Surges in power system

[Subbu68]

Many write about "voltage surges" damaging their household and AV equipment. These are not surges but temporary overvoltages or voltage swells. Some may get a surge when lightning strikes their premises.

This article briefly explains what is a surge in power system parlance and how we EEs protect the system

Overvoltage Protection | Electrical4U

Electrical power system can experience abnormal overvoltages due to various reasons like sudden interruption of heavy loads, lightning impulses, and switching impulses. These overvoltage stresses can damage the insulation of equipment and insulators. Even if not all overvoltages are strong...

www.electrical4u.com

 

[Hari Iyer]

Many write about "voltage surges" damaging their household and AV equipment. These are not surges but temporary overvoltages or voltage swells. Some may get a surge when lightning strikes their premises.

This article briefly explains what is a surge in power system parlance and how we EEs protect the system

Overvoltage Protection | Electrical4U

Electrical power system can experience abnormal overvoltages due to various reasons like sudden interruption of heavy loads, lightning impulses, and switching impulses. These overvoltage stresses can damage the insulation of equipment and insulators. Even if not all overvoltages are strong...

www.electrical4u.com

I have faced voltage surge issues in my monoblocks during turn-on almost 50% of my usage time. This was due to high VA of my R-core power transformers which has a primary DCR of just 3.5 ohms. This will cause my 10 A MCB to always trip causing me nuisance for usage.

Only last month I have added a thermistor in series which has a cold DCR of 8 ohms. Since then i have not encountered any turn- on surge issues.

 

[Subbu68]

I have faced voltage surge issues in my monoblocks during turn-on almost 50% of my usage time. This was due to high VA of my R-core power transformers which has a primary DCR of just 3.5 ohms. This will cause my 10 A MCB to always trip causing me nuisance for usage.

Only last month I have added a thermistor in series which has a cold DCR of 8 ohms. Since then i have not encountered any turn- on surge issues.

That is an overcurrent issue, isn't it? When transformers are switched in there will be an inrush and second harmonics that cause MCB to trip. With thermistors you have achieved a "soft start".

I suppose I can go a bit technical. In transmission or large distribution transformers we have very sensitive differential current protection. primary and secondary currents are balanced in the relay in normal load by proper transformation with current transformer. In case of a fault in the transformer, there will be large flow from one end - usually primary causing an unbalance in the relay and causing it to trip the trafo. The same relay acts when we switch on the trafo from dead state due to second harmonics generated due to magnetising current. We have a filter that detects this and restraints the relay from tripping. If the trafo is switched on at zero crossing of voltage wave we escape.

We cannot have such sophisticated systems in even lower down the distribution or utilisation. You have done the right thing adding the thermistor. During switch on thermistor offers high resistance and you can control the inrush. When current picks up in the trafo the thermistor resistance goes down to permit normal operation of the trafo at rated voltage. I have an article somewhere on this. I'd try to post it.

If you saw the power supply design of Nelson Pass F5 you can see this and I think he has explained this.

 

[Subbu68]

@Hari Iyer

A typical explanation from Amphenol on usage of Thermistors for in-rush current limiting attached.

Another issue is the MCBs used in the household are perhaps not slow acting types. These are C curve MCBs that trip when current exceeds 5 to 10 times the rated current. One can up the rating but then you'd loose the protection for the equipment short circuits.

What is needed is slow acting D type for inductive loads - transformers are primarily inductive during switching.

 

[Hari Iyer]

Initially i had added a 6uf capacitor in parallel with the transformer for mitigation of current and voltage lead/ lag, but they didn't work.

 

[Subbu68]

Initially i had added a 6uf capacitor in parallel with the transformer for mitigation of current and voltage lead/ lag, but they didn't work.

It is not the lead or lag of current that matters. It is the Lenz's law and magnetic saturation at work when you switch a trafo. You can't combat it that way.

Yes, on EHV systems we use what is called a zero crossing detector to switch the breaker at the zero crossing of voltage wave with respect to time. Any maloperation of the protection due to inrush will result in even a blackout. So the risk is too much there.

V-I angle is at steady state not in transient.