Offshore Wind and Energy Storage are two of our primary focus areas in renewable energy. We cover development through to execution.

Offshore Wind Trends in Development and Execution

Over the last ten years, we have witnessed considerable changes in offshore wind farm development and execution. Farms of low power capacity close to shore, used used fixed platforms and high voltage AC (HVAC) as the means of power transmission. New farms of several hundred MW are now being designed, and the trend is to move to high voltage DC (HVDC) as the preferred form of power transmission. The next phase in the evolutionary process is to more further offshore, to floating platforms. This step change introduces many new challenges.

Main components of offshore support structures: a) monopile; b) jacket; c) gravity base; d) suction bucket (1 – tower, 2 – work platform, 3 – transition, 4 – intermediate platform, 5 – boat landing, 6 – external J tubes, 7 – scour protection, 8 – grouted connection, 9 – monopile, 10 – skirt, 11 – concrete layer, 12 – suction bucket, 13 – shaft, 14 – pile sleeve, 15 – mud mat)

Typical offshore substation structure [Source: Bull. Pol. Ac.: Tech. 66(4) 2018}

Main components of offshore support structures

Offshore Substation Layout

The topside is typically a box-shaped structure placed on top of the substructure that contains the electrical equipment included in the offshore substation, providing for some or all of the platform’s functions. The topside layout is mainly determined by:

1. Electrical substation layout. This layout results from the type of the power substation implemented in the offshore grid, the capacity of the offshore wind farm and electrical parameters (voltage, type of current (AC or DC) of the submarine transmission system). Topsides for high voltage DC (HVDC) substations are generally larger than their high voltage AC (HVAC) counterparts.

2. Additional requirements, which include:

the operating conditions of electrical equipment (cooling systems, isolating clearances, etc.);
the working conditions for the substation’s services or personnel (personnel rooms, air conditioning, accommodation, etc.).
safety, navigational, signaling and rescue systems (fire systems, lightning systems, evacuation, etc.);

3. Adopted solution for the support structure. Other factors that can impact the topside structure are topside port requirements due to the fact that the topside structure is made onshore and then transported to the installation location. The topside is composed of decks divided into rooms and halls with the lowest deck being the cable deck, whereas the topmost deck accommodates signaling equipment and a helipad (if present in design). Electrical equipment on OS offshore platforms is arranged in either of the following two ways:

vertical placement of equipment – used mainly in substations for OWFs with low capacities of up to 100 MW, where the support structure is gravity-based or monopile;
horizontal placement of equipment – used in substations for OWFs with high capacity in excess of 100 MW, where – due to the quantity and weight of substation elements, a jacket multi-pile support structure is necessary.

Racebank

Sandbank

Offshore Substation Electrical Equipment

Electrical equipment of the offshore substation depends on the type of electrical current (AC – Alternating Current, or DC – Direct Current) used for the transmission of electrical energy. Due to the type of electrical current used, HVAC and HVDC offshore substations can be distinguished. The reasons for choosing HVDC instead of HVAC to transmit power from offshore to onshore systems are often numerous and complex. But in most cases the transmission distance to the shore is the key factor. Basic electrical equipment of HVAC substations includes:

transformers,
auxiliary transformers,
earthing transformers,
GIS high and medium voltage switchgear,
backup generators,
earthing resistors,
reactors,
AC filters

The main purpose of HVDC substations is to convert AC current into DC current. Hence, additional elements at HVDC substations as compared to HVAC substations include:

IGBT converters with a cooling system,
smoothing coils,
DC filters.

Kriegers Flak offshore substation: a) lower deck; b) upper deck;
1 – transformer 220/33 kV,
2 – 220 kV GIS switchgear,
3 – 33 kV switchgear,
4 – Diesel generator,
5 – emergency switchgear,
6 – workshop and storage,
7 – SCADA,
8 – 0.4 kV switchgear,
9 – accumulator battery,
10 – social rooms,
11 – heat and ventilation

Kriegers Flak offshore substation: lower deck

Kriegers Flak offshore substation: upper deck

Large Scale HVDC Windfarm

A large scale installation, such as DolWin 2 comprises the following main elements:

Wind turbine generators
Array cabling
Offshore substation (and converter if built as a single unit)
Subsea cables HVAC to converter platform
Subsea HVDC export cables to onshore converter station
Onshore converter station
Onshore substation
Grid interconnection.

In the case of HVAC transmission, the offshore and onshore converter modules are omitted, and reactive compensation (reactors, STATCOM - static compensation added). The HVAC offshore substation would connect to the onshore substation (simplified manner)

The WTG typically operates at 3.3kV and is fed into a Generator Step Up (GSU) transformer, to the array distribution voltage, typically 33kV or 66kV. A number of WTGs are connected in series arrangement to for a string.

A string might comprise 6 to 10 WTG depending on the configuration.

In the case of the DolWin2, each bus was capable of handling 4 strings, with a rated bus current of 4,000A.

WTG Single line diagram

Overall Single Line Diagram

Optimization and Modularization

The general trend is to build the offshore substation (OSS) and offshore converter platform (OCP) as a single installation. In such as case, we eliminate the HVAC cables between the two functional modules.

We can observe from the diagram, that the design incorporates a certain amount of redundancy. For example:

OCP has redundant valve strings
OSS has 4 transformers, each feeding a bus.
OSS has 2 general service transformers etc.

One of the key objectives is to test as early as possible, in controlled conditions. This will allow us weed out and resolve many small issues, and also prevent worker congestion in the final integration yard, and perhaps offshore.

Areas of optimization include, the design and build of:

Two separate valve halls
220kV GIS switchgear module
66kV GIS switchgear module, potentially comprising "power island" per bus
One General Service modules. comprising transformers, switchgear, UPS and battery systems, and either an integrated or separate:
Integrated Control and Safety System / Control Room.

It can easily be recognized, that all monitoring, control and protection functions must be made available and tested for each module.

By designing and building the modules for autonomous testing, we are also able to reduce the number of integrated cabling between modules. Cabling between modules is limited to power cabling, certain hard-wire cables, and network cabling.

Offshore Substation / Converter Platform Single Line Diagram

Modularization will be key for offshore substations and converter platforms, in order to reduce delivery time, cost, and performance predictability, in an aggressive market.

Our areas of expertise

We offer valuable expertise in a number of areas, however our primary focus is on Offshore Electrical Service Platform and Onshore Substation Engineering, Procurement and Construction within the Electrical, Instrument and Communications disciplines.

Command Center for all unmanned remote control and monitoring of turbines, Electrical Service Platforms, and Substations.

Commissioning of Medium Voltage Switchgear. Similar experience in 35kV Hybrid switchgear, and Gas Insulated Switchgear up to 232kV

Network Controllers, for Integrated Control Command and Safety (ICSS) for the electrical system.