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 Available Short Circuit Current is based on the % impedance of a hot transformer. It is mainly limited by the reactance of the transformer.
The peak offset current is limited by the resistance of the transformer.
There is a noticeable difference between the cold and hot resistance of the windings.
The worst case is energizing a cold transformer with a fully offset fault current.

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

 

[cuky2000]

FLC=3000kVA/(1.73x115kV)=15.06

Generally accepted inrush & load pickup currents for dist. transf.

Inrush current:
25xFLC for 0.01 seconds............(25x150.06=3765.3A)

Hot load pickup:
12x to 15x full load for 0.1 second.(12x150.06= 1807.04)

Cold load pickup:
6x full load for 1 second.:..........(6x150.06= 903.07)
3x full load for 10 seconds .........(3x150.06= 451.8)
2x full load for 100 to 300 seconds ..(2x150.06= 301.2)

 

[cuky2000]

Correction. FLC=30000kVA/(1.73x115kV)=150.06A

 

[Mbrooke]

Really needed that hot load. Any idea if this is based on PSC compressors?

 

[cuky2000]

I am not sure if this help but here are some general ideas related to hot load based on PSC compressors:

1) Motor with Permanent Split Capacitor (PSC) have less starting torque than the regular motors ant the inrush starting current (LRA).
2) PSC is used in small motor up to 1 HP usually single phase.
3) Hot load pickup current occurs when the protective device trips and recloses maintaining the diversity factor.
4) Therefore, the number of pre-outage cyclic loads is essentially the same as the number of post-outage cyclic loads.

Can you describe the specific application of this post?

 

[waross]

Are we discussing large transformers or small PSC compressors?

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

 

[Mbrooke]

@Waross- use your imagination- don't assume I'm working off apophenia. Such assumptions have always stifled human progress. Google "fault induced delayed voltage recovery (FIDVR)"


Most residential AC units and refrigerators use PSC motors meaning when a voltage dip or brief loss of power occurs the compressor will stall and not start turning again when voltage resumes to normal due to the high refrigerant head pressure. This causes the motor to draw its locked rotor amps until the thermal protector opens 5-20 seconds latter.

This mean a residential air conditioner may draw 8-13 amps when running on a hot day, but 80-120 amps when the motor stalls.

Now imagine 100s or 1000s of AC units and refrigerators running on a typical distribution substation near capacity. Source trips for 5 seconds and recloses. Every PSC refrigeration motor will draw 8-12 times its RLA (running load amps). The power fuses protecting the transformer must handle this surge without blowing.

 

[jghrist]

I'm not sure of the relevance of the discussion of hot transformers and PSC compressors to the OP question.

As indicated on the graph, the inrush and cold load pickup points are simply based on generally accepted multiples of full load current at different times 25x at 0.01 sec, 12x at 0.1 sec, 6x at 1 sec, 3x at 10 sec and 2x at 900 sec. These can be changed in the program to other values if you have better information for a particular case.

 

[Mbrooke]

Yes- but stalled AC motors can either add or deduct those values depending on how these commonly accepted values are derived. In short am asking how they were derived- I'm clueless.

 

[prc]

Those figures are for thermal overload capacity. It is assumed that average winding temperature after a short time loading shall not exceed 250C for oil filled transformers with copper winding. (IEC 60076-5)With aluminum, temperature shall be limited to 200C. The assumption is extra I2R heat generated from over current is completely stored in the copper of the winding. For periods > 2sec, lower limits are considered.

 

[Mbrooke]

Thanks

Before its transferred to the cooler windings and oil- does this assume cold core or hot core?

 

[stevenal]

The words "hot" and "cold" modify the word "load". Nothing was derived here, the numbers come from experience (generally accepted). Keep protection above these values to avoid nuisance tripping. Adjust if needed due to the nature of your load, but keep protection below the lower portion of the damage curve. See Cuky's and jghrist's posts.

 

[Mbrooke]

So I want to ask- how are these specific generally accepted values derived? How do they vary from what other members have seen in practice. My opinion? They are on the extreme side.

 

[stevenal]

Reclose and watch the current. Note the values at 0.01 and 0.1 s. Repeat as needed to capture the worst case offset.
Turn load off long enough to lose diversity, and close. Note the values at 1 and 10 s. Repeat.
Adjust all values for load growth and security margin.

I haven't seen the 900s point used before, and that one does seem rather extreme as your graph shows.

Are you really using fuses on a 115 kV 30 MVA unit?

 

[Mbrooke]

Noted and agree.

What would be an argument against fuse protection for a 30MVA unit? Increased probability of catastrophic failure?

 

[waross]

@Waross- use your imagination- don't assume I'm working off apophenia.

Point taken.
But without explanation it did look as if you had high-jacked your own thread.
From "Transformer Magnetizing Inrush" of large transformers to PSC motors is a bit of a jump without an explanation.
First event, "Transformer Magnetizing Inrush". This is the first recovery of voltage, almost vertical on the graph of FIDVR.
Followed by "Fault-Induced Delayed Voltage Recovery (FIDVR)".
Two different effects, one much shorter than the other.

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

 

[Mbrooke]

Don't think someone can hijack what they are steering- but I do understand that an on topic thread is easier to follow.

I thought with all the concern of stalled AC units and their effects on blackouts and near miss blackouts in the last 25 years that it would have been self-evident- but then again thats from my vantage point. Something I think about... Anyway youd be surprised how much stalled motors load down a feeder after a re-close.

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.

 

[stevenal]

Yes. Fuses can't do differential protection or restricted earth fault protection. They can't sense zero or negative sequence current for load insensitive tripping, and cannot be set to trip with instantaneous or definite time settings. In Blackburn's day, 10 MVA was the cutoff. With microprocessor relays putting all the functions into a single box, it makes sense to move that down some.

Is 30 the self or forced cooled rating?

 

[Mbrooke]

Forced cool rating. 18/25/30 MVA


What drives the cutoff? I can fuse up to 60MVA.


The way I see it if a unit fails it fails and needs to be taken out of service. A secondary bus fault will clear the primary before damage can result to the trafo.