Transformer VT/PT Selection — Electromagnetic & Capacitive Types, Accuracy Classes & Ferroresonance Damping

Introduction

Voltage transformers (VTs), also called potential transformers (PTs), provide scaled-down voltage signals for protection relays, energy meters, synchronizing equipment, and SCADA. While less failure-prone than current transformers, a VT that resonates ferroelectrically with system capacitance can destroy itself within seconds — producing smoke, fire, and a busbar fault. Selecting the right VT type, accuracy class, and ferroresonance mitigation strategy is essential for reliable transformer protection. This article covers VT selection based on IEC 61869-3 and IEC 61869-5.

1. VT Types: Electromagnetic vs. Capacitive

1.1 Electromagnetic (Inductive) VT

Property	Value / Range
Voltage range	≤ 245 kV (conventional); up to 420 kV with cascade design
Principle	Wire-wound transformer — same as a power transformer but optimized for accuracy
Secondary	Typically 100/√3 V or 110/√3 V (phase-to-ground); 100 V or 110 V (phase-to-phase)
Ferroresonance risk	Yes — inductive VT with system capacitance forms a resonant circuit
Frequency response	Good at 50/60 Hz; poor at high frequencies
Cost (relative)	Lower at MV; comparable to CVT at 132 kV+

1.2 Capacitive Voltage Transformer (CVT)

Property	Value / Range
Voltage range	66 kV to 1200 kV
Principle	Capacitive divider (C1, C2) reduces voltage; intermediate transformer (IT) further steps down
Secondary	Same as electromagnetic VT
Ferroresonance risk	Yes — IT forms resonant circuit with capacitor divider; damping circuit required
Frequency response	Compromised by the capacitor divider + IT circuit — transient response lags by 1–2 cycles
Additional function	Provides carrier-frequency coupling for power line carrier (PLC) communication
Cost	Lower at HV/EHV by eliminating heavy primary winding

1.3 Selection Guide

Voltage Level	Recommended Type	Reason
≤36 kV	Electromagnetic VT	Lowest cost, adequate insulation
36–132 kV	Electromagnetic or CVT	CVT cost-competitive above 66 kV
132–245 kV	CVT (preferred)	Lighter, cheaper, doubles as PLC coupler
≥420 kV	CVT	Only practical option; cascade EM-VT exists but rare

2. Accuracy Classes

2.1 Metering Accuracy

Class	Voltage Error (±%)	Phase Error (±minutes)	Frequency Range
0.1	0.1	5	99–101% Vn
0.2	0.2	10	99–101% Vn
0.5	0.5	20	99–101% Vn
1.0	1.0	40	99–101% Vn

2.2 Protection Accuracy

Class	Voltage Error (±%)	Phase Error (±minutes)	Voltage Range
3P	3.0	120	5%–Vf (rated voltage factor × Vn)
6P	6.0	240	5%–Vf

2.3 Rated Voltage Factor (V_f)

The voltage factor indicates how long the VT can withstand overvoltage:

Vf	Duration	Application
1.2	Continuous	Solidly earthed systems
1.5	30 s	Non-effectively earthed systems
1.9	8 h	Isolated neutral / resonant-earthed systems

For transformer protection: Select a VT with V_f appropriate for the system earthing. A 110 kV solidly-earthed system needs V_f = 1.2. A 35 kV resonant-earthed system needs V_f = 1.9.

3. Burden and Wiring

3.1 VT Burden

VT burdens are specified in VA at rated secondary voltage:

Connected Device	Burden per Phase (VA)
Digital multifunction relay	0.1–0.5
Electromechanical relay	3–10
Analog meter (moving iron)	2–5
Energy meter (electronic)	0.05–0.2
Synchroscope	5–10

3.2 Total Burden

S_total = √((ΣP)² + (ΣQ)²)

Typical VT rated burden: 10–50 VA (metering), 50–200 VA (protection with electromechanical relays).

Modern installations with digital relays: Even a 10 VA VT is oversized. The concern shifts from burden capacity to wiring adequacy.

3.3 Voltage Drop in Secondary Wiring

Unlike CTs where lead burden is critical, VT secondary cables carry voltage signals at low current (~1 A in worst case). The concern is not burden but voltage drop for accurate metering:

ΔU% = (2 × L × I_max) / (σ × A × U) × 100%

For a 100 V circuit, 4 mm² Cu, 100 m, 1 A load:

ΔU% = (2 × 100 × 1) / (56 × 4 × 100) × 100 = 0.89%

This is acceptable for protection but borderline for Class 0.2 metering. Use separate VT cores or increase cable cross-section for long metering runs.

4. Ferroresonance — The Silent VT Killer

4.1 The Phenomenon

Ferroresonance occurs when a VT's nonlinear magnetizing inductance resonates with system capacitances (cable charging capacitance, grading capacitance of circuit breakers). The resulting overvoltage and overcurrent can destroy the VT within seconds.

Typical triggering events:

• Single-phase switching (one or two poles of a breaker close before the third)
• Ground fault clearing (sudden voltage recovery)
• Busbar energization with VT connected (energization inrush)

4.2 Recognizing Ferroresonance

Oscillation Mode	Frequency	Voltage	Current	Duration
Fundamental	50/60 Hz	Up to 3 p.u.	Saturated	Sustained
Sub-harmonic (1/3)	16.7/20 Hz	Up to 2 p.u.	Very high	Sustained
Quasi-periodic	Non-harmonic	Up to 4 p.u.	Moderate	Intermittent

Operating personnel can often hear ferroresonance — the sub-harmonic mode produces a characteristic deep humming sound (20 Hz in 60 Hz systems) distinctly different from the normal 60 Hz buzz.

4.3 Mitigation Measures

Method	How It Works	Effective For
Damping resistor	Connected to open-delta tertiary; dissipates energy	All modes
Anti-ferroresonance VT	Linearized magnetizing characteristic; saturation at >1.9 p.u.	Fundamental mode
Load resistor on secondary	Permanently connected resistor that detunes the resonant circuit	Small systems
Simultaneous 3-pole switching	Prevents single-phase energization	Energization-triggered
CVT built-in damping	Ferroresonance suppression circuit (FSC) in CVT	CVT-specific

4.4 Damping Resistor Sizing

The open-delta tertiary winding (da-dn) of a three-phase VT set can feed a damping resistor:

R_damp = U_tertiary² / P_damp

Where P_damp is typically 50–200 W for MV VTs and 200–500 W for HV VTs. The resistor must be rated for continuous energization during sustained ferroresonance.

5. Fuse Protection

5.1 Primary Fuses

VT primary fuses protect the upstream system from a VT internal fault. They must:

• Clear the fault current without causing the primary system protection to operate
• Withstand VT inrush current without nuisance blowing
• Have adequate interrupting capacity for the system fault level

System Voltage	Fuse Type	Typical Rating
11–36 kV	HV current-limiting	2–3.15 A
66–132 kV	HV expulsion or current-limiting	2–6.3 A

5.2 Secondary Fuses / MCBs

Secondary-side overcurrent protection (MCB or fuse) protects the VT from wiring faults:

• Rated at 2–6 A (adjust for actual burden)
• Use MCBs with auxiliary contacts for alarm/trip on loss of VT supply
• For protection circuits, fuse-failure supervision (ANSI 60) must block protection functions that could maloperate on VT supply loss (e.g., distance, directional overcurrent)

FAQ

Q: When should I use a CVT instead of an electromagnetic VT?

Use a CVT at voltages ≥ 132 kV where the cost and weight advantage becomes significant. A 220 kV electromagnetic VT weighs 800–1200 kg and requires heavy oil-filled porcelain insulation. An equivalent CVT weighs 200–400 kg and uses a dry-type intermediate transformer. Also use CVTs when power line carrier (PLC) communication is needed — the coupling capacitor serves both functions.

Q: What are the downsides of CVTs for protection applications?

CVTs have slower transient response than electromagnetic VTs. During a sudden voltage collapse (fault), a CVT's output may take 1–2 power frequency cycles to settle, during which the protection relay receives an inaccurate voltage magnitude and phase angle. This affects distance and directional relays that need accurate voltage within 1 cycle. For EHV transmission line protection, this CVT transient characteristic must be modeled in the relay algorithm.

Q: Can I use a single-phase VT for three-phase voltage measurement?

Yes — three single-phase VTs connected in wye (Y) provide phase-to-ground voltages. However, this configuration creates a path for zero-sequence current and can support ferroresonance. Alternatively, two single-phase VTs in open-delta (V-V) connection provide three phase-to-phase voltages but not phase-to-ground. The V-V connection is inherently ferroresonance-immune because the open-delta path for zero-sequence flux is absent.

Q: What is the purpose of the open-delta tertiary winding?

The open-delta (broken-delta) tertiary of a three-phase VT set or three single-phase VTs sums the three phase-to-ground voltages: V_da-dn = V_a + V_b + V_c = 3V₀. Under normal balanced conditions, this sum is near zero. During a ground fault, the residual voltage 3V₀ appears and can be used for (1) ground fault detection (ANSI 59N/64), (2) ferroresonance damping via a connected resistor, and (3) synchronizing check by measuring zero-sequence voltage.

Q: How do I test a VT for ferroresonance susceptibility in the field?

The definitive test is a primary-side switching test: energize the busbar with the VT connected through a circuit breaker. Use a high-speed recorder (≥10 kHz sampling) to capture the VT secondary voltage for 5 seconds after energization. If the waveform shows sustained sub-harmonic oscillation or distorted fundamental with peak voltages >1.5× rated, the VT is ferroresonant in that configuration and requires damping. This test should be performed at commissioning, especially for systems with significant cable capacitance.

Q: What is the effect of VT accuracy on transformer differential protection?

Voltage transformers do not directly feed the differential element (87T), which uses CT secondary currents. However, VTs are essential for the voltage-restrained and voltage-controlled overcurrent elements (51V) used as backup protection. A VT error of 3% (Class 3P) combined with a CT error of 5% (Class 5P) could theoretically shift the voltage-restrained pickup characteristic by up to 8%. For this reason, use Class 1.0 or better VTs for backup protection when tight coordination margins exist.

References & Standards

Document	Title	Relevance
IEC 61869-3	Additional requirements for inductive voltage transformers	Electromagnetic VT specification
IEC 61869-5	Additional requirements for capacitive voltage transformers	CVT specification and testing
IEC 61869-1	General requirements for instrument transformers	Common requirements for all IT types
IEEE C57.13	Standard requirements for instrument transformers	US equivalent standard
CIGRE TB 569	Ferroresonance in power systems	Ferroresonance analysis and mitigation
IEC 60044-2	Inductive voltage transformers (superseded)	Legacy reference

*Du Fu, ZY POWER Production Engineer — A VT must be accurate when the voltage is normal and stable when the voltage is anything but.*