What’s wrong with relay test?
Stan Thompson - Product manager
While user requirements for protective relay test sets have of course, evolved over the years particularly with the advent of substations designed around the new IEC 61850 networking system, the basic factors driving developments today remain the same as they were three decades ago.
Users want smaller, lighter test sets that are versatile and easy to use, and that provide ever higher power output capabilities. They also want rugged construction so that the test sets will provide a long reliable service life under tough on-site conditions. It’s interesting to see how developments in test sets over the years have addressed these requirements, and to speculate what can be expected from the next stage in test set evolution.
Effective programmable electronic test sets for protective relays have been available since the early 1980s, and even in those days, the equipment was portable – but only just. Test sets of that era were invariably modular, with a module typically providing a single-phase AC voltage and current source. A minimum of three of these modules were therefore, needed for three-phase testing, plus another module to provide DC voltage and current outputs.
Each module weighed in at around 30 kg, which meant that in practice two people were needed simply to get the test set out of the vehicle used to transport it to site and to set it up. And a large, strong table was needed to accommodate the equipment while it was in use!
These test sets were usually capable of providing in the region of 25 A per phase at an output power level of 100 VA. Users could parallel the outputs to get 75 A, but many still wanted more current and power per phase. Test set manufacturers responded by adding high power modules to their range, but these were even larger and heavier than the standard modules. A module that weighed in excess of 50 kg really did need two strong people to carry it around!
As well as their size and weight, another shortcoming of these early units was that they were often not sufficiently robust for regular use in the field. It was not at all unusual to find that after just a year to two of service, the enclosures needed to be repaired extensively or even replaced.
By the late 1980s, advances in technology had made it possible to develop much lighter modules than those previously offered, and also to provide them with much more robust enclosures. The weight of a typical module was cut by 10 kg, giving a 40 kg reduction overall for a four-module system.
This reduction made the equipment much more manageable, but users were by then asking for a different approach. What they now wanted was a complete three-phase test set in a single box, light enough to be carried by one person, and small enough to fit in the boot of a car. And it goes almost without saying that they wanted at least as much output power as they were getting from the large modular test sets.
One of the first pieces of test equipment made available to meet these new user demands was the Pulsar Universal Test System in October 1992. This offered variable output frequency, together with variable DC voltage and current, as well as the usual AC outputs. It also, for that time, had a remarkably high output capacity: with three outputs in parallel it could deliver 90 A at 450 VA. It was housed in a durable polyethylene enclosure and, best of all, it weighed less than 30 kg. In other words, this complete three-phase test set weighed about the same as a single module from one of the first-generation test sets.
The Pulsar systems were so well matched to user requirements that they remained in production for over a decade. During this time, however, user expectations moved on, and the demands were still there for smaller, lighter test sets that could do more. In particular, at this time there was a specific need for easy-to-use facilities for manual testing.
February 2004 saw the launch of the MPRT test system, which is capable of providing 120 A at 800 VA. The original versions of the MPRT weighed less than 23 kg and the current versions, which use a new type of enclosure, weigh only 19 kg. Another important innovation was the introduction of on-board computer controls, and the Touch View interface (TVI).
Even the earliest test sets had been designed for computer control, but they had all required the use of a separate PC during testing. With the MPRT system, computing power is built in, enabling most tests to be performed without the need for an external PC. The TVI, which is a hand-held colour touch screen device, provides users with a convenient and intuitive way of accessing the computing power of the MPRT to set up and perform tests, either automatically or manually.
Overall, very significant progress has been achieved – today’s test set is only one-sixth of the weight of a first generation unit and less than a quarter of the volume, but it has far greater output capacity and, thanks to the TVI, it is much easier to use. The latest versions even have integral support for the testing of IEC 61850 devices. But nothing stands still in protective relay testing, so what can users expect from the next generation of testers?
As always, they will be even smaller and lighter than the current units, yet their output capacities will be further enhanced – the latest technologies are making it possible to deliver more than 120 A from a compact lightweight test set. And, of course, these next-generation testers will have been designed from the outset for use with the intelligent electronic devices (IEDs) or “smart relays” that are playing an increasingly important role in the design of new smart grid substations.
IEC 61850 support can be taken for granted, and there are certain to be advances in the user interface. The touch-screen technology, which is already a favourite with users, is likely to be further enhanced to provide, for example, higher screen resolution that makes it easier to read graphical displays, and allows more information to be presented on a single screen.
As we’ve seen in this short review, test sets for protective relays have come a long way over the last three decades, but developments are on-going and I can tell you in confidence that the best is yet to come!
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