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.

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.
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.
- Load Flow Study
- Short Circuit Study
- Harmonics Study
- Insulation Coordination Study
- Electromagnetic Transient Studies
- HV Export Network Transient Studies
- Flicker and Voltage Fluctuation Study
- Dynamic Stability Study
- Safety Earthing Study
- Neutral Grounding Study
- Protection Coordination Study
- Electro Magnetic Field (EMF) Study
- 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
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
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
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.
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
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.
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)
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 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
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.
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


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
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:
- Calculation of required cross section for different components of earthing system with regard to thermal stress.
- Determination of tolerable touch voltages.
- To keep tolerable limits given in Standards IEEE, IEC, BS.
- To control dissipation of fault currents to ground.
- Determination of impedance to earth of the earthing system.
- Calculation of ground potential and Hot Zone.
Suggested Guide: Practical Earthing Handbook for Power Engineers
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:
- 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
- Calculation of zero sequence current contribution of transformer neutrals
- 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
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:
- Design of the protection philosophy and selection of the individual protection devices
- Dimensioning of current transformers
- 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


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:
- Magnetic flux density at the power frequency (e.g. DC / 50 Hz)
- 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
9. Attachment (PDF): Power System Analysis and Design Handbook
Download: Power System Analysis and Design Handbook (for premium members only):
Reference: Guidelines for the Design and Construction of AC Offshore Substations for Wind Power Plants by Working Group B3.26
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