Transformer Inrush Current
Transformer Inrush Current
(OP)
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?






RE: Transformer 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>
RE: Transformer Inrush Current
RE: Transformer Inrush Current
Cold load inrush:
2X at 100
3X at 10
6X at 1
And hot load inrush:
12X at 0.1
25X at 0.01
RE: Transformer Inrush Current
RE: Transformer Inrush Current
"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."
"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."
RE: Transformer Inrush Current
RE: Transformer Inrush Current
RE: Transformer Inrush Current
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>
RE: Transformer Inrush Current
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.
RE: Transformer Inrush Current
RE: Transformer Inrush Current
RE: Transformer Inrush Current
RE: Transformer Inrush Current
RE: Transformer Inrush Current
RE: Transformer Inrush Current
RE: Transformer Inrush Current
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
RE: Transformer Inrush Current
http://geocities.com/pschimpf/temporary_stuff/transformer_inrush_simulation.htm
RE: Transformer Inrush Current