A Single-Stage Grid-Connected PV Micro-Inverter
A Single-Stage Grid-Connected PV Micro-Inverter
A Single-Stage Grid-Connected PV Micro-Inverter
II. CIRCUIT TOPOLOGY AND OPERATION Next, the presented flyback converter topology based
The circuit topology of the studied micro-inverter is shown micro-inverter in DCM operation has a perfect PFC
in Fig. 3. The circuit is considered a single-stage power characteristic will be shown.
conversion design because only the DC-DC converter is
performing PWM. The PWM contains a fixed-frequency but
variable duty cycle high-frequency AC signal that is half-
wave symmetrical with respect to the line frequency. The DC-
DC converter contains two flyback converters interleaving
each other to reduce the high-frequency noise back to the
Fig. 5 Illustration of real power output (PF=1) for the micro-inverter.
source. Primary switches S1 and S2 are the main switches
performing the PWM function, and D1 and D2 are the
secondary silicon carbide diode that used to avoid diode
reverse recovery loss. The output of the flyback converter is
high-voltage AC and is rectified to obtain half-wave
symmetry sinusoidal PWM voltage, which is then unfolded to
sinusoidal AC output with four power MOSFET switches, S H1,
SL1, SH2, and SL2. Switch pair SH1 and SL2 turn on during
positive half line cycles, and SH2 and SL1 turn on during
negative line cycles. Fig. 4 shows the current waveforms of Fig. 6 Magnetizing current of one phase flyback converter.
the interleaved flyback converter topology. Each phase is 180o
phase shifted in one switching period to achieved ripple It can be seen from Fig. 6, the grid current is
cancellation, and a lower output filter inductance can be used. I pk
1
igrid =< isec >= d1 × (1)
2 N
VPV V
where, I pk = d × Ts ; d1 = PV Nd .
Lm vgrid
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Assume the conversion efficiency of unity and power
balance relationship 0.5 × Vgrid I grid = PPV , then the constant K
can be found as
1 Lm
K= 4 PPV (5)
Ts
VPV
IPV
VPV
G1
G2
When MPPT is performed, the average duty cycle with
maximum power transfer as
1 Lm
d= 4 PPV × sin ωt (6)
VPV Ts
where, VPV and PPV are maximum power point and correspond
with PV voltage.
(a)
Though the flyback converter can achieve unity power
factor when average duty cycle is designed as igrid=Kvgrid, the
operating in DCM is necessary. Now, the critical
magnetization inductance design equation is derived to ensure
flyback converter operation in DCMs. The necessary
condition in DCM is
DMAX + D1, MIN < 1 (7)
TABLE I
POWER STAGE PARAMETERS OF THE PROTOTYPE [2ms/div]
Fig. 8 Measured waveforms of DC bus voltage [100V/div] and gate signals of
Parameter Values/Part No. low-side H-bridge unfolder switches (SL1, SL2) [5V/div].
Input capacitor bank (Cin) 1800uF/63V 4pcs
Magnetizing inductance (Lm1, Lm2) 6uH The system block diagram of the micro-inverter includes of
Transformer turn-ratio(N1, N2) Ns1/Np1=Ns2/Np2=7 power stage, driver circuits, signal sensing and feedback
Primary power MOSFET (S1, S2) IPB072N15N3G circuits. The power stage is constructed based on the
Secondary power diode (D1, D2) C2D05120E interleaved flyback converter operating in DCM. In this
H-bridge unfolder (SH1, SL1, S H2, S L2) SPB17N80C3 system, a dsPIC33FJ16GS504T microcontroller (MCU) is
used to control the output voltage of the interleaved flyabck
Bus filter capacitor (Cbus) 22nF/310V 2pcs
converter to have the shape of a rectified sinusoid. This
Output filter Inductance (Lo1, Lo2) 150uH
rectified sinusoid waveform is then inverted into a full
Output filter Inductance (Co) 0.33uF/330V sinusoidal waveform using an H-bridge unfolder that
interfaces to the grid voltage.
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Fig. 9 MPP tracking performance of the constructed micro-inverter.
In Fig. 8, it can be seen that the DC bus voltage is Fig. 10 shows the tracking operation for the maximum
modulated to a rectified sinusoidal waveform on bus filter power point of 250W PV input. As can be seen that, when the
capacitor Cbus that is connected with grid system via the power tracking mechanism is started, the PV voltage drops
polarity selector (i.e. unfolder) and the output filter. Fig. 8 also from the open circuit voltage Voc of 34.8V to the voltage Vmp
shows the measured gate signals for the low-side H-bridge of 29.67V; the PV current increases from zero to 8.33A, the
unfolder switches. For convenience in the experiments, solar maximum power of 250W is achieved with the tracking
array simulator Chroma 62100H-600S is used as the power response time of 9 seconds. Fig. 11 shows the steady-state
source. Fig. 9 shows the monitoring screen of MPP tracking waveforms of grid connected operation for full load output of
performance of the constructed micro-inverter. In this paper, 230W, the grid current harmonic is below 5%. The maximum
the perturbation and observation (P&O) method is adopted for conversion efficiency of the built prototype is about 93%
PV maximum power tracking control. The test result verifies
the excellent maximum power point tracking performance of V. CONCLUSIONS
the inverter for the specific I-V and P-V curves. A prototype with input power rating of 250W and output
power rating of 230W/220Vac is constructed and compatible
with 60 cell and 72 cell solar modules applications.
Experimental results verify the feasibility of the developed
micro-inverter which delivers greater energy harvest, higher
reliability and more intelligent operation.
ACKNOWLEDGMENT
This study was supported by the Ministry of Science and
Technology (formally National Science Council), Taiwan,
R.O.C. under Grant NSC 102-2218-E-033-004.
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