Reliability testing on existing electrical equipment

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The testing and maintenance of existing and aged equipment is a complex process rarely prescribed by Australian standards. Baoying Tong and David Lister lay out a short series of testing guidelines based on their experience.

Electrical testing aims to quantify signs of deterioration early by assessing the data collected during the test and comparing it with previous values or accepted criteria. Testing on existing electrical equipment primarily focuses on the testing for degrading insulation performance, wear or component damage that may impact continued operational safety or reliability. Compared with testing of new equipment, which is normally prescribed clearly by standards, testing existing and aged equipment can be different and more complicated.

First, the type of tests that need to be carried out on existing equipment are not prescribed in Australian standards and are normally agreed between the engineer and facility managers based on various factors. Second, the tests should avoid imposing too much stress on older equipment, as it may result in catastrophic failure. Third, testing on existing equipment requires it to be taken out of service, which disrupts the daily operations of the facility.

Based on our experience, this article aims to provide a short summary of tests recommended on existing electrical equipment.

Selected testing methodology

Functional and mechanical test

Functional tests such as tripping test on circuit breakers and locking test on interlocks are one of the first tests that should be carried out. Torque tests should also be carried out on busbars and cable terminals etc to confirm the tightness is sufficient as per manufacturers’ recommendations.

Insulation – resistance test

An insulation resistance test is carried out by measuring the insulation resistance between each phase and ground. The test is to prevent shock hazards and fire hazards from short-circuits caused by insulation break-down and equipment damage1. For low voltage equipment, 500V d.c. testing voltage is generally suggested1. Given the insulation needs time to ‘charge up’, reading may be recorded after one minute applying the voltage2, if the reading continues to fluctuate after applying the testing voltage.

For high voltage equipment, it is suggested in many industry sources that the minimum insulation resistance should be at least one megohm per 1000V of equipment voltage rating3. The criteria calculated may be arguably low. In past projects, the authors have observed 10 megohms per 1000V of equipment voltage rating used as a rule of thumb. Some statutory requirements also set the threshold higher for high voltage equipment, e.g. minimum 100 megohms for existing underground high voltage cables is required in some standards4.

Insulation – time test

The insulation resistance measurement, by itself, may not be a good indication of insulation quality. First, the test returns absolute values, making it hard to establish condition deterioration trends without historical records to compare with. Second, difference in ambient conditions (e.g. temperature and humidity) during tests at different testing time, will influence the reading and the comparison result may be misleading5. Time-based insulation resistance tests can provide better indication of the quality of the insulation by measuring the insulation resistance change over time. By applying voltage continuously, readings of resistance are recorded and ratios of insulation resistance are calculated. There are Dielectric Absorption Index (DAI) and Polarisation Index(PI). The mathematical expressions are shown below:

Both methods do not require calibration of temperature5 and provide better indication of the insulation quality. The table below shows the criteria5.

Insulation – voltage withstand test

The voltage withstand test should only be undertaken after the insulation resistance test to avoid HV testing failure. By applying the high voltage on the testing subject, e.g. on cables and busbars, and hence stressing the insulation, it simulates a transient over-voltage scenario to make sure the equipment insulation does not break down6. Caution has to be taken as voltage withstand tests may worsen the condition of the insulation particularly on aged equipment.

The purpose of the test is not to take measurement, but to assess if insulation breakdown/deterioration happens during the test6. This can be observed, whether by a surge of current, or by the insulation resistance test after applying the voltage.

Dissolved gas analysis

When mineral oil is used as insulation, AS60422 provides good guidance on the type of oil tests that may be considered. For evaluation of oil in service, it may be classified as ‘good’, ‘fair’ or ‘poor’ against each of the oil properties assessed in those tests7. By analysing the oil, the condition of the actual electrical equipment can be evaluated. For example, dissolved gas analysis (DGA) is one of the most important tests to undertake. By determining different types of combustible gas concentrations dissolved in the oil sample, trend of concentration changes over time can be established and historical faults can be identified.

Current injection test

The current injection test aims to verify the correct electrical protection tripping characteristics of the circuit protection devices by injecting current to simulate a fault scenario. There are two types of injection tests – primary and secondary injection.

A primary injection test requires heavy-current generator and disconnection of existing cables to access the primary circuit. In contrast, when there is an electronic protection relay on the protective device, testing current can be directly injected into the secondary side. It should be noted that though primary injection requires more equipment and greater disruption than a secondary injection test, it also tests the whole over-current sensing current transformers, protection tripping relay and the circuit breaker actuation, which would not be tested as part of a secondary injection test.

Contact resistance test

High contact resistance generally indicates damaged circuit breaker contact surfaces, which may lead to increased voltage drop and local overheating of the contact.

Values of contact resistances measured should be compared against manufacturer’s data. When not available, 50 percent deviation is maximum tolerances between the poles8.

Ultrasonic and thermal scanning

Ultrasonic and thermal scanning are both on-load tests to holistically check the equipment as it can detect loose connections and faulty equipment/components. The main difference lies in thermal scanning relies upon heat patterns associated with faults such as hot cable joints. In comparison, ultrasonic scanning detects non- or low-heat pattern faults, especially on high voltage switchgear, such as corona and tracking9.

Assessment and recommendations

After testing data is analysed, a recommendation report is produced, which includes identification of problems, risk ratings, reviewing both personal risks and equipment failure and prioritised remedial actions.

Conclusion

Testing on existing equipment is an effective way to identify potential asset failure risks and provides guidance on the reliability of electrical equipment as part of the facility management protocol. The testing methods, conditions and criteria should be determined in accordance with overall equipment conditions and equipment manufacturers’ recommendations, and discussed and agreed with facility managers.

Baoying Tong is an electrical engineer at AECOM. David Lister is technical director (electrical) at AECOM.

 

References

  • 1 AS/NZS 3017 Electrical installation – verification guidelines, 2007
  • 2 ANSI/NETA Standard for maintenance testing specifications for electrical power equipment and systems, 2011
  • 3 Gregorec, The Basics of Insulation Testing, 2006
  • 4 Ausgrid, NS161 Specification for Testing of Underground Cables, 2018
  • 5 A stitch in time – the complete guide to electrical insulation testing, 2016
  • 6 Nute, Dielectric Strength Testing: Transient Over-voltage Withstand Test, 2015
  • 7 AS 60422:2017 Mineral insulating oils in electrical equipment – supervision and maintenance guidance, 2017
  • 8 ANSI/NETA Standard for maintenance testing specifications for electrical power equipment and systems, 2011
  • 9 Infrared and Ultrasound Technologies – The Perfect Match for Metal-clad Switchgear Inspections, 2016

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