Hybrid on-load tap changer and a method of operating the same

1. A hybrid on-load tap changer, for use in high voltage alternating current power transmission, comprising:

a selector including two electromechanical isolating switches;

a diverter including two legs defining respective current paths, each leg including a pair of opposed first and second semiconductor switches, each leg including a first inductor arranged in series between each pair of first and second semiconductor switches and the selector, and each leg including a voltage surge arrestor in parallel with each of the electromechanical isolating switches of the selector;

a controller configured to switch on one of the first or second semiconductor switches of a given leg at a predetermined point within the alternating current cycle so as to commutate off a desired semiconductor switch in the other leg; and

a snubber arranged in parallel with each pair of first and second semiconductor switches, wherein

each of the electromechanical isolating switches includes a second inductor arranged in series therewith, each of the second inductors being arranged in series with the respective first inductors, and each of the electromechanical isolating switches and the respective second inductors being arranged in parallel with the corresponding voltage surge arrestor of each leg.

2. The hybrid on-load tap changer according to claim 1 , wherein each leg further includes a capacitor in parallel with a corresponding each of the electromechanical isolating switches of the selector.

3. The hybrid on-load tap changer according to claim 1 , wherein the two electromechanical isolating switches selectively isolate a respective leg of the diverter so as to bypass the semiconductor switches therein.

4. A method of operating a hybrid on-load tap changer, during high voltage alternating current power transmission, comprising:

(i) providing a selector including two electromechanical isolating switches;

(ii) providing a diverter including two legs, each defining a respective current path;

(iii) providing each leg with a pair of opposed first and second semiconductor switches, providing a first inductor arranged in each leg in series between each pair of first and second semiconductor switches and the selector, and providing each leg with a voltage surge arrestor in parallel with each of the electromechanical isolating switches of the selector;

(iv) selectively switching on one of the first or second semiconductor switches of a given leg at a predetermined point within the alternating current cycle so as to commutate off a desired semiconductor switched in the other leg; and

(v) providing each of the electromechanical isolating switches with a second inductor arranged in series therewith, each of the second inductors being arranged in series with the respective first inductors, and each of the electromechanical isolating switches and the respective second inductor being arranged in parallel with the corresponding voltage surge arrestor of each leg,

wherein the predetermined point at which the semiconductor switches are operated is just prior to zero voltage appearing across tap terminals.

5. The method of operating a hybrid on-load tap changer according to claim 4 , further comprising (vi) providing a capacitor in parallel with a corresponding each of the electromechanical isolating switches of the selector.

6. The hybrid on-load tap changer according to claim 1 , wherein each of the second inductors are arranged to be directly connected to the respective first inductors, when the corresponding electromechanical isolating switches are on.

7. The hybrid on-load tap changer according to claim 1 , wherein each leg further includes a capacitor in parallel with a corresponding each of the electromechanical isolating switches of the selector, each capacitor being in parallel with the respective voltage surge arrestors and the respective second inductors.