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IEC 61869-2 Certified 11kV Cast-Resin Current Transformer ZJC-10Q for Metering & Protection Applications

Introduction to the ZJC-10Q Current Transformer

The ZJC-10Q is a high-reliability, cast-resin insulated current transformer (CT) engineered for precision metering and robust protection functions in medium-voltage power systems operating at 11kV (IEC standard) or 10kV (domestic Chinese system). 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 windings, ensuring superior dielectric strength, environmental resilience, and long-term operational stability.

Operating Principle of Cast-Resin Insulation

Cast-resin insulation in the ZJC-10Q employs a two-stage VPI process where the primary conductor, secondary windings, and grain-oriented electrical steel (GOES) core are first placed in a mold under vacuum to remove air pockets. Epoxy resin is then pressure-injected to fully saturate all internal components before curing at elevated temperatures. This results in a monolithic, void-free solid insulation system with a dielectric strength exceeding 30 kV/mm. Unlike oil-filled alternatives, the resin matrix provides inherent resistance to partial discharge (PD levels < 5 pC at 1.2 × Um/√3), eliminating fire hazards and environmental contamination risks. The thermal conductivity of the cured epoxy (~0.8 W/m·K) also facilitates efficient heat dissipation during continuous operation or short-circuit events, maintaining core performance within design limits.

Advantages Over Oil-Immersed Designs

Compared to traditional oil-immersed CTs, the ZJC-10Q offers significant operational and safety benefits. Its solid insulation eliminates the need for oil containment, gaskets, or breathers—components prone to leakage and degradation over time. The absence of flammable oil makes it suitable for indoor substations, urban switchgear rooms, and confined industrial facilities where fire codes restrict combustible materials. Additionally, the cast-resin body exhibits excellent UV and ozone resistance, enabling reliable outdoor deployment without protective enclosures. Maintenance requirements are drastically reduced; no oil sampling, moisture testing, or tank inspections are needed. The mechanical rigidity of the epoxy housing also provides superior vibration damping, critical in environments with nearby rotating machinery or seismic activity. Weight is typically 30–40% lower than equivalent oil-filled units, simplifying handling and mounting.

Typical Application Overview

The ZJC-10Q is deployed across utility substations, industrial plants, renewable energy farms, and commercial distribution networks where accurate current measurement and dependable fault detection are essential. In 11kV ring-main units (RMUs) and metal-clad switchgear, it interfaces with revenue meters, protective relays (e.g., overcurrent, earth-fault), and SCADA systems. Its dual-core configuration—often one core optimized for 0.2S/0.5 class metering and another for 5P10/5P20 protection—enables simultaneous compliance with metrological and safety requirements. The transformer’s compact footprint (typically 180 mm height × 120 mm diameter) allows integration into space-constrained panels without compromising creepage distance (≥240 mm for 11kV per IEC 60664).

Technical Specifications

The ZJC-10Q delivers precise performance across a range of electrical and environmental parameters, validated through rigorous factory and type tests per IEC 61869-2.

Rated Electrical Parameters

Key electrical ratings include a highest system voltage (Um) of 12 kV, corresponding to an IEC-standard 11kV nominal system (with domestic 10kV compatibility). Standard current ratios span from 50/1 A to 3000/5 A, with multi-ratio options available via secondary tap selections. Accuracy classes comply with IEC 61869-2: Class 0.2S or 0.5 for metering cores (phase error ≤ ±10′, ratio error ≤ ±0.2% at 100% In), and Class 5P10 or 5P20 for protection cores (composite error ≤ 5% at 10× or 20× rated current). Rated burden is typically 15 VA or 30 VA per core, though custom burdens up to 50 VA can be specified. Short-time thermal current withstand is 20 kA for 1 second, while dynamic current withstand reaches 50 kA peak. Insulation levels meet IEC 60071-1: power frequency withstand voltage of 28 kV rms for 1 minute and lightning impulse withstand of 75 kV peak (1.2/50 μs wave).

Environmental and Mechanical Ratings

The ZJC-10Q operates reliably under standard service conditions defined by IEC 60060-1: ambient temperature range of –25°C to +40°C (with optional –40°C variants), relative humidity up to 100% (condensing), and installation altitude ≤1000 m above sea level (derating applies above 1000 m per IEC 60071-2). The enclosure achieves IP54 ingress protection per IEC 60529, shielding internal components from dust and water splashes. Terminal blocks are rated for 600 V and accommodate conductors up to 6 mm², with clear polarity markings (“P1” toward source, “S1” as reducing polarity output). The transformer is designed for vertical or horizontal mounting on standard busbars with diameters of 20–30 mm, secured via stainless-steel clamps.

Core and Insulation Materials

The magnetic circuit utilizes cold-rolled grain-oriented electrical steel (GOES) with a maximum specific loss of 1.0 W/kg at 1.7 T and 50 Hz, minimizing core losses and enhancing accuracy under low-load conditions. Secondary windings employ enameled copper wire (Class H insulation, 180°C rating) wound with tight pitch control to reduce leakage inductance. The VPI epoxy resin system features a glass transition temperature (Tg) of ≥120°C and a coefficient of thermal expansion closely matched to copper and steel, preventing microcracking during thermal cycling. Partial discharge inception voltage exceeds 15 kV rms, ensuring decades of stable operation even under polluted or humid conditions.

Typical Applications

The ZJC-10Q serves diverse roles in modern power infrastructure, combining metrological fidelity with protection-grade robustness.

Substation Secondary Metering

In 11kV/0.4kV distribution substations, the ZJC-10Q’s metering core (Class 0.2S) feeds revenue-grade kWh meters and power quality analyzers. Its low phase shift (< ±5′ at 20–120% In) ensures accurate billing and harmonic distortion measurement per IEC 62053-22. For example, in a municipal utility substation supplying a commercial district, three ZJC-10Q units (one per phase) provide inputs to a Class 0.5S smart meter, enabling time-of-use tariffing and demand-side management. The cast-resin housing resists tracking from salt fog in coastal areas, maintaining insulation integrity over 25+ years.

Industrial Power Distribution

Within manufacturing plants, the ZJC-10Q monitors feeders to large motors, arc furnaces, or rectifier loads. Its protection core (e.g., 5P20) interfaces with numerical relays like the SEL-751 or Siemens 7SJ62 to detect phase-to-phase or ground faults within milliseconds. In a steel mill with frequent motor startups, the CT’s high saturation point (Vk ≥ 500 V for 5P20) prevents relay misoperation during inrush currents. The compact size allows retrofitting into legacy switchgear without panel modifications.

Renewable Energy Integration

Solar PV and wind farms utilize the ZJC-10Q at the 11kV collection bus to measure exported energy and enable anti-islanding protection. During cloud-induced irradiance fluctuations, the CT’s linearity down to 1% In ensures accurate curtailment reporting to grid operators. In a 20 MW solar plant, ZJC-10Q units feed both a Class 0.5 meter for REC certification and a 5P10 core for overcurrent backup protection, complying with IEEE 1547 and local grid codes.

Rural and Suburban Distribution Networks

For rural electrification projects, the ZJC-10Q’s maintenance-free design reduces lifecycle costs in remote locations. Mounted on pole-top transformers or pad-mounted switchgear, it supports single-phase or three-phase metering with tamper-resistant terminals. In suburban ring-main units, its dual-core output enables separate metering and protection circuits without additional CTs, optimizing capital expenditure.

Harsh Environment Deployment

In mining or chemical processing sites with explosive atmospheres (Zone 2), the non-flammable resin eliminates ignition risks. The IP54 rating protects against coal dust or chemical mists, while the GOES core maintains accuracy despite DC bias from nearby traction systems.

Compliance with International Standards

The ZJC-10Q is engineered to satisfy both global and regional regulatory frameworks, ensuring interoperability and safety.

IEC 61869-2 Certification Details

IEC 61869-2 governs the performance, testing, and marking of instrument transformers. The ZJC-10Q undergoes full type testing per Clause 7, including temperature rise (≤60 K for windings at 1.2 × In), short-circuit withstand (thermal and dynamic), and accuracy verification across 1–120% In. Its nameplate includes mandatory markings: manufacturer, model, serial number, rated voltage (11 kV), rated frequency (50/60 Hz), current ratio, accuracy class per core, rated burden, and insulation level (e.g., 12/28/75). Factory routine tests (Clause 8) cover power frequency withstand, partial discharge (<10 pC at Um/√3), and polarity confirmation.

Alignment with GB/T 20840.2

GB/T 20840.2 is China’s national adoption of IEC 61869-2, with minor deviations in test durations and environmental classifications. The ZJC-10Q meets all GB-specific requirements, including extended thermal endurance testing (4 hours at 1.1 × In) and stricter creepage distance rules for polluted environments (III/IV pollution degree). Domestic utilities often require dual labeling (10 kV system voltage alongside 11 kV Um), which is accommodated on the nameplate.

Key Differences Between IEC and Domestic Standards

While IEC 61869-2 permits 60 Hz operation, GB/T 20840.2 focuses exclusively on 50 Hz systems. Burden definitions also differ slightly: IEC uses apparent power (VA), whereas older Chinese practices referenced impedance (Ω), though modern GB aligns with IEC. Most critically, GB standards mandate additional seismic testing (0.3g horizontal acceleration) for transformers installed in earthquake-prone regions like Sichuan, a requirement not explicit in IEC but incorporated into ZJC-10Q’s mechanical design.

On-Site Testing Procedures

Post-installation verification ensures the ZJC-10Q performs within specifications after transport and mounting.

Insulation Resistance Test

Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-secondary, primary-to-ground, and secondary-to-ground. Acceptance criteria: ≥1000 MΩ at 20°C. Correct for temperature using R_T2 = R_T1 × 2^(T1–T2)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement.

Turns Ratio Test

Apply a low-voltage AC (5–10 V) to the secondary winding and measure induced primary voltage. Calculate actual ratio = V_sec/V_prim. Compare to nameplate; tolerance must be within ±0.2% for metering cores and ±1% for protection cores. Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for automated validation.

Polarity Test

Verify reducing polarity using the DC kick method: briefly connect a 6 V battery between P1 and P2, observing a momentary upscale deflection on a DC milliammeter connected to S1–S2. Incorrect polarity causes relay miscoordination—critical in differential schemes.

Power Frequency Withstand Voltage Test

Apply 28 kV rms (50 Hz) between primary and grounded secondary/enclosure for 1 minute. Leakage current must remain <10 mA. This validates insulation integrity post-installation, especially after exposure to humidity during shipping.

Short-Circuit Test (for CT)

Inject 10× rated current (e.g., 500 A for 50/5 A ratio) into the primary and verify secondary output matches expected value within composite error limits (≤5% for 5P10). This confirms core linearity under fault conditions and detects winding shorts.

Preventive Maintenance Guide

Although cast-resin CTs require minimal upkeep, scheduled checks extend service life and prevent unexpected failures.

Annual Visual and Electrical Inspection

Inspect for surface cracks, tracking marks, or terminal corrosion. Clean with dry cloth—never solvents. Re-measure insulation resistance and compare to baseline. Check torque on busbar clamps (typically 25 N·m) to prevent hot spots from loose connections. Verify secondary wiring integrity, ensuring no open circuits (which can induce dangerous overvoltages).

Five-Year Comprehensive Maintenance

Perform full ratio, polarity, and excitation curve tests. Plot knee-point voltage (Vk) and compare to factory data; a >10% drop indicates core degradation. Conduct partial discharge measurement if available (acceptance: <20 pC at 1.2 × Um/√3). Replace if PD exceeds 50 pC or Vk falls below 80% of original.

Fault Diagnosis and Troubleshooting

Common issues include: (1) Ratio drift—caused by winding turn shorts; (2) High phase error—indicative of core aging or DC offset; (3) Insulation failure—evidenced by low megger readings or external flashover. Always de-energize and ground before inspection. Never operate with secondary open—use shorting links during meter replacement.

Conclusion

The ZJC-10Q 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, merging IEC 61869-2 and GB/T 20840.2 compliance with field-proven reliability. Its VPI epoxy resin insulation eliminates fire and environmental hazards inherent in oil-filled designs, while the GOES silicon steel core ensures metrological precision from 1% to 120% of rated current. Engineered for dual functionality, it simultaneously supports Class 0.2S/0.5 revenue metering and 5P10/5P20 protection duties without performance compromise. Rigorous factory testing—including partial discharge, thermal endurance, and short-circuit withstand—guarantees consistent operation across diverse environments, from coastal substations to industrial plants. With an expected service life of 25–30 years under standard conditions, the ZJC-10Q minimizes total cost of ownership through zero-maintenance operation, resistance to pollution and humidity, and immunity to mechanical vibration. Its compact form factor and IP54 rating facilitate seamless integration into modern switchgear, supporting the global transition toward smarter, safer, and more resilient power distribution networks. For engineers specifying instrumentation in 11kV (or 10kV domestic) systems, the ZJC-10Q delivers the technical excellence and regulatory assurance demanded by today’s critical infrastructure.