—Sponsored article by Megger
Traditionally, testing protection systems in power distribution networks has been a time-consuming process involving many separate steps. But an alternative approach is now available which allows all key aspects of the protection system to be tested simultaneously, leading to big time savings.
Power networks are working close to their maximum capacity, and engineers and technicians are invariably working under intense pressures to not only reduce the time they spend on each job, but to minimize the time equipment is out of service during testing.
Yet protection testing is arguably more necessary than ever, given the enormous costs that frequently result from unplanned power outages. The key is cutting the time taken to test protection systems.
So why is protection testing so time consuming? These systems are made up of multiple key components which usually include the protection relay, circuit breaker, current transformers and the tripping battery.
In conventional testing, these components are disconnected and isolated from each other before being tested individually. This involves a lot of work and time, especially since after testing, the components all have to be reconnected and the connections checked before the system can be put back into service.
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 time, cost and inconvenience.
Protection condition analyzer
There is now a new technique that overcomes these problems—testing with an on-load protection condition analyzer (PCA).
The idea is simple: without taking the circuit off line, technicians inject a test current into the protection relay in parallel with the current it’s receiving from the CTs. At the same time, they monitor not only the total current into the protection relay but other critical parts, 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 significant, with users of PCAs routinely finding it 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.
When testing is performed with a PCA, only one person is needed to test the breaker and the protection remains active right up to the point of tripping—a valuable safety benefit.
The PCA essentially provides a snapshot of the actual live operation of the protection system, which is difficult, if not impossible to capture any other way. The data the test can be expected to yield includes the protection relay operating time for over-current, 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 does have a few minor disadvantages compared with the conventional approach, however. It provides less test data on the individual components, and it also tests the operation of the protection system only in relation to over-current. Plus, 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 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 shouldn’t be taken to mean it’s easy to design and manufacture an instrument that allows PCA testing to achieve its full potential.
If the instrument is to work successfully, it’s essential the test current it injects into the protection relay has a clean, undistorted waveform, and 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.
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 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 big savings on the time taken to perform tests with minimum out-of-service time for the equipment under test.
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.
Stefan Larsson is a power product manager with Megger. Megger provides testing solutions in the most critical maintenance areas including cable fault locating, protective relay testing, and power quality testing. Megger’s product offering spans 30 distinct product groups with over 1,000 specific products. To learn more visit www.megger.com
To view additional articles from Megger, visit the Electrical Testing Success Centre.
—This article is an edited version of the original that ran in Megger’s Electrical Tester newsletter (August 2015).