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Direct power control (DPC) is widely used in grid-connected inverters. First, considering the effects of phase-locked loop (PLL), voltage outer loop, power inner loop, control delay, and frequency coupling, a broadband admittance model of the grid-connected inverter with DPC is established. The admittance characteristics of grid-connected inverters with DPC and closed-loop current control (CCC) are compared and analyzed. Due to the different inner loop control objects, the components produced by the power calculation in DPC and the Park's transformation in CCC cause the admittance differences between the grid-connected inverters with DPC and CCC. Then, the effects of grid impedance, output power, and control parameters on system stability are analyzed. It is revealed that the stability of the grid-connected inverter with DPC is very sensitive to grid impedance and easily affected by output power and inner loop bandwidth, while the grid-connected inverter with CCC has a better adaptability to weak grid, high output power, and inner loop bandwidth. Moreover, the PLL bandwidth has little effect on the stability of the grid-connected inverter with DPC, but has a great impact on the stability of the grid-connected inverter with CCC when the inverter is connected to weak grid. Finally, the correctness of the analysis is verified by experiments.

The load-side virtual synchronous machine (LVSM) enables the load to actively participate in the grid regulation, but it might still induce oscillations in a weak grid. Considering the dc-link voltage controller, power loops, ac voltage and current loops, the control delay, and sampling filters, the wideband dq -frame impedance model of the LVSM is first established, and it is found that $Z_{dd}$ exhibits negative resistor impedance within 10 Hz. In addition, the dq -frame impedance of the LVSM is approximately inductive above 10 Hz. Then, using the dq -frame impedance-based approach, the comparative study shows that the smaller the proportional gain and integral gain of the dc-link voltage controller are, the more stable the conventional voltage source rectifier (VSR) is in a weak grid. However, the larger the proportional gain is, the smaller the integral gain is, the more stable the LVSM is in a weak grid. Furthermore, the voltage feedforward decreases the stability margin of the VSR in a weak grid, while the virtual moment of inertia $J$ and the damping gain $D_{p}$ affect the stability of the LVSM, and the smaller $J$ and $D_{p}$ are, the more stable the LVSM is in weak grid. Finally, simulations and experimental results verify the impedance model and the stability analysis.

Small-signal stability problems often occur when the inverter for renewable energy generation is connected to weak grid. A small-signal transfer function integrated model reflecting the interaction of grid impedance, phase locked-loop (PLL), and current control loop is established in this paper. Based on the established model, the oscillation mechanism of the grid-connected inverter system is revealed: the inductance current flowing through the grid impedance can produce a voltage disturbance, which will eventually affect the inductance current through PLL and current control loop. Therefore, the loop composed of the grid impedance and PLL can easily lead to the oscillation of the grid-connected inverter system under weak grid condition. To suppress the oscillation, a control parameters design method of the grid-connected inverter is proposed. Without changing the control method, the proposed control parameters design method can ensure the stable operation of the grid-connected inverter system under the very weak grid condition when the short-circuit ratio (SCR) is 2. Finally, the experimental results verify the correctness of the stability analysis and the effectiveness of the control parameters design method proposed in this paper.

Phase-locked loop (PLL) and grid impedance easily negatively affect the stability of the grid-connected inverter system and even cause the system oscillation. To suppress this oscillation, a stability enhancing voltage feed-forward inverter control method is proposed to reduce the effects of PLL and grid impedance. Then, considering the effects of the frequency coupling characteristic, the admittance model of the grid-connected inverter with the proposed stability enhancing control method is established, and the admittance characteristic and the system stability of the inverters with the traditional control method and the proposed stability enhancing control method are compared and analyzed. Analysis results indicate that the proposed stability enhancing control method improves the system stability in different short-circuit ratio (SCR) conditions. What is more, the system with the proposed stability enhancing control method keeps stable under a relative larger PLL bandwidth, while the system with the traditional control method oscillates. Finally, the effectiveness of the proposed stability enhancing control method is verified by experiments.

The premise that the virtual synchronous generator (VSG) can actively support the weak grid is that the VSG can operate stably. In this paper, the small-signal sequence impedance models of the voltage-controlled VSG and the current-controlled VSG are built and the sequence impedance characteristics of these two types of VSGs are compared and analyzed. The sequence impedance of the voltage-controlled VSG, being consistent with the grid impedance characteristics, is generally inductive. By contrast, the sequence impedance of the current-controlled VSG is mainly negative impedance in the middle- and high-frequency area and the impedance amplitude is quite low. Based on the built sequence impedance models and the impedance stability criterion, the influence of the weakness of the grid on the stability of these two types of VSG grid-connected system is analyzed. The results of stability analysis show that the current-controlled VSG is prone to instability while the voltage-controlled VSG remains stable. Therefore, the voltage-controlled VSG is more suitable than the current-controlled VSG for grid-connected renewable energy generation in an ultraweak grid from the point of view of system stability. Finally, experiment results validate the stability analysis.

Adaptive modification design occupies a large proportion in modern product design, which is essentially a coordination process. Due to the conduction phenomenon, it often leads to the problem that the solution space is too large or even unsolvable. Aiming at the problems caused by the conduction phenomenon, an optimal coordination path selecting method for conduction transformation based Floyd algorithm is proposed. The product conduction transformation coordination process is considered as a path by combining the product graph theory model. The selection criteria of the start and end nodes is given. The functional contribution index, modified adaptation index and conduction impact index are described in detail. The calculation of the comprehensive coordination factor composed of them is given and used as the evaluation index of the conduction transformation coordination path. Floyd algorithm is adopted to coordination path optimization selection model, case study and system development, in order to verify the method proposed in this paper.

In the maritime VSC-HVDC system supplying power to passive network on the island, the dc side of the system is prone to oscillation and even instability, which is resulted from the negative incremental input resistance characteristic of the inverter station. First, a virtual phase-lead impedance stability control strategy aiming at rectifier station is proposed. This control strategy can effectively mitigate the dc-side oscillation of the VSC–HVDC system without affecting the load performance of the inverter station. Then, the dc impedance model of the VSC–HVDC system is built, including the output impedance of the rectifier station, dc cable impedance, and the input impedance of the inverter station. In addition, the oscillation mechanism of the VSC–HVDC system is analyzed by impedance-based Nyquist stability criterion. The reason why rectifier station fails to mitigate the dc-side oscillation using traditional virtual resistance stability control is that the output impedance of the rectifier station exhibits negative damping characteristic outside the control bandwidth of voltage outer loop. For this issue, the proposed control strategy can correct the output impedance of the rectifier station to exhibit positive damping characteristic outside the control bandwidth of voltage outer loop, thus mitigating the dc-side oscillation of the VSC–HVDC system. Finally, simulation and experiment results validate the proposed control strategy and analysis.

In Tahe Oilfield, Tarim Basin, NW China, although the carbonate reservoir displays poor porosity and permeability in matrix, the fractured-vuggy system distributes in a random, discreet and discontinuous way, which could yield high oil rates once the fractured-vuggy system is connected by production wells. The fractured-vuggy carbonate reservoirs consist a group of large caves which are inter-connected with high permeable fractures. Due to the lack of powerful and suitable IOR (Improved Oil Recovery) strategies, the depletion recovery is usually applied at the beginning for a fractured-vuggy carbonate reservoir. During the depletion process, the elastic energy of the cave formation starts to decrease, which will cause insufficient liquid supply. Then, the water was injected to improve the oil recovery when the energy (pressure) is the main reason for the insufficient production performance of the well. However, if the high water cut is the main reason, alternative IOR techniques such as the N2 huff and puff method could be utilized. This paper mainly researches the application of N2 huff and puff in a fracture-vuggy carbonate reservoir, where the critical parameters for designing the N2 huff and puff process are proposed. In addition, the proposed method is applied to Well SX to serve as a practical example. Based on the three-dimensional (3D) quantitative sculpture geological model and the dynamic production analysis, the critical parameters for N2 huff and puff are obtained by numerical simulation. In addition, the obtained parameters are validated by the field data, which proves that the proposed method can be applied to design the critical parameters for simulating the N2 huff and puff process in a fractured-vuggy carbonate reservoir.

In Tahe Oilfield, Tarim Basin, NW China, the carbonate reservoir displays poor porosity and permeability in its matrix, while the fractured-vuggy system is distributed in a random, discreet and discontinuous way, which yields significant oil rates once the fractured-vuggy system is connected by production wells. The fractured-vuggy carbonate reservoirs are consisting of a group of large carves and inter-connected by high permeability fractures. Due to lack of powerful and suitable EOR (Improved Oil Recovery) strategies, depletion-drive recovery is commonly adopted for practical production at the beginning. After a period of production, the elastic energy of the cave formation weakens, causing insufficient liquid supply, the water injected for water flooding is used to improve the oil recovery of the fractured-vuggy carbonate reservoirs. However, the water injection curves of the carbonate reservoir are different from that of the sandstone reservoir as their fundamental flow mechanisms are different. For the fractured-vuggy reservoirs with high permeability, the water injection curves can be adopted to calculate the volumes in place of crude oil and formation water in a single or multi-caves. This paper presents a new model of injection curves. Using the proposed model with multi-round water injection curves, we can initially judge reservoir type and determine the volumes of cave, crude oil and water in the caves reasonably. Besides, as testified by the practical case, the parameters interpreted by the new models are proved to be reasonable and reliable. This new model is a useful tool to estimate the crude oil reserves, to demonstrate how much remaining oil in the caves, and to identify where the water-oil contact is.

In LCL-type grid-connected inverter, an optimized design method for grid-current-feedback active damping (AD) is proposed to improve the system dynamic characteristic. By analyzing the equivalent virtual impedance model, the essential relationship between the system dynamic characteristic indicators and damping control parameters is obtained, and the damping control parameters are contradictory that difficult to optimize. Then, a parameters optimization design method combining zero-pole model and Routh stability criterion is proposed to reduce the complexity of the parameters design and improve the system dynamic. Moreover, the influence of the control delay on system stability and parameters design is analyzed in the discrete domain. Simulation and experimental results verified the validity of the proposed method.

In a single-phase grid-connected inverter, because the constructed orthogonal signals are not strictly orthogonal in broadband and the control of dq-axis is asymmetric, when the voltage disturbance with frequency of f p is injected in point of common coupling (PCC), not only the response current with frequency of f p will be generated, but also the response currents with frequencies of f p –2 f1 and f p +2 f1 will be coupled. In order to accurately establish the broadband impedance model of the single-phase grid-connected inverter, the frequency coupling mechanism is analyzed in this paper. Considering the influence of the constructed orthogonal signal, current loop, phase-locked loop (PLL) and frequency coupling, a broadband impedance model of the single-phase grid-connected inverter is established. Its broadband impedance characteristics are analyzed. The simulation results verify the accuracy of the established model.

With the continuous expansion of new energy grid-connected power generation, the utilization of a large number of power electronic inverters results in the aggravating interaction between new energy power generation and the grid, which led to critical wide-bandwidth oscillation. Therefore, this paper proposes a simplified wide-bandwidth generalized impedance model by modeling the single-phase grid-connected inverter in the polar coordinate system. Firstly, the small-signal dynamic model of the inverter in the polar coordinate system based with second-order generalized integrator (SOGI) is obtained. Then a frequency division basis of 10 Hz to 1 kHz is proposed because the dynamic response of each link varies with frequency. Finally, generalized impedance models of the inverter in the different frequency band are established respectively. The good agreement of the simulation result and the theoretical model verifies the feasibility and correctness of the model. The proposed model lays a good foundation for wide-bandwidth modeling of the three-phase system.

For lack of effective analysis methods, tools and model expression and management of conflict problems in modern product low-carbon design multi-conflict problem, low-carbon design conflict problem based on integrated TRIZ and Extenics is modeled. Firstly, an equivalent transformation model of conflict problem for low-carbon design is constructed and the core conflict factor is determined. Then the innovative solution to conflict problem for low-carbon design based on the TRIZ principle reasoning and formal extension transformation reasoning. In the end of the paper, a screw air compressor is taken as a low-carbon design case, and the conflict problem is focused on noise optimization and structural features of relevant parts of the screw air compressor. The performance data of the low-carbon design requirement is mapped into the muffler by conduction transformation. The case verified a good experimental result.

When the relative size of the measured grid-connected inverter of renewable energy power generation equipment's impedance and the grid's impedance changes in different frequencies, only using the current-injected impedance measuring device or the voltage-injected impedance measuring device can not accurately measure the wide-bandwidth frequency impedance of the measured equipment. In this paper, the basic theory of impedance measurement under single current-disturbed injection method and single voltage-disturbed injection method is introduced. Then, aiming at the problem of low measurement precision under single perturbation injection method, the impedance measurement of grid-connected three-phase LC-type inverter is taken as the research object, and a UPQC (Unified Power Quality Conditioner) -based impedance measurement device and its switching control method is proposed. The proposed method can realize the switch of the perturbation injection mode: when the impedance of the grid is higher than that of the measured equipment, the impedance measurement device works on the current injection mode; when the impedance of the grid is lower than that of the measured equipment, the device works on the voltage injection mode. So that the high proportion of the perturbation is distributed on the side of the measured equipment to improve the precision of impedance measurement. After measuring the impedance of resistance and grid-connected inverter of renewable energy power generation equipment, the effectiveness of the proposed device is verified by comparison and analysis.

In the grid-connected inverter based on the deadbeat current control, the filter inductance variation and single update PWM affect the distortion of the grid current, stability, and dynamic of the system. For this, a double update PWM method for the deadbeat current controller in three-phase grid-connected system is proposed, which not only effectively decreases the grid current distortion and control delay, but also improves the system stability and dynamic response speed due to reducing the characteristic root equation order of the closed-loop transfer function. The influence of the filter inductance deviation coefficient on the system performance is analyzed. As a conclusion, the corresponding filter inductance deviation coefficient in the system critical stability increases with increase in the parasitic resistance of the filter inductance and line equivalent resistance and decreases with increase in the sampling frequency. Considering the system stability and dynamic response, the optimal range of the control parameters is acquired. Simulation and experimental results verify the effectiveness of the proposed method.

In photovoltaic grid-connected inverter based on the deadbeat current control, the filter inductance variation and one-step-delay control affect the distortion of the grid current, stability and dynamic of the system. In this paper, a fast robust PWM method for photovoltaic grid-connected inverter is proposed, which reduces the distortion of the grid current caused by the filter inductance variation, effectively solves the delay of the one-step-delay control, and improves the stability and dynamic response speed of the system. Considering the parasitic resistance of the filter inductance and the line equivalent resistance, the influence of the filter inductance deviation coefficient on the system performance is analyzed, and the appropriate control parameter range is given. Simulation and experimental results verify the effectiveness of the proposed method.

Microgrid cluster as an interconnected system of multiple AC subgrids and DC subgrids, its complex structure increases the difficulty of power coordination control for it. To address this issue, a microgrid cluster structure and its autonomous coordination control strategy are proposed in this paper. The proposed microgrid cluster structure is composed of AC subgrids, DC subgrids, power exchange unit (PEU) and energy pool (EP). The aim of PEU is mainly to coordinate power exchange between the subgrids and EP. Meanwhile, the power coordinated control method is proposed to design the controller of PEU, which can realize mutual power support among each sub-grid and reduce the bus voltage deviation in each subgrid. Besides, EP is mainly used to balance the system power, and the dc-link voltage stability control method of EP is proposed to stabilize the dc-link voltage and to reasonably allocate the net exchange power of PEU. The proposed autonomous coordination control strategy has been verified by the simulation results.

For LCL-type distributed generation (DG) systems connected to weak grid, a novel two frequency-bandwidths real-time sensitive inverter-current-feedback (TFBRTS-ICF) control method is proposed to enhance the system reliability and robustness, which mainly includes the synchronous reference frame quasi-proportional-integral (SRFQPI) control, un-attenuated inverter-current-feedback active damping (UICFAD) control, and high-sensitivity real-time PWM algorithm. The SRFQPI control can compensate reactive power, and regulate the instantaneous current without steady-state error regardless of the fundamental frequency fluctuation. The UICFAD control can suppress the LCL-resonance peak well without requiring additional sensors. The proposed PWM algorithm can solve the robust problem of the UICFAD controller against grid-impedance variation through removing the computation delay. Meanwhile, the proposed PWM algorithm can achieve a full range of duty-ratio between 0∼1 and offer half of switching period for the inverter-current sampling and the duty-ratio calculating, which is easy to implement using a digital microcontroller. Experimental results confirm the theoretical expectations and the effectiveness of the proposed method.

Low-carbon design is a sustainable method, which coordinates the carbon footprint, cost and performance of each stage in product lifecycle from the design source to satisfying low-carbon demand. It is also a significant technology to reduce the carbon emission before manufacturing. To effectively and efficiently reduce the carbon emission, a low carbon-oriented design method is studied in product lifecycle. A product is modularized to function units for obtaining carbon footprint, which are quantified based on the framework of PAS2050. In order to identify the high-emission parts, an activity-based method is used to allocate the carbon footprint. Through the lifecycle analysis, the major parts and components characterized with high carbon emission are picked out according to allocation result. Due to the close relationship and conflict between the carbon emission and cost of product, an improved genetic algorithm with a novel selection mechanism, which used the correlation function, is presented to optimize these parts in order to reduce both the carbon footprint and cost. In the end of the paper, a vacuum pump is taken as a case to verify the allocation method and the improved genetic algorithm, and the result shows the effective reduction of carbon footprint and cost.

Low-carbon performance as well as the quality and cost of a new product are normally emphasized in the early phase of low-carbon design for products. Although the TRIZ method and Extenics theory can be applied separately to solve contradiction problems in design field, these two methods have their weaknesses in applications. The purpose of this study is to provide a novel model for accelerating the preliminary low-carbon design by integrating the TRIZ and Extenics methods. Analysis tools and knowledge base tools of TRIZ are adopted to generate generic strategies; basic-element theory and dependent function of Extenics are used to qualitatively and quantitatively describe the conflict problem in a formalized model, and detailed transformation operations are employed to achieve the feasible design solutions. Innovative design schemes for two kinds of conflict problems of the screw air compressor demonstrate the effectiveness of the proposed method.