Yiqiang Chen’s research while affiliated with Fuzhou University and other places

With the sustainable growth in the integration of distributed generation units in the distribution network, the probability of the fault current level exceeding the rating of existing components increases. Cascaded H bridge fault current limiter is widely used in the power grid due to its advantage in inhibiting surge current flexibly. In order to equilibrate the objective functions of its cost, fault current mitigation effect, and the weighted load reliability index, a novel methodology is proposed to simultaneously optimize the location and size of the limiters in the distribution network. Therein, the sensitivity factor considering the Monte Carlo fault simulation model is introduced to reduce the search space and rank candidate locations referencing the actual conditions. And then, according to the candidate locations and considering different conflicting objective functions, a multi-objective improved bat algorithm is employed to obtain the Pareto optimal solution set. Also, life cycle cost and net present value are introduced to construct an economic model to access the scheme costs and service life. The proposed approach is verified using the modified IEEE 33-bus distribution systems with DGs and IEEE 30-bus Benchmark system. The results demonstrate that the proposed method exhibits higher efficiency in finding optimum solutions and provides a new economic configuration idea for the practical engineering application.

One of the most important issues that must be taken into consideration during the planning of energy storage systems (ESSs) is improving distribution network economy, reliability, and stability. This paper presents a two-layer optimization model to determine the optimal siting and sizing of ESSs in the distribution network and their best compromise between the real power loss, voltage stability margin, and the application cost of ESSs. Thereinto, an improved bat algorithm based on non-dominated sorting (NSIBA), as an outer layer optimization model, is employed to obtain the Pareto optimal solution set to offer a group of feasible plans for an internal optimization model. According to these feasible plans, the method of fuzzy entropy weight of vague set, as an internal optimization model, is applied to obtain the synthetic priority of Pareto solutions for planning the optimal siting and sizing of ESSs. By this means, the adopted fuzzy entropy weight method is used to obtain the objective function’s weights and vague set method to choose the solution of planning ESSs’ optimal siting and sizing. The proposed method is tested on a real 26-bus distribution system, and the results prove that the proposed method exhibits higher capability and efficiency in finding optimum solutions.

This paper proposes a novel control algorithm to enhance the fault ride-through (FRT) capability of a photovoltaic (PV) system. In this method, the overcurrent of the grid-tied inverter is suppressed to a preset value using model current predictive control (MCPC) algorithm, and its DC-link overvoltage is removed using a non-maximum power point tracker (non-MPPT) algorithm. Therefore, the inverter overcurrent and its DC-link overvoltage problems can be placed under decoupling control using a DC/AC converter controller and DC/DC converter controller, respectively. Using eight switching modes for the three-phase inverter, the MCPC of DC/AC converter controller establishes a value function between the current reference value and output current value. Then, by introducing the switching mode (corresponding to the minimum value function) into the DC/AC converter controller, if the current reference is set at the rated value, the inverter’s output current can be inhibited to the rated value under fault conditions. Moreover, a balanced three-phase rated current can always be obtained under either a symmetric or asymmetric fault. However, together with the MCPC, a DC-link overvoltage will appear. To remove this DC-link overvoltage quickly, based on the grid system voltage sag level, the non-MPPT algorithm is used to calculate the adjusted power for PV arrays. Then, based on the calculated power, the amended duty cycle can be obtained and immediately introduced into the DC/DC converter controller to tune the PV arrays’ output power, which significantly decreases the power imbalance between the inverter’s AC and DC sides, thus inhibiting the DC-link overvoltage. In addition, to inhibit DC-link voltage fluctuations further, DC-link voltage feedforward compensation is introduced into the DC/DC converter controller. In particular, under an asymmetric fault condition, with the help of the voltage feedforward compensation, the second harmonic frequency components of the DC-link voltage can also be removed. Finally, based on the theoretical derivation and simulation results, it can be proved that the major problems troubling a PV system’s FRT technologies can be resolved by the proposed method, especially the problem related to the harmonic components of the DC-link voltage.

This paper proposes a hybrid coordination control strategy to improve the low voltage ride-through (LVRT) capability of microgrids. During microgrid external failure, the overcurrent and the voltage sag of the microgrid can be effectively suppressed. Compared with traditional control strategies, the microgrid transient performance can also be enhanced by adopting a generalized control algorithm, performing the active switching of the energy storage (ES) operation mode and triggering the inductance-type flux coupling fault current limiters (FCLs). Among them, according to the hierarchical control structure of the grid-connected inverter, the generalized control algorithm is applied to ES. Through the two degrees of freedom's control principle and inverse plant modeling techniques, the generalized control algorithm can make the control layer of the ES inverter operate in all control targets (i.e., PQ control, droop control and Vf control), with a single control structure. Also, it eliminates the effects of microgrid distortion voltage and ES surge current on the control system of the ES inverter. For the inverter application layer, by introducing microgrid voltage and angular frequency feedforward compensation, the voltage and frequency fluctuation of the microgrid can be suppressed efficiently under the fault conditions. Therefore, with the help of the generalized control algorithm, the active switching function of ES can be carried out successfully, which is the key technology for microgrids to realize their operational mode smooth switching. Meanwhile, the voltage sag and overcurrent of microgrids can be significantly mitigated through generating reactive power from microgrids and injecting it into inductance-type FCLs with lower cost. The related theoretical derivation and simulation analysis under various scenarios (including asymmetric faults) confirm that the proposed control strategy can not only enhance the LVRT capability, but also strengthen the microgrid transient performance.