{"schemaVersion":1,"slug":"voltage-transformer-with-residual-winding","name":"Voltage transformer set with broken-delta residual winding","kind":"symbol","abbreviation":"VT","aliases":["broken-delta VT","open-delta residual winding","residual voltage winding"],"category":"measurement-protection","drawingTypes":["SLD","Schematic"],"referenceFamilies":["IEC","Manufacturer"],"summary":"Three single-phase VTs whose residual windings are connected in broken delta to produce the zero-sequence voltage.","description":["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."],"readingNotes":["Check the primaries are earthed star so zero sequence can be measured.","Look for a damping resistor across the open corner."],"drawingChecklist":["Show broken-delta connection","Show damping resistor if fitted","Reference residual voltage relay input"],"distinction":"A broken-delta winding measures residual voltage; an open-delta (V-V) connection measures line voltages with two VTs.","facts":[{"label":"Output","value":"Residual (zero-sequence) voltage"},{"label":"Primaries","value":"Earthed star"},{"label":"Preview","value":"Independent educational concept; consult the applicable standard and project legend for the exact graphic"}],"references":[{"sourceId":"instrument-transformers","relevance":"Public manufacturer overview of indoor current and voltage instrument transformers."},{"sourceId":"pub-symbol-earth-fault","relevance":"Public context for residual quantities and neutral-earthing protection."}],"related":["residual-voltage-monitoring-relay","open-delta-voltage-transformer-connection","voltage-transformer"],"artwork":"s28-voltage-transformer-with-residual-winding","publishedOn":"2026-10-28","updatedOn":"2026-10-28","provenance":"independent-public-research","plainEnglish":{"title":"What does Voltage transformer set with broken-delta residual winding mean?","introduction":"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.","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.","drawings":"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.","terms":[{"name":"Winding","meaning":"A coil of conductor inside an electrical machine or transformer."},{"name":"Phase","meaning":"One of the related AC waveforms in a power system."},{"name":"Rating","meaning":"A value stating the conditions for which equipment is specified."}],"diagram":"flow","steps":["Identify the device and form","Read the connections and state","Check the equipment reference"],"caption":"A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.","sources":[{"label":"Indoor current and voltage transformers","url":"https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers"},{"label":"Residual current and neutral-earthing protection context","url":"https://new.abb.com/medium-voltage/digital-substations/campaigns/earth-fault-protection-portfolio/earth-fault-offering"}],"detail":"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.","profileId":"reviewed-symbol-20261028"},"url":"https://eloragrid.ai/symbols/voltage-transformer-with-residual-winding","illustration":{"url":"https://eloragrid.ai/tools/electrical-library/illustrations/voltage-transformer-with-residual-winding.svg","scope":"Original educational concept; not an official standard graphic or product footprint","license":"https://creativecommons.org/licenses/by/4.0/"},"sources":[{"id":"instrument-transformers","title":"Indoor current and voltage transformers","publisher":"ABB","url":"https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers","type":"manufacturer","scope":"Public current- and voltage-transformer product families. Ratings, terminals and dimensions depend on the model.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public manufacturer overview of indoor current and voltage instrument transformers."},{"id":"pub-symbol-earth-fault","title":"Residual current and neutral-earthing protection context","publisher":"ABB","url":"https://new.abb.com/medium-voltage/digital-substations/campaigns/earth-fault-protection-portfolio/earth-fault-offering","type":"manufacturer","scope":"Public manufacturer explanation of the equipment function and selection context.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public context for residual quantities and neutral-earthing protection."}]}