NEW THREE-PHASE SYMMETRICAL MULTILEVEL VOLTAGE SOURCE INVERTER FOR VSC HVDC

In this paper a brief review on different multilevel inverter topologies are discussed. Inverter is a power electronic device that converts DC power into AC power at desired output voltage and frequency. Multilevel Converters nowadays have become an interesting area in the field of industrial applications. Conventional power electronic converters are able to produce an output voltage that switches between two voltage levels only. Multilevel Inverter generates a desired output voltage from several DC voltage levels at its input. The input side voltage levels are usually obtained from renewable energy sources, capacitor voltage sources, fuel cells etc. The different multilevel inverter topologies are: Cascaded Hbridges converter, Diode clamped inverter, Flying capacitor multilevel inverter, and Modular Multilevel Converter (MMC).The disadvantages of MLI are the need for isolated power supplies, design complexity and switching control circuits Keywords— MLI, PWM, sinusoidal pulse width modulation, THD. Introduction: The development of new technologies and devices during the 20th century enhanced the interest in electric power systems. Modern civilization based his operation on an increasing energy demand and on the substitutions of human activities with complex and sophisticated machines; thus, studies on electric power generation and conversion devices become every day more and more important. The recent attention in environment protection and preservation increased the interest in electrical power generation from renewable sources: wind power systems and solar systems are diffusing and are supposed to occupy an increasingly important role in world-wide energy production in coming years. Not only house utilities, but industrial applications and even the electrical network requirements display the importance that energy supply and control will have in the future researches. As a consequence, power conversion and secondly control is required to be reliable, safe and available in order to accomplish all requirements, both from users and legal regulations, and to reduce the environmental impact. Voltage Source Converter (VSC) technology is becoming common in high-voltage direct current (HVDC) transmission systems (especially transmission of offshore wind power, among others). HVDC transmission technology is an important and efficient possibility to transmit high powers over long distances. The vast majority of electric power transmissions were three-phase and this was the common technology widespread. Main advantages for choosing HVDC instead of AC to transmit power can be numerous but still in discussion, and each individual situation must be considered apart. Each project will display its own pro and con about HVDC transmission, but commonly these advantages can be summarized: lower losses, long distance water crossing, controllability, limitation short circuit currents, environmental reason and lower cost. One of the most important advantages of HVDC on AC systems is related with the possibility to accurately control the active power transmitted, in contrast AC lines power flow can’t be controlled in the same direct way. However conventional converters display problems into accomplishing requirements and operation of HVDC transmission. Compared to conventional VSC technology, Modular Multilevel topology instead offers advantages such as higher voltage levels, modular construction, longer maintenance intervals and improved reliability. A multilevel approach guarantees a reduction of output harmonics due to sinusoidal output voltages: thus grid filters become negligible, leading to system cost and complexity reduction. Like in many other engineering fields, modular and distributed systems are becoming the suggested topology to achieve modern projects www.ijcrt.org © 2022 IJCRT | Volume 10, Issue 1 January 2022 | ISSN: 2320-2882 IJCRT2201333 International Journal of Creative Research Thoughts (IJCRT) www.ijcrt.org d26 requirements: this configuration ensure a more reliable operation, facilitates, diagnosis, maintenance and reconfigurations of control system. Especially in fail safe situations, modular configuration allows control system to isolate the problem, drive the process in safe state easily, and in many cases allows one to reach an almost normal operation even if in faulty conditions. The concept of a modular converter topology has the intrinsic capability to improve the reliability, as a fault module can be bypassed allowing the operation of the whole circuit without affecting significantly the performance. Many multi-level converter topologies have been investigated in these last years, having advantages and disadvantages during operation or when assembling the converters. To solve the problems of conventional multilevel converter a new MMC topology was proposed in this work describing the operation principle and performance under different operating conditions. Thus the attractive features can be summarized as follows 1. Reduced THD without filter. 2. Reduce requirement size of filters. 3. Staircase waveform which is nearer to sine wave. 4. Operates at both fundamental and high switching frequency. 5. Lower switching losses. 6. Better electromagnetic compatibility 7. Higher power quality One particular disadvantage is the need for large number of power semiconductor switches. Each switch have a related a gate driver circuit which adds complexity to the system. The overall system will be more expensive. Focuses are going on in present years to reduce the complexity of the circuit by decreasing the number of power electronic switches and gate driver circuits. This paper presents the different multilevel power converter topologies with related structures and the pros and cons of each circuit Concept of multilevel inverters: As in Figure 1, Conventional two-level inverters normally generate an output AC voltage from an input DC voltage. Pulse Width Modulation switching scheme is used to generate the AC output voltage (as shown in figure2).In the concept of Multilevel Inverter topology (MLI),several DC voltage levels are added together to create a smoother output waveform (as shown in figure3).The obtained output waveform have lower dv/dt and harmonic distortions. The circuit design is more complex with the increase in voltage levels. It needs a complicated switching controller circuit also. Figure 1Two level three phase inverter Figure 2Single leg of a three-level inverter Figure 3A three-level,five-level and seven-level output wave format fundamental switching frequency Figure2 shows the circuit of a conventional three level inverter. In a three level inverter each phase leg generate the three voltage levels (Vdc/2, 0, –Vdc/2). Three-level inverter is similar to a conventional two-level inverter, but with clamping diodes in between the two valves and are connected to the neutral between two capacitors. The capacitors act as DC bus voltage sources, each one is charged with the voltage Vdc/2.The number of levels can be increased by connecting another phase leg. Zero voltage level can be created by switching closer to the midpoint. Clamping diodes hold the voltage to zero with the neutral point. When valve pairs are more, capacitors and clamping diodes are added to generate more voltage levels in the inverter output. This results into anew topology of multilevel inverter with clamping diodes. Table 1: The difference between a 2-level and a 3level