A Multi String PV Based Multi Level Inverter for Distributed Energy ...

A Multi String PV Based Multi Level Inverter for Distributed Energy Resources

A Multi String PV Based Multi Level Inverter for Distributed Energy Resources Srinivas Mende

M V Subramanyam

M.Tech Student Scholar Department of Electrical & Electronics Engineering, Vignana Bharathi Institute of Technology, Aushapur; RangaReddy (DT); A.P, India.

M.Tech, M I E, M I S T E, (Ph.D.), Department of Electrical & Electronics Engineering, Vignana Bharathi Institute of Technology, Aushapur, RangaReddy (DT); A.P, India

Abstract- Distributed Energy Resources (DER) is systems that produce electrical power at the site where the power is needed. If only electrical power is used then the technology is called Distributed Generation (DG).Distributed energy resources systems are small power generation tools used to afford substitute solution or added enrichment of traditional electric power system. This paper is mainly concentrated on 1Ø multi-level inverter for Distributed Energy Resources. The objective of this paper is to reduce switches with increase in multi-level outputs which inherently reduces the cost & harmonics.Considering the cascaded inverter to be one unit, it can be seen that a higher number of voltage levels are available for a given number of semiconductor devices. The simplified multilevel inverter requires only six active switches instead of the eight required in the conventional cascaded H-bridge multilevel inverter. In addition, two active switches are operated at the line frequency.This MLI offers strong advances such as improved output waveforms, smaller filter size, low THD, reduced volume and cost, and lower electromagnetic interference. The simulation results are obtained using MATLAB/SIMULINK software.

conditions. Further by reducing switches & increasing level will reduce filter cost & harmonic content. 7- Level cascaded multilevel inverter [2] topology requires 12 switches but in this new multilevel inverter it requires 6 switches in which same multilevel is obtained. Invariably switching losses and cost also reduced. In reality a high step up converter is used before inverter input circuit for maintaining high constant voltage. In this paper we are going to study only about Multi level inverter circuitry.

Fig.1 Structure of Multi-level inverter for different DER application

The new multi-level inverter topology has minimum number of switching devices, improved output waveform and lower total harmonic distortion (THD).

Keywords- DC/AC power conversion, multilevel inverter.

I.INTRODUCTION II. SYSTEMCONFIGURATION OFOPERATIONPRINCIPLES

In considering the global warming and change in climate it is important to focus on emission of greenhouse gases and there reduction. The main cause of global warming is due to burning of fossil fuels for producing electricity so now everyone focusing on renewable resources. It is difficult to replace all non-renewable electricity plants into renewable one mainly considering in cost point of view. But one can reduce thermal power plant emission by adopting DER. By DER typically about 3kW – 10000kW range is possible either it can be used as an alternative or synchronized with traditional power lines depends on utility usage. To use DER’s in electrical power system we need an inverter. In case of micro grid system 1Ø multilevel inverter is usually adopted. Various research works are going on over inverter circuit configuration mainly in reducing the switches at higher voltage level. In such arrangements, utmost Distributed Energy Resources regularly supply a DC voltage that differs in a wide range according to different load

A general overview of different types of PV modules or fuel cell inverters is given in [9] and [16]. This letter presents a multi string multilevel inverter for DERs application. The multi string inverter shown in Fig. 1 is a further development of the string inverter, whereby several strings are interfaced with their own dc/dc converter to a common inverter. This centralized system is beneficial because each string can be controlled individually. Thus, the operator may start his own PV/fuel cell power plant with a few modules. Further enlargements are easily achieved because a new string with a dc/dc converter can be plugged into the existing platform, enabling a flexible design with high efficiency [9]. The single-phase multi string multi-level inverter topology used in this study is shown in Fig. 2. This topology configuration consists of two high step-up dc/dc converters connected to their individual dc-bus capacitor and a simplified multilevel inverter. Input sources, DER

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module 1, and DER module 2 are connected to the inverter followed a linear resistive load through the high step-up dc/dc converters. The studied simplified five-level inverter is used instead of a conventional cascaded pulse width modulated (PWM) inverter because it offers strong advantages such as improved output waveforms, smaller filter size, and lower EMI and THD. It should be noted that, by using the independent voltage regulation control of the individual high step-up converter, voltage balance control for the two bus capacitors Cbus1, Cbus2 can be achieved naturally.

Hence, the voltage conversion ratio of the high step-up converter, named input voltage to bus voltage ratio, can be derived as [26]

(4) B. Simplified Multilevel Inverter Stage To assist in solving problems caused by cumbersome power stages and complex control circuits for conventional multilevel inverters, this letter reports a new single-phase multi string topology, presented as a new basic circuitry in Fig. 3

Fig.2. Single-phase multi string five-level inverter topology

A. High Step-Up Converter Stage Fig.3. Basic five-level inverter circuitry

In this study, high step-up converter topology in [26] is introduced to boost and stabilize the output dc voltage of various DERs such as PV and fuel cell modules for employment of the proposed simplified multilevel inverter. The architecture of a high step-up converter initially introduced from [26], depicted in Fig. 2, and is composed of different converter topologies: boost, fly back, and a charge-pump circuit. The coupled inductor of the high step-up converter in Fig. 2 can be modeled as an ideal transformer, a magnetizing inductor, and a leakage inductor. According to the voltage–seconds balance condition of the magnetizing inductor, the voltage of the primary winding can be derived as

Referring to Fig. 2, it should be assumed that, in this configuration, the two capacitors in the capacitive voltage divider are connected directly across the dc bus, and all switching combinations are activated in an output cycle. The dynamic voltage balance between the two capacitors is automatically controlled by the preceding high step-up converter stage. Then, we can assumeVs1=Vs2=Vs. This topology includes six power switches—two fewer than the CCHB inverter with eight power switches— which drastically reduces the power circuit complexity and simplifies modulator circuit design and implementation. The phase disposition (PD) PWM control scheme is introduced to generate switching signals and to produce five output-voltage levels: 0, VS, 2VS, −VS, and and−2VS.

(1) Where Vin represents each the low-voltage dc energy input sources, and voltage of the secondary winding is

This inverter topology uses two carrier signals and one reference to generate PWM signals for the switches. The modulation strategy and its implemented logic scheme in Fig. 4(a) and (b) are a widely used alternative for PD modulation. With the exception of an offset value equivalent to the carrier signal amplitude, two comparators are used in this scheme with identical carrier signalsVtri1 andVtri2 to provide high-frequency switching signals for switchesSa1, Sb1, Sa3, and Sb3. Another comparator is used for zero-crossing detection to provide line-frequency switching signals for switchesSa2andSb2.

(2) Similar to that of the boost converter, the voltage of the charge-pump capacitor Cpump and clamp capacitor Cccan be expressed as (3)

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For convenient illustration, the switching function of the switch in Fig. 3 is defined as follows,

new topology are reduced naturally, and the overall conversion efficiency is improved.

(4) (5) Table I lists switching combinations that generate the required five output levels. The corresponding operation modes of the multilevel inverter stage are described clearly as follows 1) Maximum positive output, 2VS: Active switchesSa2, Sb1, and Sb3are ON; the voltage applied to the LC output filter is 2VS. 2) Half-level positive output, +Vs.: This output condition can be induced by two different switching combinations. One switching combination is such that active switchesSa2, Sb1, andSa3are ON; the other is such that active switches Sa2, Sa1, and Sb3 are ON. During this operating stage, the voltage applied to the LC output filter is +Vs. 3) Zero output, 0: This output condition can be formed by either of the two switching structures. Once the left or right switching leg is ON, the load will be short-circuited, and the voltage applied to the load terminals is zero. 4) Half-level negative output, −Vs: This output condition can be induced by either of the two different switching combinations. One switching combination is such that active switchesSa1, Sb2, and Sb3 are ON; the other is such that active switchesSa3, Sb1, andSb2are ON. 5) Maximum negative output, −2Vs: During this stage, active switchesSa1, Sa3, andSb2are ON, and the voltage applied to the LC output filter is−2Vs.

Fig.4.modulation logic To verify the feasibility of the single-phase five-level inverter, a widely used software program PSIM is applied to simulate the circuit according to the previously mentioned operation principle. The control signal block is shown in Fig. 4; m (t) is the sinusoidal modulation signal. BothVtri1 andVtri2 are the two triangular carrier signals. The peak value and frequency of the sinusoidal modulation signal are given as mpeak=0.7 and fm= 60 Hz, respectively. The peak-to-peak value of the triangular modulation signal is equal to 1, and the switching frequency ftri1 andftri2 are both given as 1.8 kHz. The two input voltage sources feeding from the high stepup converter is controlled at 100 V, i.e.Vs1 =Vs2 =100 V. C. Comparison with CCHB Inverter The average switching power loss Ps in the switch caused by these transitions can be defined as (7) Where tc (on) and tc (off) are the turn-on and turn-off crossover intervals, respectively; VDS is the voltage across the switch; and Io is the entire current which flows through the switch.

In these operations, it can be observed that the open voltage stress of the active power switchesSa1, Sa3, Sb1, and Sb3 is equal to input voltage VS; moreover, the main active switches Sa2andSb2are operated at the line frequency. Hence, the resulting switching losses of the

Fig.5 Five-level inverter topology of CCHB inverter

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Compared with the CCHB circuit topology as shown in Fig.5, the voltage stresses of the eight switches of the CCHB inverter are all equal to Vs. For simplification, both the proposed circuit and CCHB inverter are operated at the same turn-on and turn-off crossover intervals and at the same load Io. Then, the average switching power loss Ps is proportional to VDS and fs as (8) According to (8) and Table IV, the switching losses of the CCHB inverter from eight switches can be obtained as Considering the harmonics in the inverter output voltage VAB, the amplitude of the fundamental and harmonic components in the output voltage VAB are calculated by PSIM software. The phase-shift PWM technique is adopted for the CCHB Inverter. Both of the CCHB multilevel inverter and the studied multilevel inverter are operated in the same condition, including the same switching frequency 18 kHz, the same modulation index ma, the same input voltage VS =100 V and output LC filter, Lo = 420μH,Co =4.7μF. Tables II and III show the harmonic components and THD for the CCHB multilevel inverter and the studied multilevel inverter, respectively. It follows from Tables II and III that one can find that the studied multilevel inverter have lower THD than the CCHB multilevel inverter. It implies that the output waveform is improved and smaller filter size can be used. Finally, for further revealing the potential merits of the studied multistring multilevel inverter, voltage stress, switching losses for the CCHB multilevel inverter, andthe simplified single-phase five-level inverter.

(9) Similarly, the switching power loss of the proposed single-phase five-level inverter due to six switches can also be obtained as

(10) Because switchesSa2, Sb2can only be activated twice in a line period (60 Hz) and the switching frequency is larger than the line frequency (fs >> fm), the switching losses of the proposed circuit is approximated to 4Vsfs. obviously, the switching power loss is nearly half that of the CCHB inverter.

III. THE MULTI-STRING-CONVERTER In recent years Central PV-converters and String converters have emerged as the main competitors in the field of PV-system technology. Each converter stands forts own PV-system philosophy and both concepts are compared very often in order to identify ―the best ―concept. However, the comparison of String-converter and Central PV-converter based on the evaluation of real PV-systems has always to take into account the specific conditions of the corresponding PV-system (e.g. grade of dirt, orientation and temperature of the PV-modules) [2].Therefore a general judgment cannot be made based on comparison of real PV-systems. Consequently Fig. 8 lists the general advantages and disadvantages of both concepts from a system’s point of view. Fig. 6 shows that the Multi-String-converter summarizes the positive aspects of Central-PV-converter and Stringconverter concept and omits the drawbacks.The

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development of a PV-converter based on the new Multi String-Concept leads to a significant cost reduction of String-converters while still using the advantages of the String-converter technology.

Fig.8.shows the output waveform of Cascade H-Bridge(CHB)five-level inverter topology

Fig.6. Multi-String Technology summarizes the advantages of Central PV-Converters and String Converters

The main features of the Multi-String technology are:  optimum energy yield  optimum monitoring of strings  low specific costs of PV converter  minimum costs of PV-system installation  nominal power of converter unit not limited  modular extendibility Fig.9.shows the THD value of Cascade H-Bridge(CHB)five-level inverter topology

IV.MATLAB MODELING AND SIMULATION RESULTS Here the simulation is done in different cases 1). Cascaded H-Bridge (CHB) five-level inverter topology, 2). Proposed Topology with Distributed Generation Case 1: Cascade H-Bridge (CHB) Five-Level Inverter Topology with Multistring Inverter.

Fig.10 shows the Mat lab modeling of Single-Phase Multi-string Multilevel Inverter Topology

Fig 11.shows the output waveform of Single-Phase Multi-string Multilevel Inverter

Fig.7. shows the Mat lab modeling of Cascade H-Bridge(CHB)five-level inverter topology

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Fig.16.shows the voltage and current output waveforms of proposed method

Fig.12.shows the THD value of Single-Phase Multi-string Multilevel Inverter

Case 2: Proposed Topology with Distributed Generation

Fig.17.shows the positive phase voltage of proposed method

Fig.13.shows Mat lab modeling of proposed method

Fig.18.shows the negative phase voltage of proposed method

IV.CONCLUSION This paper presents a single-phase multistring Multi-level photovoltaic (PV) inverter topology for grid-connected PV systems with a novel H- bridge inverter as well as Multistring inverter. A newly constructed single-phase multistringmultilevel inverter topology that produces a significantreduction in the number of power devices required to implementmultilevel output for DERs. The studied inverter topology offerstrong advantages such as improved output waveforms, smallerfilter size, and lower EMI and THD. This will reduce the number of gate drivers and protection circuit requirement; this in turn reduces the cost increase the reliability. Finally a Mat lab/Simulink based model is developed and simulation results are presented. REFERENCES

Fig.14.shows output waveform of proposed method

Fig.15.shows the THD value of proposed method International Journal of Power System Operation and Energy Management ISSN (PRINT): 2231 – 4407, Volume-4, Issue-1, 2013

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AUTHORS PROFILE: M.Srinivas received B.Tech degree from Sri Kamala Institute of technology and sciences in the year 2008 and currently pursing M.Tech in Electrical power systems at Vignana Bharathi Institute of Technology, Hyderabad, Andhra Pradesh. His areas of interest are Power electronics and power systems.

M.V.Subramanyam was born in Hyderabad in 1969.He received his B.Tech(EEE) and M.Tech(ITPE) from Jawaharlal Nehru Technological University , Hyderabad, India in Electrical Engineering in 1994 and 2005 respectively. Presently he is pursuing his Ph.D. in Osmania University, Hyderabad, India. Presently he is working as Associate Professor in EEE department, Vignana Bharati Institute of Technology, Hyderabad.

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