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IEC 61869-3 Certified 11kV Voltage Transformer SZW-10K for Metering & Protection Applications
Introduction to the SZW-10K Voltage Transformer
The SZW-10K is a single-phase, dry-type, cast-resin insulated voltage transformer (VT) engineered for high-reliability operation in both metering and protective relaying circuits within medium-voltage distribution systems. Designed for nominal system voltages of 11 kV (IEC standard) or 10 kV (domestic Chinese grid equivalent), this unit leverages advanced vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings, ensuring superior dielectric strength, mechanical robustness, and environmental resilience.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the SZW-10K employs a two-component cycloaliphatic epoxy resin system cured under vacuum and pressure. This process eliminates air voids and moisture ingress pathways, resulting in a homogeneous, non-hygroscopic solid insulation matrix. The primary and secondary windings—wound on grain-oriented electrical steel (GOES)—are fully embedded within this resin, which provides a dielectric withstand capability exceeding 75 kV (1-minute power frequency test) between windings and ground. Unlike oil-filled units, the absence of liquid dielectric eliminates fire hazards, leakage risks, and maintenance-intensive oil sampling. The thermal class of the insulation system is F (155°C), allowing continuous operation at ambient temperatures up to 40°C with a 10 K temperature rise margin under rated load.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed VTs, the SZW-10K offers significant operational and safety benefits. Its dry-type construction eliminates flammability concerns, making it suitable for indoor switchgear rooms, urban substations, and confined industrial facilities where fire codes restrict oil-filled equipment. The unit requires no periodic oil testing, degassing, or conservator tank maintenance. Additionally, the cast-resin body exhibits excellent resistance to tracking, UV degradation, and pollution—critical for outdoor installations in coastal or industrial atmospheres. Mechanical stability is enhanced by the rigid resin structure, which dampens vibration-induced winding displacement and reduces acoustic noise to below 45 dB(A) at 1 m distance. Weight is also reduced by approximately 30% versus equivalent oil units, simplifying handling and mounting.
Typical Application Overview
The SZW-10K is deployed across diverse power infrastructure segments, including utility-owned primary and secondary substations, industrial plant distribution networks, renewable energy interconnection points (e.g., solar farms and wind parks), and rural electrification schemes. It interfaces directly with revenue-class energy meters (Class 0.2 or 0.5S per IEC 62053-22) and protective relays requiring accurate voltage signals under normal and fault conditions. Its dual-winding secondary configuration—one for metering (typically 0.2/3P accuracy class) and one for protection (3P)—enables simultaneous compliance with metrological and security requirements without cross-interference.
Technical Specifications
The SZW-10K adheres to stringent electrical and mechanical parameters defined by IEC 61869-3 and GB/T 20840.3, ensuring interoperability and performance consistency across global and domestic grids.
Rated Electrical Parameters
Primary rated voltage: 11 kV (system voltage); maximum system voltage: 12 kV. Standard voltage ratio: 11,000 V / √3 : 100 V / √3 (for phase-to-ground connection in three-phase systems). Alternative ratios include 11,000/110 V and 10,000/100 V for legacy 10 kV networks. Accuracy classes: 0.2/3P or 0.5/3P, where the first class applies to the metering winding (burden ≤ 30 VA) and the second to the protection winding (burden ≤ 100 VA). Rated outputs: 30 VA (metering) and 100 VA (protection) at cos φ = 0.8 lagging. Insulation level: LI 75 kV / AC 30 kV (lightning impulse / power frequency withstand). Short-time thermal current rating: 100 A for 1 second. Residual voltage ratio error at 1.9 × Un: ≤ ±3%. Phase displacement at rated burden: ≤ ±10 minutes for Class 0.2.
Environmental and Service Conditions
Designed for operation in ambient temperatures from –25°C to +40°C (with derating above 40°C per IEC 60060-1). Relative humidity tolerance: up to 100% (condensing permitted). Maximum installation altitude: 1,000 m above sea level; for altitudes up to 3,000 m, voltage correction factors per IEC 60071-2 apply. Pollution degree: III (suitable for industrial and coastal zones per IEC 60815). The unit is rated for both indoor and outdoor use, with UV-stabilized resin housing and stainless-steel hardware resistant to salt spray (ASTM B117 tested for 500 hours). Terminal markings comply with IEC 61869-3 polarity conventions (reducing polarity: H1–X1).
Mechanical and Construction Details
Housing material: self-extinguishing cycloaliphatic epoxy resin (UL 94 V-0 rated). Core material: CRGO (cold-rolled grain-oriented) silicon steel, 0.27 mm thick, annealed to minimize hysteresis loss. Winding conductor: electrolytic-tough-pitch (ETP) copper, Class 180 insulation. Primary terminals: M12 stainless-steel studs with torque specification of 25 N·m. Secondary terminals: screw-type blocks rated for 600 V, accommodating 2.5–6 mm² stranded conductors. Overall dimensions: 380 mm (H) × 220 mm (W) × 180 mm (D); weight: 28 kg. Mounting: 4×Φ14 mm holes on base plate, compatible with standard IEC 61970 mounting rails.
Typical Applications
The SZW-10K’s design versatility enables deployment across multiple critical infrastructure domains.
Substation Secondary Metering
In 11 kV/0.4 kV distribution substations, the SZW-10K supplies scaled-down voltage signals to multi-tariff energy meters for billing and load profiling. Its Class 0.2 accuracy ensures compliance with regulatory requirements for revenue metering (e.g., EN 50470-3 in Europe or DL/T 614 in China). The low phase displacement (<5 minutes at 25–100% load) minimizes reactive energy measurement errors. In ring-main units (RMUs), the VT is often mounted adjacent to vacuum circuit breakers, providing synchronized voltage inputs to digital multifunction meters such as those compliant with IEC 61850-9-2 LE.
Industrial Power Distribution Systems
Within manufacturing plants, refineries, and data centers, the SZW-10K monitors bus voltage for power quality analysis and undervoltage/overvoltage protection. Its robust cast-resin body withstands electromagnetic interference (EMI) from variable-frequency drives (VFDs) and arc furnaces. The protection winding (Class 3P) feeds voltage inputs to distance relays (e.g., IEC 61850-compliant IEDs) that detect line faults beyond the local breaker zone. In hazardous areas (Zone 2), the absence of oil eliminates ignition sources, satisfying ATEX/IECEx requirements when installed in certified enclosures.
Renewable Energy Integration
At solar photovoltaic (PV) farms and onshore wind installations, the SZW-10K interfaces with grid-tie inverters and SCADA systems. It provides precise voltage feedback for anti-islanding detection and reactive power control (Q(V) regulation per IEEE 1547). During grid disturbances, the VT maintains signal integrity down to 0.1 × Un, enabling ride-through compliance. Its fast transient response (<1 ms rise time) captures voltage sags/swells for PQ monitoring per IEC 61000-4-30 Class S.
Rural and Suburban Distribution Networks
In remote or lightly loaded feeders, the SZW-10K supports automated voltage regulation via capacitor bank controllers and line drop compensators. Its low no-load losses (<15 W) reduce standby energy consumption in sparsely populated areas. The unit’s resistance to dust and moisture ensures reliability in unattended pole-mounted or pad-mounted substations. For 10 kV legacy networks, the 10,000/100 V ratio variant maintains compatibility with existing metering infrastructure while meeting modern IEC accuracy standards.
Backup Protection in Ring Main Units
In compact RMUs used for urban cable networks, the SZW-10K provides auxiliary voltage for earth-fault detection in unearthed or compensated neutral systems. The residual voltage winding (if configured) enables calculation of zero-sequence voltage for directional earth-fault relays. Its compact footprint allows integration into SF6-insulated switchgear without compromising arc-flash safety distances.
Compliance with International Standards
The SZW-10K is engineered to satisfy both global and regional regulatory frameworks governing instrument transformers.
IEC 61869-3 Certification Requirements
Compliance with IEC 61869-3 (Part 3: Particular requirements for inductive voltage transformers) mandates rigorous validation of accuracy, thermal behavior, and dielectric performance. Key tests include: temperature rise test (≤60 K for windings at 1.2 × Un and rated burden), short-circuit withstand test (primary shorted, secondary energized to 10 × rated current for 1 s), and partial discharge measurement (<10 pC at 1.2 × Un/√3). The manufacturer must provide a type test report issued by an ISO/IEC 17025-accredited laboratory, confirming adherence to all clauses of the standard. Marking includes IEC symbol, accuracy class, rated output, and insulation level per Clause 8.
Alignment with GB/T 20840.3
For the Chinese market, the SZW-10K meets GB/T 20840.3-2013, which largely harmonizes with IEC 61869-3 but includes additional requirements for seismic withstand (horizontal acceleration 0.2 g) and higher pollution severity (creepage distance ≥25 mm/kV for 11 kV). Domestic certification by CEPREI or TÜV SÜD China validates compliance. Notably, GB/T permits a slightly wider phase error tolerance (±15 minutes for Class 0.2) compared to IEC (±10 minutes), though the SZW-10K exceeds both limits.
Differences Between IEC and Domestic Standards
While IEC 61869-3 focuses on functional performance, GB/T 20840.3 emphasizes environmental durability for China’s diverse climate zones. For instance, GB/T requires salt fog testing per GB/T 2423.17, whereas IEC references IEC 60068-2-11 generically. Additionally, GB/T specifies mandatory short-time current ratings for VTs used in resonant-grounded systems—a scenario less emphasized in IEC. Despite these nuances, the SZW-10K’s design envelope accommodates both, enabling dual-certification without model variants.
On-Site Testing Procedures
Post-installation verification ensures the SZW-10K performs within specified tolerances before commissioning.
Insulation Resistance Test
Using a 2,500 V DC megohmmeter, measure insulation resistance between primary winding and ground, secondary windings and ground, and inter-winding. Acceptance criteria: ≥1,000 MΩ at 20°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement. Perform before and after dielectric tests to detect insulation degradation.
Turns Ratio Test
Apply a low-voltage AC source (50–100 V) to the primary and measure secondary voltage with a calibrated true-RMS voltmeter. Calculate actual ratio: Vp/Vs. Compare to nameplate ratio; tolerance per IEC 61869-3: ±0.25% for Class 0.2, ±0.5% for Class 0.5. Use a dedicated turns ratio tester (e.g., Omicron CT Analyzer) for automated comparison. Deviations >1% suggest winding shorts or open circuits.
Polarity Verification
Confirm reducing polarity (H1–X1) using the DC kick method: connect a 6–12 V battery between H1 and H2; observe momentary deflection of a center-zero galvanometer connected to X1–X2. Positive kick on closure indicates correct polarity. Incorrect polarity causes 180° phase reversal, leading to metering errors or relay misoperation. Digital multimeters with phase-angle function can also verify polarity under low-voltage AC excitation.
Power Frequency Withstand Voltage Test
Apply 30 kV RMS (50 Hz) for 1 minute between primary and grounded secondary/core. Use a calibrated test transformer with overcurrent trip (≤100 mA). No flashover or disruptive discharge constitutes pass. Reduce voltage gradually post-test to avoid transient overvoltages. This test validates insulation integrity after transport and installation stresses.
Open-Circuit Characteristic Test
With secondary open, ramp primary voltage from 0 to 1.9 × Un (20.9 kV) in 10% increments. Record excitation current and secondary voltage. Plot knee-point curve; excessive current (>5% of rated primary current at 1.5 × Un) indicates core saturation or inter-turn faults. This test is critical for protection applications where VT must remain linear during overvoltage transients.
Preventive Maintenance Guide
Proactive maintenance extends service life and prevents unexpected failures.
Periodic Visual and Electrical Inspection
Conduct annual inspections: check for surface cracks, tracking marks, or discoloration on resin housing; verify terminal tightness (torque to 25 N·m); clean dust/debris with dry compressed air (≤0.3 MPa). Measure insulation resistance and compare to baseline values; a 50% drop warrants investigation. Inspect grounding continuity (<0.1 Ω resistance). In coastal areas, rinse with deionized water annually to remove salt deposits.
Long-Term Maintenance Schedule
Every 5 years, perform full diagnostic suite: turns ratio, polarity, open-circuit test, and partial discharge measurement (<20 pC at 1.2 × Un). Replace if partial discharge exceeds 50 pC or ratio error drifts beyond 0.5%. After severe system faults (e.g., close-in short circuits), conduct immediate post-event testing regardless of schedule. Maintain logbook with test results, ambient conditions, and observed anomalies.
Fault Diagnosis and Troubleshooting
Common failure modes include: (1) Secondary open-circuit—causes core saturation, overheating, and insulation breakdown; always short secondary before disconnecting loads. (2) Moisture ingress—evidenced by low insulation resistance; may require oven drying at 80°C for 24 h. (3) Core lamination damage—indicated by elevated no-load losses; irreversible, necessitates replacement. Never operate with secondary shorted—this induces excessive primary current and thermal damage.
Conclusion
The SZW-10K 11kV cast-resin voltage transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-3 and GB/T 20840.3 compliance with field-proven reliability. Its VPI epoxy resin encapsulation delivers exceptional dielectric strength, environmental resilience, and fire safety—critical advantages over oil-immersed alternatives in modern substations and industrial facilities. Engineered with GOES silicon steel cores and precision-wound copper coils, the unit achieves Class 0.2/3P accuracy across metering and protection burdens, ensuring regulatory compliance and operational integrity. Rigorous factory and on-site testing protocols guarantee performance consistency, while a structured preventive maintenance program supports a service life of 25–30 years under standard operating conditions. As power systems evolve toward digitalization and distributed generation, the SZW-10K’s robust design, compact form factor, and dual-application capability make it an indispensable component for accurate voltage sensing in next-generation grids. Its certification to both international and domestic standards further simplifies procurement and deployment across global markets, reinforcing its position as a technically superior solution for critical metering and protection functions.