# Voltage transformer set with broken-delta residual winding

Canonical URL: https://eloragrid.ai/symbols/voltage-transformer-with-residual-winding
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: VT
Drawing use: SLD, Schematic
Reference families: IEC, Manufacturer

Three single-phase VTs whose residual windings are connected in broken delta to produce the zero-sequence voltage.

## In plain English

### What does Voltage transformer set with broken-delta residual winding mean?

In this arrangement three single-phase earthed VTs each carry a residual winding, and the three residual windings are connected in series as a broken delta. In a healthy balanced system the voltage across the open corner is near zero, and during an earth fault it rises towards three times the zero-sequence voltage, which feeds residual voltage and directional earth-fault elements.

### A simple example

A damping resistor is often connected across the broken delta to suppress ferroresonance in isolated or resonant-earthed networks. The symbol does not show that resistor or the earthing of the VT primaries, both of which need explicit detail on the schematic.

### What the drawings help you do

Check the primaries are earthed star so zero sequence can be measured. Look for a damping resistor across the open corner. A broken-delta winding measures residual voltage; an open-delta (V-V) connection measures line voltages with two VTs.

### This particular reference

Three single-phase VTs whose residual windings are connected in broken delta to produce the zero-sequence voltage. A broken-delta winding measures residual voltage; an open-delta (V-V) connection measures line voltages with two VTs.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- Phase: One of the related AC waveforms in a power system.
- Rating: A value stating the conditions for which equipment is specified.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers)
- [Residual current and neutral-earthing protection context](https://new.abb.com/medium-voltage/digital-substations/campaigns/earth-fault-protection-portfolio/earth-fault-offering)


## What it represents

In this arrangement three single-phase earthed VTs each carry a residual winding, and the three residual windings are connected in series as a broken delta. In a healthy balanced system the voltage across the open corner is near zero, and during an earth fault it rises towards three times the zero-sequence voltage, which feeds residual voltage and directional earth-fault elements.
A damping resistor is often connected across the broken delta to suppress ferroresonance in isolated or resonant-earthed networks. The symbol does not show that resistor or the earthing of the VT primaries, both of which need explicit detail on the schematic.

## How to read it

- Check the primaries are earthed star so zero sequence can be measured.
- Look for a damping resistor across the open corner.

## Information to show or reference

- Show broken-delta connection
- Show damping resistor if fitted
- Reference residual voltage relay input

## Important distinction

A broken-delta winding measures residual voltage; an open-delta (V-V) connection measures line voltages with two VTs.

## Facts

- Output: Residual (zero-sequence) voltage
- Primaries: Earthed star
- Preview: Independent educational concept; consult the applicable standard and project legend for the exact graphic

## Public references

- [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers) — ABB. Public manufacturer overview of indoor current and voltage instrument transformers.
- [Residual current and neutral-earthing protection context](https://new.abb.com/medium-voltage/digital-substations/campaigns/earth-fault-protection-portfolio/earth-fault-offering) — ABB. Public context for residual quantities and neutral-earthing protection.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.