Virtual DC motor parameter control method for bidirectional DC / DC converter

Abstract

The present invention discloses a virtual DC motor parameter control method for a bidirectional DC / DC converter. According to the equivalence principle of a DC motor and a converter bidirectional DC / DC converter, the corresponding relationship of parameters is analyzed and virtual DC motor control is introduced. A parameter adaptive control method is introduced in a DC microgrid to establish an adaptive parameter virtual machine control model of a bidirectional DC / DC converter. Based on P-U droop control, the present invention proposes an adaptive parameter control method based on VDCM. By making the DC / DC converter and the DC motor equivalent in the electromagnetic induction link and the mechanical rotation link, the inertial response process of the DC motor rotor to power fluctuation is simulated, and a general control model of VDCM control is established. The adjustment and setting principles of parameters are given to realize the inertial response of the DC / DC converter output voltage to power fluctuation, improve the inertia of the DC bus voltage, and enhance the reliability of the load power supply in the grid.

Description

Technical Field

[0001] The invention belongs to the technical field of converter current control, and in particular relates to a virtual direct current motor parameter control method for a bidirectional DC / DC converter. Background Art

[0002] DC microgrid is an important part of the future intelligent power distribution system, which is of great significance to promoting energy conservation, emission reduction and sustainable development. It is easy to connect with DC power generation equipment such as photovoltaics and energy storage and DC loads, reduce the inverter link, reduce system cost and loss, and have high power supply efficiency. In addition, compared with AC power grid, DC microgrid does not have problems such as frequency stability and reactive power, and it is easy to achieve coordinated control between micro power sources.

[0003] In a DC microgrid, distributed power sources, energy storage devices, and loads are connected to the DC bus through power electronic converters. The power electronic converters themselves lack inertia and damping. Therefore, when the load power of the DC microgrid fluctuates, the instantaneous voltage shock and fluctuation will affect the stability of the DC bus voltage, which is not conducive to the reliable operation of the DC microgrid. This low inertia characteristic reduces the transient stability of the microgrid when the load increases or decreases or the power generation equipment is put into or cut out. The bus voltage of the DC microgrid will be disturbed or even collapsed. Therefore, effective measures need to be taken for the DC microgrid to ensure its stable operation.

[0004] At present, DC microgrids mostly use droop control to achieve power distribution between micro-sources. This control method realizes autonomous power current sharing, but lacks the inertia of traditional generators and has a too fast transient response speed. In order to make droop control have inertia regulation function, virtual DC machine (VDCM) control technology has been proposed, but the existing VDCM control method is limited to the simulation of DC generator characteristics, and does not consider the flexibility of power electronic converter control system. The moment of inertia and damping coefficient can be adaptively selected according to the actual application scenario. More appropriate values. Therefore, the impact of DC bus voltage dynamic characteristics and its inertial response problem need further study. Summary of the invention

[0005] The purpose of the present invention is to provide a virtual DC motor parameter control method for a bidirectional DC / DC converter, which improves the transient characteristics of a DC microgrid and then controls the stability of the DC bus voltage.

[0006] The technical solution adopted by the present invention is a virtual DC motor parameter control method for a bidirectional DC / DC converter, which is specifically carried out according to the following steps:

[0007] Step 1: According to the equivalent principle of DC motor and converter bidirectional DC / DC converter, the corresponding relationship of parameters is analyzed and virtual DC motor control is introduced;

[0008] Step 2: Introduce the parameter adaptive control method in the DC microgrid and establish an adaptive parameter virtual machine control model of the bidirectional DC / DC converter.

[0009] The present invention is also characterized in that:

[0010] In step 1, specifically:

[0011] Step 1.1, establish the DC motor rotor motion equation, as shown in formula (1);

[0012]

[0013] Where P m , P e are the input mechanical power and output electromagnetic power of the DC motor respectively, J is the rotor inertia constant, ω N is the rated angular velocity of the DC motor, ω is the angular velocity of the rotor, and D is the damping coefficient;

[0014] At the same time, the rated rotor angular velocity ω N , as shown in formula (2):

[0015] ω N =U N / k f (2);

[0016] In the formula, k f is the excitation constant of the DC motor;

[0017] In the DC microgrid, power balancing adopts droop control, and the expression is shown in formula (3);

[0018]

[0019] In the formula, with U N are the given voltage and the maximum limit voltage of the DC bidirectional DC converter, P o and P min are the DC output power and the minimum absorbed power, respectively, p is the droop coefficient in droop control;

[0020] Comparing equation (1) and equation (3), the output power P o Equivalent to the input mechanical power P of the virtual mechanical rotating link m , minimum output power P min Equivalent to the output electromagnetic power P e ; At the same time, the virtual moment of inertia J of the DC generator rotor vir The first-order differential of the angular velocity is introduced into the PU droop control, and the virtual moment of inertia J virIn power electronic devices, it is equivalent to capacitor C, and The droop control VDCM control expression is obtained as shown in formula (4):

[0021]

[0022] In step 2, specifically:

[0023] By referring to the exponential form of the differential e x Combined with the adjustment coefficient K, the parameters J and D can be increased or decreased based on the original fixed parameters. According to the required virtual inertia coefficient and virtual damping response under different working conditions, the principle of determining the values ​​of J and D in the VDCM parameter adaptive control is: When ΔU<0: The adaptive adjustment equation is shown in equations (5) and (6);

[0024]

[0025] Similarly, when ΔU>0, the adaptive adjustment equations are shown in equations (7) and (8);

[0026]

[0027] Where K and H are adjustment coefficients respectively.

[0028] The beneficial effect of the present invention is that, in view of the defect that traditional PU droop control lacks inertial response capability to power fluctuations in the grid, an adaptive parameter control method based on VDCM is proposed based on PU droop control. The DC / DC converter and the DC motor are made equivalent in the electromagnetic induction link and the mechanical rotation link, the inertial response process of the DC motor rotor to power fluctuations is simulated, a general control model of VDCM control is established, and the adjustment and setting principles of parameters are given to realize the inertial response of the DC / DC converter output voltage to power fluctuations, improve the inertia of the DC bus voltage, and enhance the reliability of power supply to loads in the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural schematic diagram of a virtual DC motor parameter control system for a bidirectional DC / DC converter according to the present invention;

[0030] Figure 2 It is a control block diagram of a virtual DC motor parameter control system for a bidirectional DC / DC converter of the present invention;

[0031] Figure 3 It is the VDCM control block diagram of the present invention;

[0032] Figure 4 It is a simplified DC microgrid simulation system architecture diagram of a virtual DC motor parameter control system for a bidirectional DC / DC converter of the present invention;

[0033] Figure 5 The method of the present invention is different from the traditional VDCM control method when the load fluctuates. load Simulation waveform comparison chart (I);

[0034] Figure 6 The method of the present invention is different from the traditional VDCM control method when the load fluctuates. load Simulation waveform comparison chart (2). DETAILED DESCRIPTION

[0035] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The present invention is used for a virtual DC motor parameter control system of a bidirectional DC / DC converter, such as Figure 1 As shown, it includes two parts: bidirectional DC / DC converter topology circuit and its control circuit. in is the battery terminal voltage, L1 is the inductance in the converter, R is the resistance, i in and i o are the battery output current and the converter output current respectively, C is the DC side capacitance, u load The specific structure of the topological circuit is as follows: the left side is connected to the battery, and the current direction is switched through two switches in the same bridge arm (that is, when S1 is turned on and S2 is turned off, energy flows from the battery to the DC side; when S2 is turned on and S1 is turned off, energy flows from the DC side to the battery), and the converter output on the right side is connected to the DC bus through the voltage stabilizing capacitor C;

[0037] The basic working principle of the bidirectional DC / DC converter is that when the DC bus voltage u load Higher than rated voltage U N When the DC bus voltage is lower than U N When , it indicates that the system power is insufficient, then the switch tube S2 is closed, S1 is opened, and the battery is discharged, that is, providing power for the DC side;

[0038] The specific control block diagram of the control circuit is as follows Figure 2 As shown, specifically, the sampling converter outputs the current i o and DC bus voltage u load Calculate the output power P of the converter o Then, the DC bus reference voltage value u is calculated according to formula (6): in_ref , and then through PI u The controller and the limiter obtain the battery output current reference value i in_ref , and the actual battery output current io After comparison, the output is transmitted to the PWM pulse signal generator through an inertia link and then a limiting link to generate a pulse signal to control the on and off of S1 and S2, thereby maintaining the stability of the DC bus voltage.

[0039] The basic principle of pulse width modulation (PWM): Its control method is to control the on and off of the switching devices of the bidirectional DC / DC conversion circuit so that the output end obtains a series of pulses with equal amplitudes, and uses these pulses to replace the sine wave or the required waveform. That is, multiple pulses are generated in half a cycle of the output waveform, so that the equivalent voltage of each pulse is a sine wave, and the obtained output is smooth and has few low-order ramp harmonics. By modulating the width of each pulse according to certain rules, the output voltage of the conversion circuit can be changed.

[0040] The virtual DC motor parameter control method for a bidirectional DC / DC converter of the present invention is specifically performed according to the following steps:

[0041] Step 1: According to the equivalent principle of DC motor and converter bidirectional DC / DC converter, the corresponding relationship of parameters is analyzed and virtual DC motor control is introduced; specifically:

[0042] Step 1.1, establish the DC motor rotor motion equation, as shown in formula (1);

[0043]

[0044] Where P m , P e are the input mechanical power and output electromagnetic power of the DC motor respectively, J is the rotor inertia constant, ω N is the rated angular velocity of the DC motor, ω is the angular velocity of the rotor, and D is the damping coefficient;

[0045] At the same time, the rated rotor angular velocity ω N , as shown in formula (2):

[0046] ω N =U N / k f (2);

[0047] In the formula, k f is the excitation constant of the DC motor;

[0048] In DC microgrids, droop control is often used for power balancing, and the expression is shown in formula (3);

[0049]

[0050] In the formula, with U Nare the given voltage and the maximum limit voltage of the DC bidirectional DC converter, P o and P min are the DC output power and the minimum absorbed power, respectively, p is the droop coefficient in droop control, the control block diagram is as follows Figure 3 As shown;

[0051] Traditional large power grids rely on the rotation of the generator rotor to provide inertia. When the motor generates ΔP=(P m -P e ) When the power fluctuates, the mechanical potential energy of the motor rotor is released instantly to maintain a smooth transition of the speed, so that the motor speed has a strong inertial response ability to power fluctuations, which ensures the stable operation of the large power grid to a certain extent.

[0052] Comparing equation (1) and equation (3), the output power P o Equivalent to the input mechanical power P of the virtual mechanical rotating link m , minimum output power P min Equivalent to the output electromagnetic power P e At the same time, the virtual moment of inertia J of the DC generator rotor vir The first-order differential of the angular velocity is introduced into the PU droop control, and the virtual moment of inertia J vir In power electronic devices, it is equivalent to capacitor C, and The droop control VDCM control expression is obtained as shown in formula (4):

[0053]

[0054] In the droop control regulation of the bidirectional DC converter, the inertia link of the DC generator is introduced, and the mechanical rotation link of the rotor is virtualized in its control, so that the inertial response to the power fluctuation in the DC system network can be achieved, thereby achieving the purpose of improving the inertia of the DC bus voltage and enhancing the reliability of the load power supply in the DC subnet. The control method is simple, reliable, easy to implement, introduces few virtual parameters, does not need to introduce additional inertia devices, and is widely applicable to power electronic facilities, so it has strong versatility.

[0055] Step 2: Introduce the parameter adaptive control method in the DC microgrid and establish an adaptive parameter virtual machine control model of the bidirectional DC / DC converter, specifically:

[0056] When the synchronous generator is connected to the grid and runs, if load fluctuation occurs, the system will generate unbalanced power in an instant. Assuming that the virtual inertia coefficient J and the damping coefficient D can be adjusted in real time according to the changes in the bus voltage during the transient process, the frequency or power oscillation can be weakened to optimize the transient response of the system. In an actual synchronous generator, the virtual inertia coefficient J and the damping coefficient D are related to the rotor characteristics and are constant constants that cannot be adjusted. However, in the virtual inertia damping control of the DC microgrid, the virtual inertia coefficient J and the damping coefficient D are both virtual parameters, which can be flexibly adjusted. Therefore, the virtual inertia coefficient J and the virtual damping coefficient D can be adjusted in real time according to the changes in voltage (or output power) to improve the system response characteristics. The VDCM adaptive control proposed in the present invention is as follows: Figure 3 As shown in the figure, the required virtual inertia coefficient and virtual damping response under different working conditions are:

[0057]

[0058] The bus voltage change rate corresponds to the bus voltage change state trend. The bus voltage change trend can be determined by determining the positive or negative value of the voltage change rate. If the bus voltage change rate is positive, the bus voltage is in an upward state. If the bus voltage change rate is negative, the bus voltage is in a downward state. If the voltage change rate is zero, it can be determined that the voltage is in a peak state. In practical applications, it is simple, convenient and easy to implement to use a differential circuit to detect the voltage change rate. By using the exponential form of the differential e x Combined with the adjustment coefficient K, the parameters J and D can be increased or decreased based on the original fixed parameters. According to the required virtual inertia coefficient and virtual damping response under different working conditions, the principle of determining the values ​​of J and D in the VDCM parameter adaptive control is: When ΔU<0: The adaptive adjustment equation is shown in equations (5) and (6);

[0059]

[0060] Similarly, when ΔU>0, the adaptive adjustment equations are shown in equations (7) and (8);

[0061]

[0062] Wherein, K and H are adjustment coefficients respectively. Due to the existence of K and H, the moment of inertia and damping can be adaptively adjusted according to the change of du / dt. By adjusting J and D as above, it is possible to optimize the transient voltage stability adjustment and provide sufficient inertia and damping support. The VDCM adaptive control proposed by the present invention is as follows Figure 3 As shown in the figure, the parameter adaptive control is mainly added to the VDCM control link, and the voltage and current regulation parts are Figure 2 Consistent.

[0063] Figure 4 The figure shows a simplified DC microgrid simulation system architecture diagram. Among them, the photovoltaic side Boost circuit adopts Maximum Power Point Tracking (MPPT) control; the battery side bidirectional DC / DC converter adopts the virtual machine control method proposed in this invention. Figure 1 , Figure 2 and Figure 3 Build a simulation in MATLAB to simulate the initial photovoltaic output power P PV is 5000W; load power P Load The initial value is 4920W.

[0064] First, the feasibility of the present invention is verified when the load power fluctuates, while keeping the light intensity and temperature constant. Figure 5 The figure is a comparison of the bus voltage drop caused by the sudden increase in load at 0.5s, the traditional fixed parameter VDCM and the adaptive parameter VDCM adjustment process proposed by the present invention. It can be seen that when the power suddenly changes at 0.5s, the output voltage of the bidirectional DC converter experiences voltage oscillation and fluctuation. The output voltage of the bidirectional DC converter drops from 404.4V to 403.7V, and then adjusts to 404V, and then slowly adjusts to the steady-state value. The voltage waveform disturbance is more obvious, and the dynamic adjustment time is longer.

[0065] The present invention adopts an adaptive parameter VDCM control method to improve the dynamic stability of the output voltage of the bidirectional DC converter. Therefore, the output voltage of the DC / DC converter is smoothly adjusted to a steady-state voltage of 402.8V, the voltage waveform oscillation adjustment time and voltage drop are significantly reduced, the steady-state voltage drop of the load voltage is smaller, and the system's anti-interference ability is enhanced.

[0066] Figure 6 The figure shows the comparison of the bus voltage rise caused by the sudden load reduction at the moment of 0.8s, and the adjustment process of the traditional fixed parameter VDCM and the adaptive parameter VDCM proposed by the present invention. When the power in the grid suddenly changes again at 0.8s, the adaptive parameter VDCM control is adopted, and the output voltage of the DC / DC converter can still smoothly reach the steady-state value. The effectiveness of the adaptive parameter VDCM method is verified. Parameters J and D are adaptively adjusted according to ΔU according to equations (5), (6), (7) and (8). The dynamic response is still fast, and the voltage fluctuation peak is reduced. On the basis of keeping the DC bus and voltage fluctuation smaller, additional inertia and damping are provided for the system, and the DC bus voltage change rate is small, the recovery process time is short and there is almost no fluctuation, thereby effectively maintaining the stability of the DC bus voltage.

Claims

1. A virtual DC motor parameter control method for a bidirectional DC / DC converter, characterized in that: Follow these steps: Step 1: Based on the equivalent principle of DC motor and bidirectional DC / DC converter, analyze the corresponding relationship of parameters and introduce virtual DC motor control; specifically: Step 1.1: Establish the DC motor rotor motion equation, as shown in formula (1); Where, P m , P e are the input mechanical power and output electromagnetic power of the DC motor respectively, J is the rotor inertia constant, ω N is the rated angular velocity of the DC motor, ω is the angular velocity of the rotor, and D is the damping coefficient; At the same time, the rated rotor angular velocity ω can be obtained N , as shown in formula (2): ω N =U N / k f (2); Where k f is the excitation constant of the DC motor; In the DC microgrid, power balancing adopts droop control, and the expression is shown in formula (3); Where, with U N are the given voltage and maximum limit voltage of the DC bidirectional DC converter, P o and P min are the DC output power and the minimum absorbed power, k p is the droop coefficient in droop control; Comparing equations (1) and (3), the output power P o Equivalent to the input mechanical power P of the virtual mechanical rotating link m , the minimum output power P min Equivalent to the output electromagnetic power P e ; At the same time, the virtual moment of inertia J of the DC generator rotor vir The first-order differential of the angular velocity is introduced into the PU droop control, and the virtual moment of inertia J vir In power electronic devices, it is equivalent to capacitor C, and The droop control VDCM control expression is obtained as shown in formula (4): Step 2: Introduce a parameter adaptive control method into the DC microgrid and establish an adaptive parameter virtual machine control model for the bidirectional DC / DC converter; specifically: By referring to the exponential form of the differential e x Combined with the adjustment coefficient K, the parameters J and D can be increased or decreased based on the original fixed parameters. According to the required virtual inertia coefficient and virtual damping response under different working conditions, the principle of determining the values ​​of J and D in the VDCM parameter adaptive control is as follows: When ΔU < 0: The adaptive adjustment equations are shown in Equations (5) and (6); Similarly, when ΔU>0, the adaptive adjustment equations are shown in Equations (7) and (8); Where K and H are regulation coefficients, and du / dt is the bus voltage change rate.

Images