Real-Time SOC Correction and Frequency Control Method for LFP Batteries Considering Ambient Temperature
Yu Jie1,2, Liao Siyang1,2, Xu Jian1,2, Yang Canran1,2, Wang Xinying3
1. Hubei Engineering and Technology Research Center for AC/DC Intelligent Distribution Network Wuhan 430072 China; 2. School of Electrical Engineering and Automation Wuhan University Wuhan 430072 China; 3. China Electric Power Research Institute Beijing 100192 China
Abstract：Lithium iron phosphate (LFP) batteries occupy an important position in the field of energy storage system (ESS) because of their excellent charge and discharge performance, better safety and cycle life. Temperature is an important factor in the application of LFP, affecting the capacity and material activity of LFP under operating conditions. The physical model of ESS involved in system frequency control is often based on conventional model data. However, the traditional mechanism model analysis and optimal control methods are difficult to meet the requirements of actual system. Therefore, this paper proposes a real-time correction method of model parameters based on environmental temperature correlation when ESS is involved in system frequency control, which can estimate the SOC of ESS more accurately and improve the reliability of frequency control. Firstly, the mechanism analysis of temperature on the performance parameters of LFP is developed, focusing on the reasons for the impact of high and low temperature on the actual capacity and charging/discharging efficiency of the battery. Secondly, based on the actual test data provided by the manufacturer, the temperature coefficient and coulomb correlation coefficient are used to correct the actual capacity and efficiency, followed by real-time correction of the SOC in battery operation. Subsequently, the time-varying droop control method of the ESS considering the corrected SOC is studied to provide fast frequency support. The SOC considering environmental factors during actual operation will be different from the SOC without considering the extreme temperature, which will also affect the arrival time of the high and low alert positions in the operation state, thus affecting the battery discharge. By making real-time corrections to the SOC, the accuracy of ESS participation in system frequency regulation can be improved. The improved IEEE13 node simulation results show that: under the step disturbance of the system by the sudden change of load, the SOC of ESS is always in the normal range within 1 000 s without considering the environment (T=25℃), the ESS always participates in system regulation, and the droop coefficient Kf_d is in a higher position due to the higher SOC, the frequency drops slowly, and the frequency regulation is better. However, high temperature (T=45℃) will lead to faster SOC decline and carry Kf_d to decrease rapidly, and the frequency regulation effect is reduced. In addition, the battery enters the low alert state near 855s, when the ESS cannot discharge for frequency control, that is, high temperature leads to the ESS to end regulation faster. Low temperature scenario (T =-10 ℃), discharge rate becomes slower, the frequency regulation effect is slightly higher than the normal temperature, and will be discharged for a longer time to participate in frequency regulation; the system is subject to extreme fluctuations in PV, the ESS has a fast response characteristics, can cooperate with the diesel to smooth out the fluctuations in PV power. The frequency regulation effect of the three ambient temperatures in the pre-disturbance period is not exactly the same, where the high temperature condition will have a smaller Kf_c due to a slightly higher SOC than the other two conditions, so the frequency regulation effect is worse and the frequency offset is larger. Besides, in the late stage of PV power fluctuation, the high temperature condition enters the high alert position at 1 738s, after which the ESS cannot continue charging thus exiting the frequency control. That is, high temperature prompted the ESS to withdraw from frequency control as soon as possible, and subsequently the system can only be supported by the diesel. The ESS under normal temperature will exit at 2 118 s, and the low temperature can all participate in frequency control within that 2 750 s. From the simulation analysis, the frequency control method of the ESS proposed can effectively estimate the SOC more accurately for extreme weather conditions after considering the environmental temperature factor and making the frequency control effect more suitable for the actual operation scenario, and improve the efficiency and reliability of ESS participation in system frequency control. The method in this paper can estimate the SOC of ESS more accurately and improve the efficiency of frequency control compared to that without considering the environmental temperature factor.
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