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Transformer rating when power flow direction may change
4

Transformer rating when power flow direction may change

Transformer rating when power flow direction may change

(OP)
C57.12.00 Section 5.4.1 states:
The rated kVA of a transformer shall be the output that can be delivered for the time specified at rated secondary voltage and rated frequency without exceeding the specified temperature-rise limitations under prescribed conditions of test, and within the limits of established standards.

C57.12.80 defines:
secondary voltage rating: The load circuit voltage for which the secondary winding is designed.
secondary winding: The winding on the energy output side.

For a 50 MVA transformer with 49 MVA of load, 49 MVA is on the energy output side and ~52 MVA is on the energy input side due to transformer losses. According to C57.12.00 the transformer is properly rated.

However, in the case where the energy output side could change, such as a BESS in which the system could be charging or discharging, is the transformer properly rated? The once secondary winding is now, by definition, the primary winding and must be rated higher than 50 MVA to account for transformer losses.

RE: Transformer rating when power flow direction may change

Quote (C57.12.80)

C57.12.80 defines:
secondary voltage rating: The load circuit voltage for which the secondary winding is designed.
secondary winding: The winding on the energy output side.
That can't be right.
Didn't Eng-Tips culture recently agree that the higher voltage side was always the primary side.
Another issue may be the voltage.
If the secondary voltage is rated at full load, then when reverse fed, the output voltage at full load will be low by a percentage closely equal to twice the regulation percentage.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Transformer rating when power flow direction may change

Personally, I'd go for dropping Primary and Secondary all together and just talk about high-side and low-side. Or, even, the H-bushing side, the X-bushing side, and occasionally the Y-bushing side.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

RE: Transformer rating when power flow direction may change

1) Transformers are rated for bi-directional flow.
2) When we refer to a 50 MVA-rated transformer in IEEE World, we are discussing the transformer's output. However, in IEC areas, it is the input to the transformer, reflecting relevant global standards.
3) In reality these are not going to affect much as transformer efficiency is above 99 %, in larger units approaching 99.9 %!

RE: Transformer rating when power flow direction may change

(OP)
2) When we refer to a 50 MVA-rated transformer in IEEE World, we are discussing the transformer's output.

Correct. Wouldn't the transformer be manufactured such that the output winding is rated for 50 MVA and the input winding is rated for 50 MVA + losses? My question is what if the input and output windings change daily in the case for BESS collectors? If the 50 MVA rated output winding all of a sudden becomes an input winding, it will be overloaded because of the transformer losses.

RE: Transformer rating when power flow direction may change

(OP)
I've answered my own question. I came across a nameplate that instead of "STEP DOWN" or "STEP UP", it said "STEP UP / STEP DOWN". So I assume the transformer was rated for bidirectional flow. And this needs to be included in any transformer spec for BESS.

RE: Transformer rating when power flow direction may change

This will only be an issue at times when the ambient temperature reaches the rating temperature.
Transformers are very tolerant of much greater overloads.
You make a good and valid point from an academic viewpoint.
In practical terms, a greater concern may be installation in an area where the ambient temperature may reach or exceed the rating temperature.
Your concern will only be realized in the event of anticipated high ambient temperatures coinciding with 100% transformer loading.
In that case, it may be prudent to derate the transformer for high ambient temperature and such derating will also compensate for the transformer's ability to safely cope with the slightly higher losses occasioned by reverse power flow.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Transformer rating when power flow direction may change

As you stated, any transformer for BESS should be specified as both step up and step down. There are enough subtle differences that it would be unfair if one bidder assumed only step down operation and another bidder assumed bidirectional operation.

When failing to specify bi-directional application and operating at 0.8 pf at full ONAF2 rating, the LV winding would have 10% higher MVA and 22% higher I^2*R losses than the design anticipated. At a more typical 0.95 pf, the LV winding would have 6% higher MVA and 13% higher I^2*R losses. The HV winding would have correspondingly lower losses.

RE: Transformer rating when power flow direction may change

I guess the BESS transformer will be with OLTC.
If yes, the voltage reference signal to AVR can be switched (by automatic / fast-acting control system) when the Battery starts discharging in to grid so that the AVR maintains grid side (of transformer) voltage, I suppose!

R Raghunath

RE: Transformer rating when power flow direction may change

Transformer are step down or step up based on the application, The winding through which transformer is energized at site is always called Primary winding, It can be LV or it Can be HV, If it is LV then you are dealing with step up and if you are dealing with HV then it is step down. Ideally the input Power (kVA) should be always equal to the output power (kVA)but in practical world losses exists and these losses are in kW not in KVA. you can convert them back in to KVA if you know the load power factor, that is the reason the efficiency equation of transformer have Cos theta in its equation. Because transformer does not have any rotary part so the losses are very less compare to input and hence efficiency are near to 99%. BESS transformer have usually of small rating like less than 10MVA, and Efficiency matter more when there is DOE compliance in these transformer which is more specifically less than or equal to 2.5MVA. All transformer are not designed for Bi directional unless it is mentioned on the nameplate. Because may be designer have utilized the max flux density limit which can cause saturation of the core in reverse flow of power.

RE: Transformer rating when power flow direction may change

For a transformer input kVA to match output kVA, both the kW losses and kvar losses must be zero. For determining input transformer kVA in practical transformers, it is the reactive power losses rather than the real power losses that drive the calculation.

RE: Transformer rating when power flow direction may change

And if you're sizing it so precisely that any of this matters, it's too small.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

RE: Transformer rating when power flow direction may change

RANS 20,
1) Transformers are designed for bidirectional power flow. IEC 60076-1 clause 3.4.6 -quote " rated power
Sr
Conventional value of apparent power assigned to a winding, which, together with the rated
voltage of the winding determines its rated current.

NOTE: Both windings of a two-winding transformer have the same rated power, which by definition is the rated
power of the whole transformer. unquote

2) However, there is an exception. Some Specific OLTC models, although rare in modern times, are designed for unidirectional power flow.

3) Let us take a 10,000 kVA transformer with a 10% percentage impedance and losses of 60 KW ( no-load and load losses). Let us assume the primary is fed from a generator to the extent of 10,000 kW. The transformer's output will be 99,940-j 1,000 (i.e. 99,940 kW and 1000 kVAR have to come from a generator, or it will be absorbed from the grid to feed transformer reactance, making the output leading pf.

4) Transformers are designed to suit 10 % overloading continuously.

5) Transformers are usually designed for 10 % continuous over-fluxing, 125 % for one minute, and 140 % for five seconds. This will be achieved if B is equal to or less than 1.72 T.

6) Even when the tap changer is provided on the HV side for HV voltage variation, the operator may change taps when LV voltage dips, though the primary grid voltage is constant, to maintain LV voltage. Then, flux density will go up over the designed value. Transformers can stand such variations in flux density in the core.


RE: Transformer rating when power flow direction may change

Please read the below article:
https://energycentral.com/system/files/ece/nodes/4...

If the transformer is designed for 1.75 Tesla then they can create issue and not able for bidirectional flow, when we say the word "Usaully" thats always have some conservative approach not accurate one.

RE: Transformer rating when power flow direction may change

PRC's example has the LV winding operating 1 PU and the HV winding operating at 1.004 PU. Per David's point, a 0.4% difference seems trivial.

However, that example is not reflective of a typical generation site in the USA required to provide 0.95 pf at the POI on the high side of the transformer. For this case the windings operate at:
HV winding 9,500 kW + 3,123 kvar = 10,000 kVA
LV winding 9,500 kW + 60 kW + 3,123 kvar + 1,000 kvar = 10,410 kVA.
A 4% difference might be considered trivial.

However, many transformers are operated at the ONAF2 rating rather than the self cooled rating. In this case the windings operate at:
HV winding 16,075 kW + 5,204 kvar = 16,667 kVA
LV winding 16,075 kW + 167 kW + 5,204 kvar + 2,777 kvar = 18,097 kVA.
An 8% difference seems more than trivial to me.

RE: Transformer rating when power flow direction may change

1) Thank you, bacon4life, for showing that the difference in loading between the HV and LV windings is significant. A star for you! But transformers are capable of handling such differences efficiently. Instead, the PF and reactance kVAR should be considered while selecting the kVA transformer rating. But as mentioned in the IEC standard, both windings shall be of the same kVA, forming the nominal kVA of the transformer.

2) When we say the transformer rating is 10 MVA, it means that at an ambient temperature of 20C, it shall transfer that much MVA without the average winning temperature exceeding 85 C.( 65+20). However, the standard allows operation up to a maximum ambient temperature of 40C. This means there will be a loss of life when the ambient temperature is above 20C. The loss of life will double with each 6C rise in temperature. It is assumed that the loss of life will be compensated when the ambient is below 20C.

RE: Transformer rating when power flow direction may change

This is one aspect where the ANSI standards differ from IEC standards. The default for ANSI specified transformer is that the transformer is only rated in a single direction. For ANSI specified transformer, the buyer must specifically request step up and/or bidirectional ratings.

A slight quibble is that the Arrhenius equations predicting insulation loss of life apply all temperatures, rather than just when the winding is above some threshold. At low temperatures the thermal related decay in the insulation system is small enough to be considered negligible. Other non-thermal factors like rust, LTC operation counts, and oil leaks then become the most life limiting factors.

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