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






RE: circulating current in 3 core cable used for single phase of t/f lv
Alternatively you can prevent these by balancing the phases by running each of the phases in each of the cables
RE: circulating current in 3 core cable used for single phase of t/f lv
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.
RE: circulating current in 3 core cable used for single phase of t/f lv
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.
RE: circulating current in 3 core cable used for single phase of t/f lv
RE: circulating current in 3 core cable used for single phase of t/f lv
God bless!
RE: circulating current in 3 core cable used for single phase of t/f lv
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?
RE: circulating current in 3 core cable used for single phase of t/f lv
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.
RE: circulating current in 3 core cable used for single phase of t/f lv
How was the physical layout of those cables?
Could you describe the cable installation and supports, if applicable?
RE: circulating current in 3 core cable used for single phase of t/f lv
RE: circulating current in 3 core cable used for single phase of t/f lv
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.
RE: circulating current in 3 core cable used for single phase of t/f lv