See more technical analysis of multilevel converters
Unified Theory Converter 1.
In the preface to his book of switching power converters Wood introduced the concept of a unified theory of the converter. He says: "Most traditional conceptions of the field seemed a bit disjointed, converters have been widely regarded as concerned only because all the semiconductor switches and use has some similarities topology. . .. The opinions expressed below (that) switching power converters are related by function and behavior, its basic characteristics does not depend on the types of switches used, or in applications that are made available, on the topologies in that are made .. According to this unified theory, all power electronic converter can be considered as an array of switches that connects its input nodes production nodes. These nodes can be continuous or alternate, and either inductive or capacitive, and the power flow can be in both directions. Two obvious limitations apply by some fundamental laws of electricity.
• If a set of nodes (input or output) is inductive, the other series should be capacitive, not to create a series of shear or current sources when the switches are closed.
• Combination of open and closed switches should never open the circuit of an inductor, or short circuit of a capacitor.
2. Inverter or rectifier? Voltage or current source?
This unified set of processors usually broken into a number of subsets. The term rectifier is used when the power flow is mainly from the port to port and AC DC inverter is used when the speed limit is essentially feed DC port to port CA. The converter uses the term, or when there is no predominant direction of power flow or as a general term to include rectifiers and investors. In a voltage converter (VSC), the port is the port, the capacitor and the DC voltage is high (ie, a large DC bus capacitor). Tensions in this converter is defined by the port and are generally considered independent of inverter operation. The value of the AC side inductance is relatively low and the modulation converter AC side currents of the coil control. If converter voltage source is responsible for controlling the bus voltage capacitors, then this tension is controlled indirectly by the current net flow control in the condenser.
The switches in such a converter must block unidirectional tension, but to be able to conduct current in either direction if bidirectional power flow is desired. The opposite is true in a current source Converter (CSC) – the DC port is inductive and the current steep. The current in this port (and thus the converter) is well defined and slow to change. The tension (especially in the port of CA) is considered the variable directly controlled by the modulation converter. From the port of CA is generally important line or inductive load, line a line of capacitors should be placed on the AC port. The switches must block any voltage polarity, but are necessary to carry the current in one direction. The GTO thyristor has naturally suits and symmetrical.
Figure 2.1. A voltage source rectifier – inverter cascade (top) and power grinder – Conversion Cascade
Since the AC line and AC motor loads are both inductive, Voltage Source Rectifier – Inverter Cascades (Fig. 2.1) are generally used for small and increasingly for large motor drives and similar applications, like the GTO and IGBT matured. converters have traditionally been major source converters power, both because it best suits the characteristics of the thyristors, and because it requires a large inductance DC bus, which was preferred to a large capacitor. Some converters do not fall easily, or can not be placed in two categories. The matrix or Venturini converter [1] is an example (Fig. 2.2). Both input and output ports AC and definition of tension or tightness in the current drive (and therefore a source of voltage or current) is somewhat arbitrary. Both input and output ports
Figure 2.2. The matrix converter, with a possible two-way switches.
3. The general multilevel converter
The next refinement is to define the meaning of various levels. The definition after a multilevel converter is proposed:
A multilevel converter can pass through either input or output nodes (or both) between multiple (more than two) levels of voltage or current. The term "two levels" will be used when necessary to refer specifically a converter that is on many levels. This simple definition is deliberately broad and inclusive, in keeping with the spirit of the converter unified theory. For example, the multi-phase converter array (Figure 2.2) is, strictly speaking, a multi-converter, according definition. Consider the third phase, three phase matrix converter with input source voltage and an inductive load. Any single output can be connected to one of three different voltage levels (phase input voltage) and in the same way, any input can be connected to one of four current levels (including zero). In the example above, the regular AC input and output nodes varying amounts so that these levels can be considered fixed for an interval much shorter than the period CA.
Figure 2.3. The current source converter (top right), a converter voltage (lower left) and single-source converter three voltage levels (below right) may be of general topology of the matrix converter
The source voltage and current source converters can be derived from the general conversion of the matrix through the establishment of a port, or a voltage two-terminal dc stiff or rigid port [70, 30]. Keep the third terminal is a classic single converter at various levels (Fig. 2.3). Note that now one of the ports have been made and the DC voltage or current waveforms rigid, the port is experienced in several staggered levels. The others still need a signal constant similar to an equivalence between the two level converter.
For example, a converter with an appropriate structure can create a voltage wave reinforced at several levels in the nodes of induction, but will always be a voltage wave, its capacitive nodes. Similarly, a converter can create a waveform different reinforced at several levels in the current node capacity, but must have a waveform of current inductive nodes.
4. The traditional multilevel converter
The traditional view of what constitutes a converter levels consistent with this narrow definition. A number of ports (two maximum) voltage or current rigid or terminal nodes, while the second port has a classic or three sets of phase terminals that are passed to these multiple levels.
Most step multilevel converters discussed in the literature between multiple voltage levels. Configuration is typically useful for high power converter, such as loss of reduced driving both processors and machines always favors the increase in tension rather than the nominal current of the converter. Even higher power levels, voltage levels input and output presented to the increase of the converter. The structures of these multilevel converters switches placed in series to share the obligation to block these tensions higher. Even, however, for existing applications, many changes can be compared with your current summed up in the inductors. When activated separately, at various levels waveform output. As expected, multilevel converters are DC-DC, DC-AC, as explained in the broadest sense, including AC-CA.
5. Multilevel topologies
Typically multilevel topologies can be divided into two groups, although in some cases, the line of demarcation is indistinct. The first approach is based on the sum of the results of a series of classic two-level converters to produce an output which is on several levels. The second group replaces the switching level and two structures with a multi-switch topology in another classic converter. These two groups are distinguished by term multi-converter bridge multilevel converter, respectively. Any of the basis of DC-DC converters (Buck, Boost, Buck-Boost, Cuk) can be extended to a topology of several levels. Often not known or perhaps even recognized as multilevel converters, but simply described as, for example, interleaved parallel converters with switching moments. Two recent examples are cited of multilevel converters used to boost power factor correction. In both examples, the switches are actually used in parallel and their contributions summarized by the inducers stimulate different. They have various levels of input current wave voltage and ripple in reduced output. Multilevel converters range DC-AC single phase and simple, the full-bridge motor with a unipolar voltage switching to complex multi-phase converters. These are the most commonly found and multilevel converters and filed and referred to in the next section. Until multi-level AC-AC matrix converters has been demonstrated in at least theoretically possible.
6. Converters source voltage phase three-level
At this point in the chapter, we will limit the focus to the three-phase voltage source multilevel converters. While this may seem a bit limited, encompasses most multilevel power converters than two in literature and in actual use. There are some examples of single-phase converters to operate as switching AC-DC rectifiers, either in tension, a computer or the power of telecommunications. The power factor correction rectifiers reduced the inherent bias and require less filtering due to multilevel topology. There are four main source of DC-AC voltage multilevel topologies have been identified here and in the literature.
These are:
Ø adding bridge using multiple processors or inductor;
Ø multiple bridge with direct serial connection;
Ø Multi-diode converter tight and
Ø flying capacitor multilevel converter.
Each of these will be discussed below. Each of the presented schemes are a five-level converter, which can produce a phase nine-level waveform of phase voltage.
7. Transformer / inductor multiple summary Bridge Converter
As the title suggests, these are just multilevel converters some of the classic two-level bridges, whose inputs or outputs are added through transformers or inductors. Multiple processors share secondary voltage between power switches (Fig. 2.4). The most common and well known example of a bridge multilevel converter is the pulse thyristor converter Twelve well covered in most books text on power electronics [49]. cancellation of harmonics in these converters is provided by the phase voltage waveforms of the star and delta transformer secondary.
Figure 2.4. Five levels of power transformer attaching several bridges, new product-level waveforms from phase to phase in the primary transformer.
"30 This gap between the secondary side of the transformer allows the same times of change and the current appear interspersed in the primary of the transformer. A series connection of HVDC is used, a parallel connection for high current applications such as electrolysis and electroplating. The technique can be and extends to many bridges are connected to each phase shift appropriate secondary transformer for harmonics cancellation Primary additional downward. With the intelligent networking of the main transformer, the exchange of current and voltage can be guaranteed.
A good example of the next step complexity and flexibility is a 10 MW plant storage of battery power. GTO converters operate in the square wave and still depend on the elimination of the transformer of the cancellation of harmonics. However, because of forced commutation is used, now both the magnitude and phase (Real and reactive power) can be ordered separately. An extension of this approach to 48 pulse operation is carried out by eight bridges GTO square wave mode on the basis of the transformation of the cancellation of harmonics. Cancellation switching harmonics can also be obtained by changing strategies, rather than relying of the secondary phase of the transformer needed change. The simplest case – the number of series or parallel PWM two bridges – was studied by several investigators. Using the PWM modulation suitable for each bridge, the odd multiples of the PWM carrier and sidebands, including the first group, were eliminated completely from the spectrum output. This improvement is better than can be achieved with only double the frequency of the carrier that the carrier amplitude is even smaller. An individual example of a bridge to six, summarized multilevel converter transformer is used as an active filter to compensate for arc furnace flicker-static [71].
The connections of these AC bridges are added separately from the transformer secondary, which allow either a serial or parallel DC. Since the processor does not provide any phase change, may seem possible remove the processor and place directly fully converters in parallel (For a parallel connection). However, if there is no difference between the desired input and components output of the two transformers, components exchange events are, by definition, exactly the opposite phase. Kirchhoff's laws would be violated if the converters are directly linked.
The solution is to use interface reactors (distribution of existing reactors) or processors to input / output interface converters. Despite these reactors see full combined power converter (and therefore have a volume of copper and similar losses), they experience the difference tension between the converters. The volt-second component of this tension is smaller and if the iron content of these reactors can be reduced compared to processors that would be required to complete isolation. Normally, the inductors are placed on the AC side, which is already using a voltage converter induction of the source. Research at the five-phase motor drive that has used this technique was carried out by Matsui et al. The outputs of the two legs and half-bridge added a reactor which can be divided to form a three-tier intermediary. This and other established three levels of production have been summarized by a third reactor to form last five-phase output level. Another solution is to add the outputs of two converters connected via a bridge source or load. The two ends of the transformer or motor winding and highlights solution must be fully floating. A converter is driven by a phase inverted signal, so that twice the desired output of the converter is impressed by the floating charge. If companies are slowly, some of the unwanted components company appears as a common mode component of the load. Of course, this technique can be applied to the converters.
In short, the transformer or inductor abstract approach has the following advantages:
• The tensions in the individual converters and switches are so well defined by transformer output voltage source high rigidity.
• If a converter module to fail, or be out of service, the converter can still operate at full power, but at a reduced intensity. • Other than the processor (inductors), the structure is modular, which simplifies maintenance and reduces parts replacement.
• The operating mode is easy to understand and, again due to its modular structure, control is easier to apply. but also the following disadvantages:
• The transformer itself, if it is not necessary for the isolation significantly increases the cost of converter and one point for maintaining and eventually to failure.
• The processor requires several secondary windings, which must be isolated from other and from the ground. This is a major problem at high voltages. It also increases the cost of the transformer.
8. Multiple series-connected insulated Bridge Converter
A second topology, which is actually a variant of the first, The bridge converters connected in series (Fig. 2.5). Each phase leg consists of a series connected single-phase full-bridge, the series connection being made directly (not through the transformer in the first case) on the CA. A three-phase converter can be constructed by the union of three of these series of one phase of channels to form a star or triangle. From this topology, each have a full bridge DC bus isolation, this link has not been considered useful until recently revised. However, this topology is proposed for applications where it is not transferring real power to active power filtering and VAR correction. While that only a flight of DC link capacitor is required for each floating DC bus.
Some other sources of energy that could easily be modular and floating battery energy storage systems for the battery (BESS) used for load leveling, or alternative sources of energy, as panels Dom Of course it is possible to feed several bridges isolated secondary transformer isolated, each with its own rectifier. Appropriate phase shift the secondaries of the transformer, the harmonic cancellation can be achieved on the primary side, as described above, and the multi-converter multi-bridge output. But the disadvantages of a transformer secondary with the return of multiple isolates. This multilevel converter structure has significant advantages, if its limits are acceptable.
Its advantage is that it has perhaps the most simple architecture and the lowest number of components. No transformer is needed, whether costs capital are low.
9. The applications of multilevel converters
At this stage, it should be clear that one of the main advantages of a multilevel converter regardless of topology, the power increases. A converter should not be limited in size by the semiconductor technology in place, and a converter allows various levels of tension and / or current is shared by a number of switches. This advantage has traditionally justified the additional complexity multilevel converters only to very high power levels for large motor drives and utility applications. As understanding and acceptance of multilevel converters has increased, these converters are used at all power levels to extend the range of useful power semiconductor switches. For example, multilevel topologies, GTO IGBT converters are difficult in traditional applications and traction motor and IGBTs MOSFETs are moving in some of the major sources of switching. The harmonic stricter law now also has the advantage multilevel converters, which produces lower spectral components of harmonic change switching frequency of certain limits.
10. Conclusion
The purpose of this chapter is to demonstrate the diversity of possible topologies of multilevel converters. Each has its own mixture of advantages and disadvantages and the application of a particular topology is more appropriate than others. Topologies are often chosen based on what happened before, but this topology can not be the best option for the application. The benefits of the research organization and expertise within the engineering community can overcome other technical problems. Despite diversity, these different topologies contain common underlying links. Generally, modulation, and to a lesser extent, control strategies can be developed independently of the topology of converter and then applied with little or no modification. In subsequent chapters, the case simple converter transformer connected Multi-bridge topology is used by default implicit multilevel converter. changes in compulsory modulation and control strategies are explained later general technique has been presented.
About the Author
Assistant professor in lord venkateswara engineering college.I am doing phd in sathyabama university, Tamil Nadu,India.
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