Mike Howell
Technical Support Specialist
Electrical Apparatus Service Association
High-potential (hipot) testing is a critical method used to evaluate the ground insulation of AC stator windings. This testing is performed during various stages, including in-process, after rewinding, and post-delivery. The primary objective of hipot testing is to ensure the insulation's adequacy against breakdown under normal operating conditions. The paper discusses the differences between AC and DC hipot testing, emphasizing the importance of sizing AC test sets appropriately when testing large windings. It also covers relevant standards, the communication of test requirements, the timing and levels of testing, and the evaluation of results.
The insulation system of stator windings serves multiple functions: it transfers heat away from conductors, electrically insulates conductors from each other and the ground, withstands environmental conditions, and minimizes vibration due to mechanical and electromagnetic forces. The paper explains the classification of materials as conductors, semiconductors, and insulators based on their resistivity. For instance, copper has a resistivity of around (10^{-8}) ohm-m, while mica has a resistivity of around (10^{13}) ohm-m, indicating that copper conducts significantly more current than mica.
Ground insulation stress is a measure of the electric field within the insulation system, typically expressed in volts per length. Proper grounding and stator core assumptions are crucial for these calculations. The paper highlights the issue of partial discharge (PD), which occurs when voids or air pockets within the insulation experience higher voltage stress, leading to repeated discharges that degrade the insulation. This condition is more prevalent in machines rated above 6 kV but can also occur in lower voltage machines fed by inverters.
Hipot testing has a long history, dating back to the late 19th and early 20th centuries. The paper provides a historical perspective, noting the evolution of standards and practices over time. It discusses the insulation resistance test, which is a preliminary DC low-potential test performed before any hipot test to ensure the winding's suitability for further testing. The DC hipot test, permissible as a substitute for the AC hipot test, is detailed, including its pass/fail nature and the guidance provided by IEEE 95.
AC hipot testing is the most common method used by manufacturers, while DC hipot testing is more prevalent in service centers due to lower equipment costs and portability. The paper explains the differences in leakage current determination between AC and DC hipot tests, with AC tests being more effective in detecting insulation defects due to the voltage distribution across insulation components.
The paper also addresses the impact of hipot testing on insulation aging and damage. While AC hipot testing does age the insulation, the reduction in life is insignificant compared to the overall life expectancy of modern insulation systems. DC hipot testing, however, can place undue stress on the end windings of machines utilizing stress control materials.
Sizing the AC hipot test set involves calculating the winding capacitance, capacitive reactance, leakage current, and apparent power. The paper provides detailed calculations and examples to illustrate this process. It also discusses the importance of performing hipot tests at appropriate levels during different stages, such as in-process, new winding acceptance, and maintenance.
In conclusion, the paper emphasizes the need for advanced diagnostic ground insulation tests for machines rated above 6 kV, while machines rated below 6 kV typically rely on AC or DC hipot tests. The effectiveness of the standard 2E+1 kV AC hipot test in detecting poorly processed stators is questioned, suggesting further exploration in this area.
Key Points Covered:
- Differences between AC and DC hipot testing
- Sizing AC test sets for large windings
- Relevant standards and communication of test requirements
- Timing and levels of testing
- Evaluation of test results
- Historical perspective of hipot testing
- Insulation resistance test
- DC hipot test details and guidance
- AC hipot testing and its effectiveness
- Impact of hipot testing on insulation aging and damage
- Sizing the AC hipot test set
- Importance of advanced diagnostic tests for high voltage machines
Key Takeaways:
- Hipot testing is essential for assessing the ground insulation of AC stator windings.
- AC hipot testing is more effective in detecting insulation defects compared to DC hipot testing.
- Proper sizing of AC test sets is crucial for accurate testing of large windings.
- Historical standards have evolved to improve hipot testing practices.
- Insulation resistance tests are preliminary steps before hipot testing.
- Advanced diagnostic tests are recommended for high voltage machines to ensure insulation quality.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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