Keep it simple for protection testing
Author: Jill Duplessis
The number of renewable energy sources that need to be connected to the national power
network is growing almost exponentially. But before they allow such a connection to be made, network operators rightly insist that the protection systems associated with the energy source must be shown to operate correctly. Marius Averitai of Megger explains how to choose the test equipment needed to meet this requirement.
The distribution network operators (DNOs) have an unenviable task: they must maintain network stability and reliability even though thousands of new energy sources are being connected to it. Officially designated non-type-tested generators, these typically take the form of renewable energy sources such as solar panel installations or small wind turbines. To prevent them destabilising the network, the DNOs insist that the energy sources, and the protection systems associated with them should meet minimum standards of performance.
In the UK, the currently applicable requirements are codified in Energy Networks Association Engineering Recommendation G59 and its soon to be implemented successor, G99. In most cases, anyone intending to make an infeed connection to the power network must not only ensure that they comply with G59 or G99 but also demonstrate this compliance to an inspector appointed by the DNO.
The challenge for electrical engineers and technicians involved with commissioning non-typetested generator systems therefore comes in two parts: first, carrying out tests to satisfy themselves that the protection arrangements are adequate and working correctly, and second, repeating these tests – or at least the most important of them – to satisfy the inspector. But long before this stage is reached a decision has to be made about the type of test equipment that should be used to meet these dual challenges.
This decision is neither trivial nor easy to make. A big part of the problem is the plethora of options available when it comes to protection relay test sets – the choice is little short of overwhelming. However, for the applications we’re discussing, the field can be reduced significantly by eliminating the sophisticated top-end test sets.
This may seem counterintuitive, but these instruments are designed for carrying out tests in large substations and on geographically extensive transmission and distribution systems. They offer everything needed for these demanding applications, but this is far more than is needed to demonstrate compliance with G59 or even G99. As a result, they’re costly, they have a very steep learning curve and, in our applications, many of their functions will never be used.
A better choice is a more straightforward test set that’s easy to learn and use, and which is, in comparison with the top-end models, relatively inexpensive. Such a test set has another big advantage that may not be immediately obvious. Since the tests are carried out sequentially under manual control, rather than in a fully automated sequence (as they would be with a high-end test set) they are much easier to demonstrate to the DNO’s inspector!
Choosing a straightforward test set is not, however, the same as choosing a test set with limited versatility. In fact, the best test sets of this type, such as the Megger SVERKER900, are exceptionally versatile and provide invaluable facilities that go beyond meeting the requirements of G59 and G99.
Let’s take a look at what can be expected from such a test set, after first making the basic decision about whether to choose a single-phase or a three-phase instrument. Single-phase types are smaller and cost less, and they are satisfactory in many applications. G59/G99 protection schemes often need a test set with three-phase capabilities however, and the use of digital protection systems that are only amenable to three-phase testing is growing.
Fortunately, there is a way to avoid the singlephase/ three-phase decision, as test sets that can be configured for single- or three-phase operation are now available. With the best of these, setting up the required configuration is easy and involves no more than using plug-in links on the instrument’s front panel. The benefit of such an instrument is that it offers full three-phase testing, but if a particular system requires an unusually high test voltage or current, this can be achieved by reverting to a single-phase configuration.
What else should the ideal instrument offer? If it supports three-phase testing, three current sources and four voltage sources are desirable. For maximum versatility, all of the sources should be floating, with an ungrounded neutral, as this will allow them to be used for primary injection. It should also be possible to connect them in series or parallel for carrying out tests that need a higher voltage or current than can be supplied by a single source.
Many users will also find it important to select an instrument with current and voltage sources that have a relatively high power rating – at least 250 VA for the current generators and 125 VA for the voltage generators – as this will make it possible to use the instrument for testing legacy electromechanical protection relays, as well as extending its usefulness in other areas of testing.
So much for the basic provisions, but what about control and usability? Traditional relay protection test sets do not, in general, have a particularly good reputation for user friendliness, but this situation has been transformed in some of the best modern units. These have in-built intelligence which eliminates the inconvenience of having to use them in conjunction with a separate computer, and they feature a user interface designed around a touchscreen, allied with a method of operation based on “virtual instruments”. The idea is straightforward – the user simply selects the “instrument” needed for a particular task from those displayed on the touchscreen and then sets up the test parameters just as if it were a real physical instrument. This method of operation is fast, simple and intuitive, and it minimises the risk of making mistakes. Full manual control will also be provided for tackling special applications.
A good test set will provide users with a choice of many virtual instruments covering a wide range of applications, but two that are particularly important for G59/G99 applications are a ramping instrument, which facilitates the testing of ROCOF (rate of change of frequency) protection relays, and the MMT (multiple timing test) instrument that allows the characteristics and settings of IDMT (inverse definite minimum time) relays to be checked and confirmed without the need to carry out multiple tests.
Other useful instruments include pre-fault and sequence instruments, and an impedance instrument that will allow relays to be tested directly from the impedance plane with conversion of from impedances to currents and voltages carried out automatically by the test set. A CT magnetisation instrument is a useful addition for many users, along with straightforward facilities for measuring voltage, current, and parameters such as phase angle; active, reactive and apparent power; power factor; resistance; reactance and frequency.
Testing power systems can be a daunting prospect and testing protection schemes is often perceived to be particularly daunting. However, as we’ve seen, this doesn’t need to be the case. Choose a straightforward – but versatile – instrument that is not loaded down with features you’ll never use, making sure that it has a modern intuitive user interface, and you’ll find that power testing challenges are easily tamed!