Testing of the main protection functions

When testing the individual protection elements in a conventional fashion, it is very important that they are the only enabled protection function (if all protection elements share the same relay output). If the IED has multiple relay outputs and different protection elements are mapped to different outputs, we need to make sure that the test device monitors the correct relay output during the test.

For a modern test system, such mappings shouldn’t be necessary. A good fault model will correctly generate a system condition that the relay should distinguish, indicate, and trip correctly for based on the enabled protection element characteristic.

If we (based on the measurement functions tests) assume that the relay measures accurately the applied current and voltage signals, the testing of the distance elements should give us an indication of what is the characteristic of the tested zone and expected relay operating time when the apparent impedance seen by the distance element based on the applied currents and voltages is within the operating characteristic.

Constant voltage and constant current methods may be used for the distance characteristics testing. This is acceptable for some microprocessor based relays that use distance elements based on the relationship of current and voltage phasors. While such tests are related to checking the distance characteristic of the relay, they may not be suitable for the testing of the relay tripping time. This is especially important for Zone 1. If the relay uses superimposed components for the fault detection, faulted phase selection or directional detection, the ramping of the current or voltage in some of the conventional test methods is not going to be seen as a fault condition and the relay under test is not going to operate. Electromagnetic transient simulation is the best way to generate the signals used for the performance testing of the distance element. Evaluation of the distance element operation for multiple points on the selected characteristic is typically required. Figure 7 shows the configuration for the testing of a distance relay with a complex characteristic.

If the results from the testing of the distance characteristics and the operating time are within the expected range, the next step is the testing of the different communications based schemes.

Testing Of Distance Protection Schemes

The testing of distance protection schemes is the final step in the testing of a distance relay and is based on the assumption that all individual protection elements – distance, overcurrent, directional, faulted phase selection, etc. have already been tested and proven to be operating correctly. An important consideration is the purpose of the test. If the test of a distance scheme is performed as part of a relay acceptance test, the complete test can be performed by the simulation of the analog and binary signals that the relay is going to measure or monitor under the specific test case conditions. However, if the test is part of the commissioning of the protection system of a transmission line before it is put in service, it may be necessary to test the complete protection system, including the communications channel. End-to-end testing using GPS synchronization is the preferred method in this case.

When communication aided schemes are used in complex system configurations, including double circuit transmission line or transmission line loops with or without mutual coupling, sequential tripping of faults on adjacent lines may result in incorrect operation of the accelerated schemes. It is required to develop test sequences simulating such conditions to verify that the protective relay is going to operate correctly.

The next step in the Distance Protection Scheme testing is the extension of the same test cases into the full operational protection system test. This is commonly referred to as End-to-End Testing or System Testing.

IEC 61850 can also be an integrated function in transmission line protection relays. It impacts the testing process by requiring the test system to be configurable using the Substation Configuration Language, as well as to be able to simulate GOOSE and sampled values, as well as to subscribe to and process GOOSE messages from the tested relay.

Transient simulation based testing

When we analyze the complexity of modern multifunctional distance protection devices, it is clear that their testing requires the use of advanced tools that can simulate the different system conditions and status of primary substation equipment and other multifunctional devices. The test system should be able to replay COMTRADE files from disturbance recorders or produced from electromagnetic transient analysis programs. The focus at this time is determining the performance of the device under realistic system conditions as required by the application. If necessary, the test cases should also include synchronous or asynchronous out of step conditions simulation to test the power swing blocking or tripping functions. Performance of the tested relay when a fault occurs during a power swing should also be included in the test plan.

The testing tools should allow easy configuration and execution of such transient simulations as part of the testing process, as well as proper evaluation and reporting of the operation of the tested device (Figure 6).

The protection of double circuit or other parallel line configurations need to be able to operate under different system conditions, evolving and cross-country faults, sequential tripping conditions and under the influence of mutual coupling. All of the above needs to be considered in the testing of such protection relays or communications based schemes.

If we are testing a relay used on a double circuit line, it is important to properly model not only the impedances of the two circuits, but also the mutual coupling between them. Generic programs such as EMTP or ATP can be used to produce such files. They require good knowledge of the software and proper configuration of the model and simulation. Specialized testing tools make this easier by providing a template for the transient simulation of different fault conditions on mutually coupled double circuit line (Figure 6).

This offers significant advantages over the testing based on a sequence of steps programmed in the software by manually entering voltage and current phasors calculated by a steady state fault analysis software which do not properly simulate the dynamic transition from one state to another.