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CCVT in Power Line Communication CCVT is also an economical choice when the transmission line is used for power line communication (refer Fig 8.6). High frequency RF signals can be coupled to the power line for communication. Filtering of this RF signal is carried out by a parallel R-L-C circuit which is also known as tuning pack. Normally, the frequency range of this RF signal is 50 kHz-400 kHz. At this frequency the drainage reactor offers a high impedance block to the RF signal; while for power frequency (50 Hz) it appears as a path to ground. The high inductance of the reactor and the transformer provides a high impedance path for the RF signal. Hence it prevents any leakage of RF signal into the transformer output at 50Hz. Ferro resonance Problem in CCVT The iron cores of the reactor and transformer will not only introduce copper and core losses but also can produce ferroresonance caused by the nonlinearity of the iron cores. Hence a ferroresonance suppression circuit is also included in the secondary of the transformer. The dangerous overvoltages caused by ferroresonance are eliminated by this circuit. But it can aggravate CCVT transients. Transient Response of CCVT As seen in the fig8. CCVT equivalent circuit is a R-L-C circuit. If transformer is considered ideal, it can be described by second order differential equation of the type, 1 t di V(t)=Ri+1/C ∫idt +L di/dt. V (t ) Ri idt L C dt Thus, any transient including faults in the voltage signal will not be faithfully reproduced by the CCVT. The response depends upon the damping α and ω and point on the voltage waveform where the fault strikes. Fig 8.7 and 8.8 shows the possible subsistence transient of CCVT. It can be seen that such a response can reduce the accuracy of distance relays. Classification of CCVTs CCVTs can be classified into two. 1. Class1 type 2. Class2 type Table 8.1 shows the maximum limit for the ratio and phase angle errors. It can be seen that errors of Class 2 type are double than that of class1 type. VT Class Maximum ratio error Maximum phase angle error Class 1 1% 40min Class 2 2% 80min