Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
  • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
  • H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
  • H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
  • H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode

Definitions

• the invention relates to an inverter for feeding electrical energy from a first and a second DC power source having a common reference potential in an AC network, the inverter being connected on the output side to a conductor and a neutral conductor of the AC mains. Furthermore, the invention relates to a method for operating the inverter.
• DC power sources such as photovoltaic cells, fuel cells, batteries, etc.
• DC power sources such as photovoltaic cells, fuel cells, batteries, etc.
• MPP maximum power point
• AC grid operators demand that inverters feed a sinusoidal current into an AC grid, whether it be a grid or a stand-alone grid.
• US Pat. No. 4,390,940 A1 specifies an inverter with which photovoltaic cells can be operated with a maximum removal power.
• the inverter typically includes a boost converter stage and an inverter stage.
• Photovoltaic systems or fuel cells require a high efficiency of the inverter for their economic use.
• the object of the invention is therefore to provide a comparison with the prior art improved inverter with high efficiency.
• this object is achieved with a
• Inverter for feeding electrical energy from a first and a second DC power source with a common reference potential in an AC network, the inverter being connected on the output side to a conductor and a neutral conductor of the AC network, and wherein
• the first DC power source has a positive potential with respect to the reference potential
• the second DC power source has a negative potential with respect to the reference potential, the reference potential of the two DC power sources is connected to the neutral conductor,
• the inverter comprises a first buck converter with which the positive potential is connected to the conductor of the AC mains, -
• the inverter comprises a second buck converter with which the negative potential is connected to the conductor of the AC mains.
• the voltages of the DC sources are at least equal to or higher than the recurring maximum expected peak voltage of the AC network.
• first and the second DC power source are designed as so-called strings of a photovoltaic system.
• Each string delivers a voltage above the peak voltage of the AC voltage network. Since photovoltaic systems are usually mounted on roofs of buildings, the connection of the reference potential of the two strings causes with the Neutral conductor, that in the building there is no disturbing electrical power frequency alternating field with earth.
• the first step-down converter comprises a first capacitor, a first switching element, a first diode connected in series with a first diode and a choke circuit and the input side with the positive potential and the reference potential of the first DC power source and the output side via a filter capacitor
• the second step-down converter comprises a second capacitor, a second switching element, a second diode connected in series with a second diode and the choke circuit and the input side with the negative potential and the reference potential of the second DC power source and the output side via the filter capacitor is connected to the conductor of the AC mains and that the neutral is continuously connected to the reference potential of the first and the second DC power source.
• This topology forms in a particularly simple manner, the two buck converter for the connection of two DC power sources to an AC network.
• the throttle circuit is divided into a first throttle element and a second throttle element, when the first step-down divider comprises the first throttle element and when the second step-down divider comprises the second throttle element. This significantly reduces the load on the switching elements.
• a further increase in the efficiency is achieved when the positive potential of the first DC power source and the negative potential of the second DC power source are connected to each other via a balance converter. This is especially important when the AC power fed current must not have a direct current component. Differences in the maximum possible power output of the two DC power sources can then be compensated for by transferring the excess power of a DC power source to the power path and thus to the potential of the other DC power source by means of a balance transformer.
• An advantageous arrangement further provides that the compensation converter comprises a third switching element and a fourth switching element connected in series, and that a
• Connection point between the third switching element and the fourth switching element via a third throttle and a means for measuring current is connected to the reference potential. This gives a simple topology of the balance converter with few components and high efficiency.
• the means for measuring the current comprises a shunt resistor.
• the current can then be measured easily.
• Other ways of measuring current such as by means of a DC-compensated magnetic transducer, are also possible.
• a third diode is arranged and when antiparallel to the second switching element, a fourth diode is arranged.
• the energy stored in the choke circuit can then be dissipated via these diodes.
• these diodes form elements of a protection circuit to protect the Weselrichters against voltage spikes in the AC network.
• connection point between the second switching element and the second throttle element via a fifth diode and a parallel circuit of a first resistor and a third capacitor to the reference potential is connected and when a connection point between the first switching element and the first throttle element via a sixth diode and a parallel circuit of a second resistor and a fourth capacitor is connected to the reference potential.
• the antiparallel to the first and second switching element arranged diodes are then constructed at voltage peaks from the AC mains current paths to the capacitors, whereby the short-term overvoltages drop across the choke circuit and thus do not burden the switching elements.
• the switching elements must therefore not be oversized and there are no additional complex filters necessary.
• Control unit comprising suitable means for controlling the switching elements and auxiliary switching elements.
• the control signals are then generated in the inverter itself, whereby an integrated design of the inverter is possible.
• Operates the inverter according to the invention by the two buck converters are driven alternately in such a way that a feed current in the form of whole sine waves results and that the positive half sine waves are trained by means of the first buck converter from the input side applied positive potential and that the negative half sine waves by means of second buck converter be formed from the input side negative potential.
• the energy from two DC sources is fed into an AC network in a simple manner.
• DC source of the momentarily maximum output power of the second DC power source is approximated.
• the two DC sources are then always taken the maximum output power and thus optimizes the overall efficiency.
• the DC component of the feed current is measured continuously and if the lower set divider a lower feed-in power is given at positive DC component and if the negative feed DC share a lower feed-in power is given to the second buck converter.
• an advantageous embodiment provides that, as of a predetermined reduction in the feed-in power of a buck converter, energy is transferred from the potential to which this buck converter is connected to the other potential by means of a compensation converter.
• the potential at which the first buck converter is connected corresponds to the positive connection of the first DC source.
• the potential at which the second buck converter is connected corresponds to the negative terminal of the second DC power source.
• control unit gives the step-down divider, which is connected to the potential to which the compensating transformer transfers energy, a higher one Feed-in performance, even before an adjustment of the performance specifications takes place via MPP tracking.
• FIG. 1 Topology of a basic circuit Fig. 2 circuit with two throttle elements Fig. 3 circuit with compensating converter
• FIG. 1 shows an exemplary circuit topology for an inverter according to the invention with two buck converters arranged in parallel.
• the first step-down converter consisting of a first capacitor Cl, a first switching element Sl (eg transistor), a first diode Dl and a choke as a choke circuit L is the input side, the first DC power source with its positive terminal as positive potential 1 and its negative terminal as a reference potential 0 connected.
• the second DC power source is connected with its positive terminal as reference potential 0 and with its negative terminal as a negative potential 2 to the second buck converter, which consists of a second capacitor C2, a second switching element S2, a second diode D2 and the throttle as a throttle circuit L.
• the first auxiliary switching element HSL is arranged in series with the first diode D1 of the first step-down converter. It is off when the second buck converter is working.
• the second auxiliary switching element HS2 is arranged in series with the second diode D2 of the second buck converter and is turned off when the first buck converter is operating.
• the circuit shown in Figure 2 differs from that shown in Figure 1 only in that are arranged as a choke circuit, two throttle elements Ll and L2, wherein the first buck converter the first
• Throttle element Ll and the second step-down divider comprises the second throttle element L2. This arrangement causes a lower load on the switching elements Sl and S2.
• the inverter On the output side, the inverter is connected to an AC network, wherein the reference potential 0 is continuously connected to the neutral conductor N network and the output of the choke circuit L to a conductor Ll network of the AC mains .
• a filter capacitor CF is additionally arranged between the neutral conductor N network and the conductor Ll network .
• the two DC sources are formed for example by two strings of a photovoltaic system.
• both strings are made of the same size panel surfaces, since the first panel surface only the first
• Downsampler supplied which feeds in the positive half-wave energy and the second panel area only supplies the second buck converter, which feeds energy in the negative half-wave.
• the same amount of energy is taken from both panel surfaces.
• the two capacitors C 1 and C 2 must be dimensioned sufficiently large since each of the two step-down dividers feeds energy into the alternating current network only during the half-cycles assigned to it and no energy is emitted therebetween.
• the capacitors C1 and C2 continue to be charged by the DC power sources during periods of lack of power output, and the voltage limits set for the circuit must not be reached.
• the inverter is operated as follows:
• the DC component of the current supplied to the AC mains is measured. This can be done for example by means of a current transformer with Hall converter.
• the residual DC current thus measured forms an input variable for the regulation of the two buck converter.
• the step-down dividers are regulated with a current setpoint specification.
• AC grid operators are required to have the current supplied to the AC grid sinusoidal, i. without power harmonics, must be.
• a sine half-wave is derived from the mains voltage and used as a model for the current form.
• a memory for example EPROM
• a sine half-cycle is stored as a table and converted into a analog signal by a DA converter during read-out in the 50 Hz rhythm.
• This solution requires the generation of a synchronizing pulse from the mains voltage to indicate the beginning of the respective half-wave and to start the read-out process from the memory.
• a load value is generated for each of the two DC sources.
• the value is to be determined at which each DC power source outputs the maximum power (Maximum Power Point, MPP).
• MPP Maximum Power Point
• MPP tracking in which the current drawn from a string and the corresponding string voltage are continuously measured and multiplied together. By slight variation of the load, it can then be determined whether an increase in output tends to be possible or the maximum has already been reached.
• MPP tracking there is an output signal that either describes the setpoint current from a string or the setpoint voltage. If the setpoint voltage is specified, the inverter must increase the current until the string voltage drops to the specified value. In the case of the invention, a setpoint voltage is more appropriate as a regulation input. Due to the connected capacitors Cl and C2, this does not change so quickly and the control becomes more stable than with current setpoints. For each of the two DC sources, MPP tracking therefore specifies a setpoint voltage as the setpoint. In this case, the voltage across the capacitor C1 or C2 is compared with the respective setpoint value for each DC power source by means of a differential amplifier.
• the control characteristic requires an integral component (for example PI controller) in order to act slowly and to keep the control deviations low.
• an integral component for example PI controller
• each buck converter To limit the power of each buck converter to protect the power components, it is expedient to provide a maximum value for the output signal of each of the two differential amplifiers. Thereby, the target current can be limited without generating current distortions and harmonics in the AC line current.
• the set current formation for controlling each buck converter is effected by multiplication of the output signals of the differential amplifier with the respective basic nominal current signal.
• Step-down converter is multiplied by the basic setpoint current signal, which is formed from the sequence of positive half sine waves.
• the output signal of the differential amplifier of the second buck converter is multiplied by the basic nominal current signal, which is formed from the sequence of negative sine half-waves.
• the switching frequency of the switching elements Sl and S2 is determined by a clock generator (for example 3OkHz).
• the current is best to measure in the drain line of the first switching element Sl; by shunt resistor or DC capable (compensated) current transformer. Due to the one-sided connection to the first capacitor Cl, the measured current signal is relatively trouble-free.
• Drain line of the first auxiliary switching element HSl Drain line of the first auxiliary switching element HSl. This current is then phase-shifted with respect to the current through the first switching element Sl. It can also be measured only the last flowed through the first switching element Sl current.
• the throttle element Ll prevents abrupt current changes, so that after switching off the first switching element Sl and the commutation of the inductor current to the first diode Dl and the first auxiliary switching element HSL at the first moment nor the last flowed through the first switching element Sl current continues to flow. Depending on the detected current value must then be intervened in the turn-on of the first switching element Sl by influencing the clock generator.
• the control of the first switching element Sl then no longer takes place according to the known current mode, since the increasing inductor current does not directly lead to switching off of the switching element Sl, but a detected at a later time current value is used.
• the second step-down divider comprising the capacitor C2, the second switching element S2, the second choke element L2, the second diode D2 and the second auxiliary switching element HS2 operate in the corresponding manner during the negative line half-cycles.
• a current sensor in the inverter's line feed line measures the AC current that is being fed.
• an integrator which consists in the simplest case of an RC element with a time constant well above the 50Hz mains frequency, a direct current flowing into the network can be detected.
• the stream of each sine half-wave may be digitized, integrated into a processor and subtracted from each other.
• a correction signal is derived from the DC signal.
• This correction signal intervenes as an additional signal in the current control of the buck converter and acts for the Tiefsetzsteiler power-limiting, which feeds the DC component in the AC mains. It is always possible to reduce the feed-in power of a buck converter, since a DC power source already running in the MPP operating point is used to compensate for
• FIG. 3 shows this circuit, whereby a choke inverter with the following elements is added by way of example to the basic circuit shown in FIG.
• the inductor inverter comprises a third inductor L3, which is connected to a first terminal via a shunt resistor RS for current measurement to the neutral conductor N network .
• the second terminal of the third choke L3 is connected via a third switching element S3 to the drain line of the first switching element Sl and via a fourth switching element S4 to the drain line of the second switching element S2.
• Antiparallel to the two switching elements S3 and S4 of the inverter freewheeling diodes are optionally arranged (eg MOSFETs or IGBTs with freewheeling diodes).
• a bidirectional transducer e.g., flyback converter
• AW equalizing transformer
• a separate differential amplifier is arranged in addition to the differential amplifier, which defines the feed-in current
• Direct current control DC component in the feed this initially causes a reduction in the current drain from the more powerful DC power source. This causes the DC source to move away from the MPP operating point as the voltage increases, causing the compensating converter to respond and boost current.
• the other buck converter is then supplied, whereby the DC component in the feed current drops while the power increases at the same time.
• the compensating transducer AW can transmit energy in both directions, its activation must check during the startup of the inverter or in the case of strongly fluctuating power outputs of the DC sources (eg photovoltaic panels in the case of very variable cloud cover) another criterion. If the request for energy transfer from both buck-boosters comes to the other, the control must block the compensation transformer AW. In this case, there is no stable state and both buck converters must first increase the feed-in power until one has reached the MPP operating point. For systems with technical differences in the two DC sources, the use of a digital controller leads to a further improvement of the control dynamics. This is the case, for example, if a direct current source formed by a string has smaller panel areas in a photovoltaic system.
• the digital control then enables a detection of the power difference of the two DC power sources over several hours and the formation of an average value, which dictates the equalization power to be transmitted from the equalizing converter AW when the system is switched on again from the beginning. Reaching the control equilibrium takes less time in this way.
• FIG. 4 shows a basic circuit with a compensating converter and additional circuit elements for deriving voltage peaks occurring in the AC network. Voltage peaks are triggered, for example, by switching operations and reach pulse voltage levels of up to a few kilovolts on a 230V / 400V line. In order to protect the electronics of an inverter against such voltage spikes, over-dimensioning of the circuit elements is usually required. In addition, elaborate filters are necessary.
• additional current paths are provided with two additional capacitors C3 and C4. This ensures that the voltage loading of the circuit elements does not increase significantly above the maximum operational load by passing the current through the inductors Ll and L2 into the four capacitors Cl, C2, C3 and C4. It is important to ensure that the time constants of the LC elements formed from the throttle elements Ll and L2 with the capacitors Cl, C2, C3, C4 are greater than the maximum expected duration of a network overvoltage pulse.
• the additional circuit elements are arranged so that a connection point between the second switching element S2 and the second throttle element L2 via a fifth diode D5 and a parallel circuit of a first resistor Rl and a third capacitor C3 is connected to the reference potential 0. Furthermore, the reference potential is 0 via a parallel connection of a fourth capacitor C4 and a second resistor R2 and further via a sixth diode D6 with a connection point between the first
• the anti-parallel divider D3 and D4 are arranged to the switching elements Sl and S2, e.g. through MOSFETs with integrated diodes.
• the first capacitor C1 is charged to the peak value of the mains voltage by the preceding mains period. (Since the capacitor Cl has a high-resistance discharge resistance, there is almost no recharging during normal operation and thus harmonic distortion of the sinusoidal current.)
• a positive voltage pulse current then flows through the first inductor Ll and the third diode D3 arranged antiparallel to the first switching element Sl first capacitor Cl, whereby the voltage at the Verschaltungs Vietnamese between the first switching element Sl and the first diode Dl is limited to the voltage of the first capacitor Cl plus the diode threshold of the third diode D3.
• a current flows through the second throttle element L2, via the fifth diode D5 in the third capacitor C3, whereby the voltage at the connection point between the second switching element S2 and the second diode D2 to the voltage of the third capacitor C3 plus
• Diode threshold of the fifth diode D5 is limited. If the mains voltage peak fades again, the signals are demagnetized both throttle elements Ll and L2 continue in the capacitors Cl and C3 and are finally de-energized.
• the capacitors C1, C2, C3 and C4 are dimensioned such that they are not charged to an unacceptably high level for the maximum expected line overvoltage with a maximum expected duration.
• the third and fourth capacitors C3 and C4 discharge again via the first and second resistors R1 and R2 connected in parallel.

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• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Inverter Devices (AREA)
• Dc-Dc Converters (AREA)
• Supply And Distribution Of Alternating Current (AREA)

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• 2005
  • 2005-09-28 DE DE102005046379A patent/DE102005046379B4/de not_active Expired - Fee Related
• 2006
  • 2006-07-14 CN CN2006800362504A patent/CN101278471B/zh not_active Expired - Fee Related
  • 2006-07-14 KR KR1020087009968A patent/KR20080066701A/ko not_active Application Discontinuation
  • 2006-07-14 JP JP2008532690A patent/JP4705682B2/ja not_active Expired - Fee Related
  • 2006-07-14 WO PCT/EP2006/064250 patent/WO2007036374A2/de active Application Filing
  • 2006-07-14 EP EP06764169A patent/EP1929617A2/de not_active Withdrawn

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