Question

Private house owner is planning PV system roof top installation. System dimensioning should fulfill following SENER regulations:

• Grid voltage should not change more than 4 % at connection point
• Nominal power of the inverter should be less than S_k/25, where S_k is the short circuit apparent power at PCC (Point of common coupling).
• Maximum fuse for inverter is 3x20A, since main fuse at the switchboard is 3x25A, and selectivity must be kept (PV inverter fuse must be smaller than main board fuse).

The low voltage grid, to which the house is connected, is supplied by a 20/0.4 kV 100 kVA transformer, which has relative short circuit impedance 4% ( can be assumed fully inductive). The length of the low voltage cable between the house and transformer is 1 km. The cable type is AMCMK 4x50/160, which has resistance per phase is 0.77 ohm/km and inductance per phase is 0.27 mH/km. The short circuit impedance at medium voltage side is so small, that it can be neglected.  

a)     Calculate short circuit current and apparent power at PCC (low voltage side)

(Give the answer with accuracy of at least three significant digits.)

Short circuit impedance for the transformer:

Answer Ω

Short circuit reactance for cable:

Answer Ω

Total short circuit impedance:

Answer Ω

Short circuit current:

Answer A

Short circuit apparent power       

Answer kVA