HARMONIC MITIGATION ON WIND TURBINE ENERGY CONVERSION SYSTEMS USING PASSIVE TRAP FILTERS

Fernando Soares dos Reis*, Member, IEEE, Kelvin Tan Student Member, IEEE, and Syed Islam, Senior Member, IEEE

Department of Electrical and Computer EngineeringCurtin University of Technology

GPO Box U1987, Perth, Western Australia 6845AUSTRALIA

e-mail: s.islam@ece.curtin.edu.au

* FENG - Department of Electrical EngineeringPontifical Catholic University of Rio Grande do Sul

90619-900, Av Ipiranga, 6681, Porto Alegre, RSBRAZIL

e-mail: f.dosreis@ieee.org

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. A feasibility study to make use of harmonic trap filters applied to minimize this problem is presented in this paper. Simulation results will be presented and discussed.

Index Terms — Harmonic mitigation; harmonic trap filters; permanent magnet synchronous generator; wind energy converters system; variable speed operation.

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 converters 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 developed by Tan et al in [1] will be used in this work for extracting desired output power from the WTG over a wide range of wind speeds. 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 full bridge rectifier 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.

Figure 1. Wind Energy Conversion System

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 the well-known Harmonic Trap Filters approach to harmonic mitigation in three-phase AC-DC energy conversion systems. Using this approach is possible to minimize the PMSG output current harmonic content. A software simulation model developed in [1] using PSIM software, which allows easy performance evaluations is used to estimate the behaviour of this simple scheme. Simulation results showed the possibility of achieving maximum power tracking, output voltage regulation and harmonic mitigation simultaneously for a specific operation point.

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 power factor rectifiers; and a VS-CCI. The entire system is shown in Fig. 1. A brief description of each element of the system is given below.

HARMONIC MITIGATION ON WIND TURBINE ENERGY CONVERSION SYSTEMS USING PASSIVE TRAP FILTERS

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 . 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 [r/sec].

(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)

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. Therefore, the controller has to be designed to keep the operating point inside the desired region. For a variable speed wind turbine with pitch control, optimum power can easily be obtained using appropriate control. However, for small machines that use a fixed pitch, this mechanism is not possible. The current paper looks at fixed pitch machines. The use of pitch machines control may, however, interfere with the control system modeled in this paper.

B. 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.

III.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 12m/sec, 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.

IV. HARMONIC ANALISYS

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 using a

HARMONIC MITIGATION ON WIND TURBINE ENERGY CONVERSION SYSTEMS USING PASSIVE TRAP FILTERS

conventional six-pulse rectifier shown in Fig. 3 which, is normally employed in PMSG WECS is presented, 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.

Figure 4. Predicted DC power characteristics the WECS.

A. 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 andline 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 5th 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

Looking for this classical power electronics problem a first design could be to use passive HTF as shown in Fig. 8 in fact a reduction of the 5th and 7th harmonic could be enough to turn the THD to acceptable levels. If the main idea is to found a compromise solution to track maximum power point at the best wind condition, the performance of the trap filters must be investigated. A design of two trap

HARMONIC MITIGATION ON WIND TURBINE ENERGY CONVERSION SYSTEMS USING PASSIVE TRAP FILTERS

filters was made to minimise the 5th and 7th harmonic at nominal power. The designed components are C5 = 135F, C7 = 69F and L5 = L7 = L_F1 = 3mH. As the focus of this work is in the field of the losses minimization is important to remark that there is no ideal components.

Figure 8. Passive harmonic trap filters.

Hence, the resistance series equivalent (RSE) of each filter component must be considered. The RSE of each branch were estimated equal to the armature resistance (RSE = 0.432 ) once the branch inductances are equal to the armature inductance. The simulated results are shown in figures 9 to 12. A remarkable improvement in the current and voltages waveforms was obtained using this simple solution. Figure 9 shows the PMSG output currents and the line to line output voltage (divided by 5), which are almost sinusoidal. A current THD less than 2.5% was obtained as shown in Fig. 10. However, this solution implies in bulky components and is matched only for the maximum power condition, which occurs at 12 m/s wind speed. In figure 11 the PMSG output line to line voltage div. by 5 is shown in the frequency domain.

Figure 9. Three-phase PMSG output currents and output line to line voltage div. by 5 using 5th and 7th harmonic trap filters.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1 2 3 4 5 6 7 8 9 10 1112 1314 1516 1718 1920 2122 2324 25

THD = 2.26 %

Harmonic amplitude in percent of the fundamental component

Figure 10. Harmonic content of the PMSG output current using 5th and 7th harmonic trap filters.

From these figures is possible to observe an improvement in the PMSG output line-to-line voltage waveform in relationship with the first case without any filter shown in figure 5. The voltage THD was drastically reduced from 29.15% to 9.20%. On the other hand its RMS value has increased from 293 V to 343 V respectively. This occurs because the chosen comparison parameter, which was: the constant output power (20 kW). The resistive load is adjusted in order to keep the output power constant. Once the harmonic core losses are related with the harmonic content of the voltage [5, 6] this characteristic can not be neglected. In order to evaluate the system losses is necessary to know the RMS value of the currents in PMSG output ia, HTF currents ia5th and ia7th and the BR diodes current i_Ret_a. Figure 12 shows the time domain evolution of these currents. The high current values will generate important losses in the system. On the other hand the copper losses inside the PMSG will be slightly reduced since the HTF has change the WECS operation point increasing the line-to-line voltage and decreasing the line current. Unfortunately in the real WECS, the wind speed is constantly varying and hence the PMSG produces variable-voltage and variable-frequency output. Therefore, an infinity number of the trap filters would be necessary. To solve this problem an active solution is needed, there are basically two actives approaches to solve this power electronics problem the first solution is to use the factor correctors (PFC) and the second possible solution is to use active power filters. The study of the active power filters applied to PMSG WECS is out of the scope of this paper. However, is under study at the moment.

HARMONIC MITIGATION ON WIND TURBINE ENERGY CONVERSION SYSTEMS USING PASSIVE TRAP FILTERS

Figure 11. Harmonic content of the PMSG output line-to-line voltage using 5th and 7th harmonic trap filters.

Figure 12. PMSG output, bridge rectifier and 5th and 7th harmonic trap filters current at 20 kW.

Unfortunately in the real WECS, the wind speed is constantly varying and hence the PMSG produces variable-voltage and variable-frequency output. Therefore, ideally, an infinity number of the trap filters would be necessary to keep the THD low at any wind speed. To solve this problem an active solution is needed, there are basically two actives approaches to solve this power electronics problem the first solution have different names as: factor correctors (PFC), power factor rectifiers (PFR), power factors preregulators (PFP), PWM rectifiers or resistor emulators the second possible solution is to use active power filters. The study of these approaches applied to PMSG WECS is out of the scope of this paper. However, is under study at the moment.

VI. 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 (PCU) 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 ith harmonic component of the current Ia and Ra is stator equivalent resistance. [5, 6]. However, operating at higher

current level resulted in temperature rise. The change of R a

due to temperature rise was not included in the calculations. The high frequency flux changing generated by the harmonics causes hysteresis (Ph) and eddy current power losses (Pe) in the core the equation (9) represents these losses [6].

(8)

(9)The influence of the voltage harmonic content in the magnetic losses can be evaluated using equations (10) and (11) proposed by Kaboli et al in [5].

(10)

(11)

Where: ke and kh are constants, Bmax is peak flux density, f is the rated frequency, weight represents the core and copper weight, Ph1, Pe1 and V1 are the hysteresis and eddy current power losses and the PMSG line to line output voltage respectively at the nominal condition resistive load without harmonics, i is the harmonic order and Vi are the amplitude of the harmonic components of the PMSG line to line output voltage. In order to evaluate the influence of the different harmonic mitigation approaches in the PMSG power losses. The PMSG power losses as well as the system losses were obtained and the results are summarized in tables 1 and 2 respectively.

Table 1. PMSG losses (W) and efficiency (%).

Table 2. WECS losses (W) and efficiency (%).

When the simple BR scheme is used the total losses in the PMSG increases about 30% in relationship to the rated condition using resistive loads directly connected to the machine. This significant extra power loss will imply in premature aging and consequently reduction of the PMSG lifetime. Using the HTF there is a reduction in the extra losses, but these losses remain high around 20% and the

HARMONIC MITIGATION ON WIND TURBINE ENERGY CONVERSION SYSTEMS USING PASSIVE TRAP FILTERS

conversion efficiency presents the worst results. An amazing result is obtained using the PFC once the losses on the PMSG are reduced about 10% in relationship to the rated condition. This improvement will reduce the internal PMSG temperature and therefore will increase its lifetime. Another positive point comes from table 2 in it is shown that the system efficiency () remains practically the some using BR or PFC schemes.

VII CONCLUSION

This digest presents a study on harmonic mitigation in three phase rectifiers using passive trap filters. Preliminary simulation results of a prototype variable speed PMSG WECS were presented. A possible harmonic mitigation solution was discussed. It was observed that using harmonic trap filters is possible to maximize the PMSG WECS efficiency by mitigation of the harmonics losses. Notwithstanding, this solution is completely effective just at one specific operation point since the wind is constantly changing as well the frequency; it is possible to choose a compromise design point.

In this paper two well-known harmonic mitigation solutions were applied to PMSG WECS AC to DC conversion. They were the HTF and the PFC. Harmonic trap filters are easily implemented by passive components but they are normally implemented with bulk components. Notwithstanding the HTF had presented the best THD results they are not the best solution once they are a matched solution for a specific operation point (wind speed and output power). The losses study also has demonstrated that the PMSG efficiency () remains practically constant and system is the lowest when the HTF are used. For these reasons, it is not a recommended way out to obtain harmonic mitigation on PMSG WECS. On the other hand, the single-switch three-phase boost rectifier has presented encouraged results. Such as: low current and voltage THD, simple power topology and control circuit, can work in all wind conditions and presents a real reduction of the PMSG total losses. Which allow expecting an increasing in the PMSG lifetime without reduction of the power capability. The main drawbacks of this topology are a) the conduction losses in the BR diodes and switch Q1 (Fig. 13 and 14) since the high RMS current value caused by the DCM operation and b) the high output voltage 1 kV. Both problems could be minimized using proper diodes and switch like IGBT. With the actual technology these problems could be easily solved.

ACKNOWLEDGMENTS

The authors wish to thank Pontifical Catholic University of Rio Grande do Sul, Curtin University of Technology and The Brazilian National Council for Scientific and Technological Development (CNPq) for supporting the

research work. Dr. Fernando Soares dos Reis have been supported by a scholarship from CNPq.

VIII. REFERENCE

[1] K. Tan, S. Islam, and H. Tumbelaka, "Performance comparison of a current controlled and line commutated inverter in maximum energy conversion," presented at International Power Engineering Conference IPEC'2003, Singapore, 2003.

[2] B. S. Borowy and Z. M. Salameh, "Dynamic response of a stand-alone wind energy conversion system with battery energy storage to a wind gust," IEEE Transactions on Energy Conversion, Vol. 12, No. 1, March 1997, pp. 73-78.

[3] J. Y. Chen and C. V. Nayar, "A low speed, high torque, direct coupled permanent magnet generator for wind turbine application" presented at Proceedings of Solar'97 - Australian and New Zealand Solar Energy Society, 1997, pp. 1-6.

[4] L. J. Borle, M. S. Dymond, and C. V. Nayar, "Development and testing of a 20kW grid interactive photovoltaic power conditioning system in western Australia," IEEE Transactions on Industry Applications, Vol. 33, No. 2, March/April 1997, pp. 502-508.

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