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11kV Cast-Resin Current Transformer LZBJ-10 for Metering and Protection – IEC 61869-2 Standard
Introduction to the LZBJ-10 Current Transformer
The LZBJ-10 is a high-reliability, indoor-type cast-resin current transformer (CT) engineered for 11kV (IEC-rated) or 10kV (domestic nominal) medium-voltage switchgear applications. Designed in strict compliance with IEC 61869-2 and GB/T 20840.2, this instrument transformer delivers precise current transformation for both metering and protective relaying functions in modern power distribution networks.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LZBJ-10 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, providing superior dielectric strength and long-term environmental stability. The resin matrix exhibits a relative permittivity of approximately 3.8–4.2 and volume resistivity exceeding 1×1014 Ω·cm at 20°C. Unlike oil-filled alternatives, the solid dielectric requires no maintenance, poses no fire hazard, and maintains consistent performance across temperature cycles from –25°C to +40°C ambient. The absence of liquid insulation also simplifies transportation, handling, and installation in confined switchgear compartments.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the LZBJ-10’s cast-resin construction offers significant operational and safety benefits. It is inherently non-flammable, eliminating fire risks in indoor substations—a critical requirement under IEC 62271-200 for metal-enclosed switchgear. The compact footprint (typically 180 mm height × 120 mm width × 80 mm depth) allows integration into space-constrained ring main units (RMUs) and cubicle-type switchboards. Additionally, the solid insulation prevents oil leakage, which can degrade contact resistance in adjacent busbar joints or compromise arc-flash containment. Long-term reliability is enhanced by the resin’s resistance to tracking and erosion under partial discharge activity; typical partial discharge levels are below 10 pC at 1.2×Um/√3 (where Um = 12 kV). These attributes make the LZBJ-10 ideal for urban substations, commercial buildings, and industrial facilities where safety, space efficiency, and zero-maintenance operation are paramount.
Typical Applications Overview
The LZBJ-10 serves dual roles in electrical infrastructure: accurate energy metering and dependable fault detection. In utility-owned 11kV distribution feeders, it interfaces with revenue-class kWh meters (accuracy class 0.5 or 0.2S) to ensure billing integrity. Simultaneously, its protection-class windings (e.g., 5P10 or 5P20) supply scaled current signals to overcurrent relays, differential protection schemes, and earth-fault detectors. Common deployment environments include primary substations feeding industrial parks, hospital backup power systems, data center UPS inputs, and renewable energy interconnection points (e.g., solar PV inverters tied to 11kV grids). Its robust design supports continuous operation under harmonic-rich conditions—critical in modern networks with variable-frequency drives and power electronics.
Technical Specifications
The LZBJ-10 is engineered to meet stringent electrical and mechanical requirements for 11kV systems. Below is a representative specification table based on standard configurations:
| Parameter | Value |
|---|---|
| Rated Voltage (Ur) | 11 kV (IEC), 10 kV (GB) |
| Maximum System Voltage (Um) | 12 kV |
| Primary Current (Ip) | 50 A to 3000 A (standard steps) |
| Secondary Current (Is) | 1 A or 5 A |
| Accuracy Classes | Metering: 0.2S, 0.5S, 1 Protection: 5P10, 5P15, 5P20 |
| Rated Output (Burden) | 2.5 VA to 30 VA (per winding) |
| Short-Time Thermal Current (Ith) | 20×Ip for 1 s (min.) |
| Dynamic Withstand Current (Idyn) | 50×Ip peak |
| 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 Epoxy Resin + Filler |
| Magnetic Core | Grain-Oriented Electrical Steel (GOES), CRGO grade |
Standard Service Conditions
The LZBJ-10 is rated for indoor installation under normal service conditions as defined in IEC 61869-2: ambient temperature range of –25°C to +40°C (24-hour average ≤ +35°C), relative humidity up to 95% at 25°C, and altitude not exceeding 1000 m above sea level. For installations above 1000 m, dielectric strength must be derated by 1% per 100 m increment. The transformer is designed for continuous operation at rated frequency (50 Hz or 60 Hz) with total harmonic distortion (THD) ≤ 5%. Mechanical vibration limits follow IEC 60068-2-6 (sinusoidal, 0.7 g from 2–9 Hz, 0.01 g²/Hz from 9–500 Hz). These parameters ensure stable performance in harsh industrial environments such as mining operations or chemical plants.
Core and Winding Configuration
Each LZBJ-10 unit typically incorporates two to four independent secondary windings wound on separate GOES cores within a single resin block. Metering windings use high-permeability CRGO steel (Bmax ≈ 1.7 T at 50 Hz) to minimize excitation current and phase error, achieving ratio error within ±0.2% for 0.2S class. Protection windings employ lower-permeability steel with larger cross-sections to avoid saturation during high-magnitude fault currents (e.g., 20×Ip). Secondary terminals are labeled S1/S2 per IEC 60617, with reducing polarity (dot convention) clearly marked. Terminal blocks accommodate 2.5–6 mm² stranded copper conductors with screw-type clamps rated for 10 A continuous current. All windings are individually insulated with Class F (155°C) enamel, ensuring thermal endurance beyond the 25-year design life.
Typical Applications
The versatility of the LZBJ-10 enables deployment across diverse power systems requiring accurate current measurement and reliable protection coordination.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the LZBJ-10 provides input to Class 0.5S or 0.2S kWh meters for utility revenue metering. Its low phase displacement (< ±10 minutes at 1–120% In) ensures compliance with EN 50470-1 for active energy accuracy. For example, a 600/5 A, 0.2S CT feeding a MID-certified meter in a commercial building guarantees billing errors below ±0.3% under load profiles ranging from 5% to 100% of rated current. The cast-resin housing resists electromagnetic interference from nearby switchgear operations, maintaining signal fidelity even during capacitor bank switching transients.
Industrial Power Distribution
Heavy industries—such as steel mills, cement plants, and refineries—rely on the LZBJ-10 for motor protection and load monitoring. A typical 1250/1 A, 5P20 configuration supplies current to numerical relays (e.g., Siemens 7SJ62) for 1 MW induction motors. The 5P20 rating ensures the composite error remains ≤5% at 20× rated current (25 kA), enabling precise time-overcurrent coordination with upstream breakers. The transformer’s high short-circuit withstand capability (Ith = 25 kA/1s) prevents mechanical damage during bolted faults, a common occurrence in high-inertia motor circuits.
Renewable Energy Integration
Solar and wind farms connecting to 11kV distribution networks use the LZBJ-10 for both generation metering and anti-islanding protection. In a 5 MW solar plant, dual-winding CTs (e.g., 800/5 A with 0.5S + 5P10) feed SCADA systems and ROCOF (Rate-of-Change-of-Frequency) relays. The CT’s linear response under DC-offset fault currents—common during grid disturbances—is critical for accurate fault detection. Cast-resin insulation also withstands UV exposure and thermal cycling in outdoor combiner boxes when housed in IP4X enclosures, though the base model is intended for indoor use.
Rural and Suburban Distribution Networks
Utility companies deploy the LZBJ-10 in pole-mounted or pad-mounted switchgear serving residential and agricultural loads. Here, cost-effective 400/5 A, 1.0 class CTs support AMR (Automatic Meter Reading) systems while 5P10 windings enable sectionalizing via reclosers. The transformer’s immunity to moisture ingress prevents accuracy drift in humid climates—a frequent issue with older paper-oil CTs. In China’s rural grid upgrades, the GB/T 20840.2-compliant version ensures interoperability with State Grid Corporation metering infrastructure.
Data Center and Critical Facility Backup Systems
Uninterruptible power supply (UPS) inputs and generator tie-breakers in Tier III/IV data centers utilize the LZBJ-10 for real-time load profiling and ground-fault detection. A 2000/1 A, 0.2S + 5P20 dual-core unit monitors 1.6 MW of IT load while triggering differential protection if internal busbar faults exceed 30 kA. The compact size allows mounting directly on generator output busbars without external CT cabinets, reducing footprint and installation cost.
Compliance with International Standards
The LZBJ-10 is certified to both global and regional standards, ensuring interoperability and regulatory acceptance across markets.
IEC 61869-2 Compliance Details
IEC 61869-2 governs the performance, testing, and marking of instrument transformers for AC systems >1 kV. The LZBJ-10 meets all mandatory clauses, including: ratio error and phase displacement limits per Table 2 (e.g., ±0.2% ratio error for 0.2S at 100% In); thermal stability under 1.2×Um/√3 for 8 hours; and short-circuit withstand per Clause 6.6 (Ith ≥ 20×Ip). Type tests include temperature rise (Δθ ≤ 60 K for windings), impulse voltage (75 kV peak), and partial discharge (<10 pC at 1.2×Um/√3). Routine tests on every unit cover power frequency withstand (28 kV/1 min), turns ratio verification (±0.25% tolerance), and polarity confirmation.
Alignment with GB/T 20840.2
China’s national standard GB/T 20840.2 mirrors IEC 61869-2 but includes localized requirements: altitude correction factors for installations above 1000 m (e.g., Xining at 2200 m requires 12% higher test voltage), and mandatory seismic qualification per GB/T 13540 (horizontal acceleration 0.2g). The domestic 10kV system voltage (vs. IEC’s 11kV) necessitates adjusted insulation coordination—though the same physical unit serves both ratings due to the 12 kV Um design margin. Chinese utilities require type test reports from CNAS-accredited labs, which the LZBJ-10 possesses.
Key Differences Between IEC and Domestic Standards
While technically harmonized, practical differences exist. IEC 61869-2 permits 1 A or 5 A secondaries universally, whereas GB/T 20840.2 historically favored 5 A in legacy systems (though 1 A is now accepted). Accuracy class definitions are identical, but Chinese procurement often specifies “0.2S” exclusively for smart metering, while European projects may accept “0.5S.” Environmental testing under GB includes salt fog resistance (48 h, 5% NaCl) for coastal regions—a non-mandatory IEC add-on. Certification bodies also differ: IEC units carry CB Scheme marks (e.g., TÜV), while GB versions require CQC certification.
On-Site Testing Procedures
Post-installation verification ensures the LZBJ-10 performs within specified tolerances before energization.
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 at 20°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings (<100 MΩ) indicate moisture absorption or resin cracking—requiring drying or replacement. Perform before and after high-voltage tests to detect insulation degradation.
Turns Ratio Test
Apply a low-voltage AC source (e.g., 10 V at 50 Hz) to the primary and measure secondary voltage. Calculate actual ratio: Nactual = Vp/Vs. Compare to nameplate ratio (Nrated). Tolerance: ±0.25% for metering classes, ±0.5% for protection. Example: 600/5 A CT should yield Vs = 0.0833×Vp ±0.25%. Use a dedicated CT analyzer (e.g., Omicron CT Analyzer) for automated error plotting across 1–120% In.
Polarity Test
Verify reducing polarity using the DC kick method: connect a 6 V battery between P1 (+) and P2 (–), and a DC millivoltmeter between S1 (+) and S2 (–). A momentary positive deflection confirms correct polarity. Incorrect polarity reverses relay operating torque—potentially causing miscoordination. Digital relays may auto-detect polarity, but physical verification remains mandatory per IEC 60255-27.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (50 Hz) between primary and grounded secondary/enclosure for 1 minute. Use a calibrated test transformer with overcurrent trip set at 100 mA. No flashover or disruptive discharge is permitted. Ramp voltage at 2 kV/s to avoid transient overstress. This test validates insulation integrity after transport-induced microcracks.
Excitation (Saturation) Characteristic Test
For protection-class windings, plot excitation curve by applying increasing AC voltage to secondary (primary open). Record current at knee-point voltage (Vk). For 5P20, Vk must satisfy Vk ≥ 20×In×(Rct + Rb), where Rct = CT winding resistance, Rb = burden resistance. Example: 5 A, 5P20 CT with Rct=0.3 Ω, Rb=1.0 Ω requires Vk ≥ 130 V. Saturation below this indicates core defects.
Preventive Maintenance Guide
Although cast-resin CTs are maintenance-free by design, periodic checks extend service life and prevent unexpected failures.
Annual Visual and Electrical Inspection
Inspect for surface tracking, cracks, or discoloration on the resin housing—indicative of partial discharge or UV degradation. Clean with dry cloth; never use solvents. Verify terminal tightness (torque: 1.5 N·m for M4 screws). Re-measure insulation resistance annually; a 50% drop from baseline warrants further investigation. Check secondary wiring for loose connections that could cause open-circuit hazards.
Five-Year Comprehensive Maintenance
Every 60 months, perform full ratio, polarity, and excitation tests as per commissioning procedures. Compare results to baseline data; ratio shift >0.5% suggests core lamination damage. Thermographic scan under 50% load should show no hot spots (>10 K above ambient) at terminals. Update maintenance logs with test certificates traceable to ISO/IEC 17025 labs. Replace units exhibiting partial discharge >20 pC during offline testing.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicator |
|---|---|---|
| Annually | Visual inspection, IR test | Cracks, carbon tracks, IR < 500 MΩ |
| 5 Years | Full electrical tests | Ratio error >0.5%, Vk drop >15% |
| After Fault | Dynamic withstand verification | Mechanical deformation, relay misoperation |
Common failure modes include secondary open-circuit (causing core saturation and overheating) and moisture ingress at terminal seals. Immediate de-energization is required if arcing sounds or ozone odor are detected.
Conclusion
The LZBJ-10 11kV cast-resin current transformer represents a benchmark in reliability, accuracy, and compliance for modern medium-voltage infrastructure. Its VPI epoxy resin encapsulation eliminates the fire and environmental hazards associated with oil-filled designs, while the GOES core ensures metrological precision across metering and protection applications. Certified to both IEC 61869-2 and GB/T 20840.2, it seamlessly integrates into global switchgear systems—from European utility substations to Chinese industrial parks. Rigorous type testing guarantees performance under extreme electrical stresses, including 28 kV power frequency withstand and 75 kV lightning impulses. With a design life of 25–30 years and minimal maintenance requirements, the LZBJ-10 reduces total cost of ownership while enhancing grid safety and measurement integrity. Its compact form factor and dual-winding flexibility make it indispensable for smart grid deployments, renewable integration, and critical facility power systems where failure is not an option. Engineers selecting this CT gain a field-proven solution that meets today’s standards and tomorrow’s challenges.
Frequently Asked Questions (FAQ)
Q1: Can the LZBJ-10 be used in 10kV domestic systems despite being rated 11kV?
Yes. The 11kV rating refers to the IEC standard system voltage (Ur = 11 kV), while the maximum system voltage Um = 12 kV provides sufficient insulation margin for 10kV domestic networks (where Um = 11.5 kV per GB standards). The same physical unit is certified under both IEC 61869-2 and GB/T 20840.2, making it interchangeable in 10kV/11kV applications without derating.
Q2: What happens if the secondary circuit is accidentally opened during operation?
An open secondary creates extremely high voltage (several kV) across the terminals due to unopposed magnetizing current, risking insulation breakdown and personnel hazard. The LZBJ-10’s resin housing contains internal arcing, but repeated incidents degrade the core. Always short-circuit secondary terminals with a link before disconnecting meters or relays. Modern numerical relays include open-circuit detection, but physical safeguards remain essential.
Q3: How does harmonic distortion affect LZBJ-10 accuracy?
Harmonics increase core losses and may cause saturation in protection windings. For metering classes (0.2S/0.5S), IEC 61869-2 specifies error limits only at fundamental frequency. However, the GOES core’s high permeability minimizes additional errors up to THD=10%. For applications with known harmonics (e.g., VFDs), specify extended accuracy or use harmonic-filtering meters. Protection windings are less affected as relays typically filter harmonics digitally.
Q4: Is the LZBJ-10 suitable for outdoor installation?
The base model is rated for indoor use only (IP00). For outdoor applications, it must be housed in an IP43 or higher enclosure to protect against rain and dust. Direct UV exposure degrades epoxy resin over time, causing surface chalking. Duomatech offers an outdoor variant with UV-stabilized resin and hydrophobic coating—consult technical datasheet DMT-LZBJ-10-OUT.
Q5: What burden values are compatible with 5P20 protection class?
The 5P20 rating assumes a specific burden (e.g., 15 VA). If actual burden exceeds this, the CT may saturate prematurely. Calculate total burden: Rb = Rwire + Rrelay + Rcontact. For 5 A secondary, 15 VA implies Rb ≤ 0.6 Ω. Use 4 mm² copper wire for runs >10 m to limit Rwire. Always verify Vk ≥ 20×In×(Rct + Rb) during design.
Q6: How to select between 1 A and 5 A secondary current?
1 A reduces burden (P = I²R), enabling longer cable runs with smaller conductors—ideal for large substations. 5 A provides higher signal-to-noise ratio for electromechanical relays but increases burden quadratically. Modern digital relays work equally well with both. Choose 1 A for new installations >20 m cable length; 5 A for retrofits matching legacy equipment.
Q7: Does the LZBJ-10 require demagnetization after testing?
Only if excitation tests drove the core into deep saturation. Residual flux can cause ratio errors in subsequent measurements. Demagnetize by applying AC voltage at 50 Hz, gradually increasing to Vk, then slowly decreasing to zero over 30 seconds. Most routine tests do not require this step.
Q8: What is the lead time for custom ratios or accuracy classes?
Standard ratios (e.g., 400/5, 600/1) ship within 2 weeks. Non-standard configurations (