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LV 400V AC armoured cable glands are geeting overheat(Temp. measured more than 60 deg C)

Hello Electrical Team.

I would like to discuss an issue being facing in our Power plant(90MW). Our plant generation voltage is 11KV & Plant auxiliary & MCC voltage is 400V AC 50Hz. We have two sections of Power plant PP1 & PP2. In PP2 we are facing issue in LV MCC Panels.

In LV system we have two BUS A & B. Both BUS's are getting power from their individual T/F(11KV/400V AC) & from here power is distributed to various MCC panels in plant.

Booster no. 1 MCC have two LV breakers(Q1 & Q2) which are getting power from BUS A & B.

During last week we were found that Q1 breaker incoming cable insulation was melted around the cable gland area & checked the other glands also & they are also found overheated with abnormal temperature. then we took changeover to Q2 breaker but this is also now getting overheat. In main LV distribution panel side also cable glands are getting overheat which are connected to Q1 & Q2 breakers in MCC.

In one phase two cable are laid & connected, size is 400 sq.mm Armoured cable. R phase(L1 & L2), Y Phase(L1 &L2) & B Phase(L1 & L2). The current also not equal in two length of each phase.Pl see the below readings.

Line

Without Armoured

With Armoured

R Phase

L11

195 A

328A

115A

174A

L12

137 A

72A

Y Phase

L21

103A

332A

145A

306A

L22

167A

175A

B Phase

L31

150A

328A

120A

317A

L32

183A

199A

without armoured = Cable outside of MCC before entry in panel.

with armoured = After gland inside the panel.

Team kindly go through the above said issue & your kind comments will be highly appreciated.

Parents

0 mapj1 over 6 years ago

Is the current flowing in and out under each gland compensated ? I am not too sure from your description if each gland surrounds 3 phases, and there are 2 glands in paralllel, or if there are 3 glands, each carrying two cores but of the same phase each.

For best results you need flow and return currents within any one hole to be near zero sum, ie. all 3 phases - if not then two effects occur -

The first is magnetic heating of any magnetic metal that the cable passes through - typically problems arise with cables entering thick walled steel boxes. A simple saw cut connecting the holes for flow and return current fixes this. If need be that slot can be back filled with epoxy or braze metal, anything interrupts the magnetic path forming a loop around an un-cancelled current in the steel

Nor is it clear what the armour of the cables is made of - steel armour suffer the same problem.

The second problem is one of induced currents - if an un-cancelled current flows in cable inside a metal sheath, and it does not need to be magnetic, this introduces a voltage along the braid/ armour . Put two such armoured cables carrying different currents, and connect the armours at both ends, and a one turn air cored transformer is made, where the primary is the wanted current, and the armour forms a shorted secondary - the voltage is very low, but so is the resistance around the loop, especially with aluminium armour, The trick is either to have insulated bushes under the load end glands on all but one cable, so the armour is earthed at the origin but the shorted loop is broken. (or an insulated plate for the glands to bolt to, and then a separate earth connection - often the glanding boxes have a removable panel to make this easier.)

Or one can break the armour in the middle, and cross link the armours from different phases, so that the induced voltages more or less cancel or at least the total voltage on each armour is a sum of so many metres of phase 1 and so many metres of phase 2 and so many metres of phase 3.
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0 mapj1 over 6 years ago

Is the current flowing in and out under each gland compensated ? I am not too sure from your description if each gland surrounds 3 phases, and there are 2 glands in paralllel, or if there are 3 glands, each carrying two cores but of the same phase each.

For best results you need flow and return currents within any one hole to be near zero sum, ie. all 3 phases - if not then two effects occur -

The first is magnetic heating of any magnetic metal that the cable passes through - typically problems arise with cables entering thick walled steel boxes. A simple saw cut connecting the holes for flow and return current fixes this. If need be that slot can be back filled with epoxy or braze metal, anything interrupts the magnetic path forming a loop around an un-cancelled current in the steel

Nor is it clear what the armour of the cables is made of - steel armour suffer the same problem.

The second problem is one of induced currents - if an un-cancelled current flows in cable inside a metal sheath, and it does not need to be magnetic, this introduces a voltage along the braid/ armour . Put two such armoured cables carrying different currents, and connect the armours at both ends, and a one turn air cored transformer is made, where the primary is the wanted current, and the armour forms a shorted secondary - the voltage is very low, but so is the resistance around the loop, especially with aluminium armour, The trick is either to have insulated bushes under the load end glands on all but one cable, so the armour is earthed at the origin but the shorted loop is broken. (or an insulated plate for the glands to bolt to, and then a separate earth connection - often the glanding boxes have a removable panel to make this easier.)

Or one can break the armour in the middle, and cross link the armours from different phases, so that the induced voltages more or less cancel or at least the total voltage on each armour is a sum of so many metres of phase 1 and so many metres of phase 2 and so many metres of phase 3.
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LV 400V AC armoured cable glands are geeting overheat(Temp. measured more than 60 deg C)

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