Problem Statement and Motivations:

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 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.

In the case of MMC, 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 multi-level converter a new MMC topology was proposed in this proposed work describing the operation principle and performance under different operating conditions.

Objective:

The main objective of proposed work is the analysis of a Modular Multilevel Converter (MMC) and the development of a control scheme. The analysis is based on the use of a simplified circuit constituted by a single leg of the converter where all the modules in each arm are represented by a single variable voltage source. Design and simulation of proposed MMC topology is carryout using commercially available software package MATLAB/SIMULINK.

Methodology:

In this proposed work, analysis, modeling and control of a Modular Multilevel Converter need to investigate. Methodology of proposed work is summarized as follows

Understanding of various Multilevel Converter operation principles.

Understanding of Modular Multilevel Converter operation principles

Multilevel Converter modeling using MATLAB/SIMULINK software package

MMC modeling using MATLAB/SIMULINK software package.

Analysis of simulation results and comparison of MMC with other Multilevel Converter topologies.

Control Strategies Analysis and Implementation for MMC.

Proposed Modular Multilevel Converter topology:

Figure 1 – Schematic of a three-phase Modular Multi-level Converter

Figure 2 – Schematic of one phase of MMC

Figure 3 – Schematic of Sub Module of MMC

An MMC is a circuit that can be used to convert AC to DC voltage and convert DC to AC voltage. The MMC is made up of two multi-valves, one for the positive voltages called the upper multi-valve and the other for the negative voltages called the lower multi-valve. These two multi-valves as shown in Figure 1 are connected together through two inductors to control the power flow. Each multi-valve contains multiple submodules that are cascaded together. The more submodules in each multi-valve the higher the voltage it can convert and the smoother the AC waveform will be. Figure 2 shows one leg of three phase MMC.

A submodule contains two switches in series that will be fired in a complementary fashion, connected to a capacitor that will charge and discharge when the switches open and close (Figure 3). The capacitor voltages will be added together from the submodules to obtain the required voltage at the output of the system. Across each switch is a reverse diode that is used to control the current flow in the submodule. The switches need to be fired with a large enough dead time between them to ensure that one switch is able to fully turn off before the second switch turns on.

Literature Survey:

Shri harsha J, Voltage Source Converter Based HVDC Transmission, International Journal of Engineering Science and Innovative Technology (IJESIT), Volume 1, Issue 1, September 2012

High Voltage Direct Current system based on Voltage Source Converters (VSC-HVDC) is becoming a more effective, solution for long distance power transmission especially for off-shore wind plants and supplying power to remote regions. Since VSCs do not require commutating voltage from the connected ac grid, they are effective in supplying power to isolated and remote loads. Due to its advantages, it is possible that VSC-HVDC will be one of the most important components of power systems in the future. Power transmission using AC system over the past years has proven to be robust and efficient. One main problem with respect to AC power transmission is the complexities involved in precise power controllability. This problem may be overcome by using VSC based HVDC system of transmission. In this paper, possibility of using VSC based HVDC transmission using two level converter for evacuating power is explored.

Mario Marchesoni, Diode-Clamped Multilevel Converters: A PracticableWay to Balance DC-Link Voltages, IEEE transactions on industrial electronics, vol. 49, no. 4, august 2002

The converter topologies identified as diode-clamped multilevel (DCM) or, equivalently, as multipoint clamped (MPC), are rarely used in industrial applications, owing to some serious drawbacks involving mainly the stacked bank of capacitors that constitutes their multilevel dc link. The balance of the capacitor voltages is not possible in all operating conditions when the MPC converter possesses a passive front end. On the other hand, in ac/dc/ac power conversion, the back-to-back connection of a multilevel rectifier with a multilevel inverter allows the balance of the dc-link capacitor voltages and, at the same time, it offers the power-factor-correction capability at the mains ac input. An effective balancing strategy suitable for MPC conversion systems with any number of dc-link capacitors is presented in this paper.

Figure 4 – Schematic of a diode clamped Multi-level Converter

5.3 Liangzong He, Chen Cheng, Flying-Capacitor-Clamped Five-Level Inverter Based on Bridge Modular Switched-Capacitor Topology, IEEE transactions on industrial electronics, 2016.

A novel flying-capacitor-clamped five-level inverter based on bridge modular switched-capacitor (BMSC) topology is proposed in the paper. The inverter features the switched-capacitor circuit with DC-DC boosting conversion ability and the multilevel inverter circuit with flying-capacitor-clamped performance. With the special composite structure, the number of components is cut down compared to the topology of conventional cascaded multilevel inverter. Meanwhile, part of switches can be operated under line voltage frequency, resulting in switching loss reduction. Hence, the potential of system efficiency and power density is released due to embed switched-capacitor circuit.

Figure 5 – Schematic of a flying capacitor clamped Multi-level Converter

5.4 Dr. Asha Gaikwad, Study of Cascaded H-Bridge Multilevel Inverter, IEEE 2016 International Conference on Automatic Control and Dynamic Optimization Techniques (ICACDOT) International Institute of Information Technology (I²IT), Pune.

In this paper cascaded h-Bridge multilevel inverter with SPWM technique is presented. Multilevel inverter have high power application ; low harmonics due to these applications it is used widely in the area of control and energy distribution. This paper includes performance of 3-Level,5-Level ; 7-Level cascaded H-Bridge multilevel inverter.

Figure 6 – Schematic of a cascaded h bridge Multi-level Converter

5.5 Anjali Krishna R, A Brief Review on Multi Level Inverter Topologies, IEEE 2016 International Conference on Circuit, Power and Computing Technologies ICCPCT.

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 H-bridges converter, Diode clamped inverter, and Flying capacitor multilevel inverter. Multilevel inverters nowadays are used for medium voltage and high power applications.

Figure 7 – Schematic of a two-level Converter

Figure 8 – Schematic of a three-level Converter

Possible Outcome:

Design and simulation of conventional multilevel converter and proposed Modular Multilevel Converter will be done using MATLAB/SIMULINK software package. Analysis of simulation results to investigate pros and cons of conventional multilevel inverter and Modular Multilevel Converter will be done, which helps to arrive at conclusion about selection of suitable converter for VSC based HVDC and AC motor drive applications.

Applications:

VSC based HVDC transmission.

AC motor drive.

References

Shri harsha J, Voltage Source Converter Based HVDC Transmission, International Journal of Engineering Science and Innovative Technology (IJESIT), Volume 1, Issue 1, September 2012

Mario Marchesoni, Diode-Clamped Multilevel Converters: A PracticableWay to Balance DC-Link Voltages, IEEE transactions on industrial electronics, vol. 49, no. 4, august 2002

Liangzong He, Chen Cheng, Flying-Capacitor-Clamped Five-Level Inverter Based on Bridge Modular Switched-Capacitor Topology, IEEE transactions on industrial electronics, 2016.

Dr. Asha Gaikwad, Study of Cascaded H-Bridge Multilevel Inverter, IEEE 2016 International Conference on Automatic Control and Dynamic Optimization Techniques (ICACDOT) International Institute of Information Technology (I²IT), Pune.

Anjali Krishna R, A Brief Review on Multi Level Inverter Topologies, IEEE 2016 International Conference on Circuit, Power and Computing Technologies ICCPCT.

S.Shalini, Voltage Balancing in Diode Clamped Multilevel inverter Using Sinusoidal PWM, International Journal of Engineering Trends and Technology (IJETT) – Volume 6 Number 2 – Dec 2013

Kazunori Hasegawa, Low-Modulation-Index Operation of a Five-LevelDiode-Clamped PWM Inverter Witha DC-Voltage-Balancing Circuit for a Motor Drive, IEEE transactions on power electronics, vol. 27, no. 8, august 2012

MULTILEVEL INVERTER TOPOLOGIES

Diode-Clamped Multilevel Inverter

The most commonly used multilevel topology is the diode clamped inverter, in which the diode

is used as the clamping device to clamp the dc bus voltage so as to achieve steps in the output voltage. The neutral point converter proposed by Nabae, Takahashi, and Akagi in 1981 was essentially a three-level diode-clamped inverter 15. A three-level diode clamped inverter consists of two pairs of switches and two diodes. Each switch pairs works in complimentary mode and the diodes used to provide access to mid-point voltage. In a three-level inverter each of the three phases of the inverter shares a common dc bus, which has been subdivided by two capacitors into three levels. The DC bus voltage is split into three voltage levels by using two series connections of DC capacitors,C1 and C2. The voltage stress across each switching device is limited to Vdc through the clamping diodes Dc1 and Dc2. It is assumed that the total dc link voltage is Vdc and mid point is regulated at half of the dc link voltage, the voltage across each capacitor is Vdc/2 (Vc1=Vc2=Vdc/2). In a three level diode clamped inverter, there are three different possible switching states which apply the stair case voltage on output voltage relating to DC link capacitor voltage rate. For a three-level inverter, a set of two switches is on at any given time and in a five-level inverter, a set of four switches is on at any given time and so on. Fig-2.2 shows the circuit for a diode clamped inverter for a three-level and a five-level inverter. Switching states of the three level inverter are summarized in table-1.

Table-2.1. Switching states in one leg of the three-level diode clamped inverter

Switch Status

State

Pole Voltage

S1=ON,S2=ON

S1’=OFF,S2?=OFF S=+ve

Vao=Vdc/2

S1=OFF,S2=ON

S1’=ON,S2?=OFF

S=0

Vao=0

S1=OFF,S2=OFF

S1’=ON,S2?=ON

S=-ve

Vao=-Vdc/2

Fig 2.2: Topology of the diode-clamped inverter (a) three-level inverter, (b) five -level inverter.

Fig 2.3 shows the phase voltage and line voltage of the three-level inverter in the balanced condition. The line voltage Vab consists of a phase-leg a voltage and a phase-leg b voltage. The resulting line voltage is a 5-level staircase waveform for three-level inverter and 9-level staircase waveform for a five-level inverter. This means that an N-level diode-clamped inverter has an N-level output phase voltage and a (2N-1)-level output line voltage. In general the voltage across each capacitor for an N level diode clamped inverter at steady state is Vdc/ (N-1). Although each active switching device is required to block only a voltage level of Vdc, the clamping diodes require different ratings for reverse voltage blocking

Fig: 2.3 .Output voltage in three-level diode- clamped inverter (a) leg voltage

b) output phase voltage

In general for an N level diode clamped inverter, for each leg 2(N-1) switching devices, (N-1) * (N-2) clamping diodes and (N-1) dc link capacitors are required. By increasing the number of voltage levels the quality of the output voltage is improved and the voltage waveform becomes closer to sinusoidal waveform. However, capacitor voltage balancing will be the critical issue in high level inverters. When N is sufficiently high, the number of diodes and the number of switching devices will increase and make the system impracticable to implement. If the inverter runs under pulse width modulation (PWM), the diode reverse recovery of these clamping diodes becomes the major design challenge.

.Though the structure is more complicated than the two-level inverter, the operation is straightforward.

Operation of DCMLI.

Fig 2.2(a) shows a three-level diode-clamped converter in which the dc bus consists of two capacitors, C1, C2. For dc-bus voltage Vdc, the voltage across each capacitor is Vdc/2 and each device voltage stress will be limited to one capacitor voltage level Vdc/2 through clamping diodes. To explain how the staircase voltage is synthesized, the neutral point n is considered as the output phase voltage reference point. There are three switch combinations to synthesize three-level voltages across a and n.

1) Voltage level Van= Vdc/2, turn on the switches S1andS2.

2) Voltage level Van= 0, turn on the switches S2 and S1? .

3) Voltage level Van= – Vdc/2 turn on the switches S1?,S2?.

Fig. 2.2(b) shows a five-level diode-clamped converter in which the dc bus consists of four capacitors, C1, C2, C3, and C4. For dc-bus voltage Vdc, the voltage across each capacitor is Vdc/4 and each device voltage stress will be limited to one capacitor voltage level Vdc/4 through clamping diodes.

Switching states of the five level inverter are summarized in table-2.2.

Table-2.2. Switching states in one leg of the five-level diode clamped inverter

Voltage Vao

Switch state

S1 S2 S3 S4 S1? S2? S3? S4? Vao=Vdc

Vao=Vdc/2

Vao=0

Vao=-Vdc/2

Vao=-Vdc

1

0

0

0

0 1

1

0

0

0 1

1

1

0

0 1

1

1

1

0 0

1

1

1

1 0

0

1

1

1 0

0

0

1

1 0

0

0

0

1 To explain how the staircase voltage is synthesized, the neutral point n is considered as the output phase voltage reference point. There are five switch combinations to synthesize five level voltages across a and n.

Voltage level Van= Vdc; turn on all upper switches S1 , S2 , S3 and S4.

Voltage level Van= Vdc/2, turn on the switches S2, S3, S4 and S1?.

Voltage level Van= 0, turn on the switches S3, S4, S1? and S2?.

Voltage level Van= – Vdc/2 turn on the switches S4, S1?, S2?, S3?.

Voltage level Van= – Vdc; turn on all lower switches S1?, S2? ,S3? and S4?.

Four complementary switch pairs exist in each phase. The complementary switch pair is defined such that turning on one of the switches will exclude the other from being turned on. In this example, the four complementary pairs are (S1 –S1?), (S2- S2?), (S3 – S3?), and (S4 – S4?).

Although each active switching device is only required to block a voltage level of Vdc/ (m-1), the clamping diodes must have different voltage ratings for reverse voltage blocking. Using D1? of Fig. 2.2(b) as an example, when lower devices S??- S4?, are turned on, D1?needs to block three capacitor voltages, or 3Vdc/4, and D1 needs to block Vdc/4. Similarly, D2 and D2? need to block 2Vdc/4, and D3 needs to block 3Vdc/4. Assuming that each blocking diode voltage rating is the same as the active device voltage rating, the number of diodes required for each phase will be (m-1)*(m-2). This number represents a quadratic increase in m.

There are some complementary switches and in a practical implementation, some dead time is inserted between the gating signals and their complements meaning that both switches in a complementary pair may be switched off for a small amount of time during a transition. However, for the discussion herein, the dead time will be ignored.

Features of Diode clamped MLI

1) High-Voltage Rating Required for Blocking Diodes:

Although each active switching device is only required to block a voltage level of Vdc/(m – l), the clamping diodes need to have different voltage ratings for reverse voltage blocking Using D1? of Fig. 2 (5-level diode clamped inverter) as an example, when all lower devices, S1?-S4? are turned on, D1? needs to block three capacitor voltages, or 3Vdc/4. Similarly, D2 and D2? need to block 2Vdc/4, and D3 needs to block 3Vdc/4. Assuming that each blocking diode voltage rating is the same as the active device voltage rating, the number of diodes required for each phase will be (m – 1) x (m – 2). This number represents a quadratic increase in m. When m is sufficiently high, the number of diodes required will make the system impractical to implement.

2) Unequal Device Rating,

In figure-2 it can be seen that switch S1 conducts only during Vao = Vdc, while switch S4 conducts over the entire cycle except during Vao = 0. Such an unequal conduction duty requires different current ratings for switching devices. When the inverter design is to use the average duty for all devices, the outer switches may be oversized, and the inner switches may be undersized. If the design is to suit the worst case, then each phase will have 2 x (m – 2) outer devices oversized. In comparison with the traditional transformer coupling multipulse converters using six-step operation for each converter, such unequal conduction duty is indeed an advantageous feature because the six-step operation needs maximum duty in each device and circulating currents between converters through transformers

3) Capacitor Voltage Unbalance:

In most applications, a power converter needs to transfer real power from ac to dc (rectifier operation) or dc to ac (inverter operation). When operating at unity power factor, the charging time for rectifier operation (or discharging time for inverter operation) for each capacitor is different. Such a capacitor charging profile repeats every half cycle, and the result is unbalanced capacitor voltages between different levels. The voltage unbalance problem in a multilevel converter can be solved by several approaches, such as replacing capacitors by a controlled constant dc voltage source such as pulse-width modulation (PWM) voltage regulators or batteries. The use of a controlled dc voltage will result in system complexity and cost penalties. With the high power nature of utility power systems, the converter switching frequency must be kept to a minimum to avoid switching losses and electromagnetic interference (EMI) problems. When operating at zero power factor, however, the capacitor voltages can be balanced by equal charge and discharge in one-half cycle. This indicates that the converter can transfer pure reactive power without the voltage unbalance problem.

Advantages and Disadvantages of DCMLI

Advantages:

1. All of the phases share a common dc bus, which minimizes the capacitance requirements of the converter. 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.

2. The capacitors can be pre-charged as a group.

3. Efficiency is high for fundamental frequency switching.

4. When the number of levels is high enough, harmonic content will be low enough to avoid the need for filters

Disadvantages:

1. Real power flow is difficult for a single inverter because the intermediate dc levels will tend to overcharge or discharge without precise monitoring and control.

2. The number of clamping diodes required is quadratically related to the number of levels 1, which can be cumbersome for units with a high number of levels.

2:Flying Capacitor

The capacitor clamped inverter alternatively known as flying capacitor was proposed by Meynard and Foch in 1992 18. The structure of this inverter is similar to that of the diode-clamped inverter except that instead of using clamping diodes, the inverter uses capacitors in their place. The flying capacitor involves series connection of capacitor clamped switching cells. This topology has a ladder structure of dc side capacitors, where the voltage on each capacitor differs from that of the next capacitor. The voltage increment between two adjacent capacitor legs gives the size of the voltage steps in the output waveform. Figure 2.4 shows the three-level and five-level capacitor clamped inverters respectively

Fig.2.4 Capacitor-clamped multilevel inverter circuit topologies,

Operation of FCMLI.

In the operation of flying capacitor multi-level inverter, each phase node (a, b, or c) can be connected to any node in the capacitor bank (V3, V2, V1). Connection of the a-phase to positive node V3 occurs when S1 and S2 are turned on and to the neutral point voltage when S2 and S1? are turned on. The negative node V1 is connected when S1? and S2?are turned on. The clamped capacitor C1 is charged when S1 and S1? are turned on and is discharged when S2 and S2? are turned on. The charge of the capacitor can be balanced by proper selection of the zero states. In comparison to the three-level diode-clamped inverter, an extra switching state is possible. In particular, there are two transistor states, which make up the level V3. Considering the direction of the a-phase flying capacitor current Ia for the redundant states, a decision can be made to charge or discharge the capacitor and therefore, the capacitor voltage can be regulated to its desired value by switching within the phase. As with the three-level flying capacitor inverter, the highest and lowest switching states do not change the charge of the capacitors. The two intermediate voltage levels contain enough redundant states so that both capacitors can be regulated to their ideal voltages.

Similar to the diode clamped inverter, the capacitor clamping requires a large number of bulk capacitors to clamp the voltage. Provided that the voltage rating of each capacitor used is the same as that of the main power switch, an N level converter will require a total of (N-1) * (N- 2) / 2 clamping capacitors per phase in addition to the (N-1) main dc bus capacitors.

Unlike the diode-clamped inverter, the flying-capacitor inverter does not require all of the switches that are on (conducting) in a consecutive series. Moreover, the flying-capacitor inverter has phase redundancies, whereas the diode-clamped inverter has only line-line redundancies 1, 3. These redundancies allow a choice of charging/discharging specific capacitors and can be incorporated in the control system for balancing the voltages across the various levels.

The voltage synthesis in a five-level capacitor-clamped converter has more flexibility than a diode-clamped converter. Using Fig. 2.4(b) as the example, the voltage of the five-level phase-leg „a? output with respect to the neutral point n (i.e. Van), can be synthesized by the following switch combinations.

1) Voltage level Van= Vdc/2, turn on all upper switches S1 – S4 .

2) Voltage level Van= Vdc/4, there are three combinations.

a) Turn on switches S1 , S2 , S3 and S1?.(Van= Vdc/2 of upper C4?s – Vdc/4 of C1?s).

b) Turn on switches S2 , S3 , S4 and S4?.(Van= 3Vdc/4 of upper C3?s – Vdc/2 of C4?s).

c) Turn on switches S1 , S3 , S4 and S3?. (Van= Vdc/2 of upper C4?s – 3Vdc/4 or C3?s + Vdc/2 of upper C2„ ).

3) Voltage level Van= 0, turn on upper switches S3 , S4 , and lower switch S1?, S2?.

4) Voltage level Van= -Vdc/4, turn on upper switch S1 and lower switches S1?, S2?and S3?.

5) Voltage level Van= -Vdc/2, turn on all lower switches S1?, S2?, S3? and S4?.

Features of FCMLI

The major problem in this inverter is the requirement of a large number of storage capacitors. Provided that the voltage rating of each capacitor used is the same as that of the main power switch, an m-level converter will require a total of (m – 1) x (m – 2)/2 auxiliary capacitors per phase leg in addition to (m – 1) main dc bus capacitors. With the assumption that all capacitors have the same voltage rating, an m-level diode-clamp inverter only requires (m – 1) capacitors.

In order to balance the capacitor charge and discharge, one may employ two or more switch combinations for middle voltage levels (i.e., 3Vdc/4. Vdc/2, and Vdc/4) in one or several fundamental cycles. Thus, by proper selection of switch combinations, the flying-capacitor multilevel converter may be used in real power conversions. However, when it involves real power conversions, the selection of a switch combination becomes very complicated, and the switching frequency needs to be higher than the fundamental frequency. In summary, advantages and disadvantages of a flying capacitor multilevel voltage source converter are as follows

Advantages and Disadvantages of (FCMLI).

Advantages

Compared to the diode-clamped inverter, this topology has several unique and attractive features as described below:

i) Added clamping diodes are not needed.

ii) It has switching redundancy within the phase, which can be used to balance the flying capacitors so that only one dc source is needed.

iii) The required number of voltage levels can be achieved without the use of the transformer. This assists in reducing the cost of the converter and again reduces power loss.

iv) Unlike the diode clamped structure where the series string of capacitors share the same

voltage, in the capacitor-clamped voltage source converter the capacitors within a phase leg are charged to different voltage levels.

v) Real and reactive power flow can be controlled.

vi) The large number of capacitors enables the inverter to ride through short duration outages and deep voltage sags.

Disadvantages

i) Converter initialization i.e., before the converter can be modulated by any modulation scheme the capacitors must be set up with the required voltage level as the initial charge. This complicates the modulation process and becomes a hindrance to the operation of the converter.

ii) Control is complicated to track the voltage levels for all of the capacitors.

iii) Precharging all of the capacitors to the same voltage level and startup are complex.

iv) Switching utilization and efficiency are poor for real power transmission.

v) Since the capacitors have large fractions of the dc bus voltage across them, rating of the capacitors are a design challenge.

vi) The large numbers of capacitors are both more expensive and bulky than clamping diodes in multilevel diode-clamped converters.

vii) Packaging is also more difficult in inverters with a high number of levels.

Cascaded multilevel inverter

Fig2.5: Single phase structures of Cascaded inverter (a) 3-level, (b)5-level, (c) 7-level

One more alternative for a multilevel inverter is the cascaded multilevel inverter or series H-bridge inverter. The series H-bridge inverter appeared in 197514. Cascaded multilevel inverter was not fully realized until two researchers, Lai and Peng. They patented it and presented its various advantages in 1997. Since then, the CMI has been utilized in a wide range of applications. With its modularity and flexibility, the CMI shows superiority in high-power applications, especially shunt and series connected FACTS controllers. The CMI synthesizes its output nearly sinusoidal voltage waveforms by combining many isolated voltage levels. By adding more H-bridge converters, the amount of Var can simply increased without redesign the power stage, and build-in redundancy against individual H-bridge converter failure can be realized. A series of single-phase full bridges makes up a phase for the inverter. A three-phase CMI topology is essentially composed of three identical phase legs of the series-chain of H-bridge converters, which can possibly generate different output voltage waveforms and offers the potential for AC system phase-balancing. This feature is impossible in other VSC topologies

utilizing a common DC link. Since this topology consists of series power conversion cells, the voltage and power level may be easily scaled. The dc link supply for each full bridge converter is provided separately, and this is typically achieved using diode rectifiers fed from isolated secondary windings of a three-phase transformer. Phase-shifted transformers can supply the cells in medium-voltage systems in order to provide high power quality at the utility connection.

Operation of CMLI.

The converter topology is based on the series connection of single-phase inverters with separate dc sources. Fig. 2.5 shows the power circuit for one phase leg of a three-level , five-level and seven-level cascaded inverter. The resulting phase voltage is synthesized by the addition of the voltages generated by the different cells. In a 3-level cascaded inverter each single-phase full-bridge inverter generates three voltages at the output: +Vdc, 0, -Vdc (zero, positive dc voltage, and negative dc voltage). This is made possible by connecting the capacitors sequentially to the ac side via the power switches. The resulting output ac voltage swings from -Vdc to +Vdc with three levels, -2Vdc to +2Vdc with five-level and -3Vdc to +3Vdc with seven-level inverter. The staircase waveform is nearly sinusoidal, even without filtering.

For a three-phase system, the output voltage of the three cascaded converters can be connected in either wye (Y) or delta (?) configurations. For example, a wye-configured 7-level converter using a CMC with separated capacitors is illustrated in the fig. 2.6.

Fig 2.6 Three-phase 7-level cascaded multilevel inverter (Y-configuration)

Features of CMLI

For real power conversions, (ac to dc and dc to ac), the cascaded-inverter needs separate dc sources. The structure of separate dc sources is well suited for various renewable energy sources

such as fuel cell, photovoltaic, and biomass, etc.

Connecting separated dc sources between two converters in a back-to-back fashion is not possible because a short circuit will be introduced when two back-to-back converters are not switching synchronously.

In summary, advantages and disadvantages of the cascaded inverter based multilevel voltage source converter can be listed below.

Advantages and Disadvantages of CMLI.

Advantages:

The regulation of the DC buses is simple.

Modularity of control can be achieved. Unlike the diode clamped and capacitor clamped inverter where the individual phase legs must be modulated by a central controller, the full-bridge inverters of a cascaded structure can be modulated separately.

Requires the least number of components among all multilevel converters to achieve the same number of voltage levels.

Soft-switching can be used in this structure to avoid bulky and lossy resistor-capacitor-diode snubbers.

Disadvantages:

i) Communication between the full-bridges is required to achieve the synchronization of reference and the carrier waveforms.

ii) Needs separate dc sources for real power conversions, and thus its applications are somewhat limited

Modular Multi-Level Converter

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 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.

In the case of MMC, 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 multi-level 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 harmonic distortion

2. Higher no. of voltage level

3. Staircase waveform quality

4. Operates at both fundamental and high switching frequency pwm

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

Description and principle of operation of MMC

An MMC is a circuit that can be used to convert AC to DC voltage and convert DC to AC voltage. The MMC is made up of two multi-valves, one for the positive voltages called the upper multi-valve and the other for the negative voltages called the lower multi-valve. These two multi-valves as shown in Figure 7 are connected together through two inductors to control the power flow. Each multi-valve contains multiple sub modules that are cascaded together. The more sub modules in each multi-valve the higher the voltage it can convert and the smoother the AC waveform will be. Figure 8 shows one leg of three phase MMC.A sub module contains two switches in series that will be fired in a complementary fashion, connected to a capacitor that will charge and discharge when the switches open and close (Figure 9). The capacitor voltages will be added together from the sub modules to obtain the required voltage at the output of the system. Across each switch is a reverse diode that is used to control the current flow in the sub module. The switches need to be fired with a large enough dead time between them to ensure that one switch is able to fully turn off before the second switch turns on.

Figure 1 – Schematic of a three-phase Modular Multi-level Converter

With reference to the SM shown in Fig. 2, the output voltage UO is given by,

UO = UC if T1 is ON and T2 is OFF

UO = 0 if T1 is OFF and T2 is ON

where UC is the instantaneous capacitor voltage

The configuration with T1 and T2 both ON should not be considered because it determines a short circuit across the capacitor. Also the configuration with T1 and T2 both OFF is not useful as it produces different output voltages depending on the current direction. Fig. 4 shows the current flows in both useful states.

In a MMC the number of steps of the output voltage is related to the number of series connected SMs. In order to show how the voltage levels are generated, in the following, reference is made to the simple three level MMC configuration shown in

Figure 2- Schematic of Sub Module of MMC

Figure 3 – States of SM and current paths

Figure 4 – Schematic of one phase of MMC

In this case, in order to get the positive output, +UD/2, the two upper SMs 1 and 2 are bypassed. Accordingly, for the negative output, – UD/2, the two lower SMs 3 and 4 are bypassed. The zero state can be obtained through two possible switch configurations. The first one is when the two SMs in the middle of a leg (2 and 3) are bypassed, and the second one is when the end SMs of a leg (1 and 4) are bypassed. It has to be noted that the current flows through the SMS that are not by passed determining the charging or discharging of the capacitors depending on the current direction. Therefore, in order to keep the capacitor voltages balanced, both zero states must be used alternatively. The voltage waveform generated by the three level converters is shown in Fig. 6.

Figure 5 – Voltage waveform of a Three-Level Converter

The principle of operation can be extended to any multi-level configuration as the one represented in Fig. 6

Figure 6- Schematic of one phase of Multi-Level Converter

In this type of inverter, the only states that have no redundant configurations are the two states that generate the maximum positive and negative voltages, + UD/2 and –UD/2. For generating the other levels, in general there are several possible switching configurations that can be selected in order to keep the capacitor voltages balanced. In MMC of Fig. 6, the switching sequence is controlled so that at each instant only N SMs (i.e. half of the 2N SMs of a phase leg) are in the on-state. As an example, if at a given instant in the upper arm SMs from 2 to N are in the on-state, in the lower arm only one SM will be in on-state. It is clear that there are several possible switching configurations. Equal voltage sharing among the capacitor of each arm can be achieved by a selection algorithm of inserted or bypassed SMs during each sampling period of the control system. A typical voltage waveform of a multi-level converter is shown in Fig. 7

Figure 7 – Voltage waveform of a Multi-Level Converter

PWM Techniques

The fundamental methods of pulse-width modulation (PWM) are divided into the traditional voltage-source and current-regulated methods. Voltage-source methods more easily lend themselves to digital signal processor (DSP) or programmable logic device (PLD) implementation. However, current controls typically depend on event scheduling and are therefore analog implementations which can only be reliably operated up to a certain power level. In discrete current-regulated methods the harmonic performance is not as good as that of voltage-source methods. A sample PWM method is described below.

Fig. 1 Pulse-width modulation.

Inverter output voltage, VA0 = Vdc/2, When vcontrol ; vtri, and VA0 = -Vdc/2, When vcontrol ; vtri .

PWM frequency is the same as the frequency of vtri . Amplitude is controlled by the peak value of vcontrol and Fundamental frequency is controlled by the frequency of vcontrol .

Modulation Index (m) is given by :

Where (VA0)1 is the fundamental frequency component of VA0

Simulation results and discussion:

Two level inverter:

Figure 1-Two level converter

Figure 11-Two level converter output voltage and current

Figure 3-FFT analysis

In two level converter output voltage is square wave switch between +Vdc and –Vdc. From the FFT analysis it is observed that output voltage contains more harmonics and THD = 48%. Hence it require large size filter to eliminate lower order and higher order harmonics.

Cascaded 5 level inverter:

Figure 4-Five level cascaded inverter

Figure 5-Five level cascaded inverter output voltage

Figure 6-FFT analysis

In five level cascaded converter output voltage is step wave (+V, +2V, 0, -V, -2V). From the FFT analysis it is observed that output voltage contains lesser harmonics than two level converter and THD = 42%. Hence it requires lesser size filter compared to two level inverter to eliminate lower order and higher order harmonics.

Cascaded 7 level inverter

Figure 7-Seven level cascaded inverter

Figure 8-Seven level cascaded inverter outptut voltage

Figure 9-FFT analysis

In seven level cascaded converter output voltage is step wave (+V, +2V, +3V, 0, -V, -2V, -3V). From the FFT analysis it is observed that output voltage contains lesser harmonics than five level cascaded converter and THD = 27.5%. Hence it requires lesser size filter compared to five level cascaded inverter to eliminate lower order and higher order harmonics.

7 level MMC converter:

Figure 10-Sub Module of Seven level MMC inverter

Figure 11-Seven level MMC inverter

At any given time, only six SMs out of twelve SMs of a phase conduct. The number of SMs to be ‘on’ in the upper arm (nup) and the number of SMs to be ‘on’ in the lower arm (nlow) is determined by the desired output voltage level in the phase. SM capacitor voltage is regulated at Vdc/6, where, Vdc is the rated DC link voltage.

Table 1- Switching table

Figure 12-Seven level MMC inverter output voltage

Figure 13-FFT analysis

In seven level MMC converter output voltage is step wave (+V/2, +V/3, +V/6, 0, -V/6, -V/3, -V/2). From the FFT analysis it is observed that output voltage contains lesser harmonics than five level cascaded converter and THD = 28.5% with improved fundamental component voltage when compared to seven level cascaded converter.

Conclusion:

The paper presents a brief discussion on basic multi-level inverter topologies. Fundamental multilevel converter structures including the advantages and disadvantages of each technique have been discussed. The main advantage of MMC is that it finds a solution to the problems of total harmonics distortion, EMI, and dv/dt stress on switch. In industrial and commercial market areas, more and more product are available that depends on the multi-level inverter topologies. Research works are in progress considering the structure complexity and control circuits. This helps to reduce the power electronics components and improve total harmonics profile and total cost of the system.