AC-DC-AC Converter for Wind Turbines under Unbalanced Voltage and Frequency Conditions

Muhammad Azeem Sheikh
Muhammad Azeem Sheikh

Rana Arslan Ahmed
Rana Arslan Ahmed

Muhammad Amir Raza
Muhammad Amir Raza

Ch 1:-Introduction:
1.1  Wind Energy
Wind energy had been harnessed by and its potential known to man a long time ago. The use of sails on the masts of ships, of wind mills to ground grain and cereals in American Mid-West and Australian barren outstretches predates to Medieval Ages. With the discovery of electricity, wind power has found a new area of application. Its primary use now in the modern world is to provide lighting to rural areas which are too remote to be connected to national grid. Now these turbines can charge lead acid accumulators coupled to an inverter for a rural farming community or even generate Giga watt hours (GWh) when erected in a wind farm inside a feasible off shore wind corridor.
1.2  Wind Turbine
Wind Turbines are used to convert kinetic energy of wind to electrical energy. It consists of aerodynamically designed turbine blades and generators which are driven by rotation of those blades thus generating electrical energy.
1.3  Horizontal Axis Turbine

Turbine mounted over a high mast is called Horizontal Axis Turbine. This design is common in case of wind farms, a term explained in following text, and is generally used in feasible ‘Wind Corridors’. These turbines can’t change their blades direction with wind and hence energy is generated only by wind blowing in blade direction. Although modern small HAWTs are incorporating tail-vanes which align the blades to the wind direction.
1.4  Vertical Axis Turbine
Vertical Axis Turbines for house roof installation are also now common. These are mounted on roof tops. An edge offered by this design is that the blades are oriented in such a way that wind blowing in any direction can generate electricity. It can feed the house completely or can be used in partial conjunction with any other renewable source or electric utility grid.
1.5  Wind Farm
 Normally, a single turbine is not significant enough to produce substantial amount of electrical energy. A huge array of wind turbines is arranged in form of a wind farm. Surplus energy obtained from such a farm is then fed into electric grid using a transmission network. More than 25% of electric energy in Scandinavian Countries is being generated by using wind turbines. A proper meteorological survey is conducted to point out feasible ‘Wind Corridors’ for erection of wind farms. A wind turbine output is random and depends upon the only variable wind speed.
1.6  On Shore Wind Farm
An on shore wind farm is a collection of wind turbines which are located at least 2 miles inside the nearest coastal line. These farms tend to gain advantage of the acceleration wind undergoes while crossing over geographical terrain of a ridge. As far as maintenance of such farms is concerned, it does not offer a big challenge to the technical staff.
1.7  Off Shore Wind Farm
An off shore wind farm is located inside the water away from the coastal land. These farms tend to gain advantage of the higher average wind speeds than on land. These farms offer a higher CF (Capacity Factor) ratio than that of on shore farms. However maintenance of off shore wind farms is relatively tougher than that of on shore wind farms.    

1.8  Capacity Factor:
Capacity Factor of a wind turbine is defined as the ratio of actual electric energy generated by wind over a year to the electric energy which could have been generated if the generators had been running on full name plate capacity over a year.
Maximum value of CF ranges up to 0.5 under optimum air speed and blade designing and continues to increase. Capacity Factor depends upon aerodynamics of turbine blades and potential of air corridor.
1.9  Motivation and Objectives
 Wind energy is truly a carbon footprint free, inexhaustible and safe energy source which reduces our dependence on conventional fossil fuels. Recent Geo-Political turmoil and territorial conflicts because of attempts to control energy sources advocate investing in abundant and globally available renewable energy sources. Economic uncertainty because of energy crisis can be averted. Wind farms take a lot of area but the portion used by wind turbines is low. Redundant land is used for crop cultivation. Quite contrary to conventional hydroelectric or thermal power houses, a wind farm does not require a couple of years to be operational.
1.10  Problems Encountered
The most common problems encountered in using electricity generated by wind turbines are as:
An automatic disconnect switch is necessary on a wind turbine to ensure isolation of alternator in case of an electrical malfunctioning or lightning flash.
An inverter is necessary so that domestic appliances can be made to operate on output of accumulators which are charged by wind turbine.
A transfer switch and IO two-way meter must be present on wind turbine to purchase/sale electricity to local grid if turbine is used for commercial purposes.

   1.11  Block Diagram

1.12  Aims of Investigation

Domestic appliances are designed keeping a standard RMS value and frequency of electric energy in mind. Wind turbine output is oscillatory and varies as a function of both blades rpm and generator swept area. This results in a variation in both frequency and amplitude values. This random output cannot be used directly to run electrical appliances.
In this project, we will simulate an effective way of using wind turbine output to operate a standard rate device. To achieve this goal, we will follow these steps:
Firstly, variable frequency and amplitude electrical output of turbine would be converted into variable DC voltage by using 3 phase bridge rectifier.
Secondly, a DC regulator is used to produce a constant DC output which could either be fed directly for charging battery bank.
Last, a 3 phase PWM inverter would be used to convert DC voltage into desired AC voltage at a controlled frequency.
Ch 2:- Literature Survey:

2.1  First Generation Wind Turbines:

In the first generation wind turbines, alternator was only a squirrel cage induction generator but it had a problem. The fluctuations caused in wind speed were directly transferred to grid in form of power pulsation.
The problem was overcome by using aerodynamic control. Pitch and Stall control with a combination of gear box was used to compensate for variation in wind speed. It ensured running of generator rotor at a nearly constant speed. This system required a reactive compensator present on site.
2.2  AC to AC Converters:
AC to AC converters had been used in case of first generation fixed speed wind turbines. The output of wind turbine was fed to a step up transformer which would increase it to grid level. Direct conversion gives an advantage of no requirement of any energy storing batteries or capacitors in between grid side and generator side. Because of this absence, rectification and inversion losses can be prevented which pays off in the form of an increased efficiency.
2.3  Exclusive DC Generation:
Instead of using an AC generator, a DC generator can be used in a wind turbine as well. Such a generation system cannot be used commercially to feed electricity to a community or local grid. It can only be used to charge batteries to produce lightning or for some prototype demonstration only.
2.4  Second Generation Wind Turbines:
Second generation wind turbines incorporated a partial semi-conductor power electronic control on the system.
That was the time when power electronics technology started its job. It was installed to provide an interfacing between random generation side and standard consumption side.
2.5  Device Selection Criterion:
The factors which had been and still are considered while developing a power electronics device are:
  • Reliability of Power Electronic Device
  • Efficiency of Power Electronic Device
  • Cost of Power Electronic Device

2.6  Modern Semi-Conductor Material:
Conventionally, power electronic devices had been fabricated out of Silicon or Germanium but current trend is following the fabrication of devices on Silicon Carbide which allows high rated device but still in compact dimensions. These devices can perform faster, in a robust way and with very low power losses thus allowing optimized and dynamic conversion and control in wind electricity generation.
2.7  Types of Wind Turbines:
On the basis of alternator construction, wind turbines can be divided into two types which are discussed as:
  • Fixed Speed Wind Turbine
  • Variable Speed Wind Turbine
2.7.1  Fixed Speed Wind Turbine:

In this type of generation, power limitation was provided by stall, active stall or pitch control. These systems required a soft starter to avoid the problem of current inrush or voltage flickers. Reactive power needed by the grid was compensated by the turbine by switching a capacitor bank present on site depending upon the nature of the demand. This method did not use any power electronic control in its generation. It was effective and did not cost much.

2.7.2  Variable Speed Wind Turbine:
Variable speed wind turbines were designed to extract energy of the wind even at a less velocity. Variable speed generation can be obtained by either synchronous or induction generator. Power convertors are always present in a variable speed wind turbine. Converter is used first to rectify the voltage to DC which is then inverted to AC at desired frequency and voltage.
2.8  Induction Generation:
Most commonly used generation scheme now is induction. It is simple, rugged and inexpensive. Two types of induction generators are used to generate electricity in a wind turbine. They are given as:
  • Squirrel Cage Induction Generator
  • Wound Rotor Induction Generator
2.9  Rectifier Development:
Rectification is done in order to convert an alternating voltage into a unidirectional voltage. This unidirectional voltage is usually pulsating and a capacitor is mounted in parallel to pulsating output. The time constant of the capacitor is adjusted such that as long as it does not decay to 37% in one period of the pulse, a constant DC voltage is obtained which is used to charge storage batteries and as an input to the inverter.
Conventionally, diode rectifiers had been used but modern 3 phase six pulses full wave rectifiers offer highest conversion efficiency and are therefore preferably used in wind turbine electric generation.
2.10  Inverter Action:
In order to achieve variable frequency generation, the DC voltage generated by rectifier is filtered out of ripples and stored in a battery bank. Then the DC voltage is changed into AC voltage at grid frequency and voltage by using Inverter operation. Inverter conversion is obtained by using PWM switching. IGBTs are dominating as switching devices in inverters these days.

Generation of sinusoidal waves from DC voltage by using PWM switching is most commonly used simple method of inverter operation. DC voltage is applied to center tape transformer with a two position electronic switch. The switch is connected rapidly from one point to second and the switching is done several thousand times per second. This allows DC connection to two alternate paths of center taped transformer and continuously changes the direction of it at the same time. From secondary of the transformer, a quasi-sine wave form is obtained which is filtered out to give pure sine wave without any harmonics.
2.11  Wind Turbine Topologies:
Different wind turbine topologies and their relative percentage shares in the market are shown as:
  • Fixed Speed Type
  • Dynamic Slip Control Type
  • Doubly Fed Induction Generator Type
  • Direct Driven Type
Ch 3:- Simulation
Ch 4:- Results and Discussions

The insatiable demands of energy can be met by wind energy if it could be harvested properly. The current problem is the efficiency of a wind turbine. The maximum possible value of energy which can be extracted out of a wind turbine is given by a German physicist Albert Betz in the form of a law.
4.1  Betz’s Law:
Betz’s law state that no wind turbine in the world can extract more than 59% of the total kinetic energy of the wind flowing across it. The constant factor of 59% is also called as Betz’s coefficient.
4.2  Practical Efficiency:
The practical turbines running at optimum speed recommended by the manufacturer can give about 70% of the theoretical maximum output which could be achieved by Betz’s Law. This means about 0.59*0.70*100 = 41%. This is considerably higher efficiency than solar panels which convert utmost 21.5% of incident sunlight into electrical energy in modern products. The remaining 78.5% of energy is wasted as heat.
In-efficiency in wind turbine lies primarily because of friction between rotor blades and shaft bearings, gearing losses, and slightly because of power electronic devices. These losses tend to increase with usage time.
Current research in materials sciences and power electronics is contributing a significant portion in increase of efficiencies. New materials having less friction coefficients and power electronic devices at high power rating and switching frequency with low losses are being incorporated in modern machines.
4.3  Future Trends:
The innovations in wind turbine have been pursued for a long time. Because world is running out of oil, gas and other energy sources, we are bound to deduce methods which ensure maximum extraction of energy out of wind. Some of these interesting innovative concepts are given as
4.3.1  Airborne Wind Turbines:
Helium filled balloons can be coupled with wind generators. This allows them to reach an altitude in excess of 10,000 feet. At this altitude, stronger and consistent wind currents can easily interact with the blades and appreciably more energy can be extracted from a wind corridor than on the ground.
4.3.2  Reciprocating Wind Harvester:
These designs use aerodynamically designed horizontal foils as can be observed on the fuselage of an air plane. This design can generate electricity at very low speeds of wind as well as well as on high speed gusts. This dual operation mode is not supported by currently deployed turbines.
4.3.3  Wind Stalks:
Wind stalks have been developed to eradicate dangers associated with rotating blades. There have been some issues of bird mortality and close encounter of helicopters with wind turbines. This design has hollow poles having piezoelectric crystals packed inside them. Piezoelectric crystals have a strange property. They develop an electric field across its edges when crystal lattice subjected to a deformation. When poles swing under the influence of wind, current is generated by relative displacement of piezoelectric crystals displacement.
4.3.4   Green Island Concept:
Wind energy will be auxiliary source in case of green island concept. Electricity is generated using pumped hydroelectric generation method. This concept allows pumping water from low reservoir to high reservoir during off peak hours and then allowing it to fall under influence of gravitational potential energy. This pumping is done by utilizing energy generated by an off shore wind farm near the island. Electricity is then generated in a hydroelectric generation method.
4.3.5   Wind Lenses:
Wind lenses are nothing but just a slight modification to existing wind turbines. This concept involves encircling the rotor blades in a specially designed brim. This brim or ring allows the existence of a low pressure zone in the exhaust region of the blades by diverting the path of wind. This low pressure zone forces more wind to pass through the turbine and thus generate more electricity. Researchers are sure that this design can increase the generation of an existing wind turbine two or three folds.
4.4  Wind Power in Pakistan:
Pakistan has feasible wind corridors along its coastal line. Currently, wind farms have been erected in areas like Jhimpir, Keti Bandar and Gwadar. Wind power is a consideration of the government to help in overcoming energy crisis.
Jhimpir Wind Farm was the first wind farm developed in Pakistan by the help of Turkey in 2002. It is still operational and has a generation capacity of 50 MW.
FFC has also obtained an LOI and is building a wind farm having a capacity of 100 MW as well.
Currently, the Government of Pakistan is planning to achieve electric power generation of 2500 MW from wind farms by the end of 2015.







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