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effect of low frequency on power transformer ?
3

effect of low frequency on power transformer ?

effect of low frequency on power transformer ?

(OP)
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 ?

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

Yep.

RE: effect of low frequency on power transformer ?

Good to know that ;)

RE: effect of low frequency on power transformer ?

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

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

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.

RE: effect of low frequency on power transformer ?

Quote (Mbrooke)

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.

RE: effect of low frequency on power transformer ?

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.

RE: effect of low frequency on power transformer ?

(OP)
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 .

RE: effect of low frequency on power transformer ?

2
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..

RE: effect of low frequency on power transformer ?

(OP)
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 ?

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

(OP)
Thank Gents ,

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

RE: effect of low frequency on power transformer ?

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

RE: effect of low frequency on power transformer ?

I have seen underfrequency protection used a lot of times on the main breakers of large industrial plants. I suspect that this serves two purposes with its primary purpose of detecting a problem with the utility source and perhaps a second purpose of protecting transformers, motors, etc... in the plant from an underfreqncny condition essentially acting as a master V/Hz protection to avoid saturation for these devices if they do not have local protection.

I have typically seen these settings at the main breaker set to about 59.5Hz (60Hz system) with a time delay ranging from .16 to .5 seconds.

Underfrequncy protection is also required in Co-Generation applications in order to prevent islanding either within the plant or having part of the utility system included as an island with the plant when an upstream utility source is disconnected.

RE: effect of low frequency on power transformer ?

As an example of the effect of saturation on a transformer let us assume a transformer with an X:R ratio of 8. The impedance will be about 8.06 time the resistance. Round it off to 8 times for simplicity.
The impedance is the result of both reactance and resistance.
Up to the saturation point, the excitation current is limited by the impedance of 8 times the resistance.
Past saturation the excitation current is limited by resistance only.
So, as an example, if the applied voltage is twice the voltage at saturation, the excitation current will be about (1 + 8) / 2 = 4.5 or 4.5 time normal excitation current. I2R will be about 20 times normal. Add to this the load current and the excess current of any loads that saturate at a lower voltage than the transformer.
A more dramatic and more often seen display of exceeding the V/Hz ratio is the case where high voltage is applied to a motor that is connected for the lower voltage. Failure often occurs within minutes.

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

RE: effect of low frequency on power transformer ?

Waross, your analysis is correct. But in reality much before such a stage is reached, the metallic structures near to core will be overheated to such an extent by the over flowing flux, that burning will start. When flux enters metallic parts, eddy loss heat up metal so rapidly that resistivity of metal also goes up reaching a thermal rundown. Copper winding near to core is also affected similarly by overflowing flux. Some of the symptoms of failure from over fluxing in transformers is blistering of painting out side transformers, melting or overheating on tie plate on core, burning of paper covering on copper conductors in winding near to core.

Kakil, To know the time Vs V/F, please refer the Electra paper mentioned in the thread mentioned by waross. You can also have some typical curves of manufacturers in Fig 30 of C37.91- Guide for Transformer Protection. Please note these are all very approximate only and this phenomena cannot be encircled in a straight formula.

RE: effect of low frequency on power transformer ?

Quote (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.

Actually, the government doesn't write the NERC rules, although there is government (FERC) oversight, direction, and approval. Industry stakeholders write and ballot the rules. Still a lot of overreach in my opinion, but UFLS regulations are good to have. Proper UFLS should arrest what would otherwise cascade into an extensive outage. Just the same, I'm sure Davidbeach had UFLS in place well before the 2007 date the standards became mandatory.

RE: effect of low frequency on power transformer ?

UFLS is long established. The 2007 FERC standards changed some tracking and some documentation, but the equipment and settings are long standing.

RE: effect of low frequency on power transformer ?

Is it safe to say that;
Under Frequency Load Shedding to maintain the system stability will generally be set at 58 Hz or higher (on a 60 Hz system).
Under Frequency Protection to avoid over fluxing a GSU will generally be set at 55 Hz or lower (on a 60 Hz system).

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

RE: effect of low frequency on power transformer ?

(OP)
Dear Waross,

yes , due to a void over flux .
but can u tell me howmany delay time setting you need to trip(I mean it should be the GSU work for some time in under freq. condition
then sending trip signal ?







RE: effect of low frequency on power transformer ?

Kakil, here is a typical setting as given in relay manual :
V/F Time (seconds)
112 600
119 60
126 20
133 8
140 4

RE: effect of low frequency on power transformer ?

(OP)
HELLO PRC,
what does it mean , 112,119,126 ....by which calculation u got it ?


RE: effect of low frequency on power transformer ?

112, etc are V/f ie overfluxing factor. 600 etc setting time. I took it from ABB Relay manual

RE: effect of low frequency on power transformer ?

Quote (waross)

... let us assume a transformer with an X:R ratio of 8. The impedance will be about 8.06 time the resistance. Round it off to 8 times for simplicity.

X/R references the leakage inductance to resistance ratio, not magnitizing inductance to resistance.

Under severe over-excitation, the impedance approaches leakage impedance. Given how low leakage impedance typically is, current can rise to very high levels.

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