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A Current-Source Grid-Connected Converter Topology

for Photovoltaic Systems

G. Ertasgin, D.M. Whaley, N. Ertugrul and W.L. Soong

School of Electrical and Electronic Engineering

The University of Adelaide

gurhan@eleceng.adelaide.edu.au

ABSTRACTThis paper investigates the performance of a grid-

connected current-source converter topology for PV

cells. The constant current source is realised by a large

DC link inductor connected in series with the PV panel.

A boost switch (named as current waveshaper) is used to

produce a modulated output current that resembles the

rectified grid voltage, which is in-phase with the grid.

An H-bridge inverter with line-frequency commutated

thyristors unfolds the output of the current waveshaper

to produce a sinusoidal AC output current. The

proposed converter concept is verified with simulationsand preliminary experimental results.

1. INTRODUCTIONRenewable energy sources such as wind, photovoltaic

(PV) and geothermal have received much attention

recently as alternative means of generating electricity.

In particular, small scale PV systems are increasing in

numbers due to decreasing costs, and efficiency

improvements [1], which are convenient for local power

generation.

The generated power in PV cells can be used in a stand-alone system or can be fed to the AC main grid. In stand-

alone systems, the output power of the PV system can

also be stored in batteries. However, the battery systems

are expensive, bulky and require high maintenance.

Where utility power is also available, another solution isto feed the power into the grid, which requires a grid-

connected inverter (GCI). With a GCI, excess power is

bought and credited by the utility, and grid power is

available at times when the local demand exceeds the PV

system output.

Although GCIs are more expensive than inverters for

motor drives or stand-alone systems, this is primarilydue to lower sales volumes and also the complexity of

meeting the strict grid requirements, such as powerquality (harmonic content) and safety standards.

This paper considers an alternative grid-connected

converter topology to offer solutions for small scale PV

systems, which can be cost effective and can meet thegrid requirements.

The layout of the paper is as follows: Section 2 discusses

the existing PV system topologies. The proposed

converter circuit is explained in Section 3, and the

subsections of this circuit and associated modelling

issues are described in Section 4. In Section 5, the

computer simulation of the entire converter is given. The

paper concludes with experimental results to verify the

models developed.

2. BACKGROUND TO PVGCITOPOLOGIESAn early type of GCI is the current-source inverter (CSI)as shown in Figure 1a. In this GCI, a DC link inductor

was utilised to act as a current source and a line-

frequency commutated inverter accommodated to

produce a square-wave output current. Although, this

concept is simple, it requires substantial filtering at the

output stage to meet the grid harmonic standards.

Figure 1: Existing converter topologies for PV systemsa) CSI topology b) VSI topology c) Two-stage VSI with

DC-DC boost converter.

The most common GCI configuration uses a voltage-

source inverter (VSI), as shown in Figure 1b. Unlike the

CSI topology, a large DC link capacitor is utilised to

produce a constant input voltage. Then, a pulse-width

modulated (PWM) inverter is used to generate a

sinusoidal AC output current. The VSI topology tends to

be slightly cheaper and more efficient than the CSI

topology as the DC link capacitor has lower losses and

may be lower cost than the DC link inductor [2]. Itshould be noted here that the power grid can also be

treated as a voltage source.

Figure 1c shows a two-stage converter topology which

consists of a DC-DC boost converter and a VSI inverter.

In this circuit, the boost converter performs maximum

power point tracking (MPPT) for the PV panel, while

delivering a constant DC input voltage to the VSI that is

controlled to produce a sinusoidal output current [1].

Although single-stage converters result in size and

weight reductions compared with two-stage converters

[2], they require more complex control algorithms to

operate correctly, especially when employing MPPT [3].On the contrary, two-stage converters often suffer in

terms of efficiency and reliability due to the increased

number of switching components [1].

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3. PROPOSED PVCONVERTERTOPOLOGYA desirable primary feature of a GCI circuit is to feed a

sinusoidal current into the grid, which is in phase with

the grid voltage, hence a maximum power factor can be

achieved and the power grid will not be polluted. In

addition, it is also desirable that the GCI system should

be efficient, low cost and high power density.

The proposed two-stage PV GCI topology is based on acurrent-source inverter and illustrated in Figure 2. This

circuit topology is an extension of a Switched-Mode

rectifier (SMR) circuit that was originally proposed for

automotive applications [4], where it acted as a DC-DC

converter. The SMR concept was investigated for usewith a small-scale wind turbine in [6] as a current-source

inverter. This paper examines the use of the SMR circuit

topology with a PV cell, to operate as a grid-connected

inverter.

The circuit uses a DC link inductor (L) in series with the

PV panel to produce a constant-current source (Figure

2). A boost switch (will be named as a current wave-shaper, WS in this paper) is used to produce a PWM

output current that resembles a rectified sinewave that isin-phase with the grid. The thyristor based H-bridge

inverter in the circuit unfolds the output of the current

wave-shaper to produce a sinusoidal AC output current.

An output LC filter (CF and LF) is used to remove the

PWM switching components (Figure 2).

Figure 2: Proposed current source inverter topology forthe grid connected PV systems

In the proposed circuit given above, when supplied froma DC current source, the boost switch produces an output

current which is proportional to (1 d), where d is thePWM duty-cycle of the switch. Though the boost switch

(WS) resembles a boost converter, under these

circumstances it operates as a current divider, or current

waveshaper. An important feature of the circuit is that,

due to the current control scheme implemented, the

control algorithm does not require the use of an output

current sensor.

The H-bridge inverter (unfolding circuit) in the circuit is

controlled by a microcontroller, which is also used to

detect zero-crossings of the mains voltage and to control

the duty-cycle of the WS switch. In addition, the

microcontroller stores a look-up-table (LUT) thatincludes the reference current waveform. As can be seenin Figure 2, the thyristor switching is determined from

the zero-crossing of the mains voltage (Vref), which

ensures an output current that is synchronised with the

grid voltage.

It should be emphasised here that although the thyristors

in the H-bridge commutate at zero-currents, the on

resistance of the WS switch creates a current divider

with the load; thus the thyristors can only commutateproperly if the on resistance of the switch is sufficiently

low. This ensures that the load current is less than thelatching current of the thyristor.

It should be reported here that due to the addition of an

external inductor in the circuit proposed, the operation of

the converter circuit is similar to the concept developed

in [4], where the alternator itself had a large winding

inductance. Furthermore, it can be noted that unlike the

permanent magnet generator implemented in [5], the PV

cell application has a much greater constant current

region (hence a wider power range), as seen in Figure 3.

0 20 40 60 800

5

10

15

20

DCC

urrent(A

)

DC Output Voltagte (V)

200rpm

400rpm

600rpm

800rpm1000rpm

0 5 10 15 20 25

0

1

2

3

4

5

Voltage (V)

Current(A)

0C

25C

50C

75C

Figure 3: Current-Voltage Curves of a high-inductancePM generator (left) [6] and a PV cell (right).

4. PVARRAY CHARACTERISTICS/MODELLING4.1. CONVENTIONAL PVMODELSA PV cell can be modelled by various equivalent

circuits. Figure 4 illustrates two of these circuits: one-diode and two-diode models. Although the two-diode

model provides greater accuracy, the one-diode model is

sufficient to simulate a PV cell with a resistive load [1].

Figure 4: Two different PV cell electrical models

The PV module used in this study is manufactured byBP Solar

(BP380J). The simulation studies in this paper

are based on the one-diode model as given in equation

(1). This equation accommodates the effect of solar

irradiance and cell temperature variations.

Here I, IPH, and V are the output current, the lightinduced current, and the output voltage respectively, Rs

is the total series resistance, Ns is the number of series

resistances,A is the ideality factor, and VT is the thermalvoltage.

Constant

Current Region

(1)1

S OC

TSPH

V IR V

N A V I I e

+

=

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The simulated current-voltage (IV) locus for the PV

module under test is given in Figure 5. A set of

measured test results taken under strong sunlightconditions is also shown in the same figure, which

corresponds to the IV curve of 850 W/m2

at 50C.

0 5 10 15 20 250

1

2

3

4

5

Voltage (V)

Current(A)

1000W/m2

850W/m2

600W/m2

400W/m2

200W/m2

0 5 10 15 20 25

0

1

2

3

4

5

Voltage (V)

Current(A)

25oC

0oC

50oC

75oC

Figure 5: Simulations of the BP Solar 380 PV cell current-

voltage loci, showing the dependence on (left) solarirradiation, and (right) cell temperature.

4.2. DARKIVMEASUREMENTSIn this paper, an alternative measurement techniquecalled the dark IV measurement [7] was used to

simulate sunlight operation of the PV cell. This method

involved covering the PV cell (to eliminate the light

induced current) and using an external constant current

source to simulate the light induced current. Figure 6a

demonstrates this mode of operation.

For faster simulation purposes, a four diode model,

based on diodes with an idealised fixed voltage drop,

shown in Figure 6b, was developed in the paper to model

the PV cell characteristics. The calculated characteristicsusing the model in Figure 5, and the measured

characteristics based on the dark IV tests (Figure 6a) are

all given in Figure 7.Rs

Rp

Figure 6: a) The equivalent circuits of the dark IVtechnique, and b) the simulation model.

0 5 10 15 20 250

1

2

3

4

5

Voltage (V)

Current(A)

MATLAB Model

4 Diode Model

Dark IV measured

0 5 10 15 20 25

0

10

20

30

40

50

60

70

80

90

Voltage (V)

Current(A)

MATLAB Model

4 Diode Model

Dark IV measured

Figure 7: The IV locus (left), and power-voltage locus(right), based on the conventional model, simplified four

diode model, and the measured dark IV data.

5. CONVERTERSIMULATIONS5.1. DCLINKINDUCTORSIZINGIn a single-phase inverter, instantaneous input and output

power which fluctuate twice the AC mains frequency

(100Hz at 50Hz supply). Besides, the PV cell supplies

maximum output power when its output voltage andcurrent are constant. As stated earlier, a DC link inductor

is used as an intermediate energy storage element. The

analogy between an inductor in a current source and an

input capacitor in a voltage-source inverter can be used

for sizing an inductor.

In the proposed topology, the difference between the

instantaneous inverter input power (at 100 Hz) and the

DC power of PV module is supplied or absorbed by the

inductor. Therefore, this results in a 100 Hz current

ripple in the inductor current and hence the PV cell

current. The ripple current can be minimised byselecting a sufficiently large inductor. Guidelines for

calculating a suitable inductance value are shown below

in (2)-(5). If the average power delivered by the PV cell

is given as

Pavg= Pcell (2)

The input power to the H-bridge inverter is sum of the

PV cell power and the instantaneous power, see below

Pinput= Pavg+Pavgsin(2100t) (3)

Therefore, the stored energy in the inductor can be

calculated easily by integrating the termPcell Pinput.

E =(Pcell Pinput) dt = PavgK (4)

In the above equations, Pavg is the average value of theinstantaneous inverter input power, Pcell is the PVmodule power,Pinput is the inverter input power,Eis the

peak energy which has to be stored by the inductor. Kisthe integration constant andL is the inductance. Figure 8illustrates a typical variation of current ripple in an

inductor and the value of inductance as a function of the

ripple current (Figure 8b).Therefore, the inductor valuecan be calculated by equalising the change in the

inductor energy and the required energy in (4), see (5).

L=2min2max

2

II

KPavg

(5)

Time

Current

I

I

t=DT

WS is on

IL

max

min

IL

0 0.1 0.2 0.3

0

1

2

3

4

Inductance (H)

RippleCurrent(A)

Figure 8: a) The variation of current ripple in an inductorand b) the value of inductance as a function of the ripple

current.

In Figure 8a,iL is the ripple current. As seen in Figure8b, the inductor current ripple is inversely proportional

to the inductance. The larger the current ripple

amplitude, the more the solar cell output power is

reduced below its ideal maximum power point value.

Therefore, in the selection of an inductor, a size trade-off

must be made, as the inductor size, cost, and losses

increases with increasing inductance. Using the above

criteria and the computer simulation studies, the value of

the DC link inductor is chosen 82mH.

5.2. PROPOSED TOPOLOGY SIMULATIONSThe inverter is simulated using PSIM

(a simulation tool

designed for power electronics and dynamic systems).

Figure 9 shows the basic components of the simulation

a) b)

a) b)

The value

of L used

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model, including the PV cell that is represented by the

subcircuit PV model(Figure 6b). In the simulationmodel, two switches were also included to allow the userto easily switch between various loads: pure resistive,

resistive + voltage source, and pure voltage source

(grid). Similar switches were also accommodated in the

experimental test setup as will be described later.

In this study, as the inverter output is expected to be low(due to the PV cell), the voltage source was simulated as

a low voltage, grid frequency AC voltage source.

Figure 9: PSIM schematic of the converter topology.

5.2.1. RESISTIVE LOAD SIMULATIONSFigure 10 shows the constant input current, the unfoldingcircuit output current, and the filtered inverter output

current, aimed to demonstrate the converter concept. In

the figure, the inverter output currents for three different

modulation indices, i.e. 100, 75, and 50% are also

shown. As shown in the results that the output current

magnitude is linearly related to the modulation index.

Figure 10: Resistive load simulation results.TOP: input current (IPV), and unfolded output current

(IUNF); BOTTOM: filtered inverter output currents (I_INV)for various modulation indices.

5.2.2. GRID-CONNECTED SYSTEM SIMULATIONSIn this section, the inverter shown in Figure 9 is loaded

by the resistor connected in parallel to the voltage

source. The simulated inverter and grid currents, as well

as the sum of the two (that is, the load resistor current)

are shown in Figure 11. This stage of the simulation is

considered as a useful intermediate step where the

inverter is still supplying the power to the resistive loadand the grid current is ideally zero. It is expected that

the load resistor current (sum of inverter and grid) will

be sinusoidal, of which the inverter supplies most.

However due to the distortion in the inverter outputcurrent, the grid (voltage source) supplies the necessary

current to yield an undistorted sinusoidal load current.

Figure 11: Simulated grid connected currents.TOP: inverter current (I_INV), grid current (I_SC);

BOTTOM: the sum of the inverter and grid currents.

In the final stage of the simulation, the load resistor was

removed, which allows the inverter to feed power intothe grid only. Figure 12 shows both the inverter current

(top) and voltage (bottom) waveforms. As can be seen in

the figure, the inverter current waveform contains a

degree of harmonic distortion. The increased distortion

level in the current is likely to be caused by the lowerload impedance in the grid-connected case compared tothe resistive + voltage source load case given earlier.

Figure 12: Simulated inverter current (top), and voltage(bottom) waveforms for the pure grid-connected system.

6. EXPERIMENTAL SET-UP AND RESULTSThe grid-connected inverter and the PV module were

tested in the laboratory. In this study, a low-voltage solar

panel (22.1V) is utilised to demonstrate the operation of

the topology proposed. The inverter operation was

synchronised with the grid using a step-up line

transformer. Two photos of the experimental setup are

given in Figure 13. Firstly, the PV module was covered

and the dark IV method was used to simulate a solarirradiance of 1kW/m

2.

Figure 13: Various components of the converter system.

(1) current wave-shaper, (2) thyristor and MOSFET drivercircuits, (3) microcontroller, (4) unfolding circuit, (5)

variable capacitor bank, (6) variable resistive load, (7)isolation transformer, and (8) auto-transformer.

grid currentinverter current

load (grid + inverter) current

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Preliminary experiments were carried out using a

variable resistive-capacitive load. This load comprised of

parallel combinations of various non-polarised

electrolytic capacitors (100F- 1000F) and a variableresistor load bank (0.1-300). A reduced voltage ACvoltage source was achieved using an autotransformerand an isolation transformer.

6.1. RESISTIVE LOAD EXPERIMENTAL RESULTSThe measured input current, the unfolding circuit output

current, and the filtered inverter output current are

shown in Figure 14 for the resistive load case. These

results match those seen in Figure 10, verifying that eachstage of the inverter operates correctly. Figure 15 shows

the inverter current, voltage, and instantaneous power;

the scales are 10:1, 10:1, and 100:1, respectively.

Figure 14: Measured grid-connected inverter currentwaveforms showing PV output, wave-shaper output,

unfolding circuit output and load currents.

Figure 15: Measured waveforms (current, voltage andpower) in the test setup, at low output power (left) and at

the maximum inverter output power (right).

As stated previously, the inverter output voltage is in

phase with the output current, which is demonstrated in

Figure 15 (left), together with the output power of the

inverter. Although this current waveform is sinusoidal at

low output powers, it was observed that the distortion

level increases at higher output powers. Figure 15 (right)

shows the distorted current waveform at the maximum

power operating point. The current deformation is

caused by the increasing output voltage which causes the

PV cell to move away from the constant current region

of the IV locus (Figure 3, right), and increases the input

current ripple.

In the results provided above, the load resistance and

capacitance were the only variables adjusted to control

the inverter output power at 100% modulation index.

The effect of varying the modulation index to control theoutput power was also studied. As the PV cell operates

mainly in the constant current region, the inverter output

power is proportional to the modulation index squared,

due to the current magnitude being directly proportional

to the modulation index. This power relationship is

shown in Figure 16, where the modulation index was setto 100%, 71% and 50%.

Figure 16: Measured resistive load results for threemodulation indices of 100%, 71% and 50%.: inverter

output powers (left), and input currents (right).

It should be emphasised here that a reduction in the

modulation index reduce the inverter current magnitude

(and hence power), and also reduces the input current

ripple and pushes the PV cell towards the constant

current region. This was demonstrated in the test setup,when the input voltage exceeded the maximum PV cell

power point for a modulation index of 100%. By

reducing the modulation index, the PV cell was pushed

towards the constant current region, which increased the

PV input, and thus inverter output power. Figure 17shows that the maximum (PV) power was observed at a

modulation index of 87%. Although this reveals a low

system efficiency, this is largely related to the prototype

developed, which did not consider an optimal selection

of the switching devices, and inductor (Rinductor= 1.26).

50 60 70 80 90 1000

20

40

60

80

100

Modualtion Index(%)

Power(W)

Input Power

Output Power

PV MaximumPower Point

Inductorlosses

Semiconductorlosses

Figure 17: Demonstration of PV cell maximum power

point tracking by varying the modulation index.

6.2. GRID-CONNECTED RESULTSIn this test, the inverter was synchronised first with a

resistive load and then connected to the AC mains

voltage source keeping the resistive load in place.

Following this the resistive load was removed to obtain apure grid connection. The result given in Figure 18 (left)

demonstrates that the converter matches the magnitude

and phase of the AC mains voltage. Figure 18 (right)

shows that once the grid connection is performed, the

inverter voltage is forced to follow the voltage source.

However, the inverter output current, in Figure 19a,

shows high levels of distortion. The grid provides the

necessary current such that the load sees a sinusoidalcurrent, as shown in the bottom trace. Similarly, Figure

19b shows the inverter, the grid, and the load currentwaveforms for the purely grid-connected case. As

expected, the sum of the currents (resistive load) is zero.

input current

wave-shaper current

unfolding circuit current

inverter current

mn = 100%

mn = 50%

mn = 71%

mn = 100%

mn = 71% mn = 50%

current

voltage

power

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Figure 18: The inverter and the grid voltages prior to grid-connection (left), and after grid connection (right).

Figure 19: Measured current waveforms: Inverter andgrid current (top), and the sum of each (bottom), for a)resistive load parallel to the grid, and b) the pure grid.

Figure 20: Comparison of the inverter output currentwaveforms for resistive load (top), resistive load parallelto the grid (middle), and grid-connected load (bottom).

6.3. SUMMARY OF THE TEST RESULTSAs stated previously, the inverter was connected to

various load configurations, which are compared inFigure 20. For each load configuration, the total

harmonic distortion (THD) was measured (Table 1) for

the inverter current and voltage waveforms. As expected,the current distortion increased and the voltage distortion

decreased when connecting the grid to the inverter (for a

given load resistance and capacitance). However, lower

distortion was observed for higher currents, and also for

the maximum power cases.

Table 1: THD summary of inverter under various loads.Inverter load Current THD Voltage THD

Resistive : low output power 6.26% 5.08%

Resistive : max output power 9.27% 10.82%

Resistive + Grid : (max power) 16.06% 4.37%Resistive : (Figure 20) 9.03% 9.49%

Resistive + Grid : (Figure 20) 17.71% 5.98%

Grid : (Figure 20) 21.21% 5.07%

7. CONCLUSIONThis paper has described a current-source converter

topology for a grid-connected PV system, which was

simulated and also realized in the laboratory. As shown,

the topology is low-cost and accommodates simple

control. The key results are :

the DC link inductor must be sized carefully to

minimize voltage and current ripples seen by the PVmodule, hence to maximize output power;

the control of the current wave-shaper and the H- bridge are simple, as the output current is linearly

related to duty-cycle;

sinusoidal output currents can be obtained without theuse of a grid current sensor;

maximum power point tracking can be performed bymonitoring the solar cell output power.

Considering the promising preliminary test results, it is

aimed to perform future modelling and implementation

studies over a range of values of solar irradiance on the

topology proposed.

The future works will incorporate high voltage PV

arrays and will study the performance of transformerless

operation.

8. REFERENCES[1] Kjaer, S.B.; Pedersen, J.K.; Blaabjerg, F., "A review of

single-phase grid-connected inverters for photovoltaicmodules," IEEE Transactions on Industry Applications,vol. 41, no. 5, pp. 1292-1306, Sept.-Oct. 2005.

[2] Calais, M.; Myrzik, J.; Spooner, T.; Agelidis, V.G.,"Inverters for single-phase grid connected photovoltaicsystems-an overview," IEEEPower Electronics Specialists

Conference, vol. 4, pp. 1995-2000, 2002.

[3] Kuo, W.C.; Liang, T.J.; Chen, J.F., "Novel maximum-

power-point-tracking controller for photovoltaic energy

conversion system," IEEE Transactions on Industrial

Electronics, vol. 48, no. 3, pp.594-601, Jun 2001.

[4] Soong W.L. and Ertugrul N., Inverterless High PowerInterior Permanent Magnet Automotive Alternator, IEEE

Transactions on Industry Applications, 2004, Vol. 40, no.4, July/Aug, pp. 1083-1091.

[5] Whaley, D.M.; Soong, W.L.; Ertugrul, N., "Investigationof switched-mode rectifier for control of small-scale windturbines," IEEE Industry Applications Conference, 2005,

vol. 4, pp. 2849-2856.

[6] Whaley, D.M,; Ertasgin, G.; Soong, W.L.; Ertugrul, N,;Darbyshire, J.; Dehbonei, H.; Nayar, C.V., Investigation

of a Low-Cost Grid-Connected Inverter for Small-ScaleWind Turbines Based on a Constant-Current Source PM

Generator Submitted toIEEE IECON Conference, 2006.

[7] King, D.L.; Hansen, B.R.; Kratochvil, J.A.; Quintana,M.A., "Dark current-voltage measurements on photovoltaicmodules as a diagnostic or manufacturing tool," IEEEPhotovoltaic Specialists Conference, 1997,pp. 1125-1128.

grid voltage

inverter voltage inverter voltage = grid voltage

inverter current

load (inverter + grid) current

grid current

inverter, grid, load current

resistive

grid

resistive + grid

a) b)