Tfmr impedance

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Any one know the impedance of this transformer? I tried their website, but no luck for me.
It is likely only found on the 'engraved nameplate inside of the transformer.
Whatever you find is probably going to be an estimate or typical value. Transformers this small are rarely tested for actual values unless that was part of the procurement specifications.
 

Carultch

Senior Member
Location
Massachusetts
Any one know the impedance of this transformer? I tried their website, but no luck for me.
Copy down the serial number, catalog number, and overall specifications of the transformer (so that it is in text form instead of photo form, and therefore easier to copy and paste for them). Then write to the manufacturer (Howard Industries) and ask them for a document that shows the content of the nameplate.

The nameplates are inside the secondary cabinet, so you would need to open it with the pentagonal transformer wrench to see it. Plus it is most likely energized.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
It is likely only found on the 'engraved nameplate inside of the transformer.
Whatever you find is probably going to be an estimate or typical value. Transformers this small are rarely tested for actual values unless that was part of the procurement specifications.
The POCO engineer wondered aloud why they had placed one so large.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Copy down the serial number, catalog number, and overall specifications of the transformer (so that it is in text form instead of photo form, and therefore easier to copy and paste for them). Then write to the manufacturer (Howard Industries) and ask them for a document that shows the content of the nameplate.

The nameplates are inside the secondary cabinet, so you would need to open it with the pentagonal transformer wrench to see it. Plus it is most likely energized.
I have a wrench, but no key to the padlock. Not interested in That route anyway. I’ll let POCO get me the FC tomorrow.
 

Russs57

Senior Member
I'm used to thinking of the impedance of a transformer as the square of the turns ratio (absolute impedance). I'm assuming you are more concerned with the following:


The per-unit impedance describes that percentage
of the rated voltage required to produce full load
current while the transformer output is shorted.

I'm gathering that you have no desire to short the winding's and ramp up voltage until full load amps happens?
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
If its a POCO transformer, let them give you the impedance. The number they give you will usually allow for a replacement with a different Z, if this transformer goes bad. For example, they might give you a range for a 100KVA transformer like 3%-5%, or a minimum number like 3%.

I'm pretty sure our local POCO just has a table that lists %Z ranges for each size transformer. I've asked for a copy of this "table" but no luck. They make me call and wait for every single transformer and service we ever do.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
I have this table from 1997-I think-from General Electric Application Information. Short-circuit Current Calculations-I am not sure. I did not find this in internet now.
 

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ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I'm used to thinking of the impedance of a transformer as the square of the turns ratio (absolute impedance). I'm assuming you are more concerned with the following:


The per-unit impedance describes that percentage
of the rated voltage required to produce full load
current while the transformer output is shorted.

I'm gathering that you have no desire to short the winding's and ramp up voltage until full load amps happens?
Ah, No. Not in this lifetime.
 

kwired

Electron manager
Location
NE Nebraska
If its a POCO transformer, let them give you the impedance. The number they give you will usually allow for a replacement with a different Z, if this transformer goes bad. For example, they might give you a range for a 100KVA transformer like 3%-5%, or a minimum number like 3%.

I'm pretty sure our local POCO just has a table that lists %Z ranges for each size transformer. I've asked for a copy of this "table" but no luck. They make me call and wait for every single transformer and service we ever do.
Worst case fault current level is fine when selecting withstand rating of equipment, may not give you worst case incident energy level for arc flash events though as response time of overcurrent protection can/will vary depending on actual current level.
 

kingpb

Senior Member
A typical impedance, Z, for this transformer would be 3.7%. This comes from The Industrial Power System Handbook by Beeman, which specifies typical data for transformers with ratings not larger than 500 kVA and primary voltage not higher than 12.47 kV. ANSI C57.12.10 specifies standard impedances for transformer larger than 500kVA. The tolerance for a new transformer is +/- 7.5%, but since this is an existing transformer the tolerance would be zero. However, because you don't know the impedance, you might want to use the -7.5% if you are looking at 3ph fault current.
This equates to 13.5MVAsc and 14.5MVAsc, respectively. Probably not going to make a difference on the rating of equipment. Utilities typically use transformers with lower impedances, so it could be down to around 2.4%, or 20.8MVAsc. Impact on equipment ratings could be significant.
 

kwired

Electron manager
Location
NE Nebraska
A typical impedance, Z, for this transformer would be 3.7%. This comes from The Industrial Power System Handbook by Beeman, which specifies typical data for transformers with ratings not larger than 500 kVA and primary voltage not higher than 12.47 kV. ANSI C57.12.10 specifies standard impedances for transformer larger than 500kVA. The tolerance for a new transformer is +/- 7.5%, but since this is an existing transformer the tolerance would be zero. However, because you don't know the impedance, you might want to use the -7.5% if you are looking at 3ph fault current.
This equates to 13.5MVAsc and 14.5MVAsc, respectively. Probably not going to make a difference on the rating of equipment. Utilities typically use transformers with lower impedances, so it could be down to around 2.4%, or 20.8MVAsc. Impact on equipment ratings could be significant.
Single phase "pole pigs" seem to tend to be lower impedance from my observations, but you usually also have at least 25 or more feet of conductor to help lower the available fault current at any customer gear in most cases
 
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