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Home / Technical Articles / Twelve power system studies obligatory for the designing the offshore AC substation

Power Systems Studies

This technical article describes the offshore AC substation design studies that involve more than one component or even the complete system. This involves reliability, availability and maintenance issues as well as system properties like total substation power, reactive power management, applied voltages and harmonics. The focus is on the electrical system.

Twelve power system studies obligatory for the designing the offshore AC substation
Twelve power system studies obligatory for the designing the offshore AC substation

The purpose is not to provide standards nor solutions for the design issues, but to provide guidance in the considerations that need to be taken into account when designing an offshore substation. The article describes a list of the studies which will normally be required.

The completed studies will lead to the final single line diagram and provide some of the key parameters for the primary plant to be installed on the offshore substation.

In order to bring together all the relevant aspects in the complex process of designing large offshore wind power plants, several system studies need to be carried out.

The Design aspects can be summarized as follows:

  • Grid Code compliance
  • Reactive power
  • Harmonic Performance
  • Static and Dynamic Stability Performance
  • Wind Power plant and export circuit component ratings
  • Protection and Safety

All of these aspects should be addressed through comprehensive system design studies. These studies are listed below followed by their main objectives.

Table of Contents:

  1. Load Flow Study
  2. Short Circuit Study
  3. Harmonics Study
  4. Insulation Coordination Study
  5. Electromagnetic Transient Studies
  6. HV Export Network Transient Studies
  7. Flicker and Voltage Fluctuation Study
  8. Dynamic Stability Study
  9. Safety Earthing Study
  10. Neutral Grounding Study
  11. Protection Coordination Study
  12. Electro Magnetic Field (EMF) Study
  13. Attachment (PDF) 🔗 Download ‘Power System Analysis and Design Handbook’

1. Load Flow Study

The main objectives of a Load Flow study should be:

Objective #1 – Reactive power capability

To determine reactive power capability requirements at the Point of Common Coupling (PCC) and identify the requirements for a reactive compensation plant in order to comply with the Grid Code.

Objective #2 – Current ratings

To check current ratings of cables and transformers for violations of their limits in order to establish correct cable cross section and transformer ratings.

Objective #3 – Voltage limits

To calculate voltages at various points in the Wind Power plant to ensure they are within acceptable limits.

Objective #4 – Power losses

To calculate active power losses throughout the Wind Power plant

Objective #5 – Tap changer settings

To determine tap changer range of both onshore and offshore transformers.

HOW TO: Load flow study example

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2. Short Circuit Study

The main objectives of a Short Circuit study should be:

Objective #1 – Max. short circuit

To calculate maximum Short Circuit currents in order to determine the required rating of cables and switchboards at different voltage levels and locations within the Wind Power plant network.

In case these switchboards are already selected the maximum short circuit currents are used to check whether their ratings are exceeded or not.

Objective #2 – Overcurrent current settings

To determine the settings of overcurrent protection devices by calculating maximum and minimum Short Circuit currents in order to correctly detect symmetric and asymmetrical faults at any location within the Wind Power plant network.

Objective #3 – Phase to ground currents

To calculate maximum and minimum single phase to ground currents within the Wind Power plant in order to determine ground fault protection settings.

Objective #4 – Max. short circuit

To determine maximum short circuit contribution from the wind turbine generators to the point of common coupling (PCC).

HOW TO: Short circuit study example

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3. Harmonics Study

The very first and the main objective of this study is to assess the voltage harmonics to be expected at the grid connection point when the Wind Power plant is connected in order to demonstrate conformity with the Grid Code.

Additional also important objectives are:

Objective #2 – Resonance behaviour

To derive the resonance behaviour given by the cables, transformers, reactors and WTGs connected to the network.

Objective #3 – Harmonic impedance frequency

To calculate harmonic impedance frequency scans for various system configurations to identify any resonance problems.

Objective #4 – Resonance mitigation

To identify countermeasures to eliminate / mitigate the encountered resonance problems.

Objective #5 – Limiting harmonic distortion

To verify and provide the necessary measures to limit the harmonic distortion at the Point of Common Coupling (PCC).

Suggested Guide: How to detect and manage harmonics in power system

How to detect and manage harmonics in power system


Objective #6 – Checking WTG converter

To evaluate wind turbine generator (WTG) converter interactions with the grid as well as existing control systems, in order to examine possible resonance conditions that may cause instability and unexpected tripping.

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4. Insulation Coordination Study

The main purpose of insulation coordination study is to specify the necessary insulation withstand levels for all the primary components within the Wind Power plant.

Further objectives are:

Objective #2 – Max. voltage stresses

To calculate maximum voltage stresses on the Wind Power plant components through EMT simulations.

Objective #3 – Protective measures

To identify and specify protective measures such as surge arresters in order to avoid dangerous transient overvoltages that can cause equipment damage.

Further study: Insulation coordination study for lightning overvoltages in 420 kV power substation

Insulation coordination study for lightning overvoltages in 420 kV power substation

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5. Electromagnetic Transient Studies

Detailed electromagnetic transient simulation (EMT) studies are required for each particular case. There is a reasonable amount of information and guidelines available regarding the modeling of MV and HV equipment for EMT studies IEC60071‐4 ‘Computational guide to insulation coordination and modelling of electrical networks’ is a good source.

The credibility of these studies relies on the quality of information available, which is often limited, and the models being used.

The magnitude and wave shape of the overvoltages depend upon the instant that the investigated event (Energisation, disconnection, fault, load rejection, switching impulses etc.) occurs, therefore many simulations should be made with different instants of occurrence for the same event.

There are usually many more scenarios (they can easily reach hundreds) required for offshore wind power plants than for onshore because of the different configurations of the wind park network.

Since TOV such as overvoltages caused by transformer energisation are largely determined by system resonances, scanning the wind park impedance helps identifying the most critical conditions with respect to the TOV.

Recommended: Analysis of the effect of capacitor switching transient on a grid substation (case study)

Analysis of the effect of capacitor switching transient on a grid substation (case study)

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6. HV Export Network Transient Studies

The configurations chosen for the connection of the export cables to shore will determine the nature of transients to consider. Vacuum circuit breakers are not employed on the HV network, so chopping and re‐ignition are less of a concern, however the capacitance trapped in the long cables and high operating voltage generate some onerous condition when energised.

The nature of the transients generated during switching or faults in a predominantly cabled network will be quite different in shape and amplitude to that of an overhead line network (longer in duration and possible higher in energy).

The combination of transformers and cables provide a very harmonic and resonance rich spectrum of transients that could affect protection, particularly if it is based on traditional OHL design principles.

The key points to consider include:

Key point #1 – Inrush currents

The size of inrush currents and the energy handling requirements of surge arresters, prior to specifying any surge arresting equipment.

Key point #2 – Transients impact

The impact of transients where export cables connect to the mature onshore network – The studies should identify the profile of the transients generated within the onshore substation, existing protection settings sensitivity should be checked particularly with regard to the duration of inrush currents and overvoltages associated with resonance.

Where point on wave is considered, the impact of incorrect VT wiring or timing failure should be examined on the adjacent plant.

Further study: Exciting and inrush currents in transformers that often make protection relays go crazy

Exciting and inrush currents in transformers that often make protection relays go crazy

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7. Flicker and Voltage Fluctuation Study

During continuous operation each Wind Turbine Generator (WTG) in the wind power plant experiences a continuously changing mechanical input power. This is caused by the variability of the wind itself, as well as by phenomena such as tower shadow (mechanical torque pulsation caused by the blades passing the tower) and wakes.

Voltage flicker is the distortion of the voltage waveform resulting from these variations in mechanical input power.

The study aims to assess the voltage flicker emission at the point of common coupling (PCC) resulting from continuous operation and from switching actions of the WTG in the wind power plant.

Calculations of the summarized flicker emission at the PCC should take into account IEC standards 61400-21 and 61000‐3‐7.

Additional objectives of the study are:

Objective #1 – Voltage drop

To calculate the voltage drop at the Point of Common Coupling (PCC) resulting from the energisation procedure of the wind power plant’s main electrical components and check for Grid Code compliance.

Objective #2 – Voltage fluctuations

To identify and specify measures (e.g. Point on wave switching, pre‐inserted resistor switchgears) in order to comply with Grid Code requirements for voltage fluctuations at the Point of Common Coupling (PCC).

Take the course: Voltage Drop and Fault Current Analysis Course For Power Engineers

Voltage Drop and Fault Current Analysis Course For Power Engineers
Voltage Drop and Fault Current Analysis Course For Power Engineers

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8. Dynamic Stability Study

The main objectives of a Dynamic stability study should be:

Objective #1 – Reactive power output

Simulate how reactive power output of the wind power plant module(s) reacts to positive and negative voltage changes at the Point of Common Coupling (PCC).

Objective #2 – Steady‐state value/dynamic response

To determine whether the simulated reactive power output of the wind power plant module(s) is Grid Code compliant with regard to steady‐state value and dynamic response.

Objective #3 – Reactive power limits

To determine whether the wind power plant module(s) behave(s) stable for voltage steps beyond the reactive power limits.

Objective #4 – Symmetrical/asymmetrical voltage sags

To simulate the response of the wind power plant module(s) to symmetrical and asymmetrical voltage sags at the Point of Common Coupling (PCC) of various depths and duration.

Objective #5 – Stable post‐fault behaviour

To assess whether the wind power plant module(s) are able to ride through these voltage sags and show a stable post‐fault behaviour (in accordance with Grid Code Fault Ride Through requirements).

Further study: Synchro Check Schemes: Key Techniques and Considerations for Power System Stability

Synchro Check Schemes: Key Techniques and Considerations for Power System Stability

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9. Safety Earthing Study

High voltage installations require an earthing system to protect human life against excessive touch voltages and to keep transferred potential to a minimum.

Therefore the main objectives of the Earthing Study should be:

  1. Calculation of required cross section for different components of earthing system with regard to thermal stress.
  2. Determination of tolerable touch voltages.
  3. To keep tolerable limits given in Standards IEEE, IEC, BS.
  4. To control dissipation of fault currents to ground.
  5. Determination of impedance to earth of the earthing system.
  6. Calculation of ground potential and Hot Zone.

Suggested Guide: Practical Earthing Handbook for Power Engineers

Practical Earthing Handbook for Power Engineers

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10. Neutral Grounding Study

The study should recommend adequate types of neutral grounding after investigating current stress and voltage stress as well as interconnection to the onshore HV network.

The main objectives of the study are:

  1. To check the design parameters concerning earthing of HV, MV transformers and earthing transformers if applicable. Alternatively, design parameters can be determined considering required limitation of short circuit currents or voltage stress or other requirements out of specification
  2. Calculation of zero sequence current contribution of transformer neutrals
  3. Calculation of power frequency voltage stress during 1_phase short circuit

Further study: How to select grounding point(s) and how many generator or transformer neutrals to use

How to select grounding point(s) and how many generator or transformer neutrals to use

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11. Protection Coordination Study

The protection coordination study is of great importance to personnel and equipment safety. The main objectives of a protection coordination study should be:

  1. Design of the protection philosophy and selection of the individual protection devices
  2. Dimensioning of current transformers
  3. Determination of the settings of the individual protection devices

Take the course: ETAP Power System Design and Analysis Course: Learn To Resolve Power System Issues

ETAP Power System Design and Analysis Course: Learn To Resolve Power System Issues
ETAP Power System Design and Analysis Course: Learn To Resolve Power System Issues

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12. Electro Magnetic Field (EMF) Study

The main objective of this study is to evaluate electromagnetic fields at the station(s) with respect to human exposure. It should give a quantitative description of the levels of electromagnetic fields associated with the operation of the station(s).

The levels of electromagnetic fields should cover:

  1. Magnetic flux density at the power frequency (e.g. DC / 50 Hz)
  2. Electric field strengths at the power frequency (e.g. 50 Hz)

The study should describe the field sources, the levels of electromagnetic fields in the areas under consideration and the assessment of the field strengths with respect to requirements concerning human exposure.

Useful tool: Calculation of electro magnetic field (EMF) around T&D overhead lines

Calculation of electro magnetic field (EMF) around T&D overhead lines

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9. Attachment (PDF): Power System Analysis and Design Handbook

Download: Power System Analysis and Design Handbook (for premium members only):

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Reference: Guidelines for the Design and Construction of AC Offshore Substations for Wind Power Plants by Working Group B3.26

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Edvard Csanyi - Author at EEP-Electrical Engineering Portal

Edvard Csanyi

Hi, I'm an electrical engineer, programmer and founder of EEP - Electrical Engineering Portal. I worked twelve years at Schneider Electric in the position of technical support for low- and medium-voltage projects and the design of busbar trunking systems.

I'm highly specialized in the design of LV/MV switchgear and low-voltage, high-power busbar trunking (<6300A) in substations, commercial buildings and industry facilities. I'm also a professional in AutoCAD programming.

Profile: Edvard Csanyi

One Comment


  1. Ayoub H. Hamad
    Jul 20, 2025

    I highly appreciate your effort in providing such extensive information on electrical engineering

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