Xiaoqing Han

Reactive Power Aspects in Reliability Assessment of Power Systems

Reactive power plays a significant role in power system operation. However, in reliability evaluation, attention has seldom been paid to reactive power. In conventional power system reliability evaluations, the fixed maximum and minimum values are applied as the reactive power limits of generators. Failures of reactive power sources are rarely considered. The detailed causes of network violations for a contingency are also seldom studied. Real power load shedding is usually used to alleviate network violations without considering the role of reactive power. There are no corresponding reliability indices defined to represent the reactive power shortage in the existing techniques. Reactive power shortage and the associated voltage violations due to the failures of reactive power sources are considered in this paper. New reliability indices are proposed to represent the effect of reactive power shortage on system reliability. The reliability indices due to reactive power shortages have been defined and are separated with those due to real power shortages. Reactive power limits determined by real power output of a generator using P-Q curve have been studied. A reactive power injection technique is proposed to determine possible reactive power shortage and location. The IEEE 30-bus system has been modified and analyzed to illustrate the proposed technique. The results provide system planners and operators very important information for real and reactive power management.

Four-Dimensional Wind Speed Model for Adequacy Assessment of Power Systems With Wind Farms

With high wind-power penetration, power-system operators require accurate wind-power output from wind farms for system operation. A more accurate four-dimension wind speed model is proposed in this paper to simulate the effects of terrain and time delay of wind speed. Two coordinate transformation matrices are proposed to easily incorporate wind direction in wind-speed simulation. The effects of random failures and wake of wind turbine generators on wind-speed distribution in a wind farm are also considered in the proposed model. A sequential Monte Carlo simulation technique is developed to evaluate adequacy of power systems with wind farms using the proposed model. The modified IEEE RTS is analyzed to illustrate the models and technique. The simulation results show that those factors have significant impact on power output of a wind farm and system reliability.

Reliability assessment of power systems considering reactive power sources

Reactive power plays a significant rule in power system reliability and security. Reactive power is considered as the network constraint in conventional reliability evaluation techniques. The impact of the failures of reactive power sources such as synchronous condensers and compensators on system reliability has not been considered in the existing reliability techniques. This paper presents a technique to evaluate system and load point reliability indices of power systems considering reactive power shortages due to the failures caused by reactive power sources. The reliability indices due to the reactive power shortages are separated from those due to the real power shortages. Two reliability indices related to reactive power shortage are proposed. The IEEE 30-bus system is modified and analyzed to illustrate the proposed technique. The results provide very important information for system planners and operators for reactive power management.

Adequacy study of wind farms considering reliability and wake effect of WTGs

The wake effect of a wind turbine generator (WTG) may affect output of a wind farm. Wake effect depends on wind speed, and the size and location of WTGs in a wind farm. When one or more WTGs fail in a wind farm, the wind speed distribution in the farm will change, which may lead to different wind power output from the farm. This paper proposes a Monte Carlo simulation technique is used to evaluate adequacy of a wind farm with considering the reliability and location of WTGs, wind speed and the wake effect of WTGs. A quadratic interpolation method is used to study the optimal distribution of WTG in a wind farm. Reliability of a wind farm with and without the optimal WTG distribution is investigated by using the proposed technique. The results show that, the optimal layout of a wind farm with considering wake effect should be considered in wind farm adequacy studies.

A Fast Contingency Screening Technique for Generation System Reliability Evaluation

In most existing reliability evaluation techniques, the stopping rules applied for contingency enumeration are either a given number of states with the highest probabilities or the states up to a specified failure order. However, the probability of a high order outage state may be larger than that of a low order outage state due to different reliabilities of the failed components. On the other hand, a low probability state with high outage capacity may have larger impact on system reliability than a high probability state with low outage capacity. These effects have not been considered in the existing techniques for contingency filtering or screening. A fast contingency screening technique (FCST) for generation system reliability evaluation is proposed in this paper. In this technique, contingency states are searched and ranked based on the severity coefficients determined by both state probabilities and outage capacities of the failed units. Several severity coefficients for a unit are defined and examined based on reliability and capacity factors. The replaceable neighboring states of a state and the minimum set of the neighboring states are defined to further reduce computation time. The merits of the proposed techniques and severity coefficients are validated by evaluating the East-China Power Grid.

Investigation of a microgrid with vanadium redox flow battery storages as a black start source for power system restoration

Microgrids (MGs) in distribution networks have been dramatically increasing due to integration of distributed renewable sources. With connection of MGs, utility grid and customer supply becomes more reliable. An emergency modes for MGs must be envisaged after a general black out. Considering operating and control flexibility, capability of grid-tied, and autonomous operation, MGs with energy storages can provide cranking power and ensure smooth load pick up during the system restoration process. This paper investigates the technical characteristics of MGs containing vanadium redox flow battery (VRB) as a black start (BS) source. The procedure for power system restoration is developed to minimize the total restoration time by maximizing the total capacity of the BS sources. The proposed restoration process is tested through simulations.

Transient stability studies of doubly-fed induction generator using different drive train models

Three equivalent drive train models of a wind turbine generator (WTG) are presented in this paper. The flexibility of the blades bending and the drive train are considered to analyze its transient stability under a three-phase fault. The simulations show that the fifth order lumped-mass model provides too optimistic estimation while the ninth order three-mass model can observe the motion of shaft system, which is more suitable for transient stability analysis. When a WTG is perturbed, a large torque may be produced in the drive train which may result in torsional oscillations between different parts of the WTG shaft system, and thereby reduce the fatigue life of the shaft due to bearing the large torque repeatedly.

Operational Reliability and Economics of Power Systems With Considering Frequency Control Processes

With high renewable power penetration, uncertainty and intermittence of renewable sources become major concerns of power system planning and operation. Large and fast wind speed change may cause great variation of active power generation, which may lead to system stability and reliability problems. The response speeds of the committed conventional generators (CGs) for frequency regulations are critical for system reliable and stable operation. The slow response of the committed CGs may result in power shortage or surplus, which may affect system frequency. This paper proposes a technique to evaluate operational reliability and efficiency problems of power systems with high wind power penetration from frequency aspect. Energy unnecessarily consumed and less supplied during system frequency control processes are modeled in detail. The reliability and economic indexes are formulated with considering system dynamic frequency control processes. The IEEE-RTS79 is used to verify the proposed models and method.

Fusion modeling for wind power forecasting based on redundant method elimination

In general, an individual forecasting model for wind power cannot always attain a sufficient degree of precision. To further improve the forecasting accuracy, this paper develops a new wind power forecasting model using a fusion model based on the concept of optimal selection and the Shapley Value. The fusion modeling method first establishes an individual forecasting model base. Next, higher-precision models are optimally selected using grey incidence analysis based on the collective evaluation index, and the redundancy of these models is judged on the basis of the information matrix of forecasting errors. This allows redundant models to be eliminated so as to simplify the establishment of the fusion model. Eliminating redundant models avoids negative degree of fusion. Finally, the fusion degree of each optimally selected individual model is calculated according to the Shapley Value. Models, which include the proposed model, seven individual models, and two other combined models, are tested to forecast t...

Advanced dynamic voltage restorer to improve power quality in microgrid

A dynamic voltage restorer (DVR) based on photovoltaic (PV) generation/battery units is proposed to improve voltage quality in a microgrid. The restorer is connected with the grid by a rectifier, which is in series with the point of common coupling (PCC). Using energy management system (EMS), the proposed apparatus could be operated under dynamic voltage restorer mode, uninterrupted power supply mode and micro-source mode, and be switched smoothly among them. The dynamic voltage restorer could handle voltage sag or surge, and thus to improve the power quality in microgrid. The uninterrupted power supply could enhance the power supply reliability. And the micro-source could reduce the energy absorbed from utility grid so as to achieve energy conservation and emission reduction. The simulation and experimentation show the validity of the proposed DVR.