480V ungrounded Delta

[Selva_51]

Hi All, I have a hybrid power plant with PV, Battery and DG connecting to a 13.2 kV grid via a step-up transformer. the 480V side is delta which is ungrounded, how do I size a grounding transformer? the L-G fault is 3.7kA,

 

[waross]

neutral-earth link

What on earth is a neutral-earth link as applied to a delta configuration?

 

[davidbeach]

What waross asked…

 

[unclebob]

You need a zigzag transformer that will provide you a neutral that you will connect through a grounding resistor. The size of this resistor will determinate the size of the zz transformer.

 

[waross]

You need a zigzag transformer that will provide you a neutral that you will connect through a grounding resistor. The size of this resistor will determinate the size of the zz transformer.

Or a wye:delta transformer.
The size of this resistor will determinate the size of the wye:delta transformer.

 

[Selva_51]

Or a wye:delta transformer.
The size of this resistor will determinate the size of the wye:delta transformer.

Thanks for your reply. If it is a solidly grounded system. What will be the size of the wye: delta transformer

 

[Selva_51]

You need a zigzag transformer that will provide you a neutral that you will connect through a grounding resistor. The size of this resistor will determinate the size of the zz transformer.

Thank you for your reply. How is the resistor calculation done and then based on this what will the transformer size if I am to use a zig-Zag transformer. Could you please share a sample calculation if you have it would be really helpful

 

[waross]

How is the resistor calculation done

Ohm's Law.

Spoiler

Ohm's law
Not just a good idea;
It's the LAW!

 

[mparenteau]

Consider the withstand time of the resistor. A 400A resistor is pretty common, having a 10 second withstand. Extended time ratings means more $$$.


Mike

 

[waross]

Hint 1: The resistor limits the ground fault current. At what current do you want to limit the ground fault current?

 

[dpc]

Wait. It sounds like you want to establish a ground on the 480 V side of the transformer? First question is why? Perhaps the ungrounded delta was chosen for a reason??

If you want to ground a 480 V system, it needs to be either solidly-grounded, or high-resistance grounded (5 A or less). Low-resistance grounding systems are not safe at 480 V. Don't do this.

The 3.7 kA of SLG fault current is presumably on the 13.2 kV side. This is more or less irrelevant for 480 V grounding due to the delta winding.

Respectfully suggest that you engage the services of a EE with experience in grounding of distributed generation systems. If the 13.2 kV system belongs to the utility, you should also discuss this with them sooner rather than later.

Good luck.

 

[waross]

Why a delta?
When a single phase unbalance is applied to a generator through a wye:wye transformer, the neutral is connected for voltage stability.
All of the unbalanced current flows through one phase of the generator.

When a single phase unbalance is applied to a generator through a wye:delta transformer, half the current flows through the phase winding associated with the unbalanced phase,
The other half of the unbalanced current flows through the other two generator phase windings, albeit at 50% power factor.
Before grounding check that your inverters are capable of operating with a ground on the delta.
480 Volt generators often have three 277 Volt windings in wye.
The wye point is often grounded at the generator by default.
Your generator may already be providing a ground path when it is on-line.

 

[Selva_51]

Wait. It sounds like you want to establish a ground on the 480 V side of the transformer? First question is why? Perhaps the ungrounded delta was chosen for a reason??

If you want to ground a 480 V system, it needs to be either solidly-grounded, or high-resistance grounded (5 A or less). Low-resistance grounding systems are not safe at 480 V. Don't do this.

The 3.7 kA of SLG fault current is presumably on the 13.2 kV side. This is more or less irrelevant for 480 V grounding due to the delta winding.

Respectfully suggest that you engage the services of a EE with experience in grounding of distributed generation systems. If the 13.2 kV system belongs to the utility, you should also discuss this with them sooner rather than later.

Good luck.

it will be a solidly grounded system to detect earth faults and the 3.7kA is on the 480V side and not on the 13.2kV side. The 13.2 kV side grid is also built by us and we are the only utility in the rural area.

 

[Selva_51]

Hi All, I have a hybrid power plant with PV, Battery and DG connecting to a 13.2 kV grid via a step-up transformer. the 480V side is delta which is ungrounded, how do I size a grounding transformer? the L-G fault is 3.7kA,

There was also scheme proposed like this with a balancing transformer and a neutral-earth link? what are your thoughts on this

 

[waross]

Each separately derived system when grounded, shall have the neutral bonded to earth at one location only.
That would be your neutral to earth link.
The transformer must withstand fault current until the system protection clears the fault. That is why a resistor is used to limit the fault current.
The resister must be in the neutral conductor of the balancing transformer.

 

[Selva_51]

Each separately derived system when grounded, shall have the neutral bonded to earth at one location only.
That would be your neutral to earth link.
The transformer must withstand fault current until the system protection clears the fault. That is why a resistor is used to limit the fault current.
The resister must be in the neutral conductor of the balancing transformer.

thank you

 

[jghrist]

Why not connect to the 13.8 kV with a Dyn transformer that would provide the grounding of the 480 V system?

 

[dpc]

How do you get 3700 A of ground fault current on an ungrounded system?

 

[waross]

Probably the ASCC David.
Or not.

 

[mparenteau]

Consider the withstand time of the resistor. A 400A resistor is pretty common, having a 10 second withstand. Extended time ratings means more $$$.


Mike

Thanks to dpc for clarifying. Yes...400A resistor on a 480V system is no bueno. My suggestion is for a MV system.