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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 equivalent). 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 deliver superior dielectric performance, environmental resilience, and long-term operational stability without the fire hazards or maintenance demands associated with oil-filled alternatives.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LB-10W utilizes a thermosetting epoxy compound that fully encapsulates the primary conductor, secondary windings, and magnetic core under vacuum and pressure conditions. This VPI process eliminates air voids and moisture ingress pathways, resulting in a homogeneous dielectric barrier with high tracking resistance (>600 V per IEC 60587), excellent partial discharge performance (<5 pC at 1.2 × Ur), and stable mechanical properties across thermal cycles from -40°C to +40°C ambient. The resin matrix also provides structural rigidity, preventing winding displacement during short-circuit events and ensuring consistent magnetic coupling over decades of service.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LB-10W’s dry-type construction eliminates flammability risks, making it suitable for indoor substations, urban switchgear rooms, and environmentally sensitive zones. It requires no oil sampling, level monitoring, or periodic reconditioning. The absence of liquid insulation also prevents leakage-related failures and reduces lifecycle costs by up to 30% over a 25-year horizon. Additionally, the compact footprint—enabled by higher dielectric strength of epoxy versus mineral oil—facilitates retrofitting into space-constrained panels without compromising safety clearances.
Typical Applications Overview
The LB-10W is deployed across utility distribution networks, industrial plants, renewable energy interconnection points, and commercial complexes where precise metering and dependable fault detection are critical. Its dual-purpose design supports both revenue-grade energy metering (Class 0.5S or 0.2S) and high-speed protection relaying (Class 5P or 10P), enabling a single unit to serve multiple secondary circuits. Common installations include ring main units (RMUs), metal-clad switchgear, and outdoor pole-mounted configurations in 10kV/11kV radial or looped feeders.
Technical Specifications
The LB-10W current transformer is engineered to meet stringent performance criteria under defined service conditions. All parameters align with IEC 61869-2:2012 and GB/T 20840.2-2014, ensuring interoperability in global and domestic grids.
Rated Electrical Parameters
Key electrical ratings include a highest system voltage (Um) of 12 kV, corresponding to an IEC-standard 11kV nominal system (with 10kV as the common domestic operating voltage). Primary current ratings range from 50 A to 3150 A in standard steps (e.g., 100/5, 400/1, 1200/5). Secondary outputs are typically 1 A or 5 A, with rated burdens of 5 VA to 30 VA depending on accuracy class. Accuracy classes include 0.2S, 0.5S for metering and 5P10, 5P20, 10P10, 10P20 for protection. The instrument security factor (FS) is ≤5 for metering cores, while the accuracy limit factor (ALF) for protection cores ranges from 10 to 30. Short-time thermal current withstand is 20 kA for 1 second, and dynamic withstand current is 50 kA peak.
Insulation and Environmental Ratings
The LB-10W features a power frequency withstand voltage of 28 kV rms for 1 minute between primary and earth, and 3 kV rms between secondary terminals and earth. Lightning impulse withstand voltage is 75 kV peak (1.2/50 µs waveform). The creepage distance exceeds 240 mm for pollution degree III environments (IEC 60815), supporting outdoor use in coastal or industrial atmospheres. Standard service conditions assume ambient temperature from -40°C to +40°C, relative humidity ≤95%, and altitude ≤1000 m above sea level. For altitudes up to 2000 m, derating factors per IEC 60071 apply.
Magnetic Core and Construction Details
The magnetic circuit employs grain-oriented electrical steel (GOES) with low hysteresis loss (<1.0 W/kg at 1.7 T, 50 Hz) and high permeability (>1500 at 0.5 T), ensuring minimal phase error and ratio deviation under load variations. Cores are annealed post-winding to relieve mechanical stress and stabilize magnetic characteristics. Secondary windings use enameled copper wire with Class F (155°C) insulation, embedded within the epoxy matrix to prevent vibration-induced fatigue. Terminal blocks comply with IEC 61869-1 for secure screw-type connections rated for 600 V.
Typical Applications
The LB-10W’s dual-certification and robust design make it suitable for diverse power system architectures requiring precision and reliability.
Substation Secondary Metering
In 11kV/10kV primary substations, the LB-10W provides Class 0.2S or 0.5S current signals to revenue meters, SCADA RTUs, and power quality analyzers. Its low phase displacement (<±10 minutes at 1–120% In) ensures accurate kWh and kVARh integration over extended billing cycles. For example, in a municipal utility substation feeding a commercial district, three LB-10W units (one per phase) supply data to a multi-tariff metering system, enabling time-of-use billing with <0.3% cumulative error over 12 months.
Industrial Power Distribution
Heavy industries such as steel mills, chemical plants, and data centers deploy the LB-10W on 10kV feeder breakers to monitor load profiles and trigger overload protection. Here, the 5P20 or 10P20 protection cores ensure reliable operation during motor start-up inrush (up to 8×In) while maintaining saturation margin during downstream faults. The cast-resin housing resists chemical vapors and dust, complying with IP54 when mounted in switchgear compartments.
Renewable Energy Integration
Solar PV farms and wind parks connecting to 11kV distribution grids use the LB-10W for both generation metering and anti-islanding protection. During cloud-induced irradiance fluctuations, the CT’s linear response down to 1% of rated current enables accurate curtailment control. Protection-class windings interface with directional overcurrent relays to detect reverse power flow during grid outages, satisfying IEEE 1547 and IEC 61727 requirements.
Rural and Suburban Distribution Networks
Pole-mounted LB-10W units serve rural feeders where maintenance access is limited. Their maintenance-free design and UV-stabilized resin housing withstand decades of exposure to rain, ice, and temperature swings. In suburban ring-main networks, they support differential protection schemes for cable sections, with matched ratio tolerance (±0.1%) ensuring minimal spill current during external faults.
Harmonic-Rich Environments
In facilities with variable-frequency drives (VFDs) or rectifier loads, the LB-10W’s GOES core minimizes additional losses from harmonic currents (up to 13th order). Special low-leakage designs reduce eddy current heating in the resin, preventing thermal runaway even under 20% THD conditions.
Compliance with International Standards
The LB-10W is certified to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and aligned with China’s GB/T 20840.2-2014, ensuring global acceptance and local regulatory compliance.
IEC 61869-2 Certification Requirements
Compliance mandates rigorous testing of ratio error, phase displacement, temperature rise, short-circuit performance, and insulation integrity. For metering classes, ratio error must not exceed ±0.2% (Class 0.2S) at 20–120% of rated current and ±0.75% at 1%. Protection classes require ALF verification: e.g., a 5P20 CT must maintain error ≤±1% at 20×In. Temperature rise is limited to 55 K for windings (measured by resistance method) under continuous rated current. Partial discharge levels are verified at 1.2 × Um/√3 with thresholds per Table 102 of IEC 61869-2.
Alignment with GB/T 20840.2
GB/T 20840.2 mirrors IEC 61869-2 but includes supplementary clauses for Chinese grid practices, such as mandatory 4-second short-time current rating (vs. 1–3 s in IEC) and stricter creepage distances for polluted regions (≥31 mm/kV vs. IEC’s 25 mm/kV). The LB-10W exceeds both, offering 4-second thermal withstand of 16 kA and creepage of 280 mm (23.3 mm/kV at 12 kV Um).
Testing and Certification Process
Each production batch undergoes type tests (once per design), routine tests (100% units), and sample tests (per IEC 60060). Type tests include temperature rise, short-circuit, and impulse tests conducted at accredited labs like KEMA or CESI. Routine tests cover polarity, turns ratio, insulation resistance (>1000 MΩ at 500 V DC), and power frequency withstand. Certificates of conformity reference test reports with unique serial numbers traceable to manufacturing lots.
On-Site Testing Procedures
Post-installation verification ensures the LB-10W performs within specification before energization.
Insulation Resistance Test
Using a 500 V DC megohmmeter, measure insulation resistance between all primary terminals shorted together and earthed tank, and between secondary terminals and earth. Acceptance criterion: ≥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.
Turns Ratio Test
Apply a low-voltage AC source (e.g., 10 V at 50 Hz) to the secondary winding and measure induced primary voltage. Calculate actual ratio as Vsec/Vprim. Compare to nameplate; tolerance must be within ±0.2% for metering, ±1% for protection. Alternatively, use a dedicated CT analyzer injecting 1–5 A into primary and measuring secondary current.
Polarity Test
Verify reducing polarity per IEC 61869-1: momentarily connect a 1.5 V battery between P1 and P2 (primary); a DC voltmeter across S1 and S2 (secondary) should show a positive kick on connection and negative on disconnection. Incorrect polarity causes metering errors and relay misoperation.
Power Frequency Withstand Voltage Test
Apply 28 kV rms at 50 Hz for 1 minute between primary and earthed enclosure. Use a calibrated HV test set with overcurrent trip (≤100 mA). No flashover or disruptive discharge is permitted. For secondary circuits, apply 3 kV rms for 1 minute between terminals and earth.
Short-Circuit Performance Verification
While full short-circuit testing isn’t feasible on-site, verify mechanical integrity by inspecting for cracks, loose hardware, or displaced cores after transport. Confirm nameplate short-circuit ratings match system prospective fault levels. If available, perform a low-current excitation test to check for core saturation anomalies.
Preventive Maintenance Guide
Although cast-resin CTs are largely maintenance-free, periodic checks extend service life and prevent unexpected failures.
Annual Visual and Electrical Inspection
Inspect for surface tracking, UV degradation (chalky resin), or terminal corrosion. Clean with non-abrasive cloth and isopropyl alcohol. Measure insulation resistance and compare to baseline. Check torque on terminal screws (typically 2.5 N·m for M6). Ensure secondary circuits are never left open during operation—use shorting links during meter replacement.
Five-Year Comprehensive Maintenance
Every 60 months, perform partial discharge measurement using a portable PD detector at 1.2 × Um/√3. Acceptable level: <10 pC. Re-verify ratio and polarity. Inspect mounting hardware for galvanic corrosion, especially in coastal areas. Update asset records with test results for predictive analytics.
Fault Diagnosis and Replacement Criteria
Common failure modes include secondary winding opens (causing dangerous overvoltages), core saturation due to DC offset, or resin delamination from thermal cycling. Replace if: insulation resistance drops below 100 MΩ, ratio error exceeds twice the class limit, or visible cracks appear. Always de-energize and ground primary conductors before handling.
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 hazards and maintenance overhead inherent in oil-filled designs, while the GOES silicon steel core ensures metrological accuracy across metering and protection applications—from urban substations to remote renewable sites. With a rated thermal short-circuit withstand of 20 kA/1s and lightning impulse resilience up to 75 kV, the LB-10W maintains integrity under extreme grid transients. Rigorous factory and on-site testing protocols guarantee performance within tight tolerances: ratio error ≤±0.2% for Class 0.2S, phase displacement ≤±10 minutes, and partial discharge <5 pC. When installed and maintained per guidelines, the LB-10W delivers a service life of 25–30 years, making it a cost-effective, future-proof solution for modern power systems operating at 11kV (IEC) or 10kV (domestic). Its dual-role capability—supporting both revenue metering and high-speed protection in a single unit—reduces panel complexity and lifecycle costs, affirming its position as a preferred choice for utilities and industrial operators worldwide.