Large-scale wind power cluster with distributed wind farms has generated the active power dispatch and control problems in the power system. In this paper, a novel hierarchical model predictive control (HMPC) strategy based on dynamic active power dispatch is proposed to improve wind power schedule and increase wind power accommodation. The strategy consists of four layers with refined time scales, including intra-day dispatch, real-time dispatch, cluster optimization and wind farm modulation layer. A dynamic grouping strategy is specifically developed to allocate the schedule for wind farms in cluster optimization layer. In order to maximize wind power output, downward spinning reserve and transmission pathway utilization are developed in wind farm modulation layer. Meanwhile, a stratification analysis approach for ultra-short-term wind power forecasting error is presented as feedback correction to increase forecasting accuracy. The proposed strategy is evaluated by a case study in the IEEE network with wind power cluster integration. Results show that wind power accommodation has been enhanced by use of the proposed HMPC strategy, compared with the conventional dispatch and allocation methods.

This paper studies the dynamic behaviors of weak-grid-tied VSCs with simplified transfer functions, which provides an accurate stability analysis and useful indications for tuning system parameters. A reduced-order multi-input multi-output (MIMO) transfer function that contains four single-input single-output (SISO) transfer functions for the weak-grid-tied VSC is first presented. It is found that the four SISO transfer functions share the same equivalent open-loop transfer function, i.e., the same stability conclusion. The Bode plots of the equivalent open-loop transfer function show that the inner current loop behaves as a band-pass filter whose maximum gain is approximately at the frequency of the PLL's bandwidth. By stability criterion, the harmonic amplification and instability occur when its maximum gain exceeds 0dB caused by high PLL's bandwidth, large grid impedance or high active power. It is also found that the target system is less stable when it works as an inverter than as a rectifier, due to the risk of the local positive feedback in the inverter mode. An effective criterion is further proposed to guide the selection of a proper PLL's bandwidth to ensure the stability of the VSC system. Simulation results validate the correctness of the analysis and the efficacy of the criterion.

This letter analyzes the mechanism of the AC bus overvoltage at the LCC-HVDC rectifier station caused by the sending-end AC faults. Firstly, the influence of commutation failure on relevant electrical quantities is analyzed. After the commutation failure disappears, the inverter switches to constant current control. Under the joint action of the constant current control of the rectifier, the DC current and the trigger angle of the rectifier decrease rapidly, which reduces the reactive power absorbed by the rectifier. With the recovery of HVDC transmission capacity, the sending end AC bus voltage sharply rises, which increases the reactive power output of the filter. Reactive power redundancy occurs in the sending end AC bus leading to overvoltage. The suppression method proposed in this letter can effectively increase the reactive power absorbed by the LCC-HVDC rectifier during fault recovery. Besides, the method can also significantly enhance the minimum value of the inverter extinction angle and avoid possible commutation failure. The effectiveness and applicability of the strategy are confirmed through Electromagnetic Transient simulation.

Modern bulk power systems are developing into AC-DC hybrid systems with high penetration of renewables. There has been a lack of representative benchmarks to study stability and security problems of modern power systems. This paper presents an attempt to provide a set of four test models, addressing problems of voltage stability, rotor angle stability, frequency stability, and power frequency overvoltage under large disturbance. On the four stability/security problems, the models consider both their conventional features and distinct features induced by renewables. All the data of the models are extracted from real power systems, with modifications to establish a near-future scenario with over 50% renewable penetration. The test models are proposed by China Electric Power Research Institute, and acknowledged by Chinese Society for Electrical Engineering (CSEE). They are expected to serve educators, researchers and practitioners in related areas.

In order to solve the problem that the inertia estimation method based on the rate of change of frequency (RoCoF) is only applicable to the centre of inertia (COI) frequency, a novel inertia estimation method is proposed based on the maximum frequency deviation. This method utilizes the property that the maximum frequency deviation of each generator is basically consistent and easy to obtain. Firstly, the inertia support power of the system is quantified according to the disturbance power and frequency, and then the inertia support energy of the disturbance process is obtained. Finally, the system inertia is estimated by the ratio of the inertia support energy to the maximum frequency deviation. The proposed estimation method is suitable for non‐COI frequency, which makes it unnecessary to obtain the COI frequency and RoCoF. On this basis, a simplified estimation method is proposed based on the linearization of regulation power, which improves the estimation accuracy and simplifies the estimation process. According to the proposed method, the governor test data is used to estimate, which increases the estimation times and provides more information for system stable operation. The effectiveness and accuracy of the proposed method are validated by some cases.

The introduction of switched system theory opens new perspectives for the control design of dc/dc converters. Regarding the classical two-level converters, two switching control design methodologies have been developed based on the common quadratic Lyapunov function (CQLF) or piecewise quadratic Lyapunov function (PQLF). However, the CQLF-based design method is not applicable for the multilevel dc/dc converter (MC). As for the PQLF-based design method, it encounters the problem that the control design is very complex, involving a nonlinear optimization problem. To further advance the application of the switching system theory in power converters, a design framework based on the polynomial Lyapunov function and the sum-of-squares approach is proposed in this article. The proposed design method generalizes the CQLF-based one. Moreover, compared with the PQLF-based design method, the mathematical design procedure can be significantly simplified. Experimental validations on an MC, i.e., the three-level flying capacitor converter and a two-level Buck-Boost converter illustrate the versatility and the effectiveness of the proposed design framework. Moreover, comparative tests show the good control performance of the designed switching control strategy. The currently presented works are preliminary but could constitute an interesting direction for developing control solutions of power converters.