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Predicting Voltage drop #128840 02/04/04 12:07 PM
Joined: Oct 2003
Posts: 10
T
Thomas Burton Illig Offline OP
Member
I have another question, and here it is.
In my area the pole transformers in an average neighborhood range from 25kva to (i think) 67 or 69 kva. Alot of times there are 6-8 houses tied to each.
The question has to do with starting current for Air Conditioners, and the percent line drop during starting.
I realize that these transformers are suppose to be "infinite Buss" but there are limitations especially if someone has a good size A/C unit on one of these.
I was just wondering if anyone has run into this problem and what was the solution. I have read of cases where some starting would cause a line drop as large as 23 % of the available line voltage although momentary it still can be an annoyance to the other customers with blinking lights or motors taking a hit during the start up of these large units.
Tom


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Re: Predicting Voltage drop #128841 02/04/04 10:16 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
Broom Pusher and
Member
Hello Tom, glad to see you again!

As to your Q', I'll address this in a few parts below.

Quote

In my area the pole transformers in an average neighborhood range from 25kva to (i think) 67 or 69 kva. Alot of times there are 6-8 houses tied to each.

I have seen PoCo owned Pole Mounted 1Ø 3 Wire Transformers as low as 10 KVA, and as high as 75 KVA.
The %Z for these (I believe) is between 2% and 5%.
The Primary fuses are typically 600% FLA
On Pad Mount Transformers for Commercial Customers' use, one PoCo in my area considers the Transformer calculated continuous load to be 130% maximum. Higher than this warrants a Transformer KVA size upgrade.
Unsure where the maximum LCL load for Pole Mounted 1Ø Pots becomes crucial for the same PoCo.

Quote

The question has to do with starting current for Air Conditioners, and the percent line drop during starting.

Last Residential project I designed + installed Electrical Systems was an addition to existing 1800 Ft² House - resulting in >3000 Ft² Conditioned space.
With this came (of course) the installation of a new larger capacity Service + Underground Feeders.

PoCo's Design Engineer used my load calcs and specs for HVAC equipment, and applied this data to the existing 25 KVA 2.75% Z Pole Mounted Transformer.
Results were a "Flicker Factor" of 4.5 maximum.
Try entering "Flicker Factor" into a search engine for more information.

Quote

I realize that these transformers are suppose to be "infinite Buss" but there are limitations especially if someone has a good size A/C unit on one of these.

Actually, they are Infinite Bus when applying a Fault Current Calculation for worst-case scenario (like when performing a "Point-to-Point" SCA calc).
PoCo's Flicker Factor calcs do not consider Primaries as Infinte Bus (AFAIK).
At Hef's Playboy Mansion West (Bel Aire, CA.), the Primary Feeders are so overloaded that when Hef' throws one of those infamous parties, the Primary Feeder's OCPD trips several times per evening. They now run on Generator power to avoid the resulting power loss.

Quote

I was just wondering if anyone has run into this problem and what was the solution. I have read of cases where some starting would cause a line drop as large as 23 % of the available line voltage although momentary it still can be an annoyance to the other customers with blinking lights or motors taking a hit during the start up of these large units.

Have run into this problem on a few occasions.
Soft Start methods / equipment are the most common fix on the user's side.
If the problem effects multiple users - and there are not "Huge" unnecessarily large loads being used by the Customers, then the PoCo might consider upgrading the Transformer's KVA size.
If the PoCo can keep the Transformer size low, the problem of frying Service Feeders and excessive SCA can be reduced.

Hope this is helpful information.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Predicting Voltage drop #128842 02/04/04 10:56 PM
Joined: Apr 2002
Posts: 2,527
B
Bjarney Offline
Moderator
It’s hard to say whether it is or is not a problem with the limited information given. For a given load, the two most significant influences on the utility side are transformer characteristics of size and impedance, but also secondary lateral gauge and length. Determining voltage drop is an iterative process, and some estimation will always be involved as there is a wide combination of loads at any given point in time.

What has the utility said? Some lamp flicker is unavoidable without a pricey lateral and transformer resize. You may want to contact your state’s public utilities commission for their rules on acceptable voltage drop. Keep in mind that their obligations end at the meter, and branch-circuit losses are not their responsibility. Also, do not forget that if you or someone else takes measurements, that the test equipment must ultimately have recent, documented NIST traceability, and published errors must always be accounted for.

Re: Predicting Voltage drop #128843 02/06/04 10:39 AM
Joined: Dec 2002
Posts: 110
W
wocolt Offline
Member
Hi, Tom
The way I look at it is this. The percent line drop is a function of Motor starting MVA and available Short Circuit MVA available at the terminals.
Or
% VD = (Motor Start MVA)/(available SC MVA) X 100 this will give you a percent line drop for starting. however, in this case KVA would be appropriate.
For example a 10 horsepower motor at 240 volts code letter G would have a starting KVA of 63KVA
A 25 KVA transformer with a 5-7 % impedance would have a SC kva or 500kva to 357kva between the two the line drop would be between 13 to 17 % during starting.
This is just the beginning of the problem. With the extent of the voltage dip and allowing for further voltage drop due to the size of the feeder to the motor,the voltage at the motor terminals is now seriously depressed. since motor torque is proportional to the square of the motor terminal voltage the starting torque will be well below normal starting torque. This in turn will cause longer starting times for the motor/compressor. That is acceleration time will be longer and the motor will be under duress (severe stress) for longer than necessary.
As Scott pointed out, 'if this is the case then the power company should be involved and maybe some type of soft start should be used to help lessen these effects.'

woc


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