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IEC 61869-3 Certified 11kV Voltage Transformer SZF-3 for Metering & Protection Applications

Introduction to the SZF-3 Voltage Transformer

The SZF-3 is a single-phase, indoor/outdoor-rated cast-resin voltage transformer (VT) engineered for precise 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 encapsulate its magnetic core and windings, eliminating the fire hazards and environmental risks associated with traditional oil-immersed designs.

Operating Principle of Cast-Resin Insulation

Cast-resin insulation in the SZF-3 employs a thermosetting epoxy resin system that fully encapsulates the primary and secondary windings along with the grain-oriented electrical steel (GOES) core. During manufacturing, the assembly undergoes vacuum degassing followed by pressure impregnation to eliminate air voids and moisture, ensuring homogeneous dielectric strength throughout the insulation matrix. This process yields a solid, monolithic structure with excellent partial discharge resistance (<5 pC at 1.2 × Ur), high tracking resistance (CTI > 600 V), and superior mechanical stability under thermal cycling. The absence of liquid insulation eliminates leakage risks and simplifies handling during transport and installation, particularly in confined switchgear compartments or elevated outdoor platforms.

Advantages Over Oil-Immersed Designs

Compared to conventional oil-filled VTs, the SZF-3 offers significant operational and safety benefits. Its dry-type construction eliminates fire hazards, making it suitable for indoor substations, commercial buildings, and underground installations where flammable materials are prohibited. Maintenance requirements are drastically reduced—no oil sampling, level checks, or gasket replacements are needed over its 25–30-year service life. Additionally, the compact footprint (typically 30% smaller than equivalent oil units) enables higher equipment density in modern GIS and RMU applications. Environmental compliance is enhanced as the unit contains no PCBs or hydrocarbons, aligning with RoHS and WEEE directives. Thermal performance remains stable under continuous 1.2 × Ur overvoltage conditions due to the high thermal conductivity (~0.8 W/m·K) of the filled epoxy resin.

Typical Application Overview

The SZF-3 is deployed across utility distribution networks, industrial plants, renewable energy farms, and commercial facilities requiring accurate voltage transformation for metering (Class 0.2 or 0.5) and protection (Class 3P or 6P). Common configurations include single-pole mounting on 11kV busbars, integration into ring main units (RMUs), or use in three-phase banks for line-to-ground or line-to-line measurements. Its robust design withstands harsh environments—from coastal zones with salt fog (IP54 rating achievable with optional sheds) to high-altitude sites up to 2000 m above sea level without derating.

Technical Specifications

The SZF-3 voltage transformer delivers precision and reliability through rigorously defined electrical and mechanical parameters aligned with international standards.

Parameter	Value
System Voltage (Ur)	11 kV (IEC) / 10 kV (GB)
Primary Voltage (Up)	11,000 / √3 V (for phase-to-ground)
Secondary Voltage (Us)	100 / √3 V or 100 V (user-selectable)
Voltage Ratio	11,000/√3 : 100/√3 V or 11,000/√3 : 100 V
Accuracy Class (Metering)	0.2, 0.5
Accuracy Class (Protection)	3P, 6P
Rated Output (per burden)	10–100 VA (standard); up to 150 VA optional
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), M4 grade
Insulation System	VPI Epoxy Resin (Class F, 155°C)
Ambient Temperature Range	–25°C to +40°C (–40°C optional)
Relative Humidity	≤95% non-condensing
Altitude Limit	≤2000 m (derating required above)
Weight	Approx. 45 kg
Mounting	Flange-mounted (M12 bolts, 4-hole pattern)

Standard Service Conditions

The SZF-3 is rated for continuous operation under standard IEC 61869-3 service conditions: ambient temperature from –25°C to +40°C, relative humidity up to 95% without condensation, and installation altitude not exceeding 2000 meters. At altitudes above 1000 m, the external insulation withstand voltage must be corrected per IEC 60071-2 using the atmospheric correction factor Ka = e^(0.00012(H–1000)), where H is altitude in meters. For example, at 1500 m, Ka ≈ 1.06, requiring a test voltage of 28 kV × 1.06 ≈ 29.7 kV for the power frequency withstand test. The unit maintains accuracy within ±0.2% for metering class even under harmonic distortion levels up to 5% THD, thanks to low core losses (<0.8 W/kg at 1.7 T, 50 Hz) in the GOES laminations.

Electrical Performance Parameters

Voltage error and phase displacement are tightly controlled per IEC 61869-3 Table 2. For a Class 0.2 metering winding at 100% rated voltage and 25–100% burden, voltage error must not exceed ±0.2%, and phase error must be ≤±10 minutes. Protection windings (Class 3P) allow ±3% voltage error and ±120 minutes phase error at 5%–100% of rated voltage under specified burden. The rated thermal short-time current withstand is 10 kA for 1 second, ensuring survival during downstream faults. Burden compatibility includes inductive, resistive, and mixed loads, with maximum permissible burden impedance calculated as Zb = (Us²)/Srated (e.g., 100 VA at 100 V → Zb = 100 Ω).

Typical Applications

The SZF-3 serves critical roles across diverse power infrastructure sectors due to its dual-purpose accuracy and rugged construction.

Substation Secondary Metering

In 11kV primary substations, the SZF-3 provides scaled-down voltage signals to revenue-class energy meters (e.g., IEC 62053-22 compliant devices). Its Class 0.2 accuracy ensures billing precision within ±0.2% over a wide load range (10–120% of rated voltage). The transformer is typically installed on each phase of a three-phase busbar, with secondary windings connected in open-delta or wye configuration to feed multi-tariff meters and SCADA RTUs. In urban substations with space constraints, the compact cast-resin design allows side-by-side mounting without clearance concerns, unlike bulkier oil units.

Industrial Power Distribution

Heavy industries—such as steel mills, chemical plants, and data centers—rely on the SZF-3 for both process monitoring and protective relaying. Motor protection relays (e.g., ANSI 27/59 functions) require stable VT output during voltage sags or swells; the SZF-3’s low magnetizing current (<0.5% of rated primary current) minimizes waveform distortion under transient conditions. In arc furnace environments with high harmonic content, the GOES core’s high permeability maintains linearity up to the 13th harmonic, preventing relay misoperation. Ground-fault detection in high-resistance grounded (HRG) systems also depends on accurate residual voltage measurement from open-delta-connected SZF-3 units.

Renewable Energy Integration

Solar PV and wind farms utilize the SZF-3 at the point of interconnection (POI) for grid compliance monitoring. Grid codes (e.g., ENTSO-E, GB/T 19964) mandate voltage ride-through (VRT) capability verification, which requires precise voltage measurement during fault-induced dips. The SZF-3’s fast response time (<20 ms) and low phase shift ensure accurate capture of voltage magnitude and angle for synchrophasor (PMU) applications. In containerized substations common in utility-scale solar projects, the dry-type design eliminates fire risks near lithium-ion battery storage systems.

Rural and Suburban Distribution Networks

For rural electrification projects, the SZF-3’s maintenance-free operation reduces lifecycle costs in remote locations with limited technical staff. Mounted on pole-top platforms or pad-mounted switchgear, it supplies voltage signals to distribution automation terminals (DATs) for feeder reclosing and sectionalizing. Its ability to operate reliably at –25°C supports deployment in northern climates, while UV-stabilized resin formulations prevent surface degradation in high-sunlight regions. In China’s “New Infrastructure” initiative, SZF-3 units are standard in 10kV smart distribution transformers for state grid companies.

Commercial and Institutional Facilities

Hospitals, airports, and university campuses integrate the SZF-3 into their 11kV/10kV switchgear for critical load monitoring. Emergency generator synchronization requires precise phase-angle matching; the SZF-3’s Class 0.5 accuracy ensures sync check relays (ANSI 25) operate within ±1° tolerance. Fire safety codes (e.g., NFPA 70, GB 50016) often prohibit oil-filled equipment indoors, making the cast-resin SZF-3 the only compliant option for basement or rooftop substations.

Compliance with International Standards

The SZF-3 is engineered to meet stringent global and regional standards, ensuring interoperability and regulatory acceptance.

IEC 61869-3 Compliance Details

IEC 61869-3 specifies requirements for electromagnetic voltage transformers for measurement and protection. The SZF-3 complies with all clauses, including insulation coordination (Section 5), accuracy definitions (Section 6), and type tests (Section 7). Key verifications include: power frequency withstand voltage (28 kV for 1 min), lightning impulse withstand (75 kV peak, 1.2/50 μs wave), temperature rise test (≤60 K for resin, measured by resistance method), and short-circuit withstand (10 kA for 1 s). Accuracy verification follows IEC 61869-3 Annex B, using calibrated reference standards traceable to national metrology institutes. Partial discharge testing is performed per IEC 60270 at 1.2 × Ur, with results documented in the test certificate.

Alignment with GB/T 20840.3

GB/T 20840.3 is the Chinese national adoption of IEC 61869-3, with minor deviations primarily in labeling and documentation language. The SZF-3 meets all technical requirements of GB/T 20840.3, including domestic voltage rating (10kV system vs. 11kV IEC), Chinese character labeling on nameplates, and submission to CMA-accredited labs for type testing. Notably, GB/T 20840.3 mandates additional seismic testing (0.3g horizontal acceleration) for units destined for earthquake-prone regions like Sichuan, which the SZF-3 passes due to its monolithic resin structure damping vibrations effectively.

Key Differences Between IEC and Domestic Standards

While technically harmonized, practical differences exist. IEC 61869-3 uses 11kV as the nominal system voltage (Um = 12 kV), whereas GB/T 20840.3 references 10kV (Um = 11.5 kV). Consequently, insulation levels differ slightly: IEC requires LI/AC = 75/28 kV, while GB may accept 70/24 kV for 10kV systems—but the SZF-3 is built to the higher IEC level for global compatibility. Accuracy class notation is identical (0.2, 3P, etc.), but GB standards emphasize local calibration traceability via provincial metrology bureaus. Documentation must include both English and Chinese in dual-market units.

On-Site Testing Procedures

Post-installation verification ensures the SZF-3 performs within specifications before energization.

Insulation Resistance Test

Using a 2500 V DC megohmmeter, measure insulation resistance between primary winding and ground, secondary winding and ground, and primary-to-secondary. Acceptance criteria: ≥1000 MΩ at 20°C. Correct for temperature using R₂₀ = Rₜ × 2^((20–t)/10). Low readings (<100 MΩ) indicate moisture ingress or resin cracking—requiring drying or replacement. Perform before and after the power frequency withstand test to detect insulation degradation.

Turns Ratio Test

Apply a low-voltage AC signal (50–100 V) to the primary and measure secondary voltage with a calibrated true-RMS meter. Calculate ratio as Up/Us and compare to nameplate value. Tolerance: ±0.2% for metering windings, ±0.5% for protection. Use a dedicated turns ratio tester (e.g., Omicron CT Analyzer) for automated comparison. Deviations beyond tolerance suggest winding shorts or incorrect tap selection.

Polarity Test

Verify reducing polarity per IEC 61869-3 Figure 3. Connect a 6–12 V DC source momentarily to the primary (H1+ to H2–). Observe secondary voltage polarity with a DC voltmeter: if X1 shows positive deflection when H1 is energized, polarity is correct. Incorrect polarity causes 180° phase reversal, leading to metering errors or relay miscoordination in differential schemes.

Power Frequency Withstand Voltage Test

Apply 28 kV RMS at 50 Hz between primary and ground (secondary shorted and grounded) for 1 minute. Use a calibrated test transformer with overcurrent trip (≤1 A). No flashover or breakdown should occur. For field tests, reduce duration to 10 seconds per IEC 60270 if full-minute test is impractical, but document justification. Always discharge windings post-test.

Open-Circuit Characteristic Test

With secondary open, gradually increase primary voltage from 10% to 120% of rated (13.2 kV) while measuring excitation current. Plot Iₑ vs. Uₚ. The knee point should exceed 1.5 × Ur (16.5 kV) to avoid saturation during overvoltages. Excessive magnetizing current (>1% of rated) indicates core damage or shorted turns. This test validates linearity for protection applications.

Preventive Maintenance Guide

Although maintenance-free by design, periodic checks extend service life and ensure reliability.

Periodic Inspection Protocol

Conduct visual and thermographic inspections annually. Check for: surface cracks, tracking marks, discoloration (indicating overheating), loose mounting bolts, and corrosion on terminals. Use an infrared camera to detect hot spots (>10 K above ambient) at connections. Clean dust and salt deposits with dry compressed air (≤0.5 MPa). Verify secondary wiring integrity—loose neutrals cause floating potentials and measurement errors. Record findings in asset management software for trend analysis.

Maintenance Intervals and Fault Diagnosis

While no scheduled internal maintenance is required, a 5-year interval is recommended for comprehensive electrical testing (insulation resistance, ratio, polarity). Common faults include:
– Secondary open-circuit: Causes core saturation, overheating, and insulation failure. Install secondary shorting links during meter replacement.
– Moisture ingress: Appears as white powder (hydrolysis) on resin surface. Replace if IR drops below 500 MΩ.
– Harmonic resonance: Occurs with capacitive burdens; add damping resistor (50–100 Ω, 100 W) across secondary.
Maintenance intervals are summarized below:

Interval	Activities
Annual	Visual inspection, thermography, terminal torque check
5 Years	Full electrical tests (IR, ratio, polarity, PD if available)
After Fault	Post-fault inspection, insulation resistance, ratio verification
End-of-Life (25–30 yrs)	Replace regardless of condition due to resin embrittlement

Conclusion

The SZF-3 11kV cast-resin voltage transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-3 and GB/T 20840.3 compliance with robust engineering for demanding metering and protection duties. Its VPI epoxy resin encapsulation eliminates fire risks and environmental liabilities inherent in oil-filled alternatives, while the GOES silicon steel core ensures exceptional accuracy (Class 0.2) and linearity under harmonic distortion. Field-proven in applications ranging from urban substations to remote renewable sites, the SZF-3 delivers reliable performance across –25°C to +40°C ambient ranges and up to 2000 m altitude without derating. On-site commissioning is streamlined through standardized testing procedures—insulation resistance, turns ratio, polarity, and withstand voltage—with clear acceptance criteria ensuring grid readiness. Preventive maintenance is minimal yet effective, centered on annual visual inspections and quinquennial electrical verification, supporting a design life of 25–30 years. As power systems evolve toward smarter, safer, and more sustainable infrastructure, the SZF-3’s dry-type, compact, and standards-compliant architecture positions it as a future-proof solution for global utilities and industrial operators alike. Its seamless integration into modern switchgear, coupled with rigorous adherence to international test protocols, guarantees long-term operational integrity and measurement fidelity essential for revenue accuracy and system protection.