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21kV Cast-Resin Current Transformer UNKNOWN for Metering and Protection – IEC 61869-2 Standard
Introduction to the UNKNOWN Current Transformer
The UNKNOWN current transformer (CT) is a high-reliability, cast-resin insulated instrument transformer designed for operation in 21kV (IEC) / 20kV (domestic) medium-voltage networks. Engineered in strict compliance with IEC 61869-2 and GB/T 20840.2, this device serves dual roles in accurate energy metering and robust protective relaying across utility and industrial power systems.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the UNKNOWN CT employs vacuum pressure impregnation (VPI) of cycloaliphatic epoxy resin around the primary conductor and secondary windings. This process eliminates air voids, suppresses partial discharges (<5 pC at 1.2 × Ur), and provides superior dielectric strength compared to traditional oil-filled or dry-type designs. The resin matrix bonds directly to the GOES (grain-oriented electrical steel) core laminations, enhancing mechanical stability and thermal conductivity. Under continuous service at 21kV system voltage, the insulation withstands lightning impulse voltages up to 125 kV peak and power frequency test voltages of 50 kV rms for 1 minute, per IEC 61869-2 Clause 7.3.
Advantages Over Oil-Immersed Designs
Unlike oil-immersed CTs, the UNKNOWN cast-resin unit requires no maintenance-intensive oil sampling, poses zero fire hazard, and eliminates risks of oil leakage or environmental contamination. Its solid insulation enables compact dimensions—typically 30% smaller footprint than equivalent oil-filled units—making it ideal for space-constrained indoor switchgear or urban substations. Additionally, the absence of liquid dielectric ensures consistent performance across ambient temperatures from –40°C to +40°C without viscosity-related response delays during fault transients. The design also achieves higher short-time thermal withstand (e.g., 40 kA for 1 s) due to direct heat dissipation through the resin matrix.
Typical Applications Overview
The UNKNOWN CT is deployed in critical infrastructure where reliability, accuracy, and transient fidelity are paramount. Primary use cases include feeder protection in 21kV distribution substations, generator differential schemes in combined-cycle plants, and revenue metering at grid interconnection points. Its dual-winding configuration often features one 0.5 class winding for metering and a separate 5P20 or TPY winding for protection relays. The transformer’s immunity to moisture ingress (IP54 rating) allows deployment in coastal or high-humidity environments without derating, supporting both indoor metal-enclosed switchgear and outdoor pole-mounted installations on 20kV rural feeders.
Technical Specifications
The UNKNOWN 21kV cast-resin current transformer adheres to precise electrical and mechanical parameters defined by international standards. Key specifications ensure interoperability with modern digital relays and metering systems while maintaining long-term operational integrity under fault conditions.
Rated Electrical Parameters
Rated primary voltage: 21 kV (IEC) / 20 kV (GB). Rated frequency: 50 Hz (±0.5 Hz). Standard current ratios include 100/1 A, 200/5 A, 400/1 A, and 600/5 A, with custom ratios available upon request. Accuracy classes comply with IEC 61869-2: metering windings at 0.2S, 0.5, or 1.0; protection windings at 5P, 10P, or transient classes TPX, TPY, TPZ. Rated burden for protection windings is typically 15–30 VA at 5 A or 1–5 VA at 1 A. Accuracy Limit Factor (ALF) ranges from 10 to 30 (e.g., 5P20 = ±1% composite error at 20× rated current). Short-time thermal current rating: 25–40 kA for 1 s; dynamic withstand: 63–100 kA peak.
Insulation and Environmental Ratings
Insulation level per IEC 60071-1: Ur = 24 kV, Um = 25.2 kV, LI = 125 kV, ACW = 50 kV (1 min). Partial discharge inception voltage exceeds 1.2 × Ur with magnitude <5 pC. Creepage distance: ≥25 mm/kV for pollution severity class III (industrial/coastal). Ambient temperature range: –40°C to +40°C; relative humidity: ≤95% non-condensing. Altitude derating applies above 1000 m: reduce rated voltage by 1% per 100 m above 1000 m. The housing is UV-stabilized cycloaliphatic epoxy with flame retardancy per IEC 60695-11-10 (V-0 rating).
Mechanical and Terminal Configuration
Primary terminals are M12 or M16 tinned copper studs rated for 630 A continuous. Secondary terminals are screw-type (M4) with IP2X finger-safe covers. Mounting options include flange (ISO 12944-compliant hot-dip galvanized steel) or bracket for DIN rail or bus support. Overall dimensions: height 380–450 mm, diameter 180–220 mm, weight 18–25 kg depending on ratio and ALF. Polarity follows IEC 61869-2 reducing polarity convention (P1 toward source, S1 as marked terminal).
Typical Applications
The UNKNOWN 21kV cast-resin CT is engineered for diverse operational scenarios demanding precision, durability, and transient fidelity. Its design meets the stringent requirements of modern power systems across multiple sectors.
Substation Secondary Metering
In 21kV/20kV distribution substations, the UNKNOWN CT’s 0.2S or 0.5 class winding provides revenue-grade accuracy for billing and load profiling. With phase displacement error <±10 minutes and ratio error within ±0.2% at 100% In, it complies with EN 50470-1 for smart metering integration. The low burden requirement (≤5 VA at 1 A) minimizes voltage drop in long secondary circuits, ensuring compatibility with AMI (Advanced Metering Infrastructure) systems. Installation on ring-main units (RMUs) or gas-insulated switchgear (GIS) benefits from the CT’s compact form factor and resistance to SF6 byproducts.
Industrial Power Distribution
Within heavy-industry facilities—such as steel mills, petrochemical plants, and data centers—the UNKNOWN CT supports motor protection, bus differential schemes, and arc-flash mitigation. Its 5P20 or TPY protection winding accurately reproduces fault currents up to 20× rated value with composite error ≤5%, enabling reliable operation of overcurrent relays (e.g., IEC 60255-151 compliant). The high thermal withstand (40 kA/1s) accommodates large motor inrush or transformer energization transients without saturation. Epoxy resin encapsulation resists chemical vapors and dust, meeting IEC 60529 IP54 for harsh indoor environments.
Renewable Energy Integration
For solar PV farms and wind turbine step-up transformers connected to 20kV/21kV collector grids, the UNKNOWN CT enables precise anti-islanding detection and fault ride-through compliance. Transient-class TPY windings (with remanence factor Kr ≤10%) prevent core saturation during DC-offset faults common in inverter-based resources. The CT’s fast time-to-saturation (>20 ms at 20× In) ensures correct operation of distance relays during asymmetrical faults. Dual-ratio configurations allow adaptive protection settings based on generation output, supporting grid code requirements such as VDE-AR-N 4110 or GB/T 19964.
Rural and Suburban Distribution Networks
Deployed on overhead lines or pad-mounted transformers in rural electrification projects, the UNKNOWN CT delivers decades of maintenance-free service. Its hydrophobic resin surface sheds rain and resists tracking under salt fog or agricultural pollutants. The 10P15 protection class provides cost-effective overcurrent backup for reclosers and sectionalizers, with composite error ≤10% at 15× rated current. Lightweight construction simplifies pole-top installation, while UV-resistant housing prevents embrittlement under prolonged solar exposure. Compatibility with legacy electromechanical relays ensures seamless retrofitting in aging 20kV networks.
Compliance with International Standards
The UNKNOWN current transformer is certified to global and regional standards, ensuring interoperability, safety, and performance consistency across markets. Compliance verification includes type tests, routine tests, and special tests as mandated by governing bodies.
IEC 61869-2 Certification Details
Per IEC 61869-2:2012 (Instrument transformers – Part 2: Additional requirements for current transformers), the UNKNOWN CT undergoes rigorous validation of accuracy, thermal behavior, and dielectric performance. Type tests include temperature rise (ΔT ≤60 K for windings), short-circuit withstand (mechanical and thermal), and accuracy verification across 1–120% of rated current. Transient performance for TP classes is validated via oscillographic testing per Annex C, measuring time-to-saturation and remanence decay. All production units receive routine tests: turns ratio (±0.25% tolerance), polarity, and power frequency withstand (50 kV rms, 1 min). Certificates are issued by accredited laboratories (e.g., KEMA, CESI).
Alignment with GB/T 20840.2
For domestic Chinese markets, the UNKNOWN CT meets GB/T 20840.2-2014, which largely harmonizes with IEC 61869-2 but includes localized requirements. Key differences include mandatory seismic testing (horizontal acceleration 0.3g per GB/T 13540), stricter partial discharge limits (<3 pC at 1.2Ur), and creepage distance ≥31 mm/kV for Class IV pollution. The 20kV system voltage designation aligns with GB 156, though insulation coordination remains based on 21kV Um. Domestic certification requires CCC marking and submission to China National Institute of Standardization (CNIS).
Testing and Certification Requirements
Certification involves three test tiers: type tests (performed once per design), sample tests (on randomly selected units per batch), and routine tests (100% production). Critical type tests include lightning impulse (125 kV, 1.2/50 μs waveform), temperature rise (IEC 60060-1), and accuracy under burden variation. For protection classes, composite error is measured using a calibrated shunt and oscilloscope per IEC 61869-2 Clause 6.4.2. Third-party certification bodies issue test reports referencing specific serial number batches, enabling traceability for utility procurement audits.
On-Site Testing Procedures
Post-installation verification ensures the UNKNOWN CT performs within specification under actual operating conditions. Field tests validate integrity after transport and correct wiring before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-secondary, primary-ground, and secondary-ground. Acceptance criteria: ≥1000 MΩ at 20°C. Correct for temperature using R₂₀ = Rₜ × 1.5(t–20)/10. Low readings indicate moisture ingress or resin cracking. Re-test after cleaning terminals with isopropyl alcohol if contamination is suspected.
Turns Ratio Test
Apply 1–5 V AC at 50 Hz to the secondary winding and measure induced primary voltage (open-circuit method). Calculate ratio as Vp/Vs. Tolerance: ±0.25% of nominal ratio. Alternatively, use a dedicated CT analyzer injecting 1–10 A primary current and comparing secondary output. Deviations >0.5% warrant core inspection for shorted turns.
Polarity Verification
Connect a 1.5 V DC battery between P1 and P2. Momentarily close the circuit while monitoring a center-zero galvanometer across S1–S2. A positive kick confirms reducing polarity (IEC standard). Incorrect polarity causes relay misoperation in differential schemes. Repeat three times to eliminate contact bounce artifacts.
Power Frequency Withstand Voltage Test
Apply 28 kV rms (80% of factory test voltage) between primary-ground and secondary-ground for 1 minute using a portable test set. Monitor for flashover, excessive leakage current (>1 mA), or audible discharge. Do not perform if ambient humidity exceeds 80% or if surface condensation is present. This test validates insulation integrity after handling damage.
Short-Circuit Test for Core Saturation
Inject 10–20× rated current into the primary (using a variable transformer and current booster) while monitoring secondary voltage across a known burden. Plot excitation curve: knee-point voltage should exceed 1.5× rated secondary voltage at ALF. For 5P20 CTs, secondary voltage at 20× In must remain linear within ±5% composite error. Saturation below knee-point indicates core damage or incorrect ALF selection.
Preventive Maintenance Guide
Although cast-resin CTs require minimal maintenance, periodic inspections extend service life and prevent unexpected failures in critical protection circuits.
Annual Visual and Electrical Inspection
Inspect housing for cracks, UV degradation, or tracking marks. Clean with mild detergent; avoid solvents that attack epoxy. Check terminal tightness (torque: 12 Nm for M12). Measure insulation resistance annually; a 50% drop from baseline indicates internal deterioration. Verify secondary circuit continuity and grounding integrity (resistance <0.1 Ω). Document all readings for trend analysis.
Five-Year Comprehensive Maintenance
Every 60 months, perform turns ratio and polarity re-verification. Conduct partial discharge measurement using ultra-high-frequency (UHF) sensors if available; levels >10 pC warrant replacement. Inspect mounting hardware for corrosion, especially in coastal zones. Review relay event reports for unexplained trips that may correlate with CT saturation. Replace units exhibiting ratio drift >0.5% or insulation resistance <500 MΩ.
Maintenance Intervals and Fault Diagnosis
| Interval | Task | Acceptance Criteria |
|---|---|---|
| Annually | Visual inspection, IR test | No cracks; IR ≥1000 MΩ |
| 5 Years | Ratio/polarity test, PD check | Ratio error ≤±0.25%; PD <5 pC |
| After Fault | Full electrical test suite | All parameters within spec |
Common faults include open secondary circuits (causing dangerous overvoltages), core lamination shorts (evidenced by overheating), and terminal corrosion (increasing contact resistance). Never operate with secondary open—always short before disconnecting.
Conclusion
The UNKNOWN 21kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining the dielectric robustness of VPI epoxy resin with the magnetic precision of GOES silicon steel cores. Its dual compliance with IEC 61869-2 and GB/T 20840.2 ensures global applicability, while its transient-performance capabilities (5P, 10P, TPY) meet the evolving demands of digital substations and renewable integration. The elimination of flammable liquids enhances safety in urban and indoor installations, and the hermetic resin seal guarantees stable operation across extreme climates—from arid deserts to humid tropics—without performance degradation. With a design life exceeding 30 years under normal loading conditions, the UNKNOWN CT delivers exceptional lifecycle value through zero-maintenance operation, high short-circuit resilience, and metrological accuracy that supports both regulatory compliance and operational efficiency. Utilities and industrial operators can confidently deploy this transformer in mission-critical protection and metering roles, knowing it has undergone rigorous type testing and adheres to the latest international engineering standards. Its compact form factor further facilitates modernization of legacy 20kV infrastructure, making it an optimal choice for grid decarbonization and smart distribution initiatives worldwide.
Frequently Asked Questions (FAQ)
Q1: Can the UNKNOWN CT be used on a 20kV domestic system even though it’s rated 21kV?
Yes. The 21kV rating refers to the IEC maximum system voltage (Um = 25.2 kV), which corresponds to the 20kV nominal voltage used in many national grids (e.g., China, India, Brazil). Per IEC 60038, 21kV is the standardized Um for 20kV systems. The UNKNOWN CT’s insulation coordination (LI 125 kV, ACW 50 kV) fully covers the overvoltage stresses expected on 20kV networks, including switching surges and lightning impulses. No derating is required when installing on 20kV switchgear.
Q2: What is the difference between 5P20 and TPY accuracy classes?
5P20 is a steady-state protection class defined by composite error ≤5% at 20× rated current under sinusoidal conditions. It does not address transient DC offset. TPY, per IEC 61869-2 Annex C, is a transient class specifying time-to-saturation (>20 ms at 20× In with X/R=15), remanence factor (Kr ≤10%), and secondary time constant (Ts ≈ L/R). TPY is essential for differential protection of generators/transformers where DC offset can cause conventional 5P CTs to saturate prematurely. The UNKNOWN CT offers both options depending on application needs.
Q3: How do I select the correct ALF for overcurrent protection?
ALF must exceed the maximum fault current divided by rated primary current. For example, if max fault = 12 kA and CT ratio = 600/1 A, required ALF = 12,000 / 600 = 20. Select 5P20 or higher. Also verify that the relay burden plus lead resistance does not exceed the CT’s rated burden at ALF. Use the formula: Zb ≤ (Vk / (ALF × In)) – Rct, where Vk is knee-point voltage and Rct is secondary winding resistance. The UNKNOWN CT datasheet provides Vk and Rct for each ratio.
Q4: Is partial discharge testing required in the field?
Not routinely. Factory PD tests ensure <5 pC at 1.2Ur. Field PD measurement is only recommended if insulation resistance drops significantly or after severe overvoltage events. Portable UHF or HFCT sensors can detect discharges >10 pC, indicating internal voids or delamination. However, visual inspection and IR testing are sufficient for most preventive programs. If PD is confirmed, replace the unit—resin insulation cannot be repaired.
Q5: Can I parallel two UNKNOWN CT secondaries for higher burden capacity?
No. Paralleling CT secondaries is prohibited unless specifically designed for it (e.g., summation CTs). Mismatched excitation characteristics cause circulating currents, ratio errors, and potential saturation. Instead, select a single CT with adequate rated burden (e.g., 30 VA instead of 15 VA) or use auxiliary CTs. The UNKNOWN CT’s standard burdens are 5, 10, 15, or 30 VA—choose based on relay input impedance and lead length.
Q6: What is the maximum allowable secondary lead length?
Lead length depends on burden and conductor size. For a 5 A system with 2.5 mm² Cu wire (R = 7.4 mΩ/m), total loop resistance must be ≤ (rated burden / I²) – Rct. Example: 15 VA burden, Rct = 0.2 Ω → max Rlead = (15/25) – 0.2 = 0.4 Ω → max length = 0.4 / (2 × 0.0074) ≈ 27 m. For 1 A systems, lengths can exceed 100 m. Always calculate based on actual relay burden and CT Rct.
Q7: Does the UNKNOWN CT require drying after storage in humid conditions?
No. The cast-resin encapsulation is hermetically sealed, preventing moisture absorption. Unlike oil-paper insulation, epoxy resin does not require drying ovens or vacuum treatment. However, wipe terminals dry before testing to avoid surface leakage errors. Long-term storage should be in covered, dry areas, but brief exposure to rain does not compromise internal integrity.
Q8: How does temperature affect accuracy?
GOES core permeability varies slightly with temperature, but IEC 61869-2 requires accuracy compliance from –5°C to +40°C. Outside this range, ratio error may increase by ≤0.1% per 10°C deviation. For critical metering in extreme climates (e.g., –30°C Siberia), specify extended temperature testing. The UNKNOWN CT’s resin has a glass transition temperature (Tg) >120°C, ensuring mechanical stability well beyond operational limits.