A VSC-based Model for Power Flow Assessment of Multi-terminal VSC-HVDC Transmission Systems

This paper puts forward a new practical voltage source converter(VSC)based AC-DC converter model suitable for conducting power flow assessment of multi-terminal VSCbased high-voltage direct current(VSC-MTDC)systems.The model uses an advanced method to handle the operational limits and control modes of VSCs into the power flow formulation.The new model is incorporated into a unified framework encompassing AC and DC power grids and is solved by using the Newton-Raphson method to enable quadratically convergent iterative solutions.The use of complementarity constraints,together with the Fischer-Burmeister function,is proposed to enable the seamless incorporation of operational control modes of VSC and automatic enforcement of any converter’s operational limits that become violated during the iterative solution process.Thus,a dedicated process for checking limits is no longer required.Furthermore,all existing relationships between the VSC control laws and their operational limits are considered directly during the solution of the power flow problem.The applicability of the new model is demonstrated with numerical examples using various multi-terminal AC-DC transmission networks,one of which is a utility-sized power system.

A Novel Pilot Protection for VSC-HVDC Transmission Lines Based on Correlation Analysis

The control system of voltage source converter HVDC (VSC-HVDC) is complex and its fault tolerance ability is not sufficient, and correct rate of line protection device is not high. A novel pilot protection for VSC-HVDC transmission lines based on correlation analysis is proposed in this paper. In the principle, external fault is equivalent to a positive capacitance model, so the correlation coefficient of the current and voltage derivative is 1;while the internal fault is equivalent to a negative capacitance model, so the correlation coefficient of the current and voltage derivative is -1. Internal faults and external faults can be distinguished by judging the correlation coefficient. Theoretical analysis and PSCAD simulation experiments show that the new principle, which is simple, not affected by transition resistance, control type and line distributed capacitance current, can identify internal faults and external faults reliably and rapidly, having certain practical value.

Preventing DC Over-voltage in Multi-terminal HVDC Transmission

This paper is concerned with power reduction control which is used to avoid DC over-voltage for multiterminal HVDC transmission of offshore wind power.Voltages and frequencies of offshore AC wind farm networks are used for transmitting control signals for the power reduction control.These methods do not require fast communication.Power reduction sharing among the offshore wind farms using the different control signals is analysed.The control systems are also compared against the DC chopper method to prevent a DC overvoltage.Simulation and experiments are carried out to evaluate the control systems.

Architecture of Nan'ao Multi-terminal VSC-HVDC System and Its Multi-functional Control

This paper presents a framework of a multi-terminal HVDC transmission system and its multi-functional control strategy.The framework possesses the basic characteristics of the DC-grid and is suitable in integrating distributed power sources.The paper proposes the first architecture for a multiterminal HVDC transmission system using the VSC technology.Its control strategy offers various functionalities that include controls for operation mode,start-up and shutdown,DC voltage,and station online re-connecting,which are significantly different from the control of point-to-point VSC-HVDC systems.The framework has not only been evaluated in real-time simulation studies,but has also been implemented onsite for the first time via the China Southern Grid's Nan'ao Multi-terminal VSCHVDC(VSC-MTDC)project.This paper gives a brief review of the current research and engineering achievements in this field,which includes four aspects:the architecture of the VSCMTDC system,the structure of the control and protection system,simulation verification tests setting,and the results of real-time hardware in hardware in loop(HIL)simulation studies and onsite tests.

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 review of the protection for the multi-terminal VSC-HVDC grid

The multi-terminal VSC-HVDC grid is believed to be widely applied in the future power system. The dc line protection is the key technique for operation security and power supply reliability of the dc grid. In this paper, the single-ended protections, namely, the traveling-wave based protection and transient-variable based protection, as well as the pilot protections, mainly including the directional pilot protection and current differential protection, are discussed in detail. With the analyzed protections, the effective main and back-up protection strategy can be configured for the dc line in multi-terminal VSC-HVDC grid.

Fault analysis method of integrated high voltage direct current transmission lines for onshore wind farm

Voltage source converter(VSC) based high voltage direct current(HVDC) transmission is most suited for the wind farm as it allows flexibility for reactive power control in multi-terminal transmission lines and transmits low power over smaller distance. In this work, a new method has been proposed to detect the fault, identify the section of faults and classify the pole of the fault in DC transmission lines fed from onshore wind farm. In the proposed scheme, voltage signal from rectifier end terminal is extracted with sampling frequency of 1 k Hz given as the input to the detection, classification and section discrimi-nation module. In this work, severe AC faults are also considered for section discrimination. Proposed method uses fuzzy inference system(FIS) to carry out all relaying task. The reach setting of the relay is 99.9% of the transmission line. Besides, the protection covers and discriminates the grounding fault with fault resistance up to 300 Ω.Considering the results of the proposed method, it can beused effectively in real power network.

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.

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.

Research on fast solid state DC breaker based on a natural current zero-crossing point

The DC fault characteristics of voltage source converter based high voltage direct current(VSC-HVDC)systems are analyzed in this paper.The phenomenon whereby the capacitor on DC side discharges quickly during a DC fault contributes to a large short-circuit fault current.Neither traditional DC breakers nor DC switches can cut off the fault current under this condition.A fast solid state DC breaker design method is proposed in this paper.This method is based on the fault current characteristics of the inverter in multi-terminal HVDC systems(MTDC),where a fault current appears at the natural zerocrossing point near the inverter.At this point,by coordinating the AC breakers near the rectifier,the DC breaker could reliably cut off the DC fault current and protect the system.A detailed model for this fast solid state DC breaker and its operation sequence are studied,based on this design method.Simulations modeling a five-terminal meshed DC grid and a fast DC breaker were carried out with PSCAD/EMTDC using this design method.The results from the simulations confirmed the validity of the design method.

A DC Current Flow Controller for Meshed Modular Multilevel Converter Multiterminal HVDC Grids

This paper proposes the design of a novel DC current flow controller(CFC)and evaluates the control performance of balancing and regulating the DC branch currents using the DC CFC in a meshed multi-terminal HVDC(MTDC)grid.The DC CFC consists of two identical full bridge DC-DC converters with the capacitors of the two converters being connected in parallel.The scalability of the DC CFC is easily achievable due to the identical bridge converter topology;the cost of this DC CFC is also relatively low due to its simple physical structure and low voltage ratings.The control performance of the DC CFC is tested on a meshed 3-terminal(3-T)HVDC grid,which is based on modular multilevel converters(MMC).The DC branch current control in the meshed MTDC grid is achieved using the proposed control strategy of the DC CFC,and is verified through case studies on the real-time digital simulator(RTDS).