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JWD-10 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified

Introduction to the JWD-10 Current Transformer

The JWD-10 is a medium-voltage, cast-resin insulated current transformer (CT) engineered for high-accuracy current measurement and dependable protective relay coordination in 11kV (IEC standard) or 10kV (Chinese domestic grid equivalent) power systems. Designed for both indoor and outdoor substation environments, this device leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and secondary windings, ensuring long-term dielectric integrity, mechanical robustness, and resistance to environmental stressors such as moisture, dust, and thermal cycling.

Operating Principle of Cast-Resin Insulation

Cast-resin insulation in the JWD-10 employs a two-stage VPI process where the primary conductor, toroidal GOES (grain-oriented electrical steel) core, and secondary windings are fully encapsulated in cycloaliphatic epoxy resin under vacuum and pressure. This eliminates air voids and microcavities that could lead to partial discharges—a critical failure mode in high-voltage insulation systems. The resulting monolithic structure provides a uniform electric field distribution, with a typical partial discharge level below 10 pC at 1.2 × U_m/√3 (where U_m = 12 kV). The resin’s high tracking index (>600 V per IEC 60112) ensures surface flashover resistance even under polluted conditions, making it suitable for coastal or industrial atmospheres.

Advantages Over Oil-Immersed Designs

Unlike traditional oil-filled CTs, the JWD-10 eliminates fire hazards, oil leakage risks, and maintenance-intensive sealing systems. Its solid insulation requires no periodic oil sampling, gas analysis, or breather replacement. The absence of flammable materials allows installation in confined spaces such as switchgear rooms without fire suppression mandates. Additionally, the thermal time constant of cast-resin is superior for short-circuit duty—withstanding 40 kA for 1 second without degradation—while maintaining dimensional stability across –40°C to +70°C ambient ranges. Weight is reduced by ~30% compared to equivalent oil units, simplifying handling and mounting on bus supports or cable terminations.

Typical Application Overview

The JWD-10 serves dual roles: precision metering (Class 0.2S or 0.5S) and protective relaying (Class 5P10 or 5P20). It is commonly deployed in 11kV ring-main units, pad-mounted transformers, utility substations, and renewable energy interconnection points. Its compact footprint (typically 220 mm height × 150 mm diameter) enables retrofit into legacy bays originally designed for older CT models. With IP54 ingress protection and UV-stabilized resin housing, it operates reliably in both sheltered indoor switchgear and exposed outdoor pole-top installations.

Technical Specifications

The JWD-10 adheres strictly to IEC 61869-2 and GB/T 20840.2, with verified performance parameters validated through type tests at accredited laboratories. Key specifications include:

Parameter	Value
Rated Voltage (Ur)	11 kV (IEC), 10 kV (GB)
Maximum System Voltage (Um)	12 kV
Primary Current (Ip)	50–3000 A (standard); up to 5000 A optional
Secondary Current (Is)	1 A or 5 A
Accuracy Classes	Metering: 0.2S, 0.5S; Protection: 5P10, 5P20
Rated Output (VA)	2.5–30 VA (per burden class)
Short-Time Thermal Current	40 kA for 1 s (Ith)
Dynamic Withstand Current	100 kA peak (Idyn)
Power Frequency Withstand Voltage	28 kV rms, 1 min (phase-to-earth)
Lightning Impulse Withstand	75 kV peak (1.2/50 μs)
Insulation Material	VPI cycloaliphatic epoxy resin
Core Material	GOES silicon steel, grain-oriented
Ambient Temperature Range	–40°C to +70°C
Altitude Limit	≤1000 m (derating above 1000 m per IEC 60071-2)
Ingress Protection	IP54 (terminal box)

Standard Service Conditions

The JWD-10 is rated for continuous operation under IEC 60071-1 standard atmospheric conditions: ambient temperature from –40°C to +70°C, relative humidity ≤95% (non-condensing), and altitude not exceeding 1000 m above sea level. For installations above 1000 m, the power frequency withstand voltage must be derated by 1% per 100 m increment. In coastal regions with salt fog (IEC 60850 pollution severity Class III), the creepage distance is ≥25 mm/kV (phase-to-earth), achieved via ribbed shed design on the resin housing. Humidity exposure during storage must not exceed 80% RH to prevent moisture absorption in terminal blocks.

Accuracy and Burden Characteristics

Metering accuracy (Class 0.2S) requires composite error ≤±0.2% at 20–120% of rated primary current, with phase displacement ≤±10 minutes. Protection accuracy (5P20) mandates composite error ≤±5% at 20× rated current. Burden compatibility is critical: for a 5 A secondary, a 15 VA burden implies maximum loop impedance of 0.6 Ω (including relay, wiring, and contact resistance). Mismatched burdens cause saturation during fault conditions, compromising relay operation. The JWD-10’s knee-point voltage (V_k) exceeds 300 V for 5P20 cores, ensuring linear response up to 20× I_n.

Typical Applications

The JWD-10’s dual-function design supports diverse deployment scenarios across utility and industrial sectors.

Substation Secondary Metering

In 11kV distribution substations, the JWD-10 feeds revenue-grade meters (e.g., IEC 62053-22 Class 0.2S) for billing and load profiling. Its low phase error (<5 arcmin at 100% I_n) ensures accurate kWh integration over monthly cycles. Installed on outgoing feeders, it enables real-time monitoring of customer demand, transformer loading, and loss allocation. For example, in a 10 MVA urban substation, three JWD-10 units (one per phase) provide inputs to a SCADA system, enabling dynamic voltage regulation and outage detection with <1% measurement uncertainty.

Industrial Power Distribution

Within manufacturing plants operating at 10kV (domestic Chinese standard), the JWD-10 interfaces with multifunction protection relays (e.g., overcurrent, earth-fault, differential). Its 5P20 rating ensures reliable tripping during bolted faults while avoiding nuisance operations during motor inrush (typically 6–8× I_n for 0.2 s). In a steel mill with harmonic-rich loads (THD >15%), the GOES core minimizes hysteresis losses, maintaining accuracy despite waveform distortion. Installation on bus ducts or switchgear incomers allows selective coordination with downstream fuses.

Renewable Energy Integration

For solar farms or wind parks connecting to 11kV grids, the JWD-10 monitors export power and provides anti-islanding protection. During cloud-induced irradiance swings, rapid current transients (di/dt up to 500 A/ms) are accurately captured due to the core’s low remanence (<0.8 T). In a 20 MW PV plant, JWD-10 CTs on the MV collector feeders supply data to a power quality analyzer, verifying compliance with GB/T 19964 grid code requirements for reactive power support and fault ride-through.

Rural and Suburban Distribution Networks

In remote areas with limited maintenance access, the JWD-10’s maintenance-free design reduces lifecycle costs. Mounted on pole-top transformers or sectionalizing switches, it enables automated feeder reconfiguration via FTUs (Feeder Terminal Units). For instance, in a rural 10kV radial network, JWD-10 units detect single-line-to-ground faults (as low as 5% of I_n) using sensitive earth-fault elements, minimizing outage duration. The UV-resistant resin housing withstands decades of direct sunlight without embrittlement.

Compliance with International Standards

The JWD-10 is certified to IEC 61869-2:2012 (Instrument transformers – Part 2: Additional requirements for current transformers) and harmonized with China’s GB/T 20840.2-2014.

IEC 61869-2 Compliance Details

IEC 61869-2 defines performance criteria for accuracy, thermal rating, and insulation coordination. The JWD-10 meets all mandatory clauses: rated insulation levels (28 kV power frequency, 75 kV lightning impulse), temperature rise limits (<60 K for windings), and short-circuit withstand (40 kA/1 s). Type tests include temperature rise (Clause 7.3), short-circuit (Clause 7.4), and accuracy verification (Clause 7.6). Routine tests per Clause 8 cover power frequency withstand, polarity check, and turns ratio validation within ±0.5% tolerance. Partial discharge testing at 1.2 × U_m/√3 confirms levels <10 pC, satisfying IEC 60270.

Alignment with GB/T 20840.2

GB/T 20840.2 mirrors IEC 61869-2 but includes localized requirements: rated voltage of 10 kV (vs. 11 kV IEC), and mandatory seismic qualification for Zone II installations (horizontal acceleration 0.2g). The JWD-10’s core lamination is optimized for 50 Hz operation (vs. 60 Hz in some IEC markets), with core loss <1.2 W/kg at 1.5 T, 50 Hz. Domestic certification requires CCC marking and type test reports from CEPREI or similar CNAS-accredited labs. Notably, GB permits 10% higher thermal current density than IEC, which the JWD-10 accommodates via oversized copper conductors.

Key Differences Between IEC and GB Standards

While both standards share core principles, critical divergences exist. IEC uses 11 kV as nominal system voltage (U_n), whereas GB specifies 10 kV—requiring dual labeling on nameplates. GB mandates stricter pollution performance (creepage ≥31 mm/kV for heavy industrial zones vs. IEC’s 25 mm/kV). Additionally, GB requires earth-fault accuracy verification at 5% I_n for protection CTs, a test not explicitly required in IEC 61869-2. The JWD-10 is engineered to satisfy both regimes simultaneously, enabling global deployment without redesign.

On-Site Testing Procedures

Post-installation verification ensures the JWD-10 performs within specification before energization.

Insulation Resistance Test

Using a 2500 V DC megohmmeter, measure insulation resistance between primary conductor and secondary terminals/ground. Acceptance criterion: ≥1000 MΩ at 25°C. Correct for temperature using R_T2 = R_T1 × 2^(T1–T2)/10. Values below 500 MΩ indicate moisture ingress or resin cracking. Perform before and after power frequency withstand test to detect insulation degradation.

Turns Ratio Test

Apply 1–5 V AC to secondary winding and measure induced primary voltage (open-circuit method). Calculate ratio as V_p/V_s; compare to nameplate (e.g., 600/5 = 120). Tolerance: ±0.5% for metering, ±1% for protection classes. Alternatively, use a dedicated CT analyzer injecting 1–10% I_n into primary and measuring secondary current. Deviations >1% suggest winding shorts or incorrect tap selection.

Polarity Test

Verify reducing polarity per IEC 61869-2 Figure 3. Connect a 1.5 V battery momentarily between P1 (primary input) and P2. Observe secondary voltage on S1–S2 with a DC voltmeter: positive deflection at S1 confirms correct polarity. Incorrect polarity causes 180° phase shift, leading to false differential relay trips or negative meter readings. Mandatory for all protection and metering circuits.

Power Frequency Withstand Voltage Test

Apply 28 kV rms, 50 Hz between primary and grounded secondary/enclosure for 1 minute. Use a calibrated test transformer with overcurrent trip (≤1 A). No flashover or sustained discharge permitted. For field tests, reduce voltage to 80% (22.4 kV) if equipment age exceeds 10 years. Always discharge primary capacitance post-test via grounding stick.

Short-Circuit Test (for CT)

Inject 100–200% of rated primary current (e.g., 1200 A for 600/5 CT) and verify secondary current within ±1%. Monitor secondary voltage: should not exceed knee-point (e.g., 300 V for 5P20). Excessive voltage indicates core saturation due to excessive burden or incorrect accuracy class selection. Record excitation curve if discrepancies arise.

Preventive Maintenance Guide

Although cast-resin CTs are largely maintenance-free, periodic checks extend service life beyond 25 years.

Periodic Inspection Protocol

Conduct annual visual inspections: check for resin cracks, terminal corrosion, or tracking marks. Clean housing with non-abrasive cloth and isopropyl alcohol if contaminated. Verify torque on M10 primary bolts (25 N·m) and M6 secondary terminals (8 N·m). Use infrared thermography during peak load: hotspot differentials >10°C above ambient indicate loose connections or internal faults. Document findings in asset management systems.

Maintenance Intervals and Fault Diagnosis

Every 5 years, perform insulation resistance and turns ratio tests. If ratio error exceeds 1%, investigate for winding damage. Common failure modes include: (1) moisture ingress at terminal seals—evidenced by low IR and elevated tan δ; (2) core lamination shorting—causing increased excitation current; (3) mechanical stress from busbar movement—leading to resin fractures. Replace unit if partial discharge exceeds 50 pC during offline testing. Below is a recommended schedule:

Interval	Action
Annually	Visual inspection, IR thermography, terminal torque check
5 Years	Insulation resistance, turns ratio, polarity verification
10 Years	Partial discharge measurement (if test equipment available)
After Fault	Full suite: IR, ratio, withstand voltage, excitation curve

Conclusion

The JWD-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 compliance with ruggedized construction for demanding substation environments. Its VPI epoxy resin encapsulation eliminates the fire and environmental risks associated with oil-filled alternatives, while the GOES silicon steel core delivers exceptional accuracy across metering (0.2S) and protection (5P20) applications. Validated to withstand 40 kA short-circuit currents and operate reliably from –40°C to +70°C, the JWD-10 ensures decades of maintenance-free service—typically exceeding 25–30 years in well-maintained installations. Dual compliance with IEC and GB standards facilitates global deployment, whether in urban utility substations, industrial complexes, or renewable energy interconnects. With precise adherence to international test protocols and preventive maintenance best practices, the JWD-10 provides the measurement fidelity and protection dependability essential for modern power systems.