Need Help - Undervoltage at the Bus

what is the voltage rating?

The exact problem indentification needs some more details about the current carrying capacities of the system components. However one of the major reasons of high voltage drop could be the under capacity of incoming cables to the bus.Just by enhancing this capacity you may achieve the voltage drop withing the specified limits. The excessive voltage drop could also be due to DOL starting on load.

Priyadarshan

Quote "My network has 5 big chiller motor's ( 1.32 MW each ) feed by individual 2 MVA transformer"

I agree with Priyadarshan. You do not have capacity to run 4 chiller motors. 4 motors (1.32 kw) x 4 = 5.28 mw/pf of .8 = 6.6 mva. Maybe I am reading your post incorrectly. Do you have more that one transformer? Capacitors would help but I would ask the mfg for a recommendations as to the correct size. Before I would do that I would rather see you add 2 more transformers if they are not already in place.

thanks for your kind answers the voltages are 6.6 KV And i have mentioned that each motor is fed by a separated transformer, or in other words each motor has its own transformer. and main reason of the voltage dip is all the motors are DOL, because of that heavy inrush current flows which cause a voltage dip for about 12 Sec's or so. We thought of installing the soft starters as well, but we are also facing the problem of low power factor as well, so we want to install capacitor banks to over come both problems with one solution. but here my main concern is the consultant solution, is it feasible to install 4.5 Mvar capacitor bank for each motor ???

4.5MVAr per motor is far to much.

4.5MVAr installed on the main bus would be over compensating if say you only had 2 motors running.

1.1MVAr switched with each motor should do it.

Maybe the suggestion is a misprint, and the recommendation is for 0.45 Mvar for each capacitor set?

(At least that would fall into line with some installations I've seen.)

NO, its not a typo, its 4.5 Mvar per motor.

What is the PF that occurs now, and what PF are you trying to attain?

In rough numbers only, I have seen PF correction capacitors with kvar around 0.15-0.30 times motor hp (which would be about 0.12-0.24 times motor kw).

How do you know that 4.5 Mvar is not a misprint?

brother thanks for your help, i really appreciate it.

rite now the pf of the motors hover between 0.85 to 0.8, i want to attain 0.95, for that purpose 500 Kvar capacitor would be enough, but 500 Kvar means the voltage rise of only aprrox 1.4 %. you think that would be enough for my requirements ??

I know its not a misprint because i have a report from consulatant stating 4.5 Mvar numerous times and i have asked inperson as well.

As the motors are started DOL, the starting inrush may be causing the dip ?

sorry but i didnt got ur point ?? but you can review the below link, may be, it will help you understanding the situation. thanks http://www.electricalengineering-book.com/starting-of-squirrel-cage-induction-motors.html

What i meant was that with three motors already running, the 6x to 8x starting current must be causing the 18% dip most likely. Your link also mentions this. The bottom line is that either the trafos are under-rated or the busbars are too small, and have a high drop.

Hi,

Check for below points:

1. What is the voltage dip during working condition - If it is more, then, source capacity / impedance is high. You need to increase source capacity (which feeds all the motors).

2. Please note, starting of motors with unit trafo., you need to consider inrush of transformer, which is usually more than motor starting. Possibly, source capacity was checked for motor starting only but not for transformer inrush. (Transformer might be considered for reducing system fault current). If the dip is due to transformer inrush, you need to install soft starter for energizing transformer. Otherwise,

3. Do the 2 MVA trafo. has online tap change? Set the tap to get lower starting voltage which will also reduce the starting current.

4. Check cable size too.

5. All the above, since you are dealing with existing system, what is the exact problem if bus voltage exceeds 15% - Is the motor not picking up the load? / Is downstream system affected? / Is the motor controls has any problem?

For more detailed feedback send the SLD of the switchgear. Possibly, adding Capacitor may not be the only solution.

Hello sir,

1) Voltage dip is about 18% in normal working condition, i have monitored and measured it my self.

2) Transformers are always in energised condition, so i think the dip is not because of transformers inrush currents.

3) no, the transformer dnt have the OLTC, but its equiped with offload tap changer. i will try that and let you know about the result.

4) cables are perfectly designed and sized.

5) if the voltage drop exceed more then 15 %, the surrounding busbars experience the voltage dip and i have some electronic load connected to those buses, who are highly sensitive to voltage fluctation, so very often they get trip.

I assume that the voltage dip you refer to is in the "primary"- 1250 amp at 13.2 kV- bus bar. That rated size should have no problem with these loads, including start up inrush currents of ONE chiller.

That would imply that the power feed to the bus bar is under capacity, NOT the feeder bus bars OR the transformers OR the low voltage bus bars or cables.

I think that you need to go "upstream" and check the higher voltage connections and/or transmission transformers/feeders.

By the way- I also agree that smaller capacitors would HELP, but I also believe that you have a bigger problem since the TOTAL electrical service should be designed for 125% of the TOTAL power draw of all 5 chillers running- which is why the start up current of the 4th causing a problem makes no sense.

Dear guest,

As per your write-up I'll give you following comments.

1) First check the busbar and cable rating matching the load requirements. Bus bar rating should be such that it should be able to carry total connected load with 10% overload margin.

2) Secondly identify the rating of transformer with the total connected load on the Transformer with its operating p.f

3) Installing 1-1.5MVAR capacitor bank across the motor should be sufficing your requirement, in such a way that it should be charged only when, motor gets the command.

We now have several opinions suggesting around 0.4 to 1.1 Mvar capacitors for each motor. Is there a calculation that justifies the 4.5 Mvar selection? (Which is additionally suspect for introducing overvoltage.)

I agree with the questions about cable sizing, both from the common bus that presumably feeds all the transformers [?] as well as from each transformer to starter to motor. A detailed SLD (single line diagram, including cable lengths) might help.

I just been provided with ETAP study report thats also with only one mode of operation ( thats 3 motor running at full load and 4th one starting) i have asked them to provide me the study for the starting up of each motor turn-by-turn ( with and with out capacitor banks) still waiting for it.

the common bus is of 1250 Amps at 13.8 KV , that feeds all the transformers, then on the secondary side i have 630 Amps bus at 6.6 KV and cable lenghts are less then 100 meters.

i will try to upload the SLD.

1. I tried to deal with most of the numbers in post #35, please refer.
2. Bearing in mind all the comments about busbar and cable resistance, I would like to point out that, if the transfo upstream of 13.8 kV has impedance of 10% on 10 MVA rating, which seems probable; then adding the (unlikely, inefficient) value of 4% resistance to 10% reactance gives impedance square root (4x4 + 10x10) = 10.8%. A more reasonable 2% resistance gives only 0.2% increase of impedance. Less than 1% increase, which does not explain 3% drop.
3. Please Guest, do you have main step-down transfo numbers/rating ??/13.8 kV and its impedance? Must be on diagram for any study of volt drop.
4. Please Guest, do you have any records of past and present 13.8 kV busbar voltages and loads (no. of motors running) to give any indication of decline in source voltage?
5. Posts about 1 motor having fault have some merit, a check of starting/running current for each motor and starting time would cover this. But a starting motor is like a transfo with short-circuit secondary, most reactance is in stator and resistance of a few bars would do little to current - the effect on start time would be most convincing.

You need a capacitor for reducing the starting current and another for PFC. Refer to motor manufacturer for recommended sizes of capacitors. Also 2MVA transformer should have OLTC. If you experience this problem only when you start the 4th motor you should upgrade the power supply which is inadequate. What is the present capacity(MVA) of your connection?. Replacing DOL with reduced current starting should be considered if it is OK for the connected load.

This may be a good point, but with a separate starting capacitor, there will need to be an additional contactor to switch it in and out. Hopefully this can be avoided.

Also, we haven't yet determined if the chiller control scheme ensures that the compressors are unloaded while being started. This is another possible item for the OP to check.

Check the cable and bus bar sizing. Seems they are under sized. Capacitor is not the right solution

Shivaram

Check the % impedance of Tfr and design code of motor(LRC),inform manufacturer.If you have software you can do it

If you have a power analyser suitable for 6.6kV or 13.8kV you can record variation of current,voltage,pf etc during a typical day and make your decision,but it is always advisable to consult motor/transformer manufacturer

Hi Guest,

Please check the capacity of transformer is matching with full load current rating of the motor at rated voltage; also please consider starting current( DOL START as stated) of the motor feeding from individual transformer. Check the Busbar size, cable size and proper firm terminations( no loose contacts in terminations). Is the motor is started through vacuum contactor or VCB? If so please check condition of vacuum bottles. Remember that at the time of starting of sq. cage induction motor, generate low power factor and what is the type of coupling used?( if fluid coupling is used pl. check the oil filling angle of the fluid coupling).If you want to install capacitor bank across the motor thumb rule is the KVAR rating of the capacitor bank will be within 90% no-load KVA of the motor to avoid over compensation. Donot forget to install surge diverter to bypass the surge generated at the time of starting.

Kindly confirm from your consultant that as the rating of Capacitor 4.5MVAR will be for common busbar at Main substaion supplying to all the chiller motor for 1.32MW(through MVA transformer).

it wont be suggested for each motor pl check it

Is this capacitor for starting(very low pf) or for running(normal pf)

I am going to ask a silly question:

Have you tried to vary the motors that you are starting?

Example: Motor 1 running; Motor 2 running; Motor 3 running; Motor 4 starting

Motor 1 starting; Motor 2 running; motor 3 running; Motor 4 running

ad. nauseum.

Can you vary the starting sequence?

Perhaps there is a problem with the last starting motor so that the LRA is excessive.

Do not be afraid of that! motors should take at least 20% voltage increase for at least an hour. Buy yourself an insurance and instal everything the consultant has written in his expertise. There is no chance that motors would burn. No chance at all.

What kind of strart up do you use?

Oversized capacitors can result in leading, rather than lagging, power factor; and system instability.

It would be interesting to know the voltage and FLC (full load current) of these motors. (In a quickie review, I didn't see that, but maybe I missed it.)

Post 32 is dangerously incompetent. Ignore it. (If it's a joke, it's horrible.)

Also had a similar problem, tried fitting a softstarter, then we couldnt get the motor to start at all, the softstarter, stopped on startup. eventually between softstarter manufacturer and motor manufacturer we found the motor had some broken rotor bars. DOL would not pick this up, but the softstarter did. It might help to check this out, it worked for us

Please check from the transformer manufacturer whether you can start a big motor(1.32MW) DOL in his transformer and whether the service life of transformer become reduced.