High voltage direct current (HVDC) transmission is an economical option for transmitting a large amount of power over long distances. Initially, HVDC was developed using thyristor-based current source converters (CSC). With the development of semiconductor devices, a voltage source converter (VSC)-based HVDC system was introduced, and has been widely applied to integrate large-scale renewables and network interconnection. However, the VSC-based HVDC system is vulnerable to DC faults and its protection becomes ever more important with the fast growth in number of installations. In this paper, detailed characteristics of DC faults in the VSC-HVDC system are presented. The DC fault current has a large peak and steady values within a few milliseconds and thus high-speed fault detection and isolation methods are required in an HVDC grid. Therefore, development of the protection scheme for a multi-terminal VSC-based HVDC system is challenging. Various methods have been developed and this paper presents a comprehensive review of the different techniques for DC fault detection, location and isolation in both CSC and VSC-based HVDC transmission systems in two-terminal and multi-terminal network configurations.

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A comprehensive review of DC fault protection methods in HVDC transmission systems

A novel nonunit transient-based protection scheme for bipolar high-voltage direct current (HVDC) lines is proposed. The protection scheme is composed of a starting unit, a boundary unit, a directional unit, a faulty pole identification unit, and a lightning disturbance identification unit. The prototype based on the proposed scheme is developed and its performance is evaluated through a hardware-in-the-loop test using real-time digital simulator (RTDS) and real HVDC control devices. Better than the presently used line protection, the proposed protection not only can detect faults on HVDC lines more rapidly and accurately in different HVDC operation modes, but is also more effective in detecting high-impedance grounding faults. In addition, the novel protection is designed to identify lightning disturbances from short-circuit faults correctly, which avoids the unnecessary restart of HVDC systems due to protection misjudgment.

Development of a Novel Protection Device for Bipolar HVDC Transmission Lines

A novel differential protection scheme for HVDC transmission lines

HVDC transmission systems have found wide applications in modern power systems and have produced a useful influence on the stability and security of power systems. The currently used protections for HVDC systems operate on the principles which have poor reliability and inadequate sensitivity. To overcome these problems, this paper presents a new whole-line quick-action protection principle for HVDC transmission lines. The proposed method, based on the boundary characteristics of the line, can distinguish internal faults from external ones using single-terminal current measurements only. The criterion of this protection has been formulated. Simulations on frequency-dependent parameter-line model and the field-recorded data of a $\pm$ 500-kV HVDC system show that this method is valid and feasible. The proposed protection, sound and clear in principle and easy to implement, can be expected to become a primary protection of HVDC transmission.

A New Whole-Line Quick-Action Protection Principle for HVDC Transmission Lines Using One-End Current

Multi-terminal HVDC is an important new technology for bulk power transfer from large sources of remote renewables to large load centers. Recently MTDC grids have become more technically feasible due to improvements in VSC based converters, which can reverse currents and generally have greater control than LCC. Furthermore, emerging DC breakers technology unlocks the possibility of de-energizing only the faulted section of an MTDC without losing the whole grid. The protection relaying in such grids is not trivial: it must be very reliable, robust to the converter control system׳s effect on the post fault transients and act quickly, before the fault current develops beyond the presently modest interrupt capability of DC breakers. Within this challenging environment, this paper discusses in detail the important considerations for designing MTDC protection, including accurate modeling of the converter topology, associated control scheme and the DC link itself, supported by the results of novel transient simulation studies. This context is then used to appraise the suitability of the existing protection of HVDC and MTDC grids and suggests future work in the area.

Design of protection schemes for multi-terminal HVDC systems

Smoothing reactors, together with the DC filter banks installed at both ends of the HVDC transmission line constitute the ‘natural boundary’, which leads to significant difference in attenuation characteristic of different frequency components of fault transient signals detected by protection. By means of Hilbert-Huang Transform, which is a kind of non-stationary transient signal processing method, the energy of high frequency components(2000Hz∼10kHz) is extracted based on the attenuation characteristic of the line boundary, then a novel algorithm for the non-unit transient-based protection is acquired. Moreover, a protection threshold that is adapt to various lengths of transmission line is obtained by extensive simulation work. Experimental results indicate that the new criteria can identify any internal fault on the whole line and clear it rapidly and credibly, what's more important, the accuracy and reliability of the criteria will not be influenced by the length of the transmission line

Realization of the transient-based boundary protection for HVDC transmission lines based on high frequency energy criteria

Appropriate modeling of dynamic loads is of primary importance in voltage stability studies, especially increasing use of induction machines in both consumption and generation. This paper is mainly related to short-term voltage instability depending on time domain simulation. The actuality of voltage instability research is summarized, and the mechanisms of short- term voltage stability are explained. A simple power system will simulate the load dynamics showing how a short-term voltage collapse occurs. Based on simulations, this paper analyzes frequency decay and voltage decay of power system with large generating unit out of service depending on the load model and system condition. It is useful to realize the impact of load characteristics on voltage stability and low frequency of power system, and properly design the under-frequency load shedding and under-voltage load shedding schemes to improve the power system stability.

Time-Domain Simulation Investigates Short-Term Voltage Stability with Dynamic Loads

In this paper, a method, which is essential in initiating emergency control measures, is proposed for fast and accurate online detection of transient instability based on real-time measurement. Under a certain disturbance, the candidate critical cluster of power system is predicted and refreshed at each sampling step. The method identifies the loss of synchronism at each time step by observing the geometric characteristic of the post-fault trajectory of the generalized one-machine equivalent system. A new structure of the emergency control scheme relies on SIME (for Single Machine Equivalent). It is unnecessary to calculate the unstable equilibrium point and some limitations caused by the time-invariant assumption are also eliminated. Simulations are carried out on the IEEE 50 generator test system and a real power system. The results indicate that the proposed method has the features of reliability and effectiveness in preventing the system from the loss of synchronism.

Study on closed-loop transient stability emergency control based on wide area measurement

It is difficult for existing HVDC line protections to distinguish internal fault from external fault rapidly and accurately, and the protective setting work encounters problems and difficulties. As a result, these protections have a low correct action rate. According to the feature of HVDC transmission system, this paper studies the characteristics of the natural boundary composed of the smoothing reactor and the DC filters. It is proved that this boundary has obvious differences in attenuation characteristics of different frequency components of fault transient signals. Consequently, a non-unit transient-based protection (NUTBP) is proposed. Wavelet multi-resolution signal decomposition technique is used to analyze the transient current caused by the faults. Extensive simulation results indicate that non-unit transient protection can clear all faults on the whole line quickly and credibly.

Study of Non-Unit Transient-Based Protection for HVDC Transmission Lines

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Papers

Realization of the transient-based boundary protection for HVDC transmission lines based on high frequency energy criteria

Time-Domain Simulation Investigates Short-Term Voltage Stability with Dynamic Loads

Study on closed-loop transient stability emergency control based on wide area measurement

Study of Non-Unit Transient-Based Protection for HVDC Transmission Lines