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Model DEL-11385 11kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the DEL-11385 Current Transformer
The DEL-11385 is a medium-voltage cast-resin current transformer (CT) engineered for reliable operation in 11kV distribution systems per IEC standards, with compatibility for domestic 10kV networks. Designed in strict compliance with IEC 61869-2 and GB/T 20840.2, this instrument transformer leverages vacuum pressure impregnation (VPI) epoxy resin technology to provide superior dielectric strength, environmental resilience, and long-term stability without the fire or leakage hazards associated with oil-filled alternatives.
Operating Principle and Core Construction
The DEL-11385 functions on the fundamental electromagnetic induction principle: the primary conductor—typically a busbar passing through the central aperture—carries system current (Ip), which magnetizes a toroidal core made of grain-oriented electrical steel (GOES). This core induces a proportional secondary current (Is) in the wound secondary coil, scaled by the turns ratio (e.g., 100:1, 200:5 A). The GOES material ensures low core loss (<0.8 W/kg at 1.5 T, 50 Hz) and high permeability, critical for maintaining accuracy under both steady-state and transient fault conditions. The entire magnetic assembly is fully encapsulated in flame-retardant, UV-stable epoxy resin via VPI, eliminating air voids and preventing partial discharge inception below 10 pC at rated voltage.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the DEL-11385 offers significant operational and safety benefits. Its solid insulation eliminates risks of oil leakage, fire propagation, and environmental contamination—making it ideal for indoor switchgear, urban substations, and environmentally sensitive zones. The epoxy housing provides excellent mechanical rigidity, resisting vibration and seismic loads up to 0.5g. Additionally, the absence of oil simplifies maintenance, reduces lifecycle costs, and enables vertical or horizontal mounting without orientation constraints. Thermal performance is enhanced through uniform heat dissipation across the resin body, supporting continuous operation at ambient temperatures from –40°C to +55°C without derating.
Typical Deployment Scenarios
This CT is commonly deployed in 11kV/10kV radial and ring-main distribution networks where space, safety, and reliability are paramount. Typical installations include metal-enclosed switchgear (e.g., RMU or AIS configurations), pad-mounted transformers, and renewable energy interconnection points (solar farms, wind substations). Its dual-winding secondary design supports simultaneous metering (Class 0.5 or 0.2S) and protection (Class 5P10 or 5P20) functions, enabling compact system architectures without compromising measurement integrity or relay coordination during faults.
Technical Specifications
The DEL-11385 is engineered to meet stringent performance criteria under defined service conditions. Below is a comprehensive specification table followed by environmental and operational parameters.
| Parameter | Value |
|---|---|
| System Voltage (IEC) | 11 kV |
| System Voltage (Domestic) | 10 kV |
| Rated Insulation Level (Um) | 12 kV |
| Power Frequency Withstand Voltage (1 min) | 28 kV rms |
| Lightning Impulse Withstand Voltage (1.2/50 µs) | 75 kV peak |
| Primary Current Ratings | 50 A to 3000 A (standard); up to 4000 A (custom) |
| Secondary Current | 1 A or 5 A |
| Accuracy Classes | Metering: 0.2S, 0.5; Protection: 5P10, 5P20 |
| Rated Burden (per winding) | 5 VA, 10 VA, 15 VA, 30 VA (at cos φ = 0.8 lag) |
| Short-Time Thermal Current (1 s) | 25 × Ipn (min. 12.5 kA) |
| Dynamic Withstand Current | 62.5 kA peak |
| Core Material | Grain-Oriented Electrical Steel (GOES), CRGO grade |
| Insulation System | VPI Epoxy Resin, UL 94 V-0 rated |
| Ambient Temperature Range | –40°C to +55°C |
| Maximum Altitude | 1000 m above sea level (derating required >1000 m) |
| Relative Humidity | Up to 95% non-condensing |
Standard Service Conditions
The DEL-11385 is rated for standard service conditions as defined in IEC 61869-2: ambient temperature between –40°C and +55°C, relative humidity ≤95% (non-condensing), and installation altitude not exceeding 1000 m. At altitudes above 1000 m, the power frequency withstand voltage must be reduced by 1% per 100 m increment. The transformer is suitable for both indoor and outdoor use due to its IP54-rated terminal box and UV-resistant resin formulation, which prevents surface tracking and chalking under prolonged solar exposure.
Electrical Performance Tolerances
Current error and phase displacement are tightly controlled per IEC 61869-2. For a Class 0.2S metering winding at 100% rated current, current error must not exceed ±0.2%, and phase displacement ≤±10 minutes. At 1% of rated current, the error tolerance widens to ±0.75%. Protection windings (5P20) guarantee that composite error remains ≤5% at 20 times rated current with rated burden connected. All windings undergo factory calibration with traceable standards, and test reports include actual measured errors at 5%, 20%, 100%, and 120% of Ipn.
Typical Applications
The DEL-11385 serves diverse roles across modern power infrastructure, combining metrological precision with robust fault-current handling.
Substation Secondary Metering
In 11kV/10kV distribution substations, the DEL-11385 provides accurate revenue-grade metering for utility billing and load profiling. Its Class 0.2S winding interfaces with digital energy meters compliant with IEC 62053-22, ensuring compliance with regulatory accuracy requirements even at light loads (down to 1% In). The CT’s low phase error minimizes reactive energy measurement drift, critical for power factor correction schemes. Installation within SF6 or air-insulated switchgear bays leverages its compact footprint (diameter: 180 mm; height: 220 mm) and standardized M12 secondary terminals.
Industrial Power Distribution
Heavy industrial facilities—such as cement plants, steel mills, and data centers—rely on the DEL-11385 for both process monitoring and protective relaying. The dual-winding configuration allows one output to feed a multifunction meter (e.g., for kW, kVAR, harmonics) while the other connects to overcurrent relays (e.g., IEC 60255-compliant devices). The 5P20 rating ensures relay operation remains reliable during high-magnitude faults (e.g., 20×400 A = 8 kA), with saturation avoided due to the high knee-point voltage (>200 V at 5 A burden).
Renewable Energy Integration
Solar photovoltaic (PV) and wind farm collector substations utilize the DEL-11385 to interface generation assets with grid operators. During islanding events or rapid irradiance changes, the CT maintains linearity under asymmetric or harmonic-rich currents (up to 15% THD). Its fast response time (<20 ms to 90% of final value) supports anti-islanding protection schemes. The epoxy resin’s resistance to salt fog and dust makes it suitable for coastal or desert installations without additional enclosures.
Rural and Suburban Distribution Networks
In remote or semi-urban grids with limited maintenance access, the DEL-11385’s maintenance-free design ensures decades of uninterrupted service. Mounted on pole-top transformers or underground vaults, it feeds SCADA RTUs for remote load monitoring. The 1 A secondary option reduces copper losses over long cable runs (>100 m), improving signal integrity. Compatibility with legacy electromechanical relays (via 5 A output) facilitates seamless upgrades in aging infrastructure.
Compliance with International Standards
The DEL-11385 is certified to global and regional standards, ensuring interoperability and regulatory acceptance.
IEC 61869-2 Certification Details
IEC 61869-2 governs the performance, testing, and marking of inductive current transformers. The DEL-11385 complies with all mandatory clauses, including insulation coordination (Section 5), accuracy verification (Section 6), and short-circuit withstand (Section 7). Type tests include temperature rise (≤60 K for windings), impulse voltage (75 kV), and partial discharge (<10 pC at 1.2 Um/√3). Routine tests—performed on every unit—include power frequency withstand (28 kV/1 min), turns ratio verification (±0.25% tolerance), and polarity check. Each unit bears an IEC-compliant nameplate with Um, Ipn, accuracy class, and burden.
Alignment with GB/T 20840.2
For Chinese markets, the DEL-11385 meets GB/T 20840.2 (equivalent to IEC 61869-2 but with localized requirements). Key differences include a slightly lower lightning impulse level (70 kV vs. 75 kV) and mandatory seismic testing (0.3g horizontal acceleration). The domestic 10kV system voltage is accommodated via identical physical dimensions and insulation levels, allowing single-stock management for multinational utilities. Factory test reports include both IEC and GB markings for customs clearance.
Testing and Certification Documentation
Certification is issued by accredited third-party laboratories (e.g., KEMA, CESI, or CEPREI). Documentation includes type test reports, routine test certificates, and material safety data sheets (MSDS) for the epoxy resin. All units undergo 100% production testing, with data logged for traceability (batch number, core ID, test date). Optional witnessed testing is available for critical projects.
On-Site Testing Procedures
Post-installation verification ensures the DEL-11385 performs as specified under field conditions.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-secondary, primary-to-ground, and secondary-to-ground. Acceptance criterion: ≥1000 MΩ at 20°C. Correct for temperature using RT = R20 × 2(20–T)/10. Low readings indicate moisture ingress or resin cracking—requiring drying or replacement.
Turns Ratio Test
Apply a low-voltage AC source (e.g., 120 V) to the secondary winding and measure induced primary voltage (open-circuit). Calculate ratio as Vs/Vp. Tolerance: ±0.25% of nominal ratio. Alternatively, use a dedicated CT analyzer injecting 1–5 A into primary and measuring secondary current. Deviations >0.5% suggest turn-to-turn shorts.
Polarity Verification
Confirm reducing polarity using the DC kick method: connect a 6–12 V battery to P1–P2 and observe secondary voltage spike on a DC voltmeter across S1–S2. A positive deflection at S1 indicates correct polarity. Incorrect polarity causes 180° phase shift, leading to metering errors or relay misoperation.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (50 Hz) for 1 minute between primary and grounded secondary/enclosure. Monitor for flashover or excessive leakage current (<1 mA). This test validates insulation integrity after transport and installation stresses. Perform only if factory test records are unavailable or after major maintenance.
Short-Circuit Test (Saturation Check)
Inject 120% of rated primary current into the CT with rated burden connected. Measure secondary current waveform on an oscilloscope. Absence of clipping or distortion confirms the core operates below saturation. For protection windings, repeat at 20×In to verify 5P20 performance. Knee-point voltage should exceed √2 × In × (Rb + Rct).
Preventive Maintenance Guide
Although cast-resin CTs require minimal upkeep, scheduled inspections 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 M4 screws) and corrosion on secondary connections. Verify secondary circuits are never left open during operation—a fused shorting link must be installed during meter replacement. Use thermal imaging to detect abnormal heating (>10 K above ambient) indicating internal faults.
Five-Year Dielectric and Accuracy Recalibration
Every 60 months, perform insulation resistance and turns ratio tests as baseline diagnostics. For revenue metering CTs, recalibrate accuracy per IEC 61869-2 using a portable CT tester. Record results in asset management software to trend degradation. If current error exceeds 150% of original values, investigate core aging or winding damage.
Maintenance Intervals and Fault Diagnosis
| Interval | Action | Fault Indicators |
|---|---|---|
| Annually | Visual inspection, terminal torque check | Cracks, arcing marks, loose lugs |
| 5 Years | Insulation resistance, ratio test, recalibration | Rins < 500 MΩ, ratio error >0.5% |
| After Fault | Full suite of on-site tests | Relay misoperation, meter anomalies |
Common failure modes include secondary open-circuit (causing dangerous overvoltages) and moisture-induced partial discharge. Immediate replacement is required if PD exceeds 50 pC during energized testing.
Conclusion
The DEL-11385 11kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, merging IEC 61869-2 compliance with practical field reliability. Its GOES core ensures metrological fidelity from 1% to 120% of rated current, while the VPI epoxy resin housing delivers unmatched environmental and fire safety—eliminating the operational liabilities of oil-filled alternatives. Engineered for dual metering and protection roles, it supports modern grid demands including renewable integration, smart metering, and automated fault response. With a design life of 25–30 years under standard service conditions, the DEL-11385 minimizes total cost of ownership through zero-maintenance operation, robust short-circuit withstand (25×In thermal, 62.5 kA dynamic), and global standard alignment (IEC and GB). Utilities and industrial operators benefit from consistent performance across temperature extremes, high humidity, and polluted atmospheres, backed by rigorous factory and field-test protocols. As distribution networks evolve toward higher reliability and digitalization, the DEL-11385 provides a future-proof sensing foundation that meets today’s accuracy, safety, and longevity requirements without compromise.