HOW DOES CAPACITIVE VOLTAGE TRANSFORMER WORK?

Capacitive Voltage Transformers (CVTs), also known as Capacitor Voltage Transformers, are used to step down high transmission voltages to measurable low levels for metering, protection, and control in power systems. They are a cost-effective alternative to traditional electromagnetic Potential Transformers (PTs) in extra-high voltage (EHV) applications above 110 kV.

Capacitive Voltage Transformers (CVTs) use capacitive divider circuit and a tuned reactor for accurate voltage measurement. A CVT consists of capacitor stack housed inside a high-voltage insulator, an electromagnetic unit (EMU), and supporting elements for precise voltage measurement in EHV systems.

Principle of operation

CVTs operate through capacitive voltage divider, followed by electromagnetic potential transformer. A high voltage capacitor C1 is connected in series with high value low voltage capacitor C2. Capacitor C1 is connected to transmission line, while C2 is connected to ground. Voltage across C2 is C1/(C1+C2). This provides an intermediate voltage (say from 110+ kV to 11 kV), further brought down to low standard voltage of say, 110V, by a potential transformer.

The series-connected and tapped capacitor is housed inside a porcelain or composite housing. High-voltage capacitor   (0.5-2 nF, line-to-tap) connects to the transmission line, while low-voltage capacitor   (5-20 nF, tap-to-ground) completes the divider, yielding intermediate voltage output of 5-15 kV.

This divider provides capacitive reactance much lower than inductive types, enabling compact insulation for 220-765 kV ratings and also serving as a coupling capacitor for PLCC signals via HF terminal. A residual voltage arm (C1+C2) supports carrier frequencies (30-500 kHz).

An inductive reactor is used in series with this auxiliary transformer to tune the circuit to fundamental line frequency (50/60 Hz), minimizing phase shift and ferroresonance risks. The capacitive divider impedance drops due to high capacitance values near ground, ensuring low-voltage output stability independent of burden variations.

Construction

CVTs comprise a capacitor column, electromagnetic unit (EMU), and base assembly. The capacitor stack uses oil impregnated paper/PP capacitors in porcelain or composite insulators for high dielectric strength up to 765 kV.

• Capacitor Voltage Divider: C1 (line-to-divider tap, ~0.5-2 nF) and C2 (tap-to-ground, higher capacitance ~5-20 nF) form the divider.
• Auxiliary Transformer: Primary winding connects to divider tap; secondary provides isolated low voltage. Core is designed for minimal saturation.
• Tuning Reactor: High-impedance inductor prevents ferroresonance and compensates capacitance.​
• Damping Circuit: Resistor or ferroresonance suppression unit (e.g., 200-500 ohms) help dampen transients.​
• HF Terminal: For PLCC coupling capacitor functionality.​

Working Mechanism

High voltage line voltage (e.g. 400 kV) is brought down to 10-15 kV at divider tap (V2= V1 x C1/(C1+C2). The reactor tunes the divider-reactor parallel resonance to line frequency. This boosts the voltage slightly for transformer input.

The auxiliary transformer turns ratio (N1:N2, often 1000:1) steps it down to 110V, phase-matched via reactor compensation. Burden (VA load on secondary) has minimal effect on accuracy due to capacitive nature, unlike inductive PTs prone to saturation.

Additional Elements

• HF Coupling Capacitor: Integrated in C1 top for PLCC injection.​
• Secondary Terminals/Burden Box: For metering/protection connections, with fuse links.​
• Partial Discharge Monitoring Points: For tan δ and PD test

Component	Primary Function	Typical Specs
C1 (HV Cap)	Voltage division, PLCC coupling	0.5-2 nF, 400-765 kV
C2 (LV Cap)	LV divider output, low impedance path	5-20 nF
Tuning Reactor	Resonance at 50/60 Hz	L = 1/(ω²C_eq), 10-50 H
Aux Transformer	Isolation, final step-down	n=100:1, Burden 0-100 VA
Damping Unit	Ferroresonance suppression	200-500 Ω resistor

CVTs deliver PT-like accuracy (0.2/3P class) economically for high-voltage applications.​

The tuning reactor, also called the compensating reactor or inductive element, is a critical series inductor in Capacitive Voltage Transformers (CVTs) that ensures accurate voltage measurement by achieving resonance at power frequency. Positioned between the capacitor divider tap and the auxiliary transformer primary, it precisely counters capacitive effects for phase alignment and voltage stability.

Resonance Compensation Role

Capacitors   and   in series create a capacitive reactance  , causing nearly 90° phase lead in the tap voltage   relative to line voltage  . The tuning reactor’s inductance   provides opposing reactance  , tuned exactly for resonance:  .

At resonance, the parallel LC circuit presents high impedance to off-fundamental frequencies but minimum reactance at line frequency, yielding   with near-zero phase shift. This makes CVT secondary voltage   mimic an inductive PT’s in-phase output, essential for metering (0.2 accuracy class) and relays. Capacitors   and   in series create a capacitive reactance  , causing nearly 90° phase lead in the tap voltage   relative to line voltage . The tuning reactor’s inductance   provides opposing reactance  , tuned exactly for resonance:

At resonance, the parallel LC circuit presents high impedance to off-fundamental frequencies but minimum reactance at line frequency, yielding   with near-zero phase shift ( ). This makes CVT secondary voltage   mimic an inductive PT’s in-phase output, essential for metering (0.2 accuracy class) and relays.

Voltage Drop Minimization

Without L, divider impedance causes   a drop proportional to burden current, degrading ratio accuracy (up to 5-10% error at full load). Resonance boosts   by quality factor  , stabilizing output against secondary burdens (0-100 VA). Thevenin equivalent shows   at tune, independent of load.​

Parameter	Role Impact
Inductance Value	Exact resonance:
Q-Factor	>50 for low error, high burden stability
Damping Pairing	Suppresses ferroresonance (<5% risk)
Frequency Select.	Passes 50/60 Hz, blocks RF/subharmonics

Precise tuning is vital for high-voltage CVT reliability work, calibrated via capacitance measurements during commissioning.​

Ferroresonance Prevention

CVTs risk ferroresonance—chaotic oscillations—from interaction with PTG capacitance or grading capacitors during switching/light loads. The reactor’s high impedance blocks subharmonics (1/3, 1/2 fundamental) while passing 50/60 Hz, complemented by parallel damping resistor 

 (200-500 Ω). Non-linear saturable designs activate only during overvoltages.

Additional Functions

• HF Signal Blocking: High   at PLCC frequencies (30-500 kHz) isolates carrier signals injected via   top terminal, preventing interference with metering circuits.​
• Transient Response Aid: Limits initial capacitive surge currents, reducing overreach (20-50% fault magnification) duration vs. untuned dividers.​
• Loss Management: Iron core introduces minor copper/core losses (tanδ contribution <0.3%), but air-core variants minimize saturation.​

Advantages Over PTs

CVTs are used in EHV/UHV due to lower insulation costs.

Feature	CVT	PT
Voltage Range	110-1200 kV	Up to 145 kV (typically <66 kV)
Size/Weight	Compact, lighter	Bulky, heavier
Cost (EHV)	Lower insulation expense	Higher due to core/insulation
Saturation Risk	None (capacitive)	High under faults/transients
PLCC Compatibility	Built-in coupling	Requires separate capacitors
Frequency Response	Faster transients	Slower
Burden Independence	High accuracy	Degrades with load

Applications in Power Systems

CVTs serve metering (revenue/energy), protection (relays like distance/differential), synchronization, and control, useful in 220-765 kV transmission. These are suitable for series-compensated lines and renewable integration grids needing precise voltage sensing.​

They double as coupling capacitors for PLCC, injecting HF signals (30-500 kHz) for telemetry/teleprotection without extra hardware. In substations, CVTs provide galvanic isolation, reducing secondary wiring insulation needs.