Study On Limiting The Dc Link Voltage Fluctuation Engineering Essay

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Abstract- This paper describes a Wind turbine coupled doubly-fed induction generator (DFIG) for modern wind energy conversion scheme. The limitation of dc-link voltage fluctuation for a back-to-back converter is analysed. An improved control strategy with the instantaneous rotor power feedback is proposed to limit the fluctuation range of the dc-link voltage. A decoupled P-Q controller to regulate the DC link voltage is implemented using MATLAB/SIMULINK, and the effect of variations in grid side voltage on the converter performance is examined.

Index Terms- Doubly fed induction generator (DFIG), dc-link voltage, d-q vector control, instantaneous power feedback, grid side controller.

introduction

With growing concerns over environmental pollution and energy shortage, great efforts have been taken around the world to implement renewable energy based programs. Especially with wind energy, that has been undergoing a significant development with improving techniques, reducing costs and low environmental impacts. The requirements of power controllability during the normal operation to allow some degree of control over the real and reactive power production have increased the scope for the development of a new generation called wind energy conversion system (WECS).

A doubly-fed induction generator (DFIG) is an adjustable-speed induction machine widely employed in modern wind power industry [1, 2]. Wind turbine manufacturers are increasingly moving to variable speed concepts because of the following reasons. 1) Variable speed wind turbines offer a higher energy yield in comparison to constant speed turbines. 2) The reduction of mechanical loads and simpler pitch control can be achieved by variable speed operation. 3) Variable speed wind turbines offer extensive controllability of both active and reactive power. 4) Variable speed wind turbines show less fluctuation in output power [3, 4]. However, the operation and performance of a DFIG depends not only on the induction machine but also on the two back to back converters as well as how they are controlled, requiring integrating all the components together for an investigation. The most preferred means of conversion of AC-DC-AC is done with a DC-link. However, the operation and performance of a DFIG depends not only on the induction machine but also on the two back to back converters as well as how they are controlled, requiring integrating all the components together for an investigation. The most preferred means of conversion of AC-DC-AC is done with a DC-link. The presence of the DC link decouples the rectifier and the inverter, and thus each can be operated independently using standard PWM techniques. These systems also provide regenerative capacity which allows excess energy from the load side to be fed into the grid which makes it useful in applications like a Doubly Fed Induction Generator [5, 6] where bidirectional power flow is required as shown in Fig. 1. With the aid of the frequency converters, a four-quadrant operation is being demonstrated [7]. The grid-side converter controls the dc-link voltage and the reactive power absorbed from the grid by the converter. The general control technique for the grid-side converter control, which is widely used in wind power industry, is a decoupled d-q control approach that uses the direct axis current component for real power control and quadrature axis current component for reactive power control [8, 9].

Fig. 1 Schematic of wind turbine coupled DFIG

doubly-fed induction generator and controls

A doubly-fed induction generator is a standard, wound rotor induction machine with its stator windings directly connected to the grid and its rotor windings connected to the grid through an AC/DC/AC frequency converter. In modern DFIG designs, the frequency converter is built by two self-commutated PWM converters, a rotor-side converter and a grid-side converter, with an intermediate DC voltage link. By controlling the converters on both sides, the DFIG characteristics can be adjusted so as to achieve maximum of effective power conversion or capturing capability for a wind turbine and to control its power generation with less fluctuation.

Many different d-q control algorithms have been proposed and used for controlling the DFIG rotor- and grid-side converters for certain dynamic and transient performance achievements of DFIGs. Most of them are based on a real and reactive power control concept, a popular DFIG converter control mechanism used in modern wind turbines. This control configuration is usually divided into rotor- and grid-side converter controls. The rotor-side converter controls the active and reactive power of the DFIG independently, and the grid-side converter is controlled in such a way as to maintain the DC-link capacitor voltage in a set value and to maintain the converter operation with a desired power factor [10].

The rotor side controller, consisting of a reactive power controller and an active power controller, is commonly a two-stage controller as shown in Fig. 2. It operates in either stator-flux or stator-voltage oriented reference frame and hence the q-axis current component represents active power and the d-axis component represents reactive power. The two controllers in the rotor-side controller determine inverter d- and q-voltages by comparing the d- and q-current set points to the actual d- and q- currents of the induction machine.

Fig. 2 Schematic of Rotor side converter

The grid-side converter regulates DC-voltage and reactive power. It is also a two-stage controller operating in a grid AC voltage reference frame as shown by Fig. 3. Traditionally, the d-axis current component is used for active power control, and the q-axis current component is used for reactive power control. The two feed-forward paths in the grid-side controller determine converter d- and q-voltages by comparing the d- and q-current set points to the actual d- and q-currents supplied to the grid.

Fig. 3 Schematic of Grid side converter

conventional d-q vector control mechanism for dfig grid-side converter

The conventional control mechanism for DFIG grid-side converter is based upon the decoupled d-q vector control concept [11, 12]. Usually, a decoupled d-q vector control approach is used, with the d-q reference frame oriented along the stator (or supply) voltage vector position.

Fig. 4 shows the schematic of the grid-side converter for doubly fed induction generator using back-to-back PWM converters [7]. In the figure, a DC-link capacitor is on the left and a three-phase grid voltage is on the right. The voltage balance across the inductors is

(1)

Fig. 4 Schematic of Grid Side Converter

where L and R are the line inductance and resistance of the transformer or the grid filter. When transforming (1) to the d-q reference frame that has the same speed as that of the grid voltage, (1) becomes (2) where ωs is the angular frequency of the grid voltage

(2)

In the d-q reference frame, the active and reactive power absorbed from the grid in per unit is

(3)

Aligning the d-axis of the reference frame along the stator-voltage position Vq is zero and since the amplitude of the supply voltage is constant, Vd is constant. Therefore, the active and reactive power will be proportional to id and iq respectively. This is the conventional foundation for the decoupled d-q controls, where the grid-side converter is current regulated, with the direct axis current used to regulate the DC-link voltage and the quadrature axis current component used to regulate the reactive power.

The strategy for the conventional decoupled d-q control of the grid-side converter is illustrated in Fig. 5. When d-q reference frame has the same speed as that of the grid voltage, θe=ωst, the transfer function for the current control loops is obtained from (2) and is given in (4). This transfer function could be different depending on how the controller is designed. The d and q reference voltages, V*d1 and V*q1 are computed from the error signals of the d and q currents, respectively, as shown in Fig. 5. The α and β reference voltages, V*α1 and

Fig. 5 Decoupled d-q vector control structure for grid-side converter

V*β1, are obtained from the d-q reference voltages, correspondingly, through a vector rotation of ejωst. The two α and β voltages together are then used to generate the three-phase sinusoidal reference voltage signal for control of the grid-side PWM converter [12]. Note that this control configuration actually intends to control the real and reactive powers through the decoupled d and q reference voltages, respectively. Although there is an iq component in V*d1 equation, it is not contributed in the way of typical close-loop control concept. The same is true for id component in V*q1 equation.

= . + (4)

, (5)

The control of the grid-side converter depends on the d and q reference voltages V*d1 and V*q1, that are obtained from the error signals of the d and q currents as shown in Fig. 5. The combined d and q reference voltages affect the converter output phasor voltage on the grid side by varying its amplitude and delay angle [10, 12]. This converter-injected voltage is linearly proportional to the three-phase sinusoidal drive signal in normal converter linear modulation mode. Normally, the grid-side converter needs to be controlled in such a way as to maintain a constant dc-link voltage, which requires that the real power outputted from one converter (rotor- or grid-side converter) should equal the power entered in another converter (grid- or rotor-side converter) when assuming no loss in the PWM converters.

By adjusting the firing scheme of the converter, we can operate converter in both rectifier and inverter mode. If α = 0, the converter behaves as an uncontrolled converter whose output voltage cannot be adjusted. When α = 90, the average voltage of the converter becomes zero. If the delay angle is increased further, the average value of the DC voltage becomes negative. In this mode of operation, the converter operates as an inverter. By adjusting the firing angle, the converter behaviour as a load can be studied as shown in the table 1below.

TABLE I

CONVERTER BEHAVIOUR AS A LOAD

Delay angle

Converter behaviour as a load

α = 0

Behaves like a resistive load

0< α <90

Behaves like a resistive/inductive load and absorbs active power

α = 90

Behaves like an inductive load with no active power drawn

α > 90

Behaves like an inductive load but is also a source of active power

simulation and analysis

This chapter deals with study on the decoupled d-q vector control techniques for control of DFIG back-to-back PWM converter both analytically and through simulation.

Fig. 6 Wind power characteristics

The wind power characteristic shows that as the velocity of wind increases, the power output of turbine also increases. By adjusting the blade pitch angle, we can achieve an optimum output power. As we increase the supply voltage, the DC link voltage also increases. From the Fig. 8 it is clear that the voltage level settles at 550 Volts, hence the DC link reference voltage is set as 550 Volts respectively.

Fig. 7 Pitch angle control

Fig. 8 Variation of DC link voltage with supply voltage

Fig. 9 Real and reactive power with normal load

Fig. 10 Real and reactive power with PQ load

TABLE II

REACTIVE POWER VS DC LINK VOLTAGE

DC link voltage

in Volts

Real power in Watts

Reactive power

in VAR

500

2.03 x 106

1.21 x 106

420

1.15 x 106

6.42 x 105

250

5.06 x 105

3.02 x 105

To maintain a constant dc-link voltage, the power passed to the rotor-side converter from DFIG rotor should equal to the power delivered to the grid from the grid-side converter when neglecting the converter losses. Thus, while a real power flows from the DFIG rotor to the rotor-side converter, the grid-side converter should be operated as an inverter and controlled in such a way as to deliver the same amount of the real power to the grid.

Fig. 11 First quadrant operation of real & reactive power

Fig. 12 Second quadrant operation of real & reactive power

Fig. 13 Third quadrant operation of real & reactive power

Fig. 14 Fourth quadrant operation of real & reactive power

Two different techniques are being adopted for converters namely SPWM and SVM. In comparison to SPWM and SVM techniques, it was noted that the SVM technique yields better performance than SPWM as shown in the table 3.

TABLE III

COMPARISON OF CONVERTER TECHNIQUES

Technique adopted

THD in %

Power factor

SPWM

35.58

0.862

SVM

26.18

0.918

conclusion

A simulation study on the power generation characteristics of integrated DFIG and its back-to-back converter with SPWM and SVM techniques is carried out. The general purpose of grid-side converter control is to keep the dc-link capacitor voltage constant by balancing the real power at rotor-side and grid-side converter, and to compensate DFIG reactive power as much as possible. Thus, while a real power flows from the DFIG rotor to the rotor-side converter, the grid-side converter should be operated as an inverter and controlled in such a way as to deliver the same amount of the real power to the grid. While a real power flows from the rotor-side converter to the DFIG rotor, the grid-side converter should be operated as a rectifier and controlled in such a way as to receive the same amount of the power from the grid. Thus in order to obtain synchronization, it is necessary to maintain appropriate reactive power across the grid.

appendix

PARAMETERS OF DFIG

Rated Power

1.5 MW

Stator Voltage

575 V

RS (Stator Resistance)

0.0071 p.u.

Rr (Rotor Resistance)

0.005 p.u.

LS (Stator Inductance)

0.171 p.u.

Lr (Rotor Inductance)

0.156 p.u.

Lm (Magnetizing Inductance)

2.9 p.u.

Number of pole pairs

3

Inertia Constant

5.04

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