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LSZY-10 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the LSZY-10 Current Transformer
The LSZY-10 is a high-accuracy, indoor-type cast-resin current transformer (CT) engineered for reliable operation in 11kV (IEC nominal) or 10kV (domestic system) medium-voltage networks. Designed per IEC 61869-2 and GB/T 20840.2, it serves dual roles in revenue-grade metering and protective relaying within substations, industrial facilities, and renewable energy interconnection points. Its construction leverages vacuum pressure impregnation (VPI) epoxy resin technology combined with grain-oriented electrical steel (GOES) cores to ensure long-term stability, minimal phase error, and exceptional thermal endurance.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LSZY-10 employs a two-stage VPI process where the wound secondary coil and GOES core are first evacuated to remove air pockets, then impregnated under pressure with cycloaliphatic epoxy resin. This creates a monolithic, void-free dielectric barrier with a relative permittivity (εr) of approximately 4.2 and volume resistivity exceeding 1×1014 Ω·cm at 20°C. Unlike oil-filled CTs, this solid insulation eliminates fire hazards, environmental leakage risks, and maintenance-intensive oil sampling. The resin’s coefficient of thermal expansion closely matches that of copper and steel, minimizing mechanical stress during thermal cycling from -40°C to +105°C. Dielectric strength exceeds 20 kV/mm, enabling reliable operation under transient overvoltages up to 75 kV peak (per IEC 61869-2 Table 3).
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LSZY-10 offers significant operational and safety benefits. The absence of flammable insulating oil reduces fire load in indoor switchgear rooms—a critical consideration under IEC 61439-2 for low-voltage assemblies adjacent to MV equipment. Maintenance intervals extend from annual (oil CTs) to once every five years due to no oil degradation, moisture ingress, or gas accumulation concerns. Weight is reduced by 30–40%, simplifying handling during installation. Furthermore, the cast-resin body provides superior resistance to partial discharge (PD), with inception voltage typically >25 kV RMS—well above the 11kV system’s maximum continuous operating voltage of 12 kV. This ensures decades of stable performance without insulation aging acceleration.
Typical Application Overview
The LSZY-10 is predominantly deployed in 11kV/10kV distribution substations where space constraints, fire safety codes, and accuracy demands favor solid-insulated CTs. It interfaces directly with digital multifunction meters (e.g., IEC 61850-compliant devices) for kWh billing and with numerical relays (e.g., overcurrent, earth-fault) for feeder protection. Common installations include ring main units (RMUs), metal-clad switchgear, and pad-mounted transformers. In renewable applications, it monitors export/import currents at solar farm grid connections. Its compact footprint (typically 180 mm height × 120 mm width) allows retrofitting into legacy panels originally designed for older oil CTs.
Technical Specifications
The LSZY-10 adheres strictly to IEC 61869-2:2012 and GB/T 20840.2-2014, ensuring interoperability across global and domestic grids. Key parameters are validated through type tests at accredited laboratories.
| Parameter | Value |
|---|---|
| Rated Voltage (Um) | 11 kV (IEC), 10 kV (GB) |
| Primary Current (Ip) | 50 A to 3000 A (standard ratios) |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Class (Metering) | 0.2S, 0.5S (per IEC 61869-2) |
| Accuracy Class (Protection) | 5P10, 5P20, 10P10 (saturation factor ≥10 or 20) |
| Rated Output (Sn) | 5 VA, 10 VA, 15 VA, 30 VA (at rated Is) |
| Insulation Level (LI/AC) | 75 kV / 28 kV (1.2/50 μs lightning impulse & 1-min power frequency) |
| Short-Time Thermal Current | 25 kA for 1 s (Ith) |
| Dynamic Withstand Current | 62.5 kA peak (Idyn) |
| Ambient Temperature Range | -40°C to +40°C (storage to +70°C) |
| Relative Humidity | ≤95% non-condensing |
| Altitude Limit | ≤1000 m (derating required above 1000 m per IEC 60071-1) |
Electrical Performance Characteristics
The LSZY-10 achieves composite error ≤±0.2% at 20–120% of rated primary current for 0.2S class, meeting stringent revenue metering requirements. Phase displacement is limited to ±10 minutes at 100% Ip. For protection classes (e.g., 5P20), the transformer maintains linear response up to 20× rated current with composite error ≤±5%. Core saturation is prevented by selecting GOES laminations with Bmax ≤1.7 T under fault conditions. Burden compatibility is verified per IEC 61869-2 Annex C: at 30 VA burden and 5 A secondary, voltage drop remains within ±0.5% tolerance. Frequency response is flat from 45 Hz to 65 Hz, accommodating grid variations in both 50 Hz and 60 Hz systems.
Environmental and Mechanical Ratings
Designed for indoor use only, the LSZY-10 complies with IP00 enclosure rating per IEC 60529, requiring installation within grounded metal switchgear. The cast-resin housing exhibits UL 94 V-0 flammability rating and withstands thermal shock from -25°C to +70°C without cracking. Primary conductor is a tinned copper busbar with cross-section sized for continuous current (e.g., 60×10 mm² for 1250 A). Secondary terminals are screw-type (M6) with creepage distance ≥25 mm between phases. Vibration resistance meets IEC 60068-2-6 (5–500 Hz, 0.7 g), suitable for seismic zones up to 0.3g ground acceleration.
Typical Applications
The LSZY-10’s dual-accuracy design enables versatile deployment across utility and industrial sectors.
Substation Secondary Metering
In 11kV distribution substations, the LSZY-10 feeds data to AMI (Advanced Metering Infrastructure) systems for accurate energy billing. Installed on outgoing feeders, its 0.2S class ensures compliance with EN 50470-1 for MID certification. For example, a 630/5 A ratio unit with 10 VA output connects to a Class 0.5S kWh meter via 2.5 mm² Cu cable (total loop resistance ≤0.4 Ω). This setup maintains metering error within ±0.3% even at 5% load—critical for detecting energy theft or unbalanced loading.
Industrial Power Distribution
Within manufacturing plants, the LSZY-10 monitors motor feeder currents for both energy management and motor protection. A typical 800/1 A, 5P10 unit supplies signals to a multifunction relay (e.g., setting Ipickup = 1.2×FLA, tdelay = 0.3 s). The 5P10 rating guarantees the relay receives a proportional current up to 8 kA fault level, enabling precise coordination with upstream breakers. Cast-resin construction resists chemical fumes common in petrochemical facilities, unlike oil CTs which may degrade seals.
Renewable Energy Integration
At solar PV substations, the LSZY-10 measures bidirectional power flow for grid compliance (e.g., IEEE 1547). A 1000/5 A, 0.5S/5P20 dual-core variant provides one winding for SCADA telemetry and another for anti-islanding protection. During cloud transients causing rapid current swings (di/dt >100 A/ms), the low-leakage inductance (<5 μH) prevents waveform distortion. Installation near inverters requires EMI shielding—achieved via the grounded resin housing acting as a Faraday cage.
Rural and Suburban Distribution Networks
In compact RMUs serving residential areas, the LSZY-10’s small size allows integration into SF6-insulated switchgear. A 400/5 A, 0.5S unit monitors transformer loading on pole-top substations, transmitting data via cellular RTUs. Its immunity to humidity prevents condensation-related failures common in tropical climates—unlike oil CTs prone to water saturation lowering breakdown voltage. Maintenance crews benefit from visual inspection windows showing secondary terminal status without opening enclosures.
Compliance with International Standards
The LSZY-10 is certified to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and harmonized with China’s GB/T 20840.2-2014.
IEC 61869-2 Certification Details
IEC 61869-2 defines performance criteria for accuracy, thermal stability, and dielectric integrity. The LSZY-10 undergoes full type testing including temperature rise (ΔT ≤60 K for windings), short-circuit withstand (25 kA/1s without deformation), and accuracy verification across 1–120% Ip. Composite error is measured using a calibrated comparator bridge with uncertainty ≤0.02%. Partial discharge is tested at 1.2×Um/√3 (7.6 kV) with levels <10 pC—well below the 50 pC limit. Certification includes test reports from ISO/IEC 17025 labs, valid for 5 years unless design changes occur.
Alignment with GB/T 20840.2
GB/T 20840.2 adopts IEC 61869-2 with minor national deviations. Key differences include: (1) altitude derating starts at 1000 m (vs. 2000 m in IEC), requiring 1.1× insulation margin above 1000 m; (2) dynamic withstand current is specified as 2.5×Ith (vs. IEC’s 2.5×Ith for Ith≤25 kA); (3) accuracy verification includes an additional point at 1% Ip for 0.2S class. The LSZY-10 meets both standards simultaneously through conservative core sizing and resin formulation.
Testing and Certification Requirements
Before market release, each LSZY-10 batch undergoes routine tests per IEC 61869-2 Clause 7: power frequency withstand (28 kV/1 min), winding continuity, and polarity check. Type tests (per Clause 6) are repeated every 5 years or after material/process changes. Certification marks include CE (EU), CCC (China), and SONCAP (Nigeria). Test certificates list serial numbers, test dates, and lab accreditation IDs.
On-Site Testing Procedures
Post-installation verification ensures the LSZY-10 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: ≥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 replacement. Perform before and after dielectric tests to detect insulation damage.
Turns Ratio Test
Apply 1–5 V AC to the secondary winding and measure induced primary voltage. Calculate ratio as Vs/Vp. Tolerance must be within ±0.2% of nameplate ratio (e.g., 600/5 A = 120:1 → measured ratio 119.76–120.24). Use a precision ratio tester (e.g., Omicron CT Analyzer) for automated verification. Deviations >0.5% suggest shorted turns—requiring core remanence demagnetization followed by retest.
Polarity Test
Verify reducing polarity per IEC 61869-2 Figure 4. 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 confirms correct polarity. Incorrect polarity causes 180° phase shift—leading to false tripping in differential protection schemes. Document results with oscillograms if using automated testers.
Power Frequency Withstand Voltage Test
Apply 28 kV RMS (50 Hz) for 1 minute between primary and grounded secondary/enclosure. Leakage current must remain <3 mA. Use a calibrated HV test set with overcurrent trip (≤100 mA). Gradually ramp voltage (≤1 kV/s) to avoid transient overstress. Failure indicates voids in resin or contamination—unit must be scrapped. Perform only once in lifetime to prevent insulation aging.
Short-Circuit Test (for CTs)
Inject 10–20× rated secondary current (e.g., 50–100 A for 5 A CT) into secondary winding while primary is shorted. Monitor secondary voltage—should not exceed 5 V (indicating core saturation). For 5P20 class, saturation voltage Vk must satisfy Vk ≥ 20 × In × (Rct + Rb), where Rct = CT winding resistance, Rb = burden resistance. Typical Vk for LSZY-10 is 120–180 V.
Preventive Maintenance Guide
Proactive maintenance extends service life beyond 25 years.
Periodic Inspection Protocol
Annual inspections include: (1) visual check for cracks, discoloration, or tracking on resin surface; (2) torque verification of secondary terminals (M6 screws: 5–6 N·m); (3) insulation resistance retest; (4) cleaning with dry cloth (no solvents). Record ambient temperature and humidity during tests. Any resin chalking or carbon tracks warrants immediate replacement—these indicate UV degradation or partial discharge activity.
Maintenance Intervals and Fault Diagnosis
Follow this schedule:
| Interval | Action |
|---|---|
| 1 year | Visual inspection, IR test, terminal check |
| 5 years | Full accuracy verification, ratio test, PD measurement |
| 10 years | Dielectric spectroscopy (tan δ vs. frequency) |
Common faults: (1) Open secondary circuit—causes dangerous overvoltage (>10 kV); always short secondary before disconnecting loads. (2) Core remanence—leads to ratio errors; demagnetize with decaying AC current. (3) Moisture absorption—lowers IR; bake at 80°C for 24 h if <500 MΩ. Never operate with secondary open—permanent core damage occurs within seconds.
Conclusion
The LSZY-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2-certified accuracy with the robustness of modern epoxy insulation technology. Its dual functionality for metering (0.2S/0.5S) and protection (5P/10P) eliminates the need for separate CTs, reducing lifecycle costs in substation retrofits and new builds. The use of GOES cores ensures minimal hysteresis loss and stable performance across temperature extremes, while VPI resin casting provides superior dielectric integrity compared to older polyester designs. Compliance with both international (IEC) and Chinese (GB) standards facilitates global deployment without redesign. With a proven service life of 25–30 years under standard conditions—and immunity to fire, leakage, and environmental degradation—the LSZY-10 delivers unmatched reliability for critical power monitoring applications. Its rigorous factory and field testing protocols guarantee that installed units meet exacting accuracy and safety requirements throughout their operational lifespan, making it a preferred choice for utilities prioritizing grid resilience and measurement fidelity.