effect of low frequency on power transformer ? 3

[kakil]

Hello Engineers ,

I wana to understand what will happen for the 200MVA power transformer if it worked under nominal frequency for some time ?
I mean why there is UF protection ? and how many time is safe for it ?

 

[itsmoked]

Under-frequency leads to magnetic core saturation where the magnetic circuit part of the transformer simply cannot carry further magnetic field. This results in non-linear transformer action that creates non-sinusoidal waveforms and current that is no longer limited by inductance. This means greater current than expected will flow thru the windings causing I²R losses to increase rapidly. The transformer efficiency plummets resulting in greater heat rejection that overheats everything associated with the transformer; oil, windings, insulation system, containment, etc.

The harmonics associated with the non-sinusoidal waveforms also stresses other power equipment subjected to it.

As for UF protection I cannot answer that other than to say, depending on the actual loading of a transformer the extra heat caused by under frequency could be no problem or could push it into destructive thermal areas.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Under frequency protection on a power transformer may be a form of load shedding to assist the system to recover from overloads.
In this case the protection setting may be at a level that does not pose a threat to the transformer but does indicate a grid problem.
Under frequency protection on a Generator Step Up transformer is to protect the transformer from saturation in the event that the generator is overexcited.
The issue is not so much the frequency as the Volts per Hertz ratio. If under frequency allows the Volts per Hertz ratio to rise too far, the transformer may saturate and overheat due to excess magnetizing current. Transformer burn out may result.


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

 

[Mbrooke]

So 138kv 60Hz transformer can operate at 115kv 50Hz?

 

[davidbeach]

Yep.

 

[Mbrooke]

Good to know that

 

[waross]

A side question David. Do you use under frequency load shedding on your system?


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

 

[davidbeach]

Yes, we have an extensive UFLS program, that's a NERC compliance requirement. It has never actually needed to shed any load though.

 

[HamburgerHelper]

I know this is a sidetrack but load though does help in reducing the excitation/saturation of a transformer.

 

[waross]

Thank you David.
I saw under frequency load shedding used on the main 140 kV service to a large mine mill on Vancouver Island. The island was fed by a combination of hydro power and undersea cables from the mainland.
In the event that the island became "islanded" the load may exceed local generating capacity. If the island became islanded and the frequency started to drop, the main incoming breaker for the mill would be tripped. A further drop in frequency would cause a lockout with a more stringent protocol for going back online.

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

 

[Mbrooke]

Why is NERC so strict? Can't load be shed manually via central control? With automatic shedding you could end up shedding to much load or at the incorrect frequency.

 

[davidbeach]

Why is NERC so strict?

Because FERC says so. Our UFLS scheme is part of a coordinated wide area plan among many utilities, it includes multiple steps and a certain amount of auto restored load if the scheme overshoots. Where we are a major wide area problem is likely to leave us in an overfrequency condition and the Chief Joe brake will slow the system as generation is dropped.

 

[Mbrooke]

Its better when governments don't make the rules imho lol. But I'll take your word the study has the the load shedding fine tuned.

 

[kakil]

Thanks Engineers , for the kind reply .
But why the UF is limited by a time ? I wana the equation that approve the time setting for this protection?
to be the transformer not expose more to UF .

 

[prc]

Voltage E= 4.44x flux volume in core x frequency x number of turns. Since voltage remains same, but frequency goes down means flux volume increases (since core area remains same, B,flux density goes up) When B moves above 1.7T ,normal flux density,losses in core increases not linearly but many fold. As B reach 2 T, core cannot contain flux any more and overflows in to tank ,core structure and copper winding near to core. This creates severe overheating due to eddy losses that can even melt metals. It is seen that time of over fluxing (ie % reduction in F) is limited by 125 % for 1 minute and 140 % for 5 seconds. More than that time, stored heat in metals will up the temperature above permissible limits..

 

[kakil]

Thanks PRC ,

do u have a curve between the time and the freq. ?

because for example if the rated freq is 50Hz ,and the the transformer if expose to 47 Hz How I can know the time it should not be more than for example 5 sec ?

OR is there any formula that give me the time limit for UF ?

 

[waross]

That is only 6% low. That is within the safety factor for the transformer. The protection setting should be determined by the requirement to protect the system and should coordinate with the settings of other UF protection devices in the system.
When Uf protection is applied to Power Transformers it is to protect the grid or system, not the transformer.
Generator Step Up transformers are a different case altogether. GSUs are often direct connected to the generator. There have been instances where slow speed of the prime mover and over excitation of the generator has destroyed Generator Step Up transformers.
This may happen very rapidly. There is no way to disconnect the transformer from the generator. Protection must act by reducing the excitation of the generator.


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

 

[waross]

kakil
Look at this thread:
Generator and Transformer V/Hz Capability - standard curve? 2
thread238-407663
And particularly this post by prc:

prc (Electrical)
3 May 16 04:18
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.

DOES NOT APPLY TO POWER TRANSFORMERS.

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

 

[kakil]

Thank Gents ,

I need the formula that tell me that the time delay is:5 sec for example not more to protect the GSU ?

 

[waross]

We both posted at the same time.
One of the differences between a power transformer and a GSU is the type of protection needed.
Check the thread that I referred to.

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