The majority of voltage transformer (VT) failures occur due to electrical stresses or manufacturing and installation errors.
Typically electrical stresses are caused by:
• Ferro-resonances effects
In case of (re-)commissioning of substations VT circuits should also be checked. Verifying the VT´s nameplate data helps to identify damages of the VT or wrong connections.
With a voltage output of up to 2 000 VAC the Omicron CPC 100 Multi-functional Primary Test System can be used to test VT ratio, polarity and burden.
By injecting voltage into the primary side, ratio can be measured. Thereby the phase angles of high-voltage output and voltage measurement input are also measured. Thus the correct VT polarity can be verified.
Applying voltage to the secondary VT circuits and measuring the load current in amplitude and phase allows the actual burden to be measured, ensuring that it is within the VT´s specification data.
The VT‘s secondary signal may be difficult to measure if it is small in amplitude – especially if neighboring parts of the substation are in operation. In case of strong disturbances, the user can select a frequency different to that of the power system and utilizes the “frequency selective measurement” function. Thus only the VT‘s output signal with this particular frequency is measured while all other signals are filtered out.
Omicron CPC 100 Multi-functional Primary Test System Voltage Transformer testing:
• VT ratio up to 2 kVAC | polarity and burden
• VT burden up to 130 VAC | secondary
• VT secondary voltage withstand test up to 2 kVAC
• Polarity check with CPOL up to 2 kVAC
• VT electronics up to 2 kVAC
• IEC 61850 Sampled Values testing
• Power / dissipation factor (tan δ) test up to 12 kV, 300 mA | with CP TD1
This is part of the Omicron CPC 100 All-In-One Test System Series. Check out the other posts!