HK1205297B

HK1205297B - Static variable compensator and voltage-control method

Info

Publication number: HK1205297B
Application number: HK15105564.8A
Authority: HK
Inventors: 宇敷修一; 大岛正明
Original assignee: 欧利生电气株式会社
Priority date: 2012-08-30
Filing date: 2013-08-13
Publication date: 2018-05-04

Description

Static reactive power compensation device and voltage control method

Technical Field

The present invention relates to a static reactive power compensator for adjusting a voltage of a power system by controlling a reactive power and a voltage control method of a single-phase ac power system.

Background

Generally, in an electric power system, the voltage of the system end away from the power supply side drops. However, when a distributed power source using renewable energy such as solar power generation (PV) or wind power generation is connected to a power system, the voltage at the end of the system may increase. When a substation supplies power in a conventional manner, a low-voltage distribution voltage in a power system may deviate from an appropriate voltage range. Therefore, it is known to connect a static reactive power compensator to a power grid in order to keep a low-voltage distribution voltage in the power grid within an appropriate voltage range (see, for example, non-patent document 1). Such Static Var Compensator is also called "STATCOM (Static Synchronous Compensator)" or "self-excited SVC (Static Var Compensator).

Non-patent document 1: STATCOM for smart grid beneficial to effective utilization of renewable energy resources, Toshiba comment Vol.66 No.12(2011) P36-39

In the future, with the expansion of smart grids, PV is expected to spread in general households supplied with single-phase ac power. Therefore, voltage regulation in the single-phase ac power system is required, but the conventional static reactive power compensator adjusts the three-phase ac voltage as in the STATCOM of non-patent document 1, and cannot adjust the voltage in the single-phase ac power system.

Disclosure of Invention

The invention aims to provide a static reactive power compensation device and a voltage control method which can adjust voltage in a single-phase alternating current power system.

In order to achieve the above object, a static reactive power compensator according to the present invention generates a second axis voltage command based on a difference between a dc voltage detected at a dc terminal side of the static reactive power compensator and a set dc voltage command.

Specifically, the present invention provides an electric power system comprising: the high-voltage distribution line is used for distributing three-phase high-voltage alternating current; a plurality of transformers connected to the high-voltage distribution line, and converting the three-phase high-voltage alternating current into a single-phase low-voltage alternating current having a voltage lower than that of the three-phase high-voltage alternating current; a plurality of low-voltage distribution lines for respectively distributing the single-phase low-voltage alternating current converted by the transformer; and any number of static reactive power compensation devices, an alternating current terminal being connected to the low-voltage distribution line, the electric power system being characterized in that the static reactive power compensation device includes: a single-phase voltage type ac/dc conversion circuit that has an internal electromotive force and an internal equivalent impedance when viewed from the ac terminal, and converts between single-phase ac power and dc power from a single-phase ac power system connected to the ac terminal, in accordance with a pulse width of a gate signal generated based on a PWM command; a voltage command circuit that outputs a first shaft voltage command that is a target value of the amplitude of the single-phase ac voltage at the ac terminal, sets a dc voltage command value that is higher than a peak value of the single-phase ac voltage, detects the dc voltage converted by the single-phase voltage type ac/dc conversion circuit, calculates a difference between the dc voltage command value and a dc voltage detection value, and outputs a second shaft voltage command; a phase difference generation circuit having a phase-delayed single-phase ac generator that delays a phase of a single-phase ac voltage at the ac terminal to generate a delayed single-phase ac, the phase difference generation circuit generating a voltage corresponding to a phase difference between the single-phase ac voltage at the ac terminal and the internal electromotive force of the single-phase voltage type ac/dc conversion circuit, based on the delayed single-phase ac; an upper-level voltage control circuit that outputs a voltage command signal generated such that an amplitude of a single-phase ac voltage at the ac terminal approaches the first shaft voltage command and a frequency command signal generated such that a voltage corresponding to the phase difference from the phase difference generation circuit approaches the second shaft voltage command, based on the first shaft voltage command and the second shaft voltage command from the voltage command circuit, a voltage corresponding to the phase difference from the phase difference generation circuit, and the single-phase ac voltage at the ac terminal; a frequency control circuit that sets a reference frequency that is a reference of a frequency of a single-phase alternating current at the alternating-current terminal, and determines an electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit and generates a generated electrical angle, based on the reference frequency, a frequency command signal from the upper voltage control circuit, and a voltage corresponding to the phase difference generated by the phase difference generation circuit; and a lower voltage control circuit to which a reference voltage is set as a reference of a voltage amplitude of the single-phase alternating current at the alternating current terminal, adding a voltage command signal from the upper voltage control circuit to a value obtained by multiplying the reference voltage by the signal for generating the electrical angle from the frequency control circuit to obtain the internal electromotive force, and outputting a difference between the internal electromotive force and the single-phase alternating-current voltage as the PWM command, the power system is controlled so as to lower the voltage of the AC terminal when the voltage of the low-voltage AC power rises, based on the difference between the voltage of the single-phase low-voltage AC power of the low-voltage distribution line and the internal electromotive force, the voltage of the alternating current terminal is raised when the voltage of the low-voltage alternating current is lowered, and reactive current is transmitted between the low-voltage distribution lines through the alternating current terminal, so that the voltage of the single-phase low-voltage alternating current of the low-voltage distribution lines is controlled within a proper voltage range.

The present invention also provides a voltage control method of causing a voltage of a single-phase ac electric power system to be within an appropriate voltage range using a single-phase voltage type ac/dc conversion circuit that has an internal electromotive force and an internal equivalent impedance as viewed from an ac terminal and converts between single-phase ac electric power and dc electric power from the single-phase ac electric power system connected to the ac terminal in accordance with a pulse width of a gate signal generated based on a PWM command, the voltage control method performing the steps of: a voltage command step of outputting a first shaft voltage command that is an amplitude target value of a single-phase alternating-current voltage at the alternating-current terminal, setting a direct-current voltage command value that is higher than a peak value of the single-phase alternating-current voltage, detecting a direct-current voltage converted by the single-phase voltage type alternating-current/direct-current conversion circuit, calculating a difference between the direct-current voltage command value and a direct-current voltage detection value, and outputting a second shaft voltage command; a phase difference generation step of generating a delayed single-phase alternating current, which delays a phase of a single-phase alternating voltage at the alternating-current terminal, by a phase-delayed single-phase alternating current generator, and generating a voltage corresponding to a phase difference between the single-phase alternating voltage at the alternating-current terminal and the internal electromotive force of the single-phase voltage type alternating current-direct current conversion circuit, based on the delayed single-phase alternating current; an upper level voltage control step of outputting a voltage command signal generated so that the amplitude of the single-phase ac voltage at the ac terminal approaches the first axis voltage command and a frequency command signal generated so that the voltage corresponding to the phase difference generated at the phase difference generation step approaches the second axis voltage command, based on the first axis voltage command and the second axis voltage command output at the voltage command step, a voltage corresponding to the phase difference generated at the phase difference generation step, and the single-phase ac voltage at the ac terminal; a frequency control step of determining an electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit and generating a generated electrical angle, based on a reference frequency that is a frequency reference of a single-phase ac at the ac terminal, the frequency command signal output in the upper-level voltage control step, and a voltage corresponding to the phase difference generated in the phase difference generation step; and a lower level voltage control step of adding the voltage command signal output in the upper level voltage control step to a value obtained by multiplying a reference voltage, which is a reference of a voltage amplitude of the single-phase alternating current at the alternating-current terminal, by the signal generated based on the generated electrical angle generated in the frequency control step, as the internal electromotive force, and outputting a difference between the internal electromotive force and the single-phase alternating-current voltage as the PWM command, thereby keeping a voltage effective value at the alternating-current terminal constant.

The static reactive power compensator and the voltage control method using the same according to the present invention generate the second axis voltage command from the difference between the detected dc voltage value and the dc voltage command value. An internal electromotive force is generated from the second shaft voltage command and a first shaft voltage command that is a voltage target value of the ac terminal, and a PWM command is generated from a difference between the internal electromotive force and a voltage of the ac terminal, thereby controlling a voltage on the ac terminal side of the inverter (single-phase ac/dc conversion circuit). For example, when the voltage of the single-phase ac power system (the voltage at the ac terminal) increases, the difference between the voltage at the ac terminal and the internal electromotive force increases, and therefore the stationary reactive power compensation device performs PWM control so as to decrease the voltage at the ac terminal side of the single-phase voltage type ac/dc conversion circuit. Since the voltage of the single-phase voltage type ac/dc conversion circuit is lower than the voltage of the single-phase ac power system, a reactive current flows from the single-phase ac power system into the static type reactive power compensation device, and the voltage of the single-phase ac power system can be reduced.

Therefore, the present invention can provide a static reactive power compensation device and a voltage adjustment method capable of performing voltage adjustment in a single-phase ac power system.

The first shaft voltage command output by the voltage command circuit of the static reactive power compensator according to the present invention may be a fixed value set in advance. Reactive power may be calculated and monitored.

Further, the static reactive power compensation device of the present invention further includes: an alternating current detection circuit that detects an alternating current at the alternating current terminal; and an ac power measuring circuit that measures reactive power at the ac terminal, wherein the voltage command circuit may calculate a difference between the reactive power measured by the ac power measuring circuit and a preset reactive power command value to generate the first shaft voltage command.

In the voltage control method of the single-phase ac power system according to the present invention, the ac terminal of at least one of the static reactive power compensation devices is connected to the single-phase ac power system, and the voltage of the single-phase ac power system is controlled to be within an appropriate voltage range. By connecting a plurality of static reactive power compensation devices of the present invention to a single-phase ac power system, it is possible to more effectively bring the low-voltage distribution voltage in the power system within an appropriate voltage range.

The invention provides a static reactive power compensation device and a voltage adjustment method capable of adjusting voltage in a single-phase alternating current power system.

Drawings

Fig. 1 is a schematic configuration diagram of a static reactive power compensator according to the present invention.

Fig. 2 is a diagram showing a control flow of the static reactive power compensator according to the present invention.

Fig. 3 is a schematic configuration diagram of a single-phase voltage type ac/dc conversion circuit provided in the static type reactive power compensation device of the present invention.

Fig. 4 is a schematic configuration diagram of a single-phase voltage type ac/dc conversion circuit provided in the static type reactive power compensation device of the present invention.

Fig. 5 is a schematic configuration diagram of a single-phase ac filter circuit and a single-phase voltage-type ac/dc conversion unit in a single-phase voltage-type ac/dc conversion circuit included in the static reactive power compensation device according to the present invention.

Fig. 6 is an equivalent circuit as viewed from the ac terminal of the static type reactive power compensator of the present invention.

Fig. 7 is a diagram illustrating a command value calculation circuit provided in the static reactive power compensator according to the present invention.

Fig. 8 is a diagram illustrating a command value calculation circuit provided in the static reactive power compensator according to the present invention.

Fig. 9 is a diagram illustrating a command value calculation circuit provided in the static reactive power compensator according to the present invention.

Fig. 10 is a diagram illustrating simultaneous control of the dc voltage and the reactive power of the static reactive power compensator according to the present invention.

Fig. 11 is a schematic configuration diagram of a static reactive power compensator according to the present invention.

Fig. 12 is a schematic configuration diagram of a phase difference generation circuit provided in the static reactive power compensation device of the present invention.

Fig. 13 is a schematic configuration diagram of a static reactive power compensator according to the present invention.

Fig. 14 is a schematic configuration diagram of an ac power measuring circuit provided in the static reactive power compensator of the present invention.

Fig. 15 is a schematic configuration diagram of an ac power measuring circuit provided in the static reactive power compensator of the present invention.

Fig. 16 is a diagram illustrating a single-phase ac power system of the present invention.

Description of the reference numerals

11: static reactive power compensation device

21: AC terminal

30: phase difference generating circuit

31: alternating voltage detection circuit

33-1 to 33-3: terminal with a terminal body

34: output current detection circuit

35: phase delay single-phase alternating current generator

36: phase difference voltage generator

38: current transformer

40: single-phase voltage type AC-DC conversion circuit

40-1, 40-2: single-phase voltage type AC-DC conversion circuit

41: gate signal generator

42: single-phase voltage type AC-DC conversion part

43: current detection circuit

44: voltage detection circuit

45: single-phase alternating current filter circuit

50: frequency control circuit

51: reference frequency setting device

53: loop filter

55: time integrator

56: second adder

57: generating electrical angles

58: third adder

60: lower voltage control circuit

61: reference voltage setting device

62: first adder

63: third subtracter

64: voltage controller

65: second multiplier

66: filter current compensator

67: PWM current deviation compensator

68: feed forward amplifier

69: fourth adder

70: upper voltage control circuit

71 a: first subtracter

71 b: second subtracter

72 a: first upper control amplifier

72 b: second upper control amplifier

73: first multiplier

121: limiting device

140: alternating current power measuring circuit

141: reference frequency circuit

142: voltage phase delay circuit

143: current phase delay circuit

144: electric power arithmetic circuit

145: active power value measuring circuit

146: reactive power value measuring circuit

147-1, 147-2, 147-3, 147-4: multiplier and method for generating a digital signal

148-1: adder

148-2: subtracter

149-1, 149-2: low-pass filter

150-1, 150-2: voltage instruction circuit

161: DC voltage detection circuit

B1: voltage command

B2: upper level voltage control

B3: frequency control

B4: lower voltage control

B5: generating a gate signal

B6: main switch

Detailed Description

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, but the present invention is not limited to the embodiments described below. The embodiments are merely illustrative, and the present invention can be implemented in various modifications and improvements based on knowledge of those skilled in the art. In the present specification and the drawings, the same components are denoted by the same reference numerals and the same configurations are denoted by the same reference numerals.

[ static reactive power compensator ]

Fig. 1 is a schematic configuration diagram showing a static reactive power compensator according to the present embodiment, and fig. 2 is a diagram showing a control flow illustrating the static reactive power compensator. As shown in fig. 2, the static reactive power compensation device of the present embodiment generates a first axis voltage command and a second axis voltage command by a voltage command circuit (B1), generates a voltage command signal and a frequency command signal by an upper level voltage control circuit so that the amplitude and the frequency of a single-phase ac voltage are close to those of the first axis voltage command and the second axis voltage command (B2), determines the electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit by the frequency control circuit (B3), generates a current command for fixing the ac voltage of the single-phase voltage type ac/dc conversion circuit by a lower level voltage control circuit (B4), generates a gate signal by a gate signal generator (B5), and operates a main switch (B6). Here, the single-phase voltage source ac/dc conversion unit included in the single-phase voltage source ac/dc conversion circuit 40 of fig. 1 corresponds to the main switch B6 of fig. 2.

Next, the control flow shown in fig. 2 will be described in further detail.

The static type reactive power compensation device 11 shown in fig. 1 includes: a single-phase voltage type ac/dc conversion circuit 40 that has an internal electromotive force and an internal equivalent impedance as viewed from the ac terminal 21, and converts between single-phase ac power and dc power from a single-phase ac power system connected to the ac terminal 21, based on a pulse width of a gate signal generated based on a PWM command; a voltage command circuit 150-1 that outputs a first shaft voltage command that is a target value of the amplitude of the single-phase ac voltage at the ac terminal 21, sets a dc voltage command value that is higher than the peak value of the single-phase ac voltage, detects the dc voltage converted by the single-phase voltage type ac/dc conversion circuit 40, calculates the difference between the dc voltage command value and the dc voltage detection value, and outputs a second shaft voltage command; a phase difference generation circuit 30 having a phase-delayed single-phase ac generator that delays the phase of the single-phase ac voltage at the ac terminal 21 to generate a delayed single-phase ac, and generating a voltage corresponding to a phase difference between the single-phase ac voltage at the ac terminal 21 and the internal electromotive force of the single-phase voltage type ac/dc conversion circuit 40, based on the delayed single-phase ac; an upper-level voltage control circuit 70 that outputs a voltage command signal generated so that the amplitude of the single-phase ac voltage at the ac terminal 21 approaches the first shaft voltage command and a frequency command signal generated so that the voltage corresponding to the phase difference from the phase difference generation circuit approaches the second shaft voltage command, based on the first shaft voltage command and the second shaft voltage command from the voltage command circuit 150-1, the voltage corresponding to the phase difference from the phase difference generation circuit 30, and the single-phase ac voltage at the ac terminal; a frequency control circuit 50 that sets a reference frequency that is a reference of the frequency of the single-phase ac at the ac terminal 21, determines an electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit 40 based on the reference frequency, the frequency command signal from the upper voltage control circuit 70, and the voltage corresponding to the phase difference generated by the phase difference generation circuit 30, and generates a generated electrical angle; and a lower voltage control circuit 60 that sets a reference voltage as a reference of the voltage amplitude of the single-phase alternating current at the alternating-current terminal 21, adds a voltage command signal from the upper voltage control circuit 70 to a value obtained by multiplying a signal based on the electrical angle from the frequency control circuit 50 by the reference voltage to obtain an internal electromotive force, and outputs the difference between the internal electromotive force and the single-phase alternating current voltage as the PWM command.

Fig. 3 and 4 are schematic configuration diagrams of the single-phase voltage type ac/dc conversion circuit 40.

The single-phase voltage type ac/dc conversion circuit 40-1 shown in fig. 3 includes: a single-phase voltage type ac/dc conversion unit 42 having an internal equivalent impedance as viewed from the ac terminal 21, for converting the single-phase ac power and the dc power at the ac terminal 21 in accordance with the pulse width of the gate signal; a current detection circuit 43 that detects a current between the single-phase voltage type ac/dc conversion unit 42 and the single-phase ac filter circuit 45 and outputs a signal generated in accordance with the magnitude of the single-phase ac current; a gate signal generator 41 for generating and outputting a gate signal such that a difference between the PWM command and the output from the current detection circuit 43 approaches zero; and a single-phase ac filter circuit 45 that removes a high-frequency component caused by the gate signal in the single-phase voltage source ac/dc conversion unit 42 from the single-phase ac voltage in the single-phase voltage source ac/dc conversion unit 42 and communicates with the system.

Further, the single-phase voltage source ac/dc conversion circuit 40-2 shown in fig. 4 includes a voltage detection circuit 44, and the voltage detection circuit 44 detects a single-phase ac voltage of the single-phase voltage source ac/dc conversion section 42 in place of the current detection circuit 43 in fig. 3, and outputs a signal generated in accordance with the magnitude of the single-phase ac voltage. In this case, the gate signal generator 41 generates and outputs a gate signal in such a manner that the difference between the PWM command and the output from the voltage detection circuit 44 approaches zero.

Here, fig. 5 is a schematic configuration diagram of the single-phase voltage type ac/dc conversion unit 42 and the single-phase ac filter circuit 45 in fig. 3 and 4.

The single-phase voltage type ac/dc conversion section 42 shown in fig. 5 includes four automatic arc extinguishing type switches 46g, 46h, 46k, 46l and four diodes 46a, 46b, 46e, 46 f. The automatic arc-extinguishing switches 46g, 46h, 46k, and 46l are elements for switching on/off of the switches in accordance with on/off of an input signal, and may be MOSFETs (field effect transistors) or IGBTs (insulated gate bipolar transistors), for example. The gate signal input from the gate signal generator 41 shown in fig. 1 is an input signal of the single-phase voltage type ac/dc conversion section 42. The single-phase voltage type ac/dc conversion unit 42 switches between ac and dc by switching on/off of four switches by using pulse signals and gate signals by four automatic arc extinguishing type switches 46g, 46h, 46k, and 46l, respectively. In addition, the ac terminals 21-1, 21-2 in fig. 5 correspond to the ac terminal 21 in fig. 1. Further, the series capacitor 47g is connected in parallel with the four switch groups. Instead of the series capacitor 47g, a capacitor such as a battery or an electric double layer capacitor may be connected.

The single-phase ac filter circuit 45 shown in fig. 5 is composed of an inductor 45a, a resistor 45b, and a capacitor 45c, and can be connected to the system by removing a high-frequency component caused by a gate signal in the single-phase voltage source ac/dc conversion unit 42 from the single-phase ac voltage in the single-phase voltage source ac/dc conversion unit 42. Depending on the operating conditions, the resistor 45b may not be connected.

Fig. 6 is an equivalent circuit as viewed from the ac terminal of the static reactive power compensator shown in fig. 3 and 4, and more specifically, as viewed from the point of arrow a excluding the inductance 45a of the single-phase ac filter circuit 45 in the circuit diagram shown in fig. 5, the equivalent circuit on the dc side is viewed. In fig. 6, vco (t) is an internal electromotive force, and the internal equivalent impedance is a parallel circuit of a resistance component Ri and an inductance component Li.

The single-phase voltage type ac/dc conversion unit 42 shown in fig. 3 and 4 may have an internal equivalent impedance by a control variable in the static type reactive power compensation device 11 of fig. 1, or the single-phase voltage type ac/dc conversion unit 42 shown in fig. 3 and 4 may have an internal equivalent impedance by connecting a resistor, a reactor, a single-phase transformer, or a combination thereof to the output of the single-phase voltage type ac/dc conversion circuits 40-1 and 40-2 of fig. 3 and 4. For example, the single-phase outputs of the single-phase voltage type ac/dc conversion circuits 40-1 and 40-2 may be connected in series with a resistor or a reactor, respectively, and when the resistor is connected, the reactor may be connected in series with the latter stage of the resistor, respectively. In addition, the single-phase output of the single-phase voltage type AC/DC conversion circuits 40-1, 40-2 may be connected to a single-phase transformer. In addition, when the single-phase outputs of the single-phase voltage type ac/dc conversion circuits 40-1 and 40-2 are connected to reactors, respectively, a single-phase transformer may be connected to a rear stage of the reactor. Further, when the single-phase outputs of the single-phase voltage type ac/dc conversion circuits 40-1 and 40-2 are connected to resistors, respectively, and reactors are connected in series to the rear stages of the resistors, respectively, the rear stages of the reactors may be connected to a single-phase transformer. Thus, by providing the single-phase voltage type ac/dc conversion circuit 40 with an internal equivalent impedance, the static type reactive power compensator 11 of fig. 1 can be connected to the power system as a load capable of changing the reactive power, and the ac voltage of the power system can be adjusted.

Further, by detecting the current or voltage between the single-phase ac filter circuit 45 and the single-phase voltage type ac/dc conversion unit 42 in the current detection circuit 43 or the voltage detection circuit 44 and generating the gate signal so that the difference between the PWM command and the output from the current detection circuit 43 or the voltage detection circuit 44 approaches zero in the gate signal generator 41, it is possible to control the current error within the allowable range or to change the output voltage in accordance with the PWM command.

[ Voltage instruction ]

The voltage command circuit 150-1 of fig. 1 sets or inputs the voltage (appropriate voltage) of the ac terminal 21 from the outside as the first shaft voltage command value V in advance₁ ^*And outputs the above value as a first shaft voltage command V₁ ^*. Further, the dc voltage detection circuit 161 detects the dc voltage converted by the single-phase voltage type ac/dc conversion circuit 40, and the detected dc voltage value is inputted to the voltage command circuit 150-1 and calculated by the adder circuit 154 as the dc voltage command value V_D ^*The second shaft voltage command V is generated by a command value calculation circuit 152 described later₂ ^*And output.

Fig. 7 to 9 are diagrams for explaining the operation of the instruction value operation circuit 152. The command value calculation circuit 152 in fig. 7 calculates the difference between the dc voltage command value and the dc voltage detection value by the low-pass characteristic circuit 152 a. The low-pass characteristic circuit 152a has a characteristic of several 1 s.

[ number 1]

Wherein, V₂ ^*Is the second shaft voltage command, V_D ^*Is a direct current voltage command value, V_DIs a DC voltage detection value, s is a variable of Laplace transform, Kdc is a proportional gain, T_KdcIs the first delay time constant, # denotes the Laplace transform. This command value calculation circuit has a better transient response than a command value calculation circuit using an integration circuit described later.

The command value calculation circuit 152 in fig. 8 calculates the difference between the dc voltage command value and the dc voltage detection value by the integration circuit 152 b. The integration circuit 152b has a characteristic of a number 2.

[ number 2]

Wherein, V₂ ^*Is the second shaft voltage command, V_D ^*Is a direct current voltage command value, V_DIs a DC voltage detection value, s is a variable of Laplace transform, T_dcIs the integral time constant, # denotes the Laplace transform. The present command value arithmetic circuit has a smaller dc voltage deviation in a steady state than the command value arithmetic circuit using the low-pass characteristic circuit.

The command value calculation circuit 152 in fig. 9 calculates the difference between the dc voltage command value and the dc voltage detection value by using a parallel circuit 152c in which a low-pass characteristic circuit 152a and an integration circuit 152b are connected in parallel. The parallel circuit 152c has a characteristic of a number 3.

[ number 3]

Wherein, V₂ ^*Is the second shaft voltage command, V_D ^*Is a direct current voltage command value, V_DIs a DC voltage detection value, s is a variable of Laplace transform, Kdc is a proportional gain, T_KdcIs a time constant of one time delay, T_dcIs the integral time constant, # denotes the Laplace transform. In the present command value arithmetic circuit, the low-pass characteristic circuit and the integrating circuit are connected in parallel, so that both transient response and steady-state deviation can be achieved.

Here, the voltage command circuit 150-1 shown in fig. 1 is used as the first shaft voltage command V₁ ^*The voltage (appropriate voltage) of the ac terminal 21 is set in advance or inputted from the outside as the first shaft voltage command value V₁ ^*However, as shown in the voltage command circuit 150-2 in fig. 10, the ac current detection circuit 34 and the ac power measurement circuit 140 may be provided, and a value calculated using the reactive power measured by the ac power measurement circuit 140 may be output as the first axis voltage command V₁ ^*。

The ac current detection circuit 34 shown in fig. 10 detects a single-phase ac current at the ac terminal 21 via the converter 38, and outputs the detected single-phase ac current to the ac power measurement circuit 140. The reactive power value of the single-phase output power at the ac terminal 21 calculated by the ac power measuring circuit 140 is input to the voltage command circuit 150-2, and the reactive power command value Q is calculated by the adder circuit 153^*The first shaft voltage command V is generated by a command value arithmetic circuit 151, which will be described later, while the difference from the reactive power value of the single-phase output power of the ac terminal 21 is obtained₁ ^*And output. Here, the reactive power command value Q^*Is a command value set in advance or input from the outside.

The instruction value arithmetic circuit 151 is advantageous in that it is similar to the instruction value arithmetic circuit 152Calculating reactive power command value Q by using combination circuit of low-pass filter characteristic circuit and integrating circuit^*And the difference of the reactive power value and generates a first shaft voltage command.

For example, if the command value arithmetic circuit 151 and the command value arithmetic circuit 152 are configured by a circuit in which a low-pass filter characteristic circuit and an integrating circuit are connected in parallel, the first shaft voltage command and the second shaft voltage command can be calculated by the following equations.

Wherein, V₁ ^*: first shaft voltage command [ V ]]，

V₂ ^*: second shaft Voltage instruction [ V ]]，

Q^*: reactive power instruction [ var ]]，

Q: the value of the reactive power [ var ],

V_D ^*: DC voltage command value V]，

V_D: DC voltage detection value [ V ]]，

K_Q: the gain of the first delay of the Q control,

T_KQ: q controlled primary delay time constant [ s ]]，

T_Q: integration time constant [ c ] of Q control]，

K_dc: the first delay gain of the dc voltage control,

T_Kdc: primary delay time constant [ s ] of DC voltage control]，

T_dc: integration time constant [ s ] of DC voltage control]，

Λ denotes the pralace transform.

Limiter 121 determines a first shaft voltage command V₁ ^*Upper limit and lower limit of (d), preventing input of an excessive first shaft voltage command V to the upper voltage control circuit 70₁ ^*。

In the following description, the static reactive power compensator including the voltage command circuit 150-1 will be described, but the same applies to the static reactive power compensator including the voltage command circuit 150-2.

The ac voltage detection circuit 31 in fig. 1 detects a single-phase ac voltage at the ac terminal 21 and outputs the detected voltage to the phase difference generation circuit 30, the lower voltage control circuit 60, and the upper voltage control circuit 70, respectively. Further, a low-pass filter may be provided in a preceding stage of the ac voltage detection circuit 31, and the single-phase ac voltage of the ac voltage detection circuit 31 may be detected by the low-pass filter. The PWM component can be removed from the single-phase ac voltage, thereby stabilizing the control of the static type reactive power compensation device 11. Further, it is possible to have a low-pass filter in the latter stage of the alternating voltage detection circuit 31, through which the output voltage from the alternating voltage detection circuit 31 is output. The control of the static reactive power compensator 11 can be stabilized by removing the PWM component from the output voltage from the ac voltage detection circuit 31.

[ Upper voltage control Circuit ]

The first shaft voltage command V from the voltage command circuit 150-1 is input to the upper voltage control circuit 70 in fig. 1₁ ^*And a second shaft voltage command V₂ ^*The generated electric angle 57 from the frequency control circuit 50 described later, the phase difference voltage from the phase difference generation circuit 30, and the single-phase ac voltage at the ac terminal 21. The upper level voltage control circuit 70 outputs the first axis voltage command V having the amplitude and frequency of the single-phase ac voltage at the ac terminal 21 close to the first axis voltage based on these inputs₁ ^*And a second shaft voltage command V₂ ^*The voltage command signal and the frequency command signal generated in the manner of (1). Can not directly control the upper voltageThe circuit 70 inputs the first shaft voltage command V₁ ^*And a second shaft voltage command V₂ ^*By determining a first shaft voltage command V₁ ^*And a second shaft voltage command V₂ ^*And a limiter 121 for the upper and lower limits of (d). Specifically, as shown in fig. 11, the first multiplier 73 multiplies a value obtained by multiplying √ 2 by the sine value of the generated electrical angle 57 from the frequency control circuit 50, and the first axis voltage command V₁ ^*A multiplication calculation is performed. The first subtractor 71a subtracts the single-phase alternating-current voltage at the alternating-current terminal 21 from the signal from the first multiplier 73. The first upper control amplifier 72a approaches the first shaft voltage command V with the single-phase ac voltage at the ac terminal 21₁ ^*The signal from the first subtractor 71a is amplified and output as a voltage command signal. Further, the second subtractor 71b derives the second shaft voltage command V from the second shaft voltage command V₂ ^*The phase difference voltage from the phase difference generation circuit 30 is subtracted from the value multiplied by √ 2. The second upper control amplifier 72b approaches the second shaft voltage command V at the frequency of the single-phase ac voltage at the ac terminal 21₂ ^*The signal from the second subtractor 71b is amplified and output as a frequency command signal.

Thus, even if the amplitude and frequency of the power system vary, errors of the amplitude and frequency of the single-phase ac voltage of the static reactive power compensator 11 with respect to the amplitude and frequency can be detected. Here, the first upper control amplifier 72a and the second upper control amplifier 72b may add a low-pass filter element to the outputs from the first subtractor 71a and the second subtractor 71 b. This stabilizes the feedback loop. Further, a limiter may be provided at a stage subsequent to the first and second upper control amplifiers 72a and 72b, and outputs from the first and second upper control amplifiers 72a and 72b may be outputted through the limiter. It is possible to prevent over-output and stabilize control.

[ phase difference generating Circuit ]

The phase difference generating circuit 30 of fig. 1 generates a single-phase alternating current with the alternating current terminal 21Voltage V_FIL(t) a phase difference voltage corresponding to a phase difference between the internal electromotive force of the single-phase voltage type ac/dc conversion circuit 40 and the phase difference voltage. Fig. 12 is an example of a schematic configuration diagram of the phase difference generation circuit 30. The phase difference generation circuit 30 includes: a phase-delayed single-phase ac generator 35 that generates a delayed single-phase ac by delaying the single-phase ac voltage input from the terminal 33-1 by a predetermined phase; a phase difference voltage generator 36 that generates a phase difference voltage from the single-phase ac voltage input from the terminal 33-1, the voltage of the delayed single-phase ac from the phase-delayed single-phase ac generator 35, and the value input from the terminal 33-3; and a terminal 33-2 outputting a phase difference voltage. In fig. 12, the phase-delayed single-phase alternating current generator 35 delays the phase of the delayed single-phase alternating current by about 90 °, but may be any angle as long as the phase is not delayed by 0 ° and 180 °.

The single-phase AC voltage V detected by the AC voltage detection circuit 31 is input to the terminal 33-1_FIL(t) of (d). The generated electrical angle 57 generated by the frequency control circuit 50 described later is input to the terminal 33-3. Single-phase ac voltage V at ac terminal 21_FIL(t) is represented by the number 5.

[ number 5]

Wherein, ω is_s: angular frequency [ rad/s]，θ_s: phase angle [ rad ]]，E_s: effective value [ V ]]. The phase angle is based on the internal electromotive force.

Angular frequency omega of single-phase AC voltage at AC terminal 21_sAnd the reference angular frequency omega of the single-phase voltage type AC/DC conversion circuit 40_coEqual, single-phase AC voltage V_FIL(t) and phase-delayed single-phase AC voltage V "_FIL(t) has a phase difference of 90 DEG, and the phase-delayed single-phase alternating voltage V generated by the phase-delayed single-phase alternating current generator 35 "_FIL(t) is represented by the number 6.

[ number 6]

The phase difference voltage generator 36 generates a single-phase alternating voltage V from the alternating current_FIL(t) phase-delayed single-phase AC voltage V "_FIL(t) outputting the phase difference voltage V from the generated value generated by the frequency control circuit 50_q(t) of (d). Phase difference voltage V_q(t) is represented by the number 7.

[ number 7]

If theta is greater than theta_iAngular velocity of and ω_sIf equal, equation 7 becomes a constant. Theta_sIs the phase difference of the voltages at two ends of the internal equivalent impedance, so the phase difference is generally smaller. Thus, V_q(t) may be approximated by the number 8.

[ number 8]

The phase difference generation circuit 30 outputs the generated phase difference voltages to the frequency control circuit 50 and the upper voltage control circuit 70, respectively. In addition, here, only ω is shown_sAnd omega_coThe same approximate solution can be obtained even when the values are not equal, and no problem exists in practical use.

[ frequency control Circuit ]

The frequency control circuit 50 determines the electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit 40 based on the reference frequency of the single-phase ac output frequency of the ac terminal 21, the frequency command signal from the upper voltage control circuit 70, and the output signal from the phase difference generation circuit 30. Specifically, as shown in fig. 11, the second adder 56 adds the frequency command signal from the upper voltage control circuit 70 and the phase difference voltage from the phase difference generation circuit 30. The loop filter 53 filters the frequency component of the signal output from the second adder 56, and passes a low-pass component, which is a component related to the frequency difference of the single-phase ac voltage. The low-pass filter element added to the loop filter 53 is a delay element such as a primary delay element, for example. This stabilizes the feedback loop.

The third adder 58 adds the reference frequency output from the reference frequency setter 51 and the output value of the loop filter 53. The time integrator 55 time-integrates the output from the third adder 58. The output from the third adder 58 is time-integrated by the time integrator 55, and the intrinsic angle θ can be obtained as_iGenerates an electrical angle 57.

The generated electrical angle 57 becomes an electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit 40 by the second multiplier 65 of the lower voltage control circuit 60. This makes it possible to vary the rotation angle according to the frequency of the power system.

Here, as described above, the phase difference generation circuit 30 outputs the phase difference voltage corresponding to the phase difference between the single-phase ac voltage at the ac terminal 21 and the internal electromotive force of the single-phase voltage type ac/dc conversion circuit 40. Therefore, the signal processing in the phase difference generating circuit 30 corresponds to phase comparison processing for comparing the single-phase ac voltage with the phase of the generated electrical angle 57 from the frequency control circuit 50. The signal processing for adding and integrating the reference frequency from the reference frequency setter 51 and the output value from the loop filter 53 corresponds to the signal processing of a Voltage Controlled Oscillator (Voltage Controlled Oscillator), and the value of the generated electrical angle 57 is variable according to the output Voltage from the loop filter 53. Therefore, the phase difference generation circuit 30 and the frequency control circuit 50 perform the PLL operation of synchronizing the generated electrical angle 57 with the frequency of the single-phase ac voltage at the ac terminal 21 as a whole.

[ Next Voltage control Circuit ]

The lower-level voltage control circuit 60 in fig. 1 outputs, as a PWM command, a signal generated so that the amplitude, frequency, and phase of the single-phase ac voltage approach the reference voltage of the single-phase ac voltage at the ac terminal 21, and the synthesized value of the voltage command signal and the electrical angle command signal, based on the single-phase ac voltage at the ac terminal 21, the electrical angle command signal including the generated electrical angle 57 of the frequency control circuit 50, and the voltage command signal from the upper-level voltage control circuit 70. The reference voltage is set in advance by the reference voltage setting unit 61. The reference voltage is a reference for the amplitude of the single-phase ac voltage at the ac terminal 21.

Specifically, as shown in fig. 11, the reference voltage setter 61 sets and outputs a reference voltage. The second multiplier 65 multiplies the value obtained by multiplying √ 2 by the sine value of the generated electrical angle 57 from the frequency control circuit 50 by the reference voltage from the reference voltage setter 61. The first adder 62 adds the voltage command signal from the upper voltage control circuit 70 and the signal output from the second multiplier 65, and outputs the result. The third subtractor 63 subtracts the signal from the ac voltage detection circuit 31 from the signal output from the first adder 62. The voltage controller 64 controls the signal output from the third subtractor 63 so that the single-phase ac voltage at the ac terminal 21 approaches the combined value of the reference voltage, the voltage command signal, and the electrical angle command signal, and outputs the signal as a PWM command.

This makes it possible to compensate for the error detected by the upper voltage control circuit 70, and to control the amplitude and phase of the static reactive power compensator 11 so that the amplitude and phase of the single-phase ac voltage of the static reactive power compensator 11 match the amplitude and phase of the power system. The voltage controller 64 may use, for example, an amplifier. Here, a low pass filter may be further provided between the third subtractor 63 and the voltage controller 64, and an output from the third subtractor 63 may be output through the low pass filter. The control of the voltage controller 64 can be stabilized. Further, a voltage limiter may be further provided between the third subtractor 63 and the voltage controller 64 (between the low-pass filter and the voltage controller 64 when the low-pass filter is provided at this position), and the output from the third subtractor 63 may be output through the voltage limiter. It is possible to suppress a transient change in the output voltage at the time of starting the static type reactive power compensation device 11.

Fig. 13 is a schematic configuration diagram of a static reactive power compensator according to another embodiment.

The static type reactive power compensation device 11 of fig. 13 further adds the outputs from the smoothing current compensator 66, the PWM current deviation compensator 67 and the feed forward amplifier 68 to the output from the voltage controller 64 of the static type reactive power compensation device 11 shown in fig. 11 by a fourth adder 69. In this case, any one of the single-phase voltage source ac/dc conversion circuits 40-1 and 40-2 described with reference to fig. 3 or 4 may be used as the single-phase voltage source ac/dc conversion circuit 40. Therefore, fig. 13 shows that the single-phase voltage type ac/dc conversion circuits 40-1 and 40-2 in fig. 3 or 4 are used.

The filter current compensator 66 outputs a current compensation value that is defined so as to compensate for a current loss portion of the single-phase ac filter circuit 45 (fig. 3 or 4) in the single-phase voltage type ac/dc conversion circuit 40. Thus, in the static reactive power compensator 11, the filter current compensator 66 is provided with a current loss portion in the single-phase ac filter circuit 45 shown in fig. 3 or 4 in advance, and adds the current loss portion to the output vector from the voltage controller 64, thereby compensating for the current loss portion. The PWM current deviation compensator 67 outputs a current deviation compensation value that is defined to compensate for a current deviation portion of the single-phase ac current from the single-phase voltage type ac/dc conversion circuit 40. Thus, in the static reactive power compensator 11, the PWM current deviation compensator 67 is provided with a current deviation portion in the single-phase voltage type ac/dc converter circuit 40 when the PWM command is set to zero, and the current deviation portion can be compensated by adding the current deviation portion to the output vector from the voltage controller 64. The value of the single-phase ac current detected by the ac current detection circuit 34 is input to the feed-forward amplifier 68, amplified by a predetermined feed-forward gain so as to compensate for the current corresponding to the load on the ac terminal 21, and output. Thus, in the static reactive power compensator 11, the single-phase ac current at the ac terminal 21 is detected by the ac current detection circuit 34, and the detected value is added to the output value from the voltage controller 64 by the feed forward amplifier 68, whereby a stable output voltage can be generated even if the load current changes.

The single-phase ac voltage value at the ac terminal 21 detected by the ac voltage detection circuit 31 and the single-phase ac current value at the ac terminal 21 detected by the ac current detection circuit 34 are input to the ac power measurement circuit 140 of fig. 10, and the active power value and the reactive power value of the single-phase output power at the ac terminal 21 are calculated.

Specifically, as shown in fig. 14, in ac power measuring circuit 140, a multiplier 147-1 multiplies the voltage and the current at the power measuring point measured by voltage detecting circuit 31 and current detecting circuit 34, respectively, and the product of the multiplication is passed through low-pass filter 149-1, and active power value is measured by active power value measuring circuit 145. Further, a function for shifting the current phase of the power measurement point by 90 degrees by the current phase delay circuit 143 is generated, the multiplier 147-2 multiplies the voltage of the power measurement point by the function, the product of the multiplication is passed through the low-pass filter 149-2, and the reactive power value is measured by the reactive power value measurement circuit 146.

Further, the ac power measuring circuit 140 may have the configuration shown in fig. 15. The ac power measurement circuit 140 includes: a reference frequency circuit 141 that generates a reference frequency; a voltage phase delay circuit 142 that delays the phase of a measured ac voltage, which is an ac voltage at a power measurement point, based on a reference frequency from the reference frequency circuit 141 to generate a delayed ac voltage; a current phase delay circuit 143 that delays the phase of a measured ac current, which is an ac current at a power measurement point, based on a reference frequency from the reference frequency circuit 141 to generate a delayed ac current; and a power arithmetic circuit 144. In the electric power operation circuit 144, the adder 148-1 adds the value of the product of the measurement ac voltage and the measurement ac current multiplied by the multiplier 147-1 to the value of the product of the delay ac voltage from the voltage phase delay circuit 142 and the delay ac current from the current phase delay circuit 143 multiplied by the multiplier 147-2, and the value of the addition is passed through the low-pass filter 149-1 to measure the active power value by the active power value measurement circuit 145. Further, the subtraction is performed by the subtractor 148-2, the product value of the measurement ac voltage multiplied by the delayed ac current from the current phase delay circuit 143 is subtracted by the multiplier 147-3 from the product value of the measurement ac current multiplied by the delayed ac voltage from the voltage phase delay circuit 142 by the multiplier 147-4, and the reactive power value is measured by the reactive power value measurement circuit 146 by passing the subtraction calculation value of the subtraction through the low pass filter 149-2. By adding the product value of the delayed ac voltage and the delayed ac current to the product value of the measured ac voltage and the measured ac current, the frequency component included in the active power value can be reduced by two times. Further, by subtracting the product value of the measured alternating voltage and the delayed alternating current from the product value of the measured alternating current and the delayed alternating voltage, the frequency component included in the reactive power value can be reduced by two times. Therefore, the measurement accuracy of the active power value and the reactive power value can be improved, and the active power value and the reactive power value can be controlled with high accuracy.

[ examples ]

Next, an example in which the static var compensator 11(STATCOM) and the low-voltage household connection on the single-phase side of the power distribution system are required will be described with reference to fig. 16. Fig. 16 (a) is a diagram showing the distance between the high-voltage distribution line voltage and the distribution substation. FIG. 16 (B) shows the flow direction on the side of the cross flow with the single phaseEach system (n)₁、n₂、···n_m) The low voltage of the connection requires a diagram of an example in which a plurality of STATCOM's are provided in the home. Fig. 16 (C) is a diagram showing the distance between the low-voltage distribution line voltage and the low-voltage transformer disposed in the high-voltage distribution line.

The high-voltage distribution line voltage shown in fig. 16 (a) is a value converted into a low-voltage distribution line voltage. Since no power is supplied from the PV at night or in rainy days, the voltage is lower the farther from the distribution substation. On the other hand, since power is supplied from the PV during good weather, the voltage of a portion away from the distribution substation may rise. For example, as shown in fig. 16 (B), if there are a plurality of houses in which Power Conditioners (PCS) for solar power generation (PV) are installed in each system on the single-phase current side, the high-voltage distribution line voltage may exceed 107V, which is the upper limit value of 101 ± 6V that is an appropriate voltage range, at the end of the system away from the distribution substation. For example, in a distribution system (n) leaving a distribution substation_m) In the case where the voltage of the high-voltage distribution line corresponds to 107V in low-voltage conversion, the voltage of the low-voltage distribution line may further increase to a value greater than 107V at the end of the single-phase ac that is separated from the high-voltage distribution line.

When the voltage of the power system changes in this way, a difference occurs between the voltage at the ac terminal 21 and the internal electromotive force, and therefore the static reactive power compensator 11 performs PWM control to decrease or increase the voltage on the ac terminal side of the single-phase voltage type ac/dc conversion circuit 40. For example, when the voltage of the power system rises, the stationary reactive power compensator 11 lowers the voltage on the ac terminal side of the single-phase voltage source ac/dc conversion circuit 40, so that the voltage of the single-phase voltage source ac/dc conversion circuit 40 becomes lower than the voltage of the single-phase ac power system, and the reactive current flows from the single-phase ac power system into the stationary reactive power compensator 40, whereby the voltage of the single-phase ac power system can be lowered.

Generally, when the STATCOM is provided on the high-voltage distribution side, the voltage rise on the high-voltage distribution line side can be suppressed within an appropriate voltage range, but it is difficult to suppress the voltage rise on the high-voltage distribution line side to be within an appropriate voltage rangeThe voltage rise in each system on the low voltage single phase current side is locally controlled. In the above embodiment, since the STATCOM can be provided on the single cross flow side of the low-voltage side, each system (n) on the low-voltage single cross flow side can be locally controlled₁、n₂、···n_m) Voltage in (2) rises.

Claims

1. An electrical power system comprising: the high-voltage distribution line is used for distributing three-phase high-voltage alternating current;

a plurality of transformers connected to the high-voltage distribution line, and converting the three-phase high-voltage alternating current into a single-phase low-voltage alternating current having a voltage lower than that of the three-phase high-voltage alternating current;

a plurality of low-voltage distribution lines for respectively distributing the single-phase low-voltage alternating current converted by the transformer; and

any number of static reactive power compensation devices, alternating current terminals connected to the low voltage distribution line,

the power system is characterized in that the static type reactive power compensation device includes:

a single-phase voltage type ac/dc conversion circuit that has an internal electromotive force and an internal equivalent impedance when viewed from the ac terminal, and converts between single-phase ac power and dc power from a single-phase ac power system connected to the ac terminal, in accordance with a pulse width of a gate signal generated based on a PWM command;

a voltage command circuit that outputs a first shaft voltage command that is a target value of the amplitude of the single-phase ac voltage at the ac terminal, sets a dc voltage command value that is higher than a peak value of the single-phase ac voltage, detects the dc voltage converted by the single-phase voltage type ac/dc conversion circuit, calculates a difference between the dc voltage command value and a dc voltage detection value, and outputs a second shaft voltage command;

a phase difference generation circuit having a phase-delayed single-phase ac generator that delays a phase of a single-phase ac voltage at the ac terminal to generate a delayed single-phase ac, the phase difference generation circuit generating a voltage corresponding to a phase difference between the single-phase ac voltage at the ac terminal and the internal electromotive force of the single-phase voltage type ac/dc conversion circuit, based on the delayed single-phase ac;

an upper-level voltage control circuit that outputs a voltage command signal generated such that an amplitude of a single-phase ac voltage at the ac terminal approaches the first shaft voltage command and a frequency command signal generated such that a voltage corresponding to the phase difference from the phase difference generation circuit approaches the second shaft voltage command, based on the first shaft voltage command and the second shaft voltage command from the voltage command circuit, a voltage corresponding to the phase difference from the phase difference generation circuit, and the single-phase ac voltage at the ac terminal;

a frequency control circuit that sets a reference frequency that is a reference of a frequency of a single-phase alternating current at the alternating-current terminal, and determines an electrical angle of the internal electromotive force of the single-phase voltage type ac/dc conversion circuit and generates a generated electrical angle, based on the reference frequency, a frequency command signal from the upper voltage control circuit, and a voltage corresponding to the phase difference generated by the phase difference generation circuit; and

a lower-level voltage control circuit that sets a reference voltage as a reference of a voltage amplitude of a single-phase alternating current at the alternating-current terminal, adds a voltage command signal from the upper-level voltage control circuit to a value obtained by multiplying the reference voltage by the signal based on the generated electrical angle from the frequency control circuit to obtain the internal electromotive force, and outputs a difference between the internal electromotive force and the single-phase alternating current voltage as the PWM command,

the power system controls the voltage of the single-phase low-voltage alternating current of the low-voltage distribution line to fall within an appropriate voltage range by controlling the voltage of the alternating-current terminal to fall when the voltage of the low-voltage alternating current rises and to rise when the voltage of the low-voltage alternating current falls, and transmitting a reactive current between the low-voltage distribution lines through the alternating-current terminal, based on a difference between the voltage of the single-phase low-voltage alternating current of the low-voltage distribution line and the internal electromotive force.

2. The power system according to claim 1, wherein the first shaft voltage command output by the voltage command circuit is a fixed value set in advance.

3. The power system of claim 1,

further comprising:

an alternating current detection circuit that detects an alternating current at the alternating current terminal; and

an alternating current power measurement circuit that measures reactive power at the alternating current terminal,

the voltage command circuit calculates a difference between the reactive power measured by the ac power measurement circuit and a preset reactive power command value to generate the first shaft voltage command.