Efficiency standard for Distribution Transformer
Efficiency standard for Distribution Transformer
(OP)
I have a general question on efficiency on distribution transformers. In our specification, we specifiy 20000/25000kVA transformers to be factory tested to the standard of NEMA TP 1. In this case, according to the standard, which specifies the minimum efficiency levels for NEMA Class I liquid filled distribution transformers, it says for for transformers 2500kVA and beyond, the minimum efficiency shall be 99.4%. This is from Table 4-1 in Section 4, which listed reference condition for Load Loss at 55% and at 50% nameplate loading.
The manufacturer factory test reports came out to, as an example, 99.37%/99.36%/99.35% forone transformer and 99.35%/99.34%/99.32% for another, etc. The results are close to 99.4%, but not quite.
When I asked the manufacturer to justify these test results, and to clarify what standard was used in conducting these tests, the reply was they categorize this as a power transformer vs a distribution transformers, the standard does not apply. The efficiency tested just happens to come out close to NEMA TP-1, even though the standard does not apply.
FUrther, the manufacture says that ANSI, NEMA nor IEEE do not specify efficiency for power transformers above 25000kVA for the reason that hihger efficiencyies require superior core steel and that may not be readily available. Current standards on power transformers govern only safety and construction but not on efficiency. The Mfr tried to optimize the design of power trans taking into account material cost that result in the least costs of ownership to the end-user under normal conditions....
This sounds like a standard industry response. My questions are:
1. Is it true, that efficiency does not exist for large transformers similiar to what we used?
2. Is 2500kVA the maximum limit on transformer capacity where one can truly comment on efficiency, and anything else larger than that is merely luck?
3. Even if it is true no standards exist for such large transformers, there must be a minimum efficiency you can use to determine the level of construction we are getting from the mfr?
4. One last thing, am I being too picky? Is perhaps 99.32% considered acceptable? Afterall, the standard only goes up to 2500kVA. It must be for a reason.
I like to understand more of the economic as well as technical justifications based on what was discussed above. If anyone has any comments, I will be glad to hear them. Thanks.
The manufacturer factory test reports came out to, as an example, 99.37%/99.36%/99.35% forone transformer and 99.35%/99.34%/99.32% for another, etc. The results are close to 99.4%, but not quite.
When I asked the manufacturer to justify these test results, and to clarify what standard was used in conducting these tests, the reply was they categorize this as a power transformer vs a distribution transformers, the standard does not apply. The efficiency tested just happens to come out close to NEMA TP-1, even though the standard does not apply.
FUrther, the manufacture says that ANSI, NEMA nor IEEE do not specify efficiency for power transformers above 25000kVA for the reason that hihger efficiencyies require superior core steel and that may not be readily available. Current standards on power transformers govern only safety and construction but not on efficiency. The Mfr tried to optimize the design of power trans taking into account material cost that result in the least costs of ownership to the end-user under normal conditions....
This sounds like a standard industry response. My questions are:
1. Is it true, that efficiency does not exist for large transformers similiar to what we used?
2. Is 2500kVA the maximum limit on transformer capacity where one can truly comment on efficiency, and anything else larger than that is merely luck?
3. Even if it is true no standards exist for such large transformers, there must be a minimum efficiency you can use to determine the level of construction we are getting from the mfr?
4. One last thing, am I being too picky? Is perhaps 99.32% considered acceptable? Afterall, the standard only goes up to 2500kVA. It must be for a reason.
I like to understand more of the economic as well as technical justifications based on what was discussed above. If anyone has any comments, I will be glad to hear them. Thanks.






RE: Efficiency standard for Distribution Transformer
2 through 4: How much are you willing to pay for a high efficiency transformer? At some point, the economics don't work when the extra first cost exceeds the value of the losses saved over the transformer lifetime. You need to evaluate losses with a dollar figure that will allow a manufacturer to design a unit that will suit. ht
RE: Efficiency standard for Distribution Transformer
Chalk it up to experience.
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer
I was thinking about what OPMGR1 has said earlier, too. I really like to know can you determine exactly what the upfront cost is vs exactly what efficienty you are going to get. That is, can you design your transfomrer construction so well, that the manufacturing can give you exactly what efficieny you asked for? Or is some allowance/tolerance permitted at the end?
Earlier in this discussion by stevenal, he said that minimum is not required by standards. Is this statement true? I thought what NEMA TP 1 is specifying is the minimumm efficiency for the specific class of transforms?
Just like to hear what everyone's opinion is?
Andrew
RE: Efficiency standard for Distribution Transformer
Knowing the efficiency at a single load point is not as useful as knowing the no-load and load losses. With the no-load and load losses, you can determine the efficiency and any load and the loss costs for any load profile.
RE: Efficiency standard for Distribution Transformer
Evaluation of losses is the best approach. This requires you to estimate what the energy loss costs actually are to you.
RE: Efficiency standard for Distribution Transformer
Because losses and efficiency are related, one calcuates the other, I assumed they mean the same thing.
Our spec did say that if the efficiency is lower than the NEMA Class 1 Eff, the product is unacceptable. Based on what we discussed above, guess we have a defective spec.
Switching the topic, how do you determine what is the optimal losses you should have from a manufacturer viewpoint? What is the design criteria to optimize ownership cost vs upfront cost of construction?
h
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer
You can calculate efficiency at 50% load from the losses, but you cannot calculate the no-load and load losses separately from the efficiency at 50% load.
See stevenal's 12/16 link or opmgr1's reference for the method to optimize ownership costs. You need to ask a manufacturer how to optimizing losses from their standpoint; I just know it can be done.
RE: Efficiency standard for Distribution Transformer
Using the same formula as for the standard;
eff = 0.5*KVA/(0.5*KVA+0.25*LOADLOSS+NLL)*100
We get for the following transformer sizes, the following efficiencies. A number of each MVA transformer were calculated. They are all 3 phase core type.
120MVA - 99.68%
50MVA - 99.60%, 99.58%
33MVA - 99.54%
19MVA - 99.72%, 99.71%, 99.7%
Hope they are of interest to the forum.
ausphil
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer
I'm guessing that a10jp's 20000/25000kVA unit is an OA/FA transformer. The overall efficiency will change for the load regions where the cooling fans are or are not operating. The same holds true for an FOA transformer. The efficiencies in each region should be calculated and compared to the expected load profile to determine the economic costs of losses.
RE: Efficiency standard for Distribution Transformer
ausphil
RE: Efficiency standard for Distribution Transformer
RE: Efficiency standard for Distribution Transformer