Reliable testing of transformer wiring – A poor relation?

Reliable testing of transformer wiring – A poor relation?
Published: 11 August 2019 - 8:22 a.m.
By: Baset Asaba

Every protection system must work safely, quickly and in a targeted manner. The aim of this requirement is to deliver a supply of energy that is as stable as possible and, most importantly, to ensure the safety of electrical energy facilities.

In the field of relay testing, various suppliers offering well-proven devices together with a generally accepted set of methods regarding how testing is to be carried out have been around for many years. However, even the most thoroughly tested protection relay will not be doing its job if it is unable to detect an event caused by a one-off wiring error.

Wiring testing is, therefore, vital - especially when commissioning a new or modified installation - and should be a process carried out according to a standard that is likewise broadly accepted.

Operators of electrical energy facilities have shown there does not appear to be an established standard for wiring testing. Most companies 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 propose 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 and malfunction of a relay test plug or relay test connector.

Other elements to be looked for include break in the secondary wiring of current or voltage transformers, polarity error in the secondary wiring, phase reversal in the secondary wiring and installation direction of current transformer incorrectly set in relay.

Suitable Equipment for Reliable Testing

In principle, any test set that can output a current and a voltage is suitable for the 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 they have major disadvantages when used for such tests because some errors - for example, auto-reclosing of the testing switch - are impossible to detect.

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 expensive. If this is not done, multiple errors will sometimes not be detected.

OMICRON has launched the COMPANO 100 a new device that, among other things, is optimised to test the wiring of transformer circuits.

What makes the COMPANO 100 useful for wiring testing is that at 10 kg, it is significantly lighter than comparable devices, battery-powered, therefore highly portable and allows special DC-free polarity check signal.

It has a special circuit for detecting breaks in current transformer circuits, for example, when checking test plugs or test switches, allows regulated outputs for direct adjustment of set points and it is equipped with an emergency stop switch for the highest levels of safety.

The sources and inputs also enable the device to be employed for numerous other tests, such as simple relay tests, micro-resistance measurements, and the ground resistance measurements of small grounding systems.

Proposal for a Reliable Testing Method

Once the transformers have been energised and grounded according to the five safety rules, a primary injection is used to verify the grounding of the secondary circuits, followed by the transformation ratios (if required) and the polarities of the transformers. The details will not be set out here, but this step will detect the errors numbered 1, 3 and 4 in the previous list.

The remaining tests can be carried out more easily using a secondary injection. The test set can remain 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. This involves injecting a saw-tooth signal into the current and voltage transformer circuits.

This signal has no DC component, so magnetization 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. 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 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 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 or her device. The duration of the break will be also shown on the display.

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.

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.

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