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How to design your substation

A quick guide to taking full advantage of designing your substation in pvDesign.

Our software pvDesign will automatically design the basic engineering of the step-up substation of your PV plant. In a few minutes, you will have a complete single line diagram and a fully detailed report. Each substation's component is calculated based on the project characteristics while fulfilling the requirements of international electrical standards or IEEE standards upon your choice. 

Switching and breaking station

The switching and breaking station or sectioning station consists of a number of cubicles that allow modularity. A modular design provides the capacity to extend and adapt the station to the development of the network and to replace the modules without interrupting the supply. Each module will include different protection or measurement devices depending on the type of cubicle.

In general, a switching and breaking station has some cubicles that are designed for a feeder function and others that are designed for either measuring the voltage and the current (metering function) or for protecting the station against overvoltages or faults (circuit breaker or fuse protection function).

SBS

When should I use this switching and breaking station?

The switching and breaking station is the perfect solution to connect the PV plant to medium voltage distribution grids when it is not necessary to install step-up power transformers. The station performs at the same voltage level. Hence, the PV plant is at the same MV level as the distribution grid.

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The voltage level of the distribution grid depends on local parameters, but it is common to use a switching and breaking station when the interconnection level is: 

  • 24 kV
  • 36 kV

Can't use the switching and breaking station option. How do I enable it?

There are two conditions required in order to enable the switching and breaking station option:

  1. The installed capacity of the PV plant must be lower than 60 MVA.

  2. The total current of the PV plant must be lower than 1250 A. You can calculate the total current of your PV plant by dividing the capacity (in VA) by the MV level (in V) and by the square root of 3.

  3. The medium voltage level is lower than 50 kV.

In the table below, the maximum installed capacities per medium voltage system that enables the switching and breaking station are shown.

Medium voltage system Installed capacity
 11.5 kV Up to 24.5 MVA
16 kV Up to 34.5 MVA
20 kV Up to 43 MVA
24 kV Up to 51 MVA
30 kV Up to 60 MVA
36 kV Up to 60 MVA
45 kV Up to 60 MVA

My PV plant fulfills both criteria, so why is the switching and breaking station still disabled?

In some cases, despite the PV plant fulfilling these two criteria, the switching and breaking station's card won't be enabled. In general, this happens when the perimeter of the PV plant is irregular or rotated with respect to the N-S direction, and the total current of the plant is close to 1250 A. 

Still confused regarding this last point, well then, let's see the following project!

The installed capacity that we check in the layout tab is 38.5 MVA and the MV level is 20 kV by default. By applying the simple formula mentioned before, our total current is around 1110 A. The two criteria are fulfilled but the card is still disabled. Why is that so?

Short answer, maximum capacity is selected. Yes, when you choose the maximum capacity option, pvDesign cannot figure out the exact installed capacity. What it does instead is to estimate a capacity based on the type of structure, the pitch distance, and the total area. This estimation causes the switching and breaking station option to be disabled despite its two requirements being fulfilled.

Don't worry though, there is a simple solution for this!

Just go to the electrical tab, select Specific Capacity, and introduce the number of inverters you need in your PV plant.

This way, pvDesign can know the exact installed capacity of your PV plant, and the switching and breaking station card is thus enabled!

Substation

To connect a solar PV plant to distribution or transmission networks (66 - 400 kV), it is necessary to step-up the voltage level from medium to high voltage. The purpose of a substation is to convert low voltages to high voltages, or vice versa, using power transformers.

An important decision that is made at the planning/concept stage is the type of equipment that will be used for the substation. Three types of equipment are available to choose among:

  • Air-insulated switchgear (AIS)
  • Gas-insulated switchgear (GIS)
  • Mixed-technology switchgear (MTS)

At this moment, pvDesign's solution will define an air-insulated substation (AIS)

In addition, pvDesign will automatically choose the type of substation´s arrangement between the following ones:

  • Line-to-transformer. The line-to-transformer substation connects the PV plant to the grid directly without the use of a busbar. It stands out for being the simplest substation layout.
  • Single busbar. There will be a busbar to allow operational and flexible bay configurations. 
  • Double busbar. There will be two busbars to allow operational and flexible bay configurations, to increase the security of supply, and to improve the availability during maintenance periods.

When should I use a substation?

The substation is used to connect the PV plant to high voltage distribution or transmission grids. The voltage level of the PV plant will be stepped up to a high voltage level to facilitate the transmission of the electric current.

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The preferred high voltage level of distribution and transmission networks will depend on the country of the project. Nonetheless, it is common to use a substation when the interconnection level is: 

  • 66 kV
  • 132 kV
  • 220 kV
  • 400 kV

When I choose the substation, how can I know the type of arrangement?

You have the possibility to manually choose between a single busbar or a double busbar substation arrangement.

However, there is an option that defines automatically the substation arrangement. But, how do you know what arrangement is chosen by pvDesign?

It is not straightforward.

The substation arrangement will depend on several criteria such as the admissible and short-circuit currents that could flow per each potential transformer bay, the short-circuit impedance of the power transformers and, the current-carrying capacity of the cables.

However, a simple estimation can be carried out to find out the type of arrangement. Here is our so-called "2500 A method".

This method serves as an approximation but can still give us a quick idea of ​​what type of arrangement we have.

MV Level 20 kV 20 kV 20 kV
Installed Capacity 80 MVA 100 MVA 275 MVA
Total current 2300 A < 2500 A 2500 A < 2880 A < 3 * 2500 A 3 * 2500 A < 7930 A 
Type of arrangement Line-to-transformer Single Busbar Double busbar

Note. In order to compute the total current, here is an example for the 80 MVA PV plant

80 MVA * 10^6 / (20 kV * 10^3 * square root(3)) = 80 MVA * 10^6 / (35 * 10^3 ) = 2300 A

Conclusion

Can't remember all these details, just stay with the following ideas in mind: 

  • You can size the interconnection facility according to the IEC or the IEEE standard.
  • Use the switching and breaking station when the PV plant is interconnected to MV distribution networks up to 36 kV.
  • To enable the switching and breaking station, the installed capacity of the PV plant must be lower than 60 MVA, its total current must be lower than 1250 A, and medium voltages lower than 50 kV. 
  • If the switching and breaking card is disabled, go to Electrical > Power Requirements and define a Specific Capacity instead of using the Maximum Capacity option.
  • Connect your PV plant to HV distribution or transmission networks from 66 kV to 400 kV by selecting the substation.
  • Finally, the arrangement of the substation is automatically defined in the most optimal way. However, to have an idea of your substation's arrangement, apply our so-called "2500 A method".

For any other questions or for more information, you can contact us at the following email: support@ratedpower.com