An energy management method suitable for a hybrid power supply system of a more electric aircraft

Abstract

The application provides an energy management method suitable for a hybrid power supply system of a more electric aircraft, belongs to the technical field of a more energy storage direct current micro grid, and solves the problems that an existing distributed energy management strategy cannot effectively deal with the optimal distribution of load power among three different characteristic power supply sources, the design of control parameters, the state of charge adjustment of energy storage devices, and the feedback of load renewable energy, and comprises the following steps: step 1, building a hybrid power supply system of a more electric aircraft; step 2, setting the distribution relationship of load current among the power supply sources according to the output characteristics of the power supply sources and designing the corresponding distributed control strategy of each power supply source branch; and step 3, designing virtual impedance parameters and inner loop control parameters based on the power supply source parameter design criteria and in combination with the distribution relationship of load current among the power supply sources, so as to ensure the stable operation of the hybrid power supply system of the more electric aircraft.

Description

TECHNICAL FIELD

[0001] The application relates to an energy management method suitable for a hybrid power supply system of a more-electric aircraft, and belongs to the technical field of a more-energy direct-current micro-grid. BACKGROUND

[0002] At present, the development of the more-electric aircraft power system presents an overall trend of light weight, high efficiency, economy and green. The direct-current system has the advantages of high transmission efficiency, high power density and high reliability, and has been increasingly widely applied. With the continuous growth of actual demand, the electrical load is becoming more and more complex. In addition to the conventional resistive load and constant power load, there are also pulse loads such as radars. The pulse load has the characteristics of high peak-to-average ratio, strong randomness, volatility and impact. The more-electric aircraft power system has the characteristics of low inertia and small capacity, and the pulse load effect is easy to induce the direct-current bus voltage fluctuation problem, so the more-electric aircraft power supply system should have the ability to provide and absorb instantaneous pulsating power.

[0003] The generator has slow dynamic response and poor climbing ability, and is difficult to meet the instantaneous power demand of the pulse load. The pulsating power suppression technology based on the hybrid power supply system (HPSS) is a mainstream method for the more-electric aircraft to reduce fuel consumption, reduce weight and smooth fluctuations. In order to achieve the control objectives of autonomous allocation of wide frequency domain strong pulsating load power, voltage stabilization under the action of pulse load, green environmental protection and economic benefit improvement, an energy management strategy suitable for the direct-current power supply and distribution system of the more-electric aircraft with high-power pulse load needs to be designed.

[0004] At present, the research on the energy management strategy of the hybrid power supply system at home and abroad can be divided into centralized control and decentralized control. Most of the existing researches adopt the centralized control strategy based on the central control unit and the communication network, including fuzzy control, model predictive control and artificial neural network intelligent control methods. However, the centralized control has the problems of single point failure, communication delay, high cost and poor scalability, and gradually cannot meet the development requirements of the more-electric aircraft power supply end, such as easy expansion, strong fault tolerance and high robustness.

[0005] For the decentralized control, most of the current domestic and foreign researches focus on the improvement of the traditional droop control, and are mostly applied to the wind-solar-storage direct-current micro-grid and electric vehicle power supply system, and there are few research cases in the more-electric aircraft power system. In addition, most of the research objects are based on two different characteristic power supply units, and there are few researches on the optimal allocation of load power among three different characteristic power supply units. In addition, the design of control parameters, the state of charge adjustment of energy storage devices, the life cost problem, the feedback problem of load renewable energy and other aspects still need further research. SUMMARY

[0006] The application is a kind of energy management method suitable for a hybrid power supply system of a more electric aircraft.

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

[0008] Step 1: building a hybrid power supply system of a more electric aircraft;

[0009] The hybrid power supply system of the more electric aircraft includes power supply sources, a hybrid energy storage system, power electronic converters, a high-voltage DC bus and a load-related model.

[0010] Step 2: setting the distribution relationship of the load current among the power supply sources according to the output characteristics of the power supply sources and designing the corresponding decentralized control strategies of the power supply source branches;

[0011] The decentralized control strategies of the power supply sources include the design of the dynamic and efficient distribution strategy of the power of each power supply source and the state of charge recovery strategy of the hybrid energy storage system.

[0012] Step 3: designing the virtual impedance parameters and the inner loop control parameters based on the power supply source parameter design criteria and the distribution relationship of the load current among the power supply sources to ensure the stable operation of the hybrid power supply system of the more electric aircraft.

[0013] Preferably, the power supply sources in step 1 include fuel cells, lithium batteries and super capacitors, the lithium batteries and the super capacitors form the hybrid energy storage system, the fuel cells, the lithium batteries and the super capacitors are connected to the DC bus through corresponding converters, and the load side includes resistive loads, constant power loads and pulse loads.

[0014] Preferably, step 2 specifically includes:

[0015] Step 2.1: extending and expanding the hybrid droop control method based on the output characteristics of the fuel cells, the lithium batteries and the super capacitors and the principle of the traditional virtual resistance control, wherein the hybrid droop control method includes adopting virtual inductance droop control for the fuel cells, adopting virtual resistance droop control for the lithium batteries and adopting virtual capacitance droop control for the super capacitors.

[0016] Step 2.2: under ideal conditions, calculating the droop characteristic equation of the fuel cell converter, the lithium battery converter and the super capacitor converter based on the hybrid droop method;

[0017] Step 2.3: calculating the distribution relationship of the load current among the fuel cells, the lithium batteries and the super capacitors based on the droop characteristic equation of the fuel cell converter, the lithium battery converter and the super capacitor converter.

[0018] Step 2.4: On the basis of the mixed droop control method, the center frequency f0 and bandwidth Δf of the band-pass filter are reasonably designed according to 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 distributed to the fuel cell, the medium-frequency part is distributed to the lithium battery, and the high-frequency part is distributed to the super capacitor, and the design of the power effective distribution strategy of the power supply is completed;

[0019] Step 2.5: After any power supply in the fuel cell, lithium battery and super capacitor is disconnected from the multi-electric aircraft hybrid power supply system due to failure, the remaining two power supplies are controlled based on the power effective distribution strategy of the power supply, to ensure that the power can be effectively distributed after any power supply fails and exits the multi-electric aircraft hybrid power supply system;

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

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

[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 the bus voltage, L νfc is the virtual inductance, i ofc is the output current of the fuel cell, and s is a complex frequency variable;

[0024] The droop characteristic equation expression of the lithium battery is:

[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, and i ob is the output current of the lithium battery;

[0027] The droop characteristic equation expression of the super capacitor is:

[0028]

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

[0030] The expression of the distribution relationship of the generator load current among the fuel cell, lithium battery and super capacitor is:

[0031]

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

[0033] Preferably, step 2.5 specifically comprises:

[0034] Step 2.5.1: After the fuel cell is disconnected from the hybrid power supply system of the more-electric aircraft due to system failure, the lithium battery and the super capacitor are controlled to bear the load pulse power through the power effective distribution strategy of the power supply source, and a new transfer function G B1 (s) corresponding to the lithium battery is calculated. After the fuel cell is disconnected, the low-frequency power required to be borne by the fuel cell is borne by the lithium battery, and the super capacitor still bears the high-frequency power according to the original cutoff frequency of the power supply source, and the lithium battery and the super capacitor respectively realize power frequency division as a new first-order low-pass filter and a first-order high-pass filter;

[0035] Step 2.5.2: After the super capacitor is disconnected from the hybrid power supply system of the more-electric aircraft due to system failure, the fuel cell and the lithium battery are controlled to bear the load pulse power through the power effective distribution strategy of the power supply source, and a new transfer function G FC2 (s) corresponding to the fuel cell is calculated. After the super capacitor is disconnected, the boundary frequency of the fuel cell distribution load power rises to provide more low-frequency power, the high-frequency power borne by the super capacitor is borne by the lithium battery, and the load feedback current is all absorbed by the lithium battery, and the fuel cell and the lithium battery respectively realize power frequency division as a new first-order low-pass filter and a new first-order high-pass filter;

[0036] Step 2.5.3: After the lithium battery is disconnected from the hybrid power supply system of the more-electric aircraft due to system failure, the power effective distribution strategy of the power supply source is improved, a virtual impedance R vsc is connected in parallel in the branch of the super capacitor, and a new transfer function G FC3(s), after the lithium battery is disconnected, the boundary frequency of the fuel cell to distribute load power rises, more low-frequency power is provided, the medium-frequency power borne by the lithium battery is borne by the super capacitor, combined with the parallel virtual resistance branch, the super capacitor absorbs all feedback currents, the fuel cell and the super capacitor respectively serve as a new second-order low-pass filter and a new second-order high-pass filter to implement power frequency division;

[0037] The new transfer function G B1 (s) is expressed as:

[0038]

[0039] In formula (5), the new transfer function G B1 (s) presents a standard form of a first-order low-pass filter, and the cutoff frequency is equal to the passband width value Δf;

[0040] The new transfer function G FC2 (s) is expressed as:

[0041]

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

[0043] The new transfer function G FC3 (s) is expressed as:

[0044]

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

[0046] Preferably, step 2.6 specifically comprises:

[0047] Step 2.6.1: based on the capacitance characteristics of the super capacitor, the state of charge of the super capacitor is calculated, and an indirect means is used to control the voltage V sc of the super capacitor to adjust the state of charge of the super capacitor;

[0048] Step 2.6.2: set a super capacitor charge-discharge switching point SOC sw as an input quantity of the mode selector based on hysteresis control, by applying a feedforward voltage compensator at the front end of the super capacitor droop control branch, combining the mode selector based on hysteresis control, the layered SOC recovery control of the super capacitor is realized, and the compensation V rec output by the feedforward voltage compensator is realized.

[0049] The expression of the state of charge of the super capacitor is:

[0050] ΔQ = ∫i sc dt = C sc · ΔV sc (8);

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

[0052] The expression of the compensation V rec is:

[0053]

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

[0055] Preferably, step 2.6 specifically comprises:

[0056] Step 2.6.1: based on the capacitance characteristics of the super capacitor, the state of charge of the super capacitor is calculated, and the state of charge of the super capacitor is adjusted by using an indirect means to control the voltage V sc of the super capacitor;

[0057] Step 2.6.2: set a super capacitor charge-discharge switching point SOC sw as the input quantity of the mode selector based on hysteresis control, and through the application of a feedforward voltage compensator at the front end of the super capacitor droop control branch, in combination with the mode selector based on hysteresis control, the hierarchical SOC recovery control of the super capacitor is realized, and through the compensation V rec output by the feedforward voltage compensator, the hierarchical management function is realized;

[0058] The expression of the state of charge of the super capacitor is:

[0059] ΔQ = ∫i sc dt = C sc · ΔV sc (8);

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

[0061] The expression of the compensation V rec is:

[0062]

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

[0064] Preferably, step 3 specifically comprises:

[0065] Based on the transfer function, the algebraic relationship between the virtual impedance parameter and the cutoff frequency ω c , the damping ratio is converted into the algebraic relationship with the center frequency f0 and the bandwidth Δf.

[0066] In combination with the virtual impedance parameter being the algebraic relationship with the center frequency f0 and the bandwidth Δf and the distribution relationship of the generator load current among the fuel cell, the lithium battery and the super capacitor, a relationship expression of the virtual impedance parameter with the center frequency and the bandwidth is calculated;

[0067] According to the engineering actual demand, the parameter of the virtual resistance R vb is calculated, the parameter of the virtual resistance R vb is substituted into the relationship expression of the virtual impedance parameter with the center frequency and the bandwidth to obtain the parameters of the virtual inductance L vfc and the virtual capacitance C vsc , and the setting of the virtual resistance-inductance-capacitance parameters is completed.

[0068] The relationship expression of the virtual impedance parameter with the center frequency and the bandwidth is:

[0069]

[0070] The present application has the following advantages:

[0071] The present application provides an energy management method suitable for a hybrid power supply system of a more-electric aircraft, realizes dynamic autonomous optimal distribution of wide frequency domain strong pulsating load power among a fuel cell, a lithium battery and a super capacitor and voltage maintenance of a bus under the action of a pulse load, without a central controller and a communication network. In addition, the hybrid power supply system under the control strategy can realize efficient recovery of load renewable energy, adjustment of the state of charge of an energy storage device and stable operation of the system under partial failure of a power supply source, and has the performance advantages of high energy utilization rate, high durability, high flexibility and high robustness. BRIEF DESCRIPTION OF DRAWINGS

[0072] Figure 1 The present application provides a system architecture diagram of a hybrid power supply system of a more-electric aircraft;

[0073] Figure 2A flow chart of an energy management method suitable for a hybrid power supply system of a more-electric aircraft is provided in the present application.

[0074] Figure 3 A load characteristic diagram of a high-power radar pulse load is provided in the present application.

[0075] Figure 4 A simplified equivalent circuit topology based on virtual resistance-inductance-capacitance droop control is provided in the present application.

[0076] Figure 5 A virtual capacitance droop control block diagram containing a feed-forward voltage compensator and a mode selector is provided in the present application.

[0077] Figure 6 A supercapacitor state-of-charge recovery control flow chart is provided in the present application.

[0078] Figure 7 A power demand diagram of the load in each flight phase is provided in the present application.

[0079] Figure 8 A DC bus voltage waveform and output power of each power supply when the load is stable is provided in the present application.

[0080] Figure 9 A power demand diagram of the load in each flight phase when the pulse load is working is provided in the present application.

[0081] Figure 10 A DC bus voltage waveform and output power of each power supply when the pulse load is working is provided in the present application.

[0082] Figure 11 A DC bus voltage waveform and output power of each power supply when the fuel cell is disconnected from the system due to a fault is provided in the present application.

[0083] Figure 12 A DC bus voltage waveform and output power of each power supply when the supercapacitor is disconnected from the system due to a fault is provided in the present application.

[0084] Figure 13 A DC bus voltage waveform and output power of each power supply when the lithium battery is disconnected from the system due to a fault is provided in the present application.

[0085] Figure 14 Output power of each power supply before and after introducing the supercapacitor state-of-charge recovery strategy is provided in the present application. DETAILED DESCRIPTION

[0086] COMBINATION Figures 1-14To achieve the efficient and reliable distribution of pulsating load power among power supplies, the embodiment is designed to realize Figure 2 The steps of the energy management method for the hybrid power supply system of the more electric aircraft include

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

[0088] The hybrid power supply system of the more electric aircraft built by the embodiment is shown in Figure 1 The multi-source hybrid power supply system includes fuel cells, batteries and super capacitors, and the output ends are connected in parallel through one-way DC-DC converters and bidirectional DC-DC converters, and the control loop is composed of an internal control loop and an external control loop, the external loop uses a designed virtual droop controller, and the internal loop uses a voltage-current double closed loop.

[0089] The loads of the more electric aircraft are divided into three categories according to the output characteristics: resistive load, constant power load and pulse load, which are all connected to the DC bus. Among them, the load characteristics of the high-power radar pulse load are shown in Figure 3 A task sequence of the pulse load is composed of random pulses with different periods, different duty cycles and different peak powers. The pulse load has periodicity and overall high peak-to-average ratio and strong step characteristics, and the bus voltage impact problem and transient power supply problem caused by the pulse load cannot be ignored.

[0090] S2: Set the distribution relationship of the load current among the power supplies according to the output characteristics of the power supplies and design the distributed control strategy for the power supply branches;

[0091] S201: Use virtual inductance, virtual resistance and virtual capacitance droop control for fuel cells, lithium batteries and super capacitors respectively to realize high, medium and low frequency distribution of power among energy storage units. The simplified equivalent circuit diagram based on virtual resistance-inductance-capacitance droop control is shown in Figure 4 Under the healthy working condition of each power supply, the virtual switch T1 of the super capacitor branch is disconnected.

[0092] According to Kirchhoff's current law, the relationship of the bus current 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 the fuel cell, lithium battery and super capacitor respectively;

[0095] Since the virtual droop coefficient is much larger than the line impedance, the voltage drop on the line can be neglected compared with the voltage drop caused by the droop control. Under the proposed hybrid droop control strategy, the output V-I characteristics of the fuel cell, lithium battery and super capacitor converters can be expressed as:

[0096]

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

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

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

[0100] By combining equations (1)-(3), the current distribution relationship among the fuel cell, lithium battery and super capacitor can be obtained:

[0101]

[0102] The transfer functions G FC (s), G B (s) and G SC (s) in equation (4) act as second-order low-pass, band-pass and high-pass filters, respectively. By reasonably setting the upper and lower limit frequencies of the band-pass filter, the autonomous dynamic distribution of the load power among the fuel cell, lithium battery and super capacitor can be realized, in which the low-frequency power is borne by the fuel cell, the middle-frequency part is borne by the lithium battery, and the high-frequency part is borne by the super capacitor.

[0103] In the energy feedback process, the fuel cell cannot absorb the regenerated energy of the system due to the irreversibility of the chemical reaction, so it needs to be absorbed by the lithium battery and super capacitor. According to Figure 4 the distribution relationship of the load backflow current among the lithium battery and super capacitor power supplies can be derived:

[0104]

[0105] According to formula (5), in the energy feedback process, the super capacitor absorbs the high-frequency load feedback current, and the lithium battery absorbs the low-frequency load feedback current, and the transfer function G LPF (s) and G HPF (s) are first-order high-pass filter and first-order low-pass filter respectively, and do not affect the service life of the lithium battery and the super capacitor; According to the above analysis, in the energy feedback stage, the system under the proposed control strategy can not only realize the recovery of renewable energy and improve the energy utilization efficiency of the system, but also can realize power frequency recovery by considering the output characteristics of the lithium battery and the super capacitor.

[0106] When the multi-electric aircraft hybrid power supply system is in a partial failure state, that is, a power supply source is disconnected from the system due to failure, the proposed strategy can still realize the autonomous optimization distribution of the pulsating load power. Three fault states of fuel cell power supply source, lithium battery power supply source and super capacitor power supply source are analyzed, including:

[0107] S20101: When the fuel cell power supply source is disconnected from the system due to failure, the lithium battery and the super capacitor bear the load pulse power, and the new output V-I characteristic can be represented as:

[0108]

[0109] According to formula (6), the new distribution relationship of the load current between the lithium battery and the super capacitor power supply source can be derived as:

[0110]

[0111] According to formula (7), after the fuel cell is disconnected, the new transfer function G B1 (s) presents a standard form of first-order low-pass filter, and the cutoff frequency is equal to the passband width value. Therefore, after the fuel cell is disconnected, the low-frequency power required by the fuel cell is borne by the lithium battery, and the super capacitor still bears the high-frequency power according to the proposed strategy. On the basis of not changing the original control strategy of the normal operation power supply source, the lithium battery and the super capacitor can be used as a new low-pass filter and a high-pass filter respectively to realize power frequency division.

[0112] S20102: When the super capacitor is disconnected from the hybrid power supply system due to failure, the fuel cell and the lithium battery bear the load pulse power, and the new output V-I characteristic can be represented as:

[0113]

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

[0115]

[0116] The new transfer function G FC2 (s) corresponding to the fuel cell power supply in formula (9) can be known that when the super capacitor is disconnected due to failure, the fuel cell corresponding transfer function G FC2 (s) presents a standard form of a first-order low-pass filter, and the new cut-off frequency is Therefore, after the super capacitor is disconnected, the boundary frequency of the fuel cell for distributing load power slightly rises, which will provide more low-frequency power. The lithium battery bears 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 is disconnected from the hybrid power supply system due to failure, the virtual switch T1 is immediately closed, and the super capacitor branch is connected in parallel with the virtual impedance R vsc At this time, the V-I characteristic of the power supply system composed of the fuel cell and the super capacitor is:

[0118]

[0119] It can be deduced from formula (10) that the new distribution relationship of the load current between the fuel cell and the super capacitor power supply is:

[0120]

[0121] It can be known from formula (11) that the new transfer function G FC3 (s) presents a standard form of a second-order low-pass filter, and the corresponding cut-off frequency is f0; therefore, after the lithium battery is disconnected, the boundary frequency of the fuel cell for distributing load power slightly rises, which will provide more low-frequency power; the super capacitor bears the remaining high-frequency power, and benefits from the parallel virtual resistance branch, the super capacitor can absorb all the feedback current.

[0122] S202: Limited by the volume and weight of the more electric aircraft, the capacity of the energy storage device is greatly limited, and in actual application, the energy storage device inevitably exists in the case that the remaining energy cannot meet the load power demand or the capacity is full and cannot absorb the backflow energy on the load side. In addition, the high peak-to-average ratio, strong step and strong impact pulse load on the load side generates instantaneous high power demand when working, which puts forward great requirements on the energy supply of the energy storage device.

[0123] For the more electric aircraft direct current microgrid studied in the embodiment, the energy of the lithium battery is much higher than the energy capacity of the super capacitor. In addition, when the more electric aircraft is in the steady state running stage, the fuel cell mainly provides low-frequency load power. Therefore, in actual operation, the SOC change of the lithium battery is much smaller than the SOC change of the super capacitor. Therefore, the present application focuses on the super capacitor for the state of charge recovery strategy.

[0124] In order to make the super capacitor work in normal mode, the present embodiment proposes an improved super capacitor SOC recovery strategy. According to the capacitance characteristics of the super capacitor, the following can be obtained:

[0125] ΔQ = ∫i sc dt = C sc · ΔV sc (12);

[0126] In formula (12), ΔQ is the amount of charge released by the super capacitor, ΔV sc is the bias value of the super capacitor terminal voltage and the voltage reference, the super capacitor terminal voltage V sc is directly related to the state of charge, so the state of charge of the super capacitor can be controlled by controlling V sc and thus achieve the purpose of controlling the state of charge of the super capacitor.

[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 super capacitor; when the super capacitor is in normal working mode, the voltage compensator stops working. When the state of charge of the super capacitor deviates from the normal working interval, the voltage compensator is applied on the basis of the virtual capacitor control loop. By controlling the compensation voltage V rec , the partition management function is realized, and the compensation voltage V rec is set as:

[0128]

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

[0130] The virtual capacitor droop control block diagram with a feedforward voltage compensator is shown in Figure 5 . When the power supply is in a non-power fluctuation compensation state, i.e. the multi-electric aircraft is in a cruise mission phase and the pulse load is not working, the working mode of the feedforward voltage compensator is changed based on the state of charge of the super capacitor; when the SOC of the super capacitor is in the normal working range, the feedforward compensator is in an idle state; when the SOC of the super capacitor is lower than the threshold, the compensation voltage V rec is positive, and the HPSS provides power to charge the super capacitor. Similarly, when the SOC of the super capacitor is higher than the threshold, the compensation voltage V rec is negative, and thus a part of the electric energy of the super capacitor is discharged to enter the normal working mode.

[0131] To avoid the problem of super capacitor switching between normal mode and charge-discharge mode, this paper adopts a mode selector based on hysteresis control. Combined with the actual engineering needs, a super capacitor charge-discharge switching point SOC sw (35%≤SOC sw ≤80%) is set to ensure the duration of super capacitor in charge-discharge mode. Through the cooperation of feedforward voltage compensator and mode selector based on hysteresis control, layered regulation of super capacitor SOC can be realized, and its control flow chart is shown in Figure 6 .

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

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

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

[0135] S30102: To realize accurate power sharing, the parameter design is carried out on the basis of ignoring the line impedance, so the virtual impedance parameter needs to be much larger than the line impedance parameter to eliminate its influence on power sharing, which is designed as:

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

[0137] S30103: When the lithium battery fails due to failure, the super capacitor needs to bear a certain 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, when designing the parallel virtual resistance parameter of super capacitor branch, it needs to meet:

[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 in steady-state operation.

[0140] S302: Virtual impedance parameter design:

[0141] The transfer function G FC (s) of the fuel cell, lithium battery and super capacitor B (s) and G SC (s) is rewritten into a standard form:

[0142]

[0143] According to formula (16), the natural frequency ω c and the relationship between the damping ratio ζ and the virtual impedance can be obtained:

[0144]

[0145] The transfer function G B (s) presents a standard form of a second-order band-pass filter, so the algebraic relationship between the virtual impedance parameters and the cut-off frequency ω c , the damping ratio can be converted into the algebraic relationship with the center frequency f0 and the bandwidth Δf.

[0146]

[0147] According to formula (18), the relationship between the virtual impedance parameters and the center frequency f0 and the bandwidth Δf can be obtained:

[0148]

[0149] The virtual resistance corresponding to the lithium battery is given by the following formula:

[0150]

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

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

[0153]

[0154] According to the electrical standard of the more electric aircraft, the deviation value of the bus voltage is not more than 10%, and on the basis of substituting the system parameters to solve the virtual resistance R vb , the parameters of the virtual resistance R vb are substituted into formula (19) to solve the parameters of the virtual capacitor and the virtual inductor.

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

[0156] The virtual resistance R vb has the following parameters:

[0157]

[0158] The parameters of the virtual resistance R vb are substituted into equation (19) to obtain the parameters of the virtual capacitance and virtual inductance, which are 4.59 F and 0.02 H respectively. According to engineering experience, the virtual resistance R vsc in the super capacitor branch is 0.007 Ω.

[0159] Specific implementation method two: combining Figures 7-14 the specific implementation method one, in order to verify the technical effect of the specific implementation method one, the specific implementation method two uses Matlab / Simulink software for simulation verification. In order to simulate the actual working condition of the more electric aircraft as much as possible, the power demand of the load side in the specific implementation method two is set according to five task stages of take-off, climbing, cruising, initial descent and landing, and a disturbance is added to verify the rapid response ability of the control strategy. The simulation time is set to 2.1 s in proportion, and the power demand of the load in each flight stage is as shown in Figure 7 .

[0160] When the load is stable, the bus voltage waveform and the output power of each power supply are as shown in Figure 8 . It can be seen that, during the stable flight process, the bus voltage deviation value can be controlled within 0.3 V, and the voltage fluctuation rate is less than 0.22%; during the flight mode switching stage, the bus voltage deviation can be controlled within 1.5 V, which meets the electrical standard of the more electric aircraft; in addition, when the load power demand changes with the flight phase, the super capacitor responds immediately, the lithium battery and the fuel cell bear the medium frequency component and the low frequency component respectively, and the power automatic optimal distribution between the three different power supplies is realized, thereby ensuring the stability of the DC bus. The simulation results verify the effectiveness of the proposed energy management strategy.

[0161] The pulse load represented by high-power radar switches between full load and no load in a certain time sequence, so the ideal pulse load can be simulated by square wave. In different working conditions, the period, duty cycle and peak power of the pulse load will change, and the load power demand of the system with pulse load is shown in Figure 9 , and the waveform of bus voltage under the action of pulse load is shown in Figure 10 . 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 control strategy can realize the suppression of pulse load impact.

[0162] When the fuel cell is disconnected from the system due to failure at simulation time 0.36s, it can be seen from Figure 11 that the proposed strategy can still realize the automatic power distribution between the remaining two power supplies without changing the controller. When the fuel cell is disconnected, the super capacitor quickly responds 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 super capacitor is disconnected from the system due to failure at simulation time 0.36s, it can be seen from Figure 12 that the proposed strategy can still realize the automatic power distribution between the remaining two power supplies without changing the original controller. When the super capacitor is disconnected, the lithium battery and the fuel cell quickly respond to the change, and the bus voltage can be maintained at 268.5V, with a voltage deviation rate of 0.556%, meeting the electrical standard of more electric aircraft. Although the lithium battery and the super capacitor can maintain the normal operation of the system and ensure the power supply of the key load after the super capacitor is disconnected, the fluctuation of the bus voltage is significantly intensified, and the stability of the system is reduced. In addition, the lithium battery and the fuel cell bear a large amount of high-frequency power, and short-term over-discharge will reduce their service life. Therefore, the system cannot run for a long time after a power supply is disconnected, and the corresponding power supply should be replaced or repaired as soon as possible.

[0164] When the lithium battery is disconnected from the system due to failure at simulation time 0.36s, it can be seen from Figure 13 that under the improved control strategy, the automatic power distribution between the remaining two power supplies can still be realized. When the lithium battery is disconnected, the fuel cell and the super capacitor quickly respond to the change, and the bus voltage quickly recovers to 270V within 0.005s after falling to 268.4V, which can realize the regulation of bus voltage and the suppression of pulse power, meeting the electrical standard of more electric aircraft.

[0165] The power output of each power supply under the recovery of the state of charge of the super capacitor is shown in Figure 14As shown, it can be seen that the proposed supercapacitor SOC recovery strategy can separate the recovery process from the high-frequency power fluctuation compensation process in the time scale on the basis of realizing supercapacitor SOC regulation, and realizes hierarchical control. Figure 14 It can be seen from the above that, under the premise that the multi-electric aircraft is in the cruising stage and the pulse load stops working, the system judges that the supercapacitor state of charge is lower than the set threshold value, at this time the feedforward voltage compensator starts to work, and the fuel cell and the lithium battery increase the output to charge the supercapacitor. When the multi-electric aircraft flight task stage changes at 1.9s, the feedforward voltage compensator stops working, and the supercapacitor still normally works according to the set strategy.

[0166] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above with reference to the preferred embodiment, it is not intended to limit the present application. Any person skilled in the art can make some changes or modifications to the above disclosed technical content without departing from the technical solution of the present application, and the equivalent embodiments with equivalent changes are equivalent to the embodiments. Any simple modification, equivalent replacement and improvement of the above embodiments, as long as it does not depart from the technical solution of the present application, is within the protection scope of the present application.

Claims

1. An energy management method suitable for a hybrid power supply system of a more electric aircraft, characterized in that, The steps of the energy management method suitable for the hybrid power supply system of the more electric aircraft include the following steps: Step 1: building a hybrid power supply system of the more electric aircraft; The power supply sources in step 1 include fuel cells, lithium batteries and supercapacitors, the lithium batteries and the supercapacitors form a hybrid energy storage system, the fuel cells, the lithium batteries and the supercapacitors are connected with a direct-current bus through corresponding converters, and the load side includes resistive loads, constant power loads and pulse loads; The hybrid power supply system of the more electric aircraft includes power supply sources, a hybrid energy storage system, power electronic converters, a high-voltage direct-current bus and a load-related model; Step 2: setting a distribution relationship of load currents among the power supply sources according to output characteristics of the power supply sources and designing corresponding decentralized control strategies of the power supply source branches; The decentralized control strategies of the power supply sources include design of a dynamic and efficient power distribution strategy of each power supply source and a state of charge recovery strategy of the hybrid energy storage system; The step 2 comprises step 2.4: on the basis of the mixed droop control method, the center frequency of the band-pass filter is reasonably designed in combination with engineering practice And bandwidth The load current is automatically decoupled into low-frequency, medium-frequency and high-frequency parts, the low-frequency part of the load current is distributed to the fuel cell, the medium-frequency part is distributed to the lithium battery, and the high-frequency part is distributed to the super capacitor, so that the design of the power effective distribution strategy of the power supply is completed. An expression of the distribution relationship of the generator load current among the fuel cells, the lithium batteries and the supercapacitors is as follows: （4）； In equation (4), is a second-order low-pass filter, is a second-order band-pass filter, is a second-order high-pass filter, is a load-side current, is a virtual resistance, is a virtual inductance, is a virtual capacitance, s is a complex frequency domain variable, is an output current of the fuel cell, is an output current of the lithium battery, is an output current of the super capacitor; Step 3: designing virtual impedance parameters and inner loop control parameters based on power supply source parameter design criteria and in combination with the distribution relationship of the load currents among the power supply sources, so as to ensure stable operation of the hybrid power supply system of the more electric aircraft.

2. The energy management method for a hybrid power supply system of a more electric aircraft according to claim 1, wherein, Step 2 specifically includes the following steps: Step 2.1: extending and developing a hybrid droop control method based on output characteristics of the fuel cells, the lithium batteries and the supercapacitors and principles of traditional virtual resistance control, wherein the hybrid droop control method includes virtual inductance droop control for the fuel cells, virtual resistance droop control for the lithium batteries and virtual capacitance droop control for the supercapacitors; Step 2.2: in an ideal case, calculating a droop characteristic equation of the fuel cell converter, the lithium battery converter and the supercapacitor converter based on the hybrid droop method; Step 2.3: calculating the distribution relationship of the load current among the fuel cells, the lithium batteries and the supercapacitors based on the droop characteristic equation of the fuel cell converter, the lithium battery converter and the supercapacitor converter; Step 2.5: after any power supply source among the fuel cells, the lithium batteries and the supercapacitors is disconnected from the hybrid power supply system of the more electric aircraft due to failure, controlling the remaining two power supply sources based on the power efficient distribution strategy of the power supply sources, so as to ensure that the power can be efficiently distributed even if any power supply source exits from the hybrid power supply system of the more electric aircraft due to failure; Step 2.6: setting a state of charge recovery strategy of the hybrid energy storage system; An expression of the droop characteristic equation of the fuel cells is as follows: （1）； In equation (1), is the output voltage of the fuel cell inverter, is the reference value of the bus voltage, is the virtual inductance, is the output current of the fuel cell, s is the complex frequency domain variable; An expression of the droop characteristic equation of the lithium batteries is as follows: （2）； In equation (2), is the output voltage of the converter connected to the lithium battery, is the virtual resistance, is the output current of the lithium battery; An expression of the droop characteristic equation of the supercapacitors is as follows: （3）； In equation (3), is the output voltage of the supercapacitor, is the virtual capacitance, is the output current of the supercapacitor.

3. The energy management method for a hybrid power supply system of a more electric aircraft according to claim 2, wherein, Step 2.5 specifically includes the following steps: Step 2.5.1: After the fuel cell is disconnected from the hybrid power supply system of the more electric aircraft due to system failure, the lithium battery and the super capacitor are controlled by the power effective distribution strategy of the power supply source to bear the load pulse power, and the new transfer function of the lithium battery is calculated After the fuel cell is disconnected, the low-frequency power required by the fuel cell is borne by the lithium battery, and the super capacitor still bears the high-frequency power according to the original cutoff frequency of the power supply source. The lithium battery and the super capacitor respectively serve as a new first-order low-pass filter and a first-order high-pass filter to realize power frequency division. Step 2.5.2: After the super capacitor is disconnected from the hybrid power supply system of the more electric aircraft due to system failure, the fuel cell and the lithium battery are controlled to bear the load pulse power through the power effective distribution strategy of the power supply source, and a new transfer function corresponding to the fuel cell is calculated After the super capacitor is disconnected, the boundary frequency of the fuel cell distributing load power rises, more low-frequency power is provided, the high-frequency power borne by the super capacitor is borne by the lithium battery, and the load feedback current is all absorbed by the lithium battery, and the fuel cell and the lithium battery respectively act as a new first-order low-pass filter and a new first-order high-pass filter to realize power frequency division; Step 2.5.3: After the lithium battery is disconnected from the hybrid power supply system of the more electric aircraft due to system failure, the power effective distribution strategy control is improved by connecting a virtual impedance in parallel with the super capacitor branch The new transfer function corresponding to the fuel cell is calculated After the lithium battery is disconnected, the boundary frequency of the fuel cell to distribute the load power rises, providing more low-frequency power, and the medium-frequency power originally borne by the lithium battery is now borne by the super capacitor, in combination with the parallel virtual resistance branch, the super capacitor absorbs all the feedback current, and the fuel cell and the super capacitor respectively act as a new second-order low-pass filter and a new second-order high-pass filter to realize power frequency division; New transfer function The expression for the new transfer function is （5）； In equation (5), the new transfer function The standard form of a first-order low-pass filter is now presented, with a cutoff frequency equal to the passband width value ; New transfer function The expression for the new transfer function is （6）； In equation (6), the fuel cell corresponding transfer function The standard form of a first-order low-pass filter is presented, with the new cutoff frequency ; New transfer function The expression for the new transfer function is （7）； In equation (7), the new transfer function The standard form of a second-order low-pass filter is presented, with a center frequency corresponding to the cutoff frequency .

4. The energy management method for a hybrid power supply system of a more electric aircraft according to claim 1, wherein, Step 2.6 specifically includes the following steps: Step 2.6.1: Calculate the state of charge of the supercapacitor based on the capacitance characteristics of the supercapacitor, control the voltage of the supercapacitor by indirect means to adjust the state of charge of the supercapacitor; Step 2.6.2: Set a super capacitor charge-discharge switching point according to engineering practice As the input quantity of the mode selector based on hysteresis control, by applying a feedforward voltage compensator at the front end of the super capacitor droop control branch, combined with the mode selector based on hysteresis control, the layered SOC recovery control of the super capacitor is realized, and the compensation output by the feedforward voltage compensator is controlled Implement layered management functions; An expression of the state of charge of the supercapacitors is as follows: （8）； In Equation (8), Q is the amount of charge released by the super capacitor, V is the bias value of the voltage at the super capacitor terminal and the voltage reference compensation The expression for the compensation is: （9）； In formula (9), is the compensation voltage corresponding to the feedforward compensator, and is the PI controller parameter of the voltage compensation loop.

5. The energy management method for a hybrid power supply system of a more electric aircraft according to claim 4, wherein, Step 2.6.2 specifically includes the following steps: When the power supply is in a non-power fluctuation compensation state, the working mode of the super capacitor charge state based feedforward voltage compensator is changed; when the super capacitor SOC is in a normal working range, the feedforward compensator is in an idle state; when the super capacitor SOC is lower than a threshold value, the compensating voltage is positive, the HPSS provides power to charge the super capacitor, when the super capacitor SOC is higher than a threshold value, the compensating voltage is negative, the control super capacitor releases extra power to enter the normal working mode.

6. The energy management method for a hybrid power supply system of a more electric aircraft according to claim 1, wherein, Step 3 specifically includes the following steps: Transforming an algebraic relationship of virtual impedance parameters and cutoff frequency , damping ratio based on transfer function into an algebraic relationship with center frequency and bandwidth ​ Combining virtual impedance parameters with the center frequency and bandwidth The algebraic relationship and the distribution of generator load current among fuel cells, lithium batteries and supercapacitors are used to calculate the expression for the relationship between virtual impedance parameters and center frequency and bandwidth. The virtual resistance is calculated based on the actual needs of the project. The parameters will be used to determine the virtual resistance. Substituting the parameters into the expression relating the virtual impedance parameters to the center frequency and bandwidth yields the virtual inductance. Virtual capacitor The parameters are used to complete the setting of virtual resistance, inductance, and capacitance parameters; An expression of the relationship between the virtual impedance parameters and the center frequency and the bandwidth is as follows: （10）。

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