A ventilation calculation method and ventilation control device for power distribution rooms

Abstract

This invention provides a ventilation calculation method and ventilation control device for power distribution rooms, including acquiring the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room; calculating the heat generated by each type of heat source per unit time based on the operating parameters, wherein if the distribution transformer and the outgoing line cabinet are within the mutual influence range and / or the outgoing line cabinet and the switch cabinet are within the mutual influence range, the heat of influence between the corresponding devices per unit time is calculated; calculating the total heat in the power distribution room based on the heat of various heat sources and the heat of influence; and calculating the ventilation volume of the power distribution room based on the total heat and the temperature difference between the inside and outside of the power distribution room. This invention achieves the purpose of reducing the indoor ambient temperature by calculating the ventilation volume of the power distribution room in real time, thereby improving the operational safety of equipment and reducing power consumption; it also enhances the heat dissipation capacity of equipment and reduces eddy current heating through effective means, ensuring the long-term safe operation of the power distribution room equipment and eliminating potential equipment hazards.

Description

Technical Field

[0001] This invention belongs to the field of ventilation control technology for power distribution rooms, specifically relating to a ventilation calculation method and a ventilation control device for power distribution rooms. Background Technology

[0002] A power distribution room is an indoor electrical distribution facility with low-voltage loads, primarily supplying power to low-voltage users. It is equipped with medium-voltage incoming lines, distribution transformers, and low-voltage distribution devices. According to power distribution room safety regulations, maintaining a suitable temperature in the power distribution room is essential for the safe operation of the electrical equipment.

[0003] Poor ventilation is a common problem in current power distribution rooms, leading to equipment overheating. Especially during peak summer load periods, indoor temperatures exceed 55°C, with transformers and switchgear exceeding 70°C, and high-current cabinets reaching temperatures as high as 100°C in some areas. To ensure the healthy operation of electrical equipment, an ambient temperature of 40°C is generally required. Therefore, measures need to be taken to reduce the operating temperature of equipment in power distribution rooms. Summary of the Invention

[0004] In view of this, the present invention provides a ventilation calculation method and a ventilation control device for a power distribution room, which aims to improve the ventilation capacity of the power distribution room so that the temperature inside the power distribution room meets the requirements for the safe operation of power equipment.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] In a first aspect, the present invention provides a ventilation calculation method for a power distribution room, comprising the following steps:

[0007] Obtain the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room;

[0008] The heat generated per unit time for each type of heat source is calculated based on the operating parameters. If the distribution transformer and the outgoing cabinet are within the range of mutual influence and / or the outgoing cabinet and the switch cabinet are within the range of mutual influence, the heat generated per unit time between the distribution transformer and the outgoing cabinet and / or the outgoing cabinet and the switch cabinet is calculated.

[0009] The total heat in the power distribution room is calculated based on the heat and the heat of various heat sources.

[0010] The ventilation volume of the power distribution room is calculated based on the total heat and the temperature difference between the inside and outside.

[0011] Furthermore, if the distance between the distribution transformer and the outgoing switchgear is within the range of mutual influence, the heat generated per unit time between the distribution transformer and the outgoing switchgear shall be calculated according to the following formula:

[0012] Q 出变 =I 2R / (n(1-ɑ 2-r1 ))+I 2 R / (n(1-ɑ 2-r2 ))+I 2 R / (n(1-ɑ 2-r3 ))

[0013] In the formula, Q 出变 The heat generated per unit time between the distribution transformer and the outgoing switchgear is represented by I, the current in the distribution transformer busbar, R, the resistance of the distribution transformer outgoing busbar, n, the number of outgoing switchgear, r1, r2, and r3, which are different distances between the outgoing switchgear and the distribution transformer, respectively, and α is the experimental constant.

[0014] Furthermore, the mutual influence range between the distribution transformer and the outgoing switch cabinet is such that the distance between them is no more than 2m.

[0015] Furthermore, if the distance between the outgoing line cabinet and the switch cabinet is within the range of mutual influence, the heat generated per unit time between the outgoing line cabinet and the switch cabinet shall be calculated according to the following formula:

[0016] Q 出开关 =I n 2 R / (nm(1-ɑ 1-r ))

[0017] In the formula, Q 出开关 The heat generated per unit time between the outgoing line cabinet and the switch cabinet is represented by I, the current of the distribution transformer busbar is represented by R, the resistance of the distribution transformer outgoing line busbar is represented by n, the number of outgoing line cabinets is represented by m, the number of switch cabinets corresponding to the current outgoing line cabinet is represented by r, the distance between the current outgoing line cabinet and the nearest switch cabinet is represented by α, and α is the experimental constant.

[0018] Furthermore, the mutual influence range between the outgoing line cabinet and the switch cabinet is such that the distance between them is no more than 1m.

[0019] Furthermore, the range of the experimental constant is [0.83, 0.87].

[0020] Furthermore, the total heat in the power distribution room is the sum of the heat from various heat sources and the heat affecting the heat.

[0021] Furthermore, the ventilation volume of the power distribution room is calculated according to the following formula:

[0022] G = Q / C·(tg-th)

[0023] In the formula, G is the air volume provided by the forced fan, i.e. the ventilation volume of the power distribution room, Q is the total heat of the power distribution room, C is the specific heat of the air in the surrounding environment, and tg-th is the temperature difference between the inlet and outlet of the fan.

[0024] In a second aspect, the present invention provides a ventilation control device for a power distribution room, which implements control based on a ventilation calculation method for a power distribution room according to the first aspect, including:

[0025] The operation monitoring module is used to acquire the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room;

[0026] The ventilation control module is used to calculate the heat generated by each type of heat source per unit time based on operating parameters. Specifically, if the distribution transformer and the outgoing cabinet are within the mutual influence range and / or the outgoing cabinet and the switch cabinet are within the mutual influence range, the module calculates the heat of influence between the distribution transformer and the outgoing cabinet and / or the outgoing cabinet and the switch cabinet per unit time. It is also used to calculate the total heat in the power distribution room based on the heat of each type of heat source and the heat of influence. Furthermore, it is used to calculate the ventilation volume of the power distribution room based on the total heat and the temperature difference between the inside and outside.

[0027] In summary, this invention provides a ventilation calculation method and ventilation control device for power distribution rooms, including acquiring the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room; calculating the heat generated by each type of heat source per unit time based on the operating parameters, wherein if the power distribution transformer and the outgoing line cabinet are within the mutual influence range and / or the outgoing line cabinet and the switch cabinet are within the mutual influence range, then calculating the heat of influence between the power distribution transformer and the outgoing line cabinet and / or the outgoing line cabinet and the switch cabinet per unit time; calculating the total heat in the power distribution room based on the heat of various heat sources and the heat of influence; and calculating the ventilation volume of the power distribution room based on the total heat and the temperature difference between the inside and outside of the power distribution room. This invention achieves the purpose of reducing the indoor ambient temperature by calculating the ventilation volume of the power distribution room in real time, thereby improving the operational safety of equipment and reducing power consumption; and by effectively enhancing the heat dissipation capacity of equipment and reducing eddy current heating, it ensures the long-term safe operation of the power distribution room equipment and eliminates potential equipment hazards. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A flowchart illustrating a ventilation calculation method for a power distribution room provided in an embodiment of the present invention;

[0030] Figure 2 This is a schematic diagram of a ventilation control device for a power distribution room provided in an embodiment of the present invention. Detailed Implementation

[0031] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] A power distribution room is an indoor electrical distribution facility with low-voltage loads, primarily supplying power to low-voltage users. It is equipped with medium-voltage incoming lines, distribution transformers, and low-voltage distribution devices. According to power distribution room safety regulations, maintaining a suitable temperature in the power distribution room is essential for the safe operation of the electrical equipment.

[0033] Poor ventilation is a common problem in current power distribution rooms, leading to equipment overheating. Especially during peak summer load periods, indoor temperatures exceed 55°C, with transformers and switchgear exceeding 70°C, and high-current cabinets reaching temperatures as high as 100°C in some areas. To ensure the healthy operation of electrical equipment, an ambient temperature of 40°C is generally required. Therefore, measures need to be taken to reduce the operating temperature of equipment in power distribution rooms.

[0034] Based on this, the present invention provides a ventilation calculation method and a ventilation control device for a power distribution room.

[0035] The following is a detailed description of an embodiment of the ventilation calculation method for a power distribution room according to the present invention.

[0036] like Figure 1 As shown in the figure, this embodiment provides a ventilation calculation method for a power distribution room, including the following steps:

[0037] S100: Obtain the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room.

[0038] First, the calculation of ventilation and heat in the power distribution room is as follows:

[0039] Q = G·C·(tg-th)

[0040] In the formula, Q represents heat consumption; G represents the air volume provided by the forced fan; C represents the specific heat of the ambient air; and tg-th represents the temperature difference between the inlet and outlet of the fan, which can be considered as the temperature difference between the outdoor and indoor environments in a power distribution room. This formula demonstrates that heat and air volume have a linear relationship.

[0041] For heat sources, the power distribution room includes equipment such as distribution transformers, switchgear, outgoing line cabinets, and lighting, all of which generate heat during operation. Except for the distribution transformers, the other heat sources are current-driven heating devices, meaning the magnitude of the current directly determines the heat generated by the equipment. Therefore, to calculate the heat generation of different equipment, it is necessary to obtain the operating parameters of different heat sources.

[0042] S200: Calculate the heat generated per unit time for each type of heat source based on the operating parameters. If the distribution transformer and the outgoing cabinet are within the range of mutual influence and / or the outgoing cabinet and the switch cabinet are within the range of mutual influence, then calculate the heat generated per unit time between the distribution transformer and the outgoing cabinet and / or the outgoing cabinet and the switch cabinet.

[0043] For a typical power distribution room, the total heat source is represented by the following formula:

[0044] Q 热源 =Q 配变 +Q 开关柜 +Q 出线柜 +Q 照明

[0045] Among them, Q 配变 Heating is caused by a mixture of current and voltage. During operation, the voltage remains relatively stable, while the change in heat is determined by the current, the magnitude of which is determined by the load. Generally, the low current of distribution transformers ranges from 50-500A, a wide range. Its Q... 配变 =I 2 R is the resistance of its outgoing busbar, and I is the current in the distribution transformer busbar. During normal operation, part of the main heat-generating components of the distribution transformer are exposed, and the heat is directly dissipated into the indoor space. Another part enters the outgoing cabinet, where it interacts with the heating elements. The remaining heat is voltage-induced; when the voltage changes, it also affects Q. 配变 This has an impact; after testing, the Q-matching becomes: Q 配变 =IR*I V / VN In the formula, V is the current voltage and VN is the rated voltage. From the formula, it can be seen that when the current voltage equals the rated voltage, Q... 配变 =I 2 R.

[0046] The distribution transformer busbar is typically connected to the outgoing line cabinet, and then branches off. The outgoing line cabinet is heated by current, i.e.:

[0047] Q 出线柜 =I 2 R+I 出 2 R 出

[0048] Thermal analysis of switchgear, such as outgoing line cabinets:

[0049] Q 开关柜=I 开关 2 R 开关

[0050] The heat analysis of the lighting power supply is the same as above.

[0051] Q 照明 =I 照明 2 R 照明

[0052] That is, the heat generated per unit time by distribution transformers, outgoing line cabinets, switch cabinets, and lighting equipment is calculated from the current flowing through the corresponding equipment and its own resistance.

[0053] However, although the heat generated in the power distribution room is simply the sum of the heat generated by each heat source, the equipment in the room operates in a closed environment, and without ventilation, heat dissipation is poor. Furthermore, the size of the room, the relative positions of the equipment, and the ambient temperature and humidity all affect heat dissipation; in reality, the total heat and the heat generated by each device are not a simple linear relationship.

[0054] Of the four heat sources mentioned above, Q 照明 This heat source is located far from other heat sources, so there is virtually no mutual influence, and the lighting is generally off during operation. Therefore, this heat source can be ignored when calculating ventilation. The other three heat sources, however, all have mutual influence.

[0055] First, determine the correlation between the heat generated by the distribution transformer and the heat generated by the outgoing line cabinets and switchgear. Generally, there are 1-3 outgoing line cabinets. Their heat generation consists of busbar heating and outgoing line heating. The total heat generated by the busbar is consistent with that of the distribution transformer busbar, while the outgoing line heating is nQ. 出线 =nI 出 2 R 出 , where n is the number of outgoing switchgear. The interaction between the outgoing switchgear and the distribution transformer heat varies with distance.

[0056] In one embodiment, when the distance between the outgoing line cabinet and the distribution transformer is greater than 2 meters, their heat generation does not affect each other. When the distance between the outgoing line cabinet and the distribution transformer is less than 2 meters, the equivalent heat generation due to their mutual influence is as follows:

[0057] Q 出变 =I 2 R / (n(1-ɑ 2-r1 ))+I 2 R / (n(1-ɑ 2-r2 ))+I 2 R / (n(1-ɑ 2-r3 ))

[0058] Where r1, r2, and r3 represent the distances between different outgoing switchgear and the transformer. n is the number of outgoing switchgear. If there is only one outgoing switchgear, then n = 1. α is the experimental constant, ranging from 0.83 to 0.87.

[0059] The transformer and the switchgear are far apart, so their heat does not affect each other.

[0060] One outgoing line cabinet corresponds to several switch cabinets, and the current in the outgoing line section of the outgoing line cabinet is relatively small.

[0061] In one embodiment, when the distance between the outgoing line cabinet and the switch cabinet is greater than 1m, they do not affect each other. When the distance is less than 1m, the equivalent heat of mutual influence between the outgoing line cabinet and the switch cabinet is as follows.

[0062] Q 出开关 =I 2 R / (nm(1-ɑ 1-r ))

[0063] n represents the number of outgoing cabinets, and m represents the number of switch cabinets corresponding to the current outgoing cabinet. α is an experimental constant, ranging from 0.83 to 0.87. Since each outgoing cabinet will only be close to one switch cabinet, it will only have an equivalent heat effect with the nearest switch cabinet.

[0064] The mutual thermal influence between switch cabinets can be ignored.

[0065] S300: Calculates the total heat in the power distribution room based on the heat and the heat influencing various heat sources.

[0066] Based on the heat distribution of each heat source calculated in the preceding steps, and the heat distribution that influences each other, the total heat of the power distribution room can be calculated as follows:

[0067] Q 热源 =Q 配变 +Q 开关柜 +Q 出线柜 +Q 照明 +Q 出变 +Q 出开关

[0068] S400: Calculate the ventilation volume of the power distribution room based on the total heat and the temperature difference between inside and outside.

[0069] That is, the required ventilation volume of the power distribution room under the current environment is calculated based on the relationship between heat and air volume.

[0070] This embodiment provides a ventilation calculation method for a power distribution room, including acquiring the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room; calculating the heat generated by each type of heat source per unit time based on the operating parameters, wherein if the power distribution transformer and the outgoing line cabinet are within the mutual influence range and / or the outgoing line cabinet and the switch cabinet are within the mutual influence range, the heat of influence between the power distribution transformer and the outgoing line cabinet and / or the outgoing line cabinet and the switch cabinet per unit time is calculated; calculating the total heat in the power distribution room based on the heat of various heat sources and the heat of influence; and calculating the ventilation volume of the power distribution room based on the total heat and the temperature difference between the inside and outside of the power distribution room. This invention achieves the purpose of reducing the indoor ambient temperature by calculating the ventilation volume of the power distribution room in real time, thereby improving equipment operating safety and reducing power consumption; it also enhances the heat dissipation capacity of the equipment through effective means, reduces eddy current heating, and ensures the long-term safe operation of the power distribution room equipment, eliminating potential equipment hazards.

[0071] The above is a detailed description of an embodiment of the ventilation calculation method for a power distribution room according to the present invention. The following will provide a detailed description of an embodiment of the ventilation control device for a power distribution room according to the present invention.

[0072] This embodiment provides a ventilation control device for a power distribution room, which implements control based on a ventilation calculation method for a power distribution room provided in the foregoing embodiment, including:

[0073] The operation monitoring module is used to acquire the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room;

[0074] The ventilation control module is used to calculate the heat generated by each type of heat source per unit time based on operating parameters. Specifically, if the distribution transformer and the outgoing cabinet are within the mutual influence range and / or the outgoing cabinet and the switch cabinet are within the mutual influence range, the module calculates the heat of influence between the distribution transformer and the outgoing cabinet and / or the outgoing cabinet and the switch cabinet per unit time. It is also used to calculate the total heat in the power distribution room based on the heat of each type of heat source and the heat of influence. Furthermore, it is used to calculate the ventilation volume of the power distribution room based on the total heat and the temperature difference between the inside and outside.

[0075] It should be noted that the ventilation control device for power distribution rooms provided in this embodiment is used to achieve ventilation control based on the ventilation calculation method for power distribution rooms provided in the aforementioned embodiment. The specific settings of each module are based on the complete implementation of the method, and will not be repeated here.

[0076] In an optional embodiment, such as Figure 2As shown, the operation monitoring module can be composed of several current monitoring modules. Each current monitoring module includes a low-pass filter, a processing module, and a power supply. The processing module specifically includes an AD converter, an MCU, and a UART-to-wireless module. Each current detection module is connected to the current transformer of the corresponding device. The ventilation control module includes a processing module, fan control, and a power supply. The processing module includes a UART-to-wireless converter, an MCU, and a relay. The ventilation control module is connected to the fan in the power distribution room.

[0077] Each device in the power distribution room has a current transformer. The transformer collects the current of the current device, filters out noise using a low-pass filter, and then converts it into a digital signal via an AD converter. The MCU then formats the digital signal and sends it to the ventilation control module located near the fan. The various current signals are then processed by the MCU of the ventilation control module according to Q... 热源 =Q 配变 +Q 开关柜 +Q 出线柜 +Q 照明 +Q 出变 +Q 出开关 Calculations are performed, and ventilation is then calculated using Q = G·C·(tg-th), where the tg-th temperature difference is determined by the indoor thermometer and the outdoor ambient temperature. After the calculations are complete, the ventilation control circuit is adjusted via a relay to regulate the fan's ventilation volume.

[0078] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A ventilation calculation method for a power distribution room, characterized in that, Includes the following steps: Obtain the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room; Based on the operating parameters, the heat generated by each type of heat source per unit time is calculated. If the distribution transformer and the outgoing cabinet are within the range of mutual influence and / or the outgoing cabinet and the switch cabinet are within the range of mutual influence, then the heat of influence between the distribution transformer and the outgoing cabinet and / or the outgoing cabinet and the switch cabinet per unit time is calculated. The total heat in the power distribution room is calculated based on the heat from the various heat sources and the heat affecting them. The ventilation volume of the power distribution room is calculated based on the total heat and the internal and external temperature difference; If the distance between the distribution transformer and the outgoing switchgear is within the range of mutual influence, the heat generated per unit time between the distribution transformer and the outgoing switchgear shall be calculated according to the following formula: Q 出变 =I 2 R / (n(1-ɑ 2-r1 ))+I 2 R / (n(1-ɑ 2-r2 ))+I 2 R / (n(1-ɑ 2-r3 )) In the formula, Q 出变 The heat generated per unit time between the distribution transformer and the outgoing switchgear is represented by I, the current in the distribution transformer busbar, R, the resistance in the distribution transformer outgoing busbar, n, the number of outgoing switchgear, r1, r2, and r3, which are the distances between different outgoing switchgears and the distribution transformer, respectively, and α is the experimental constant. If the distance between the outgoing line cabinet and the switch cabinet is within the range of mutual influence, the heat generated per unit time between the outgoing line cabinet and the switch cabinet shall be calculated according to the following formula: Q 出开关 =I 2 R / (nm(1-ɑ 1-r )) In the formula, Q 出开关 The heat generated per unit time between the outgoing line cabinet and the switch cabinet is denoted by m, where m is the number of switch cabinets corresponding to the current outgoing line cabinet, and r is the distance between the current outgoing line cabinet and the nearest switch cabinet.

2. The ventilation calculation method for a power distribution room according to claim 1, characterized in that, The mutual influence range between the distribution transformer and the outgoing line cabinet is such that the distance between them is no more than 2m.

3. The ventilation calculation method for a power distribution room according to claim 1, characterized in that, The mutual influence range between the outgoing line cabinet and the switch cabinet is such that the distance between them is no more than 1m.

4. The ventilation calculation method for a power distribution room according to claim 1, characterized in that, The range of the experimental constant is [0.83, 0.87].

5. The ventilation calculation method for a power distribution room according to claim 1, characterized in that, The total heat in the power distribution room is the sum of the heat from various heat sources and the heat affecting the room.

6. The ventilation calculation method for a power distribution room according to claim 1, characterized in that, The ventilation volume of the power distribution room is calculated according to the following formula: G = Q / C·(tg-th) In the formula, G is the air volume provided by the forced fan, i.e. the ventilation volume of the power distribution room, Q is the total heat of the power distribution room, C is the specific heat of the air in the surrounding environment, and tg-th is the temperature difference between the inlet and outlet of the fan.

7. A ventilation control device for a power distribution room, comprising a ventilation calculation method for a power distribution room according to any one of claims 1-6, characterized in that, include: The operation monitoring module is used to acquire the operating parameters of various heat sources in the power distribution room and the temperature difference between the inside and outside of the power distribution room; The ventilation control module is used to calculate the heat generated by each type of heat source per unit time based on the operating parameters. Specifically, if the distribution transformer and the outgoing line cabinet are within the mutual influence range and / or the outgoing line cabinet and the switch cabinet are within the mutual influence range, the module calculates the heat generated by the interaction between the distribution transformer and the outgoing line cabinet and / or the outgoing line cabinet and the switch cabinet per unit time. The module is also used to calculate the total heat in the power distribution room based on the heat generated by each type of heat source and the heat generated by the interaction. Furthermore, the module calculates the ventilation volume of the power distribution room based on the total heat and the internal and external temperature difference. If the distance between the distribution transformer and the outgoing switchgear is within the range of mutual influence, the heat generated per unit time between the distribution transformer and the outgoing switchgear shall be calculated according to the following formula: Q 出变 =I 2 R / (n(1-ɑ 2-r1 ))+I 2 R / (n(1-ɑ 2-r2 ))+I 2 R / (n(1-ɑ 2-r3 )) In the formula, Q 出变 The heat generated per unit time between the distribution transformer and the outgoing switchgear is represented by I, the current in the distribution transformer busbar, R, the resistance in the distribution transformer outgoing busbar, n, the number of outgoing switchgear, r1, r2, and r3, which are the distances between different outgoing switchgears and the distribution transformer, respectively, and α is the experimental constant. If the distance between the outgoing line cabinet and the switch cabinet is within the range of mutual influence, the heat generated per unit time between the outgoing line cabinet and the switch cabinet shall be calculated according to the following formula: Q 出开关 =I 2 R / (nm(1-ɑ 1-r )) In the formula, Q 出开关 The heat generated per unit time between the outgoing line cabinet and the switch cabinet is denoted by m, where m is the number of switch cabinets corresponding to the current outgoing line cabinet, and r is the distance between the current outgoing line cabinet and the nearest switch cabinet.

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