Menu
Search
HV Capacitive Voltage Transformers
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 type of the CVT is always a single-pole one, thus the connection is between phase and earth. The higher the voltage level is, the more price-competitive the capacitive type becomes.

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

A typical application would be to utilize the CVTs for power line carrier (PLC) high-frequency signal interface units. For the voltage measurement purposes, the behavior and the data specification of CVTs follow the same guide lines as the inductive 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 is based. The trimming windings are used for fine tuning the output signal to correspond with the required accuracy class requirements. The compensating reactor compensates the phase angle shift caused by the capacitive voltage divider.

Capacitive voltage transformer’s principal construction
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 wound transformer (inside the electromagnetic unit EMU), constitutes a tuned resonance circuit. Unlike with the inductive type of voltage transformers, the CVTs usually have the ferroresonance damping circuit inbuilt in the CVT itself, as shown in the previous figure.

Capacitor voltage transformer (CVT) nameplate
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-harmonic frequencies, resulting in voltage transformer heating (finally damages) and non-selective operations of protective relaying 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 of the secondary cores. During ferroresonance conditions, high voltages appear, saturating the reactor and turning the damping resistor on to effectively mitigate the parasitic voltage. During normal system conditions, the reactor presents high 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 (CVT)
Capacitive Voltage Transformers (photo credit: haikudeck.com)

Reference // ABB’s Distribution Automation Handbook

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

2 Comments


  1. Steven Xu
    Jun 02, 2016

    Now we design out High voltage Screw Ceramic Capacitor,

    500PF 24KVAC, ø25x44mm
    Withstand Voltage 50KVAC
    Lightning impulse up to 120KVAC

    This capacitor is important components of capacitive coupler.

    Datasheet can be send on request.


  2. ajay vikrant
    Dec 27, 2015

    Dear Sir,

    Now, Coupling capacitor of CVT in EHV system are not of any use as State Power Utilities are preferring OPGW cable for communication and lightning protection of transmission line, therefore, now we should go for VT instead of CVT or not as function of Voltage Transformers now end with protection and metering. What are the demerits if we prefer VT instead of CVT at line terminating point. Also clarify whether CVTs are costly than Vts or not.

    Requesting for an early reply.

Leave a Comment

Tell us what you're thinking... we care about your opinion!

Time limit is exhausted. Please reload CAPTCHA.

Get PDF