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JDZW-35 21kV Cast-Resin Voltage Transformer for Substation Metering and Protection – IEC 61869-3 Certified
Introduction to the JDZW-35 Voltage Transformer
The JDZW-35 is a single-phase, indoor/outdoor-rated cast-resin insulated voltage transformer (VT) designed for accurate voltage measurement and reliable protective relay operation in medium-voltage power systems operating at nominal system voltages of 21kV (IEC standard) or 20kV (domestic Chinese grid equivalent). This device transforms high primary voltages to standardized secondary outputs—typically 100 V or 100/√3 V—for interfacing with metering instruments, revenue-grade energy meters, and protective relays.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the JDZW-35 employs vacuum pressure impregnation (VPI) technology using cycloaliphatic epoxy resin systems. The primary and secondary windings, wound on grain-oriented electrical steel (GOES) cores, are fully encapsulated under vacuum to eliminate air voids and moisture ingress. This monolithic structure provides superior dielectric strength, tracking resistance, and mechanical rigidity compared to traditional oil-paper or air-insulated designs. The resin’s high glass transition temperature (>110°C) ensures thermal stability during transient overvoltages and continuous operation at elevated ambient temperatures. Partial discharge levels are maintained below 10 pC at 1.2 × Um/√3, per IEC 61869-3 requirements, ensuring long-term insulation integrity even in polluted environments.
Advantages Over Oil-Immersed Designs
Unlike oil-immersed VTs, the JDZW-35 eliminates fire hazards, environmental contamination risks, and maintenance-intensive oil sampling. Its solid insulation requires no periodic oil testing, degassing, or leakage monitoring. The compact footprint—enabled by higher dielectric strength of epoxy resin versus mineral oil—reduces substation spatial requirements. Additionally, cast-resin construction offers superior seismic resilience (withstanding 0.5g horizontal acceleration) and immunity to altitude effects up to 2000 m without derating. These attributes make it ideal for urban substations, indoor switchgear, and environmentally sensitive installations where flammability and spill containment are critical concerns.
Typical Applications Overview
The JDZW-35 serves dual roles in power systems: precision metering (accuracy classes 0.2, 0.5, or 1.0) and protective relaying (accuracy class 3P or 6P). It is commonly deployed in 21kV distribution substations for revenue metering, fault detection in radial feeders, synchronization checks in generator tie-lines, and voltage supervision in capacitor bank controls. Its robust design supports both indoor metal-enclosed switchgear and outdoor pole-mounted configurations, making it versatile across utility, industrial, and renewable energy applications.
Technical Specifications
The JDZW-35 adheres to stringent electrical and environmental parameters defined by IEC 61869-3 and GB/T 20840.3. Below is a comprehensive specification table:
| Parameter | Value |
|---|---|
| System Voltage (Um) | 24 kV (IEC), 21 kV rated |
| Domestic System Voltage | 20 kV |
| Primary Voltage | 21,000 / √3 V (for grounded systems) |
| Secondary Voltage | 100 / √3 V (metering), 100 V (protection) |
| Voltage Ratio | 21,000/√3 : 100/√3 V or 21,000/√3 : 100 V |
| Accuracy Class | Metering: 0.2, 0.5; Protection: 3P, 6P |
| Rated Output (per burden) | 10–100 VA (metering), 50–200 VA (protection) |
| Insulation Level (LI/AC) | 125 kV / 50 kV (1.2/50 μs impulse & 1-min power freq) |
| Short-Time Thermal Withstand | 1 second at 10× rated primary current |
| Ambient Temperature Range | –25°C to +40°C |
| Relative Humidity | ≤95% (non-condensing) |
| Altitude Limit | ≤2000 m above sea level |
| Core Material | Grain-Oriented Electrical Steel (GOES), 0.27 mm thickness |
| Insulation System | VPI cycloaliphatic epoxy resin, UL 94 V-0 flame rating |
Standard Service Conditions
The JDZW-35 is engineered for continuous operation under IEC 60060-1 standard atmospheric conditions: ambient temperature from –25°C to +40°C, daily average not exceeding +35°C. Relative humidity may reach 95% provided condensation does not occur—a critical consideration in coastal or tropical climates. At altitudes above 1000 m, external clearances must be increased per IEC 60071-2, though the internal cast-resin insulation remains unaffected up to 2000 m. The transformer withstands solar radiation exposure (up to 1000 W/m²) when installed outdoors with appropriate UV-resistant coatings on external surfaces.
Electrical Performance Tolerances
Voltage error and phase displacement are tightly controlled per accuracy class. For a 0.2-class unit, voltage error must not exceed ±0.2% and phase displacement ≤±10 minutes at 25–100% of rated burden. In protection applications (3P class), composite error must remain within ±3% at 5%–100% of rated voltage and up to 100% of rated burden. Burden compatibility ranges from 2.5 VA (minimum) to 200 VA (maximum), with linear performance verified across all load points during factory testing.
Typical Applications
The JDZW-35’s dual-certification and robust construction enable deployment across diverse power infrastructure segments.
Substation Secondary Metering
In 21kV primary substations, the JDZW-35 supplies scaled-down voltages to revenue meters, SCADA RTUs, and power quality analyzers. Its 0.2-class accuracy ensures compliance with regulatory billing standards (e.g., EN 50163, DL/T 448). For three-phase systems, three single-phase units are connected in wye configuration to provide phase-to-ground and phase-to-phase measurements. The low phase displacement minimizes reactive energy measurement errors, critical for industrial tariff structures involving kVARh billing.
Industrial Power Distribution
Large manufacturing facilities with 20/21kV internal distribution networks use the JDZW-35 for motor protection, bus undervoltage tripping, and harmonic monitoring. Its high short-circuit withstand capability protects downstream relays during feeder faults. In arc furnace or rolling mill plants with significant harmonic distortion, the GOES core’s low hysteresis loss maintains accuracy even under non-sinusoidal waveforms up to the 13th harmonic.
Renewable Energy Integration
Solar PV and wind farms operating at 21kV interconnection voltages rely on the JDZW-35 for grid synchronization, anti-islanding detection, and reactive power control. During cloud transients or wind gusts, rapid voltage fluctuations demand VTs with minimal response lag—the JDZW-35’s low leakage inductance (<0.5%) ensures faithful waveform reproduction for fast-acting inverters and STATCOMs.
Rural and Suburban Distribution Networks
Pole-mounted JDZW-35 units monitor voltage profiles along 20kV rural feeders, enabling utilities to optimize tap changer settings and reduce technical losses. Their maintenance-free nature reduces outage frequency in remote areas. In suburban ring-main units (RMUs), they support automated sectionalizing schemes that isolate faults within 100 ms, minimizing customer minutes lost (CML).
Capacitor Bank Voltage Control
For automatic switching of shunt capacitor banks, the JDZW-35 provides real-time voltage feedback to controllers. Its ability to operate continuously at 1.2 × Un accommodates temporary overvoltages during bank energization, preventing nuisance tripping while maintaining measurement fidelity for VAR management algorithms.
Compliance with International Standards
The JDZW-35 meets the rigorous requirements of both global and Chinese national standards, ensuring interoperability and safety.
IEC 61869-3 Certification Details
IEC 61869-3 specifies performance, safety, and testing criteria for inductive voltage transformers. The JDZW-35 undergoes type tests including temperature rise (≤60 K for resin, ≤50 K for windings), short-circuit withstand (10× In for 1 s), and partial discharge (≤10 pC at 1.2 × Um/√3). Routine tests include turns ratio verification (±0.25% tolerance), polarity check (reducing polarity confirmed), and power frequency withstand (50 kV for 1 min). All tests are witnessed by accredited third-party laboratories with certificates traceable to ILAC-MRA signatories.
Alignment with GB/T 20840.3
GB/T 20840.3 mirrors IEC 61869-3 but includes China-specific provisions such as mandatory seismic testing (horizontal acceleration ≥0.3g) and stricter pollution degree requirements (IV for coastal zones). The JDZW-35 exceeds these with a 0.5g seismic rating and creepage distance of ≥25 mm/kV (vs. 20 mm/kV minimum), achieved through optimized shed geometry on its porcelain or composite bushings.
Key Differences Between IEC and Domestic Standards
While IEC focuses on functional performance, GB/T emphasizes environmental durability for China’s diverse climate zones—from Gobi Desert aridity to Pearl River Delta humidity. GB/T also mandates local calibration traceability to NIM (National Institute of Metrology), whereas IEC accepts any ISO/IEC 17025 lab. Notably, GB/T requires dual secondary windings (one for metering, one for protection) as standard, which the JDZW-35 provides via separate, magnetically decoupled coils to prevent burden interaction.
On-Site Testing Procedures
Post-installation verification ensures the JDZW-35 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 RT2 = RT1 × 2(T1–T2)/10. Values below 500 MΩ indicate moisture ingress or resin cracking requiring investigation.
Turns Ratio Test
Apply 100–200 V AC to the primary and measure secondary voltage with a calibrated voltmeter. Calculate ratio as Vp/Vs. Tolerance: ±0.25% of nameplate value. For a 21,000/√3 : 100/√3 VT, expected ratio is 210:1; measured value must be 209.475–210.525. Deviations suggest winding shorts or incorrect tap selection.
Polarity Test
Connect a 6–12 V battery momentarily between primary terminals (H1–H2). Observe secondary voltage polarity with a DC voltmeter across X1–X2. A momentary positive deflection confirms reducing polarity (standard for IEC VTs). Reversed polarity causes 180° phase shift, leading to incorrect wattmeter readings or relay misoperation.
Power Frequency Withstand Voltage Test
Apply 50 kV RMS at 50 Hz between primary and grounded secondary/core for 1 minute. Leakage current must remain <10 mA. Use a calibrated test transformer with overcurrent trip set at 15 mA. This verifies insulation integrity after transport-induced microcracks.
Open-Circuit Characteristic Test
Gradually increase primary voltage from 0 to 1.5 × Un while measuring secondary voltage and excitation current. Plot Vs vs. Iexc. Knee point should occur above 1.2 × Un; premature saturation indicates core damage. Excitation current at Un must be ≤0.5% of rated primary current.
Preventive Maintenance Guide
Although cast-resin VTs are largely maintenance-free, periodic checks extend service life beyond 30 years.
Annual Visual and Functional Inspection
Inspect for surface tracking, UV degradation (chalky residue), or mechanical damage on bushings. Verify terminal tightness (torque: 15 N·m for M8 bolts). Perform insulation resistance and ratio tests annually. Clean surfaces with isopropyl alcohol if salt or dust deposits exceed 0.1 mg/cm² (per IEC 60815). Check secondary wiring for corrosion—especially in coastal areas—and ensure burden impedance matches nameplate ratings.
Five-Year Comprehensive Maintenance
Every 60 months, conduct partial discharge measurement using IEC 60270 methods. Acceptable level: ≤15 pC at 1.2 × Um/√3. Re-calibrate accuracy against a reference standard (uncertainty ≤0.05%). Inspect mounting hardware for galvanic corrosion if dissimilar metals are used. Replace silica gel in terminal boxes if present (though most JDZW-35 models are fully sealed).
Fault Diagnosis and Troubleshooting
Common issues include blown secondary fuses (indicating short-circuit burden), erratic meter readings (winding insulation failure), or overheating (excessive burden). If secondary voltage drops >5% under load, suspect core lamination shorting. Use thermal imaging to detect hotspots >10 K above ambient. Never operate with open secondary circuits—this induces dangerous overvoltages (>10 kV) due to capacitive coupling.
| Maintenance Interval | Activities |
|---|---|
| Annual | Visual inspection, IR test, ratio verification, terminal torque check |
| 5 Years | Partial discharge test, accuracy recalibration, mounting hardware inspection |
| After Fault | Full suite of on-site tests plus oil-free dielectric spectroscopy if available |
Conclusion
The JDZW-35 21kV cast-resin voltage transformer represents a benchmark in reliability, accuracy, and compliance for modern medium-voltage networks. Its VPI epoxy resin insulation eliminates the fire and environmental risks associated with oil-filled alternatives, while the GOES core ensures metrological precision across metering and protection burdens. Certified to both IEC 61869-3 and GB/T 20840.3, it bridges international and domestic requirements without compromise. Field-proven in harsh environments—from desert heat to industrial harmonic pollution—the JDZW-35 delivers consistent performance with minimal lifecycle costs. With a design life exceeding 30 years under standard service conditions, it offers exceptional total cost of ownership for utilities and industrial operators alike. Its compact form factor facilitates retrofitting into legacy switchgear, supporting grid modernization initiatives worldwide. As power systems evolve toward smarter, more resilient architectures, the JDZW-35 remains a foundational component for trustworthy voltage sensing.
Frequently Asked Questions (FAQ)
Q1: Can the JDZW-35 be used in a 20kV Chinese distribution system?
Yes. While rated for 21kV per IEC standards, the JDZW-35 is fully compatible with 20kV domestic systems (Um = 24 kV). The 5% voltage margin accommodates normal system fluctuations (19–21 kV), and GB/T 20840.3 certification ensures regulatory acceptance.
Q2: What is the maximum allowable secondary burden for protection applications?
For 3P-class protection windings, the maximum burden is 200 VA at 100 V secondary. Exceeding this increases composite error beyond ±3%, potentially causing relay miscoordination. Always verify burden impedance (Zb = V²/S) matches the VT’s rated output.
Q3: Is outdoor installation permitted without additional weatherproofing?
Yes. The JDZW-35 features IP54-rated terminal boxes and UV-stabilized resin, suitable for direct outdoor mounting. However, in heavy pollution zones (IEC 60815 Class D), periodic cleaning of bushings is recommended to maintain creepage distance effectiveness.
Q4: How does temperature affect accuracy?
Accuracy drift is ≤±0.05% per 10°C deviation from 20°C reference. Within the –25°C to +40°C range, total error remains within class limits. For critical metering, install in temperature-controlled enclosures if ambient exceeds +40°C.
Q5: Can multiple secondary windings share a common core?
No. The JDZW-35 uses separate, magnetically isolated cores for metering and protection windings to prevent burden interaction. This design ensures a short circuit on the protection circuit does not distort metering accuracy—a requirement per IEC 61869-3 Clause 7.4.