Article Content
11kV Cast-Resin Current Transformer LJM-1 for Metering and Protection – IEC 61869-2 Standard
Introduction to the LJM-1 Current Transformer
The LJM-1 is a medium-voltage cast-resin current transformer (CT) engineered for reliable operation in 11kV (IEC standard) or 10kV (domestic Chinese system) electrical networks. Designed in strict accordance with IEC 61869-2 and GB/T 20840.2, this instrument transformer provides accurate current transformation for metering, protection relaying, and monitoring applications. Its robust construction ensures long-term stability under both normal load conditions and high-magnitude fault currents.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LJM-1 utilizes vacuum pressure impregnation (VPI) epoxy resin technology to fully encapsulate the primary conductor, secondary windings, and magnetic core assembly. This monolithic structure eliminates air voids and moisture ingress pathways, significantly enhancing dielectric strength and partial discharge performance. The resin matrix exhibits excellent thermal conductivity (0.2–0.3 W/m·K), enabling efficient heat dissipation during continuous operation and transient overloads. Unlike oil-filled alternatives, the solid dielectric requires no maintenance related to fluid levels or gas monitoring, making it ideal for indoor switchgear and outdoor pole-mounted installations where environmental exposure is a concern. The curing process under controlled temperature and vacuum ensures uniform mechanical integrity and dimensional stability across the operating temperature range of –25°C to +40°C ambient.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LJM-1’s cast-resin construction offers superior fire safety (non-flammable, self-extinguishing per IEC 60695), reduced environmental risk (no oil leakage or PCB concerns), and lower lifecycle costs. The absence of liquid insulation eliminates the need for conservator tanks, breather systems, or periodic oil sampling—critical advantages in urban substations and confined indoor spaces. Additionally, the compact physical footprint (typical height: 320 mm, diameter: 180 mm) allows for easier integration into modern ring main units (RMUs) and metal-enclosed switchgear. Mechanical robustness is enhanced through the resin’s high tensile strength (~50 MPa), providing resistance to vibration and seismic loads up to 0.3g. These attributes make the LJM-1 particularly suited for applications demanding minimal maintenance and high reliability over a 25–30 year service life.
Typical Applications Overview
The LJM-1 serves as a foundational component in secondary circuits across diverse power infrastructure. In utility distribution substations, it interfaces with revenue-class kWh meters (accuracy class 0.2S or 0.5S) and digital protective relays requiring 1A or 5A secondary outputs. Industrial facilities deploy it for motor protection schemes, arc flash mitigation systems, and energy management platforms. Renewable integration points—such as solar farm collector substations or wind turbine step-up transformers—rely on its linear response under harmonic-rich waveforms. Rural electrification projects benefit from its tolerance to wide ambient temperature swings and humidity variations without performance degradation. Its standardized mounting flange (M12 bolts, 120 mm pitch circle) ensures compatibility with common busbar configurations in 11kV switchgear.
Technical Specifications
The LJM-1 is engineered to meet stringent electrical and environmental performance benchmarks. Below is a representative specification table based on standard configurations; custom ratios and accuracy classes are available upon request.
| Parameter | Value |
|---|---|
| Rated System Voltage (IEC) | 11 kV |
| Rated System Voltage (Domestic) | 10 kV |
| Primary Current Rating | 50 A to 3000 A (standard steps) |
| Secondary Current | 1 A or 5 A |
| Accuracy Class (Metering) | 0.2S, 0.5S |
| Accuracy Class (Protection) | 5P10, 5P20 |
| Rated Output (Burden) | 5 VA, 10 VA, 15 VA, 30 VA |
| Short-Time Thermal Current | 20 kA for 1 s (Ith) |
| Dynamic Withstand Current | 50 kA peak (Idyn) |
| Insulation Level (LI/AC) | 75 kV / 28 kV rms |
| Partial Discharge | <10 pC at 1.2 × Um/√3 |
| Ambient Temperature Range | –25°C to +40°C |
| Altitude Limit | ≤ 1000 m above sea level |
| Relative Humidity | ≤ 95% (non-condensing) |
Core and Winding Construction
The magnetic circuit employs grain-oriented electrical steel (GOES) laminations with a maximum core loss of 1.0 W/kg at 1.5 T and 50 Hz. This material minimizes hysteresis and eddy current losses, ensuring high linearity even at 10–20% of rated primary current—critical for precision metering. Secondary windings are wound with oxygen-free copper (OFC) wire, insulated with Class F (155°C) enamel, and embedded within the epoxy matrix to prevent movement under electromagnetic forces during fault conditions. Terminal blocks feature tin-plated brass connectors rated for 16 A continuous current, with IP2X finger-safe enclosures to prevent accidental contact. The primary conductor is a solid copper bar or tubular section sized to handle rated continuous current with a temperature rise ≤ 60 K above ambient.
Environmental and Mechanical Ratings
Designed for both indoor and outdoor service, the LJM-1 housing incorporates UV-stabilized cycloaliphatic epoxy resin that resists cracking and discoloration after prolonged solar exposure. Creepage distance exceeds 240 mm for 11kV systems in pollution class III environments (IEC 60815). The unit withstands mechanical loads of 250 N applied to the primary terminals without deformation, per IEC 61869-2 clause 6.5. Vibration resistance is validated per IEC 60068-2-6 (10–150 Hz, 0.5 g amplitude). For high-altitude installations (>1000 m), derating factors apply: voltage withstand capability reduces by 1% per 100 m above 1000 m, necessitating consultation with the manufacturer for sites above 2000 m.
Typical Applications
The LJM-1’s versatility stems from its dual compliance with international and domestic standards, enabling deployment across global markets while maintaining consistent performance.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LJM-1 provides input to Class 0.2S kWh meters for billing-grade energy measurement. Its low phase displacement error (< ±5 minutes at 5% In) ensures accurate reactive energy accounting. When paired with a matching cast-resin voltage transformer (e.g., JDZJ-10), it forms a complete metering set compliant with EN 50163 and DL/T 448 requirements. The 5 VA burden rating supports cable runs up to 50 m (for 2.5 mm² Cu conductors) without exceeding composite error limits. Digital metering systems benefit from the CT’s stable ratio over time—drift is typically < 0.1% per decade due to the absence of oil aging effects.
Industrial Power Distribution
Within manufacturing plants, the LJM-1 protects critical assets like induction motors (≥100 kW) and transformers via 5P20-class protection windings. During a three-phase short circuit, it maintains accuracy up to 20× rated current, enabling precise coordination with upstream fuses or circuit breakers. Its compact size allows retrofitting into legacy switchgear without busbar modifications. In arc flash hazard mitigation, the CT feeds real-time current data to zone-selective interlocking (ZSI) relays, reducing clearing times to < 100 ms. The resin housing’s flame resistance (UL 94 V-0 equivalent) meets NFPA 70E requirements for equipment in hazardous locations.
Renewable Energy Integration
Solar photovoltaic (PV) farms utilize the LJM-1 at the 11kV collector substation to monitor string inverter output and detect ground faults. Its immunity to DC offset—common during inverter startup or grid disturbances—prevents saturation and ensures reliable operation. Wind turbine step-up transformers employ dual-core LJM-1 variants: one core for SCADA telemetry (0.5S class) and another for generator differential protection (5P10). The CT’s low remanence (< 0.2 T after demagnetization) minimizes residual flux during rapid load changes, crucial for inverters with variable power factor control.
Rural and Suburban Distribution Networks
In remote areas with limited maintenance access, the LJM-1’s sealed design eliminates concerns about oil leaks or moisture absorption. Pole-mounted installations leverage its lightweight construction (≈12 kg) and corrosion-resistant stainless steel hardware. For single-phase laterals, the CT enables accurate load profiling and theft detection when connected to AMI (Advanced Metering Infrastructure) endpoints. Its wide operating temperature range accommodates desert climates (daytime highs > 45°C) and cold regions (winter lows < –30°C), with no derating required below –25°C ambient.
Compliance with International Standards
The LJM-1 is certified to both IEC 61869-2 (Instrument transformers – Part 2: Additional requirements for current transformers) and GB/T 20840.2 (Chinese national standard for CTs). This dual compliance facilitates global procurement while meeting local regulatory mandates.
IEC 61869-2 Certification Requirements
Under IEC 61869-2, the LJM-1 undergoes rigorous type tests including temperature rise (clause 7.3), short-circuit withstand (clause 7.4), and accuracy verification across the full current range (clause 7.6). Composite error for protection class 5P must not exceed 5% at rated accuracy limit factor (e.g., 10 or 20). Partial discharge measurements are performed at 1.2 × Um/√3 (9.55 kV for 11kV system) with acceptance criteria of ≤10 pC. Routine tests per clause 8 include power frequency withstand (28 kV rms for 1 min), winding continuity, and polarity checks. Type test reports are issued by accredited laboratories (e.g., KEMA, CESI) and remain valid for five years unless design changes occur.
GB/T 20840.2 Alignment and Differences
GB/T 20840.2 closely mirrors IEC 61869-2 but includes China-specific requirements such as mandatory seismic testing (0.2g horizontal acceleration) and stricter creepage distances for coastal regions (≥31 mm/kV). While IEC permits 1A or 5A secondaries, GB/T emphasizes 5A for legacy compatibility with domestic relays. Accuracy classes align (0.2S, 0.5S, 5P10), but GB/T adds a “0.2” class without the “S” suffix for non-revenue applications. Dielectric tests use slightly higher AC voltages: 30 kV rms for 1 min versus IEC’s 28 kV. Manufacturers must obtain CCC (China Compulsory Certification) marking, which involves factory audits and sample retesting every two years.
Testing and Certification Documentation
Each LJM-1 unit ships with a test certificate listing individual results for ratio error, phase displacement, insulation resistance (>1000 MΩ at 2500 V DC), and power frequency withstand. Batch certificates cover routine tests per IEC 61869-2 clause 8. For projects requiring third-party validation, type test summaries (including short-circuit oscillograms and thermal imaging) are available upon request. Compliance with RoHS 2 (2011/65/EU) and REACH is documented for EU markets, while GB/T 26218 addresses pollution performance in Chinese coastal zones.
On-Site Testing Procedures
Post-installation and periodic field tests verify the LJM-1’s integrity and performance. All procedures assume de-energized primary circuits and proper grounding of adjacent equipment.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-secondary, primary-to-ground, and secondary-to-ground. Acceptance criteria: ≥1000 MΩ for new units; ≥500 MΩ for units in service. Readings below 100 MΩ indicate moisture ingress or resin degradation. Perform the test at 20–30°C ambient; correct values using the formula Rcorr = Rmeas × 1.5(Tref–Tmeas)/10, where Tref = 20°C. Allow 60 seconds for stabilization before recording. Discharge windings through a grounded resistor post-test to prevent residual charge hazards.
Turns Ratio Test
Apply a low-voltage AC source (5–10 V, 50 Hz) to the secondary winding and measure induced primary voltage. Calculate actual ratio as Vsec/Vprim. Compare against nameplate ratio; tolerance must be within ±0.25% for metering classes (0.2S/0.5S) and ±1% for protection classes (5P). Alternatively, use a dedicated CT analyzer injecting 1–10% of rated primary current. Ensure secondary burden does not exceed 10% of rated VA during testing to avoid loading errors. Document ratio at multiple tap points if multi-ratio configuration exists.
Polarity Verification
Confirm reducing polarity (standard for IEC/GB) using the DC kick method: connect a 6–12 V battery momentarily between P1 and P2. Observe a DC milliammeter connected to S1–S2; a positive deflection indicates correct polarity (S1 corresponds to P1). Modern CT analyzers automate this via phase angle measurement—polarity is correct if phase shift is 0° ±2°. Incorrect polarity causes watt-hour meters to register negative energy and protection relays to misoperate during external faults. Always verify polarity after terminal box servicing.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (for 11kV system) at 50 Hz between primary and grounded secondary/enclosure for 1 minute. Use a calibrated test transformer with overcurrent trip set at 1.5× expected capacitive current (typically 5–10 mA). Voltage ramp rate: 1–2 kV/s. Failure is indicated by flashover, sustained arcing, or abrupt current increase. For in-service units, perform at 80% of factory test voltage (22.4 kV) per IEC 60270. Ensure all secondary terminals are short-circuited and grounded during the test to prevent dangerous open-circuit voltages.
Short-Circuit Test (for CTs)
Unlike VTs, CTs require verification of short-circuit withstand capability indirectly. Inject 100–200% of rated primary current through the primary conductor while monitoring secondary voltage across a known burden (e.g., 1 Ω resistor). Verify that secondary current scales linearly without saturation. For protection-class validation, inject current equal to accuracy limit factor (e.g., 20× In for 5P20) and confirm composite error ≤5%. Use a portable relay test set with ≥50 A output capability. Record waveform distortion—THD should remain <3% up to 120% In.
Preventive Maintenance Guide
Proactive maintenance extends the LJM-1’s service life beyond 25 years and prevents unexpected failures. The following schedule aligns with IEEE C57.13.2 and DL/T 596 recommendations.
Annual Visual and Electrical Inspection
Inspect for surface cracks, tracking marks, or resin discoloration—indicative of UV degradation or partial discharge activity. Clean housing with isopropyl alcohol; avoid abrasive cleaners. Check terminal tightness (torque: 12 N·m for M6 screws) and corrosion on grounding lugs. Measure insulation resistance and compare to baseline values; a 50% drop warrants further investigation. Verify secondary wiring integrity—broken strands or loose connections cause metering errors. In coastal areas, inspect for salt deposits and rinse with deionized water if conductivity exceeds 10 µS/cm.
Five-Year Comprehensive Maintenance
Perform full suite of on-site tests: ratio, polarity, insulation resistance, and power frequency withstand (at reduced voltage). Demagnetize the core using a decaying AC current (start at 120% In, reduce to zero over 30 seconds) to reset remanent flux. Re-calibrate if ratio error drifts beyond half the class tolerance (e.g., >±0.125% for 0.2S). Replace terminal box gaskets if hardened or cracked. For units exposed to frequent fault currents (>5 kA), conduct dissolved gas analysis (DGA) is unnecessary (no oil), but infrared thermography can detect hot spots at connections.
Maintenance Intervals and Fault Diagnosis
| Interval | Tasks | Fault Indicators |
|---|---|---|
| Annual | Visual inspection, IR scan, insulation resistance | Cracks, overheating (>10 K above ambient), Rins < 500 MΩ |
| 3 Years | Ratio/polarity check, terminal torque verification | Metering discrepancies, relay misoperation |
| 5 Years | Full electrical tests, demagnetization, gasket replacement | Ratio drift >0.2%, THD >5% at 100% In |
| After Fault | Withstand test, core inspection via excitation curve | Visible damage, abnormal noise during operation |
Common failure modes include secondary winding opens (causing dangerous kV-level voltages), core saturation from DC offset, and terminal corrosion leading to high-resistance joints. Immediate replacement is required if partial discharge exceeds 20 pC during energized operation (detected via HFCT sensors).
Conclusion
The LJM-1 11kV cast-resin current transformer represents a mature, standards-compliant solution for medium-voltage metering and protection applications. Its epoxy resin encapsulation delivers exceptional dielectric reliability, environmental resilience, and maintenance-free operation over a projected service life of 25–30 years. By leveraging GOES core technology and precision winding techniques, it achieves accuracy class 0.2S for revenue metering and 5P20 for high-fidelity fault current reproduction—critical for modern digital substations. Dual certification to IEC 61869-2 and GB/T 20840.2 ensures global interoperability while meeting regional regulatory demands. Field-proven in diverse environments—from arid deserts to humid coastal zones—the LJM-1 minimizes lifecycle costs through robust construction and predictable performance degradation. Adherence to the outlined testing and maintenance protocols guarantees continued compliance with accuracy and safety requirements, safeguarding both personnel and connected instrumentation. As power networks evolve toward smarter, more resilient architectures, the LJM-1 remains a cornerstone component for trustworthy current measurement at the distribution level.
Frequently Asked Questions (FAQ)
Q1: Can the LJM-1 be used on a 10kV domestic system even though it’s rated 11kV?
A: Yes. The 11kV IEC rating corresponds to a maximum system voltage (Um) of 12 kV, which safely covers 10kV nominal systems (Um = 11.5 kV per GB standards). No derating is required.
Q2: What is the minimum secondary burden for accurate metering?
A: For 0.2S class, the burden must be ≥25% of rated VA (e.g., ≥1.25 VA for a 5 VA unit). Operating below this increases ratio error due to core magnetization effects.
Q3: How often should polarity be rechecked?
A: Verify polarity after any terminal box intervention, secondary wiring modification, or if metering/relay anomalies suggest incorrect phasing. Annual checks are recommended in critical protection schemes.
Q4: Is demagnetization necessary after every fault?
A: Only if the fault current exceeded 80% of the accuracy limit factor (e.g., >16 kA for a 5P20, 800/5A CT). Remanence can cause ratio errors in subsequent operations.
Q5: Can the LJM-1 be installed horizontally?
A: Yes. Cast-resin CTs have no orientation restrictions unlike oil-filled types. Ensure drainage holes (if present) face downward to prevent water accumulation.
Q6: What test voltage is used for insulation resistance on a 10kV system?
A: Use 2500 V DC regardless of system voltage (10kV or 11kV), as specified in IEC 61869-2 clause 8.3. The test voltage depends on the transformer’s insulation level, not the operating voltage.