Generator Testing Solution

For a generator manufacturer, power plant, maintenance contractor or commissioning company, generator testing is not limited to checking whether the machine can produce voltage. The generator insulation system, stator winding, rotor winding, inter-turn insulation and grounding system all need to withstand electrical, thermal and mechanical stress during operation.

A generator may operate normally during a short functional test while an insulation defect is already developing inside the stator winding. A small void, loose conductor, damaged turn insulation or localized discharge may not immediately produce an obvious fault. The problem becomes more serious when the generator is exposed to rated voltage, thermal cycling, vibration and repeated electrical stress.

UHV approaches generator testing from the perspective of high-voltage insulation verification and electrical condition assessment.

The testing scope can include:

• AC Withstand
• Partial Discharge
• Insulation Resistance
• Stator Testing
• Rotor Testing
• Inter-Turn Testing
• Tan Delta

The equipment configuration can be adapted to generator manufacturers, hydropower stations, thermal power plants, nuclear power facilities, wind power generation systems, industrial power plants, overhaul contractors and electrical testing laboratories.

A typical generator testing program can be organized as:

Generator Assembly → Visual Inspection → Stator Testing → Rotor Testing → Insulation Testing → Tan Delta → Partial Discharge → AC Withstand → Inter-Turn Verification → Final Assessment

The actual sequence and acceptance criteria should follow the generator manufacturer's technical documentation and the applicable project standards.

1. Testing Background

A generator operates under a combination of electrical, thermal, mechanical and environmental stresses.

The stator winding is exposed to high operating voltage. The rotor experiences mechanical stress from high-speed rotation and electrical stress from the excitation system. Insulation materials are repeatedly subjected to temperature changes, vibration and electromagnetic forces.

Over an extended operating period, these conditions can produce:

• Insulation aging
• Partial discharge
• Thermal deterioration
• Slot discharge
• End-winding discharge
• Inter-turn insulation deterioration
• Rotor winding faults
• Loose winding components
• Contamination
• Moisture-related insulation degradation

For this reason, generator testing is required at several stages.

Factory Testing

The generator manufacturer performs electrical and mechanical tests before shipment.

The results establish the original condition of the generator.

Commissioning Testing

After transportation and installation, the generator is tested again.

Transportation, installation and connection work can introduce:

• Mechanical damage
• Moisture
• Contamination
• Incorrect connections
• Winding displacement
• Insulation damage

The commissioning test therefore provides a baseline for the generator in its installed condition.

Maintenance Testing

During service, generator testing becomes more diagnostic.

The engineering team is usually interested in changes compared with:

• Previous test results
• Factory test records
• Similar generators
• Manufacturer limits

A single measurement rarely explains the complete condition of a generator. Trend analysis becomes particularly valuable for insulation-related parameters.

2. Testing Objects

Generator testing focuses mainly on the stator, rotor and their insulation systems, but the test scope varies according to generator construction.

2.1 Stator Winding

The stator winding is the main high-voltage winding in many large AC generators.

Testing can include:

• Insulation resistance
• Polarization index where applicable
• AC withstand
• Tan delta
• Partial discharge
• Inter-turn insulation
• Winding resistance
• Phase comparison

The purpose is to identify electrical insulation problems before they develop into a major winding failure.

2.2 Rotor Winding

The rotor winding operates at a much lower voltage than the stator in many large generators, but its mechanical operating environment is demanding.

Testing may include:

• Rotor insulation resistance
• Rotor winding resistance
• Inter-turn testing
• AC/DC-related insulation checks according to generator design
• Shorted-turn detection
• Pole-to-ground insulation checks where applicable

Rotor testing procedures differ substantially between cylindrical rotors and salient-pole rotors.

2.3 Stator Ground Insulation

The insulation between the stator winding and the stator core is critical.

Insulation resistance testing can identify:

• Moisture
• Contamination
• Surface leakage
• Insulation deterioration

However, insulation resistance alone should not be used as the only indicator of generator insulation condition.

2.4 Inter-Turn Insulation

Inter-turn insulation separates individual turns within a winding.

A defect at this level can be difficult to detect because the generator may still show normal phase-to-ground insulation resistance.

The inter-turn insulation can nevertheless be exposed to significant circulating current and localized heating.

This makes inter-turn testing particularly important for generator winding evaluation.

2.5 End-Winding Insulation

The end-winding region experiences:

• Electrical stress
• Mechanical vibration
• Thermal expansion
• Electromagnetic forces

Partial discharge testing can be useful for identifying insulation activity in this area.

3. Testing Requirements

Generator testing requirements depend on:

• Generator rated voltage
• Generator capacity
• Winding configuration
• Insulation class
• Generator type
• Factory or site testing
• New or refurbished generator
• Maintenance history
• Applicable standard
• Manufacturer requirements

A 6.6 kV industrial generator and a 20 kV hydrogenerator should not automatically use the same test procedure.

The required test voltage, duration and acceptance criteria must be determined from the generator's rated insulation level and applicable testing standard.

New Generator

A new generator normally requires a comprehensive factory and commissioning test program.

Typical tests include:

• Winding resistance
• Insulation resistance
• AC withstand
• Tan delta
• Partial discharge
• Inter-turn tests where specified
• Rotor tests

Rewound Generator

For a rewound stator, insulation testing becomes particularly important.

The testing program should establish whether:

• New insulation is correctly manufactured
• Winding installation is correct
• End-winding connections are sound
• There are localized defects
• The insulation system meets the required electrical level

Generator Overhaul

During overhaul, testing should be compared with historical records.

A significant change in:

• Tan delta
• PD activity
• Insulation resistance
• Winding resistance may justify additional investigation.

Testing Methods

4.1 AC Withstand Testing

AC withstand testing applies a specified AC voltage to the generator winding for a defined period.

The purpose is to verify the dielectric strength of the insulation system.

The general test arrangement is:

AC Test Source → Generator Stator Winding → Ground

The test voltage is increased in a controlled manner.

During the test, engineers monitor:

• Test voltage
• Leakage current
• Stability
• Discharge activity where measured
• Protection system status

If the generator has a large winding capacitance, a conventional power-frequency test source may require significant power.

For larger generators, an AC Resonant Test System can provide a more practical approach.

The resonant circuit allows the capacitive test object and inductive reactor to exchange reactive energy, reducing the active power requirement of the test source.

UHV's AC resonant test systems are designed for high-voltage AC testing of capacitive electrical equipment and can be configured according to the capacitance and required test voltage of the test object.

4.2 Partial Discharge Testing

Partial discharge is one of the important diagnostic methods for generator insulation.

Possible PD sources include:

• Internal voids
• Delamination
• Slot discharge
• End-winding discharge
• Surface contamination
• Loose conductive components
• Poor stress grading

The test can provide information about insulation activity that may not be visible from an insulation-resistance measurement.

Important parameters include:

• PD magnitude
• PD inception voltage
• PD extinction voltage
• Phase-resolved pattern
• Pulse repetition
• Discharge location where available

For generator applications, the background noise level must be controlled carefully.

Sources of interference may include:

• Excitation systems
• Motors
• Transformers
• Power cables
• Switching equipment
• Ground loops
• Radio-frequency interference

A suitable PD system should therefore provide adequate sensitivity and signal discrimination.

4.3 Insulation Resistance Testing

Insulation resistance testing is one of the basic electrical tests for generator windings.

The test measures insulation resistance between:

• Stator winding and ground
• Rotor winding and ground
• Individual circuits where applicable

The result can be affected by:

• Temperature
• Moisture
• Contamination
• Winding condition
• Test voltage
• Test duration

For this reason, test results should be recorded together with winding temperature and environmental conditions.

Polarization Index

Where applicable, polarization index can provide additional information about insulation condition.

The basic concept is to compare insulation resistance at two time points during a DC test.

The result should be interpreted according to the applicable standard and generator manufacturer's requirements.

4.4 Stator Testing

Stator testing can combine several techniques.

Winding Resistance

Used to identify:

• Connection problems
• Phase imbalance
• Poor joints
• Winding abnormalities

Insulation Resistance

Checks winding-to-ground insulation.

Tan Delta

Evaluates dielectric losses.

Partial Discharge

Detects localized insulation activity.

AC Withstand

Verifies dielectric withstand capability.

These tests answer different questions.

A stator that has acceptable insulation resistance may still show abnormal tan delta or PD behavior. Conversely, a high PD reading needs to be interpreted with the test setup and historical data rather than treated as an automatic failure.

4.5 Rotor Testing

Rotor testing depends on the generator design.

Cylindrical Rotor

Typically used in turbo-generators.

Testing may focus on:

• Rotor winding resistance
• Insulation resistance
• Inter-turn condition
• Ground insulation
• Shorted-turn detection

Salient-Pole Rotor

Common in hydrogenerators.

Testing may involve:

• Pole winding resistance
• Pole insulation
• Inter-pole connections
• Ground insulation
• Shorted turns

Rotor testing must take the actual winding configuration into account.

4.6 Inter-Turn Testing

Inter-turn testing focuses on insulation between adjacent turns.

This is important because a turn-to-turn defect can remain undetected by a conventional winding-to-ground insulation test.

The test applies a controlled electrical stress between turns or uses an appropriate surge/inter-turn testing method according to the winding design.

The purpose is to identify:

• Turn insulation weakness
• Shorted turns
• Localized winding defects
• Abnormal phase behavior

For large generator stator windings, the applicable test method must be selected carefully because excessive test stress can itself damage an insulation system.

4.7 Tan Delta Testing

Tan delta, also known as dielectric dissipation factor, evaluates dielectric losses in the insulation system.

It can provide information about:

• Moisture
• Contamination
• Aging
• Voids
• Insulation deterioration

The test is normally performed over a range of test voltages rather than relying only on one measurement.

Important observations include:

• Tan delta value
• Change in tan delta with voltage
• Capacitance
• Phase-to-phase comparison
• Historical trend

For generator insulation, the shape of the tan delta curve can sometimes provide more useful information than a single number.

5. Recommended Equipment

UHV can configure generator testing equipment according to the generator's rated voltage, capacitance, winding configuration and testing stage.

For large generators, UHV recommends configuring the equipment according to the actual test object rather than selecting instruments solely according to generator rated power.

6. Testing Procedure

A practical generator commissioning or maintenance program can be arranged as follows.

Step 1 - Collect Generator Data

Confirm:

• Generator manufacturer
• Generator model
• Rated capacity
• Rated voltage
• Rated current
• Frequency
• Stator winding configuration
• Rotor type
• Insulation system
• Generator capacitance if available
• Previous test records

Step 2 - Visual Inspection

Inspect:

• Stator winding
• End winding
• Slot areas
• Rotor
• Connections
• Grounding
• Insulation surfaces
• Cooling system interfaces

Look for:

• Contamination
• Cracks
• Loose components
• Mechanical damage
• Signs of overheating
• Moisture

Step 3 - Insulation Resistance

Measure:

Stator → Ground

and where applicable:

Rotor → Ground

Record:

• Test voltage
• Resistance
• Test duration
• Temperature
• Humidity

If historical results are available, compare the new measurements with previous records.

Step 4 - Winding Resistance

Measure the winding resistance of each phase.

Compare:

Phase A ↔ Phase B ↔ Phase C

Large differences should be investigated.

Temperature correction should be considered when comparing measurements made under different conditions.

Step 5 - Tan Delta

Connect the tan delta system to the stator winding according to the approved test configuration.

Increase the test voltage step by step.

Record:

• Capacitance
• Tan delta
• Voltage
• Current

Compare the curve between phases and with previous test results.

Step 6 - Partial Discharge

Measure background noise before applying test voltage.

Then:

• Apply the specified voltage
• Monitor PD activity
• Record PRPD patterns
• Determine PD inception behavior
• Compare phase-to-phase results
• Identify abnormal patterns

If necessary, perform additional localization.

Step 7 - AC Withstand

Connect the AC high-voltage test system.

The general sequence is:

Zero Voltage → Controlled Increase → Required Test Voltage → Hold → Controlled Reduction → Discharge → Ground

The exact voltage and duration must follow the approved test procedure.

Step 8 - Rotor Testing

After confirming the rotor configuration, perform:

• Insulation resistance
• Winding resistance
• Inter-turn/shorted-turn checks where applicable

For large rotors, mechanical safety and grounding procedures are essential.

Step 9 - Inter-Turn Test

Perform the approved inter-turn test according to the generator manufacturer's winding structure.

Compare:

• Phase behavior
• Individual winding sections
• Reference waveform
• Historical results

Step 10 - Final Assessment

The final evaluation should combine:

Insulation Resistance + Tan Delta + PD + AC Withstand + Winding Resistance + Inter-Turn Results

rather than using one test result as the sole basis for acceptance.

7. Applicable Standards

The applicable standard depends on generator type, rated voltage and testing stage.

IEC 60034 Series

The IEC 60034 series covers rotating electrical machines and is a major reference for generator design and testing.

Relevant parts can address:

• Rating
• Performance
• Insulation
• High-voltage tests
• Electrical characteristics

For a specific project, the exact applicable edition should be identified in the testing plan.

IEEE Standards

IEEE standards are widely used for generator insulation evaluation and maintenance in North American and international projects.

Depending on the generator and test purpose, relevant documents can address:

• Rotating machine insulation
• AC withstand
• Partial discharge
• Tan delta
• Field winding testing
• Maintenance testing

Manufacturer Specifications

For generator commissioning, the manufacturer's test requirements are particularly important.

The final test voltage, duration and acceptance criteria should not be taken from a generic online table.

They should be based on:

Generator Design + Rated Voltage + Insulation System + Manufacturer Requirement + Applicable Standard

8. Test Parameters

AC Withstand

The required test voltage depends on:

• Rated generator voltage
• Insulation system
• Generator manufacturer
• Test stage
• Applicable standard

Partial Discharge

Important parameters include:

• PD level
• Inception voltage
• Extinction voltage
• PRPD pattern
• Phase relationship

A PD value should not be evaluated without considering the test system's background noise and measurement configuration.

Tan Delta

Record:

• Test voltage
• Tan delta
• Capacitance
• Current

The voltage-dependent trend is often useful when assessing insulation behavior.

Insulation Resistance

Record the winding temperature because insulation resistance changes significantly with temperature.

9. Common Testing Problems

Problem 1 - Low Insulation Resistance

Possible causes:

• Moisture
• Contamination
• Surface leakage
• Insulation aging
• Incorrect grounding

The generator should be evaluated after correcting obvious moisture or contamination conditions.

Problem 2 - High Tan Delta

Possible causes include:

• Moisture
• Insulation aging
• Delamination
• Contamination
• Voids

The phase comparison and historical trend should be reviewed.

Problem 3 - Increasing PD

A rising PD trend can indicate changes in the insulation system.

Possible sources include:

• Internal voids
• Slot discharge
• End-winding discharge
• Loose components
• Surface contamination

Further diagnosis may be required before deciding on repair.

Problem 4 - AC Withstand Failure

Potential causes:

• Insulation defect
• Moisture
• Surface contamination
• Localized damage
• Incorrect test connection
• Inadequate test setup

The generator should not simply be retested repeatedly at higher stress.

Problem 5 - Phase Resistance Difference

Possible causes:

• Connection problem
• Winding damage
• Loose joint
• Tap/connection configuration
• Temperature difference

Problem 6 - Rotor Insulation Problem

Possible causes:

• Moisture
• Contamination
• Damaged insulation
• Rotor winding-to-ground defect

The rotor construction should be reviewed before selecting a further diagnostic test.

Problem 7 - Inter-Turn Test Abnormality

Possible causes:

• Turn insulation deterioration
• Shorted turn
• Winding geometry problem
• Incorrect test connection

The test waveform should be compared with a known-good reference whenever possible.

10. Equipment Selection Guide

The correct generator test equipment depends on the rated voltage, generator capacity, capacitance and test purpose.

Small / Medium Generator

A typical package may include:

• Insulation resistance tester
• Tan delta tester
• AC Hipot tester
• Winding resistance tester
• Inter-turn tester

Large Generator

A more comprehensive package can include:

• AC Resonant Test System
• Partial Discharge Test System
• Tan Delta & Capacitance Tester
• Insulation Resistance Tester
• Winding Resistance Tester
• Inter-Turn Tester
• Rotor Test Equipment

Hydrogenerator

Particular attention should be given to:

• Large stator winding
• Salient-pole rotor
• Pole insulation
• Inter-pole connections
• End-winding condition
• Partial discharge

Turbo-Generator

Particular attention should be given to:

• Cylindrical rotor
• Rotor winding
• Stator insulation
• Slot discharge
• End-winding condition
• Inter-turn condition

Equipment Selection Data

Before UHV configures the test system, provide:

• Generator Rated Voltage
• Generator Capacity
• Stator Capacitance
• Generator Type
• Stator Winding Configuration
• Rotor Type
• Required AC Test Voltage
• PD Test Requirement
• Tan Delta Requirement
• Factory / Site / Maintenance Application

This information determines the required test voltage, output capacity, reactor configuration and measurement range.

11. Case Studies

Case Study 1 - Hydropower Generator Commissioning

A newly installed hydrogenerator requires electrical commissioning before connection to the grid.

The testing sequence includes:

Insulation Resistance

→ Winding Resistance

→ Tan Delta

→ Partial Discharge

→ AC Withstand

→ Rotor Insulation

→ Inter-Turn Testing

The test results are documented as the initial operating baseline.

Case Study 2 - Generator Rewinding Project

A power plant replaces the stator winding of an aging generator.

The testing team needs to verify the new insulation system before returning the machine to service.

UHV equipment is configured for:

• Insulation resistance
• Tan delta
• Partial discharge
• AC withstand
• Winding resistance
• Inter-turn testing

The key objective is to verify the new winding before the generator is exposed to full operating conditions.

Case Study 3 - Generator PD Diagnosis

During periodic maintenance, PD measurements show a higher discharge level than previous records.

The engineering team reviews:

• PD magnitude
• PRPD pattern
• Phase behavior
• Inception voltage
• Historical trend

The result is used to determine whether additional inspection or maintenance is justified.

Case Study 4 - Rotor Testing

A turbo-generator shows abnormal excitation behavior.

The maintenance team performs:

• Rotor insulation resistance
• Rotor winding resistance
• Shorted-turn/inter-turn diagnosis where applicable

The test results help narrow the investigation toward the rotor rather than the stator insulation system.

12. FAQ

13. Related Products

AC Resonant Test System

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

Partial Discharge Test System

For detecting localized insulation discharge in stator winding systems.

Tan Delta & Capacitance Tester

For evaluating dielectric loss and capacitance changes in generator insulation.

Insulation Resistance Tester

For stator and rotor insulation measurements.

Winding Resistance Tester

For measuring stator winding resistance and identifying abnormal phase differences.

AC/DC Hipot Tester

For high-voltage insulation testing where the approved generator test procedure specifies an AC or DC test method.

Inter-Turn Test System

For evaluating insulation between turns and identifying abnormal winding behavior.

Rotor Testing Equipment

For rotor insulation, winding resistance and shorted-turn-related diagnosis according to generator design.

High Voltage Test System

For generator insulation testing applications requiring controlled high-voltage output.

UHV's high-voltage testing portfolio can be configured around generator testing requirements rather than limiting the solution to a single test instrument.

14. Request a Quote

Need a Generator Testing Solution?

UHV can configure generator testing equipment according to the actual generator specifications and test requirements.

Send the following information:

Generator Type

→ Rated Capacity

→ Rated Voltage

→ Rated Current

→ Stator Winding Configuration

→ Stator Capacitance

→ Rotor Type

→ Required AC Test Voltage

→ PD Test Requirement

→ Tan Delta Requirement

→ Inter-Turn Test Requirement

→ Factory / Commissioning / Maintenance

→ Applicable Standard

UHV Generator Testing Solution

• AC Withstand
• Partial Discharge
• Insulation Testing
• Stator Testing
• Rotor Testing
• Inter-Turn Testing
• Tan Delta

The equipment configuration can be adapted for hydrogenerators, turbo-generators, industrial generators and other large rotating electrical machines.

For large generator windings, UHV can evaluate the required AC test system according to rated voltage, winding capacitance, required test voltage and site power conditions.

A complete generator testing program should ultimately connect the individual measurements into one technical assessment:

Insulation Resistance → Winding Resistance → Tan Delta → Partial Discharge → Inter-Turn → Rotor Testing → AC Withstand

This gives the commissioning or maintenance team a much clearer picture of the generator's insulation condition than relying on a single test result.

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