Search

Abstract This paper presents various control strategies to improve operations in two interconnected areas connected by a VSC‐HVDC transmission line. The main focus is on designing a central control system (CCS) that coordinates control units in both areas. In area 1, an AC voltage control unit is connected to the CCS. In area 2, three control units including a load power control unit, a fault detection unit, and an AC voltage control unit are also connected to the CCS. The CCS receives inputs from these units and generates commands for the DC voltage and active/reactive power control units on both sides of the DC line. The first proposed strategy addresses permanent voltage drops caused by load fluctuations in area 2. It adjusts the transmitted power from area 1 based on voltage variations in area 2. The second strategy focuses on mitigating faults in area 2 by injecting active and reactive power from area 1 during such events. The third strategy resolves transient voltage oscillations in both areas by controlling the reactive power of stations on either side of the DC line. Simulations using MATLAB‐SIMULINK demonstrate that these mechanisms successfully achieve their objectives.

Abstract In the case of dispersed load consumption, Ultra‐high Voltage Direct Current (UHVDC) system in cascaded connection mode can significantly reduce the cost by building DC lines between different ends, but the communication delay caused by it brings challenges to the original control strategy. Therefore, the control strategy suitable for cascaded multi‐terminal UHVDC (CMT‐UHVDC) system is developed. In this paper, first of all, the operation characteristics, mathematical model, and adaptability of the original control strategy are discussed. Next, an autonomous master‐slave control strategy is proposed to maintain voltage balance without real‐time communication between high‐end and low‐end valves. Finally, based on the planning parameters of an actual project, the electromagnetic transient simulation model of the CMT‐UHVDC transmission system is established. Several working conditions and the practical influence of sampling deviation on the control performance are discussed and analyzed, which verifies the proposed control strategy's effectiveness.

Abstract Multi‐input converters play an important role in integrating the independent energy sources utilized in the grid‐connected system and electric vehicle (EV) applications. In this scenario, several types of multi‐input converters are presented in the literature. Most of the MICs are operated using a time‐sharing scheme. This leads to a restricted duty cycle which limits the energy sources utilization and output voltage. To overcome the aforementioned limitations, a multi‐input single‐output (MISO) boost converter is proposed. It can improve the utilization of energy sources and output voltage with a reduction in the part count. To verify the feasibility of the proposed scheme, a 200 W prototype circuit is developed; simulation and experimental results are validated.

Abstract Conventional phase‐shifting transformers (PST) have been utilized for power flow control in transmission networks for many years. Compared to the slow reacting PSTs, recently trending unified power flow controllers (UPFC) enable much faster power flow control. This paper introduces a decentralized power flow control scheme for power flow controllers (PFCs) without the need for communication between the PFCs. For each PFC, an influence area is defined. The line loading of the transmission lines in this influence area is monitored in real‐time operation and if there is an overloading of a line, a feedback control scheme is used to mitigate the overloading of the line. As the power flow control of UPFC is dependent of the network topology and the UPFC converter limit, additional control measures are introduced to circumvent potential critical scenarios. Supplementary control measures are also developed to increase the robustness of control scheme for the simultaneous operation of UPFC with slow reacting PST. Using RMS simulation in DIgSILENT PowerFactory, the developed control method is validated in the IEEE 68‐Bus NETS/NYPS test bench network. The method is also validated for UPFCs with overlapping influence areas and the combination of fast acting UPFCs and slow acting PST devices.

Abstract The classical formulation of the transmission switching problem as a mixed‐integer problem is intractable for large systems in real‐time control settings. Several heuristics have been proposed in the past to speed up the computation time, which only limits the number of switchable lines. In this paper, a real‐time switching heuristic based on neural networks that provides almost instantaneous switching actions, are presented. The findings are shown on case studies of the IEEE 118‐bus test system, and the results show that the proposed heuristic is robust to out of distribution data. Additionally, the proposed heuristic has significant computational savings while all other performance metrics like accuracy are similar to state‐of‐the‐art machine learning methods proposed for transmission switching.

As conventional power generation units are being replaced with distributed energy resources, operational practices such as voltage regulation and congestion management are expected to be challenging. To address these challenges, regular and automated intercontrol center operational coordination will be needed between transmissions and distributions system operators (TSO and DSO). In this study, the data exchange required for the near-real-time operational coordination is investigated for a reactive power management use case. A realistic ICCP/TASE 2 protocol is implemented in a laboratory environment where the physical network is simulated in a real-time simulator while optimal set-points are communicated from the control centers to simulated assets being communicated through the IEC 60870-5-104. In addition, the sufficiency of the common information model (CIM) and common grid model exchange standard (CGMES) is evaluated for TSO-DSO network data exchange. The results from the cyber-physical test setup show that the detailed DSO grid model knowledge by the TSO results in lower system losses. Such data can be difficult to prepare, exchange, and compute. However, results show that simplified equivalent models can be acceptable if they are properly tailored to the specific use case. The experiences from the CIM implementation with the CGMES profile are found out to be sufficient for such operational data exchange.

Abstract The automated identification and localisation of grid disturbances is a major research area and key technology for the monitoring and control of future power systems. Current recognition systems rely on sufficient training data and are very error‐prone to disturbance events, which are unseen during training. This study introduces a robust Siamese recurrent neural network using attention‐based embedding functions to simultaneously identify and locate disturbances from synchrophasor data. Additionally, a novel double‐sigmoid classifier is introduced for reliable differentiation between known and unknown disturbance types and locations. Different models are evaluated within an open‐set classification problem for a generic power transmission system considering different unknown disturbance events. A detailed analysis of the results is provided and classification results are compared with a state‐of‐the‐art open‐set classifier.

A unified power flow controller (UPFC) is a versatile compensator providing multi-variable control for the critical power systems (CPS). In an attempt to reduce the search space for optimal allocation of UPFC thereby standardizing the deliberation over UPFC placement, zone-wise segregation of the network into weak and healthy zones is logically executed by an advanced network power stability indicator (NPSI). The UPFC performance is ascertained by a new line flow margin index (LFMI), power loss index and its economic benefit. The latter is derived by the cash outflow reduction (COR) and the internal rate of return (IRR), calculated by practical aspects like installation and maintenance cost, considering inflation and interest on amortization. For optimization, an improved version of the wind-driven optimization algorithm is proposed with finite-mass air parcels and intra-particle forces. An improved normalization technique is adopted for the multi-ranged, multi-objective problem. To simulate the CPS, the 57-bus test system with enhanced arbitrary load is considered. A significant reduction in power losses is observed when optimizing in reduced search space and the optimal location of G-UPFC is found to be in bus 13–15, which is also a part of the weaker zone. Several other results confirmatively approve the proposed approach of isolating the most appropriate zone for UPFC allocation based on the healthy and weak regions along with the proposed techno-economical evaluators.

Short circuit fault occurrence in high-voltage DC (HVDC) systems causes extremely high currents in a fast raising time that does not experience current zero-crossing. To protect HVDC systems/grids against fault current, fast HVDC breaker is an essential equipment. This study presents the design procedure of a novel HVDC breaker based on solid-state controllable reactor which is able to reduce the fault current's rate of rise and fault current amplitude to less than grid nominal current in the breaking process. The main achievement of the proposed HVDC breaker is that not only breaker does not encounter fault current, but also none of the series HVDC equipment is influenced by the fault. The designed breaker performance is studied by PSCAD/EMTPS, and then the simulation results are validated by the developed laboratory experimental setup.

Recently, there has been significant interest in developing the modular multilevel converter (MMC) based DC grids in the power industry. However, the immature DC circuit breaker (DCCB) technology hampers the development of DC grids in overhead lines transmission field. The full controlled power electronic devices based hybrid DCCB has been proposed and researched substantially. But the hybrid type DCCB suffers from the drawback of high cost and low reliability, leading to high capital cost for DC grids that require numerous DCCBs. In this paper, a mechanical type DCCB, consisting of a mechanical switch, a circuit and an oscillation circuit, is proposed. Then, a four-terminal bipolar MMC-HVDC system implementing the mechanical DCCBs is studied. Extensive simulations have been conducted to analyse the transient performance under different fault conditions. The simulation results verify the technical feasibility of this mechanical DCCB in DC grid.

To integrate bulk wind power, using modular multilevel converter (MMC) based DC grids is an effective solution to improve the flexibility and reliability of the power grid. This paper analyses a bipolar four-terminal MMC based DC grid to integrate wind farms with overhead lines. Since the overhead lines are prone to DC faults, the hybrid MMC is adopted in the system to ride through DC faults. The fault ride-through control strategies are designed, which remain continuous operation of wind turbines under DC faults. Besides, an adaptive dissipation control strategy associated with chopper resistances is designed to absorb surplus energy. Finally, simulations based on PSCAD/EMTDC platform verify the fault ride through control of hybrid MMC based DC grid and proves the effectiveness of the energy dissipation strategies.

Existing mechanical HVDC circuit breakers (CB) are capable of interrupting HVDC currents within several tens of milliseconds, but this is too slow to satisfy the requirements of a reliable HVDC grid. HVDC CB based on semiconductors can easily overcome the limitations of operating speed, but need large number of the series (parallel) connection of electronic switching devices. To overcome these shortcomings, this paper introduces a topology of hybrid CB; this paper first describes the principle of hybrid CB; then a control strategy of fault pre-treatment is also proposed, that is, the ‘pre-action and action (or recovery)’ of the switches, in order to improve the breaking speed of the hybrid CB and the equalising circuit of its solid-state switch has been designed. The proposed topology and control strategy do not produce loss in the normal operation of CB, and instantaneous trigger solid-state switch with quickly cutting short-circuit current during short-circuit faults; at last the simulation and experiment of the hybrid HVDC CB have also been done. The results turn out when a short circuit occurs, the hybrid HVDC CB can quickly break the short-circuit current; the buffer circuit can be provided for the release of the stored energy circuit.

The dynamic performance of voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) grid for large-scale renewable energy integration is becoming a concern. This study proposes a system-level dynamic response model of wind farm (WF)-MTDC-main AC systems, and then analyses the dynamic performance based on the singular value decomposition (SVD) technique. In the modelling, an improved virtual synchronous machine control is developed, and the interaction between AC frequency and DC voltage can be readily described. Using the SVD technique, parameters of the controllers are tuned, thereby making the AC frequency and DC voltage deviations within the limitations. Some oscillation modes of the system are observed and virtual power system stabiliser is proposed to suppress the oscillations. Additionally, the oscillation can be mitigated by emulating capacitance-based inertia response. The efficiency of the proposed model and analysis is verified through the frequency-domain and time-domain results.

The fractional frequency transmission system (FFTS) can multiply increase transmission capacity by using a lower frequency to reduce the reactance of the AC transmission system. The modular multi-level matric converter (M^3C) as the new generation of the AC/AC converter is the critical equipment of FFTS. The self-start control strategy for M^3C-based FFTS is proposed and the whole start-up procedure is divided into the uncontrollable stage and controllable stage. The model of the equivalent circuit in an uncontrollable stage is built and the mathematical relation between the maximum charging current and the current-limit resistor is obtained. Finally, a digital simulation system of M^3C-based FFTS is established, and the effectiveness of the proposed control strategies is verified by simulation results.

With the increase of DC project voltage level and transmission capacity, the security and stability risk caused by DC blocking is gradually increasing. Aiming at reserve dispatch after ultra-high voltage direct current (UHVDC) block, scenarios of DC power modulation participating in dispatching are presented. Mathematical models of generator re-dispatching incorporated control of DC in AC/DC hybrid grid in three situations in terms of enough reserve capacity in the disturbed province, regional reserve capacity sufficient and insufficient to make up for the power deficiency are presented. Then, a decision-aid system for optimal reserve dispatch after UHVDC block is proposed. Simulation results of Central China power grid indicate that the proposed scheduling scheme can reduce the operation risks of security and load loss if necessary.

When the ultra-high-voltage direct current (UHVDC) transmission system operates in a monopolar ground return mode, an enormous DC current flows into earth via the DC grounding electrode in the converter station and then comes into the AC system around the converter station through the neutral earthed transformers, which will cause DC bias in AC systems and harm the transformers' safe operation. In this study, the DC current distribution in the AC systems around the Linyi converter station of the Shanghaimiao-to-Shandong UHVDC Transmission Project was studied. Based on the direct field-circuit coupled method, the DC bias calculation model was established in CDEGS. By using the model, the DC potential in the earth and the DC current distribution in the AC systems were calculated. Then, the DC current distribution in the AC systems was calculated and analysed after adopting a blocking capacitor and a resistor limiting method in a comprehensive way, and the effect of this controlling method was evaluated further. The research results have some practical engineering guiding significance for solving the DC bias problem of AC systems around the UHVDC grounding electrode.

Flexible AC transmission systems have shown effective functionalities in promoting the system operation security and service reliability. Facing with series-compensated lines, subsynchronous resonance (SSR) may jeopardise the stability and mechanical facilities. The control strategy and simulation model of a unified power flow controller (UPFC) based on the dq decoupling control are deduced and constructed. Considering the impacts of grid-connected wind farms, SSR characteristics of the series-compensated system with UPFC are studied by the complex torque coefficient method and time-domain simulation. The results verify the capability of UPFC in attenuating SSR in wind farm integrations by utilising subsynchronous damping controllers coordinated with UPFC's main controllers. It also suggests that the shunt transformer−converter has a slight effect on SSR, whereas the series transformer−converter affects the system resonance characteristics significantly. Simulations are carried out on a typical power system model integrated with wind turbines.

Synchronous generation is being displaced by renewable energy sources and high-voltage direct current (HVDC) interconnectors. These factors will lead to a diminishing provision of ancillary services. Conventional services tendered to provide network support rely on the availability of synchronous generation and because of this, future power system operators will have to develop new ways to obtain the ancillary services required. A technology rapidly gaining in popularity is voltage source converter (VSC)-based HVDC and further advances in the technology have led to the modular multilevel converter (MMC). Due to the design of an MMC there is intrinsic electrostatic energy that could potentially be utilised to provide ancillary services. This study determines the electrostatic energy available from a typical grid-scale MMC VSC for network support during disturbances and for power oscillation damping. Supporting simulations are provided to ascertain the impact that this electrostatic energy could have on network performance if utilised. This study displays that although an MMC possesses intrinsic energy, utilising it for frequency response would provide negligible system benefits. However, for POD, the available intrinsic MMC energy did improve system stability which leads to the potential of using multiple MMCs to increase the energy and further improve system stability.

In many future power systems, the power generation may be predominantly supplied from converter interfaced sources (i.e. wind, solar, HVDC interconnection). In the context of the UK, the government is aiming for a share of wind capacity in the total generation mix as high as 29 GW by 2020. For a small power system like Great Britain (GB) with no synchronous connection to continental Europe, frequency stability is identified as a major challenge. Therefore, additional primary response and inertia (or fast frequency containment response) are anticipated as being required to sustain the frequency stability of the GB system. Here, techno-economic assessment is carried out to explore the overall benefits of various frequency control strategies. Frequency regulation by VSC-HVDC system is realised as the best strategy regarding overall system benefits.

Nowadays, the hybrid high-voltage direct current (HVDC) system consisting of line commutated converter (LCC) and modular multilevel converter (MMC) has become the most competitive candidate in multi-infeed power systems. Owing to the limitation of insulated-gate bipolar transistor's withstand voltage, the DC voltage rating of MMC is not compatible with the LCC valve. To increase the DC voltage level, the number of sub-modules of each arm should be increased drastically, leading to many technical problems in communication design, optical fibre instalment, cooler system design and so on. To overcome the above problems, a hybrid HVDC system consisting of LCC and cascaded hybrid MMC valves is proposed to realise ultra-HVDC transmission. It can switch two operating modes (single valve/double valve) according to the conditions so that the system can maintain a certain power transmission when one of the valves quit operation during faults or maintenances. Meanwhile, by implementing the hybrid MMC, the system can ride through DC faults. The design of the topology and controllers, the DC fault ride-through strategy and the online switching strategy for MMCs will be presented in this study. Finally, the performance during DC faults and online switching is verified by extensive simulations.

Transmission congestion threatens the reliability and economy of power systems. It becomes more severe with the increase of load demands and peak−valley differences. In recent years, energy internet zones (EIZs) have attracted wide attention, and many demonstration projects have been developed. EIZs connect the supply and consumption of multi-energy like electricity, gas, heat, and cooling. Thus, EIZs’ electricity load demands can be flexibly adjusted by controlling their multi-energy flows. The controlling method should consider not only demand response but also heat, cooling, and gas energy balance. The integration of EIZs provides a new solution for transmission congestion management (TCM). First, an EIZ's optimal controlling strategy considering its multi-energy flow requirements was proposed. Then, the optimal re-dispatch problem for TCM was studied, taking into account EIZs' and traditional generators' adjustment costs. Re-dispatch determines the output adjustment schedules and compensation prices, while the EIZ's controlling strategy controls multi-energies' supply and consumption. A simplified Songjiang transmission system in Shanghai was adopted to illustrate the proposed methods. It was proven that the EIZs' integration could reduce the re-dispatch costs in TCM and increase EIZs' profits.

With the maturity and wide application of the voltage-source converter-based high-voltage direct current (VSC-HVDC) technology in the power grid, the fast-response characteristic of VSC helps to improve the speed of corrective control for the AC/DC hybrid system. This paper presents a corrective control model for AC/DC hybrid grids based on VSC-HVDC. The model is expressed as a typical optimisation model with both the features of the equipment and security constraints of a power network. The corrective control model is dived into an active power sub-model and a reactive power sub-model in order to improve the calculation speed. Case studies of a modified IEEE testing system have been carried out. The results demonstrate the effectiveness of the approach.

This paper proposes an additional reactive power control strategy and an emergency control strategy for ±1100 kV ultra-high voltage direct current (UHVDC) system with a hierarchical connection mode, in which the inverter station connect to two AC systems with different voltage levels. Firstly, the relationship between reactive power and DC voltage is analysed. Then, an additional reactive power control strategy is proposed to solve the voltage fluctuation caused by reactive power disturbance. Next, the reactive power emergency control strategy is proposed when a short-circuit fault occurs. Reactive power compensation devices will be inputted into the fault-free AC system to provide reactive power support through coupling relationship between two AC systems in the inverter side. Simulation results show that the additional reactive power control strategy can suppress reactive power disturbances without affecting the active power. Moreover, the reactive power emergency control strategy can reduce the probability of commutation failure in converters connected to the fault AC system while a short-circuit fault occurs.

In order to solve the problem of long-distance transmission of large-scale photovoltaic power stations, a multi-terminal voltage source converter-based high-voltage direct current (VSC–HVDC) system based on a DC voltage-active power-AC voltage control strategy, which is connected with photovoltaic power stations, is designed in this study. In order to verify the feasibility of the method, a model of a five-terminal parallel system is established by MATLAB/Simulink, which includes two photovoltaic power stations and a passive network. A series of simulations are carried out based on the model. The results show that the active power of the system can be automatically balanced when the output power of photovoltaic power stations changes. When the three-phase short-circuit fault occurs in power grid, the photovoltaic power stations can still run in a stable state and have better fault ride-through ability.

The separating pole connection (SPC) mode of ultra-high-voltage direct current (UHVDC) can significantly improve the power stability, but when the AC bus of one pole is failed and two receiving systems are near, it may cause subsequent commutation failure. In order to restrain subsequent commutation failure and accelerate system's recovery, an adaptive voltage-dependent current order limiter (VDCOL) control strategy based on granular computing is proposed. According to this VDCOL control strategy, VDCOL, which uses fuzzy rules to separate and granulate a different voltage level, can adjust DC current dynamically in a variety of environments. Also, some modifications on VDCOL characteristics are applied to reduce reactive power consumption during low-voltage situations. Then based on the MATLAB simulation platform, the simulation systems for 800 kV the UHVDC SPC system are built. Finally, the simulation example of the SPC mode with the adaptive VDCOL control strategy showed that this VDCOL control strategy can reduce the risk of subsequent commutation failure and improve the stability of the AC/DC interconnection system.

This paper proposes a multi-hierarchy control strategy in abc coordinate for modular multilevel matrix converter (M^3C) in fractional frequency transmission system (FFTS). The proposed control strategy avoids complicated coordinate transformations when the traditional control strategies based on coordinate transformation are adopted. Four hierarchies are designed to balance the capacitor voltages. In different hierarchies, the capacitor voltage ripples have different frequencies. According to this characteristic, different bandwidths of filters are adopted in different hierarchies, resulting in different control speeds among the four hierarchies. This will not only speed up the control but also assure the control accuracy. Simulation results verify the feasibility and effectiveness of the proposed strategy.

As electricity load in large cities overgrows these years, mainly due to the increasing penetration of electric vehicles, the urban transmission networks are subject to the high risk of congestion because of insufficient planning. Reconfigure the high-voltage distribution network (HVDN) is an efficient way to mitigate the urban congestion. However, the discrete load transfer amount is not precise that might cause load curtailment, and the computation burden is also heavy when the system scale is enormous. In recent years, the emergence of voltage source converter (VSC) as the core components of DC transmission technique provides a new control measurement featuring fast and flexible. Nevertheless, its adjusting capacity is limited and subject to the system topological structure. Thus this paper proposed a coordinative control model that combines the load transfer capability of the HVDN and flexible and precise control of VSC to alleviate the urban transmission congestion. Numerical results showed that the congestion lines could be efficiently relieved thus defer further investment in the transmission network.

Due to the increasing renewable penetration, there is potential for larger and faster grid frequency fluctuations increasing the risk of system instability. The National Grid Electricity Transmission, primary electricity transmission network operator in the UK, has introduced various frequency response services that are developed to provide a real-time response to deviations in the grid frequency. A battery energy storage system is a suitable choice for delivering such services. Here, a control algorithm is presented which generates a charge/discharge power output with respect to deviations in the grid frequency and the required specifications. Using the real UK electricity prices, an arbitrage control algorithm has been also developed to deliver different types of grid balancing services, while scheduling throughout the day for energy arbitrage. Simulation results show that the proposed algorithm delivers both dynamic and non-dynamic firm frequency response and also enhanced frequency response to specifications, while generating arbitrage revenue in the balancing market. Simulation results on a 1 MW/1 MWh lithium-titanate BESS are provided to verify the proposed algorithm based on the control of an experimentally validated battery model.

In recent years, large-capacity high-voltage direct currents (HVDCs) have been employed to transfer electricity from the west to the east of China. However, the frequent frequency drops due to HVDC blockings seriously threaten the short-term frequency stability of receiving-end power systems. The existing emergency frequency control strategy is not adaptive to the varying operation conditions and may cause excessive or deficient actions, which could result in inefficient control or high risk of frequency instability. Therefore, a real-time coordinated control strategy is proposed in this study based on the online-updated frequency response model. The model is designed to incorporate multiple types of generators and to reflect the dynamic frequency response of loads. The new control strategy combines multiple control resources, including the emergency demand response and HVDC power modulation, to improve the short-term frequency stability after HVDC failures. By online data preparation and cubic fitting, the nadir of the frequency is expressed as an analytic function of the control variables. A real-time optimisation of emergency controls is achieved to improve the short-term frequency dynamics. Case studies show that the proposed scheme is robust to the varying operation conditions and has lower control cost than the existing control strategy.

This work studies the application of short-time Fourier transform (STFT) to extract current harmonics of various fault types to design a protection method for meshed multi-terminal voltage source converter-high-voltage direct current (VSC-HVDC-MTDC) grids. The frequency spectrum of fault current harmonics is used to detect internal and external faults, the faulty pole and fault type. The method does not need the implementation of series inductance on the sides of the DC lines and it is also effective for lines combining overhead lines and cables. The window function and hop size play the primary role in STFT, which are investigated in a sensitivity analysis. A modified version of CIGRE meshed DC grid system is simulated in PSCAD in order to show the robustness of the method, and further signal processing analysis is done in MATLAB. The results show the accurate detection and discrimination between faulty sections using the proposed harmonic-based method and how important the STFT parameters are in order to have a robust protection algorithm.

In this study, direct current (DC) voltage margin control strategy and droop control strategy of voltage source converter multi-terminal direct current (VSC-MTDC) are introduced firstly, and according to the characteristics of two control strategies, an adaptive droop control strategy of DC voltage margin is proposed. When the power fluctuation of DC grid is relatively smaller, some of converter stations which connect to the power generations systems are involved in power coordination to balance the power margin of the entire DC grid. When the power of DC grid suddenly changes and the power fluctuation is relatively larger, the converter stations that adopt the adaptive droop control strategy adjust power and can avoid some of them to lose the adjust ability due to full load and power huge fluctuation as far as possible. Finally, the simulation based on MATLAB validates the effectiveness of the strategy.

In this study, the integration of multi-terminal high voltage direct current (HVDC) systems to weak power grids is analysed under different grid strengths and control methods. In this study, a modular multi-level converter based MTDC network is considered. The analysis showed the importance of voltage control of DC network. The voltage control of DC network needs to be performed from the converter station connected to the strongest AC network. Furthermore, the analysis shows that either droop or proportional-integral control can be used for controlling the DC voltage of the MTDC network connected to one or more weak grids. However, the droop control of DC voltage provides faster response compared to PI with larger steady state error. Finally, the analysis showed that the modular multi-level converter (MMC)-based MTDC is capable of ride-through unbalanced AC side faults even if the faults occur in a weak grid.

The modular multilevel converter (MMC) has the advantages of high efficiency, low harmonic, modular design, and easy cascade, which has been widely used in the field of high voltage and large capacity energy conversion. In order to improve the dynamic response speed of the MMC-HVDC based on the modular multi-level converter, a novel method which combines a model predictive control (MPC) of MMC-HVDC system with improved sub-module voltage balanced control strategy is proposed. The method, which utilises the prediction model, feedback correction and rolling optimisation to obtain the optimal voltage control, overcomes the difficulties in the traditional way of setting PI parameters of the internal loop current controller and the outer loop controller and tackles the problem of low dynamic response. Finally, a 21-level MMC-HVDC system simulation model is built on PSCAD-EMTDC software platform. The simulation results indicate the effectiveness and feasibility of the control strategy.

Commutation failure is an important factor on safe and stable operation of the receiving-end power grid for high voltage direct current (HVDC) project. A commutation failure event in Jinsu DC transmission project caused by a complex AC failure was taken as background, process of fault and action sequences of protection was introduced, and the reason of the commutation failure and protection action was analysed from principle. Real-time digital simulation (RTDS) was connected to the true-type device of ultra-high voltage direct current (UHVDC) control and protection to form a closed-loop simulation platform. The commutation failure event was inversed, and the simulation result was compared with the actual recorded wave. The results show that abnormal AC voltage leads to the commutation failure.

There may be a commutation failure during a fault or start-up of the UHVDC inverter side. The law of commutation failure process in UHVDC converter is studied here. First, the cause of inverter commutation failures is briefly introduced. Then, the general law for the bridge arm conduction of the inverter in the process of the commutation failure is revealed. Finally, the system debugging recorded data of some commutation failures of Taizhou UHVDC inverter in China is used to verify the law. Research shows that the commutation failure will go through three stages which has obvious features: (i) the commutation failure stage, (ii) the bypass circuit formation stage, and (iii) the commutation failure recovery stage. In the last stage, the AC currents of two conducted phases increase instantly in opposite directions, which are easy to recognise. In the second stage, only the DC current increases, and the AC currents equal 0 approximately. Because of the three stages, the commutation failure valve number is easily recognised. The study on the law of commutation failure process lay the foundation for commutation failure process analysis and identification for relay protection.

Owing to the fast control response and distinct characteristics of the voltage source converter (VSC), various types of oscillation incidents related to the VSC-HVDC systems have been observed around the world. However, the causes of such oscillations are complex due to the interactions among the components of the transmission system and the various control methods of the converter station, and yet not systematically comprehended and summarised. Therefore, this study reviews several typical VSC-HVDC oscillation incidents and focuses on the root causes of the oscillation phenomenon. The analysis indicates that the oscillations in the VSC-HVDC system can occur either in subsynchronous frequency or in high-frequency range, up to 1.2 kHz. The modelling methods, including the conventional detailed model, equivalent detailed model, and average equivalent model, are also affecting the accuracy of describing the oscillations. The control methods, including adding active control scheme, passive filter at oscillation frequency, and temporary power plant disconnections, are also discussed. The advantages and drawbacks of these modelling methods, analysis methods, and control methods are summarised based on the various system conditions. In the end, the future research perspectives and key challenges are provided for further research references.

This study presents the comparison among three control systems: Cigre Benchmark model, ABB model, and SIMENS model based on PSCAD. Firstly, the control systems which refer to Cigre Benchmark control model, ABB control model, and SIMENS control model are introduced, respectively. Analysis and comparison are done on these control systems about their design theories, control logic, and functional models, and the advantages and disadvantages of each control system are concluded. Secondly, simulations in both normal and fault conditions are done in order to verify the conclusions. On the one hand, the steady-state response characteristics of these three high-voltage direct current (HVDC) models are obtained and compared by setting different voltage of the receiving terminal AC bus. On the other hand, the transient response characteristics of these three HVDC models under different operating conditions are obtained, such as symmetrical and asymmetrical short-circuit fault, line switching on or off etc. The research results will provide experiences and references for the future optimisation of the HVDC control system.

The main difference between flexible high-voltage direct current (HVDC) system and the traditional HVDC system is that there are two control degrees of freedom in the flexible HVDC system that is the amplitude and phase angle of the output voltage. While, there is only one control degrees of freedom in the traditional HVDC system which is the amplitude of the output voltage. Therefore, compared with the traditional HVDC system, there will not be the problem of commutation failure in the flexible HVDC system, and the control systems of the rectifier side and the inverter side being operated independently make the reliability of the system greatly improved. This article analyses and compares the main methods of reliability assessment. Analysing and comparing the reliability parameters of the two systems by using the state enumeration method, and using the economic characterisation to make the comparison more intuitive.

This paper takes a close research on the impact of voltage reduction in a single phase as well as its occurring moments on commutation failures (CFs). When there is a sudden voltage reduction in a single phase, the change rule of post-fault magnitudes and phases of commutation voltages is researched theoretically. On the basis of the change characteristic, valves that are most probable to suffer from CFs are carefully elaborated. Moreover, a method for calculating critical voltage of a single phase considering different fault moments is developed. As long as the phase voltage is lower than the critical voltage, it may result in a CF. In addition, the change regularity of critical voltage along with different fault moments and phases is given. All the analysis and theoretical calculation results are validated by simulation in PSCAD/EMTDC.

With the increasing of direct current (DC) transmission capacity, high-voltage DC (HVDC) location has become more and more intensive, and commutation failure has become increasingly prominent in multi-infeed DC (MIDC） transmission systems. To reduce the probability of commutation failure in MIDC systems, this study first analyses the fundamental causes of simultaneous or successive commutation failure, and puts forward the coordinated control strategy of voltage compensation variable slope voltage dependent current order limit, which takes the advantage of the DC current characteristics while fault occurs, and then proposes the influence factors and determination method of compensation coefficient K. Meanwhile add the switch function of K to compensate voltage according to the severity of the fault. Finally, built the actual system using PSCAD/EMTDC, simulation results show that the control strategy can effectively reduce the probability of commutation failure in MIDC transmission systems, and makes all DC systems recovery orderly after fault, meanwhile ensure the good recovery performance which can greatly improve the safety and reliability in MIDC transmission systems.

DC voltage control is one of the key technologies of DC power grid operating control. Here, the local voltage measurement is replaced by the pilot voltage in traditional droop control, and the droop control based on the pilot voltage is applied to the DC power grid. When the power disturbance appears in the DC power grid, the coordination of multiple converter stations can be better realised and the voltage fluctuation of the DC power grid can be reduced. In addition, this paper also applies the concept of pilot voltage to the analysis and control of interconnected DC power grid, and analyses important significance of pilot voltage to DC power grid. The transmission of the power in DC interconnected system can be guided by the change of the pilot voltage. The simulation results verify the effectiveness of the proposed method.

Based on the hybrid HVDC transmission system, which is composed by line-commutated converter (LCC) in parallel with voltage source converter (VSC), a coordinated reactive power control strategy of hybrid-HVDC in both steady and transient states is proposed. The operation characteristics in situations like low load operation, filter switching, and de-locking process are analysed. Influence of the method to AC bus voltage and system power factor is taken into account. Three kinds of reactive control methods of VSC and LCC are studied, and the logical relationship between the filter and VSC reactive power controller is also given. Considering both the system voltage drop and the risk of commutation failure caused by AC faults, the VSC in voltage control mode is utilised to maintain AC bus voltage stability by providing emergency power support. Simulation results show the feasibility and effectiveness of presented control strategy, then the proposed strategy is applied in practical DC project, whose results achieve the desired effect by engineering field test.

Nowadays, the voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission technology is widely adopted for the integration of renewable power generations. However, the grid-connected VSC systems may also cause power system instability issues. This study focuses on the parameter stability region of VSC control system in VSC-HVDC transmission systems. The small-signal linearised model of the whole power system is firstly established and on the basis of which the modal analysis method is applied to obtain the parameter stability region of VSC control system. The results show that the region is mainly affected by both the selection of the initial operating point of VSC and the dynamic interaction between the AC system and the VSC. From these two perspectives, the parameter stability regions of the VSC control system under different operating conditions are investigated. Meanwhile, it is found in the study that when modal coupling occurs between the oscillation modes corresponding to AC system and VSC, the parameter stability regions of the VSC control system obtained in the previous analysis will be deteriorated. Consequently, in terms of stability region, an evaluated method for VSC control system is proposed and validated through case study.

The feasibility study of unified power flow controller (UPFC) application in 220 kV Shanghai Grid is conducted based on the analysis of the practical operation of the grid. The location selection method and scheme are proposed for the application of 220 kV UPFC demonstration project. The application effect of the demonstration project of UPFC is evaluated from various perspectives including enhancement of interface transfer capability, improvement of voltage stability performance of the grid and contribution to balancing of reactive power in 220 kV sectionalised systems.

Integration of renewable energy generations requires the transmission of bulky power over long distance, and high-voltage direct current (HVDC) transmission systems become a more preferable choice compared to conventional HVAC systems. For HVDC systems, one of the important concerns is the DC protection strategy which can significantly impact on the connected AC system performance, e.g. system frequency. The maximum loss-of-infeed for a AC network is highly dependent on the duration of the power outage, and the impacts of DC fault protection arrangements which result in different speed of power restoration on the connected AC system, on the system frequency, have not been properly understood. Different DC protection arrangements using DC disconnectors, fast and slow DC circuit breakers on frequency response of the connected AC networks are investigated. A three-terminal meshed HVDC system is studied to demonstrate system behaviour during DC faults.

The analysis and control design of multi-terminal VSC-HVDC systems requires a linearised model of these inherently non-linear systems. This paper develops such a model, from converter to multi-terminal system, discusses the key concepts and shows its use for some test cases. The linearised model is simulated in Matlab, and it is compared against the non-linear model results from an established DEM PSCAD model.

The interaction between renewable energy generation and high-voltage direct current (HVDC) transmission system becomes an essential cause of the subsynchronous oscillation (SSO) problem. In this study, the permanent magnet synchronous generator-based wind farm connected to HVDC transmission system is studied. The susceptivity analysis is carried out to find the main influence factors of the SSO problem. Besides, a parameter design method for subsynchronous damping controller is proposed. The configuration and the performance indicator are discussed, and then the optimisation algorithm is developed based on hybrid genetic algorithm and electromagnetic transient simulation. Finally, the proposed design method is validated by simulation in PSCAD/EMTDC.

Reducing the power loss and handling short-circuit faults in HVDC systems are crucial for economical and reliable operation of modular multilevel converters (MMC). This paper proposes a loss reduction method for AC voltage-boosted FBSM-MMC by controlling the second harmonic component in the circulating current. The relationship between converter loss and second harmonic circulating current (SHCC) is first derived for AC voltage-boosted FBSM-MMC. From the analysis, it can be seen that the converter loss can be reduced by injecting the optimal SHCC into the arm current. The topology comparison between traditional FBSM-MMC and AC voltage-boosted FBSM-MMC is carried out to show the benefit of this method. Simulation results using PSCAD/EMTDC confirm the feasibility and validity of the proposed loss reduction method for AC voltage-boosted FBSM-MMC.

With the increasing demand for power supply reliability and the transmission equipment is an important part of power generation, transmission and distribution engineering. In the study, which makes use of the advantages and the mechanism of responsibility area of D5000 system platform, in combination with other related applications, the system architecture, software architecture, and related main functions are designed for on-line monitoring application, and realised the comprehensive analysis of the on-line monitoring and collecting data of the transmission and transformation equipment, to obtain the characteristic parameters of the reaction state of the equipment, equipment fault diagnosis, analysis and judge the running state of the equipment, to fill the domestic electrical equipment condition monitoring and analysis system of electrical equipment and States unified data processing, analysis and judgment of technology gaps. The active maintenance system based on multi-dimension data analysis and fault prediction is formed from the state of equipment and referring to the running history of equipment state.

Currently, the flexible direct current transmission technique becomes a research focus around the world, and be applied in power engineering projects gradually. However, due to the topology of the modular multi-level converter (MMC), a rapidly rising fault current will appear in case of the fault on the DC side, which constraints the development of MMC-based flexible high-voltage direct current (MMC–HVDC) system. To restraint the fault current, this paper proposes a novel fault current-limiting scheme for symmetrical bipolar MMC–HVDC system, which consists of a current-limiting resistance and dozens of serial controllable power electronics in parallel with the current-limiting resistance. The current-limiting resistance is designed to reduce the fault current and the requirement for the direct current circuit breaker (DCCB) capacity. The novel current-limiting scheme configuration and operation principles are analysed. Finally, a symmetric bipolar MMC–HVDC system model is constructed, and simulation results show the feasibility of the proposed fault current-limiting scheme.