- 1. A Novel Three Phase Multi-string Multilevel Inverter with High DC-DC Closed operation for Photovoltaic System Abstract This paper presents a novel three phase multi- string multilevel inverter; this inverter reduces number power devices and high performances. Before this inverter provide a high step up DC-DC converter with PI controller for better conversion efficiency and to improve the output dc voltage of varies renewable energy sources. This multi-string multilevel inverter consists of six switches only instead of eight switches in cascaded H-bridge multilevel inverter in order to reduce conversion losses. The main objective of this paper is to save cost and size by removing any kind of transformer as well as reducing the power devices .This multi- string inverter topology have more advantages such as better output waveforms ,lower electromagnetic interference and low THD. Finally this inverter connects to three phase induction machine for analysis. Simulation and experimental results show the effectiveness of proposed solution. 1. Introduction In recent year’s electrical energy requirement very high, because of different factors like raises population, industries, colleges and hospitals, etc., conventional energy sources based on oil, coal and natural gas have proven to be highly effectives drives of economic progress, but at the same time damaging to the environment and to human health. Therefore the traditional fossil fuel based energy sources are facing increasing pressure on a host of environmental fronts, with perhaps the most serious challenge confronting the future use of coal being the greenhouse gas reduction targets. The potential of renewable energy sources (RES) is enormous as they can in principle meet many times the world’s energy demand. Renewable energy sources such as solar systems, fuel cells, micro-turbines and wind has become a more issues for delivering premium power to loads with power quality, reliability and high efficiency in converters of RES. In such systems, RES usually supply a dc voltage that varies in a wide range according to varies load conditions. Thus, a dc/ac power converting processing interface is required and is compliable with residential, industrial, and utility grid standards [1]- [2]. Various converter topologies have been developed for RESs that demonstrate effective power flow control performance whether in grid- connected or stand alone operation. Among them, solutions that employ high frequency transformers or make no use of transformers at all have been investigated to reduce size, weight, and expense. For low-medium power applications, international standards allow the use of grid-connected power converters without galvanic isolation, thus allowing so called “transformer less” architectures. Furthermore, as the output voltage level increases, the output harmonic content of such inverters decreases, allowing the use of smaller and less expensive output filters. As a result, various multilevel topologies are usually characterized by a strong reduction in switching voltages across power switches, allowing the reduction of switching power losses and electromagnetic interference (EMI). A three-phase multi-string five- level inverter integrated with an auxiliary circuit was recently proposed for dc/ac power conversion. This topology used in the power stage offers an important improvement in terms of lower component count and reduced output harmonics. Unfortunately, high switching losses in the additional auxiliary circuit caused the efficiency of the multi-string five-level inverter to be approximately 4% less than that of the conventional multi-string three-level inverter. In [3], a novel isolated single phase inverter with generalized zero vectors (GZV) modulation scheme was first presented to simplify the configuration. However, this circuit can still only operate in a limited voltage range for practical applications and suffer degradation in the overall efficiency as the duty cycle of the dc-side switch of the front-end conventional boost converter approaches unity. Furthermore, the use of isolated transformer with Koppineni R N V Subbarao1 Asst.Prof in GIET Polyt College Rajahmundry, AP, India Atti V V Srinivas 3 Asst.Prof in GIET College Rajahmundry, AP, India D.Vani 2 Asst.Prof in GIET Polyt College Rajahmundry, AP, India 39 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023
- 2. multi windings of the GZV based inverter results in the larger size, weight, and additional expense [3]. In case of single phase to overcome the aforementioned problem, the objective of this paper is to study a newly constructed transformerless five level multi-string inverter topology for RESs. In this paper, the aforesaid GZV-based inverter is reduced to a multi-string multilevel inverter topology that requires only six active switches instead of the eight required in the conventional cascaded H bridge (CCHB) multilevel inverter. In addition, among them, two active switches are operated at the line frequency. In order to improve the conversion efficiency of conventional boost converters, a high step-up converter is also introduced as a front-end stage to stabilize the output dc voltage of each RES modules for use with the simplified multilevel inverter. The newly constructed inverter topology offer strong advantages such as improved output waveforms, smaller filter size, and lower EMI and total harmonics distortion (THD). In this letter, the operating principle of the developed system is described, and a prototype is constructed for verifying the effectiveness of the topology. 2. Photovoltaic System A Photovoltaic (PV) system directly converts sunlight into electricity. The basic device of a PV system is the PV cell. Cells may be grouped to form panels or arrays. The voltage and current available at the terminals of a PV device may directly feed small loads such as lighting systems and DC motors. A photovoltaic cell is basically a semiconductor diode whose pn junction is exposed to light. Photovoltaic cells are made of several types of semiconductors using different manufacturing processes. The incidence of light on the cell generates charge carriers that originate an electric current if the cell is short circuited. Figure1: Equivalent circuit of a PV device The equivalent circuit of a PV cell is shown in figure1. In the above diagram the PV cell is represented by a current source in parallel with diode, RS and RP represents series and parallel resistance respectively. The output current and voltages from PV cell are represented by I and V. The V-I characteristic of PV cell is shown in figure2. The net cell current I is composed of the light-generated current Ipv and the diode current Id. Figure2: Characteristic V-I curve of the PV cell I=Ipv – Id (1) Where Id = Io exp (qV/akT) Io = leakage current of the diode q = electron charge k = Boltzmann constant T = temperature of pn junction a = diode ideality constant The basic equation (1) of the PV cell does not represent the V-I characteristic of a practical PV array. The basic equation of PV array requires the additional parameters as shown in figure. I = IPV – [exp (V+RS/Vta) – 1] – (V+RS/RP) (2) Where Vt = NSkT/q is the thermal voltage of the array with NS cells connected in series. 3. Proposed Concept This topology configuration for single phase consists of two high steps up dc/dc converters connected to their individual dc-bus capacitor and a simplified multilevel inverter. Input sources, PV module 1, and PV module 2 are connected to the inverter followed a linear resistive load through the high step-up dc/dc converters. For three phases consists of same as of single phase connection but each phase connected by 1200 phase difference. 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, lower THD and EMI. High step up converter introduced the output voltage is compared with the reference value. The error is given to the PI controller and the driving pulses for the converter are generated. The converter output voltage meets the reference value. 40 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023
- 3. The boosted DC voltage to Multi-string multilevel inverter is shown in figure 3. Figure 3: single phase multi-string five level inverter 3.1. High step-up converter stage In this study, high step-up converter topology is introduced to boost and stabilize the output dc voltage of various RESs such as PV and fuel cell modules for employment of the proposed simplified multilevel inverter. The coupled inductor of the high step-up converter in Fig. 4 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 Vpri = Vin * (D/1-D) Hence, the voltage conversion ratio of the high step-up converter, named input voltage to bus voltage ratio, can be derived as 𝑉𝑠𝑖 𝑉𝑝𝑟𝑖 = 2 + 𝑁𝑠 𝑁𝑝 ∗ 𝐷 (1 − 𝐷) 3.2. Multi-string Multilevel Inverter This paper reports a new single-phase and block diagram of three phase multi-string topology, presented as a new basic circuitry in Fig. 3 Figure4. Basic Single Phase Multi-string Five level inverter Figure 5. Block diagram of Three Phase Multi-string Multi Level Inverter This three phase inverter consists of three (R, Y and B) phase conductors connect to load and return conductors connected to ground. In this three phase inverter each phase conducts with 1200 difference. For convenient illustration, the switching function of the switch in Fig. 4 is defined as follows: 𝑆𝑎𝑗 = 1, 𝑆𝑎𝑗 𝑂𝑁 0, 𝑆𝑎𝑗 𝑂𝐹𝐹 , 𝑗 = 1,2,3 𝑆𝑏𝑗 = 1, 𝑆𝑏𝑗 𝑂𝑁 0, 𝑆𝑏𝑗 𝑂𝐹𝐹 , 𝑗 = 1,2,3 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 switches Sa 2, Sb 1, and Sb 3 are ON; the voltage applied to the LC output filter is 2VS. 41 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023
- 4. 2. Half-level positive output, +Vs: This output condition can be induced by two different switching combinations. One switching combination is such that active switches Sa 2, Sb 1, and Sa 3 are ON; the other is such that active switches Sa 2, Sa 1, and Sb 3 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 switches Sa 1, Sb 2, and Sb 3 are ON; the other is such that active switches Sa 3, Sb 1, and Sb 2 are ON. 5. Maximum negative output, −2Vs: During this stage, active switches Sa 1, Sa 3, and Sb 2 are ON, and the voltage applied to the LC output filter is −2Vs. Figure 6. Modulation strategy for reference signal Table1 Switching combination In these operations, it can be observed that the open voltage stress of the active power switches Sa 1 , Sa 3, Sb 1, and Sb 3 is equal to input voltage VS ; moreover, the main active switches Sa 2 and Sb 2 are operated at the line frequency. Hence, the resulting switching losses of the new topology are reduced naturally, and the overall conversion efficiency is improved. The two input voltage sources feeding from the high step up converter is controlled at 100V, i.e. Vs1 = Vs2 = 100V. The switch voltages of Sa1, Sa2, Sa3, Sb1, Sb2, and Sb3 are all shown in Fig. 6. It is evident that the voltage stresses of the switches Sa1, Sa3, Sb1, and Sb3 are all equal to 100V, and only the other two switches Sa2, Sb2 must be 200V voltage stress. For three phase inverter as similarly as single phase inverter but each phase operates with 1200 phase difference 3.3. Comparison with CCHB inverter The average switching power loss Ps in the switch caused by these transitions can be defined as 𝑃𝑠 = 0.5 𝑉𝑑𝑠 𝐼𝑜 𝑓𝑠[𝑡𝑐 𝑜𝑛 + 𝑡𝑐(𝑜𝑓𝑓)] 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. Figure 7. Five level inverter of CCHB 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 shown in table2. For three phases five level inverter of CCHB compare with proposed topology required number switches are reduced six switches and switching loss is nearly half that of CCHB inverter. 42 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023
- 5. Table 2 Comparison of two multi level inverter for single phase 3.4. DC–DC Closed loop with PI Controller In the closed loop model, the simulation is carried out to meet the reference value. The closed loop model of the DC- DC step up converter is shown in Figure 7. Figure 7. Block diagram of DC-DC closed loop with PI controller The output voltage is compared with the reference value. The error is given to the PI controller and the driving pulses for the converter are generated. Block diagram of PI controller as shown in figure 8. U* signal given to high step up converter switch, the converter output voltage meets the reference value. It can be seen that the output remains constant. This constant voltage given to five level inverter and load voltage is directly proportional to inverter input. Figure 8. Block diagram of PI controller 4. Simulation Results 4.1. Three Phase Resistor Load Simulations were performed by using MATLAB/ SIMULINK to verify that the proposed inverter topology. Three phase five level inverter with resistor load is shown in figure 9. Each phase voltage fed from two PV panels via high dc-dc converter, this dc-dc converter operates with PI controller. Figure 9. MATLAB/SIMULINK model of Three Phase Five Level Inverter with R-Load A prototype system with a high step-up dc/dc converter stage and the simplified multilevel dc/ac stage are built with the specifications of the two preceding high step-up dc/dc converters are 1) input voltage 30V; 2) controlled output voltage 100V; and 3) switching frequency 2 kHz; 4) three phase output voltage 200V.Simulation results of three phase load voltage, single phase voltage waveform and dc-dc output voltage are shown in figure 10, figure 11 and figure 12. 43 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023
- 6. Figure 10. Simulation result of three phase load Voltage waveform Figure 11. Simulation result of single phase waveform (200V Peak-Peak) Figure 12. DC-DC output voltage waveform (100V). This high DC-DC converter stage provide every time 100V is shown in figure 12, the output voltage of this converter compare with reference voltage (100V) which gives error signal every time and this error given to PI controller ( Kp = 0.001 and Ki = 0.01). After that driving pulses are generated by triangular waveform with high frequency. 4.2. Three Phase Induction Motor Load Figure 13. MATLAB/SIMULINK model of three phase five level inverter with induction motor For better analysis this new three phase multi-string five level inverter connected to induction motor is shown in figure 13. The analysis of three phase induction motor results is shown in figure 14, figure 15 and figure 16. Figure 14. Simulation result of induction motor stator current and voltage/phase Figure 15. Simulation result of a induction motor voltage and current 44 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023
- 7. Figure 16. Rotor speed and electromagnetic emf of induction motor The conversion efficiency of the implemented inverter and THD of the output voltage measured in this case are approximately 96% and 3%, respectively. The studied multilevel inverter has lower THD than the CCHB multilevel inverter. 5. Conclusion In this paper modeling and simulation of a novel three phase multi-string multi level inverter with high dc-dc closed loop topology that produces a significant reduction in the number of power devices required to implement multilevel inverter. This inverter topology has more advantages such as improved output waveforms, and lower EMI and THD. The proposed topology has minimum number of switches compare than other configuration. 6. Reference [1] C. L. Chen,Y.Wang, J. S. Lai,Y. S. Lee, andD.Martin, “Design of parallel inverters for smooth mode transfer micro-grid applications,” IEEE Trans. Power Electron., [2] C. T. Pan, C. M. Lai, and M. C. Cheng, “A novel integrated single phase inverter with an auxiliary step-up circuit for low- voltage alternative energy source application,” IEEE Trans. Power Electron, [3] C. T. Pan, W. C. Tu, and C. H. Chen, “A novel GZV-based multilevel single phase inverter,” in Proc. Taiwan Power Electron. [4] “A novel high step-up ratio inverter for distributed energy resources (DERs),” in Proc. IEEE Int. Power Electron. [5] L. M. Tolbert and T. G. Habetler, “Novel multilevel inverter carrier-based PWM method,”. [6] R. Gonzalez, E. Gubia, J. Lopez, and L.Marroyo, “Transformer less single phase multilevel-based photovoltaic inverter,”. [7] S. Vazquez, J. I. Leon, J. M. Carrasco, L. G. Franquelo, E. Galva ,M. Reyes, J. A. Sanchez, and E. Dominguez, “Analysis of the power balance in the cells of a multilevel cascaded H-bridge converter,” IEEE Trans. Ind. Electron., [8] T. Kerekes, R. Teodorescu, and U. Borup, “Transformer less photovoltaic inverters connected to the grid,” in Proc. IEEE Appl. Power Electron. [9] G. Ceglia,V. Guzm´an, C. S´anchez, F. Ib´a˜nez, J.Walter, and M. I. Gim´enez, “A new simplified multilevel inverter topology for DC–AC conversion, ”IEEE Trans. Power Electron., [10] W. Yu, C. Hutchens, J. S. Lai, J. Zhang, G. Lisi, A. Djabbari, G. Smith, and T. Hegarty, “High efficiency converter with charge pump and coupled inductor for wide input photovoltaic AC module applications,” in Proc. IEEE Energy Convers. 45 International Journal of Electrical Engineering Research & Applications (IJEERA) Vol. 1 Issue 3, August - 2013 IJEERA IJEERA www.ijeera.orgIJEERAV1IS030023