Aerostar 6 Meter Safety Features
The primary consideration when designing a wind turbine is safety. The 6 Meter wind turbine contains multiple levels of safety features. Some of these features relate to rotor overspeed control. Other features are designed to insure electrical safety.
1. The turbine has a failsafe brake which stops the rotor in 2-3 seconds if there is a power outage or if the rotor goes into overspeed. The brake is electrically released and spring applied. If utility power fails, the magnetic field which maintains the brake in its released state dissapears and springs apply the brake stopping the rotor.
2. The turbine blades have tips which are centrifugally controlled. If the rotor goes into overspeed the centrifugal force on the blade tips causes a mechanism to release which forces the blade tips to turn 90°. This decreases the effective blade area, destroys the aerodynamic lift and increases the drag causing the rotor to slow to about 10%-20% of its normal operational speed. The system is completely failsafe and not dependant on electricity or other controls. The tips will not reset unless the rotor comes to a stop. Once the rotor is stopped, or nearly so, the tips will reset automatically.
3. All electrical components in the control system are UL listed.
4. In the event of a power outage, the induction generator will not continue to produce power. This is because induction generators require reactive power supplied by the utility company in order for the generator to function. Although it has been claimed that self excitation of the generator is possible if sufficient capacitance exists, in the real world, the chances of an induction generator self exciting approach infinity. The theory of self excitation has 5 possible scenarios (courtesy of Jonathan Raab Ph.D):
-
With unity power factor for the island, if the loads on the generator are greater than the capacity of the generator’s prime mover, the generator will shut down on an under-frequency alarm. This is the most likely scenario.
-
With unity power factor for the island, if the loads are less than the generator’s kW output, the machine will speed up in order to meet it’s setpoint and will shut down on an over-frequency or over-speed alarm.
-
If the loads match exactly, but the KVAR requirement is too high for the capacitors, the machine will shut down from lack of excitation. The induction generator only provides kilowatts and no KVARs.
-
If the loads match exactly, but the KVAR requirement is too low for the capacitors, the voltage will increase to meet it’s setpoint and shut down on a high voltage alarm
-
If the loads exactly match and the capacitor loads match exactly, then self excitation could occur, but both sets of loads would have to remain constant continuously. Slight variation in power output or slight load change will destabilize this highly theoretical condition causing protective relays to disengage the machine.
It is important to note that there are no reported incidents of an induction generator self exciting and feeding the grid during a power outage. This is because:
-
Induction generators cannot control voltage and frequency--destabilizes and relay protection engages
-
Requires unity power factor of the total island (load and generator) and must exactly meet machine output---probability approaches infinity.
-
Could only occur with that customer since customer load might possibly match machine output. However, this requires simultaneous break on either side of the customer’s service drop at 2 points of the line section
5. The system incorporates a UL listed over/under voltage and frequency safety relay. This relay has adjustable limits which can be set to accomodate utility interconnection requirements. If the voltage or frequency of the system exceeds the limits, the safety relay will apply the brake and stop the turbine. The relay also communicates with the system microprocessor which will stop firing the solid state relays within a few milliseconds.
6. The control system utilizes solid state relays for connecting the generator to the mains. Solid state relays must be turned on every half cycle in order to transmit power. In the event of a power outage the control system will shut down and the thyristors in the solid state relays will not be gated. The solid state relays will not be able to conduct power. This effectively disconnects the generator from the mains.
7. Although the generator contains power factor correction capacitors, these capacitors are only connected during a very narrow range of generator speeds. Should a power outage occur, the generator will either speed up or slow down causing the capacitors to be disconnected. Without capacitors, there can be no reactive power for self excitation and the generator will stop producing power.
9. Frequency measurements in the system microprocessor are based on a real time clock which is independent of fluctuations in line frequency.
10. The system microprocessor constantly monitors generator and disconnects the generator from the mains if generator RPM is outside of its normal operating range.