About the PT / CVT Selection
Potential transformer (PT) or capacitive voltage transformer (CVT) selection determines the appropriate voltage ratio, accuracy class, and burden for measuring voltage in HT circuits of a solar plant — for protection relays, energy meters, and power quality analysers.
How to calculate it
- 1Primary voltage: system phase-to-earth voltage for star-connected PT (11,000 ÷ √3 = 6,351 V for 11 kV system); or phase-to-phase for delta-connected PT.
- 2Secondary voltage: 110 V (standard) or 110 ÷ √3 = 63.5 V for star-connected secondary.
- 3Standard ratio: 11,000 ÷ √3 / 110 ÷ √3 = 100:1 (expressed as 11,000 V ÷ √3 / 110 ÷ √3).
- 4Accuracy class for metering: class 0.2 or 0.5. For protection: class 3P.
- 5Burden: connected relay and meter VA inputs — PT rated burden must be ≥ total connected burden.
- 6CVT vs electromagnetic PT: CVT is used at 33 kV and above (more economical at high voltages); electromagnetic PT for 11 kV and below.
Worked example
An 11 kV solar plant: PT ratio 11,000/√3 : 110/√3 (i.e., 6,351 V : 63.5 V, or 100:1). Metering class 0.2S (for revenue meter), 30 VA burden. Protection class 3P, 30 VA. Total PT burden: 60 VA. Select PT rated for 100 VA at class 0.2S (to allow future additions). Verify thermal limit current: must withstand 1.9× rated voltage continuously and 1.25× for 8 hours per IEC 61869-3.
What you can calculate
- VT/CVT class
- Burden VA
- Accuracy
- IEC 61869-3
Frequently asked questions
What is the difference between a PT and a CVT?
A PT (potential/voltage transformer) is an electromagnetic device — a conventional transformer with a high primary-to-secondary turns ratio. Accurate and linear but expensive at high voltages (> 33 kV) because the primary winding must be insulated for the full line voltage. A CVT (capacitive voltage transformer) uses a capacitor divider to step down the voltage before a small electromagnetic transformer — economical at 33 kV and above. CVTs have slightly higher phase error at sub-power-factor currents, making them less suitable for class 0.2 revenue metering than electromagnetic PTs.
Why is accurate voltage measurement important in a solar plant?
The energy meter's kWh measurement depends on both voltage and current readings — a 1% error in the PT ratio translates to a 1% error in all energy billing. For a 10 MW solar plant generating 15 GWh/year at ₹3.5/kWh, a 1% metering error is ₹52.5 L/year in revenue discrepancy. DISCOM revenue meters are typically class 0.2, calibrated annually. Disputes over metering accuracy are common in large solar plants — always keep PT calibration certificates current.
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