Secondary voltage control method of virtual synchronous generator based on preset time consistency

Abstract

The present invention discloses a secondary voltage control method of a virtual synchronous generator based on preset time consistency. The topological structure of the virtual synchronous generator includes a VSG main circuit and a VSG control circuit. The VSG main circuit includes a DC side power supply, a voltage-type inverter, and an LC filter. The VSG control circuit includes an active frequency loop and a reactive voltage loop. By improving the distributed consistency control algorithm, a time base generator is introduced, and a preset time is applied to the time base generator to achieve secondary voltage control of the virtual synchronous generator with timed voltage recovery. The method only requires a small amount of communication between adjacent DGs to solve the deviation problem of primary control at the preset time. Adding a link of customizing the voltage recovery time can solve the problem of slow voltage recovery speed and the problem that the recovery time is affected by the initial capacity, parameters and quantity changes of the DG. While ensuring the timed voltage recovery, the system reliability and flexibility are enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power and energy, and in particular to a secondary voltage control method of a virtual synchronous generator based on preset time consistency. Background Art

[0002] An island microgrid is a small independent power system that integrates a distributed generator (DG), energy storage devices and loads. It can be a working mode for microgrids to cope with large power grid failures, or it can be an effective way to solve the power demand of islands and remote areas.

[0003] At present, distributed power sources generally have problems of inertia and insufficient damping. Therefore, some scholars have proposed a virtual synchronous generator (VSG) control method. By simulating the electromechanical transient characteristics and damping characteristics of synchronous generators, the microgrid's ability to accept DG is improved and the stable operation of the system is guaranteed. Since its proposal, it has received widespread attention from scholars at home and abroad. However, VSG mainly realizes the primary control of DG, provides inertia for the system and participates in the system power regulation according to the set droop characteristics. In an isolated microgrid, due to the lack of support from the main grid, the dynamic characteristics of the isolated microgrid are susceptible to disturbances. Relying solely on the primary differential control of VSG, the normal operation of the isolated microgrid cannot be guaranteed. In order to improve the control accuracy, a secondary control layer is usually introduced to eliminate the frequency, voltage deviation and power distribution error generated by the primary control. At present, the secondary control of the isolated microgrid can be divided into three types: centralized control, decentralized control and distributed control. Distributed control combines the advantages of centralized control and decentralized control, and achieves global optimization of local control decisions through local information. For a multi-machine parallel microgrid system operating in island mode, distributed consistency control can achieve a stable state through simple information exchange with neighbors, thereby affecting other branches to ensure the stable operation of the overall system.

[0004] The current distributed secondary voltage control algorithms mainly include three types: secondary control based on traditional consistency, secondary control based on finite time consistency, and secondary control based on fixed time consistency.

[0005] Among them, the secondary control method based on traditional consistency continuously updates the global information through the interactive information between distributed power sources to ensure the stable operation of the microgrid, and obtains the information of adjacent units through local units to realize the interaction and analysis of global information. However, for asymptotic consistency, the individual states in infinite time are still close to consistency. The increase in convergence speed will only make it closer to the convergence value in the same time, and the final convergence result will still remain at infinite time. In addition, the faster the convergence speed, the more adverse effects it will also bring, such as serious overshoot in the convergence dynamic process. The faster the convergence speed, the more serious the overshoot, which will affect the stability of the system; for example, convergence robustness refers to the performance of maintaining consistent convergence under some external disturbance factors such as the communication topology of the distributed power source, time delay, parameters used in the convergence process, and operating conditions. While the convergence speed is improved, convergence robustness has also become an issue that cannot be ignored.

[0006] The quadratic control method based on finite-time consistency can complete iterative convergence within a finite time, but the estimation of the finite time for the solution obviously depends on the initial conditions of the distributed power generation group of the microgrid. When the initial state of the distributed power generation is unknown or difficult to solve, it will limit the application scope of finite-time consistency in distributed collaborative control.

[0007] In addition, through continuous research on the secondary control method based on fixed time consistency, some scholars have found that fixed time consistency will produce a large initial control input due to the specific protocol structure, that is, an exponent greater than 1. The existence of the exponent means that when the initial values ​​differ too much, it will lead to excessive amplitudes, which will in turn have some impact on the microgrid equipment. In addition, due to the existence of exponential functions, the estimated time of fixed time consistency is often too conservative. Fixed time consistency control has shortcomings in terms of tight estimation of convergence time and providing good initial value input control. Summary of the invention

[0008] In order to overcome the defects of the prior art, the present invention proposes a secondary voltage control method of a virtual synchronous generator based on preset time consistency. From the perspective of distributed collaborative consistency, the method regards the micro-sources of the microgrid as nodes of multiple intelligent agents, and regards the relationships such as voltage, current, and power as communication relationships. A distributed secondary voltage control method with timed recovery of voltage and power is designed. The method can solve the voltage deviation problem caused by dynamic characteristics such as line impedance and external interference in the primary control, ensure simple communication and rapid recovery of voltage timing, and improve work efficiency.

[0009] The purpose of the present invention can be achieved by the following technical solutions:

[0010] A virtual synchronous generator secondary voltage control method based on preset time consistency, wherein the topology of the virtual synchronous generator includes a VSG main circuit and a VSG control circuit, wherein the VSG main circuit includes a DC side power supply, a voltage source inverter, and an LC filter, and the VSG control circuit includes an active frequency loop and a reactive voltage loop. By improving a distributed consistency control algorithm, a time base generator is introduced, and a preset time is applied to the time base generator, the secondary voltage control of the virtual synchronous generator voltage timing recovery is realized, thereby enhancing system reliability and flexibility;

[0011] The virtual synchronous generator secondary voltage control method comprises the following steps:

[0012] S1: Collect the inverter output current i oabc And the output voltage u oabc According to the equal amplitude transformation formula, the static three-phase coordinate current and voltage of abc are transformed into the rotating coordinate system dq. By measuring the output voltage and output current of VSG, the three-phase instantaneous active power P is calculated. e And reactive power Q e ;

[0013] S2: Calculate the active frequency loop to convert the active power P e With reference value P n The deviation is obtained by simulating the characteristic equation of the synchronous generator through VSG to obtain the active power loop output angle, and the reference voltage amplitude E is calculated according to the reactive voltage equation. m ;

[0014] S3: Calculate the abc three-phase voltage, obtain the input reference signal of the voltage and current double closed loop in the dq coordinate system through park transformation, and then obtain the inverter PWM command signal through voltage and current double closed loop control to drive the inverter to work and complete a control layer control task;

[0015] S4: Improve distributed consistency control, introduce a time base generator to achieve timing recovery characteristics, and design consistency control based on preset time;

[0016] S5: Through the improved preset time consistency, construct the VSG reactive voltage equation and design the average voltage deviation controller u Ei (t) and reactive power deviation controller u Qi (t), obtain the rated voltage U after distributed coordination consistency adjustment ni (t), the VSG secondary voltage control strategy based on the preset time consistency is obtained. When the microgrid reaches a stable state, the control target of the average voltage is restored to the rated value, and the control task of the secondary control layer is completed.

[0017] Furthermore, the method of obtaining the active power loop output angle θ by simulating the characteristic equation of the synchronous generator through VSG is shown in formula (1):

[0018]

[0019] In formula (1), J represents the moment of inertia, which is the reason why the frequency dynamic response should have inertia, D represents the damping coefficient, and P n Indicates the reference power, P e represents the VSG output power, ω n represents the reference angular frequency, ω represents the actual angular frequency;

[0020] According to the reactive voltage equation shown in formula (2), the reference voltage amplitude E is calculated m :

[0021] E m =n(Q n -Q e )+U n (2)

[0022] In formula (2), U n Represents the reference voltage signal of the reactive voltage loop, U n It is not a fixed value, but a constantly revised value obtained through the secondary control layer.

[0023] Furthermore, according to the reference voltage amplitude and the active power loop output angle θ, the abc three-phase voltage is calculated as shown in formula (3):

[0024]

[0025] Furthermore, the preset time consistency control is shown in formula (4):

[0026]

[0027] In formula (4), λ 2 (L) represents the second smallest eigenvalue of the Laplace matrix L, ξ(t) represents the introduced time base generator, δ represents the convergence error range after reaching stability, and x i (t)∈R, represents the state variable of the i-th agent, a ij Indicates reactive power communication relationship;

[0028] The design of the time base generator ξ(t) is shown in formula (5):

[0029]

[0030] In formula (5), t f Indicates the convergence time.

[0031] Furthermore, the average voltage deviation controller mainly corrects the deviation of the VSG voltage to maintain voltage stability. The design steps of the average voltage deviation controller include:

[0032] S51: Based on the preset time consistency control, the reactive voltage equation of the i-th VSG is constructed as shown in equation (6):

[0033] E mi (t) = U ni (t)+n i (Q ni (t)-Q ei (t)) (6)

[0034] Differentiating both ends of equation (6) yields equation (7):

[0035]

[0036] In formulas (6) to (7), U ni (t), Q ni (t) are the rated voltage and rated reactive power of the i-th VSG, Q ei (t) is the reactive power output by the i-th VSG; n i Represents the droop coefficient of the reactive voltage equation, E mi (t) is defined as the output voltage reference signal of the i-th distributed generation DG, u Ei (t) represents the average voltage deviation controller to be designed, u Qi (t) represents the reactive power deviation controller to be designed, The rated voltage after distributed coordination consistency adjustment is as follows:

[0037] U ni (t)=k∫[u Qi (t)+u Ei (t)]dt+E ref (8)

[0038] S52: Obtain the average voltage of the microgrid through a distributed collaborative average voltage observer designed with preset time consistency As shown in formula (9):

[0039]

[0040] S53: The average voltage and reference voltage U * Compare and generate voltage difference, design average voltage deviation controller u Ei (t), as shown in formula (10):

[0041]

[0042] In equations (8) to (10), k represents the integral term coefficient of the state controller, U * represents the reference voltage of the microgrid, m represents the weight coefficient of reactive power sharing and voltage recovery, They represent the average voltage observation values ​​of the i-th and j-th distributed power sources, C represents the voltage coupling coefficient, and E mi (t) is defined as the output voltage reference signal of the i-th distributed power supply, E ref Indicates the system rated voltage reference value.

[0043] Furthermore, the reactive power deviation controller is used to ensure the distribution of reactive power, and the design steps include:

[0044] S61: Select the condition that the droop coefficient of the inverter voltage is inversely proportional to the reactive power capacity of the distributed power source, as shown in formula (11):

[0045] n 1 Q n1 =n 2 Q n2 =…=n N Q nN (11)

[0046] It is also necessary to satisfy the requirement that reactive power is distributed according to capacity, as shown in formula (12):

[0047] n 1 Q e1 =n 2 Q e2 =…=n N Q eN (12)

[0048] S62: Based on equations (7) to (12), the reactive power deviation controller is designed as equation (13):

[0049]

[0050] In formulas (11) to (13), n N represents the droop coefficient of the reactive voltage equation of the Nth VSG, Q nN Indicates the rated reactive power of the Nth VSG, Q eN Represents the output reactive power of the Nth VSG, n j represents the reactive voltage equation droop coefficient of the jth VSG, N i Indicates that there are N VSGs connected in parallel.

[0051] Further, according to the average voltage deviation controller u Ei(t) and the design of the reactive power deviation controller u Qi (t) is designed, and the overall design of the VSG secondary voltage control strategy based on the preset time consistency is obtained, as shown in formula (14):

[0052]

[0053] Compared with the prior art, the present invention proposes preset time consistency and designs a distributed secondary control that can be restored on a timed basis, which specifically includes the following technical effects:

[0054] (1) Only a small amount of communication between adjacent DGs is required to solve the voltage deviation problem caused by dynamic characteristics such as line impedance and external interference during the primary control of VSG parallel operation, and can simultaneously achieve voltage difference-free control and reactive power distribution according to capacity;

[0055] (2) The proposed control strategy can realize the timed voltage recovery in the secondary control of virtual synchronous generators. Compared with the traditional consistency secondary control, it adds a link of customized voltage recovery time, which not only ensures the timed voltage recovery, but also enhances the system reliability and flexibility.

[0056] (3) Compared with the current distributed secondary control, the proposed control strategy can overcome the problems of slow voltage recovery and the recovery time being affected by changes in the initial capacity, parameters and quantity of DGs, as well as the influence of the initial state of the microgrid and the design parameters on the recovery speed, thereby improving work efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a typical VSG control block diagram;

[0058] Figure 2 This is the control block diagram of VSG in island mode;

[0059] Figure 3 This is the framework diagram of distributed coordinated voltage control for microgrids;

[0060] Figure 4 This is the structure diagram of the system with four DGs in parallel operation in an isolated microgrid;

[0061] Figure 5 This is a schematic diagram of the topology change when the communication failure occurs at 7s;

[0062] Figure 6 is the reactive power under primary and secondary control of VSG;

[0063] Figure 7 is the convergence performance under different numbers of distributed generation, where Figure 7 (a) is the convergence performance diagram of fixed time consistency in 2DG, 4DG, and 6DG respectively. Figure 7(b) is the convergence performance diagram of preset time consistency in 2DG, 4DG, and 6DG respectively;

[0064] Figure 8 is the reactive power and average voltage diagram under VSG secondary voltage control, Figure 8 (a) is the reactive power and average voltage diagram under fixed time consistency secondary control, Figure 8 (b) is the reactive power and average voltage diagram under the improved control strategy at the preset time of 0.3s;

[0065] Fig. 9 This is a comparison chart of the control effect of distributed secondary control on droop and VSG. Fig. 9 (a) is the droop control diagram, Fig. 9 (b) is the control diagram of the virtual synchronous generator;

[0066] Fig.10 Reactive power and voltage graphs under plug-and-play features, Fig.10 (a) is the reactive power distribution diagram, Fig.10 (b) is the voltage recovery characteristic diagram. DETAILED DESCRIPTION

[0067] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the invention, all other embodiments obtained by ordinary technicians in this field without creative work, any modifications, equivalent substitutions, improvements, etc., should be included in the protection scope of the present invention.

[0068] The typical VSG control method mainly calculates the reference voltage by calculating the active and reactive power obtained from the main circuit, generating the voltage amplitude and phase through the active frequency control part and the reactive voltage control part. The reference voltage is output through the voltage and current double closed-loop control, and the PWM generator generates the inverter working signal. The virtual synchronous generator secondary voltage control method based on preset time consistency, such as Figure 1 As shown, the topological structure of the virtual synchronous generator includes a VSG main circuit and a VSG control circuit. The DC side is mainly used for distributed power sources containing energy storage elements, using a DC power supply U dc Instead, the rest of the main circuit is mainly composed of voltage source inverter, LC filter, line impedance Z line , load and some measuring elements.

[0069] The VSG control circuit includes an active frequency loop and a reactive voltage loop, such as Figure 2As shown, the function of the active frequency loop is: the synchronous generator controls the mechanical power input of the generator through the speed governor. When the system frequency is different from the rated frequency, the speed governor adjusts the opening and closing degree of the valve to adjust the mechanical power of the prime mover to achieve the function of primary frequency regulation; the function of the reactive voltage loop is: when the output voltage amplitude of the virtual synchronous generator deviates, the excitation characteristics of the reactive voltage of the traditional generator are simulated to adjust the output voltage of the virtual synchronous generator. The phase information obtained by the active frequency loop and the reference voltage amplitude information obtained by the reactive voltage loop are synthesized into the input command signal of the voltage and current double closed loop, and the voltage and current control output command is obtained by calculation, and the working signal of the inverter is generated by the PWM generator.

[0070] By improving the distributed consistency control algorithm, introducing a time base generator, and applying a preset time to the time base generator, the secondary voltage control of the virtual synchronous generator voltage timing recovery is realized, thereby enhancing the system reliability and flexibility;

[0071] The virtual synchronous generator secondary voltage control method comprises the following steps:

[0072] S1: Collect the inverter output current i oa 、i ob 、i oc And the output voltage u oa 、u ob 、u oc According to the equal amplitude transformation formula, the static three-phase coordinate current and voltage of abc are transformed into the rotating coordinate system dq. By measuring the output voltage and output current of VSG, the three-phase instantaneous active power P is calculated. e And reactive power Q e ;

[0073] The static three-phase coordinate current and voltage of abc are transformed into the rotating coordinate system dq. The transformation process is shown in equations (S-1) and (S-2):

[0074]

[0075]

[0076] In formula (S-1) and (S-2), i a ,i b ,i c is the filter inductor current;

[0077] Calculate the input active power P of the VSG system e and reactive power Q e , as shown in formula (S-3):

[0078]

[0079] S2: Calculate the active frequency loop to convert the active power P e With reference value P n The method of obtaining the active power loop output angle θ by simulating the characteristic equation of the synchronous generator through VSG is shown in formula (S-4):

[0080]

[0081] In formula (S-4), J represents the moment of inertia, which is why the frequency dynamic response should have inertia, D represents the damping coefficient, and P n Indicates the reference power, P e represents the VSG output power, ω n represents the reference angular frequency, ω represents the actual angular frequency;

[0082] According to the reactive voltage equation shown in formula (S-5), the reference voltage amplitude E is calculated and obtained m :

[0083] E m =n(Q n -Q e )+U n (S-5)

[0084] In formula (S-5), U n Represents the reference voltage signal of the reactive voltage loop, U n It is not a fixed value, but a constantly revised value obtained through the secondary control layer.

[0085] S3: According to the reference voltage amplitude and the active power loop output angle θ, the abc three-phase voltage is calculated as shown in formula (S-6):

[0086]

[0087] After Park transformation, the input reference signal of the voltage and current dual closed loop in the dq coordinate system is obtained, and then the inverter PWM command signal is obtained through voltage and current dual closed loop control to drive the inverter to work and complete a control layer control task.

[0088] S4: Introduce distributed consistency control as shown in formula (S-7):

[0089]

[0090] In formula (S-7), x i (t)∈R represents the state variable of the ith agent; u i (t)∈R represents the control input of the ith agent. For the system x i(0), i∈R represents the initial state of the system. Under the convergence of consistency, all initial states will approach consistency. At infinite time, the deviation between units will approach zero, and the system will reach convergence consistency.

[0091] In order to achieve faster convergence speed and improve distributed consistency control, a time base generator is introduced to realize the characteristics of timing recovery, and the design is based on preset time consistency control, as shown in formula (S-8):

[0092]

[0093] In formula (S-8), λ 2 (L) represents the second smallest eigenvalue of the Laplace matrix L, ξ(t) represents the introduced time base generator, δ represents the convergence error range after reaching stability, and in order to ensure stability within the rated value range, the present invention preferably takes δ=0.01, x i (t)∈R, represents the state variable of the ith agent, including the voltage, frequency, power and other state information of the ith distributed generation, a ij Indicates reactive power communication relationship;

[0094] The design of the time base generator ξ(t) is shown in formula (S-9):

[0095]

[0096] In formula (S-9), t f Indicates the convergence time.

[0097] S5: Through the improved preset time consistency, construct the VSG reactive voltage equation and design the average voltage deviation controller u Ei (t) and reactive power deviation controller u Qi (t), obtain the rated voltage U after distributed coordination consistency adjustment ni (t), the VSG secondary voltage control strategy based on the preset time consistency is obtained. When the microgrid reaches a stable state, the control target of the average voltage is restored to the rated value, and the control task of the secondary control layer is completed.

[0098] The average voltage deviation controller mainly corrects the deviation of the VSG voltage to maintain voltage stability. The design steps of the average voltage deviation controller include:

[0099] S51: Based on the preset time consistency control, the reactive voltage equation of the i-th VSG is constructed as shown in formula (S-10):

[0100] E mi (t) = U ni (t)+n i (Q ni (t)-Qei (t)) (S-10)

[0101] Differentiating both ends of equation (S-10) yields equation (7):

[0102]

[0103] In formulas (S-10) to (S-11), U ni (t), Q ni (t) are the rated voltage and rated reactive power of the i-th VSG, Q ei (t) is the reactive power output by the i-th VSG; n i Represents the droop coefficient of the reactive voltage equation, E mi (t) is defined as the output voltage reference signal of the i-th distributed generation DG, u Ei (t) represents the average voltage deviation controller to be designed, u Qi (t) represents the reactive power deviation controller to be designed, The rated voltage after distributed coordination consistency adjustment is as follows:

[0104] U ni (t)=k∫[u Qi (t)+u Ei (t)]dt+E ref (S-12)

[0105] S52: Obtain the average voltage of the microgrid through a distributed collaborative average voltage observer designed with preset time consistency Such as formula (S-13):

[0106]

[0107] S53: The average voltage and reference voltage U * Compare and generate voltage difference, design average voltage deviation controller u Ei (t), as shown in formula (S-14):

[0108]

[0109] In formulas (S-12) to (S-14), k represents the integral term coefficient of the state controller, U * represents the reference voltage of the microgrid, m represents the weight coefficient of reactive power sharing and voltage recovery, They represent the average voltage observation values ​​of the i-th and j-th distributed power sources, C represents the voltage coupling coefficient, and E mi (t) is defined as the output voltage reference signal of the i-th distributed power supply, E refIndicates the system rated voltage reference value.

[0110] The reactive power deviation controller is used to ensure the distribution of reactive power, and the design steps include:

[0111] S61: Select the condition that the droop coefficient of the inverter voltage is inversely proportional to the reactive power capacity of the distributed power source, as shown in formula (S-15):

[0112] n 1 Q n1 =n 2 Q n2 =…=n N Q nN (S-15)

[0113] It is also necessary to satisfy the requirement that reactive power is distributed according to capacity, as shown in formula (S-16):

[0114] n 1 Q e1 =n 2 Q e2 =…=n N Q eN (S-16)

[0115] S62: Based on equations (S-15) to (S-16), the designed reactive power deviation controller is as shown in equation (S-17):

[0116]

[0117] In formulas (S-15) to (S-17), n N represents the droop coefficient of the reactive voltage equation of the Nth station, Q nN Indicates the rated reactive power of the Nth VSG, Q eN Represents the output reactive power of the Nth VSG, n i represents the droop coefficient of the reactive voltage equation, n j represents the droop coefficient of the reactive voltage equation of the jth station, N i Indicates that there are N VSGs connected in parallel.

[0118] According to the average voltage deviation controller u Ei (t) and the design of reactive power deviation controller u Qi (t) is designed, and the overall design of the VSG secondary voltage control strategy based on the preset time consistency is obtained, as shown in formula (S-17):

[0119]

[0120] In order to verify the feasibility of the above-mentioned secondary voltage control strategy with preset time consistency, Figure 4As shown, the present invention builds a simulation model of an island microgrid system on the MATLAB / Simulink software platform, simulates the output voltage and reactive power of four distributed power sources, optimizes the communication topology of the four DGs, and analyzes the results as follows.

[0121] (1) Four DGs with the same capacity were constructed, and the droop coefficient n was unified to 5×10 -4 , running in parallel island mode, initially only controlled by virtual synchronous generators, and at time 3s, a secondary voltage control strategy based on preset time consistency was introduced, and the preset time was set to 1s after the addition, in order to verify the secondary recovery performance of the proposed secondary voltage control strategy. Figure 6 It can be seen that the power under primary control deviates from the rated value due to line impedance, power generation characteristics, etc. After adding the preset time consistency for 3s, the power fluctuates slightly and approaches consistency within the preset time. The fault operation of simulating communication failure at 7s, such as Figure 5 As shown in the simulation waveform, the connection line between DG1 and DG4 is disconnected. Figure 6 As shown in the figure, under the premise of ensuring that the topology contains a spanning tree, communication failure has almost no effect on the system control performance, and the simulation waveform has almost no change at the 7s moment when the communication is disconnected. From the results described above, it can be concluded that the secondary voltage control strategy designed based on preset time consistency can effectively improve the deviation problem existing in the primary control. In addition, even if there is a communication failure problem, as long as the topology structure contains a spanning tree, it will not have a major impact on the original result. This simulation result proves that the secondary control strategy designed through distributed collaborative consistency has a stable convergence effect.

[0122] (2) 2 DGs, 4 DGs, and 6 DGs were constructed respectively, with 1000W and 2000W capacities evenly distributed, to verify the convergence speed advantage of the preset time compared with the current fixed time consistency under different numbers of DGs. Figure 7 The simulation results show that the convergence performance of fixed time consistency and preset time consistency under different number of DG structures is compared. This distributed consistency adopts the classic strategy and does not improve the convergence acceleration of consistency. Figure 7 As shown in the figure, the convergence time of fixed time consistency with 2 DGs, 4 DGs and 6 DGs is 0.35s, 0.6s and 0.8s respectively. The convergence effect of fixed time consistency will be affected by the number of DGs, while the preset time consistency used in this article can guarantee convergence within 0.3s for different numbers of DGs. It can be seen that the preset time consistency can still guarantee fast convergence when the number of DGs increases, while the power recovery speed of fixed time consistency is affected to different degrees under different numbers of DGs.

[0123] (3) Verify the convergence change state of the average voltage and reactive power of the proposed improved secondary control strategy compared with the secondary control under the current fixed time consistency. First, verify the convergence advantage of the preset time consistency, adjust the rated capacity ratio of the four DGs to 1:1:2:2, and design the droop coefficients inversely proportional to the value of the reactive droop coefficient n and the rated capacity, and set the droop coefficients of DG1 and DG2 to 5×10 -4 , the droop coefficient of DG3 and DG4 is 2.5×10 -4 At the initial moment, only Load1 with a reactive power of 6 kVar is working. At 2 seconds, Load2 with a reactive power of 3 kVar is added. By comparing and analyzing the advantages of the proposed preset time in microgrid applications. Figure 8 As shown in (a), due to the distributed cooperative consistency control, both reactive power and average voltage can be restored to a stable state, but the recovery time under the traditional strategy is 0.6s, which is different from Figure 8 Compared with the secondary control strategy based on preset time consistency shown in (b), the present invention can also achieve a stable state, but the recovery time can be stabilized within 0.3s. The results show that the secondary control designed based on preset time consistency can restore the voltage to the rated value within the preset time. Compared with the secondary control designed by the current fixed time consistency method, it has a faster and more accurate recovery performance and can achieve the control effect of timed recovery that the current secondary control does not have.

[0124] (4) Comparison of the control effects of droop control and the proposed control method. The operating conditions are that Load1 is working at the initial moment, Load2 is added at 2s, and Load2 is removed at 4s. The simulation results are shown in Fig. 9 As shown, Fig. 9 (a) The droop control is the frequency waveform of the four DGs converged at the preset 0.5s time. Fig. 9 (b) is the frequency operation waveform of DG1 when the moment of inertia parameters are 1, 2, and 4 respectively. From the simulation results, it can be seen that with the increase of the moment of inertia, the curve moves backward and the time to reach the peak increases. Therefore, the secondary control based on the virtual synchronous generator design can increase the inertia of the microgrid by increasing the moment of inertia, while the conventional droop control, although it meets the rapid recovery of the frequency under the effect of the preset time consistency, the frequency of DG quickly reaches the peak and the system inertia is very small. Therefore, the distributed secondary control based on VSG can provide the required virtual inertia for the microgrid system.

[0125] (5) Verification of the plug-and-play function: First, the four DGs are connected to Load1 only during the test time. However, at t = 2s, DG4 is disconnected from the network and reconnected at t = 4s. Fig.10As shown in the figure, after DG4 is disconnected, the capacity assumed by DG4 is redistributed to DGs1-3 in proportion, and the voltage is quickly stabilized to the rated value after adjustment by the state controller. It can be seen that after DG4 is disconnected and reconnected, both the capacity redistribution and the voltage recovery can always be restored within 0.5s under the preset time consistency control, and this time can be adjusted according to the actual situation.

[0126] The present invention aims to improve the working efficiency of the microgrid, starting from the voltage recovery time, to achieve a shorter recovery time, better recovery effect, and most importantly, controllable recovery time. The present invention is compared with existing limited time control, fixed time control and other schemes to verify the superiority and feasibility of the present invention. The secondary control based on preset time consistency proposed by the present invention is more practical and applicable.

Claims

1. A virtual synchronous generator secondary voltage control method based on preset time consistency, characterized in that: The topological structure of the virtual synchronous generator includes a VSG main circuit and a VSG control circuit. The VSG main circuit includes a DC side power supply, a voltage source inverter, and an LC filter. The VSG control circuit includes an active frequency loop and a reactive voltage loop. By improving the distributed consistency control algorithm, a time base generator is introduced, and a preset time is applied to the time base generator to realize the secondary voltage control of the virtual synchronous generator voltage timing recovery, thereby enhancing the system reliability and flexibility. The virtual synchronous generator secondary voltage control method comprises the following steps: S1: Transform the static three-phase coordinate current and voltage into a rotating coordinate system, and calculate the three-phase instantaneous active power and reactive power; S2: The active power loop output angle is obtained by simulating the characteristic equation of the synchronous generator through VSG, and the reference voltage amplitude E is calculated according to the reactive voltage equation. m ; S3: After Park transformation, the input reference signal of the voltage and current double closed loop in the rotating coordinate system is obtained, and then the inverter PWM command signal is obtained through the voltage and current double closed loop control to drive the inverter to work and complete a control layer control task; S4: Improve distributed consistency control, introduce a time base generator to achieve timing recovery characteristics, design a preset time consistency control, and set the time t f Achieve preset time consistency control; The preset time consistency control is shown in formula (1): In formula (1), u i (t) * represents the control input of the i-th distributed generation, λ2(L) represents the second smallest eigenvalue of the Laplace matrix L, ξ(t) represents the introduced time base generator, δ represents the convergence error range after reaching stability, and x i (t)∈R, represents the voltage, frequency, and power status information of the i-th distributed generation, a ij Indicates the reactive power communication relationship, Ni indicates that there are N VSGs connected in parallel; S5: Through the improved preset time consistency, the VSG reactive voltage equation is constructed, the average voltage deviation controller and the reactive power deviation controller are designed, the rated voltage after distributed coordinated consistency adjustment is obtained, and the VSG secondary voltage control strategy based on the preset time consistency is obtained to achieve the control target of restoring the average voltage to the rated value and complete the control task of the secondary control layer.

2. The method for controlling the secondary voltage of a virtual synchronous generator based on preset time consistency according to claim 1 is characterized in that: The method of obtaining the active power loop output angle θ by using the characteristic equation of the VSG simulating synchronous generator is shown in formula (2): In formula (2), J represents the moment of inertia, D represents the damping coefficient, and P n Indicates the reference power, P e represents the VSG output power, ω n represents the reference angular frequency, ω represents the actual angular frequency, and θ is the output angle of the active power loop; According to the reactive voltage equation shown in formula (3), the reference voltage amplitude E is calculated m : E m =n(Q n -Q e )+U n (3) In formula (3), n is the droop coefficient of the reactive voltage equation, U n Represents the reference voltage signal of the reactive voltage loop.

3. The method for controlling the secondary voltage of a virtual synchronous generator based on preset time consistency according to claim 2 is characterized in that: According to the reference voltage amplitude E m And the active power loop output angle θ, as shown in formula (4), calculate the abc three-phase voltage:

4. The method for controlling the secondary voltage of a virtual synchronous generator based on preset time consistency according to claim 3 is characterized in that: The design of the time base generator ξ(t) is shown in formula (5): In formula (5), t f Indicates the convergence time.

5. The method for controlling the secondary voltage of a virtual synchronous generator based on preset time consistency according to claim 4 is characterized in that: The average voltage deviation controller is used to correct the deviation of the VSG voltage to maintain voltage stability. The design steps of the average voltage deviation controller include: S51: Based on the preset time consistency control, the reactive voltage equation of the i-th VSG is constructed as shown in equation (6): E mi (t)=U ni (t)+n i (Q ni (t)-Q ei (t)) (6) Differentiating both ends of equation (6) yields equation (7): In formulas (6) to (7), U ni (t) represents the rated voltage of the i-th VSG, Q ni (t) represents the rated reactive power of the i-th VSG, Q ei (t) is the reactive power output by the i-th VSG; n i Represents the droop coefficient of the reactive voltage equation, E mi (t) represents the output voltage reference signal of the i-th distributed generation DG, u Ei (t) represents the average voltage deviation controller to be designed, u Qi (t) represents the reactive power deviation controller to be designed, The rated voltage after distributed coordination consistency adjustment can be calculated as follows: U ni (t)=k∫[u Qi (t)+u Ei (t)]dt+E ref (8) S52: Obtain the average voltage of the microgrid through a distributed collaborative average voltage observer designed with preset time consistency As shown in formula (9): S53: The average voltage and reference voltage U * Compare and generate voltage difference, design average voltage deviation controller u Ei (t), as shown in formula (10): In equations (8) to (10), k is the integral term coefficient of the state controller, U * is the reference voltage of the microgrid, m is the weight coefficient of reactive power sharing and voltage recovery, They represent the average voltage observation values ​​of the i-th and j-th distributed power sources, C is the voltage coupling coefficient, and E ref It is the system rated voltage reference value.

6. The method for controlling the secondary voltage of a virtual synchronous generator based on preset time consistency according to claim 5, characterized in that: The reactive power deviation controller is used to ensure the distribution of reactive power, and the design steps include: S61: Select the condition that the droop coefficient of the inverter voltage is inversely proportional to the reactive power capacity of the distributed power source, as shown in formula (11): n1Q n1 =n2Q n2 =…=n N Q nN (11) It is also necessary to satisfy the requirement that reactive power is distributed according to capacity, as shown in formula (12): n1Q e1 =n2Q e2 =…=n N Q eN (12) S62: Based on equations (7) to (12), the reactive power deviation controller is designed as equation (13): In formulas (11) to (13), n N represents the droop coefficient of the reactive voltage equation of the Nth VSG, Q nN Indicates the rated reactive power of the Nth VSG, Q eN Represents the output reactive power of the Nth VSG, n j N represents the droop coefficient of the reactive voltage equation of the jth VSG. i Indicates that there are N VSGs connected in parallel.

7. The method for controlling the secondary voltage of a virtual synchronous generator based on preset time consistency according to claim 6 is characterized in that: According to the design of the average voltage deviation controller and the design of the reactive power deviation controller, the overall design of the VSG secondary voltage control strategy based on the preset time consistency is obtained, as shown in formula (14):

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