Grid-connected voltage control method for photovoltaic inverter

Abstract

The invention relates to the technical field of grid-connected control, and discloses a photovoltaic inverter grid-connected voltage control method, which comprises the following steps: if the voltage of a grid-connected point is out of limit, entering a control stage; and if no grid-connected point voltage exceeds the limit, entering a recovery stage. Through combination of local reactive compensation, distributed reactive compensation, local active reduction, local reactive compensation coordination control, distributed active reduction and distributed reactive compensation coordination control, rapid regulation and control of photovoltaic grid-connected voltage out-of-limit nodes are realized, the regulation and control action adopts an exponential function form, compared with fixed-step regulation and control, the regulation and control are more accurate, and the regulation and control efficiency is improved. In addition, distributed reactive power recovery and distributed active power recovery can be carried out in sequence, group dispatching and group control of the transformer area are supported, accurate transformer area topology, impedance and other parameters and enough historical data are not needed, popularization and reuse are facilitated, and the method is quite suitable for incremental transformer area construction.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected control, and in particular to a method for controlling grid-connected voltage of a photovoltaic inverter. Background Art

[0002] As the penetration rate of distributed photovoltaics in low-voltage distribution networks continues to increase, the problem of voltage over-limit caused by random fluctuations in photovoltaic power generation has become increasingly prominent, seriously endangering the safe and stable operation of the distribution network. In order to solve the problem of voltage over-limit caused by distributed photovoltaic grid connection, methods such as droop control, optimal power flow, and data-driven have gradually attracted the attention of researchers. Among them, droop control can achieve local reactive power regulation, but does not support group regulation and control of the substation; optimal power flow control can accurately regulate voltage over-limit, and compared with other methods, it requires more accurate parameters such as substation topology and impedance; in the context of the rapid development of artificial intelligence, data-driven methods can be used to solve the problem of voltage over-limit, provided that there is sufficient historical data support. Therefore, due to factors such as incomplete substation data and distributed photovoltaic grid connection, the above methods are not easy to be applied on-site in actual projects. Summary of the invention

[0003] In view of the shortcomings and defects of the prior art, the present invention provides a photovoltaic inverter grid-connected voltage control method, which realizes rapid regulation of photovoltaic grid-connected voltage over-limit nodes by combining local reactive power compensation, distributed reactive power compensation, local active power reduction, local reactive power compensation coordinated control, distributed active power reduction, and distributed reactive power compensation coordinated control. The regulation action adopts an exponential function form, which is more accurate than fixed-step regulation. In addition, distributed reactive power recovery and distributed active power recovery can be performed in sequence, and group regulation and control of substations are supported, without considering precise substation topology, impedance and other parameters and sufficient historical data.

[0004] The purpose of the present invention can be achieved by the following technical solutions:

[0005] A photovoltaic inverter grid-connected voltage control method, if the grid-connected point voltage exceeds the limit, then enter the control stage; if the grid-connected point voltage does not exceed the limit, then enter the recovery stage.

[0006] The steps of the control stage are as follows: S21, continue local reactive power compensation; S22, if the grid connection point voltage is still over-limit when the local reactive power compensation capacity is exhausted, enter S23; if there is no grid connection point voltage over-limit, enter the recovery stage; S23, continue distributed reactive power compensation; S24, if the grid connection point voltage is still over-limit when the reactive power compensation capacity of the photovoltaic power station of the entire line of the substation is exhausted, enter S25; if there is no grid connection point voltage over-limit, enter the recovery stage; S25, continue local active power reduction and local reactive power compensation coordinated control; S26, if the grid connection point voltage is still over-limit when the local active power compensation capacity is exhausted, enter S27; if there is no grid connection point voltage over-limit, enter the recovery stage; S27, continue distributed active power reduction and distributed reactive power compensation coordinated control; S28, if the grid connection point voltage is still over-limit when the active power compensation capacity of the photovoltaic power station of the entire line of the substation is exhausted, issue a grid connection voltage control failure alarm; if there is no grid connection point voltage over-limit, enter the recovery stage.

[0007] Distributed reactive power recovery is performed in the recovery phase and distributed active power recovery is performed after the distributed reactive power recovery is completed; the termination of the recovery phase is marked by the photovoltaic output coefficient reaching a threshold.

[0008] Preferably, when performing local reactive power compensation and distributed reactive power compensation, the photovoltaic active power output is maintained at a maximum, wherein the reactive power compensation capacity limit is .

[0009] In the formula, Indicates the capacity of the photovoltaic inverter, is the rated power of the PV inverter.

[0010] Local reactive power compensation means to provide reactive power compensation to the grid-connected point where the voltage exceeds the limit the most seriously, and the reactive power compensation comes from the photovoltaic inverter at the current grid-connected point.

[0011] .

[0012] Where t is the current round of local reactive power compensation, t0 is the initial round of local reactive power compensation, Indicates the reactive power before local reactive power compensation, It represents the reactive power after the local reactive power compensation during the t-t0 round, and α is the reactive power control factor.

[0013] Preferably, the distributed reactive power compensation refers to performing reactive power compensation through photovoltaic inverters outside the grid connection point where the voltage over-limit is most serious.

[0014] .

[0015] In the formula, represents the reactive power before compensation at the grid connection point i, It represents the reactive power of grid-connected point i after round t-t0 compensation, and α is the reactive power regulation factor.

[0016] Preferably, when performing local active power reduction, local reactive power compensation coordinated control, distributed active power reduction, and distributed reactive power compensation coordinated control, reactive power compensation is coordinated and controlled while active power is reduced.

[0017] Among them, local active power reduction and local reactive power compensation coordinated control means to carry out active power reduction and reactive power compensation coordinated control at the grid connection points where the voltage over-limit is most serious.

[0018] .

[0019] .

[0020] In the formula, Indicates the active power before local active power reduction. , They represent the active power after the local active power reduction in rounds t-t0 and the reactive power after the coordinated control of local reactive power compensation, respectively. is the power factor limit, and β is the active power regulation factor.

[0021] Preferably, the distributed active power reduction and distributed reactive power compensation coordinated control refers to performing active power reduction and reactive power compensation coordinated control through photovoltaic inverters other than the grid connection point where the voltage over-limit is most serious.

[0022] .

[0023] .

[0024] In the formula, represents the active power before the grid connection point i is cut, , They represent the active power reduced by the grid connection point i after rounds t-t0 and the reactive power after compensation and coordinated control, is the power factor limit, and β is the active power regulation factor.

[0025] Preferably, the distributed reactive power recovery refers to performing reactive power recovery on all grid connection points, and the process is as follows.

[0026] .

[0027] In the formula, It represents the reactive power before the grid connection point i is restored, It represents the reactive power after the grid connection point i is restored, is the reactive power control step length, is the reactive power compensation capacity limit of grid connection point i.

[0028] Preferably, the distributed active power recovery in step 6 refers to active power recovery of all grid connection points, and the process is as follows.

[0029] .

[0030] In the formula, It represents the active power before the grid connection point i is restored. It represents the active power after the grid connection point i is restored, is the active power control step length, P Limit,i is the rated power of the photovoltaic inverter at the grid connection point i, is the active capacity limit of the PV inverter at the grid connection point i.

[0031] The beneficial effects of the present invention are as follows: by combining local reactive power compensation, distributed reactive power compensation, local active power reduction, local reactive power compensation coordinated control, distributed active power reduction, and distributed reactive power compensation coordinated control, rapid regulation of photovoltaic grid-connected voltage over-limit nodes is achieved, and the regulation action adopts an exponential function form, which is more accurate than fixed-step regulation. In addition, distributed reactive power recovery and distributed active power recovery can be performed in sequence, and group regulation and control of substations are supported. There is no need for precise substation topology, impedance and other parameters and sufficient historical data, which is convenient for promotion and reuse, and is very suitable for incremental substation construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the overall flow chart of the present invention.

[0033] Figure 2 This is a simulation topology diagram of a low-voltage power distribution system in an embodiment of the present invention.

[0034] Figure 3 Graph showing the relationship among inverter capacity, active power, reactive power and power factor in an embodiment of the present invention.

[0035] Figure 4 Schematic diagram of voltages of various nodes under different voltage regulation strategies in an embodiment of the present invention.

[0036] Figure 5 Schematic diagram of voltages of nodes under different recovery strategies in an embodiment of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention 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 invention and do not limit the present invention.

[0038] Example: Figures 1 to 5As shown, a photovoltaic inverter grid-connected voltage control method is characterized in that if the grid-connected point voltage exceeds the limit, a control stage is entered; if the grid-connected point voltage does not exceed the limit, a recovery stage is entered; the steps of the control stage are as follows.

[0039] S21, continue local reactive power compensation; S22, if the grid connection point voltage still exceeds the limit when the local reactive power compensation capacity is exhausted, enter S23; if there is no grid connection point voltage exceeding the limit, enter the recovery stage.

[0040] Among them, the voltage upper limit is 242V. When performing local reactive power compensation and distributed reactive power compensation, the photovoltaic active output is maintained at the maximum. The control process corresponds to Figure 3 The ab section, where the reactive power compensation capacity limit ; In the formula, Indicates the capacity of the photovoltaic inverter, is the rated power of the PV inverter.

[0041] Local reactive power compensation means to provide reactive power compensation to the grid-connected point where the voltage exceeds the limit the most seriously. The reactive power compensation comes from the photovoltaic inverter at the current grid-connected point.

[0042] .

[0043] Where t is the current round of local reactive power compensation, t0 is the initial round of local reactive power compensation, Indicates the reactive power before local reactive power compensation, It represents the reactive power after the local reactive power compensation during the t-t0 round, and α is the reactive power control factor. According to the above formula, the more serious the voltage deviation is, the greater the reactive power compensation will be.

[0044] S23, continue to perform distributed reactive power compensation; S24, if the grid connection point voltage still exceeds the limit when the reactive power compensation capacity of the photovoltaic power station in the entire line of the substation is exhausted, enter S25; if there is no grid connection point voltage exceeding the limit, enter the recovery stage.

[0045] Distributed reactive power compensation refers to reactive power compensation performed through photovoltaic inverters outside the grid connection point where the voltage over-limit is most serious.

[0046] .

[0047] In the formula, represents the reactive power before compensation at the grid connection point i, It represents the reactive power of grid-connected point i after round t-t0 compensation, and α is the reactive power regulation factor.

[0048] S25, continue to coordinate and control local active power reduction and local reactive power compensation; S26, if the grid connection point voltage is still over-limit when the local active power compensation capacity is exhausted, enter S27; if there is no grid connection point voltage over-limit, enter the recovery stage.

[0049] Under the common constraints of the photovoltaic inverter capacity and power factor, the reactive power is regulated while the active power is reduced. The above control process corresponds to Figure 3 The bc segment.

[0050] Among them, local active power reduction and local reactive power compensation coordinated control means to carry out active power reduction and reactive power compensation coordinated control at the grid connection points where the voltage over-limit is most serious.

[0051] .

[0052] .

[0053] In the formula, Indicates the active power before local active power reduction. , They represent the active power after the local active power reduction in rounds t-t0 and the reactive power after the coordinated control of local reactive power compensation, respectively. is the power factor limit, and β is the active power regulation factor.

[0054] S27, continue to coordinate and control distributed active power reduction and distributed reactive power compensation; S28, if the active power compensation capacity of the photovoltaic power station in the entire line of the substation is exhausted and the grid-connected point voltage is still over-limit, a grid-connected voltage control failure alarm will be issued; if there is no grid-connected point voltage over-limit, the recovery stage will be entered.

[0055] Distributed active power reduction and distributed reactive power compensation coordinated control refers to the coordinated control of active power reduction and reactive power compensation through photovoltaic inverters outside the grid connection point where the voltage exceeds the limit the most seriously. The control process corresponds to Figure 3 co segment.

[0056] .

[0057] .

[0058] In the formula, represents the active power before the grid connection point i is cut, , They represent the active power reduced by the grid connection point i after rounds t-t0 and the reactive power after compensation and coordinated control, is the power factor limit, and β is the active power regulation factor.

[0059] The present invention further illustrates a photovoltaic inverter grid-connected voltage control method proposed in the embodiment. A simulation model is built by MATLAB software. The simulation topology of the low-voltage power distribution system is as follows: Figure 2 shown. Figure 4 The voltage diagram of each node under different voltage regulation strategies is shown, where the dotted line represents the upper voltage limit, which is +10% (242V) of the normal voltage. Figure 4 It can be seen that the reactive power compensation method can play a certain role in limiting the node voltage over-limit situation, but under the limitation of inverter capacity and power factor, the reactive power regulation capability is often not enough to solve the node voltage over-limit problem. Therefore, on the basis of reactive power compensation, appropriate reduction of active power can greatly alleviate the high voltage problem.

[0060] Distributed reactive power recovery is performed in the recovery phase and distributed active power recovery is performed after the distributed reactive power recovery is completed; the termination of the recovery phase is marked by the photovoltaic output coefficient reaching a threshold.

[0061] The voltage recovery range is 205V~235V. Distributed reactive power recovery refers to the reactive power recovery of all grid-connected points. The process is as follows.

[0062] .

[0063] In the formula, It represents the reactive power before the grid connection point i is restored, It represents the reactive power after the grid connection point i is restored, is the reactive power control step length, is the reactive power compensation capacity limit of grid connection point i.

[0064] Distributed active power restoration refers to the restoration of active power at all grid-connected points. The process is as follows.

[0065] .

[0066] In the formula, It represents the active power before the grid connection point i is restored. It represents the active power after the grid connection point i is restored, is the active power control step length, P Limit,i is the rated power of the photovoltaic inverter at the grid connection point i, is the active capacity limit of the PV inverter at the grid connection point i.

[0067] Figure 5The voltage diagram of each node under different recovery strategies is shown, where the voltage upper limit is 242 V and the recovery range is 205 V-235 V. By switching on and off the load at a certain moment on the transformer side of the simulation model, the voltage of each node meets the recovery conditions, and the inverter power factor and photovoltaic output are improved through reactive power recovery and reactive and active power combined recovery.

[0068] The above embodiments are descriptions of specific implementation methods of the present invention rather than limitations of the present invention. Technical personnel in the relevant technical field may make various changes and modifications to obtain corresponding equivalent technical solutions without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. A photovoltaic inverter grid-connected voltage control method, characterized in that: If the grid-connected point voltage exceeds the limit, the control phase will be entered; if the grid-connected point voltage does not exceed the limit, the recovery phase will be entered; The steps of the control stage are as follows: S21, continue to perform local reactive power compensation; S22, if the grid connection point voltage is still over-limit when the local reactive power compensation capacity is exhausted, enter S23; if there is no grid connection point voltage over-limit, enter the recovery stage; S23, continue to perform distributed reactive power compensation; S24, if the grid connection point voltage is still over-limit when the reactive power compensation capacity of the photovoltaic power station of the entire line of the substation is exhausted, enter S25; if there is no grid connection point voltage over-limit, enter the recovery stage; S25, continue to perform local active power reduction and local reactive power compensation coordinated control; S26, if the grid connection point voltage is still over-limit when the local active power compensation capacity is exhausted, enter S27; if there is no grid connection point voltage over-limit, enter the recovery stage; S27, continue to perform distributed active power reduction and distributed reactive power compensation coordinated control; S28, if the grid connection point voltage is still over-limit when the active power compensation capacity of the photovoltaic power station of the entire line of the substation is exhausted, a grid connection voltage control failure alarm is issued; if there is no grid connection point voltage over-limit, enter the recovery stage; Distributed reactive power recovery is performed in the recovery phase and distributed active power recovery is performed after the distributed reactive power recovery is completed; the termination of the recovery phase is marked by the photovoltaic output coefficient reaching a threshold.

2. A photovoltaic inverter grid-connected voltage control method according to claim 1, characterized in that: When performing local reactive power compensation and distributed reactive power compensation, the photovoltaic active power output is maintained at the maximum, and the reactive power compensation capacity limit is ; In the formula, Indicates the capacity of the photovoltaic inverter, is the rated power of the PV inverter; Local reactive power compensation means to compensate the grid connection point with the most serious voltage over-limit, and the reactive power compensation comes from the photovoltaic inverter at the current grid connection point; ； Where t is the current round of local reactive power compensation, t0 is the initial round of local reactive power compensation, Indicates the reactive power before local reactive power compensation, It represents the reactive power after the local reactive power compensation during the t-t0 round, and α is the reactive power control factor.

3. A photovoltaic inverter grid-connected voltage control method according to claim 2, characterized in that: The distributed reactive power compensation refers to reactive power compensation performed by photovoltaic inverters outside the grid connection point where the voltage over-limit is most serious; ； In the formula, represents the reactive power before compensation at the grid connection point i, It represents the reactive power of grid-connected point i after round t-t0 compensation, and α is the reactive power regulation factor.

4. A photovoltaic inverter grid-connected voltage control method according to claim 2, characterized in that: When performing local active power reduction, local reactive power compensation coordinated control, distributed active power reduction, and distributed reactive power compensation coordinated control, reactive power compensation is coordinated and controlled while active power is reduced; Among them, local active power reduction and local reactive power compensation coordinated control means to carry out active power reduction and reactive power compensation coordinated control at the grid connection point with the most serious voltage over-limit; ； ； In the formula, Indicates the active power before local active power reduction. , They represent the active power after the local active power reduction in rounds t-t0 and the reactive power after the coordinated control of local reactive power compensation, respectively. is the power factor limit, and β is the active power regulation factor.

5. A photovoltaic inverter grid-connected voltage control method according to claim 2, characterized in that: The distributed active power reduction and distributed reactive power compensation coordinated control refers to the coordinated control of active power reduction and reactive power compensation through photovoltaic inverters outside the grid connection point where the voltage over-limit is most serious; ； ； In the formula, represents the active power before the grid connection point i is reduced, , They represent the active power reduced by the grid connection point i after rounds t-t0 and the reactive power after compensation and coordinated control, is the power factor limit, and β is the active power regulation factor.

6. A photovoltaic inverter grid-connected voltage control method according to claim 1, characterized in that: The distributed reactive power recovery refers to the reactive power recovery of all grid connection points. The process is as follows: ； In the formula, represents the reactive power before the grid connection point i is restored, It represents the reactive power after the grid connection point i is restored, is the reactive power control step length, is the reactive power compensation capacity limit of grid connection point i.

7. A photovoltaic inverter grid-connected voltage control method according to claim 1, characterized in that: The distributed active power recovery in step 6 refers to the active power recovery of all grid connection points, and the process is as follows: ； In the formula, It represents the active power before the grid connection point i is restored. It represents the active power after the grid connection point i is restored, is the active power control step length, P Limit,i is the rated power of the photovoltaic inverter at the grid connection point i, is the active capacity limit of the PV inverter at the grid connection point i.

Images