Control of a wind turbine for grid fault detection in stand-alone or grid-connected modes

Mouna Rekik1, Lotfi Krichen2

1 National Engineering School of Sfax Tunisia, research unit ACEM, e-mail: mouna_ing@hotmail.fr 2National Engineering School of Sfax Tunisia, research unit ACEM, e-mail: lotfi.krichen@enis.rnu.tn

ABSTRACT

In this study, a wind generation is considered as a unit able to supply an isolated load or to be integrated into a network coupled with classical alternators. This generator operates without auxiliary source and participates in the production-consumption balance by adjusting the provided amplitude and frequency of the voltages, through the generation or the absorption of active and reactive powers. For a smooth transition between different operation modes, a control strategy is proposed to receive measurements and operates in grid-connected or stand-alone modes with automatic switches. The transition from first to second mode corresponds to an islanding one. In this context, an algorithm, based on surveillance of the wind power, the voltage and frequency of the grid, is suggested to detect islanding in case of defects and to participate in the grid stabilization by reducing the voltage and frequency fluctuations across the injection of active and reactive powers.

Index Terms— Wind generator, grid stability, islanding, fault detection, consumption-production balance.

1. INTRODUCTION

The increased consumption of electricity generated

by fossil or nuclear energy and the nascent ecological consciousness increased significantly the interest of using renewable energy.

The wind energy is one of the largest and most promising sources of renewable energy around the world in terms of development because it is clean and economically viable.

The production of wind energy can be used under an autonomous power supply (isolated site) or connected to the electricity grid [1,2]. The connection and disconnection of distributed generation is a major issue brought forward at a particular defect of the grid: islanding [3, 4].

We talk about islanding when a generator is found disconnected from the main network and continuous supplying loads, in this event many risks may arise: Degradation of electrical equipment in case of drift voltage and high frequency, problems of phase shift between the electrical grid and decentralized generator during the reclosing after a defect, security problems for the public and maintenance personnel. So, it is essential to detect islanding situation and reduce the operating time of the islanded system.

In this context, our study is focused, in the first part, on the modeling of wind conversion chain that plays a dual role. On the one hand, it is used to supply a balanced load characterized by its active and reactive powers. On the other hand, it contributes to control and to stabilize the voltage and frequency of the electrical

network through the generation or absorption of active and reactive powers [5]. In this paper, the wind chain and the load constitute a micro grid which can operate in three modes:

- Grid connected mode: type low-voltage. - Stand-alone mode: it may be intentional or accidental. - Synchronization mode to recover the connection to the

main network without damaging our system. Then, in the second part, an "anti-islanding" algorithm

applied to detect stand-alone mode (islanding detection) is presented. This technique is based on the supervision of these parameters: wind power, voltage and frequency of the grid. Moreover, this method is used for other purposes, for example, it can be used to protect equipment for not reaching values of voltage and frequency that could damage, also it can participate in stabilization of the main network by generation or absorption of active and reactive powers (PA , QA ) 6 . Finally, for a flexible and soft transition between different operation modes, a "power control" program is proposed to receives all the measurements and sets the operation modes of our micro-grid.

2. OPERATING MODES OF A WIND GENERATOR

The structure of the studied wind generation system is depicted in figure 1.

Figure1. Structure of the studied wind generation system

2.1. Grid connected mode

In this operating mode the wind is connected to the grid through a voltage inverter and local load. The voltage at the common connection point (PCC) is synchronized with the network (the switch K is closed) and there is no condition of islanding (A = 1). Under these conditions, the amplitude and the phase of V are equal to those of the main network.

In addition, the outputs of anti islanding block, PA and QA are activated to reduce the fluctuations in voltage and frequency at the connection point to the main grid.

2.2. Stand-alone Mode

In case of islanding, the anti-islanding signal A is turned immediately to 0 causing the opening of the switch

and then the tow voltages and are not synchronized. Once the micro network working in stand-alone mode, the additional powers PA and QA have no meaning, so they are null.

During this mode, the anti-islanding continues the analysis of amplitude and frequency of V in order to detect the recovery time to the main network in case where the troubleshooting is assured. When this happens, the synchronization mode will be activated and the micro grid connects again with the main network by closing the switch .

2.3. Synchronization mode

When the network defect is recovered, the transition from stand-alone mode to the connected grid mode cannot be assured in any conditions so a resynchronization is required. The objective is to reduce

the difference of phase and amplitude between the two voltages V and V . Under these conditions, the reconnection is performed by the switch on of with a minimum of transient current.

3. WIND GENERATION SYSTEM MODELING

3.1. Wind turbine modeling

The wind power across the circular surface (radius ) swept by the turbine is defined as follows: . .2 (1)

The aerodynamic power appearing at the turbine rotor is then [7]: . ( , ). . .2 (2)

Cp: represents the power coefficient. According to the theory of Betz, the maximum possible value for this parameter is 0.592 [8]. ( , ) 0.53. 151 0.58. 0.002. . 10 . ( 18.4) (3)110.02. 0.0031 (4)Each wind turbine is defined by its own expressed in terms of the tip speed ratio( ) , which is the ratio between the linear speed at the end of the rotor blades and the wind speed , for a radius and a rotation speed ( / ). This TSR is expressed by: . (5)

: The rotation speed of wind turbine.

PWM

1Power Control

1 Anti-

islanding

Wind generator

control

Synchronisation system

3.2. PMSG modeling

The permanent magnet synchronous machine considered is by hypothesis a machine type radial magnetization with smooth poles. It can be modeled by using the simulation tool MATLAB Simulink. The equations used to model the PMSG in the Park reference frame are [7, 9]:

1 . ( . . . . ) (6)1 . ( . . . . . . ) (7). ( . . . ) (8)- : the stator resistance. - : are the inductances of the generator on the d and q axis. - : Is the flow induced by the permanent magnets, - : are the direct and quadratic components of stator voltages, respectively. - : are the direct and quadratic components of the stator currents, respectively. - p: Is the number of pole pairs. 3.3. DC bus modeling

The DC bus voltage changes according to the following equation: 1 ( _ _ ) (9)

3.4. Filter modeling in Park reference

The equivalent model of the LC filter in the d-q reference frame is given by the following equations: 1 1

(10) 1 1 (11) 1 1 (12) 1 1 (13)

: The direct and quadratic components of

modulated voltages. : The direct and quadratic components of the load

voltages. : The components of direct and quadratic currents

through the inductance. i : The direct and quadratic components of the supplied currents to the grid and the load.

4. CONTROL STRATEGY

The control strategy comprises the PMSG converter control, the DC bus voltage control and the Grid converter control.

4.1. PMSG converter control

Among the strategies of vector control is applied to , the one which consists to impose a reference of direct current equal to zero. The direct current is regulated to a null value because it does not used to calculate the torque. The PMSG control used in this study is depicted in figure 2.

Figure2. PMSG control.

The voltages V and V applied at the machine terminals are defined by the following expressions: V V p. Ω . L . I (14) V V p. Ω . L . I p. Ω . Φ (15) V L dIsddt R . I (16) V L dIsqdt R . I (17)

The controlled system of the stator currents is

given in the following figure: Figure.3. Control system of the stator currents.

The block diagram of the control currents is

determined using the equations 16 and 17. The regulations loops of direct and quadratic voltage are given in figures 4 and 5, respectively.

_ _

_ _ _ __ _

_

Generation

1 11 .(1 ) ( ) , ,

Figure.4. Determination of the control voltage .

Figure.5. Determination of the control voltage .

After determining the control voltages and , it remains to determine the control signals applying to the converter as shown by the following equations: 2 (18)

2 (19)

4.2. Regulation of the DC bus voltage

The objective of this regulation is to have a constant DC bus voltage. This regulation is ensured by the balance between the generated power and the required power.

The employed technique is to apply a reference torque at the terminal of equivalent to the sum of resistant torques, in such cases the wind generator debits only the quantity of power needed to supply loads and stabilize grid. The principle is as shown in Figure 6.

The adjustment of the DC bus voltage is effected by means of a corrector which generates the reference current to be injected into the capacitor . The stored power in the capacitor is expressed by:

(20)

The transmitted power to supply loads and stabilize network is expressed by: (21) The power generated by wind turbine is then:

(22)

It is noted that the losses in the capacitor, the converters and the filter are negligible compared with the transmitted power. Under these conditions, the reference electromagnetic torque is given by the following equation:

_ (23)

Figure.6. Control strategy of the DC bus.

4.3. Grid converter control

In this paper an algorithm built to the inverter control is proposed. During the three operating modes, this algorithm receives the wind power, the voltage and the frequency of the grid and plays four roles following:

• Islanding detection in case of default. • Surveillance and regulation of the generated and

consumed powers. • Surveillance and control of the network voltage

by injection reactive power . • Surveillance and control of the network

frequency by injection of active power .

The operating principle of this algorithm is detailed in Figure 7.

Figure.7. Anti-islanding algorithm. and : are the active and reactive powers of

the load.

: : :

> 0 |∆ | > |∆ ||∆ | > |∆ |

>

|∆ | > |∆ |

_

_

__

. . .

( ) + _ _ +

( ) . . .

+ _ +

. . + _

|∆ | > |∆ |

4.3.1. Power monitoring As mentioned previously, the wind generated power

is used to supply load or to adjust voltage and frequency of the grid.

* : in such case, the aero generator is operating in network connection, it is used to cover the load demands and to regulate voltage and frequency of the grid.

* < : The wind generator is unable to participate in the service system, but it is able to ensure the load demands. In this case, the micro-grid switches to stand-alone mode to protect load from voltage or frequency fluctuations.

* < : The wind power is insufficient to meet load demands which results the reconnection to the grid to recompense this amount of power.

4.3.2. Grid voltage control To develop the grid voltage control strategy, voltage

variations are defined as follows: ∆ (24) ∆ (25) ∆ (26) With: : The actual grid voltage. : The nominal grid voltage. : The grid voltage from which amplitude regulation is enabled. : The maximum grid voltage from which there is islanding detection.

*If ∆ < ∆ it is the grid connecting mode where, A 1 and QA 0 , the rms voltage of the network V is adjusted to its nominal value.

*If ∆ ∆V < ∆V : 1 no islanding condition. For this, an anti-islanding system integrated in the inverter control serves as a voltage regulator by calculating the amount of reactive power to be injected to the network to recover the disturbance. This amount of reactive power is calculated by the following equation: (∆ (∆ ). ∆ ) (27)Where: QA >0 in case of under voltage and QA <0 in case of overvoltage. KVQ : is an adjustable gain to still working in the stabilization area.

*If ∆V ∆V : an islanding condition has been detected, A is immediately turned to 0 and the inverter is disconnected from the main network: this is the stand-alone mode, the additional injection of QA has no meaning, so it is null during this mode.

4.3.3. Grid frequency control

To develop the grid frequency control strategy, frequency variations are defined as follows: ∆ (28) ∆ (29) ∆ (30)

With: : The actual grid frequency. : The nominal grid frequency. The grid frequency from which phase regulation is enabled. : The maximum grid frequency from which there is islanding detection.

*If ∆f < ∆f : A 1 mode network connection, in which case, there is no injection of active power because the measured frequency is included in the acceptable operating domain.

*If ∆ ∆f < ∆f : there is no islanding condition (A 1), the inverter provides an amount of active power to be injected to the grid for ensuring the frequency regulation. This amount of active power is calculated as follows: (∆ (∆ ). ∆ ) (31)

Where: PA > 0 in case of under frequency and PA < 0 in case of over frequency. Kfp : is an adjustable gain to still working in the operating margin.

*If ∆f ∆f : an islanding condition has been detected (A 0), the inverter is disconnected from the main network: this is the stand-alone mode, PA has no meaning so it is null during this mode.

4.3.4. Power control system

The block diagram of the power control system is illustrated in figure 8.

Figure.8. Principle of power system control.

P/ QPPA reg(PI)

wt

∆V ∆wt

VQAQ

iVC

Φ VdqCalculation

PL

reg(PI) QL

C

C

1 2

21

VdVq

V V

The adjustment variables are the rms value and the phase of v at the common connection point (PCC). This control allows the injection of the desired active and reactive powers in order to regulate voltage and frequency at the connection point during connected mode, and to impose rms value and phase of v in stand-alone mode. This setting is made using a PI controller as shows the following equations [1, 2]: . ( ) (32) . ( ) (33)

Where the values of depend to the operating modes. The operating state of the switches C1 and C2 is indicated by the following table 2:

Table 2. The switches C1 and C2 states.

Modes operating C1 and C2 switches

Grid-connected mode

Position 1

Stand-alone mode Position 2

Synchronization mode

Position 1

With: (2 ) (34)

Table 2. Studied system parameters.

Wind turbine Blade radius Number of blades

23

DC bus Capacitance Maximal voltage

2200μ400

PMSG Number of pole pairs Nominal power Stator winding inductance Stator winding resistance Friction coefficient Inertia

43.8515.1 0.82 10_99.10_ kgm

Filter Filter resistance Filter inductance Filter capacitance

0.0232200 μ 5. SIMULATION RESULTS

The performance of the proposed control strategies was evaluated by computer simulation using MATLAB SIMULINK. This system was tested under the following conditions:

- Variable wind speed ranging between 9 and 12 m/s, this signal is depicted in figure 9.

- Voltage and frequency fluctuations of the grid, these fluctuations are shown in figures 11 and 12, respectively.

-Variable load power as given by figure10.

Figure.9. Wind speed.

Figure.10. Load power.

Figure.11. Grid voltage.

Figure.12. Grid frequency

Some simulation results from the wind turbine generator are presented. The figure 13 shows the DC bus voltage that it is constant at 400V regardless of the perturbations which may occur during operation .The mechanical speed is represented by figure 14 that it is sensitive according to the previous constraints. Under the two figures 15 and 16, it is remarkable that the wind turbine generates the amount of power needed to supply load and to stabilize grid in order to ensure the balance between the generated power and the required power which proves the effectiveness of implemented control strategies.

Figure.13. DC bus

voltage. Figure.14. Mechanical

speed.

Figure.15. Generated and injected powers.

Figure.16. Electromagnetic torque.

To respond to previous fluctuations the anti-islanding

system provides the following outputs:

0 2 4 6 8 10 12 14 16 18 208

8.5

9

9.5

10

10.5

11

11.5

12

12.5

13

Times (s)

Win

d sp

eed

(m/s

)

0 2 4 6 8 10 12 14 16 18 20800

1000

1200

1400

1600

1800

2000

2200

2400

Times (s)

Loa

d po

wer

(W)

0 2 4 6 8 10 12 14 16 18 20205

210

215

220

225

230

235

240

245

250

Times (s)

Gri

d vo

ltag

e (V

)

0 2 4 6 8 10 12 14 16 18 2048

48.5

49

49.5

50

50.5

51

51.5

52

52.5

53

Times (s)

Gri

d fr

eque

ncy

(HZ

)

0 2 4 6 8 10 12 14 16 18 20390

392

394

396

398

400

402

404

406

408

410

Times(s)

DC

bus

vol

tage

(V

)

0 2 4 6 8 10 12 14 16 18 20

160

180

200

220

240

260

Temps (s)

Ωm

ec (

rad/

s)

0 2 4 6 8 10 12 14 16 18 20-1000

-500

0

500

1000

1500

2000

2500

3000

3500

4000

Times (s)

Pow

ers

(w)

power generated (w)

power injected (w)

0 2 4 6 8 10 12 14 16 18 20-20

-15

-10

-5

0

5

Times(s)

Ele

ctro

mag

neti

c to

rque

(N

m)

Figure.17. Anti-islanding signal.

Figure.18. Anti-

islanding active power

Figure.19. Anti-islanding reactive

power. The curves of the efficient voltage and frequency after

regulation at the common connection point to the grid (PCC) are given by Figures 20 and 21.

Figure.20. Grid voltage

after regulation. Figure.21. Grid frequency after

regulation.

6. CONCLUSION

Through this paper, a wind conversion chain operating in three modes (grid-connected mode, stand-alone mode, and synchronization mode) is presented. A control strategy is designed with two control interfaces: one for grid converter operating and the other for wind generator converter. An anti-islanding algorithm, which is responsible to detect islanding mode and to stabilize the main network by generation or absorption of active and reactive powers (PA , QA ) is proposed. The transition between the three operating modes is assured by "power control" program. The simulation results showed that the islanding detection is distinguished when there are significant variations in grid voltage’s amplitude or phase which can damage loads. In addition, it is shown that the response of the proposed control schemes is able to maintain the voltage and the frequency within permissible domains during grid connected and stand-alone operation modes. Finally, this control strategy ensures the continuous supply of the load during the three operating

modes and participates in the grid stabilization without help from storage system which proves the effectiveness of implemented control.

REFERENCES

[1] J. C. Vasquez, J. M. Guerrero, A. Luna, P. R Teodorescu,

“Adaptive Droop Control Applied to Voltage-Source Inverters Operating in Grid-Connected and Islanded Modes’’, IEEE transactions on industrial electronics, vol. 56, no. 10, october 2009.

[2] J. C. Vásquez, J. M. Guerrero , E. Gregorio, P. Rodríguez,

R. Teodorescu, F. Blaabjerg, “ Adaptive Droop Control Applied to Distributed Generation Inverters Connected to the Grid’’, Spanish Ministry of Science and Technology.

[3] Shyh-Jier Huang, Fu-Sheng Pai, “ A New Approach to

Islanding Detection of Dispersed Generators with Self-Commutated Static Power Converters’’, IEEE transactions on power delivery, vol. 15, no. 2, april 2000.

[4] Irvin J. Balaguer, Qin Lei, Shuitao Yang, Uthane

Supatti, Fang Zheng Peng ,“Control for Grid-Connected and Intentional Islanding Operations of Distributed Power Generation”, IEEE transactions on industrial electronics, vol. 58, no. 1, january 2011.

[5] A. A. Salam, A. Mohamed and M. A. Hannan “Technical

challenges on microgrids” ARPN Journal of Engineering and Applied Sciences,vol. 3, no. 6, december 2008 issn 1819-6608.

[6] J. M. Guerrero, J. Matas, L. G. Vicuña, M. Castilla, J.

Miret, “Wireless-Control Strategy for Parallel Operation of Distributed-Generation Inverters”, IEEE ransactions on industrial electronics, vol. 53, no. 5, october 2006.

[7] D. C. Aliprantis, “Modelling and control of a variable

speed wind turbine with a permanent magnet synchronous generator”, Diploma Thesis, NTUA, 1999.

[8] J. G. Slootweg, S. W. H. de Haan, H. Polinder, W. L.

Kling, “General Model for Representing Variable Speed Wind Turbines in Power System Dynamics Simulations”, IEEE Transactions on Power Systems, Vol. 18, No.1, Feb. 2003.

[9] S. B. Papaefthimiou, “Simulation and control of a variable

speed wind turbine with synchronous generator”, Diploma Thesis, NTUA, September 2005.

[10] J. M. Guerrero, J. C. Vasquez, J. Matas, L. García de

Vicuña, M. Castilla, “Hierarchical Control of Droop-Controlled AC and DC Microgrids—A General Approach Toward Standardization’’, IEEE transactions on industrial electronics, vol. 58, no. 1, january 2011.

0 2 4 6 8 10 12 14 16 18 20-1.5

-1

-0.5

0

0.5

1

1.5

Temps (s)

Ais

l

0 2 4 6 8 10 12 14 16 18 20-300

-200

-100

0

100

200

300

Temps (s)

P Ais

l(W)

0 2 4 6 8 10 12 14 16 18 20-300

-200

-100

0

100

200

300

400

500

Temps (s)

QA

isl(V

AR

)

0 2 4 6 8 10 12 14 16 18 20210

212

214

216

218

220

222

224

226

228

230

Times (s)

Gri

d vo

ltag

e re

gula

ted

(V)

0 2 4 6 8 10 12 14 16 18 2049.5

49.6

49.7

49.8

49.9

50

50.1

50.2

50.3

50.4

50.5

Times (s)

Gri

d fr

eque

ncy

regu

late

d (H

z)