Documentation, Nameplate, and Site Readiness Review

Before any electrical test is performed, the commissioning team should close out all document and site-condition gaps. This step prevents mismatched specifications from reaching the energization stage.

Procurement and factory test document check

Collect and verify the factory test reports, routine and—where contracted—type/special test certificates, general arrangement drawings, foundation and anchoring drawings, terminal boxes/wiring diagrams, OLTC control drawings, cooling control diagrams, SCADA point list, material certificates, and operation/maintenance manuals. For Xinda/Xinda Transformer export packages, the factory scope can cover 10 kV, 35 kV, 110 kV and 220 kV oil-immersed transformers, 10 kV SCB/SGB dry-type units, amorphous oil/dry units, special transformers such as rectifier/furnace/mining units, plus switchgear and prefabricated substations; the accepted document set should match the ordered scope rather than a generic transformer model .

Compare the nameplate with the purchase order for rated power, HV/LV voltages, frequency, vector group, tapping range and step number, impedance guarantee, cooling class, insulation levels, and applicable standards such as IEC 60076 or the contract’s IEEE/GB/local-grid equivalent. For 110 kV projects, Xinda’s public qualification references include State Grid prequalification for 110 kV transformers up to 63 MVA and 35 kV transformers up to 31,500 kVA/20,000 kVA depending on the prequalified subgroup; these credentials support tender compliance review but do not replace project-specific factory and site acceptance reports .

Site civil, grounding, and safety readiness

Confirm foundation leveling, load capacity, oil containment or dry-type ventilation as applicable, cable trench/sealing, fire protection, lighting, and safe access clearance. Verify station earth grid continuity, transformer tank earthing, neutral earthing arrangement, and CT/relay earthing according to the protection single-line diagram. Complete LOTO procedure, permit-to-work, hazard briefing, calibrated instrument certificates, and emergency response setup before applying any test voltage.

Visual and Mechanical Inspection Before Electrical Testing

Electrical diagnostics should follow a clean visual and mechanical pass. Transport damage not visible in paperwork can invalidate later test interpretation.

Tank, bushings, and sealing inspection for oil-immersed units

For oil-immersed transformers—including 10 kV S11-M/S13-M.RL, 35 kV S11/SZ11, 110 kV and 220 kV power units—inspect tank surface, radiator/fin joints, conservator, breather silica gel condition, oil level vs ambient temperature, bushing porcelain/polymer surfaces, flange gaskets, valve positions, pressure/vacuum gauges, and Buchholz/pressure-relief accessories where fitted. Check for oil traces at welds, gaskets, bushing bases, and tap-changer compartment. For sealed or nitrogen/dry-air transported units, verify transport pressure records, gas replacement procedure, and manufacturer-approved atmosphere checks before opening any cover .

Dry-type, amorphous, and indoor-unit inspection

For 10 kV SC(B)10 cast-resin or SG(B)10 non-encapsulated dry-type transformers, amorphous dry-type SCBH units, and indoor packages, inspect resin/insulation surfaces for cracks, moisture stains, dust, and mechanical impact. Verify enclosure IP rating, ventilation path, fan/filter status for forced-cooled units, terminal box sealing, core grounding point, and temperature-sensor routing. Dry-type commissioning follows IEC 60076-11 for dry-type construction in addition to IEC 60076-1 general requirements; cast-resin and VPI/non-encapsulated units may have different moisture, cleaning, and withstand limitations, so the manufacturer’s installation manual governs site retests .

Tap-changer, cooling, and accessory mechanical check

For off-circuit tap changers, confirm mechanical position vs nameplate required service tap. For on-load tap changers—relevant to 35 kV SZ11 and many 110/220 kV regulating units—verify tap-position indicator, motor drive, limit switches, manual/auto selector, counter, control voltage, transition resistor/reactor compartment condition, and oil compartment separation if applicable . For forced-oil/forced-air units, run fans/pumps manually, check phase rotation, thermal switch setpoints, and starter interlocks; for conservator units, confirm oil-level indicators and alarm contacts.

Core Electrical Acceptance Tests for All Transformer Types

Field acceptance usually repeats or verifies factory routine tests after transport and installation. The exact test list depends on voltage class, fluid type, OLTC presence, and contract standard.

Insulation resistance and polarization index

Measure IR between each winding and earth and between windings using a megger selected for the insulation system voltage. Record temperature, humidity, applied DC voltage, 1-minute value, and—where moisture/dryness assessment is required—10-minute value to calculate PI. IR/PI limits should be taken from the project standard, manufacturer’s acceptance criterion, and comparable factory baseline; do not energize further dielectric tests if values indicate moisture, contamination, or uncontrolled site atmosphere. For site work on Xinda 110/220 kV units, post-handling IR, PI, and further high-voltage tests should be scoped with the manufacturer’s technical representative because impulse, induced-voltage, and partial-discharge expectations differ by voltage level and transport condition .

Winding DC resistance at all relevant taps

Inject DC current and measure each phase/terminal pair for HV and LV windings, at every OLTC position for regulating transformers and at all off-circuit taps where applicable. Correct readings to a common temperature using copper/aluminum coefficients and compare with factory records. This test verifies winding continuity, busbar/terminal joints, brazed/bolted connections, and OLTC contact condition. For SZ11 on-load units, perform tap-by-tap resistance trends to detect selector/diverter contact deviation; for 110/220 kV multi-winding units, follow the manufacturer’s terminal measurement matrix rather than a simplified three-phase average only .

Turns ratio, polarity, and vector-group verification

Use a TTR/injection set to measure ratio at all tap positions and compare with nameplate and factory report. Verify polarity for two-winding checks and full vector group for three-phase units—for example Dyn11, Yyn0, YNd11, or project-specified connection—through phase-angle/displacement testing. Ratio error, wrong vector group, or reversed phase termination can cause differential-protection maloperation at first energization. Contract acceptance tolerance should follow IEC 60076-1 or IEEE C57.12.90 as specified; export projects commonly record all-tap ratio deviation, principal-tap deviation, and polarity confirmation separately .

No-load loss, excitation current, load loss, and impedance

Factory routine tests normally cover no-load loss/current at rated frequency and voltage and load loss/impedance at agreed tap. Site acceptance may repeat no-load excitation where transport/core suspicion exists, or verify load-loss/impedance through contracted SAT rather than full-load trial. For energy-efficient tenders—Xinda S11/S13/S18/S20/S22 oil series or SCB10–SCB18/SGB dry series—request guaranteed loss values, efficiency class, and test tolerance in the purchase specification; site results should be judged against those contracted guarantees and corrected test conditions, not generic catalogue numbers .

Dielectric withstand, induced voltage, impulse, and partial discharge

AC applied-voltage withstand, induced overvoltage, lightning impulse, and PD tests are scheduled by voltage class and contract. IEC 60076-3 covers dielectric test procedures; PD is commonly specified for MV/HV factory acceptance and may be repeated on site for 110/220 kV units under controlled conditions. Site AC withstand should not be executed if IR, ratio, resistance, oil condition, or moisture status are unsatisfactory. For Xinda 110/220 kV units, the factory test infrastructure includes comprehensive test benches, impulse generators, regulators, intermediate transformers, frequency-doubling sets, and HV withstand systems; site SAT should be limited to confirming transport/installation impacts and agreed witness points, not duplicating full type testing unless the contract requires it .

Tan delta, SFRA, and condition-based special tests

Power-factor/tan-delta identifies moisture or insulation degradation in windings and bushings; SFRA provides a frequency-response fingerprint to detect winding deformation, bracing looseness, or transit mechanical shift. SFRA baseline is valuable before unloading, after major handling, and after any impact/shock record. Additional special tests—core-grounding check, zero-sequence impedance for multi-winding or grounding studies, CT/VT ratio and polarity, and dissolved-gas screening—are included by project risk, transformer type, and utility acceptance rules .

Oil-Immersed Transformer Acceptance: Fluid Quality and Auxiliary Function Tests

Oil testing is a separate work stream from winding electricals because fluid condition changes with storage, vacuum filling, and site handling.

BDV, moisture, acidity, resistivity, and dissolved gas

For new mineral oil, verify acceptance against IEC 60296 and in-service judgment against IEC 60422; for ester or alternative fluids, use the specified fluid standard and supplier limits. Perform dielectric breakdown per IEC 60156, moisture by Karl Fischer per applicable IEC/ASTM method, acidity, resistivity, interfacial tension, and DGA where unit size or fault suspicion justifies it. Sample from approved valves after settling/filtration, label by tap-compartment/tank/main-tank separation, and compare with factory oil certificate. Xinda site procedures require filtered, tested oil of the specified type, controlled vacuum filling, recorded oil temperature/vacuum/volume, and post-fill electrical tests scoped by project standard .

OLTC functional, Buchholz, thermals, and cooling interlocks

For OLTC, verify auto/manual control, tap travel time, position feedback, alarm/fault contacts, motor-protection, and—where relevant—vacuum/resistor transition timing per the tap-changer manual. For oil units with Buchholz, test float/alarm, trip, sudden-pressure element, and gas-collection function without introducing unsafe gas simulation beyond approved procedure. Calibrate/verify OTI/WTI or RTD/PT100 sensors, cooling-group staging, high-temperature alarm, and SCADA mapping. For 35 kV SZ11, separate-tap-compartment checks and drive-function tests should be documented; for 110/220 kV regulating transformers, include manufacturer witness points in the SAT plan .

Dry-Type and Special-Application Commissioning Differences

Dry-type units eliminate oil BDV/DGA but add solid-insulation and enclosure-environment controls.

Dry-type routine site verification

For SC(B)10/SCB11–SCB18 and SG(B)10 non-encapsulated units, verify IR, winding resistance, ratio/vector group, no-load loss/current where contract requires, AC separate-source withstand per IEC 60076-3, induced voltage where applicable, temperature-sensor continuity, fan/control function, and enclosure cleanliness. Cast-resin and VPI units have different humidity/partial-discharge sensitivity; follow IEC 60076-11 and the OEM drying/cleaning instructions before applying high voltage . Do not use oil-transformer moisture limits for dry-type commissioning.

Special transformers: rectifier, furnace, mining, and renewable packages

Rectifier, electric-furnace, mining-general, high-overload, and renewable interface transformers may require phase-number-specific ratio checks, harmonic/loading loss verification, special CTs, forced cooling validation, and protection-parameter matching. Xinda’s special-transformer scope includes rectifier and furnace units; site acceptance must use the individual factory test protocol, not a standard distribution checklist, because lead arrangement, regulating windings, and cooling interfaces vary by application .

Integrated Substation Acceptance: Switchgear, Protection, and SCADA

A transformer handover is incomplete if breakers, relays, metering, and remote signals are not validated together.

MV/LV switchgear and prefabricated substation checks

Where the package includes KYN61-40.5/KYN28A-12/HXGN-12/XGN2-12 MV cabinets or GCK/GCS/MNS/GGD LV switchgear, verify breaker/contactors, interlocking, earthing switch, protection CT/VT wiring, cable termination torque, insulation resistance of secondary circuits, and functional operation with the transformer de-energized. For YB-12 or metal-clad prefabricated substations, check civil-electric interface, ventilation/heating, lighting, door interlocks, fire/system sensing, and manufacturer-approved cable entry sealing .

Protection relay, differential, and first-energization sequence

Perform CT ratio/polarity/knee-point as required, VT ratio/polarity/burden, differential 87T balance injection, overcurrent/earth-fault setpoint verification, Buchholz/pressure/thermal trip wiring, and SCADA alarm/event mapping using Modbus, IEC 60870, or IEC 61850 as contracted. First energization should follow a staged plan: isolate test equipment, remove temporary grounds, set OLTC to agreed initial tap, apply HV source gradually, record inrush, no-load voltage at all secondary windings, neutral/grounding status, and monitor temperature, noise, and protection alarms for the agreed observation period. Do not load the transformer until all electrical, oil/thermal, protection, and documentation exceptions are closed.

Acceptance Report Format and Supplier Handover Record

The final SAT dossier should be audit-ready and usable for warranty, O&M, and future retrofit.

Minimum record package for exporter buyers

Include site environmental log, impact/transport records, visual exception list, nameplate-vs-order reconciliation, IR/PI values with temperature correction, DC resistance tables per tap, TTR/vector report, no-load/load-loss and impedance certificates, dielectric/impulse/PD records as contracted, oil test certificate pack, OLTC operation log, Buchholz/thermal/cooling functional results, CT/VT/relay secondary-injection report, SCADA point acceptance, outstanding-items register, and signed SAT protocol. For Xinda/Xinda shipments, request factory routine/test certificates aligned with the ordered 10/35/110/220 kV platform, dry-type class, or special-transformer scope, and confirm that site tests reference the same serial number, drawing revision, and standards edition used at factory acceptance.