UNIT III PROTECTION OF TRANSFORMER OCTOBER 2 (2).ppt
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This document discusses the protective schemes and devices for power transformers, emphasizing the importance of reliable protection tailored to the size and type of transformer. It details various protection methods, including percentage differential protection for large transformers, thermal relays for overload conditions, and specialized relays for detecting incipient faults and magnetizing inrush currents. Additionally, it covers protection against overfluxing, earthing transformers, and detailed schemes for generator-transformer units and three-winding transformers.
UNIT III PROTECTION OF TRANSFORMER OCTOBER 2 (2).ppt
- 1. 20EE023 PROTECTION AND SWITCHGEAR UNIT-III Apparatus Protection
- 3. Introduction • The power transformer is a major and very important equipment in a power system. It requires highly reliable protective devices. • The protective scheme depends on the size of the transformer. • The rating of transformers used in transmission and distribution systems range from a few kVA to several hundred MVA. • For small transformers, simple protective device such as fuses are employed. • For transformers of medium size, over current relays are used. • For large transformers differential protection is recommended.
- 4. Types of Faults Encountered in Transformers • External Faults • For external faults, time graded over current relays are employed as back-up protection. • In case of sustained overload conditions, the transformer should not be allowed to operate for long duration. Thermal relays are used to detect overload conditions and give an alarm. • Internal Faults • The primary protection of transformers is meant for internal faults. Internal faults are classified into two groups. • (i) Short circuits in the transformer winding and connections: These are electrical faults of serious nature and are likely to cause immediate damage. Such faults are detectable at the winding terminals by unbalances in voltage or current. This type of faults include line to ground or line to line and interturn faults on H.V. and L.V. windings. • (ii) Incipient faults: Initially, such faults are of minor nature but slowly might develop into major faults. Such faults are not detectable at the winding terminals by unbalance in voltage or current and hence, the protective devices meant to operate under short circuit conditions are not capable of detecting this type of faults. Such faults include poor electrical connections, core faults, failure of the coolant, regulator faults and bad load sharing between transformers.
- 5. Percentage Differential Protection • Percentage differential protection is used for the protection of large power transformers having ratings of 5 MVA and above. • This scheme is employed for the protection of transformers against internal short circuits. It is not capable of detecting incipient faults. • Figure shows the schematic diagram of percentage differential protection for a Y – Δ transformer. • The direction of current and the polarity of the CT voltage shown in the figure are for a particular instant. The current entering end has been marked as positive. The end at which current is leaving has been marked negative.
- 7. • O and R are the operating and restraining coils of the relay, respectively. • The connections are made in such a way that under normal conditions or in case of external faults, the current flowing in the operating coil of the relay due to CTs of the primary side is in opposition to the current flowing due to the CTs of the secondary side. • Consequently, the relay does not operate under such conditions. • If a fault occurs on the winding, the polarity of the induced voltage of the CT of the secondary side is reversed. Now the currents in the operating coil from CTs of both primary and secondary side are in the same direction and cause the operation of the relay. • To supply the matching current in the operating winding of the relay, the CTs which are on the star side of the transformer are connected in delta. The CTs which are on the delta side of the transformer are connected in star.
- 8. • Moreover, zero sequence current flowing on the star side of the transformers does not produce current outside the delta on the other side. • Therefore, the zero sequence current should be eliminated from the star side. • This condition is also fulfilled by CTs connection in delta on the star side of the transformer. • In case of star/star connected transformer CTs on both sides should be connected in delta. • In case of star/star connected transformer, if star point is not earthed, CTs may be connected in star on both sides. • If the star point is earthed and CTs are connected in star, the relay will also operate for external faults. • Therefore, it is better to follow the rule that CTs associated with star-connected transformer windings should be connected in delta and those associated with delta windings in star. • The relay settings for transformer protection are kept higher than those for alternators. The typical value of alternator is 10% for operating coil and 5% for bias. The corresponding values for transformer may be 40% and 10% respectively.
- 9. • The reasons for a higher setting in the case of transformer protection are. • (i) A transformer is provided with on-load tap changing gear. The CT ratio cannot be changed with varying transformation ratio of the power transformer. • The CT ratio is fixed and it is kept to suit the nominal ratio of the power transformer. • Therefore, for taps other than nominal, an out of balance current flows through the operating coil of the relay during load and external fault conditions. • (ii) When a transformer is on no-load, there is no load current in the relay. Therefore, its setting should be greater than no-load current.
- 10. Overheating Protection • Sustained overload is not allowed • If the ambient temperature is equal to the assumed ambient temperature. At lower ambient temperature, some overloading is permissible. • The maximum safe overloading is that which does not overheat the winding. • The maximum allowed temperature is about 95°C. • Thus the protection against overload depends on the winding temperature which is usually measured by thermal image technique. • In the thermal image technique, a temperature sensing device is placed in the transformer oil near the top of the transformer tank. • A CT is employed on LV side to supply current to a small heater. • Both the temperature sensing device and the heater are placed in a small pocket.
- 11. • The heater produces a local temperature rise similar to that of the main winding. • The temperature of the sensing element is similar to that of the winding under all conditions. • In a typical modern system the heat sensitive element is a silicon resistor or silistor. • It is incorporated with the heating element and kept in a thermal molded material. • The silistor is used as an arm of a resistance bridge supplied from a stabilized dc source. An indicating instrument is energized from the out of balance voltage of the bridge. • Also the voltage across the silistor is applied to a static control circuit which controls cooling pumps and fans, gives warning of overheating, and ultimately trips the transformer circuit breakers.
- 12. Protection against Magnetising Inrush Current • When an unloaded transformer is switched on, it draws a large initial magnetising current which may be several times the rated current of the transformer. This initial magnetising current is called the magnetising inrush current. • As the inrush current flows only in the primary winding, the differential protection will decide this inrush current as an internal fault. • The harmonic contents in the inrush current are different than those in usual fault current. • As the second harmonic is more in the inrush current than in the fault current, this feature can be utilised to distinguish between a fault and magnetising inrush current. • Figure shows a high speed biased differential scheme, incorporating a harmonic restraint feature. • The relay of this scheme is made insensitive to magnetic inrush current. • The operating principle is to filter out the harmonics from the differential current, rectify them and add them to the percentage restraint.
- 14. • The tuned circuit XC, XL allows only current of fundamental frequency to flow through the operating coil. • The DC and harmonics, mostly second harmonics in case of magnetic inrush current, are diverted into the restraining coil. • The relay is adjusted so as not to operate when the second harmonic (restraining) exceeds the fundamental current (operating). • The minimum operating time is about 2 cycles. • The DC offset and harmonics are also present in the fault current, particularly if CT saturates. • The harmonic restraint relay will fail to operate on the occurrence of an internal fault which contains considerable harmonics due to an arc or saturation of the CT. • To overcome this difficulty, an instantaneous over current relay (the high set unit) is also incorporated in the harmonic restraint scheme. • This relay is set above the maximum inrush current. It will operate on heavy internal faults in less than one cycle. • In an alternative scheme, known as harmonic blocking scheme, a separate blocking relay whose contacts are in series with those of a biased differential relay, is employed.
- 15. Buchholz Relay • It is a gas actuated relay. It is used to detect incipient faults which are initially minor faults but may cause major faults. Buchholz relay cannot detect short circuits within the transformer or at the terminals. • When a fault develops slowly, it produces heat, thereby decomposing solid or liquid insulating material in the transformer. The decomposition of the insulating material produces inflammable gases. • The operation of the Buchholz relay gives an alarm when a specified amount of gas is formed. • The analysis of gas collected in the relay chamber indicates the type of the incipient fault.
- 16. Buchholz Relay • There is a chamber to accommodate Buchholz relay, in between the transformer tank and the conservator as shown in Fig. • When gas accumulates, the oil level falls down and thus the float also comes down. It causes an alarm to sound and alert the operator. • For reliable operation, a mercury switch is attached with the float. • When the oil level falls because of gas accumulation, the bucket is filled up with oil. Thus, the force available to operate the contacts is greater than with hollow floats. • The accumulated gas can be drawn off through the petcock via a pipe for analysis to know the type of fault. If there is a severe fault, large volumes of gases are produced which cause the lower float to operate. It finally trips the circuit breakers of the transformer
- 17. Buchholz relay
- 18. • The buchholz relay is a slow acting device, the minimum operating time is 0.1 s, the average time 0.2 s. • Too sensitive settings of the mercury contacts are not desirable because they are subjected to false operation on shock and vibration caused by conditions like earthquakes, mechanical shock to the pipe, tap changer operation and heavy external faults. • This can be reduced by improved design of the mercury contact tubes.
- 19. Oil Pressure Relief Devices • An oil pressure relief device is fitted at the top of the transformer tank. • In its simplest form, it is a frangible disc, located at the end of an oil relief pipe protruding from the top of the transformer tank. • In case of a serious fault, a surge in the oil is developed, which bursts the disc, thereby allowing the oil to discharge rapidly. This avoids the explosive rupture of the tank and the risk of fire. • The drawback of the frangible disc is that the oil which remains in the tank after rupture is left exposed to the atmosphere.
- 20. Oil Pressure Relief Devices • This drawback can be overcome by employing a more effective device: a spring controlled pressure relief valve. • It operates when the pressure exceeds 10 PSI but closes automatically when the pressure falls below the critical level. • The discharged oil can be ducted to a catchment pit where random discharge of oil is to be avoided. • The device is commonly employed for large power transformers of the rating 2 MVA and above but it can also be used for distribution transformers of 200 kVA and above.
- 21. Rate of Rise of Pressure Relay • This device is capable of detecting a rise of pressure, rather than absolute pressure. • Its operation is quicker than the pressure relief valve. • It is employed in transformers which are provided with gas cushions instead of conservators. • Figure shows a modern sudden pressure relay which contains a metallic bellows full of silicone oil. • The bellows is placed in the transformer oil. The relay is placed at the bottom of the tank where maintenance jobs can be performed conveniently. • It operates on the principle of rate or increase of pressure. It is usually designed to trip the transformer.
- 23. Over current Relays • Overcurrent relays are used for the protection of transformers of rating 100 kVA and below 5 MVA. • An earth fault tripping element is also provided in addition to the overcurrent feature. Such relays are used as primary protection for transformers which are not provided with differential protection. • Overcurrent relays are also used as back-up protection where differential protection is used as primary protection. • For small transformers, overcurrent relays are used for both overload and fault protection. • An extremely inverse relay is desirable for overload and light faults, with instantaneous overcurrent relay for heavy faults. A very inverse residual current relay with instantaneous relay is suitable for ground faults.
- 24. Earth Fault Relays • A simple over current and earth fault relay does not provide good protection for a star connected winding, particularly when the neutral point is earthed through an impedance. • Restricted earth fault protection, as shown in Fig. provides better protection. • This scheme is used for the winding of the transformer connected in star where the neutral point is either solidly earthed or earthed through an impedance. • The relay used is of high impedance type to make the scheme stable for external faults.
- 25. • For delta connection or ungrounded star winding of the transformer, residual overcurrent relay is employed. The relay operates only for a ground fault in the transformer. • The differential protection of the transformer is supplemented by restricted earth fault protection in case of a transformer with its neutral grounded through resistance. • For such a case only about 40% of the winding is protected with a differential relay pick-up setting as low as 20% of the CT rating.
- 26. Over fluxing Protection • The magnetic flux increases when voltage increases. • This results in increased iron loss and magnetising current. The core and core bolts get heated and the lamination insulation is affected. • Protection against overfluxing is required where overfluxing due to sustained overvoltage can occur. • The reduction in frequency also increases the flux density and consequently, it has similar effects as those due to overvoltage. • The expression of flux in a transformer is given by Therefore, to control flux, the ratio E/f is controlled. When E/f exceeds unity, it has to be detected. Electronic circuits with suitable relays are available to measure the E/f ratio. Usually 10% of overfluxing can be allowed without damage. If E/f exceeds 1.1, overfluxing protections operates. Overfluxing does not requires high speed tripping and hence instantaneous operation is undesirable when momentary disturbances occur. But the transformer should be isolated in one or two minutes at the most if overfluxing persists.
- 27. Protection of Earthing Transformer • The function of an earthing transformer is to provide a grounding point for the power system where machines have delta connection. • An earthing transformer is connected either in star-delta or zig-zag fashion. • When a fault occurs only zero sequence current flows from the earthing transformer to the grounding point. • Positive or negative sequence currents can flow only towards the earthing transformer and not away from it. • An earthing transformer can be protected by IDMT overcurrent relays fed by delta connected CTs, as shown in Fig.
- 28. • The CTs are connected in delta and zero sequence currents circulate in it. • An overcurrent relay with time delay is inserted in this delta. The time setting of this relay is selected to coordinate with the time setting of the earth fault relays. This relay is used as a back-up relay for external faults.
- 30. Protection of Three-Winding Transformer • In a three-winding transformer, one of the three windings is connected to the source of supply. • The other two windings feed loads at different voltages. One line diagram is shown in Fig. for the protection of a three-winding transformer.
- 31. • When a three-winding transformer is connected to the source of supply at both primary and secondary side, the distribution of current cannot readily be predicted and there is a possibility of current circulation between two sets of paralleled CTs without producing any bias. • Figure shows protective scheme for such a situation. • In this case, the restraint depends on the scalar sum of the currents in the various windings.
- 32. Generator-Transformer Unit Protection • In a modern system, each generator is directly connected to the delta connected primary winding of the power transformer. • The star connected secondary winding is HV winding and it is connected to the HV bus through a circuit breaker. • In addition to normal protection of the generator and transformer, an overall biased differential protection is provided to protect both the generator and transformer as one unit. • Figure shows an overall differential protection. • Usually harmonic restraint is not provided because the transformer is only connected to the busbar at full voltage. • However, there is a possibility of a small inrush current when a fault near the busbar is cleared, suddenly restoring the voltage.
- 33. • Differential protection of generator transformer unit
- 35. Differential Current Protection The operating principle is based on Kirchhoff’s law. The algebraic sum of all the currents entering and leaving the busbar zone must be zero, unless there is a fault therein. The relay is connected to trip all the circuit breakers. In case of a bus fault the algebraic sum of currents will not be zero and relay will operate.
- 36. Differential Current Protection The main drawback of this type of differential scheme is that there may be a false operation in case of an external fault. This is due to the saturation of one of the CT of the faulted feeder. When the CT saturates, the output is reduced and the sum of all the CT secondary currents will not be zero. To overcome this difficulty, high impedance relay or biased differential scheme can be employed.
- 37. High Impedance Relay Scheme A sensitive dc polarised relay is used in series with a tuning circuit which makes the relay responsive only to the fundamental component of the differential (spill) current of the CTs. The tuning circuit makes the relay insensitive to dc and harmonics, thereby making it more stable on heavy external faults.
- 38. High Impedance Relay Scheme To prevent excessive voltages on internal faults, a non-linear resistance (thyrite) and a high set overcurrent relay, connected in series with the non-linear resistance are employed. The high set relay provides fast operation on heavy faults. Its pick-up is kept high to prevent operation on external faults.
- 39. High Impedance Relay Scheme This is more favoured for indoor than outdoor installations. This is applicable to metal clad type switchgear installations. This scheme is most effective in case of isolated-phase construction type switchgear installations in which all faults involve ground.
- 40. High Impedance Relay Scheme To avoid the undesired operation of the relay due to spurious currents, a check relay energised from a CT connected in the neutral of the system is employed. An instantaneous overcurrent relay is used in the frame leakage protection scheme if a neutral check relay is incorporated. If neutral check relay is not employed, an inverse time delay relay should be used.