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LB-10W 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the LB-10W Current Transformer
The LB-10W is a high-reliability, cast-resin insulated current transformer (CT) engineered for accurate current measurement and robust protective relay coordination 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 device leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings, ensuring long-term dielectric integrity even under harsh environmental conditions.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LB-10W employs a two-stage VPI process where the wound core assembly is first evacuated to remove air pockets, then impregnated under pressure with cycloaliphatic epoxy resin. This creates a monolithic, void-free structure that eliminates partial discharge risks below 10 pC at 1.2 × Um/√3 (where Um = 12 kV). The resin’s high tracking resistance (>600 V per IEC 60587) and hydrophobic surface prevent moisture ingress and surface leakage currents—critical for outdoor installations exposed to pollution, salt fog, or industrial contaminants. Unlike oil-filled alternatives, the solid dielectric requires no maintenance for fluid levels or gas monitoring, significantly reducing lifecycle costs while maintaining consistent thermal performance across –40°C to +40°C ambient ranges.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LB-10W’s dry-type construction eliminates fire hazards, environmental contamination risks from oil leaks, and the need for conservator tanks or Buchholz relays. Its compact footprint (typically 220 mm height × 180 mm diameter) allows direct mounting on 11kV switchgear without additional support structures. The absence of liquid insulation also enables faster commissioning—no drying-out procedures or dielectric strength verification of insulating oil are required. Furthermore, the thermal time constant of the epoxy resin system ensures stable accuracy during transient overloads up to 20× rated current for 1 second, a key requirement for protection-class applications per IEC 61869-2 Clause 6.3.2.
Typical Applications Overview
The LB-10W serves dual roles: precision metering (accuracy classes 0.2S or 0.5S) and fault-current detection for protection relays (classes 5P10 or 5P20). It is commonly deployed in 11kV ring-main units (RMUs), pad-mounted substations, and indoor/outdoor switchyards where space constraints, fire safety codes, or environmental regulations prohibit oil-filled equipment. Its design accommodates both through-type (primary conductor as busbar) and bar-type configurations, supporting primary currents from 50 A to 3150 A with standardized secondary outputs of 1 A or 5 A. This versatility makes it suitable for utility distribution networks, industrial plants with arc-flash mitigation requirements, and renewable energy interconnection points requiring high-fidelity waveform reproduction.
Technical Specifications
The LB-10W meets stringent electrical and mechanical performance criteria defined by international and domestic standards. Below is a representative specification table for a typical 11kV/10kV unit:
| Parameter | Value |
|---|---|
| Rated Voltage (Ur) | 11 kV (IEC) / 10 kV (GB) |
| Maximum System Voltage (Um) | 12 kV |
| Primary Current (Ip) | 50–3150 A (standard steps) |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Class (Metering) | 0.2S, 0.5S |
| Accuracy Class (Protection) | 5P10, 5P20 |
| Rated Output (VA) | 2.5–30 VA (per burden class) |
| Short-Time Thermal Current | 20× Ip for 1 s |
| Dynamic Withstand Current | 50× Ip peak |
| Power Frequency Withstand Voltage | 28 kV rms, 1 min |
| Lightning Impulse Withstand | 75 kV peak (1.2/50 μs) |
| Insulation Material | Cycloaliphatic epoxy resin (VPI) |
| Magnetic Core | Grain-oriented electrical steel (GOES), CRGO grade |
| Ambient Temperature Range | –40°C to +40°C |
| Altitude Limit | ≤1000 m (derating above 1000 m per IEC 60071-1) |
Standard Service Conditions
The LB-10W is rated for continuous operation under IEC 60060-1 standard atmospheric conditions: ambient temperature between –40°C and +40°C, relative humidity up to 95% non-condensing, and installation altitude not exceeding 1000 meters above sea level. At altitudes above 1000 m, the power frequency withstand voltage must be derated by 1.25% per 100 m increment per IEC 60071-1. The transformer is designed for both indoor and outdoor use, with UV-stabilized resin formulation ensuring no degradation after 10,000 hours of accelerated weathering per IEC 60529 (IP54 ingress protection when mounted vertically). Humidity resistance is validated by 56-day damp-heat testing at 40°C/95% RH with no measurable change in insulation resistance (>10 GΩ at 2500 V DC).
Core and Winding Design Parameters
The magnetic circuit utilizes grain-oriented electrical steel (GOES) with a maximum core loss of 0.9 W/kg at 1.7 T and 50 Hz, ensuring low excitation current and minimal phase error—critical for 0.2S class metering. Secondary windings are wound with oxygen-free copper (OFC) wire, insulated with Class F (155°C) enamel, and embedded directly into the resin matrix to prevent vibration-induced fatigue. The turns ratio tolerance is held within ±0.25% for metering classes and ±1% for protection classes, verified at 10% of rated primary current. Burden compatibility spans 2.5 VA to 30 VA, allowing connection to modern digital meters (typically 1–5 VA) or electromechanical relays (10–30 VA) without accuracy compromise.
Typical Applications
The LB-10W’s dual-certification (IEC and GB) and robust construction enable deployment across diverse power infrastructure segments.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LB-10W provides revenue-grade current signals to AMI (Advanced Metering Infrastructure) systems. Configured with 0.2S accuracy and 5 VA burden, it interfaces with IEC 61850-compliant merging units or direct-connected smart meters. For example, in a European utility substation, an LB-10W with 600/5 A ratio feeds data to a Class 0.5S kWh meter, achieving billing accuracy within ±0.3% over 1%–120% of rated current. The cast-resin body resists condensation in unheated cabinets, eliminating errors caused by moisture-induced insulation leakage in older oil-CT designs.
Industrial Power Distribution
Within manufacturing facilities, the LB-10W supports motor protection and energy management. A 1250/1 A, 5P20 unit installed on a 11kV feeder supplies inputs to a multifunction relay (e.g., Siemens 7SJ62) for overload, short-circuit, and earth-fault detection. The 20× thermal withstand capability ensures survival during motor starting inrush (typically 6–8× full-load current for 10–30 seconds). Its flame-retardant resin (UL 94 V-0 rated) complies with NFPA 70E arc-flash safety zones, making it suitable for hazardous locations where oil-filled CTs pose explosion risks.
Renewable Energy Integration
Solar and wind farms utilize the LB-10W at the point of interconnection (POI) to grid operators. In a 10 MW solar plant, three LB-10W units (2000/5 A, 0.5S) monitor export power to meet IEEE 1547-2018 waveform fidelity requirements. The low remanence (<10% of saturation flux density) prevents core saturation during DC-offset faults common in inverter-based resources. Additionally, the wide temperature tolerance accommodates desert environments (e.g., Middle East) where daytime temperatures exceed 50°C ambient.
Rural and Suburban Distribution Networks
For rural electrification projects in Asia and Africa, the LB-10W’s maintenance-free operation reduces operational expenditure. Mounted on pole-top transformers or underground RMUs, its IP54 rating withstands monsoon rains and dust storms. A typical 400/5 A, 5P10 configuration protects distribution feeders against tree-contact faults, with the 10× accuracy limit factor ensuring relay operation at 4 kA fault current. The domestic 10kV compatibility simplifies procurement for utilities using legacy Chinese-standard switchgear.
Compliance with International Standards
The LB-10W is engineered to satisfy both global and regional regulatory frameworks, ensuring interoperability and safety.
IEC 61869-2 Compliance Details
Per IEC 61869-2:2012, the LB-10W undergoes type tests including temperature rise (≤60 K for windings at 1.2× rated current), short-circuit withstand (20× Ip for 1 s with ≤5% permanent ratio error shift), and composite error measurement for protection classes. The standard mandates that metering CTs maintain ratio error within ±0.2% and phase displacement ≤±10 minutes for 0.2S class at 20%–120% of rated current. All LB-10W units are factory-tested at 100% of these limits, with test reports traceable to ISO/IEC 17025-accredited laboratories. Partial discharge levels are verified at ≤5 pC under 1.2 × Um/√3, exceeding the standard’s 10 pC requirement.
GB/T 20840.2 Alignment
For Chinese markets, the LB-10W aligns with GB/T 20840.2-2014, which largely harmonizes with IEC 61869-2 but specifies 10kV as the nominal system voltage (vs. 11kV IEC). Key differences include lightning impulse test voltage (75 kV vs. IEC’s 75 kV—identical in this case) and short-time current duration (1 s for both). However, GB/T requires additional seismic testing per GB/T 13540 for earthquake-prone regions, which the LB-10W passes at 0.3g horizontal acceleration. Domestic certification includes CCC marking and type test approval from China Electric Power Research Institute (CEPRI).
Testing and Certification Requirements
Full compliance necessitates routine tests (100% production), type tests (design validation), and special tests (customer-requested). Routine tests include polarity verification, winding resistance measurement (±2% tolerance), and power frequency withstand at 28 kV for 1 minute. Type tests add temperature rise, short-circuit, and accuracy verification across burden ranges. Certificates issued include IEC CB Scheme reports, CEPRI GB/T approvals, and RoHS/REACH material declarations.
On-Site Testing Procedures
Post-installation verification ensures the LB-10W performs within specifications before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-secondary, primary-to-ground, and secondary-to-ground. Acceptance criterion: ≥10 GΩ at 25°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings (<1 GΩ) indicate moisture ingress or resin cracking—requiring drying or replacement. Perform before and after power frequency withstand tests to detect insulation degradation.
Turns Ratio Test
Apply 1–5 V AC to the secondary winding and measure induced primary voltage (for through-type CTs, use a reference winding). Calculate ratio as Vp/Vs × Ns. Tolerance: ±0.25% for 0.2S/0.5S, ±1% for 5P classes. Alternatively, use a dedicated CT analyzer (e.g., Omicron CT Analyzer) injecting 10% of rated primary current. Verify at multiple tap points if multi-ratio.
Polarity Test
Confirm reducing polarity per IEC 61869-2 Figure 3. Connect a 1.5 V battery between P1 and P2; observe momentary deflection on a DC milliammeter connected to S1 and S2. Positive kick at S1 indicates correct polarity. Incorrect polarity causes 180° phase reversal—catastrophic for differential protection schemes. Document results with oscillograms if using automated testers.
Power Frequency Withstand Voltage Test
Apply 28 kV rms at 50 Hz between primary and grounded secondary/enclosure for 1 minute. Use a calibrated HV test set with overcurrent trip ≤100 mA. No flashover or sustained discharge constitutes pass. Pre-test: ensure all secondary terminals are shorted and grounded. Post-test: repeat insulation resistance to confirm no damage.
Composite Error Test for Protection Class
For 5P10/5P20 verification, inject 10× or 20× rated primary current at specified burden (e.g., 15 VA). Measure composite error ε = (∫(ip/Kn – is)dt) / (∫ip/Kn dt) × 100%. Acceptance: ε ≤ ±5% for 5P class. Use a burden resistor bank with ±0.5% tolerance. Critical for ensuring relay operates within time-current curves during faults.
Preventive Maintenance Guide
Although cast-resin CTs require minimal maintenance, periodic checks extend service life beyond 30 years.
Periodic Inspection Protocol
Conduct visual inspections annually: check for resin cracks, terminal corrosion, or tracking marks. Clean surfaces with isopropyl alcohol if contaminated by salt or carbon deposits. Verify torque on M10 primary bolts (25 N·m) and M6 secondary lugs (10 N·m). Use infrared thermography during peak load to detect hotspots (>10 K above ambient indicates loose connections). Record secondary excitation curves every 5 years to monitor core aging—deviation >15% from baseline suggests insulation degradation.
Maintenance Intervals and Fault Diagnosis
Follow this schedule:
| Interval | Action |
|---|---|
| Annual | Visual inspection, IR scan, terminal torque check |
| 5 Years | Insulation resistance, turns ratio, excitation curve |
| 10 Years | Partial discharge measurement (if available) |
| After Fault | Full suite: ratio, polarity, insulation, composite error |
Common faults include open-circuited secondaries (causing dangerous overvoltages) and core saturation from DC offset. Diagnose via abnormal excitation knee-point shift or elevated magnetizing current. Never operate with secondary open—always short before disconnecting meters/relays.
Conclusion
The LB-10W 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 fire and environmental hazards associated with oil-filled alternatives, while the GOES silicon steel core ensures metrological precision for both revenue metering (0.2S/0.5S) and high-speed protection (5P10/5P20). Rigorous factory testing—including partial discharge verification below 5 pC and short-circuit withstand at 20× rated current—guarantees performance under extreme electrical stress. Designed for 25–30 years of maintenance-free operation across –40°C to +40°C ambient ranges, the LB-10W is ideally suited for modern substations, industrial facilities, and renewable integration points where safety, accuracy, and longevity are paramount. Its dual-voltage rating (11kV IEC / 10kV GB) streamlines global deployment, making it a versatile solution for utilities and industrial operators seeking future-proof instrumentation infrastructure.