TX backfeed inrush currents? 3

[pavement]

I was speaking to a colleague yesterday, when he mentioned that it is possible for a TX to have a greater inrush mutiplier when backfeeding through it, in order provide emergency measures.Has anybody else encountered this over there experience and any reason why it happens that the inrush multiplier is higher say 10, than normal inrush currents i.e 3-5??

 

[pavement]

Sorry in addition to the above post, just to clarify if you were using non-directional overcurrent highset overcurrent protection, im i right in saying that you would have to account for both situations i,e normal and reverse if carrying out the protection cordination studies? Just for future reference in case i come across it myself.


Many Thanks

 

[dpc]

Normal inrush current is 8 to 10 times full load current for about 0.1 seconds as a rule of thumb. Doesn't really matter which side you energize, except if you really want to be able to energize from the low side, both the low and high side protection need to be able to handle the inrush. That will crimp your use of instantaneous OC protection.

 

[Rodmcm]

Our experience is that the magnetising inrush is about 20-30% of load rating, we have recently back livened an 11kV/415V 1500kVA via a 415V generator and the maximum inrush was about 30% with th 11kV open

 

[ctpt]

Was the inrush 30% of load current @415V?

 

[digitrex]

Do you have highset overcurrent protection at both primary and secondary side? I thought normally the instantaneous highset OC protection is only applied at the primary side.

But anyway, set your highset OC protection to a value 1.5x normal inrush current, i.e. 12..15x rated current.

Rodmcm's experience(inrush current = 1.3x FLC) may be true in a way, if he reads that inrush current from a panel ammeter. A panel ammeter cannot display actual inrush current that lasts for less than0.1s.

 

[GOswitch]

Just to get the understanding of the inrush phenomenon see the link:

http://www.allaboutcircuits.com/vol_2/chpt_9/12.html

The page explains the inrush phenomenon beautifully.

 

[Rodmcm]

Digitrex, you are correct, it was from a panel meter, however, in terms of generator response and protection tripping the inrush in the first few cycles is almost meaningless.
To ctpt, yes the current was measured at the 415V side.

 

[cuky2000]

QUESTION 1: Why transformers have a greater inrush multiplier when back feeding through it….

ANSWER 1: According with the transformer committee of IEEE/PES, there are significant inrush current differences when back feeding a transformer. Below is a summary of the inrush range multipliers based in the nominal full load rated current.

Peak Inrush Current in PU of Nominal Current for Power Transformer
Without External Impedance.
-----------------------------------------------------------------------------------------
DESCRIPTION STEP-UP STEP-DOWN
-----------------------------------------------------------------------------------------
Delta connected primary 30-45 19-25

Grounded Y Primary 50-60 30-35
-----------------------------------------------------------------------------------------



Please notice that the above values are for step-up and step-down units. However, the similarity with a single transformer still applies.

QUESTION 2: ….the inrush multiplier is higher say 10, than normal inrush currents i.e 3-5??

ANSWER 2: Beware that inrush is a random factor that primarily depends on the point of the voltage waveform at which the switch/breaker closes, as well as on the sign and value of the residual magnetic flux in the transformer core.

The rms inrush current is a fraction of the peak values determined in a particular period of time. For instance, the “standard” rms inrush points multipliers of transformer (rated) full-load current are: 35x, 12x, 6x, 3x, at 0.01, 0.1, 1.0, 10 seconds respectively.

QUESTION 3: Any reason why it happens (Why transformers have a greater inrush multiplier when back feeding through it…).

ANSWER 3: The above results shows that the inrush current are higher if the transformer is energizing from the LV side regardless the type of connections with higher values for grounded Y primary configuration.

One possible reason is the core will be saturated easier if energized from the low voltage side were the high current occur. Keeping in mind the magnetic flux increase with the current.