POWER QUALITY ISSUES IN A WIND TURBINE DRIVEN INDUCTION GENERATOR AND
DIESEL HYBRID AUTONOMOUS GRID
Hari Sharma* Syed Islam** Trevor Pryor* C. V. Nayar**
*Murdoch University Energy Research Institute (MUERI), Murdoch University, WA
**Centre for Renewable Energy and Sustainable Technologies Australia (CRESTA),
Curtin University of Technology, WA
Abstract:
Power quality issues of wind-diesel hybrid systems have been discussed in this paper. Fixed pitch wind turbine
coupled with induction generator has been considered for this study. The rotor blade passing frequency effect has
been included in the wind turbine model. Dynamic response to random wind speed, voltage deviation due to
continuously varying load, reactive power limit is shown in the results. Effect of varying X/R ratios has also
been investigated on system voltage deviation.
1. INTRODUCTION The system shown in figure (1) is a typical wind-
diesel hybrid system. A fixed pitch wind turbine is
Wind energy offers the possibility of generating coupled with an induction generator through a gear
substantial amounts of energy. Consumer load is box (1:29). WindMaster 150 kW wind turbine data
continuous and varies throughout the day. Power has been used in this study. Both wind turbine
available from wind generator is variable due to generator (WTG) and diesel driven synchronous
fluctuating wind velocity. Wind-diesel hybrid systems generator (DG) are connected to the AC bus. This
are generally used for remote power supply. These hybrid power system is feeding some load through a
systems are often classed as weak grid systems as they transmission line. Simple models of fixed pitch wind
have limited reactive support. A power fluctuation turbine, induction generator, compensating capacitor
problem has been experienced when the wind banks, diesel engine, diesel engine governor,
generator system uses an induction generator for synchronous generator, automatic voltage regulator
energy conversion. This problem could be because of (AVR), and loads [1,2,3,4] are discussed briefly in
the turbulent nature of wind velocity, and the reactive this paper.
power drawn by these induction generators. Power
quality and reliability are some of major concerns in a A constraint of minimum 40% loading on diesel has
wind-diesel hybrid system. Dynamic analysis of these been applied in the model. Minimum 40% power has
wind-diesel hybrid systems has been performed in this to be supplied by the diesel and the rest will come
paper to study the effect of disturbances like random from the wind generator. In case of higher wind speed,
wind variation, network disturbances like load the output power may exceed the actual load demand.
changes, and the effect of various X/R ratios on the This may lead to instability because of perturbations
transient performance and system voltage. Voltage in voltage and frequency. A dump load has been
and power fluctuations resulting from random wind incorporated in the model to dump excess power. In
velocity and load changes can be a problem, this paper, the load in most of the case studies has
particularly where fault levels are low. Remote area been taken as P=80 kW and Q=60 kVAR.
power supplies are characterised by low inertia, low
damping and poor reactive power support. 2.1 Wind Turbine Model:
2. SYSTEM MODEL In practice, it is sometimes difficult to get all the
parameters required for the dynamic models. To this
Iqs, Ids Iq_ig, Id_ig
AC Bus Load Bus (Grid)
effect, the wind turbine model has been simplified.
Vt VGrid
The quantities on the high-speed side have been
Induction
Gear Box
Generator
Ic referred to the low speed side. The losses in the wind
Wind
Turbine
turbine and the induction generator have also been
RTx , XTx
incorporated in this model. Simple models of a wind
Iq, Id
turbine system are discussed by Wilkie [5]. The
Pload , Qload
torque equation for the wind turbine-induction
Load
Diesel Synchronous generator system is given below where the induction
Engine Generator
generator inertia is referred to the low speed side:
Fig 1: Block diagram of wind diesel hybrid system
The equations governing the dynamics of the
dÉr
2
Ttur = RgTe + CLossÉr + [Jtur + Rg Je]
induction machine [2,6] are:
dt
2
= RgTe + [CLoss + RgCLoss ]Ér .
e
tur
sIqs = h1Vqs - (rsh1)iqs - (Ée + h2LmÉm)ids +
2
+ [Jtur + Rg Je]sÉr (1)
(rrh2)iqr - (ÉmLmh1)idr (4)
2
Where, CLoss = CLoss + RgCLoss
sIds = (Ée + h2LmÉm )iqs - (rsh1)ids + (ÉmLmh1)
e
tur
The induction generator losses are also referred to the
" iqr + (rr h2 )idr
(5)
low speed side. On further simplification, equation (1)
is solved for turbine rotor speed Ér :
sI = -h V + (r h )i + (É L h )i -
qr 2 qs s 2 qs m s 2 ds
1
Ér = [Ttur - RgTe -
r L h
2J r s 1
s[Jtur + Rg e] ( ) i + ( L h É - É )i (6)
qr s 1 m e dr
L
r
2
{CLoss + RgCLoss }Ér ] (2)
e
tur sI = -(É L h )i + (r h )i - (L h É - É )
dr m s 2 qs s 2 ds s 1 m e
Where, Ttur = Wind turbine torque in Nm, Te =
rr Lsh1
Induct. generator torque in Nm, Rg = Gear box ratio,
" iqr - ( )idr (7)
Lr
Jtur =Moment of inertia of wind-turbine in kg m2 ,
Lr Lm
Je = Moment of inertia of Ind. Gen. in kg m2
Where h1 = , h2 =
2 2
(Ls Lr - Lm ) (Ls Lr - Lm )
CLoss = Wind turbine loss coefficient, CLoss =
e
tur
The standard symbols have been used for these
Induction generator loss coeff., Ér = Wind gen. rotor
equations for d, q reference frame. These equations
are derived assuming that q-axis is aligned with the
angular velocity in rad/sec, Ém = Mech. Shaft-speed
stator terminal voltage phasor (i.e. Vds=0). The
of machine in rad/sec, Ée = Angular velocity of
electrical torque from the induction generator can be
synch. ref. frame (electrical frequency) in rad/sec
computed as:
Te _ ind = Lm (iqsidr - idsiqr ) (8)
2.1.1 Rotor blade passing frequency effect
Only 75% of the reactive power requirement of
When the three rotor blades passes the tower, there
induction generator (at no load) has been compensated
may be some fluctuation in the speed. In this model,
with the shunt capacitors to take care of the constant
 1P and  3P fluctuations have been added and the
reactive power requirement whereas the reactive
resulting turbine torque is given as:
power which varies with the load will be drawn from
the network. The current components from the WTG
TWTG=Ttur+(0.2Ttur) Sin (Ér t)+(0.4Ttur) Sin (3Ér t) (3)
generator, after the capacitor, can be expressed as:
2.2 Induction Generator Model
Iq_ig= Iqs+ É C Vd (9)
Id_ig= Ids- É C Vq (10)
2.3 Diesel Engine and Governor
Diesel generator model used in this study consists of
diesel engine, governor control and inertia block. The
details of the model can be found in [1,3]. The
electrical angle of the rotor ´ is related to the
electrical angular velocity by:
d´
= É - Éo = "É (11)
dt
The mechanical motion equation in pu is:
Éo
Fig.2: Equivalent circuit of induction generator (d,q dÉ D
= (TDm - TDe - "É ) (12)
reference frame)
dt 2H Éo
2.4 Synchronous Generator Model Where "P= ("V Cos ¸ )2 / RTx (20)
The equations of the synchronous generator are "Q= ("V Sin ¸ )2 / XTx (21)
obtained from Park equations after some
simplifications. The dynamic equations for the The change in voltage is given by:
synchronous generator [7] used in this paper are:
(RTx Pload + XTxQload )
"V = pu
' ' VGrid
Vd = Ed - xq I - rI (13)
q d
Where Pload and Qload = active and reactive load
' '
demand, ¸ = tan-1(XTx/RTx)
Vq = Eq + xd I - rI (14)
d q
1
' ' ' Varying wind velocity, perturbation in load (Pload,
Eq = [E - Eq + (xd - xd )I ] (15)
fd d
Qload) and power electronics load (e.g. variable speed
'
sTdo
drives) can cause voltage fluctuation in weak grids
and where fault levels are low. Effect of varying X/R
1
' ' '
Ed = [-Ed - (xq - xq )I ] (16)
ratios and load has been investigated in this paper.
q
'
sTqo Voltage fluctuation profile on mains can be variable
and can be a mixture of step, ramp, sinusoidal or
random, which largely depend upon the source of the
The three phase pu electrical power output of a
fluctuations [9]. ICC recommended limits for the
synchronous generator (two axis model) on a 3 phase
frequency range 0.7-2.5 Hz (1P to 3P fluctuations)
power base is given by:
are 0.9-0.65% [10].
' ' ' '
Pe = Ed Id + EqIq + (xd - xq )Id Iq (p.u.) (17)
4. SIMULATION RESULTS
The simulations of wind-diesel dynamic model have
Where Ed ', Eq' = Synchronous Generator voltages
been performed in Matlab/Similink [11]. The wind
diesel hybrid system transient behaviour has been
behind the transient reactances xd ' and xq' (pu),
investigated as it undergoes various disturbances such
as random wind speed, change in load, reactive
Tdo',Tqo' = Synch. Gen. open circuit transient time
generation limit, as has the effect of changes in X/R
constant of direct/quadrature axis (sec.), xi' ,q =
=d
ratios and load on system voltage.
Synch. Gen. transient reactances of direct or
quadrature axis (pu), Vi, Ii (i=d,q) =Voltage,
current in d/q axis, Efd= Field voltage, TDm, TDe=
Diesel and Synchronous Generator torque, H, D=
inertia constant and damping factor
The most commonly used IEEE type 1 AVR (auto-
matic voltage regulator) model [8] has been used.
3. VOLTAGE FLUCTUATION
As mentioned earlier, WTG and diesel generator are
feeding a load through a transmission line having
impedance of RTx+j XTx as shown in figure (1). Vt is
the voltage at AC bus connected to WTG and DG.
VGrid is the voltage at load bus where the load is
actually connected. There will be some power loss
("P, "Q) in the transmission lines and transformer.
Fig. 3: Block diagram of Wind Diesel hybrid system
The Wind-Diesel hybrid system will have to supply
in Matlab/Simulink
the following load:
The block diagram of the dynamic model of combined
P/ = Pload + "P (18)
wind-diesel hybrid system has been shown in figure
(3) where both, wind generator and the diesel
Q/ = Qload + "Q (19)
generators, are connected to a common bus feeding
the load. Initially the total load is supplied by the In wind-diesel hybrid system, induction generator
synchronous generator while the wind generator is reactive power requirement varies with the load. Only
idling at 6 m/sec, so that both the models settle down 75% of the reactive power requirement of induction
after initial transients. At t=350 sec, the wind turbine generator (at no load) has been compensated with the
generator (WTG) is connected to the system in such a shunt capacitors. Diesel generator is supplying the rest
manner that the diesel generator feeds only the load of the reactive power to induction generator in
unmet by the WTG. addition to active/reactive power of the system load.
At t=350 sec, WTG is connected to the system to
4.1 Effect of Blade Passing Frequency on Wind share the load with DG. In the beginning, wind speed
Turbine Torque is increased in steps so as to increase reactive demand
on DG. At t= 400 sec, wind speed u increased from 6
to 8 m/sec, at t=450, u increased from 8 to 11 m/sec
and at t=500, u increased from 11 to 14 m/sec. After
this, the wind speed is kept constant at 14 m/sec.
During this time, the load was kept constant (Pload=80
kW and Qload=60 kVAR). At t=550, 600 and 650 sec,
the reactive load was increased from 60 kVAR to 63
kVAR, 66 and 69 kVAR in steps respectively to
further increase the reactive power demand on diesel
generator.
In figures (5) and (6), the transients in voltage and
Fig. 4: Wind turbine torque for blade passing
frequency are clearly visible because of the frequent
frequency effect
changes in reactive power demand. After 500 seconds,
a small voltage deviation can be seen, when reactive
As mentioned earlier, the rotor blade passing effect
demand from induction generator (IG) increased. This
has been included in the wind turbine model. The
voltage fluctuation is further increased after 600
effect of this can be seen in the wind turbine torque as
seconds, when the reactive load was also increased.
shown in figure (4). The variation in torque is shown
Finally the voltage becomes unstable when the diesel
in the plot due to 1P and 3P frequency variations.
generator hits its reactive generation limit and the
reactive power demand is still increasing. This may
4.2 Effect of Reactive Power (Q) Variation
result in poor power quality and reliability due to
rapid fluctuations in voltage and frequency resulting
from the changes in reactive power demand.
Thyristorised compensation can be used to overcome
this problem, which may further increase the cost of
remote area power systems.
4.3 Random Wind Speed Variation
The transient response of this wind diesel hybrid
system has also been investigated for a more realistic
variable wind speed, as shown in figure 7(a). After
350 seconds, wind velocity is suddenly increased from
6 m/sec to around 14 m/sec and thereafter the wind
Fig. 5: Diesel generator voltage plot
velocity is undergoing rapid changes. As a result of
that, the transients in system voltage and frequency
can be observed in plots 7(b) and (c). Voltage
deviation is shown in fig 7(d), which varies between
0.0368 to 0.0384 pu (approximately 3.8%). Similar
transients can be seen in the power plots in fig 7 (e),
(f), (g) and (h) for synchronous generator active,
reactive power, induction generator active and reactive
power contributions. At 350 seconds, when WTG is
connected to this rural autonomous grid, the
contribution from diesel generator drops down. As the
wind speed increases, the contribution from WTG
increases and vice versa. Transients in wind power
Fig. 6: Frequency plot
plots are visible in figure 7(g) due to rapidly varying 5, 2,1 and 0.5. For one X/R ratio (say 7), the active
wind speed and the WTG power is trying to follow the load demand has been decreased in steps from 0.4 pu
same pattern. to 80%, 60%, 40%, 20% and 0. This step decrease in
active load started at t=400 seconds after the wind
turbine is connected to wind-diesel bus and thereafter
at every 50 seconds, the active power has been
reduced. This was repeated for the various X/R ratios.
The reactive demand was kept constant at 0.4 pu.
Voltage deviation for different X/R ratios has been
plotted in figure (8).
Fig. 8: Plots for "V (in d, q frame) for different X/R
ratios for change in active power
It can be seen in figure (8) that "V for X/R ratio of 7
is low whereas "V is high for X/R ratio of 0.5. It
indicates that when the wind turbine generator is
connected to strong network, voltage fluctuations are
less compared to the case when WTG+DG are
connected to a weak network having low X/R ratio.
Fig. 7: Plots for random wind speed
When the wind power and diesel power generation
exceed the load demand, excess power goes into the
dump load as shown in figure 7(i) [plotted for 400 sec.
onwards]. In response to random wind speed (fig 7a),
the wind turbine torque is shown in figure 7(j). Wind
turbine torque plot is changing rapidly due to varying Fig. 9: Plots for "V (in d, q frame) for different X/R
rotor speed and the blade passing frequency effect. It ratios for change in reactive power
can be commented here that the power quality of wind
diesel hybrid system can be improved by using storage A similar procedure was used to plot voltage deviation
or a converter/inverter. for various X/R ratios, with reactive power demand
reduced in steps as in the previous case. During this
4.4 Effect of X/R Ratio process, active load demand was kept constant at 0.4
pu. The plots for "V in figure (9) indicate that the
In this case, voltage deviation ("V) in the system has voltage variations are more for the case when the
been investigated by varying the X/R ratios from 7 to reactive power demand on wind-diesel system is
varied. Voltage becomes unstable when the reactive (ACRE). ACRE' is funded by the Commonwealth's
demand was reduced to zero around 600 seconds. It Cooperative Research Centres Program.
can be commented here that the voltage fluctuation is
more sensitive to reactive power variation. The 7. REFERENCES
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6. ACKNOWLEDGEMENTS
The authors wish to acknowledge the support of
program 5.21 on system integration within the
activities of Australian CRC for Renewable Energy