This study introduces power quality (PQ) state space representation to describe continuous PQ disturbances. A tolerance boundary is designed to represent the integrated disturbance tolerance of all equipment connected to electric power network, and the uncertainty of integrated disturbance tolerance is described with the cloud model. Electromagnetic compatibility and incompatibility degree indices are proposed to reflect the severity of PQ disturbances affecting the equipment. The influence degree index is defined to quantify the severity and impact scope of PQ disturbances. The uncertainty of equipment fault state is expressed by a fuzzy membership function. Subsequently, PQ performance is assessed according to equipment compatibility/incompatibility degree to PQ disturbances. Case studies on a 14-bus distribution system are performed and analysed. Results reveal the effectiveness and reasonableness of the proposed indices in assessing equipment compatibility/incompatibility degree to PQ disturbances, influence of PQ disturbances on the equipment, and PQ performance.