T&D World Magazine
Distance Relays: How to Get Accurate Settings

Distance Relays: How to Get Accurate Settings

The results of a false operation of a distance relay can be extremely severe. Non-operation may lead to damage or instability of the system and incorrect tripping leads to unnecessary outages with all the problems that result. A common cause of such problems is the incorrect setting of impedance values of the relay. The positive sequence impedance is relatively easy to calculate because the parameters of the transmission line are known. By contrast, the impedance of the return path through the ground, or zero sequence, is much more difficult to estimate. This is because a typical impedance value is used but the ground is not homogenous and the influence of objects such as pipes, underground cables and rocks can significantly change its value.

As the calculated values for zero sequence impedance are unreliable, a far more effective approach is to measure the impedance of the fault loop directly. Traditionally this would require big mobile generators in order to deliver the power necessary to allow a measurement to be made against interference from the power system. This made such measurements costly and they were rarely performed.

A more elegant approach to this problem is to use the OMICRON CPC 100 with the CP CU1 accessory. This combination allows the injection and measurement of values at variable frequencies so that the effects of power system frequency can be eliminated. As a result this compact, lightweight combination can perform accurate measurements of the line and ground impedances. This provides the data to allow the distance relays protecting the line to be set precisely with impedance values and the calculated k-factor.

To ensure operational safety, the CP CU1 comes with the CP GB1 grounding unit. This unit can divert currents of up to 30 kA if its voltage limit value is exceeded due to unexpected induced potentials on adjacent lines.

In order to guarantee highest safety levels within and around substations CPC 100 and CP CU1 can also be used to measure the ground impedances inside a substation as well as step and touch voltages on its periphery. This can confirm the effectiveness of the substation’s grounding system.

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