Article Content
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 (MV) cast-resin current transformer (CT) rated for 11kV systems per IEC standards, with a domestic system equivalent of 10kV. Engineered for both revenue metering and protective relaying applications, this instrument transformer integrates advanced vacuum pressure impregnation (VPI) epoxy resin technology with grain-oriented electrical steel (GOES) core laminations to deliver exceptional accuracy, thermal stability, and long-term reliability. Unlike traditional oil-immersed CTs, the LZX-10 eliminates fire hazards, environmental leakage risks, and maintenance-intensive fluid management, making it ideal for indoor substations, urban distribution networks, and industrial facilities where safety and space constraints are critical.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LZX-10 utilizes a two-stage VPI process wherein dry-wound windings and GOES cores are first evacuated to remove moisture and air, then impregnated under controlled pressure with cycloaliphatic epoxy resin. This resin cures into a monolithic, void-free solid that provides superior dielectric strength (≥30 kV/mm), excellent tracking resistance (CTI ≥600 V), and high mechanical rigidity. The absence of internal voids prevents partial discharge inception even under continuous overvoltage conditions up to 1.2 × Um. Thermal conductivity of the cured resin (~0.8 W/m·K) ensures efficient heat dissipation from the core during overload or fault conditions, maintaining stable magnetic characteristics across the operating temperature range of –25°C to +40°C ambient.
Advantages Over Oil-Immersed Designs
Compared to oil-filled CTs, the LZX-10 offers significant operational and safety benefits. It is inherently non-flammable (IEC 60695 glow-wire tested at 960°C), eliminating fire propagation risks in confined spaces such as switchgear rooms or underground vaults. The solid insulation requires no periodic oil sampling, degassing, or level checks, reducing lifecycle costs by an estimated 30–40% over 25 years. Additionally, the compact footprint—enabled by higher dielectric strength of epoxy versus mineral oil—allows for denser panel layouts in modern metal-enclosed switchgear. Environmental compliance is enhanced as the unit contains no PCBs or hydrocarbons, satisfying RoHS and REACH directives without special disposal protocols.
Typical Applications Overview
The LZX-10 is deployed across diverse MV infrastructure where precision and dependability are paramount. In utility-owned zone substations, it supplies Class 0.2S metering signals to AMI systems while simultaneously feeding Class 5P20 protection relays for feeder overcurrent schemes. Industrial plants use it for motor protection coordination and energy billing at 10kV busbars. Renewable integration points—such as solar farm collector substations—leverage its low remanence and high saturation margin to ensure accurate fault current measurement during grid transients. Its robust design also supports deployment in harsh environments including coastal areas (salt fog resistant) and high-altitude sites (up to 2000 m above sea level without derating).
Technical Specifications
The LZX-10 adheres to stringent dimensional, electrical, and thermal parameters defined by IEC 61869-2 and GB/T 20840.2. Key specifications include a highest voltage for equipment (Um) of 12 kV, power frequency withstand voltage of 28 kV rms for 1 minute, and lightning impulse withstand of 75 kV peak. Standard current ratios span from 50/5 A to 3000/5 A, with dual-ratio options available (e.g., 600/1 A and 1200/1 A on a single core). Accuracy classes comply with IEC 61869-2 Table 102: Class 0.2S for metering (phase error ≤±10′, ratio error ≤±0.2% at 1–120% In), and Class 5P20 for protection (composite error ≤±5% at 20× In).
Rated Electrical Parameters
Rated primary current (Ip) ranges from 50 A to 3000 A, with standard secondary currents of 1 A or 5 A. Rated burden is specified at 5 VA, 10 VA, 15 VA, or 30 VA depending on application; for example, 10 VA is typical for digital meters, while 30 VA may be required for electromechanical relays with long cable runs. The short-time thermal current rating is 20 kA for 1 second (Ith = 20 kA, t = 1 s), and dynamic withstand current is 50 kA peak (Idyn = 50 kA). Core material is CRGO (cold-rolled grain-oriented) silicon steel with thickness of 0.23 mm, ensuring low core loss (<0.8 W/kg at 1.5 T, 50 Hz) and high permeability (>15,000 µr at low flux densities).
Environmental and Mechanical Ratings
The LZX-10 operates reliably under standard service conditions per IEC 61869-1: ambient temperature –25°C to +40°C, relative humidity ≤95% (non-condensing), and altitude ≤1000 m. For installations above 1000 m, dielectric tests must be corrected per IEC 60071-2 (e.g., at 2000 m, test voltage reduced by 12%). The housing is UV-stabilized cycloaliphatic epoxy with IP00 rating (intended for enclosed switchgear); however, optional IP2X terminal covers are available for exposed secondary terminals. Creepage distance exceeds 240 mm/kV (medium pollution, System B per IEC 60815), suitable for industrial atmospheres with moderate chemical exposure.
Accuracy and Performance Characteristics
Metering cores achieve Class 0.2S per IEC 61869-2 Annex C, validated at 1%, 5%, 20%, 100%, and 120% of rated current. Ratio error tolerance is ±0.2% at 100% In, tightening to ±0.15% at 120% In. Phase displacement remains within ±10 minutes of arc across the full range. Protection cores meet Class 5P20, meaning composite error does not exceed 5% when subjected to 20 times rated current at rated burden. Saturation factor (FS) is ≥5 for metering cores and ≥20 for protection cores, ensuring linear response during faults. Remanence is limited to <10% of saturation flux density due to controlled core annealing during manufacturing.
Typical Applications
The LZX-10 serves as a foundational component in modern MV power systems, enabling precise energy measurement and reliable fault detection across multiple sectors.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LZX-10 provides Class 0.2S current signals to smart meters for billing-grade energy accounting. Its low phase error ensures accurate reactive power (kVARh) and power factor calculations, critical for utility revenue integrity. For example, in a European DSO’s ring-main unit, dual-core LZX-10 units supply one core to a MID-certified meter and another to a SCADA RTU, with burdens matched to 5 VA each. The cast-resin construction prevents moisture ingress in humid climates, avoiding the insulation degradation common in older oil-paper CTs that cause drift in ratio error over time.
Industrial Power Distribution
Within manufacturing facilities operating at 10kV (domestic equivalent), the LZX-10 protects critical loads such as large induction motors (≥500 kW) and rectifier transformers. A typical configuration uses a 1200/5 A, 5P20 CT connected to a multifunction relay (e.g., SEL-751) for instantaneous overcurrent, thermal overload, and ground-fault protection. The high dynamic withstand (50 kA) ensures survival during bolted three-phase faults on busbars fed by utility transformers with 250 MVA short-circuit capacity. The absence of oil simplifies compliance with NFPA 70E arc-flash safety protocols during maintenance.
Renewable Energy Integration
Solar photovoltaic (PV) and wind farms utilize the LZX-10 at the point of interconnection (POI) to monitor export/import and enable anti-islanding protection. During grid disturbances, inverters inject reactive current per IEEE 1547, requiring CTs with minimal saturation to accurately measure asymmetric fault currents. The LZX-10’s GOES core and air-gap-free design maintain linearity up to 30× In, supporting precise operation of directional overcurrent relays in microgrid islanding schemes. Its compact size allows mounting directly on PV combiner boxes or wind turbine step-up transformers without external enclosures.
Rural and Suburban Distribution Networks
In remote or semi-urban feeders, the LZX-10 enables cost-effective automation through integration with reclosers and sectionalizers. A 400/1 A, 0.5/5P10 dual-core unit might feed a revenue meter and a recloser control module simultaneously. The cast-resin body resists rodent damage and vandalism better than porcelain-housed alternatives, while the wide operating temperature range accommodates desert (–30°C to +50°C ambient with derating) or alpine conditions. Utilities report 99.98% field reliability over 10 years in such deployments, attributed to the hermetic seal of the epoxy matrix against dust and moisture.
Compliance with International Standards
The LZX-10 is certified to IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) and harmonized Chinese standard GB/T 20840.2–2014. Both standards define performance, testing, and marking requirements, though key differences exist in test methodologies and environmental classifications.
IEC 61869-2 Certification Details
Compliance with IEC 61869-2 mandates rigorous type tests including temperature rise (≤60 K for windings at 1.2× In), short-circuit withstand (thermal and dynamic), and accuracy verification across extended current ranges. The LZX-10 undergoes partial discharge testing at 1.2 Um/√3 with inception levels <5 pC, well below the 20 pC limit. Markings include rated voltage (11 kV), accuracy class (e.g., 0.2S/5P20), rated output (e.g., 10 VA), and polarity dots per IEC 61869-1 Clause 7. The manufacturer’s test certificate includes measured ratio error, phase displacement, and excitation curves traceable to national metrology institutes.
Alignment with GB/T 20840.2
GB/T 20840.2 adopts IEC 61869-2 with minor modifications for the Chinese market. Notably, it specifies a higher power frequency withstand voltage (32 kV rms for 1 min vs. IEC’s 28 kV) for 10kV-class equipment, reflecting historical practice in China’s 10kV networks. The LZX-10 meets this requirement through reinforced resin wall thickness (minimum 18 mm between primary and secondary). Additionally, GB/T mandates salt fog testing (1000 hours, 5% NaCl) for outdoor-rated units—though the base LZX-10 is indoor-only, an optional hydrophobic coating satisfies this for hybrid applications. Accuracy class definitions remain identical, ensuring global interoperability.
Key Differences Between IEC and Domestic Standards
While IEC 61869-2 uses Um = 12 kV for 11kV systems, Chinese standards reference nominal system voltage (10 kV) directly. This leads to different insulation coordination: IEC bases lightning impulse on Um (75 kV), whereas GB historically used 10 kV nominal (75 kV remains common, but some legacy specs cite 60 kV). The LZX-10 bridges this gap by exceeding both requirements. Another divergence lies in burden notation: IEC uses VA (e.g., 10 VA), while older Chinese drawings may specify impedance (e.g., 0.4 Ω for 5 A secondary). Modern GB/T 20840.2 now aligns with IEC on VA-based burden, simplifying international procurement.
On-Site Testing Procedures
Post-installation verification ensures the LZX-10 performs within specification. All tests follow IEC 61869-2 Annex E and IEEE C57.13.6 guidelines.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary winding and earthed frame, and between secondary windings and earth. Acceptance criterion: ≥1000 MΩ at 20°C. Temperature correction applies per IEEE 43: Rcorr = Rmeas × 2(40–T)/10. Values below 500 MΩ indicate moisture ingress or resin cracking, requiring drying or replacement. This test is mandatory after transportation in humid conditions or prolonged storage.
Turns Ratio Test
Apply a low-voltage AC source (50–100 V) to the primary and measure secondary voltage. Calculate actual ratio as Vp/Vs; compare to nameplate. Tolerance per IEC 61869-2: ±0.2% for metering cores, ±1.0% for protection cores. For a 600/5 A CT, measured ratio must be 120 ±0.24 (metering) or 120 ±1.2 (protection). Use a dedicated ratio tester (e.g., Omicron CT Analyzer) for automated comparison against factory excitation curves.
Polarity Test
Verify reducing polarity using the DC kick method: connect a 6–12 V battery momentarily across primary terminals (H1 to H2). Observe secondary voltage polarity with a DC voltmeter (X1 to X2). A positive kick confirms correct dot convention. Incorrect polarity causes 180° phase shift, leading to false tripping in differential or directional schemes. This test is critical for multi-CT installations like busbar protection.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (IEC) or 32 kV rms (GB) at 50 Hz between primary and grounded secondary/frame for 1 minute. Leakage current must remain <10 mA. Partial discharge should not exceed 10 pC. This test validates insulation integrity after handling; it is typically performed only during commissioning or after major maintenance, not routinely.
Excitation (Saturation) Characteristic Test
For protection CTs, plot secondary excitation voltage (Ve) versus magnetizing current (Ie) up to 150% of knee-point voltage (Vk). Per IEC 61869-2, Vk must exceed 2× rated burden voltage at 20× In. For a 5P20, 10 VA CT, Vk ≥ 2 × (10 VA / 5 A) × 20 = 80 V. Low Vk indicates core damage or excessive air gaps, risking saturation during faults.
Preventive Maintenance Guide
Although cast-resin CTs require minimal maintenance, periodic checks extend service life beyond 30 years.
Annual Visual and Functional Inspection
Inspect for surface cracks, tracking marks, or discoloration on the resin housing. Check terminal tightness (torque: 2.5 N·m for M6 screws) and corrosion on secondary connections. Verify secondary circuits are never left open during operation—a shorting link must be engaged if disconnected. Measure secondary loop resistance annually; a >20% increase suggests loose connections or degraded wiring. Clean surfaces with isopropyl alcohol if contaminated with conductive dust.
Five-Year Comprehensive Maintenance
Every 60 months, repeat insulation resistance and turns ratio tests. Perform thermographic scanning under load (≥30% In) to detect hot spots indicating internal defects. If ratio error drifts >0.1% from baseline (for metering cores), investigate core aging or winding deformation. Replace units showing partial discharge >20 pC at 1.2 Um/√3 during off-line testing. Maintain records per ISO 55000 asset management standards.
Maintenance Intervals and Fault Diagnosis
| Interval | Task | Acceptance Criteria |
|---|---|---|
| Annually | Visual inspection, terminal torque check | No cracks, tracking, or overheating signs |
| 5 Years | Insulation resistance, ratio test | R ≥1000 MΩ; ratio error within spec |
| 10 Years | Partial discharge measurement | <10 pC at 1.2 Um/√3 |
| As needed | Thermography under load | ΔT ≤10°C vs. adjacent phases |
Common faults include open-circuited secondaries (causing dangerous overvoltages), moisture-induced surface leakage (reducing insulation resistance), and mechanical stress cracks from improper handling. Immediate replacement is required if the resin shows UV degradation (chalky surface) or impact fractures.
Conclusion
The LZX-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 compliance with practical engineering robustness. Its vacuum pressure impregnated epoxy resin insulation eliminates the fire, environmental, and maintenance liabilities of oil-filled alternatives, while the GOES silicon steel core ensures metrological precision for both revenue metering (Class 0.2S) and high-integrity protection (Class 5P20). Validated across diverse applications—from urban substations to renewable integration points—the LZX-10 delivers consistent performance under thermal, electrical, and environmental stressors. With a design life exceeding 25–30 years and minimal preventive maintenance requirements, it offers a compelling total cost of ownership advantage. Utilities and industrial operators can deploy the LZX-10 with confidence in its ability to support grid modernization, smart metering, and resilient protection schemes in compliance with global standards. Its adherence to both IEC and GB frameworks further facilitates international procurement and interoperability in multi-vendor environments.