HARMONIC MITIGATION IN WIND TURBINE ENERGY CONVERSION SYSTEMS
Fernando Soares dos Reis, Member, IEEE, Kelvin Tan Student Member, IEEE, and Syed Islam, Senior Member, IEEE
*School of Electrical and Computer Engineering
Curtin University of Technology
GPO Box U1987, Perth, Western Australia 6845
Australia
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. This paper presents a comparative simulation study between three different approaches applied to harmonic mitigation on PMSG WECS. The studied techniques are: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase boost type PWM rectifier;
Keywords:
Harmonic mitigation; PFC; PWM rectifier; permanent magnet synchronous generator; wind energy converters system; variable speed operation.
Corresponding author:
Fernando Soares dos Reis
School of Electrical and Computer Engineering
Curtin University of Technology
GPO Box U1987, Perth, Western Australia 6845 - Australia
E-mail:
HARMONIC MITIGATION IN WIND 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. This paper presents a comparative simulation study between three different approaches applied to harmonic mitigation on PMSG WECS. The studied techniques are: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase boost type PWM rectifier;
Keywords
Harmonic mitigation; PFC; PWM rectifier; permanent magnet synchronous generator; wind energy converters system; variable speed operation.
1.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 three well-known approaches to harmonic mitigation in three-phase AC-DC energy conversion systems [2, 3]: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase boost type PWM rectifier;
Using these approaches is possible to minimize or to eliminate the 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 these three different schemes associated with the PMSG WECS. Simulation results showed the possibility of achieving maximumpowertracking, output voltage regulation and harmonic mitigation simultaneously.
2.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.
2.1.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 modelled in this paper.
2.2.PMSG model
Theoretical models for generator producing power from a wind turbine have been previously developed [6, 7]. The outer rotor 20kW CRESTA PMSG described in [8] 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) [7]:
(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 ρ = d/dt. r=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)
2.3.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
3.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.
Figure 4. Predicted DC power characteristics the WECS.
4.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 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. A complete harmonic analysis of the two three-phase PFC presented previously will be presented in the final paper.
4.1.Three-Phase Full Bridge Rectifier
A detail of the PMSG WECS output currents, for the maximum power deliver situation at 12 m/sec wind speed, is shown in Fig. 5. In order to evaluate the quality of these currents an objective study was made using the Fourier analysis, the harmonic content and the total harmonic distortion (THD) of the output PMSG current were obtained, the results are summarized in Fig. 6 and Fig. 7. From these figures it is possible to observe that the 5th, 7th, 11th, 13th, 17th and 19th harmonics are significant. The obtained total harmonic distortion was THD = 13.47 % which is quite high.
Figure 5. Three phase PMSG output current at 20 kW.
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.
Figure 6. Harmonic content of the PMSG output current.
In Fig. 7 a detail of the harmonic content amplitude of the PMSG output current in percent of the fundamental is shown. The fundamental component was omitted in this figure, in order to, remark the harmonic content.
Figure 7. Harmonic content of the PMSG output current.
The amplitude of the 5th harmonic is 12.4% of the fundamental, which is greater than the 10% allowed by IEEE 519 standard. Of course, the IEEE 519 standard it is not applicable to this situation but it is a guideline.
5.0.HARMONIC MITIGATION
The classical passive trap filters are always associated to the idea of harmonic mitigation. Also the PFC schemes will need high frequency filters to mitigate the high frequency current ripple generated by them selves. A high frequency filter design criterion will also be presented.
5.1.Passive Harmonic Trap Filters
Looking for this classical power electronics problem the first solution is to use passive harmonic trap filters 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 at the best wind condition the trap filters could be used.
Figure 8. Passive harmonic trap filters.
A design of two trap filters was made to minimise the 5th and 7th harmonic the results are shown in Figs. 9 and 10.
Figure 9. Three-phase PMSG output current at 20 kW using 5th and 7th harmonic trap filters.
A remarkable improvement in the current wave form was obtained using this simple solution. A THD less than 3% was obtained as shown in Fig. 10. The fundamental component was omitted in this figure, in order to, remark the low harmonic content. Note that this solution implies in bulky components and is matched only for the maximum power condition, which occurs at 12 m/s wind speed.
Figure 10. Harmonic content of the PMSG output current using 5th and 7th harmonic trap filters.
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 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 the active power filters applied to PMSG WECS is out of the scope of this paper. However, is under study at the moment.
5.2.Single-Switch Three-Phase Boost Rectifier (AC-DC converter)
One of the selected power factor corrector (PFC) approaches, suitable to implement the input rectifier, the black block shown in Fig. 3, was the three-phase DCM boost rectifier proposed by Prasad and Ziogas [2]. Because this converter presents many advantages such as: a very low THD in the input current, a simple structure using just one controlled device and could be controlled using a simple control strategy easily implemented with general-purpose PWM integrated circuits (IC) controllers. The transistor can operate at constant switching frequency. The control of the DC output power is easily made by duty cycle control. Only a single active device such as a MOSFET or IGBT is needed. The main disadvantages of this implementation are the increasing of the power losses in the devices in comparison with the conventional three-phase AC-DC converter and the need of an additional input filter to remove the high-frequency harmonic components of the input currents [2, 4]. Therefore, the implementation of the input rectifier using a single-switch three-phase boost rectifier resulting in harmonic mitigation on the PMSG output currents. The complete schematic diagram of the three-phase DCM boost rectifier is shown in Fig. 11.
Figure 11. Single-switch three-phase boost rectifier.
Inductors L1, L2, and L3 have the some small value, such that they operate in the discontinuous conduction mode in conjunction with diodes D1 – D6. At the end of the transistor Q1conduction subinterval, the inductor currents reach peak values, which are also proportional to the applied three-phase line-to-neutral voltages. When transistor Q1 turns off, then diode D7becomes forward-biased and the inductors release their stored energies to the DC output. Since the peak input currents are proportional to the applied input line-to-neutral voltages, then the average values of the input currents are also approximately proportional to the input line-to-neutral voltages. Approximate three-phase input resistor emulation is obtained. The three-phase DCM boost rectifier does generate a modest amount of low frequency input current harmonics. But, increasing the DC output voltage is possible to reduce the THD [4].