Article Content
LAJ-10Q 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the LAJ-10Q Current Transformer
The LAJ-10Q is a high-reliability, indoor-type cast-resin current transformer (CT) engineered for accurate current measurement and robust protection in medium-voltage power systems operating at 11kV (IEC standard) or 10kV (domestic Chinese grid equivalent). Designed in strict accordance with IEC 61869-2 and GB/T 20840.2, this instrument transformer leverages vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings, ensuring long-term dielectric integrity, mechanical stability, and resistance to environmental stressors such as humidity, dust, and partial discharge.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LAJ-10Q employs a two-component cycloaliphatic epoxy resin system cured under controlled vacuum and pressure conditions. This VPI process eliminates air voids and microcavities that could initiate partial discharges under sustained electrical stress. The resin fully encapsulates the primary conductor (typically a solid copper bar rated for continuous thermal duty) and the secondary winding wound around a grain-oriented electrical steel (GOES) core. The resulting monolithic structure provides uniform electric field distribution, high tracking resistance (>600 V according to IEC 60112), and excellent thermal conductivity (0.2–0.3 W/m·K), enabling efficient heat dissipation during overload conditions. Unlike oil-filled CTs, the solid dielectric eliminates fire hazards, leakage risks, and maintenance-intensive oil sampling, making it ideal for indoor substations, commercial buildings, and industrial facilities where safety and space constraints are critical.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LAJ-10Q offers significant operational and safety benefits. Its dry-type construction eliminates flammability concerns, satisfying stringent fire codes in urban substations and high-occupancy buildings. The absence of liquid insulation removes the need for conservator tanks, breather systems, and periodic oil testing, reducing lifecycle costs by up to 40% over a 25-year service life. Additionally, the compact footprint—enabled by higher dielectric strength of epoxy resin (~20 kV/mm vs. ~10 kV/mm for mineral oil)—allows for denser switchgear layouts. The LAJ-10Q also demonstrates superior performance in seismic zones due to its rigid, non-sloshing internal structure, and it exhibits negligible aging under normal operating temperatures (−25°C to +40°C ambient).
Typical Applications Overview
The LAJ-10Q is deployed across diverse power infrastructure segments requiring precise current transformation for metering accuracy classes (0.2S, 0.5) and protection reliability (5P10, 5P20). Primary use cases include 11kV/10kV ring main units (RMUs), metal-clad switchgear panels, renewable energy interconnection points (e.g., solar farm collector feeders), and industrial motor control centers. Its dual-winding configuration supports simultaneous connection to revenue metering circuits and protective relays, ensuring compliance with both billing accuracy requirements and fault-clearing speed mandates. The transformer’s low remanence and high saturation margin make it particularly suitable for applications involving frequent inrush currents, such as transformer energization or capacitor bank switching.
Technical Specifications
The LAJ-10Q is engineered to meet exacting electrical and environmental performance criteria. All parameters align with IEC 61869-2 (2012) and GB/T 20840.2 (2012) standards, ensuring global interoperability and domestic regulatory acceptance.
| Parameter | Value |
|---|---|
| Rated System Voltage (Um) | 12 kV (IEC); 11.5 kV (GB) |
| Maximum Operating Voltage | 12 kV |
| Primary Current (Ip) | 50 A to 3150 A (standard ratios) |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Class (Metering) | 0.2S, 0.5 |
| Accuracy Class (Protection) | 5P10, 5P20 |
| Rated Burden (VA) | 2.5 VA to 30 VA (per winding) |
| Short-Time Thermal Current (Ith) | 20×Ip for 1 s (min.) |
| Dynamic Withstand Current (Idyn) | 2.5×Ith |
| Power Frequency Withstand Voltage | 28 kV rms, 1 min (phase-to-earth) |
| Lightning Impulse Withstand Voltage | 75 kV peak (1.2/50 μs) |
| Insulation Material | VPI cycloaliphatic epoxy resin |
| Magnetic Core | Grain-Oriented Electrical Steel (GOES), M6 grade |
| Ambient Temperature Range | −25°C to +40°C |
| Relative Humidity | ≤95% (non-condensing) |
| Altitude Limit | ≤1000 m above sea level |
Standard Service Conditions
The LAJ-10Q is rated for operation under standard IEC 60060-1 environmental conditions. Ambient temperature must not exceed +40°C, with a 24-hour average not exceeding +35°C. Relative humidity may reach 95% provided condensation does not occur on the housing surface—a condition ensured by the hydrophobic nature of the epoxy resin. Installation altitude is limited to 1000 m; for sites above this elevation, derating factors per IEC 60071-1 apply (e.g., 1.5% reduction in withstand voltage per 100 m above 1000 m). The transformer is designed for continuous operation at rated primary current without exceeding a 60 K temperature rise above ambient, verified through thermal endurance tests per IEC 61869-2 Annex B.
Core and Winding Configuration
Each LAJ-10Q unit features one or two independent secondary windings wound on a single GOES core. The core material exhibits a maximum specific loss of 1.0 W/kg at 1.7 T and 50 Hz, minimizing no-load losses and enhancing accuracy at low load levels—critical for 0.2S class metering. Windings are precision-layer wound with enameled copper wire (Class F insulation, 155°C rating) and embedded within the resin matrix to prevent movement-induced errors. Terminal blocks are made of flame-retardant polyamide (UL94 V-0) with clearly marked polarity dots (reducing polarity per IEC 61869-2 Clause 6.3). Primary terminals accept bolted connections up to M12 size, rated for 3150 A continuous current.
Typical Applications
The LAJ-10Q serves as a foundational component in modern medium-voltage power systems, delivering reliable current transformation across diverse operational contexts.
Substation Secondary Metering
In 11kV/10kV distribution substations, the LAJ-10Q provides high-fidelity current signals to revenue-class meters (e.g., IEC 62053-22 compliant). Its 0.2S accuracy class ensures billing precision even at 1% of rated current—a requirement for smart grid implementations and time-of-use tariff structures. For example, in a 10 MVA urban substation feeding commercial loads, LAJ-10Q units with 600/5 A ratio and 10 VA burden supply data to AMI (Advanced Metering Infrastructure) systems with composite error < ±0.35% across 1–120% Ip. The cast-resin housing resists electromagnetic interference from adjacent switchgear, maintaining signal integrity without shielding.
Industrial Power Distribution
Within manufacturing plants and data centers, the LAJ-10Q interfaces with multifunction protection relays (e.g., IEC 61850-compliant devices) to enable fast fault detection. A typical 11kV motor feeder might employ an LAJ-10Q with dual windings: one 5P20 winding (15 VA) for overcurrent and earth-fault protection, and a 0.5 winding (5 VA) for energy monitoring. The transformer’s high saturation point (≥20×Ip) prevents core saturation during motor starting inrush (typically 6–8×In), ensuring relay stability. Its compact dimensions (e.g., 180 mm height × 120 mm width) facilitate retrofit into legacy switchgear cabinets.
Renewable Energy Integration
Solar and wind farms utilize the LAJ-10Q at collector substation incomers to monitor power export and provide anti-islanding protection. In a 20 MW solar plant, LAJ-10Q units (1250/1 A, 0.2S/5P10) feed signals to SCADA systems and distance relays. The low phase displacement error (< ±10 minutes at 100% Ip) ensures accurate vector measurements for synchrophasor applications. The dry-type design is preferred in remote locations due to zero maintenance and immunity to temperature cycling that degrades oil seals.
Rural and Suburban Distribution Networks
For utility pole-mounted or pad-mounted RMUs serving residential areas, the LAJ-10Q delivers cost-effective metering and sectionalizing protection. A common configuration uses 400/5 A ratio with 5P10 class for recloser coordination and 0.5 class for load profiling. The transformer’s IP2X-rated terminals prevent accidental contact, while UV-stabilized resin ensures decades-long outdoor durability when installed in ventilated enclosures. Its lightweight (≈8 kg) simplifies manual handling during rural grid upgrades.
Harmonic-Rich Environments
In facilities with nonlinear loads (e.g., variable frequency drives, UPS systems), the LAJ-10Q’s GOES core minimizes harmonic distortion effects. Testing per IEC 61869-2 Annex D confirms ratio error remains within class limits up to the 13th harmonic when burden impedance is resistive-inductive. This makes it suitable for data center PDU monitoring where THD can exceed 25%.
Compliance with International Standards
The LAJ-10Q’s design, testing, and certification strictly adhere to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and its Chinese counterpart, GB/T 20840.2-2012. These standards define performance, safety, and verification protocols for instrument transformers used in AC systems.
IEC 61869-2 Compliance Details
Key IEC 61869-2 requirements met by the LAJ-10Q include: (1) Accuracy verification under sinusoidal conditions per Clause 7.3, with composite error measured at 1%, 5%, 20%, 100%, and 120% of rated current; (2) Thermal stability demonstrated via 1-second short-circuit test at 20×Ip without exceeding 250°C hotspot temperature; (3) Dielectric tests including 28 kV power frequency withstand and 75 kV lightning impulse; (4) Partial discharge levels < 10 pC at 1.2 Um/√3. The transformer’s marking includes Ipn, Isn, accuracy class, burden, and polarity per Clause 6.2.
Alignment with GB/T 20840.2
GB/T 20840.2 mirrors IEC 61869-2 but incorporates China-specific requirements: (1) Altitude correction factor for installations >1000 m; (2) Mandatory type tests for fire resistance (vertical burning test per GB/T 5169.16); (3) Stricter moisture absorption limit (<0.15% after 24h immersion). The LAJ-10Q’s resin formulation meets all GB criteria, enabling use in State Grid and China Southern Power Grid projects. Domestic voltage designation is 10kV (Um=11.5 kV), though the physical design accommodates 11kV IEC systems without modification.
Testing and Certification Requirements
Certification requires successful completion of type tests (per IEC 61869-2 Table 2), routine tests (every unit), and sample tests (per batch). Type tests include temperature rise, short-circuit, and accuracy verification. Routine tests—performed on 100% of units—comprise: (1) Power frequency withstand (28 kV, 1 min); (2) Turns ratio and polarity check; (3) Insulation resistance (>1000 MΩ at 2500 V DC). Third-party certification by bodies like KEMA or CESI validates compliance.
On-Site Testing Procedures
Post-installation verification ensures the LAJ-10Q performs within specified tolerances. All tests follow IEC 61869-2 Clause 8 and IEEE C57.13.6 guidelines.
Insulation Resistance Test
Measure insulation resistance between primary-to-secondary and primary-to-ground using a 2500 V DC megohmmeter. Acceptance criterion: ≥1000 MΩ at 20°C. Correct for temperature using RT2 = RT1 × 2(T1−T2)/10. Low readings indicate moisture ingress or resin cracking—requiring replacement. Perform before and after dielectric tests to detect insulation degradation.
Turns Ratio Test
Apply low-voltage AC (e.g., 120 V) to the secondary winding and measure induced primary voltage. Calculate actual ratio = Vs/Vp. Compare to nameplate ratio; tolerance per IEC 61869-2 is ±0.25% for 0.2S class, ±0.5% for 0.5 class, and ±1% for 5P class. Use a dedicated turns ratio tester (e.g., Omicron CT Analyzer) for automated verification across multiple taps.
Polarity Test
Verify reducing polarity using the DC kick method: connect a 6–12 V battery to primary terminals (positive to P1), and a center-zero galvanometer to secondary (positive to S1). A momentary positive deflection confirms correct polarity. Incorrect polarity causes 180° phase shift, leading to metering errors or relay misoperation. Document results with timestamped oscillograms if using AC methods.
Power Frequency Withstand Voltage Test
Apply 28 kV rms, 50 Hz between primary and grounded secondary/housing for 1 minute. Use a calibrated test transformer with overcurrent trip (≤5 mA). No flashover or disruptive discharge is permitted. Reduce test voltage by 20% for repeat tests within 24 hours. Ensure all secondary windings are short-circuited and grounded during the test to avoid core saturation.
Short-Circuit Test (for CT)
Inject primary current at 100% and 120% Ip into a known burden (e.g., 1 Ω resistor) and measure secondary current. Verify ratio error and phase displacement against accuracy class limits. For 5P20 class, confirm linear response up to 20×Ip with composite error ≤10%. Use a high-current test set (e.g., Doble F6100) capable of 5000 A output. Record waveform distortion to assess core linearity.
Preventive Maintenance Guide
Although cast-resin CTs require minimal maintenance, periodic inspections extend service life and prevent unexpected failures.
Periodic Inspection Protocol
Conduct annual visual and electrical checks: (1) Inspect housing for cracks, tracking, or discoloration; (2) Verify terminal tightness (torque: 15 N·m for M8 bolts); (3) Measure insulation resistance; (4) Check secondary circuit continuity. Clean surfaces with dry cloth—never use solvents that degrade resin. In coastal or industrial atmospheres, inspect every 6 months for salt or chemical deposits.
Maintenance Intervals and Fault Diagnosis
Replace units exhibiting: (1) Insulation resistance <100 MΩ; (2) Ratio error exceeding twice the class limit; (3) Audible humming (indicating loose core laminations). While the LAJ-10Q has no user-serviceable parts, secondary terminal blocks can be replaced if corroded. Maintain records of all test results to trend performance degradation.
| Interval | Action |
|---|---|
| Annually | Visual inspection, IR scan, insulation resistance |
| Every 5 Years | Full accuracy verification, dielectric test |
| After Fault | Ratio, polarity, and withstand voltage tests |
Conclusion
The LAJ-10Q 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 and GB/T 20840.2 compliance with robust engineering for demanding metering and protection duties. Its VPI epoxy resin insulation eliminates the operational liabilities of oil-filled alternatives, offering fire-safe, maintenance-free performance in confined or sensitive environments. The use of high-permeability GOES core material ensures exceptional accuracy at low loads—critical for modern smart metering—and high saturation margins for reliable protection during fault transients. With a design life exceeding 30 years under standard service conditions, the LAJ-10Q delivers long-term value through reduced lifecycle costs, space efficiency, and immunity to environmental degradation. Its rigorous type testing—including partial discharge levels below 10 pC and thermal endurance at 20× rated current—validates suitability for critical infrastructure from urban substations to renewable energy hubs. As power systems evolve toward digitalization and distributed generation, the LAJ-10Q’s precision, reliability, and standards alignment position it as an essential component for secure and accurate current measurement across global markets.
Frequently Asked Questions (FAQ)
Q1: Can the LAJ-10Q be used in 10kV domestic systems?
Yes. While rated for 11kV IEC systems (Um=12 kV), the LAJ-10Q is fully compatible with 10kV Chinese distribution networks (Um=11.5 kV) per GB/T 20840.2. No derating is required.
Q2: What is the maximum number of secondary windings available?
The standard LAJ-10Q offers up to two independent secondary windings (e.g., 0.2S for metering + 5P20 for protection). Custom configurations with three windings are available upon request.
Q3: Is outdoor installation permitted?
The base model is rated for indoor use only. For outdoor applications, specify the LAJ-10Q-O variant with UV-resistant resin and IP54-rated terminals.
Q4: How do I select the correct burden for my relay?
Calculate total burden as Zb = (Rwire + Rrelay) + jXwire. Ensure Zb ≤ rated VA / Is². For 5 A systems, keep loop resistance <0.6 Ω for 15 VA burden.
Q5: What happens if the secondary is left open-circuited?
Never operate with open secondary—it induces dangerous overvoltages (>3 kV) and saturates the core, causing overheating and permanent damage. Always short-circuit unused secondaries with approved links.
Q6: Does the LAJ-10Q support digital output (e.g., IEC 61850-9-2)?
No. The LAJ-10Q is an analog CT. For digital substations, pair it with a merging unit (MU) that samples the analog output per IEC 61869-9.