A voltage balancing unit (330; 431 -433; 500) for a symmetric monopole high voltage direct current, HVDC, transmission line (303; 407, 408) being arranged for interconnecting a first voltage source converter (203; 301; 401), VSC, and a second VSC (203; 302; 402, 403), the transmission line comprising: a first pole (304; 409; 505) and a second pole (305; 410; 506), and two pairs of first arresters, a first pair (321, 322; 421, 422, 425, 426) being arranged at a first end of the transmission line and a second pair (323, 324; 423, 424, 427, 428) being arranged at a second end of the transmission line, a first arrester (321, 323; 421, 423, 425, 427) of either pair being connected between the first pole and ground and a second arrester (322, 324; 422, 424, 426, 428) of either pair being connected between the second pole and ground, the first arresters having a first switching impulse protective level, SIPL, characterized in that the voltage balancing unit is arranged to perform voltage balancing for a voltage unbalance caused by an earth fault (210; 311; 411) and comprises: a pair of second arresters (331, 332; 501, 502) having a second SIPL which is less than the first SIPL, and switching means (333, 334; 503, 504) being arranged for: connecting, through being closed after the earth fault has been cleared and in response to determining that the voltage unbalance exists between the first pole and the second pole, a first arrester (331; 501) of the pair of second arresters between the first pole and ground, and a second arrester (332; 502) of the pair of second arresters between the second pole and ground, and disconnecting, in response to determining that the voltage unbalance is cleared, the second arresters. a first pole (304; 409; 505) and a second pole (305; 410; 506), and two pairs of first arresters, a first pair (321, 322; 421, 422, 425, 426) being arranged at a first end of the transmission line and a second pair (323, 324; 423, 424, 427, 428) being arranged at a second end of the transmission line, a first arrester (321, 323; 421, 423, 425, 427) of either pair being connected between the first pole and ground and a second arrester (322, 324; 422, 424, 426, 428) of either pair being connected between the second pole and ground, the first arresters having a first switching impulse protective level, SIPL, characterized in that the voltage balancing unit is arranged to perform voltage balancing for a voltage unbalance caused by an earth fault (210; 311; 411) and comprises: a pair of second arresters (331, 332; 501, 502) having a second SIPL which is less than the first SIPL, and switching means (333, 334; 503, 504) being arranged for: connecting, through being closed after the earth fault has been cleared and in response to determining that the voltage unbalance exists between the first pole and the second pole, a first arrester (331; 501) of the pair of second arresters between the first pole and ground, and a second arrester (332; 502) of the pair of second arresters between the second pole and ground, and disconnecting, in response to determining that the voltage unbalance is cleared, the second arresters. a pair of second arresters (331, 332; 501, 502) having a second SIPL which is less than the first SIPL, and switching means (333, 334; 503, 504) being arranged for: connecting, through being closed after the earth fault has been cleared and in response to determining that the voltage unbalance exists between the first pole and the second pole, a first arrester (331; 501) of the pair of second arresters between the first pole and ground, and a second arrester (332; 502) of the pair of second arresters between the second pole and ground, and disconnecting, in response to determining that the voltage unbalance is cleared, the second arresters.