Why Synchronous Generators 6

[JFCWTR]

Hello,

I have a decent idea what the answer is but here is my question:

Why do large scale power generators (hydro, oil, coal, gas) use synchronous generators and not asynchronous (induction) generators?

Thanks

 

[dpc]

One of the main reasons is that an induction generator requires an external source of exciting current and will always operate at a poor power factor. Synchronous generators can operate over a wide range of power factors, providing necessary lagging vars to the power system.

 

[JFCWTR]

Thank you for your help. This was one of the reasons that I had in mind.
What about efficiency? Fault-ride through capability? Dynamic Stability? Cost?

 

[unclebob]

May be the rotation speed a factor? In hydro power, the flow of water may be to slow to rotate the induction generator to high rpm, for exemple 900 rpm for a 4 pole motor, thus the need for using transfer box. This extra item means extra money in purchasing, maintenance...

 

[JFCWTR]

Has anyone any other suggestions?

 

[varri79]

Hi JFCWTR,
I agree with Mr dpc & Mr unclebob. Here is some more info.. Synchronous generators are standard in electrical power generation and are used in most power plants. Both Synchronous & Asynchronous(induction) generators are available in three-phase or single-phase systems. System capacity, type of load and length of the transmission or distribution network dictate whether a single or three phase generator should be used.

Induction generators are generally appropriate for smaller systems. They have the advantage of being rugged and cheaper than synchronous generators. The induction generator is a standard three-phase induction motor, wired to operate as a generator. All generators must be driven at a constant speed to generate steady power at the frequency of 50/60 Hz. The number of poles in the generator determines the speed.

The cost of the generator is more or less inversely proportional to the speed; the lower the speed, the larger the frame size needs to be for equivalent power output. For this reason, generators that operate at less than 1000 rpm become costly and bulky. In order to match the speed of the generator to the low speed of the turbine, a speed increaser such as belt and/or gearbox might be needed.

If we talk about efficiencies, synchronous generators vary from 75 to 90 percent, depending on the size of the generator. Larger generators are more efficient, and three-phase generators are generally more efficient than single-phase ones. The efficiency will be reduced by a few percentage points when being used at part load.
Efficiency of induction generators is approximately 75 percent at full load and decreases to as low as 65 percent at part load.


Srinivas

 

[rcw retired EE]

Another point- induction generators cannot operate by themselves. They must be connected to a utility system or other synchronous generator that sets the system frequency and provides the reactive power needed for the loads and for excitation of the induction generator.

An induction generator cannot control its output voltage, frequency or power factor, only its output power. An induction generator cannot supply fault current after the first few cycles, so it can't stay on line to help down stream protection clear a fault.

 

[davidbeach]

I'm not sure I entirely agree with varri79.

Basically, a synchronous generator runs at synchronous speed and can produce both voltage and power; while an induction generator runs at negative slip (rotor faster than synchronous speed), and can only produce power. A synchronous generator receives its excitation from its field, while an induction generator (not counting doubly feed induction machines) requires an external source of excitation.

I've not priced generators, but I doubt that generator cost is inversely proportional to speed. 3600rpm machines require much more work to overcome windage losses than slower machines. My guess is that, for the generator only, the lowest cost in the 1200 - 1800 rpm range (for 60Hz machines). All large hydro units are directly driven, no speed increasers used, and the generator is built with the number of poles necessary to run at the turbine speed. Gear boxes are used on gas turbine units simply because there is no way of running the shaft at only synchronous speed.

 

[ScottyUK]

Addendum to David's post:

It is entirely possible to run a gas turbine at synch speed of 3000 or 3600 rpm for a two-pole turbo-alternator. It really depends on the turbine design. Aero-derivative engines often have two shafts - one very high speed turbine running co-axially with a low speed turbine which is direct coupled to the generator. For example, the GE LM6000 is in this class. Industrial gas turbines use a single shaft with compressor, power turbine and generator all running at synch speed, for example the Westinghouse W501 and W701 series are in this class.




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