Control method for controlling current ripple of DC-DC converter and electrical system

Abstract

The embodiment of the invention relates to a control method for controlling a current ripple of a DC-DC converter and an electrical system. An electrical system includes a DC link (2), an energy storage device (12), and a DC-DC converter (10) coupled between the DC link and the energy storage device. The DC link has an adjustable DC link voltage UDC, and the energy storage device has a DC energy storage voltage UES. A current ripple control (110) is provided to dynamically adjust the DC link voltage UDC such that the voltage ratio UES / UDC is controlled to an optimal value or range of values that causes a desired current ripple level at the energy storage device.

Description

Technical Field

[0001] The present invention relates to DC-DC converters, and more particularly to a DC-DC converter coupled between an electrical energy storage device and a DC link. Background Art

[0002] Electrical energy storage (EES) is becoming increasingly important for electrical systems such as power grids, electric and hybrid vehicles, electric motor drives, uninterruptible power supplies, etc. Depending on the application, short-term or long-term energy storage or both may be required. Typically, short-term storage can be ensured via classical storage systems (such as batteries and supercapacitors), for example to ensure the stability of the power grid in the event of load and production fluctuations, or to store braking energy (regenerative braking) in motor drives and electric traction vehicles. Energy storage devices can provide a storage buffer when the supply is greater than the demand (or the vehicle is braking), and discharge the stored energy during peak demand periods (or the vehicle is accelerating). In contrast, long-term energy storage devices can be batteries or fuel cells, or electrolyzers that produce hydrogen via hydrogen electrolysis (EL). This produced hydrogen can then be stored in tanks in various forms. The fuel cell can then use the hydrogen stored in the tank to provide power to the electrical system.

[0003] A DC-DC converter (also called a DC chopper) is commonly used in electrical systems as an interface between an electrical energy storage device and a DC intermediate circuit of the rest of the electrical system so that the energy storage device can be discharged or charged with the desired power. The DC intermediate circuit is also called a DC link or a DC bus. If the electrical energy storage device is used directly in the power converter DC link, only a portion of the total available energy can be used. In this case, the minimum voltage level of the energy source is determined by the grid voltage. With the help of a DC-DC converter, the energy storage device does not have to operate at the same voltage level as the DC link, but the energy storage device can just be provided with the desired voltage level. When the energy storage device is connected to the DC link through a DC-DC converter, the nominal voltage of the energy storage device is usually lower than the DC link voltage. Therefore, in order to discharge energy into the DC link, the voltage needs to be increased (boost operation), and in order to charge energy from the DC link, the voltage needs to be reduced (buck operation). Such a converter may be necessary not only for voltage step-down or step-up, but also for voltage regulation, because the voltage of the hydrogen electrolyzer and the fuel cell may have nonlinear behavior.

[0004] Electrical energy storage requires clean DC current and places high demands on the performance of DC-DC converters. The larger the current ripple of the energy storage device, the greater the power loss and heat generated in the energy storage device, which leads to an increase in the need for cooling or a shortened life of the energy storage device. Electromagnetic interference caused by current ripple must also be considered when designing the system and verifying interference-free operation. Unfortunately, there is always a high-frequency current ripple associated with the power switches used in the power converter coupled to the DC link. The frequency of this current ripple is the same as the switching frequency used for the gated power switch, typically 10kHz or higher. In addition, due to AC-DC and / or DC-AC conversion in the electrical system, there may be low-frequency current ripple (twice the AC frequency, such as 2×50Hz, or six times the AC frequency in a three-phase system). However, there are ways to minimize current ripple. Typically, lower DC current ripple can be achieved through stronger passive filtering, that is, by increasing the inductance in the output filter of the DC-DC converter or adding filter capacitors in the output filter. The disadvantage is that high-power filters can become bulky and expensive, and in addition, it introduces additional losses, which reduces the efficiency of the entire electrical system. On the one hand, the inductance requirements can be reduced by increasing the switching frequency of the converter. However, as the switching frequency increases, the switching losses of the power switches are much higher, and the losses of the inductor core may also increase depending on the core material and frequency range, which greatly reduces the energy efficiency. In addition, more expensive semiconductors with higher current rates are required. Lower conduction losses will compensate for the increased switching losses. Alternatively, semiconductor components with lower switching losses, such as silicon carbide semiconductors, can be used, but this usually results in poor EMC performance. Another approach is the active ripple reduction method. An example of this is a multiphase DC-DC converter, also known as an interleaved converter. Although DC current has no phases, current ripple has phases. With multiple evenly spaced ripple phases, the current ripple can be reduced to 1 / N, where N is the number of phases. The number of achievable phases is limited because more phases represent more complexity and more switching losses.

[0005] There is a need to further reduce the current ripple of energy storage devices without increasing the losses, price or size of the system. Summary of the invention

[0006] The object of the invention is to further reduce or minimize the current ripple at the electrical energy storage device. This object is achieved by the method and the electrical system recited in the independent claims. Preferred embodiments of the invention are disclosed in the dependent claims.

[0007] One aspect of the present invention is a method of controlling a current ripple caused by a DC-DC converter coupled between a DC link and an energy storage device, the DC link having a DC link voltage U DC, and the energy storage device has a DC energy storage voltage U ES , the method comprises dynamically adjusting the DC link voltage U DC The ratio U ES / U DC Controlled to an optimal value or range of values ​​that results in a desired current ripple level at the energy storage device.

[0008] In one embodiment, dynamically adjusting includes:

[0009] Determine the energy storage voltage U ES The voltage or voltage range,

[0010] Determine the DC link voltage U DC The optimal value or value range of the DC link voltage U DC The optimum value or value range for the energy storage voltage U ES The determined voltage or voltage range gives the ratio U ES / U DC the optimal value or range of values ​​of , and

[0011] The DC link voltage U DC Adjust to the optimum value or value range for the energy storage voltage U ES The determined voltage or voltage range obtains the desired current ripple level.

[0012] In one embodiment, the desired current ripple is equal to or less than a predetermined target ripple or within a predetermined current ripple range, preferably zero or close to zero.

[0013] In one embodiment, the determination includes: if the DC link voltage U DC If there are multiple possible optimal values ​​or value ranges, the DC link voltage U is selected DC The highest possible optimal value or range of values ​​of to enable as much power as possible to be transferred.

[0014] In one embodiment, the method comprises: DC The DC link is discharged before changing from one optimal value or value range to another optimal value or value range.

[0015] In one embodiment, the DC-DC converter is an interleaved multiphase DC-DC converter, and wherein the method comprises dynamically adjusting the DC link voltage U DC The ratio U ES / U DC Controlled to a value or value range corresponding to one of the zero points of the current ripple.

[0016] In one embodiment, the DC-DC converter is an interleaved three-phase DC-DC converter, and wherein the method comprises regulating the DC link voltage to reduce the ratio U ES / U DC Controlled to be approximately or close to a value or value range of one of 1 / 3, 2 / 3 and 1.

[0017] In one embodiment, the DC-DC converter is an interleaved six-phase DC-DC converter, and wherein the method comprises regulating the DC link voltage to reduce the ratio U ES / U DC Controlled to be approximately or close to a value or value range of one of 1 / 6, 1 / 3, 1 / 2, 2 / 3, 5 / 6, 1.

[0018] In one embodiment, the DC-DC converter is an interleaved multiphase DC-DC converter, preferably and wherein the method includes: selectively shutting down one or more phases of the multiphase interleaved DC-DC converter to obtain a zero point of the current ripple at a desired point in the energy storage voltage range, preferably so that the phase shift between the remaining phases is 180 degrees or 90 degrees and there is a zero point of the current ripple at the midpoint of the energy storage voltage range.

[0019] In one embodiment, the DC-DC converter is a single-phase DC-DC converter, and the method comprises:

[0020] Set a limit value for the current ripple level of the energy storage device,

[0021] Determine the current ripple level,

[0022] The DC link voltage U DC The primary adjustment is a first predetermined value, and

[0023] If the current ripple level exceeds the limit value, the DC link voltage U DC The level of is adjusted from the first predetermined value to the second predetermined value, wherein the second predetermined value is smaller than the first predetermined value, and the second predetermined value is greater than the value U ES / U DC The control is to limit the current ripple level to a set limit value or a smaller value or value range.

[0024] In one embodiment, the energy storage device includes one or more of a rechargeable energy storage device, a hydrogen electrolyzer, and a fuel cell.

[0025] In one embodiment, the DC-DC converter is bidirectional.

[0026] Another aspect of the present invention is an electrical system comprising:

[0027] A DC link having a DC link voltage U DC ,

[0028] Energy storage device, the energy storage device having a DC energy storage voltage U ES ,

[0029] a DC-DC converter coupled between the DC link and the energy storage device,

[0030] Regulating component, used to adjust the DC link voltage U DC ,as well as

[0031] A control component configured to control an electrical system to perform a method according to any one of claims 1 to 12.

[0032] In one embodiment, the electrical system includes at least one additional converter coupled to a DC link, the DC link being coupled to a power source or a load, wherein the at least one additional converter includes: one or more of an AC-DC converter connected to an AC power source or an AC load, a DC-AC converter connected to an AC power source or an AC load, and an additional DC-DC converter connected to a DC power source or a DC load, wherein the AC power source preferably includes one or more of an AC grid and a wind power plant, wherein the DC power source preferably includes one or more of a battery, a fuel cell, and a solar power plant, wherein the load preferably includes an AC motor or a DC motor.

[0033] In one embodiment, the regulating component comprises an adjustable voltage source, preferably a DC-DC converter or at least one further converter.

[0034] In one embodiment, the control component comprises a separate controller unit, a controller of the DC-DC converter or at least one further converter, a higher level controller in the electrical system, or any combination thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In the following, the invention will be described in more detail by means of exemplary embodiments with reference to the accompanying drawings, in which:

[0036] Figure 1A , Figure 1B , Figure 1C and Figure 1D An exemplary electrical system is shown in which current ripple control according to an embodiment of the present invention may be applied;

[0037] Figure 2 An exemplary single-phase bidirectional DC-DC converter is shown;

[0038] Figure 3 An example of a multi-phase interleaved DC-DC converter is shown;

[0039] Figure 4 An example of peak-to-peak current ripple versus duty cycle for a single-phase DC-DC converter is shown;

[0040] Figure 5 shows the energy storage device current I for the three-phase interleaved DC-DC converter ES And the phase current I p1 ,I p2 and I p3 Example:

[0041] Figure 6 The peak-to-peak phase current ripple I is shown when the number of phases N is increased from single phase to three phases and then to six phases. p2p relative changes in

[0042] Figure 7 An example of peak-to-peak current ripple for a three-phase interleaved DC-DC converter is shown;

[0043] Figure 8 shows that when the energy storage voltage U ES The peak-to-peak current ripple varies with the DC link voltage U DC Examples of changing behaviors;

[0044] Fig. 9 The peak-to-peak current ripple is shown as a function of the energy storage voltage U in an exemplary embodiment of current ripple control of a three-phase interleaved DC-DC converter. ES Changing behavior;

[0045] Fig.10 shows a flow chart of an exemplary current ripple control process according to an embodiment;

[0046] Fig.11 shows an exemplary current ripple control arrangement according to an embodiment;

[0047] Fig.12 FIG. 4 shows the peak-to-peak current ripple and the optimal DC link voltage versus energy storage voltage U in another exemplary embodiment of current ripple control of a three-phase interleaved DC-DC converter. ES Changing behaviors; and

[0048] Fig.13 1 shows the peak-to-peak current ripple and the optimal DC link voltage versus energy storage voltage U in an exemplary embodiment of current ripple control of a single-phase DC-DC converter. ES Changing behavior. DETAILED DESCRIPTION

[0049] The present invention may be applied to any DC-DC converter, also referred to as a DC chopper, where there is a need to control (eg optimize or minimize) the current ripple at the output of the DC-DC converter connected to a DC link.

[0050] The term "DC link" (also called "DC bus" or "intermediate circuit") refers to a connection or connection that is provided between two conversion stages and effectively acts as an adaptive intermediate level. As is well known, a DC link can be formed by a pair of conductive rails, cables, or similar structures for connecting devices. The so-called DC link capacitors (which may include one or more capacitors) are connected in parallel between the positive rail and the negative rail to smooth the voltage and current ripples and form a stable energy and power supply for the inverter. In addition, energy can also be temporarily stored in the DC link, where the energy can still be used on demand. It should be noted that the implementation of the DC link is irrelevant to the present invention, and embodiments of the present invention are intended to be limited to any specific implementation of the DC link.

[0051] The invention is particularly applicable to a DC-DC converter for an electrical system between an energy storage device of the electrical system and a DC link of the rest of the electrical system so that the energy storage device can be discharged or charged with a desired power.

[0052] Figure 1A-Figure 1D An exemplary electrical system in which current ripple control according to an embodiment of the present invention can be applied is shown. Generally, a DC-DC converter 10 is coupled between a DC link 2 and an energy storage device 12. In addition, at least one further converter 6 is coupled to the DC link, the other side of which is coupled to a power source or a load, such as Figure 1A As shown. There are several additional converters of the same or different types connected to the common DC link 2. The additional converter 6 can be an AC-DC converter (e.g., a rectifier) ​​or a DC-AC converter (e.g., an inverter), for example, if the additional converter 6 is connected to an AC power source or an AC load. For example, if the additional converter 6 is connected to a DC power source or a load, the additional converter 6 can also be a DC-DC converter. The DC-DC converter 10 can be a unidirectional converter (e.g., a buck or boost converter) or a bidirectional converter (e.g., a buck-boost converter). Here, the DC voltage on the DC link side is referred to as the DC link voltage U DC or input voltage, the DC voltage on the energy storage device side is called the energy storage voltage U ES or output voltage, and the DC current on the energy storage device side is called the energy storage current I ES .

[0053] The terms "energy storage device" and "electrical energy storage device" as used herein refer to any energy storage device with a DC voltage, to which electrical energy can be charged from a DC link to be stored as electrical energy or converted to another form (such as hydrogen) for storage, and / or electrical energy can be discharged from the energy storage device to a DC link through a DC-DC converter. Examples of energy storage devices include one or more rechargeable batteries or accumulators, super / ultracapacitors, fuel cells, and hydrogen electrolyzers, or a combination thereof. All of these can be used for energy storage, but each is suitable for different applications and load conditions. Batteries have high energy density and relatively low power density, making them more suitable for providing lower levels of power over longer periods of time. On the other hand, capacitors are more suitable for high and fast-cycling loads, such as applications such as active load compensation or dynamic (regenerative) braking. Batteries are more suitable for lower, more stable loads, such as propulsion, traction, and uninterruptible power supplies (UPS). Supercapacitors (SC) bridge the gap between batteries and conventional capacitors. They can store more energy than capacitors and provide energy at a higher power output than batteries. In hydrogen electrolysis, water is split into hydrogen and oxygen by applying a voltage. In the electrolyzer, electricity is converted into hydrogen, which can then serve as an energy carrier and be used for other hydrogen applications. Fuel cells are used to convert the chemical energy stored in hydrogen, ethanol, methanol, etc. directly into electrical energy. For example, they can be used as vehicle traction batteries, energy storage devices for renewable electricity (e.g. wind or solar), load levels of the power grid, etc.

[0054] Figure 1B An exemplary electrical system is shown, in which the further inverter 6 is an AC-DC converter (e.g., a rectifier, a controlled bridge rectifier) ​​connected to a power supply network (grid) or another type of power source 4 and capable of transferring electrical energy from the grid 4 to the DC link 2 and vice versa. A unidirectional or bidirectional DC-DC converter 10 is connected between the DC link 2 and an energy storage device 12 (such as a hydrogen (H2) electrocracker unit), which may be associated with a hydrogen tank, a fuel cell or a similar H2 storage device. The DC-DC converter 10 can convert the DC link voltage U DC Step down to energy storage voltage level U ES , and the electrolyzer unit converts the electricity into stored hydrogen. In the reverse direction, the stored hydrogen can be converted into electricity in a fuel cell or the like, and the bidirectional DC-DC converter can convert the energy storage voltage level U ES The converted power is stepped up to the DC link voltage, and then the power is transmitted back to the grid 4 through the AC-DC converter 6. For example, the energy storage device 12 can store excess power capacity when it is available, so as to be released to the load or the grid 4 later when needed.

[0055] Figure 1C Another exemplary electrical system is shown, in which the additional converter 6 is an AC-DC converter (e.g., rectifier, controlled bridge rectifier) ​​connected to the power supply network (grid) 4 or another type of AC power source, and is capable of transferring energy from the grid 4 to the DC link, and vice versa. The DC-AC converter 8 (e.g., inverter, controlled bridge inverter) is connected between the DC link 2 and the AC load 9 (e.g., AC electric motor), and is capable of transferring energy from the DC link 2 to the AC load 9, and / or vice versa. For example, the DC-AC converter 8 can be an electric motor drive that drives the electric motor 9 using power transmitted from the grid 4. The DC-DC converter 10 is connected between the DC link 2 and the energy storage device 12, preferably one or more rechargeable batteries or supercapacitors. The DC-DC converter 10 can be capable of charging the energy storage device 12 when needed. For example, when there is excess available energy in the DC link 2, electrical energy can be stored. For example, from the perspective of the DC link, the motor 9 has two main operating modes: an electric mode and a generating mode. In electric mode, the motor 9 rotates the machine. Energy flows from the AC grid 4 to the motor 9 through the AC-DC converter 6, the DC link 2 and the DC-AC converter 8. In power generation mode, the machine rotates the motor 9. This occurs, for example, when the hoisting motor of a crane lowers a load (overhauls a load) or decelerates. In order to keep the speed stable or reduce the speed, the motor 9 brakes. During braking, the motor 9 will generate energy back to the DC-AC converter 8, which then further delivers the energy to the DC link 2. This surplus energy can be stored in the energy storage device 12. The DC-DC converter 10 is also capable of feeding power from the energy storage device 12 to the DC link 2 when needed (for example, in the case where the power consumption of the equipment connected to the DC link is higher than the feed from the AC-DC converter 6). In an embodiment, if the grid 4 and the AC-DC converter 6 are not connected, the energy storage device 12 can be used as the main power source. In an embodiment, the DC-DC converter 10 and the energy storage device can be used as the main power source, and the power from the AC-DC converter is used to charge the energy storage device 12.

[0056] Figure 1D Another exemplary electrical system is shown, which may be similar to the Figure 1C The electrical system discussed above differs in that now the further converter 6 is a DC-DC converter connected to a DC power source 14 such as a battery, a fuel cell, a photovoltaic array or the like.

[0057] There are many different topologies for DC-DC converters, but what they have in common is that they require a choke inductor to slow down the rate of change of the current, and a semiconductor switch that can be used to reduce the DC voltage (step-down), such as when power is transferred from a DC link to an energy storage device, or increase the DC voltage (step-up), such as when power is transferred from an energy storage device to a DC link or from an energy storage device to a DC link. Exemplary embodiments with a basic step-down or buck converter, a step-up or boost converter, or a buck-boost converter are described herein, but embodiments of the invention are not intended to be limited to any particular topology of a DC-DC converter.

[0058] Figure 2 An exemplary single-phase bidirectional DC-DC converter 10 is shown connected between a DC link 2 and an electrical energy storage device 12. The DC link 2 includes: a DC capacitor C DC , the positive DC link rail 22, the negative DC link rail 24 and the DC link voltage U between the rails DC The exemplary buck-boost converter 10 has a half-bridge topology including a pair of main or power switching devices S1 and S2 coupled in parallel to DC link rails 22 and 24 of the DC link 2. The first main switching device S1 may have a first terminal electrically coupled to the positive DC link rail 22 and a second terminal electrically coupled to the output node 110. The second main switching device S2 has a first terminal coupled to the output node 110 and a second terminal coupled to the negative DC link rail 24. A first anti-parallel diode D1 is connected across the first upper main switching device S1 between the positive DC link rail 22 and the output node 110, and a second anti-parallel diode D2 is connected across the second lower main switching device S2 between the output node 110 and the negative DC link rail 24. In the buck mode of operation, in response to (multiple) pulse width modulation (PWM) gate control signals G1 received from the control and driver circuit system, the lower switch device S2 is continuously turned off and the upper main switch device S1 is operable to turn on and off, and thereby connect and disconnect the DC link rail 22 and the output node 110, respectively. As a result, electrical energy is transferred from the DC link 2 and charged to the energy storage device 12. In the boost mode of operation, in response to (multiple) pulse width modulation (PWM) gate control signals G2 received from the control and driver circuit system, the upper main switch device S1 is continuously turned off and the lower main switch device S2 is operable to turn on and off, and thereby connect and disconnect the DC link rail 24 and the output node 110, respectively. As a result, electrical energy is discharged from the energy storage device 12 and transferred to the DC link 2. The output 110 of the DC-DC converter 10 can be connected to the DC link 2 by the inductor L o coupled to the energy storage device 12 so that the DC energy storage current IES is output to the energy storage device 12 or is input from the energy storage device 12. Energy storage voltage U ES exists across the energy storage device 12. The purpose of the inductor is to reduce (filter) the energy storage current I ES Filtering can also include capacitors.

[0059] In an embodiment, the switching devices S1 and S2 may be insulated gate bipolar transistors (IGBTs), or another type of semiconductor switching device such as an integrated gate commutated thyristor (IGCT), a metal oxide semiconductor field effect transistor (MOSFET), or a silicon carbide (SiC) MOSFET.

[0060] Figure 3 An example of a multiphase DC-DC converter 10, also referred to as an interleaved converter, is illustrated. The interleaved converter 10 may include several (i.e., N) parallel DC-DC converter branches (e.g., from N=2 to N=6) sharing a common DC link 2. The pulse width modulation (PWM) gate control signals G1 / G2 are offset from each other (i.e., 360 degrees / N). When several phase branches are connected together on an energy storage device, the interleaved topology reduces the energy storage device current ripple, thereby allowing a reduction in the output inductor L. o size. Although DC current does not have phases, current ripple does. With multiple evenly spaced ripple phases, current ripple can be reduced to 1 / N, where N is the number of phases. The number of phases that can be achieved is limited because more phases represent more complexity. Figure 3 The exemplary converter 10 shown is a bidirectional three-phase (N=3) interleaved buck-boost converter including a three-phase half-bridge having control of the phases interleaved with a 120 degree phase shift. In one embodiment, the three-phase half-bridge may include three parallel single-phase converters connected between rails 22 and 24 of a common DC link 2, such as Figure 2 The node 110 of the parallel converter is connected through the corresponding output inductor L o The output inductor L is coupled to the energy storage device 12. o This can be achieved by a separate inductor or a magnetically coupled inductor. The energy storage current I output to or input from the energy storage device 12 ES is the individual phase current I p The sum of the energy storage voltage U ES Exists across energy storage device 12 .

[0061] When discharging the energy storage device (boost mode), the phase current (I p )Ripple I p2p The size of is calculated as follows:

[0062] I p2p =(U ES / L)*D*T s (1)

[0063] Where D = 1-(U ES / U DC ).

[0064] Equation (1) yields

[0065] I p2p =(U ES / L)*(1-(U ES / U DC ))*T s (2)

[0066] L = output inductor L o Inductance

[0067] T s =Switching period = 1 / f S , where f S is the switching frequency of the converter.

[0068] When charging the energy storage device (buck mode), the phase current (I p )Ripple I p2p The size of is calculated as follows:

[0069] I p2p =((U DC -U ES ) / L)*D*T s (3)

[0070] Where D = (U ES / U DC ).

[0071] Equation (3) yields

[0072] I p2p =((U DC -U ES ) / L)*(U ES / U DC )*T s (4).

[0073] Ratio U ES / U DC In this article, it is called pulse ratio or duty cycle. During the charging and discharging period with a duty cycle of 0.5, the single-phase current ripple I p2p reached the maximum value. Figure 4 Picture shows Figure 2 The current ripple I of a single-phase DC-DC converter p2pExample of the relationship with duty cycle, where fixed U DC =750V, variable U ES =0...750V, L=300uH, f s =4kHz, I p =200A. Current ripple I p2p The peak value is about 156A.

[0074] In the multi-phase interleaved DC-DC converter 10, the energy storage current I ES is the sum of the phase currents, such as Figure 5 The phase current I of the three-phase interleaved DC-DC converter in p1 ,I p2 and I p3 Therefore, the energy storage current I ES The ripple of is also the sum of the ripples of the phase currents, and when the number of phases is N, the peak-to-peak current ripple I at the energy storage device 12 is p2p The ripple of the single-phase configuration will be reduced to at least about 1 / N, where N = a positive integer. Ideally, when the ripples of different phases cancel each other, the peak-to-peak current ripple I p2p will decrease to zero. Figure 6 The interleaving of the phase current ripple I p2p The impact of Figure 6 The peak-to-peak phase current ripple I is shown when the number of phases N increases from single phase to three phases and then to six phases. p2p relative change. Figure 7 Picture shows Figure 3 The peak current ripple I of the three-phase interleaved DC-DC converter p2p , whose parameter values ​​are Figure 4 The parameter values ​​of the single-phase DC-DC converter in are the same. Figure 7 In the example, the maximum ripple is reduced to Figure 4 The maximum ripple of the single-phase example shown is reduced by 1 / 3, ie, from about 156 A to about 52 A. However, although interleaving effectively reduces the current ripple, the number of phases that can be realized is limited, for example, to 6 phases, because more phases represent more complexity.

[0075] As mentioned above, the DC current ripple of the DC-DC converter is usually further reduced by stronger passive filtering, that is, by increasing the inductance in the output filter of the DC-DC converter or adding filter capacitors in the output filter. The disadvantage is that the high-power filter may become bulky and expensive, and it will also introduce additional losses, thereby reducing the efficiency of the entire electrical system. In addition, the slow rise rate of the current (i.e., high inductance) may cause the system to respond slowly compared to systems with lower inductance or higher current ripple, because the current cannot be increased or decreased quickly due to the low inductance. On the one hand, the inductance requirements are reduced by increasing the switching frequency of the converter. However, as the switching frequency increases, the switching losses of the power switches are much higher, and the losses of the inductor core may also increase depending on the core material and frequency range, thereby greatly reducing the energy efficiency. In addition, more expensive semiconductors with higher current rates are required. Lower conduction losses will compensate for the increased switching losses. Alternatively, semiconductor elements with lower switching losses, such as silicon carbide semiconductors, can be used, but this generally results in poor EMC performance.

[0076] According to one aspect of the present invention, the DC link voltage U DC is dynamically adjusted to change the ratio U ES / U DC is controlled at an optimal value that results in a desired current ripple of the energy storage device. In an embodiment, the energy storage device comprises one or more of a rechargeable energy storage device, a hydrogen electrolyzer, and a fuel cell. The energy storage voltage U ES Typically, this is determined by the specific energy storage device 12 used. Conventionally, the DC link voltage U DC is also substantially fixed, for example, determined by the AC voltage of the grid 4. DC When fixed, the operating point of the DC-DC converter 10 may not be optimal from the perspective of energy storage current ripple. ES / U DC The duty cycle is changed, and thus the current ripple is affected by the DC link voltage U DC In other words, by adjusting the DC link voltage U DC , a better ratio U can be used ES / U DC To reduce or minimize the current ripple of the DC-DC converter.

[0077] Figure 8 shows that when the energy storage voltage U ES When fixed, the peak current ripple varies with the DC link voltage U DC The energy storage device may be a hydrogen electrolyzer 12, such as Figure 1BEnergy storage device in the electrical system configuration shown. In this example, U ES =500V, I ES =250A, and the DC link voltage U DC The DC link voltage U is provided by an AC-DC converter (eg, an active front end rectifier or an IGBT power supply unit) 6 coupled to a 500V AC voltage grid. DC It is adjustable from 707 to 799V. Figure 8 It can be clearly seen that by reducing the DC link voltage U DC Adjusting to an optimal value or range of values ​​(eg, approximately 750 V) can significantly reduce the current ripple.

[0078] In an embodiment, the DC-DC converter is an interleaved multiphase DC-DC converter, and the level of the DC link voltage is dynamically adjusted to reduce the ratio U ES / U DC The energy storage current I ES The ripple at duty cycle U ES / U DC There are N peaks and N+1 zeros (zero values) at intervals of 1 / N. Figure 6 As shown in the example shown, for a three-phase interleaved DC-DC converter, there are four (N+1) zero points (zero values) at duty cycles 0, 1 / 3, 2 / 3, and 1, and for a six-phase interleaved DC-DC converter, there are seven (N+1) zero points (zero values) at duty cycles 0, 1 / 6, 1 / 3, 1 / 2, 2 / 3, 5 / 6, and 1. Therefore, when the DC link voltage U DC is adjusted so that the duty cycle U ES / U DC When the current ripple is at one of the zero points, it is minimized to zero. In an embodiment, it may be desirable to set the DC link voltage U DC is adjusted to a level as high as possible to enable the transmission of as much power as possible. DC It may not be regulated down to the value corresponding to a duty cycle of 1, but close to it, since no power is transferred when the duty cycle is 1. A duty cycle of 0 represents the energy storage voltage U ES is 0V.

[0079] Exemplary embodiments of single-phase, three-phase and six-phase DC-DC converters according to the present invention are described herein. However, the present invention can be similarly applied to any number N phases, such as a 2-phase or 4-phase DC-DC converter. As the number of phases increases, the advantages of the current ripple control according to the present invention become increasingly significant. However, compared to prior art methods, single-phase DC-DC converters have achieved a significant reduction in current ripple. It should be understood that the ripple current control according to the present invention is not limited to any specific DC-DC converter topology presented as an example herein, but can be applied to all converter topologies, where the magnitude of the current ripple generated by the DC-DC converter depends on the energy storage voltage U ES and DC link voltage U DC ratio.

[0080] It will be appreciated that the ripple current control according to the present invention is not limited to any particular control embodiment presented herein as an example, but may be implemented in various alternative ways within the scope of the appended claims that will be apparent to a person skilled in the art after reading this specification.

[0081] refer to Fig. 9 An exemplary embodiment of current ripple control for a three-phase interleaved DC-DC converter is described. The topology of the DC-DC controller 10 may be as follows: Figure 3 The energy storage device may be a hydrogen electrolyzer 12, such as Figure 1B Energy storage device in the electrical system configuration shown. For example, the energy storage voltage U ES (e.g., the cell voltage) can vary in the range from 150 V to 720 V. In this example, the DC link voltage U DC The DC link voltage U may be provided by an AC-DC converter (eg, rectifier) ​​6 coupled to a 500V AC voltage grid. DC It can be obtained from the minimum value U DC,min =707V to U DC,max = Adjustable within the range of 799V. Fig. 9 The diagram shows the DC,min and U DC,max The peak current ripple I p2p With energy storage voltage U ES The peak current ripple I p2p It is zero at duty cycle 1 / 3 and 2 / 3. Let us assume that the desired current ripple is 40% or less of the maximum (peak) ripple value. The desired current fluctuation is given by Fig. 9 The threshold I p2p,th Indicates. At a fixed DC link voltage U DC (e.g. 707V), the energy storage voltage U at and below and above the zero duty cycle 1 / 3ES (210V...260V) within about 50V, and the energy storage voltage U at and below and above the zero duty cycle 2 / 3 ES Within the range of about 50V (450V...500V), the expected current ripple will be achieved. At zero duty cycle 1 / 3, only the energy storage voltage U ES Zero current ripple can be achieved at a value of (about 230V), and at the zero duty cycle of 2 / 3, only the energy storage voltage U ES At another value of (about 465V), zero current ripple can be achieved. On the other hand, by ES Adjustable DC link voltage U in the range from 707V to 799V DC , and thereby adjust the duty cycle, the energy storage voltage U of the desired current ripple can be achieved ES The range will extend to about 80V (210V...290V) at and around the duty cycle 1 / 3, and to about 115V (450V...565V) at and around the duty cycle 2 / 3. In these ranges, at the zero point duty cycle 1 / 3, for the energy storage voltage U ES The energy storage voltage U ES The duty cycle U ES / U DC The optimal value of energy storage voltage U ES The minimum current ripple of the DC link voltage U DC In this paper, the optimal value of the DC link voltage is called U DC,opt .

[0082] refer to Fig.10 and Fig.11 , an exemplary current ripple control process may be performed as follows. Measuring or otherwise determining the energy storage voltage U ES The current value of ( Fig.11 Optionally, the DC link voltage U is also measured or otherwise determined. DC and / or the current ripple I p2p In an embodiment, the DC-DC converter 10 may be configured to perform these measurements and provide a measurement value U ES.meas , and optional U p2p.meas and / or U DC.meas . For energy storage voltage U ES The current value of the DC link voltage U is calculated or otherwise determined and selected. DC,opt(Step 104). In an embodiment, these calculations and selections may be performed in a current ripple control unit 110. The control unit 110 may be implemented in a dedicated controller device, or it may be part of a controller in one of the converters 6, 8, 10 or 16, or in any other higher level controller in a particular electrical system (such as a motor control), or the control unit 110 may be distributed in one or more controllers. DC Adjust to the optimal value U DC,opt (Step 116). Preferably, for the energy storage voltage U ES This process is repeated for each new measured value of . Typically, the DC link voltage U DC The voltage U can be provided by any adjustable voltage source connected to the DC link 2. In an embodiment, one of the converters 6, 8, 10 and 16 connected to the DC link 2 is controllable to adjust the DC link voltage U DC Adjust to the optimal value U DC,opt Typically, the measurement of the current DC link voltage U DC.meas The current ripple control 110 may be configured to set the optimized voltage value U DC,opt The adjustable voltage source or converter is provided as a reference or required voltage value to which the DC link voltage should be regulated. In an embodiment, the AC-DC converter 6 connected to the grid 4 is controlled to regulate the DC link voltage U DC Examples of suitable AC-DC converters include the IGBT power supply units (ISUs) ACS800 and ACS880 manufactured by ABB plc. If the DC link voltage band needs to be swapped, this may need to be done actively in the supply unit, for example by discharging the DC link 2 to the grid 4 for a short period of time to achieve a rapid voltage change.

[0083] refer to Fig.12 Another exemplary embodiment of current ripple control for a three-phase interleaved DC-DC converter is described. In this exemplary embodiment, for the energy storage voltage U ES The current ripple can be controlled to zero or close to zero over the entire range of values. The topology of the DC-DC controller 10 can be as follows: Figure 3 The energy storage device may be a battery or a supercapacitor 12, such as Figure 1C or Figure 1D Energy storage device in the electrical system configuration shown. For example, the energy storage voltage U ES (e.g., battery or supercapacitor voltage) can vary in the range of 0 V to 800 V. In this example, the DC link voltage U DCIt can be provided by an AC-DC converter (e.g., a rectifier, an active front end rectifier, or an IGBT power supply unit) 6 coupled to the AC voltage grid and capable of regulating the DC link voltage. It should be understood that the AC voltage level U AC Not relevant to the present invention but can be freely varied according to each specific application. The DC link voltage U provided by the AC-DC converter DC The minimum value of For example, for a grid voltage of 230VAC, the minimum DC link voltage U DC,min will be approximately 324V, and for a grid voltage of 500VAC, the minimum DC link voltage U DC,min The AC-DC converter can convert the desired DC link voltage value U DC For example, when the grid voltage is 230V, the DC link voltage U DC Can be boosted to higher than U DC,min = any voltage of 324 V. Therefore, the DC link voltage U DC At the minimum value U DC,min To the maximum value U DC,max Adjustable within the range. Fig.12 The peak current ripple I p2p With energy storage voltage U ES Line I p2p,750V shows a fixed DC link voltage U DC =750V, i.e., without dynamic ripple control. The dashed line shows the current ripple at a fixed DC link voltage U DC =750V. Line I p2p,opt shows the optimized DC link voltage U under dynamic control DC The current ripple at the time, that is, with dynamic ripple control. DC,opt shows the optimal DC link voltage U DC,opt , the voltage is adjusted dynamically, so the duty cycle U ES / U DC The energy storage voltage U ES Other parameters in this example may be L = 300uH, f s =4kHz, I p =200A. Fig.12 A general example is shown in which the DC link voltage U is optimized DC,opt The regulation range is shown starting from 0V. In practice, the optimal DC link voltage U DC,opt The regulation range can be from the minimum DC link voltage U available in the specific application DC,min Start, for example, from 324V or 707V.

[0084] In an embodiment, the current ripple control can be achieved by, for example, referring to Fig.10 and Fig.11 The described process and control device are implemented.

[0085] In an exemplary embodiment, Fig.12 The optimal value of the DC link voltage U in the example DC,opt The calculation and selection of can be as follows. Preferably, the optimized value of the DC link voltage U DC,opt .

[0086] The optimum value of the DC link voltage for a duty cycle of 1 / 3 can be calculated as

[0087] U DC，(1 / 3) =U ES / (1-(1 / 3)) (5).

[0088] The optimum value of the DC link voltage for a duty cycle of 2 / 3 can be calculated as

[0089] U DC，(2 / 3) =U ES / (1-(2 / 3)) (6).

[0090] The optimum value of the DC link voltage for a duty cycle close to 1 or less can be calculated as

[0091] U DC，(＜1) =U ES +5V (7).

[0092] Duty cycle 1 is not used because no power is transferred at this duty cycle. Now, among these three calculated optimal values ​​of the DC link voltage, one optimal value can be selected as the optimal value of the DC link voltage U DC,opt , as shown below

[0093] U DC，opt =IF(MIN(U DC，(1 / 3) , U DC，(2 / 3) )>900,U DC，(＜1) , IF(U DC，(2 / 3) >900), U DC，(1 / 3) , U DC，(2 / 3) )) (8).

[0094] In other words, if the DC link voltage value U is optimized DC,(1 / 3) and U DC,(2 / 3) If both are greater than 900V, select U DC,(<1) As the optimal value of DC link voltage U DC,opt , that is U DC,opt =U DC,(<1) This applies to Fig.12Energy storage voltage U above 600V ES If U DC,(2 / 3) Greater than 900V, but U DC,(1 / 3) If not, select U DC,(1 / 3) As the optimal value of DC link voltage U DC,opt , that is U DC,opt =U DC,(1 / 3) This applies to Fig.12 Energy storage voltage U equal to or less than 300V ES Finally, if the DC link voltage value U is optimized DC,(1 / 3) and U DC,(2 / 3) If both are less than 900V, select U DC,(2 / 3) As the optimal value of DC link voltage U DC,opt , that is U DC,opt =U DC,(2 / 3) This applies to Fig.12 Energy storage voltage U above 300V and up to 600V ES This choice also ensures that the highest possible DC link voltage U is always used. DC To transfer as much power as possible.

[0095] refer to Fig.13 Another exemplary embodiment of current ripple control for a single-phase DC-DC converter is described. The topology of the DC-DC controller 10 may be as follows: Figure 2 The energy storage device may be a battery or a supercapacitor 12, such as Figure 1C or Figure 1D Energy storage device in the electrical system configuration shown. For example, the energy storage voltage U ES (e.g., battery or supercapacitor voltage) can vary in the range from 0 V to 750 V. In this example, the DC link voltage U DC It can be provided by an AC-DC converter (eg, rectifier) ​​6 coupled to a 500VAC voltage grid. The DC link voltage U DC It can be obtained from the minimum value U DC,min =707V to maximum value U DC,max = Adjustable within the range of 900V. Fig.13 The peak current ripple I p2p With energy storage voltage U ES The dotted line I p2p,900V shows a fixed DC link voltage U DC = Current ripple at 900V, i.e., without dynamic ripple control. Dashed line I p2p,707V shows a fixed DC link voltage U DC = 707V, i.e., without dynamic ripple control.p2p,th shows that the maximum allowable peak current ripple I p2p The control threshold or limit 145A. Solid line I p2p,opt shows the optimal DC link voltage U for dynamic control DC The current ripple at this time, that is, with dynamic ripple control. Other parameters in this example can be L = 300uH, f s =4kHz. Current ripple control mainly chooses to optimize the DC link voltage U DC 900V, for energy storage voltage value U ES , the resulting peak current ripple I p2p,900V below the limit 145 A. This applies to energy storage voltage values ​​U from 0 V to approximately 230 V and from approximately 680 V to 750 V ES , where the current ripple follows Fig.13 The dotted line I p2p,900V . For the peak current ripple I p2p,900V The energy storage voltage value U that exceeds the limit of 145A ES , current ripple control will optimize the DC link voltage U DC Change to 707V so that the current ripple follows Fig.13 The dotted line I p2p,707V This applies to energy storage voltage values ​​U from approximately 230 V to approximately 680 V. ES Therefore, the current ripple I p2p The energy storage voltage U ES Keep below the limit of 145A.

[0096] In an embodiment, one or more phases of the multi-phase interleaved DC-DC converter can be selectively turned off to store energy within the energy storage voltage range U ES The zero point of the current ripple is obtained at the desired point of the energy storage voltage U, preferably, so that the phase shift between the remaining phases is 180 degrees or 90 degrees, and the energy storage voltage U ES The zero point of the current ripple exists at the 50% point of the energy storage voltage U ES The current ripple can also be controlled at or near a desired point (such as the 50% point) within the range of Fig. 9 As shown, the three-phase interleaved DC-DC converter has an energy storage voltage U ES There is no zero point of the current ripple at the midpoint of . Therefore, the control of the current ripple cannot be achieved near this midpoint. On the other hand, when one phase of the three-phase interleaved DC-DC converter is selectively turned off, we will effectively make the two-phase interleaved DC-DC converter have a 180-degree phase shift between phases and the energy storage voltage U ESThe midpoint of the current ripple has a zero point. Therefore, the current ripple can also be controlled at and near the midpoint voltage. Of course, one or more phases can only be turned off when the load current is low enough so that the remaining phases are not overloaded.

[0097] The current ripple control techniques described herein may be implemented in various ways. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or a combination thereof. For firmware or software, implementation may be performed by modules (e.g., procedures, functions, etc.) that perform the functions described herein. The software code may be stored in any suitable processor / (multiple) computer-readable data storage medium or memory unit and executed by one or more processors / computers. The data storage medium or memory unit may be implemented within a processor / computer or external to a processor / computer, in which case it may be communicatively coupled to the processor / computer via various means known in the art. In addition, the components of the system described herein may be rearranged and / or supplemented by additional components to facilitate the implementation of the various aspects, objectives, advantages, etc. described therewith, and are not limited to the precise configurations listed in a given figure, as understood by those skilled in the art.

[0098] The description and related drawings are intended only to illustrate the principles of the present invention by way of example. Based on this description, various alternative embodiments, modifications and variations will be apparent to those skilled in the art. The present invention is not intended to be limited to the examples described herein, but the present invention may vary within the scope and spirit of the appended claims.

Claims

1. A method comprising: Controlling the current ripple caused by a DC-DC converter coupled between a DC link having a DC link voltage U and an energy storage device DC , and the energy storage device has a DC energy storage voltage U ES , wherein the control comprises Dynamically adjust the DC link voltage U DC The ratio U ES / U DC Controlled to an optimal value or range of values ​​that results in a desired current ripple level at the energy storage device.

2. The method according to claim 1, wherein the dynamically adjusting comprises: Determine the energy storage voltage U ES The voltage or voltage range, Determine the DC link voltage U DC The optimal value or value range of the DC link voltage U DC The optimal value or value range of the energy storage voltage U ES The ratio U is given by the determined voltage or voltage range ES / U DC the optimal value or range of values ​​of , and The DC link voltage U DC is adjusted to the optimal value or value range to adjust the energy storage voltage U ES The determined voltage or voltage range obtains the desired current ripple level. 3 . The method according to claim 1 , wherein the desired current ripple is equal to or less than a predetermined target ripple or within a predetermined current ripple range, preferably zero or close to zero.

4. The method of claim 1, wherein the determining comprises: If the DC link voltage U DC If there are multiple possible optimal values ​​or value ranges, the DC link voltage U is selected DC The highest possible optimal value or range of values ​​of to enable as much power as possible to be transferred.

5. The method according to claim 1, comprising: DC The DC link is discharged before changing from one optimal value or value range to another optimal value or value range.

6. The method of claim 1 , wherein the DC-DC converter is an interleaved multiphase DC-DC converter, and wherein the method comprises dynamically adjusting the DC link voltage U DC To convert the ratio U ES / U DC Controlled to a value or value range corresponding to one of the zero points of the current ripple.

7. The method of claim 1, wherein the DC-DC converter is an interleaved three-phase DC-DC converter, and wherein the method comprises regulating the DC link voltage to reduce the ratio U ES / U DC Controlled to be approximately or close to a value or value range of one of 1 / 3, 2 / 3 and 1.

8. The method of claim 1, wherein the DC-DC converter is an interleaved six-phase DC-DC converter, and wherein the method includes regulating the DC link voltage to reduce the ratio U ES / U DC Controlled to be approximately or close to a value or value range of one of 1 / 6, 1 / 3, 1 / 2, 2 / 3, 5 / 6, 1.

9. The method according to claim 1, wherein the DC-DC converter is an interleaved multiphase DC-DC converter, preferably and wherein the method comprises: One or more phases of the multi-phase interleaved DC-DC converter are selectively shut down to obtain a zero point of the current ripple at a desired point in the energy storage voltage range, preferably so that the phase shift between the remaining phases is 180 degrees or 90 degrees and there is a zero point of the current ripple at the midpoint of the energy storage voltage range.

10. The method of claim 1, wherein the DC-DC converter is a single-phase DC-DC converter, and the method comprises setting a limit value for the current ripple level of the energy storage device, Determine the current ripple level, The DC link voltage U DC The primary adjustment is a first predetermined value, and If the current ripple level exceeds the limit value, the DC link voltage U DC The level of the ratio U is adjusted from a first predetermined value to a second predetermined value, wherein the second predetermined value is smaller than the first predetermined value, and the second predetermined value reduces the ratio U ES / U DC The control is to limit the present current ripple level to a value or a value range that is a set limit value or smaller.

11. The method of claim 1 , wherein the energy storage device comprises one or more of a rechargeable energy storage device, a hydrogen electrolyzer, and a fuel cell.

12. The method of claim 1, wherein the DC-DC converter is bidirectional.

13. An electrical system comprising: A DC link having an adjustable DC link voltage U DC , An energy storage device having a DC energy storage voltage U ES , A DC-DC converter coupled between the DC link and the energy storage device, A regulating component for regulating the DC link voltage U DC ,as well as A control component configured to control the electrical system to perform the method according to any one of claims 1 to 12.

14. The electrical system of claim 13, comprising at least one further converter coupled to the DC link, the DC link being coupled to a power source or a load, wherein the at least one further converter comprises: One or more of an AC-DC converter connected to an AC power source or an AC load, a DC-AC converter connected to an AC power source or an AC load, and an additional DC-DC converter connected to a DC power source or a DC load.

15. The electrical system of claim 14, wherein the AC power source comprises one or more of an AC grid and a wind power plant.

16. The electrical system of claim 14, wherein the DC power source comprises one or more of a battery, a fuel cell, and a solar power plant.

17. The electrical system of claim 14, wherein the load comprises an AC motor or a DC motor.

18. An electrical system according to claim 13 or 14, wherein the regulating component comprises an adjustable voltage source.

19. An electrical system according to claim 13 or 14, wherein the regulating component comprises the DC-DC converter or the at least one further converter.

20. The electrical system of claim 13 or 14, wherein the control component comprises a separate controller unit, a controller of the DC-DC converter or the at least one further converter, or a higher level controller in the electrical system, or any combination thereof.

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