MV Current & Voltage Transformers: Saturation & Open-CT

1. The Sensory Organs of MV Switchgear: Scaling 36,000 Volts and 25,000 Amps for Digital Relays

Inside a Medium Voltage (1 kV – 36 kV) switchgear bay, microprocessor-based Intelligent Electronic Devices (IEDs / protection relays) and revenue meters cannot interface directly with 34.5 kV primary busbars or 25 kA short-circuit fault currents. Epoxy-resin cast Instrument Transformers—Current Transformers (CTs) and Voltage Transformers (VTs/PTs)—provide galvanic isolation while scaling primary values down to standardized secondary signals (1A / 5A and 100V / √3 or 110V / √3).

Governed by IEC 61869-2 (Current Transformers) and IEC 61869-3 (Inductive Voltage Transformers), these two components operate under completely opposite electromagnetic boundary conditions:

• Current Transformer (CT): Connected in SERIES with the primary power conductor. Its primary winding consists of a single bar or a few heavy turns, while the secondary has hundreds of fine turns. Under normal service, a CT operates into the near-zero impedance of relay current coils—effectively in a continuous short-circuit state.
• Voltage / Potential Transformer (VT / PT): Connected in PARALLEL (shunt) between phase and earth (or phase-to-phase). Its primary consists of thousands of high-voltage turns. Under normal service, a VT feeds high-impedance voltage inputs—operating in a near open-circuit state.

2. Electromagnetic Physics: Why Does an Open-Circuited CT Explode?

During normal closed-loop operation, primary load current (I₁) establishes a powerful primary magnetomotive force (N₁ · I₁). By Lenz's Law, the induced secondary current (I₂) circulates through the connected relay burden and generates an opposing counter-magnetomotive force (N₂ · I₂) shifted by nearly 180 degrees. The two opposing fields cancel each other out, leaving only a tiny net excitation flux (Φ_m) in the silicon-steel core.

What happens physically if a technician accidentally disconnects a secondary terminal block while 400 Amps of primary load current is flowing (making I₂ = 0)?

1. Collapse of Counter-MMF: With I₂ = 0, the demagnetizing counter-flux vanishes instantly. All 400 Amps of primary current now act purely as magnetizing excitation current (I_m = I₁).
2. Extreme Core Saturation: Driven by hundreds of times its rated excitation force, the magnetic core slams into deep saturation. Instead of a smooth sine wave, core magnetic flux (Φ) flattens into a clipped trapezoidal wave that only changes polarity during the brief AC current zero-crossings.
3. Faraday's Law & Lethal Voltage Spikes: According to Faraday's Law of Induction (e₂ = -N₂ · dΦ/dt), secondary voltage is proportional to the rate of change of magnetic flux. As the saturated flux snaps from +Φ_sat to -Φ_sat across a few microseconds at each zero-crossing, dΦ/dt skyrockets—inducing razor-sharp voltage spikes reaching 3,000 V to 15,000 V across the open secondary terminals!
4. Dielectric Flashover & Thermal Rupture: These kilovolt spikes flash over terminal blocks and electrocute personnel, while massive eddy-current and hysteresis core losses bake the epoxy insulation until the CT violently ruptures.

// CURRENT TRANSFORMER MMF BALANCE & OPEN-SECONDARY SPIKE PHYSICS
// 1. Normal Closed-Loop Operation (Secondary Shorted Through Relay Burden):
N_1 * I_1 - N_2 * I_2 = N_1 * I_m  (I_m ~ 1%, Net Core Flux Phi is Minimal -> V_2 < 5V)

// 2. Open-Circuit Secondary Fault (I_2 = 0):
N_1 * I_1 - 0 = N_1 * I_m          (100% of Primary Current Drives Core Saturation!)

// 3. Peak Induced Secondary Voltage at Current Zero-Crossing (Faraday's Law):
V_peak_open = - N_2 * ( dPhi_sat / dt_zero_crossing )  --> 5,000 V to 15,000 V Spikes!

N_1, N_2 : Primary and Secondary Turn Counts (e.g., N_1 = 1, N_2 = 400)
Phi_sat  : Saturated Core Magnetic Flux (Webers)
dPhi/dt  : Rate of Flux Reversal at Zero-Crossing (Wb/s)

  [CT CORE FLUX WAVEFORM vs. LETHAL OPEN-SECONDARY VOLTAGE SPIKES]

  Magnetic Flux (Phi) & Secondary Voltage (V2)
   +10 kV |          |                   |          <-- Lethal Voltage Spikes (Max dPhi/dt!)
          |          |   _________       |   ______
    +Phi  |         /|  /         \     /|  /      \  <-- Clipped Trapezoidal Saturated Flux
       0  |--------/-|-/-----------\---/-|-/--------\-------- (Time - ms)
    -Phi  |_______/  |/             \_/  |/          \_______
          |          |                   |
   -10 kV |          |                   |          (10 kV+ Needle Pulses Every Half-Cycle!)

3. Metering Cores (0.2S FS5) vs. Protection Cores (5P20): Dual-Core CT Specification

MV switchgear CTs typically house two or three physically separate magnetic cores cast inside a single epoxy housing. Specifying the wrong core for the wrong device leads to burned meters or failed relay tripping:

• Metering Core (e.g., Class 0.2S or 0.5 FS5): Engineered with high-permeability Nickel-Iron (Mu-metal) to deliver pinpoint accuracy from 1% to 120% of rated current. Crucially, the FS5 (Instrument Security Factor = 5) rating guarantees that the core intentionally saturates above 5× rated current, shielding delicate revenue meters from destructive short-circuit currents.
• Protection Core (e.g., Class 5P20 or 10P20): Built from grain-oriented silicon steel with a high saturation knee-point voltage. In a 5P20 rating, the "20" (ALF — Accuracy Limit Factor) certifies that the core will NOT saturate up to 20× rated primary current, keeping composite error below 5% so overcurrent relays (ANSI 50/51) accurately trip the breaker during severe faults.

4. The Silent Killer of Voltage Transformers: Ferroresonance & Open-Delta Damping Resistors

In isolated-neutral or impedance-earthed MV networks, three single-pole inductive Voltage Transformers connected phase-to-earth (Star/Wye) frequently explode due to Ferroresonance—a non-linear chaotic oscillation between the saturable iron-core inductance (L_m) of the VT and the zero-sequence phase-to-earth capacitance (C₀) of MV XLPE cables and busbars.

Triggered by circuit breaker switching, fuse blowing, or intermittent earth faults, core saturation causes L_m to drop abruptly until it matches the capacitive reactance of the switchgear cables (ωL_m = 1 / ωC₀). Primary magnetizing current surges up to 100× normal levels, thermally destroying the VT within minutes.

The Engineering Cure (Broken-Delta Winding + Damper Resistor): Phase-to-earth VTs are specified with two secondary windings:

1. Primary Measurement Winding (100/√3 V — Star Connected): Feeds phase voltage inputs on relays and meters (Class 0.5 / 3P).
2. Residual Earth-Fault Winding (100/3 V — Open-Delta / Broken-Delta): The three auxiliary windings (da–dn) are wired in series to form a broken triangle. Under balanced 3-phase conditions, the vector sum of voltages equals zero (V_R + V_S + V_T = 0 V). Whenever an earth fault or ferroresonance occurs, zero-sequence voltage (3V₀ = 100V) appears across the open-delta terminals. Connecting a heavy-duty 22 Ω – 47 Ω (500W – 1000W) Anti-Ferroresonance Damping Resistor across the open delta immediately absorbs the chaotic resonant energy as heat, saving the VTs from destruction.