Problems connected with Commissioning of Power Transformers
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1 Problems connected with Commissioning of Power Transformers ABSTRACT P Ramachandran ABB India Ltd, Vadodara, India While commissioning large Power Transformers, certain abnormal phenomena were noticed. Some of these are arcing across the tank flanges with burning of paint, mal operation of differential relay protection and excessive current from star connected winding neutral to earth. Case histories are given describing the problems, causes for the same and the mitigation measures taken to avoid repetition of the problems. INTRODUCTION Large Power Transformers are commissioned (energized first time) at site after installation and electrical testing. Energisation is normally done from high voltage (HV) side in case of step down transformers and from low voltage (LV) side for generator step up transformers (GSU). In rare cases, commissioning will be done from opposite side(from LV side in case of step down transformers and from HV side in case of GSUs) This may be necessary when the station is not ready but the transformer healthiness is to be ensured in advance. While commissioning power transformers, certain problems were noticed. The phenomenon or failure mode, cause of failure, rectification and preventive measures for such occurrences are explained. Some of the peculiar phenomena noticed are arcing across the transformer tank flange, mal-operation of differential relay and excessive neutral current in star connected windings. The case studies reported below are from sites and the mitigation measures suggested may be useful to engineers involved in the installation and commissioning of power transformers. CASE HISTORIES Arcing across the tank flange (curb) Joints MVA 3 phase generator transformer with dual LV, 570/ MVA, 132/ kv, with HV side OLTC, flux density in core = 1.649T Leakage flux density =0.409T at 370 MVA,Tank Length= 11.7 meters, Inrush current Peak =6.9 ka, Impedance= 25/46/16.7% on 570 MVA base(hv-lv1,hv-lv2,lv1-lv2) 2. Transformer is energized from HV side, and synchronized to generator voltage on LV using Generator circuit breaker. Tank cover and lower tank are connected together by flexible copper wires (70 mm 2 ) at two positions, diagonally opposite corners. In addition, six numbers LV flexible copper links are provided on LV side of tank connecting top cover to lower tank, to take care of induced currents from LV leads to turret. Tank is grounded to earth using 240 mm 2 copper cables at two positions on opposite sides. 3. During energisation, flash over is noticed occasionally between cover and lower tank causing burning of paint at some bolt positions. Arc flash over is momentary and occurs only at the time of energisation of transformer.
2 Paint Burning at Tank Curb Bolt Figure1 4 Cause of the Phenomenon: During energisation, magnetizing inrush currents cause leakage flux to flow out of winding to tank and cover causing difference in induced voltages between cover and tank, resulting in arc flash over. This will happen only when inrush current is high or maximum. Maximum inrush current will occur when transformer is energized exactly when the voltage sine wave is crossing the zero value. This is the reason why flash over is not noticed during all energisation events as inrush current may not be that high on all occasions. Carbonization on painting due to such flashover is rarely noticed during energisation. In this particular case, it may be due to the high rating of transformer or tank with longer length. When transformers are short circuit tested at High Power laboratories by pre-set method, arc flash over at tank curb is noticed during voltage application. This is due to controlled switching adopted to create maximum asymmetrical over current from inrush current. 5 Mitigation measures: Three additional 95 mm 2 insulated copper cables are connected between cover to ground directly. 6 Preventive measures: In case of large transformers with longer tank, two numbers shorting copper links may not be adequate.it is better to provide more number of connecting links distributed along the tank curbs or remove painting at some bolt positions to get tank to cover galvanic connection through bolts and then repaint over it to avoid corrosion. Adopt controlled switching for the HV breaker to minimize inrush current, thereby leakage flux. Controlled switching (also referred as point on wave switching or point on cycle switching) of a circuit breaker involves operation of the switching device at a specific pre-determined point in relation to the power frequency current or voltage. For reducing the inrush current in power transformers, it involves switching on at peak of voltage of sine wave (to reduce inrush current) and switching off at current zero (to reduce residual flux in core) 2-6
3 Back Charging of Generator Transformers 1. Back charging (energizing from HV side with LV open) of generator transformers(gt) is necessary in power stations to ensure that GT is healthy, well before the generator / turbine installation is completed. This energisation is onerous than charging from LV side due to the high transferred surges to LV from HV side and the sudden application of full rated HV voltage, rather than building up voltage slowly from generator. 2. Since the transferred voltages to LV may exceed the Basic Insulation Level (BIL) of LV, back charging shall be done only with Lightning Arrester (LA) and surge absorber of the generator in circuit. When generator circuit breaker (GCB) is used, make sure that on the transformer side of the breaker, surge absorber and LA are connected. On some models of GCB, LA and capacitor on transformer side are optional items and hence shall be checked at the time of ordering of GCB. 3. Back charging shall be done at maximum tap with the entire tap winding in circuit. This will slightly reduce inrush current as the flux density will be less than at rated tap. It will also result in increased 2 nd harmonic content in inrush current reducing chances of mal operation of differential relay. This will avoid part winding resonance in the floating tap winding and consequent insulation failure in tap winding. This type of resonant over voltages can occur when natural frequency of regulating winding is near to the frequency components in the switching surge. Similar failure of dielectric breakdown in regulating winding was reported in generator transformers when back charged at rated tap, with 10 % regulating winding floating. 3-6
4 Mal-operation of Differential Relay 1. Recently in new transformers it is seen that differential relay trips during back charging of large generator transformers. This is due to the mal-operation of the harmonic restraining feature of the relay. Harmonic restraining feature (i.e. blocking the operation of relay when there is 2 nd harmonics in the overcurrent) was incorporated in differential relays around 1930 s. This was to prevent the relay from operation during magnetizing inrush current (which can be 5~15 times of full load current) and for relay operation only during over currents from internal faults. Normally inrush current used to have second harmonic content of 20~30% of the peak value of inrush current and the fault current will be with less 2 nd harmonics content. So the relay sleeps when 2 nd harmonic content is more than 10~15% and will operate when 2 nd harmonic current in unbalanced current is less. Due to the improvements in core material (Hi-B & Domain refined cold rolled grain oriented silicon steel-crgo) and core construction (step lap construction), 2 nd harmonic content in inrush current of modern large capacity transformers is less than 10%. In such cases, the differential relay will operate during back charging as the 2 nd harmonic content in inrush current is less than the setting time of relay. 2. If we reset relay for a lower 2 nd harmonic percentage, then the fault clearing time for a real internal fault will increase. Hence engineers are developing new differential relays based on new principle of operation such as wave form blocking criterion (a pattern recognition algorithm that searches for intervals within each fundamental power system cycle). 3. Sometimes it is necessary to reduce the inrush current, especially in large units, to avoid adverse effects on transformer windings from over currents. Inrush current will be minimum when the switching is done at the peak of the applied voltage sine wave. Special devices (preinsertion resistors, point on wave switching relay) are available to go with circuit breakers to achieve this end. In India, controlled switching is provided in all 765kV circuit breakers and in some of the 400kV circuit breakers used for transformer switching. This reduces the magnetizing inrush current. High neutral current When the neutral of star connected HV winding is earthed, a small current flow to earth is normal. But sometimes this may be quite high, more than 10 % of full load current, resulting in mal operation of restricted earth fault (REF) protection. In such cases, the cause is to be investigated and countermeasures are to be taken. 1. In a substation feeding to arc furnace transformer, continuous neutral current was noticed. Transformer was 80 MVA 220/33 kv of YNynd11 connection with a stabilizing delta tertiary winding. HV transformer neutral continuously showed 6-7 A current flow and to avoid tripping REF relay had to be set at 20 %. Table 1 Phase Currents and Neutral Current Circuit R Phase Y Phase Current B Phase current Star neutral current(a) (A) (A) current(a) HV under 10% Load LV Under 10% load
5 The above condition can occur in case of an unbalance in primary HV phase voltages which can be verified by measuring phase to neutral voltages. Phase angle errors can be checked by measuring phase to phase voltages. Reason for the unbalanced voltages can be lack of transpositions on the transmission line feeding the substation. 2. In a 100 MVA 11/138 KV YNd11 generator transformer with 19 tap on-load tap-changer (OLTC) neutral current was nearly 10 % of the load current when operated at certain taps and the same increased proportional to the load current. The transformer was provided with three single pole OLTC (MR type 3xMI 802) on HV side. When tested, the voltage ratio was tallying at all taps when measured under sequence of 19 to 1 taps. But when measured under 1 to 19 tap sequences, ratio and winding resistance in U phase was coming identical at certain taps viz 3&4, 7&8, 11&12 and 15 &16.High neutral current was also noticed when operated at these taps. Tap No At MW load, neutral current displayed in REX 521 relay was 21.3 A at tap no.12, 2.3 A at tap no.11 and nil at tap no.9 Actual Ratio U Phase 1 >19 tap Table 2 Measured Turns Ratio U Phase 19 > 1 tap V phase 1 >19 tap V phase 19 > 1 tap W phase 1 >19 tap W phase 19 > 1 tap On investigation, it was found that there was a rotational lag adjustment required in U phase between the tap changer and motor drive. When this was corrected by decoupling the drive and tap selector shaft, the ratio error came correct on both directions and neutral current disappeared. 3 Sometimes large transient neutral current can occur due to DC content in the magnetizing inrush current. This can cause spurious operation of earth fault protection. The above phenomena can occur due to the non-simultaneous closing of three phases of the circuit breaker.(ref 1) CONCLUSION When commissioning large power transformers certain abnormal phenomena can occur which may delay or prolong the commissioning process.some of the problems noted at site are reported with the reasons for such occurrence and mitigation measures to be adopted. REFERENCES [1] Pender, J.T and Kirkland, I. Large Transient neutral currents during energisation of 3 phase transformers Proceedings IEEE Vol 122 No.4 April 1975 Pages
6 BIOGRAPHY P Ramachandran has been employed at ABB India since 1999, and currently works as Technical Advisor, Power Products. He is involved in Transformer Engineering since He represented India in CIGRE study committee A2 during He received his Bachelor of Science in Electrical Engineering from the University of Kerala and Master of Business Administration from Cochin University, Kerala. 6-6
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