| 1. Unified Converter Theory | | | | inductor summing; |
| | | | | Ø Multiple bridge using direct series |
| In the preface of his book Switching Power | | | | connection; |
| Converters, Wood introduces the concept of a unified | | | | Ø Multilevel diode-clamped converter; and |
| converter theory. There he states: “Most traditional | | | | Ø Multilevel flying capacitor converter. |
| views of the field have seemed somewhat disjointed; | | | | |
| converters were largely regarded as related only | | | | Each of these will be examined in turn. Each of the |
| because they all use semiconductor switches and | | | | diagrams presented are of a five-level converter, |
| have certain topological similarities. . . . the view | | | | which can produce a nine-level phase to phase |
| expounded herein (is that) switching power converters | | | | voltage waveform. |
| are related by function and behavior; their basic | | | | |
| characteristics do not in any way depend on the | | | | |
| types of switches used, nor on the applications to | | | | 7. Transformer/Inductor summed Multiple Bridge |
| which they are put, nor on the topologies in which | | | | Converter |
| they are realized.”. According to this unified theory, | | | | |
| any power electronic converter can be viewed as a | | | | As the title suggests, these multilevel converters are |
| matrix of switches which connects its input nodes to | | | | simply a number of conventional two-level bridges, |
| its output nodes. These nodes may be either DC or | | | | whose inputs or outputs are summed using |
| AC, and either inductive or capacitive; and the | | | | transformers or inductors. The multiple transformer |
| power flow may be in either direction. Two obvious | | | | secondary’s force voltage sharing between the |
| restrictions are enforced by some basic laws of | | | | switches (Fig. 2.4). The most common and well known |
| electricity. | | | | example of a multi-bridge converter is the twelve pulse |
| | | | | thyristor converter, well covered in most power |
| • If one set of nodes (input or output) is inductive, | | | | electronic textbooks [49]. Harmonic cancellation in |
| the other set must be capacitive, so as not to create | | | | these converters is achieved through the phase |
| a cut set of voltage or current sources when | | | | displacement of the voltage waveforms of the star |
| the switches are closed. | | | | and delta transformer secondary’s. |
| • The combination of open and closed switches | | | | Figure 2.4. A five-level Transformer coupled multiple |
| should never open circuit an inductor, or short circuit a | | | | bridges, which produces nine level phase-phase |
| capacitor. | | | | waveforms on the transformer primary. |
| | | | | |
| 2. Inverter or Rectifier? Voltage or Current Source? | | | | This 30? phase shift between transformer |
| | | | | secondaries allows identical secondary switching |
| This unified set of converters is generally broken into a | | | | instants and current waveforms to appear interleaved |
| number of subsets. The term rectifier is used when | | | | on the transformer primary. A series connection is |
| the power flow is predominately from the AC port to | | | | used for HVDC; a parallel connection for high current |
| the DC port and the term inverter is used when power | | | | applications such as electrolysis and electro-plating. |
| flow is predominately from the DC port to the AC | | | | The technique can and is extended to many bridges |
| port. The term converter is used either when there is | | | | each with a transformer secondary connection of the |
| no predominant direction of power flow or as a | | | | appropriate phase shift to achieve cancellation of the |
| general term to encompass both rectifiers and | | | | further low order harmonics in the primary. By clever |
| inverters. In a Voltage Source Converter (VSC), the | | | | connection of the transformer primaries, current as |
| DC port is the capacitive port and is voltage stiff (i.e. a | | | | well as voltage sharing can be ensured. |
| large DC bus capacitor). The voltages in such a | | | | |
| converter are well defined by this port and are | | | | A good example of the next degree of complexity |
| generally considered independent of the | | | | and flexibility is seen in a 10 MW battery energy |
| converter’s operation. The value of the AC side | | | | storage plant. The GTO converters operate in square |
| inductance is comparatively small and modulation of | | | | wave mode and still rely on the transformer phasing |
| the converter controls these AC side inductor currents. | | | | for harmonic cancellation. However because forced |
| Should the voltage source converter be responsible | | | | commutation is used; now both the magnitude and the |
| for the control of the DC bus capacitor voltage, then | | | | phase (real and reactive power) can be separately |
| this voltage is indirectly controlled by controlling the net | | | | controlled. An extension of this approach to 48 pulse |
| current flow in the capacitor. | | | | operation is achieved by eight GTO bridges operating |
| | | | | in square wave mode, with reliance on the transformer |
| The switches in such a converter must block a | | | | for harmonic cancellation. The cancellation of switching |
| unidirectional voltage, but be able to conduct current in | | | | harmonics can also be achieved by switching |
| either direction if bidirectional power flow is desired. | | | | strategies, rather than relying on the transformer |
| The converse is true in a Current Source Converter | | | | secondary’s for the necessary phase shifting. The |
| (CSC) — the DC port is inductive and current stiff. | | | | simplest case — the series or parallel connection of |
| The current in this port (and hence the converter) is | | | | two PWM bridges — has been investigated by a |
| well defined and slow to change. The voltage | | | | number of researchers. By the use of appropriate |
| (particularly at the AC port) is considered the variable | | | | PWM modulation for each bridge, the odd multiples of |
| directly controlled by the converter modulation. Since | | | | the PWM carrier and sidebands, including the first |
| the AC port usually has significant line or load | | | | cluster, were entirely removed from the output |
| inductance, line to line capacitors must be placed on | | | | spectrum. This improvement is better than can be |
| the AC port. The switches must block either voltage | | | | achieved by merely doubling the carrier frequency as |
| polarity, but are only required to conduct current in one | | | | the carrier which remains has lower amplitude. A |
| direction. This naturally suits thyristors and symmetrical | | | | particularly good example of a six bridge, transformer |
| GTOs. | | | | summed multilevel converter is used as an active filter |
| Figure 2.1. A voltage source rectifier - inverter cascade | | | | for arc furnace static flicker compensation [71]. |
| (top) and a current source rectifier - inverter cascade | | | | The AC connections of these bridges are summed by |
| | | | | separate transformer secondaries, which allow either a |
| Since the AC line and AC motor loads are both | | | | series or parallel DC connection. Since the transformer |
| inductive, Voltage Source Rectifier – Inverter | | | | no longer provides phase shifting, it may seem possible |
| cascades (Fig. 2.1) are usually used for small and now | | | | to remove the transformer entirely and place the |
| increasingly for large motor drives and similar | | | | converters directly in parallel (for a parallel connection). |
| applications, as GTOs and IGBTs have matured. | | | | However, while no difference exists between the |
| Larger converters have traditionally been current | | | | desired input and output components of the two |
| source converters, both because this best suits the | | | | converters, the undesired switching components are |
| characteristics of the thyristors and because it requires | | | | by definition exactly out of phase. Kirchhoff’s laws |
| a large DC bus inductor, which was preferred to a | | | | would be violated if the converters were directly |
| large capacitor. Some converters do not easily fall, or | | | | connected. |
| cannot be placed into either category. The matrix or | | | | |
| Venturini converter [1] is one example (Fig. 2.2). Both | | | | The solution is to use inter-phase reactors (current |
| input and output ports are AC, and the definition of | | | | sharing reactors) or interphase transformers on either |
| voltage stiff or current stiff (and hence voltage or | | | | the input or output of the converters. Although these |
| current source) becomes somewhat arbitrary. Both | | | | reactors see the full combined converter current (and |
| input and output ports are | | | | so have similar copper volume and copper losses), |
| | | | | they only experience the difference in voltage |
| Figure 2.2. The matrix converter, with one possible | | | | between the converters. The volts-second component |
| implementation of the bidirectional switches. | | | | of this voltage is smaller and so the iron content of |
| | | | | these reactors can be reduced in comparison to the |
| 3. The General Multilevel Converter | | | | transformers which would be required for full isolation. |
| | | | | Normally the inductors are placed on the AC side, |
| The next refinement is to define the meaning of | | | | which is already the inductive port of a voltage source |
| multilevel. The following definition of a multilevel | | | | converter. Research on a five level three-phase motor |
| converter is offered: | | | | drive which used this technique was conducted by |
| A multilevel converter can switch either its input or | | | | Matsui et al . The outputs of two half bridge legs were |
| output nodes (or both) between multiple (more than | | | | summed with a current sharing reactor to form a |
| two) levels of voltage or current. The term | | | | three level intermediate output. This and another |
| “two-level” will be used where it is necessary to | | | | similarly formed three level output were summed by a |
| refer specifically to a converter which is not multilevel. | | | | third reactor to form the final five level phase output. |
| This simple definition is deliberately quite broad and | | | | One further solution is to sum the outputs of two |
| inclusive, in keeping with the spirit of the unified | | | | converters across a bridge connected source or load. |
| converter theory. For example, the multi-phase matrix | | | | Both ends of the transformer or motor winding are |
| converter (Fig. 2.2) is, strictly speaking, a multilevel | | | | brought out and the winding must be fully floating. One |
| converter, according to this definition. Consider the | | | | converter is driven with a phase inverted signal, so that |
| three phase to three phase matrix converter, with | | | | twice the desired converter output is impressed |
| voltage source inputs and an inductive load. Any single | | | | across the floating load. If the carriers are appropriately |
| output can be switched to one of three different | | | | phased, part of the undesired carrier component will |
| voltage levels (the voltages of the three input phases) | | | | appear as a common mode component to the load. |
| and similarly, any input can be switched to one of four | | | | Of course, this technique can only be applied for two |
| current levels (including zero). In this preceding example, | | | | converters. |
| both the input and the output nodes are AC periodic | | | | |
| varying quantities and so these levels can only be | | | | To summarize, the transformer or inductor summed |
| considered stationary for an interval much shorter than | | | | approach has the following advantages: |
| their AC period. | | | | |
| Figure 2.3. The current source converter (top right), | | | | • The voltages within the individual converters and |
| voltage source converter (bottom left) and a simple | | | | thus across the switches are well defined by the stiff |
| three level voltage source converter (bottom right) can | | | | voltage source output of the transformer secondaries. |
| all be derived from the general topology of the matrix | | | | • Should a converter module fail, or be removed for |
| converter | | | | service, the converter may continue operating at full |
| | | | | voltage, but at reduced current. • Other than the |
| Both the voltage source | | | | transformer (inductors), the structure is modular, which |
| and current source converters can be derived from | | | | allows easier maintenance and reduced spares. |
| the general matrix converter by setting one port to be | | | | • Its mode of operation is easily understood and, |
| either a two terminal DC voltage stiff or DC current | | | | again because of its modular structure, control is more |
| stiff port [70, 30]. Retaining the third terminal leads to a | | | | easily applied. but also the following disadvantages: |
| simple and more conventional multilevel converter (Fig. | | | | • The transformer itself, if not needed for isolation, |
| 2.3). Note that now one of the ports has been made | | | | adds significantly to the cost of the converter and is |
| DC and voltage or current stiff, only one port will | | | | one more item to maintain and potentially, to fail. |
| experience the multilevel stepped waveforms. The | | | | • The transformer requires multiple secondary |
| other will still have a continuous waveform similar to | | | | windings, which must be isolated from one another and |
| that of an equivalent two level converter. | | | | from ground. This is a significant problem at high |
| | | | | voltages. This also increases the cost of the |
| For example, a converter with an appropriate structure | | | | transformer. |
| may create a stepped multilevel voltage waveform at | | | | |
| the inductive nodes, but will always have a continuous | | | | 8. Series Connected Isolated Multiple Bridge |
| voltage waveform at its capacitive nodes. Similarly a | | | | Converter |
| different converter may create a stepped multilevel | | | | |
| current waveform at its capacitive nodes, but must | | | | A second topology, which is really only a variation on |
| have a continuous current waveform at its inductive | | | | the first, is that of series connected bridge |
| nodes. | | | | converters (Fig. 2.5). Each phase leg |
| | | | | consists of series connected single phase full bridges, |
| | | | | the series connection being made directly (not by |
| 4. The Traditional Multilevel Converter | | | | transformer as in the first case) on the AC side. A |
| | | | | three phase converter can be constructed by |
| The traditional understanding of what constitutes a | | | | connecting three of these single phase series strings |
| multilevel converter follows this more narrow definition. | | | | to form a star or delta. Since this topology requires |
| One of the ports has multiple (more than two) voltage | | | | each full bridge to have an isolated DC bus, this |
| or current stiff DC nodes or terminals, while the | | | | connection has not been considered useful until |
| second port has a conventional single or three phase | | | | recently re-examined. Now this topology is being |
| set of terminals which are switched to these multiple | | | | considered for applications where no real power |
| levels. | | | | transfer is involved, such as for active power filtering |
| Most multilevel converters discussed in the literature | | | | and VAR correction. Then only a floating DC bus |
| step between multiple voltage levels. This is usually the | | | | capacitor is required on each floating DC bus. |
| most useful configuration for a high power converter, | | | | |
| as reducing conduction losses in both converter and | | | | Some other sources of power which could easily be |
| machines will always favour increasing the voltage | | | | made modular and floating are batteries for battery |
| rating rather than the current rating of the converter. | | | | energy storage systems (BESS) used for load leveling, |
| Also as power levels increase, the input and output | | | | or alternative energy sources such as solar panels. It is |
| voltage levels presented to the converter increase. | | | | of course possible to power the isolated bridges from |
| The structures of these multilevel converters place the | | | | multiple isolated transformer secondaries, each with |
| switches in series to share the duty of blocking these | | | | their own rectifier . By appropriate phase shifting of the |
| higher voltages. Equally however, for high current | | | | transformer secondary windings, harmonic cancellation |
| applications, many switches can be placed in parallel, | | | | can be achieved on the primary side, as described |
| with their current summed by inductors. When | | | | previously, as well as at the multilevel output of the |
| switched separately, multilevel current waveforms | | | | multi-bridge converter. However the disadvantages of |
| result. As expected, multilevel converters can be | | | | a transformer with multiple isolated secondaries return. |
| DC-DC, DC-AC and as explained, in the broadest | | | | This multilevel converter structure has some very |
| sense, even AC-AC. | | | | significant advantages, if its limitations are acceptable. |
| | | | | |
| 5. Multilevel Topologies | | | | Its advantage is it has perhaps the simplest |
| | | | | architecture and the lowest component count. No |
| Generally multilevel topologies can be divided into two | | | | transformer is needed, so capital costs are low. |
| groups, although in some cases the dividing line is | | | | |
| indistinct. The first approach relies on summing the | | | | 9. Applications of Multilevel Converters |
| outputs of a number of conventional two-level | | | | |
| converters, to produce a resultant multilevel output. The | | | | At this point it should be clear that one of the major |
| second group replaces the two-level switch structure | | | | advantages of a multilevel converter, regardless of |
| with a multilevel switch topology within an otherwise | | | | topology, is increased power rating. A converter need |
| conventional converter. These two groups will be | | | | not be limited in size by the prevailing semiconductor |
| distinguished by the terms multi-bridge converter and | | | | technology, since a multilevel converter allows the |
| multilevel converter respectively. Any of the basic | | | | voltage and/or the current to be shared among a |
| DC-DC converters (buck, boost, buck-boost, Cuk) can | | | | number of switches. This advantage has traditionally |
| be extended to a multilevel topology. Often these are | | | | justified the extra complexity of multilevel converters |
| not called or perhaps even recognized as multilevel | | | | only at very high power levels, for large motor drives |
| converters, but rather simply described as, for | | | | and utility applications. As the understanding and |
| example, paralleled converters with interleaved | | | | acceptance of multilevel converters has increased, |
| switching instants. Two recent examples cited are | | | | these converters are being used at all power levels to |
| multilevel boost converters used for power factor | | | | extend the useful power range of semiconductor |
| correction. In both of these examples, the switches are | | | | switches. For example, using multilevel topologies, |
| effectively placed in parallel and their contributions | | | | IGBTs are challenging traditional GTO converters in |
| summed by separate boost inductors. They present | | | | motor drive and traction applications and MOSFETs |
| multilevel current waveforms to the input and reduced | | | | are displacing IGBTs in some larger Switch Mode |
| voltage ripple at the output. Multilevel DC-AC | | | | Power Supplies. The more stringent harmonic |
| converters range from the simplest single phase, full | | | | standards now being legislated also advantage |
| bridge driven with unipolar voltage switching to | | | | multilevel converters, since they produce lower |
| complex multi-phase converters. These are the most | | | | switching harmonic spectral components for a given |
| commonly recognized and reported multilevel | | | | switching frequency limit. |
| converters and will be further categorized and | | | | |
| referenced in the next section. Even multilevel AC-AC | | | | 10. Conclusion |
| matrix converters have been shown to be at least | | | | |
| theoretically possible. | | | | The aim of this chapter has been to demonstrate the |
| | | | | diversity of possible multilevel converter topologies. |
| 6. Three Phase Multilevel Voltage Source | | | | Each has its own mixture of advantages and |
| Converters | | | | disadvantages and for any one particular application, |
| | | | | one topology will be more appropriate than the others. |
| At this point in the chapter, we will narrow the focus to | | | | Often, topologies are chosen based on what has gone |
| that of three phase voltage source multilevel | | | | before, even if that topology may not be the best |
| converters. Although this may seem somewhat limiting, | | | | choice for the application. The advantages of the body |
| it encompasses most of the higher power multilevel | | | | of research and familiarity within the engineering |
| converters both in the published literature and in actual | | | | community may outweigh other technical |
| use. There are some examples of single phase | | | | disadvantages. Despite the diversity, these different |
| converters functioning as AC-DC switching rectifiers, | | | | topologies contain common underlying links. Usually the |
| either in traction, computer or telecommunications | | | | modulation and, to a lesser extent, control strategies |
| power supplies. These Power Factor Correction | | | | can be developed independently of the converter’s |
| rectifiers have lower inherent distortion and require less | | | | topology and then subsequently applied with little or no |
| filtering because of their multilevel topology. There are | | | | modification. In subsequent chapters, the simplest case |
| four main voltage source DC-AC multilevel topologies | | | | of the transformer connected multi-bridge converter |
| which have been distinguished here and in the literature. | | | | will be used as the implied default multilevel converter |
| | | | | topology. Required variations on modulation and control |
| | | | | strategies will be explained after the general technique |
| These are: | | | | has been presented. |
| Ø Multiple bridge using transformer or | | | | |