Simulation for solar fed cascade multilevel inverter
To improve power quality
Krishna D Tiwari1, Sanjay. A. Deokar2
M. E. Student, Department of Electrical Engineering, Zeal College of Engineering and Research, Narhe, Pune, Maharashtra1
Associate Professor, Department of Electrical Engineering, Zeal College of Engineering and Research, Narhe, Pune, Maharashtra2
Abstract: The simulation for 15 level multilevel inverter to diminish the number of switches is presented in our topology. The modulation includes CMLI based topologies suitable for changing input sources from solar PV. This topology attempt to outline both controlling circuitry and power circuitry to provide different output voltage levels by different switching sequences. Hence to obtain a required level of inverter, the current method requires more switches and filter circuits whereas in our topology only 10 switches are selected to get required levels. The advantage is to reduce the degree of harmonics by increasing the levels and hence improve power quality. Simulation is done in MATLAB and comparisons were made.
Inverters convert DC voltage to variable magnitude, variable frequency AC voltage’s.Multilevel inverter is define as the multiple or no of connections of individual inverters which is termed as stages to provide the output voltage with required levels. As the number of levels increases at o/p it goes more nearer to sinusoidal. Multi-level Inverters are found to be of extreme importance in industrial applications now a days. It has found more applicable in medium and high power applications. Further it has promised in providing fewer disturbances than conventional inverter. And voltage stresses across the switches has also been reduced. Multi-level inverter provides a suitable solution for medium and high power systems to synthesize output voltage which allows a reduction of harmonic content in voltage and current waveforms. There are three approaches such as Flying Capacitor, Cascaded Multilevel Inverters and Neutral-point Clamped are used for different applications in industries, power sector and defence depending upon its different working principle and analysis . Among these three approaches, CMLI is highly preferred for the interconnection in case of renewable energy systems because of the advantages such as:-
[I] Absence of voltage unbalance problem,
II] Reduction in system cost.
III] Absence of clamping capacitors and diodes.
DC-MLI, C-MLI, FC-MLI topologies are proposed in . PWM technique for a 5-level CMLI [3-4] and seven-level CMLI for photovoltaic system is addressed which boost converter, auxiliary circuitry and multiple reference indicator for need pulse generation . A 5-level current CMLI for a single phase grid connected PV system given in requires the redesign of LC ﬁlter to reduce the resistive and inductive losses for higher levels of power . The three control loop maximum power point (MPP) for a 5-level inverter in  employs an output transformer between inverter and grid. A 5-level inverter with four DC sources is proposed in . The drawback of this method is that in conventional inverters up to 9 levels can be generated.
Fig. 1: Solar fed C-MLI system
A topology is proposed in this for CMLI to achieve higher levels, which in-turn eliminates the dis-advantages (as given above) by minimising number of switches without the use of bipolar switches, ﬁltering components, detailed table contents and production transformers . The topology used are Modified Multilevel Connections’ (MMC) mode for 15 level using 10 switches as shown in fig 1.The mode used for Solar fed CMLI has following advantages:-
1] Reduction in number of switches.
2] Increases the number of levels.
3] Reduces Harmonic distortion which in turn improves the power quality.
Rahim, N.A & Selvaraj, J presents multilevel inverter with five-level topology for grid-connected photovoltaic systems with a pulse width-modulated (PWM) control scheme. Three PV panels are used together in parallel configuration and connected to a five-level inverter to produce output voltage in five levels .
Villanueva, E & Correa, P. Rodriguez presents a cascaded MLI converter for a grid-connected photovoltaic (PV) system. The multilevel topology consists of different Half-bridge cells connected in series; each one connected to a string of Photovoltaic modules .The adopted control circuit gives the full control over every dc-link voltage that enable in this way, the tracking of the maximum power point for each string of PV panels.
Albert Alexander & Manigandan Thathan presents the power quality improvement for a solar fed CMLI with minimum number of semiconductor switches is analyzed in this paper. The required level output is achieved with large no of switches in binary and trinary mode. In this THD is between 5 to 16% .
In this MMC mode, the modification is made in power circuit to obtain 15 levels with only 10 switches. Fig No.1 shows the power circuit diagram of MMC approach. In this mode input scaling is not mandatory the input voltages kept here is 48 v, 96 v and 192v.The switches S1, S2 and S3 are controlled according to get desired voltage level at the output. Fig No.3 shows switches S1, S2 and S3. To get particular voltage level at output switches S1, S2 and S3 are made ON. T1 and T4 are ON for positive half cycle and T2 and T3 are ON for negative half cycle
Fig 2.power circuit diagram (MMC mode)
Table 1 shows the switching state for MMC mode as 10 switches are used according to that output voltage is obtained as required.
Table.1. Switching States for MMC mode
IV. PROJECT SIMULATION RESULTS
This paper focuses on modelling photovoltaic panels composed of several basic cells. The word array used for any photovoltaic device composed of several basic cells. A photovoltaic system convert’s sunlight into electricity.The basic of a photovoltaic system is the photovoltaic cell. The electricity available at the terminals of a photovoltaic panel or array may directly feed small loads such as bulb, fans and DC motors. The simulation and the corresponding harmonics analysis for this topology is carried out in MATLAB.As shown below is the proposed simulation circuit for mmc mode and the output waveform as shown in fig 3 and fig 4 respectively.
Table no. 2: Parameters of the solar panel at 25◦C,
1.5AM, 1000 W/m2
Ki 8 A/K
Fig 3: Simulation for 15 levels MMC mode
Fig 4: Output waveform and corresponding harmonic MMC mode
(a) Output voltage waveform for 15 levels
(b) FFT analysis for 15 levels CMLI
C-MLI technique used is better because, there is absence of voltage unbalance problem. Absence of transformer and filter circuitry help in minimizing complexity. There is absence of clamping capacitor and diodes .The topology proposed: 1). Minimise number of switches, which minimise switching.2).Increase the number of levels.3.minimise harmonic distortion which in turn improves the power quality, apart from this there is reduction in cost, complexity and space. We have made comparison based on previous data collected and the data collected in this topology so based on that we concluded that in MMC mode we have achieved 15 level by using 10 switches. as shown in table 3  comparison of switches. Also fig 4(a) shows the reduction in overall harmonic distortion as obtained is 2.12%.
Table 3: Comparison of switches
No of stages No of switches No of level
Conventional Binary Trinary MMC
1 4 3 3 3 15
2 8 5 7 9 with
3 12 7 15 27 10
4 16 9 31 81 switches
5 20 11 63 243
6 24 13 127 729
7 28 15 255 2187
The power quality improvement for CMLI with minimum number of semiconductor switches is analysed. The required 15-level output is achieved with only 10 switches in MMC mode. Solar PV source is given as input to the inverter stages. A detailed simulation study is carried out for various levels and comparison has been made. The proposed method provides the multiple advantages which include reduced THD, less system cost, simple design and maintenance, less computational complexity and the absence of transformers, boost converters, detailed look-up table and filter circuit. Cascade multilevel inverter can be implemented using renewable energy. Dc input has to be replaced by solar or PV cell, But for implementing C-MLI using PV cell charge controller, batteries and solar panels will be required which will increase the cost and make system complex.
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