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IEC 61869-3 Certified 11kV Voltage Transformer SZW-6 for Metering & Protection Applications
Introduction to the SZW-6 Voltage Transformer
The SZW-6 is a single-phase, indoor/outdoor-rated cast-resin voltage transformer (VT) designed for accurate voltage measurement and reliable protective relay operation in medium-voltage networks operating at 11kV (IEC standard) or 10kV (domestic Chinese system). This instrument transformer leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings, providing superior dielectric strength, mechanical robustness, and environmental resistance compared to traditional oil-immersed alternatives.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the SZW-6 employs a two-component cycloaliphatic epoxy resin system processed under vacuum and pressure to eliminate voids and moisture entrapment. The primary and secondary windings—typically wound with high-purity electrolytic copper—are embedded within this solid dielectric matrix along with a grain-oriented electrical steel (GOES) core. This monolithic structure ensures uniform electric field distribution, suppresses partial discharges below 5 pC at rated voltage (per IEC 61869-3), and provides excellent thermal conductivity for heat dissipation. Unlike oil-filled units, the absence of liquid insulation eliminates fire hazards, leakage risks, and maintenance associated with oil sampling or level checks, making it ideal for urban substations, indoor switchgear, and environmentally sensitive installations.
Advantages Over Oil-Immersed Designs
The SZW-6’s cast-resin construction offers multiple engineering advantages. First, its solid insulation enables compact dimensions—typically 30–40% smaller footprint than equivalent oil-immersed VTs—facilitating integration into space-constrained ring main units (RMUs) or metal-clad switchgear. Second, the unit exhibits exceptional short-circuit withstand capability due to rigid winding fixation within the resin, preventing deformation under dynamic electromagnetic forces during system faults. Third, the design supports higher pollution degree ratings (up to III per IEC 60664), allowing deployment in coastal or industrial atmospheres without derating. Finally, the absence of oil simplifies end-of-life disposal and complies with stringent environmental regulations such as RoHS and REACH, reducing total lifecycle costs.
Typical Applications Overview
The SZW-6 is engineered for dual-purpose use: precision metering (accuracy classes 0.2 or 0.5) and protective relaying (accuracy class 3P or 6P). It is commonly deployed in 11kV/10kV distribution substations feeding commercial complexes, hospitals, data centers, and manufacturing plants. Its robust design also suits renewable energy interconnection points—such as solar farms or wind turbine step-up transformers—where harmonic distortion and transient overvoltages demand stable VT performance. Additionally, rural electrification projects benefit from its maintenance-free operation and tolerance to wide ambient temperature swings (-40°C to +40°C).
Technical Specifications
The SZW-6 adheres to strict dimensional, electrical, and thermal parameters defined by IEC 61869-3 and GB/T 20840.3. Below is a comprehensive specification table followed by service condition details.
| Parameter | Value |
|---|---|
| System Voltage (IEC) | 11 kV |
| System Voltage (Domestic) | 10 kV |
| Rated Primary Voltage | 11 / √3 kV (for phase-to-ground connection) |
| Rated Secondary Voltage | 100 / √3 V or 100 V (user-selectable) |
| Voltage Ratio | 11000/√3 : 100/√3 V or 11000/√3 : 100 V |
| Accuracy Class (Metering) | 0.2, 0.5 |
| Accuracy Class (Protection) | 3P, 6P |
| Rated Output (Burden) | 10 VA, 15 VA, 30 VA (per accuracy class) |
| Insulation Level (LI/AC) | 75 kV / 28 kV (1 min, 50 Hz) |
| Partial Discharge | < 5 pC at 1.2 × Ur |
| Core Material | Grain-Oriented Electrical Steel (GOES), 0.27 mm thickness |
| Insulation System | VPI Epoxy Resin (Class F, 155°C) |
| Creepage Distance | ≥ 25 mm/kV (pollution degree III) |
| Weight | Approx. 28 kg |
Standard Service Conditions
The SZW-6 is rated for continuous operation under IEC 60060-1 standard atmospheric conditions: ambient temperature range of -40°C to +40°C, relative humidity up to 95% (non-condensing), and installation altitude ≤ 1000 m above sea level. For altitudes exceeding 1000 m, a derating factor of 1.0% per 100 m applies to the power frequency withstand voltage. The unit is suitable for both indoor (IP2X terminal box) and outdoor (IP54 housing) environments when mounted vertically with adequate clearance around bushings to prevent localized heating.
Electrical Performance Tolerances
Voltage error and phase displacement must comply with IEC 61869-3 limits. For a 0.2-class metering winding at 100% rated voltage and burden, voltage error shall not exceed ±0.2%, and phase error shall be ≤ ±10 minutes. Under 25% to 100% of rated voltage, errors remain within specified bands. Protection windings (3P) permit ±3% voltage error and ±120 minutes phase error at rated voltage and burden. These tolerances ensure compatibility with modern digital relays and revenue-grade meters requiring high waveform fidelity.
Typical Applications
The SZW-6 serves critical roles across diverse power infrastructure segments, leveraging its dual accuracy classes and rugged construction.
Substation Secondary Metering
In 11kV primary substations, the SZW-6 supplies scaled-down voltage signals to revenue meters, SCADA systems, and power quality analyzers. Its 0.2-class winding ensures billing accuracy compliant with regulatory standards (e.g., EN 50163). For example, in a 10 MVA urban distribution substation, three SZW-6 units (one per phase) feed a three-phase kWh meter with 10 VA burden, maintaining error within ±0.15% under 80–120% load variation. The cast-resin body resists condensation in unheated enclosures, eliminating drift caused by oil viscosity changes.
Industrial Power Distribution
Heavy industries—such as steel mills or chemical plants—use the SZW-6 for motor protection and process control. Here, the 3P protection winding interfaces with overvoltage, undervoltage, and directional earth-fault relays. During arc-furnace operation, which introduces significant harmonics, the GOES core’s low hysteresis loss minimizes saturation, preserving VT linearity. Installation in metal-enclosed switchgear benefits from the unit’s non-flammability, meeting NFPA 70E arc-flash safety requirements.
Renewable Energy Integration
Solar photovoltaic (PV) farms connect to 11kV grids via inverters that generate high-frequency transients. The SZW-6’s solid insulation dampens these oscillations, preventing false tripping of anti-islanding relays. In a 5 MW solar plant, SZW-6 VTs monitor grid voltage at the point of common coupling (PCC), enabling reactive power control per IEEE 1547. The unit’s wide temperature tolerance accommodates desert climates where daytime highs exceed 45°C.
Rural and Suburban Distribution Networks
Remote areas with limited maintenance access rely on the SZW-6’s 25–30 year service life and zero routine servicing. Pole-mounted or pad-mounted installations use the outdoor-rated version with silicone rubber sheds for enhanced pollution performance. In China’s rural grid upgrade programs, SZW-6 units replace aging oil-filled VTs in 10kV lines, improving reliability while reducing fire risk near wooden structures.
Backup Power and Critical Facilities
Hospitals and data centers employ the SZW-6 in automatic transfer switch (ATS) schemes to detect utility failure. Its fast response time (< 20 ms) ensures seamless switchover to backup generators. The VT’s immunity to vibration makes it suitable for seismic zones (Zone 2 per IEC 60068-2-6), where oil units might suffer bushing fractures.
Compliance with International Standards
The SZW-6 is engineered to satisfy both global and regional regulatory frameworks, ensuring interoperability and safety.
IEC 61869-3 Certification Details
IEC 61869-3 governs electromagnetic voltage transformers for measurement and protection. The SZW-6 meets all clauses regarding insulation coordination, accuracy verification, temperature rise (≤ 60 K for windings), and short-circuit withstand (250 × In for 1 s). Type tests include power frequency withstand (28 kV, 1 min), lightning impulse (75 kV, 1.2/50 μs), and partial discharge measurement. Routine tests—performed on every unit—include ratio verification (±0.1% tolerance), polarity check, and insulation resistance (> 1000 MΩ at 2500 V DC).
Alignment with GB/T 20840.3
GB/T 20840.3 is China’s national adoption of IEC 61869-3, with minor deviations. Key differences include creepage distance requirements (GB mandates ≥ 20 mm/kV vs. IEC’s performance-based approach) and labeling in Chinese. The SZW-6 complies with both by incorporating extended shed profiles for domestic deployments. Additionally, GB/T 20840.3 requires a 1.9 × Ur overvoltage test for 8 hours—successfully passed by the SZW-6’s resin system without degradation.
Testing and Certification Requirements
Certification involves third-party laboratories accredited to ISO/IEC 17025. Tests include:
- Type Tests: Full suite per IEC 61869-3 Annex A (conducted once per design)
- Routine Tests: Performed on 100% of production units
- Sample Tests: Random batch testing for thermal stability and impulse strength
Certificates issued include IEC CB Scheme reports and CQC marks for Chinese market access.
On-Site Testing Procedures
Post-installation verification ensures the SZW-6 performs within specifications before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary winding and ground, and between primary and secondary windings. Acceptance criterion: ≥ 1000 MΩ at 25°C. Correct for temperature using RT2 = RT1 × 2(T1−T2)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement.
Turns Ratio Test
Apply 100–200 V AC to the primary and measure secondary voltage. Calculate ratio as Vp/Vs. Tolerance: ±0.1% of nominal ratio. Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for accuracy. Deviations >0.2% suggest inter-turn faults or incorrect tap selection.
Polarity Test
Verify reducing polarity per IEC 61869-3 Figure 3. Connect a 6–12 V battery across primary terminals (H1+, H2−). Momentarily close the circuit while monitoring secondary with a DC voltmeter (X1+, X2−). A positive kick confirms correct polarity. Incorrect polarity causes metering errors and relay misoperation.
Power Frequency Withstand Voltage Test
Apply 28 kV RMS at 50 Hz between primary and ground for 1 minute. Secondary windings are short-circuited and grounded. No flashover or disruptive discharge is permitted. This test validates insulation integrity after transport-induced stress.
Open-Circuit Characteristic Test
Gradually increase primary voltage from 20% to 120% of Ur while measuring secondary voltage and excitation current. Plot Vs vs. Iexc. Knee-point voltage should exceed 1.5 × Ur. Excessive excitation current at rated voltage indicates core saturation—often due to remanence or manufacturing defect.
Preventive Maintenance Guide
Although cast-resin VTs require minimal upkeep, periodic checks extend service life and prevent unexpected failures.
Annual Visual and Functional Inspection
Inspect for:
- Cracks, tracking, or UV degradation on resin surface
- Loose terminal connections or corrosion in secondary box
- Dust accumulation on bushings (clean with dry cloth if >2 mm layer)
Perform insulation resistance and ratio tests annually. Record trends—resistance dropping >20% year-over-year warrants investigation. Ensure secondary circuits are never left open during operation to avoid dangerous overvoltages.
Five-Year Comprehensive Maintenance
Every 60 months, conduct:
- Partial discharge measurement (should remain < 5 pC)
- Thermal imaging under load to detect hot spots (>10 K above ambient)
- Torque verification of mounting bolts (15 N·m for M10 studs)
Replace units showing PD > 10 pC or visible resin delamination.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicator |
|---|---|---|
| Annually | IR test, visual check | IR < 500 MΩ → moisture ingress |
| Biennially | Ratio & polarity test | Ratio error >0.3% → winding fault |
| 5 Years | PD test, thermal scan | PD > 8 pC → internal voiding |
| After Fault | Full retest | Distorted waveform → core damage |
Conclusion
The SZW-6 11kV cast-resin voltage transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-3-certified accuracy with the durability of solid insulation technology. Its dual-winding configuration—supporting both 0.2-class metering and 3P protection—enables versatile deployment across utility, industrial, and renewable sectors without compromising performance. The use of GOES cores minimizes losses and enhances linearity under harmonic distortion, while VPI epoxy resin ensures decades of maintenance-free operation even in harsh environments. With a design life of 25–30 years, the SZW-6 significantly reduces total cost of ownership compared to oil-immersed alternatives, eliminating fire hazards, oil handling, and frequent servicing. Compliance with both IEC 61869-3 and GB/T 20840.3 guarantees global acceptance and seamless integration into existing protection and metering architectures. For engineers specifying reliable, future-proof voltage transformation in 11kV/10kV networks, the SZW-6 delivers uncompromised technical excellence, safety, and longevity.