A Study on Wind Power Converter Topologies for Permanent ... · SUJITH M et al: A STUDY ON WIND POWER CONVERTER TOPOLOGIES FOR PERMANENT MAGNET … DOI 10.5013/IJSSST.a.17.32.36 36.2

SUJITH M et al: A STUDY ON WIND POWER CONVERTER TOPOLOGIES FOR PERMANENT MAGNET …

DOI 10.5013/IJSSST.a.17.32.36 36.1 ISSN: 1473-804x online, 1473-8031 print

A Study on Wind Power Converter Topologies for Permanent Magnet Synchronous Generators

Sujith M

Department of EEE

IFET College of Engineering, Villupuram-605108

[email protected]

Padma S

Department of EEE Sona College of Technology,

Salem- 636 005 [email protected]

Abstract - Power Electronic Converters plays an important role in integrating the renewable energy source to the grid. In this paper, permanent magnet synchronous generator based Wind Energy Conversion Systems (WECS) is considered for evaluating the different power converter topologies and control strategy. The comparative study carried out in Matlab/Simulink for various power converter topologies. It impact the study of converter and proposed control strategy help to find the suitable converter for improving the power quality in compare with other power converter topology. The results of three simulations for back-to-back Voltage Source Converter (VSC) converter, multilevel Neutral Point Clamped (NPC) converter and matrix converter topology are presented and compared. Keywords - permanent Magnet Synchronous motor, Voltage Source Converter, Neutral Point Clamped, Power Quality.

I. INTRODUCTION

Due to increasing demand of power, renewable energy plays a vital role in energy production in a profitable way [1]. Thus renewable energy has an alternative way to meet the energy demand in clean with respect to power quality. In recent days , wind energy generation is increased and penetrated in to grid due to their reliability and security on power supply, as various renewable energy sources are combined together to meet the demand in peak periods [2,3]. The various power converter topologies are employed for integrating the generated power to the energy grid. By use of power converters in PMSG wind turbine allows the generator to operate under variable speed operation and enhanced power extraction. The proposed control schemes are employed to extract the maximum output at variable speed of wind turbine [4].

In this proposed work, various topologies of power converter are discussed along with the control strategy. This paper summarizes the comparison of various converter topologies. In compare with other generators, PMSG has more advantages and paid more attention due to simple structure, no need of gearbox and high efficiency. The PMSG equipped with converters for the purpose of ensuring constant frequency operation at variable speed by decoupling the speed from grid frequency. In this proposed work, the analyses of various converters are discussed and identify the suitable converter by comparing their performance employed for direct drive permanent magnet synchronous generator [5,6]. In section I describe the modeling of wind turbine. In section II deals with the various converter topologies are analyzed with their performance for selecting suitable converter for WECS

stand alone system. The Matlab simulation results for proposed converter topologies are discussed in section III and to determine the feasibility of the proposed system.

II. MODELLING OF PMSG

The performance of a PMSG can be discussed through mathematical model derived between the speed of the wind and the power extracted from turbine can be expressed as [6] follows:

2 30.5 ( ) (1)w w pP r V C

Where Pw-Power extraction through wind, ρ-density of the air, r-radius of blade, CP –tip speed ratio coefficient, β-pitch angle. The coefficient of the tip speed ratio can be calculated by mathematical expression as follows,

21

1

116( , )=0.5176 0.4 5 e +0.0068 (2)pC

The output power of the wind turbine is considered to be maximum for calculating the maximum power coefficient Cpmax as expressed as

3

2 1 max max

3 max

3

2max

P =0.5 r C

P = (3)

0.5 r C (4)

wt popt

wt opt i

opt popt

rw

k w

rk



Where Kopt is optimal tip speed ratio , ωt is angular speed. The maximum speed of the turbine is to be achieved by selecting MPPT Method.

In recent days, direct driven PMSG has been employed due to gearless operation and energy efficient operation and reliability of the generator. The model is limited to impact of wind fluctuations due to simulated application, so it is considered as single shaft mass model to derive the model for mathematical drive train.

The modeling of mechanical drive train are derived as

1

1= (T -T -T -T ) (5)

1= (T -T -T -T ) (6)

tt dt at as

gts dg ag g

g

d

dt J

d

dt J

The numerical approximations are involved to model the

generator by using the following park transformation in dq plane.

1= (U -P L i -R i ) (7)

1= (U -P (L i +Mi )-R i ) (8)

dd wg q q d d

d

qq wg d d f q q

q

di

dt L

di

dt L

The generated power Pg is given by:

=[u u u ] [ i i i ] (9) Tg d q f d q fP

To neglect the demagnetization in PMSG, a stator

current is considered to be null i.e. id=0. A. Wind Energy Conversion System

Wind Energy Conversion Systems are composed of direct drive PMSG and Power Electronics Converters. The Conversion of mechanical energy to electrical energy takes place in PMSG and the generated energy is to be matched with actual requirement of grid or battery with the help of converters. In compared with doubly fed Induction Generator, PMSG are completely decoupled from grid. PMSG does not require excitation current, gearless, low maintenance and efficiency [6,7]. The proposed system is full scale converter system shown in fig.1.

Figure 1.Full scale Power Electronics converter System.

III. TOPOLOGIES OF POWER ELECTRONICS CONVERTER

A. Back to Back Converter

The Generator Side Voltage Source Converter (VSC) is controlled by PWM Controller and the maximum torque is obtained by d-axis current held to zero. The maximum power point tracking is used to determining the optimum speed to extract the maximum rotor speed. The DC link voltage is controlled with the help of PI Controller. The grid

side VSC controls and maintains the line current to be sinusoidal. Fig.2 shows the topology of Back to Back converter used for PMSG-WECS.

The controlled converter reduces the input current harmonic losses. The Grid side converter (GSC) enables to control active and reactive power to the load / grid and improve the power quality performance by reducing harmonics distortion (THD). The Machine Side converter (MSC) enables to control the magnetization and maintain the constant rotor speed of the generator [8,9].



Figure 2. Conversion system based on Back to Back converter

For the switching function, the variable rk is used to

identify the state of the converter [10, 11], k is the leg of the

converter. The index i with 1,2,3k

identifies the leg of the converter. Two switching conditions are employed for the leg k.

1 2

1 2

1 S 1 and S 0 (10)

0 S 0 and S 1

1,2,3,4,5,6

k kk

k k

k

The leg K is expressed as

2

1

1 k (1,2,3,4,5,6) (11)ik

i

S

The DC Voltage is modeled by 3 6

1 1

1= (12)dc

k k k k

k k

dVr i r i

dt C

B. Matrix Converter

Matrix converter is converting variable AC to Constant AC for the purpose of connecting the wind energy conversion system (WECS) to the grid. The main advantages of the converter do not require any bulky storage or DC link. The other unique feature of matrix converter does not require any intermediate DC Conversion [12]. For avoiding commutation and harmonics generation on input side, the filter is placed [13]. The control of stator side power flow is achieved by regulating d-axis current, on other side q-axis regulating active power on stator side. Figure 3 shows the topology of matrix converter is used for PMSG-WECS.

Figure 3.Conversion System Based on Matrix Converter.

The output side function is derived by:

=V Cos( t)

=V Cos( t-2 /3) (13)

=V Cos( t-4 /3)

a m

b m

c m

V

V

V

The switching function is given by:

(t)=1(S is Closed), if it is 0(S is open) (14)ij ij ijS

Always only one switch connected to output phase:

S +S +S =1 (15)aj bj cj The voltage and current vectors are derived as follows



4

3

2

30 0 0

4 4

3 3

2 4

3 30 0 0

2V = =V e

3

2V = =V e (16)

3

2i = =I e

3

2I = =I e

3

j

j

j

j

j

i a b i i

j

A B

j j

i a b c i i

j j

A B C

V V e

V V e

i i e i e

i i e i e

(17)

The required duty cycles for selected switching

functions is given by

0 1

0 1

01

01

cos cos2 3 3d =(-1) q (18) cos3

cos cos2 3 3d =(-1) q (19) cos3

i

i

iS S

i

iS S

i

0 1

0 1

01

01

cos cos2 3 3d =(-1) q (20) cos3

cos cos2 3 3d =(-1) q (21) cos3

i

i

iS S

i

iS S

i

Whereas So is the output vector, Si is the input vector, α0 is the input displacement vector and β0 are the angles of input and output vectors.

C. Multilevel Converter

The multilevel converter is either operated as rectifier or

inverter mode. The various topologies are discussed in past few decades, for achieving high voltage by using low DC Voltage sources, the multilevel converter concepts are proposed. In particularly neutral point clamped (NPC) based multilevel converters are used to feed the wind generated power directly to the gird. The different topologies of multilevel converter are reported in literature survey (14,15). The weight, complexity, size is increased due to large number of capacitors and the performance and lifetime is increased (16). By using NPC converter, the ground leakage current is minimized by connecting the neutral point to midpoint of DC link. To improve the transient response and steady state in NPC converter, the various control schemes are proposed in [17, 18]. Fig.4 shows the topology of multilevel converter is used for PMSG-WECS.

Fig.4 Conversion system based on Multilevel Converter.

The condition for switching functions is given by

1 2 3 4

2 3 1 4

3 4 1 2

1, ( and S ) 1 and (S or S )=0

0 ( and S ) 1 and (S or S )=0 (21)

1 ( and S ) 1 and (S or S )=0

j j j j

j j j j

j j j j

S

S

S

The multilevel topology restriction for leg k is

1 2 2 3 3 4( . S )+( . S )+( . S ) (22)

1,2,3,4,5,6

k k k k k kS S S

wherek

The state equations is given by

3 6

1 1

1 4

3 6

2 2

1 4

1 = +

1 (23)

dck k k k

k k

k k k k

k k

dVi i

dt C

i iC

IV. SIMULATION RESULTS AND DISCUSSION

The proposed topology of back to back voltage sourec converter, matrix converter and multilevel neutral point clamped converter is implemented using Matlab/Simulink with PWM and PI control Strategy.



The back to back voltage source converter based PMSG wind energy conversion system is shown in figure 5.

Figure 6 illustrates the output load phase voltage, phase voltage of voltage source converter (VSC 2), and Modulation Index of VSC 2.

Figure 5: Proposed Back-to-Back VSC Converter based PMSG-WECS.

Figure 6: Output of Voltage Source converter2 and Load Voltage.

The matrix converter based PMSG wind energy conversion system is shown in figure.7. Figure 8 & 9 depicts the ouput phase voltage and line current of matrix

converter. Figure 10 depicts the phase voltage, line current, RMS voltage, RMS current of RL load.

Figure 7: Proposed Matrix Converter based PMSG-WECS



Figure 8: Output Voltage of Matrix Converter.

Figure 9: Output Current of Matrix Converter.

Figure 10: RMS & Output Load Voltage and Current.

The Neutral point Clamped Multilevel converter based PMSG wind energy conversion system is shown in figure.11.Figure 12 depicts the input phase voltage and

modulation index value of multilevel converter. Figure 13 shows the inverted output voltage from converter and phase voltage of RL load.



Figure 11: Proposed Neutral point clamped (NPC) multilevel Converter based PMSG-WECS.

Figure 12: Input voltage and modulation index of Multilevel NPC Converter.

Figure 13: Inverted Output voltage and Load Characteristics.

The Proposed Back to Back VSC topology is achieved the minimum THD compared with conventional converters and the percent THD values of RL loads are depicted in figure 18, 19 & 20. The Comparison table for

PWM and PI controlled based proposed converter topology is shown in Table 1.



Figure 18: THD Analysis of Back to Back Converter Figure 19: THD Analysis of Matrix Converter

Figure 20: THD Analysis of Multilevel NPC.

=== TABLE 1: COMPARISON OF RESULT Converter Topology PMSG Generator

Power (W) Maximum Wind

Power (W) Active power

P/Phase (Kw/Kvar) Reactive Power

Q/Phase (Kw/Kvar) THD

Analysis

Matrix Converter 4577 6255 3.29 -0.22 0.64%

NPC MLI Converter 4577 6255 2.99 -0.50 1.30%

Back-to-Back Converter

4577 6255 4.78 -0.69 0.52%

V. CONCLUSION

The wind energy conversion system are more popular in Renewable energy generation, the solution for integrating the WECS to grid has faced more problem and needs development. Power electronics converter plays an important role in integrating the wind generator and power grid. This Paper summarize the above discussion with various converter topology implemented for PMSG based Wind energy conversion System. The Study concluded that Back to Back voltage source converter topology is suitable for AC to AC power conversion. The Back to back converter topology having more advantages over other converter topology because it is suitable for standalone based PMSG. The THD analysis of back to back converter shows that the lower percent of 0.52% utilized for RL Load in compared with other converters. For handling high voltage and power ratings of wind

generator based grid connected systems, matrix and multilevel converters can be employed for PMSG Generator turbine. However, the tradeoffs between the converter price and configuration, it is hard to conclude the wind power converters for universal applications. From the study view point, PMSG-WECS based back to back converter operated in continuous mode and maintains the unity power factor and achieves low harmonic distortion.

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APPENDIX

Specification of PMSG Wind Generator Stator Phase Resistance Rs(ohm)

Armature Inductance (H) Flux linkage

Voltage Constant (V) Torque Constant (Nm)

0.425Ohm 0.835mH

0.433 392.6876

3.2475 Load Type=RL

Vrms Nominal Frequency

Active Power

400V 50Hz 5KW

FFT Analysis Parameter Setting Start time

No of Cycles Maximum frequency

Fundamental Frequency THD Computation

1 1

1000 50

Nyquist

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A Study on Wind Power Converter Topologies for Permanent ... · SUJITH M et al: A STUDY ON WIND POWER CONVERTER TOPOLOGIES FOR PERMANENT MAGNET … DOI 10.5013/IJSSST.a.17.32.36 36.2