Article Content

IEC 61869-3 Certified 21kV Voltage Transformer UNKNOWN for Metering & Protection Applications

Introduction to the UNKNOWN Voltage Transformer

The UNKNOWN 21kV cast-resin voltage transformer (VT) is engineered for high-reliability metering and protective relaying in medium-voltage power systems operating at nominal system voltages of 21kV (IEC standard) or 20kV (domestic equivalent). This instrument transformer leverages advanced vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings, eliminating the need for flammable insulating fluids and enabling safe deployment in both indoor switchgear and outdoor substations.

Operating Principle of Cast-Resin Insulation

Cast-resin insulation in the UNKNOWN VT employs a two-component cycloaliphatic epoxy resin system cured under vacuum and pressure. This process fully saturates the interstices between the primary and secondary windings and the grain-oriented electrical steel (GOES) core, creating a monolithic, void-free dielectric structure. The absence of air pockets prevents partial discharge inception below 10 pC at 1.2 × Um/√3 (14.5 kV), as verified per IEC 61869-3. The resin’s high tracking resistance (>600 V per IEC 60112) ensures long-term surface integrity even under pollution conditions. Thermal conductivity of the cured resin (~0.8 W/m·K) facilitates efficient heat dissipation from copper losses, maintaining winding temperature rise within 60 K limits during continuous operation at rated burden.

Advantages Over Oil-Immersed Designs

Compared to traditional oil-filled VTs, the UNKNOWN cast-resin design offers significant operational and safety benefits. It is inherently fire-resistant (compliant with IEC 60695 glow-wire tests at 850°C), eliminating explosion hazards in confined spaces such as urban substations or industrial plants. Maintenance requirements are drastically reduced—no oil sampling, degassing, or leakage monitoring is needed. The solid insulation system exhibits superior mechanical robustness against seismic loads (tested to 0.5g horizontal acceleration per IEC 60068-2-57) and resists moisture ingress even at 95% relative humidity. Additionally, the compact footprint (typically 30–40% smaller than oil equivalents) simplifies integration into space-constrained switchgear bays.

Typical Application Overview

The UNKNOWN VT is deployed across utility distribution networks, industrial facilities, and renewable energy interconnection points where accurate voltage transformation is critical. Primary applications include revenue metering (accuracy class 0.2 or 0.5), overvoltage/undervoltage protection (class 3P or 6P), and synchro-check functions in generator synchronization schemes. Its dual secondary windings—one optimized for metering (e.g., 15 VA at 0.2 class), another for protection (e.g., 50 VA at 3P)—enable simultaneous compliance with metrological and reliability requirements without cross-interference.

Technical Specifications

The UNKNOWN 21kV cast-resin voltage transformer adheres strictly to IEC 61869-3 and GB/T 20840.3, ensuring interoperability across global power systems. Key parameters are defined for both international (21kV) and domestic (20kV) operating contexts.

Rated Electrical Parameters

Primary rated voltage: 21/√3 kV (phase-to-ground) for 21kV systems; alternatively rated at 20/√3 kV for 20kV domestic grids. Standard secondary voltage: 100/√3 V or 110/√3 V (configurable). Voltage ratio tolerance: ±0.1% at rated frequency (50 Hz or 60 Hz). Accuracy classes: 0.2, 0.5 for metering; 3P, 6P for protection. Rated outputs: 10–100 VA per secondary winding, with thermal stability up to 150% of rated burden for 8 hours. Insulation level: Lightning impulse withstand voltage (LIWV) = 125 kV peak; Power frequency withstand voltage (PFWD) = 50 kV rms for 1 minute. Partial discharge magnitude: ≤5 pC at 1.2 × Um/√3 during factory testing.

Environmental and Service Conditions

Designed for ambient temperatures from –25°C to +40°C (extended range –40°C to +55°C available). Relative humidity tolerance: up to 95% non-condensing. Maximum installation altitude: 1,000 m above sea level; derating factor of 1.1% per 100 m applies beyond this limit per IEC 60076-11. Pollution degree: III (creepage distance ≥25 mm/kV for outdoor use). Seismic rating: Zone 2 per IEEE 693 or 0.3g horizontal ground acceleration. The transformer is suitable for both indoor (IP2X enclosure) and outdoor (IP54 with optional rain shield) installations.

Construction and Materials

Magnetic core: CRGO (cold-rolled grain-oriented) silicon steel, M4 grade, laminated at 0.23 mm thickness to minimize hysteresis and eddy current losses (<0.8 W/kg at 1.5 T, 50 Hz). Windings: High-purity electrolytic tough pitch (ETP) copper, Class F insulation (155°C thermal rating). Encapsulation: Cycloaliphatic epoxy resin with silica filler (CTE ≈ 50 × 10⁻⁶/K), molded under 10 mbar vacuum and 8 bar pressure. Terminal bushings: Epoxy-silica composite with integrated corona shields. Mounting: Flanged base with M16 stainless steel bolts, compatible with standard IEC 61869 mounting dimensions.

Typical Applications

The UNKNOWN 21kV VT serves diverse roles in modern power infrastructure, leveraging its dual-winding architecture and high accuracy to meet stringent operational demands.

Substation Secondary Metering

In transmission and distribution substations, the UNKNOWN VT provides precise voltage signals to revenue-class kWh meters and power quality analyzers. With accuracy class 0.2 and phase error <10 minutes at 20–100% of rated voltage, it ensures billing compliance per IEC 62053-22. The low burden requirement (as low as 2.5 VA) minimizes loading effects on the secondary circuit, preserving measurement fidelity even with long cable runs. Integration with digital metering systems via IEC 61850-9-2 LE is facilitated by low harmonic distortion (<0.5% THD at rated load).

Industrial Power Distribution

Within heavy-industry facilities—such as steel mills, petrochemical plants, and data centers—the UNKNOWN VT supports motor protection relays, arc-flash detection systems, and automatic transfer switches. Its 3P-class protection winding delivers linear output up to 190% of rated voltage (40 kV phase-to-ground), enabling reliable detection of ferroresonance or single-phase grounding faults. The fire-safe cast-resin construction meets NFPA 70 (NEC Article 450) requirements for equipment in hazardous locations.

Renewable Energy Integration

For solar PV farms and wind turbine substations, the UNKNOWN VT enables grid-code-compliant voltage regulation and fault ride-through (FRT) functionality. During low-voltage events, its fast response time (<20 ms to 90% of final value) allows inverters to maintain reactive power support per ENTSO-E Grid Code. The VT’s immunity to DC offset (tested with 10% DC component superimposed on AC waveform) prevents saturation during asymmetrical faults common in inverter-based resources.

Rural and Suburban Distribution Networks

In remote or lightly loaded feeders, the UNKNOWN VT’s low no-load losses (<15 W) reduce standby energy consumption compared to oil-filled alternatives. Its robust pollution performance (withstands salt fog per IEC 60068-2-11) makes it ideal for coastal or agricultural regions. Dual secondaries allow one winding to serve local SCADA RTUs while the other feeds customer-side meters, optimizing capital expenditure.

Generator Synchronization Systems

During islanded operation or black-start sequences, the UNKNOWN VT supplies synchro-check relays with phase-angle and magnitude data accurate to ±0.2° and ±0.1%, respectively. This precision prevents out-of-phase closing that could damage prime movers. The VT’s low remanence (<5% of saturation flux density) ensures rapid recovery after fault clearance, critical for microgrid stability.

Compliance with International Standards

The UNKNOWN VT is certified to IEC 61869-3:2011 (Instrument transformers – Part 3: Additional requirements for inductive voltage transformers) and harmonized with China’s GB/T 20840.3-2013 standard.

IEC 61869-3 Certification Details

Certification encompasses type tests, routine tests, and special tests per Clause 7 of IEC 61869-3. Type tests include temperature rise (verified via resistance method, Δθ ≤ 60 K for windings), short-circuit withstand (withstanding 25 × rated current for 1 s without damage), and electromagnetic compatibility (EMC immunity to 10 V/m RF fields per IEC 61000-4-3). Routine tests performed on every unit: polarity check, turns ratio verification (±0.1% tolerance), insulation resistance (>10 GΩ at 2,500 V DC), and partial discharge measurement. The certification is issued by an ISO/IEC 17025-accredited laboratory, with test reports traceable to national metrology institutes.

Alignment with GB/T 20840.3

GB/T 20840.3 adopts IEC 61869-3 with minor national deviations. Key alignments include identical accuracy class definitions (0.2, 0.5, 3P, 6P), same insulation levels for 20kV systems (LIWV = 125 kV, PFWD = 50 kV), and equivalent thermal performance criteria. Differences are limited to labeling requirements (Chinese characters mandatory) and altitude correction factors (GB uses 1,000 m base vs. IEC’s 1,000 m). The UNKNOWN VT meets both standards simultaneously through dual-rating of primary voltage (20/21 kV) and bilingual nameplates.

Testing and Certification Requirements

Full compliance requires submission of type test reports covering all clauses of IEC 61869-3 Annex A. Critical tests include:
– **Accuracy verification**: Performed at 20%, 50%, 80%, 100%, and 120% of rated voltage with burdens from 25% to 100% of rated VA.
– **Capacitive divider effect test**: Ensures ratio error remains within limits when connected to capacitive burdens (e.g., CVT simulation).
– **Damp heat test**: 56 days at 40°C/95% RH followed by insulation resistance and PD checks.
Certification validity is maintained through annual surveillance audits and batch sampling per IEC 61869-1 Clause 8.

On-Site Testing Procedures

Post-installation verification ensures the UNKNOWN VT performs within specifications under actual site conditions.

Insulation Resistance Test

Measure insulation resistance between primary winding and ground, and between primary and secondary windings, using a 2,500 V DC megohmmeter. Acceptance criterion: >10 GΩ at 20°C. Correct for temperature using R₂₀ = Rₜ × 1.5^(t–20)/10. Low readings indicate moisture ingress or resin cracking—investigate immediately.

Turns Ratio Test

Apply 100–500 V AC to the primary and measure secondary voltage with a calibrated voltmeter (accuracy class 0.1). Calculate ratio error: [(Vₚ/N) – Vₛ]/Vₛ × 100%, where N = nominal ratio. Tolerance: ±0.1% for metering windings, ±3% for protection windings. Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for automated results.

Polarity Test

Verify reducing polarity per IEC 61869-1 Figure 3. Connect a 6–12 V battery across primary terminals (H1+, H2–). Momentarily close the circuit and observe secondary voltage polarity with a DC millivoltmeter. A positive kick at X1 confirms correct polarity. Incorrect polarity causes 180° phase shift, leading to relay misoperation.

Power Frequency Withstand Voltage Test

Apply 50 kV rms (for 21kV class) at 50 Hz between primary and grounded parts for 1 minute. Ramp up at 2 kV/s. No flashover or disruptive discharge permitted. For in-service units, perform at 80% of factory test voltage (40 kV) per IEC 60270. Use a calibrated HV test set with overcurrent trip (≤100 mA).

Open-Circuit Characteristic Test

With secondary open, apply 10–120% of rated primary voltage and record excitation current. Plot V vs. I curve—knee point should exceed 150% of rated voltage. Excessive magnetizing current (>5% of rated primary current at 100% V) indicates core degradation or shorted turns.

Preventive Maintenance Guide

Proactive maintenance extends service life and prevents unexpected failures in the UNKNOWN VT.

Periodic Inspection Protocol

Conduct visual inspections quarterly: check for surface cracks, tracking marks, or discoloration on the resin housing. Verify terminal tightness (torque: 25 N·m for M12 bolts). Annually, clean external surfaces with isopropyl alcohol to remove conductive dust. Measure insulation resistance and compare to baseline—decline >20% warrants further investigation. In coastal areas, inspect for salt deposits monthly during monsoon seasons.

Maintenance Intervals and Fault Diagnosis

Follow this schedule:
– **Annual**: Insulation resistance, visual inspection, terminal torque check.
– **5-year**: Full ratio and polarity test, partial discharge scan (using HFCT sensors), thermal imaging under load.
– **10-year**: Dielectric frequency response (DFR) analysis to detect moisture or aging.

Common faults and remedies:
– **High ratio error**: Likely shorted turns—replace unit.
– **Low insulation resistance**: Dry out with hot air blower (max 70°C for 24 h); if unresolved, replace.
– **Excessive noise**: Loose core laminations—requires factory repair.

Conclusion

The UNKNOWN 21kV cast-resin voltage transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-3 compliance with robust engineering for demanding metering and protection applications. Its VPI epoxy resin insulation eliminates fire risks and maintenance overhead associated with oil-filled designs, while the GOES silicon steel core ensures exceptional accuracy and thermal stability. With dual secondary windings rated for both 0.2-class metering and 3P-class protection, it satisfies concurrent regulatory and operational requirements across utility, industrial, and renewable sectors. Rigorous factory testing—including partial discharge levels below 5 pC and lightning impulse withstand up to 125 kV—guarantees reliability under transient overvoltages. Adherence to both IEC 61869-3 and GB/T 20840.3 facilitates global deployment, and the transformer’s design supports a service life of 25–30 years when maintained per the recommended schedule. For engineers specifying instrumentation in 20/21kV networks, the UNKNOWN VT delivers precision, safety, and longevity without compromise.