Yanbing Jia

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.

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.

An improved state selection technique for power system reliability evaluation

Most existing reliability techniques determine credible system outage states by considering system states up to a certain contingency level. The probability of a high level outage may be larger than those of low level outage states since reliabilities of components are different. The significant impact of these high level outage states may be neglected when considering up to certain level of contingencies. An improved fast sorting algorithm (IFSA) for generating system reliability evaluation is proposed in this paper. In this technique, the sequence of units is decided by not only outage probability but also available capacity of a unit in descending order. In order to accelerate the convergence, i.e. minimizing the number of computations and comparisons, a replaceable neighboring states and minimum neighboring states set are proposed to reduce the selected states. The merit of the algorithm is validated by its application to a five units system and East-China power grid. The results show that the efficiency of reliability evaluation has been improved.

Battery Energy Storage Selection Based on a Novel Intermittent Wind Speed Model for Improving Power System Dynamic Reliability

Owing to intermittency of wind power and slow ramp rates of conventional generators, a considerable amount of wind energy cannot be effectively utilized during frequency control processes. This paper proposes a technique for power system planners and operators to select or commit power capacity and energy capacity of battery energy storage system (BESS) for mitigating the effects of intermittent wind speeds on reliability and economics. A novel procedure is proposed to determine intermittent wind speeds based on the chaos theory and intermittency-related parameters. The frequency-related reliability and economic indices of a power system with BESS are formulated based on frequency control processes. The effects of BESS on dynamic reliability and economics of a practical power system are investigated using the proposed techniques and models.

Coordinated control strategy for improving fault ride-though performance of photovotaic/battery microgrid

In this paper, a new coordination photovoltaic/battery micro-grid control strategy is proposed to improve the micro-grid fault ride-through capability. Under normal condition, the proposed strategy can restrain the influence of load fluctuations. When symmetrical grid fault happens, the micro-grid can ride through it by the proposed strategy. The DC-link bus voltage and AC current could be kept stable with the help of the energy storage system. Meanwhile, the grid-connected inverter can provide a certain reactive power to support the voltage at the PCC. The detailed control structure is discussed and the simulation model of photovoltaic/battery micro-grid system is built. Simulation results demonstrate that the proposed strategy can improve the micro-grid fault ride-through capability.

Frequency aspects of power system operational reliability

In an autonomous micro grid or a power system with high renewable power penetration, the intermittency of renewable energy resources becomes major concern of system operation. Dynamic process of frequency response and control due to unit failures and resource variation may have significant impacts on system operational reliability because the outputs of committed conventional generators have to be frequently adjusted for frequency recovery. This paper proposes a technique to investigate system operational reliability from the aspect of frequency variation and control. The time durations of primary and secondary frequency controls are considered in the proposed technique. Frequency related reliability indices are defined. The IEEE-RTS is used to illustrate the applications of the proposed method.

Fatigue reliability analysis of wind turbines shafts caused by sub-synchronous oscillations during power system fault

Fatigue and destruction in the shafts of a wind turbine caused by sub-synchronous oscillations during power system faults is critical for wind turbine reliability. This paper investigates a three-mass shaft model according to a MW class doubly-fed wind generator shaft mechanical structure and obtains the electromagnetic torque of a wind turbine by PSCAD simulation when sub-synchronous oscillation occurred in power system. The equivalent load spectrum is obtained via the rain flow counting method. The finite element model of the gear is set up by Solidworks software. And then it is meshed by ANSYS software and finally the equivalent Von-Mises stress is achieved. The fatigue damage of transmission gear caused by sub-synchronous torsional vibration loads was assessed through Ncode software based on the S-N theory and Miner theory, and then it carries on the analysis of reliability. To sum up, the paper provided an analysis of the fatigue damage of wind turbine shafts caused by sub-synchronous oscillations of various durations during power system faults. The results are helpful to predict the fatigue life of wind turbine gear, to better maintain wind turbine shafts in time and to improve the safety and reliability of wind power system.

Reliability and efficiency-based energy storage sizing from the aspect of system frequency

A major concern for wind farm connection to power systems is large variation of power output caused by the variability and unpredictability of wind speed. For a power system with high wind power penetration, the frequency control process of conventional generators (CGs) to match load and wind power variation becomes an important operation issue. Energy storage systems play an important role in solving the problem. This paper proposes an analytical technique to select the optimal size of battery storage system (BSS) for a power system based on operational reliability analysis and frequency control process. According to optimal size of BSS, the optimal size of state of charge (SOC) and depth of discharge (DOD) are selected to achieve the minimal frequency variation for the fixed wind farm.