R&D and application of voltage sourced converter based high voltage direct current engineering technology in China

As a new generation of direct current(DC)transmission technology,voltage sourced converter(VSC)based high voltage direct current(HVDC)has been widely developed and applied all over the world.China has also carried out a deep technical research and engineering application in this area,and at present,it has been stepped into a fast growing period.This paper gives a general review over China’s VSC based HVDC in terms of engineering technology,application and future development.It comprehensively analyzes the technical difficulties and future development orientation on the aspects of the main configurations of VSC based HVDC system,topological structures of converters,control and protection technologies,flexible DC cables,converter valve tests,etc.It introduces the applicable fields and current status of China’s VSC based HVDC projects,and analyzes the application trends of VSC based HVDC projects both in China and all over the world according to the development characteristics and demands of future power grids.

Damping controller design based on FO-PID-EMA in VSC HVDC system to improve stability of hybrid power system

Wind energy sources have different structures and functions from conventional power plants in the power system.These resources can affect the exchange of active and reactive power of the network.Therefore,power system stability will be affected by the performance of wind power plants,especially in the event of a fault.In this paper,the improvement of the dynamic stability in power system equipped by wind farm is examined through the supplementary controller design in the high voltage direct current(HVDC)based on voltage source converter(VSC)transmission system.In this regard,impacts of the VSC HVDC system and wind farm on the improvement of system stability are considered.Also,an algorithm based on controllability(observability)concept is proposed to select most appropriate and effective coupling between inputs-outputs(IO)signals of system in different work conditions.The selected coupling is used to apply damping controller signal.Finally,a fractional order PID controller(FO-PID)based on exchange market algorithm(EMA)is designed as damping controller.The analysis of the results shows that the wind farm does not directly contribute to the improvement of the dynamic stability of power system.However,it can increase the controllability of the oscillatory mode and improve the performance of the supplementary controller.

A Similarity Comparison Based Pilot Protection Scheme for VSC-HVDC Grids Considering Fault Current Limiting Strategy

In the voltage source converter based high-voltage direct current(VSC-HVDC)grids,fast and reliable protections are the key technologies.The traditional protection schemes are easily affected by fault resistance,line distributed capacitance,etc.Meanwhile,the influence of fault current limiting strategy(FCLS)has not been fully considered.In this paper,the fault characteristics under FCLS and the feasibility of traditional travelling wave protections are analyzed.To improve the reliability and sensibility,a similarity comparison based pilot protection scheme is proposed,which focuses on the relationship between the fault characteristics and the state of the protected transmission line,with the establishment of a precise frequencydependent transmission line model.The criteria based on the similarity comparison calculated by cross-wavelet can identify the fault effectively.Meanwhile,the protection scheme can also endure the influence of error synchronization.Finally,the protection performance is verified in the PSCAD/EMTDC under different fault conditions.

Reclosing Current Limiting for DC Line Faults in VSC-HVDC Systems

The problem of reclosing current limiting in voltage source converter based high-voltage direct current(VSCHVDC)systems is becoming more and more serious.A soft reclosing scheme for DC permanent faults is presented in this paper.Because the converter voltages of stations at both terminals of the disconnected faulty line may be different,the choice of which terminal to reclose first will affect the reclosing overcurrent.A method for selecting the terminal to reclose first is investigated to achieve a minimum peak overcurrent during the reclosing process.In order to ensure that the hybrid DC circuit breaker(HDCCB)adapts to the needs of the reclosing process better,the traditional HDCCB is improved by adding a soft reclosing module(SRM).The energy dissipated in the arresters is significantly reduced when using the improved HDCCB.The improved HDCCB will be able to reclose multiple times safely and thus increase the possibility of successful reclosing.Moreover,the recovery time after the HDCCB is successfully reclosed is very short with the improved HDCCB and its control principles.Simulation results show that this proposed scheme is capable of limiting the reclosing overcurrent when the fault still exists.

Frequency Regulation of VSC-MTDC System with Offshore Wind Farms

Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current(VSC-MTDC)system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system.However,it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system.Moreover,it is difficult for the frequency control of the wind farms to manage changes in wind speed;and the risk of wind-turbine stalls is high.Thus,based on the kinetic energy of wind turbines and the power margin of the converters,the frequency-regulation capability of wind turbines is evaluated,and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance.A power adaptation technique at variable wind speeds is developed;the active power in the frequency-support stage and restoration stage is switched according to the wind speed.A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms.The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.

A Comprehensive Power Flow Approach for Multi-terminal VSC-HVDC System Considering Cross-regional Primary Frequency Responses

For the planning,operation and control of multiterminal voltage source converter(VSC)based high-voltage direct current(HVDC)(VSC-MTDC)systems,an accurate power flow formulation is a key starting point.Conventional power flow formulations assume the constant frequencies for all asynchronous AC systems.Therefore,a new feature about the complex coupling relations between AC frequencies,DC voltages and the exchanged power via VSC stations cannot be characterized if VSC-MTDC systems are required to provide cross-regional frequency responses.To address this issue,this paper proposes a comprehensive frequency-dependent power flow formulation.The proposed approach takes the frequencies of asynchronous AC systems as explicit variables,and investigates the novel bus models of the interlinking buses of VSC stations.The proposed approach accommodates different operation modes and frequency droop strategies of VSC stations,and considers the power losses of VSC stations.The effectiveness and generality of the developed approach are validated by a 6-terminal VSC-HVDC test system.The test system presents the characteristics of the coexistence of numerous VSC operation modes,the absence of slack buses in both AC and DC subsystems,and diversified grid configurations such as point-to-point integration of renewable energy sources and one AC system integrated with multiple VSC stations.

A sensorless and simple controller for VSC based HVDC systems

Voltage source converter high-voltage direct current （VSC-HVDC） is a new power transmission technology pref- erable in small or medium power transmission. In this paper we discuss a new control system based on space vector modulation （SVM） without any voltage line sensors. Using direct power control （DPC） SVM and a new double synchronous reference frame phase-locked loop （DSRF-PLL） approach, the control system is resistant to the majority of line voltage disturbances. Also, the system response has accelerated by using a feed forward power decoupled loop. The operation of this control strategy was verified in a SIMULINK/MATLAB simulation environment. To validate this control system, a 5 kV.A prototype system was constructed. Compared to the original controllers, the current total harmonic distortion （THD）, the active and reactive deviations and the DC voltage overshoot were lowered by 2.5%, 6.2% and 8%, respectively. The rectifier power factor in the worst condition was 0.93 and the DC voltage settling time was 0.2 s.

An End-to-end Transient Recognition Method for VSC-HVDC Based on Deep Belief Network

Lightning is one of the most common transient interferences on overhead transmission lines of high-voltage direct current(HVDC)systems.Accurate and effective recognition of faults and disturbances caused by lightning strokes is crucial in transient protections such as traveling wave protection.Traditional recognition methods which adopt feature extraction and classification models rely heavily on the performance of signal processing and practical operation experiences.Misjudgments occur due to the poor generalization performance of recognition models.To improve the recognition rates and reliability of transient protection,this paper proposes a transient recognition method based on the deep belief network.The normalized line-mode components of transient currents on HVDC transmission lines are analyzed by a deep belief network which is properly designed.The feature learning process of the deep belief network can discover the inherent characteristics and improve recognition accuracy.Simulations are carried out to verify the effectiveness of the proposed method.Results demonstrate that the proposed method performs well in various scenarios and shows higher potential in practical applications than traditional machine learning based ones.

Application of wavelet-based active power filter in accelerator magnet power supply

Since modern accelerators demand excellent stability to magnet power supply （PS）, it is necessary to decrease harmonic currents passing magnets. Aiming at depressing the rappel current from the PS in the Beijing electron-positron collider Ⅱ, a wavelet-based active power filter （APF） is proposed in this paper. An APF is an effective device to improve the quality of currents. As a countermeasure to these harmonic currents, the APF circuit generates a harmonic current, countervailing harmonic current from PS. An active power filter based on wavelet transformation is proposed. Discrete wavelet transformation is used to analyze the harmonic components in the supply current, and an active power filter circuit works according to the analysis results. Our simulation and experiment results are given to prove the effect of the APF.

Small-signal Stability Analysis and Improvement with Phase-shift Phase-locked Loop Based on Back Electromotive Force Observer for VSC-HVDC in Weak Grids

Voltage source converter based high-voltage direct current(VSC-HVDC)transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources.However,connecting a voltage source converter(VSC)to an AC weak grid may cause the converter system to become unstable.In this paper,a phase-shift phaselocked loop(PS-PLL)is proposed wherein a back electromotive force(BEMF)observer is added to the conventional phaselocked loop(PLL).The BEMF observer is used to observe the voltage of the infinite grid in the stationaryαβframe,which avoids the problem of inaccurate observations of the grid voltage in the dq frame that are caused by the output phase angle errors of the PLL.The VSC using the PS-PLL can operate as if it is facing a strong grid,thus enhancing the stability of the VSC-HVDC system.The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL,which makes it easy to implement.In addition,because it is difficult to obtain the exact impedance of the grid,the influence of shortcircuit ratio(SCR)estimation errors on the performance of the PS-PLL is also studied.The effectiveness of the proposed PSPLL is verified by the small-signal stability analysis and timedomain simulation.

Low Impedance Fault Identification and Classification Based on Boltzmann Machine Learning for HVDC Transmission Systems

Identification and classification of DC faults are considered as fundamentals of DC grid protection.A sudden rise of DC fault current must be identified and classified to immediately operate the corresponding interrupting mechanism.In this paper,the Boltzmann machine learning(BML)approach is proposed for identification and classification of DC faults using travelling waves generated at fault point in voltage source converter based high-voltage direct current(VSC-HVDC)transmission system.An unsupervised way of feature extraction is performed on the frequency spectrum of the travelling waves.Binomial class logistic regression(BCLR)classifies the HVDC transmission system into faulty and healthy states.The proposed technique reduces the time for fault identification and classification because of reduced tagged data with few characteristics.Therefore,the faults near or at converter stations are readily identified and classified.The performance of the proposed technique is assessed via simulations developed in MATLAB/Simulink and tested for pre-fault and post-fault data both at VSC1 and VSC2,respectively.Moreover,the proposed technique is supported by analyzing the root mean square error to show practicality and realization with reduced computations.

Multi-port Network Modeling and Stability Analysis of VSC-MTDC Systems

The voltage source converter based multi-terminal high-voltage direct current(VSC-MTDC)system has attracted much attention because it can achieve the interconnection between AC grids.However,the initial phases and short-circuit ratios(SCRs)of the interconnected AC grids cause the steady-state phases(SSPs)of AC ports in the VSC-MTDC system to be different.This can lead to the issues such as mismatches in multiple converter reference frame systems,potentially causing inaccuracies in stability analysis when this phenomenon is disregarded.To address the aforementioned issues,a multi-port network model of the VSC-MTDC system,which considers the SSPs of the AC grids and AC ports,is derived by multiplying the port models of different subsystems(SSs).The proposed multi-port network model can accurately describe the transmission characteristics between the input and output ports of the system.Additionally,this model facilitates accurate analysis of the system stability.Furthermore,it identifies the key factors affecting the system stability.Ultimately,the accuracy of the proposed multi-port network model and the analysis of key factors are verified by time-domain simulations.