Power Quality Assessment of Large Motor Starting and Loading for the Integrated Steel-Making Cogeneration Facility

This paper presents the power quality assessment of large synchronous motor starting and loading in the integrated steel-making cogeneration facility. To execute the transient stability analysis, the proper mathematical models, and the accurate parameters of the cogeneration units, excitation systems, governor systems, load, and static var compensators (SVCs) are investigated in detail. Four case studies with or without considering the connection of the power grid, the installation of autotransformer (AT) starter, and SVC are performed to demonstrate the dynamic responses of system frequency, voltage, and cogeneration units due to motor starting and loading. Also, the voltage sag ride-through curve of sensitivity load has been included, and a power quality index (PQI) due to voltage variation in the assessment period has been proposed to find the impact of motor starting and loading on the power quality of the cogeneration system. It is concluded that the system dynamic responses and PQI values have better performance if the AT starter is applied with either the regulation of the SVC system or connection to the bulk power grid

I. Introduction

The Cogeneration facility to be studied in this paper is a large integrated steel-making plant, which will be built in the near future [1]. It will produce steel through iron-making, steel-making, and rolling, thus converting raw iron into many kinds of products. From the economic and security viewpoints, the energy efficiency and power system reliability can be greatly improved if the cogeneration units are installed. The process byproduct gases, such as coke oven gas, blast furnace gas, and basic oxygen furnace gas, are mixed with coal or oil as multiple fuels for boilers, which generate steam for the cogeneration units. The whole project plans to fulfill the plant expansion through four stages of construction. Although the cogeneration facility can provide electric power to its own load demand, the electric power system has to be connected to the utility so that the backup power can be obtained in case of outage or maintenance of the cogeneration units. The load demand of the steel plant may vary dramatically and irregularly due to the stochastic load characteristics. The rapid and frequent changes of load, such as in rolling mills, may result in serious voltage and frequency fluctuation phenomena. In addition, the large motor starting can draw several times of their full load current that result in the significant voltage sags, which may cause the magnetic contactors to dropout and disrupt sensitive equipment [1], [2]. This paper discusses a large synchronous motor starting and loading by executing the transient stability analysis. To alleviate the motor starting impact on the cogeneration system, the suitable tap autotransformer (AT) starter can be used to reduce the starting current and the voltage sag. Also, the static var compensator (SVC) systems have been widely used in the steel plant to reduce voltage fluctuation by quick response of reactive power compensation with power electronic devices [3]. It is also important for the system planners to know the effectiveness of the power grid on motor starting and loading. The large motors are planned to be started and loaded under various operation scenarios, as described previously, by executing transient stability analysis. The dynamic responses of system frequency, voltage, and cogeneration units are examined carefully for each case study. Besides, a power quality index (PQI) based on the root-mean-square (rms) voltage level has been proposed and applied to evaluate the impact of voltage variation due to motor starting and loading. By comparing the dynamic response and PQI for each case study, the most suitable operation case is therefore selected and adopted by the cogeneration facility.