This thesis investigates and proposes novel solutions to enhance voltage quality in medium networks with increased distributed generation (DG) penetration, focusing on voltage level variations and harmonic distortion. A new hierarchical distributed voltage control structure is proposed for DG to facilitate autonomous integrated Volt-VAr control in medium networks with DG. It is implemented especially in converter-connected DG, and in addition to enhancing voltage quality, the proposed controller also reduces the need to install new compensation devices. A decentralised active voltage control is developed for DG units to provide short and long-term voltage compensation by manipulating their reactive power output. Local controllable zones (LCZs) are introduced and used to determine the voltage control boundaries for each DG unit, and a methodology is introduced to allow LCZ adapt and follow network changes in real time. The performance and value of the proposed LCZ identification method and voltage control approaches are demonstrated based on load-flow and transient simulations, under various network operating scenarios conducted in DigSILENT PowerFactory. A droop control and a coordinated controller between DG units and other voltage controllable devices is presented to enhance voltage controllability whilst minimizing voltage interaction between devices, in case more than one DG unit, in the same LCZ, supports voltage control at the same time. The proposed coordinated controller applies to DG units, and also to DG units and other voltage compensators such as the modern on-load tap changer (OLTC) and the solid-state transformer (SST). In this context, DG provides primary support, and the other devices concentrate on providing secondary voltage support. Also, a DG structure combining energy storage systems (ESS) is proposed to enhance voltage quality by controlling not only the reactive power output from the DG unit, but also either active or reactive power output of ESS, thus allowing the voltage control to be more effective in a wider range of scenarios. The control and operation of DG in cooperation with a voltage regulator (VR) is also investigated. The voltage regulator is assumed as the secondary voltage support to provide slow voltage control to a wider area, including LCZs, located behind its secondary side. Furthermore, the substation's OLTC can give the slow voltage support similar to the voltage regulator but it provides the wide-area voltage control to cover the whole network including those buses which do not belong to any LCZs. The coordinated voltage controller approach iv presented is demonstrated under various network operating scenarios using case studies based on the IEEE 33-bus radial distribution network. Harmonic distortion across the network when the number of converter-connected DG is increased is also investigated in the thesis. It is found that the increase of this type of DG can raise the level of harmonic distortion to above the statutory limits. A generic approach is adopted to reduce the harmonic injection from converter-connected DG units and non-linear loads, using a phase-shifting technique. By changing the vector group between transformers that connect either converter-connected DG or non-linear loads is operated off-line for selfharmonic cancellation, without physically modifying the existing converter unit. The case study is based on the typical 17-bus medium voltage Dutch network. The performance of the proposed harmonic mitigation solution is examined under various scenarios based on harmonic load flow calculations performed on the distribution network modelled in the harmonic domain.