Table 3: Required values of air gaps under switch impulse overvoltage for a 4-terminal MMC-HVDC project.

LinesSwitching overvoltage level (p.u.)Required 50% switching impulse discharge voltageAltitude (m)
010002000

A-D1.9 p.u.11302.733.133.63
C-D1.93 p.u.11472.793.213.73
B-C1.9 p.u.11302.733.133.63
A-B1.7 p.u.10112.42.753.2