Janitza AHF
34
5.5.1 Current transformer selection
The current transformer (CT) is of considerable importance in the operation with the AHF,
which means that the selection of an external current transformer (CT) is very important.
3-phase conductor system without N-conductor
:
Two CTs are required and they must be installed on phase A (L1) and C (L3) respectively.
3-phase conductor system with N-conductor:
Three CTs are required and they must be installed in the current circuits of phases A (L1), B
(L2) and C (L3).
The current transformer ratio must be selected to suit the load current. To achieve more
accurate compensation, the present magnitude of the load current must be taken into
account when selecting the CT ratio. A selection of 1.5 times the maximum current in
operation is recommended to ensure more accurate harmonic compensation, whereby a
resonable tolerance is also recommended.
Example: If the maximum detected load current is 1000 A, the best selection for the CT
ratio will be between 1500:5 to 2000:5.
The accuracy rating of the external CT should be at least 0.2 (closed loop) or 0.5
(open loop). Poorer ratings can have a negative effect on the compensation accuracy.
The CT can be installed as an accessory part either in closed or open loop configuration.
• When selecting a CT, be sure the CT ratio is supported.
• Before commissioning, check to be sure that the set-up of the “
CT Ratio
” on the inter-
face of the monitor is identical to the current CT ratio (see chapter 4).
NOTE
Minimum permissible ratio of an external CT:
150:5
Maximum permissible ratio
: 10,000:5
