circulating current in 3 core cable used for single phase of t/f lv

[pradtt]

hi
we recently used 3 - 3 core cables ( one cable for each phase - for ampacity issues ) for the lv of a 33/6.6 kV T/F and there was a resultant current circulating through the armour of the cable which was connected to the T/F lv cable box and the earth bar of the switch gear.
can anyone say why?
prad

 

[walk6341]

Have you used one three core cable per phase? If so you have effectively run single cores. When running single core cables you shound gland the armour at each end to a non-ferrous gland plate (sorry written from IEC perspective, not ANSI conduits etc) to prevent circulating currents at the gland plate.

Alternatively you can prevent these by balancing the phases by running each of the phases in each of the cables .

 

[busbar]

From an ANSI/NEC {310-4} standpoint, each cable with ferrous-metal armor should have one of each phase in each cable. Overheating and poor voltage regulation may otherwise result.

 

[rbulsara]

I agree and strongly recommend that (also required by NEC)to install parallel conductors in metal raceway or cable armour in this case, such that metal enclousre (armour) contains the complete set of 3 phases, ground(and neutral where applicable), to avoid the exact problem you are having.

The currents you are seeing are induced currents casued by alternatig magentic field of the single phase. When all three phases (and neutral where applicable) are enclosed in the same metal enclosure, the net magnetic field cancels out and does not induce current in the enclosure.

 

[madura]

If you use 3 core cables in a single core configuration, insulate cable sheath/armour at one end.

 

[mvcjr]

In the same manner, you will induce heating of a metallic conduit if only one phase of a three-phase system is run through it.

God bless!

 

[uno]

Madura- You may not be able to insulate armour at one end . Sometimes the armour is used as a PE Conductor which needs to be earthed at both ends. Armour earthing also allows safe disconnection times when it is suitable to withstand the specified short circuit levels.

It is the shield/screen which is recommended to be earthed at one end and insulated at the other to prevent circulating currents.

Any more suggestions?

 

[madura]

Uno,
It is a common practice, in short runs of cables, (interconnection between a transformer and switchgear), to earth cable sheaths at one end. This arrangement do not compromise protection.

For long runs, the induced voltages under fualt condition will become too high and the cables sheats are crossbonded and transposed.

 

[jbartos]

Question: What kind of grounding have you applied, at one end or at both ends?
How was the physical layout of those cables?
Could you describe the cable installation and supports, if applicable?

 

[hsj]

The current in the cable produces magnetic field and the armour acts as a conductor loop. A conductor in magnetic field means voltage is produced and current flows. Better method is to use one cable for three phases.

 

[uno]

I fully agree with the practice of using one cable for three phase. However since the cables are already laid , we need to find a solution.

I also agree with sheath grounding at one end to stop the circulating currents. However the cable manufacturer recommends the following:

Quote "VOLTAGE RISE IN OPEN SHEATHS
Eliminating the problem of circulating sheath currents by operating the circuit with grounding at one end results in a voltage being induced in the sheath. This voltage rise is proportional to distance from the ground point and the phase current. As covered under sheath losses the voltage rise should be limited to approximately 120 volts under normal operating conditions." Unquote.

 

[jghrist]

If it is totally impracticable to reconnect the cables to have 3 phases in each armored cable, you have to be concerned not only with circulating currents (if grounded at both ends) or high voltage (if grounded at one end), but also with heating caused by eddy currents in the armor. I question whether analytical methods are readily available to confidently predict heating or circulating effects in this unusual application. The cable manufacturer or cable specialists should be consulted.