Traditional cable system design requires developing the ampere capacity, or ampacity, rating of the cables. Methods for rating calculations are based on the 1957 Neher-McGrath paper and subsequent standards such as IEC 60287. These methods require knowledge of the cable system construction from a manufacturer’s cut sheets, expected circuit loading patterns (daily load and loss factors), as well as information about the installation conditions including trench geometry, circuit separation, type of special backfill used around the cable pipes and native soil characteristics.

Often, conservative assumptions are made because the cable engineer has incomplete and imperfect information about the characteristics. Some parameters such as ambient temperature change on a seasonal basis, while circuit loading may change rapidly over the course of a few hours, affecting both load shape and preload conditions for emergency ratings. For typical static book ratings supplied to operators, worst-case assumptions are applied to combat these unknowns, often at the cost of sacrificing some usable capacity on the cable system.

Real-time thermal rating (RTTR) systems monitor key cable system parameters, including circuit loading and measured ambient temperature, to avoid the overly conservative rating calculations and optimize the allowable power-transfer capability of the cable asset. PEPCO employed this technology with a LIOS-supplied system using a real-time rating engine from Cyme International.

In conjunction with defining the cable temperature measurement zones, unique installation conditions for all these locations were configured in the real-time rating engine model. The calculations incorporated the measured earth ambient temperature, measured load and real-time feedback from the measured DTS fiber temperatures near the cable pipes. Emergency ratings consider the circuit preload, and the real-time engine calculates new emergency values based on preload conditions, often providing a significant increase particularly for short-duration emergencies.

PEPCO selected normal ratings and several emergency durations that were programmed into the RTTR system. Calculated ratings were transferred from the RTTR computer to PEPCO’s SCADA system, where they could be used by system operators and engineers to evaluate the performance of the cable circuits and make market decisions on power transfer availability through the 230-kV pipe-type circuit.

As part of the configuration process, ratings calculated with the RTTR system were compared to book ratings to verify the system had been modeled correctly. The system was allowed to run to evaluate reliability and stability of the calculation results under varied conditions.

PEPCO, real-time thermal

Poise Under Pressure

Dielectric fluid pressurization is maintained by a shipping-container-sized pumping plant to accommodate dielectric liquid expansion and contraction. MAC Products manufactured the pressurization plants. Old-school pumping plants used electromechanical control systems to regulate when pressurization pumps or relief valves activated as dielectric liquid in the pipes expanded and contracted with load cycling. While these types of systems are reliable, any type of alarm generally required personnel to go to the pumping plant locations to determine whether immediate intervention was required.

PEPCO’s Benning-Ritchie pressurization plants were designed with specialized solid-state programmable logic controller (PLC)-based monitoring systems that permit remote monitoring. When a trouble call comes in at any time, day or night, responsible personnel can check the entire state of the pressurization system remotely using a portable computer or by logging on to PEPCO’s energy management system.

PEPCO, real-time thermal