Distribution Transformers Replacement Strategy

[Adam1980]

Dear all,

is there any literature regarding recommendations for transformer replacement (time-line).

The transformers installed currently are 13.8/0.22 kV and serving a residential area (about 1000 units with different ratings). These are being replaced becausee the utility is changing the low voltage level to 0.4 kV.

The transformers are located in different geographical areas (neighbourhoods or residential blocks) and connected to different feeders so logically those geographically close and connected to the same feeders should be replaced in the same week/month/year.

But is there any systematic approch for such an activity?

Thank you.

 

[magoo2]

I'm not aware of any papers on this.

You can approach it as a mathematical oprimization problem where you're trying to minimize the number of trips. There's a "wandering salesman" example in the annals of mathematics that you can problem find some reference.

You can also look at the reliability aspects. Assume the 0.22 kV transformers are more likely to fail than the ones use on 0.4 kV. What are the consequences?
- Could look at the impact on your reliability indices. This would favor going after clusters of transformers rather than single ones.
- might also look at loss of revenue. This might guide you to replace those that have the highest revenue stream over others thant are lightly loaded.

 

[Adam1980]

Thanks for the reply. Appart from the "time crunching" issue is there anything that should be considered?

In principle:
- The equipment of the LV distribution should be able to handle the voltage change as these are usually rated to 1 kV.
- Breakers and protection settings should be re-calculated since higher voltage will lead to smaller currents however transformers with larger rating could lead to more short-circuit contribution.
- Is there a certain way to optimize the change, i.e. disconnect available transformer, move it out, move in new transformer connect. During this time a mobile diesel generator can supply the load. How long would this process take?
- Any other issues?

Thank you.

 

[magoo2]

Tell me more about your system since I'm in the U.S. and guess you are not.

Are these mainly three-phase padmount transformers? If so what are typical ratings? 300-500 kVA?

The 0.22 kV seems like a single-phase voltage. The 0.4 kV would be a 400 V Y/230 V connection where the customer gets 0.23 kV instead of 0.22 kV. Correct?

Most of our residential transformers are single-phase and we would just take one out and put in its replacement. Even where we have three-phase transformers, the practice is the same. We wouldn't have a generator. It is normal for our customers to expect to lose power until the transformers are swapped out.

 

[Adam1980]

The system is in Saudi Arabia the old 220 V or 208 V 3 phase system is still being used there and now there is a plan to change to 400 V. Some transformers are still single phase but I believe everything should be 3 phase as per international standards. The rating of the transformers varies a lot from 50 kVA till 500 kVA. so there is also some work to be done for using certain size as standard.

But the main idea now is will replacing the transformers be just a matter of simple switching based on crews working on a certain area or is there any hidden "technical issues".

Thanks

 

[jghrist]

Your secondary system probably would not have to change much, as you have indicated. The 220V equipment is probably rated for 600V or 1 kV. Currents would decrease for the same kW load, and conductor sizes would be more than adequate. Fusing sized to protect conductor would not have to change if the conductor size didn't change.

It would seem that the main impact would be on the customer. Single phase customers that were served with phase-to-phase voltage and had 208V or 220V appliances would now be served with 230V phase-to-neutral voltage and would have minimal changes. Any customers that had 120V phase-to-neutral connected equipment or 208V or 220V three-phase equipment would have to install step-down transformers or change their equipment. Timing the transformer replacement may be more a political than technical decision. Which customers do you want to inconvenience first?

 

[waross]

Don't forget to check the customer's voltage before changing out each transformer/bank. A utility was changing transformers because of a change in primary voltage in an area. They removed a bank supplying 120/208 Volts and replaced it with a bank connected for 240/416 Volts. Fortunately I had previously installed phase loss/ reversal and under/over voltage protection equipment for the refrigeration equipment.
Unfortunately everything else let the smoke out. Lighting ballasts, electronic scales, telephone equipment, all the air conditioners and anything else that was connected in. I was the hero for having saved the refrigeration equipment. A total loss of product would have increased the cost of damages many times.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Adam1980]

Hello,
thank you all for the replies.

The consumers should have their appliances changed forehand.

I would like to ask if someone knows what would be the time needed to change one transformer if this is done by a professional crew. Should there be testing for the grounding system following the installation to know if it can handle the new fault currents.

thanks