Reliable by design: The power of EMT-type design studies for offshore wind substation development
August 25, 2023

Jenny Zhou & Iman Kaffashan 

Offshore wind energy is taking off! With various projects underway, taking the necessary steps to effectively test, develop, and operate the electricity power system is vital. EMT-type design studies empower developers to ensure their offshore substations (OSS) are safe, reliable, and resilient.

One of the critical components of offshore wind farms is the substation, which is responsible for converting generated power and transmitting it to the grid.

Of course, this requires meticulous planning.

EMT-type design studies play a vital role in OSS development. The simulations test the substation’s resiliency under transient stresses, enabling developers to select the right equipment and optimize the system’s design.

This article explores how EMT-type design studies can help with equipment rating and grid connection compliance and why they’re essential in OSS development.

OSS EMT-type design studies

So, what exactly does an EMT-type design study bring to the table? Let’s break things down before we dive into the details.

OSS layout

Typically, an OSS connects to the wind turbines via offshore 66 kV Inter-array Cabling (IAC) systems. The IAC system aggregates power from the wind turbines and funnels it to the OSS. An OSS is generally an indoor gas-insulated switchgear (GIS), and all medium- and high-voltage platform connections are made by gas-insulated bus duct. OSS also includes main power transformers and export cables that connect to the onshore power system.

Types of studies

EMT-type design studies involve complex simulations that analyze the system’s response to short-duration disturbances such as faults, switching operations, and dynamic stability and control interaction to confirm that the offshore wind power plant meets technical interconnection requirements.

There are three main types of studies in offshore wind substations:

  1. Switching transients for insulation coordination analysis
  2. Switching transients to support equipment, such as breaker specification
  3. Dynamic studies for stability and control interactions

Switching transients for insulation coordination analysis

The insulation coordination study for OSS includes switching transient analysis, very fast transient (VFT) analysis, and effectively grounded analysis. The studies verify the ability of surge arrestors to maintain the overvoltage levels below 80% of the equipment’s basic insulation level (BIL).

Besides modeling the system with a detailed three-phase representation of transformers, surge arresters, cables, shunt reactors, harmonic filters, and other equipment and system equivalents, an offshore collector system is represented in detail based on IAC systems layout arrangement for switching transient analysis. The events that are studied include the switching transients like transformer energization, cable energization, and fault clearance/load rejection. The fault is applied at different locations within the OSS and at different locations of all IAC systems. This study considers different operating conditions that evaluate the worst stress on the arresters.

There’s also a need to develop a separate model to represent the GIS station equipment and the main transformers for the GIS VFT study. The model used for the VFT study is developed based on the GIS layout drawings and surge impedance provided. The study evaluates the VFT overvoltage as the result of the opening or closing of a disconnect switch and other events, such as the operation of a circuit breaker, the closing of a grounding switch, or the occurrence of a fault.

A neutral grounding study needs to be performed to enable developers to lay out an appropriate neutral grounding strategy for the OSS. The study determines that the maximum voltages on the unfaulted phases in the system during line-to-ground faults will not exceed limits as well as ensuring the system is effectively grounded for all agreed operational configurations.

Switching transients to support breaker specification

Switching transients to support breaker specification include transformer energization, transient recovery voltage (TRV), rate of rise of recovery voltage (RRRV) analysis, sympathetic inrush during offshore cable energization, and breaker zero-missing evaluations. The analysis helps to determine the performance requirements of circuit breakers and protective devices, ensuring the safe and reliable operation of the substation.

The main power transformers’ energization is used to evaluate the overall system impact due to the high inrush current drawn from the connected AC system. In this study, transformer saturation and residual magnetism are represented, and sympathetic inrush is analyzed by energizing one transformer with another already energized transformer. The study ultimately determines if a point-on-wave controller or pre-insertion resistor for energization breakers is required.

For breaker TRV and RRRV analysis, the fault is applied at different locations within the OSS, as well as at different locations of all offshore export cables and various locations of IAC systems. The TRVs and RRRVs are then recorded for all three circuit breaker poles. The highest values are evaluated and compared with the minimum circuit breaker capabilities specified in the IEEE standards.

An OSS typically includes multiple offshore feeder cables. The sympathetic inrush during offshore cable energization, as well as outrush for nearby fault conditions, is the point of focus. The inrush and outrush currents are evaluated and compared with related circuit breaker or switcher specifications and IEEE standards.

Reactive power analysis may determine the need for a shunt reactor at the OSS. Depending on the switching instance of the voltage waveform, when energizing the export cable together with the shunt reactor or transformer, the breaker may not see current zero crossings due to slow DC offset decay. The potential for occurrence of breaker zero-missing phenomena for energizing offshore export cables is also part of this analysis.

Dynamic stability and control interactions analysis

Dynamic stability analysis is performed to confirm that the project’s power plant meets the utility voltage and frequency ride-through requirements. The characteristics of the wind turbine model are validated through steady state, fault, and dynamic response under indicative short circuit ratio (SCR) conditions. This study also evaluates reactive power output in reaction to positive and negative voltage changes at the point of interconnection (POI), and whether the reactive power output is compliant with interconnection requirements.

In addition to and in coordination with the dynamic stability analysis, a control interaction analysis needs to be performed to ensure no adverse interactions are present as a result of the offshore wind plant operation. This analysis would also consider the potential presence of sub-synchronous resonance (SSR) phenomenon and mitigation techniques.

Even though an initial analysis may be performed with generic converter and controller models (e.g., WTGs, MVAr controller, and STATCOM, if applicable), the final analysis should be done with validated vendor-specific models of the actual equipment to be installed in the project.

Harnessing the wind with EMT-type design studies

With so many factors to consider in OSS development, a little foresight goes a long way. EMT-type design studies play a pivotal role in the successful development of offshore wind substations.

Engineers can design robust and reliable substation infrastructures by thoroughly analyzing electrical systems’ transient behavior and response. The insights gained from these studies allow for the implementation of effective protection measures, ensuring the seamless integration of offshore wind energy into the grid.

As the world continues to embrace renewable energy sources like offshore wind energy, investing in EMT-type design studies becomes ever more critical for a sustainable energy future.

Related content: Check out this blog post from author’s Jenny Zhou and Iman Kaffashan, “Increasing substation resiliency with EMT studies.”

Do you have a project underway or about to start? The experts at PSC have the tools, insights, and experience to ensure your OSS is structurally sound and reliable.