Electrical Power Systems

Automatic Generation Control: Conventional Scenario 311
12.7 LOAD SHARING BY PARALLEL GENERATING UNITS
If two or more generating units with drooping governor characteristics are connected to a power
system, there must be a unique frequency at which they will share a load-change. ig. 12.7
shows the droop characteristics of two generating units. Initially they were operating at nominal
frequency f 0 , with outputs P g1 and P g2 . An increase of load DP L causes the generating units to
slow down and the governors increase the output until they reach a new common operating
frequency f c .
Power output
generating unit-1
ig. 12.6: Load sharing by two prallel generating units
with drooping governor characteristics.
The amount of load picked up by each unit depends on the droop characteristic:
Df
¢
DPg1 = Pg1 – Pg1
=
R1 ¢ Df
DPg2 = Pg2 – Pg2
=
R2 Pg1
2
...(12.2)
...(12.3)
D
Hence,
=
DPg2
R
... (12.4)
R1
If the percentages of regulation of the units are nearly equal, the change in the outputs of
each generating unit will be nearly in proportion to its rating.
12.8 CONTROL O POWER OUTPUT O GENERATING UNITS
The relationship between frequency and load can be adjusted by changing an input shown as
"load reference setpoint u in ig. 12.7.
(a) Schematic diagram of governor and turbine
Power output
generating unit-2