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WHEN and WHERE to connect capacitors for PFC relating to ATS?

- It is forbidden to connected capacitors between ATS and motor.
 
- In any case capacitors must be connected upstream ATS (in the supply network). The further capacitors from ATS are, the better.
 
- Some drawback of capacitor bank connected to network:
1) When capacitor bank is connected to network, overvoltage in the network (at the place where capacitors are connecting) or resonance can occur due to resonant circuit by inductance of the cables with these capacitors. Overvoltage in supply network is not good for thyristors. Thyristor voltage level is 1600V (ATS...Q)
2) Furthermore, synchronization for firing thyristor can be lost. Picture attached shows bad example when cable inductance and capacitor battery are close to resonance.

SOLUTIONS
 
If capacitors for PFC are required, there are two solutions (A,B):
 
A. Connect capacitors to network after bypassing ATS. Starting process should consist of following sequence:
a) Disconnect all capacitors from network.
b) connect supply to ATS
c) Start the motor via ATS
d) After start finishes, bypass ATS using bypass contactor
e) connect capacitors to the network.
 
B. Connect capacitors to the network before you connect voltage on ATS. Thus, you will reduce overvoltage on ATS or risk of losing synchronization during connecting capacitors. Be sure that PFC capacitors are not very close to ATS to avoid excessive current peaks to PFC capacitors due to presence of harmonics during acceleration/deceleration.
 
In both cases you will need at least two contactors (one for ATS supply and second for capacitor battery). If you decide for solution 1, you need one contactor more (for ATS bypassing). As I wrote in the beginning, the
further capacitors from ATS are, the lower risk of losing synchronization is.
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