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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) engineered for accurate voltage measurement and reliable protective relay operation in medium-voltage power systems operating at 11kV (IEC standard) or 10kV (domestic Chinese system). Designed in strict accordance with IEC 61869-3 and GB/T 20840.3, this instrument transformer leverages advanced vacuum pressure impregnation (VPI) epoxy resin technology to deliver superior dielectric performance, environmental resilience, and long-term stability without the fire hazards or maintenance demands associated with oil-filled alternatives.

Operating Principle of Cast-Resin Insulation

The SZW-6 employs a dry-type construction where the primary and secondary windings—wound on a grain-oriented electrical steel (GOES) core—are fully encapsulated in high-purity epoxy resin under vacuum and pressure. This VPI process eliminates air voids and moisture ingress pathways, ensuring uniform electric field distribution and preventing partial discharge inception even under transient overvoltages. The resin matrix provides mechanical rigidity, thermal conductivity for heat dissipation, and resistance to tracking, UV degradation, and chemical exposure. Unlike oil-immersed VTs, the cast-resin design requires no conservator tanks, breathers, or oil sampling, significantly reducing lifecycle operational complexity.

Advantages Over Oil-Immersed Designs

Compared to traditional oil-filled voltage transformers, the SZW-6 offers critical advantages in safety, maintenance, and environmental compatibility. Its non-flammable epoxy resin eliminates fire risk, making it suitable for indoor substations, commercial buildings, and confined urban installations where fire codes prohibit combustible insulation. The hermetically sealed structure prevents moisture absorption, ensuring stable ratio and phase angle error over decades of service. Additionally, the absence of oil simplifies disposal logistics and avoids regulatory burdens related to PCBs or hydrocarbon leaks. Weight is reduced by approximately 30%, facilitating easier handling and mounting on switchgear panels or support structures.

Typical Applications Overview

The SZW-6 is deployed across diverse power infrastructure segments requiring precise voltage transformation for metering accuracy class 0.2 or 0.5 and protection class 3P or 6P per IEC 61869-3. Primary use cases include utility-owned 11kV/10kV distribution substations, industrial plant switchyards, renewable energy interconnection points (e.g., solar farms feeding into 11kV grids), and railway traction power systems. Its robust design supports continuous operation under polluted or humid conditions (up to 95% relative humidity) and altitudes up to 1,000 meters above sea level without derating.

Technical Specifications

The SZW-6 voltage transformer is engineered to meet stringent performance criteria defined by international and national standards. Below are its core technical parameters, validated through type tests and routine production verification.

Rated Electrical Parameters

The SZW-6 features a primary rated voltage of 11kV (line-to-earth), corresponding to a system voltage of 12kV. The domestic equivalent operates at 10kV (system voltage 11.5kV). Standard secondary voltages are 100/√3 V or 100 V for three-phase connections, with optional 110 V configurations available upon request. Accuracy classes include 0.2 and 0.5 for metering applications, and 3P or 6P for protection functions. Rated outputs range from 10 VA to 100 VA per secondary winding, with thermal limits verified at 1.2 × Un continuous and 1.9 × Un for 8 hours. The insulation level adheres to Um = 12kV, with a lightning impulse withstand voltage of 75 kV peak and a power frequency wet test voltage of 28 kV rms for 1 minute.

Construction and Materials

The magnetic core is fabricated from high-permeability grain-oriented electrical steel (GOES), laminated to minimize eddy current losses and hysteresis heating. Windings use oxygen-free copper with Class F (155°C) thermal rating, embedded in cycloaliphatic epoxy resin via VPI. The resin formulation includes silica fillers to enhance thermal conductivity (≥0.8 W/m·K) and reduce coefficient of thermal expansion mismatch. Terminal blocks are made of flame-retardant PBT with IP2X finger-safe protection. The housing incorporates integrated lifting lugs and M12 grounding studs compliant with IEC 61869-1 clause 5.104.

Standard Service Conditions

The SZW-6 is rated for ambient temperatures from –25°C to +40°C, with a maximum daily average of +35°C. Relative humidity tolerance extends to 95% non-condensing. Installation altitude is limited to 1,000 m; for sites between 1,001–2,000 m, a 1% reduction in test voltage per 100 m above 1,000 m is applied per IEC 60060-1. The transformer is suitable for both indoor switchgear compartments and outdoor pole-mounted or pad-mounted enclosures when fitted with optional weatherproof terminal covers. Pollution degree is classified as III per IEC 60664-1, supporting creepage distances ≥20 mm/kV.

Typical Applications

The SZW-6 voltage transformer serves critical roles in modern power systems where measurement fidelity and protection reliability are non-negotiable.

Substation Secondary Metering

In 11kV/10kV distribution substations, the SZW-6 provides scaled-down voltage signals to revenue-class energy meters, demand recorders, and SCADA RTUs. With accuracy class 0.2, it ensures billing precision within ±0.2% error across 80–120% of rated voltage and 25–100% of rated burden. Its low phase displacement (<10 minutes at 0.2 class) minimizes reactive energy measurement errors. The transformer’s stable thermal characteristics prevent drift during load cycling, crucial for utilities implementing time-of-use tariffs or distributed generation net metering.

Industrial Power Distribution

Within manufacturing facilities, data centers, and mining operations, the SZW-6 interfaces with multifunction protection relays (e.g., overvoltage, undervoltage, directional earth fault) requiring class 3P or 6P accuracy. Its compact footprint allows integration into metal-enclosed switchgear (e.g., RMU or AIS panels), while the cast-resin body resists contamination from dust, oil mist, or chemical vapors common in industrial environments. The 100 V secondary output standardizes relay input requirements across global OEM platforms.

Renewable Energy Integration

Solar photovoltaic and wind farms connecting to 11kV distribution networks rely on the SZW-6 for grid synchronization, anti-islanding detection, and power quality monitoring. During rapid irradiance changes or cloud transients, the VT’s low remanence and fast response (<20 ms settling time) ensure accurate voltage waveform capture for inverters and synchrocheck relays. Its immunity to DC bias—critical in systems with unbalanced PV injection—prevents core saturation and measurement distortion.

Rural and Suburban Distribution Networks

In remote or cost-sensitive deployments, the SZW-6’s maintenance-free operation reduces O&M expenses over its 25–30 year service life. Mounted on poles or ground pads, it supplies voltage signals to automated feeder switches, capacitor bank controllers, and fault indicators. The 10kV domestic variant aligns with China’s rural electrification standards (DL/T 726), while the 11kV version supports export projects in Africa, Southeast Asia, and Latin America adhering to IEC frameworks.

Compliance with International Standards

The SZW-6 is certified to the latest editions of IEC 61869-3 (Instrument transformers – Part 3: Additional requirements for inductive voltage transformers) and GB/T 20840.3 (identical adoption of IEC 61869-3 in China).

IEC 61869-3 Compliance Details

Compliance encompasses all mandatory clauses: accuracy verification under defined burden ranges, temperature rise limits (≤60 K for resin, ≤50 K for windings), short-circuit withstand capability (withstanding 25 × In for 1 s without damage), and partial discharge levels (<10 pC at 1.2 × Um/√3). Type tests include temperature rise, short-time current, impulse voltage, and accuracy performance across frequency bands (48–52 Hz). Routine tests performed on every unit include turns ratio, polarity, insulation resistance (>1,000 MΩ at 2,500 V DC), and power frequency withstand.

GB/T 20840.3 Alignment and Regional Adaptations

While GB/T 20840.3 mirrors IEC 61869-3 structurally, minor differences exist in labeling (Chinese characters required), terminal marking conventions (A, N vs. H1, H2), and domestic voltage ratings (10kV vs. 11kV). The SZW-6 is dual-marked to satisfy both regimes, enabling seamless deployment in China’s State Grid and Southern Grid projects as well as international tenders. Notably, GB/T 20840.3 mandates additional seismic testing (0.3g horizontal acceleration) for transformers installed in earthquake-prone zones—a requirement the SZW-6 meets via finite element analysis-validated structural design.

Testing and Certification Requirements

Certification involves third-party witness testing at accredited laboratories (e.g., KEMA, CESI, or China Electric Power Research Institute). Key deliverables include a type test report, routine test records per unit, and a declaration of conformity signed by the manufacturer’s authorized representative. For projects requiring grid code compliance (e.g., ENTSO-E, CIGRE TB 724), supplementary tests such as harmonic distortion response (<0.5% THD up to 13th harmonic) and DC offset immunity may be conducted.

Conclusion

The SZW-6 11kV cast-resin voltage transformer represents a benchmark in reliability, accuracy, and compliance for modern medium-voltage networks. By integrating GOES core technology with VPI epoxy resin encapsulation, it achieves exceptional dielectric strength, thermal stability, and environmental resilience—eliminating the operational liabilities of oil-filled alternatives. Its dual compliance with IEC 61869-3 and GB/T 20840.3 ensures seamless deployment across global markets, from European distribution grids to Chinese rural electrification projects. Rigorous factory and on-site testing protocols guarantee performance within tight tolerances for both metering (class 0.2/0.5) and protection (class 3P/6P) duties. With a design life exceeding 25–30 years under standard service conditions, the SZW-6 delivers unmatched total cost of ownership through zero-maintenance operation, fire safety, and consistent metrological performance. As power systems evolve toward smarter, more distributed architectures, this voltage transformer remains a foundational component for accurate measurement, dependable protection, and regulatory compliance in 11kV/10kV infrastructure worldwide.