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Stay Online With Partial Discharge Testing

How to maintain continuous power supply while measuring for weak spots.

Fortnightly Magazine - October 2006

Failures in medium-voltage power cables and their components cause a large proportion of annual power service interruptions, especially in high-density urban areas. Locating and repairing weak areas in cables at an early stage can improve the reliability of the energy supply considerably.

Analytical methods for asset management that depend on cable-specific failure models work best when the condition of the cable is known. Partial discharge testing is a proven condition assessment/test methodology. A partial discharge (PD) is a short pulse that originates from weak spots along a cable.

In its current form, PD testing occurs offline and involves some drawbacks in terms of cost, risk of damage to the cable, safety, and operational impact. However, a new measuring technique has been developed that can diagnose and locate PD activity on a medium-voltage power cable while the cable remains in service. The patented system, which was created by KEMA in partnership with the Eindhoven University of Technology in the Netherlands, uses inductive sensors to detect, diagnose, and locate partial discharge activity along a medium-voltage power cable while the cable remains online.

The PD Detection Online With Localization (PD-OL) system provides significant benefits over the current offline test systems. While the offline systems require disconnecting a cable from the grid and energizing it with a separate power supply to generate PDs and are performed on an occasional basis, PD-OL enables continuous on-line cable monitoring, allowing utilities the benefits of:

• Maintaining continuous power supply during measurement;

• Testing the cable system under exact operating conditions, including over voltages and load variations;

• Cost-effective operation with minimal personnel effort required after installation;

• Continuous registering of data that captures time-related information, such as variation of PD activity in time, the effects of sudden over-voltages, and cable temperature variations; and

• Detecting PDs occurring shortly before failure.

Improving Asset Management and Reliability

Many of North America’s critical customer areas are served through underground distribution systems, at voltages between 5 kV and 35 kV, using paper insulated, lead covered or solid dielectric (polymeric) cables. Although these systems have given good service in the past, wear and tear at several utilities are driving up failure rates, operating problems, and service interruptions as the systems age. Repair and replacement costs, as well as service quality problems due to circuit and equipment outages, are expected to escalate in the future unless preventative action is taken.

An asset-management strategy that includes some form of proactive cable replacement, life extension, condition assessment, and continuous monitoring to manage cable failures is essential to avoid these future problems. In some cases, load reduction, switching, dynamic ratings, and temporary overloads also can be used as strategies to reduce failure rates and mitigate reliability impacts of aging cables. However, an overall cable-management strategy is essential for long-term improvement in cable-system operation and electric-system reliability to