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TURBINE SPEED CONTROL
The turbines are of the stall regulated
type, but, as they do not connect to a large fixed frequency network a
different form of speed governing is required. The load management system
devised for the original scheme has shown excellent long term reliability,
and was a natural choice for use in the new scheme. This operates simply by
using precision frequency sensitive static relays to increase the load
applied to the turbine output in steps as the speed rises and, conversely,
to disconnect these loads in steps as the speed falls.
Each turbine has a "dump load", capable of
accepting at least 150% of the nominal turbine rating, provided adjacent to
the turbine control panel. The dump load control equipment is housed in the
turbine control panel, and is so placed to allow the turbine to be run, and
its output measured, before its power is switched to the island network. For
this governing system to be stable, it is very important that the rate of
increase of load with increasing frequency is set to be significantly
greater than the increase in turbine power with the corresponding increase
in rotor speed. The response time of the dump load control must also be
fast, to avoid hunting, which will interact with the generator's automatic
voltage regulator and may cause lights on the system to flicker. The control
system used is triplicate, one chain for each phase, and the switches are
reset every cycle. Thus the dump load is adjusted some one hundred and fifty
times each second.
The concept of using power generated in
excess of demand for heating, that was developed for the 1982 scheme, was
commercially successful and popular. This is a distributed form of dump
load, and uses remote frequency sensitive static relays to control heating
appliances in the individual buildings of the community. These remote relays
are programmable, and sealed within a special distribution box at the meter
board in each building. The programme settings are controlled by the system
operator, as it is desirable to control the slope of the load vs. frequency
characteristic and to maintain a reasonable balance between phase loadings.
The original scheme provided for three one kilowatt heating appliances per
household. One of these, normally the first on, was used for water heating
via a changeover thermostat, which rerouted that channel to a storage
radiator when the water was up to the required temperature. This system has
been extended as a necessary part of the project to provide up to six
frequency channels per household, each rated to a maximum load of two
kilowatts.
The frequency settings range for the
domestic load control relays are chosen within the band from 50.2 Hz. - 52.4
Hz. The settings used for the turbine dump load relays are chosen within the
band from 52.5 Hz. - 54.6 Hz. It follows that all the automatic heating
circuits on the island have an opportunity to accept power before this
energy is dumped.
One
consideration that was nearly overlooked in the new scheme, is the need
to
balance the settings for
the dump load relays between the two turbines. If this is not done, there is
no problem up to the point where the turbines are soft coupled or
synchronised. Then, if there is energy to be dumped, it will go to that dump
load with the lowest selected frequency and, in extreme cases, surplus power
from one turbine . may transmit across the network to the other dump load.
In practice, a
perfect match is not necessary. However, a reasonable match between the dump
load control profiles does reduce the circulating current in the network and
must be beneficial in reducing losses.
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