Inductive and Capacitive Voltage Transformers: Testing

What a voltage transformer is

The voltage transformer (VT, PT or TP) is an instrument transformer whose secondary voltage is proportional to the primary with a phase displacement close to zero. It delivers to relays and meters a low, standardized voltage, of the order of 100 to 120 V phase to phase depending on the standard and the country, and isolates them from the high voltage.

Voltage-dependent protections, such as distance 21, directional, undervoltage, overvoltage, frequency and synchronism check, are only as good as their voltage signal.

Main parts

Inductive voltage transformer (IEC 61869-3):

• Magnetic core with a primary winding of many turns, connected phase to ground or phase to phase.
• One or more secondary windings: metering, protection and, sometimes, a residual winding for open-delta connection.
• Paper-and-oil, SF6 or resin insulation, depending on the voltage level.
• At medium voltage, primary protection fuses.

Capacitor voltage transformer, CVT (IEC 61869-5):

• Two-section capacitive divider (C1 and C2) that steps the voltage down to an intermediate value.
• Electromagnetic unit: an intermediate inductive transformer and a compensating reactor tuned with the divider at the system frequency.
• Ferroresonance suppression circuit.
• High-frequency terminal and drain coil when the CVT is used as a coupling device for power line carrier.

How it works

The inductive VT works as a small transformer running almost unloaded. Its accuracy depends on the secondary burden being within the rated one.

The CVT divides the voltage with the capacitors and brings it to a level that an inductive transformer with less insulation can handle. The compensating reactor cancels the reactance of the divider at rated frequency, so its accuracy depends on the frequency and on the stability of the capacitances. On a sudden voltage drop, the energy stored in the tuned circuit produces a transient that can last a few cycles.

VTs are also specified by their rated voltage factor: the overvoltage they withstand for a defined time, which depends on how the system neutral is grounded.

With three residual windings connected in open delta, the residual voltage is obtained, which is three times the zero-sequence voltage. Under balanced conditions it is almost null and it grows on a ground fault. In ungrounded systems a damping resistor is usually connected in that delta to attenuate ferroresonance; its value is given by the VT manufacturer.

Accuracy classes

According to IEC 61869-3 and -5, the most used metering classes are 0.1, 0.2, 0.5 and 1, and the protection classes are 3P and 6P. One VT can have windings with different classes. IEEE C57.13 defines its own classes and standard burdens.

Types

• Extra-high and high voltage: almost always CVT, for cost and for their additional use as a power line carrier coupling device; inductive VTs when better transient response or revenue-metering accuracy is needed, depending on the specification.
• GIS: inductive VTs encapsulated in SF6.
• Medium voltage and industry: inductive VTs in switchgear, often withdrawable and fused; phase-to-ground, phase-to-phase or V connections.
• Dividers and low-power sensors, defined in the IEC 61869 series, in modern switchgear and in digital substations.

Related protection

• Distance 21, directional 67 and 67N, power 32.
• Undervoltage 27, overvoltage 59 and neutral overvoltage 59N, which usually takes the residual voltage from an open delta.
• Frequency 81 and synchronism check 25.
• Loss-of-potential supervision (fuse failure or VT supervision), which blocks voltage-dependent functions when a secondary phase is lost.

Ferroresonance in detail

ABB’s technical guide on instrument transformers (English, 2004) explains the mechanism. An ungrounded system is always connected to ground, however poorly, through the capacitance and resistance of its insulation. A voltage transformer connected between a phase and ground is thus in parallel with a capacitor, and a parallel resonance can appear between the two that depends on the partial saturation of the iron core: hence the name. The result can be a very high voltage and the destruction of the transformer. As a clue, if a failed VT shows its primary winding burned uniformly from end to end, overexcitation and ferroresonance are the first suspects.

The prevention ABB describes is to load the secondary with a resistor of sufficient power or to connect a resistor in the open delta. The value depends on the voltage class and the manufacturer, so it must be taken from the equipment documentation. The guide adds two rules: a VT connected phase to ground in an ungrounded system is never used for precision metering (those connected phase to phase are used), and the secondaries of three VTs connected in wye must not be closed in delta. The specific standard is technical report IEC TR 61869-102:2014, on ferroresonance oscillations in substations with inductive voltage transformers. More real cases in ferroresonance in voltage transformers.

The secondary cable counts

The ratio and phase angle errors of a VT are determined at its terminals. If the cables to the burden are long, their impedance adds voltage drop and error. According to the ABB guide, the ratio error increases in proportion to the secondary current and to the combination of cable resistance and reactance, depending on the burden angle. The phase angle error grows in a similar way. That is why the cable is included in the burden calculation and in the accuracy test, which must reproduce the real burden with its conductors. For revenue metering, calculating with the real cable impedance is part of the design.

The IEEE C57.13 class 0.3 is defined by a parallelogram of ratio correction factor and phase angle, with the total correction factor between 0.997 and 1.003. Its limits are met between 90 % and 110 % of rated voltage and also at zero burden. There are also classes 0.6 and 1.2, with permitted errors twice and four times larger.

Primary fuses

At medium voltage, the ABB guide explains that primary fuses serve mainly to take a VT with an internal fault off the line, so that it does not become a phase-to-ground fault that forces the breaker to interrupt service. They also protect against a partial short circuit in the winding and against a secondary short circuit, and some utilities choose fuses that do not operate for the latter. Fuses have a maximum interrupting current, and current-limiting fuses have higher ratings. The use of secondary fuses is uncommon; if they are used and blow without anyone noticing, the measurement or protection is lost, so the loss-of-potential supervision must cover them.

Testing in commissioning and maintenance

The current and voltage transformer tests include, for the VT:

• Ratio and polarity of each winding; see the polarity check on VTs.
• Winding resistance of the primary and secondaries.
• Insulation resistance of the primary and secondaries.
• Capacitance and dissipation factor of the divider sections in CVTs, compared with the nameplate and the factory report.
• Real secondary burden of the circuit, including cables.
• Verification at energization: magnitudes, phase sequence and angles at each relay and meter, and residual voltage close to zero in the open delta.
• Anti-ferroresonance device and damping resistors, where they exist.

There is equipment dedicated to these tests, such as the VOTANO 100, which qualifies inductive and capacitor voltage transformers without an external reference.

Common failures

• Open secondary fuses or miniature circuit breakers, which the relay interprets as a fault if supervision is not active.
• Short circuit in the secondary, which destroys the VT or its fuses.
• Ferroresonance in inductive VTs of ungrounded systems or with breakers that have grading capacitors.
• Punctured capacitor elements in a CVT, which change the ratio little by little; it shows up as drift of the phase-to-phase secondary voltage.
• Crossed phases or a badly connected open delta.
• Moisture and insulation deterioration in oil-filled equipment.

Testing and commissioning with Tiza Group

At Tiza Group we test inductive and capacitor voltage transformers and verify their secondary circuits with the protections in service. Field work is done in Colombia, or elsewhere on request. Write to us and we will propose the scope.

Related guides

• OMICRON VOTANO 100: voltage transformer testing
• Capacitor voltage transformer (CVT)
• Ferroresonance in voltage transformers
• Loss-of-potential alarm: VT fuse failure
• Current transformer (CT): parts, functions and testing
• CT terminals and secondaries: P1, P2, S1, S2 and taps
• VT and CT supervision 60 (fuse failure): how it works

References

• IEC 61869-1, Instrument transformers: general requirements.
• IEC 61869-3, Additional requirements for inductive voltage transformers.
• IEC 61869-5, Additional requirements for capacitor voltage transformers.
• IEEE C57.13, Requirements for instrument transformers.
• Blackburn and Domin, Protective Relaying: Principles and Applications.
• ABB, Instrument Transformers: Technical Information and Application Guide, 1VAP420003-TG, rev. A, 2004: ferroresonance, secondary cables, class 0.3, primary fuses.
• IEC TR 61869-102:2014, Instrument transformers - Ferroresonance oscillations in substations with inductive voltage transformers.

Frequently asked questions