Closing onto a Dead Bus with Leading
PF Load
It is possible to have a power system
configuration where a bus might have capacitive loading.
• Static capacitors connected to it.
• Energizing a long high voltage
transmission line. Note HV lines inherently appear like capacitors,
which are able to supply MVARs.
In the capacitive loading situations
the generator would have to absorb these MVARs. If the Automatic
Voltage Regulator is in the Auto mode, the generator excitation is
automatically decreased to cause the generator to take in the
required MVARs and to hold the terminal voltage.
If the Automatic Voltage Regulator is
in the manual mode, the excitation is constant and the leading power
factor current which is required for the generator to take in MVARs
could cause the generator terminal voltage to go very high.
Closing onto a Dead Bus with Lagging
PF Load
Inductive loading can take the form of:
• Connected power transformers
• Motor Loads
Inductive loading will cause a
significant voltage drop when the generator breaker is closed, due to
the load absorbing MVARS.
Closing onto a Faulted Bus
Closing the generator output breakers
onto a bus, which has a short circuit fault, can cause generator
damage because of high winding currents, stresses and possible pole
slipping.
Closing onto a Dead Bus with no
Connected Loads
This should not present a problem as
long as the bus has been proven to be free of faults or working
grounds.
GENERATOR LOADING
Closing onto a Finite vs Infinite
System
When we enter into the topic of
generator loading we must consider whether or not the connected
electrical system is very large and hence strong or smaller and
weaker. The first is classed as infinite and the second finite.
A generator connected to a very large
(infinite bus) electrical system will have little or no effect on its
voltage or frequency. In contrast, a generator connected to a finite
bus does have a substantial effect on voltage and frequency.
It is normally assumed that when a
generator has a capacity of greater than 5% of the system size, then
with respect to this generator, the system does not behave as an
infinite bus. For example, when an 800 MW generator is loaded onto a
grid having a capacity of l0,000 MW, the system voltage and frequency
can vary and the system will behave
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