PD for PQ

If one searches for power quality tools on the internet,- the most common results will be for equipment, such as digital multimeters, harmonic analyzers and, most often, power quality analyzers or monitors. Instruments dedicated to measuring the quality of the electrical supply have been around for more than 40 years, evolving from rotary switch settings with a thermal printer paper tape output to color liquid crystal display (LCD) touchscreens with multiple digital signal processors making thousands of calculations per cycle. In the past decade, infrared thermography has been added to the tool kit, enabling the user to perform a thermal scan for hot breakers, wiring or other components that may indicate a power quality problem, such as high current harmonic levels.

One tool that has been around for more than 50 years may be worth adding to the power quality tool box. Though partial discharge testing is most often associated with medium- and high-voltage systems, it has the right criteria for being another diagnostic tool for determining potential or even the source of power quality-related problems. The real goal of power quality monitoring is to prevent minor problems from growing into catastrophic ones that shut down the processes. The processes often can’t be shut down readily to make the connections for the measurements. Traditional power quality monitoring can’t readily detect insulation failures, and infrared testing can only detect heating at visible points. Stresses from transients and harmonics can contribute to this degradation. According to the IEEE Gold Book, insulation failures are a significant source of equipment failures in many of the types of electrical equipment that can affect power quality, as extracted from Table 36 (right).

According to an Iris Power Engineering case study, “industry statistics by IEEE and the Electrical Power Research Institute (EPRI) indicate that approximately 40 percent of all air-cooled high-voltage motor and generator failures occur due to failure of the stator winding insulation.” Switchgear, lightning arrestors and capacitors can also benefit from partial-discharge (PD) testing.

PD activity is a predecessor to insulation failure and equipment failure. PDs are “micro-sparking” within or across the surface of the insulation or into the air and are caused by partial failures in the insulation. The sparking or arcing creates an electrical and acoustic signature, the source of which specialized instruments can detect, analyze and even locate. The severity of these PDs is an indication of the condition of the insulation system. It usually occurs in voids, cracks or other defects in the insulation that occurred during manufacture, installation or operation.

Though more prevalent in higher voltage systems and equipment that creates more electrical stress, PDs can occur in equipment over 600V, typically 2.4 kV to 25 kV. Though this voltage level may not be one that electricians typically work in, the testing equipment used in PD allow for nonintrusive testing. PD can be detected several ways, and the equipment ranges from around $10,000 for a handheld survey tool to $200,000 for more comprehensive monitoring and analysis systems. Detection is made using sensors for radio frequency interference and electromagnetic, acoustic and/or ultraviolet detection. A database of other equipment and historical data on the equipment under test provide the information for software programs to indicate the health and whether failure is imminent. Though online testing is effective in determining the levels of PD, offline testing can be used to accurately locate the source of the PD, especially in cables. PD testing also differs from conventional testing (such as hipot tests) in that no potentially damaging over-potentials are applied to the equipment being tested. It wouldn’t make the failure occur sooner, but will point to how soon it may fail.

The benefits to the equipment owners can be huge, as it allows for repair or replacement before catastrophic failure and loss of production, as well as cost avoidance by not rewinding or replacing equipment that doesn’t need it. Survey tests can be done quickly and safely with the handheld testers. Test fees range from $2,000–3,000, depending on level of analysis. Training classes on the use of such equipment run less than $1,000 for a two-day course. Coupled with the instrument investment, it is a significant expense, but the return on investment period can be less than a year. It may not be a tool for every electrician, but it is one that can help improve the quality of the electrical supply by finding hidden problems before they result in an interruption to the process and costly repairs. For more on insulation testing, turn to page 94.

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.

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