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Utilities install and maintain power system protection with the intent to maintain or increase power system reliability. Therefore, protection engineers and technicians should align their testing strategy with this goal. Yet with limited testing time and resources, we have to invest our efforts where we can actually prevent misoperations.

Which raises the question: what is causing today's misoperations?

In the early days of electromechanical (EM) protection, the cause for misoperations was often found in the relay itself. Temperature, vibration, and other influences caused the relay threshold to drift or resulted in complete mechanical failure. Thus, testing procedures were in place to commission and periodically maintain the protection relays.

The goal for the majority of test cases is still to verify threshold values and timing. Since the introduction of the first digital relays, however, the power and complexity of the relays have grown. Affordable communication technology has made the relay part of a bigger protection scheme and, according to the NERC (North American Electric Reliability Corporation) misoperation study, the causes for misoperations have shifted towards communication failures and settings, logic, and design errors.

The challenge is that our testing has not adapted. If the target settings have errors, a threshold test can still pass. This only proves that the relay works correctly with the wrong settings. It is time to rethink our testing procedures.

Our field experience showed that, after commissioning with conventional testing tools, we regularly found errors with a system-based test approach (which is our definition of testing a protection system against its system-requirements). These system-requirements are the foundation for the design process of the protection and often describe a real-world fault scenario and how the protection system is supposed to respond. These system requirements remain true no matter what settings have been calculated for the protection. As such, a system-based test can validate if the given protection actually fulfills the system requirement; for example, if a fault on the line is tripped instantaneously. Similarly, a wrong target setting would fail the test. In other words, we validated if the protection does its job for the specific fault scenario.

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End-to-end testing can be kind of a system-based test. This is where the test case is derived from a power system simulation of a real-world fault scenario. This principle was further improved and developed into a tool to be applicable for every protection scheme with minimal effort. This tool integrates a power system simulation to define the real world scenarios with the capability to test a system of relays with multiple distributed test sets.

Experience of many different field tests collected with this tool proved the importance of system-based testing. Virtually all protection systems were commissioned with the conventional approach and we found errors in every third system-based test that would have otherwise gone into operation. This why system-based testing should be part of every modern commissioning.

Looking into the future, the demand for system-based testing will only increase and, for some technology, there even won’t be an alternative way to test. The digitalization of the protection system communication will result in even more distributed logic, which will require realistic input signals at many distributed nodes. Modern time-domain algorithm will also require a signal based on realistic power system simulations to operate correctly.

Finally, protection engineers and technician can only contribute to an increased system reliability if they follow a clear testing strategy. This strategy must have the goal to find as many errors as possible before they go into operation. System-based testing is a principle that contributes to this goal by testing if the protection actually works under realistic conditions.

The technologies and processes around commissioning are only evolving. To adapt, we must understand the increasingly-complex causes of misoperations and adapt our testing procedures accordingly. This is where events like the upcoming Canada Protection Symposium are critical for keeping engineers and technicians informed and connected to industry developments. Here, companies like OMICRON can share perspectives with fellow utilities and learn how they have implemented some of these new technologies. Additionally, attending CPS allows us to gain new insights from industry experts who will be presenting their case studies, lessons learned, and best practices.

Omicron plans to join those industry experts and encourage all industry stakeholders to do the same. The Canada Protection Symposium runs December 5-6 at the Westin Hotel in Toronto. A complete agenda is available online at www.omicronenergy.com/2017CPS.

Christopher Pritchard is a Product Manager for OMICRON electronics Corp. For more, visit www.omicronenergy.com.

 

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