Voltage transformer (potential transformer)

01

Set the accuracy class, burden and voltage factor

RITZ's medium-voltage instrument transformer catalogue defines limits for voltage error and phase displacement that must hold between 80% and 120% of rated voltage with burdens from 25% to 100% of rated burden at power factor 0.8: class 0.2 allows ±0.2% and ±10 minutes, class 0.5 ±0.5% and ±20 minutes, and class 1 ±1% and ±40 minutes. It states that "in single pole insulated transformers, it is common practice to use a rated voltage factor of 1,9 x the rated voltage for a load duration of 8 hours", and 1.2 × for all other types. [1]

Its VES indoor single-pole VTs for 12, 17.5 and 24 kV take primary voltages from 10,000/√3 to 22,000/√3 V, give 100/√3 or 110/√3 V secondaries plus 100/3 or 110/3 V earthing windings, and are rated 20 VA in class 0.2, 50 VA in class 0.5 and 100 VA in class 1, with a 6 A thermal limit current also at 1.9 × Un for 8 hours. A VT can only be reconnected to another primary voltage by secondary tapping; the class stays the same but the rated burden falls by roughly the square of the voltage reduction. [1]

Editorial calculation: a 10,000/√3 V primary is about 5,774 V phase to earth, so 1.9 × for 8 hours is about 10,970 V — close to full line voltage, which a phase-to-earth VT can see during an earth fault on an ungrounded network. [1] [2] A 100/√3 V secondary is about 57.7 V and a 100/3 V earthing winding about 33.3 V; halving the primary voltage by tapping would cut a 50 VA rated burden to about 12.5 VA. [1]

Give the supplier the highest voltage for equipment and insulation level, rated primary and secondary voltages, single- or double-pole insulation, network grounding and rated voltage factor, the number of secondary windings with class and burden for each (metering and protection), any residual (open-delta) winding and its thermal output, frequency, indoor or outdoor installation and altitude, fuse or touch-proof design, partial discharge limits, mounting dimensions, and quantity.

02

Ground it correctly and stop ferroresonance

RITZ warns that a VT must never be short-circuited on its secondary side, that the grounded "N" end of a single-pole VT's primary must not be removed in service, and that in a V-connection of two double-pole VTs only one transformer's secondary may be grounded. Ferroresonance can occur with single-pole VTs on an ungrounded (insulated neutral) network after switching surges: the earth capacitance and VT inductance oscillate, the core saturates and overheats, and the resin body can flash over and burst. A residual winding in open delta with a damping resistor, reactor or both prevents this, provided the open delta is grounded only once. [1]

RITZ's damping-unit note explains that in three-phase ferroresonance the open-delta voltage is about 70 to 90% of phase-to-earth voltage at roughly half the system frequency, and that the damping load must still let the network run with an earth fault for 8 hours without exceeding the residual winding's thermal limit. For a winding rated 6 A, a damping resistor at 110 V must not be below 20 Ω, which limits its damping power; a reactor that saturates during ferroresonance drops to about 1 Ω and damps far harder but less so at small amplitudes, so RITZ combines both. [2]

RITZ's insulation table pairs 12 kV equipment with 28 kV power-frequency and 75 kV lightning-impulse withstand, 24 kV with 50 and 125 kV, and 36 kV with 70 and 170 kV, and gives the example that a 75 kV BIL at 2,500 m altitude must be raised to 90 kV. Partial discharge is tested above 3.6 kV rated voltage, at 1.2 × Um/√3 with a 20 pC limit for single-pole VTs and 1.2 × Um with 20 pC for double-pole VTs; indoor units are designed for −5 to +40 °C. [1]

03

Compare voltage transformer types and suppliers

Arteche lists medium-voltage VTs for primary and secondary air-insulated switchgear, generation and capacitor banks, including support types up to 72.5 kV, fused support types up to 36 kV, touch-proof types up to 52 kV, and units supplying auxiliary power up to 3 kVA. [3] At transmission level Trench lists eco-fluid inductive VTs up to 550 kV with hermetic bellows [4] and capacitive VTs up to 1,200 kV that also couple power-line carrier and damp ferroresonance. [5]

Suppliers linked to this page list cast-resin indoor and outdoor voltage transformers, low-voltage VTs, fused VTs and metering units; ask each for the class and burden per winding, rated voltage factor, insulation level and test reports, and ferroresonance damping options.

04

Trace the voltage transformer construction

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your transformer

A voltage transformer quotation needs values for your network. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the network, windings and grounding together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Network: [Um; insulation level; frequency; grounding]
Voltages: [primary; secondary; taps]
Windings: [class and burden each; residual winding]
Voltage factor: [1.2 continuous; 1.9 × 8 h]
Ferroresonance: [damping device]
Installation: [indoor/outdoor; altitude; fused or touch-proof]
Tests: [type and routine; partial discharge]

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