Electrical Protection of Synchronous Condenser 9

[sori]

Hello,
Our company is interested to change a synchronous generator 464 MVA to a synchronous condenser.
Do we need to change the generator electrical protections?
Thank you

 

[RRaghunath]

Yes, I suppose. As Synchronous condenser, the generator will be drawing Watts (active power) from the grid and supplying VArs to the grid.
If yes, the reverse power and Low forward power protections have to be reviewed.
I don't think other protections will be affected.

 

[dpc]

How will you start it? Starting may require additional protection consideration. You are basically changing a generator to a motor.

 

[edison123]

Hydro generator? Turbo generator? How do you plan to start it and bring it up to synch speed?

This is a big project which can't be done via internet forums. Take professional services.

Muthu

http://www.edison.co.in

 

[sori]

Thank you for your reference to the subject.
It's about GE turbo generator 40 years old . We plan to start it with a pony motor.
We have 2 REG216*4 that protect the generator.
RRaghunath :"...the reverse power and Low forward power protections have to be reviewed. I don't think other protections will be affected..." - O.K.
dpc :"...Starting may require additional protection consideration. You are basically changing a generator to a motor..." - additional protection what did you mean (the pony motor will come with his protection)?
edison123:"...This is a big project which can't be done via internet forums. Take professional services..." - You are absolutely right, we will definitely take it.

 

[crshears]

Pony motor . . . hmmm . . . why?

If the required electrical equipment is already a/v considerable cost savings can be realized by going this route.

If not, there may well be more cost-effective ways of running up the machine, using one of several different solid-state devices.

A number of years ago I repeatedly had the delightful job of running up and synchronizing a pair of 160 MX synchronous condensers; these were 1960s vintage decoupled generators, formerly connected to steam-driven dual shaft cross-compounded turbines. Run-up used a six-pole-set wound rotor induction motor with salt-water liquid starting resistor driving a five-pole-set generator so full synchronous speed could be obtained.
With this background I can well visualize what it is you hope to do, hence the question.


CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

Is it a mothballed 2 pole TG set? If you plan to use a pony motor, then the generator settings should not need much change for condenser operation since it is mostly going to provide VAR load. Hydro generators are run often as condensers with the same generator settings.

Out of curiosity, what is the design HP of the pony motor? Will it be VFD driven or with a fluid coupling?


Muthu

http://www.edison.co.in

 

[cranky108]

I suppose the pony motor will be to reduce the start up current/voltage dip. This works well verses a very large VFD, and preserves the fault current contribution of the condenser. As in some places the amount of inverter based generation has so reduced the amount of fault current, that a source needs to be inserted in the system.

We thought of that, but public view of a coal plant is seen as better to tear it down. But with a hydro unit, it needs to be investigated if the auxiliaries need the water for cooling.

 

[Hoxton123]

Pony motor will work, need to engineer the mechanical drive side. Low slip, oversized motor will be good.

Make sure you have field failure / pole slipping / field failure protection enabled.

Lots of GE Frame V sets were SYNCON enabled with a clutch in the gearbox, so nothing new here. Very often we shut down the GT and let the generator run on in case it was needed for compensation, since the losses were relatively low.

 

[Gr8blu]

sori: A synchronous condenser is essentially an synchronous motor operating at no (mechanical) load, with a very large power factor to allow generation of significant reactive power.

Your original generator rating has some power factor associated with the MVA rating. In effect, it already operates with a combination of active and reactive power (most likely). As a condenser, the intent is to presumably operate at a different power factor which shifts the balance to more reactive power than currently available. That (generally) means FAR more field current to the synchronous rotor winding - and your protection should account for this scenario. As Rragunath mentioned, be aware of power flow (direction) and adjust protective settings appropriately.

Converting energy to motion for more than half a century

 

[edison123]

CR


Interesting way to use slow speed machines to run up the generator. Quick questions.

1. Wouldn't six pole set motor driving five pole set generator produce only 5/6th frequency? It should be six pole set generator?

2. Once the main generator reaches synch frequency, how do you transfer to the mains from the pony MG set?

Muthu

http://www.edison.co.in

 

[jraef]

Pony motor . . . hmmm . . . why?

I imagine it might be that having been built as a generator that would be connected to a prime mover, the amortisseur winding is not capable of accelerating it as an induction motor perhaps?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[dpc]

the amortisseur winding is not capable of accelerating it as an induction motor perhaps?

That would certainly be my assumption without some evidence to the contrary, jraef.

 

[crshears]

Sorry all, I wrote that from memory on a busy day shift and got the pole numbers reversed; edison is correct, it was a five pole induction motor @ 60 Hz that only needed to run up to 600 rpm [five-sixths of its synchronous speed] so the six-pole generator could make 60 Hz.

High [steam] pressure generator [3600 rpm] and low [steam] pressure generator [1800 rpm] were not originally intended to ever start as motors and therefore had no amortisseur windings that I ever heard of - then again, they may well have been there only for system stability purposes, but nobody mentioned their existence or absence.

HP and LP generator output windings [~18 kV] were connected in common via "isolated phase bus" to two transformers, one being the Main 0utput Transformer [step-up to 115 kV] and the other being the unit station service transformer [step down to 4160V]. Run-up was via backfeed into the unit station service transformer.

Decoupled HP and LP machines were placed on turning gear for 24 hours prior to run-up to mitigate rotor sagging/eccentricity.

Required lube oil pressures and flows and bearing and lube oil cooling water flows were established.

Motor driven exciters were started, field breakers closed, and HP and LP excitation slowly applied until the HP and LP machines locked into synchronism. Since the sub-synchronous speed on turning gear were slightly different, one turning gear [usually the HP] would trip off and the other [usually the LP] would then drive both machines.

Pony generator output breaker was closed; IIRC it had amortisseur windings that pulled it up close to lock-in speed, excitation was then applied to it so it locked into synchronism with the HP/LP combo.

With liquid resistor at maximum, pony run-up motor was energized from 4 kV station service and run-up began.

As run-up progressed, pony motor current would drop, and liquid resistor prongs were lowered into the solute to decrease wound rotor path resistance and increase stator current back to maximum. Also excitation on the HP and LP machines and the pony set generator would be increased to maintain the V/Hz ratio within an acceptable range.

When the aggregate speed was near synchronous, the condenser would be synchronized to the system by closing the 115 kV breaker between the station buswork and the high side of the unit's MOT, and the run-up gear disconnected from the isolated phase bus. The condenser was then available for VAR dispatch.

 

[edison123]

CR - Thanks for an excellent explanation of the process, which I could totally visualize. Using barring/turning gears for break away torque, which is the most pita, is brilliant. As is using slow speed MG set as a sorta VFD.

I know the cylindrical rotors do have embedded damper windings for speed swings correction but didn't know they are start duty rated. Thanks.

By any chance, do you still remember the nameplate ratings of the pony motor and the generator?

I wish I could h/t both of your posts. Silly ET rules.

Muthu

http://www.edison.co.in

 

[crshears]

Hi Muthu,

I don't have to remember; I kept a paper copy of the SLD for the scheme, along with a set of the start-up and shutdown procedures, and have fetched same from my locker.

I can therefore not only correct my earlier mis-remembrances but supply additional information.

Original units full load output when steam driven was 200 MW; unit rating as a synchronous condenser was from 40 MX in to 168 MX out.

There were actually two unit station service transformers connected to the isolated phase bus; the latter operated @ 13.8 kV, not 18 kV.

Run-up set pony motor was rated @ 3300 IHP; pony generator was rated @ 2400 kVA. The pony set was equipped with an overspeed trip set at 660 rpm.

Addition via edit: The pony generator had a squirrel cage winding so that when its output breaker was closed it would spin up slowly from rest to whatever sub-synchronous speed was for a prevailing isolated phase bus frequency < 1 Hz. Field excitation would then slowly be applied to it until it locked into synchronism with the HP and LP machines. Both the pony motor and generator had jacking [oil lift] pumps, [virtually?] eliminating any sticktion at spin-up from rest.

The external liquid resistor for the wound rotor induction pony motor contained washing soda dissolved in distilled water. Today of course one would use regenerative circuitry to recoup the energy formerly lost in heating up the liquid resistor by instead converting it back into an AC output and returning the power to the system.

Maximum allowable start-up sequence time was thirty minutes, but it could normally be completed in under twenty-five.

Using movable plug-in pin contacts, the run-up set could be configured to start C5, C6, C7 and C8, although only C6 and C7 were ever fully commissioned.

Question for the OP: was your protection concern only for the unit when on line and deployed as a VAR resource, or were you also looking at a protection package for the run-up gear?




CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

Thanks, CR.

2.4 MVA pony generator to start up 200 MW generators is pretty neat.

Is there a reason why the start-up sequence should not exceed 30 minutes?

Muthu

http://www.edison.co.in

 

[Gr8blu]

edison123 Probable reason for 30 minute limit was heat generation in the squirrel cage winding of the pony motor, particularly at the bar-to-ring joint. (Note that this is the usual reason for line-start timing for any AC machine (induction or synchronous) spinning up under induction to line frequency. The limit does NOT exist for operation with a variable frequency source, provided the source can ramp up from < 1 HZ.)

Converting energy to motion for more than half a century

 

[crshears]

What Gr8blu wrote sounds about right; if I find the stated answer I'll provide it. - addition via edit: went through the entire document; no reason for time limit given, only additional item is that operator would receive an "approaching maximum start time" annunciation one minute before actual trip would occur.

Somebody sure did some careful calculations to size the pony set properly; I remember the initial acceleration rate seemed almost breathtaking, but as the windage of the hydrogen blowers built with the square of speed, near synchronous the acceleration rate fell off significantly. Indeed if one kept an eagle eye and had the unit's terminal voltage close one could synch the units on the fly, meaning catching the wave just as the units accelerated through synchronous speed.

 

[crshears]

edison123 wrote:

I wish I could h/t both of your posts.

That sounds very complimentary, Muthu . . . but what does it mean?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]