Transformer Inrush Current 5

[redtrumpet]

Anybody know of any references for estimating or calculating transformer inrush current for purposes of protective device coordination? Rule of thumb I have seen is 12 times full-load current for 0.1 s. I have also seen other recommendations of 3 times FLC for 10 s and 25 times FLC for 0.01s. Are these generally adequate estimations?

 

[nbucska]

I don't think you have to worry about TRF's inrush current.
Any transient is faster than the 60 Hz -- so on the same inductance you will have LESS current.

So, inductors don't have inrush current as capacitors or motors.
<nbucska@pcperipherals.com>

 

[peterb]

12x FLC for 0.1 sec is a well established rule of thumb and should be adequate for most situations.

 

[stevenal]

My reference is the Cooper Fusing Manual.
Cold load inrush:

2X at 100
3X at 10
6X at 1

And hot load inrush:

12X at 0.1
25X at 0.01

 

[redtrumpet]

Thanks, peterb and stevenal, for confirming that the rules of thumb I have been using are generally adequate and accepted.

 

[redtrumpet]

nbucska - transformers do have an inrush current. A good explanation is given in the old Westinghouse Applied Protective Relaying book:

&quot;The maximum inrush current occurs if the transformer is energized when the voltage wave is near zero. At this time, the current and flux should be a maximum in the highly inductive circuit, and a half wave of voltage requires a flux change of twice the maximum flux. If there is a residual flux left in the bank from the previous energization, it may either add or subtract from the total flux, thereby increasing or decreasing the inrush current. Power transformers are operated normally near the knee of the saturation curve, so the additional flux requirement of zero-voltage-wave energization plus any residual flux will definitely saturate the iron and increase the magnetizing current components.&quot;

&quot;The inrush current decays rapidly for the first few cycles, and then very slowly, sometimes taking 4 or 5 seconds to subside, where the resistance is low. The time constant of the circuit (L/R) is not a constant because L is variable because of the transformer saturation. During the first few cycles, the saturation is high; hence, L is low. As the losses damp the circuit, the saturation becomes less and L increases.&quot;

 

[stevenal]

The Westinghouse explanation applies to what Cooper calls hot load. The longer cold-load inrush is when the transformer and comnnected load are reenergized after an outage. Loss of diversity and motor starting cause the current to remain high. You still have the short duration hot load magnetizing inrush whether the transformer load is actually hot, cold, or disconnected. Cooper's term may be confusing this way.

 

[jbartos]

Suggestion: The transformer inrush current will vary depending on the transformer parameters, load, surge capacitors, feeder length, upstream source internal impedance(s), power supply quality, etc. The safety code is giving one some minimum safety requirements that you may exceed, if you are concerned with the transformer safety and lifecycle.

 

[nbucska]

Redtrumpet :
You are right, I neglected the transient. On the other hand
the inrush current of the load ( cold incandescent lamps
and starting motors ) will be much more -- and I neglegted this, too.

<nbucska@pcperipherals.com>

 

[cuky2000]

The Inrush current in power transformers occurs when the polarity and magnitude of the residual flux do not agree with the polarity and magnitude of the instantaneous value of steady-state flux in the transformer core.

Values of inrush current had been register during energization of the unit varying between 8 to 30 time the full load current with duration from 10 cycles in small units to 1 min in large transformers.

Inrush current had been identified to be the root cause of false tripping in 75% of survey response performed in various companies by IEEE. Some of them those who reported not difficulties with Inrush current used relatively long time (1/3 to ½ sec.) or decrease sensitivity, or both.

There are two approach commonly used in the industry to draw the protective device selectivity coordination curve passing between the equipment damage curve and the Inrush criteria describe as follow:
1- One Inrush point:
At 0.1 second for example the Inrush current will be Kx FLC of the power transformer
Were: K= 12 for sizes above 3MVA selft cooling rating or K=8 for <3 MVA rating. Many engineers choose use K= 12 regardless the size of transformer.

2- Set of Inrush points or Inrush curve:
Redtrumpet and Stevenal describe typical sets of point on posted on 9/701.
For modeling purposes the following relation provide an approximated Inrush transformer curve as follow:

For t<5 Ipu=6.t^-0.31.
t>5 Ipu=5.t^-0.6 – 1.8.

If a fine accuracy is required for special applications, the transformer may be modeled using computer program such as PSPICE, EMTP, etc. to determine the magnetizing current and other core parameters. Other alternative is to work with the manufacturer to provide this information from the testing or design parameter such as of steel used on transformer, core design, etc.

 

[sawsan]

Does group charging of a number of transformers all at same time through one larger size tranformer harms the transformer due to higher inrush current

 

[coingy]

The rule of thumb I've always used is 6 x for large transformers, say, 2MVA and above. The smaller transformeres due to the larger inductances can be 14 x.

 

[Sajith]

Here's a small side question. This is a practical necessity. Can we load a 6.3 MVA 11/3.3 kV transformer from a 6 MW Turbo generator already loaded to 3.5 MW ? Will the TG trip on the inrush current ?

 

[gordonl]

If the transformer is already energized and your synchronizing in on it there will be no in-rush. If your picking up the transformer from dead the generator protection will have to be co-ordinated with the inrush if it is the only source energizing the transformer.

 

[Sajith]

The generator does not have an instantaneous Over current relay. So I hope there'll be no problem if the AVR can cope with the situation, which is equipped with series compounding diodes.

 

Can anybody provide some links for the transformer inrush current topics on-line? Thanks.

 

[electricpete]

http://selinc.com/techpprs/6025.pdf

(got to page 6)

http://www.geindustrial.com/industrialsystems/pm/notes/artsci/art11.pdf

(pages 15 and 16)

Also take a look at some of the transformer items at:

http://www.geindustrial.com/industrialsystems/pm/notes/index.htm

 

[electricpete]

I have posted here a simulation of transformer inrush current which I believe shows the math why the inrush looks like it does. Previous versions of this page had a garbled file... I fixed that.

http://geocities.com/pschimpf/temporary_stuff/transformer_inrush_simulation.htm

 

[StevieC]

I must apologise in advance for having very little transformer starting knowledge. I have a problem with a 4225 kVA gas engine generator at 11kV trying to 'Black Start' a large factory after a power outage. The gas engine can only take 25% load step in one hit. The site has 8 x 1000 kVA 11 kV to 433 V transformers and 6 x 1500 kVA 11 kV to 433V transformers. I need to try to find out if this one gas engine generator can start (magnetize) all 14 trans- formers when their secondaries are disconnected or alternatively how many transformers can be started (magnetized) if the secondaries cannot be disconnected. Does any one have any advice on this or can you point me in the right direction.