Designing a wind farm - Part II

Part I of this topic (in the July 2009 edition of Wind Directions) was focused on wind farm design from the point of view of the physical parameters influencing its design: the resource, visual influence, noise and turbine loads. In Part II, the “hardware” aspects of infrastructures (split between civil works, electrical works and the control system (SCADA), construction issues, and commissioning, operation and maintenance), are described.

Infrastructure

CIVIL WORKS

The largest part of civil works are the wind turbine foundations, which must be suitable to support the wind turbine under extreme loads. The evaluation of these loads is therefore a first step for foundation design. For security reasons, these loads are calculated from the strongest wind speed which could happen once every 50 years. A typical foundation will be a block of reinforced concrete, 13m across hexagonal form, and 1-2m deep.

ELECTRICAL WORKS

The turbine generator is low voltage (usually below 1 kV, exceptionally 3 kV, and often 690 V). This voltage level is not high enough for direct interconnection to other turbines. Therefore, it is necessary for each turbine to have a transformer to step up to Medium Voltage (MV - 10 to 35 kV). In most cases, turbines are interconnected by

underground cables. The design requirements for the wind farm electrical system are set out in the connection agreement, a ‘grid code’ or something similar.

The electrical system shall:

• meet local electrical safety requirements and be capable of being operated safely;

• achieve an optimum balance between capital and operating costs (electrical losses) and reliability;

ensure that the wind farm satisfies the technical requirements of the electricity network operator; and

• ensure that the electrical requirements of the turbines are met.

SCADA AND INSTRUMENTS

The SCADA system acts as a ‘nerve centre’ for the project. It connects the individual turbines, the substation and meteorological stations to a central computer. This system allows the operator to supervise the behavior of all the wind turbines and also the wind farm as a whole. It will keep a record of all the activity and allows the operator

to determine what corrective action, if any, needs to be taken. It also records energy output, availability and error signals, which will act as a basis for any warranty calculations and claims. The SCADA system also has to implement any requirements in response to instructions from the network operator.

Construction

Newcomers to the wind industry tend to think of a wind farm as a power station. However, a conventional power station is one large machine, which will not generate power until its construction is complete. It will often need a substantial and complicated civil structure, and construction risk will be an important part of the project assessment. The construction of a wind farm is more akin to the purchase of a fleet of trucks. The turbines will be purchased at a fixed cost agreed in advance and a delivery schedule will be established exactly as it would be for a fleet of trucks. In a similar way, the electrical infrastructure can be specified well in advance. There may

be some variable costs associated with the civil works which are small compared to the cost of the project as a whole. The construction time is also very short, as a 10 MW wind farm can easily be built within a couple of months.

Commissioning, operation and maintenance

Commissioning refers to the activities after all components of the wind turbine are installed. It takes about two days, involves standard electrical tests for the electrical infrastructure and the turbine and inspection of routine civil engineering quality records. It usually takes about six months for the wind farm to reach full, mature,

commercial operation; that means the availability will increase from 80-90% after commissioning to the long-term level, on average, of 97% or more. Once a wind farm is working, usually 40 hours per year per turbine have to be budgeted for routine maintenance work, which is usually covered by a two- to five-year warranty from the turbine supplier.

Reference: Wind Direction by EWEA, September 2009