Simply stated, a wind turbine works the opposite of a fan. Instead of using electricity to make wind, a turbine uses wind to make electricity.

The wind turns the blades, which spin a shaft, which connects to a generator and makes electricity The electricity is sent through transmission and distribution lines to a substation, then on to homes, business and schools.

High-tech turbines equal low environmental impact. That’s why wind power is gaining public approval and generating increased awareness.

It is also becoming economically competitive with more conventional power sources – a fact that’s greatly improving its prospects as a viable energy source.

Components of Wind Energy Systems

If you plan on building a horizontal axis wind turbine, you will need a tower on which to mount the turbine (vertical axis turbines are usually built on the ground).

An important factor in how much power your wind turbine will produce is the height of its tower. The power available in the wind is proportional to the cube of its speed. This means that if wind speed doubles, the power available to the wind generator increases by a factor of 8 (2 x 2 x 2 = 8). Since wind speed increases with height, increases to the tower height can mean enormous increases in the amount of electricity generated by a wind turbine.

Picking the Best Location for a Wind Turbine

Where you choose to build your wind turbine is important. Remember that if nearby houses, tree lines and silos obstruct the full force of the wind from your wind turbine, you will not be able to generate as much power.

Wind speeds tend to be higher on the top of a ridge or hill, and for that reason it is a good idea to locate wind turbines at hilly locations Just remember to keep your turbine away from high turbulence. Neighbours must also be taken into consideration when picking a spot to build your turbine. The farther your wind turbine site is from neighbouring houses, the better.

Do not expect your wind turbine to generate the same amount of power all the time. The wind speed at a single location may vary considerably, and this can have a significant impact on the power production from a wind turbine. Even if the wind speed varies by only 10%, the power production from a wind turbine can vary by up to 25%!

Figure 3. Graph showing wind speed distibution by hour of the day.

Example of wind speed distribution by hour of the day.
Values shown are monthly averages of measurements made by anemometers.
(Source: US Department of Energy)

Types of Wind Turbines

There are two basic types of wind turbines: horizontal axis wind turbines and vertical axis wind turbines. Horizontal axis turbines (more common) need to be aimed directly at the wind. Because of this, they come with a tailvane that will continuously point them in the direction of the wind. Vertical axis turbines work whatever direction the wind is blowing, but require a lot more ground space to support their guy wires than horizontal axis wind turbines.

Figure 4. A schematic of a horizontal axis and a vertical axis wind turbine.

Two basic wind turbines, horizontal axis and vertical axis.
(Source: Ontario Ministry of Energy)

Choosing an appropriate wind turbine size

To determine the appropriate size of wind turbine to use, review your monthly electricity consumption in kilowatt-hours (kWh). To do this look at your electricity bills for the last year, add the kilowatt-hours you consumed, and divide by 12. Then compare this total to estimates of the power production for different wind turbines, a figure available from a wind turbine dealer.

To get a preliminary estimate of the performance of a particular wind turbine, use the formula below:

By making your home or farm more energy efficient and reducing the size of your peak demand electrical loads, you can reduce the size of wind turbine you'll need, thereby decreasing the purchase cost.

Small wind systems are used for individual homes and businesses that are off-grid and grid-tie. A small off-grid wind system consists of a wind turbine, which generates electrical power, a controller, a battery bank that stores the power, an inverter and a tower. In many locations hybrid systems that include solar panels and a generator are used to ensure a “round-the-clock” electricity supply.

A grid-tie wind system consists of a wind turbine, a grid-tie inverter and a tower. Depending on the amount of power being used by the household, electricity is either fed to or accepted from the utility. They can be set up with or without emergency back-up power (will have a controller if using a battery back-up).

Batteryless grid-tie systems may see increased performance (sometimes dramatically) from the wind turbine compared to battery-based systems. This is because the inverter´s electronics can match the wind´s load more exactly, running the turbine at optimum speed, and extracting the maximum energy. A grid-tie wind power system can have almost exactly the same components as the off-grid system except that the inverter is a special inverter that connects directly into the public utility grid. These inverters tend to be much more expensive due to their complexity.

How loud might a wind turbine be?

At a distance of 250 m, a typical wind turbine produces a sound pressure level of about 45 dB(A) (decibels). As the picture shows, this sound level is below the background noise level produced in a home or office. Most small wind turbines, in fact, make less noise than a residential air conditioner.

Small wind turbines:

The blades rotate at an average range of 175-500 revolutions per minute with some as high as 1150 rpm. Large turbines turbine blades rotate in the range of at 50-15 rpm at constant speed, although an increasing number of machines operate at a variable speed.

Comparison of decibel levels from a hypothetical wind turbine
(from 250 m away) with other sources of noise. (Source: American Wind Energy Association)

Grid tie inverter

A grid-tie inverter, or a (GTI) is an electrical device that allows solar & wind power users to complement their grid power with solar or wind power. It works by regulating the amount of voltage and current that is received from the direct current solar panels (or other D.C. energy source) and converting this into alternating current. The main difference between an Inverter (electrical) and a grid-tie inverter is that the latter also ensures that the power supplied will be in phase with the grid power. This allows individuals with surplus power (wind, solar, etc) to sell the power back to the utility. This is sometimes called "spinning the meter backwards" as that is what literally happens.

Inverter for grid connected PV

On the AC side, these inverters must supply electricity in sinusoidal form, synchronized to the grid frequency, limit feed in voltage to no higher than the grid voltage including disconnecting from the grid if the grid voltage is turned off.

On the DC side, the power output of a module varies as a function of the voltage in a way that power generation can be optimized by varying the system voltage to find the 'maximum power point'. Most inverters therefore incorporate 'maximum power point tracking'.

The inverters are designed to connect to one or more strings.

For safety reasons a circuit breaker is provided both on the AC and DC side to enable maintenance. The AC output usually goes through across an electricity meter into the public grid.

The meter must be able to run in both directions.

In some countries, for installations over 30kWp a frequency and a voltage monitor with disconnection of all phases is required.

Typical Operation

Inverters work by taking the 12 or 24 volt DC voltage from the source, such as solar panels or micro hydroelectric generators and 'chopping' by turning it on and off at grid supply frequency (e.g. 60 Hz) using a local oscillator and a power transistor. This chopped DC signal is then filtered to make it into a sine wave (removing the upper 3,5,7 harmonics that make up the square wave and then applying it to a transformer to up the voltage to 120 or 240 to supply the needs of load.

A grid tie inverter does the same but has two key differences. Firstly the frequency has to be matched in phase to the grid. This means the local oscillator has to be in sync with the grid. Secondly the voltage of the inverter output needs to be variable to allow it to be slightly higher than the grid voltage to enabling current to flow out to the grid. This is done by sensing current flow and raising the voltage on the output (or duty cycle of the transformer input) until the current flow results in the resulting output power matching the input power from the DC supply.

Effects on Grid Power Quality

In order for grid tie inverters to comply with utility electrical standards, the output power needs to be clean, undistorted and in phase with the AC grid. Typical modern GTI's have a fixed unity power factor, which means its output voltage and current are perfectly lined up, and its phase angle is within 1 degree of the AC power grid. The inverter has an on board computer which will sense the current AC grid waveform, and output a voltage to correspond with the grid.

Inverter (electrical)

An inverter is an electrical or electro-mechanical device that converts direct current (DC) to alternating current (AC). Inverters are used in a wide range of applications, from small switching power supplies in computers, to large electric utility applications that transport bulk power. The inverter is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC. The inverter performs the opposite function of a rectifier.

Applications

The following are examples of inverter applications.

DC power source utilization

Inverter designed to provide 115 VAC from the 12 VDC source provided in an automobile

An inverter converts the DC electricity from sources such as batteries, solar panels, or fuel cells to AC electricity. The electricity can then be used to operate AC equipment such as those that are plugged in to most house hold electrical outlets.

Uninterruptible power supplies

An uninterruptible power supply is a device which supplies the stored electrical power to the load in case of raw power cut-off or blackout. One type of UPS uses batteries to store power and an inverter to supply AC power from the batteries when main power is not available. When main power is restored, a rectifier is used to supply DC power to recharge the batteries.

Induction heating

Inverters convert low frequency main AC power to a higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then changes the DC power to high frequency AC power.

High-voltage direct current (HVDC) power transmission

With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to AC.

Variable-frequency drives

Main article: variable-frequency drive

A variable-frequency drive controls the operating speed of an AC motor by controlling the frequency and voltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that DC power for the inverter can be provided from main AC power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters.

Electric vehicle drives

Adjustable speed motor control inverters are currently used to power the traction motor in some electric locomotives and diesel-electric locomotives as well as some battery electric vehicles and hybrid electric highway vehicles such as the Toyota Prius. Various improvements in inverter technology are being developed specifically for electric vehicle applications. In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a generator) and stores it in the batteries.

Basic designs

In one simple inverter circuit, DC power is connected to a transformer through the centre tap of the primary winding. A switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other. The alternation of the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit.

The electromechanical version of the switching device includes two stationary contacts and a spring supported moving contact. The spring holds the movable contact against one of the stationary contacts and an electromagnet pulls the movable contact to the opposite stationary contact. The current in the electromagnet is interrupted by the action of the switch so that the switch continually switches rapidly back and forth. This type of electromechanical inverter switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been used in door bells, buzzers and tattoo guns.

As they have become available, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs.

Square waveform with fundamental sine wave component, 3rd harmonic and 5th harmonic

Output waveforms

The switch in the simple inverter described above produces a square voltage waveform as opposed to the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of sine waves. The sine wave that has the same frequency as the original waveform is called the fundamental component. The other sine waves, called harmonics, that are included in the series have frequencies that are integral multiples of the fundamental frequency.

The quality of the inverter output waveform can be expressed by using the Fourier analysis data to calculate the total harmonic distortion (THD). The total harmonic distortion is the square root of the sum of the squares of the harmonic voltages divided by the fundamental voltage:

$\mbox{THD} = {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1}$

The quality of output waveform that is needed from an inverter depends on the characteristics of the connected load. Some loads need a nearly perfect sine wave voltage supply in order to work properly. Other loads may work quite well with a square wave voltage.

Advanced designs

H-bridge inverter circuit with transistor switches and antiparallel diodes

There are many different power circuit topologies and control strategies used in inverter designs. Different design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used.

The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used to filter the waveform. If the design includes a transformer, filtering can be applied to the primary or the secondary side of the transformer or to both sides. Low-pass filters are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide power at a fixed frequency, a resonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency.

Since most loads contain inductance, feedback rectifiers or antiparallel diodes are often connected across each semiconductor switch to provide a path for the peak inductive load current when the switch is turned off. The antiparallel diodes are somewhat similar to the freewheeling diodes used in AC/DC converter circuits.

Fourier analysis reveals that a waveform, like a square wave, that is antisymmetrical about the 180 degree point contains only odd harmonics, the 3rd, 5th, 7th etc. Waveforms that have steps of certain widths and heights eliminate or “cancel” additional harmonics. For example, by inserting a zero-voltage step between the positive and negative sections of the square-wave, all of the harmonics that are divisible by three can be eliminated. That leaves only the 5th, 7th, 11th, 13th etc. The required width of the steps is one third of the period for each of the positive and negative voltage steps and one sixth of the period for each of the zero-voltage steps.

Changing the square wave as described above is an example of pulse-width modulation (PWM). Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating or adjusting an inverter's output voltage. When voltage control is not required, a fixed pulse width can be selected to reduce or eliminate selected harmonics. Harmonic elimination techniques are generally applied to the lowest harmonics because filtering is more effective at high frequencies than at low frequencies. Multiple pulse-width or carrier based PWM control schemes produce waveforms that are composed of many narrow pulses. The frequency represented by the number of narrow pulses per second is called the switching frequency or carrier frequency. These control schemes are often used in variable-frequency motor control inverters because they allow a wide range of output voltage and frequency adjustment while also improving the quality of the waveform.

Multilevel inverters provide another approach to harmonic cancellation. Multilevel inverters provide an output waveform that exhibits multiple steps at several voltage levels. For example, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the inverter output terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped waveform is generated at the inverter output. This is an example of a three level inverter: the two voltages and ground.

Three phase inverters

3-phase inverter with wye connected load

Three-phase inverters are used for variable-frequency drive applications and for high power applications such as HVDC power transmission. A basic three-phase inverter consists of three single-phase inverter switches each connected to one of the three load terminals. For the most basic control scheme, the operation of the three switches is coordinated so that one switch operates at each 60 degree point of the fundamental output waveform. This creates a line-to-line output waveform that has six steps. The six-step waveform has a zero-voltage step between the positive and negative sections of the square-wave such that the harmonics that are multiples of three are eliminated as described above. When carrier-based PWM techniques are applied to six-step waveforms, the basic overall shape, or envelope, of the waveform is retained so that the 3rd harmonic and its multiples are cancelled.

3-phase inverter switching circuit showing 6-step switching sequence and waveform of voltage between terminals A and C

To construct inverters with higher power ratings, two six-step three-phase inverters can be connected in parallel for a higher current rating or in series for a higher voltage rating. In either case, the output waveforms are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an 18-step inverter is obtained with three inverters etc. Although inverters are usually combined for the purpose of achieving increased voltage or current ratings, the quality of the waveform is improved as well.

History

Early inverters

From the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron.

The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be "mechanically rectifed AC". Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run backwards", converting DC to AC. Hence an inverter is an inverted converter.

Controlled rectifier inverters

Since early transistors were not available with sufficient voltage and current ratings for most inverter applications, it was the 1957 introduction of the thyristor or silicon-controlled rectifie (SCR) that initiated the transition to solid state inverter circuits.

12-pulse line-commutated inverter circuit

The commutation requirements of SCRs are a key consideration in SCR circuit designs. SCRs do not turn off or commutate automatically when the gate control signal is shut off. They only turn off when the forward current is reduced to zero through some external process. For SCRs connected to an AC power source, commutation occurs naturally every time the polarity of the source voltage reverses. SCRs connected to a DC power source usually require a means of forced commutation that forces the current to zero when commutation is required. The least complicated SCR circuits employ natural commutation rather than forced commutation. With the addition of forced commutation circuits, SCRs have been used in the types of inverter circuits described above.

In applications where inverters transfer power from a DC power source to an AC power source, it is possible to use AC-to-DC controlled rectifier circuits operating in the inversion mode. In the inversion mode, a controlled rectifier circuit operates as a line commutated inverter. This type of operation can be used in HVDC power transmission systems and in regenerative braking operation of motor control systems.

Another type of SCR inverter circuit is the current source input (CSI) inverter. A CSI inverter is the dual of a six-step voltage source inverter. With a current source inverter, the DC power supply is configured as a current source rather than a voltage source. The inverter SCRs are switched in a six-step sequence to direct the current to a three-phase AC load as a stepped current waveform. CSI inverter commutation methods include load commutation and parallel capacitor commutation. With both methods, the input current regulation assists the commutation. With load commutation, the load is a synchronous motor operated at a leading power factor.

As they have become available in higher voltage and current ratings, semiconductors such as transistors that can be turned off by means of control signals have become the preferred switching components for use in inverter circuits.

Rectifier and inverter pulse numbers

Rectifier circuits are often classified by the number of current pulses that flow to the DC side of the rectifier per cycle of AC input voltage. A single-phase half-wave rectifier is a one-pulse circuit and a single-phase full-wave rectifier is a two-pulse circuit. A three-phase half-wave rectifier is a three-pulse circuit and a three-phase full-wave rectifier is a six-pulse circuit.

With three-phase rectifiers, two or more rectifiers are sometimes connected in series or parallel to obtain higher voltage or current ratings. The rectifier inputs are supplied from special transformers that provide phase shifted outputs. This has the effect of phase multiplication. Six phases are obtained from two transformers, twelve phases from three transformers and so on. The associated rectifier circuits are 12-pulse rectifiers, 18-pulse rectifiers and so on.

When controlled rectifier circuits are operated in the inversion mode, they would be classified by pulse number also. Rectifier circuits that have a higher pulse number have reduced harmonic content in the AC input current and reduced ripple in the DC output voltage. In the inversion mode, circuits that have a higher pulse number have lower harmonic content in the AC output voltage waveform.

ELECTRIC POWER AROUND THE WORLD

The first safety issue that comes to everyone's mind for small customer-sited systems is a condition called islanding. Islanding is where a portion of the utility system that contains both loads and a generation source is isolated from the remainder of the utility system but remains energised. When this happens with a distributed power system, it is referred to as supported islanding. The safety concern is that if the utility power goes down (perhaps in the event of a major storm), a distributed generation system could continue to unintentionally supply power to a local area.

While a utility can be sure that all of its own generation sources are either shut down or isolated from the area that needs work, an island created by a residential system can be out of their control. There are a number of potentially undesirable results of islanding. The principal concern is that a utility line worker will come into contact with a line that is unexpectedly energised. Although line workers are trained to test all lines before working on them, and to either treat lines as live or ground them on both sides of the section on which they are working, this does not remove all safety concerns because there is a risk when these practices are not universally followed.

Fortunately, static inverter technology developed for grid-interactive systems is now specifically designed so that there is practically no chance of an undesired supported island stemming from an interconnected residential or small commercial systems. This feature is referred to as anti-islanding. Grid-tied inverters monitor the utility line and cease to deliver power to the grid as quickly as necessary in the event that abnormalities occur on the utility system. Such performance requirement is generally described in both the inverter and the interconnection standards.

An external manual lockable disconnect switch ("manual disconnect") in the interconnection context is a switch external to a building that can disconnect the generation source from the utility line. The requirement for a manual disconnect, stems from utility safe working practices that require disconnecting all sources of power before proceeding with certain types of line repair.

Whether a manual disconnect for small systems, such as photovoltaic (PV) systems, using certified inverters should be required has been the source of considerable debate. In strict safety terms, a manual disconnect is not necessary for most modern systems because of the inverter¡¯s built-in automatic disconnect features as discussed in the previous section. Both the Canadian Electrical Code (CE Code) and the National Electrical Code (NEC) in the United States, refers to the need for an additional switch that is (1) external to the building, (2) lockable by utility personnel, and (3) offers a visible-break isolation from the grid.

As such, a manual disconnect is an additional means of preventing an islanding situation. And, the key from the utility perspective is that the switch is accessible to utility personnel in the event of a power disruption when utility line workers are working on proximate distribution system lines. In addition, in many situations, utility line workers can provide redundant protection against islanding by removing a customer's meter from the meter socket. Still, many utilities require a separate, external manual disconnect.

While the cost of installing such a switch is not large relative to the overall cost of a micropower system, a PV system for example, when compared to expected energy savings from the system, such a switch is relatively expensive. Also, for systems located on the top of tall buildings, such a switch becomes very
expensive.

In the USA, some state-level net metering and interconnection rules require that the utility pay for the installation of a manual disconnect. In New Mexico, use of the meter is an optional alternative to a separate switch while in many other states a manual disconnect is not required, at least for small systems; that is the case of California, New Jersey, Washington, and Nevada. Also, some utilities, such as those owned by the New England Electric System, have established their own interconnection guidelines that do not require an external manual disconnect for small systems.

Wind farms are clusters of turbines that generate electricity. Wind is a free and renewable resource that produces clean energy - no emissions, no waste products. Wind farms are located in areas with reliably favorable wind speeds.

What causes wind?

The wind that turns the turbine blades is a form of solar energy. The sun warms the earth's atmosphere unevenly, causing the air to move and swirl, creating wind.

For centuries, wind movement has been converted into mechanical power for low-tech jobs like watering cattle. Now, we can use it to efficiently turn high-tech turbines for electrical generation.

Why use wind power?

For centuries, wind has been harnessed to power ships, grind grain, run sawmills and pump water.

Today, high-tech wind turbines are becoming an increasingly familiar part of the landscape around the world, cropping up in cornfields, on wind-swept deserts and along breezy mountain passes.

As the need for clean, safe, reliable energy grows, wind is taking on a new role. Wind power is a free, non-polluting, renewable resource. No matter how much is used, there will still be a plentiful supply in the future. That's why wind is the world's fastest growing energy source!

What are the benefits of wind power?

Thanks to two decades of innovative technical developments, modern wind turbines are highly reliable and cost effective. They run quietly with little or no direct impact on the environment.

In many parts of our country, wind-powered electric generating projects are becoming a preferred way to develop safe, new sources of energy for a variety of reasons:

Wind projects create little or no interference with existing farming or ranching operations - livestock graze among the turbines as though they weren't there.

Small wind turbines can range in size from 90 - 10,000 watts (10 KW), and range in price from $800 - $37,000. With these turbines you need towers, which vary in size and price as much as the turbines. Some turbines can connect directly to the utility (Grid-tie) and some are designed to charge batteries. If you go with a battery system you will need batteries and an inverter to convert the DC power from the batteries to AC power for use in your home.

Basically, asking how much a wind system costs is like asking how much a car costs - it depends on many factors and can vary widely.

The only way to put this question into perspective is to ask another question:

• What's the payback period on your car?

*However...typically a small complete system depending on the size and cost of the system, will take about 3-8 years to pay for itself. Larger systems will take longer...approximately 6 - 15 years.

Components of Wind Energy Systems

Picking the Best Location for a Wind Turbine

Types of Wind Turbines

Choosing an appropriate wind turbine size

How loud might a wind turbine be?

Small wind turbines:

Grid tie inverter

Typical Operation

Effects on Grid Power Quality

Inverter (electrical)

Applications

DC power source utilization

Uninterruptible power supplies

Induction heating

High-voltage direct current (HVDC) power transmission

Variable-frequency drives

Electric vehicle drives

Basic designs

Output waveforms

Advanced designs

Three phase inverters

History

Controlled rectifier inverters

Rectifier and inverter pulse numbers

Sine wave or modified sine wave?

Battery charging, and other goodies

ELECTRIC POWER AROUND THE WORLD

What causes wind?

Why use wind power?

What are the benefits of wind power?

All disconnects and fuses have to be rated for DC

Types of Solar Panel Mounts

Flush Mounts

Roof-Ground (Universal) Mounts

Pole Mounts

Solar Water Heating