% impedence

[bejoyengineer]

hi,
i had read in one of the forums that"An increasing transformer impedance has the effect of reducing the fault current, and also reducing the voltage regulation.

From a voltage regulation perspective, you want to have a very low impedance. If you want a low fault current you want to have a high impedance.
Generally, in most applications, a low impedance is preferable"

so now i would like to know why a low impedence is preferable because both voltage regulation and fault currents are important factors when designing a transformer?
Thanks in advance

 

[LarryFine]

so now i would like to know why a low impedence is preferable because both voltage regulation and fault currents are important factors when designing a transformer?

From a non-engineer's point of view, because the voltage regulation, as it affects the performance of the load(s), is more important to the end user, even at the expense of accommodating the higher available fault currents.

 

[steve66]

Fault currents normally only require one to specify a breaker or fuse that can handle the available fault current. I've never seen anyone try to design for low fault currents, except in very rare cases. You normally just accept the calculated available fault current, and move on.

I believe a higher impedence might also adversly affect the efficency of a transformer.

 

[bejoyengineer]

% Impedence

% Impedence

Thanks for the Reply and information

 

[kingpb]

Fault currents normally only require one to specify a breaker or fuse that can handle the available fault current. I've never seen anyone try to design for low fault currents, except in very rare cases. You normally just accept the calculated available fault current, and move on.

I believe a higher impedence might also adversly affect the efficency of a transformer.

Rare? Every power plant we design has the transformer impedance adjusted to keep the fault levels to acceptable levels; while maintaining motor starting voltage regulation.

 

[steve66]

Rare? Every power plant we design has the transformer impedance adjusted to keep the fault levels to acceptable levels; while maintaining motor starting voltage regulation.

Every "power plant"?? What do you mean by a power plant? A large industrial service?? A power generation station?? Or are you talking more general services??

What size of services are you talking about, and what do you consider an acceptable level?

Steve

 

[steve66]

Rare? Every power plant we design has the transformer impedance adjusted to keep the fault levels to acceptable levels; while maintaining motor starting voltage regulation.

How do you adjust a transfomer impedence??

 

[jghrist]

How do you adjust a transfomer impedence??

During design, not after construction.

 

[kingpb]

Every "power plant"?? What do you mean by a power plant? A large industrial service?? A power generation station?? Or are you talking more general services??

What size of services are you talking about, and what do you consider an acceptable level?

Steve

Power plant, as in generating station, as in generator producing MW and MVA RS through a switchyard and onto a transmission system.

Due to cost of MV switchgear, typically it is best to keep the fault current below 50kA @ 4.16kV and 25KA @ 13.8kV. Generator could be as large as 900MVA, which requires multiple auxiliary system transformers and multiple MV busses.

As stated by another poster, you select the impedance during design phase, and purchase what you need, while considering trade-offs such as MV switchgear costs, voltage regulation, transformer losses, transformer purchase price, manufacturing limits, etc.; and of course what the client desires.

 

[steve66]

Power plant, as in generating station, as in generator producing MW and MVA RS through a switchyard and onto a transmission system.

Due to cost of MV switchgear, typically it is best to keep the fault current below 50kA @ 4.16kV and 25KA @ 13.8kV. Generator could be as large as 900MVA, which requires multiple auxiliary system transformers and multiple MV busses.

As stated by another poster, you select the impedance during design phase, and purchase what you need, while considering trade-offs such as MV switchgear costs, voltage regulation, transformer losses, transformer purchase price, manufacturing limits, etc.; and of course what the client desires.

Like I said, this would be one of the rare cases.

 

[kingpb]

I deal with selection of transformer impedances quite frequently, so do substation engineers, protection engineers (relays), distribution engineers, system planners and especially transformer manufacturer's. I would say that without people paying attention to this, contractors would not have power at the building for their service installation.

Are MV systems rare because someone only wires houses? Are process controls rare because the individual only does power wiring? I'd say not. I could say 120/240V systems are rare because I don't utlizie them for what I do, the big box stores obviously don't think they are rare.

Just because it does not affect the work done by some, does not make it a rare occurrence. It may be rare to one group, but quite common to many others.