Hydroelectric powerstation with Asynchronous or synchronous generator

[vampslayer]

Let's say we have small hydropower plant which can operate in on-grid and in islanded mode too.
I understand a principles of frequency and active power control in a case we have synchronous generator, but what would be diff in a case of Asynchronous generator? How do we controll frequency in a case of A.G?

Since the Asynchronous generator needs reactive power to work, how can hydropower plant with Asynchronous generator work in ISLANDED mode then?

 

[jim hardy]

It can't, with the most common type of asynchronous generator, the plain old induction machine.. You must provide such a generator with exciting power to its stator.

Remember that an induction machine will be a generator if it's spun > synchronous speed
and it will be a motor if it's spun < synchronous speed.
The speed-torque curve is symmetrical about synchronous speed.

With no external voltage there's nothing for it to be synchronous with, or asynchronous either.

Perhaps it'd be better to call the two types of machine by their other names, "Wound Rotor" and Induction".

 

[Windadct]

Hello Vamp- and sorry Jim... I must disagree with "It can't", this is used in wind generation frequently, and known as doubly fed induction generator. ( https://en.wikipedia.org/wiki/Doubly_fed_electric_machine )

However in islanded mode - this would take some tricks in the controls - but should be possible, I am thinking a rather large DC Capacitance in the converter.

Of course Asynchronus can also mean a PM Generator - just not in sync, that can be handled by a power converter. ( Rectify then Inverter to pure AC) - for a smaller power system, this would be the way to go due to efficiency. Up to 100KW or so this is often referred to as micro-hydro.

 

[jim hardy]

No disagreement here. I should have said 'it's not nearly so straightforward as with a synchronous machine'


Here's a link to a pretty good paper I stumbled across a while back
https://www.calvin.edu/~pribeiro/courses/Power%20Systems%20Interim/wind-generator-types.pdf

with generic generator descriptions...

 

[vampslayer]

Thx both of you

 

[SirAskalot]

The below is related to a squirrel-cage IM in island mode:

If you draw the torque-speed curve for motor and generator operation, you will notice how the slip changes according to loading (active power). This gives a direct correlation between the electrical frequency and the rotor speed.

As for reactive power, which is closely related to voltage, is also depends on the active and reactive loading of the generator. A fixed capacitor bank connected to the terminals of the machine provides reactive power in relation to the voltage squared. Increasing the reactive power loading decreases the voltage, which could have a severe effect on the system. In order to increase the voltage, more capacitor must be switched in. A SVC (static VAR compensator) or STATCOM could control the reactive power in a "step-less" manner.

Small portable generators may have a simple IM with fixed capacitor bank, and voltage is in such system may vary to a large extent.