JP2017169385A - System interconnection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress in-rush current in a system interconnection device at the time of system voltage reset.SOLUTION: A system interconnection device 1 includes a DVS function 8 for supporting the system voltage at the time of voltage drop of a system 3, and rate of change limitation means 9 for limiting the rate of change of a system voltage inputted to the DVS function 8. The rate of change limitation means 9 performs rate of change limitation processing for the system voltage Vat the time of system voltage reset, and outputs the system voltage Vhaving a limited rate of change to the DVS function 8. By performing rate of change limitation processing, output of reactive current is continued several cycles after system voltage reset, and an in-rush current flowing into a transformer 10 at the time of system voltage reset is cancelled by a reactive current supplied by the DVS function 8.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a grid interconnection device corresponding to an operation continuation (FRT: Fault Ride Through) requirement (hereinafter referred to as FRT requirement) during an accident. In particular, the present invention relates to a grid interconnection device having a control technique for suppressing an inrush current of a transformer that flows at the time of recovery from a voltage drop.

  In solar power generation systems and wind power generation facilities, instantaneous voltage drops due to grid transmission line accidents, instantaneous frequency increases, large-scale power supply disconnection, and frequency fluctuations due to system separation may cause simultaneous disconnection or continued output decrease. There is a possibility of having a great influence on the overall voltage and frequency maintenance of the power system. Therefore, a system that satisfies the FRT requirement is required so that the operation can be continued if the system voltage drop level and the voltage drop duration are within the specified range even when the instantaneous voltage drop due to a system fault ( For example, Non-Patent Document 1). Non-Patent Document 1 discloses a rule regarding operation continuity during voltage reduction in a grid interconnection device.

  Further, although not defined in Non-Patent Document 1, a function of outputting a delayed reactive current to compensate for a reactive current (output from a generator or the like) that occurs during a voltage drop and to support a system voltage (DVS function: Dynamic Voltage Support) (for example, Patent Documents 1 and 2). The DVS function is mainly stipulated in overseas interconnection regulations.

  In FIG. 14, the control block diagram of the grid connection apparatus 14 (PCS: Power Conditioning System) which has the DVS function 8 is shown.

The grid interconnection device 14 links a DC power source 2 such as a solar cell or a fuel cell and the grid 3, and includes an inverter 4 (INV), a filter circuit 5 (Z _FIL ) including a reactor, a capacitor, and the like, and a PWM signal. An output unit 6, a control unit 7 (ACR: Automatic Current Regulator) for controlling the current (I _INV ) of the inverter 4, and a DVS function 8 are provided. A transformer 10 (TF) for insulation and transformation is provided between the system 3 and the inverter 4.

  The inverter 4 includes a switching device and a free-wheeling diode connected in reverse parallel to the switching device, and converts DC power into AC power by switching the direction in which the DC current flows with the switching device. The PWM signal output means 6 generates an on / off signal for a switching device in the inverter 4.

DVS features 8 refers to the system voltage V _AC, the system voltage V _AC is a predetermined value (for example, V ₁ as shown in the characteristic diagram of FIG. 15) control means the reactive power command Q _REF when lower than 7 to supply reactive power from the inverter 4 to the system 3. That is, a reactive current flows from the inverter 4 to the system 3. At this time, if a current exceeding the current limit amount of the inverter 4 is about to flow into the inverter 4, it is necessary to control the effective current for protecting the inverter 4. In this case, the active power command P _REF is lowered.

Figure 15 is a diagram showing an example of input-output characteristics of DVS features 8 to changes in system voltage V _AC. As shown in FIG. 15, the gradient of the reactive power command Q _REF when the system voltage recovery is dependent on the gradient of the effective value of the system voltage V _AC. If normal DVS features 8, it is desirable that the reactive current = 0 to follow immediately against return of the system voltage V _AC.

JP-A-11-262187 JP 2014-053975 A

NEC Technical Standards Committee, “System Interconnection Rules JEAC 9701-2012”, Grid Connection Special Subcommittee, NEC Corporation, 2012

  However, in the grid interconnection device corresponding to the FRT requirement, an inrush current is generated due to the magnetic saturation of the transformer when power is restored due to an instantaneous voltage drop or the like. The occurrence of the inrush current is passive due to the phase difference between the system voltage at the time of voltage drop and at the time of recovery, and it is difficult to avoid the occurrence by control.

  The inrush current is generally more than several times the transformer rated current and the device rated current, and it can be suppressed because there is a possibility of an increase in the breaking capacity in the host system or a device shutdown due to the breaking operation of the grid interconnection device. desirable.

  As a general method for suppressing the inrush current, there is a method of setting a current-limiting reactor. However, since these measures are factors for increasing costs, it is necessary to consider other measures.

  This invention is made | formed in view of the said situation, and it aims at suppressing the rush current at the time of the grid voltage return in a grid connection apparatus.

  One aspect of the grid interconnection device of the present invention that achieves the above object is ineffective for an inverter connected to a power system via a transformer and a control means for controlling the inverter according to a decrease in the system voltage of the power system. DVS means for outputting a power command and causing the inverter to output a reactive current; and a reactive current output continuation means for outputting the reactive power command to the DVS means for a predetermined period after system voltage recovery of the power system. It is characterized by providing.

  Another aspect of the grid interconnection device of the present invention that achieves the above object is that in the grid interconnection device, the reactive current output continuation means limits the rate of change of the grid voltage after the grid voltage recovery. The system voltage having a limited rate of change is output to the DVS means.

  Another aspect of the grid interconnection device of the present invention that achieves the above object is that in the grid interconnection device, the reactive current output continuation means inserts a time delay into the grid voltage after the grid voltage recovery. It outputs to the said DVS means, It is characterized by the above-mentioned.

  According to the above invention, the rush current at the time of the grid voltage return in a grid connection apparatus is suppressed.

It is a control block diagram of the grid connection apparatus which concerns on 1st Embodiment of this invention. It is a figure which shows the input-output characteristic of a change rate limiting means. It is an apparatus block diagram of the grid connection apparatus of a built-in transformer. It is an apparatus block diagram of the grid connection apparatus which has a connection transformer outside. It is a control block diagram of the grid connection apparatus which concerns on 2nd Embodiment of this invention. It is a figure which shows the input / output characteristic of a time delay means. It is the schematic of the grid connection apparatus which has a transformer. It is a figure which shows the reactive current output at the time of a system | strain voltage fall and reset (when a compensation reactive current output is performed but there is no inrush current of a transformer). It is a figure which shows the reactive current output at the time of a system voltage fall and reset (when a compensation reactive current is not output only by the inrush current of a transformer). It is a figure which shows the reactive current output at the time of a system | strain voltage fall and reset at the time of using a rate-of-change restriction | limiting. It is explanatory drawing explaining the inrush current suppression effect by a compensation reactive current at the time of using a rate-of-change restriction | limiting. It is a figure which shows the reactive current output at the time of the system voltage fall and reset at the time of using a time delay. It is explanatory drawing explaining the inrush current suppression effect by a compensation reactive current at the time of using a time delay. It is a control block diagram of the grid connection apparatus which has a DVS function. It illustrates input and output characteristics of DVS features to changes in system voltage V _AC.

  A grid interconnection apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

  In the grid interconnection device, the inrush current that flows through the transformer when the system voltage is restored is a reactive current that flows into the excitation circuit of the transformer, and is a reverse component of the reactive current supplied by the grid interconnection device through the DVS function. Therefore, unlike the normal case, the grid interconnection device according to the embodiment of the present invention has a DVS function that continues to output a reactive current for several cycles (predetermined time) after the grid voltage is restored. The inrush current flowing in the current is offset by the reactive current supplied by the DVS function, and the peak value of the inrush current is suppressed.

[First Embodiment]
FIG. 1 is a control configuration diagram of the grid interconnection device 1 according to the first embodiment of the present invention. In addition, the same code | symbol is attached | subjected about the same structure as the grid connection apparatus 14 based on the prior art shown in FIG. 14, and a different structure is demonstrated in detail.

The grid interconnection device 1 links a DC power source 2 such as a solar cell or a fuel cell and a grid 3, and includes an inverter 4 (INV), a filter circuit 5 (Z _FIL ) including a reactor, a capacitor, and the like, and a PWM signal. An output means 6, a control means 7 (ACR) for controlling the current (I _INV ) of the inverter, a DVS function 8, and a change rate limiting means 9 are provided. A transformer 10 (TF) for insulation and transformation is provided between the system 3 and the inverter 4.

The change rate limiting means 9 performs a change rate limiting process on the system voltage V _AC and outputs the voltage V _AC1 after the change rate limitation to the DVS function 8. By performing such a change rate limiting process, the output of the reactive current for several cycles is continued after the system voltage is restored.

FIG. 2 shows an example of input / output characteristics of the change rate limiting means 9 when the system voltage is restored. The upper graph in FIG. 2 is an input characteristic diagram, and the lower graph is an output characteristic diagram. Note that V ₁ in FIG. 2 corresponds to V ₁ in FIG.

Voltage V _AC1 after the change ratio limitation for use as an input of the reactive power amount calculation of DVS features 8, the voltage change at the time of system voltage recovery by performing the change rate limiting process becomes slower than the voltage change of the system voltage V _AC Accordingly, the output of the reactive power command Q _REF and the supply of the reactive current are continuously performed until the time T ₀ (shown in the lower graph of FIG. 2). Therefore, the inrush current flowing through the transformer 10 when the system voltage is restored is offset by the reactive current supplied by the DVS function 8, and control for suppressing the peak value of the inrush current is realized.

  In addition, even if the equipment configuration of the grid interconnection device 1 according to the first embodiment is a grid interconnection device 1A with a built-in transformer as illustrated in FIG. 3, the interconnection transformer 11 is externally disposed as illustrated in FIG. Even if it has the grid connection apparatus 1a and 1b which has, there is no difference in a control structure, and it is good also as a structure of either grid connection apparatus.

  In the grid interconnection device 1A shown in FIG. 3, the inverter 4 and the built-in transformer 10 are shown. In practice, however, the grid interconnection device 1A is a necessary device such as a filter circuit, a circuit breaker, or other auxiliary equipment. It has. In such a configuration, the amount of reactive power output when the system voltage drops is generally 1 pu or less, and in this case, the peak value of the inrush current can be reduced by about 1 pu at the maximum.

  In addition, the grid interconnection devices 1a and 1b shown in FIG. 4 have a device configuration having a linkage transformer 11 for linking to the high voltage system 3 at the upper level, but the grid interconnection devices 1a and 1b are only inverters 4. Necessary equipment such as built-in transformer, filter circuit, circuit breaker, and other auxiliary equipment is provided. In such a configuration, the total capacity of the grid interconnection devices 1a and 1b ≦ the capacity of the interconnection transformer 11, and the reactive power output when the system voltage decreases is smaller than the capacity of the interconnection transformer 11, but the inrush current This peak can be reduced in the same manner as in the transformer interconnection type grid interconnection device 1A.

[Second Embodiment]
After the system voltage returns, several cycles (predetermined time) DVS features 8 can be continued output of the reactive current, not only by performing the change rate limiting process of the system voltage V _AC, inserting a time delay when the system voltage returns It can also be realized by such a method. Therefore, in the grid interconnection device according to the second embodiment, the time delay means is provided to suppress the peak value of the inrush current when the grid voltage is restored. That is, the grid interconnection apparatus according to the second embodiment is provided with a time delay means instead of the change rate limiting means 9 of the grid interconnection apparatus 1 of the first embodiment. Therefore, the same components as those of the grid interconnection device 1 are denoted by the same reference numerals, description thereof is omitted, and different components are described in detail.

As shown in FIG. 5, the grid interconnection device 12 according to the second embodiment of the present invention links the DC power supply 2 and the grid 3, and includes an inverter 4 (INV) and a filter circuit 5 (Z _FIL ). , A PWM signal output means 6, a control means 7 (ACR) for controlling the inverter current (I _INV ), a DVS function 8, and a time delay means 13. A transformer 10 (TF) for insulation and transformation is provided between the system 3 and the inverter 4.

The time delay means 13 inserts a delay time of a predetermined time into the system voltage V _AC at the time of system voltage recovery, and outputs the voltage V _AC2 with the inserted delay time to the DVS function 8.

FIG. 6 shows an example of input / output characteristics of the time delay means 13 when the system voltage is restored. The upper graph in FIG. 6 is an input characteristic diagram, and the lower graph is an output characteristic diagram. V _{1 in} FIG. 6 corresponds to V ₁ in FIG. By this time delay function, the reactive power command Q _REF is output and the reactive current is continuously supplied until the time T ₀ (shown in the lower graph of FIG. 6).

  Similar to the grid interconnection device 1 according to the first embodiment, the equipment configuration of the grid interconnection device 12 according to the second embodiment is the configuration of a transformer interconnection type grid interconnection device 1A as shown in FIG. Even in the grid interconnection devices 1a and 1b having the interconnection transformer 11 as shown in FIG. 4, there is no difference in the control configuration, and the configuration of either grid interconnection device may be used.

[Principle of inrush current suppression]
The principle of inrush current suppression of the grid interconnection device 1 according to the first embodiment and the grid interconnection device 12 according to the second embodiment will be described.

FIG. 7 shows the flow of reactive current when the grid voltage is restored in the grid interconnection device 1 having the transformer 10. The arrows in FIG. 7 indicate the reactive current (hereinafter referred to as compensation reactive current) and the inrush current that are output from the inverter 4 to support the system voltage when the system voltage _VAC decreases.

  FIG. 8 is a time series diagram of the effective current and the reactive current output when the system voltage is lowered / returned, and shows a case where the compensated reactive current is output by the DVS function 8 and there is no inrush current of the transformer 10. In FIG. 8, since there is no inrush current of the transformer 10, reactive current = compensation reactive current.

  The compensation reactive current is a component for maintaining the system voltage when the voltage of the system 3 is reduced and canceling out the reactive current output from the generator separately connected to the system 3. As shown in FIG. 8, when the system voltage is lowered, control is performed to switch the active current that has been output until then to the compensation reactive current, and when the system voltage is restored, the active current is quickly restored to the active current.

  FIG. 9 is a time series diagram of the effective current and the reactive current output when the system voltage is lowered / returned, and shows a case where the DVS function 8 does not operate. That is, the reactive current shows only the inrush current of the transformer 10 and no compensation reactive current is output.

  The inrush current of the transformer 10 flows when the phase when the system voltage decreases and the phase when the system voltage returns. In FIG. 9, an envelope in which an inrush current can flow is indicated by a dotted line. However, since a current whose peak value is several times the rated current flows immediately after the return as described above, the influence on other devices is great. This current becomes a delayed reactive current when viewed from the grid 3, but becomes an advanced reactive current when viewed from the grid interconnection device 1.

  That is, since the inrush current is a reactive current that is advanced as viewed from the grid interconnection device 1, it can be canceled by flowing a reactive current that is delayed at the same timing. By using the compensation reactive current by the DVS function 8 as the reactive current of this delay, the inrush current can be suppressed without function failure.

  FIG. 10 is a time series diagram of the effective current and the reactive current output when the system voltage decreases / returns in the grid interconnection device 1 having the change rate limiting means 9 and shows a case where there is no inrush current of the transformer 10. ing. In addition, the dotted line part in FIG. 10 has shown the effective current and the reactive current when not using change rate restriction | limiting. In FIG. 10, since there is no inrush current of the transformer 10, reactive current = compensated reactive current.

As shown in FIG. 10, by using the rate of change limitation when the system voltage is restored, the output of the compensation reactive current for a predetermined period is continued after the system voltage is restored. The time for which the change rate limiting means 9 continues to output the compensation reactive current by limiting the change rate of the system voltage V _AC cannot be delayed indefinitely. For example, in Non-Patent Document 1, since the output recovery characteristic after voltage recovery is defined, the rate of change within that range is limited. However, since the inrush current is particularly problematic only for several cycles after voltage recovery, a sufficient effect can be expected. In FIG. 10, a period T in which the effective current and the reactive current are changed is a range in which the change rate is limited. The reactive power command Q _REF output by the DVS function 8 is synchronized with the time change of the reactive current in FIG.

  FIG. 11 shows the effect of suppressing the inrush current of the transformer due to the return delay of the compensation reactive current of the grid interconnection device 1. The dotted line is the reactive current after voltage recovery, and has a waveform in which the dotted line in FIG. 9 and the reactive current (compensation reactive current) in period T in FIG. 10 are superimposed. When a change rate limitation is inserted when switching the compensation reactive current to the active current, the output of the compensation reactive current for several cycles continues after the system voltage is restored, so that there is no change rate limiting means 9 (for example, FIG. 9). Inrush current (indicated by the dotted line in the figure) is suppressed more than the case indicated by the dotted line in FIG.

  FIG. 12 is a time series diagram of the effective current and the reactive current output when the system voltage decreases / returns in the grid interconnection device 12 having the time delay means 13, and shows a case where there is no inrush current of the transformer 10. Yes. In FIG. 12, since there is no inrush current of the transformer 10, reactive current = compensated reactive current.

By inserting a time delay at the time of switching the compensation reactive current to the effective current at the time of voltage recovery, the output of the compensation reactive current is continued for a predetermined period after the system voltage recovery. This amount of delay time is not infinitely limited. For example, in Non-Patent Document 1, since the output recovery characteristic after voltage recovery is specified, a delay within that range is inserted. However, the inrush current is a major problem only in the first few cycles, and since the convergence is fast, a sufficient effect can be expected. T in FIG. 12 is a range where the delay time is applied. The reactive power command Q _REF output by the DVS function 8 is substantially synchronized with the time change of the reactive current in FIG.

  FIG. 13 shows the effect of suppressing the inrush current of the transformer 10 due to the return delay of the compensation reactive current of the grid interconnection device 12. The dotted line is a reactive current after voltage recovery, and has a waveform in which the dotted line in FIG. 9 and the reactive current (compensation reactive current) in period T in FIG. 12 are superimposed. By inserting a time delay when switching the compensation reactive current to the active current, the inrush current is suppressed as compared with the case where there is no time delay means 13 (for example, shown by the dotted line in FIG. 9).

  Since the grid interconnection devices 1 and 12 are generally regarded as current sources, the inrush current of the transformer 10 is supplied from the grid 3 which is a voltage source. Therefore, the inrush current of the transformer 10 as viewed from the grid interconnection devices 1 and 12 is almost always a reactive current, and even in the case of the built-in transformer 10 shown in FIG. 3, it is connected to the outside shown in FIG. Even when the transformer 11 is provided, the inrush current can be suppressed by applying the control for continuing the output of the compensation reactive current for a predetermined period after the system voltage is restored.

  In the grid interconnection devices 1 and 12 according to the embodiment of the present invention as described above, in the grid interconnection devices 1 and 12 corresponding to the FRT requirement, at the time of system voltage recovery of the function of outputting a reactive current when the system voltage is reduced, By inserting a change rate limit or a time delay, it is possible to suppress an inrush current of the transformer 10 (or the interconnected transformer 11) that occurs at the time of recovery from a voltage drop. Thereby, the apparatus stop by the circuit breaker operation | movement etc. of the grid connection apparatus 1 and 12 which generate | occur | produces due to an inrush current is lost, and the reliability of the grid connection apparatuses 1 and 12 improves.

  The inrush current is a reactive current with a delay flowing from the system 3 due to a sudden decrease in excitation impedance due to magnetic saturation of the transformer. Further, since the inrush current is attenuated to such an extent that it does not become a problem in several cycles, it is important to suppress the inrush current during the period of several cycles. Therefore, in the grid interconnection devices 1 and 12 according to the embodiment of the present invention, the reactive current is canceled and the inrush current is suppressed by continuing to output the compensated reactive current for a predetermined period when the system voltage is restored.

  Moreover, since the breaking capacity in the higher system of the grid interconnection devices 1 and 12 can be reduced by suppressing the inrush current, there is an effect of downsizing the system.

  In addition, the grid connection apparatus of this invention is not limited to embodiment, A design deformation | transformation is possible suitably in the range which does not impair the characteristic.

DESCRIPTION OF SYMBOLS 1,12 ... Grid interconnection device 1A ... Grid interconnection device 1a, 1b ... Grid interconnection device 2 ... DC power supply 3 ... Grid (power system)
4 ... Inverter 5 ... Filter circuit 6 ... PWM signal output means 7 ... Control means 8 ... DVS function (DVS means)
9. Change rate limiting means (reactive current output continuation means)
10 ... Transformer 11 ... Interconnected transformer 13 ... Time delay means (reactive current output continuation means)

Claims (3)

An inverter connected to the power system via a transformer;
DVS means for outputting a reactive power command to a control means for controlling the inverter in response to a decrease in the system voltage of the power system, and causing the inverter to output a reactive current;
A grid interconnection device comprising: a reactive current output continuation unit that causes the DVS unit to output the reactive power command for a predetermined period after the system voltage of the power system is restored. 2. The reactive current output continuation means limits a change rate of the system voltage after the system voltage is restored, and outputs a system voltage with the change rate limited to the DVS means. Grid interconnection device. 2. The grid interconnection apparatus according to claim 1, wherein the reactive current output continuation unit inserts a time delay into the system voltage after the system voltage recovery and outputs the system voltage to the DVS unit.

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