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Module 2 : Current and Voltage Transformers

Lecture 8 : Introduction to VT

8.2	CCVT in Power Line Communication

		The capacitance potential divider also serves the dual purpose of providing a shunt path to high frequency signal used in power line carrier communication. Normally, CCVT is used in HV/EHV systems where carrier line communication is used. High frequency i.e. Radio Frequency (RF) signals (50 - 400 kHz) can be coupled to power line for communication. At high frequency, the capacitive shunt impedance is very small and hence these signals can be tapped by the potential divider. To block the path to ground for the RF signal, a small drainage reactor is connected in series with the capacitance divider. At power frequencies, it has a very small impedance. Thus, the role of capacitance potential divider at power frequency is not compromised. On the other hand, at RF, the impedance of drainage reactor is large and it blocks the RF signal.

	Also, compensating reactor and transformer leakage reactance by their inductive nature, block the path of RF signal. This signal is then tapped by a tuning pack which provides low impedance to the RF signal.
8.2.1	Ferro Resonance Problem in CCVT
	The iron cores of the reactor and transformer will not only introduce copper and core losses but it can also 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. Unfortunately, it can aggravate CCVT transients.