Similarly, the PMS may have to take into account the
impact of sudden loss of electrical load on the exhaust heat
recovery systems, particularly for HRSG’s. Power system
transient disturbances have much less affect on the response
of auxiliary steam systems, but a longer term reduction in the
electrical load will result in a shortfall in the heat input to an
HRSG, for example. In this case, generally the steam control
system has to implement changes to make up this shortfall,
which can be by increasing auxiliary/supplementary firing in
the HRSG (if installed), or increasing output from
independently fired boilers. Typically, a HRSG will have a
minimum operating heat and mass flow rate, and operating
below this limit may not be possible. Hence if the electrical
load falls below this equivalent threshold, the GTG exhaust
gases may be diverted into an exhaust stack and the HRSG
may be switched into an independently fired mode of
operation. The system must be designed to accept rapid
changeover from exhaust gas mode to completely
independent firing to minimize the potential impact on the
steam system. To support this, the PMS may be required to
make steady state governor adjustments to increase the
electrical load on the generators supplying the HRSG’s, if
other system operating conditions can be maintained within
design limits.