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

 

[edison123]

Gr8blu - CR said it's a slipring motor with external resistance used as a sort of speed control. In today's world, a cage motor with VFD would do it, as you say.

CR - It means that I could not give more stars for all your posts. ET's silly rule restricts it to only one star in a thread. These synch condensers were hydrogen cooled? That would explain such low levels of pony MG set since I expect windage loss with air to be much higher.

Muthu

http://www.edison.co.in

 

[crshears]

Hello sori, I realize I'm going beyond the scope of your OP, but more information would help.

For example, you state:

We plan to start it with a pony motor.

This does not in any way say how you plan to go about this . . .

Would a retired prime mover be partially dismantled to make room for the pony motor needed to be permanently mechanically coupled to the prospective condenser it would drive to synchronous speed?

What kind of pony motor? What rating?

Or would your pony motor be driving a run-up generator similar to what I used to operate?

Or some other approach?

Depending what's already on site, a far cheaper approach could be to segregate a portion of the station's high voltage switchgear so that a two-shaft gas turbine generator rated at, say, 10 to 20 MVA could be electrically coupled to the prospective condenser and used to run it up to synchronous speed.

Alternatively a hydraulic unit at some other site could be segregated onto one circuit or feeder and electrically coupled to the machine for the same purpose.

One electrical area I am aware of found itself in need of a frequency changer between 25 and 60 Hz, and a very innovative approach was developed to address this need, to wit:

Since a great deal more water had been made available to this plant, it was twinned [although its twin had more than double the capacity]. There was therefore now more generating capacity available within the pair of plants than there was water to run them.

So one unit was decommissioned as a generator; the 25 Hz alternator on top was lifted out of the way, the turbine cavity was gutted, and the scroll case inlet and draft tube outlet sealed off with concrete. A new air-cooled 60 Hz alternator small enough to fit into the cavity was then installed, cooling water piped to its bearings and stator cooling rads [to reject the heat from the recirculated cooling air, while eliminating the requirement for a great deal of jack-hammering of structural concrete], and appropriate electrical connections made. The 25 Hz machine was then re-installed atop it and mechanically coupled up.

Starting was accomplished by separating the adjacent hydraulic unit and one outgoing circuit onto their own little synchronizing bus, starting the jacking oil pumps, getting the hydraulic unit creeping, applyiong excitation to both units until the two locked into synchronism, then running them up as a pair, maintaining the appropriate V/Hz ratio on the way to synchronous speed.

It worked beautifully.

Going one step further, the system design engineers did not want to be limited to starting only from the 25 Hz side, thus the ability to perform starts was specified for the 60 Hz machine as well so the unit could be started from the adjacent 60 Hz plant.

Addition via edit: since the frequency changer was kept i/s 24/7 and very rarely shut down, there was little additional burden imposed by system reconfiguration for these infrequent starts, and therefore very little financial incentive to develop a different starting means such as adding really beefy amortisseur windings to either of the machines so as to enable DOL starts.

Bottom line is, there may be any number of ways to not needlessly spend money and still accomplish your desired objective . . . more information yields better answers!

 

[crshears]

edison 123 wrote;

CR - It means that I could not give more stars for all your posts. ET's silly rule restricts it to only one star in a thread.

Then I guess all you can do is to try and talk others into giving me more LPSs !!! [ written tongue in cheek while displaying soot-eating grin ]

CR

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

 

[cranky108]

May or may not apply, but some GT sets are started by using a sort of VFD on the generator windings to bring it up to a speed where the GT can ignite. This scheme could be used to start a synchronous condenser to a speed, then attempt to catch it.
Just a thought.

 

[Gr8blu]

replying to edison123 post from 20 AUG Even a wound rotor induction (actual rotor winding connected to slip ring and from there to external resistance, usually supplied by liquid rheostat and mechanical movement of electrodes) still starts up "direct-on-line", just like any other squirrel cage induction or synchronous machine not operating on a drive. That means there is a thermal limit on the conductors of the windings and the connections between them, just like the other AC machines. All the external resistance does is limit the current being applied during the acceleration - thereby limiting the available torque through the rotor shaft.

The reasoning behind that magic 30 minute window still applies to a wound rotor motor design.

Converting energy to motion for more than half a century

 

[edison123]

gr8blu - A slipring rotor starting on line rarely sees two times the rated current as LRA. Since 30 minutes is already continuous duty, this may have been a special purpose motor designed to handle that current continuously. Anyway, my post you referred was to point out it wasn't a cage motor.

Muthu

http://www.edison.co.in

 

[crshears]

Cranky108 wrote:

May or may not apply, but some GT sets are started by using a sort of VFD on the generator windings to bring it up to a speed where the GT can ignite. This scheme could be used to start a synchronous condenser to a speed, then attempt to catch it.

Hi cranky, that was what I generalized about when I referenced "several different solid-state devices"; LCI was one of the approaches I had in mind, I just didn't want to get into the nitty-gritty details and possibilities too early into the thread.

Also, edison asked:

These synch condensers were hydrogen cooled? That would explain such low levels of pony MG set since I expect windage loss with air to be much higher.

Yup, hydrogen cooled, with rotor-mounted blowers/fans to circulate the H[sub]2[/sub] through coolers with raw water flowing through them. I remember early on in my working life when I was an auxiliary plant operator in a similar coal-fire generating station operating these cooling systems, the seal oil system that kept the hydrogen inside the generator where it belonged, maintaining the internal casing pressure at ~30 psig, purging out hydrogen with CO[sub]2[/sub] and then purging that out with air when overhauls were about to commence, and on the way back from overhaul displacing the air with CO[sub]2[/sub] then top-filling with H[sub]2[/sub], operating the associated desiccant systems, and so on and so forth; fun times.

CR

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

 

[cranky108]

One of the problems we saw with using a sort of VFD on the generator to start the GT, was that the unusual frequency caused the PT fuses to degrade, and blow for normal loading. But for the life of me I don't know why we need current limiting fuses when the high impedance neutral would limit the fault current to a very low value.
Anyway, different types of fuses, and now they are saying fuse orientation is an issue.
Looks like a bad design by GE, but we keep paying to fix it.

I was told that GE means 'Good Enough'.

 

[thermionic1]

I was told that GE means 'Good Enough'.

I used to work for GE and we said the same thing.

 

[crshears]

This link

https://www.google.com/search?rlz=1...QIHfk0AzkQ7Al6BAgTEDQ&biw=1912&bih=1055&dpr=1

is to pictures of what the control room became for just a short while almost thirty years after I used to run up and control synchronous condensers there . . .

CR

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

 

[edison123]

cranky - What was the rating of that VFD/LCI?

CR - Was hydrogen cooling used in condenser mode too? Those restaurant pictures are so dystopian.

Muthu

http://www.edison.co.in

 

[crshears]

Edison123 asked:

CR - Was hydrogen cooling used in condenser mode too?

Yes, it was; units at full rotative speed could not have handled a gas of any greater density withut blower and other modifications, plus a fairly healthy current would be flowing in both the rotor and stator windings at 80 MX per generator @ ~14 kV [ unfortunately I have no documented information about whether these units also had stator winding cooling with demineralized water / raw water heat exchanger, and my memory isn't THAT good . . . ].

CR

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

 

[edison123]

Thanks, CR. Your memory is damn good.

Muthu

http://www.edison.co.in

 

[cranky108]

cranky - What was the rating of that VFD/LCI?

I never thought it important to look into it. Should I?

I try to keep my nose out of things I don't need to be involved with. I.E. irritate other engineers that don't need the disruption.

 

[waross]

I have been away for awhile. crshears just brought this thread to my attention and asked for my comments.
First, my thoughts on motors and generators.
For most industrial machines, there is only a few RPM or Volts difference between a motor and a generator.
A an induction motor, synchronous motor, or a DC motor, a fairly minor increase in RPM and the motor is now a generator.
An induction motor rated for 1760 RPM becomes an induction generator when driven over synchronous speed. It will be pushing close to full rated current into the grid if over-driven at 1840 RPM. (Think; 1800 - 40 RPM, 100% loaded motor, 1800 + 40 RPM, 100% loaded induction generator.
A synchronous motor, needs only a few electrical degrees advance to become a generator.
A DC motor is similar to the induction motor in that if it is over-driven a few RPM it becomes a generator.
These motors generate back EMFs that are close to the applied voltage.
If the field is increased so that the back EMF of a synchronous motor rises above the line voltage the motor becomes a VAR generator.
I haven't seen it dome for many years but once upon a time and long long ago, plants would drive a large load with a synchronous motor that was two or more times the HP that was needed.
Then when in operation, it would be run over excited so as to both drive a load as a motor and function as a VAR generator to help the plant PF.

Consider a DC motor coupled to a drive of fixed speed. (This is a class-room demonstration of DC machine characteristics, and not intended to be a practical field application.)
Neglecting the field current and windage and friction, (shunt connected motor) if the field is adjusted so that the back EMF is equal to the applied voltage, the motor will idle and draw no armature current. With any increase in field strength, the motor will become a generator and export power to the source.
If the field is decreased, the back EMF will drop below the applied voltage and the machine will start to export mechanical power.
Now, if the field is held constant, so that the back EMF equals the applied voltage, and then the speed is reduced, the back EMF will drop and the machine will become a motor.
Likewise, if the speed of the drive is increased, the back EMF will rise above the applied voltage and the machine will become a generator.

To sum up.it can be said that there may be only a few RPM or a few volts difference between a generator and a motor.

Back on topic:
I have not seen mention of a synchro-scope in the above posts. If I missed it, I am sorry and I tender my regrets.
You may have two choices here;
1. You may use a synchro-scope and synchronize the machine as if it were a power generator.
2. You may be able to use a polarized field frequency relay and sync the machine on line similar to starting a synchronous motor.
If I saw someone about to close the breaker without any kind of sync check, despite friendly warnings, I may just move back and start recording. There is a fairly good chance of excitement with an out of phase closure.

The other point that I would mention is that heat is your enemy. I would not be running either the field or the armature above rated current. Motor or generator, I^2R doesn't care. Current is current and heat is heat.
If the field must be run at higher than rated current, then consider de-rating or reducing the armature current.
How much? It depends. A rule of thumb may be to calculate the heat developed (I^2R) in both the field and the armature windngs at rated currents and then again at the new proposed currents. Reduce the calculated maximum armature current somewhat to allow for hot spots and unforeseen consequences. You may consider that as the ultimate maximum. (If you find someone with proven experience in this matter, listen to him, and forget what I say.)


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Hi again, Bill, and thanks for weighing in.

I readily admit that I only mentioned synchroscopes in passing, almost tangentially if you will, when I wrote:

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.

So just to be clear, yes, in the application I mentioned we synchronized the condenser to the grid using the then-conventional fully manual, perhaps now dated, synchroscope and voltmeters to match voltages, minimize slip frequency, and close the unit breaker just as the needle on the 'scope was approaching zero degrees [ 0° ] / twelve o'clock, meaning giving the breaker close impulse sufficiently in advance that by the time the 115 kV oil circuit breaker contacts actually closed zero degrees phase angle difference across them had been reached.

I was sometimes criticized for taking too long at this task, but as you allude to, I was not and am still not a fan of that kind of "excitement".

CR

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

 

[panter]

Sori ,
I am interested in the price of the taken reactive energy for your factory and positive results of converting a synchronous motor into condenser .
Often the price of reactive energy is low in relation to the active power you have to use to idle the synchronous motor and there is no economic interest in engaging in such activities

 

[cranky108]

Vars have no value, until your utility imposes a Power Factor penalty on your bill.
But Vars do consume transformer and lead capacity, and can cause over heating.
That extra current can cause voltage drop, which can lead to motors over heating, and starting problems.

So reactive power is important, but likely not to those who over simplify their plant electrical systems.

 

[crshears]

Unfortunately explaining this to the bean counters can be problematic; you need to find one who has

[1] the willingness and vision to look beyond the limitations of, and information provided on, their spreadsheets so as to see the bigger picture, and

[2] the authority to direct others to incorporate these end facts into their thinking, fiscal structures, and financial decisions.


This is not always an easy task.

CR

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

 

[waross]

The value of a VAR:
Buying or selling?
It is common for consumers to be forced to buy VARS, to compensate for a poor power factor.
Rather than a direct, fixed cost per VAR, it is common to be charged a penalty for excess VAR consumption.
The worst case that I am aware of was an installation being charged a 90% penalty for VAR consumption.
That is, the penalty was 90% of the KWHr charges.
Think: Significant negative value.

On the other hand, I know of an instance where the main transmission line feeding a large city was being loaded beyond its capacity.
Note: On some transmission lines, the capacity is not limited by conductor ampacity but rather be the ability of the On Load Tap Changers to compensate for line loss.
In this instance, the line capacity was increased by mitigating the line losses to some extent by injecting VARs at the load end of the line.
This was done by taking an old diesel generating plant out of moth balls and putting it back online, over excited and under powered.
Running over excited, the sets injected VARs into the system.
Running at a low power setting resulted in little kW generation and low fuel costs.
I was never aware of the billing formula, but those VARs surely had value.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!