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 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
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