Test & measurement - Putting engineers back in control


IEC 61850, the new standard for substation data networks, is creating a lot of interest and  excitement. It's also creating more than a few challenges, says Romain Douib of Megger, not least for substation control engineers who spend their lives creating and working on interlocking schemes

One of the biggest challenges substation control engineers face, is not how to implement interlocking schemes based on IEC 61850, but how to test them. The problem is particularly acute, because at present IEC 61850 is being more widely used for interlocking than it is in protection applications.
Of course, options do exist for testing IEC 61850 interlocking schemes. However, these almost always involve the use of protective relay test set that supports IEC 61850. This approach, however, is far from ideal. The first concern is that, in most cases, control engineers are not protection engineers. They are unlikely, therefore, to be familiar with the operating a protective relay test set. They could, of course, learn, but that's a pretty steep learning curve for something that is not central to their work.
Another issue is protective relay test sets are necessarily costly, since they incorporate high-performance precision amplifiers and other elements that are expensive to develop and produce. Yet these are not needed for testing interlocking schemes, so using a relay test set in this application is not only overkill, it also needlessly ties up expensive capital equipment.

It's clear there is a pressing need for a reasonably priced instrument that is simple to use and provides all of the facilities needed for testing IEC 61850 interlocking schemes, but does not incorporate the expensive extras needed for protective relay testing.

It's not difficult, in principle at least, to imagine how such a test set would work. First of all, it would monitor the Goose messages IEC 61850 installations use to communicate and it would convert them to the ordinary type of on/off binary signal that control engineers are used to working with in non-networked installations.

The test set would also be capable of working in the opposite direction. That is, it should take signals from ordinary contacts and convert them into appropriate Goose messages. In effect, a test set of this kind is simply an interface between the Goose messages on the bus and the electromechanical world of the control engineer.

Of course, there's rather more to be considered than this very basic overview initially suggests. For example, the conversion between Goose messages and binary signals must be fast enough so as not to materially affect the timing of the interlocking system. In practice, a conversion time of less than a millisecond, which is achievable with careful design, will be fast enough to satisfy the most demanding of requirements.

Next, it is clearly necessary to be able to associate particular Goose messages with specific inputs and outputs on the test set. This is best accomplished with software but, if it is to be intuitive and easy to work with, the software needs to be carefully designed. Further refinements can also be envisaged. For example, LEDs that provide instant visual confirmation of the state of the instruments binary inputs and outputs would be an important benefit for users.

The ideas mentioned in this article have driven the development of Megger's new Goose Message Interface.. This embodies a number of unique technical features for which patents are pending, and offers the most efficient and cost-effective solution currently available to the challenge of testing IEC 61850-based substation interlocking schemes.

That is, however, by no means the limit of the capabilities of the Goose Message Interface. While it may not be particularly interesting to control engineers, the unit can also be used to adapt a conventional protection relay test set so that it can be used to test IEC 61850 protection schemes. This is a big benefit for users that already have protection relay test sets - whether they are units supplied by Megger or by others - as it is offers a very straightforward and cost-effective upgrade path.
It also creates an attractive option for consultants and smaller organisations who can now purchase a Goose Message Interface and a modestly priced relay test set, to cover all their relay and interlocking test requirements for both conventional and IEC 61850 schemes.

Equipment that allows convenient and dependable testing of IEC 61850 interlocking schemes has, until now, been difficult or even impossible to find. This situation has now been addressed by Megger's Goose Message Interface, an instrument that provides the added bonus of facilitating the testing of IEC 61850 protection schemes.

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