A novel control strategy is proposed for the recent proposed electric springs (ESs), which are connected in series with noncritical loads to form smart loads for enhancing stability of smart grid with renewable energy sources. δ control is the key concept in this paper, which is realized by controlling the phase angle of the predefined reference in a proportional resonant controller. Four critical operating functions of the ESs are analyzed with different critical loads such as resistive, inductive and capacitive types, where vector diagrams and geometric relationships are explored for δ calculation with which the ac mains voltage is regulated to the predefined value and the phase angle between the ES voltage and current is determined. With the proposed δ control, the operating modes of the ES can also be determined automatically as the input voltage varies. Operating limitations and constraints of the ESs and guidelines on how to distribute the ESs in the distributed systems are provided with δ control. Both simulation and experiment are carried out to verify the effectiveness of the proposed control strategy and theoretical analysis for the ESs.

Aiming at effective power management in microgrids with high penetration of renewable energy sources (RESs), this paper proposes a simple power control for the so-called second-generation, single-phase electric springs (ES-2), which overcomes the shortcomings of the existing ES control methods. By the proposed control, the unpredictable power generated from RESs is divided into two parts, i.e., the one absorbed by the ES-2 that still varies and the other injected into the grid that is controllable by a simple and accurate signal manipulation that works both at steady-state and during RES transients. It is believed that such a control is suitable for the distributed power generation, especially at domestic homes. In this paper, the proposed control is supported by a theoretical background. Its effectiveness is at first validated by simulations and then by experiments. To this purpose, a typical RES application is considered, and an experimental setup is arranged, built up around an ES-2 implementing the proposed control. Testing of the setup is carried out in three steps and proves not only the smooth operation of the ES-2 itself, but also its capability in running the application properly.

The modular multilevel converter (MMC) is attractive for medium or high-power applications because of the advantages of its high modularity, availability, and high power quality. Reliability is one of the most important challenges for the MMC consisting of a large number of submodules (SMs). The capacitor monitoring is one of the important issues in the MMC. This paper proposed a reference submodule (RSM)-based capacitor monitoring strategy for the capacitance estimation in the MMC, where the capacitances in the monitoring SMs can be estimated based on the capacitor voltage relationship between the RSM and the monitoring SMs. The proposed monitoring strategy does not rely on the information of all capacitor voltage and current, which effectively simplifies the algorithm for capacitance estimation. The simulation studies with the time-domain professional tool PSCAD/EMTDC are conducted and a down-scale MMC prototype is also tested in the laboratory with the proposed capacitor monitoring strategy. The study results confirm the effectiveness of the proposed capacitor monitoring strategy.

The modular multilevel converter (MMC) is attractive for medium/high-voltage and high-power applications because of the advantages of its high modularity, availability and high power quality. Due to chemical process, aging effect, etc., the capacitor in the submodule (SM) of the MMC would gradually deteriorate and the capacitance would drop, which would cause unbalanced SM power losses distribution in the same arm of the MMC and affect the reliability of the MMC. This paper proposed an equivalent-reference (ER) control method, which can effectively realize balanced SM power losses distribution in the same arm of the MMC through the voltage-balancing control for the virtual capacitor voltages in the MMC under the capacitor deterioration. The proposed ER control can effectively improve the reliability of the MMC with the balanced SM power losses distribution in the MMC under capacitor deterioration. The simulation studies with the time-domain professional tool PSCAD/EMTDC are conducted and a down-scale MMC prototype is also tested with the proposed control strategy. The study results confirm the effectiveness of the proposed control strategy.

The capacitor voltage balancing is one of the most important issues for safe and reliable operation of the MMC. This paper proposes a currentless submodule individual voltage balancing control (SMIVBC) to realize capacitor voltage balancing in the MMC. Through regulating the dc component in each SM capacitor current by modulation index m-based SMIVBC or phase angle m-based SMIVBC, each SM capacitor voltage can be individually controlled to follow reference value, which can realize capacitor voltage balancing in the MMC. The proposed SMIVBC not only requires no sorting technique to select the SMs in the MMC avoiding sorting algorithm, but also requires no knowledge of current in the MMC, which reduces the sensors and sampling signals, saves the cost and improves the reliability. Simulation studies with professional tool PSCAD/EMTDC and experiment studies with a down-scale prototype in the laboratory are both conducted and their results confirm the effectiveness of the proposed SMIVBC for the MMC.

Permanent magnet synchronous motor (PMSM) has been widely used in industry in recent years because of its simple structure, high power density, and high efficiency. Accurate electromagnetic parameters of PMSM are of great significance in many fields, such as control system design, fault diagnosis, and condition monitoring. However, when the motor is in steady-state operation, the traditional identification method will have the issues, such as model accuracy, rank deficient, and the incompletely observable. Therefore, a parameter identification method based on biased triangular wave injection and current differential term is proposed in this article to solve them. In addition, the influence of triangular wave injection and current differential term on motor parameter identification is comparatively analyzed and verified by comparative experiments. Finally, the experimental results show that the proposed method can realize the multiparameter identification of motor resistance, inductance, and flux linkage in steady-state operation.