The cable body/accessory heterogeneous insulation at HVDC cable installation sites is the system's weakest point. The internal space charge accumulation, especially at the interface, readily cause electrical field distortion, leading to partial discharge and even breakdown. Therefore, research on the charge accumulation mechanism and regulation is crucial for the safe operation of the cable line. This paper focuses on the space charge accumulation in the heterogeneous insulation of HVDC cable body/accessory. First, the charge transport in the heterogeneous insulation is introduced, and it is decomposed into two key processes: interface contact charge transfer, and carrier migration and interaction with interface charge. The importance of the interface is pointed out, and the existing interface trap evaluation methods are analyzed. Then, the theoretical models of charge transport in heterogeneous insulation are summarized. Subsequently, the charge accumulation regulation methods are sorted from the two perspectives: improving the bulk characteristics and optimizing the interface/surface characteristics. Finally, the current research status and the key problems to be solved in the future are summarized and prospected from the perspectives of interface trap characterization, charge transport mechanism, and charge accumulation regulation methods

Partial discharge (PD) under high dV/dt square wave voltage is one of the main causes resulting in power module packaging insulation aging and failure. However, the evolution of PD during aging and the mechanism of its effects on package insulation are still unknown. The optical spectral distribution of PD could characterize various microscopic physical processes, whereas the topic is seldom investigated for power module packaging insulation. Therefore, this paper focuses on the PD multispectral characteristics of the power module packaging insulation during PD. A multispectral PD detection system based on the silicon photomultiplier (SiPM) is established. The multispectral signals of PDs during aging at different temperatures and square wave voltage frequencies are investigated. The results indicate that PD mainly produces optical signals in the near-ultraviolet (UV) band, and the magnitudes of the visible and IR lights are lower. The aging process can be divided into initial, developing, and late stages based on the generation situation of long-wavelength visible and near-infrared (IR) optical signals. In the initial stage, UV and near-UV visible lights are generated. In the developing stage, after the formation of the gas cavity, long-wavelength visible and near-IR optical signals are generated. In the late stage, the magnitude of each band increases, and the signal magnitude and generation probability of the long-wavelength visible and near-IR optical signals increase significantly until the insulation breakdown and failure. The average magnitude of light signals in all bands increases with rising temperature, and the generation probability of photons in the B3 and B4 bands increases significantly. At higher voltage frequencies, photons in the B3 and B4 bands are more likely to be generated. The evolution of multispectral characteristics can act as helpful indicators for packaging insulation diagnosis.

Modular multilevel converter (MMC) has been widely used in DC power transmission, motor control, etc. Sub-modules (SMs) capacitor voltage fluctuation is the main factor affecting the performance and capacitance design of MMC. Besides, harmonic injection is a popular approach to achieve fluctuation suppression of the SMs capacitor voltage, but it is prone to the increase of common-mode voltage. In this paper, a novel series-capacitor-type modular multilevel converter (SCTMMC) is proposed. SCTMMC introduces a new intermediate module between the midpoint of the Traditional MMC arm and the output. With this module, the AC output-side common-mode voltage is eliminated through harmonic injection, the capacitor voltage balance is ensured by a dedicated control scheme, and the quality of the AC current is improved. The effectiveness of the proposed structure and control method in suppressing the arm energy fluctuation is verified by analyzing the mathematical model of SCTMMC. In addition, the SCTMMC has fewer power electronic devices, and the control of the intermediate module is simpler, which showed better compatibility for the improvement of the existing harmonic voltage injection control method. Finally, the performance of SCTMMC was verified by simulation and experiment results.

The space charge accumulation in the heterogeneous insulation composed of cross‐linked polyethylene (XLPE) cable and silicone rubber (SiR) accessory poses a serious threat to the safe operation of the high voltage direct current (HVDC) cable. When the cable is in heavy load, the charge transport behaviour in XLPE/SiR becomes more complicated due to the high temperature. In order to investigate the charge transport characteristics of XLPE/SiR under heavy load condition, the simultaneous measurement of space charge and relaxation current is performed on XLPE/SiR at both 70°C and 30°C with different polarities. The results show that the polarity of the interface charges in XLPE/SiR is always consistent with that of the SiR side electrode, and the influence of high temperature (70°C) caused by heavy load on the interface charge accumulation of XLPE/SiR is reversed at different polarities. The interface trap depth of XLPE/SiR is consistently greater than the bulk trap depths in both XLPE and SiR. When at high temperature of 70°C, the depth and density of interface traps increase, and the bulk traps in XLPE and SiR also exhibit increased depth. The component of polarisation relaxation current associated with space charge activity increases and exhibits longer decay time at 70°C, indicating more active and complex charge trapping‐detrapping activities under heavy load condition. In this paper, an advanced simultaneous measurement is used to correlate the internal charge distribution with the external current for analysis, and the charge transport characteristics of XLPE/SiR under heavy load condition is revealed. The results can provide reference for the operation and maintenance of HVDC cable, and can also provide a basis for the space charge regulation of heterogeneous insulation at HVDC cable accessories.

The reliability of power module packaging insulation is critical for the stable and safe operation of high-voltage power devices. Within the module packaging, the interface between the encapsulation and the ceramic substrate represents a critical weak point, where dielectric discontinuities under applied voltage can cause interfacial charge accumulation, distort the electric field, and, in severe cases, compromise insulation performance. Understanding the interfacial charge dynamics is therefore essential for ensuring long-term reliability. This study investigates the spatiotemporal distribution of interfacial charges in power module silicone-direct bond copper (DBC) packaging under DC and unipolar square wave voltages using a non-invasive electrostatic induction method. The results show that DC voltage induces approximately twice as much interfacial charge accumulation as square wave conditions. However, under square wave voltage, the charge decay rate is more slowly. Subsequently, the possible generation of interfacial charges are discussed, including electrode injection, impurity ionization, and Maxwell-Wagner polarization. Moreover, two types of trap states are identified. Shallow traps are likely related to intrinsic defects and present similar depth despite of waveform and field strength. In contrast, the formation of deep traps is more complex. Although a DC electric field results in a larger amount of charge being trapped, a square-wave electric field drives the charges into deeper trap states. These findings highlight the combined influence of Maxwell-Wagner polarization and space-charge effects on interfacial charge dynamics, providing key insights into the mechanisms governing insulation aging and strategies for improving insulation reliability.

This study systematically analyzed the regulatory mechanisms of nanodoping on the electronic structure and properties of LDPE through quantum chemical calculations and simulations. DFT and first-principles calculations were employed to construct models of LDPE molecular chains as well as nanoparticles of $\text{MgO}, \text{SiO}_{2}$, and $\text{TiO}_{2}$, respectively, to simulate the effects of nanodoping on the band structure of LDPE. The energy levels of all three types of nanoparticles are relatively dispersed, and the incorporation of nanoparticles into LDPE introduces deeper trap energy levels, thereby capturing more holes and charges. Additionally, the study simulated the band structure variations of LDPE under a gradient in the number of nano MgO clusters, revealing that as the doping concentration increases, the number of energy levels in the LDPE/MgO system gradually increases.

The modular multilevel converter (MMC) open-circuit fault diagnosis under full power condition has become a challenging problem. Especially, when MMC operates under low-power conditions, there are significant fluctuations in capacitor voltage and the existing fault diagnosis strategies face difficulties in selecting appropriate thresholds. This article proposes a multiple open-circuit fault diagnosis strategy for MMC using the feature reconstruction-recurrent neural network (FR-RNN). In this method, the dominant characteristics of the theoretical and actual capacitor current of the submodules (SMs) are presented as the preferred feature set. The auxiliary mathematical feature based on capacitor current was screened using maximal information coefficient (MIC), and then, the selected auxiliary mathematical feature and the preferred feature set were inputted into the RNN for FR to realize fault diagnosis based on the optimal feature set. The proposed strategy can directly achieve accurate fault localization and fault-type judgment of SM faults without additional detection steps and diagnostic delay. In addition, the determined optimal feature set greatly simplifies the network computation, thus avoiding deviation caused by multidimensional and redundant features on the diagnosis outputs. Simulation and experimental studies confirm the effectiveness of strategy.

Modular multilevel converter (MMC)-based wind power converter exhibit large submodule (SM) capacitor voltage fluctuation due to the low-frequency output characteristics of wind turbines. Harmonic injection (HI) can effectively suppress the SM capacitor voltage fluctuation, but its injection value should be adjusted according to the change in MMC operating conditions. This implies that it is challenging to determine the optimal injection value. To address these challenges, we propose an online HI (OHI) strategy for the MMC operating in a wide frequency range. The strategy comprises second-order harmonic current injection (SHCI) and third-order harmonic voltage injection (THVI), where the amplitude of the second-order harmonic current is calculated automatically to dynamically change with wind speed changes in offshore wind farms. The fluctuation characteristics of the SM capacitor voltage over a wide frequency range are evaluated. The mathematical model of the proposed OHI strategy is then established as a unified calculation principle. The proposed strategy can suppress SM capacitor voltage fluctuations and reduce the MMC power loss over a wide range of wind speeds. The SM capacitance can also be reduced by 27% using the proposed method. The effectiveness of the proposed OHI strategy is validated by Simulink simulations and hardware experiments.

The key to the stable operation of the FID (flexible interconnection device) is the voltage stability of the DC (direct current) bus. However, when transient events occur on the DC side of the flexible interconnection device and the operation mode needs to be switched, the traditional master-slave control strategy can not be switched in time due to a certain delay of the communication system, resulting in the damage of the device. To solve this problem, this paper proposes a smooth switching strategy based on virtual synchronous control for flexible interconnection devices, which can stabilize the DC bus voltage within the range of power grid operation when transient events occur in the system. When the operation mode of the flexible interconnection device is switched, the voltage and current on the DC side can be smoothly transitioned, and the inertia and damping support can be provided for the power grid during steady operation. The proposed strategy is verified by the simulation of flexible interconnection device under three conditions: switching from two-terminal power supply to two-terminal isolated power supply, active command step in single-terminal power supply, and frequency reduction in double-terminal power supply.

The composite insulation composed of cross-linked polyethylene (XLPE) and silicone rubber (SiR) is common in high voltage direct current cable accessory installation. However, the space charge accumulation, especially the interfacial charge accumulation of XLPE/SiR, poses a serious threat to the safe operation of cable accessories, and its charge transport mechanism is still unclear, especially at the micro-scale. In order to investigate the charge transport mechanism of XLPE/SiR, simultaneous measurement of space charge and relaxation current is performed on XLPE/SiR at various electric fields with different polarities, and the electronic energy levels of XLPE and SiR are determined by quantum chemical calculation (QCC). The results of QCC show that both the hole traps and the electron traps in XLPE are mainly shallow traps. As for SiR, the hole traps are shallow traps, while the electron traps are deep traps. The results of simultaneous measurement show that the interfacial charge accumulation and the composite conductivity of XLPE/SiR are significantly different under different polarities, that is, there is an obvious polarity effect. Based on the results of QCC, the electronic energy levels of XLPE/SiR system are constructed considering the interface between XLPE and SiR, as well as the interfaces between the materials and the electrodes. On this basis, the charge transport mechanism of XLPE/SiR is discussed in detail, and the polarity effect is well explained, which is believed to be related to the differences in the charge injection barrier and the interfacial barrier under different polarities, as well as the electron/hole traps in XLPE and SiR.

The growth characteristics of electrical tree under AC and DC voltages are very different. In order to study the growth characteristics of electrical tree at the interface of cable accessories, in this paper, the growth and partial discharge (PD) characteristics of electrical tree at the interface between cross-linked polyethylene (XLPE) and silicone rubber (SIR) double-layer specimens were investigated under AC and DC voltages, respectively. The experimental results show that when under the AC voltage, the electrical tree channels gradually change from non-conductive to conductive during the growth towards the interface, and then breakdown occurs along the XLPE-SIR interface. When under the DC voltage, the electrical tree is a typical non-conductive tree with a small amount of partial discharge. When the electrical tree grows near the interface, some small lateral branches will be developed and no longer grow towards the interface. The results provides some experimental support for improving the safe and reliable operation of cable joints.

The space charge characteristics of cross-linked polyethylene (XLPE) can be improved to some extent by doping the appropriate amount of nano-MgO. In this study, in order to explore the influence of nano-MgO on the space charge and electric field distributions of the composite insulation of high voltage direct current (HVDC) cable joints, the effect of nano-MgO concentration on the depth and density of the deep traps in MgO/XLPE was first analyzed. On this basis, the charge transport simulation model of a 320 kV HVDC cable joint was established with MgO/XLPE as the cable insulation, and the space charge and electric field distributions of the cable joint under different temperature conditions were simulated. It was found that the radial charge distribution in the joint shows different trends with the change of nano-MgO concentration. There is a significant difference in the charge density on both sides of the (MgO/XLPE)/EPDM interface, and the difference first decreased and then increased with the increase of concentration. When the nano-MgO concentration was 0.5 wt%, the number of charges in the radial direction is the fewest, and the maximum value is only 0.42 C/m−3. The radial electric field changed abruptly at the (MgO/XLPE)/EPDM interface, and it was homogenized to a certain extent with time. It was found that the highest electric field of the interface is at the root of the stress cone, which is the weakest point of the joint insulation. When the nano-MgO concentration was 0.5 wt%, the electric field at the root of the stress cone was found to be the lowest, with a value of 13.38 kV/mm. A comprehensive comparison shows that the joint can maintain better insulation when the concentration is 0.5 wt% compared to other concentrations. The results can provide a basis for further improving the insulation properties of HVDC cable joints through nano doping technology.

The interfacial charge of silicone rubber (SiR) has been an issue of widespread interest. In the case of industrial production, the introduction of chemical functional groups intensifies the charge accumulation, so it is significant to study the effect of the introduction of different functional groups on the internal trap depth of SiR. In this paper, the energy bands and surface electrostatic potentials of SiR small molecules containing double bonds at both ends and side chains attached to a benzene ring were calculated separately using density functional theory(DFT). On this basis, the causes of the formation of such traps and their possible sites are determined, and the trap depths formed by both are analyzed. The results indicate that the chemical defects are caused by these small molecular groups, and at the same time, these small molecular groups introduce more localized states, dominated by electronic traps. This suggests that these small molecular groups may be responsible for exacerbating the space charge accumulation.

LCL grid-connected inverter is good at suppressing the high frequency current, but the inherent resonance frequency of the filter will lead to the resonance peak. Therefore, the active damping is commonly used to suppress the resonance effect. Since the weighted average current control can provide better bandwidth for the system with high frequency and has the characteristics of system reduction, it is often used to replace the traditional dual-loop current control in the application of LCL grid-connected inverter. However, the reduction of current resonance peak at the grid side by the weighted average current control is limited, so it is necessary to combine the grid voltage and capacitor current feedback loop to achieve the ideal results. In weak grid, the newly added feedback path would easily lead to the loss of system reduction, which results in the existence of transient variables in the transfer function affecting the stability of the control strategy. To solve this technical problem, this paper addresses a new control method of LCL grid-connected inverter based on weighted average current control. By adding a capacitor current feedback path, pole-zero cancellation of the control system is realised as to reduce the systematic order and improve the robustness of the controller under the time-varying weak grid.

在高压直流电缆接头绝缘中,携带被深陷阱捕获电荷的分子链在库伦力作用下会发生位移,导致聚合物绝缘中深陷阱能级发生变化,进而对电荷输运造成影响.该文基于分子链动力学对传统双极性电荷输运模型进行改进,在温度梯度下分析高压直流电缆接头不同界面的深陷阱能级、空间电荷及电场分布,探讨在温度梯度下深陷阱能级变化对电缆接头绝缘界面电荷分布的影响.研究表明:基于分子链动力学改进的双极性电荷输运模型中,复合绝缘界面以及介质内部深陷阱能级增大,导致电荷在介质内部的扩散和迁移受到阻碍,大量电荷在界面积聚;界面电荷分布规律与界面深陷阱能级分布一致;相较于接头内部整体高温,接头两侧存在较大的温差更容易使接头绝缘面临更严峻的挑战,电缆接头在10K的温差下能够保持较为良好的电气绝缘和运行状态.所得结果为进一步理清电缆接头绝缘电荷输运特性提供了支撑.

To satisfy the fast-growing electricity demand, high-voltage power systems with higher performance and better stability are required, which raises challenges including the design of power systems, the control of high-power electronics, the synthesis and preparation of high-performance insulation materials, evaluation of the insulation conditions, dielectric insulation tests, material modification, environment protection, etc [...]

In recent years, the modular multi-level converter (MMC) has been widely used in high and medium voltage DC transmission systems because of its topological advantages. However, for an MMC with a two-stage model predictive control (TSMPC) method, it is difficult to precisely and reasonably design the weighting factor in the cost function. Here, an improved TSMPC method is proposed which not only can avoid choosing the weighting factor for both first and second stage control but also can raise the output voltage level (OVL) to 2N + 1 without increasing computation burden. The discrete-time mathematical model of the MMC is first derived. Two circulating current factors are introduced to calculate the optimal number of submodules (SMs) of the upper and lower arms in the first stage control. Secondly, the second stage control calculates the optimal number of SMs through the superior control and forms the optimisation array by adding or subtracting one SM. Then the objective function is developed, and the SMs with the minimum value of the objective function are selected for the final input. Finally, the algorithm of reducing switching frequency (RSF) is applied to achieve the balance of the SM capacitor voltage. The simulation and experimental results verify the effectiveness of the proposed method.

Model Predictive Control (MPC) methods are suitable for modular multilevel converters (MMCs) due to their multi-objective control capability and fast dynamic response. But the weighting factors of the cost function are difficult to tune appropriately and need extensive simulation and experimental tests. This paper proposes a model predictive control strategy based on hierarchical feedback. By establishing the feedback mechanism between alternating current and circulating current control, the number of inserted submodules (SMs) of upper and lower arms are quickly determined to realize the optimal control of multiple objectives, and the tuning process of the weighting factors is eliminated. The computation burden of the cost function is reduced by the preselection of the inserted SMs combination. MPC with the Hierarchical Feedback control can generate 2N+1 output voltage level in the MMC while suppressing the circulating current. Finally, an MMC-HVDC system has been developed to verify the validity and effectiveness of the proposed method.

Simultaneous measurement at various temperatures of space charge and relaxation current is performed on cross-linked polyethylene (XLPE)/ethylene-propylene-diene monomer (EPDM) double layers with and without silicone grease. The simultaneous measurement correlates the charge distribution and current information for the same sample and avoids discrepancy, due to the separate measurement on different samples. It is found that the polarity of interfacial charges change to the opposite with the increase in temperature in most samples, which is related to both Maxwell-Wagner polarization and enhanced charge injection and migration at higher temperatures. Bipolar charges appear in the vicinity of interface in some XLPE/EPDM double layers with silicone grease. This phenomenon can be explained by Maxwell-Wagner polarization at two interfaces of XLPE/silicone grease and EPDM/silicone grease. The addition of silicone grease decreases the conductivity of XLPE/EPDM double layers. Based on the proposed three-component exponential decay model, it is found that the interfacial charge movement affects the polarity of induced relaxation current, and the addition of silicone grease decreases the interface trap depth.

Charge transport in high-voltage direct-current (HVDC) cable joint with complicated structure has not been profoundly investigated. In this study, a two-dimensional electrothermal coupled charge transport model combining finite element method (FEM) and bipolar charge transport algorithm is proposed, and the effects of temperature gradient and stress cone tilt angle on charge transport and electric field distribution in a 320 kV HVDC cable joint is investigated by this model. In the simulation, the interfacial area between the cross-linked polyethylene (XLPE) cable insulation and the ethylene-propylene-diene monomer (EPDM) joint insulation is specially modeled with different trap parameters from XLPE and EPDM. To obtain the simulation parameters, simultaneous measurements of space charge and relaxation current are performed on XLPE/EPDM double-layer samples, XLPE single-layer samples, and EPDM single-layer samples. It is found that with the increase of temperature gradient, more positive charges are injected and migrate from the inner side of XLPE insulation to the interface, thus, the charges accumulated at the interface change from negative to positive under large temperature gradient. At the same time, the position of the maximum electric field changes from the inner side of XLPE insulation to the root area of the stress cone at the interface. With the increase of the stress cone tilt angle, both the positive charges in XLPE insulation and the negative charges near the stress cone become less, and the overall maximum electric field first decreases and then increases under 20°C temperature gradient.