Storms blew through Austin the week before last knocking out power to our building. It reminded me of the criticality of reliable power to conduct business in this information age. If you’re responsible for maintaining the power distribution facility at your plant, you know how critical it is to avoid unplanned power outages.
I caught up with Emerson’s Cliff Kirby, a manager in the Electrical Reliability Services business. Cliff described some advances to their partial discharge (PD) testing and monitoring services that have expanded to include switchgear.
Now, for those not steeped in an electrical engineering background, partial discharges are small electrical sparks that occur within a cable’s insulation, or on the surface of the insulation of medium and high voltage electrical equipment. These sparks result in the electrical breakdown of a small portion of the insulation surface or in an air pocket within the insulation. Over time, these partial discharges will erode the insulation and can result in a complete breakdown.
According to the National Fire Protection Association (NFPA 70B), the leading cause of electrical failures is insulation breakdown. The National Electrical Code (NEC) states that these partial discharges are the first indication of insulation deterioration. Research from the Recommended Practice for the Design of Reliable Industrial & Commercial Power Systems (IEEE Gold Book), Table 36, indicates that cables, switchgear, and transformers suffer the greatest losses from insulation failure.
Cliff indicated that it’s impractical for most process manufacturers to de-energize their power system to perform testing. He noted that partial discharge testing and monitoring service could be performed online (while the electrical equipment is energized.) There are a lot of factors to consider in performing partial discharge testing including whether it’s practical to de-energize the system, the age of your electrical assets, the material composition of the insulation, weather conditions, etc. Online testing can range from the use of a simple handheld detection unit, to periodic testing with special capacitive or high-frequency current transformers (HFCT) sensors, to continuous monitoring for the most critical or hard-to-access electrical assets.
As implied by the term “offline,” this type of partial discharge testing can only be performed when the system is de-energized-such as when the installation is new and has not been released to operation or during a plant turnaround.
In the United States, more than ten industry standards provide information about field-testing medium voltage cables and components. The IEEE 400-2001 standard defines six tests and their advantages and disadvantages. One of the well-established tests, DC Hipot testing, may cause damage and premature failure in certain types of cables including EPR and XLPE cables.
In addition to online and offline PD testing, Cliff listed some other tests that the electrical reliability team performs: Ultrasonics, Tan Delta (dissipation factor testing), and Very Low Frequency (VLF) testing. Every plant has unique circumstances and selecting the right mix of test methods and technologies helps provide the diagnostic information necessary to analyze the data to spot problems well before an unplanned shutdown occurs.
These tests are usually incorporated into an ongoing program to help improve the maintenance plan and length of service of the electrical assets.