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International Journal on Electrical Engineering and Informatics - Volume 7, Number 4, Desember 2015
Effect of Grid-Connected Wind Turbine Generators on Power
System Transient Stability
Shaon Ahmed1, Mohd Abdur Rashid
2, Shamshul Bahar YAAKOB
3,
Adawati Yusof3, and Mohd Fareq
3
1Department of Electrical and Electronic Engineering, Hamdard University Bangladesh, Bangladesh
2Faculty of Design Arts and Engineering Technology, Universiti Sultan Zainal Abidin, Kuala Terengganu, Malaysia 3School of Electrical Systems Engineering, Universiti Malaysia Perlis, Kubang Gajah, Perlis, Malaysia
Abstract: A huge number of wind turbine generators will be integrated into the existing
power systems in the near future because it has been identified as one of the most
promising field of energy industry. Wind turbines are required to remain connected to
the grid during a fault condition so as to support constant power supply. It is therefore
necessary to investigate the impact of wind turbine generators on the stability of power
systems. This paper presents a comparative study of available wind generator types and
examines the effect of penetration level on the power system. Simulations have been
performed to compare and demonstrate the transient behavior of a typical 5-machine 22-
bus system with and without wind power integration. The simulation results reveal that
the transient behavior of wind generators has significant effect on the overall stability of
power systems and the increase in penetration level may induce the instability into the
systems.
Keywords: Wind Energy Conversion System, Wind Generator, Transient Response,
Transient Stability Index
1. Introduction
Over the past few decades the electrical power generations from wind turbines has received
substantial interest from manufacturers and researchers in order to reduce fossil fuel
dependency and rising environment concerns. Global wind energy council (GWEC) estimates
that, as of 2013, the global wind power production has reached 318 MW. The market forecast
suggests that it could go as high as 600GW by 2018, which would mean that almost 8%
world’s total electrical energy consumption would be supplied by wind turbines[1-2]. As a
result a large number of wind turbines are going to be connected to the existing network in the
near future.
One of the significant decisions in designing the wind energy conversion (WECS) is what
type of generators is to be used along with the wind turbine. Induction generators (IG) have
been used widely due to their robustness, low cost and rugged construction. However, several
other types of generators have also been trialed and used on wind turbines. So, researchers are
often forced to mark some subtle indicators to decide among the available options. The
transient stability of a particular generator type and the impact it has on the power system
stability often serves as one of those key indicators.
A power system is said to be transiently stable if it is capable of maintaining synchronism
even after sustaining a large disturbance. Often, these faults are caused by equipment outage,
sudden load change of equipment faults and the time frame of a transient study is usually 3-
5seconds [3]. These faults results in a change of rotor angle position and impedes natural
operating conditions. The ability to restore equilibrium between mechanical torque and
electrical torque of a machine in interconnected system is determined by the rotor angle of the
machine. The transient stability problem of generators and machines is a thoroughly
investigated and now a fully understood topic in power system study. However transient
stability study of wind generator is still an evolving issue, since the nature of problem posed by
these generators are not same as that of conventional generators. Unlike conventional
generators the transient fault behavior of a wind generator depends largely on power electronic
Received: November 17th
, 2014. Accepted: November 16th
, 2015
DOI: 10.15676/ijeei.2015.7.4.8
644
controllers along with the inertial response. As a result, a grid fault into the power system does
not always accompanied by loss synchronism.
The dynamic characteristic of a power system is largely influenced by the presence of wind
turbines into it. A fault at the grid causes change in electromagnetic torque of a wind machine.
This change in electromagnetic torque can be characterized as synchronizing torque and
damping torque. If the synchronizing torque is reduced by fault current machine will show
non-oscillatory instability. One the other hand, if damping torque is reduced it will show
oscillatory instability. The impact of wind generator on the power system is determined by the
response of their controller. The oscillatory instability of a wind generator is the prime concern
of this paper.
There are several reports investigating the transient stability of individual wind generators
[4-8]. The effect of wind generators on an interconnected power system has also been analyzed
[9]. However none of them had focused on the comparative study of available types of wind
generators. This paper presents a comparative analysis of transient behavior of two of the most
popular types of wind generators, the squirrel cage induction generator (SCIG) and
synchronous generators (SG). The impact of increased penetration of wind power on an
interconnected power system has also been analyzed. Firstly, the characteristics and typical
model of wind turbine is presented. Then the ground for comparative study has been
established. Four different fault scenarios’ has been modeled in order to understand and
scrutinize the transient response of both generator types. The simulation is performed on
PSS/U. After the simulation, it is found that SG’s performs better in faulty condition.
2. Characteristics and Modeling of Wind Generators
The proper use of power electronic controllers allows several generator types to be
employed in wind power production. The main distinction between available wind generator
types is, whether they are fixed speed or variable speed. The variable speed configuration of
wind generator has been popular over the last decade due to its capability of running at
optimum power coefficient for a wind range of wind speed. Although conventional Squirrel
Cage Induction Generators (SCIG) has the advantage of robustness and simple mechanical
configuration, the converter driven Synchronous Generators (SG) and doubly fed induction
generators have been widely used as wind generators in recent years since they provide more
control over power conversion [10]. The present paper, however, only considers the effect of
SCIG and SG in transient stability of the grid.
The basic configuration of a wind turbine driving SG is shown in Figure 1 [11]. The
generator is decoupled from the grid by a power converter that is actually connected to the
grid. The SCIG configuration is similar to that but; it has a mechanical drive train with one
high speed shaft and a low speed shaft between turbine and generator [12]. The advantage of
SG configuration is that it does not require external excitation sources and at the same time it
can eliminate the mechanical loss associated with gearbox [13-17].
In order to find the overall stability behaviour of a power system both steady state analysis
and dynamic analysis has to be performed. The power flow analysis provides the initial
conditions for dynamic analysis of a grid connected wind turbine. Although the total wind
power generation from the wind farm is distributed among small single unit of wind generator,
the collective power from them is still connected at a single point to the main grid line. So, the
wind farm is modelled as a single equivalent machine of same MW rating as the summation of
the MW rating of the individual machine. Including large number of individual wind turbines
into the simulation will increase unnecessary computational complexity. The assumption of
single large unit is more reasonable when the power system under consideration is relatively
large and penetration level of wind power is adjusted according to the size of the total system.
The dynamic behaviour of the wind turbine is influenced by several components as listed
below:
Pitch control; it controls the mechanical power delivered to the generator shaft. In this
study a PI system is used to control the pitch of the blade.
Shaon Ahmed, et al.
645
Aerodynamic model of the turbine; aerodynamic model of the turbine can be defined by a
three dimensional curve of power coefficient-pitch angle and tip speed ratio [18]. The
performance of a wind turbine is characterized by the non-dimensional curves of power co-
efficient, Cp. The power coefficient of wind turbine is a function of both tip speed ratio and
the blade pitch angle. The tip speed ratio is defined as the ratio of linear speed at the tip of
blades to the speed of the wind. It can be expressed by Equation (1).
𝜆 =𝑅𝛺𝑤
𝑉𝑤. (1)
Where, 𝑉𝑤. is wind speed, 𝛺𝑤 is the angular velocity of the turbine and R is the radius of
the blade. The pitch angle of the turbine blade is expressed as β and is depended on the
angle of attack of the wind.There are quite few mathematical expressions of Cp although the
theoretical basis remains similar for all [19]. For the wind turbine used in this study, the
following form approximates Cp as a function of 𝜆 and β.
𝐶𝑝(𝜆, 𝛽 ) = 𝑐1 (𝑐2
𝜆𝑖−𝑐3𝛽−𝑐4) 𝑒−
𝑐5𝜆𝑖 + 𝑐6𝜆 (2)
Here, c1=0.5167, c2=116, c3=0.4, c4=5, c5=21 and c6= 0.0068. Tip speed ratio and pitch
angle are related by the following equation:
1
𝜆𝑖=
1
𝜆+0.08𝛽−
0.035
𝛽3+1 (3)
The wind speed is assumed to be constant during the transient analysis to avoid complexity.
The mechanical power converted by the wind turbine is given by the following equation,
𝑃𝑚𝑒𝑐ℎ =1
2⍴𝜋𝑅2𝑉𝑤
3𝐶𝑝(𝜆, 𝛽 ) (4)
here, ⍴ is the air density and is a function of air pressure and temperature.
Figure 1. Typical outfit of a wind turbine
Dynamic model of the shaft; here a two mass shaft is considered, one mass represents
the turbine blade and another represents generator. The motion equation of shaft is
given by equations (5) to (7).
Effect of Grid-Connected Wind Turbine Generators on Power
646
2 𝐻𝑡𝑑𝜔𝑡
𝑑𝑡= 𝑇𝑚 − 𝐷𝑡𝜔𝑡 − 𝐷𝑡𝑔(𝜔𝑡−𝜔𝑟) − 𝑇𝑡𝑔 (5)
2 𝐻𝑔𝑑𝜔𝑟
𝑑𝑡= 𝑇𝑡𝑔 − 𝐷𝑔𝜔𝑟 − 𝐷𝑡𝑔(𝜔𝑡−𝜔𝑟) − 𝑇𝑒 (6)
𝑑𝑇𝑡𝑔
𝑑𝑡 = 𝐾𝑡𝑔 (𝜔𝑡−𝜔𝑟) (7)
Where, 𝜔𝑡 and 𝜔𝑟 are the turbine and generator rotor speed, respectively; 𝑇𝑚 and Te
are the mechanical torque applied to the turbine and the electrical torque of the
generator, respectively; 𝑇𝑡𝑔 is an internal torque of the model; 𝐻𝑡 and 𝐻𝑔 are the
inertia constants of the turbine and the generator, respectively; 𝐷𝑡 and 𝐷𝑔 are the
damping coefficients of the turbine and the generator, respectively; 𝐷𝑡𝑔 is the damping
coefficient of the shaft.
Electrical control; it regulates active/reactive power and maximizes the power
absorption from wind energy.
protection relay settings
The European grid code requirement for wind power integration defines the tolerance for
under voltage and overvoltage. According to regulations, a fault sustaining 250ms at rated
power generation will increase the frequency by 1.9 Hz. If the AC network does not recover by
the time generator over speed limit is reached, the turbine should trip [12, 19]. In this paper it is
assumed that the protection relays are set according to the mandate and the bus fault created for
simulation purpose is also kept below the estimated time limit so that the protection relays are
not triggered.
3. The Proposed Approach
The relative position of rotor axis with respect to synchronously rotating magnetic field is
considered as one of the parameters to examine stability of the system. The angle between the
two is known as the power angle or torque angle.
Figure 2. Single line diagram of 5-machine 22-bus test system with a large wind farm
Under normal operating condition and fixed load, the rotor angle should stay constant.
When a fault occurs into the system, the machine will either accelerate or decelerate depending
with respect to the synchronously rotating field. Thus a relative motion between the two starts
and this is described by the swing equation. After this oscillatory period the rotor angle must
Shaon Ahmed, et al.
647
go back to normal operating condition, if not, the machine will drive into an unstable condition.
If the fault is created by making a change in the generation, load or network condition, the
machine may come to new angular position and still behave stable. Observing the severity and
contingency of the trajectory of rotor angle following a faulty condition, it is possible to
conclude on system stability. This is measured by the Transient Stability Index (TSI). The TSI
is found in transient security assessment tool which calculates the angle margin by the
following equation [10].
𝑇𝑆𝐼 = 360−𝛿𝑚𝑎𝑥
360+𝛿𝑚𝑎𝑥× 100, −100 < 𝑇𝑆𝐼 < 100 (8)
In the above equation,𝛿𝑚𝑎𝑥 represents the maximum angle separation between any two
generators of the system after a fault has occurred. If TSI>0, it can be concluded that the
system has sustained the fault condition and remained stable. Similarly, a negative value for
TSI would imply that the system has become unstable due to sudden rise of fault current.
In this paper some scenarios are designed to investigate the transient stability of a power
system that has significant wind power penetration. The angle response from each generators
of the system would be examined. Figure 3 shows the single line diagram of the test system. It
is a 22-bus system with 5 different type of plant. It includes two nuclear power plant and one of
diesel plant, hydro power plant, wind farm, coal plant. The total generation capacity of the
system is 3258 MW with a wind power penetration of is 3.06%. The estimated active power
load is 3200MW. Figure 3 shows the swing bus -3011 and a large wind farm of 100 MW
connected to bus-3018.
Figure 3. Single line diagram of swing bus (3011) and wind bus (3018) of the test system
The four different scenarios are applied on bus-3018 as described below:
Case A constitutes of a scenario where the system under consideration has no wind
farm into it. The generator connected to bus-3018 is a normal diesel generator of
100MW rating.
In Case B, the diesel generator in the previous scenario is replaced by a wind farm of
same MW rating. Here, all the wind turbines are of variable speed configuration and
are coupled with a squirrel cage induction machine.
Case C is constituted by replacing the induction type wind generators by synchronous
types.
In Case D the penetration level of wind farm is increased by another 3% to observe if
that has any effect on Case C. This increase in wind power production is compensated
Effect of Grid-Connected Wind Turbine Generators on Power
648
by concomitant reduction from other plants so that the total power generation is kept
constant.
The objective of transient response analysis here is to inspect if any of the above mentioned
scenarios are can be triggered by a large bus fault of 100ms duration at bus-3011. All the
scenarios are scrutinized for transient response in time domain. The electrical control system of
the wind turbines are designed to operate with the specified power factor and the machines
active power settings is used to set the reactive power limit.
4. Results for Transient Stability Analysis
The system under consideration is designed in Siemens PSS/U interface. Simulations are
conducted for a bus fault at bus-3011. The fault is created after five seconds of stable run and
cleared after 100ms. If the disturbance causes the electrical torque to fall behind mechanical
torque, the rotor will try to increase the speed and move the position of the flux vector in the
positive direction. An increase in rotor angle will result in an increase in generator load torque.
For stable run the generator torque has to meet the turbine torque. The following scenarios are
designed to understand the transient response of wind generators:
Case A:
In Case A, the presence of wind firm is ignored and a conventional diesel generator is
placed at bus-3018. In Case A, the presence of wind firm is ignored and a conventional diesel
generator is placed at bus-3018. The rotor angle response to the transient fault of the all the
generators are shown in Figure4. As seen from the figure, the generators have been operating
for five seconds in stable condition before the fault has occurred. The generators respond to the
bus fault by increasing the speed and attempts to catch up to fault current. However, the as
fault is cleared within next 100ms, the generators goes back to its normal operating condition
within next three seconds. But the fault is created by causing a change in the network, therefore,
although the generators are still behaving stable, a shift of rotor angle has appeared in all the
generators. The maximum angle separation between the generators is 50 degrees, hence
TSI=0.75. So, it can be concluded that the Case A is stable.
Figure 4. Rotor angle response of the system to transient fault without wind turbine
Case B:
a wound rotor induction generator at bus-3018. The fault response is shown in Figure 5.
From the figure it is seen that the wind generator goes to stability but five seconds after the
fault has occurred. The maximum angle separation is found to be 65 degree and TSI is equal to
0.69. The oscillation produced in the supplied power by the fault current is shown in Figure 6.
Angle
(D
egre
es)
Time (seconds)
Shaon Ahmed, et al.
649
Figure 5. Rotor angle response of the system to transient fault with Induction wind generator
Figure 6. Power curve of Induction wind generator showing post fault performance
Case C:
Figure 7. Rotor angle response of the system to transient fault with Synchronous wind
generator
Figure 8. Change of rotor angle in synchronous wind generator due to a large
fault at the system
Angle
(D
egre
es)
Pow
er
(PU
)
Angle
(D
egre
es)
Angle
(D
egre
es)
Time (seconds)
Time (seconds)
Time (seconds)
Time (seconds)
Effect of Grid-Connected Wind Turbine Generators on Power
650
In Case C the generator at 3018 is a permanent magnet synchronous wind generator
(PMSWG). The transient response of this configuration is shown in Figure 7. From the figure it
is can be seen that a PMSWG responded much better than any other cases described above.
The effect of transient fault on the wind generator is very little and although there is an initial
disruption in stable condition, it goes to normal operation as soon as the fault is cleared. The
TSI level in this case is 0.6. Figure 8 is the zoomed view of the angle response of PMSWG.
The associated active and reactive power of the generator is shown in Figure 9.
Case D:
In Case D, the generation capacity of the wind generator is increased by another 100MW,
whereas the generation from other sources was reduced to keep the total active power
generation constant. This is done to investigate that whether scenario C can be triggered by
increasing the penetration level. So, the wind power penetration level is now increased to
6.13%. The rotor angle response is presented in Figure 10. As seen from the figure, a fault after
five seconds of stable run has caused all the generators to decelerate except the wind generator.
Although the TSI value is still in the stable region, the trajectory of the rotor angle is still
unconvincing. So the simulation is continued for another 10 seconds. This is shown in Figure
11.
Figure 9. Active and reactive power of Synchronous wind generator
Figure 10. Rotor angle response of the system to transient fault with increased penetration level
(run time 15seconds)
Figure 11 shows that the generator speed is still decelerating even after twenty seconds of
clearing the fault and eventually drives the system into instability. So, the increase in
penetration level of wind power has derived the system into instability.
Pow
er
(PU
)
Angle
(D
egre
es)
Angle
(D
egre
es)
Time (seconds)
Time (seconds)
Shaon Ahmed, et al.
651
Figure 11. Rotor angle response of the system to transient fault with increased penetration level
(run time 25seconds)
5. Conclusion
This paper presents a simulation analysis of transient stability of a grid integrated wind
turbine. A systematic approach has been designed to demonstrate the effect of wind generators
on power system. Two different types of conventional generators are considered and their
response to a large grid fault has been analyzed and compared. The TSI has been considered as
the telling factor for transient stability. The effect of increased penetration of wind power has
also been observed. From the analysis, it can be concluded that although both induction and
synchronous generators can be used as wind generators, the synchronous generators provides
better post fault operating conditions for the power system. However, the increase in wind
power penetration may drive the system into instability. With increasing in penetration level,
the system experiences major changes in dynamics and operating conditions. In this case, when
the wind power penetration was increased from 100 MW to 200 MW, the system became
vulnerable to the grid even though it was operating stably prior to that. The doubly fed
induction generators have also become popular in the recent past but they were not included
into this study since the stability of a doubly fed induction wind generator depends mostly on
controller settings rather than on other system parameters.
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Shaon Ahmed obtained B.Sc in Electrical and Electronic Engineering from
Khulna University of Engineering and Technology, Bangladesh in 2012 and
M.Sc in Electrical Systems engineering from Unversity Malaysia Perlis,
Malaysia in 2015. He is currently working as Lecturer at the Department of
Electrical and Electronic Engineering, Hamdard University Bangladesh,
Narayangonj, Bangladesh. His research interests include Integration of
Renewable energy, Wind energy, Hybrid Power System and Power System
Reliability and stability.
Shaon Ahmed, et al.
653
Mohd Abdur Rashid received his B.Sc. and M.Eng degrees in Electrical &
Electronics Engineering from BIT Khulna, Bangladesh in 1991 and
University of the Ryukyus, Japan in 2000 respectively. He obtained his Ph.D.
in Electrical and Information Engineering from University of the Ryukyus,
Japan in 2003. He is currently working as Associate Professor at the Faculty
of Design Arts and Engineering Technology, Universiti Sultan Zanial Abidin,
Kuala Terengganu, Malaysia. Dr. Rashid has authored more than 70 technical
papers in the international journals and conferences. He is involved in
multidisciplinary research fields including mathematical modeling, renewable energy,
electronic devices, biomedical engineering and power systems. He is a regular member of
IEEE, IEICE, IAENG and IEB.
Shamshul Bahar YAAKOB received a Bachelor’s degree in Computer
Engineering from Shizuoka University, Japan and a MEng. degree in
Electrical and Electronic Systems from Nagaoka University of Technology,
Japan. Currently he is an Associate Professor in the School of Electrical
System Engineering, Universiti Malaysia Perlis, Perlis, Malaysia. His
research interests include soft computing, reliability optimisation, and multi-
criteria optimisation.
Adawati Yusof obtained Bachelor of Industrial Electronic Engineering (Hons)
in 2009 from University Malaysia Perlis (UniMAP), and MSc in Industrial
Electronic and Control in 2011 from University of Malaya (UM), Malaysia.
She is currently a full time lecturer at the School of Electrical System
Engineering, University Malaysia Perlis (UniMAP), Perlis, Malaysia. Her
research interest includes power system efficiency, renewable energy, and
communication system. She is also a member of Board of Engineer Malaysia
(BEM) and International Association of Engineering (IAENG).
Mohd Fareq bin Abdul Malek obtained PhD in Electrical Engineering
(Radio Frequency and Microwave), University of Liverpool, UK. He
received MSc (Eng) Microelectronic Systems and Telecommunications
(Distinction), University of Liverpool, UK. He got BEng (Hons) Electronic
and Communication Engineering, University of Birmingham, UK. He is
currently working as Associate Professor and holding the position of Dean
in the School of Electrical System Engineering, UniMAP. His research
interests include communication, antenna theory, electromagnetics and so
forth.
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