Voltage balancing of symmetric HVDC monopole transmission lines after earth faults

1. A voltage balancing unit for a symmetric monopole high voltage direct current, HVDC, transmission line being arranged for interconnecting a first voltage source converter, VSC, and a second VSC, the transmission line comprising:

a first pole and a second pole, and two pairs of first arresters, a first pair being arranged at a first end of the transmission line and a second pair being arranged at a second end of the transmission line, a first arrester of either pair being connected between the first pole and ground and a second arrester of either pair being connected between the second pole and ground, the first arresters having a first switching impulse protective level, SIPL,

wherein the voltage balancing unit is arranged to perform voltage balancing for a voltage unbalance caused by an earth fault and comprises:

a pair of second arresters having a second SIPL which is less than the first SIPL, and

a switching device configured to:

connect, 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 of the pair of second arresters between the first pole and ground, and a second arrester of the pair of second arresters between the second pole and ground, and

disconnect, in response to determining that the voltage unbalance is cleared, the second arresters.

2. The voltage balancing unit according to claim 1 , wherein the pair of second arresters is arranged at either of the ends of the transmission line.

3. A high voltage direct current, HVDC, power transmission unit comprising:

a first voltage source converter, VSC, comprising an alternating current, AC, side and a direct current, DC, side, the AC side of the first VSC being connectable to a first AC network,

a symmetric monopole transmission line being arranged for interconnecting the DC side of the first VSC and a DC side of a second VSC, an AC side of the second VSC being connectable to a second AC network, the transmission line comprising a first pole and a second pole, and

a voltage balancing unit according to claim 2 .

4. The voltage balancing unit according to claim 1 , wherein the first SIPL is in the range of 1.8 to 2.0 per unit, p.u., and the second SIPL is in the range of 1.15 to 1.20 p.u.

5. A high voltage direct current, HVDC, power transmission unit comprising:

a first voltage source converter, VSC, comprising an alternating current, AC, side and a direct current, DC, side, the AC side of the first VSC being connectable to a first AC network,

a symmetric monopole transmission line being arranged for interconnecting the DC side of the first VSC and a DC side of a second VSC, an AC side of the second VSC being connectable to a second AC network, the transmission line comprising a first pole and a second pole, and

a voltage balancing unit according to claim 4 .

6. The voltage balancing unit according to claim 1 , wherein the switching device comprises:

a pair of high speed switches, HSSs, a first HSS being connected in series with the first arrester of the pair of second arresters and a second HSS being connected in series with the second arrester of the pair of second arresters, the HSSs being in an open state under normal operation, and

a pair of disconnectors, a first disconnector being connected in series with the first arrester of the pair of second arresters and a second disconnector being connected in series with the second arrester of the pair of second arresters, the disconnectors being in a closed state under normal operation,

wherein the switching means is further configured to:

close, in response to determining that a voltage unbalance exists, the HSSs, and

in response to determining that the voltage unbalance is cleared:

open the disconnectors,

open the HSSs, and

close the disconnectors.

7. A high voltage direct current, HVDC, power transmission unit comprising:

a first voltage source converter, VSC, comprising an alternating current, AC, side and a direct current, DC, side, the AC side of the first VSC being connectable to a first AC network,

a symmetric monopole transmission line being arranged for interconnecting the DC side of the first VSC and a DC side of a second VSC, an AC side of the second VSC being connectable to a second AC network, the transmission line comprising a first pole and a second pole, and

a voltage balancing unit according to claim 6 .

8. A high voltage direct current, HVDC, power transmission unit comprising:

a first voltage source converter, VSC, comprising an alternating current, AC, side and a direct current, DC, side, the AC side of the first VSC being connectable to a first AC network,

a symmetric monopole transmission line being arranged for interconnecting the DC side of the first VSC and a DC side of a second VSC, an AC side of the second VSC being connectable to a second AC network, the transmission line comprising a first pole and a second pole, and

a voltage balancing unit according to claim 1 .

9. The HVDC power transmission unit according to claim 8 , further comprising an AC circuit breaker being arranged for:

disconnecting, in response to determining that a voltage unbalance exists, the first VSC from the first AC network, and

reconnecting, in response to determining that the voltage unbalance is cleared, the first VSC to the first AC network.

10. The HVDC power transmission unit according to claim 8 , further comprising a pair of DC circuit breakers being arranged for:

disconnecting, in response to determining that a voltage unbalance exists, the transmission line from the first VSC, and

reconnecting, in response to determining that the voltage unbalance is cleared, the transmission line to the first VSC.

11. A method of voltage balancing in a symmetric monopole high voltage direct current, HVDC, transmission line being arranged for interconnecting a first voltage source converter, VSC, and a second VSC, the transmission line comprising:

a first pole and a second pole, and two pairs of first arresters, a first pair being arranged at a first end of the transmission line and a second pair being arranged at a second end of the transmission line, a first arrester of either pair being connected between the first pole and ground and a second arrester of either pair being connected between the second pole and ground, the first arresters having a first switching impulse protective level, SIPL,

the method being performed for a voltage unbalance caused by an earth fault and comprising the steps of:

providing a pair of second arresters having a second SIPL which is less than the first SIPL,

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 of the pair of second arresters between the first pole and ground, and a second arrester 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.

12. The method according to claim 11 , wherein the pair of second arresters is connected at either of the ends of the transmission line.

13. The method according to claim 11 , wherein the first SIPL is in the range of 1.8 to 2.0 per unit, p.u., and the second SIPL is in the range of 1.15 to 1.20 p.u.

14. The method according to claim 11 , further comprising:

providing a pair of high speed switches, HSSs, a first HSS being connected in series with the first arrester of the pair of second arresters and a second HSS being connected in series with the second

arrester of the pair of second arresters, the HSSs being in an open state under normal operation, and

providing a pair of disconnectors, a first disconnector being connected in series with the first arrester of the pair of second arresters and a second disconnector being connected in series with the second arrester of the pair of second arresters, the disconnectors being in a closed state under normal operation,

wherein the step of connecting the second arresters comprises:

closing the HSSs, and

wherein the step of disconnecting the second arresters comprises:

opening the disconnectors,

opening the HSSs, and

closing the disconnectors.

15. The method according to claim 11 , wherein the first VSC is connectable to a first AC network, the method further comprising the steps of:

disconnecting, in response to determining that a voltage unbalance exists, the first VSC from the first AC network, and

reconnecting, in response to determining that the voltage unbalance is cleared, the first VSC to the first AC network.

16. The method according to claim 11 , further comprising the steps of: disconnecting, in response to determining that a voltage unbalance exists, the transmission line from the first VSC, and

reconnecting, in response to determining that the voltage unbalance is cleared, the transmission line to the first VSC.