Voltage Regulation Problem

Try balancing the single phase loads better across the phases, first.

I agree with the comments about good load balancing and the need for 3 phase voltage regulation. You have not mentioned whether the problem is worse when parts of the load are switched in or out.

We have found that in general for an internal combustion engine powered generator (diesel, petrol or gas) the capacity of the generator must be at least twice the load for a fully balanced load to give a stable supply.

If the load is unbalanced a higher ratio is preferred.

The larger engine / generator capacity will also have greater inertia and give better frequency control.

I believe that 250 kVA for a 170 kVA load is too small.

If significant portions of the load are switched independently the inrush current of attached equipment must also be allowed for.

can you publish the study showing why you need such a large engine and alternator.

Have to agree with Pnaban's call mate.

100% overhead is ridiculously oversized, even with starting currents considered.

What you say may be true for small machines under 10kVA but not a generator with enough mechanical inertia for the start surges.

Running a DG at only 50% load long term leads to other issues for the primemover, alternator doesn't care.

"We" have found that a genset loading of around 80% steady state is fine. eg 800kVA load has a 1000kVA DG pushing it. OP's 70% loading is OK too.

Dear Mr. manvendra,

Have you measured the current in each phase, and it should be near to equal in each phase and if not try to shift the load to the other phase and narrow down the difference.

Pl. check the AVR whether it is functioning properly, it appears - response time for the AVR is much slow. If so this response time should be improved.

What is the Power Factor, if it is less than 0.8, try to improve to 0.9 or so.

Stabilizer will be expensive for your Load. Replacing AVR will be economical option.

Pl. inform this Forum, how the prolem was sorted out.

DHAYANANDHAN.S

good morning sir, thanking for your comments

sir normally as my observation it is happening in night time normally and I think it it due to unbalance load but the use of load is not in my hand so as normal I have balance the load but use of load I can not decide sir because it has other user also so if stabilizer is the solution so I will think about after economical analysis. avr is ok sir, I think avr is not responsible for balance the voltge, it is for excitation of generator for voltage regulation

sorry sir it may be the topic of title is not convening my problem.

What I understand from the information you gave is that your power supply from Electricity Board is varying from 250 V, 3 phase to 420 volts depending on load conditions and voltage in all the three phases are almost balanced. It is presumed that your local generator is regulated correctly at 420 Volts. The other information we will need to know is are you using the local generator only as a stand by power supply to take over when the Electricity board's supply is not available. Please confirm these points so that proper advice can be given.

sir,I am using generator when state board power is not available

and one things sir generator has also different other load which is balance but all load are not used at a time in each phase so some time it is also vary from 300 to 400 volts while we need 5%max regulation.

thanking you

Sorry to "drop a stone in the pond", but if you have 250 kVA generator and 170 kVA load which is unbalanced a supplementary voltage regulator on a phase which is at 280V when it should be 400V will not help. You will just increase the current on the overloaded phase and bring its volts down even lower and risk tripping the current overload device or damaging the winding.

You have not given any examples of voltage and current on each phase in your undervoltage events - so we cannot judge if there really is a potentially damaging current overload.

You may have to try telling those users who are overloading at night that you may have to cut them off to prevent generator damage - which would leave them without power for ??? till the generator is repaired. They should only use essential loads. You are replacing an inadequate mains supply with your generator - the users cannot expect miracles.

67model

Where are you measuring these voltages? At source or at the load?

Is the neutral grounded/earthed/decked?

You have already been told what the nature of the problem is, please stop wasting our time ignoring it and purchase an adequately sized UPS system, it is the quickest and most reliable solution to your problems short of finding an old antiquated ferroresonant voltage stabilizer.

I would find a voltage regulator large enough to keep your power supply clean enough to keep what you service constant (ahead of the generator). The load your generator is suppling will take care of the load.

In my opinion, your problem is if your stand-by generator will be your single source- if no supply from the transformer is available.

In this case if you'll try to start a big induction motor-mainly D.O.L. it will be a problem: the dip of 120 [400-280] V it could happened if you try to start D.O.L. an induction motor of-let's say- 75 kW if the only source is this generator.

Starting the same motor on transformer the voltage will stay 400 V.

In my opinion, no AVR will help you. Try to fit a VFD on this big motor.