VT FAT must prove the complete voltage chain: primary phase and polarity, transformer ratio, secondary winding/fuse, single earthing point, selector/test link, relay/meter scaling and residual-voltage circuit. A correct transformer ratio does not prevent a swapped phase, open fuse or miswired neutral from defeating protection.
This guide covers inductive voltage-transformer ratio, polarity and secondary-circuit checks for MV switchgear under IEC 61869-1/3. It explains primary versus secondary injection, multi-winding/open-delta circuits, voltage/frequency limits, burden, fuses/MCBs, withdrawable VTs, ferroresonance damping, earthing, LPIT interfaces, safety, records and restoration.
Executive conclusions
- Unlike a CT, a VT secondary must not be short-circuited; it is a voltage source and a short can cause destructive current.
- Confirm the exact VT type, primary connection, ratio, winding, class/burden, voltage factor, polarity and phase before testing.
- Primary-side ratio testing proves the transformer and installed primary path; secondary injection proves downstream wiring/relay/meters but not the VT ratio.
- Maintain safe voltage-to-frequency conditions. Excessive V/f can overexcite an inductive VT even when applied voltage seems modest.
- Test every used secondary winding and tap independently, including residual/open-delta winding.
- Verify phase sequence and polarity end to end with signed phasors; magnitude alone cannot detect every swap/reversal.
- Locate the single intentional secondary neutral/phase earth point and search for unintended duplicate earths.
- Verify fuse/MCB poles, ratings, discrimination, blown-fuse alarm and safe isolation/test facilities.
- Ferroresonance damping components and winding connections are functional scheme elements, not optional accessories.
- Record all temporary disconnects and independently restore VTs, fuses, earths, selectors and relay scaling before energisation.
1. VT data and connection baseline
- manufacturer/type/serial and IEC 61869 declarations;
- highest/rated primary voltage and rated secondary voltage(s);
- primary connection: phase-earth, phase-phase, star or other design;
- secondary winding/tap designations and intended function;
- accuracy class, rated burden and voltage factor/time;
- rated frequency and insulation level;
- primary/secondary terminal markings and polarity;
- residual-voltage/open-delta winding data;
- fuse/disconnector/withdrawable mechanism and interlocks;
- damping resistor/ferroresonance circuit where provided;
- secondary neutral/phase earth point and external destinations.
Reconcile nameplate, VT schedule, single-line, secondary schematic, relay settings and meter scaling. A 100/√3 V winding and a 100/3 V residual winding are not interchangeable.
2. Safety differences from CT testing
| CT | VT |
|---|---|
| Secondary must not be opened under primary current | Secondary must not be shorted under primary voltage |
| Shorting test block is critical | Fused/isolating test block is critical |
| Primary source approximates current source | Secondary behaves as voltage source |
| Burden increase can drive saturation/error | Excess burden causes voltage drop/error/heating |
- De-energise/isolate/earth the MV primary and prove dead.
- Prevent backfeed from secondary injection into the primary by approved isolation.
- Withdraw/open primary fuses/disconnectors and verify position as required.
- Isolate relay/meters/SPDs that cannot accept the test signal.
- Use voltage-rated leads, barriers and safe discharge/earthing.
- Do not bypass VT truck/door interlocks casually.
- Restore fuses, links, neutral earth and secondary protection with independent sign-off.
3. Test-method comparison
| Method | What it proves | Limitation |
|---|---|---|
| Primary AC injection | VT ratio/polarity plus installed primary/secondary path | Source voltage/power and HV safety |
| Secondary AC injection | Fuses/selector/wiring/relay/meter scaling and phase mapping | Does not test transformer ratio/primary connection |
| Model-based low-signal analyser | Equivalent parameters, ratio/phase/class under validated method | Separate end-to-end wiring test still needed |
| Continuity/resistance | Open winding/fuse/path and comparative resistance | Does not establish ratio/class |
| Phasor/phase-sequence test | Phase identity, polarity and rotation | Requires known reference and channel calibration |
OMICRON’s official VOTANO 100 description states that it uses low secondary-side signals and an equivalent-circuit method for inductive VTs/CVTs, with ratio, phase, polarity and class assessment. Such analysis complements installed primary/secondary circuit verification.
4. Ratio equations
Nominal transformation ratio:
Kn = Upn / Usn
Measured ratio:
Kmeas = Up / Us
- Use RMS values and stable frequency/waveform.
- Account for phase-earth versus phase-phase definitions and √3 factors.
- Use IEC 61869 method for formal ratio/phase-displacement accuracy.
- Test all phases/windings/taps under comparable burden.
- Confirm relay/meter primary/secondary scaling independently.
- Do not claim class accuracy from a low-voltage ratio check alone.
5. Voltage-to-frequency control
Magnetic flux is approximately related to voltage/frequency:
Φmax ∝ V / (fN)
- Do not apply rated 50/60 Hz voltage at a much lower frequency.
- Respect manufacturer limits for primary/secondary injection and winding under test.
- Use a source with adequate VA, low distortion and current protection.
- Monitor magnetising current; stop on abnormal rise/noise/heating.
- Account for connected burden/damping circuits.
- Record actual voltage and frequency at the winding terminals.
6. Primary ratio and polarity test
- Confirm primary isolated/earthed and test boundaries approved.
- Set VT/disconnector/truck/fuse state to include the intended primary path.
- Connect known AC voltage/reference to the designated primary terminals.
- Connect simultaneous secondary/reference channels to every winding tested.
- Apply voltage gradually at rated/approved frequency.
- Record primary and secondary RMS magnitude and phase angle.
- Verify ratio, polarity, phase identity and winding/tap destination.
- Reduce to zero, de-energise/discharge/earth before rewiring.
- Repeat every phase and used winding.
OMICRON CPC 100 official data identifies VT testing and voltage output applications; boosters may extend capability. Equipment availability does not determine the test voltage—use the approved VT/test-set procedure.
7. Polarity and phase sequence
- Use simultaneous phasor measurement against a known primary/reference phase.
- Verify terminal marking direction through fuse/MCB/test link to relay input.
- Confirm L1/L2/L3 secondary phase rotation.
- Check phase-earth and phase-phase relay channels.
- Verify residual/open-delta polarity mathematically and by injection.
- Check synch-check/under-overvoltage/directional functions see correct phase.
- Record signed phasors; magnitude-only screenshots can hide reversal.
8. Secondary continuity and protection
- Trace each winding terminal through primary/secondary fuses, MCBs, selector/test blocks and terminals.
- Verify both poles where required and neutral treatment.
- Confirm fuse/MCB rating, breaking capacity, characteristic and discrimination.
- Test blown-fuse/VT MCB trip alarm and loss-of-potential logic.
- Check VT circuit cannot be paralleled unintentionally through selector switches.
- Verify isolation creates no unsafe floating/hidden backfeed.
- Measure loop continuity/resistance only with safe source and isolated electronics.
- Restore covers/seals and normal switch positions.
9. Secondary injection end to end
- Isolate VT winding/primary path so test voltage cannot backfeed.
- Connect a calibrated three-phase voltage source at the defined test block.
- Inject balanced nominal secondary phasors.
- Verify relay/meter phase magnitudes, angles, frequency and phase sequence.
- Vary one phase to prove channel mapping and VTS/fuse-failure logic.
- Test selectors, bus/line VT changeover and synch-check inputs.
- Verify SCADA scaling, units, deadbands, alarms and SOE.
- Remove source and restore VT circuit before clearing test mode.
10. Secondary earthing
- Identify the intentional star-point/phase earth from the approved design.
- Verify terminal and conductor identification/section.
- Temporarily lift under de-energised conditions and test for hidden duplicate earths.
- Check relay internal earth, test equipment, screens, marshalling and external circuits.
- Restore before energising/injection.
- Keep functional screen earth distinct from the winding circuit earth.
- Check each galvanically separate secondary winding individually.
11. Burden and voltage drop
For a secondary load:
Sburden = Us × Is and ΔU = Is × Zloop
- Include relay/meter inputs, leads, fuses/terminals and damping load.
- Compare operating burden/power factor with VT rated class burden.
- Long small-section secondary cables increase voltage drop/ratio error.
- Do not leave temporary test meters as permanent burden.
- Assess multiple parallel devices and spare cores/terminals.
- Formal class verification requires IEC method, not a DC resistance estimate.
12. Residual/open-delta winding
- Verify winding voltage designation and series/open-delta polarity.
- Confirm damping resistor value, duty, connection and thermal mounting.
- Inject balanced phase voltages and check near-zero expected residual within method limits.
- Apply a controlled zero-sequence/unbalanced pattern and verify 3U0 magnitude/polarity.
- Test earth-fault/59N function, alarm and trip path.
- Check one open fuse/phase produces the expected VTS versus earth-fault discrimination.
- Record relay residual phasor and physical winding connections.
12A. Ferroresonance and damping verification
Inductive VTs in isolated or resonant-earthed networks can participate in ferroresonant oscillation under certain switching/unbalance/capacitance conditions. FAT cannot reproduce every network case, but it can verify the engineered mitigation:
- correct VT connection and voltage factor for the earthing system;
- open-delta/tertiary damping resistor type, resistance, power/thermal rating and ventilation;
- contact/relay used to switch damping load, including fail/alarm state;
- no fuse, test switch or terminal arrangement leaves the damping circuit unintentionally open;
- relay logic distinguishes sustained residual voltage, VT fuse failure and transient conditions;
- warning/maintenance instructions and replacement part are documented.
Do not add or resize a resistor during FAT from trial-and-error. Ferroresonance mitigation requires a system/VT manufacturer study because excessive loading can overheat the winding/resistor and degrade measurement accuracy.
12B. VT fuse-failure and loss-of-potential scenarios
- Open each primary/secondary fuse or MCB pole using the approved simulator/test block.
- Verify affected phase voltage, negative/zero-sequence quantities and relay VTS pickup/delay.
- Confirm voltage-dependent/directional/distance functions block or restrain exactly as designed while current-based backup remains available.
- Check alarm text identifies the correct VT set/phase and SCADA/SOE timestamp.
- Restore voltage and verify hysteresis/reset without a spurious trip.
- Test bus/line VT selector failure and two valid sources accidentally paralleled/selected where the design prevents it.
13. Withdrawable VT truck and interlocks
- Verify VT type/rating and primary fuse orientation.
- Test service/test/disconnected positions and earth contact sequence.
- Check shutters, doors, racking force and mechanical coding.
- Verify secondary plug/position switches and fuse alarm in every relevant state.
- Prove VT cannot be racked/withdrawn under prohibited energised conditions.
- Check primary fuse striker/indicator and spare fuses/tools.
- Repeat ratio/continuity if primary/secondary contacts are adjusted.
14. Low-power voltage sensors
LPVT/voltage sensors may provide low-level analogue or digital output rather than a conventional 100/110 V secondary. Use the applicable IEC 61869 part and manufacturer interface:
- rated transformation ratio and phase error;
- connector/channel identity and polarity;
- screen/functional earth and cable length/type;
- IED/merging input compatibility and scaling;
- no application of conventional VT burden/hipot test to electronics;
- end-to-end phase/GOOSE/SV/SCADA verification as applicable.
15. Test record and restoration
- VT type/serial/phase/winding/tap/nameplate;
- primary/secondary connection diagrams and device states;
- test method, source/reference, calibration and software;
- applied voltage/frequency, measured ratio and phase/polarity;
- secondary continuity, fuses/MCBs, alarm and earth point;
- burden/voltage-drop calculation or measurement;
- relay/meter/SCADA scaling and residual circuit results;
- interlock/truck results;
- NCR/retest and before/after values;
- fuses, earths, selectors, plugs and protection restored with sign-off.
Common mistakes
- Shorting a VT secondary as if it were a CT.
- Using secondary injection as proof of VT ratio.
- Ignoring V/f and overexciting the VT.
- Checking magnitude but not polarity/phase sequence.
- Missing √3 or residual-winding scaling.
- Leaving duplicate/no secondary earth.
- Skipping blown-fuse/VTS logic.
- Omitting the damping resistor/open-delta test.
- Applying conventional VT tests to an LPVT interface.
- Forgetting a primary fuse, selector or earth restoration.
Official standards and primary references
- IEC 61869-1:2023 — current general instrument-transformer requirements.
- IEC 61869-3:2011 — additional requirements for inductive voltage transformers.
- IEC 62271-200:2021+AMD1:2024 — current MV switchgear integration and routine requirements.
- OMICRON VOTANO 100 official information — model-based VT/CVT ratio, phase, polarity and class assessment example.
- OMICRON CPC 100 official information — primary-injection/VT testing source example.
- IEC 60255-1:2022 — protection-equipment requirements relevant to connected voltage inputs.
Engineering note: Formal VT accuracy/class verification follows IEC 61869. FAT must additionally prove the actual winding, fuse, earth, phase, scaling and residual circuit delivered to protection and metering. Repeat disturbed primary contacts, fuses, secondary links, external cables and SCADA/relay phasors at SAT. Site comparison should retain the factory voltage, frequency, burden and connection diagram; otherwise a different result cannot be assigned confidently to transport, installation or test method. Preserve signed phasor screenshots together with numeric exports, since pictures without channel names and reference direction are weak evidence.