Generator Under Voltage

I send to a friend, digg this and send to del-icious. wait for an answer,

Do not worry about the AVR!! It is very difficult to lose one phase of a generator without great flashes and bangs!! Check voltage at incoming terminals from generator (then at generator!!) before guessing generator faults. My best guess is that red phase of under-volt relay and red phase lamp are both fed through a common control fuse which has blown. The under-volt alarm is not "armed" until unit has been running some seconds. Perhaps fuse blew because the lamp shorted. Recommend you change failed lamp for an LED type, all I installed 19 years ago (permanently energised) still work. Few lamps above 50V have an ON life over 1 month.

To expand on my first post. Any AVR fault will affect all phases, the rotor "magnets" rotate past every stator phase coil in succession, the inductive time constant of rotor field is too long for field current to change much in the 20/3 milliseconds taken to move from one stator phase to another. Even if you checked the fuses, check voltage at lamp and under-volt relay - wires do break if stretched too much, installers crimp wires badly, wires look to be in place but are not. I remember an intermittent fault in a string of dozens of looped terminals (with a no-volt alarm relay at the end) which was finally traced by pulling hard on every wire until one finally caused the alarm - it was a ring crimp on a stud with several crimps which looked OK, but actually it was only pinched-in and the stud did not pass through the ring! Letting go the wire restored the circuit because the insulation on the crimps was springing it into place!

Thank you. I'll will check the lamp. Yesterday, what we did was changing the relay for the under voltage and and also some other relays which I think is related to the under voltage and voltage coming from the generator, but it did not solve the problem.

We already checked all the fuses inside the control panel and trace the connections. So far, we havent find anything wrong with the wiring connections.

Post #5 has experience and good idea. From his description, it seems the AVR is bolted to the generator unit. Also his writing, in last paragraph, that he has done this resetting "many times" makes me think that vibration is shifting the setting potentiometers (pots). One fault of designers is that they make one model fit a wide range of machines. The result is that the "pots" for STAB, VOLTAGE etc have a very wide range of adjustment, a small movement has a big effect. And pots are less reliable (and usually less stable in value with changing temperature, vibration etc) than fixed resistors. It is worth measuring the resistance of each Pot at a good working setting. Record these resistances. Modern digital multimeters have very good resolution and repeatability and such low measurement current that only resistance is measured (no spurious error from diode conduction when you reverse the ohmmeter leads). If problem recurs, measure pot again , to see if value changed. If resistance changed, move pot till you get original value. A more permanent fix may be to put a fixed resistor in parallel with the pot. Suppose it is a 10 kohm maximum pot, but in-situ value is 3 kohm, then 1) move pot till it reads 6 kohm. solder a 6 kohm fixed metal film resistor in parallel with pot. Re-measure resistance, adjust pot to get 3 kohm. You have now halved the sensitivity to changes in the pot value.

If there is no more to turn/adjust the knob???

The simple and practical answer is that it is not the AVR problem as if you adjusted all with no response, then go to another possible area like checking all phase terminal ensuring that all are in placed and tight. also from alternator main winding could some un-insulated part caused by scratching due to minor vibration.

Thanks

Roman

I didn't get you properly, you are saying after few second it means without load or after putting some load and how many times it was happened.

I hope the generator was run without any issue prior to shutdown, if it is correct the problem might not be in AVR setting no need to change anything.

Hopefully you or somebody put on the load before the generator stabilized to normal operation; I mean the engine not reached to rated speed.

So first UV relay was activated and trip the breaker then the load was released suddenly the frequency went up.

"Hopefully you or somebody put on the load before the generator stabilized to normal operation; I mean the engine not reached to rated speed."

If that is the case Under frequency will be the priority fault indication and not under voltage, beside as normal condition you can't expect that the breaker will automatically closed as it should be set with few minutes warm up to at least 60 deg C before the breaker will close and load automatically. In about 2 hours ago, I just finished and fixed my generator with the same problem as OP encountered. I found the under voltage fault indication and when i do few start up attempt I just do the same procedure as mentioned on # 5 reply and DONE!!

Happy new year!!!

Roman

Hi sukumar75, We ran the generator without load and about 5-10 seconds, its automatically shutdown with generator under voltage indication.

Oh! Boy! You have also the engine running the generator. 55 Hz relates to speed of the diesel that should be adjusted to 3000 rpm. The engine probably has 3100 rpm.

The low voltage is caused by not enough current in the exciter of generator. Probably there is a potentiometer to adjust that. Do both generators connect to the same bus-bars on the switchboard? These should be synchronized. Or one is stand-by and another is working?

The exciter measures output from generator and converts it into DC signal using diode bridge by comparing it to a set point. The set point is adjustable using a potentiometer that adjusts the output from op-amplifier, one input of which is on Zener diode and another is on the signal from geny. You can full around these or you can replace the whole box of the controller and use the old box to learn.

Actually, both of the generators are standby unit. 1 generator is actually sufficient to cater all loads. That is why I have time to check and learn where the problem is, instead of calling for experts (specialist) helps ( which will cost a lot of money) and we cannot learn a lot.

Yesterday, we check voltage at the generator ( i think at the the alternator terminal) and the chargeman said the values are ok. Then, we check voltage at bus-bar( at the back of control panel) and found that voltage for red-neutral phase only give ~90V instead of 240V. Voltage for all other phases are ok.

When the generator start, the red light (at control panel) actually light up for 2 or 3 seconds. There is some voltage but not enough.

If the AVR need to be set, the voltage for all phases should be affected ( all voltage phases not stable), but now the symptom is that only the red-neutral phase are under voltage. So does the AVR still need to be set according to post #5?

Tq

What did I say in my first answer that is right, if the generator out put voltage is correct in generator side there is no problem in generator and AVR. The problem is in the cabling, check your R phase cable tightness on both ends and check for lug were gripped properly. Then check your control wiring tightness in your control panel there might be loose contact some where.

Hi Zawa,

There is nothing wrong by doing post # 5 suggestion, you can easily put it back with the same set up anyway. However, NEVER play around adjusting AVR when there is load. Also try to check/inspect alternator main winding terminal lugs. Sometimes it is not really visible that one of the lugs was cut off and you can get reading of less 100 volts in one phase, if I can remember one time was in Neutral terminal and the second time was on live terminal. it was happened to my generator way back two years ago for twice in three months interval (terminal lugs poor quality).

Good luck

Roman

Hi Zawa

Try to start the genset without the control modul and with open breaker, then check tension and frequency at alternator conectors. The genset should not shut down and you can take the measures. If both values are right, follow the wiring verifying tension, somewhere voltage will be low (or zero).

Hi all,

Tq for all the replies.

We checked the voltage at the genset and it gives ~240v for red, yellow and blue phase (even voltage for all phases at the generator and from this we conclude that the generator and AVR are in good condition).

Then we check the voltage at the busbar (at the back of control panel) and the voltage are red=~90v, yellow=~333V, blue= ~333V. So what we did was checked the condition of all the cables from the generator to the busbar and megger for all cables. All the cables are ok. We also checked if there are any leakage to earth, but it was confirmed that there are no leakage to earth.

Now we are still tracing the control wiring inside the control panel.

What a mistery!!

I agree the alternator and AVR are fine.

Did you mesure the busbar voltage when it is disconnected from the load?

Did you test the resistance of all the 3 cables from generator to busbar. If they are normal (low resistance) then you must get voltage at busbar same as at generator. Pull on the cables while measuring - you may have bad joint. If voltage still low on RED then you are wrong to think there is just a cable between the two points, something is losing voltage! Did you check busbar at point where generator cables connect?? There could be a loose nut and bolt at a busbar junction.

would suggest you check all the sensing cables ensuring you have input voltage from the output plug to the avr

Hi all,

Tq for all the replies. The source of the problem has been identified.

The problem was: there is an ACB (air circuit breaker) connected to the neutral line. It was located at somewhere else (although still in the same room). The circuit breaker was not drawn in the schematic diagram (it was installed later) and we actually wondered why there is ACB located there and what is it for. Before this, I assume, it supplies to some secret area of the building because at LV MSB panel, there are few ACB that we do not know where the supplies goes ( as there are some secret area which is not listed or drawn for security purpose).

The problem actually arises when the ACB does not automatic close (connecting) the neutral line to the busbar at the back of the panel. Now if the generator start, we need to close the ACB manually and the generator runs just fine.

Me and my team learned a lot during finding this problem. I appreciate very much all the comment and suggestions that have been given to me and my team.

Have a great year ahead. Happy New Year.

Glad you found the "joker in the pack" (of playing cards). I recommend you read the several threads about "neutral earthing" in CR4. In particular question :-

Ground Resistor for Generators

12/16/2010 3:14 AM

Posts #4, #6, #7.

Dear Mr.zawa,

My doubt is in the Excitation System/Circuit. It may be in-sufficient DC current, Field Winding problem.

In my opinion the high speed is not a problem as for as power generation is concerned - but while in parallel operation, the common frequency will decide the speed of the system.

DHAYANADHAN.S