JP5753738B2 - Reactive power compensation device, reactive power compensation method, and reactive power compensation program - Google Patents

Description

  The present invention relates to a reactive power compensation device, a reactive power compensation method, and a reactive power compensation program.

  In recent years, from the viewpoint of environmental problems, the introduction of distributed power sources using natural energy, such as solar power generation systems, has progressed, and is expected to increase in the future. In a distribution system, when a large number of these distributed power sources are introduced and a reverse power flow from the distributed power source occurs, it is expected that the system voltage will increase and deviate from the specified voltage.

  A reactive power compensator such as STATCOM (STATic synchronous COMPensator) is known as a technique for compensating for such voltage fluctuations in the distribution system. Since this STATCOM can control reactive power at high speed and continuously, it can cope with a rapid change in system voltage at high speed.

  STATCOM has a circuit configuration in which a circuit module called a cell in which a capacitor and a switching element are connected is connected in multiple stages to each phase of a distribution system (so-called MMC (Modular Multilevel Converter)). In Non-Patent Document 1, in a circuit configuration in which cells are connected in multiple stages to each phase of a distribution system, secondary batteries such as nickel metal hydride batteries and lithium ion batteries are further provided in parallel with capacitors of each cell, and wind power generation and the like are distributed. A cascade PWM converter has been proposed in which the power generated by the power source can be stored in the secondary battery of each cell. In Non-Patent Document 1, a zero-phase voltage is superimposed on an AC voltage output from a converter, and unbalanced power is allowed to flow into and out of each phase cell, thereby remaining the secondary battery in each phase cell. A technique for performing capacity balance control is described.

Shigenori Inoue et al., "6.6 kV Transless Power Storage System Using Cascade PWM Converter and Secondary Battery", IEEJ Transactions D, 2009, Vol. 129, P67-P76

  By the way, when STATCOM outputs an unbalanced negative-phase current to compensate for the negative-phase voltage of the system due to voltage unbalance, a large unbalance occurs in the voltage of the cell capacitor between the phases. Thus, when a large unbalance occurs in the voltage of the capacitor between the phases, STATCOM needs to increase the breakdown voltage by using a high breakdown voltage element or increasing the separation distance between various elements. Become.

  In Non-Patent Document 1, when a negative phase current is output, unbalanced power is formed by the negative phase current and the positive phase current. Therefore, the AC voltage to be output does not include a negative phase component.

  The present invention has been made in view of the above, and an object of the present invention is to provide a reactive power compensator that can suppress voltage imbalance generated in capacitors of each phase due to the output of reverse-phase current.

  In order to solve the above-described problems and achieve the object, the reactive power compensator of the present invention accumulates and discharges power for each phase of the distribution system according to the switching element and the switching element being turned on / off. One or more circuit modules each having a capacitor are provided in series, and a compensation power output unit that outputs compensation power including a reverse-phase current that compensates for a reverse-phase AC voltage generated in the distribution system, and the distribution system And a zero-phase voltage generator for generating a zero-phase voltage in each phase of the distribution system so that the amount of active power entering and exiting the circuit module becomes zero before and after one cycle of AC power flowing through .

  ADVANTAGE OF THE INVENTION According to this invention, the imbalance of the voltage which generate | occur | produces in the capacitor | condenser of each phase by the output of a negative phase current can be suppressed.

FIG. 1 is a diagram illustrating an example of a conceptual configuration of a distribution system including a reactive power compensator. FIG. 2 is a diagram illustrating an example of a schematic configuration of STATCOM. FIG. 3 is a diagram showing the relationship between the instantaneous value current i _αβ and ΔE _αβ . FIG. 4 is a diagram illustrating an example of a carrier wave and a modulated wave. FIG. 5 is a diagram illustrating an example of a change in the zero-phase voltage A _VO when the magnitude of the reverse-phase current A _I2 and the phase θ _I2 are changed. FIG. 6 is a diagram illustrating an example of a change in the zero-phase voltage when the magnitude and phase of the positive-phase current are changed. FIG. 7A is a diagram showing the relationship between the instantaneous value current i _αβ and ΔE _αβ when STATCOM outputs only the positive phase current. FIG. 7B is a diagram showing the relationship between the instantaneous value current i _αβ and ΔE _αβ when STATCOM outputs positive and negative phase currents of different magnitudes. FIG. 7C is a diagram illustrating a relationship between the instantaneous value current i _αβ and ΔE _αβ when STATCOM outputs a positive phase current and a negative phase current having the same magnitude. FIG. 8 is a diagram illustrating an example of a schematic configuration of the zero-phase voltage generation unit. FIG. 9 is a block diagram showing the flow of reactive power compensation control. FIG. 10 is a flowchart showing the procedure of reactive power compensation control processing. FIG. 11 is a diagram illustrating a schematic configuration of a power distribution system used in the simulation. FIG. 12 is a diagram showing the main specifications of STATCOM for which simulation was performed. FIG. 13A is a diagram illustrating a change in voltage of a capacitor of each phase of STATCOM. FIG. 13B is a diagram illustrating a change in current output from each phase of STATCOM. FIG. 13C is a diagram illustrating a change in voltage of each phase of the distribution system. FIG. 14A is a diagram illustrating a change in voltage of a capacitor of each phase of STATCOM. FIG. 14B is a diagram illustrating a change in current output from each phase of STATCOM. FIG. 14C is a diagram illustrating a change in voltage of each phase of the distribution system. FIG. 15A is a diagram illustrating a change in voltage of a capacitor of each phase of STATCOM. FIG. 15B is a diagram illustrating a change in current output from each phase of STATCOM. FIG. 15C is a diagram illustrating a change in voltage of each phase of the distribution system. FIG. 16 is a diagram illustrating an example of a circuit configuration of the delta connection MMC. FIG. 17 is a diagram illustrating an example of a circuit configuration of a double Y-connection bridge cell type MMC. FIG. 18 is a diagram illustrating an example of a circuit configuration of a double Y-connection chopper cell type MMC. FIG. 19 is a diagram illustrating a computer that executes a reactive power compensation program.

  Embodiments of a reactive power compensator according to the present invention will be described below in detail with reference to the drawings. In the following, STATCOM will be described as a reactive power compensator, but the present invention is not limited to this embodiment.

  The configuration of the power distribution system according to the first embodiment will be described. FIG. 1 is a diagram illustrating an example of a conceptual configuration of a distribution system including a reactive power compensator. In the example shown in FIG. 1, a STATCOM 10 is connected to the end of a power distribution system 12 extending from a power distribution substation 11 that transforms power for power distribution. Note that the STATCOM 10 only needs to be provided in any of the power distribution systems 12, and is not necessarily provided at the terminal.

  The distribution substation 11 transforms the electric power supplied from the host system into a predetermined distribution voltage (for example, 6600 V) and supplies it to the distribution system 12. Consumers and distributed power sources (not shown) are connected to the power distribution system 12. In the power distribution system 12, voltage fluctuation may occur due to power from the distributed power source. The STATCOM 10 is a device that can continuously supply and consume reactive power using a self-excited semiconductor element, and compensates for voltage fluctuations generated in the distribution system. Since the STATCOM 10 can control reactive power continuously at high speed, it can cope with a rapid change in the voltage of the power distribution system of the power distribution system 12 at a high speed.

  Next, the configuration of the STATCOM 10 according to the present embodiment will be described. FIG. 2 is a diagram illustrating an example of a schematic configuration of STATCOM. In the example illustrated in FIG. 2, the STATCOM 10 includes a compensation power output unit 20 that outputs compensation power including a negative phase current that compensates a negative phase AC voltage generated in the distribution system. The compensation power output unit 20 has a so-called Y-connection MMC circuit configuration in which a series circuit in which circuit modules 21 called cells are connected in series in three stages is Y-connected for each phase of the distribution system 12. . The circuit module 21 has a so-called bridge cell type circuit configuration in which two series circuits 24 in which two switching elements 23 are connected in series are connected in parallel to the capacitor 22. Each switching element 23 is provided with a diode 25 connecting both ends.

  In the example illustrated in FIG. 2, the STATCOM 10 further includes a zero-phase voltage generation unit 30. The zero-phase voltage generation unit 30 controls the ground voltage of the connection part where the circuit module 21 is Y-connected by controlling the ON / OFF timing of each switching element 23 provided in the circuit module 21 of each phase. Then, a zero-phase voltage that compensates for the voltage imbalance of the capacitor 22 of each phase of the compensation power output unit 20 is generated.

Here, the voltage imbalance of the capacitor 22 of each phase generated in the STATCOM 10 according to the present embodiment will be described. When the STATCOM 10 outputs a reverse phase current, the voltage of the capacitor 22 of each phase of the compensation power output unit 20 becomes unbalanced. Here, each phasor, the zero-phase voltage compensating power output unit 20 outputs a V _0, a positive phase voltage and V _1, a reverse-phase voltage and V _2, the A-phase voltage is V _a, B-phase the voltage is _{V b,} the C-phase voltage and V _c. Also, in each phasor display, the zero-phase current output from the compensation power output unit 20 is I ₀ , the positive-phase current is I ₁ , the reverse-phase current is I ₂ , the A-phase current is I _a , and the B-phase current It was a _{I b,} the C phase current to I _c.

Thereby, it can express like the following formula | equation (1)-(4).

Here, the compensation power output section 20 is intended to not output the zero-phase current I _0, the zero-phase current I ₀ and 0. This is based on the idea that since the distribution system 12 is ungrounded, the zero-phase current hardly flows even if the compensation power output unit 20 outputs the zero-phase voltage V ₀ . In fact, due to the ground capacity of the system, etc., when the compensated power output unit 20 outputs the zero-phase voltage, a small amount of zero-phase current flows through the system, so it is confirmed that the flowing zero-phase current is not given to the system. There is a need. If the flowing zero-phase current is so large that it cannot be ignored, the STATCOM 10 needs to be connected to the system via a transformer.

When the zero-phase current I _{0 is set} to 0 based on the above idea, it can be expressed as the following equations (5) to (10).
V _a = V ₀ + V ₁ + V ₂ (5)
V _b = V ₀ + a ² · V ₁ + a · V ₂ (6)
V _c = V ₀ + a · V ₁ + a ² · V ₂ (7)
I _a = I ₁ + I ₂ (8)
I _b = a ² · I ₁ + a · I ₂ (9)
I _c = a · I ₁ + a ² · I ₂ (10)

Here, the apparent power of the A-phase power distribution system and S _a, the apparent power of the B-phase when the S _b, the apparent power of the C phase was S _c, as shown in the following expression (11) - (13) I can express.

The active power and reactive power flowing through the A phase, B phase, and C phase of the distribution system are the real part and the imaginary part of the apparent power. For this reason, in the STATCOM 10 of Y connection, the energy balance for each phase is different, and the voltage of the capacitor 22 of each phase is unbalanced. However, when the compensation power output unit 20 outputs only the positive phase voltage and the positive phase current, the following equation (14) is obtained, and the voltage of the capacitor 22 of each phase is not unbalanced.

  The normal distribution system has a voltage unbalance rate that is not so large, and the distribution system voltage is mostly a positive phase voltage. For this reason, when the compensation power output unit 20 outputs only the positive phase current, the capacitor voltage imbalance hardly occurs.

  However, the actual compensated power output unit 20 gradually unbalances the voltage of the capacitor 22 of the circuit module 21 of each phase due to variations in elements, a slight amount of reversed phase, and the like. For this reason, it is necessary to perform interphase voltage control.

  When an imbalance occurs in the interphase voltage of the capacitor, the interphase imbalance of the DC capacitor voltage can be eliminated by controlling the zero phase voltage of the STATCOM 10. The basic concept of control is as follows. In addition, the following is the content regarding the instantaneous value of each phase of the distribution system, and in order to make it easy to distinguish from the above-described content regarding the effective value, each phase of the distribution system is described as an expression regarding the R phase, the S phase, and the T phase. .

The R-phase capacitor voltage of the distribution system is E _r , the S-phase capacitor voltage is E _s , and the T-phase capacitor voltage is E _t . Further, when the average value of the capacitor voltage is E _ave and the deviation of the voltage from the average value E _{ave of} the capacitor voltages E _r, E _s, E _t is ΔE _r, ΔE _s, ΔE _{t ,} respectively, 15) to (19) are established.
E _r + E _s + E _t = 3E _ave (15)
E _r −E _ave = ΔE _r (16)
E _s −E _ave = ΔE _s (17)
E _t −E _ave = ΔE _t (18)
ΔE _r + ΔE _s + ΔE _t = 0 (19)

When three-phase to two-phase conversion is performed on the voltage deviations ΔE _r, ΔE _{s and} ΔE _t from the average value E _ave , the following equations (20) and (21) are obtained.

Here, ΔE _αβ is defined as a vector having ΔE _{α and} ΔE _β as a pair. Further, _assuming that the R phase current of the compensation power output unit 20 is i _r , the S phase current is i _s , and the T phase current is i _t , the R phase current i _{r, the} S phase current i _s, Doing three-phase two-phase conversion on the T-phase current i _t, made similarly to the following equation (22).

Here, i _αβ is defined as a vector with i _{α and} i _β as a pair, and this is called an instantaneous current. If the _zero- phase voltage V ₀ is output to each phase while the instantaneous value current i _αβ is flowing, electric power represented by the following equation (23) is generated in the R phase, the S phase, and the T phase.

  It is assumed that the power distribution system is ungrounded and that the zero-phase current flowing into the system is negligible even when the zero-phase voltage is output from the STATCOM 10. In this case, in the distribution system, the line voltage does not change even if the zero-phase voltage is superimposed. Therefore, even if the zero-phase voltage is superimposed, the current flowing from the STATCOM 10 to the system is not affected. However, if the flowing zero-phase current becomes so large that it cannot be ignored, such as the ground capacity of the distribution line is large, it is necessary to consider grid interconnection through a transformer.

When the electric power generated by the superimposed zero-phase voltage V ₀ is three-phase to two-phase converted, the following equation (24) is obtained.

That is, by outputting the zero-phase voltage V ₀ , the capacitor voltage can be corrected in the current i _αβ direction. FIG. 3 is a graph showing the relationship between the instantaneous value current i _αβ and ΔE _αβ . The instantaneous value current i _αβ circulates at a period of current flowing through the distribution system (for example, 50 Hz or 60 Hz). When this instantaneous value current i _αβ is directed in the same direction as ΔE _αβ , a negative zero-phase voltage is superimposed, and when the instantaneous value current i _αβ is directed in the opposite direction to ΔE _αβ , a positive zero-phase is applied. By superimposing the voltage, the interphase imbalance of the capacitor voltage can be canceled. In this embodiment, the STATCOM 10 phase voltage control is performed based on the above principle.

Next, how this interphase voltage control is performed will be described. In the phasor display, when the average power value of the three-phase apparent power output from the STATCOM 10 is S _ave and the deviation of the power of each phase from the power average value S _ave is ΔS _a , ΔS _b , ΔS _c , From (11) to (13), conversion can be performed as in the following formulas (25) to (28).

Here, when three-phase to two-phase conversion is performed on the power shifts ΔS _a , ΔS _b , ΔS _c of each phase, the following equation (29) is obtained.

Here, if ΔS _α and ΔS _β have a real part of 0, an unbalance of active power does not occur, and an unbalance does not occur in the voltage between the capacitors 22 of each phase.
Re [ΔS _α ] = 0 (30)
Re [ΔS _β ] = 0 (31)

Therefore, if V ₀ satisfying the above equations (30) and (31) exists, compensation is possible. Here, V ₀ satisfying the following relationship may be obtained from the equations (29), (30), and (31).

From equations (32) and (33), V ₀ is obtained as in equation (34) below.

As shown in the equation (34), the superposed zero-phase voltage V ₀ has a solution except when I ₁ = I ₂ . That is, negative phase compensation is possible in principle except when the magnitude of the positive phase current I ₁ is equal to the magnitude of the negative phase current I ₂ .

Also, the zero phase component of the distribution system voltage is V _S0 , the positive phase component of the distribution system voltage is V _S1, and the reverse phase component of the distribution system voltage is V _S2 . In general, since the resistance at the connected reactor is much smaller than the resistance due to the inductance, the following equations (35) to (37) are assumed if ignored.
V ₀ = V _S0 (35)
V ₁ = V _S1 + j · X · I ₁ (36)
V ₂ = V _S2 + j · X · I ₂ (37)
Here, X represents the impedance of the connected reactor connected between the STATCOM 10 and the system. X is a real number.

Then, from the equations (29), (30), (31), the following equations (38), (39) are obtained.

Here, the real part becomes 0 as in the following equations (40) and (41).

Thereby, the following expressions (42) and (43) are obtained, and expressions (32) and (33) and expressions (42) and (43) have the same form.

Therefore, the equation (34) relating to the superimposed zero-phase voltage is obtained by replacing the portions of the positive phase voltage V ₁ and the negative phase voltage V ₂ output from the compensation power output unit 20 with the positive phase voltage V _{S1 of} the distribution system and the negative phase of the distribution system. This can be achieved by replacing the voltage V _S2 .

Next, the level of the zero phase voltage V ₀ necessary for canceling the unbalance caused by the reverse phase current will be examined. In the following, the magnitude of the superposed zero-phase voltage V ₀ under the assumptions shown in the following (1) to (3) will be examined. This study is for analyzing the range of the zero-phase voltage V ₀ under assumptions close to reality. When the actually superposed zero-phase voltage V ₀ is used for inter-phase voltage control of the STATCOM 10, the equation (34) is used. ) Is used.
(1) The positive phase voltage is used as a phase reference. That is, V ₁ is a real number.
(2) Operate as a reactive power compensator. That is, I ₁ is a pure imaginary number. Here, it is assumed that the loss generated in the STATCOM 10 is very small.
(3) The voltage of the distribution system is assumed to be normal phase voltage V ₁ >> reverse phase voltage V ₂ . That is, the term relating to V ₂ is ignored.

Based on the above assumptions, V ₀ , V ₁ , V ₂ , I ₁ , and I ₂ are expressed as the following formulas (44) to (48).

From the equations (44) to (48), the magnitude of the zero phase voltage A _VO can be expressed as the following equation (49).

By the way, the compensation power output unit 20 performs PWM (pulse width modulation) control that determines whether the switching element 23 of the circuit module 21 is ON or OFF by comparing the carrier wave and the modulated wave. FIG. 4 is a diagram illustrating an example of a carrier wave and a modulated wave. In the compensation power output unit 20, in order to effectively use the element, the waveform of the modulation wave is usually determined so that the peak is around 1. However, when the compensated power output unit 20 determines the waveform of the modulated wave so that the peak is near 1, when the large zero-phase voltage is superimposed, the magnitude of the modulated wave exceeds 1, and normal PWM Control cannot be performed. For this reason, the zero-phase voltage V ₀ needs to be suppressed to a certain level. Here, there is a margin for superimposing the zero-phase voltage by the difference between the peak of the carrier wave and the peak of the modulated wave. The zero-phase voltage can be superimposed by a margin of this modulation degree.

Here, the magnitude of the zero-phase voltage A _VO will be examined using Equation (49). FIG. 5 is a diagram illustrating an example of a change in the zero-phase voltage A _VO when the magnitude of the reverse-phase current A _I2 and the phase θ _I2 are changed. In the example of FIG. 5, when the magnitude of the positive phase current A _I1 is 1 [pu], the magnitude of the negative phase current A _I2 is 0.1, 0.2, 0.3, 0.5 [pu]. ] _Shows a result of _{obtaining the} zero-phase voltage A _VO from the equation (49) by changing the phase θ _I2 . The vertical axis in FIG. 5 is the magnitude of the superimposed zero-phase voltage when the distribution system voltage (for example, 6600 V) is 1 [pu], and the horizontal axis in FIG. 5 is the phase of the negative-phase current output from the STATCOM 10. represents _θI2 . If the phase θ _I2 of the negative phase current is arbitrary, the magnitude of the zero phase voltage increases as the proportion of the negative phase current output by the STATCOM 10 increases. When the modulation degree margin is 0.2, the negative phase current that can be output by the STATCOM 10 is about 20% of the positive phase current.

FIG. 6 is a diagram illustrating an example of a change in the zero-phase voltage when the magnitude and phase of the positive-phase current are changed. In the example of FIG. 6, when the magnitude of the negative phase current A _I2 is 1 [pu], the magnitude of the positive phase current A _I1 is 0.0, 0.1, 0.2, 0.3 [pu]. ] _Shows a result of _{obtaining the} zero-phase voltage A _VO from the equation (49) by changing the phase θ _I2 . The vertical axis in FIG. 6 is the magnitude of the superimposed zero-phase voltage when the voltage of the distribution system (for example, 6600 V) is 1 [pu], and the horizontal axis in FIG. 6 is the phase of the negative-phase current output from the STATCOM 10. Represents. When STATCOM10 purely flows only a reverse-phase current (when the positive-phase current A _I1 = 0), the magnitude of the zero-phase voltage A _VO is 1.0 [pu], which is the same magnitude as the system voltage of the distribution system Zero phase voltage must be output. That is, the DC voltage rating of the STATCOM 10 needs to be about twice the normal. As the ratio of the positive phase current A _I1 output from the STATCOM 10 increases, the magnitude of the zero phase voltage further increases, and the DC voltage rating of the STATCOM 10 needs to be larger than twice the normal value.

  The zero-phase voltage has a solution as long as the positive-phase current and the negative-phase current are not equal, but the magnitude of the zero-phase voltage that can be superimposed is limited by the DC voltage of the capacitor 22. Therefore, in practice, it is necessary to reduce the magnitude of the zero-phase voltage or to increase the DC voltage of the capacitor 22 more than usual.

Due to the above constraints, the STATCOM 10 can be designed in the following two ways (1) and (2), for example.
(1) The DC voltage design of the STATCOM 10 is doubled as usual so that the negative phase can be compensated even if the STATCOM 10 outputs a pure negative phase current. Then, PWM control is performed so that the magnitudes of the positive phase current and the negative phase current output from the STATCOM 10 are not equal. For example, the magnitude of the negative phase current is changed by changing the frequency of the modulation wave.
(2) The DC voltage of the STATCOM 10 is limited to a normal design or slightly higher than that, and the negative phase current output for the negative phase voltage compensation is also limited to about a few percent of the positive phase current.

  By the way, the equation (34) diverges infinitely when the positive phase current output from the STATCOM 10 is equal to the negative phase current. This is considered as follows.

As described above, when the STATCOM 10 outputs the reverse phase current and the voltage of the capacitor 22 of each phase is unbalanced, ΔE _αβ has a magnitude and can be changed in the direction of i _αβ .

FIG. 7A is a diagram showing a relationship between the instantaneous value current i _αβ and ΔE _αβ when the STATCOM 10 outputs only the positive phase current. When the STATCOM 10 outputs only the positive phase current, the trajectory of i _αβ draws a circle that rotates at the speed ωt. Therefore, in the long term, cancellation is possible regardless of the direction of ΔE _αβ .

On the other hand, FIG. 7B is a diagram showing the relationship between the instantaneous value current i _αβ and ΔE _αβ when the STATCOM 10 outputs positive and negative phase currents of different magnitudes. When a reverse phase component exists in i _αβ , a reverse phase component rotating at a speed −ωt is superimposed, so that the trajectory of i _αβ becomes an ellipse. As the antiphase component increases, the major axis of the ellipse becomes longer, but the minor axis of the ellipse becomes shorter. At this time, the voltage imbalance of the capacitor 22 is easily corrected in the major axis direction of the ellipse, but is difficult to correct in the minor axis direction of the ellipse.

On the other hand, FIG. 7C is a diagram showing the relationship between the instantaneous value current i _αβ and ΔE _αβ when the STATCOM 10 outputs the positive phase current and the reverse phase current of the same magnitude. When the magnitudes of the positive phase current and the negative phase current are the same, the trajectory of i _αβ draws a straight line instead of a circle. Therefore, ΔE _αβ can be canceled only in one direction drawn by i _{αβ, and} there are components that cannot be canceled by the zero-phase voltage control. Thus, when the positive phase current and the negative phase current are equal, there is a voltage unbalance component that cannot be canceled. Therefore, in the STATCOM 10 having the Y-connection MMC configuration, it is necessary to prevent the positive phase current and the reverse phase current from being the same.

  Next, the configuration of the zero-phase voltage generator 30 according to the present embodiment will be described. FIG. 8 is a diagram illustrating an example of a schematic configuration of the zero-phase voltage generation unit. In the example illustrated in FIG. 8, the zero-phase voltage generation unit 30 includes a system state detection unit 40, a compensation power detection unit 41, a control unit 42, and a zero-phase voltage generation control unit 43. The system state detection unit 40 detects the state of the power distribution system, and detects the AC power of the power distribution system. The compensation power detection unit 41 detects the current of each phase of the compensation power output unit 20 and the capacitor voltage across the circuit module 21 connected for each phase. The control unit 42 performs reactive power compensation control based on detection results by the system state detection unit 40 and the compensation power detection unit 41, and derives a zero-phase voltage to be generated. The zero phase voltage generation control unit 43 generates the zero phase voltage derived by the control unit 42 in the compensation power output unit 20. For example, the zero-phase voltage generation control unit 43 changes the voltage level of the constant voltage superimposed on the modulation wave supplied to the circuit module 21 of each phase of the compensation power output unit 20 to change the voltage level of the entire modulation wave to 3 A zero-phase voltage is generated in the compensation power output unit 20 by performing control to change the ON / OFF period of the switching element 23 of the circuit module 21 of each phase by raising and lowering the phases collectively.

Next, reactive power compensation control by the control unit 42 according to the present embodiment will be described. FIG. 9 is a block diagram showing the flow of reactive power compensation control. As described above, in the STATCOM 10, when an unbalanced negative phase current is output in order to compensate for the negative phase voltage of the distribution system, a large unbalance occurs in the voltage of the capacitor 22 of the circuit module 21 of each phase. Therefore, in the control unit 42 according to the present embodiment, the zero-phase voltage is superimposed on the output voltage so as not to cause an imbalance of the voltage of the capacitor 22 of each phase even when the current output from the STATCOM 10 includes a reverse-phase current. Feedforward control is performed. In this feedforward control, a zero-phase voltage is generated in the circuit module 21 of each phase of the distribution system so that the amount of active power flowing into and out of the circuit module 21 becomes zero around one cycle of AC power flowing through the distribution system. Specifically, the AC power of the distribution system detected by the system state detection unit 40 is subjected to effective value conversion to obtain the normal phase voltage V ₁ and the reverse phase voltage V ₂ existing in the distribution system. Further, the positive phase current I ₁ and the negative phase current I ₂ are obtained from the current of each phase of the compensation power output unit 20 detected by the compensation power detection unit 41. Then, using the above equation (34), the effective value V ₀ of the zero phase voltage is obtained from the positive phase voltage V ₁ , the negative phase voltage V ₂ , the positive phase current I ₁ , and the negative phase current I ₂ , and the effective value V _The instantaneous value V _0f is derived by performing conversion to an instantaneous value of ₀ .

Further, in the STATCOM 10, even if the above-described feedforward control is performed, the balance of the voltages of the capacitors 22 in each phase may be slightly lost due to control errors, individual differences in element characteristics, and the like. Therefore, in the zero-phase voltage generation unit 30 according to the present embodiment, when an unbalance occurs in the voltage of the capacitor 22 of each phase, the zero-phase voltage that cancels the unbalance of the voltage of the capacitor 22 is also superimposed on the output voltage. Control is also performed. Specifically, an average voltage E _ave of the voltages E _r , E _s , E _{t is obtained} from the voltages E _r , E _s , E _t of the entire circuit module 21 detected by the compensation power detector 41. Then, a voltage deviation from the average value E _{ave of} the voltages E _r, E _s, E _t is obtained and three-phase to two-phase conversion is performed to obtain voltages ΔE _α and ΔE _β . Then, a phase gamma from the calculation of _{^{γ = tan -1 (ΔE β /}} ΔE α), | ΔE αβ | = (ΔE α 2 + ΔE β 2) 1/2 from the arithmetic | Delta] E _.alpha..beta | a seek. Further, each phase current I _r of the compensation power output unit 20 detected by the compensation power detector 41, I _s, determine the current I _alpha, I _beta performs three-phase two-phase converted to _{I t,} [delta] = tan ^The phase δ is obtained from the calculation of ⁻¹ (I _β / I _α ). Then, | ΔE _αβ | is multiplied by a predetermined gain K, and further multiplied by cos (δ−γ) to obtain an instantaneous value V _0b . Then, the instantaneous value V _0b is added to the instantaneous value V _0f to derive the instantaneous value V _0, and the derived instantaneous value V ₀ is output to the compensation power output unit 20.

The compensation power output unit 20 superimposes the zero-phase voltage of the instantaneous value V ₀ on the output voltage. In the STATCOM 10, this feedback control can cancel the voltage imbalance of the capacitors 22 of each phase due to control errors, individual differences in element characteristics, and the like.

  The control unit 42 may realize reactive power compensation control with a circuit configuration using an integrated circuit such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA). In addition, a storage unit that stores a program for performing reactive power compensation control and an electronic circuit such as a CPU (Central Processing Unit) and an MPU (Micro Processing Unit) is provided, and reactive power compensation control is performed by executing processing of the program by the electronic circuit. May be realized.

  Next, a processing flow of the control unit 42 according to the present embodiment will be described. FIG. 10 is a flowchart showing the procedure of reactive power compensation control processing. This reactive power compensation control process is always executed after the control unit 42 is activated.

As shown in FIG. 10, the control unit 42 uses the compensation power detection unit 41 to control the currents I _r , I _s , and I _t of each phase of the compensation power output unit 20 and the voltages E _r and E _{s in the} entire circuit module 21. , _Et are detected (step S100). The control unit 42 detects the alternating current power of the power distribution system by system condition detecting portion 40, the positive-phase voltages V ₁ present in the distribution system by performing the effective value conversion, detects the negative-phase voltage V ₂ (step S101 ). Then, the control unit 42 obtains each phase of the current I _r of the detected compensating power output section 20, I _s, perform effective value conversion for I _t positive sequence current I _1, the negative sequence current I _2, normal phase The above feedforward control is performed using the current I ₁ , the negative phase current I ₂ , the positive phase voltage V ₁ , and the negative phase voltage V ₂ to derive the instantaneous value V _0f (step S102). Further, the control unit 42 _obtains an average voltage E _ave of the voltages E _r , E _s , E _t from the detected voltages E _r , E _s , E _t , and determines the voltages E _r , E _s , E _t , E _ave, and each phase currents I _r, I _s, carries out an operation of the above-described feedback control derives the instantaneous values V _0b using _{I t} (step S103). Then, the control unit 42 derives the instantaneous values V ₀ by adding the derived instantaneous value V _0b and the instantaneous value V _0b, and outputs the instantaneous value V ₀ derived to zero-phase voltage generator control unit 43 (step S104 ). The zero-phase voltage generation control unit 43 changes the voltage level of the constant voltage superimposed on the modulation wave supplied to the circuit module 21 of each phase of the compensation power output unit 20 according to the derived instantaneous value V _0. Thus, control is performed to change the ON / OFF period of the switching element 23 of the circuit module 21 of each phase. As a result, a zero-phase voltage is generated in the compensation power output unit 20. The control unit 42 determines whether the end of the process has been instructed by another control unit that controls the entire STATCOM 10 (step S105). When the process end is instructed (Yes at Step S105), the control unit 42 ends the process. On the other hand, when the process end is not instructed (No at Step S105), the control unit 42 proceeds to Step S100.

  Next, the result of simulating the operation of the STATCOM 10 according to the present embodiment will be described. FIG. 11 is a diagram illustrating a schematic configuration of a power distribution system used in the simulation. FIG. 12 shows the main specifications of the STATCOM 10 that has been simulated.

Normally, the STATCOM 10 is transformerlessly connected to a 6600 V distribution system, so that the DC voltage applied to the capacitor 22 of the circuit module 21 of each phase is designed to be about 2000 [V]. However, in the simulation, the design value of the DC voltage applied to the capacitor 22 is more than double the normal value so that the STATCOM 10 can also output a pure negative phase current, for example, 4500V. The AC voltage output from each circuit module 21 is obtained as 1270 [V] from the following equation (50) because there are three stages of circuit modules 21 in each phase.
(6600 / √3) / 3 = 1270 [V] (50)

  Further, the STATCOM 10 shifts the phase of the carrier wave or the modulated wave for each phase circuit module 21 connected in series, so that the switching frequency is equivalently 2N times (N is the number of stages of the circuit module 21). . In the simulation, in order to set the switching frequency to 20 [kHz] exceeding the audible range, the frequency of the carrier wave input to each circuit module 21 is set to 3.3 [kHz].

  The positive phase current component of the output current of STATCOM 10 is set to 0.0 [pu], the negative phase current component is set to 1.0 [pu], and the phase angle of the negative phase current component based on the positive phase voltage is 0 [deg]. A simulation was performed. FIGS. 13A to 13C show examples of simulation results. FIG. 13A is a diagram illustrating a change in voltage of the capacitor 22 of each phase (R phase, S phase, T phase) of STATCOM. FIG. 13B is a diagram illustrating a change in current output from each phase of STATCOM. FIG. 13C is a diagram illustrating a change in voltage of each phase of the distribution system. However, since the entire capacitor DC voltage control is operating, the loss generated in the STATCOM 10 outputs a positive phase current. As shown in FIG. 13B, since the output current of STATCOM 10 is not in the phase order of R → S → T, but in the phase order of R → T → S, STATCOM 10 outputs a substantially pure negative phase current. ing. Further, as shown in FIG. 13A, it can be seen that there is almost no imbalance in the voltage of the capacitor 22 of each phase, and the reactive power compensation control is operating as expected. The slight deviation exists because only proportional control is performed in the feedback control. The remaining deviation is steadily generated in the voltage of the capacitor 22, but is small as compared with the fluctuation of the system cycle twice and does not cause a problem here.

  In order to check whether an imbalance occurs in the voltage of the capacitor 22 of each phase under various other conditions, the positive phase current component of the output current of the STATCOM 10 is set to 0.7 [pu], and the negative phase current component is set to The simulation was performed with 0.35 [pu] and the phase angle 0 [deg] of the negative phase current component with respect to the positive phase voltage. 14A to 14C show examples of simulation results. FIG. 14A is a diagram illustrating a change in voltage of the capacitor 22 of each phase of the STATCOM 10. FIG. 14B is a diagram illustrating a change in current output from each phase of the STATCOM 10. FIG. 14C is a diagram illustrating a change in voltage of each phase of the distribution system. Even under this condition, as shown in FIG. 14A, the voltage unbalance of the capacitors 22 of each phase is a small value, and it can be seen that the reactive power compensation control is operating normally.

  Further, the positive phase current component of the output current of STATCOM 10 is 0.7 [pu], the negative phase current component is 0.35 [pu], and the phase angle with respect to the positive phase voltage is 45 [deg], 90 [ In this case, no imbalance occurs in the voltage of the capacitor 22 of each phase.

  14A to 14C, the STATCOM 10 sets the positive phase current component of the output current to 0.7 [pu], the negative phase current component to 0.35 [pu], and sets the positive phase voltage to The simulation was performed by setting the phase angle of the negative-phase current component as a reference to 0 [deg] and setting the output of the feedforward control to 0 after 0.1 [s]. FIGS. 15A to 15C show examples of simulation results. FIG. 15A is a diagram illustrating a change in voltage of the capacitor 22 of each phase of the STATCOM 10. FIG. 15B is a diagram illustrating a change in current output from each phase of the STATCOM 10. FIG. 15C is a diagram illustrating a change in voltage of each phase of the distribution system. As shown in FIG. 15A, after 0.1 [s] when the output of the feedforward control is set to 0, the voltage of the capacitor 22 of each phase has a large imbalance, and if there is no feedforward control, the capacitor voltage The imbalance indicates that it is difficult to suppress. From this, it can be seen that the feedforward control is effective when the STATCOM 10 performs the reverse phase control. In the example of FIG. 15A, the feedback control without the integration element remains even when the feedforward control is stopped. Therefore, the voltage of the capacitor of each phase does not necessarily diverge, and when the unbalance progresses to some extent, it is not more than that. It is not balanced.

[Effect of Example 1]
As described above, the STATCOM 10 according to the present embodiment is a circuit including the switching element 23 and the capacitor 22 that stores and discharges power according to the on / off of the switching element 23 for each phase of the power distribution system 12. Modules 21 output compensation power including a reverse phase current that compensates for a reverse phase AC voltage generated in the power distribution system 12 from a plurality of compensation power output units 20 provided in series. Then, the STATCOM 10 applies a zero-phase voltage to each phase of the distribution system 12 from the zero-phase voltage generation unit 30 so that the amount of active power entering and exiting the circuit module 21 becomes zero around one cycle of the AC power flowing through the distribution system 12. Generate. In this way, by generating a zero-phase voltage in the circuit module 21 of each phase of the distribution system 12 so that the amount of active power flowing into and out of the circuit module 21 around one cycle of AC power is zero, The output can suppress voltage imbalance generated in the capacitor 22 of the circuit module 21 of each phase.

In addition, the STATCOM 10 according to the present embodiment detects the positive phase voltage V ₁ , the negative phase voltage V ₂ , the positive phase current I ₁ output from the compensation power output unit 20, and the negative phase current I ₂ existing in the distribution system 12. Thus, by performing the calculation of the above equation (34), the superposed zero-phase voltage V ₀ can be obtained except for the case where the positive phase current I ₁ = the negative phase current I ₂ .

  Although the embodiments related to the disclosed apparatus have been described above, the present invention may be implemented in various different forms other than the above-described embodiments. Therefore, another embodiment included in the present invention will be described below.

  In the above embodiment, the compensation power output unit 20 is illustrated as having a circuit configuration in which the circuit modules 21 are connected in three stages in series for each phase of the power distribution system 12 as shown in FIG. However, the apparatus is not limited to this. For example, a circuit configuration in which one circuit module 21 is provided for each phase may be employed. Moreover, it is good also as a circuit structure which connected the circuit module 21 for each phase 2 steps | paragraphs or 4 steps | paragraphs or more in series.

  In the above embodiment, the compensation power output unit 20 has a so-called Y-connection MMC circuit configuration in which a series circuit in which the circuit modules 21 are connected in series for each phase of the distribution system 12 is Y-connected. Although illustrated, the disclosed apparatus is not limited thereto. For example, the compensation power output unit 20 may have a so-called delta connection MMC circuit configuration in which a series circuit in which bridge cell type circuit modules 21 are connected in series for each phase of the distribution system 12 is connected in delta. FIG. 16 is a diagram illustrating an example of a circuit configuration of the delta connection MMC. In addition, a circuit configuration of a so-called double Y-connection bridge cell type MMC in which Y connection of a series circuit in which a bridge cell type circuit module 21 is connected in series for each phase of the distribution system 12 is doubled may be employed. . FIG. 17 is a diagram illustrating an example of a circuit configuration of a double Y-connection bridge cell type MMC.

  In the above-described embodiment, the circuit module 21 is a so-called bridge cell in which two series circuits 24 each having two switching elements 23 connected in series are connected in parallel to the capacitor 22 as shown in FIG. Although the example of the case of the type circuit configuration is illustrated, the disclosed apparatus is not limited to this. For example, the circuit module 21 is connected in parallel with one series circuit 24 in which two switching elements 23 are connected in series to the capacitor 22, and a portion between the two switching elements 23 and an end portion of the series circuit 24 are connected. A chopper cell type may be used as an input terminal and an output terminal. FIG. 18 is a diagram illustrating an example of a circuit configuration of a double Y-connection MMC (so-called double Y-connection chopper cell type MMC) using a chopper cell type circuit module 21.

  In the STATCOM 10, when the circuit module 21 is operated at the same DC voltage, the ratio of the number of elements required for the circuit module 21 is such that the Y connection MMC, the delta connection MMC, the double Y connection chopper cell type MMC, and the double Y connection bridge cell type. In the order of MMC, 1: √3: 2: 2. Therefore, when considering the number of necessary elements, the circuit configuration of the Y connection MMC is an effective option.

  Thus, the circuit configuration of the Y-connection MMC has an advantage that the number of cells and the number of semiconductor elements can be reduced compared to other circuit configurations. However, conventionally, since the circuit configuration of the Y-connection MMC cannot pass a circulating current as compared with other circuit configurations, only the positive phase reactive current can be compensated, and the voltage imbalance of the capacitor 22 of each phase cannot be compensated. It was thought. However, by using the technique of this embodiment, it is possible to compensate for the voltage imbalance of the capacitor 22 of each phase.

  On the other hand, each circuit configuration of the delta connection MMC, the double Y connection chopper cell type MMC, and the double Y connection bridge cell type MMC is configured so that when an unbalance occurs in the voltage of the capacitor 22 of each phase, The voltage of the phase capacitor 22 can be controlled so as not to cause an unbalance. However, a loss occurs in electric power by flowing a circulating current. Therefore, the STATCOM 10 has a circuit configuration capable of eliminating the voltage imbalance of the capacitor 22 of each phase by flowing a circulating current such as a delta connection MMC, a double Y connection chopper cell type MMC, a double Y connection bridge cell type MMC. Even in some cases, by using the technique of the present embodiment, it is possible to compensate for the voltage imbalance of the capacitor 22 of each phase without flowing a circulating current.

  Further, depending on various loads and usage conditions, the processing at each step of each processing described in the embodiment may be arbitrarily divided or combined, or the processing order may be changed. For example, in the reactive power compensation control process shown in FIG. 10, the processes in steps S100 and S101, and steps S102 and S103 may be interchanged.

  Further, each component of each illustrated apparatus is functionally conceptual, and does not necessarily need to be physically configured as illustrated. In other words, the specific state of distribution / integration of each device is not limited to the one shown in the figure, and all or a part thereof may be functionally or physically distributed or arbitrarily distributed in arbitrary units according to various loads or usage conditions. Can be integrated and configured. For example, the system state detection unit 40 and the compensation power detection unit 41 illustrated in FIG. 8 may be integrated. Further, the control unit 42 may be divided into finer processing units.

[Reactive power compensation program]
The various types of processing of the control unit 42 described in the above embodiment can also be realized by executing a program prepared in advance on a computer system. Therefore, in the following, an example of a computer that executes a reactive power compensation program having the same function as the control unit 42 described in the above embodiment will be described with reference to FIG. FIG. 19 is a diagram illustrating a computer that executes a reactive power compensation program.

  As shown in FIG. 19, the computer 300 includes a CPU (Central Processing Unit) 310, a ROM (Read Only Memory) 320, and a RAM (Random Access Memory) 340. These units 300 to 340 are connected via a bus 400.

  The ROM 320 stores in advance a reactive power compensation program 320a that exhibits the same function as the control unit 42 described in the first embodiment. That is, the reactive power compensation program 320a is stored in the ROM 320 as shown in FIG. The reactive power compensation program 320a may be separated as appropriate.

  Then, the CPU 310 reads the reactive power compensation program 320a from the ROM 320 and executes it.

  The reactive power compensation program 320a is not necessarily stored in the ROM 320 from the beginning.

  For example, the program is stored in a “portable physical medium” such as a flexible disk (FD), a CD-ROM, a DVD disk, a magneto-optical disk, or an IC card inserted into the computer 300. Then, the computer 300 may read and execute the program from these.

  Furthermore, the program is stored in “another computer (or server)” connected to the computer 300 via a public line, the Internet, a LAN, a WAN, or the like. Then, the computer 300 may read and execute the program from these.

10 STATCOM
DESCRIPTION OF SYMBOLS 12 Distribution system 20 Compensation power output part 21 Circuit module 22 Capacitor 23 Switching element 30 Zero phase voltage generation part 40 System state detection part 41 Compensation power detection part 42 Control part 43 Zero phase voltage generation control part

Claims (4)

For each phase of the power distribution system, one or more circuit modules each having a switching element and a capacitor for storing and discharging power according to the on / off of the switching element are provided in series, and are generated in the power distribution system. A compensation power output unit for outputting compensation power including a negative phase current for compensating a negative phase AC voltage;
In each phasor display, the positive phase voltage existing in the distribution system is V ₁ , the negative phase voltage existing in the distribution system is V ₂ , the positive phase current output by the compensation power output unit is I ₁ , A reactive power compensator comprising: a zero-phase voltage generator configured to generate a zero-phase voltage based on V ₀ obtained from the following equation when the negative-phase current output by the compensation power output unit is I ₂ : .

The reactive power compensator according to claim 1, wherein the compensation power output unit is Y-connected to a circuit in which one or more circuit modules for each phase of the distribution system are provided in series. For each phase of the power distribution system, one or more circuit modules each having a switching element and a capacitor for storing and discharging power according to the on / off of the switching element are provided in series, and are generated in the power distribution system. A compensation power output unit that outputs compensation power including a negative phase current that compensates for a negative phase AC voltage is represented by a phasor display, and the positive phase voltage existing in the distribution system is V ₁ and exists in the distribution system. When the negative phase voltage to be output is V ₂ , the positive phase current output from the compensation power output unit is I _1, and the negative phase current output from the compensation power output unit is I ₂ , V ₀ obtained from the following equation: A reactive power compensation method comprising a zero-phase voltage generation step of generating a zero-phase voltage based on

For each phase of the power distribution system, one or more circuit modules each having a switching element and a capacitor for storing and discharging power according to the on / off of the switching element are provided in series, and are generated in the power distribution system. A compensation power output unit that outputs compensation power including a negative phase current that compensates for a negative phase AC voltage is represented by a phasor display, and the positive phase voltage existing in the distribution system is V ₁ and exists in the distribution system. When the negative phase voltage to be output is V ₂ , the positive phase current output from the compensation power output unit is I _1, and the negative phase current output from the compensation power output unit is I ₂ , V ₀ obtained from the following equation: A reactive power compensation program for causing a computer to execute a zero-phase voltage generation procedure for generating a zero-phase voltage based on the above .

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