A Multi-Input Single-Control (MISC) Battery Charger for DC N anogrids

Arun Sankar U. Student Member, IEEE, Soumya Shubhra Nag, Student Member, IEEE, and Santanu K. Mishra, Senior Member, IEEE

Dept. of Electrical Engineering, Indian Institute of Technology Kanpur, U.P., India arunsu@iitk.ac.in, soumyasn@iitk.ac.in, santanum@iitk.ac.in

Abstract- Renewable power and storage have made DC

based domestic distribution an attractive alternative for future

homes. Due to low power rating of the system, it is very

important to reduce number of converter stages and use the

generated energy efficiently. Therefore, instead of using different

dedicated converters from various uni-directional renewable

sources, this paper proposes a single stage boost converter with

multiple inputs that can efficiently decimate generated energy to

charge a battery. This philosophy of interfacing renewable

sources will have commercial value when some of the additional

sources are not large enough to mandate a dedicated converter.

The converter is called Multi-Input-Single Control (MISC)

converter. The converter varies the duty cycle based on optimum

operation of the largest power source (e.g., MPP in case of solar

panel), where as the other smaller unidirectional sources act as

slave. As per the characteristic of the source, the proposed

converter works under various operating modes which are

discussed in this paper. The concept is validated using a

laboratory prototype for different operating scenarios with a

solar panel as a master source working under MPPT.

Keywords-DC Nanogrid, Multi Input Single Control(MISC),

Battery Charging ,Boost Converter,Multi-port

I. [NTRODUCTION

DC nanogrid is a small residential supply system (at hundreds of Watts to few kWatts level) which uses a DC voltage based power distribution to power various domestic loads. The reason DC based nanogrids are an attractive alternative for future domestic power system design is due to the inclusion of more and more renewable and emerging power sources for domestic consumption. Fig. I shows the block diagram of a DC nanogrid in standalone application. An input solar source (Vp) is interfaced to a DC distribution bus (Vd) using a DCIDC converter. It uses a DC based distribution with various loads connected to it through dedicated point-of-Ioad converters [ 1-4]. The idea is basically inspired from computer power system. The renewable power sources are generally DC. Due to the intermittent nature of renewable sources, even though the generated power is AC or DC (e.g., solar power is DC and wind power is AC), they are converted to a DC and interfaced with storage before further processing [5].

For continuity in supply of power with renewable sources, a storage element is a must. [t is advantageous to make the distribution voltage and battery voltage to be the same in a small power system like a nanogrid to improve efficiency of power usage. This will also eliminate an additional stage of conversion between the battery and the DC nanogrid. Another

978-1-4799-0482-2/13/$31.00 ©2013 IEEE 304

factor that improves efficiency is: more the distribution voltage (Vd) less is the current for the same power level. Therefore, a boost stage between the solar panel and distribution bus is advantageous.

In [ 1], a 380 V based distribution is used and in [2-4] a 144 V based distribution is used in realizing the nanogrid. The choice of voltage level is dependent on the power level of the nanogrid and available area for renewable installation (size of a house, etc.). Never the less, in most implementations, a boost converter is used to interface the source with the nanogrid. This is because, from MPPT point of view, in a smaller installation, the solar panels (generally, with open circuit voltage of 12-36 V) are not preferred to be connected in series. They are paralleled as this arrangement has less impact on power generation [6]. An important distinction is that a commercial grid connected renewable sources (e.g., solar farms) are always connected in series to produce a high voltage which is used by the VSI to produce the grid voltage. This need not be done for DC based Nanogrids.

From load point of view, most future loads are compatible with DC, e.g., LED lights, Flat panel TV. [n summary, a DC nanogrid is preferred because: (a) Renewable sources are DC (b) future loads are more compatible with DC supply.

Solar Panel

Vbat Vd (DC Bus)

Fig. 1. Concept of DC Nanogrid

Future DC based nanogrid will have various sources interfaced to the DC-distribution bus as shown in Fig. 2. Unidirectional Renewable Sources are interfaced directly to the converter from which power is supplied to the distribution bus. When bi-directional sources are used it is directly interfaced to the distribution bus through a dedicated converter. Renewable Sources have varied characteristics, e.g., a solar array is a current source and a fuel cell is a voltage source [7]. In order to interface all these sources to a single domestic supply grid, a converter has to be interfaced between the respective nodes. The converter takes care of the regulation and protection which arise because of the interfacing.

[n [8], a multiport converter is described which uses multiple boost converters with common output as interfaces between various sources and the DC bus. [n [9], a multi-input series output concept is used to realize this interface which works at ZVS. [n [ 10], the authors use various sources with its full bridge and the output of the full-bridges are connected to an AC link and then rectified. All the converters used to interface the sources and the grid are individually controlled.

VI

Mise

Fig. 2. Multi-Input Converter Application

Most of the researches have addressed the multi-source power utilization problem by using individually controlled converters. That means, the source and grid have a dedicated converter between them. This option is quite good for higher power installations. However, in a nanogrid where the power generation is limited and the sources have limited output, these options may not be the most efficient ones as more converters will degrade efficiency and reliability.

This paper looks at an alternate circuit to realize a power electronic interface for nanogrids, with multiple unidirectional input sources and a single output. The converter structure is realized using a single master control with multiple sources of relatively smaller power rating. The output of the converter is directly connected to a battery to save an additional stage as well as MPPT control. Thus, the battery bus and the DC distribution bus are the same.

The organization of paper is as follows Section II gives an overview of general topology of proposed Converter. Section III describes the possible modes of operation due to interfacing of different input sources. Section [V and V describes the use of converter in nanogrid applications and basic design criteria. [n Section V[ experimental verification of proposed topology is done. Section VII presents some concluding remarks of this paper.

[I. MULTI-[NPUT SINGLE CONTROL (M[SC) TOPOLOGY

The overall topology is derived from boost Converter as shown in Fig. 3. There is a Master source S 1 (can be voltage or current source) and N-l slave sources S2 to Sn are interfaced to the drain of the control switch through dedicated inductors. The converter topology is referred to as Multi Input Single Control (MISC) due to the fact that the master source is always in control of the duty. The Master Source S 1 fixes the duty cycle of switch Ml based on the MPP operating point of the input source, if input is a solar panel. [n this case the current is used to charge the battery as well as supply the loads connected to this bus. [n case of a voltage source, the output current of the boost can be controlled. The loads are connected to the battery

305

bus directly or through an interface converter. As the converter topology is based on boost converter output voltage is always greater than input voltage for the operation of the converter.

Ln �� ysn

T : NSets

�t I ' rm�f:�

!t�" T� ---19"' :.- - -' .. - -- . - -.1 !

:Master Source -:-o

nVbat ""

� � J I

Fig. 3. Multi Input Single Control (MISC) Boost Converter

Depending upon the characteristics of the sources connected to the converter it can be classified either as a voltage or a current source. The possible combinations for a two input MISC Converter with current and voltage sources are as shown in Fig. 4.

Fig. 4. (a) Current and Voltage Source Inputs, (b) Current and Current Source Inputs, (c) Voltage and Voltage Source Inputs, and (d) Voltage and Current Source Inputs.

[t is important to note that all the common renewable sources are unidirectional in nature and should be interfaced with a diode. When a current source is interfaced to the converter, a capacitor is used at the input terminal as can be seen from Fig. 4. This capacitor is required to meet the ripple requirements of the converter input current. When the source has a voltage source property, an input capacitor is avoided as it will invariably force the capacitor voltage to become equal to the master source terminal voltage due to duty constraint.

III. MODES OF OPERATION

The different modes of operation of M[SC converter is as outlined in Table I. All the modes given in Fig. 4 are valid if the basic constraint of output voltage being greater than input voltage is satisfied. Note that the output of the converter is

connected to a battery in parallel to a resIstIve load. The resistive load represents the load on the DC bus. All the operating modes explained below are very much dependent on the design of input inductor. This design philosophy will be covered in the next section.

TABLE!. MODES OF OPERATION

Mode Modes of Operation

Master Source Slave Source I (CSVS) Current Source(CS) Voltage Source(VS)

II (CSCS) Current Source(CS) Current Source(CS)

III (VSVS) Voltage Source(VS) Voltage Source(VS)

IV (VSCS) Voltage Source(VS) Current Source (CS)

A. Mode I: Current Source and Voltage Source(CSVS)

As shown in Fig. 4 (a), when a current source is the master source and voltage source acts as a slave source there are two operating scenarios possible which is explained below as two sub-modes. The duty ratio for these modes is fixed by the current source.

1) Sub-Mode l(A): VI > V2: When the input voltage of the master source is greater than that of the slave sources there will be 3 operating intervals for MISC converter, as shown in Fig. 5 (a), Fig. 5 (b), and Fig. 5 (c). For a properly designed inductor, the current through the inductor (LI) of master source will be in CCM. Therefore, for a similar or smaller value of inductor (L2), the current from voltage source is forced to be in DCM. Assuming the L2 to be smaller is

reasonable as the power rating of the slave source is not high. The inductor Current waveforms ILl and IL2 are as shown in Fig. 6 (a).

L2 L2

� �;-� fD�-��l J�; }�?tD]J �D�

iJ>1:n�

I 1 j T I I 1 j T I � w � 00

L2

fD2·-""'��� -do- Ll

rD-;T?'" -;-rl '" i� TV1 .Ml :1 �� I 1 I T I

o (e) Fig. 5. Operating Intervals when Current and Voltage Sources are used: (a) when Ml is on,(b) when Diode D is on, and (c) Input Diode D2 is ofr

2) Sub-Mode l(B): VI :s V2: When the master source terminal voltage is equal to that of the slave source, both the input currents are ideally in CCM. However, this may not be possible all the time, as the second source is assumed smaller.

306

This may lead to the second source working under current limit. Similarly, if slave source voltage is larger, for same duty cycle as the master, its current will invariably reach current limit. Therefore, all that can be said about this mode is that, in a practical scenerio, the slave source works under current limit.The inductor current waveform IL2 indicated in Fig. 6 (b) indicates the state of operation before the source current reaches its limit. When the slave source reaches current limit the inductor current waveform depend on the charecteristics of source such as type of source, source impedance etc.

celVI C1U'roe-nt

(a) (b)

Fig. 6. Inductor current Waveforms (a) Sub-Mode I(A), and (b) Sub-Mode 1(8). B. Mode II: Current Source and Current Source(CSCS)

In this mode both inputs of the two input MISC converter are connected to current sources (shown in Fig. 4 (b)).The terminal voltages VI and V2 will be forced to become equal by the duty ratio of the switch controlled by master source. The equivalent circuits under this mode of operation are as shown in Fig. 7 (a) and Fig. 7 (b). For normal operation of MISC converter the inductor current waveforms are as shown in Fig. 8 (a).The normal operation of the converter is valid when input inductors and capacitors are selected properly. The selection criteria of inductors and capacitors will be discussed later in section IV.

Fig. 7. Operating Intervals when both inputs are current sources: (a) when Ml is on, and (b) when Diode D is on.

Fig. 8 (b) shows the operation of the converter under high ripple due to the improper selection of the inductor and capacitance value. It can be seen that the current waveform IL2 has a negative portion. The average current drawn from the current source 12 is positive but the negative portion of the current indicates that inductor current ILl is charging the input capacitor of slave source 12 every switching cycle. Ideally current IL I drawn from the source should be transferred only to the output battery and therefore this operating scenario is not efficient. To prevent this mode of operation the proper selection of Inductance and capacitance value is mandatory.

CCMCllli'Pllt

(,) (b)

Fig. 8. Inductor Current waveforms (a) Normal Operation, and (b) High Ripple operation.

C. Mode III: Voltage Source and Voltage Source(VSVS)

The inputs of MISC converter are connected to voltage sources as shown in Fig. 4 (c). In this mode of operation there are two sub-modes possible which are given below. The duty ratio for this mode is fixed by the master source.

1) Sub-Mode lll(A): VI > V2: When input voltage of master source is greater than the slave source voltage there will be three operating intervals as shown in Fig. 9 (a), Fig. 9 (b), and Fig. 9 (c). The slave voltage source will be forced to operate in OCM as the gain required is more. As the power generated from the source is not very high, interfacing a separate converter for power extraction is not necessary and OCM operation can be used for extraction of the available power. The input current waveforms is as shown in Fig. 10 (a).

L2 L2

��t fn;��� � Ll � Ll

� �;-��'" D en "'a! '; + --r�-��""

----+;n.t '; VI MI � � � .MI .

_ �

l j J I -l- T I 6 W -- 00

12

� .���

+ D2 2

� Ll . 0 --o __ "'nT'.c.::! __ :'w ----+ .... nat + IDl ii? D

"'"

� .!VIl ._

� 1- j T I

(c) Fig. 9. Operating Intervals when both inputs are voltage sources: (a) when MI is on, (b) when D is on, and (c) when input diode D2 is off.

2) Sub-Mode III(B): VI ::; V2: In this sub-mode of operation the input inductor currents I L I and I L2 ideally should operate in CCM.The corresponding operating intervals of MISC converter is as shown in Fig. 9 (a) and Fig. 9 (b). As the duty ratio is fixed by master source, when input voltage of master source is less than or equal to the slave source voltage it will have a higher gain than required due to higher duty ratio. Output current from slave source increases till it reaches

307

its current limit which is the equilibrium point of operation for the source. The input inductor IL2 current waveform before the slave source reaches the current limit is as shown in Fig. I 0 (b). When source reaches the current limit the operating waveform will depend of source characteristics such as source type,source impedance etc.

CC�II CUl'l'E'nt CC�'I CUlTPlIt

(,) (h)

Fig. 10. Inductor Current Waveforms (a) Sub-Mode III(A), and (b) Sub-Mode 111(8). D. Mode IV: Voltage Source and Current Source(VSCS)

When a voltage source is the master source and a current source acts as a slave (as shown in Fig. 4 (d)), the duty ratio of switch Ml is controlled by voltage source. The terminal voltage V2 of the current source will be forced to be equal to terminal voltage VI and the current that can be supplied by the source is drawn from the source. There will be only two operating intervals as shown in Fig. 1 1 (a) and Fig. 1 1 (b).

Fig. II. Operating Intervals when voltage and current sources are used: (a) when M I is on, and (b) when D is on.

Under Normal operating conditions, when the input inductor and capacitors are selected properly, both the converters work in CCM operation and the input inductor current waveforms are as shown in Fig. 12 (a).

Fig. 12. Inductor Current Waveforms (a) Normal operation, and (b) High ripple mode of operation.

Under high ripple operation which is caused when low value of inductance is selected and proper value of capacitance is not used to maintain the ripple the input current waveforms are as shown in Fig. 12 (b). This mode of operation is not preferred as the there is a negative portion of current which

indicates the charging of the slave source input capacitor by master source which reduces the amount of current flowing to the output.

IV. DESIGN OF MISC COMPONENTS

The design of components is done for the operating modes Mode I and Mode II which is of primary importance in nanogrid interconnection of the MISC converter. This is due to the fact that solar panels working under MPP are assumed to be the master source and any other smaller sources work as slave. The design procedure described below is a general procedure and hence can be used for all modes of operation.

A. Selection of input inductor Ll

The ripple in main inductor is kept as low as possible so as to operate the converter in CCM operation for different insolation levels of solar panel. As the ripple is reduced the value of inductance required increases. So a tradeoff between the value of inductance and ripple is necessary. The inductor is designed at the MPP point of solar panel hence Vin and IL 1 for a particular insolation level is known.

Let �h represent the ripple in inductor current I L 1.

;}'1I = x * ILl 100

Where x is the ripple percentage of current IL I Input inductance LI is calculated using (2)

Where,

LI = Vin* D*T, MI

Vin represents the MPP voltage of master source

Duty ratIo, D = 1 - -

. (VlI1) Viml

Vbat- Terminal voltage of battery

Ts- Time period of switching pulse.

B. Selection of input capacitor Cl

The selection of input capacitance is based on (4)

Where,

CI= MI*T, 8*�V

�h is the ripple in inductor current given by ( 1)

Ts- Time period of switching pulse

. y * Vi,,· Input Voltage npple �V = --

100

(I)

(2)

(3)

(4)

(5)

V oc is the open circuit voltage of panel, y is percentage voltage ripple

C. Selection of input inductor L2

The cost involved in making of inductor forces the use of lower value inductor which causes more ripple in input current. As the value of inductance is reduced we get less no of turns

308

and lower size of inductor. Using a lower inductance value can cause the operation of source in DCM which is allowed as the power level of slave source is not high.

From ( 1) we can get the amount of ripple in current based on the value of x used. To reduce the inductance value amount of ripple is increased by using higher value of x (50% ).F or this higher value of ripple inductor is designed based on (2). Where Vin = V2 the terminal voltage of slave source, as the duty ratio is fixed by master source the value of 0 from (3) is used.

D. Selection of input capacitor C2

The input inductance L2 used is reduced as the cost of making inductor is higher than the cost of capacitance used. As the inductance value is reduced the capacitor should be able to sustain more amount of ripple. To sustain more current ripple higher capacitor value has to be used which is calculated using (4). Higher inductance causes high dc series resistance (OCR) with inductor which increases loss in system. If we use higher capacitor value it will reduce the effective series resistance (ESR) as capacitors are connected in parallel.

E. Selection o{Switches

The main switch Ml and Diode D is rated to carry l.5 times (factor of safety) the sum of peak currents of all the source currents.

V. MASTER SOURCE CONTROL

As DC nanogrid is a small residential supply system which uses DC voltage based power distribution to various domestic loads the major supply sources used will be renewable energy sources which are interfaced to MISC converter. The source with highest power rating will be designated as the master source and all other low power sources will be used as slave sources.

When MISC converter is used to supply power in DC nanogrid the master source is selected as a solar panel which is modeled as a current source. The characteristics of solar panel are as shown in Fig. 13 for a particular solar insolation level. The point represented by MPP (Maximum Power Point) is the ideal point of operation of the solar panel.

1, P I -Venne Ise +

Voe

Fig. 13. Characteristics of Solar panel

With the variation in insolation level MPP shifts and hence a controller is necessary to track the shifting point. The tracking of MPP point is done by the varying the duty cycle of

M[SC converter. There are various methods of tracking MPP available in literature. The tracking of MPP is done with the help of Perturb & Observe (P&O) algorithm. The MPP point is calculated using P&O algorithm by observing the change in power for the variation of duty cycle. The direction of change of duty cycle is based on the direction of change in power.

The slave sources that can be connected to M[SC are as shown in Fig. 2. For Mode I operation a low power fuel cell can be interfaced as slave source. Depending on the terminal voltage sub modes I (A) or I (B) exists as the operating mode.

For Mode II operation a lower power solar panel is interfaced as the slave source to the converter. If the panel is selected such than the MPP of slave source matches the MPP of master source in normal insolation level we can extract maximum power from both the panels with a single control switch. The input inductor and capacitor value is properly designed so as to prevent high ripple operation of the converter.

VI. EXPERIMENT AL RESULTS

An experimental prototype is developed to test Mode [ and Mode II operation of MISC as shown in Fig. 4 (a) and Fig. 4 (b) respectively. The component specifications of prototype are as shown in Table [I. The master source used is a solar panel. MPPT is implemented using a digital controller. A second voltage source or a solar panel is used as the slave source depending on the mode of operation under test.

TABLE II. COMPONENT SPECIFICATIONS USED IN PROTOTYPE

Component Specifications Used in Prototype Parameter Attributes

LI 1.35mH

L2 620uH

2 Input MISC Cinl,Cin2 820uF Converter

Co 3.3 mF

F switching 10 kHz

Vbatl Rating 24V/2*12 V, 32Ah

battery in series

The pnmary solar panel rated 34WNoc=2 1V and Isc=2.IA (at STC) is used. P&O algorithm is implemented on a Xilinx FPGA (Spartan 3A Board) during testing. Fig. 14 (a) and Fig. [4 (b) shows the MPP test results. Two black panther 32Ah, 12 V batteries were used in series at the output.

fl

' [r�'� : Power..I XTracted � .k� ,?pera.ringPoinT �. ClIPP) . . . . . .. ...... , ..... , ..... , .................. , ..... ,................... .. p l ' . . I--�������--�� ��I--�. �--������ Panel Current

Panel roltage rr Chi 10.0V � 1.0011 M,20.0ms Al. Ch3 f 5.2011 Math c::::Jl).ovv tt!i!ill

(,) � c:::TIi.ovv IOOmsi

M1100msJ Al. ChJ f 4.80 V

(b)

Fig. 14. Master source under MPP operation. (a) Startup waveform, and (b) Operating point on PV characteristics.

309

The experimental results obtained for Mode [ (A) operation as represented in Fig. 4 (a) is shown in Fig. 15 (a) and Fig. 15 (b). The master source is a solar panel under MPP operation giving a terminal voltage VI = 14 V at MPP operating point and the dc source used has a terminal voltage of V2 =[0 V. As explained in Section III under Mode I (A) operation the current through slave source operates in DCM. The DCM operation of slave source provides the higher gain requirement.

When two solar panels are interfaced to M[SC converter as shown in Fig. 4 (b ).Mode II operation is observed as explained in Section III. The passive components used and the power ratings of the master and slave panels are as indicated in TABLE III.

TekSto

Chi 20.0 II �Ch2 10.0 \I M,40.01olS A[ eM f 2.12 A ChJ .0

0 .... FT60. ooons (,) Fig. 15. Mode LA operation. (a) Master source waveforms, and (b) Slave Voltage Source waveforms.

TABLE III. MODE II OPERATION PARAMETERS

Mode II Operation Parameters Normal Mode High Ripple

Master Source : PMPP = Master Source : PMPP = 29. 8 W, IMPp=1.85 A 17.36 W, IMPP=1.24 A

2 Input MISC Slave Source : Slave Source : P2-4.2

Converter P2=25.43 W, h=I.62 A W, h=0.26A

Ll=1.35 mH Ll=1.35 mH

L2=620 /lH L2=620 /lH

C1=C2=820 /lH C1=C2=820 /lH

Under Mode II operatIOn If the mductor and capacitor values are properly selected it results is normal operation mode which is proved experimentally in Fig. [6 (a) and Fig. [6 (b). Both the inductor currents are in CCM and maximum power is drawn from the master source which is under MPP operating point. The slave panel supplies the power that can be extracted at the particular point.

(,) (b)

Fig. 16. Mode II Normal operation. (a) Master Source waveforms, and (b) Slave Current Source waveforms.

[f the inductor value used is very low which results in high ripple current, a high input capacitor has to be used to supply the required ripple. [f the selection of capacitor value is not proper it results in high ripple mode of operation which is as seen in Fig. [7 (a) and Fig. 17 (b). The current [L2 passing through slave source has a negative portion which indicates the flow of current through inductor in opposite direction which is facilitated by the presence of input capacitor.

Th�k S�"� __ ������ ____ � �' �k S�"� __ ������ ____ r-,

I " "-LrU

Fig. 17. Mode II High Ripple Operation. (a) Master Source waveforms, and (b) Slave Current source waveforms.

VII. CONCLUSION

This paper proposed a Multi-Input-Single Control (MISC) converter. [t accepts multiple inputs and boosts the voltage to charge a battery using a single control. A fixed duty cycle based on the highest power source (Master source) characteristics is used to control the circuit. The entire smaller sources act as slave and supply the power that can be extracted from it. Depending upon the interfaced source characteristics different modes of operations are possible. The modes are explained and the concept is experimentally validated using a prototype for different operating scenarios with a solar panel as a master source.

ACKNOWLEDGEMENT

The Authors would like to thank Department of Science and Technology (DST), Government of India for providing funding to support the work presented in this paper under the grant no. SRlS3IEECE/0 187/20 12.

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