:::renewable energy education:::solar energy education:::environmental education::: Wind Glossary::::WindEdu:::::SolarEdu::::::PVApps

SolarBestValue's

School of Terrestrial Wind Energy

invites you to consider issues concerning

:: Wind Energy Conversion System Controls::

Efficient wind turbines need to be able to run unattended, regardless ofweather or load conditions, sensing voltage overloads and sorting out thedifference between gusting high winds and continuous gale drafts. The adventof solid state electronics has made it possible to incorporate many importantcontrols into the design of wind systems.

In an AC system, voltage and frequency regulations are imperative. Housecurrent is 120V at 60 hertz, no more and no less. In order to use currentdirectly, voltage and frequency must be regulated. In addition, utilityinterconnect and disconnect, vibration shutdown, redundancy in the overspeedshutdown, use of AC and/or DC are a few more of the important controls whichhave become a reality through the use of solid state electronics.

Voltage and frequency regulators are absolutes in wind energy conversionto electricity. Output voltage tends to naturally fluctuate due to varyingtorque input to the generator. Besides standard "automotive type"regulators, wind system manufacturers are incorporating logic circuitryto sense RPM and/or frequency fluctuations which may in turn signal theactivation of aerodynamic or dynamic controls. The generally accepted rule-of-thumbis that any voltage imbalance should not exceed 3 percent continuously or5 percent for short periods of time.

Vibration shutdown controls are crucial to the long life of any medium tolarge size wind plant. Most quality systems utilize solid state controlsto sense vibration or imbalance by applying the brakes automatically atthe critical point. Vibration sensing is especially important in downwindmachines where tower shadow may cause uneven stresses on opposing rotorblades and in large Darrius ("eggbeater") type machines whereuneven icing may cause severe imbalance.

Controls open up many options to the owner of a wind system. The controlscan tie the system into the utility grid directly, or feed direct to a load,can run AC current when there is an immediate need for it, and automaticallyswitch to the DC storage bank when there is no AC load. The owner can leavethe wind system unattended for long periods of time because monitoring ofvoltage, frequency and RPM is all provided for in the control box.

INVERTERS

An inverter is a system which converts Direct Current (DC) into AlternatingCurrent (AC) for use in AC devices. When an electrical appliance that isdesigned to operate solely from alternating current at 60 Hz is to be usedwith a DC storage system (i.e., deep cycle batteries,) an inverter is necessary.The inverter takes the DC output from the battery storage system and througha conversion process changes the waveform to alternating current at thedesired voltage and frequency.

The direct current (DC) from the batteries may be used for many generalpurposes, however, some devices require AC power for correct and safe operation.Depending on the overall requirements and uses of the wind system, either,none, some or all of the power is inverted to AC Since all inverters useup some power to operate themselves, only those appliances which operatesolely on AC should operate through the inverter. (Note: many devices operateon either AC or DC)

There are three basic categories of inverters available and each differswidely in cost and efficiency ratings. The three categories are: Rotary,Vibratory, and Solid State.

This inverter system utilizes a DC motor to operate a governed AC generatorto produce the desired AC power. The system generally exhibits excellentwaveform output and voltage, but its efficiency may run as low as 50%, usingconsiderable power to operate the inverter. Furthermore, since the rotaryinverter is a rotating device with brushes, it requires regular maintenanceand repair.

Vibrator-type Inverters
These inverters are also electromechanical devices but contain less movingparts than the rotary inverter. The vibrator assembly is driven by a DCpower source which commutates the waveform through a transformer-filternetwork into an AC signal at constant frequency. Efficiencies run up toapproximately 75%. These inverters generally require little maintenance.The vibrator assembly is replaced once every 1,000 to 1,500 hours. Vibrator-typeinverters are a good choice for low power applications and are relativelyinexpensive, compared to the solid state inverters.

Solid State Inverters
Solid state inverters are generally either constant frequency output invertersor synchronous inverters. Constant frequency output inverters provide anominal 120V 60 hertz signal when used independently of the utility grid.A synchronous inverter matches the voltage level, frequency and power outputto the utility lines to provide AC output. A synchronous inverter allowsthe wind plant to interface with the utility line to sell excess electricityback to the utility company. Because it is interfaced with the utility grid,there is no need for a storage system.

There are two general types of solid state inverters, one of which producesa square (or semi-square) wave output, and the other a sine wave. The square(or semi-square) wave units are less expensive and in many applicationswork very well. However, they should not be used to power devices wherea good quality sine wave is required (stereos, television, etc.)

Modern, high quality sine wave units are generally more expensive but willbe very sensitive to sine wave characteristics where a high quality AC signalis essential to performance of the load.

Though the true-sine-wave, solid state inverter is generally more expensivethan the other two types, it has the highest efficiency rating (up to 90%)when operated at rated capacity, and is virtually maintenance-free.

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Wind Glossary::::WindEdu:::::SolarEdu::::::PVApps