A Practical Alternative for Increasing Power Flows

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Dynamic feeder rating (DFR) systems are an important alternative to analytical methods for determining real-time conductor operating temperatures and corresponding real-time cable ratings.

DFR systems can be used to enhance the current-carrying capacity of power cables and eliminate the risk of overheating.

Data acquisition system

Kinectrics' Dynamic Feeder Rating (DFR) system is configured to link several remote terminal units (RTU) positioned at strategic locations along the cable route to a main computing unit (CPU), that is generally installed at a substation. The system is based on an open architecture, relying on proven third party sub-components. Designed for both new and retrofit cable installations, an overview of the Kinectrics DFR is shown in Figure 1.

The system is controlled by the CPU, typically an industrial grade unit that can maintain continuous service. Hardware is powered by an uninterruptible power supply with provision for safe shutdown protocols in case of extended power outages. To overcome possible malfunction of the system and maintain remote operation capability, the CPU can be restarted at will by a simple telephone command.

The key performance characteristics of the RTUs provide reliable acquisition and storage of cable and ambient temperatures and load current data. These data are transferred at regular intervals to the CPU under typical field conditions, including transient potential rises. Alternatively, cable and soil temperature data can be acquired by a distributed fiber optic measurement system capable of collecting temperature information along the entire cable route.

The distributed RTU units are commercial data acquisition units requiring a power supply and a link for communication to the CPU. The units are capable of long-term operation (1-2 weeks) based solely on battery power to allow uninterrupted collection of data. The units have sufficient memory to retain upwards of 500k points of data. To overcome possible malfunction of the remote system and maintain continuous operation, the RTUs can be restarted directly by the CPU. The RTUs have a broad operating range, typically from -40° C to +70° C. The technology has proven highly reliable in monitoring the mechanical performance of transmission equipment for over 10 years outdoors in Northern Ontario.

RTU power can be provided from a conventional ac source or, where appropriate, from a solar battery pack to minimize installation costs. To minimize telephone installation costs, the communications link can be established via cellular phone or dedicated wireless transmitter. Depending on requirements, RTUs can accommodate over 100 thermocouple inputs.

DFR software

The DFR software program can perform calculations of cable conductor temperature, steady state and emergency ratings in real-time. The user specifies the relevant maximum operating temperatures and the duration of the emergency. The software computes the maximum current that the cable can carry for the specified conditions.

Calculation of the steady-state rating can be performed in two modes. In one case, standard Neher/McGrath or IEC 287 calculations are performed with the user-specified load-loss factor. In the second case, the program evaluates recorded load variations over a 24-hour period and assumes the same variations during the following period. Duration of this period is such that the steady-state conditions are achieved.

Time-dependent ratings are based on the method described in the IEC Standards 853-1 and 2. Similarly, as in the steady-state case, two modes of load representation can be considered during the emergency period: (1) a step function can be applied, or (2) the load curve from the last 24 hours can be scaled.

During the steady state and time-dependent calculations, the thermal and electrical parameters of the model are continuously adjusted so that the computed and measured cable surface temperatures match.

Enhanced capacity

DFR systems can be used to enhance current-carrying capacity of power cables and to eliminate risk of overheating. They allow utilization of cable systems to their maximum capabilities and are particularly useful when a deferment of costly capital programs is desirable.

System Specifications

Ratings

• User definable (e.g., 20 min, 4 h, 24 h, etc.)
• Steady-state, time to specified over-temperature
• Conductor temperature

Cable types

• 5 through 500 kV
• Paper insulated (HPFF, SCFF), extruded, gas
• In air, directly buried, submarine, ducts, tunnels
• Single, multiple cables & circuits
• Custom systems

Input

• Load current
• Ambient & cable surface temperatures
• Conventional thermocouples, fiber optic DTS

General characteristics:

• Windows™ NT/2000 operating environment main system access by phone (PC Anywhere), LAN
• CPU – RTU communications: phone, wireless
• RTU: ac, dc, solar power
• Alarm capabilities
• Access to historical data
• Data download to local WS
• Dedicated single circuit, multiple circuits
• One year warranty

For more information, contact

Steve Cress, 416.207.6000 x6557, steve.cress@kinectrics.com

George Anders 416.207.6000 x6251, george.anders@kinectrics.com