System-based transformer protection testing

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 over excitation.

System-based testing increases the protection testing quality and simplifies the process of commissioning and testing of all kinds of protection systems at the same time. Let know more how to simplify a transformer protection system test using system-based testing with OMICRON RelaySimTest.

The transformer differential application presenting 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 over excitation. The included transformer model can be parametrised 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 2^nd harmonic content, the energisation 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.

Images

Topology with the power transformer model

Fault incident during energization

Sympathetic inrush due to energization of parallel transformer

CT Saturation during external fault

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
  feeder protection systems
• line protection systems
  
• and logic schemes
  

The simulation capabilities included in RelaySimTest continue to evolve.