A generic small-signal model of modular multilevel converter (MMC) based DC grid is established and a DC virtual impedance damping control to suppress the resonance and instability is proposed. The ...averaged-value model (AVM) of MMC is employed to derive the Thévenin equivalent model of the converter from its DC side using the power-balancing principle considering the dynamics of phase-locked loop (PLL) of the interconnected ac system. The single section π-typed line model is selected to develop the nodal admittance equations of the DC network in s domain. The stability criterion of the DC grid is given after establishing the generic linearization model. The key factors affecting the DC grid's stability are identified using the root locus method and participation factors analysis. The parameters and the performances of the damping controller are designed and studied. Electromagnetic transient simulation model and RT-LAB real-time simulation are used to validate that the proposed damping control can suppress the instability of the DC grid and improve its operating performance.
A dc-dc converter is essential for interconnecting HVdc networks of different voltage levels. This letter introduces a new family of dc-dc converters consisting of semiconductor switches and stack of ...submodules, which is named as hybrid-cascaded dc-dc converter (HCDC). This concept shows very attractive capital costs and low power losses. In this letter, the detailed operating principle of the HCDC is analyzed. Simulation results are obtained from PSCAD/EMTDC to verify the feasibility of the converters. The performances of the converters are evaluated in detail.
Voltage source converter (VSC)-based high-voltage direct current (HVDC) and multi-terminal (MT)/DC grid technologies are the new HVDC transmission technologies after ultra-high voltage alternative ...current (UHVAC) and UHVDC transmission technologies which have been widely used in China. The application of the new technologies has resulted in a rapid increase in the number of schemes in construction and planning worldwide. This has been stimulated by the greater level of functionality available from the VSC technology, which makes it suitable for a wide variety of applications. These include the integration of off-shore wind farms, embedded links within AC networks and interconnectors, especially where the AC networks are relatively ‘weak’. VSC technology has renewed interest in MT DC systems, which may ultimately lead to wide area DC grids. This study outlines the research and application on MT and DC grids in China with respect to VSC-HVDC key technologies and DC grid key technologies based on the presentation given in the International Workshop on Next Generation Power Equipment held on 23 September 2016 in Xian, China. The briefing details of the VSC-HVDC projects constructed and to be constructed in China are summarised in this study.
An unbalance voltage between a positive pole and negative pole will exist for a long time in an ungrounded modular multilevel converter (MMC) HVDC transmission system. A zig-zag transformer has been ...proposed to construct an artificial neutral point and keep voltage balance between the positive pole and the negative pole. An efficient grounding system consists of the zig-zag transformer and the neutral grounding resistor. In this paper, the different impedances, including positive-, negative-, zero-sequence impedance, and dc current impedance, have been analyzed, respectively. Then, the computer simulations have been performed to evaluate the characteristic of the zig-zag transformer under normal voltage, zero-sequence voltage and unbalanced dc voltage, respectively. The simulation results show that the zig-zag transformer can effectively balance the unbalanced voltage in dc side of the MMC under different work conditions; the zero-sequence impedance is low and zero-sequence current is convenient for the detection of a single-phase fault; the power consumption of the neutral grounding resistor is of a light rating, and the cooling of the resistor is unnecessary. The method presented in this paper proves to be an efficient grounding method for the MMC-HVDC transmission system.
Using voltage-source converter high-voltage direct current (VSC-HVDC) to interconnect weak ac power systems has drawn much attention. Under the traditional vector control for the VSC that the ...response of AC voltage loop cannot match that of the active power loop may limit power transmission capability of the VSCs that interconnect with weak power systems. To enhance the power transmission capability and stability, this paper proposes a power compensation control that is based on the analysis of characteristics of power-voltage (PV) curves. This method can improve the damping control performance of the active power and the ac voltage by adjusting the compensated values adaptively. A small-signal model of the integrated system is established to study the performance of the proposed control method. The impact of the compensation coefficients and steady-state operating points on the system stability is studied. Motivated by an on-going back-to-back project in China, corresponding electromagnetic transient simulation models are constructed in MATLAB/Simulink environment to verify the effectiveness of the proposed method on the performance of operating limits, damping performance, disturbance withstanding capability, and fault ride-through capability.
Grid-forming control (GFC) is promising for power electronics based power systems with high renewable energy penetration. Naturally, the impedance modeling for GFC is necessary and has gained ...significant attention recently. However, most of the impedance analyses for GFC are based on a two-level converter (TLC) rather than a modular multilevel converter (MMC). MMC differs from TLC with respect to its dominant multi-frequency response. It is necessary to analyze the impedance of GFC-based MMC owing to its superiority in high-voltage direct current (HVDC) transmission to interlink two weak AC systems with high renewable energy penetration. As the main contribution, this paper presents the AC- and DC-side impedance analyses for the GFC-based MMC with both power and DC voltage control using the harmonic transfer function (HTF), and compares the impedances of GFC-based MMC and TLC. It is inferred that although the impedance is mainly influenced within 200 Hz, the instability still could occur owing to negative resistance triggered by relatively larger parameters. The difference in AC-side impedance with power and DC voltage control is not apparent with proper parameters, while the DC-side impedance differs significantly. The generalized Nyquist criterion is necessary for AC-side stability owing to the relatively large coupling terms under GFC. Moreover, the coupling between AC- and DC-side impedances is noneligible, especially considering the DC-side resonance around the system resonant peak. The effects of parameters, system strength, and virtual impedance on the impedance shaping are analyzed and verified through simulations.
Monascus is one of the most essential microbial resources in China, with thousands of years of history. Modern science has proved that Monascus can produce pigment, ergosterol, monacolin K, ...γ-aminobutyric acid, and other functionally active substances. Currently, Monascus is used to produce a variety of foods, health products, and pharmaceuticals, and its pigments are widely used as food additives. However, Monascus also makes a harmful polyketide component called citrinin in the fermentation process; citrinin has toxic effects on the kidneys such as teratogenicity, carcinogenicity, and mutagenicity (Gong et al., 2019). The presence of citrinin renders Monascus and its products potentially hazardous, which has led many countries to set limits and standards on citrinin content. For example, the citrinin limit is less than 0.04 mg/kg according to the Chinese document National Standard for Food Safety Food Additive Monascus (GB 1886.181-2016) (National Health and Family Planning Commission of the People's Republic of China, 2016), and the maximum level in food supplements based on rice fermented with Monascus purpureus is 100 µg/kg in the European Union (Commission of the European Union, 2019).
The hybrid multilevel voltage-source converter, combining semiconductor switches and power module cells, is a new family for VSC-HVDC applications. The hybrid multilevel converter in parallel ...arrangement (PHMC), in terms of converter investment, is very adaptable for cable transmission systems. However, the dc voltage of the converter, synthesized by the output voltage of three-phase cell stacks, is strongly coupled with ac voltage variations, which influences the stability of converter operation. This paper proposes a dc voltage compensation strategy for the parallel hybrid multilevel converter. This strategy, implemented by introducing an isolation period, can effectively cope with the dc voltage deviation, especially under ac network faults. Simulation results in PSCAD/EMTDC verify the effectiveness of the proposed strategy under various circumstances.
The use of overhead lines for power transmission in the future high-voltage and large-capacity voltage-source converter (VSC)-based direct current (DC) grid will significantly increase the ...probability of temporary faults. To eliminate potential adverse impacts such as erroneous protection, line-insulation failure, and even damage to power electronic devices resulting from a DC breaker reclosing operation with the traditional sequential auto-reclosing strategy, a novel sequential auto-reclosing strategy for hybrid HVDC breakers (HHBs) in VSC-based DC grids is proposed. This strategy is based on the step-by-step operation of the transfer branch in the HHB. As a result, d
u
/d
t
resulting from the HHB reclosing operation is greatly reduced, and therefore those potential negative impacts can be eliminated. Several other advantages are also presented. The feasibility and validity of the proposed strategy are verified in a four-terminal annular VSC-based DC grid electromagnetic transient model.
Monascus
produces a kind of mycotoxin, citrinin, whose synthetic pathway is still not entirely clear. The function of
CtnD
, a putative oxidoreductase located upstream of
pksCT
in the citrinin gene ...cluster, has not been reported. In this study, the
CtnD
overexpressed strain and the
Cas9
constitutively expressed chassis strain were obtained by genetic transformation mediated by
Agrobacterium tumefaciens
. The
pyrG
and
CtnD
double gene–edited strains were then obtained by transforming the protoplasts of the
Cas9
chassis strain with in vitro sgRNAs. The results showed that overexpression of
CtnD
resulted in significant increases in citrinin content of more than 31.7% and 67.7% in the mycelium and fermented broth, respectively. The edited
CtnD
caused citrinin levels to be reduced by more than 91% in the mycelium and 98% in the fermented broth, respectively. It was shown that
CtnD
is a key enzyme involved in citrinin biosynthesis. RNA-Seq and RT-qPCR showed that the overexpression of
CtnD
had no significant effect on the expression of
CtnA
,
CtnB
,
CtnE
, and
CtnF
but led to distinct changes in the expression of acyl-CoA thioesterase and two MFS transporters, which may play an unknown role in citrinin metabolism. This study is the first to report the important function of
CtnD
in
M. purpureus
through a combination of CRISPR/Cas9 editing and overexpression.
PDF
