does anyone know how to size a zig zag transformer where the secondary voltage is 120/208 3 phase appx 385 amps/phase and a short ckt rating of the switchgear of 85,000 amps.. size of the solar we'd be tying into the switchgear is appx 160 kW... the utility requires a zig zag on the secondary side... i'm assuming we'd tie it into (parallel) the switchgear buss'.
zig zag transformer sizing
- Thread starter JohnI
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Uggg.....is this national grid? This topic has come up here before, and no one can seem to figure out what the purpose of this zigzag transformer is, so it's kind of hard to say how to size it.
the zig zag, mainly, is to minimize unbalance on the neutral and provide ground fault protection mainly for the utility.. but i wonder if you can provide protection better and less expensively like Europeans do, i.e. monitor insulation degradation.
If it's for providing a ground path of that system, choose the current at which you will limit the ground fault to be (Io)--> size your grounding resistor!
- Take that fault current that passes through the wye point of the zigzag transformer and compute for the line current passing thru the transformer phases (Ip = Io/3).
- Remember that you will also need the amount of time your transformer is designed to withstand during the fault (the time the fault persists before relays will trip your breaker). To find the final transformer size, find the correct factor.
- Your transformer size will be = (Ip )(VL_N )(3) X factor. Say you will have the fault to be isolated no longer than 1 minute, Use a factor of 0.104.
| time | factor |
| 10 sec | 0.064 |
| 1 min | 0.104 |
| 2 min | 0.139 |
| 3 min | 0.170 |
| 4 min | 0.196 |
| 5 min | 0.220 |
I hope that helps.
so is Ip = the short ckt rating of the switchgear n(85,000 amps)_and is the 85,000 amps indicative of the "wye point" as you've described.. or is Ip determined by the utility to protect their service transformer? there is no cut out fuse for that transformer... and i would think it'd be cheaper to put a fuse in all three phases.
If your zigzag transformer is solidly grounded at the wye-point, your ground-fault current will be = VL_N/ transformer impedance--> dependent on the impedance of your zigzag transformer (assuming a ground fault on the wye-point!
You will choose a ground fault current level that you intend to have in your design, say for example you wanted to choose a high impedance grounding system, the ground fault current recommended is below 10A! 10A will be the current that you allow to flow from your wye point to the ground. That can be achieved by inserting a grounding resistor between the wye point and the ground equal in resistance to R=VL-N/10 amperes! If you have a secondary line-to-line voltage of 480V, your ground resistor R= 480/(1.732 x 10) = 27.7 ohms~ 28 ohms (impedance of transformer neglected)!
Or you could choose a low-resistance grounding where the allowed ground-fault window is 100A to 1000A, 400A being the typical design amps!
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Topgone,. Can you elaborate on why you would use a zig zag on a system that's already solidly grounded? I am not clear on that part.
The use of a grounding transformer is necessary when one inherits an ungrounded system and you want a way of providing a ground point on the system and protecting your installation from ground faults as well. But, you have the choice to either solidly ground it or employ low-resistance, high-resistance, or reactive impedance grounding! I had that experience in my green years where our ungrounded 6.9kV feeder line made up of stranded Cu wire was broken (or intentionally cut) which resulted in a fatality--> the feeder breaker did not trip and the line was on the ground; a person accidentally (or intentionally, as in wire thievery) touched the live line and died. I was the one who helped calculate the needed grounding transformer, why I replied to this post.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
But is there ever a reason to use a zig zag on a system that's already solidly grounded? I am not sure about the specifics of the op's situation, but here we have a utility that requires zig zag transformers on systems that are ALREADY solidly grounded.
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
My (very weak) understanding is that these 'effective grounding' requirements have nothing to do with the customer side of things, but rather with maintaining proper line-neutral voltages on the utility side of things during fault events.
From the customer side, solidly grounding just requires a low impedance between L and G at a single point to create a solidly grounded system.
My (very weak) understanding is that the utility also wants low impedance between their distribution L and G otherwise a fault between one phase and ground can raise the other phases voltage, much like a fault on a delta system. I think that one of the techniques used is for the utility to provide a wye:wye service. The wye:wye transformer reflects one side to the other, so the customer then provides a zig-zag transformer to provide the low impedance to ground.
-Jon
From the customer side, solidly grounding just requires a low impedance between L and G at a single point to create a solidly grounded system.
My (very weak) understanding is that the utility also wants low impedance between their distribution L and G otherwise a fault between one phase and ground can raise the other phases voltage, much like a fault on a delta system. I think that one of the techniques used is for the utility to provide a wye:wye service. The wye:wye transformer reflects one side to the other, so the customer then provides a zig-zag transformer to provide the low impedance to ground.
-Jon
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