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High-Performance 21kV Cast-Resin Voltage Transformer JDJ-3 for Metering & Protection per IEC 61869-3
Introduction to the JDJ-3 Voltage Transformer
The JDJ-3 is a single-phase, indoor/outdoor-rated cast-resin voltage transformer (VT) engineered for reliable operation in medium-voltage electrical networks operating at a system voltage of 12 kV, with an IEC-standard rated insulation level of 21 kV. Designed in strict accordance with IEC 61869-3 and GB/T 20840.3, this instrument transformer reduces high primary voltages to standardized secondary levels—typically 100 V or 110 V—for safe interfacing with metering instruments, protective relays, and monitoring systems. Its robust construction and precision performance make it indispensable in modern power infrastructure.
Operating Principle of Cast-Resin Insulation
The JDJ-3 employs vacuum pressure impregnation (VPI) epoxy resin technology to fully encapsulate its magnetic core and windings. This process eliminates air voids and moisture ingress by saturating the coil assembly under vacuum before curing the resin under controlled pressure and temperature. The resulting monolithic structure provides superior dielectric strength, tracking resistance, and mechanical stability. Unlike oil-filled counterparts, the solid insulation system prevents leakage, fire hazards, and environmental contamination. The epoxy matrix also exhibits excellent thermal conductivity, enabling efficient heat dissipation during continuous operation at rated load. Dielectric tests confirm that the cast-resin body withstands lightning impulse voltages up to 170 kV peak and power frequency withstand voltages of 50 kV RMS for 1 minute—critical for 21 kV class equipment per IEC 61869-3.
Advantages Over Oil-Immersed Designs
Cast-resin VTs like the JDJ-3 offer significant operational and safety advantages over traditional oil-immersed units. First, they are maintenance-free—no oil sampling, degassing, or level checks are required. Second, their non-flammable construction meets stringent fire safety codes (e.g., IEC 60695), making them suitable for indoor switchgear and confined substations. Third, the absence of liquid insulation eliminates risks of oil degradation, sludge formation, and bushing leaks. Additionally, the compact footprint and lighter weight simplify transportation and mounting on bus supports or switchgear panels. Environmental resilience is enhanced: the JDJ-3 operates reliably from -40°C to +40°C ambient temperature and withstands humidity up to 95% without condensation-related failures. These attributes reduce lifecycle costs and improve grid reliability, particularly in urban or industrial settings where space and safety are constrained.
Typical Applications Overview
The JDJ-3 is deployed across diverse medium-voltage infrastructure where accurate voltage transformation and long-term reliability are paramount. Primary use cases include utility substations for revenue metering and feeder protection, industrial plants requiring precise energy monitoring, renewable integration points (e.g., solar farms feeding into 12 kV grids), and rural distribution networks with limited maintenance access. Its dual compliance with international (IEC) and Chinese national (GB) standards ensures interoperability in global projects. The transformer’s high accuracy class (0.2 or 0.5 for metering; 3P or 6P for protection) supports both fiscal-grade billing and fast-acting relay coordination. With a typical service life exceeding 25 years under standard conditions, the JDJ-3 represents a cost-effective, future-proof solution for critical voltage sensing applications.
Technical Specifications
The JDJ-3 voltage transformer is engineered to meet exacting electrical and environmental parameters defined by IEC 61869-3 and GB/T 20840.3. Below is a comprehensive specification table followed by detailed contextual explanations.
| Parameter | Value |
|---|---|
| Primary Rated Voltage (Up) | 21 kV (IEC), 20 kV (Domestic) |
| Secondary Rated Voltage (Us) | 100 V or 110 V (single secondary); optional dual secondaries available |
| Voltage Ratio | 21,000 / √3 : 100 / √3 V or 20,000 / √3 : 110 / √3 V (for grounded systems) |
| Accuracy Class | Metering: 0.2, 0.5; Protection: 3P, 6P |
| Rated Output (per secondary) | 10 VA, 15 VA, 30 VA, 50 VA (at specified accuracy class) |
| Insulation Level (IEC) | 21/√3 kV (phase-to-earth); LI 170 kV; AC 50 kV (1 min) |
| Core Material | Grain-oriented (GOES) silicon steel, 0.27 mm thickness |
| Burden Power Factor | 0.8 lagging (standard) |
| Temperature Rise Limit | ≤ 60 K (windings, measured by resistance method) |
| Ambient Operating Temp. | -40°C to +40°C |
| Altitude Limit | ≤ 1,000 m above sea level (derating required above) |
| Humidity | Up to 95% RH, non-condensing |
| Polarity | Reducing polarity (standard marking: H1–X1) |
Electrical Performance Parameters
The JDJ-3’s voltage ratio tolerance is tightly controlled to ±0.2% for 0.2-class units and ±0.5% for 0.5-class, ensuring metrological integrity in revenue metering. Phase error remains within ±10 minutes for 0.2 class and ±20 minutes for 0.5 class at 80–120% of rated voltage. For protection applications, the 3P class guarantees composite error ≤3% at 5% to 100% of rated voltage and up to 100% of rated burden, critical for distance and overvoltage relays. The GOES core minimizes hysteresis and eddy current losses, achieving no-load loss ≤15 W and short-circuit impedance ≤0.3%. Secondary terminals are rated for 10 A continuous current, with thermal withstand capability of 100 A for 1 second during fault conditions. All terminals comply with IEC 61869-1 terminal marking conventions (H1, H2 for primary; X1, X2 for secondary).
Environmental and Mechanical Ratings
Designed for both indoor and outdoor service, the JDJ-3 features a UV-stabilized epoxy housing with hydrophobic surface properties to resist pollution-induced flashovers. Creepage distance exceeds 25 mm/kV (medium pollution class III per IEC 60815), totaling ≥525 mm for 21 kV systems. Mounting is via M12 stainless steel bolts on a standardized flange (dimensions per IEC 61869-3 Annex B). The unit weighs approximately 45 kg and measures 420 mm (H) × 280 mm (W) × 220 mm (D). At altitudes above 1,000 m, the dielectric strength must be derated by 1% per 100 m increment; for example, at 2,000 m, the power frequency withstand voltage reduces to 45 kV. Condensation is prevented through integrated silica gel breathers in outdoor variants or sealed construction in indoor models.
Typical Applications
The JDJ-3 voltage transformer serves as a foundational component in modern power systems requiring precise, reliable voltage measurement and protection signaling. Its design flexibility accommodates a wide range of operational environments.
Substation Secondary Metering
In 12 kV/21 kV utility substations, the JDJ-3 provides the reference voltage for kilowatt-hour meters, power quality analyzers, and SCADA RTUs. Installed on each phase of the busbar, it enables three-phase metering with vector group Yn/yn. For revenue-grade applications, 0.2-class units ensure billing accuracy within ±0.2%, meeting regulatory requirements such as EN 50470-3. The low phase displacement (<10 arcmin) prevents reactive energy errors in tariff calculations. In digital substations, the secondary output interfaces directly with IEC 61850-9-2 LE sampled value networks via merging units. In coastal zones, inspect for salt creepage every 6 months.
Five-Year Comprehensive Assessment
Every 60 months, perform:
– **Partial discharge (PD) measurement**: Using IEC 60270 methods, PD should be <10 pC at 1.2× Um/√3. Elevated PD indicates internal voids.
– **Dielectric spectroscopy**: Assess moisture content via tan δ measurement at 0.1–100 Hz. Tan δ >0.5% suggests aging.
– **Burden verification**: Confirm connected load does not exceed rated VA (e.g., 30 VA). Overburden causes ratio errors.
Replace units showing irreversible degradation. Most JDJ-3 units require no intervention beyond year 10 if operated within ratings.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicators |
|---|---|---|
| Annually | Visual inspection, IR test, ratio check | Cracks, IR <500 MΩ, ratio error >1% |
| 5 Years | PD test, tan δ, burden audit | PD >20 pC, tan δ >1%, overheating terminals |
| 10+ Years | Full re-certification or replacement | Recurring ratio drift, visible carbon tracking |
Common failure modes include:
– **Terminal corrosion**: Caused by galvanic mismatch; use tin-plated lugs.
– **Resin delamination**: From thermal cycling; mitigated by proper ventilation.
– **Secondary short-circuit**: Blows fuses; install 10 A HRC fuses per IEC 60282.
Conclusion
The JDJ-3 cast-resin voltage transformer represents a benchmark in medium-voltage instrumentation, combining precision engineering with rugged reliability for 21 kV (IEC) / 20 kV (domestic) systems. Its VPI epoxy encapsulation eliminates the fire, environmental, and maintenance liabilities inherent in oil-filled alternatives, while grain-oriented silicon steel cores ensure metrological accuracy across metering (0.2/0.5 class) and protection (3P/6P) applications. Full compliance with IEC 61869-3 and GB/T 20840.3 guarantees global acceptance and interoperability, supported by rigorous type and routine testing protocols. Field-proven in substations, industrial plants, renewable farms, and rural networks, the JDJ-3 delivers consistent performance under extreme temperatures, humidity, and pollution. With a design life of 25–30 years and minimal preventive maintenance requirements, it offers exceptional lifecycle value. Engineers specifying the JDJ-3 gain a future-ready solution that meets today’s grid demands for safety, accuracy, and sustainability—making it a cornerstone of resilient power infrastructure worldwide.