Insulation Class for Generator windings 2

[SENGUTTUVAN]

For one of the Emergency DG set ( 1600 KW), the specification indicates Class F insulation and Overtemperature rise class B.

One of the vendor has offered for Class H insulation and Overtemperature rise class B.

Class H insulation will give better life than Class F . Is it OK to stick on to Class F insulation or based on the KW rating is it advisable to go for Class H insulation. How much will be approx cost impact between the two.

Thanks to clarify

 

[edison123]

What is the voltage ?

 

[electricpete]

In my opinion if you intend to operate the machine within it's rating, then Class Finsulation with Class B temperature rise already has PLENTY of thermal margin. On top of that remember thermal aging is a function of run-time, and Emergency DG is not an application that I expect to see a lot of run-time. From what you've told us I don't see any reason to pay extra for class H myself.

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[Hoxton]

We must not get confused between class of insulation material and class of permitted temperature rise.

I think that a higher class insulation material automatically meets that required for a lower class. (So a machine rated to Class F can used Class H material, and so on).

So a Class H material can be classified as Class F, as long as Class F temperature permitted rises are not exceeded.

But you cannot use class F material with class H temperature rise (except by agreement)

I say this with caution, since I do not have access at the moment to the latest Standards on this.

For your emergency diesel set, I assume that a mass produced generator is used. These are usually wound with Class H insulation material, for short term standby service.

If a user wants some margin, then they may specify a lower class of temperature rise.

Say Class F rise or Class B rise. The lower temperature gives a longer anticipated insulation life. The other benefit can be lower generator losses (better efficiency, because of the way the efficiency curve works) in relation to the lower rating.

The generator manufacturer does not use lower standard materials if you specify Class F insulation, he offers a standard, machine, wound with Class H materials, but selected for Class B rises for the KVA specified.

He can declare it Class F material, he is just using better material and does not have to tell.

I think your vendor has just offered his standard product, but rated for the lower temperature rise specified.

I hope this ramble is clear!

 

[ScottyUK]

Hoxton,

Yes, I agree that a class H machine is likely the standard design. A true class F machine would likely be a 'special' and would probably attract a price premium in spite of being a lower spec. If a vendor offered a class H winding with a class B rise I wouldn't be concerned - the chances are the other vendors are also offering a class H winding but have chosen to declare the rating so as to meet the class F spec you requested rather than be seen to exceed it.

There are plenty other things to worry about on a diesel set rather than whether the winding is class F or class H when you are so far below either limit - the quality and performance of the AVR and the engine for a start!


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[SENGUTTUVAN]

Thanks to electricpete , hoxton & scottyuk for replying and I had got very useful tips from all. The voltage is 415V.

Thank you very much hoxton for your detailed reply and I have clearly understood now.

ScottyUk, as rightly pointed by you , while reviewing the offer the cost aspect is also one of the main factor. As indicated by you , can you please give me the checklist to review the offer for an alternator and the diesel engine. This will be verymuch helpto me for reviewing the technical offers. Thanks to help.

 

[ScottyUK]

The spec you issued would be the first thing to review - upload the spec and we'll have a look. Often it isn't what the vendor declares as spotting all the little things they 'forgot' to declare which is the key when reviewing a spec.

Who's engine are you looking at and what model is it - there are couple of guys who frequent this forum who can likely comment on specifics of the engine design if you tell them what model it is.


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

 

[SENGUTTUVAN]

Thanks ScottyUK. I have uploaded the filled in data sheet by one of the bidder. In this pl highlight which are the details to be looked into carefully for evaluation .This will be helpful for me to check all other offers also.

With regard to the make of the alternator and the engine there is no specific mark given. Different Vendors have quoted for Engines of MTU, Cummins & Mitsubishi and for alternators - Marelli & Leroy somer make.

Thanks to help and for your efforts

 

[ScottyUK]

A few thoughts on the spec:

The rated temperature is variously quoted as 54, 55, and 57 Deg. C. Which one is it?

Control voltage of 110V DC is unusual for a small generator; I'd expected 24V DC. 24V DC for the starter at least gives you the chance to jump-start the generator from a truck / crane / forklift if you are in an emergency situtation (last time this happened at site we used a crane ). 110V DC doesn't give you the option.

Is the AVR a 3-phase averaging type and tolerant of non-linear loads? I expect it will be on a machine this size but worth checking.

Brushless excitation isn't as tolerant of transient loads as a PMG type. If the AVR draws upon the generator output for power then it will have (relatively) poor field forcing during fault conditions.
This is one reason for the poor recovery after a step load - 5 seconds to 95% voltage isn't a particularly impressive response for a small set.

No mention of the engine / governor arrangement. I would be concerned about the engine in such a hot and dusty environment: filters for combustion air and cooling air; fin-fan cooler for circulating water; etc.

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

 

[edison123]

Scotty

How do you field force in a brushless excitation ?

 

[ScottyUK]

One technique uses a grossly oversized excitation rectifier and adds a signficant resistance into the exciter field, thus modifying the L/R time constant. Under dynamic conditions the AVR can achieve a fast response by operating on the steep initial part of the L/R curve. It is very lossy, especially on bigger machines.


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