Seeking advice for proper grounding

[BlackEagle]

Hi,
Our house is 150m away from the street. We installed a 600V step-up/down system to bring electricity to it in order to avoid excessive losses and very large cables. The 240V/600V 50KVA transformers are rated NEMA-3R enclosures, located one 10ft above ground and the second one 5ft above ground.
Last week we have got a nasty storm (heavy rain, strong winds and lightning everywhere). Of course, one bolt hit the installation and both transformers blew-up. They have been replaced and I decided to analyze the system. Below is the schematic as I could make it. I am wondering if there is a grounding issue.
Regarding the causes of the blow up, I think the combination of NEMA-3R, the heavy rain and the wind allowed water to enter the step-down transformer. Then the lightning strike created an arc that have been maintained by the utility company power feed. The main breaker did not trip fast enough. We heard the transformer humming like crazy for about one second. The aluminum windings have melted. I suspect the breaker tripped because of over current (>200A), not because of ground fault. This may be the reason it took so long to trip.
In the diagram the black lines are what exists today. The red lines is what I think is missing.
Any advice?
Thanks in advance.

 

[oscar21]

Ok I'll bite, I dont think the primary and secondary neutral/earth plates should be linked. From what I can gather in the UK the HV and LV earth rods/plates at the transformer are totally separate, usually running in different directions.

 

[BlackEagle]

Ok I'll bite, I dont think the primary and secondary neutral/earth plates should be linked. From what I can gather in the UK the HV and LV earth rods/plates at the transformer are totally separate, usually running in different directions.

The Canadian electrical code requires ground wire continuity. This is the reason, as my understanding, the red lines are necessary. Without them, the step-up transformer enclosure and the meter enclosure are not connected and may not be at the same potential, creating a safety hazard for anyone touching them both.

 

[davesparks]

Ok I'll bite, I dont think the primary and secondary neutral/earth plates should be linked. From what I can gather in the UK the HV and LV earth rods/plates at the transformer are totally separate, usually running in different directions.

HV/LV earths are usually linked in the UK as long as the resistance to earth is low enough (known as a cold site AFAIK)

 

[pc1966]

HV/LV earths are usually linked in the UK as long as the resistance to earth is low enough (known as a cold site AFAIK)

Not sure if that is allowed for new sites, and from memory it was below 1 ohme Ra which is often difficult anyway.

 

[pc1966]

Ok I'll bite, I dont think the primary and secondary neutral/earth plates should be linked. From what I can gather in the UK the HV and LV earth rods/plates at the transformer are totally separate, usually running in different directions.

That is usually true, but strictly speaking 600V is not HV!

 

[pc1966]

At the feed end I am surprised that N is connected, you only really need L1 - L2 as the transformer is a 240V load.

At the load end you definitely need N connected as you need the 120V - 0V - 120V support.

I am surprised that a 50kVA 240V transformer was not protected by the 200A breaker so something seems a little unhappy to me (as no doubt to you). Are you sure those transformers are really rated for 50kVA continuous power? If they are undersize and rated for intermittent duty as many cheaper / portable transformers are then you have some sort of an answer.

 

[pc1966]

This might seem like a dumb question, but is there a 200A breaker feeding this transformer, not just a 200A isolator switch?

It is possible the red-line ground might have helped, in the sense it might have reduced the risk of over-voltage from initiating an arc in one or both transformers, but you might also want to consider adding some SPD at the feed end as well as at you house panel.

 

[BlackEagle]

At the feed end I am surprised that N is connected, you only really need L1 - L2 as the transformer is a 240V load.

At the load end you definitely need N connected as you need the 120V - 0V - 120V support.

I am surprised that a 50kVA 240V transformer was not protected by the 200A breaker so something seems a little unhappy to me (as no doubt to you). Are you sure those transformers are really rated for 50kVA continuous power? If they are undersize and rated for intermittent duty as many cheaper / portable transformers are then you have some sort of an answer.

Agree with you that the N at the feed end is not strictly needed. It was connected, so I left it connected. No harm in doing so.
I confirm the transformer is rated 50KVA continuous (I am attaching the datasheet - DA2050V from Delta).
The fact that the breaker did not trip immediately surprised me. This is why I reviewed the entire cabling to understand what was the cause. The best guess I am having today is the combination of water+lightning+utility feed. Even if the grounding was continuous, I might have experienced the same issue.
The grounding continuity has been added as well as a canopy over the transformers. Fingers crossed...

 

[BlackEagle]

This might seem like a dumb question, but is there a 200A breaker feeding this transformer, not just a 200A isolator switch?

It is possible the red-line ground might have helped, in the sense it might have reduced the risk of over-voltage from initiating an arc in one or both transformers, but you might also want to consider adding some SPD at the feed end as well as at you house panel.

The drawing show a breaker. The breaker is in the enclosure of the meter.
The red line was not there at the time of the incident.
There is an SPD on the main panel in the house (EATON CHSPT2ULTRA). On the feed side, a 600V spark gap device could definitely help.