Apparatus and method for determining whether or not a transformer is overloaded

Abstract

To appropriately determine whether an overload operation of a transformer for a low-voltage solar power generation system on which a load equal to or greater than the rating is applied for a long time period, does not go beyond an allowable limit.SOLUTION: On the basis of a panel capacity in a transformer bank and a PCS rated output, a PCS output predicted from a solar power generation curve is calculated. On the basis of the calculated PCS output, a time period of an overload operation exceeding the rated capacity of a transformer 7 is calculated. For the calculated time period of an overload operation, a previously estimated allowable load factor and an actual overload factor of the transformer 7 are compared. When the overload factor is equal to or less than the allowable load factor, it is determined that the overload operation of the transformer 7 is not beyond an allowable limit. When the overload factor is greater than the allowable load factor, it is determined that the overload operation of the transformer 7 is beyond the allowable limit. An overload operation allowability determination considering a time period of an overload operation of the transformer 7 obtained by introducing a solar power generation curve, can be made. A proper determination suitable for a pole transformer for a low-voltage solar power generation system can be made.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an apparatus and method for determining whether or not overload operation of a pole transformer for low-voltage photovoltaic power generation grid connection is within a tolerable range. [Background technology]

[0002] Conventionally, the overload limit for pole transformers installed on distribution lines has been set at 160% for transformer capacities of 30 kVA or less, except in special cases (see, for example, Non-Patent Document 1). Even nowadays, when solar power generation has become widespread, such standards for overload limits are often adopted as is. In addition, the conditions under which a transformer can continue to operate without significantly reducing its lifespan even when subjected to a load greater than the rated capacity are disclosed in the Institute of Electrical Engineers' technical report, "Guidelines for Operation of Oil-Immersed Transformers." [Prior art documents] [Patent documents]

[0003] [Non-Patent Document 1] "Facility Formation Rules (High and Low Voltage Edition)", Hokuriku Electric Power Co., Inc., p.11 [Non-Patent Document 2] "Oil-immersed Transformer Operation Guidelines (from the Institute of Electrical Engineers of Japan Technical Report Part I No. 143)" Summary of the Invention [Problem to be solved by the invention]

[0004] In photovoltaic power generation systems, the spread of photovoltaic power generation equipment, in which the panel capacity exceeds the PCS capacity, has led to a phenomenon known as "overloading," and there have been an increasing number of cases in which the transformer utilization rate exceeds 100% (the transformer overload rate exceeds 1.0) and continues for long periods of time in an overloaded state.

[0005] The previous overload limit standard (160%) was set on the assumption that overload operation times for ordinary households, shops, factories, etc. would be on the order of a few minutes to a few tens of minutes. Therefore, when a load above the rated capacity is applied for a long period of time, such as in transformers for connecting photovoltaic power generation systems, if the permissible overload operation was determined based on the previous overload limit standard, there was a concern that the insulating oil, windings, etc. would deteriorate and eventually burn out. Therefore, it would be possible to make a judgment on the permissibility of overload operation of a transformer based on the conditions specified in the "Guidelines for Operation of Oil-Immersed Transformers." However, since it is not possible to accurately grasp how long overload operation will continue, it is not possible to make an appropriate judgment on the permissibility of overload operation.

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and its main objective is to provide a transformer overload operation tolerance determination device and a transformer overload operation tolerance determination method that are capable of appropriately determining whether overload operation is within the permissible limit for a transformer for low-voltage solar power generation grid connection that is subjected to a load above the rated load for long periods of time. [Means for solving the problem]

[0007] In order to achieve the above object, a transformer overload operation allowance determination device according to the present invention comprises: An input means for inputting a rated capacity of a transformer, a panel capacity of a solar panel in the bank of the transformer, and a rated output of a PCS; A PCS output calculation means for calculating a PCS output expected from a photovoltaic power generation curve based on the panel capacity and the PCS rated output; an overload operation time calculation means for calculating an overload operation time during which the transformer is overloaded beyond a rated capacity based on the PCS output calculated by the PCS output calculation means; an allowable judgment means for comparing an overload rate determined by the ratio of the rated output of the PCS to the rated capacity of the transformer with an allowable load rate previously evaluated for the overload operation time for a transformer of the same type as the transformer, and judging that the overload operation of the transformer is within an allowable range if the overload rate is equal to or less than the allowable load rate, and judging that the overload operation of the transformer is outside the allowable range if the overload rate exceeds the allowable load rate; The present invention is characterized by the following:

[0008] The solar power generation curve may be a predetermined representative power generation curve, may be obtained from the amount of solar radiation on a past day corresponding to the determination date, may be obtained from the amount of solar radiation immediately prior to the determination date, or may be obtained from the average value of the amount of solar radiation over a specified period of time.

[0009] Therefore, the PCS output calculation means calculates the PCS output expected from the solar power generation curve, and the overload operation time calculation means calculates the overload operation time that will result in an overload that exceeds the rated capacity of the transformer based on the calculated PCS output, and the tolerance judgment means compares the allowable load rate previously assessed for the calculated overload operation time with the actual overload rate of the transformer, and if the overload rate is below the allowable load rate, it is judged that the overload operation of the transformer is within the allowable range, and if the overload rate exceeds the allowable load rate, it is judged that the overload operation of the transformer is outside the allowable range.Therefore, an tolerance judgment is made that takes into account the overload operation time of the transformer, making it possible to make an accurate judgment that is appropriate for a pole-mounted transformer for low-voltage solar power generation grid connection.

[0010] Here, it is preferable that the PCS output expected from the solar power generation curve is calculated based on a corrected curve in which the solar power generation curve is adjusted so that the solar panel has a rated output at noon. By using such a correction curve, the time when the amount of solar radiation is greatest is considered to be noon, and a state in which the panel output is rated at this time is created, so that the transformer overload is not underestimated. Therefore, by introducing such processing, it is possible to reliably avoid the risk of the transformer burning out due to overload.

[0011] In addition, input through the input means is performed by selecting an input value from an operation screen or by inputting an arbitrary input value, and the result determined by the tolerance determination means is displayed on the operation screen. In this way, the judgment result is displayed on the input screen, making it possible to grasp the judgment result together with the input value, and thus making it possible to avoid erroneous judgment. Effect of the Invention

[0012] As described above, the overload operation time of the transformer is calculated from the PCS output estimated from the photovoltaic power generation curve, the overload rate of the transformer is compared with the allowable load rate previously evaluated for the overload operation time, and if the overload rate of the transformer is equal to or lower than the allowable load rate, it is determined that the overload operation of the transformer is within the allowable range, and if the overload rate of the transformer exceeds the allowable load rate, it is determined that the overload operation of the transformer is outside the allowable range, so that even if the transformer is connected to a low-voltage photovoltaic power generation system, it is possible to appropriately determine whether the overload state of the transformer is within the allowable range. This makes it possible to avoid accidents such as burnout of the transformer used for photovoltaic power generation. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the schematic configuration of a low-voltage solar power generation facility and a pole-mounted transformer connected to the power generation facility. [Diagram 2] FIG. 2 is a diagram showing a schematic configuration of a transformer overload operation allowance determination device according to the present invention. [Diagram 3] FIG. 3 is a flowchart showing an example of a calculation process in the control unit of FIG. [Figure 4]Figure 4 is a table showing various data for the solar power generation curve, the modified curve resulting from modifying the solar power generation curve, the PCS output when not overloaded, the solar panel output, the PCS output, the capacity of the transformer when operating at 100%, the capacity of the transformer when operating at 160%, and the overload operating time when the transformer is operated beyond its rated capacity, assuming that the transformer rated capacity is 30 kVA, the panel capacity is 80 kW, and the PCS rated output is 47.4 kW. [Diagram 5] FIG. 5 is a table showing the relationship between overload operation time and allowable load (allowable load rate) in the Oil-immersed Transformer Operation Guidelines (Technical Report of the Institute of Electrical Engineers of Japan). [Figure 6] FIG. 6 is a diagram showing a transformer overload judgment sheet when the rated capacity of the transformer is 30 kVA, the panel capacity is 80 kW, and the PCS rated output is 47.4 kW. [Figure 7] Figure 7 is a table showing various data for the solar power generation curve, the modified curve resulting from modifying the solar power generation curve, the PCS output when not overloaded, the solar panel output, the PCS output, the capacity of the transformer when operating at 100%, the capacity of the transformer when operating at 160%, and the overload operating time when the transformer is operated beyond its rated capacity, assuming that the transformer rated capacity is 43 kVA, the panel capacity is 80 kW, and the PCS rated output is 47.4 kW. [Figure 8] FIG. 8 is a diagram showing a transformer overload judgment sheet when the rated capacity of the transformer is 43 kVA, the panel capacity is 80 kW, and the PCS rated output is 47.4 kW. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0015] An example of a low-voltage solar power generation system is shown in Fig. 1. This solar power generation system 1 has a solar cell array 2a constituting a solar panel 2, and a power conditioner (hereinafter referred to as PCS) 4 that converts DC power generated by the solar panel 2 into AC power. The AC power output from the PCS 4 is supplied to a pole-mounted transformer (hereinafter referred to as transformer) 7 via a distribution board 5, a watt-hour meter 6, etc., and is supplied to a power system (electric line) 8 via the transformer 7. The solar cell array 2a includes, for example, a plurality of solar cell strings 2b each including a plurality of solar cell modules connected in series.

[0016] When one PCS 4 is connected directly below the transformer 7 as described above, the rated output of the solar panel 2 (solar cell array 2a) connected to this PCS 4 becomes the panel capacity (panel rated output) in the transformer bank, and the rated output of one connected PCS 4 becomes the PCS rated output in the transformer bank. Note that when multiple PCSs are connected directly below the same transformer 7, the sum of the rated outputs of the solar panels connected to each PCS becomes the panel capacity in the transformer bank, and the sum of the rated outputs of each PCS becomes the PCS rated output in the transformer bank.

[0017] Incidentally, the panel capacity of the solar panel 2 is often set to exceed the rated output of the PCS 4 (to be overloaded) in order to increase the operating time at the rated output of the PCS 4. Furthermore, when attempting to install a PCS 4 having a rated output greater than the capacity of the transformer 7, the rated output of the PCS 4 is often set to less than 160% of the transformer capacity so that the current transformer can handle it, rather than changing the transformer to a larger capacity one, because a standard was conventionally used that set the allowable overload limit of the transformer at less than 160%.

[0018] For example, if the panel capacity is 80kW and the pole transformer capacity is 30kVA, the PCS capacity is selected to be less than 48kW (e.g., 47.4kW) in order to keep the overload rate below 160%. Setting it in this way satisfies the standard for the overload limit of the transformer 7, and also meets the standard that the Electricity Business Act requires that the generating capacity be less than 50kW in the case of small-scale power generation via low-voltage interconnection.

[0019] However, since the overload tolerance limit of 160% for transformer 7 is set on the assumption that the overload operation time of pole transformers supplying general households, shops, factories, etc. is several minutes to several tens of minutes, in the case of pole transformers for low-voltage solar power generation grid connections that are subjected to a load above the rated capacity for a long period of time, if the overload operation is judged as permissible based on the same criteria as before, there is a concern that the deterioration of insulating oil, windings, etc. will progress, leading to damage. For this reason, when reviewing the permissible judgment of overload operation of transformer 7, it is desirable to judge the permissible overload operation of transformer 7 taking into consideration not only the overload rate of transformer 7 (the ratio of the rated output of PCS4 to the rated capacity of transformer 7), which indicates the rate at which the rated output of PCS4 exceeds the rated capacity of transformer 7, but also the duration of overload operation (the time during which the output of PCS4 exceeds the rated capacity of transformer 7).

[0020] Therefore, in the present invention, data on a reference solar power generation curve is introduced, and the introduced solar power generation curve is used to estimate the PCS output. This estimated PCS output is compared with the transformer capacity to calculate the time during which the transformer utilization rate exceeds 100% (transformer overload operation time), and based on this, it is determined whether the transformer overload state is within the acceptable range.

[0021] As one example, as shown in Figure 2, the transformer overload operation tolerance determination device inputs the rated capacity of the transformer to be determined, the rated output of the solar panels in the transformer bank (panel capacity), and the rated output of the PCS in the transformer bank (PCS rated output) from an input / output screen 10 of a personal computer, mobile terminal, etc., and the control unit 11 calculates the overload operation time of the transformer based on this information and taking into account the solar power generation curve, and determines whether the overload operation of the transformer 7 is within the tolerance range, taking into account this overload operation time, and displays the result on the input / output screen 10.

[0022] 3 shows a specific example of calculation processing in the control unit 11 as a flowchart, and below, the process of determining whether or not the transformer 7 is allowed to operate overload will be described based on this flowchart. In addition, in order to provide this description, each calculation data is shown in FIG. 4 when the rated capacity of the transformer 7 is 30 kVA, the rated output (panel capacity) of the solar panels in the transformer bank is 80 kW, and the rated output (PCS capacity) of the PCS in the transformer bank is 47.4 kW.

[0023] First, the control unit 11 takes in each piece of data of the rated capacity of the transformer 7 to be judged, the rated output of the solar panels in the transformer bank (panel capacity), and the rated output of the PCS in the transformer bank (PCS capacity) (step 100). Thereafter, data on the solar power generation curve for one day (data on "Sunlight" in FIG. 4) that serves as the basis for the calculation is acquired (step 102).

[0024] For this reference solar power generation curve, it is preferable to set power generation data corresponding to the judgment date in the area where the transformer 7 is installed for each time period (e.g., every 30 minutes), but representative power generation data in a predetermined representative area may be used. In this case, the power generation data of the solar power generation curve may be obtained from solar radiation data on the same past day corresponding to the judgment date, from solar radiation data immediately before the judgment date, or from the average value of solar radiation data for a predetermined period. Since the power generation model of these solar power generation curves is based on solar radiation data, data from the NEDO solar radiation database (METPV-11) 12, for example, may be used.

[0025] The captured photovoltaic power generation curve data is then corrected by multiplying it by a predetermined coefficient so that the efficiency is maximum (power generation efficiency is "1") at noon, and a corrected curve (data of "corrected curve" in FIG. 4) is calculated (step 104). This is because we want to estimate the situation in which the panel output in the transformer bank is maximum at noon (the panel output of the photovoltaic panel 2 is the rated output). In this way, by considering noon as the time when the amount of power generation is greatest and assuming a state in which the panel output at this time is the rated output, it is possible to avoid underestimating the overload state of the transformer, and to more reliably prevent burnout of the transformer.

[0026] Then, based on this correction curve (power generation curve in which the power generation efficiency at noon is set to 1), the panel output for one day (the data for "Panel Output" in FIG. 4) is calculated (step 106).

[0027] Thereafter, this panel output is compared with the rated output of PCS4, and the PCS output peak-cut by PCS4 (the "PCS output" data in FIG. 4) is calculated (step 108). Since the time during which the PCS output exceeds the rated capacity of the transformer (100% operation) is the time during which overload operation occurs, that time is tallied to calculate the overload operation time (step 110). In the example of Fig. 4, the time during which the PCS output exceeds the transformer capacity of 30 kVA is calculated to be 9 hours (0.5h x 18 = 9h) from 7:30 to 4:30. Through the series of operations described above, the overload operation time of transformer 7 is calculated from the PCS output estimated from the solar power generation curve.

[0028] After the overload operation time has been calculated in this manner, the overload rate of the transformer 7 used in the solar power generation equipment (rated output of PCS / rated capacity of transformer 7) is then compared with the corresponding portion of the allowable load (allowable overload rate) evaluated in relation to the overload operation time of the same type of transformer according to, for example, the Oil-Immersed Transformer Operation Guidelines (Technical Report of the Institute of Electrical Engineers) shown in Figure 5, and it is determined whether the overload rate of the transformer 7 is within the range of this allowable overload rate (step 112).

[0029] If the result of the judgment is that the overload rate of the transformer is equal to or less than this allowable load (allowable overload rate), it is judged that the overload state of the transformer is within the allowable range, and the display unit is made to show "Good." Conversely, if the overload rate of the transformer exceeds this allowable load (allowable overload rate), it is judged that the overload state of the transformer exceeds the allowable limit, and the display unit is made to show "Overload" (steps 114, 116).

[0030] For example, if the rated output of the PCS is 47.4kW, the panel capacity (rated capacity) of the solar cell array is 80kW, and the rated capacity of the transformer is 30kVA (time constant 2.5, ambient temperature 40°C), the overload factor of the transformer is 1.58 (47.4 / 30). If the overload operation time is calculated from the PCS output calculated based on the above solar power generation curve, it is 9 hours, and the allowable load at that time is 1.09 (109 as a percentage), which is the allowable load when exceeding 8 hours according to Table 4.2 of the "Oil-Immersed Transformer Operation Guidelines," so the current overload factor of 1.58 is greater than the allowable load. For this reason, the overload state of the transformer exceeds the allowable limit (outside the allowable range), and is displayed as "overloaded."

[0031] Therefore, with the conventional method of determining overload tolerance, there would have been no problem if the PCS capacity was less than 48 kW (for example, 47.4 kW). However, according to the above method, which was calculated by introducing the solar power generation curve, if the PCS was selected at 47.4 kW, the load on the transformer would be an overload that exceeded the tolerance limit, and measures would be needed to avoid the transformer burning out.

[0032] Conversely, by using the method described above, if the rated output of PCS4 is 47.4 kW and the panel capacity (rated capacity) of solar panel 2 is 80 kW, it is possible to find the transformer capacity that can tolerate an overload condition by changing the transformer capacity and simulating.

[0033] For example, if the transformer capacity is selected to be 43kVA, it will look as shown in Figures 7 and 8, and the overload operation time of the transformer will be calculated as 7.5 hours using the same method as above. The overload factor of the transformer is 1.1 (47.4 / 43), and the allowable load (percentage of rated load) assessed for the same type of transformer by the Oil-Immersed Transformer Operation Guidelines for the calculated overload operation time is 1.15 (115% as a percentage), so the transformer overload is within the allowable range and the display will show "Good." Therefore, from these results, it can be seen that a transformer that can tolerate an overload state should be selected that has a capacity at least greater than 43kAV (for example, 50Kw).

[0034] Therefore, by introducing a photovoltaic power generation curve, the overload operation time during which the output of PCS4 exceeds the rated capacity of transformer 7 is calculated, and the allowable overload rate, which is the permissible limit, is calculated from this overload operation time. This allowable overload rate is compared with the actual overload rate of the transformer to determine whether the overload operation state is within the permissible range, making it possible to easily determine whether the transformer overload is within the permissible range in relation to the overload operation time.

[0035] In addition, by using this evaluation method, it is also possible to simulate the transformer capacity for which the overload condition will be within the acceptable range for the rated output and panel capacity of the selected PCS. By selecting a transformer that is evaluated as "good" using this method, it is possible to reliably avoid accidents such as burnout of transformers for low-voltage solar power generation grid connection. [Explanation of symbols]

[0036] 1. Transformer overload operation tolerance determination device 2. Solar panels 4 Power conditioner (PCS) 7. Transformers

Claims

1. An input means for inputting a rated capacity of a transformer, a panel capacity of a solar panel in the bank of the transformer, and a rated output of a PCS; A PCS output calculation means for calculating a PCS output estimated from a photovoltaic power generation curve based on the panel capacity and the PCS rated output; an overload operation time calculation means for calculating an overload operation time during which the transformer is overloaded beyond a rated capacity based on the PCS output calculated by the PCS output calculation means; an allowable judgment means for comparing an overload rate determined by the ratio of the rated output of the PCS to the rated capacity of the transformer with an allowable load rate previously evaluated for the overload operation time for a transformer of the same type as the transformer, and judging that the overload operation of the transformer is within an allowable range if the overload rate is equal to or less than the allowable load rate, and judging that the overload operation of the transformer is outside the allowable range if the overload rate exceeds the allowable load rate; A transformer overload operation tolerance determination device comprising:

2. The transformer overload operation tolerance determination device as described in claim 1, characterized in that the PCS output estimated from the solar power generation curve is calculated based on a correction curve in which the solar power generation curve is adjusted so that the solar panel has rated output at noon.

3. The input by the input means is performed by selecting an input value from an operation screen or by inputting an arbitrary input value; 3. The transformer overload operation permissibility judgment device according to claim 1, wherein the result judged by said permissibility judgment means is displayed on said operation screen.

4. A PCS output calculation step of calculating a PCS output estimated from a photovoltaic power generation curve based on the panel capacity of the solar panels in the bank of the transformer and the PCS rated output; an overload operation time calculation step of calculating an overload operation time during which the overload exceeds a rated capacity of the transformer based on the calculated PCS output; an allowable judgment step of comparing an overload rate determined by the ratio of the rated output of the PCS to the rated capacity of the transformer with an allowable load rate evaluated in relation to the overload operation time for the same type of transformer, and judging that the overload operation of the transformer is within an allowable range if the overload rate is equal to or less than the allowable load rate, and judging that the overload operation of the transformer is outside the allowable range if the overload rate exceeds the allowable load rate; A method for determining whether a transformer is overloaded, comprising:

5. The method for determining whether or not a transformer is overloaded is characterized in that the PCS output estimated from the solar power generation curve is calculated based on a correction curve in which the solar power generation curve is adjusted so that the solar panel has a rated output at noon.

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