STATIC EXCITERS

[xxjohnh]

Does anyone have any technical reason to specify static exciters instead of PMG generators. Don't think it is a function of size and it would seem harder to maintain with brushes etc, but folks are still specifying them.

 

[ScottyUK]

A static exciter can have a much faster response than a PMG-pilot exciter-main exciter arrangement because each exciter stage introduces at least a first-order lag into the transfer function. Depending on the grid code it may be necessary to have a static exciter to meet the response requirements.

The down side is that a static exciter is usually fed from the machine output, so in the event of a close-in fault which drags down the machine terminal voltage the AVR struggles to boost excitation at the very time it is needed most.


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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

Scotty what have you been seeing in the UK?

 

[waross]

Hi Scotty;
I had never given this issue much thought (Static versus rotary exciters). Now that you mention it, the biggest difference may be response time.
I have only encountered a couple of static exciters on the many smaller sets I have seen. They have been of an age that probably predates the common use of brushless exciters. That being the case, the static exciter, while using slip rings,would eliminate a commutator so that the brush maintenance issue may be a plus rather than a minus.
I would imagine that the response of rotary exciters would be similar for brushed with Automatic Voltage Regulators, brushless with AVR, and brushless with the AVR powered by a Permanent Magnet Generator. The PMG would have the edge in block loading and motor starting (that's why we use them) and a big advantage in voltage collapse situations (fault conditions). I have serviced installations that in addition to brushless exciters with conventional AVRs, used series boost panels. These panels used Current Transformers on the load conductors to use part of the fault current to supply a boost to the field under fault conditions and mitigate voltage collapse.
I don't know if this technique would be suitable on your big machines, Scotty.
I agree with your suggestion that the reason for static exciters may be response time. Field forcing may be used with a static exciter to improve response time even more.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

In the UK it tends to be whatever GE or Siemens want to offer as their CCGT power island, which typically means it's a static exciter. The UK power industry is so fragmented that the OEMs now have more influence over station design than the utilities themselves, and also more technical expertise. Middle East is pretty much all static exciters in recent years, and for similar reasons.


Bill,

On a big machine there is at least one additional pilot exciter machine between the PMG and the main field, and that slows response down. The motor starting performance is no doubt pretty good but not many people DOL start a motor big enough to trouble a 500MW set. At least, they don't in this part of the world. I've seen the power CTs used to reinforce the field power on machines in the 10MW class, but it's not really an option for a static exciter which is typically consuming a couple of MW in its own right.

With a static excitation system and a four quadrant rectifier the system is able to regenerate the energy stored in the field back into the supply so it can suppress the field very quickly. A standard exciter can't compete with that response time.


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If we learn from our mistakes I'm getting a great education!

 

[edison123]

Nice post Scotty.

I fully agree that big OEM's now have only take it or leave it attitude since there are only so few of them in that MW range. I'm in Middle East now on business and hear a lot of grumbling from owners about this.

Muthu

http://www.edison.co.in

 

[waross]

Thanks Scotty.
I always enjoy learning the contrasts between little sets and big sets.
Re PMGs; I haven't seen PMGs used as anything except a supply for the AVR. The AVR then controls the field of the pilot exciter. Typically, a PMG will be mounted on a shaft extension outside the housing, and the pilot exciter (usually a brushless exciter) will be shaft mounted inside the housing.
This works great for small machines but I can see how the lag time both increasing and decreasing a large field will be excessive.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[esee135]

+ 1 on the first order lag. The static exciters are nice units these days. I could foresee that the owner would have to be fairly creative in trying to retro a brush less exciter onto some generators.

Brush gear can be an art, but slip ring brush gear is generally not a large issue as compared to having a DC exciter comm brush gear setup causing field grounds on top of the slip ring gear.

Static exciters can be setup under station service feed as well as shunt installations. There are several studies showing that as the unit voltage collapses, the exciter supply suffers, however, the results of close in faults are generally improved in comparison to the previous slower excitation systems, and hence requirements for I2T OEL limiters and monster specified ceiling voltages and currents. It depends on the where the fault is, and what the owner is attempting to achieve during a fault. System intact, or post fault recovery are the usual suspects.

In general, the static exciters have made a good use of abusing the gen field to survive power system disturbances and aide post disturbance recovery. These are factors where the time lag and ability to crank the synchronizing torque while deploying a PSS tends to show improved results. These benefits were known back in the late 60s to early 80's, with analog electronic static exciters.

It is an owner decision, fast is nice, but some owners maybe incapable of performing good brush maintenance, and others may not care about the speed and control benefits. Not all companies have endless funds. Economics unfortunately alters the technical decision process.

 

[msquared48]

Are these static exciters things anything like centerfolds?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[oldfieldguy]

Early in my career I worked on some small (<10 MW) steam turbine generators with rotary exciters AND carbon-pile voltage regulation. The positive side of such an arrangement is that a couple of decent analog meters and a mechanical toolbox was all one needed to maintain and troubleshoot it.

Sometimes I wish things were that simple again.

old field guy

 

[esee135]

I agree oldfieldguy, there is something to be said about simplicity.

In actual deployment, the upgrade to static excitation generally turns exciter apparatus and maintenance into an engineering function versus a maintenance technician function. This is generally accepted since the control improvements gained are significant. In the eyes of practicality, it is just another black box on the powerhouse floor containing throw away parts.

Technicians are often surprised when the majority of the static exciter maintenance is to change the door lamps, and vacuum the cabinets. Versus, carbon segment replacement, or card maintenance.

 

[burnt2x]

My previous employer retrofitted an old 26MW unit with a new static exciter(just when I was separated); only to bust the rotor! Field windings were toast and have to be shipped-out for rewind. Over-excitation limiters were not quick enough to react and much money was spent fixing the unit! I was told the proponent engineer was sent packing his things as a result of that incident.
esee135 is correct in saying

"In general, the static exciters have made a good use of abusing the gen field to survive power system disturbances and aide post disturbance recovery.

but only to the point when the words "gen field" ends on that specific case!
As with any other retrofit, care must be taken when implementing changes to equipment setups especially with old units.

 

[esee135]

Actually a good note to know is often GSU transformer taps can be locked (fear of detanking a transformer to repair a tap changer from never moving it) and the unit is forced to run in the as-is GSU tap.

In these cases, the OEL which I have seen 160% of rated field for 10sec, pulled back for 30 sec cooling cycle to 105% rated field for the I2T. When the GSU tap is not in the correct position, the unit will reach theoretical capability curve limits at wrong/unexpected field currents. In other words, if things are unideal, you can reach PQ cap curve limits before the OEL is even active. Actually I have seen it where you can push so deep into OE that you still won't reach the OEL. In my mind, the OEL and UEL still have room for improvement. They work, but they are not the most intelligent limiters.

Burnt2x said it all, not everyone has a Class F pole winding, and not everyone has G11 stator wedges. Imagine the rotor attempting to tear out the stator winding from it's slots. That's called field forcing folks.