Preface

Power systems are currently undergoing significant changes. Support for environmental protection and advocacy of sustainable economic development have led to a rapid growth of renewable energy-generating capacity in recent years, primarily in the form of wind and solar generation resources. Wind and solar power can provide huge benefits since they are plentiful, widely distributed, and clean. The massive deployment of these renewable energy resources also drives their per-unit cost down so that wind and solar power can now compete in the marketplace with conventional energy production. Power system operators will inevitably face challenges in adapting to this environment. Wind and solar energy resources exhibit notably different characteristics from their fossil fuel-burning counterparts. Since the output of wind and solar generators is variable and uncertain across multiple timescales, they are often referred to as variable energy resources (VERs). Numerous integration studies and operational experiences have implied that as a large amount of VERs is integrated into an electric grid, it could fundamentally change how the grid is planned and operated. Thus, grid operators are concerned with the economics and reliability of a power grid where VERs are added. To overcome this concern, successful solutions have been developed to mitigate the adverse effects brought to the grid by the VERs. The operational experiences gained worldwide also provide useful guidance to renewable integration. However, most of the past efforts were focused on studies assuming a low or medium penetration level of VERs. Due to the assumptions made in these studies, some integration issues may not manifest themselves if the penetration level of VERs is not high enough. The issues associated with integrating VERs are also complicated and multifaceted, spanning from long-term planning to short-term operations. This requires comprehensive studies to be performed to ensure that the whole spectrum of issues is examined. Today, interconnection studies of VERs are evolving at a fast pace, and the focus is on scenarios with a high penetration level of VERs. As we continue on the path of increasing the installed capacity of renewable electricity, many areas that have set goals to incorporate high levels of VERs in their power systems are quickly approaching those penetration levels. For example,