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Thermal control of semiconductor devices in the submodules (SMs) is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). During normal operation, significant thermal differences can arise among SMs due to the manufacturing tolerance and aging effect of capacitors, which can affect the lifespan of semiconductor devices and even lead to equipment failure. To address the problem, this paper presents a method for achieving thermal balancing among SMs with different capacitances by adjusting their switching principle. Firstly, the relationship between the degree of capacitance degradation and the junction temperature difference among SMs is revealed. Then, by setting data windows to capture the dynamic losses of power devices, which can reflect the trend of junction temperature. The switching principle of thermal balance among SMs is studied based on the trend of junction temperature in power devices. The thermal balancing method has advantages including not requiring high-precision temperature sensors and eliminating the reliance on capacitance monitoring technology. Finally, the simulation and experimentation results demonstrate that the proposed method can effectively achieve balanced junction temperatures of power devices among different SMs.

• In this paper, the loss and thermal stress distribution among SMs of MMC have been investigated which under different capacitance. The relationship between the degree of capacitance degradation and the junction temperature difference among SMs is revealed. • The switching principle of thermal balancing control is studied by analyzing the trend of junction temperature changes in power semiconductor devices. • A thermal balancing method is proposed, offering advantages such as reducing the demand for temperature sensors and eliminating the reliance on high-precision capacitance monitoring technology. The proposed method can effectively achieve balanced junction temperatures of power semiconductor devices among different SMs under capacitor deterioration. Thermal control of semiconductor devices in the submodules (SMs) is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). During normal operation, significant thermal differences can arise among SMs due to the manufacturing tolerance and aging effect of capacitors, which can affect the lifespan of semiconductor devices and even lead to equipment failure. To address the problem, this paper presents a method for achieving thermal balancing among SMs with different capacitances by adjusting their switching principle. Firstly, the relationship between the degree of capacitance degradation and the junction temperature difference among SMs is revealed. Then, by setting data windows to capture the dynamic losses of power devices, which can reflect the trend of junction temperature. The switching principle of thermal balance among SMs is studied based on the trend of junction temperature in power devices. The thermal balancing method has advantages including not requiring high-precision temperature sensors and eliminating the reliance on capacitance monitoring technology. Finally, the simulation and experimentation results demonstrate that the proposed method can effectively achieve balanced junction temperatures of power devices among different SMs. [ABSTRACT FROM AUTHOR]

• Improvement of grid-connected wave power plants and wave energy converters. • A new MMC-based strategy for offshore HVDC grid-side converter. • Fast control of grid-side HVDC-MMC converters. Modular multilevel converter (MMC) is a type of electronic converter currently used as voltage source in real HVDC transmission systems. This paper presents, as an alternative, a control system designed to operate an MMC as a nonlinear vector current source (NLVCS). From the design point of view, it features advantages such as constant switching frequency, robustness against grid parameters changes and that the algorithm is entirely designed from space vectors. From the application point of view, the main advantages are its fast dynamic response combined with its capability to handle large amounts of electric power. Together, these two features allow the MMC to deal with highly variable renewable energies. To show the advantages of the proposed control system, it has been chosen a very demanding application: wave energy. In this type of applications, the random nature of the sea produces large and fast variations of the generated power, that enters the DC link and that the grid side converter has to handle very fast, injecting it into the grid in order to maintain the DC voltage constant. The capability of the NLVCS for MMC to handle the energy generated has been tested in two different wave energy applications. Firstly, on a single wave energy converter (WEC), a point absorber. This case is characterized by smaller and faster power changes. Secondly, on a wave farm made up of 200 WECs connected to the grid through an MMC. This case is characterized by much larger but smoother power changes. Wave farms, WECs and MMC are large and sophisticated facilities or devices that are almost impossible to gather in the same laboratory. Consequently, the features of the new control system for MMC and the advantages that provide in the connection of wave energy power plants and point absorbers to the grid has been simulated using detailed models. The results demonstrated the capability of the MMC operating as NLVCS to deal with the fast and random power changes present in renewable energy sources. [ABSTRACT FROM AUTHOR]

• Proposes novel dc grid device based on capacitive energy storage. • Reduces fault current and voltage drop of voltage source converters under dc faults. • Reduces overvoltages under pole-to-ground faults in symmetrical monopole grids. • Improves power flow security by preventing converter blocking. • Improves transient dc grid stability. A major and very important challenge in dc grid development is maintaining continuous converter operation under dc faults. This paper proposes a novel capacitive energy storage device which improves security of dc grids by avoiding terminal blocking. The device provides current from the capacitor bank during dc faults, reducing fault current contribution and voltage drop of dc grid converters. Moreover, the device also helps in balancing pole voltages which is of particular significance during pole-to-ground faults in symmetrical monopole systems. Other benefits like improved transient grid stability are also demonstrated. Device's design and performance is assessed using theoretical analysis and verified on a three-terminal offshore dc grid model in PSCAD. The cost of the device's electronics is minimized and the total cost and weight estimation are also shown. [ABSTRACT FROM AUTHOR]

• This article proposed a method to calculate DC-side harmonic currents in hybrid HVDC transmission systems. • DC filter that can mitigate low frequency ripple and accelerate the response time of filter was designed. • The influence of partial element decay on the filter's transfer characteristic was analyzed and appropriate partial component dissipation was adopted. Hybrid high-voltage direct current (HVDC) transmission is a new transmission structure that uses line commutated converter (LCC) at the rectifier side and voltage source converter (VSC) at the inverter side. This study proposes a method to calculate DC side harmonic currents of hybrid HVDC transmission systems and design filters to mitigate DC-side low-frequency voltage pulsations. First, an equivalent harmonic analysis model of modular multilevel converter (MMC) is deduced according to the relationship among different harmonic components. The harmonic voltage expressions at DC side of 12-pulse LCC are deduced by combining the three-pulse harmonic voltage source model and FFT decomposition. On this basis, the expressions for harmonic currents are derived by multiplying the harmonic voltage transfer function and transfer admittance equation of DC network and are solved for the inverse Laplace transform. Two different types of DC filters are designed according to the requirements of DC filter on decaying specific order harmonic currents. The influence of partial element decay on the transfer characteristic of filter is analyzed, and appropriate partial component dissipation parameters are applied. The decay pole near the harmonic resonance frequency is configured to achieve reasonable DC-filtering results. Simulation results of a hybrid HVDC transmission system demonstrate the effectiveness of the proposed analysis model and filter design method. [ABSTRACT FROM AUTHOR]

Highlights • An improved average value model (AVM) of modular multilevel converter (MMC) for large-scale system simulation. • Accurate performance in all types of faults and blocked state of MMC. • Easy modelling in commonly used software packages. Abstract Detailed model (DM) of modular multilevel converter (MMC) brings about a high computational burden for electromagnetic transient (EMT) simulation. Therefore averaged value model (AVM) of MMC is proposed for fast simulation of large-scale multi-terminal HVDC system. No direct electrical contact exists between AC and DC system in conventional AVM and therefore the coupling relationship among electric quantities in fault analysis could not be accurately modelled. In order to solve the aforementioned problem, an improved AVM (IAVM) of half-bridge (SB) MMC topology is proposed. All capacitor voltages of AVM are constant and an arm is directly equivalent to a controlled voltage source. The proposed AVM can be applied for the DC faults and blocking conditions by introducing a group of switches connecting AC and DC sides. Two extra controlled voltage sources are introduced for correct DC voltage offset in AC voltages under DC line-to-ground (LTG) faults. Simulation results verify the accuracy and efficiency of the IAVM. IAVM could be implemented in commonly used software packages and is suitable for power system fault analysis. [ABSTRACT FROM AUTHOR]

The overhead dc line has higher fault probability, especially the nonpermanent fault probability, than the dc cables. The conventional restart strategy of flexible high voltage dc (HVDC) transmission system deblocks the converter directly, causing a severe secondary damage to the system in the case of permanent faults. For improving the power supply reliability and security, the paper proposed a novel restart control strategy which can identify the fault property (permanent or nonpermanent) correctly for the modular multilevel converter (MMC) based HVDC overhead transmission system. By utilizing the uncontrolled rectifier operation mode and existing starting resistor, the proposed restart strategy is effective to reduce the secondary damage to the system significantly, including the overcurrent damage and overvoltage damage. Additionally, the delay time is introduced to eliminate the influence of the line distributed capacitor charging current on the proposed method, with the setting principle of this delay time also being discussed. The proposed strategy is very simple to be implemented, thus being more valuable for engineering application. Simulation cases based on the PSCAD/EMTDC were carried out finally to verify the feasibility and superiority of the proposed strategy. [ABSTRACT FROM AUTHOR]

It is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5–20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50–60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented. [ABSTRACT FROM AUTHOR]

• Power converter dominated system has highly controlled fault characteristics. • Current differential protection not operate reliably in converter dominated system. • Coordinated converter control with protection ensures the reliable operation. The modular multilevel converter (MMC) based high-voltage dc (HVDC)-connected offshore wind farm (OWF) is power electronic converter dominated power system, where the conventional current differential protection may not operate effectively. Based on the basic operation principle of the current differential relay, this paper points out that the phase difference (ϕ diff) of fault currents from MMC and OWF should be limited within a specific range, i.e. (- ϕ diffmax , ϕ diffmax), to guarantee the reliable tripping of the current differential relay. Yet, the highly control-dependent fault currents from the MMC and OWF could yield an arbitrary ϕ diff that might beyond (- ϕ diffmax , ϕ diffmax), which leads to the possible malfunction of the current differential relay. To tackle this challenge, the coordinated control method of MMC is proposed to align the phase angle of its fault current with that of the OWF, such that ϕ diff ∈ (- ϕ diffmax , ϕ diffmax) can be always guaranteed, so as the reliable operation of the current differential relay. Finally, the proposed coordinated control is verified by the electromagnetic transient (EMT) simulations in PSCAD and the real-time digital simulator (RTDS). [ABSTRACT FROM AUTHOR]

Modular multilevel converter based multi-terminal HVDC (MMC-MTDC) technology is increasingly adopted as a promising option in the power transmission and distribution fields. However, how to isolate dc fault currents and ensure the continuous operation of the healthy parts of the MMC-MTDC system under dc faults presents a huge challenge. Based on a comprehensive analysis on the dynamic behavior of the dc fault current of a MMC, along with the operation characteristics of hybrid dc circuit breakers (CBs), a protection strategy will be developed. It coordinates the system parameters to ensure the hybrid dc CBs cut off the dc fault current before triggering MMC blocking in bipolar dc faults. The mathematical approximation of the dc fault current without converter blocking is achieved through theoretical analysis and numerical calculation, and the smoothing inductance at the dc side of MMC station has shown a major influence on the dynamic behavior of the dc fault currents. The smoothing inductance can be determined based on the mathematical expression of the dc fault current, as well as the breaking capacity of hybrid dc CBs. Data analysis and simulation are performed based on a three-terminal dc test system to validate the effectiveness of the coordination protection strategy. [ABSTRACT FROM AUTHOR]

The Modular Multi-level Converter (MMC) topology is one of the most important switch mode converter topology and provides indispensable potential applications in the power systems industry, specifically, for High Voltage Direct Current (HVDC) transmission systems. Recent applications of MMC in HVDC transmission systems has posed problems such as circulating currents in the converter legs, which threatens the safe operation and also deteriorates the system’s performance. The circulating currents occur mainly due to the fluctuations in the capacitor voltages in the Sub Modules (SMs) of the MMC. Hence, in order to nullify the SM’s unbalanced voltage, this article aims to use a new counting sort based technique in conjunction with Carrier Phase Disposition Pulse Width Modulation Technique (CPD PWM). The increased harmonic content in the output voltage and current waveforms may be attributed to the unpredictable dynamics of capacitors. To address this, the article also proposes a cascaded voltage and current H ∞ repetitive control scheme under normal and fault conditions. This entire scheme has been applied for back to back connected HVDC systems for mitigating the Total Harmonic Distortion (THD) in current and voltage respectively. Further, stability analysis for the proposed controllers has been performed to prove its robust operation and finally all the results have been explored. [ABSTRACT FROM AUTHOR]

Meshed offshore grids (MOGs) present a viable option for a reliable bulk power transmission topology. The station-level control of MOGs requires faster dynamics along with multiple objective functions, which is realized by the model predictive control (MPC). This paper provides control, and protection design for the Modular Multilevel Converter (MMC) based multi-terminal DC (MTDC) power system using MPC. MPC is defined using a quadratic cost function, and a d q z rotating frame voltage inputs are represented using Laguerre orthonormal functions. MPC has been applied for the control of both grid forming and grid following converters in a four-terminal MTDC setup, implemented for real-time Electromagnetic Transient (EMT) simulation. By applying numerous time-domain simulations, the advantages of the MPC when dealing with AC and DC side disturbances are investigated. The investigation highlights the MPC's inherent feature of fast response and high damping during- and post-disturbance, which is compared to the traditional PI controller performance. The analysis provides a comprehensive insight into the transient behavior of the MTDC during disturbances. • A standalone MPC algorithm runs in a Real-Time Digital Simulator environment. • Control interoperability between the MPC and PI controlled MTDC grid. • The reliability and resilience of the designed MPC under transient disturbances. • A digital twin model of offshore AC-DC grid through HiL and SiL setup. [ABSTRACT FROM AUTHOR]

• This paper proposes a novel design of circulating current control target to minimize the requirement of SM capacitance in MMC. • The phase angle of circulating current is designed to reduce the fundamental and second-harmonic capacitor voltage ripple at the same time. • The magnitude of circulating current is proposed to achieve the optimal phase angle or magnitude coordination between the components of SM capacitor voltage ripple. • Compared with the available methods, the proposed method can further reduce the requirement of SM capacitance by 16.4%, which can contribute to the lighter design of MMC on the offshore wind farm side. In an MMC station, the submodule (SM) capacitor constitutes a high proportion in both the footprint size and construction cost, which greatly limits the application of MMC in some scenarios, such as offshore wind power transmission system. Since the SM capacitor voltage ripple has a close interaction with the phase circulating current in MMC, an appropriate management of circulating current is widely acknowledged to reduce the SM capacitance. This paper proposes a novel design of circulating current control target to minimize the requirement of SM capacitance in MMC. Firstly, the phase angle of circulating current is designed to significantly reduce the fundamental and second-harmonic capacitor voltage ripple at the same time. Secondly, the magnitude of circulating current is determined to achieve the optimal phase angle or magnitude coordination between the components of SM capacitor voltage ripple. Thus, under the proposed design method, the peak-peak capacitor voltage ripple is minimized and the requirement of SM capacitance can be effectively reduced. Finally, an MMC-HVDC case system connecting to the offshore wind farm is built on PSCAD/EMTDC to validate the effectiveness of the proposed design method. [ABSTRACT FROM AUTHOR]

• In this paper, the sub-module switching process and the capacitor voltage sequence of a modular multilevel converter arm is analyzed. • A method to predict the capacitor voltage convergence in the next control cycle based the voltage sequence is proposed, which can remarkably improve the utilization of switching occurrences and reduces the average switching frequency. • Based on the predicted voltage sequence, an improved capacitor voltage balancing method is proposed, which can significantly reduce the additional switching loss and improve the operating efficiency of the converter system. Modular multilevel converter (MMC) is one of the most promising topologies in high/medium-voltage applications, but high switching losses impair system economy and reliability. For the high-voltage MMC with the nearest level modulation (NLM), there is usually high frequency and uncertain switching actions among submodules (SMs) in each arm, due to the traditional sorting based capacitor voltage balancing control method, which brings additional power losses. In order to solve this problem, the SM switching process and the capacitor voltage sequence of a MMC arm is analyzed, and then a method to predict the capacitor voltage convergence in the next control cycle is proposed in this paper. Based on the predicted voltage sequence, an improved capacitor voltage balancing method is proposed, which can significantly reduce the additional switching loss and improve the operating efficiency of the converter system. MATLAB/simulink simulation and RT-LAB based HIL test results verify the proposed method under different operating conditions. [ABSTRACT FROM AUTHOR]

High-voltage direct current (HVDC) transmission system based on modular multilevel converter (MMC) technology is considered as an attractive solution for the interconnection of two AC grids to enhance the security of supply. Several studies have concluded that the AC grid impedance is a key parameter for the control system design of the MMC. Such important design parameter, however, is subject to wide variations due to the time-varying nature of AC grid and the future modifications and expansions. Therefore, the grid impedance has to be considered as an uncertain parameter during control design stage. To this end, this paper suggests a robust multivariable controller which is able to maintain system stability and performance for an extended range of grid impedance uncertainties. To ensure that the controller operates robustly in a wide range of grid impedances, structured singular value of the system (known as μ) is minimized via an iterative procedure. In such procedure, the uncertainties are separated from the nominal linear model of the system and the weighing functions are tuned to fulfill the robustness criteria defined by the small-gain theorem. The effectiveness of the suggested controller and the design procedure is validated by means of simulation results. • A robust multivariable controller is suggested for MMC-based HVDC system. • Robust controller guarantees system stability under AC grid impedance uncertainties. • The robust design is based on the minimization of structured singular value. • Internal energy of MMC is used to achieve robust performance and stability. [ABSTRACT FROM AUTHOR]

Multi-terminal High-Voltage Direct-Current (MT-HVDC) Voltage Source Converter (VSC) based grids are arising as an extension of point-to-point connections, aiming at boosting the integration of renewable energy resources through a more interconnected power system. This paper investigates and compares different energy-based control structures for Modular Multilevel Converters (MMC) operating in DC voltage droop control mode, identifying the most suitable ones in terms of stability. Then, a systematic, automatic and optimal control design based on H ∞ for a generic and bidirectional MT-HVDC system, including multiple DC voltage droop controlled terminals, is developed, based on both steady-state and transient requirements under different contingencies. • DC voltage droop control structures comparison for MMCs in multi-terminal HVDC networks. • Systematic optimal control tuning method based on H ∞. • Combination of steady-state and transient requirements. • Converters with different power ratings and bidirectional operation (generation, load). [ABSTRACT FROM AUTHOR]

• In consideration of cost, stability, transient characteristics and parameter matching of the DC SFCL, an effective optimization design method consisting of electromagnetic theory and intelligent algorithm is proposed, which can obviously promote the utilization efficiency and stability of YBCO coated conductors. • Combined with three operation states of the DC SFCL, a corresponding FE model is established to simulate and evaluate the performance of superconducting coil in the DC SFCL, which further reveal the AC loss characteristics of super-conducting coil during current limiting process. With the increase of DC transmission voltage and capacity, large dc fault current becomes an important issue due to the bottleneck of the capacity of the DC breaker. In order to solve the problem, we have proposed a novel hybrid type DC superconducting fault current limiter (DC SFCL) in our previous papers. Based on a series of previous researches, some improvements are made to design the hybrid type DC SFCL for application in a MMC system in this paper. The target is to limit short circuit current and avoid converter valves blocked when the DC short circuit occurs. In consideration of cost and stability, combined with the transient characteristics and parameter matching of the DC SFCL, an effective optimization design method consisting of electromagnetic theory and intelligent algorithm is proposed, which can obviously promote the utilization efficiency and stability of YBCO coated conductors. According to the optimization design method, a design scheme of the DC SFCL is put forward. Combined with three operation states of the DC SFCL (normal state, current limiting state I and current limiting state II), a corresponding finite element model is established to simulate and evaluate the performance of superconducting coil in the DC SFCL, which further reveal the AC loss characteristics of superconducting coil during current limiting process. [ABSTRACT FROM AUTHOR]

• A novel AT-fed co-phase TPSS is proposed, providing continuous power with high PQ. • The mathematic model of the novel system is built and the PQ problems can be solved. • A hierarchical control method coordinating PQ compensation and PFC control is given. • The novel system can reduce the PFC capacity by 59% compared to the existing scheme. A large unbalanced current injected into the power gird and the neutral sections at the exits of traction substations have been the outstanding obstacles in conventional single-phase 25 kV AC traction power supply systems (TPSS). In order to meet the requirements of high-speed and heavy haul transportation systems, a novel autotransformer (AT)-fed co-phase TPSS is proposed, which can provide continuous power for the trains with high power quality (PQ). In this research, the electrical configuration of the new TPSS is presented, which is structured with Vx-connection traction transformer (TT) and power flow controller (PFC) adopting the modular multilevel converter (MMC) topology. Furthermore, the mathematic model of the novel system is built. The unbalanced current and the reactive power caused by traction load can be compensated effectively based on power balance principle. Moreover, a hierarchical control strategy is designed including upper optimized PQ compensation algorithm and low-layer PFC control, which can coordinate dynamic characteristics between different working patterns. Finally, the validity of the control method and the performance of the system are demonstrated by case studies. The results indicate that compared to the conventional co-phase TPSS, the proposed co-phase TPSS is able to reduce the compensation capacity by up to 59%. [ABSTRACT FROM AUTHOR]

This paper proposes a new circuit topology of Modular Multilevel Converter (MMC) for grid connected applications. The power cells in each lower arm of conventional MMC are replaced by a passive filter in the proposed topology. The passive filter is an LCL filter and is designed in such a way that it restricts the flow of fundamental current through it. However, it allows the DC and second harmonic component (generated in the upper arm due to power oscillation) to flow through it. As the power cells in the lower arms are removed, the total number of power cells required in the new topology also reduces to half as compared to conventional MMC. Additionally, DC bus voltage requirement is reduced to half to produce the same quality and magnitude of AC voltage waveform as conventional MMC. The theoretical analysis is validated by simulation and a downscaled experimental prototype and the results confirm the proper operation of the proposed converter. [ABSTRACT FROM AUTHOR]

• A voltage balancing method for three phase multistring PV system based on Modular multilevel converter is proposed. • The voltage balancing method is capable of balancing the SMs capacitor voltages. • This method inherently generate a DC and fundamental frequency components of the circulating currents. • These circulating current components are capable of equally distributing the total generated power among the three phases. • A balanced three phase currents are injected into the grid despite the inhomogeneous irradiance. When it comes to renewable energy applications, the Modular Multilevel Converter (MMC) is considered as one of the most promising converter topologies. In this paper, the MMC is used for integrating the Photovoltaic (PV) strings to the grid. The PV strings are connected to the MMC submodules (SMs) through a DC-DC converter, which facilitates the independent MPPT implementation. For satisfactory operation of MMC, it is important to maintain a balanced voltage across the SMs. In addition, the total generated power by the PV strings should be uniformly distributed among the three phases. These requirements are considered as challenging tasks during different inhomogeneous irradiation conditions. The proposed method is capable of balancing the SMs capacitor voltages and at the same time inherently generating a DC and fundamental frequency components of the circulating currents inside the MMC without using a controller. These circulating current components are responsible for equally distributing the total generated power by the PV strings among all phases. As a result, balanced three phase currents are injected into the grid. The proposed method is verified through extensive simulation during different irradiation conditions. [ABSTRACT FROM AUTHOR]