Re energizing problem

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dcv

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texas
Below is an email I recieved from a Electrician who is having a problem on a college campus where he works any ideas or suggestions will surely help. thanks:
Craig,
The new cables was installed by your company and tested on , or around 10/31/2010 By ------ testing Co. They left after testing that evening. We brought power back up and It was on for and 10 min. with no problem, we were in the process of bringing the buildings back on line, when the circuit blew while bringing up one of the building. It took out the C phase pole-top fuse, (200 amp ), and one lighting arrestor in our central plant gear. We replaced the lighting arrestor and refused the pole top. This time the circuit held.
The next outage was to have Absolute remove parts from our Ball/Anderson pad mount switch for repairs, check the capacitor bank, and 4-1 air switch. The air switch was at this point not operable and we decided to re-energize by the pole top switches. We brought up C phase first, B phase second, and when we brought in A phase the B, and C fuses blew taking out the lighting arrestors. We replaced all three arrestors,(10kv), and refused,(200 amp), we brought up A phase, B phase, and C phase in that order. The circuit held.
Last month we brought the circuit down to re-install the parts to the ball/Anderson pad mount switch parts, that were removed on the previous outage. Again while re-energizing the circuit, this time A phase first, B phase blew when energized, also taking out the B phase lighting arrestor. We replace the arrestor, and fuse. Re-energizing went without any problem?
We only have three transformers that become energized without load during this time. I believe all are oil cooled Delta-wye.
We will have an outage on August 20th. To replace the 4-1 air switch, and would like to have this problem resolved by then. Do you have any suggestions as to what might be causing this?

Thanks

John
 

dcv

Member
Location
texas
Also I want to add the location of this college campus is a costal town, rich with salt air and we are in the middle of a drout so rain is not washing the power lines
 

zog

Senior Member
Location
Charlotte, NC
Hard to tell what and how this testing company did anything without details of the system. Was this system properly tested per ANSI/NETA testing standards by certified testing techs?
 

dcv

Member
Location
texas
Hard to tell what and how this testing company did anything without details of the system. Was this system properly tested per ANSI/NETA testing standards by certified testing techs?
I feel The new section Of Wire can be ruled out as they have been energized for almost a year without incident, However here is some history on the section I replaced.
A section of an old duct bank had to be relocated: new 15Kv 2/0 copper Medium Voltage (MV 105) cable 133% insulation w/ copper shield grounded at both ends and continues on to existing equipment. Lenth of runs are 3cables x 488' underground between two 6'x6'x 8'deep j-boxes also underground. Inside each box is where the new splices to the old cables. Cable bodies(cooper 600-30) and juction bars (cooper 600-42)have same spec ratings 600A 15/25 bolt deadbreak connections 900A Continous 19kv discharge. Without being able to post the test report all I can say about the cable testing is they were tested by a third party testing company and High Potential or hi pot test was performed for 19 minutes voltage brought up slowly to 5,000dcv which is where the test was over and each cable with cable Bodies install at each end were tested and was determined to have passed and safe to energize. After this, another test was performed on the entire lenth (olded connected to new) at a much lower voltage to avoid damaging the old cables.
 

zog

Senior Member
Location
Charlotte, NC
Well for starters DC hipot tests should not be performed on any service aged (5 years) MV cable as it is considered a destructive test by every standard (IEEE, ICEA, ANSI, NETA) that covers MV cables. And even if a DC test was performed and did not damage the cable DC hipot tests may not show any issues besides "gross installation failures", so passing a DC hipot test does not necessarliy mean it is a good cable.

There could be several issues here but I would start with testing with some more modern methods and equipment.
 

dcv

Member
Location
texas
My knowleged of MV work is limited my main stay is 480v and below,the test preformed on the older cable was a much lower voltage I am not sure how low but much lower, now I do know that a hi pot test causes damage on new and old in fact MV Cable come from the factory already damaged on a microscopic level at least that what the films and classes i sat through taught me. DC hi pot test is the only test I am Aware of used to test MV cables. Do u know of another test that would tell me these are good cables?
 

zog

Senior Member
Location
Charlotte, NC
My knowleged of MV work is limited my main stay is 480v and below,the test preformed on the older cable was a much lower voltage I am not sure how low but much lower, now I do know that a hi pot test causes damage on new and old in fact MV Cable come from the factory already damaged on a microscopic level at least that what the films and classes i sat through taught me.
Exactly, the problem of those small imperfections is more relevant in extruded insulation such as XLPE. The issue with DC is the application of potential in a single direction for a period of time that causes those imperfections to grow into "trees", which will lead to cable failure eventually.

DC hi pot test is the only test I am Aware of used to test MV cables. Do u know of another test that would tell me these are good cables?
Most common test (That most testing companies now use) would be VLF, better tests for condition assesment would be Tan Delta or Partial Discharge.
 

dcv

Member
Location
texas
System KV, arrestor type, & type of system grounding would be useful information to obtain.

These factors can be used to determine if the KV of the arrestor is correct.

The system is 12.47 KV, Phase to phase, 7.2KV to ground. Arrestors are 10KV. Not sure what you are wanting for the grounding info.
Thanks
 

dcv

Member
Location
texas
The arrestors are hooked phase to ground.

The entire 4-1 circuit run is app. 2662 feet.
1. From the pole to the first transformer at the Health Science is app. 1349 feet.( 750kva D-Y)
2. From the Health Science transformer to the Music building transformer is app. 550 feet.(300kva D-Y)
3. From the Music building transformer to the Child Care transformer is app. 763 feet.(300kva D-Y)
I talked to an Engineer friend of mine and he suggested that when we were bringing the pole-top switches up one at a time, the time between each switch could be enough for the energized portion of the transformers to act like auto transformers causing the excessive inrush on the 2nd or third switch and taking out the arrestors. When the arrestor is replaced it has sufficient strength when new; however it would weaken it, each time the spike happens causing failure on the next operation. When the air switch was working all three phases came on and did not allow the system to operate as a auto transformer. What are your thoughts?
Thanks
 

SG-1

Senior Member
The system is 12.47 KV, Phase to phase, 7.2KV to ground. Arrestors are 10KV. Not sure what you are wanting for the grounding info.
Thanks
Provided the system is solidly grounded 10KV arrestors seem reasonable applied L-G. If the system were low resistance or high resistance grounded 10KV would be too low. It is painfully apparent that they are seeing much more voltage than they should.

I am using a Joslyn polmer distribution & station arrestor chart as reference.
 

wirenut1980

Senior Member
Location
Plainfield, IN
The arrestors are hooked phase to ground.

The entire 4-1 circuit run is app. 2662 feet.
1. From the pole to the first transformer at the Health Science is app. 1349 feet.( 750kva D-Y)
2. From the Health Science transformer to the Music building transformer is app. 550 feet.(300kva D-Y)
3. From the Music building transformer to the Child Care transformer is app. 763 feet.(300kva D-Y)
I talked to an Engineer friend of mine and he suggested that when we were bringing the pole-top switches up one at a time, the time between each switch could be enough for the energized portion of the transformers to act like auto transformers causing the excessive inrush on the 2nd or third switch and taking out the arrestors. When the arrestor is replaced it has sufficient strength when new; however it would weaken it, each time the spike happens causing failure on the next operation. When the air switch was working all three phases came on and did not allow the system to operate as a auto transformer. What are your thoughts?
Thanks
This scenario meets the conditions for a ferroresonance condition to occur. Switching all 3 phases in together is the fix. I tried to attach a PDF, but the size was too large.:blink:
 

dcv

Member
Location
texas
This scenario meets the conditions for a ferroresonance condition to occur. Switching all 3 phases in together is the fix. I tried to attach a PDF, but the size was too large.:blink:
Ferroresonance can occur when an unloaded 3-phase system consisting mainly of inductive and capacitive components is interrupted by single phase means. In the electrical distribution field this typically occurs on a medium voltage electrical distribution network of transformers (inductive component) and power cables (capacitive component). If such a network has little or no resistive load connected load and one phase of the applied voltage is then interrupted, ferroresonance can occur. If the remaining phases are not quickly interrupted and the phenomenon continues, overvoltage can lead to the breakdown of insulation in the connected components resulting in their failure.
The phenomenon can be avoided by connecting a minimal resistive load on the transformer secondaries or by interrupting the applied voltage by a 3-phase interrupting device such as a ganged (3 pole) circuit breaker.

Thanks
I was already leaning in that direction, and the following definition is also supportive of that conclusion. I believe when we get the air switch operative it will take care of the problem. The last statement is suggesting that the problem can be avoided by leaving a load on the secondaries of the transformers, which we didn?t have except in the Child Care Building.
Thanks
John
 
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