Sunday, October 28, 2012

CVT Principle of Operation Illustrated

CVT Principle of Operation
Capacitor Voltage Transformers
Main Components


The main components are the capacitor divider, the intermediate step-down transformer and the series
reactor.


The step-down transformer and series reactors are connected to the intermediate voltage tap between C1 & C2. The series reactor is manufactured so its impedance cancels the impedance of the capacitor; therefore, the full intermediate voltage is delivered to the terminals of the step-down transformer, in phase with the primary line voltage. The series reactor and primary winding of the step-down transformer are manufactured with taps to enable ratio and phase angle adjustment. These are factory preset and do not require alteration after delivery unless a capacitor unit is changed.


Auxiliary Components


The choke coil assembly prevents the grounding of the carrier signal when the voltage tap ground switch is closed.
The harmonic suppression filter prevents sustained ferroresonance oscillations. It consists of a resistor in series with a saturable reactor and a parallel resistor.


The reactor is designed to saturate above the highest over-voltage rating to form a loading circuit, which will dampen sub-harmonic ferroresonant oscillations.


The protective gap  is a voltage sensitive device wired in series with a loading resistor on a secondary winding of the series reactor. The protective gap is normally open circuit, but goes in short circuit mode when the intermediate voltage exceeds the overvoltage factor of the CVT, or when the secondary current causes the thermal burden rating  to be exceeded. This has the effect of de-tuning the CVT and limiting the secondary current available during overvoltage and external short circuit conditions. The protective gap also serves to further limit ferroresonance
oscillations. If the fault condition persists for more than about thirty seconds, the protective gap will not reset from the short circuit mode and must be replaced. Its location inside the low voltage terminal box facilitates replacement.


The drain coil, gap and carrier ground switch are supplied if the CVT is to function as a coupling capacitor for power line carrier.




















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