Stefan Larsson - Product manager, power products
Traditionally, testing protection systems in power distribution networks has been a time-consuming process involving many separate steps. Now however, an alternative approach is available which allows all key aspects of the protection system to be tested simultaneously, leading to big time savings.
In these days when staffing levels in almost every sector have been cut to the bone and power networks are working close their maximum capacity, engineers and technicians whose work involves testing protection equipment are invariably working under intense pressures not only to reduce the time they spend on each job, but also to minimise the time for which equipment is out of service during testing. Yet protection testing is arguably more necessary than ever, given the enormous costs that now frequently result from unplanned power outages.
In view of these issues, it is clear that any way of cutting the time taken to test protection systems would be very advantageous. But before exploring how this might be achieved, it’s useful to consider just why protection testing is so time consuming.
Underlying the answer is the simple fact that protection systems are made up of multiple key components which usually include the protection relay, the circuit breaker, the current transformers and the tripping battery. In conventional testing, these components are disconnected and isolated from each other before being tested individually.
It’s easy to see that this involves a lot of work that takes a lot of time – especially when it is recalled that, after testing, the components will all have to be reconnected and the connections checked before the system can be put back into service. It’s also worth noting that, in many instances, the individual components will need to be tested by different people – a relay expert for the relays, for example, and a circuit-breaker specialist for the breakers. This adds further to the time, cost and inconvenience of the process.
Now, however, there is a new technique that overcomes these problems – testing with an on-load protection condition analyser (PCA). The idea behind these instruments is simple: without taking the circuit off line, inject a test current into the protection relay in parallel with the current it is receiving from the CTs while monitoring not only the total current into the protection relay but also other critical parts of the circuit, such as the status of the circuit breaker main contacts, and the current in the circuit breaker trip coil.
The test current is increased until the protection relay operates and the breaker trips. Since all key parameters are being monitored, this single operation tests the CTs, the protection relay, the circuit breaker, the tripping battery and more, all without the need to disconnect and reconnect the individual components.
The time saved compared with conventional testing is very significant, with users of PCAs routinely finding that this amounts to several hours per equipment bay. And that’s not the only benefit. Because the equipment remains in service until the moment of tripping, this form of testing captures the valuable “first trip” data for circuit breakers that may have not tripped for months or even years. This data is an invaluable guide to a whole range of potential problems, such as degraded lubricants in the circuit breaker mechanism, which may not be clearly revealed with tests carried out at subsequent trip operations.
Further, when testing is performed with a PCA, only one person is needed to test the breaker and the protection and – a valuable safety point – the protection remains active right up to the point of tripping.
What the PCA test provides, in fact, is a snapshot of the actual live operation of the protection system, which is difficult if not impossible to capture in any other way. The data that the test can be expected to yield includes the protection relay operating time for overcurrent, the breaker operating time and the operating time for the auxiliary contacts, the trip coil current profile, battery condition information, and verification of the integrity of the CT circuit and the overall protection system wiring.
PCA testing, however, does have a few minor disadvantages compared with the conventional approach. It provides, for example, less test data on the individual components, and it also tests the operation of the protection system only in relation to over current. In addition, automatic analysis of the breaker relates only to the phase into which the fault current was injected.
Nevertheless, in the majority of cases the most important objective in testing is to verify that the overall protection system works correctly, which PCA testing does quickly, conveniently and cost-effectively. In the comparatively rare instances where more detailed test data is needed, the facilities provided by the PCA instrument can still be used to facilitate conventional component-by-component testing.
While the method of operation of a PCA instrument may be easy to explain and understand, this should not be taken to mean that it is easy to design and manufacture an instrument that allows PCA testing to achieve its full potential.
If the instrument is to work successfully, it is essential that the test current it injects into the protection relay has a clean, undistorted waveform, and that it is accurately phase matched to the current being sent to the relay from the CT. This makes digital synthesis of the test current highly desirable. It is also important for the total current flowing into the protection relay to be continuously monitored throughout the test, to ensure that the results are not affected by an unexpected change in the current contributed by the CT.
Another essential feature is a convenient method of recording detailed test data at high speed – in effect, the PCA needs to operate as a high-speed recording oscillograph that allows the data to be recalled and analysed as required. This process is greatly facilitated if the instrument has a large memory capacity and if it records data in the widely used COMTRADE format.
As we have seen, PCA testing is considerably more convenient than conventional protection system testing, but careful design of the PCA test set can add yet another layer of convenience. With the best instruments, no direct connections are needed to the equipment under test. Instead all currents are monitored with Hall-effect sensors. This means that there are no electrical connections to be made or broken and that test blocks are not needed.
For hard-pressed engineers and technicians working on distribution networks, PCA testing is an invaluable option combining, as it does, big savings on the time taken to perform tests with minimum out-of-service time for the equipment under test. Furthermore, the best PCA test sets, such as the new PCA2 from Megger, are readily portable, easy to use and they allow simultaneous monitoring of a wide range of protection system parameters without the need for direct electrical connections.
There’s no doubt that component-by-component testing of protection systems still has its place, particularly during commissioning and when trying to diagnose unusually intransigent faults. However, for the routine testing of in-service systems, PCA testing offers decisive benefits and, because of the enormous time saving it makes possible, the cost of the equipment needed will be quickly recovered.
Want to know more about the PCA2, click here