Investigation of a Low-Cost Grid-Connected Inverter for Small-Scale ...

Investigation of a Low-Cost Grid-Connected Inverter for Small-Scale Wind Turbines
Based on a Constant-Current Source PM Generator
David M. Whaley 1 * , Gurhan Ertasgin 1 , Wen L. Soong 1 , Nesimi Ertugrul 1 , James Darbyshire 2 ,
Hooman Dehbonei 2 , Chem V. Nayar 2
Abstract – This paper describes a novel low-cost grid-connected
inverter for small-scale wind turbines based on a highinductance
PM generator operating in a constant-current
output mode. The PM generator is connected to a switchedmode
rectifier (SMR) which consists of an uncontrolled rectifier
and a switch. The combination of the high-inductance
generator and uncontrolled rectifier produces a constant dc
current. The switch is used to modulate this current and
produces an output current whose fundamental component is a
full-wave rectified sinewave and is in-phase with the grid
voltage. A line-frequency commutated H-bridge inverter and
LC output filter is used to produce a sinusoidal output grid
current. The concept is verified using simulations and
experimental results.
I. INTRODUCTION
1 University of Adelaide, Adelaide
2 Curtin University of Technology, Perth
AUSTRALIA
* dwhaley@eleceng.adelaide.edu.au
A. Small-Scale Grid-Connected Wind Turbines
The growing demand for electricity is of grave concern,
when considering pollution and fossil fuel exhaustion issues;
as such, alternative renewable energy sources must be
considered, of which wind power is especially promising.
Small-scale wind turbines (

etween the rectifier and inverter to maintain a constant dc
link voltage, even at lower wind conditions, but this also
adds extra cost and complexity.
Fig. 2. Grid-connected inverter topologies with conventional low-inductance
PM generators, showing (a) early grid-connected square-wave CSI,
(b) current-controlled VSI.
D. Proposed Inverter Topology
The proposed concept is based on using a high-inductance
PM generator which acts as a current source, with a
switched-mode rectifier (SMR) and inverter (see Fig. 3). A
current source output from the generator is attractive as the
grid-connected inverter output is a controlled current.
Fig. 3. Proposed grid-connected inverter topology, showing circuit diagram
and control block diagram.
The SMR consists of an uncontrolled rectifier and a
switch. The combination of the uncontrolled rectifier and
high inductance generator acts as a dc constant current source
(see Fig. 1). With the constant current input, the switch acts
as a current chopper (or wave-shaper) in which the output
current is linearly related to the switch duty-cycle.
High-inductance generators have been used in the past
with SMRs to generate a controllable dc output current.
Examples include the use of high-inductance wound-field [4]
and interior PM [5] generators for automotive applications,
and a surface PM generator [1] for a small wind turbine. In
all these cases a dc current output was desired for battery
charging purposes and so the duty-cycle of the SMR was
kept constant under steady-state conditions.
For this grid-connected inverter application, the duty-cycle
of the SMR is modulated to produce an output current which
has the shape of a full-wave rectified sinewave and is
synchronised with the grid voltage. This output current is
then fed through a line-frequency commutated inverter (also
known as an “unfolding” circuit) and an LC filter, to produce
the desired sinusoidal output grid current (see Fig. 3).
The proposed topology has similarities to that given in [6],
which describes a three-phase PWM CSI. The main
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difference is that a high inductance PM machine is used as
the current source, avoiding the need for a bulky dc link
inductor.
A simple control algorithm is proposed based on using the
grid voltage as a reference. The switching of the unfolding
circuit and the wave-shaper PWM signal is controlled by the
zero-crossings of the grid voltage.
The main advantages of the proposed inverter are:
simplified circuit topology and control requirements; the
elimination of large, expensive inductors and unreliable dc
link capacitors; and the potential to operate over a wide wind
speed range.
E. Simulation Model
The inverter system was simulated using PSIM ® and the
grid-connected circuit model is shown in Fig. 4. A MOSFET
was used for the SMR switch and thyristors for the output
inverter H-bridge.
Fig. 4. PSIM inverter circuit model, showing the PM generator, rectifier,
current wave-shaper, H-bridge inverter, voltage-source load, PWM signal
generator, modulation index controller, and zero-crossing detector.
F. Paper Layout
The layout of the paper is as follows: section II describes
the experimental test arrangement, and sections III and IV
describe the resistive load and grid-connected results,
respectively.
II. EXPERIMENTAL TEST ARRANGEMENT
This section describes the PM generator, and the power
electronics and controls hardware used to obtain the
experimental results.
A. PM Generator and Dynamometer
An outer-rotor surface PM machine used in Fisher &
Paykel washing machines was selected for the test
generator. These have high inductance due to their use of a
concentrated stator winding. Its torque rating is consistent
with a several hundred watt wind generator. The generator
used had been rewound to produce an output voltage
compatible with 24 V battery charging. An example of a
Fisher & Paykel machine is shown in Fig. 5 (a). The
machine shown is similar but not identical to the test
generator. During the testing, the high-inductance PM
generator was driven by a 5 kW dc machine.
Figure 5 (b) shows the measured and calculated PM
generator’s dc output current versus output voltage
characteristics. This was measured at various speeds when

operating into a resistive load through a three-phase rectifier.
The calculated results were obtained based on the measured
parameters of the delta-connected PM generator as shown in
Table 1. These parameters were used in the generator and
rectifier models shown in Fig. 4, and the rectifier was loaded
with a variable resistive load. The calculated results show a
good correspondence with the experimental results and give
confidence in the simulation model. They are also similar to
the idealised characteristics shown previously in Fig. 1.
The generator is a reasonable approximation to a constantcurrent
source when the dc output current is above about 18
A. This corresponds to a peak output voltage limit of about
20 V at 600 rpm, and 40 V at 1000 rpm.
DC Current (A)
25
20
15
10
5
1000rpm
800rpm
600rpm
400rpm
200rpm
0
0 25 50 75
DC Voltage (V)
100
(a) (b)
Fig. 5. (a) Photograph, and (b) current-voltage locus of PM generator. Solid
lines show simulations, whilst points represent measured experimental data.
TABLE 1
MEASURED PM GENERATOR PROPERTIES
Parameter Value
Stator Connection Delta
Pole Pairs 24
Short-Circuit Line Current 16 Arms
Back-EMF constant, k 0.0668 V/rpm
Phase Inductance, L 2.87 mH
Phase Resistance 0.519 Ω
Maximum Torque 9.08 Nm
B. Power Electronics and Controls Implementation
The SMR was constructed using a line frequency, threephase
uncontrolled rectifier and a high-current MOSFET. As
the generator stator currents are continuous at high speeds,
the rectifier does not need a high-frequency switching
capability. The unfolding circuit was constructed using
thyristors and pulse transformers were used to drive the
gates. The semiconductor devices are rated at 40A. Sections
of the inverter are shown below in Fig. 6.
1
3
2
5
Fig. 6. Sections of the inverter hardware, showing the: (1) wave-shaper, (2)
rectifier, (3) thyristor / MOSFET driver circuits, (4) micro-controller, (5)
thyristor H-bridge inverter (unfolding circuit).
4
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The inverter was controlled by a micro-controller which
used the zero-crossings of the grid voltage to generate the
wave-shaper PWM signal and thyristor trigger pulses. A
PWM switching frequency of 4 kHz was used. The PWM
signal was based on a duty-cycle look-up table whose pointer
was reset at positive grid zero-crossings. Adjustment of the
modulation index (ma) of the PWM waveform allows the
output current magnitude and hence power to be controlled.
III. RESISTIVE LOAD OPERATION
The simulation and experimental testing was conducted in
two parts. In this section, testing of the inverter with a
resistive load is described. In the next section, testing with a
voltage source load was then performed.
A. Resistive Load Simulations
Figure 7 shows the simulation results using the model
shown in Fig. 4 for the PM generator at 500 rpm for an RC
load of 0.93 Ω // 2000 µF. The waveforms shown include the
input, wave-shaper output, unfolding circuit output, and the
inverter output currents. These waveforms indicate the
correct operation of each inverter stage.
Fig. 7. Calculated inverter currents showing the input, wave-shaper,
unfolded, and output current.
The effect of the inverter modulation index (ma) variation
is observed in Fig. 8, where the output current is directly
proportional to ma. With a voltage source load the output
power would be proportional to ma. Figure 8 also shows that
the current THD is increased by reducing ma.
100%
75%
50%
Fig. 8. Calculated inverter output current for modulation indices of 100%,
75% and 50%.

B. Preliminary Testing
Preliminary tests on the circuit were carried out with two
power sources. Firstly, a dc power supply and series
inductance was used to simulate an ideal constant current
source, and secondly, the PM generator was used.
Figure 9 shows various inverter current waveforms using
both the dc power supply with series inductor (a), and the
PM generator (b), as constant current sources, under the same
test conditions shown in Fig. 7. The waveforms seen are the:
(1) rectifier, (2) wave-shaper, (3) unfolding circuit, and (4)
inverter output currents. The measured results show a good
correspondence to the calculated waveforms of Fig. 7, and
give confidence in the simulation model.
(1)
(2)
(3)
(4)
(a) (b)
Fig. 9. Measured current waveforms (a) using a dc power supply and series
inductance, (b) using the PM generator at 500 rpm. Scales are (a) 2 A/div,
and (b) 20 A/div. The zero position for each waveform is indicated by the
small arrow on the left.
The waveforms seen in Fig. 7 illustrate the key principles
of the inverter, i.e. the power source acts as a current source,
and the wave-shaper acts as a PWM current chopper which
produces a waveform whose fundamental component is a
full-wave rectified sinewave. The action of the H-bridge
inverter is also illustrated, and the final output current is
closely sinusoidal.
The PM generator’s rectified output current has a high
frequency ripple component due to the bridge rectifier. It
also shows a lower frequency 100 Hz ripple component due
to the output voltage variation causing an input current
variation (see Fig. 5b). The effect of the input current
variations is to cause the amplitude of the PWM wave shaper
output to be rather noisy and to cause some distortion of the
output current.
Figure 7 also shows that the input current ripple in the PM
generator case does not greatly affect the load current nor the
current THD; see Table II for a detailed comparison of the
two current source cases.
TABLE II
INVERTER PERFORMANCE FOR EACH CURRENT SOURCE
dc power PM generator PM generator
supply (500 rpm) (1000 rpm)
RMS current input 2.03 A ≈ 21 A ≈ 22 A
Current ripple (input) 0 A 8.8 A 4.4 A
RMS current output 1.26 A 12.8 A 13.8 A
Output power 1.52 W 152 W 178 W
Current THD 3.7 % 4.0 % 3.0 %
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C. Constant Input Current Assumption
For high quality grid output current waveforms it is
necessary that the input current be relatively constant at the
peak output voltage. This condition is satisfied when the
peak output voltage is much less than the open-circuit
voltage.
Consider a load resistance of 1 Ω. With ma = 100%, Fig.
10 indicates a peak current of about 17 A at low output
voltages. The peak output voltage is thus about 17 V. The
voltage-current loci indicate that changing the PM generator
speed from 500 rpm to 1000 rpm, increases the open-circuit
voltage from 45 V to 90 V, but has little effect on the shortcircuit
current.
DC Current (A)
25
20
15
10
5
R L =1Ω
R L =5Ω
1000rpm
0
0 20 40
500rpm
60 80 100
DC Output Voltage (V)
Fig. 10. Simulated IV locus of generator for speeds of 500 and 1000 rpm
(from Fig. 5b) including output load lines for two values of load resistance.
Figure 11 illustrates the measured input and output
currents for these two speeds. Increasing the speed clearly
makes the input current more constant. Note that the peak
input current does not change much, but the ripple is
reduced. This slightly increases the average input current
and hence the output current. The higher speed also
significantly improves the output current THD, from 4% to
3% (see Table II).
scale: 10A/div scale: 10A/div
(a) (b)
Fig. 11. Measured inverter input (top) and filtered output (bottom) currents,
for generator speeds of (a) 500 rpm, and (b) 1000 rpm. Scales are 10A/div.
Consider an example where the constant current
assumption breaks down. Let us increase the load resistance
by a factor of five to 5 Ω (see Fig. 10). With an ideal
constant current source input, the peak output voltage should
increase by a factor of five to about 85 V. However at 500
rpm the open-circuit voltage is only 45 V and so the constant
current input assumption will break down. This case is
illustrated in Fig. 12 showing both simulated and measured
results for the input and output current waveforms for a load
resistance of 5 Ω, which show a good correspondence. Note
the filter capacitor was reduced by a factor of 5 to maintain
the same filter time-constant.

scale: 10A/div
(a) (b)
Fig. 12. Inverter input (top) and output (bottom) current for RL = 5 Ω // Cf =
100 µF. (a) shows the simulation and (b) shows the measured results.
IV. EXPERIMENTAL GRID-CONNECTED TESTING
In this section the grid-connected operation of the inverter
was tested. Due to the low voltage generator used, this was
experimentally simulated by using an autotransformer to
provide a reduced-voltage ac voltage source load. The
autotransformer’s leakage reactance also conveniently
provided some inductance to filter the output current (see
Fig. 3).
The grid-connection was performed in two stages. The
inverter was initially operated into a resistive load. The
magnitude of the output voltage of the inverter and
autotransformer were made equal and it was verified that the
two voltages had the same phase. The intermediate gridconnection
stage involved connecting the autotransformer in
parallel with the resistive load. In this case, ideally the
autotransformer should provide zero current and the output
power of the inverter should still be absorbed by the load
resistance. The pure grid-connection stage involved
removing the load resistor and hence feeding the inverter
output power into the grid.
A. Preliminary Grid-Connected Inverter Results
Figure 13 (a) shows the inverter, grid and resistive load
currents for the intermediate grid-connected stage. The
resistive load current waveform is a scaled version of the
sinusoidal grid voltage waveform shown in Fig. 13 (b).
Ideally the grid current should be zero in this situation,
however in practice it supplies the difference between the
slightly distorted inverter output current waveform and the
resistive load current waveform.
(a) (b)
Fig. 13. Intermediate grid-connected case measurements, (a) shows the
inverter and grid currents (top) and the resistive load current (bottom), and
(b) shows the grid voltage. Scales are (a) 10A/div, (b) 5V/div.
The pure grid-connected case was obtained by removing
the load resistance. The resulting grid voltage and current
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waveforms are given in Fig. 14. Removing the load
resistance causes the inverter current to flow through the
autotransformer’s leakage inductance into the mains. The
extra inductance causes the inverter current to lag the voltage
by about 27° (power-factor ~ 0.89). This can be corrected by
appropriately adjusting the control timing. The current
waveform also shows greater harmonic distortion. This
could be partly related to resonance between the filter
capacitance and the transformer leakage reactance.
voltage current
Fig. 14. Measured inverter output voltage and current for the pure gridconnected
case. Scales are 10V/div (voltage), and 5A/div (current).
The effect of varying the filter capacitance, grid voltage
and modulation index are studied in the following sections.
B. Effect of Grid Voltage Variation
The inverter current was monitored for rms grid voltages
of 11, 13, and 15 V, see Fig. 15. It is seen that increasing the
grid voltage causes the output current to decrease in
magnitude and the THD to increase. This is because the
generator is shifted toward the voltage source region and its
output current shows increasing ripple. The grid voltage
(secondary of autotransformer) THD may be related to the
current THD due to the transformer leakage reactance.
Fig. 15. Inverter output currents (left) and grid voltages (right) for the purely
grid-connected case, showing the effect of increasing the grid voltage. The
grid rms voltages are 11 V (top), 13 V (middle) and 15 V (bottom). Scales
are 10A/div.
Figure 16 shows the inverter current THD as the grid
voltage is increased. This experiment was first performed
whilst maintaining a constant filter capacitance (Fig. 16a),
and was repeated whilst adjusting the filter capacitance (Fig.
16b) to obtain the lowest THD for each grid voltage. The
large current THDs occur in the constant capacitance case, as
the filter inductance is increased by increasing the grid
voltage (increasing turns ratio). Hence the filter capacitance
was varied during the repeated tests, to reduce the THD.

THD (%)
25
20
15
10
5
Current
0
0 5 10 15
Volatge
20 25
Grid Voltage (V)
THD (%)
25
20
15
10
5
Current
Voltage
0
0 5 10 15 20 25
Grid Voltage (V)
Fig 16. Inverter current and voltage THD vs. grid voltage, for (a) constant
filter capacitance (600 µF), and (b) various capacitances (300-2000 µF) to
minimise current THD.
C. Effect of Modulation Index Variation
The PWM modulation index was varied for the pure gridconnected
case, and the effect on the inverter current is seen
below in Fig. 17. The filter capacitance and grid voltage were
kept constant, and hence the output power should be linearly
proportional to ma.
Fig. 17. Measured inverter output current for modulation indices of 100%
(top), 80% (middle) and 60% (bottom) for the pure grid-connected case.
The effect of the modulation index variation is seen in Fig.
18. Figure 18 (a) shows the current and voltage magnitude
response to the ma variation, whereas Fig. 18 (b) shows the
resulting current and voltage THD.
For an ideal constant current source, the inverter current is
expected to vary linearly with the ma; however Fig 18(a)
shows that the measured inverter output current is slightly
larger than expected, and that the grid voltage decreases
slightly with decreasing inverter current. The increased
current at low values of ma is caused by the PM generator
output current increasing slightly with reduced loading. The
inverter voltage reduction is likely related to the regulation of
the auto-transformer.
Current & Voltage (PU)
1
0.8
0.6
0.4
0.2
Voltage
scale: 10A/div
Current
0
0 20 40 60 80 100
Modulation Index (%)
THD (%)
15
10
5
Current
Voltage
0
0 20 40 60 80 100
Modulation Index (%)
Fig. 18. (a) measured and expected inverter current and grid voltage
magnitudes (normalised to values at 100% modulation index), and (b)
measured current and voltage THD, vs modulation index.
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VI. CONCLUSIONS
This paper proposed a novel grid-connected inverter for a
small-scale wind turbine based on a high inductance PM
generator combined with a switched-mode rectifier (SMR)
and line-frequency commutated H-bridge inverter stage. The
key results are:
• the high-inductance PM generator must be designed to
have a rated output voltage which is much smaller than
its open-circuit voltage at maximum speed, in order for it
to act as a constant current source;
• the control of the SMR is straightforward as its output
current is linearly related to duty-cycle;
• it was experimentally demonstrated that a controllable
magnitude ac sinusoidal output current could be
obtained when operating into both a resistive and voltage
source load (simulating grid-connected operation);
• simulations and experimental testing was used to
examine the effect of varying the filter capacitance and
output power on the inverter current THD.
In future, it is planned to use a higher voltage PM
generator to demonstrate rated voltage grid-connected
inverter operation, to integrate a maximum power point
tracker in the controller, and to perform field testing on a
small-scale wind turbine. During this study, the control and
LC filter time-constant will be optimised, to minimise the
THD, and maximise efficiency over the expected turbine
operating range.
ACKNOWLEDGMENT
This work was supported by an Australian Research
Council Discovery grant, DP0342874. The technical support
from the Electrical and Electronic Engineering workshop
staff in the construction of the test rigs and machine set-up is
gratefully acknowledged.
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