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CT-10K 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the CT-10K Current Transformer
The CT-10K is a high-reliability, cast-resin insulated current transformer engineered for precision metering and robust protection functions in medium-voltage (MV) power systems operating at 11kV (IEC standard), equivalent to 10kV in domestic Chinese networks. Designed in strict accordance with IEC 61869-2 and GB/T 20840.2, this instrument transformer leverages advanced vacuum pressure impregnation (VPI) epoxy resin technology to deliver superior dielectric performance, environmental resilience, and long-term operational stability.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the CT-10K employs a thermosetting epoxy system processed under vacuum and pressure to eliminate air voids and moisture ingress. This VPI technique ensures homogeneous encapsulation of the primary conductor and secondary windings around a grain-oriented electrical steel (GOES) core. The resulting monolithic structure provides excellent partial discharge resistance (<5 pC at 1.2 × Um/√3), high tracking index (>600 V), and mechanical rigidity. Unlike oil-filled alternatives, the solid dielectric eliminates fire hazards, leakage risks, and maintenance-intensive fluid management, making it ideal for indoor switchgear and densely populated urban substations where safety and space constraints are critical.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed current transformers, the CT-10K offers significant operational and safety benefits. Its dry-type construction eliminates flammability concerns, complies with stringent fire codes (e.g., IEC 60695), and requires no oil sampling or gas monitoring. The epoxy matrix exhibits low thermal expansion coefficients, minimizing stress during thermal cycling between -40°C and +55°C ambient conditions. Additionally, the absence of liquid insulation reduces weight by approximately 30%, simplifying handling and mounting on busbars or support insulators. Electromagnetic performance remains stable over decades due to the hermetic seal against humidity—critical in coastal or high-humidity regions where condensation can degrade paper-oil insulation systems.
Typical Applications Overview
The CT-10K serves dual roles in MV infrastructure: accurate energy metering (Class 0.2S or 0.5S) and fast-acting protective relaying (Class 5P or 10P). It is commonly deployed in 11kV ring main units (RMUs), pad-mounted substations, and utility distribution feeders. In industrial settings, it interfaces with digital multifunction meters and numerical relays for overload, earth-fault, and differential protection schemes. Its compact footprint and IP54-rated terminal box facilitate integration into both new-build and retrofit projects across utilities, manufacturing plants, and renewable energy interconnection points.
Technical Specifications
The CT-10K delivers precise current transformation under defined electrical and environmental parameters, ensuring compatibility with modern secondary systems.
Electrical Ratings and Performance Parameters
| Parameter | Value |
|---|---|
| Model | CT-10K |
| System Voltage (Um) | 11 kV (IEC), 10 kV (GB) |
| Primary Current (Ip) | 50 A to 3150 A (standard ratios) |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Class (Metering) | 0.2S, 0.5S per IEC 61869-2 |
| Accuracy Class (Protection) | 5P10, 5P20, 10P10, 10P20 |
| Rated Output (VA) | 2.5 VA to 30 VA (at rated burden) |
| Short-Time Thermal Current (Ith) | 20 kA for 1 s (at 50 Hz) |
| Dynamic Withstand Current (Idyn) | 50 kA peak |
| Insulation Level (LI/AC) | 75 kV / 28 kV (1 min, 50 Hz) |
| Partial Discharge | <5 pC at 1.2 × (11/√3) kV |
All accuracy classes comply with composite error limits: ≤0.2% for 0.2S at 20–120% of rated current, and ≤10% composite error for 5P20 at 20× rated current.
Environmental and Service Conditions
The CT-10K is rated for continuous operation under the following service conditions per IEC 60060-1 and GB/T 11022:
- Ambient Temperature: -40°C to +55°C
- Relative Humidity: Up to 100% (condensing)
- Altitude: ≤1000 m above sea level (derating required above 1000 m)
- Pollution Degree: III (creepage distance ≥20 mm/kV)
- Installation Orientation: Vertical or horizontal (primary conductor axis)
For altitudes exceeding 1000 m, the power frequency withstand voltage must be reduced by 1% per 100 m increment above 1000 m. The terminal box is sealed to IP54, protecting secondary connections from dust and water splashes during routine maintenance.
Typical Applications
The CT-10K’s dual-certification and robust design enable deployment across diverse power infrastructure segments.
Substation Secondary Metering Systems
In 11kV utility substations, the CT-10K provides revenue-grade metering when configured with 0.2S accuracy class. It interfaces directly with static kWh meters compliant with IEC 62053-22, ensuring billing accuracy within ±0.2% across load profiles from 1% to 120% of rated current. The low phase displacement error (<10 minutes at 100% In) minimizes reactive energy measurement drift. In smart grid deployments, its stable ratio enables seamless integration with AMI (Advanced Metering Infrastructure) data concentrators without recalibration over 15-year intervals.
Industrial Power Distribution Protection
Manufacturing facilities using 10kV/11kV internal distribution rely on the CT-10K’s 5P20 class for motor feeder and transformer protection. During fault conditions (e.g., 20× rated current), the core remains unsaturated, delivering linear secondary output to numerical relays like SEL-751 or Siemens 7SJ62. This ensures trip times within 20 ms for short-circuit faults while avoiding nuisance tripping during motor inrush (typically 6–8× In for 0.5 s). The high thermal withstand rating (20 kA/1s) accommodates upstream breaker coordination in complex radial networks.
Renewable Energy Integration Points
At solar PV or wind farm interconnection substations, the CT-10K monitors bidirectional power flow and provides anti-islanding protection. Its symmetrical hysteresis loop (due to GOES core) ensures consistent accuracy during reverse power conditions. For grid-code compliance (e.g., IEEE 1547 or NB/T 32004), the transformer supports harmonic-rich environments up to the 13th order without significant ratio shift—critical during inverter-based generation transients.
Rural and Suburban Distribution Networks
In remote 10kV feeders with limited maintenance access, the CT-10K’s maintenance-free design reduces lifecycle costs. Its UV-stabilized resin housing resists degradation under prolonged solar exposure, while hydrophobic surface properties prevent flashovers during fog or light rain. Utilities deploy it on pole-top reclosers for sectionalizing faulted segments, leveraging its 10P10 class to distinguish permanent faults from temporary tree contacts.
Compliance with International Standards
The CT-10K meets rigorous global and regional requirements for safety, performance, and interoperability.
IEC 61869-2 Certification Details
IEC 61869-2 governs electromagnetic instrument transformers, specifying test methods for accuracy, thermal behavior, and dielectric strength. The CT-10K undergoes type tests including temperature rise (≤60 K for windings), short-circuit withstand (20 kA/1s), and accuracy verification across burden ranges (2.5–30 VA). Composite error for protection classes is validated at specified multiples of rated current (e.g., 20× for 5P20). Every production unit receives routine tests: power frequency withstand (28 kV/1 min), partial discharge (<5 pC), and polarity check. Certification is issued by accredited labs (e.g., KEMA, CESI) with traceable calibration chains.
Alignment with GB/T 20840.2
For the Chinese market, the CT-10K complies with GB/T 20840.2—2014, which harmonizes with IEC 61869-2 but includes localized requirements. Key differences include mandatory seismic testing (0.3g horizontal acceleration) and stricter creepage distance (≥25 mm/kV for pollution degree IV in industrial zones). The domestic 10kV system voltage aligns with GB 1094.1, though the transformer’s insulation level (75/28 kV) exceeds minimum GB requirements (70/28 kV), providing additional safety margin.
Testing and Certification Documentation
Each CT-10K unit ships with a test report detailing:
- Ratio and polarity verification (per IEC 61869-1 Annex B)
- Dielectric tests (LI 75 kV, AC 28 kV/1 min)
- Partial discharge magnitude at 1.2 × (Um/√3)
- Accuracy class validation at 1%, 5%, 20%, 100%, and 120% In
Certificates of Conformity reference both IEC 61869-2:2012 and GB/T 20840.2—2014, enabling cross-border project acceptance.
On-Site Testing Procedures
Post-installation verification ensures the CT-10K performs within specification before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-ground, secondary-to-ground, and primary-to-secondary. Acceptance criteria: ≥1000 MΩ at 20°C. Correct for temperature using RT2 = RT1 × 2(T1−T2)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement. Perform before and after dielectric tests to detect insulation degradation.
Turns Ratio Test
Apply low-voltage AC (5–10 V) to the secondary winding and measure induced primary voltage (open-circuit). Calculate actual ratio: Np/Ns = Vs/Vp. Tolerance must be within ±0.2% of nameplate ratio for metering classes, ±0.5% for protection. Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for automated comparison across multiple taps.
Polarity Verification
Confirm reducing polarity using a 1.5 V DC battery and analog ammeter. Momentarily connect (+) to P1 and (−) to P2; the ammeter needle should kick positive when connected to S1(+) and S2(−). Incorrect polarity causes 180° phase shift, leading to relay misoperation or meter reversal. Document results in commissioning reports.
Power Frequency Withstand Voltage Test
Apply 28 kV RMS at 50 Hz between primary and grounded secondary/core for 1 minute. Leakage current must remain below 10 mA. Gradually ramp voltage (≤1 kV/s) to avoid transient overstress. This test validates insulation integrity after transportation-induced microcracks. Never perform if ambient humidity exceeds 80%.
Short-Circuit Test (for CT)
Inject 10–20× rated secondary current (e.g., 50 A for 5 A secondary) into the secondary winding with primary shorted. Monitor for abnormal heating or saturation. The test verifies thermal stability under fault conditions. Duration: ≤10 seconds. Core temperature rise should not exceed 5 K above ambient. This simulates relay burden during high-current events.
Preventive Maintenance Guide
Although cast-resin CTs are largely maintenance-free, periodic checks extend service life beyond 30 years.
Annual Visual and Electrical Inspection
Inspect for:
- Surface cracks, tracking, or UV discoloration on resin housing
- Corrosion on primary terminals or mounting hardware
- Loose secondary connections in terminal box (torque: 1.5 N·m for M4 screws)
- Moisture accumulation inside terminal box (use desiccant if needed)
Perform insulation resistance and ratio tests annually in critical applications (e.g., hospital or data center feeders). Record trends—declining insulation resistance may signal end-of-life.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicators |
|---|---|---|
| 1 year | Visual inspection, IR test | Cracks, discoloration, IR <500 MΩ |
| 5 years | Full accuracy retest, PD measurement | Ratio error >0.5%, PD >10 pC |
| 10+ years | Thermal imaging under load | Hot spots >10 K above ambient |
Common failure modes include secondary winding open-circuit (causing dangerous overvoltage) and core lamination damage from mechanical shock. Always de-energize and ground before maintenance.
Conclusion
The CT-10K 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 the fire and environmental risks associated with liquid-filled alternatives, while the GOES silicon steel core ensures metrological stability across decades of service. With dual functionality for 0.2S metering and 5P20 protection, it supports the evolving demands of smart grids, industrial automation, and renewable integration. Rigorous type testing—including 75 kV lightning impulse withstand and 20 kA thermal short-circuit endurance—validates performance under extreme conditions. When installed and maintained per manufacturer guidelines, the CT-10K delivers an expected service life of 25–30 years with minimal intervention. Its standardized dimensions and terminal configuration simplify retrofitting into legacy 10kV/11kV infrastructure, making it a future-proof investment for utilities and industrial operators seeking accuracy, safety, and regulatory compliance in a single package.