Improved Reactive Power Control Method for a Multi-Machine Parallel System of Impedance-Based Virtual Synchronous Machines

Abstract

The present invention discloses an improved reactive power control method for a multi-machine parallel system of virtual synchronous machines based on impedance, which relates to the technical field of virtual synchronous machines. By using low-bandwidth communication to obtain the reactive power information output by the inverter, and through the multi-machine parallel connection and control strategy of virtual synchronous machines with adaptive virtual impedance, the system frequency and voltage stability are ensured, the power quality of the virtual synchronous machines connected to the power grid is improved, the damping inertia of the power grid is enhanced, the accurate distribution of reactive power loads is realized, and more control freedoms are provided for the safe operation of the active distribution network.

Description

Technical Field

[0001] The invention belongs to the technical field of virtual synchronous machines, and particularly relates to an improved reactive power control method for a multi-machine parallel system of virtual synchronous machines based on impedance. Background Art

[0002] Distributed generation technology based on renewable energy sources (such as wind energy, solar energy, etc.) is one of the important means for humans to cope with energy crises and environmental pollution. A large number of distributed power sources are connected to the power grid through power electronic devices such as grid-connected inverters, gradually reducing the proportion of traditional synchronous generators, thus causing a relative reduction in the rotational reserve capacity and moment of inertia of the power system; at the same time, due to the intermittency and uncontrollability of primary energy sources, the grid stability problem has become increasingly severe. Therefore, how to enable distributed power sources to be grid-connected in a friendly manner, reduce the negative impact of distributed power sources on the safe and high-quality operation of the grid, and improve the grid's acceptance capacity for distributed power sources is a major problem that urgently needs to be solved in intelligent distribution networks.

[0003] In such a background, the concept of virtual synchronous generators is proposed, that is, by introducing corresponding control algorithms into grid-connected inverters, distributed power sources based on grid-connected inverters can simulate or partially simulate the frequency and voltage control characteristics of synchronous generators from the external characteristics, while enhancing the grid connection damping and inertia of distributed energy sources, thereby improving the stability of distributed systems.

[0004] The networking operation of multiple virtual synchronous generator sets has great advantages in terms of system capacity, redundancy, reliability, flexibility, etc. However, the distributed energy sources in the system are geographically dispersed, and many factors such as the mismatch of line impedance, the structural parameters of inverters, and the types of controllers will cause differences in the output voltages of inverters, resulting in it being difficult to accurately distribute the reactive power of the parallel system according to the capacity ratio, generating reactive power circulation. The reactive power circulation is transmitted between inverters, increasing the current stress and thermal stress of the power devices, lines, and output filters in the inverters. In severe cases, it will cause the inverters to trip due to overload or overcurrent protection, resulting in the parallel system being unable to operate. Summary of the Invention

[0005] The purpose of the invention is to provide an improved reactive power control method for a multi-machine parallel system of virtual synchronous machines based on impedance, thereby overcoming the disadvantages of inappropriate reactive power distribution and circulation distribution during the parallel operation of existing multiple virtual synchronous machines.

[0006] To achieve the above purpose, the invention provides an improved reactive power control method for a multi-machine parallel system of virtual synchronous machines based on impedance, including the following steps:

[0007] The central controller obtains the reactive power information output by each VSG unit in the multi-machine parallel system of virtual synchronous generators through low-bandwidth communication, constructs an adaptive virtual impedance according to the reactive power information, thereby compensating for the difference in the reference value of the inverter output voltage caused by unequal feeder voltage drops, and achieving accurate distribution of reactive power loads;

[0008] Among them, the multi-machine parallel system of virtual synchronous generators includes a local controller;

[0009] The central controller constructs a control strategy according to the adaptive virtual impedance, and the control strategy includes the following steps:

[0010] The central controller obtains the reactive power information. Before the local controller receives a reactive power load reference value that is different from the actual output, the integral control signal is 0, and the output of the integrator remains in the state of the previous adjustment. If the load does not change, the reactive power load output by the VSG unit stably follows the reactive power load reference value, and the reactive power distribution of the system is good. The control strategy continues until the load changes and proceeds to the next step;

[0011] When the load changes, the reactive power load output by the VSG unit changes accordingly. The central controller synchronously updates the reactive power load reference value according to the new reactive power load situation. When the local controller detects a deviation between the real-time reactive power load and the new reactive power load reference value, it sets the integral control signal from 0 to 1 to enable the integrator to start the adjustment of reactive power. When the reactive power output by the VSG unit stably follows the new reactive power load reference value, the adjustment process ends, the integral control signal is reset to 0, and it returns to the previous step for control.

[0012] Preferably, the central controller obtains the reactive power information regularly.

[0013] Preferably, the central controller obtains the reactive power information regularly as follows:

[0014] The frequency of the synchronization signal is 5 Hz, and the central controller calculates and updates the reactive power load reference value every 0.2 seconds.

[0015] Preferably, the control strategy further includes: when the central controller is interrupted from the multi-machine parallel system of virtual synchronous generators, the local controller obtains the load. If the load does not change, the multi-machine parallel system of virtual synchronous generators will continue the steady state of accurate reactive power distribution; if the load changes, the output of the integrator remains in the previous adjustment state.

[0016] Preferably, when the central controller is interrupted from the multi - machine parallel system of virtual synchronous generators (VSGs), the local controller records the update time of the last reactive power reference value. When it detects that the update time has timed out, it determines that there is a communication failure in the multi - machine parallel system of VSGs, sets the integral control signal of the multi - machine parallel system of VSGs to 0. At the same time, the central controller stops updating the reactive power signals of all VSG units until the communication is restored.

[0017] Preferably, the expression of the adaptive virtual impedance is:

[0018]

[0019] Wherein,

[0020]

[0021] In the above formula, X v is the virtual reactance for decoupling reactive power and active power in the low - voltage system; Q j_rate is the rated reactive power of the j - th VSG unit; Q Load is the total power of the reactive power load of the multi - machine parallel system of VSGs, obtained by summing the reactive powers output by all VSG units; Q i * is the reactive power reference value that the corresponding micro - source should share, calculated by the central controller according to the total reactive power load power and the rated capacity weights of each VSG unit; K i is the integral control gain; G k is the integral control signal, generated by the logic circuit in the local controller of the VSG unit. When it detects a reactive power distribution deviation, that is, when the reactive power reference value received by the VSG is not equal to the actual output reactive power, the G k signal is 1, enabling the integrator; when it detects that the reactive power distribution deviation is eliminated or a communication interruption occurs, the G k signal is 0, disabling the integrator enable, and Q i is the actual output reactive power received by the i - th VSG unit.

[0022] Compared with the existing technologies, the present invention has the following beneficial effects:

[0023] The improved reactive power control method for the multi - machine parallel system of impedance - based virtual synchronous generators provided by the present invention uses low - bandwidth communication to obtain the reactive power information output by the inverter. Through the multi - machine parallel connection of virtual synchronous generators with adaptive virtual impedance and the control strategy, it ensures the stability of the system frequency and voltage, improves the power quality of the virtual synchronous generators connected to the grid, enhances the grid damping inertia, realizes the accurate distribution of reactive power loads, and provides more control degrees of freedom for the safe operation of the active distribution network. Description of the Drawings

[0024] To more clearly illustrate the technical solution of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only one embodiment of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0025] Figure 1 It is a schematic structural diagram of the multi-machine parallel system of the virtual synchronous machine of the present invention;

[0026] Figure 2 It is a schematic diagram of the power distribution principle of the parallel virtual synchronous generator;

[0027] Figure 3 It is a schematic diagram of the equivalent principle of the multi-machine parallel system of the virtual synchronous machine with the introduction of virtual impedance;

[0028] Figure 4 It is a circulation control block diagram of the voltage-current ratio after the introduction of virtual impedance;

[0029] Figure 5 It is a schematic structural diagram of the simulation model of the multi-machine parallel system of the virtual synchronous machine in the embodiment of the invention;

[0030] Figure 6 It is a schematic diagram of the simulation result when three parallel VSG units adopt traditional virtual synchronous control;

[0031] Figure 7 It is a schematic diagram of the simulation result of the control method of the present invention;

[0032] Figure 8 It is a schematic diagram of the simulation result of the active and reactive power distribution of three VSGs;

[0033] Figure 9 It is the simulation result of the power distribution of the multi-machine parallel system of the virtual synchronous machine during communication interruption;

[0034] Figure 10 It is a flowchart of an improved reactive power control method for a multi-machine parallel system of a virtual synchronous machine based on impedance according to the present invention. Specific embodiments

[0035] The following will clearly and completely describe the technical solutions in the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0036] Figure 1A virtual synchronous machine multi-machine parallel system is shown. The virtual synchronous machine multi-machine parallel system includes multiple parallel virtual synchronous machines (VSG units). The VSG unit includes a DC source, a grid-connected inverter, and a filter connected in sequence. Each virtual synchronous machine is connected to the AC bus through a feeder and jointly supplies power to the load. The load directly connected to the virtual synchronous machine is the local load, and the rest is the network load. Through the static switch K at the Point of Common Coupling (PCC), the system's grid connection / disconnection operation is completed.

[0037] The virtual synchronous generator technology essentially simulates the power-frequency static characteristics of traditional synchronous generators and introduces the inertia and damping unique to mechanical devices such as generators into the control loop to enhance the stable operation of the system. Its power distribution principle is as Figure 2 shown. The parallel virtual synchronous machines respectively use the frequency and amplitude of their respective output voltages as the measurement criteria for active and reactive power outputs. According to the preset active-frequency curve and reactive-voltage curve, they automatically participate in the proportional distribution of power to achieve load sharing without communication.

[0038] And the system frequency, as a global quantity, will ultimately be synchronized. The active power can always be accurately distributed according to the preset distribution ratio. However, the reference value of the amplitude of the output voltage of the virtual synchronous machine is approximately the output voltage amplitude of the inverter, and its value is affected by the differences in the characteristics of the corresponding network ports of each inverter and is difficult to unify. This results in the parallel VSG units being unable to output reactive power according to the ratio of their rated capacities, thereby generating system reactive power circulation.

[0039] Traditional virtual impedance loop control:

[0040] The principle of introducing virtual impedance is as Figure 3 shown. By introducing a virtual impedance Z i between the virtual output voltage E oi of the inverter and the actual port voltage U viri , the equivalent impedance of the system with respect to the relative virtual voltage Ei meets the requirement of being inversely proportional to the inverter capacity, that is, the ratio of the sum of the virtual impedances Z viri of each VSG unit and the feeder impedance Z fi is inversely proportional to the capacity. Thus, the mismatched feeder impedance is corrected so that the voltage drops generated on the equivalent impedance are the same, that is, ΔE1 = ΔE2, and further E1 = E2 is satisfied to achieve accurate reactive power distribution of the parallel virtual synchronous machines.

[0041] Using the output current value feedback, the loop control block diagram of the voltage-current ratio after introducing virtual impedance is as Figure 4 shown. Figure 4 In it, E ref is the voltage reference modulation signal after introducing virtual impedance:

[0042]

[0043] According to the circulation control block diagram, it is obtained that:

[0044] u o (s) = G v (s)E ref (s) - Z o (s)i o (s) (2)

[0045] By using equations (1) and (2), it is obtained that:

[0046]

[0047] In equation (2), Z0'(s) is the equivalent output impedance after the introduction of the virtual impedance:

[0048] Z' o (s) = G v (s)Z vir +Z o (s)

[0049] At the fundamental frequency, |G v (s)| ≈ 1 and |Z o (s)| ≈ 0 hold. Therefore, at the fundamental frequency, approximately Z' o (s) = Z vir ;

[0050] Therefore, after ignoring the influence of the inverter equivalent output impedance, the value of the introduced virtual impedance only needs to make the ratio of the system equivalent impedance satisfy the capacity inverse ratio according to the feeder impedance parameter information.

[0051] After analysis, it is found that the traditional virtual impedance method is mainly based on the physical circuit model, which requires accurate knowledge of the feeder impedance parameter information and has high requirements for engineering measurement; moreover, this method does not consider the influence of local load on the feeder voltage drop. When the local load changes, the established virtual impedance cannot match the new operating point, and the current sharing of the parallel system cannot be achieved well.

[0052] As Figure 10 shown, to improve the reactive power distribution performance of the virtual impedance, this paper proposes an improved reactive power control method for a multi-machine parallel system of virtual synchronous machines based on impedance by means of communication, which specifically includes the following steps:

[0053] S1. The central controller obtains the reactive power information output by each VSG unit in the multi-machine parallel system of virtual synchronous machines through low-bandwidth communication, constructs an adaptive virtual impedance according to the reactive power information, thereby compensating for the difference in the inverter output voltage reference value caused by unequal feeder voltage drops, and achieving accurate distribution of reactive power loads;

[0054] Among them, the multi - machine parallel system of virtual synchronous machines includes local controllers.

[0055] The expression of the adaptive virtual impedance is:

[0056]

[0057] Among them,

[0058]

[0059] In the above formula, X v is the virtual reactance for realizing the decoupling of reactive and active power in the low - voltage system; Q j_rate is the rated reactive power of the j - th VSG unit; Q Load is the total power of the reactive load of the multi - machine parallel system of virtual synchronous machines, obtained by summing the reactive powers output by all VSG units; Q i * is the reactive reference value that the corresponding micro - source should share, calculated by the central controller according to the total reactive load power and the rated capacity weights of each VSG unit; K i is the integral control gain; G k is the integral control signal, generated by the logic circuit in the local controller of the VSG unit. When a reactive power distribution deviation is detected, that is, when the reactive reference value received by the VSG is not equal to the actual output reactive power, the G k signal is 1, enabling the integrator; when the reactive power distribution deviation is eliminated or a communication interruption occurs, the G k signal is 0, prohibiting the integrator from being enabled, and Q i is the actual output reactive power received by the i - th VSG unit.

[0060] Figure 3 is the control block diagram of the adaptive virtual impedance. Among them, the central controller periodically sends out synchronization signals to collect the reactive load information of the system, and calculates and sends the reactive reference value Q i * that each VSG unit should share according to the above formula. The local controller receives Q i * and then adjusts the reactive power output by the VSG unit through the integrator, finally making the actual output reactive power of the micro - source consistent with the reference value, realizing the accurate sharing of reactive power. Note that the communication only involves the central controller and the local controller, and there is no need for communication between local controllers, still retaining the plug - and - play characteristic of the inverter. Therefore, to solve the limitations existing in the reactive load:

[0061] S2. The central controller constructs a control strategy according to the adaptive virtual impedance, and the control strategy includes the following steps:

[0062] S21. The central controller periodically obtains the reactive power reference value Q i * , before the local controller receives a reactive power load reference value different from the actual output, the integral control signal G k is 0, and the output of the integrator remains in the state of the previous adjustment. If the load does not change, the reactive power load output by the VSG unit stably follows the reactive power load reference value, the reactive power distribution of the system is good, and the control strategy continues until the load changes and proceeds to the next step;

[0063] S22. When the load changes, the reactive power load output by the VSG unit changes accordingly, and the central controller synchronously updates the reactive power load reference value Q according to the new reactive power load situation i * . When the local controller detects a deviation between the real-time reactive power load and the new reactive power load reference value, it sets the integral control signal G k from 0 to 1, enabling the integrator to start the adjustment of reactive power. When the reactive power output by the VSG unit stably follows the new reactive power load reference value, the adjustment process ends, and the integral control signal is reset to 0, returning to the previous step for control.

[0064] In step S21, the central controller periodically obtains the reactive power information as follows: the frequency of the synchronization signal is 5 Hz, and the central controller calculates and updates the reactive power load reference value every 0.2 seconds. Considering the inherent delay of communication, the time for each inverter to receive the synchronous reactive power reference signal may be inconsistent. A relatively slow adjustment process can reduce the "sensitivity" to communication consistency, thereby greatly reducing the requirements for the communication link bandwidth and rate. However, too long an adjustment time will also make the compensation effect too small to be obvious. Considering that the change in system load demand is not very frequent, such as the change in residential load is usually very slow, the adjustment process can be reasonably selected as 1 - 2 s. In this way, the adjustment time of reactive power is much longer than the update period of Q i * . When a load change occurs during the adjustment process, the central controller can update Q in a timely manner according to the new load situation i * , ensuring the reactive power distribution accuracy and the stability of the system.

[0065] During the operation of the virtual synchronous generator multi-machine parallel system, a communication interruption fault may occur. The control strategy further includes step S23. When the central controller is interrupted from the virtual synchronous generator multi-machine parallel system (local controller), the local controller obtains the load. If the load does not change, the virtual synchronous generator multi-machine parallel system will continue the steady state of accurate reactive power distribution; if the load changes, the output of the integrator remains in the previous adjustment state.

[0066] Specifically, when the central controller is interrupted from the multi - machine parallel system of virtual synchronous generators (local controller), the local controller records the update time of the most recent reactive power reference value Q i * , and when it detects that the update time has timed out, it determines that a communication failure has occurred in the multi - machine parallel system of virtual synchronous generators, and sets the integral control signal G k of the multi - machine parallel system of virtual synchronous generators to 0. At the same time, the central controller stops updating the reactive power signals of all VSG units until the communication is restored.

[0067] The improved reactive power control method for the multi - machine parallel system of virtual synchronous generators based on impedance of the present invention is simulated and verified to enable those skilled in the art to better understand the present invention:

[0068] A simulation model of the multi - machine parallel system of virtual synchronous generators is built on the Matlab / Simulink software platform. As Figure 5 shown, the system consists of three virtual synchronous generators VSG1 - VSG3 with a capacity of 9 kVA each, local loads Load1 - Load3 of each virtual synchronous generator, and common loads Load4 - Load6. The feeder lengths between each node are set as shown in the figure, and their impedances are calculated according to the typical feeder parameters of a low - voltage microgrid. Among them, the unit feeder reactance is 0.083 Ω / km, and the unit feeder resistance is 0.642 Ω / km. The main circuit and control parameters of the VSG are shown in Table 1, and the initial load parameters of the multi - machine parallel system simulation model of virtual synchronous generators are shown in Table 2.

[0069] Table 1 Main circuit and control simulation parameters of the VSG unit

[0070]

[0071] Table 2 Initial system load parameters

[0072]

[0073]

[0074] Example 1: Traditional virtual synchronous control method

[0075] Figure 6 is the simulation result when three parallel VSG units adopt traditional virtual synchronous control. It can be seen that due to the characteristic differences of the network ports corresponding to each VSG unit, it is difficult to unify the amplitude reference of the inverter output voltage, and there are obvious reactive power distribution deviations because the reactive power cannot be evenly divided. And since each inverter operates at the same frequency in the steady state, the active power can be evenly divided well. The current output of each VSG unit at the initial operating point is as Figure 6(c), it can be seen that due to the inability to evenly distribute reactive power, there are significant amplitude differences in the output current amplitude and phase of each VSG. The simulation results show that traditional Q-E control has limitations in the even distribution of reactive power.

[0076] Example 2: Using the adaptive virtual impedance method of the present invention

[0077] After adopting the improved reactive power control method for a multi-machine parallel system of virtual synchronous generators based on impedance proposed in this paper, the power distribution of the VSG units is as Figure 7 shown. At 2 s, the integrator is enabled to start regulating the reactive power. It can be seen that the reactive power deviation of each VSG output gradually decreases to zero, and finally the accurate distribution of reactive power loads is achieved. Figure 7 In (b), there are slight disturbances in the active power output by each VSG unit at the beginning of regulation and load application, but the active power sharing effect is always good. Figure 7 (c) shows the output current waveforms of each inverter at the initial operating point. Comparing Figure 7 (c), due to the improvement of the reactive power distribution accuracy after regulation, the VSG output currents almost completely overlap, and the current sharing effect of the system is significantly improved.

[0078] Example 3: Communication link delay situation

[0079] To detect the influence of communication link delay on the effect of the proposed control strategy, delays of 0.05 s and 0.1 s are respectively applied to the reactive power update signals of VSG2 and VSG3. To reflect the inconsistency of communication, the update signal of VSG1 remains normal. The simulation results of the active and reactive power distribution of the three VSGs are as Figure 8 shown. It can be seen from the figure that due to the communication link delay, when the load suddenly changes at 6 s, VSG2 and VSG3 cannot update the reactive power reference value in a timely manner according to the new operating point, and slight disturbances occur in both the active and reactive powers, affecting the dynamic performance of power distribution. However, when the system is in steady-state operation, the active and reactive power outputs are stable and the sharing effect is good. Therefore, communication delay does not affect the steady-state performance of reactive power distribution.

[0080] Example 4: Communication interruption situation

[0081] Figure 9 The simulation results of the system power distribution during communication interruption are given. At t = 5 s, a communication fault occurs. The receiver in the local control detects the timeout of the active power update signal, sets Gk to 0, and disconnects the integrator input. From Figure 9As can be seen from (a), when there is a communication fault and the load remains unchanged, the accuracy of reactive power distribution will not be affected; when the load changes, there is a deviation in the equal sharing of reactive power. However, compared with Case 1 where only traditional virtual synchronous control is adopted, the reactive power distribution error of each VSG unit is significantly reduced. When the communication is restored at t = 8 s, the reactive power distribution error gradually decreases, and a good equal sharing effect is restored. Figure 9 In (b), the active power output of each VSG is always the same, so the communication interruption has no effect on the distribution of active power.

[0082] In conclusion, the improved reactive power control method for the multi-machine parallel system of virtual synchronous machines based on impedance proposed by the present invention uses low-bandwidth communication to obtain the reactive power information output by the inverter. Through the multi-machine parallel connection and control strategy of virtual synchronous machines with adaptive virtual impedance, it ensures the stability of the system frequency and voltage, improves the power quality of the virtual synchronous machines connected to the grid, enhances the grid damping inertia, realizes the accurate distribution of reactive power loads, and provides more control freedoms for the safe operation of the active distribution network.

[0083] The specific embodiments disclosed above are only for the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention can easily think of changes or variations, which should all be covered within the protection scope of the present invention.

Claims

1. An improved reactive power control method for a multi - machine parallel system of impedance - based virtual synchronous machines, characterized in that, It includes the following steps: The central controller obtains the reactive power information output by each VSG unit in the multi - machine parallel system of virtual synchronous generators through low - bandwidth communication, constructs an adaptive virtual impedance according to the reactive power information, thereby compensating for the difference in the inverter output voltage reference value caused by unequal feeder voltage drops, and achieving accurate distribution of reactive power loads; Among them, the multi - machine parallel system of virtual synchronous generators includes a local controller; The central controller constructs a control strategy according to the adaptive virtual impedance, and the control strategy includes the following steps: The central controller obtains the reactive power information. Before the local controller receives a reactive power load reference value different from the actual output, the integral control signal is 0, and the output of the integrator remains in the state of the previous adjustment. If the load does not change, the reactive power load output by the VSG unit stably follows the reactive power load reference value, and the reactive power distribution of the system is good. The control strategy continues until the load changes and enters the next step; When the load changes, the reactive power load output by the VSG unit changes accordingly. The central controller synchronously updates the reactive power load reference value according to the new reactive power load situation. When the local controller detects a deviation between the real - time reactive power load and the new reactive power load reference value, it sets the integral control signal from 0 to 1 to enable the integrator to start the adjustment of reactive power. When the reactive power output by the VSG unit stably follows the new reactive power load reference value, the adjustment process ends, and the integral control signal is reset to 0, and it returns to the previous step for control; The expression of the adaptive virtual impedance is: Wherein, In the above formula, X v is the virtual reactance for decoupling reactive and active power in the low-voltage system; Q j_rate is the rated reactive power of the j-th VSG unit; Q Load is the total power of the reactive load in the multi-machine parallel system of virtual synchronous generators, obtained by summing the reactive powers output by all VSG units; Q i * is the reference reactive power that the corresponding micro-source should share, calculated by the central controller based on the total reactive load power and the rated capacity weights of each VSG unit; K i is the integral control gain; G k is the integral control signal, generated by the logic circuit in the local controller of the VSG unit. When the reactive power distribution deviation is detected, that is, when the received reactive power reference value of the VSG is not equal to the actually output reactive power, G k the signal is 1 to enable the integrator; when the reactive power distribution deviation is eliminated or a communication interruption occurs, G k the signal is 0 to prohibit the integrator from being enabled, Q i is the actually output reactive power received by the i-th VSG unit.

2. The improved reactive power control method for a multi - machine parallel system of impedance - based virtual synchronous machines according to claim 1, characterized in that, The central controller regularly obtains the reactive power information.

3. The improved reactive power control method for a multi - machine parallel system of impedance - based virtual synchronous machines according to claim 2, characterized in that, The central controller regularly obtaining the reactive power information is: The frequency of the synchronization signal is 5Hz, and the central controller calculates and updates the reactive power load reference value every 0.2 seconds.

4. The improved reactive power control method for a multi - machine parallel system of impedance - based virtual synchronous machines according to claim 1, characterized in that, The control strategy also includes: when the central controller is interrupted from the multi - machine parallel system of virtual synchronous generators, the local controller obtains the load. If the load does not change, the multi - machine parallel system of virtual synchronous generators will continue the steady state of accurate reactive power distribution; if the load changes, according to the output of the integrator remaining in the previous adjustment state.

5. The improved reactive power control method for a multi - machine parallel system of impedance - based virtual synchronous machines according to claim 4, characterized in that, When the central controller is interrupted from the multi - machine parallel system of virtual synchronous generators: the local controller records the update time of the most recent reactive power reference value. When it detects that the update time has timed out, it judges that a communication fault has occurred in the multi - machine parallel system of virtual synchronous generators, sets the integral control signal of the multi - machine parallel system of virtual synchronous generators to 0. At the same time, the central controller stops the update of the reactive power signals of all VSG units until the communication is restored.

Images