Grid connection point
      Back to home
      1. Knowledge base
      2. Grid connection point

      How to define the interconnection schema between PV plant and grid?

      Learn how to define the interconnection facilities and the lines connecting them, to optimally evacuate the PV plant power to the grid.

      In this article you'll learn:

      • The different types of interconnection facilities you can define
      • The different interconnection schemas you can achieve
      • The output documentation
      • Common errors

      If you want to learn the different options you can choose from to evacuate the power through the defined schema, give a look to this article.

      The interconnection facilities

      There are 3 types of interconnection facilities you can define:

      1. Switching and breaking station (SBS): it's used to collect the power from different lines coming from the power stations (PS) on the field reducing the number of outgoing lines.
      2. Substation (ST): It's used to step up the voltage from the level used on the field (determined by the power stations' transformers) to a higher level.
      3. Pooling Substation (ST): it's used to further step up the voltage from the substation level to the one required to connect to the grid.

      A substation and a pooling substation have different input circuit bays.

      • A substation has indoor MV input circuits
      • A pooling substation has outdoor HV input circuits

      Learn more about each type of facility in this article.

      The limits in the number of facilities you can define in a site are the following:

      • 20 SBS polygon
      • 20 ST polygons including both substations and pooling substation

      The interconnection schema options

      Combining different interconnection facilities and power lines, you can achieve different interconnection schemas according to your needs. In this paragraph, we'll explain the following schemas:

      • Single interconnection facility schema
      • Two levels of facilities schema
      • Three levels of facilities schema

      Single interconnection facility schema


      This is the simplest schema. It consists of just one facility, that can be placed either inside or outside the available area (AA). It can either be a switching and breaking station (SBS) or a substation (ST). 

      5

      • An MV point must be defined inside the interconnection facility.
      • Remember to draw only one facility to follow a single interconnection facility schema.

      Here is a simplified block diagram of the examples above.

      Schema 1 level

      The power stations (PS) are always placed automatically inside the available area (AA) by RatedPower.

      Two levels of facilities schema


      This schema allows you to select two different levels of facilities to connect to the grid. Let's see the requirements for each level:

      1. Lowest voltage level: this level can consist of either SBS(s) or ST(s). This level interconnects the transformer stations directly connected to your generation assets: the power stations (PS) in the PV field and the power conversion stations (PCS) of the AC-coupled storage system (if present).

      • Each lowest voltage level facility must be identified by an MV point placed inside its perimeter.
      • Only one type of lowest-level facility is allowed! This means you can't have both a SBS and a ST at the same level.
      • It's possible to have one or more facilities of the same type at this level.
      • A maximum of one interconnection facility can overlap with a given AA polygon. 

      The connection between available areas (AA) and these lowest-level facilities will be a straight line between the AC point of the AA and the MV point of the facility. If you wish, you can customize the path by:

      • placing an MV point inside the AA: the connection will depart from the AA's MV point instead of the AC point, connecting the two MV points (facility and AA) in a straight line
      • drawing a custom MV cable between the polygons: the connection will follow this path

      2. Highest voltage level: it consists of one facility collecting the power from the facilities at the previous level and stepping up the voltage. This facility is always identified as an "ST" polygon and we will refer to it as:

      • Substation if the lowest voltage level facilities are SBS 
      • Pooling substation if the lowest voltage level facilities are ST 
      • The connection between the lowest voltage level facilities and the highest voltage level facility must be defined by the user. The type of line you can choose depends on the type of interconnection facility selected at the lowest voltage level.
        • To connect an SBS to an ST, you can use either an MV cable (underground) or an overhead line (OHL).
        • To connect a substation to a pooling substation only overhead lines (OHL) are accepted.
      • An MV point is not needed for the highest voltage level facility.
      • Optional: a final OHL path can be defined starting from within the highest voltage level ST polygon and finishing wherever the interconnection point with the grid is.
      • The highest voltage level facility cannot overlap with any AA polygon.

      Here follows an example of a 2-level facility schema, with 2 SBS (in green) at the lower level: one outside the AAs and one inside one AA. No custom MV cable is defined between the outside SBS and AA. Both SBS are connected to an ST (in purple) using MV cables (in orange). An OHL (in light blue) is used to evacuate the power from the substation to the point of interconnection (POI).

      7

      Here is a simplified block diagram of the same example:

      20-1

      If the lowest voltage level facilities were substations the simplified block diagram would be:

      21-2

      Three levels of facilities schema

      This schema allows you to select three different levels of facilities to connect to the grid:

      1. Lowest voltage level: it consists of SBS directly connected to the transformer stations in the field (PS and PCS). The lowest voltage level facilities in a three-level schema must be SBS.

      • Each lowest voltage level facility must be identified by an MV point which must be placed inside the SBS polygon(s).
      • It's possible to have one or more SBS at this level.

      As mentioned for the two levels schema, the connection between available areas (AA) and SBS can be customized by the user or automatically determined by the software.

      2. Intermediate voltage level: it consists of one or more substations (ST) collecting the power from the SBS and stepping up the voltage.

      • The connection between the lowest voltage level facilities (SBS) and the intermediate voltage level facilities (ST) must be defined by the user using MV cables (underground) or OHL (overhead).
      • The number of intermediate voltage level facilities (ST) must be less than or equal to the number of lowest voltage level facilities (SBS).
      • Each SBS polygon must be connected to only one ST polygon
      • An MV point is not needed for the intermediate voltage level facilities.
      • No intermediate voltage level facility can overlap with any AA polygon.

      3. Highest voltage level: it consists of one pooling substation (ST) collecting the power from intermediate-level substations (ST) and stepping up the voltage.

      • The connection between the intermediate voltage level facilities (ST) and the pooling substation (ST) must be defined by the user using OHL.
      • Only one pooling substation is accepted.
      • An MV point is not needed for the highest-level facility.
      • Optional: a final OHL path can be defined starting from the pooling substation (ST) to the point of interconnection (POI).
      • The highest voltage level facility cannot overlap with any AA polygon.

      Here is an example of a 3-level facility schema with a) one SBS per AA (in green), b) two STs (in purple) collecting the power from a group of 2 and 3 SBS respectively through MV cables (in orange), and c) a single pooling substation (in purple) collecting the power coming from the two previous substations via OHL (in light blue). Another OHL is used to evacuate the power from the pooling substation to the grid (POI). 

      8

      Here is a simplified block diagram of the same example.

      22

      The documentation

      There's a package of documents dedicated to the interconnection schema, it's downloadable as a .zip file.

      18

      Inside, you'll find for each facility:

      • Interconnection Facility Report
      • Interconnection Facility Layout
      • Interconnection facility Single Line Diagrams (SLDs)

      and for each overhead line (OHL):

      • Overhead Line Report
      • Overhead Line Layout

      All the information about the selected MV cables and their sizing can be found in the "Listing of Cables"

      Common errors when creating a site with flexible schema

      RatedPower identifies the lowest voltage level facilities through the MV points. The following facilities must be connected to the lowest level facilities with custom MV paths. If these elements are missing, RatedPower won't be able to understand the schema and will yield an error. Here are some common errors:

      • One or more of the lowest voltage level facilities are missing the MV point.
      • The MV paths between facilities has not been defined.
      • You are trying to mix substations (ST) and switching and breaking stations (SBS) as the lowest voltage level facilities. Remember that only one type can be used.
      • You are trying to have more than one facility at the highest voltage level. Remember that at the highest voltage level, only one facility can exist.

      Any questions? Check our Interconnection Facility methodology for more information or contact us at support@ratedpower.com