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Novel Three-Level Current-Source PWM Inverter
for Photovoltaic Power Conditioner with Grid Connection
Suroso, and Toshihiko Noguchi (Nagaoka University of Technology)
Abstract- This paper presents a new configuration of a three-level current-source PWM inverter (CSI) with a fully common-source topology
in terms of all FET switching devices used as a grid connected inverter. Using this common-source CSI, the number of gate drive power
supplies can be reduced by using only a single power supply instead of using a conventional bootstrap circuit or isolated power supplies. As aresult, it can eliminate expensive transformers and capacitors in the drive circuits of the switching devices. In addition, the circuit can beoperated at a higher switching frequency and a higher dv/dt rate because of its common-source topology. The effectiveness of the proposed
topology is experimentally verified by using a laboratory prototype set up. Finally, the new inverter circuit is tested for grid connected
operation. The experimental results show that the inverter works properly, injecting a sinusoidal current with unity power factor operation.
Keyword: current source inverter, multi-level, power grid

1. INTRODUCTIONCdc

LinI1

Q2 Q3
Cf

R_in Vdc Q1
PVLoadRecently, the distributed renewable power generation isbecoming more and more popular from the viewpoint ofenvironmental issue and energy conservation.
Thephotovoltaic (PV) solar energy is a very interestingalternative to supplement the generation of electricity
among green power sources because it can be utilized either
at remote regions as stand alone apparatus or at urban
applications with a grid interactive connection [1], [4].
A voltage-source inverter (VSI) such as a three-level
neutral-point clamped inverter is conventionally used as aPV power conditioner either for the grid interactive or thestandalone operation. This system has limitation whenused for a high-voltage and high-switching-frequency
application because of a dv/dt problem in the circuit. On
the other hand, a current-source inverter (CSI) has someadvantages compared with the voltage-source inverter suchas no need of an interconnection inductor when used for thegrid interactive application, direct controllability of theoutput current, inherent short-circuit current protection
capability and longer lifetime of its reactor if comparedwith an electrolytic capacitor in the VSI.
Currently, for further practical, reduced dv/dt problem
and longer lifetime requirement of the inverter, innovative
PV power conditioner circuits should be newly created forthe stand alone or the grid interactive operation. Generalcircuit of multilevel CSI has been proposed in [3].
Reference [2] reported the multilevel CSI driven by single
gate power supply only at the inverter side. However, thepower switches of its chopper still need isolated drivecircuits.
In this paper, a new three-level current-source PWM
inverter used as a grid connected inverter, which is fully

I2
Q4Q5LinFig. 1. Proposed common-source three-level CSI circuit.

common-source topology at both of the chopper and theinverter circuit is presented. Using this new topology, thenumber of gate drive power supplies can dramatically bereduced to a single to drive all power switches without
isolated power supplies or a conventional bootstrap circuit.
In addition, the circuit can overcome the dv/dt problem of
the conventional inverters; hence it can be operated at
higher voltage and higher switching frequency for better
quality of the output current.
2. CIRCUIT TOPOLOGY AND OPERATION PRINCIPLE
One of the main goals of the PV power conditioner is toprocess the PV power into a sinusoidal current to be
injected into the utility grid. Such a low distorted current
is obtained using proposed three-level current-source
inverter as shown in Fig. 1. The current source, at theinput of the dc-to-dc converter, is composed by some PV
arrays connected to input inductors (Lin). As the optimum
current of PV is determined by its short-circuit current,
photovoltaic is modeled by a simple DC voltage source[I - 363]

平成 20年電気学会産業応用部門大会

1-80(Vdc) connected in series with its internal resistance (R_in)[5]. A power decoupling capacitor Cdc is connected acrossthe output terminals of the PV arrays to reduce the voltage
ripple of the PV arrays. Blocking diodes are placed
between the input inductors and the negative terminal of thedecoupling capacitor in order to prevent the current fromflowing back during positive and negative cycles.

Carrier waveform

Reference signal+-Carrier waveform

2.1. Chopper Circuit+

Gate drivers

Synchronizer
Triangular waveform
Comp.

Iref+

PI

Filter

I1-Gatedriver+Ipwm

Q1

I2Chopper

1/2+ I1-Sensing
andscaling+
I2

Fig. 2. Block diagram of current controller.

ID=ILin

V DC

ID=0

IL=0

I Lin

VDC
toQ1
ID=ILin

I Lin

IPWM

Q1
ILin

SPWM
Inverter

IL=0

IL=ILinILin
L o adV pv

SPWM
Inverter

IL=ILin

ID=0Charging mode

Discharging mode

Fig. 3. Operation modes of chopper circuit.

I1

Q2(1)

I pwm

IloadCfQ3Q4I2

Rin

−Rint

(1 − e Lin ),(2)where Vpv and Rin are the terminal output voltage and theinternal resistance of photovoltaic system, respectively.
The current increases exponentially to its maximum value,depending on the terminal output voltage of thephotovoltaic (Vpv) and its internal resistance (Rin). This
current is the short-circuit current of the photovoltaic. In
this calculation, the internal resistance of the input inductor
is neglected because its value is much smaller than that ofthe photovoltaic arrays (Rin). The energy is stored in amagnetic field of the input inductor, and when the chopper
switch Q1 is off, during (1-DT), the stored energy isdischarged to the load via the inverter circuit as a PWM
current, where D is the duty cycle of the chopper. TheLoad-Q5

In case of a pure resistive load RL, the input inductor current
ILin is expressed asI Lin =ILoad

Inverter

Load

12
W = Lin I Lin .
2

Q2Q3Q4 Q5

L oad

For an analytical purpose of the buck chopper circuitry,
we can consider this circuit comprises two buck chopperswith commonly controlled switch (Q1). The chopper
behaves as a current source where during positive andnegative cycle, a part of the input inductor current (ILin) willflow back through the blocking diode (ID) while the switch
Q1 is on and the remaining current (IL) will flow into theinverter during an off-state of the switch Q1.
A current controller is applied to regulate the currents
flowing into the input inductors (Lin) and amplitude of theoutput current (Ipwm) simultaneously. Making the input
inductors current (ILin) and the output current (Ipwm) follow
the reference current (Iref) is the objective of this current
controller. The switching gate signal of the switch Q1(dc-to-dc switch) is generated by comparing the error signalof the average value of detected steady state current flowing
through both of the input inductors I1 and I2, and atriangular carrier wave after passing through a proportional
integral (PI) regulator. This signal is used to control theduty cycle of the chopper to obtain balanced input inductor
currents I1 and I2. In order to avoid uncontrolled state ofthe output current by modulation signal during the on-stateof the switch Q1, the switching time between switch Q1 andthe inverter switches must be synchronized as shown in Fig.
2.
When the chopper switch is on for a period of DT, theinput inductor is charged by photovoltaic energy to its
maximum energy W expressed asFig. 4. Equivalent circuit diagram of proposed inverter.

Carrier signal 1

Reference signal

Carrier signal 2
Gating signal

Gating signal

Fig. 5. Sinusoidal PWM for three-level current generation.

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平成 20年電気学会産業応用部門大会

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on-state circuit or the inductor’s energy charging mode andenergy discharging mode of the input inductor in this circuit
can be represented in Fig. 3. From this figure, the current
flowing into the inverter (IL) can be expressed as−RLt

I L = iL 0 e Lin ,TABLE I
CIRCUIT PARAMETERS AND TEST CONDITIONS
Component(3)

where iLo is the initial current flowing through the input
inductor. From this equation, it can be found that thecurrent decreases, depending on the time constant of input
inductance (Lin) and the load resistance (RL).
2.2. Inverter Circuit and Modulation Strategy
The inverter circuit consists of four controlled switches
and six diodes as shown in its equivalent circuit of Fig. 4,
where I1 and I2 represent the steady state input inductor
currents. The diodes connected in series with switching
devices are used to protect them from transient voltage dueto the load current switching. Sinusoidal pulse widthmodulation (SPWM) is used to generate the three-level
output current pattern with line-frequency (50 or 60 Hz).
The gating signals of the inverter switches are generatedby using a sinusoidal reference wave modulated with two
triangular carrier waves with the same frequency but with
opposite offset values as shown in Fig. 5. This strategy isable to perform the sinusoidal PWM of the inverter. Thesesignal patterns are used to control the inverter switches Q2,Q3, Q4, and Q5. The frequency of the reference signal
determines the output current frequency of the inverter andits magnitude can be controlled by the amplitude of thefundamental current component.
The required three-level output current (positive, zero andnegative levels) are generated as follows:
1) Positive-level current: Q5 is on, connecting the input
inductor current (I1) to the load. Q4 is on, making thecurrent path for input inductor current (I2). Q2 andQ3 are off.
2) Zero-level current: Q2 and Q4 are on, making thecurrent path for current I1 and I2, respectively.
Switches Q3 and Q5 are off.
3) Negative-level current: Q3 is on, connecting thecurrent I2 to the load. Q2 is on, making the current
path for input inductor current (I1). Q4 and Q5 areoff.
The inverter switching frequency is chosen to achieve low
noise as well as small harmonic components of the output
current and a smaller size of an output current filter bypushing the switching harmonic component to a higher
frequency range.
3. EXPERIMENTAL RESULTS
In order to verify and to prove the proposed configuration,a laboratory prototype was set up with 300-V 30-A
F K 3 0 S M- 6 p o w e r MO S F E Ts a n d 1 2 0 0 - V 1 6 -A
HFA16PB120 fast recovery diodes. The experimentalcircuit specifications are listed in TABLE I. Fig. 6 showssome experimental waveforms, where the three-level output

Type or value

Input voltage

120 V

PV internal resistance

10 Ω

Input inductor

100 mH

Decoupling capacitor

220 µF

Chopper switching frequency

10 kHzInverter switching frequency

30 kHzFilter capacitor (Cf)220 µF
RL = 5.5 Ω, LL = 3.4 mH

Load (RL,LL)
Modulation Index

0.9

TABLE IIGRID CONNECTED TEST CONDITIONS
Input voltage

DC 200 V

Input inductor
Output filter

100 mHC = 2 μF, L = 870μH, andR = 2ΩPower gridAC 100 V

current waveform is indicated in Fig. 6(a). Its FFTanalysis result is shown in Fig. 6(b). As can be seen inthis result, most of the harmonic amplitudes are less than3%. The total harmonic distortion of this three-level
output current is 5.48%. Fig. 6(c) show the load current
after filtering. The current waveform is almost perfectsinusoidal with very small distortion (THD = 0.67%). Fig.
7 shows the current flowing in both the input inductor 1 andthe input inductor 2.
4. GRID CONNECTED OPERATION TESTIn this section, the experimental results of proposed
inverter during grid connected operation are presented.
The experimental condition is listed in TABLE II. Fig. 8
shows the inverter output current and the voltage of power
grid during experiment. The inverter injects a sinusoidal
output current into the power grid with unity power factor
operation. Fig. 9. shows the harmonic profile of theinjected inverter output current during grid connected
operation.
5. CONCLUSION
In this paper, a three-level current-source inverter with afully common-source topology at both of the chopper andinverter sides is experimentally verified. Using this new
current source inverter topology, the number of the gate
drive power supplies can dramatically be reduced to only asingle to drive all power switches. The inverter works
well injecting sinusoidal current with unity power factor
during grid connected operation.

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86

6

4

4Current (A)2Current (A)20-2

0-2-4-4-6
200-6-0.01

0.00

0.01

1000.02

Voltage (V)-8-0.02

Time (s)(a) Three-level output current waveform.

0-100

FFT of Three-level output current (%)

100-200- 0 .0 2- 0 .0 1

0.1

0 .0 20200400600

800

FFT of inverter output current (%)

100

1000 1200 1400 1600 1800 2000

Frequency (Hz)(b) Frequency spectra of three-level output current.
864Current (A)0 .0 1

Fig. 8. Inverter output current and grid voltage

10.01

0 .0 0T im e ( s )101010.1

20200

4006008001000 1200 1400 1600 1800 2000

Frequency (Hz)0-2

Fig. 9. Harmonic profile of inverter injected current.

-4-6-8-0.02

-0.01

0.00

0.01

REFERENCES

0.02

T im e (s)(c) Load current waveform.
Fig. 6. Experimental result of three-level common-source CSI.

8Current (A)64208[1] P. G. Barbosa, H. A. C. Braga, M. C. Barbosa and E. C. Teixeria,“Boost current multilevel inverter and its application on single phasegrid connected photovoltaic system,” IEEE Transaction on PowerElectronic, Vol. 21, No.4, July 2006.
[2] 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.
[3] F. L. M. Antunes, A. C. Braga and I. Barbi, “Application of ageneralized current multilevel cell to current source inverters, IEEE
Transaction on Power Electronic,” Vol. 46, No.1, February 1999.
[4] S. B. Kjaer, J. K. Pedersen and F. Blaajerg, “A review of single phasegrid connected inverters for photovoltaic modules,” IEEE
Transaction on Industry Application, Vol. 41, No.5,
September/October 2005.
[5] T. Noguchi, S. Togashi, and R. Nakamoto, “Short current pulse based
maximum power point tracking method for multiple photovoltaic andconverter module system,” IEEE Transaction on Industrial
Electronics, Vol. 49, No.1, February 2002.

Current (A)6420- 0 .0 2

- 0 .0 10 .0 00 .0 10 .0 2

T im e ( s )Fig. 7. Input inductor current 1 and 2 of inverter.

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