4/30/2015
Composite Regulating Characteristic of
Power Systems
• In the analysis of load-frequency controls (LFCs), we
are interested in the collective performance of all
generators in the system.
• The inter-machine oscillations and
transmission
system performance are therefore not considered.
• We tacitly assume the coherent response of all
generators to changes in system load and represent
them by the equivalent generator.
• The equivalent generator has an inertia constant Meq
equal to the sum of the inertia constants of all the
generating units and is driven by the combined
mechanical outputs of the individual turbines.
• The composite power/frequency characteristic of a
power system depends on the combined effect of
the droops of all generator speed governors.
• It is also depends on the frequency characteristic of
all the loads in the system.
• For a system with n generators and a composite loaddamping constant of D, the steady state frequency
deviation following a load change is given by,
• The effect of governor speed droop and frequency
sensitivity of load on the net frequency change are
illustrated below.
System equivalent for LFC analysis
• where
• Thus, the composite frequency characteristic of
the system is
• Normally expressed in MW/Hz.
• Sometimes referred to as the stiffness of the
system
• The composite regulating characteristic of the
system is equal to
-: Example 2 :Two generators rated 200 MW and 400 MW are
operating in a parallel. The droop characteristics of their
governors are 4% and 5%, respectively. From no load to
full load. Assuming that the generators are operating at
50 Hz at no load,
• How would a load of 600 MW be shared between
them ?
• What will be the system frequency at this load ?
Composite governor and load characteristic
1