Advances in Computational Sciences and Technology ISSN 0973-6107 Volume 10, Number 4 (2017) pp. 599-611 © Research India Publications http://www.ripublication.com

Hybrid Multi Output Converter for Microgrid

Applications

Syed Bilal Qaiser Naqvi1, Digvijay Singh2 and Adil Sarwar3

1 ,2 ,3 Electrical Engineering Department, Aligarh Muslim University Aligarh-202002, Uttar Pradesh, India.

Abstract

The paper proposes a single stage hybrid multi-output converter with two independent DC sources (e.g. DC battery, PV cell, Fuel cell etc.) having same voltage and three isolated outputs (two DC, one AC). The proposed converter topology does away with extra switches / conversion units which would otherwise be present in case of conventional converter systems supplying both ac and dc loads. This would reduce the cost of energy conversion and overall efficiency of system increases. The AC output obtained is of high frequency which is suitable for loads like fluorescent lighting and induction cooking. Such a conversion system can be of great utility in microgrids and various industrial applications. The simulation as well as hardware realization of this proposed topology is presented.

Keywords: Hybrid multi output converter, Pulse Width Modulation (PWM), Design Parameters for Converter, Small Signal Analysis of Buck Converter, High Frequency AC loads

I. INTRODUCTION

The world’s electricity demand has been growing rapidly for the past few years. Availability of reliable power supply is a major goal of all nations. For meeting this goal, renewed focus is being given to Distributed Generation (DG). The advancements in power electronics and renewable energy, free trade of power, and environmental problems associated with conventional generation have led to increased impetus towards having stand-alone power systems which supply the local loads. The energy sources of

600 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar

such a network may be green sources like solar energy, wind energy, fuel cell etc. Such a network of distributed energy sources and loads, coupled using power electronic converters, forms a microgrid, as shown in Fig. 1. A microgrid can be integrated with the conventional grid using Point of Common Coupling (PCC). The microgrids can be classified into:

Dc microgrid Ac microgrid Hybrid microgrid

DC microgrid provides higher system efficiency than ac microgrid due to absence of losses due to no-load equipment and skin effect. Frequency stability and reactive power issues are not present in dc distribution systems [1-3]. However, various loads are present in a microgrid. Thus, there is power conversion at various stages to meet the requirements of different loads, as shown in Fig. 2.

acdc

Sensitive dc load

dcdc

dcdc

dcdc

acdc

DG with

storage

dcdc

Normal dc load

ac load

acdc

Main Grid

Bidirectional

dc/ac

converter PCC: Point of

common coupling

Fuel CellsSolar PVWind Turbine

Battery

Fig. 1. A typical dc microgrid

Hybrid Multi Output Converter for Microgrid Applications 601

dc/ac

Inverterac/dc dc load

dc/dc

Converterdc load

dc

(Energy

Storage

System)

ac dc

ac load

Fig. 2. Multi stage conversions in a dc microgrid

There may be a battery supplying power to an inverter which then supplies ac as well as dc loads. Thus, there is decrease in efficiency due to multiple conversion stages. A separate dc to dc converter may be used to supply loads like small dc fans, dc motors etc. Multistage conversion for most of the domestic and industrial ac/dc loads running on dc power supply adds to the cost of the entire system (since more power electronic converters are needed) and reduces the overall efficiency. A single stage energy conversion would reduce cost of the system and increase its efficiency.

Hybrid output converters are the converters which can supply simultaneous ac and dc loads from a single dc input. They require lesser number of switches, have increased reliability, greater power processing density. They are very useful for microgrid applications having both ac and dc loads [4-8].

Power electronic converters provide energy conservation at reduced operating cost and increased safety in residential and industrial applications, which include space heating, air conditioning, water heating, cooking, lighting, clothes washing and drying etc. Some of the most important applications of high frequency ac supply are fluorescent lighting and induction cooking. Energy efficiency of fluorescent lamps can be increased by almost 20-30% when they are operated at high frequency compared to their line frequency counterparts [9]. Induction based cooking systems utilize fraction of electrical energy compared to the thermal cooking based system. They employ high frequency ac supply for inducing circulating current in the cooking pan placed on top of the induction coil.

The proposed converter is a single stage hybrid multi output converter, with dc energy storage systems which can be charged through the distributed energy sources present in the microgrid, or using conventional grid during off peak hours. This paper consists of seven sections. Section I is the introduction. In section II, proposed Hybrid multi output converter topology has been discussed. In section III, modes of operation have been discussed. Section IV discusses design and simulation of the converter. In section V, small signal analysis of the converter is presented. Section VI discusses experimental setup and experimental results. Finally section VII concludes the paper.

602 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar

II. PROPOSED HYBRID MULTI OUTPUT CONVERTER

The proposed hybrid multi output converter is shown in Fig. 3, having two dc sources, each dc source having a potential difference of Vdc/2. The dc sources are three terminal dc sources. The proposed converter circuit topology consists of three parts; one half bridge inverter with one ac output and two buck converters with one dc output each. This configuration is arranged in such a way so as to minimize the number of switches. This configuration uses four switches (S1,S2,S3,S4) and two dc sources, rather than six switches and three dc sources needed for independent converters (one switch and one dc source for each buck converter and four switches and one dc source for full bridge inverter). The first part consists of buck converter having switch S3, diode D1, capacitor C1 and inductor L1. The dc load 1 is connected across this buck converter. The second part consists of buck converter having switch S2, diode D2, capacitor C2 and inductor L2. The dc load 2 is connected across this buck converter. The third part is half bridge inverter having switches S1 and S4. An ac load is connected across this inverter. The gate pulses for switches S1, S2, S3 and S4 are shown in Fig.4. A dead-band of 10 microseconds is provided between switching of S1 and S4 to avoid short circuit of battery which may damage the switches as well as battery. This converter operates in two modes.

Vdc/2

Vdc/2

D1

D2

S1

S4

S3

S2

L1

L2

C1

C2

AC Load

DC

Lo

ad

1D

C L

oad

2

Fig. 3. Circuit topology

S1

S2

S3

S4

Mode 1 Mode 2

T1 T2

Fig.4. Gate Signals

Hybrid Multi Output Converter for Microgrid Applications 603

III. MODES OF OPERATION

Mode 1: Switch S3 and S4 are turned on. Dc voltage is obtained across the first dc-dc buck converter and also negative half cycle of ac voltage is obtained across the ac load. The inductor L2 supplies current to the dc load 2 and diode D2 is forward biased. The switching diagram for mode 1 operation is shown in Fig.5, and output voltage is shown in Fig.7.

Vdc/2

Vdc/2

D1

D2

S1

S4

S3

S2

L1

L2

C1

C2

AC Load

DC

Lo

ad 1

DC

Lo

ad 2

Fig.5. Switching diagram for Mode-1 operation Mode 2: Switch S1 and S2 are turned on. Dc voltage is obtained across the second dc-dc buck converter and also positive half cycle of ac voltage is obtained across the ac load. The inductor L1 supplies current to the dc load 1 and diode D1 is forward biased. The switching diagram for mode 2 operation is shown in Fig.6, and output voltage is shown in Fig.7.

Vdc/2

Vdc/2

D1

D2

S1

S4

S3

S2

L1

L2

C1

C2

AC Load

DC

Lo

ad 1

DC

Lo

ad 2

Fig.6. Switching diagram for Mode-2 operation

604 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar

IV. DESIGN AND SIMULATION

When the switches S3 and S4 are ON, the power source is connected to the upper buck converter. Diode D1 is reverse biased. The current flows through switch S3, inductor L1, and dc load 1. The current through inductor L1 increases as long as switch S3 is ON. In the lower buck converter, switch S2 is OFF. The inductor L2 supplies current to the dc load 2 and the diode D2 is forward biased. The current through inductor L2 keeps decreasing during this time. The ripples in the inductor currents ΔiL1 and ΔiL2 during time interval T1 (turn-on time of switch S3) are given by [10]

∆𝑖𝐿1 =(𝑣𝐷𝐶 2⁄ −𝑣𝑜)

𝐿1𝑇1 (1)

∆𝑖𝐿2 =

(−𝑣𝑜)

𝐿2𝑇1 (2)

When the switches S1 and S2 are ON, the power source is connected to the lower buck converter. Diode D2 is reverse biased. The current flows through switch S2, inductor L2, and dc load 2. The current through inductor L2 increases as long as switch S2 is ON. In the upper buck converter, switch S3 is OFF. The inductor L1 supplies current to the dc load 1 and the diode D1 is forward biased. The current through inductor L1 keeps decreasing during this time. The ripples in the inductor currents ΔiL1 and ΔiL2 during time interval T2 (turn-on time of switch S2) are given by [10]

∆𝑖𝐿1 =(−𝑣𝑜)

𝐿1𝑇2 (3)

∆𝑖𝐿2 =(𝑣𝐷𝐶 2⁄ −𝑣𝑜)

𝐿2𝑇2 (4)

The ripples in the output voltage as well as current should be low for better performance [10-12]. For desired performance parameters, the value of inductor used can be derived as [10],

𝐿 =𝑉𝑑𝑐/2𝐷(1−𝐷)

𝑓𝑠∆𝑖 (5)

Where D is the duty ratio of the switch, Vdc/2 is the battery potential difference, fs is the switching frequency of the switch, Δi is the maximum current ripple allowed in the inductor, Δvo is the maximum allowed ripple in output voltage. The output voltage ripple is equal to the capacitor voltage ripple. The value of capacitor can be derived as [10],

𝐶 =∆𝑖

8𝑓𝑠∆𝑣𝑜 (6)

The values of inductor and capacitor for buck converter are calculated for load current 1A, Δvo=5%, Δi=10%, Vdc/2=12V, switching frequency=1000 Hz, duty ratio of switch=0.5 . Therefore, Δi= 0.1A and Δvo=0.3V. The parameter values obtained from (5) and (6) are L= 30mH and C=42µF.

Hybrid Multi Output Converter for Microgrid Applications 605

Following parameters (Table I) have been used for simulation

Table I. Parameters used for simulation

Parameters/Components Attributes

Input Voltage (𝑉𝐷𝐶 2⁄ ) 12 V Switching Frequency (𝑓𝑠) 1 kHz

Inductor (L1 and L2) 30 mH Capacitor (C1 and C2) 42 µF

dc Load 1 6 Ω dc Load 2 6 Ω ac Load 12 Ω

MATLAB/SIMULINK has been used for simulation work. Open loop operation has been developed which can be extended to closed loop operation, in case of tightly regulated output supply using weighted voltage mode control strategy.

The simulation results are shown in Fig.7. From Fig. 7, we can observe that when switch S3 and S4 are ON, the voltage across dc load 1 is increasing, voltage across dc load 2 is decreasing and negative half cycle of ac voltage is obtained. When switch S1 and S2 are ON, the voltage across dc load 2 is increasing, voltage across dc load 1 is decreasing and positive half cycle of ac voltage is obtained.

Fig.8 shows the output voltage and current of buck converter 1. From this figure, we can observe that the ripples in current and voltage are within the desired range. The voltage ripple is less than 0.3 V, and current ripple is less than 0.1 A.

Fig. 7. Simulation output with design values

0.06 0.0605 0.061 0.0615 0.062 0.0625 0.0635

5.5

Time (Seconds)

Vo

lta

ge

(V

)

0.06 0.0605 0.061 0.0615 0.062 0.0625 0.0635

5.5

Time (Seconds)

Vo

lta

ge

(V

)

0.06 0.0605 0.061 0.0615 0.062 0.0625 0.0630

0.51

1.5

Time (Seconds)

S3

-S4

0.06 0.0605 0.061 0.0615 0.062 0.0625 0.0630

0.51

1.5

Time (Seconds)

S1

-S2

0.06 0.0605 0.061 0.0615 0.062 0.0625 0.063-20

0

20

Time (Seconds)

Vo

lta

ge

(V

)

DC Load 1 Output Voltage

DC Load 2 Output Voltage

Gating Pulses of Switch S3 and S4

Gating Pulses of Switch S1 and S2

AC Load Output Voltage

606 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar

Fig. 8. Output of upper buck converter

V. SMALL SIGNAL ANALYSIS OF PROPOSED CONVERTER

State space averaging is used to describe a circuit that changes over a switching cycle. Two sets of state equations: one set for the switch closed and other set for switch open are averaged over the switching period [10,13,15]. Steady state and small signal analyses are separated by the assumption that the variables are perturbed around the steady state operating point. In the proposed topology, there are two buck converters. The small signal analyses is shown for one of these identical buck converters. The state variables are inductor current and voltage across capacitor. For switch closed, the state space representation is

�̇� = 𝐴1𝑥 + 𝐵1𝑉𝑆 (7) 𝑣𝑜 = 𝐶1

𝑇𝑥 (8)

Where , from [13],

𝐴1 = [

−𝑟𝐶𝐿

−1

𝐿1

𝐶

−1

𝑅𝐶

]

𝐵1 = [1

𝐿0

]

𝐶1

𝑇 = [𝑟𝐶 1] Where rc is the equivalent series resistance (ESR) of the capacitor, L is the inductance in the cuck converter circuit, C is the capacitance of the buck converter circuit, Vs is the supply voltage, vo is the output voltage, and d is the duty ratio of the switch. For switch open,

�̇� = 𝐴2𝑥 + 𝐵2𝑉𝑆 (9)

0.06 0.061 0.062 0.063 0.064 0.065 0.066 0.0673.5

4

4.5

5

5.5

6

Time (Seconds)

Vo

lta

ge

(V

olts)

0.06 0.061 0.062 0.063 0.064 0.065 0.066 0.067

0.5

0.6

0.7

0.8

0.9

Time (Seconds)

Cu

rre

nt (A

mp

ere

s)

Current Ripple < 0.1 Ampere

Voltage Ripple < 0.3 V

Hybrid Multi Output Converter for Microgrid Applications 607

Here, A1=A2 , B2=0, C1T=C2

T. Weighing the state variables over one switching period gives

𝑥�̇� = 𝐴1𝑥𝑑 + 𝐵1𝑉𝑆𝑑 (10) And,

�̇�(1 − 𝑑) = 𝐴2𝑥(1 − 𝑑) + 𝐵2𝑉𝑆(1 − 𝑑) (11)

The steady state output is Vo=VsD, where Vo, Vs and D are the steady state values of output voltage, supply voltage and duty ratio respectively. The small signal transfer characteristic is developed as

�̇̃� = 𝐴�̃� + 𝐵𝑉𝑆�̃� (12)

𝑣�̃� = 𝐶𝑇�̃� (13) Where x̃, d̃, . o, x̃̃̇ represent the small signal values. The transfer function of output to variations in the duty ratio (when rc<<R) is obtained as [12],

𝑣�̃�(𝑠)

�̃�(𝑠)=

𝑉𝑆

𝐿𝐶[

1+𝑠𝑟𝐶𝐶

𝑠2+𝑠(1 𝑅𝐶⁄ +𝑟𝐶 𝐿⁄ )+1 𝐿𝐶⁄] (14)

For the design values obtained in Section IV, i.e. L=30mH, C=42µF, R=6Ω, rc=0.5Ω, Vs=12V,

𝑣�̃�(𝑠)

�̃�(𝑠)=

200𝑠 + 9523810

𝑠2 + 3985𝑠 + 793651

For L=10mH, C=10µF, R=50Ω ,rc=0.5Ω, Vs=12V,

𝑣�̃�(𝑠)

�̃�(𝑠)=

600𝑠 + 120000000

𝑠2 + 3985𝑠 + 793651

The bode plots for above transfer functions are shown in Fig.9. The Phase margins are 68.2 deg and 14.4 deg, with gain crossover frequecies of 2.18e+03 rad/sec and 1.13e+04 rad/sec for L=30mH, C=42µF and L=10mH, C=10µF respectively.

608 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar

Fig. 9. Bode plots for different parameters.

VI. EXPERIMENTAL SETUP

The experimental setup for realizing the proposed converter is shown in Fig.10. An experimental power electronic bed consisting of six IGBT legs with twelve IGBTs and six power diodes is used. Four IGBTs and two diodes have been used for realizing the proposed converter circuit. Capacitor is not connected across load. The gating signals for the IGBT’s have been generated using TMS320F28335 digital signal controller [14]. However, the output of Digital Signal Controller is not sufficient to drive the IGBT’s. Thus, a TLP 250 based driver circuit has been used to enhance the voltage level of gating pulses and to isolate the controller from the power circuit. Following parameters (Table II) have been used for experimental study.

Table II. Parameters used for experimental study

Parameters/Components Attributes

Input Voltage (𝑉𝐷𝐶 2⁄ ) 12 V Switching Frequency (𝑓𝑠) 1 kHz

Inductor (L1 and L2) 10 mH dc Load 1 50 Ω dc Load 2 50 Ω ac Load 50 Ω

The gate signals generated by the driver circuit are shown in Fig.11. The switches operate in pairs (S1-S2 and S3-S4) with a duty ratio of 0.5 . While giving gating pulses

-100

-80

-60

-40

-20

0

20

40

Ma

gn

itu

de

(d

B)

Bode Diagram

Frequency (rad/s)101

102

103

104

105

106

107

-180

-135

-90

-45

0

Ph

ase

(d

eg

)

L=30mH,C=42uF

L=10mH,C=10uF

PM: 68.2 deg

PM: 14.4 deg

Hybrid Multi Output Converter for Microgrid Applications 609

to the switches, to avoid short circuit, a dead-band of 10 micro-seconds is given between alternating between the two pair of switches. The experimental output of proposed circuit is shown in Fig.12. Two chopper outputs (pink and blue), and one inverter ac output (yellow) are obtained. The frequency of ac output obtained is 1 kHz.

Fig. 10. Experimental Setup

Fig. 11. Driver circuit output

610 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar

Fig. 12. Experimental Result VII. CONCLUSION

In this paper, a single stage hybrid multi output converter is proposed. The desired design values of inductor and capacitor are calculated. The simulation of the above converter is shown using MATLAB/SIMULINK for the calculated design values. The simulated output is in accordance with the design parameters. The small signal analysis of the proposed converter is also shown. For the experimental setup, gating pulses are generated using TMS320F28335 Digital Signal Controller, and the converter experimental results are obtained. This converter can be used for domestic/industrial applications, where both dc and ac loads are present. The ac output is at a high frequency, which is suitable for applications requiring high frequency ac input, like induction heating etc. REFERENCES

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[12] Ma D., Ki W.H., and Tsui C.Y., “A pseudo-CCM/DCM SIMO switching converter with freewheel switching,” IEEE J. Solid-State Circuits, vol. 38, no. 6, pp. 1007–1014, Jun. 2003.

[13] Hart D.W., “Power Electronics”, 1st Edition, McGraw-Hill, 2010 [14] “TMS320F28335, TMS320F28334, TMS320F28332 TMS320F28235,

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612 Syed Bilal Qaiser Naqvi, Digvijay Singh and Adil Sarwar