Neutral Current for CAPACITOR Unbalance Protection 1

[afzumannu]

Dear Engineers,

We have Filter bank line voltage is 33 KV ( 33000V), ungrounded double star connection. capacitors are connected series parallel group. each phase one row is connected to star1 and other row is connected to star2 , unbalance CT is connected in between both star point. i want to know what is the formula for neutral current flow so can understand the % of unbalance.
i know one formula that is Iunb = ( I1+I2+I3)-(I4+I5+I6), I1,I2,I3 is one star side each phase, while I4,I5,I6 is other star side. also Iunb = Un/Xc, so i will apply the values in this formula which. I have recorded the values for Xc1 = 27.2231µF, Xc2 = 27.2499, Xc3 = 26.6247 ( one group), Xc4 = 26.6499,Xc5 = 26.7749, Xc6 = 26.7747(2nd group), hence Xc=1/2πfc, here i want to know from you guys What is Un here is it 33000 V supply voltage? or individual Capacitor name plate rating, 15000V, 459 KVAR, or someone can calculate the neutral current here, any other formula let me know.

 

[beyond86]

afzumannu, I took your parameters, 33 kV supply voltage, 50 Hz supply frequency, capacitance with your screen, inductance 21.66mH and just put these parameters in this model.

 

[afzumannu]

@beyond,

Except Matlab not possible to do?

please if you know the calculation formula let me know

 

[afzumannu]

Dear @ beyond

Give me that formula so I can calculate as how u prepared the model

 

[beyond86]

I don't know any formula. You should read more information about Matlab and what you can do in this programm.

 

[afzumannu]

@ Beyond
Is it free or subscription required?

 

[afzumannu]

Dear All,
As Mr. Beyond given the Calculated current it is correct,
can anyone here tell me the Formula. @ Argotier can you explain about it.

 

[afzumannu]

I am still behind this, please help to solve this. Thanx

 

[afzumannu]

@ Beyond. we have change one capacitor so the value in star 1 R phase is 26.8744 micro farad before it was 27.2231. Please can you calculate the unbalance current now.

 

[beyond86]

afzumannu, 2.038A primary.

 

[afzumannu]

This time it's not matching. I mean not near to our recorded value.may be measurement factor. May be it's original value is 26.9247 not the 26.8744.
Anyway still closer, but without matlab is it possible? Please reply someone.

 

[beyond86]

If we take capacitance 26.9247 for phase R of 1 star, unbalance will be 2.069A primary.

 

[afzumannu]

May be our measurement error, @ beyond actually I already calculated the impedance then phase current etc. Then I did the vector sum for neutral current finding..but somewhere I m missing results are not coming correct..can you give ur contact number Mr beyond..
I have to solve it by today my boss is crying now.

 

[beyond86]

afzumannu, I don't know how I can help you. You should study Matlab and after you will able to calculate current by yourself.
How often you replace capasitor? I don't understand why you need to calculate current often. It makes one time for chose pick up current of protection.

 

[afzumannu]

Actually we have around 15 filter bank and 3 no.s of plant all having the same capacitors..my main question is when u are making the model in matlab sure there you are using the input formula etc..same calculation I want..

 

[beyond86]

I didn't use any formula. When you create model in Matlab you take virtual capasitor, reactor, network etc and after that you should put your parameters in these virual elements. After that you can run calculation and get result.

 

[crshears]

My utility uses two different values of monitored neutral current in its cap banks; the first lower threshold alarm is set to annunciate upon loss of one capacitor can as either a neutral overcurrent or neutral current unbalance alarm [ it is my understanding that despite the intitiating circumstance being identical, different Protection and Control departments chose slightly different alarm text wording ].

Upon first receipt of this alarm an immediate dispatch is made to have the capacitor bank inspected; note that the results of inspection are tracked, and provided there is no significant damage to the cap bank its availability for voltage control / reactive supply system deployment is not constrained pending repair.

The second and significantly higher current threshold is set to operate the capacitor trip protection scheme.



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[afzumannu]

@ Crshears you are telling correct. But I need the calculation of neutral current which can calculate based on impedance. We have lot of things to do.sometime capacitors blown we have to balance the circuit before restarting. Please give me the calculation formula.

 

[afzumannu]

@ beyond
can you send the SLX File again.
i donwloaded the matlab simulink, but file opening time message displaying invalid block name etc.
please send it again

 

[afzumannu]

 

[afzumannu]

@beyond I am waiting for your slx file once again as it is not opening please