wineyard.in_Abstract_mtech_Electronics_bp_14PE3.pdf

The 2014 International Power Electronics Conference

Coupled Inductor Based Current-Fed Switched

Inverter for Low Voltage Renewable Interface Soumya Shubhra Nag and Santanu Kumar Mishra

Department of Electrical Engineering, Indian Institute of Technology, Kanpur, V.P., India

E-mail:- [email protected] and [email protected]

Abstract: This paper presents a novel coupled inductor based

high boost inverter topology which can be utilized in low voltage

renewable systems where high voltage step-up is needed to

interface with 110 Vt220 V AC systems. The proposed inverter

possesses high boost ability with superior EMI immunity

compared to a traditional voltage source inverter (VSI). Unlike

the traditional VSI, the proposed inverter does not need dead­

time circuit for its switching signals as it utilizes shoot-through

state of the inverter in its single-stage configuration. Insertion of

shoot-through state also helps it to achieve high boost operation

essential for renewable energy applications. The proposed

inverter is derived from Current-Fed Switched Inverter topology.

Apart from topology derivation, this paper describes the steady­

state analysis of the inverter and establishes the relation between

input, DC-link, and AC output. An experimental prototype is

built to validate the proposed inverter circuit. A 220 V (rms) AC

is obtained from 52 V DC input to demonstrate its boost mode of

operation.

Index Terms - ZSI, SBI, Coupled inductor, Shoot through state,

EMI immunity.

I. INTRODUCTION

Voltage source inverters are widely used in UPS, motor drives, grid connected and stand-alone renewable systems, etc. The main limitations of traditional VSI are:

1) The output AC voltage cannot be more than its input DC voltage as VSI is a buck inverter. Due to this reason a DC­DC boost converter stage is needed prior to the VSI to achieve step-up DC-AC inversion when the input DC voltage is limited like in the case of solar PV, fuel cell, etc. Commercially available solar PV panel voltage ranges from 12 V to 48 V typically whereas for fuel cells, it is typically between 24 V to 56 V. For this reason, a high step-up inversion is needed to connect the renewable sources to 110 V / 240 V AC systems which cannot be obtained from a VSI.

2) The upper and lower switching devices of any leg of the VSI cannot be turned on simultaneously thus requiring for a dead-time circuit which in turn contributes to waveform distortion. Although, adding dead-time in the switching signals cannot alleviate the chances of mis-gating or shoot­through due to spurious signals or EM! noise [1].

To eliminate these drawbacks of VSI, inverters like Z­Source Inverter (ZSI) [1], Quasi-ZSI (q-ZSI) [2], Switched Boost Inverter (SBI) [3]-[5], Boost-Derived Hybrid Converter (BDHC) [6], Trans-ZSI (T-ZSI) [7], etc., were proposed. These new-age inverters present single stage DC-AC inversion with high boost capability and utilize the shoot-through phenomenon in the inverter legs to provide superior EMI immunity. In the lines of these inverters, Current-Fed Switched Inverter (CFSI) was proposed [8]-[9] which

978-1-4799-2705-0/14/$31.00 ©2014 IEEE 3587

provided high gain (same as ZSI) with the low passive component count. Due to the presence of input inductor, CFSI provided continuous input current property which is necessary for renewable applications. In all of the above mentioned inverters, shoot-through state imposes some restriction on the modulation index which limits them to achieve high overall input DC to output AC gain. Thus, in recent years, there is a constant effort among the researchers to increase the overall DC-to-AC conversion ratio of these shoot-through inverters by

a) Modifying the pulse width modulation scheme so that the constraint on modulation index can be minimized. It has resulted in invention of new modulation techniques like Constant Boost Control, Maximum Boost Control schemes, etc.

b) Improving the boost factor (input DC-to-inverter input gain) of the inverters by using either passive network (switched capacitor, switched inductor etc.) or magnetic (tapped inductor, coupled inductor etc.) network.

Nevertheless, inverters with low component count, continuous input current, low device stress are always an attractive option owing to their high efficiency, ease of integration with renewable sources, low device cost and device footprint.

This paper presents a coupled inductor based high boost inverter topology derived from Current-Fed Switched Inverter (CFSI) which is named as Coupled Inductor based Current-Fed Switched Inverter (Trans-CFSI) as it utilizes energy transfer through the transformer action of the coupled inductor to achieve high boost. Like SBI, the proposed inverter uses an active network between the DC input and inverter bridge with one LC-filter pair. In the next section, CFSI topology is reviewed. Derivation of Trans-CFSI topology from CFSI is discussed in section III along with its steady-state characteristics. In section IV, the PWM control scheme of Trans-CFSI is described. The proposed inverter is verified with experimental results in section V. Section VI presents some conclusions.

II. REVIEW OF CFSI TOPOLOGY

The circuit schematic of Current-Fed Switched Inverter (CFSI) is shown in Fig. 1 (a). CFSI provides high-boost operation similar to ZSI and q-ZSI utilizing the shoot through state of the inverter legs. The operating states of the CFSI can be broadly categorized into i) Shoot through state and ii) Non­Shoot through state, the later can be further be divided into active state (power interval of the inverter) and zero state (zero interval of the inverter). The equivalent circuit of the CFSI is shown in Fig. 1 (b). In the shoot through interval (or duty

interval D) switch S is turned on along with both the switches of any inverter leg. In this interval source V g and capacitor Co charges inductor L together. In non-shoot through interval ((1-D) interval or D' interval), switches S is turned off which forces diodes Da and Db to tum on, and the inductor charges Co and power is delivered to the AC-Ioad through the inverter. The equivalent circuits of CFSI in D and D' intervals are shown in Fig. 1 (c). From Fig. 1 (c), the voltage across inductor L in one switching period of Ts is given by (1) (assuming small ripple approximation) from which the boost conversion ratio of CFSI can be obtained as shown in (2).

During D.T,

During(l- D).T,

L J�l Da Jfz + .... 1. -

iL +

+

(1)

(2)

L

VL - + iD: Co 1+

--==-J� J�

+ is J�

ic -=x:: 19 L------17.Jf'---3 ----'

(b)

v;

�

L

+ vL -+

J�

(c)

(a)

ic Co

ic Co

i;

Fig. 1. Schematic of (a) Current-Fed Switched Inverter (CFST), (b)

equivalent circuit of CFSI, and (c) equivalent circuit of CFSI in shoot though and non-shoot through state.

Although CFSI provides high boost output, use of shoot through state restricts the modulation index to a value always less than (1-D) in simple boost control method. This also imposes higher stress on the inverter switches. In the next section, a coupled inductor based CFSI topology (Trans-CFSI) will be derived which will mitigate the problems of CFSI as stated above.

III. DEVELOPEMENT OF TRANS-CFSI

TOPOLOGY

The coupled inductor based CFSI topology, namely Trans­CFSI, is shown in Fig. 2. It utilizes energy transfer through the transformer action of the coupled inductor to achieve high voltage boost which depends on the turns-ratio nj:n2.

The 2014 International Power Electronics Conference

3588

+

Fig. 2. Schematic of Trans-CFST topology

The equivalent circuit diagram of Trans-CFSI is shown in Fig. 3 which is obtained by replacing the two windings of the coupled inductor with an ideal transformer and a mutual inductance Lm on the primary side.

+

+ Co 1 J-c ic v;

• i; +

�Jg

Fig. 3. Equivalent circuit of Trans-CFSl.

In the shoot through duty interval (D interval), switch S is turned on with the inverter leg being shorted while both the diodes remain reverse biased (as shown in Fig. 4 (a)). The inductor voltages in this interval can be written as in (3). In the non-shoot through duty interval ((1-D) interval), switch S is turned off and the inverter operates either in active or zero state. In this interval both the diodes remain in conduction (as shown in Fig. 4 (b)). The inductor voltages in (1-D) interval can be written as in (4).

+ -=-v.. - '"

+ -= v.. '"

=n�1=== n�2 = L2

i;

(a)

(b)

Fig. 4. Equivalent circuit of Trans-CFST in (a) D-interval (shoot-through state), and (b) in (I-D)-interval (non-shoot through state).

VL, + VL2 =VL =Vg -Vc v =_n_1 -(V -V) During(l-D).T,

M g c nl +n2

(3)

(4)

Applying volt-second balance [10] using (3) and (4), the boost factor of Trans-CFSI can be obtained as,

(Vg +VJ.D+(_n'-(Vg -VJ] . (l-D)= 0 l n, +n2

� B Vc =_I_+_n_D __

Trans-CFS/ Vg 1-(2 + n)D (5)

Where coupled inductor turns-ratio n=n,:n2' The switch­node voltages of Trans-CFSI, viz. Vs/, Vsb Vs3 are shown Fig. 5 (a) in along with shoot through switching signal and input current (current through the primary winding of the coupled inductor) of the inverter. The boost factor (VjVg) of Trans­CFSI for different values of coupled inductor turns-ratio is plotted in Fig. 5 (b).

IV, PWM CONTROL SCHEME OF TRANS-CFSI To incorporate shoot-through state in the PWM control,

the traditional PWM technique for VSI is modified accordingly. The modified PWM scheme for Trans-CFSI is developed based on the traditional sine-triangle PWM with unipolar voltage switching, Sinusoidal modulation signals VnltJ and -VnltJ and high frequency carrier signal Vlrlt) are shown in Fig. 6 (a).

DTs D'Ts DTs S �.. 12' . . . . I IIVg+'� � .)

'�l 01 c=J 1'�"1+iI t 10 � � ____ L __ ,� �

'''? oj ri [--on_'� � 75 SM :: :: t" u:

'�30ii ii .. � R Ron

.. !'" -,� <!l 2.5

iL � o . : . : .. 0.05 0.1 0.15 0.2 0.25 03 � Ts---.l I Shoot TIll'ough Duty Ratio (D)

(a) (b)

Fig. 5. (a) Switch-node voltages along with shoot through duty and inverter

input current of Trans-CFSI, and (b) boost factor of Trans-CFSI.

Shoot-through constant voltages Vsr and -Vsr, and Gate signals Gs, Gs/, Gs]' GS3, GS4 of the modified modulation scheme for positive and negative half cycles of the sinusoidal modulation signal Vm{t) is shown in Fig. 6 (a). Shoot-through signals STl and ST2 are generated by comparing Vsr and -Vsr with carrier signal as shown in Fig. 6 (b) and Fig. 6 (c) for the positive and negative half-cycle of the modulation signal, respectively. In this half cycle, the shoot-through interval is inserted in the gate signals GS3 and GS4' Gate signals GS3, GS4, and Gs are generated using the following logic.

The 2014 International Power Electronics Conference Likewise, in the negative half-cycle (VnltJ < 0) of the modulation signal, gate signals GS3 and GS4 are generated by comparing the sinusoidal modulation signals -VnltJ and VnltJ with carrier signal Vlr;{t). The shoot-through interval is inserted in gate signals Gs/ and GS2. Gate signals Gs/, Gs]' and Gs are generated using the following logic equation.

3589

The PWM control signals Gs/, Gs]' Gs3, and GS4 are shown in Fig. 6 (d) for the positive half-cycle of Vnlt).

(a)

(b) (c)

Tek J'L • Stop W Pos: 34.40»s MEASURE ...

�'. Gsl L: CH3

4.l 0.:4 .. 1 \. I" Period 66.64»s?

CH3

s.l .... l r Pos Width

I 43.00»s? G.d CH4

Pk-Pk

'\ _J- lS.6Y Gs2 CH2

2· Mean 8ASY

CH2 10.0Y W 10.0»s 1/8

CH3 10.OY CH4 10.OV (d)

Fig. 6. (a) Generation of PWM control signals, (b) PWM control scheme

when V,.,(t»O, (c) PWM control scheme when V,.,(t)<O, and (d) PWM control signals for the positive half-cycle of the modulation signal.

The mathematical relation between D and VST can be written as,

(6)

In order to ensure that the shoot-through duty D interval doesn't overlap with the power interval of the inverter, D should be chosen such that

D$.I-M (7)

Where M is the modulation index of the inverter. The relation between the AC output and DC input (Vg) is

VACpeak = Brrans-CFSI x M x Vg Mx l+nD

V 1-(2+n)D g

V. EXPERIMENTAL VERIFICA nON

(8)

A prototype is built to test the proposed Trans-CFSI topology. The PWM control for the inverter is developed in Texas Instruments TMS320F28335 DSP. The design specifications are tabulated in Table I.

TABLE I

PROTOTYPE SPECIFICATION

Parameter/ Component

Modulation Frequency (fm)

Carrier Frequency (ftr;)

Coupled Inductor (Ll> L2, n)

Capacitor (Co)

Output Filter Inductor (Lf)

Output Filter Capacitor (Cf)

Output Power

Steady-State Operation

Attributes

50 Hz

25 kHz

1.78 mH, 1.78 mH, 1

940 uF

4.6 mH

10 llF

225 W

Fig. 7 (a) shows converter switch node voltages along with the DC-link capacitor voltage. Fig. 7 (b) shows the inverter circuit operation of Trans-CFSI in boost mode where a 110 V AC output (at 50 Hz) is obtained from 230 V DC-link voltage with input voltage of 28 V.

Fig. 8 (a) shows the high boost operation of Trans-CFSI where a 220 V AC output is obtained from an input voltage of 52 V DC at D=0.285, M=0.68, n=1 with DC-link capacitor voltage (Ve) of 450 V. The output power is 225 Watt at unity power factor (resistive load). Steady-state operation of Trans­CFSI in buck mode is shown in Fig. 8 (b) where 36 V AC output is generated from an input voltage of 60 V DC with DC-link capacitor voltage (Ve) of 59 V with D=O, M=0.87, n=l.

The 2014 International Power Electronics Conference

3590

Tek JL M POI: -40,OO)JS MEASURE + CH1 """' ..... -"""'-_ .................. --"1" ..... - PH�

8.-�W-�W-v-: 2���

472V

CH2 Cye RMS

111V?

CH3 Pk-P� 236V

CH3 Freq

43,34�Hz

CH4 Mean 211V

CH1 200VE\oJ CH2 200VE\oJ M 1O.0)JS CH3 f 30,OV

CH3 200VE\.! CH4 50,OVE\.! <10Hz

Tek JL • Stop M POI: -140,O)J' MEASURE +

Vc CH1 Mean .p' v.:: f r I I 228V

� g .... .. CH2

J Cye RMS

873mA

::AAV CH3

P�-P� 320V

CH3 Cye RMS

103V

CH4 Mean 27,7V

CH1 100VE\.! CH2 1.00AE\.! M 5,OOm, CH3 f 2,80V

CH3 100VE\oJ CH4 10,OVE\oJ <10Hz

(a)

(b)

Fig. 7. (a) Steady-state switch-node voltages and DC-link voltage (Ve) of Trans-CFSI, and (b) steady-state output AC voltage (Vae) and output current

(lac) with DC-link voltage (Ve) and DC-input voltage (Vg) for 110 V AC systems.

Tek JL • Stop +

CH1 200VE\oJ CH2 2,OOAE\oJ M 5,OOm,

CH3 200VE\oJ CH4 50,OVE\.!

Tek JL • Stop +

CH1 50,OVE\.! CH2 2,OOAE\.! M 5,OOm,

CH3 20,OVE\oJ CH4 50,OVE\.!

M POI: -100,O)J' MEASURE

CH1 Mean 447V

CH2 Cye RMS

1.03A?

CH3 Cye RMS

222V

CH3 Freq

43,80Hz

CH4 Mean 52,OV

CH3 '- 176V

<10Hz (a)

M POI: -100,O)J' MEASURE

CH1 Mean 59,OV CH2

Cye RMS 532mA?

CH3 Cye RMS

36,OV

CH3 Freq

50,OOHz

CH4 Mean 60,OV

CH3 '- 6,40V

<10Hz (b)

Fig. 8. Steady-state operation of Trans-CFSI in (a) high-boost mode: DC-link

capacitor voltage (Ve), input DC voltage (Vg), output AC voltage (Vae), and

output AC current (iae) waveforms for 220 V AC systems with D=0.285, M=0.68, n=l, (b) buck mode: DC-link capacitor voltage (Ve), input DC

voltage (Vg), output AC voltage (Vae), and output AC current (iae) waveforms

with 0=0, M=0.87, n=l.

Effect of De-link Fault Due to EMf

Due to the presence of shoot-through state, Trans­CFSI provides better EMI immunity than traditional VSI. To prove the EMI immunity of Trans-CFSI, an EMI noise of 1-ms duration is applied to the inverter bridge which makes all the gate switching signals high. The operation of the circuit under this test is shown in Fig. 9 (a). From the result, it can be seen that the output AC voltage and DC-Link capacitor voltage remains at their previous voltage levels and does not respond to the noise. Although the input current, iim (im is the input current drawn by the converter after placing an input filter capacitor) of the inverter rises to about 4 times of the normal value, it comes back to steady-state within a short period of time (3-ms). The Trans-CFSI circuit is also tested for a long duration DC-bus fault test with 80 ms fault duration which is shown in Fig. 9 (b). It can be seen from the test result that there is no damage to the inverter and the circuit regains steady-state after the fault is cleared.

Tek JL • Aeq Complete M POI: 11.00ms ...

' (tf\; 4·V <

� � �

\<==;;r\ " ..., c � ' 1 . '---r--'�--y-

' in 2'

MEASURE CH1

Mean 63,8V

CH2 Mean 1,72A

CH3 Off Cye RMS

CH3 Off freq

CH4 Cye RMS

20,5V

CH1 50,OV CH2 2,00A M 5,00ms CH2 .I 4.56A

CH4 20,OV <10H,

Tek JL • Aeq Complete M POI: 150,Oms ... Fault Initiated

Vac

4·M ���wNVWNvW �Fault Cleared

MEASURE CH1

Mean ?

CH2 Mean

? CH3 Off

Cye RMS

CH3 Off freq

CH4 Cye RMS

? CH1 50,OV CH2 20,OA M 50 ,Oms

CH4 50,OV

CH2 .I 9,60A

(a)

(b)

Fig. 9, (a) EMI testing on Tran-CFSI with I ms duration EMI pulse, and (b) DC-bus fault testing on Trans-CFSI with 80 ms fault duration.

VI. CONCLUSION

This paper proposed a coupled inductor based high boost inverter, named Trans-CFSI, which exhibits improved EMI noise immunity similar to the ZSI, SBI etc. The high gain of the inverter is obtained by the transformer action of the coupled inductor and insertion of shoot-through state. In this paper the development of Trans-CFSI topology is described in details along with its steady-state characteristics and PWM switching scheme. The proposed inverter is tested on a laboratory prototype and verified. The inverter is also tested

The 2014 International Power Electronics Conference

3591

for EMI and DC-bus fault which shows that the inverter shows EMI immunity and can sustain DC-bus fault.

ACKNOWLEDGEMENT

This work was supported by Science and Engineering Research Board (SERB), Govt. of India under grant no. SRlS3/EECE/0 187/2012.

VII. REFERENCES

[1] F.Z. Peng, "Z-Source Inverter," IEEE Transaction on Industrial

Applications, Volume. 39, No. 2, 2003, pp, 504-510, [2] J. Anderson, F.z. Peng, "Four Quasi-Z-Source Inverters" in

IEEE Power Electronics SpeCialists Coriference, PESC 2008, pp. 2743 - 2749.

[3] S. Mishra, R. Adda, and A. Joshi, "Inverse Watkins-Johnson Topology-Based Inverter," IEEE Transactions on Power

Electronics, Volume: 27, Issue: 3, 2012, pp. 1066 - 1070. [4] R. Adda, 0, Ray, S. Mishra, and A. Joshi, "Synchronous

Reference Frame Based Control of Switched Boost Inverter for Standalone DC Nanogrid Applications," IEEE Transactions on

Power Electronics, Volume: 28, Issue: 3, 2013, pp. 1219 - 1233. A. Ravindranath, S. Mishra, and A. Joshi, "Analysis and PWM Control of Switched Boost Inverter," IEEE Trans. Ind.

Electron., vol. 60, no. 12, pp. 5593-5602, Dec. 2013.

[5]

[6]

[7]

[8]

[9]

Olive Ray and Santanu Mishra, "Boost-Derived Hybrid Converter with Simultaneous DC and AC Outputs," accepted for publication in iEEE Trans. on Indus. Applications, 2013. DOl 1O.11091TIA.2013.2271874. Wei Qian, F. Z. Peng, H. Cha, "Trans-Z-Source Inverters" IEEE

Transactions on Power Electronics, Volume: 26, Issue: 12, 20 II, pp. 3453 - 3463. Soumya Shubhra Nag and Switched Inverter," iEEE

Transactions on indus.

10,1109ITIE.2013.2289907.

Santanu Mishra, "Current-Fed Early Access Article, iEEE

Electronics, 2014, DOl

Soumya Shubhra Nag, Ravindranath Adda, Olive Ray, Santanu Mishra, "Current-Fed Switched Inverter Based Hybrid Topology for DC Nanogrid Application, " in 39th Annual Coriference of

IEEE Industrial Electronics SOCiety, IECON, 2013, pp. 7146-7151.

[10] R. W, Erickson and D, Maksimovic, Fundamentals of Power Electronics, 2nd Edition, Springer science+ business media Inc., NY, 2001.