Important aspect in periodical testing is to secure that at the end of the testing all parts of the installation are restored to the original state. For example, if some test requires disconnecting relay voltage measurements from busbar voltage transformer secondary circuits, the connections must be reestablished at the end of that testing. It’s highly recommended that the test engineer keeps a record of all possible temporary modifications within the installation, to help him/her to revert to the original set-up.
In general, a good rule is to perform the tests using a method which is as close as possible to the normal operational conditions of the installation. This is one way to reduce the risk of leaving some temporary connection or setting unrestored to original position at the end of the testing.
Periodical testing is performed with the protected feeder de-energized, circuit breaker truck withdrawn to test position or in case of a fixed circuit breaker, bus- and line-disconnectors in open position. In case the other feeders within the installation remain energized and supplying loads, it’s important to isolate the relay under testing from the rest of the system to avoid accidentally sending upstream trip commands during the testing.
Protection relay periodical testing is normally carried out as secondary injection testing. The currents and voltages normally measured via sensors or instrument transformers are replaced with adjustable analogue signals from the test set. In case the installation is equipped with relay test blocks, the separation of the relay from the measurement and trip circuits happens automatically when the test adaptor is inserted into the test block.
If test blocks are not present, the connection point of the test set is the LV-compartment’s terminal blocks. These terminal blocks typically offer the possibility to open the terminal mid bridge connection and that way separate the relay under testing from the measurement and trip circuits. Each protection function is then tested one by one to confirm the functionality in respect of:
- start value level
- operate value level
- reset ratio value
- operate time, typically done with an infeed of 1,5 times the operate value setting
The results are compared to the setting values and the test records from the commissioning and the latest periodical test results. In case discrepancies outside the manufacturer’s stated accuracy limits are found, the corresponding test shall be repeated. If the second test results are still outside the limits, deeper considerations are needed. If the deviation is considerable, recommendation is to replace the relay with a spare relay. In case of a slight deviation the operation until the next periodical test round can be considered.
The secondary test set will also introduce certain level of inaccuracies to the tests, especially if the test set has not been calibrated as per the manufacturer’s instructions. As a final task for each protection function testing, it’s highly recommended to do final operate test including the circuit breaker operation. In case the circuit breaker is equipped with two trip coils, the condition of both coils shall be verified.
During the testing of protection functions, the related measurements and events shall be verified from the local HMI. The verification, if possible, is recommended to be extended all the way to the upper-level systems like Scada or process control system.
When testing individual protection functions, there will be situations where some other protection function might interfere the testing of the selected one. REX615 HMI under menu “Tests” offers various support for testing activities. When the relay is returned to Normal mode, the functions will regain their original status automatically.
The correct function and the related external circuitry of IRF-relay (internal relay fault) output shall be checked. This can be performed using dedicated functionality in the HMI. Under the menu “Tests” there is a tab called “Internal fault test”. By selecting position “test on”, the IRF-relay will be de-energized, and the function of the related circuitry can be verified.
Arc flash sensors connected to REX615 relays must be tested by high intensity light like strong camera flash. This functionality will indicate if the connected sensor’s or lens, optical circuit is intact.