Rising energy costs and energy-conservation concerns have made efficient performance of an electrical system important. One crucial aspect of the performance is maintaining a high power factor (PF) on the system. To gain top performance, a large electrical power user has to know about the causes of low PF and the ways to correct it.
In one case, when power factor problems caused concern at an industrial complex-and threatened to get worse-engineers initiated a two-stage program to improve the facility's power factor. By combining capacitors with a synchronous condenser, they were able to cut electric costs considerably and improve overall power quality.
The Timken Co., a leading international manufacturer of highly engineered bearings and alloy steels with sales of about $2.6 billion, has five manufacturing plants within a three-mile radius of its headquarters in Canton, Ohio. Two of the plants, along with a high-tech research facility, are within the city limits. During peak production times, these operations, in aggregate, use enough electric energy to light eight cities the size of Cleveland, and much of the energy serves electric arc furnaces.
Timken's Harrison steel mill occupies about 7-million sq ft and is served at 23 kV from three separate utility feeders to a primary distribution bus. It also distributes power at the primary voltage to a second plant (Gambrinus facility) and power at 2300 V (secondary voltage) to two other manufacturing facilities-a bearing plant and the Dueber Avenue plant.
Power consumption at these industrial sites varies constantly. When the Dueber Avenue plant began experiencing fluctuating voltage and loads started dropping out, plant personnel complained to the staff at the Harrison steel mill. The voltage fluctuation problem was also causing failure among the array of seven power-factor-correcting capacitors, which at that time were manually switched. A Timken supplier recommended the company install digital ac power meters that use a PLC interface to replace the original analog meters installed at the substations.
To achieve true RMS measuring this type of ac power meter uses an advanced digital signal processing function. Made by Bitronics Inc., these devices allow Timken engineers to have real time monitoring and automated control because of the communications feature. The Timken facility uses an instrumentation and automation system that includes a Modicon D984-385 programmable logic controller (PLC). The PLC provides overall intelligence and communications. The communications backbone for the system is a Modicon Modbus Plus 1.0 Mbps peer-to peer LAN using a single twisted pair cable.
In the first stage of the project, six Multi-Comm power meters were installed on incoming feeder circuit breakers. The outputs of the meters-networked together-are sent to PLCs, which automatically control three synchronous condensers (motors) to maintain voltage on the secondary power bus within a setpoint range and to maintain power factor as necessary.
Since the installation, the Harrison plant is able to keep voltage within 4% of setpoint, and the power meters have virtually eliminated maintenance of the capacitor banks. At the same time, by accessing the Modbus Plus network, process system users now can monitor how much power is distributed to each plant site on a real-time basis. This capability further motivated engineers to design new screens for Timken's application software, which now allows for cost center accounting. Information also is being put into a database in anticipation the arrival of retail wheeling. A total of 25 meters are now installedat the Harrison plant, and more will be added.
Improving PF is done by installing PF capacitors or synchronous machines to compensate for the flow of reactive energy. Capacitors, which are usually placed in the power distribution system in a shunt configuration, serve as an energy storage device or "var" generator. When used to correct a low PF, a synchronous machine is also called a synchronous condenser. A synchronous condenser generates reactive power that leads real power by 90 deg in phase to offset or cancel normal lagging reactive power consumed by inductive and nonlinear loads.
Operation of the synchronous condenser reduces power factor penalties by generating expensive kilovars from relatively inexpensive kilowatts. As an example, say a facility pays about 3 cents per kilowatt. A kilovar can cost as much as $1.38 per kilowatt demand, depending on power factor. In addition, the condenser absorbs high-frequency transients and noise, in effect "scrubbing" the electric system power at its primary distribution source.