## 1 Introduction
From an environmental point of view, the peak-shaving costs are not only related to units' coal-fired cost, oil, and wear-and-tear cost, but also related to some additional costs such as the environmental cost [4; 16; 17; 18]. When related to the economic dispatch with large-scale wind power integration, the reported results usually evaluated the economy of the coal-fired units from the standpoint of power grids and power plants during deep peak shaving. In [19], an economic dispatch model was proposed for coal-fired units, photovoltaic system, and wind energy generation system, and the economic effect was investigated from the perspectives of thermal power, wind power, and solar power plants. In [20], a wind power generation cost index was introduced to form the dynamic economic dispatch model from the view of wind power plants and thermal power plants. Prof. [PERSON] of [21] presented an economic dispatch model for a renewable generation penetrated power system with energy storages, and optimized the heat and water consumption to raise the system operational economics. Based on the following observations; (1) though the use of retrofitted coal-fired units in deep peak shaving has become the mainstream, there is still a paucity of economic analysis of those units during deep peak shaving; and (2) most references such as those reviewed above all neglect to consider environmental pollution in deep peak shaving when conducting economy analysis; it is very necessary to investigate the economy of retrofitted coal-fired units with the consideration of environmental benefits in deep peak shaving. The retrofits of coal-fired units include adding the external facilities and improved combustion technology. (1) For the external facilities, it is mainly to install electric boilers and large hot water tanks, which can convert excessive electric energy into heat energy and release it when necessary. This measure is usually with a large front-end investment and well applicable for heating units; (2) the improved combustion technology mainly includes the plasma ignition, tiny-oil ignition, and oxygen-enriched combustion strategy, etc. Since the plasma ignition technology is the most promising technology applied in coal-fired units, in this paper, we will take the coal-fired unit with plasma ignition (UPI) as a representative to analyze its economy in deep peak shaving and compare it with coal-fired units with oil injection (UOI). The main contributions of this paper are twofold as follows:  
1. A peak-shaving costs function representing the operation characteristics of UPI and UOI in three peak-shaving stages is proposed, and an economic dispatch model is presented to reflect the peak-shaving costs. 2. An environmental efficiency index is defined to evaluate the environmental benefits; the multi-angle economic analysis of UPI and UOI in deep peak shaving is investigated from perspectives of thermal power plants, wind power plants, and the environment. The paper is organized as follows: In Section 2, based on different load levels and power consumption characteristics of coal-fired units, the peak shaving process is divided into three phases: RPS, DPSW, and deep peak shaving with combustion supporting (DPSC), and the main factors influencing the deep peak shaving procedure are also analyzed. In Section 3, taking the coal consumption cost, wear-and-tear cost, combustion-supporting cost, and environmental cost into account, the peak-shaving costs function is proposed for UPI and UOI coal-fired units, and afterwards an economic dispatch model with an environmental efficiency index is presented to evaluate the environmental benefits. Then, economic analysis of UPI and UOI in deep peak shaving is investigated in Section 4 from perspectives of thermal power plants, wind power plants, and the environment. Finally, the conclusions are given in Section 5.