Energy and Power Engineering, 2013, 5, 746-750
doi:10.4236/epe.2013.54B144 Published Online July 2013 (http://www.scirp.org/journal/epe)
Analysis of Nonlinear Characteristics of Turbine
Governor and Its Impact on Power System Oscillation
Ge Jin1, Hanting Yan2, Shaoxiang Deng1, Xia omei Chen2, Zexiang Cai2
1Electric Power Research Institute, Guangdong Power Grid, Guangzhou, China
2School of Electric Power, South China University of Technology, Guangzhou, China
Email: yanhanting@126.com
Received March, 2013
ABSTRACT
In this paper, the model of turbine governor based on the physical principles is constituted to improve the accuracy of
power system dynamic simulation, making the results of simulation consistent with the actual situation. The unit and
grid coupling model which reflects the interaction between thermal system of power plant and power system is built
using EMTDC/PSCAD. The influence of nonlinear characteristics on the valve opening and the steam turbine me-
chanical power is also analyzed in this paper. The results of simulation show that the improper setting of parameters
reflecting nonlinear characteristics o f turbine governor can lead to the cyclical oscillatio ns of the valve open ing and the
steam turbine mechanical power, and even can lead to the power oscillation persistently. The research achievements in
this paper have a certain reference value on exploring the causes of power oscillation from the prime mover.
Keywords: Turbine Governor; Modeling; Nonlinear Characteristics; Power System Oscillation
1. Introduction
In the analysis of power system stability, the response
speed of traditional governor is slow and the time con-
stant is large, the mechanical power of prime mover can
be approximately considered as a constant, so the dy-
namic characteristics of governor and prime mover can
be ignored. However, with the rapid development of
power system, the response speed of governor has been
improved greatly. So, when the power system is under
disturbance, it is necessary to consider the response cha-
racteristics of the governor [1]. The power system dy-
namic process of the unit and grid coordination is very
complicated. The governor model of prime mover for
dynamic simulation is very different in different time
domain, but the current governor models are simplified,
which results in the decrease of simulation precision, and
a greater difference between the simulation results and
the accident records. Therefore, the modeling of gover-
nor specific to different dynamic process is critical to the
power system dynamic simulation.
Steam turbine governor system mostly adopts the dig-
ital electro-hydraulic control system (DEH), which is a
dynamic system composed of many elements. It occupies
a very important position in the turbo-generator unit.
Therefore, building complete mathematical model of the
turbine governor, understanding its working mechanism
and analyzing its influence on the power system dynamic
simulation are a very meaningful research subject. In
reference [2], a conclusion is drawn through the simula-
tion that if the speed variatio n ratio is locally too small, a
power system low frequency oscillation with resonance
mechanism may occur. In reference [3], improper selec-
tion of the governor parameter will provide negative
damping to exacerbate low frequency oscillation of the
system.
This paper will be organized as follows. First, the cha-
racteristics of the digital electro-hydraulic control system
(DEH) are analyzed deeply. Secondly, the model of tur-
bine governor considering the nonlinear characteristics is
constituted. Last, the research of the nonlinear character-
istics and their impacts on the power system are per-
formed. Research shows that the nonlinear characteristics
of the governor have a certain influence on the power
system oscillation.
2. Modeling of Turbine Governor
Considering Nonlinear Characteristics
At present, in the power system dynamic simulation,
governor models are simplified ignoring the nonlinear
characteristics of the governor, which results in the de-
crease of simulation precision, and a greater difference
between the simulation results and the accident records.
However, in fact, because of factors such as machinery
manufacturing, the governor has some nonlinear charac-
teristics, such as the rate-limit characteristic, ampli-
tude-limit characteristic, dead band characteristic, clear-
Copyright © 2013 SciRes. EPE
G. JIN ET AL. 747
ance nonlinearity characteristic, etc [4]. These nonlinear
characteristics may lead to the cyclical fluctuation of
regulating valve, which will cause cyclical fluctuation of
the steam turbine mechanical power.
DEH system is composed of pure electric and hydrau-
lic servo system, with the digital computer as the con-
troller of the DEH system, the electro-hydraulic conver-
sion mechanism and the oil motor as its actuator. It is th e
ideal governor system of steam turbine at present [5].
The DEH system usually has speed control, load control,
over-speed protection, steam turbine generator parame-
ters monitoring, as well as on-off sequence control func-
tion. The modeling of DEH governor can be divided into
two parts, the modeling of the control part and the mod-
eling of the actuator.
The control part of DEH has 3 kinds, namely the valve
position control, load control and the regulating pressure
control. Actuator adjusts the valve to the specified loca-
tion according to the turbine control valve instructions
received from the control part. Actuator itself is a closed
loop system. It accepts instruction from the governor
system, and then forms control signals according to the
deviation of current opening and the received instruction,
some add the PID control law in order to achieve the
rapidity and accuracy requirements. Electro-hydraulic
converter is one of the pivotal components in the digital
electro-hydraulic control system (DEH). Electro-hydrau-
lic converter connects the part of electric with the hy-
draulic actuator, and at the same time, enlarge weak
electrical signal into hydraulic signal. Finally, the hy-
draulic actuator with strong power controls the regulating
valve, achieving automatic adjustment. The hydraulic
actuator is composed of relay, wrong throttle and oil
motor. Due to friction, clearance, the insensitive area of
the detecting element, machinery manufacturing and
other factors, the governor has some nonlinear character-
istic [6].
Steam turbine governor system generally adopts the
slide valve oil motor as its actuator. In order to reduce
the interference that pulse pressure fluctuation bring to
the oil motor, slide valve need to have a certain overlap-
ping degree, which cause the slide valve is no t sensitive.
At the same time, the slide valve overcomes the dry fric-
tion during sliding movement. The friction force is al-
ways in the opposite direction with the valve movement,
which also leads to the insensitive of governing system.
In practice, because of the overlapping degree and the
dry friction of slide valve, the turbine governor has the
dead band characteristic. In addition, the load of electric
power system is random, so the frequency of power sys-
tem has natural fluctuation characteristic. The fluc tuation
range is about 0.2 Hz, in which governor system is not
expected to work, lest the frequency fluctuation of power
system would increase. When the frequency of Power
system changes, the unit with electro-hydraulic regulat-
ing system would grab the load due to the high sensitiv-
ity, and the unit acted as basic load is not expected to
participate in the load distribution, and the load is not
expected to change frequently while rate changes
[7].Therefore, in the power system simulation, in order to
stabilize the load, reduce the frequency of adjustment,
the governor has the dead band characteristic.
Hydraulic actuator has a certain rate limit when con-
trols the movement of regulating valve. If the rate limit is
near to the setting value of regulating valve, the regulat-
ing rate of valve wo uld not be proportional to the rate of
the linear system [8]. Therefore, the turbine governor has
the rate-limit characteristic.
The turbine governor has the amplitude-limit charac-
teristic, when the flow of slide valve or the displacement
of oil motor reaches the boundary. In the case of small
disturbance, the system performs in the linear area of
amplitude-limit characteristic of the slide valve and the
oil motor. In the case of great disturbance (such as load
shedding, etc.), the flow of slide valve and the position of
oil motor reach the limit, so, the non linear area of ampli-
tude-limit characteristic works, limiting the response
amplitude of system [9]. As a result, it can prev ent over-
load, reduce the gain of the system and improve the sta-
bility of the system.
Because of the slip ring of the governor need to over-
come factors such as the friction, clearance, and the
overlapping degree of wrong throttle when moving. The
shift of oil motor piston forms the hysteresis; as a result,
the turbine governor has the clearance nonlinear charac-
teristic [10].
To sum up, the model of the turbine governor consid-
ering nonlinear characteristics can be built, as shown in
Figure 1.
w
0
w
1
1
R
TS
K
Amplitude-limit
dead band
P
K
D
K
S
/
I
K
S
1
1
K
TS
GV
PID
1
C
T
1
S
1
1
CH
TS
CV
P
Valve opening
Servo actua tor
rate-limit
m
P
mechanical power
Steam tu rbine
Clearance
GV
valve-regulating
instruction
dead band
Figure 1. The model of turbine governor considering nonlin-
ear characteristics.
Copyright © 2013 SciRes. EPE
G. JIN ET AL.
748
3. Analysis of the Influence of Nonlinear
Characteristics
3.1. The Influence of Dead Band Characteristic
The setting of the dead band has a greater impact on the
primary frequency regulation of the unit. When the fre-
quency of power system fluctuates in a small area, the
dead band is to prevent the unsafe operation of equip-
ments caused by unnecessary working of the turbine
control valve frequently. To analyze the influence of
dead band, this paper take no account of the rate-limit
characteristic, amplitude-limit characteristic, clearance
nonlinear characteristic, and adjust the value of dead
band under 0.1pu external load disturbance. When the
setting value of dead band is less than 0.057 Hz, valve
opening degree and mechanical power have no obvious
fluctuation. When the setting value of dead zone is more
than 0.057 Hz, valve opening degree and mechanical
power have obvious fluctuation, as shown in Figure 2.
3.2. The Influence of Rate-limit Characteristic
The typical parameters reflecting the rate-limit charac-
teristic of the turbine digital electro-hydraulic control
system are as follows: upper to 1.0, lower to -3.0, which
requires the rise rate (the turning up rate of valve adjust-
ment) of the governor less than 1.0. The upper limit is
looser. If the rate limit is near to the setting value of re-
gulating valve, the regulating rate of valve would not be
proportional to rate of the linear system. Therefore, it is
not suitable for the analysis of real system. To analyze
the influence of rate-limit characteristic, this paper take
no account of the dead band characteristic, amplitude-
limit characteristic, clearance nonlinear characteristic,
and adjust the value of rate limit under 0.1pu external
load disturbance, making it close to the rate of actual
valve adjustment slowly.
When the upper limit decreases to 0.29, we can find
that the steam valve fluctuates obviously, and the ampli-
tude increases slowly. When the upper limit is 0.28, the
wave of steam valve oscillates persistently, as shown in
Figure 3. At this moment, the upper limit reflecting the
nonlinear rate-limit characteristic is below the actual ad-
justment rate of the valve. To sum up, if the upper limit
reflecting the nonlinear rate-limit characteristic of the
governor is set inappropriately, making it near to the ac-
tual regulating rate of the adjusting valve, it may lead to
the cyclical fluctuation of valve opening degree and the
steam turbine mechanical power.
3.3. The Influence of Amplitude-limit
Characteristic
The amplitude limit of the general thermal power gener-
ating unit is ±6% - ±10%. To analyze the influence of
Valve opening /pu
/Timet s
Dead band 0.057Hz
Dead band 0.058Hz
Mechanical power Pm/pu
/Timets
Dead band 0.057Hz
Dead band 0.058Hz
Figure 2. Valve opening degree and mechanical power un-
der different value of dead band.
Valve opening /pu
/Timet s
Rate limt 0.29
Rate l imt 0.28
Mechan ical power P m/pu
/Timets
Rate limt 0.29Rate limt 0.28
Figure 3. Valve opening degree and mechanical power un-
der different value of rate limit.
amplitude-limit characteristic, this paper take no account
of the dead band characteristic, rate-limit characteristic,
clearance nonlinear clearance, and adjust the value of
amplitude limit under 0.1pu external load disturbance
(operating v alue set as 106%). When the upper and low er
limits decrease to ±5%, the valve opening degree and
mechanical power have no obvious fluctuation. When the
upper and lower limits decrease to ±4%, the valve open-
ing degree and mechanical power have obvious fluctua-
Copyright © 2013 SciRes. EPE
G. JIN ET AL. 749
tion, as shown in Figure 4.
3.4. The Influence of Clearance Nonlinearity
Characteristic
In the electro-hydraulic servo system, the clearance non-
linearity is mainly produced by torque motor. This kind
of nonlinear characteristic mainly affects the sensitivity
and the static error of system. The effect on the dynamic
characteristics is a fixed phase shift. It has no serious
influence on the stability of the system. The graph and
mathematical expressions of clearance nonlinearity cha-
racteristic are shown in Figure 5.
Valve openi ng /pu
/Timets
4%
amplitude limit
5%
amplitude limit
Valve openi ng /pu
/Timets
4%
amplitude limit
5%
amplitude limit
Figure 4. Valve opening degree and mechanical power un-
der different amplitude limit.
0
()uk
()uk

  
 
  
 

1
,1
1
,1
1,
uk uk
Ku kRukuk
ukuk
ukKukR ukuk
u kothers





Figure 5. The graph and mathematical expressions of clear-
ance nonlinear.
4. The Impact of Nonlinear Characteristics
of Turbine Governor on Power System
Oscillation
The power system unit and grid simulation environment
is established in EMTDC/PSCAD, including the steam
turbine, the governor considering the nonlinear charac-
teristics, the generator, the excitation and the load model,
as shown in Figure 6. With the single machine infinite
system as an example, this paper analyzes the impact of
nonlinear characteristics of turbine governor on the pow-
er system oscillation.
When the nonlinear characteristics of the governor
system give rise to the cyclical fluctuations of the regu-
lating valve, the mechanical power of the unit may fluc-
tuate cyclically. What is more, the active power of
transmission line may also engender cyclically oscilla-
tion.
T
P
ste a m tu r b in e
governor
generator
M
P
excitation
power grid
load
w
T
ref
w
w
re
f
U
e
P
e
Q
t
U
L
P
L
Q
f
E
steam pressure
Figure 6. Structure diagram of machine-grid coupling model.
L
Active power of transmission line P/pu
/Timets
generator /Angle ofrotorpu
/Timets
Figur e 7. Active pow er of transmission line and the angle of
generator rotor.
Copyright © 2013 SciRes. EPE
G. JIN ET AL.
Copyright © 2013 SciRes. EPE
750
Taking the dead band characteristic as an example,
when the value of dead band is greater than 0.057 Hz, the
valve opening degree and mechanical power will have
obvious fluctuations, meanwhile, the active power of
transmission line and the angle of generator rotor also
have cyclically oscillations, as shown in Figure 7.
5. Conclusions
The nonlinear characteristics of turbine governor actually
exist in the real system, but the governor model of BPA
and other simulation software does not take into account
the nonlinear characteristics, leading to the simulation
results inconsistent with the actual situation in some cas-
es. Therefore, the modeling of turbine governor consid-
ering the nonlinear characteristics is very necessary. The
nonlinear characteristics have a certain influence on the
stability of the system, the parameters reflecting the dead
band characteristic, the rate-limit characteristics and the
amplitude-limit characteristics set improperly can cause
the regulating valve degree and the mechanical power to
fluctuate cyclically, and then give rise to the power os-
cillations of transmission line.
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