Equivelent Thevinin impedance mddel for Load Flow and Motor Starting

[rockman7892]

Related to my other post I am looking into replacing an existing unit substation consisting of a 12.47kV primary, 5MVA transformer, and 2.4kV secondary switchgear.

The secondary 2.4kV switchgear feeds several large motors as well as some new motors that are being added so I wanted to evaluate new transformer size based on load flow and motor starting (5MVA size stated above is rough estimate for now).

This unit substation is downstream in the distribution system in the customers plant so I wanted to include upstream source impedance for voltage drop across transformer for motor starting as well as voltage drop impact on buses on primary of transformer for voltage starting.

I have an old copy of the system study from which I was going to use upstream fault contribution to model source equivalent but the portion of the study I was given only had the equipment evaluation table showing interrupting and momentary calculations. These interrupting and momentary calculations have multiplying factors applied to them based on ANSI SC standard so I am questioning how useful these values would be for using to determine an upstream equivalent impedance.

Would using these values to model upstream source impedance be accurate for determining voltage drop at secondary of transformer as well as voltage impact on primary system buses? I'm not sure how I would necessarily model the primary buses to evaluate impact since these would be somehow combined in that source impedance?

 

[7anoter4]

In my opinion, the short-circuit aparent power of 12.47 kV System it is between 200 to 500 MVA.
There are - I think -2 problems -in order to maintain 5% voltage drop in the steady state case, and 15% in the case of the largest motor start:
1)If the load is 5 MVA you need a 7-8 MVA rated transformer.
2)preferable do not start D.O.L. more than one 0.5 MW induction motor.

 

[krisys]

When you have interrupting and momentary values and the multiplication factor, source impedance and the X/R value can be easily evaluated. Thus you have the parameters you are looking for.

 

[rockman7892]

krisys

The excerpt from the report I am looking at only gives the momentary and interrupting SC values but does not give the multiplying factors. Per ANSI standard the momentary multiplication factor is based on X/R ratio. So without knowing multiplying factor or X/R ratio is not possible to evaluate source impedance? Am I missing something?

Would it be reasonable to use ANSI multiplying factor of 1.6X for asymmetrical momentary value to back calculate E/Z momentary network fault current to use for evaluating source impedance?

7anoter4 - does that 15% voltage drop for motor starting only apply to bus where starting motor is connected or does it also apply to all buses in system? I had always thought that starting motor terminal voltage can tolerate 30% voltage drop however all other system buses(where motors are running in steady state) should have no more that 15% voltage drop?

 

[7anoter4]

You are right,rockman7892.This was only an example.
As standard for voltage limits for motors according MG-1 and IEC 60034-1 a minimum +/- 10% is
ok.This it could be at switchgear voltage-but namely at any of running motor terminal when other motor is starting.
For the motor starting terminal voltage no standard- as far as I know-it states the drop limit.
The higher the engine is required, the higher the voltage it has to provide.
If we see the starting T=f(speed) curve the pull-up torque has to be larger than the load torque at this speed. The torque is direct proportional with the square of voltage so the pull-up torque it is also.
If we take from IEC 60034-12 Table 1 Minumum values of torque for design N the pull-up values
and we try by reducing voltage so that the resulted pull-up torque will be more or less 40% of the rated then we get the following table:
up to 12.5 hp 30%, up to 50 hp 25%,up to 75 hp 20%,up to 200 hp 16% and above 10% allowable voltage drop.

 

[krisys]

If the momentary and interrupting SC values are available, the ratio of momentary SC value / interrupting SC value gives an approximate value of X/R ratio. The refer the ANSI X/R typical value for comparison. If it is comparable, you can use the derived value of X/R.