CN111463837B - Distributed power distribution method for multi-source hybrid power system - Google Patents

Abstract

The invention discloses a distributed power distribution method for a multi-source hybrid power system, which comprises the following steps of 1) constructing the multi-source hybrid power system; 2) designing a hybrid droop control strategy of the multi-source hybrid power system, wherein the hybrid droop control strategy comprises the steps of performing power distribution on a fuel cell unit, a storage battery unit and a super capacitor unit by adopting a virtual droop controller; 3) and designing internal control loops of virtual droop controllers of the fuel cell unit, the storage battery unit and the super capacitor unit respectively, controlling the fuel cell converter as a current source, and controlling the storage battery converter and the super capacitor converter as voltage sources so as to regulate the bus voltage and provide required power for a load. The invention not only realizes the automatic distribution of the load power among different power supply units so as to meet different requirements of the system under different operating conditions; and under the condition of not adding an additional controller, the automatic recovery of the charge state of the super capacitor in a steady state is realized.

Description

Distributed power distribution method for multi-source hybrid power system

Technical Field

The invention relates to the field of fuel cells, electrified traffic systems and direct-current micro-grids, in particular to a power distribution method of a multi-source hybrid power system.

Background

In recent years, demand for fossil fuels such as petroleum has been rapidly increasing due to the rapid development of the world economy. On the one hand, however, the large-scale use of fossil fuels is likely to trigger a new energy crisis due to its non-renewable nature. On the other hand, various types of pollution due to the use of fossil fuels have serious negative effects on the environment. In order to solve the above problems, the use of renewable energy is gradually increasing.

In the field of transportation, the requirements for energy conservation and emission reduction are becoming more and more urgent. Fuel Cell (FC) power systems are becoming increasingly popular due to high energy density, energy efficiency and zero emissions. However, since the dynamic response of the fuel cell is slow, it cannot accommodate sudden changes in load current. During the new steady state establishment, the life of the fuel cell may be affected by excessive temperature, pressure and humidity. Furthermore, in some conditions of the vehicle or aircraft, a large amount of regenerative energy will be generated to be fed back to the power supply, but the fuel cell cannot store it, so an additional unloading resistor needs to be installed to dissipate it, which will inevitably increase the volume and weight of the power system. Therefore, it is difficult to satisfy new demands for electric loads in renewable energy vehicles or airplanes only by using a fuel cell. In order to improve energy utilization efficiency and extend the life of the fuel cell, the combination of the fuel cell with other auxiliary energy storage devices with fast dynamic response (e.g., batteries, Supercapacitors (SCs), or a combination of both) has become a potential solution.

With the currently available fuel cells, the energy flow is irreversible, i.e. it is only possible to output energy and not to recover, whereas the auxiliary power supply must be able to recover braking energy. The super capacitor has high power density, fast dynamic response and low energy density. And the storage battery has higher energy density although the power density is lower and the dynamic response is slower. Existing solutions typically use one of a super capacitor or a battery as an auxiliary power source to supply power according to specific requirements. This has the advantage that the problem can be solved in a targeted manner, but the disadvantages of the fuel cell cannot be completely compensated for. Therefore, the present invention contemplates treating fuel cell/battery/supercapacitor hybrid systems as the ultimate solution for an application.

For a hybrid system consisting of a fuel cell, a battery and a super capacitor, the super capacitor has the characteristics of high power density and fast dynamic response compared to the fuel cell and the battery, which allows the super capacitor to buffer fast changing or pulsating loads. Meanwhile, the fuel cell and the storage battery can make up for the defect of low energy density of the super capacitor. In addition, the storage battery and the super capacitor can recover the braking energy which cannot be absorbed by the fuel cell. The hybrid power system combines the advantages of the three, and can simultaneously realize rapid dynamic performance, high power supply and large energy supply and energy recovery. In order to properly distribute the load power and fully utilize the advantages of the three power sources, a proper energy management strategy is required.

Existing energy management strategies mainly include two types: a centralized power allocation strategy and a decentralized power allocation strategy. The centralized power distribution strategy mainly comprises model prediction control, multi-objective optimization based on a neural network, fuzzy control and the like. However, the centralized processing is prone to problems such as communication delay, single point failure, etc., and the amount of computation is multiplied when adding the power supply unit, which greatly affects the scalability of the system.

Therefore, at present, a distributed energy management strategy suitable for a multi-source hybrid power system needs to be researched urgently, so that the requirements of high power density and high energy density can be met simultaneously, and quick dynamic response and braking energy recovery can be realized.

Disclosure of Invention

In view of the above, the present invention is directed to a distributed power distribution method for a multi-source hybrid power system, so as to solve the technical problem that the existing energy management strategy cannot meet the requirements of a fuel cell, a storage battery and a super capacitor hybrid power system for achieving high power density and high energy density of the multi-source hybrid power system, and the requirements for fast dynamic response and braking energy recovery.

The invention relates to a distributed power distribution method for a multi-source hybrid power system, which comprises the following steps:

1) constructing a multi-source hybrid power system, wherein the multi-source hybrid power system comprises a fuel cell unit, a storage battery unit and a super capacitor unit; the fuel cell unit comprises a fuel cell and a fuel cell converter which connects the fuel cell and the direct current bus, and the fuel cell converter is a unidirectional DC/DC converter; the storage battery unit comprises a storage battery and a storage battery converter for connecting the storage battery and the direct current bus, and the storage battery converter is a bidirectional DC/DC converter; the super capacitor unit comprises a super capacitor and a super capacitor converter which is connected with the super capacitor and a direct current bus, and the super capacitor converter is a bidirectional DC/DC converter;

2) designing a hybrid droop control strategy for a multi-source hybrid powertrain system, comprising:

i) the method comprises the following steps that a virtual droop controller is adopted to distribute power to a fuel cell unit, a storage battery unit and a super capacitor unit, the fuel cell unit adopts a virtual inductance droop controller, the storage battery unit adopts a virtual resistance droop controller, and the super capacitor unit adopts a virtual capacitor droop controller;

the output voltage-current characteristic of the virtual inductance droop controller is as follows:

the output voltage-current characteristic of the virtual resistance droop controller is as follows:

the output current-voltage characteristic of the virtual capacitance droop controller is as follows:

in the formula, V_nomIs the nominal voltage of the direct current bus; i.e. i_oFC，i_oB，i_oSCOutput currents of the fuel cell unit, the storage battery unit and the super capacitor unit are respectively; v. of_oFC，v_oB，v_oSCOutput voltages of the fuel cell unit, the storage battery unit and the super capacitor unit are respectively; i.e. i^* _oFC，v^* _oB，v^* _oSCRespectively represent i_oFC，v_oB，v_oSCA measured value of (a); l is_vFCIs a virtual inductance, R, of a fuel cell converter_vBAs a virtual resistance of the battery converter, C_vSCThe virtual capacitor is a virtual capacitor of the super capacitor converter; s represents the laplacian operator;

ii) designing virtual droop control parameters:

obtaining a transfer function of a band-pass filter used as a battery converter according to the hybrid droop controller designed in step i)

In the formula, A_uIs the passband gain, omega₀At the center angular frequency, Q is the quality factor, where:

from transfer function G_BLower limit frequency f of(s)_LAnd an upper limit frequency f_HObtaining the center frequency f₀And the bandwidth Δ f is:

Δf＝f_H-f_L (6)

through f₀And deltaf to obtain the quality factor Q and the natural frequency omega₀：

According to the rated power P of the system_eNominal voltage V of dc bus_nomAnd determining the virtual resistance R by the voltage ripple factor ∈_vBThe value of (A) is as follows:

simultaneous calculation of virtual inductance L from (5) to (9)_vFCAnd a virtual capacitor C_vSCTaking the value of (A);

3) the internal control loop of each virtual droop controller of the fuel cell unit, the storage battery unit and the super capacitor unit consists of a voltage control loop and a current control loop, wherein a voltage controller in the voltage control loop and a current controller in the current control loop are cascaded to control the fuel cell converter as a current source, and the storage battery converter and the super capacitor converter are controlled as voltage sources to adjust the bus voltage and provide required power for a load.

Further, the voltage controller and the current controller in the step 3) both adopt PI controllers, and the parameters of the two PI controllers are designed by adopting the following formula,

wherein k is_vp-xAnd k_vi-xProportional gain and integral gain of the voltage PI controller respectively; k is a radical of_ip-xAnd k_ii-xProportional gain and integral gain of the current PI controller respectively; omega_vxIs the desired bandwidth, ω, of the voltage PI controller_ixIs the desired bandwidth of the current PI controller; η is a constant between 1/10 and 1/5; v_xIs the rated voltage of each power supply, D_xIs the duty cycle d_xSteady state value of (L)_xIs the filter inductance of the source converter, C_xIs the output filter capacitor of the source converter, and subscript x is written FC, B, SC, respectively, indicating the fuel cell, the battery cell, and the supercapacitor cell.

The invention has the beneficial effects that:

the invention relates to a distributed power distribution method for a multi-source hybrid power system, which is characterized in that on the basis of a traditional droop control method, a fuel cell converter adopts a virtual inductance droop controller, a storage battery converter adopts a virtual resistance droop controller, and a super capacitor converter adopts a virtual capacitor droop controller; and the control of the output voltage and the input current of the converter is realized by cascading two PI controllers to form a double loop.

The invention not only realizes the automatic distribution of the load power among different power supply units, the low-frequency slowly-varying power is provided by the fuel cell, the medium-frequency load power is provided by the storage battery, and the high-frequency pulsating power is provided by the super capacitor, so as to meet different requirements of the system under different operating conditions; and the automatic recovery of the state of charge (SoC) of the super capacitor in a steady state is realized without adding an additional controller. In addition, when a certain power supply unit is controlled, information or common information of other power supply units does not need to be detected, so that the load power distribution method and the system do not need a communication network when distributing the load power, and the diversification of the optimized distribution control of the load power is realized.

Furthermore, in the case of a sudden disconnection of the fuel cell power supply unit, the battery can assume the function of a low-pass filter, instead of the fuel cell, and distribute it to the low-frequency part of the load power. Therefore, the invention realizes reasonable distribution of energy among the three power supply units and optimizes the overall performance and working efficiency of the multi-source power supply system.

Drawings

FIG. 1 is an equivalent circuit diagram of a multi-source hybrid power system based on virtual droop control;

FIG. 2 is an overall control block diagram of a multi-source hybrid power system based on hybrid droop control;

FIG. 3 shows a band-pass filter G in the designed droop control power distribution strategy_B(s) Bode plot;

FIG. 4 is a graph of the transfer function G before fuel cell unit turn-off in a designed droop control power distribution strategy_BBode plot (solid line in the figure) of(s) and G 'after disconnection'_B(s) comparison of Bode plots (dashed lines in the figure);

FIG. 5 shows the simulation results of the system under two states of current ramp-up (a) and ramp-down (b) when the load is constant current;

FIG. 6 shows the simulation results of the system under two states of current ramp-up (a) and ramp-down (b) when the load is pulsating;

fig. 7 is a simulation result of the FC unit being turned off halfway in the normal operation of the constant current load.

In fig. 5, fig. 6, and fig. 7, the terminal current, the bus voltage, the load power, and the state of charge (SoC) are shown in the simulation results of each sub-diagram from top to bottom.

Detailed Description

The invention is further described below with reference to the figures and examples.

The distributed power distribution method for a multi-source hybrid power system in the embodiment is used for the distributed power distribution method for the multi-source hybrid power system, and comprises the following steps:

1) building a multi-source hybrid power system, wherein the multi-source hybrid power system comprises a fuel cell unit, a storage battery unit and a super capacitor unit, and the fuel cell unit, the storage battery unit and the super capacitor unit are connected in parallel at the output end; the fuel cell unit comprises a fuel cell and a fuel cell converter which connects the fuel cell and the direct current bus, and the fuel cell converter is a unidirectional DC/DC converter; the storage battery unit comprises a storage battery and a storage battery converter for connecting the storage battery and the direct current bus, and the storage battery converter is a bidirectional DC/DC converter; the super capacitor unit comprises a super capacitor and a super capacitor converter which is connected with the super capacitor and a direct current bus, and the super capacitor converter is a bidirectional DC/DC converter.

2) Designing a hybrid droop control strategy for a multi-source hybrid powertrain system, comprising:

i) the virtual droop controller is adopted to distribute power to the fuel cell unit, the storage battery unit and the super capacitor unit, the fuel cell unit adopts the virtual inductance droop controller, the storage battery unit adopts the virtual resistance droop controller, and the super capacitor unit adopts the virtual capacitance droop controller.

The current relationship on the DC bus is

i_o＝i_oFC+i_oB+i_oSC (1)

Wherein i_oIs the output current of the power supply system, which can be positive or negative i_oFC，i_oB，i_oSCThe output currents of the fuel cell unit, the storage battery unit and the super capacitor unit are respectively.

The voltage-current characteristics of a VRD controller, i.e., a virtual resistance droop controller, may be expressed as

v_oB＝V_nom-R_vB·i_oB (2)

Wherein V_nomIs a DC bus voltage V_busNominal value of v_oBAnd i_oBIs the output voltage and current, R, of the accumulator unit_vBIs a corresponding virtual resistanceIs given by

Wherein Δ V_maxIs the maximum allowable bus voltage deviation, I_oBmaxIs the rated current of the battery cell.

By replacing the virtual resistance in (2) with a virtual inductance and a virtual capacitance, the voltage-current relationship of the virtual inductance droop controller (VID), the virtual resistance droop controller (VRD) and the virtual capacitance droop controller (VCD) of the equivalent circuit can be expressed as

Wherein v is_oFC，v_oBAnd v_oSCRespectively representing the output voltages of the fuel cell unit, the battery unit and the supercapacitor unit. L is_vFC，R_vBAnd C_vSCThe virtual inductance, the virtual resistance and the virtual capacitance are respectively adopted, and s represents a Laplace operator.

By combining (1) and (4), the current distribution relationship among the fuel cell, the storage battery and the super capacitor can be obtained:

here:

wherein the transfer function G_FC，G_BAnd G_SCActing as a Low Pass Filter (LPF), a Band Pass Filter (BPF) and a High Pass Filter (HPF), respectively. Writing (6) to standard form gives (7):

in which the natural frequency omega_nAnd damping ratio ζ expressed as

As a second-order band-pass filter, G in (7) can be used_BIs shown as

Wherein A is_uIs the passband gain, omega₀Is the central angular frequency, Q is the quality factor, which is easily obtained:

thus, VID, VRD and VCD achieve an automatic decoupling of the load current into low, medium and high frequency components, which are distributed to the fuel cell unit, the accumulator unit and the supercapacitor unit, respectively.

ii) designing virtual droop control parameters:

according to the distribution rule given in i), we can derive: only the transfer function G needs to be determined_B(s), then G can be deduced_FCAnd G_SCTherefore, there is no need to know G additionally_FCAnd G_SCI.e. a final virtual droop control power allocation scheme may be determined.

a) Determination of the transfer function: the original parameters of the energy source and the load of the multi-source hybrid power system of the embodiment can be obtained from the following system parameter table. The transfer function G is then determined_BLower limit frequency f of(s)_LAnd an upper limit frequency f_HThen the center frequency f₀And the bandwidth Δ f can be calculated as:

Δf＝f_H-f_L (11)

quality factor Q and natural frequency omega₀Can be expressed as f₀And Δ f:

simultaneous (8), (10) and (13), the upper and lower limit frequency values substituted into the system parameter table are obtained (21):

the transfer function G can be calculated by substituting the data obtained in (10) and (14) into (9)_B：

b) Design of control parameters: according to P in the system parameter table_eAnd V_nomRated output current value I_omaxThe following were used:

the maximum ripple voltage delta V is obtained by taking the ripple coefficient of the voltage to be epsilon-3 percent_maxAs shown in (17):

ΔV_max＝V_nom·∈＝15V (17)

simultaneous (16) and (17) to obtain

R in (18)_vBBy substituting (14) into the value of (D), L can be obtained_vFCAnd C_vSC：

System parameter table

3) The internal control loop of each virtual droop controller of the fuel cell unit, the storage battery unit and the super capacitor unit consists of a voltage control loop and a current control loop, wherein a voltage controller in the voltage control loop and a current controller in the current control loop are cascaded to control the fuel cell converter as a current source, and the storage battery converter and the super capacitor converter are controlled as voltage sources to adjust the bus voltage and provide required power for a load. The voltage controller and the current controller in the step both adopt PI controllers, and the parameters of the two PI controllers are designed by adopting the following formula,

wherein k is_vp-xAnd k_vi-xProportional gain and integral gain of the voltage controller, respectively; k is a radical of_ip-xAnd k_ii-xProportional gain and integral gain of the current controller, respectively; omega_vxAnd ω_ixThe expected bandwidths of the voltage controller and the current controller are respectively, and the values of the expected bandwidths are shown in a control parameter table; η is a constant between 1/10 and 1/5, and for a large damping ratio of the system, η is 0.1. V_xIs the rated voltage of each power supply; d_xIs the duty cycle d_xA steady state value of; l is_xIs the filter inductance of the source converter; c_xIs the output filter capacitor of the source converter, and the values of the parameters are shown in a system parameter table. Subscript x is written asFC, B, SC denote a fuel cell, a battery cell, and a supercapacitor cell, respectively. The proportional and integral gains of each PI controller in the inner control loop can be determined from the values of the bandwidths ω and η in the control parameter table.

Control parameter table

The distributed power distribution method for the multi-source hybrid power system in the embodiment can still realize effective distribution after the fuel cell unit is disconnected.

If the fuel cell unit is disconnected, the distribution relation of the load current between the storage battery unit and the super capacitor unit is as follows according to the preset parameters:

then the new transfer function G'_B(s) and G'_SC(s) can be deduced as

It is easy to know from equation (23) that the load current is automatically added to the low pass filter when being distributed to the storage battery branch and is automatically added to the high pass filter when being distributed to the super capacitor branch. And the load power can be automatically divided and dynamically distributed to different power supply units, so that the optimal distribution of the load power is realized.

If the fuel cell unit is disconnected, the new transfer function of the battery converter can be deduced as

Wherein the cut-off frequency can be expressed as

According to (24) and (25), the new transfer function G'_B(s) is represented by

A cut-off frequency f' of

It can be seen from (27) that after disconnecting the fuel cell unit, f' is still very close to the upper limit frequency f before disconnecting the fuel cell unit_HI.e. 1 Hz. That is, in the implemented subject system, after disconnecting the fuel cell unit, the battery and supercapacitor unit can still decompose the load power into low and high frequency parts with approximate cut-off frequencies.

The effectiveness of the distributed power distribution method for the multi-source hybrid system in this embodiment is verified using Matlab/Simulink.

In order to verify the feasibility and the correctness of the designed strategy, a fuel cell/battery/super capacitor hybrid power system simulation model is built in Matlab/Simulink. Simulation verification is carried out under the two states of constant current and pulsation, and related parameters are given by the system parameter table and the control parameter table.

In order to verify the feasibility and the effectiveness of the adopted energy management strategy, a system load is set to be in a constant current state (direct current 100A), and simulation verification is carried out under two states of sudden rise and fall of the load current; similarly, the system load is set to be in a pulsating state (dc 100A + ac 10A), and simulation verification is performed in two states, namely, a sudden rise and a sudden fall of the load current, and it can be known from the simulation results shown in fig. 5 to fig. 7 that the system can reasonably distribute power as expected in the two states, and the SoC of the super capacitor can be automatically recovered. In addition, the fuel cell unit is suddenly disconnected in the normal operation of the system to form a storage battery/super capacitor hybrid power system, and the storage battery is distributed to a low-frequency part instead of the fuel cell at the moment, so that the system can still realize the function of power distribution. The simulation result is consistent with the theoretical analysis result, so that the correctness and feasibility of the strategy are verified.

Finally, the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting, although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all of them should be covered in the claims of the present invention.

Claims (2)

1. A decentralized power distribution method for a multi-source hybrid powertrain system, comprising the steps of:

1) constructing a multi-source hybrid power system, wherein the multi-source hybrid power system comprises a fuel cell unit, a storage battery unit and a super capacitor unit; the fuel cell unit comprises a fuel cell and a fuel cell converter which connects the fuel cell and the direct current bus, and the fuel cell converter is a unidirectional DC/DC converter; the storage battery unit comprises a storage battery and a storage battery converter for connecting the storage battery and the direct current bus, and the storage battery converter is a bidirectional DC/DC converter; the super capacitor unit comprises a super capacitor and a super capacitor converter which is connected with the super capacitor and a direct current bus, and the super capacitor converter is a bidirectional DC/DC converter;

2) designing a hybrid droop control strategy for a multi-source hybrid powertrain system, comprising:

i) the method comprises the following steps that a virtual droop controller is adopted to distribute power to a fuel cell unit, a storage battery unit and a super capacitor unit, the fuel cell unit adopts a virtual inductance droop controller, the storage battery unit adopts a virtual resistance droop controller, and the super capacitor unit adopts a virtual capacitor droop controller;

the output voltage-current characteristic of the virtual inductance droop controller is as follows:

the output voltage-current characteristic of the virtual resistance droop controller is as follows:

the output current-voltage characteristic of the virtual capacitance droop controller is as follows:

in the formula, V_nomIs the nominal voltage of the direct current bus; i.e. i_oFC，i_oB，i_oSCOutput currents of the fuel cell unit, the storage battery unit and the super capacitor unit are respectively; v. of_oFC，v_oB，v_oSCOutput voltages of the fuel cell unit, the storage battery unit and the super capacitor unit are respectively; i.e. i^* _oFC，v^* _oB，v^* _oSCRespectively represent i_oFC，v_oB，v_oSCA measured value of (a); l is_vFCIs a virtual inductance, R, of a fuel cell converter_vBAs a virtual resistance of the battery converter, C_vSCThe virtual capacitor is a virtual capacitor of the super capacitor converter; s represents the laplacian operator;

ii) designing virtual droop control parameters:

obtaining a transfer function of a band-pass filter used as a battery converter according to the hybrid droop controller designed in step i)

In the formula, A_uIs the passband gain, omega₀At the center angular frequency, Q is the quality factor, where:

from transfer function G_BLower limit frequency f of(s)_LAnd an upper limit frequency f_HObtaining the center frequency f₀And the bandwidth Δ f is:

Δf＝f_H-f_L (6)

through f₀And deltaf to obtain the quality factor Q and the natural frequency omega₀：

According to the rated power P of the system_eNominal voltage V of dc bus_nomAnd determining the virtual resistance R by the voltage ripple factor ∈_vBThe value of (A) is as follows:

simultaneous calculation of virtual inductance L from (5) to (9)_vFCAnd a virtual capacitor C_vSCTaking the value of (A);

3) the internal control loop of each virtual droop controller of the fuel cell unit, the storage battery unit and the super capacitor unit consists of a voltage control loop and a current control loop, wherein a voltage controller in the voltage control loop and a current controller in the current control loop are cascaded to control the fuel cell converter as a current source, and the storage battery converter and the super capacitor converter are controlled as voltage sources to adjust the bus voltage and provide required power for a load.

2. The decentralized power distribution method for a multi-source hybrid system according to claim 1, wherein: the voltage controller and the current controller in the step 3) both adopt PI controllers, and the parameters of the two PI controllers are designed by adopting the following formula,

wherein k is_vp-xAnd k_vi-xProportional gain and integral gain of the voltage PI controller respectively; k is a radical of_ip-xAnd k_ii-xProportional gain and integral gain of the current PI controller respectively; omega_vxIs the desired bandwidth, ω, of the voltage PI controller_ixIs the desired bandwidth of the current PI controller; η is a constant between 1/10 and 1/5; v_xIs the rated voltage of each power supply, D_xIs the duty cycle d_xSteady state value of (L)_xIs the filter inductance of the source converter, C_xIs the output filter capacitor of the source converter, and subscript x is written FC, B, SC, respectively, indicating the fuel cell, the battery cell, and the supercapacitor cell.

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