A coordinated operation control method for multi-source system of electric vehicle

Abstract

The present invention discloses a method for coordinated operation control of a multi-source system of an electric vehicle, and belongs to the field of power distribution and energy management technology in vehicle operation control technology. In the prior art, most operation control strategies have problems such as unbalanced release and absorption of energy by supercapacitors, frequent reaching of upper and lower limits of supercapacitor state of charge (SoC) and inability to automatically recover, and uncontrolled DC bus voltage. The present invention proposes a method for coordinated operation control of a multi-source system of an electric vehicle with self-recovery of supercapacitor SoC and a DC bus voltage inner loop. The method utilizes the characteristics of a second-order filter to realize the autonomous recovery of the supercapacitor SoC before and after the dynamic process, without setting a specific charging time, thereby avoiding the supercapacitor SoC from frequently touching the upper and lower limits. On the basis of the traditional single current loop control, a "current outer loop-voltage inner loop" control loop is constructed by adding a DC bus voltage inner loop, which effectively suppresses the DC bus voltage fluctuation and maintains it at a reasonable level.

Description

Technical Field

[0001] The present invention belongs to the technical field of power distribution and energy management in vehicle operation control technology, and specifically relates to a method for controlling the coordinated operation of a multi-source system of an electric vehicle. Background Art

[0002] In modern society, electric vehicles have greatly facilitated people's travel and life. Electric vehicles use electricity as a driving energy source. They are light, fast, green, low-carbon, energy-saving and environmentally friendly. They have become the primary travel choice for many people. Although chemical power batteries such as lithium batteries and lead-acid batteries have high energy density, their low power density is a serious flaw. Electric vehicles have greater power requirements during starting and acceleration, and the power density of batteries currently on the market is far from the power density required by electric vehicles. The motors of electric vehicles cannot obtain sufficient torque and dynamic output characteristics. Therefore, it is difficult for electric vehicles to achieve rapid starting and acceleration in short-term power output situations. At the same time, frequent changes in power throughput will cause the power battery to age faster, reduce capacity, shorten life, and affect the cruising range.

[0003] To solve the above problems, many scholars have proposed using power batteries and supercapacitors to build a multi-source system as the energy source of electric vehicles. Supercapacitors have the advantages of high power density, long charge and discharge life, wide operating temperature range, high reliability, etc., and form a good complementary relationship with chemical batteries. The two energy storage devices play to their strengths and avoid their weaknesses. The fast-changing high-frequency power is borne by the supercapacitor, and the slowly changing low-frequency power is borne by the power battery. At the same time, domestic and foreign scholars have proposed a series of new wide-gain bidirectional DC-DC converters (BDC) for supercapacitor applications, which increases the availability of supercapacitors under large terminal voltage changes.

[0004] In recent years, domestic and foreign scholars have conducted extensive research on the operation control strategy of multi-source systems. The operation control strategy of multi-source systems includes the bottom-level BDC control strategy, the middle-level power allocation strategy, and the upper-level energy management strategy. For the bottom-level BDC control strategy, if the BDC branch is controlled as a voltage source, a voltage and current dual closed-loop control is adopted; if the BDC branch is controlled as a current source, a single current loop control is adopted, but the single current loop lacks the regulation of the DC bus voltage. For the middle-level power allocation strategy, [1] the logic threshold control method is used to allocate power, but frequent switching between different modes is prone to occur, affecting the stability of the system. [2] The fuzzy logic control method is used, but the setting of internal parameters requires a large amount of empirical data. The traditional first-order filter power allocation method is widely used [3-4]. This method divides high and low frequency power into separate ones, and the structure is simple, but the supercapacitor has the problem of unbalanced release and absorption of energy. The supercapacitor SoC is prone to frequently reach the upper and lower limits, affecting the operation stability of the multi-source system.

[0005] In summary, the traditional filtering power allocation method used in the prior art has the problem of unbalanced release and absorption of energy in the dynamic process of supercapacitors, and the supercapacitor SoC is prone to frequently reach the upper and lower thresholds. Secondly, the supercapacitor is exhausted after releasing energy many times, and it is necessary to charge the supercapacitor specifically within a specific time, which affects the continuity of the operation of the electric vehicle. Moreover, under the traditional single current loop control, the DC bus voltage cannot be effectively controlled.

[0006] [1] B.Wang, Q.Hu and Z.Wang, "Improving power output of battery and moderating frequency based on real-Time average power method for multi-modehybrid energy storage system in electric vehicles," IEEE Access, vol.8, pp.34654-34663, 2020.

[0007] [2] Q.Zhang and G.Li, "Experimental study on a semi-active battery-supercapacitor hybrid energy storage system for electric vehicle application," IEEE Trans.Power Electron., vol.35, no.1, pp.1014-1021, Jan.2020.

[0008] [3]H.Zhou, T.Bhattacharya, D.Tran, TTSSiew, and AMKhambadkone, "Composite energy storage system involving battery and ultracapacitor with dynamic energy management in microgrid applications," IEEE Trans.PowerElectron., vol.26, no.3, pp.923–930, Mar.2011.

[0009] [4] W.Na, T.Park, T.Kim and S.Kwak, "Light fuel-cell hybrid electricvehicles based on predictive controllers," IEEE Trans.Veh.Technol., vol.60, no.1, pp.89-97, Jan.2011. Summary of the invention

[0010] In view of the defects existing in the prior art, the present invention discloses a method for controlling the coordinated operation of a multi-source system of an electric vehicle. The method utilizes the characteristics of a second-order filter to realize the self-recovery of the supercapacitor SoC before and after the dynamic process, that is, each time the electric vehicle undergoes a dynamic process such as starting, accelerating, and decelerating, the supercapacitor charge state can be restored to the state before releasing the peak current, thereby avoiding the supercapacitor SoC from frequently reaching the upper and lower limits. On the basis of the traditional single current loop control, the "current outer loop-voltage inner loop" control loop is constructed by adding a DC bus voltage inner loop, thereby realizing auxiliary control of the system DC bus voltage.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is a coordinated operation control method of a multi-source system of an electric vehicle, wherein the multi-source system of the electric vehicle is composed of an energy source, a bidirectional DC-DC converter, a DC bus, a load, a controller and a sensor, wherein the energy source is composed of a power battery and a supercapacitor, and the structure of the multi-source system is a semi-active structure of a bidirectional DC-DC converter with a supercapacitor interface. The control method comprises the following steps:

[0012] (1) A second-order high-pass filter is used to divide the load power demand into a high-frequency part and a low-frequency part. These two parts of the power demand are converted into given currents that the battery and supercapacitor need to respond to respectively;

[0013] (2) Using the “current outer loop” to track the given current of the supercapacitor in real time;

[0014] (3) Use the “voltage inner loop” to assist in regulating and controlling the system DC bus voltage.

[0015] Further, in step (1), the second-order high-pass filter is expressed as:

[0016]

[0017] Among them, H 2-HPF (s) is the second-order high-pass filter transfer function, and T1 and T2 are the filter time constants.

[0018] Furthermore, in step (1), the conversion process of the high frequency part and the low frequency part is as follows:

[0019] (1) Load power requirement P req After passing through a second-order high-pass filter, the high-frequency component P is extracted SC-req ;

[0020] (2) The extracted high-frequency power component P SC-req With DC bus voltage U bus Divide to get the given current I of the supercapacitor branch high-SC-ref ;

[0021] (3) Load current I load Subtract the given current I of the supercapacitor branch high-SC-ref The resulting difference becomes the current the battery needs to respond to.

[0022] Furthermore, in step (2), the loop form of the "current outer loop" is: the given value I high-SC-ref The high-side current feedback value I high-SC The current difference ΔI is used to generate a DC bus voltage given by U through the controller. bus-ref .

[0023] Furthermore, in step (3), the loop form of the “voltage inner loop” is: the DC bus voltage is given by U bus-ref With DC bus voltage U bus The voltage difference ΔU is compared to construct a voltage inner loop, and the controller generates a pulse width modulation (PWM) signal to drive the bidirectional DC-DC converter to work.

[0024] Furthermore, in the control loop of "current outer loop" - "voltage inner loop", proportional-integral-differential (PID) controllers are used to respectively control the outer loop current error and the inner loop voltage error.

[0025] Furthermore, in the control loop of "current outer loop-voltage inner loop", the integrators and total output ends of the two PID controllers use limiting links to limit the output signal amplitude. If the accumulated value reaches the upper and lower limits allowed by the integrator or the output end, the value of the integrator and the output end are maintained unchanged.

[0026] The effects of the present invention are:

[0027] 1. The supercapacitor SoC can recover autonomously without setting a specific charging time.

[0028] 2. The SoC of supercapacitors will hardly reach the lower limit, and the operation stability is higher.

[0029] 3.Using a supercapacitor with a smaller capacity can meet the operating requirements.

[0030] 4. The number of continuous discharges is greatly increased.

[0031] 5. Auxiliary control of DC bus voltage. bus When a large fluctuation occurs due to load changes, the voltage loop can be limited by limiting the voltage. bus Keep it at a reasonable level.

[0032] 6. If the power battery operates abnormally or the power battery terminal voltage is lower than the discharge cut-off voltage (the minimum limit set in the controller) due to increased discharge depth, the supercapacitor interface bidirectional DC-DC converter branch can support the DC bus voltage and maintain the normal operation of the electric vehicle for a period of time. At this time, the supercapacitor branch works in the voltage source state.

[0033] 7.U bus It can characterize the change of system power. Compared with the single current loop control, the bidirectional DC-DC converter high voltage side current outer loop and bus voltage inner loop use I high-SC and U bus As a feedback quantity, it can reflect the power change of the system more comprehensively and accurately, and compensate for the control error caused by the current collection deviation. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a structural diagram of a semi-active multi-source system of a supercapacitor interface bidirectional DC-DC converter.

[0035] Figure 2 It is the logarithmic amplitude-frequency characteristic comparison curve of the proposed second-order high-pass filter and the traditional first-order high-pass filter.

[0036] Figure 3 This is the current step response curve of a traditional first-order high-pass filter.

[0037] Figure 4 is the current step response curve of the proposed second-order high-pass filter.

[0038] Figure 5 This is a schematic diagram of the changes in supercapacitor SoC under the traditional control strategy.

[0039] Figure 6 It is a schematic diagram of the changes of supercapacitor SoC under the proposed control strategy.

[0040] Figure 7 It is the control block diagram of the proposed “converter high-voltage side current outer loop-DC bus voltage inner loop” based on the second-order filter.

[0041] Figure 8 It is the experimental waveform of the traditional first-order high-pass filter under the loading and unloading conditions of electric vehicles.

[0042] Fig. 9 It is the experimental waveform of the second-order high-pass filter proposed and designed under the loading and unloading conditions of electric vehicles. DETAILED DESCRIPTION

[0043] The technical solution and implementation effects of the present invention will be further described in detail below with reference to the accompanying drawings.

[0044] The structure of the semi-active multi-source system of the supercapacitor interface bidirectional DC-DC converter (BDC) is as follows: Figure 1 As shown. The electric vehicle multi-source system consists of an energy source, a bidirectional DC-DC converter, a DC bus, a load, a controller and a sensor. Among them, the energy source is composed of a power battery and a supercapacitor. The topological structure of the electric vehicle energy source system is a semi-active structure of a supercapacitor interface bidirectional DC-DC converter. Among them, the power battery is directly connected to the DC bus, and it directly clamps the DC bus voltage as a voltage source. The supercapacitor is connected to the DC bus through a bidirectional DC-DC converter. The inverter and the motor are connected to the DC bus as loads. The battery and the supercapacitor provide energy to the load as energy sources. The quantities that need to be collected in the system include: DC bus voltage U bus , supercapacitor voltage U SC , battery current I bat , the high voltage side current of the converter connected to the supercapacitor I high-SC 、Load current I load .

[0045] To achieve power control of the power battery and supercapacitor, the supercapacitor branch needs to be controlled as a current source. At this time, the control target of BDC is the output current I of the supercapacitor. high-SC , then based on the relatively stable DC bus voltage U bus , the output power P of the supercapacitor SC Becomes a controllable quantity. Since the load side power demand P req The output power P of the power battery is determined. bat Equal to the load side demand power P req Subtract the supercapacitor output power P SC , the output current of the power battery I bat Equal to the output power P of the power battery bat Divide by the DC bus voltage. Therefore, the load side power requirement P req (or load current I load ) is used as the feedback of BDC to realize power distribution between the two energy sources of power battery and supercapacitor.

[0046] The constructed second-order filter is expressed as:

[0047]

[0048] H 2-LPF (s) is the second-order low-pass filter transfer function, H 2-HPF (s) is the transfer function of the second-order high-pass filter, where a, b, and c are the coefficients of the filter.

[0049] If a second-order filter is used for power division, the system requires power P req The high-frequency power obtained after high-pass filter is given as P SC-ref The energy accumulated by integrating over time is W SC-ref for:

[0050]

[0051] From the above calculation, we can know that if the coefficient of the first-order term in the numerator of the second-order high-pass filter used is b=c, that is, the numerator of the filter transfer function expression has only a quadratic term, the system high-frequency power P SC-ref The final value of the integral over time is 0. At this point, the process from the occurrence of power fluctuations in the system to the system reaching a new steady state is P SC-ref The energy emitted by the controlled supercapacitor is equal to the energy absorbed, and the total energy inside the supercapacitor does not change before and after the power change. Also because:

[0052]

[0053] and:

[0054] SOC SC ∝U SC 2

[0055] This means that the SoC of the supercapacitor remains unchanged before and after the power change, that is, the supercapacitor SoC can achieve self-recovery in the system demand power change of "steady state-dynamic-steady state".

[0056] When b = c, the transfer function of the second-order low-pass / high-pass filter is as follows:

[0057]

[0058] The logarithmic amplitude-frequency characteristic curves of the proposed second-order filter and the first-order filter are compared as follows: Figure 2 As shown in the figure, by comparing the two, it can be seen that the second-order high-pass filter has a faster attenuation rate in the low-frequency band and has a better filtering effect on the low-frequency power demand of the system.

[0059] When T1=T2=T, the second-order high-pass filter has only one turning frequency 1 / T. In the frequency range below this turning frequency, the logarithmic amplitude-frequency curve decays at a rate of -40dB, and in the frequency range above this turning frequency, the logarithmic amplitude-frequency characteristic curve is a horizontal straight line. This means that the low-frequency power demand has a very fast decay rate at the 1 / T frequency, so when designing a second-order high-pass filter, the two turning frequencies can be set to equal values ​​to make the filter more frequency-selective.

[0060] At this time, the transfer function of the second-order filter is as follows:

[0061]

[0062] When T=1, the current step response curves of the traditional first-order HPF and the proposed second-order HPF are as follows: Figure 3 and Figure 4 As shown. Both the first-order filter and the second-order filter can divide the required power on the load side into a low-frequency part and a high-frequency part. The low-frequency part is borne by the battery, and the high-frequency part is borne by the supercapacitor. However, the supercapacitor current I SC After the output peak current, there is a stage where the current crosses zero and becomes negative. In this stage, the supercapacitor absorbs energy from the battery to enable the supercapacitor SoC to self-recover.

[0063] The schematic diagrams of supercapacitor SoC changes under the traditional control strategy and the proposed control strategy are shown in Figure 2. Figure 5 and Figure 6 As shown. Under the traditional control strategy, the supercapacitor is only responsible for responding to peak current demands during the startup and acceleration phases. Therefore, each time the supercapacitor responds to a startup or acceleration command, it means a reduction in the energy reserves of the supercapacitor. Until the SoC of the supercapacitor drops to the specified lower limit, the supercapacitor can no longer bear the peak current demand. The traditional control strategy is to charge the supercapacitor separately in a specific time period when the supercapacitor energy is insufficient, and charge the supercapacitor to a full charge state at one time. The traditional control method always goes through the cycle of "SoC reaches the lower limit - detects the lower limit state - switches the working mode - charges". The disadvantages of the traditional strategy are:

[0064] (1) The SoC of the supercapacitor frequently reaches the lower limit, which has an adverse effect on the operating stability of the electric bicycle;

[0065] (2) It always needs to be charged separately after the energy is exhausted, and the timing of supercapacitor charging is uncertain;

[0066] (3) The number of times a supercapacitor can be discharged is very limited;

[0067] (4) The supercapacitor capacity required to maintain multiple discharge capabilities is relatively large, and the corresponding weight and volume are also large.

[0068] The core of the new control method is that after the supercapacitor releases peak current each time, the battery replenishes the supercapacitor's energy, and the supercapacitor SoC after replenishment can be restored to the state before the peak current is released. This replenishment process is completed automatically. The advantages of the new strategy are:

[0069] (1) The supercapacitor SoC can self-recover, without the need to set a specific charging time or complicated control;

[0070] (2) The SoC of the supercapacitor will hardly reach the lower limit, there is no mode switching, and the operation stability is higher;

[0071] (3) The number of continuous discharges is greatly increased;

[0072] (4) Due to the self-healing characteristics of the supercapacitor SoC, when designing the supercapacitor capacity, it is only necessary to consider the maximum drop of the SoC during a single output peak current, so a smaller capacity can meet the operating requirements.

[0073] The structure diagram of the control strategy based on second-order filtering, BDC high-voltage side current outer loop and bus voltage inner loop is as follows: Figure 7 The specific control logic is as follows:

[0074] (1) Load power requirement P req After passing through a second-order high-pass filter, the high-frequency component P is extracted SC-req ;

[0075] (2) The extracted high-frequency power component P SC-req With DC bus voltage U bus Divide to get the given current I of the supercapacitor branch high-SC-ref ;

[0076] (3) Load current I load Subtract the given current I of the supercapacitor branch high-SC-ref The resulting difference becomes the current the battery needs to respond to.

[0077] (4) The loop form of the “current outer loop” is: The given value I high-SC-ref The high-side current feedback value I high-SC The current difference ΔI is used to generate a DC bus voltage given by U through the controller. bus-ref .

[0078] (5) The loop form of the “voltage inner loop” is: DC bus voltage given by U bus-ref With DC bus voltage U busThe voltage difference ΔU is compared to construct a voltage inner loop, and the controller generates a pulse width modulation (PWM) signal to drive the bidirectional DC-DC converter to work.

[0079] (6) In the control loop of "current outer loop" and "voltage inner loop", proportional-integral-differential (PID) controllers are used to control the outer loop current error and inner loop voltage error respectively. The integrators and total output ends of the two PID controllers use a limiting link to limit the output signal amplitude. If the accumulated value reaches the upper and lower limits allowed by the integrator or the output end, the value of the integrator and the output end are maintained unchanged.

[0080] The parameters of a specific embodiment of the present invention are shown in Table 1 below.

[0081] The experimental results of first-order and second-order HPF control under load-addition and load-reduction (simulating uphill and downhill) conditions are as follows: Figure 8 and Fig. 9 As shown. Initially, the load current is 1.2A, all provided by the power battery. At the moment of increased load, the supercapacitor branch quickly responds to the power demand and outputs transient current, and the power battery current slowly rises to the load current. At this time, the load current is 4.0A. At the moment of reduced load, the supercapacitor branch also takes the lead in responding to the power demand and inputs transient current, and the power battery current slowly decreases to the load current. At this time, the load current is 1.2A.

[0082] For the first-order HPF, the voltage at the supercapacitor terminal drops from 24.5V to 23.6V during the process of increasing the load, and rises from 23.6V to 24.5V during the process of reducing the load. This shows that in the process of "steady state-dynamic-steady state", the supercapacitor SOC of the latter steady state under the control of the first-order HPF cannot be restored to the state of the previous steady state. For the second-order HPF, the supercapacitor terminal voltage is 27.0V before and after the dynamic fluctuation during the process of increasing and reducing the load. The experiment verifies that the method proposed in this paper can make the supercapacitor SOC self-recovering.

[0083] Table 1 Bidirectional Buck / Boost converter parameters

[0084]

[0085] The present invention can be implemented in other specific forms without departing from its spirit or essential characteristics. The described embodiments are considered to be illustrative and non-restrictive in all aspects, for example:

[0086] 1) The bidirectional DC-DC converter used in the multi-source system is not limited to the topology type and parameter configuration in the embodiment;

[0087] 2) The voltage, capacity and other parameters of the battery and supercapacitor in the multi-source system are not limited to those used in the embodiment;

[0088] 3) The control parameters such as the filter time constant used in the collaborative control method are not limited to those used in the embodiment.

[0089] The scope of the invention is therefore indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalent technical solutions of the claims are embraced within their scope.

Claims

1. A method for controlling the coordinated operation of a multi-source system of an electric vehicle, wherein the multi-source system of the electric vehicle is composed of an energy source, a bidirectional DC-DC converter, a DC bus, a load, a controller and a sensor, wherein the energy source is composed of a power battery and a supercapacitor, and the structure of the multi-source system is a semi-active structure of a bidirectional DC-DC converter with a supercapacitor interface; characterized in that: The control method comprises the following steps: (1) A second-order high-pass filter is used to divide the load power demand into a high-frequency part and a low-frequency part. These two parts of the power demand are converted into given currents that the battery and supercapacitor need to respond to respectively; (2) Using the "current outer loop" to track the given current of the supercapacitor in real time; (3) Use the "voltage inner loop" to assist in regulating and controlling the system DC bus voltage; In step (1), the second-order high-pass filter is expressed as: Among them, H 2-HPF (s) is the second-order high-pass filter transfer function, T1 and T2 are the filter time constants; The conversion process of the high frequency part and the low frequency part is as follows: ① Load demand power P req After passing through a second-order high-pass filter, the high-frequency component P is extracted SC-req ; ② The extracted high frequency power component P SC-req With DC bus voltage U bus Divide to get the given current I of the supercapacitor branch high-SC-ref ; ③Load current I load Subtract the given current I of the supercapacitor branch high-SC-ref The resulting difference becomes the current the battery needs to respond to.

2. The electric vehicle multi-source system coordinated operation control method according to claim 1, characterized in that: In step (2), the loop form of the "current outer loop" is: the given value I high-SC-ref The current feedback value I of the high side of the bidirectional DC-DC converter high-SC By comparison, the current outer loop is constructed, and the current difference ΔI is generated by the controller to generate a DC bus voltage given by U bus-ref .

3. The electric vehicle multi-source system coordinated operation control method according to claim 1, characterized in that: In step (3), the loop form of the "voltage inner loop" is: DC bus voltage given U bus-ref With DC bus voltage U bus The voltage difference ΔU is compared to construct a voltage inner loop, and the controller generates a pulse width modulation (PWM) signal to drive the bidirectional DC-DC converter to work.

4. A method for controlling coordinated operation of a multi-source system of an electric vehicle as claimed in claim 2 or 3, characterized in that: In the control loop of "current outer loop" - "voltage inner loop", proportional-integral-derivative (PID) controllers are used to control the outer loop current error and the inner loop voltage error respectively.

5. The electric vehicle multi-source system coordinated operation control method according to claim 4, characterized in that: In the control loop of "current outer loop-voltage inner loop", the integrators and total output ends of the two proportional-integral-differential (PID) controllers use limiting links to limit the output signal amplitude. If the accumulated value reaches the upper and lower limits allowed by the integrator or the output end, the value of the integrator and the output end are maintained unchanged.

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