Using PWM Boost Rectifier for Harmonic Mitigation inWind Turbine Energy Conversion Systems

Abstract—Permanent magnet synchronous generators (PMSG) wind energy conversion system (WECS) using variable speed operation is being used more frequently in low power wind turbine application. Variable speed systems have several advantages over the traditional method of operating wind turbines, such as the reduction of mechanical stress and an increase in energy capture. To fully exploit the last mentioned advantage, many efforts have been made to develop maximum power point tracking (MPPT) control schemes for PMSG WECS. To allow the variable speed operation of the PMSG WECS a conventional three-phase bridge rectifier with a bulky capacitor associated with voltage source current controlled inverter (VS-CCI) is used. This simple scheme introduces a high intensity low frequency current harmonic content into the PMSG and consequently increases the total loses in it. Subsequently, decreases the power capability of the system. In this paper a simulation study using a three-phase boost type PWM rectifier applied to harmonic mitigation in these systems is presented.

I. INTRODUCTION

The amount of energy capture from a WECS depends not only on the wind at the site, but depends on the control strategy used for the WECS and also depends on the conversion efficiency. Permanent magnet synchronous generators (PMSG) wind energy conversion system (WECS) with variable speed operation is being used more frequently in low power wind turbine application. Variable speed systems have several advantages such as the reduction of mechanical stress and an increase in energy capture. In order to achieve optimum wind energy extraction at low power fixed pitch WECS, the wind turbine generator (WTG) is operating in variable-speed variable-frequency mode. The rotor speed is allowed to vary with the wind speed, by maintaining the tip speed ratio to the value that maximizes aerodynamic efficiency. The PMSG load line should be matched very closely to the maximum power line of the WTG. MPPT control is very important for the practical WECS systems to maintain efficient power generating conditions irrespective of the deviation in the wind speed conditions. To achieve optimal power output, a sensor-less scheme including a wind turbine model was developed by Tan et al in [1]. The developed wind turbine model will be used in this work in order to evaluate the different harmonic mitigation approaches. In spite of, all this complex control theory to get MPPT on PMSG WECS the standard way to implement a grid connected PMSG WECS at variable speed is using two conversion stages: the first one an AC-DC stage and the second one a DC-AC stage. To realize the first one a classical three phase bridge rectifier (BR) associated to a bulky capacitor is used and the second stage could be

implemented by two types of converters schemes Voltage source current controlled inverter (VS-CCI) and Line commutated inverter (LCI) as shown in Fig. 1. This paper has the main focus in the first energy conversion stage the AC-DC converter, which is responsible by an injection of a high harmonic current content into the PMSG. The circulation of these currents into the machine will generate losses. This work applies a well-known approach to harmonic mitigation in three-phase AC-DC converters to WECS [2, 3, 4]: The three-phase boost type PWM rectifier. Using this converter is possible to minimize the current harmonic content.

Fig. 1. Wind Energy Conversion System.

A software simulation model developed in [1] using PSIM software, which allows easy performance evaluations is used to estimate the behaviour of these two different schemes associated with the PMSG WECS. Simulation results showed the possibility of achieving maximum power tracking, output voltage regulation and harmonic mitigation simultaneously.

II. WECS MODEL

The WECS considered in this work consists of a PMSG driven by a fixed pitch wind turbine; an AC-DC energy conversion stage implemented using two different approaches and a VS-CCI. The entire system is shown in Fig. 1. A brief description of each element of the system is given below.

A. Power from wind turbine

The output mechanical power of the wind turbine is given by the usual cube law equation (1). Where Cp is the power coefficient, which in turn is a function of tip speed ratio and blade angle . This relationship is usually provided by the turbine manufacturer in the form of a set of non-dimensional curves, the Cp curve for the wind turbine used in this study is shown in Fig. 2. The tip speed ratio is given by equation (2). A= wind turbine rotor swept area [m2], Uw= wind speed [m/s], = air density [kg/m3], r= radius of the rotor [m], m= mechanical angular velocity of the generator [rad/s].

(1)

(2)

Figure 2. Power coefficient vs. Tip seed ratio with =0.

It can be seen that if the rotor speed is kept constant, then any change in wind speed will change the tip-speed ratio (), leading to change of Cp as well as the generated power out of the wind turbine. If the rotor speed is adjusted according to the wind speed variation, then the tip-speed can be maintained at the optimum points, which yield maximum power output from the system. Cpmax is the maximum torque coefficient developed by the wind turbine at the optimum tip-speed ratio max. The rate of the rotor speed is proportional to the inverse of the inertia and difference between mechanical torque (Tm) produced by the wind turbine and the electrical torque (Te) load from the generator (3).

(3)

The wind turbine output mechanical torque is affected by the Cp. In order to maximize the aerodynamic efficiency, the Te of the PMSG is controlled to match with the wind turbine Tm to have maximum possible Cpmax. With a power converter, adjusting the electrical power from the PMSG controls the Te; therefore, the rotor speed can be controlled. For the system to operate at maximum power at all wind speeds, the electrical output power from the power converter controller must be continuously changed so that under varying winds speed condition the system is matched always on the maximum power locus. From the power curve of the wind turbine, it is possible to operate the wind turbine at two speeds for the same power output. In practice, the operating range at region 1 is unstable as the rotor speed of the WTG belongs to the stall region. Any decrease in the tip speed region will cause a further decrease until the turbine stops.

B. PMSG model

Theoretical models for generator producing power from a wind turbine have been previously developed. The outer rotor 20kW CRESTA PMSG is used in this WECS mathematical model. The model of electrical dynamics in terms of voltage and current can be given as (4) and (5) [1]:

(4)

(5)

Where, R and L are the machine resistance and inductance per phase. vd and vq are the 2-axis machine voltages. id and iq are the 2-axis machine currents. m is the amplitude of the flux linkages established by the permanent magnet and r is the angular frequency of the stator voltage. The expression for the electromagnetic torque in the rotor is written as:

(6)

The relationship between r and m may be expressed as:

(7)

C. Input Bridge Rectifier (AC-DC converter)

The complete grid connected sensor-less PMSG WECS scheme using a well-known three-phase six-pulse bridge rectifier and two bulky capacitors are shown in Fig. 3.

Figure 3. Implemented sensor-less VS-CCI WECS.

D. Power Variation of the PMSG Wind Turbine

The loading characteristic of the PMSG WECS can be easily simulated by connecting an adjustable load resistor to the PMSG and rectifier terminal. Fig. 4 shows the calculated corresponding output power of the PMSG for wind speeds ranging from 4 to 12 [m/s], where the generator maximum power curves show the different operating DC voltages and currents over a range of wind speeds. In order to extract the peak power from the WTG at a given wind speed, the WECS has to match closely to the maximum power curve.

Figure 4. Predicted DC power characteristics the WECS.

III. HARMONIC ANALYSIS

First of all it is necessary to understand why this study is important. Therefore, a briefly remark of the problem is

presented. To do this job a study case is presented showing the PMSG output currents at full load condition (20 kW resistive load) using a conventional BR shown in Fig. 3, which is normally employed in PMSG WECS. The wind speed in this case is 12 m/s. Harmonic characterization of these abnormal currents is obtained and the results are presented in the following section. A complete harmonic analysis of the two three-phase harmonic mitigation approaches mentioned previously will be presented in the following sections.

IV. THREE-PHASE FULL BRIDGE RECTIFIER

A detail of the PMSG WECS output current and line-to-line voltage (divided by 4), for the rated power deliver situation at 12 m/sec wind speed, is shown in Fig. 5.

Figure 5. PMSG output currents and line to line voltage div. by 4.

In order to evaluate the quality of current and voltage an objective study was made using the Fourier analysis, the harmonic content and the total harmonic distortion (THD) of the output PMSG current and voltage were obtained, the results are summarized in Fig. 6 and Fig. 7.

Figure 6. Harmonic content of the PMSG output current.

The fundamental components were omitted in these figures, in order to, remark the harmonic content. From these figures it is possible to observe that the 5 th, 7th, 11th, 13th, 17th and 19th harmonics are significant. The obtained total harmonic distortion was THD = 10.68 % and 29.15 % for current and voltage respectively, which are quite high. At full load, the harmonic content of the output current is minimized by the influence of the machine stator equivalent inductance and resistance which are L_F1 = 3 mH and R_S1 = 0.432 respectively. Unfortunately this effect is not so noticeable when the available wind decreases and therefore, the maximal output power decreases and the THD increases. The amplitude of the 5 th

current harmonic is 9.2% of the fundamental, which is

greater than the 4% allowed by IEEE 519 standard. Of course, the IEEE 519 standard it is not applicable to this situation but it is a guideline.

Figure 7. Harmonic content of the PMSG output voltage.

V. HARMONIC MITIGATION

The classical passive trap filters are always associated with the idea of harmonic mitigation, but they are not a good solution for this application once the frequency of the generator changes with the wind. In this context an active solutions like the three-phase boost type PWM rectifier will play an import role.

A. Three-Phase Boost type PWM Rectifier (AC-DC converter)

As mentioned before the behavior of the Three-Phase Boost type PWM Rectifier associated to PMSG WECS is studied in this work focused in losses reduction on PMSG by harmonic mitigation. The three-phase boost type PWM rectifier has the capability of to work as an ideal PFC, with very low THD at any operation point. Hence, it is investigated in this work. The converter topology has six two-quadrant controlled switches each one implemented by an IGBT associated to a diode. The inductors and capacitor filter the high-frequency switching harmonics, and have little influence on the low frequency AC components of the waveforms. The switches of each phase are controlled to obtain input resistor emulation. To obtain undistorted line current waveforms, the DC output voltage V must be greater than or equal to the maximal peak line-to-line AC PMSG output voltage. The three-phase boost type PWM rectifier shown in Fig. 8 has several attributes the AC input currents are non-pulsating because the converter works in continuous conduction mode (CCM), and hence very little additional input EMI filtering is required. The converter is capable of bidirectional power flow. This converter also has some disadvantage like: requirement for six active devices, since the rectifier has a boost characteristic working in CCM it requires a complex control scheme using either a multiplying controller scheme employing average current control, or with some other approach like estimation [3, 4].

Fig. 8. Three-phase boost type PWM rectifier.

The three-phase bridge rectifier input currents are shown in figure 9 from this figure is easy to observe the CCM and the very good results obtained.

Figure 9. Three-phase bridge rectifier input currents.

The PMSG output current harmonic content is shown in figure 10.

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

THD = 0.06 %

Harmonic amplitude in percent of the fundamental component

Figure 10. Harmonic content of the PMSG output current using Three-Phase Boost Rectifier.

VI. PMSG POWER LOSSES CALCULATIONS

Basically the total power losses generated into the machine can be divided into two big groups: a) copper losses and b) core losses. The copper power losses are produced in the stator winding as function of RMS current in it according to equation (8), where Ia_I is the RMS value of the i th

harmonic component of the current Ia and Ra is stator equivalent resistance. [9]. However, operating at higher current level resulted in temperature rise. The change of Ra

due to temperature rise was not included in the calculations. The high frequency flux changing generated by the harmonics causes hysteresis and eddy current power losses in the core the equation (9) represents these losses [9, 10].

(8)

(9)

In order to evaluate the influence of the different harmonic mitigation approaches in the PMSG power losses a normalized study is presented in the final paper.

VII. CONCLUSION

In this paper a well-known harmonic mitigation solution was successfully applied to the PMSG WECS AC to DC conversion system. The Three-phase bridge rectifier has presented encouraged results, such as: low current THD, simple power topology and control circuit and can work in all wind conditions. Which allow expecting an increasing in the PMSG lifetime without reduction of the power capability. The main drawbacks of this topology are: a) the control complexity and b) the converter cost. With the actual technology these problems could be minimized using propers switches like IGBT and DSP processors. The losses study has demonstrated that the PMSG losses decrease when the Three-phase bridge rectifier is used. The PMSG efficiency () increases about 2% and the system efficiency increases about 1%, losses study will be presented in the final paper.

VIII. REFERENCES

[1] K. Tan and S. Islam, "Optimum Control Strategies in Energy Conversion of PMSG Wind Turbine System Without Mechanical Sensors", IEEE Transactions on Energy Conversion, Vol. 19, No. 2, June 2004, pp. 392-400.

[2] Phipps, J.K.;"A transfer function approach to harmonic filter design", Industry Applications Magazine, IEEE , Volume: 3 , Issue: 2 , March-April 1997, pp.:68 – 82.

[3] A. R. Prasad, P. D. Ziogas, and S. Manias, “An active power factor correction technique for three-phase diode rectifiers,” in PESC’89 Rec., pp. 58–65.

[4] Robert W. Erickson, “Some Topologies of High Quality Rectifiers” Keynote paper, First International Conference on Energy, Power, and Motion Control, May 5-6, 1997, Tel Aviv, Israel, pp. 1-6.

[5] Kaboli, Sh.; Zolghadri, M.R.; Homaifar, A., “Effects of sampling time on the performance of direct torque controlled induction motor drive”, IEEE International Symposium on Industrial Electronics, 2003. ISIE '03, Volume: 2 , June 9-11, 2003, pp.:1049 – 1052

[6] Yao Tze Tat, "Analysis of Losses in a 20kW Permanent Magnet Wind Energy Conversion System", in the Department of Electrical and Computer Engineering. Western Australia: Curtin University of Technology, October 2003. pp. 101.