IMPACT OF LARGE SCALE WIND POWER ON POWER SYSTEM STABILITY

7 Pages·00.42 MB·English

by  Dr. Eng. Mohamed EL-Shimy (Ain Shams University)

I L S W P
MPACT OF ARGE CALE IND OWER ON
P S S
OWER YSTEM TABILITY
M. EL-Shimy*, M.A.L Badr**, and O.M.RASSEM***
present use either one of the following systems, depicted in
Abstract-- This paper presents a detailed analysis of the impact Fig. 2: (a) squirrel cage induction generator, (b) doubly fed
of large scale wind power generation on both the dynamic voltage (wound rotor) induction generator, (c) direct drive
stability and the transient stability of electric power systems. The synchronous generator.
following problems have been analyzed: different penetration of
wind power impact on transient stability and on voltage stability
(dynamic voltage stability) following a major fault in the
transmission system, determination of the acceptable wind power
penetration level in power system without deteriorating both
transient stability and voltage stability of the system. Moreover,
the effect of SVC on the system stability is studied.

Index Terms—Wind energy, wind turbine, transient stability,
dynamic voltage stability, penetration level.

Fig. 1: Basic Principle of Wind Power Generation
I. INTRODUCTION

TH E main advantages of conventional generation are the The main subsystems of a constant speed wind turbine are a
rotor and a squirrel cage induction generator. It has, however,
relatively moderate price of the generated electricity and
repeatedly been argued in the literature that a representation of
the controllability and flexibility of the generator output.
the low speed wind turbine shaft, which connects the wind
On the other hand, the main advantages of renewable power
turbine rotor to the gearbox, should be included in the model,
generation are the usage of an infinitely available primary
particularly for transient stability studies [4-6]. The shaft of
energy source (such as sunlight, wind, and biomass) and the
conventional synchronous generators is normally neglected in
less severe environmental consequences. The wind is an
power system dynamics simulations, because the torsional
infinite primary energy source. Further, other environmental
resonance frequencies tend to lie above 10 Hz, the upper limit
impacts of wind power are limited as well [1]. Wind has
of the investigated frequency band [7]. However, this is not
proven to be one of the most successful of all available sources
true for constant speed wind turbines. Due to the softness of
of renewable energy offering relatively high capacity, with
the low speed shaft between the turbine rotor and the gearbox,
generation costs competitive with conventional energy sources.
its resonance frequency is in the order of 2 Hz and thus well
Therefore, the technologies for generation of electrical energy
within the bandwidth of interest. Therefore, the shaft is also
from renewable energy sources, especially wind energy, have
represented in the constant speed wind turbine model
evolved in recent years [2, 3].
presented here. The resonance frequencies of the gearbox and
The working principle of a wind turbine encompasses two
the high speed shaft are well above 10 Hz [8]. These are
conversion processes, which are carried out by its main
therefore neglected.
components: the rotor, which extracts kinetic energy from the
The behavior of a power system is for the largest part
wind and converts it into a mechanical torque, and the
determined by the behavior and the interaction of the
generating system, which converts this torque into electricity.
generators that are connected to it. The grid itself consists
This general working principle is depicted in Fig. 1.
mainly of passive elements, which hardly affect the behavior
Currently, three main wind turbine types are on the market.
of the system, and as for the loads, only those in which directly
The main differences between the three concepts are the
grid coupled motors are applied have a significant impact on
generating system and the way in which the aerodynamic
the behavior of the system [9].
efficiency of the rotor is limited during high wind speeds. As

for the generating system, nearly all wind turbines installed at

* Mohamed EL-Shimy Mahmoud, PhD, is with Ain Shams University,
Faculty of Engineering, Cairo, Egypt (emails: [email protected] and
[email protected] )
** Mohamed Abd Latef Badr, Prof., is with Ain Shams University, Faculty of
Engineering, Cairo, Egypt (emails: [email protected])
*** Omnia Rassem, Eng., is with TELECOM Egypt, and is an M.Sc
candidate (email: [email protected])
978-1-4244-1933-3/08/$25.00 ©2008 IEEE 630
Fig. 2: Wind Turbine Generator (WTG) Types

Fig. 3: The general structure of the constant speed wind turbine model

In wind turbines, generating systems differ from the different penetration of wind power impact on transient
conventional directly grid coupled synchronous generator stability and on voltage stability (dynamic voltage stability)
which is traditionally used in power plants. Due to their following a major fault in the transmission system,
different characteristics, these generating systems interact determination of the acceptable wind power penetration
differently with the power system than synchronous level in power system without deteriorating both transient
generators. The change in this interaction determined stability and voltage stability of the system. Moreover, the
mainly by the degree of penetration of wind power effect of SVC on the system stability is studied.
generation in the system [10, 16].
In the analysis of wind generation-system interaction a II. CONSTANT SPEED WIND TURBINE
distinction is made between local and system wide impacts (CSWT)MODEL
of wind power [15-18]. Local impacts of wind power are It was one of this research objectives to investigate
impacts that occur in the (electrical) vicinity of a wind whether models of the constant speed wind turbines that can
turbine or wind park that can be attributed to a specific be used for power system dynamics simulations are
turbine or park, i.e. of which the cause can be localized. available already. The aim was to identify existing models
These effects occur at each turbine or park, independently that can be used for the investigations, possibly with
of the overall wind power penetration level in the system as adaptations. If no usable models was not found, the
a whole. Wind power locally has an impact on the objective becomes to develop these models.
following aspects of a power system: branch flows and node The presented CSWT model, for simplicity, assumes an
voltages, protection schemes, fault currents and switchgear algebraic relationship between the wind speed and the
ratings, harmonics, and flicker. System wide impacts, on the mechanical power extracted from the wind i.e. a quasistatic
other hand, are impacts of which the cause can not be rotor model. The disadvantages of using a quasistatic
localized. They are a consequence of the application of approach are a reduced accuracy and a neglect of the
wind power that can, however, not be attributed to dynamic nature of the conversion of wind speed to
individual turbines or parks. Nevertheless, they are strongly mechanical torque. The models of the generator in the wind
related to the penetration level in the system as a whole. turbine system is derived assuming that magnetic saturation
However, in opposition to the local effects, the level of is neglected, flux distribution is sinusoidal, all losses are
geographical spreading of the wind turbines and the applied neglected, except for copper losses, the sum of the stator
wind turbine type are less important. Apart from the local currents equals zero. Other assumptions will be indicated
impacts, wind power also has a number of system wide when appropriate.
impacts, because it affects: dynamics and stability, reactive The general structure of the constant speed wind turbine
power generation/voltage control possibilities, and system model is depicted in Fig. 3. From the left, first, a wind
balancing: frequency control and dispatch of the remaining speed model is depicted of which the output is a wind speed
conventional units. sequence. The wind speed sequence is converted into
This paper presents a detailed analysis of the impact of mechanical power by the rotor model. This mechanical
large scale wind power generation on both the dynamic power serves as an input for the model of the shaft or the
voltage stability and the transient stability of electric power drive train, of which the second input is the rotational speed
systems. The following problems have been analyzed: of the generator. The outputs of the shaft model are the
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wind turbine rotor speed and the mechanical generator voltage control at the connected node and outside the
power. The inputs of the generator model are the operating limits the SVC susceptance is locked.
mechanical power from the rotor model and the grid voltage
and frequency. Its outputs are the active and reactive power
supplied to the grid.
For each are of the blocks depicted in Fig. 3, models are
already available in the literature [19-37]. However, most of
them are incomplete. Hence, a complete mathematical
model for CSWT that can be used in power system stability
analysis are shown in the appendix.

III. STUDY SYSTEM DESCRIPTION Fig. 5: Wind Farm Interconnection to The Original syste

The IEEE 9-bus system shown in Fig. 4-a represents the
Table 1: Induction Generator Parameters (Sb = 100MVA)
interconnected power system used for this study. This Valu
Parameter Unit
system is considered the “original system” and consists of 9 e
buses, 3 synchronous generator, 6 lines, 3 transformers, and Stator Resistance, r 0.01 p.u
s
3 load centers. Each generator is equipped with an AVR of Stator Reactance, xs 0.10 p.u
the standard IEEE model 1 type with its block diagram Rotor Resistance, rr 0.01 p.u
Rotor Reactance, x 0.08 p.u
shown in Fig. 4-b. The original system data on a 100 MVA r
Magnetizing Reactance, x 3.00 p.u
base are given in [24, 38]. m
Inertia Constant, H 2.5 kWs/kVA
wr
Inertia Constant, H 0.5 kWs/kVA
m
Shaft stiffness, K 0.3 p.u
s
No. of Poles, p 4 -
Gear Box Ratio, η 1/89 -
GB
Blade length 75 m
No. of Blades 3 -

Fig. 4-a The Original System

Fig. 6: SVC Regulator

The block diagram of the SVC regulator is shown in Fig.
6. The SVC regulator parameters on a 100 MVA base are:
the regulator time constant, T = 10 sec.; the regulator gain,
r
K = 100; the reference voltage, V = 1.00 p.u.; the
r ref
maximum susceptance, b = +1 p.u., the minimum
max
susceptance, b = -1 p.u.
min
This study system can be considered as a simple model
for studying impact of large scale wind power on power
system stability. After the load flow analysis, the focus is to
perform dynamic analyses to verify that the system remains
transiently stable under different contingencies and different
levels of wind power produced. The system loads are kept

Fig. 4-b: Standard IEEE Model 1 AVR fixed at the values utilized in the load flow analyses.

The wind farm has been modeled as an induction
generator that is connected to the original system at bus 7 IV. SIMULATION RESULTS
via the two transformers and a line as shown in Fig. 5. The
First of all to set a base for comparison between the
induction generator data are shown in Table 1.
system transient response with and without wind generation,
Additionally, an SVC unit at bus 8 has been included as
a 3 cycle (0.05 sec, for 60 Hz) duration fault is simulated on
the reactive power supplier. The SVC is modeled in the
line 5-7 at bus 7 (started at t = 0.5 sec) in the original IEEE
simulation program [24] as a limited linear controlled
9-bus system (0.0 % penetration level, and no SVC). The
susceptance. The regulator has an anti-windup limiter, thus
power angles, and bus voltage magnitude (for buses 7 and 8
within the operating limits the SVC performs automatic
only to avoid crowding of plots) variations are shown in
Fig. 7 and Fig. 8. Based on these figures, following the
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applied fault the system is dynamically stable and the of the SVC increases the settling time of the power angles,
system is able to restore the voltage. in other words, reduces the system damping. Based on Fig.
8, and Fig. 11, the dynamic voltage stability of the system is
enhanced due to the existence of the SVC.

Fig. 7: Original System (0.0 % penet. level, and no SVC) Power Angle
Variations
Fig. 11: Variations of b, V8, Q8 (0.0 % penet. level, and with SVC)

Now, the impact of wind power generation on the system
transient and voltage stability is considered. Different
penetration levels of wind power generation are considered
such that a part of the power produced by the synchronous
generator at bus 2 is replaced by an equal amount of wind
power generation at bus 12 and the system is simulated for
the previously described fault location and fault duration.
The question to be answered is: what is the limit of wind

power penetration level to keep the system transient and
Fig. 8: Original System (0.0 % penet. level, and no SVC) Bus Voltage
voltage stability for the considered contingency?
Magnitude Variations
The following cases are considered (which are important
As a matter of investigation, the simulation is performed
samples of the full study cases):
with the same fault location and fault duration but with the • Case 1: No SVC, 0.63 p.u wind power
SVC connected at bus 8 (0.0 % penetration level, and with
generation (24.55 % penetration level)
SVC). The power angles, and bus voltage magnitude • Case 2: SVC, 0.63 p.u wind power generation
variations are shown in Fig. 9 to Fig. 10. Moreover, the
(24.55 % penetration level)
variations of the SVC susceptance, voltage magnitude at
• Case 3: SVC, 1.40 p.u wind power generation
bus 8, and the net reactive power injected to bus 8 are
(77.94% penetration level)
shown in Fig. 11.
The power angles, and bus voltage magnitude variations

for each of these cases are shown in Fig. 12 to Fig. 17.

Fig. 9: Power Angle Variations (0.0 % penet. level, and with SVC) a) Fig. 12: Case 1, Power Angle Variations

Fig. 13: Case 1, Bus Voltage Magnitude Variations

Fig. 10: Bus Voltage Magnitude Variations (0.0 % penet. level, and with Based on the above responses of cases 1 and 2, it is clear

SVC)
that both the transient and voltage stability of the system are

Based on these figures, following the applied fault the enhanced with the SVC installed in the system. In both

system is transient stable and the system is able to restore cases the system is stable.
the voltage. However, the angle response of the system is
quite different (Based on Fig. 7, and Fig. 9). The existence
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Fig. 14: Case 2, Power Angle Variations
Fig. 19: Case 3, Variations of b, V8, Q8.

V. CONCLUSION
This paper presents a detailed analysis of the impact of
large scale wind power generation on both the dynamic
voltage stability and the transient stability of electric power
systems.
Different penetration levels of wind power generation are
considered such that a part of the power produces by
Fig. 15: Case 2, Bus Voltage Magnitude Variation
conventional synchronous generators is replaced by an

equal amount of wind power generation and the system is
simulated for a prescribed perturbation. This kind of study
is not only intended to investigate the impact of wind
power generation on system stability, but also in the
determination of the acceptable wind power penetration
level in power system without deteriorating both transient
stability and voltage stability of the system. Moreover, the
effect of SVC on the system stability is studied.

Fig. 16: Case 3, Power Angle Variations
VI. APPENDIX: CSWT MODEL
ROTOR MODEL: The mechanical power extracted from
the wind is calculated from the following equation:

ρ
P = c (λ)A v3 (1)
w p r w
2
where P the mechanical power extracted from the wind, ρ
w
is the air density, c the performance coefficient or power
p
Fig. 17: Case 3, Bus Voltage Magnitude Variations coefficient, λ the tip speed ratio and A the rotor swept area.
r
With penetration level of 77.94% (case 3), the system The speed tip ratio λ is the ratio between the blade tip
becomes unstable both in transient stability and voltage speed vt and the wind upstream the rotor vw and is given by:
stability. The instability phenomena can be viewed as v 2Rω
λ= t =η ωr
(2)
reactive power compensation insufficient, this can be GB
v pv
detected form the variations of the SVC susceptance w ω
(shown in Fig. 18 and Fig. 19 for cases 2, and 3 where ηGB is the gear box ratio, p the number of poles of
respectively). Based on Fig. 19, it is clear that the SVC the induction generator and R the rotor radius. Finally, the
susceptance hits its upper limit and locked at that value the cp(λ) curve is approximated as follows:
which means that the reactive power limits of the SVC are 16.5
125  −
reached. Therefore, the system instability in this situation is cp =0.44 λ −6.94e λi (3)
a voltage instability phenomenon in nature which is clear  
i
from Fig. 17.
with
Based on the above study, the maximum penetration
1
level is less than 77%. λ= (4)
i 1 +0.002
λ

SHAFT MODEL: Figure 20 shows a two mass
representation of the rotating part a wind turbine. When the
shaft damping is neglected, the shaft is described by the
following equations:

Fig. 18: Case 2, Variations of b, V8, Q8.
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v =Vsin(−θ)
r (8)
v =Vcos(θ)
m
and the power absorptions are given by:
P =v i +v i
r r m m
( ) (9)
Q =v i −v i +b v2 +v2
m r r m c r m
where b is the fixed capacitor conductance which is
c

determined at the initialization step.
Fig. 20: A Two Mass Representation of The Rotating Part A W
The differential equations in terms of the voltage behind the

dω T −K γ stator transient impedance r+j x’ are:
wr = wr s s
dt 2H e' −v = ri −x'i
wr r r s r m (10)
dω K γ−T e' −v = ri +x'i
m = s e (5) m m s m r
dt 2H whereas the link between voltages, currents and state
m
variables is as follows:
dγ
=Ω(ω −ω ) de' ( ) ( ( ) )
dt b wr m r =Ω 1−ω e' − e' − x −x' i /T'
b m m r o m o
dt
where Ωb = 2πf, and f is the nominal grid frequency (Hz), T (11)
is torque (p.u.), γ is the angular displacement between the de' ( ) ( ( ) )
m = −Ω 1−ω e' − e' + x −x' i /T'
two ends of the shaft (electrical radians), ω is rotational b m r m o r o
dt
speed (p.u.), H is the inertia constant (s) and K is the shaft
s where x , x’, and T can be obtain from the generator
stiffness (p.u. torque/electrical radians). The indices wr, m o o
parameters, that are represented in Fig. 22 , and are given
and e mean wind turbine rotor, generator mechanical and
by:
generator electrical respectively.
x = x +x
o s m
The mechanical torque is: x x
P x' = x + r m (12)
T = w (6) s x +x
wr ω r m
wr x +x
To simulate the tower shadow effect [4], a periodic torque T' = r m
pulsation is added to T , whose frequency depends on the o Ωr
wr b r
rotor speed ωr, the gear box ratio ηGB, and the number of The electrical torque Te is given by:
blades nb, as follows T =e'i +e' i (13)
~   Ωω  e r r m m
Twr =Twr1+0.025sinηGB b wr t (7)
  n 
b
where the torque pulsation amplitude is fixed to 0.025
according to [4].
GENERATOR ELECTROMAGNETIC MODEL: The simplified

electrical circuit used for the squirrel cage induction
Fig. 22: 1st order Induction Generator Mode
generator is the same as the one for the single-cage

induction motor the only difference with respect to the
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