Directional Protection with the OSM Recloser

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The OSM recloser has full directional protection capabilities as a standard feature.

When enabled, the recloser has the ability to operate on Earth Fault, Sensitive Earth Fault and Overcurrent protection settings in either power flow direction. This makes it ideal for applications where power flow may reverse such as ring main circuits or distribution system automation schemes.

For a recloser to determine power flow direction, it must first be able to measure both voltage and current. The OSM uses six Rogowski current sensors and six capacitively coupled voltage sensors to achieve this. Why six current sensors? Three sensors on one side of the device are used to measure phase currents, while the three on the other side are connected in series and used to measure residual earth currents. The Rogowski sensors are also capable of measuring down to 4A allowing it to provide directional SEF protection as a standard feature.

The OSM uses voltage and current measurements to calculate the phase angle between the positive sequence voltage and current. The user sets separate torque angles for overcurrent, earth fault and sensitive earth fault protection elements. Each element can have separate forward and reverse protection settings to ensure proper grading occurs in either power flow direction.

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.

Where: A1 = Phase angle between positive sequence voltage U1 and current I1

At = Preset torque angle

In this example the torque angle is set to 30 degrees for Earth Fault directional protection

The OSM recloser implements full directional protection. The protection elements applied during a fault depends entirely on the phase angle between voltage and current measured during the fault. This means, if the fault itself causes the power flow to reverse, the OSM will correctly apply protection settings for that power flow direction. This is an important point because devices that apply protection settings based on the last measured power flow direction may not be able to correctly operate when power flow suddenly reverses during a fault.

The example below is a simple example of directional protection in a feeder with two power sources. Recloser R2 will operate using positive power flow protection settings when Fault A occurs. If Fault B occurs the fault current reverses the direction of power flow at R2 causing it to operate using negative power flow protection settings.

Directional protection is easily activated on the OSM recloser by checking a box in TELUS for each element. Adjustments to protection curves can then be made using the built in graphical curve editor.

Directional protection is an involved subject and cannot be fully explained in the space provided here. For further Information on the OSM's directional protection features contact NOJA Power or your local distributor.