Apparatus and method for calculating candidate set values

Abstract

To reduce a voltage deviation of a power distribution system within the limited number of times of tap operation even in a system into which a large number of photovoltaic generation operations is introduced.SOLUTION: A setting value candidate calculation apparatus applying a setting value of a tap position to a power transformer with a tap of a process control system, which is installed to a power distribution system and for which a timing for adjusting the tap position where voltage of the power distribution system is set to a set voltage is previously determined, comprises: first means of estimating an optimal value of the voltage of the power distribution system of a date as a prediction object; second means of calculating the tap value at the time within an optimal reference voltage obtained by adding and subtracting a coefficient to / from the optimal value; third means of calculating the tap value at a reference time unit for a reference date to the tap value at an initial time in a time sequence; and fourth means of determining a tap operation content that the change frequency of the tap value in the reference data becomes the reference frequency or less by sequentially changing the coefficient. The tap operation content determined in the fourth means is applied as a setting value of the tap position to the transformer with the tap.SELECTED DRAWING: Figure 4

Description

Technical Field

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[0001] The present invention relates to a setting value candidate calculation device, and particularly to a setting value candidate calculation device and method that enable voltage control of a power distribution system in which a large number of renewable energies such as solar power generation devices are connected in series.

Background Art

[0002] In recent years, the connection of solar power generation devices to the power distribution system has been increasing. However, in the power distribution system, when the power generation amount of the solar power generation device increases, there is a phenomenon that the voltage around the installation point of the solar power generation device rises. To avoid this, the solar power generation device is equipped with a function to suppress the power generation amount when the self-terminal voltage rises above the specified voltage. With this function, the power generation amount of the solar power generation device will be limited. <00000 -> 10>

[0003] On the other hand, the voltage of the power distribution system is controlled by tap switching of a load tap changing transformer (LRT) installed in the distribution substation and tap switching of an automatic voltage regulator (SVR) installed on the distribution line.

[0004] [[ID= -> 19]] To avoid the suppression of the power generation amount in the solar power generation device described above, it is important to adjust the voltage of the power distribution system with a voltage regulating device (load tap changing transformer LRT or automatic voltage regulator SVR) to avoid output suppression. For this purpose, it is necessary to appropriately perform tap control according to the power generation amount of the solar power generation device.

[0005] As a control method for a voltage regulating device (load tap changing transformer LRT or automatic voltage regulator SVR), the following methods are known.

[0006] For example, in a normal automatic voltage regulator SVR, a method of determining the tap value from the secondary side voltage at the self-terminal, the passing current, and the power factor is known.

[0007] Patent Document 1 describes a control method for selecting the output voltage of a voltage regulator so that the center value of the voltage fluctuation range, which is the sum of the voltage rise from the output voltage of the voltage regulator transformer to the voltage at the highest voltage point and the voltage drop from the output voltage of the transformer to the voltage at the lowest voltage point, becomes a specified value. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] WO2014 / 207849 [Overview of the project] [Problems that the invention aims to solve]

[0009] On the other hand, voltage regulators such as load-tap changing transformers (LRT) and automatic voltage regulators (SVR) are set by calculating the setting value using past measurement data acquired in advance from substations, etc., and then setting it on the voltage regulator via communication or manually on-site. In the future, a method could be considered in which measurement values ​​from sensor measurement points are acquired and stored using a communication network, the setting value is calculated on a server, and then set via the communication network.

[0010] There are two control methods for on-load tap-changing transformers (LRTs): the LDC method and the programmer control method (hereinafter referred to as the PRC method), with the PRC method being the most common. The LDC method operates a voltage relay 90Ry to control the tap position of the automatic voltage regulator SVR so that the line voltage drop is equivalent to the voltage drop at the load center point. The PRC method, on the other hand, schedules setting values, including the reference voltage, for each time period. The LDC method allows for fine-grained control according to the load current, making it superior when controlling on a feeder-by-feeder basis.

[0011] In the method described in Patent Document 1, when the voltage rise is avoided by suppressing the output of the solar power generation system, it is not possible to properly set the voltage regulator (load tap changer LRT or automatic voltage regulator SVR), which presents a problem in that the suppression of solar power generation output cannot be avoided.

[0012] In particular, with the increasing interconnection of mega solar power plants to the end of the distribution grid and the expansion of general consumer solar power generation equipment interconnection to the low-voltage side, the terminal voltage of solar power generation equipment will rise, potentially leading to output curtailment for certain consumers' solar power generation equipment and resulting in unequal losses of electricity sales opportunities. This raises concerns that voltage adjustment operations by power companies will become more complicated.

[0013] Furthermore, depending on the location where solar power generation is installed, the influence of solar radiation on solar power output varies due to factors such as the tilt and orientation of the solar panels, and differences in scattered light due to geographical conditions. This situation cannot be ignored, as it may prevent proper voltage regulation.

[0014] Based on the above, the present invention focuses on a pro-control method that is effective when setting banks collectively, and provides a setting value candidate calculation device and method that enables control to eliminate output suppression by lowering the voltage at the installation point of a solar power generation device when the output of a solar power generation device installed in a power distribution system is suppressed by dynamic setting that takes into account voltage fluctuations due to solar power generation and load conditions. [Means for solving the problem]

[0015] Based on the above, the present invention is described as "a setting value candidate calculation device that provides a setting value for a tapped transformer using a pro-con system, which is installed in a power distribution system and has a predetermined timing for adjusting the tap position to set the voltage of the power distribution system to a set voltage, comprising: a first means for estimating the optimal value of the voltage of the power distribution system for a date and time to be predicted; a second means for finding the tap value when it is within the range of an optimal reference voltage obtained by adding or subtracting a coefficient to the optimal value; a third means for finding the tap value in a reference time unit in chronological order with respect to the tap value at the initial time for a reference day; and a fourth means for sequentially changing the coefficient to find the tap operation content such that the number of changes in the tap value on the reference day is less than or equal to a reference number, wherein the setting value candidate calculation device provides the tap operation content found by the fourth means to the tapped transformer as the setting value for the tap position."

[0016] Furthermore, the present invention is defined as "a method for calculating candidate setting values ​​for tapped transformers using a pro-con system, in which the timing for adjusting the tap position to set the voltage of the distribution system to a set voltage is predetermined, and which is installed in a power distribution system, and which provides a set value for the tap position, characterized in that it estimates the optimal value of the voltage of the power distribution system for the date and time to be predicted, finds the tap value when it is within the range of the optimal reference voltage obtained by adding or subtracting a coefficient to the optimal value, finds the tap value in a time series for the reference day with respect to the tap value at the initial time in reference time units, finds the tap operation content such that the number of changes in the tap value on the reference day is less than or equal to a reference number by sequentially changing the coefficient, and provides the tap operation content to the tapped transformer as the set value for the tap position." [Effects of the Invention]

[0017] The present invention provides a candidate setting value calculation device and control method that can reduce voltage deviations in the distribution system within a limited number of tap operations, even in systems where a large amount of solar power generation is introduced. [Brief explanation of the drawing]

[0018] [Figure 1]A diagram showing a general power distribution system to which a setting value candidate calculation device according to an embodiment of the present invention is applied and an overview of a control system. [Figure 2] A diagram showing a general procone method in a load tap-changing transformer LRT. [Figure 3] A diagram showing a hardware configuration example of the setting value candidate calculation device 6. [Figure 4] A diagram showing a processing flow in the setting value candidate calculation device 6. [Figure 5] A diagram showing the relationship between the optimum value α and the optimum reference voltage Vref. [Figure 6] A diagram showing LRT tap position candidates within the range determined by the optimum value α. [Figure 7] [[ID=18ID=18]]A diagram showing tap positions by time zone before and after application of the present invention. [Figure 8] A diagram showing voltages by time zone before and after application of the present invention. [Figure 9] A diagram showing an example of a monitor screen display of the setting value candidate calculation device 6.

Embodiments of the Invention

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Examples

[0020] FIG. 1 is a diagram showing a general power distribution system to which a setting value candidate calculation device according to an embodiment of the present invention is applied and an overview of a control system.

[0021] First, regarding a general power distribution system, FIG. 1 illustrates a power distribution system from the power source on the left side of the figure to the load on the right side of the figure. A load tap-changing transformer LRT is installed at a substation on line L, and an automatic voltage regulator SVR is arranged in series on line L. In addition, a photovoltaic power generation device PV is arranged at an appropriate location in the power distribution system. Further, the power distribution system may include a power transmission system.

[0022] In contrast, the control system is configured to include a programmer 4 and a set value candidate calculation device 6 according to the present invention. Of these, the programmer 4 is installed, for example, on the on-load tap-changing transformer LRT2. Various system state quantities (voltage, current, etc.) are collected from various sensors 2 installed at various points in the power distribution system and sent to the programmer 4 via the slave station 3 and communication network 5. The programmer 4 calculates the ideal voltage using the various system state quantities and then sets the tap position of the on-load tap-changing transformer LRT2 so that it stays within the upper limit of the number of tap operations.

[0023] Furthermore, the control system is connected to a setting value candidate calculation device 6 according to the present invention via a communication network 5. The setting value candidate calculation device 6 calculates a setting value from past measurement data acquired in advance from a slave station, etc., and transmits the calculated setting value to the control device of the voltage regulator (load tap changing transformer LRT or automatic voltage regulator SVR) (procon 4 in the case of load tap changing transformer LRT2) via the communication network 5, or sets it to the voltage regulator manually. Here, we will describe an example in which measurement values ​​from the measurement point of sensor 2 are acquired and stored via the communication network 5, the setting value candidate calculation device 6, which is a server, calculates the setting value, and sets it via the communication network 5.

[0024] The on-load tap-changing transformer LRT in Figure 1 is controlled by a control device (procon) 4 that receives a set value from a set value candidate calculation device 6, employing a procon control method. In this procon method, the timing for adjusting the tap position of the on-load tap-changing transformer LRT to bring the voltage of the power distribution system within a predetermined range is pre-scheduled, and the tap adjustment is performed at that timing. There are various approaches to pre-scheduling the timing for adjusting the tap position, and the present invention is not limited to this point, but it is desirable that the number of tap adjustments within a unit period be kept below a predetermined number from the viewpoint of suppressing transformer tap fatigue.

[0025] Figure 2 illustrates a typical programming method for an on-load tap-changing transformer (LRT). In Figure 2, the vertical axis represents each month of the year, and the horizontal axis represents the days within each month. For each day determined by the month and day, the tap positions for each 24-hour period are predetermined.

[0026] In this way, the setting method for load-tap changing transformers (LRTs) using the pro-controller method involves scheduling the pro-controller settings for one year, and using a time relay to set the tap position (target voltage) for up to six time periods each day. The scheduling settings for changing the tap position can be set, for example, to be higher under heavy load and lower under light load relative to a pre-determined load curve.

[0027] On the other hand, because the system is operated with the same set values ​​throughout the year (dividing the day into 6 time periods and setting them accordingly), there may be times when voltage fluctuations cause large errors. Therefore, in this invention, we believe that advanced voltage management is possible by setting the programmer settings, calculated based on voltage fluctuations caused by solar power generation equipment (PV) and loads, on a daily basis.

[0028] Figure 3 shows an example of the hardware configuration of the setting value candidate calculation device 6. The setting value candidate calculation device 6, which is composed of a computer, communicates with the sensor 2 and slave station 3 via the communication network 5 from the communication unit 61 to exchange data, accepts input from input means 66 such as a keyboard operated by the operator via the input unit 67, and converts the processing results into visualization information in the image creation unit 64 and displays it on the screen 90 of the monitor 65, which is externally connected. In addition to the programmer 4, the setting value candidates determined by the setting value candidate calculation device 6 can also be sent to other load-on tap-changing transformers LRT1 and automatic voltage regulators SVR for use.

[0029] The set value candidate calculation device 6, which is composed of computers, shares various information through the connection of its database DB and CPU 63 (which is the calculation unit) to the bus 62, in addition to the communication unit 61, input unit 67, and image creation unit 64. The database DB stores the data necessary for processing by the programmer 4, but there is no compelling reason why this database DB must be installed within the set value candidate calculation device 6; it may obtain the appropriate information from an externally installed database DB.

[0030] Furthermore, the system configuration shown in Figure 3 includes a solar radiation acquisition means that obtains information from a measurement means for solar radiation in the power distribution system, and a wind speed acquisition means that obtains information from a measurement means for wind speed. The database DB should record information from the solar radiation acquisition means, the wind speed acquisition means, information on the configuration of the target power distribution system, information on the arrangement and capacity of the solar power generation equipment, information on the load pattern, and information on the arrangement data of tapped transformers. It is even more desirable to take this information into account when deriving the optimal voltage using power flow calculations, which will be discussed later.

[0031] Figure 4 shows the processing flow in the setting value candidate calculation device 6, which determines the tap position to be set on the program controller 4 of the load-tap switching transformer LRT. This process mainly describes the processing performed in the CPU 63, which is the calculation unit in Figure 3.

[0032] Furthermore, the programmable settings for the load-tap changing transformer LRT require determining tap positions for up to six time periods. Therefore, it is necessary to appropriately determine tap values ​​for these six time periods to account for voltage fluctuations such as weekdays, holidays, seasons, and solar power generation equipment. In addition, when determining the optimal tap position for the load-tap changing transformer LRT for each time period, it is necessary to determine the optimal reference voltage Vref and simultaneously provide a daily division pattern. In this case, there are a huge number of patterns for dividing the day into six sections. Therefore, by setting an optimal value α (upper and lower limits) for the coefficient and determining the tap setting within the range of α, the optimal tap setting can be extracted with a small number of steps.

[0033] The flow shown in Figure 4 will be explained below. The following process is shown as an example of being performed for 365 days a year. In the first processing step S301 of this flow, the setting value candidate calculation device 6 acquires measured values ​​measured by sensors 2 (including switches with sensors) installed in the power distribution system. Next, in processing step S302, power flow calculations are performed in the feeder based on these values, and voltage calculations are performed. This process is performed for each time section, such as in one-hour increments.

[0034] In processing step S303, based on these results, the ideal voltage of the distribution substation output voltage, i.e., the optimal reference voltage Vref, is calculated so as to ensure the greatest voltage margin with respect to the upper and lower voltage limits determined from the tap map at each node of the feeder. This can be done using multiple regression analysis.

[0035] Next, in processing step S304, the coefficient α is increased from 0 by an upper limit and step size set in advance by the operator, and in processing step S305, tap values ​​within the range of Vref±α are extracted.

[0036] Next, in processing step S306, the tap that has the nth voltage deviation is selected for the initial time period of 0:00:00. This nth value is a parameter that is changed, and it is possible to avoid the voltage deviation at tap positions in subsequent time periods becoming large due to the initial tap position.

[0037] Therefore, in processing step S307, the following series of processes are executed. First, in processing step S307a, a determination is made as to whether the tap extracted one hour earlier is within ±α. If it is within the range, the process moves to processing step S307b and the tap is selected. Otherwise, the process moves to processing step S307c and the tap with the smallest voltage deviation is selected.

[0038] In processing step S307d, this process is carried out up to the 24:00:00 cross-section, and the process proceeds to processing step S307e, indicating that the tap position has ended. If the process does not reach the 24:00:00 cross-section, the process returns to the beginning of processing step S307 and repeats the process up to the 24:00:00 cross-section. If the process is completed up to the 24:00:00 cross-section, in processing step S307e, tap pattern extraction is performed for all initial positions.

[0039] In processing step S308, the number of tap settings is checked, and it is determined whether it falls within the maximum number of tap operations set by the operator. For example, if the maximum number of tap operations is 6 times / day, a tap pattern with 6 or fewer tap operations is selected. In processing step S309, the tap pattern with the smallest voltage deviation is finally determined.

[0040] This flow allows for voltage adjustment while staying within the daily limit on the number of tap operations, making it possible to understand and manage the lifespan due to tap operations. In this embodiment, the daily limit on the number of tap operations was set, but it is also possible to set an annual limit on the number of tap operations and determine the tap pattern for the entire year within that range. This would relax the daily limit on the number of tap operations, which is expected to have the effect of enabling voltage adjustment that takes into account fluctuations in solar power generation.

[0041] To summarize the above process and express it as a processing means to realize it, it can be said that a setting value candidate calculation device for a tapped transformer using a pro-con system, which is installed in a power distribution system and has a predetermined timing for adjusting the tap position to set the voltage of the power distribution system to a set voltage, is configured as follows: a first means for estimating the optimal value of the voltage of the power distribution system for the date and time to be predicted; a second means for finding the tap value when it is within the range of the optimal reference voltage obtained by adding or subtracting a coefficient to the optimal value; a third means for finding the tap value in reference time units in chronological order for the reference day with respect to the tap value at the initial time; and a fourth means for sequentially changing the coefficient to find the tap operation content such that the number of changes in the tap value on the reference day is less than or equal to a reference number, and the tap operation content obtained by the fourth means is given to the tapped transformer as the setting value of the tap position.

[0042] Furthermore, while the above results and historically accumulated data will be stored in a database (DB), it is desirable that this database record the total number of tap operations per day, the maximum number of tap operations per day, the total number of tap operations per year, and the maximum number of tap operations per year separately.

[0043] Figure 5 shows the relationship between the optimal value α and the optimal reference voltage Vref, with time on the horizontal axis and voltage on the vertical axis. Figure 6 shows the candidate tap positions within the range determined by this optimal value α. Here, as shown in Figure 5, the optimal value (α) is calculated for each time period (every hour), and the upper and lower limits of the optimal value (Vref ± α) are set. Then, candidate taps are extracted from the range of the upper and lower limits Vref ± α of the optimal values ​​from 0:00 to 23:00 (24 times), and the tap value that minimizes the deviation is determined from the extracted candidate taps, taking into account the constraints on the number of taps to be set. This sets the tap positions in the program controller so that the deviation from the optimal value is minimized.

[0044] Furthermore, Figure 6, which shows candidate LRT tap positions within the range determined by the optimal value α, shows the error from the optimal value on the horizontal axis and the voltage and tap position on the vertical axis. According to these candidate LRT tap positions, for the time period before 7:00:00, the error from the optimal value when tap position 5 is 6600(V) is 15(V), and the upper and lower limits of the optimal value, Vref±α, are between tap position 4, 6700(V) and tap position 6, 6500(V). In contrast, even at the time period of 7:00:00, the error from the optimal value when operating under the same conditions (tap position 5, 6600(V)) is 30(V), but the error from the optimal value within the upper and lower limits of the optimal value, Vref±α, is 130(V) at tap position 4, 6700(V) and 70(V) at tap position 6, 6500(V). The tap position at the time period of 7:00:00 is not appropriate and there is room for improvement.

[0045] At 8:00:00, the tap position was changed from 5 to 4 based on these results, but the error from the optimal value increased to 40(V), actually widening the gap. However, at 9:00:00, the error from the optimal value decreased to 10(V) under the same operating conditions, and remained around 20(V) thereafter, so the same tap conditions were maintained until 12:00.

[0046] Figure 6 shows that, subsequently, due to the widening error from the optimal value in the previous time period, tap changes are made at 15:00:00, 19:00:00, and 21:00:00.

[0047] Figure 7 is a graph showing the tap positions by time of day before and after applying the flow for determining the tap positions to be set on the LRT procontroller shown in Figure 4. The horizontal axis shows time and the vertical axis shows the tap position. The solid line shows the period after application of the present invention, and the dotted line shows the period before application. Before application, a total of 8 tap operations should have been performed at 5:00, 7:00, 8:00, 9:00, 11:00, 14:00, 17:00, and 23:00, while after application, the number of taps was limited to 5:00, 7:00, 11:00, 14:00, and 18:00.

[0048] Figure 8 shows time on the horizontal axis and voltage on the vertical axis, with solid lines indicating the period after application of the present invention and dotted lines indicating the period before application. The time segments A1, A2, and A3 enclosed by dotted lines indicate that the tap operation was performed to change the voltage in the direction of the arrow (changing the voltage from dotted lines to solid lines). The arrow indicated by B1 shows the phenomenon where the voltage rise caused by the photovoltaic (PV) solar power generation system was reduced and improved by control.

[0049] As described above, using the candidate setting values ​​calculated by the setting value candidate calculation device 6, the program control is performed as shown in Figure 7, and it is expected that the system voltage will be improved as shown in Figure 8.

[0050] According to conventional methods, when setting the tap value of a load-tapped transformer (LRT) to minimize the deviation from the optimal value, the tap setting may exceed the constraint conditions of the programmable transformer (maximum 6 zones). In this regard, by applying the present invention method shown in Figure 4, it was confirmed that the tap setting can be configured to satisfy the constraint conditions even when the tap constraint is exceeded.

[0051] Figures 7 and 8 show the results of an investigation into August (summer), a period of significant load fluctuations throughout the day. It can be seen that when the tap value that minimizes the deviation is set in August, the number of tap settings exceeds the constraint. However, by performing optimal setting of the load-dependent tap-changing transformer (LRT) based on the flowchart shown in Figure 4, it is possible to determine the tap position even if the number of tap settings is within the constraint. Furthermore, it was confirmed that the taps were appropriately controlled in the time-series voltage transition, addressing voltage drops in the morning and evening, as well as voltage increases from solar power generation equipment (PV). Similarly, good control was confirmed for other seasons, demonstrating its effectiveness.

[0052] Figure 9 shows an example of the monitor screen display of the set value candidate calculation device 6. In this example displayed on the monitor screen 90, items are selectively displayed on the left, and the selection results are visualized on the right, for example, as numerical values ​​or status. First, check the "Month and Day Selection" item on the left and select the month and day for which you want to determine the tap pattern from the displayed date table. In this process, you can also check the "One Year at Once" item to determine the tap pattern for one year at once.

[0053] Then, with the measured values ​​acquired, press "Execute Setting" to perform the setting. As a result, the "Optimal Voltage Calculation Result" is displayed based on the power flow calculation. Furthermore, the operator sets the "Upper Limit and Step Size for Optimal Voltage α". As a result, the process in Figure 4 determines a range of ±α for the "Optimal Voltage Calculation Result", and the result of searching for tap positions within that range is displayed as the "Setting Result".

[0054] The above explanation primarily describes the use of the set value candidate calculation device 6 for predictive processing of future dates and times in offline processing, but it can also be applied online for same-day control.

[0055] This invention can be used as a voltage regulator to adjust the voltage of a power distribution system. Furthermore, it can be used in power distribution systems as a voltage maintenance measure and a measure to improve the utilization rate of power distribution equipment, in response to the addition of distributed power sources such as solar power generation systems (PV). [Explanation of Symbols]

[0056] LRT: On-load tap changer transformer SVR: Automatic Voltage Regulator 3: Solar power 4: Pro Controller 5: Communication Network 6: Set value candidate calculation device 61: Communications Department 62: Bus 63:CPU 64: Image Creation Department 65: Monitor 66: Input method 67: Input section 90: Screen DB: Database

Claims

1. A tapped transformer using a programmer-type control system, installed in a power distribution system, in which the timing for adjusting the tap position to set the voltage of the power distribution system to a set voltage is predetermined, is a device for calculating candidate setting values ​​for the tap position, which provides setting values ​​for the tap position. A set value candidate calculation device comprising: a first means for estimating the optimal voltage of the power distribution system at a date and time to be predicted; a second means for determining the tap value when it is within the range of an optimal reference voltage obtained by adding or subtracting a coefficient to the optimal value; a third means for determining the tap value in a reference time unit in a time series with respect to the tap value at an initial time for a reference day; and a fourth means for determining the tap operation content such that the number of changes in the tap value on the reference day is less than or equal to a reference number by sequentially changing the coefficient, wherein the tap operation content determined by the fourth means is provided to the tapped transformer as the set value of the tap position.

2. A device for calculating candidate set values ​​according to claim 1, The device for calculating candidate set values ​​is characterized by sequentially increasing the coefficient from an initial value and confirming the number of times the tap value has been changed each time.

3. A device for calculating candidate set values ​​according to claim 1, The power distribution system setting value candidate calculation device is characterized in that the coefficient is set such that the total number of tap operations per day on the reference day is less than or equal to the maximum number of tap operations per day, or the total number of tap operations per year is less than or equal to the maximum number of tap operations per year.

4. A device for calculating candidate set values ​​according to claim 1, A power distribution system setting value candidate calculation device characterized by setting the initial tap value to a variable value and determining the number of times the tap value has been changed at that time.

5. A device for calculating candidate set values ​​according to claim 1, A power distribution system setting value candidate calculation device characterized by, when determining the tap value in a reference time unit with respect to the tap value at the initial time, selecting the same tap as the tap before the reference time if that tap is within the coefficient, and otherwise selecting the tap that minimizes the voltage deviation.

6. A method for calculating candidate setting values ​​for tapped transformers using a pro-con system, in which the timing for adjusting the tap position to set the voltage of the distribution system to a set voltage is predetermined, wherein the setting value for the tap position is provided. A method for calculating candidate setting values, characterized by estimating the optimal voltage of the power distribution system for a date and time to be predicted, determining the tap value when it is within the range of the optimal reference voltage obtained by adding or subtracting a coefficient to the optimal value, determining the tap value in a reference time unit for the reference day in chronological order with respect to the tap value at the initial time, determining the tap operation content such that the number of tap value changes on the reference day is less than or equal to a reference number by sequentially changing the coefficient, and providing the tap operation content to the tapped transformer as the setting value of the tap position.

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