A Flexible Interconnection and Cooperative Control Method for Improving the Penetration Rate of New Energy

Abstract

The present invention discloses a flexible interconnection collaborative control method for improving the penetration rate of new energy, including: flexibly interconnecting a DC microgrid and a distribution network through a PET to construct a flexible interconnection collaborative control method with a two-level VSC at the front stage and a DAB at the rear stage; controlling the virtual inertia of the front-stage VSC using AVSG to obtain the control equation of AVSG; controlling the virtual inertia of the rear-stage DAB using a virtual capacitor to obtain the expression of the virtual capacitor current; evaluating the flexible interconnection collaborative control method through a simulation software using the transfer function of the load current and the two-stage output voltage to obtain the quantitative evaluation result of the flexible interconnection collaborative control method. The present invention relates to the technical field of new energy power system control, and solves the technical problems of poor stability of the single-phase shift control of the rear-stage DAB, resulting in an enlarged difference in the dynamic response of the two stages and limited penetration rate of new energy.

Description

Technical Field

[0001] The present invention belongs to the field of new energy power system control, relates to cooperative control technology, and specifically is a flexible interconnection cooperative control method for improving the penetration rate of new energy. Background Technique

[0002] With the promotion of the country's "dual carbon" goal, new energy technologies such as photovoltaics and wind power have matured, and building a new power system with new energy as the main body has become a trend. The high proportion of new energy and electric vehicles are connected to the distribution network, changing the composition of power sources and loads. The research on DC microgrids has emerged and is expected to become the main form of distributed power sources connected to the distribution network, and it has attracted much attention because of its important significance in the efficient utilization of new energy.

[0003] As a key device for coordinating the energy flow between the distribution network and the DC microgrid, the power electronic transformer (PET) is composed of fully controlled power electronic switching devices and high-frequency transformers and can replace traditional transformers. However, the transient stability of the DC microgrid is affected by the voltages before and after the PET. The topologies and control strategies of the converters before and after it are different. The independent control with low inertia in the front stage is likely to cause the superposition of voltage fluctuations and may lead to system instability.

[0004] Many existing literatures have studied the control of the front and rear stages of the PET. In terms of the front-stage control, methods include load current feedforward, establishing the relationship between the inverter and the current loop, energy feedforward, etc.; for the rear-stage control, there are double active bridge input voltage feedforward, load current feedforward, control strategies based on power balance, etc. Although these studies have a certain effect on suppressing the output voltage fluctuations of the front and rear stages of the PET, they all have limitations. Whether it is single-stage optimization or two-stage coordinated control, only the stability of one side of the PET DC link or output voltage is improved, and the anti-interference performance of the bus voltages on both sides is not improved simultaneously. The reason for the fluctuation of the front-stage output voltage lies in the different dynamic power characteristics of the two-stage converters. When only improving the dynamic response characteristics of the rear-stage converter to enhance the output voltage stability, it will inevitably exacerbate the voltage fluctuation of the front stage. At the same time, with the large-scale access of frequently fluctuating electric vehicle loads and high-penetration distributed power sources, the rotational inertia of the entire power grid decreases, which is not conducive to the stability of the system dynamic characteristics and reduces the ability of the power grid to access new energy. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art; for this purpose, the present invention proposes a flexible interconnection cooperative control method for improving the penetration rate of new energy, which is used to solve the technical problems that the single-phase shift control stability of the rear-stage DAB is poor, resulting in the expansion of the two-stage dynamic response difference and the limitation of the new energy penetration rate.

[0006] To achieve the above object, the present invention provides a flexible interconnection cooperative control method for improving the penetration rate of new energy, including:

[0007] S1. Flexibly interconnect the DC microgrid and the distribution network through a PET, and construct a flexible interconnection and coordinated control method with a two-level VSC at the front stage and a DAB at the rear stage;

[0008] S2. Control the virtual inertia of the front-stage VSC using AVSG to obtain the control equation of AVSG;

[0009] S3. Control the virtual inertia of the rear-stage DAB using a virtual capacitor to obtain the expression of the virtual capacitor current;

[0010] S4. Evaluate the flexible interconnection and coordinated control method through a simulation software using the transfer function of the load current and the two-stage output voltage to obtain the quantitative evaluation result of the flexible interconnection and coordinated control method.

[0011] Based on the above technical process, the present invention realizes the flexible interconnection between the DC microgrid and the distribution network through a two-stage virtual inertia control strategy. By combining the front-stage AVSG control and the rear-stage virtual capacitor control, the coordinated stability of the front-stage output DC voltage and the rear-stage DC bus voltage is achieved, effectively coping with the dynamic response differences of the two-stage converters, reducing voltage fluctuations, and avoiding the limitations of traditional methods that only optimize unilaterally. At the same time, the AVSG control and the virtual capacitor control introduce virtual inertia into the system, enabling the system to maintain stability during new energy fluctuations and load changes, improving the anti-interference ability, and thus increasing the new energy consumption rate.

[0012] It should be noted that the VSC (Voltage Source Converter) in the present invention represents a voltage source converter, which is the front-stage part of the power electronic transformer (PET);

[0013] DAB (Dual Active Bridge) represents a dual-source bridge, which is the rear-stage part of the PET;

[0014] AVSG (Analogous Virtual Synchronous Generator) represents an analogous virtual synchronous generator, which is used to control the virtual inertia of the front-stage VSC of the PET.

[0015] Furthermore, the acquisition method of the control equation of the AVSG includes:

[0016] By analogizing the DC voltage and the output current to the motor speed and the electromagnetic power, introduce virtual inertia into the control algorithm of the AVSG, and the control equation of the AVSG is obtained as: I set -i1-D b (u * dc -U N )=C v1 (du *dc / dt); where I set represents the reference value of the pre-stage output current, i1 represents the input current of the post-stage DAB, and u * dc represents the reference value of the pre-stage output DC voltage, and U N represents the rated value of the pre-stage output DC voltage, and C v1 represents the size of the virtual capacitor for AVSG control, and D b represents the damping coefficient, and t represents time.

[0017] By referring to concepts such as inertia, rotational speed, and electromagnetic power during motor operation, the control strategy can be more intuitively understood and designed, making the control of complex power electronic systems easier to analyze and optimize, and providing a new perspective and method for solving the stability and dynamic response problems of DC microgrids. At the same time, introducing virtual inertia into the control algorithm, through the term C v1 (du * dc / dt) in the control equation, the inertia of the pre-stage output DC voltage is enhanced, enabling the voltage to change more smoothly when the system faces external disturbances (such as load mutations, fluctuations in new energy output, etc.), avoiding sharp voltage fluctuations, thereby improving the stability of the system and laying a foundation for the stable access and efficient utilization of new energy in the microgrid.

[0018] Furthermore, the control strategy of the AVSG includes:

[0019] A1, the outer loop control uses the DC voltage reference value u * dc as the control target;

[0020] A2, the voltage loop participates in the control using the transfer function of the pre-stage voltage loop PI controller;

[0021] A3, the current loop participates in the control using the transfer function of the pre-stage current inner loop PI controller.

[0022] Furthermore, the voltage loop participates in the control using the transfer function of the pre-stage voltage loop PI controller, including:

[0023] A2-1, subtracting the actual pre-stage output DC voltage u dc from the reference value u * dc to obtain the voltage error;

[0024] A2-2, operating the voltage error through the transfer function G u1 of the pre-stage voltage loop PI controller: G u1 =k up1 +k ui1 / s to obtain the voltage loop control signal idref , namely the input reference value of the current loop; where k up1 , k ui1 respectively represent the proportional and integral coefficients of the front-stage voltage loop PI controller. After passing through G u1 operation, the obtained value is the input reference value of the current loop, and s represents the differential process;

[0025] A2-3, using the voltage control signal to adjust the output voltage of the front-stage converter.

[0026] The application of the transfer function of the front-stage voltage loop PI controller further improves the dynamic performance of the system. The proportional coefficient k up1 can quickly respond to the voltage error, enabling the system to make immediate adjustments when the voltage deviates from the reference value, thus accelerating the response speed of the system; the integral coefficient k ui1 is used to eliminate the steady-state error, ensuring the accuracy of the voltage output during the long-term operation of the system. The two work together to enable the voltage loop to achieve fast, accurate, and stable voltage control under different working conditions, effectively improving the dynamic performance and stability of the system.

[0027] Furthermore, the current loop participates in the control by using the transfer function of the front-stage current inner loop PI controller, including:

[0028] A3-1, subtracting the actual value i d of the current inner loop from the reference value i dref of the current inner loop to obtain the current error;

[0029] A3-2, passing the current error through the transfer function G i1 of the front-stage current inner loop PI controller: G i1 =k ip1 +k ii1 / s for operation to obtain the output signal of the current inner loop; where k ip1 , k ii1 respectively represent the proportional and integral coefficients of the front-stage current loop PI controller. After passing through G i1 operation, the obtained value is the output signal of the current inner loop;

[0030] A3-3, the output signal of the current inner loop undergoes equivalent gain processing to obtain the control signal of the front-stage converter;

[0031] A3-4, using the control signal of the front-stage converter to control the front-stage VSC.

[0032] The current loop and the voltage loop work closely together. By quickly processing the current error and comprehensively acting with the voltage control signal, the stability and response speed of the system are further enhanced. When the load current changes, the current loop can quickly sense and adjust the output current of the front-stage VSC, while assisting the voltage loop to stabilize the voltage. This dual-loop collaborative control mechanism enables the system to quickly balance power, maintain stable operation, and improve the system's adaptability to dynamic changes when facing complex and variable loads and new energy fluctuations.

[0033] Further, the formula for the equivalent duty cycle β is: β = 3u d / (2u dc ); where u d represents the amplitude of the power supply voltage on the AC side of the front-stage converter.

[0034] Further, the method for obtaining the virtual capacitor current expression includes:

[0035] Based on the charging and discharging mechanism of the DC capacitor, a virtual capacitor is introduced to improve the dynamic response of the DC bus output by the rear-stage DAB, and the virtual capacitor current expression of the rear-stage DAB of the PET is obtained: ; where T represents the unit sampling time, u o0 represents the DC bus voltage sampled in the previous sampling period, U on represents the reference value of the bus voltage, C v2 represents the rear-stage virtual capacitor, and u o represents the DC bus voltage sampled at the current moment.

[0036] By simulating the charging and discharging process of a real capacitor with the virtual capacitor current, additional inertia and energy buffering are provided for the DC bus output by the rear-stage DAB. When the system faces dynamic changes such as sudden load changes or new energy output fluctuations, the virtual capacitor can temporarily store or release energy like an actual capacitor, thereby effectively improving the dynamic response of the rear-stage DC bus voltage. At the same time, the relationship between the current bus voltage, the voltage in the previous sampling period, and the reference value of the bus voltage is considered in the virtual capacitor current expression. By comparing the voltage differences in adjacent sampling periods, the system can predict the direction and amplitude of voltage changes in advance, so as to adjust the control signal more timely and effectively, optimize the control effect of the rear-stage DAB, further reduce voltage fluctuations, and improve the dynamic performance of the system.

[0037] Further, the control strategy of the virtual capacitor includes:

[0038] B1, the virtual capacitor current is passed through the transfer function G u2 of the DAB voltage loop PI controller: G u2 = k up2 + k ui2 / s, and the small-signal transfer function G of the DC current compared with the phase shift DI2 : G DI2 = nu dc (1 - 2D0) / (2f s L1), acting together, to obtain the virtual phase shift ratio generated by the virtual capacitor ; where k up2 and k ui2 are respectively the proportional coefficient and integral coefficient of the PI controller of the post-stage voltage loop; D0 represents the steady-state value of the phase shift ratio, f s represents the DAB switching frequency, n represents the turns ratio of the DAB high-frequency transformer, and L1 represents the equivalent leakage inductance of the DAB;

[0039] B2, combines the virtual phase shift ratio with the total phase shift ratio signal D to obtain the post-stage control signal;

[0040] B3, uses the post-stage control signal to control the post-stage DAB.

[0041] The DAB voltage loop PI controller adjusts the control signal according to the voltage error, and its proportional and integral actions can ensure that the voltage is stable near the reference value; the small-signal transfer function of the DC current compared with the phase shift ratio reflects the relationship between the internal current and the phase shift ratio of the DAB. By the combined action of the two, the required virtual phase shift ratio can be accurately calculated according to the actual operating state of the system (such as voltage, current, etc.), so as to achieve precise control of the power transmission of the post-stage DAB and improve the power conversion efficiency and stability of the system.

[0042] Further, the load current and two-stage output voltage transfer function includes:

[0043] The transfer function of the pre-stage output voltage and the load current: u dc / -i o =(1 / n)(1 / C1s + G u1 G i1 β)G io_i2 (1 + G u1 G i G cv1 β); where i o represents the load current, s represents the differential process, G cv1 represents the transfer function from the input current i1 of the post-stage DAB to the DC voltage reference value u * dc generated by the pre-stage AVSG, and G cv1 = 1 / (D b + C v1 s), D b is the damping coefficient of the AVSG control loop, and G io_i2 represents the load current i oTransfer function to the DAB output current i2, and G io_i2 =[C v2 s / (Ts + 1)+G u2 G DI2 / [C2s + C v2 s / (Ts + 1)+G u2 G DI2 , G DI2 represents the transfer function of the phase shift ratio D to the DAB output current i2, C2 represents the DAB output capacitor;

[0044] Transfer function of the post - stage output voltage and the load current: u o / -i o =1 / [C2s+(C v2 s / Ts + 1)+G D12 G u2 .

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] Aiming at the problems that the single - phase - shift control of the post - stage dual - active bridge (DAB) of the traditional power electronic transformer (PET) is difficult to improve the DC voltage stability, and a series of problems caused by improving the post - stage output voltage stability, the present invention proposes the post - stage DAB virtual capacitor control and the front - stage converter - type virtual synchronous generator control respectively. The front - stage VSC provides inertia for the DC micro - grid to reduce the DC voltage fluctuation, and the post - stage DAB improves the stability of the front - stage output voltage and reduces the dynamic response performance gap between the two - stage converters, so that when the system faces complex situations such as load mutation and frequent fluctuations of new energy, the two - stage voltages can remain stable, avoid over - limit, and enhance the stability of the DC voltage. At the same time, the traditional PET control method has no inertia and no damping, which is easy to cause system instability, while the two - stage virtual inertia control of the present invention introduces virtual inertia into the system, effectively improving the system's ability to maintain stability and anti - interference, ensuring the stable operation of the system under various disturbances, and laying a solid foundation for the access and consumption of new energy;

[0047] By improving the dynamic response of the DC micro - grid voltage, the present invention effectively overcomes the limitation of the reduction of the grid rotational inertia and other adverse factors on the new - energy access ability, enables the grid to better adapt to the volatility of new energy, and improves the penetration rate of new energy in the interconnected system of the distribution network and the DC micro - grid; moreover, under working conditions such as load mutation, the method of the present invention can provide virtual inertia power support for the system, reduce the adverse impact of voltage fluctuation on new - energy consumption, and improve the system's acceptance and utilization ability of new energy. Brief Description of the Drawings

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0049] Figure 1 This is the DC microgrid topology structure based on PET provided by the present invention;

[0050] Figure 2 This is the control strategy block diagram of the front-stage VSC provided by the present invention;

[0051] Figure 3 This is the control block diagram of the front-stage AVSG provided by the present invention;

[0052] Figure 4 This is the control strategy block diagram of the rear-stage DAB provided by the present invention;

[0053] Figure 5 This is the control block diagram of the rear-stage DAB based on virtual capacitance control provided by the present invention;

[0054] Figure 6 This is the two-stage inertia control block diagram based on PET provided by the present invention;

[0055] Figure 7 This is the comparison diagram of the simulation results of the two-stage inertia control method and the traditional control method of the present invention under the load mutation scenario;

[0056] Figure 8 This is the flow schematic diagram of a flexible interconnection and collaborative control method for improving the new energy penetration rate provided by the present invention. Specific embodiments

[0057] The following will clearly and completely describe the technical solutions of the present invention in combination with the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0058] Please refer to Figures 1 - 8 , the first aspect embodiment of the present invention provides a flexible interconnection and collaborative control method for improving the new energy penetration rate, including:

[0059] S1, flexibly interconnect the DC microgrid and the distribution network through PET, and construct a flexible interconnection and collaborative control method with a two-level VSC in the front stage and a DAB in the rear stage;

[0060] S2. The virtual inertia of the front - stage VSC is controlled by AVSG to obtain the control equation of AVSG.

[0061] S3. The virtual inertia of the rear - stage DAB is controlled by a virtual capacitor to obtain the expression of the virtual capacitor current.

[0062] S4. The flexible interconnection and cooperative control method is evaluated by using the load current and the transfer function of the two - stage output voltage through simulation software to obtain the quantitative evaluation results of the flexible interconnection and cooperative control method.

[0063] In this embodiment, as shown in Figure 1 , the DC microgrid and the distribution network are flexibly interconnected through a power electronic transformer (PET) to ensure that power can flow bidirectionally between the distribution network and the DC microgrid, and a flexible interconnection and cooperative control architecture with a two - level VSC at the front - stage and a DAB at the rear - stage is constructed. In the figure, u ga , u gb , u gc are the grid - side voltages respectively, i ga , i gb , i gc are the grid - side currents respectively, L g is the AC filter inductor, C1 is the DC - link capacitor, and C2 is the DAB output capacitor; i dc , i1 are the DC voltages on both sides of the DC - link capacitor C1, i2, i o are the DC currents on both sides of the output capacitor C2, n is the turns ratio of the DAB high - frequency transformer, L1 is the equivalent leakage inductance of the DAB, and i1 = i2 / n, R g represents the AC grid - side resistance, and u o represents the output DC bus voltage.

[0064] The virtual inertia of the PET front - stage VSC is realized by the control of an analogous virtual synchronous generator (AVSG). By comparing the control of the virtual synchronous generator (VSG), the DC voltage and the output current are analogized to the motor speed and the electromagnetic power, so that the virtual inertia is introduced into the control algorithm of the converter to strengthen the inertia of the front - stage output DC voltage and suppress the front - stage voltage fluctuation. The control equation of AVSG is: I set -i1 - D b (u * dc -U N ) = C v1 (du * dc / dt); where, I set represents the reference value of the front - stage output current, u * dc represents the reference value of the front - stage output DC voltage, U N represents the rated value of the front - stage output DC voltage, Cv1 Indicates the size of the virtual capacitance controlled by AVSG, D b Indicates the damping coefficient, and t represents time.

[0065] As Figure 2 Shown is the pre-stage VSC control strategy based on AVSG. Among them, i dref Indicates the current reference value of the current inner loop of AVSG; the outer loop is the DC voltage reference value obtained from the AVSG control loop, and then the converter control signal is obtained through the voltage-current double closed loop.

[0066] Specifically, the outer loop takes the DC voltage reference value u * dc obtained through the AVSG control loop as the control target, and always makes the actual pre-stage output DC voltage u dc track and approach this reference value u * dc , to ensure the stability and accuracy of the pre-stage output voltage;

[0067] In the voltage loop control link, the actual pre-stage output DC voltage u dc is collected in real time, and it is subtracted from the DC voltage reference value u * dc to obtain the voltage error ;

[0068] Then the voltage error is input into the transfer function G u1 =k up1 +k ui1 / s of the pre-stage voltage loop PI controller for operation. Among them, the proportional coefficient k up1 determines the fast response ability of the controller to the voltage error. It multiplies the voltage error by k up1 to obtain the proportional term output, enabling the controller to quickly adjust to the voltage deviation. The integral coefficient k ui1 is used to eliminate the steady-state error. It integrates the voltage error and then multiplies it by k ui1 to obtain the integral term output. The proportional term and integral term outputs are added together to finally obtain the voltage control signal G u1 ; s represents the differential process;

[0069] The voltage control signal G u1 is used to adjust the output voltage of the pre-stage converter:

[0070] According to the magnitude and polarity of the voltage control signal G u1 , the drive signal of the power switch device is adjusted to make the actual output DC voltage u dc towards the voltage reference value u *dc Approximate, thereby reducing the voltage error. That is, if the voltage error is positive (i.e., udc > u * dc ), the output voltage is reduced by adjusting the duty cycle; conversely, if the voltage error is negative, the output voltage is increased, and continuous iterative adjustment is performed to stabilize the actual voltage near the reference value.

[0071] In the current loop control link, the actual value i of the inner current loop d is subtracted from its reference value i of the inner current loop determined according to the system operating state and control requirements dref to obtain the current error ;

[0072] The current error is input into the transfer function G of the PI controller of the pre-stage inner current loop i1 =k ip1 +k ii1 / s for operation. Similarly, the proportional coefficient k ip1 enables the controller to quickly respond to the current error and make timely adjustments to the current change. Its calculation method is to multiply the current error by k ip1 to obtain the proportional term output. The integral coefficient k ii1 is used to eliminate the current steady-state error. By integrating the current error and multiplying by k ii1 to obtain the integral term output, and adding the proportional term and integral term outputs to obtain the current control signal G i1 . The current control signal G i1 can accurately control the input current of the pre-stage VSC according to the situation of the current error, making it track the reference value i of the inner current loop dref , ensuring the power balance and stable operation of the system;

[0073] The voltage control signal G u1 and the current control signal G i1 are comprehensively processed. First, determine the weighting coefficients of the two according to the system design requirements (for example, determine the weights according to the emphasis on voltage stability and current response speed), and perform weighted summation on G u1 and G i1 according to the weighting coefficients to obtain the intermediate control signal. Then, multiply the intermediate control signal by the equivalent duty cycle β = 3u d / (2u dc ) (where u d is the grid-side voltage amplitude and u dc is the actual output DC voltage of the pre-stage). The equivalent duty cycle takes into account the relationship between the grid-side voltage and the pre-stage output DC voltage and can dynamically adjust the control signal according to the actual operating voltage of the system. Finally, multiply the result by the equivalent gain KPWM , the equivalent gain K PWM is related to factors such as the characteristics of the power switch device and the circuit topology of the front-stage VSC. It further adjusts the amplitude of the control signal to ensure that the control signal can accurately control the power switch device of the front-stage converter, and obtains the final control signal of the front-stage converter.

[0074] Finally, the front-stage VSC is controlled by using the control signal of the front-stage converter. The control signal is converted into the drive signal of the power switch device (such as IGBT). According to the level and pulse width of the drive signal, the on and off moments and the duration of the power switch device are controlled. By precisely controlling the switching state of the power switch device, the accurate regulation of the input current and output voltage of the front-stage VSC is realized, so that the front-stage VSC can stably output the DC voltage according to the system requirements, and achieve good power matching and collaborative work with the rear-stage DAB, ensuring the stable operation of the entire DC microgrid system. During the operation of the system, the output DC voltage u dc , current i d and other parameters are continuously monitored, and the control signal of the front-stage converter is continuously adjusted according to the parameter changes to dynamically optimize the control effect of the front-stage VSC to adapt to various working conditions such as system load changes and new energy output fluctuations.

[0075] In this embodiment, the virtual inertia of the rear-stage DAB of the PET is realized by virtual capacitor control. Imitating the charging and discharging effect of the DC capacitor, by introducing the virtual capacitor C v2 the distribution network transmits more supporting power to improve the dynamic response of the rear-stage output DC bus. The specific operation process is as follows:

[0076] Obtain the current DC bus voltage u o and the DC bus voltage u o0 sampled in the previous sampling period. At the same time, obtain the bus voltage reference value U on and the value of the rear-stage virtual capacitor C v2 to determine the unit sampling time T;

[0077] Calculate according to the virtual capacitor current expression ; The control strategy of the rear-stage converter with an additional virtual capacitor is as Figure 4 shown. The virtual capacitor current generated by the virtual capacitor releases virtual inertial power for the DC microgrid, thereby reducing voltage fluctuations;

[0078] It should be noted that Figure 4 in, u oref represents the reference value of the rear-stage DC bus voltage.

[0079] The virtual capacitor current passes through the transfer function G of the DAB voltage loop PI controlleru2 : G u2 = k up2 + k ui2 / s, and the small-signal transfer function G of the DC current compared with the phase shift DI2 : G DI2 = nu dc (1 - 2D0) / (2f s L1), acting together, to obtain the virtual phase shift ratio generated by the virtual capacitor ; where k up2 and k ui2 are respectively the proportional coefficient and the integral coefficient of the PI controller of the secondary voltage loop; D0 represents the steady-state value of the phase shift ratio, f s represents the DAB switching frequency, n represents the turns ratio of the DAB high-frequency transformer, and L1 represents the equivalent leakage inductance of the DAB;

[0080] Combine the virtual phase shift ratio with the total phase shift ratio signal D to obtain the secondary control signal;

[0081] Use the secondary control signal to control the secondary DAB. The power switching devices (such as IGBTs, etc.) in the secondary DAB adjust their on and off times according to the secondary control signal, thereby changing the phase shift ratio of the DAB. By precisely controlling the phase shift ratio, the control of the power transmission of the secondary DAB is realized, and then the DC bus voltage u o of the secondary is stabilized. During the operation of the system, continuously monitor the DC bus voltage u o , the virtual capacitor current and other parameters, and dynamically adjust the virtual phase shift ratio and the secondary control signal according to the changes of these parameters to ensure that the secondary DAB can adapt to various working conditions such as system load changes and new energy output fluctuations, and stably output DC voltage to provide a reliable power supply for the loads in the DC microgrid.

[0082] Finally, through the simulation software, use the transfer function of the load current and the two-stage output voltage to evaluate the flexible interconnection and coordinated control method, and obtain the quantitative evaluation result of the flexible interconnection and coordinated control method;

[0083] Among them, the transfer function of the load current and the two-stage output voltage includes:

[0084] The transfer function of the primary output voltage and the load current: u dc / -i o = (1 / n)(1 / C1s + G u1 G i1 β)G io_i2 (1 + G u1 G i G cv1 β); where s represents the differential process, Gcv1 Represents the transfer function of the post-stage DAB input current \(i_1\) to the DC voltage reference value \(u\) generated by the pre-stage AVSG, and \(G\) * dc is \(\frac{1}{(D cv1 + C b s)\), where \(D v1 is the damping coefficient of the AVSG control loop, and \(G b represents the transfer function of the load current \(i io_i2 to the DAB output current \(i_2\), and \(G o is \(\frac{[C io_i2 s / (Ts + 1)+G v2 G u2 G DI2}{[C2s + C v2 s / (Ts + 1)+G u2 G DI2}\), and \(G DI2 represents the transfer function of the phase shift ratio \(D\) to the DAB output current \(i_2\), and \(C2\) represents the DAB output capacitor;

[0085] Transfer function of the post-stage output voltage and the load current: \(u o / -i o =\frac{1}{[C2s+(C v2 s / Ts + 1)+G D12 G u2}\).

[0086] In this embodiment, a DC microgrid simulation model as shown in Figure 1 is built using MATLAB / Simulink simulation software. The simulation system parameters are shown in the following table:

[0087] System parameters Value <![CDATA[Preamplifier output DC voltage U n / v]]> 750 <![CDATA[Rated value of DC bus voltage U on / v]]> 400 <![CDATA[Equivalent inductance L on the AC side g / H]]> 1.1e-3 AC side equivalent resistance R / Ω 0.135 <![CDATA[Grid-connected voltage amplitude u d / v]]> 311 <![CDATA[Capacitance C1, C2 / F]]> 2500e-6 <![CDATA[DAB switching frequency f s / Hz]]> 50000 <![CDATA[DAB equivalent inductance L1 / H]]> 0.13e-3 <![CDATA[Preamplifier virtual capacitor C v1 / F]]> 0.1 <![CDATA[Prefix damping coefficient D b > 150 <![CDATA[Post-stage virtual capacitor C v2 / F]]> 0.5 Sampling frequency T / s 0.05

[0088] To verify the effectiveness of the proposed PET two-stage virtual inertia control strategy, compared with the traditional PET control method where the pre-stage uses voltage-current double closed-loop control and the post-stage uses single-phase-shift voltage control, a load mutation scenario is set to verify the ability of inertial power transmission to suppress DC voltage mutations. When the system runs to 1 s, the DC load suddenly decreases from 80 kw to 8 kw, and the voltage fluctuation simulation results of the front and rear stages are as shown in Figure 7 ;

[0089] As can be seen from Figure 7 , compared with the traditional control method, the proposed two-stage virtual inertia control (i.e., the flexible interconnection and collaborative control method of the present invention) can provide virtual inertial power support for the system during load mutation, thereby greatly reducing voltage fluctuations, enhancing DC voltage stability, and improving the new energy accommodation rate.

[0090] Some of the data in the above formula are calculated by removing the dimension and taking their numerical values. The formula is the one that is closest to the actual situation obtained through software simulation of a large amount of collected data. The preset parameters and preset thresholds in the formula are set by those skilled in the art according to the actual situation or obtained through simulation of a large amount of data.

[0091] The working principle of the present invention:

[0092] Step S1 constructs the basic topological structure for the interconnection of the DC microgrid and the distribution network;

[0093] In step S2, the control equation is obtained through AVSG control, determining the basis for calculating the control signal according to the system state; and through links such as the voltage loop, current loop, equivalent duty cycle, and equivalent gain, the control equation is converted into a control signal that can directly operate the VSC power switch device, thereby achieving precise control of the front-stage VSC.

[0094] In step S3, the virtual capacitance control of the rear-stage DAB cooperates with the front-stage VSC control to jointly achieve the stable and optimized operation of the entire DC microgrid system, and the precise control of the front-stage VSC also provides good input conditions for the stable operation of the rear-stage DAB;

[0095] In step S4, the entire flexible interconnection collaborative control method is evaluated using the load current and the transfer function of the two-stage output voltage, and the evaluation results can be used to further optimize the control strategy of the front-stage VSC to improve the performance of the entire system and achieve the goal of increasing the new energy penetration rate.

[0096] The above embodiments are only used to illustrate the technical method of the present invention and not to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical method of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical method of the present invention.

Claims

1. A flexible interconnection collaborative control method for improving the penetration rate of new energy, characterized in that: include: S1, the DC microgrid and the distribution network are flexibly interconnected through PET to build a flexible interconnection collaborative control method with a two-level VSC at the front stage and a DAB at the back stage; S2, the virtual inertia of the preceding VSC is controlled by AVSG, and the control equation of AVSG is obtained; S3, the virtual inertia of the subsequent DAB is controlled by a virtual capacitor, and the virtual capacitor current expression is obtained; S4, evaluating the flexible interconnection collaborative control method by using a load current and a two-stage output voltage transfer function through simulation software to obtain a quantitative evaluation result of the flexible interconnection collaborative control method; Among them, PET stands for power electronic transformer, VSC stands for voltage source converter, DAB stands for dual source bridge, and AVSG stands for quasi-virtual synchronous generator; The control equation of the AVSG is obtained by: By analogizing the DC voltage and output current to the motor speed and electromagnetic power, virtual inertia is introduced into the control algorithm of AVSG, and the control equation of AVSG is obtained as follows: set -i1-D b (u * dc -U N )=C v1 (du * dc / dt); where I set represents the reference value of the output current of the front stage, i1 represents the DAB input current of the rear stage, u * dc Indicates the reference value of the DC voltage output by the previous stage, U N Indicates the rated value of the DC voltage output of the previous stage, C v1 Indicates that AVSG controls the size of the virtual capacitor, D b represents the damping coefficient, and t represents the time; The load current and two-stage output voltage transfer function includes: Transfer function of the output voltage and load current of the previous stage: u dc / -i o =(1 / n)(1 / C1s+G u1 G i1 β)G io_i2 (1+G u1 G i G cv1 β); among them, u dc Indicates the actual output DC voltage of the front stage, i o Represents the load current, G i represents the front-stage current inner loop PI controller, n represents the DAB high-frequency transformer ratio, G cv1 Indicates the DC voltage reference value u generated by the rear-stage DAB input current i1 to the front-stage AVSG * dc The transfer function of G cv1 =1 / (D b +C v1 s), D b is the damping coefficient of the AVSG control loop, G io_i2 Indicates the load current i o The transfer function to the DAB output current i2, C1 represents the DC link capacitance, s represents the differential process, G u1 represents the transfer function of the front-stage voltage loop PI controller, and β represents the equivalent duty cycle; The transfer function of the output voltage and load current of the next stage: u o / -i o =1 / [C2s+(C v2 s / Ts+1)+G D12 G u2 ]; where u o Indicates the DC bus voltage sampled at the current moment, C v2 represents the virtual capacitor of the next stage, T represents the unit sampling time, G DI2 represents the transfer function from the phase shifter D to the DAB output current i2, C2 represents the DAB output capacitor, G u2 represents the DAB voltage loop PI controller transfer function; The load current i o The transfer function G to the DAB output current i2 io_i2 The expression of G is: io_i2 =[C v2 s / (Ts+1)+G u2 G DI2 ] / [C2s+C v2 s / (Ts+1)+G u2 G DI2 ].

2. According to claim 1, a flexible interconnection collaborative control method for improving the penetration rate of new energy is characterized in that: The control strategy of the AVSG includes: A1, outer loop control with DC voltage reference value u * dc as a control target; A2, the voltage loop uses the transfer function of the previous voltage loop PI controller to participate in the control; A3, the current loop uses the transfer function of the previous current inner loop PI controller to participate in the control.

3. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 2, characterized in that: The voltage loop uses the transfer function of the previous voltage loop PI controller to participate in the control, including: A2-1, the actual front stage output DC voltage u dc With reference value u * dc Perform subtraction to obtain the voltage error; A2-2, the voltage error is transferred through the previous voltage loop PI controller transfer function G u1 : G u1 =k up1 +k ui1 / s to calculate and obtain the voltage loop control signal, that is, the current inner loop reference value i dref ; where k up1 , k ui1 Respectively represent the proportional and integral coefficients of the front-stage voltage loop PI controller, and s represents the differential process; A2-3, using the voltage control signal to adjust the output voltage of the previous stage converter.

4. According to claim 3, a flexible interconnection collaborative control method for improving the penetration rate of new energy is characterized in that: The current loop uses the transfer function of the previous current inner loop PI controller to participate in the control, including: A3-1, the actual value of the current inner loop i d and the current inner loop reference value i dref Perform subtraction to obtain the current error; A3-2, the current error is transferred through the previous current inner loop PI controller transfer function G i1 : G i1 =k ip1 +k ii1 / s to calculate and obtain the current inner loop output signal; where k ip1 , k ii1 It indicates the proportional and integral coefficients of the PI controller of the previous current loop; A3-3, the current inner loop output signal is processed by equivalent duty cycle β and equivalent gain to obtain the control signal of the previous stage converter; A3-4, using the control signal of the previous stage converter to control the previous stage VSC.

5. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 4, characterized in that: The formula of the equivalent duty cycle β is: β = 3u d / (2u dc ), where u d Indicates the power supply voltage amplitude on the AC side of the previous converter.

6. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 5, characterized in that: The method of obtaining the virtual capacitor current expression includes: Based on the charging and discharging mechanism of DC capacitors, virtual capacitors are introduced to improve the dynamic response of the DC bus output by the subsequent DAB, and the virtual capacitor current expression of the subsequent DAB of PET is obtained: Δi vir =C v2 (du o / dt)=-(C v2 / T)[(U on -u o )-(U on -u o0 )]; where T represents the unit sampling time, u o0 Indicates the DC bus voltage sampled in the previous sampling cycle, U on Indicates the bus voltage reference value, C v2 Indicates the subsequent virtual capacitor, u o Indicates the DC bus voltage sampled at the current moment.

7. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 6, characterized in that: The control strategy of the virtual capacitor includes: B1, virtual capacitor current Δi vir The DAB voltage loop PI controller transfer function G u2 : G u2 =k up2 +k ui2 / s, and the small signal transfer function G of DC current and shift DI2 : G DI2 =nu dc (1-2D0) / (2f s L1), work together to obtain the virtual displacement ΔD generated by the virtual capacitor vir ; where k up2 , k ui2 They represent the proportional coefficient and integral coefficient of the PI controller of the subsequent voltage loop, D0 represents the steady-state value of the phase shift ratio, and f s represents the DAB switching frequency, n represents the DAB high-frequency transformer ratio, and L1 represents the DAB equivalent leakage inductance; B2, compare the virtual shift to ΔD vir Combined with the total shift phase signal D, the subsequent control signal is obtained; B3, uses the post-stage control signal to control the post-stage DAB.

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