Why Alternator Mfgs Fit PMGs to Non-Black Start Alternators

[Jobrag]

Why do alternator manufacturers fit PMGs to alternators that are not black start machines.
On site today we are doing some maintenance on a 27MW alternator, for this machine to run there must be electrical power available (to run oil pumps fans etc), so why do we need a PMG when we could supply power for excitation from another source?

 

[SirAskalot]

Not quite sure what type of excitation you are talking about. But by using permanent magnets directly or indirectly to supply the rotor field winding, the excitation current which again controls reactive and to some degree active power are independent of the terminal voltage.

This is an important property when it comes to fault ride trough and supporting the grid when a fault appears. In case the excitation where supplied from the terminals and a short circuit appears near the terminals the voltage would drop substantially and the field winding would have trouble providing enough current to increase induced voltage in the armature winding's (EMF).

On the other hand, pumps, fans etc. have auxiliary power supply like UPS, battery banks or even diesel generators, so they can run without being connected to the grid.

If you are talking purely permanent magnet synchronous generators, permanent magnets give the possibility of building larger generators without increasing the weight, increasing the efficiency (no I^2 R losses in the field winding) etc.

 

[jim hardy]

OOPS disregard this post I mis-read the question and answered a different question than was asked..


[STRIKE]I have wondered the same thing for decades. No reason not to put a small magnet there so as to make residual magnetism enough to bootstrap the solid state regulator.[/STRIKE]

Automobiles use a resistor behind the "ALT" light to provide a trickle of field current from the battery and that's why you can't push start today's cars with the battery removed .

old jim

 

[jim hardy]

Your question was:

so why do we need a PMG when we could supply power for excitation from another source?

If black start is NOT a requirement then you can get by without a PMG.
It is not uncommon to see excitation taken from machine terminals, with a small source to provide excitation for startup.
Two reasons to provide PMG excitation are:
1. What happens when somebody opens a breaker that's feeding your non-pmg exciter?
The machine immediately trips offline from loss-of-field.
So reliability of the generating machine is one answer.

2. What happens when a fault on the transmission lines close to generator lowers terminal voltage?
The source for your excitation is now reduced so you are limited on how much excitation you can provide, just at the instant you need max excitation to "get under" that fault and deliver lots of current to burn it clear .
So reliability of the system to which the generating machine is connected is another answer.

The main generator at my power station used a shaft driven 420 hz PMG for source, and the voltage regulator itself was magnetic amplifiers. The 420hz power source gave the magamps quicker response than possible with 60 hz.

So put yourself in generator manufacturer's shoes. You'd offer customer the most robust and reliable machine you can build. If customer decides to sacrifice reliability of shaft driven excitation for simplicity of separate excitation that's up to him.

I apologize for my mistaken answer above. Haste makes waste.

Utilities go to what seems extreme lengths to get reliability. Our emergency diesels had huge cooling radiators with shaft driven fans and pumps so they'd be independent of any other cooling water pumps. You recall at Fukushima the only diesel they had left was air cooled, like ours.
Your system is only as reliable as its weakest link.

I should have repeated your question at top of that other post. Then i'd have realized Sir Askalot already answered it. (chagrin icon)old jim