Generator Loadings

[johnlp]

I have a Deisel generator which is 408Kw with a maximum load of 709 Amps.

How can i tell what load the generator will take at startup, as obviously until fully run up the generator will not take its full load.

 

[jbartos]

Eng-tip: It depends how it is specified, manufactured, and "nameplated." There are gensets that start on the 50% load block. The genset can be manufactured to start on the 100% load block. Contact the genset manufacturer tech support and check the genset literature.

 

[alehman]

Standby generator sets manufactured for compliance with NFPA 110 for life safety systems (most in the U.S.) must accept 100% of rated load in one step and within 10 seconds of the start signal. Special provisions are needed to assure this can happen such as engine jacket heaters.

Many manufacturers include "starting kVA" in their specifications which provides an indication of the motor starting capability. Starting kVA is usually based on 30% voltage dip.

 

[BBB609]

In past step load (NFPA 100%)testing I have seen voltage regulator (static) stall engine. Change to volts per hertz regulator on same unit and it passed. Calcs on starting KVA are some what subjective and a lot of the variables can be manipulated. The sequence the loads are started also matters as does slight time delays. Field testing may produce much different results than calcs if you need to make an existing unit work. If you have luxury of trying different seqences a loaded unit may have stiffer voltage for starting motors.

 

[RajT]

Johnlp,

I assume that your are referring to a diesel set.
In a normal application of gen sets,it is not wise to design a system where the set is started with the load (ie motor loads) connected to it on the onset.
The reason being that the large starting kVA drawn by the motors will pull the gen volts too low for the motor to accelerate properley to reach the synchronese speed which would result in excessive run up time and thus damaging the motor due to insufficient cooling and subject to the size of the motor even stall the set.
In your case of the gen rated at 400kW, the max motor rating thet can be started with the set at rated speed would be about 110kW as a rule of thumb ( ie the gen has to be 3 1/2 times the motor rating to accelerate safely while maintaining a volt drop not exceeding -25% with a nominal cable leangth).

So the biggest motor you can have connected to your system
( allowing for say a 100m cable run volt drop ) would be 110kW for normal application.

Now assume that your set is only designed to connect to one motor only and that this motor is 200kW, so how do you start this motor, as if you did switch the motor on with the set, at rated speed, then the gen set or the motor would stall as explained earlier.The only solution then would be to start the gen ( with the motor connected) and increase the excitation in manual slowly ( trial and error)on a ramp up time such that the motor thermal time is not exceeded with the motor accelerating to its synch speed without stalling.

Contarary to previous poasts,diesel sets are NOT designed to take full(100%) gen kW rating even when the set is running at full speed and no manufacturer will guarantee it either.Not only that but there are no standards(any country)which requires that a set of a unit rating must be capable of starting (or take on 100% load when at rated speed) of the same rating in one unit step, (though it can obviously shed the load of the same rating). The normal "step on load " is limited to 60% of gen rating in kW or pure active load, ie if the figure of 60% is exceeded then a larger set to suit should be specified.

 

[alehman]

RajT,
In the US, life safety rated gensets (most units supplied) must be capable of accepting and field test for 100% load block per NFPA 110 (there is no votlage dip criteria however). We routinely peroform this test on new units and rarely experience failure. The justification for requiring the test is obviously open to question but it required.

NFPA 110-2002:
7.13.7 A full load test shall be initiated immediately after the cooling time specified in 7.13.5 by any method that starts the prime mover and, immediately upon reaching rated rpm, picks up 100 percent of the nameplate kW rating on one step, less applicable derating factors for site conditions.

 

[jbartos]

Comment: Some DG sets are designed to accept 100% step load. If there is a standby generator and a preferred power source suddenly becomes unavailable, then the standby generator may be specified to accept and power load of 100% of its rating. The genset may also have a flywheel.