Influences of ±800 kV Yunnan-Guangdong HVDC System on Security and Stability of China Southern Power Grid

The interaction mechanism between AC and DC systems in a hybrid AC-DC transmission grid is discussed with PSS/E software. Analysis shows that receiving-end AC faults may cause much more damage on the HVDC system operation than the sending-end AC faults in a multi-infeed HVDC system, and the damage severity depends on the power recovering rate of the HVDC systems. For HVDC systems with slow power recovering rate, the receiving-end AC faults may probably be a critical factor to constrain power transfer limits. Larger capacity of HVDC system means not only higher power transfer-limit of the parallel connected AC-DC transmission grid, but also more expensive stabilizing cost.

Application of Risk-based Flexibility Assessment Methods to Evaluate System Expansion Plans

This paper proposed a flexibility assessment approach based on and risk assessment methodology. System planners prioritize each planning scheme with consideration of three aspects: reliability, economics, and flexibility. In the past, there is lack of quantitative indices to measure flexibility of a power system. This paper proposes applying probabilistic risk assessment method to quantify system flexibility. The proposed approach is demonstrated to compare two transmission planning schemes during Guangdong expansion planning process.

Delay-dependent Wide-area Damping Controller Synthesis Approach Using Jensen’s Inequality and Evolution Algorithm

A time-delay-dependent wide-area damping controller synthesis approach,based on Jensen’s integral inequality and evolution algorithm,is developed to suppress the adverse effect of time delay on the supplemental control of high-voltage direct current(DC)transmission systems.Initially,the state-space model of hybrid AC/DC systems with time delay is derived and the delay-dependent criteria for the stability of the closed-loop system are provided based on Jensen’s integral inequality.Subsequently,initial solutions are randomly generated to overcome the difficulty of solving the nonlinear matrix inequality.Finally,the time-delay stability upper bound of the controller is optimized using the differential evolution algorithm.In comparison to popular time-delay stable controller design methods,such as the free-weighting-matrix approach,the proposed method based on output feedback realization requires fewer decision variables and is more suitable for large-scale hybrid AC/DC systems.Three examples are introduced to verify the effectiveness of the proposed method.

Day-ahead Network-constrained Unit Commitment Considering Distributional Robustness and Intraday Discreteness:A Sparse Solution Approach

Quick-start generation units are critical devices and flexible resources to ensure a high penetration level of renewable energy in power systems.By considering the wind uncertainty and both binary and continuous decisions of quickstart generation units within the intraday dispatch,we develop a Wasserstein-metric-based distributionally robust optimization model for the day-ahead network-constrained unit commitment(NCUC)problem with mixed-integer recourse.We propose two feasible frameworks for solving the optimization problem.One approximates the continuous support of random wind power with a finite number of events,and the other leverages the extremal distributions instead.Both solution frameworks rely on the classic nested column-and-constraint generation(C&CG)method.It is shown that due to the sparsity of L_(1)-norm Wasserstein metric,the continuous support of wind power generation could be represented by a discrete one with a small number of events,and the rendered extremal distributions are sparse as well.With this reduction,the distributionally robust NCUC model with complicated mixed-integer recourse problems can be efficiently handled by both solution frameworks.Numerical studies are carried out,demonstrating that the model considering quick-start generation units ensures unit commitment(UC)schedules to be more robust and cost-effective,and the distributionally robust optimization method captures the wind uncertainty well in terms of out-of-sample tests.

Unified probabilistic gas and power flow

The natural gas system and electricity system are coupled tightly by gas turbines in an integrated energy system. The uncertainties of one system will not only threaten its own safe operation but also be likely to have a significant impact on the other. Therefore, it is necessary to study the variation of state variables when random fluctuations emerge in the coupled system. In this paper, a multislack-bus model is proposed to calculate the power and gas flow in the coupled system. A unified probabilistic power and gas flow calculation, in which the cumulant method and Gram–Charlier expansion are applied, is first presented to obtain the distribution of state variables after considering the effects of uncertain factors. When the variation range of random factors is too large, a new method of piecewise linearization is put forward to achieve a better fitting precision of probability distribution. Compared to the Monte Carlo method, the proposed method can reduce computation time greatly while reaching a satisfactory accuracy.The validity of the proposed methods is verified in a coupled system that consists of a 15-node natural gas system and the IEEE case24 power system.

Frequency Control at the Power Sending Side for HVDC Asynchronous Interconnections Between Yunnan Power Grid and the Rest of CSG

For a large-scale high voltage direct current (HVDC)asynchronous interconnected power grid, the frequency issue atthe power sending side under DC faults is a crucial problem.To solve this problem, based on rotor motion equations, theeffect of unbalanced power on the system frequency under DCfaults is analyzed. The characteristics and dynamic developmentprocess of frequencies after the injection of disturbances areanalyzed. In addition, the actions and coordinated strategies ofvarious frequency control measures are also investigated. Basedon the testing projects of an asynchronous interconnection in theChina Southern Power Grid (CSG), the frequency features arestudied according to the measured PMU data. The outcome showsthat the frequency problem of the Yunnan Power Grid after anasynchronous interconnection can be solved and controlled. Italso shows that the frequency limit control (FLC) is importantfor the frequency regulation of large scale HVDC asynchronousinterconnected DC power grids. As demonstrated, DC FLC caneffectively suppress the deviation of the transient frequency.However, reasonable frequency regulation parameters shouldbe set and other area frequency control measures should becoordinated to maintain the frequency stability of the system.

Modeling nitrate aerosol distributions and its direct radiative forcing in East Asia with RAMS-CMAQ

The geographical and seasonal characteristics in nitrate aerosol and its direct radiative forcing over East Asia are analyzed by using the air quality modeling system RAMS-CMAQ coupled with an aerosol optical properties/radiative transfer module. For evaluating the model performance, nitrate ion concentration in precipitation, and mixing ratios of PMl0, and some gas precursors of aerosol during the whole year of 2007 are compared against surface observations at 17 stations located in Japan, Korea, and China, and the satellite retrieved NO2 columns, The comparison shows that the simulated values are generally in good agreement with the observed ones. Simulated monthly averaged values are mostly within a factor of 2 of the measurements at the observation stations. The distribution patterns of NO2 from simulation and satellite measurement are also similar with each other. Analysis of the distribution features of monthly and yearly averaged mass concentration and direct radiative forcing （DRF） of nitrate indicates that the nitrate aerosol could reach about 25-30% of the total aerosol mass concentration and DRF in Sichuan Basin, Southeast China, and East China where the high mass burden of all major aerosols concentrated. The highest mass concentration and strongest DRF of nitrate could exceed 40 μg/m^3 and -5 Wire2, respectively. It also indicates that other aerosol species, such as carbonaceous and mineral particles, could obviously influence the nitrate DRF for they are often internally mixed with each other.