ABSTRACT: The effect of linear imbalances and nonlinear loads on the voltage balance of the neutral-point clamped converter is described here. The Neutral-Point-Clamped inverters are used in the multilevel inverters for high power applications. A three level NPC inverter that can accommodate with solar photovoltaic (PV) and battery storage in a grid connected system is described here. This paper presents the Fuzzy Logic controller design philosophy of the proposed configuration and the theoretical framework of the proposed modulation technique. An incipient Fuzzy Logic controller for the proposed system is additionally presented in order to control the power delivery between the solar PV, battery and grid, which simultaneously provides Maximum power point tracking (MPPT) operation for the solar PV. A simulation model for the solar energy system has been developed using MATLAB/SIMULINK. The energy system performances under different scenarios, including battery charging and discharging with different levels of solar irradiation has been verified by carrying out simulation studies.
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Rapid increase in energy prices and recent geopolitical events renewable energy sources such as solar PV energy and wind energy generation are becoming more promising alternatives to replace conventional generation.there are two conversion methods to transfer the power from renewable energy resource to grid ; single stage energy conversion and double stage energy conversion. In the double stage energy conversion two converters are required to convert dc to ac usually in the first stage dc to dc converter is used in the second stage dc- ac inverter is used, the function of the dc to dc converters to facilitate the MPPT of the PV array to produce appropriate dc voltage for the dc to ac inverter. In the single stage energy conversion one converter is sufficient to convert the dc into ac these can reduce the cost and improves the overall efficiency .however more complex control is required. solar PV system is integrated with three level NPC inverter having a capability of MPPT and ac side current control,and also ability of controlling the
In three-phase applications, two types of power elec- tronic configurations are commonly used to transfer power from the renewable energy resource to the grid: single-stage and double-stage conversion. In the dou- ble-stage conversion for a PV system, the first stage is usually a dc/dc converter and thesecond stage is a dc/ ac inverter. The function of the dc/dc converter is to fa- cilitate the maximum power point tracking (MPPT) of the PV array and to produce the appropriate dc voltage for the dc/ac inverter. The function of the inverter is to generate three-phase sinusoidal voltages or currents to transfer the power to the grid in a grid-connected solar PV system or to the load in a stand-alone system –. In the single-stage connection, only one con- verter is needed to fulfill the double-stage functions, and hence the system will have a lower cost and higher efficiency, however, a more complex control method will be required. The current norm of the industry for high power applications is a three-phase, single stage PV energy systems by using a voltage-source converter (VSC) for power conversion .One of the major con- cerns of solar and wind energy systems is their unpre- dictable and fluctuating nature. Grid-connected renew- able energy systems accompanied by battery energy storage can overcome this concern. This also can in- crease the flexibility of power system control and raise the overall availability of the system . Usually, a con- verter is required to control the charging and discharg- ing of the battery storage system and another convert- er is required for dc/ac power conversion; thus, a three phase PV system connected to battery storage will require two converters. This paper is concerned with the design and study of a grid-connected three-phase solar PV system integrated with battery storage using only one three-level converter having the capability of MPPT and ac-side current control, and also the ability of controlling the battery charging and discharging.
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In theory, multiple-input converters (e.g.three-port converters) can provide a single-unit solution interfacing multiple energy sources and common loads. They perform better than traditional two-port converters due to their lower part count and smaller converter size. In particular, the isolated three-port converter (ITPC) has become an attractive topology for various applications owing to their multiple energy source connection, compact structure, and low cost. In this topology, a simple power-flow management scheme can be used since the control function is centralized. A high-frequency transformer, can provide galvanic isolation and flexible voltage conversion ratio. The ITPC is usually integrated into an individual converter such as forward, push–pull, full- bridge, and Flyback converters. The ITPC utilizes the triple active bridges (TAB) with inherent features of power controllability and ZVS. Their soft switching performance can be improved if two series-resonant tanks are implemented. An advanced modulation strategy which incorporates a phase shift (PS) and a PWM to extend the operating range of ZVS.
There are two main modulation strategies for the three-level NPC inverter, namely space vector pulse width modulation (SVPWM) and carrier-based PWM [14, 15]. The nearest three vectors (NTV) modulation is one of the common SVPWM methods . It conforms to the principle in which the reference vector is synthesized by the nearest three vectors and can prevent overlapping of the level layers in the line to line voltages (analyzed in ), as well as reducing the total harmonic distortion (THD). However, the essence of the NTV modulation method is that a third-order harmonic is injected into the three-phase sinusoidal modulation waves, and its fluctuation-suppression for NPP is limited (this is analyzed in detail in Section III. A). Therefore, the nearest three virtual vectors (NTV 2 ) method was proposed to enhance the capability
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This structure wasfirst proposed by Nabae et. al in 1980 (Krug et. al., 2004; Marchasoni and Mazzucchelli, 1993). Figure1 shows the 3-level NPC inverter.Basically, NPC multilevel inverters synthesize the small stepof stair- case output voltage fromseveral levels of DC capaci- tor voltages. An m-level NPC inverter consists of (m-1) capacitors on theDC bus, 2(m-1) switching devices per phase and 2(m-2) clamping diodes per phase. Figure 1 shows thestructure of 3-level NPC. The DC bus voltage is split into 3 levels by using 2 DC capacitors, C1 andC2. Each capacitor has Vdc/2 volts and each voltage stress will be limited to one capacitor level throughclamping diodes (Chaturvedi et. al., 2005; Lai and Peng, 1996). The number of levels can be extended to a higher level by additional switching devices andwith these additions, the inverter will be able to achieve higher AC voltage, producing more voltagesteps that will be approaching sinusoidal with minimum harmonics dis- tortion. During inverteroperations, the switches near the centre tap are switched onfor a longer period com- pared to theswitches further away from the centre tap as given in the switching states in Table 1. As the switch isfurther away from the centre tap the switching time is shorter.Another difference between theconvention- al 2-level and multilevel NPC is the clamping diode. In case of 3-level NPC inverter,clamping diode, D1 and D4 clamped the DC bus voltage into three voltage level, +Vdc/2, 0 and -Vdc/2 Diode, D4 balances out the volt- age sharing between S4in and S4out, with S4in block- ing the voltage acrossC1 and S4out blocking the volt- age across C2
This paper proposes a high performance three-level inverter Neutral Point Clamped (NPC) structure for photovoltaic system. The proposed configuration which can boost the low voltage of photovoltaic (PV) array, can also convert the photovoltaic DC power into high quality AC power. Attention has been paid to the problem of neutral point potential variation. In this way, a Direct Torque Control (DTC) technique has been applied and the estimated value of the Neu- tral Point Potential (NPP) is used, which is calculated by motor currents. This control strategy uses the redundancy presented by the inverter for selecting appropriate switching state through a switching table to achieve the control of NPP. This study shows the effect of the stability problem of the DC voltages and good static and dynamic performances were obtained in simulation of the proposed cascade “photovoltaic cell-three-level NPC VSI-induction motor”.
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The Z-source inverter  topology was proposed to overcome the above limitations in traditional inverters. The Z-source concept can be applied to all dc-to-ac , ac-to-dc , ac-to-ac ,,, and dc-to-dc , power conversion whether two-level or multilevel. The Z-source concept was extended to the NPC inverter in , where two additional Z-source networks were connected between two isolated dc sources and a traditional NPC inverter. In spite of its effectiveness in achieving voltage buck-boost conversion, the Z-source NPC inverter proposed in  is expensive because it uses two Z-source networks, two isolated dc sources and requires a complex modulator for balancing the boosting of each Z-source network. To overcome the cost and modulator complexity issues, the design and control of an NPC inverter using a single Z-source network was presented in . The operational analysis and optimal control of the Reduced Element Count (REC) Z-source NPC inverter was subsequently described in .
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In Phase Distortion PWM (PDPWM), Figure 6 all carrier waves are in phase. A great acknowledgment for this technique is that it is generally accepted as the method that creates the lowest harmonic distortion in line-to-line voltage. When used for an NPCMLI with m number of voltage levels, m−1 number of triangular carrier waves is used. These carrier waves have the same frequency and are arranged on top of each other, with no phase shift, so that they together span from maximum output voltage to minimum output voltage. The carrier waves amplitudes should be modulated with aspect of the current voltage magnitude for each respective voltage level, each carrier wave is connected to a specific output voltage level. If the carrier waves are not modulated in this way the correct output voltage will not be achieved if the sources voltage levels change from their supposed value (get unbalanced). If the sources voltage amplitudes change without that the carrier waves are modulated with that change the correct output voltage will not be generated during the during the correct time spans.
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In view of later, to retrieve the demerits of traditional inverters we know about the multilevel technology and the merits it offer. Multilevel converters are a good choice for power applications due to the fact that, it can be achieves high power using medium-power inverter technique. Practically, multilevel inverters present more advantages comparing with traditional converter. These advantages are basically focused on betterments in the output signal quality (Voltage & Current) and power increase in the inverter . These advantages make multilevel converters very attractive to the commercially and, these days, researchers all over the world are making tremendous efforts and trying to improve multilevel inverter characteristics performances such as the regulation modification and the performance of many optimization algorithms technique in order to improve the total harmonic distortion  of the output signals, the controlling of the dc capacitor voltage , and the ripple of the currents, harmonic mitigation to fulfill a certain grid codes, the development of new multilevel inverter model (hybrid or new ones), and regulation strategies . However, before introducing about the multilevel converters, we are making an overview about the traditional converters and their problems. To address the problems of traditional converters, one should have an idea about the Mean to high-power range converters and related challenging issues. Some of the facts summarized.
The SVM technique has been presented for two level inverter. Also, the proposed SVPWM method for the three level inverter has been described in detail. Calculation of dwelling times for voltage vectors have been conducted same as the two-level SVPWM. Thus the explained method minimizes the processing time of three-level SVPWM. This technique can be implemented to the multi-level SVPWM method above four-level. The effectiveness of the presented SVPWM method is demonstrated and verified by experimental results.
Multilevel inverters are promising; they have nearly sinusoidal output voltage waveforms, output current with better harmonic profile, less stressing of electronic components owing to decreased voltages, switching losses that are lower than those of conventional two-level inverters, a smaller filter size, and lower EMI, all of which make them cheaper, lighter, and more compact .
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We now have several kinds of control algorithms of the three-level inverter, which include SPWM, SHEPWM and SVPWM. Among these algorithms, as the result of these characteristics such as the high voltage utilization ratio, the low harmonic content of output waveform, and the easiness in digital realization, the space vector module method(SVPWM) become the relatively popular control algorithm.
Power electronics converters is a technology that enables the efficient and flexible interconnection of different sources to the electric power system. These converters are to provide stable output voltage in spite of unstable input variables at the highest efficiency, lowest cost and minimum size. This has led to the development of many new interface power electronics converters. Most of the power converter topologies employed in PV applications are characterized as two-stage converters. The two-stage converter topology uses a boost dc/dc converter to minimize the required kVA rating of the inverter and boost the wide range input voltage to a constant desired output value . However, this solution is more complex and difficult to control because of the two-stage power conversion.
references, respectively. In Fig. 17, the measured and reference active power are shown when the reference value changes in three different values, P= 800w, P= 1,200w and P= 1,000w. In the same way, different changes in the reference reactive power are simulated as shown in Fig. 18. In this figure, the measured reactive power tracks the reference reactive power value in Q=100, Q=-100 and Q=0, which is the desired value for system. As seen in Figs. 17 and 18, the used controller allows the reference active and reactive power proper tracking. Important to mention that the maximum active power injected to the grid depends on the panel photovoltaic capabilities.
Fig.5 shows the two-level inverter pole voltage and three-level inverter pole voltage and output voltage. As gating pulses for two- level inverter are produced by using two carrier signals, pole voltage of two-level inverter is almost similar to three-level inverter output voltage. Fig. 6(a) shows harmonic spectrum of two-level inverter pole voltage where harmonics are present near to first center band which is equal to switching frequency. Similarly Fig. 6(b) shows the harmonic spectrum of three-level inverter pole voltage where harmonics are present near to first center band. As the modulating signals of two-level inverter and three-level inverter are phase shifted by 180 0 , all odd center band harmonics are also phase shifted by 180 0 . As effective voltage is the
The advantages of NPC inverter are that the entire phases share a common dc bus, which minimizes the capacitance requirements of the inverter. For this reason, a back-to-back topology is not only possible but also practical for uses such as a high-voltage back-to-back inter-connection or an adjustable speed drive. The capacitors can be recharged as a group. Efficiency is high for fundamental frequency switching -.
inverter (Seyedi, 2009). A crowbar is actualized to control SC (hamper) the wind power framework that outcomes in high present and high potential. The RSC converters working at the slip recurrence and its regulation of the transition in the DFIG wind turbine. The power rating of the RSC is computed by the most extreme dynamic and receptive power controls capacity. The RSC might fill in as a current-controlled voltage sources converter. The GSC worked at a framework recurrence and directs the voltage and current level of the DC-interface circuit. The wind power plant withstands voltage plunges to settled estimation of the ostensible voltage and for a settled time
Since the introduction of three-level inverters in 1981 , , they have been widely used in several applications, suchinverter space vector diagram for balanced dc-link capacitors as: motor drives, STATCOM, HVDC, pulse width modulation (PWM) rectifiers, active power filters (APFs), and renewable energy applications , . Fig. 1(a) shows a typical three phase three-level neutral-point- clamped (NPC) inverter circuit topology. The converter has two capacitors in the dc side Normally., since it has been reported that unbalance capacitor voltages can affect the ac side voltages and can produce unexpected behavior on system parameters such as even-harmonic injection and power ripple , . Several methods have discussed by using fuzzy control f balancing these capacitor voltages in variousapplications
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In this topology there are mainly two pairs of switches and also two diodes are used in a three-level diode clamped multi level inverter. The DC bus voltage is divided into three different voltage levels with the help of two series connections of DC capacitors i.eC1 and C2 as shown in the figure 1. The voltage across each capacitor is Vdc/2 i.e voltage across capacitor C1 and C2 is equals to Vdc/2 .There are mainly twelve active combinations were taken and using these switching states twelve active voltage vectors are produced. There are nonzero voltage vectors and are from V1 to V12. Fig.1. shows the Three Level Diode Clamped Multilevel Inverter (MLI). Table I shows the switching states for one leg of the three-Level Diode Clamped Multilevel Inverter.
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