KR20080016520A - Inverter - Google Patents

Abstract

An inverter (1,1') having one or more phases, comprising:-a first and a second DC capacitor (Cl, C2), which may be supplied by a photovoltaic generator (30), constituting an intermediate DC circuit (2,2'), whereby the common connection point of the DC capacitors represents a common earth connection (8), and-at least one subcircuit (12) for generating one phase of an AC voltage at an AC node (16), each subcircuit comprising:-a series connection of a first and second power switch (Sl, S2),-a series connection of a first and second auxiliary switch (S3,S4). In case of a three-phase AC voltage, a star-star connected transformer (32) is attached to the output of the inverter (1'), the inverter being configured to produce phase voltages, comprising each a wave of fundamental frequency and a wave of third harmonic frequency, across the secondary coil of the transformer.

Description

Inverter {INVERTER}

The present invention relates to an inverter for converting a DC voltage into an AC voltage having one or more phases. The invention also relates to a solar energy system comprising an inverter and a solar generator.

Inverters are generally well known. Inverters, for example, are energy generated by a photovoltaic generator and can be used in the form of direct current and voltage, independent of an AC grid such as the European power grid or in the form of alternating current and voltage. Used to feed a stand-alone grid. Such grid feeding may be single phase or multiphase, in particular three phase. Inverters designed for this purpose have an intermediate DC circuit in which energy to be fed to the grid is supplied in the form of a DC voltage.

One intermediate circuit is connected to the intermediate DC circuit for each phase of the generated AC voltage. In a "full bridge", each subcircuit has first and second AC terminals to which the voltage of the intermediate circuit is supplied with different pulse widths and variable polarities. As a result, there is a pulsed voltage signal between the first and second AC terminals. By connecting one or more coils, approximately sinusoidal current is generated from the pulsed AC voltage signal when the appropriate pulse is selected. The frequency of the pulsed voltage is generally several times higher than the frequency of the sinusoidal current.

One drawback of such prior art is the generation of negative voltage pulses during the positive half-wave of the current and positive voltage pulses during the negative phase of the current and thus reactive power which results in power loss in the inverter. power) Another drawback is that the pulsation voltage applied to the choke provided between the AC connection terminals and the power grid supplied are at levels in the range up to the voltage in the intermediate DC circuit. The choke must be quite large at design time. In a "full bridge", especially when the inverter is being used for a photovoltaic generator, the switching necessary to generate a voltage pulse also results in a potential shift to an abnormal ground. Photoelectric generators typically have high capacity coupling to ground due to their large area. As a result of the aforementioned potential shifts, in particular potential jumps, stray currents leak to ground by capacitive coupling.

In order to eliminate or at least mitigate the aforementioned problems, there is a known inverter in which an intermediate DC circuit is divided into grounded midpoint and three point circuits as shown in FIG. The DC voltage (U _DC ) of the intermediate circuit is divided into two halves, i.e. one half has a positive voltage and the other half has a negative voltage (+ ½U _DC and -½U _DC ) to ground. In addition, only one AC node is provided which generates a pulsed voltage to ground. For the set value set for the positive half wave, the positive voltage of the intermediate DC circuit is pulsed by the switches SA and SB to the AC node A disposed between the switches SB and SC. Correspondingly, the negative voltage of the intermediate DC circuit is pulsed to the AC node A for negative half wave by the switches SC and SD.

For a positive half wave, this specifically means that the positive intermediate circuit voltage is pulsed to AC node A by switch SA. When this occurs, the switch SB is in the closed position. The pulsed voltage signal induces a continuous, partially positive current in the choke DRA, and that positive current passes through the AC node A from the positive side of the intermediate circuit, through the switches SA and SB, and through the choke D. Flow into the grid. However, this current path is interrupted by an open switch SA for pulsed. At present, it is possible for the current still being driven through the choke D to continue to flow from the common ground connection E through the diode DE and the switch SB. To generate a negative voltage halfwave, positive switches SA and SB are open, and the negative voltage of the intermediate DC circuit is pulsed to AC node A by switches SC and SD. In the latter case, the switch SB must be open since the short-circuit current flows from the common ground connection for the intermediate circuit E to the AC node A through the diode DE and the switch SB in the latter case.

A known drawback of three-point circuits is that current flows from the positive terminal of the intermediate circuit to AC node A through two switches SA and SB, or from AC node A to two switches SC and SD. To the negative terminal of the DC circuit. Since the switches SA to SD are conventionally configured as solid switches such as IGBTs, significant power loss occurs in the switches. In addition, the use of six diodes for the half bridge makes the circuit very complex and expensive.

One particular problem that arises when supplying energy from the photoelectric generators to the grid is that these generators can often only supply a weak DC voltage. Thus, using the inverter to supply current to the power grid, in particular the European grid network, it is necessary to raise the DC voltage from the photoelectric generator to the intermediate DC circuit of the inverter before grid feeding. Another option is to use a lower voltage in the intermediate DC circuit, but convert the generated alternating voltage to the desired voltage level by the transformer, in particular the voltage of the powered grid. In both cases, additional equipment is needed, making the purchase and / or operation of the inverter more expensive and financially useless.

It is therefore an object of the present invention to at least reduce the aforementioned problems, to provide an inverter with less power loss compared to known half bridges and in particular requiring fewer components.

The invention proposes an inverter as defined in claim 1. The invention also proposes an inverter according to claim 11, a solar energy system according to claim 14 and a method for converting a DC voltage according to claim 16 to an AC voltage.

The inverter is preferably configured as shown in claim 2, whereby the inverter is an intermediate DC circuit having first and second DC capacitor units connected in series between the first and second terminals of the intermediate DC circuit, and It has a common ground connection of the circuit. As the DC capacitor unit, a single capacitor can be used. DC voltage is applied to the first and second terminals of the intermediate DC circuit, and in operation a direct current is supplied to the circuit, for example from a photoelectric generator. Between the two DC capacitors, a ground terminal is arranged in which the two DC capacitor units are also connected to each other. The DC voltage applied to the intermediate DC circuit is consequently divided into two halves, so that half the positive intermediate circuit voltage is applied to the first terminal of the intermediate DC circuit, and the negative intermediate circuit voltage to half the ground is the intermediate DC. Is applied to the second terminal of the circuit.

At least one subcircuit is connected to the intermediate DC circuit, and each such subcircuit generates an one-phase alternating voltage. The first and second auxiliary switches, which are connected in parallel with the diodes respectively, are connected in series with the common ground connection of the intermediate circuit. The other terminal of this series circuit having two auxiliary switches is connected to an AC node to which a pulsed voltage is supplied when the inverter is operated. The diodes of the auxiliary switches are connected in opposite directions to each other so that current cannot flow between the common ground connection and the AC node whenever both auxiliary switches are open. If one auxiliary switch is closed, current can only flow in the direction allowed by a diode connected in parallel to the other open switch. Only when both auxiliary switches are closed can current flow in both directions.

To supply the pulsed voltage, two power switches, each having a diode connected in parallel, are connected in series between the first and second terminals of the intermediate DC circuit. The connection point of the two power switches, and thus two diodes connected in parallel of the power switch, is connected to the AC node. The diodes of the power switch are connected in the same forward direction to each other, preventing the diodes from flowing current from the first terminal of the intermediate DC circuit to the second terminal of the intermediate DC circuit.

By this circuit configuration, a pulsed voltage can be generated at the AC node of each subcircuit. To generate a positive voltage half wave, a positive half of the intermediate circuit voltage is pulsed to the AC node by the first power switch and the first auxiliary switch. For negative voltage halfwaves, the negative half of the intermediate circuit voltage is similarly pulsed to the AC node by the second power switch and the second auxiliary switch.

Current is generated when an inductive load is connected between the AC node and ground, such as a choke or a transformer. When generating a positive voltage half wave and a positive current, the current flows from the first terminal of the intermediate DC circuit through the first power switch to the AC node when the first power switch is closed. Thus, the current flows through only one power switch, thereby making it possible to reduce power loss. When a positive voltage half wave occurs, when the first power switch is opened, current can continue to flow from the common ground connection to the AC node through the two series connected auxiliary switches, but this is arbitrary from the common ground connection toward the AC node. It is subject to the fact that the auxiliary switch with a parallel-connected diode which blocks the current flow of is closed.

By using the zero state in the switching cycle, in particular when both the current and the set voltage value are positive or negative, the invention provides that reactive power is generated and transmitted through the power switch and diode and through any filter choke connected. Makes little or no possible. Power loss can also be reduced compared to power loss in conventional three point circuits by making only one power switch conductive at any time except the zero state.

In this way, the overall efficiency of the inverter can be improved. This may include improving the degree of efficiency, achieving a more compact configuration, and / or saving weight, because at least the filter choke will be reduced due to smaller pulse levels compared to those required for known full bridges. This is because only minimally operatively induced stray currents occur as compared to that at full bridge. In addition, the present invention requires fewer diodes than conventional half bridges.

The two DC capacitor units preferably have the same capacitance and the intermediate DC circuit is symmetrical. When a DC voltage is applied between the first and second terminals of the intermediate DC circuit, the common ground connection of the intermediate circuit maintains the intermediate voltage between the first and second terminals of the intermediate DC circuit.

In another embodiment of the invention, auxiliary and / or power switches are used which can conduct current in only one forward direction, which are connected in parallel to each diode in the opposite forward direction. The forward direction for such switches is the result of using solid state switches such as transistors or thyristors. In particular IGBTs, MOSFETs and GTOs can be used.

Each subcircuit includes an AC output and a choke, which choke is advantageously connected between the AC node and the AC output. By choking the voltage appropriately, it is possible to obtain a sinusoidal current at the AC node or to maintain such current when other inductive components are present. Chokes are provided especially when the inverter is operated without a transformer. In the inverter according to the invention, such a choke can be smaller in size than in a prior art inverter with a full bridge, for example because only half of the maximum pulsed intermediate circuit voltage is applied to the choke.

An additional capacitor is preferably connected to the AC output of each subcircuit, in particular a single capacitor connected to ground. Two connection points of the powered power grid are connected in parallel to such a capacitor. The voltage across this additional capacitor unit thus matches the grid voltage of each phase. Thus, this additional capacitor unit smoothes the voltage, so that a time-voltage waveform advantageous for grid feeding can be obtained. The above-described choke and additional capacitor units are combined to create an LC smoothing circuit.

According to a preferred embodiment, a controller is provided for controlling the auxiliary and / or power switches. The inverter is advantageously configured to generate a pulse modulated voltage signal at an AC node of each subcircuit to generate an AC voltage. The inverter topology of the present invention can be used to generate one or more pulse modulated voltage signals that generate an alternating voltage having one or more corresponding phases. The auxiliary and power switches are controlled by a controller to obtain a switching sequence that generates a pulsed voltage. Regardless of the interaction that defines the particular pulse pattern and hence the desired current or voltage waveform, the switching state is already included in the inverter, for example in the form of suitable software. Voltage amplitude, frequency and phase can be externally determined and / or depend on the grid to which it is powered.

According to another embodiment of the invention, the AC output or AC node of each subcircuit is connected to a transformer. The inverter according to the invention is basically suitable for supplying power to the grid even without a transformer, but it may actually happen that the transformer is connected to the inverter on the output side. Such a case is, for example, when a small photoelectric generator is used at the input side and only a small intermediate circuit voltage is available, and / or when power is supplied to the grid at the high voltage at which the transformer is required. Depending on the particular application, it is possible to omit the choke and additional capacitor units when the transformer is used.

Due to its topology, the inverter, in particular according to the invention, is basically suitable for single phase grid feeding. One subcircuit is sufficient in such a case. According to yet another embodiment, the inverter is configured with three subcircuits feeding a three phase power grid. This means that a separate subcircuit is provided for each phase of the alternating voltage to be generated. Each subcircuit provides its AC node or its AC output with a voltage that feeds a single phase power grid. Preferably, all subcircuits are connected to one single intermediate DC circuit.

An inverter according to claim 11 is also proposed. The inverter has three AC terminals which can correspond to an AC node when three subcircuits are used. Three-phase transformers with star connections on both sides of the primary and secondary are connected to the AC terminal. The primary coil side may be connected to the AC terminal, and the neutral point on the primary side may be connected to the common ground terminal of the intermediate DC circuit of the inverter. A voltage signal is currently supplied to each of the three AC terminals and to each line on the primary side of the transformer. As a result, a voltage signal, that is, a three-phase voltage, is generated in each corresponding line on the secondary side of the transformer. By pulsing the inverter accordingly, the three-phase voltage has a fundamental wave which is a sinusoidal wave having a fundamental frequency superimposed by a third harmonic having a frequency three times the fundamental frequency. Due to the pulsed, although not considered relevant, additional smaller signals, especially harmonics, can also overlap. It is preferable that the fundamental wave is in phase with the third harmonic, and the zero crossover of each rising edge of the fundamental wave coincides with the zero crossover of the rising edge of the third harmonic. The fundamental waves of the phase voltages are also shifted by 120 ° from each other. Due to the star connection on the secondary side of the transformer, the line voltage occurring between the first and second lines is the difference between the phase voltages on the first and second lines. While the two fundamental waves of the first and second phase voltages are 120 ° out of phase with each other, each superimposed harmonic has three times the frequency of the fundamental wave so that they are not phase shifted from each other. As a result, since the third harmonic is eliminated by subtraction, the voltage difference between the two phase voltages that are not sinusoidal due to the superimposed third harmonics is nevertheless sinusoidal for each line voltage.

This means that even if a highly distorted signal is supplied and transmitted to the primary side, a sinusoidal three-phase system can be generated on the secondary side. Due to the superposition of the 3rd harmonics on the fundamental wave, with the 3rd harmonic in phase, the distortion of the fundamental wave is achieved so that its maximum value is reduced. This kind of distortion signal can thus be generated in inverters with intermediate DC circuits by lower intermediate circuit voltages. Thus, by adding third harmonics, a sinusoidal three-phase AC voltage of sufficient amplitude can still be generated on the secondary side of the transformer even when the intermediate circuit voltage is reduced.

The added third harmonic can basically have any amplitude, but the amplitude should be set according to the boundary conditions such as the saturation behavior, in particular the available intermediate circuit voltage and the transformer used. The preferred value of the amplitude of the third harmonic is in the range between 1/4 and 1/8 of the fundamental wave amplitude. The amplitude of the third harmonic is preferably 1/6 of the fundamental wave amplitude, which is the optimal value for minimizing the intermediate circuit voltage.

It is preferred that the solar energy system comprises an inverter and a photoelectric generator according to the invention. In such a configuration, the photoelectric generator supplies the DC voltage supplied to the intermediate DC circuit of the inverter directly or using a boost converter. The inverter then generates a single phase or polyphase alternating voltage from the intermediate circuit voltage. The inverter and the solar energy system as a whole are configured to supply current to the power grid. To achieve this, the solar energy system must be accessible to the power grid, and in particular be able to synchronize the magnitude and phase of the voltage supplied.

The invention will be described in more detail with reference to the drawings below.

1 shows a prior art inverter circuit with a grid connected to a half bridge.

2 shows an inverter circuit according to the invention for supplying a single phase voltage to a power grid.

3 shows a schematic diagram of an inverter with a transformer connected by a star connection.

4 shows some time-voltage waveforms in a transformer.

The prior art half bridge of FIG. 1 has four solid state switches SA-SD, each having anti-parallel connected diodes DA-DD. With the help of these switches, voltages ½U _DC and -½U _DC are pulsed at node A. When the voltage pulses are positive, that is, the switches SA and SB are closed and the switches SC and SD are open, and when the positive current flows through the choke DRA, the current is also switched by the switches SA and SB. You can see that flowing through. Thus, when there is a negative current flowing through choke DRA, the current flows through power switches SC and SD when switches SA and SB are open and switches SC and SD are closed. .

In order to understand how the inverter according to the invention works, the circuit operation is described in detail with reference to FIG.

The inverter 1 has an intermediate DC circuit 2, which has first and second terminals 4, 6 and a common ground connection 8. The common ground connection 8 is grounded through terminal 10. The intermediate DC circuit 2 comprises two series connected capacitors C1 and C2.

The subcircuit 12 is connected to the intermediate DC circuit 2 for single phase feeding to the power grid 14. The subcircuit 12 has an AC node 16 provided with a pulsed voltage. To achieve this, first and second power switches S1 and S2 are connected between the respective first and second terminals 4 and 6 of the intermediate DC circuit 2 and the AC node 16. A series circuit comprising first and second auxiliary switches S3 and S4 is connected between the common ground connection 8 and the AC node 16. All power and auxiliary switches are configured as solid state switches and can only conduct current in one forward direction. The diodes D1 to D4 are connected to each of the power source and the auxiliary switches S1 to S4 in an anti-parallel connection, that is, in parallel but in the opposite forward direction. An LC smoothing circuit 18 having a choke 20 and an additional capacitor C3 is connected to the AC node 16. Capacitor C3 is connected in parallel to power grid 14. Capacitor C3 and power grid 14 are connected between AC output 22 and an additional ground terminal 24.

In order to generate a sinusoidal alternating current voltage in the capacitor C3 and sinusoidal alternating current in the choke 20, the following circuit operation is performed. There should be a basic distinction between the four operating periods below. They depend on whether the current i1 is a positive half wave or a negative half wave and whether the positive or negative voltage is pulsed at the AC node 16 (+ ½U _DC or -½U _DC ) when the current is delayed above the voltage. .

In the first operating period, there is a positive half wave of current i1 and a positive pulsed voltage at AC node 16 through choke 20. In order to generate a positive voltage value at the AC node 16, the power switch S1 is closed and a current flows from the first terminal 4 of the intermediate circuit through the power switch S1 to the choke 20. The auxiliary switch S3 is closed during the first operation period. When power switch S1 is currently open, the voltage at AC node 16 drops to approximately zero volts. However, due to the choke 20, the current i1 is substantially maintained and flows from the current ground connection 8 to the choke 20 through the diode D4 and the auxiliary switch S3.

When the predetermined nominal voltage drops to zero at the end of the first operating period for the switching operation of the inverter, the inverter enters the second operating period so that the current i1 is still positive but the negative voltage is equal to the AC node ( In 16). During the transition from the first to the second operating period, both power switches S1 and S2 are initially opened. As long as the auxiliary switch S3 is closed, current continues to flow from the common ground connection 8 to the AC node 16 through the diode D4 and the closed auxiliary switch S3, where the voltage is approximately zero. When the auxiliary switch S3 is currently open, the current is interrupted, at which time only positive current i1 can flow from the second terminal 6 to the choke 20 through D2. Thus, the voltage at AC node 16 drops to a value approximately equal to the negative value -½U _DC of the intermediate circuit voltage. In order to terminate such a negative voltage pulse at the AC node 16, the auxiliary switch S3 is closed again, so that the pulsed is affected by the auxiliary switch S3.

In the third operating period, the current i1 passing through the choke 20 drops to zero and then changes its polarity. The auxiliary switch S3 is then opened and the auxiliary switch S4 is closed, and both switches remain in this state for a third period of operation. Negative voltage is now pulsed to AC node 16 by power switch S2. When the power switch S2 is closed, a negative voltage is applied to the AC node, and the current i1 is then from the choke 20 through the AC node 16 and the power switch S2 to the intermediate DC circuit 2. Substantially flows into the second terminal 6. By opening switch S2, the negative voltage pulse is terminated and AC node 16 obtains a voltage of approximately 0 volts. The current i1 can flow to the common ground connection 8 and from there to the ground terminal 10 via the diode D3 and the auxiliary switch S4 connected thereto. The switching operation of the power switch S2 and the auxiliary switch S4 thus takes place in this third operation period similar to the switching operation of the power switch S1 and the auxiliary switch S3 during the first operation period.

When the set point of the time-voltage waveform at AC node 16 drops toward zero and becomes negative, in other words, when the positive voltage is pulsed at AC node 16 but current i1 is still negative, Four operating periods are disclosed, wherein the power switches S1 and S2 and the auxiliary switch S3 are opened and the positive voltage is controlled in a manner similar to the switching operation of the auxiliary switch S3 during the second operation period. Pulsed to the AC node by S4). When the current i1 rises toward zero and becomes positive toward the end of the fourth operating period, the first operating period resumes.

The inverter 1 ′ shown in schematic form in FIG. 3 has an intermediate DC circuit 2 ′ having a center 8 ′. The intermediate DC circuit 2 'is connected to a solar system 30 that supplies a DC voltage. On its output side, a three-phase inverter 1 'is connected to the transformer 32. Transformer 32 has a star connection on both its primary side connected to inverter 1 'and its secondary side 36 connected to power grid 38.

The inverter 1 'outputs the phase voltages u1' to u3 'on the primary side 34 of the transformer 32, and the phase voltage results in the phase voltages u1 to u3 on the secondary side. The line voltage results from the difference between each phase voltage. The line voltage u12 is the difference between the phase voltages u2 and u1, that is, u12 = u2-u1.

4 shows the first harmonic u0 of the phase voltage u2 and the line voltage u12. Although the phase voltage u1 is not shown, only the fact that it is 120 ° ahead of the phase voltage u2 is different. Phase voltages u1 and u2 comprise fundamental waves and have overlapping third harmonics of the same phase. The comparison of the phase voltage u2 and its fundamental wave u0 indicates that superposition with the third harmonic causes flattening of each half wave. When the difference between the two phase voltages u2 and u1 is formed, the third harmonic overlapping the two phase voltages is eliminated, so that the sinusoidal waveform is shown as the waveform of the line voltage u12 without any further overlap. Is generated together.

Claims (16)

An inverter that converts a DC voltage into an AC voltage having one or more phases, An intermediate DC circuit having first and second DC capacitor units connected in series between the first and second terminals of the intermediate DC circuit and a common ground connection, and At least one subcircuit generating one phase of said alternating voltage, each subcircuit comprising: First and second auxiliary switches, each having a diode connected in parallel, and An AC node for supplying a pulsed voltage, The first and second auxiliary switches are connected in series between the common ground terminal of the intermediate circuit and the AC node, The diodes of the power switch are connected in opposite forward directions. The method of claim 1, wherein each of the subcircuits A first and a second power switch each having a diode connected in parallel, said two power switches In series between the first and second terminals of the intermediate DC circuit, Is connected to a common connection point at the AC node, The diodes of the power switch are connected in the same forward direction to each other. The method according to claim 1 or 2, And the two DC capacitor units have the same capacitance. The method according to any one of claims 1 to 3, The auxiliary and / or power switch can conduct current in only one forward direction and the switches are connected in parallel to each diode in the opposite forward direction. The method according to any one of claims 1 to 3, Each of the subcircuits AC output and An choke connected between said AC node and said AC output. The method according to claim 5, Each of said subcircuits having an additional capacitor unit connected between said AC output and ground. The method according to any one of claims 1 to 6, And a controller for controlling said auxiliary and / or power switch. The method according to any one of claims 1 to 7, The inverter is configured to generate a pulse modulated voltage signal at the AC node of each subcircuit to generate an AC voltage in each subcircuit. The method according to any one of claims 1 to 8, The AC node or the AC output of each of the subcircuits is connected to a transformer. The method according to any one of claims 1 to 9, And three subcircuits for supplying said output to a three-phase grid. As an inverter for generating a three-phase AC voltage according to any one of claims 1 to 10, 3 AC terminals A three-phase transformer connected in star connection to said AC terminal by a primary side coil and in star connection by a secondary side coil, said inverter comprising: Generating a pulsed voltage signal at each of the AC terminals such that a phase voltage having a sinusoidal fundamental wave of the fundamental frequency and a third harmonic of three times the fundamental frequency is generated in the secondary coil of the transformer Inverter characterized in that. The method according to claim 11, The fundamental is in phase with the third harmonic The fundamental waves of the phase voltage are shifted by 120 ° from each other. The method according to claim 11 or 12, The amplitude of the fundamental wave at each phase voltage is 4 to 8 times, in particular 6 times, the amplitude of the third harmonic. An inverter according to any of claims 1 to 13 and A solar energy system, including a photoelectric generator. The method according to claim 14, The voltage generated by the photoelectric generator is supplied to the intermediate DC circuit, The solar energy system is configured to connect to a power grid to supply current to the grid. A method for converting a DC voltage to an AC voltage by pulsing the DC voltage to at least one AC node, characterized in that an inverter according to any of the preceding claims is used.

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