Xi Liu

Renewable Energy Certificates (RECs) have emerged as critical market-based policy instruments to promote renewable energy development worldwide. This comprehensive review examines the theoretical foundations, market mechanisms, policy effectiveness, and challenges of global REC systems based on extensive international experiences spanning over two decades. RECs function by separating the environmental attributes of renewable electricity from its physical energy, creating flexible trading mechanisms that effectively channel private investment toward renewable energy projects while providing compliance tools for renewable portfolio standards. Our analysis reveals significant variations in design and implementation across major markets, including the United States, European Union, China, India, Australia, and emerging economies. Despite their widespread adoption with over 50 countries implementing various forms of REC mechanisms, these markets face persistent challenges including price volatility, limited liquidity, regulatory inconsistencies, and ongoing debates about their environmental additionality. Recent technological developments, particularly blockchain-enabled tracking systems and digital platforms, are reshaping REC markets by enhancing transparency, reducing transaction costs, and enabling smaller-scale participation. This review proposes corresponding recommendations from the dimensions of optimizing market design, promoting digital transformation and product diversification, and establishing international coordination mechanisms.

This article presents a modified double phase shifted pulse width modulation (MDPS-PWM) strategy designed to suppress circulating current in parallel three-level inverters. In contrast to the double phase shifted pulse width modulation (DPS-PWM), the proposed MDPS-PWM introduces a deliberately chosen phase shift angle for circulating current suppression. By incorporating this targeted phase shift, the strategy effectively reduces the amplitude of the difference in common-mode voltage. This modification leads to a significant decrease in circulating current without compromising the overall output current quality. Furthermore, a sawtooth carrier-based modulation (SCBM) strategy is employed for simplified implementation of carriers. Both simulation and experimental results validate the effectiveness and superiority of the proposed MDPS-PWM method.

The hybrid cascaded high voltage direct current (HC-HVDC) technology, which combines the advantages of the line commutated converter-based HVDC (LCC-HVDC) and the modular multilevel converter-based HVDC (MMC-HVDC) technologies, is one of the most cost-effective solutions for long-distance transmission of large-scale renewable energy. Therefore, a topology of the HC-HVDC transmission system is constructed in this paper for direct-drive wind farms in desert and gobi areas where there is no synchronous power support in the sending-end system. Then a grid-forming control strategy for the sending-end hybrid cascade converter is designed to provide AC voltage support. Besides, an active power balancing control strategy of the sending-end system based on the additional DC voltage control of the MMC is proposed to adapt to the stochastic fluctuation characteristics of wind power. After that, a small signal dynamic model of the whole system is established, and the influence of important parameters on the small signal stability of the system is analyzed. Finally, the time-domain simulations based on PSCAD/EMTDC confirm the correctness and validity of the proposed control strategies and the stability analysis conclusions.

DC-link voltage fluctuation and average load torque characteristic are significant concerns for the small film capacitor interior permanent magnet synchronous motor (IPMSM) based compressor drives. The quantitative mathematical model of fluctuated dc-link voltage is fully investigated, and hence the available voltage margin is obtained, which consists of the constant and fluctuation part. The two parts are related to the minimum and mean value of the dc-link voltage, respectively. Meanwhile, the average load torque characteristic considering operating frequency and temperature variation is established based on the measurements. To achieve full use of dc-link voltage and torque promotion, a dual stage voltage margin allocation strategy is proposed namely constant voltage limit (CVL) and constant current limit (CCL) stage, which well corresponds to the constant and fluctuation part of the dc-link voltage. The essence of dual stage reveals that the motor current vector can be reshaped on dq-plane with the extra voltage margin, and the principle is illustrated step by step based on current vector trajectory. Therefore, the power region can be expanded compared with the conventional strategy. In addition, the special CVL stage that six step operation (SSO) mode is introduced on account of severely reduction of dc-link capacity. Experimental results and industrial tests have been presented to confirm the effectiveness of the proposed method.

For three-level T-type inverter (3LT2I), the unbalanced neutral-point (NP) voltage is required in some photovoltaic (PV) generation systems. Meanwhile, the unbalanced NP voltage easily leads to the fault of power switches. However, the existing fault tolerant control (FTC) strategies are only capable of the balanced NP voltage conditions. In order to simultaneously realize the FTC and unbalanced NP voltage control (UNPVC), a half cycle FTC (HC-FTC) strategy and a whole cycle FTC (WC-FTC) strategy are proposed in this paper. The carrier is modified to avoid using the impossible switching states and realize FTC. Meanwhile, the mathematical models of NP current under both normal and fault conditions are established based on volt-second balance principle, and thereby the zero-sequence component (ZSC) derived by the model can be used to realize the UNPVC whether the switch is fault or not. In addition, the ZSC is selected in detail to improve the NP voltage control ability in HC-FTC. Furthermore, the application range of HC-FTC is obtained and the WC-FTC with wider application range is proposed. The two proposed strategies can be selected according to the requirements. Finally, experimental results validate the effectiveness of the proposed strategies.

Waste-to-energy (WtE) incineration is a feasible way to respond to both the municipal solid waste management and renewable energy challenges, but few studies have been carried out on its environmental and economic impact in fast-developing southeastern Asian countries. To fill such a research gap, this study innovatively conducted a holistic assessment of WtE incineration application potential in Java Island, Indonesia. Here, we have established a life cycle assessment model for WtE incineration in Java, and have estimated the environmental impact, electricity generation potential, and techno-economic feasibility of implementing incineration by 2025. We have revealed that global warming potential, terrestrial ecotoxicity potential, eutrophication potential, and acidification potential are the major environmental impacts stemming from incineration activities. Moreover, we have estimated that promoting incineration in Java could reduce CO₂ emissions by 41% on average. The electricity generated from incineration could contribute to 3.72% of Indonesia’s renewable energy target for the electricity grid mix by 2025. The cumulative energy production potential from incineration is estimated to reach 2,316,523 MWh/year in 2025 and will increase by 14.3% in 2050. The techno-economic assessment of incineration implementation in Java cities has been enumerated as feasible. The levelized cost of electricity from incineration (0.044 USD/kWh) is competitive with the current Indonesian electricity price (0.069 USD/kWh). Policies of minimizing incineration pollution, providing financial support guarantees, and overcoming social barriers have been proposed to facilitate the application of WtE incineration.

This article proposes a double phase shifted pulsewidth modulation (PWM) strategy to improve the quality of total output current in multiple parallel converters for the photovoltaic application with the advantage of being easy to implement/extend. Different from phase shifted PWM (PS-PWM), the additional modified angles are deduced and added in the initial phase shifted angles of carriers. The better total output current with lower total harmonic distortion can be achieved by modifying the phase shifted angles of carriers in each converter twice. The harmonic spectrum analysis shows that the proposed method could provide superior total output line voltage and total output current quality than conventional PS-PWM. The correctness and effectiveness of the proposed method are verified by the experiments in parallel three-phase two-level and three-level converters.

AC/DC power converter is the momentous interface of the power router, which is responsible for maintaining the DC bus voltage stability. However, under unbalanced and distorted grid voltage conditions, there are voltage ripples in the DC bus, and grid currents will also be severely distorted. Accordingly, this paper proposes an improved passivity-based direct power control (IPBDPC) strategy based on extended reactive power theory in the $\alpha \beta$ axis, which can effectively suppress DC voltage ripples and current harmonics of power converter. Meanwhile, with the proposed IPBDPC, the fundamental grid voltages and those orthogonal components are injected into the improved passivity model, so that the controller can further reduce the THD of the grid current when unbalanced and distorted grid conditions coexist. Compared with the traditional modified DPC, the IPBDPC strategy has a simple structure and strong robustness, does not require complex positive and negative sequence separation, and power compensation value calculation. Moreover, the sequence impedance model of IPBDPC is constructed based on the harmonic linearization method to verify the robustness of the control system against parameters uncertainty. The simulation and experimental results based on a three-phase three-level converter system further demonstrated the effectiveness of the proposed control strategy.

The hybrid cascade converters is a potential solution to achieve harmonic current compensation in medium and high voltage applications due to the characteristics of having high frequency module. However, the high switching frequency inevitably leads to an increase of undesirable switching loss. In addition, since the reduction of power loss and the balance of clustered voltage are mutually interrelated, the existing modulation and control schemes cannot cope with them decently. To address this problem, the HCC is divided into a high-frequency module and a low-frequency module, and a corresponding hybrid modulation scheme is proposed to reduce the power loss while maintaining a high harmonic compensation performance. Then, based on the analysis of the system voltage constraints, the dc-link voltage and threshold voltage under different parameters are redesigned. Furthermore, since the power flow between modules has been changed with the hybrid modulation scheme, the conventional clustered voltage control strategy is not capable of clustered control. Therefore, based on the remodeling of the power flow, a negative-sequence voltage injection control strategy is proposed to achieve voltage balance. Finally, simulation and experimental results are demonstrated to verify the effectiveness of the proposed scheme.

At present, shunts are widely used in current measurement of DC Switch Cabinet. In view of the problems of large volume, narrow measurement range and inability to realize electrical isolation, a busbar current measurement method based on differential magnetic field is proposed. The differential magnetic field is generated by adding shunt slots on the busbar and the back-end circuit is designed to reduce the influence of interference magnetic field on measurement. It has small volume and wide measurement range, strong anti-interference ability, non-contact and other characteristics. Firstly, the measurement method and anti-interference principle are analyzed theoretically, and then the bus simulation model is established in COMSOL to simulate and verify the measurement method, and the transfer relationship between magnetic field and current is calibrated by finite element method. Finally, an experimental platform for busbar current measurement is built, and the experimental results show that the error of the sensor prototype is less than 1.2%, and the influence of the interference magnetic field generated when the interference source is 25mm or more from the sensor can be ignored, which proves the accuracy and anti-interference of the measurement method.