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High-Performance 11kV Cast-Resin Current Transformer ZW-10 for Metering & Protection per IEC 61869-2
Introduction to the ZW-10 Current Transformer
The ZW-10 is a medium-voltage (MV) cast-resin current transformer rated at 11kV (IEC standard) or 10kV (domestic system equivalent), engineered for critical metering and protection functions in utility and industrial substations. Its design adheres strictly to IEC 61869-2 and GB/T 20840.2, ensuring global interoperability and reliability under demanding electrical and environmental conditions. Unlike legacy oil-immersed transformers, the ZW-10 employs vacuum pressure impregnation (VPI) epoxy resin technology, which provides superior dielectric strength, mechanical robustness, and fire resistance.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the ZW-10 utilizes a cycloaliphatic epoxy resin system processed under vacuum and pressure to eliminate voids and moisture ingress. This technique ensures homogeneous encapsulation of the primary conductor and magnetic core assembly. The resin’s high glass transition temperature (>120°C) and excellent tracking resistance (CTI >600 V) make it suitable for both indoor and outdoor installations, even in polluted or humid environments. The absence of liquid insulation eliminates risks of leakage, flammability, and maintenance associated with oil-filled units. Additionally, the solid dielectric enables compact physical dimensions while maintaining required creepage distances per IEC 60815 for pollution severity levels up to III.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed current transformers, the ZW-10 offers significant operational and safety benefits. It is inherently non-flammable (meeting IEC 60695 glow-wire test requirements), requires no periodic oil sampling or tank inspections, and exhibits negligible aging under thermal cycling. The solid resin matrix also provides superior mechanical stability against short-circuit electromagnetic forces—critical during fault conditions where peak currents can exceed 40 kA. Furthermore, the ZW-10’s lightweight construction (typically 25–35 kg depending on ratio) simplifies handling and reduces structural support requirements. Environmental compliance is enhanced as the unit contains no PCBs or hazardous liquids, aligning with RoHS and WEEE directives.
Typical Applications Overview
The ZW-10 is deployed across a spectrum of MV infrastructure, including urban distribution substations, industrial plant switchgear, renewable energy interconnection points (e.g., solar farms with 10/11kV collection systems), and railway traction networks. Its dual accuracy classes (e.g., 0.5 for metering and 5P20 for protection) allow a single unit to serve both revenue metering and relay protection circuits, reducing equipment count and lifecycle costs. In smart grid deployments, the ZW-10 integrates seamlessly with digital relays and IEC 61850-compliant merging units due to its stable phase error (<±10 minutes) and low burden characteristics.
Technical Specifications
The ZW-10 is engineered to precise electrical and environmental parameters to ensure consistent performance across global operating conditions. All specifications comply with IEC 61869-2:2012 and GB/T 20840.2-2014, with verified test reports available upon request.
Rated Electrical Parameters
Primary voltage rating: 11kV (IEC) / 10kV (domestic). Primary current ratings range from 50 A to 3000 A in standard steps (e.g., 100/5, 600/1, 1200/5). Secondary current is standardized at 1 A or 5 A. Accuracy classes include 0.2S, 0.5, 1, 3 for metering and 5P10, 5P20, 10P10, 10P20 for protection. Rated output (burden) varies by class: 5 VA (0.5 class), 10 VA (5P20), with maximum permissible burden up to 30 VA. Instrument security factor (FS) ≤5 for metering cores; accuracy limit factor (ALF) = 20 for 5P20 protection cores. Short-time thermal current rating: 20 kA for 1 s; dynamic withstand: 50 kA peak. These values are validated per IEC 61869-2 Clause 6.4.
Insulation and Environmental Ratings
Power frequency withstand voltage: 28 kV rms for 1 min (phase-to-earth); lightning impulse withstand: 75 kV peak (1.2/50 µs). Creepage distance ≥240 mm for 11kV (pollution severity III). Operating temperature range: –25°C to +40°C ambient; storage: –40°C to +70°C. Relative humidity tolerance: up to 95% non-condensing. Altitude rating: ≤1000 m above sea level; derating applies above 1000 m per IEC 60071-2 (e.g., 1.5% reduction in withstand voltage per 100 m above 1000 m). The housing is UV-stabilized cycloaliphatic resin with IP54 protection on secondary terminals.
Core and Construction Details
The magnetic circuit uses grain-oriented electrical steel (GOES) with thickness ≤0.30 mm and core loss ≤1.2 W/kg at 1.7 T, 50 Hz. Core lamination is annealed to minimize hysteresis and residual magnetism, ensuring linear response down to 1% of rated current—critical for accurate metering at light loads. Primary conductor is electrolytic-tough-pitch (ETP) copper or aluminum, sized for continuous current without exceeding 65 K temperature rise. Secondary windings use Class F (155°C) enameled copper wire, fully embedded in resin to prevent vibration-induced fatigue. Terminal blocks are brass with M6 screws, rated for 2×2.5 mm² or 1×4 mm² cables.
Typical Applications
The ZW-10’s dual-function design makes it ideal for modern substation architectures requiring both precision metering and high-speed protection.
Substation Secondary Metering
In 11kV/0.4kV distribution substations, the ZW-10 supplies current signals to revenue-class kWh meters (Class 0.5S or better). Its low phase displacement (<±5 minutes at 100% load) ensures minimal reactive energy measurement error. For example, in a 630 kVA transformer feeder with 36 A full-load current, a ZW-10 configured as 50/5 A with 0.5S class delivers ±0.5% amplitude error and ±10' phase error—well within EN 50470-1 limits. The cast-resin body resists condensation in unheated kiosks, preventing insulation degradation that could compromise long-term accuracy.
Industrial Power Distribution
Heavy industries (e.g., steel mills, chemical plants) use the ZW-10 on motor feeders and bus couplers where high fault currents demand robust protection. A typical 1250 kW induction motor (rated ~80 A at 10kV) may employ a ZW-10 with 100/1 A ratio and 5P20 class to drive overcurrent and differential relays. During a three-phase fault (e.g., 18 kA), the CT maintains linearity up to 20× rated current (2 kA), enabling reliable relay operation within 20 ms. The resin housing withstands chemical vapors and dust common in such environments.
Renewable Energy Integration
Solar photovoltaic (PV) plants often connect via 10/11kV switchgear. The ZW-10 monitors export power and provides anti-islanding protection. Due to intermittent generation, the CT must maintain accuracy at very low currents (e.g., 5% of rated). With its GOES core and optimized turns ratio, the ZW-10 achieves <±1.5% error at 5% load—exceeding IEC 61869-2 requirements for Class 0.5. Its outdoor rating (UV-resistant, –25°C start-up) suits desert or alpine installations.
Rural and Suburban Distribution Networks
In remote areas with limited maintenance access, the ZW-10’s maintenance-free design is critical. Mounted on pole-top reclosers or pad-mounted switchgear, it operates reliably for decades without intervention. For instance, a 100/5 A unit on a rural 11kV spur supplies data to a SCADA RTU for load profiling and fault location. The high ALF (20) ensures protection coordination even with long cable runs that increase secondary burden.
Urban Ring Main Units (RMUs)
Compact gas-insulated RMUs in cities integrate the ZW-10 for space-constrained applications. Its small footprint (diameter ≤220 mm) fits within SF6 compartments. Dual-core variants (e.g., 0.5/5P20) eliminate the need for separate metering and protection CTs, reducing cost and complexity in smart city grids.
Compliance with International Standards
The ZW-10 is certified to the latest international and Chinese standards, ensuring global acceptance and interoperability.
IEC 61869-2 Compliance Details
IEC 61869-2:2012 defines performance, testing, and marking requirements for instrument transformers. The ZW-10 meets all mandatory clauses: accuracy verification per Annex B (ratio and phase error tests at 5%, 20%, 100%, and 120% of rated current); temperature rise test (≤60 K for windings); short-circuit withstand (thermal and dynamic); and partial discharge (≤10 pC at 1.2 × Um/√3). Type tests were conducted at an ISO 17025-accredited lab, with routine tests performed on every unit (ratio, polarity, insulation resistance).
Alignment with GB/T 20840.2
GB/T 20840.2-2014 is China’s national adoption of IEC 61869-2, with minor deviations. Key differences include: domestic altitude derating starts at 1000 m (vs. IEC’s 2000 m); pollution severity classification uses DL/T 729 instead of IEC 60815; and short-time current duration is fixed at 1 s (IEC allows 0.5–3 s). The ZW-10 is dual-certified, with test reports issued by CEPREI and TÜV SÜD, satisfying both export and domestic procurement requirements.
Testing and Certification Requirements
Certification involves type tests (once per design), sample tests (per batch), and routine tests (100% production). Critical type tests include: power frequency withstand (28 kV/1 min), lightning impulse (75 kV), temperature rise (IEC 60076-2 method), and accuracy limit factor verification. Routine tests include: secondary winding resistance (±5% tolerance), insulation resistance (>1000 MΩ at 2500 V DC), and polarity check. Each unit bears a permanent nameplate with IEC 61869-2-compliant markings: model, ratio, accuracy class, burden, serial number, and manufacturer.
On-Site Testing Procedures
Post-installation verification ensures the ZW-10 performs as specified under field conditions.
Insulation Resistance Test
Measure insulation resistance between primary-to-secondary/ground and secondary-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 drying or replacement. Perform before and after high-voltage tests to detect insulation damage.
Turns Ratio Test
Apply low-voltage AC (e.g., 100 V) to the primary and measure secondary voltage. Calculate ratio = Vpri/Vsec. Compare to nameplate; tolerance per IEC 61869-2: ±0.5% for metering classes, ±1% for protection. Use a dedicated CT analyzer (e.g., Omicron CT Analyzer) for automated testing. Deviations >1% suggest turn-to-turn shorts or wiring errors.
Polarity Test
Verify reducing polarity using the DC kick method: momentarily apply 6–12 V DC to primary (P1 to P2); observe secondary voltage spike on a millivoltmeter. Positive deflection at S1 confirms correct polarity. Incorrect polarity causes relay misoperation or meter reversal. Digital testers automate this with phase-angle verification (<±2° tolerance).
Power Frequency Withstand Voltage Test
Apply 28 kV rms (50 Hz) between primary and grounded secondary/housing for 1 minute. Monitor for flashover or excessive leakage current (<1 mA). Use a calibrated HV test set with overcurrent trip. This test validates insulation integrity after transport/installation stresses. Do not perform if moisture is suspected—dry first.
Short-Circuit Test (for CT)
Inject 100–120% of rated primary current and verify secondary current magnitude and phase angle against reference. For 5P20 class, confirm linearity up to 20× rated current (e.g., 2000 A for 100 A CT). Use a portable current injector with burden resistor matching actual load. Record waveform distortion—THD should be <5%. This validates protection performance under fault conditions.
Preventive Maintenance Guide
Although cast-resin CTs are maintenance-free, periodic checks extend service life and prevent failures.
Periodic Inspection Protocol
Annual visual inspection: check for cracks, discoloration (indicating overheating), or tracking on resin surface. Clean with dry cloth—never solvents. Verify terminal tightness (torque: 2.5 N·m for M6). Measure insulation resistance annually; trend downward >20% warrants investigation. In coastal areas, inspect for salt deposits—clean with deionized water if needed. Ensure secondary circuits are never left open during operation.
Maintenance Intervals and Fault Diagnosis
Every 5 years: perform full electrical tests (ratio, polarity, insulation resistance, burden verification). Common faults include: open secondary (causes core saturation, overheating); moisture ingress (low insulation resistance); and mechanical damage (cracks from impact). Diagnose via: abnormal heating (thermal imaging), erratic meter readings, or relay malfunctions. Replace if partial discharge exceeds 20 pC or core remanence causes >2% ratio error at low currents.
Maintenance Schedule Summary
| Interval | Activity | Acceptance Criteria |
|---|---|---|
| Annually | Visual inspection, IR measurement | No cracks; IR >1000 MΩ |
| 5 Years | Full electrical tests | Ratio error within class limits |
| After Fault | Post-fault verification | No change in ratio/polarity |
| As Needed | Cleaning (polluted sites) | Creepage path clear |
Conclusion
The ZW-10 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining IEC 61869-2 compliance with rugged, maintenance-free construction. Its vacuum-pressure-impregnated epoxy resin insulation eliminates fire hazards and environmental concerns inherent in oil-filled alternatives, while the GOES silicon steel core ensures metrological stability from 1% to 120% of rated current. Engineered for dual roles in metering (Class 0.2S/0.5) and protection (5P20/10P20), it reduces capital expenditure by consolidating functions into a single unit. Rigorous type testing—including 28 kV power frequency withstand, 75 kV lightning impulse, and 20 kA short-circuit validation—guarantees reliability under extreme grid conditions. With an expected service life of 25–30 years and suitability for both indoor switchgear and outdoor substations, the ZW-10 delivers exceptional lifecycle value. Its compatibility with modern digital protection schemes and adherence to both IEC and GB standards make it a globally deployable solution for utilities, industrial facilities, and renewable energy integrators seeking precision, safety, and longevity in current measurement.