Shunt reactors are connected in parallel with the rest of the power network. Shunt reactor can be treated as a device with the fixed impedance value. Therefore the individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z).
Thus during external fault condition, when the faulty phase voltage is lower than the rated voltage , the current in the faulty phase will actually reduce its value from the rated value.
Depending on the point on the voltage wave when external fault happens the reduce current might have superimposed dc component. Such behavior is verified by an ATP simulation and it is shown in Figure 17.
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| Figure 17: External Phase A to Ground Fault, Reactor Phase Currents |
As a result, shunt reactor unbalance current will appear in the neutral point as shown in Figure 18. However, this neutral point current will typically be less than 1 pu irrespective of the location and fault resistance of the external fault.
Similarly during an internal fault the value of the individual phase currents and neutral point current will depend very much on the position of the internal fault. Assuming that due to the construction details, internal shunt reactor phase-to-phase faults are not very likely, only two extreme cases of internal phase to ground fault scenarios will be presented here.
In the first case the Phase A winding to ground fault, 1% from the neutral point has been simulated in ATP. As a result the phase currents on the HV side (i.e. in reactor bushings) will be practically the same as before the fault as shown in Figure 19.
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| Figure 18: External Phase A to Ground Fault, Reactor Zero-sequence Currents |
In the first case the Phase A winding to ground fault, 1% from the neutral point has been simulated in ATP. As a result the phase currents on the HV side (i.e. in reactor bushings) will be practically the same as before the fault as shown in Figure 19.
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| Figure 19: Internal Phase A Winding to Ground Fault, Phase Currents |
However phase A current at the shunt reactor star point and common neutral point current will have very big value due to so-called transformer effect. These currents can be so high to even cause CT saturation as shown in Figure 20 for the common neutral point current.
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| Figure 20: Internal Phase A Winding to Ground Fault, Zero-sequence Currents |
In the second case the Phase A to ground fault, just between the HV CTs and shunt reactor winding (i.e. shunt reactor bushing failure) has been investigated. In this case the currents have opposite properties. The phase A current on the HV side is very big (limited only by the power system source impedance and fault resistance), while the phase A current in reactor star point will have very small value due to a fact that phase A winding is practically short-circuited.
As a result, shunt reactor unbalance current will appear in the neutral point. However, this neutral point current will typically have a value around 1 pu (i.e. similar value as during external ground fault).
That type of the internal fault (i.e. shunt reactor bushing failure) shall be easily detected and cleared by the differential, restricted ground fault or HV side overcurrent or residual ground fault protections. Neutral point ground overcurrent protection can operate with the time delay.
For internal ground fault in some other location in-between these two positions the shunt reactor currents will have values somewhere in the range limited by this two extreme cases.