dynamic response of doubly fed induction generator connected to wind system...

Dynamic Response of Doubly Fed Induction Generator

connected to wind system under unbalance condition

1P.Nagasekhar Reddy

Associate Professor

Department of EEE,

Mahatma Gandhi Institute of Technology, Hyderabad, India

E-Mail: [email protected]

Abstract: Wind power plants have been playing an important role for electricity production around the

world. The Doubly fed induction generator (DFIG) based wind farm is today the most widely used

concept. This paper presents dynamic response of DFIG based wind system under various fault

conditions. The recent evolution of power semiconductors and variable frequency drive technology has

aided the acceptance of variable speed generation systems. The goal of the work is to study the dynamic

response of DFIG based wind system under fault condition using MATLAB simulation. The behavior of

large wind turbine system under variable condition may affect the grid stability. The detailed model of

doubly fed induction generator coupled to wind turbine system using MATLAB is is presented in the

paper for various fault condition and the significant result of the analysis is also shown and being

compared with the existing literature to validate approach.

Index: induction generator, doubly fed induction generator, power system, modeling and simulation

interconnection, MATLAB, variable speed drives and wind energy system.

I. INTRODUCTION:

Wind energy generation equipment is most often installed in remote, rural areas. These remote areas

usually have weak grids, often with voltage unbalances and under voltage conditions. When the stator

phase voltages supplied by the grid are unbalanced, the torque produced by the induction generator is not

constant. Instead, the torque has periodic pulsations at twice the grid frequency, which can result in

acoustic noise at low levels and at high levels can damage the rotor shaft, gearbox, or blade assembly.

Also an induction generator connected to an unbalanced grid will draw unbalanced current. dynamic

behavior of grid should not get affected by operation of wind farm. But when grid is attributed to fault

and voltage dips, the disconnection of the wind farm creates shedding of loads resulting in unreliable

power supply. Therefore according to the magnitude of voltage at point of interconnection, the fault ride

through capability is specified to withstand voltage dips without load shedding. These unbalanced current

tend to magnify the grid voltage unbalance and cause over current problems as well [1- 2]. Wind energy

has been the subject of much recent research and development. In order to overcome the problems

associated with fixed speed wind turbine system and to maximize the wind energy capture, many new

wind farms will employ variable speed wind turbine [3]. DFIG (Double Fed Induction Generator) is one

of the components of Variable speed wind turbine system. The stator is directly connected to the grid and

the rotor is fed to magnetize the machine [4]. The reason for the world wide interest in developing wind

generation plants is the rapidly increasing demand for electrical energy and the consequent depletion

reserves of fossil fuels, namely, oil and coal. Many places also do not have the potential for generating

hydel power[5-6]. To study these issues, dynamic model of Wind Turbine has been developed. For the

present study DFIG Wind Turbine is considered. This Wind Turbine is connected to grid through step up

transformer. The Grid Side Converter and Rotor Side Converter are connected back to back to control

generator output parameters in both normal & abnormal conditions. The Rotor Side Converter is current

controlled & Grid Side Converter is voltage controlled. The variable speed generator system is the

preferred design for large wind turbines, providing more efficient utilization of power and the ability to

reduce the mechanical stress on the system under changing wind conditions [7-9]. There are two types of

Science, Technology and Development

Volume IX Issue IV APRIL 2020

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mailto:[email protected]

variable-speed generator used in large wind power plant: synchronous generator with full power

converter and DFIG. About two-thirds of the wind turbines are operated with a DFIG, while about a third

of them are operated with a high pole synchronous generator [10-11]. The DFIG has many advantages

compared to the synchronous generator includes wider speed range, lower cost of power electronic and

better efficiency [12-13]. In modern DFIG designs, the frequency converter is built by self-commutated

PWM converters, a machine-side converter, with an intermediate DC voltage link. Variable speed

operation is obtained by injecting a variable voltage into the rotor at slip frequency. The injected rotor

voltage is obtained using DC/AC insulated gate bipolar transistor based voltage source converters (VSC),

linked by a DC bus [14]. By controlling the converters, the DFIG characteristics can be adjusted so as to

achieve maximum of effective power conversion or capturing capability for a wind turbine and to control

its power generation with less fluctuation [15]. In this paper, modeling and simulation of DFIG is

performed in order to analyze the behavior of the system. The behavior of DFIG is described in

mathematical equations and followed by modeling in the form of block diagrams and simulation using

MATLAB/Simulink.

II. MODELLING OF DOUBLY FED INDUCTION GENERATOR (DFIG):

During the recent years, with development of power electronic devices, large variable wind turbine

equipped with doubly fed induction generator (DFIG) are considered as the most effective and popular

configuration for electricity generation due to its advantages.

The air gap flux linkages Ψqm and Ψdm can be expressed as

Ψqm= Lm (iqs +iqr') and Ψdm= Lm (ids +idr

')

The mechanical power and the stator electric power output are computed as follows:

Pm = TmωrPs = Temωs. (1)

For a lossless generator the mechanical equation is:

Jdωrdt

=Tm−Tem. (2)

In steady-state at fixed speed for a lossless generator

Tm= Tem and Pm= Ps+ Pr. (3)

It follows that:

Pr=Pm−Ps=Tmωr−Temωs=−Tmωs−ωrωsωs=−sTmωs=−sPs, (4)

where s is defined as the slip of the generator: s= (ωs–ωr)/ωs.

Generally the absolute value of slip is much lower than 1 and, consequently, Pr is only a fraction of Ps.

Since Tm is positive for power generation and since ωs is positive and constant for a constant frequency

grid voltage, 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). For super-

synchronous speed operation, Pr is transmitted to DC bus capacitor and tends to rise the DC voltage. For

subsynchronous speed operation, Pr is taken out of DC bus capacitor and tends to decrease the DC

voltage.

III. DYNAMIC RESPONSE OF DOUBLY FED INDUCTION GENERATOR (DFIG):

This section will detail the AC-DC-AC converter used on the rotor which consists of two voltage-sourced

converters, i.e., rotor-side converter (RSC) and grid-side converter (GSC), which are connected “back-to-

back.” Between the two converters a dc-link capacitor is placed, as energy storage, in order to keep the



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voltage variations (or ripple) in the dc-link voltage small. With the rotor-side converter it is possible to

control the torque or the speed of the DFIG and also the power factor at the stator terminals, while the

main objective for the grid-side converter is to keep the dc-link voltage constant regardless of the

magnitude and direction of the rotor power. The grid-side converter works at the grid frequency (leading

or lagging in order to generate or absorb a controllable magnitude of reactive power). A transformer may

be connected between the grid-side inverter or the stator, and the grid. The rotor-side converter works at

different frequencies, depending on the wind speed. The back-to-back arrangement of the converters

provides a mechanism of converting the variable voltage, variable frequency output of the generator (as

its speed changes) into a fixed frequency, fixed voltage output compliant with the grid. The DC link

capacitance is an energy storage element that provides the energy buffer required between the generator

and the grid. At the current state of development, most DFIG power electronics utilise a two-level six-

switch converter. Two-level refers to the number of voltage levels that can be produced at the output of

each bridge leg of the converter. A two-level converter can typically output zero volts or Vdc, where Vdc is

the voltage of the dc link.

IG

T

GB – Gearbox IG – Induction Generator

RSC – Rotor-Side Converter GSC – Grid Side Converter T – Transformer

RSC GSC

GB

Fig.1. Typical control of doubly fed induction generator:

Fig. 1 shows two such converters connected in a back-to-back arrangement with a DC link between the

two converters. The switching elements in higher power converters are likely to be Insulated-gate Bipolar

Transistors (IGBTs). The six-switch converter can synthesise a three-phase output voltage which can be

of arbitrary magnitude, frequency and phase, within the constraint that the peak line voltage is less than

the DC link voltage. The converter is capable of changing the output voltage almost instantaneously – the

limit is related to the switching frequency of the pulse-width modulated switching devices, and delays

introduced by any filtering on the output (typical on the grid-side converter). The converter switches are

switched ON and OFF with a fixed frequency but with a pulse-width that is varied in order to control the

output voltage.



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wm

1500

w1

w

rotorpwm

Vdc

Sabc

iabc

ur_abc

I_inv

rotor controller

ird_ref

¦Øm _ref

¦Ør

ir_abc

¦×s

Ur_abc

reactive current

8

powergui

Continuous

Vdc

150

Three -Phase

V-I Measurement

VabcA

B

C

a

b

cThree -Phase

Programmable

Voltage Source

N

A

B

C

Te

Scope 1

Scope

PWM genrator

UabcSabc

Ia

-K-

-K-

Ef=150 V

E=150 V1

Demux

DFIG

us_abc

ur_abc

w1

Tm

Te

wr

is_abc

ir_abc

phis

DC_Motor

5 hp ; 240 V; 15 .54 A; 1750 rpm

TL m

A+

F+

A-

F-

dc

BL

0 .2287

1 MVA

A

B

C

A

B

C

Fault

A

B

C

A

B

C

Fig 2 Simulink diagram of the proposed system of DFIG connect to wind system

IV. SIMULATION RESULTS:

Fig.3. Free Acceleration Characteristics of Current Component (ids, iqs, idr, iqr)



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Fig. 4. Speed Torque Characteristics

Fig.5. Stator Currents (ias, ibs, ics) During Balance Condition



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Fig.7. Rotor Currents (iar, ibr, icr) During Balance Condition

Fig.7. Speed and Torque (ωr, Te) During Balance Condition



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Induction generator rating:

3hp (2.23kw), 440 v, 1500 rpm. RS= 0.435 ohm

Rr= 0.816 ohm, Xs= 0.446 ohm, Xm= 0.43 ohm, Xs= 0.446 ohm, Number of pole pair (p) = 2, J = 0.08 kg.

m2.

Wind turbine data: Air density ρ = 1.2 kg/ m3, Wind speed v = 10 m/s.

When the induction generator is started, initially it shows transients and this region of operation is called

as unstable region of operation due to inverting rotor voltage. After some time torque increases and a

steady state is reached. Free acceleration with the reference frame of rotating in synchronism with the

electrical speed of the applied voltage is shown in fig. 3 here the zero position of the reference is selected

so that vqs is the amplitude of the stator applied phase voltages and vds =0. The torque verses speed

characteristics during free acceleration shown in fig..4. We also note from the currents plots shown in

fig.5 and fig. 6 that the envelope of the machine currents varies during transient period. It is shown in a

subsequent that this due to the interaction of the stator and rotor electric transients.

Fig.8. Active Power (P) and Reactive Power (Q) During Balance Condition



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Fig 9 Stator Voltages vas, vbs, vcs During Grid Fault

Fig.10. Stator Currents (ias, ibs, ics) During Grid Fault



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Fig.11. Rotor Currents (iar, ibr, icr) During Grid Fault

Fig 12. Speed and Torque (ωr, Te) During Fault Condition



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The dynamic performance of the induction generator is shown respectively in fig.8. to Fig 12. during a 3

phase fault at terminals. Initially generator is operating at essentially rated condition with a load torque to

base torque. The 3-phase fault at the terminals is simulated by setting vas, vbs, vcs to zero at the instant vas

passes through zero going positive. After few cycle the source voltage reapplied.

Fig.13. Stator Currents (ias, ibs, ics) During Unbalance Condition

Fig.14. Rotor Currents (iar, ibr, icr) During Unbalance Condition



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Fig.15. Speed and Torque (ωr, Te) During Unbalance Condition

Fig.16. Active Power (P) and Reactive Power (Q) During Unbalance Condition



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The dynamic performance of the induction generator coupled with dc machine is shown in fig 13. To

Fig.16 respectively during a 3 phase fault and step changes in load.

V. CONCLUSION:

This paper presents a study of the dynamic performance of variable speed DFIG coupled with either wind

turbine or a dc motor and the power system is subjected to disturbances; such as voltage sag, unbalanced

operation or short circuit faults. The dynamic behavior of DFIG under power system disturbance was

simulated both using MATLAB. the dynamic DFIG performance is presented for both normal and

abnormal grid conditions. The control performance of DFIG is satisfactory in normal grid conditions and

it is found that, both active and reactive power maintains a study pattern in spite of fluctuating wind

speed and net electrical power supplied to grid is maintained constant.

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