A secondary control method for a networked virtual synchronous machine based on a predetermined time

Abstract

The present invention discloses a secondary control method of a meshed virtual synchronous machine based on a predetermined time, including constructing a meshed AC microgrid including DGs, and each DG includes a virtual synchronous machine. The secondary control method of a meshed virtual synchronous machine based on a predetermined time constructs a secondary control equation of a virtual synchronous machine within a predetermined time by calculating a frequency error and a coordinated control input signal error, and through the constructed control model, the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is distributed according to the proportional demand. Compared with the prior art, it can not only solve the frequency error problem caused by the virtual synchronous machine in the isolated meshed AC microgrid, so that the frequency is kept within the error range and synchronized to the rated frequency, but also solve the problem of uncertainty in the convergence time caused by the input of a new DG, thereby realizing control within a predetermined time.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid control, and in particular relates to a secondary control method of a network-building virtual synchronous machine based on a predetermined time. Background Art

[0002] With the continuous increase in the penetration rate of new energy in my country, the randomness, volatility and uncontrollable characteristics of new energy power generation have brought great impact on the stable operation of the power system. The existing isolated microgrid control mainly adopts the droop control method. Since power electronic equipment such as inverters have high sensitivity and extremely fast response speed, they have almost no rotational inertia and cannot provide frequency support for the distribution network containing distributed power generation units, nor can they provide the necessary damping for the microgrid. When the system is disturbed, random fluctuations such as load changes or power output will cause changes in power and frequency, causing system instability. Therefore, the virtual synchronous machine technology enables the power electronic converter to access the distributed power supply process, so that the entire distributed power supply simulates the operating characteristics of the synchronous generator and gives the power grid the characteristics of rotational inertia and damping.

[0003] The existing virtual synchronous machine technology simulates the primary frequency modulation characteristics and electromagnetic transient characteristics of synchronous generators, but it fails to play its due supporting role in isolated microgrids. Its dynamic characteristics need to be adjusted and the response speed is slow. Therefore, it is necessary to design a secondary control method to improve the dynamic performance of microgrids. At present, the mainstream virtual synchronous machine control strategy adds finite time control and fixed time control, so that the microgrid can quickly restore the frequency to the rated frequency value within a certain time range when responding to a rapidly changing environment, and the active power can be accurately distributed according to a certain proportion when the load changes suddenly or a new distributed generation unit is introduced. However, the current finite time control effect is sensitive to the initial state of the system. If the initial state deviates too much from the predetermined stable value, it may cause the control process to fail or become unstable, and the expected stability cannot be achieved within the specified time. For the application of virtual synchronous machines, the uncertainty of initial conditions may cause instantaneous fluctuations in frequency or power. Although the fixed time is not affected by the initial state, the calculated steady-state time is too conservative and has a great difference from the actual recovery time. In addition, the existence of protocol parameters will also force the recovery time to change, and the newly invested distributed generation units need a long time to stabilize.

[0004] Although progress has been made in the research of time-varying functions of scheduled time, the practical application of secondary control of virtual synchronous machines still faces many challenges: 1) The control effect of secondary control based on scheduled time is not obvious in terms of accuracy and its practicality is poor; 2) The convergence time of the system is uncertain when it exceeds the scheduled time. Summary of the invention

[0005] The purpose of the present invention is to solve the problem raised in the background technology and to propose a secondary control method of a meshed virtual synchronous machine based on a predetermined time.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] The present invention proposes a method for secondary control of a networked virtual synchronous machine based on a predetermined time, comprising: A meshed AC microgrid with DGs, and each DG contains a virtual synchronous machine;

[0008] The output angular frequency of each DG is obtained based on the active frequency control equation of the primary control of the virtual synchronous machine;

[0009] Calculate the output frequency of each DG according to the output angular frequency of each DG, and calculate the frequency error of each DG;

[0010] Based on the secondary control of the virtual synchronous machine, the cooperative control input signal error of each DG is calculated, and the secondary control equation of the virtual synchronous machine within a predetermined time is constructed;

[0011] A control model is constructed based on the active frequency control equation and the secondary control equation of the primary control of the virtual synchronous machine. Through the control model, the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is distributed according to the proportional demand.

[0012] Preferably, after being connected in parallel, the DGs in the meshed AC microgrid are connected to the load, and the DGs in the meshed AC microgrid are communicatively connected according to a preset communication topology.

[0013] Preferably, the formula of the active frequency control equation of the primary control of the virtual synchronous machine is as follows:

[0014] ;

[0015] in, represents the virtual inertia of the virtual synchronous machine, represents the damping coefficient, Indicates the output power reference value, Indicates The output power of each DG is Indicates The output angular frequency of a DG, represents the desired angular frequency of the virtual synchronous machine, .

[0016] Preferably, the step of calculating the output frequency of each DG according to the output angular frequency of each DG, and calculating the frequency error of each DG, comprises:

[0017] No. The output frequency of the DG The calculation formula is as follows:

[0018] ;

[0019] No. Frequency error of DG The calculation formula is as follows:

[0020] ;

[0021] in, Indicates the rated frequency of DG.

[0022] Preferably, the secondary control based on the virtual synchronous machine calculates the cooperative control input signal error of each DG and constructs the secondary control equation of the virtual synchronous machine within a predetermined time, including:

[0023] The calculation formula of the coordinated control input signal error of each DG is as follows:

[0024] ;

[0025] in, Indicates The coordinated control input signal error of the DGs, Indicates The secondary control input signal of a DG, Indicates The secondary control input signal of a DG, Indicates The set of all distributed neighbors of a DG, Indicates The output power ratio of each DG is required to allocate the parameters, Indicates The ratio of the output power of each DG is required to allocate parameters;

[0026] The formula of the quadratic governing equation is as follows:

[0027] ;

[0028] in,

[0029] ;

[0030] in, For the scheduled time control system, is a custom function, and , is a preset positive integer, and These are the control parameters of the scheduled time control system. is the preset recovery time. Indicates the current time. is the first preset value, and , is the preset start time of the secondary control. Represents a time-varying function, which is a monotonically increasing continuous function. There is a limit inside, and ,when Approaching hour, tend to ,when Greater than or equal to , equal .

[0031] Preferably, the calculation formula of the control model is as follows:

[0032] ;

[0033] The secondary control input signal is compensated to the output power end of each DG, so that the secondary control of the virtual synchronous machine only transmits the frequency error, so that the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is distributed according to the proportional demand.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This secondary control method of the virtual synchronous machine in the networked AC microgrid based on the scheduled time calculates the frequency error and the error of the cooperative control input signal, constructs the secondary control equation of the virtual synchronous machine in the scheduled time, and through the constructed control model, the output frequency of each DG is restored to the rated frequency within the scheduled time, and the output power is distributed according to the proportional demand. Compared with the existing technology, it can not only solve the frequency error problem caused by the virtual synchronous machine in the isolated networked AC microgrid, so that the frequency is kept within the error range and synchronized to the rated frequency, but also solve the convergence time uncertainty problem caused by the new DG input, thereby realizing the control within the scheduled time. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a flow chart of the secondary control method of the networked virtual synchronous machine based on the predetermined time of the present invention;

[0037] Figure 2 Schematic diagram of each DG circuit of the present invention;

[0038] Figure 3 Schematic diagram of electrical connection of each DG of the present invention;

[0039] Figure 4 A communication topology diagram of each DG of the present invention;

[0040] Figure 5 It is a curve diagram of the output frequency of each DG in the experimental process of the present invention;

[0041] Figure 6 It is a curve diagram of the output power of each DG during the experimental process of the present invention;

[0042] Figure 7 is a graph of the output frequency of each DG using a predetermined time coordinated control scheme in the prior art;

[0043] Figure 8 It is a curve diagram of the output power of each DG in the predetermined time coordinated control scheme in the prior art of the present invention. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0045] In one embodiment, Figure 1-Figure 2 As shown, a secondary control method of a networked virtual synchronous machine based on a predetermined time is provided, comprising:

[0046] Step 1: Build the package A grid-type AC microgrid with DGs ( DGs are divided into 1 to denoted by ), and each DG contains a virtual synchronization machine;

[0047] Among them, DG is Distributed Generation (DG for short), and a DG is a distributed generation unit. After the DGs in the grid-type AC microgrid are connected in parallel, they are connected to the load, and the DGs in the grid-type AC microgrid are connected in communication according to the preset communication topology. Figure 3 As shown, in this embodiment, four DGs are taken as an example for explanation. After the four DGs are connected in parallel, they are connected to the load to supply power to the load, wherein the load may specifically be a user with power supply demand. The communication topology structure can be set according to actual needs, such as Figure 4 In the middle, four DGs form a square-shaped communication topology; the meshed AC microgrid is in island mode;

[0048] in Figure 2For each DG circuit diagram, it includes the main circuit of DG and the control circuit of virtual synchronous machine. The main circuit includes DC voltage source, inverter, LC filter circuit, etc. The control circuit is the core, including active-frequency control and reactive-voltage control. The grid-type AC microgrid simulates the operating characteristics of synchronous generators. Damping and rotation inertia links are introduced in active-frequency control. The inverter output current and voltage are collected for power calculation. , output reference voltage through active-frequency control and reactive-voltage and reference phase angle , obtain the voltage and output frequency required for AC power, and finally control the inverter through sinusoidal pulse width modulation technology.

[0049] Step 2: Obtain the output angular frequency of each DG based on the active frequency control equation of the primary control of the virtual synchronous machine;

[0050] The formula of the active frequency control equation of the primary control of the virtual synchronous machine is as follows:

[0051] ;

[0052] in, represents the virtual inertia of the virtual synchronous machine, represents the damping coefficient, Indicates the output power reference value, Indicates The output power of each DG is Indicates The output angular frequency of a DG, represents the desired angular frequency of the virtual synchronous machine, .

[0053] Step 3, calculating the output frequency of each DG according to the output angular frequency of each DG, and calculating the frequency error of each DG;

[0054] No. The output frequency of the DG The calculation formula is as follows:

[0055] ;

[0056] No. Frequency error of DG The calculation formula is as follows:

[0057] ;

[0058] in, Indicates the rated frequency of DG.

[0059] Step 4: Calculate the cooperative control input signal error of each DG based on the secondary control of the virtual synchronous machine, and construct the secondary control equation of the virtual synchronous machine within a predetermined time;

[0060] The calculation formula of the coordinated control input signal error of each DG is as follows:

[0061] ;

[0062] in, Indicates The coordinated control input signal error of the DGs, Indicates The secondary control input signal of a DG, Indicates The secondary control input signal of a DG, Indicates The set of all distributed neighbors of a DG (i.e. The set of all DGs that have direct communication connections with each DG). Indicates The output power ratio of each DG is required to allocate the parameters, Indicates The output power ratio of each DG is required to allocate parameters; each DG only needs to communicate with the adjacent DGs, which reduces a lot of communication, greatly reduces communication costs, and improves economic benefits;

[0063] The formula for the quadratic governing equation is as follows:

[0064] ;

[0065] in,

[0066] ;

[0067] in, A control system with a predetermined time (to achieve convergence of the secondary control within the predetermined time), is a custom function, and , is a preset positive integer, and These are the control parameters of the scheduled time control system. is the preset recovery time. Indicates the current time. is the first preset value, and (in, is a small constant used to control the control accuracy of the scheduled time. The closer it is to 0, the higher the control accuracy of the scheduled time, and the better the effect of the grid-forming AC microgrid). is the preset start time of secondary control (once the moment starts, the grid-forming AC microgrid will recover within the preset time range if it is disturbed, the load suddenly changes, or a new DG is connected). Represents a time-varying function, which is a monotonically increasing continuous function. There is a limit inside, and ,when Approaching hour, tend to ,when Greater than or equal to , equal ; Setting the time-varying function in this way can make Set it to the fastest recovery value to speed up the convergence time of transient time.

[0068] Step 5: Construct a control model based on the active frequency control equation and the secondary control equation of the primary control of the virtual synchronous machine. Through the control model, the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is distributed according to the proportional demand.

[0069] The calculation formula of the control model is as follows:

[0070] ;

[0071] The secondary control input signal is compensated to the output power end of each DG, so that the secondary control of the virtual synchronous machine only transmits the frequency error, so that the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is allocated according to the proportional demand (that is, the output power is allocated according to the demand of the load, and the output power is output according to the power required by the load). In this embodiment, the rated frequency is 50Hz.

[0072] In another embodiment, in order to intuitively verify the effect of the virtual synchronous machine secondary control method based on the predetermined time proposed in the present invention, a system communication topology structure built on the Matlab / Simulink software platform and a meshed AC microgrid including four DGs in parallel are constructed, such as Figure 1 As shown, the experimental results are used to illustrate:

[0073] In this method, 4 DGs form a 220V, 50Hz meshed AC microgrid. Each DG is denoted as DG1, DG2, DG3, and DG4. The electrical connection method is as follows: Figure 3 As shown (LOAD represents load), the communication topology is as follows Figure 4 As shown, the communication topology is a ring structure, DG1, DG2, DG3 and DG4 form a ring in sequence and are connected in the first place, wherein the dotted arrows on both sides of DG4 represent the disconnection or connection of DG4;

[0074] In this experiment, the operating data of each DG is first sampled (as shown in Table 1) to obtain its current output frequency, and then the frequency error and the cooperative control input signal error are obtained. The secondary control of the virtual synchronous machine within the predetermined time is constructed to achieve that the output frequency of each DG is restored to the rated frequency within the predetermined time, and the output power is distributed according to the proportional demand.

[0075] Table 1

[0076]

[0077] According to the data in Table 1, the output frequency curve and output power curve of the secondary control method of the virtual synchronous machine based on the predetermined time are obtained (such as Figure 5 and Figure 6 As shown), the secondary control starts at 1s, and the predetermined recovery time is set. The output frequency returns to the rated frequency of 50Hz within 1.5s. Set the output power ratio demand allocation parameter of DG1, DG2, DG3 and DG4 to the value of ( , , , The proportional demand allocation parameters are DG1, DG2, DG3, and DG4 respectively), and the output power converges within 1.5s and is accurately allocated according to the proportional demand allocation parameters.

[0078] The performance of the virtual synchronous machine secondary control method based on the scheduled time is further tested by plug-and-play test. The output frequency is restored to the rated frequency after the secondary control is started. After the output power is accurately distributed according to the proportional demand distribution parameters, DG4 is disconnected at 2s. Figure 5 and Figure 6 It can be seen that the secondary control method of the virtual synchronous machine based on the scheduled time is still working, and the stabilization is completed ahead of schedule. The output frequency quickly recovers to 50Hz, and the remaining 3 DGs follow The proportional demand allocation parameters are allocated; DG4 is reconnected at 3s, and the output frequency and output power converge to stable values ​​in advance and remain consistent with the state before disconnection. Effective control can be performed even when the number of DGs is changed. It can be seen that changing the number of DGs can still achieve the control target, and it has a plug-and-play function.

[0079] The output frequency and output power waveforms of the predetermined time coordinated control scheme in the prior art are as follows: Figure 7 and Figure 8 As shown, set the scheduled recovery time The same is 0.5s, and the secondary control starts at 1s. Figure 7 and Figure 8 , it can be seen that the overshoot and oscillation are large at 1s, the fluctuation caused by the different DG damping coefficients cannot be eliminated after 0.5s, the convergence time is uncertain, and in the plug-and-play test, the fluctuation is large, the secondary control of the initial startup does not work for the subsequent equipment disconnected system recovery, the convergence time is uncertain, and the scheduled time switch needs to be reset, which is very troublesome. In comparison, this network-based virtual synchronous machine secondary control method based on scheduled time has a better oscillation suppression effect on the transient process of output frequency and output power, and the disturbance outside the scheduled time range can also be restored to stability within the scheduled time.

[0080] This secondary control method of the virtual synchronous machine in the networked AC microgrid based on the scheduled time calculates the frequency error and the error of the cooperative control input signal, constructs the secondary control equation of the virtual synchronous machine in the scheduled time, and through the constructed control model, the output frequency of each DG is restored to the rated frequency within the scheduled time, and the output power is distributed according to the proportional demand. Compared with the existing technology, it can not only solve the frequency error problem caused by the virtual synchronous machine in the isolated networked AC microgrid, so that the frequency is kept within the error range and synchronized to the rated frequency, but also solve the convergence time uncertainty problem caused by the new DG input, thereby realizing the control within the scheduled time.

[0081] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims.

Claims

1. A method for secondary control of a networked virtual synchronous machine based on a predetermined time, characterized in that: The secondary control method of the networked virtual synchronous machine based on the predetermined time includes: Build includes A meshed AC microgrid with DGs, and each DG contains a virtual synchronous machine; The output angular frequency of each DG is obtained based on the active frequency control equation of the primary control of the virtual synchronous machine; Calculate the output frequency of each DG according to the output angular frequency of each DG, and calculate the frequency error of each DG; Based on the secondary control of the virtual synchronous machine, the cooperative control input signal error of each DG is calculated, and the secondary control equation of the virtual synchronous machine within a predetermined time is constructed; A control model is constructed based on the active frequency control equation and the secondary control equation of the primary control of the virtual synchronous machine. Through the control model, the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is distributed according to the proportional demand; The step of calculating the output frequency of each DG according to the output angular frequency of each DG and calculating the frequency error of each DG includes: No. The output frequency of the DG The calculation formula is as follows: ； No. Frequency error of DG The calculation formula is as follows: ； in, Indicates the rated frequency of DG; The secondary control based on the virtual synchronous machine calculates the coordinated control input signal error of each DG and constructs the secondary control equation of the virtual synchronous machine within a predetermined time, including: The calculation formula of the coordinated control input signal error of each DG is as follows: ； in, Indicates The coordinated control input signal error of the DGs, Indicates The secondary control input signal of a DG, Indicates The secondary control input signal of a DG, Indicates The set of all distributed neighbors of a DG, Indicates The output power ratio of each DG is required to allocate the parameters, Indicates The ratio of the output power of each DG is required to allocate parameters; The formula of the quadratic governing equation is as follows: ； in, ； in, For the scheduled time control system, is a custom function, and , is a preset positive integer, and These are the control parameters of the scheduled time control system. is the preset recovery time. Indicates the current time. is the first preset value, and , is the preset start time of the secondary control. Represents a time-varying function, which is a monotonically increasing continuous function. There is a limit inside, and ,when Approaching hour, tend to ,when Greater than or equal to , equal .

2. The method for secondary control of a networked virtual synchronous machine based on a predetermined time as claimed in claim 1, characterized in that: After being connected in parallel, the DGs in the meshed AC microgrid are connected to the load, and the DGs in the meshed AC microgrid are communicatively connected according to a preset communication topology.

3. The method for secondary control of a networked virtual synchronous machine based on a predetermined time as claimed in claim 1, characterized in that: The formula of the active frequency control equation of the primary control of the virtual synchronous machine is as follows: ； in, represents the virtual inertia of the virtual synchronous machine, represents the damping coefficient, Indicates the output power reference value, Indicates The output power of each DG is Indicates The output angular frequency of a DG, represents the desired angular frequency of the virtual synchronous machine, .

4. The method for secondary control of a networked virtual synchronous machine based on a predetermined time as claimed in claim 1, characterized in that: The calculation formula of the control model is as follows: ； The secondary control input signal is compensated to the output power end of each DG, so that the secondary control of the virtual synchronous machine only transmits the frequency error, so that the output frequency of each DG is restored to the rated frequency within a predetermined time, and the output power is distributed according to the proportional demand.

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