What Insulation Tests Are Required for 12kV Ring Main Units?

The power frequency withstand voltage test verifies the RMU's main insulation capability under AC power frequency voltage. Per GB/T 11022, the 12kV RMU must undergo the following 4 sub-tests:

Phase-to-Phase and Phase-to-Ground Power Frequency Withstand Voltage (42kV/1min)

During this test, the switch is in the closed position. A 42kV AC voltage is applied to each phase (A/a, B/b, C/c) sequentially for 1 minute, while the remaining phases and the metal frame are grounded. Below is the actual type test data:

Circuit Breaker Gap Power Frequency Withstand Voltage (42kV/1min)

The circuit breaker is in the open position. A 42kV AC voltage is applied to the incoming side (A, B, C) and outgoing side (a, b, c) breaker gaps sequentially for 1 minute each. This verifies the dielectric strength across the open breaker contacts.

Isolating Gap Power Frequency Withstand Voltage (48kV/1min)

The isolating switch is in the open position. A 48kV AC voltage is applied across the isolating gap for 1 minute. The isolating gap test voltage is 14% higher than the phase-to-ground value (48kV vs. 42kV), reflecting the enhanced safety requirement for visible isolation gaps that protect maintenance personnel.

Auxiliary and Control Circuit Power Frequency Withstand Voltage (2kV/1min)

Although auxiliary and control circuits (secondary circuits) are not part of the main current path, their insulation integrity is critical for protection system reliability. A 2kV AC voltage is applied for 1 minute to verify secondary circuit insulation. While the voltage level seems low, secondary circuit insulation failure can cause protection device maloperation or refusal to operate, leading to serious incidents.

The lightning impulse withstand voltage test simulates the effect of lightning overvoltage surges on RMU insulation. It uses the standard lightning impulse wave of 1.2/50μs (wavefront time 1.2μs ±30%, wavetail time 50μs ±20%). Each test point receives 15 positive and 15 negative polarity impulses - no flashover or breakdown is permitted.

Phase-to-Phase and Phase-to-Ground Lightning Impulse Withstand Voltage (75kV)

The switch is in the closed position. A 75kV lightning impulse is applied to each phase sequentially. Below is the actual test data:

Circuit Breaker Gap Lightning Impulse Withstand Voltage (75kV)

With the circuit breaker open, a 75kV lightning impulse is applied across the breaker gap to verify its insulation capability under lightning surge conditions.

Isolating Gap Lightning Impulse Withstand Voltage (85kV)

With the isolating switch open, an 85kV lightning impulse is applied across the isolating gap. The isolating gap lightning impulse voltage is 13% higher than the phase-to-ground value (85kV vs. 75kV), consistent with the same pattern seen in power frequency testing.

Notation: A, B, C = incoming side three phases; a, b, c = outgoing side three phases; F = metal frame. Both positive and negative polarities were applied, 15 impulses each, totaling 30 impulses per test point.

The partial discharge (PD) test is the key test for evaluating long-term insulation reliability of the RMU. By measuring partial discharge at 1.1 times the rated voltage, latent defects in insulation materials - such as internal voids, cracks, and impurities - can be detected. These defects may not surface during power frequency or impulse withstand tests but can progressively deteriorate over years of operation, ultimately leading to insulation failure.

Partial Discharge Limits by Insulation Type

Different insulation technologies have different PD limits:

Actual Partial Discharge Test Data

Test conditions: Atmospheric pressure 100.9kPa, dry-bulb temperature 20.0°C, wet-bulb temperature 16.0°C, relative humidity 63.8%, atmospheric correction factor K=0.996, gas chamber relative pressure 0.04MPa, background noise 0.8pC. Technical requirement: PD ≤20pC. Pre-stress voltage: 15.6kV, duration ≥10s, test voltage: 13.2kV (i.e., 1.1 × 12kV rated voltage).

Phase-to-Phase and Phase-to-Ground Partial Discharge

Circuit Breaker Gap Partial Discharge

Isolating Switch Gap Partial Discharge

Notably, the insulation resistance test was not listed in the above type test report conclusion. However, per GB 3906 and GB/T 11022, insulation resistance testing is a mandatory routine test and site acceptance test item that must not be omitted.

Insulation Resistance Test Requirements

The insulation resistance test serves three critical purposes:

• Pre-test screening: Confirms insulation condition before power frequency withstand testing, preventing further damage to already compromised insulation from high-voltage application
• Post-transport acceptance: After long-distance shipping, RMU insulation may be affected by moisture or mechanical damage - insulation resistance testing is the first checkpoint at site commissioning
• Predictive maintenance: Periodic testing tracks insulation aging trends, enabling preventive maintenance before failure occurs

The insulation tests for 12kV ring main units are governed by the following standards:

Pre-Test reparation

• Remove non-withstand components: Surge arresters, potential transformers (PTs), electronic instruments, and other components that cannot withstand test voltages must be disconnected before testing to prevent damage
• Clean insulation surfaces: Ensure insulation surfaces are clean and dry - surface contamination can cause flashover during testing
• Verify gas chamber pressure: For gas-insulated RMUs, confirm SF₆ gas pressure is within the rated range before testing
• Safety grounding: Set up safety barriers around the test area and ensure all non-energized parts are reliably grounded

• Focus on isolating gaps: Isolating gap test voltages are higher than phase-to-ground values (48kV vs. 42kV, 85kV vs. 75kV). Improper assembly of isolating switches is a common failure mode - inspect carefully
• PD test environment: The ≤20pC limit requires a high-sensitivity PD detector with electromagnetic shielding. Background noise should be below 1pC
• Atmospheric correction: Record atmospheric pressure, temperature, and humidity during testing to calculate correction factor K. Altitude correction is additionally required for installations above 1000m
• Bipolarity impulse testing: Lightning impulse tests must include both positive and negative polarities (15 each). In SF₆ gas insulation, negative polarity breakdown voltage is typically lower - pay special attention to negative polarity results

Insulation testing is the most critical quality verification step for 12kV ring main units before factory dispatch and commissioning. It covers power frequency withstand voltage (4 items), lightning impulse withstand voltage (3 items), and partial discharge (1 item) - totaling 8 test items, supplemented by the mandatory insulation resistance test, forming a comprehensive insulation performance verification system.

For purchasers and project engineers, key points to verify during selection and acceptance include:

• The type test report covers all 8 insulation test items with "Passed" conclusions
• Actual partial discharge values are well below the limit (premium products ≤5pC; compliant products ≤20pC)
• Isolating gap test voltages meet the enhanced requirements of 48kV/85kV
• Factory and site acceptance tests include insulation resistance measurement

Selecting a 12kV ring main unit that has passed comprehensive insulation testing is the first step toward ensuring safe and reliable power distribution network operation. For more technical specifications, type test reports, or custom solutions, contact us for professional support.