10, 03:20 PM
  5. survey about generator rotor earth fault experiment
    بواسطة hossam allam في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 0
    آخر مشاركة: 30-06-06, 04:41 AM

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