Conventional
methods of system load shedding are too slow and do not effectively calculate
the correct amount of load to be shed. This results in either excessive or
insufficient load reduction. In recent years, load shedding systems have been
repackaged using conventional under-frequency relay and/or breaker interlocks
schemes integrated with Programmable Logic Controllers to give a new look to an
antiquated load preservation methodology.
A truly
modern and intelligent load shedding system with a computerized Power
management system should provide fast and optimal load management by utilizing
system topology and actual operating conditions tempered with knowledge of past
system disturbances. This paper demonstrates the need for a modern load
shedding scheme and introduces the new technology of intelligent load shedding.
Comparisons
of
intelligent load shedding with conventional load shedding methods are made from
perspectives of system design, system engineering, project implementation, and
system operation. A case study of the application of an intelligent load
shedding scheme in a large industrial facility is provided.
INTRODUCTION AND BACKGROUND
In general, load shedding can be defined as the amount of load
that must almost instantly be removed from a power system to keep the remaining
portion of the system operational. This load reduction is in response to a
system disturbance (and consequent possible additional disturbances) that
results in a generation deficiency condition. Common disturbances that can
cause this condition to occur include faults, loss of generation, switching
errors, lightning strikes, etc.
When a power system is exposed to a disturbance, its dynamics
and transient responses are mainly controlled through two major dynamic loops.
One is the excitation (including AVR) loop that will control the generator
reactive power and system voltage. Another is the prime-mover loop,which will
control the generator active power and system frequency. A brief discussion of
these two dynamic loops is given below.
A. Excitation /
Generator – Reactive Power – Voltage
During a fault condition, one of the direct effects of a fault
current is the drainage of reactive power from the system.This reactive power
is essential for the transfer of mechanical energy to electrical energy (and
vice versa) in the rotating machines (generators and motors). After the fault
clearance, system is faced with partially collapsed flux energy in the rotating
machines and has to balance its generation and load levels while rebuilding its
magnetic energy. During this time, depending on the motor residual back emf , the system is also faced with an
additional reactive power demand from the motor loads under reacceleration
conditions. The voltage regulation and operating voltage of the overall system
will directly depend on the amount of reactive power that the generators could
deliver to the system. On severe disturbances, the generators may automatically
call upon its over-excitation capability (ceiling voltage), which help in
recovering the system stability.
B. Prime Mover /
Generator – Real Power - Frequency
Turbine governors
and the type of prime movers also have a dramatic impact on the performance of
the power system during major disturbances. The frequency conditions of the
overall system directly depend on the amount of real power that the generator
prime movers can deliver to the system. Also, the mechanical energy available
to help the generators prime mover ride through a fault or other disturbances
plays an important role on the system behavior. This stored energy varies
dramatically between that of a gas turbine, steam turbine, and hydro units. As
a consequence, the performance of power systems supplied by different types of
prime movers and governors will behave very differently under both steady state and transient conditions.In addition to
system upsets caused by faults, there are disturbances caused by switching
surges or lightning strikes. As an example, some switching disturbances can
result in a loss of generation or cause a system to separate from the utility
grid (system is landing condition). This condition can cause the power system to
collapse and will be adversely impacted by inappropriate load reduction caused
by an improper load shedding scheme. For some switching disturbances (that
results in a loss of generation or system is landing condition), the cascading
effects may be of the primary concern if the load shedding action is not set
correctly and/or timed properly.
Moreover, the type
of disturbance impacts the dynamic response of the prime mover. For instance, a
short circuit at the power station bus bar may result in acceleration of the
generator prime mover. When this occurs the speed regulator will then initiate
closing of the fuel or gas inlet valve. After the fault has been cleared, the
turbines face the impact of the load still connected. At this time their fuel
or gas inlet valves are closed resulting in difficult re acceleration conditions.
