INTRODUCTION

As we can see from the History articles in the magazine, different forms of protection have been used in electric power systems for more than a century. With the increase in the importance of protection, it has been necessary to ensure that the relays are able to perform their functions as expected, when necessary, and do not operate when not required. Throughout the years different testing tools have been used successfully with electromechanical, solid-state and microprocessor based relays. Protection engineers and testing specialists from around the world have developed confidence in traditional methods such as “constant current” or “constant voltage” that have served them well for over a century. These methods were more or less the things you could do with the test devices available through most of the twentieth century:

• Fix the current at a specific level and then start reducing the voltage until the relay operates.
• Fix the voltage and increase the current.

This works great with electromechanical relays, but unfortunately does not help us if we need to test advanced protection IEDs. This brings us to the issue of Questions.

As with any human activity, before we start, it is important first to ask ourselves some questions that will help us do a better job, i.e. not only to get the job done, but also to do it in the most efficient way possible.

Why are we testing?

This is the first question that we should ask. The reason is, because the requirements for testing can be very different, for example if a manufacturer is performing type testing of a new relay or a user is considering the acceptance of a multifunctional protection device, a certain set of things will need to be tested:

• Characteristics of every function
• Performance of different functions under various operating conditions
• Performance of different functions under various non-operating conditions

These tests can be performed also with settings of the tested functions within the whole available range. There is a significant difference if we are testing a multifunctional protection IED during commissioning. In this case it will be sufficient to do testing only of the used functions with their application specific settings. Testing of the complete characteristic is not necessary. Checking of a couple of points just to ensure that the relay has the right settings and the wiring between the test switch and the relay is correct, is sufficient.

Things get more interesting when we are testing substation automation systems. There is a difference in the requirements, methods and tools if we are doing factory acceptance testing versus site acceptance testing.

What are we testing?

This second question has a lot to do with the complexity of the new world of protection, automation and control. And there are different answers to it. The requirements for testing tools and methods will depend on what we are testing:

• Electromechanical relay
• Multifunctional IED
• Distributed protection scheme
• Substation Automation System
• Communications based protection scheme in the lab
• Communications based protection scheme in the field

If we look at some of the examples above, testing of an electromechanical relay can be performed using a “constant current” or “constant voltage”, while the testing of an advanced multifunctional IED may require the use of electromagnetic transient simulation.

How are we testing?

This third question is related to the selection of the testing tools. It is clear that we can not answer it if we don’t already have the answers to the first two questions. But this is not sufficient. We need to also know very well the capabilities of the test equipment available, as well as the functionality of the available testing tools.

For example, if we are testing a high-burden electromechanical ground overcurrent protection relay, we will need to use a test set that can deliver the required current at the necessary compliance voltage. But if we are testing a relay using the IEC 61850 sampled values and GOOSE messages – a test device with communications only capabilities and support of the protocol may be used. Manual testing may be OK if we are just verifying the connections of a relay during commissioning, while execution of large, object-oriented test plans is needed for acceptance testing.

We will try to explain in mode details the answers to the above questions and discuss some modern methods and tools that can be used for twenty first century testing.