A unified power electronic controller for wind driven grid connected wound rotor induction generator using line commutated inverter

Abstract

The implementation of a simple power converter for a wound rotor induction generator employing a three phase diode bridge rectifier and a line commutated inverter in the rotor circuit for super synchronous speeds has been proposed. The detailed working of the system in power smoothing mode and maximum power point tracking mode is presented. The current flow in the rotor circuit is controlled (by controlling the firing angle of the line commutated inverter) for controlling the stator power in both the modes. An 8 bit PIC microcontroller has been programmed to vary the firing angle of the line commutated inverter. Experiments have been carried out on a 3-phase, 3.73 kW, 400 V, 50 Hz, 4-pole, 1500 r/min wound rotor induction generator and the results obtained with the generator supplying power in both the modes are furnished. The complete scheme has been modeled using MATLAB/SIMULINK blocks and a simulation study has been conducted. The experimental waveforms are compared with the simulation results and a very close agreement between them is observed.

Keywords

line commutated inverter / MPPT / power smoothing / wound rotor induction generator

Introduction

Due to the fast depletion of fossil fuels, increased emphasis is being given to alternate energy sources such as solar and wind power. The cost of installation of wind energy conversion systems (WECS) is much less compared to that of solar energy systems. Squirrel cage induction generators (SCIGs) have been largely used for both stand alone and grid connected WECS. There is an increasing trend towards using permanent magnet synchronous generators too for WECS. However, these systems require a power electronic converter connected between the stator terminals of the generator and the utility grid. In addition, the rating of the converter is as high as that of the generator. In view of this drawback, wound rotor induction machines (WRIM) are recommended for grid connected wind applications. In these generators, the power electronic converters are shifted to the rotor circuit, thus allowing less rating for the converters. Depending on wind speed, a wound rotor induction generator (WRIG) based variable speed wind turbine is capable of operating in super-synchronous or sub-synchronous mode to supply power to the grid via both stator and rotor circuits. A simple microprocessor based controller for double output induction generator (DOIG) has been developed [1,2] for super-synchronous operation with no much detail of maximum power point tracking (MPPT). It is quite advantageous to operate the wind generator system in both sub-synchronous and super-synchronous regions to enhance the operating speed range. To operate the system in both modes, back-to-back line (or naturally) commutated converters were preferred in the early days. The overriding problem while operating at near synchronous speed with such a scheme has been commutation failure [3-5]. In view of this limitation, a forced commutated rotor side converter is more suitable for a DOIG driven by a wind turbine. But the commutation equipment of such a converter configuration is more costly and complex [6]. A super-synchronous DOIG based wind power conversion system with the advantages of back-to-back converters [7,8] and a method of tracking peak power points in both sub-synchronous and super-synchronous speeds [9] have already been proposed. A generalized modeling of machines and a meagre idea of MPPT is presented in Ref. [7]. But a vast idea of MPPT and pitch control of the wind turbine above rated wind velocity have been proposed [8,9] to maintain the generator power limit. No idea of power smoothing from cut-in-velocity to cut-out-velocity as required by the grid is presented in the above references. A different configuration with rotor connected to battery bank through converters is presented [10]. This scheme is found to be attractive, but the batteries used for frequent charging and discharging are expensive with the increase in capacity of the wind generator. Besides, the complexity involved in the design of the controller is high.

With variation in wind speed, the generators can be made to work either in MPPT mode [9] or power smoothing mode [10-14] or both. The power smoothing mode in which a required amount of constant power is fed to the grid assumes importance whenever the grid faces evacuation problem, i.e., at times of overloading on the transmission lines, the grid can receive only a specified amount of power. At present in India there is approximately 30% power shortage and the installed wind power is not fully tapped due to evacuation problem. The design and implementation of power smoothing controllers will alleviate such power shortage problem to a great extent.

The proposed work aims at MPPT and power smoothing operations using a very simple control technique by employing an uncontrolled diode bridge and a line commutated inverter (LCI) of 3-phase SCR bridge in the rotor circuit of the WRIG. By means of fixed gear ratio between the wind turbine and the generator, the generator is made to run at speeds more than its synchronous speed from the cut-in wind velocity to the rated wind velocity. This shifts the operating region of the wind generator in the super synchronous mode. A suitable torque control operates the WRIG in the MPPT mode or the power smoothing mode for a wide speed variation. This is achieved by automatically controlling the firing angle of LCI using a PIC microcontroller. Further, a comprehensive steady state analysis with extensive experimental validation is presented. The enhancement of power factor for the proposed scheme is also demonstrated. The controller used is simple and robust, the switching devices being SCRs.

To incorporate the proposed control strategy, it is essential to know the model of the wind turbine. Variable speed wind turbines can extract more power and operation at the MPP is more viable than fixed speed wind turbines [15]. Generally, a minimum of three control parameters, i.e., system yaw, pitch angle and generator torque, must be considered for developing a control strategy. Yaw control is necessary in any turbine to change the direction in accordance with the wind. Pitch controller is a slow-acting device [16] and hence it can never respond fast to turbulent wind velocities. In variable speed wind turbines, pitch control is normally employed above rated velocity. As wind velocities are not constant, pitch control introduces hard control non-linearities. In another word, it is possible to increase the pitch in the positive direction to reduce the torque during high winds, but it is difficult to drive the blades in the negative direction to increase the torque during low winds. Fixed pitch angle turbines will exhibit good response [17]. Fixed pitch improves the aerodynamic efficiency as no extra effort is being taken to turn the blades. Usually, turbines with variable-pitch blades are expensive and complex, especially when the turbines are larger [18,19]. This paper considers a variable speed, fixed pitch wind turbine. The power contained in the wind is given by P_{\omega }, and

(1)P_{\omega }={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}{\rho }_{air}{A}_{blade}{C}_p(\beta ,\lambda ){\nu }^{\mathrm{3}},

where the power coefficient C_p(\beta ,\lambda ) is a function of the pitch angle of the blades \beta and tip speed ratio (TSR) \lambda; {\rho }_{air}is the air density; \nu is the wind velocity, and A_{blade}is the area swept by the blades. The torque developed by a wind turbine is given by

(2)T_a={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}R{\rho }_{air}{A}_{blade}{C}_p(\beta ,\lambda ){\nu }^{\mathrm{2}},

where R is the radius of the rotor. This torque may be suitably controlled based on the operating region of the wind turbine. The various regions in which a wind turbine can operate are shown in Fig. 1. Point A corresponding to a wind velocity of 5 m/s is the cut-in-velocity. Region 1 extends from 0 to Point A, Region 2 from Point A to Point B, and Region 3 from Point B to Point C. Points B and C correspond to rated velocity and cut-out-velocity respectively. As it is well known, the operation of the turbine in Region 1 is generally inhibited. In Region 2, the operating modes like MPPT and power smoothing are possible and have been carried out in the proposed work. Here the pitch angle is normally unaltered and made approximately zero to have a maximum value of C_p. In conventional systems, the operation in Region 3 is done by adjusting the pitch angle in accordance with the wind speed to reduce the torque. In the proposed system, the pitch angle remains unchanged in the entire range of operating wind speeds. In Region 3, power control is achieved through proper torque control in the machine via power converters. This will help the machine run at full load. MPPT in this region is invalid.

Proposed scheme

The proposed WRIG scheme for super-synchronous mode of operation is illustrated in Fig. 2 which consists of a WRIM and an ac-dc-ac power electronic converter. In this mode of operation, the rotor side converter (RSC) is a three-phase uncontrolled bridge rectifier which converts the ac power of the WRIM at slip frequency into dc power, while the grid side converter (GSC) is an LCI (with firing angle α>90°), which enables the power flow from the dc link to the three phase grid. The stator winding is connected directly to the 50 Hz grid while the rotor is connected through the ac-dc-ac converter via slip rings to allow the WRIG to operate at variable speeds in response to the changing wind speeds.

(1) Mode 1 (MPPT mode)

The firing angle of the LCI is adjusted automatically to change the dc link current to track the MPP. The automatic adjustment is performed till the actual dc link current I_act equals the reference current I_ref which is a function of wind velocity. The slip power generated by the rotor, P_r is transferred to the grid through the dc-link and the GSC. In the steady-state, for a lossless ac-dc-ac converter, the rotor power delivered to the grid, P_g is equal to the rotor generated power, P_r. This is determined by the speed of the wind turbine. And the power delivered to the grid increases with the increase in the speed of the wind turbine.

(2) Mode 2 (power smoothing mode)

In this mode, the reference current I_ref is the function of the grid power demand. In Region 3 of Fig. 1, any constant power less than or equal to the rated machine power can be delivered as demanded by the grid. However, there are possibilities when the power demanded by the grid cannot be completely supplied during low winds. Now the controller responds in such a manner to deliver the available maximum power to the grid.

Steady state analysis

Figure 3 depicts the per-phase steady state equivalent circuit of the WRIG. Although Fig. 3 can be used to determine the performance of an induction machine, the formulation of the resulting equations must be modified to suit the fact that the rotor current, rather than the stator current is the controlled variable.

Referring to the per phase equivalent circuit of the WRIG in Fig. 3, with all the parameters referred to the stator, the rotor current I_r^1can be given as

(3)I_{\mathrm{r}}^{\mathrm{1}}={\displaystyle \frac{\left|E_r^1\right|-\left|V_r^1\right|}{\sqrt{{(R_r^1/s)}^{\mathrm{2}}+{(X_r^1)}^{\mathrm{2}}}}}.

In Fig. 2, if the rectifier in the rotor terminals and the LCI are assumed ideal, the dc voltage V_dc referred to the stator terminals (V_{dc}^1) and the dc link voltage V_av referred to the phase voltage of the grid (V_{av}^1) can be respectively given by

(4)V_{dc}^1={\displaystyle \frac{\mathrm{3}\sqrt{\mathrm{6}}\left|V_{\mathrm{r}}^{\mathrm{1}}\right|}{\pi }}

and

(5)\left|V_{av}^1\right|={\displaystyle \frac{\mathrm{3}\sqrt{\mathrm{6}}\left|V_{rg}^1\right|}{\pi }}\cos \alpha =I_{dc}^{\mathrm{1}}{R}_d^{\mathrm{1}}-V_{dc}^1,

where V_{rg}^1is the phase value of the grid voltage referred to stator.

Substituting for V_{dc}^1from Eq. (4) into Eq. (5) yields

(6)\left|V_{\mathrm{r}}^{\mathrm{1}}\right|={\displaystyle \frac{\pi I_{dc}^1{R}_d^1-\mathrm{3}\sqrt{\mathrm{6}}\left|V_{rg}^1\right|\cos \alpha }{\mathrm{3}\sqrt{\mathrm{6}}}},

I_{dc}^1can be written in terms of I_r^1 as

(7)I_{dc}^1=\sqrt{{\displaystyle \frac{\mathrm{3}}{\mathrm{2}}}}{I}_r^1.

Substituting Eq. (7) into Eq. (6) and then Eq. (6) into Eq. (3) and solving \alphagives

(8)\alpha ={\cos }^{-\mathrm{1}}\left({\displaystyle \frac{I_r^1(\mathrm{6}\times \sqrt{{(R_r^1/s)}^{\mathrm{2}}+{(X_r^1)}^{\mathrm{2}}}+\pi R_d^1)-\mathrm{6}\left|E_r^1\right|}{\mathrm{6}{V}_{rg}^1}}\right).

In Eq. (8), as slip s is the only variable, \alpha varies as a function of s. The controller is designed in such a manner to vary the value of \alpha between 90° and 120° (inverter action) for a change in slip in order to change the dc link current in the MPPT mode and constant dc link current in the power smoothing mode. Thus for feeding a specified amount of stator power, the firing angle delay required at any rotor speed can be predicted using Eq. (8).

Line commutated inverter (LCI)

The power circuit of the LCI is displayed in Fig. 4 where V_dc represents the dc voltage output from the rotor circuit. When the firing angle delay \alpha is greater than 90°, the power flows from the rotor side to the grid side. L_d is the DC link inductance used to reduce the ripple in the dc link current. The value of L_d also dictates the DC link current to be continuous or discontinuous. The average output voltage V_av is hence given by

(9)V_{av}={\displaystyle \frac{\mathrm{3}\sqrt{\mathrm{6}}}{\pi }}{V}_{rg}\cos \alpha .

The source current has a quasi square waveform when sufficiently large inductance is connected to the dc link. The Fourier analysis [20] of this current gives the total harmonic distortion (THD) as

(10)THD=\sqrt{\left({\left({\displaystyle \frac{I}{I_{\mathrm{1}}}}\right)}^{\mathrm{2}}-\mathrm{1}\right)}=\mathrm{0.3108}=\mathrm{31.08}\%.

Details of experimental set-up

Figure 5 exhibits the closed loop scheme of the proposed controller, which consists of a wound rotor induction machine, an LCI, a diode bridge rectifier module, a dc motor, a PIC microcontroller and pulse amplifier circuits. In order to generate the firing pulses, the rotor side grid voltage (V_RB = 60 V) is stepped down to 6 V and fed to the opamp zero crossing detector (ZCD) circuit. The ZCD output is fed to a level translator to get a single-ended+5 V pulsed signal which is given as the input to pin 11 of the microcontroller. The low to high transition of the ZCD pulse at pin 11 which represents the zero crossing point of V_RB is sensed by the microcontroller by an appropriate program and after a delay of \alpha (>90°), the firing pulses are generated on pins 21 to 26 for the six thyristors in the LCI. The firing pulses thus generated from the microcontroller are fed to the gate circuit of the SCRs through the pulse amplifier circuits. For firing angles above 90°, the sequence of the thyristors to be fired for every 60° is T₅T₆, T₆T₁, T₁T₂, T₂T₃, T₃T₄, T₄T₅, so that no zero crossing point of line voltage V_RB will be missed out in each cycle [21].

The available maximum power in wind varies as a cube of its velocity. This power corresponding to various shaft speeds is predetermined and the corresponding I_ref values are stored in the look-up-table of the microcontroller. The shaft speed is converted to appropriate dc voltage by means of tacho-generator. The microcontroller senses the shaft speed via the ADC block for the MPPT operation in Region 2 of Fig. 1. In Region 3, the controller is adjusted in such a way that the rated power of the machine is delivered to the grid unless power smoothing command is given, by appropriately adjusting the firing angle of LCI. While the I_ref for the MPPT mode is taken from the look-up table, the I_ref for the power smoothing mode is generated based on the grid power demand. The DC link current I_dc is sensed using a Hall current sensor LA 55-P and fed as the input to pin 3 of the microcontroller. This current I_act is compared with reference current I_ref in the PIC microcontroller and the difference between these two signals is used for adjusting the firing angle\alpha. Care has been taken to set the value for initial firing angle at 120º and the gate pulses in appropriate sequence are generated. The power circuit of the LCI has been fabricated in the laboratory using 1200 V, 48 A, BTW48 SCRs and the diode bridge using Semikron’s MD8TU6012 rectifier module and is mounted on the heat sink.

As the speed varies, the rotor power delivered to the grid is varied but the stator power is maintained constant in the power smoothing mode and varied to track the MPP in the MPPT mode. In the practical setup, the variation in rotor speed (slip) is obtained by varying the speed of the dc motor by the field control method. The power smoothing operation in Region 2 in Fig. 1 is also carried out when demanded by the grid by automatically adjusting the firing angle of the LCI. The flow chart depicting the steps involved in firing pulse generation is presented in Fig. 6.

With the rated voltage (400 V) applied to the stator, the open circuit rotor voltage per phase on stand-still, (

V_{r_0}

) was 69.3 V at 50 Hz. The rotor voltage per phase for a given slip of 25% will be

(11)V_{\mathrm{r}}=s{V}_{{\mathrm{r}}_{\mathrm{0}}}=\mathrm{17.32} \mathrm{V},

and the corresponding rectified dc output voltage will be

(12)V_{dc}={\displaystyle \frac{\mathrm{3}\sqrt{\mathrm{6}}{V}_r}{\pi }}=\mathrm{40.5} \mathrm{V}\mathrm{.}

When I_dc = 0, V_dc = V_av. If V_rg is the per phase rotor side grid voltage,

(13)V_{av}=V_{dc}={\displaystyle \frac{\mathrm{3}\sqrt{\mathrm{6}}{V}_{rg}}{\pi }}\cos \alpha .

Upon substitution of the value of V_dc (= 40.5 V) and \alpha=120º in Eq. (13), the line to line rotor side grid voltage will be 60 V. The rated rotor current of the chosen WRIG is 22 A. So the three phase step-up transformer at the rotor side should also be rated at 60 V (L-L) and 22 A. Therefore, the rating of the three-phase step-up transformer in the rotor circuit has to be approximately 2300 VA, and consequently the same is true of the rating of the power converter. This is nearly 44% of the rating of the stator.

Experimental investigations and comparison

Experiments have been conducted on a 3-phase, 3.73 kW, 400 V, 50 Hz, 4-pole, 1500 r/min WRIM driven by a dc motor of 3.73 kW, 220 V. Initially the dc motor was started and the speed was adjusted to a value a little above the synchronous speed. Then the rated voltage of the machine was applied to the stator with the rotor terminals opened, allowing the machine to draw magnetizing currents only. Now the grid was closed and the firing pulses were applied to the LCI. The reference current corresponding to the desired value of stator power as demanded by the grid is set and the automatic adjustment of \alpha makes the dc link current equal to I_ref. For instance, it is desired to deliver a constant power of 1440 W from the stator to the grid in spite of variations in the rotor speed. The dc link current corresponding to this power is 10 A and the experimental results obtained for this current are furnished in Table 1. The variation of the firing angle obtained for a constant stator power maintained at 1440 W and the predicted values of \alpha under this condition using Eq. (8) are given in Fig. 7.

Similarly, in the MPPT mode, the reference current I_ref is retrieved from the look-up-table based on the speed of the machine. The firing angle gets adjusted to maintain a maximum power delivery at this current. The experimental results corresponding to the MPPT at various wind speeds are furnished in Fig. 8.

The predicted performance values of the machine for various slips can be determined using the analysis present in Section 2.1. For example, the stator current I_s and rotor current I_r at 1800 r/min for a constant I_dc of 10 A are obtained as

(14)\overline{I_s}=\overline{I_m}+k\overline{I_r},

\left|I_s\right|=\mathrm{3.2}\angle -\mathrm{90}+\mathrm{0.3}\times \mathrm{8.2}\angle -\mathrm{28}=\mathrm{4.86} \mathrm{A}\mathrm{.}

(15)I_r=\sqrt{{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}}{I}_{dc}=\mathrm{8.16} \mathrm{A}\mathrm{.}

The stator and rotor power factors calculated for a constant dc link current of 10 A for various values of slip as listed in Table 1 are approximately 0.467 and 0.23 respectively. To compensate for such poor power factor values, a number of methods based on active compensation have been suggested [22-25]. To study the effect of power factor correction (PFC) capacitor on various load conditions, experiments were conducted on the proposed system with a fixed capacitor bank of 20 µF per phase. This gave a much improved power factor greater than 0.9 for loads above 30 % of full load. The variation of power factor with and without capacitors is shown in Fig. 9.

The experimental waveforms of various parameters of the proposed scheme such as dc link current, dc link voltage, stator current, rotor current and rotor voltage as observed in a digital storage oscilloscope are compared with the corresponding simulation results. The complete simulation model has been developed in MATLAB and the simulation study is conducted with the same circuit parameters used in the experimentation. The parameters of the WRIG used are given in the Appendix.

Figure 10 shows the dc link current waveforms and Fig. 11 shows the dc link voltage waveforms at N = 1800 r/min. The corresponding experimental and simulated waveforms of the stator current are demonstrated in Fig. 12. The oscillogram of the rotor current at the output of LCI which is being fed to the grid is given in Fig. 13. Figure 14 shows the rotor voltage waveform taken between the R and Y phases.

Conclusions

A very simple and easy to implement configuration of WRIG with MPPT and power smoothing for wind driven applications has been presented. The analysis of the WRIG in feeding the required power to the grid with the variation in rotor speed is carried out. Implementation of super-synchronous mode of a 3.73 kW, 1500 r/min, 400 V, WRIG has been brought out in this paper. A controller consisting of diode rectifier, LCI and pulse amplifier circuits has been fabricated in the laboratory and an 8-bit PIC microcontroller has been programmed for the close loop operation of the system.

Experiments have been conducted by varying the rotor speed of the 3.73 kW induction machine from 1550 to 1800 r/min through a dc drive. The simulation and experimental results in both power smoothing mode and MPPT mode are presented. It is to be further mentioned that there is only a small change in the firing angle range for maintaining the stator power at higher values. The fact that the power factor correction could be easily achieved with fixed bank of capacitor is an added advantage of the proposed system.

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