Validating Current and Voltage Transformer Wiring

In essence, electrical substations consist of power transformers, circuit breakers, ... ... And secondary systems for protection, load control and metering. The input signals for these secondary systems are provided by the CTs and VTs ...

... Which transform the primary currents and voltages into smaller signals. High primary currents are transformed into low currents of up to 1 or 5 amps, and high primary voltages into low voltages of up to 100 or 110 volts. The protection system uses these signals to continuously scan for faults, which enables it to quickly disconnect a faulted section from the grid.

The signals are also displayed in the control room, where they provide the data used for various switching actions. They are also needed for revenue metering. The polarity of these signals is of utmost importance, as a reversed polarity would give the secondary systems a false interpretation of the energy flow. In energy metering, a false interpretation of the energy flow based on reversed polarity ...

... Would cause the utility to pay for the energy that it delivers instead of receiving revenues. Polarity faults can lead to a protection system malfunction. There can be many reasons for improper polarity: The CT might have been installed with the wrong mechanical orientation.

A CT could have an internal wiring error. A single instrument transformers secondary wires may have been crossed ... Somewhere along the way to the secondary system. The secondary wires from two of the instrument transformers in a three phase system ...

... May have been swapped. The settings in the relay, meter, or control room device may have been set incorrectly. This is why the secondary systems correct wiring has to be verified ...

... Either during substation commissioning or after a refurbishment. Primary injection is used to validate the wiring. High currents of up to about a 1000 amps can be generated by larger testing equipment.

Smaller currents of about a 100 amps can be generated by smaller and more portable test sets. When a saw tooth signal with a distinct polarity has been used for injection,   a handheld device can be used to validate the polarity throughout the system From the secondary side of the CT,   along the secondary wiring   all the way to the last terminal. Since this test is done phase by phase, all of the potential wiring errors or phase swaps can be detected. Testers should be able to move around the substation easily.

So ideally, they need light-weight, battery-operated equipment. The process of verifying voltage signal polarity is the same thing in principle, except that voltage is applied instead. If the wiring is correct, the secondary systems should run without a problem. COMPANO 100.

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