Developing the holistic approach - from single element testing to system validation

Authors: T. Schossig and C. Pritchard, OMICRON electronics Gmbh, Austria

Evolution of Protection Testing

History of protection testing: The history of protection testing is covered in the current issue of the magazine and will continue.
Test sets had to be multifunctional (Figure 5). They had to be portable (Figure 6).
Automation started already at this time. The most important was all the time to have fun while testing, as can be seen in Figure 7.
With the digital relays a new era in testing started (Figure8).

History of SAS testing:

 Substation Automation Systems have been available for a long time. They have been put into operation and tested in detail during commissioning and site acceptance ("bit test".)  Routine or maintenance testing was not common practice.

 Why are we testing protection? Utilities install and maintain power system protection with the intent to maintain or increase power system reliability. To achieve this goal utilities, standardization committees and federal agencies developed rules, processes and best practices that shaped state-of-the-art testing today. This can lead us to sometimes forget the goal of system reliability and only test for compliance, and in order to avoid legal claims. Therefore, we must constantly realign the testing strategy to the goal of improving the power system reliability and to reshape state-of-the-art protection testing.

Let's make this goal a little more tangible.  A well-designed protection system maintains an appropriate balance between selectivity, dependability, security and speed. If the protection is not working correctly during a system event, we call it a misoperation. By testing the protection, we try to find errors before they cause a misoperation. Therefore, with limited testing time and resources, we must invest our efforts where we can prevent misoperations. Which raises the question: what is causing today's misoperations?

The causes for misoperations in modern protection systems: In the early days of electromechanical (EM) protection, the cause for misoperations was very often found in the relay itself. Temperature, vibration and other influences caused the relay threshold to drift or caused complete mechanical failure. To avoid such relay failures, testing thresholds – for example over-current pick-up values and time delays – is a proven technique.
Since the introduction of the first microprocessor-based relays the power and complexity of the relays has continued to grow. Modern relays have more than 30 protection elements and approximately 1000 setting values plus a freely programmable logic. Due to now affordable communication technology these relays are part of a bigger protection scheme. At the same time less mechanical parts, suggest that the causes for misoperations is shifting. The NERC misoperation study proofs this hypothesis with numbers. (See Figure 10).

This study shows the reported main cause for misoperations in North America during the timespan of a year. Relay failures cause 20% of all misoperations, which still justifies conventional testing methods to some extent, though it must be considered that a significant amount of EM relays are still in operation in North America. As almost every transmission system relies on line differential protection, transfer tripping schemes or both, it is no surprise that communication failures are ranked 3rd as a cause for misoperations. With more IEC 61850 substations being built, protection engineers will start taking advantage and more often design schemes like breaker-failure, fast bus transfer etc.  This will relatively increase the misoperations caused by communication.

But what really underlines the shift in misoperations is that the most common cause for misoperations are setting, logic & design errors. These errors are usually assigned to protection engineers, who should ideally avoid making such errors. But looking at this study, we must accept that, setting, logic & design errors will sneak through the engineering process. Fortunately, this is why we test! Ideally the field test would act as a last safety net, finding the setting, logic & design errors before the system goes into operation.

With the goal of protection testing in mind and knowing the cause for misoperations, we can come up with a better testing strategy. We identified three sub-goals:

  • Making sure the protection is operating correctly under real-world scenarios and realistic current and voltage signals
  • Testing a system of IEDs simultaneously, to make sure the communication and coordination are working correctly
  • Increase efforts for testing protection automation and logic
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