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IEC 61869-2 Certified 11kV Current Transformer LZX-10 for Metering & Protection Applications
Introduction to the LZX-10 Current Transformer
The LZX-10 is a high-reliability, cast-resin insulated current transformer (CT) engineered for accurate current measurement and robust protection functions in 11kV (IEC-rated) or 10kV (domestic system) medium-voltage networks. Designed in strict accordance with IEC 61869-2 and GB/T 20840.2, this instrument transformer leverages advanced vacuum pressure impregnation (VPI) epoxy resin technology to deliver exceptional dielectric strength, environmental resilience, and long-term operational stability. Unlike traditional oil-immersed CTs, the LZX-10 eliminates fire hazards, oil leakage risks, and complex maintenance protocols, making it ideal for both indoor switchgear and outdoor substations.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LZX-10 employs a two-component cycloaliphatic epoxy resin system cured under vacuum and pressure (VPI process). This technique fully encapsulates the primary conductor, secondary windings, and magnetic core—typically constructed from grain-oriented electrical steel (GOES)—within a homogeneous, void-free solid dielectric matrix. The absence of air pockets prevents partial discharge inception even under transient overvoltages, ensuring stable performance across the transformer’s 25–30 year service life. The resin’s high tracking resistance (>600 V per IEC 60587) and hydrophobic surface properties further enhance reliability in polluted or humid environments. Thermal expansion coefficients are carefully matched between resin, copper, and steel components to minimize mechanical stress during thermal cycling from –40°C to +40°C ambient conditions.
Advantages Over Oil-Immersed Designs
Compared to oil-filled counterparts, the LZX-10 offers significant operational and safety benefits. It is inherently non-flammable (complying with IEC 60695 flammability tests), eliminating fire propagation risks in confined switchgear rooms. Maintenance requirements are drastically reduced—no oil sampling, degassing, or level checks are needed. The solid insulation provides superior mechanical rigidity, resisting vibration-induced winding displacement during short-circuit events (rated up to 20 kA/1s). Additionally, the compact, hermetically sealed design prevents moisture ingress, ensuring consistent dielectric performance without desiccant breathers or nitrogen blankets. Weight is typically 30% lower than equivalent oil units, simplifying handling and installation in retrofit projects.
Typical Application Overview
The LZX-10 serves dual roles: precision metering (accuracy classes 0.2S/0.5S) and protective relaying (classes 5P10/5P20). It is commonly deployed in 11kV ring main units (RMUs), pad-mounted transformers, utility substations, and industrial power distribution systems. Its robust construction supports continuous operation under harmonic-rich loads (e.g., variable frequency drives) without saturation, thanks to optimized core cross-sections and air-gap-free magnetic circuits. The transformer accommodates standard primary currents from 50 A to 3150 A with multiple secondary taps (e.g., 1A/5A), enabling flexible configuration for revenue metering, fault detection, and load monitoring.
Technical Specifications
The LZX-10 is engineered to meet stringent performance criteria under defined service conditions. Key parameters are validated through type, routine, and special tests per IEC 61869-2.
| Parameter | Value |
|---|---|
| Rated Voltage (Ur) | 11 kV (IEC) / 10 kV (GB) |
| Primary Current (Ip) | 50–3150 A (standard); custom up to 4000 A |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Class (Metering) | 0.2S, 0.5S |
| Accuracy Class (Protection) | 5P10, 5P20 |
| Rated Output (Burden) | 5–30 VA (metering); 10–50 VA (protection) |
| Insulation Level (LI/AC) | 75 kV / 28 kV rms (1 min) |
| Short-Time Thermal Current | 20 kA for 1 s (Ith) |
| Dynamic Withstand Current | 50 kA peak (Idyn) |
| Ambient Temperature Range | –40°C to +40°C |
| Altitude Limit | ≤1000 m (derating required above) |
| Relative Humidity | ≤95% (non-condensing) |
| Core Material | GOES (Grain-Oriented Electrical Steel), M6 grade |
| Insulation System | VPI Epoxy Resin, Class F (155°C) |
Standard Service Conditions
The LZX-10 is rated for normal service per IEC 61869-1: 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. For installations above 1000 m, voltage withstand levels must be derated by 1% per 100 m increment. The transformer assumes a maximum system frequency deviation of ±2% (i.e., 49–51 Hz for 50 Hz systems). Pollution degree is classified as III per IEC 60664-1, suitable for industrial atmospheres with conductive dust or moderate chemical exposure. Continuous operation is permitted at 1.2× rated primary current without exceeding thermal limits.
Electrical Performance Tolerances
Metering accuracy adheres to IEC 61869-2 Table 3: for class 0.2S, ratio error ≤±0.2% and phase displacement ≤±10 minutes at 20–120% of rated current. Protection class 5P20 guarantees composite error ≤±5% at 20× rated current with specified burden. Burden tolerance is ±10% of nominal value. Insulation resistance between windings and earth must exceed 1000 MΩ at 25°C (measured with 2500 V DC megger). Partial discharge magnitude is limited to ≤10 pC at 1.2× Ur/√3 during factory testing.
Typical Applications
The LZX-10’s dual functionality enables deployment across diverse power infrastructure segments.
Substation Secondary Metering
In 11kV utility substations, the LZX-10 provides revenue-grade current signals to kWh meters and SCADA systems. Its 0.2S accuracy ensures billing compliance even at low load levels (down to 1% of rated current). The cast-resin housing resists ozone degradation near SF6-insulated switchgear, while the compact footprint allows integration into space-constrained RMUs. Secondary terminals feature IP54-rated enclosures with screw-type connectors compatible with 2.5–6 mm² copper conductors, minimizing contact resistance and thermal drift.
Industrial Power Distribution
Within manufacturing facilities, the LZX-10 monitors motor feeder currents and supplies inputs to digital protective relays (e.g., overcurrent, earth-fault). Its 5P20 class maintains linearity during motor inrush (6–8× rated current) and downstream short circuits. The transformer’s high dynamic withstand (50 kA peak) prevents mechanical damage during bus faults. In arc-flash mitigation schemes, fast-saturating cores can be optionally specified to limit secondary current during extreme faults, enhancing personnel safety.
Renewable Energy Integration
Solar farms and wind parks utilize the LZX-10 at the 11kV collection point for generation metering and anti-islanding protection. The CT’s low remanence (<10% of saturation flux) ensures rapid reset after grid disturbances, critical for synchronism check relays. Harmonic immunity (tested up to 13th order per IEC 61000-4-7) prevents false tripping from inverter switching transients. Outdoor versions include UV-stabilized resin and drainage grooves to prevent water pooling on the housing.
Rural and Suburban Distribution Networks
For pole-mounted or pad-mounted transformers in rural grids, the LZX-10 delivers reliable metering despite wide load variations (e.g., agricultural irrigation pumps). Its maintenance-free design reduces outage time in remote areas. Dual-ratio secondaries (e.g., 5 A for local metering, 1 A for telemetry) support legacy and modern communication systems simultaneously. The transformer complies with IEEE C57.13 for interchangeability in North American-influenced markets.
Commercial Building Infrastructure
Large commercial complexes employ the LZX-10 in main distribution boards for tenant submetering and demand-side management. The low thermal time constant (<15 minutes) enables accurate peak demand capture. Fire safety certifications (e.g., UL 94 V-0) facilitate approval in high-occupancy buildings.
On-Site Testing Procedures
Post-installation verification ensures the LZX-10 performs within specifications.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure resistance between primary-to-secondary, primary-to-earth, and secondary-to-earth. Acceptance criterion: ≥1000 MΩ at 25°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement. Perform before and after power frequency withstand tests.
Turns Ratio Test
Apply 1–5 V AC to the secondary winding and measure induced primary voltage (primary open-circuited). Calculate ratio as Vp/Vs. Tolerance: ±0.5% for metering classes, ±1% for protection. Alternatively, use a dedicated CT analyzer injecting 1–10% of rated primary current. Verify all tap combinations if multi-ratio.
Polarity Test
Confirm reducing polarity (IEC standard): momentary DC applied to primary (P1→P2) should induce positive voltage at secondary S1 relative to S2. Use a battery and analog voltmeter or digital polarity checker. Incorrect polarity causes meter reversal or relay misoperation—critical in differential protection schemes.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (50 Hz) for 1 minute between primary and grounded secondary/earth. Leakage current must remain <10 mA. Gradually ramp voltage (≤1 kV/s) to avoid transient overstress. This test validates insulation integrity after transport or installation stress. Do not perform if partial discharge exceeds 20 pC during pre-test screening.
Short-Circuit Test (for CT)
Not applicable as a field test—short-circuit performance is verified during factory type testing. On-site, ensure secondary windings are never left open-circuited during primary energization, as this induces dangerous overvoltages (>10 kV). Always short secondary terminals before disconnecting meters/relays.
Preventive Maintenance Guide
Although maintenance-free by design, periodic checks extend service life.
Periodic Inspection Protocol
Annual visual inspections should verify: (1) housing integrity (no cracks, tracking, or discoloration), (2) terminal tightness (torque: 2.5 N·m for M6), (3) grounding continuity (<0.1 Ω resistance), and (4) secondary wiring insulation condition. Clean surfaces with dry cloth—never solvents. Check for abnormal heating via infrared thermography during peak load (ΔT <10 K vs. ambient).
Maintenance Intervals and Fault Diagnosis
Every 5 years, perform insulation resistance and ratio tests. Suspect core saturation if ratio error exceeds +2% at 120% load—indicative of mechanical shock during installation. Erratic meter readings may stem from loose secondary connections (resistance >10 mΩ). Complete failure modes include open secondary (causing insulation breakdown) or moisture-induced partial discharge (audible buzzing, ozone smell). Replace if insulation resistance drops below 500 MΩ after cleaning and drying.
| Interval | Action |
|---|---|
| Annually | Visual inspection, IR thermography, terminal torque check |
| 5 Years | Insulation resistance, turns ratio, polarity verification |
| After Fault | Full electrical retest including withstand voltage |
| End of Life (25+ yrs) | Replace regardless of condition—resin embrittlement risk |
Conclusion
The LZX-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 certified accuracy with rugged, maintenance-free construction. Its VPI epoxy resin encapsulation and GOES core deliver exceptional dielectric strength, thermal stability, and immunity to environmental stressors—critical for uninterrupted operation in substations, industrial plants, and renewable energy sites. By eliminating the fire hazards and upkeep demands of oil-filled alternatives, the LZX-10 reduces total cost of ownership while ensuring decades of reliable service. With dual metering (0.2S/0.5S) and protection (5P10/5P20) capabilities, it supports modern grid requirements including harmonic resilience, precise low-load measurement, and fast fault response. Rigorous compliance with both international (IEC) and domestic (GB) standards guarantees seamless integration into global power systems. When installed and maintained per guidelines, the LZX-10 consistently achieves its designed service life of 25–30 years, making it a technically sound and economically prudent choice for critical current sensing applications.