Zero voltage switching with an input resonant circuit
One method of switching 80 voltage in an inverter is to create a voltage with high -frequency ripple on the the dc side. This entails the excitement of series or parallel oscillatory circuits periodic switching so as to produce oscillation of the input voltage, which passes through zero at a resonant frequency of the input circuit. Consider this approach in more detail for the example of the circuit shown in figure.
We assume that the load at the circuit output is a current source. This source may be represented in the equivalent circuit as a load inductance or the output filter off the inverter phase, which considerable exceeds the inductance L, the oscillatory input circuit contains a reactor and a capacitor C, resistor R, represent the total active losses in the system. Create oscillation in the circuit at the resonant frequency and assume that R=0, the capacitor voltage will vary with frequency from 0 to E, and the mean value of current in the reactor L will be . With no losses, this process is undamped as shown in figure.
Taking account of the actual losses in the circuit, we see that energy equal to the losses at equivalent resistor Rmust be introduced in order to sustain the oscillatory process in the system.
docility process in the circuit shown in figure are sustained by periodically switching transistor VT on and off within an near zero. As a result, the inverter input voltage falls periodically to zero. At zero voltage, the switching losses of the inverter components will be minimum. Obviously, then,the switching strategy should focus on moments when the input voltage passes through the zero. That limits the scope for the control of the output voltage. As shown in figure, a three- phase inverter bridge circuit with in input resonant link based on transistors and L,C circuit. Inverter output voltage is determined by the number of pulses in each phase when the switches VT1-VT6 are conducting. The switching frequency of transistor VT0 in that case considerable exceeds the frequency of the inverter output voltage as in figure. It is is evident from upper figure that this system has a following serious deficiencies:
the maximum voltage at the inverter switches is greater than the DC input voltage.
Additional output voltage harmonics appears under the action of the voltage pulsations in the circuit.
The range of output voltage regulation is limited and discrete.
These problems can be eliminated by modifying the resonant circuit, for example,by using a configuration in which the reactor and the capacitor of the oscillator circuit are in parallel.
Multilevel, modular and multicell converter topologies
In this chapter, we understand a module to be structurally and functionally complete power -electronics device. In modular systems,some additional connection and components of particular type can be used. A cell is understood to be a structurally complete device consisting of a standard structures. using this term, we consider modular design,which permits the connection of functionally different devices, for example,the frequency converter is made by connecting a rectifier and an inverter.
The use of modular and multi circuit apologies enables to minimise the time and production cost in the design of one -time converters. In addition,this approach is well suited to reducing the higher harmonics of the current and voltage and it can also be used for reservation. In general, this approach can be used for the following purposes:
Increasing system Power limited parameters of the available components.
Decrease in the design period of new power electronics devices, with distinctive voltage and current characteristics.
Reservation of devices and their components with output parameters continuity.
Reduction of higher harmonics in input and output current or voltage.
Matching of the input and output current and voltage.
Unification and standardization of component.
modular and multicell converters where first used in secondary power sources of an auto nomous devices, primarily aeroplanes. This approach is based on the theory of structural-algorithmic synthesis of secondary power sources. The same design principles are widely used in the power engineering for the design of DC power system. Various approximations of the harmonic signals were analysed in in (Mytsk,1989). In particular, a signal with Nstages was considered as shown in figure . Obviously, the distortion in the case depends on two variables and Intel numerical solution of a system of Transcendental equations. The solutions obtained do not lend themselves to technical realizations, . Therefore, in practice,the relative level is assumed to be a multiple of the minimum level of the first stage. A better method is to combine pulse amplitude modulation with pulse width modulation (PWM). In that case, the modulation frequency is minimised, and the number of stages is selected so as to have the best utilisation of the component or determined in accordance with technical and economic optimization.
The most widely used techniques of modular design are following:
Parallel connection of AC or DC converter.
Shunt connection of self- commutated inverter.
multi-cell technologies of diode- capacitor rectifier circuits of voltage multiples and voltage divider.
Multilevel converter topologies.
Cascades converter configuration.
Parallel connection of rectifiers and DC to DC converter
Search circuit enables to obtain the following advantages:
Increase in power.
Reservation of devices.
Improvement of the converter output and input parameters.
Increase in modulation of frequency of DC to DC converter.
The first and second options basically involve the parallel operations of converter with the DC output. not that it is simpler to organise parallel operation for DC to DC converter then for a AC to DC converter, as it involves regulating off 12 parameters of the output voltage (or the output current, where necessary. Therefore, we focus on parallel operation of AC to DC converter. The requirement on the parallel converter will depend on the required function. Thus, for complete (one operational unit and one auxiliary unit), it is sufficient to ensure stable operation of two converter at a common bus without any constant on the load power distribution between these models. This follows from the complete reservation principle,when the maximum power of the consumer does not exceed the maximum Power capacity of which converter. Depending on the type of consumers, the structure with parallel models for reservation function can be replaced by a structure in which the output buses of one of the module is operating in cold or hot reserve can we switch special command. With parallel reservation (example, two operational units and one reserve unit) Orange modular circuit with function of power increase, the power distribution between the parallel converters must not lead to our load of a converter.
In general, there are several types of parallel operations:-
Operations head common buses with an arbitrary power distribution between the separate converter, provided that the load power does not exceed the rated power of each converter. (Sometimes known as combined operation rather than a parallel operation).
Operation at common buses with a distribution of the load power, such that its proportional to the rated power of each converter. In that case of equal converter power, the load distribution will be balanced.
Operation at common buses with an arbitrary load power distribution between the separate converters, exceptional that the load of each converter is limited to the rated overload power (power capability.