Qinyong Zhou

Wind farm model with DFIG for small signal stability study

Appropriate models of wind farm are necessary for the power system stability study with large scale wind farm penetration. In this paper, an aggregated wind farm model with doubly fed induction generator (DFIG) based wind turbine for small signal stability study is proposed to deal with the situation that the wind turbines operate receiving different incoming wind speeds. The different control strategies of wind turbines are considered for modelling wind farms. It is demonstrated that the wind turbines with the identical dynamic parameters and control strategies can be aggregated as one equivalent wind turbine, even though the operating condition for each wind turbine is not the same. Through the calculation of eigenvalues and participation factors, the aggregated wind farm model is verified by comparing with the full wind farm model. From the simulations, it comes to the conclusion that the improved aggregated model considering the different control strategies and wind speed can effectively track the changes of the dominant oscillation mode.

A probabilistic load flow method based on modified Nataf transformation and quasi Monte Carlo simulation

To expose operational risk of large-scale wind power integration system, probability distribution functions (PDFs) of input variables are required to model accurately in probabilistic load flow (PLF) analysis. Unfortunately, PDFs are difficult to obtain in reality. Therefore, a PLF method based on modified Nataf transformation and quasi Monte Carlo simulation is proposed in this paper. This method is able to establish PDF of input variables by their first several orders of moments with the employment of spline reconstruction, then quasi Monte Carlo simulation based on Sobol sequence is adopted to obtain the probability distribution of the output variables. Simulation on IEEE 30 bus system and a real power system demonstrate the validity of the proposed method. The results suggest that the proposed method not only has the advantages of modelling input variables accurately and fast convergence, but also can deal with correlation with convenience.

Reliability assessment of a power system with high penetration of wind and cascade hydropower plant considering wind correlation

This paper presents a method to assess the impact on system reliability of wind integration, considering coordinated operation with cascade hydropower. The seasonal characteristics of river flows and correlation amongst different wind farms are considered. Time sequential Monte Carlo simulation and Latin Hypercube Sampling techniques are used to preserve the characteristics of the time series of the wind speed, water inflows, load, etc. Reliability indices are developed and used to assess, the impact of wind energy production on system reliability. The proposed method is applied to northwest grid of China. Results show the validity of the method.

The power system dispatching rolling optimization model based on the coordination of day-ahead and within-day wind-power prediction

Because of the intermittent and uncertainties of wind speed, the difficulty of absorbing wind power increases with wind power penetration growing continuously. The prediction error of wind farm output is large, and it varies with the wind speed and fluctuation, therefore, different reserve capacity is needed to meet power supply and demand balance. Due to above issue, this paper presents a gradual optimization of day-ahead and within-day dispatching schedule. The wind power and predication error band is involved during the implementation of dispatching schedule, and the dispatching schedule of remaining time is adjusted according to the newest predication message to improve the accuracy of dispatching schedule gradually, and the adjustment includes the output power and reverse capacity of conventional generators. Compared to the implementation of dispatching schedule base on general prediction information, this model can reflect the impact of wind power predication error on dispatching schedule more effectively, and the reserve capacity can be adjusted accurately. The conclusion is demonstrated with the simulation on IEEE 30-bus system.

Determining a critical contingency set using probabilistic performance indexes

This paper proposed a method for determining a critical contingency set for detailed power system analysis studies. The K-means clustering is applied to reduce operation scenarios in a power system. The performance indices with and without probabilities of system states are used to rank contingencies. The top contingencies ranked by the proposed performance indices are combined with additional extreme contingencies specified by experienced engineers to establish the critical contingency set. The proposed method is applied to the IEEE-RTS system and a real grid in Jiangxi province of China. The results demonstrate the effectiveness of the proposed method.

A new type of MW and MVar dispatch index for meeting voltage stability margin criteria based on normal vector of limit surface

This paper presents a new type of index for improving the power system economical dispatch from a voltage stability margin perspective. The proposed index is based on normal vector of voltage stability limit surface, which are supplement and enhancement of participation factor. Firstly, the rationality of this index is proved mathematically. Then numerical simulation on IEEE 39 system demonstrate the validity of the proposed index. Compared to the traditional indexes, proposed index has a wider range of application since it can deal with the critical point of voltage stability, both saddle node bifurcation and limit induced bifurcation. The proposed index can be utilized in active/reactive dispatch for normal operation, and also minimum load shedding strategies in case of critical contingencies.

Transmission network expansion planning based on coordinated economy and security

Transmission network expansion planning (TNEP) is a key component of power system planning. Two important aspects to be considered in TNEP are economy and security factors, which are always contradictory and affect the benefits of both investment and power demand side, so the balance between these two factors needs to be focused. In this paper, a TNEP model based on coordinated economy and security is proposed so as to reconcile these two contradictory factors. The objective function of proposed TNEP model is to minimize the annual synthetic cost of transmission system, which contains investment cost, operation cost, maintenance cost and load shedding cost. The annual synthetic cost of transmission system is calculated with multistep load model established by load clustering using improved K-means algorithm. Then, an improved niche genetic algorithm is utilized to solve the TNEP model and an optimal scheme is obtained. Furthermore, the applicability and effectiveness of proposed TNEP model and algorithm are verified by case study on IEEE 24-bus system.

Reliability evaluation of wind integrated composite power system based on third-order polynomial normal transformation

To overcome the drawback that wind speed distribution is assumed in the reliability evaluation of wind farms integrated power system, the third-order polynomial normal transformation (TPNT) based non-sequential Monte Carlo simulation method, is proposed to evaluate the reliability of generation and transmission system incorporating wind farms in this paper. When historical data or distribution functions of wind speed are given, the relationship between wind speed distribution and standard normal distribution can be established with the TPNT. Then correlated wind speed samples, which are employed in the process of reliability evaluation, can be produced through TPNT and correlated normally distributed samples. The accuracy of TPNT to simulate variables and applicability of the proposed method were verified by case study. Based on that, impacts of wind farms on composite power system are studied from four aspects, including wind speed correlation, location, rate capacity and wind source intensity.

Comprehensive risk assessment method of power planning scheme considering natural disasters

In order to hedge the small-probability high-risk events which power system may face against, it is essential to consider the impact of natural disasters in the power system planning stage. First, this paper establishes the component damage probability model under natural disasters. Second, the system risk assessment based on analytical method is discussed, and the indicators and algorithm for rapid line risk identification are proposed to alleviate the computation burden of the analytical method. Finally, a comprehensive risk assessment method considering natural disaster is proposed based on PSD-BPA transient stability program. The case study performed in the actual systems verifies that the proposed indicators and method are accurate and effective.

Research on quantitatively evaluating system acceptance capacity of electric vehicles considering coordinated strategy

By studying on the coordination between the system reliability level and the EV grid-integration scale, the system acceptance capacity of EVs can be assessed from a new point of view. In this paper, a model for the charging loads of three types of EVs and a coordinated strategy with time-of-use price (TOU) are established. Then, the maximum acceptable capacity, reliability sensitivity and other indicators are proposed to quantitatively characterize the system ability to accommodate EVs. Subsequently, by combining the acquired EV charging load algorithm with the traditional reliability assessment method, the quantitative evaluation approach of the system acceptance capacity of EVs considering coordinated strategy is proposed. In order to improve the simulation efficiency, the Latin hypercube method is used to sample the system state. The feasibility of proposed indicators and method is verified with test results of the IEEE24-RTS system. Furthermore, different cases are comparatively analyzed with and without the coordinated strategy, respectively. Results prove that the coordinated strategy can improve the system reliability level, and greatly enhance the system acceptance capacity of EVs.