Testing multi-ratio CTs
David Milner - Product manager
Because of the crucial role that current transformers (CTs) play in metering and protection schemes, CT testing is a routine task for every power engineer. It is also a task that can be very time consuming, particularly when multi-ratio CTs are involved.
It’s not hard to see why. Testing CTs in line with IEC or IEEE requirements involves measuring several key characteristics, including ratio, knee point, saturation and polarity. In addition, the CT will typically have to be demagnetised as part of the test procedure. Carrying out all of the tests that are needed on a single-ratio CT takes a significant amount of time and, for multi-ratio CTs, the situation is even worse as the test leads have to be repositioned and the tests repeated for every ratio.
That’s the position with traditional CT test sets. The latest generation of automated test sets, however, make life much easier for power engineers. With these new test sets, the full test sequence, including all of the prescribed tests, is initiated by a single button press. Even better, the best of these test sets can be directly connected to multi-ratio CTs and will automatically carry out the tests on all taps without the need to change the test connections.
The CTs can be tested in their normal equipment configuration – that is, while mounted in transformers, oil circuit breakers or other switchgear – which saves even more time.
A good example of a modern CT test set that offers all of these facilities and more is the new MCT1605 from Megger. Based on the proven technology used in the MCT1600 range of instruments, this new test set performs a CT saturation test and calculates the rated knee point at the touch of a button. Tests can be performed at 50 Hz or 60 Hz, and the knee point calculations can be carried in line with IEEE C57.13.1, IEC 60044-1 or IEC 60044-6.
While the saturation test is being performed, the tester plots the CT saturation curve on a large graphical display. To cater for CTs with multi-ratio secondaries, up to 10 saturation curves can be plotted and displayed simultaneously.
To accurately determine the ratio of the CT, the test set injects a voltage into the secondary, and measures resulting voltage in the primary. Normally, this test is performed automatically at the same time as the saturation test, but provision is also made for it to be carried out manually.
The polarity of the CT is displayed as a simple “correct” or “incorrect” indication, accompanied by the measured phase angle.
The MCT1605 also offers the option of automatically demagnetising the CT under test. Demagnetisation prior to saturation testing helps to ensure that accurate results are obtained, and this procedure is recommended in ANSI C57.13.1.
As a further aid to fast, convenient and comprehensive testing of CTs, the MCT1605 incorporates a 500/1000 V insulation resistance test system, which ensures that the CT secondary winding and wiring are properly insulated as required by ANSI C57.13.1. The test set automatically switches the test connections to perform all of the required insulation tests, including H-L, H-G, and L-G. For maximum user convenience, the MCT1605 catalogues and stores all of the test results for later retrieval. Configurable test plans can be associated with individual CTs and stored along with the results, which greatly simplifies ongoing monitoring and profiling.
The instrument stores results in special xml schema so that PowerDB can read it. A particularly versatile option, however, is to transfer the data to Megger’s PowerDB Lite software package, which has powerful facilities for generating reports in industry-standard fomats. PowerDB Lite can also be used to control the MCT1605 without the need for operator intervention, thereby allowing it to be used as a fully automated computer-controlled CT test system.
Today’s ever-increasing commercial pressures mean that power engineers now have less time available than ever before for carrying out routine yet essential tasks such as testing CTs. Fortunately, as we’ve seen, the latest CT test sets deliver big time savings complemented by greatly enhanced convenience, thereby providing at least a partial solution to this enduring problem.
