Thesis Summary
Line commutated HVDC systems are widely used due to their high power ratings. However,
one of the disadvantages of such HVDC systems is the high risk of commutation failures when
AC disturbances arise. These failures normally develop in the inverter station. When failed
commutation occurs, the LCC HVDC system is greatly disturbed resulting in loss of power
transmission. Moreover, the rapid increase in the direct current during unsuccessful
commutation results in additional stresses on the thyristor valves.
In an attempt to reduce the probability of unsuccessful commutation, a commutation failure
prevention function is added to the HVDC system controls. When an AC system disturbance
is detected, this function is activated with the aim of altering the firing order at the inverter
station. Since the angle contribution from the function is independent of the minimum inverter
extinction angle, this approach possesses limitations under certain AC faults.
In the thesis, a commutation failure prevention function based on voltage-time area
contribution was designed and implemented. Simulation results show that both the proposed
and existing functions are equally ineffective in mitigating the first commutation failures when
three phase faults are applied. However, the proposed function is more effective in mitigating
the first commutation failure when single phase faults are applied compared to the existing
function. In 17% of the investigated cases, improvements were registered when the proposed
function was utilised. Moreover, in 25% of the cases when three phase faults were applied, the
proposed function reduced the occurrence of multi-valve commutation failures.