Load Frequency Control Slides _ Part Two

Load Frequency Control

1. GOVERNOR CHARACTERISTICS OF A SINGLE GENERATOR
•Prime movers are fitted with governors, regarded
as primary control elements.
•Governors sense the change in a speed control
mechanism to adjust the opening of steam valves.
•Load increases -Speed drops from no load (N
o
) to
its full-load value (N) is (N
o
−N ).
•Steady-state speed regulation in per unit is given
by
•Value of R varies from 2% to 6% for any
generating unit.
Characteristics of a typical governor of
a steam turbine

2. GOVERNOR CHARACTERISTICS OF A SINGLE GENERATOR
•Since the frequency and speed are directly related
load to the rated frequency of the unit:
•If the generation is increased by ΔP
G
due to a
static frequency drop of Δf, then the speed
regulation can be defined as the ratio of the
change in frequency to the corresponding
change in real-power generation:
•Unit of R is taken as Hz per MW. I
•Power is measured in per unit and hence R is in
Hz/p.u. MW.
Characteristics of a typical governor of
a steam turbine

3. GOVERNOR CHARACTERISTICS OF A SINGLE GENERATOR
•Let ‘A’ be the initial operating point of the turbine at 50%
load
•If the load is dropped to 25%, the speed increases to
99%.
•To keep the speed at 25% of the load same as at ‘A’, the
governor setting has to be changed by changing the
spring tension in the fly-ball type of governor.
•Thistypeofshiftingthespeedorfrequency
characteristicparalleltoitselfisknownas
supplementarycontrol.
•Itisadoptedinon-linecontroltoensureproperload
divisionamongtherunningunitsandtomaintainthe
systemfrequency.
Speed Governing System

4. GOVERNOR CHARACTERISTICS OF A SINGLE GENERATOR
•There is another method of changing the slope of the
governor characteristics.
•ThisisachievedbychangingtheratiooftheleverL(see
speedcontrolmechanism)ofthegovernorandthereby
adjustingtheparameterRtoensureproperco-
ordinationwiththeotherunitsofthesystem.
•This adjustment can be made during the off-line
condition only.
Speed Governing System

5. ADJUSTMENT OF GOVERNOR CHARACTERISTIC OF
PARALLEL OPERATING UNITS
•Twogeneratorsarerunninginparallel,
•Governorcharacteristicofthefirstunit
(Line1)isshowntowardstheright,
•Secondunit(Line2)isshowntowards
theleftofthefrequencyaxis.
•Correspondingtotheratedfrequencyf
r
,
thetwounitscarryloadsP
1
andP
2
so
thatthesystemloadP
D
=P
1
+P
2
.
•Ifthesystemloadisnowincreasedto
P′
D
,thesystemfrequencywilldropdown
tof′,sincetheunitscanonlyincrease
theiroutputbydecreasingthespeed.
Sharing of load by two units (parallel) with a
speed-governor characteristics setting