Testing Protective Relays

As power grids get bigger and carry more power, the need for quick, reliable disconnection when faults occur becomes more and more urgent. The purpose of protective relay equip-ment is to sense fault states and trip circuit breakers. If a fault is not corrected early, personal injuries and serious damage can occur.

Disconnection must take place selectively, i.e. it must be limited to the faulty part of the power grid. This is why the protective relay equipment must be able to distinguish between permissible heavy load conditions and hazardous operational disturbances. To avoid unjustifiable interruptions, the protective relay equipment must not react to disturbances beneath a specified level called the pick-up value.

Electric power grid protection systems guard extremely valuable equipment, and protective relay equipment plays a vital role in this protection chain. To ensure consistent reliability, protective relay equipment must be checked by testing at regular intervals.

These tests must make certain that the protective relay equipment is operating according to its preset settings. The test equipment supplies the relay protection equipment with inputs that correspond to different faults and different operating situations. Pick-up values are approached by gradually changing the magnitudes of these inputs. Quick, selective disconnection in the event of a fault also requires correct operating times. These can be measured by supplying the protective relay equipment with inputs that exceed by a wide margin the pick-up value while simultaneously mea-suring the time that elapses prior to tripping.

There are two main principles for testing protective relay equipment. For primary injection testing, high current is injected on the primary side of the current transformer. The entire chain – current transformer, conductors, connection points, relay protection and sometimes circuit breakers as well – is covered by the test. The system being tested must be taken out of operation during primary injection testing (usually conducted in connection with commissioning and also when secondary circuits are not accessible).

For secondary injection testing, the protective relay equipment is disconnected from the measuring transformers and the circuit breaker. Current and voltage is fed directly to the protective relay equipment, and the system being tested does not have to be taken out of operation.

If a relay’s curves/characteristics are to be tested at many points or angles, repeated manual adjustment of the test equipment is time consuming. Test equipment that can conduct a test automatically in accordance with a plan drawn up in advance is much faster and far more convenient. Moreover, the time during which the protective relay equipment is out of operation is minimized and the test can be conducted in exactly the same way every time it is run.

Protective relay equipment must sometimes handle unusual faults that involve distortion, transients and harmonics. These unusual disturbances can be handled by test equipment having a DC-coupled amplifier and a program that gene-rates suitable disturbances or plays back information previously stored on a disturbance recorder. This permits nearly all forms of waveforms and transients to be generated.

Current transformers have different cores for protection devices and energy measurement equipment. Measurement cores are highly accurate, but will go into saturation at high fault currents. The protective relay equipment must be connected to the correct core in order to be able to operate pro-perly when a fault is present. This can be checked by plotting an excitation curve. The relay’s connection to the current transformer is measured using an AC voltage that is increased until the current transformer becomes saturated. Voltage is then plotted as a function of current, and the knee of the curve indicates saturation. Since the knee is much higher for the relay core than the core used for measuring purposes you can easily see whether or not the relay is connected to the correct core.

Current transformers must also have the correct transformation ratio. This can be tested by injecting high currenton the primary side, while simultaneously measuring the current in the secondary winding. Current transformers are tested, for the most part, in connection with commissioning. Since automatic testing proceeds at high speed and can be conducted repeatedly in exactly the same way, the time and effort devoted to preparations made before the first test are well worthwhile.