A protection relay is a live device during testing: it needs DC power for its electronics, and its binary inputs and outputs must be wetted with the correct voltage so that trip and close commands are genuine. In the substation that voltage comes from the station battery (typically 110 V or 220 V DC). On the test bench there is no station battery - so the relay test set must replace it.
Relay power supply on site - an integrated DC auxiliary output powers the relay under test directly from the test set, removing the need to carry a separate bench supply or battery.
110 V / 220 V DC selection - the output is continuously adjustable around the standard station-battery levels, so the same set tests relays rated for 110 V DC and 220 V DC (and lower-voltage logic such as 24/48 V).
Independent adjustment - the auxiliary channel is galvanically separated and adjusted independently of the test currents and voltages, so it can simulate undervoltage of the DC supply while AC quantities remain constant - an important check for DC-supervision elements.
Realistic contact verification - because trip/close circuits are energized with real DC, the test set sees actual contact performance (including arcing and bounce) instead of a logic-level approximation.