The power system control and complexity increases further
when GT driven generators are also coupled with an exhaust
heat recovery system. Most heat recovery systems are largely
de-coupled from the electrical power system, even though the
amount of GT waste heat recovered is proportional to the
electrical load. This de-coupling is achieved by accepting the
variable heat input from the GT exhaust and supplementing it
with inbuilt auxiliary-fired burners. Alternatively separately
fired boilers or local heating systems may be provided. Time
constants associated with steam system disturbances tend to
be much longer than those associated with electrical power
system disturbances, hence the heat recovery control
management system has time to respond and initiate
secondary heat sources. However, some exhaust heat
recovery systems, such as HRSG’s require a minimum GT
exhaust heat and mass flow for stable operation. Thus some
form of PMS controlled secondary governor regulation may
also be required to ensure the GT’s are loaded above a
minimum load, as well as the other requirements for load
sharing, frequency control and GT DLN operating limits.