Over-Voltage for capacitive Load

Thread Starter

Ali_ibra

Joined May 28, 2018
4
Hi everyone,

Could any one provide me in detail why over-voltage occur on secondary of transformer in case of capacitive load (Light load and Power factor correction capacitors are in service)??!!!

Thanks in advance
 

MrAl

Joined Jun 17, 2014
11,276
Hi everyone,

Could any one provide me in detail why over-voltage occur on secondary of transformer in case of capacitive load (Light load and Power factor correction capacitors are in service)??!!!

Thanks in advance
Hello,

Not enough information really. For example, the leakage inductance of the transformer could be resonating with the capacitor. Too hard to tell for sure though without more information.
 

Thread Starter

Ali_ibra

Joined May 28, 2018
4
MrAI ,

thanks for your response , we have the case of a light load occurring during night time , over-voltage happens -I know- because the total load turns into capacitve due to existing of power factor correction capacitors.My question is why over-voltage occurs in detail ?

what kind of additional information would you need?

thanks in advance
 

MrAl

Joined Jun 17, 2014
11,276
MrAI ,

thanks for your response , we have the case of a light load occurring during night time , over-voltage happens -I know- because the total load turns into capacitve due to existing of power factor correction capacitors.My question is why over-voltage occurs in detail ?

what kind of additional information would you need?

thanks in advance
Hello,

What kind of system are we dealing with, 120vac, or other, or an inverter/converter, etc.
What kind of load.
When the over voltage occurs: is it after 1 second, 2 seconds, 10 seconds, 20 minutes, etc. Especially if it happens when the load is first disconnected.

Use your imagination. Imagine you are asking someone a question about yout system who has no idea what kind of system it is or how you are operating it. The more information you can think to give, the better.
 

Thread Starter

Ali_ibra

Joined May 28, 2018
4
MrAI ,

thanks for your response , A distribution transformer 1600 KVA (11 KV/0.38 KV) supplying a no of loads consists of induction motors ,IT equipment and service lighting and power loads.A Power factor correction capacitor bank (manual ,5 steps each 100 KVAR ) is installed so that at demand load PF=0.95 .

at night ,most of motors are off and the load turns into capacitive with lead power factor, after less than one minute the voltage starts to rise before we try to disconnect some capacitor bank steps so load turns back to be inductive , I don't have a problem with this issue.

My question is why over-voltage occurs in case of capacitive load on transformer secondary?
 

MrAl

Joined Jun 17, 2014
11,276
MrAI ,

thanks for your response , A distribution transformer 1600 KVA (11 KV/0.38 KV) supplying a no of loads consists of induction motors ,IT equipment and service lighting and power loads.A Power factor correction capacitor bank (manual ,5 steps each 100 KVAR ) is installed so that at demand load PF=0.95 .

at night ,most of motors are off and the load turns into capacitive with lead power factor, after less than one minute the voltage starts to rise before we try to disconnect some capacitor bank steps so load turns back to be inductive , I don't have a problem with this issue.

My question is why over-voltage occurs in case of capacitive load on transformer secondary?
Hello,

Ok for now i will take your reply of "less than one minute" to means actually: "less than one minute but more than 30 seconds".
I make this change, anticipating your possible correction, because i need to know approximately what time window we are looking at here. If the time was very short like 1 second, it could easily be inductive kickback, but if it is more than 30 seconds then it would make more sense that something is causing resonance with the line or with something else connected that you have little or no information about.

So that's my best guess at the moment, that the caps are causing a resonance with some impedance that has at least some inductance and that causes the voltage to gradually ramp up.
It could also be that when the load is removed, the acting series resistive components in the system have time to cool off after a heavy load, which would mean more voltage available at the output with light load, and that could take the 30 seconds to 1 minute or more to show up. That would also be apparent if the voltage 'creeps' up slowly as the resistive parts that act in series cool off (transformer windings, etc).
Because when you remove caps the voltage drops though, it sounds more like resonance, or it could be both.

However, i think you found the solution which is to either remove caps or perhaps apply a SMALL load to see if you can eat up that extra voltage a little. Sometimes even a small load will eat up voltage caused by some inductance.
 
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