Voltage Transformer (VT) Selection: Ratio, Accuracy Class and Ferroresonance
voltage transformer selection: A voltage transformer steps medium-voltage system voltage down to the 100 V or 110 V level that measuring and protection devices can use. Selection is governed by the transformation ratio, accuracy class (0.5 for metering, 3P for protection), rated burden, voltage factor and the ferroresonance risk that follows from the system earthing arrangement.
Contents
- What does a voltage transformer do and how does it differ from a CT?
- Connection arrangements: phase-to-phase, phase-to-earth and residual winding
- How are ratio, accuracy class and burden determined?
- Voltage factor and the risk of ferroresonance
- Protection, sealing and application on the panel side
- Key takeaways
- Frequently asked questions
- Related articles
While a current transformer appears on almost every feeder in a board, a voltage transformer is usually fitted once or twice per busbar, so selection mistakes are noticed less often but have wider consequences. The wrong accuracy class affects revenue metering, the wrong connection affects earth-fault protection, and the wrong voltage factor affects the transformer itself. This article looks at ratio, accuracy, burden, voltage factor and ferroresonance from the point of view of medium-voltage panel design.
What does a voltage transformer do and how does it differ from a CT?
A voltage transformer reduces primary circuit voltage by a defined ratio and delivers a safe level on the secondary side for instruments, meters and protection relays. A current transformer operates with its secondary short-circuited, whereas a voltage transformer secondary may be left open but must never be short-circuited; that is the most fundamental operating difference between the two. Structurally a voltage transformer resembles a small power transformer: the primary winding is connected in parallel with the system and the secondary load is limited by the rated burden. At medium voltage typical ratios are expressed as a step-down from several thousand volts on a 34.5 kV or 36 kV class system to 100 V or 110 V.
Connection arrangements: phase-to-phase, phase-to-earth and residual winding
A single-phase voltage transformer connected phase to phase is sufficient for line voltage measurement and synchronisation only. A phase-to-earth connection, used as a three-phase group, makes both phase voltages and the zero-sequence voltage available, which is the basis of earth-fault detection in isolated or compensated neutral systems. For residual voltage measurement three secondary windings are connected in open delta and the voltage that appears during a fault is fed to the protection relay. When choosing this topology, the system earthing arrangement, the magnitude of the fault current and the input signal the relay needs must be considered together.
How are ratio, accuracy class and burden determined?
Accuracy class follows the application: class 0.2 or 0.5 for revenue metering, class 1.0 for panel indication and class 3P or 6P for protection windings. The rated burden is the sum of all loads connected to the secondary plus cable resistance; loading far below the rated burden also degrades accuracy, so the range must be checked. In large installations the secondary cable length produces a noticeable voltage drop and effectively loses the accuracy class, which is why conductor sizing is part of the measuring circuit design. A voltage transformer may have more than one secondary winding, with measuring and protection functions fed from separate windings.
Voltage factor and the risk of ferroresonance
| Parameter | Metering application | Protection application |
|---|---|---|
| Accuracy class | 0.2 or 0.5 (revenue), 1.0 (indication) | 3P or 6P |
| Secondary voltage | 100 V or 110 V | 100 V or 110 V, divided value for a residual winding |
| Rated burden | Meter and analyser load, typically small | Relay input impedance, usually smaller still |
| Voltage factor | 1.2 times continuously | Up to 1.9 times during an earth fault, time limited |
| Winding arrangement | Separate measuring winding | Separate protection winding plus residual winding if needed |
The voltage factor defines how long the transformer can operate above its rated voltage: 1.2 times continuously, and during an earth fault up to 1.9 times for a limited period such as thirty seconds or eight hours depending on the system earthing. In isolated neutral systems ferroresonance can develop between the saturated iron core of a phase-to-earth connected voltage transformer and the cable capacitance, resulting in overvoltage and overheating of the transformer. The countermeasure is a damping resistor on the residual voltage winding, or a core with a higher saturation point. The risk is more pronounced in compact medium-voltage installations with long cable runs.
Protection, sealing and application on the panel side
The primary side of a voltage transformer is protected by medium-voltage fuse links that isolate the transformer on a short circuit and are positioned so they can be replaced easily inside the panel. On the secondary side each winding is given its own miniature circuit breaker or fuse, and the metering circuit is separated by a terminal arrangement suitable for sealing. Panel design should provide a dedicated compartment for the voltage transformer and, where required, a lockable withdrawable mechanism, giving safe access during maintenance while the busbar remains live. Labelling and circuit numbering should clearly separate the measuring circuit from the power circuit.
Key takeaways: voltage transformer selection
- Never short-circuit the secondary — unlike a current transformer, a VT secondary may be left open.
- Accuracy class follows the application — 0.2 to 0.5 for revenue metering, 3P or 6P for protection.
- Voltage factor depends on system earthing — isolated neutral systems see higher voltage for the fault duration.
- Ferroresonance is prevented by damping — add a resistor on the residual winding or change the core selection.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Why must I not short-circuit a VT secondary?
A voltage transformer behaves like a source connected in parallel; with the secondary short-circuited it draws a very large current from the primary and the winding burns out. With a current transformer the situation is reversed: leaving the secondary open produces dangerous overvoltage. The operating rules of the two devices must not be confused.
Can one voltage transformer feed both a meter and a relay?
Yes, if separate secondary windings are provided in the same housing. Feeding a revenue meter and a protection relay from a single winding compromises both the accuracy class and the sealing arrangement. The correct solution is separate windings and separate secondary protective devices for measuring and protection.
How is ferroresonance recognised?
The symptoms are abnormally high secondary voltage under no-load conditions, overheating and humming of the voltage transformer, and unexplained oscillation in measurements. The risk increases in isolated or high-resistance earthed systems and in installations with long cables; a damping resistor resolves it.
Related articles
- Current Transformer (CT) Selection: Ratio, Accuracy Class and Burden Calculation
- Metering Panel: Grid Connection, Sealing and Measurement Accuracy
- What is a Medium Voltage (MV) Switchgear? Metal-Clad and RMU Switchgear.
- All technical articles
In Devpan medium-voltage panels the voltage transformer connection, accuracy class, secondary protection and sealing arrangement are designed to the project specification. Talk to our engineering team to separate your measuring and protection requirements correctly. Related Devpan solution: medium voltage switchgear.
