Electrical System Design Methodology

For details of the code implementation, please see Electrical System Design API.

Overview

Below is an overview of the process used to design an export cable system and offshore substation in ORBIT using the ElectricalDesign module. This module is to be used in place of both the ExportSystemDesign module and the OffshoreSubstationDesign module as it codesigns the export cables and offshore substation. Depending on whether HVAC or HVDC cables are selected, different components will contribute to the final BOS. For more detail on the helper classes used to support this design please see Cabling Helper Classes, specifically Cable and CableSystem.

Number of Required Cables

The number of export cables required for HVAC is calculated by dividing the windfarm’s capacity by the configured export cable’s power rating and adding any user defined redundnacy as seen below.

$num\_cables = \lceil\frac{plant\_capacity}{cable\_power}\rceil + num\_redundant$

For HVDC cables (both monopole and bipole), the number of cables is twice the number as calculated abpve because HVDC systems require a pair of cables per implementation. The equation for this calculation is shown below.

$num\_cables = 2 * \lceil\frac{plant\_capacity}{cable\_power}\rceil + num\_redundant$

Export Cable Length

The total length of the export cables is calculated as the sum of the site depth, distance to landfall and distance to interconnection multiplied by the user defined :py:attr`percent_added_length` to account for any exclusions or geotechnical design considerations that make a straight line cable route impractical.

$length = (d + distance_\text{landfall} + distance_\text{interconnection}) * (1 + length_\text{percent_added})$

Cable Crossing Cost

Optional inputs for both number of cable crossings and unit cost per cable crossing. The default number of cable crossings is 0 and cost per cable crossing is $500,000. This cost includes materials, installation, etc. Crossing cost is calculated as product of number of crossings and unit cost.

Number of Required Power Transformer, Tranformer Rating, and Cost

The number of main power transformers (MPT) required is assumed to be equal to the number of required export cables. The transformer rating is calculated by dividing the windfarm’s capacity by the number of MPTs. MPTs are only required if the export cables are HVAC. The default cost of the MPT is $2.87m per HVAC cable. Therefore, the total MPT cost is proportional to the number of cables. Note: Previous versions may have used curve-fits to calculate total MPT cost based on the windfarm’s capacity. The MPT unit cost ($/cable) can be ovewritten by the user by setting (mpt_unit_cost) to the desired cost. If the export cables are HVDC, then the cost of power transformers will be $0.

Number of Shunt Reactors, Reactive Power Compensation, and Cost

The shunt reactor cost is dependent on the amount of reactive power compensation required based on the distance of the substation to shore. This model assumes one shunt reactor for each HVAC export cable. An HVDC export systems do not require reactive power compensation. The default cost rate of the shunt reactors is $10k per HVAC cable. The total cost is proportional to the number of cables multipled by a cable-specific compensation factor. The default cost rate can be overwritten by the user by setting (shunt_unit_cost) to the desired cost. The shunt reactor cost is $0 for HVDC systems.

Number of Required Switchgears and Cost

The number of switchgear relays required is assumed to be equal to the number of required export cables. Switchgear cost is only necessary if HVAC export cables are chosen. The default cost is $4m per cable for HVAC. The default cost can be overwritten by the user by setting (switchgear_cost) to the desired cost. Switchgear cost is equal to $0 for HVDC export cables.

Number of Circuit Breakers and Cost

The number of circuit breakers required is assumed to be equal to the number of required export cables. Breakers are only necssary if HVDC export cables are chosen. The default cost is $10.6m per HVDC cable. The default cost can be overwritten by the user by setting (dc_breaker_cost) to the desired cost. Breaker cost is $0 for HVAC cables.

Number of Required ACDC Converters and Cost

ACDC converters are only required for HVDC export cables. The number of converters is assumed to be equal to the number of HVDC export cables.

Ancillary System Cost

Costs are included such as a backup generator, workspace cost, and miscellous to capture any additional features outside the main components. The user can define each variable by setting (backup_gen_cost), (workspace_cost), and (other_ancillary_cost).

Assembly Cost (On Land)

The majority of the electrical components are located on the offshore substation platform, but they must be assembled on land. Therefore, an assembly factor of 7.5% is added to the components cost. Those components include switchgear, shut reactors, and ancillary costs. The user can change the factor by setting (topside_assembly_factor) to the desired percentage.

Substation Topside Mass and Cost

We assume that the topside design cost is a fixed amount based on the export cables (either HVDC or HVAC). The user can specify the topside cost by setting (topside_design_cost). The mass of the topside is determined by a curve fit.

Substation Substructure Mass and Cost

The mass and cost associated with the substructure of the offshore substation are based on curve fits. The topside mass will drive the mass/size of the substructure. Then, the cost of the substructure is determined by its mass. The substructure has a default cost rate of $3000 per ton of steel. The value can be overwritten by setting (oss_substructure_cost_rate) to the desired cost rate.

Onshore Cost

The onshore cost is considered to be the minimum cost of interconnection. This includes the major required hardware for a cable connection onshore. For HVDC cables, it includes the converter cost, DC breaker cost, and transformer cost. For HVAC, it includes the transformer cost and switchgear cost. The onshore costs may or may not be included in the BOS of the wind farm. Therefore, this cost is not included in the total system_capex calculated by ProjectManager.

Design Result

The result of this design module (design_result) includes the specifications for both the export cables and offshore substation. This includes a list of cable sections and their lengths and masses that represent the export cable system, as well as the offshore substation substructure and topside mass and cost, and number of substations. This result can then be passed to the export cable installation module and offshore substation installation module to simulate the installation of the export system.