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Transformer protection

Transformer Protection.

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Transformer protection

  1. 1. Presented by Dr.Selvarasu Ranganathan Assistant Professor School of Electrical Engineering and Computing Adama Science and Technology University, Adama. . TRANSFORMER PROTECTION
  2. 2. Transformers • Transformers are the major power system components which are used at various voltage levels with the capacity varying from 1kVA to >600 MVA. • Transformers are widely used devices to transfer power from one circuit to another circuit magnetically with out physical coupling. • Any damage to this component results in complete loss of power to one system. • The transformer should be protected from the faults to avoid break down of the transformer which may lead to major interruption in power transformation. • To protect it from the fault the typical faults in the fault should be known.
  3. 3. Possible Transformer Faults Transformer Faults Internal Faults (Winding faults) Phase to Phase fault Phase to earth fault Inter-turn fault External Faults (Through faults)
  4. 4. Transformer Internal Faults •Breakdown of the insulating oil. •Insulation failure of winding. •Over heat generated in the coils due to failure of insulation etc.
  5. 5. Transformer External Faults High Voltage Disturbance in Power Transformer are of two kinds 1.Transient Surge Voltage •Arcing ground if neutral point is isolated. •Switching operation of different electrical equipment. •Atmospheric Lightening Impulse. 2. Power Frequency Over Voltage As we know that, voltage V = 4.44Φ.f.N ⇒ V ∝ Φ, increased voltage causes proportionate increase in the working flux. This therefore causes, increased in iron loss and dis-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.
  6. 6. Transformer External Faults Contd… Protection against lighting surges lighting arrester can be provided.
  7. 7. Transformer Types There are different kinds of transformers such as •Two winding or three winding power transformer, •Auto transformer, •regulating transformers, •Earthing transformer, •Rectifier transformers etc. Different transformers demand different schemes of transformer protection depending upon their importance, winding connections, earthing methods and mode of operation etc. •It is common practice to provide Buchholz relay protection to all 500 kVA and above transformers. •While for all small size distribution transformers, only high voltage fuses are used as main protective device. • For all larger rated and important distribution transformers, over current protection along with restricted earth fault protectionis applied. •Differential protection should be provided in the transformers rated above 5 MVA
  8. 8. Typical Protections in Transformer Typically transformer protections are classified as two types. 1) Main protections 2) backup protections. Main protections: These are immediate fault clearance protections which acts instantaneously to clear the faults in the transformer. Differential protection Over current protection Back up protection: These protections will acts as a back up to the main protections i.e. if the main protection fails corresponding back up protection will act and safe guard the device from fault. Over voltage Volts/hertz Negative phase sequence relay
  9. 9. Differential Protection •Generally Differential protection is provided in the electrical power transformer rated more than 5MVA. The Differential Protection of Transformer has many advantages over other schemes of protection. •The faults occur in the transformer inside the insulating oil can be detected by Buchholz relay. But if any fault occurs in the transformer but not in oil then it can not be detected by Buchholz relay. •Any flash over at the bushings are not adequately covered by Buchholz relay. •Differential relays can detect such type of faults. Moreover Buchholz relay is provided in transformer for detecting any internal fault in the transformer but Differential Protection scheme detects the same in more faster way. •The differential relays normally response to those faults which occur inside the differential protection zone of transformer.
  10. 10. Differential Protection Contd… Basic Considerations for Transformer Differential Protection: • Line Current Transformer Primary Ratings •Current Transformer Connections •The neutrals of C.T star and power transformer stars are grounded. •Selection of C.T and Power transformer tapings.
  11. 11. Differential Protection Contd… Line Current Transformer Primary Ratings Consider a two winding, 11kV/132kV, 30MVA Power Transformer. The rated line current on the 11kV side of the Transformer is: Irated = MVA / Line Voltage = 30000000 / (11000)(sqrt 3) = 1574.59A Thus, choose a CT with a primary rating of 1600/1. The line current on the 132kV side of the Transformer is: Irated = 30000000 / (132000)(sqrt 3) = 131.22A Thus choose a CT with a primary rating of 200/1. Notice that in both cases, the current that the Bias Coils in the relay is 1A at both ends. Any deviation of this 1A will causes an imbalance within the relay, which will consequently cause protection to operate and isolate the Transformer.
  12. 12. Differential Protection Contd… Current Transformer Connections
  13. 13. Merz-Price Protection for Star-Delta Transformer
  14. 14. Merz-Price Protection for Star-Delta Transformer Contd…
  15. 15. Merz-Price Protection for Star-Star Transformer
  16. 16. BUCHHOLZ RELAY •Buchholz relay is a type of oil and gas actuated protection relay universally used on all oil immersed transformers having rating more than 500 kVA. • Buchholz relay is not provided in transformer having rating below 500 kVA from the point of view of economic considerations.
  17. 17. Why Buchholz relay is used in transformers? Buchholz relay is used for the protection of transformers from the faults occurring inside the transformer. Short circuit faults such as inter turn faults, incipient winding faults, and core faults may occur due to the impulse breakdown of the insulating oil or simply the transformer oil. Buchholz relay will sense such faults and closes the alarm circuit. Working principle Buchholz relay relies on the fact that an electrical fault inside the transformer tank is accompanied by the generation of gas and if the fault is high enough it will be accompanied by a surge of oil from the tank to the conservator Whenever a fault occurs inside the transformer, the oil in the transformer tank gets overheated and gases are generated. The generation of the gases depends mainly on the intensity of fault produced. The heat generated during the fault will be high enough to decompose the transformer oil and the gases produced can be used to detect the winding faults. This is the basic principle behind the working of the Buchholz relay. Construction Buchholz relay can be used in the transformers having the conservators only. It is placed in the pipe connecting the conservator and the transformer tank. It consists of an oil filled chamber. Two hinged floats, one at the top of the chamber and the other at the bottom of the chamber which accompanies a mercury switch each is present in the oil filled chamber. The mercury switch on the upper float is connected to an external alarm circuit and the mercury switch on the lower is connected to an external trip circuit. Buchholz Relay Contd..
  18. 18. Buchholz Relay Contd..
  19. 19. Buchholz relay is employed for the following types faults, • Core bolt insulation failure • Local-overheating • Entrance of air into the oil • Less of oil due to leakage • Short circuited lamination due to burns. Buchholz Relay Contd..
  20. 20. Advantages of Buchholz relay • The advantage of using a Buchholtz relay is that they indicate incipient faults. For example, It indicates the faults between turns or heating of core which enables us to take the transformer out of service before severe damage occurs. • Buchholtz relay gives an alarm when the oil level reduces below a certain level due to leakage of oil from transformer. • Buchholtz relay gives an audible warning which informs the operator that there is some fault in the transformer. • Type of insulation failure of the transformer can also be detected by testing the gas. Buchholz Relay Contd..
  21. 21. Limitation of Buchholz relay 1. This type of relay can be used only for transformers with conservators. 2. The relay does not operate when the faults are above the oil level. 3. Buchholz relay does not protect the cables associated with the transformers below. A separate protection has to be employed for the cables. 4. Due to economic considerations, this relay is not employed for transformers above 500 kVA. 5. Setting of the mercury switch cannot be too sensitive, otherwise there can be a false operation due to vibrations,earthquakes. mechanical shocks to the pipe. 6. The operation of this new is unsatisfactory due to slow operating time. (Nearly 0.2 to 0.1 seconds). Buchholz Relay Contd..