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ZJC-10Q 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the ZJC-10Q Current Transformer
The ZJC-10Q 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 per IEC 61869-2 and GB/T 20840.2, it leverages vacuum pressure impregnation (VPI) epoxy resin technology to provide superior dielectric strength, environmental resilience, and long-term operational stability without the fire hazards or maintenance demands associated with oil-immersed alternatives.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the ZJC-10Q utilizes a thermosetting epoxy resin matrix that fully encapsulates the primary conductor, secondary windings, and magnetic core under vacuum conditions. This VPI process eliminates air voids and moisture ingress pathways, ensuring uniform electric field distribution and preventing partial discharge even under transient overvoltages. The resin compound exhibits a relative permittivity (εr) of approximately 4.2 and volume resistivity exceeding 1×1014 Ω·cm at 20°C. Unlike oil-filled CTs, which rely on liquid dielectric properties that degrade over time, the solid epoxy structure maintains consistent insulation performance across temperature cycles from –40°C to +100°C. This design also enables vertical or horizontal mounting without leakage concerns, making it ideal for compact indoor switchgear and exposed outdoor installations.
Advantages Over Oil-Immersed Designs
The ZJC-10Q eliminates critical drawbacks of traditional oil-immersed CTs. First, it is inherently non-flammable—complying with IEC 60695 flammability class V-0—making it suitable for urban substations, underground vaults, and industrial facilities with strict fire codes. Second, its sealed construction requires no periodic oil sampling, degassing, or level checks, reducing lifecycle maintenance costs by an estimated 40–60% over a 25-year service life. Third, the absence of liquid dielectric prevents environmental contamination risks during faults or aging-related leaks. Additionally, the compact footprint (typically 180 mm height × 120 mm diameter for single-ratio units) allows higher equipment density in ring-main units (RMUs) and pad-mounted switchgear. Thermal performance is enhanced through the resin’s thermal conductivity (~0.8 W/m·K), which efficiently dissipates heat generated by continuous load currents up to 1.2× rated value.
Typical Application Overview
The ZJC-10Q serves dual roles in modern power systems: precision metering for revenue-grade energy billing and fast-response protection for fault detection. In utility substations, it interfaces with digital multifunction meters (e.g., IEC 61850-9-2 LE sampled values) requiring 0.2S or 0.5S accuracy classes. Simultaneously, its 5P or 10P protection windings feed overcurrent relays (e.g., IEC 60255-compliant devices) with linear response up to 20× rated current. Common deployment scenarios include 11kV feeder bays, generator step-up transformers, capacitor bank monitoring, and renewable interconnection points where harmonic distortion from inverters demands low remanence cores. Its robust mechanical design withstands seismic loads up to 0.5g (per IEC 60068-2-57), supporting use in earthquake-prone regions.
Technical Specifications
The ZJC-10Q adheres to stringent electrical and environmental parameters defined by international and Chinese standards. Key specifications are summarized below:
| Parameter | Value |
|---|---|
| Rated System Voltage (Um) | 11 kV (IEC) / 10 kV (GB) |
| Rated Insulation Level | 12/28/75 kV (Urms/LIpeak/SIpeak) |
| Primary Current Ratings | 10 A to 4000 A (standard); custom up to 6300 A |
| Secondary Current | 1 A or 5 A (IEC 61869-2 compliant) |
| Accuracy Classes | Metering: 0.2S, 0.5S; Protection: 5P10, 5P20, 10P10, 10P20 |
| Rated Output (Burden) | 2.5 VA to 30 VA (at 5 A); 1 VA to 15 VA (at 1 A) |
| Short-Time Thermal Current | 25 kA for 1 s (Ith) |
| Dynamic Withstand Current | 63 kA peak (Idyn) |
| Core Material | Grain-Oriented Electrical Steel (GOES), CRGO grade, 0.27 mm thickness |
| Insulation System | VPI epoxy resin, UL 94 V-0 rated |
| Ambient Temperature Range | –40°C to +40°C (storage: –50°C to +70°C) |
| Relative Humidity | Up to 95% non-condensing |
| Maximum Altitude | 1000 m above sea level (derating required >1000 m) |
Electrical Performance Parameters
The ZJC-10Q achieves metering accuracy within ±0.2% error at 100% rated current for 0.2S class, with phase displacement ≤±10 minutes. At 1% rated current, composite error remains ≤±0.75%, enabling accurate billing even under light-load conditions typical of rural distribution. Protection windings maintain linearity up to 20× rated current with composite error ≤±5% for 5P20 class. The core’s knee-point voltage (Vk) exceeds 300 V for 5 A secondary units, ensuring sufficient saturation margin during external faults. Burden tolerance is ±10% of nominal value; exceeding this may cause ratio errors beyond class limits. Dielectric tests include 28 kV AC for 1 minute (power frequency) and 75 kV lightning impulse (1.2/50 μs waveform), both with zero flashover or disruptive discharge.
Environmental and Mechanical Ratings
Designed for both indoor and outdoor service, the ZJC-10Q features UV-stabilized resin housing resistant to tracking (CTI ≥600 V per IEC 60112). Creepage distance meets IEC 60815 pollution severity Class III (≥25 mm/kV for 11 kV), suitable for coastal or industrial atmospheres. Mechanical strength includes a cantilever load rating of 500 N applied to primary terminals without permanent deformation. Terminal blocks accommodate 16 mm² to 120 mm² copper conductors with torque specs of 12–15 N·m. Weight ranges from 3.2 kg (100/5 A) to 8.7 kg (3000/1 A), facilitating manual handling during installation. No auxiliary cooling is required; natural convection suffices for continuous operation at 1.2× In.
Typical Applications
The ZJC-10Q’s dual-metering/protection capability makes it versatile across multiple infrastructure segments.
Substation Secondary Metering Systems
In 11kV/0.4kV distribution substations, the ZJC-10Q provides inputs to revenue meters compliant with MID or DLMS standards. For example, a 400/5 A, 0.2S unit with 10 VA burden feeds a three-phase smart meter measuring kWh, kVArh, and power quality parameters (THD, harmonics up to 50th order). The low phase error ensures accurate reactive energy billing, while the high saturation point prevents core distortion during motor-start surges. Integration with SCADA via Modbus TCP or IEC 61850 GOOSE messaging enables real-time load profiling and theft detection.
Industrial Power Distribution Networks
Heavy industries (e.g., steel mills, data centers) deploy ZJC-10Q units on 11kV motor control centers (MCCs) to monitor large induction motors (500–5000 kW). Here, 5P20 protection windings trigger instantaneous overcurrent relays within 20 ms of a phase-to-phase fault, minimizing equipment damage. The CT’s ability to withstand 63 kA dynamic current protects against arc-flash incidents. In variable-frequency drive (VFD) applications, the GOES core’s low hysteresis loss reduces heating from harmonic currents (notably 5th, 7th, 11th), maintaining accuracy despite non-sinusoidal waveforms.
Renewable Energy Integration Points
At solar PV or wind farm interconnection points, the ZJC-10Q monitors bidirectional power flow. During islanding events, its fast response (<30 ms) supports anti-islanding protection schemes per IEEE 1547. The 0.5S metering winding accurately captures exported energy for feed-in tariffs, while the 10P10 protection winding coordinates with reclosers to isolate DC-side faults. Resin encapsulation resists salt fog in coastal solar farms, eliminating corrosion issues seen in older porcelain-housed CTs.
Rural and Suburban Distribution Feeders
For overhead line applications in remote areas, the ZJC-10Q’s maintenance-free design reduces outage durations. Mounted on poles with polymer insulators, it feeds RTUs that report load data via cellular networks. A typical 200/1 A, 5P10 unit detects high-impedance faults (e.g., tree contact) by sensing residual currents as low as 5% of rated value. Its wide temperature tolerance ensures reliability in desert (50°C ambient) or alpine (–30°C) climates without recalibration.
Compliance with International Standards
The ZJC-10Q is certified to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and GB/T 20840.2-2014 (Chinese national standard).
IEC 61869-2 Compliance Details
IEC 61869-2 mandates rigorous testing for accuracy, thermal endurance, and dielectric integrity. The ZJC-10Q passes all type tests: temperature rise ≤55 K at 1.2× In; short-circuit withstand per IEC 60076-5; and partial discharge <10 pC at 1.2 Um/√3. Accuracy verification follows IEC 61869-2 Annex B, using calibrated reference CTs traceable to national labs. The standard defines “rated instrument limit factor” (FS) for metering (e.g., FS5 = saturation at 5× In) and “accuracy limit factor” (ALF) for protection (e.g., ALF20 = linear up to 20× In). All ZJC-10Q units undergo routine tests per clause 12, including polarity check and inter-turn insulation test at 3 kV AC.
Alignment with GB/T 20840.2
While GB/T 20840.2 mirrors IEC 61869-2 structurally, key differences exist. GB specifies 10 kV as nominal system voltage (vs. 11 kV in IEC), requiring adjusted insulation levels (12/42/75 kV vs. IEC’s 12/28/75 kV for 11 kV). Chinese standards also mandate additional seismic testing (GB/T 13540) and stricter creepage distances for polluted environments. However, the ZJC-10Q’s dual-rating design satisfies both: its 11 kV IEC rating covers global exports, while 10 kV GB compliance enables domestic Chinese grid approval. Certification includes CQC mark and State Grid Corporation of China (SGCC) type approval.
Testing and Certification Protocols
Third-party validation is performed by accredited labs (e.g., KEMA, CESI, or China Electric Power Research Institute). Tests include:
- Temperature rise test: 6-hour duration at 1.2× In, measured via resistance method
- Short-circuit test: 25 kA for 1 s, verifying no mechanical damage or insulation failure
- Impulse test: 75 kV standard lightning impulse, positive and negative polarity
- Accuracy test: Performed at 5%, 20%, 100%, and 120% of rated current
Certificates include IEC CB Scheme reports and CCC (China Compulsory Certification) documentation.
On-Site Testing Procedures
Post-installation verification ensures the ZJC-10Q performs within specification before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-secondary, primary-ground, and secondary-ground. Acceptance criteria: ≥1000 MΩ at 20°C. Values below 500 MΩ indicate moisture ingress or resin cracking. Corrective action includes drying at 80°C for 24 hours or replacement if irreversible damage is suspected.
Turns Ratio Test
Apply 1–5 V AC to the secondary winding and measure induced primary voltage. Calculate actual ratio = Vsecondary / Vprimary. Tolerance must be within ±0.5% of nameplate ratio (e.g., 400/5 A = 80:1 ±0.4). Deviations >1% suggest inter-turn shorts or incorrect tap selection.
Polarity Test
Verify reducing polarity per IEC 61869-2 Figure 101. Apply momentary DC to primary (H1+ to H2–); secondary should show positive kick at X1. Incorrect polarity causes relay misoperation—e.g., differential protection tripping on through-faults. Use a dedicated polarity tester or oscilloscope for confirmation.
Power Frequency Withstand Voltage Test
Apply 28 kV AC (rms) at 50 Hz between primary and grounded secondary/core for 1 minute. Monitor for flashover, excessive leakage current (>1 mA), or audible discharge. This validates insulation integrity after transport-induced microcracks.
Short-Circuit Test (for CTs)
Unlike VTs, CTs require short-circuit validation. Inject 10× rated current into primary with secondary shorted; measure secondary current. Error must comply with protection class (e.g., ≤±5% for 5P). Open-circuiting the secondary during this test is prohibited—it induces hazardous kV-level voltages.
Preventive Maintenance Guide
Although cast-resin CTs are largely maintenance-free, periodic checks extend service life.
Annual Inspection Protocol
Visually inspect for:
- Surface cracks or tracking marks on resin housing
- Loose terminal connections (torque recheck at 12 N·m)
- Corrosion on grounding lugs
- Abnormal heating via infrared thermography (ΔT >10 K above ambient warrants investigation)
Clean with dry cloth; avoid solvents that degrade epoxy.
Five-Year Maintenance Schedule
| Interval | Action |
|---|---|
| Year 1 | Baseline insulation resistance and ratio test |
| Year 3 | Repeat ratio and polarity tests; verify burden connections |
| Year 5 | Full dielectric test (28 kV AC); compare results to baseline |
| Every 5 years thereafter | Repeat Year 5 protocol |
Replace if insulation resistance drops >50% from initial value or ratio error exceeds class limits.
Fault Diagnosis and Troubleshooting
Common issues include:
- High ratio error: Caused by core saturation (excessive burden) or inter-turn shorts. Reduce connected burden or replace CT.
- Overheating: Indicates loose terminals or harmonic overload. Tighten connections; verify THD <5%.
- Ground fault alarms: May stem from degraded insulation. Perform megger test immediately.
Never operate with open secondary—this risks core saturation and insulation breakdown.
Conclusion
The ZJC-10Q 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2-certified accuracy with rugged, maintenance-free construction. Its VPI epoxy resin encapsulation eliminates fire and environmental hazards inherent in oil-filled designs, while the GOES silicon steel core ensures metrological precision across diverse loading conditions—from light rural loads to industrial motor inrush currents. Dual-winding configurations support simultaneous metering (0.2S/0.5S) and protection (5P/10P) functions, streamlining substation architecture and reducing lifecycle costs. Rigorous compliance with both international (IEC) and Chinese (GB) standards guarantees interoperability in global grids, and its 25–30 year design life aligns with modern asset management strategies. Field-proven in extreme climates and high-pollution environments, the ZJC-10Q delivers unwavering reliability for critical power system monitoring and protection—making it the preferred choice for utilities, industrial operators, and renewable energy developers seeking future-proof instrumentation solutions.