950kVA Generator cannot pick up load 1

[zak]

Hi,

I have a standy generator set rated 950kVA 415V 50Hz. Recently we tried to commission the genset with the biggest load being 160kW (first block). However, the load cannot be transfered to the genset. The load is using Direct-on-line (DOL) starter. The allowable voltage dip during starting is 15%. We believe the load(with DOL starting) is too big for the genset to pick up. Is it really? The load was supposed to be 132kW during design and procurement stage. Questions:-

a) Is there anything wecan modify ( on the genset) to make the genset pick up the load?
b) We are looking at other options of motor starting to reduce the starting current ( say, 300%)so that the genset can pick it up. What would be the most either convenient, cost effective and practical solutions
c) If we choose a starting method to reduce the starting current what are the things that we need to consider

Thanks in advance for your prompt reply.

rgds,

Zak

 

[rhatcher]

Can you provide more info on what is preventing you from using this generator? Is it tripping out and, if so, is it an overload trip or an instantaneous trip? Is it being paralleled or is it a stand-alone unit?

 

[zak]

Hi rhatcher & Others,

When we simulated a power failure, the genset kicked in and changed over from normal switchboard to emergency without a problem. Then when we manually ( no auto restart/reacceleration) energised the 160kW load breaker, the load breaker remained in contact for 1-2 seconds only before it dropped of. We believe, the load with DOL starter, drew a high current which caused the voltage to drop and as a result the breaker disengaged. What do you think happened?

Thanks alot for your prompt responses.

rgds,

Zak

 

[jack6238]

Zak
Ther is a lot of information you left out of the problem definition, but here is a quick voltage drop calculation for your problem. I will assume that the motor has an inrush KVA of 6 times full load KVA. You gave the load in KW and you need to specify the load in KVA. I will assume that the 160 KW laod (~200KVA) has a starting KVA of 6 times and that the transient reactance of the machine is appromimately 20 % on the machine base. Given this information the approximate voltage drop is

% Voltage Drop = Start KVA/(Short Circuit KVA+Start KVA) *100
For your system, Start KVA is 1200
Short Circuit KVA= 950/.2=4,750 KVA

% Voltage Drop= 1200/(4750+1200)*100=20%

This should not be a problem but I do not have sufficient data on your system to evaluate the system. I have started systems where the voltage drop at start has been as much as 30 % with an accel time approaching 15 seconds. You stated that the circuit breaker opened after 1-2 seconds. Is this a circuit breaker or a starter? Most circuit breakers are not equipped with under voltage trip devices. Did you measure the voltage drop when the unit started? What type of load is the 16o KW?A fan? Pump?
Most manufacturers of gensets will furnish voltage drop data for the unit you purchased. Why not get the machine reactance data, the motor starting data and make the calculations? What type of voltage regulator is supplied with the unit? Is it working properly?
Motor starting problems on gensets is a var supply problem and is unrelated to the engine response. The var supply of the generator is directly related to the exciter and its response
Also if the the voltage drop is 20 % then you have a reduced voltage starter built into the system. The current will be less thanit would be from the normal system so it cannot be tripping from high current.
That's all
Jack

 

[jbartos]

Suggestions:
1. Soft start of large motor
2. Genset flywheel
3. Different motor with lower LRA

 

[catkw]

Using a generator sizing program, a 160 KW block load with across-the-line start and allowing only a 15% voltage dip; indicates a genset requiremet of 1100 KW. You didn't indicate if yours is diesel or natural gas. If gas,motor starting capabilities are even worse. I would try reduced voltage starter.

 

[RRaghunath]

zak,

What protection has caused breaker trip (Is it undervoltage, prolonged start or any other)?.

I think 950kVA generator is not bad for DOL starting of 160kW motor. Please check the rated starting current of the motor (PE motors have higher starting currents (say 700%), also please check that the AVR is in droop mode and not in power factor control mode. It also helps if you ensure that the motor is the first equipment to be put on the generator (after the supply changeover).

Hope it is helpful.

raghunath_n00@rediffmail.com

 

[e5fornow]

I assume if this is commissioning. The genset is under warranty. I would have the engine manf. verify gov and voltage regulator settings. It might be as simply as clogged fuel filter. Was block loading with load banks part of comissioning? I would check voltage and freq. dips. You should also verify that the voltage regulator has V per Hz regulations and that the knee freq. is properly set.

Hopefully this helps.

 

[DanDel]

Does anyone realize that this thread was started on March 13, 2001?
zak has not logged in to the forum for almost two years; I doubt he will respond to anyone's questions.