Wind energy has grown from a non-conventional to a more widely used form of energy over the last 30 years. This has led to an increase in the research and development of wind turbine technology. Many wind turbine designs were evaluated in the 1980s to create a cost effective architecture for wind turbines. The ¨Danish wind turbine¨ concept was one of the more popular designs developed in the 1980s. The design was successful because of its simple architecture, which included a three bladed, stall regulated, fixed speed wind turbine. However as the size of wind turbines increased, new technologies such as variable speed operation, full span blade control, advanced loads mitigation and alternative materials emerged.

Hansen et al in their study have classified wind turbines into 4 major categories. 

Type A: Fixed speed wind turbine concept (Danish concept). Since this concept always draws reactive power from the grid, it uses a capacitor bank.
Type B: Variable speed wind turbine, with variable rotor resistance (Optislip concept). These turbines typically have a dynamic speed control range of 0-10 per cent above synchronous speed. This concept also needs capacitor banks.
Type C: Variable speed wind turbine with partial scale frequency converter (DFIG concept). Typical variable speed range for this kind of turbine is +/-30 per cent of synchronous speed. This concept provides reactive power compensation and is attractive in terms of economic returns to the end user.
Type D: Variable speed wind turbine with full scale frequency turbine. This concept has full control of synchronous speed. It supports reactive power and smooth grid transition.

More than 50 per cent of the installed wind turbines in the last 5 years belong to Type C category.

Section II of this article discusses key design criteria for wind turbines designed for low wind speed markets such as India with a focus on Type C and Type D turbines. In section III, we review technologies available to optimise energy capture from a wind farm. In section IV, we describe how technology can help wind farm owners to operate turbines with low maintenance. In section V, we will address the future trends in wind generation in India.

Design Criteria For Low Wind Speed Turbines
Majority of the sites in India have an average wind speed of 7 m/s or lower at standard hub height of 80 metres. This poses challenges in terms of extracting maximum energy from such sites.

Power extracted from wind can be described in terms of air density, wind speed, rotor diameter and turbine efficiency.

Where the density of the air, D is rotor diameter, Cp is the power coefficient and V is the wind speed.

Air density depends on the altitude and temperature of the project site. Higher wind power can be captured by improving the power coefficient, by increasing rotor diameter and by ¨increasing the wind speed¨.

While significant research has gone into improving the power coefficient (Cp), it has a theoretical limit of 0.593. Most of the modern wind turbines have a Cp of up to 0.52. Wind turbine blades have undergone major developments in order to achieve a high Cp. Wind turbine blade design was originally based on airfoils used in aircraft industries and was not optimised for higher angle of attack frequently employed by a wind turbine blade. Today manufacturers are using airfoils optimised for higher angles of attack.

Increasing rotor diameter is a key factor to capture more energy especially at low wind speeds. Each one metre increase in blade length will allow the wind turbine to capture approximately 2 per cent more energy. Increasing rotor diameter poses certain challenges to the wind turbine design. Total rotor mass increases significantly as rotor diameter increases. This increase in blade weight is approximately 2-4 tons for every five metres incincrease in blade length.

The additional blade weight can be managed using controls strategies which will help to mitigate increased gravitational and inertial loads. Most of the modern wind turbines today use advanced controls technologies which reduce rotor loads on wind turbines. In addition, new lightweight materials for blade technologies such as carbon can also be used to reduce blade weight. This will help to reduce overall weight of the turbine, transportation costs and installation challenges.

Increasing hub height (taller tower) is another option which will help to increase energy capture from a wind turbine, as it increases the wind speed incident on the wind turbine. However this is purely a function of wind shear at a particular site. Studies show that taller towers up to 100 metres and above are more economical for sites with a wind shear of 0.20 and above.

Energy Optimisation 
Net energy produced by a wind farm mainly depends on the following factors:

• Wake losses
• Turbine availability
• Grid availability
• Transmission losses and
• Other uncertainties

Wake losses
Wake losses in a wind farm can easily be mitigated by smart micrositing. Traditional micrositing approach of following 5D x 7D is intended to reduce wake losses in a wind farm and additional mechanical loads it imparts on the turbines. However this approach directly impacts opportunity to ¨site¨ optimum number of turbines in the wind farm and eventually may not be a good solution for the end customer. Manufactures who can offer multimodal optimisation techniques can help the customer to extract maximum energy from the wind farm by optimising turbine locations. Using optimisation techniques and wake management, modern wind farms are operating with turbines located as close as 2.5D from each other. Studies conducted on micrositing optimisation showed that net energy output from a wind farm can be increased by up to 6 per cent using such techniques. Based on the terrain, wind pattern and environmental conditions, there are significant opportunities for micrositing optimisation on Indian wind farms.

Availability
According to IEEE standards, availability is defined as the percentage of time a wind power plant is available to provide service. Availability calculation should include all forced, planned outage hours and maintenance hours. For high wind seasons in India, generally wind turbines are expected to be running with an availability of 97 per cent or above.

Turbine reliability and availability are well correlated. While availability is mainly operational focused, reliability focuses on product quality. Till date, reliability criterion was not given its due importance while selecting or operating wind turbines. However this trend is changing. Additional focus on reliability from customers will force wind turbine manufacturers to design wind turbines with up to 99 per cent reliability. This will ensure that all wind turbine components are well tested and proven before their commercial introduction.

Grid availability
Reducing grid outages was considered outside the scope of wind turbine design during the early years. However this is not the case anymore. Grid codes in most countries with well-developed wind markets dictate that a wind farm operate like a conventional power plant. The ability to control wind farm MW output is critical to grid operators. This will help to maintain stable grid frequency, as well as thermal and stability limits of transmission lines. Many wind turbine manufacturers offer the control strategy which can help to provide reactive power, even when a turbine is not producing any active power. With the new ¨ride-through¨ technologies, a wind turbine can remain online and feed reactive power to the grid through major disturbances. These technologies allow wind turbines to meet transmission reliability standards similar to thermal generators. As wind farms increase in size and contribute a large share of the power portfolio, the ability to provide uninterrupted power is key to the overall grid stability. With today´s technological advances, wind turbines can also offer inertial response capabilities similar to conventional synchronous generators during under frequency grid events.

Optimising Maintenance On A Wind Farm
Many customers in emerging wind markets are still reluctant to opt for a condition based monitoring system (CBM). However case studies have shown that CBM is an essential tool to reduce the overall maintenance cost of a wind turbine. This is most evident in case of a drive-train, which consists of shafts, bearings and gears. Benefits of the CBM system can be easily realised when one compares the cost of a CBM system installation with the cost of potential failures if the system is not installed. As Abramson points out, CBM is a diagnostic tool, which helps to drive asset performance higher and enables best in class operation.

For example, one type of CBM system is designed to gather acceleration data from the drive-train to understand the operating characteristics of bearings and gears. Looking at their vibration signatures, the CBM system can predict any potential issues with the drive-train components, which in turn can help with preventive maintenance planning.

Future Trends In Wind Generation In India
India has an installed wind generation capacity in excess of 20 GW. India´s wind energy potential, which has been estimated by various organisations, is at least 100 GW. Majority of the undeveloped sites in Indian States are low wind speed sites with the average mean wind speed between 4.5-7.5 m/s.

Many of these sites also pose challenges in terms of power evacuation infrastructure and smooth transportation of large wind turbine components. As the penetration of wind power grows, the grid will also need to be stabilised through forecasting and scheduling techniques. As the generation-based policies get established, there will be added focus on performance and reliability of wind turbine assets.

We have already addressed the low wind speed turbine development (including performance and reliability) earlier in the article. Technology development would also be critical to tackle some of the other challenges, thereby allowing the Indian wind industry to achieve its true potential.

Conclusion
Technical improvements over the last few years in wind turbines have helped manufacturers to develop cost effective wind turbines for low wind speed sites. This has opened up new markets for wind turbines. Technologies have also been developed to improve operation and maintenance of the wind turbines; as well as to minimise losses from a wind farm. As a result, in today´s environment, focusing on technology and reliability will help customers capture more value from their wind turbines.

The authors of the article are Renjith Viripullan (Engineering Manager with GE Power & Water, Renewable Engineering, GE India Technology Centre) and Nitin Bhate (Strategic Marketing Manager with GE Power & Water).