The stability and sensitivity test of the transformer before the final trip is an important and necessary test to verify the calculated and measured parameters of the power transformer and current transformer as per the data available on the nameplate of the transformers, Also another important aspect of the test is to verify that the differential relay should not operate in normal condition(stability) while it should operate in fault condition(sensitivity). important safety precautions need to be taken to conduct this test and are discussed.
There are several ways to perform a transformer stability test, however, I will explain one of the ways we choose to perform this test. Inject from the LV busbar and measure the results in the transformer protection and transformer LCP panel. But before injecting make sure to prevent false tripping and unnecessary availability of voltage in the HV bus bar, which we must take into consideration. Calculate the required LV and HV currents of the power transformer for differential and restricted earth faults and also calculate the required CT secondary results for verification of these parameters during injection.
LV Side:
Since we are going to inject into the LV busbar from the outgoing panel, the corresponding CT link must be shorted. Closed both CB of outgoing and incomer feeder so that power reaches LV terminal of transformer and make sure to rack out VT. Make MCB off the trip circuit and connect the cable from the generator to any outgoing feeder from the LV busbar.
HV side:
On the HV side voltage should reach to CT of the transformer bay, therefore, open the high-speed earth switch & close the circuit breaker to provide a path for the current, we don't need current up to the HV busbar, so make sure to close the earth switch after circuit breaker so that small HV current after passing through CB direct toward earth.