Transformer Magnetizing Inrush 5

[Mbrooke]

Does anyone know how S&C derives transformer magnetizing inrush and hot/cold load inrush values? They seem to be on the extreme side, but then again they may have a point in terms of a feeder trip and reclose with residual magnetism in the core.

 

[waross]

The magnetizing inrush will be the close to the same regardless of the rating.
There are exceptions but the duration of the magnetizing inrush may be measured in cycles.
The magnetizing inrush may be much greater than the load inrush.
Again there are exceptions but the the load inrush follows the magnetizing inrush and is much less.
The load inrush may be measured in seconds. (How long does it take for the thermal trips an a PSC motor to operate.?)
Your fuse must with stand both a short high current peak followed by a lower but longer load inrush.

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

 

[Mbrooke]

Typically within a few seconds, but in theory with depressed voltage longer.

 

[cuky2000]

It is understood that transformer protection is a compromising between the above factors, economics, reliability, tradition among other factors.

Many utility in the US prefer protect distribution transformer in a substation with circuit breaker and relays, some of them with circuit switchers and less common today with fuses particularly for application above 15 kV.

Transformer protection with fuses for 69 kV and above are unusual in modern installations.

Please let us know your thoughts in this matter.

 

[Mbrooke]

Why is fuse protection of lesser preference? Why go through the trouble of relays, batteries, circuit switcher, ect?

FWIW a lot of old US installation just used an air break switch and relied on direct transfer trip or fault induced tripping. Larger substations like 345kv to 138kv as an example TX relaying would just trip the busbar.

 

[crshears]

Normally the trafo in question would see only half load being part of a normally open tie, but loaded to its max rating for the other unit out of service pushes protection to its limits.

Not only that, but if the correct operation of downstream components is predicated on there being two paralleled HV sources of supply and one of them is out of service, the reduced fault current infeed capability due to the impedance of the supply being doubled may simply not provide enough fault current for relay schemes to operate effectively.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[stevenal]

The nameplate impedance, and therefore the damage curve and inrush are all based on the base (self cooled) transformer rating in IEEE/ANSI land. Suggest entering 18000 in Coordinade.

Fuses, being single phase devices, are likely to single phase a transformer and the served loads.
Yes a failed transformer needs to be taken out of service. Do you then repair or scrap it? See IEEE C37.91.

 

[Mbrooke]

I agree, but it shifts the damage curve into unrealistic boundaries.

 

[crshears]

Hi Mbrooke, I forgot to mention earlier: I like that word "apophenia"; I had to look that one up...and soon realized I've been apophenic myself on more than one occasion...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

Sometimes it can be a blessing. Great human discoveries have taken place connecting two seemingly irrelevant concepts together.

 

[crshears]

Hmmm...does the reality that they seemed irrelevant to each other but were in fact not render them anapophenic?

Just askin'...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

Hmmmm- that is a good question... perhaps yes in that a specific case was presented where the presenter knew it was correlated... but then again our minds often choose not to make connections or ignore them entirely... I mean if we saw meaning in everything whether it real or not I'd think our brains would go into information processing overload...


You make me ponder

 

[jrostron]

There are cases where the inrush current exceeds that calculated by transformer impedance. This is when there is residual magnetism in the core. This residual magnetism is a common occurrence but if frequently ignored because it usually is complicated to simulate accurately. It is real however.

In this case, the inrush currents can exceed the transformer through fault magnitude by several times. The magnitude can approach that of the fault current in the circuit since for half the time, it is only the air core inductance of the transformer limiting current.

Here is a link for estimating the resultant voltage drop that occur when this occurs. It is quite easy to use for a first look.

http://www.southernstatesllc.com/applications/connecting-solar-to-the-grid

These voltage dips are a big deal for solar farm connection agreements according to IEEE 1547.

 

[Mbrooke]

Thanks

Any idea what the worse case value might look like and how long it may last? I'd certainly hate for three $3000 fuse units to blow from reclosing or switching.

 

[waross]

One rule of thumb for magnetizing inrush is 20 to 25 times FLC.
Up to the saturation region, the current is limited by the combined resistance and inductive reactance.
That part of the voltage above saturation causes a current that is limited by the combined resistance and the air core inductive reactance.
Thus the resulting very high peak current.

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

 

[jrostron]

Yes these peaks indeed can be very high indeed. They can last several seconds depending on circuit conditions although these inrush currents are single polarity rather than a true RMS waveshape. The longest times they exist are where there are multiple transformers of similar size on the same circuit. Here there is resonant condition that exists and low circuit resistance that enables them to persist quite long.

What circuit voltage and currents are your applications?

 

[Mbrooke]

115kv, 30MVA units.

 

[jrostron]

Is you issue blowing fuses or reducing voltage dips? I would suggest looking and trying out the link in my earlier post. It is focused on evaluating voltage dips.

One customer had the issue of blowing fuses and this was resolved with the Transwitcher of Southern States. This link will show some possible alternatives to consider to limit inrush.

 

[Mbrooke]

Blowing fuses. I don't want a transmission line trip and reclose to take out a 200E fuse.

 

[jrostron]

It is starting to sound like it is more than just transformer inrush causing the problem. If motors are stalled, they do indeed have high starting currents and they can be there for several seconds, hence causing FIDVR as was discussed earlier in this thread.

Is it just transformer inrush or is your problem starting stalled motors from a high speed reclosing operation? They are significantly different problems to solve.

 

[waross]

I'd certainly hate for three $3000 fuse units to blow from reclosing or switching.

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