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Introduction to the DZC-3 Current Transformer
The DZC-3 is a 11kV class, indoor-rated, cast-resin insulated current transformer (CT) engineered specifically for high-accuracy metering and robust protection functions in medium-voltage (MV) power substations. Unlike conventional oil-immersed or dry-type air-insulated CTs, the DZC-3 leverages advanced epoxy resin casting technology to deliver superior dielectric integrity, mechanical resilience, and long-term operational stability under demanding electrical and environmental conditions.
Cast-Resin Insulation Technology Principles
Cast-resin insulation in the DZC-3 is achieved through vacuum pressure impregnation (VPI) and high-temperature curing of cycloaliphatic epoxy resins around the primary conductor, secondary windings, and magnetic core assembly. This process eliminates voids and moisture entrapment—critical failure points in traditional insulation systems. The resulting monolithic structure provides homogeneous electric field distribution, high partial discharge inception voltage (PDIV > 20 kV), and excellent resistance to tracking and erosion under polluted or humid conditions.
Key Advantages over Oil-Immersed Designs
- Fire Safety: Eliminates flammable insulating oil, complying with IEC 61482 arc flash safety requirements and reducing fire load in indoor switchgear rooms.
- Maintenance-Free Operation: No oil sampling, degassing, or leakage monitoring required over its 30+ year design life.
- Environmental Resilience: Immune to oil oxidation, moisture ingress, and temperature-induced expansion/contraction cycles.
- Compact Footprint: Higher dielectric strength of epoxy resin allows reduced creepage and clearance distances versus oil-filled units.
DZC-3 Specific Innovations
The DZC-3 incorporates grain-oriented electrical steel (GOES) cores with low hysteresis loss (< 0.8 W/kg at 1.5 T, 50 Hz) and precisely wound secondary coils optimized for minimal leakage inductance. Its dual-core configuration—one dedicated to 0.2S/0.5S class metering, the other to 5P10/5P20 protection—ensures independent performance optimization without cross-interference. Additionally, integrated thermal monitoring terminals and shielded secondary terminals mitigate electromagnetic interference (EMI) in digital relay environments.
Technical Specifications and Design Parameters
The DZC-3 is designed to operate reliably within the IEC-defined 11kV system voltage class (Um = 12 kV). Its electrical and mechanical parameters are rigorously aligned with both international and Chinese national standards.
Rated Voltage, Current, and Frequency
| Parameter | Value |
|---|---|
| System Voltage (Urms) | 11 kV |
| Highest System Voltage (Um) | 12 kV |
| Rated Primary Current (Ip) | 50–4000 A (standard steps) |
| Rated Secondary Current (Is) | 1 A or 5 A |
| Rated Frequency | 50 Hz / 60 Hz |
Insulation Levels
The DZC-3 meets the following insulation coordination levels per IEC 60071-1:
- Power Frequency Withstand Voltage (1 min): 28 kV rms
- Lightning Impulse Withstand Voltage (BIL): 75 kV peak
- Creepage Distance: ≥ 25 mm/kV (suitable for pollution degree III per IEC 60815)
Accuracy Classes
Dual-core design enables simultaneous compliance with distinct accuracy requirements:
- Metering Core: Class 0.2S (for revenue metering) or 0.5S (for general metering), per IEC 61869-2 Table 101.
- Protection Core: Class 5P10 or 5P20, ensuring ≤5% composite error at 10× or 20× rated current during fault conditions.
Thermal and Dynamic Performance
The DZC-3 is rated for continuous thermal current (Ith) of 1.2 × Ip and short-time thermal withstand current (Ith) of 25 kA for 1 second (or 40 kA for 0.5 s, depending on primary rating). Its dynamic withstand current (Idyn) exceeds 63 kA peak, satisfying mechanical stress requirements during asymmetrical faults per IEC 61869-2 Clause 6.5.
IEC 61869 Compliance and Standards
Compliance with IEC 61869-2:2012 (“Instrument transformers – Part 2: Additional requirements for current transformers”) forms the foundation of the DZC-3’s design validation framework.
IEC 61869-2 Specific Requirements
Key mandates addressed include:
- Definition of rated insulation level (Um = 12 kV)
- Accuracy class verification under defined burden (e.g., 15 VA for 0.2S)
- Temperature rise limits: ≤ 60 K for windings (measured by resistance method)
- Marking requirements: primary/secondary polarity, accuracy class, burden, and standard reference
Testing and Verification Procedures
All DZC-3 units undergo type tests including:
- Temperature rise test (per IEC 61869-2 §7.3)
- Short-circuit withstand test (§7.5)
- Partial discharge measurement (≤ 10 pC at 1.2 Um/√3)
- Accuracy verification across 1–120% of rated current
Routine production tests include power frequency withstand, winding resistance, polarity check, and ratio verification.
Comparison with GB/T 20840 Standards
GB/T 20840.2-2014 aligns closely with IEC 61869-2 but includes region-specific provisions:
- Stricter requirement on secondary terminal short-circuit duration (≥3 s)
- Mandatory seismic qualification for earthquake-prone regions (Class II per GB/T 13540)
- Enhanced marking in Chinese characters for domestic deployment
The DZC-3 is dual-certified to both standards, facilitating global deployment.
International Certification Requirements
Beyond IEC and GB, the DZC-3 meets:
- KEMA-KEUR (Netherlands)
- UL 61010-1 (North America safety)
- CISPR 11 Class B (EMC for industrial environments)
Third-party certification from TÜV or SGS is available upon request.
Installation Guidelines and Best Practices
Proper installation is critical to ensure long-term reliability and measurement integrity.
Site Preparation and Environmental Requirements
The DZC-3 is rated for indoor use only, with ambient temperature range of –25°C to +40°C and relative humidity ≤95% (non-condensing). Installation must occur in clean, dust-free environments free from corrosive gases (SO₂, H₂S) exceeding IEC 60721-3-3 Class 3C2 limits.
Mounting Procedures
The unit features standardized M12 threaded inserts on the base flange, compatible with IEC 61936-1 mounting dimensions. Torque specification: 25 N·m ±10%. Vertical orientation is mandatory; horizontal mounting induces mechanical stress on the resin body and alters magnetic flux paths, degrading accuracy.
Electrical Connections and Grounding
- Primary conductor must be centered within the aperture to minimize magnetic asymmetry.
- Secondary terminals must be connected using shielded twisted-pair cables (min. 2.5 mm² Cu), with shield grounded at one end only (typically at the relay panel).
- Frame grounding lug must be bonded to the substation earthing grid with ≤0.1 Ω resistance.
Safety Precautions During Installation
Never energize the CT with open-circuited secondary windings—this induces dangerous overvoltages (>10 kV) due to core saturation. Always short-circuit secondaries with approved test blocks before disconnecting loads. Use insulated tools and follow LOTO (Lockout-Tagout) procedures per IEC 61984.
Operation and Performance Characteristics
Load Behavior and Burden Considerations
The DZC-3’s performance is burden-dependent. For metering cores, total connected burden (including lead resistance) must not exceed the rated value (e.g., 10 VA). Exceeding burden increases ratio and phase errors beyond class limits. Protection cores tolerate higher burdens but require verification that knee-point voltage (Vk) exceeds relay setting voltage under maximum fault current.
Transient Response Characteristics
During DC-offset fault currents, the protection core’s remanence factor (Kr) is < 0.1 due to GOES core geometry and controlled air gaps. This ensures rapid core reset and prevents saturation during successive faults—critical for distance and differential relays.
Temperature Rise and Thermal Management
Under 1.2× rated current, measured temperature rise is typically 50–55 K, well below the 60 K limit. Epoxy resin’s thermal conductivity (~0.8 W/m·K) facilitates heat dissipation to ambient air via convection. Forced cooling is unnecessary.
Partial Discharge Performance
Factory-measured PD levels are consistently <5 pC at 1.2 Um/√3 (6.9 kV), indicating absence of internal voids or delamination. Field PD testing using IEC 60270 methods is recommended during commissioning to establish baseline data.
Testing Procedures and Quality Assurance
Factory Acceptance Testing (FAT)
FAT includes:
- Ratio and polarity verification (±0.1% tolerance)
- Insulation resistance (>1000 MΩ at 2500 V DC)
- Power frequency withstand (28 kV, 1 min)
- Partial discharge mapping
Test reports are provided per unit with traceable calibration certificates.
Site Commissioning Tests
Post-installation tests should include:
- Secondary loop resistance measurement
- Insulation resistance between windings and earth
- Excitation curve test (to verify Vk and detect core damage)
- Ratio check under low-current injection (e.g., 1 A primary)
Use relay test sets capable of 0.1% accuracy for metering validation.
Routine and Type Tests per IEC 61869-2
Type tests (performed once per design) validate:
- Short-circuit performance
- Temperature rise
- Accuracy over extended current/frequency ranges
Routine tests (100% production) cover dielectric, ratio, and polarity checks.
Diagnostic Testing Methods
For aging assessment:
- Tan δ measurement (not applicable—resin is non-polar)
- Time-domain reflectometry (TDR) for internal crack detection
- Infrared thermography under load to identify hot spots
No routine oil or gas analysis is needed.
Maintenance and Troubleshooting
Preventive Maintenance Schedules
The DZC-3 is maintenance-free under normal conditions. However, visual inspections every 3 years are recommended to check for:
- Surface tracking or UV degradation (indoor units rarely affected)
- Loose terminal connections
- Corrosion on grounding lugs
No scheduled cleaning or re-torquing is required.
Common Fault Diagnosis
- High ratio error: Check for excessive burden or damaged secondary winding.
- Overheating: Verify primary conductor alignment and absence of eddy currents in nearby steel structures.
- Intermittent signal: Inspect terminal blocks for oxidation or loose screws.
Insulation Resistance Testing
Perform annually using a 2500 V DC megohmmeter:
- Primary-to-secondary: >1000 MΩ
- Windings-to-frame: >1000 MΩ
Values below 100 MΩ indicate moisture ingress or surface contamination.
When to Replace vs Repair
Cast-resin CTs are not field-repairable. Replace if:
- Cracks or fractures are visible in the resin body
- Insulation resistance drops irreversibly below 50 MΩ
- Accuracy cannot be restored after burden correction
Core replacement is uneconomical due to monolithic construction.
Application Scenarios and System Integration
Substation Metering Applications
The 0.2S-class core interfaces directly with revenue-grade kWh meters (e.g., IEC 62053-22 compliant). Its low phase error (< ±10 minutes) ensures accurate reactive energy measurement for tariff billing.
Protection Relay Coordination
The 5P20 core supplies inputs to overcurrent (50/51), earth-fault (50N/51N), and differential (87) relays. Its high Vk (>200 V) prevents saturation during external faults, maintaining security of differential schemes.
Integration with SCADA Systems
When paired with merging units (IEC 61850-9-2 LE), analog outputs can be digitized for sampled-value transmission. Ensure anti-aliasing filters are applied if sampling rate < 4 kHz.
Case Studies and Field Experience
In a 110/11kV urban substation in Guangdong, 24 DZC-3 units have operated continuously since 2019 with zero failures. Accuracy drift over 5 years was <0.05%, verified by portable calibrators. Another deployment in a mining facility (high harmonic content) showed no degradation due to the core’s low-loss characteristics.
FAQ1: Can the DZC-3 be used outdoors?
No. The DZC-3 is designed exclusively for indoor, sheltered environments. Outdoor applications require IP54-rated or porcelain-housed variants with UV-stabilized resin.
FAQ2: What is the maximum allowable secondary burden for the 0.2S core?
Typically 10 VA or 15 VA, as marked on the nameplate. Exceeding this compromises accuracy class compliance. Always include lead resistance in burden calculation: R_lead = 2 × ρ × L / A (ρ = 0.0178 Ω·mm²/m for Cu).
FAQ3: How does the DZC-3 handle harmonics?
The GOES core exhibits linear B-H characteristics up to 3 kHz, minimizing harmonic distortion. However, accuracy classes apply only at fundamental frequency (50/60 Hz); harmonic measurements require specialized wideband CTs.
FAQ4: Is the DZC-3 suitable for arc flash mitigation systems?
Yes. Its fast transient response (rise time < 100 µs) and high di/dt tolerance make it compatible with arc flash detection relays requiring sub-cycle fault current data.
FAQ5: Can multiple secondary loads be connected to one core?
Only if the sum of all burdens (including wiring) remains within the rated VA. Parallel connection of metering and protection devices on the same core is prohibited—it violates accuracy and security requirements.
FAQ6: What is the expected service life?
Design life exceeds 30 years under IEC 60060-1 environmental conditions. Accelerated aging tests (150°C, 1000 h) show <5% change in key parameters, confirming long-term stability of the epoxy matrix.