Comparative study of Feed Forward and SPWM control Technique for DC to DC Dual Active Bridge Convert

IJIRST –International Journal for Innovative Research in Science & Technology| Volume 3 | Issue 01 | June 2016 ISSN (online): 2349-6010

Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter Digvijay B. Kanase Assistant Professor Department of Electrical Engineering Annasaheb Dange College of Engineering & Technology.

Ashta 416301 India.

Swapnil S. Amale Assistant Professor Department of Electrical Engineering Annasaheb Dange College of Engineering & Technology.

Ashta 416301 India.

Yakub A. Makandar Assistant Professor Department of Electrical Engineering Annasaheb Dange College of Engineering & Technology.

Ashta 416301 India.

Dhanyakumar A. Patil Assistant Professor Department of Electrical Engineering Annasaheb Dange College of Engineering & Technology.

Ashta 416301 India.

Abstract A Dual active bridge converter is a high efficiency power converter. This converter is provided isolation with use of solid state transformer. DAB converter consists of two H-bridges of active power control along with one high-frequency transformer. High frequency transformer (SST) provides galvanic isolation and act as energy storage device. The leakage inductance of transformer at primary and secondary side acts as energy storage component. DAB converter has considerable amount of harmonics. The aim is to reduce the harmonics. This paper discusses sinusoidal pulse width modulation technique (SPWM) to achieve harmonics reduction. Solid state transformer is a high frequency transformer can be used as power electronic converter. The three stage SST configuration includes ac to dc rectifier, isolated dc to dc dual active bridge converter, dc to ac inverter. The switching frequency of inverter part is greater than DAB part so inverter stage is model for double line frequency i.e. at 120 Hz. The second harmonics are dominant at this frequency. For better performance of DAB converter PI control is used but at 120 Hz it has limited gain. To proposed methods are PI plus feed forward and PI plus Resonant controller.

Keywords: Dual Active Bridge converter (DAB), Sinusoidal Pulse Width Modulation (SPWM), THD, Feed Forward Controller _______________________________________________________________________________________________________ I.

INTRODUCTION

Dual Active Bridge converter is designed in (1). It is basically a buck and boost dc to dc converter, and is isolated by high frequency transformer. The main advantage of this type of converter is we can achieve bidirectional power flow. These DAB converters (buck and boost dc–dc converter) are important components in implementing power electronics interfaces for future power systems based on smart-grid technologies. The applications of these type of converter includes, interconnecting grid and various storage systems, example fuel cell, batteries, photovoltaic systems etc it also has application in hybrid systems and electrical and hybrid vehicles so as to deal with different voltage and power level. Main aim is to eliminate the losses present in the power electronics. The main object consists of minimizing circulating currents, conduction losses and to operate the converter under soft switching to minimize switching losses. Conventional modulation strategy (CMS) can be use to achieve soft switching, minimizes power losses depending upon voltage conversion ratio and on the output current [1]. This might be one of the drawbacks for some application, where converter operates for variable load, this reduces the efficiency.

Inverter is basic device that is used to convert DC electrical energy into AC. The main purpose of inverter is to take DC power from battery and convert into AC. DC to AC inverter has wide application such as uninterruptible power supply (UPS), AC motor drives. Now a day’s inverter play important role in renewable source applications, as it is used to connect renewable energy sources such as photovoltaic system, wind energy system etc to the grid. Control algorithm used for inverters are similar to those used in DC-DC converters. Current control and voltage control method can be applied to the converters as per the applications. Dc to AC inverter uses Pulse width modulation control (PWM) [3]. This PWM method is simple to implement and is very advanced where the gate signal or pulses can be controlled by various mechanisms. This method is used to control the output voltage of inverter to the rated value as per the requirement of its application.

In conventional type inverter as the load varies so does the output voltages vary. So as to deal with this problem PWM technique, adjust the width of the pulses. Thus the output depends upon the switching frequency and the pulse width that is adjusted according to the output required so that constant output is obtained. Inverter operates in pulse width modulated (PWM)

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043)

way and switch between different circuit topologies, this means that inverter is nonlinear system or circuit. UPS systems are used at almost all places in order to provide emergency power to critical loads which cannot afford utility failure. The core circuit of UPS system is CVCF inverter [2]. High-quality output voltage is required for these types of inverters. SPWM technique can prove to be great control algorithm so as to obtain improved output voltage [4].

The dual active bridge converter is used to interface these sources to grid. Dual active bridge converter interconnects medium voltage distribution and low voltage distribution network [1]. The SST is important element for future renewable electric energy.

SST manages the renewable energy transfer from grid to distribution side [3]. The distribution transformer is fundamental component and is inexpensive, highly reliable and efficient [2]. SST provides active power flow control. But due to presence of power converter there are conduction and switching losses [4]. The output of DAB converter is dc whereas the output at inverter side is ac so the power fluctuates at 120 Hz. So in order that DAB converter to drive such inverter it is necessary to select the capacitor that can absorb such 120 Hz harmonics. As the second order harmonics is less, to compensate it, capacitor bank should be connected at the output of DAB, but the system becomes ineffective also cost increases. This method is used in various applications because it is easy to implement, low cost and reliable operation [5]. This paper discusses implementation of novel sinusoidal pulse width modulation and Feed Forward control technique for dc to dc Dual Active Bridge converter (DAB) to eliminate harmonics present at the output of inverter.

A Dual active bridge converter is a high power; high-efficiency power converter has the isolation provided with the use of SSTs. It has two H-bridges of active power switching devices and one high-frequency transformer. This transformer provides the galvanic isolation and energy storage capability in the winding leakage inductance. The H Bridge is set and operated at fixed 50% duty ratio. Depending upon the available configuration of switching devices there are dc-dc DAB converters and ac-ac DAB converters. Circuit of DAB converter consists of two bridges with one high-frequency transformer. Two bridges are made of four high-frequency switching devices (MOSFETs or IGBTs) this switching device is connected in an H-bridge connection.

This connection is similar to that used in full-bridge dc-dc converters. This configuration uses two active bridges instead of diodes on the other side of the transformer. So the name “Dual Active Bridge� is given to this type of converters [7].

The high frequency transformer which is connected between two active H bridges is used for providing galvanic isolation of a DAB converter. The high-frequency transformer reduces the weight and volume of the magnetic core. If we compare the conventional converters with DAB converters, Dual Active Bridge converters has more number of silicon devices (whose price is continuously going down) and has less copper and smaller magnetic core (whose price is continuously going up). Apart from galvanic isolation, the high-frequency transformer has small amount of leakage inductance in primary and secondary windings.

This leakage inductance has severs two purposes: (1) it acts as energy storage in DAB converters (2) it used to reduce dv/dt across switching devices at time of commutation transients, it ensures the soft switching, and reduces switching losses. Figure shows SST configuration, first stage is ac to dc rectifier. Second part is isolated dc to dc dual active bridge converter, power is transferred between two dc buses using transformer in this part. The third stage is dc to dc inverter [8].

Fig. 1: Multi-stage configuration of SST

Fig. 2: Power transfer using multistage SST

Figure 2 shows the novel power flow of DAB converter. The solid line indicates the active power flow and the dashed line is double frequency line. The output power from rectifier and inverter is average power (dc) and ripple power (second order harmonics 120Hz) [6].

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043)

(1) p  2V sin  w s t  2 I sin  w s t   VI 1  cos 2 w s t  The main aim is design of DAB converter in SST following assumptions is made (1) rectifier stage is designed to provide stable dc input to DAB converter (2) inverter is designed such that output voltage is sinusoidal nearly equal to 120 V (3) switching frequency of inverter is more than converter. In this study the switching frequency of DAB converter is 5 KHz and that of inverter is 20 KHz. This is because the voltage at input of DAB converter is higher than the inverter at distribution level. By separating the load from the source, the disturbances at grid side will be absent because it is compensated by the SSTs. Thus solid state transformer help is providing the isolation between the two sides of the converter and to reduce the disturbances occurring [10] II. DUAL ACTIVE BRIDGE MODEL CONFIGURATION For modeling the dc-dc DAB converter a full order discrete time model technique is used. This model provides accurate control to output transfer function [11] in matrix form written as

(2) (3)

y  Cx

Where,

y   v o 0 

x   v o 0  1   RC o  2 sin( D  )  A   Lt  2 cos( D  )   Lt 

 it 1 I 

 it 1 R

4 sin( D  )





(4)

T

C o Rt

(5) 

Lt  ws

4 cos( D  )   C o   ws   Rt   Lt 

(6)

 4    I sin( D  )  I cos( D  ) oI oR   C  o  2V o 0   B cos( D  )   Lt   2V o 0    sin( D  ) L t  

(7)

C  1 0

(8)

0

T

Where v o 0 output voltage of DAB converter, L t is leakage inductance of transformer of DAB converter, R t is lumped equivalent resistance of transformer winding. w s  2 fs , fs  5000 Hz. The Transfer Function of converter is given as

G vd  s   C  sI  A  B 1

The power transferred by DAB converter is v v P  s o d 1  d  2 f s Lt

(9)

(10)

III. CONTROLLER DESIGN Various methods for controlling the output voltage of DAB converter is discussed in the paper in detail viz, PI plus feed forward, Sinusoidal pulse width modulation controller. The inverter stage is model at double frequency as that of inverter stage. So in this type of converter second order harmonics are dominant. Using PI controller has limitation of low gain at 120 Hz, to overcome

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043)

this problem [13] Feed forward and SPWM methods are used. Control of DAB converter consists of (I) to regulate output voltage (II) to reduce the ripples present at the output of inverter.

Fig. 3: MATLAB SIMULINK model of Dual Active Bridge Converter

IV. PI PLUS FEED FORWARD CONTROLLER PI controller only regulates output dc voltage and control output average power, while feed forward portion takes care of disturbances at 120 Hz from inverter side and tracks 120 Hz ripples. After voltage of DAB converter is sampled low pass filter whose pass band is below 120 Hz [14]. The cut-off frequency of Low pass filter is less and PI controller only regulates dc voltage, so band width of PI controller is not necessary to be large. The phase angle of inverter output is controlled by the controller used at inverter. According to equation (1) the power angle is given by

 p  2 v 

 2

(11) Feed forward portion provides excellent ripple rejection for unity power factor loads. But the performance of converter is affected for unity power factor load. So to improve its performance PI controller is combined with resonant controller.

Although the above solution is able to track the 120 Hz power by inverter, the analysis and parameter selection is not straightforward [15]. Also, if the load power factor is non-unity, the feed forward term may not properly track the output power ripple.

Fig. 4: Schematic diagram of Feed forward controller

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043)

Fig. 5: MATLAB SIMULINK model of Feed Forward controller

V. SIMULATION RESULTS 300

Power [W]

250 200 150 100 50 0 -50 0.5

0.55

0.6

0.65

0.7

Time 0.75

0.8

0.85

0.55

0.6

0.65

0.7

Time 0.75

0.8

0.85

0.9

0.95

1

200

Voltage [V]

100

0

-100

-200 0.5

0.9

0.95

1

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043)

Ripple (duty ratio)

0.4 0.3 0.2 0.1 0 -0.1 0.5

0.55

0.6

0.65

0.7

Time 0.75

0.8

0.85

0.9

0.95

1

Fig. 6: Simulation results of PI plus feed forward controller for pure resistive load of 30Ω. (a) Inverter power (b) output voltage (c) ripple (duty ratio). (d) FFT Analysis

VI. SINUSOIDAL PULSE WIDTH MODULATION TECHNIQUE SPWM control has numbers of output pulses [16] for duration of per half cycle and these pulses are of different width. The width of these pulses changes according to the amplitude of a sine wave; the gating signals are produced by comparing the sinusoidal signal with a high frequency triangular signal. Referring to fig (7), dc link voltage gives us the information of transfer of active I power between source and grid. At output of dc link, voltage regulator gives active current m . [19] [20] by multiplying active current component (

Im

) with unity grid voltage vector templates (

Ua

) it gives reference grid current (

Ia

*

). The grid neutral

current is set to zero. The synchronizing angle (  ) obtained from phase locked loop (PLL) is used to generate unity vector template as U a  Sin   (12) The dc link voltage is then supplied through first-order low pass filter (LPF) that reduces switching ripples present in dc-link voltage also present due to reference current signal [17] [18]. The difference (dc-link voltage and reference dc-link voltage) is given to a discrete-PI regulator. Constant dc-link voltage is obtained under varying load conditions. The error dc-link voltage is given as:

Vdcerr The output of PI is

n   Vdc * n   Vdc n 

Im n   Im n  1  K p

vdc

(13)

Vdcerr n   Vdcerr n  1  K I vdcVdcerr n 

(14)

The reference current is computed as *

I a  Im* U a

Equation (15) is compared with actual grid current

(15)

Ia

to as to calculated current error as

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043) *

Iaerr  I a  I a

(16)

This current is compared with triangular wave to generate the pulse signal.

Fig. 7: MATLAB SIMULINK model of SPWM controller

VII. SIMULATION RESULTS 300

Power [W]

250 200 150 100 50 0 0.7

0.75

0.8

0.75

0.8

Time 0.85

0.9

0.95

0.85

0.9

0.95

1

150

Voltage [v]

100 50 0 -50 -100 -150 0.7

Time

1

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Comparative Study of Feed Forward and SPWM Control Technique for DC to DC Dual Active Bridge Converter Driving Single Phase Inverter (IJIRST/ Volume 3 / Issue 01/ 043)

Fig. 8: Simulation result of DAB converter using SPWM method (a) Inverter Power (b) Output Voltage (c) FFT analysis.

Table – 1 Result Comparison Controller Total Harmonic Distortion (THD) Feed Forward Controller 21.19% SPWM Controller 3.60%

VIII. CONCLUSION DC to DC Dual Active Bridge Converter driving a single phase inverter has dominant second order harmonics. To improve the performance of DAB converter by using conventional technique like PI control has some limitation, to have ripple free output voltage at the inverter, the phase margin has to be increased but this may destabilize the feedback loop. This limitation can be overcome by using Feed forward and Sinusoidal pulse width modulation control method. Simulation results show that THD can be reduced more as compared to feed forward Controller (Feed Forward-21.19% and SPWM-3.60%). The future scope may extend to the analysis of dc-dc DAB converter by using non-linear control technique such as sliding mode control technique.

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