CHAPTER 4 MODELING OF PERMANENT MAGNET SYNCHRONOUS GENERATOR BASED WIND ENERGY CONVERSION SYSTEM

Transcription

1 47 CHAPTER 4 MODELING OF PERMANENT MAGNET SYNCHRONOUS GENERATOR BASED WIND ENERGY CONVERSION SYSTEM 4.1 INTRODUCTION Wind energy has been the subject of much recent research and development. The only negative point that degrades the performance of Wind Energy Conversion System (WECS) in terms of maximum utilization of available power is high variation in wind-velocity (ranging from 3 m/s to15 m/s and above) ( Shilpa Mishra, 2014). Now a day s wind system operation is widely being worked out so as to extract maximum active power at all possible wind speeds with least detrimental effects on overall performance. In a fixed speed wind turbine system, the generator rotates at an almost constant speed for which it is designed regardless of variation in wind speed. As a result, the turbine will be the most efficient in extracting the maximum power from the wind for only one particular wind speed and wasted significant amount of energy. Also as turbine is forced to operate at constant speed, it is necessary for the turbine to be extremely robust to withstand a significant amount of mechanical stress due to the wind speed fluctuations. On the other hand, with variable speed wind turbine systems the rotor of the generator is allowed to rotate freely. Thus, it is possible to control the rotor speed by means of power electronics to maintain the optimum tip speed ratio at all times under varying wind conditions. Several different configurations are researched and

2 48 developed like fixed speed system with a Squirrel Cage Induction Generator (SCIG),variable speed system with Permanent Magnet Synchronous Generator (PMSG) and Doubly Fed Induction Generator (DFIG) to improve the efficiency While recent research has considered larger scale designs, the economics of large volumes of permanent magnet material has limited their practical application. But now a day s as cost of magnet has fallen down in global market significantly, PMSGWT has become most preferred system for wind generation. The primary advantage of Permanent Magnet Synchronous Generators (PMSG) is that they do not require any external excitation current. A major cost benefit in using the PMSG is the fact that a thyristor bridge rectifier may be used at the generator terminals since no external excitation current is needed. Further, the elimination of the gear box and brushes can increase the efficiency of wind turbine by 10%. Hence wind turbines generators based on Permanent Magnet Synchronous Generators (PMSG) without gear box is more useful over electrically excited machines. The Permanent Magnet Synchronous motor is a rotating electric machine where stator is a classic three-phase Induction Motor and rotor has permanent magnets. In this respect Permanent Magnet synchronous motor is similar to induction motor expects the rotor magnet field in case of PMSG is produced by permanent magnets. The use of permanent magnet to generate a substantial air gap magnetic flux makes it possible to design highly efficient PM motors. The permanent magnet motors are classified based on type of back EMF induced. Permanent magnet synchronous motor has sinusoidal back EMF and Brushless DC motors have trapezoidal back EMF. The features of PMSG motor are, 1. Medium construction complexity, multiple fields. High reliability (no brush wear), even at very high achievable speeds

3 49 2. High efficiency 3. Low EMI 4. Driven by multi-phase inverter controllers. Sensor less speed control possible 5. Appropriate for position control 4.2 MODELING AND DESIGN OF WIND TURBINE Wind turbine is applied to convert the wind Energy to mechanical torque (Prechanon, 2015). Equation (4.1) and Equation (4.3) is used to describe the power and torque characteristics of wind turbine. Here, A is the turbine rotor cover area in meter square (m 2 ), is the air density that is 1.225kg/m 3 at normal temperature, U w is the wind speed in meter per second (ms -1 ), C p is the power coefficient of the wind turbine and is the tip-speed ratio, m is the rotor angular speed in rads -1, R is turbine radius in meter (m), H is turbine height in meter (m), C p is power coefficient and ѳ is pitch angle. The maximum value of C p is 0.59 as per Betz law. For VAWT the pitch angle is nearly 0, therefore at null pitch angle the maximum power coefficient is The design of turbine in SIMULINK is given in

4 50 Figure 4.1. The Figure 4.2 shows the wind turbine parameters. The fan radius is 1.5 m the air density is Area of wind turbine is 5 m 2 and the pitch angle is 0. Figure 4.1 Modeling of wind turbine Figure 4.2 Wind turbine parameters

5 DESIGN AND MODELING OF CONVENTIOANL PMSG The computations associated with the PMSG modeling in abc reference frame are complicated and lengthy. Usually, the dq or Park transformation is applied in the PMSG modeling (Ye, 2013). The voltage equations of PMSG are shown in Equation (4.5) and (4.6). The electromagnetic torque equation is given by ( ( ) ) Where, L q = q axis inductance L d = d axis inductance R s = Resistance of the stator windings i q = q axis current i d = d axis current v q = q axis voltage v d = d axis voltage = Angular velocity of the rotor a = Amplitude of flux induced p =Number of pole pairs The dynamic equations are given by,

6 52 Where, J = Inertia of rotor; F = Friction of rotor; θ r = Rotor angle. Figure 4.3 Modeling of PMSG using mathematical equations for electromagnetic torque Figure 4.4 Modelling of PMSG using mathematical equations for rotor speed and rotor angle

7 53 Figure 4.3 shows the modelling of DRCRPMSG using mathematical equations for electromagnetic torque and Figure 4.4 shows the modeling of PMSG using mathematical equations for rotor speed and rotor angle Mathematical Inverse Park and Clark Transform A practical generator produces 3 phase AC power. For this reason, the inverse Park and Clarke transforms are introduced to implement the 3 phase AC output from the generator model. Figure 4.5 Inverse park transform Figure 4.5 shows the transform from the stator axis reference frame (α, β) to the rotating reference frame (d-q) is called the Park transform. The Clarke transform is the transformation of the 3-phase reference frame to the 2- phase orthogonal stator axis (αβ). Figure 4.5 illustrate, assumes the αβ frame has an angle θ field with the dq frame, the inverse Park transform (dq - αβ) which can be expressed as, [ ] [ ] [ ]

8 54 The mathematical inverse Clarke transform is, [ ] [ ] [ ] Figure 4.6 PMSG parameters Figure 4.6 shows the conventional PMSG parameters and Figure 4.7 shows the overall modeling of the conventional PMSG. In conventional type PMSG is connected with the 1.5KW load.

9 55 Figure 4.7 Conventional PMSG modelling Figure 4.8 shows the load current of the three phases and the Figure 4.9 shows the load voltage of three phases. Figure 4.10 shows the characteristics of conventional PMSG and the Power characteristics of conventional PMSG is as shown in the Figure Figure 4.8 I abc load currents

10 56 Figure 4.9 V abc load voltages Figure 4.10 Performance characteristics of PMSG

11 57 Figure 4.11 Power characteristics of PMSG 4.4 DESIGN AND MODELING OF PROPOSED DUAL ROTOR COUNTER ROTATION PMSG ` As the name implies, it has dual rotor operating at counter rotation and it has one stator winding. Equations (4.12) and (4.13) are voltage equation of three phase winding respectively where u sd, u sq, i sd, i sq, are the voltages and currents of stator winding respectively, R s is the stator resistance, L sd and L sq are the inductance components in the d- and q- axis, Ψ f is rotor flux produced by permanent magnet and r is the rotor electrical angular speed. The entire simulation was designed in terms of park transformation analysis. Park transform is a vector representation of AC circuit (three phase) models in a dq reference coordinates which is given Equation (4.15) and

12 58 Equation (4.16). [ ] [ ] [ ] ( ) ( ) [ ] ( ) ( ) [ ] Swings equation portrays the physical features of a turbine and explains how a common drive shaft of the turbine drives the generator rotor. It serves as the coupling element between the turbine and the generator. Here, J is the total moment of inertia of the rotor mass in kgm 2, T m is the mechanical torque supplied by the turbine in Nm, T e is the electrical torque output of the generator in Nm, e is the mechanical speed of the rotor, θ r is the angular position of the rotor in radian. Figure 4.12 shows the proposed Dual rotor counter rotation PMSG based wind energy conversion system. The motor parameters are as shown in the Figure 4.13 and the motor modeling is as shown in the Figure 4.14.

13 Figure 4.12 Modeling of proposed dual rotor PMSG 59

14 60 Figure 4.13 Proposed dual rotor PMSG parameters Figure 4.14 Modeling of PMSG

15 SIMULATION RESULTS ` Figure 4.15 show the load current of the three phases and the Figure 4.16 shows the load voltage of three phases. Figure 4.17 shows the characteristics of proposed dual rotor PMSG and the Power characteristics of proposed dual rotor PMSG is as shown in the Figure Figure 4.15 Load current I abc Figure 4.16 Load voltage V abc

16 62 Figure 4.17 Performance characteristics of dual rotor PMSG Figure 4.18 Power characteristics of dual rotor PMSG 4.6 SUMMARY The modeling of wind turbine, conventional PMSG and the proposed dual rotor counter rotation PMSG has been analyzed in this chapter. From the performance characteristics and power characteristics, confirmed the proposed dual rotor counter rotation PMSG rotated more speedily and produced more power in the output.