Page 452 - Handbook of Electrical Engineering

COMPUTER BASED POWER MANAGEMENT SYSTEMS 441

• Measurement of active power at the generator terminals.

• Measurement of gas-turbine operating temperature.
• Measurement of the power system frequency.

When a plant is heavily loaded with a load factor above 90% it is necessary to ensure that all
the generators are equally loaded. The equalisation of load is often left to the droop settings of each
governor, or by manual trimming if suitable controls are available. Automatic load sharing can also
be included in the PMS, see sub-sections 2.5.4 and 2.5.5. It is also necessary to maintain the gas
turbines in a ‘clean’ state and not let them become widely mismatched in this respect. If mismatches
in operating electrical power and engine cleanliness exist at the same time, then it is possible that, for
example, say one of the gas-turbine generators will be operating very close to its upper limits. This
will be seen as an excessively high operating temperature. Under very steady load conditions this
excessive temperature could be tolerated for a long time. If the plant has a number of large motors,
comparable in rating to that of one of the generators, then the starting of such a motor will cause a
significant power disturbance at the main busbars. It is common practice with offshore platforms to
start these motors direct-on-line. It is less common to do this with onshore plants. The disturbance
will be shared amongst the generators, and may last for 0.5 to 20 seconds, depending upon the
run-up time of the motor. The disturbance will consist of the static power characteristic of the driven
machine and the necessary accelerating power for the rotating inertia. Hence the disturbance may be
large enough and long enough to cause the operating temperature of the highest loaded generator to
exceed its tripping limit. This generator will then shut down.
The PMS will receive this shut-down signal from one of the 86 lock-out relays, and will
respond in exactly the same way as with the high-speed load shedding, see sub-section 16.3.1.
The low-speed load shedding will be more active when the ambient temperature is high i.e.
near to the site high limit.

16.3.4 Inhibiting the Starting of Large Motors

The volt-drop V at the main busbars can be given by the approximation:

2
X g (KS m − S g S L sin φ L )
| V |= per unit (16.1)
2
S g + KS m X g
Where the prestarting busbar voltage V is unity, and

X g is the transient reactance X in per unit of the generator
d
S g is the KVA rating in per unit of the generator
S m is the KVA rating in per unit of the motor
S L is the KVA rating in per unit of the standing load
cos φ L is the power factor of the standing load
φ L is the power factor angle, hence sin φ L can be found.
K is the starting current to running current ratio of the motor