Overload capacity of Generator

[sakaran51]

Hi everyone,
I would like to know whether the overload capacity of the generator is considered while sizing the Diesel Engine Generator by the DG manufacturers in their software programs.
As per IEC standard, generator shall be capable of withstanding 1.5 times the full load current for 30 seconds.Or only 15 seconds? For Prime rated engine 10% overload capacity is considered.But for generator? .

 

[waross]

A standby set does not have overload capacity. A prime power set is sized at 110% and has 10% overload capacity. The kW overload capacity depends on the capacity of the prime mover. Draw too many kW and the engine will slow down. Rated current is limited by heating. Rather than overload capacity, the software predicts the voltage dip When a given load is connected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sakaran51]

Starting of a highest rated motor with already running base load will result in excessive over current than FLC of generator.I was wondering whether the over current capability of generator is also considered apart from starting voltage dip in order not to over size the generator.

 

[waross]

When a heavy load is dumped onto a generator the generator impedance changes dynamically in response to the load current and the action of the Automatic Voltage Regulator.
Generator sizing software considers the resulting voltage dip caused by motor starting.
The old school rule of thumb is that a reserve capacity of three times the rated motor current will provide acceptable starting performance.
The sizing software, based on voltage drop, often sizes the generator somewhat larger.
In extreme cases, a reserve of 2.5 times the rated motor current will start the motor but the voltage dip may be objectionable.
The smaller the motor starting load in relation to the base load, the lower the voltage dip will be.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[squeeky]

For protection and tripping we look to the AVR. Our preferred AVR has two additional settings. Very high for up to 10 seconds and high up to 120 seconds. The rating is a percentage of the defined excitation current. The high allows you to start the large motor etc., so it will boost the excitation current for a specific duration for a specific value. This allows you to start a star delta motor without a reduction in voltage. The very high setting allows the voltage to remain high to generate sufficient fault current for the protection to operate. This is so that under fault conditions, the high current does not lead to a voltage drop which leads to a lower current, lower voltage etc., etc., and then there is not enough current to trip the protection.

Selecting a generator AC to meet your requirements requires a control philosophy where the starting sequence of plant is stated along with their kilowatts. A simple addition of kilowatts may lead to short falls or over compensation.