The transformer differential application presents several challenges and numerous features are employed in relay algorithms to compensate for these challenges. As a result, developing appropriate test quantities and properly quantifying results can be challenging with traditional settings-based testing.
The System-based Test Setup
In system-based testing, a system topology with a power transformer model is used to simulate the relevant power system scenarios, such as faults, inrushes or overexcitation. The included transformer model can be parametrized from nameplate data and it can simulate all relevant phenomena with sufficient accuracy.
Compared to settings-based testing, the test setup is much simpler, as all of the details of the simulation are taken care of by the transformer models included in the software. The only required data are the CT ratios and the transformer nameplate data:
- Vector group
- Rated voltages
- Rated power
- Short circuit voltage or short circuit impedance
- No-load current
- Core-type
Defining the Test Scenarios
Now all the test scenarios can be defined in a much simpler way thanks to the power transformer model:
- For instance, instead of defining a 2nd harmonic content, the energization of the power transformer at various closing angles can be simulated
- Or instead of defining a differential & restraining current, through-faults and internal differential zone faults can be simulated
All scenarios can be defined within a single-line diagram of the transformer and the surrounding power system.
- The differential relay is not supposed to trip for normal load current or faults outside of the differential zone. Thus, a test often begins with a test case for stability. In order to define such a fault in the software, the fault is simply dragged from a toolbar and dropped on the bus.
- To test the differential trip, the fault can be dropped on the power transformer.
For a turn-to-ground fault close to the starpoint (< 5%) a differential element might not be sensitive enough, therefore a Restricted Earth Fault (REF) element, if applied, will pick up the fault.
Same for turn-to-turn faults with both terminal close to each other, the differential element might not be sensitive enough, therefore a negative sequence element may pick up this type of fault.
- To test stability during inrush both breakers are in an open state. Within the test case an event will close the breaker, which will cause an inrush condition. By changing the closing angle, the amount of inrush in each phase can be altered.
Apart from these major test cases, many more real-world scenarios can be tested e.g. sympathetic inrush, fault during inrush, external faults with CT saturation and overexcitation due to overvoltage.
Since RelaySimTest can control multiple test sets from one PC software, three-winding transformers or REF protection can be tested without rewiring the test setup. Protection schemes for phase shifting transformers can be tested the exact same way, without adding complexity. Line protection systems, where the transformer is within the protected zone, can also be easily tested.
System-based Testing of other Protection Schemes
Even tests where the relays are in different or far away locations can be executed with ease from within a single instance of RelaySimTest.
The simulation capabilities included in RelaySimTest currently include:
- accurate simulation of busbar protection systems
Watch a video about busbar protection commissioning, https://youtu.be/bsK3fHHUNHQ
- feeder protection systems,
- line protection systems
Watch a video on testing a distance relay teleprotection scheme https://youtu.be/7mNAalfSTVY
- and logic schemes
Watch a video about testing the autoreclosing function https://youtu.be/a_lRgJ9_Gcc
The simulation capabilities included in RelaySimTest continue to evolve. Readers may download the most recent edition of the software for free here.
For more information: www.omicronenergy.com/en/products/relaysimtest/#download
