Sizing the generator-set for the core loop test. 3

[kh2]

How do you size a three phase 480 volt gen-set in KVA (KW) based on loading only one phase of the gen-set? The current required to perform the core loop test (in this case) is 615 amps single phase at 480 volts.

Appreciate your expertize and valuable responses.

 

[ScottyUK]

kh2,

At our site I've successfully (ab)used three-phase hire gensets to provide single-phase test current when carrying out primary injection tests on large transformers. I have sized the genset at approximately 1.5x current rating, which suggests a 750 or 800kVA set for your application, although the sizing is based upon handed down advice rather than on calculation. The rotor will see a serious negative sequence heating effect due to the load imbalance.

You should consider that the core loop test will present a low power factor load to the generator and this may require a further increase in rating because the generator capability will be limited in this region by rotor heating, and the rotor will already be seeing large NPS currents due to the unbalanced load. Most European gensets of this size don't have a NPS protection relay, so you arguably can get away with this kind of abuse provided it does not go on for too long. Don't do it with your own generator!




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If we learn from our mistakes,
I'm getting a great education!

 

[kh2]

ScottyUk,

Thank you for your important input. The Contractor decided to lease a 1000 kW, 3-phase, 480 volt gen-set. I will keep you informed of the behavoir of the gen-set durig the test specially that the full flux will be applied and stayed for 60 minutes. The gen-set should be stable all that time.

Regards, Kh2

 

[kh2]

ScottyUK,

The core loop test was performed on 11/6/04 using a gen-set rated 1,250 kVA, 3-phase, 480 volts and an auto transformer 1,875 kVA, 480-600 volts. Full flux density of 82 kL/square inch was applied at 570 volts, 5 turns, 103.2 volts (search coil voltage), and 554 Amps, loop current. The test was run for 40 minutes and discontinued because the core temperature reached 10C rise over the frame temperature. No hot spots were detected. The generator was stable and the test was completed without any problems. As you see the current was 554 amps at 570 volts. This is a two phase load or 554 x 570/1000 = 315.7 kVA. Three phase load = 1.5 x 315.7 = 473.67 kVA. Apprecaite any comments from you and from the other experts.

Regards, Kh2

 

[edison123]

kh2

What is the capacity and speed and core dia of your generator on which the loop test was done ?

The 3 ph rating of a generator is for a balanced loading. For heavy unbalanced loads like yours, as Scotty says, rotor overheat due to -ve sequence currents will reduce the usable KVA. At the end of the test, did you check the stator and rotor temps of the hired genny ? And may be 40 minutes was too short a thermal time to develop any trouble.

 

[kh2]

Hi edison123

The generator is 85 MVA, 0.95 PF, 13. 8 kV, 400 rpm. The core length is 62 inches, and effective length is 53 inches. The outside diameter D1 = 187 inches and the inside diameter D2 = 169 inches.

No check of the temperature of the stator and the rotor of the leased generator. But nothing was reported unusual with regard to the generator performance. You have a good point here and was missed.

The core loop test was performed on 5 steps;
Step 1: Applied voltage 477, number of turns 24, search coil 19.9 volts, loop current 17.4 amps.
Step 2: 477/13/36.5/45.7
Step 3: 476/7/58.4/108
Step 4: 470/6/75.9/212
Step 5: 570/5/103.2/554.
The duration of the test for steps 1 through 4 was 10-15 minutes. The frame temperature was about 19 C and the core reached about 22 C and back to 19.5 before the next step.
The last step (step 5) the temperature of the frame reached 20.3C after 40 minutes and the core temperture reached 31.2C. The test procedure states that the test for step 5 will last 60 minutes or until the mean core temperature reaches 10 dgrees C over the frame temperture, which happenend and discontinued the test. After the 40 minutes the supply votage was 565 volts, the supply current 620 amps, and the secondary turn voltage 102.4 volts.
An infrard camera was used during the test to detect any hot spots. The core passed the test with no hot spots.
The five points were plotted on a curve with the Ampere-turns on the Horizental axis, and the Volt/turn on the Vertical axis. It is interesting to note that only about 350 kVA was required to saturate the core and that the core temperature only increased by 10 degrees C during the 40 minute test.

Appreciate you input.

Regards, kh2

 

[edison123]

Thx kh2 for your detailed feedback. Looking at AT, I figured it must have been a large dia/slow speed genny. It is interesting that you did the test in steps. I used to go direct to the calculated AT. May be, I will follow your method in future to see what happens.

The reason for low AT and low core temp rise is that you use a single phase flux which is only pulsating and generally is restricted only to the teeth unlike a 3 ph flux, which is circulating and enters back iron also. With a 3 ph flux, I would expect about 20-30 deg C rise in core temp for a machine of this size. You can check this by doing open circuit characteristic (OCC) test during commissioning when only the iron loss is present in the stator.

I will be shortly doing this core loop test in a 25 MW, 14 pole hydro genny with a core length/dia of 1.1 M/3.0 M. Will let you know how that goes.

 

[edison123]

kh2. The feedback I promised.

Core eff length - 940 mm Dia - 2890 mm.

Loop turns - 10 Current - 200 A Turn volt - 34 V

Average core temp rise after 60 minutes - 6 deg C (from 27 to 33 deg C)

Hot spot temp - 34 deg C

 

[kh2]

edison123,

Thank you for the information. On our Core Test Procedure, the Contrator has stated that the test will last 60 minutes or until the mean core temperature reaches 10 degrees C over the frame termperature. Our test was discontinued after 40 minutes at full flux density because the core temperature reached the 10 degrees C above the frame temperature. The final core temperature was 31 degrees C and the final frame temperature was 20.2 degrees C.

Did you record the frame temperature during your test? And if so, was the difference less or more than 10 degrees C? Thanks again.

 

[edison123]

kh2,

We did measure the frame temp before and after test. It stayed the same at 26 deg C.

Our AT was 2000 and yours was 2216. For your dia and length (more than mine), I would have anticipated more AT to produce a 10 deg C. Was yours tested with the winding in place ? My test was on an empty core so quciker to heat. What was your back iron depth (below the slot)?

 

[ScottyUK]

edison and kh2,

I'm am still following this thread and learning from you both. It is good to see some practical experience of performing this sort of test on large machines, especially when it is backed up by well-reasoned discussion and well-documented results. The difference between estimated and actual current drawn from the test source is interesting, as is the likely cause highlighted by Edison. Your notes may well be invaluable in years to come.




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If we learn from our mistakes,
I'm getting a great education!

 

[kh2]

The core was tested with the new winding in place. The iron depth below the slot is 26.87 inches. This figure is reached as follows: Core OD = 186.7 inches; ID = 156.8 inches; Slot depth = 6.032 inches. Additional information: Core length = 62 inches; Number of ducts = 33; Width of ducts = 0.188 inches; Stacking factor = 0.95; Effective lenght = 53 inches; and Flux density = 82 kL/sq.inch. This the latest and more accurate data of the machine.
The frame temperature at start of the test was 19.1 C and after 40 minutes was 20.2 C. The core temperature at start was 21.9 C and after 40 minutes was 31 C.

We still did not receive the justification, from the Contractor, for the 10 dgrees C difference between the frame and the core to stop the test. Do you have an explaination?

Thanks.

 

[edison123]

I presume yours is a 18 pole, 60 Hz m/c.

Are you sure about the back iron depth of 26.87 inches ? Given your ID/OD/Slot depth, I get it as 8.918 inches which is reasonable for your 18 poles.

With 570 V, I get 6 turns with about 800 A loop current for 85 KL/sq.inch.

Standards do not talk about frame temp vs core temp. The acceptance criteria - Less than 10 deg C between the hottest core temp and the average core temp at the end of the test.

 

[kh2]

You are right edison 123. The back iron depth is 8.918 inches as follows: (186.7 - 156.8)/2 = 14.95 - 6.032 = 8.918 inches. The gnenerator is 18 poles, 400 rpm, 60 hz.

The test report showed that the number of turns was 5, the voltage 515 volts, and the current was 617 amps. The step up transformer used was 480/570 volts. The volt per turn = 103 volts, and the flux density = 82 kL/sq. inch. Thank you for your comment that let me review the information given on the report.

Please give me the name and section number of the Standard that provided the acceptance criteria to be less than 10 deg C between the hottest core temp and the average core temp at the end of the test. This informantion could very much settle the issue with the Contractor.

Thanks again for your valuable input.

 

[Sargardani]

KH2

I have been following the thread and would like to thank you folks for very thorough and sound discussion. The 10C temperatuire difference between the core and the frame may not be defined in a standard, yet, but apparently the logic behind it is to avoid exposing the core to stress, which potentially can cause damage to the core and or frame. The core loop test is a good test until and unless its done as safely as possible. Besides from your above discussions its evident that you have a very good core as no hot spots were found and I am not sure what further achievements can be made if the test is continued for 60 minutes or beyond?

Sarg

 

[kh2]

Sarg,

You are right, there was no hot spots, but to discontinue the test because of reaching 10 deg C difference between the frame and the core, this is not going to cause damage to the stator. It may be alarming if the difference reched 25 - 30 deg C. The explaination of the one hour is given on EPRI Report EL-5036, Volume 16 - Section 5.1.5.1 which stated "defects deep in the core may take up to an hour to show as high temperature, on the observed surfaces, because the surrounding 'healthy' sections of core act to dissipate the heat. Also, you may refer to IEEE Standard 56-1977 Appendix "Test of Laminar Insulation in Stator Core" under A4, is stated that a final heat run of 1 to 3 h should be made after all repairs are completed. And, this is not a new stator core. There is no mention on the IEEE Standards (to the best of my knowledge) of a maximum temperature differential between the core surface and the stator frame during the core loop test.

Kh2

 

[itsmoked]

What's a core loop test? And how is it done?