Impact of Reactive Power on Stable Production of Wind Farms

Majlesi Journal of Energy Management

Vol. 4, No. 2, June 2015

Impact of Reactive Power on Stable Production of Wind Farms Ashkan Edrisian1, Mohsen Hajian2, Mostafa Kermani3, Mahmoud Ebadian4 1- Department of Electrical Engineering, Islamic Azad University (IAU), Science and Research branch of Birjand, Birjand, Iran Email: [email protected] (Corresponding author) 2- Department of Electrical Engineering, Islamic Azad University (IAU), Bojnourd branch, Bojnourd, Iran Khorasan Regional Electric Company (KREC.), Esfarayen, Iran Email: [email protected] 3- Technical Institute of Ebn-Hesam, Birjand, Iran Email: [email protected] 4- Assoc. Prof. Department of Electrical Engineering, University of Birjand, Birjand, Iran Email: [email protected]

Received September 2014

Revised November 2014

Accepted December 2014

ABSTRACT: Increasing the share of wind energy in supplying load demand of power grid has created new challenges in the field of power system stability. So that, the stable operation of each units of wind power producer on interaction with grid could be affected by power system instability. Among them we can mention the phenomenon of voltage instability in power system which eventually will lead to voltage collapse if stability controls are not applied. In this paper the impact of reactive power control on improvement of SCIG-based wind farms operation has been studied. With regard to the asynchronous operation of induction machine, the increment of wind power generation will be possible only by absorption of more reactive power. Increasing in the power reactive consumption of induction generator leads to reduction of voltage in point of common connection (PCC) which the wind farm connects to the grid. Ultimately, this process in lack of proper reactive power control can be lead to instability in whole of power system. The conducted studies in this paper are in framework of quasi-static time-domain simulations (QSTDS) and continuation power flow (CPF) algorithm. The results indicate with increment of wind power penetration in power system if the proper support of reactive power is not used, the stable performance of the wind farm will be threatened and caused to over speed of SCIGs. Another task done in this paper is the employment of SVC and STATCOM as a practical solution in improvement of wind farms-based power system stability. KEYWORDS: Squirrel Cage Induction Generator (SCIG), Reactive Power, Quasi-Static Time-Domain simulation (QSTDS), Continuation Power Flow (CPF) 1. INTRODUCTION In recent years, environmental concerns and the rising cost of fossil fuels and petroleum products has caused a fundamental change in the mechanism of energy production. So that the use of cleaner and cheaper sources of energy have been on the agenda of manufacturers. Among the policies taken in this context are the use and development of renewable energies. Considering the long background of wind energy in the energy supply as one of the practical aspects of utilization of renewable resources along with other energy sources such as solar energy, it has enjoyed considerable growth. The production capacity of wind turbines and number of Installed units in wind farms per year has had witnessed a growth of 20%. The share of wind power

in 2020 is expected to reach 12 % of total world energy [1]. Figure (1) shows the increase of at least 4 times in the annual production capacity of wind turbines. The assessment of the connecting impact of wind farms on power system stability issue and conditions of network-connecting of these units, especially in remote areas (where the sufficient infrastructures are not developed enough and in terms of network topology are poorly known) is one of the facing significant problems of wind energy expansion [2]. As the penetration level of wind power in the power system increases, the stable operation of power system will significantly be affected by characteristics of the wind turbines.

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Fig. 1. Annual capacity increment of wind turbines during the years 1980 to 2006 [3] The stability issue comes from this principle that these turbines have often used of induction generators to convert the mechanical torque into electricity which potentially lead to a voltage drop and a tendency to absorb the reactive power and voltage instability [2]. Whereas, wind farms are connected to grid via weak buses (from voltage point of view) and distribution network, a considerable amount of reactive current during the instability event ranging from small-signal or large-signal instability are absorbed [4]. Nowadays utilization of variable speed wind turbine with power electronic interconnection such as DFIG and PMSG [5] and [6] have solved the stability problem of power system, especially voltage stability. Nonetheless, still 30% of installed capacity of wind turbines is dedicated to SCIG-based wind farms. SCIGs are fixed speed type of generators that are connected to grid directly [7]. Among the benefits of SCIG which lead to its attraction, are simplicity, robustness, reliability [8] and its economic utilities [9], [10]. These characteristics are caused to needless of SCIGs to complicated equipment of exciter, voltage regulator and frequency controllers [10]. With regard to incapability of SCIG in regulation and controlling of voltage of system and also the absorption of reactive power, these generators appear as source of voltage fluctuations [2]. On the other hand, considering fixed speed operation of SCIGs, the wind speed fluctuation appears in format of mechanical torque and finally output electrical power fluctuations. Consequently, due to the close relationship between reactive power and active power in SCIGs, the fluctuation of active power led to variation of reactive power absorption. The importance of power system stability issue, especially in the voltage stability discussion so far as goes that the most of recent researches have been focused on voltage stability of the power system in presence of wind farms. In order to cope with load fluctuating and output power fluctuation of wind farms that are changed in influencing by wind variation nature, reference [11] has introduced combination of energy storage and reactive

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Vol. 4, No. 2, June 2015 power compensator as a proper alternative solution. And has discussed to how interaction between energy storage system and compensator. The use of power electronic converters to interface the generator to grid is one of another approach which has been addressed in reference [9]. Connecting the SCIG-based wind farm to grid with respect to the random nature of wind speed can lead to instability and voltage fluctuation, especially when the connected network is weak in voltage point of view. Hence, providing the local support and as much as possible close to the level of demand for reactive power compensation is necessary [11]. Predominantly the study topics, such as [2, 4, 12, 13, 14], have been focused on using FACTs devices for stable operation of wind farms. In this paper the reactive power control and its impact on stable operation of SCIG-based wind farms has been investigated. Another task done in this paper is the effect of upward increasing the production capacity of wind farms on voltage stability. The main focus of studies has been on small signal voltage stability, and employment of CPF method which is an advanced numerical technique, coincide with QSTDS have been led to the plausible and complementary results concerning the interplay of power system and wind farms. The results which presented in this study introduce subject of reactive power control as the determining factor in the improvement of working conditions of grid and wind farm, with expression the reaction of power system for connection of wind farm. Finally, compensation of reactive power in the use of compensator devices such as SVC and STATCOM is modeled. 2. CHARACTERISTIC OF WIND TURBINE AND INDUCTION GENERATOR The wind energy through the rotor of wind turbine is converted into rotational energy and this extracted rotational energy by either a gearbox or directly with no mechanical interconnection is connected to the generator [15]. This section explains the characteristics of the wind turbine and induction generator, as well as how these characteristics influence on voltage stability problem are described. 2.1. Performance characteristics of wind turbine: Wind power generation is intimately depended to wind speed so that every change in wind speed would lead to considerable variations in outlet electrical power. The extracted power from an air mass that is blowing with speed v on the turbine swept area (A) is calculated as follows:

1 Pwind  . air . A.v 3 2

(1)

Where, ρair is the air density, v is the wind speed and Ar


Vol. 4, No. 2, June 2015

is equivalent to the area swept by turbine blades. Wind energy potential is not fully achievable, attainable optimal power of the wind for the first time was discovered by Betz in 1926 [16]. According to Betz theory, the maximum of extracted power from wind would be:

PBetz 

1 1 Av 3C Betz  Av 3  0.59 2 2

Fig.3. Equivalent circuit of squirrel cage induction generator (SCIG)

The fraction of the wind energy which can be achieved by wind turbine is determined via performance coefficient of power (CP). In practical cases CP will not exceed of 48%. Eventually, the output power of turbine is calculated as follows:

According to equivalent circuit shown, the equations describing the active and reactive power are as follows:

Pm  C p  ,  

Vr 

A 2

3 v wind

(2)

C  C 5 C p  ,    C1  2  C3   C4  e  i  C6  i  1 1 0.035   3 i   0.08   1

(4)

(5)

Figure 2 indicates the generated mechanical power of turbine as a function of wind speed and generator shaft speed for pitch angle of β=0 (regardless of pitch control system). 1.2

output power [p.u.]

1.5

1

1

0.8

0.5

0.6

0

0.4

-0.5 14

0.2 10 8 6 4 wind speed [m/s]

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 -0.2

rotor speed [p.u.]

Fig. 2. The mechanical power of wind turbine versus changes in wind speed and rotor speed. As can be seen in Fig.3 by varying the wind speed in range of 5m/s to 25m/s, the active power production and subsequently the absorption of reactive power has changed. 2.2. Characteristics of Squirrel Cage Induction Generator (SCIG) In this section, using the steady-state equivalent circuit of a SCIG indicated in figure (3), the equations describing the reactive power and active power are extracted.

Pm 

(6)

2

 1  s   2  s  R2  Re   X e     21 s  Vs   R2  s 

(3)

R  v

12

1 s  Vs   R2  s 

2

(7)

 1  s   2  s  R2  Re   X e   

Pm2 Pe  Pm  Re 2 Vr

(8)

Pm2 Vs2 Qe   X e 2  Vr Xm

(9)

With placement the equations (6) and (7) in equations (8) and (9) the final form of the active and reactive power would be as follows: 2  1 s  Vs ( ) R2  Re   s  Pe  2  1  s   2  s  R2  Re   X e   

2

Qe  

(10)

2

Vs Vs  Xe 2 Xm  1  s   2  s  R2  Re   X e   

(11)

The close relationship of voltage and slip of speed with active power, and also same relationship on reactive power assesses that the reactive power has direct effect on the active power generation of induction machine, clearly. And even can play a role in controlling or limiting. The figures (4)-a and (4)-b indicate the characteristics of the active and reactive powers of induction generator versus the variation of rotor slip at different voltage levels. In other word these tow figure are the graphically description of above equations.

3

Majlesi Journal of Energy Management 4

Electrical Power [p.u.]

3 2

Vol. 4, No. 2, June 2015

Vs=1 p.u. Vs=0.75 p.u. Vs=0.5 p.u.

1 0 -1 -2 -3 -4 -5 -6 -0.2

-0.15

-0.1

-0.05

0

slip

0.05

0.1

0.15

0.2

Fig. 4. a. The power-slip characteristics of induction machine. 0

Electrical Reactive Power [p.u.]

-1

Vs = 1 p.u. Vs = 0.75 p.u. Vs = 0.5 p.u

-2 -3 -4

3. THE EFFECT OF INCREASED PRODUCTION In order to modeling the increase of generation and penetration of wind power in power system and also studying how characteristic of wind turbines impact on stable operation of power system the quasi-static timedomain simulation (QSTDS) has been used. Also, the necessity and the role of reactive power in providing the optimal and stable performance of wind turbine by using the continuation power flow (CPF) algorithm have been studied. 3.1. Continuation Power Flow (CPF) Among the most common tools to study the voltage stability is CPF which the most salient feature of CPF is that it remains well-conditioned at near of the point of voltage collapse and divergence caused by the singularity of the Jacobian matrix at the critical point would not happen [18]. The procedure stages of the CPF have been shown in figure 5.

-5

Start

-6 -7 -8 -9 -10 -0.2

-0.15

-0.1

-0.05

0

slip

0.05

0.1

0.15

0.2

Fig. 4. b. The reactive power-slip characteristics of induction machine. According to these tow figures, the increase of active power production is associated with rise of slip and it leads to increasing in reactive power consumption. Consuming the reactive power as a criterion of voltage stability leads to voltage sag in the bus connecting the wind farm to grid which This phenomenon in turn reduces the active power output of the wind farm. In a constant mechanical torque due to constant wind speed with regard to the inability of SCIG to control the input mechanical power [4], reduction of output power resulted of voltage drop leads to an imbalance between the input powers of generator. The difference due input powers imbalance appears in form of kinetic energy and resulted in an increase in rotor speed of SCIG. Under these conditions, due to the raise of rotor slip and the reactive power increment, the voltage of terminal and output active power reduce continuously [4]. In the lack of rapid and timely recovery of voltage the process increasing the rotor speed (slip) will continued [17]. This increase in speed to over of a critical value steers the generator to instability region. Hence, the turbine work must be prevented by using the high-speed protection equipment.

Run conventional power flow on base case Specify continuation parameters

Calculate target vector

Check to see critical point has been passed

Yes Stop

No Choose continuation parameter for next step

Predict solution

Preform correction

Fig. 5. The stages of CPF In implementing this method, after each stage of the

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Vol. 4, No. 2, June 2015

implementation of load flow, the load-demand would be affected by load parameter λ and increased. To simulate a load change, the active and reactive power (PLi and QLi) must be broken into tow components. One component corresponds to the original load at bus i and the other component will represent a load change brought about by a change in the loading parameter λ [18]. Thus: PLi  PLio   k Li S base cos i  (12)

QLi  QLio   k Li S base sin i 

(13)

The load parameter λ must be chosen in bellow range where λ=0 corresponds to base load and λ= λCritical corresponds to the point of voltage collapse or (Critical Load): 0    Critical (14) In addition, the term of active power generation can be reformed to: PGi  PGio 1  kGi  (15) Where PGio is the active generation at bus i, kGi and kLi are constant coefficient to specify the rate of change in generation and load at bus i as changes, respectively. The value of kGi , kLi and Ѱi can be specified for every bus of power system [18]. 3.2. Quasi-Static Time-Domain Simulation (QSTDS) In this approach the wind power production will be gently increased as a function of time. The salient distinction of this approach in comparison with conventional continuation methods is that the power generation and consumption in whole grid is fixed and only the increase of wind power production is observable. This increase in production will be in format of wind speed increment and will be simulate as a time- dependent function [13, 19]. The rotor slip and consequently the associated reactive power increase with wind speed increasing, synchronism of these incidents would lead to voltage drop at the point of common connection (PCC) of wind farm to grid which will result in the deterioration of voltage stability. Equation (16) which is available by some of the calculations in equations (2) and (7), demonstrates the relationship between voltage, wind speed and induction generator slip: 2 3 1  s R2 Vs  AC Pv R2 Re   s ACPv3 R22 (16)



4.1. Static Var Compensator This type of compensators, as the one of effective and simple ways to compensate the reactive power [11], includes controlled thyristor switches, capacitor banks and parallel reactors placed among the FACTs devices with parallel connection. Among the common type of SVCs, the thyristor controlled reactor with fixed capacitor can be pointed out, which also are recognized as TCR/FC. The figure (6) shows the single-line diagram of the TCR/FC.



AC v R R  R V   2AC v  R X 2 2

3

P

power in order to achieve stable production in wind farms, providing reactive power demand from local sources as close as possible to the place of use is essential [11]. One of simple approaches to achieve sustainable production and providing the needs of induction generator in wind farms is the use of fixed capacitors which is considered as corrective and economical solution [10], but with regard to the fixed impedance nature of this circuit element and direct relationship of reactive power injected by capacitor with voltage quadrate (QC α V 2), voltage drop for any reasons influences reactive power injected and leads to lack of it [14]. Consequently, the network is forced to compensate the reactive power shortage, while it leads to reduction of stability margin of voltage. On the other hand, reactive power absorption of SCIG, which influences by changes in wind speed and power generated, varies and applying fixed capacitors with static switching because of their transient conditions and step changes doesn’t seem plausible. Hence, the use of more flexible devices such as SVC and STATCOM, which are able to control the reactive power dynamically and consistently, have been put on the agenda [4]. The use of such these equipment due to the high economic costs and internal consumption leads to a slight decrease in output power of generators and wind farms [20].

2

e

2

3 2

s

P

2 2

2 e

 Re2



ACPv R 3

2 2

4. REACTIVE POWER CONTROL As be noted, considering to the importance of reactive

Fig. 6. The single-line diagram of the TCR/FC 4.2. Static Synchronous Compensator (STATCOM) This compensator is placed among the parallel 5

Majlesi Journal of Energy Management compensators similar to SVC and a voltage source converter has been used instead of parallel capacitors and reactors. The STATCOM is also known with alias names like GTO-SVC or advanced SVC. The figure (7) shows the STATCOM single-line diagram.

Vol. 4, No. 2, June 2015 5. THE SIMULATION RESULTS The simulations presented in this paper, have been conducted by PSAT toolbox of MATLAB software. For more scientific matching of results the IEEE 9- bus system as the case study has been used. The desired wind farm is connected to 5th bus of case study system through a transformer with capacity of 100 MVA and a transmission line with line impedance of 5.25×10 -7 + j 9.23×10-3. The wind farm simulated consist of 43 units of wind turbine equipped SCIGs with total capacity of 28.38Mw. The figure 8 shows the topology of IEEE 9bus system in presence of wind farm. With CPF Implementing on the base state of system (without the wind farm connection), the bus number 5 is identified as the weakest bus from voltage stability point of view and also selected to connect the wind farm (Figure 9). The main reason for this choice is that by connecting the wind farm to weakest bus, the worst case of voltage collapse occurrence will be simulated [4], [21].

Fig. 7. The single line diagram of STATCOM

Fig. 8. IEEE 9-bus system connected to wind farm

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Vol. 4, No. 2, June 2015

1.4

1

1.2

0.8

VBus W PBus W QBus W

1

0.6

0.8

[p.u.]

Voltage amplitude [p.u]


0.6

0.4 0.2

0.4

0 0.2

-0.2 0

1

2

3

4

5

6

7

Bus Number

8

9

-0.4 0

Fig. 9. Voltage amplitude achieved by CPF implementation for IEEE 9- bus system in absence of wind farm

6

8

10

12

14

16

Fig. 10-b. The voltage, active power and reactive power of wind farm with 43 wind turbine. 1.4

VBus W PBus W

1.2

QBus W

1 0.8 0.6 0.4 0.2 0 -0.2 0

5

10

15

20

25

30

35

40

time [s]

Fig. 10-c. The voltage, active power and reactive power of wind farm with 25 wind turbine. 0.6 0.4 0.2 0

wind speed

Cp

45

4

time (s)

[p.u.]

The 43- unit wind farm studied, along with the QSTDS implementation according to figure 10- a to 10- c, can be seen the raise of wind speed of its nominal speed (12 m/s ) as a rate function (figure 10-a) leads to lose the stable performance of wind farm in a wind speed lower than 15m/s. This instable operation is due to the inability of system to provide the reactive power demand of induction generators sets in wind farm. As the figure 10- b reveals, continuity of increasing in wind speed for over 15m/s leads to a coercive voltage collapse in the whole network. By reducing the number of units from 43 to 25 for same wind farm, according to figure 10-c, as the wind speed increases, even close to 37 m/s, the sufficient reactive power is supplied by case study system. In figure 10- c, the decrease in output power of wind farm is due to the characteristic of wind turbine, so that in a constant voltage the CP factor of turbine will be decreased by increasing in wind speed till above nominal wind speed (Figure 11). In this simulation, to achieve higher speeds the cut-out limit of wind turbine has been ignored.

2

-0.2 -0.4

40

wind speed [m/s]

-0.6 35

-0.8

30

-1

25

-1.2 5

10

15

20

25

30

35

wind speed

20

Fig. 11. The CP characteristic of wind turbine versus wind speed

15 10 5 0

5

10

15

20

25

time (s)

30

35

40

45

Fig. 10- a. The rise of wind speed as a rate function

In order to study of the margin of voltage stability, the CPF algorithm in presence of wind farm (with 43 wind turbine) in case study system has been implemented. The analysis of results and P-V curves achieved by CPF, reveals that the connection of reactive power compensators caused to stability improvement of power system connected to wind farm. As can be seen in figure 12, fixed capacitor despite of its satisfactory

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Vol. 4, No. 2, June 2015

performance in terms of stable operation, is unable to adapt its injected reactive power with consumption of induction generators when increasing reduction of voltage occurs. And eventually causes the instability of system and wind turbine connected to it. While the use of SVC and STATCOM, with respect to their flexible capability in reactive power generation, create more reliable stability margin and more stable voltage. 1

Voltage Amplitude [p.u.]

0.95

0.9

0.85

0.8

Capacitor No Comp. SVC STATCOM

0.75

0.7

1

1.1

1.2

1.3 1.4 1.5 Loading Parameter (Lambda) [p.u.]

1.6

REFERENCES

1.7

1.8

Fig. 12. The P-V curves of the connecting bus of wind farm to power system in presence of compensator devices The figure 13 indicates the amount of reactive power transmitted from wind farm to the network via transmission line. Transmitted Reactive Power \ From: Bus 15 \ To: Bus 5

1.2

1

0.8

0.6

0.4

0.2

0

-0.2

Capacitor SVC STATCOM 1

1.1

1.2

1.3 1.4 1.5 1.6 Loading Parameter (Lambda) [p.u.]

1.7

1.8

Fig. 13. Reactive power transmitted from bus 15 (the secondary bus of wind farm transformer) versus loading parameter (λ) increasing. According to this picture, due to constant impedance nature of fixed capacitors, the induction generators absorb reactive power from network. While SVC and STATCOM, in addition to providing the wind farm demand, by injecting their surplus reactive power are able to meet the needs of the network.

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6. CONCLUSION In this paper, the importance of reactive power control to achieve the stable operation of wind farms in connection of power system was studied. The results demonstrated that an increase in the turbine numbers of wind farm despite of desirable impact on power quality context will be led to reduction of voltage stability margin of power system. This is due to rise of reactive power absorption of wind farm. Thus the development and increasing the capacity of wind farms requires accurate and detailed studies on the ability of power system to meet the reactive power needs of induction generators of wind farms. The P-V curves analysis clearly illustrated the advantages of using reactive power compensators. Among all simulated compensators the STATCOM, with respect to the use of electronic converters, displays better performance. [1] N., Amutha, Kalyan, B., Kumar, “Improving fault ride-through capability of wind generation system using DVR”, International Journal of Electrical Power & Energy Systems, pp. 326-333, 2013. [2] L.,Ching-Yin, C., Li-Chieh, T., Shao-Hong, L.,WenTsan, and W., Yuan-Kang, “The Impact of SCIG Wind Farm Connecting into a Distribution System”, Power and Energy Engineering Conference, APPEEC. Asia-Pacific, 2009. [3] J., Ernest, T., Wizelius, “Wind Power Plant and Project Development”, PHI Learning, New Delhi, 2011. [4] O., Noureldeen, M., Rihan, and B., Hasanin, “Stability improvement of fixed speed induction generator wind farm using STATCOM during different fault locations and durations”, Ain Shams Engineering Journal, pp. 1-10, 2011. [5] Z., Chen, J. M., Guerrero, and F., Blaabjerg, “A review of the state of the art of power electronics for wind turbines”, IEEE Trans. on Power Electronics, Vol. 24, No. 8, pp. 1859-1875, Aug. 2009. [6] Y., Lei, A., Mullane, and G., Lightbody, “Modeling of the wind turbine with a doubly fed induction generator for grid integration studies”, IEEE Trans. on Energy Conversion, Vol. 21, No. 1, pp. 257-264, Mar. 2006. [7] G. L., Johnson, Wind energy systems. Manhattan: Prentice-Hall, 2004. [8] Phan Dinh Chung, “Comparison of Steady-State Characteristics between DFIG and SCIG in Wind Turbine”, International Journal of Advanced Science and Technology, Vol. 51, Feb. 2013. [9] M., Molinas, B., Naess, W., Gullvik, and T., Undeland, “Cage Induction Generators for Wind Turbines with Power Electronics Converters in the Light of the New Grid Codes”, European Conference on Power Electronics and Applications, Dresden, 2005.

Majlesi Journal of Energy Management [10] T. C.,Tsao, N. F., Tsang, “The squirrel-cage induction generator for power generation”, IEEE J. Elect. Eng., Vol. 70, No. 9, pp. 793-795, Sept. 1951. [11] E., Muljadi, C. P., Butterfield, R., Yinger, and H., Romanowitz, “Energy Storage and Reactive Power Compensator in a Large Wind Farm”, AIAA Aerospace Sciences, Meeting and Exhibit, Reno, Nevada, Jan 2004. [12] V., Salehi, S., Afsharnia, and S., Kahrobaee, “Improvement of voltage stability in wind farm connection to distribution network using FACTS devices”, IECON 2006-32nd Annual Conference on IEEE Industrial Electronics, Nov. 2006, pp. 42424247, 2006. [13] A. Edrisian, A. Goudarzi, and M. Ebadian “Investigating the effect of high level of wind penetration on voltage stability by quasi-static time domain simulation (QSTDS)”, International Journal of Renewable Energy Research (IJRER), Vol. 4, No. 2, pp. 355-362, 2014. [14] A. W., Oyekanmi, G., Radman, A. A. Babalola, and L. O., Uzoechi, “Effect of Static VAR Compensator positioning on a Grid-connected wind turbinedriven Squirrel Cage Induction Generator”, IEEE International Conference on Emerging & Sustainable Technologies for Power & ICT in a Developing Society (NIGERCON), pp. 247–252, 14-16 Nov. 2013. [15] Yu Zou, M., Elbuluk, and Y., Sozer, “A Complete Modeling and Simulation of Induction Generator

Vol. 4, No. 2, June 2015 Wind Power Systems”, IEEE Industry Applications Society Annual Meeting (IAS), Houston, pp. 1-8, Oct. 2010. [16] L., Huan-ping, Y., Jin-ming, “The Performance Research of Large Scale Wind Farm Connected to External Power Grid”, 3rd International Conference on Power Electronics Systems and Applications (PESA), Hong Kong, pp.1-5, 2009. [17] T., Ackermann, “Wind Power in Power Systems”, John Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England [18] V. K., Ajjarapu, C., Christy, “The Continuation Power Flow: A Tool for Steady State Voltage Stability Analysis”, IEEE Transactions on Power Systems, Vol. 7, pp. 416–423, Feb. 1992. [19] F. Milano, “Assessing Adequate Voltage Stability Analysis Tools for Networks with High Wind Power Penetration”, Third International Conference on Electric Utility Deregulation and Restructuring and Power Technologies (DRPT), Nanjuing, China, pp. 2492-2497, 6-9 April 2008. [20] M. J. Hossain, H. R. Pota, and R. A. Ramos, “Robust STATCOM control for the stabilisation of fixed-speed wind turbines during low voltages”, Renewable Energy 36, pp. 2897-2905, 5 May 2011. [21] A. Kun Yang, C. S., Garba, K. L., Tan, Lo, “The impact of the wind generation on reactive power requirement and voltage profile”, RDPT2008, Nanjing, China, pp. 866-871, April 2008.

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