transient suppression on MV side of transformer

[electrofelon]

I know this a more complex topic than can be answered in a few sentences, but does a transformer "damp out" certain type of transients? Does transient suppression on the MV side of an isolation transformer help suppress transients on the other side? Say I have a step up step down transformer arrangement with 1000' of MV conductors all underground. The low voltage side at the load end feeds a PV system so I have lots of area exposed and inverters (thinking lightning induced surges). On the load end (but on the MV side of the transformer), I am not using the loop feed bushing. Should I throw an arrestor elbow on there? Cheap insurance or waste of money?

 

[gar]

180129-2417 EST

electrofelon:

If you were to look at a transformer for all frequency values, voltage, and waveform conditions then it would be difficult to come up with any reasonable equivalent circuit.

However in many power conditions it can be modeled as an ideal transformer (used for voltage change), a series inductor with resistance between input and output, and a shunt inductor from line to line.

Further simplification and you have a single series inductor and resistor between input and output.

Many unwanted transient signals will propagate thru the transformer modified on one side or the other by the impedances on either side, the transformer impedance, and the source of the transient.

MOVs or other nonlinear devices on one or both sides of a transformer could be useful.

.

 

[electrofelon]

Thanks Gar, interesting. Im sure its a pretty complex topic, lots of variables.

 

[gar]

180128-2534 EST

electrofelon:

See https://www.vishay.com/docs/29079/varintro.pdf for a discussion on MOVs. Look at the V-I curves. Very high impedance until a threshold voltage region, then switches to a moderately low impedance.

From one sample of a particular part number to another sample there can be considerable variation in the voltage threshold. In addition to this variation one has to pick a unit that has a sufficently high threshold that normal line voltage (the peak voltage) variations do not exceed the threshold.

.

 

[ron]

Should I throw an arrestor elbow on there? Cheap insurance or waste of money?

Absolutely cheap insurance

 

[electrofelon]

So when I select a unit, do I want one with a MCOV just over my max system voltage? Do I lose protection if I get, say, a 12.7K version and the system is 2400?
http://www.cooperindustries.com/con...ces/library/235_SurgeArresters/CA235025EN.pdf

 

[Besoeker]

180129-2417 EST

electrofelon:

If you were to look at a transformer for all frequency values, voltage, and waveform conditions then it would be difficult to come up with any reasonable equivalent circuit.

However in many power conditions it can be modeled as an ideal transformer (used for voltage change), a series inductor with resistance between input and output, and a shunt inductor from line to line.

Further simplification and you have a single series inductor and resistor between input and output.
.

FWIW, I used the Stienmetz model.

 

[ron]

So when I select a unit, do I want one with a MCOV just over my max system voltage? Do I lose protection if I get, say, a 12.7K version and the system is 2400?
http://www.cooperindustries.com/con...ces/library/235_SurgeArresters/CA235025EN.pdf

There are some standard selections based on voltage class and nominal ratings as you saw from Table 2 and 3.
http://apps.geindustrial.com/publibrary/checkout/Arresters?TNR=White Papers|Arresters|generic

 

[Ingenieur]

very complex subject
here's a good reference
very simple equivilent ckts can be derived for hand calcs
a modelling program for more complex systems