LONDON — In offshore wind development, the safety and reliability requirements for medium voltage (MV) equipment are even higher than for the onshore environment. As access is more limited at sea, turbine reliability and its safety features become vital to operational success. Guaranteeing maximum and continuous supply during operation, and personnel safety when inside an offshore turbine, are thus serious considerations during construction.
As a consequence of this new reality, MV switchgear has undergone developmental changes. Now swicthing equipment must accomplish higher insulating levels and operate under much harder salinity corrosion, humidity and temperature conditions than those established by exisiting onshore international standards.
The positioning of switchgear in the tower may require design modifications
The following describes the main developments with MV switchgear, as well as the particular tests performed, which reproduce sea transportation conditions and the environmental conditions of offshore wind turbines. Part of the performance assessment is achieved by means of accelerated aging tests in salinity chambers, which has allowed certification of MV switchgear for use in offshore wind turbines, as well as on wind farms destined to operate in other extreme climate conditions, such as extreme cold.
Driving mechanisms are the devices responsible for opening and closing a turbine’s three-position switch-disconnector and circuit breaker. These systems would only be opened when a fault occurs within the wind farm’s MV grid, or during maintenance or operation checks. Nonetheless, the reliability of this device is vital from a safety and performance point of view.
In places like the U.S. and Canada, for example, temperatures at a wind farm can be as low as -22°F (-30°C) during operation and -40°F (-40°C) while equipment is being stored. In these conditions, operation of the driving mechanisms must be verified according to the mechanical endurance category they are listed under, and verified for leakage levels from the MV gas tank so they don’t increase, losing insulating properties.
A modular switch unit undergoing testing on a vibration platform
In one case study of this nature, the circuit breaker and switch-disconnector driving mechanisms were operated inside a climatic chamber for 12 hours at -40°F. This was done to ensure that the number of operating sequences was achieved and the speed (rad/s), torque operation value (Nm), and rebounding (mechanical degrees) remained within the required parameters.
A related issue is corrosion. International standards for indoor installed switchgear states: ‘Air must not be significantly contaminated with dust, smoke, corrosive/explosive gas, steam, or salt.’ However, the environmental conditions in any offshore wind farm cannot guarantee the above statement due to high levels of salinity in the atmosphere. Therefore, it is necessary to apply special surface treatments to the galvanised steel elements and, most importantly, to extend the driving mechanism’s lifecycle against corrosion, ensuring it will without fail accomplish the number of operations it promises.
A specific test performed on driving mechanisms and small components (such as tripping coils, motors, relays, and steel enclosures) is to place them in a salt-spray chamber for 720 hours at 95°F (35°C), and spray a solution of five percent NaCl by weight. The goal attained: to achieve a high-corrosion resistance classification, according to international standards.
Transportation of the Switchgear
Every wind project developer knows that one of the most important parameters to optimise during construction of a wind farm is the installation time because of the high costs associated with the required resources. That’s why it’s common to install the MV switchgear horizontally inside a turbine tower and then transport the tower to its offshore platform in a special vessel. However, transporting the MV switchgear under these conditions adds another challenge, as the equipment must withstand horizontal forces due to the positioning of the tower. It also must withstand vibrations during sea transportation.
To reproduce these conditions on MV switchgear, a horizontal axis 3D vibration test has been used, for example. In this case the device applies a 4 Hz frequency and an acceleration of 0.875 m/s2 with a maximum amplitude of 27.2 mm. The test was fixed at a low frequency, and performed on a group of three modular functional units. Afterwards, the copper circuit resistance was measured to ensure the values did not change during testing.
In this case, all joint connections, screw drive connections, and the welding successfully passed.
Power & Design
The power capacity of wind turbines and wind farms has increased dramatically over the last years. The power capacity of the first turbines ranged between 200 kW and 300 kW, whereas today most offshore turbines are 3-MW machines or more. Some experimental wind farms are even being developed with 6 MW machines and still large machines are under development.
To decrease the electrical loses within a MV wind farm’s grid, there’s a tendency to increase distribution voltage levels, which requires higher insulation levels than those established by international standards. This is why a certain amount of wind farms use 40.5-kV units, which determine insulating levels of 95 kV for one minute at industrial power frequencies (routine tests in all compartments of the MV unit) and 185 kV for the lightning impulse test (BIL).
A switch driving mechanism following a 720-hour salt-spray chamber test
The particular position in which the MV switchgear is installed inside the wind turbine tower requires different designs to maximise safety in the case of an internal arc fault inside the switchgear. Some turbine manufacturers choose a layout where the switchgear is installed above ground level. This means that an alternative design is necessary for the gas release duct from the switchhousing.
In one example, keeping this potential requirement in mind during testing, the bottom of the unit was sealed (the cable compartment), and a backside gas-release duct was designed and installed to guarantee that nobody either in front of or underneath the unit could be harmed during its operatuion. In this case the testing of this special design was performed according to the IEC & IEEE standards, accomplishing IAC AFL (R as an option) arc fault resistance classification for a 21 kA short circuit current.
Wind energy technology and the construction of new wind farms in places where the operational requirements are higher due to environmetal conditions such as salinity, temperature, or humidity, require MV switchgear which is adapted to those particular conditions if it is to operate safely and successfully — and which international standards do not fully take into consideration currently.