Substation Testing Solution

A substation is not commissioned by testing one piece of equipment at a time and checking whether every instrument produces a value within its range. A complete substation is an interconnected electrical system. Transformers, GIS, circuit breakers, CTs, PTs, cables, relay protection, batteries and grounding systems all have different operating functions, but their performance is closely related once the substation is energized.

A transformer may have acceptable insulation resistance while a CT secondary circuit is incorrectly connected. A circuit breaker may have good contact resistance but incorrect operating timing. A cable may pass a withstand test while its sheath has been damaged during installation. A protection relay may pass an individual functional test but still fail to operate correctly because of incorrect CT polarity or secondary wiring.

For this reason, UHV approaches substation testing from the perspective of the complete electrical system.

UHV provides testing equipment and integrated solutions for:

• Transformer Testing
• GIS Testing
• Circuit Breaker Testing
• Relay Protection Testing
• CT/PT Testing
• Cable Testing
• Battery Testing
• Grounding Testing

The equipment can be configured for substation construction, commissioning, factory acceptance, site acceptance testing, preventive maintenance, troubleshooting and periodic inspection.

A typical UHV substation testing program can be structured as:

Equipment Inspection → Primary Equipment Testing → Secondary System Testing → DC System Testing → Cable Testing → Grounding Testing → Functional Verification → Energization Readiness

The exact sequence depends on the voltage level, substation architecture, equipment manufacturer and project specification.

1. Testing Background

Modern substations contain a combination of high-voltage equipment, protection and control systems, measurement transformers, communication interfaces and auxiliary power systems.

A typical high-voltage substation may include:

• Power transformers
• GIS or AIS
• Circuit breakers
• Disconnectors
• Earthing switches
• Current transformers
• Potential transformers
• Power cables
• Busbars
• Relay protection systems
• DC battery systems
• Chargers
• Grounding grids
• Control and monitoring equipment

Each section has its own testing requirements.

However, the most important commissioning question is not whether each individual device has passed its own test.

The question is:

Will the complete substation operate correctly when it is energized and exposed to an actual fault condition?

That requires coordination between primary and secondary systems.

Primary Equipment

Primary equipment carries or switches high voltage and high current.

Typical testing includes:

• Transformer testing
• GIS testing
• Circuit breaker testing
• CT/PT testing
• Cable testing
• Grounding testing

Secondary System

The secondary system measures electrical conditions and controls protection and switching.

Typical testing includes:

• Relay protection
• CT/PT secondary circuits
• Trip circuits
• Control circuits
• Alarm circuits
• Interlocks
• Communication interfaces

DC System

The station battery supplies power to protection, control, tripping and closing circuits when the AC auxiliary supply is unavailable.

Typical testing includes:

• Battery discharge
• Battery capacity
• Internal resistance
• Charger performance
• DC insulation
• DC voltage
• Cell balance

A failure in any one of these systems can affect the availability of the complete substation.

2. Substation Types & Testing Objects

UHV substation testing solutions can be configured for different substation architectures.

Testing Objects

A complete substation testing project can be divided into eight technical areas.

1. Transformer

Turns ratio

• Winding resistance
• Insulation resistance
• Tan delta
• Excitation current
• Oil BDV
• Partial discharge
• AC withstand where applicable

2. GIS

• AC withstand
• Partial discharge
• SF6 purity
• Dew point
• Decomposition products
• Circuit breaker characteristics
• Contact resistance

3. Circuit Breaker

• Opening time
• Closing time
• Phase synchronization
• Travel
• Speed
• Coil current
• Contact resistance

4. Relay Protection

• Current protection
• Voltage protection
• Differential protection
• Distance protection
• Overcurrent protection
• Directional protection
• Trip logic
• Protection sequence

5. CT/PT

• Ratio
• Polarity
• Excitation characteristics
• Winding resistance
• Insulation
• Burden
• CT saturation
• PT performance

6. Cable

• Insulation
• VLF withstand
• AC withstand
• Sheath
• Partial discharge
• Fault location

7. Battery

• Capacity
• Discharge
• Internal resistance
• Cell voltage
• DC voltage
• Charger performance

8. Grounding

• Ground resistance
• Ground grid continuity
• Ground conductor connection
• Step voltage
• Touch voltage where required

3. Testing Requirements

The testing requirements of a substation depend heavily on the project stage.

Factory Acceptance

Equipment manufacturers perform factory testing before shipment.

The objective is to verify that the equipment conforms to its approved design and contractual specifications.

Typical factory tests include:

• Transformer routine tests
• GIS routine tests
• Circuit breaker tests
• CT/PT tests
• Cable tests
• Relay functional tests
• Battery tests

Factory test records become important references during site commissioning.

Site Acceptance

Once equipment reaches the substation, transportation and installation can introduce new problems.

Typical risks include:

• Mechanical impact
• Incorrect wiring
• Loose terminals
• Moisture
• Incorrect grounding
• Damaged cable insulation
• Incorrect CT polarity
• Incorrect relay settings
• Circuit breaker mechanism adjustment problems

The site acceptance test therefore verifies the equipment after installation.

Commissioning

Commissioning goes one step further.

The objective is to verify that different systems work together.

For example:

CT → Relay → Trip Circuit → Circuit Breaker

must operate correctly as one protection chain.

Similarly:

PT → Relay → Measurement System

must provide the correct voltage information.

A transformer differential protection system also requires correct:

CT Ratio → CT Polarity → Secondary Wiring → Relay Settings → Trip Logic

A substation can therefore pass individual equipment tests and still fail an integrated functional test if the secondary system has been wired incorrectly.

Maintenance Testing

For operating substations, testing is generally based on:

• Previous test records
• Manufacturer recommendations
• Operating history
• Fault records
• Environmental conditions
• Equipment age
• Maintenance schedule

Trend analysis is particularly useful for transformers, batteries, circuit breakers and insulation systems.

4. Testing Methods

4.1 Transformer Testing

Transformer testing evaluates the electrical and insulation condition of the transformer.

UHV's transformer testing solutions cover:

Turns Ratio

Confirms the actual transformation ratio and vector relationship.

Winding Resistance

Identifies abnormal resistance differences between phases and potential problems with winding connections or tap changer contacts.

Insulation Resistance

Checks insulation between windings and between windings and ground.

Tan Delta

Evaluates dielectric losses and capacitance.

Excitation Current

Provides information about the transformer's magnetic circuit.

Oil BDV

Checks the dielectric strength of insulating oil.

Partial Discharge

Detects localized insulation defects.

AC Withstand

Verifies insulation withstand capability where required.

The individual tests provide different information and should be evaluated together.

4.2 GIS Testing

GIS testing covers both the insulation system and the mechanical switching components.

Main tests include:

• AC withstand
• Partial discharge
• SF6 purity
• SF6 dew point
• SF6 decomposition products
• Gas leakage
• Circuit breaker timing
• Contact resistance
• Interlock testing

For high-voltage GIS, UHV can configure AC resonant test systems based on GIS capacitance and required test voltage.

4.3 Circuit Breaker Testing

A circuit breaker has two distinct aspects that need to be verified.

Electrical Path

The closed breaker should have low and stable contact resistance.

Mechanical Operation

The mechanism must operate at the specified speed and timing.

UHV circuit breaker analyzers can record:

• Opening time
• Closing time
• Phase difference
• Travel
• Speed
• Coil current
• Operating sequence

This allows maintenance engineers to compare the actual operating curve with the manufacturer's reference data.

4.4 Relay Protection Testing

Protection testing is fundamentally different from primary equipment testing.

The objective is to simulate fault conditions and verify the complete protection response.

A relay protection test system can inject controlled:

• AC current
• AC voltage
• Frequency
• Phase angle
• Harmonic components
• Digital signals where supported

Typical protection functions include:

• Overcurrent
• Overvoltage
• Undervoltage
• Differential
• Distance
• Directional
• Zero-sequence
• Negative-sequence
• Frequency
• Transformer protection

The test should verify not only whether the relay operates, but also:

Pickup → Timing → Logic → Trip Output → Breaker Operation → Alarm/Indication

This is particularly important during final commissioning.

4.5 CT/PT Testing

CT and PT errors can directly affect protection and measurement accuracy.

CT Testing

Typical tests include:

• Ratio
• Polarity
• Winding resistance
• Excitation curve
• Saturation point
• Insulation
• Burden

CT saturation is particularly important for protection applications.

If the CT saturates prematurely during a fault, the secondary current waveform may no longer represent the primary fault current accurately.

This can affect differential, distance and overcurrent protection.

PT Testing

PT testing can include:

• Ratio
• Polarity
• Insulation
• Secondary voltage
• Burden
• Accuracy

Correct polarity and phase relationship are important when PT signals are used by directional and distance protection.

4.6 Cable Testing

Substation cable systems can include:

• MV power cables
• HV power cables
• Transformer cables
• GIS cable terminations
• Control cables
• Auxiliary power cables

For power cables, UHV can provide:

• VLF testing
• AC Hipot
• AC resonant testing
• Sheath testing
• PD testing
• Cable fault location

For long high-voltage cables, AC resonant systems can be selected according to cable capacitance and required test voltage.

4.7 Battery Testing

The station battery is often overlooked because it does not operate the primary circuit directly.

In an actual fault, however, the battery may be required to supply:

• Protection relays
• Circuit breaker trip coils
• Closing coils
• Control systems
• Communication equipment
• Emergency lighting
• Monitoring equipment

Battery testing can include:

• Discharge test
• Capacity test
• Internal resistance
• Cell voltage
• Total DC voltage
• Charger output
• DC insulation

UHV's battery discharge testers support different battery technologies and voltage systems, including VRLA, AGM, GEL and Ni-Cd applications, depending on model configuration.

4.8 Grounding Testing

The grounding system provides the low-impedance path required for fault current and helps control dangerous potential differences.

Grounding tests can include:

• Ground resistance
• Ground continuity
• Ground grid resistance
• Step voltage
• Touch voltage
• Ground conductor continuity

The measured value should be interpreted according to the substation design, soil conditions, grid geometry and applicable standard.

A low ground resistance alone does not guarantee that every part of the substation has acceptable touch and step voltage conditions.

5. Recommended Equipment

UHV can build a substation testing equipment package according to the project's voltage level and testing scope.

For a complete substation project, UHV can combine these into a project-specific test package.

6. Testing Procedure

A practical substation commissioning procedure can be organized into several stages.

Stage 1 - Documentation

Collect:

• Single-line diagram
• Protection schematic
• Wiring diagram
• Transformer data
• GIS data
• Breaker data
• CT/PT data
• Cable data
• Battery specifications
• Grounding design
• Factory test reports

Stage 2 - Primary Equipment

Test:

Transformer

→ Ratio
→ Winding Resistance
→ Insulation
→ Tan Delta
→ Oil
→ PD where required

GIS

→ Gas
→ Contact Resistance
→ PD
→ AC Withstand

Circuit Breaker

→ Timing
→ Travel
→ Coil Current
→ Contact Resistance

CT/PT

→ Ratio
→ Polarity
→ Excitation
→ Resistance
→ Insulation

Stage 3 - Cable System

Perform:

Insulation Resistance

→ Sheath Test

→ VLF / AC Test

→ PD / Diagnostic Test where required

Stage 4 - Secondary System

Test:

• Protection relay
• CT secondary
• PT secondary
• Trip circuit
• Closing circuit
• Control circuit
• Alarm
• Interlock
• Remote control

Stage 5 - DC System

Verify:

• Battery voltage
• Cell voltage
• Capacity
• Charger
• DC insulation
• DC distribution

Stage 6 - Grounding

Verify:

• Ground resistance
• Ground continuity
• Equipment grounding
• Grid connections
• Step/touch voltage where required

Stage 7 - Integrated Functional Test

This is one of the most important commissioning stages.

Simulate the relevant fault condition and verify:

Fault Simulation

→ CT/PT Signal

→ Relay Recognition

→ Protection Logic

→ Trip Output

→ Circuit Breaker Opening

→ Alarm / SCADA Indication

A complete chain test provides information that cannot be obtained from isolated relay or breaker testing.

Stage 8 - Final Energization Check

Before energization, confirm:

• All test results approved
• Temporary grounding removed where required
• Test leads removed
• Protection settings approved
• CT/PT circuits correct
• Breaker operating correctly
• Battery system available
• SF6 parameters acceptable
• Grounding system complete
• Interlocks functional

7. Applicable Standards

The applicable standards depend on the equipment and project.

Transformers

Relevant standards can include:

• IEC 60076 series
• IEEE C57 series
• Applicable GB/T and DL/T standards

GIS

IEC 62271-203 is a key standard for AC gas-insulated metal-enclosed switchgear above 52 kV. (webstore.iec.ch)

Circuit Breakers

IEC 62271-100 applies to AC circuit breakers above 1 kV. (webstore.iec.ch)

SF6

Relevant standards include:

• IEC 60376
• IEC 60480
• IEC 62271-4

IEC 62271-4:2022 addresses gas handling during installation, commissioning, repair, operation and end-of-life activities. (webstore.iec.ch)

Cables

Depending on cable type and voltage:

• IEC 60502
• IEC 60840
• IEC 62067
• IEEE 400 series

Protection

Protection testing may reference:

• IEC 60255 series
• IEEE standards
• Utility-specific protection requirements

Grounding

Grounding requirements may be based on:

• IEEE 80
• IEC requirements
• National standards
• Utility specifications

The actual commissioning procedure should identify the exact standard edition and project-specific acceptance criteria.

8. Test Parameters

The parameters required for substation equipment selection include:

For high-voltage AC testing, GIS and cable capacitance are particularly important.

For transformer testing, rated capacity and winding configuration determine the appropriate instrument.

For CT testing, the required excitation voltage and knee-point test range should be considered.

For battery testing, both nominal voltage and capacity determine the required discharge capability.

9. Common Testing Problems

Transformer Problems

Abnormal Winding Resistance

Possible causes:

• Tap changer contact
• Loose connection
• Winding deformation
• Incorrect temperature correction

Abnormal Tan Delta

Possible causes:

• Moisture
• Aging
• Contamination
• Test connection error

GIS Problems

AC Withstand Failure

Possible causes:

• Metallic particles
• Spacer contamination
• Internal insulation defect
• Incorrect assembly

High PD

Possible causes:

• Floating particle
• Spacer defect
• Protrusion
• External interference

High SF6 Moisture

Possible causes:

• Improper evacuation
• Moisture ingress
• Poor gas handling

Circuit Breaker Problems

Slow Opening

Possible causes:

• Mechanism wear
• Lubrication
• Coil issue
• Hydraulic/pneumatic problem

High Contact Resistance

Possible causes:

• Contact wear
• Poor pressure
• Connection problem
• Oxidation

CT/PT Problems

Incorrect Ratio

Possible causes:

• Wrong test connection
• Incorrect nameplate data
• Winding problem

Incorrect Polarity

• This can have serious consequences in differential and directional protection.

Relay Protection Problems

A common problem is that the relay itself passes its test but the complete trip chain does not.

Possible causes include:

• Wrong CT polarity
• Wrong PT phase sequence
• Incorrect protection setting
• Incorrect terminal wiring
• Trip relay problem
• Breaker control circuit problem

Cable Problems

VLF Test Cannot Reach Voltage

Check cable capacitance and equipment load capability.

Sheath Failure

Check for mechanical damage and moisture ingress.

Fault Location Is Inaccurate

Check cable length, propagation velocity and fault resistance.

Battery Problems

Capacity Is Below Requirement

Possible causes:

• Cell aging
• Sulfation
• Uneven charging
• High internal resistance
• Long-term undercharging

Grounding Problems

A grounding system can show acceptable resistance while still requiring investigation of touch and step voltage.

The complete grounding design must therefore be considered.

10. Equipment Selection Guide

The equipment package should be determined by four factors:

• Substation Voltage Level
• Substation Architecture
• Testing Scope
• Project Stage

Distribution Substation

A typical package:

• Transformer tester
• Circuit breaker tester
• CT/PT analyzer
• Relay protection tester
• Cable VLF tester
• Battery tester
• Ground resistance tester

HV GIS Substation

Recommended:

• Transformer testing system
• GIS AC resonant system
• PD tester
• Circuit breaker analyzer
• Contact resistance tester
• SF6 analyzer
• Relay protection tester
• CT/PT analyzer
• Battery tester
• Grounding tester

EHV Substation

A larger integrated package may be required:

• Transformer Testing
• GIS AC Resonant Testing
• GIS PD
• Circuit Breaker Analysis
• CT/PT Testing
• Protection Relay Testing
• Cable Testing
• Battery Testing
• Grounding Testing

The final configuration should be calculated from the actual technical data rather than based solely on voltage class.

11. Case Studies

Case Study 1 - New GIS Substation Commissioning

A new high-voltage GIS substation is ready for energization.

UHV's testing plan covers:

SF6 Testing

→ Breaker Testing

→ Contact Resistance

→ CT/PT Testing

→ PD Testing

→ AC Withstand

→ Relay Protection

→ Battery

→ Grounding

The commissioning team creates a complete test record for each GIS bay and transformer.

Case Study 2 - Utility Transformer Substation

A utility requires periodic testing of a power transformer.

The UHV testing package includes:

• Turns ratio
• Winding resistance
• Insulation resistance
• Tan delta
• Excitation current
• Oil BDV

The results are compared with the factory report and previous maintenance records.

Instead of relying on a single measurement, the maintenance team evaluates the transformer through several independent parameters.

Case Study 3 - Protection System Commissioning

A newly constructed substation requires verification of transformer differential protection.

The commissioning process checks:

CT Ratio

→ CT Polarity

→ Secondary Wiring

→ Relay Setting

→ Differential Protection Logic

→ Trip Output

→ Circuit Breaker

The complete chain is verified using a relay protection test system and circuit breaker analyzer.

Case Study 4 - Underground Cable Substation Connection

A new HV cable connects the substation to an external transmission line.

The test sequence includes:

Insulation Check

→ Sheath Test

→ AC Resonant / VLF Test

→ PD Diagnostic

→ Final Acceptance

The testing system is selected based on cable length, capacitance and required test voltage.

Case Study 5 - Station Battery Maintenance

A substation battery system has been in operation for several years.

The maintenance team checks:

• Cell voltage
• Internal resistance
• Total voltage
• Discharge capacity
• Charger performance

The battery discharge test identifies whether the battery can still provide the required backup energy for protection and breaker operation.

12. FAQ

13. Related Products

Transformer Testing Equipment

• Transformer Turns Ratio Tester
• Winding Resistance Tester
• Insulation Resistance Tester
• Tan Delta Tester
• Transformer Oil BDV Tester
• Excitation Current Tester
• Partial Discharge Tester
• Transformer Comprehensive Test Bench

GIS Testing Equipment

• AC Resonant Test System
• GIS Partial Discharge Test System
• SF6 Gas Analyzer
• SF6 Dew Point Meter
• SF6 Leakage Detector
• SF6 Density Relay Tester

Circuit Breaker Testing Equipment

• Circuit Breaker Analyzer
• Contact Resistance Tester
• Circuit Breaker Dynamic Resistance Tester where applicable

Relay Protection Testing Equipment

• Relay Protection Test Set
• Three-Phase Relay Test System
• Secondary Current/Voltage Injection System

CT/PT Testing Equipment

• CT/PT Analyzer
• CT Analyzer
• PT Analyzer
• CT Excitation Tester
• Transformer Ratio Tester

Cable Testing Equipment

• VLF Hipot Tester
• AC Hipot Tester
• AC Resonant Test System
• Cable Sheath Tester
• Partial Discharge Tester
• Cable Fault Locator

Battery Testing Equipment

• Battery Discharge Tester
• Battery Internal Resistance Tester
• Battery Capacity Tester

Grounding Testing Equipment

• Ground Resistance Tester
• Earth Resistance Tester
• Ground Grid Tester
• Grounding Continuity Tester

14. Request a Quote

Need a Complete Substation Testing Solution?

UHV approaches substation testing from the perspective of the complete electrical system.

Instead of selecting equipment separately for each test, UHV can configure the test package around the actual substation equipment list and commissioning requirements.

Provide the Following Information

Substation Voltage

→ Transformer Capacity

→ GIS/AIS Configuration

→ Circuit Breaker Type

→ CT/PT Specifications

→ Cable Voltage & Length

→ Battery Voltage & Capacity

→ Grounding System

→ Required Test Items

→ Required Test Voltage

→ Applicable Standards

→ Factory / Site Application

UHV Substation Testing Solution

• Transformer Testing
• GIS Testing
• Circuit Breaker Testing
• Relay Protection Testing
• CT/PT Testing
• Cable Testing
• Battery Testing
• Grounding Testing

The final testing architecture can then be configured around the actual substation.

For a new substation, the recommended workflow is:

Factory Test Records

→ Site Installation Inspection

→ Primary Equipment Testing

→ Secondary System Testing

→ DC System Testing

→ Cable Testing

→ Grounding Testing

→ Integrated Protection Test

→ GIS/Transformer Dielectric Test

→ Final Commissioning

→ Energization

UHV provides high-voltage testing equipment and electrical testing systems for transformer, GIS, cable, circuit breaker, relay protection, CT/PT, battery and grounding applications.

Send your substation technical data to UHV and request a project-specific testing configuration and quotation.