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平成 19 年電気学会産業応用部門大会

Development of New Three-Level Current-Source Inverterfor Grid Connected Photovoltaic System
Suroso and Toshihiko Noguchi (Nagaoka University of Technology)
Abstract This paper discusses a single-phase three-level current source inverter (CSI) driven by a single gate-drive power supply in bothchopper and inverter, and its feasibility on grid connected photovoltaic system application. Using this new topology, the complexity of thechopper and inverter circuitry is dramatically reduced by using single gate-drive power supply for all power switches. The circuit works
properly at a unity power factor operation when the quasi-sinusoidal current is injected into the grid. The simulation results provide firm
evidences about the practical feasibility of the proposed system.
Keywords : current-source inverter (CSI), grid connected photovoltaic system

1. INTRODUCTION
Solid state inverters allow to put photovoltaic (PV) systems into
the power grid with interconnecting circuit [1]. However, notmany PV systems have so far been placed into the grid due to therelatively high cost of power converters. Therefore, cost reduction
per inverter watt is important to make such PV systems more
attractive [2]. Focus has, therefore, been placed on new, cheap,
and innovative inverter solutions.
Multilevel CSI has been proposed in [3], but this converter
needs two large balance inductors to achieve intermediary
three-level current and the balanced current sharing among theswitches. Reference [4] reported the multilevel CSI driven bysingle gate power supply in an inverter side only.
This paper proposes a new three-level current-source inverter
and its feasibility on a grid connected PV system application. Thisnew topology implements common emitter topology of allswitches in the chopper and the inverter sides. Therefore, it makes
possible to drive all switches using only a single gate-drive power
supply. The effectiveness of this topology has been studied
through simulations using PSIM software.

2.

comparing the error signal of the average value of the detectedcurrents I1 and I2 flowing through both input inductors and atriangular carrier wave after passing through a PI regulator as
shown in Fig.2. This signal is used to control the duty cycle of thechopper according to the optimal current produced by PV.
For analysis purpose of the buck chopper circuitry, we canconsider this circuit comprises two buck choppers with commonswitch (Q1). The chopper behaves as a current source. When theswitch is closed for a period DT, the input inductor current rises
and energy is stored in the input inductor. If the switch is opened
during time (1-D)T, the stored energy is transferred into theinverter, where D is the duty cycle of chopper.
The input inductor produces current IL, a part of it will flow
through diode (Id) during the switch on and the other part (IO) willflow into the inverter during the switch off. The input inductor
current (IL), gating signal (Vgate) and output current (IO) flowing
into the inverter are shown in Fig. 3.

CIRCUIT CONFIGURATION
I1
I2

2.1. CHOPPER CIRCUIT
The main goal of a grid connected PV energy converter is toprocess the PV power and inject a sinusoidal current into theutility grid. Such a low distorted current is obtained using
three-level current-source inverter. The current source, in the inputof the dc-to-dc converter, is composed with the PV arrays
connected to input inductors (Lin) as shown in Fig.1. As theoptimum current of the PV is determined by a short circuit currentof the PV, where photovoltaics are modeled with simple DC
voltage sources connected in series with their internal resistance.
Power decoupling capacitors C are inserted between the PV arrays
and the input inductors to reduce the voltage ripple in the PV
arrays. Two diodes are placed between the input inductors and thecapacitors in order to keep the current to flow only in one
direction when inverter is connected with the grid.
Current control mode is applied to control the output currentof each chopper. Making the output current follow the reference
current I* is the objective of this current controller. The switchinggate signals of switch Q1 (dc-to-dc switch) is generated byFig. 1. Buck chopper connected to inverter with link inductors.

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I∗

Fig. 2. Gating signal generation of chopper switch.

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平成 19 年電気学会産業応用部門大会

IL(a).

DTt(1-D)T

VGate(b).

Fig. 4. Output current and grid voltage waveform.
TontToffIO△i(c).

DT(1-D)Tt

Fig. 3. (a). Input inductor current during switch on (DT) and switch
off (1-D)T, (b). Gating signal of switch (c). Output current flowing
into inverter.
Fig. 5. Harmonic spectrum of the output current.
2.2. INVERTER CIRCUITThe inverter circuit comprises of four switches and six diodes
as shown in Fig.1. Sinusoidal PWM is used to produce athree-level current with a grid frequency. Switching frequency ischosen to achieve low switching losses and small harmonicamplitude of odd harmonic component of the output current at thesame time.
The gating signals of the inverter are produced by using asinusoidal reference wave modulated with two triangular carrier
waves with the same frequency but with opposite offset. This
strategy is able to perform the sinusoidal PWM of the inverter.
Output current is IO when switches Q2 Q3 are off and Q4 Q5 areon. When Q3 Q5 are off and Q2 Q4 are on, no current will flow
into the grid and the reverse current -IO will flow when Q4 Q5 areoff and Q2 Q3 are on.

3. SIMULATION RESULTThe simulation results show that the output current of theinverter is in phase with the grid voltage. Thereby, the high power
factor operation is achieved. From FFT analysis result, it showsthat harmonics appear as sidebands around multiples of thechopper switching frequency. Hence, it can be easily eliminatedusing simple low pass filter or operating with a higher switchingfrequency.
These results provide evidences about the practical feasibility ofthe proposed system.

4. CONCLUSION
In this paper, a three-level current-source inverter with achopper driven by a single gate-drive power supply has beenstudied as a grid connected photovoltaic energy converter. The
results have shown that the proposed strategy works properly andis quite effective to simplify the PV system with a grid connection.

This section describes the simulation results of the proposed
topology when connected to the power grid of 100 V 50 Hz. It
should be noted that even the input inductor was chosen as Lin=
100 mH to get enough current as determined by the reference
current signal, the circuit also works for smaller input inductor.
The decoupling capacitor used in this simulation is 100 μF, hence
it makes possible to use film capacitors whose lifetime is much
longer than electrolytic capacitors.
The inverter injects a three-level current directly into the grid
which is modeled as an ideal sinusoidal power source. A 10 μH
inductor is used to connect the chopper and the inverter sides. The
switching frequency has been chosen to be 10 kHz for the dc-to-dc
converter and 30 kHz for the inverter. Phase relationship between
output current and the grid voltage is shown in Fig.4. Fig.5 shows
the fourier transform (FFT) of the output current.

References(1). S. B. Kjaer, J. K. Pedersen and F. Blaajerg : “A review of single phase gridconnected inverters for photovoltaic modules,” IEEE Transaction on Industry
Application, Vol. 41, No.5

(2005)(2). P. G. Barbosa, H. A. C. Braga, M. C. Barbosa and E. C. Teixeria : “Boostcurrent multilevel inverter and its application on single phase grid connected
photovoltaic system,” IEEE Transaction on Power Electronic, Vol. 21, No.4
(2006)(3). F. L. M. Antunes, A. C. Braga and I. Barbi : “Application of a generalized
current multilevel cell to current source inverters”, IEEE Transaction on PowerElectronic, Vol. 46, No.1 (1999)(4). K. Iwaya and T. Noguchi : “Novel Current source multi level inverter driven
by single gate drive power supply,” IEEJ Trans. IA, Vol. 126, No.1 (2006)[I - 238]