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LZX-10 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the LZX-10 Current Transformer
The LZX-10 is a medium-voltage cast-resin current transformer (CT) rated for 11kV systems (IEC standard), with a domestic system equivalent of 10kV. Engineered for both metering and protection functions in utility and industrial power networks, it complies fully with IEC 61869-2 and GB/T 20840.2. Its design leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate high-permeability grain-oriented electrical steel (GOES) cores and copper windings, ensuring long-term dielectric integrity, mechanical stability, and resistance to environmental stressors.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LZX-10 employs a two-stage VPI process where dry-wound coils are placed in a mold, evacuated to remove air, and then impregnated under pressure with cycloaliphatic epoxy resin. This eliminates voids and microcavities that could initiate partial discharges under operating voltage stress. The cured resin forms a monolithic, hydrophobic barrier around conductors and magnetic cores, providing uniform electric field distribution and preventing moisture ingress. Unlike oil-filled transformers, this solid insulation system eliminates fire hazards, reduces maintenance, and enables safe indoor or outdoor deployment without containment requirements. The dielectric strength exceeds 20 kV/mm, supporting reliable operation at 11kV RMS with a lightning impulse withstand level of 75 kV peak.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LZX-10’s cast-resin construction offers superior safety, environmental compatibility, and lifecycle economics. There is no risk of oil leakage, flammability, or PCB contamination, making it suitable for urban substations, commercial buildings, and environmentally sensitive zones. The solid insulation exhibits negligible aging under thermal cycling (–40°C to +40°C ambient), with no need for periodic oil sampling or gas monitoring. Additionally, the compact form factor—enabled by higher dielectric strength of epoxy versus oil—reduces footprint by up to 30%. Mechanical robustness allows direct mounting on busbars or switchgear without vibration dampers, and the absence of breathing mechanisms prevents internal condensation during humidity swings.
Typical Application Overview
The LZX-10 serves dual roles: high-accuracy revenue metering (Class 0.2S or 0.5S) and fast-acting protection (Class 5P10 or 5P20). It is commonly deployed in 11kV primary distribution feeders, ring main units (RMUs), pad-mounted substations, and renewable energy interconnection points. In utility applications, it interfaces with digital relays (e.g., overcurrent, earth-fault) and smart meters via standardized secondary outputs (typically 1A or 5A). Industrial users leverage its high saturation characteristics to maintain accuracy during motor inrush or fault transients. The transformer supports single-ratio or multi-tap configurations, enabling flexible scaling across load profiles without hardware replacement.
Technical Specifications
The LZX-10 is engineered to precise electrical and environmental parameters to ensure interoperability and reliability across global power systems.
| Parameter | Value |
|---|---|
| Rated System Voltage (IEC) | 11 kV |
| Domestic Equivalent Voltage | 10 kV |
| Rated Primary Current (Ip) | 50 A to 3150 A (standard steps) |
| Rated Secondary Current (Is) | 1 A or 5 A |
| Current Ratio Options | Single or multi-tap (e.g., 600/1, 600/5, 1200/1/5) |
| Metering Accuracy Class | 0.2S, 0.5S (per IEC 61869-2) |
| Protection Accuracy Class | 5P10, 5P20 |
| Rated Output (Burden) | 2.5 VA to 30 VA (at rated current and power factor 0.8 lag) |
| Short-Time Thermal Current | 25 kA for 1 s (symmetrical) |
| Dynamic Withstand Current | 62.5 kA peak |
| Power Frequency Withstand Voltage | 28 kV RMS, 1 min (between HV and ground) |
| Lightning Impulse Withstand Voltage | 75 kV peak (1.2/50 µs wave) |
| Insulation Material | Cycloaliphatic epoxy resin (VPI process) |
| Magnetic Core Material | Grain-Oriented Electrical Steel (GOES), CRGO grade |
| Ambient Temperature Range | –40°C to +40°C |
| Maximum Installation Altitude | 1000 m above sea level (derating required above) |
| Relative Humidity | Up to 95% non-condensing |
Electrical Performance Parameters
The LZX-10 achieves metering accuracy through low core loss and precisely controlled excitation characteristics. At 0.2S class, composite error must not exceed ±0.2% at 20% to 120% of rated current, with phase displacement ≤ ±10 minutes. For protection, the 5P20 rating ensures the ratio error remains within ±5% even at 20 times rated current, critical for relay coordination during high-magnitude faults. The burden tolerance is defined at power factor 0.8 lagging; exceeding rated VA causes increased ratio error and potential saturation. Standard secondary terminals accommodate 6 mm² to 16 mm² copper conductors with M6 stainless steel bolts, rated for 130°C continuous operation.
Environmental and Mechanical Ratings
Designed for both indoor switchgear panels and outdoor pole-mounted installations, the LZX-10 features UV-stabilized resin housing with IP54 ingress protection. The creepage distance exceeds 25 mm/kV (minimum 275 mm for 11kV), satisfying pollution degree 3 environments per IEC 60664. Mounting brackets support horizontal or vertical orientation with torque specifications of 15 N·m for M10 bolts. Thermal stability is validated through 1000 thermal cycles (–25°C to +70°C) without cracking or delamination. The transformer weighs 18–24 kg depending on ratio, facilitating manual handling during installation.
Typical Applications
The LZX-10’s dual functionality makes it indispensable across diverse power infrastructure segments.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LZX-10 provides revenue-grade current signals to AMI (Advanced Metering Infrastructure) systems. Installed on outgoing feeders, it feeds Class 0.2S data to billing meters while simultaneously supplying protection-class signals to feeder relays. Its low phase error (<5 minutes at 100% In) ensures accurate kWh integration over time, minimizing commercial disputes. The compact size allows retrofit into legacy RMUs without panel redesign.
Industrial Power Distribution
Manufacturing plants with large induction motors require CTs that maintain linearity during inrush currents (6–8× rated). The LZX-10’s 5P20 core saturates only above 20× In, enabling protective relays to distinguish between transient inrush and genuine faults. In arc furnace or welding facilities with high harmonic content, the GOES core minimizes eddy current losses, preserving accuracy even with 15% THD. Secondary outputs are often routed to SCADA systems for real-time load profiling and demand-side management.
Renewable Energy Integration
Solar farms and wind parks connect to the grid via 11kV collection feeders. The LZX-10 monitors export/import power for net metering and detects islanding conditions through vector shift relays. Its fast response time (<20 ms) supports anti-islanding schemes compliant with IEEE 1547. During cloud-induced solar ramp events, the CT’s wide dynamic range captures rapid current changes without distortion, ensuring accurate curtailment control. Outdoor-rated housings withstand coastal salt spray and desert dust.
Rural and Suburban Distribution Networks
In remote areas with limited maintenance access, the LZX-10’s maintenance-free design reduces operational costs. Deployed on pole-top transformers or underground vaults, it enables remote fault location via differential current analysis. Multi-tap models allow utilities to adjust ratios as load grows—from 200/5 for initial rural electrification to 800/5 after village expansion—without replacing hardware. The 1A secondary option minimizes copper losses over long cable runs to centralized control rooms.
Compliance with International Standards
The LZX-10 is certified to harmonized international and Chinese national standards, ensuring global acceptance.
IEC 61869-2 Compliance Details
IEC 61869-2 governs instrument transformer performance, testing, and marking. The LZX-10 meets all clauses for electromagnetic CTs: accuracy verification per Annex B (ratio and phase error tests at 5%, 20%, 100%, and 120% In), temperature rise limits (<60 K for resin, <50 K for windings), and short-circuit withstand per Clause 6.5. Dielectric tests include 1-min AC withstand (28 kV) and lightning impulse (75 kV) with ≤3 partial discharge pulses >10 pC. Markings include rated voltage, ratio, accuracy classes, burden, and manufacturer ID per Clause 8.
Alignment with GB/T 20840.2
GB/T 20840.2 is China’s adoption of IEC 61869-2 with minor deviations. Key differences include: (1) domestic systems reference 10kV instead of 11kV nominal voltage, though insulation levels remain identical; (2) GB permits slightly wider ratio error tolerances at 5% In for metering classes; (3) mandatory type tests include additional seismic qualification (0.3g horizontal acceleration). The LZX-10 is dual-marked for both standards, allowing seamless use in Belt and Road projects requiring IEC certification and domestic Chinese grid tenders requiring GB compliance.
Testing and Certification Requirements
Type tests per IEC 61869-2 include temperature rise, short-circuit, accuracy, and dielectric tests conducted in accredited labs (e.g., KEMA, CESI). Routine tests on every unit cover: (1) visual inspection, (2) winding resistance measurement (±2% tolerance vs. prototype), (3) ratio and polarity verification, and (4) 1-min AC withstand at 80% of type test voltage (22.4 kV). Certificates include test reports traceable to ISO/IEC 17025, with serial-number-specific data available via QR code on the nameplate.
On-Site Testing Procedures
Post-installation verification ensures the LZX-10 performs within specification before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure resistance between primary winding and ground, and between primary and secondary windings. Acceptance criterion: ≥1000 MΩ at 20°C. Correct for temperature using RT = R20 × 2(20–T)/10. Values below 500 MΩ indicate moisture ingress or resin degradation, requiring drying or replacement. Perform before and after dielectric tests to detect insulation damage.
Turns Ratio Test
Apply low-voltage AC (5–10 V) to the secondary winding and measure induced primary voltage. Calculate actual ratio as Vp/Vs. Tolerance: ±0.2% for metering taps, ±1.0% for protection taps. Alternatively, use a dedicated CT analyzer injecting 1–5 A into the primary and measuring secondary current. Deviations beyond tolerance suggest turn-to-turn shorts or incorrect tap selection.
Polarity Test
Verify reducing polarity using the DC kick method: connect a 6 V battery momentarily between P1 (primary input) and P2. Observe secondary voltage on S1–S2 with an analog voltmeter. A positive deflection confirms correct polarity (S1 is positive when P1 is positive). Incorrect polarity causes relay misoperation or negative meter readings. Digital multimeters with min/max capture can also be used.
Power Frequency Withstand Voltage Test
Apply 28 kV RMS at 50 Hz between primary and grounded enclosure for 1 minute. Use a calibrated test transformer with overcurrent trip set at 10 mA. No flashover or sustained arcing is permitted. Leakage current should remain stable (<5 mA). This test validates insulation integrity after transport and installation stresses. Reduce voltage gradually post-test to avoid resonant overvoltages.
Short-Circuit Test (for CT)
Unlike VTs, CTs undergo short-circuit validation by injecting 10× rated current into the primary with secondary shorted. Monitor secondary current waveform for distortion indicating core saturation. For 5P20 class, the ratio error at 20× In must be ≤5%. This test is typically performed during factory type testing but may be repeated on-site using portable high-current injectors for critical protection circuits.
Preventive Maintenance Guide
Although cast-resin CTs are largely maintenance-free, periodic checks extend service life and prevent unexpected failures.
Periodic Inspection Protocol
Conduct annual visual inspections: check for surface tracking, cracks, or discoloration on the resin housing; verify terminal tightness (torque to 15 N·m); inspect for corrosion on mounting hardware. Clean with dry cloth or mild detergent—never solvents. Use infrared thermography during peak load to detect abnormal heating (>10 K above ambient at terminals), which may indicate loose connections or internal faults. Record findings in asset management systems for trend analysis.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicators |
|---|---|---|
| Annually | Visual + IR scan | Cracks, overheating, discoloration |
| Every 5 Years | Insulation resistance + ratio test | R<500 MΩ, ratio error >1% |
| After Major Fault | Full electrical retest | Relay misoperation, meter drift |
Common failure modes include: (1) terminal oxidation causing contact resistance rise—clean with abrasive pad and re-torque; (2) partial discharge from internal voids—detected via ultrasonic sensors; (3) core lamination shorting due to mechanical shock—identified by elevated excitation current. Replace if insulation resistance drops below 200 MΩ or ratio error exceeds 2%.
Conclusion
The LZX-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining metrological precision with rugged protection performance. Its compliance with IEC 61869-2 and GB/T 20840.2 ensures global interoperability, while the VPI epoxy resin encapsulation delivers unmatched environmental resilience and fire safety. The use of GOES cores enables low-loss operation across a wide current range—from 1% for tamper detection to 2000% for fault clearing—making it equally suited for revenue metering and high-speed relay schemes. With a design life of 25–30 years under standard service conditions, the LZX-10 minimizes total cost of ownership through zero fluid maintenance, compact installation, and resistance to pollution and humidity. Utilities and industrial operators benefit from its dual-accuracy architecture, which eliminates the need for separate metering and protection CTs on the same circuit. As power systems evolve toward digitalization and distributed generation, the LZX-10’s robust signal fidelity and long-term stability provide a reliable foundation for smart grid applications, ensuring accurate energy accounting and rapid fault isolation for decades.
Q1: Can the LZX-10 be used in 10kV domestic systems?
Yes. While rated at 11kV per IEC standards, the LZX-10 is fully compatible with 10kV domestic systems (e.g., China, India). Insulation levels (28 kV AC withstand, 75 kV LIWV) exceed requirements for both voltages, ensuring safe operation.
Q2: What is the maximum altitude rating without derating?
The standard LZX-10 is rated for altitudes up to 1000 m. Above this, external clearance and creepage must increase per IEC 60071-2; consult the manufacturer for high-altitude variants.
Q3: Is the secondary winding inherently short-circuit proof?
No CT is inherently short-circuit proof, but the LZX-10’s design limits thermal stress. Never leave the secondary open-circuited during primary energization, as this induces dangerous overvoltages. Always short secondary terminals during maintenance.
Q4: How many accuracy classes can be provided in one unit?
Dual-core models offer one metering (0.2S/0.5S) and one protection (5P10/5P20) core in a single housing. Single-core units support only one function per winding.
Q5: What is the partial discharge level at rated voltage?
Partial discharge is ≤5 pC at 1.2 × Um/√3 (8.8 kV) and ≤10 pC at Um (11 kV), well below the IEC 61869-2 limit of 20 pC.