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CT / VT (PT) characteristics and saturation

ANSI C-class CTs (C400, C800), accuracy classes, burden, saturation effects on differential / distance / overcurrent protection. Inductive VT vs CCVT. Ferroresonance on ungrounded systems. Modern relay tolerance.

Senior ~12 min

Step 1 — CTs and VTs scale primary I and V for relays

Animation speed 0.55×

class — burden — state —

Reference notes

Current transformers (CTs) and voltage transformers (VTs / PTs) scale primary current and voltage to relay-compatible levels (5 A or 1 A; 120 V or 67.5 V). Their accuracy classes, burden ratings, and saturation behavior directly affect protection performance — particularly differential schemes. Use Next → to walk through CT accuracy classes, saturation effects, VT and CCVT characteristics, and best-practice CT/VT specification.

CT accuracy classes (ANSI)

Class	Meaning
C-class	Protection-class accuracy. Number = secondary voltage at 20× rated primary current into rated burden. E.g., C400 → 400 V at 100 A → 4 Ω burden equivalent.
T-class	Like C-class but explicitly includes DC offset effects.
Metering 0.3 / 0.6 / 1.2	Accuracy class for revenue metering, expressed as percent error at rated current.

Standard burden classes: B-1 (1 Ω), B-2, B-4, B-8 (highest). Higher burden makes CT more likely to saturate during faults.

CT saturation

Iron core has flux limits. When secondary current × burden requires more flux than the core can sustain, the CT saturates: secondary waveform distorts (peaked, asymmetric), RMS reading drops below true scaled value, phase angle shifts.

Symmetric saturation: occurs at high AC currents above the CT's design limit. For C400 into 4 Ω burden, ~20× rated current.
DC offset worsens it: real fault currents have a decaying DC component proportional to system X/R ratio. DC flux adds to AC flux requirement, causing earlier saturation than symmetric calculation predicts.
Industry rule: design CT burden so flux stays within 50 % of saturation at maximum fault current with DC offset.

Effect on protection

Differential protection (87T / 87B / 87L) — most vulnerable. Through-fault currents that should balance among CTs become slightly different when one CT saturates before another → apparent differential → potential false trip. Mitigation: percentage-restraint slope, higher CT class, high-impedance differential schemes (CTs paralleled across a series stabilizing resistor).
Distance protection (21) — saturated CT under-reads I, so calculated Z = V/I appears larger than actual → relay UNDER-REACHES, may miss a Zone 1 fault. Modern numerical relays detect saturation via harmonics and apply correction.
Overcurrent (51) — saturated CT delivers less secondary current → relay sees lower fault → operates slower. 30 % CT under-read can extend trip time by 40 %.

Voltage transformer (VT / PT) characteristics

Inductive VT — small transformer to secondary. ANSI accuracy classes 0.3, 0.6, 1.2 (% ratio error). Used up to ~138 kV. Shunt burden (relays drawing current in parallel with the VT secondary) causes voltage droop and phase shift if excessive.
CCVT (capacitor-coupled VT) — capacitor voltage divider + magnetic VT on lower-voltage tap. Used above 138 kV where inductive VTs become uneconomic. Transient response: stored energy in capacitors → secondary takes 10-50 ms to follow primary voltage collapse → affects distance Zone 1 nearby-fault timing.
Ferroresonance — nonlinear LC resonance between VT magnetizing reactance and stray capacitance. Particularly problematic on UNGROUNDED systems. Mitigation: damping resistor on open-delta winding.

CT testing methods

Ratio test — apply known primary current, verify secondary scaling.
Polarity test — critical for differential protection. Flash test or DC pull method.
Excitation curve — apply variable V to secondary with primary open; identify knee-point voltage (saturation onset).
Insulation resistance — Megger test between primary, secondary, ground.

Modern numerical relay tolerance

Sample at 1-4 kHz, apply digital filters.
Recognize saturation by harmonic content (3rd, 5th harmonics during saturated cycles).
Apply correction algorithms for distance impedance calculation during DC offset.
Adaptive percentage-restraint with dual-slope characteristic for differential.
Oscillographic recording captures sub-cycle waveforms — post-event review verifies protection operation.

Best practices for high-fault-level installations

Spec C800 or higher at sites with high fault duty (138 kV+).
Keep secondary burden low — use modern microprocessor relays (~0.5 VA) and minimize wiring runs.
Match CT ratios across feeders in differential schemes.
Specify DUAL-CORE CTs — separate cores for protection and metering so heavy relay burden doesn't compromise metering accuracy.
On-site test at commissioning AND periodically per IEEE C37.230 (annual to biannual).

Take-away. CT C-class (e.g., C400, C800) = secondary V at 20× rated current into rated burden — higher number = more capacity before saturation. DC offset on fault current makes saturation worse. Saturation distorts waveform (peaked, lower RMS, phase shift) — most vulnerable scheme is differential. Inductive VT for ≤138 kV; CCVT above, with capacitor-transient effect on Zone 1 distance. Best practice: high C-class + low burden + dual-core + periodic testing. Modern numerical relays detect and partially correct for saturation via harmonic analysis.