Motor starting with genset 4

[reactive]

Some comments from those of you with experience on gensets would be appreciated.

Client has bought an AVK diesel generator with the following ratings:
6,6kV
5300kVA @ pf 0,83
1500rpm
Unfortunately I do not have any further detail at this stage as the whereabouts of the manuals are a mystery.

Based on the above limited data is it possible to determine whether the gen will start a 1650kW synchronous motor DOL (also do not have much detail at this stage)?

While I can see that the gen kVA (current) rating would be exceeded for a short time the real power requirement should be OK.

Also, what would be the effect of regeneration from the motor (part of a Ward Leonard minewinder set) if the winder was the only load connected to the generator.

 

[TomG33]

Mr Reactive,
I am no help to you with the Genset part of this question, that is not in my area of experience.

Regarding the Regeneration question, You should specify if the mine winder is a "Balanced Winder" or an un balanced winder.

Usually a balanced winder has two skip cars, one on either side of the drum so that when One comes up filled with product, the other empty one is going down. In this situation under normal conditions the winder will almost always be doing motoring work , with regenreation just transiently with speed changes and such. A GenSet would possibly be able to handle this.

an unbalanced winder usually has the one cage. there is a counter weight but the value is fixed.

In this situation the winder is motoring to make the cage go one way and regenerating all the other way. ( which way is which, depends if the counterweight is heavier or lighter then the maximum load)

From What I have heard, Diesel Gensets are not all that good at absorbing Regenerative power. Especially sustained regenreation such as encountered by the unbalanced winder application.

Regards
Tom

 

[bigamp]

Reactive,

There is not sufficient data to be able to predict the performance. The problem will be voltage dip during start-up. From "Diesel Generator Handbook" by LLJ Mahon, the initial dip is calculated as follows:

Vd(%) = 100Xd'/(Xd'+c)

where:

Xd' = generator unsaturated transient reactance in per unit
c = ratio of generator kVA rating / Impact Load kVA

So, you need to know the generator transient reactance and the motor starting current. The initial dip will occur irrespective of generator excitation system and AVR, but the AVR should be able to take care of it fairly well. If possible, check with the generator manufacturer, who is normally able to provide good data on motor starting performance for his generators.

Could speculate using "typical" data but it will not really mean much as there is a wide range of possible starting current and generator Xd'. You really need to get the data.

I agree with what you say about the kW load, the governor should be able to handle it no problem.

 

[rbulsara]

Refer to faq237-766 posted by me for approximation. For exact analysis use a computer software like SKM.

It is also important to know the load inertia to predict the acceleration time.

However judging rules of thumb, you would be better off and almost need a reduced voltage starter.

 

[jbartos]

Suggestion: Reference:
IEEE Std 141-1993 Red Book
Section: 3.11.6 Effect of Motor Starting on Generators
Figure 3-14 Typical Generator Voltage Behavior due to Full-Voltage Starting of a Motor

 

[jbartos]

Suggestion: The generator manufacturer tech support may provide the most accurate information as to what size of a synchronous motor may be started by the generator. The motor nameplate data will be required to provide the generator manufacturer or whoever will model this application on a computer.

 

[reactive]

Thanks everyone. Trying to lay my hands on manufacturer's data.

Any comments on the regen? Winder is balanced so regen only occurs during braking.

Obviously generators are rated by kW and kVA. Is there a standard PF that they are rated at?

I would assume that the kW rating cannot be exceeded (limited by mechanical strength and engine output?) but that the kVA output (thermal)can be exceeded for a limited time as indicated in rbulsara's FAQ.

Practically how would you control say 8 generators in parallel (operating as an island), PV method? Would there need to be a centralised control system or could the gensets be set to control themselves independently?

 

[rbulsara]

I would suggest you engage services of good consulting engineering firm.

There is quite a few items requiring proper engineering and planning for proper parallel operation of so many generators.

It includes but not limited to:

Protective Relaying, Load control/shedding scheme, alarms, what to monitor and control (SCADA system), design proper maintainability (access for service), battery system and not to mention your site specific requirements.

 

[SEENA]

Reactive,

You could also refer to "The Amps Guide to ISO 8528 (BS 7698)"
RECIPROCATING INTERNAL COMBUSTION ENGINE DRIVEN ALTERNATIVE CURRENT GENERATING SET - Part 5: Specification for generating sets.

Which has typical calculation with examples but again as said by bigamp you need the generator manufacturer data to arrive at the right figures.

 

[jbartos]

Suggestions to reactive (Electrical) Feb 11, 2004 marked ///\\
Any comments on the regen?
///The rotating machinery of larger sizes have to have two-sided VFD. The two-sided means four-quadrant operational VFD. Some manufacturers also name it Active Front End (AFE) motor drives. It is justified if there is enough energy to be return to the power supply. There is also higher cost involved in procuring 4-quadrant motor drives, e.g. 60% over the standard, e.g. 6-pulse motor speed drives.\\ Winder is balanced so regen only occurs during braking.
///This is promising; especially, if there is a lot of energy dissipated during brakings.\\Obviously generators are rated by kW and kVA. Is there a standard PF that they are rated at?
///The standard pf is .08=PF. Other PFs are available and must be specified, e.g. 0.85.\\I would assume that the kW rating cannot be exceeded (limited by mechanical strength and engine output?) but that the kVA output (thermal)can be exceeded for a limited time as indicated in rbulsara's FAQ.
///The generators are permitted to run about 10% over their rated kW or kVA for a limited time stated by the genset manufacturers, based on temperature limitations.\\Practically how would you control say 8 generators in parallel (operating as an island), PV method? Would there need to be a centralised control system or could the gensets be set to control themselves independently?
///Both are feasible. Decentralized controls has its advantages as well as centralized controls. It would be nice to be able to procure both.\\

 

[GerH]

I would guess that this genset will start the motor with only minor commissioning setup.

The X'd Transient Reactance of a generator of this size will be about 20%. So you could expect up to about 40% voltage reduction at the instant of switching on.

Risk of genset overload => underspeed trip depends on the run-up time for the drive.

Starting will be improved if you can pre-load the genset to say 25%+. In fact this might be essential. It affects both voltage and engine speed - the internal generator voltage will start from a high value and the fuel control will start part open.

 

[reactive]

Thanks all

I am not planning to design a control/protection system for 8 generators in parallel. This was for my own interest only and is actually a different job to my first post (sorry if I confused you). I only have to design PFC for the genset network.

My brief is to design PFC to bring the load close to unity (this is for the mine with 8 sets). I have not yet started the study but am told that they have four sets rated 6kV and four rated 6,6kV which they run in parallel. I can only assume that the unity load would help the 6kV sets achieve the 6,6kV voltage with more stability. Sounds darn untidy to me to run a generator 10% above it's rated voltage. Any comments.

PS The site is in Angola so I would assume that for a time anything went as long as it worked.

Also, can anybody comment on my query in my first post regarding absorbtion of regen by a single generator.