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11kV Cast-Resin Voltage Transformer SZK-12 for Metering and Protection – IEC 61869-3 Standard
Introduction to the SZK-12 Voltage Transformer
The SZK-12 is a single-phase, indoor-type cast-resin voltage transformer (VT) engineered for accurate voltage transformation in 11kV (IEC-rated) or 10kV (domestic system) medium-voltage networks. Designed in strict compliance with IEC 61869-3 and GB/T 20840.3, this instrument transformer reduces primary system voltage to standardized secondary levels—typically 100 V or 110 V—for use in metering, protective relaying, and control circuits. Its core function is to provide galvanic isolation while maintaining high accuracy under normal and transient operating conditions.
Cast-Resin Insulation Technology
The SZK-12 employs vacuum pressure impregnation (VPI) epoxy resin casting technology, wherein both primary and secondary windings are fully encapsulated in a homogeneous, void-free epoxy matrix. This process eliminates air pockets that could lead to partial discharges under high electric stress. The resin formulation includes silica fillers to enhance thermal conductivity and mechanical strength, enabling stable operation across ambient temperatures from –25°C to +40°C. Unlike oil-filled alternatives, the cast-resin design is non-flammable, requires no maintenance of insulating fluid, and exhibits superior resistance to environmental contaminants such as dust, moisture, and chemical vapors. Partial discharge levels are maintained below 10 pC at 1.2 × Um/√3, per IEC 61869-3 requirements, ensuring long-term dielectric integrity even in polluted indoor environments.
Advantages Over Oil-Immersed Designs
Compared to traditional oil-immersed VTs, the SZK-12 offers significant operational and safety benefits. The absence of flammable oil eliminates fire hazards, making it suitable for confined spaces like urban substations, commercial buildings, and underground installations. Its compact footprint—approximately 30% smaller than equivalent oil units—optimizes switchgear bay utilization. Additionally, the solid insulation system prevents leakage, aging, or moisture ingress issues common in oil-filled units, thereby reducing lifecycle costs. Thermal performance is enhanced through direct heat conduction from windings to the outer casing, allowing continuous operation at 1.2 × rated voltage without derating. These attributes make the SZK-12 particularly advantageous in applications where space, safety, and reliability are critical.
Typical Deployment Scenarios
The SZK-12 is predominantly deployed in indoor 10kV/11kV distribution substations serving industrial plants, data centers, hospitals, and municipal infrastructure. It interfaces directly with digital multifunction meters, revenue-class energy meters, and numerical relays requiring precise voltage inputs. In ring-main units (RMUs) and metal-enclosed switchgear, its robust mechanical design withstands electromagnetic forces during short-circuit events up to 20 kA for 1 second. The transformer also supports synchronization checks in generator tie-in panels and provides voltage reference signals for automatic voltage regulators (AVRs) in distributed generation systems. Its standardized mounting dimensions facilitate retrofitting into legacy switchgear without structural modifications.
Technical Specifications
The SZK-12 is engineered to deliver consistent performance under defined electrical and environmental parameters. All specifications adhere to IEC 61869-3 (2011) and GB/T 20840.3 (2013), ensuring global interoperability and regulatory acceptance.
Rated Electrical Parameters
The primary rated voltage is 11 kV (Up), corresponding to a system voltage of 10 kV (Un). Standard secondary voltages include 100 V, 100/√3 V, 110 V, and 110/√3 V, selected based on regional metering standards. Common voltage ratios are 11000/100 V, 11000/√3 / 100/√3 V, and 10000/100 V for domestic compatibility. Accuracy classes are available in 0.2, 0.5, 1, and 3P/6P for metering and protection, respectively. Rated outputs range from 25 VA to 100 VA per secondary winding, with thermal stability verified at 1.5 × rated burden for 8 hours. The insulation level follows IEC 60071-1: LI 75 kV / AC 42 kV (1 min), ensuring withstand capability against lightning impulses and power-frequency overvoltages. Winding resistance is temperature-compensated to 75°C, with typical values of 120 Ω (primary) and 0.15 Ω (secondary).
Environmental and Service Conditions
The SZK-12 is rated for indoor installation only, with an ambient temperature range of –25°C to +40°C and relative humidity up to 95% (non-condensing). Maximum installation altitude is 1000 m above sea level; for altitudes between 1000–2000 m, a derating factor of 1.1% per 100 m applies to the insulation withstand voltage. The transformer is designed for continuous operation at system frequency of 50 Hz or 60 Hz, with harmonic distortion tolerance up to 5% THD. Mechanical vibration resistance meets IEC 60068-2-6 (5–100 Hz, 0.7 g amplitude). Terminal markings comply with IEC 61869-3 polarity conventions: A (high-voltage terminal), N (grounded terminal), a and n (secondary terminals), with reducing polarity standard.
Construction and Materials
The magnetic core is fabricated from grain-oriented electrical steel (GOES) with a maximum specific loss of 1.0 W/kg at 1.7 T and 50 Hz, minimizing no-load losses and enhancing phase-angle accuracy. Primary windings use enameled copper wire with Class F (155°C) insulation, while secondary windings employ double-insulated stranded copper conductors for flexibility and solderability. The entire assembly is cast under vacuum in cycloaliphatic epoxy resin, which offers UV resistance and hydrophobic surface properties. The housing includes integrated lifting lugs, grounding studs (M10 thread), and cable entry knockouts compatible with PG13.5 or M20 glands. Weight is approximately 28 kg, with overall dimensions of 320 mm (H) × 220 mm (W) × 180 mm (D).
Typical Applications
The SZK-12 serves as a foundational component in modern medium-voltage infrastructure, enabling precise voltage monitoring and system protection across diverse sectors.
Substation Secondary Metering Systems
In utility-owned 10kV/11kV distribution substations, the SZK-12 supplies voltage signals to Class 0.2S or 0.5S revenue meters for accurate energy billing. Its low phase displacement (< ±10 minutes at 0.2 class) ensures minimal error in kWh measurement, even under light-load conditions. When paired with matching current transformers (e.g., LZZBJ9-12), it forms a complete metering set compliant with MID or ANSI C12.20 standards. Digital meters sample the 100 V secondary output at 4 kHz or higher, enabling harmonic analysis and power quality monitoring. The VT’s thermal stability prevents ratio drift during summer peak loads, maintaining billing accuracy within ±0.2% over a 10-year interval.
Industrial Power Distribution Networks
Within manufacturing facilities, the SZK-12 integrates into motor control centers (MCCs) and process automation panels to feed voltage inputs to protective relays such as overvoltage (59), undervoltage (27), and directional power (32) functions. For example, in a cement plant with 10kV induction motors, the VT enables undervoltage ride-through logic that prevents nuisance tripping during grid sags. Its 3P accuracy class guarantees < ±3% error at 5% of rated voltage, critical for reliable relay operation during fault conditions. The cast-resin body resists oil mist and airborne particulates common in heavy-industry environments, eliminating the need for sealed enclosures.
Renewable Energy Integration Points
At solar PV or wind farm interconnection points, the SZK-12 provides grid-synchronization voltage to inverters and SCADA systems. During anti-islanding tests, its fast response time (< 20 ms) ensures rapid detection of grid loss. In microgrids operating in islanded mode, the VT supports droop control algorithms by delivering stable voltage feedback to energy management systems. Its ability to withstand temporary overvoltages up to 1.9 × Un for 1 second (per IEC 61869-3 Annex B) protects downstream electronics during capacitor bank switching transients.
Rural and Suburban Distribution Feeders
In remote areas with limited maintenance access, the SZK-12’s maintenance-free design ensures decades of reliable service. Installed in pole-mounted or pad-mounted switchgear, it enables remote voltage monitoring via RTUs using the 100 V output. Utilities leverage its consistent ratio accuracy to detect feeder voltage drops indicative of conductor degradation or unbalanced loading. The transformer’s immunity to rodent damage and vandalism—due to its solid construction—makes it preferable over oil-filled units in exposed locations.
Compliance with International Standards
The SZK-12 is certified to the latest international and Chinese national standards governing instrument transformers, ensuring global acceptance and interoperability.
IEC 61869-3 Certification Details
IEC 61869-3:2011 specifies requirements for inductive voltage transformers for measurement and protection. The SZK-12 undergoes type tests including temperature rise (≤ 60 K for windings), short-circuit withstand (not applicable for VTs, but verified via open-circuit stability), and accuracy verification across 25–100% of rated voltage. Routine tests per clause 10 include power-frequency withstand (42 kV, 1 min), partial discharge (< 10 pC at 1.2 × Um/√3), and ratio/polarity checks. The manufacturer provides a test report traceable to ISO/IEC 17025-accredited laboratories, confirming compliance with all mandatory clauses. Notably, the standard mandates that protection-class VTs (3P/6P) maintain specified errors under 5% to 100% of rated voltage—a criterion the SZK-12 meets with margin.
Alignment with GB/T 20840.3
GB/T 20840.3-2013 is the Chinese national adoption of IEC 61869-3, with minor deviations in labeling and terminal marking. The SZK-12 complies fully, including the requirement for dual voltage labeling (10kV system / 11kV equipment). Chinese grid codes (e.g., State Grid Q/GDW 1374) mandate additional surge testing with 1.2/50 μs impulses at 75 kV peak, which the SZK-12 passes without insulation failure. The standard also specifies creepage distance ≥ 240 mm for pollution degree 3, achieved via ribbed epoxy surfaces that extend tracking paths. All units shipped to China include bilingual (Chinese/English) nameplates and test certificates approved by CEPREI or similar bodies.
Harmonization Between IEC and Domestic Requirements
While IEC 61869-3 uses 11kV as the standard highest voltage for equipment (Um), Chinese practice often references 10kV as the nominal system voltage. The SZK-12 bridges this gap by being rated for both: its insulation coordination is based on Um = 12 kV (per IEC 60071), providing a safety margin over the 11kV rating. Accuracy class definitions are identical, but Chinese utilities may require extended calibration points at 1% and 120% of rated voltage—tests the SZK-12 routinely undergoes. This dual-compliance strategy allows seamless deployment in export markets and domestic projects alike.
On-Site Testing Procedures
Post-installation verification ensures the SZK-12 performs within specification before energization. All tests follow IEC 61869-3 Annex D and IEEE C57.13 guidelines.
Insulation Resistance Test
Using a 2500 V DC megohmmeter, measure insulation resistance between primary-to-secondary, primary-to-ground, and secondary-to-ground. Acceptance criteria: ≥ 1000 MΩ at 20°C. Correct for temperature using RT = R20 × 2(20–T)/10. Values below 500 MΩ indicate moisture ingress or resin cracking and require drying or replacement. Perform before and after power-frequency withstand tests to detect insulation degradation.
Turns Ratio Verification
Apply 100–200 V AC to the primary and measure secondary voltage with a calibrated voltmeter (accuracy class 0.1 or better). Calculate actual ratio and compare to nameplate. Tolerance: ±0.2% for 0.2 class, ±0.5% for 0.5 class, ±1% for 1 class, and ±3% for 3P class at 100% voltage. Repeat at 25%, 50%, and 100% of rated voltage to verify linearity. Deviations beyond tolerance suggest winding shorts or incorrect tap selection.
Polarity Confirmation
Connect a low-voltage AC source (e.g., 12 V) between A and N. Simultaneously connect a center-zero galvanometer between a and n. Momentary application should produce a positive kick, confirming reducing polarity (IEC standard). Incorrect polarity causes 180° phase reversal, leading to metering errors or relay misoperation. This test is mandatory for all new installations and after any terminal rewiring.
Power-Frequency Withstand Voltage Test
Apply 42 kV RMS at 50 Hz between primary and grounded parts (secondary shorted and grounded) for 1 minute. Use a calibrated test transformer with overcurrent trip set at 10 mA. No flashover or disruptive discharge constitutes pass. For field testing, reduced voltage (80% of factory test) may be used if agreed with the utility. Always discharge the unit through a grounding stick post-test.
Open-Circuit Characteristic Test
With secondary open, gradually increase primary voltage from 0 to 150% of rated (16.5 kV) while recording excitation current. Plot V vs. I curve; knee point should exceed 120% of rated voltage. Excessive magnetizing current (> 5% of rated primary current at 100% voltage) indicates core saturation or inter-turn faults. This test validates core integrity and is especially recommended after transportation.
Preventive Maintenance Guide
Although cast-resin VTs are largely maintenance-free, periodic inspections extend service life and prevent unexpected failures.
Annual Visual and Functional Inspection
Inspect for surface cracks, tracking marks, or discoloration on the epoxy housing. Verify terminal tightness (torque: 15 N·m for M8 bolts) and check for corrosion on grounding connections. Clean dust accumulation using dry compressed air (< 3 bar). Functionally, compare secondary voltage readings against historical data; deviations >1% warrant diagnostic testing. Ensure secondary circuits are never left open during operation—a fused shorting link must be installed during meter replacement.
Five-Year Diagnostic Schedule
Every 60 months, perform insulation resistance, turns ratio, and partial discharge tests (if PD detector available). Partial discharge magnitude should remain <15 pC at 1.2 × Um/√3. Replace units exhibiting PD growth >5 pC/year. Also inspect mounting hardware for fatigue, especially in seismically active zones. Update maintenance logs with test results for lifecycle tracking.
Fault Diagnosis and Replacement Criteria
Common failure modes include secondary winding opens (causing infinite ratio error), core delamination (evidenced by elevated no-load losses), and terminal overheating (from loose connections). If ratio error exceeds twice the class limit or insulation resistance drops below 200 MΩ, replace the unit. Never attempt field repair of cast-resin VTs—epoxy encapsulation is irreversible. Maintain spare units with identical ratio and accuracy class for rapid swap-out.
| Maintenance Interval | Activities |
|---|---|
| Initial Commissioning | All on-site tests (ratio, polarity, insulation, withstand) |
| Annually | Visual inspection, terminal torque check, secondary voltage verification |
| Every 5 Years | Full diagnostic suite (including partial discharge if feasible) |
| After Fault Events | Insulation resistance and ratio test post-system short-circuit |
Conclusion
The SZK-12 11kV cast-resin voltage transformer represents a benchmark in reliability, accuracy, and safety for medium-voltage instrumentation. By leveraging advanced VPI epoxy encapsulation and GOES core technology, it delivers stable performance across demanding industrial and utility environments while eliminating the fire risks and maintenance burdens associated with oil-filled alternatives. Its strict adherence to IEC 61869-3 and GB/T 20840.3 ensures seamless integration into global power systems, supporting both precision metering (0.2 class) and dependable protection (3P/6P class) functions. Field-proven in thousands of installations, the SZK-12 maintains ratio and phase-angle accuracy within tight tolerances over decades of service, even under thermal cycling and harmonic distortion. With a design life exceeding 25–30 years and minimal preventive maintenance requirements, it offers a compelling total cost of ownership for distribution network operators. As grids evolve toward digitalization and distributed generation, the SZK-12’s robust signal fidelity and transient resilience position it as a critical enabler of smart substation architectures. For engineers specifying instrumentation in 10kV/11kV networks, the SZK-12 provides a technically superior, standards-compliant solution that balances performance, safety, and longevity.