The most frequent causes of protection misoperations are setting, logic and design errors, caused, for the most part, by the increasing complexity of modern protection systems. Line differential protection is a case in point. To date, dedicated test plans and sequences have been created for every test set and every test case, all of which had to be calculated and evaluated individually. By combining the three core functions of the test process in a software package, OMICRON electronics is raising the security and dependability of system test results to a new level.
When commissioning a protection system, utilities always endeavor to test relays in a way that reflects how they will later be used in operation. Line differential protection fulfills its function using two or more relays that communicate with each other. The relays are distributed across various substations.
The testing of line differential protection under operational conditions, or as realistically as possible, requires the extremely precise time-synchronised simulation of a fault scenario at each end of the protected line, also referred to as an end-to-end test.
To date, a separate test sequence was created for each end of the line. In the facility, a phone call is made to a colleague at the opposite end so that the respective pair of test cases can be started at exactly the same instant. This methodology has stood the test of time.
However, what is evident is that the costs involved and the susceptibility to errors of this approach – in its preparation, execution and troubleshooting – are very high. This has led to the scope of the test being kept to a minimum and even avoiding end-to-end testing altogether, relying instead on the self-tests carried out by the relays. But it is precisely the end-to-end test that guarantees that the protection is working securely, dependably and selectively.
An innovative solution has, therefore, been developed that enables this important test to be carried out at a reasonable cost while still satisfying the testing prerequisites.
The end-to-end test as a system test
The main purpose of an end-to-end test is to validate the communications connection. However, it can also be used as a system test.
Studies such as the NERC Study have shown that most protection misoperations are caused by incorrect settings, or logic, or design errors. This is due to the complexity of the power systems and the increasing demands, resulting in an ever-growing number of protection and logic functions, not to mention relays.
A system test can be a great help in detecting the faults that can occur as a result. If a threshold has already been calculated incorrectly in the design, a conventional test is only able to check if the relay picks up at the incorrect threshold.
This fault can be detected by simulating the power system while calculating the end-to-end test. If we calculate the test signals by simulating a fault at 70 per cent of the line, it is possible to validate whether the settings would in fact trip a fault at 70 per cent instantaneously.
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