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 ?

 

[rockman7892]

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.

 

[waross]

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

 

[prc]

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.

 

[stevenal]

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.

 

[davidbeach]

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

 

[waross]

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

 

[kakil]

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 ?

 

[prc]

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

 

[kakil]

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

 

[prc]

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

 

[MatthewDB]

... 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.

 

[unclebob]

See solar storms, solar activity or solar flares. These cause much low frequency currents.

http://www.solar-storm-warning.com/solar_storm_cycle_24.html