COVER FEATURE On-site Calibration of Voltage Transformers The concept of modelling a VT allows for a detailed view of the transformer’s design and its physical behaviour. The model-based approach allows calculation of the accuracy class in general, by inversely applying the Möllinger-Gewecke Diagram. For this method, knowledge of the parameters of the equivalent circuit is necessary. The parameters are determined by electrical measurements from the low- voltage and high-voltage side of the VT. The method uses low frequencies for the measurement to enable low testing power level and low voltages. For an accurate modelling approach, the total losses of the VTs have to be determined. The losses are separated into: a. Primary and secondary leakage reactance (secondary stray losses) b. Primary and secondary winding resistance (primary stray losses) c. Excitation losses (iron core losses) Figure 2 indicates the equivalent circuit diagram of an inductive VT with five secondary windings, whereas the fifth winding is a residual voltage winding (da-dn). All voltages, currents and impedances are referred to the secondary winding 1a-1n. Fig.1: VOTANO 100 – a small and lightweight (15 kg) VT Analyser V oltage 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 14 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 high- voltage 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 required accuracy, or are incapable of taking all of the different burdens into account. OMICRON has developed a new way of testing VTs. THE MODELLING APPROACH OMICRON’s new VT test device, VOTANO 100, uses a modelling approach. Refer to Figure 1. Fig. 2: Equivalent circuit diagram of an inductive VT Figure 3 indicates the equivalent circuit diagram of a capacitive VT with five secondary windings, whereas the fifth winding is a residual voltage winding (da-dn). The capacitive divider consists of C1 and C2. All voltages, currents and impedances are referred to the secondary winding 1a-1n. In addition to the equivalent circuit diagram of an inductive voltage transformer, the equivalent circuit diagram of a CVT has the following parameters: • Xc1’’ impedance of top capacitor ESI AFRICA ISSUE 1 2015