Article Content
LSZY-35 33kV Cast-Resin Voltage Transformer for Substation Metering and Protection – IEC 61869-3 Certified
Introduction to the LSZY-35 Voltage Transformer
The LSZY-35 is a single-phase, outdoor-rated cast-resin voltage transformer (VT) designed for accurate voltage measurement and protective relaying in 33kV (IEC) / 35kV (domestic) medium-voltage distribution systems. It employs vacuum pressure impregnation (VPI) epoxy resin technology to fully encapsulate its magnetic core and windings, providing superior dielectric strength, mechanical robustness, and environmental resistance compared to traditional oil-immersed or dry-type alternatives.
Operating Principle of Cast-Resin Insulation
Cast-resin insulation in the LSZY-35 utilizes a two-component cycloaliphatic epoxy resin system cured under vacuum and pressure. This process eliminates air voids and moisture ingress pathways, ensuring uniform dielectric properties throughout the insulation matrix. The primary and secondary windings—typically wound with high-purity electrolytic copper—are embedded within this monolithic resin structure alongside a grain-oriented electrical steel (GOES) core. The resulting solid insulation system exhibits excellent partial discharge performance (<5 pC at 1.2 × Ur), high thermal conductivity (≈0.8 W/m·K), and resistance to tracking and erosion under polluted conditions. Unlike oil-filled units, cast-resin VTs require no maintenance of insulating fluid and pose zero fire or environmental hazard, making them ideal for indoor substations, urban installations, and environmentally sensitive areas.
Advantages Over Oil-Immersed Designs
Compared to oil-immersed voltage transformers, the LSZY-35 offers significant operational and safety benefits. Its solid insulation eliminates risks associated with oil leakage, flammability, and PCB contamination. The absence of an oil expansion tank simplifies mechanical design and reduces overall footprint. Cast-resin construction provides higher short-circuit withstand capability due to rigid winding fixation, minimizing deformation during fault transients. Additionally, the LSZY-35 demonstrates superior performance in high-humidity and coastal environments where oil degradation and moisture absorption can compromise insulation integrity. Maintenance intervals are extended from annual (for oil units) to 5–10 years, reducing lifecycle costs. The unit’s lightweight design (typically 85–110 kg) also facilitates easier handling and installation without specialized lifting equipment.
Typical Applications Overview
The LSZY-35 is engineered for critical roles in utility and industrial power systems. Primary applications include revenue metering at 33kV/35kV feeders, synchronization of generators to the grid, and input to protective relays such as overvoltage, undervoltage, and directional earth-fault schemes. Its high accuracy class (0.2 or 0.5 for metering; 3P or 6P for protection) ensures reliable data for billing and system stability. The transformer is commonly deployed in compact secondary substations, renewable energy interconnection points (e.g., solar farms with 35kV collection systems), and industrial plants requiring precise voltage monitoring. Its IP54-rated terminal box and UV-stabilized resin housing support reliable outdoor operation across diverse climatic zones.
Technical Specifications
The LSZY-35 adheres to stringent electrical and mechanical parameters defined by IEC 61869-3 and GB/T 20840.3, ensuring interoperability and performance consistency across global markets.
Rated Electrical Parameters
Key electrical ratings for the LSZY-35 include: primary voltage of 33/√3 kV (IEC standard) or 35/√3 kV (Chinese domestic system); standard secondary voltages of 100/√3 V or 100/3 V for three-phase configurations; rated frequency of 50 Hz (60 Hz optional); accuracy classes of 0.2, 0.5, 1, 3P, or 6P per IEC 61869-3; and rated outputs ranging from 25 VA to 100 VA per secondary winding. The transformer features a thermal short-time current rating of 100 A for 1 second and a dynamic withstand current of 2.5 kA peak. Insulation levels comply with Um = 36 kV, with a lightning impulse withstand voltage of 170 kV (1.2/50 μs) and a power frequency wet test voltage of 70 kV RMS for 1 minute. Burden limits are strictly defined to maintain accuracy—e.g., a 0.2-class winding must operate within 25–100% of rated burden to meet tolerance.
Environmental and Mechanical Ratings
The LSZY-35 is rated for outdoor installation with an ambient temperature range of –40°C to +40°C and relative humidity up to 100%. It is suitable for altitudes up to 1,000 m above sea level without derating; above this, output must be adjusted per IEC 60060-1 altitude correction factors. The housing is constructed from UV-resistant, hydrophobic cycloaliphatic epoxy resin with integrated sheds for pollution performance (creepage distance ≥25 mm/kV for light pollution, ≥31 mm/kV for heavy). Mounting is via a standardized M16 or M20 threaded stud base compatible with IEC 61869 mechanical interfaces. Secondary terminals are housed in an IP54 enclosure with screw-type connectors rated for 10 mm² copper conductors. The unit weighs approximately 95 kg and measures 680 mm (H) × 320 mm (W) × 280 mm (D).
Core and Winding Construction
The magnetic circuit employs high-permeability grain-oriented electrical steel (GOES) laminations, typically grade M4 or equivalent, with a core loss below 1.2 W/kg at 1.7 T and 50 Hz. Windings are concentrically arranged with primary on the outside for optimal electric field distribution. The secondary winding uses double-insulated enameled copper wire with Class F (155°C) thermal rating. Interlayer and interwinding insulation consists of Nomex® or polyester film, fully impregnated during the VPI process. Partial discharge inception voltage exceeds 1.5 × Ur, and measured PD levels remain below 3 pC at rated voltage—well within IEC 61869-3 limits of 10 pC for resin-cast VTs.
Typical Applications
The LSZY-35 serves diverse roles in modern power infrastructure, leveraging its accuracy, reliability, and environmental resilience.
Substation Secondary Metering
In 33kV/35kV distribution substations, the LSZY-35 provides precise voltage signals to revenue-class energy meters. Installed on outgoing feeders or bus sections, it enables accurate kWh and kVArh billing for commercial and industrial consumers. With accuracy class 0.2 and burden stability from 25–100% load, it meets regulatory requirements in most jurisdictions. For example, in a typical Chinese 35kV urban substation, LSZY-35 units feed multi-tariff meters that track peak/off-peak consumption, ensuring compliance with NDRC tariff structures. The cast-resin design prevents drift due to temperature cycling—a common issue in oil-filled VTs—maintaining calibration over decades.
Industrial Power Distribution Systems
Large manufacturing facilities often operate internal 35kV networks for efficient power distribution. Here, the LSZY-35 supplies voltage inputs to motor protection relays, capacitor bank controllers, and power quality analyzers. In a steel mill application, LSZY-35 units monitor furnace transformer inputs, enabling undervoltage lockout to prevent equipment damage during grid sags. The transformer’s high saturation point (>2.0 × Ur) ensures linear response during ferroresonance events common in lightly loaded industrial feeders. Its immunity to vibration and dust makes it suitable for harsh plant environments where oil units would require frequent maintenance.
Renewable Energy Integration
Solar and wind farms frequently use 35kV collector systems to aggregate generation before step-up to transmission voltage. The LSZY-35 provides synchronization voltage for inverters and grid-tie relays, ensuring phase alignment during connection. In a 50 MW solar project in Inner Mongolia, LSZY-35 VTs feed synchrocheck relays that verify ΔV < 5%, Δf < 0.1 Hz, and Δδ < 10° before closing the main breaker. The unit’s low capacitive coupling minimizes false tripping during rapid irradiance changes. Additionally, its non-flammable construction satisfies fire codes for PV combiner stations located near vegetation.
Rural and Suburban Distribution Networks
In remote or mountainous regions, compact 35kV pole-mounted substations rely on LSZY-35 VTs for basic protection and metering. These units endure wide temperature swings (–30°C to +45°C) and high humidity without performance degradation. For instance, in Sichuan province, LSZY-35 transformers enable single-phase earth-fault detection in isolated neutral systems using residual voltage measurement (open-delta configuration). Their maintenance-free nature is critical where access is limited during rainy seasons. The standardized IEC interface allows interchangeability with legacy electromagnetic VTs during grid upgrades.
Grid Monitoring and SCADA Systems
Modern distribution automation schemes use LSZY-35 outputs for real-time voltage profiling. Connected to RTUs or IEDs, they provide data for volt/VAR optimization and outage management. In smart grid pilots, LSZY-35 units with dual secondaries—one for metering (0.2 class), one for protection (3P)—feed both billing systems and fault recorders. The consistent phase angle error (<10 minutes for 0.2 class) ensures accurate power factor calculation, essential for demand-side management programs.
Compliance with International Standards
The LSZY-35 is certified to both international and Chinese national standards, ensuring global acceptance and local regulatory compliance.
IEC 61869-3 Certification Requirements
IEC 61869-3 specifies performance, testing, and marking requirements for inductive voltage transformers. The LSZY-35 undergoes type tests including temperature rise (≤60 K for resin), short-circuit withstand (thermal and dynamic), and accuracy verification across burden and frequency ranges. Routine tests per clause 12 include turns ratio (±0.25% tolerance), polarity (reducing polarity confirmed), and power frequency withstand (70 kV RMS for 1 min). Special tests like partial discharge (≤10 pC at 1.2 × Ur/√3) and capacitance/tan δ measurements validate insulation quality. Marking includes IEC voltage designation (33/√3 kV), accuracy class, rated output, and manufacturer ID per clause 15.
Alignment with GB/T 20840.3
GB/T 20840.3 is China’s national adoption of IEC 61869-3 with minor modifications. Key differences include: domestic system voltage reference of 35kV instead of 33kV; creepage distance requirements based on Chinese pollution zones (e.g., 25 mm/kV for Zone II); and mandatory short-time current rating of 100 A/1s for all 35kV VTs. The LSZY-35 meets both standards simultaneously by designing for the more stringent requirement—e.g., using 31 mm/kV creepage for export models. Chinese certification (CQC mark) requires additional factory audits and sample testing at authorized labs like China Electric Power Research Institute (CEPRI).
Testing and Certification Documentation
Each LSZY-35 unit ships with a test report detailing routine test results traceable to national standards. Type test certificates (valid for 8 years) are available upon request, covering all IEC 61869-3 clauses. For projects requiring third-party validation, units can be witness-tested per client specifications. Compliance documentation includes material declarations (RoHS, REACH), fire resistance test reports (IEC 60695 glow-wire), and seismic qualification data (if applicable).
On-Site Testing Procedures
Field commissioning of the LSZY-35 requires verification of insulation integrity, ratio accuracy, and polarity to ensure safe and reliable operation.
Insulation Resistance Test
Perform insulation resistance measurement between primary-to-ground, secondary-to-ground, and primary-to-secondary using a 2,500 V DC megohmmeter. Acceptance criteria: minimum 1,000 MΩ for primary circuits and 100 MΩ for secondary circuits at 20°C. Correct for temperature using RT2 = RT1 × 2(T1–T2)/10. Low readings indicate moisture ingress or surface contamination—clean terminals with isopropyl alcohol and retest. Never perform this test immediately after rain; allow 24 hours of dry weather.
Turns Ratio Test
Apply a low-voltage AC source (50–100 V) to the primary and measure secondary voltage with a calibrated true-RMS meter. Calculate ratio as Vp/Vs and compare to nameplate. Tolerance per IEC 61869-3: ±0.25% for accuracy classes ≤0.5, ±0.5% for protection classes. Example: for 33,000/√3 : 100/√3 V ratio, expected secondary is 100 V at 33,000 V primary. Deviations >0.5% suggest winding faults or incorrect tap selection.
Polarity Verification
Confirm reducing polarity using the DC kick method: connect a 6–12 V battery to primary (H1+, H2–) and a center-zero galvanometer to secondary (X1, X2). Momentary closure should produce a positive kick. Alternatively, use an automatic ratio tester with polarity indication. Incorrect polarity causes 180° phase shift, leading to metering errors or relay misoperation—especially critical in differential and directional schemes.
Power Frequency Withstand Voltage Test
Apply 70 kV RMS (50 Hz) between primary and grounded secondary/core for 1 minute. Use a calibrated test transformer with overcurrent protection set at 100 mA. The test verifies insulation integrity after transport. Partial discharge monitoring during the test is recommended—any PD >20 pC indicates voids or delamination. Do not perform if ambient humidity >80% or temperature <5°C to avoid condensation errors.
Open-Circuit Characteristic Test
For voltage transformers, the open-circuit test assesses core excitation characteristics. Gradually increase primary voltage from 0 to 1.5 × Ur while measuring secondary voltage and excitation current. Plot Vs vs Iexc; knee point should exceed 1.9 × Ur. Excessive excitation current at rated voltage (>0.5% of rated secondary current) indicates core saturation or shorted turns. This test is critical after exposure to ferroresonance or switching surges.
Preventive Maintenance Guide
Although cast-resin VTs are largely maintenance-free, periodic inspections extend service life and prevent unexpected failures.
Annual Visual and Electrical Inspection
Inspect annually for: cracks or tracking on resin housing; corrosion on mounting hardware; loose secondary terminals; and signs of overheating (discoloration at connections). Clean sheds with deionized water if pollution layer exceeds 0.1 mm thickness. Perform insulation resistance and ratio tests as baseline data. Check grounding continuity—resistance from base to substation grid must be <0.1 Ω. Document all findings for trend analysis; sudden drops in insulation resistance warrant immediate investigation.
Five-Year Comprehensive Maintenance
Every 5 years, conduct partial discharge measurement using IEC 60270 methods. Acceptable levels: <5 pC at 1.2 × Ur/√3. Also verify secondary burden impedance matches connected loads—mismatched burdens cause accuracy drift. Inspect terminal box gaskets for hardening; replace if compression set exceeds 20%. Torque secondary terminals to 2.5 N·m (per IEC 61869-1). If the unit has been exposed to short-circuit currents >50% of rated dynamic withstand, perform a full open-circuit test to check for core damage.
Fault Diagnosis and Troubleshooting
Common failure modes include: secondary open-circuit (causing dangerous overvoltage on primary), moisture ingress at terminal seals, and external flashover due to pollution. Symptoms: blown secondary fuses, erratic meter readings, or audible humming. Diagnose by isolating the VT and performing ratio and insulation tests. If secondary voltage is zero but primary is live, suspect an open secondary winding—replace immediately. For persistent partial discharge, infrared thermography may reveal hot spots indicating internal defects.
Conclusion
The LSZY-35 33kV cast-resin voltage transformer represents a mature, reliable solution for metering and protection in modern medium-voltage networks. Its VPI epoxy resin insulation delivers exceptional dielectric performance, environmental resilience, and fire safety—addressing key limitations of oil-immersed alternatives. Compliance with both IEC 61869-3 and GB/T 20840.3 ensures global applicability while meeting stringent Chinese grid requirements. With accuracy classes down to 0.2 and protection ratings up to 6P, it supports critical applications from revenue metering to renewable integration. The transformer’s robust GOES core and precision-wound coils maintain performance across temperature extremes and electrical transients. When subjected to recommended maintenance protocols—annual visual checks and 5-year electrical tests—the LSZY-35 achieves a service life of 25–30 years, significantly reducing total cost of ownership. Its standardized interfaces and proven field reliability make it a preferred choice for utilities upgrading aging infrastructure or deploying new smart grid assets. As distribution networks evolve toward higher automation and distributed generation, the LSZY-35’s stable, low-maintenance operation will continue to underpin grid accuracy and security.
Frequently Asked Questions (FAQ)
Q1: Can the LSZY-35 be used on a 35kV system even though it’s rated 33kV?
A: Yes. The 33kV rating follows IEC 60038 standard voltages, while 35kV is the Chinese domestic system voltage. The LSZY-35 is designed with Um = 36 kV insulation level, making it fully compatible with 35kV maximum operating voltage (typically 38.5 kV during emergencies).
Q2: What is the maximum allowable secondary burden for a 0.2-class LSZY-35?
A: The burden must be within 25–100% of the rated output (e.g., 25–100 VA for a 100 VA unit). Exceeding 100% burden increases ratio and phase errors beyond 0.2-class limits per IEC 61869-3 Table 5.
Q3: Is partial discharge testing required during routine maintenance?
A: Not routinely, but it is recommended every 5 years or after exposure to severe overvoltages. Acceptable levels are <5 pC at 1.2 × Ur/√3. Portable PD detectors with HFCT sensors can perform this test without disconnecting the VT.
Q4: Can the LSZY-35 be installed indoors?
A: Yes. Although rated for outdoor use, its non-flammable, non-toxic construction makes it suitable for indoor GIS or switchgear rooms. Ensure ambient temperature does not exceed +40°C and ventilation prevents condensation.
Q5: What causes ratio error drift over time?
A: Primary causes include core aging (minimal in GOES), secondary burden changes, or temperature effects. Annual ratio testing detects drift; if error exceeds ±0.25%, investigate burden mismatches or internal faults.
Q6: How do I verify reducing polarity in the field?
A: Use the DC kick test: apply momentary DC to H1-H2 and observe galvanometer deflection on X1-X2. Positive kick confirms reducing polarity. Modern multifunction testers automate this with clear pass/fail indication.