Wind.Turbine.Driven.Induction.Generator.pdf

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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
coupled with an induction generator through a gear
box (1:29). WindMaster 150 kW wind turbine data
has been used in this study. Both wind turbine
generator (WTG) and diesel driven synchronous
generator (DG) are connected to the AC bus. This
hybrid power system is feeding some load through a
transmission line. Simple models of fixed pitch wind
turbine, induction generator, compensating capacitor
banks, diesel engine, diesel engine governor,
synchronous generator, automatic voltage regulator
(AVR), and loads [1,2,3,4] are discussed briefly in
this paper.
Wind energy offers the possibility of generating
substantial amounts of energy. Consumer load is
continuous and varies throughout the day. Power
available from wind generator is variable due to
fluctuating wind velocity. Wind-diesel hybrid systems
are generally used for remote power supply. These
systems are often classed as weak grid systems as they
have limited reactive support. A power fluctuation
problem has been experienced when the wind
generator system uses an induction generator for
energy conversion. This problem could be because of
the turbulent nature of wind velocity, and the reactive
power drawn by these induction generators. Power
quality and reliability are some of major concerns in a
wind-diesel hybrid system. Dynamic analysis of these
wind-diesel hybrid systems has been performed in this
paper to study the effect of disturbances like random
wind variation, network disturbances like load
changes, and the effect of various X/R ratios on the
transient performance and system voltage. Voltage
and power fluctuations resulting from random wind
velocity and load changes can be a problem,
particularly where fault levels are low. Remote area
power supplies are characterised by low inertia, low
damping and poor reactive power support.
A constraint of minimum 40% loading on diesel has
been applied in the model. Minimum 40% power has
to be supplied by the diesel and the rest will come
from the wind generator. In case of higher wind speed,
the output power may exceed the actual load demand.
This may lead to instability because of perturbations
in voltage and frequency. A dump load has been
incorporated in the model to dump excess power. In
this paper, the load in most of the case studies has
been taken as P=80 kW and Q=60 kVAR .
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
effect, the wind turbine model has been simplified.
The quantities on the high-speed side have been
referred to the low speed side. The losses in the wind
turbine and the induction generator have also been
incorporated in this model. Simple models of a wind
turbine system are discussed by Wilkie [5]. The
torque equation for the wind turbine-induction
generator system is given below where the induction
generator inertia is referred to the low speed side:
Iqs, Ids
Iq_ig, Id_ig
AC Bus
Load Bus (Grid)
V t
V Grid
Induction
Generator
Gear Box
Ic
Wind
Turbine
R Tx , X Tx
Iq, Id
P load , Q load
Load
Diesel
Engine
Synchronous
Generator
Fig 1: Block diagram of wind diesel hybrid system
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The equations governing the dynamics of the
induction machine [2,6] are:
ω
d
2
r
T
=
R
T
+
C
ω
+
[
J
+
R
J
]
g
r
g
e
tur
tur
Loss
dt
2
=
R
T
+
[
C
+
R
C
]
ω
.
g
g
r
Loss
Loss
e
tur
sI
=
h
V
−
(
r
h
)
i
−
(
ω
+
h
L
ω
)
i
+
qs
1
qs
1
qs
e
2
m
ds
2
+
[
J
+
R
J
]
s
ω
(1)
g
e
r
tur
(
r
h
)
i
−
(
ω
L
)
i
2
qr
m
1
dr
(4)
2
Where,
C
=
C
+
R
C
g
Loss
Loss
Loss
sI
=
(
ω
+
h
L
ω
)
i
−
(
r
h
)
i
+
(
ω
L
h
)
e
tur
ds
e
2
m
m
qs
1
ds
m
1
The induction generator losses are also referred to the
low speed side. On further simplification, equation (1)
is solved for turbine rotor speed
∗
i
+
(r
h
)
i
qr
r
2
dr
(5)
ω
:
sI
=
−
h
V
+
(
r
h
)
i
+
(
ω
L
h
)
i
−
r
qr
2
qs
2
qs
m
s
2
ds
1
ω
=
[
T
−
R
T
−
r
L
h
r
g
tur
2
r
s
1
s
[
J
+
R
J
]
(
)
i
+
(
L
h
ω
−
ω
)
i
(6)
tur
g
e
qr
s
1
m
e
dr
L
r
2
{
C
R
C
}
]
(2)
+
ω
g
r
Loss
Loss
e
tur
sI
=
−
(
ω
L
h
)
i
+
(
r
h
)
i
−
(
L
h
ω
−
ω
)
dr
m
s
2
qs
2
ds
1
m
e
Where,
tu T = Wind turbine torque in Nm,
T =
r
L
h
Induct. generator torque in Nm,
R = Gear box ratio,
1
)
∗
i
−
(
)
i
(7
qr
dr
L
m ,
J
=Moment of inertia of wind-turbine in kg
tur
L
L
r
m
2
m
J = Moment of inertia of Ind. Gen. in kg
Where
h
=
,
h
=
1
2
2
2
(
L
L
−
L
)
(
L
L
−
L
)
C
= Wind turbine loss coefficient,
C
=
s
r
m
s
r
m
Loss
Loss
e
tur
The standard symbols have been used for these
equations for d, q reference frame. These equations
are derived assuming that q-axis is aligned with the
stator terminal voltage phasor (i.e. V ds =0). The
electrical torque from the induction generator can be
computed as:
Induction generator loss coeff.,
ω
= Wind gen. rotor
r
ω
angular velocity in rad/sec,
= Mech. Shaft-speed
m
ω
of machine in rad/sec,
= Angular velocity of
synch. ref. frame (electrical frequency) in rad/sec
e
)
T
=
L
(
i
i
−
i
i
(8)
e
_
ind
m
qs
dr
ds
qr
2.1.1 Rotor blade passing frequency effect
Only 75% of the reactive power requirement of
induction generator (at no load) has been compensated
with the shunt capacitors to take care of the constant
reactive power requirement whereas the reactive
power which varies with the load will be drawn from
the network. The current components from the WTG
generator, after the capacitor, can be expressed as:
When the three rotor blades passes the tower, there
may be some fluctuation in the speed. In this model,
“1P” and “3P” fluctuations have been added and the
resulting turbine torque is given as:
T WTG =T tur +(0.2T tur ) Sin (
ω
r t)+(0.4T tur ) Sin (3
ω
r t) (3)
2.2 Induction Generator Model
I q_ig = I qs +
ω
C V d (9)
I d_ig = I ds -
ω
C V q (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
δ
is related to the
electrical angular velocity by:
d
δ
=
ω
−
ω
=
∆
ω
(11)
o
dt
The mechanical motion equation in pu is:
Fig.2: Equivalent circuit of induction generator (d,q
reference frame)
ω
d
ω
D
o
=
(
T
−
T
−
∆
ω
)
(12)
Dm
De
dt
2
H
ω
o
781072933.011.png 781072933.012.png 781072933.013.png 781072933.014.png 781072933.015.png 781072933.016.png 781072933.017.png 781072933.018.png
 
Where ∆P= (∆V Cos θ ) 2 / R Tx (20)
2.4 Synchronous Generator Model
∆Q= (∆V Sin θ ) 2 / X Tx (21)
The equations of the synchronous generator are
obtained from Park equations after some
simplifications. The dynamic equations for the
synchronous generator [7] used in this paper are:
The change in voltage is given by:
(
R
P
+
X
Q
)
Tx
Tx
load
load
∆
V
=
pu
'
'
V
Grid
V
=
E
−
x
I
−
rI
(13)
d
d
q
q
d
Where P load and Q load = active and reactive load
demand,
'
'
= tan -1 (X Tx /R Tx )
θ
V
=
E
+
x
I
−
rI
(14)
q
q
d
d
q
1
Varying wind velocity, perturbation in load (P load ,
Q load ) and power electronics load (e.g. variable speed
drives) can cause voltage fluctuation in weak grids
and where fault levels are low. Effect of varying X/R
ratios and load has been investigated in this paper.
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
fluctuations [9]. ICC recommended limits for the
frequency range 0.7-2.5 Hz (1P to 3P fluctuations)
are 0.9-0.65% [10].
'
'
'
E
=
[
E
−
E
+
(
x
−
x
)
I
]
(15)
q
fd
q
d
d
d
'
sT
do
1
'
'
'
E
=
[
−
E
−
(
x
−
x
)
I
]
(16)
d
d
q
q
q
'
sT
qo
The three phase pu electrical power output of a
synchronous generator (two axis model) on a 3 phase
power base is given by:
'
'
'
'
P
=
E
I
+
E
I
+
(
x
−
x
)
I
I
(p.u.)
(17)
q
q
q
q
4. SIMULATION RESULTS
d
d
d
d
The simulations of wind-diesel dynamic model have
been performed in Matlab/Similink [11]. The wind
diesel hybrid system transient behaviour has been
investigated as it undergoes various disturbances such
as random wind speed, change in load, reactive
generation limit, as has the effect of changes in X/R
ratios and load on system voltage.
' ,
'
Where
E
d E
= Synchronous Generator voltages
q
'
d x and
'
x (pu),
behind the transient reactances
' , qo
'
T
do T
= Synch. Gen. open circuit transient time
'
constant of direct/quadrature axis (sec.),
=
Synch. Gen. transient reactances of direct or
quadrature axis (pu), V i , I i (i=d,q) =Voltage,
current in d/q axis, E fd = Field voltage, T Dm , T De =
Diesel and Synchronous Generator torque, H, D=
inertia constant and damping factor
x
=
d
, q
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 R Tx +j X Tx as shown in figure (1). V t is
the voltage at AC bus connected to WTG and DG.
V Grid 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.
The Wind-Diesel hybrid system will have to supply
the following load:
Fig. 3: Block diagram of Wind Diesel hybrid system
in Matlab/Simulink
The block diagram of the dynamic model of combined
wind-diesel hybrid system has been shown in figure
(3) where both, wind generator and the diesel
generators, are connected to a common bus feeding
P / = P load + ∆P (18)
Q / = Q load + ∆Q (19)
781072933.019.png 781072933.020.png 781072933.021.png
 
the load. Initially the total load is supplied by the
synchronous generator while the wind generator is
idling at 6 m/sec, so that both the models settle down
after initial transients. At t=350 sec, the wind turbine
generator (WTG) is connected to the system in such a
manner that the diesel generator feeds only the load
unmet by the WTG.
In wind-diesel hybrid system, induction generator
reactive power requirement varies with the load. Only
75% of the reactive power requirement of induction
generator (at no load) has been compensated with the
shunt capacitors. Diesel generator is supplying the rest
of the reactive power to induction generator in
addition to active/reactive power of the system load.
At t=350 sec, WTG is connected to the system to
share the load with DG. In the beginning, wind speed
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 (P load =80
kW and Q load =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.
4.1 Effect of Blade Passing Frequency on Wind
Turbine Torque
In figures (5) and (6), the transients in voltage and
frequency are clearly visible because of the frequent
changes in reactive power demand. After 500 seconds,
a small voltage deviation can be seen, when reactive
demand from induction generator (IG) increased. This
voltage fluctuation is further increased after 600
seconds, when the reactive load was also increased.
Finally the voltage becomes unstable when the diesel
generator hits its reactive generation limit and the
reactive power demand is still increasing. This may
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.
Fig. 4: Wind turbine torque for blade passing
frequency effect
As mentioned earlier, the rotor blade passing effect
has been included in the wind turbine model. The
effect of this can be seen in the wind turbine torque as
shown in figure (4). The variation in torque is shown
in the plot due to 1P and 3P frequency variations.
4.2 Effect of Reactive Power (Q) Variation
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
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. 5: Diesel generator voltage plot
Fig. 6: Frequency plot
781072933.022.png
plots are visible in figure 7(g) due to rapidly varying
wind speed and the WTG power is trying to follow the
same pattern.
5, 2,1 and 0.5. For one X/R ratio (say 7), the active
load demand has been decreased in steps from 0.4 pu
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
rotor speed and the blade passing frequency effect. It
can be commented here that the power quality of wind
diesel hybrid system can be improved by using storage
or a converter/inverter.
Fig. 9: Plots for ∆V (in d, q frame) for different X/R
ratios for change in reactive power
A similar procedure was used to plot voltage deviation
for various X/R ratios, with reactive power demand
reduced in steps as in the previous case. During this
process, active load demand was kept constant at 0.4
pu. The plots for ∆V in figure (9) indicate that the
voltage variations are more for the case when the
reactive power demand on wind-diesel system is
4.4 Effect of X/R Ratio
In this case, voltage deviation (∆V) in the system has
been investigated by varying the X/R ratios from 7 to
781072933.023.png

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