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VT-36K 33kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the VT-36K Current Transformer
The VT-36K is a high-reliability, cast-resin insulated current transformer engineered for accurate measurement and robust protection in medium-voltage power systems operating at 33kV (IEC standard) or 35kV (domestic Chinese grid equivalent). Designed in strict compliance with IEC 61869-2 and GB/T 20840.2, this instrument transformer leverages vacuum pressure impregnation (VPI) epoxy resin technology to deliver exceptional dielectric strength, environmental resilience, and long-term operational stability. Unlike traditional oil-immersed CTs, the VT-36K eliminates fire hazards, reduces maintenance overhead, and supports both indoor and outdoor deployment without risk of leakage or degradation under UV exposure.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the VT-36K utilizes a two-component cycloaliphatic epoxy system cured under vacuum and pressure to fully encapsulate the primary conductor, secondary windings, and magnetic core. This process eliminates air voids and moisture ingress pathways, achieving a homogeneous dielectric barrier with a relative permittivity (εr) of approximately 3.8–4.2 and volume resistivity exceeding 1×1014 Ω·cm. The VPI method ensures complete impregnation of the GOES (grain-oriented electrical steel) core laminations, minimizing partial discharge activity to below 5 pC at 1.2 × Um/√3 (where Um = 36 kV). This construction provides superior tracking resistance (CTI > 600 V) and withstands thermal cycling from –40°C to +40°C without cracking or delamination.
Advantages Over Oil-Immersed Designs
Compared to oil-filled CTs, the VT-36K offers significant operational and safety benefits. It is inherently non-flammable (meeting IEC 60695 flammability class V-0), requires no periodic oil sampling or gas monitoring, and exhibits negligible aging under continuous load. Its solid insulation eliminates the risk of bushing leaks, making it ideal for urban substations, underground installations, and environmentally sensitive zones. Additionally, the compact footprint—enabled by higher dielectric strength of epoxy versus oil—reduces spatial requirements by up to 30%. The absence of liquid also simplifies transportation and commissioning, as no vacuum drying or oil filling procedures are needed.
Typical Applications Overview
The VT-36K is deployed across utility transmission networks, industrial complexes, and renewable energy interconnection points where precision metering and fast fault detection are critical. It supports dual secondary windings—one optimized for 0.2S class revenue metering and another rated 5P20 for overcurrent and differential protection relays. Common use cases include 33kV ring main units (RMUs), switchgear panels in 110/33kV substations, and feeder monitoring in solar farms connected to the grid via 35kV collection systems. Its robust design ensures stable performance even under harmonic distortion (THD ≤ 15%) and transient overcurrent conditions up to 20× rated current.
Technical Specifications
The VT-36K current transformer is engineered to meet stringent electrical and mechanical requirements for 33kV systems. Below is a comprehensive specification table aligned with IEC 61869-2 and GB/T 20840.2:
| Parameter | Value |
|---|---|
| System Voltage (Um) | 36 kV (IEC); 35 kV (GB) |
| Rated Primary Current (Ip) | 50 A to 3150 A (standard ratios: 100/5, 200/1, 400/5, etc.) |
| Rated Secondary Current (Is) | 1 A or 5 A |
| Metering Accuracy Class | 0.2S (per IEC 61869-2, error ≤ ±0.2% at 1–120% In) |
| Protection Accuracy Class | 5P20 (composite error ≤ 5% at 20× In) |
| Rated Output (Burden) | 15 VA (metering), 30 VA (protection) |
| Insulation Level (LI/AC) | 170 kV / 70 kV (1.2/50 μs lightning impulse; 1 min power frequency) |
| Short-Time Thermal Current | 25 kA for 1 s (Ith) |
| Dynamic Withstand Current | 63 kA peak (Idyn) |
| Ambient Temperature Range | –40°C to +40°C |
| Maximum Altitude | 1000 m above sea level (derating required above 1000 m) |
| Relative Humidity | ≤ 95% (non-condensing) |
| Core Material | GOES silicon steel, 0.3 mm thickness, grain-oriented |
| Insulation System | VPI cycloaliphatic epoxy resin, UL 94 V-0 rated |
Standard Service Conditions
The VT-36K is rated for continuous operation under IEC 60060-1 standard atmospheric conditions: temperature range –40°C to +40°C, relative humidity up to 95% (non-condensing), and altitude not exceeding 1000 m. At altitudes between 1000 m and 2000 m, the power frequency withstand voltage must be derated by 1% per 100 m increment above 1000 m. For example, at 1500 m, the AC test voltage becomes 70 kV × (1 – 0.05) = 66.5 kV. The transformer is designed for three-phase systems with balanced loading and assumes a maximum system frequency deviation of ±0.5 Hz from nominal 50 Hz.
Accuracy and Burden Characteristics
The 0.2S metering winding maintains phase error within ±10 minutes and ratio error within ±0.2% across 1% to 120% of rated current, satisfying IEC 62053-22 for smart metering applications. The 5P20 protection winding delivers a saturation factor (FS) ≥ 20, ensuring linear response up to 20× rated current with composite error ≤ 5%. Burden compatibility is verified at 15 VA (0.2S) and 30 VA (5P20) using standard burdens (e.g., ZB = 3 Ω for 5 A secondary at 30 VA). Exceeding rated burden increases ratio error nonlinearly; for instance, at 40 VA on the protection winding, the 5P20 rating may degrade to 5P15.
Typical Applications
Substation Secondary Metering
In 110/33kV grid substations, the VT-36K’s 0.2S winding interfaces directly with revenue-class kWh meters and SCADA RTUs. Its low phase displacement (< 10′ at 100% In) ensures accurate reactive energy measurement, critical for power factor billing. For example, in a 33kV feeder supplying a 10 MVA industrial load, the CT enables ±0.2% billing accuracy over a dynamic range from 100 kW (1% load) to 10 MW (100% load). The dual-winding design allows simultaneous connection to billing meters and demand-side management systems without cross-interference.
Industrial Power Distribution
Large manufacturing facilities operating at 35kV utilize the VT-36K for motor protection and energy auditing. In steel mills or chemical plants, the 5P20 winding feeds numerical relays (e.g., SEL-751) that detect phase-to-phase faults within 20 ms. The CT’s high dynamic withstand (63 kA) accommodates inrush currents from large induction motors (up to 8× In for 0.5 s). Its cast-resin housing resists chemical fumes and dust, meeting IP54 ingress protection when mounted in switchgear.
Renewable Energy Integration
Solar and wind farms connecting to 33kV/35kV distribution networks rely on the VT-36K for grid compliance monitoring. During cloud-induced irradiance fluctuations, PV inverters cause rapid current swings; the CT’s low remanence (< 10% of saturation flux density) prevents core saturation during asymmetrical transients. The 0.2S output feeds into power quality analyzers that verify THD limits per IEEE 519, while the 5P20 winding triggers anti-islanding protection during grid faults.
Rural and Suburban Distribution Networks
In remote 35kV radial feeders, the VT-36K enables automated sectionalizing via recloser controls. Its extended temperature tolerance (–40°C) ensures reliability in alpine or desert climates. For instance, in a 50-km rural line with distributed generation, the CT provides fault current data to adaptive relays that adjust pickup settings based on DG contribution. The maintenance-free design reduces outage time in areas with limited technician access.
Urban Underground Substations
Compact city substations benefit from the VT-36K’s fire-safe construction. Installed in SF6-free switchgear, it eliminates oil containment requirements and complies with municipal fire codes. The reduced partial discharge (< 5 pC) minimizes electromagnetic interference with nearby communication cables in dense duct banks.
Compliance with International Standards
IEC 61869-2 Certification Details
The VT-36K is fully certified to IEC 61869-2:2012 (Instrument transformers – Part 2: Additional requirements for current transformers). This includes verification of accuracy classes under defined burdens, temperature rise tests (ΔT ≤ 60 K for windings), and short-circuit performance per Clause 12. The type test report includes lightning impulse (170 kV), power frequency (70 kV/1 min), and partial discharge measurements at 1.2 × Um/√3 = 24.9 kV. The 0.2S class validation follows IEC 61869-10 Annex B, requiring error measurement at 1%, 5%, 20%, 100%, and 120% of In.
Alignment with GB/T 20840.2
For domestic Chinese markets, the VT-36K meets GB/T 20840.2-2014, which largely harmonizes with IEC 61869-2 but specifies 35kV as the nominal system voltage (vs. 33kV IEC). Key differences include slightly higher short-time thermal current requirements (31.5 kA/1s in GB vs. 25 kA in IEC for similar ratings) and mandatory seismic testing (0.3g horizontal acceleration). The CT undergoes additional salt fog testing (1000 h per GB/T 2423.17) for coastal deployments.
Testing and Certification Requirements
Certification involves type tests (performed once per design), routine tests (100% production), and sample tests (per batch). Type tests include temperature rise (IEC 60060-2), short-circuit (IEC 61869-2 Clause 12), and accuracy verification. Routine tests cover polarity, turns ratio (±0.5% tolerance), insulation resistance (> 1000 MΩ at 2500 V DC), and power frequency withstand (70 kV/1 min). All test records are traceable to national metrology institutes (e.g., NIM China, PTB Germany).
On-Site Testing Procedures
Insulation Resistance Test
Perform with a 2500 V DC megohmmeter between primary-to-secondary/ground and secondary-to-ground. Acceptance criterion: ≥ 1000 MΩ at 20°C. Correct for temperature using RT = R20 × 2(20–T)/10. Low readings (< 100 MΩ) indicate moisture ingress or resin cracking—requiring drying or replacement.
Turns Ratio Test
Apply 1–5 V AC (50 Hz) to secondary winding; measure induced primary voltage. Calculate ratio as Vp/Vs. Tolerance: ±0.5% of nameplate ratio. For a 400/5 CT, measured ratio must be 79.6–80.4. Deviations suggest turn shorts or incorrect tap selection.
Polarity Verification
Use DC kick method: connect battery (+) to P1, (–) to P2; connect galvanometer between S1 and S2. Momentary closure should produce positive deflection. Confirms reducing polarity (IEC 61869-2 Figure 101). Incorrect polarity causes relay misoperation.
Power Frequency Withstand Voltage Test
Apply 70 kV RMS (50 Hz) for 1 minute between primary and grounded secondary/core. Leakage current must remain < 10 mA. Partial discharge should not exceed 10 pC. Failure indicates insulation defects.
Secondary Winding Resistance and Short-Circuit Test
Measure DC resistance of secondary windings (e.g., 0.15 Ω ±10% for 5 A winding). Then perform short-circuit test: inject 10–20% In into primary; verify secondary current matches ratio within ±1%. Ensures no open circuits or high-resistance joints.
Preventive Maintenance Guide
Periodic Inspection Protocol
Conduct annual visual inspections for surface tracking, cracks, or terminal corrosion. Clean with dry cloth; avoid solvents. Check torque on M10 secondary terminals (25 N·m). Verify grounding continuity (< 0.1 Ω). Use infrared thermography during peak load to detect hot spots (> 10 K above ambient indicates internal fault).
Maintenance Intervals and Fault Diagnosis
Replace only if insulation resistance drops below 100 MΩ after cleaning/drying, or if ratio error exceeds ±1.0%. Common faults: moisture ingress (low IR, high PD), core saturation (distorted secondary waveform), or winding opens (infinite resistance). No scheduled internal maintenance is required due to hermetic resin sealing.
| Interval | Action |
|---|---|
| Annually | Visual inspection, IR scan, terminal torque check |
| Every 5 Years | Full electrical tests (ratio, polarity, IR, withstand) |
| After Fault | Post-fault ratio and insulation tests before re-energizing |
Conclusion
The VT-36K 33kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining metrological precision with rugged protection capabilities. Its dual-winding architecture—featuring 0.2S class accuracy for revenue metering and 5P20 performance for high-speed relaying—ensures compliance with both commercial and safety-critical grid requirements. Constructed with GOES silicon steel cores and VPI epoxy resin, the unit delivers exceptional dielectric integrity (partial discharge < 5 pC), thermal stability (ΔT ≤ 60 K), and environmental resilience across –40°C to +40°C. Certified to IEC 61869-2 and GB/T 20840.2, it bridges international and domestic grid standards, supporting seamless integration in 33kV IEC systems and 35kV Chinese networks alike. The elimination of flammable oil enhances safety in urban and confined installations, while the maintenance-free design reduces lifecycle costs. With a proven service life of 25–30 years under standard operating conditions, the VT-36K offers utilities and industrial operators a reliable, future-proof solution for accurate energy accounting and dependable fault protection. Its robust short-circuit withstand (25 kA/1s thermal, 63 kA dynamic) and immunity to harmonic distortion further solidify its role in modernizing aging infrastructure and integrating distributed energy resources.