Relay Protection Academy Module 06 of 25
Module 06 Intermediate

Instrument
Transformers

⌛ ~2 hours 📚 IEC 60044 / IEEE C57.13 📑 12 slides

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Module 06 / 6.1

Measuring vs Protection ITs

Measuring ITs
  • High accuracy near rated load (Class 0.2, 0.5)
  • Core saturates quickly on overload - protects instruments
  • Accuracy degrades intentionally at fault magnitudes
Protection ITs
  • Must stay accurate up to the Accuracy Limit Factor (ALF = 10x to 30x rated)
  • Core must not saturate under fault conditions
  • IEC: Class 5P, 10P; IEEE: C100, C200, C400, C800
Composite error (IEC) RMS difference between ideal and actual secondary current, as % of rated. Class 5P = 5% error at ALF; Class 10P = 10%.
Never mix classes Using a measuring CT in a protection circuit is a common and dangerous mistake - it will saturate and give zero output at fault current levels.

Module 06 / 6.2

VT Errors: Ratio and Phase

Ratio Error
  • Exciting current causes both ratio and phase errors
  • Turns compensation winds extra secondary turns to keep ratio error within limits
  • Phase error is in minutes of arc - critical for power metering
⎓
Diagram needed

VT equivalent circuit secondary side. Source Vp, series leakage Zs, parallel exciting branch Ze (Rc parallel Xm), secondary Vs feeding burden Zb. Label Ip, Ie, Is. Dark background, engineering style.

Class factor VTs used with broken-delta earth fault schemes must be rated for sustained phase-to-earth voltage: class factor 1.2 (solidly earthed) or 1.9 (isolated neutral).

Module 06 / 6.3

CT Classes and Knee-Point

Accuracy Limit Factor (ALF)
Multiple of rated current up to which composite error is met. Class 5P20 = 5% error maintained to 20x rated.
IEC Class PX
Specified by knee-point e.m.f. , max exciting current , and max winding resistance . Used for unit protection with engineered saturation margins.
IEC Knee-Point
Point where +10% secondary e.m.f. requires +50% exciting current. An internal e.m.f. (winding drop included).
Anti-remanence types
TPY (max 10% remanence, small air gap) and TPZ (near-zero remanence, large air gap) prevent flux build-up across successive fault shots.
Unit protection All CTs in a differential zone must be Class PX with matched, high . Asymmetric saturation during a through-fault creates a false differential current and will false-trip.

Module 06 / 6.4

IEC vs IEEE Knee-Point

IEC Class PX
  • Linear excitation curve
  • +10% voltage / +50% current tangent point
  • is an internal e.m.f.
IEEE C-class
  • Log-log excitation curve
  • 45-degree tangent point on log-log plot
  • is a terminal voltage at 20x rated
Conversion formula
Never equate directly IEC 400 V does not meet IEEE C400. The conversion always reduces available terminal voltage.

Module 06 / 6.5

Lead Burden: 1A vs 5A

Lead burden scales with the square of secondary current.

Formulas
  • Lead loop >50 m: always specify 1A secondary
  • Modern numerical relays: <0.05 VA - leads dominate regardless
  • Always use loop resistance (both conductors)
200 m leads, Rlead = 3 Ω
5A:
1A:

1A rating cuts lead burden by a factor of 25.

Module 06 / 6.6

Transient Factor (TF)

Formula
= primary time constant (cycles); worst case assumes full DC offset.
Knee-point requirement
Most common CT sizing error Sizing for steady-state AC fault only, ignoring TF. Near generators X/R > 30, so TF > 31 - the CT will saturate on the first cycle of a fully asymmetric fault.
  • CT saturation clips the secondary waveform
  • High-speed differential protection may fail to trip or false-trip
  • Specify TPY or TPZ cores where TF is high

Module 06 / 6.7

Broken Delta VT

  • Balanced: , open-end output = zero
  • Phase-to-earth fault: output =
  • Solidly earthed fault: of rated secondary
  • Primary rated for phase-to-earth voltage (class factor 1.2 or 1.9)
Five-limb cores required Three-limb VTs cannot support independent zero-sequence flux. Use five-limb or three separate single-phase VTs.
⎓
Diagram needed

Three single-phase VTs in broken delta. Star primaries on three-phase system. Secondary Va, Vb, Vc head-to-tail open delta. Relay residual element bridges the open ends. Label 3V0 output. Dark background, clear labels.

Module 06 / 6.8

Capacitor Voltage Transformers

Transient response
Fault inception excites the LC resonant circuit, producing a decaying oscillation on the secondary. High-speed distance relays may see an incorrect voltage for the first few cycles.
Ferro-resonance
Non-linear core interacts with the series capacitance, sustaining sub-harmonic oscillation (16.7 Hz on 50 Hz). Sustained overvoltages of 25-50% and severe waveform distortion result.
Suppression
Permanent resistive burden damps ferro-resonance but worsens transient response - a direct trade-off. Inductive auxiliary VTs on CVT secondaries are a common trigger for ferro-resonance.
Secondary protection
Fuse or MCB as close to the VT as possible. Primary fuses cannot clear a secondary short circuit - secondary currents can reach catastrophic levels.

Module 06 / 6.9

Non-Conventional Instrument Transformers

Rogowski Coil
Air-cored toroidal winding. No saturation, no remanence. Output is the derivative of current - requires electronic integration before relay use.
Magneto-Optic CT
Polarised light rotation (Faraday effect) proportional to current. Passive fibre at HV - inherent galvanic isolation, no saturation.
Hall-Effect Sensor
Semiconductor in iron core air-gap. Primarily for DC/low-frequency measurement. Temperature-sensitive; not standard for power-frequency protection.
Process Bus Output
Digital sampled-value stream (IEC 61850-9-2). No open-circuit hazard. Protection IEDs must support SV inputs.
Why use them? No saturation, lighter weight, smaller footprint. Ideal for GIS where space is at a premium.

Module 06 / Worked Example

CT Sizing: EHV Switchgear

Given , relay 10 VA, , , cycles, ALF=20,

Step A: Total burden

Step B: Transient Factor

Step C: IEC to IEEE conversion

Result IEC 400 V does not meet IEEE C400. Specify IEEE C400 (next standard step above 360.95 V).

Module 06

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