This article explains how pvDesign's cabling topology works
pvDesign's cabling topology aims to reduce the total cable length inside the PV plant. This article aims to explain how it achieves that.
LV system topology
In pvDesign, the modules are connected in series following a leapfrog configuration as shown in the image below. This connection allows the positive and negative cables to have the same length, which reduces the total length of the string cables.
Note: The length shown in the BOQ that pvDesign generates considers both the positive and negative cables.
In pvDesign, we represent a string by a block of modules defined by the user through the number of modules per string parameter. For example, for a 2V tracker with a configuration of 2 strings and a number of modules per string of 20, each string is represented as shown by the blue rectangle in the image below.
Combiner box/string inverter location
String cables are connected vertically through trays to the combiner boxes that are generally located at the structures' edge. The only exception is in the case of 1V trackers with three or more strings where pvDesign will locate the combiner box at the middle of the tracker with the aim of reducing the string cabling.
As for the fixed structures, the software will always place the combiner box at the edge except when the structure has three or more strings. In this case, it will place it in the middle of the structure.
Other LV configuration types
Among the LV configuration types that pvDesign suggests, you will find a string box L2 configuration. This later includes two combiner boxes of which the second one can be defined either in the field or inside the power station. In the case of choosing the string box L2 inside the field, the secondary box will be placed at the edge of the structure.
In addition, pvDesign offers the DC bus system configuration where a number of string cables, that you can define in the electrical configurations' table, will all connect through a DC bus that reaches the corresponding power station.
MV system topology
pvDesign's algorithm locates the power stations at the center of their corresponding block of structures with the objective of minimizing the length of the LV cables.
pvDesign connects the power stations using a radial configuration. This results in a lower cost of cabling and more simplicity in the design. The following image illustrates this radial configuration:
The software automatically connects up to 7 power stations together based on their capacity and proximity. If the total load current of a certain line of power stations exceeds 630A, fewer power stations will be connected together.
If you would like to have more information regarding how pvDesign defines the electrical configuration from defining the number of modules per string all way to the power stations, please refer to the article How to define your electrical configuration.
The LV trenches will drive the LV cables from the combiner boxes/string inverters to the power stations. These may vary depending on the chosen road layout.
In the case of horizontal roads, the LV trenches will first be installed horizontally following the edges of the structures and then vertically to reach the power stations (below trackers), which can be seen in the image below.
As for the case of vertical road layout, the LV trenches will first be installed vertically and then horizontally to reach the power stations (both, vertical and horizontal trenches will be installed below trackers).
Note: For fixed structures, the trenches are installed in a similar way but in the case of horizontal roads, all the trenches are below the structures, and for the vertical roads, only the horizontal trenching will be below.
Keep in mind that when the uniform alignment option is selected, LV trenching will be defined in a regular grid. On the contrary, when border adaptation is selected, LV trenching will be tangled and less uniform.
Power stations are connected together through MV trenches. These are usually installed following the roads. If the power stations are separated by the DC field, the MV trenching will cross the field below the structures to reach the power stations.
In conclusion, pvDesign's algorithm tends to group the structures in a way that minimizes the cables' length. This objective results in connecting and placing the electrical elements in the way mentioned above.
The software uses leapfrog to connect the positive and negative module wires, it places the combiner boxes either at the edge or in the middle of the structure depending on the structure type and the string configuration. Moreover, the software uses a radial configuration to connect the power stations and installs the LV and MV trenches depending on the road layout and the structure type.
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