Traditional passive distribution networks can not sufficiently handle the voltage stability issues brought by the increasingly integrated PV systems, while an active distribution network, which features active management of distributed energy resources, can flexibly utilise PV inverters to provide a volt/var control (VVC) function to regulate the network voltage. However, PV inverters are vulnerable power electronics devices and utilising them for additional VVC support can further degrade their reliability, leading to shortened inverter lifetime and impaired economic benefits. In this regard, the thesis focuses on addressing the PV inverter reliability issues in VVC methods, via assessing the PV inverter reliability in VVC and proposing advanced PV inverter based VVC methods considering inverter reliability enhancement. The thesis consists of four stages of my research. Firstly, a comprehensive PV inverter reliability assessment approach is developed to evaluate inverter lifetime when used for VVC functions, and the impacts of the PV inverter based VVC on inverter lifetime are successfully quantified. Secondly, a PV inverter reliability constrained VVC method is proposed in which the constraints to enhance inverter reliability are developed with a restriction factor to regulate inverter apparent power outputs. This method can efficiently minimise network power loss and curtailed PV power, while guaranteeing long inverter lifetime. Thirdly, a PV inverter reliability constrained VVC approach with power smoothing is proposed, in which an inverter power smoothing scheme with high control flexibility is developed by utilising a power smoothing factor to constrict variations of inverter apparent power outputs. Additionally, a penalty convex-concave procedure (CCP) solution method is developed to solve the non-convex optimisation problem with high computing efficiency. Fourthly, a multi-objective PV inverter based VVC method is proposed to simultaneously minimise network power loss and inverter apparent power output, and a Pareto front analysis method is developed to select a solution to achieve efficient power loss reduction with expected inverter lifetime. All the proposed methods apply advanced network operating models and optimisation methods to address uncertainties. These methods have been successfully demonstrated and tested through comprehensive case studies, and numerical simulation results verified the feasibility and high efficiency of the proposed methods.