Multi-source self-adaption STATCOM and line commutated converter (SLCC) technology can overcome the inherent shortcomings of conventional line commutated converter (LCC) based high-voltage direct current (HVDC) transmission technology. In the "embedded" scenario, the topology and control strategy of a symmetrical unipolar SLCC-HVDC is proposed, which possesses significant technical and economic advantages. Then, the operating principles of SLCC are analyzed. During the non-commutation process, the SLCC can directly control the grid current through the static synchronous compensator (STATCOM) supplementing current. During the commutation process, the SLCC need to passively absorb the commutation current from the grid, while the grid current can still be indirectly controlled through changing the commutation angle. Finally, the operation characteristics of the traditional LCC and the SLCC under the symmetrical unipolar topology are compared. Both the commutation angle and the commutation voltage drop of the SLCC-HVDC are smaller than the LCC-HVDC and the reactive power of the SLCC-HVDC can be adjusted smoothly. Under transient condition, the fluctuation of the commutation angle is smaller, taking less risk of commutation failure. For the harmonic characteristics, SLCC-HVDC obviously decreases the harmonics compared with the LCC-HVDC during the unsymmetrical operation, thus is supposed to possess more flexible operation modes.

When current transformers are not installed on the delta winding of a wye-delta transformer， the differential protection must utilize currents measured outside the delta winding to calculate the differential current. To address the issue of reduced differential protection sensitivity during single-phase faults under this configuration， this paper proposes an optimal transformer differential protection method based on zero-sequence differential current characteristics. Firstly， the corrected currents on each side are calculated using a method that does not subtract the zero-sequence component from the delta side currents， from which the three-phase differential currents are derived. Secondly， based on the characteristics of the differential current during single-phase faults， an estimated value of the zero-sequence differential current is obtained. This is used to eliminate the influence of the zero-sequence component， yielding a corrected differential current. Subsequently， the protection operation decision is made by combining this corrected differential current with the percentage differential characteristic， and fault phase selection is achieved based on the operated phase. Finally， simulations and actual waveform tests verify the effectiveness of the proposed method. The results demonstrate that the method increases the differential current in the faulted phase during single-phase faults while eliminating the impact of the zero-sequence component on the healthy phases， thereby improving both the sensitivity of the differential protection and the accuracy of fault phase selection.

The application of embedded high voltage direct current (HVDC) technology provides innovative ideas for improving the power transmission capacity in limited transmission corridors and enhancing the flexibility and controllability of power grids. It adopts a special symmetrical monopole topology structure, which makes the conventional protection principles no longer fully applicable. This paper analyses the adaptability of traditional open line test (OLT) protection principles in embedded HVDC systems and finds that there are performance deficiencies such as refusal-action of protection and long action time. Based on fault characteristics, an improved overcurrent criterion and a new voltage imbalance criterion of OLT protection are proposed, and a new OLT protection strategy is formed to deal with DC grounding faults. The simulation results show that the proposed protection strategy can correctly act on DC grounding faults on each pole and greatly shorten the protection action time, comprehensively improving the reliability and speed of OLT protection, and effectively ensuring the safety of embedded HVDC systems.

As the core converter device in high-voltage direct current (HVDC) transmission systems, modular multilevel converters (MMCs) face severe challenges from the typical fault of DC bipolar short circuits in engineering applications, which urgently require focused research and resolution. To address this issue, this paper first provides a detailed introduction to a hybrid MMC topology based on Crossing Thyristor Branches (CTB). This structure connects three-phase upper arm reactors via thyristor branches, and this key design aims to effectively suppress DC fault currents. Subsequently, the paper conducts an in-depth analysis of the bipolar short-circuit fault characteristics of CTB-MMC, exploring the electrical behaviors and laws of the topology under fault conditions. Finally, to comprehensively verify the protective performance of the proposed CTB-MMC under bipolar short-circuit fault conditions, the research team constructs a complete sending-end and receiving-end MMC-HVDC transmission system based on the PSCAD/EMTDC simulation platform, laying a solid foundation for subsequent performance testing and analysis.

Thyristor controlled phase shifting transformer (TCPST) is a new type of power flow control equipment. Due to its series and shunt coupling topology and special connection to the power grid, it is necessary to consider the handling strategy in case of power grid faults. Based on the structure and principle of TCPST, the influence of grid fault on TCPST is analyzed in this paper. It is clarified the distribution of fault current and characteristics of overcurrent and overexcitation within TCPST. On this basis, the control strategy of TCSPT for deal with grid fault impact is proposed, and the fault state identification criterion and control sequences for the TCPST is constructed. By identifying the faults within and outside the zone of TCPST and the recovery state of power grid faults with the electrical quantities, the out of service and auto-restart strategy for TCPST are realized. The temporarily withdraw method for TCPST is used to protect equipment safety in the event of severe power grid faults. Finally, some simulations are conducted for the proposed fault ride-through strategy of TCPST. It is shown that the reliable ride-through of TCPST during power grid faults can be achieved by using the strategy, which has a good application for feasibility and effectiveness of TCPST. It is benefit to take advantage of the potential of power flow regulation function of TCPST.

Based on the topology and equivalent circuit of thyristor controlled phase shifting transformer (TCPST), the current amplitude equal characteristic of TCPST is deduced in this paper. The differential protection criterion of series transformer and excitation transformer is proposed based on the flux balance principle. The differential protection criterion of TCPST winding is proposed based on Kirchhoff's current law. The main relay protection strategy of TCPST is constructed. Furthermore, the transient characteristics of bypass switch stealing and thyristor control circuit fault under abnormal conditions are analyzed, and the protection strategy for abnormal conditions and the backup protection scheme of winding overvoltage and overcurrent are formulated. Finally, the fault at different locations is simulated to verify the completeness and effectiveness of the protection strategy.

The AC/DC transmission system is an important component of the power system, and the cross-circuitry Fault diagnosis of the AC/DC transmission system plays an important role in ensuring the normal operation of power equipment and personal safety. The traditional AC/DC transmission detection methods have the characteristics of complex detection processes and low fault line identification rates. Aiming at such problems, this paper proposes a new method of cross-circuitry Fault diagnosis based on the AC/DC transmission system based on a blind signal separation algorithm. Firstly, the method takes the typical cross-circuitry Fault scenario as an example to construct the topology diagram of the AC/DC power transmission system. Then, the electrical signals of the AC system and the DC system of the AC/DC power transmission system are collected, and the collected signals are extracted by the blind signal separation algorithm. Then, aiming at the cross-circuitry Fault problem of the DC system, the electrical quantities of the positive and negative poles on the rectifier side and the inverter side are collected, and the characteristics of the electrical quantities are analyzed by wavelet to determine the fault. At the same time, aiming at the problem of the cross-circuitry Fault of the AC system, three fault types of cross-circuitry Fault, ground fault, and intact fault are set up, and the electrical quantities of A, B, and C are collected on the same side, and the characteristics of three-phase electrical quantities are analyzed by wavelet. Finally, the cross-circuitry Fault judgment interval of the AC/DC system is set as the basis of fault judgment. After experimental verification, the relative error of the model is 1.4683%. The crossline fault identification method of the AC/DC transmission system based on the blind source separation algorithm proposed in this paper can accurately identify the crossline fault location and identify the fault type. It also provides theoretical and experimental support for power system maintenance personnel to maintain equipment.

With the fast development of the new power system, uneven power flow distribution restricts the enhancement of transmission capacity in certain corridors, and the issue of accommodating new energy sources becomes increasingly prominent. Traditional methods for controlling power flow, such as constructing new transmission lines or adjusting generator outputs, are costly and have limited effectiveness. Phase shifter is an economic compact trend control device, can fully tap the power supply capacity of the grid, has a broad application prospect, many places in the country are carrying out phase shifter selection and construction projects. This paper condenses China's first set of transmission grid phase shifter technology research experience, describes the phase shifter siting and capacity, operation control, protection and transformation of key application technologies, and provides an overview of engineering applications and effectiveness data, in order to provide reference for the subsequent development of phase shifter technology and engineering applications.

In multi-terminal modular multilevel converter (MMC)-high voltage direct current (HVDC) systems, fault current caused by DC fault is prone to damage power electronic devices, posing a serious threat to the safe and stable operation of the system. As a new type of MMC topology, the crossing thyristor branch (CTB) MMC is characterized by low cost and excellent fault ride-through capability, and has shown significant application potential in multi-terminal flexible HVDC systems. In this paper, the topological composition and working mechanism of the multi-terminal CTB-MMC are systematically elaborated firstly. On this basis, a control method for fault converter isolation and healthy converter fault ride-through suitable is proposed, and its technical advantages in the fault clearing process are analyzed in depth. Finally, a verification model is built through the power systems computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) tool, and the experimental results effectively confirm the feasibility and effectiveness of the proposed strategy.

Transforming the existing key HVAC transmission lines into High Voltage Direct Current (HVDC) transmission systems is a new type of transmission capacity expansion scheme that has been applied in power systems in Germany, the United Kingdom and other regions. After the occurrence of AC/DC intersystem faults, the fault characteristics are complex, and the protection adaptability will be affected. At present, there is no specific protection scheme for AC/DC intersystem faults. In this paper, a protection scheme based on the same side current similarity characteristics of AC and DC transmission lines is proposed, and the Hausdorff distance algorithm is introduced to measure two sets of current waveforms under different fault scenarios. The proposed protection scheme can complete the fault identification within a few milliseconds after the fault and has good rapidity and application prospects, and the effective value of the scheme is verified on the simulation platform.

Phase-shifting transformers play a crucial role in power grid stability and efficiency. They adjust phase differences between loads, improve transmission efficiency, and balance loads during large-scale power transmission and grid integration. However, traditional mechanical phase-shifting transformers use fixed-tap designs with limited taps, preventing continuous and precise adjustments. This discrete adjustment method affects control accuracy and optimal tap position selection for proper power flow. This paper proposes a hybrid open-loop and closed-loop control strategy. This strategy maintains the phase-shifting transformer at its optimal tap position, enhancing system regulation precision and control effectiveness.

The valve-side single-phase-to-ground (SPG) fault is one of the key challenges for hybrid modular multilevel converter (HMMC)-based bipolar high voltage direct current systems. In this article, the valve-side SPG fault characteristics of HMMCs are analyzed in detail, revealing that not only the submodules (SMs) in upper arms are severely overcharged, but also the full-bridge SMs in lower arms suffer unbearable overvoltage, which may destroy the devices. A series thyristor branches-based protection strategy is proposed in this article, which can handle the faults with the advantages of low SM capacitor overvoltage, low semiconductor cost, low ac-side overcurrent, and no arm overcurrent. Electromagnetic transient simulation studies using PSCAD/EMTDC tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of the proposed strategy.

The increasing penetration of power electronic devices has elevated the possibility of sub-synchronous oscillation (SSO) in the power grid. At the same time, the continuously adjustable phase-shifting transformer (CAPST) exhibits a certain capability for SSO suppression while regulating power flow. To clarify the SSO suppression mechanism and enhance its suppression capability, this study first analyzes the oscillation suppression mechanism of the CAPST, quantifies the boundaries of its oscillation suppression performance, and reveals inherent limitations of CAPST, including inadequate SSO suppression capability and the inability to simultaneously achieve effective power flow regulation and SSO suppression. To further extend the SSO suppression performance boundary, a capacitive series-compensated phase-shifting transformer (CSCPST) is introduced, accompanied by a design method for the series compensation capacitor. Additionally, to address the associated issue of power-flow reversal, a hybrid open-closed-loop power flow regulation strategy is proposed. Finally, simulation studies are conducted on both the CAPST and the CSCPST. The results verified the limitations of CAPST in SSO suppression capability and confirmed the superior performance of the CSCPST in enhancing SSO suppression and simultaneously achieving effective power flow regulation and SSO suppression.

DC-side short-circuit fault is one of the key challenges for the modular multilevel converter (MMC) based high-voltage direct current systems. In this article, a thyristor branch loop (TBL) based hybrid MMC (HMMC), where the thyristor branches are adopted to connect the upper arm inductors in three phases, is proposed to handle the dc faults. The protection operation for the TBL-HMMC in case of dc faults is also proposed, where the TBL-HMMC can handle the faults with the advantages of short dc current interruption time, short ac current interruption time, and low ac-side overcurrent. In addition, the proposed TBL-HMMC requires much fewer unipolar full-bridge submodules than the conventional HMMC, which results in low semiconductor cost and low power loss. Electromagnetic transient simulation studies using power systems computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of proposed TBL-HMMC and its operation under dc faults.

In heavily loaded regional power grids, some AC transmission lines are confronting escalating pressures due to excessive short-circuit currents. To optimize AC channels, most research advocates for retrofitting existing AC lines into multi-line-commutated converter-based high-voltage direct current (LCC-HVDC) lines. However, there is a contradiction between limited land area for AC stations and the relatively large footprint of passive filters in LCC-HVDC; this paper introduces self-adapted LCC (SLCC) by replacing passive filter groups with a static var generator (SVG). Secondly, the reactive power compensation, harmonic filtering control methods of SVGs, and operation characteristics of the SLCC system are explored, and the harmonics of the grid-side current are reduced by nearly 14.6%. Then, to fill the gap of previous studies on solely AC or AC-DC line touching, inspired by emerging DC line-touching risks in double-circuit (LCC and SLCC) lines on the same tower, the equivalent models are formulated to elucidate the evolution mechanisms of voltage/current and extract fault features in various line-touching faults; it finds that the longitudinal differential current during line-touching faults can be capitalized. Based on the current feature, an effective protection algorithm tailored for the identification of DC line-touching faults is proposed. Finally, simulations are conducted to validate the efficacy of proposed control and protect methods, demonstrating the potential to enhance the reliability of AC to DC conversion projects.

The valve-side single-phase-to-ground (SPG) fault is one of the key challenges for the MMC-based bipolar HVdc system. In this article, a grounded thyristor loop (GTL)-based hybrid MMC (HMMC) is proposed to protect the bipolar HVdc system under valve-side SPG faults. In the GTL-HMMC, the thyristor branches are proposed to connect the different arm inductors and the ground. The operation for the GTL-HMMC in case of valve-side SPG faults is also proposed, where the GTL-HMMC can handle the faults with the advantages of short-fault current interruption time, low capacitor overvoltage, low arm overcurrent, low semiconductor cost, and low power loss. Electromagnetic transient (EMT) simulation studies using Power system computer aided design / electromagnetic transients including DC (PSCAD/EMTDC) tool and experimental studies with a laboratory scaled hardware prototype are conducted to confirm the effectiveness of the proposed technique.

Aiming at the problem of imbalance load of feeder lines and reverse photovoltaic power flow in distribution network, the flexible interconnection scheme based on full-power converter is the key to realize intelligent control of distribution network. In this paper, a power control strategy for flexible interconnection system is proposed, which including heavy-load limiting control for the rational power distribution when some feeder lines are heavy-loaded, and power balance control for that all feeders are heavy-loaded to relieve the power supply pressure of flexible interconnection devices (FIDs). Case study based on real load data verifies that the proposed strategy can cope with different seasonal distribution network scenarios with different load characteristics and different degrees of reverse photovoltaic power flow. The proposed can suppress forward and reverse heavy load flow, and improve the power supply safety and efficiency of the distribution network.

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) systems have found rapid growth in power grids. To reduce the inrush current, it is necessary to precharge the submodule (SM) capacitors to the rated voltage before normal operation. In this paper, an optimized pre-charging strategy for MMC is proposed based on power balancing . First, the charging power is obtained by a designed closed-loop control of the SM capacitors. Then the total power released by the AC or DC side could be calculated by power conservation and feedback by controlling the charging current. At the same time, the power difference between the upper and lower bridge arm of each phase and the internal voltage balancing of bridge arms are also considered by the proposed bridge arm and SM voltage algorithms. By comparing with conventional closed-loop pre-charging strategies, the pre-charging speed is greatly enhanced as the achievement of synchronous charging of upper and lower bridge arm SMs. Moreover, as the same voltage and current double closed-loop control applied in both MMC pre-charging and normal state, the state transition time could be reduced. Finally, a series of simulations and prototype experiments are conducted to verify the feasibility of the proposed pre-charging strategy.

The crossing thyristor branches (CTBs) based hybrid modular multilevel converter (HMMC) can effectively protect high-voltage direct current system under dc-line short-circuit fault with advantages of low percentage of unipolar full-bridge (UFB) submodules (SMs), short dc current interruption time, and high efficiency. However, the faulty UFB-SMs being bypassed would affect dc fault current interruption performance of the CTB-HMMC, which may cause large capacitor voltage increment of UFB-SMs and deteriorate CTB-HMMC. This article first analyzes the capacitor voltage increment of the CTB-HMMC with bypassed UFB-SMs under dc-line short-circuit fault. And then, this article proposes a capacitor voltage increment suppression control (CVISC) for the CTB-HMMC with bypassed UFB-SMs under dc-line short-circuit fault, where the capacitor voltage increment of UFB-SMs during dc-line short-circuit fault can be significantly suppressed through bypassing suitable number of UFB-SMs according to the proportion of faulty UFB-SMs being bypassed in each arm, and therefore the proposed CVISC effectively protects the CTB-HMMC under dc-line short-circuit fault. The simulation and experiment are conducted, and their results verify the effectiveness of the proposed CVISC.

At present, the droop control strategy is widely used in modularized multilevel converter stations in multi-terminal flexible DC interconnection system, and the voltage and active power can be adjusted autonomously according to the droop characteristic curve. However, this strategy can barely handle the abnormal DC side voltage fluctuation caused by power flow reversal and outage of the converter station. Therefore, the linear model of converter station based on droop control strategy is firstly established in this paper, which shows that droop control increases the difficulty of meeting the demand of emergent conditions. On this basis, an improved droop control strategy based on auxiliary power compensation was proposed to solve the problem of DC side voltage mutation under emergent conditions. Compared with traditional droop control, this strategy does not need to change the control parameters of converter station. The reaction speed is faster, indicating stronger voltage fluctuation suppression capability.Meanwhile, the performance of the DC interconnection system to cope with sudden working conditions is optimized. Simulation results verify the effectiveness of the proposed control strategy.