Operation Risk Assessment And Reserve Strategy Of Power System Considering Wind Power Ramp Events

In recent years, with the increasing penetration of wind power in China, a series of new potential problems have emerged in power system brought by large-scale wind power connected to the power grid. Especially in extreme weather conditions, active power output of wind farm may change dramatically in a short time, the possibility of forming a high risk "ramp event" is increasing. Wind power high risk ramp event is a low probability event, but once it occurs, it will bring great challenges to the safe and stable operation of power system. Therefore, this paper studies the risk assessment of power system and the corresponding reserve strategies considering wind power high risk ramp events, so as to effectively respond to wind power high risk ramp events.Firstly, a risk assessment system of power system considering wind power ramp events is established. Wind power ramp events are classified and the illustration of the model considering all the ramp scenes is given based on short-term wind power prediction, thus the model of wind power ramp events is established; quantitative assessment of the risk of wind power curtailment and loss of load of the system is done, risk indicators of the system at short time scales are formed, thus the risk of system when wind power positive and negative ramp events arrive is comprehensively assessed. As is shown in numerical example, the difference of start time of wind power ramp events will cause changes in the indicators of risk, the tendency is closely related to the daily load level; risk index is bigger correspondingly with the larger ramp amplitude and shorter ramp duration. This is because when the wind power ramp event occurs, the ramp rate and the reserve capacity of conventional units are insufficient so that the regulation ability of the conventional units are not able to track the change of the net load, thus lead to the increase of risk indices.Subsequently, the traditional reserve strategy to respond to wind power ramp events at sub-hourly time-scale is established. The expected energy not supplied and the expected wind power curtailment are set to satisfy the system allowable maximum limit at 15-min time-scale, the cost of load loss or wind power curtailment caused by wind power ramp events is converted into the risk cost and is included in the objective function, thus the rational spinning reserve and conventional units’ output is optimized to prevent wind power ramp events. As is shown in numerical example, dispatch strategy at sub-hourly time-scale is more suitable for wind power ramp events than at hourly time-scale, thus respond to wind power changes in each 15 minutes; security and stability of power system is guaranteed, reserve shortage or redundency at some dispatch periods under wind power ramp event is avoided, the prevention cost responding to wind power ramp event is reduced correspondingly.Lastly, the flexibility envelope model to respond to wind power ramp events at multi time scale is established. The reserve optimization problem respond to wind power ramp events is converted into the optimal flexibility envelope tracking problem, using flexibility envelope to determine the reserve deployment and the unit output adjustment when wind power ramp event arrives, thus balance the reliability and security of the system operation. At the same time, adopt corresponding reserve strategies according to the characteristics of wind power ramp events, first-order dynamic ramp model of conventional units is used to respond to the wind power ramp event which is more severe, corresponding to the decision making at 5-min time-scale. As is shown in numerical example, flexibility envelope model at 5-min time-scale is better at accurately deploying reserve and making adjustments of units’ output than traditional reserve strategies at 15-min or hourly timescales; first-order dynamic ramp model ensures the ramping ability of units to respond to wind power ramp event which is more severe in 1h, the sharp rise on net load caused by wind power ramp event can be tracked accurately via flexibility envelope model, which ensures the flexibility and reduces operational risk of power system, thus ensures the safe and stable operation of power system.