Transient Performance of Capacitor Voltage Transformers (CVT) Introduction A CVT system can be represented by its Thevenin equivalent circuit. In steady state, the secondary volt- The analysis of a simplified L-C-R age of a capacitor voltage trans- circuit for the case of a sudden former mirrors the primary voltage. collapse of the primary voltage Because of the energy-storage (assuming a given set of initial elements used, the transient out- conditions) produced the following put, caused by sudden change in conclusions: primary voltage, is no longer a rep- lica of the primary. This distorted 1 If the equivalent capacitance Ce output may last for a few cycles of is larger and assuming a fixed the supply frequency. These errors value of the burden, the magni- may be termed as the CVT linear tude of the residual transient transient since they would still voltage becomes smaller. The occur in the event that all of the intermediate voltage should also components of the CVT were line- be maximized in order to achieve ar. In addition to this, ferroreson- a high value of the equivalent ance may occur resulting in volt- capacitance. age measurement error. Ferroreson- ance is caused by the saturation 2 The effect of the burden is so of the electromagnetic unit or profound that it can dominate inductive burden. Unlike the linear the CVT behavior. The burden transient it does not occur with configuration is insignificant every change in primary voltage. since it determines the way in which energy can be stored and the path through which it can Linear Transient Error discharge. Pure resistive burden (Transient Response) dissipates and does not store energy. Stored energy burdens The linear transient error is mainly (lagging or leading) create harm- influenced by the following factors: ful effect on the transient response producing oscillations 1 The Thevenin equivalent capaci- of low frequency nature. tance referred to the secondary voltage level, Ce. The oscillograms shown below 2 The burden value and configura- illustrate the variation in the tion. residual voltages as a function of burden at crest fault initiation. At 3 The point of wave voltage inter- zero fault initiation, the transient ruption. response worsens. E 214.60 3 During the sudden application or sudden disappearance of primary voltage, the distribution of energy stored in the CVT is a major factor in understanding the effect of the point of wave voltage interruption. The tran- sient performance in those cases is similar with the transi- ents in the case of the primary short circuit. The amount of energy accumula- ted by the capacitor at zero in- itiation is equivalent to the energy stored by the series reactor at crest voltage initiation. The discharge time constant will vary depending upon where the maximum energy is stored. A typical example of transient res- ponse ( see below) indicates the worse case with the maximum energy on the capacitor at the time the primary voltage wave is going through the zero voltage interruption.
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