Electrician Talk banner
1 - 16 of 16 Posts

·
Registered
Joined
·
200 Posts
Discussion Starter · #1 ·
I've learned multiple definitions of which side is which, in terms of a transformer primary and secondary. In the most common applications, they are all consistent. However, I often work with cases where these definitions are inherently inconsistent, and therefore I'd like to get a clarification, if at all possible. I'd like to be able to use the proper terms on drawings and equipment placards, rather than making my own judgement call as I see fit.

Definition 1: the source of energy is on the primary side. Power flows from the primary side to the secondary side.

Definition 2: the primary side is the side that is connected to the utility. The secondary side is the side that is connected to the customer.

Definition 3: the primary side operates at the higher voltage than the secondary side.


Certainly, for the transformer of my house, all three of these definitions are consistent. My house is the secondary customer side, the secondary 240V side compared to the kV primary, and the secondary side in terms of energy flow direction.

One particularly tough example, is a photovoltaic system that I'm working with. I'm using US sourced inverters, at 480 volts. And the transformer is built to step this voltage to 400 volts, such that it matches the local grid at the job site. This is overseas, hence the exotic situation.

By definition 1, the 480V inverter side is primary.
By definition 2, the 400V building side is primary.
By definition 3, the 480V inverter side is primary.

It looks like the 480 inverter side is the primary by the logic of "best two out of three", so that is my judgement call for it.

Anyone else have thoughts?
 

·
Donuts > Fried Eggs
Joined
·
17,042 Posts
The "primary" is the side to which power is being supplied, the "secondary" is the side to which the load is connected. But transformers are not directional, so in facilities capable of generating their own power the "primary" and "secondary" switch sides depend on whether the facility is consuming power from the grid or shipping it out.

In my industry we work on a ton of generation so for clarity we don't use "primary" and "secondary" but rather "high side" and "low side" because that's a fixed designation based on the number of transformer turns.
 

·
Registered
Joined
·
2,969 Posts
1 & 2 are correct !
But 3 not always so !
In most domestic grid supply situations
Good chance it is so !
But trannys can also be used to step UP voltage !
 

·
Registered
Joined
·
37 Posts
Also you may see references to HV bushing and LV bushing, meaning high and low voltage respectively. Thats what we use at my workplace. Usually that keeps things pretty unambiguous. Also, the most important thing is to ensure that you place protection in the right areas, ie. Over current protection, like a circuit breaker, on the load side. So after a while the primary and secondary definitions don't really become necessary as long as the correct protection and controls are in place

Sent from my Nexus 5 using Tapatalk
 

·
animal lover /rat bastard
Joined
·
13,547 Posts
3 has nothing to do with primary/secondary. remove it from the list.
 

·
Donuts > Fried Eggs
Joined
·
17,042 Posts
Number 2 really doesn't, either. A transformer primary has nothing to do with utility.

I think the confusion for both those comes from the fact that utility guys use the slang "primaries" to refer to all high voltage.
 

·
Registered
Joined
·
37 Posts
I think that being a step up or step down transformer changes that designation anyways. Primaries on a step up transformer will be lower voltage (I'm picturing a terminal station near a generating site). Secondaries would be transmission lines side in that case, at a much higher voltage.

Sent from my Nexus 5 using Tapatalk
 

·
Bilge Rat
motors and controls.........
Joined
·
8,849 Posts
The "primary" is the side to which power is being supplied, the "secondary" is the side to which the load is connected. But transformers are not directional, so in facilities capable of generating their own power the "primary" and "secondary" switch sides depend on whether the facility is consuming power from the grid or shipping it out.

In my industry we work on a ton of generation so for clarity we don't use "primary" and "secondary" but rather "high side" and "low side" because that's a fixed designation based on the number of transformer turns.
Same here; it's high side or low side.
 

·
Registered
Joined
·
287 Posts
I would consider it line side and load side
Not sure I have ever heard someone call it that :eek: If you walked up to a 100MVA tx for maintenance and started talking about line and load like a breaker I would probably look at you like you have 3 eyes.
 

·
Registered
Joined
·
32,059 Posts
"Line and load" suffers from the same problems as "primary and secondary." It's not a fixed designation and changes based on the direction of power flow.
Which is why I like standard terminology, Neutral not a noodle, transformer not a tranny........We have enough local terms, 1900 in lieu of 4 square, as the country shrinks in size due to rapid movement of the population and the internet standardization of terms should a priority.
 

·
Donuts > Fried Eggs
Joined
·
17,042 Posts
Which is why I like standard terminology, Neutral not a noodle, transformer not a tranny........We have enough local terms, 1900 in lieu of 4 square, as the country shrinks in size due to rapid movement of the population and the internet standardization of terms should a priority.
When I first moved to New England and asked for a 1900 box at a supply house, they looked at me like I was speaking Mandarin. :laughing:
 
1 - 16 of 16 Posts
Top