We’ve discussed the testing of protection relays many times in Electrical Tester. There is, however, never any shortage of questions on this interesting and sometimes challenging topic, and the technology adopted in both the relays themselves and in the equipment used to test them is developing rapidly. We make no apology, therefore, for taking another look at questions our technical support team has recently been asked about relay test.
Q: Modern protection relays are becoming increasingly complicated and intelligent, which means that it’s often very difficult or even impossible to test them with single-phase voltage and current. Three-phase test sets are, however, usually complicated to use and typically offer features that are only needed by the most advanced users. Is there a better solution?
A: Indeed there is! A new type of three-phase test set has recently become available which has been designed specifically to address this problem. These instruments have essentially been designed as an engineer’s “multifunction test box”, and they combine a straightforward and intuitive user interface with extensive facilities for three-phase testing. They provide a wide range of pre-configured virtual test instruments so that all of the most commonly used protection relay tests can be set up and performed quickly and easily. Full manual control and configuration options are also provided to cater for special testing requirements. In addition, the instruments offer other useful functions, which include an analogue ammeter and an analogue voltmeter together with facilities for measuring a wide range of parameters such as phase angle; active, reactive and apparent power; power factor; resistance; reactance and frequency.
Q: What are the functions needed to test protection systems that are used in distributed generation applications?
A: When connecting generating installations with a capacity greater than 16 A to the supply network in the UK, they should now comply with Engineering Recommendation G59/3 issued by the Energy Networks Association. Requirements in most other countries are likely to be broadly similar but it is, of course, important to check local regulations. G59/3 dictates the type of network protection to be provided and the tests that need to be carried out on the protection relays. These include under and over-voltage, under- and over-frequency, stability tests, and protection from the loss of mains supply by step change of phase or rate of change of frequency (ROCOF).
Q: Sometimes it’s necessary to test single-phase relays with higher currents or higher voltages than are directly available from a single channel of a three-phase test set. How can this be done?
A: The most convenient and cost-effective solution is to choose a three-phase test set that allows output channels to be connected in parallel to provide higher currents or in series to provide higher voltages. This isn’t a particularly new idea and there are several test sets on the market that offer this option. In most cases, however, the series and parallel connections have to be made using cables. This is fiddly and time consuming and it’s very easy to make mistakes. Megger’s new easy-to-use three-phase Sverker 900 test set provides a better solution. Series and parallel connections are set up using rigid plug-in links rather than flexible cables. The links and the sockets they plug into have a patented configuration that has been specially designed so that it is physically impossible to make an incorrect connection. As a result, swapping from three-phase testing to high-current or high voltage single-phase testing takes only a couple of minutes and the risk of errors is completely eliminated.