Energy and Power Engineering, 2013, 5, 363-367
doi:10.4236/epe.2013.54B070 Published Online July 2013 (http://www.scirp.org/journal/epe)
Analysis o n Effect of P a rameters of Different Wind
Generator on Power Grid Transient Stability*
Zhi-wei Wen1, Li Ding 2, Shi-en He3
1Gansu Electric Power Research Institute, Lanzhou, China
2Beijing Electric Research Institute of Economics and Technology, Beijing, China
3Wind Power Technology Center of Gansu Electric Power Corporation, Lanzhou, China
Received January, 2013
ABSTRACT
To analyze the factors which affecting transient stability of power system, the dynamic model of doubly-fed induction
generator and direct-drive PM synchronous generator has been built using PSCAD. Impact of different wind farm inte-
gration on grid typically in China has been presented. The influence of the variations of transient reactance, negative
sequence reactance and rotary inertia on critical clearing time of power system transient stability is analyzed by
time-domain simulation. Mixture operation of DFIG and PMSG to optimize the stability of system has been analyzed
firstly. The digital simulation results show that doubly-fed induction wind turbines is a better choice to meet the re-
quirement of system instability due to large wind farm integration in comparison with direct-drive PM synchronous
wind turbines. With a rather large rotary inertia, the proper ratio of direct-drive PM synchronous wind turbines used in
wind farm could be comprehensive planning by optimized the stability of system. Analysis of this paper should be pro-
vided as academic reference for improving design of wind farm system.
Keywords: Wind Farm; Doubly-fed Induction Generator; Direct-drive PM Synchronous Generator; Parameter;
Transient Stability
1. Introduction
Unlike coal and other conventional resources used for
power generation, some renewable energy, such as wind
power, is random, intermittent and uncontrollable. Acci-
dent of grid or the tripping of wind farm will increase the
difficulty of system recovery with significant penetration
of wind power, even leading to the grid collapse. During
the past decades, wind power penetration limit has been
exceeded from about 5 percent to 20 percent in many
countries based on the reactive power compensators and
reinforcement of networks[1]. The choice of wind gen-
erator has been an important technical and economic
target to power system because analysis results show that
parameters of induction motor, such as stator reactance,
initial rotor slip, have influence on power system tran-
sient stability[2]. Effect of mechanical parameters of
induction generator (IG) and doubly-fed induction gen-
erator (DFIG) has been analyzed in conference[3]. Com-
parative analysis between IG, DFIG and direct-drive
permanent magnetism synchronous generator (PMSG) on
power system transient stability shows that variable
speed wind generator is a better choice to meet the re-
quirement of system in stability due to large wind farm
integration in comparison with wind generator of con-
stant speed[4]. Because the effect of converter, it is dif-
ficult to research the contrast between different type of
wind generator.
The main motive of this paper is to develop a simula-
tion platform of the integrated wind farm, and to evaluate
the performances of the transient characteristic with dif-
ferent t type of wind generator. Models of different type
of type of wind generator connected with grid are estab-
lished. Furthermore, some factors of generator which
affect the permissible power flow of the grid, such as
transient reactance, negative sequence reactance and ro-
tary inertia, have been studied. Comparative analyze of
the critical clearing time (CCT) have been proposed. Es-
pecially, mixture operation of DFIG and PMSG to opti-
mize the stability of system has been analyzed that can
provide fundamental data for future research of the wind
energy development.
2. Model of Grid
2.1. Model of Grid-connected System
To investigating the transient stability of power system
connected with a large wind power, a simulation system
*This work was supported by Science and Technology Research Pro-
j
ect of Gansu Electric power Corporation (No. 2013101022).

Z.-W. WEN ET AL.
364
was developed. The simulation system consists of three
parts: the wind power generator, the transmission system
and the consumer load.
The transmission system of HVAC is the most popular
transmission system, which is shown in Figure 1. It
needs support power grid and reactive power compensa-
tion equipment to meet the requirement of voltage and
frequency regulation[5].
2.2. Model of Grid-connected Wind Farm
A typical wind power system includes generator, three-
blade rotor, gearbox, and control devices of wind speed,
blade angle and voltage. Accessory system includes
convertor and protection devices. DFIG is the main type
of wind generator used in grid-connected wind farm be-
cause of active and reactive power flow of grid can be
decoupling controlled by convertor of wind farm to im-
proved electric power quality and power factor of wind
farm[6]. But in recent decade, more and more PMSG
based on full power convert have been used in wind
farms to realize the flexible control of operation[7]. On
the other hand, price fluctuation of magnet material will
affect the future interest of manufacture and the cost of
using PMSG. Diagram of wind power system with DFIG
and PMSG are shown in Figures 2 and 3, respectively.
2.3. Equivalent Circuit of Generator
Using equivalent T-circuit of DFIG, equations of static
electromagnetism analysis can be described as [8]:


sss ssrm
sr
rrs rm
UIrjx IIjx
UrjX III jx
ss



 



(1)
where Us and Is stand for the voltage and current of the
stator, respectively (p.u.); Ur and Ir stand for the voltage
and current of the rotor, respectively(p.u.); s is the slip
factor; rs and xs stand for the resistance of the stator and
synchronizing reactor of generator, respectively (p.u.);
xm stand for the field reluctance of rotor(p.u.) ; rr and xr
stand for the resistance and reactor of rotor, respec-
tively(p.u.).
Using equivalent Γ-circuit of PMSG, equations of
static electromagnetism analysis can be described as[9]:
0
1
11
q
da
de qd
ddd
qad
qe d
qqq
L
dir iiu
dt LLL
di RL
ii
dt LLLL


 



 



q
q
u
(2)
where id and iq stand for the d-axis current and q-axis
current of stator, respectively(p.u.); Ld and Lq stand for
the d-axis induction and q-axis induction of stator, re-
spectively(p.u.); r
a stand for the resistance of the stator;
ωe stand for the angular frequency; λ0 stand for the mag-
netic flux linkage; ud and uq stand for the d-axis voltage
and q-axis voltage of stator, respectively(p.u.).
Figure1. Model of transmission system based on HVAC.
Figure 2. Model of wind turbine with DFIG.
Figure 3. Model of wind turbine with PMSG.

Z.-W. WEN ET AL. 365
2.4. Model of Wind Farm and Stability Limit of
Power System with Multi Generators
In most conference, we always assume a wind farm con-
nected with the grid as an equivalent generator to simpli-
fied exponent number of model of wind farm. To analyze
power distribution and interrelationship of different type
of generator, a mixture operation model of wind farm
with DFIG and PMSG has been discussed in this paper.
Transient stable region of power system with multi
generators defined as[10]:
A(Tm)={(ω1, ω2,…, ωm)|-Tei(ω1, ω2,…, ωm)>-Tmi,
i=1,2,…m} (3)
where A is stable region of power system operated on
balance point after the fault; Tmi is the input mechanical
torque of the ith generator; Tei is the electromagnetic
torque of the ith generator.
To analyze the stability limit of power system with
multi generators, such as calculating the critical clearing
time (CCT), the CCT of power system is the time when
any one of generators could not meet the requirement of
equation (3).
3. Simulation
3.1. Diagram of Power System
A typical single-load infinite-bus power system connected
with a group of wind farm been built using PSCAD is
shown in Figure 4. To analyze the effect to the transient
stability of grid, only value of one parameter of generator
in the wind farm G1, such as transient reactance, would
be changed linearly to calculate the CCT of power sys-
tem when two-phase short circuit fault occurred in one
transmission line. The voltage of load, transmission
power and other parameters of grid will keep constant.
Parameters of two typical wind generators used in
wind farm are shown in Table 1.
3.2. Result of Simulation
Firstly, the ratio of PMSG used in the wind farm G1
would be changed linearly to analyze the effect of mix-
ture operation. Results of simulation shown in Figure 5
are that value of initial power-angle get smaller with the
increasing the ratio of PMSG used in wind farm. It
means that failure recovery procedure of power system
be different with different wind generator used in the
same wind farm. Neglecting the effect of control strategy,
the CCT also get smaller is shown in Figure 6. It means
that transient stability of power system maybe get worse
whit more PMSG connected with grid.
As we all know, there are two main factors of genera-
tor used in wind farm that affect the transient stability of
grid: main electric parameters and mechanical parame-
ters.
M
240 MW
10.5 kV
cosφ=0.80
300 MVA
10.5/242 kV
Uk%=14%
280 MVA
220/121 kV
Uk%=14%
Y0Y0
G1 T-1T-2L
115 kV
P0=220 MW
cosφ=0.98
230 km
Figure 4. Diagram of single-load infinite-bus power system.
Table 1. Parameters of a wind power generator system.
Item Value Value
Type DFIG PMSG
PN(MW) 2.0 2.0
UN (V) 690 660
fN (Hz) 50 50
rs (p.u.) 0.0043 0.0052
xs (p.u.) 0.1 0.26
rr (p.u.) 0.012 /
xr (p.u.) 0.1 /
xm(p.u) 3.5 0.414(xad)
HT (s) 4.02 4.54
HG (s) 1.4 3.5 Figure 5. Curve of power-angl.

Z.-W. WEN ET AL.
366
Figure 6. CCT of power system.
Figure 7. Curve of CCT with change of reactance.
Aim to analyze the effect of electric parameter, the
va
riation of calculating CCT with changing rotary
in
riation of calculating CCT when change transient re-
actance and negative sequence reactance of generator is
shown in Figure 7. Results of simulation show that CCT
of system get smaller when increasing the transient reac-
tance of wind generator. On the contrary, the calculating
CCT of system get a little bigger when increasing the
negative sequence reactance of wind generator. Com-
parative simulation shows that the effect of transient re-
actance is more significantly than negative sequence re-
actance.
The va
ertia of generator to analyze the effect of mechanical
parameter is shown in Figure 8. Results of simulation
show that CCT of system get larger when increasing the
transient reactance of wind generator. One reason of de-
velopment of PMSG is to raising the pull-in torque of
generator to improve the utilized coefficient of wind en-
ergy, improving of transient stability of power system
would also be ascribed to this electromagnetic and me-
chanical character.
Figure 8. Curve of CCT with rotary inertia.
Results of simulation show that transient stability of
po
convert in both DFIG
an
4. Conclusions
lation of a grid-connected wind farm
wer system would be affected by electromagnetic pa-
rameters and mechanical parameters of generator in a
complementary way. For PMSG, on the one hand the
bigger would contribute to the transient stability; On the
other hand, the bigger transient reactance would worsen
the transient stability of power system. Considering all
these factors, the result of transient stability of power
system should be as the basis of choosing the type of
generator or defining the number of PMSG which has
some promising features, such as high torque and low
speed are desired in wind farm.
Because the power control of
d PMSG been neglected, results of simulation in this
paper just revealing some possible reason based on elec-
tromagnetic and mechanical parameters of generator. For
further application of wind generator, a study of optimum
design of generator should also be needed.
In this paper, a simu
equipped with doubly fed induction generator and di-
rect-drive permanent magnetism synchronous generator
has been established. Comparative analysis of post-fault
transient performance of grid with different type of gen-
erator has been proposed. The digital simulation results
of critical clearing time of two-phase short circuit fault
show that both electric parameters and mechanical pa-
rameters should be affect the stability of grid with large
amounts of wind power. Because the increase of number
of permanent magnetism synchronous generator would
decrease the CCT of power system, the dynamic stability
of grid connected with a wind farm must be analyzed
with full academic. Analysis of this paper should be pro-
vided as academic conference for improve design of in-
terconnected wind farm system.

Z.-W. WEN ET AL.
367
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