Protection of LV Generators with LV Molded Case Breakers

[rockman7892]

I wanted to hear the approach that others take when settings adjustable LV molded case breaekrs in order to protect LV 480V generators. I have always been told that the breaker should be set so that the breaker curve falls below the generator damage point on the TCC and so that the breaker short time range crosses the generators decrement curve at some point. The damage curve and decrement curve that I am refering to are the ones created automatically be commercial sofware such as SKM, etc.. depending on generator size and other paramaters.

Does anyone have a specific methodology or reference to how these breakers should be set in order to protect generator and allow in most cases selective coordination with downstream breakers. Do most people use the decrement curve and damage point created by commercial software, or do they build custom damage curves and decrement curves based of generator datasheets?

 

[waross]

The sets that I am familiar with, up to about 1600 KVA, have the breaker sized to carry full load current. (Often 125% of the set current rating). The instantaneous setting should be as low as possible.
Prime power sets allow a 10% overload for one hour and this may be considered in sizing the breaker.
Under conditions of heavy overload most sets tend to stall the engine. This drops the frequency and the UFRO becomes active, reducing the voltage. This reduces the current.
Short circuits: Often a close in fault will cause voltage collapse. This drops the current to the point that reliable tripping of the over current protection may not be assured.
With voltage collapse the frequency may remain high and UFRO will not operate.
There are accessories to prevent voltage collapse, such as "Permanent Magnet Generator" options and CT derived AVR boost.
Note: With very low voltage the current will be low and the resulting I²R losses in the generator will be due to the low I and the low R of the generator.
Note 2: Small single phase sets, are often three phase sets that have been reconnected with no other changes. As a result single phase sets above about 20 KVA may be overpowered and be less elf protecting on heavy overloads than three phase sets. At 480 Volts this will not be an issue.

Bill
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