Brushless Excitation Field Flash 2

[electrageek]

Normally in a Brushless excitation system we have a PMG (Permanent Magnet Generator). Because it is a independent source of power it will provide the Voltage Regulator system all the power it needs to control the generator voltage. However if there is no PMG and you must use a self excited source like a PPT (Power Potential Transformer), connected to the Generator Terminals, then if there is not enough residual voltage on the Generator output the voltage regulator cannot startup. Does anyone have any experience with this problem? I know how to use the field flash option in a full static voltage regulator, but have not had to use one in a brushless excitation system. I would assume it is the same concept only smaller.

 

[waross]

Step one; Reverse the field leads. It is unusual for a generator to lose all residual magnetism.
Step two; disconnect the AVR and run the set with a twelve volt automotive battery for excitation. Shut down and reconnect the AVR.
Note one; A free wheeling or discharge diode in the circuit is a good idea.
Note two; If it doesn't come up now, see step one.
Step three. Connect a 1.5 volt "D" cell in series with the AVR and start the set. Polarity matters. See step one.
Note, you may try two "D" cells in series but I don't like to see any more than abut 3 volts added to the AVR circuit.
DON'T TRY TO "FLASH" THE FIELD WITH THE AVR CONNECTED. That will let the smoke out of the AVR faster than you can say "OOPS".

Normally in a Brushless excitation system we have a PMG (Permanent Magnet Generator).

Must be nice. When I was doing a lot of generator work PMGs were a nice option that many customers would not pay for. I spec'ed them when I could, but any gen-set bought on price would not have a PMG.
I was called to more than one set because "The other guy" had let the smoke out of the AVR trying to flash the field.

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

 

[GTstartup]

In my experience (units larger than 80MW)

It's no different than static excitation with the excitation transformer fed from the generator terminals.

For the first 50% or so of voltage build up, the excitation is powered from another source and then automatically switched to line suppy when the voltage is sufficient to feed the AVR.

 

[waross]

Hi GT;
I have worked on smaller unis, up to about 1.2 MW. These have enough residual magnetism to "bootstap" themselves. I have had failures in either the PMG or the PMG specific AVR. I have been able to run on a non-PMG AVR until parts arrived.

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

 

[Hoxton]

We had some ac generators where we used a battery to flash the field as you describe. There were when we went from units with a pmg driven off the free end of the exciter, to units where the free end was not available (either units with a turbine at each end , or a turbine at one end and a start motor at the other). In either case we used a battery to flash the brushless exciter field to get the initial build up. This was carefully engineered, with a resistor in circuit, to avoid overcurrent etc. It worked, but there was a relay control circuit, to guard from back feeding the AVR and damaging it.

The we developed a suitable pmg generator and went away from the battery system.

This was for two pole (3000/ 3600 rpm) generators which have less remanent magnetism than salient pole machines.

Generally, salient pole generators have some magnetic steel in the rotors, the remanent magnetisn left in the steel after the last run is sufficient to build up the excitation.

On salient pole machines (motors or generators) it was common to have to manually flash the field for the first factory test, when the rotor was not magnetised.

Occasionally this had to be repeated during commissioning, or if the magnetism was lost in operation.

 

[dcset]

hi
I used to do on this way with a decs 200 but it´s same for other units, , and about the control, the plc control the moment flashing goes on and the time its on, all the time in paralell operation with the decs 200, and for others avr, you need to control voltage rise and switch off flashing, i mean the flashing go on paralell with avr and with a diode to control back voltage to the batteries..

 

[electrageek]

dcset,

Thanks that is what I was thinking too. Worst case if I have trouble I can use a 2 pole contactor with a diode isolation and current limiting resistor from my 24Vdc battery bank and give it a flash to get started. I do have a DECS200 VR as my regulator and an AB ControlLogix PLC. The original design had a PMG but it is gone now so am just trying to prepare for the case that I may need a field flash to get up and going.

 

[rhatcher]

electrageek,

waross's suggestions are good. For me, I usually skip to my variation of his step two: "disconnect the AVR and run the set with a twelve volt automotive battery for excitation. Shut down and reconnect the AVR."

For my variation, I have found that for most small generators (<1250kW or so) that a 6V lantern battery works just as well, it is easier to carry, and it does not require jumper leads that are so long as necessary to reach from the engine batteries to the generator voltage regulator that is sometimes some distance away from the batteries since it is located in the generator junction box or sometimes in the switchboard.

I would also add that if at this point the generator is not producing some voltage greater than the unexcited residual voltage then this is usually an indication that some other problem exists.

If this doesn't work and the generator engine is battery started by 12V or 24V batteries (versus air start), I usually repeat step two using the engine batteries.

I have never tried waross's "step three", mostly because I wasn't smart enough to think of it before I read his post. Nice suggestion waross, I'll add that to my 'bag of tricks.'

My usual "step three" is what waross does not recommend. That being said, I would not recommend it myself unless the recommendation was given to someone that had experience and someone who was very careful and if I did, it would be as a "last resort".

The last resort is to flash the field while the generator is running and the voltage regulator is connected. This flashing is only momentary, literally just a "touch" of one or two seconds of the battery to the field terminals. The danger that I have considered in doing this is not that the voltage regulator would be damaged but that the battery would be damaged and/or explode once the voltage regulator activated and applies a large (ie. up to 50VDC) voltage to the small (ie. 6VDC) battery. I have done this successfully many times. However, this is a method that is not recommended to persons who are not very experienced and that it is a procedure that is inherently is risky..."do not try this at home".

 

[waross]

A couple more notes:
1> I run the set when flashing. Flashing with the set stopped just magnetizes the brushless exciter. Flashing with the engine running magnetizes the main field also.
2> Flashing with the starting batteries. I have cleaned up a couple of installations where the other guy did this without disconnecting both field leads from the AVR and let the smoke out.
3> Some AVR instruction pages give instructions for flashing with a battery and diode with the AVR connected. Be wary, using a grounded battery may still damage some AVRs.

My procedure is to first reverse the field leads and try again. This works surprisingly often.
Second, disconnect the AVR and conect a 12 Volt battery. Start the set and shut it down. Reconnect the AVR, same polarity as the battery. If the set does not now come up to voltage you probably have a problem (PS, remember step #1). Possibly the AVR, possibly the set.

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