Generator and Transformer V/Hz Capability - standard curve? 2

[Truant]

Hello Everyone,

I am working on confirming some generator and transformer protection coordination and I am having trouble finding manufacturer "V/Hz capability" curves for some of our older equipment.

Do any of you know if there is an industry standard for building a standard type curve?

I have looked through IEEE searching for 'V/Hz' and 'over-fluxing' capability, but haven't found anything.

Thanks for the help!


EDIT: I found plenty of protection setting recommendations (eg. in "C37.102 IEEE Guide for AC Generator Protection", but the NERC standard says specifically to coordinate with generator and transformer V/Hz capability. Which I don't have.

 

[waross]

The rated voltage and the rated frequency determine the Volts per Hertz ratio. On grid supplied applications the saturation point is of most concern. If a motor or generator is to be used with a different frequency, the Volts per Hertz ratio is calculated and the ratio is then used to determine the proper voltage at the new frequency.
A note on generators. Generator windings may be capable of 115 Volts, 120 Volts and 138 Volts on 60 Hz systems.
Similarly the individual winding rating may be a selection of possible voltages on 50 Hz systems. Use the highest V/Hz figure at either frequency to determine the safe maximum voltage.
Example; 138 Volts is often the highest rated voltage for a single winding in a 60 Hz generator. (137.5 V x 2 = 277 V. Used for 277:480 volt generators.
138V/60Hz = 2.3 Volts per Hertz. 2.3 V/Hz at 50 Hz = 115 Volts. In the event that the generator is rated for a voltage higher than 115 Volts at 50 Hz, Use that voltage and frequency to calculate the maximum safe V/Hz ratio. That ratio may now be applied to determine the maximum safe voltage at 60 Hz.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Generator transformer over fluxing (V/F) capability is generally 110 % continuous, 125 % for one minute and 140 % for 5 seconds with a working flux density of 1.72 T at rated voltage and frequency. Frankly nobody has tested this but for the past 50 years we are considering like that based on calculations and simulations etc. In case the working flux density is different, the with stand factors should be modified to the same extent, percentage wise.

The first "capability curve" was prepared by GE Pittsfield engineers ( see G W Alexander, Influence of design and operating practices on excitation of generator step up transformers, AIEE Vol PAS 85-8,1966 Aug, Pages 901-909 Later CIGRE Electra ( 1973 No 31 December) published a paper adding a curve from old VDE 0532/8.64 plus AIEE curve. Updated one is in 1983, May Electra "Final Report of study committee 12" page 42.

CEGB during 1970's was adopting the following settings for GTs- relay actuation at 1.12 & 1.06 of rated V/f for transformer rated flux density of 1.7 T &1.8 T. The relay used to actuate within 4 sec and trip out at 20 sec. So the settings should be much below the "capability" curves.

 

[waross]

With respect for your excellent post, prc may I add that the rated voltage and frequency, and most notable the rated voltage may be considerably higher than the nameplate rating.
I see generators name-plated at 120 Volts, but the voltage on which your over-fluxing percentages are based on may be around 140 Volts.
This to add to your post, not to contradict it in any way. The name-plated voltage may not be the voltage of interest.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

waross, The point you raised is not clear to me. The over fluxing settings shall be based on the rated voltage of transformer ie same as the indication in name plate of transformer. Understand that in US, rated LV voltage of GTs is some times less than that of generator.(See C57.116-2014 Transformers directly connected to Generators) Hence my clarification.

 

[waross]

I am considering generators not transformers. Specifically smaller multi voltage sets.
The same generator end may have different rated nameplate voltages applied.
A set may be capable of both 120:208 Volt service and 277:480 volt service. (With reconnection of course.)
The nominal 120 Volt winding will be subject to 277 Volt / 2 = 138 Volts when used for 480 Volt service.
I was pointing out that in the case of generators, unlike transformers, the voltage of interest may be greater than the nameplate voltage.
The original question was to do with both transformers and generators.
Respectfully.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Truant]

Sorry guys, not quite what I'm looking for.

For larger transformers and generators 100MVA+, there is a specific capability called 'over-fluxing' which results from either an increase in voltage, a decrease in system frequency, or a combination. Obviously a decease in system frequency only occurs during a massive, system wide stability even, or an islanding event.

The V/Hz is usually expressed in PU, so 1, and has protection controls normally set at 1.05 and 1.1. But, the new NERC standard says specifically to coordinate with the manufacture supplied capability curves, which I don't have.

 

[ScottyUK]

Without the manufacturer's curves you seem a bit stuck TBH. You will almost certainly find that the GSU transformer V/Hz limit dominates that of the generator, so if you can find data for the transformer then in my opinion you won't need to worry about the generator while it is connected to the GSU transformer. If the generator and GSU transformer aren't close-coupled and you have a circuit breaker at generator voltage rather than transmission voltage then the generator's V/Hz protection will protect it while the machine is not synchronised to the grid.

 

[Truant]

Thanks Scotty,

I have 1 example where I found the generator and GSU V/Hz curves on a 200MVA unit and you are right, the GSU is more sensitive than the generator.

Ill try my best to find the capability curves of the GSU, but hopefully standard settings will suffice to meet compliance.

 

[bacon4life]

All of our transformer are specified per IEEE C57.12.00, which includes the 1.05 and 1.10 V/Hz limits as part of the definition of normal service for transformers. My reading is that PRC-024 only requires documenting more detailed manufacturer recommendations if the transformer V/Hz results in violating the limits on the attachment-1 graph.

As a side note, a during a transformer factory acceptance test, the manufacturer tested excitation current all the way up 1.15 PU voltage as part of their normal testing routine. I remember the voltage staying at 1.15 PU for several minutes, but there was no official documentation from them of any capability beyond the C57.12.00 requirements.

 

[Truant]

Hi Bacon,

I am actually referring to PRC-019, which states

"Equipment limits, types of limiters and protection functions which could be coordinated include:

Volts per hertz limiter and associated protection functions.

Generator and transformer volts per hertz capability"

The first bullet point is easy, standard protection coordination, but the second bullet point is where I am getting hung up on. We have standard settings that follow IEEE C57.12 but no manufacture capability curves.

 

[prc]

Truant, you were reading from a 'manufacturer' who is giving such capability curves for large transformers for the past 50 years and knows well how other manufacturers are giving this! Earlier(ie 20 years back) in US, manufacturers were adopting a rated flux density of 1.7 to 1.8 T. Nowadays it is limited to 1.7 T or less due to steep loss capitalization rates. So on safer side assume your GSU is with 1.8 T and adopt settings accordingly. All these concerns on over fluxing of GSU came after a few large GSUs (more than 300 MVA) failed in US during 1960's during the first station start up run when excitation went hay wire with lower frequency causing over fluxing and transformer breakdown.

 

[ScottyUK]

Hi prc, thanks for the historical background on the origins of this requirement.

 

[Outdoors]

Truant,
I just joined this forum after reading your thread. I am in your same situation working with PRC-019 and have been looking for Generator, GSU and UAT V/Hz manufacture curves. Have you found anything for your unit's or what have you done?

 

[edison123]

If the V/Hz values are in PU basis and standards specify these PU values for protection settings, why would one need the manufacturer's curves?

prc - a lps.

Muthu

http://www.edison.co.in

 

[waross]

The V/Hz is usually expressed in PU, so 1, and has protection controls normally set at 1.05 and 1.1. But, the new NERC standard says specifically to coordinate with the manufacture supplied capability curves, which I don't have.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Outdoors]

Waross,
Thank you for your feedback. We have both Westinghouse and GE Generators and Transformers at our site. I have some contacts with GE that we are working with to find information. Do you know of anyone that I could contact about the Westinghouse units? These are 1950's and 1960's vintage Generators and Transformers.

 

[waross]

Hello Outdoors;
As this thread progressed, it became apparent that my experience is with much smaller sets than the sets in question.
A large purpose built generator and unit transformer have different design factors than a small multi voltage set.
A large set will be designed for a specific voltage. The V/Hz limit will be based on the design voltage.
A small multi voltage set will often have nominal 120 Volt windings that are actually capable of generating about 139 Volts to allow use as a 480 Volt set. The V/Hz limit will be based on the higher voltage rating.
In a large set, over fluxing and exceeding the V/Hz capability may result in damage to the GSU.
In a small set, over fluxing at lower speeds/frequencies is prevented by the UFRO feature on the AVR of smaller sets.
In the case of low speed operation and over fluxing with an AVR that does not have the UFRO feature or in which the UFRO has been disabled or configured incorrectly, the point of failure is almost always the AVR.
Before UFRO became a common feature, AVR failure at low speeds was common on smaller sets.
The point is;-
You are best guided by the other posters who have direct experience with larger sets and access to the appropriate standards.
I am in the wrong ball park here.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Outdoors,

Siemens took over Westinghouse in the late 1990s. After trading as Siemens-Westinghouse they're now listed as:

Siemens Energy, Inc.
4400 Alafaya Trail
Orlando, US
phone: +1 (407) 736-2000
support.energy@siemens.com

Their records are reasonably good in my experience.

 

[prc]

Scotty, are you referring to transformers or generators? Transformer was taken over by ABB in 1980's (eg Muncie Plant in USA). Since outdoors are referring to GSUs of 1950&60 vintage, better to be pessimistic as those days a higher flux density was usual in GSUs.