When are Step Up Transformers need?

[bendesa]

Hi all,

PV Plants produce AC low voltage electricity. Most of the time it will be connected to a low voltage network as well.

When we look at the big scale utility solar farms the most of thetime will be connected to medium or high voltage
networks.

It's my understanding that in such a case you need a step up transformer in between the PV Plant en the point of connection.

Is that correct or are there any other situations where you need a step up transformer?

Mant thx in advance

Kind regards

Ben

 

[Hv&Lv]

Say you have a small manufacturing plant that is served 208Y/120.
you want 277/480 (wye or delta).
step up transformer..

 

[winnie]

You need a step up transformer any time you need to change AC voltage and your source of excitation is the low voltage side of the transition.

For example a power plant feeding a long distance transmission network. Power is better transmitted long distance at high voltage, so the generator voltage needs to be stepped up.

Interestingly, many PV systems are connected to the grid using step down transformers.

All transformers are bidirectional, what differentiates step up and step down transformers are secondary aspects such as location of voltage changing taps, optimization for startup inrush current, wye vs delta coil arrangement, etc.

The power is going through many PV transformers from low voltage to high voltage, but the excitation source is the high voltage side. So even though in operation the transformer is acting in a step up fashion, by design and during initial energizing it is a step down transformer.

Jon

 

[pv_n00b]

Hi all,

PV Plants produce AC low voltage electricity. Most of the time it will be connected to a low voltage network as well.

When we look at the big scale utility solar farms the most of thetime will be connected to medium or high voltage
networks.

It's my understanding that in such a case you need a step up transformer in between the PV Plant en the point of connection.

Is that correct or are there any other situations where you need a step up transformer?

In large-scale PV plants, you will generally have two voltage levels you need step-up transformers for, the MV collection system and the HV substation to connect to the utility.
When designing long AC runs someone might choose to use a pair of step up and step down transformers to save on conductor.

 

[ggunn]

When designing long AC runs someone might choose to use a pair of step up and step down transformers to save on conductor.

We did that a couple of times for 1000' 240V AC runs, but between the losses in the transformers and what they cost at the end of the day it was pretty much a wash.

 

[pv_n00b]

We did that a couple of times for 1000' 240V AC runs, but between the losses in the transformers and what they cost at the end of the day it was pretty much a wash.

Did you go up to 4,160V or higher?

 

[ggunn]

Did you go up to 4,160V or higher?

No, just to 2000V because none of us had much (any) MV experience. Higher voltage might have been more advantageous.

 

[pv_n00b]

Yeah, I would go to 4,160V on a run like that. That's the minimum step up voltage to make it worthwhile.

 

[infinity]

We use 208-4160 step ups in high rise apartment buildings for tenant power. Service is 208Y/120 step up to 4160 and then step back down to 208Y/120 up in the building.

 

[pv_n00b]

We use 208-4160 step ups in high rise apartment buildings for tenant power. Service is 208Y/120 step up to 4160 and then step back down to 208Y/120 up in the building.

Yeah, gravity really increases voltage drop when going upstairs.

 

[electrofelon]

No, just to 2000V because none of us had much (any) MV experience. Higher voltage might have been more advantageous.

Yeah, I would go to 4,160V on a run like that. That's the minimum step up voltage to make it worthwhile.

There may well be a "sweet spot" for a 2kv system. The primary advantage would be one could use extremely affordable 2KV PV wire. Another big advantage is there could be no MV type connections to deal with which many may not be comfortable making....Although I claim there isnt much to putting on a MV elbow connector, you just need a few special tools (BG crimper and semi-con stripper), watch a video, and practice a few times- any competent electrician can do it.

I went with a 2KV PV wire system at my house, but I do run it at 2.4KV which allowed me access to the surplus transformer market rather then ordering custom new units.

If you are going to go above 2KV, a 2400 system might be an option using relatively affordable 5KV L-824 aircraft lighting cable, using 310.10(E) exception #1, although 300.50(A)(3) seems to be absurdly strict and could be a deal breaker making it worth just going up to a 15KV concentric neutral cable at 7200 or 7620. Pre Covid, #2 15KV AL CN cable was only $1.90/foot but is about double that now.

 

[electrofelon]

Hi all,

PV Plants produce AC low voltage electricity. Most of the time it will be connected to a low voltage network as well.

When we look at the big scale utility solar farms the most of thetime will be connected to medium or high voltage
networks.

It's my understanding that in such a case you need a step up transformer in between the PV Plant en the point of connection.

Is that correct or are there any other situations where you need a step up transformer?

Mant thx in advance

Kind regards

Ben

One way to think about it is that there is always a transformer, it just depends on who owns it. The largest systems i have experience with are 2-3 MW, and all those were basically just a "normal" 277/480 electrical service provided by the utility. Sure there was a transformer there, usually 15KV class, but it was the POCO's and we had nothing to do with it. Larger systems would likely have multiple utility supplied 277/480 services, and very large systems would probably have a customer owned MV system that fed into a utility substation at HV.

There are several other reasons for transformers. One that was mentioned is if the customer has a long way to send the power and needs a "step up/step back down" system. Presumable there would be some reason to step the voltage back down rather than leave it at MV, for example a utility may not offer primary services below a certain size.

Another situation where one would need a transformer is where you need to match the inverter voltage to the buildings existing voltage. Although matching the inverter to the building's available voltage seems most logical, there is not much choice in large 208 or 240 V inverters like there is at 480, so it is often worth designing a 480V PV system and using a transformer to connect it to a 208V supply.

 

[Phil Timmons]

There may well be a "sweet spot" for a 2kv system. The primary advantage would be one could use extremely affordable 2KV PV wire. Another big advantage is there could be no MV type connections to deal with which many may not be comfortable making....Although I claim there isnt much to putting on a MV elbow connector, you just need a few special tools (BG crimper and semi-con stripper), watch a video, and practice a few times- any competent electrician can do it.

I went with a 2KV PV wire system at my house, but I do run it at 2.4KV which allowed me access to the surplus transformer market rather then ordering custom new units.

If you are going to go above 2KV, a 2400 system might be an option using relatively affordable 5KV L-824 aircraft lighting cable, using 310.10(E) exception #1, although 300.50(A)(3) seems to be absurdly strict and could be a deal breaker making it worth just going up to a 15KV concentric neutral cable at 7200 or 7620. Pre Covid, #2 15KV AL CN cable was only $1.90/foot but is about double that now.

Is that actually kosher? Not asking as a nag, but rather just curious.

I understand it is your own house, and I would certainly never want a legit inspector in mine, either . . . . (3 phase electric lawn mower running on 14-3 romex and tray cable, for example).

Just wondering the 2K PV Cable is intended for DC (only?) and do you shield and terminate it like a typical MV cable?

 

[electrofelon]

Is that actually kosher? Not asking as a nag, but rather just curious.

I understand it is your own house, and I would certainly never want a legit inspector in mine, either . . . . (3 phase electric lawn mower running on 14-3 romex and tray cable, for example).

Just wondering the 2K PV Cable is intended for DC (only?) and do you shield and terminate it like a typical MV cable?

IMO the conductors have plenty of dielectric strength. Check in in another 10 years and I'll let you know if it's failed or not

2k PV wire is just RHW-2, and UL44, the standard that covers it only goes to 2000V FWIW. It terminates into a load break elbow. You have to jam more conductor or solder it in to the coppertop compression connector because they aren't available bigger than number 8 (I'm using #12 and#10).

 

[ggunn]

Just wondering the 2K PV Cable is intended for DC (only?) and do you shield and terminate it like a typical MV cable?

The voltage ratings on conductors are for AC or DC.

 

[electrofelon]

IMO the conductors have plenty of dielectric strength.

The voltage ratings on conductors are for AC or DC.

Just wanted to comment a bit more on the dielectric strength topic. Something interesting in case no one has noticed: The voltage rating is far from black and white. Most of the insulation on conductors is for physical strength not dielectric strength. For example, #12 THHN has an insulation thickness of 15 mils. #1 THHN has an insulation thickness of 50 mils, more than three times that of #12. Obviously we need more insulation to prevent damage from the much heavier unwieldly conductor. So we can conclude that 40 volts per mil is adequate from a dielectric standpoint. #14 thru #10 PV wire has an insulation thickness of 75 mils. Using that same 40 volts/mil on the PV wire would equate to a rating of 3000 volts. Now of course when you get into MV, you get into needing shielding to equalize voltage stress and special constructions like semiconducting layers and careful manufacturing to prevent air gaps where corona discharge can happen, but the general consensus is that shielding is typically not necessary at and below 2.4 KV. Of course I am not recommending re-engineering voltage rating in a professional capacity, I just wanted to explain my reasoning for this system for myself at my property.

 

[capncheets]

Another situation where one would need a transformer is where you need to match the inverter voltage to the buildings existing voltage. Although matching the inverter to the building's available voltage seems most logical, there is not much choice in large 208 or 240 V inverters like there is at 480, so it is often worth designing a 480V PV system and using a transformer to connect it to a 208V supply.

This is a very common reason in my experience, once your system size starts to reach a certain point your options become either using multiple lower voltage inverters or a 480v inverter with a transformer, with the transformer route typically being the cheaper option. A SE50k 208v inverter for instance costs about the same as a SE120k 480v inverter

 

[ggunn]

This is a very common reason in my experience, once your system size starts to reach a certain point your options become either using multiple lower voltage inverters or a 480v inverter with a transformer, with the transformer route typically being the cheaper option. A SE50k 208v inverter for instance costs about the same as a SE120k 480v inverter

I have done this many times; the larger the PV system the more it makes sense. 208V delta primary to 480/277V wye secondary transformers are often off the shelf items.