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For Substation Metering & Protection: JDJ-3 11kV Cast-Resin Voltage Transformer per IEC 61869-3
Introduction to the JDJ-3 Voltage Transformer
The JDJ-3 is a single-phase, indoor-type cast-resin voltage transformer (VT) engineered for precise voltage transformation in 11kV (IEC-rated) or 10kV (domestic system) medium-voltage networks. Designed in strict accordance with IEC 61869-3 and GB/T 20840.3, it serves dual roles in revenue-grade metering and protective relaying applications. Its construction leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate high-purity grain-oriented electrical steel (GOES) cores and copper windings, ensuring exceptional dielectric integrity, mechanical robustness, and long-term thermal stability.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the JDJ-3 employs a two-component cycloaliphatic epoxy resin system cured under controlled vacuum and pressure conditions. This VPI process eliminates air voids and moisture ingress pathways, resulting in a homogeneous solid dielectric with a relative permittivity (εr) of approximately 3.8–4.2 and volume resistivity exceeding 1×1014 Ω·cm at 20°C. Unlike oil-filled counterparts, the solid resin matrix provides intrinsic fire resistance (compliant with IEC 60695 glow-wire tests), zero risk of leakage, and immunity to environmental contaminants such as dust, humidity, and chemical vapors. The resin’s coefficient of thermal expansion closely matches that of copper and steel, minimizing mechanical stress during thermal cycling from –25°C to +40°C ambient operation.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed VTs, the JDJ-3 offers significant operational and safety benefits. It requires no oil containment systems, eliminating fire hazards and environmental remediation risks—critical for indoor substations near public infrastructure. Maintenance is drastically reduced: no oil sampling, degassing, or level checks are needed. The compact footprint (typically 280 mm height × 180 mm width × 150 mm depth) enables space-efficient integration into metal-enclosed switchgear (e.g., KYN28A-12). Furthermore, the absence of liquid insulation prevents partial discharge (PD) activity under normal operating voltages; factory PD levels are maintained below 5 pC at 1.2 × Um/√3, per IEC 61869-3 Clause 10.3. This enhances reliability in polluted or high-humidity environments where oil degradation accelerates.
Typical Application Overview
The JDJ-3 is predominantly deployed in 10kV/11kV distribution substations across urban, industrial, and renewable energy sectors. Its primary function is to step down system voltage to standardized secondary values (typically 100 V or 100/√3 V) for interfacing with watt-hour meters, power quality analyzers, and protective relays (e.g., overvoltage, undervoltage, and directional earth-fault schemes). Due to its high accuracy class (0.2 for metering, 3P for protection), it supports both fiscal metering compliance and fast, dependable fault detection. Common installations include ring main units (RMUs), pad-mounted transformers, and utility-owned secondary substations feeding commercial complexes or manufacturing plants.
Technical Specifications
The JDJ-3 voltage transformer is engineered to deliver consistent performance under defined service conditions while meeting stringent international accuracy and insulation requirements. Below is a comprehensive specification table followed by environmental and operational parameters.
| Parameter | Value |
|---|---|
| Primary Rated Voltage (Up) | 11 kV (IEC) / 10 kV (GB) |
| Secondary Rated Voltage (Us) | 100 V or 100/√3 V |
| Voltage Ratio | 11000/100 V or 11000/100/√3 V |
| Accuracy Class (Metering) | 0.2 (per IEC 61869-3) |
| Accuracy Class (Protection) | 3P (composite error ≤ 3% at 5%–100% rated voltage) |
| Rated Output (Burden) | 30 VA (metering), 50 VA (protection) |
| Insulation Level (LI/AC) | 75 kV / 28 kV (1 min, 50 Hz) |
| Short-Time Thermal Withstand | 1 s at 16 kA (symmetrical) |
| Core Material | Grain-Oriented Electrical Steel (GOES), M4 grade |
| Insulation System | VPI Epoxy Resin, UL 94 V-0 rated |
| Ambient Temperature Range | –25°C to +40°C |
| Relative Humidity | ≤ 95% (non-condensing) |
| Maximum Altitude | 1000 m above sea level (derating required >1000 m) |
Standard Service Conditions
The JDJ-3 is rated for continuous operation under standard service conditions defined in IEC 61869-3 Clause 5. These include an ambient temperature range of –25°C to +40°C, with a 24-hour average not exceeding +35°C. Relative humidity may reach 95% provided condensation does not occur—a critical consideration for coastal or tropical installations. At altitudes exceeding 1000 m, the dielectric strength of air decreases by approximately 1% per 100 m; thus, for installations at 2000 m, the power frequency withstand voltage must be derated to 25.2 kV (90% of 28 kV). The transformer is designed for three-phase systems with neutral grounding via Petersen coils or low-resistance grounding, ensuring stable secondary voltage even during single-line-to-ground faults.
Accuracy and Burden Characteristics
Accuracy performance is tightly controlled across the specified burden range. For the 0.2 class, voltage error must remain within ±0.2% and phase displacement within ±10 minutes at 80%–120% of rated voltage and 25%–100% of rated burden. The 3P protection class guarantees composite error ≤3% when subjected to voltages between 5% and 100% of rated value under burdens up to 50 VA. Exceeding the rated burden (e.g., connecting multiple meters without load calculation) increases magnetizing current, leading to saturation and accuracy degradation. Therefore, total connected burden—including wiring resistance—must be verified during commissioning using a burden tester calibrated to IEC 61869-3 Annex D.
Typical Applications
The JDJ-3 voltage transformer is a versatile component in modern medium-voltage infrastructure, supporting both precision measurement and system protection across diverse sectors.
Substation Secondary Metering
In utility-owned 10kV/11kV distribution substations, the JDJ-3 provides the reference voltage signal for revenue metering cabinets. Connected to class 0.2S or 0.5S kWh meters, it ensures billing accuracy compliant with national regulatory frameworks (e.g., China’s DL/T 448). The transformer’s low phase displacement (<8 minutes at 100% burden) minimizes reactive energy measurement errors. Installation typically involves mounting directly onto the busbar compartment of metal-clad switchgear, with secondary leads routed through shielded cables to prevent electromagnetic interference from adjacent current transformers or circuit breakers.
Industrial Power Distribution Systems
Large manufacturing facilities often operate their own 10kV internal distribution networks. Here, the JDJ-3 feeds voltage inputs to multifunction power meters (e.g., for demand monitoring) and protection relays such as SEL-351 or Siemens 7SJ62. In motor control centers (MCCs), it enables undervoltage release coordination during grid disturbances. The cast-resin design is particularly advantageous in environments with high ambient temperatures (e.g., steel mills) or chemical exposure (e.g., petrochemical plants), where oil-filled units would degrade rapidly. Thermal stability up to 130°C hotspot temperature ensures decades of service without insulation breakdown.
Renewable Energy Integration
Solar photovoltaic (PV) and wind farms frequently connect to the grid via 10kV/35kV step-up transformers. At the point of interconnection, JDJ-3 units supply voltage signals to synchrophasors (PMUs) and anti-islanding protection relays. During grid faults, the VT must maintain output fidelity to enable accurate fault location and ride-through compliance per GB/T 19964. The 3P accuracy class ensures that voltage sag detection remains reliable even at 5% of nominal voltage—a critical requirement for low-voltage ride-through (LVRT) schemes. Additionally, the absence of flammable materials aligns with fire safety codes in containerized inverter stations.
Rural and Suburban Distribution Networks
In remote or semi-urban areas, pole-mounted or pad-mounted 10kV transformers often incorporate JDJ-3 VTs for local metering and feeder protection. These installations benefit from the unit’s maintenance-free operation—essential where access is limited. The compact size allows integration into compact secondary substations serving residential clusters. During seasonal load variations (e.g., agricultural irrigation peaks), the VT’s linear response across 20%–120% voltage range ensures consistent metering accuracy without recalibration. Furthermore, its resistance to pollution (IP2X rating on terminals) prevents flashovers in dusty or saline coastal regions.
Compliance with International Standards
The JDJ-3 voltage transformer is certified to both global and Chinese national standards, ensuring interoperability and regulatory acceptance across markets.
IEC 61869-3 Compliance Details
IEC 61869-3:2011 specifies requirements for inductive voltage transformers for measurement and protection. The JDJ-3 meets all essential clauses, including insulation coordination (Clause 7), temperature rise limits (≤60 K for resin, measured by resistance method), and short-circuit withstand capability. Dielectric tests include 1-minute AC withstand at 28 kV and lightning impulse at 75 kV (1.2/50 μs waveform). Accuracy verification follows Annex C procedures using calibrated ratio bridges with uncertainty ≤0.02%. Partial discharge measurements are conducted at 1.2 × Um/√3 = 7.6 kV, with acceptance criteria of ≤5 pC—well below the 20 pC limit for resin-insulated VTs.
Alignment with GB/T 20840.3
GB/T 20840.3-2013 is China’s adoption of IEC 61869-3 with minor national deviations. Key differences include stricter altitude derating rules (mandatory correction factor application above 1000 m) and additional type tests for seismic resilience (required for installations in earthquake-prone zones like Sichuan). The JDJ-3 undergoes supplementary testing per DL/T 726, including 1000 thermal cycles (–25°C to +70°C) to validate resin-core adhesion. Accuracy classes use identical nomenclature (0.2, 3P), but burden definitions reference VA at cosφ = 0.8 lagging—consistent with IEC practice. Certification is issued by authorized bodies such as CEPREI or TÜV Rheinland China.
Testing and Certification Requirements
Type testing per IEC 61869-3 includes 12 mandatory tests: temperature rise, short-circuit, insulation, accuracy, PD, impulse, wet test (for outdoor types), and more. The JDJ-3, being indoor-only, is exempt from wet tests but must pass IP2X ingress protection verification on terminals. Routine tests on every unit include turns ratio (±0.2% tolerance), polarity (reducing polarity confirmed), and 3-second AC withstand at 3 kV on secondary windings. Factory test reports include traceable calibration certificates for ratio and phase error. Third-party certification (e.g., CE marking under LVD 2014/35/EU) requires annual surveillance audits of the manufacturing facility’s quality management system (ISO 9001).
On-Site Testing Procedures
Post-installation verification ensures the JDJ-3 performs within specifications before energization. All tests must follow IEC 60060-1 and site safety protocols (lockout/tagout, grounding).
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary winding and ground, and between primary and secondary windings. Acceptance criterion: ≥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 and require investigation. Perform before and after AC withstand tests to detect insulation damage.
Turns Ratio Test
Apply a low-voltage AC signal (e.g., 100 V) to the primary and measure secondary voltage with a calibrated true-RMS voltmeter. Calculate actual ratio and compare to nameplate (e.g., 110:1). Tolerance: ±0.2% for 0.2-class VTs. Deviations >0.5% suggest winding shorts or incorrect tap selection. Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for automated comparison against factory data.
Polarity Test
Verify reducing polarity using the DC kick method: briefly apply 6–12 V DC to primary (H1+ to H2–); observe momentary deflection on a center-zero galvanometer connected to secondary (X1 to X2). Positive kick at X1 confirms correct polarity. Incorrect polarity causes 180° phase reversal, leading to metering errors or relay misoperation. Document results with oscillograms if digital relays are used.
Power Frequency Withstand Voltage Test
Apply 28 kV AC (RMS, 50 Hz) between primary and grounded enclosure for 1 minute. Monitor for flashover, excessive current (>1 mA), or audible discharge. Use a calibrated HV test set with overcurrent trip. Reduce voltage gradually post-test. This validates insulation integrity after transport and installation stresses. Do not perform if insulation resistance is <500 MΩ.
Open-Circuit Characteristic Test
With secondary open, apply 0–120% of rated primary voltage in 10% increments. Record excitation current and secondary voltage. Plot Vs vs. Iexc; knee point should exceed 150% rated voltage. Excessive excitation current at 100% voltage indicates core saturation due to manufacturing defect or mechanical damage. Compare curve to factory baseline—deviation >10% warrants rejection.
Preventive Maintenance Guide
Although cast-resin VTs are largely maintenance-free, periodic inspections extend service life and prevent unexpected failures.
Annual Visual and Electrical Inspection
Annually, inspect for physical damage (cracks, discoloration), terminal corrosion, and loose connections. Clean dust with dry, oil-free compressed air (≤0.3 MPa). Measure insulation resistance and secondary burden impedance. Verify secondary fuse continuity (typically 2 A slow-blow for 30 VA burden). Check for abnormal heating using infrared thermography—hotspots >10 K above ambient indicate high-resistance joints or core issues. Document findings in asset management software.
Five-Year Comprehensive Maintenance
Every five years, perform full electrical retesting: turns ratio, polarity, insulation resistance, and open-circuit characteristics. Inspect mounting hardware for torque relaxation (re-torque to 15 N·m per M8 bolts). Examine cable glands for seal integrity. If installed in high-pollution areas (e.g., near cement plants), clean resin surface with isopropyl alcohol to remove conductive deposits. Replace secondary terminal blocks if oxidation exceeds 0.1 mm depth. Update test records and compare trends against baseline.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicator |
|---|---|---|
| Annually | Visual check, IR scan, insulation test | Cracks, hotspots >10 K, Rins < 500 MΩ |
| 5 Years | Full electrical retest, terminal cleaning | Ratio error >0.5%, excitation current ↑20% |
| After Fault | Post-fault inspection, PD check if possible | Flashover marks, burnt odor, relay misoperation |
Common faults include secondary short circuits (causing core saturation and overheating) and primary insulation failure due to prolonged overvoltage. If accuracy degrades suddenly, suspect winding turn-to-turn shorts—confirmed by elevated excitation current at low voltage.
Conclusion
The JDJ-3 11kV cast-resin voltage transformer represents a mature, reliable solution for metering and protection in medium-voltage networks worldwide. Its VPI epoxy resin encapsulation delivers superior dielectric performance, fire safety, and environmental resilience compared to legacy oil-filled designs. Engineered to IEC 61869-3 and GB/T 20840.3, it guarantees accuracy class 0.2 for revenue metering and 3P for dependable protection coordination. With a typical service life of 25–30 years under standard conditions, the JDJ-3 minimizes lifecycle costs through zero fluid maintenance, compact installation, and robust fault withstand capability. Its compatibility with modern digital substations—via stable analog outputs for merging units—ensures relevance in evolving grid architectures. For utilities and industrial operators seeking a maintenance-free, code-compliant VT for 10kV/11kV systems, the JDJ-3 remains a technically sound and economically justified choice.
Frequently Asked Questions (FAQ)
Q1: Can the JDJ-3 be used outdoors?
No. The JDJ-3 is rated for indoor use only (IP00 on body, IP2X on terminals). Outdoor applications require a weatherproof enclosure or an outdoor-rated VT model.
Q2: What is the maximum allowable secondary burden?
For metering (0.2 class), max burden is 30 VA at cosφ = 0.8 lagging. For protection (3P), it is 50 VA. Exceeding these values degrades accuracy and may cause saturation.
Q3: How do I verify polarity during installation?
Use the DC kick test: apply momentary DC to primary (H1+, H2–); a positive deflection on a galvanometer connected to X1–X2 confirms reducing polarity.
Q4: Is the JDJ-3 compatible with digital relays?
Yes. Its stable secondary output (100 V or 100/√3 V) interfaces directly with conventional inputs of digital relays like SEL-421 or GE UR series.
Q5: What happens if the secondary is left open during operation?
Unlike CTs, VT secondaries can be safely open-circuited. However, always ensure fuses or miniature circuit breakers protect against short circuits.
Q6: Can I install the JDJ-3 vertically or horizontally?
The JDJ-3 must be mounted vertically with primary terminals upward, as specified in the installation manual. Horizontal mounting affects heat dissipation and may void warranty.
Q7: What is the recommended torque for primary busbar connections?
Apply 15 N·m to M8 bolts. Under-torquing causes hotspots; over-torquing risks cracking the resin housing.
Q8: Does the JDJ-3 require periodic oil sampling?
No. Being cast-resin insulated, it contains no oil and requires no fluid maintenance—only visual and electrical inspections per the preventive maintenance schedule.