The utility tie does not have sufficient reactive power to
maintain the site power system voltage within the required
tolerance. Thus when operating in the connected mode, an
additional criterion is to keep the reactive power import close
to zero. The per unit voltage on the plant side of the utility
transformers is kept close to the nominal value by the plant
generators connected to the utility transformer bus. The perunit
voltage on the utility side of these transformers could be
quite different than the nominal utility voltage. The only way to
ensure that the flow of reactive power through the utility
transformers is close to zero and to keep the plant voltage
within tolerance is to ensure that the turns ratio of the utility
transformer equals the ratio of the voltages on the utility and
plant sides. This will cause the voltage drop across the utility
transformers to be very small leading to a very low flow of
reactive power through the transformers. In order to achieve
the correct turns ratio, the PMS will send voltage set point
adjustments to the utility transformer AVRs. These set point
adjustments will result in the tap position being changed until
such time that the correct turns ratio is achieved. In
accordance with the principles of robust control described
earlier, only voltage set point changes are sent to the
transformer AVRs – no attempt is made by the PMS to directly
control the tap position of the utility transformers. Should a
large GTG trip, the utility transformer AVRs will control the
OLTCs in order to supply reactive power to the plant in order
to try to maintain the voltage at a correct level.