PRACTICAL CONSIDERATIONS FOR CONTROLLED SWITCHING OF POWER TRANSFORMERS

Transcription

1 Seminar / Workshop on Controlled Switching Possible Benefits for Transformers Applications PRACTICAL CONSIDERATIONS FOR CONTROLLED SWITCHING OF POWER TRANSFORMERS Esteban Portales Yvon Filion André Mercier St-Petersburg, Florida - May

2 Transformers Controlled Switching : Programme 1. Introduction 2. Transformer energization and resulting Temporary Overvoltages (TOVs) 3. Mitigation means 4. Reasons for using CSS on transformers 5. Examples of cases where CSS can advantageously be used: a) Integration of power plant b) System restoring c) Energization of transformer near sensible equipment d) Replacement or complete refurbishment of CB 6. Optimal closing strategy a) Importance of the residual flux b) Switching algorithm c) Simulations with Controlled Switching solution d) Means for measuring the residual flux 7. Means for measuring the residual flux 8. Conclusions 9. References St-Petersburg, Florida - May

3 1: Introduction Transformer energization produces an inrush current whose magnitude depends on the transformer characteristics, the point on wave closing time and residual flux. Inrush current has a rich harmonic content, among which 2 nd, 3 rd, 4 th harmonics are the most important. Inrush current harmonics may cause temporary overvoltages on the system due to resonance prevailing under some operating conditions. St-Petersburg, Florida - May

4 2: Transformer energization and resulting TOVs Because generated temporary-overvoltages on the system at transformer energization depend on: Network configuration Operating condition Transformer characteristics Circuit breaker characteristics Etc. Each case has to be studied closely (simulations performed, etc.) in order to select the right solution and to specify the right characteristics for associated equipments St-Petersburg, Florida - May

5 2: Transformer energization and resulting TOVs example - Network TOV produced at the energization of a kv transformer from 315 kv without controlled switching St-Petersburg, Florida - May

6 2: Transformer energization and resulting TOVs example - Inrush current produced at the energization of a kv transformer from 315 kv without controlled switching St-Petersburg, Florida - May

7 3: Mitigation means - Reasons for using them to reduce the overvoltages (TOV) resulting from the energization of large power transformers [1], [2], [3]. to reduce the mechanical and electrical stresses on XFOS in order to increase their remaining life [4], [5] St-Petersburg, Florida - May

8 3: Mitigation means list of possible solutions Use of pre-insertion resistors Use of controlled switching system (CSS) Generator voltage ramping restoration procedure; Energization of transformers under load on the sub-transmission side (if possible). Apply restrictions on the operating conditions (less flexibility for the operators, may be critical for system restoring scenarios) St-Petersburg, Florida - May

9 3: Mitigation means pre-insertion resistors - Inserer les resultats de simulation avec 8 et 14 ms St-Petersburg, Florida - May

10 4: Use of CSS with optimal closing strategy A. Importance of the residual flux B. Switching algorithm C. Simulations with Controlled Switching solution D. Means for measuring the residual flux St-Petersburg, Florida - May

11 4: a) Importance of the residual flux Optimal closing strategy for 3-phase transformers equipped with a secondary or tertiary delta winding was found to be first closing the phase having the highest residual flux, and then closing the two remaining phases simultaneously: For the first phase to close, the optimal closing time is determined by the residual flux relative to the instantaneous voltage; Closing time of the two other phases is optimal upon zero crossing of the voltage induced onto these phases. St-Petersburg, Florida - May

12 4: a) Importance of the residual flux Accurate knowledge of the residual flux (magnitude and polarity on each phase) of the transformers to be switched-in is a key parameter allowing to determine the best strategy for minimizing the inrush current and thus limit system overvoltages and transformer stresses. St-Petersburg, Florida - May

13 4: b) Switching Algorithm The first closing instant is set when the instantaneous voltage is equal to: v =V * 2 3 ( ) *sinα α = arccos ϕ * 3 * 2 * ω r V St-Petersburg, Florida - May

14 4: b) Switching Algorithm Graphical example of the algorithm application Phase B Phase A Phase C St-Petersburg, Florida - May

15 5. Practical cases where CSS can advantageously be used. 5.1 Integration of power plant Energizing at power plant S/S Energizing at integrating S/S 5.2 System restoring 5.3 Energization of transformer near equipment sensible to system disturbances St-Petersburg, Florida - May

16 5.1 Integration of power plant Power plant substation Integrating substation St-Petersburg, Florida - May

17 5.1.1 Energizing of transformer at power plant s/s Integrating substation Power plant substation St-Petersburg, Florida - May

18 5.1.1 Energizing of transformer at power plant s/s (Next Next) Temporary overvoltages (tov) were observed when energizing the step-up transformers located at the power plant substation (refer to fig. 1) Protection systems may operate under such voltage disturbances, possible damages for the equipment For the cases where important TOVs are observed, it is necessary to reduce overvoltages using controlled switching or pre-insertion resistors. St-Petersburg, Florida - May

19 Example of TOVs observed in the power plant s/s during energization of transformer at the Power plant S/S without any mitigation means Fig 1 - TOVs and inrush currents observed when energization takes place at power plant S/S St-Petersburg, Florida - May

20 4: c) Simulations with Controlled Switching solution Practical case 1: Energizing power transformers at SM -3 3 (with ( controlled switching) St-Petersburg, Florida - May

21 5.1.2 Energization at remote s/s (particular operating conditions when power plant is islanded on the radial sub-transmission system) St-Petersburg, Florida - May

22 5.1.2 Situation at the remote s/s Overvoltages due to transformer energization at remote s/s with the power plant islanded on the subtransmission system may lead to even higher overvoltages. (refer to fig. 2) solutions contemplated to mitigate overvoltages : Pre-insertion resistors on breakers; Controlled switching with some adaptions for this particular application; Generator voltage ramping restoration procedure; Energization of transformers under load on the subtransmission side. St-Petersburg, Florida - May

23 5.1.2 Energization at remote s/s (w/o any mitigation means) Fig 2 - TOVs and inrush currents observed whwn energization takes place at integrating S/S St-Petersburg, Florida - May

24 5.2 System restoring 1. Start generator groups Integrating S/S Power plant S/S Power plant S/S 2. Energize Xfo at integrating S/S 735 kv system St-Petersburg, Florida - May

25 5.2 System restoring - results Similar case as for energizing at remote S/S Can result in very important TOVs Can cause damage to equipment St-Petersburg, Florida - May

26 5.2) System restoring another scenario - St-Petersburg, Florida - May

27 5.2 System restoring example with 2 Groups - 1 Line St-Petersburg, Florida - May

28 5: System restoring - Simulations with Controlled Switching solution Practical case 2: Sw.-in at 315 kv: 2Groups-1Line with css St-Petersburg, Florida - May

29 5.3 Energization of transformer near equipment sensible to system disturbances Islanded network Energizing one of these xfo w/o mitigation mean may cause some commutation failure of HVDC converter St-Petersburg, Florida - May

30 6: Measuring the residual flux Theory Sensors Real case measurement St-Petersburg, Florida - May

31 6: measuring the residual flux: theory To achieve the magnetic flux Φ measurement without being intrusive, the value has to be derived from a more common (or easily accessible) signal: the voltage E at the transformer input.. 1 Φ = E + ϕ r N Measuring and integrating the voltage of each phase just before and during de-energization of the transformer in the steady state determines the residual flux, which is the end value of the integrated voltage. When an event occurs, a time window is set (just before and during de-energization of the transformer) and the integration of the voltage signal of each phase of the unloaded transformer is performed. With this method, only a small number of cycles is used for integration to prevent that even a small offset error in the voltage measurement lead to a large error in the results. St-Petersburg, Florida - May

32 6: Measuring M the residual flux: sensors For an accurate magnetic flux measurement, the system needs 3 main items: Transformer voltage measurement Measurement of the transformer input current and a software package St-Petersburg, Florida - May

33 6: Measuring M the residual flux: sensors Transformer voltage measurement St-Petersburg, Florida - May

34 6: Measuring M the residual flux: sensors Transformer voltage measurement St-Petersburg, Florida - May

35 6: Measuring M the residual flux: sensors Accurate measurement of the transformer input current ranging from the very low magnetizing current up to the high inrush current values St-Petersburg, Florida - May

36 6: Measuring M the residual flux: sensors Transformer input current St-Petersburg, Florida - May

37 6: Measuring M the residual flux: sensors Software package St-Petersburg, Florida - May

38 6: Measuring M the residual flux: real case For integrating the new Sainte-Marguerite-3 hydroelectric powerhouse (SM-3) in TransÉnergie s 315-kV grid, the following challenges were successfully overcome: Extensive EMTP simulation studies Development of voltage and current sensors Development and installation of 2 prototypes Preliminary tests St-Petersburg, Florida - May

39 6. Measuring the residual flux: real case Measured/calculated residual flux after switching-out the transformer St-Petersburg, Florida - May

40 6. Measuring M the residual flux: real case Measured current at the transformer energization St-Petersburg, Florida - May

41 6. Measuring the residual flux: real case Measured network voltage at the transformer energization St-Petersburg, Florida - May

42 7. Economical considerations List of points to be considered Cost analysis (PIR replacement vs CSS) Maintenance costs (frequent switching) Technical feasibility of CSS with existing CB characteristics (RDDS, time scattering,) Reliability issue and consequences for unavailability of the equipment St-Petersburg, Florida - May

43 8. Conclusions Energization of power transformer can result in severe TOVs on the system mainly depending on: The system conditions Transformer characteristics Instant of energizing on the point on wave in relation with the residual flux. System simulation studies are necessary to evaluate the risk of havingtovs following XFo energization For the cases with TOVs, mitigation means are needed to avoid dammages to equipment. St-Petersburg, Florida - May

44 8. Conclusions Laboratory tests and on-site tests confirmed the accuracy of residual flux measurement. An optimal closing strategy was validated for 3-phase transformers having a secondary or tertiary delta connected winding. Accurate residual flux measurement enables energization of unloaded transformers without generating inrush current or TOV on any phase. Controlled switching of transformers is feasible, reliable, cost-effective and less stressful for the transformer and the electrical network. St-Petersburg, Florida - May

45 1: References Hydro Québec Simulation studies Other references 1) CIGRE WG13.07, Controlled Switching of HVAC Circuit Breakers Guide for application, Part 1: ELECTRA, No 183, April 1999, pp Part 2: ELECTRA, No 185, August 1999, pp ) CIGRE WG 13.07, Controlled Switching of HVAC Circuit Breakers Planning, Specification and Testing of controlled switching systems, ELECTRA, No 197, August ) CIGRE 2002, report , «Analysis, simulation and testing of transformer insulation failures related to switching transients overvoltages» 4) IEEE, Transactions on power delivery, M. Steurer, K. Fröhlich, The impact of inrush currents on the mechanical stress of high voltage power transformer coils St-Petersburg, Florida - May

46 1: References????? «Worldwide many transformer insulation failures have been reported caused by switching operations, while those transformers had previously passed all the standard tests and complied to all quality requirements.» «The problem is generally associated to the high frequency overvoltages produced by the re-strikes and pre-strikes during the opening or closing of a switching device.» St-Petersburg, Florida - May

47 1: References «From failure experience on power transformers very often it was suspected that inrush currents, occurring when energizing unloaded transformers, where reason for damage.» «Although inrush currents normally are smaller the forces can have similar amplitudes as those at short circuit however with longer exposure time.» St-Petersburg, Florida - May

48 Seminar / Workshop on Controlled Switching Possible Benefits for Transformers Applications Thank you for your attention St-Petersburg, Florida - May