Voltage control system and voltage control method

Abstract

To suppress voltage fluctuations at specific locations on the power circuit. [Solution] The power generation system 10 is connected to a power company grid 2 at a connection point PT1 where the inductance component x is greater than the resistance component r, and is a voltage control system that adjusts the voltage at a predetermined location on the circuit by controlling the reactive power that flows into the power company grid 2, and comprises a constant power factor control unit 125 that calculates reactive power based on the active power that flows into the power company grid 2, and a droop control unit 124 that calculates reactive power based on the voltage of the power company grid 2, and flows reactive power based on the sum of the reactive power calculated by the constant power factor control unit 125 and the reactive power calculated by the droop control unit 124 into the power company grid 2.

Description

Technical Field

[0001] The present invention relates to a voltage control system and a voltage control method.

Background Art

[0002] A system connection system that connects a DC power source such as wind power generation and solar power generation to a power system is known. This type of system connection system has, for example, a power conversion device that converts DC power supplied from a DC power source into AC power. For example, Patent Document 1 discloses a power conversion device having an inverter that converts DC power generated from renewable energy such as sunlight into AC power.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in a system connection system that connects a DC power source using renewable energy (for example, wind power generation, etc.) to a power system, it is often necessary to lay a long-distance transmission line. When the length of the transmission line is long, the impedance of the transmission line becomes high, so the voltage on the transmission line easily fluctuates due to changes in the amount of power output from the DC power source. Therefore, it is required to suppress voltage fluctuations at a predetermined location on the electric circuit. In particular, voltage fluctuations at the connection point of the DC power source to the power system are required to be within the range defined by the system connection regulations. In consideration of the above circumstances, one aspect of the present invention aims to suppress voltage fluctuations at a predetermined location on the electric circuit.

Means for Solving the Problems

[0005] A voltage control system according to a preferred embodiment of the present invention is connected at an interconnection point to a power system in which the inductance component is greater than the resistance component, and adjusts the voltage at a predetermined location on the circuit by controlling the reactive power flowing through the power system, comprising: a first control unit that calculates reactive power based on the active power flowing through the power system; and a second control unit that calculates reactive power based on the voltage of the power system, wherein the system flows reactive power to the power system based on the sum of the reactive power calculated by the first control unit and the reactive power calculated by the second control unit.

[0006] Furthermore, a voltage control method according to a preferred embodiment of the present invention is a voltage control method that is connected at an interconnection point to a power system where the inductance component is greater than the resistance component, and adjusts the voltage at a predetermined location on the circuit by controlling the reactive power flowing through the power system, wherein the reactive power is calculated based on the active power flowing through the power system, the reactive power is calculated based on the voltage of the power system, and the reactive power calculated based on the sum of the reactive power calculated based on the active power flowing through the power system and the reactive power calculated based on the voltage of the power system is flowed through the power system. [Brief explanation of the drawing]

[0007] [Figure 1] This is an explanatory diagram illustrating an example of a grid-connected system including a power generation system according to an embodiment. [Figure 2] Figure 1 is an explanatory diagram illustrating an example of the configuration of each PCS. [Figure 3] This is an explanatory diagram illustrating an example of the operation of the droop control unit 124. [Figure 4] This is an explanatory diagram illustrating an example of the operation of a constant power factor control unit. [Figure 5] This is an explanatory diagram illustrating the general characteristics of voltage fluctuations in power transmission lines. [Figure 6] This is an explanatory diagram to illustrate the problems with constant power factor control. [Figure 7] This is an explanatory diagram illustrating the operation of each PCS (Power Conditioning System) when the power generation output of the power generation system is at a moderate level. [Figure 8]This is an explanatory diagram illustrating the operation of each PCS (Power Conditioning System) when the power generation output of the power generation system is high. [Figure 9] This is an explanatory diagram illustrating the effects obtained by controlling the reactive power supplied to the power company grid using both droop control and constant power factor control. [Figure 10] This is an explanatory diagram illustrating an example of a grid connection system related to the first modified example. [Modes for carrying out the invention]

[0008] Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. In each drawing, the dimensions and scale of each part have been appropriately changed from those of the actual parts. Furthermore, the embodiments described below are preferred specific examples of the present invention and are subject to various technically preferred limitations, but the scope of the present invention is not limited to these embodiments unless otherwise stated in the following description.

[0009] A. Embodiment Embodiments of the present invention will be described below. First, an example of the general outline of a grid-connected system 1 including a power generation system 10 according to an embodiment will be described with reference to Figure 1. In this embodiment, the voltage control system will be described using the power generation system 10 as an example.

[0010] Figure 1 is an explanatory diagram illustrating an example of a grid connection system 1 including a power generation system 10 according to an embodiment.

[0011] The grid interconnection system 1 comprises a power company system 2, which is a power system in which the inductance component x is greater than the resistance component r; a power generation system 10; a transformer 30; and a transmission line PL1. Note that in Figure 1, the transmission towers supporting the transmission line PL1, etc., have been omitted for clarity.

[0012] The power company system 2 includes, for example, a system power source 20 approximated by an infinite busbar, and a transmission line PL2 which is a circuit from the system power source 20 to the interconnection point PT1. The transmission line PL2 is, for example, a circuit in which the inductance component x is greater than the resistance component r. The interconnection point PT1 is the point where the responsibilities of the power company and the operator of the power generation system 10 diverge. For example, the power generation system 10 is connected to the interconnection point PT1 via a transformer 30. In this way, the grid interconnection system 1 connects the power generation system 10 (more specifically, each of the multiple DC power sources 100 described later) to the system power source 20 included in the power company system 2 via the interconnection point PT1. In this embodiment, it is assumed that the system including transmission lines PL1 and PL2 has an inductance component x greater than the resistance component r. The transmission line PL1 is, for example, a privately owned line. The power company system 2 is an example of a "power system".

[0013] The power generation system 10 comprises a plurality of DC power sources 100 and a plurality of power conditioner systems (hereinafter also referred to as PCS) 120, each corresponding to one of the DC power sources 100. In Figure 1, to distinguish the plurality of DC power sources 100 from one another, a lowercase letter (a, b, c, d, or e) is appended to the end of the code of each DC power source 100. Similarly, each PCS 120 has the same lowercase letter appended to the end of its code as the lowercase letter appended to the code of the corresponding DC power source 100. PCS 120 is an example of a "power converter." PCS 120a is an example of a "second power converter," and PCS 120b, 120c, 120d, and 120e are examples of "first power converters." The power generation system 10 is also an example of a "voltage control system." Furthermore, a group of multiple PCS120 components within the power generation system 10 may be considered as a "voltage control system."

[0014] In this embodiment, it is assumed that the plurality of DC power sources 100 are wind power generation devices. Note that the DC power source 100 is not limited to a wind power generation device. For example, the DC power source 12 may be a solar cell or a fuel cell. Alternatively, the DC power source 12 may be a storage battery. Also, some or all of the plurality of DC power sources 100 may be of different types (for example, a wind power generation device and a solar cell, etc.). Also, the number of DC power sources 100 is not limited to the example shown in FIG. 1. For example, the number of DC power sources 100 may be one or more and four or less, or six or more.

[0015] Each DC power source 100 is connected to the transmission line PL1 via a PCS talk about 120 corresponding to the DC power source 100. For example, the DC power source 100a is connected to the node Na of the transmission line PL1 via the PCS120a. Also, the DC power source 100b is connected to the node Nb of the transmission line PL1 via the PCS120b, and the DC power source 100c is connected to the node Nc of the transmission line PL1 via the PCS120c. The DC power source 100d is connected to the node Nd of the transmission line PL1 via the PCS120d, and the DC power source 100e is connected to the node Ne of the transmission line PL1 via the PCS120e.

[0016] For example, the node Na of the transmission line PL1 is connected to the connection terminal CTa of the PCS120a, the node Nb of the transmission line PL1 is connected to the connection terminal CTb of the PCS120b, and the node Nc of the transmission line PL1 is connected to the connection terminal CTc of the PCS120c. Also, the node Nd of the transmission line PL1 is connected to the connection terminal CTd of the PCS120d, and the node Ne of the transmission line PL1 is connected to the connection terminal CTe of the PCS talk about 120e.

[0017] [[ID=I1]] It seems there is a minor error in the original text where "PCS talk about 120" and "PCS talk about 120e" are mentioned. I've translated it as best as possible with the given text. If this was a typo and should be something else, please correct the original text for a more accurate translation.The PCS120 is an inverter that converts DC power into AC power. For example, each PCS120 converts DC power into AC power by switching on and off a plurality of switching elements included in an inverter circuit (not shown) that each PCS120 has. Each PCS120 converts the DC power output from the DC power source 100 corresponding to that PCS120 into AC power, and outputs the converted AC power to the transmission line PL1. Specifically, for example, the PCS120a converts the DC power output from the DC power source 100a into AC power, and outputs the converted AC power to the node Na of the transmission line PL1.

[0018] The power output to the transmission line PL1 is supplied to the grid power source 20 (infinite bus) via, for example, the connection point PT1 and the transmission line PL2. That is, the power output from the power generation system 10 is supplied to the grid power source 20 via the transmission line PL1, the connection point PT1, and the transmission line PL2. Hereinafter, the amount of power output from the power generation system 10 may be referred to as the power generation amount. In a system where the inductance component x is larger than the resistance component r, as the power generation amount of the power generation system 10 increases, the voltages of the transmission lines PL1 and PL2 tend to decrease. In the present embodiment, by outputting reactive power from the power generation system 10, a decrease in the voltages of the transmission lines PL1 and PL2 is suppressed. For example, in the present embodiment, the voltage at a predetermined location on the circuit (for example, the voltage at the connection point PT1) is adjusted by controlling the reactive power flowing into the power company system 2. In the present embodiment, unless otherwise specified, capacitive reactive power is taken as positive. Hereinafter, the operation of the PCS120 and the like will be described mainly with respect to the control of reactive power.

[0019] Further details are explained in Figure 2, but in this embodiment, the method of controlling reactive power differs between PCS120a and the other PCS120s (120b, 120c, 120d, and 120e). For example, PCS120a outputs reactive power by droop control, while PCS120b, 120c, 120d, and 120e each output reactive power by constant power factor control. In droop control, reactive power is calculated based on the droop characteristics that show the relationship between the voltage of the power company system 2 and the reactive power. In constant power factor control, reactive power is calculated based on the active power output to the transmission line PL1 so that the power factor remains constant at a predetermined value.

[0020] Reactive power flows through node N1 of transmission line PL1, based on the sum of reactive power calculated at multiple PCS120s.

[0021] In this embodiment, in each PCS120, a positive power value means the power output from the PCS120 to the transmission line PL1, and a negative power value means the power input from the transmission line PL1 to the PCS120. For example, in PCS120a, if the reactive power calculated based on the droop characteristics is negative, outputting the reactive power calculated based on the droop characteristics corresponds to the reactive power being input to PCS120a.

[0022] Note that the configuration of the grid interconnection system 1 is not limited to the example shown in Figure 1. For example, multiple power generation systems 10 may be connected to node N1 of transmission line PL1. In this case, each of the multiple power generation systems 10 connected to node N1 may control the reactive power supplied to the power company grid 2 using both droop control and constant power factor control, similar to the power generation system 10 shown in Figure 1. In addition, one or more transformers may be provided between node N1 of transmission line PL1 and interconnection point PT1, in addition to transformer 30. Furthermore, transformers may be provided between each PCS 120 and transmission line PL1. Also, in Figure 1, the depiction of various measuring instruments such as voltage measuring units for measuring the voltage of transmission line PL1 is omitted.

[0023] Next, we will describe an example of the configuration of each PCS120, referring to Figure 2.

[0024] Figure 2 is an explanatory diagram illustrating an example of the configuration of each PCS120 shown in Figure 1. In Figure 2, the elements of each PCS120 (for example, the processing unit 122 and the storage device 126, etc.) also have the same lowercase alphabet appended to the end of their code as the lowercase alphabet appended to the end of the code of the PCS120.

[0025] Each PCS120 has a processing unit 122 and a storage device 126 that stores various information such as a control program PG.

[0026] The processing unit 122 is a processor that controls the entire PCS120 and is configured to include, for example, one or more CPUs (Central Processing Units). For example, the processing unit 122 functions as an element that controls an inverter circuit (not shown) of the PCS120 by reading a control program PG from the storage device 126 and executing the read control program PG. In this embodiment, the processing unit 122 also functions as a first control unit or a second control unit that calculates reactive power by executing the control program PG read from the storage device 126. For example, the droop control unit 124a (hereinafter also referred to as the droop control unit 124), which calculates reactive power based on the voltage of the power company system 2, corresponds to the second control unit, and the constant power factor control unit 125, which calculates reactive power based on the active power flowing through the power company system 2, corresponds to the first control unit.

[0027] For example, the processing unit 122a of PCS120a functions as a droop control unit 124a by executing the control program PGa read from the storage device 126a. Similarly, the processing unit 122b of PCS120b functions as a constant power factor control unit 125b by executing the control program PGb read from the storage device 126b. The processing unit 122c of PCS120c functions as a constant power factor control unit 125c by executing the control program PGc read from the storage device 126c. The processing unit 122d of PCS120d functions as a constant power factor control unit 125d by executing the control program PGd read from the storage device 126d. The processing unit 122e of PCS120e functions as a constant power factor control unit 125e by executing the control program PGe read from the storage device 126e. The control program PG may be transmitted from another device via a network or the like. The droop control unit 124 will be explained in Figure 3 below, and the constant power factor control unit 125 will be explained in Figure 4 below.

[0028] Furthermore, for example, if the processing unit 122 is configured to include multiple CPUs, some or all of the functions of the processing unit 122 may be realized by these multiple CPUs cooperating and operating according to a program such as a control program PG. Also, the processing unit 122 may be configured to include hardware such as a GPU (Graphics Processing Unit), DSP (Digital Signal Processor), or FPGA (Field Programmable Gate Array) in addition to one or more CPUs, or in place of some or all of the one or more CPUs. In this case, some or all of the functions of the processing unit 122 may be realized by hardware such as a DSP.

[0029] The storage device 126 includes, for example, one or both of the following: volatile memory such as RAM (Random Access Memory) that functions as a work area for the processing unit 110, and non-volatile memory such as EEPROM (Electrically Erasable Programmable Read-Only Memory) that stores various information. The storage device 126 may be detachable from the PCS 120. Specifically, the storage device 126 may be a storage medium such as a memory card that is detachable from the PCS 120. Alternatively, the storage device 126 may be a storage device (e.g., online storage) that is connected to the PCS 120 via a network or the like for communication. Furthermore, if all the functions of the processing unit 122 are implemented by hardware, the storage device 126 may be omitted.

[0030] In this embodiment, focusing on reactive power, PCS120a outputs the reactive power calculated by the droop control unit 124a to node Na of transmission line PL1. PCS120b outputs the reactive power calculated by the constant power factor control unit 125b to node Nb of transmission line PL1, and PCS120c outputs the reactive power calculated by the constant power factor control unit 125c to node Nc of transmission line PL1. PCS120d outputs the reactive power calculated by the constant power factor control unit 125d to node Nd of transmission line PL1, and PCS120e outputs the reactive power calculated by the constant power factor control unit 125e to node Ne of transmission line PL1.

[0031] Note that the configuration of each PCS120 is not limited to the example shown in Figure 2. For example, some or all of the multiple PCS120s may have different configurations. Specifically, for example, PCS120b may have a power factor constant control unit 125b implemented by hardware such as an FPGA, and PCS120c may have a power factor constant control unit 125c implemented by a processing unit 122c that executes a control program PGc.

[0032] Furthermore, the number of PCS120 having a droop control unit 124 and the number of PCS120 having a constant power factor control unit 125 are not limited to the example shown in Figure 2. For example, of the five PCS120, each of two PCS120 may have a droop control unit 124, and each of the remaining three PCS120 may have a constant power factor control unit 125. However, it is preferable that the droop control unit 124 and the constant power factor control unit 125 are allocated such that the maximum total reactive power that can be supplied by multiple PCS120 having a constant power factor control unit 125 is greater than the maximum total reactive power that can be supplied by multiple PCS120 having a droop control unit 124. Therefore, it is preferable that the maximum reactive power that can be supplied to the power company system 2 based on the reactive power calculated by the constant power factor control unit 125 is greater than the maximum reactive power that can be supplied to the power company system 2 based on the reactive power calculated by the droop control unit 124. For example, if the upper limit of reactive power that can be output is the same for multiple PCS120, it is preferable that the number of PCS120 having a constant power factor control unit 125 is greater than the number of PCS120 having a droop control unit 124. In other words, it is preferable that the number of PCS120 that send the reactive power calculated by the constant power factor control unit 125 to the power company grid 2 is greater than the number of PCS120 that send the reactive power calculated by the droop control unit 124 to the power company grid 2.

[0033] Furthermore, it is preferable that PCS120a, which attempts to maintain a constant voltage through droop control, be positioned such that node Na, which is the connection node to the power transmission line PL1, is close to the interconnection point PT1. For example, it is preferable that PCS120a having a droop control unit 124a be positioned such that the distance between PCS120a and the interconnection point PT1 is shorter than the distance between PCS120 having a constant power factor control unit 125 and the interconnection point PT1.

[0034] Next, an example of the operation of the droop control unit 124 will be described with reference to Figure 3.

[0035] Figure 3 is an explanatory diagram illustrating an example of the operation of the droop control unit 124. Figure 3 shows the droop characteristics representing the relationship between reactive power Qv [pu] and voltage V [pu]. The horizontal axis of Figure 3 shows the voltage V [pu] at the connection terminal CTa of PCS120a, and the vertical axis shows the reactive power Qv [pu] to be output from PCS120a. Note that the unit [pu] for voltage V indicates a value relative to a predetermined base value (for example, a reference voltage V0). Similarly, the unit [pu] for reactive power Qv indicates a value relative to a predetermined base value.

[0036] In the example shown in Figure 3, if the voltage V at the connection terminal CTa of PCS120a is the reference voltage V0, the reactive power Qv that should be output from PCS120a is 0[pu]. Note that in Figure 3, if the reactive power Qv is negative, it means that the same amount of reactive power as Qv is input to PCS120a. Here, for example, the voltage of the circuit from PCS120a to node Na may be considered as the voltage V at the connection terminal CTa of PCS120a.

[0037] The droop control unit 124 calculates the reactive power Qv based, for example, the droop characteristics shown in Figure 3 and the voltage V at the connection terminal CTa of the PCS120a.

[0038] In this embodiment, as shown in Figure 3, upper and lower limits are set for the reactive power Qv. In the example shown in Figure 3, the upper limit of the reactive power Qv is set to upper limit reactive power Qu, and the lower limit of the reactive power Qv is set to lower limit reactive power Ql. Also, as shown by the dotted line in Figure 3, a dead zone DZ may be set for the droop characteristic voltage V (voltage V at the connection terminal CTa of PCS120a). In the example shown by the dotted line in Figure 3, the dead zone DZ is set to a range from a voltage V1 that is smaller than the reference voltage V0 to a voltage V2 that is larger than the reference voltage V0. First, we will explain the droop characteristic in which no dead zone DZ is set (the droop characteristic shown by the solid line in Figure 3).

[0039] The droop characteristic shown by the solid line in Figure 3 is a characteristic in which the reactive power Qv to be output from PCS120a is reduced in response to an increase in the voltage V at the connection terminal CTa of PCS120a. Therefore, the droop control unit 124a controls PCS120a so that the reactive power Qv to be output from PCS120a is reduced in response to an increase in the voltage V at the connection terminal CTa of PCS120a. However, in the example of the droop characteristic in which no dead zone DZ is set as shown in Figure 3, when the voltage V at the connection terminal CTa of PCS120a is less than or equal to voltage VL1, the reactive power Qv is maintained at the upper limit reactive power Qu. Also, when the voltage V at the connection terminal CTa of PCS120a is greater than or equal to voltage VH1, the reactive power Qv is maintained at the lower limit reactive power Ql.

[0040] In droop characteristics where no dead zone DZ is set, the reactive power Qv is expressed by equation (1) or equation (2) using the droop gain Gv, etc. For example, when the voltage V at the connection terminal CTa of PCS120a is less than or equal to the reference voltage V0, the reactive power Qv is expressed by equation (1) using the droop gain Gv, voltage V0, the voltage V at the connection terminal CTa of PCS120a, and the function min() which returns the minimum value in parentheses. Also, when the voltage V at the connection terminal CTa of PCS120a is greater than or equal to voltage V0, the reactive power Qv is expressed by equation (2) using the droop gain Gv, voltage V1, the voltage V at the connection terminal CTa of PCS120a, and the function max() which returns the maximum value in parentheses.

[0041] Qv = min(Gv·(V0-V),Qu) …(1) Qv = max(Gv·(V0-V),Ql) …(2)

[0042] The droop gain Gv is a proportionality constant that represents the ratio of the change in reactive power Qv to the change in voltage V at the connection terminal CTa of the PCS120a, and is also called the droop coefficient. The droop gain Gv corresponds to the slope of the droop characteristic. Also, voltage V0 is an example of a "reference voltage".

[0043] For example, the droop control unit 124a sets the reactive power Qv to be output from PCS120a if the result of multiplying the voltage V at the connection terminal CTa of PCS120a by the droop gain Gv is greater than or equal to the lower limit reactive power Ql and less than or equal to the upper limit reactive power Qu. Furthermore, the droop control unit 124a sets the upper limit reactive power Qu as the reactive power Qv to be output from PCS120a if the result of multiplying the voltage V at the connection terminal CTa of PCS120a by the droop gain Gv is greater than or equal to the upper limit reactive power Qu. For example, if the voltage V at the connection terminal CTa of PCS120a is voltage VL1, the reactive power Qv calculated by equation (1) becomes the upper limit reactive power Qu. Furthermore, the droop control unit 124a sets the lower limit reactive power Ql as the reactive power Qv to be output from PCS120a if the result of multiplying the voltage V at the connection terminal CTa of PCS120a by the droop gain Gv is less than or equal to the lower limit reactive power Ql. For example, if the voltage V at the connection terminal CTa of PCS120a is voltage VH1, the reactive power Qv calculated by equation (1) becomes the lower limit reactive power Ql. That is, if the voltage V at the connection terminal CTa of PCS120a is within the range from voltage VL1 to voltage VH1, the reactive power Qv is expressed as "Qv = Gv·(V0-V)".

[0044] Furthermore, in a droop characteristic with a dead zone DZ (shown by the dotted line in Figure 3), the reactive power Qv is expressed by equation (3) or equation (4) using the droop gain Gv, etc. For example, when the voltage V at the connection terminal CTa of PCS120a is less than or equal to the lower limit voltage V1 of the dead zone DZ, the reactive power Qv is expressed by equation (3) using the droop gain Gv, voltage V1, the voltage V at the connection terminal CTa of PCS120a, the upper limit reactive power Qu, and the function min(). Also, when the voltage V at the connection terminal CTa of PCS120a is greater than or equal to the upper limit voltage V2 of the dead zone DZ, the reactive power Qv is expressed by equation (4) using the droop gain Gv, voltage V1, the voltage V at the connection terminal CTa of PCS120a, the lower limit reactive power Ql, and the function max(). Note that when the voltage V at the connection terminal CTa of PCS120a is within the range of the dead zone DZ, the reactive power Qv is 0 [pu].

[0045] Qv = min(Gv·(V1-V),Qu) …(3) Qv = max(Gv·(V2-V),Ql) …(4)

[0046] In a droop characteristic with a dead zone DZ, if the voltage V at the connection terminal CTa of PCS120a is within the range of voltage VL2 to voltage VH2 and outside the dead zone DZ, the reactive power Qv decreases as the voltage V at the connection terminal CTa of PCS120a increases. Voltage VL2 is the voltage V at the connection terminal CTa of PCS120a where the reactive power Qv calculated by equation (3) is the upper limit reactive power Qu, and voltage VH2 is the voltage V at the connection terminal CTa of PCS120a where the reactive power Qv calculated by equation (4) is the lower limit reactive power Ql. That is, when the voltage V at the connection terminal CTa of PCS120a is voltage VL2, the upper limit reactive power Qu is obtained by multiplying the value obtained by subtracting the voltage V at the connection terminal CTa of PCS120a from voltage V1 by the droop gain Gv. Furthermore, if the voltage V at the connection terminal CTa of PCS120a is voltage VH2, the lower limit reactive power Ql is obtained by subtracting the voltage V at the connection terminal CTa of PCS120a from voltage V2 and multiplying the result by the droop gain Gv. Note that voltages V1 and V2 are other examples of "reference voltages".

[0047] In this way, the droop control unit 124a calculates the reactive power Qv to be output from PCS120a based on a predetermined droop characteristic representing the relationship between reactive power Qv and voltage V, and the voltage V at the connection terminal CTa of PCS120a. For example, the droop control unit 124a acquires voltage information indicating the voltage V at the connection terminal CTa of PCS120a, and calculates the reactive power Qv to be output from PCS120a based on the voltage V indicated by the acquired voltage information and the predetermined droop characteristic. Note that the voltage information indicating the voltage V at the connection terminal CTa of PCS120a may be acquired, for example, by measuring the voltage V at the connection terminal CTa of PCS120a, or by measuring the voltage of the circuit from PCS120a to node Na. Furthermore, since the voltage V at the connection terminal CTa of PCS120a is the voltage output to the power company system 2, calculating the reactive power Qv based on the voltage V at the connection terminal CTa of PCS120a is equivalent to calculating the reactive power Qv based on the voltage of the power company system 2. For example, the reactive power Qv calculated by the droop control unit 124a is a reactive power proportional to the difference between the voltage of the power company system 2 and the reference voltage (for example, voltage V0).

[0048] Furthermore, the droop characteristics used for droop control are not limited to those showing the relationship between the reactive power Qv to be output from PCS120a and the voltage V at the connection terminal CTa of PCS120a. For example, droop characteristics showing the relationship between the reactive power Qv to be output from PCS120a and the voltage near node N may be used for droop control.

[0049] Next, an example of the operation of the constant power factor control unit 125 will be described with reference to Figure 4.

[0050] Figure 4 is an explanatory diagram illustrating an example of the operation of the constant power factor control unit 125. In Figure 4, the operation of the constant power factor control unit 125 is explained using the constant power factor control unit 125b of PCS120b as an example. Figure 4 shows an example of the relationship between reactive power Qp [pu] and active power P [pu]. The horizontal axis of Figure 4 shows the active power P [pu] output from PCS120b, and the vertical axis shows the reactive power Qp [pu] that should be output from PCS120b. The unit [pu] for active power P indicates a value relative to a predetermined base value. Similarly, the unit [pu] for reactive power Qp indicates a value relative to a predetermined base value. In the example shown in Figure 4, if the active power P output from PCS120b is 0 [pu], the reactive power Qp that should be output from PCS120b is 0 [pu]. Hereafter, the active power P output from PCS120b may be referred to as the active power P of PCS120b.

[0051] The constant power factor control unit 125b calculates the reactive power Qp based, for example, on the relationship between the reactive power Qp[pu] and the active power P[pu] shown in Figure 4, and the active power P of the PCS120b.

[0052] As shown in Figure 4, the reactive power Qp to be output from PCS120b increases in accordance with the increase in the active power P of PCS120b. However, in this embodiment, as shown in Figure 4, an upper limit reactive power Qu is set for the reactive power Qp.

[0053] The reactive power Qp is expressed by equation (5) using the gain Gp, which is a proportionality constant representing the ratio of the change in reactive power Qp to the change in active power P of PCS120a, the active power P, the upper limit of reactive power Qu, and the function min().

[0054] Qp = min(Gp·P,Qu) …(5)

[0055] For example, the constant power factor control unit 125b sets the result of multiplying the active power P of PCS120b by the gain Gp (the multiplication result) as the reactive power Qp to be output from PCS120b if the result is less than or equal to the upper limit reactive power Qu. Furthermore, the constant power factor control unit 125b sets the upper limit reactive power Qu as the reactive power Qp to be output from PCS120b if the result of multiplying the active power P of PCS120b by the gain Gp is ​​greater than or equal to the upper limit reactive power Qu.

[0056] In this way, the constant power factor control unit 125b calculates the reactive power Qp to be output from PCS120b based on the gain Gp and the active power P of PCS120b. For example, the constant power factor control unit 125b acquires power information indicating the active power P of PCS120b, and calculates the reactive power Qp to be output from PCS120b based on the active power P indicated by the acquired power information and a predetermined gain Gp. The acquisition of power information indicating the active power P of PCS120b may be done by acquiring the measurement result of the active power output from PCS120b, or by acquiring the active power command value used when converting the DC power output from DC power supply 100b to AC power. Furthermore, since the active power P of PCS120b is the active power output to the power company grid 2, calculating the reactive power Qp based on the active power P of PCS120b is equivalent to calculating the reactive power Qp based on the active power flowing to the power company grid 2. For example, the reactive power Qp calculated by the constant power factor control unit 125b is proportional to the active power output to the power company grid 2 in the PCS 120b within the range from 0 [pu] to the upper limit reactive power Qu.

[0057] Next, referring to Figures 5 to 9, we will explain an example of the effects obtained by controlling the reactive power flowing to the power company grid 2 using both droop control and constant power factor control. First, referring to Figure 5, we will explain the overview of voltage fluctuations in transmission lines PL1 and PL2. Hereafter, transmission lines PL1 and PL2 may be collectively referred to as transmission line PL.

[0058] Figure 5 is an explanatory diagram illustrating the general overview of voltage fluctuations in a transmission line PL. Figure 5 shows an example of the relationship between the distance from the busbar (e.g., system power source 20) and the voltage of the transmission line PL at a corresponding location. The horizontal axis of Figure 5 represents the distance from the busbar, and the vertical axis represents the voltage of the transmission line PL. Furthermore, the graph shown in Figure 5 is plotted ignoring the impedance of the transformer 30 to facilitate the explanation of voltage fluctuations in the transmission line PL. In addition, as a comparative example, Figure 5 shows the voltage fluctuations of a circuit in a system where the resistance component r is greater than the inductance component x, indicated by a dotted line. An example of a system where the resistance component r is greater than the inductance component x is a distribution system.

[0059] As shown by the dotted line in Figure 5, in a power distribution system where the resistance component r is greater than the inductance component x, the voltage in the circuit at the position corresponding to the distance from the busbar increases as the distance increases. In other words, in a power distribution system where the resistance component r is greater than the inductance component x, the voltage increases as you approach the end of the system. In this embodiment, when the term "system" is used, unless otherwise specified, it refers to the system from the power grid 20 to the power generation system 10. Also, in this embodiment, when the term "end of the system" is used, unless otherwise specified, it refers to the part of the system from the power grid 20 to the power generation system 10 that is near the power generation system 10.

[0060] Furthermore, as shown by the solid line in Figure 5, in a power system where the inductance component x is greater than the resistance component r (for example, a system including transmission lines PL1 and PL2), as the distance from the busbar, for example, the distance from the power source 20, increases, the voltage of transmission line PL at the position corresponding to that distance decreases. In other words, in a power system where the inductance component x is greater than the resistance component r, the voltage decreases as you approach the end of the system, for example, the power generation system 10. Also, focusing on the amount of power generated, in a power system where the inductance component x is greater than the resistance component r, the voltage drop at the end of the system tends to increase as the amount of power generated increases.

[0061] In a power system where the inductance component x is greater than the resistance component r, reactive power is output from the power generation system 10 to the transmission line PL to suppress a voltage drop in the transmission line PL. The effect of the voltage increase due to the reactive power output from the power generation system 10 to the transmission line PL decreases as you get closer to the busbar, that is, as you get further away from the end of the system (for example, the power generation system 10). Constant power factor control is a known method for controlling the reactive power output from the power generation system 10 to the transmission line PL. However, if all PCS 120s of the power generation system 10 control the reactive power using constant power factor control, there is a problem, as explained in Figure 6.

[0062] Figure 6 is an explanatory diagram illustrating the problems of constant power factor control. Figure 6 shows an example of the relationship between the impedance of the circuit from the busbar (e.g., grid power source 20) and the voltage of the transmission line PL when reactive power is controlled using constant power factor control. The horizontal axis of Figure 6 represents the impedance of the circuit from the busbar, and the vertical axis represents the voltage of the transmission line PL. The impedance of the circuit from the busbar includes the impedance of the transformer 30. Furthermore, the impedance of the circuit from the busbar increases as it approaches the end of the grid. Here, "Medium Power Generation" in Figure 6 shows the relationship between the impedance of the circuit from the busbar and the voltage of the transmission line PL when the power generation of the power generation system 10 is moderate. "High Power Generation" in Figure 6 shows the relationship between the impedance of the circuit from the busbar and the voltage of the transmission line PL when the power generation of the power generation system 10 is high. The interconnection point PT1 shown in Figure 6 represents a point on the horizontal axis corresponding to the impedance of the circuit from the busbar to the interconnection point PT1. Note that the horizontal axis in Figure 6 corresponds to the distance from the busbar, assuming that the impedance of the transformer 30 is ignored.

[0063] If the power factor is made too small, excessive reactive power is output to the transmission line PL, causing the voltage near the end of the grid to rise excessively. As a result, as shown in "Medium Power Generation" in Figure 6, when the power generation of the power generation system 10 is moderate, the voltage near the end of the grid becomes too high. In other words, when the power generation of the power generation system 10 is moderate, the voltage near the end of the grid becomes higher than the upper limit of the specified range.

[0064] If the power factor is made too large, the effect of voltage increase due to reactive power may not be sufficient to counteract the voltage drop due to active power, depending on the distance from the busbar, i.e., the impedance of the circuit from the busbar. For example, as shown in "High Power Generation" in Figure 6, when the power generation of the power generation system 10 is large, the effect of voltage increase due to reactive power is small in the vicinity of the interconnection point PT1, and the voltage drop is not sufficiently suppressed. In other words, when the power generation of the power generation system 10 is large, the voltage in the middle of the system (for example, near the interconnection point PT1) falls below the lower limit of the specified range. Note that the middle of the system is, for example, the part of the system from the grid power source 20 to the power generation system 10 that is at a certain distance from both the grid power source 20 and the power generation system 10, and does not have to be exactly in the middle.

[0065] There is a trade-off between keeping the voltage near the end of the grid within a specified range when the power generation of the power generation system 10 is moderate, and keeping the voltage in the middle of the grid within a specified range when the power generation of the power generation system 10 is high. For this reason, it is difficult to keep the voltage of the transmission line PL within a specified range when all PCS 120 of the power generation system 10 control reactive power with constant power factor control.

[0066] Therefore, in this embodiment, among the multiple PCS120s of the power generation system 10, some PCS120 control the reactive power supplied to the power company grid 2 using droop control, while the remaining PCS120 control the reactive power supplied to the power company grid 2 using constant power factor control. This makes it possible in this embodiment to keep the voltage near the end of the grid within a specified range when the power generation amount of the power generation system 10 is moderate, and to keep the voltage in the middle of the grid within a specified range when the power generation amount of the power generation system 10 is high.

[0067] Figure 7 is an explanatory diagram illustrating the operation of each PCS 120 when the power generation of the power generation system 10 is at a moderate level.

[0068] For example, if the voltage near the end of the grid (e.g., node Na) rises due to excessive reactive power Qb, Qc, Qd, and Qe output by PCS120b, 120c, 120d, and 120e respectively, PCS120a will operate to lower the voltage based on the droop characteristics. Specifically, for example, the droop control unit 124a of PCS120a calculates the reactive power Qv based on the voltage V at the connection terminal CTa of PCS120a and the droop characteristics in order to lower the voltage near the end of the grid. In Figure 7 and Figure 8, which will be described later, the operation of PCS120a will be explained using the case where droop control is performed based on the droop characteristics shown by the solid line in Figure 3 (droop characteristics without a dead zone DZ set) as an example for the sake of clarity.

[0069] For example, if PCS120b, 120c, 120d, and 120e output excessive reactive powers Qb, Qc, Qd, and Qe respectively, causing the voltage V at the connection terminal CTa of PCS120a to exceed the reference voltage V0, the reactive power Qv calculated by equation (2) explained in Figure 3 will be a negative value. Therefore, PCS120a operates to absorb the reactive power Qa corresponding to the reactive power Qv calculated by equation (2). As a result, the voltage V at the connection terminal CTa of PCS120a approaches the reference voltage V0. In other words, PCS120a operates to lower the voltage near the end of the system (e.g., node Na) that has risen due to the output of excessive reactive powers Qb, Qc, Qd, and Qe by PCS120b, 120c, 120d, and 120e. As a result, in this embodiment, when the power generation amount of the power generation system 10 is moderate, it is possible to suppress the voltage near the end of the grid from rising outside the specified range.

[0070] Furthermore, when the power generation of the power generation system 10 is moderate, the amount of reactive power supplied by each of the PCS120b, 120c, 120d, and 120e is small. Therefore, by increasing the droop gain Gv of the droop characteristics, even if the number of PCS120s performing droop control is small, the excess reactive power can be reliably absorbed by the PCS120s performing droop control. For example, as shown in Figure 7, the excess reactive power can be reliably absorbed by a single PCS120a.

[0071] Thus, in this embodiment, when the power generation amount of the power generation system 10 is moderate, the PCS120a can absorb excess reactive power, and as shown in Figure 9 later, the voltage near the end of the grid can be kept within a specified range.

[0072] Next, referring to Figure 8, we will explain the operation of each PCS 120 when the power generation of the power generation system 10 is large.

[0073] Figure 8 is an explanatory diagram illustrating the operation of each PCS 120 when the power generation amount of the power generation system 10 is large.

[0074] In this embodiment, as explained in Figure 7, PCS120a performs droop control, which suppresses the voltage near the end of the grid from rising outside the specified range when the power generation of the power generation system 10 is moderate. Therefore, in this embodiment, the power factors of each of PCS120b, 120c, 120d, and 120e can be made smaller compared to the case where all PCS120s of the power generation system 10 perform constant power factor control. As a result, the amount of reactive power supplied by each of PCS120b, 120c, 120d, and 120e increases when the power generation of the power generation system 10 is high. This makes it possible to suppress the voltage drop in the middle of the grid (for example, near the interconnection point PT1) when the power generation of the power generation system 10 is high.

[0075] Furthermore, in the PCS120 that performs droop control, increasing the reference voltage V0 will result in a larger reactive power Qv, calculated by equation (1), compared to the case where the reference voltage V0 is small. Therefore, in the PCS120 as well, increasing the reference voltage V0 will allow a larger reactive power Qa to be output to the transmission line PL1.

[0076] Thus, in this embodiment, when the power generation amount of the power generation system 10 is large, a large amount of reactive power can be output from each PCS 120 to the transmission line PL1, and as shown in Figure 9 later, the voltage in the middle of the grid can be kept within a specified range.

[0077] Figure 9 is an explanatory diagram illustrating the effects obtained by controlling the reactive power flowing to the power company system 2 using both droop control and constant power factor control. Similar to Figure 6, Figure 9 shows an example of the relationship between the impedance of the circuit from the busbar (e.g., system power source 20) and the voltage of the transmission line PL. The horizontal axis of Figure 9 represents the impedance of the circuit from the busbar, and the vertical axis represents the voltage of the transmission line PL. In Figure 9, as in Figure 6, the impedance of the circuit from the busbar includes the impedance of the transformer 30 and increases as it approaches the end of the system. Furthermore, the interconnection point PT1 shown in Figure 9, similar to Figure 6, represents a point on the horizontal axis corresponding to the impedance of the circuit from the busbar to the interconnection point PT1. Note that, similar to Figure 6, the horizontal axis of Figure 9 corresponds to the distance from the busbar when the impedance of the transformer 30 is ignored.

[0078] Note that the dotted line in Figure 9 (without droop control) is a comparative example and shows the voltage fluctuation when all PCS120s in the power generation system 10 perform constant power factor control. The solid line in Figure 9 (with droop control) shows the voltage fluctuation when PCS120a performs droop control and PCS120b, 120c, 120d, and 120e each perform constant power factor control. The allowable range of voltage fluctuation is predetermined, for example, as a specified range.

[0079] For example, voltage VLMl1 indicates the lower limit of the first specified voltage range RA1 at the interconnection point PT1, and voltage VLMh1 indicates the upper limit of the first specified voltage range RA1 at the interconnection point PT1. In other words, at the interconnection point PT1, the voltage of the transmission line PL is required to fall within the first specified range RA1, which ranges from voltage VLMl1 to voltage VLMh1.

[0080] Furthermore, voltage VLM12 indicates the lower limit of the second specified voltage range RA2 at locations other than the interconnection point PT1, and voltage VLMh2 indicates the upper limit of the second specified voltage range RA2 at locations other than the interconnection point PT1. In other words, at locations other than the interconnection point PT1, the voltage of the transmission line PL is required to fall within the second specified voltage range RA2, from voltage VLM12 to voltage VLMh2. Generally, the permissible range of voltage fluctuation at the interconnection point PT1 (first specified range RA1) is smaller than the permissible range of voltage fluctuation at locations other than the interconnection point PT1 (second specified range RA2).

[0081] In the following, the mode in which all PCS120s of the power generation system 10 perform constant power factor control will also be referred to as the comparative mode.

[0082] As shown in Figure 9 under "No Droop Control," in the comparative configuration, when the power generation of the power generation system 10 is moderate, the voltage near the end of the grid becomes higher than the voltage VLMh2 (upper limit of the second specified range RA2). Also, in the comparative configuration, when the power generation of the power generation system 10 is high, the voltage at the interconnection point PT1 becomes lower than the voltage VLM11 (lower limit of the first specified range RA1). Thus, in the comparative configuration where all PCS120s of the power generation system 10 perform constant power factor control, the voltage of the transmission line PL may not fall within the specified range (first specified range RA1 or second specified range RA2).

[0083] In contrast, in this embodiment, as shown in Figure 9 under "with droop control," when the power generation of the power generation system 10 is moderate, the voltage of the transmission line PL can be made lower compared to the comparative embodiment by droop control. As a result, in this embodiment, for example, when the power generation of the power generation system 10 is moderate, it is possible to suppress the voltage near the end of the system from becoming higher than the voltage VLMh2 (upper limit of the second specified range RA2).

[0084] Furthermore, in this embodiment, as shown in Figure 9 under "Droop control enabled," when the power generation amount of the power generation system 10 is large, the amount of reactive power supplied can be increased compared to the comparative embodiment, and therefore the voltage of the transmission line PL can be increased compared to the comparative embodiment. As a result, in this embodiment, for example, when the power generation amount of the power generation system 10 is large, it is possible to suppress the voltage in the middle part of the system (for example, near the interconnection point PT1) from falling below the voltage VLM1 (lower limit of the first specified range RA1).

[0085] Thus, in this embodiment, by applying droop control to some of the multiple PCS120, it is possible to supply a large amount of reactive power when the power generation amount of the power generation system 10 is large, while suppressing the supply of reactive power when the power generation amount of the power generation system 10 is moderate. In other words, in this embodiment, the trade-off that is a problem in the comparative embodiment in which all PCS120 of the power generation system 10 perform constant power factor control can be improved. As a result, in this embodiment, it is possible to keep the voltage near the end of the grid within a specified range when the power generation amount of the power generation system 10 is moderate, and to keep the voltage in the middle of the grid within a specified range when the power generation amount of the power generation system 10 is large. In other words, in this embodiment, voltage fluctuations of the transmission line PL can be suppressed.

[0086] In droop control, voltage and reactive power interact with each other. Therefore, if multiple PCS120s perform droop control, oscillation and resonance may occur. Consequently, it is preferable to have a small number of PCS120s that perform droop control. For example, in the multiple PCS120s of the power generation system 10, it is preferable that the number of PCS120s that perform droop control is less than the number of PCS120s that perform constant power factor control. In this case, since the number of PCS120s that perform droop control can be kept to a small number, it is possible to suppress the difficulty in achieving a stable design that suppresses the occurrence of oscillation and resonance. In particular, in this embodiment, since the number of PCS120s that perform droop control is one, a stable design that suppresses the occurrence of oscillation and resonance can be easily realized.

[0087] Furthermore, the inventors of this application have confirmed through simulations that the configuration in which both droop control and constant power factor control are performed can suppress voltage fluctuations in the transmission line PL even more effectively than the configuration in which only droop control is performed. For example, in the configuration in which only droop control is performed, the amount of reactive power supplied when the power generation amount of the power generation system 10 is large is insufficient compared to the configuration in which both droop control and constant power factor control are performed, resulting in a larger voltage drop in the middle of the grid. In other words, in this embodiment, voltage fluctuations in the transmission line PL can be suppressed even more effectively than in the configuration in which all PCS 120 of the power generation system 10 perform droop control.

[0088] Furthermore, in this embodiment, even when the interconnection point PT1 is not in the middle of the grid, it is possible to obtain a voltage fluctuation suppression effect equivalent to or better than that of the constant power factor control only or the droop control only. In other words, in this embodiment, even when the interconnection point PT1 is not in the middle of the grid, it is possible to suppress the voltage of the transmission line PL from falling outside the specified range.

[0089] Furthermore, in this embodiment, it is possible to suppress not only voltage fluctuations at the interconnection point PT1 but also voltage fluctuations throughout the entire system. This provides the effect of protecting equipment such as the transformer 30 and PCS120, as well as the effect of suppressing voltage fluctuations in the power company system 2, where maintaining power quality is required.

[0090] In this embodiment, the power generation system 10 is connected at the interconnection point PT1 to a power company grid 2 where the inductance component x is greater than the resistance component r, and is a voltage control system that adjusts the voltage at a predetermined location on the circuit by controlling the reactive power flowing to the power company grid 2. The system comprises a constant power factor control unit 125 that calculates reactive power based on the active power flowing to the power company grid 2, and a droop control unit 124 that calculates reactive power based on the voltage of the power company grid 2, and the system flows reactive power to the power company grid 2 based on the sum of the reactive power calculated by the constant power factor control unit 125 and the reactive power calculated by the droop control unit 124.

[0091] Thus, in this embodiment, the power generation system 10 supplies reactive power to the power company grid 2 based on the sum of reactive power calculated based on the active power flowing to the power company grid 2 and reactive power calculated based on the voltage of the power company grid 2. This improves the trade-off that is a problem in the embodiment where only reactive power calculated based on active power is supplied to the power company grid 2. The trade-off is the relationship between keeping the voltage near the end of the grid below the upper limit of a specified range when the power generation amount of the power generation system 10 is moderate, and keeping the voltage in the middle of the grid above the lower limit of a specified range when the power generation amount of the power generation system 10 is high. In this embodiment, since the above-mentioned trade-off can be improved, it is possible to keep the voltage near the end of the grid within a specified range when the power generation amount of the power generation system 10 is moderate, and to keep the voltage in the middle of the grid within a specified range when the power generation amount of the power generation system 10 is high. As a result, in this embodiment, voltage fluctuations at predetermined locations on the power line PL and other circuits can be suppressed.

[0092] Furthermore, in this embodiment, the maximum value of reactive power that can be supplied to the power company grid 2 based on the reactive power calculated by the constant power factor control unit 125 may be greater than the maximum value of reactive power that can be supplied to the power company grid 2 based on the reactive power calculated by the droop control unit 124. In this case, the reactive power based on the sum of the reactive power calculated by the constant power factor control unit 125 and the reactive power calculated by the droop control unit 124, i.e., the reactive power supplied to the power company grid 2, can be set to an appropriate amount of energy.

[0093] Furthermore, in this embodiment, the power generation system 10 includes a plurality of PCS120b, 120c, 120d, and 120e that send the reactive power calculated by the constant power factor control unit 125 to the power company grid 2, and a smaller number of PCS120a than the plurality of PCS120b, 120c, 120d, and 120e that sends the reactive power calculated by the droop control unit 124 to the power company grid 2. In this way, in this embodiment, the number of PCS120 that send the reactive power calculated by the droop control unit 124 to the power company grid 2 can be kept to a small number. When the number of droop control units 124 increases, oscillation and resonance tend to occur more easily due to the relationship between the interacting voltages and reactive power. In this embodiment, as described above, since the number of PCS120 that send the reactive power calculated by the droop control unit 124 to the power company grid 2 can be kept to a small number, it is possible to suppress the difficulty in designing a stable system that suppresses the occurrence of oscillation and resonance. In particular, when there is only one PCS 120 that supplies the reactive power calculated by the droop control unit 124 to the power company grid 2, a stable design that suppresses oscillation and resonance can be easily achieved.

[0094] Furthermore, in this embodiment, the power generation system 10 includes a plurality of PCS 120b, 120c, 120d, and 120e that include only the constant power factor control unit 125 out of the constant power factor control unit 125 and droop control unit 124, and that send the reactive power calculated by the constant power factor control unit 125 to the power company grid 2, and a PCS 120a that includes only the droop control unit 124 out of the constant power factor control unit 125 and droop control unit 124, and that sends the reactive power calculated by the droop control unit 124 to the power company grid 2. Thus, in this embodiment, each of the plurality of PCS 120 has only one of the constant power factor control unit 125 and droop control unit 124. In other words, in this embodiment, each PCS 120 does not need to have both the constant power factor control unit 125 and the droop control unit 124. For this reason, in this embodiment, the complexity of the configuration of each PCS 120 can be suppressed. In other words, in this embodiment, each PCS 120 can be realized with a simple configuration.

[0095] Furthermore, in this embodiment, the reactive power calculated by the droop control unit 124 is proportional to the difference between the voltage of the power company system 2 and the reference voltage V0. Therefore, in this embodiment, the reactive power can be appropriately controlled by appropriately setting the reference voltage V0, etc.

[0096] Furthermore, in this embodiment, the reactive power calculated by the constant power factor control unit 125 is the reactive power proportional to the active power output to the power company grid 2 in the multiple PCS 120b, 120c, 120d, and 120e. This makes it possible to suppress deviations of the power factor from a predetermined value in this embodiment.

[0097] B: Modification The embodiments illustrated above can be modified in various ways. Specific examples of modifications that can be applied to the aforementioned embodiments are given below. Two or more embodiments arbitrarily selected from the following examples may be combined to the extent that they do not contradict each other.

[0098] B1: First variation In the embodiment described above, the grid connection system 1 may have a facility (for example, a factory 14 shown in Figure 10) that receives power from the transmission line PL1 as a private line.

[0099] Figure 10 is an explanatory diagram illustrating an example of a grid connection system 1 according to the first modified example. This modified grid connection system 1 is the same as the grid connection system 1 shown in Figure 1, except that it has a factory 14 that is supplied with power from the transmission line PL1.

[0100] Factory 14 has, for example, an electrical circuit (not shown) to which power is supplied from transmission line PL1. In this modified example as well, the power generation system 10 controls the reactive power flowing to transmission line PL using both droop control and constant power factor control. Therefore, in this embodiment, voltage fluctuations in the electrical circuits within factory 14 can also be suppressed.

[0101] In this modified example, the same effects as those of the embodiment described above can be obtained. Furthermore, in this modified example, voltage fluctuations in the electrical circuits within facilities such as the factory 14, to which power is supplied from the power transmission line PL1, can also be suppressed.

[0102] B2: Second variation In the embodiments and modifications described above, if the power generation system 10 has one PCS120, the one PCS120 in the power generation system 10 has a droop control unit 124 and a constant power factor control unit 125. For example, the processing unit 122 of the PCS120 functions as the droop control unit 124 and constant power factor control unit 125 by executing a control program PG stored in the storage device 126. In this modification, the PCS120 outputs reactive power to the transmission line PL1 based on the sum of reactive power Qv calculated by the droop control unit 124 and reactive power Qp calculated by the constant power factor control unit 125.

[0103] As described above, the same effects as those of the embodiments and modifications described can be obtained in this modified example as well.

[0104] B3: Third variation In the embodiments and modifications described above, the number of PCS120 having a droop control unit 124 may be greater than the number of PCS120 having a constant power factor control unit 125. In this modification as well, the same effects as in the embodiments and modifications described above can be obtained, except for the effect obtained by limiting the number of PCS120 having a droop control unit 124 to a small number.

[0105] B4: Fourth variation In the embodiments and modifications described above, an example was given in which PCS120a, which has a droop control unit 124, is connected to the transmission line PL1 at the position closest to node N1 among the multiple PCS. However, the present invention is not limited to this embodiment. For example, PCS120, which has a constant power factor control unit 125, may be connected to the transmission line PL1 at the position closest to node N1 among the multiple PCS.

[0106] As described above, the same effects as those of the embodiments and modifications described can be obtained in this modified example as well. [Explanation of Symbols]

[0107] 1...Grid interconnection system, 2...Power company grid, 10...Power generation system, 14...Factory, 30...Transformer, 100a, 100b, 100c, 100d, 100e...DC power supply, 120a, 120b, 120c, 120d, 120e...PCS, 122a, 122b, 122c, 122d, 122e...Processing unit, 124a...Droop control unit, 125b, 125c, 125d, 125e...Constant power factor control unit, 126a, 126b, 126c, 126d, 126e...Memory device, PL1, PL2...Transmission line.

Claims

1. A voltage control system that is connected at a connection point to a power system where the inductance component is greater than the resistance component, and adjusts the voltage at a predetermined location on the circuit by controlling the reactive power flowing through the power system, A first control unit that calculates reactive power based on the active power flowing through the power system, A second control unit that calculates reactive power based on the voltage of the power system, Equipped with, The reactive power calculated by the first control unit and the reactive power calculated by the second control unit is used to supply reactive power to the power system. A voltage control system characterized by the following:

2. The maximum amount of reactive power that can be supplied to the power system based on the reactive power calculated by the first control unit is greater than the maximum amount of reactive power that can be supplied to the power system based on the reactive power calculated by the second control unit. The voltage control system according to claim 1, characterized in that it is as described above.

3. A plurality of first power converters that supply the reactive power calculated by the first control unit to the power system, The reactive power calculated by the second control unit is supplied to the power system, and the number of second power converters is fewer than the number of first power converters. Having, The voltage control system according to claim 2, characterized in that it is as described above.

4. One or more first power converters, which include only the first control unit among the first and second control units, and which supply the reactive power calculated by the first control unit to the power system, A second power converter comprising only the second control unit among the first control unit and the second control unit, which supplies the reactive power calculated by the second control unit to the power system, Having, The voltage control system according to claim 1, characterized in that it is as described above.

5. The reactive power calculated by the second control unit is a reactive power proportional to the difference between the voltage of the power system and the reference voltage. The voltage control system according to claim 1, characterized in that it is as described above.

6. The system includes one or more power converters that supply the reactive power calculated by the first control unit to the power system. The reactive power calculated by the first control unit is a reactive power proportional to the active power output to the power system by the one or more power converters. The voltage control system according to claim 1, characterized in that it is as described above.

7. A voltage control method that is connected at a connection point to a power system where the inductance component is greater than the resistance component, and adjusts the voltage at a predetermined location on the circuit by controlling the reactive power flowing through the power system, Based on the active power flowing through the aforementioned power system, the reactive power is calculated. Based on the voltage of the aforementioned power system, the reactive power is calculated. The reactive power supplied to the power system is the sum of the reactive power calculated based on the active power flowing through the power system and the reactive power calculated based on the voltage of the power system. A voltage control method characterized by the following:

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