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KZB-0 33kV Cast-Resin Current Transformer for Substation Metering and Protection – IEC 61869-2 Certified
Introduction to the KZB-0 Current Transformer
The KZB-0 is a high-reliability, cast-resin insulated current transformer (CT) engineered for medium-voltage power systems operating at a nominal system voltage of 35 kV, with an IEC-standard rated voltage of 33 kV. Designed in strict compliance with IEC 61869-2 and GB/T 20840.2, this instrument transformer delivers precise current replication from primary to secondary windings under both steady-state and transient fault conditions. Its robust construction and advanced insulation system make it suitable for demanding indoor and outdoor substation environments.
Operating Principle of Cast-Resin Insulation
The KZB-0 employs vacuum pressure impregnation (VPI) epoxy resin technology to encapsulate its magnetic core and windings in a homogeneous, void-free solid dielectric matrix. This process eliminates air pockets that could lead to partial discharges under high electric stress. The resin formulation exhibits excellent tracking resistance (CTI > 600 V), thermal stability up to 130°C, and mechanical rigidity that withstands seismic loads up to 0.5g. Unlike oil-filled alternatives, the cast-resin design ensures no risk of leakage, fire hazard, or environmental contamination. The insulation system provides a uniform electric field distribution around the primary conductor, minimizing local stress concentrations and extending service life beyond 30 years under standard operating conditions.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed CTs, the KZB-0 offers significant operational and safety benefits. Its maintenance-free nature eliminates the need for periodic oil sampling, dielectric testing, or gasket replacement. The absence of flammable oil reduces fire load in substations, aligning with modern safety codes such as NFPA 70E. Additionally, the compact footprint—typically 30% smaller than equivalent oil units—enables space-efficient switchgear integration. The cast-resin body demonstrates superior resistance to humidity ingress (IP54 rating when mounted vertically), making it ideal for coastal or high-humidity regions where condensation compromises oil-paper insulation over time. Thermal performance is enhanced by the resin’s high thermal conductivity (~0.2 W/m·K), which facilitates efficient heat dissipation during overload conditions.
Typical Applications Overview
The KZB-0 is primarily deployed in utility substations, industrial power distribution networks, and renewable energy interconnection points requiring accurate current measurement for metering and protective relaying. It supports dual secondary windings—one optimized for 0.5 class metering accuracy and another for 5P20 protection class—enabling simultaneous revenue-grade energy recording and fast fault detection. Common installations include ring main units (RMUs), gas-insulated switchgear (GIS) interfaces, and outdoor pole-mounted configurations in rural distribution grids. Its standardized mounting dimensions (per IEC 61869-2 Annex B) ensure compatibility with legacy and modern switchgear frames without adapter hardware.
Technical Specifications
The KZB-0 current transformer is engineered to meet stringent electrical and environmental performance criteria. Below is a comprehensive specification table followed by detailed subsections on service conditions and output characteristics.
| Parameter | Value |
|---|---|
| Rated System Voltage (IEC) | 33 kV |
| Domestic System Voltage | 35 kV |
| Primary Current Rating | 50–3000 A (standard steps) |
| Secondary Current | 1 A or 5 A (user-selectable) |
| Accuracy Classes | Metering: 0.2S, 0.5; Protection: 5P10, 5P20 |
| Rated Output (Burden) | 5–30 VA per winding (at rated current) |
| Insulation Level (Um/√3) | 36/√3 kV = 20.8 kV RMS |
| Power Frequency Withstand Voltage | 70 kV RMS, 1 min |
| Lightning Impulse Withstand Voltage | 170 kV peak (1.2/50 μs) |
| Short-Time Thermal Current | 25 kA for 1 s (Ith) |
| Dynamic Withstand Current | 62.5 kA peak (Idyn) |
| Ambient Temperature Range | −40°C to +40°C |
| Altitude Limit | ≤ 1000 m above sea level |
| Core Material | Grain-Oriented Electrical Steel (GOES), M4 grade |
| Insulation System | VPI Epoxy Resin, UL 94 V-0 rated |
Standard Service Conditions
The KZB-0 is rated for continuous operation under IEC 60060-1 standard atmospheric conditions: ambient temperature between −40°C and +40°C, relative humidity up to 95% non-condensing, and installation altitude not exceeding 1000 meters. At altitudes above 1000 m, derating factors apply per IEC 60071-2: for every 100 m increase, the power frequency withstand voltage must be reduced by 1%. The transformer is designed for vertical or horizontal mounting with primary conductor orientation parallel to ground. In coastal areas with salt fog density exceeding 0.1 mg/cm²/day, supplementary silicone grease coating on external surfaces is recommended to prevent surface tracking.
Accuracy and Burden Characteristics
Each KZB-0 unit undergoes factory calibration to ensure composite error remains within ±0.5% for 0.5-class metering windings at 5–120% of rated primary current. Protection windings comply with 5P20 specification, meaning the composite error does not exceed 5% at 20 times rated current with specified burden. The maximum permissible burden is 30 VA for 5 A secondary circuits and 15 VA for 1 A circuits to maintain accuracy. Exceeding these burdens increases phase displacement error beyond ±10 minutes, potentially compromising directional relay coordination. All windings are tested at 120% of rated current for 1 hour to verify thermal stability without exceeding 60 K temperature rise over ambient.
Typical Applications
The KZB-0 current transformer serves critical roles across diverse power infrastructure segments, combining metrological precision with rugged reliability.
Substation Secondary Metering
In transmission and distribution substations, the KZB-0 provides revenue-grade current signals to kWh meters and SCADA systems. Its 0.2S-class secondary winding ensures billing accuracy even at low load levels (down to 1% of rated current), meeting regulatory requirements in markets like China (DL/T 448) and Europe (MID Annex MI-003). For example, in a 35 kV urban substation feeding commercial districts, two KZB-0 units per feeder enable three-phase energy measurement with redundant metering paths. The cast-resin housing resists electromagnetic interference from adjacent busbars, preventing signal distortion in digital metering systems using IEC 61850-9-2 LE protocols.
Industrial Power Distribution
Heavy industries—such as steel mills, chemical plants, and data centers—deploy KZB-0 CTs for motor protection and load monitoring. In a typical arc furnace installation, the transformer’s 5P20 protection winding feeds differential relays that detect internal winding faults within 20 ms. The high dynamic withstand current (62.5 kA) accommodates inrush currents during capacitor bank switching, while the GOES core minimizes remanence after fault clearance, ensuring rapid reset of protection schemes. The IP54-rated enclosure withstands airborne particulates common in industrial settings without degradation of insulation resistance.
Renewable Energy Integration
Solar and wind farms utilize the KZB-0 at point-of-interconnection (POI) switchyards to monitor export power and provide anti-islanding protection. During cloud-induced irradiance fluctuations, the CT’s linear response down to 5% of rated current enables accurate reactive power control per IEEE 1547-2018. In a 20 MW solar plant connected to a 35 kV grid, KZB-0 units feed synchrophasor measurements to PMUs for grid stability analysis. The absence of oil prevents contamination risks in environmentally sensitive sites, and the lightweight design simplifies crane-less installation on tracker-mounted structures.
Rural and Suburban Distribution Networks
For utilities managing extensive rural feeders, the KZB-0’s maintenance-free operation reduces lifecycle costs. Mounted on poles or pad-mounted transformers, it supplies current signals to reclosers and sectionalizers for automated fault isolation. In a 35 kV overhead line serving agricultural communities, the CT’s wide temperature tolerance (−40°C to +40°C) ensures reliable operation during seasonal extremes. Its standardized 120 mm × 120 mm base plate fits common crossarm brackets, enabling retrofit replacements of aging oil-filled units without structural modifications.
Compliance with International Standards
The KZB-0 current transformer is certified to global and regional standards governing instrument transformer performance, safety, and interoperability.
IEC 61869-2 Compliance Details
Per IEC 61869-2:2012, the KZB-0 meets all requirements for electromagnetic instrument transformers used in AC systems. Key compliance aspects include: rated insulation levels (Um = 36 kV), short-circuit performance (Ith ≥ 25 kA/1s), and accuracy verification under defined burden conditions. The standard mandates that protection-class CTs exhibit saturation voltage (Vk) ≥ 2 × If × Zb, where If is fault current and Zb is burden impedance. For a 5P20 CT with 15 VA burden at 5 A, Vk must exceed 120 V—verified during factory excitation tests. Additionally, the standard requires terminal markings per IEC 60617: primary terminals labeled P1/P2, secondaries S1/S2 with reducing polarity.
GB/T 20840.2 Alignment
The Chinese national standard GB/T 20840.2-2014 closely mirrors IEC 61869-2 but includes localized requirements. Notably, GB/T specifies a higher lightning impulse test voltage (185 kV vs. IEC’s 170 kV) for 35 kV class equipment and mandates partial discharge levels ≤ 10 pC at 1.2 × Um/√3. The KZB-0 exceeds these criteria through optimized resin curing cycles that minimize microvoids. Furthermore, GB/T requires nameplate information in Chinese characters alongside English, including manufacturer code, production batch, and QR-code traceability—features integrated into all units shipped to mainland China.
Key Differences Between IEC and Domestic Standards
While IEC 61869-2 focuses on functional interoperability, GB/T 20840.2 emphasizes manufacturing process controls. For instance, GB/T mandates ultrasonic testing of resin integrity post-curing—a step not required by IEC. Conversely, IEC places greater emphasis on transient response characterization via TPS (transient performance specification) parameters, which the KZB-0 documents in its type test report. Both standards agree on core material requirements (GOES with B8 ≤ 1.0 T at 50 Hz), but GB/T adds coercivity limits (Hc ≤ 80 A/m) to ensure low hysteresis loss. These harmonized yet distinct requirements necessitate dual-certification testing at accredited labs like KEMA or CESI.
On-Site Testing Procedures
Post-installation verification ensures the KZB-0 operates within design tolerances before energization.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-ground, secondary-to-ground, and primary-to-secondary. Acceptance criteria: ≥ 1000 MΩ at 20°C. Correct for temperature using RT2 = RT1 × 2(T1−T2)/10. Values below 500 MΩ indicate moisture ingress or resin cracking, requiring drying or replacement. Perform this test after cleaning terminals with isopropyl alcohol to remove conductive dust.
Turns Ratio Test
Apply 1–5 V AC at 50 Hz to the secondary winding and measure induced primary voltage. Calculate ratio as Vp/Vs; compare to nameplate value (e.g., 600:5 = 120:1). Tolerance: ±0.25% for metering classes, ±0.5% for protection. Use a calibrated ratio bridge (e.g., Omicron CT Analyzer) for accuracy. Deviations >1% suggest turn-to-turn shorts or incorrect tap selection.
Polarity Test
Verify reducing polarity using the DC kick method: connect a 6 V battery between P1 (+) and P2 (−), and a center-zero galvanometer between S1 and S2. A momentary positive deflection confirms correct polarity. Incorrect polarity reverses phase angle by 180°, causing watt-hour meters to register negative energy and directional relays to misoperate. Document results with timestamped oscillograms.
Power Frequency Withstand Voltage Test
Apply 70 kV RMS at 50 Hz between primary and grounded tank for 1 minute. Monitor for flashover, excessive leakage current (>1 mA), or audible discharge. Use a calibrated sphere gap for voltage measurement. This test validates insulation integrity after transportation stresses. Never perform if ambient humidity exceeds 80% to avoid surface flashover masking internal defects.
Excitation (Saturation) Characteristic Test
Gradually increase AC voltage on secondary (open-circuited primary) while measuring current. Plot Vs vs. Is to determine knee-point voltage (Vk). For 5P20 CTs, Vk must satisfy Vk ≥ (If/In) × Zb × In × √2. Example: 600:5 CT, Zb=10 Ω, If=12 kA → Vk ≥ 170 V. A 10% drop in Vk versus factory data indicates core damage.
Preventive Maintenance Guide
Although cast-resin CTs require minimal upkeep, scheduled inspections extend service life and prevent unexpected failures.
Periodic Inspection Protocol
Conduct annual visual and electrical checks: inspect for surface cracks, UV degradation (chalky residue), or tracking marks; clean with mild detergent; verify terminal tightness (torque: 15 N·m for M10 bolts); retest insulation resistance. In polluted environments (ESDD > 0.1 mg/cm²), increase frequency to semi-annual. Record all findings in a CMMS with photo documentation. Replace units showing >20% decline in insulation resistance over three consecutive tests.
Maintenance Intervals and Fault Diagnosis
Adhere to the following schedule:
| Interval | Action |
|---|---|
| 1 year | Visual inspection, IR thermography, insulation resistance |
| 5 years | Full electrical tests (ratio, polarity, excitation curve) |
| 10 years | Partial discharge measurement (if available) |
| After fault | Immediate post-fault testing before re-energization |
Common faults include: (1) Open secondary circuit—causes dangerous overvoltage; always short-circuit secondaries before disconnecting meters. (2) Core saturation—indicated by distorted secondary waveform during faults; verify burden compatibility. (3) Moisture ingress at terminal seals—evidenced by low IR values; replace gaskets with fluorosilicone O-rings.
Conclusion
The KZB-0 33kV cast-resin current transformer represents a benchmark in medium-voltage instrumentation, combining decades of engineering refinement with modern materials science. Its VPI epoxy resin insulation system eliminates the fire, environmental, and maintenance liabilities associated with oil-filled alternatives, while delivering metrological accuracy that meets stringent international standards. Engineered with GOES silicon steel cores and rigorously tested to IEC 61869-2 and GB/T 20840.2, the KZB-0 ensures reliable operation across diverse applications—from urban substations to remote renewable sites—under extreme thermal, electrical, and environmental stresses. With a design life exceeding 30 years and minimal lifecycle costs, it provides utilities and industrial operators with a future-proof solution for accurate metering and dependable protection. The transformer’s standardized interfaces and dual-winding flexibility further enhance its adaptability in evolving grid architectures, including smart substations leveraging IEC 61850 communication protocols. As power systems grow more complex and distributed, the KZB-0’s proven performance and compliance pedigree make it an indispensable component for safe, efficient, and accurate current measurement at the 33/35 kV level.