Patent Application
20150043254
The present invention relates to a network feed device for feeding electrical energy from a direct-current energy source into an AC or three-phase supply network, where the DC voltage of a DC intermediate circuit is converted by means of at least one inverter unit into an AC or three-phase voltage and is fed by means of a transformer device into an AC supply network. Subsidiary aspects of the invention relate to an energy feed system for feeding DC voltage from a photovoltaic source, fuel cell, battery source or AC generator followed by a rectifier into an AC supply network, to an operating method for operating such a network feed device, and to the use of a network feed device for operating an AC consumer, in particular an AC motor. The device in accordance with the invention can in principle also be used for the reverse direction of energy flow, so that a DC consumer can be supplied with an adjustable intermediate circuit voltage.
Network feed devices with which the electrical energy from DC voltage sources such as batteries, photovoltaic cells, fuel cells or similar DC voltage sources can be converted into AC or three-phase energy and fed into a supply network are known from the prior art. Usually, a single inverter unit is used for this purpose, with the maximum three-phase output voltage being limited by the DC voltage made available by the DC voltage source. For example, the DC voltage is determined in photovoltaic feed systems by the voltage at the maximum power point (MPP) of the solar cell arrangement, i.e. that point on the current/voltage diagram of a solar cell at which the greatest power can be extracted, i.e. at which the product of current and voltage adopts a maximum value, and depends on the operating mode and type of a solar cell. An optimum efficiency can be achieved for the use of photovoltaic cells by exploiting the voltage at the MPP and the corresponding current, where on the other hand a high flexibility of the voltage configuration is wanted in the DC intermediate circuit of a network feed device. Since the highest possible AC voltage for feeding into the supply network results in a reduction in the magnitude of the current and is desirable for reasons of efficiency, the physical size and the costs of the components involved, so that the currents for the same power are correspondingly reduced by a high DC intermediate circuit voltage, thereby allowing the components to be longer-lasting, more economical and of smaller dimensions, it is desirable to try to maximize the deliverable AC voltage depending on the magnitude of the DC intermediate circuit voltage.
In the case of the three-phase inverter units known from the prior art, a transformer is supplied from a three-phase inverter, where as a rule the following relation applies to the maximum three-phase voltage in the photovoltaic field: UAC<UMPPmin/√{square root over (2)}·0.9, where UMPPmin is the minimum voltage at the MPP of the solar cell, where a 10% reserve is provided for regulation and for the compensation of tolerances, and a third harmonic zero sequence component is injected into the modulation. Typically, in an application in the photovoltaic field, a minimum DC intermediate circuit voltage of 450 V is provided, so that a maximum output voltage of about 290 V results for the line-to-line three-phase voltage. A three-phase voltage of this type is insufficient to feed a typical 400 V three-phase network, so that the network-side transformer, a boost converter or other measures for adjusting the voltage have to be used. As a result, in the case of a 100 kW system for example, there is a necessity for all the components on the AC voltage side, in other words the consumer network side, i.e. transformers, filters, chokes and output stages, to be dimensioned for a network current of up to 200 A, i.e. a comparably high current load, which makes the feed circuit correspondingly expensive, complex and less efficient. In addition, the high open-circuit voltage of photovoltaic cells usually means that semiconductors of the next higher voltage class have to be employed in the inverter, since it is not otherwise possible to modulate the inverter at the high intermediate circuit voltage that results.
The switching behaviour of inverter topologies known to date for network feeding via a network transformer causes harmonics in the output phases which act on the primary windings of a network transformer, resulting in high losses in the transformer and feeding a voltage into the network with a higher harmonic component. The losses lead to unwanted heating and reduce the efficiency of the network feed circuit. The service life of the circuit is reduced by a high power loss. For this reason, complex network or sine wave filters are frequently employed at the output of the inverter; these are relatively expensive, are of large physical size, and increase the susceptibility to failure and the costs of the network feed circuit.
Starting out from the prior art mentioned above, the object of the invention is to reduce the disadvantages of the known methods.
The above-named disadvantages are overcome by a device, a method and a use of a network feed device according to the independent claims. Advantageous developments of the invention are the subject matter of the dependent claims.
In a first inventive aspect, a network feed device for feeding electrical energy from at least one DC energy source into an AC or three-phase supply network is proposed, where the DC voltage of a DC intermediate circuit is converted by means of at least one inverter unit into an AC or three-phase voltage and is fed into an AC supply network by means of a transformer device comprising three network transformer windings. For this purpose, it is proposed in accordance with the invention that a first winding terminal of the network transformer winding is connected to a half-bridge of a first inverter unit, and a second winding terminal of the network transformer winding of the transformer device is connected to a half-bridge of a second inverter unit. By operation of the network transformer windings between the half-bridges of two inverting units, the full range of the DC intermediate circuit voltage can be exploited in order to convert the DC voltage into a three-phase voltage. By operation of the primary-side transformer windings at the two inverter devices, the latter can be operated at an amplitude of the output voltage, averaged over a switching period, from DC+ to DC−. If this winding were to be alternatively operated with an inverter unit according to the prior art, then in the case of a star circuit (Y-circuit) only the amplitude averaged over a switching period (DC+−DC−)/√{square root over (3)} is available to each winding, and in the case of a delta circuit the inverter unit must supply a current that is greater by the factor than what the winding of the transformer device accepts. These increased currents or reduced voltages can be avoided in the device in accordance with the invention, so that the AC output voltage can be increased by the factor √{square root over (3)}.
It is thus proposed in accordance with the invention that a transformer with primary winding terminals that are brought out separately is used, so that in the transformer device in particular one independent potential, and preferably three, is externally accessible through the opened transformer windings with six terminals. Each of the first terminals of the primary windings of the transformer device is connected to a first inverter, and the second terminals of the respective primary windings are connected to a second frequency converter. By a coordinated operation of the two inverter units, it is for example possible to exploit the MPP voltage range of a solar cell arrangement flexibly, in order to utilize the maximum generator power. This optimizes the network feed power of the DC energy feed source. With the device in accordance with the invention, it is possible to provide a maximum AC output voltage, so that the corresponding power semiconductor components can be dimensioned for a lower current load, i.e. can have smaller dimensions and therefore be manufactured more economically. The lower currents in the inverter units result in reduced conduction and switching losses while improving the efficiency of the feed device. With the double inverter topology in accordance with the invention, the voltage potentials of the output phases L1U, L1V and L1W of the first inverter unit and the output phases L2U, L2V and L2W of the second inverter unit can be switched in finer stages independently of one another, whereby smaller voltage variations result and the harmonic component together with the associated losses can be lowered. The output voltage approaches a sinusoidal curve.
In an advantageous development of the invention, the first inverter unit can be operated synchronously with the network, in particular at 50 Hz or 60 Hz, to determine the polarity of the supply network voltage to be delivered, and the second inverter unit can be operated at a higher-frequency PWM (pulse width modulation) cycle, in particular greater than 500 Hz, and preferably at least 4 kHz, for modulation. Switching frequencies of 8 kHz, 16 kHz or above are also conceivable and advantageous. It is proposed in this development that the first inverter unit determines the polarity of the supply network voltage to be delivered, i.e. the positive or negative half-waves, and in this process is operated synchronously with the network at the required supply network frequency, usually at 50 Hz or 60 Hz, or, when driving a motor, at the required motor rotation speed. The second inverter unit is clocked at a frequency that is higher, for example by a factor of 10 to 100, and emits PWM signals for shaping a required signal form of the supply network voltage, in particular a sinusoidal signal form. These are used for modulation and matching the emitted voltage to the network voltage or to the required voltage form. Only low switching losses occur in the first inverter unit, so that the cooling system and the semiconductors can be designed using less expensive elements. Alternatively, both inverter units can be operated with a higher-frequency PWM clock, in particular greater than 500 Hz, preferably 4 kHz, for modulation of the supply network voltage to be delivered. In this case, the quality of the output voltage delivered can be further improved, in particular if the two inverter units are clocked with an offset of one half of a clock period. Thanks to the high-frequency switching operation of the second inverter unit, the output phase voltage between the associated phases L1U-L2U, L1V-L2V and L1W-L2W can be matched substantially more closely to a sinusoidal voltage curve, whereby harmonics can be further reduced, transformer and switching losses lowered and the service life extended. The network feed voltage provided closely approaches an ideal sinusoidal curve. This allows complex sinusoidal filters to be dispensed with, saving costs and space and reducing the susceptibility to faults. Instead, economical LC filters and smoothing units can be used in the output phases of the inverter units to the primary windings of the transformer device, or filtering measures can be dispensed with entirely. As a result of the higher voltages and reduced current of a double inverter rather than a single inverter topology, the filter and smoothing unit can be designed for lower current magnitudes.
According to an advantageous development of the invention, the transformer unit can be a three-phase transformer with separable primary winding terminals accessible from outside, where the two connecting terminals of each primary winding can be connected from outside, and each primary winding is preceded by a smoothing capacitor and a filter inductor. In this development, it is proposed that the usually present star node (neutral node) of the primary windings can be opened up, so that each primary winding can be connected in a manner accessible from outside. This enables each of the first connecting terminals to be connected to the first inverter unit and the second terminal of each primary winding to be connected to the second inverter unit. Each primary winding is preceded by a smoothing unit which usually comprises a filter capacitor and a filter inductor, in order to convert the generally binary switching signals emitted by the inverter units into a harmonically sinusoidal oscillation, in that the stored magnetic and electrical energies of the capacitor and the inductor are used to smooth the PWM signals. In this way, by opening up a star node of a transformer device known from the prior art, it can be made suitable for the connection of a network feed device in accordance with the invention. Depending on the design of the transformer it may be possible to dispense# with the filter device, or it may also be possible in some circumstances to connect a filter device on the secondary side.
It is recommended to connect an intermediate circuit capacitor, which is able to smooth and stabilize the intermediate circuit DC voltage in the DC intermediate circuit, between the at least one DC energy source and the two inverter units. This improves the stability of the DC intermediate circuit voltage, reduces harmonics and reduces interference losses.
In single inverter operation, and also in double inverter operation with two inverter units at a single DC energy source, harmonics and unwanted loop currents can occur in operation between the output phases of the inverter unit, increasing the current load and losses. A higher current load and the harmonic component reduce the service life of the circuit arrangement, while the losses on the one hand require an increase in the cooling for the power elements and transformer device, and on the other hand require cables and contacts with larger cross sections, lowering the overall efficiency of the circuit arrangement. In the final analysis, this creates a need for more powerful switching elements, entailing increased installation space and higher costs. Finally, the network feed voltage does not correspond to an ideal sinusoidal curve, hence leading to disturbances in the current network.
According to one advantageous embodiment, a filter and smoothing unit preferably comprising at least one filter capacitor and at least one filter inductor can be incorporated between the inverter units and the transformer device, in particular before each primary winding. The filter and smoothing unit is used to significantly reduce the harmonic component and to suppress loop currents, and so to increase the efficiency and the service life of the circuit arrangement and to approximate the output voltage to an ideal sinusoidal curve. The filter and smoothing unit can preferably be inserted into the supply lines to the transformer windings of the transformer device, and is designed in particular to suppress common-mode currents and to attenuate high-frequency oscillations. By the use of a simple LC network as a filter circuit, it is possible to dispense with complex network or sinusoidal filters having a large number of passive components such as X and Y capacitors.
According to one advantageous embodiment, a filter inductor of the filter and smoothing unit can be arranged in series with each primary winding. The filter inductor can be connected in an output phase of the first or second inverter unit and can be used to suppress harmonics, since the inductor provides an increased impedance for higher frequencies.
On this basis, the filter inductor can furthermore comprise two chokes that are inserted into the forward and reverse phases L1U-L2U, L1V-L2V, L1W-L2W of the primary winding. The above-mentioned filter inductor can hence be divided into two chokes and integrated into the two mutually corresponding drive phases of the two inverter units, whereby a simple parallel connection of inverter units can generally be implemented for increasing or scaling up the power.
Building in turn on this, the chokes of the filter inductor of the forward and return phases L1U-L2U, L1V-L2V, L1W-L2W can have a current-compensated design. This can be achieved in that, for example, the coils of the forward and return lines are wound in opposite phase on a common coil carrier, so that the magnetic fields of the forward and reverse lines cancel out, although common-mode components give rise to a high magnetic field and hence to a high impedance, as a result of which inefficient loop currents are heavily attenuated and a low-loss parallel connection of inverter units can be implemented.
In the proposed filter and smoothing unit, a filter inductor is generally assigned to each primary winding of the transformer device. The at least three filter inductors can advantageously be wound together on a multi-leg coil carrier, in particular a three-leg coil carrier, whereby on the one hand the magnetic circuits of the current flows can be linked together advantageously, and on the other hand a single, space-saving inductor component can be provided which, with small physical dimensions, can be integrated into an assembly of a double inverter device.
It is therefore possible in accordance with the preceding embodiments for the filter and smoothing unit to be advantageously a single-phase choke for each phase L2U, L2V, L2W or a divided and preferably current-compensated choke for the forward and return phases L1U-L2U, L1V-L2V, L1W-L2W. Preferably, the current-compensated choke can comprise a multi-leg filter choke for current-compensated attenuation of loop currents between the inverter units. A current-compensated choke has a plurality of identical windings through which current flows in opposite directions, so that the magnetic fields forming of the forward and return lines can compensate each other in order to eliminate interference currents. A common-mode current which can be caused by loop currents in the parallel operation of two inverter units at a single DC voltage source however generates a high inductance in the current-compensated choke, which has an attenuating effect, i.e. forms a high impedance with respect to the loop currents. It is desirable for this purpose that the current-compensated choke generates a high stray inductance; this can be achieved by the advantageously three-leg construction of the magnetic-field-carrying coil body, which in most cases consists of laminated metal sheet, where the forward and reverse lines of the three phases of the two inverter units L1U-L2U, L1V,-L2V, L1W-L2W are passed in pairs over one leg each. The stray inductance reduces the effect of harmonics due to the switching effects of the inverter units and attenuates disturbing loop currents.
According to one advantageous embodiment, two or more inverter units can be connected in parallel and operated synchronously to form an inverter unit, where the synchronized output phases L11U & L12U, L11V & L12V, L11W & L12W, L21U & L22U, L21V & L22V and L21W & L22W can be coupled to one another via chokes, preferably via a multi-leg choke for the suppression of harmonics and the reduction of loop currents. The single-phase chokes mentioned above can hence be divided between the forward and return lines, i.e. one choke is integrated into each of the L1U, L1V and L1W branches connecting the first inverter unit to the transformer device, and a corresponding choke into the L2U, L2V and L2W branch connecting the second inverter unit to the transformer device. This makes it possible to connect two or more inverter units in parallel to form one inverter unit, and to operate them in parallel, without the need for additional circuit measures to be taken to protect the inverter units. This allows for economical cascading for increasing the feed power, where a three-leg structure of the chokes with marked stray inductance as discussed above permits a compact construction. The required inductance of the LC network can be divided between two chokes which can each be advantageously arranged on one leg of a three-leg coil yoke. Ultimately it is advantageous and helpful in the case of parallel operation for the PWM clocking of the parallel-connected inverter units of each inverter unit to be synchronized, either by means of parallel wiring of the switching lines of the semiconductor switching units employed in the inverter unit, or by means of synchronized clocking of the PWM control units (pulse width modulation control units) associated with the two parallel-connected inverter units.
According to one advantageous embodiment of the parallel operation of inverter units, it is possible for at least the inverter units of a double inverter device and the associated output-side filter and smoothing units, i.e. the filter conductors and capacitors of the LC stage, to be brought together in a common assembly of a double inverter device. This makes it possible for two or more double inverter devices of this type to be connected in parallel, where the respective output phases L1U, L1V, L1W and L2U, L2V and L2W of the two inverter units included in the double inverter devices are connected to one another. The filter inductors here substantially permit the parallel operation and suppression of loop currents. The structural integration of the inverter units in one double inverter device allows a compact and economical scalability to be achieved. If necessary, appropriate circuit means can be used to connect a double inverter in parallel, where a high fault redundancy is also achieved and the delivered power can be scaled.
According to one advantageous development, at least the second inverter unit can be an inverter unit with 3 or more stages with a centre node. Such an inverter unit with 3 or more stages permits the output of at least three different voltage magnitudes in each polarity direction between the two inverter units, i.e. a zero voltage, an intermediate voltage and a maximum voltage close to the voltage magnitude of the DC intermediate circuit in the positive and negative direction, so that at least five different voltage magnitudes can be delivered, and hence a finer control of the PWM signals and an improved quality of the AC voltage to be delivered can be achieved. Such a 3-level inverter unit improves the efficiency. As a rule, both inverter units here share one intermediate circuit capacitor. To determine a zero voltage, the 3-stage inverter unit comprises a centre node if it is, for example, 3-stage inverter unit of the NPC type (neutral point clamped three level inverter), which can make a defined zero voltage available. Also conceivable and moreover advantageous is however the use of a multi-stage inverting unit which is however correspondingly expensive and can be operated with expensive technical control equipment.
As an alternative to a 3-or-more stage inverter unit with standard power conductors (IGBT switching transistors), the high PWM clock frequencies can be implemented by employing a two-point, three-point or higher-stage inverter unit with switching elements which have significantly lower conduction and/or switching losses, e.g. 2-point or 3-point inverter units based on silicon carbide (SiC) semiconductor elements or gallium arsenide (GaAs) semiconductor elements. These can also implement the required advantages of the invention when operated at a higher frequency than the inverter unit operated synchronously with the network; in particular, power semiconductor components of this type have very low switching losses and low conduction resistances.
On the basis of the above exemplary embodiment, it is possible according to an advantageous development to connect, in the DC intermediate circuit between the at least one DC energy source and the two inverter units, an intermediate circuit capacitor between the positive and negative intermediate circuit potentials and the centre node of the intermediate circuit respectively. It is hence proposed that two separate intermediate circuit capacitors are connected between the positive intermediate circuit potential and the centre node and between the negative intermediate circuit potential and the centre node, in order to provide an improved smoothing of the intermediate DC potential and a delivery of voltages from the 3-stage inverter unit with stable voltages. Both inverter units are connected to the same DC intermediate circuit. Dimensioning of the common intermediate circuit capacitance can be reduced as compared with the total of two separate DC intermediate circuits, so achieving a cost saving.
In one advantageous development, a switching device for decoupling from the inverter unit and/or for a star and/or delta connection of the winding terminals can be provided at the first and/or second winding terminals of the network transformer windings. A decoupling switching device separates the winding terminals from the corresponding inverter unit, for example in the event of a fault in the inverter unit or a collapse of the DC supply voltage of the inverter unit. In order to continue to operate the transformer device, the winding terminals are connected to one another in a star circuit by means of a star switching device, or in a delta by means of a delta switching device. Redundancy can be created in this way, increasing the security of the network feed device against failure. In a star circuit, the winding terminals that are connected to one of the inverter units are short-circuited. The associated inverter here is either electrically disconnected from the winding terminals, or it must be ensured that the associated inverter unit does not modulate shortly before and during the short-circuiting. The transformer device is hence changed over to a star circuit. This is advantageous at high intermediate circuit voltages, which sporadically occur in photovoltaic generators as a result of the MPP tracking. By switching over to a star circuit, the losses of the short-circuited inverter unit are avoided, and hence the overall efficiency of the network feed device can be further increased. This short-circuiting is also advantageous in the event that the associated inverter fails as a result of a fault, since this measure allows the system to continue in operation. The short-circuiting of the ends of the primary windings to the inverter unit can in particular be achieved by means of contactors, relays or electronic switches. In a delta circuit, the windings are connected in series, in particular by means of contactors, relays or electronic switches. In this case, some of the time only one of the two inverter units, which is fitted with semiconductors able to withstand higher voltages, is in operation. In this operating mode, the network feed device can be operated with intermediate circuit voltages which may not be appropriate for the second inverter unit. This solves, for example, the problem of the high open-circuit voltages of the photovoltaic generators.
In an advantageous development, the first inverter unit can be connected to a first DC energy source, and the second inverter unit to a second DC energy source. This makes it possible to use inverter units dimensioned for lower power in order to feed in a higher power. If one of the two DC energy sources fails, the second continues to be available to supply the network with a reduced power. Increased redundancy and security against failure are achieved. The two inverter units are therefore connected in a configuration in which each inverter unit is assigned a separate DC source or DC consumer and a separate intermediate circuit unit. The two inverter units are connected to one another on the DC side by a maximum of one connection. In this configuration, two DC sources such as photovoltaic generators, batteries or fuel cells can be operated separately from one another, and their operating point adjusted. In particular, two photovoltaic generators can thus be independently optimized in respect of their MPP points.
In a subsidiary aspect, an energy feed system is proposed comprising a DC energy source, for example a photovoltaic energy source, a fuel cell energy source, a battery energy source, a mechanically operable generator with rectifier equipped with a network feed device according to one of the previously mentioned exemplary embodiments. The DC energy source can be a photovoltaic source, a fuel cell source, a battery source or a generator, preferably a synchronous generator with attached rectifier. In particular, during operation as a photovoltaic energy source it is possible to enable an increased efficiency by operation at the MPP voltage at which the highest power of the photovoltaic cell can be achieved.
In a further subsidiary aspect of the invention, a method for operating a network feed device is proposed in which the first and second inverter units operate in a coordinated manner such that the voltage acting on each primary coil of the transformer device can be adjusted to an amplitude between 0 V up to the intermediate circuit potential DC+/DC−. A voltage can hence be provided between the first and second inverter units with an amplitude averaged over one switching period that varies adjustably between zero and the DC intermediate circuit voltage. The total voltage variation of the intermediate circuit is available for modulation of the respective half-wave. A high-frequency operation of the second inverter unit for modulation of the AC voltage to be delivered is particularly advantageous here.
On the basis of the above-mentioned method, it can be advantageous if the first inverter unit is operated for determining the polarity of the supply network voltage to be delivered synchronously with the network frequency, in particular in a 50 Hz or 60 Hz cycle, or in a cycle corresponding to the rotation speed of the motor, and for the second inverter units to be operated at a higher frequency, in particular for PWM clocking in a range that as a rule is at least 10 times higher in frequency, in particular greater than 500 Hz, preferably 4 kHz, for modulation of the supply network voltage to be delivered. The first inverter unit therefore determines the polarity of the AC voltage to be delivered as a positive or negative half-wave, while the second inverter unit can, by PWM modulation, perform a modulation of a sinusoidal AC voltage curve to be delivered. It is favourable for this purpose for the second inverter unit to be switched at high frequency, in particular at a clock frequency of 4 kHz, 8 kHz or 16 kHz, in order to generate as few harmonic components as possible and to provide a harmonic curve of the AC voltage.
In a further advantageous implementation of the above-mentioned method, it is possible for at least the second inverter unit to be switched in at least three voltage stages for PWM modulation of the supply network voltage. For this purpose, the second inverter unit is implemented as an inverter unit with 3 or more stages, in order to achieve the most accurate and exact influence possible on the AC voltage to be delivered. As a result, the form factor of the alternating voltage to be delivered is improved in respect of an ideal sinusoidal oscillation, harmonics are reduced and thus, in combination with a lower current load and lower switching losses, the overall efficiency is increased, so that smaller, more economical and longer-lasting semiconductor components, transformer components, cables, filters and heatsinks can be employed.
In a further aspect, the invention relates to the use of a network feed device for coupling a photovoltaic source, a fuel cell source, a battery source or a mechanically operable generator with rectifier, preferably a synchronous generator, to an AC supply network, or for operating an AC consumer, in particular an AC motor, where preferably the operation of the inverter unit is performed with a variable frequency corresponding to the speed of rotation of the AC motor. It is possible here to use the above-mentioned network feed device for operating a synchronous or asynchronous three-phase motor, where the first and second inverter units are to be operated in a correspondingly frequency-variable manner depending on the power and rotation speed that the AC motor is to provide. In the most favourable case, the first inverter unit here is operated corresponding to the speed of rotation that the motor is to provide, and the second inverter unit clocked at a higher frequency, for example a frequency higher by a factor of 10 to 100 than the first inverter unit, for modulation of the AC voltage to be delivered. In this way, an improved efficiency, lower switching losses and an efficient utilization of the voltage potential of the DC intermediate circuit is achieved.
When using an above-mentioned network feed device, a direct energy feed on the secondary side into a medium-voltage network at 1-30 kV is particularly advantageous. As a rule, a transformer is not employed with low-voltage networks, the energy being fed directly. As a rule, feed is not performed directly from DC energy sources into high-voltage networks, which are first connected to a medium-voltage network in order to assemble sufficiently high energy capacity. For this reason, a preferred field of application of the proposed invention is to feed into a medium-voltage network in which the transformers have the appropriate properties.
Further advantages emerge from the following description of the drawings. Exemplary embodiments of the invention are represented in the drawing. The drawing, the description and the claims contain a combination of numerous features. The person skilled in the art will also expediently consider the features individually and group them into useful further combinations.
The drawing shows in:
Components that are identical or similar are given the same reference numbers in the figures.
The maximum output power on the alternating current side from such a network feed device 50 is limited by the maximum current and the AC output voltage of the inverter unit. The lowest MPP voltage that occurs in the DC intermediate circuit 24 limits the possible AC output voltage. Considerations of the efficiency and the physical size, and hence of the cost required for the power switching components 20, 22, 34, 36 and 38 that are involved, show that the highest possible three-phase AC voltage is desirable, since as a result of a higher voltage the losses for the same power undergo a squared reduction since the losses are above all a result of the current. With a network feed device known from the prior art comprising one inverter unit 14 in which each half-bridge 30 is associated with one network transformer winding 38 with a common star node or with delta connection, it is only possible to provide an effective line-to-line three-phase voltage UAC<UMPPmin/√{square root over (2)}·0.9, where the 0.9 provides a 10% reserve for the regulation of tolerances, and at least one third harmonic zero sequence component is injected into the modulation. If such a network feed device 50 is used in typical photovoltaic systems which have for example a minimum MPP voltage of 450 V, then only an output voltage of about 290 V can be delivered on the three-phase side, which means that an upward transformation with corresponding transformation losses is necessary in order to feed, for example, into a 400 V three-phase supply network. In an exemplary 100 kW system, all the components on the three-phase side, i.e. inverter unit 14, network transformer device 16, smoothing unit 18, must be designed for a current load of up to 200 A, in order to provide the required electrical power. This makes the corresponding components relatively expensive and entails high conversion losses.
It follows that the first inverter unit 14a is connected by the transformer supply line 44a with the winding ends of the network transformer windings 38 to the negative intermediate circuit potential 24− during the positive half-wave of the output voltage and to the positive intermediate circuit potential 24+ during the negative half-wave. The inverter unit 14a is switched, synchronously with the network, at a clock rate of 50 Hz, and negligible switching losses occur. The second inverter unit 14b performs the modulation of the sinusoidal voltage U1 at a higher clock frequency of, usually, 4 kHz. Since the three winding ends of the transformer device 16 are not connected via a star node to the other windings, but are connected to 24− or to 24+ depending on the half wave, the inverter unit 14b makes available the full voltage swing of the DC intermediate circuit 24 in each half wave for the modulation of the voltage for each individual phase. The maximum achievable effective output voltage is given by UAC<UMPPmin·√{square root over (3)}/√{square root over (2)}·0.9, and is hence greater by a factor of √{square root over (3)} than the output voltage of the network feed device 50 according to the prior art illustrated in
The exemplary embodiment of
The embodiment of a network feed device according to
Connecting together the half-bridge outputs of two inverter units operated in parallel is permitted by the coupling via the current-compensated chokes 58, where harmonics are attenuated and unwanted loop currents and common-mode components can be suppressed. By a parallel operation of two or more inverter units, the feed-in power can be increased by the use of economical inverters of limited power, redundancy is created and the susceptibility to faults lowered. An inverter that is connected in parallel can also be switched in or out as required, so minimizing switching losses in order to make a needs-based inverter capacity available that corresponds to a quantity of energy to be fed in. By the use of low-power, economical inverters operating in parallel, cascading is created, and the feed-in power of existing systems can be scaled up or subsequently expanded, where parts of an already existing network feed circuit can continue to be used.
b and 12c shows schematically possible structural forms of three-leg coil carrier 62 of a current-compensated three-phase choke 58 through each of which currents for the feed and return lines of a primary coil 38 of a transformer device 16 can pass. The phase outputs of the corresponding half-bridges of the inverter units 14a, 14b are each wound in opposite directions around a single leg 64 of the three-leg coil carrier 62, so that common-mode loop currents can be suppressed. Each leg 64 carries coils for the feed line to a primary winding 38 of the network transformer device 16. The coil carrier 62 comprises a series of laminated metal layers in order to suppress eddy currents and to be able to provide a high stray magnetic inductance.
The circuit variants illustrated in
The illustrated network feed device with double inverter topology results in a marked increase in efficiency, since the ohmic losses in the transformer device 16 and in the smoothing inductors 36 have a square relation to the current I. The inverter losses of the second inverter unit 14b can be markedly reduced. However, further inverter losses are added by the use of a pair of inverter units 14a, 14b, but are correspondingly low, as conduction losses, when the first inverter unit 14a is clocked at a low frequency of 50 Hz corresponding to the network frequency. The reduction in losses in the rest of the components and in the overall system significantly over-compensates for these additional losses. An overall improvement in efficiency of typically between 0.3 and 1% results. The second inverter unit 14b can usually be designed as a standard 2-stage inverter unit. The variant as a 3-stage inverter unit offers the advantage that the switching losses can be significantly reduced, and hence the overall system achieves an even higher efficiency.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 101 156.7 | Feb 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/052568 | 2/8/2013 | WO | 00 |
