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High-Performance 21kV Cast-Resin Voltage Transformer JDZW-20 for Metering & Protection per IEC 61869-3
Introduction to the JDZW-20 Voltage Transformer
The JDZW-20 is a single-phase, indoor/outdoor rated, cast-resin insulated voltage transformer (VT) designed for accurate voltage transformation in medium-voltage power systems operating at a nominal system voltage of 21kV (IEC standard), corresponding to the domestic 20kV network classification. This device serves critical roles in both revenue metering and protective relaying applications within distribution substations, industrial facilities, and renewable energy interconnection points.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the JDZW-20 employs vacuum pressure impregnation (VPI) technology using cycloaliphatic epoxy resin systems. The primary and secondary windings—constructed from high-purity electrolytic copper—are wound concentrically around a grain-oriented electrical steel (GOES) core. The entire assembly is then encapsulated under vacuum to eliminate air voids, followed by pressure curing to ensure complete resin penetration. This process yields a monolithic, void-free dielectric structure with superior partial discharge resistance (<5 pC at 1.2 × Um/√3), excellent thermal conductivity (0.8–1.2 W/m·K), and high mechanical strength. Unlike oil-filled units, the solid insulation eliminates fire hazards, environmental contamination risks, and the need for periodic oil testing or tank maintenance.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed VTs, the JDZW-20 offers significant operational and safety benefits. Its dry-type construction enables installation in confined spaces such as switchgear rooms, underground vaults, or urban substations where flammability codes restrict oil-filled equipment. The absence of liquid insulation eliminates leakage concerns, reduces lifecycle costs, and simplifies disposal at end-of-life. Thermally, the epoxy matrix provides stable performance across –40°C to +40°C ambient ranges with a temperature rise limit of ≤60 K above ambient under rated load. Additionally, the compact footprint (typically 30% smaller than equivalent oil units) facilitates retrofitting into existing bays without structural modifications.
Typical Applications Overview
The JDZW-20 is engineered for dual-purpose use: Class 0.2 or 0.5 accuracy windings support precision energy metering for billing compliance, while Class 3P or 6P windings provide robust fault-level signals for overvoltage, undervoltage, and directional protection schemes. Common deployment scenarios include 20/21kV ring main units (RMUs), pad-mounted transformers, solar farm collector substations, and industrial motor control centers. Its robust design withstands transient overvoltages up to 125 kV peak (BIL rating), making it suitable for networks with frequent switching surges or lightning exposure.
Technical Specifications
The JDZW-20 adheres strictly to IEC 61869-3 and GB/T 20840.3, ensuring global interoperability and performance consistency. Below are its core electrical and mechanical parameters.
Rated Electrical Parameters
| Parameter | Value |
|---|---|
| System Voltage (Um) | 21 kV (IEC), 20 kV (domestic) |
| Primary Voltage (Up) | 21/√3 kV = 12.12 kV (phase-to-ground) |
| Secondary Voltage(s) | 100/√3 V (metering), 100 V (protection) |
| Voltage Ratio | 12120 / (100/√3) : 100 |
| Accuracy Classes | Metering: 0.2, 0.5; Protection: 3P, 6P |
| Rated Output (per winding) | 30 VA (0.2 class), 50 VA (0.5 class), 100 VA (3P/6P) |
| Burden Power Factor | 0.8 lagging (standard) |
| Insulation Level (LI/AC) | 125 kV / 50 kV (1 min) |
| Partial Discharge | <5 pC at 1.2 × Um/√3 |
Standard Service Conditions
The JDZW-20 is rated for continuous operation under the following environmental conditions per IEC 60060-1: ambient temperature range of –40°C to +40°C, relative humidity up to 95% (non-condensing), and installation altitude not exceeding 1000 m above sea level. For altitudes between 1000–3000 m, derating factors apply: 1.25% reduction in rated voltage per 100 m above 1000 m. The unit is suitable for both indoor (IP2X) and outdoor (IP54 with optional rain shield) installations. Seismic withstand capability meets IEC 60068-2-57 (0.3g horizontal acceleration). Terminal clearances comply with IEC 61439-1 for safe live-part separation.
Construction and Materials
The magnetic core uses CRGO (Cold-Rolled Grain-Oriented) silicon steel laminations (M4 grade, 0.3 mm thickness) with flux density limited to 1.6 T at rated voltage to minimize hysteresis losses. Windings feature Class F (155°C) insulation with double-layer enamel coating. The epoxy resin system includes UV stabilizers for outdoor durability and hydrophobic additives to resist surface tracking. Primary terminals are tinned copper studs rated for 630 A short-time current; secondary terminals use screw-type connectors compatible with 2.5–6 mm² stranded copper conductors. Overall dimensions: height ≈ 680 mm, base footprint ≈ 220 × 220 mm, weight ≈ 45 kg.
Typical Applications
The JDZW-20’s dual-accuracy windings and robust insulation make it ideal for diverse medium-voltage infrastructure.
Substation Secondary Metering
In 20/21kV distribution substations, the JDZW-20 supplies scaled-down voltages to revenue-class kWh meters and power quality analyzers. The 0.2-class secondary winding ensures measurement uncertainty below ±0.2% at 20–100% of rated burden, meeting EN 50163 and IEEE C57.13 requirements for billing accuracy. For example, in a municipal utility substation feeding commercial districts, three JDZW-20 units (one per phase) connect to a three-phase meter via shielded twisted-pair cables (min. 2.5 mm² cross-section) to minimize capacitive coupling errors. Grounding of the secondary neutral at a single point prevents circulating currents that could distort readings.
Industrial Power Distribution
Within heavy-industry facilities—such as steel mills or chemical plants—the JDZW-20 interfaces with multifunction relays (e.g., SEL-751, Siemens 7SJ62) for undervoltage lockout during motor restart sequences. The 3P-class protection winding maintains ratio error within ±3% and phase displacement under ±120′ at 5%–100% of rated voltage, even during deep sags. In arc-furnace environments with high harmonic distortion, the low-leakage-reactance design (<0.5%) minimizes resonance with capacitor banks. Units are typically mounted on bus duct supports with vibration-damping brackets to mitigate mechanical stress from nearby transformers.
Renewable Energy Integration
Solar photovoltaic (PV) farms operating at 21kV utilize the JDZW-20 for grid-synchronization and anti-islanding protection. During cloud transients, rapid voltage fluctuations require VTs with low remanence (<0.2 T) to avoid saturation-induced waveform distortion. The JDZW-20’s GOES core and air gaps (if applicable) ensure linear response down to 0.1 × Un. In a 10 MW solar plant, JDZW-20 units feed synchrocheck relays that verify grid voltage magnitude, frequency, and phase angle before closing the main breaker. Secondary circuits include surge arresters (e.g., 275 V MOVs) to clamp induced transients from DC-side switching.
Rural and Suburban Distribution Networks
In remote areas with long feeder lines, the JDZW-20 supports voltage regulation via automatic tap changers (ATCs) and capacitor bank controllers. Its extended thermal stability allows continuous operation at 110% of rated voltage during light-load periods—a common condition in agricultural zones. For pole-top installations, the unit mounts directly to crossarms using stainless-steel clamps, with secondary leads routed through conduit to ground-level meter cabinets. The IP54 rating protects against dust ingress and driving rain, critical in monsoon-prone regions.
Backup Protection in Ring Main Units
Within SF6-insulated RMUs, the JDZW-20 provides voltage signals for earth-fault detection in compensated (Petersen coil) networks. The 6P-class winding delivers sufficient output at 5% residual voltage to trigger sensitive earth-fault relays (e.g., 59N). Its compact size fits within standard RMU compartments alongside load-break switches, eliminating the need for external VT cubicles. Secondary wiring follows IEC 61850-9-2 LE protocols when interfacing with digital relays via merging units.
Compliance with International Standards
The JDZW-20 is certified to both international and Chinese national standards, ensuring compatibility across global markets.
IEC 61869-3 Compliance Details
IEC 61869-3 specifies performance, testing, and marking requirements for inductive voltage transformers. The JDZW-20 meets all clauses, including: accuracy verification under sinusoidal waveforms (Clause 6.3), temperature rise limits (Clause 6.5), short-circuit withstand (Clause 6.7), and electromagnetic compatibility (Clause 6.10). Key tests include ratio error measurement at 20%, 50%, 80%, 100%, and 120% of rated voltage, with results documented in calibration certificates traceable to national labs. Markings include polarity dots (reducing polarity), accuracy class, rated output, and serial number per Clause 8.
GB/T 20840.3 Alignment
GB/T 20840.3 is China’s adoption of IEC 61869-3 with minor national deviations. The JDZW-20 complies fully, including the requirement for a 1-minute power-frequency withstand test at 50 kV (vs. IEC’s 46 kV for 24 kV class). Domestic certification (CQC mark) mandates additional salt-fog testing (IEC 60068-2-11) for coastal installations and stricter partial discharge limits (<3 pC) for Class 0.2 units used in state-grid metering. All materials meet RoHS 2 (EU 2015/863) and China RoHS II restrictions on hazardous substances.
Key Differences Between IEC and Domestic Standards
While harmonized, notable differences exist: GB/T requires secondary terminal short-circuit duration of 3 seconds (vs. IEC’s 1 second), reflecting higher fault current expectations in Chinese grids. Also, GB/T mandates a 1.9 × Un/√3 overvoltage test for 30 seconds to simulate ferroresonance conditions—absent in IEC. Conversely, IEC places greater emphasis on EMC immunity (IEC 61000-4 series), requiring immunity to 10 V/m RF fields and 4 kV ESD. The JDZW-20 incorporates ferrite beads on secondary leads and grounded electrostatic shields to satisfy both regimes.
On-Site Testing Procedures
Post-installation verification ensures the JDZW-20 performs within specification before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary winding and ground, secondary windings and ground, and primary-to-secondary. Acceptance criteria: ≥1000 MΩ at 20°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings indicate moisture ingress or resin cracking. Perform before and after the power-frequency withstand test 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 voltmeter (±0.1% accuracy). Calculate ratio error: [(Vp/Vs)measured – (Vp/Vs)nominal] / (Vp/Vs)nominal × 100%. Tolerance: ±0.2% for 0.2-class, ±0.5% for 0.5-class, ±3% for 3P-class at rated burden. Use a dedicated turns ratio tester (e.g., Omicron CT Analyzer) for automated comparison across multiple taps.
Polarity Test
Verify reducing polarity per IEC 61869-3 Figure 3. Connect a 6–12 V DC battery momentarily between primary terminals (H1+, H2–). Observe secondary voltage polarity with a DC voltmeter: positive deflection at X1 indicates correct reducing polarity. Incorrect polarity causes 180° phase shift, leading to relay misoperation or meter reversal. Document results with oscillograms if using AC methods.
Power Frequency Withstand Voltage Test
Apply 50 kV RMS (for 21kV class) at 50 Hz between primary and ground/secondary for 1 minute. Ramp voltage at 2 kV/s to avoid transient overstress. Monitor for flashover, excessive leakage current (>1 mA), or audible discharge. Post-test, repeat insulation resistance measurement—values must not drop below 70% of pre-test reading. Use a calibrated test transformer with overcurrent trip set at 1.5 × expected capacitive current.
Open-Circuit Characteristic Test
With secondary open, gradually increase primary voltage from 0 to 190 V (≈1.5 × rated secondary equivalent). Plot excitation current vs. voltage. Knee-point voltage should exceed 1.5 × Un (i.e., >150 V). Excessive magnetizing current at low voltages indicates core lamination damage or shorted turns. Compare curves to factory baseline; deviation >10% warrants further investigation.
Preventive Maintenance Guide
Regular maintenance extends service life beyond 25 years and prevents unexpected failures.
Periodic Inspection Protocol
Conduct visual and thermographic inspections annually. Check for: surface cracks or tracking marks on resin housing, corrosion on terminals, loose mounting bolts (torque: 25 N·m), and discoloration indicating overheating. Use an infrared camera to scan under 50% load—hot spots >10 K above ambient suggest poor connections or internal faults. Clean housing with non-abrasive detergent; avoid solvents that degrade epoxy. Verify secondary circuit integrity: no open neutrals, proper grounding (≤0.1 Ω resistance), and intact shielding.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicators |
|---|---|---|
| Annual | Visual inspection, IR scan, terminal torque check | Cracks, discoloration, hot spots |
| 5 Years | Insulation resistance, ratio/polarity retest | Ratio error drift, low IR |
| 10 Years | Partial discharge measurement (offline) | PD >10 pC at Um/√3 |
| As Needed | Secondary burden verification | Metering inaccuracies, relay false trips |
Common failure modes include: (1) Resin delamination due to thermal cycling—detected by ultrasonic testing; (2) Secondary winding open-circuit from vibration fatigue—identified by infinite resistance; (3) Core saturation from DC offset—diagnosed via distorted secondary waveform on oscilloscope. Replace unit if PD exceeds 20 pC or ratio error exceeds twice the accuracy class limit.
Conclusion
The JDZW-20 21kV cast-resin voltage transformer represents a technically mature solution for modern medium-voltage networks requiring reliable, accurate, and maintenance-free voltage transformation. Its compliance with IEC 61869-3 and GB/T 20840.3 ensures consistent performance across international and domestic grids, while the VPI epoxy resin insulation provides inherent safety advantages over oil-filled alternatives. With dual-accuracy windings supporting both Class 0.2 metering and Class 3P/6P protection, the JDZW-20 eliminates the need for separate VTs in many applications, reducing capital expenditure and panel space. Rigorous factory testing—including partial discharge, ratio error, and power-frequency withstand—guarantees reliability under transient overvoltages and continuous thermal stress. When installed and maintained per the guidelines outlined, the JDZW-20 delivers a service life exceeding 25–30 years, even in harsh industrial or outdoor environments. Its compact design, seismic resilience, and compatibility with digital substation architectures (via analog outputs to merging units) position it as a future-proof choice for utilities investing in grid modernization. For engineers specifying VTs in 20/21kV systems, the JDZW-20 offers an optimal balance of precision, durability, and lifecycle cost efficiency.