Capacitive Voltage Transformers (CVTs) have been widely used within transmission power systems for applications ranging from high-voltage to ultra-high-voltage. The Capacitive Voltage Transformer and Coupling Capacitive Voltage Transformers used in transmission systems are ranging from 72kV to 1200kV.
CVTs are primarily used for voltage measurement, providing voltage signals to metering units,
protection relay devices, and automatic control devices. Before going into the actual topic let’s see the types of voltage transformers.
Voltage transformers or Potential transformers are used for measurement and protection. Voltage transformers are necessary for voltage, directional, and distance protection.
The primary side of the voltage transformer is connected directly to the power circuit between phase and ground depending upon rated voltage and application.
Three types of Voltage Transformers:
- Electromagnetic Voltage Transformers: EMVT
- Capacitive Voltage Transformer: CVT
- Coupling Capacitor Voltage Transformer: CCVT
1. Electromagnetic Voltage Transformers: EMVT
In which primary and secondary wound on magnetic core like in usual transformers. The secondary voltage is a reduced proportional voltage used for measurement or protection.
2. Capacitive Voltage Transformer: CVT
CVT is a combination of a Capacitive voltage divider and an Electromagnetic Voltage Transformer. In which the primary voltage is applied to a series capacitor group. The voltage across one of the capacitors is taken to Electromagnetic Voltage Transformer. The secondary of EMVT is taken for measurement or protection.
3. Coupling Capacitor Voltage Transformer: CCVT
CCVT is a combination of a coupling capacitor and CVT. A coupling capacitor is used for carrier current protection and communication purposes.
Power Line Carrier Current PLCC equipment is used for high-frequency carrier communication, protection, and control.
Construction of Capacitive Voltage Transformer: CVT
CVT has three major components.
- Capacitance Voltage Divider- consists of a series group of capacitors. It has one or more sections depending on the voltage level.
- Electromagnetic Unit- consists of a step-down transformer and a compensating reactor.
- Ferro Resonant Damping Circuit- consists of an Iron core reactor and a resistor
1. Capacitance Voltage Divider:
Series connected capacitor elements, housed in porcelain shells, each hermetically sealed, are
referred to as capacitor sections. The dielectric of the capacitor elements is made up of high-quality
polypropylene film/paper and impregnated with highly processed synthetic fluid. Each capacitor
section is equipped with a stainless steel bellow which will allow the synthetic fluid to expand and
contract with changes in ambient operating temperature while maintaining the hermetic sealing.
A capacitance voltage divider is used to take the line to ground voltage applied to the capacitor and reduce it to an intermediate voltage typically 5 to 20 kV.
2. Electromagnetic Unit:
A tap voltage (approximately 5-12 kV depending on the type) is taken from the lowest capacitor section and fed to an electromagnetic circuit in the cast aluminum base box.
The base box contains the intermediate transformer which will provide the final output voltages via multiple tapped secondary windings for protection and metering purposes.
A series compensating reactor is used to compensate the capacitive reactance of the capacitor voltage divider. The base box is filled with dried mineral oil, protecting the components from environmental deterioration.
The Capacitor voltage transformer secondary is protected by HRC cartridge-type fuses for
all the windings.
3. Ferro Resonant Damping Circuit:
The base box contains a Ferro resonance damping circuit. Placing capacitance in series and shunt to the natural inductance of the power system leads to resonant circuits and RC time constants. This circuit can be brought into resonance that may saturate the iron core of the transformer by various disturbances in the network. This phenomenon can also overheat the electro-magnetic unit, or lead to insulation breakdown.
Resonance also causes the generation of transients and these transients affect the performance of numerical relays.
Hence it is required for CVT to eliminate this resonance by using a damping circuit.
The damping circuit is connected in parallel with one of the secondary windings. The damping circuit
consists of a reactor with an iron core and a series resistor. Under normal use, the iron core of the
damping reactor is not saturated, yielding a high impedance, so that practically no current is flowing
through this circuit.
Construction of Coupling Capacitor Voltage Transformer: CCVT
A coupling capacitor voltage transformer CCVT is similar to CVT except for the addition of a carrier coupling unit known as PLCC equipment as shown in the above schematic diagram.
CCVTs are used to couple power line carrier technology to the power system for communications purposes. CCVTs are designed for the carrier frequency range of 40 kHz to 500 kHz.
In general, CVT contains an HF terminal for connecting PLCC equipment. The HF terminal shall be kept earthed when not used for PLCC purposes.
Hence CCVT transformers serve as a combined voltage transformer and Power Line Carrier Current PLCC equipment.