kVA rating discripancy 2

[DmanD]

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

 

[davidbeach]

High difference? I don't see anything out of line there. Of much more interest would be the kW comparison, but 2:1 is not at all out of line, probably lots of inrush to consider and somebody didn't want too much voltage sag on motor starting.

 

[DmanD]

PLease explain DAvid, the power going to motors through drives. The max power delivered to drive is through rectifier only. So the max power calculated from max rectifier should take care of it. Why does it need to be double?

 

[ScottyUK]

Controlled rectifiers are notoriously awkward loads for generators to deal with. Small gensets usually have a harder time than large ones because the larger sets typically have a better AVR than their smaller cousins. That's just down to economics - a decent AVR is an expensive item of kit, and it is easier to justify on a large high value set. Small generators typically don't have much headroom in the magnetic design for non-ideal operating conditions such as crappy power factor and harmonic-rich loads. Again economics works against small sets - the market is much more competitive, so any money saved in the core and windings is important to those manufacturers.


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Sometimes I only open my mouth to swap feet...

 

[DmanD]

I have MX321 AVR in my genset and it is the best I believe. Does the controlled rectifier have very high harmonics? Still that would be included in its overload capacity of 125% isn't it?

Still pondering
D'man

 

[Skogsgurra]

Part of my job is to explain to people why the upgrade to higher power levels in harbour cranes doesn't work. The reason is always (I think at least four cases over the last ten years) that the impedance of the generator is so high that the commutation notches from the controlled rectifier interferes with operation. Or, in other words, the harmonics contents gets too high. Not the harmonics from the delayed firing but harmonics from the commutation of the rectifier. I have some recordings from such a crane. Will try to find them and show them in a following posting. They are quite revealing.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Here it is:

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

skogs; Not following you...
[highlight]"Part of my job is to explain to people why the upgrade to higher power levels in harbour cranes doesn't work."[/highlight]

Followed by:
[highlight]"A stiffer mains (or a much larger generator) would not give you this kind of problem."[/highlight]

Would you please clarify this a little.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

OK. But it will be a lengthy one...

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ozmosis]

Are we sure this is a controlled rectifier? Reading this thread, the first mention of a controlled rectifier came from Scotty. It is possible the drive system could be on an uncontrolled rectifier with inverters drives (in the true sense of the name 'inverter'i.e. DC to AC control) connected on the common DC bus with the Brake Unit in situ on the bus. It is highly possible I'm way off mark here but there could be different interpretations to a 'rectifier drive': Controlled rectifier thyristers (SCR) and AFE (transistors) as well as uncontrolled (diode bridge) with inverters + brake chopper/resistors etc. Each method would have different effects on the supply generator.

 

[Skogsgurra]

Good catch! The mentioning of a DC bus in the OP points to an inverter. D'man has the answer.

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Hi sed2,

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...

 

[ozmosis]

Certainly no crtitism going your way Scotty or to Gunnar for his interesting points.
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.

 

[DmanD]

Hi All,
[tab]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

[tab]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

[tab]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

 

[DmanD]

Hi,
[tab]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

 

[cbarn24050]

Hi, yes it does to some extent. Your rectifier diodes only conduct for 120 degrees so each phase can only supply power for 2/3 of the time at full load. The notches noted above "overlap period" are caused by the supplys source reactance the higher the reactance the wider they become. With simple diodes these notches move to the begining of the wavform and so reduce the maximum conduction period of each diode.

 

[DmanD]

Hi,
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!