Energy management method suitable for hybrid power supply system of multi-electric aircraft

Abstract

The invention provides an energy management method suitable for a hybrid power supply system of a multi-electric aircraft, and belongs to the technical field of multi-energy-storage direct-current micro-grids. The problem that an existing distributed energy management strategy cannot effectively process optimal distribution of load power among three power supplies with different characteristics, the design problem of control parameters, the charge state adjustment problem of an energy storage device and the feedback problem of load renewable energy are solved. The method comprises the steps that 1, a multi-electric aircraft hybrid power supply system is built; 2, according to the output characteristics of the power supplies, the distribution relation of load currents among the power supplies is set, and a distributed control strategy corresponding to each power supply branch is designed; and step 3, based on a power supply parameter design criterion, designing a virtual impedance parameter and an inner loop control parameter in combination with the distribution relation of the load current among the power supplies, and ensuring stable operation of the hybrid power supply system of the more-electric aircraft.

Description

Technical Field

[0001] The invention relates to an energy management method suitable for a hybrid power supply system of a multi-electric aircraft, and belongs to the technical field of multi-energy storage DC microgrids. Background Art

[0002] Nowadays, the development of the power system of more-electric aircraft shows an overall trend of lightweight, high efficiency, economy and greenness. Among them, the DC system has the advantages of high transmission efficiency, high power density and high reliability, and has been more and more widely used. With the continuous growth of actual demand, electrical loads are becoming more and more complex. In addition to conventional resistive loads and constant power loads, there are pulse loads such as radar. Pulse loads show the characteristics of high peak-to-average ratio, with strong randomness, volatility and impact. The power system of more-electric aircraft has the characteristics of low inertia and small capacity. The pulse load effect can easily induce the problem of DC bus voltage fluctuation. Therefore, the power supply system of more-electric aircraft should have the ability to provide and absorb instantaneous pulsating power.

[0003] The generator has slow dynamic response and poor climbing ability, making it difficult to meet the instantaneous power demand of pulse loads. The pulsation power smoothing technology based on the hybrid power supply system (HPSS) is the mainstream method for reducing fuel consumption, weight and fluctuations in more-electric aircraft. In order to achieve the control objectives of autonomous power distribution of strong pulsating loads in a wide frequency domain, stabilization of bus voltage under pulse loads, green environmental protection, and improved economic benefits, it is necessary to design an energy management strategy for the DC power supply and distribution system of more-electric aircraft with high-power pulse loads.

[0004] At present, the research on energy management strategies of hybrid power supply systems at home and abroad can be divided into two categories: centralized control and decentralized control. Most of the existing studies adopt centralized control strategies based on central control units and communication networks, including fuzzy control, model predictive control, and artificial neural networks and other intelligent control methods. However, centralized control has problems such as single point failure, communication delay, high cost, and poor scalability, which gradually makes it difficult to meet the development requirements of easy scalability, strong fault tolerance, and high robustness of the power supply end of more-electric aircraft.

[0005] For distributed control, most of the current research at home and abroad focuses on the improvement of traditional droop control, and is mostly applied to wind, solar, storage, DC microgrids and electric vehicle power supply systems. There are few research cases in multi-electric aircraft power systems. In addition, most of the research objects are based on two power supply units with different characteristics, and there are relatively few studies on the optimal allocation of load power among three power supply units with different characteristics. In addition, there is still room for in-depth research in the design of control parameters, the state of charge adjustment of energy storage devices, life cost issues, and the feedback of load renewable energy. Summary of the invention

[0006] The present invention aims to solve the problems that the existing distributed energy management strategies cannot effectively handle the optimal distribution of load power among three power supplies with different characteristics, the design of control parameters, the charge state adjustment of energy storage devices and the feedback of load renewable energy, and further proposes an energy management method suitable for a hybrid power supply system of a multi-electric aircraft.

[0007] The technical solution adopted by the present invention to solve the above problems is: the steps of the present invention include:

[0008] Step 1: Build a hybrid power supply system for a multi-electric aircraft;

[0009] The hybrid power supply system of the more-electric aircraft includes power supply, hybrid energy storage system, power electronic converter, high-voltage DC bus and load-related models;

[0010] Step 2: Set the distribution relationship of the load current between the power supplies according to the output characteristics of the power supply and design the distributed control strategy corresponding to each power supply branch;

[0011] The distributed control strategy of power supply includes the design of dynamic and efficient power allocation strategy of each power supply and the charge state recovery strategy of hybrid energy storage system;

[0012] Step 3: Based on the power supply parameter design criteria and combined with the load current distribution relationship among the power supplies, the virtual impedance parameters and inner loop control parameters are designed to ensure the stable operation of the hybrid power supply system of the more-electric aircraft.

[0013] Preferably, the power supply in step 1 includes a fuel cell, a lithium battery and a supercapacitor, the lithium battery and the supercapacitor constitute a hybrid energy storage system, the fuel cell, the lithium battery and the supercapacitor are connected to the DC bus through corresponding converters, and the load side includes a resistive load, a constant power load and a pulse load.

[0014] Preferably, step 2 specifically includes:

[0015] Step 2.1: Based on the output characteristics of fuel cells, lithium batteries and super capacitors and the principle of traditional virtual resistance control, a hybrid droop control method is extended and expanded, wherein the hybrid droop control method includes virtual inductance droop control for fuel cells, virtual resistance droop control for lithium batteries and virtual capacitance droop control for super capacitors;

[0016] Step 2.2: Under ideal conditions, the droop characteristic equations of the fuel cell, lithium battery and supercapacitor converter are calculated based on the hybrid droop method;

[0017] Step 2.3: Based on the droop characteristic equations of the fuel cell, lithium battery and supercapacitor converter, the distribution relationship of the load current among the fuel cell, lithium battery and supercapacitor is calculated;

[0018] Step 2.4: Based on the hybrid droop control method, the center frequency f0 and bandwidth Δf of the bandpass filter are reasonably designed in combination with engineering practice, the load current is automatically decoupled into low-frequency, medium-frequency and high-frequency parts, and the low-frequency part of the load current is allocated to the fuel cell, the medium-frequency part is allocated to the lithium battery, and the high-frequency part is allocated to the supercapacitor, completing the design of the power efficient allocation strategy of the power supply;

[0019] Step 2.5: After any power source among the fuel cell, the lithium battery and the supercapacitor is disconnected from the hybrid power supply system of the more-electric aircraft due to a fault, the remaining two power sources are controlled based on the power effective allocation strategy of the power sources to ensure that effective power division can still be achieved after any power source fails to exit the hybrid power supply system of the more-electric aircraft due to a fault;

[0020] Step 2.6: Set the hybrid energy storage system charge state recovery strategy;

[0021] The droop characteristic equation of the fuel cell is expressed as:

[0022] V ofc =V ref -L νfc s.i ofc (1);

[0023] In formula (1), V ofc is the output voltage of the fuel cell converter, V ref is the reference value of bus voltage, L νfc is the virtual inductance, i ofc is the output current of the fuel cell, s is a complex frequency domain variable;

[0024] The droop characteristic equation of lithium battery is expressed as:

[0025] V ob =V ref -R νb ·i ob (2);

[0026] In formula (2), V ob is the output voltage of the converter connected to the lithium battery, R νb is the virtual resistance, i ob is the output current of the lithium battery;

[0027] The droop characteristic equation of the supercapacitor is expressed as:

[0028]

[0029] In formula (3), V osc is the output voltage of the converter connected to the supercapacitor, Cνsc is the virtual capacitor, i osc is the output current of the supercapacitor;

[0030] The distribution relationship of the generator load current among the fuel cell, lithium battery and supercapacitor is expressed as:

[0031]

[0032] In formula (4), G FC (s) is a second-order low-pass filter, G B (s) is a second-order bandpass filter, G SC (s) is a second-order high-pass filter, i o is the load side current.

[0033] Preferably, step 2.5 specifically includes:

[0034] Step 2.5.1: When the fuel cell is disconnected from the hybrid power supply system of the multi-electric aircraft due to a system failure, the power distribution strategy of the power supply is used to control the lithium battery and supercapacitor to bear the load pulse power, and the new transfer function G corresponding to the lithium battery is calculated. B1 (s), after the fuel cell is disconnected, the low-frequency power required by the fuel cell is transferred to the lithium battery, and the supercapacitor still bears the high-frequency power according to the original cut-off frequency of the power supply. The lithium battery and the supercapacitor respectively act as a new first-order low-pass filter and a first-order high-pass filter to achieve power division;

[0035] Step 2.5.2: When the supercapacitor is disconnected from the hybrid power supply system of the multi-electric aircraft due to a system failure, the fuel cell and lithium battery are controlled to bear the load pulse power through the power supply power effective allocation strategy, and the new transfer function G corresponding to the fuel cell is calculated. FC2 (s), after the supercapacitor is disconnected, the boundary frequency of the fuel cell's load power distribution increases, providing more low-frequency power. The high-frequency power borne by the supercapacitor is transferred to the lithium battery, and the load feedback current is all absorbed by the lithium battery. The fuel cell and the lithium battery act as a new first-order low-pass filter and a new first-order high-pass filter respectively to achieve power division;

[0036] Step 2.5.3: When the lithium battery is disconnected from the hybrid power supply system of the multi-electric aircraft due to a system failure, the power distribution strategy of the power supply is improved by connecting a virtual impedance R in parallel to the branch of the supercapacitor. vsc , calculate the new transfer function G corresponding to the fuel cell FC3(s), after the lithium battery is disconnected, the boundary frequency of the fuel cell's load power distribution rises, providing more low-frequency power. The intermediate-frequency power borne by the lithium battery is transferred to the supercapacitor. Combined with the parallel virtual resistance branch, the supercapacitor absorbs all feedback current. The fuel cell and supercapacitor respectively act as a new second-order low-pass filter and a new second-order high-pass filter to achieve power division;

[0037] The new transfer function G B1 The expression of (s) is:

[0038]

[0039] In formula (5), the new transfer function G B1 (s) The standard form of a first-order low-pass filter is shown, with a cut-off frequency equal to the passband bandwidth Δf;

[0040] The new transfer function G FC2 The expression of (s) is:

[0041]

[0042] In formula (6), the fuel cell corresponds to the transfer function G FC2 (s) presents the standard form of a first-order low-pass filter, and the new cutoff frequency is

[0043] The new transfer function G FC3 The expression of (s) is:

[0044]

[0045] In formula (7), the new transfer function G FC3 (s) presents the standard form of a second-order low-pass filter, with the corresponding cutoff frequency being the center frequency f0.

[0046] Preferably, step 2.6 specifically includes:

[0047] Step 2.6.1: Calculate the state of charge of the supercapacitor based on the capacitance characteristics of the supercapacitor, and use indirect means to control the supercapacitor V sc To adjust the state of charge of the supercapacitor;

[0048] Step 2.6.2: Set a supercapacitor charge and discharge switching point SOC based on actual project needs sw As the input of the mode selector based on hysteresis control, a feed-forward voltage compensator is applied at the front end of the supercapacitor droop control branch, combined with the mode selector based on hysteresis control, to realize the hierarchical SOC recovery control of the supercapacitor, and the compensation V output by the feed-forward voltage compensator is controlled. rec Realize hierarchical management function;

[0049] The expression of the state of charge of a supercapacitor is:

[0050] ΔQ=∫i sc dt=C sc ΔV sc (8);

[0051] In formula (8), ΔQ is the charge released by the supercapacitor, ΔV is sc is the bias value between the supercapacitor terminal voltage and the voltage reference;

[0052] Compensation V rec The expression is:

[0053]

[0054] In formula (9), V rec is the compensation voltage corresponding to the feedforward compensator, k prec and k irec are the PI controller parameters of the voltage compensation loop.

[0055] Preferably, step 2.6 specifically includes:

[0056] Step 2.6.1: Calculate the state of charge of the supercapacitor based on the capacitance characteristics of the supercapacitor, and use indirect means to control the supercapacitor V sc To adjust the state of charge of the supercapacitor;

[0057] Step 2.6.2: Set a supercapacitor charge and discharge switching point SOC based on actual project needs sw As the input of the mode selector based on hysteresis control, a feed-forward voltage compensator is applied at the front end of the supercapacitor droop control branch, combined with the mode selector based on hysteresis control, to realize the hierarchical SOC recovery control of the supercapacitor, and the compensation V output by the feed-forward voltage compensator is controlled. rec Realize hierarchical management function;

[0058] The expression of the state of charge of a supercapacitor is:

[0059] ΔQ=∫i sc dt=C sc ΔV sc (8);

[0060] In formula (8), ΔQ is the charge released by the supercapacitor, ΔV is sc is the bias value between the supercapacitor terminal voltage and the voltage reference;

[0061] Compensation V rec The expression is:

[0062]

[0063] In formula (9), V rec is the compensation voltage corresponding to the feedforward compensator, k prec and k irec are the PI controller parameters of the voltage compensation loop.

[0064] Preferably, step 3 specifically includes:

[0065] Based on the transfer function, the virtual impedance parameter is related to the cutoff frequency ω c , the algebraic relationship of the damping ratio is transformed into the algebraic relationship with the center frequency f0 and the bandwidth Δf;

[0066] Combining the algebraic relationship between the virtual impedance parameter and the center frequency f0 and the bandwidth Δf and the distribution relationship between the generator load current of the fuel cell, the lithium battery and the supercapacitor, the relationship expression between the virtual impedance parameter and the center frequency and the bandwidth is calculated;

[0067] The virtual resistance R is calculated according to the actual engineering needs. vb The virtual resistor R vb Substitute the parameters into the relationship between the virtual impedance parameters and the center frequency and bandwidth to obtain the virtual inductance L vfc 、Virtual capacitor C vsc Parameters to complete the setting of virtual RC parameters;

[0068] The relationship between the virtual impedance parameter and the center frequency and bandwidth is expressed as:

[0069]

[0070] The beneficial effects of the present invention are:

[0071] The present invention proposes an energy management method suitable for a hybrid power supply system of a multi-electric aircraft, which realizes the dynamic and autonomous optimization distribution of wide-band strong pulsating load power among fuel cells, lithium batteries and supercapacitors, and the stabilization of bus voltage under pulse loads, without the need for a central controller and a communication network. In addition, the hybrid power supply system under the proposed control strategy can simultaneously realize the efficient recovery of load renewable energy, the adjustment of the charge state of the energy storage device, and the stable operation of the system under partial failure of the power supply, and has the performance advantages of high energy utilization, high durability, high flexibility and high robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 A system architecture diagram of a hybrid power supply system for a multi-electric aircraft provided by the present invention;

[0073] Figure 2A flow chart of an energy management method applicable to a hybrid power supply system of a more-electric aircraft provided by the present invention;

[0074] Figure 3 A schematic diagram of load characteristics of a high-power radar pulse load provided by the present invention;

[0075] Figure 4 A simplified equivalent circuit topology diagram based on virtual RC droop control provided by the present invention;

[0076] Figure 5 A block diagram of a virtual capacitor droop control including a feed-forward voltage compensator and a mode selector provided by the present invention;

[0077] Figure 6 A supercapacitor charge state recovery control flow chart provided by the present invention;

[0078] Figure 7 A schematic diagram of the power demand of the load provided by the present invention at each flight stage;

[0079] Figure 8 A schematic diagram of the DC bus voltage waveform and the output power of each power supply when the load is stable provided by the present invention;

[0080] Fig. 9 A schematic diagram of the power demand of the load at each flight stage when the pulse load provided by the present invention is working;

[0081] Fig.10 A schematic diagram of the DC bus voltage waveform and the output power of each power supply when the pulse load provided by the present invention is working;

[0082] Fig.11 A schematic diagram of the DC bus voltage waveform and the output power of each power supply after the fuel cell provided by the present invention is disconnected from the system due to a fault;

[0083] Fig.12 A schematic diagram of the DC bus voltage waveform and the output power of each power supply after the supercapacitor provided by the present invention is disconnected from the system due to a fault;

[0084] Fig.13 A schematic diagram of the DC bus voltage waveform and the output power of each power supply after the lithium battery provided by the present invention is disconnected from the system due to a fault;

[0085] Fig.14 A schematic diagram of the output power of each power supply before and after the introduction of the supercapacitor charge state recovery strategy provided by the present invention. DETAILED DESCRIPTION

[0086] Combination Figure 1-14The present embodiment is described, and the present embodiment aims to realize efficient and reliable distribution of pulsating load power among various power supplies, such as Figure 2 As shown, the steps of an energy management method applicable to a hybrid power supply system of a more-electric aircraft described in this embodiment include:

[0087] S1: Build a hybrid power supply system for more-electric aircraft;

[0088] The hybrid power supply system of the multi-electric aircraft constructed in this embodiment is as follows Figure 1 As shown, the multi-source hybrid power supply system includes a fuel cell, a battery and a supercapacitor. The output ends are connected in parallel through a unidirectional DC-DC converter and a bidirectional DC-DC converter respectively. The control loop is composed of an internal control loop and an external control loop. The external loop adopts the designed virtual droop controller, and the internal loop adopts a voltage and current double closed loop.

[0089] According to the output characteristics, the loads of multi-electric aircraft are divided into three categories: resistive loads, constant power loads and pulse loads, all of which are connected to the DC bus. Among them, the load characteristics of high-power radar pulse loads are as follows: Figure 3 As shown in Figure 1, a task sequence of pulse load consists of several random pulses with different periods, different duty cycles and different peak powers. Pulse load is periodic, with high peak-to-average ratio and strong step characteristics. The bus voltage impact and instantaneous power supply problems caused by it cannot be ignored.

[0090] S2: setting the distribution relationship of the load current between the power supplies according to the output characteristics of the power supply and designing the distributed control strategy adopted by the power supply branch;

[0091] S201: Virtual inductance, virtual resistance and virtual capacitance droop control are used for fuel cells, lithium batteries and supercapacitors respectively to achieve high, medium and low frequency power distribution among energy storage units. The simplified equivalent circuit diagram based on virtual resistance, inductance and capacitance droop control is shown in the figure below: Figure 4 As shown, under the healthy working conditions of each power supply, the virtual switch T1 of the supercapacitor branch is disconnected.

[0092] According to Kirchhoff's current law, the relationship between the bus currents is:

[0093] i o =i ofc +i ob +i osc (1);

[0094] In formula (1), i o is the load side current; i ofc 、i ob 、i osc are the output currents of fuel cells, lithium batteries and supercapacitors respectively;

[0095] Since the virtual droop coefficient is much larger than the line impedance, the voltage drop on the line is negligible compared to the voltage drop caused by the droop control. Under the proposed hybrid droop control strategy, the output VI characteristics of the fuel cell, lithium battery and supercapacitor converter can be expressed as:

[0096]

[0097] In formula (2), V ofc 、V ob and V osc are the output voltages of the fuel cell, lithium battery and supercapacitor converter respectively; V ref is the reference value of bus voltage; L vfc , R vb and C vsc are virtual inductance, virtual resistance and virtual capacitance respectively, and s is a complex frequency domain variable.

[0098] Since each power supply is connected in parallel to the DC bus, ignoring the voltage drop on the line, it can be considered that the output voltage of the converter is equal to the bus voltage V bus :

[0099] V ofc =V ob =V osc =V bus (3);

[0100] The current distribution relationship between the fuel cell, lithium battery and supercapacitor can be obtained by combining formulas (1) to (3):

[0101]

[0102] The transfer function G in formula (4) FC (s), G B (s) and G SC (s) act as second-order low-pass, band-pass and high-pass filters respectively. By properly setting the upper and lower limit frequencies of the band-pass filter, the load power can be autonomously and dynamically distributed among the fuel cell, lithium battery and supercapacitor, where the low-frequency power is borne by the fuel cell, the medium-frequency part is borne by the lithium battery, and the high-frequency part is borne by the supercapacitor.

[0103] During the energy feedback process, the fuel cell cannot absorb the regenerative energy of the system due to the irreversibility of the chemical reaction, so it needs to be absorbed by the lithium battery and supercapacitor. Figure 4 The distribution relationship of load return current between lithium battery and supercapacitor power supply can be derived:

[0104]

[0105] According to formula (5), in the process of energy feedback, the supercapacitor absorbs the high-frequency load feedback current, and the lithium battery absorbs the low-frequency load feedback current. The transfer function G LPF (s) and G HPF (s) are first-order high-pass filters and first-order low-pass filters, respectively, and do not affect the service life of lithium batteries and supercapacitors. From the above analysis, it can be seen that in the energy feedback stage, the system can not only realize the recovery of renewable energy under the proposed control strategy, thereby improving the energy utilization efficiency of the system, but also take into account the output characteristics of lithium batteries and supercapacitors to realize power division recovery.

[0106] When the hybrid power supply system of a more-electric aircraft is in a partial failure state, that is, a certain power supply is disconnected from the system due to a failure, the proposed strategy can still achieve autonomous optimization of the pulsating load power distribution. The three fault states of fuel cell power supply, lithium battery power supply and supercapacitor power supply are analyzed respectively, including:

[0107] S20101: When the fuel cell power supply is disconnected from the system due to a fault, the load pulse power is borne by the lithium battery and supercapacitor. At this time, the new output VI characteristic can be expressed as:

[0108]

[0109] From formula (6), the new distribution relationship of load current between lithium battery and supercapacitor power supply can be derived as follows:

[0110]

[0111] From equation (7), we can see that after the fuel cell is disconnected, the new transfer function G B1 (s) presents a standard form of a first-order low-pass filter, with a cutoff frequency equal to the bandwidth. Therefore, after the fuel cell is disconnected, the low-frequency power required by the fuel cell is transferred to the lithium battery, and the supercapacitor still bears the high-frequency power according to the proposed strategy. Without changing the original normal operating power supply control strategy, the lithium battery and supercapacitor can be used as new low-pass filters and high-pass filters to achieve power division.

[0112] S20102: When the supercapacitor fails and exits the hybrid power supply system, the fuel cell and lithium battery bear the load pulse power. At this time, the new output VI characteristic can be expressed as:

[0113]

[0114] The new distribution relationship of load current between the fuel cell and lithium battery power supply can be derived as follows:

[0115]

[0116] The new transfer function G corresponding to the fuel cell power supply in formula (9) is FC2 (s) It can be seen that when the supercapacitor is disconnected due to failure, the corresponding transfer function of the fuel cell is G FC2 (s) presents the standard form of a first-order low-pass filter, and the new cutoff frequency is Therefore, after the supercapacitor is disconnected, the boundary frequency of the fuel cell's load power distribution increases slightly, and it will provide more low-frequency power. The lithium battery will bear the remaining high-frequency pulsating power of the load, and the load feedback current is all absorbed by the lithium battery.

[0117] S20103: When the lithium battery fails and exits the hybrid power supply system, the virtual switch T1 is immediately closed, and the supercapacitor branch is connected in parallel with the virtual impedance R vsc At this time, the output VI characteristics of the power supply system composed of fuel cells and supercapacitors are:

[0118]

[0119] From formula (10), the new distribution relationship of load current between the fuel cell and the supercapacitor power supply can be derived as follows:

[0120]

[0121] From formula (11), we can know that the new transfer function G FC3 (s) presents the standard form of a second-order low-pass filter, corresponding to a cutoff frequency of f0; therefore, after the lithium battery is disconnected, the boundary frequency of the fuel cell's load power distribution increases slightly, and it will provide more low-frequency power; the supercapacitor takes on the remaining high-frequency power, and thanks to the parallel virtual resistance branch, the supercapacitor can absorb all the feedback current.

[0122] S202: Limited by the size and weight of the more electric aircraft, the capacity of the energy storage device is greatly limited. In practical applications, the energy storage device inevitably has the situation where the remaining energy does not meet the load power demand or the capacity is full and can no longer absorb the return energy on the load side. In addition, the pulse load with high peak-to-average ratio, strong step and strong impact on the load side generates instantaneous high power demand during operation, which puts forward great requirements on the energy supply of the energy storage device.

[0123] For the DC microgrid of the multi-electric aircraft studied in this embodiment, the energy of the lithium battery is much higher than the energy capacity of the supercapacitor. In addition, when the multi-electric aircraft is in the steady-state operation stage, the low-frequency load power is mainly provided by the fuel cell. Therefore, in actual operation, the SOC change of the lithium battery is much smaller than the SOC change of the supercapacitor. Therefore, the present invention focuses on the supercapacitor in the state of charge recovery strategy.

[0124] In order to make the supercapacitor work in normal mode, this embodiment proposes an improved supercapacitor SOC recovery strategy. According to the capacitance characteristics of the supercapacitor, it can be obtained that:

[0125] ΔQ=∫i sc dt=C sc ΔV sc (12);

[0126] In formula (12), ΔQ is the charge released by the supercapacitor, ΔV sc is the bias value between the supercapacitor terminal voltage and the voltage reference, the supercapacitor terminal voltage V sc It is directly related to the state of charge, so it can be controlled by V sc Thereby achieving the purpose of controlling the charge state of the supercapacitor.

[0127] Under the premise that the power supply is in a non-power fluctuation compensation state, the working mode of the feedforward voltage compensator is controlled according to the current operating state of the supercapacitor; when the supercapacitor is in a normal working mode, the voltage compensator stops working. When the supercapacitor charge state deviates from the normal working range, the voltage compensator is applied on the basis of the virtual capacitor control loop. By controlling the compensation voltage V rec Realize the partition management function and compensate the voltage V rec Set to:

[0128]

[0129] In formula (13), V rec is the compensation voltage corresponding to the feedforward compensator; ΔV sc is the deviation between the supercapacitor terminal voltage and the reference voltage; k prec and k irec are the PI controller parameters of the voltage compensation loop.

[0130] The block diagram of virtual capacitor droop control with feedforward voltage compensator is as follows: Figure 5 When the power supply is in a non-power fluctuation compensation state, that is, the multi-electric aircraft is in the cruise mission stage and the pulse load is not working, the working mode of the feedforward voltage compensator is changed based on the supercapacitor charge state; when the supercapacitor SOC is within the normal working range, the feedforward compensator is in an idle state; when the supercapacitor SOC is lower than the threshold, the compensation voltage V rec is positive, HPSS provides power to charge the supercapacitor. Similarly, when the supercapacitor SOC is higher than the threshold, the compensation voltage V rec is negative, thereby controlling the supercapacitor to release an additional portion of electrical energy to enter the normal working mode.

[0131] In order to avoid the problem of supercapacitors switching back and forth between normal working mode and charge and discharge mode, this paper adopts a mode selector based on hysteresis control. sw (35%≤SOC sw ≤80%) to ensure the duration of the supercapacitor in the charge and discharge mode. Through the cooperation of the feedforward voltage compensator and the mode selector based on hysteresis control, the hierarchical adjustment of the supercapacitor SOC can be achieved. The control flow chart is as follows: Figure 6 shown.

[0132] S3: Based on the power supply parameter design criteria, the virtual impedance parameters and inner loop control parameters are designed in combination with the load current distribution relationship among the power supplies to ensure the stable operation of the hybrid power supply system of the more-electric aircraft.

[0133] S301: Considering the actual situation of the power supply and distribution system of the more-electric aircraft, the corresponding design criteria are proposed:

[0134] S30101: Considering that the pulse load has a high-frequency power demand of 50-500ms under multiple time series, and the peak-to-average power ratio is greater than 5:1, the cutoff frequency of the hybrid power supply system of the more-electric aircraft is selected as 2π×0.2rad / s;

[0135] S30102: To achieve accurate power sharing, this implementation mode designs parameters on the basis of ignoring line impedance. Therefore, the virtual impedance parameter needs to be much larger than the line impedance parameter to eliminate its influence on power sharing. The design is:

[0136] R vb >>10r b (14);

[0137] S30103: When the lithium battery fails due to a fault, the supercapacitor needs to bear a certain amount of low-frequency power in addition to the high-frequency pulse load power. In order to ensure that the low-frequency power is mainly provided by the fuel cell, the following conditions must be met when designing the supercapacitor branch parallel virtual resistance parameters:

[0138] R vsc ≥5R vb (15);

[0139] S30104: The DC bus voltage must meet the aircraft electrical standard MIL-STD-704F, which requires it to be limited to V busmin =250V and V busmax =280V.

[0140] S302: Virtual RC parameter design:

[0141] The transfer function G corresponding to the fuel cell, lithium battery and supercapacitor FC (s), G B (s) and G SC (s) Rewrite in standard form:

[0142]

[0143] According to formula (16), the natural frequency ω corresponding to the transfer function can be obtained c And the relationship between the damping ratio ζ and the virtual resistance, inductance and capacitance:

[0144]

[0145] Transfer function G B (s) presents the standard form of a second-order bandpass filter, so the virtual impedance parameter can be compared with the cutoff frequency ω c The algebraic relationship between the damping ratio and the center frequency f0 is transformed into the algebraic relationship between the center frequency f0 and the bandwidth Δf.

[0146]

[0147] Combining formula (18) we can get the relationship between the virtual impedance parameter, the center frequency f0 and the bandwidth Δf:

[0148]

[0149] Among them, the virtual resistance value corresponding to the lithium battery is given by the following formula:

[0150]

[0151] In formula (20), V dev is the deviation value of bus voltage, I obe is the rated current of the lithium battery.

[0152] According to the rated power P of the load e and bus reference voltage V ref , lithium battery rated output current value I obe The calculation is as follows:

[0153]

[0154] According to the electrical standards of multi-electric aircraft, the deviation of bus voltage does not exceed 10%. Substituting the system parameters into the virtual resistance R vb Based on the virtual resistor R vb Substituting the parameters into equation (19) can solve the parameters of virtual capacitor and virtual inductor.

[0155] The specific parameters of the DC power supply and distribution system of the multi-electric aircraft in this embodiment are as follows: Load rated power Pe is 100kW; DC bus reference voltage V ref The voltage of the fuel cell terminal at the power supply end is 270V; FC is 150V, the lithium battery terminal voltage V B is 150V, the supercapacitor terminal voltage V SC is 200V, the supercapacitor capacitance is C SC is 18F; the converter inductance value is 60μH, the output capacitance value is 1880μF, and the switching frequency f s is 20kHz; the lower limit frequency f of the bandpass filter L is 0.05Hz, the upper limit frequency f H is 1Hz.

[0156] Combined with formula (20), the virtual resistance R can be calculated vb The parameters are:

[0157]

[0158] The virtual resistor R vb Substituting the parameters into formula (19), we can obtain the parameters of virtual capacitance and virtual inductance, which are 4.59F and 0.02H respectively. Based on engineering experience, the supercapacitor branch is connected in parallel with a virtual resistor R vsc Take 0.007Ω.

[0159] Specific implementation method 2: Combination Figure 7-14 This implementation is explained. To verify the technical effect of the first implementation, this implementation uses Matlab / Simulink software for simulation verification. In order to simulate the actual working conditions of the multi-electric aircraft as much as possible, the power demand on the load side is set according to the five mission stages of takeoff, climb, cruise, initial descent and landing, and disturbances are added to verify the rapid response capability of the control strategy. The simulation duration is proportionally set to 2.1s. The power demand of the load in each flight stage is as follows: Figure 7 shown.

[0160] When the load is stable, the bus voltage waveform and the output power of each power supply are as follows: Figure 8 As shown in the figure, it can be seen that during stable flight, the bus voltage deviation can be controlled within 0.3V, and the voltage fluctuation rate is less than 0.22%; during the flight mode switching stage, the bus voltage deviation can be controlled within 1.5V, meeting the electrical standards of more electric aircraft; in addition, when the load power demand changes with the flight phase, the supercapacitor responds immediately, and the lithium battery and fuel cell bear the medium-frequency component and low-frequency component respectively, realizing the automatic optimal power distribution among the three different power supplies and ensuring the stability of the DC bus. The simulation results verify the effectiveness of the proposed energy management strategy.

[0161] Pulse loads represented by high-power radars exhibit the characteristic of switching back and forth between full load and no load in a certain time series, so square waves can be used to simulate ideal pulse loads. Under different working conditions, the period, duty cycle, and peak power of the pulse load will change. When the system is loaded with pulse loads, the load power demand is as follows: Fig. 9 As shown; the waveform of bus voltage under pulse load is as follows Fig.10 As shown in the figure, when the pulse load is working, the bus voltage fluctuation can be controlled within 9V, and the recovery time after the pulse load stops working is 0.9ms. The proposed control strategy can achieve the smoothing of the pulse load impact.

[0162] When the fuel cell is disconnected from the system due to a fault at simulation time 0.36s, Fig.11 It can be seen that the proposed strategy can still achieve automatic power distribution between the remaining two power supplies without changing the controller. When the fuel cell is disconnected, the supercapacitor responds quickly to the change, and the bus voltage quickly rises to 269V within 0.015s, with a voltage deviation rate of 0.37%, proving that the strategy still has the ability of dynamic power distribution and bus voltage regulation.

[0163] When the supercapacitor is disconnected from the system due to a fault at simulation time 0.36s, Fig.12 It can be seen that the proposed strategy can still achieve autonomous power distribution between the remaining two power supplies without changing the original controller. When the supercapacitor is disconnected, the lithium battery and fuel cell respond quickly to the change, and the bus voltage can be maintained at 268.5V, with a voltage deviation rate of 0.556%, which meets the electrical standards of more-electric aircraft. Although the lithium battery and capacitor can maintain the normal operation of the system and ensure the power supply of critical loads after the supercapacitor is disconnected, the fluctuation of the bus voltage is significantly aggravated and the stability of the system is reduced. In addition, lithium batteries and fuel cells bear a large high-frequency power, and short-term over-discharge will reduce their service life. Therefore, when a power supply is disconnected, the system cannot operate for a long time, and the corresponding power supply should be replaced and repaired as soon as possible.

[0164] When the lithium battery is disconnected from the system due to a fault at simulation time 0.36s, Fig.13 It can be seen that the automatic power distribution between the remaining two power supplies can still be achieved under the proposed improved control strategy. When the lithium battery is disconnected, the fuel cell and supercapacitor respond quickly to the change. After the bus voltage drops to 268.4V, it quickly recovers to 270V within 0.005s, which can achieve the regulation of the bus voltage and the smoothing of the pulse power, meeting the electrical standards of more-electric aircraft.

[0165] When the supercapacitor charge state is restored, the power output of each power supply is as follows: Fig.14As shown in Figure 2, it can be seen that the proposed supercapacitor state of charge recovery strategy can separate the recovery process from the high-frequency power fluctuation compensation process on a time scale based on the supercapacitor SOC regulation, and realize hierarchical control. Fig.14 It can be seen that when the multi-electric aircraft is in the cruise phase and the pulse load stops working, the system determines that the supercapacitor charge state is lower than the set threshold. At this time, the feedforward voltage compensator starts to work, and the fuel cell and lithium battery increase their output to charge the supercapacitor. When the flight mission phase of the multi-electric aircraft changes at 1.9s, the feedforward voltage compensator stops working, and the supercapacitor still works normally according to the set strategy.

[0166] The above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent replacement and improvement made to the above embodiments without departing from the content of the technical solution of the present invention, based on the technical essence of the present invention, within the spirit and principles of the present invention, still fall within the protection scope of the technical solution of the present invention.

Claims

1. An energy management method applicable to a hybrid power supply system of a more-electric aircraft, characterized in that: The energy management method applicable to the hybrid power supply system of a more-electric aircraft comprises the following steps: Step 1: Build a hybrid power supply system for a multi-electric aircraft; The hybrid power supply system of the multi-electric aircraft includes a power supply, a hybrid energy storage system, a power electronic converter, a high-voltage DC bus and a load-related model; Step 2: Set the distribution relationship of the load current between the power supplies according to the output characteristics of the power supply and design the distributed control strategy corresponding to each power supply branch; The power supply distributed control strategy includes the design of a dynamic and efficient power allocation strategy for each power supply and a charge state recovery strategy for the hybrid energy storage system; Step 3: Based on the power supply parameter design criteria and combined with the load current distribution relationship among the power supplies, the virtual impedance parameters and inner loop control parameters are designed to ensure the stable operation of the hybrid power supply system of the more-electric aircraft.

2. The energy management method applicable to the hybrid power supply system of a more-electric aircraft according to claim 1, characterized in that: In step 1, the power supply includes a fuel cell, a lithium battery and a supercapacitor. The lithium battery and the supercapacitor form a hybrid energy storage system. The fuel cell, the lithium battery and the supercapacitor are connected to the DC bus through corresponding converters. The load side includes a resistive load, a constant power load and a pulse load.

3. The energy management method applicable to a hybrid power supply system of a more-electric aircraft according to claim 1, characterized in that: Step 2 specifically includes: Step 2.1: Based on the output characteristics of fuel cells, lithium batteries and super capacitors and the principle of traditional virtual resistance control, a hybrid droop control method is extended and expanded, wherein the hybrid droop control method includes virtual inductance droop control for fuel cells, virtual resistance droop control for lithium batteries and virtual capacitance droop control for super capacitors; Step 2.2: Under ideal conditions, the droop characteristic equations of the fuel cell, lithium battery and supercapacitor converter are calculated based on the hybrid droop method; Step 2.3: Based on the droop characteristic equations of the fuel cell, lithium battery and supercapacitor converter, the distribution relationship of the load current among the fuel cell, lithium battery and supercapacitor is calculated; Step 2.4: Based on the hybrid droop control method, the center frequency f0 and bandwidth Δf of the bandpass filter are reasonably designed in combination with engineering practice, the load current is automatically decoupled into low-frequency, medium-frequency and high-frequency parts, and the low-frequency part of the load current is allocated to the fuel cell, the medium-frequency part is allocated to the lithium battery, and the high-frequency part is allocated to the supercapacitor, completing the design of the power efficient allocation strategy of the power supply; Step 2.5: After any power source among the fuel cell, the lithium battery and the supercapacitor is disconnected from the hybrid power supply system of the more-electric aircraft due to a fault, the remaining two power sources are controlled based on the power effective allocation strategy of the power sources to ensure that effective power division can still be achieved after any power source fails to exit the hybrid power supply system of the more-electric aircraft due to a fault; Step 2.6: Set the hybrid energy storage system charge state recovery strategy; The droop characteristic equation of the fuel cell is expressed as: V ofc =V ref -L νfc s·i ofc (1); In formula (1), V ofc is the output voltage of the fuel cell converter, V ref is the reference value of bus voltage, L νfc is the virtual inductance, i ofc is the output current of the fuel cell, s is a complex frequency domain variable; The droop characteristic equation of lithium battery is expressed as: V ob =V ref -R νb ·i ob (2); In formula (2), V ob is the output voltage of the converter connected to the lithium battery, R νb is the virtual resistance, i ob is the output current of the lithium battery; The droop characteristic equation of the supercapacitor is expressed as: In formula (3), V osc is the output voltage of the converter connected to the supercapacitor, C νsc is the virtual capacitor, i osc is the output current of the supercapacitor; The distribution relationship of the generator load current among the fuel cell, lithium battery and supercapacitor is expressed as: In formula (4), G FC (s) is a second-order low-pass filter, G B (s) is a second-order bandpass filter, G SC (s) is a second-order high-pass filter, i o is the load side current.

4. The energy management method applicable to a hybrid power supply system of a more-electric aircraft according to claim 3, characterized in that: Step 2.5 specifically includes: Step 2.5.1: When the fuel cell is disconnected from the hybrid power supply system of the multi-electric aircraft due to a system failure, the power distribution strategy of the power supply is used to control the lithium battery and supercapacitor to bear the load pulse power, and the new transfer function G corresponding to the lithium battery is calculated. B1 (s), after the fuel cell is disconnected, the low-frequency power required by the fuel cell is borne by the lithium battery, and the supercapacitor still bears the high-frequency power according to the original cut-off frequency of the power supply. The lithium battery and the supercapacitor respectively act as a new first-order low-pass filter and a first-order high-pass filter to achieve power division; Step 2.5.2: When the supercapacitor is disconnected from the hybrid power supply system of the multi-electric aircraft due to a system failure, the fuel cell and lithium battery are controlled to bear the load pulse power through the power supply power effective allocation strategy, and the new transfer function G corresponding to the fuel cell is calculated. FC2 (s), after the supercapacitor is disconnected, the boundary frequency of the fuel cell's load power distribution increases, providing more low-frequency power. The high-frequency power borne by the supercapacitor is transferred to the lithium battery, and the load feedback current is all absorbed by the lithium battery. The fuel cell and the lithium battery act as a new first-order low-pass filter and a new first-order high-pass filter respectively to achieve power division; Step 2.5.3: When the lithium battery is disconnected from the hybrid power supply system of the multi-electric aircraft due to a system failure, the power distribution strategy of the power supply is improved by connecting a virtual impedance R in parallel to the branch of the supercapacitor. vsc , calculate the new transfer function G corresponding to the fuel cell FC3 (s), after the lithium battery is disconnected, the boundary frequency of the fuel cell's load power distribution rises, providing more low-frequency power. The intermediate-frequency power borne by the lithium battery is transferred to the supercapacitor. Combined with the parallel virtual resistance branch, the supercapacitor absorbs all feedback current. The fuel cell and supercapacitor respectively act as a new second-order low-pass filter and a new second-order high-pass filter to achieve power division; The new transfer function G B1 The expression of (s) is: In formula (5), the new transfer function G B1 (s) The standard form of a first-order low-pass filter is shown, with a cut-off frequency equal to the passband bandwidth Δf; The new transfer function G FC2 The expression of (s) is: In formula (6), the fuel cell corresponds to the transfer function G FC2 (s) presents the standard form of a first-order low-pass filter, and the new cutoff frequency is The new transfer function G FC3 The expression of (s) is: In formula (7), the new transfer function G FC3 (s) presents the standard form of a second-order low-pass filter, with the corresponding cutoff frequency being the center frequency f0.

5. The energy management method applicable to a hybrid power supply system of a more-electric aircraft according to claim 3, characterized in that: Step 2.6 specifically includes: Step 2.6.1: Calculate the state of charge of the supercapacitor based on the capacitance characteristics of the supercapacitor, and use indirect means to control the supercapacitor V sc To adjust the state of charge of the supercapacitor; Step 2.6.2: Set a supercapacitor charge and discharge switching point SOC based on actual project needs sw As the input of the mode selector based on hysteresis control, a feed-forward voltage compensator is applied at the front end of the supercapacitor droop control branch, combined with the mode selector based on hysteresis control, to realize the hierarchical SOC recovery control of the supercapacitor, and the compensation V output by the feed-forward voltage compensator is controlled. rec Realize hierarchical management function; The expression of the state of charge of a supercapacitor is: ΔQ=ωi sc dt=C sc ·ΔV sc (8); In formula (8), ΔQ is the charge released by the supercapacitor, ΔV is sc is the bias value between the supercapacitor terminal voltage and the voltage reference; Compensation V rec The expression is: In formula (9), V rec is the compensation voltage corresponding to the feedforward compensator, k prec and k irec are the PI controller parameters of the voltage compensation loop.

6. The energy management method applicable to a hybrid power supply system of a more-electric aircraft according to claim 5, characterized in that: Step 2.6.2 specifically includes: When the power supply is in a non-power fluctuation compensation state, the working mode of the feedforward voltage compensator is changed based on the supercapacitor charge state; when the supercapacitor SOC is within the normal working range, the feedforward compensator is in an idle state; when the supercapacitor SOC is lower than the threshold, the compensation voltage V ref is positive, HPSS provides power to charge the supercapacitor. When the supercapacitor SOC is higher than the threshold, the compensation voltage V ref When it is negative, the supercapacitor is controlled to release additional energy to enter the normal working mode.

7. The energy management method applicable to a hybrid power supply system of a more-electric aircraft according to claim 1, characterized in that: Step 3 specifically includes: Based on the transfer function, the virtual impedance parameter is related to the cutoff frequency ω c , the algebraic relationship of the damping ratio is transformed into the algebraic relationship with the center frequency f0 and the bandwidth Δf; Combining the algebraic relationship between the virtual impedance parameter and the center frequency f0 and the bandwidth Δf and the distribution relationship between the generator load current of the fuel cell, the lithium battery and the supercapacitor, the relationship expression between the virtual impedance parameter and the center frequency and the bandwidth is calculated; The virtual resistance R is calculated according to the actual engineering needs. vb The virtual resistor R vb Substitute the parameters into the relationship between the virtual impedance parameters and the center frequency and bandwidth to obtain the virtual inductance L vfc 、Virtual capacitor C vsc Parameters to complete the setting of virtual RC parameters; The relationship between the virtual impedance parameter and the center frequency and bandwidth is expressed as:

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