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IEC 61869-2 Certified 11kV Current Transformer LFS-10 for Metering & Protection Applications
Introduction to the LFS-10 Current Transformer
The LFS-10 is a medium-voltage cast-resin current transformer (CT) rated for 11kV systems per IEC standards, with a domestic system equivalent of 10kV. Engineered for dual functionality in both revenue-class metering and protective relaying, the LFS-10 leverages vacuum pressure impregnation (VPI) epoxy resin technology to deliver exceptional dielectric strength, mechanical robustness, and long-term environmental stability. Unlike traditional oil-immersed CTs, which pose fire hazards and require periodic oil sampling, the LFS-10’s solid insulation eliminates leakage risks and enables maintenance-free operation in both indoor switchgear and outdoor substations.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LFS-10 is achieved through a VPI process where high-purity cycloaliphatic epoxy resin fully encapsulates the magnetic core and secondary windings under vacuum conditions. This eliminates air voids and moisture ingress pathways, resulting in a homogeneous dielectric structure with a relative permittivity (εr) of approximately 4.2 and volume resistivity exceeding 1×1014 Ω·cm at 20°C. The resin matrix provides a creepage distance of ≥25 mm/kV for 11kV systems, satisfying pollution degree 3 requirements per IEC 60664-1. Thermal conductivity of the cured resin (~0.2 W/m·K) ensures efficient heat dissipation during continuous or transient overcurrent events, maintaining core temperature within safe limits even under 20× rated current for 1 second.
Advantages Over Oil-Immersed Designs
The LFS-10 eliminates the operational and safety drawbacks inherent in oil-filled CTs. There is no risk of flammable fluid leakage, making it suitable for confined spaces such as urban substations or industrial facilities with strict fire codes. The solid insulation exhibits superior resistance to partial discharge—typically <5 pC at 1.2×Um/√3—compared to oil-paper systems that degrade over time due to cellulose aging. Additionally, the LFS-10 requires no oil level monitoring, gas chromatography, or dielectric loss testing, reducing lifecycle costs by an estimated 30–40% over a 25-year service period. Its compact footprint (typically 180 mm height × 120 mm diameter) also facilitates retrofitting into legacy switchgear panels originally designed for smaller CT footprints.
Typical Application Overview
Deployed across utility distribution networks, industrial plants, and renewable energy interconnection points, the LFS-10 serves critical roles in energy metering (accuracy class 0.2S or 0.5S) and protection (class 5P or 10P). In 11kV ring main units (RMUs), it provides inputs to digital multifunction meters and overcurrent relays. In solar farms, it interfaces with SCADA systems for generation monitoring while simultaneously feeding fault-current data to protection relays during grid disturbances. Its dual-winding configuration (e.g., 0.5S/5P20) allows a single unit to satisfy both fiscal and safety requirements, minimizing equipment count and installation complexity.
Technical Specifications
The LFS-10 is engineered to precise electrical and environmental parameters aligned with international and domestic standards. Below is a representative specification table for a typical dual-ratio, dual-accuracy model:
| Parameter | Value |
|---|---|
| Rated Voltage (Un) | 11 kV (IEC) / 10 kV (domestic) |
| Maximum System Voltage (Um) | 12 kV |
| Primary Current (Ip) | 50–3000 A (standard); up to 6000 A optional |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Classes | Metering: 0.2S, 0.5S; Protection: 5P10, 5P20, 10P10, 10P20 |
| Rated Output (Burden) | 5–30 VA per winding (e.g., 15 VA @ 0.5S, 30 VA @ 5P20) |
| Short-Time Thermal Current | 20×Ip for 1 s (e.g., 60 kA for 3000 A primary) |
| Dynamic Withstand Current | 50×Ip peak (e.g., 150 kA peak) |
| Insulation Level (LI/AC) | 75 kV / 28 kV (1 min power frequency) |
| Ambient Temperature Range | –40°C to +55°C |
| Altitude Limit | ≤1000 m (derating required above 1000 m) |
| Relative Humidity | ≤95% non-condensing |
| Core Material | Grain-Oriented Electrical Steel (GOES), 0.23–0.27 mm thickness |
Standard Service Conditions
The LFS-10 is rated for normal service conditions as defined in IEC 61869-2: ambient temperature from –40°C to +55°C, relative humidity up to 95% without condensation, and installation altitude not exceeding 1000 meters above sea level. At altitudes between 1000 m and 2000 m, the power frequency withstand voltage must be reduced by 1% per 100 m increment. The transformer is designed for continuous operation at 1.2× rated primary current without exceeding a temperature rise of 60 K for the resin surface and 75 K for internal windings (measured by resistance method). These thermal limits ensure long-term insulation integrity and prevent premature aging of the epoxy matrix.
Electrical Performance Parameters
Key performance metrics include composite error ≤±0.5% at 100% Ip for 0.5S class, and ≤±10% at 20× Ip for 5P20 protection class. The knee-point voltage (Vk) for protection windings typically exceeds 300 V, ensuring adequate saturation margin during high-magnitude faults. Magnetizing current at Vk is limited to ≤0.5 A to maintain relay coordination accuracy. For metering windings, phase displacement is held within ±10 minutes at 100% Ip, critical for vector-based tariff calculations in three-phase systems. Burden tolerance is ±10% of rated VA, with linear response maintained down to 1% of rated current for 0.2S class units.
Typical Applications
The LFS-10’s dual-certification and robust design make it suitable for diverse power system environments requiring precision measurement and reliable fault detection.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LFS-10 provides high-accuracy current signals to revenue meters compliant with IEC 62053-22. Installed on outgoing feeders, its 0.2S or 0.5S winding ensures billing accuracy even under light-load conditions (e.g., 5% of rated current). The cast-resin housing resists tracking from salt fog or industrial pollutants, maintaining insulation integrity in coastal or chemical plant environments. Integration with AMI (Advanced Metering Infrastructure) systems is seamless due to low phase error and stable ratio over temperature cycles.
Industrial Power Distribution
Within manufacturing facilities, the LFS-10 monitors motor feeder currents for both energy management and motor protection. Its 5P20 winding drives electromagnetic or digital overcurrent relays that trip circuit breakers during locked-rotor or ground-fault events. The transformer withstands frequent switching transients from variable-frequency drives (VFDs) due to its high partial discharge inception voltage (>25 kV). In arc furnace plants, it operates reliably despite severe harmonic distortion (THD up to 25%), thanks to GOES core lamination that minimizes eddy current losses.
Renewable Energy Integration
Solar photovoltaic (PV) and wind farms utilize the LFS-10 at the point of common coupling (PCC) to meet grid code requirements for generation monitoring and anti-islanding protection. During cloud-induced ramp events, the CT accurately tracks rapid current changes (di/dt > 100 A/ms) without saturation. Dual outputs allow one winding to feed a SCADA RTU for real-time telemetry while the other triggers directional overcurrent relays during backfeed faults. The unit’s UV-stabilized resin housing ensures 25-year outdoor durability without yellowing or embrittlement.
Rural and Suburban Distribution Networks
In pole-mounted or pad-mounted transformers serving residential areas, the LFS-10 enables load profiling and theft detection. Its compact size fits within standard 11kV cutout assemblies, and the absence of oil eliminates environmental liability during pole-top fires. Accuracy at low currents (e.g., 0.1 A secondary) supports detection of unauthorized tapping. In regions with wide temperature swings (e.g., –30°C to +50°C), the coefficient of thermal expansion mismatch between copper windings and epoxy resin is minimized through controlled curing, preventing microcracks.
Compliance with International Standards
The LFS-10 is certified to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and fully harmonized with China’s GB/T 20840.2-2014 standard.
IEC 61869-2 Compliance Details
Compliance encompasses dimensional, electrical, and test requirements. The standard mandates that metering CTs exhibit composite error ≤±0.2% for 0.2S class at 20–120% of rated current, verified via comparison method using a reference standard of at least 0.05 class. Protection CTs must maintain error ≤±10% at specified multiples (e.g., 20×) of rated current under maximum burden. Dielectric tests include 1-minute AC withstand at 28 kV and lightning impulse at 75 kV (1.2/50 μs waveform). Partial discharge levels are measured per IEC 60270, with acceptance criteria of ≤10 pC at Um/√3 for new units.
GB/T 20840.2 Alignment
While GB/T 20840.2 closely mirrors IEC 61869-2, key differences exist in short-circuit performance validation. The Chinese standard requires dynamic withstand verification via actual short-circuit testing or validated simulation, whereas IEC permits calculation-based compliance. Additionally, GB/T specifies a minimum creepage distance of 20 mm/kV for indoor use and 25 mm/kV for outdoor—identical to IEC—but mandates salt fog testing per GB/T 2423.17 for coastal deployments. The LFS-10 meets both frameworks, enabling global deployment without redesign.
Testing and Certification Requirements
Each LFS-10 unit undergoes type, routine, and sample tests. Type tests (performed once per design) include temperature rise, short-circuit, and impulse tests. Routine tests (100% production) cover ratio, polarity, insulation resistance (>1000 MΩ at 2500 V DC), and power frequency withstand. Sample tests (per batch) verify partial discharge and accuracy. Certification is issued by accredited bodies such as TÜV Rheinland or CEPREI, with test reports traceable to national metrology institutes.
On-Site Testing Procedures
Post-installation verification ensures the LFS-10 performs within specifications under field conditions.
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: ≥1000 MΩ at 20°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings indicate moisture ingress or resin cracking—requiring replacement. Perform before and after dielectric tests to detect insulation degradation.
Turns Ratio Test
Apply a low-voltage AC source (e.g., 120 V) to the secondary winding and measure induced primary voltage. Calculate ratio as Vs/Vp; compare to nameplate (e.g., 3000:5 = 600:1). Tolerance: ±0.2% for metering, ±1% for protection classes. Use a dedicated CT analyzer (e.g., Omicron CT Analyzer) for automated excitation curve and ratio sweep from 1% to 120% of rated current.
Polarity Test
Verify reducing polarity per IEC 61869-2: momentarily apply DC voltage to primary (P1 positive, P2 negative); observe secondary voltage spike direction with an analog voltmeter. Positive deflection at S1 confirms correct polarity. Incorrect polarity causes 180° phase shift, leading to metering errors or relay misoperation. Digital testers automate this via pulse injection and zero-crossing detection.
Power Frequency Withstand Voltage Test
Apply 28 kV RMS at 50 Hz between primary and grounded secondary/enclosure for 1 minute. Monitor for flashover or excessive leakage current (>1 mA). Use a calibrated HV test set with overcurrent trip. This validates insulation integrity after transportation and installation stresses. Do not perform if insulation resistance is below 1000 MΩ.
Excitation (Saturation) Characteristic Test
For protection windings, plot excitation curve by applying increasing voltage to secondary and measuring magnetizing current. Determine knee-point voltage (Vk) per IEEE C57.13: point where 45° tangent intersects extrapolated linear region. Vk must exceed relay setting voltage (e.g., >200 V for 5P20). Low Vk indicates core damage or incorrect turns—requiring unit replacement.
Preventive Maintenance Guide
Although cast-resin CTs are largely maintenance-free, periodic checks extend service life and prevent unexpected failures.
Periodic Inspection Schedule
Conduct visual inspections annually: check for surface cracks, tracking marks, or discoloration on the resin housing. Verify terminal tightness (torque: 2.5 N·m for M6 screws) and corrosion on primary busbar contact surfaces. Clean with dry cloth or mild detergent—never solvents. In polluted environments (e.g., cement plants), inspect every 6 months for dust accumulation that could bridge creepage distances.
Maintenance Intervals and Fault Diagnosis
Every 5 years, perform insulation resistance and ratio tests as baseline comparisons. A >20% drop in insulation resistance or >0.5% ratio drift indicates internal degradation. Common failure modes include: (1) core lamination delamination due to thermal cycling—detected by elevated magnetizing current; (2) secondary winding open-circuit during installation—causing dangerous overvoltage; (3) moisture ingress at terminal seals—evidenced by white powder (hydrolysis byproducts). Replace units exhibiting any of these symptoms immediately.
| Interval | Action |
|---|---|
| Annually | Visual inspection, terminal torque check |
| Every 5 Years | Insulation resistance, ratio, polarity tests |
| After Fault Event | Full suite of on-site tests (Sections 5.1–5.5) |
| End of Life (25+ Years) | Replace regardless of test results |
Conclusion
The LFS-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 and GB/T 20840.2 compliance with proven field reliability. Its VPI epoxy resin insulation eliminates fire hazards and maintenance burdens associated with oil-filled alternatives, while the GOES core ensures high accuracy across metering (0.2S/0.5S) and protection (5P/10P) applications. Rigorous factory and on-site testing protocols guarantee performance under thermal, electrical, and environmental stressors typical of 11kV distribution networks. With a design life of 25–30 years—even in harsh industrial or outdoor settings—the LFS-10 delivers exceptional total cost of ownership. Its dual-winding architecture reduces equipment count in modern digital substations, supporting both smart metering infrastructure and adaptive protection schemes. As power systems evolve toward greater decentralization and automation, the LFS-10’s robustness, precision, and standards alignment ensure continued relevance in next-generation grid architectures.
Q1: Can the LFS-10 be used in 10kV domestic systems?
Yes. While rated at 11kV per IEC standards, the LFS-10 is fully compatible with 10kV nominal systems commonly used in China and other regions. The insulation level (12kV Um) provides adequate margin for 10kV operation.
Q2: What is the maximum altitude rating without derating?
The LFS-10 is rated for altitudes up to 1000 meters. Above this, the power frequency withstand voltage must be reduced by 1% per 100 meters of elevation increase.
Q3: Is the LFS-10 suitable for outdoor installation?
Yes. The UV-stabilized cycloaliphatic epoxy resin housing is rated for outdoor use with IP54 protection, resisting rain, dust, and solar radiation without degradation.
Q4: What accuracy class is required for revenue metering?
For fiscal metering, IEC 62053-22 requires 0.2S or 0.5S class. The LFS-10 offers both, with 0.2S providing superior accuracy at low loads (down to 1% of rated current).
Q5: How is polarity marked on the LFS-10?
Primary terminals are labeled P1 (line side) and P2 (load side). Secondary terminals are S1 and S2, with S1 corresponding to P1 under reducing polarity convention per IEC 61869-2.