Operational Risk Assessment And Early Warning Of Large-Scale Wind Power Ramp Events

With the increase of installed wind power capacity in China, the development and utilization of wind energy is characterized by large scale and high concentration. The penetration rate of wind power is growing and thus the influence on power system operation is increasingly important. The variability and randomness of wind energy make the output of wind farm fluctuate with time, causing a series of operational problems. Especially when some extreme meteorological phenomena including storms, cyclones and frontal passages occur in wind farm area, the wind power will change sharply and ramp events will happen. Wind power ramps will raise big threats to the balance of power generation and demand. Nowadays, developing renewable energies has become popular globally under the pressure of resources and environment and a high renewable energy penetration power system will be realized by 2050 in China. At that time, the penetration level of wind power will be higher and the operational risk will be more challengeable. Besides, the high frequency of extreme weather caused by global climate change will increase the probability of occurrence of high risk wind power ramp events. It can thus be concluded that wind power ramps will be an important threat to power system operation in the future and risk assessment strategies of wind power ramps must be made in order to provide system operators with risk information for decision-making. This paper provides some in-depth studies and helpful explorations in the area of risk assessment and risk early warning of wind power ramps considering their influences on power system.Firstly, this paper proposes a new short-term output model of wind farm considering the extreme weather conditions, which takes icing weather for an example. This model considers the ice accretion on wind turbines and transmission lines. The power output models of wind turbines with ice on blades are modified based on existing studies. The outage of facilities in wind farm is also considered in this model. The outage probability of wind turbines and transmission lines under bad conditions is estimated and a time varying model is established. Considering the wake effect, the states of wind turbines are sampled sequentially. Combined with the output level of wind turbines and the states of transmission lines, the power output of wind farm is calculated. Simulation result shows that the proposed model can reflect the short-term characteristics of wind farm output and can be used for short-term reliability assessment of power system.Secondly, this paper provides some conclusions on wind power ramps studies and details the basic concept, definitions, occurrence mechanism, forecasting methods and influence mechanism on power system. A wind power ramps model is established considering the characteristics of wind power ramps and an expression of wind power ramps is derived. The event characteristic of ramps is detailed and a rule-based sampling method for scenario generation of wind power time series containing ramps is provided. A neighborhood-based K-medoids clustering algorithm is adopted to reduce the size of wind power time series scenario pool. Thus some typical ramps scenarios can be generated for risk assessment of power system operation.At last, this paper proposes a method for operational risk assessment and early warning of wind power ramps based on their impacts on power system operation and current risk theories. In the risk assessment process, a time-varying outage model of general units, a Sequential Monte Carlo (SMC) sampling method, ramping risk indexes and system power balance analysis method are adopted. The operation risk caused by wind power ramps is analyzed by using Utility Theory and Prospect Theory and the warning levels of risk are determined using a fuzzy comprehensive evaluation method. The proposed method is verified in a case study using IEEE RTS-79 test system. Simulation results show that the system risk increases with ramp events and rising wind power penetration rate. The risk analysis methods based on Utility Theory and Prospect Theory can get assessment results involving operators’judgments and the warning level classification method based on Fuzzy Comprehensive Evaluation Theory can provide effective information for system operators.