Method for controlling a DC charging system for charging battery storage systems in motor vehicles using a photovoltaic unit and charging system for this purpose

Abstract

Method for controlling a DC charging system (101) for charging battery storage systems in motor vehicles (1) by means of a photovoltaic unit (30), with an MPPT unit (21), a DC / DC converter (11, 12) and a DC charging unit (40) connectable to a charging port of the motor vehicle (1), wherein: - the inputs of the MPPT unit (21) and the DC / DC converter (11) are connected to the DC outputs of the photovoltaic unit (30) and - the outputs of the DC / DC converter (11) are connected to the inputs of the DC charging unit (40); and wherein the power drawn via the DC charging unit (40) is controlled with current I and voltage U at the DC / DC converter (11) by at least the following steps: - adjusting the voltage UPV of the photovoltaic unit (30) to the voltage UCH of the DC charging unit (40) by means of the MPPT unit (21);- Check whether the excess power of the photovoltaic unit (30) at a given voltage UPV is greater than the charging power of the DC charging unit (40). - If no: Increase the voltage UCH of the DC charging unit (40) until the excess power of the photovoltaic unit (30) equals the charging power of the DC charging unit (40). - Draw a constant current IPV from the photovoltaic unit (30) that is independent of the voltage UPV of the photovoltaic unit (30). - Check for the presence of a power maximum at the photovoltaic unit (30) via the derivative dIPV / dUPV = 0. - Vary the voltage UPV until a maximum in the current IPV is found.

Description

The invention relates to a method for controlling a DC charging system for charging battery storage devices in motor vehicles by means of a photovoltaic unit and charging system therefor.With the wide spread of battery electric motor vehicles (motor vehicles), the owners are available considerable electricity storage capacities. At the same time, the demand for electrical energy increases considerably. The distribution of renewable energies, in particular photovoltaics, can cover this energy requirement. The local consumption of this energy can be increased by the high storage capacity of the motor vehicle batteries. This reduces the individual costs for energy and at the same time reduces the burden on public power grids. However, there has been a lack of a possibility to efficiently and fully utilize the fluctuating power of the renewable energies. Specifically, the photovoltaics offer an efficient energy transfer to the motor vehicle battery, which can be charged with DC, due to the generation of direct current (DC).Hitherto, the direct current generated in a photovoltaic unit is converted into alternating current (AC) via an inverter. The alternating current is then transmitted to an AC charging station for the motor vehicle or fed into the grid. There, rectification takes place via the battery charger of its own motor vehicle. This double conversion entails several disadvantages. On the one hand, losses occur during each conversion, which can amount to up to 20% of the primary energy. On the other hand, each conversion step requires electronic components to add cost.A particular disadvantage is that the double conversion results in switch-on thresholds below which charging of the motor vehicle battery is not possible and which consequently have to be overcome. Motor vehicle chargers usually require at least 6A (AC current) on each phase in order to be able to charge the motor vehicle batteries, as a result of which at least about 4 kW of photovoltaic power must be available in the case of the customary 3-phase chargers. Such a power is often not available in smaller roof systems on residential buildings and / or in the case of low lighting.The object of the invention is thus to specify a charging system and a charging method for charging electric motor vehicles with a photovoltaic DC voltage source, which charging system and charging method make it possible to charge a motor vehicle battery without the intermediate step of conversion into alternating current.This object is achieved by a charging system having the features of claim 1.The following advantages result according to the invention: 1. the direct charging of the motor vehicle battery with DC reduces the energy losses to the percentage range and is in particular only approximately 2%. 2. fewer electronic components are required, so that a more cost-effective construction results. 3. a switch-on threshold at the charging power can be avoided. As a result, the utilization level of the renewable energies can be significantly increased. 4. Due to low investment costs and additional savings in energy, substantial financial advantages are provided for the users of the invention and the removal of the public motor vehicle charging infrastructure can be increased.The technical solution of the invention is compatible with existing systems and allows the further use of renewable energies for AC networks. For this purpose, the device for charging the motor vehicle battery must be connected in parallel with an inverter at the DC output of the photovoltaic system.For this purpose, photovoltaics (PV) will be exemplified below as a DC voltage source of the renewable energies. In the same way, however, a fuel cell operated by hydrogen or a wind power plant, for example, where rectification is generally carried out first for grid frequency adaptation, can serve as a DC voltage source.The combined charging system (CCS) standard is also exemplary below for a DC voltage charging standard for motor vehicles. Alternative compounds such as CHADENO or TESLA SUPERCHARGER may be used analogously.The technical implementation requires the solution of two technical aspects. On the one hand, the DC power supplied to the motor vehicle must meet the limit of the PV power and the motor vehicle charging power, and on the other hand, depending on the design option, the device must cooperate with an existing PV system and inverter in order to maximize the use of renewable energies. In this case, there are different DC voltage levels and regulating mechanisms which can conflict with one another.For this purpose, the invention provides in particular the following designs of a charging system:Design 1: A stand-alone charging system without inverter and without connection to the public AC mains, but with its own MPPT control unit;Design 2: A stand-alone charging system in combination with an existing inverter, which may comprise a battery storage, with connection to the public alternating current network;Design 3: A charging system as part of an inverter and with connection to the public alternating current network;Design 4: A charging system as part of a hybrid inverter; with connection to the public alternating current network and with an additional battery storageThe designs according to numbers 2 to 4 can also be designed as a bi-directional variant, so that it is possible for the motor vehicle battery to feed the inverter like a battery storage.For existing PV systems, the design 2 offers an expansion possibility without replacing the existing inverter technology.The designs 3 and 4 offer the possibility of integration into a (hybrid) inverter according to the prior art and thus an economic integration possibility for new installations.In the designs 2 to 4, a series and parallel connection with the energy source is possible in principle:In the case of the motor vehicle battery being charged via the CCS terminal, a current must flow via the inverter in a series connection. In the bidirectional variant, in the case of a series connection, in the case of the feedback, a significantly higher voltage must be present at the inverter than usual.A parallel connection is therefore advantageous.When multiple PV strings are used, they are coupled in multistring PV inverters to supply the CCS port.All designs can be combined with an AC charging possibility according to the prior art via a changeover switch in order to enable the user to change smoothly from DC charging to grid-based AC charging. The motor vehicle can thus also be charged when no renewable energy is available, without a separate charging cable having to be used or plugged in by the user. The changeover switch can be controlled manually, via a minimum charge level, an automatic yield prediction of the renewable energy or the like. By way of example, for this purpose, the combination of the CCS charging standard and the Type 2 charging standard in a plug is possible, among other things.The trouble-free cooperation with a maximum power point tracker control unit (MPPT control unit) required for the use of photovoltaic energy requires, in the case of the DC / DC converter of the motor vehicle charging connection, a control of the power drawn via the CCS connection with current I and voltage U.A DC / DC converter in the device according to the invention converts the PV voltage to the voltage level required for charging the motor vehicle. The PV voltage varies according to temperature and irradiation intensity; the motor vehicle charging voltage of the CCS terminal varies according to the temperature and charge state of the motor vehicle battery. The DC / DC converter continuously adjusts the voltage ratios accordingly. The charging power of the motor vehicle is controlled via voltage or current limit values of the CCS terminal.Since the power maximum of a PV system is mathematically defined by the derivative dI / dU=0, a possible MPPT algorithm is a variation of the PV voltage until a maximum in the current has been found. For this purpose, the DC / DC converter in the device according to the invention must draw a constant current on the PV side which is independent of the PV voltage. The extraction of a constant current makes this additional load invisible to a dI / dU control and thus allows the linking of second control loops, the MPPT with the charging device according to the invention, in the same circuit. The CCS power accordingly varies slightly with the PV voltage. The CCS power can be adjusted by discrete variation of the current drawn.For alternative algorithms, a corresponding control mechanism is required, e.g. in the case of an MPPT which is based on power variation or conductance, the extracted CCS power must be kept constant, i.e. dP / dU=0. In general, this requires two independent control variables in order to avoid coupling of the control circuits, which are both present in the same circuit, and thus a harmful competition of the control mechanisms.In the following, the dI / dU=0-based MPPT algorithm is assumed by way of example, i.e. a quasi-constant current setpoint value is predefined. This first control loop for regulating the PV current drawn from the PV unit for the CCS charging current must be at least as fast as the scan interval of the MPPT, namely on the order of a few milliseconds or more rapidly, in order not to simulate an incorrect power maximum for the MPPT when the PV voltage varies. This can be done, with the current setpoint value of the CCS-side DC / DC converter being specified, via electronic assemblies known per se, such as an electronic load, inverse converters, split-pi converters, Cuk converters, SEPIC converters, etc.A second, slower control loop must cover the typical rate of variation of the renewable energy source, such as pre-over vaulting, to dynamically adjust the charging power of the CCS port to the renewable energy source supply. This second control loop defines the quasi-constant current value that the first control loop draws from the renewable energy source. In PV, a control interval of the order of 1 s is typically sought for the second loop.The invention is explained in more detail below with reference to the exemplary embodiments of charging systems shown in the drawings. The figures show in detail: FIG. 1 shows a charging system according to a first embodiment of the invention in a schematic overview; FIG. 2 shows a charging system according to a second embodiment of the invention in a schematic overview; FIG. 3 shows a charging system according to a third and fourth specific embodiment of the present invention in a schematic overview; FIG. 4 is a flow chart of a charging method for the charging system of FIG. 2.FIG. 1 shows a first, simple embodiment of a DC charging system 101 according to the invention, which is configured as a stand-alone solution without the inclusion of a public alternating current grid and also without other consumers or energy stores such as buffer batteries etc. For the sake of simplicity of the drawing, all circuits are shown with only one solid line which is intended to symbolise at least two wires of a line. The dashed lines represent the control connections of the respective circuits and / or modules to a controller 50.A PV module 30 is connected to an MPPT control unit 21; a PV circuit 31 is formed therebetween. The MPPT control unit 21 continuously optimizes the operating point of the PV module 30, since the optimum operating point is not constant in the PV module 30, but depends on changing factors such as the irradiance, the temperature at the PV module 30 and the type of solar cells.When searching for the power maximum, the MPPT control unit 21 first increases the DC voltage in the PV circuit 31 of the PV module 30 in the usual manner and checks whether the current changes to the effect that the power increases as the product of voltage and current in the PV circuit 31, for example by searching for the maximum dI / dU=0. If a power maximum is exceeded, the power starts to decrease again with a higher voltage, which serves as a termination criterion for the search. This iterative method is carried out periodically by a microprocessor in the MPPT control unit 21, so that even with changing irradiation conditions, operation at the maximum power point always occurs.A motor vehicle 1 is connected to a CCS charging station 40 for charging. The CCS charging station 40 and the MPPT regulating unit 21 are connected via a DC / DC converter 11 so that a different voltage level can be set in a direct current circuit 41 between the DC / DC converter 11 and the CCS charging station 40 than in the PV circuit 31 of the PV module 30.The voltage set on the side of the PV module 30 is converted via the DC / DC converter 11 to the CCS charging voltage of the CCS charger 40 necessary to charge the battery in the automobile 1. This charging voltage depends, inter alia, on the state of charge of the battery. Without the DC / DC converter 11, the voltage at the PV module 30 would always have to be higher than the CCS charging voltage, which cannot be achieved under all conditions or at least hinders the optimization of the PV power by the MPPT control unit 21 or reduces the efficiency of the PV. This obstacle is eliminated in the charging systems according to the invention by the additionally provided DC / DC converter 11. Nevertheless, specifications for the respective voltage exist both on the side of the power-generating unit, here the PV module 30, and on the side of the motor vehicle battery, such that the voltage is already predefined as one of the parameters influencing the charging power.Both direct current circuits 31, 41 are monitored and influenced by continuous measurement of current intensity and voltage via a controller 50.In order to achieve the optimum yield at the PV module 30 on the one hand for given environmental conditions and also to optimize the charging power for the battery of the motor vehicle 1 on the other hand, a charging method implemented in the controller 50 is carried out according to the invention.For this purpose, it is checked in particular continuously in the controller 50 whether excess power is available in the PV circuit 31. As a function of this, the regulation is carried out as follows: 1. if excess power is available in the PV circuit 31, the current setpoint value of the DC / DC converter 11 on the side of the CCS charging station 40 is increased until excess power is no longer present. 2. if the PV power is too small, the current setpoint value of the DC / DC converter 11 on the side of the CCS charging station 40 is reduced until either a small delta of control reserve of excess power is present again or-if the PV power is too low-charging power is no longer transmitted to the CCS charging station 40. 3. The power set according to the numbers 1. and 2. must not exceed the maximum accepted CCS charging power of the motor vehicle 1.FIG. 2 shows a second embodiment of a charging system 102 according to the invention, which is configured to be integrated in an inverter for the inclusion of a public alternating current grid 2 and a buffer battery 4 etc. (design 4). This system represents an extension of a hybrid inverter according to the prior art by a further DC line for charging the motor vehicle 1. Via corresponding DC-DC converters 11 and 23, the motor vehicle can function as an additional storage battery for supplying the AC power grid 2.Here, too, the PV module 30 is initially connected to the MPPT control unit 21. The DC / DC converter 11 with the associated controller 50 is connected between the MPPT control unit 21 and the CCS charging station 40. On the load side of the MPPT control unit 21, the DC / DC converter 11, another DC / DC converter 23 for charging the storage battery 4 and a DC / AC converter 24 for connection to the AC power supply 5 are connected in parallel. All units are connected to the controller 50.FIG. 3 shows a third embodiment (design 2) of a charging system 103 according to the invention, to which two PV module strings 30 are connected. As in the charging system 102 according to FIG. 2, a connection to the alternating current grid 5 and a storage battery 4 are also provided. Two MPPT control units 21, 22, the DC / DC converter 23 for charging the buffer battery 26 and the DC / AC converter 24 are part of an inverter 20 which is configured as a multistring device known per se. For each PV direct current circuit 31, 33 of a PV module 30 and 32, an associated MPPT control unit 21, 22 is provided in the inverter 20.The charging methods depicted in the respective controller 50 and adapted to at least one of the charging systems 101, 102, 103 are explained below. All charging methods for the charging systems 101, 102, 103 have in common the interaction, controlled by the controller 50, of the MPPT control unit 21, the DC / DC converter 11 and the CCS charging station 40.In the charging system 103 according to FIG. 3, the second MPPT control unit 22, the second DC / DC converter 12 and the CCS charging station 40 additionally cooperate, otherwise the function is the same as in the charging system 101, 102.A charging method according to the invention for the charging system 102 depicted in FIG. 2 is described by way of example with reference to the flow chart shown in FIG. 4.All CCS powers, voltages and currents are to be understood as actual values. The (+Δ>0) is to be understood as a small control interval in the form of a technically conditioned interval width of the control that can be selected in terms of its design. The magnitude of the Δ depends on the accuracy of the power measurement and the steps of adjusting the power. If, for example, the CCS charging power can be adjusted only in 1 A steps at a charging voltage of 400 V due to the system, Δ≥400 W in order to prevent the excess power from becoming less than zero when increasing by one step.Since the CCS charging power must never exceed the excess PV power and the power limit, this interval ensures, within the framework of cycle time, measurement accuracy, and system step size, that the excess PV power never becomes truly less than zero and the power limit never becomes truly less than the CCS power.The cycle begins in step 201 with the measurement of the relevant values, namely the current CCS charging power and the current PV surplus power. The CCS power limit is defined by the motor vehicle and will typically represent the instantaneous maximum charging power of the motor vehicle battery and is considered herein to be a known value from communication via the charging port.It is calculated in step 202 whether the sum of the CCS charging power and the PV surplus power is greater than the CCS power limit:If the CCS power limit has not yet been reached, the internal power limit is set to the CCS power limit in step 203.Otherwise, the PV power is set as an internal power limitRegardless of this internal power limit, the DC circuit 31 of the PV module 30 continues to attempt to increase the power by means of the MPPT control unit 21 until the power maximum is reached there.In step 205, a check is made as to two conditions, namely whether there is any excess PV power at all and whether the internal power limit previously defined in step 204 is greater than the CCS charging power. In other words, this means that there must be an offer on the energy generator side and at the same time there is still a need on the CCS charging station side.If both conditions are met, the CCS charging voltage or the CCS charging current is increased in step 206, depending on technical possibilities, in order to increase the CCS charging power; a jump is then made before the beginning of the cycle (step 201).Only if both of the above-mentioned conditions from step 205 are not fulfilled is a differentiated evaluation carried out in step 207:If the internal power limit defined in step 204 is less than the CCS charging power, the CCS charging voltage or the CCS charging current is reduced depending on technical possibilities (step 208).If the PV surplus power is less than zero, the CCS charging voltage or the CCS charging current is also reduced (step 208).If excess PV power is no longer available and at the same time the power limit is exactly equal to the instantaneous CCS charging power or, if appropriate, greater, then the instantaneous CCS charging power is evaluated as optimized (step 209), so that no further measures are required any longer. The cycle starts from the beginningThe target value of the CCS charging power or of the CCS current or of the CCS voltage determined with this method is transferred to the controller 50 for DC-DC converters. There, this serves as an instantaneous setpoint value for the output variables of the DC-DC converter 11, which corresponds to the charging power of the motor vehicle 1 at the DC charging unit 40.In the case of the design 2 and the charging system 103, the actual CCS charging power is regulated by the controller 50 in priority around this setpoint value in order to achieve a quasi-constant input current with dI / dU=0 for decoupling from the MPPT regulation. If the actual CCS charging power leaves a technically defined interval around the target value of the CCS charging power, the input current of the DC-DC converter 11 is then readjusted to again satisfy the target value. In the case of energy consumption or feed specifications in the AC grid 2 on the output side of the inverter 25, this power can be subtracted from the setpoint value of the CCS charging power in order to prioritize the consumption in the AC grid 2 and to reduce the CCS charging power accordingly, provided this power is not already contained in the term of the PV excess power used above.Reference Number:1 Motor vehicle 2 AC power supply 4 Battery storage 101, 102, 103 Charging systems 11, 12 DC / DC converters 20 Inverters 21, 22 MPPT control unit 23 DC / DC converters 24 DC / AC converters 25 Inverters 30, 32 Photovoltaic units (PV) 31, 33 PV direct current circuits 40 DC charging unit 41, 42 Direct current circuits 50 Controller

Claims

Method for controlling a DC charging system (101, 102, 103) for charging battery storage devices in motor vehicles (1) by means of a photovoltaic unit (30, 32), at least comprising: - a maximum power point tracker (MPPT) unit (21, 22) for controlling the photovoltaic unit (30, 32); - a DC / DC converter (11, 12); - a DC charging unit (40) which can be connected to a charging connection of the motor vehicle (1); wherein: - the inputs of the MPPT unit (21, 22) and of the DC / DC converter (11, 12) are connected to the DC outputs of the photovoltaic unit (30), and - the outputs of the DC / DC converter (11, 12) are connected to the inputs of the DC charging unit (40); and wherein the power with current I and voltage U drawn from the DC charging unit (40) at the DC / DC converter (11, 12) is regulated with at least the following steps: - adaptation of the voltage U PV of the photovoltaic unit (30, 32) to the voltage U CH of the DC charging unit (40) by means of the MPPT unit (21, 22); checking whether the excess power of the photovoltaic unit (30, 32) is greater than the charging power of the DC charging unit (40) at a given voltage U PV - if no: increasing the voltage U CH of the DC charging unit (40) until the excess power of the photovoltaic unit (30, 32) is equal to the charging power of the DC charging unit (40); extraction of a constant current I PV from the photovoltaic unit (30, 32), which is independent of the voltage U PV of the photovoltaic unit (30, 32), - testing for the presence of a maximum power at the photovoltaic unit (30, 32) via the derivative dI PV / dU PV=0; - variation of the voltage U PV, until a maximum has been found in the current I PV.A DC voltage charging system (100) for carrying out the method according to claim 1, at least comprising: - a photovoltaic unit (30) having an MPPT control unit (21); - a DC / DC converter (11), the inputs of which are connected to the DC outputs (31,..., 34) of the photovoltaic unit (30); - a DC charging unit (40), which can be connected to a charging connection of the motor vehicle (1), the inputs of which are connected to the outputs at the DC / DC converter unit (12).DC voltage charging system (100) according to claim 2, characterized in that the DC outputs of the photovoltaic unit (30, 32) are additionally connected to the DC inputs of an inverter (20), which is connectable to the public AC power supply (3).DC voltage charging system (100) according to claim 3, characterized in that the at least one MPPT unit (21, 22) is part of the inverter (20).DC voltage charging system (100) according to Claim 3 or 4, characterized in that the inverter (20) is connected to a battery store (4) via additional DC input connections.DC voltage charging system (100) according to Claim 5, characterized in that the battery storage unit (4) is formed by the motor vehicle battery (1), the poles of the motor vehicle battery being connected to the DC input connections of the inverter (20).

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