Power Transformer Protection Author Application Guide R Nylen Senior Application Engineer
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A GO3-5005 E March 1988 Power Transformer Protection Author Application Guide R Nylen Senior application engineer ) ABB Relays Power transformer protection AGO3-5005 E Page 2 List of contents 1. INTRODUCTION 2. CONDITlONS LEADING TO FAULTS 4.4 Grou~d fault protection 2.1 Insulation breakdown 4.4.1 General 2.2 Aging of insulation 4.4.2 Low irrnpedance residual overcurrent relay 2.3 Overheating due to overexcitation 4.4.3 Harmonic restraint relay 2.4 Oil contamination and leakage 4.4.4 High impedance restricted relay 2.5 Reduced cooling 4.4.5 Low impedance restricted relay 3. FAULT CURRENT 4.4.6 Tank protection 3.1 Ground faults in a solidly grounded 4.4.7 Residual voltage relay star-connected secondary winding 4.5 Overexcitation protection 3.2 Ground faults in a high impedance grounded star-connected 4.6 Flashover and ground fault protections for low voltage systems secondary winding 3.3 Ground faults in a delta-connected 4.6.1 Systems without rectifiers or frequency converters winding 3.4 Turn-to-turn faults 4.6.2 Systems with rectifiers and frequency converters without 3.5 Phase-to-phase faults pulse'width-modulation 4. PROTECTIVE RELAVS 4.6.3 Systems with rectifiers and pulse- 4.1 General width-modulated frequency converters 4.2 Differential relays 4.2.1 General 5. MONITORS ) 4.2.2 Differential relays for fully insulated 5.1 Gener,al transformers 5.2 Gas d~tector relay 4.2.3 Differential relays for auto- 5.3 Temperature monitoring transformers 5.4 PresslUre relay for on-load tap- 4.3 Overcurrent protection and changsrs impedance relays 5.5 PresslUre relief valve 4.3.1 Time-overcurrent relays 5.6 Oil level monitor 4.3.2 Under-impedance relays 5.7 Silica gel dehydrating breather 4.3.3 Distance relays 6. SUMMARY 7. REFERENCES List of illustrations Fig. 1 Permissible short time Fig.11 Distanlce relay used as a overexitation. transfQrmer protection. Fig.2 Ground fault current in a solidly Fig.12 Connection of ground fault grounded Y-connected winding. overclilrrent relays. Fig. 3 Ground fault current in a high Fig.13 Conn~ction of arestricted ground impedance grounded Y-connected fault relay for a Y-connected winding. winding. Fig.4 Theoretical inrush currentlm. Fig.14 Connection of arestricted ground fault relay for a D-connected Fig. 5 Recorded inrush current for a 60 winding and agrounding MV A transformer 140/40/6.6 kV. transf<i>rmer. connected YNyd. Fig.15 Transformer differential and Operating time. Fig.6 restricted ground fault relays on the Fig.7 Through faun restraint. same CT cores. Fig.8 Magnetizing current at Fig.16 Flash0ver relay. overexitation. Fig.17 Grou1d fault relay. Fig.9 Transformer differential relay for Grou~d fault and flashover relay. autotransformer. Fig.18 Fig.10 Differential relay RADHA or RADSG for autotransformer. ASS Relays Power transformer protection AGO3-5005 E Page 3 1. INTRODUCTION A power transformer is a very valuable and To prevent faults and to minimize the damage vitallink in a power transmission system. High in case ot a fault, transformers are equipped reliability of the transformer is therefore with both protective relays and monitors. The essentiai to avoid disturbances in trans- choice af protective equipment varies mission of power. depending, on transformer size, voltage level, etc. A high quaiity transformer properly designed and supplied with suitable protective relays ASEA RELA VS is the largest manutacturer of and monitors is very reliable. Less than one protective relays in the world, leading the fault in 100 transformer years can be developm~nt of relays with microprocessors. expected. The relays are built up in a modularized plug-in system called COMBIFLEX@, a sys- When a fault occurs in a transformer, the tem characterized by great flexibility and re- damage is normal ly severe. The transformer Iiability. Tl11erelays in the COMBIFLEX sys- has to be transported to a workshop and tem have l:iJeenoptimized with respect to their repaired, which takes considerable time. To quaiity, dimension and Gast. operate a power transmission system with a transformer out of service is always difficult. All COMBIFLEX relays can be tested by a Frequently, the impact of a transformer fault is test system called COMBITEST by plugging a more serious than a transmission line outage. test-handla inta a built-in test switch. By this it is possible to carry out a sate and easy The operation and maintenance of a injection test of a relay. The road current transformer can be a contributory cause of a through a relay in service can be measured fault. If a transformer is operated at too high by an ammeter connected to a current mea- temperature, too high voltage, or exposed to suring plug with a built-in overvoltage pro- an excessive number of high current external tection. If the am meter circuit is open by faults etc, the insulation can weaken to the mistake the plug will be short-circuited by point of breakdown. the overvoltage protection. By this the current On-road tap-changers should be checked in a CT -cirrcuit can be measured without any and maintained according to the operating risk of getling an open CT -circuit. A very instructions to prevent any faults. A fault in a important safety feature. tap-changer with a separate housing can cause too high a pressure in the housing. A pressure relay can be used to trip the circuit breakers at a certain set pressure, see point 5.4 2. 2.1 2.2 CONDITlONS Insulation breakdown Aging of insulation LEADING TO FAULTS A breakdown of the insulation results in short Aging or deterioration of insulation is a circuits or ground faults, frequently causing function of time and temperature. The part of severe damage to the windings and the the windin~ which is operated at the highest transformer core. Furthermore, a high gas temperature (hot-spot) will ordinarily pressure may develop, damaging the trans- undergo the greatest deterioration and gets former tank. the shorte$t length of life. However, it is not possible tO! accurately predict the length of life Breakdown of the insulation between windings as a functibn of temperature and time under or between windings and the core can be caused by: constant Gontrolled conditions, much less under widely varying service conditions. .aging of insulation due to overtempe- In case a transformer gets too hot, Improve rature during long time. the cooling if possible or reduce the load in .contaminated oil order to avoid accelerated aging of the insulation. A temporary moderate over- .corona discharges in the insulation temperature can be allowed as it takes a .transient overvoltages due to thunder- considerablle time to age the insulation. storms or switching in the network .current forces on the windings due to external faults with high current. A flash-over between the primary and se- condary windings usually results in a break- down of the insulation between the secondary winding and ground. ASS Relays Power transformer protection AGO3-5005 E Page 4 2.3 2.4 I Overheating due to overexcitation Oil contaminatipn and leakage According to lEG 76-1 guidelines, trans- The oil in a transformer constitutes an elect- iformers shall be capable of delivering rated rically insulating medium and also a cooling icurrents at an applied voltage equal to 105% medium. The service reliability of an oil- lot the rated voltage. Transformers may be immersed transformer therefore depends to a !specified for operation at a voltage up to great extent on the quaiity of the oil. The oil 110% of rated voltage. should fulfill the l1equirements of lEG 296. When a transformer is operated at too high The dielectric sttength of the oil is the most voltage or at too low frequency, the important propert y of the oi!. If the dielectric Itransformer core gets overexcited. The flux is strength of the <Dil is reduced by water and then forced through surrounding steel parts impurities etc, a breakdown of the insulation such as the sheet metal of the tank and other can occur. Testing of the dielectric strength of lunlaminated parts of the transformer. These the oil is normally conducted on site to get a iparts are heated up in an unacceptable way quick check of the purity of the oil. and the transformer can be damaged. As a transformer loaded with rated current can The oillevel must be monitored, a breakdown withstand on ly 105% of rated voltage of the insulation occurs if the oil level gets to continuously, the transformer has to be low. disconnected if the voltage is too high or the Oil immersed transformers with an oil frequency too low. According to IEEE general conservator should therefore be provided with guide for permissible short-time over- both a silica gel breather and an oil level excitation of power transformers, see tig 1, monitor. transformers can only with stand overex- citation a short time. pspecially turbo-generator transformer units can be exposed to overvoltage and under- frequency conditions. They should be provided with an overexcitation relay opera- 2.5 ting when the ratio between voltage and rrequency (V/Hz) gets too high. Reduced cooling To get a correct representation of the flux, the Forced cooling systems must be supervised. and an alarm sh all be given if the cooling overexcitation relays must be connected to a system stops. The oil temperature can then potential transformer, measuring the voltage ?f an untapped transformer winding. be watched and appropriate action taken before the transformer becomes overheated. -1l-lLL IEEE'GEN'ER~L I G~;D~ ~~OR PE~MISSI~LEIJJ 11.5 SHORT-TIME OVEREXCITATIONOF 140 POWER TRANSFORMERS \.