We were looking for a method to determine the individual capacitance values of a capacitive divider for Capacitor Voltage Transformers concerning accuracy and state determination, and we have recently met this goal. Here, we provide a short overview of the motivation behind developing and designing this new patented measurement method. Voltage transformers come in an inductive and capacitive design. A Capacitor Voltage Transformer (CVT/CCVT = capacitor voltage transformer/coupling capacitor voltage transformer) is formed from a capacitive voltage divider, a compensation coil, and a medium-voltage transformer (Graphic 1). The voltage divider is often designed as a stack of smaller capacitor elements connected in series. Capacitance C1 is usually smaller than capacitance C2. The power supply voltage (Graphic 1, connection A/ H1) is split into a smaller voltage in the range of 8–20 kV at capacitance C2, which reduces the required degree of insulation of the inductive voltage transformer in the CVT. This design has been used for decades and is generally cheaper than pure inductive voltage transformers. CVTs are used from a power supply voltage of 40 kV and up, depending on the region. The capacitances may change due to moisture entry, oil leakage, or aging, which, among other things, influences transmission accuracy. A survey in the CIGRE 394 brochure showed that aging CVTs are one of the leading causes of explosions. This is illustrated in image 1, which shows a voltage transformer. Therefore, it is essential to monitor the capacitance values of a CVT throughout its life cycle. Voltage transformers worldwide have been effectively diagnosed using the VOTANO 100s model-based approach for years. VOTANO 100s modeling can produce highly accurate measurements without large equipment. However, the capacitance values of a CTs capacitive divider have always been a significant parameter that remains unsolved. However, the overall transfer ratio of a CVT can be determined when it comes to its capacitive voltage dividers; only the ratio of the two capacitances can be determined, not their values. The advantage of this new method is that the capacitance values can now be determined regardless of the CVT design. In the standard high-voltage test, certain preconditions related to the CVT design must be met to determine the individual capacitances. By combining the determination of the individual capacitances with the DETERMINING THE C-DIVIDER CAPACITANCES OF A CAPACITOR VOLTAGE TRANSFORMER Developing a new measurement method A/H1 N NHF/CAR 1n/X2 1a/X1 FSC C1 Lcomp C2 Graphic 1: CVT model Magazine | Issue 1 2024 38
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