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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 high-reliability, cast-resin insulated current transformer (CT) engineered for accurate current measurement and robust protection functions in medium-voltage power systems operating at 11kV (IEC standard) or 10kV (domestic Chinese system voltage). Designed in strict accordance with IEC 61869-2 and GB/T 20840.2, this instrument transformer leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and primary winding, ensuring long-term dielectric integrity, mechanical stability, and environmental resistance.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the JWD-10 employs a two-component epoxy resin system processed under vacuum and pressure to eliminate air voids and moisture ingress. This VPI (Vacuum Pressure Impregnation) technique fully saturates the glass-fiber reinforced winding assembly and GOES (Grain-Oriented Electrical Steel) core laminations, creating a monolithic, non-hygroscopic solid insulation structure. Unlike oil-filled or gas-insulated alternatives, the cast-resin design eliminates fire hazards, leakage risks, and maintenance-intensive sealing systems. The resulting composite exhibits a dielectric strength exceeding 20 kV/mm, thermal class F (155°C) endurance, and excellent tracking resistance (CTI > 600 V), making it suitable for both indoor switchgear and outdoor pole-mounted installations. The absence of liquid or gaseous media also ensures consistent performance across wide temperature ranges without pressure compensation mechanisms.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed current transformers, the JWD-10 offers significant operational and safety benefits. First, it is inherently fire-retardant—critical in urban substations and industrial facilities where flammability codes restrict oil-filled equipment. Second, the solid resin body provides superior mechanical rigidity, resisting vibration-induced winding displacement during short-circuit events (withstanding up to 40 kA for 1 second). Third, there is no need for periodic oil sampling, degassing, or conservator tank maintenance, reducing lifecycle costs by an estimated 30–40% over a 25-year service life. Additionally, the compact footprint (typically 250 mm height × 180 mm diameter) enables retrofitting into legacy switchgear bays originally designed for smaller oil units. Environmental resilience is another key advantage: the JWD-10 operates reliably in humidity levels up to 95% RH and altitudes up to 1,000 meters without derating, whereas oil units may suffer from moisture absorption and reduced breakdown voltage under similar conditions.
Typical Applications Overview
The JWD-10 serves dual roles in power systems: precision metering for billing and energy management, and high-speed fault detection for protective relaying. In 11kV distribution substations, it interfaces with revenue-class kWh meters (accuracy class 0.2S or 0.5S) and microprocessor-based relays (e.g., overcurrent, earth-fault, differential). Its robust saturation characteristics (knee-point voltage ≥ 1.5× rated secondary voltage under 20× rated current) ensure reliable operation during external faults without compromising relay coordination. Beyond utility networks, the JWD-10 is deployed in industrial plants for motor protection, renewable energy interconnection points (e.g., solar farm collector feeders), and railway traction supply systems where harmonic distortion and DC offset demand stable magnetization curves. Its dual-core configuration—often one core for metering (0.2S/5 VA) and another for protection (5P10/15 VA)—enables simultaneous compliance with metrological and protection requirements without cross-interference.
Technical Specifications
The JWD-10 is engineered to meet stringent electrical and environmental performance criteria defined by international and national standards. Below is a comprehensive specification table followed by detailed service condition parameters.
| Parameter | Value |
|---|---|
| Rated System Voltage (IEC) | 11 kV |
| Rated System Voltage (Domestic) | 10 kV |
| Insulation Level (IEC 60071-1) | 12/28/75 kV (Ur/Up/LI) |
| Primary Current Ratings | 50 A to 3000 A (standard); custom up to 4000 A |
| Secondary Current | 1 A or 5 A |
| Accuracy Classes | Metering: 0.2S, 0.5S; Protection: 5P10, 5P20, 10P10 |
| Rated Burden (per core) | Metering: 2.5–15 VA; Protection: 10–30 VA |
| Short-Time Thermal Current | 40 kA for 1 s (Ith) |
| Dynamic Withstand Current | 100 kA peak (Idyn) |
| Core Material | GOES (Grain-Oriented Electrical Steel), 0.3 mm lamination |
| Insulation Material | VPI Epoxy Resin, UL 94 V-0 flame rating |
| Polarity | Reducing Polarity (per IEC 61869-2) |
Standard Service Conditions
The JWD-10 is rated for continuous operation under the following ambient conditions as per IEC 60060-1 and GB/T 11022: ambient temperature range of –25°C to +40°C (with optional –40°C low-temp variant), relative humidity up to 95% (non-condensing), and installation altitude not exceeding 1,000 meters above sea level. For installations above 1,000 m, a derating factor of 1.0% per 100 m applies to the rated voltage due to reduced air density affecting external creepage. The transformer is designed for three-phase systems with nominal frequency of 50 Hz or 60 Hz, with harmonic content not exceeding 5% THD. Mechanical loads on primary terminals are limited to 500 N axial force to prevent stress on the resin housing. These conditions ensure stable accuracy and insulation performance over the expected 25–30 year service life.
Electrical Performance Parameters
Key electrical characteristics include a magnetizing impedance of ≥ 20 Ω at 10% of rated current for metering cores, ensuring minimal ratio error under light-load conditions. For protection cores, the knee-point voltage (Vk) is guaranteed to exceed 1.5 times the rated secondary voltage when subjected to 20 times rated primary current, preventing saturation during external faults. Ratio error tolerance is ±0.2% for 0.2S class at 20–120% of rated current, and phase displacement ≤ ±10 minutes. The secondary winding resistance is typically 0.15–0.45 Ω (for 5 A output), enabling burden calculations for connected devices. Transient response meets IEC 61869-2 Annex C requirements, with remanence factor Kr ≤ 0.1 for protection cores to minimize DC offset effects during breaker reclosing.
Typical Applications
The JWD-10’s dual functionality makes it indispensable across diverse power infrastructure segments. Its design accommodates both precision metrology and high-reliability protection, fulfilling critical roles in modern grid operations.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the JWD-10 supplies current signals to Class 0.2S or 0.5S revenue meters for accurate energy billing. Installed on outgoing feeders or main incomers, it ensures compliance with regulatory metrology standards (e.g., MID in Europe, DL/T 614 in China). The low-ratio-error performance across 1–120% load range captures consumption during off-peak hours without under-registration. For example, in a commercial building substation with variable HVAC loads, the JWD-10 maintains ±0.35% error at 5% load—critical for detecting energy theft or inefficient equipment. Its cast-resin housing resists electromagnetic interference from adjacent switchgear, preserving signal fidelity to digital meters via shielded twisted-pair cables terminated at IP20-rated terminal blocks.
Industrial Power Distribution
Heavy industries such as steel mills, chemical plants, and data centers deploy the JWD-10 for motor protection and load monitoring. On 10kV motor feeders (e.g., 1,250 kW induction motors), the protection core (5P10/15 VA) interfaces with thermal overload relays, providing instantaneous tripping during locked-rotor or phase-loss conditions. The transformer withstands frequent inrush currents (6–8× rated) without residual magnetism buildup due to its low-remanence GOES core. In arc furnace applications with severe harmonics (up to 15% 3rd/5th order), the JWD-10’s linear B-H curve prevents saturation-induced waveform distortion, ensuring correct operation of harmonic-filtered relays. Its IP54-rated enclosure option protects against conductive dust in foundry environments.
Renewable Energy Integration
Solar photovoltaic (PV) and wind farms utilize the JWD-10 at the point of common coupling (PCC) for grid compliance monitoring. On 11kV collector feeders aggregating multiple inverters, it measures active/reactive power flow for SCADA systems and anti-islanding protection. During cloud transients or gust events, rapid irradiance/wind speed changes cause sub-second current fluctuations; the JWD-10’s fast transient response (< 20 ms rise time) captures these dynamics for accurate ramp-rate control. Its dual-core design allows one core to feed a Class 0.5S meter for feed-in tariff calculations while the other drives a directional overcurrent relay for fault ride-through (FRT) compliance per IEC 61400-21. The resin insulation resists UV degradation in desert or coastal sites, unlike polymer-jacketed alternatives.
Rural and Suburban Distribution Networks
In rural electrification projects, the JWD-10 is mounted on pole-top switchgear or pad-mounted transformers for feeder automation. Its compact size fits within space-constrained RMU (Ring Main Unit) compartments, enabling remote fault location via FTUs (Feeder Terminal Units). For single-wire earth return (SWER) systems operating at 10kV, the JWD-10’s high insulation coordination (75 kV LI) withstands lightning surges common in exposed overhead lines. In suburban residential feeders with high EV charging penetration, the metering core accurately records time-of-use consumption even with pulsed DC loads, thanks to its flat frequency response up to 2 kHz. Maintenance-free operation reduces truck rolls in remote areas, lowering OPEX for distribution utilities.
Compliance with International Standards
The JWD-10 is certified to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and aligns with the Chinese national standard GB/T 20840.2-2014, which adopts IEC 61869-2 with minor national deviations.
IEC 61869-2 Compliance Details
IEC 61869-2 defines performance, testing, and marking requirements for instrument transformers. The JWD-10 meets all mandatory clauses, including:
– Accuracy verification per Clause 6.3 (ratio/phase error tests at 5%, 20%, 100%, and 120% of rated current)
– Temperature rise limits per Clause 6.5 (≤ 60 K for resin, measured by resistance method)
– Short-circuit withstand per Clause 6.6 (40 kA/1s thermal, 100 kA dynamic)
– Insulation tests per Clause 6.7 (power frequency 28 kV rms for 1 min, lightning impulse 75 kV peak)
– Marking requirements per Clause 8 (model, ratio, accuracy class, polarity dot, standards)
Certification is issued by accredited third-party labs (e.g., KEMA, CESI) with test reports traceable to SI units. The transformer’s design includes a secondary terminal cover with IP2X finger-safe protection, satisfying IEC 61439-1 safety mandates for switchgear integration.
GB/T 20840.2 Alignment and National Deviations
GB/T 20840.2 mirrors IEC 61869-2 but introduces specific Chinese requirements:
– Mandatory short-time current rating of 31.5 kA/3s for 10kV systems (vs. IEC’s 1s basis)
– Creepage distance ≥ 240 mm/kV for polluted environments (Class III)
– Additional partial discharge test limit of ≤ 10 pC at 1.2× Ur/√3
The JWD-10 exceeds these with 40 kA/1s (equivalent to 23 kA/3s thermally) and 300 mm/kV creepage. It also complies with GB/T 17215.321 for metering accuracy in smart grid deployments. Notably, GB/T permits 10kV as the nominal system voltage, whereas IEC uses 11kV; the JWD-10’s insulation level (12 kV Ur) covers both, ensuring interoperability in export and domestic markets.
Testing and Certification Requirements
Type tests per IEC 61869-2 include:
– Temperature rise test (winding heated to 155°C, stabilized within 8 hours)
– Short-circuit test (40 kA applied for 1s, post-test ratio error shift ≤ 50% of class limit)
– Lightning impulse test (75 kV, 1.2/50 μs wave, 15 shots positive/negative)
– Partial discharge test (< 5 pC at 1.2× Ur/√3)
Routine tests performed on every unit:
- Power frequency withstand (28 kV rms, 1 min)
- Turns ratio and polarity verification
- Insulation resistance (> 1,000 MΩ at 2,500 V DC)
– Secondary winding continuity
All test data is archived for 10 years, supporting warranty claims and grid code audits.
On-Site Testing Procedures
Post-installation verification ensures the JWD-10 performs within specifications. All tests follow IEC 60270 and IEEE C57.13 guidelines.
Insulation Resistance Test
Using a 2,500 V DC megohmmeter, measure insulation resistance between primary-to-secondary, primary-to-ground, and secondary-to-ground. Acceptance criterion: ≥ 1,000 MΩ at 20°C. Correct for temperature using RT = R20 × 2(20–T)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement. Perform before and after power frequency tests to detect insulation degradation.
Turns Ratio Test
Apply 1–5 V AC (50/60 Hz) to the secondary winding and measure induced primary voltage. Calculate ratio as Vp/Vs; compare to nameplate. Tolerance: ±0.2% for 0.2S class, ±1% for 5P10. Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for automated error calculation. Verify at multiple tap points if multi-ratio.
Polarity Test
Connect a 1.5 V battery to primary terminals (H1+, H2–). Momentarily close the circuit while monitoring secondary with a DC voltmeter (X1+, X2–). A positive kick confirms reducing polarity. Incorrect polarity causes wattmeter reversal or relay misoperation—critical in differential schemes. Repeat three times for consistency.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (50 Hz) between primary and grounded secondary/enclosure for 1 minute. Use a calibrated HV test set with overcurrent trip (≤ 10 mA). No flashover or disruptive discharge permitted. Ramp up/down at 1 kV/s to avoid transient overstress. Conduct in dry conditions (RH < 80%).
Short-Circuit Test (for CT)
Inject 10–20× rated primary current (e.g., 1,000 A for 50/5 A unit) using a portable current generator. Monitor secondary voltage; for 5P10 cores, Vsec must remain linear up to 10× In. Saturation indicates core damage or incorrect burden. Record excitation curve to verify knee-point voltage ≥ 1.5× (In × Zb).
Preventive Maintenance Guide
Although cast-resin CTs require minimal maintenance, periodic checks extend service life and prevent unexpected failures.
Periodic Inspection Schedule
Conduct visual and electrical inspections annually:
– Check for surface cracks, tracking, or discoloration on resin housing
– Verify terminal tightness (torque: 15 N·m for M10 studs)
– Measure insulation resistance (trend analysis: >20% drop warrants investigation)
– Inspect grounding continuity (< 0.1 Ω resistance)
Biannually in coastal or industrial zones with high pollution (IEC 60815 Class E).
Long-Term Maintenance Intervals
Every 5 years, perform:
– Full ratio and polarity retest
– Partial discharge measurement (if portable PD detector available; limit: < 20 pC)
- Thermal imaging under load (hot spots > 10 K above ambient indicate loose connections)
– Review historical fault current exposure (cumulative I²t vs. rated Ith²t)
Replace if cumulative short-circuit duty exceeds 80% of rating. Expected service life is 25–30 years with proper maintenance.
Fault Diagnosis and Troubleshooting
Common issues and remedies:
– **Open secondary circuit**: Causes dangerous overvoltage (> 2 kV). Always short CT secondaries before disconnecting meters. Install shorting switches per IEC 61869-1.
– **Ratio drift**: Indicates core aging or mechanical shock. Compare excitation curves to baseline.
– **Moisture ingress**: Visible as white powder (hydrolysis) on terminals. Replace if insulation resistance drops below 500 MΩ.
– **Harmonic distortion**: Use true-RMS meters; verify burden impedance matches nameplate.
Maintain a logbook recording all test results for predictive analytics.
Conclusion
The JWD-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 and GB/T 20840.2 compliance with field-proven reliability. Its VPI epoxy resin encapsulation eliminates fire and environmental hazards inherent in oil-filled designs, while the GOES silicon steel core delivers exceptional accuracy for metering (0.2S class) and robust saturation resistance for protection (5P20). Engineered for 25–30 years of maintenance-free operation, the JWD-10 withstands harsh conditions—from desert heat to industrial pollution—without performance degradation. Key technical advantages include a compact form factor for retrofit applications, dual-core flexibility for simultaneous metering and protection, and rigorous type testing covering thermal, dielectric, and short-circuit stresses. With precise adherence to international standards and documented test protocols, the JWD-10 ensures grid operators achieve both regulatory compliance and operational resilience. Its lifecycle cost efficiency, driven by zero fluid maintenance and minimal inspection needs, makes it the optimal choice for modern substations, industrial facilities, and renewable energy integrations worldwide.