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Capacitive Voltage Transformers (CVT) For HV Measurements

HV Capacitive Voltage Transformers (photo credit: gegridsolutions.com)

66 kV and upwards //

Capacitive voltage transformers (CVTs) are used on higher voltage levels, starting from 66 kV and upwards. The typeof the CVT is always a single-pole one, thus the connection is between phase and earth. The higher the voltage levelis, the more price-competitive the capacitive type becomes.

One of the advantages the capacitive type has, in comparison to the inductive type, is the possibilityto use capacitive voltage transformers as high-frequency coupling units towards the primarysystem (over headlines).

A typical application would be to utilize the CVTs for power line carrier (PLC) high-frequency signal interface units. Forthe voltage measurement purposes, the behavior and the data specification of CVTs follow the same guide lines as theinductive ones.

In addition, the possibility for high-frequency signal coupling calls for a specified value for rated capacitance (Cn) .

This value is chosen considering the following issues //

Voltage magnitude to be measured

Demands from PLC system (frequency, bandwidth, connections)

Capacitive voltage transformer manufacturing considerations

The construction of capacitive voltage transformers

The figure above shows the principle of a capacitive voltage divider on which the capacitive voltage transformer isbased. The trimming windings are used for fine tuning the output signal to correspond with the required accuracy classrequirements. The compensating reactor compensates the phase angle shift caused by the capacitive voltage divider.

Figure 1 – Capacitive voltage transformer’s principal construction

All capacitive voltage transformers require some sort of ferroresonance damping circuit.

The capacitance in the voltage divider, in series with the inductance of the compensating reactor and the woundtransformer (inside the electromagnetic unit EMU), constitutes a tuned resonance circuit. Unlike with the inductivetype of voltage transformers, the CVTs usually have the ferroresonance damping circuit inbuilt in the CVT itself, asshown in the previous figure.

Capacitor voltage transformer (CVT) nameplate (photo credit: technosources.blogspot.rs)

At higher system voltages, the resonance phenomenon usually takes place on fundamental or on sub-harmonicfrequencies, resulting in voltage transformer heating (finally damages) and non-selective operations of protectiverelaying possible protective relaying non-selective operations.

The modern CVTs are utilizing the so-called “adaptive” damping circuits.

The circuit consists of a saturable series reactor and a loading resistor. This circuit is connected in parallel to one ofthe secondary cores. During ferroresonance conditions, high voltages appear, saturating the reactor and turning thedamping resistor on to effectively mitigate the parasitic voltage. During normal system conditions, the reactor presentshigh reactance, effectively “switching off” the damping resistor.

Possible triggering factors for the ferroresonance phenomena could be //

Planned primary switchings in the system

Circuit breaker trippings caused by primary fault

High-speed autoreclosing

Capacitive Voltage Transformers (photo credit: haikudeck.com)

Reference // ABB’s Distribution Automation Handbook