Thermal generating
plants are designed and constructed to convert energy from fuel
(coal, oil, gas, or radiation) into electric power. The actual
conversion is accomplished by a turbine-driven generator.
Thermal generating
plants differ from industrial plants in that the nature of the
product never changes. The plant will always produce electric energy.
The things that may change are the fuel used (coal, oil, or gas) and
environmental requirements.
Many plants that
were originally designed for coal were later converted to oil,
converted back to coal, and then converted to gas. Environmental
requirements have changed, which has required the construction of air
and water emissions control systems.
Plant electrical
systems should be designed to allow for further growth. Sizing of
transformers and buses is at best a matter of guesswork. The plant
electrical system should be sized at 5 to 10% the size of the
generating unit depending on the plant configuration and number of
units at the plant site.
Plant Auxiliary System
Selection of Auxiliary System Voltages
The most common plant auxiliary system
voltages are 13,800 V, 6900 V, 4160 V, 2400 V, and 480 V. The highest
voltage is determined by the largest motor. If motors of 4000 hp or
larger are required, one should consider using 13,800 V. If the
largest motor required is less than 4000 hp, then 4160 V should be
satisfactory.
Auxiliary System Loads
Auxiliary load consists of motors and
transformers. Transformers supply lower level buses which supply
smaller motors and transformers which supply lower voltage buses.
Generation plants built before 1950 may have an auxiliary generator
that is connected to the main generator shaft. The auxiliary
generator will supply plant loads when the plant is up and running.
Auxiliary System Power Sources
The power sources for a generating
plant consist of one or more off-site sources and one or more onsite
sources. The on-site sources are the generator and, in some cases, a
black start diesel generator or a gas turbine generator which may be
used as a peaker.
Auxiliary System Voltage Regulation
Requirements
Most plants will not require voltage
regulation. A load flow study will indicate if voltage regulation is
required. Transformers with tap changers, static var compensators, or
induction regulators may be used to keep plant bus voltages within
acceptable limits. Switched capacitor banks and overexcited
synchronous motors may also be used to regulate bus voltage.
Plant One-Line Diagram
The one-line diagram is the most
important document you will use. Start with a conceptual one-line and
add detail as it becomes available. The one-line diagram will help
you think about your design and make it easier to discuss with
others.
Do not be afraid to get something on
paper very early and modify as you get more information about the
design. Consider how the plant will be operated. Will there be a
start-up source and a running source? Are there on-site power
sources?
Plant Equipment Voltage Ratings
Establish at least one bus for each
voltage rating in the plant. Two or more buses may be required
depending on how the plant will be operated.
Grounded vs. Ungrounded Systems
A method of grounding must be
determined for each voltage level in the plant.
Ungrounded
Most systems will be grounded in some
manner with the exception for special cases of 120-V control systems
which may be operated ungrounded for reliability reasons. An
ungrounded system may be allowed to continue to operate with a single
ground on the system. Ungrounded systems are undesirable because
ground faults are difficult to locate. Also, ground faults can result
in system overvoltage, which can damage equipment that is connected
to the ungrounded system.
Grounded
Most systems 480 V and lower will be
solidly grounded.
Low-Resistance Grounding
Low-resistance grounding systems are
used at 2400 V and above. This system provides enough ground fault
current to allow relay coordination and limits ground fault current
to a value low enough to prevent equipment damage.
High-Resistance Grounding
High-resistance grounding systems limit
ground fault current to a very low value but make relay coordination
for ground faults difficult.
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