Coordinated voltage control of power systems with integration of renewable energy sources

Abstract

It is anticipated that more and more renewable energy sources (RESs) will parallel the traditional power system generation at all voltage levels. In particular, due to the economic and convenience aspects, the number of RESs connected to the subtransmissmion (medium voltage) and distribution (low voltage) networks is growing dramatically. However, the characteristics of medium or low voltage networks, with the high R/X ratio, bring challenges to the integration of RESs. Among these challenges, voltage issues are the main problems that limit the penetration level of RESs. How to handle these new voltage problems in subtransmission and distribution networks through coordinated voltage control is the focus of this thesis. In distribution networks, in order to handle voltage fluctuations and violations, a fast distributed control framework is proposed to regulate bus voltages by coordinating reactive power contributions of distributed energy resources (DERs) in the network. In this control framework, based on the linearized DistFlow model, the control problem is firstly formulated as a quadratic convex optimization problem and then solved in a fast distributed manner by employing dual decomposition and accelerated gradient projected techniques. In subtransmission networks, the large number of DERs dispersed within a small number of distribution networks are utilized to support the voltage control. To solve the control challenge brought by the huge number of DERs, each distribution network with DERs is modeled and regarded as a distributed energy resource cluster (DERC) which serves as the intermediary between DERs and the subtransmission voltage controllers. Based on the concept of DERC, two different coordinated voltage control strategies are proposed to regulate the bus voltages in subtransmission networks. In the first voltage control strategy, only DERCs are utilized as voltage controllers. In this control strategy, a fully distributed control framework is proposed to handle voltage violations through a two step communications with different purposes. In the second voltage control strategy, DERCs, on-load tap changers (OLTCs), and capacitors are utilized as voltage controllers. In order to handle the problem of optimal coordination of continuous controllers with quick responses (i.e. DERCs) and discrete controllers with slow responses (i.e. OLTCs and capacitors), a novel hierarchical control architecture is proposed in this thesis by regulating these two different type of controllers separately in a complementary manner. The detailed management scheme for each DERC to support voltage control in subtransmission networks is also investigated in this thesis. For each DERC, a novel distributed control framework is proposed, which consists of two modules: a command following module and a power quality management module. The former one aims to rapidly track the active power change command from the subtransmission voltage controllers by coordinating the active power of DERs in an optimal distributed manner. The latter one aims to improve the power quality within the distribution network by coordinating the reactive power of DERs also in an optimal distributed manner. The advantage and effectiveness of all the voltage control strategies proposed in this thesis have been verified through various test systems under different scenarios.