kVA rating discripancy
kVA rating discripancy
(OP)
Hi,
This question is baffling me ever since I am working in this crane.
This crane lifts containers of max. load 40T. It uses rectifier-drive for control of all motors. The kVA rating of the diesel generator set is 670kVA. CAlculating the max kVA load including all auxillary drives, it comes to 301kVA.
The only thing that I have some doubt is - when the load is lowered the regenerated energy is diverted to a resistor bank to be dissipated as heat. But the resistor bank gets the dc from dc bus only as the rectifier cannot send it back to the ac input side.
So, this high difference between generator capacity and load is due to what reason?
D'man
This question is baffling me ever since I am working in this crane.
This crane lifts containers of max. load 40T. It uses rectifier-drive for control of all motors. The kVA rating of the diesel generator set is 670kVA. CAlculating the max kVA load including all auxillary drives, it comes to 301kVA.
The only thing that I have some doubt is - when the load is lowered the regenerated energy is diverted to a resistor bank to be dissipated as heat. But the resistor bank gets the dc from dc bus only as the rectifier cannot send it back to the ac input side.
So, this high difference between generator capacity and load is due to what reason?
D'man





RE: kVA rating discripancy
RE: kVA rating discripancy
RE: kVA rating discripancy
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Sometimes I only open my mouth to swap feet...
RE: kVA rating discripancy
Still pondering
D'man
RE: kVA rating discripancy
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kVA rating discripancy
What you see is the three "sine" shaped mains voltages. This is under heavy lifting and the generator was a little more than half loaded. If loading more, the internal supervising circuit tripped the controlled rectifier saying "mains problem".
A stiffer mains (or a much larger generator) would not give you this kind of problem.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kVA rating discripancy
"Part of my job is to explain to people why the upgrade to higher power levels in harbour cranes doesn't work."
Followed by:
"A stiffer mains (or a much larger generator) would not give you this kind of problem."
Would you please clarify this a little.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: kVA rating discripancy
The controlled rectifier switches from one phase to another. The switching (delay angle) determines what voltage is output to the motor. 30 - 90 degrees produce a positive (hoisting) voltage and 90 - 150 degrees lowers the load (negative voltage).
A conducting thyristor cannot be switched off in any other way than taking current down below holding current. There are several ways of doing it. In a single-phase system, you just wait for voltage to reverse or (if load is inductive) current to become zero. In a three-phase system, things get more complicated. If you fire the next thyristor while the actual thyristor is conducting (it always is when you have continuous current in the armature) then you get a short across the phases.
This short carries as much current as you have in the armature (inductive, constant current for the duration of the short) and the current in the conducting thyristor is reduced while the current in the recently fired thyristor is building up. The time it takes to switch from one thyristor to the next is called "commutation time" and during this time, the two phases are short-circuited. The resulting voltage is the mean voltage of the two voltages involved. That is why you get the deep notches shown in the picture.
The commutation time, the width of the notches, is determined by the impedance of the sourcing grid. The impedamnce is mainly inductive (generator X') and a high value makes the notch wider than a small value. The width is usually controlled by putting commutating reactors before the rectifier. In many cranes, the generator X' serves as reactors. It is so high that additional reactors are not needed or possible.
The supervising circuitry in the controlled rectifier has a limit as to what it can accept before saying "Enough!" and shutting the system down. Since the depth of the notch is independent of current and reactance, it is the width (which determines both higher component harmonics contents and RMS of the resulting voltage) that is critical.
By using a generator with higher rated output, the X' is reduced, the notch width reduced and the rectifier is happier. That's why a higher rated generator helps. Or one with a lower X' - but usually not possible to reduce X' for a given generator size.
There are also other things to consider - a crane usually needs to handle dynamic overloads and if the generator cannot handle that as good as motor and rectifier can, then the crane is not going to be popular with the harbour people. It may ultimately result in the harbour losing customers. It happened in the harbour where the picture in the earlier posting was taken.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kVA rating discripancy
RE: kVA rating discripancy
Anyhow, with all power factors and efficiences told, a slightly oversized generator is not a bad thing to have. Sometimes the next smaller one is too small for reliable operation. I see no problem with the configuration D'man has. Cranes are usually "semi-standard" with chassis, wheels, power system, control cabin etcetera kept the same and it is possible that this platform can be equipped with drives with a somewhat higher capacity (newton-wise or velocity-wise) - and than the generator will definitely not be oversized. Not sure that is true in this case, but possible.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: kVA rating discripancy
Yes, guilty as charged! I don't know many crane applications where an AC drive has been employed, but cranes aren't my forte by any means.
Nice post skogs - even if it turns out to not be directly relevant to this thread, it made for good reading.
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Sometimes I only open my mouth to swap feet...
RE: kVA rating discripancy
From our company's point of view, the preferred choice would be AFE+inverter(s) on both ship-shore and also the motorised container handlers where inverters connected on a common DC bus from AFE would be also driving the wheels.
RE: kVA rating discripancy
It has become a great topic due to your super contribution. Kudos to you all. There is a flaw in my explanation that has caused you some confusion. EXTREMELY SORRY skogsgurra, scottyuk and sed2developer
The rectifier gets 3-Phase AC supply from the alternator(DGset). The output of rectifier is sent to the Hoist, Trolley and Gantry drives(inverters) by a common DC bus. On this DC bus, the chopper is connected which diverts the regenerated power to be dissipated in the form of heat energy. You are on bullseye sed2developer
Since, power source of the drives is only rectifier, I must be correct in calculating max power input of the rectifier. Given the explanation of skogsgurra is correct, the alternator output being more than double of max rated expenditure is a biiig lump difficult to gulp.
Thanks a lot
D'man
RE: kVA rating discripancy
Your conclusions are absolutely correct Gunnar Englund. The waveforms that I got were similar in four of my cranes having rectifier drives.
Special Thanks Gunnar
But does it hold true for the rectifiers which supply dc to the dc bus common to three drives? Because in this case the rectifier is not of responsive type.
D'man
RE: kVA rating discripancy
RE: kVA rating discripancy
I got a smoother waveform with notches of shorter time period which is considered within the 25%percent overload capacity of the rectifier.
I guess the answer to my question belongs to some other aspect of the crane design - its gross nature!