ELECTRIC POWER
149
standby or emergency generator system which was required by local building codes. This invested capital was standing idle, waiting for a power failure that hardly ever occurred. In addition, the engine's waste heat could be used to heat water or make steam, that could be used and meet the PURPA requirements for a Cogeneration system. It then made simple economic sense to install these Cogeneration systems to reduce overall operating costs. In all of these systems, overall energy conversion efficiencies often are in the 60-70 percent range, whereas the standard fossil fuel fired electric utility plant rarely exceeds 35 percent energy conversion efficiency.
9.4 VARIABLE SPEED ELECTRIC DRIVE SYSTEMS. Variable speed electric drive systems are an emerging technology that has its roots in several areas, among these are: energy conservation; microprocessor controls; SCADA or DCS systems; improved process control needs; and reductions in operating costs. Many process systems do not operate at constant rates, and as a result the process does not operate as efficiently as theoretically possible. This is primarily due to the limitations of the mechanical equipment used in the process, such as pumps, compressors, blowers, fans, control valves, skimmers, agitators, mixers, etc. Improvements in power semiconductor technology have allowed steadily increasing currents and voltages to be used on transistors, thyristors, and silicon controlled rectifiers. In most industrial operations, the induction motor,is the preferred driver, due to its low initial cost, high reliability, and relatively smaller size compared to other motor and engine drives. The disadvantage of the electric motor has always been the difficulty of speed control; for alternating current motors, the speed is directly proportional to the power supply frequency, which could not usually be varied. Thus other means had to be used to vary the process output of the driven equipment, since the speed of the equipment usually was fixed. Control valves were used in fluid control applications, and the results worked well, but were not energy efficient. In air flow application, dampers had to be used, which were often unreliable. In some applications, a bypass flow stream resulted in fluid constantly being recycled through the same pump, resulting in higher fluid operating temperatures than was desirable. Many times, operating controls could not be properly adjusted for changing operating conditions, possibly due to the operating range being wider than that of the control equipment's capability, or in some cases, due to lack of automation of the control process. All of these problems are solvable with a variable speed drive system. The installation of variable speed drive systems is usually economically justified based on energy cost savings over the installation of a fixed speed drive, however, the other factors mentioned above also can be used as the justification for their use. A modern variable speed drive system will consist of a synchronous or induction motor driven by a solid-state adjustable frequency power supply package. The system will be controlled by an integral microprocessor based control system that usually interfaces to a remote DCS. The heart of the system is the adjustable frequency power supply. This subsystem consists of: appropriate input power conditioning, filtering and transformation equipment; a rectification section to convert the AC supply power to DC; an inverter section to convert the DC to an adjustable AC output; output power conditioning, filtering and transformation equipment; and a control system to monitor and produce the continuously variable AC output, based on the required load characteristics and any external control signals from the process. The rectification section usually consists of silicon controlled rectifiers. The inverter section for small systems consists of power transistors, while larger systems use thyristors or silicon controlled rectifiers. The output is usually formed from a six or twelve pulse generating bridge, with the pulse widths being controlled by the microprocessor to simulate a sinusoidal wave shape. The frequency is adjusted by the control system based on the changing process conditions. Use of microprocessor based controls allows very stable frequencies to be produced, as well as allowing very small changes to occur as required by the process. Suitable output inductors, capacitors and filters are used to smooth the wave shape as necessary for the output motor device to utilize. Harmonic filtering may be required on the input side, so that higher order harmonics are not transmitted to the rest of the electrical system. Variable speed drive systems of the above types are now available in sizes ranging from fractional horsepower to over 10,000 horsepower, and in speed ranges from as low as a few rpm to over 6,000 rpm. Of course, the entire speed range is not available in any one drive system, as usually drive systems, especially the motors are designed for a smaller speed range, e.g., a large drive might be designed for from 3,600 to FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554621