500KV autotransformer

Type :                                                         Single-phase, outdoor
Altitude:                                                          ≤ 1000 m above sea level
Average ambient temperature :                    30
Maximum Ambient temperature:                  40
Cooling:                                                         Non  inhibited  mineral  oil
                                                                     (Nynas   Libra   or   Shell
                                                                         Diala D or equivalent)
Cooling class :                                                  OFAF
Rated power :                                                    340 MVA
Voltage vector group :                                           YN, a0
Insulation:                                                              Not uniform
Neutral grounding:                                                    solidly earthed
Rated voltages:
HV1 winding :                                                       231 kV (400/√3 kV)
HV2 winding :                                                        289 kV (500/√3 kV)
HV1 (400 kV) winding rated insulation level:
Highest voltage for equipment :                                Um = 420 kV
Switching impulse withstand voltage :                           consequence of HV2
Lightning impulse withstand voltage :                       1300 kV
Power frequency withstand voltage:                            140 kV
HV2 (500 kV) winding rated insulation level:
Highest voltage for equipment :                                         Um = 550 kV
Lightning impulse withstand voltage :                               1425 kV
Switching impulse withstand voltage :                               1175 kV
Power frequency withstand voltage :                                      140 kV
Neutral terminal rated insulation level:
Highest voltage for equipment :                                          72.5 kV
Power frequency withstand voltage :                                140 kV
Short circuit impedance
at 75  and rated voltage (400/500kV):                         12%
Temperature-rise limits at rated power:
for windings :                                                                     65 
for oil:                                                                                  60 
On-load tap-changer :                                                   “Vacutap” MR, Germany
tapping range :                                                                  + 12 %
number of steps :                                                        + 15 x 0,8%
Sound pressure level:                                                     < 70 dB (A)