Design of Interline Power Flow Controller for Grid Connected Wind ...

International Journal of Electronics, Electrical and Computational System IJEECS ISSN 2348-117X Volume 5, Issue 6 June 2016

Design of Interline Power Flow Controller for Grid Connected Wind Energy Conversion System Arun Kumar R, PG Scholar, Department of Electrical and Electronics Engineering, Sri Ramakrishna Institute of Technology, Coimbatore, Tamilnadu, India,

Sangeetha S, Department of Electrical and Electronics Engineering, Sri Ramakrishna Institute of Technology, Coimbatore, Tamilnadu, India,

Abstract- Now-a-days non-conventional energy sources are considered as a major field of power generation. Among the various non-conventional energy sources, wind has got highest potential for power generation. For this advantage, wind energy conversion system is connected to the grid in order to provide the exponentially increasing electricity demand. The grid integration involves various stability and power quality issues. To overcome these issues modern FACTS devices like UPFC and IPFC are used in the grid connected wind energy conversion system. This paper focuses on the Inter line power flow controller (IPFC) connected to the grid connected wind energy conversion system. From the common dc link, any inverter within the IPFC is able to transfer real and reactive power flow between the lines, enhances the stability of the system and damps the oscillation. This paper narrates the improvement of stability and power quality using necessary waveforms with different controllers obtained by the simulation done with Matlab/Simulink. I. INTRODUCTION Today, utilities are facing several power system stability problems such as voltage instability, transient instability, voltage sag, voltage swell, etc. Existing system consists of clearing the power system stability problems by connecting FACTS devices such as STATCOM, SSSC etc. at the distribution side using the concept of distributed generators (DGs) by connecting renewable energy as its source of supply. The proposed paper uses the IPFC in wind energy system for the voltage stability enhancement of the power system. The IPFC injects both real and reactive power to the transmission line thereby compensating the voltage instability produced by the different loads. The emergence of wind source of generation is the leading renewable energy in the power industry, wind farms totalling hundreds, even thousands, of MW are now being considered. DFIG is the main type of wind generation currently in use due to their variable speed operation, four-

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Arun Kumar R, Sangeetha S, Brinda Devi A

Brinda Devi A, PG Scholar, Department of Electrical and Electronics Engineering, Sri Ramakrishna Institute of Technology, Coimbatore, Tamilnadu, India,

quadrant active and reactive power capability, lowconverter cost and reduced power losses. In the paper, the modeling of wind energy based IPFC has been discussed. The wind energy system with IPFC is modeled by using DFIG based wind energy source connected to the rectifier and then to the inverter through a common dc link. The IPFC is modeled by connecting two or more SSSC’s through its common dc-link. The pulses to the inverter of the IPFC are given through PWM generator. The simulation results of Wind Energy Based IPFC for Voltage Stability Enhancement in the test system is concluded. In other FACTS controllers there are two or more VSCs coupled together via a common DC bus which increases not only the controllability but also the complexity. For IPFC two series VSCs connect to each other at the DC bus so one of them(assumed as the master VSC) can control both line active and reactive power and the other one(assumed as the Slave VSC) can only regulate line active power supporting sufficient active power to the Master VSC through the DC tie. The major components of a typical wind energy conversion system include a wind turbine, generator, interconnection apparatus and control systems. Wind turbines can be classified into the vertical axis type and the horizontal axis type. Most modern wind turbines use a horizontal axis configuration with two or three blades, operating either down-wind or up-wind. A wind turbine can be designed for a constant speed or variable speed operation. Variable speed wind turbines can produce 8% to 15% more energy output as compared to their constant speed counterparts, however, they necessitate power electronic converters to provide a fixed frequency and fixed voltage power to their loads. Most turbine manufacturers have opted for reduction gears between the low speed turbine rotor and the high

International Journal of Electronics, Electrical and Computational System IJEECS ISSN 2348-117X Volume 5, Issue 6 June 2016

speed three-phase generators. Direct drive configuration, where a generator is coupled to the rotor of a wind turbine directly, offers high reliability, low maintenance, and possibly low cost for certain turbines. Several manufacturers have opted for the direct drive configuration in the recent turbine designs. At the present time and in the near future, generators for wind turbines will be synchronous generators, permanent magnet synchronous generators, and induction generators, including the squirrel cage type and wound rotor type. For small to medium power wind turbines, permanent magnet generators and squirrel cage induction generators are often used because of their reliability and cost advantages. Induction generators, permanent magnet synchronous generators and wound field synchronous generators are currently used in various high power wind turbines. Interconnection apparatuses are devices to achieve power control, soft start and interconnection functions. Very often, power electronic converters are used as such devices. Most modern turbine inverters are forced commutated PWM inverters to provide a fixed voltage and fixed frequency output with a high power quality. Both voltage source voltage controlled inverters and voltage source current controlled inverters have been applied in wind turbines. For certain high power wind turbines, effective power control can be achieved with double PWM (pulse width modulation) converters which provide a bi-directional power flow between the turbine generator and the utility grid. The block diagram of wind energy conversion system as shown in Fig. 1.

Rotor Model

Gear Box

Induction Generato r

External Grid

Fig. 1. Block diagram of wind energy conversion system

WIND ENERGY CONVERSION SYSTEM Introduction The wind energy conversion system (WECS) includes wind turbines, generators, control system, interconnection apparatus. Wind Turbines are mainly classified into horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT). Modern wind turbines use HAWT with two or three blades and operate either downwind or upwind

II.

A.

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Arun Kumar R, Sangeetha S, Brinda Devi A

configuration. This HAWT can be designed for a constant speed application or for the variable speed operation. Among these two types variable speed wind turbine [1] has high efficiency with reduced mechanical stress and less noise. Variable speed turbines produce more power than constant speed type, comparatively, but it needs sophisticated power converters, control equipments to provide fixed frequency and constant power factor . The generators used for the wind energy conversion system mostly of either doubly fed induction generator (DFIG) or permanent magnet synchronous generator (PMSG) type. DFIG have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and grid. In DFIG the converters have to process only about 25-30 percent of total generated power (rotor power connected to grid through converter) and the rest being fed to grid directly from stator. Whereas, converter used in PMSG has to process 100 percent power generated, where 100 percent refers to the standard WECS equipment with three stage gear box in DFIG. Majority of wind turbine manufacturers utilize DFIG for their WECS due to the advantage in terms of cost, weight and size. The wind energy conversion system which will be modelled as shown in Fig. 1 may not be optimal for extracting maximum energy from the resource and hence various optimization techniques are used to achieve the goal. An entire wind energy system can be sub divided into following components: 1. Model of the wind, 2. Turbine model, 3. Shaft and gearbox model, 4. Generator model and 5. Control system model. The first three components form the mechanical part of the wind turbine generator. The generator forms the electro-mechanical link between the turbine and the power system and the control system controls the output of the generator. The control system model includes the actuator dynamics involved, be it the hydraulics controlling the pitch of the blades, or the converters controlling the induction generator. This chapter describes the mathematical modelling of the various components of the wind system.

International Journal of Electronics, Electrical and Computational System IJEECS ISSN 2348-117X Volume 5, Issue 6 June 2016

Model of the Wind The model of the wind should be able to simulate the temporal variations of the wind velocity, which consists of gusts and rapid wind speed changes. The wind velocity 𝑉𝑤 can be written as Eq. 1 𝑉𝑤 =𝑉𝑤𝐵 +𝑉𝑤𝐺 +𝑉𝑤𝑅 (1) where, 𝑉𝑤 = Tital wind velocity, 𝑉𝑤𝐵 = Base wind velocity, 𝑉𝑤𝐺 = Gust wind component and 𝑉𝑤𝑅 = Ramp wind component. The base wind speed is a constant and is given by Eq. 2 𝑉𝑤𝐵 = 𝐶1 ; 𝐶1 = constant (2) The gust component is represented as a (1-cosine) term and is given by Eq. 3 0 𝑡 < 𝑇1 B.

𝑡−𝑇

𝑉𝑤𝐺 = 𝐶2 1 − 𝑐𝑜𝑠 𝜋 𝑇 −𝑇1 2

1

𝑃𝑤 = 2 𝜌𝐴𝑅 𝑉1 2 − 𝑉3 2 𝑉2

(7)

According to Betz (2006), the maximum wind turbine power output is given by Eq. 8 𝑃𝑚 = 𝑉2 =

16 𝜌 𝐴 𝑉3 27 𝑅 2 1 2 𝑉 and 𝑉3 3 1

(8) =

1 𝑉 3 1

(9)

The turbine model represents the power capture by the turbine. The power in the wind (𝑃𝑤 ) in an area is given by Eq. 10 1

𝑃𝑤 = 2 𝜌𝐴𝑣𝑤3 , 𝐴𝑅 = 𝐴

(10)

The Power Coefficient of wind turbine model is given in Fig. 2.

𝑇1 ≤ 𝑡 ≤ 𝑇2

1

0

𝑡 ≥ 𝑇2 (3) Where 𝐶2 is the maximum value of the gust component and 𝑇1 and 𝑇2 are the start and stop times of the gust, respectively. The rapid wind speed changes are represented by a ramp function, which is given by Eq. 4 0 𝑡 < 𝑇3 𝐶3

𝑉𝑤𝑅 =

𝑡−𝑇3 𝑇4 −𝑇3

𝑇3 ≤ 𝑡 ≤ 𝑇4

(4)

0 𝑡 ≥ 𝑇4 where 𝐶3 is the maximum change in wind speed caused by the ramp and 𝑇3 and 𝑇4 are the start and stop times of the ramp, respectively. C. Turbine Model The noise component of the wind speed is not modelled, as the large turbine inertia does not respond to these high frequency wind speed variations. The Wind Power is given by Eq. 1 𝑑 𝑉𝑎

𝑃𝑤 =

1 2

𝜌 𝑉1 2 −𝑉3 2 𝑑𝑡

(5)

An air volume flow in the rotor area is given by Eq. 6 𝐴2 = 𝐴𝑅 of

𝑑𝑉𝑎 𝑑𝑡

= 𝐴𝑅 𝑉2

(6)

yields in the quasi-steady state,

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Arun Kumar R, Sangeetha S, Brinda Devi A

Fig. 2. Power Coefficient of wind turbine model where 𝑣𝑤 is the wind velocity. However, the turbine captures only a fraction of this power. The power captured by the turbine (𝑃𝑚 ) can be expressed as Eq. 11 𝑃𝑚 = 𝑃𝑤 × 𝐶𝑝 (11) where 𝐶𝑝 is a fraction called the power coefficient. The power coefficient represents a fraction of the power in the wind captured by the turbine and has a theoretical maximum of 0.55 (David Richard et al 1993). The power coefficient can be expressed by a typical empirical formula as Eq. 12 1

𝐶𝑝 = 2 𝛾 − 0.022𝛽2 − 5.6 𝑒 −0.17𝛾

(12)

where 𝛽 is the pitch angle of the blade in degrees and 𝛾 is the tip speed ratio of the turbine, defined as Eq. 13 γ =ɷ

𝑣𝑤 𝑚𝑝 ℎ 𝑟𝑎𝑑 𝑠 −1

𝑏

(13)

ɷ𝑏 = Turbine angular speed Equations describe the power captured by the turbine and constitute the turbine model. The simulink model of the wind turbine is shown in Fig. 3.

International Journal of Electronics, Electrical and Computational System IJEECS ISSN 2348-117X Volume 5, Issue 6 June 2016

using the general principle that the under loaded lines are to provide help, in the form of appropriate real power transfer, for the overloaded lines. The IPFC model is shown in Fig. 4.

Fig. 3. Simulink model of the wind turbine INTERLINE POWER FLOW CONTROLLER The Interline Power Flow Controller proposed is a new concept for the compensation and effective power flow management of multi-line transmission systems. In its general form, the IPFC employs a number of inverters with a common dc link, each to provide series compensation for a selected line of the transmission system. Because of the common dc link, any inverter within the IPFC is able to transfer real power to any other and thereby facilitate real power transfer among the lines of the transmission system. Since each inverter is also able to provide reactive compensation, the IPFC is able to carry out an overall real and reactive power compensation of the total transmission system. A. Principle Of Operation In its general form the Interline Power Flow Controller employs a number of dc to ac inverters each providing series Compensation for a different line. In other words, the IPFC comprises a number of Static Synchronous Series Compensators. However, within the general concept of the IPFC, the compensating inverters are linked together at their dc terminals, as illustrated in Fig 3.1. With this scheme, in addition to providing series reactive compensation, any inverter can be controlled to supply real power to the common dc link from its own transmission line. Thus, an overall surplus power can be made available from the underutilized lines which then can be used by other lines for real power compensation. In this way, some of the inverters, compensating overloaded lines or lines with a heavy burden of reactive power flow, can be equipped with full twodimensional, reactive and real power control capacity, similar to that offered by the UPFC. Evidently, this arrangement mandates the rigorous maintenance of the overall power balance at the common dc terminal by appropriate control action,

III.

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Arun Kumar R, Sangeetha S, Brinda Devi A

Fig. 4. Interline Power Flow Controller IPFC In Wind Energy Conversion System The wind energy based IPFC is modeled by using DFIG based wind energy source connected to the rectifier and then to the inverter through a common dc link. The IPFC is modeled by connecting two or more SSSC’s through its common dc-link. A 1.5 MW wind turbine connected to a Wind Energy based SSSC is modelled using voltage source converter consists of a rectifier and an inverter. The output from wind turbine is rectified using three phase thyristor controlled rectifier. A common dc link is provided to eliminate the ripples in rectified output. The output from rectifier is again converted to AC voltage using three phase IGBT based inverter.IPFC is modelled by connecting two SSSC’s through a common dc link. The Single line diagram of the pu Wind Energy based IPFC is given in Fig. 5. B.

Fig. 5. Schematic diagram of WECS with IPFC Wind turbines using a Doubly-fed induction generator (DFIG) consist of a wound rotor induction generator and an AC/DC/AC IGBT-based

International Journal of Electronics, Electrical and Computational System IJEECS ISSN 2348-117X Volume 5, Issue 6 June 2016

converter modelled by voltage sources. The stator winding is connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on the turbine during gusts of wind. D. SIMULATION AND RESULTS The Simulink model of WECS with and without IPFC is shown in Fig. 6 and Fig. 9.

Fig. 9. Simulink model of WECS with IPFC

Fig. 10. Voltage waveform of the grid connected WECS without IPFC

Fig. 6. Simulink model of WECS without IPFC Voltage waveform and the THD analysis of the grid connected WECS with and without IPFC is shown in Fig. 7, Fig. 8, Fig. 10 and Fig. 11.

Fig. 11. THD analysis of the grid connected WECS with IPFC Fig. 7. Voltage waveform of the grid connected WECS without IPFC

Fig. 8. THD analysis of the grid connected WECS without IPFC

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Arun Kumar R, Sangeetha S, Brinda Devi A

TABLE I. COMPARITION OF THE GRID CONNECTED WECS WITH AND WITHOUT IPFC Case

THD Value (%)

With IPFC

31.77

Without IPFC

53.55

Comparition of the grid connected WECS with and without IPFC is shown in TABLE I.

International Journal of Electronics, Electrical and Computational System IJEECS ISSN 2348-117X Volume 5, Issue 6 June 2016

CONCLUSION The design of IPFC for grid connected WECS for voltage stability enhancement and harmonic reduction is thus performed by using Matlab Simulink. The voltage instability is created by connecting different loads to the test system. The voltage instability created is compensated by using IPFC connected to the system. IPFC like other FACTS Controller contribute to the optimal system operation by reducing the power loss and improving the voltage profile. The IPFC is a kind of combined compensators, which combines at least two SSSCs via a common DC voltage link. This DC voltage link provides the device with an active power transfer path among the converters, which enables the IPFC to compensate multiple transmission lines at a given substation. This is a very attractive feature of this FACTS device. E.

3.

4.

5.

6.

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