Voltage transformer (VT/PT) selection and wiring: ratio, burden, accuracy class, residual and open-delta connections and how to specify one for metering.
A voltage transformer (VT), still widely called a potential transformer (PT), does for voltage what a current transformer does for current: it isolates and scales a high system voltage down to a safe, standardized secondary so that meters, protective relays and transducers can measure it without ever touching the live conductor. For medium-voltage switchboards this is not a convenience — it is the only safe way to present 11 kV or 33 kV to a 0.5 W panel meter. This guide covers what a buyer or panel builder should actually specify.
The primary of a VT connects directly across the line (phase-to-phase or phase-to-neutral). Its secondary delivers a fixed, accurate fraction of that voltage — almost always 100 V or 110 V — to the instrument. Because the VT is a precision wound transformer operated well below saturation, its secondary voltage stays proportional to the primary across the正常 load range. A multifunction meter then reads the 100 V secondary and, knowing the ratio, displays the true system voltage. Get the ratio or the burden wrong and every meter and relay downstream reads a scaled error.
The VT ratio is written as primary : secondary, e.g. 11000/100 V or 33000/110 V. The secondary standard is set by regional practice — 100 V in IEC territories, 110 V in parts of North America — so pick the secondary your meters expect or nothing will agree. Choose the primary rating to match the nominal system voltage, not the maximum possible, because a VT operated far below its nameplate voltage loses accuracy just like a CT.
| System voltage | Typical VT ratio | Secondary |
|---|---|---|
| 3.3 kV | 3300/100 V | 100 V |
| 11 kV | 11000/100 V | 100 V |
| 22 kV | 22000/100 V | 100 V |
| 33 kV | 33000/110 V | 110 V |
These are planning values; always confirm the ratio against the exact system voltage and the meter's rated input.
The burden is the total impedance the VT secondary drives — the voltmeters, relay coils, transducers and the wiring between them — expressed in volt-amperes (VA) at the rated secondary voltage. A VT is only accurate while its actual burden stays at or below the rated burden on the nameplate. A 50 VA VT feeding a single 2 VA voltmeter and 3 m of wire is comfortable; the same VT asked to drive six relays and a long cable run can exceed its burden and sag in accuracy. Accuracy classes for metering VTs run 0.2, 0.5 and 1.0; protection VTs use the 3P / 6P classes with an accuracy limit factor that guarantees the secondary stays within tolerance at rated voltage even under fault-induced overvoltage.
A single-phase VT gives you one ratio. For three-phase metering and protection you combine VTs in standard arrangements:
Keep polarity (the marked terminal) consistent across every VT in the bank, or the vector sums are wrong and protection mis-operates.
Below roughly 66 kV an electromagnetic (inductive) VT is small, cheap and accurate. At EHV levels a capacitor voltage transformer — a stack of capacitors that taps the voltage, buffered by a small inductive element — is used because a wound VT at 132 kV would be enormous. CVTs introduce a phase error and a slower transient response, so they are specified for the protection classes they can meet, not by default.
ELEHPD supplies indoor and outdoor voltage transformers and potential transformers in 3.3 kV through 33 kV ratios with metering and protection classes, matched to our voltmeters, multifunction power meters and transducers for complete MV metering panels.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.