Using Directional Protection To Protect Isolated Neutral Networks

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It is necessary to consider special requirements when installing reclosers onto electrical distribution networks with isolated neutrals. Phase to ground faults in particular can have capacitively coupled fault currents on adjacent feeders when a phase to ground fault occurs. As a result any network protection scheme needs to be able to detect the direction of the fault.

In the example below, a fault is shown on the bottom feeder downstream of recloser R4. This has resulted in capacitively coupled fault currents in the adjacent feeders to start flowing in the opposite direction into the fault through the substation medium voltage bus connection. If R1, R2 and R3 were not able to detect the actual direction of the power flow, then they may trip erroneously. This would result in customers losing their supply unnecessarily.

The best solution would be to have just R4 trip, while the other reclosers remain closed. This is achieved by utilising reclosers with full directional protection. When the fault occurs, R1, R2 and R3 detect that the power flow is in the opposite direction and use this to block the fault so they do not trip. R4 detects the power flow is flowing downstream and trips normally, according to its earth fault settings.

All NOJA Power reclosers feature full directional protection. To do this they measure voltage and current on all three phases using built-in Rogowski current sensors and capacitive voltage sensors. The controller then calculates positive and zero sequence voltage and current to determine the phase angle and direction of power flow. The positive current calculation is used for overcurrent protection while zero sequence is used for earth fault.

In order to use directional protection on the OSM the user must program a torque angle into the controller which then determines when power flow is forward or reverse.

When a fault occurs, the phase angle between positive sequence voltage and current is calculated. If this angle is within +/- 90 degrees of the torque angle then the power flow is in a forward direction and the positive sequence protection settings will be applied. If the phase angle is outside of this area then the power flow is in a reverse direction and the negative sequence protection settings are used instead.

Using reclosers with directional protection capabilities on isolated neutral networks has distinct advantages. This method can eliminate the need for substation circuit breakers to operate to clear the fault while at the same time isolating just the faulted section. In addition, the reclosers provide the added ability to automatically restore the supply if the fault is transient.

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