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Thread: Eskom Power Supply

  1. #1
    Email problem Whyburd's Avatar
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    Eskom Power Supply

    This is a new installation from the Eskom transformer to the 90 KW Diaken chiller screw compressor type
    Eskom transformer : 500 kva , 22kv, 415 volts .
    HV side Delta , LV side Star .
    Make and model unknown of transformer
    Supply voltage on LV side of Transformer : 400 , 399 , 400 . 230 , 230 , 230 .
    The problem I have is that on line 2 the amperage is out by 20 amps when I am connected to eskom supply , when I switch over the the genset , 300 kva
    the amperage imbalance is gone . Got eskom out to redo lose connections at the metering box hoping to resolve problem . Still the same fault , they then put a second transformer in place which was faulty .Without load on the LV side the transformer blew all the HV fuses on the line .Why not megger , I ask ? To test is to know. Anyway the old transformer is now back with the same problem happening.
    Distance between genset and load the same as transformer and load
    Voltage on genset and voltage on transformer : same
    The load is 3 phase without a neutral and there is no single phase equipment that can cause that amp imbalance on line 2
    Eskom supply on : volts correct amps are out
    Genset supply on : volts correct amps correct
    Anybody with ideas ?
    Eskom , any ideas ?
    This has gone on for a month now ..

  2. #2
    Diamond Member AndyD's Avatar
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    When you say the current is 'out' is it high or is it low.

    If it's high it might be a negative sequence harmonic issue. It's difficult to even guess without more info. Is there PFC equipment installed? What are the actual run currents on the 3 phases? Have you IR tested the compressor?
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    Diamond Member Justloadit's Avatar
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    If the current imbalance of the phase in question, is less than 5% out of the average of the sum off the current of all three phases, then the compressor should have a healthy life, and will not over heat due to the imbalance.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    Email problem Whyburd's Avatar
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    The chiller has pumps connected for circulation which are three phase motors, depending at what percentage the chiller is at that moment the amperage will go up when loading and down when unloading depending on the water set point. There are two smaller diaken chillers 40 kw which are also , on line 2 , out by 20 amps . when on genset power none of the chillers have that amperage imbalance.Chiller motors have been meggered before startup when cold and also when they have run for awhile to see if any of the ohm readings alter . No earth readings at all and motor winding readings remain constant.
    Line 2 is 20 amps higher than line 1 or line 3
    The 90 kw chiller will read line 1 : 80 amps , line 2 : 100 amps , line 3 : 80 amps with a constant voltage supply of 400 volts
    The compressor is not overheating as a result of the PC board protection protecting unit from amperage imbalance
    I have not done an IR test as yet but will have to get this done to keep on record

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    Diamond Member Justloadit's Avatar
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    I have experienced a similar problem in Nelspruit many years ago, where we had two 45Kw pumps in the river. In this case, we could feel the White phase cable from the transformer vibrating in the control box. After complaining to ESKOM, they replaced the transformer, and the problem went away.
    In your case, it seems that the transformer may be hurt on the primary side, probably from a surge on the HV side. There is no solution but to replace the transformer. According to you, it seems that ESKOM did attempt to replace the transformer but it had a fault, so they reverted back to the old transformer. You have no choice but to keep on at ESKOM to replace the transformer. Unfortunately these big transformers are not easily available at the local electrical wholesaler, and undoubtedly will require to be specifically manufactured, and can take up to 6 months depending on the manufacturers work load.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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    A 3 phase power analyser would record voltage dips or surges as the the transformer loaded and unloaded, which would also help to determine wether the problem in from the plant or transfomer, but if there isnt a problem when the generator runs , then it can only be the transformer, unless there is a wiring issue or a contact issue at the changeover switch or further back, but once again mere speculation without looking at the plant.

    When i get these kind of issues i normally, connect the PQA at the incoming supply point, then take a piece of paper or my netbook and make up a schematic of the wiring, earthing, list all the components. then take a mA current clamp and check readings on earth cables, on many occassions i have found the problem to be something far from what everyone expected.

    The problem i had in newcastle, engineers where looking at the 11 transfomers feeding the building, supply cables and switchgear, some people even suggested fitting a resistor across the circuit to reduce the "nusance tripping", i slept on it that night, woke up in the morning, checked the recorder (fluke 435), the VSDs were new and had gear to reduce harmonics and PFC all built in. While all the engineers walked around puzzling, i took my mA meter and traced the fault to the earth grid, i climbed under the building and found that the earth cables had been cut short pieces out the earth grid and stolen by the last contractor which had worked under the building.

    Sometimes a simple thing like a bridge wire missing can cause huge headaches, i learnt this at mondi control room, a simple fluorescent dimming system went horribly wrong when the electrician tried to get smart and bridged a wire instead of installing the wire from the control unit to the pre heat transfomer, it cost them R25000 for that little oops.

    My point is sometimes we need to open our minds and look at the big picture.

    I had a similar problem to what you are having in durban, also with a chiller plant, i will check my records and see how we resolved the issue.
    Last edited by ians; 09-Jan-13 at 01:09 PM.

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    Diamond Member Justloadit's Avatar
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    Another simple test, is to roll the phases on the chillers (ensure phase sequence is maintained!). If the discrepancy moves, then it is load driven.
    Victor - Knowledge is a blessing or a curse, your current circumstances make you decide!
    Solar and LED lighting solutions - www.microsolve.co.za

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