40220140505011

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 110-120 © IAEME
110
FAULT TOLERANCE AND POWER QUALITY STUDY OF DFIG BASED
WIND TURBINE SYSTEM
1
Partha Das, 2
Sushabhan Biswas
1, 2
(Electrical Engineering, JIS College of Engineering, Kalyani, Nadia, India)
ABSTRACT
Wind power is the fastest growing renewable source of electrical energy hence it has become
necessary to address problems associated with maintaining a stable electric power system. The wind
generated power is always fluctuating due to its time varying nature and causing stability problem.
Unified Power Flow Controller (UPFC) is used to control the power flow in the transmission
systems by controlling the impedance, voltage magnitude and phase angle. This controller offers
advantages in terms of static and dynamic operation of the power system. A Unified Power Flow
Controller (UPFC) is an electrical device for providing fast-sensation on high-voltage electricity
transmission networks. The measurement and assessment of power quality characteristics of grid
connected wind turbines are specified in International Electro-technical Commission standard IEC-
61400-Part 21. Another effort is given in this report to make a comparative study of the fault current
and voltages of a DFIG (Doubly Fed Induction Generator) system with using a relay protection
system and incorporating UPFC in the system. The UPFC control scheme for the grid connected
wind energy generation system for mitigating power quality issues is simulated using
MATLAB/SIMULINK environment, and a DFIG is also used to study the response of the system
during grid disturbances.
Keywords: Doubly Fed Induction Generator (DFIG), Fault Tolerance, Grid Connected DFIG
System, Relay Protection System, Unified Power Flow Controller (UPFC).
1. INTRODUCTION
In recent years, world vigorously development clean energy especially wind power in order
to carry out and practicable the sustainable development measure an implement the economy and
environment coordinated development, and with the wind power generation techniques increasingly
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 5, May (2014), pp. 110-120
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mature, large scale capacity wind power growingly dive into the network. Wind power solves the
energy crisis problem in a certain extent, but the large scale wind power dive into power system has
brought a series of technical problems, one of the problems is that the impaction on the protection
configuration can't neglect. To increase the system efficiency, high efficiency devices based on
power electronics equipments have been increasingly used in many applications. This causes
increasing harmonic levels on power systems and concerns about the future impact on system
capabilities. So, if there is any fault in the subsystems there will be disturbances, disruptions and the
other effects, which decrease the power quality in the system. To maintain the power quality and to
improve the fault protection techniques of a wind energy system, several research scholars all over
the world are trying to find a solution. [1] presents a control strategy to improve the low-voltage
ride-through capability of a doubly fed induction generator and the necessary simulations and
experiments are carried out, but the application of FACTS devices are not revealed in the work. [2]
proposes a new converter protection method, based on Series Dynamic Resistor (SDR) and also the
conventional crowbar protection method. Performance comparison of crowbar protection and SDR
has been analyzed with simulation, but still this paper does not produce enough details of the power
quality. [3] demonstrates the power quality problem due to wind turbine installation with the grid. In
that proposed scheme, Unified Power Quality Conditioner (UPQC) is used with a battery energy
storage system (BESS) to palliate the power quality issues, but the paper does not illuminate the fault
protection issues. In [4] STATCOM is used with energy storage system to reduce the power quality
problems. The paper clearly shows that the power quality problem exists due to wind turbine
installation with the grid. [5] employs the Pulse Width Modulation (PWM) inverter scheme for the
grid connected wind energy generation for power quality improvement and simulation is done using
power system block set in MATLAB/SIMULINK. All the approaches definitely elucidated the
power quality issues and the various fault protection schemes of a wind energy system, but the need
of a combined approach can still be felt. The present work proposes a grid connected DFIG system
using UPFC which will produce a better quality of power than the conventional DFIG system. Also,
a relay protection system has been used in the proposed model to minimize the effect of fault in any
grid disturbances. The simulation has been carried out using MATLAB software.
2. THEORY
A doubly fed induction generator is basically a standard, wound rotor induction generator
with its stator windings directly connected to the grid and its rotor windings connected to the grid
and its rotor windings connected to the grid through a converter. The AC to DC and DC to AC
Converter is divided into two parts: the rotor side converter and the grid side converter. Force
commutated power electronic devices are used in these voltage source converters to synthesize an
AC Voltage from a DC source. A capacitor connected on the DC side acts as the DC voltage source.
The grid side converter is connected to the grid by using a coupling inductor. The three phase rotor
winding is connected to the rotor side converter by slip rings and brushes and the three phase stator
windings are directly connected to the grid. The power of the wind turbine, the DC voltage and the
reactive power or the grid terminal voltage are controlled by the pitch angle command and the
voltage command signals Vr and Vgc generated by the control system for the rotor and grid side
converters respectively.

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Fig.1: Power Flow in DFIG
Fig.1 shows the flow of power in a Doubly Fed Induction Generator. The absolute value of
slip is generally much lower than 1 and accordingly the electrical power output of the rotor Pr is only
a fraction of the real power output of the stator Ps. Ws is positive and constant for a constant
frequency grid voltage and also the electromagnetic torque Tm is positive for power generation, so
the sign of Pr is a function of the slip sign. Pr is positive for negative slip (speed greater than
synchronous speed) and it is negative for positive slip (speed lower than synchronous speed). Pr is
transmitted to DC bus capacitor and tries to set up the DC voltage for super synchronous speed
operation. Pr is taken out of the DC bus capacitor and tries to reduce the DC bus voltage for sub
synchronous speed operation. The DC voltage is kept constant by generating or absorbing the grid
electrical power Pgc from the grid side converter. For a lossless AC/DC/AC converter Pgc is equal to
Pr in steady state and the wind turbine speed is determined by the absorbed or generated power Pr by
the rotor side converter. The voltage measured at the grid terminals can be controlled by controlling
the grid side converter DC bus voltage of the capacitor can be regulated and by controlling the rotor
side converter. The phase sequence of the AC voltage generated by the rotor side converter is
positive for sub synchronous speed and negative for super synchronous speed. The frequency of this
voltage is equal to the product of the grid frequency and the absolute value of the slip. The reactive
power or the voltage at the grid terminals can be controlled by the capability of the rotor and grid
side converter of generating or absorbing reactive power. The rotor side converter is used to control
the wind turbine output power and the voltage (reactive power) measured at the grid terminals. The
voltage regulation of the DC bus capacitor is done by using the grid side converter. The rotor power
converter as a vulnerable part of the DFIG power converter, which has a restricted over-current limit,
needs special attention especially during faults in the grid. When faults occur and cause voltage dips,
subsequently the current flowing through the power converter may be very high (over-current).
During this situation, it is common to block the converter to avoid any risk of damage, and then to
disconnect the generator from the grid. Motivated by the reason above, this report provides a study
of the dynamics of the grid connected wind turbine with DFIG. In steady-state at fixed turbine speed
with a lossless DFIG system, the mechanical power from the aerodynamic system is balanced by the
DFIG power. Pm = Ps + Pr. It follows that
Pr =Pm-Ps=Tmωr- Tmωs= -Tm((ωs-ωr)/ ωs) * ωs= -s
Tmωs= -sPs where s is defined as the slip of the generator.

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2.1 POWER QUALITY ISSUES
Power quality problem is any power problem manifested in voltage, current, or frequency
deviation that results in failure or malfunctioning of customer equipment. Power quality is a two-
pronged issue, with electronic equipment playing both villain and victim. The causes and
consequences of power quality problems can be traced to a specific type of electrical disturbance. In
most of industry, more than 90% of the electric motor with inverter driven application .Poor power
quality causes trouble in receptacle/transmission equipment and electronic equipment malfunctions /
Failure. Power quality is a common problem for both electric power suppliers and users. It is not
easy to identify whether the cause of poor power supply quality is at the supplier’s system or the
user’s system. V oltage fluctuations are changes or swings in the steady-state voltage above or below
the designated input range for a piece of equipment. Fluctuations include both sags and swells and it
causes large equipment start-up or shut down; sudden change in load.
2.2 FAULT PROTECTION ISSUES
The rotor power converter as a vulnerable part of the DFIG power converter, which has a
restricted over-current limit, needs special attention especially during faults in the grid. When faults
occur and cause voltage dips, subsequently the current flowing through the power converter may be
very high (over-current). During this situation, it is common to block the converter to avoid any risk
of damage, and then to disconnect the generator from the grid. Actuated by the above reason, this
paper furnishes a study of the dynamics of the wind turbine which is grid connected with DFIG. The
report begins with the development of a wind turbine model with DFIG in Matlab, Simulink,
followed by simulations of the model during grid disturbance. In the simulation, the ability of the
DFIG to recover terminal voltage after grid disturbance is introduced. The reaction of the DFIG to
faults and subsequent action of the over-current protection is described. Two different operation
modes, i.e. sub-synchronous and super-synchronous operation, are cared for individually. The results
from the two operation modes are then evaluated. The inclusion of the saturation effect in the
generator to provide better prediction of current magnitude is included as well. The relay protection
system block consists of following:
Instantaneous AC Overcurrent, AC Overcurrent (positive-sequence), AC Current unbalance,
AC Undervoltage (positive-sequence), AC Overvoltage (positive-sequence), AC Voltage Unbalance
(Negative-sequence), AC Voltage Unbalance (Zero-sequence), DC Overvoltage, Under Speed, Over
Speed.
The parameters and ratings are given in the table-1.
Table-1
Parameters Ratings
Fundamental Frequency 60 Hz
Instantaneous AC Overcurrent (pu) 10
Maximum AC Current[ I1max(pu), Delay(s) ] 1.1, 5
Maximum AC Current Unbalance [ I2/I1max(pu), Delay(s) ] 0.4, 0.2
AC Under/Over Voltage [ V1min(pu), V1max(pu), Delay(s) ]: 0.75, 1.1, 0.1
Maximum Voltage Unbalance [ V2/V1max(pu), 0.05, 0.05, 0.2
V0/V1max(pu), Delay(s)]
Maximum DC Voltage [ Vmax(V), Delay(s) ] 1900,0.001
Under/Over Speed [ Speedmin (pu), Speedmax (pu), Delay(s)] 0.3, 1.5, 5
Start time for protection system (sec) 5

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2.3 OPERATING PRINCIPAL OF UPFC
Line outage, congestion, cascading line tripping, power system stability loss are the major
issues where capability and utilization of FACTS are noticed. Example of the terminal generation of
FACTS devices is the Unified Power Flow Controller (UPFC). The UPFC is a gimmick which can
control simultaneously all three parameters of line power flow (line impedance, electric potential and
phase angle). Such "new" FACTS device combines together the features of two "old" FACTS
devices: the Static Synchronous Compensator (STATCOM) and the Static Synchronous Series
Compensator (SSSC).
The shunt inverter is used for voltage regulation at the degree of connection injecting an
opportune reactive power flow into the lineage and to balance the actual power flow exchanged
between the series inverter and the transmission line. The series inverter can be applied to curb the
real and reactive line power flow inserting an opportune voltage with controllable magnitude and
phase in series with the transmission line.
The series inverter is controlled to inject a symmetrical three phase voltage system of
controllable magnitude and phase angle in series with the line to control active and reactive power
flows along the transmission line. So, this inverter will exchange active and reactive power with the
line. The reactive power is electronically provided by the series inverter, and the active power is
channeled to the DC terminals. The shunt inverter operates in such a fashion as to demand this DC
terminal power (positive or negative) from the line keeping the potential difference across the
storage capacitor Vdc constant. So, the net real power absorbed from the crease by the UPFC is
equal only to the losses of the two inverters and their transformers. The remaining content of the
shunt inverter can be employed to exchange reactive power with the line and then to provide a
voltage regulation at the point of connection.
Fig 2: UPFC Link in Transmission line.
3. PROPOSED MODEL
In the proposed Simulink model of wind farm we have introduced-
Unified Power Flow Controller (UPFC) mode, when the shunt and series converters are
interconnected through the DC bus. When the disconnect switches between the DC buses of the
shunt and series converter are opened, two additional modes are available i.e. Shunt converter

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operating as a Static Synchronous Compensator (STATCOM)controlling voltage and Series
converter operating as a Static Synchronous Series Capacitor (SSSC) controlling injected voltage,
while keeping injected voltage in quadrature with current. Unified Power Flow Controller (UPFC)
are being used extensively in power systems because of their ability to provide flexible power flow
control. The main motivation for choosing SVC in wind farms is its ability to provide bus bar system
voltage support either by supplying and/or absorbing reactive power into the system. As a main
stream configuration for large wind turbines, DFIG wind turbines are required to remain grid
connected during grid faults so that they can contribute to the stability of the power transmission
system. This raises problems in terms of generator/converter protection and control. In the case of
grid faults, the controllability of the DFIG variable speed wind turbine embraces both the wind
turbine control for preventing over-speeding of the wind turbine and the control and protection of the
power converter during and after grid faults.
A Unified Power Flow Controller (UPFC) is used to control the power flow in a 120 kV /25
kV transmission systems. The system connected in a loop configuration consists essentially of three
buses (B575, B25, B120) interconnected through transmission line (L1) and two transformer banks.
The model of DFIG based 9MW wind farm with UPFC is designed using MATLAB/ SIMULINK is
shown in Fig.3.
Fig.3: Simulink model of the system with UPFC & Relay protection system
The UPFC located at the right end of the 30-km line L1 between the 120 kV buses B25 and
B575 is used to control the active and reactive powers flowing through bus B25, while controlling
voltage at bus B575. The UPFC consists of two 100-MVA, three-level, 48-pulse GTO-based
converters, one connected in shunt at bus B25 and one connected in series between wind B575 and
B25. The shunt and series converters can exchange power through a DC bus.

116
This pair of converters can be run in three ways:
Unified Power Flow Controller (UPFC) mode, when the shunt and series converters are
interconnected through the DC bus. When the disconnect switches between the DC buses of the
shunt and series converter are opened, two additional modes are available. Shunt converter operating
as a Static Synchronous Compensator (STATCOM) controlling voltage at bus B1.
Series converter operating as a Static Synchronous Series Capacitor (SSSC) controlling
injected voltage, while keeping injected voltage in quadrature with current.
4. SIMULATION RESULTS
Initially started simulation, wind speed is set at 12 m/s, then at t = 10s, wind speed increases
suddenly at 16 m/s. Start simulation and observe the signals along the "Wind Turbine" scope
monitoring the wind turbine voltage, current, generated active and Reactive powers, DC bus voltage
and turbine speed.
Now unfold the "Fault" block menu and select "Phase A Fault". Find out that the fault is
programmed to apply a 9-cycle, single-stage to earth break in t = 10s. We noticed that when the wind
turbine is in "Voltage regulation" mode, the positive-sequence voltage at wind-turbine terminals
(B575) drops to 0.8 pu during the break, which is above the under voltage protection threshold (0.75
pa for at > 0.1 s). The wind farm therefore stays in service.
Fig.4: Fault tolerance at time of fault
The voltage (Vabc_25) and current (Iabc_25) at bus B25 without UPFC controller and with
UPFC controller are shown in Fig5 and Fig6 respectively.

117
Fig.5: The voltage, current at bus B25 without UPFC
Fig.6: The voltage, current at bus B25 with UPFC

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Now, the fault voltages and currents characteristics of the simulink diagram of fig.7 is shown
below:
Fig.7: Fault Voltages & currents with UPFC & relay protection system in the proposed model
5. CONCLUSION
This paper presents the grid connected wind energy system for power quality improvement by
using UPFC. The power quality problems, its consequences and their mitigation techniques are
presented here. In this proposed scheme to eliminate the harmonic content of the load current the
UPFC-based control system is used. So that power quality is maintained at the point of common
coupling. Therefore, to control the power from one end to another end, this concept of power flow
control and voltage injection is applied. Modeling the system and studying the results have given an
indication that UPFC are very useful when it comes to organize and maintain power system. The
UPFC is a device which can control simultaneously all three parameters of line power flow (line
impedance, voltage and phase angle). The UPFC controller mitigates the harmonic distortion that
caused by the nonlinear load where all values of THD for voltage and current at all AC buses are
decreased to values within allowable limits of IEEE standard. Wind power generation with DFIG
provides better performance for terminal voltage recovery after fault clearance owing to its ability to
control reactive power. However DFIG is sensitive to severe voltage dips that result in an excessive
stator and rotor current, which leads to the rotor converter being blocked. The shaft oscillation
caused by the fault should be considered when examining the dynamic response of the DFIG.
Special attention should be paid to the blocking that occurs when the DFIG operates at far below
synchronous speed. Since, in this case, the abrupt change in the rotor speed has more serious impacts
on the electrical response of the system. As regards the saturation effect during fault, it can be seen
in the simulation that the peak value of the stator and rotor current in the model with saturation is
higher than in the model without saturation. Therefore it is important to take the saturation effect into
account, especially when designing a protection setting. However, the prediction of the current
magnitude in the model with saturation is characterized by the saturation curve of the generator.

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When UPFC is only present in the proposed system, the oscillations are created in the fault voltages
and currents, that is undesirable. With using only the relay protection system, the oscillations are
absent, but the effect of fault is greater in the fault voltages and currents, that is not acceptable. So,
the model with both UPFC and relay protection system is acceptable in terms of a better fault
tolerance capacity and power quality.
6. FUTURE SCOPE
The scope can be extended by using other FACTS device and also a fault detection system
based on microcontroller can be used to detect and minimize the effect of fault in the overall
performance of the system.
7. ACKNOWLEDGEMENT
We are acknowledging to the TEQIP grant scheme of JIS College of Engineering.
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