SUPPLEMENT FEATURE TEST & MEASUREMENT ONE-STOP PROTECTION
Traditionally, testing protection systems in power distribution networks has been very time consuming. Now an alternative approach allows all key aspects of the protection system to be tested simultaneously, leading to big time savings. Stefan Larsson of Megger explains
I
n these days of reduced staffing levels and power networks that are working
close to maximum capacity, engineers who test protection equipment are invariably under pressure to reduce the time they spend on each job, and to minimise the time equipment is out of service during testing. Yet protection testing is more necessary than ever, given the high costs of unplanned power outages. Cutting the time needed to test protection systems would clearly be advantageous. However, before exploring how this might be achieved, let’s consider why protection testing is so time consuming. Underlying the answer is the fact that protection systems have multiple key components such as the protection relay, the circuit breaker, the current transformers and the tripping battery. Conventionally, these components are disconnected and tested individually. This is a lot of time consuming work and, after testing, the components have to be reconnected and the connections checked before the system is put back into service. In many instances, the various components are tested by different people, adding even more time and cost.
A NEW TECHNIQUE Testing with an on-load protection condition analyser (PCA) addresses these problems. The idea is simple - without
S16 OCTOBER 2014 | TEST & MEASUREMENT
Above: cutting the time needed to test protection systems would clearly be advantageous. However, before exploring how this might be achieved, it’s important to consider why protection testing is so time consuming
taking the circuit offline, inject a test current into the protection relay in parallel with the current from the CTs, while monitoring the total current into the protection relay and also other critical parts of the circuit, such as the status of the circuit breaker contacts and the circuit breaker trip coil current. 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, with no component disconnection and reconnection. The time saved compared with conventional testing is very significant and, because the equipment remains in service until the moment of tripping, valuable ‘first trip’ data for circuit breakers is captured.
PCA TESTING
Right: the new PCA2 from Megger
The PCA test provides a true snapshot of the operation of the protection system. The test data will typically include protection relay operating time for overcurrent, breaker operating time, operating time for auxiliary contacts, trip coil current profile, battery condition information, and verification of the protection system wiring integrity. PCA testing does have a
few minor disadvantages. It provides less test data on individual components, and it tests the operation of the protection system only for overcurrent. Also, automatic analysis of the breaker relates only to the phase into which the fault current is injected. Nevertheless, in most cases the key objective is verifying that the overall protection system works correctly, which PCA testing does conveniently and cost effectively. While the operation of a PCA
instrument is easy to explain, it isn’t easy to produce an instrument that allows PCA testing to achieve its full potential. To work successfully, the test
current the instrument injects must have an undistorted waveform, and it must be accurately phase matched to the current sent to the relay from the CT. The total current flowing into the protection relay must also be continuously monitored, to ensure results are not affected by an unexpected change in the current from the CT.
Another essential feature is high speed recording of detailed test data - the PCA must operate as a high speed recording oscillograph that allows the data to be recalled and analysed. This is facilitated if the instrument has a large memory and if it records data in the widely used COMTRADE format. As we’ve already seen, PCA testing is
more convenient than conventional testing, but it’s possible to add even more convenience. With the best PCA instruments, no direct connections are needed to the equipment under test - instead currents are monitored with Hall effect sensors. This means no electrical connections have to be made or broken and test blocks aren’t needed.
For hard pressed engineers,
PCA testing is an invaluable option that combines big savings in test time with minimum out of service time for equipment under test. Furthermore, the best PCA test sets are portable, easy to use and they allow simultaneous
monitoring of multiple protection system parameters
without direct electrical connections. Component by component testing of protection systems still has its place, particularly during commissioning. However, for routine testing of in-service systems, PCA testing offers decisive benefits and, because of the timesaving it makes possible, the cost of the equipment will be quickly recovered.
Megger
www.megger.com T: 01304 502 101
Enter 241 / ELECTRICALENGINEERING
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