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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 high-reliability, cast-resin insulated current transformer (CT) engineered for accurate current measurement and robust protection functions in medium-voltage power systems operating at 11kV (IEC standard) or 10kV (domestic nominal voltage). Designed in strict accordance with IEC 61869-2 and GB/T 20840.2, this device leverages advanced vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings, ensuring long-term dielectric integrity and mechanical stability under harsh environmental conditions.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LFS-10 employs a two-component cycloaliphatic epoxy system processed under vacuum and pressure to eliminate air voids and moisture ingress. This VPI technique fully saturates the glass-fiber reinforced winding assembly and high-permeability grain-oriented electrical steel (GOES) core, creating a monolithic, non-hygroscopic solid insulation structure. Unlike oil-filled CTs, the resin matrix provides uniform electric field distribution, suppresses partial discharges below 5 pC at 1.2 × Um/√3, and eliminates fire hazards or environmental contamination risks. The thermal class of the insulation system is rated at 105°C (Class A), supporting continuous operation at ambient temperatures up to +40°C with a 25 K temperature rise limit under rated load.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed current transformers, the LFS-10 offers significant operational and safety benefits. Its solid insulation eliminates the need for oil containment, gaskets, or conservator tanks, reducing maintenance requirements and enabling safe deployment in indoor substations, urban switchgear rooms, and confined industrial spaces. The absence of flammable liquid enhances fire safety compliance per IEC 61439 and local building codes. Additionally, the rigid resin housing provides superior mechanical strength against short-circuit electrodynamic forces—withstanding peak currents up to 63 kA for 1 second without deformation. The design also exhibits excellent resistance to tracking and erosion under polluted or humid conditions (IV pollution degree per IEC 60815), making it suitable for coastal or industrial environments.
Typical Application Overview
The LFS-10 is deployed across utility distribution networks, industrial plants, renewable energy interconnection points, and commercial facilities requiring precise current transformation for revenue metering (accuracy class 0.2S or 0.5S) and protective relaying (class 5P or 10P). Its compact footprint allows integration into ring main units (RMUs), metal-clad switchgear, and pad-mounted transformers. With dual or triple secondary windings, it simultaneously serves metering, protection, and monitoring circuits—ensuring functional segregation per IEC 61869-2 clause 7.3. The transformer supports both through-type and bar-type primary conductor configurations, accommodating busbars up to 60 × 10 mm cross-section.
Technical Specifications
The LFS-10 current transformer delivers precision performance under standardized service conditions, with parameters rigorously validated per IEC 61869-2 test protocols.
Rated Electrical Parameters
Key electrical ratings include: primary voltage 11kV (Um = 12kV), primary current range from 50 A to 3150 A, secondary current standardized at 1 A or 5 A, and accuracy classes of 0.2S, 0.5S (metering) and 5P10, 10P10, 10P20 (protection). Rated burden ranges from 2.5 VA to 30 VA depending on class and secondary current. The insulation level complies with IEC 60071-1: power frequency withstand voltage of 28 kV rms for 1 minute and lightning impulse withstand voltage of 75 kV peak (1.2/50 μs waveform). Short-time thermal current rating is 25 kA for 1 s or 31.5 kA for 3 s, with dynamic withstand current of 63 kA peak. Polarity is reducing (dot convention), verified during factory testing.
Environmental and Mechanical Ratings
The LFS-10 operates reliably under standard service conditions defined by IEC 60060-1: ambient temperature range of –25°C to +40°C, relative humidity up to 100% (condensing permitted), and installation altitude ≤ 1000 m above sea level (derating required above 1000 m per IEC 60071-2). The housing is UV-stabilized cycloaliphatic epoxy resin with IP54 ingress protection when mounted vertically. Primary terminals are tinned copper with M12 or M16 threaded inserts; secondary terminals are screw-type in an IP65-rated terminal box with cable gland entries. Creepage distance exceeds 240 mm for 11kV systems in pollution class III environments.
Core and Winding Configuration
The magnetic circuit utilizes CRGO (cold-rolled grain-oriented) silicon steel laminations with thickness of 0.23–0.27 mm, annealed to minimize hysteresis loss and ensure low excitation current (< 0.5 A at 100% rated voltage equivalent). Windings are wound with oxygen-free copper (OFC) conductors, insulated with Class H polyester-imide enamel. For multi-ratio models, taps are accessible via external terminal blocks without opening the resin casing. Core saturation flux density is limited to 1.6 T under maximum fault current to maintain linear response in protection windings. Each secondary winding is individually screened and grounded to mitigate capacitive coupling interference.
Typical Applications
The LFS-10’s dual functionality in metering and protection makes it indispensable across diverse power infrastructure segments.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LFS-10 provides high-accuracy current signals to revenue-class energy meters (e.g., IEC 62053-22 compliant). With 0.2S class accuracy over 1–120% of rated current, it ensures billing precision even under light-load conditions typical of residential areas. The transformer’s low phase displacement error (< ±10 minutes at 100% In) minimizes reactive energy measurement inaccuracies. Installed in ring main units feeding urban neighborhoods, it enables remote AMR/AMI data collection while maintaining traceable metrological performance per national calibration standards.
Industrial Power Distribution
Within manufacturing facilities, the LFS-10 monitors feeders supplying motors, transformers, and process loads. Its 5P10 or 10P20 protection windings interface with overcurrent relays (e.g., IEC 60255-compliant devices) to detect phase faults, ground faults, and overload conditions. The high knee-point voltage (> 150 V for 5 A secondary) ensures relay operation remains unaffected during high-magnitude faults. In arc flash mitigation schemes, the CT’s fast response time (< 20 ms saturation delay) supports zone-selective interlocking logic. The resin housing resists chemical vapors and dust prevalent in cement, mining, and chemical plants.
Renewable Energy Integration
At solar PV or wind farm collector substations, the LFS-10 interfaces with SCADA systems and protection relays managing bidirectional power flow. Its symmetrical core design maintains accuracy during reverse current injection from distributed generation. For anti-islanding protection, the CT supplies signals to rate-of-change-of-frequency (ROCOF) relays requiring stable harmonic response up to the 13th order. The transformer’s wide temperature tolerance accommodates desert or alpine climates, while UV-resistant resin prevents degradation under prolonged solar exposure.
Rural and Suburban Distribution Networks
In overhead line applications, pole-mounted LFS-10 units feed single-phase or three-phase metering panels in villages and peri-urban zones. Their maintenance-free design reduces outage frequency in remote areas lacking skilled technicians. With 0.5S class accuracy, they support equitable tariff structures while minimizing technical losses. The high short-circuit withstand capability protects against frequent lightning-induced faults common in tropical regions. Compatibility with legacy electromechanical relays ensures seamless retrofitting in aging grids undergoing modernization.
Compliance with International Standards
The LFS-10 is engineered to satisfy stringent global and regional regulatory frameworks governing instrument transformer performance and safety.
IEC 61869-2 Certification Requirements
Full compliance with IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) mandates verification of ratio error, phase displacement, temperature rise, short-circuit performance, and insulation coordination. The LFS-10 undergoes type tests including temperature rise (≤ 60 K for windings), short-time current (25 kA/1s), and impulse voltage (75 kV). Routine tests per clause 10 include polarity check, turns ratio verification (±0.25% tolerance for 0.2S class), and power frequency withstand (28 kV/1 min). Partial discharge levels are measured at 1.2 × Um/√3 and must remain below 10 pC—typically achieving < 5 pC in production units.
Alignment with GB/T 20840.2
The Chinese national standard GB/T 20840.2-2014 mirrors IEC 61869-2 but includes supplementary requirements for domestic grid compatibility. Key alignments include identical accuracy class definitions, burden notation (VA), and insulation levels for 10kV systems (Um = 12kV). However, GB/T 20840.2 specifies additional seismic withstand testing (0.3g horizontal acceleration) and mandates flame retardancy per GB/T 5169.16 (equivalent to IEC 60695-11-10). The LFS-10 meets all GB-specific criteria, enabling deployment in State Grid and China Southern Power Grid projects without modification.
Harmonization of IEC and Domestic Standards
While IEC 61869-2 governs international trade, domestic standards like GB/T 20840.2 address region-specific grid characteristics—such as higher harmonic distortion in Chinese industrial networks or stricter pollution performance in Indian coastal zones. The LFS-10’s design incorporates these nuances: its core cross-section is oversized by 8% to accommodate 15% THD without accuracy drift, and creepage distance exceeds 25 mm/kV for 11kV (vs. IEC’s 20 mm/kV minimum). Certification documentation includes dual test reports—one per IEC protocol, another per GB/T—facilitating customs clearance and utility acceptance worldwide.
On-Site Testing Procedures
Post-installation verification ensures the LFS-10 performs within specification before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary and secondary windings, and between windings and earth. Acceptance criterion: ≥ 1000 MΩ at 20°C. Correct for temperature using RT = R20 × 2(20–T)/10. Low readings 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 (or vice versa using a precision current source). Calculate actual ratio and compare to nameplate. Tolerance limits: ±0.25% for 0.2S, ±0.5% for 0.5S, ±1% for 5P/10P classes. Use a ratio bridge or digital CT analyzer with ±0.05% basic accuracy. Verify all tap positions if multi-ratio.
Polarity Test
Connect a 3–6 V DC battery across the primary (positive to P1). Momentarily close the circuit while observing a center-zero galvanometer connected to S1–S2. A momentary positive deflection confirms reducing polarity. Incorrect polarity causes metering errors and relay misoperation—requiring immediate correction of secondary wiring.
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 (≤ 10 mA). No flashover or disruptive discharge is permitted. Reduce voltage gradually post-test. This verifies insulation integrity after transport and installation stresses.
Excitation (Knee-Point) Test
For protection windings, perform open-circuit secondary excitation: incrementally apply voltage to secondary while measuring current. Plot V-I curve; knee-point is where slope decreases by 45°. Minimum knee-point voltage must exceed relay setting voltage (e.g., > 100 V for 5P10 at 20× In). Low knee-point indicates core saturation risk during faults.
Preventive Maintenance Guide
Although cast-resin CTs are largely maintenance-free, periodic checks extend service life and prevent unexpected failures.
Annual Visual and Functional Inspection
Inspect for surface cracks, tracking marks, or discoloration on the resin housing—indicative of UV degradation or partial discharge activity. Check terminal tightness (torque: 2.5 N·m for M4 screws) and corrosion on primary contacts. Verify secondary wiring continuity and grounding integrity (resistance < 0.1 Ω). Review historical load data for abnormal heating signatures. Clean housing with non-abrasive detergent; avoid solvents that may attack epoxy.
Five-Year Comprehensive Maintenance
Every 60 months, repeat insulation resistance and turns ratio tests. Perform thermographic scanning under 70% load to detect hot spots (> 10 K above ambient). Measure contact resistance of primary terminals (< 50 μΩ). If installed in high-pollution areas, verify creepage distance hasn’t been compromised by salt or cement deposits. Update asset management records with test results for lifecycle analysis.
Fault Diagnosis and Replacement Criteria
Common failure modes include secondary winding open-circuit (causing dangerous overvoltage), core delamination (increased ratio error), or terminal corrosion (intermittent connections). Replace the unit if: ratio error exceeds twice the class limit, insulation resistance drops below 100 MΩ, or physical damage compromises dielectric strength. Never operate with an open secondary—always short-circuit before disconnecting instruments.
Conclusion
The LFS-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining metrological precision for revenue metering with rugged reliability for critical protection functions. Its VPI epoxy resin encapsulation eliminates the operational liabilities of oil-filled alternatives, offering fire-safe, maintenance-free performance across diverse climatic and electrical environments. Full compliance with IEC 61869-2 and GB/T 20840.2 ensures global interoperability, while rigorous factory and field testing protocols guarantee consistent accuracy under real-world loading conditions—including harmonics, transients, and extreme temperatures. Engineered with GOES silicon steel cores and oxygen-free copper windings, the LFS-10 maintains ratio error within ±0.1% for 0.2S class over its entire operating range, supporting smart grid initiatives requiring high-fidelity data. With a design life exceeding 30 years under standard service conditions, it delivers exceptional total cost of ownership for utilities and industrial operators alike. Its compact form factor facilitates retrofitting into legacy switchgear, accelerating grid modernization without costly infrastructure overhaul. As power systems evolve toward greater decentralization and digitalization, the LFS-10’s dual-winding architecture and robust electromagnetic compatibility position it as a foundational component for next-generation substation automation.