Primary Transformer Compatability Problem 3

[powerproblems]

In checking into power problems in (2) differant rural schools. They both suffer a grea deal of electronic equipment failure (typically after a power outage or primary phase loss). I have found that both had delta wye configurations on the primary transformers. This is unusual in our area as the utility is a wye configured system. I feel that this is causing the problems but I am having trouble finding the documentation I need to support this. Any help would be appreciated.

 

[dpc]

The only issue with the delta-wye transformers is the slight risk of ferro-resonance. This can occur if the transformer is very lightly loaded, the primary is a long run of underground cable and one of the primary phases is lost. This could cause overvoltage conditions.

If you get lightning in your area, you might also ask utility to check the transformer grounding to make sure it is intact. Surges on the primary can create problems on the secondary side if you don't have a good local ground reference.

Aside from the ferro-resonance problem, which can be solved by using three-pole switching and protection, a delta-wye configuration is really to be preferred over a wye-wye, at least for the customer.

 

[mc5w]

Lightning protection will only be as good as the ground. There is somebody at this forum who is with a Colorado electrical utility and he says that on the portions of their system that use 17 foot ground rods they have ZERO lightning damage. Maybe sometime in the next 10 years they will have a big enough stroke to toast the 17 foot rod. This uses ground rod couplings and there are now threadless as well as threaded couplings. If you cannot drive the rod 16 to 20 feet deep you might want to consider a Porous Pipe (R) irrigation system to supply a few gallons of water every night.

One of the top electrical inspectors in northeast Ohio says tht only the bottom 2 feet of an 8 foot rod driven from the surface will act as a grounding electrode. Most soils around here are clay that retains moisture. He liked to drive ground rods below basement floors when he was an electrical contractor.

If the rod is installed in the backfill around the pole or is within 3 feet of the pole ground resistance will be increased by loose soil. Even if you use power compacting equipment the soil will take 20 years to compact enough to work well.

One time a cable plant technician for Ohio Bell told me that 97% of their lightning damage comes from silent lightning and invisible lightning. He then said that when a Big One strikes it has about 100 side strikes over a 100 yard radius.

We have also installed copper foil underneath floor tile to provide a static electricity path using the equipment grounding wires in the electrical outlets. If a person has a 100,000 volt or even 25,000 volt electric charge on them that is just like silent lightning.

 

[NYedison]

Powerproblems
Can you elaborate “primary transformers” ? Do you have your own secondary transformers?

Do you attribute the failure to lightening, Overvoltage and grounding ?

Is the primary side high voltage and Delta? And secondary Wye?
Is the secondary Wye 4 wires?
Do they have TVSS, Movs and other overvoltage protection devices on the Load side? You have more scope to work on Load side.
Marvin

 

[mc5w]

One thing that you might want to consider for delta primary wye secondary when using open cutout fuses is to use 4 primary fuses. This reduces risk of funny voltages as the transformers can be energized 1 at a time.

One of the few things that Cleveland Public Power does right is to use 4 primary fuses on delta-delta transformer banks.

 

[powerproblems]

In answer to NYedison: One of these schools were built within the last ten years and the other was built this summer (2004). They both have almost identical configurations fed from the utility overhead 12.kv 3 phase at the road. It then is metered. At this point, the customer then owns the primary cables and transformers. Then it enters a 3 compartment S/C switch. It then continues undergound to another S/C switch where it branches to (3) seperate 12.4KV delta/480-277Wye transformers. The primary cable is wye configured from the utility and feeds the (3) wire delta wound primary side of the transformer. The secondary side of the transformer is 4 wire wye. The wye from the utility is bonded to the tank as well as the 480 volt service ground.
There are TVSS units on and MOV on the service as well as all of the building panels. In the older building (10 yrs) the Cutler Hammer MOV TVSS unit has expired and needs to be replaced. After an outage on the Max/Min panel meter that when they lost 2 of the phases the other phase increased to 528 volts. They did not lose equipment in this event, but it may illustrate what could be happening.
As far as load on the secondary side it is a school. It is overbuilt, lightly loaded and probably never a balanced load.