We perform this test according to IEC 62271-203 and IEC 60060. For a GIS, the basic insulation level generally corresponds to,
33 kV System
| Item | Typical IEC Value |
|---|---|
| Rated voltage | 36 kV |
| Power frequency withstand voltage | 70 kV rms |
| Lightning impulse withstand voltage | 170 kV peak |
| Switching impulse withstand voltage | Not applicable |
132 kV System
| Item | Typical IEC Value |
|---|---|
| Rated voltage | 145 kV |
| Power frequency withstand voltage | 275 kV rms |
| Lightning impulse withstand voltage | 650 kV peak |
| Switching impulse withstand voltage | Not applicable |
220 kV System
| Item | Typical IEC Value |
|---|---|
| Rated voltage | 245 kV |
| Power frequency withstand voltage | 460 kV rms |
| Lightning impulse withstand voltage | 1050 kV peak |
| Switching impulse withstand voltage | 850 kV peak |
400 kV System
| Item | Typical IEC Value |
|---|---|
| Rated voltage | 420 kV |
| Power frequency withstand voltage | 680 kV rms |
| Lightning impulse withstand voltage | 1425 kV peak |
| Switching impulse withstand voltage | 1050 kV peak |
Since it is three-phase encapsulated, the dielectric test procedure differs slightly from single-phase encapsulated GIS.
Procedure for Power Frequency Voltage withstand Test
Before testing, we assemble all GIS compartments, to ensure the test is conducted on the final electrical geometry. Then we fill SF6 gas to rated pressure. SF6 acts as the main insulation medium at the prescribed pressure. Therefore, incorrect pressure may affect the dielectric strength. The next step is earthing. This is extremely important. Here, we solidly earth all the metal enclosures. Also, we connect the conducting portion of all the non-tested phases to the ground. Then, we short circuit and earth all the CT secondaries of the GIS. The earthing arrangements prevent voltage induction on the non tested parts during the high voltage dielectric test.
Also, we need to disconnect surge arresters, CVTs/VTs, and electronic sensors. Because their insulation level may be lower than GIS insulation level. Hence, if they remain connected, they may fail before GIS insulation before completion of dielectric test of GIS.
If inspection windows exist, we need to cover accessible side with grounded metallic foil. This arrangement prevents abnormal electric field concentration.
Test Circuit Connection
Now, we connect the HV testing equipment. For the purpose of test voltage application we use a high voltage test transformer. Also we fit partial discharge detector for PD monitoring and measuring during the test. Example:
Connection Arrangement
| Part | Condition |
|---|---|
| Tested Phase | Energized |
| Other Phases | Grounded |
| Enclosure | Grounded |
| CT Secondaries | Grounded |
Applied Voltage
For 33 kV, 132 kV, 220 kV, and 400 kV GIS, we apply the power frequency withstand voltage corresponds to rated voltage level. We gradually rise the voltage from zero to rated power frequency withstand voltage and stay there for 1 minute and then decrease to zero again. During 1 minute test duration we observe, leakage current, sound, flashover and PD activity.
Acceptance Criteria
GIS passes the test if there is no flashover, no puncture, no collapse of voltage, and no abnormal discharge observed during the test.
Lightning Impulse Test of GIS
This test simulates lightning surges. We apply the standard impulse of 1.2/50 μs wave form. This means the impulse voltage takes 1.2 μs to rise from zero to its peak value. 50 μs is the instant when the voltage falls to 50% of the peak value on the tail portion.
Applied Voltage
The peak value of the 1.2/50 μs impulse waveform shall be applied according to the standard values specified in the relevant IEC standards listed in the tables above.
Connection Arrangement
| Part | Condition |
|---|---|
| Tested phase | Energized |
| Other phases | Grounded |
| Enclosure | Grounded |
| CT Secondaries | Grounded |
Impulse Application Sequence
Typically, we apply 5 positive full waves and 5 negative full waves.
Acceptance Criteria
The GIS passes if we observe on oscillogram, no insulation breakdown, no flashover, no waveform collapse. In other words the oscillogram remains normal.
Tests Across Open Contacts
Now we repeat the test across the open contacts of the circuit breaker and open contacts of disconnectors. This ensures dielectric stability across contact gaps during lightning surges.
Partial Discharge (PD) Test
This test follows IEC 62271-203 Clause 6.2.101. Measurement partial discharge detects microscopic insulation defects.
PD Test Voltage
Typically, we use PD test voltage as\[U_{PD}=1.2\times\frac{U_r}{\sqrt{3}}\]Where Ur is the rated voltage of the GIS.
PD Measurement Process
We apply the voltage gradually. PD detector measures, apparent charge, pulse magnitude, and pulse repetitions. It shows partial discharge in pC (pico-coulomb). If the measured value is under 5 pC generally we accept.
Important IEC Clauses
| Requirement | IEC Clause |
|---|---|
| Dielectric tests | IEC 62271-203 Clause 6.2 |
| Test conditions | Clause 6.2.3 |
| Test voltage application | Clause 6.2.5 |
| PD test | Clause 6.2.101 |
| Impulse methods | IEC 60060 |
| Common requirements | IEC 62271-1 |