Elimination of Transformer Inrush Currents When Energizing Unloaded Power Transformers John H. Brunke

1. Elimination of Transformer Inrush Currents When Energizing Unloaded Power TransformersJohn H. Brunke Thank you. The topic of my presentation is the elimination of transformer inrush currents when switching unloaded power transformers. Thank you. The topic of my presentation is the elimination of transformer inrush currents when switching unloaded power transformers.

2. Presentation Outline Basics of transformer inrush transients Three phase transformer core flux transients based on model studies New controlled closing strategies Statistical performance

3. Transformer Core Characteristics Transformers use ferro-magnetic cores to couple the flux between windings. These cores exhibit properties which include hysteresis and saturation. These characteristics are most typically represented by a B-H curve (magnetic flux density , magnetic field intensity), can also be represented by a flux - current curve. (B wb/m2), (A*t/m). The characteristic shown here is such a curve for the model transformer at the power frequency. The sinusoidal voltage applied to the transformer winding produces a sinusoidal flux in the core and a very non-sinusoidal magnetizing current. Inductance is proportional to coil flux divided by current, so the slope of this characteristic is proportional to winding impedance. On interruption, assuming no current chopping, the magnetizing current will be interrupted at a current zero and a high residual flux, which is a permanent magnetization of the core, will result. The magnitude of the residual flux is dependent upon many factors, but is most largely effected by the capacitance of, and connected to, the transformer. EXPLAIN ON PLOTTransformers use ferro-magnetic cores to couple the flux between windings. These cores exhibit properties which include hysteresis and saturation. These characteristics are most typically represented by a B-H curve (magnetic flux density , magnetic field intensity), can also be represented by a flux - current curve. (B wb/m2), (A*t/m). The characteristic shown here is such a curve for the model transformer at the power frequency. The sinusoidal voltage applied to the transformer winding produces a sinusoidal flux in the core and a very non-sinusoidal magnetizing current. Inductance is proportional to coil flux divided by current, so the slope of this characteristic is proportional to winding impedance. On interruption, assuming no current chopping, the magnetizing current will be interrupted at a current zero and a high residual flux, which is a permanent magnetization of the core, will result. The magnitude of the residual flux is dependent upon many factors, but is most largely effected by the capacitance of, and connected to, the transformer. EXPLAIN ON PLOT

4. Energizing a Transformer When energizing a transformer the initial condition of the core flux is the residual flux. The point on the voltage wave where the transformer is energized determines the instantaneous applied flux. This can result in an asymmetrical flux in the core which results in the core being saturated, in this case in the negative direction. This is a worst case energization for any negative residual flux condition and produces saturation of the core in the negative direction. I would also like to define a term here..prospective flux. The flux shown is a virtual flux, flux does not exist until the instant the circuit is energized. It is the flux that the voltage source has the prospect to provide. This will be explained further. EXPLAIN When energizing a transformer the initial condition of the core flux is the residual flux. The point on the voltage wave where the transformer is energized determines the instantaneous applied flux. This can result in an asymmetrical flux in the core which results in the core being saturated, in this case in the negative direction. This is a worst case energization for any negative residual flux condition and produces saturation of the core in the negative direction. I would also like to define a term here..prospective flux. The flux shown is a virtual flux, flux does not exist until the instant the circuit is energized. It is the flux that the voltage source has the prospect to provide. This will be explained further. EXPLAIN

5. Transformer Inrush Current The core flux saturation produces the inrush current shown here. The peak inrush current was over 2200 amperes for the 250 MVA 230/115 kV transformer modeled. This current obviously has a large DC component and is rich in harmonics. These currents can persist for very long time. Times of over one minute have been reported. This transient current can cause the false operation of protective relays and fuses, and can cause sympathetic inrush currents to be generated in other nearby transformers. The high magnitude currents can stress the transformers windings and cause a reduction of the power quality on the system. The best method to prevent the generation of these transients is controlled closing. The core flux saturation produces the inrush current shown here. The peak inrush current was over 2200 amperes for the 250 MVA 230/115 kV transformer modeled. This current obviously has a large DC component and is rich in harmonics. These currents can persist for very long time. Times of over one minute have been reported. This transient current can cause the false operation of protective relays and fuses, and can cause sympathetic inrush currents to be generated in other nearby transformers. The high magnitude currents can stress the transformers windings and cause a reduction of the power quality on the system. The best method to prevent the generation of these transients is controlled closing.

6. Optimal controlled closing The basic theory of controlled closing has been understood for decades, although the technology has not been available for it to be achieved. If energization occurs when the residual flux is equal to the magnitude of the applied flux at the instant of energization, no asymmetry is produced. Note that this waveform could also be for the controlled energization of a simple capacitor from a power frequency voltage source. The dotted section would be the source voltage before energization. For optimal controlled energization of a capacitor, the correct closing instant is when the source voltage is equal to the voltage trapped charge voltage on the capacitor. For the transformer if this source �voltage� is a virtual source flux, the prospective flux, we have a duality. For optimal controlled energization of a transformer, the correct instant is when the magnitude of the residual flux is equal to the prospective flux. Note that there are two optimal instants per cycle. If we have a three phase transformer with single phase cores and only grounded windings each phase is independent and, with a measurement of the residual flux which can be made by integration of the winding voltage, optimal energization is possible.The basic theory of controlled closing has been understood for decades, although the technology has not been available for it to be achieved. If energization occurs when the residual flux is equal to the magnitude of the applied flux at the instant of energization, no asymmetry is produced. Note that this waveform could also be for the controlled energization of a simple capacitor from a power frequency voltage source. The dotted section would be the source voltage before energization. For optimal controlled energization of a capacitor, the correct closing instant is when the source voltage is equal to the voltage trapped charge voltage on the capacitor. For the transformer if this source �voltage� is a virtual source flux, the prospective flux, we have a duality. For optimal controlled energization of a transformer, the correct instant is when the magnitude of the residual flux is equal to the prospective flux. Note that there are two optimal instants per cycle. If we have a three phase transformer with single phase cores and only grounded windings each phase is independent and, with a measurement of the residual flux which can be made by integration of the winding voltage, optimal energization is possible.

7. Three Phase Transformers For the case of a three phase transformer switched from one winding with an other winding delta connected, or 3 legged core, the situation is more complex. Fluxes within the cores must sum to zero, but only the first phase to be energized is independent. When the second phase is energized full voltage is applied to the third phase from the delta connected winding. Therefore what every strategy is employed, the last two phase must be closed simultaneously. For the case of a three phase transformer switched from one winding with an other winding delta connected, or 3 legged core, the situation is more complex. Fluxes within the cores must sum to zero, but only the first phase to be energized is independent. When the second phase is energized full voltage is applied to the third phase from the delta connected winding. Therefore what every strategy is employed, the last two phase must be closed simultaneously.

8. Core Flux - No residual If first it is assumed that there is no residual core flux in the transformer, which is commonly assumed in the literature and state of the art, and the first phase to close is energized at the optimal point on the prospective flux wave (prospective flux zero), the delta connected winding will apply voltage to and generate flux in the other two cores is 180 degrees out of phase with the first phase, and each is 1/2 the magnitude. Now the flux in these two cores is not a static residual flux, but a time varying dynamic flux, which his the term I will apply to it, dynamic core flux. If first it is assumed that there is no residual core flux in the transformer, which is commonly assumed in the literature and state of the art, and the first phase to close is energized at the optimal point on the prospective flux wave (prospective flux zero), the delta connected winding will apply voltage to and generate flux in the other two cores is 180 degrees out of phase with the first phase, and each is 1/2 the magnitude. Now the flux in these two cores is not a static residual flux, but a time varying dynamic flux, which his the term I will apply to it, dynamic core flux.

9. Prospective and Dynamic Flux Now to the same flux time plot, the prospective flux is added. At the instant shown by the arrow, the dynamic core flux and the prospective fluxes are equal in magnitude on both of the last two phases, and energization at this instant will not produce flux asymmetry and is optimal. This is the state of the art.Now to the same flux time plot, the prospective flux is added. At the instant shown by the arrow, the dynamic core flux and the prospective fluxes are equal in magnitude on both of the last two phases, and energization at this instant will not produce flux asymmetry and is optimal. This is the state of the art.

10. Three phase residual flux If a transformer configured as the one I have just shown is de-energized, and interruption in each phase occurs at a current zero (no current chopping), a pattern of residual fluxes is typically formed. The fluxes must sum to zero, and the resulting pattern has zero residual flux on one phase and +/- some finite value on the other two. This pattern was observed in field tests, laboratory tests, model studies, and in the literature. Laboratory tests and studies showed this pattern could be modified by current chopping, but the fluxes still must sum to zero.If a transformer configured as the one I have just shown is de-energized, and interruption in each phase occurs at a current zero (no current chopping), a pattern of residual fluxes is typically formed. The fluxes must sum to zero, and the resulting pattern has zero residual flux on one phase and +/- some finite value on the other two. This pattern was observed in field tests, laboratory tests, model studies, and in the literature. Laboratory tests and studies showed this pattern could be modified by current chopping, but the fluxes still must sum to zero.

11. Core flux with residual flux If there is residual flux, and assuming for this explanation that the residual flux has formed an expected pattern on the previous de-energization, although this analysis is still valid for other residual flux patterns under the constraint that the residual fluxes sum to zero, then for this example closing the phase with zero residual flux at the optimal point again results in voltage at the delta connected winding (tertiary). This voltage is applied again to the other two windings connected in series, but this time it is not divided evenly. The phase with the negative value of residual is very quickly driven to the knee of the saturation curve, and the impedance of this winding is reduced rapidly. The voltage applied to the phase with a positive residual flux is driven away from saturation. In this series circuit the voltage across this winding is much higher and therefore the change in flux is much greater. This phenomena continues until both of these two phases which are energized in series are operating of the linear portion of the flux current characteristic. This results in the rapid elimination of the residual flux.If there is residual flux, and assuming for this explanation that the residual flux has formed an expected pattern on the previous de-energization, although this analysis is still valid for other residual flux patterns under the constraint that the residual fluxes sum to zero, then for this example closing the phase with zero residual flux at the optimal point again results in voltage at the delta connected winding (tertiary). This voltage is applied again to the other two windings connected in series, but this time it is not divided evenly. The phase with the negative value of residual is very quickly driven to the knee of the saturation curve, and the impedance of this winding is reduced rapidly. The voltage applied to the phase with a positive residual flux is driven away from saturation. In this series circuit the voltage across this winding is much higher and therefore the change in flux is much greater. This phenomena continues until both of these two phases which are energized in series are operating of the linear portion of the flux current characteristic. This results in the rapid elimination of the residual flux.

12. Prospective and Dynamic Flux Again, adding prospective flux and changing the time scale this plot is generated. First, the state of the art�if closure had occurred on the phase with zero residual, then there is only a small error 1/4 cycle later�.but if the phase with a non zero residual had been closed first a high flux asymmetry would result on that phase, and would alter the dynamic flux wave significantly. The optimal closing instants can be seen. Depending upon the polarity of the residual flux, the optimal closing point 1) or 2) is possible. The polarity of the first phase closure can select these. This solution to this problem is called the rapid closing strategy. It requires a detailed knowledge of the transformer characteristics�a look up table based on model studies is suggested. It should be noted the the accuracy achieved on the first phase closure greatly effects the existence of an optimal second optimal point. Another possible solution is can also been seen here. Independent pole control is required to implement the rapid closing strategy, but is residual fluxes are high in magnitude, then a near optimal solution for a three pole close is available.Again, adding prospective flux and changing the time scale this plot is generated. First, the state of the art�if closure had occurred on the phase with zero residual, then there is only a small error 1/4 cycle later�.but if the phase with a non zero residual had been closed first a high flux asymmetry would result on that phase, and would alter the dynamic flux wave significantly. The optimal closing instants can be seen. Depending upon the polarity of the residual flux, the optimal closing point 1) or 2) is possible. The polarity of the first phase closure can select these. This solution to this problem is called the rapid closing strategy. It requires a detailed knowledge of the transformer characteristics�a look up table based on model studies is suggested. It should be noted the the accuracy achieved on the first phase closure greatly effects the existence of an optimal second optimal point. Another possible solution is can also been seen here. Independent pole control is required to implement the rapid closing strategy, but is residual fluxes are high in magnitude, then a near optimal solution for a three pole close is available.

13. Prospective and Dynamic Flux The same plot is here made for a longer time interval. Core flux equalization creates a near optimal closing opportunities which do not require knowledge of transformer characteristics and are not affected by closing error on the first phase. Also only the residual flux on the first phase needs to be determined. This is called a delayed closing strategy.The same plot is here made for a longer time interval. Core flux equalization creates a near optimal closing opportunities which do not require knowledge of transformer characteristics and are not affected by closing error on the first phase. Also only the residual flux on the first phase needs to be determined. This is called a delayed closing strategy.

14. Closing Strategies Rapid closing strategy Requires detailed transformer data and look up table Delayed Closing strategy Generalized approach Three phase strategy Limited to high residual flux scenarios

15. Verification - Laboratory TestsRapid Closing Strategy

16. Verification - Laboratory TestsDelayed Closing Strategy

17. Effect of Closing Error

18. Prestrike

19. Statistical Studies Benchmarks - linear bias runs with statistical pole span 1000 studies/run Controlled studies - 200 studies/run 0.5, 1.0, 1.5, 2.0 ms timing error ranges

20. Statistical Performance

21. Transformer Model

22. Effect of Capacitance

23. State of the Art

24. Delayed Performance

25. Three Phase Performance

26. Rapid Closing Performance

27. Conclusions It is possible, by considering residual flux together with the appropriate closing strategy, to eliminate transformer inrush currents in most transformer configurations With consideration of breaker closing scatter, a reduction of typically 90% of worst, can be achieved