excitation system for an old generator set 2

[dcset]

Hi, I´m changing an excitation system from a old to a new system and i´ll like to know, some resistance values, cause i don´t know how to calculate it, cause i´m not experienced,
please cheek the drawings

http://files.engineering.com/getfil...4-1c14e70b05f7&file=moncabril_plano_final.pdf

this is the idea i got, but i´ll like to know in the old system the values of the resistences numbered as,

http://files.engineering.com/getfil...5-25bd-4d5d-bb0d-1785ab341eda&file=Planos.pdf

the resistence is for discharging the main rotor, and it´s numbered as R202 and the other is for charging the avr, i think this must be a similar value to the field of the dc generator...

if somebody have some idea of how much resistence and power must be R202 and R203 on the old system, or for a normal system, thanks a lot

 

[waross]

What over voltage can you withstand? The field current times the sum of the field resistance and the resister value will give the inductive "Kick" voltage from the field.
Consider a free wheeling diode, or a MOV.

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

 

[dcset]

warross could you explain with a little example? cause i don´t understand exactly what do you mean, i would like to expect no more than a 10% on the generator output from full load to no load... what could be the normal values? and does somebody know the typical excitation times for this kind of generators... I know i have to sum the Tdo of the inductor and the Tdo of the generator.... maybe i´m asking too much.

thanks for your time

 

[rhatcher]

dcset, R202 and R203 appear to be field discharge resistors of the generator field and the exciter field. Since you have an original design to copy, why can't you measure the original design resistors and copy them?

About the Tdo..I am guessing that you mean 'T'ime 'D'elay before turning 'O'n?? I would say that this is determined by the mass/inertia of the generator coming up to speed. It would appear that the unit you are looking at is 5Mva and 600rpm and that is has a separate rotary exciter. This sounds like an older design hydropower unit to me. Even if it is not hydropower, the speed of 600rpm suggests that the rotor inertia is large and that the accel time would be also large. By large, I mean on the order of 30s-60s minimum to come up to 'synchronous speed'.

Again though, I have to ask. If you are copying an original design to upgrade the voltage regulator, why can't you determine the necesary values by measuring the existing design?

 

[dcset]

I copy it, but this for a identical generator on other place, and here it´s not possible to measure anything cause the cabinet explote...i just wondering how much this resistences use to be, and yes you are right about the power of the group.

thanks for your time

 

[waross]

I was talking about the inductive kick voltage coming back from the field(s) when the supply is interrupted. The field is an induction that tends to keep the current flowing. If there is 50 Amps flowing in the field, then the induction will try to keep 50 amps flowing.
The kick voltage depends on the resistance of the discharge path.
If the open circuit resistance of the field circuit is ten meg-ohm, then by Ohm's law the kick voltage will be (10000000Ohm/50Amps = 200,000 Volts). The insulation may break down at a lower voltage and provide a lower resistance and so lower the kick voltage.
To calculate the Kick voltage use the value of field current at the moment of circuit opening and the sum of the field resistance and the discharge resistor resistance. If the calculated kick voltage is too high for the field insulation, reduce the value of the discharge resistor and try again.
A discharge diode will allow both the voltage and current to decay exponentially from normal working values to zero.
An MOV will provide better voltage clamping than a conventional resistor.

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

 

[ratz1]

The typical values of the discharge resitors are 3 to 5 times the rotor resistance. On this type of the generator i expect to found the rotor resistance somewhere between 0.5 and 1 Ohm, so the "good" value for the R202 discharge resistor could be 3-4 Omhs.
Tricky thing is to determinethe power rating. If you know the rated rotor current assume that will decay exponentailly once the resistor is applied. Let say 500A (for this size of generator could be the right value) decaying exponentially in 3-5 seconds. This is enough to the resistor manufacturer to design it.
For the other resisstor (R203) use the same criteria, but is expected to be higher resistance and certainly lower power rating.

 

[dcset]

And about the circuit breaker to open the circuit on the main rotor, as I disconnect at the same time excitation for the inductor, does i need to size the circuit breaker for the nominal current or could i size for a lower current???

 

[rasevskii]

For dcset:

It appears that you are upgrading the old system as there was a fault (damage) to the old BBC system. Is this correct?

On the BBC the field discharge resistor (R 202) is connected in only after the field breaker is open via a makebefore break back contact. This is standard practice also on the old ASEA systems. The Field breaker is a DC breaker with arc chute. You cannot use an ordinary AC contactor as it will burn up if opened under DC current. Unless- The DECS 200 Basler regulator can be so programmed to lower the field current to zero, BEFORE the contactor is allowed to open (aka"phase retard").

In your design the R 202 is permanently connected and will therefore dissipate power continuously. If as suggested 3 to 5 times the rotor resistance that will be a some KW all the time.

On start-up, usually the excitation is applied at 90% speed only, not less.

At commissioning, see to it that the generator overvoltage protection is in service and that it trips the excitation, test this first...before proceeding...

regards, Rasevskii

 

[rasevskii]

For decs:

Correction. I see that in fact there is no more R 202 in the new diagram, rather a De-excitation module and another connected module, likely an MOV module. The De-excitation module seems to include a normally closed contactor, a proper DC breaker I assume. This opens on de-excitation during a normal stop or only upon a trip of the generator protection?

regards, Rasevskii

 

[dcset]

Yes Rasevskii, i use a dc breaker, i would open this breaker and close the resistence on parallel with the main rotor, I was calculating with this values, cause my system is a little diferent, less power, and the values used are
58 A 35 V, 0,641 ohms, this means that resistence must be 2,6 ohmos with a power of 5 kw, (exactly must be 3,8 kw) what do you think, i think i understand and calculate with a normal trip, but what about the reverse voltage coming from the rotor cause of induction of the current of stator on a trip???

 

[dcset]

and the circuit breaker must be at less 75 to 100 Amps....

 

[rasevskii]

For dcset:

Yes OK. In principle the discharge resistor has to be the same as the old R202 before. The KW values that you calculate are likely the same as the old resistor. Remember this is only for discharging the main field energy for a few seconds.

The DC breaker has to be sized to interrupt the maximum field current that the exciter can put out. Remember that this is a DC machine with commutator and brushes, and cannot be hugely overloaded without danger of flashover.

Probably you should rate this breaker at least 1.5 times the DC ampere output of the exciter as it says on its nameplate, to be safe.

You will also have to set the new regulator conservatively so as not to overload the exciter.

On a normal shutdown the excitation is removed by opening the exciter field only, or causing the new regulator to go to zero output via external command, it appears. In this case the main field breaker stays closed all the time and the discharge resistor is never connected. The main field discharges slowly via the DC exciter armature. Is this correct? Many old units worked this way originally.

It seems that this unit was previously upgraded before with an analog BBC regulator that replaced the original mechanical regulator, possibly the field breaker was added then.

In case of a protection trip the field breaker has to open and de-excite the machine quickly. If there is a generator differential protection provided, this is essential to protect the stator if it had an internal fault.

Much depends on the operating procedure to be used. In case this is a hydro unit being automated and upgraded then I would assume the protections are also being upgraded.

Please tell us a bit more about the project at hand.

regards, rasevskii

 

[dcset]

First of all, thanks for your help rasevskii,
I don´t know the value of the old resistor, but as the resistance of the main rotor is 0,645, i we multiply by 4, we get a resistance of 2,616 ohms, i wonder why to multiply by 4 as ratz said, if somebody know if please explain me.
About the kw values, the nominal volts and currents to get the full power and cos fi 0,8, is 54 A and 35 V dc, and if we get the kw V*A, is 1,907 kw , I wonder if this way of calculation is ok to calculate the power for the resistence, cause i don´t know how much current will flow through this resistence, or how much volts i´ll get, maybe somebody could help me on this calculation, i think we have to scenes, one on a normal trip that we only see a high voltage on the stator if we didn´t discharge the rotor and cut excitation, cause as it´s a hydro unit, speed go ups and the magnetic flux is inside of the machine... on this situation, i think my calculations are all right, but i wonder how much current & volts will flow in example on a short circuit, or an overload, when the stator induces volts onto the rotor, i think on this situation the current will be AC current?? and polarity oppositive?? maybe is better to short circuit it with a big diode???
Yes about the dc breaker, i wonder why if i´ll limit the current on my AVR to control the field of the dc exciter, i could size to interrupt at the maximun current given to the main rotor, this means 34 V * 1,5,
Why not to open the exciter field and the main rotor field all times we disconect excitation??? this means when we stop machine or on a trip. Do you have some documentation or know some place on internet where i could study this question?? I know i´m asking too much.

I don´t know the history of this unit, and yes we are upgrading the protections of the generator, but we are thinking on de-excite the machine always as soon as posible, cause i´m too scare about overvoltage problems on the load rejections, do you see any problems on this?

ok. i think i have done a lot of questions, and thanks for your nice help....rasevskii

 

[rasevskii]

for dcset:

I think that I have identified the hydro plant that you are dealing with (won`t mention the name here)and it has 3 x 14.4 MVA pelton units. Installed around 1951. Correct?

If this is so, you should engage the services of the original manufacturer (BBC now ABB)to recommend on all the technical aspects of the upgrade especially on the protection, control, and excitation. That would be the safest approach of course.

Perhaps your organization is tendering on this or you already have a contract with the plant owner, and are manufacturing the new panels already.

If my assumption is incorrect and it is some other plant, then please inform on this.

Getting to the specifics, you are confusing the de-excitation of the exciter field (35V at 54A) with the discharge of the main rotor which is around possibly 500A at some hundred volts at full output.

On a pelton unit (if that is so) the speed rise on 100% load rejection is only a few percent due the fast operation of the deflectors, if it is not a pelton then you can expect a rise of about 30% speed for some tens of seconds.

A good AVR properly set up should nevertheless keep the generator voltage within a couple of percent even at full load rejection. On a recorder or oscillogram record, hardly noticeable. (ignoring the droop if used)

The discharge of the main field from the back contact of the FCB is best done as said by others above by an MOV.
The generator manufacturer has to recommend on this, and supply their standard discharge device. Usually this is something rather proprietary.

Yes it is usual to open the FCB on a normal stop on modern units in which case the discharge device is then connected in.

regards, rasevskii

 

[nawao]

You are upgrading the system from low resistance grounded to floating neutral:
1) You may have issues with your ground fault protection.
2) Voltage instability may difficult synchronisation