Reliable testing of transformer wiring: A poor relation?

Operators of many electrical energy facilities have expressed that there does not appear to be an established standard for wiring testing. Practically, all the companies surveyed by Omicron rely on an in-house process for wiring testing that has been developed over the years from their respective experiences. The basic idea behind this approach addresses the question of what errors should be found (or, ideally, not found) when testing wiring. In this article, the authors proposed a reliable method for wiring testing that has been developed on the basis of this question and the findings from the interviews. It can be carried out with minimal outlay while providing as high a level of safety as possible. To keep the length of this article within bounds, the various steps are listed, but not all of them are examined in detail.

If we look at the configuration that crops up most frequently in the field, that is, a three-phase system with three current and three voltage transformers, the following errors might occur. They should, therefore, be looked for every time a test is carried out:
• Incorrect polarity in the current transformer.
• Current transformer installed in wrong direction.
• Current transformer circuit not grounded.
• Additional unintentional ground connections.
• Malfunction of a relay test plug or relay test connector.
• Break in the secondary wiring of current or voltage transformers.
• Polarity error in the secondary wiring.
• Phase reversal in the secondary wiring.
• Installation direction of current transformer incorrectly set in relay.

This last item is, of course, not a wiring error in the strict sense of the term, but in traditional protection testing it is this aspect in particular that is not especially easy to test. This is why it is good practice to eliminate this possible error while carrying out the wiring test.

Suitable equipment for reliable testing
In principle, any test set that can output a current and a voltage is suitable for the overwhelming majority of wiring tests. It should also have at least two inputs for measuring current and/or voltage. However, a closer look at many of the test sets already available on the market reveals that they have major disadvantages when used for such tests as some errors, for example, auto-reclosing of the testing switch, are impossible to detect at all. Polarity checking with traditional sources and measuring inputs is of course possible, but a cable always has to be run from the measuring point in the installation to the source, something that is very expensive. If this is not done, multiple errors will sometimes not be detected.

OMICRON has, therefore, now launched the COMPANO 100 (Figure 1), a new device that, among other things, is optimised to test the wiring of transformer circuits.

The following list itemises some of the major benefits that make the COMPANO 100 ideal for wiring testing:
• At 10 kg, it is significantly lighter than comparable devices.
• Battery-powered, hence highly portable.
• Special DC-free polarity check signal.
• Special circuit for detecting breaks in current transformer circuits, for example, when checking test plugs or test switches.
• Regulated outputs for direct adjustment of setpoints.

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Proposal for a reliable testing method

The remaining tests can be carried out more easily using a secondary injection. The test set can remain right next to the transformers for all these tests.

Polarity checking of the secondary wiring is performed using the CPOL method familiar from other OMICRON devices (Figure 2). This involves injecting a saw-tooth signal into the current and voltage transformer circuits (shown in red and blue respectively in Figure 2). This signal has no DC component, so magnetisation of the transformer is never a problem. The small, battery-powered CPOL polarity checker can then be used to carry out measurements anywhere in the installation – both in the current and voltage circuits – without requiring a connection to a source (Figure 3). It shows if the measurement is being carried out in the right circuit and whether the polarity of the circuit is correct. This approach will detect errors 6, 7, and 8 in the list.

Test switch malfunctions pose danger to personnel
Another interesting feature of the COMPANO 100 is its ability to detect breaks in the current circuit. It is well known that relay test switches must short when the current transformer circuit is activated before they disconnect the relay from the circuit. This is an area where not all test switches have proved reliable, a situation that can pose a danger to personnel. Checking this function by means of a primary injection is, however, not advisable (in fact it can be extremely dangerous), as doing so would, if the test switch were to malfunction during the test, generate precisely these excessively high voltages. By detecting breaks in the secondary circuit, the COMPANO 100 performs this test in a straightforward manner without exposing the tester to any danger.

If the test switch is some distance away from the source, the device can be configured to disconnect the output automatically if a break is detected in the circuit. The tester can then operate the relevant test switch a couple of times in succession without having to be anywhere near the test set, and in doing so establish whether a break has occurred without having to check the display on his device. The duration of the break will be also shown on the display. Item 5 in the list of errors shown above can now be reliably detected.

The remaining sources can now be checked for possible polarity errors by simultaneous injection in the current and voltage transformers and reading off the sign of the active power indicator on the relay display. This method can reveal the remaining possible polarity errors (numbers 2 and 9), especially in the case of primary injection in the current transformer.

Other COMPANO 100 features
The sources and inputs present in the COMPANO 100 enable the device to be employed for a range of other tests, such as simple relay tests, micro-resistance measurements and the ground resistance measurements of small grounding systems, such as transformer distribution stations.

In developing the COMPANO 100, OMICRON has looked very closely at the whole subject of wiring testing. We have noticed that companies have adopted a variety of interesting methods over the years, which is why we are currently inviting interested parties to discuss the pros and cons of these methods directly with our product developers. Simply visit www.facebook.com/compano100 to find out more.

With the COMPANO 100, OMICRON has developed a test set that is optimised to test the wiring from the transformers to the relays, meters, or control room. All the most important errors that might occur in such applications can be detected using this device. Its light weight and battery-powered operation simplify the testing process. New methods, such as the detection of auto-reclosing in the current circuit, are genuine innovations that also offer increased safety for testing and operating personnel.

Authored by__
Ulrich Klapper, Product manager for protection and measuring system testing solutions at OMICRON Electronics, Austria

Klaus Jotz, Marketing Communications Engineer at OMICRON Electronics, Austria