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

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

When to Test

Routine testing? - > check for changes in the system:
Another important consideration, other than the testing tools and methodology, is the lifecycle of a protection system. the testing tools and methodology described so far can be applied to the following stages in the Lifecycle:

  • Pre-qualification testing
  • Factory acceptance testing
  • Site acceptance testing
  • Commissioning
  • Maintenance or Routine Testing

Generally, it can be said, the later the stage where an error is found, the costlier it will be. Contrary, the costs can even be reduced by eliminating redundancy in the testing process. To discuss every stage in this process in detail would go beyond the scope of this article, except for the topic of routine testing, which must be re-evaluated.
The main goal for most routine tests today, is to verify the correct functioning of the relay asset. The interval of testing mainly depends on the technology being used – electromechanical relays must regularly be re-adjusted and even microprocessor-based relays can now and then have component failures. Digital substations though can take advantage of the fact that GOOSE and Sampled Value communication has a life beat by design. As soon as an IED would fail, for example because of DC supply issues, its communication would drop out, which could raise an alarm. This would further extend the interval for a routine test for most IEDs. Only the end points like standalone merging units and switch control units would still require a shorter interval.
By contrast the intervals in which the power system itself is changing, is getting shorter. These contrary facts can lead to a situation that before the first routine test is performed, the setting and design are not appropriate anymore for the new power system status. Therefore, we must consider that the conventional routine test will become a re-commissioning, checking if the present protection scheme is still doing its job. For a re-commissioning the system-based test used during commissioning can easily be reused. Only the power system data must be updated, before the test plan can be re-executed.

Testing the SAS after firmware update, security:  With the introduction of complex substation automation systems, distributed functionality in combination with protection and updates because of security measures, the topic became more important. In the meantime, testing communication and automation is estimated as essential and time consuming. Testing solutions are available (PAC World 2018-03) (See Figure13).

Testing Together
As structures in the utility are developed further, it is no longer possible to distinguish between protection testing and control testing in all cases. Whoever is testing the protection should be testing the communication to the HMI as well. Changing communication might have an impact protection setup of the IED and demand a new protection test. A slightly different configuration revision in IEC 61850 GOOSE (confRev) can cause failed subscription of GOOSE and make a breaker failure scheme for instance not working any more. So, the protection testing solution must be capable of working with IEC 61850 GOOSE, set devices under test to test mode and utilize sampled measured values. Testing the SAS with the original protection system delivers trustworthy results- from the source to the local and remote HMI.

Outlook
As we see, protection, automation and control and their testing are growing together. Testing the PAC system as a system becomes reality.  Modern testing solutions have to take this into account.

BeijingSifang June 2016