I. INTRODUCTION:

As this is an age of technological development of power generation where distributed generation can play significant role to meet the energy requirements of areas specially at remote areas. Penetration of distributed generation has not yet reached significant levels[1]. However that situation is changing rapidly and requires attention to issues related to high penetration of distributed generation within the distribution system.

India ranks fifth in the installed capacity of Grid connected electricity in the world as indicated in the Fig.1. While WEGs have varied type of innovative conceptual designs, there are two main types one is horizontal axis wind turbines (HAWT) and the other type is vertical axis wind turbine (VAWT). The former type has several technological and economical advantages. Further discussions will be on some design features of GRID connected application of Horizontal Axis Wind Turbines (HAWT).

Indian Scenario:

Issues in Wind Power Developments: Availability of accurate wind potential data all over India is one of the issues. C-WET has assessed more than 620 Stations spread all over the Country by continuous monitoring of wind as a resource and identified more than 233 locations as economically viable and wind potential.

As on date, in India a site which has more than 200 watts per square meter as Wind Power Density (WPD) which deemed to be economically viable so far is removed. With recently picked up micro wind generation i.e to exploit urban wind areas and low wind areas and to facilitate remote village electrification domestic wind mills of .3 kW to 50 kW are being adopted. This micro wind generation as on date has a low market demand since it has been mostly developed as a standalone system which is often not grid connected. The grid interface system which will have similar concepts such as net metering along with exporting to grid and importing from the grid in a house connection is likely to be possible soon. It has fast track implementation of wind power projects given by Government having fiscal and financial initiative[1].

Another important issue in any country involved in wind power development is development of infrastructure facility in wind farmable areas such as roads and logistics for larger machine components to reach windy Regions.

Other infrastructural facilities for establishment of human habitation in wind farm and establishment of electrical grid for evacuation of the power generated by the wind turbines. Most of the State and Central Government have been concentrating on these infrastructural developments and hence wind power in India has become more viable and the growth is sustained at the global rate of more than 20% every year.Wind Generation is cumulative over the years hence higher than even in Tamil Nadu where 40% of Wind power is installed. The other extreme is Madhya Pradesh where machines are getting installed every year but the generation is not happening, which may be due to low wind, inappropriate WEG efficiency , generation data update errors, or machine or grid failures related to operation and maintenance issues..While steady growth of WUI is indicative of orderly development any extreme as well as fluctuations the regulators/Grid operators and the investors need to be concerned to set the wind generation more useful.

Engineering Design of HAWT:

Engineering design of horizontal axis wind turbines requires an interdisciplinary approach involving composite material and meteorological sciences, aeronautical, mechanical, electrical, electronic control and civil engineering. Before we proceed with the engineering design of a turbine, we have to identify a windy location and assess the resource i.e. wind.

However the maximum available power, when the wind flows past a wind turbine’s rotor would be limited by a power coefficient, which will depend on the diameter of the rotor, frequency and speed of the rotor, aerodynamic characteristics of the rotor blades, and above all the wind speed at 5 to 6 diameters in front of the rotor and the turbulence of wind and terrain conditions of the site[2].

Criteria for the rotor and blade design:

The most important prime mover of the wind turbine is the rotor which consists of one to three blades assembled into a hub at the tip of a rotating shaft. The blade cross-section along the length is normally of aerofoil sections stacked with smooth twist and transition. Some details of such section are discussed later in the blade structural design lecture. The important criteria for the blade configuration are light weight, high strength, excellent fatigue endurance and robustness for operational dynamic stresses, and durability for the entire design life of the wind turbine[3]. The aerodynamic controls that would be required for the rotor would also govern the sectional modifications of the blade tip, which would serve as aerodynamic brake at times of need by design. The other parameters will be the constant/variable RPM of rotor, upwind/downwind functioning of the turbine, large deflection characteristics of the blade, natural frequencies of blades and rotor. Mostly composite materials are adopted for blade analysis, design and manufacturing.

Requirements of blade manufacturing:

Most of the wind turbine blades are made of fibre reinforced composites, with multiple layers of glass fibre or carbon fibre embedded in epoxy/polymer resins.

Blade Testing Facility:

The commercial blades are of size ranging from 11m to 45m in India. A blade test facility would require a hard test bed with anchorages for fixing the blade root. The loading is either applied by pulley and guy arrangement or using hydrodynamic jack systems with computerised controls for applying the required load to the blade and simultaneously measure the stresses in the blade under static and cyclic loading conditions.

Rotor design needs:

The rotor in the case of HAWT is an assembled hub consisting of 2 or 3 blades fitted to a low speed shaft. These blades derive an optimum lift force spread along their longitudinal axis from the root to the tip of the blade by their intrinsic aerodynamic shape and rotates the rotor.

Since the wind force is proportional to the square of velocity, as the wind speed increases the rotor while rotating has to resist that drag force which acts as a wind- frictional brake pressing the rotor to stop. So, most rotors can harness wind power efficiently only from about 3 m/s (cut-in) to 25m/s (cut-out) wind speeds beyond which the rotor has to come to a halt (stalled and feathered-out of wind) [4]. To reduce the fatigue stresses at blade root can be hinged to the hub or rotor may have teethered (a ball and Socket type of hinge) connection at the shaft tip.

Machine design needs:

The machine design depends on the type of wind turbine adopted to convert the kinetic energy in Wind to mechanical energy and finally to that of electrical energy of constant voltage and frequency. In India it is 220 volts with 50 Hz. The conventional technique is to convert the low rotational energy through a system of step-up gears and then the high speed shaft is then connected to the generator. Some of the latest use direct drive techniques which means the wind rotor is connected through gearless interface to generator or back to back mounted generator.

Drive train Characteristics:

The drive train is an interface between the wind rotor and the generator, the design of which is governed by the active/passive controls such as blade stall/pitch that are needed to the rotor for efficient power capture in the wind. Some modern wind machines are directly driven without gears.

Generator characteristics:

There are four types of generators in use in the grid connected application of wind electric generators. These can be grouped mainly into two, one having the old fixed speed induction type, and the other with variable speed multi-pole/permanent magnet-based.

The later ones have better connectivity for grid connected application of WEGs. In India we use 220 volts with 50 Hz in general. The usual low speed rotor speed (revolutions per minute, 10 to 40 RPM) needs to be stepped up (50 Hz = 50 cycles/s = 3000 cycles/minute = 1500 RPM per pole in the case of a 2-pole generator) to interface with the generator whose output needs to be synchronised to the GRID for public utility.

II. SINGLE TURBINE AND MULTIPLE TURBINES:

A. Single Turbine Representation (STR)

In this section, we look at a wind power plant represented by one group of wind turbines. This is the worst-case assumption because we assume that all the wind turbines in this group are synchronized. Thus, the same wind fluctuations and tower shadow effects will affect the output power of the wind power plant and the power quality at the PCC[5].

B. Multiple Turbine Representation (MTR)

In this section, we focus on the aggregation impact on the wind power plant output at the PCC. We use the same wind turbulence intensity and the same impedance of the transmission line. We measured the real and reactive power fluctuations, the voltage fluctuations, and at the PCC of a wind power plant. We quantified the difference in power and voltage fluctuations level if we treat a wind power plant as a single turbine or as multiple groups of turbines. The flicker level measurement can be implemented using design specification in IEC 61000-4-15 [1]. Ideally we would like to model every wind turbine on the wind power plant. Unfortunately, a large wind power plant can have more than 100 wind turbines on site. Therefore, it is not possible to represent all the turbines simultaneously, because the computing time would be excessive. To closely represent a real wind power plant without simulating each wind turbine, we made the following assumptions: A large wind power plant (200 turbines) is divided into several groups of wind turbines.· The wind speed is uniform for each group of wind turbines.

C. Comparison between STR and MTR

To start, consider the time series of wind speed shown in Figure 3. In an STR, the wind speed is applied to a single turbine and the output of the single wind turbine is multiplied by the number of the turbines within the wind power plant. In an MTR, the time series of the wind speed is subdivided into several sections and each subdivision is applied to a different group of turbines. For example, for the figure shown, the time series of wind speed is divided into four different files with the starting time (t=0) at w1, w2, w3 and w4. This assumption is an approximation of the time it takes for the wind speed to travel from one group of turbines to another group of turbines down wind. Although this assumption is not perfect, by assuming that the wind speed has a characteristic of frozen turbulence, and that the turbulence does not change as it passes a wind turbine, we can more closely simulate the real situation. Let us consider the output of STR and MTR and place the two graphs next to each other for a better comparison. Figure 3 shows variation of real power for both representations taken at the point of interconnection.

The time scale is changed to make an easier observation of the nature of power fluctuations within a short time frame. In these particular traces, the trace of tower shadow is very visible. Tower shadow effect is the effect of power fluctuations due to power production deficit every time a blade passes the turbine tower. Usually the tower shadow has a frequency 3 per revolution. This effect is commonly known as 3 p effects. Besides the tower shadow, the power variation is also caused by the wind speed variations with time. For the STR, the power fluctuation reflects the power fluctuation of a single turbine. It is amplified by the number of turbines within the wind power plant.

For the MTR, the power fluctuation is the collective behavior of several groups of wind turbines with each group fed by a different time series as illustrated in Figure 3.The label WP1G is a single-group representation and WP16G is a 16-group representation. Comparing the two graphs, it is obvious that there is some smoothing effect in the power fluctuations if we consider that the wind power plant consists of sixteen different groups of wind turbines. Figure 3 shows the voltage fluctuations as the wind speed varies with two different representations. The STR obviously shows very large variations of the voltage at the point of interconnection as the wind speed varies while the output voltage for 16 groups representation shows a much smaller voltage swings[5].

Control functions and their needs:

Wind energy cannot be fully captured by any wind machine as on date but it should start generating power when wind speed exceeds cut-in ( 3m/s ) and continue to produce power till cut-out ( 25m/s). It will stop rotation when the wind speed goes higher than cut-out, 25 m/s under such conditions the drag force on the rotor dominates acting like a wind brake on the rotating blades. Hence one has to design the rotor to loose some wind energy even during operation within limits to maintain the permitted RPM levels of the rotor.

This has been originally achieved by the passive twist in the blade and by aero-elastically on the aerofoil section of the blade. Now the technology is shifting from the stall regulation to pitch regulation. It has been further improved with active pitch regulation giving the advantage of using the fluctuations in the natural wind, for more efficient power generation. Apart from rotor controls, upwind WEGs would require a yawing system with controls which would make the rotor seek the wind as it changes directions.

Requirements of power electronics:

If one tries to use the directly connected wind rotor to a induction generator the generated electricity is of AC (alternating current) ; but the output will vary as the wind varies. Our electrical gadgets will be flickering as the wind power varies. So, to connect the wind generated electricity to any utility grid (transmission and distribution system) the designer needs get the generator output in the grid voltage and frequencies (220V/50Hz in India and 110V/60Hz in USA). This primary need forces one to use sophisticated power electronics for all the control functions and output power conditioning especially when it is variable speed operation. Grid interfacing of Wind electric Generators will be covered in a special lecture in this course.

Alternative torque converters to power electronics:

Power electronics in some low temperature areas has functional difficulties for ensuring reliability of operation of wind turbines and also needs special requirements for onshore and offshore applications. Recently some of the automobile spare manufacturers have demonstrated applications of variable speed couplers in combination with torque converters which have wide applications under adverse temperature conditions as well as onshore/offshore wind turbines without altering the design conditions.

The basic idea being the replacement of power electronics demand for variable speed operation of wind turbine due to fluctuating nature of wind to a fluid coupled variable torque to constant torque converter thus ensuring the use of conventional induction generators more reliably in varying wind and grid conditions.

Modern Accessories:

The traditional squirrel cage induction generators which convert AC-AC as the wind varies has been shifting to synchronised AC-DC-AC type of generators ( doubly fed induction generators (DFIG), wound rotor generators, back mounted direct drive generators and permanent magnet generators (PMG)) for efficient power capture from the wind.

Modern developments like torque converters using variable speed fluid couplers, magnetic levitation based rotating systems are likely to become cost effective bringing down the cost of wind generated electricity to lower levels.

In the aero dynamic controls, the shift is taking place from traditional stall control to pitch control and modern machines have active controls independently for stall as well as pitch mechanism. These sophisticated active controls require power electronics and hydraulics interfacing with various systems. Some of the latest WEGs use electro-magnetic/mechanical control systems instead of hydraulics.

Wind electric Generation: A scenario

Globally installed capacity of WEGs has crossed 200 GW. Indian Energy Industries have a total installed capacity of 184 GW of Electricity generations as on date, of which about 19 GW is from all Renewable Energy Sources in which about 14 GW is from Wind Energy. Wind is one of the fastest and most viable Renewable energy technologies. In India, the annual capacity addition per year , in 2010-11 is currently about 2300 MW. India also has about 1MW of wind- solar hybrid domestic system which are mostly used as standalone applications.

India has got an ambitious plan to exploit in full, the wind energy potential in the Country which is estimated to be 49 GW of which only 20% has been exploited as on date.

However, this amounts to about 3 to 5% of net electricity generated in India.

Project Development Procedure:

Steps to follow for a wind power project are as follows:

· Wind Resource Assessment through measurements, micro surveying and potential site identification

· Choice of the capacity and the number of the wind turbines for the identified sites for wind farming

· Micrositing of the wind turbines in a particular wind farm

· Erection and commissioning of wind turbines

· Establishment of continuous monitoring systems like SCADA

· Grid connection of wind turbines

· Power quality measurements and feed back

· Wind resource prediction/forecasting and load scheduling to load demand and Generation Management

· Power trading options across inter-state boundaries

Wind Electricity Generation (World):

Worldwide there is growing interest to harness this free, not-needing storage, not-needing transport, abundantly available renewable clean and green energy.

Wind electricity generation has been steadily increasing in some countries and world leaders being, Denmark 21% energy penetration, Portugal 18%, Spain 16% Germany 9%, India 3-4%, USA 2% and China 1.2%. Wind as a fuel for electricity generation is free, pollution free, devoid of the need to transport and to store. More than everything it is renewable.

Infirmity and Sustainability of Wind Power: (i)Infirmity:

It is well known that wind is natural and is infirm power and the implications of infirmity are:

• Wind power cannot be scheduled

• Needs a spinning reserve or alternate power

• Wind generators bank energy and demand during peak hours

• Electricity Board pays for both base power purchase and wind generation

• Grid stability and low voltage, high frequency due to wind farm operations

• Needs research for economic storage to avoid wastage of free wind power

Weather triggered random wind and wind generation can be forecast to make the infirm power as dispatchable firm power. Some successful energy storage techniques such as pumped storage hydel (PSH), compressed Air energy storage systems and flow battery (NaS) systems at world’s wind farms are proven to be practical solutions.

Sustainability:

It is well known that wind is certainly an inexhaustible abundant source of energy which is caused by the differential solar radiation on the Earth’s geo-diverse surfaces, having different degrees of absorption/ reflection/ refraction/convection/transmission. Wind power is not only a renewable green source of energy; but also results in significant saving of potable/drinking water, which is much needed for human survival.

III. MICROGRID CONCEPT:

To realize the emerging potential of distributed generation one must take a system approach which views generation and associated loads as a subsystem or a “microgrid” [4]. During disturbances, the generation and corresponding loads can separate from the distribution system to isolate the microgrid’s load from the disturbance (and thereby maintaining service) without harming the transmission grid’s integrity.

The difficult task is to achieve this functionality without extensive custom engineering and still have high system reliability and generation placement flexibility. To achieve this we promote a peer-to-peer and plug-and-play model for each component of the microgrid. The peer-to-peer concept insures that there are no components, such as a master controller or central storage unit that is critical for operation of the microgrid. This implies that the microgrid can continue operating with loss of any component or generator. With one additional source (N+1) we can insure complete functionality with the loss of any source. Plug-and-play implies that a unit can be placed at any point on the electrical system without re- engineering the controls.

Plug-and-play functionality is much akin to the flexibility one has when using a home appliance. That is it can be attached to the electrical system at the location where it is needed. The traditional model is to cluster generation at a single point that makes the electrical application simpler. The plug-and-play model facilitates placing generators near the heat loads thereby allowing more effective use of waste heat without complex heat distribution systems such as steam and chilled water pipes.

This ability to island generation and loads together has the potential to provide a higher local reliability than that provided by the power system as a whole. Smaller units, having power ratings in thousands of watts, can provide even higher reliability and fuel efficiency. These units can create higher reliability and fuel efficiency. These units can create microgrid services at customer sites such as office buildings, industrial parks and homes. Since the smaller units are modular, site management could decide to have more units (N+) than required by the electrical/heat load, providing local, online backup if one or more of the operating units failed. It is also much easier to place small generators near the heat loads thereby allowing more effective use of waste heat.