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CT-35K 33kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified

Introduction to the CT-35K Current Transformer

The CT-35K is a high-reliability, cast-resin insulated current transformer engineered for accurate metering and dependable protection in medium-voltage electrical systems rated at 33 kV (IEC standard), corresponding to domestic 35 kV distribution networks. Designed in strict compliance with IEC 61869-2 and GB/T 20840.2, this device leverages vacuum-cast epoxy resin technology to deliver superior dielectric performance, mechanical robustness, and long-term environmental stability.

Operating Principle of Cast-Resin Insulation

Cast-resin insulation in the CT-35K employs vacuum pressure impregnation (VPI) of cycloaliphatic epoxy resin around the primary conductor and secondary windings. This process eliminates air voids and moisture ingress pathways, ensuring uniform electric field distribution and preventing partial discharges under continuous operating voltage (U_m = 36 kV). The resin matrix exhibits a relative permittivity (ε_r) of approximately 3.8 and volume resistivity exceeding 1×10¹⁴ Ω·cm at 20°C. Thermal class F (155°C) insulation allows operation under peak load conditions without degradation. Unlike oil-filled alternatives, the solid dielectric eliminates fire hazards, leakage risks, and maintenance-intensive oil sampling, making it ideal for indoor switchgear and environmentally sensitive outdoor substations.

Advantages Over Oil-Immersed Designs

Compared to traditional oil-immersed current transformers, the CT-35K offers significant operational and safety benefits. Its solid insulation system eliminates flammability concerns, satisfying stringent fire codes in urban substations and industrial facilities. The absence of liquid dielectric removes the need for conservator tanks, breather systems, and periodic oil testing—reducing lifecycle costs by up to 40%. Additionally, the monolithic epoxy structure provides excellent resistance to UV radiation, salt fog (per IEC 60068-2-52), and pollution (creepage distance ≥ 900 mm for 33 kV, Class IV pollution). Mechanical strength is enhanced through fiber reinforcement, enabling withstand forces of 63 kA/1s short-circuit current without deformation. These attributes make the CT-35K particularly suited for compact GIS installations and coastal or high-altitude environments where oil viscosity and gas evolution pose reliability challenges.

Typical Applications Overview

The CT-35K serves dual roles in power systems: precision energy metering (accuracy classes 0.2S/0.5S) and protective relaying (classes 5P10/5P20). It is commonly deployed in 33/35 kV transmission-distribution interface substations, industrial plant switchyards, and renewable energy collector systems. In utility metering applications, its low phase displacement (< ±10 minutes at 1–120% I_n) ensures billing accuracy compliant with ISO/IEC 17025 requirements. For protection, the high saturation point (≥ 20× I_n for 5P20) guarantees reliable fault detection during high-magnitude short circuits. The unit’s modular design supports single or multi-ratio configurations, accommodating future load growth without hardware replacement.

Technical Specifications

The CT-35K is engineered to meet rigorous electrical and environmental performance criteria. All parameters align with IEC 61869-2 (2012) and GB/T 20840.2 (2013) standards, ensuring interoperability across global power infrastructure.

Electrical Ratings and Performance Parameters

Primary voltage rating: U_m = 36 kV (system voltage 33 kV IEC / 35 kV domestic). Primary current ratings range from 50 A to 3150 A, with standard ratios including 100/5, 200/1, 600/5, and 1200/1 A. Secondary current is standardized at 1 A or 5 A. Accuracy classes include 0.2S, 0.5S for metering, and 5P10, 5P20 for protection. Rated burden is 15–30 VA (at cos φ = 0.8 lagging). Insulation level: BIL = 170 kV (lightning impulse), power frequency withstand = 70 kV rms for 1 min. Short-time thermal current: 25–40 kA for 1 s (depending on ratio). Dynamic withstand current: 63 kA peak. Phase displacement at rated current: ≤ ±10′ for 0.2S class. Temperature rise: ≤ 60 K for windings (measured by resistance method per IEC 61869-2 Annex C).

Environmental and Service Conditions

Designed for operation in ambient temperatures from –40°C to +40°C (with derating above +40°C per IEC 60060-1). Relative humidity tolerance: up to 100% (condensing). Maximum installation altitude: 1000 m above sea level; for altitudes up to 3000 m, voltage withstand values are reduced by 1% per 100 m above 1000 m. Pollution degree: Heavy (Class IV per IEC 60815), requiring minimum creepage distance of 25 mm/kV × U_m = 900 mm. The housing features hydrophobic silicone sheds to suppress surface tracking under wet-polluted conditions. Vibration resistance complies with IEC 60068-2-6 (5–500 Hz, 0.7 g acceleration), suitable for seismic Zone 2 installations.

Construction and Materials

The magnetic core is fabricated from grain-oriented electrical steel (GOES) with thickness of 0.23–0.27 mm, achieving core loss ≤ 1.1 W/kg at 1.7 T and 50 Hz. Secondary windings use electrolytic-tough-pitch (ETP) copper, insulated with Class F enamel and embedded in the resin matrix. Primary conductor is either solid copper bar (for ≤630 A) or tubular aluminum (for >630 A), silver-plated at contact surfaces to minimize contact resistance (< 20 µΩ). Terminal blocks are made of flame-retardant polyamide (UL 94 V-0) with IP2X finger-safe protection. The entire assembly undergoes post-curing at 120°C for 8 hours to relieve internal stresses and optimize cross-linking density of the epoxy resin.

Typical Applications

The CT-35K’s dual functionality enables deployment across diverse power system architectures requiring both revenue-grade metering and high-speed protection.

Substation Secondary Metering Systems

In 33/11 kV grid substations, the CT-35K supplies current signals to kWh meters, demand recorders, and SCADA RTUs. Its 0.2S accuracy class ensures measurement error remains within ±0.2% from 1% to 120% of rated current—critical for utility billing compliance under EN 50470-1. For example, in a 20 MVA substation with 600/5 A CTs feeding a Class 0.5S meter, annual energy discrepancy is limited to < 0.3%, translating to financial accuracy of ±$1,200/year at $0.10/kWh. The low burden requirement (15 VA) minimizes voltage drop in long secondary circuits, preserving signal fidelity over distances up to 200 m.