Patent Application
20120057378
The present invention relates to a method for regulating the power in an alternating current electrical network using a static conversion system.
The international community is devoting increasing resources to innovation in medium and high power static conversion, up to medium voltage levels, in response to the potential of new applications such as energy generation from renewables and the increasing requirement for improved conversion and compatibility in conventional applications such as the supply of electric motors in existing installations.
The common factor in all these activities can be summarized as the application of the regulating properties of PWM (Pulse Width Modulation), made possible by forced switching, replacing the more limited properties of natural switching of the controlled diode type, while providing a radical solution to the problems of compatibility and efficiency posed by the prior art PWM structures.
Specifically, the prior art of ‘two-level’ forced switching is represented by simple circuit structures in which the electrical quantities are generated and regulated by pulses at two values or levels which delimit the range of regulation. Forced switching between these extreme values or levels produces a quantity of dissipated energy which, at the high switching frequency required by PWM methods, has a dominant effect in energy terms on the overall efficiency.
The reduction of the switching times, and therefore of energy losses, is not achievable because of the dynamic limits of the semiconductors or physical layout limitations, leading to the emission of electromagnetic interference which makes a converter incompatible unless it is provided with complex partially dissipating filters to ensure electromagnetic compatibility, which would make the converter bulky.
The compromise between switching performance and electromagnetic compatibility, which is fundamental for the widespread adoption of industrial electrical drives, is also based on the opportunity of using new electrical machines, or conventional machines redesigned on an ad hoc basis, which can withstand the dynamic stresses of switching in order to exploit the inductive reactive properties as a functional part of the conversion.
In this context, it is evident that generation applications in an electrical network cannot be equated to industrial drive applications. For generation applications, it is necessary to attenuate the discontinuities of PWM switching before they reach the converted electrical circuit, by providing filters (or reactive dipole filters) which have a major effect on the volume, weight, efficiency and cost of a complete PWM converter.
At the present time, most international research appears to be devoted to the demonstration of the functions of new and more complex conversion structures, having in common what is called the “multi-level” approach, which can provide multiple different instantaneous voltage levels between the extreme values required by the application. This multiplicity of levels is achieved by the serial connection of multiple elementary substructures which can switch amplitudes which are a fraction of the total range of values. The multiphase composition of these elementary substructures also gives rise to the distinctive properties and redundancies of the resulting vector components.
The term “multi-level” summarizes the difference between these systems and the “two-level” prior art.
Multi-level splitting increases the resolution and makes it possible to reduce the amplitude of the individual discontinuities required for mean value regulation over the whole specified range of regulation. The reduction of the individual discontinuities reduces switching losses and improves compatibility; in other words it requires (reactive) filters which are cheaper in proportion to the amplitude of the individual discontinuities.
However, this multi-level multiplicity is achieved by means of a series of multiple switches, which greatly increase the losses in the semiconductors due to the conduction of the same current component.
However, multi-level structures may also be indispensable, for example in medium-voltage applications where it would be either inconvenient or impossible to produce elementary switches to withstand the full voltage. In this sense, the development of multi-level structures is following a route imposed by necessity, which is used to justify a degree of complexity, cost, and operational instability which would otherwise be considered unacceptable. Some of these drawbacks which are related to the cascaded substructures may be mentioned here:
In conclusion, multi-level conversion requires a multiplicity of transformer power supplies and/or energy stores in flying capacitor banks, the numbers of these devices increasing with the number of levels.
In view of the requirements and costs of each substructure and the need to protect the whole conversion system from failure or individual loss of service, the weight of the “power supplies” explains the limited take-up of multi-level conversion, as opposed to the expectations raised by the reduction in cost of the input and output filters.
However, the problem of competitiveness of multi-level converters is demonstrated in a more straightforward way by the increase in power losses due to conduction, which is correlated with the number of components in series or of cascaded structures which have to conduct the same current in order to provide multi-level performance.
One object of the present invention is to propose an innovative approach to static conversion, based on the recognition of the properties of components which are characteristic of alternating electrical quantities, in support of which the conversion structures implicitly having “multi-level” performance are defined as being of the single type because they are specific to the converted AC system.
A further object is to propose an approach to static conversion which requires circuit structures which are highly simplified, economical and highly efficient because of the smaller number of cascaded components or structures with respect to the degree of resolution of the multi-level regulation.
These and other objects are achieved according to the invention by means of a method for the regulation of power in an alternating current electrical network using a conversion system having at least two for connection to the network, wherein terminals, in which each of said terminals has a corresponding electrical potential.
Each of the potentials of said terminals has an upper and a lower envelope, the difference between which, called the “envelope voltage”, is a variable positive continuous quantity having a maximum value called the “envelope amplitude”.
The method comprises the operations of:
The invention enables new conversion properties to be obtained by distinguishing between the energy effects and the properties of values characteristic of AC quantities.
The invention provides for the use of a power conversion system which is variable about a specified continuous mean value, which can be correctly coupled to a DC network, in relation to which it operates as an inverter to regulate the power in the AC network. The resulting structure is therefore of the DC/AC type. The range of values of potential on the DC side which is necessary for regulation towards the connections to the AC network corresponds to the difference between the upper and lower envelope potentials of all the electrical potentials of the terminals for connection to the AC network, this difference being called the envelope voltage.
The envelope voltage is generally a variable, which is positive by definition, whose maximum value is called the envelope amplitude, or more simply the amplitude, this variable being the dominant variable in respect of the design of the conversion system as a whole.
In an elementary embodiment of the invention, the envelope amplitude is divided into two ranges of values, one of which is called the “upper range” or “envelope range” or “type E range”, while the other is called the “lower range” or “transition range” or “type T range”.
This distribution or division, referred to hereafter as the “ET” division, in its various configurations, has corresponding dedicated conversion substructures, each specifically designed for type E or type T components, so that conversion structures made according to the present invention can generally be referred to as “DC/ETAC”, referring to the E and T components of the AC quantities.
Further features and advantages of the invention will be made clear by the following detailed description, provided purely by way of non-limiting example, with reference to the appended drawings, in which:
The example of an AC voltage in any multi-phase network, present between two terminals of the network connected to a converter which has to regulate it, will be considered.
It can be demonstrated that, in all cases, most of the active power in any single period is converted for values close to the values of maximum and minimum amplitude of the waveform of this voltage, in other words values close to the maximum value of the modulus of the voltage, while the remainder of the voltage waveform defines the passage or transition modalities between the sub-intervals of the AC period in which the active power conversion takes place to a greater extent.
By way of a typical example, the sinusoidal alternating waveforms of
The instantaneous power p(t), the product of the two waveforms v(t) and i(t), is a second harmonic cosinusoid which is cancelled twice in the period of v(t) and i(t) and has a mean value P=0.4, equal to half of the peak power which is 0.8 in the case of
By integral calculus it can be demonstrated that:
In other words, the present invention is based on the recognition that voltage regulation in the range from 0.707 {circumflex over (V)} to 1.00 {circumflex over (V)} (that is to say, in a range equal to less than 30% of the amplitude {circumflex over (V)}) makes it possible to regulate what is clearly a majority proportion (82%!) of the active power; similarly, voltage regulation in the range from 0.866 {circumflex over (V)} to 1.00 {circumflex over (V)} (that is to say, in a range equal to less than 15% of the amplitude {circumflex over (V)}) makes it possible to regulate a proportion of the active power which is still preponderant (65%!).
With reference to the definition of the invention provided in claim 1, in the numerical examples considered above, either the range [0.707:1]{circumflex over (V)} or the range [0.866:1]{circumflex over (V)} can be used as the upper range or envelope range.
With reference to the sinusoidal case of a single-phase network with two connections,
With reference to the single-phase case of
In the case of
The case of
Each switch of the array SWM has double multiplicity to allow the two-way connection of each of the three terminals (EP,TB,EN) of the ET conversion to the two terminals of the single-phase network ACNW, making a total of six two-way switches or twelve one-way switches.
Because of the claimed definition of the sub-range ER of values, the type E conversion system, in other words the ES converter, has the best properties of conversion of the active power exchanged with the AC network ACNW.
Most of the active power should therefore be conveniently converted by this system ES, which by definition has better properties in terms of efficiency than PWM regulation, simply because the width VE of the range of regulation ER is very small compared to the envelope amplitude VEA:
The complementary system TS, for the transition conversion (T), can be designed in a different way, for example in order to reduce costs, or can include auxiliary or secondary system functions, since it will have less effect on the overall performance.
The converter TS can even be omitted, in specific cases where only the active power is of interest, in other words cases in which distortion of the AC current waveform is acceptable.
A multiphase system comprises multiple alternating components which have the property of being offset with respect to each other during the period, regularly or with small errors of phase shift or relative symmetry.
Independently of the number of phases, the waveform and the symmetry properties, all the quantities of a multiphase AC network can conveniently be regulated by a specific application of the method of ET division of each AC quantity.
This is because the range of values of the envelope voltage of any multiphase system, including two-phase orthogonal systems, meets the requirements of ET division and defines the range of regulation of the envelope conversion system which enables it to be used continuously, being switched towards the phases of the multiphase AC network.
By definition, the range of values of the envelope voltage, of the potentials of the multiphase AC network, forms the range ER of ideal “continuous” regulation of the converter, or of the conversion subsystem, of type E.
Because of this continuity, the efficiency properties provided by the general principle of ET efficiency can be used “continuously”; above all, however, it provides most of the competitive cost advantage of ET conversion: a single sub-converter or conversion sub-system ES, of type E, regulates each AC electrical quantity in the sub-interval of the AC period in which the individual AC quantity coincides with the multiphase envelope.
In the case of sinusoidal quantities of a regular multiphase system, the characteristic values for ET conversion are defined, because for these the envelope voltage has a maximum value equal to the envelope amplitude and a minimum value which is:
With reference to the symmetrical sinusoidal three-phase case of
The typical ranges of ET regulation are shown in
As in the case of the single-phase inverter of
In
The elementary circuits described above are conceptually and technically incomplete. The perfect contiguity between the regulation ranges gives rise to problems of application, for example in real multiphase systems characterized by non-symmetry between the phase quantities, and in the use of the “natural” switching properties of real thyristors.
The theoretical solution is to create a regulation range which overlaps the two contiguous E and T ranges, and which is therefore called “ETR”. In particular, this ETR range is preferably provided by the transition converter TS, according to the general principle of cost and efficiency.
By contrast with the ideal case shown in
In particular,
For the case of
Retaining the definition of the envelope amplitude as VEA=VE+VT, it should be noted that, in some phases, the transition component comprises values greater than VT; in other words, it partially “enters” the envelope regulation range ER which has the ideal width VE.
In conclusion, with reference to
Clearly, this overlap range ETR increases the design complexity of the conversion system, but does not necessarily increase its structural complexity.
This confirms that it is preferable to limit any necessary overdimensioning to the transition conversion system TS, so that the cost of the envelope conversion system ES can be minimized by designing it for the minimum necessary width VE. This is because, by definition, VET<VE<<VT, and therefore the inclusion of VET in the total width of the transition regulation range, which would be increased from VT to VET+VT, would increase the cost of the transition conversion by an amount which was insignificant in percentage terms.
Additionally, it may be useful in practice to explicitly generate the voltage VET only if VET<<VE; otherwise, if these values are similar, as in the case of
With widely differing design values and criteria, the application of this overlap principle permits the control of the switching modalities of the switches that have to switch the regulated overlapping currents. For example, in this way it is possible to use more efficient and less costly thyristors, suitable for natural switching only.
According to the overlap principle, it is clearly useful to be able to regulate and control the connection switching between the ET and AC terminals, which takes place in the area of both the envelope potentials. For this and other reasons, it appears preferable to use structures having properties of symmetry, especially for the symmetrical control of the envelopes and their properties.
The point and corresponding potential of symmetry of the envelopes is called the midpoint ‘M’, the potential of which is defined at any instant as the intermediate potential between the potentials of the upper and lower envelopes of the potentials of the connections to the regulated AC network.
With reference to the sinusoidal three-phase network which is non-typical because of the inverse sequence component, the second graph of
Each definition relating to the division and the ET regulation ranges is applicable to the individual envelopes by simply dividing by two the result for the total envelope voltage, particularly the result for the definition of the overlap range ET.
For the purposes of visualization,
The division of the voltage applied to the transition converter TS can be used to define the midpoint, but it is not necessary unless a power connection, by homopolar conversion for example, which is not included in the diagram of
In the multiplicity of connections to a multiphase AC network, it is possible to distinguish one in particular, in cases in which the neutral point is present and the conversion system is required to control it.
In all cases concerned with power applications, the neutral potential has no effect on the definition of the properties of the ET division, since it always lies between the two envelope potentials. Additionally, the current of the neutral connection to be regulated, when required, is always much lower than the line current. Ultimately, any homopolar converter, in other words a specialized converter for regulating the neutral current, is characterized by much smaller dimensional requirements than those of the multiphase converter.
In particular, the homopolar converter can be configured in a simple way if the midpoint “M”, defined above with respect to the potential envelopes of the multiphase AC network, is available in the multiphase conversion system.
In the case in
For example, it can be demonstrated that in all three-phase systems of practical interest the proposed combination is advantageous compared with the overdimensioning of the inverter PCS which would otherwise be necessary for the neutral regulation without a dedicated homopolar converter O-CONV.
Such a homopolar converter O-CONV can always be added without modifying the multiphase conversion structure of any conversion circuit without a neutral connection, including ET.
Because of the overlap of the regulation ranges of the two types, ET conversion also makes it possible to control the current derivatives of the switches included in the switched connection array SWM.
This solves one of the problems of the use of thyristors, but another problem arises from the need to limit the voltage derivative applied to the thyristor, with rather low values where the most efficient, least costly and therefore “slow” thyristors available on the market are used.
In innovative applications, it is recognized that it is useful, if not necessary, to provide a capacitive filter to ensure a non-discontinuous supply regardless of the discontinuities of PWM regulation. In these cases, ET conversion, which offers multi-level performance and therefore the possibility of efficient high-frequency modulation, can benefit to the greatest extent from an incorporation of the filter and snubber functions associated with the thyristors in the ET/AC switched connection array SWM.
In particular, a snubber capacitor, CSN, can be connected directly in parallel with each switch, without diodes or resistors, because the switching can be controlled by the ET converters having overlap ranges.
Clearly, in this case the inductances for the control of the envelope and transition currents in PWM mode have to be interposed between the ES and TS converters and the array SWM provided with snubber capacitors, as shown within the rectangle drawn in broken lines in
It is also possible to provide a further set of three filter and snubber capacitors, connected between the envelope terminals EP and EN and the transition terminal TB.
It should be noted that the ET conversion in overlap of the two types of regulation range makes it possible to control the derivatives of current and voltage, particularly by bringing the single switch to the zero cut-off voltage drop, thus enabling it to switch to the closed position without any discontinuity or pulse, and therefore making it unnecessary to provide the conventional resistors in series with the snubber capacitors.
As suggested by
Solutions with Multiphase Multiplicity of the Transition Converters
One of the problems of ET conversion arises from the need to regulate the DC voltages which define the ER and TR regulation ranges. In some applications this regulation is useful in step-up mode, whereas it would not be desirable to provide it at low or zero values.
These cases can be avoided by sacrificing some of the cost advantages of ET conversion, by multiplying the number of transition converters TS by the number of regulated AC connections, so as to provide multiple possibilities of step-down regulation of the AC network with constant values of DC voltage.
An interesting case is the three-phase system shown in
Using the conduction of the switches of SWM is much more efficient than the possible alternative of PWM TS transition conversion. One phase is therefore connected to one envelope and the other two phases are regulated in PWM mode by two of the three inverter legs provided for TS transition conversion.
The current which can be regulated in these conditions of reduced voltage amplitude may be less than that which can be conducted when the envelope conversion is fully operational, but in many applications this is a requisite, or is at least acceptable.
Another interesting case is that of the complete integration of the transition function and the function of switched connection to the AC network shown in
This solution is characterized by high efficiency and the low cost of the filter reactances, but the overall cost of the semiconductors is not reduced, and all the components must be able to withstand the very high voltage derivatives created by the MOSFETs in forced switching to provide the PWM regulation.
In the application of the ET division, the transition conversion is by far the most costly conversion because of the width of the regulation range, which is smaller but close to the envelope amplitude, and therefore if this is multiplied by the number of AC connections (as in
However, ET division offers a different possibility of step-down regulation of the AC network without increasing the cost of the semiconductors, for the further benefit of all cases in which the DC link has to be regulated in step-up mode, as in all types of DC generators or accumulators, for example.
This is because the output of the transition converter TS is of the AC type and is therefore bidirectional, and can always be conveniently broken down into the two constituent unidirectional structures. By controlling them independently of each other, these structures can regulate two current components of the network ACNW separately, the only constraint being that of opposing directionality. However, this constraint can be disregarded in AC networks, in which the sum of the currents is zero or very small.
It will certainly be necessary to provide a modified switched connection array SWM, having two independent transition connections of opposing directionality for each bidirectional transition connection provided for connection to the multiphase network ACNW.
For a three-phase AC network there is a single transition component and the array SWM provides a switched bidirectional connection for this component, as in
To connect mutually independent unidirectional components to the AC network, it is simply necessary to separate the common connections of the switches having the same directionality.
In the case of
In the simplest three-phase case, the transition converter TS is single and is bidirectional, formed by an inverter leg. However, this inverter leg is actually composed of two typical cells of opposing directionality.
Therefore, when the inverter leg is broken down into the two unidirectional typical cells regulated in a mutually independent way, as also shown in
The phase selection derives directly from the principle of ET division, since at least one of the two phases which have the same current sign generates one of the two envelopes, this phase being suitably connected to this envelope by the 4×3 array SWM (
In the other cases, in which the two phases of identical current sign both generate the envelopes, there is a further degree of freedom in the selection of which of the two envelopes is to be connected to the AC network by means of the 4×3 array, while keeping the third phase, which has the opposite current sign, in the regulation range of the transition conversion.
Envelope conversion can conveniently be carried out by an envelope conversion system ES comprising a dedicated PWM regulator with a mean power which is very small (as in the bidirectional cells 1 and 2 of
This is possible, and may be very convenient, in all cases in which PWM envelope regulation of this kind can be added to a separate regulation of the active power only, which is obtained in a different way and is suitable for, or already provided by, the application.
An example of considerable practical interest is the adaptation of real DC voltage sources such as batteries, fuel cells and photovoltaic modules.
In particular, there is now widespread agreement in favour of using a DC/DC step-up conversion stage with substantially constant power equal to the active power to be converted into the AC network, or into an electric motor which is usually three-phase.
In all these cases, ET division can be used in a specific form, in order to obtain conversion properties which are so distinctive, in terms of the achievable efficiency, that this conversion process can even be justifiably referred to as “direct envelope conversion”. In the following text, this conversion will also be referred to as “direct EE/AC” conversion.
However, it should be noted that these “direct” conversion properties relate to the conversion of some active components of the power, namely the dominant components produced by the envelope component, but not to the regulation of waveforms or reactive or distortion power.
An extreme case of direct ETAC conversion is illustrated in the diagram of
The diagram in
The diagram in
In particular, the voltage denoted “VET” at the terminals of each of the two capacitors CET can always be added to the envelope voltage switched by the IGBT transition cells, by appropriately switching the unidirectional low-voltage MOSFET cells supplied at the voltage “VET”. This solution conforms to the principle of ET overlap which is required for non-symmetrical AC networks, and, more particularly, can control the transition current and consequently the current of the thyristors in the array SWM in the time intervals required for the switching of the AC phases carried out by said ET/AC array (SWM).
The normal equilibrium condition of the voltages at the terminals of the capacitors CCL and CET is established by the ripple of the envelope voltage present at the EE link, through the Schottky rectifier diodes which connect the capacitor CCL to the EE link and to the two capacitors CET arranged as in
To sum up, the circuit of
The circuit of
However, in most cases the active power required in the EE link from the ETAC conversion is not perfectly constant, owing to the distorting terms which may be due to the properties of the network ACNW, and as a result of the transients required for the control of the switching of the thyristors of the ET/AC array.
In other words, it is generally preferable to have independence, at least in dynamic terms, between the power regulation towards the input source and towards the network ACNW, as is implicit in the prior art in intermediate DC link systems with adequate energy storage in the DC link.
This concept represents the natural evolution of direct envelope conversion, which therefore provides an independent envelope regulation contribution at a mean power level which is practically zero.
The circuit of
To sum up, the circuit of
The conversion is direct, but may correctly be called quasi-direct, in spite of the requirement for zero mean power, when the current rating of the intermediate envelope converter is of the same order of magnitude as the amplitude of the envelope current.
Quasi-Direct AC/ET/AC Conversion Between AC Networks Having Equal Amplitude
In ETAC a possible singularity of design and performance is recognized when conversion is required between networks which are entirely different in their frequency, power factor, neutral connection type and even number of phases, but which are characterized by identical voltage amplitudes, or, more generally, by equivalent values for the purposes of ET division.
The case we are concerned with corresponds to ranges of ET regulation of the two networks which are identical or very similar, making it particularly useful to attempt to overlap or unify them.
This can be done by a special application of the principle of ET overlap, by defining a new range of envelope regulation, “EER”, formed by combining the ranges of envelope regulation of all the networks interconnected by the ET conversion.
In other words, the resulting voltage VEE represents the ideal, necessary and sufficient width of the regulation range EER required for the envelope regulation of all the interconnected networks.
The voltage value denoted VT, on which the transition conversions are based, continues to be similarly defined by the smallest possible value of all the multiphase envelopes. Finally, the width VET of the overlap range ETR is again defined in such a way that the value (VT+VET) can allow the continuous regulation of all the transition components, for all the networks interconnected by the ET conversion.
To sum up, the principle of quasi-direct AC/ET/AC conversion between AC networks of equal amplitude is manifested in the considerable simplification of a common DC link, called an ET link in this case, for both ETAC conversions, and in a specific application of the general principle of ET division of non-symmetrical AC networks and of the corresponding principle of ET overlap of the envelope and transition regulation ranges.
In ETAC conversion it is recognized that the transition converter TS has a regulation range TR which is much wider than the range ER known as the envelope range. Consequently, as a specific application of the general principle of cost and efficiency, in high voltage applications the transition converter TS alone may need to benefit from the multi-level splitting of its regulation range. In particular, in ETAC conversion it is possible and useful to split the regulation range TR so as to constantly assign the task of PWM regulation to a single conversion stage.
In the quasi-direct AC/ET/AC conversion of
The same configuration using a common DC link (ET link), which is very convenient for the simplification of a multi-level structure, can be retained by using the generalized transition splitting proposed in
The general concept of the splitting of the transition range means that (
Because of the contribution [±½(VET+ΔV)] of PWM regulation of the cascade unit “ET+Δ”, at the transition level switch “MLT−Δ” the width of the range VT is suitably reduced by two quantities ½ΔV towards both ends of the range VT. The residual range “VT−2(½ΔV)” is therefore correctly split into “NL” different levels, where NL is the number (a positive integer) of capacitors between which the voltage VT−ΔV is distributed, and therefore:
V
T=(1+NL)ΔV.
In the special and non-exclusive case of
In any case, the interesting property arises from the fact that the incremental ET overlap regulation range VET also becomes available to assist the individual level switching operations executed by the multi-level transition switch “MLT−Δ”, for example by reducing the current conducted in the switching time intervals to suitable values for switching, and even cancelling this current if necessary, in order to enable simple thyristors to be used.
Realistically, however, it will be necessary to implement suitable relationships between the switching times of the switches and the PWM period in order to achieve the theoretically desirable results.
One of the more interesting aspects of the multi-level performance in ET conversion arises from the simplification of the independent capacitor banks required. In particular, it appears to be very useful to be able to provide the multi-level performance without the need for banks of “flying capacitors” (FC), which generally constrain the prior art multi-level structures, especially since they require transformer-based power supply systems for high frequencies, including reversible and regenerative systems.
A specific application of the general principle of multi-level and PWM splitting of the transition regulation range resolves the problem in a radical way by using such high-frequency transformers directly for the construction of the cascade-connected unit such as the unit “ET+Δ” of
Additionally, the flow design of these transformers for the units “ET+Δ” benefits from the specific waveform of the transition voltage, of the quasi-triangular type at a frequency which is a multiple of the frequency of the network magnitudes.
Clearly, the practical embodiments can benefit in many ways from the principle of FC-less conversion. For example, it is possible:
The specific case shown in
In this case,
It will be recalled that VT is 3/2 times the amplitude {circumflex over (V)}ƒN of the three-phase phase to neutral components, and therefore the components of “MLT−Δ” are designed for ¾ of the amplitude {circumflex over (V)}ƒN.
The maximum flow design at the secondary winding of a single transformer, as in
Clearly, the dimensions and cost of such transformers can be reduced further as NL increases.
A notable property of ET/AC conversion arises from the implicit possibility of providing the AC magnitudes in modular mode for each of the envelope and transition components.
This property is of considerable practical usefulness for high power or current converters, and ET conversion benefits from it in a particular way by establishing simple functional redundancy which also provides:
Each of the envelope and transition current components can be converted by the same DC link or ET link, as the sum of any number of typically equal parts, converted by modules designed for this current fraction, each switched in the AC network by dedicated thyristors or diodes of SWM or ET/AC arrays in the same multiplicity of individual modules. If a module fails, it can be isolated, by an active method and/or by fuses, either from the DC link or from the AC network, while keeping the properties of all the other modules unchanged.
A very simple example of current modularity is shown in
Specialization of ET Converters with the Power Factor
By applying the principle of ET division it is possible to reduce the cost of the semiconductor power components in many cases in which the specific features of the application are associated with constraints on the direction of the converted power and limits on the phase offset between voltage and current or on the power factor.
The simplest example, which is of considerable practical usefulness, is that of an AC/DC supply from a three-phase network, replacing simple diode rectifiers in order to achieve sinusoidal current absorption. In this case, characterized by a unitary power factor, the transition current component is composed of curves of the sinusoid in phase with the transition voltage, the maximum value of which is half of the amplitude of the line currents of the AC network.
Consequently, with reference to
The input stage PCS 1 in
In practice, the input stage PCS1 of
The output stage PCS2 is compatible with the input stage in respect of the direction of the power and therefore of the currents. However, in order to simplify the envelope part of the array by using simple thyristors without diodes in antiparallel, it is necessary to assume a current offset within a range of±30 degrees, in other words | cos φ|>0.866, as indicated in
In conclusion, the specified variability of | cos φ| does not provide any benefit in respect of the dimensions of the envelope and transition converters, but it enables the envelope switches of the array to be simplified.
One of the problems of multi-level conversion is the partial use, which is discontinuous in some areas, of the voltage supply units with their banks of source filter capacitors required for the generation of the multilevel potentials. The result is always a considerable overdimensioning of the supply units and/or the capacitors with respect to what would be sufficient with a greater degree of simultaneity of use of a single DC source supplying a plurality of converters simultaneously.
ET conversion makes it possible to use a single DC link, split into ranges which depend on the specific application of the ET division, which in itself simplifies the regulation of the load distribution and the corresponding dimensions of the auxiliary conversion means. However, it should also be noted that the voltage sources for supplying PWM converters must be regulated and protected according to more restrictive criteria than those which are suitable or sufficient in the generation of voltages switched by a plurality of simple switches, such as natural switching or controlled turn-off thyristors.
To sum up, in order to achieve a simpler and more efficient mode of reliable operation and corresponding failure-mode analysis, it would ideally be preferable to have a voltage source, or no more than two sources, for the supply of all the PWM converters, which would therefore be constructed by the same technology, and would be equipotential to each other for further savings in respect of insulated mounting, cooling, driving and control.
This can be achieved in ET conversion by a specific application of the general principle of what is known as FC-less multi-level transition conversion, in other words conversion using transformer-based cascaded PWM regulation, as for the single generic unidirectional module of
The regular splitting of the voltage in the DC link, in other words its splitting into voltages which are all equal or nominally equal, can be very useful in making it possible to use only one type of capacitor, and to simplify the load balancing by regulation of the corresponding voltages.
The minimum number required for this equally distributed split is seven, because of the choice of a non-symmetrical structure; otherwise this minimum number would be doubled. The width of the envelope regulation range, equal to approximately 1/7.5 of the envelope amplitude of a single sinusoidal three-phase network, is increased to 1/7 for better coverage of both envelopes of the two AC networks. The remaining 6/7 represents the transition conversion base, to which the cascaded overlap range must be added. The cascaded contribution is also extended to the term ±½ΔV which completes the capacity of PWM regulation in cascade in application of the FC-less principle; clearly, ½ΔV is made equal to 1/7 of the envelope amplitude, in other words ⅙ VT, for the correct equal distribution of the DC link, which is better defined as an “equally distributed ET link”.
With reference to the waveforms of
Incorporation of the ET Overlap Range into PWM Switch Drivers for Transition Converters
The principle of ET overlap of the envelope and transition ranges can be applied in a particularly simple and economical way when the necessary voltage, called VET, is equal or similar to the supply voltage of the drivers of the transistors used for the PWM regulation of the transition component. The voltage VET defines the range which can be regulated in overlap for contiguous envelope and transition ranges. In application of the criterion of cost and efficiency, this range is added to the transition range, and therefore the regulation in overlap is provided by the transition converter in the envelope regulation range.
This can be provided in a useful way, for example in ET/AC conversion to a single AC network, for which the ET overlap range is designed for the control of the derivative of current in the natural switching thyristors used in the array SWM (ET/AC), of a type with or without integrated snubber capacitors. In this case, the triggering of the transition thyristors must be “advanced”, to allow the “natural” switch-off of the envelope thyristors, controlled by the envelope conversion system in its regulation range. In other words, to achieve continuity of the AC current regulation, at least one transition converter must be able to regulate the current “in advance” to permit the advanced switch-off of an envelope current and consequently the natural switching of the thyristor which conducts it in the ET/AC array.
The low voltage VET provided by the ETSW circuit is sufficient in some special cases, typically when a PWM transition conversion at very high frequency is used in a conversion to an AC network whose frequency is low or much lower than the PWM frequency. Evidently, this possibility is also available in applications using low AC voltage, including domestic and some industrial applications.
ET conversion, designed for AC networks, imposes specific requirements for DC/DC conversion in the construction of DC/AC converters, commonly called inverters. This is because the DC links for supplying the voltages required for ET conversion, called ET links because they define the ET regulation ranges, have to be supplied in a different way for each capacitor of the ET link, which is suitably adapted to form a filter and an energy store capable of making the conversions between the different converted networks independent of each other.
However, this requirement does not make it necessary to provide multiple DC/DC regulation converters, one for each voltage of the capacitors. In fact, one of the properties of an ET link is that its filter capacitors have a common connection, thus facilitating the construction of integrated DC/DC conversion structures for the ET link, these structures therefore being called DC/ET conversion structures.
The DC/ET conversion structures of practical interest are defined by the characteristics of the DC energy sources, which typically have the properties of real voltage generators. Therefore, a suitable DC/ET converter, of any type (step-down, step-up, etc.), initially regulates a whole inductive state variable, in other words the current in an inductance, to regulate the converted DC power. Now, this inductive current can be “distributed” in PWM between the different terminals of the ET link, with structures conveniently integrated to enable their multi-level properties to be used.
The step-up output is available in
The current “distributor” is the unidirectional cell “DCELL”, designed for a very low voltage, equal to the width VE of the higher or envelope regulation range.
The optimal operation of the DC/ET step-up conversion conveniently comprises three stages in each PWM period:
This sequence defines the convenient phase synchronization of the switching of the various typical cells used for a step-up DC/ET conversion. The same principle can be applied, for example, to a symmetrical envelope conversion system, simply by doubling the “distribution” cells for users at opposite ends of the ET link, in a similar way to the arrangement shown in
The case of step-down conversion is even simpler and has more degrees of freedom. In this case, the MOSFET “distributor” on the terminals of VE could act continuously to distribute the inductive current regulated by a step-down DC/DC converter. Consequently this case will not be described further.
It should be noted that DC/ET structures use the components which are also used in ET/AC conversion, with dimensions suitable for the same regulation ranges, as shown in
With reference to
Naturally, the principle of the invention remaining the same, the forms of embodiment and the details of construction may be varied widely with respect to those described and illustrated, which have been given purely by way of non-limiting example, without thereby departing from the scope of the invention as defined in the attached claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| TO2009A000367 | May 2009 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB10/52002 | 5/6/2010 | WO | 00 | 10/27/2011 |
