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Friday, July 23, 2010

Setting or resetting of Frequency Relay at Electric Power Substation

Yesterday was the scheduled setting of Frequency Relay on our substation. Frequency Relay is a device that monitors the increase and decrease of frequency which can isolate a certain feeder if the frequency is unstable.
Normally, under frequency conditions will result in isolating of loads since low frequency means low power supply on the generation side. The Frequency Relay is very important to ensure that the generating plants will not overload in case the demand is greater than the supply. potential transformer inputs are usually required for frequency relays and this relay has a number of contacts that can be wired to a power circuit breaker tripping circuit, or to different alarm indications. Usually the power supply of these relays is 125 VDC supply. The relationship of voltage to frequency and Under frequency condition is usually monitored by the relay rather than over frequency conditions so my focus is under frequency setting. There are usually two settings of the frequency relay, one is the instantaneous setting and another is the time delay setting. The instantaneous setting will automatically trip the desired breaker/s or feeders once the set frequency has been reached. on the other hand, time delay setting will isolate the desired feeders or breaker/s if the desired frequency is maintained at a certain duration. Usually the value of instantaneous trip is lower than the time delay trip. Setting of the relay can be done through a test equipment, one good example is the Megger brand.
First, isolate the tripping signal by sliding the terminal contacts or lifting the wires. The PT inputs can be isolated and replaced by the generated voltage of the test equipment by serting a prewired test block on the test terminal.Then ensure that the test leads are properly connected from the test equipment. Usually tehse test equipment has markings to avoid error connections. The test equipment is very easy to use. It is just a step by step instruction until you go to the menu where you can just input the desired frequency setting and measure if the relay will actuate at the desired time setting. Usually, Frequency relays have +-accuracy which can affect the setting of frequency value. For example, the desired frequency is set 59.3Hz at 12 seconds. But during the actual measurement, the time for 59.3Hz is 12.5 seconds, which is not good. So, the frequency is adjusted to a lower frequency but near the desired value, say 59.29Hz, then measure again the time. if the measured time is 11.98 seconds already, which is also near the desired time, then that will be the prevailing setting. After all has been set, then return back the isolated terminals and remove the test plug to restore the normal operating condition of the feeders/ breakers.

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