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Modulation strategy plays a key role in the operation of modular multilevel converters (MMC). Although many studies have been carried out on its optimization in MMC, the limitations of hardware implementation are seldom considered. This paper presents a novel and easy‐to‐implement modulation strategy that simplifies both hardware design and application. From the analysis in this paper, the widely‐used modulation methods have the problems of pulse width modulation (PWM) signal feedback and complex carrier configuration in hardware implementation. By setting up a flexible insertion and removal submodule and proposing a capacitor voltage balance algorithm based on duty ratio allocation, the proposed modulation method can allow for the separate design of the Algorithm Unit and the PWM Unit in the hardware, which solves the problem of PWM feedback and reduces the hardware requirement of the trigger system. Moreover, the proposed method can broaden the selection range of processor chips. Finally, a downscaled MMC prototype is built; the proposed method is verified by both simulation and experiment. [ABSTRACT FROM AUTHOR]

A large scale of distributed photovoltaics is accessed through a low voltage distribution network in a single‐phase or two‐phase, which causes three‐phase unbalance and over‐voltage problems. These problems can be solved by using a voltage source converter to realize AC/DC interconnection and flexible power transfer between lines. How to utilize the characteristics of voltage source converters to optimize the power qualities, and fully promote distributed PV systems consumption in low voltage distribution network is an important research point. This paper proposes a power regulation model of VSC. The goal of this model is to adjust and optimize over‐voltage and three‐phase unbalance problems. By constructing a voltage‐power sensitivity calculation model for a three‐phase four‐wire system and integrating different control modes and adjustment capabilities of voltage source converters, a control strategy based on voltage source converters is proposed to address three‐phase unbalance and over‐voltage issues. Finally, the simulation results demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Finite control set model predictive control (FCS‐MPC) has been widely used in the control of three‐phase voltage source converter (VSC), but its control performance declines sharply at low sampling frequencies. This paper proposes an optimal control set expansion method with slip window selection and online parameter modification to improve the performance and robustness of FCS‐MPC. First, the performance of different control set expansion methods is compared. Then, an optimal control set expansion method that carries 12 virtual vectors is proposed. Last, a slip window selection strategy and an online parameter modification algorithm are proposed to reduce the computational burden and improve the controller's robustness respectively. Experimental result show that the proposed method significantly improves the control accuracy and carries good robustness and anti‐disturbance ability. Compared with existing typical MPC strategies, the proposed method obtains better current quality with lower switching frequency and requires less calculation. Moreover, the design of the proposed method is simple and there is no weighting factor to be tuned, which increases the method's practicality. [ABSTRACT FROM AUTHOR]

The increasing penetration of the distributed energy resources (DER) in the power grid, which, while having significant advantages, also pose significant challenges. The behaviors of DERs differ from those of synchronous generators, particularly in abnormal conditions. For this reason, the power grid enforces grid codes to ensure that DERs perform properly in different conditions. For instance, short circuit faults and unbalanced grid voltage are severe transient events that inverters need to be able to pass through without disconnecting from the grid. Furthermore, the inverters are required to support the grid voltage by regulating the active and reactive power injections. This article proposes a voltage support control scheme to support grid voltage during asymmetrical voltage drop by utilizing an optimization problem. In this optimization problem, the active and reactive powers injected into the grid will be obtained optimally by considering constraints such as instantaneous active and reactive power oscillation magnitudes and peak current limitation. To aid in this purpose, the corresponding mathematical formulations such as instantaneous active and reactive power oscillation magnitudes will be obtained by using the currents and voltages in stationary reference frame. The proposed scheme will be verified by simulating it in MATLAB/Simulink under three different scenarios and tested on a real‐time experimental Opal‐RT platform. [ABSTRACT FROM AUTHOR]

Abstract Modulation strategy plays a key role in the operation of modular multilevel converters (MMC). Although many studies have been carried out on its optimization in MMC, the limitations of hardware implementation are seldom considered. This paper presents a novel and easy‐to‐implement modulation strategy that simplifies both hardware design and application. From the analysis in this paper, the widely‐used modulation methods have the problems of pulse width modulation (PWM) signal feedback and complex carrier configuration in hardware implementation. By setting up a flexible insertion and removal submodule and proposing a capacitor voltage balance algorithm based on duty ratio allocation, the proposed modulation method can allow for the separate design of the Algorithm Unit and the PWM Unit in the hardware, which solves the problem of PWM feedback and reduces the hardware requirement of the trigger system. Moreover, the proposed method can broaden the selection range of processor chips. Finally, a downscaled MMC prototype is built; the proposed method is verified by both simulation and experiment.

Abstract A large scale of distributed photovoltaics is accessed through a low voltage distribution network in a single‐phase or two‐phase, which causes three‐phase unbalance and over‐voltage problems. These problems can be solved by using a voltage source converter to realize AC/DC interconnection and flexible power transfer between lines. How to utilize the characteristics of voltage source converters to optimize the power qualities, and fully promote distributed PV systems consumption in low voltage distribution network is an important research point. This paper proposes a power regulation model of VSC. The goal of this model is to adjust and optimize over‐voltage and three‐phase unbalance problems. By constructing a voltage‐power sensitivity calculation model for a three‐phase four‐wire system and integrating different control modes and adjustment capabilities of voltage source converters, a control strategy based on voltage source converters is proposed to address three‐phase unbalance and over‐voltage issues. Finally, the simulation results demonstrate the effectiveness of the proposed strategy.

Abstract Finite control set model predictive control (FCS‐MPC) has been widely used in the control of three‐phase voltage source converter (VSC), but its control performance declines sharply at low sampling frequencies. This paper proposes an optimal control set expansion method with slip window selection and online parameter modification to improve the performance and robustness of FCS‐MPC. First, the performance of different control set expansion methods is compared. Then, an optimal control set expansion method that carries 12 virtual vectors is proposed. Last, a slip window selection strategy and an online parameter modification algorithm are proposed to reduce the computational burden and improve the controller's robustness respectively. Experimental result show that the proposed method significantly improves the control accuracy and carries good robustness and anti‐disturbance ability. Compared with existing typical MPC strategies, the proposed method obtains better current quality with lower switching frequency and requires less calculation. Moreover, the design of the proposed method is simple and there is no weighting factor to be tuned, which increases the method's practicality.

Abstract The increasing penetration of the distributed energy resources (DER) in the power grid, which, while having significant advantages, also pose significant challenges. The behaviors of DERs differ from those of synchronous generators, particularly in abnormal conditions. For this reason, the power grid enforces grid codes to ensure that DERs perform properly in different conditions. For instance, short circuit faults and unbalanced grid voltage are severe transient events that inverters need to be able to pass through without disconnecting from the grid. Furthermore, the inverters are required to support the grid voltage by regulating the active and reactive power injections. This article proposes a voltage support control scheme to support grid voltage during asymmetrical voltage drop by utilizing an optimization problem. In this optimization problem, the active and reactive powers injected into the grid will be obtained optimally by considering constraints such as instantaneous active and reactive power oscillation magnitudes and peak current limitation. To aid in this purpose, the corresponding mathematical formulations such as instantaneous active and reactive power oscillation magnitudes will be obtained by using the currents and voltages in stationary reference frame. The proposed scheme will be verified by simulating it in MATLAB/Simulink under three different scenarios and tested on a real‐time experimental Opal‐RT platform.

Abstract The weak AC grid in a modular multilevel converter modular multilevel converter based high voltage direct current (MMC‐HVDC) receiving‐end system requires faster reactive power support from the receiving‐end MMC. However, under the MMC traditional constant AC voltage dual‐loop control strategy, the increase in MMC's reactive power support speed will reduce the system stability. The reactive power cascade control strategy is a novel MMC control strategy. The analysis in this paper demonstrates that it possesses advanced performance under faster reactive power support, which is required by the MMC connected to a weak AC grid, but, some of its control links and operations can cause impacts exceeding that a weak AC grid can undertake. To solve this problem, based on the original control strategy, this paper proposes an improved reactive power cascade control strategy for weak AC grid conditions. The complete framework and the detailed design of primary control links of the improved cascade control strategy are displayed in this paper. Finally, a digital simulation has been conducted to validate the analysis and the proposed improved control strategy in this paper.

Abstract Constrained by the AC voltage amplitude modulated by a modular multilevel converter‐based static synchronous compensator (MMC‐STATCOM), its reactive power output may be subject to oscillations under grid contingencies, posing a threat to the grid stable operation. To solve this problem, this paper proposes a variable DC voltage (VDCV)‐based reactive power enhancement scheme for MMC‐STATCOM. In this scheme, a novel variable DC voltage control is designed, which can increase the DC voltage in a transient state for relaxing the constraint of the AC voltage amplitude modulated by MMC‐STATCOM and improving its reactive power output capability (RPC). At the same time, to make full use of the improved RPC of MMC‐STATCOM, a VDCV scheme also proposes an optimisation algorithm of its reactive current‐AC voltage droop coefficient using the established reactive power model of the MMC‐STATCOM. Based on small signal modelling and analysis, the key parameters of the proposed VDCV scheme are optimised. The performance and reactive power enhancement of the VDCV scheme is evaluated through the hardware‐in‐the‐loop experiment under grid disturbances.

Abstract This paper proposes an extended state observer (ESO)‐based second‐order sliding mode control (SMC) for dynamic voltage restorers (DVRs). Unlike the conventional first‐order SMC and some second‐order SMC (SOSMC) methods that suffer from chattering, the proposed control method can alleviate chattering and achieve finite‐time convergence. Chattering suppression (i.e. eliminating discontinuities) is achieved via continuous control input which is also used to generate the pulse width modulation signals. However, while removing the discontinuities in the control input, the performance of the control method is degraded when it is subjected to disturbances. Therefore, an active disturbance rejection (ADR) based on an ESO is proposed to enhance the performance. In addition, an advanced single‐phase phase‐locked loop (PLL) using linear observer and quasi‐type‐1 PLL is also proposed. The effectiveness of the proposed method is verified through simulation and experimental results which are compared with the results of the existing SMC methods applied to DVR.

The weak AC grid in a modular multilevel converter modular multilevel converter based high voltage direct current (MMC‐HVDC) receiving‐end system requires faster reactive power support from the receiving‐end MMC. However, under the MMC traditional constant AC voltage dual‐loop control strategy, the increase in MMC's reactive power support speed will reduce the system stability. The reactive power cascade control strategy is a novel MMC control strategy. The analysis in this paper demonstrates that it possesses advanced performance under faster reactive power support, which is required by the MMC connected to a weak AC grid, but, some of its control links and operations can cause impacts exceeding that a weak AC grid can undertake. To solve this problem, based on the original control strategy, this paper proposes an improved reactive power cascade control strategy for weak AC grid conditions. The complete framework and the detailed design of primary control links of the improved cascade control strategy are displayed in this paper. Finally, a digital simulation has been conducted to validate the analysis and the proposed improved control strategy in this paper. [ABSTRACT FROM AUTHOR]

This paper proposes an extended state observer (ESO)‐based second‐order sliding mode control (SMC) for dynamic voltage restorers (DVRs). Unlike the conventional first‐order SMC and some second‐order SMC (SOSMC) methods that suffer from chattering, the proposed control method can alleviate chattering and achieve finite‐time convergence. Chattering suppression (i.e. eliminating discontinuities) is achieved via continuous control input which is also used to generate the pulse width modulation signals. However, while removing the discontinuities in the control input, the performance of the control method is degraded when it is subjected to disturbances. Therefore, an active disturbance rejection (ADR) based on an ESO is proposed to enhance the performance. In addition, an advanced single‐phase phase‐locked loop (PLL) using linear observer and quasi‐type‐1 PLL is also proposed. The effectiveness of the proposed method is verified through simulation and experimental results which are compared with the results of the existing SMC methods applied to DVR. [ABSTRACT FROM AUTHOR]

Abstract Grid‐connected inverters with an inductor–capacitor–inductor (LCL) filter usually require the implementation of damping in the filter to suppress the resonance associated with the filter and hence ensure internal stability. The value for the damping resistor is commonly chosen as one‐third of the capacitive reactance at the resonance frequency of the LCL filter. However, this commonly‐used principle does not take into account the external stability of the inverter, i.e. the potential for an interaction between the inverter output impedance and the grid impedance. This work discusses a new design for the damping resistor to assure the external stability of the voltage source inverter as well as its internal stability. The analysis shows that the proposed design can ensure the external stability of the voltage source inverter with different grid impedances as well as robustness against the variations of the filter parameters while maintaining low power losses and an acceptable attenuation rate for high‐frequency harmonics. To validate the theoretical analysis, a 1 kW, 110 V laboratory prototype is implemented. The obtained results confirm the applicability of the proposed design approach.

Abstract This paper demonstrates that the switching of one grid‐connected virtual synchronous generator (VSG) between two control modes, namely constant voltage control (CVC) and current limiting control (CLC) happens at the derived two switching lines. Based on the current limiting inequality, the traditional current‐switched model is transferred to the angle‐switched model proposed here, and the system can be studied as one switched dynamic system. Based on this model, the transient stability of VSG with controller limits is investigated. This is achieved by constructing Lyapunov functions for each subsystem and deriving the relationship between the values of Lyapunov functions constructed under different conditions at the switching moment. The stability of the system is ensured when the Lyapunov function of each subsystem presents a decreasing trend in two consecutive switching intervals. On this basis, the stability boundary of the switching system is derived. Further analysis shows that optimal adjustment of the saturation current angle can make the system reach the maximum stability boundary. Finally, the numerical simulations and experimental tests verify the correctness of the proposed analysis.

Grid‐connected inverters with an inductor–capacitor–inductor (LCL) filter usually require the implementation of damping in the filter to suppress the resonance associated with the filter and hence ensure internal stability. The value for the damping resistor is commonly chosen as one‐third of the capacitive reactance at the resonance frequency of the LCL filter. However, this commonly‐used principle does not take into account the external stability of the inverter, i.e. the potential for an interaction between the inverter output impedance and the grid impedance. This work discusses a new design for the damping resistor to assure the external stability of the voltage source inverter as well as its internal stability. The analysis shows that the proposed design can ensure the external stability of the voltage source inverter with different grid impedances as well as robustness against the variations of the filter parameters while maintaining low power losses and an acceptable attenuation rate for high‐frequency harmonics. To validate the theoretical analysis, a 1 kW, 110 V laboratory prototype is implemented. The obtained results confirm the applicability of the proposed design approach. The value for the damping resistor of the inductor–capacitor–inductor (LCL) filter resonance is commonly chosen as one‐third of the capacitive reactance at the resonance frequency of the LCL filter. However, this commonly‐used principle does not take into account the external stability of the inverter, i.e. the potential for an interaction between the inverter output impedance and the grid impedance. This work discusses a new design for the damping resistor to assure the external stability of the voltage source inverter as well as its internal stability. [ABSTRACT FROM AUTHOR]

This paper demonstrates that the switching of one grid‐connected virtual synchronous generator (VSG) between two control modes, namely constant voltage control (CVC) and current limiting control (CLC) happens at the derived two switching lines. Based on the current limiting inequality, the traditional current‐switched model is transferred to the angle‐switched model proposed here, and the system can be studied as one switched dynamic system. Based on this model, the transient stability of VSG with controller limits is investigated. This is achieved by constructing Lyapunov functions for each subsystem and deriving the relationship between the values of Lyapunov functions constructed under different conditions at the switching moment. The stability of the system is ensured when the Lyapunov function of each subsystem presents a decreasing trend in two consecutive switching intervals. On this basis, the stability boundary of the switching system is derived. Further analysis shows that optimal adjustment of the saturation current angle can make the system reach the maximum stability boundary. Finally, the numerical simulations and experimental tests verify the correctness of the proposed analysis. [ABSTRACT FROM AUTHOR]

Abstract Sub‐synchronous oscillation of grid‐connected voltage source converter (VSC) dominated by phase locked loop (PLL) is an important stability issue in nowadays power systems. The complex torque coefficient method has been used to investigate the issue because of its advantage of giving insights into source of damping and contributions of different components on damping. However, it is found that the classic complex torque coefficient method is not entirely applicable for stability analysis of VSC because of the proportional term in PLL. This paper proposes a corrected complex torque coefficient method and defines synchronous and damping coefficients. Based on the proposed method, a small‐signal stability criterion is obtained, in which the system stability depends on the sign of corrected damping coefficient. The impacts of different elements on system damping can be quantitatively evaluated through the method. It is drawn that current control loop and line dynamics contribute negative damping, whereas power control loop contributes positive damping. Moreover, the impacts of operating conditions and controller parameters on damping of different elements are analysed. The eigenvalue analyses and electromagnetic transient simulations have verified the theoretical analysis.

Abstract To solve the commutation failure (CF) issues in high voltage direct current (HVDC) transmission systems based on line commutated converter (LCC), this paper proposes a hybrid solution combining the LCC inverter with the continuously controlled IGCT‐based full‐bridge submodules (FBSMs). The scheme can provide the LCC inverter with an additional commutation area, and compensate for the phase‐shift of the grid voltage under faults. Therefore, it shows better CF resistance than the existing scheme. The characteristics of the proposed hybrid converter are modelled and analyzed in both normal and fault conditions. Then a decoupling control strategy is presented for it. With the ability to counteract negative‐sequence grid voltage, the strategy brings good voltage stability to the dc transmission. And when the grid fault occurs, it can switch to suppress CF by rapidly increasing the output voltage of the FBSMs and applying commutation failure prevention (CFPREV) control to the LCC. As a result, the CF issues can be maximally solved with a smaller number of FBSMs than the existing scheme. Simulations in PSCAD verify the effectiveness of the strategy. Furtherly, an experimental platform is built to show the capacity of the proposed scheme to solve the CF issues.

Abstract In voltage‐controlled voltage source inverters (VSIs)‐based microgrids (MGs), the inner control is of prime interest task for guaranteeing safe and stable operation. In this paper, an in‐depth investigation of the modelling, control design, and analysis of the voltage and current inner control loops intended for single‐phase voltage‐controlled VSIs is established. The main objective of this work is to provide a comprehensive study of the mathematical modelling, control design, and performance evaluation of the inner control's loops considering different proportional‐integral (PI) controller types with and without compensation, and to determine the optimal scheme that can offer better performance in terms of implementation simplicity, robustness, and transient and steady‐state responses. Thus, the mathematical closed‐loop models of designed outer voltage and inner current control schemes based on PI, P, and feedforward controllers with and without compensation are, first, derived. Following this, a systematic and effective control design for tuning the different PI controllers’ parameters is proposed. Furthermore, an analysis revealing the performance of the designed voltage and current control schemes is provided. This analysis enables us to choose a P controller and PI feedforward controller for the current control loop and the voltage control loop, respectively. The chosen P and PI controllers should be simple; meanwhile, they should offer a wide bandwidth. A simulation study is carried out in MATLAB/Simulink software to assess the performance of the adopted inner control scheme for both linear and non‐linear loads. In addition, an experimental setup, based on a TMS320F2837xD μC, of a single‐phase VSI supplying linear and non‐linear loads is built to verify the effectiveness and the robustness of the adopted inner controller. The results demonstrated: (1) the necessity of introducing the compensation term, which is responsible for offering control improvement against voltage perturbation, (2) the high tracking performance of the chosen controller in terms of dynamic and steady‐state responses as well as its simplicity of implementation.

Sub‐synchronous oscillation of grid‐connected voltage source converter (VSC) dominated by phase locked loop (PLL) is an important stability issue in nowadays power systems. The complex torque coefficient method has been used to investigate the issue because of its advantage of giving insights into source of damping and contributions of different components on damping. However, it is found that the classic complex torque coefficient method is not entirely applicable for stability analysis of VSC because of the proportional term in PLL. This paper proposes a corrected complex torque coefficient method and defines synchronous and damping coefficients. Based on the proposed method, a small‐signal stability criterion is obtained, in which the system stability depends on the sign of corrected damping coefficient. The impacts of different elements on system damping can be quantitatively evaluated through the method. It is drawn that current control loop and line dynamics contribute negative damping, whereas power control loop contributes positive damping. Moreover, the impacts of operating conditions and controller parameters on damping of different elements are analysed. The eigenvalue analyses and electromagnetic transient simulations have verified the theoretical analysis. [ABSTRACT FROM AUTHOR]

In voltage‐controlled voltage source inverters (VSIs)‐based microgrids (MGs), the inner control is of prime interest task for guaranteeing safe and stable operation. In this paper, an in‐depth investigation of the modelling, control design, and analysis of the voltage and current inner control loops intended for single‐phase voltage‐controlled VSIs is established. The main objective of this work is to provide a comprehensive study of the mathematical modelling, control design, and performance evaluation of the inner control's loops considering different proportional‐integral (PI) controller types with and without compensation, and to determine the optimal scheme that can offer better performance in terms of implementation simplicity, robustness, and transient and steady‐state responses. Thus, the mathematical closed‐loop models of designed outer voltage and inner current control schemes based on PI, P, and feedforward controllers with and without compensation are, first, derived. Following this, a systematic and effective control design for tuning the different PI controllers' parameters is proposed. Furthermore, an analysis revealing the performance of the designed voltage and current control schemes is provided. This analysis enables us to choose a P controller and PI feedforward controller for the current control loop and the voltage control loop, respectively. The chosen P and PI controllers should be simple; meanwhile, they should offer a wide bandwidth. A simulation study is carried out in MATLAB/Simulink software to assess the performance of the adopted inner control scheme for both linear and non‐linear loads. In addition, an experimental setup, based on a TMS320F2837xD μC, of a single‐phase VSI supplying linear and non‐linear loads is built to verify the effectiveness and the robustness of the adopted inner controller. The results demonstrated: (1) the necessity of introducing the compensation term, which is responsible for offering control improvement against voltage perturbation, (2) the high tracking performance of the chosen controller in terms of dynamic and steady‐state responses as well as its simplicity of implementation. [ABSTRACT FROM AUTHOR]

To solve the commutation failure (CF) issues in high voltage direct current (HVDC) transmission systems based on line commutated converter (LCC), this paper proposes a hybrid solution combining the LCC inverter with the continuously controlled IGCT‐based full‐bridge submodules (FBSMs). The scheme can provide the LCC inverter with an additional commutation area, and compensate for the phase‐shift of the grid voltage under faults. Therefore, it shows better CF resistance than the existing scheme. The characteristics of the proposed hybrid converter are modelled and analyzed in both normal and fault conditions. Then a decoupling control strategy is presented for it. With the ability to counteract negative‐sequence grid voltage, the strategy brings good voltage stability to the dc transmission. And when the grid fault occurs, it can switch to suppress CF by rapidly increasing the output voltage of the FBSMs and applying commutation failure prevention (CFPREV) control to the LCC. As a result, the CF issues can be maximally solved with a smaller number of FBSMs than the existing scheme. Simulations in PSCAD verify the effectiveness of the strategy. Furtherly, an experimental platform is built to show the capacity of the proposed scheme to solve the CF issues. [ABSTRACT FROM AUTHOR]

Abstract It is very important to obtain high reliability in the high‐voltage direct current (HVDC) system operation because of extremely high and huge power capacity. To guarantee the reliability IEC‐62501 standard requires valve test for the voltage source converter (VSC) HVDC system. This paper proposes a test circuit for modular multilevel converter (MMC)‐based HVDC valve. The proposed scheme has good features of adequate test current generation, considerably reduced power supply voltage, simplified test circuit topology, etc., compared to the conventional test circuits. The validity of the proposed scheme is verified through experimental results.

Abstract This paper presents a comprehensive assessment for the small‐signal voltage stability of voltage source converter (VSC)‐HVDC systems supplying industrial loads represented by the induction motors (IMs) and provides a stability enhancement control for the VSCs. First, the steady‐state voltage stability is analysed to seek the factors that affect the existence of the stable equilibrium point. Then, the small‐signal voltage stability analysis is performed based on the developed linear model. Eigenvalue analysis is carried out to capture the critical factors that induce instability phenomena correlative to the system voltage. It demonstrates that decreasing either the proportional gain K of the VSC's PI controller or the electrical distance between the VSC and IM is detrimental to voltage stability. Next, the enhancement control strategy is elaborated. It adopts virtual reactance and virtual admittance control for the VSC to improve the voltage control capability. Finally, the effectiveness and robustness of the proposed enhancement control are validated via time‐domain simulations in a multi‐IM system supplied by VSC‐HVDC.

Abstract Grid‐forming (GFM) converter is believed to be highly promising in the future power systems, due to its ability of providing voltage and frequency support. However, some recent studies have shown that the GFM converter may suffer from instability in stiff grids, which seriously hampers its application. In this paper, the mechanism of this anomalous effect is studied by using the small‐signal stability analysis in detail. First, a detailed state‐space model of a single converter grid‐tied system is established from the first principle, and by using the participation factor analysis, the interaction between the terminal voltage loop and the power synchronization loop is identified as the major cause for the system instability. Then relying on a reduced‐order model containing only these two controls and using two classical analytical methods including the Routh criterion analysis and the Phillips– Heffron model of complex torque analysis, the origin of this anomalous instability of GFM converters tied to stiff grid coming from the negative damping provided by the terminal voltage loop is well uncovered and the critical grid strength is well predicted. In addition, these results may provide ideas for subsequent control optimization and stability improvement of GFM converters under various situations.

Abstract For high‐power converters, the low switching frequency (SF) modulation represented by selective harmonic elimination pulse width modulation (SHE‐PWM) is crucial to reduce the switching loss and enhance the efficiency with a good steady‐state performance. However, since the advantages of SHE‐PWM mainly come from the fixed switching angle distribution limited by non‐linear equations, switching angles should be adjusted slowly. This requires that the bandwidth of proportional integral (PI) controllers be limited hugely, resulting in a poor dynamic response. As the most popular control to improve the dynamic response, finite‐control‐set model predictive control (FCS‐MPC) has a worse steady‐state performance under low SF because of large errors caused by a long prediction period and harmonics caused by space vector PWM (SVPWM). Therefore, a hybrid modulation strategy with an acceptable complexity is proposed in this article, which combines SHE‐PWM and FCS‐MPC. PI + SHE‐PWM is adopted in a steady state with low SF, while MPC + SVPWM is used in a transient state with high SF. By studying smooth switching from the modulation level and control level, the proposed method can eliminate the current surge when switching between PI + SHE‐PWM and MPC + SVPWM. Experiments verify that the proposed method have a good steady‐state performance and high dynamic response simultaneously under overall low SF.

Abstract As the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation

It is very important to obtain high reliability in the high‐voltage direct current (HVDC) system operation because of extremely high and huge power capacity. To guarantee the reliability IEC‐62501 standard requires valve test for the voltage source converter (VSC) HVDC system. This paper proposes a test circuit for modular multilevel converter (MMC)‐based HVDC valve. The proposed scheme has good features of adequate test current generation, considerably reduced power supply voltage, simplified test circuit topology, etc., compared to the conventional test circuits. The validity of the proposed scheme is verified through experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive assessment for the small‐signal voltage stability of voltage source converter (VSC)‐HVDC systems supplying industrial loads represented by the induction motors (IMs) and provides a stability enhancement control for the VSCs. First, the steady‐state voltage stability is analysed to seek the factors that affect the existence of the stable equilibrium point. Then, the small‐signal voltage stability analysis is performed based on the developed linear model. Eigenvalue analysis is carried out to capture the critical factors that induce instability phenomena correlative to the system voltage. It demonstrates that decreasing either the proportional gain K of the VSC's PI controller or the electrical distance between the VSC and IM is detrimental to voltage stability. Next, the enhancement control strategy is elaborated. It adopts virtual reactance and virtual admittance control for the VSC to improve the voltage control capability. Finally, the effectiveness and robustness of the proposed enhancement control are validated via time‐domain simulations in a multi‐IM system supplied by VSC‐HVDC. [ABSTRACT FROM AUTHOR]

Abstract Considering that the AC–DC hybrid microgrid (HMG) is connected to the low‐voltage distribution network (DN) where voltage faults occur most commonly, the fault ride‐through (FRT) capability and power quality performance of the HMG under abnormal DN conditions have become a critical issue. In this paper, a decentralized coordination control scheme based on an improved topology is proposed for the grid‐interactive HMG to guarantee uninterrupted and high power‐quality operation during grid faults. The method of it includes: (1) a new HMG topology is presented by adding a series transformer (ST), which is inserted between the DN and HMG; (2) an improved coordination control strategy that considers the interaction between converters of the enhanced HMG. Based on the proposed method, the operation modes of the system are introduced, and the specific design process and small‐signal model analysis of the proposed control scheme are presented. Finally, the simulated and experimental results show that the proposed method can keep both AC and DC bus voltage stable and improve the fault ride‐through capability and power quality of the HMG under abnormal DN conditions.

Abstract A transformer‐less unified power flow controller (UPFC) is a power electronic device consisting of series and shunt voltage source converters (VSCs) that are not connected to a common DC bus. It can control power flow in medium voltage (MV) distribution networks without the need for interfacing transformers. Hence there is a significant reduction in size and cost compared to a conventional UPFC. This paper investigates the operating range of a transformer‐less UPFC in an MV distribution network and the required power ratings of the series and shunt converters. The results showed that a transformer‐less UPFC is able to provide active power control using partially rated converters. However, its ability to control reactive power is limited by the current ratings of the converters. The analysis was verified using software simulation and hardware experiment.

Grid‐forming (GFM) converter is believed to be highly promising in the future power systems, due to its ability of providing voltage and frequency support. However, some recent studies have shown that the GFM converter may suffer from instability in stiff grids, which seriously hampers its application. In this paper, the mechanism of this anomalous effect is studied by using the small‐signal stability analysis in detail. First, a detailed state‐space model of a single converter grid‐tied system is established from the first principle, and by using the participation factor analysis, the interaction between the terminal voltage loop and the power synchronization loop is identified as the major cause for the system instability. Then relying on a reduced‐order model containing only these two controls and using two classical analytical methods including the Routh criterion analysis and the Phillips– Heffron model of complex torque analysis, the origin of this anomalous instability of GFM converters tied to stiff grid coming from the negative damping provided by the terminal voltage loop is well uncovered and the critical grid strength is well predicted. In addition, these results may provide ideas for subsequent control optimization and stability improvement of GFM converters under various situations. [ABSTRACT FROM AUTHOR]

For high‐power converters, the low switching frequency (SF) modulation represented by selective harmonic elimination pulse width modulation (SHE‐PWM) is crucial to reduce the switching loss and enhance the efficiency with a good steady‐state performance. However, since the advantages of SHE‐PWM mainly come from the fixed switching angle distribution limited by non‐linear equations, switching angles should be adjusted slowly. This requires that the bandwidth of proportional integral (PI) controllers be limited hugely, resulting in a poor dynamic response. As the most popular control to improve the dynamic response, finite‐control‐set model predictive control (FCS‐MPC) has a worse steady‐state performance under low SF because of large errors caused by a long prediction period and harmonics caused by space vector PWM (SVPWM). Therefore, a hybrid modulation strategy with an acceptable complexity is proposed in this article, which combines SHE‐PWM and FCS‐MPC. PI + SHE‐PWM is adopted in a steady state with low SF, while MPC + SVPWM is used in a transient state with high SF. By studying smooth switching from the modulation level and control level, the proposed method can eliminate the current surge when switching between PI + SHE‐PWM and MPC + SVPWM. Experiments verify that the proposed method have a good steady‐state performance and high dynamic response simultaneously under overall low SF. [ABSTRACT FROM AUTHOR]

This letter presents a hardware demonstrator of an all‐SiC three‐level T‐type (3LTT) inverter with the common‐mode (CM) EMI filter stages placed on the DC input instead of the AC output side, targeting, for example, high‐efficiency PV applications. The extensive experimental characterization shows that state‐of‐the‐art SiC transistors and a DC‐side CM filter enable an unprecedented peak/full‐load efficiency of 99.4% (calorimetric measurement) at 12.5 kW and a power density of 2.4 kW/dm3 (39 W/in3). The demonstrator fulfills CISPR 11 Class A EMI regulations as well as upcoming EMI standards for the frequency range of 9–150 kHz. Compared to other high‐efficiency converters, which often employ bridge legs with more output voltage levels, the described 3LTT concept thus offers a very favorable trade‐off between complexity and performance. [ABSTRACT FROM AUTHOR]

As the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation [ABSTRACT FROM AUTHOR]

Abstract Selective harmonic elimination (SHE) is a suitable pulse‐width modulation (PWM) technique for some applications of modular multilevel converters (MMC) with lower number of semiconductor submodules (SM). However, SHE can make it difficult to control circulating currents in the MMC. Moreover, the MMC is vulnerable to SM switch failures that reduce its output power magnitude and quality. This paper proposes a fault‐tolerant model predictive control strategy for MMCs operating with SHE (FT SHE‐MPC). The effects of switch failure on phase voltage waveforms are studied first. Then, a method is proposed to design the FT SHE waveform to preserve its harmonic performance. Next, the proposed FT SHE‐MPC is designed to increase the capacitor voltages during an SM failure to maintain the fundamental component of the faulty phase voltage and control the circulating currents, maintaining balance among line voltages and the output power of the MMC. Additionally, the proposed method offers as much as 60% faster FT transition time and as much as 36% lower capacitor voltage ripple. The advantages of the proposed method are verified through offline and real‐time simulation studies on a three‐phase nine‐level MMC in PSCAD/EMTDC software and RTDS.

Considering that the AC–DC hybrid microgrid (HMG) is connected to the low‐voltage distribution network (DN) where voltage faults occur most commonly, the fault ride‐through (FRT) capability and power quality performance of the HMG under abnormal DN conditions have become a critical issue. In this paper, a decentralized coordination control scheme based on an improved topology is proposed for the grid‐interactive HMG to guarantee uninterrupted and high power‐quality operation during grid faults. The method of it includes: (1) a new HMG topology is presented by adding a series transformer (ST), which is inserted between the DN and HMG; (2) an improved coordination control strategy that considers the interaction between converters of the enhanced HMG. Based on the proposed method, the operation modes of the system are introduced, and the specific design process and small‐signal model analysis of the proposed control scheme are presented. Finally, the simulated and experimental results show that the proposed method can keep both AC and DC bus voltage stable and improve the fault ride‐through capability and power quality of the HMG under abnormal DN conditions. [ABSTRACT FROM AUTHOR]

A transformer‐less unified power flow controller (UPFC) is a power electronic device consisting of series and shunt voltage source converters (VSCs) that are not connected to a common DC bus. It can control power flow in medium voltage (MV) distribution networks without the need for interfacing transformers. Hence there is a significant reduction in size and cost compared to a conventional UPFC. This paper investigates the operating range of a transformer‐less UPFC in an MV distribution network and the required power ratings of the series and shunt converters. The results showed that a transformer‐less UPFC is able to provide active power control using partially rated converters. However, its ability to control reactive power is limited by the current ratings of the converters. The analysis was verified using software simulation and hardware experiment. [ABSTRACT FROM AUTHOR]

Selective harmonic elimination (SHE) is a suitable pulse‐width modulation (PWM) technique for some applications of modular multilevel converters (MMC) with lower number of semiconductor submodules (SM). However, SHE can make it difficult to control circulating currents in the MMC. Moreover, the MMC is vulnerable to SM switch failures that reduce its output power magnitude and quality. This paper proposes a fault‐tolerant model predictive control strategy for MMCs operating with SHE (FT SHE‐MPC). The effects of switch failure on phase voltage waveforms are studied first. Then, a method is proposed to design the FT SHE waveform to preserve its harmonic performance. Next, the proposed FT SHE‐MPC is designed to increase the capacitor voltages during an SM failure to maintain the fundamental component of the faulty phase voltage and control the circulating currents, maintaining balance among line voltages and the output power of the MMC. Additionally, the proposed method offers as much as 60% faster FT transition time and as much as 36% lower capacitor voltage ripple. The advantages of the proposed method are verified through offline and real‐time simulation studies on a three‐phase nine‐level MMC in PSCAD/EMTDC software and RTDS. [ABSTRACT FROM AUTHOR]

Abstract Inverter's performance and operating mode may be negatively affected by inverter input (dc‐link) current and voltage ripple. It is a common experience that even theoretically balanced loads with perfectly balanced supply voltages, such as multiphase ac motors supplied by pulse‐width modulation (PWM) inverters, in practice show a certain degree of current unbalance, in the range of a few percent, which introduces a low‐frequency instantaneous power oscillation. This reflects in current and voltage low‐frequency ripple on the dc‐link inverter side (i.e. at the double‐fundamental frequency). A possible method to analyse this matter is through the symmetric sequence components. In particular, based on the first negative current sequence component and by considering the equivalent dc‐link impedance calculated at the dominant double‐fundamental frequency, the amplitude of the corresponding dc‐link voltage ripple component is calculated in this work for a general multiphase load. Finally, the design of the dc‐link capacitor in multiphase inverters is proposed considering requirements referred to the double‐fundamental dc voltage ripple. The feasibility of the proposed developments has been verified for three‐, five‐ and seven‐phase inverters by both numerical simulations and comprehensive experimental tests, always showing a good matching.

Abstract In this study, a design of a medium‐voltage current source inverter (CSI) and a conventional voltage source inverter (VSI) is presented for high‐power (1 MW) photovoltaic (PV) applications. The characteristics of a new 1700 V/1600 A reverse‐blocking insulated‐gate bipolar transistor (IGBT) in the CSI are compared with the same generation of IGBT device in the VSI. The passive components design, including ac‐ and dc‐side filters, are developed based on a given design procedure. Power loss analysis is demonstrated to compare the CSI efficiency with the VSI's in the specified power range. Simulation and experimental results for the operation and control of the CSI in grid‐connected PV application in the central power range show the effectiveness of the proposed CSI and the possibility of applying it as a viable topological candidate.

DC transformer plays a significant role in DC distributed network. This study proposes a modular multilevel dynamic DC transformer (MMDT), which can realise smooth control of transformer ratio, multi‐level voltage conversion, and reliable fault isolation without high‐frequency transformers and extra loss. The inner circuit, formed by the capacitors, not only realises the energy exchange between different sides but also provides the capability of fault isolation. In the proposed MMDT, the voltage transforming is realised by switching to different numbers of control units (CUs) on each side. To realise multi‐level voltage transforming, the multi‐port MMDT is developed. Furthermore, this study offers analyses of the current and voltage characteristics in steady‐state and investigates the parameter design method in detail. Finally, the simulation and experiment tests are conducted to verify the feasibility and superiority of the proposed MMDT. [ABSTRACT FROM AUTHOR]

Providing basic human needs like water and household electricity is a challenging task at remote locations. To support both needs, this study presents the development of a multipurpose battery‐assisted solar water pumping system (SWPS). The system consists of only two power electronics converter, viz., bidirectional DC–DC converter, and three‐phase voltage source inverter (VSI). The detailed control system is presented, where the DC–DC converter controls the battery power and also ensures the maximum utilisation of stand‐alone solar photovoltaic source. In pumping operation, the VSI is operated as a variable frequency motor drive and rotates an induction motor (IM) at the rated power to deliver rated water discharge. In power supply operation, the household electrical load is connected between the mid‐point of DC‐bus and the star point of an IM winding. The VSI, along with motor inductance, realises the single‐phase inverter, which is controlled with synchronous sine pulse‐width modulation to deliver single‐phase output. The motor does not rotate during this operation and the motor winding acts as a current filter. This multipurpose SWPS assures rated water discharge during the irrigation period and availability of electrical power in the remaining period. A prototype is developed and extensive experiments are performed to investigate system performances. [ABSTRACT FROM AUTHOR]

This work presents a plug‐and‐play solution for voltage source converters controlled by proportional–integral controllers to automatically operate connected to a previously unknown grid. The methodology consists of an initial operation using hysteresis band current control in order to synthesise harmonic currents, which are used for online grid parameter estimation, allowing the proper analytical tuning of the controllers using the grid's equivalent model. The method is firstly simulated and then validated through experimental results using Texas Instruments TMS320F28377S Digital Signal Processor and BOOSTXLDRV8305‐EVM static converter. [ABSTRACT FROM AUTHOR]

Power converter faults represent about 12% of electrical system faults. More than 31% of these converter faults are caused by the transistors in the converters. These transistor failures are classified into open‐gate and short‐gate faults. A fault diagnostic and tolerant control system is proposed in this study for an open‐gate fault of a three‐phase voltage‐source inverter (VSI). The system under study is an induction motor fed by a VSI. The diagnostic technique uses a detection method of positive and negative sequence components, which requires only the measurement of motor currents. The diagnostic signals are derived from the negative sequence components of the motor currents. These signals are then compared using logic functions to trigger the tolerant control. In the case of an open‐gate fault, the tolerant system converts the inverter topology from six switches to four switches where the faulty inverter leg is disconnected. The proposed control method was evaluated under steady‐state and transient conditions through simulation and experiments. The results confirm an effective and reliable performance of the motor under an open‐gate fault. [ABSTRACT FROM AUTHOR]

Recently large‐scale integration of power‐electronic‐based devices has made system‐level oscillations occur frequently, which posed a big challenge for modern power grids. Previous studies mainly treated sustained oscillations under the framework of (linear) small‐signal stability and/or by incorporating the impact of current saturation, but seldom considered system non‐linearity like phase‐locked‐loop. In this study, the authors established a sixth‐order non‐linear model for a three‐phase voltage‐source converter tied to AC grid, and studied its dynamics completely from non‐linear system theory and signal analysis technique, with all parameters including current controllers, phase‐locked‐loop, line inductance, and grid voltage. They found that the operation point usually becomes unstable by Hopf bifurcation, and the sustained oscillations are possible only for super‐critical Hopf bifurcation. Based on these observations, sustained periodic oscillation should be treated as a limit cycle even without saturation. With saturation, some other phenomena have also been found, such as saturation‐induced instability and saturation‐restricted oscillation. In addition, they discovered that for sustained oscillations, the usual purely sinusoidal three‐phase currents exhibit the form of sin⁡[2πf1t+csin⁡(2πfdqt)] or cos⁡[2πf1t+csin⁡(2πfdqt)], where f1 (fdq) denotes basic (modal oscillation) frequency with c a constant. This work could provide an improved understanding of sustained oscillations and associated system dynamics in an overall perspective manner. [ABSTRACT FROM AUTHOR]

The full controllable electronic device based railway feeder station offers better power quality and more flexible configurations than conventional transformer based stations. This study investigates a modular multilevel converter (MMC)‐based static frequency converter station with renewable energy access. Wind power generation is coupled into the station via DC link of the back to back converter. The dynamic single‐phase traction load and intermittent renewable generation bring double frequency oscillation and large deviation problems to the DC link voltage. Special design considerations and control schemes are proposed for the MMC to stabilise DC link voltage by controlling the total number of total inserted modules. The proposed control scheme resolves the voltage oscillation issue caused by single‐phase load and reduces the DC link voltage deviation under 10 MW step change. A series of device‐based simulations validate the control scheme which realises a reliable coupling interface for connecting the renewable generation to the DC bus. [ABSTRACT FROM AUTHOR]

In a DC microgrid, the anti‐interference ability of bus voltage can easily become vulnerable due to low inertia caused by a large number of power electronic converters. This has already challenged the system stability and practicability in DC girds. In this study, a flexible virtual capacitance (FVC) control strategy with multiple constraints (MCs) is proposed to guarantee the security and stability of the DC microgrid, where the system stability, dynamic characteristics, actual operation requirements and realisability are considered as MCs. The FVC controller, which mainly modifies the reference current in voltage source converter by flexibly regulating virtual capacitance value, is given to provide inertial support and improve voltage quality. The stable operating boundary calculation based on small signal model and key control parameters design are further introduced into the control loop. This can ameliorate the dynamic response of the microgrid and obtain the better engineering practicability. Finally, experiment tests on a controller‐level hardware‐in‐the‐loop simulation platform are carried out to verify the validity of the proposed strategy. [ABSTRACT FROM AUTHOR]