The Switched Capacitor Inverter As A MPPT In A PhotovoltaicApplication
CC Marouchos
Cyprus University of Technology/Electrical Engineering, Limassol, Cyprus
M. Darwish
Brunel University/School of Engineering and Design
M.S.Moghadam
Brunel University/School of Engineering and Design
DespoXenofontos
Cyprus University of Technology/Electrical Engineering, Limassol,
Christina Armefti
Cyprus University of TechnologyElectrical Engineering, Limassol, Cyprus
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Abstract-The new topology for a dc to ac converterbased on the switched capacitor circuit was introduced recently.
The promising features of low harmonic content, high efficiency, MPPT facility and voltage step up characteristics makes it a good candidate for grid connected-renewable energy sources. The harmonic content is further minimised in this paper by introducing multiple current modulators, each connected to a PV panel. It is also shown that the circuit also tracks the Maximum Power Point of the output power of the photovoltaic panel with varying irradiation from the sunand ambient temperature.The work is carried out by extensive simulations on PSIM.
Index Terms--Inverters, Multilevel inverters, Grid-connected Inverters
I.INTRODUCTION
The topology of the Switched Capacitor Inverter is based on the Switched Capacitor Circuit. A Current Modulator in the form of a boost converter is feeding a three level inverter operating at 50Hz, Fig.1.Two bands of frequencies are identified in the output voltage when the values of L and C are chosen in random. A Lower Band, very close to the fundamental and a Higher Band around the switching frequency and its multiple. It was shown [1] that with a wise choice of the values of L and C for a specific load minimises the harmonics and specifically the Lower band can be eliminated. The Higher Band of voltage harmonics is almost completely eliminated now by introducing multiple Current Modulators. The switching signal of each Current Modulator is setat the appropriate phase and at least the lower group of the Higher Band is eliminated. This is achieved either by two or more Current Modulators. The extra cost of the Current Modulators is justified in a PV Park application where the PV modules are usually arranged in panels and each panel is connected to an inverter. It is suggested to connect each panel to a Current Modulator.
The new inverter can be used as an element of a multilevel configuration [2-4]. It is demonstrated that stacking two or more Switched Capacitor Inverters, greatly reduces the output voltage harmonics.
The Current Modulator is a boost dc to dc converter and it can be utilised also for Maximum Power Point Tracking (MPPT). The ability of the circuit to track the Maximum Power Point is therefore tested. It is shown that the Maximum Power Point is tracked for varying values of sun Irradiation and a range of ambient temperature.
II.OPERATION OF THE CIRCUIT
The input dc voltage undergoes two conversion processes in the Switched Capacitor Inverter [1]. First the input dc voltage is converted to a PWM rectified sinusoidal voltage, Fig.2a at the required frequency eg. 50Hz and then it is converted into a proper sinusoidal voltage, Fig.2b. The first conversion is done by a current modulator and the second is done by a simple 3-level inverter. The current modulator is a boost inverter and the switch is controlled by a PWM circuit, Fig. 3.The 3-level converter simply reverses the even half-cycles of the rectified voltage in order to construct the proper ac waveform, switching at the very low switching frequency of 50Hz. The advantages of this topology over other topologies [2-4] are obvious since only two switches are employed in the first conversion switching at relatively high frequency, a transistor and a diode. In the next stage the losses are even lower because the 3-level inverter is switching at a low frequency, at 50Hz -for a 50Hz output voltage- and more importantly switching is taking place at almost zero voltage and current.In this way the losses of the new configuration are lower than the 3-level PWM inverter switching at a relatively high frequency with four transistors and four diodes.
Fig.1 Block Diagram of the Switched Capacitor Inverter
The inductance L and capacitance C,must be chosen for a range of load values R in order to minimise output voltage distortion[1]. The switching frequency is also a parameter and it determines the order and in conjunction with L, C and load R values, the magnitude of the harmonics.
Fig.3 Generation of the PWM modulation function
The modulation function is a PWM signal and it is produced by comparing a rectified sinusoidal voltage at 50Hz to a triangular carrier signal at a frequency which gives the switching frequency, Fig.3.
III. THE OUTPUT VOLTAGE
Two bands of frequencies are identified in the output voltage when the values of L and C are chosen in random. A Lower Band, very close to the fundamental and a Higher Band around the switching frequency, Fig.4. The presence of harmonics close to the fundamental -Lower band- is due to three main reasons. Unsmoothed inductor current, non-zero values of the voltage across the capacitor and non-unity load power factor [1]. The relative values of the inductor and the capacitor, L and C for a specific load R can be chosen in order to minimise or even eliminate the Lower Band, Fig. 5. The Higher Band is repeated at decreasing magnitudes for multiples of the switching frequency.
It is shown in this paper that the Higher Band of frequencies can be completely eliminated (at least the first set) by adding more current modulators to the inverter. This is practically possible and perhaps desirable in PV Parks where the PV panels are arranged into groups and connected to a single inverter.In order to eliminate Higher Band frequency components, the phase of the switching frequency for each current modulator is arranged accordingly. For two current modulators the phase difference is 180o and for three it is 120o etc. This is an effective way to cancel harmonics, keep the switching frequency low and hence reduce the switching losses.
Fig.2a. Top trace: Output of current amplitude modulatorVodc(t)
Fig.2b. Bottom trace: Output of Inverter, Vo(t)
Fig.4. Frequency Spectrum of the Output voltage, Vo(t)
Fig. 5.Frequency Spectrum of output voltage,Vo(t)of the new Inverter for proper choice of L and C (L=0.25 C=9uF R =15Ω switching at 1000Hz)
IV. HIGHER VOLTAGE HARMONIC ELIMINATION
The prime source of the Higher Band of voltage harmonics is the switching signal. These harmonics are present around the switching frequency and its multiples.
nm±1 (1)
Where n is an integer number and m is the ratio of the switching frequency to the frequency of the output voltage to be produced. Obviously the phase of the higher order harmonics is set by the switching signal. Therefore a way to eliminate them to some degree is to feed the dc capacitor, Cdc, of Fig.1 with two currents, each with a phase difference of 180ofor the higher order harmonics. This is partly achieved by arranging the phase of their switching signals in each current Modulator to have a phase difference of 180o.
This can be repeated for three or more Current Modulators, the phase difference of the switching signals for each current modulator is given by
(2)
Where h is the number of current modulators.
Two current modulators, Fig.6a, are feeding the dc capacitor. The modulation functions applied to each modulator are exactly the same [1] but the carrier signal, Fig.3, of each modulator is set at 180o phase difference. The higher order harmonics, the first set m±1,are cancelled completely, Fig. 6b.Three current modulators are feeding the dc capacitor in Fig.7a. Again the modulation functions applied to each modulator are exactly the same but the carrier of each modulator is set at 120o phase difference. The higher order harmonics, the first set, are eliminated Fig. 7b. and the second set is reduced.
Fig.6b Frequency spectrum of the double current modulator inverter
Fig.7a.Triple Current Modulator Inverter
Fig.7b Frequency spectrum of the triple current modulator inverter
Two inverters are stacked together feeding the same loadand all higher order harmonics are eliminated, Fig.8a. This is repeated for three inverters in series and again all higher order harmonics are eliminated Fig.8b.
Fig.8a Frequency spectrum of two inverters stacked together
Fig.8 b Frequency spectrum of three inverters stacked together
A range of options are available to the connection of PV panels to the grid either with more current modulators or more inverters stacked together.
V. MAXIMUM POWER POINT TRACKING
The circuit is tested for Maximum Point Power Tracking in two ways: for different sun irradiations and different ambient temperatures. It is demonstrated that it tracks the Maximum Point Power and precisely sets the depth of modulation accordingly.
Three different values of sun irradiations were tested 1000w/m2, 800w/m2 and 600w/m2.For each value the depth of modulation is set appropriately between0.81 and 0.82 Fig.9. Four ambient temperatures were tested 25 oC, 35 oC, 55 oC and 65 oC. For each temperature the depth of modulation is set appropriately between 0.6 and 0.7 Fig.10.
Fig. 9 Maximum power point tracking for different levels of irradiation
Fig. 10 Maximum power point tracking for ambient temperatures
IV. DISCUSSION AND FUTURE WORK
Elimination of the voltage harmonics at the output voltage of the SC inverter near the fundamental, Lower Band, is achieved by proper choice of the value of the dc capacitor C. Unfortunately the magnitudes of the Higher Band are increased when the dc capacitor value is set for zero magnitude of the Lower Band of frequency components. By introducing more Current Modulators, the HigherBand is greatly reduced. With two Current Modulators, the lower setof the Higher Band of voltage harmonics at the output of the inverter is completely eliminated. The phase of the switching signal of each Current Modulator is arranged accordingly. With three Current Modulators more components of the Upper Band are eliminated. The extra cost and complexity of Current Modulators is justified when this inverter is used in PV Parks. In PV Parks panels of PV modules are connected to individual inverters to form the total output power to the grid.
The ability of the circuit to monitor the output power and track The Maximum Power Point is demonstrated for a range of sun irradiation values and temperature. Therefore this new inverter is a good candidate for PV Parks connected to the grid. Naturally a more in depth work is necessary to optimise this inverter and compare it to the other available configurations.
Future work will also be concentrated in developing a compact form of a 3-phase circuit.
V. CONCLUSIONS
The ability of the circuit to perform maximum power point tracking for different values of sun irradiation and ambient temperatures is demonstrated. Furthermore the output voltage harmonics are greatly reduced by the introduction of more current modulators. Both characteristics make this inverter a good candidate for PV parks connected to the grid.
Future work is proposed for a 3-phase system and optimisation of the circuit.
REFERENCES
[1]A new Concept for a Multilevel Switched Capacitor sinusoidal Grid Connected Inverter, CC Marouchos M. Darwish L. Diomidou, UPEC2013, Dublin Ireland
[2]CassianoRech, Humberto, Pinheiro, Hilton and A. Griindling, HClio L,“Analysis and Comparison of Hybrid Multilevel Voltage Source Inverters”, Power Electronics Specialists Conference, 2002. pesc 02. 2002 IEEE 33rd Annual
[3]Fang Z. , “Generalized Multilevel Inverter Topology with Self Voltage Balancing”, Industry Applications Conference, 2000. Conference Record of the 2000 IEEE
[4]N Benaifa, “Parallel operatedInverers as a Multilevel Case”, Electrical and Computer Engineering, 2008. CCECE 2008. Canadian Conference o