A reactive power distribution method for voltage source inverter under weak communication conditions

Abstract

The present invention relates to inverter control technology for microgrids, and aims to provide a method for reactive power distribution of voltage source inverters under weak communication conditions. It includes: for N voltage source inverters connected in parallel to the power grid, calculating the real-time output reactive power and instantaneous weighted average reactive power of all inverters; calculating the output reactive power matching coefficient of the Nth inverter and the virtual impedance used for reactive power regulation in droop control; calculating the difference between the output reactive power of the Nth inverter and the instantaneous weighted average reactive power; if the difference is not equal to 0, iteratively processing the virtual impedance value; if the calculated difference is equal to 0, it means that the reactive power distribution adjustment in the microgrid system has been completed, and the conditions for connecting to a new distributed power source are met. In the present invention, when the inverter completes the stable operation of the networking system, the value of the virtual impedance is fixed to the local controller, and reactive power distribution will no longer rely on communication, thereby improving the stability of the networking system.

Description

Technical Field

[0001] The invention relates to a reactive power distribution method of a voltage source inverter under weak communication conditions, belonging to inverter control technology in microgrid technology. Background Art

[0002] Voltage source inverters are grid-friendly because of their voltage support capabilities. Since the capacity of a single inverter is limited, multiple inverters are usually connected in parallel to increase voltage capacity in microgrids such as photovoltaic power stations. In the process of connecting voltage source inverters in parallel, due to the difference in impedance between the inverter and the AC bus, there will be uneven distribution of reactive power between inverters, resulting in current circulation between inverters. Therefore, the proportional distribution of reactive power between inverters can fully utilize the capacity of the power generation unit and reduce power loss between loops.

[0003] The traditional reactive power distribution technology of voltage source inverter adopts centralized control, which collects reactive power information of each inverter through the central controller and sends power distribution and utilization at the same time. This control method is highly dependent on communication and has weak anti-interference ability and stability. In the case of weak communication conditions such as line faults and electromagnetic interference, it will be difficult for the power generation devices in the system to receive normal instructions, resulting in the system not being able to operate normally and stably, and even causing damage to the devices in the system.

[0004] Therefore, the present invention aims to propose a reactive power distribution method for a voltage source inverter under weak communication conditions, so as to reduce the dependence on central control communication and improve the stability of the operation of the networking system during the stable operation of the inverter. Summary of the invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a method for reactive power distribution of a voltage source inverter under weak communication conditions.

[0006] To solve the technical problem, the solution of the present invention is:

[0007] A method for reactive power distribution of a voltage source inverter under weak communication conditions is provided, comprising the following steps:

[0008] (1) For N voltage source inverters connected in parallel to the grid, calculate the real-time output reactive power Q of all inverters i And the instantaneous weighted average reactive power Q avg ;

[0009] (2) Calculate the output reactive power matching coefficient η of the Nth inverter N and the virtual impedance Z for reactive power regulation in droop control ηVN ;

[0010] (3) Calculate the reactive power Q output by the Nth inverter N and the instantaneous weighted average reactive power Q avg If the difference is not equal to 0, the virtual impedance value is iterated;

[0011] (4) If the difference calculated in step (3) is equal to 0, it means that the reactive power distribution adjustment in the microgrid system has been completed, and the conditions for connecting a new distributed power source are met.

[0012] As a preferred solution of the present invention, the instantaneous weighted average reactive power Q output by the inverter is avg The calculation method is as follows:

[0013]

[0014] In the formula, Q avg is the weighted average of the output reactive power of all inverters; N is the number of inverters in the microgrid system; Q i is the reactive power output by the i-th inverter, n i is the droop coefficient of the i-th inverter, i=1, 2...N.

[0015] As a preferred embodiment of the present invention, the reactive power matching coefficient η N The calculation method is as follows:

[0016]

[0017] Where η N n is the inverter output reactive power matching coefficient; N is the reactive power droop coefficient; Q N Output reactive power for the Nth inverter; Q avg Output reactive power for the inverter in the microgrid.

[0018] As a preferred solution of the present invention, the calculation method of the virtual impedance is shown in the following formula:

[0019] Z ηVN =k·η N

[0020] In the formula, Z ηVN is the virtual impedance of the inverter; k is the impedance adjustment coefficient; η N It is the inverter output reactive power matching factor.

[0021] As a preferred solution of the present invention, the iterative processing of the virtual impedance value specifically refers to:

[0022] (1) If the Nth inverter outputs reactive power Q N and the instantaneous weighted average reactive power Qavg If the difference is greater than 0, then the initial virtual impedance Z vN Based on the reactive power regulation virtual impedance Z ηVN Then re-execute steps (1) and (2);

[0023] (2) If the Nth inverter outputs reactive power Q N and the instantaneous weighted average reactive power Q avg The difference is less than 0, then the initial virtual impedance Z vN Based on the reactive regulation virtual impedance Z minus ηVN Then re-execute steps (1) and (2).

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] During the networking process, the present invention collects and calculates the output reactive power of each inverter to adjust the output virtual impedance of the inverter, thereby realizing proportional distribution of reactive power; when the inverter completes the stable operation of the networking system, the value of the virtual impedance is fixed to the local controller, and reactive power distribution will no longer rely on communication, thereby improving the stability of the networking system. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The flowchart of the reactive power distribution method of voltage source inverter under weak communication conditions is shown in FIG.

[0027] Figure 2 This is the microgrid simulation structure diagram in PLECS software.

[0028] Figure 3 It is the reactive power distribution effect of the traditional control method without adding virtual impedance.

[0029] Figure 4 This is the reactive power distribution effect of three inverters under the control technology of the invention. DETAILED DESCRIPTION

[0030] The technical solution of the invention is described in detail below with reference to the accompanying drawings:

[0031] like Figure 1 As shown, the reactive power distribution method of the voltage source inverter under weak communication conditions includes the following steps:

[0032] 1. For N voltage source inverters connected to the grid in parallel, calculate the real-time output reactive power Q of all inverters i And the instantaneous weighted average reactive power Q avg ;

[0033] The instantaneous weighted average reactive power Q output by the inverter avg The calculation method is as follows:

[0034]

[0035] In the formula, Q avg is the weighted average of the output reactive power of all inverters; N is the number of inverters in the microgrid system; Q i is the reactive power output by the i-th inverter, n i is the droop coefficient of the i-th inverter, i=1, 2...N.

[0036] 2. Calculate the output reactive power matching coefficient η of the Nth inverter N and virtual impedance Z for reactive power regulation of droop control ηVN ;

[0037] Reactive power matching coefficient η N The calculation method of virtual impedance is as follows:

[0038]

[0039] Z θVN =k·η N

[0040] Where η N n is the inverter output reactive power matching coefficient; N is the reactive power droop coefficient; Q N Output reactive power for the Nth inverter; Q avg The reactive power output of the inverter in the microgrid; Z ηVN is the virtual impedance of the inverter; k is the impedance adjustment coefficient; η N It is the inverter output reactive power matching factor.

[0041] 3. Calculate the reactive power Q output by the Nth inverter N and the instantaneous weighted average reactive power Q avg If the difference is not equal to 0, the virtual impedance value is iterated;

[0042] (1) If the Nth inverter outputs reactive power Q N and the instantaneous weighted average reactive power Q avg If the difference is greater than 0, then the initial virtual impedance Z vN Based on the reactive power regulation virtual impedance Z ηVN Then re-execute steps (1) and (2);

[0043] (2) If the Nth inverter outputs reactive power Q N and the instantaneous weighted average reactive power Q avg The difference is less than 0, then the initial virtual impedance Z vNBased on the reactive regulation virtual impedance Z minus ηVN Then re-execute steps (1) and (2).

[0044] 4. If the difference calculated in step 3 is equal to 0, it means that the reactive power distribution adjustment in the microgrid system has been completed, and the conditions for connecting new distributed power sources are met.

[0045] Specific application examples:

[0046] In order to verify the reactive power sharing control effect of the present invention, a three-inverter network simulation model is established in the PLECS software. The model structure in the microgrid is as follows: Figure 2 shown.

[0047] If the reactive power is not distributed, the output reactive power of the three inverters will be as follows Figure 3 As shown in the figure. First, one inverter runs alone with load, at 1.6s, the second inverter is connected to the microgrid, and at 2.5s, the third inverter is connected to the microgrid. It can be seen that the reactive power distribution effect in the system is poor during the inverter connection and load switching projects.

[0048] The reactive power distribution effect of three inverters using the control technology of the present invention is as follows: Figure 4 As shown in the figure, at 1.6s, the second inverter is connected to the island microgrid system, and at 2.1s, the reactive power distribution control scheme is put into use to realize the power distribution between the two machines; at 3.1s, the third inverter is connected to the microgrid system, and at 3.6s, the reactive power distribution control technology is put into use to realize the equal distribution of reactive power output by the three inverters.

[0049] In summary, the present invention obtains the reactive weighted average value by collecting the reactive power output by the inverter in the microgrid. A reactive matching coefficient η for measuring the reactive sharing state is proposed to evaluate the reactive matching degree of the inverter in the microgrid. The reactive power matching coefficient of the inverter in the microgrid is introduced into the virtual power regulation control, and the reactive distribution state is adjusted by adjusting the virtual size. When the network operation enters a stable state, the reactive power distribution of the inverter in the microgrid will no longer rely on communication and the central controller, thereby improving the stability and reliability of the system.

Claims

1. A reactive power distribution method for a voltage source inverter under weak communication conditions, characterized in that: The following steps are involved: (1) For parallel access to the grid N Voltage source inverters, calculate the real-time output reactive power of all inverters Q i and instantaneous weighted average reactive power Q avg ; (2) Calculate the N Output reactive power matching coefficient of each inverter η N and virtual impedance for reactive power regulation in droop control Z ηVN ; The output reactive power matching coefficient η N The calculation method is as follows: In the formula, n N is the reactive power droop coefficient; Q avg is the instantaneous weighted average reactive power; N is the number of inverters in the microgrid system, n i For the i The inverter droop coefficient is i =1, 2, ... N; Q i For the i The real-time reactive power output of each inverter, Q N For the N Real-time output reactive power of each inverter; The calculation method of the virtual impedance used for reactive power regulation in droop control is as follows: In the formula, k is the impedance adjustment coefficient; η N is the output reactive power matching coefficient of the inverter; (3) Calculate the N Real-time reactive power output of each inverter Q N The instantaneous weighted average reactive power Q avg If the difference is not equal to 0, the virtual impedance value is iterated; The iterative processing of the virtual impedance value specifically refers to: (3.1) If N Real-time reactive power output of each inverter Q N The instantaneous weighted average reactive power Q avg If the difference is greater than 0, then the initial virtual impedance Z vN Add virtual impedance for reactive power regulation in droop control Z ηVN Then re-execute steps (1) and (2); (3.2) If N Real-time reactive power output of each inverter Q N The instantaneous weighted average reactive power Q avg The difference is less than 0, then the initial virtual impedance Z vN The virtual impedance used for reactive power regulation in droop control is subtracted from Z ηVN Then re-execute steps (1) and (2); (4) If the difference calculated in step (3) is equal to 0, it means that the reactive power distribution adjustment in the microgrid system has been completed, and the conditions for connecting new distributed power sources are met.

2. The method according to claim 1, characterized in that The instantaneous weighted average reactive power output by the inverter Q avg The calculation method is as follows: In the formula, Q avg is the instantaneous weighted average reactive power of all inverters; N is the number of inverters in the microgrid system; n i For the i The inverter droop coefficient is i =1, 2, ... N; Q i For the i The real-time reactive power output of each inverter, Q N For the N The real-time output reactive power of each inverter.

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