Turn-to-turn faults in shunt reactor present a formidable challenge to the protection engineer. The current and the voltage changes encountered during such fault are very small and therefore sensitive and reliable protection against turn-to-turn faults is difficult to achieve. At the same time the longitudinal differential protection offers no protection at all for such faults. Hence special protection schemes shall be employed.
One such scheme, often used in certain countries, utilizes a fact that the HV shunt reactor winding is often made of two half-windings connected in parallel (i.e. the HV lead is brought out at the mid point of the winding, and the two neutral leads at the bottom and the top of the winding). This gives the opportunity to install two CTs in the winding star point (i.e. one in each winding part). Then so-called split phase differential protection can be utilized to detect turn-to-turn faults. However this protection scheme have the following drawbacks:
• this special CT arrangement typically causes reactor manufacturing problems
• typically low CT ratio is required, which can cause longitudinal differential protection problems during reactor switching in, if the same CTs are used for both differential protections
• this scheme can be only used if the shunt reactor is specifically ordered with these CTs
Second turn-to-turn protection scheme for shunt reactors, successfully used in some other counties, utilize the following facts:
• HV power system voltages are well balanced during normal load conditions
• Modern HV, oil immersed shunt reactors have very small manufacturing asymmetry between individual phases
• Shunt reactor winding impedance is approximately proportional to the square of the number of active turns
• Short circuit between some number of turns will cause the decrease of the winding impedance only in the faulty phase and corresponding small raise of the shunt reactor neutral point current
• Currents during turn-to-turn fault are of the small magnitude and they will not produce any sufficient unbalance voltage
• Any external cause of neutral point current (i.e. external phase to ground fault) will cause appearance of unbalance voltage which can be used to block the operation of turn-to-turn protection scheme
• In case of a bigger winding turn-to-turn fault which might cause the sufficient voltage unbalance, sensitive directional zero sequence relay connected on the shunt reactor HV side and set to look into the reactor shall be capable to detect such fault This protection scheme was developed even before multifunctional numerical relays were available. To implement such shunt reactor turn-to-turn protection scheme within multi-functional numerical relay utilizing its graphical configuration facilities, and readily available logical gates, timers etc. shall not be a big problem for a protection engineer.
In order to verify above statements, shunt reactor behavior, for phase A winding 1% turn-to-turn faults, is verified by an ATP simulation and it is shown in Figures 21 & 22. From these figures is obvious that the above-described scheme can be successfully implemented if the power system itself is well balanced.
 |
| Figure 21: Internal Phase A Winding turn-to-turn fault, Phase Currents |
 |
| Figure 22: Internal Phase A Winding turn-to-turn fault, Zero-sequence Quantities |
