GIS Testing Solution

Gas-insulated switchgear is designed for substations where high voltage, limited installation space and long-term operating reliability are all important considerations. Unlike conventional air-insulated switchgear, the main energized components of GIS are enclosed inside grounded metal compartments and insulated by gas or gas mixtures.

This construction provides a compact installation, but it also changes the way testing and commissioning are performed.

Once the GIS enclosure has been assembled, many critical components are no longer directly accessible. A small conductive particle, a damaged spacer, an incorrect connection, insufficient gas quality or a defect introduced during installation may remain hidden inside the enclosure.

For a GIS project, commissioning is therefore not simply a matter of applying a high voltage and checking whether the equipment trips.

The testing process needs to answer several separate questions:

• Can the GIS insulation withstand the specified test voltage?
• Is there any localized partial discharge?
• Are the circuit breakers operating within their specified mechanical limits?
• Is the main current path electrically sound?
• Is the insulating gas at the required purity, moisture level and pressure?
• Has assembly or transportation introduced contamination or mechanical damage?
• Is the complete GIS installation ready for energization?

UHV provides testing equipment and integrated solutions covering AC Withstand, Partial Discharge, Circuit Breaker Analysis, Contact Resistance, SF6 Gas Testing and GIS Commissioning.

The equipment can be configured for GIS manufacturers, power utilities, EPC contractors, substation commissioning companies, testing laboratories and electrical maintenance teams.

The objective is to move from individual instrument testing toward a structured process:

GIS Assembly → Electrical Inspection → Gas Inspection → Mechanical Testing → Contact Resistance → Insulation Testing → Partial Discharge → AC Withstand → Commissioning

1. Testing Background

GIS combines several high-voltage components inside one metal-enclosed system.

A typical GIS bay may contain:

• Circuit breaker
• Disconnector
• Earthing switch
• Busbar
• Current transformer
• Voltage transformer
• Surge arrester
• Cable termination
• Bushing
• Gas compartment
• Insulating spacers

The internal arrangement is compact, which is one of GIS's major advantages. However, the same compact structure means that an installation error can have consequences that are difficult to identify through visual inspection alone.

Installation Can Introduce New Defects

Even when every GIS component has passed factory testing, the final assembled system may be affected by:

• Dust
• Metallic particles
• Incorrect flange assembly
• Damaged O-rings
• Incorrect contact installation
• Poor grounding
• Loose connections
• Improper gas filling
• Moisture contamination
• Incorrect circuit breaker adjustment
• Transportation-related mechanical changes

For this reason, factory acceptance testing and site commissioning testing have different roles.

Factory testing verifies the manufactured equipment.

Site commissioning verifies the assembled GIS system.

Why Partial Discharge Matters

GIS insulation defects can be small enough that the equipment continues to withstand normal operating voltage while localized discharge activity is already present.

A metallic particle attached to an insulating spacer may not immediately cause breakdown. Under electrical stress, however, it can generate repeated discharge pulses.

Over time, this activity can damage the insulation structure and eventually develop into a more serious insulation failure.

That is why PD testing is particularly valuable during GIS commissioning.

Why SF6 Testing Matters

For SF6-insulated GIS, gas quality is directly related to insulation and switching performance.

Important gas parameters can include:

SF6 purity

Moisture / dew point

Decomposition products

Gas pressure or density

Leakage

IEC 60376:2018 specifies the quality requirements for technical-grade SF6 and complementary gases used in electrical equipment, while IEC 62271-4:2022 addresses gas handling during installation, commissioning, repair, operation and end-of-life activities.

2. GIS Types & Testing Objects

GIS testing requirements vary according to voltage level, configuration, insulation medium and application.

Main Testing Objects

A GIS commissioning program can cover:

• Primary Circuit
• Busbars
• Circuit breaker main contacts
• Disconnectors
• Earthing switches
• CTs
• VTs
• Cable compartments
• Transformer interfaces
• Insulation System
• Gas insulation
• Epoxy spacers
• Bushings
• Cable terminations
• Internal conductor insulation
• Gas compartments
• Mechanical System
• Circuit breaker mechanism
• Disconnector mechanism
• Earthing switch
• Interlocks
• Operating mechanisms
• Gas System
• SF6 purity
• Moisture
• Decomposition products
• Density
• Pressure
• Leakage

This is why GIS commissioning should be treated as a system test, not a single-device test.

3. Testing Requirements

A practical GIS commissioning plan normally starts before the first high-voltage test.

3.1 Documentation Review

Before testing, the commissioning team should collect:

• GIS single-line diagram
• General arrangement drawings
• Gas compartment layout
• Circuit breaker specifications
• Rated voltage
• Rated current
• Short-circuit rating
• Gas filling specification
• Factory test reports
• Assembly records
• Previous test results
• Manufacturer commissioning instructions

The factory test report is particularly important because it provides a reference for site measurements.

3.2 Mechanical Inspection

Before energization, inspect:

• Circuit breaker mechanism
• Disconnector
• Earthing switch
• Interlocks
• Local/remote control
• Mechanical position indicators
• Gas compartment valves
• Grounding connections
• Cable termination
• Bushing connections

The purpose is to identify assembly problems before electrical testing begins.

3.3 SF6 Inspection

Where SF6 is used, check:

• Gas pressure
• Gas density
• Purity
• Moisture
• Decomposition products where required
• Leakage

Gas handling should follow the equipment manufacturer's instructions and applicable gas-handling requirements. IEC 62271-4:2022 specifically covers handling procedures for gases used for insulation and/or switching during installation and commissioning.

3.4 Electrical Testing

The electrical testing program can include:

• Contact resistance
• Circuit breaker timing
• Circuit breaker travel
• Insulation measurements
• AC withstand
• Partial discharge
• Secondary circuit checks
• Interlock verification

Not every GIS project requires exactly the same test sequence. The final procedure should be established from the GIS manufacturer's commissioning instructions, project specification and applicable standard.

4. Testing Methods

4.1 AC Withstand Test

AC withstand testing is one of the principal dielectric tests used to verify the insulation system of GIS.

The test applies a specified AC voltage to the GIS primary circuit while monitoring the test system for abnormal current, discharge or breakdown.

The purpose is straightforward:

Verify that the assembled GIS insulation system can withstand the specified electrical stress.

For GIS, the challenge is that the test object can have substantial capacitance.

A conventional high-voltage transformer can therefore require significant reactive power.

An AC Resonant Test System can be used to address this problem.

The system typically consists of:

Variable-Frequency Power Supply → Excitation Transformer → High-Voltage Reactor → GIS

When the inductive reactance and capacitive reactance approach resonance, reactive energy circulates between the reactor and GIS.

This allows a high test voltage to be generated while the active power requirement of the source is reduced.

UHV describes AC resonant systems specifically for GIS switchgear testing, including on-site commissioning and high-voltage testing of large-capacitance equipment.

AC Withstand Testing Can Be Used For

• GIS factory testing
• Site commissioning
• High-voltage insulation verification
• Post-maintenance testing
• Major assembly verification

The test voltage and duration must be taken from the applicable test specification rather than using one generic value for every GIS.

4.2 Partial Discharge Testing

Partial discharge testing is one of the most useful diagnostic tests for GIS.

A PD event is a localized electrical discharge that does not completely bridge the insulation between conductors.

Inside GIS, possible PD sources include:

• Metallic particles
• Protrusions
• Loose conductive particles
• Spacer defects
• Poor contact structures
• Surface contamination
• Floating electrodes
• Insulation defects

Why PD Testing Is Valuable During Commissioning

A GIS may pass a basic insulation test while still containing a localized defect that produces discharge activity at operating voltage.

PD testing can therefore add information that a simple withstand test cannot provide.

The important parameters include:

• PD magnitude
• PD inception voltage
• PD extinction voltage
• Phase-resolved pattern
• Pulse repetition
• Background noise
• Location information where available
• The test environment is critical.

External interference can come from:

• Nearby switching equipment
• Radio signals
• Ground loops
• Poor shielding
• Control cables
• Corona
• Test equipment

A reliable GIS PD system must therefore combine sensitivity with effective interference suppression and signal analysis.

UHV positions partial-discharge equipment as a diagnostic technology for medium- and high-voltage equipment and identifies GIS among applications where early insulation defects need to be detected.

4.3 Circuit Breaker Testing

The circuit breaker is one of the most mechanically active components in GIS.

A breaker can have excellent insulation and low contact resistance while still suffering from a mechanical timing problem.

Circuit breaker analysis therefore focuses on dynamic operation.

Typical measurements include:

• Opening time
• Closing time
• Phase synchronization
• Contact bounce
• Reclosing sequence
• Opening-closing operation
• Closing-opening operation
• Travel
• Speed
• Overtravel
• Rebound
• Coil current
• Operating voltage

UHV's circuit breaker analyzer is designed for SF6 switches, GIS combined electrical equipment, vacuum switches and oil switches, with functions for opening/closing timing, travel measurement and waveform/data storage.

Why Timing Matters

If the three phases do not operate within the specified synchronization window, switching transients can increase and the breaker may not perform as intended.

A commissioning test should therefore compare measured timing against the manufacturer's specified values rather than relying on generic limits.

4.4 Contact Resistance

Contact resistance testing measures the resistance of the main conductive path.

A low resistance is expected when the GIS primary circuit is correctly assembled and the main contacts are in the appropriate closed position.

An abnormal result can indicate:

• Poor contact
• Loose connection
• Incorrect assembly
• Oxidation
• Damaged contact surface
• Mechanical contact pressure problem
• Internal connection issue

The measurement is generally performed using a high-current DC source.

The test current is passed through the contact path while the voltage drop is measured.

Because the resistance of GIS primary contacts can be in the micro-ohm range, the test instrument must have suitable current output and measurement resolution.

UHV's contact-resistance documentation provides circuit-breaker loop-resistance examples expressed in μΩ, illustrating the low-resistance range involved in such measurements.

4.5 SF6 Gas Testing

For SF6 GIS, gas testing should not be reduced to a single "SF6 purity" measurement.

A useful gas condition assessment can include:

SF6 Purity

Determines whether the gas composition meets the required specification.

Dew Point / Moisture

Excess moisture can affect insulation performance and may also indicate problems in gas filling or enclosure sealing.

UHV's SF6 dew-point equipment measures dew point and moisture-related parameters and is designed for field gas testing.

Decomposition Products

Following electrical arcing or abnormal thermal activity, SF6 can produce decomposition products.

Depending on the test equipment and project requirements, parameters can include:

SO₂

H₂S

HF

Other decomposition products

These measurements can provide information about abnormal switching or internal fault activity.

Gas Pressure / Density

The GIS gas system must remain within the specified operating range.

Leakage

Gas leakage should also be investigated because loss of gas pressure can eventually affect the insulation and switching environment.

IEC 60480:2019 provides criteria for reuse of SF6 and SF6 mixtures after recovery/reclaiming, while IEC 60376:2018 covers technical-grade SF6 and complementary gases for electrical equipment.

4.6 GIS Commissioning

GIS commissioning combines the individual tests into one controlled sequence.

A typical commissioning structure is:

Documentation Review

↓

Visual & Mechanical Inspection

↓

Gas Inspection

↓

Circuit Breaker Test

↓

Contact Resistance

↓

Insulation Checks

↓

Partial Discharge

↓

AC Withstand

↓

Interlock & Control Verification

↓

Final Gas Check

↓

Commissioning Report

The exact order can change according to the GIS manufacturer's instructions and project test procedure.

The key point is that high-voltage testing should not be the first test performed on a newly assembled GIS.

5. Recommended Equipment

UHV can configure the GIS testing system according to the voltage level, GIS configuration and commissioning requirements.

For large GIS projects, the recommended approach is to create an equipment package rather than selecting one instrument in isolation.

Typical UHV GIS Test Package

• AC Resonant Test System
• Partial Discharge Test System
• Circuit Breaker Analyzer
• Contact Resistance Tester
• SF6 Comprehensive Analyzer
• SF6 Dew Point Meter
• SF6 Leakage Detector

This combination covers the main dielectric, mechanical, conductive and gas-related commissioning requirements.

6. Testing Procedure

Step 1 - Review GIS Documentation

Confirm:

• Rated voltage
• Rated current
• Gas pressure
• Circuit configuration
• Test voltage
• Test duration
• Breaker specifications
• Contact resistance limits
• PD acceptance criteria
• Gas quality requirements

Step 2 - Confirm Assembly

Check:

• Flanges
• Spacers
• Busbar connections
• Grounding
• Cable connections
• Transformer connections
• CT/VT interfaces
• Mechanical interlocks

Particular attention should be paid to foreign objects and metallic particles inside accessible assembly areas.

Step 3 - Check SF6 Condition

Record:

• Pressure
• Density
• Purity
• Dew point
• Decomposition products where required

If gas quality is abnormal, the problem should be addressed before high-voltage testing.

Step 4 - Circuit Breaker Test

Perform:

Close → Open → Reclose → Open-Close-Open

according to the manufacturer's specified operating sequence.

Record:

• Opening time
• Closing time
• Phase synchronization
• Travel
• Speed
• Coil current
• Operating voltage

Step 5 - Contact Resistance Test

Close the main circuit breaker or appropriate switching path and inject high DC current.

Record resistance for each phase.

Compare:

Phase A ↔ Phase B ↔ Phase C

and compare with the factory record where available.

Step 6 - Partial Discharge Test

Establish the test circuit and measure background interference first.

Then:

• Apply the specified test voltage
• Monitor PD activity
• Record phase-resolved patterns
• Determine PD inception/extinction behavior where required
• Identify abnormal locations if the system supports localization

Step 7 - AC Withstand

Connect the resonant test system to the GIS according to the approved test configuration.

The general sequence is:

Low Voltage → Controlled Voltage Increase → Test Voltage → Hold → Controlled Reduction → Discharge → Ground

The test should be stopped immediately if abnormal discharge, breakdown or protection action occurs.

Step 8 - Interlock and Control Test

Verify:

• Electrical interlock
• Mechanical interlock
• Local control
• Remote control
• Breaker indication
• Disconnector indication
• Earthing switch indication
• Alarm functions

Step 9 - Final Gas Check

After testing and any necessary gas handling, confirm that the GIS gas compartments remain within the specified operating condition.

Step 10 - Commissioning Report

The final report should include:

• GIS identification
• Bay number
• Test equipment
• Calibration information
• Environmental conditions
• Gas parameters
• Circuit breaker data
• Contact resistance
• PD results
• AC withstand results
• Interlock results
• Abnormal observations
• Final acceptance conclusion

7. Applicable Standards

The exact standards depend on GIS voltage class, gas technology, test purpose and project specification.

IEC 62271-203

IEC 62271-203:2022 - AC gas-insulated metal-enclosed switchgear for rated voltages above 52 kV

This is one of the key standards for GIS. The current edition applies to AC gas-insulated metal-enclosed switchgear above 52 kV and up to 60 Hz. The 2022 edition also accommodates gases and gas mixtures beyond SF6 where applicable.

The standard addresses requirements including gas insulation, interfaces, gas tightness and testing.

IEC 62271-100

For the circuit breaker component, IEC 62271-100:2021, with its 2024 amendment, covers AC circuit breakers for systems above 1 kV and 50/60 Hz.

This is particularly relevant when defining:

Opening/closing tests

Mechanical characteristics

Breaking performance

Circuit breaker test requirements

IEC 62271-4

For gas handling, IEC 62271-4:2022 specifies procedures for gases used for insulation and/or switching during installation, commissioning, repair, operation and end-of-life activities.

IEC 60376

IEC 60376:2018 covers technical-grade SF6 and complementary gases used in mixtures for electrical equipment and includes gas quality requirements and relevant detection approaches.

IEC 60480

IEC 60480:2019 addresses criteria for reuse of SF6 and SF6 mixtures after recovery and reclaiming from electrical equipment.

IEEE / ANSI Standards

Projects in North America may also reference the relevant IEEE/ANSI standards for:

• GIS
• Circuit breakers
• High-voltage testing
• Partial discharge
• Switchgear commissioning

The final test procedure should always identify the exact standard edition and project acceptance criteria.

8. Test Parameters

GIS testing involves several different parameter groups.

AC Withstand

The required test voltage should be based on:

• Rated voltage
• Insulation level
• GIS manufacturer's test specification
• IEC/IEEE requirement
• Site commissioning procedure

Partial Discharge

Do not use one universal PD acceptance value for every GIS.

The evaluation should consider:

• GIS voltage class
• Test arrangement
• Background noise
• Measurement bandwidth
• PD source
• Location
• Factory reference data
• Applicable standard

Circuit Breaker

The acceptable opening and closing times are manufacturer-specific.

For this reason, the most useful comparison is:

Measured Result ↔ Factory Test Result ↔ Manufacturer Limit

rather than a generic internet value.

Contact Resistance

Contact resistance is normally measured in the micro-ohm range.

The important comparison is:

Phase Balance + Factory Reference + Manufacturer Specification

9. Common Testing Problems

Problem 1 - GIS Fails AC Withstand

Possible causes include:

• Metallic particle
• Damaged spacer
• Poor assembly
• Contamination
• Incorrect test connection
• Insufficient clearance
• Internal insulation defect

Do not immediately assume that the GIS itself is defective.

The test circuit and assembly condition should first be checked.

Problem 2 - PD Is Detected but AC Withstand Passes

This situation is technically important.

A GIS can withstand the specified test voltage while still showing localized PD.

The next step should be to determine:

• PD magnitude
• PD pattern
• PD location
• Inception voltage
• Repetition behavior

If the source is confirmed as internal GIS discharge, the relevant compartment may need to be opened and inspected according to the manufacturer's procedure.

Problem 3 - High PD Background

Typical sources:

• Poor grounding
• External electromagnetic interference
• Corona
• Test cable coupling
• Nearby high-voltage equipment
• Inadequate shielding

Measure background before applying the full test voltage.

Problem 4 - High Contact Resistance

Check:

• Main contact position
• Connection torque
• Contact pressure
• Internal contact condition
• Test lead connections
• Grounding arrangement

Compare all three phases rather than looking at only one value.

Problem 5 - Circuit Breaker Timing Is Abnormal

Possible causes include:

• Mechanism adjustment
• Lubrication problem
• Hydraulic/pneumatic pressure
• Spring mechanism condition
• Coil problem
• Mechanical resistance
• Position sensor installation

Travel and speed curves can provide more information than opening time alone.

Problem 6 - SF6 Moisture Is Too High

Possible causes include:

• Improper gas filling
• Moisture remaining after assembly
• Poor evacuation
• Moisture ingress
• Long-term leakage
• Sampling contamination

The gas system and sampling procedure should be checked before deciding that the GIS enclosure has a moisture problem.

Problem 7 - SF6 Purity Is Acceptable but Decomposition Products Are High

This can indicate that gas composition alone is not sufficient to evaluate the condition of the compartment.

Decomposition products can provide additional information about previous arcing or abnormal switching conditions.

10. Equipment Selection Guide

GIS equipment selection should start from the test object, not from the product catalog.

For GIS Commissioning

Recommended:

• Circuit Breaker Analyzer
• Contact Resistance Tester
• SF6 Gas Analyzer
• SF6 Dew Point Meter
• AC Withstand / AC Resonant System
• PD Test System

This is the most complete configuration for a high-voltage GIS commissioning project.

For GIS Factory Testing

Priority should be given to:

• AC withstand capacity
• PD sensitivity
• Measurement repeatability
• Automated data acquisition
• Test report generation
• Long-duration operation

For Site Commissioning

Priority should be given to:

• Portability
• Modular design
• Fast connection
• Field power requirements
• Safety interlock
• Gas sampling
• Noise suppression
• Automatic test records

For GIS Maintenance

A maintenance team may not need a full AC resonant system for every project.

A practical diagnostic package may include:

PD Tester + Circuit Breaker Analyzer + Contact Resistance Tester + SF6 Analyzer + Dew Point Meter

The AC withstand system can then be brought in when a specified dielectric test is required.

For High-Voltage / EHV GIS

For higher-voltage GIS, equipment selection becomes more project-specific.

The supplier needs:

• GIS rated voltage
• Test voltage
• GIS capacitance
• Number of compartments
• Test object configuration
• Required frequency
• PD requirement
• Site power supply
• Transport limitations

UHV's AC resonant technology is intended for large-capacitance high-voltage equipment, including GIS, and can be configured for factory or site testing.

11. Case Studies

Case Study 1 - GIS Site Commissioning

A newly installed high-voltage GIS bay requires complete commissioning before connection to the substation.

The UHV test sequence is configured as:

Gas Test

↓

Circuit Breaker Test

↓

Contact Resistance

↓

PD Test

↓

AC Withstand

↓

Interlock Verification

↓

Final Gas Check

The advantage of this sequence is that basic mechanical and gas conditions are verified before the GIS is subjected to the high-voltage test.

Case Study 2 - GIS AC Resonant Testing

A large GIS installation presents a substantial capacitive load.

A conventional AC test source would require a high reactive power capacity.

UHV configures:

Variable Frequency Power Supply + Excitation Transformer + HV Reactor

The reactor is selected according to the GIS capacitance and required test voltage.

At resonance, the system can generate the required high voltage with a substantially reduced active-power requirement.

UHV specifically identifies GIS switchgear among the applications of its high-voltage AC resonant test systems.

Case Study 3 - GIS PD Diagnosis

During commissioning, a GIS passes preliminary insulation checks but shows abnormal PD activity during the diagnostic test.

The engineering team compares:

• Background noise
• PD inception voltage
• PRPD pattern
• Discharge amplitude
• Phase relationship
• Measurement point

The result indicates that further investigation is required rather than treating the GIS as automatically acceptable simply because it passed a withstand check.

This demonstrates why PD and AC withstand answer different technical questions.

Case Study 4 - Circuit Breaker Maintenance

A GIS circuit breaker has developed inconsistent opening behavior during periodic maintenance.

UHV's circuit breaker analyzer records:

Opening time

Closing time

Phase synchronization

Travel curve

Speed

Coil current

The maintenance team compares the result with the original factory record.

A change in the travel curve can provide useful information about the mechanism even when the final breaker opening time remains within the specified range.

12. FAQ

13. Related Products

AC Resonant Test System

For high-voltage AC withstand testing of GIS and other large-capacitance electrical equipment.

UHV's resonant systems use variable-frequency power supply, excitation transformer and high-voltage reactor technology for high-voltage testing applications.

Partial Discharge Test System

For GIS insulation diagnostics and detection of localized discharge activity.

Circuit Breaker Analyzer

For opening/closing time, phase synchronization, travel, speed, coil current and mechanical characteristic testing. UHV provides a circuit breaker tester suitable for GIS and SF6 switch applications.

Contact Resistance Tester

For low-resistance measurement of circuit breaker and GIS primary contacts.

SF6 Comprehensive Analyzer

For SF6 purity and decomposition-related gas measurements.

SF6 Dew Point Meter

For moisture and dew-point measurement of GIS gas.

SF6 Gas Leakage Detector

For identifying gas leakage from GIS compartments, valves, joints and connection points.

SF6 Density Relay Tester

For checking SF6 density relay operating characteristics.

SF6 Recovery & Refilling Device

For controlled gas recovery, evacuation, filling and related maintenance operations.

High Voltage Tester

For AC high-voltage withstand applications where the required test object and capacity are suitable.

UHV's solution portfolio includes high-voltage AC testing, partial discharge equipment, circuit breaker analyzers and primary current injection equipment for substation testing and commissioning.

14. Request a Quote

Need a GIS Testing Solution?

GIS testing should be configured around the actual switchgear rather than selecting separate instruments from a standard catalog.

Send UHV the following information:

GIS Rated Voltage → Rated Current → GIS Configuration → Number of Bays → Required Test Voltage → GIS Capacitance → PD Requirement → SF6 Gas Type → Circuit Breaker Type → Applicable Standard → Factory/Site Application

UHV can then configure the appropriate testing package, including:

• AC Resonant Test System
• Partial Discharge Test System
• Circuit Breaker Analyzer
• Contact Resistance Tester
• SF6 Comprehensive Analyzer
• SF6 Dew Point Meter
• SF6 Leakage Detector
• SF6 Density Relay Tester

Typical UHV GIS Commissioning Solution

GIS Assembly

↓

Mechanical Inspection

↓

SF6 Purity & Dew Point

↓

Circuit Breaker Mechanical Test

↓

Contact Resistance

↓

Partial Discharge

↓

AC Withstand

↓

Interlock & Control Verification

↓

Final Gas Inspection

↓

Commissioning Report

For high-voltage and EHV GIS projects, UHV can evaluate the required AC resonant system according to the GIS capacitance, test voltage, test frequency and site power conditions.

Request a Quote for Your GIS Testing Solution.