VSD (Electric Drives) & Energy Efficiency 1

[argentox]

There are a number of variable speed drive technologies out there. While some have a uniform power factor close to unity across the operational range (such as the VSIs), others go from anywhere around 0.4 to 0.8 (such as the Cyclos).

So, is any one type of electrical variable speed drive SIGNIFICANTLY more efficient than the other?

Thank you,

Eric

http://www.MarineEngineeringManagement.com

 

[DickDV]

The energy savings is in the LOAD thru a reduction in speed. The drives and motors all waste energy thru losses that present themselves as heat.

I am only familiar with VFD's and it may be true that some technologies waste more than others but, as far as I can see, given the same speed reduction in the load, the energy savings there should be pretty similar across the technologies.

 

[argentox]

Thank you DickDV for your prompt reply. I understand the use of VFD for load reduction, or rather adjustable load rather than static. However, different types of VFDs operate across a range of pf. VSI type drives usually being the one closest to unity.

Hence, does anyone know why and by how much? Is it really worth investigating?

http://www.MarineEngineeringManagement.com

 

[LionelHutz]

I'd think the typical VFD with a diode rectifier and an IGBT inverter would be the most efficient. Those devices are quite good and have low voltage drops.

 

[argentox]

Thank you LionelHutz for your insight. I share your thought. Do we know by how much? I have a 20MW load driven by VFDs so a small difference may end up to be a big difference.

http://www.MarineEngineeringManagement.com

 

[DickDV]

20MW!!!!! Now that changes things a bit. Up to maybe 4000hp, a common VFD as described above is certainly the mainstream design and typically has between 2 and 3% of the load kw in losses.

Above that hp range, there are other designs with their own experts (not me!).

So, how you've got that 20MW divided up makes a lot of difference.

 

[cswilson]

Keep in mind that efficiency and power factor are very different things, and don't necessarily vary together.

Curt Wilson
Delta Tau Data Systems

 

[jraef]

As said in another forum you posted in, don't equate power factor to drive efficiency, they are separate issues.

It seems we have to drag information out of you. Is this a single 20MW motor or 20 1MW motors or 200 10kW motors? It makes a differnce because LSI drives are only worth discussing if the motor is (are) in the MW range.

Cycloconverters are ancient history, don't waste time on them in my opinion.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

An LCI is probably the most practical solution for a single multi-MW load, ABB's Megadrive or GE's LC2100 being a couple of examples.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[argentox]

Thank you all for your comments and help.
Yes, I have 2x10MW propulsion motors driven by a Cycloconverter and the pf going in is bad, specially at low RPM. There is not much I can do there.

Nevertheless, for future builds... I will also have 2x20MW propulsion motors and yes, I will opt for a VSI. Hence the pf will be much better.

I also agree and understand that pf and efficiency is NOT the same. Nevertheless, do we agree that a pf of 0.5 is "bad"?
I think it is bad because a pf of 0.5 is not as 'efficient' as a pf 0.9.... but please correct me if I am wrong.

So, if my thinking that a power factor of 0.5 is 'bad'... how bad is it? Can I simply calculate how many more amps I am using and calculate the losses? (I frankly do not think it is so simple).

Thank you again for your support.

 

[Skogsgurra]

I assume (sorry Jeff) that your motors are synchronous motors. The current consumption of such a motor is directly proportional to torque delivered by the motor since the excitation is input via the slip-rings.

The cyclo-converter selects 'snippets' of the grid voltage and outputs them to the motor so that the motor voltage is proportional to speed and matches the EMF produced by the rotating externally excited rotor.

The snippets of the grid voltage that are delivered to the motor will be taken close to zero crossing of the grid sine-wave if the speed is low and progressively closer to the sine's peak as voltage demand (speed) grows when the motor is accelerating. This is what makes your PF bad at low speeds and better at higher speeds.

I wouldn't say that you are using more current than needed at low speeds. You do use the current needed to produce the torque needed by the propellers and that torque is never higher at low speeds than it is when running with full steam. And the power consumed by the cyclo converter/motor combination is not a lot higher than needed to move the ship. Actually, the cyclo converter is very efficient since there is just one set of thyristors between grid and motor and the switching losses are very low because the switching takes place at a low pace.

There is no reason to bother about a low power factor in the propulsion of a ship. At least not as long as it doesn't lead to increased current at low speeds. The reason is that you are using internal (to the ship) cables, bus-bars, transformers and switch-gear and needn't pay any extra for the low PF and you do not heat your equipment extra due to the low PF.

Instead, what you should worry about is the energy (equals oil consumed) consumption of the propulsion system. And that is quite low when you are running a synchronous motor from a cyclo converter.

It is true that you, perhaps, could reduce the current draw at low speeds and thereby reduce losses in the cables and transformers. But, since the current as such is already low at low speeds (affinity laws), the losses are also low (proportional to current squared).

You have a rather one-eyed view on this. As has been said several times in this thread, PF does not have anything (not much, anyhow) with efficiency to do. And as long as you are using a local and self-contained system and there's no penalty for the 'bad' PF, you really shouldn't worry.


Gunnar Englund

http://www.gke.org

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

 

[argentox]

Thank you Gunnar for your excellent reply. I feel I am making progress. Your assumption is right, they are synchronous motors.

You said: "instead, what you should worry about is the energy (equals oil consumed) consumption of the propulsion system. And that is quite low when you are running a synchronous motor from a cyclo converter" ...how does the cyclo-synch propulsion at low speeds compare to a VSI-synch propulsion in terms of efficiency? How much better is it?

Cheers.

 

[Skogsgurra]

If you look at the cycko converter, it only has one set of thyristors between grid and motor while the VSI (voltage source inverter) has one set of diodes or (in case of an AFE) one inverter in reverse, then a DC link with capacitors and, after that, the inverter that produces PWM for the motor.

Thyristors have a rather low forward voltage drop and the switching losses are low. That makes the cyclo converter a very efficient plant.

The rectifier in the VFD input stage usually have a voltage drop at their rated current that is something like 1.2 - 1.6 volts. Two of them always work in series and that alone produces losses that are 3 times rated current.

Then, there is the ripple current in the capacitors in the DC link. That ripple current squared times the capacitor's ESR creates another component in the loss budget.

The IGBTs in a VFD switch at a much higher rate than the thyristors in the cyclo converter and, therefore, have higher losses. They also have conduction (forward) losses.

Sometimes, there are du/dt filters and (not very often) sine filters between inverter and motor. They also create losses.

Losses produce heat and that heat has to be removed. There are examples where air conditioning is being used to remove that heat and that consumes a lot of energy.

For motors in the 10+ MW range, I think that cyclo converters (or CSI) is a good choice and there is definitely no need to replace them with less efficient VSI systems.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

Yes, for multi-MW loads the LCI is probably the best choice. An example if you want to read a bit more about them would be ABB's Megadrive.

I am surprised that you don't have flicker problems on your islanded generation system if you have two large cycloconverters. Most distribution utilities would be unhappy at the prospect of two cycloconverters of this size on their system because of the likely effects on other consumers.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[LionelHutz]

Ya, Like others have posted, the power factor on the ship doesn't matter much. Power factor matters when you are being billed for have a poor power factor. The other time it matters is if you want to free up capability - if you increase the power factor then the the same transformer (for example) can transfer more real power and real power can be used to do work.

On a ship, it could easily be worth while to increase the size of some components to allow the low power factor since the gain in efficiency more than makes up for it. At 40MW of power, a gain of 1% in the efficiency is 400kW.

I agree with Gunner, the cyclo-converter should easily be the more efficient controller compared to a VSI. He did not mention the transformer or transformers in a cyclo-converter system, however a VSI that size quite likely has magnetics as well.

If you want to research a different drive topology then LCI should be the one you investigate. I don't believe a VSI is worth looking at for those power levels.

 

[VSDPRO]

I would be considering a Robicon 4 quadrant Inverter for this duty, the power factor is very close to unity, and will clean your power supply on regeneration if any at all.

 

[ozmosis]

For this level of power on a ship then LCI is often the technology to look at, for a number of reasons:
1) Efficiency is very high compared to IGBT/IGCT technology
2) 4Q operation as standard
3) more importantly on a big ship: reliability is excellent. They are bomb-proof when it comes to supply dips and short-circuits.

 

[LionelHutz]

ozmosis - that is something I didn't post but an LCI is a good choice from a reliability standpoint. My personal feeling is properly applied thyristors are many times more rugged than IGBTs.

Another to add is that a LCI and synchronous motor make a good combination.

 

[ScottyUK]

Part of the reason for the inherent reliability of an LCI is that the DC link inductor limits the rate-of-rise of fault current so that protective gear can operate to trip the incoming breaker or inhibit the front end gate drive before the current reaches a dangerous level. VSIs don't have this luxury so the semiconductor switch usually blows to protect the ultra-fast fuse.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Skogsgurra]

A short addition to what is being said above: The LCI is also called a CSI (Current Source Inverter) in some quarters. So, LCI = CSI.

Gunnar Englund

http://www.gke.org

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