Novel PV Fed Three Phase Single Power Stage System for Stand-Alone Applications

Moumita Das, and Vivek Agarwal IIT-Bombay, Powai, Mumbai, Maharashtra, 40076, India

Abstract - Renewable energy sources like solar PV are desirable to meet the increasing power demand. This paper proposes a novel single power stage, PV system designed for 3-φφφφ stand-alone applications. The single stage topology has the advantage of being simple and more efficient. The developed topology is basically a current source inverter with a single power conversion stage that directly feeds the stand-alone AC load. The inductor diode network allows the inverter to operate in buck-boost manner. This converter is especially suitable for sources such as PV and fuel cells with low voltage. A modified 120o conduction mode operation of the inverter is implemented. Maximum power point tracking algorithm based on P&O method is also implemented. Circuit analysis and simulations of the proposed system are included.

I. INTRODUCTION

Among the existing renewable energy sources, solar energy is probably the most important and abundant energy source because of endless availability of sunlight during the day time. A power electronic circuit is the heart of any renewable energy power generation scheme. The power electronics for solar PV power conversion is used to track the maximum power point of the solar PV system in order to maximize the power extraction (and hence increase the energy yield) [1] of the solar PV source. Apart from this, power electronics enables the feeding of sinusoidal voltage and sinusoidal current to the load. Conventionally, photovoltaic power generation systems are two-stage systems. First stage consists of a dc/dc converter for maximum power point tracking (MPPT) and the second stage is a dc/ac inverter for sinusoidal current generation. The two stage approach requires voltage and current measurements and logic implementation to track the MPP in the dc/dc stage and a separate dc/ac inverter for generation of sinusoidal current output. This causes reduced efficiency and the resultant system is not compact. Instead of two stages, we can use single power stage [2-4] approach to track the MPP and also to generate sinusoidal current output in order to realize simple control and to minimize the losses and cost.

PV Array

DC/DCConverter

+MPPT

DC/AC Inverter+

Filter

Load

Battery

Stage 1 Stage 2

(a)

PV Array

DC/AC Inverter+

MPPT+

Filter

Load

Battery

only one stage

(b)

Fig. 1 Power conditioning circuit for PV (a) Conventional Strategy, (b) Proposed strategy. The block diagram shown in Fig. 1(a) is for the conventional two stage approach, used for feeding sinusoidal current into the load. The proposed single stage approach is shown in Fig. 1(b). The single power stage inverter used in this work offers the following advantages: 1. Simple power stage and single power stage conversion. 2. Simple control. 3. Maximum power point tracking. 4. Low cost and high efficiency. The common two-level voltage-source inverter (VSI) is not the best solution for photovoltaic applications. When a PV module is connected directly to the VSI, the VSI requires a high input voltage for proper sinusoidal current output due to its buck type characteristic. Therefore, a current source inverter (CSI) [5,6] may be used in series with a dc inductor to

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increase the voltage gain and generate sinusoidal current output [7]. The proposed topology belongs to this category. The three-phase CSI is composed of six reverse blocking switches (S1–S6), which consist of three fast power devices such as MOSFETs (for high frequency operation) and three slow, line frequency devices. The three phase inverter circuit contains three buck-boost inductors one for each phase as main energy storage component [8], and, at the output, there are LC filters one for each phase [9], which reduce the ripple present in the current coming from the intermediate inverter stage. Maximum Power Point Tracking algorithm based on P&O algorithm [10] is also implemented. Battery shown in the Fig. 1(b) is used to store the extra power generated by solar PV and utilize that stored energy during low sunlight period or during absence of sun light (e.g. night, cloudy weather etc.). The control strategy of the proposed circuit is based on modified 120 degree mode of conduction [11,12]. Remaining of the paper is organized as follows: The single power stage, three phase inverter power circuit and its control strategy are described in section II. Simulation results of the three phase inverter are given in section III. Finally, conclusions and future scope of work are included in section IV.

II. ANALYSIS OF THE PROPOSED CONVERTER

S1

S3

S5

S4

S6

S2

Vi

L1

L2

L3

Lf1

Lf2

Lf3

Cf1

Cf2

Cf3

R1

R2

R3

D1

D2

D3

Fig. 2 Circuit diagram for single power stage three phase inverter

Fig. 2 shows the circuit topology of the proposed converter, which is based on three buck boost converter principle. This converter consists of six power switches among them three are high frequency (S1, S3, S5) and rest three are line frequency switches (S2, S4, S6). Only one high and one line frequency switch can be on at a time, low frequency switch will be on for entire on period and high frequency switch will be switching

at high frequency for that on period. The main operating principle is that buck boost inductor stores energy by switching of the high frequency switch and it transfer its energy to the output capacitor by using line frequency switch path. Vi is the input dc voltage which represents input PV voltage. D1, D2 and D3 are the feedback diodes of each phase buck-boost converter operations. The buck-boost inductors require to increase the voltage gain is L1, L2 and L3. As the proposed inverter is a CSI it requires CL filter to reduce the output ripple. The capacitors present in the circuit Cf1, Cf2 and Cf3 are filter capacitors. Lf1, Lf2 and Lf3 are the filter inductors of the single stage inverter. The resistances R1, R2 and R3 presents in the circuits fulfill the purpose of load requirement of the circuit. The maximum power point tracking of PV can be realized by varying duty cycle of the high frequency switches S1, S3, S5 of the buck- boost converter. The proposed converter requires twelve steps to complete its operation and to generate three phase sine wave. Each phase of the inverter requires four steps to generate single sine wave. The circuit operation of one phase of the three phase inverter is described as below.

S5

S2

L3

Lf1

Lf2

Lf3

Cf1

Cf2

Cf3

R1

R2

R3

D1

D2

D3

S1

S3

S4

S6

Vi

L1

L2

VO1

VO3

(a)

S5

S2

L3

Lf1

Lf2

Lf3

Cf1

Cf2

Cf3

R1

R2

R3

D1

D2

D3

S1

S3

S4

S6

Vi

L1

L2

VO3

VO1

(b)

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S5

S2

L3

Lf1

Lf2

Lf3

Cf1

Cf3

R1

R3

D1

D2

D3

S1

S3

S4

S6

Vi

L1

L2

VO1

VO2

(c)

S5

S2

L3

Lf1

Lf2

Lf3

Cf1

Cf2

Cf3

R1

R2

R3

D1

D2

D3

S1

S3

S4

S6

Vi

L1

L2 VO2

VO3

(d)

Fig. 3 Current flowing path of operating modes during one switching period at CCM operation (a) mode I, (b) mode II, (c) mode III, (d) mode IV.

A. Mode I

At t=0, S1 and S6 are turned on, S1 is operating at high frequency and S2 is operating at line frequency. When S1 is on diode D1 is turned off. The current flowing path is shown in Fig. 3(a). The voltage equation of the input side buck boost inductor is Vi=VL1. In this period the buck boost converter stores energy in inductor L1. The output capacitors Cf1 and Cf2

discharges to load resistances R1 and R3. Load side voltage equations are

VV OCf = 11 (1)

VV OCf −= 22 (2)

B. Mode II:

During this time interval switch S1 is off and diode D1 is turned on. The current flowing path is shown in Fig. 3(b). The buck boost inductor L1 transfers its stored energy to the output capacitors Cf1 and Cf2. The inductor voltage can be expressed as below

VVV CfCfL 211 += (3)

C. Mode III

During this mode S6 is turned off, S2 is turned on and S1 is turned on. The inductor voltage expression is same as the Mode I. The current flowing path is shown in Fig. 3(c). The voltage across output filter capacitors Cf1 and Cf3 can be expressed as

VV OCf 11= (4)

VV OCf −= 33 (5)

D. Mode IV

During this interval S1 is turned off. The current flowing path is shown in Fig. 3(d). The buck boost inductor L1 transfers its energy to the output filter capacitors Cf1 and Cf3. The inductor voltage can be expressed in terms of filter capacitors voltage is as below

VVV CfCfL 311+= (6)

The voltage balance equation across the inductor L1 can be written as

0)1(11 =−×+× DVDV LL (7)

D is the duty cycle of the high frequency switch S1.

Now by substitute equations (1) to (6) in equation (7) the voltage gain expression becomes

1

1

V DCf

DV i=

− (8)

Now this mode I to mode IV will repeat its operation with different switch combinations for other two phases of the three phase inverter. The control strategy of the proposed three phase inverter is based on 120 degree conduction mode of operation of the

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three phase inverter. In 120 degree mode of operation two switches operates at a time instead of three switches of 180 degree conduction mode of operation. Two switches operation at a time helps to reduce EMI effect and avoids switching overlap.

Sin(wt) comparator

Ref

120 degree pulse

Not

comparator

High frequency carrier wave

S1

S4

MPPT BlockVPV

IPV

comparator 120 degree pulse

Fig. 4 Control circuit for one of the phases of the proposed system The block diagram shown in Fig. 4 illustrates the control circuit for generation of the switching signals for one phase out of the three phases of the inverter for DCM mode of operation. In Fig. 4 sine wave is compared with a reference signal to generate 120 degree pulse. Generated 120 degree pulse multiplies with the output signal of the MPPT block and is compared with high frequency carrier wave to generate high frequency 120 degree signal for high frequency switch S1. The sine wave passed through a NOT gate and is compared with an identical reference signal to generate line frequency pulse for the line frequency switch S4. To generate a pulse for the other two high frequency switches S3 and S5, the other two phases’ sine waves are used in a similar manner. For two other low frequency switches S6 and S2 pulses can be generated by using sine waves used for S3 and S5. The 120 degree mode of conduction allows each switch to remain on for 120 degree period in one cycle. As described above two switches will be operating at one time under this mode of conduction. Among all the six switches any two switches will be on together for 60 degree period. The voltage and current during this 60 degree period for the switch S1 and S2 are described as below. Voltage equations:

21

1L

Cf

VV = ........…………… (9)

02 =CfV .........…………… (10)

21

3L

Cf

VV −= ...…………….\ (11)

Current equations:

11 LCf ii = ........…………… (12)

02 =Cfi ........…………….. (13)

13 LCf ii −= ........…………… (14)

The voltage and current equations for one switching set are shown in equations (9)-(14) are same nature for other five switching operating sets. There are six different set of switching operations require to complete the inverter operation. Also there will be six different sets of current and voltage equations exist for six different set of operations.

Table 1

Operating States

Phase A

Phase B

Phase C

1(0-600) S1, S2 0 0 2 (600-1200) S1, S6 0 0

3 (1200-1800) 0 S5, S6 0 4 (1800-2400) 0 S5, S4 0 5 (2400-3000) 0 0 S3, S4

6 (3000-3600) 0 0 S3, S2

Table 1 shows the switches’ conduction sequence for various phases with respect to the operating states. Phase A, Phase B and Phase C denote the three different phases of the three phase inverter. There are six different sets of switching operations required to complete the inverter operation. Also there will be six different sets of current and voltage equations for six different set of operations.

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Table 2

Operating States

S1 (HF)

S2 S3 (HF)

S4 S5 (HF)

S6

1 1 1 0 0 0 0 2 1 0 0 0 0 1

3 0 0 0 0 1 1 4 0 0 0 1 1 0 5 0 0 1 1 0 0

6 0 1 1 0 0 0

The operating states of the switches are shown in Table 2. The switching signals are shown in the simulation results. The high frequency switches will operate at high frequency for 120 degree period and remaining time they will be completely off.

III. SIMULATION RESULTS

The operation of the proposed converter is verified by simulation. The scheme for three phase inverter discussed is simulated in MATLAB/SIMULINK and the results of the simulation are presented in this section.

00.51

00.51

00.51

00.51

00.51

00.51

0.65 0.67 0.69

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 5 Switching pulses for three phase inverter (a) pulses for S1, (b) pulses for S4, (c) pulses for S3,(d) pulses for S6, (e) pulses for S5, (f) pulses for S2. Fig. 5 shows switching signals of the three phase inverter. High frequency switching signals for S1, S3, S5 are shown in (a), (c) and (e) and line frequency switching signals for S2, S4, S6 are shown in (b), (d) and (f).

0100200

-100-200

01

2

-1-20.65 0.66 0.67

(a)

(b)

t Fig. 6 Output waveforms of the three phase inverter: (a) Output voltage, (b) Output current The simulated voltage and current waveforms of the three phase inverter are shown in Fig. 6 (a) and (b) for R-L type of load. The circuit is also simulated for resistive load with high value of filter inductance.

02040

02040

02040

02040

0.06 0.07 0.08 0.09

(a)

(b)

(c)

(d)

t Fig. 7 Current flowing through the buck-boost inductor (a) current through inductor L1, (b) current through inductor L2, (c) current through inductor L3, (d) current flowing from source. Fig. 7 displays the current flowing through the buck boost inductors L1, L2 and L3 present in the circuit of the three phase inverter.

t

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2

-2

1

-1

Fig. 8 Voltage across the filter capacitor of one of the phases The waveform shown in Fig. 8 is the voltage waveform across the filter capacitor Cf1 with resistive load connected across it.

IV. CONCLUSION

The proposed three phase standalone system and its control strategy are analyzed and simulation results have been presented. The presence of diode, which is inherent to the “buck-boost” converter structure blocks the reverse current that might try to flow from the load side towards the PV source. Control is observed to be simple and yet able to supply high quality sinusoidal current to the load. The THD of the current flowing into the stand alone load is found to be less than 5%. The proposed configuration with P&O MPPT method has shown encouraging results. One of the major issues with this scheme is the high peak current due to DCM operation of the converter. Efforts are underway to realize a CCM control strategy which will reduce the peak current stress on the devices. Those results will be presented in a future paper.

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[4] Y. Huang, M. Shen, F. Z. Peng, and J. Wang, “Z-source inverter for residential photovoltaic systems,” IEEE Trans. Power Electron., vol.21, no. 6, pp. 1776–1782, Nov. 2006.

[5] B. Sahan, A. N. Vergara, N. Henze, A. Engler, and P. Zacharias, “A Single-Stage PV Module Integrated Converter Based on a Low-Power Current-Source Inverter,” IEEE Trans. on Industrial Electronics, vol. 55, no. 7, July 2008.

[6] S. Jain and V. Agarwal, “New current control based MPPT technique for single stage grid connected PV systems,” Energy Conversion and Management (Elsevier Science), vol. 48, pp. 625-644, May 2006.

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[8] Sachin Jain, Vivek Agarwal, “A single-stage grid connected inverter topology for solar PV systems with maximum power point tracking,” IEEE Trans. Power Electron., vol. 22, no.5, Sep. 2007, pp. 1928-1940.

[9] F. Gao, C. Liang, P. C. Loh, and F. Blaabjerg, “Buck–Boost Current-Source Inverters With Diode-Inductor Network,” IEEE Trans. Ind. App., vol. 45, no. 2, Mar. 2009.

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[12] S. D. Sudhoff, P. C. Krause, “A McMurray Inverter Controller,” IEEE Transactions on Energy Conversion, Vol. 5, No. 3, Sep. 1990.

t

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