JP2015204652A - power conversion system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve voltage stability of a power conversion system, in which a plurality of power converters are connected to a DC bus, by suppressing system stop even when a power converter performing voltage control of the DC bus has stopped.SOLUTION: Normally, a second power converter 2 of a plurality of first to fourth power converters 1 to 4 performs voltage constant control of a DC bus 7. The first, third, and fourth power converters 1, 3, 4 individually perform current control. In the case that the second power converter 2 performing the voltage constant control cannot keep voltage of the DC bus 7 constant, the voltage constant control is performed by the third power converter 3 at the time of the voltage of the DC bus 7 having been a voltage instruction value for the third power converter 3.

Description

  The present invention relates to a DC bus voltage control technique in a power conversion system in which a plurality of power converters are connected to a DC bus.

  FIG. 9 is a diagram illustrating an example of a circuit in which a plurality of power converters are connected to the DC bus 7. The first power converter 1 can transmit and receive bidirectional power to the AC and DC bus 7. As the device function of the first power converter 1, the power generated by the peak cut and the natural energy power generation (for example, the solar power generation module: hereinafter referred to as the solar power generation module) PV is supplied to the load L or the system 6. . Power storage media (for example, capacitors, secondary batteries) 8 and 9 are connected to opposite sides of the DC bus 7 of the second power converter 2 and the third power converter 3, and the second power converter 2 and the second power converter 2 3 The power converter 3 transmits and receives power to and from the DC bus 7 in both directions, and performs current control both during charging and discharging.

  Here, the second power converter 2 operates so as to maintain the voltage of the DC bus 7 at a predetermined value. Further, since the photovoltaic power generation module PV is connected to the fourth power converter 4, power is supplied to the DC bus 7 by maximum power operating point tracking control (hereinafter referred to as MPPT control).

  Temporarily, the 1st power converter 1 is fixed current command value operation (schedule operation) by a time axis, the 3rd power converter 3 is fixed current command value operation (schedule operation) by a time axis, and the 4th power converter 4 is MPPT. The instructions of the controller 5 when operating under control will be shown below.

In the fixed current command value operation (schedule operation) on the time axis, the current command value is given by time, but the output of the photovoltaic power generation module PV depends on the weather. Therefore, the following cases (1) and (2) are assumed.
(1) First power converter 1 output> PV power generation amount + discharge amount of third power converter 3 (2) First power converter 1 output <PV power generation amount + discharge amount of third power converter 3 In order to maintain the DC bus 7 at a constant voltage, it is necessary to use the following equation.

Input current (= DC current of first power converter 1) = Output current (= second power converter 2 + third power converter 3 + each DC bus current of fourth power converter 4)
* The same applies to instantaneous power. Therefore, the controller 5 detects the currents at the four points indicated by ◯ in FIG. 9 to monitor the power generation amount and output capacity of each power converter, and to satisfy the above formula. 2 Charge / discharge of the power converter 2 is controlled to keep the DC bus 7 at a constant voltage. That is, the MPPT control of the fourth power converter 4 is controlled according to the weather, and the fourth power converter 4 is not dared to control current from the outside. Since the current of the first power converter 1 is controlled by the current command value, compensation of the uncontrolled current of the fourth power converter 4 is performed by the second power converter 2.

  As shown in FIG. 10, in the case of (1) output of the first power converter 1> PV power generation amount + discharge amount of the third power converter 3, an insufficient current is generated from the storage medium 8 of the second power converter 2. Discharge. In the case of (2) output of the first power converter 1 <PV power generation amount + discharge amount of the third power converter 3, an extra current is charged in the power storage medium 8 of the second power converter 2.

  If the power storage medium 8 is considered as a capacitor, the power storage medium 8 has a limit capacity in both cases (1) and (2). Therefore, in the case of (1), there is a limit to the discharge of insufficient current. For this reason, when the discharge limit is reached, a voltage at the end of the discharge (voltage drop) is issued, and the current command value of the first power converter 1 is reduced, whereby the voltage of the power storage medium 8 of the second power converter 2 is reduced. To an appropriate voltage range. Similarly, in the case of (2), there is a limit to the charging of the extra current in the power storage medium 8. Therefore, when the limit of charging is reached, a signal at the end of charging (voltage increase) is issued, and the current command value of the first power converter 1 is increased, whereby the voltage of the power storage medium 8 of the second power converter 2 is increased. To an appropriate voltage range.

JP 2009-232675 A JP 2009-232674 A

  However, since the input current to the DC bus 7 and the output current need to be the same, if the controller 5 monitors the output of another power converter and gives an output command to the second power converter 2, the controller 5 Due to the control delay, a slight error occurs and the fluctuation range becomes large.

  Further, when the voltage control of the DC bus 7 is performed in one power converter (for example, the second power converter 2), when the power converter (second power converter 2) stops due to a failure or the like, the controller There is no device that keeps the voltage of the DC bus 7 constant from 5 until the designation of a new voltage control device. Therefore, the voltage of the DC bus 7 becomes unstable and the system may stop.

  Further, in the method in which the controller 5 monitors other currents and determines the output, the amount of data given when adding a power converter is large, so that the calculation burden increases.

  As described above, in the power conversion system in which a plurality of power converters are connected to the DC bus, even when the power converter that has performed the voltage control of the DC bus stops, the system stop is suppressed, The challenge is to improve the voltage stability of the system.

  The present invention has been devised in view of the conventional problems, and one aspect thereof includes a plurality of power converters connected to a DC bus and a controller that controls the plurality of power converters. The power conversion system outputs a voltage command value for starting constant voltage control from the controller to the plurality of power converters, and in normal operation, any one of the plurality of power converters If the voltage of the DC bus is not controlled and the other power converters perform current control, and the power converter performing the voltage constant control cannot maintain the voltage of the DC bus, the voltage of the DC bus When the voltage becomes a voltage command value of another power converter, the other power converter performs a constant voltage control.

  In addition, as one aspect thereof, different voltage command values are set in advance in the plurality of power converters.

  According to the present invention, in a power conversion system in which a plurality of power converters are connected to a DC bus, even when the power converter that has performed voltage control of the DC bus is stopped, the stop of the system is suppressed, It is possible to improve the voltage stability.

The block diagram which shows the power conversion system in embodiment. The flowchart which shows operation | movement of the power conversion system in embodiment. The block diagram which shows operation | movement of the power conversion system at the time of normal. The time chart which shows the command value of each power converter at the time of normal. The block diagram which shows operation | movement of the power conversion system at the time of a 2nd power converter failure. The block diagram which shows operation | movement of the power conversion system at the time of 2nd power converter return. The block diagram which shows operation | movement of the power conversion system at the time of a 3rd power converter failure. The block diagram which shows operation | movement of the power conversion system at the time of a 3rd power converter return. The block diagram which shows the conventional power conversion system. The time chart which shows the command value of the conventional power conversion system.

  Hereinafter, embodiments of a power conversion system according to the present invention will be described in detail with reference to FIGS.

[Embodiment]
FIG. 1 is a block diagram showing a power conversion system in the present embodiment. As shown in FIG. 1, first to fourth power converters 1 to 4 are connected to the DC bus 7. A grid 6 and a load L are connected to the first power converter 1 via switches, respectively. In addition, a power storage medium 8 and a power storage medium 9 are connected to the second power converter 2 and the third power converter 3, respectively. A solar power generation module PV is connected to the fourth power converter 4. The voltage of the DC bus 7 is detected by a voltage detector PT.

  The power conversion system according to the present embodiment controls so that the voltage of the DC bus 7 is constant in a certain power converter. The controller 5 instructs each of the first to fourth power converters 1 to 4 only with voltage command values that are different for each power converter. Here, the voltage command value indicates a voltage value switched from current control to voltage control, that is, a voltage value at which voltage control is started. Each of the power converters other than the designated power converter operates under current control, etc., and the voltage of the DC bus 7 is lowered due to a failure of the other power converter, and the voltage of the DC bus 7 is instructed to the power converter. When the voltage command value is reached, the other power converter is switched from current control to voltage control, and control is performed so as to keep the voltage of the DC bus 7 constant. Note that the voltage command value given from the controller 5 is given a different value to each of the first to fourth power converters 1 to 4, so that even when each power converter stops for some reason, the DC bus 7 automatically Control to keep the voltage constant can be performed.

Each of the first to fourth power converters 1 to 4 performs a schedule operation, MPPT control, and the like until the voltage command value set for each, and performs current control in the power converter.

  Hereinafter, specific examples will be described with reference to FIGS. FIG. 2 is a flowchart showing the operation of the power conversion system in the present embodiment. Here, it is assumed that the following voltage command value is given from the controller 5 to each power converter. 2nd power converter 2 = 360V, 3rd power converter 3 = 350V, 1st power converter 1 = 340V, 4th power converter 4 = 330V.

[Normal]
As shown in FIGS. 2, 3, and 4, in S <b> 1, constant voltage control (for example, 360 V) of the DC bus 7 is performed by the second power converter 2 on the system voltage side. The other first power converter 1 and the third power converter 3 execute a current output of a current command value (schedule operation or the like) given from the controller 5. The fourth power converter 4 performs MPPT control (peak cut operation or the like) in order to effectively use the power generated by the solar power generation module PV.

  The voltage of the DC bus 7 is kept constant by the second power converter 2 because the voltage increases if the amount of power generation is large and decreases if the amount of power generation is small.

[When DC bus 7 voltage drops: second power converter 2 failure (eg 350V)]
Since there is a limit to the amount of charge that can be stored in the second power converter 2, the second power converter 2 stops when the supplementary amount of the photovoltaic power generation module PV is large and the capacity of the power storage medium 8 is exceeded. Similarly, the second power converter 2 also stops when the supplementary amount of the solar power generation module PV is small and the energy of the power storage medium 8 is exhausted. Further, even when the second power converter 2 fails, the voltage of the DC bus 7 cannot be kept constant.

  As described above, when the second power converter 2 cannot keep the voltage of the DC bus 7 constant for some reason, the second power converter 2 is stopped as shown in FIG. 5 (S2). As a result, since there is no power converter that controls the voltage of the DC bus 7, the voltage of the DC bus 7 becomes unstable, and the voltage of the DC bus 7 decreases.

  In S3, the controller 5 automatically switches the third power converter 3 to voltage control when the voltage of the DC bus 7 decreases to the voltage command value of the third power converter 3, and the DC bus 7 Is controlled to be constant (350 V).

[When the second power converter 2 is restored]
In S4, when the second power converter 2 returns, as shown in FIG. 6, the second power converter 2 increases the voltage of the DC bus 7 so as to keep the voltage of the DC bus 7 constant at 360V. Take control. As the voltage of the DC bus 7 increases, the third power converter 3 automatically shifts to constant current control (schedule operation or the like) and returns to a steady state (S5).

[DC bus 7 When voltage drop: third power converter 3 failure (example: 340V)]
After the second power converter 2 is stopped, if it is difficult for the third power converter 3 to perform constant voltage control of the DC bus 7 for some reason, the third power converter 3 is stopped as shown in FIG. . As a result, since there is no power converter for controlling the voltage of the DC bus 7, the voltage of the DC bus 7 becomes unstable and the voltage of the DC bus 7 decreases (S6).

  In S7, when the voltage of the DC bus 7 decreases to the voltage command value of the first power converter 1, the first power converter 1 is automatically switched to voltage control, and the voltage of the DC bus 7 is kept constant. Control to keep at (340V).

[When returning to the third power converter 3]
When the third power converter 3 returns, control is performed so that the third power converter 3 increases the voltage and keeps the voltage of the DC bus 7 at 350 V as shown in FIG. 8 (S8). As the voltage of the DC bus 7 increases, the first power converter 1 automatically shifts to current control and returns to the steady state (S9).

  As described above, according to the present embodiment, in the power conversion system in which a plurality of power converters are connected to the DC bus 7, a specific power converter (this book) that performs constant voltage control of the DC bus 7. In the embodiment, when the second power converter 2) is stopped (when the voltage drops), the power converter (the third power converter 3 in the present embodiment) responsible for the voltage control of the next DC bus 7 is controlled by current control → voltage. By switching to control and performing constant voltage control of the DC bus 7, it is possible to suppress the stop of the system and improve the voltage stability of the system.

  Further, since the controller 5 only gives a voltage command value that automatically switches to constant voltage control of the DC bus 7 from the first to fourth power converters 1 to 4, switching from current control to voltage control. Can be done quickly.

  Further, conventionally, since it is necessary to make the input current to the DC bus 7 equal to the output current, the controller 5 monitors the output of another power converter and gives an output command to the second power converter 2. Due to the control delay of the controller 5, a slight error occurred and the fluctuation range was large. In this embodiment, since only the voltage command value is given from the controller 5 to the first to fourth power converters 1 to 4, a slight error occurs due to the control delay of the controller 5 and the fluctuation range becomes large. Can be suppressed.

  Furthermore, even when a device is added, since each power converter operates independently, it can be easily added.

  Although the present invention has been described in detail only for the specific examples described above, it is obvious to those skilled in the art that various changes and modifications are possible within the scope of the technical idea of the present invention. Such variations and modifications are naturally within the scope of the claims.

DESCRIPTION OF SYMBOLS 1 ... 1st power converter 2 ... 2nd power converter 3 ... 3rd power converter 4 ... 4th power converter 5 ... Controller 6 ... System | strain 7 ... DC bus 8, 9 ... Power storage medium L ... Load PV ... Sun Photovoltaic modules (natural energy power generation, etc.)

Claims (2)

A plurality of power converters connected to the DC bus;
A power conversion system comprising a controller for controlling the plurality of power converters,
A voltage command value for starting constant voltage control is output from the controller to the plurality of power converters, respectively.
During normal operation, DC power bus constant voltage control is performed by any one of the plurality of power converters, and the other power converters each perform current control.
When the voltage of the DC bus cannot be kept constant by the power converter performing the voltage constant control, the voltage of the DC bus becomes the voltage command value of the other power converter. A power conversion system characterized by performing constant voltage control.   The power conversion system according to claim 1, wherein different voltage command values are set in advance in the plurality of power converters.

Images