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Network Planning for Wind Power: Benefits of and Options for Increasing Transmission Capacity

THE NEED FOR IMPROVED NETWORKS

Liberalisation, market conditions, technology and environment create fundamental changes and challenges for the European transmission and distribution networks. One of the major drivers is the emerging internal electricity market in Europe, which requires an adequate transport capacity between control regions and Member States to enable effective competition and trade of physical electricity. Therefore, enhancing the suitability of the grid for increased trans-national and regional electricity transport is in the interest of both the wind industry and crucial for the development of the internal electricity market.

In addition, the specific nature of wind energy as a distributed and variable output generation source requires specific infrastructure investments and the implementation of new technology and grid management concepts. The impacts of wind power on transmission, as described in Chapter 2 wind power variability and impacts on power systems and Figure 2.1, are related to grid stability, congestion management, and transmission efficiency and adequacy. The large-scale integration of wind power requires a substantial increase in transmission capacity and other upgrade measures within and between the European Member States.

IMPROVING NETWORKS FOR INTEGRATING WIND POWER

The typical additional grid improvement measures required at increasing levels of wind power penetration can be classified into the following categories, in order of increasing effort and cost.

 

Soft Measures

In the short term, and at relatively low levels of wind power penetration, transmission upgrades coincide to a large extent with methods for congestion management and optimisation in the transmission system. Soft measures do not involve extensive expenditure, but rather avoid or postpone network investments.

The utilisation of existing power lines can often be increased by operating them at a higher capacity, assisted by temperature monitoring. Improving the cross-border electricity exchange procedures, and thus the manner in which power is flowing between different countries, is also a method for alleviating congestion. If controllable power plants are available within the congested area, coordinated automatic generation control (AGC) may be applied. Demand-side management, controlled according to the wind energy and transmission situation, is another option. Applying control systems that limit the wind power generation during critical hours should be considered as a last resort, because it is both environmentally and economically inefficient.

Investments Other than the Construction of New Lines

At significant penetration levels, there is a need for additional voltage management in the grids, which can be achieved by devices such as FACTS , and also by the technical capabilities of the wind farms themselves; in particular with technologies that enable expanded MVAR capabilities .

Studies in the UK have concluded that it may be preferable to insist on sufficient fault ride through (FRT)  capability from large wind power plants. In certain cases, and in order to ensure power system security at higher penetration levels, this would be more economical than modifying the power system operation and not insisting on FRT capability from wind turbines.

It can be argued that the additional costs associated with the improved wind power plant capabilities at wind farm level should also be socialised, as is the case in the 2008 amendment of the German Renewable Energy Law.

There are several ways in which the transmission capacity of the network can be increased. These include:

• Adding transformers to existing substations, thus enabling a higher load feed and, in some cases evacuating higher generated power;
• Upgrading assets, for example, operating a line at higher voltage (within its design limits), or increasing the transmission capacity of a power line by tightening the conductors and reinforcing the towers;
• Installing new facilities in grid substations to improve the distribution of power flows among different parallel paths, to fit better with the line capacities, for example, series reactors, phase shifting transformers, or devices to increase voltage support (shunt reactive devices and static VAR compensators);
• Improving the utilisation of existing assets when possible, for example,  replacing line conductors with high temperature conductors, or adding a second circuit on an existing line (within the design limit of the towers);
• Replacing existing assets with those of a higher transmission capacity, for example replacing an existing 225-kV line with a 400-kV-double-circuit line.

Construction of New Lines

Grid reinforcement is necessary to maintain adequate transmission, as wind power penetration increases. This reinforcement is preceded by extensive power system analysis, including both steady-state load flow and dynamic system stability analysis. The construction of new lines is also a prerequisite for reaching regions with a high wind resource, for example offshore locations. Nowadays, in many areas of Europe, the construction of new overhead lines may take as long as 10 to 12 years from the initial concept to implementation, mainly because of lengthy planning and permission procedures.

Several studies at national and European level are now underway to back up the plans for upgrading the European transmission system, in order to facilitate large-scale integration. The most important international studies are the European Wind Integration Study (EWIS) and TradeWind, which will provide recommendations in 2009. Initiated in 2007, EWIS investigates the grid measures necessary to enable the wind power capacity foreseen for 2015 in a cooperative effort between European TSOs. EWEA coordinated project, TradeWind, started in 2006 and investigates the grid upgrade scenarios at European level that would be necessary to enable wind energy penetration of up to 25, using wind power capacity scenarios up to 2030.

ENSURING ADEQUATE TRANSMISSION CAPACITY AND ACCESS FOR WIND POWER

From the above, it can be seen that in order to integrate wind power, sufficient transmission capacity needs to be available to carry the power to the demand centres. This capacity must be provided by transmission lines and a proper legal framework for accessing this capacity is required.

On a European level, two major initiatives contain basic elements of sch a framework (see also [c]European policy framework relevant for wind power integration):

• The proposed new European Renewable Energy Directive (2008) stipulates that national governments and TSOs should guarantee sufficient transmission capacity and fair access for renewables to the transmission network.
• The mandatory ownership unbundling of generation and transmission, as required by the proposed 3rd Energy Package (2008), should provide the legal basis to guarantee a level playing field with other generators.

In practice, the construction of the required network upgrades, especially new lines, is a very lengthy process. Also, because of the difference in speed between wind power development and transmission development, there is a need to implement fair access rules, for cases where lines have to be shared between wind and other generators. As yet, there are no established rules at European level and grid access for wind energy is presently solved in a rather pragmatic way. Some countries, such as Germany and Spain, take into account the recommendations from the 2001 RES Directive, and grant priority access to wind power to a certain extent. In practice, in cases where available grid capacity is limited, the principle of ‘connect and manage’ is often used.

INTEGRATING WIND POWER IN DISTRIBUTION NETWORKS

The addition of embedded generation, such as wind power, to distribution networks is quite common and was at the origin of wind power development in most countries. However, when wind generation reaches very high levels, it brings new challenges, for the following reasons:

• The distributed generation adds a further set of circumstances (full generation/no generation) with which the network must cope, without negatively affecting the quality of supply seen by other customers
• The direction and quantity of real and reactive power flows change, which may affect the operation of network control and protection equipment at local level
• Design and operational practices are no longer suitable and may need to be modified

In contrast to these challenges, Distributed Generation Systems (DGS) also bring benefits to distribution networks, including:

• a reduction in network losses, in many situations;
• the avoidance of network reinforcement, which would otherwise be required to achieve standards for quality of supply.

To address these issues, distribution networks may become more “actively managed”. This implies cost, and requires the development of suitable equipment and design principles. Actively managing the networks by DSOs may be assisted by introducing new concepts, such as “clusters of wind farms” that aggregate and enable the monitoring of generation (see also paragraph [d]Wind power cluster management). In the future, it is expected that distributed wind generation will be fully controlled and operated as a Virtual Power Station (VPS).