Tutorial on Wind Power

Re: Tutorial on Wind Power

a0128958 wrote: »

Some rookie questions that aren't part of solar PV systems, that I'd like to learn about include:
- do wind turbines output AC instead of DC?

In general, the world has changed from DC Generators (with brushes and commutators) to AC Alternators. Slip rings are cheaper/last longer (for field current) and Permanent Magnets can be used (more efficient).

AC is much easier to transmit/transform(er) than DC.

Small wind turbines typically turn at "optimum" speed for load/wind speed--so their RPM/Frequency changes with wind speed and load.

Many of the Large Utility Turbines are fixed speed alternators that spin, in sync, with the AC line frequency (you will see this on many wind farms--all blades turning exactly in sync).

- what's a 'break' and why is it needed?

Probably a typo from me... Brake is required to stop the turbine for maintenance and, in high winds, used to prevent turbine over speed (along with manual/auto furling--tail moving to side, blade feathering, blade fluttering, disk brakes, shorted alternator output brake, blades flying off break, etc.).

- how is the inverter different? AC to AC?

Big subject here... Short answers:

• Grid Tied Inverter--A current mode energy source that "follows the voltage/frequency" of the Utility Mains (utility is a "voltage mode" energy source). Output current/power is typically based on available power from the solar array (or other energy source), usually as determined by MPPT (maximum power point tracking) hardware/software in the inverter.
• Off Grid Inverter--A voltage mode energy source that takes energy (typically) from a constant voltage energy source (like a battery bank) and outputs the voltage/frequency needed to support AC loads.
• Hybrid Inverter--Typically connects to a battery bank and can operate in Grid Tied mode (if utility mains are present) or in Off Grid mode (if no utility power is present).
• Inverters typically take DC energy, "inverter it" to high frequency AC voltage/current (smaller inductors/capacitors needed at high frequency) then output 50/60HZ AC for loads.
• The above can be mixed and matched depending on exact needs (there are large wind turbines that use "wild frequency alternators to DC to AC using GT type technology).

- how does the power get reliably put onto the grid?

With a GT inverter, it monitors voltage, frequency, energy--if any of these are out of spec., it will shutdown for 5 minutes and then try again.

- why is a dummy load needed, and is it an electrical or mechanical 'gadget?'

A "dummy load" is use for dynamic braking of a wind turbine (or hydro, etc.). Most turbines will overspeed if unloaded. So, you need some sort of load to put the energy if your battery is fully charged.

Another way is to "short the output" of the alternator--this provides a "load" on the blades--and no dummy load needed (zero volts * any amps = zero watts). However, alternators themselves have been burned out using the shorted output method is used in very heavy winds.

- are batteries now required, even for a grid-tie only system, to 'even out' what is put onto the grid?

Can be done either way--Batteries make systems more expensive--plus the batteries need replacement every 5-10 years or so.
- etc. (unique to wind turbine system questions)

• enough wind (location, location, location). No wind, no power. Little wind, little power.
• clean wind (high tower, no obstructions
• Put a few thousand pounds of rotating equipment on top of a 60-120 foot tower
• Slip rings--to transmit power from rotating head to fixed tower (rings fail, what happens to turbine/electronics
• "brakes fail"--need multiple levels of backups to prevent runaways/self destruction/injuries on ground/etc.
• Weather--placing electro mechanical devices 100' in the air--a very tough environment (sun, wind, dust, water, ice, etc.).
• Servicing--electro mechanical systems need servicing. How to do it cost effectively (tilt-up towers, cranes, etc.).
• Towers/Infrastructure--At least for small turbines, towers and infrastructure cost way more than turbines themselves... People tend to "cheap out" on the turbine which is the heart of the system and most expensive to service.

My two cents worth.

-Bill