A power converter system comprising: first and second direct current DC buses (8, 10) connectable at a first end to an energy source and at a second end to a load; a third DC bus; a DC/DC converter comprising: first and second controllable semiconductor devices (16, 18) connected in series between the second and third DC buses (10, 12), each semiconductor device (16, 18) including a semiconductor switch (S 1, S 2) and an anti-parallel connected diode (D 1, D 2), and a reactor connected at a first end to the first DC bus and at a second end to a junction of the series-connected first and second semiconductor devices (16, 18); a DC energy store connected between the second and third DC buses (10, 12) and connected in parallel with the series-connected first and second semiconductor devices (16, 18); and a controller configured to control switching of the series-connected first and second semiconductor devices (16, 18), wherein the controller is configured to operate in a protective mode in which one of the series-connected first or second semiconductor devices (16, 18) is turned off whilst the other one of the series-connected first or second semiconductor devices (16, 18) is constantly turned on, such as to create a short circuit current path between the first and second DC buses (8, 10) of the power converter system. first and second direct current DC buses (8, 10) connectable at a first end to an energy source and at a second end to a load; a third DC bus; a DC/DC converter comprising: first and second controllable semiconductor devices (16, 18) connected in series between the second and third DC buses (10, 12), each semiconductor device (16, 18) including a semiconductor switch (S 1, S 2) and an anti-parallel connected diode (D 1, D 2), and a reactor connected at a first end to the first DC bus and at a second end to a junction of the series-connected first and second semiconductor devices (16, 18); a DC energy store connected between the second and third DC buses (10, 12) and connected in parallel with the series-connected first and second semiconductor devices (16, 18); and first and second controllable semiconductor devices (16, 18) connected in series between the second and third DC buses (10, 12), each semiconductor device (16, 18) including a semiconductor switch (S 1, S 2) and an anti-parallel connected diode (D 1, D 2), and a reactor connected at a first end to the first DC bus and at a second end to a junction of the series-connected first and second semiconductor devices (16, 18); a DC energy store connected between the second and third DC buses (10, 12) and connected in parallel with the series-connected first and second semiconductor devices (16, 18); and a controller configured to control switching of the series-connected first and second semiconductor devices (16, 18), wherein the controller is configured to operate in a protective mode in which one of the series-connected first or second semiconductor devices (16, 18) is turned off whilst the other one of the series-connected first or second semiconductor devices (16, 18) is constantly turned on, such as to create a short circuit current path between the first and second DC buses (8, 10) of the power converter system.