Essential Type, Routine, and Site Acceptance Tests for Modern Eco-Friendly RMUs
1. Type Test System and Standards
Type testing verifies the design rationality and safety of eco-friendly insulated ring main units (RMUs), based on IEC 62271-200 and GB/T 3906, and includes:
Insulation Performance: For 12kV RMUs, power frequency withstand voltage is 42kV (1 min) for main circuits and 48kV for breakers. Lightning impulse withstand is 75kV (12kV system) or 125kV (24kV system), with 15 standard impulses (1.2/50μs) per polarity. Partial discharge must be ≤10pC at 1.2× rated voltage—stricter than SF₆ units due to lower insulation strength of eco-gases (e.g., nitrogen, ~1/3 of SF₆). Gas insulation strength tests, including evaluation of the "hump phenomenon" in nitrogen, are also required.
Mechanical Performance: Circuit breakers must endure 5,000 operation cycles, isolators ≥2,000. Mechanical characteristics (timing, speed, synchronicity) are measured. Internal arc testing requires withstanding 20–50kA for 0.1–1s, with internal pressure ≤50kPa and enclosure integrity maintained. IP67-level protection is verified using double EPDM seals and stainless steel.
Environmental Adaptability: Temperature/humidity cycling (40°C/93%RH for 56 days) limits insulation resistance drop to ≤50%. Salt spray testing (IEC 60068-2-52) requires 500 hours with corrosion <0.1μm/year. High-altitude operation (1,000–1,800m) requires 5–15% derating per 1,000m. Seismic testing at 0.5g ensures structural integrity and contact resistance fluctuation <3%.
2. Routine Tests and Execution
Routine tests ensure each unit meets basic requirements:
Main Circuit Resistance: Measured via DC voltage drop or bridge method; values must comply with specifications and differ ≤20% from type test results.
Power Frequency Withstand Voltage: 42kV (12kV system) applied for 1 second; no breakdown or flashover. Auxiliary/control circuits tested at 2kV/1min.
Sealing Test: Critical for gas-insulated units. Leakage rate ≤1×10⁻⁷ Pa·m³/s (IEC 62271-200), verified by 24-hour pressure monitoring or helium leak detection for higher precision.
Mechanical Operation: 5–10 operation cycles verify flexibility and correct function of mechanical interlocks ("five prevention" rules).
Visual and Electrical Checks: Inspect appearance, coating, labels, fasteners, and electrical connections. Solid-insulated units (e.g., epoxy-coated modules) require special attention to insulation integrity (no cracks or damage).
3. Site Acceptance and Special Environmental Tests
Final verification after installation:
Insulation Resistance: >1,000MΩ (measured with megohmmeter). Critical for detecting moisture, contamination, or defects—especially important for gas-insulated units in humid environments.
Protection Function Test: Simulate overcurrent and ground faults to verify protection device response and tripping reliability.
Temperature Rise Test: At rated current, busbar temperature rise ≤70K and contact rise ≤80K (GB/T 3906). Crucial due to poor heat dissipation of eco-gases (thermal conductivity ~1/4 of SF₆).
Special Environmental Tests:
High Altitude: Derate withstand voltage (e.g., 42kV ×1.15 ≈48.3kV at 1,800m).
High Humidity: Anti-condensation testing to ensure internal dryness.
Low Temperature: Operation tests at -40°C to ensure reliable switching.
4. Gas System Specialized Tests
Key differentiator from SF₆-based units:
Sealing Test: Helium leak detection (after vacuuming and helium injection) achieves 1×10⁻⁷ Pa·m³/s sensitivity. Pressure decay method uses 24-hour monitoring.
Pressure-Insulation Relationship: For nitrogen-insulated units (0.12–0.13MPa operating pressure), test insulation performance at reduced pressure (e.g., <90% rated) and evaluate "hump phenomenon" under impulse voltage.
Gas Purity and Humidity: Moisture in dry air units must be <150ppm. Use dew point meters or humidity sensors for monitoring.
Gas Chamber Integrity: X-ray inspection for weld quality (no pores/cracks), mechanical load tests for deformation resistance, and long-term pressure monitoring for seal stability.
5. Thermal Stability and Innovations
Critical due to poor heat dissipation of eco-gases (e.g., nitrogen):
Temperature Rise Test: Long-term operation at rated current; measure busbar, contact, and joint temperatures. Must meet GB/T 3906 limits (≤70K for busbars, ≤80K for contacts).
Short-Circuit Temperature Rise Test: Apply rated short-time current (e.g., 20kA/3s); verify temperature rise and thermal distribution under compact designs.
Innovative Cooling Solutions:
Radiative Cooling Coatings: Reduce surface temperature by up to 30.9°C; durable and corrosion-resistant.
Smart Cooling/Dehumidification: Fan and dehumidifier systems reduce temperature by 40% and humidity by 58%.
Design Improvements: Optimized ventilation and high-thermal-conductivity insulation materials enhance overall heat dissipation.
