Flexible interconnection cooperative control method for improving new energy permeability

Abstract

The invention discloses a flexible interconnection cooperative control method for improving new energy permeability, and the method comprises the steps: enabling a DC micro-grid and a power distribution network to be in flexible interconnection through PET, and constructing a flexible interconnection cooperative control method of which the front stage is a two-level VSC and the rear stage is a DAB; the virtual inertia of the front-stage VSC is controlled by adopting the AVSG to obtain a control equation of the AVSG; the virtual inertia of the post-stage DAB is controlled by a virtual capacitor to obtain a virtual capacitor current expression; and evaluating the flexible interconnection cooperative control method by using the load current and the two-stage output voltage transfer function through simulation software to obtain a quantitative evaluation result of the flexible interconnection cooperative control method. The invention relates to the technical field of new energy power system control, and solves the technical problems that the two-stage dynamic response difference is enlarged and the new energy permeability is limited due to the fact that the stability of post-stage DAB single-shift control is poor.

Description

Technical Field

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

[0002] With the advancement of the country's "dual carbon" goals, new energy technologies such as photovoltaics and wind power are maturing, and building a new power system with new energy as the main body has become a trend. The access of a high proportion of new energy and electric vehicles to the distribution network has changed the composition of power sources and loads. The rise of DC microgrid research is expected to become the main form of distributed power access to the distribution network, and it has attracted much attention because of its important significance in the efficient use 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) consists of fully controlled power electronic switching devices and high-frequency transformers, which can replace traditional transformers. However, the transient stability of the DC microgrid is affected by the voltage of the front and rear stages of the PET. The topological structures and control strategies of the front and rear stage converters are different. The independent control of the front stage with low inertia is prone to voltage fluctuation superposition, which may cause system instability.

[0004] There are many existing literatures that have studied the front-end and back-end control of PET. The front-end control includes methods such as load current feedforward, building the relationship between the inverter and the current loop, and energy feedforward; the back-end control includes dual active bridge input voltage feedforward, load current feedforward, and control strategies based on power balance. Although these studies have a certain effect on suppressing the fluctuation of the output voltage of the front-end and back-end of PET, they all have limitations. Whether it is single-stage optimization or two-stage coordinated control, they only improve the stability of one side of the PET DC link or output voltage, and do not simultaneously improve the anti-interference performance of the bus voltage on both sides. The reason for the fluctuation of the front-end output voltage lies in the different dynamic power characteristics of the two-stage converter. When only improving the dynamic response characteristics of the back-end converter to improve the output voltage stability, it is bound to aggravate the fluctuation of the front-end voltage. 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 is reduced, 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; to this end, the present invention proposes a flexible interconnected collaborative control method for improving the penetration rate of new energy, which is used to solve the technical problems that the stability of the rear-stage DAB single-shift control is poor, resulting in the widening of the difference in dynamic response between the two stages and the limited penetration rate of new energy.

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

[0007] Based on the above technical process, the present invention realizes the flexible interconnection between the DC microgrid and the distribution network through a two-level virtual inertia control strategy. By combining the front-stage AVSG control with 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 difference of the two-stage converter, reducing voltage fluctuations, and avoiding the limitation of only one-side optimization of the traditional method. At the same time, AVSG control and virtual capacitor control introduce virtual inertia into the system, so that the system can maintain stability when new energy fluctuates and load changes, improve anti-interference ability, and thus improve the new energy consumption rate.

[0008] It should be noted that the VSC (Voltage Source Converter) in the present invention refers to a voltage source converter, which is the front-stage part of a power electronic transformer (PET); DAB (Dual Active Bridge) means dual active bridge, which is the post-stage part of PET; AVSG (Analogous Virtual Synchronous Generator) refers to a virtual synchronous generator, which is used to control the virtual inertia of the VSC before the PET.

[0009] Furthermore, the control equation of the AVSG is obtained in the following manner: 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 * dcIndicates 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.

[0010] By drawing on concepts such as inertia, speed, and electromagnetic power in motor operation, we can more intuitively understand and design control strategies, making complex power electronic system control 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, virtual inertia is introduced into the control algorithm, and C v1 (du * dc / dt) term, which enhances the inertia of the DC voltage output of the previous stage, so that when the system faces external disturbances (such as sudden load changes, fluctuations in the output of new energy, etc.), the voltage changes can be smoother, avoiding sharp voltage fluctuations, thereby improving the stability of the system and laying the foundation for the stable access and efficient use of new energy in microgrids.

[0011] Furthermore, 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.

[0012] Furthermore, the voltage loop utilizes 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 i dref , that is, the current loop input reference value; where k up1 , k ui1 The proportional and integral coefficients of the PI controller of the previous voltage loop are respectively represented by G u1 The result after operation is the current loop input reference value, and s represents the differential process; A2-3, using the voltage control signal to adjust the output voltage of the previous stage converter.

[0013] The application of the transfer function of the front-stage voltage loop PI controller further improves the dynamic performance of the system. up1 It can respond quickly to voltage errors, allowing the system to make adjustments immediately when the voltage deviates from the reference value, thus speeding up the system's response speed; the integral coefficient k ui1 The two functions together 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.

[0014] Furthermore, the current loop utilizes 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 The proportional and integral coefficients of the PI controller of the previous current loop are respectively indicated. i1 After the operation, the output signal of the current inner loop is obtained; A3-3, the current inner loop output signal is processed by 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.

[0015] The current loop and the voltage loop work closely together, and the stability and response speed of the system are further enhanced by the rapid processing of current errors and the combined effect with the voltage control signal. When the load current changes, the current loop can quickly sense and adjust the output current of the previous VSC, while assisting the voltage loop to stabilize the voltage. This dual-loop collaborative control mechanism enables the system to quickly balance power and maintain stable operation when facing complex and changeable loads and new energy fluctuations, improving the system's ability to adapt to dynamic changes.

[0016] Furthermore, 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.

[0017] Furthermore, the virtual capacitor current expression is obtained in the following manner: 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: ; 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.

[0018] The virtual capacitor current simulates the charging and discharging process of the real capacitor, providing additional inertia and energy buffering for the DC bus output of the subsequent DAB. When the system faces dynamic changes such as sudden load changes or fluctuations in the output of new energy sources, the virtual capacitor can temporarily store or release energy like the actual capacitor, thereby effectively improving the dynamic response of the DC bus voltage of the subsequent stage. At the same time, the virtual capacitor current expression takes into account the relationship between the current bus voltage and the voltage of the previous sampling cycle and the bus voltage reference value. By comparing the voltage difference between adjacent sampling cycles, the system can predict the direction and amplitude of voltage changes in advance, thereby adjusting the control signal more timely and effectively, optimizing the control effect of the subsequent DAB, further reducing voltage fluctuations, and improving the dynamic performance of the system.

[0019] Furthermore, the control strategy of the virtual capacitor includes: B1, virtual capacitor current 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 phase produced by the virtual capacitor. ; where k up2 , k ui2 are the proportional coefficient and integral coefficient of the PI controller of the subsequent voltage loop respectively; D0 represents the steady-state value of the phase shift ratio, 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 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.

[0020] The DAB voltage loop PI controller adjusts the control signal according to the voltage error, and its proportional and integral effects can ensure that the voltage is stable near the reference value; the small signal transfer function of the DC current and the shift phase ratio reflects the relationship between the current and the shift phase ratio inside the DAB. Through the joint action of the two, the required virtual shift phase ratio can be accurately calculated according to the actual operating status of the system (such as voltage, current, etc.), thereby achieving precise control of the subsequent DAB power transmission and improving the system's power conversion efficiency and stability.

[0021] Furthermore, 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); where i o represents the load current, s represents the differential process, 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, 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 It represents the transfer function from the phase shift ratio D to the DAB output current i2, and C2 represents the DAB output capacitance; The transfer function of the output voltage and load current of the next stage is: o / -i o =1 / [C2s+(C v2 s / Ts+1)+G D12 G u2 ].

[0022] Compared with the prior art, the present invention has the following beneficial effects: In view of the difficulty of improving the DC voltage stability with the single-shift control of the rear-stage dual active bridge (DAB) of the traditional power electronic transformer (PET), and a series of problems caused by improving the stability of the rear-stage output voltage, the present invention proposes rear-stage DAB virtual capacitor control and front-stage converter-type virtual synchronous generator control respectively. The front-stage VSC provides inertia for the DC microgrid to reduce DC voltage fluctuations, and the rear-stage DAB improves the stability of the front-stage output voltage and narrows the dynamic response performance gap between the two-stage converters, so that when the system faces complex situations such as sudden load changes and frequent fluctuations in new energy, the two-stage voltage can remain stable and avoid over-limit, thereby enhancing the stability of the DC voltage. At the same time, the traditional PET control method has no inertia and no damping, which can easily cause system instability. The two-stage virtual inertia control of the present invention introduces virtual inertia into the system, which effectively improves the system's ability to maintain stability and resist interference, ensures that the system operates stably under various disturbances, and lays a solid foundation for the access and consumption of new energy. The present invention effectively overcomes the limitation of the access capacity of new energy caused by the unfavorable factors such as the reduction of the grid's rotational inertia on the access capacity of new energy by improving the dynamic response of the DC microgrid voltage, so that the grid can better adapt to the volatility of new energy and improve the penetration rate of new energy in the interconnection system of the distribution network and the DC microgrid; and under the working conditions such as load mutation, the method of the present invention can provide virtual inertial power support for the system, reduce the adverse effects of voltage fluctuations on the consumption of new energy, and improve the system's acceptance and utilization capacity of new energy. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 The PET-based DC microgrid topology provided by the present invention; Figure 2 A block diagram of the front-stage VSC control strategy provided by the present invention; Figure 3 The front-stage AVSG control block diagram provided by the present invention; Figure 4 A block diagram of the post-stage DAB control strategy provided by the present invention; Figure 5 A block diagram of the post-stage DAB control based on virtual capacitor control provided by the present invention; Figure 6 A block diagram of a two-stage inertial control based on PET provided by the present invention; Figure 7A comparison chart of simulation results between the two-level inertia control method of the present invention and the traditional control method under the load mutation scenario provided by the present invention; Figure 8 A schematic flow chart of a flexible interconnection collaborative control method for improving the penetration rate of new energy provided by the present invention. DETAILED DESCRIPTION

[0025] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] See also Figure 1-Figure 8 The first aspect of the present invention provides a flexible interconnection collaborative control method for improving the penetration rate of new energy, including: 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, the flexible interconnection cooperative control method is evaluated by using the load current and two-stage output voltage transfer function through simulation software to obtain the quantitative evaluation results of the flexible interconnection cooperative control method.

[0027] In this embodiment, according to Figure 1 As shown in the figure, 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 to build a flexible interconnected collaborative control architecture with a two-level VSC at the front stage and a DAB at the back stage. ga 、u gb 、u gc are the grid-side voltage, i ga 、i gb 、i gc are the grid-side current, L g is the AC filter inductor, C1 is the DC link capacitor, and C2 is the DAB output capacitor; i dc , i1 is the DC voltage on both sides of the DC link capacitor C1, i2, i o is the DC current on both sides of the output capacitor C2, n is the DAB high-frequency transformer ratio, L1 is the DAB equivalent leakage inductance, and i1=i2 / n, R g Indicates the AC grid side resistance, u oIndicates the output DC bus voltage; The virtual inertia of the PET front-stage VSC is realized by using quasi-virtual synchronous motor (AVSG) control. By comparing the virtual synchronous generator (VSG) control, the DC voltage and output current are analogized to the motor speed and electromagnetic power, thereby introducing the virtual inertia into the control algorithm of the converter, strengthening the front-stage output DC voltage inertia and smoothing the front-stage voltage fluctuation. The control equation of AVSG is: set -i1-D b (u * dc -U N )=C v1 (du * dc / dt); where I set Indicates the reference value of the output current of the previous 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.

[0028] like Figure 2 The figure shows the front-stage VSC control strategy based on AVSG, where i dref It represents the current reference value of the inner loop of AVSG current; the outer loop is the DC voltage reference value obtained by the AVSG control loop, and then the converter control signal is obtained through the voltage and current double closed loop.

[0029] Specifically, the outer loop is based on the DC voltage reference value u obtained through the AVSG control loop. * dc As the control target, the actual front-stage output DC voltage u dc Track and approach this reference value u * dc To ensure the stability and accuracy of the output voltage of the previous stage; In the voltage loop control link, the actual front-stage output DC voltage u is collected in real time dc , and compare it with the DC voltage reference value u * dc Subtract and get the voltage error ; Then the voltage error Input front-stage voltage loop PI controller transfer function G u1 =k up1 +k ui1 / s is used for calculation. Among them, the proportionality coefficient k up1 Determines the controller's ability to quickly respond to voltage errors. Multiply by k up1 The proportional output is obtained, which enables the controller to quickly adjust the voltage deviation. Integral coefficient k ui1 It is used to eliminate the steady-state error, which converts the voltage error Integrate and multiply by k ui1 The integral term output is obtained. The proportional term and the integral term output are added together to finally obtain the voltage control signal G u1 ; s represents the differential process; Using the voltage control signal G u1 To adjust the output voltage of the previous converter: According to the voltage control signal G u1 The size and polarity of the power switch device are adjusted to adjust the drive signal so that the actual output DC voltage u dc Towards the voltage reference value u * dc Approximation, thereby reducing the voltage error. That is, if the voltage error is positive (that is, 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 the adjustment is made iteratively to stabilize the actual voltage near the reference value.

[0030] In the current loop control link, the actual value of the current inner loop i d It is consistent with the current inner loop reference value i determined according to the system operating status and control requirements. dref Perform subtraction to obtain the current error ; The current error Input the transfer function of the inner loop PI controller of the previous current G i1 =k ip1 +k ii1 / s is calculated. Similarly, the proportionality factor k ip1 The controller can respond quickly to the current error and make timely adjustments to the current change. The calculation method is the current error Multiply by k ip1 Get the proportional output. Integral coefficient k ii1 Used to eliminate the current steady-state error, by Integrate and multiply by k ii1 The integral term output is obtained, and the proportional term and the integral term output are added to obtain the current control signal G i1 . Current control signal G i1 According to the current error, the input current of the previous VSC can be accurately controlled to track the current inner loop reference value i dref , ensuring the power balance and stable operation of the system; The voltage control signal G u1and the current control signal G i1 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 then u1 and G i1 The intermediate control signal is obtained by weighted summation according to the weighting coefficient. Then, the intermediate control signal is multiplied by the equivalent duty cycle β=3u d / (2u dc )(where u d is the grid-side voltage amplitude, u dc is the actual output DC voltage of the previous stage), the equivalent duty cycle takes into account the relationship between the grid voltage and the output DC voltage of the previous stage, and can dynamically adjust the control signal according to the actual operating voltage of the system. Finally, multiply the result by the equivalent gain K PWM , equivalent gain K PWM It is related to the power switch device characteristics and circuit topology of the previous 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 previous converter to obtain the final control signal of the previous converter.

[0031] Finally, the control signal of the previous converter is used to control the previous VSC. The control signal is converted into a drive signal for the power switch device (such as IGBT), and the on and off time and duration of the power switch device are controlled according to the level and pulse width of the drive signal. By accurately controlling the switching state of the power switch device, the input current and output voltage of the previous VSC can be accurately adjusted, so that the previous VSC can stably output DC voltage according to system requirements, and achieve good power matching and coordinated work with the subsequent DAB to ensure the stable operation of the entire DC microgrid system. During the operation of the system, the output DC voltage u of the previous stage is continuously monitored. dc 、Current i d The control signal of the preceding converter is continuously adjusted according to the parameter changes, and the control effect of the preceding VSC is dynamically optimized to adapt to various working conditions such as system load changes and new energy output fluctuations.

[0032] In this embodiment, the virtual inertia of the PET post-stage DAB is realized by virtual capacitor control, imitating the effect of DC capacitor charging and discharging, by introducing virtual capacitor C v2 The distribution network can transmit more supporting power and improve the dynamic response of the downstream output DC bus. The specific operation process is as follows: Get the current DC bus voltage u o And the DC bus voltage u sampled in the previous sampling cycle o0 , and obtain the bus voltage reference value U on And the subsequent virtual capacitor C v2The value of determines the unit sampling time T; According to the virtual capacitor current expression Calculate; the control strategy of the subsequent converter with additional virtual capacitor is as follows Figure 4 As shown, the virtual capacitor current generated by the virtual capacitor Release virtual inertia power for DC microgrid, thus reducing voltage fluctuations; It should be noted that Figure 4 in, u oref Indicates the reference value of the downstream DC bus voltage.

[0033] Virtual capacitor current 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 phase produced by the virtual capacitor. ; where k up2 , k ui2 are the proportional coefficient and integral coefficient of the PI controller of the subsequent voltage loop; D0 represents the steady-state value of the shift phase, f s represents the DAB switching frequency, n represents the DAB high-frequency transformer ratio, and L1 represents the DAB equivalent leakage inductance; Compare the virtual Combined with the total shift phase signal D, the subsequent control signal is obtained; The subsequent stage DAB is controlled by the subsequent stage control signal. The power switch device (such as IGBT, etc.) in the subsequent stage DAB adjusts its on and off time according to the subsequent stage control signal, thereby changing the shift phase of the DAB. By accurately controlling the shift phase, the power transmission of the subsequent stage DAB is controlled, thereby stabilizing the output DC bus voltage u of the subsequent stage o During system operation, the DC bus voltage u is continuously monitored. o , virtual capacitor current Parameters such as and the subsequent control signal to ensure that the subsequent DAB can adapt to various working conditions such as system load changes, new energy output fluctuations, etc., stably output DC voltage, and provide reliable power supply for the loads in the DC microgrid.

[0034] Finally, the flexible interconnection collaborative control method is evaluated by simulation software using the load current and two-stage output voltage transfer function to obtain the quantitative evaluation results of the flexible interconnection collaborative control method. Among them, the transfer function of load current and two-stage output voltage 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); where s represents the differential process, 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, 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 It represents the transfer function from the phase shift ratio D to the DAB output current i2, and C2 represents the DAB output capacitance; The transfer function of the output voltage and load current of the next stage is: o / -i o =1 / [C2s+(C v2 s / Ts+1)+G D12 G u2 ].

[0035] In this embodiment, the following simulation software is used to build Figure 1 The DC microgrid simulation model is shown in the figure. The simulation system parameters are shown in the following table: System Parameters Numeric <![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[Equivalent inductance of DAB, 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 In order to verify the effectiveness of the proposed PET two-stage virtual inertia control strategy, the traditional PET control method, which uses voltage and current dual closed-loop control in the front stage and single-phase voltage control in the back stage, is compared with the load mutation scenario to verify the ability of inertial power transmission to suppress DC voltage mutation. When the system runs to 1s, the DC load suddenly decreases from 80kw to 8kw. The voltage fluctuation simulation results of the front and rear stages are shown in the figure. Figure 7 As shown; Depend on Figure 7 It can be seen that compared with the traditional control method, the proposed two-level virtual inertia control (i.e., the flexible interconnected collaborative control method of the present invention) can provide virtual inertia power support for the system when the load changes suddenly, thereby greatly reducing voltage fluctuations, enhancing DC voltage stability, and improving the new energy consumption rate.

[0036] Part of the data in the above formula is calculated by removing the dimension and taking its numerical value. The formula is a formula closest to the actual situation obtained by software simulation of a large amount of collected data; the preset parameters and preset thresholds in the formula are set by technical personnel in this field according to actual conditions or obtained through simulation of a large amount of data.

[0037] Working principle of the present invention: Step S1 constructs the basic topology structure of the interconnection between the DC microgrid and the distribution network; Step S2 obtains the control equation through AVSG control, and determines the basis for calculating the control signal according to the system state; and converts the control equation into a control signal that can directly operate the VSC power switch device through the voltage loop, current loop, equivalent duty cycle and equivalent gain, so as to achieve precise control of the previous VSC; In step S3, the virtual capacitor control of the rear-stage DAB cooperates with the front-stage VSC control to jointly achieve 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; In step S4, the load current and the two-stage output voltage transfer function are used to evaluate the entire flexible interconnection collaborative control method, and the evaluation results can be used to further optimize the front-stage VSC control strategy to improve the performance of the entire system and achieve the goal of increasing the penetration rate of new energy.

[0038] The above embodiments are only used to illustrate the technical method of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical method of the present invention may be modified or replaced by equivalents 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.

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 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.

3. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 2, 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.

4. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 3, 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.

5. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 4, 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.

6. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 5, characterized in that: The formula for 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.

7. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 6, 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: ; 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.

8. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 7, characterized in that: The control strategy of the virtual capacitor includes: B1, virtual capacitor current 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 phase produced by the virtual capacitor. ; 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 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.

9. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 8, characterized in that: 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); where i o Represents the load current, 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; The transfer function of the output voltage and load current of the next stage is: o / -i o =1 / [C2s+(C v2 s / Ts+1)+G D12 G u2 ]; among them, G DI2 represents the transfer function from the phase shifter D to the DAB output current i2, and C2 represents the DAB output capacitance.

10. A flexible interconnection collaborative control method for improving the penetration rate of new energy according to claim 9, characterized in that: The load current i o The transfer function G to the DAB output current i2 io_i2 The expression is: G io_i2 =[C v2 s / (Ts+1)+G u2 G DI2 ] / [C2s+C v2 s / (Ts+1)+G u2 G DI2 ].

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