DC Bus or Trunk cables allow efficient installation of DC cables above the ground. Using IPC (Insulation Piercing Connectors) or preassembled connector systems, the string cables are connected directly to the DC Bus cable. This type of system is becoming ever more common in utility scale projects that use tracker systems, and results in fewer connections, less DC cable under the ground, easier installation and lower power losses.
a) System type
PVcase GM allows designing these types of systems by selecting the "Harness - Trunk - Central Inverter" (3) (formerly "Harness - IPC - Central Inverter") in the "Electrical Devices" tab (2) of the "Electrical Design" window (1).
Note: Before being allowed to select a system type, the Select PV areas button needs to be clicked and the PVcase PV areas to be considered need to be selected in the model space.
b) System configuration
In most cases, these systems combine the use of the trunk with harnesses that combine multiple strings in parallel. Then those harnesses are subsequently connected to different locations of the bus cable (paralleling). Once all are connected to the trunk, a Load Break Disconnect (LBD) is installed for switching purposes (note: although referenced as Combiner Box, it is not combining/paralleling because that has been done previously in the trunk) and from there the DC feeder cable runs all the way to the Central Inverter.
The user can define the "String per harness" (1), "Harnesses per trunk" (2), "Trunks per Central Inverter" (3) and "AC Power" per Central Inverter (4):
Note: Some parameters are grayed out as they are calculated automatically based on the user inputs. "Required strings" refers to the number of strings required to complete one Central Inverter Block. "Harness amount", "Trunk amount" and "Central inv. amount" refer to the amounts of those objects that can be built with the remaining "Unused strings". "Missing strings" refers to the number of strings that are necessary to complete the next full Central Inverter block.
c) Placement type
Only the option to select "Trench Adjacent" is available for Trunk systems. This type of placement is the more adequate for this type of design in order to place the LBDs right after all harnesses have joined the trunk close to the frames. If any of the LBDs need to be relocated, the user can always move them manually (ideally before doing Cabling).
d) Numbering
In order to ensure proper placement of the Harness + & - connectors, it is important to define the numbering of strings with the correct priority and order. This is not as important when each harness is contained within one frame, but becomes critical when harnesses have strings on separate frames. Numbering correctly during device generation (1) can avoid tedious manual "Renumber" (2) if any changes are necessary.
e) Labeling
The labels can be left as they're shown, or modified to match the user's standard nomenclature (1). It can also be easily modified after device generation by simply clicking on "Update" (2).
f) Actions
In this example, each tracker frame has three 30 module strings (except for four frames which have two 30 module strings). Every six strings are grouped into a harness and three of those harnesses connected to a trunk. Each trunk cable runs in the E-W direction to its own Load Break Disconnect (LBD) and from there runs the feeder cable to the Central inverter, which can accommodate up to fourteen of these circuits (Note: Go to Appendix 1 of this tutorial to see how to design a system where trunk systems run parallel to the frames and perpendicular to the LBD line, instead of parallel to the LBD line as is shown here).
Depending on how regular each block is, different combinations of "Auto" and/or "Semi" electrical device generation steps can be used. In this example, three steps are used:
1) "Auto" generation of complete LBD blocks on Northern side of E-W road
2) "Auto" generation of complete LBD blocks on Southern side of E-W road
3) "Semi" generation of partial LBD blocks on Southern side of E-W road
As can be seen, LBDs are placed on the frames and as close as possible to the user-selected "Alignment lines" and to the Central Inverter. But for harness positive and negative end connectors (circle block objects placed in the "PVcase Harness Positive and Negative" layers respectively), it is important to verify the adequate location. These are automatically placed at the positive and negative ends of the strings that are closest to the Alignment line, but they can be easily moved wherever necessary (to the LBD E-W line in this example).
Harness connectors (created by GM during the device generation) and Trunk connectors (created by GM during the cabling generation), are placed based on the user-selected "Numbering" order. For this example, "Left to Right" had been selected in the "Numbering" section, so the Harness connectors and Trunk connectors were placed on the first row on the left of each harness group. If a different order is needed for different regions of the block, then the "Numbering" order would need to be changed before generating devices in each section.
Note: Click on image to enlarge
g) Cabling
For the Trunk electrical systems, there is no need to generate the AC cables, because those are run inside the Central Inverter between the inverter and transformer and are therefore already preassembled/installed by the manufacturer offsite.
For the DC system, the cable generation is done separately for the "DC Main cabling" (cables from Central Inverter to LBD and trunk cables) and "DC String cabling" (trunk to harness cables and harness to string extensions). This allows the user to define different cable parameters (depth/height, separation) for the different cable types.
In order for GM to correctly create the harness to trunk cables, it is necessary to create the "Main DC cabling" first. In this manner, the Trunk is created before the strings so they can connect to it at the proper connector locations.
g.1) DC Main cabling
Because the site is quite regular in this example, "Auto" generation will be used for the whole block in one step. This will also mean that both the cables from Central Inverter to LBD and trunk cables are generated in one step. But if necessary, "Semi" could be used in order to generate the two types of cables separately and also by sections in a more controlled fashion.
g.2) DC String cabling
Since Trunks were created in the previous step, now the software can correctly define the location for the trunk connectors and for the harness to trunk cables. In this case, "Auto" generation will be used as well, so cables from strings to harness and from harness to trunk connectors can be done for the whole block at once. But in other, more complex cases, "Semi" can be used to generate the two types of cables separately and also by sections in a more controlled fashion.
h) BoM and SLD
Now that the design is complete, the Bill of Materials (BoM) and SLD with all cables can be easily shown and exported, like is done for all other types of GM electrical systems.
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Appendix: Alternative design: Trunk cable running parallel to the frames and perpendicular to the LBD alignment line
In this example, each tracker frame has two parallel 28 module strings. Every four strings (two frames) are leapfrogged to have their +/- close to their harness, and three of those harnesses connected to a trunk which runs under the frames until they reach the LBD. The LBDs are all placed next to a trench line (N-S) perpendicular to the trunk lines (E-W). This trench line runs the feeder cables from the LBDs to the Central inverter, which accommodates up to eighteen of these circuits.
1. Generate Electrical devices
Depending on how regular each block is, different combinations of "Auto" and/or "Semi" electrical device generation steps can be used. In this example, "Semi" is used in order to ensure proper grouping of strings to harness:
Continue until all strings are linked to their harnesses and LBDs in block:
2. Cabling
In this case, the cabling process needs to be divided in three steps, two of them of DC Main Cabling and one more for DC String Cabling.
2.1 Trunk
To generate the horizontal E-W trunks to the LBD, select DC Main cabling and Semi-auto, click on Generate and when prompted, select only the Harnesses (not the LBDs) and a N-S trench placed flushed to the last harness of each row, so that it creates the horizontal trunks starting from that vertical line to the LBDs:
2.2 LBD to Central Inverter
To generate the vertical N-S feeders from the LBDs to the Inverter, select DC Main cabling and Semi-auto, click on Generate and when prompted, select only the LBDs (not the harnesses) and a N-S trench that runs next to those LBDs, all the way up to the Central Inverter:
2.3 String to Harness to Trunk
To generate the string to harness and harness to trunk cables, select DC String Cabling and Auto. Also make sure that any required vertical trenches to run strings to harnesses are preselected at the top of the window. Click on Generate and when prompted, select the Central Inverter (Transformer):