23 impact of wind farm - iaeme of...impact of wind energy on power systems is thus focused on...

Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print), ISSN 0976 –

6553(Online) Volume 3, Issue 1, January-June (2012), © IAEME

235

IMPACT OF WIND FARM OF DOUBLE-FED INDUCTION

GENERATOR (DFIG) ON VOLTAGE QUALITY

Ameer H. Abd

a* , D.S.Chavan

b

a Master Student in electrical Engineering electrical Engineering Department, Bharati

Vidyapeeth Deemed University College of Engineering , Pune ,India

Phone: +91- 8446268248

Email: [email protected]

b Professor, electrical Engineering Department, Bharati Vidyapeeth Deemed University

College of Engineering, Pune ,India

Phone : + 91- 9823977557


* Corresponding Author

ABSTRACT

The impact of large-scale grid-connected wind farms of Doubly-fed Induction Generator

(DFIG) type on power system transient stability is elaborately discussed in this paper. In

accordance with an equivalent generator/converter model, the comprehensive numerical

simulations with multiple wind farms of DFIG type involved are carried out to reveal the

impact of wind farm on dynamic behavior of existing interconnected power system.

Different load models involving nonlinear load model and induction motor model are

considered during simulations. Finally, some preliminary conclusions are summarized

and discussed.

Keywords: DFIG, Multiple Wind Farms, Wind Farm Integration,

I. INTRODUCTION

There is now general acceptance that the burning of fossil fuels is having a significant

influence on the global climate. Effective mitigation of climate change will require deep

reductions in greenhouse gas emissions, with UK estimates of a 60–80% cut being

necessary by 2050 (Stern Review, UK HM Treasury, 2006). The electricity system is

INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &

TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)

ISSN 0976 – 6553(Online)

Volume 3, Issue 1, January- June (2012), pp. 235-246

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236

viewed as being easier to transfer to low-carbon energy sources than more challenging

sectors of the economy such as surface and air transport and domestic heating. Hence the

use of cost-effective and reliable low-carbon electricity generation sources, in addition to

demand-side measures, is becoming an important objective of energy policy in many

countries (EWEA, 2006; AWEA, 2007).

Over the past few years, wind energy has shown the fastest rate of growth of any form of

electricity generation with its development stimulated by concerns of national policy

makers over climate change, energy diversity and security of supply.

Some of the advantage include of the following :

1) Emergency backup during sustained utility outages.

2) Voltage support.

3) Loss reduction.

4) Improved utility system reliability.

5) Distribution capacity release.

6) Potential utility capacity addition deferrals.

II. AIMS AND OBJECTIVES

The main aim of this project is to simulate DFIG control and observe the dynamic

behavior of turbine and effected on the voltage quality. Following are the main

objectives of this study:

To present the detailed analysis study of wind turbine model and DFIG control

To present the approach to quantify the effects of individual parameters over the dynamic

behaviour of WIND turbine on voltage quality.

To simulate the proposed approach using MATLAB

To evaluate the results from simulation studies.

To make the final conclusion based on obtained results.

III.1. IMPACT OF WIND ENERGY ON POWER SYSTEMS

Incorporation of great amount of distributed resources, such as wind energy, has a

significant impact on power network, which are mainly related to environmental,

economical and reliability aspects. Low wind penetration levels are usually

accommodated in power networks considering that the network is passively controlled

and operated. Although there are several available tools to be used for wind power

forecasting (González et al., 2004), wind energy is still considered as a non dispatch able

and not centrally planned technology.

Impact of wind energy on power systems is thus focused on several issues related to

security, stability, power quality and operation of power systems.

1- Wind energy has several impacts on power flow that could lead to reverse power flow

and, as a result, power systems operation will become more complex (Vilar, 2002).

Moreover, power injection by wind farms may cause power losses in the distribution

systems.

2- All the utilities have to keep stable and reliable the voltage supply to the customers

within specific limits of frequency and magnitude. Connection of wind farms may result

in voltage changes, consequently, some countries have defined a higher short-circuit



237

level at the connection point, normally between 20 and 25 times the wind farm capacity.

There are already some examples of successful operation of power networks with a lower

short circuit level (Jenkins et al., 2000).

3- Power quality is related to voltage variation and harmonic distortion in the network.

However, the incorporation of wind energy in power networks could affect the quality of

the supplied voltage to the customers. To reduce this impact, nowadays, variable speed

wind turbines equipped with power electronics are widely used in wind energy

conversion. Power electronics increase power quality because they raise the harmonic

distortion.

4- Protection system is also affected by wind farms since the incorporation of wind power

injection alters power flows; so that conventional protection systems might fail under

fault situations.

5- In the past, power network was passive operated and kept up stable under most

circumstances. However, this statement is no longer valid if considering an increase of

wind energy penetration. Recently, new requirements for wind units have been designed

in order to keep power networks stable under several disturbances, such as low voltage

ride through capability.

III.2. VOLTAGE STABILITY

Voltage Stability is defined as the ability of a power system to maintain steady-state

voltage at all buses in the system after being subjected to a disturbance from a given

initial operating condition (Kundur, 1994). In the literature, two voltage stability

problems are analyzed:

1- Estimation of the maximum laudability.

2- Computation of the critical power system loading that could lead to voltage collapse.

Voltage stability is usually represented by P-V curve (Fig. 3). In this figure the noise

point is called the point of voltage collapse (PoVC) or equilibrium point. At this point,

voltage drops rapidly with an increase of the power load and subsequently, the power

flow Jacobean

matrix becomes singular. Classical power-flow methods fail to converge beyond this

limit. This failure is considered as an indication of voltage instability and frequently

associated with a saddle-node bifurcation point (Kundur, 1994).

Although voltage instability is a local phenomenon, the problem of voltage stability

concerns to the whole power system, becoming essential for its operation and control.

This aspect is more critical in power networks, which are heavily loaded, faulted, or with

insufficient reactive power supply.



238

Fig. 1 P-V curve

In power networks with huge amount of wind penetration levels, the role of voltage

stability is of great importance due to the lack of reactive power contribution of many

wind generators as well as their integration into weak networks.

Wind farms equipped with variable speed are presented as a good alternative to alleviate

problems related to voltage stability. Therefore reactive power planning in large power

systems has become a particularly important point in recent years since it is necessary to

develop new techniques to solve any problem that may arise.

IV. MODELING OF DFIG

The basic configuration of a DFIG wind turbine is shown in Fig. 4.1. The wind turbine is

connected to the induction generator through a mechanical shaft system, which consists

of a low-speed shaft and a high-speed shaft and a gearbox in between. The wound-rotor

induction generator in this configuration is fed from both stator and rotor sides. The stator

is directly connected to the grid while the rotor is fed through a VFC. In order to produce

electrical power at constant voltage and frequency to the utility grid over a wide

operating range from sub synchronous to super synchronous speeds, the power flow

between the rotor circuit and the grid must be controlled both in magnitude and in

direction.

Therefore, the VFC consists of two four-quadrant IGBT PWM converters (a rotor-side

converter RSC and a grid-side converter GSC) connected back-to-back by a dc-link

capacitor. The operation of the DFIG wind turbine is regulated by a control system,

which generally consists of two parts: the electrical control of the DFIG and the

mechanical control of the wind turbine blade pitch angle. Control of the DFIG is

achieved by controlling the VFC, which includes control of the RSC and control of the

GSC, as shown in



239

FIG.2 CONFIGURATION OF A DFIG WIND TURBINE CONNECTED TO A UTILITY GRID

V. SIMULATION

1-WORK DONE

After the simulation we have got following results for this module: This case shows a

induction generator being driven by a wind turbine. The turbine is controlled by a wind

governor. The 'wind source’ is used to model wind speed fluctuations. Turbine torque

equations are included in the Mathcad file. At a wind speed of 15 [m/s], a pitch angle of

14 [deg] will result in the turbine providing the required steady state torque.

1.01358 is approximately the pu speed of the machine when delivering 1.44 MW (0.77

pu) of power to the system. This is used as the initial seep to speed up the initialization of

the simulation. If the starting transients are of interest, then the initial speed should be

zero and the machine should start in the 'Torque control' mode.

When in steady state, apply a step change (using the slider) to ES. The pitch angle will

regulate the real power to the power order (Demand) requested from the governor.

The turbine is controlled by a wind governor. The 'wind source’ is used to model wind

speed fluctuations. Turbine torque equations are included in the Math cad file. At a wind

speed of 14 [m/s], a pitch angle of 11.5 [deg] will result in the turbine providing the

required steady state torque.

At 230 V per phase, maximum L for transmitting 2 MW is 0.222 mH. 0.1 mH is used in

this example. The corresponding phase angle difference (from simple hand calculations),

is 23.3 deg (0.406 rad). This is entered as the initial machine angle for initialization.

2. RESULTS

According to the aims and objectives for this project, we presented two modules for wind

turbine connection with grid.



240

Figure 3 Matlab Simulink Overall Model Of DFIG Connected To Grid

After the simulation we have got following results for this module: This case shows a

induction generator being driven by a wind turbine. The turbine is controlled by a wind

governor. The 'wind source’ is used to model wind speed fluctuations. Turbine torque

equations are included in the Mathcad file. At a wind speed of 15 [m/s], a pitch angle of

14 [deg] will result in the turbine providing the required steady state torque.

1.01358 is approximately the pu speed of the machine when delivering 1.44 MW (0.77

pu) of power to the system. This is used as the initial seep to speed up the initialization of

the simulation. If the starting transients are of interest, then the initial speed should be

zero and the machine should start in the 'Torque control' mode.

When in steady state, apply a step change (using the slider) to ES. The pitch angle will

regulate the real power to the power order (Demand) requested from the governor.



241

Figure 4 Overall vector control scheme of the RSC.

Figure 5 Matlab/Simulink Design Of Rotor Side Controller



242

Figure 6 DFIG response to a change in wind speed from 6m/s to 14m/s



243

Figure 7



244

Figure 8 DFIG operation under grid fault condition

VI. CONCLUSION

It is an unfortunately reality that many parameters of wind turbine models are poorly

known. In order to investigate the dynamic performance of wind turbine generators,

parameter values must be assigned. Not all parameter values need to be known with the

same accuracy though. Using trajectory sensitivities, it has been shown that for a

particular disturbance, some parameters are much more influential than others. This

pattern of influential parameters may change for different disturbances.

The research and simulation results have shown that the WT-DFIG improves the voltage

profile and the voltage stability of the load bus. In addition, this impact is confirmed at

low wind speed 4m/s (low generation). During faults all the DFIG turbines have zero



245

outputs; but after fault clearance all of them experience short-term motor behavior;

however the system remains stable after that In general, the connection of WT-DFIGs

improve the stability of the systemand the load voltage. Wind power generation with

DFIG provides better performance for terminal-voltage recovery after the load connects

suddenly. The DFIG harmonic problems were also analyzed and the undesirable effects

of the rotor-side converter on the system were presented. A suitable filter was designed,

analyzed and proposed for reduction of system THD.

VII. REFERENCE

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Ameer Hakim Abd

Master Student in electrical Engineering electrical Engineering

Department, Bharati Vidyapeeth Deemed University College of

Engineering , Pune ,India

Phone: +91- 8446268248

Iraq phone:+9647902221951


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