Ferroresonance on Switching Secondary Breakers.

[charz]

The Main Power transformer at unloaded condition. As one of the secondary VCB is switched ON, the power transformer and the auxiliary transformer makes loud rumbling noise. this doesn't always happen, sometimes it occurs and sometimes it don't. The rough SLD is attached. What are the solutions to mitigate this?. It has long secondary single core cables aroud 200 meters.

 

[jghrist]

Not your typical ferroresonce situation. Normally you might suspect ferroresonce when energizing a delta winding through cables one phase at a time. In this situation the transformer winding is energized the whole time and switching is three-phase. Do you have any oscillography showing the voltage?

 

[charz]

I don't have any oscillography. Not only the Power Transformer makes noise, but also the auxiliary transformer too makes noise at the same time. Doesn't ferroresonance occur during lightly loaded conditions?

 

[prc]

The problem seems to be due to the unearthed system that you are switching in. A PT bank is seen. Can you give the specification of it? In case you have a broken delta secondary add a resistor and see the difference. Or load one of the star secondary with resistors.

 

[charz]

Hi PRC,

The PT specification is as below, I have never seen such configuration. The PT is made in russia and this type of configuration is used in russia. I'm still waiting for the translation to get done.

 

[prc]

charz, Iam not familiar with this connection. If you can put some shunt capacitor on 6.3 KV, this phenomena may disappear. That is one alternative to resistor loading of PT secondary.

 

[1capybara]

if those secondary cables are causing the resonance their probably inductive and if you can
measure their inductance you can add an equal amount of capacitance to neutralize it,
or even some small load, exactly like prc said above.

 

[charz]

Hi PRC,

Sorry, I can't understand how adding shunt capacitors would mitigate this problem. As far as loading resistor at the PT secondary, I understand it to be for avoiding ferroresonant at the PT's. Could you please kindly explain more about this.

My understanding for adding resistors at the PT secondary:

1. Due to switching/Voltage transients causes the PT to go into saturation.
2. If the PT's are saturated means it draws a huge magnetizing current. The PT's remain saturated until the magnetizing current decreases.
3. The Inductance of the PT winding changes during the saturation.
4. Now if the value of the varying Inductance of the PT's during saturation matches with the line to ground capacitance of the system, In other words, the capacitive charging currents of the cables cancels the magnetizing current of primary winding of pt. Then Ferroresonant occurs.
5. So to come out of the saturation for the PT's,the exciting current should be reduced drastically, It can be reduced by increasing the load of the PT.

 

[charz]

Hi PRC,

By shunt capacitor, do you mean Harmonic Filters?

 

[prc]

Please see thread238-418141.
It is usually called surge absorber. Simple shunt capacitors.