A medium-voltage current transformer (MVCT) is an instrument transformer that steps down high primary currents in a 5kV–35kV circuit to a safe, standardized secondary current (usually 1A or 5A) for metering and protective relaying. What this component fundamentally changes in a real installation is the boundary between lethal medium-voltage busbars and low-voltage control wiring; it converts a dangerous 1200A primary fault current at 15kV into a manageable 5A secondary signal that a microprocessor relay can use to trip a vacuum breaker in milliseconds, without exposing the relay to the medium-voltage potential.

Engineers and technicians commonly confuse MVCTs with Potential Transformers (PTs/VTs), which step down voltage rather than current. Another frequent mix-up is assuming MVCTs are simply larger versions of the split-core CTs used in residential solar monitoring. In reality, MVCTs are typically solid epoxy-cast or oil-insulated, window-type or bushing-type devices that require de-energized installation and strict adherence to burden and accuracy class limits defined by IEEE C57.13.

Core Ratings and IEEE C57.13 Accuracy Classes

Selecting an MVCT is not just about picking a current ratio. You must match the accuracy class to the application. Metering CTs are designed to be highly accurate at normal load currents but intentionally saturate during faults to protect delicate panel meters. Relaying CTs, conversely, must maintain accuracy far into the overcurrent range so protective relays see the true fault magnitude.

Spec Sheet Rule of Thumb: The letter 'C' in a relaying class (e.g., C200) means the CT is fully distributed wound with low leakage flux, allowing its performance to be calculated. The number (200) is the secondary terminal voltage rating the CT can deliver at 20 times the rated secondary current without exceeding a 10% ratio error.
IEEE C57.13 Standard Accuracy Classes for MVCTs
Application Accuracy Class Ratio Error Limit Typical Use Case
Metering 0.3 ±0.3% at 100% rated current Utility revenue billing meters
Metering 0.6 ±0.6% at 100% rated current Plant power monitoring / SCADA
Relaying C100 ±10% at 20x rated current (up to 100V) Standard overcurrent protection (50/51)
Relaying C200 ±10% at 20x rated current (up to 200V) Differential protection (87) / Distance (21)
Relaying C400 ±10% at 20x rated current (up to 400V) High-speed bus differential / close-in faults

Worked Numeric Example: Ratio, Secondary Current, and Burden VA

Let’s calculate the actual secondary output and verify the burden for a 15kV switchgear feeder protected by a Schweitzer SEL-751 overcurrent relay. This is the exact math you must perform to ensure the CT does not saturate during a fault.

The Setup:

  • CT Ratio: 1200:5A
  • CT Accuracy Class: C200 (Relaying)
  • Maximum Expected Fault Current (Primary): 18,000A
  • Secondary Wiring: 12 AWG stranded copper, 50 feet one-way run to the relay panel.
  • Relay Burden: 0.05 ohms (typical for modern microprocessor relays at 5A nominal).

Step 1: Calculate Secondary Fault Current
During an 18,000A primary fault, the secondary current is:
I_secondary = I_primary × (5 / 1200) = 18,000 × 0.004166 = 75A
This is exactly 15 times the rated 5A secondary current (15 × 5A = 75A). The C200 rating guarantees accuracy up to 20x (100A), so we are within the safe linear zone.

Step 2: Calculate Total Secondary Burden (Resistance)
Burden is the total impedance of the secondary circuit. We must account for the relay, the wire loop, and the CT's own internal winding resistance.

  • Relay Burden: 0.05 Ω
  • Wire Burden: 12 AWG copper is ~1.588 Ω per 1000 ft. The loop is 100 ft (50 ft out, 50 ft back).
    R_wire = 1.588 × (100 / 1000) = 0.158 Ω
  • CT Internal Winding Resistance: Let's assume 0.40 Ω (check the specific manufacturer test report for the exact value).
  • Total Burden (Z_total): 0.05 + 0.158 + 0.40 = 0.608 Ω

Step 3: Calculate Required Secondary Voltage
To push 75A through 0.608 Ω, the CT must generate:
V_required = I_secondary × Z_total = 75A × 0.608 Ω = 45.6V

Verdict: The CT is rated C200, meaning it can output up to 200V before exceeding the 10% error limit. Since 45.6V is well below 200V, this CT will not saturate, and the relay will see an accurate representation of the 18kA fault.

Where You Meet This in Practice

Medium-voltage CTs are physically integrated into the infrastructure wherever power is distributed between 5kV and 35kV. You will encounter them in three primary physical configurations:

  1. Metal-Clad Switchgear (Bushing/Slip-over Type): In equipment like Eaton VCP-W or Siemens GM-SG switchgear, MVCTs are often toroidal cores slipped directly over the primary busbar or the vacuum interrupter bushings inside the breaker compartment. This saves space and utilizes the existing primary insulation.
  2. Padmount Transformers and Reclosers (Window/Donut Type): On utility pole-top reclosers or ground-mounted pad transformers, you will find large epoxy-cast window CTs. The primary medium-voltage cable passes directly through the center hole. These are heavily weather-sealed and designed to withstand UV and moisture.
  3. Medium-Voltage VFDs and Motor Control (Bar-Type): For large industrial loads like 13.8kV induced draft fans in power plants, bar-type CTs are used. The primary winding is a solid copper bar cast directly into the CT body, providing extreme mechanical rigidity to withstand the magnetic forces of massive motor starting inrush currents.
Safety Warning: Never open-circuit the secondary of an energized MVCT. Without a secondary load to create a counter-magnetomotive force, the core saturates heavily. This induces lethal peak voltages (often exceeding 2kV-5kV) across the open secondary terminals, posing a fatal shock hazard and potentially causing the CT to explode from core overheating. Always short the secondary terminals before removing a relay or meter.

FAQ: Medium-Voltage CT Misconceptions

Do I need to ground the secondary circuit of an MVCT?
Yes. IEEE and NEC standards require that CT secondary circuits be grounded at exactly one point, typically at the first terminal block in the switchgear low-voltage compartment. This prevents the secondary wiring from floating up to medium-voltage potentials in the event of an internal insulation failure between the primary bus and the secondary winding. Never ground it at multiple points, as this creates a parallel ground loop that will divert fault current away from the relay.

What is the 'knee-point voltage' and why does it matter for differential protection?The knee-point voltage is the point on the CT's excitation curve where a 10% increase in voltage requires a 50% increase in exciting current—essentially, the exact threshold where the magnetic core begins to heavily saturate. For bus differential protection (ANSI 87), relays compare currents from multiple CTs. If one CT saturates during an external through-fault while the others do not, the relay sees a false 'spill' current and will nuisance-trip the entire bus. Specifying a high knee-point voltage (often requiring C400 or C800 classes) ensures all CTs remain linear during external faults.

Can I use a 5A secondary CT with long wire runs to a remote control building?
It is highly discouraged. As demonstrated in the burden calculation above, wire resistance adds directly to the CT burden. For long runs (over 50 feet), engineers specify a 1A secondary CT instead of a 5A CT. Because burden is calculated as I²R, dropping the secondary current from 5A to 1A reduces the wire burden heating and voltage drop by a factor of 25, allowing for much longer wire runs without driving the CT into saturation.