Voltage transformers (VTs) are used in electrical grids for metering and/ or protection purposes. The high requirements regarding their precision demand a calibration of the objects before installation.
The accuracy class of VTs are different for protection and metering transformers, and are classified depending on the maximum ratio and phase-angle error between the vectors of the primary voltage and the secondary voltage related to the primary side. This calibration is performed in the laboratories of the manufacturers, operators or testing institutes. Once calibrated, the VT typically operates without re-calibration for its lifetime. In some cases, such as the reconstruction of the switchyard, the accuracy of the VT is reconfirmed with a laboratory test or extensive on-site measurements.
The accuracy of the VT is dependent on the leakage inductance, the winding resistance, the turns-ratio and the excitation current at power frequency. Core or winding deformation as a result of external influences and ageing cause a change of the error of the VT. In addition, it may be interesting to obtain the excitation characteristics of the VT from measurements for ferroresonance analysis or simulation programmes.
Testing of VTs in the field
Until now, precise testing of VTs in operation was a major undertaking. Highly accurate measurement solutions are complex systems, made up of various devices that are calibrated and extremely accurate. This includes a highvoltage source, a reference transformer, a set of standard burdens, measuring bridges for comparative purposes, and a computer to evaluate all of the measured data. The time and costs involved are immense, both for on-site tests and during manufacturing. Smaller testing solutions typically lack the…