0885-8977 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPWRD.2017.2745758, IEEE Transactions on Power Delivery  Abstract—This paper presents a novel protection method for monopolar HVDC converters. This method uses the secondary and tertiary currents of the converter transformers and based on DC and harmonic contents of the currents decides for the security of the converter. The new protection method detects the location of the fault inside the converter and based on that issues a signal for single phase disconnection in the AC system to eliminate the fault. It will be shown that the new method is not only faster compared to the conventional protection methods but also can be easily applied to the existing HVDC systems without imposing any significant requirement. The Hydro–Quebec HVDC test system is used to present the simulation results and to ensure the validity and functionality of the proposed protection method. Index terms— Converter protection, HVDC protection, internal and external faults, overcurrent protection. I. INTRODUCTION IGH voltage direct current (HVDC) transmission systems are the new popular kind of transmission networks which compared to the traditional AC networks have obvious advantages such as the ability of bulk power transmission in long distances, low power losses, enhanced flexibility in control and asynchronous interconnection[1]. Due to the huge amount of energy being transferred, the reliability and security of the system has an undeniable impact on overall power system operation. There have been a considerable number of studies dealing with different types of HVDC protection systems [2]-[5]. The most important aspect of them is the subject of detecting the internal faults from the external ones and applying protective operations if there is an internal fault F. Dehghan Marvasti is with the Department of Electrical Engineering, Faculty of Engineering, Yazd University, Iran (e-mail: farzadmarv@gmail.com). A. Mirzaei is corresponding author and with the Department of Electrical Engineering, Faculty of Engineering, Yazd University, Iran (e-mail: mirzaei@yazd.ac.ir). in the system. A prominent protection system should also be reliable in case of other detrimental phenomenon such as lightening disturbances or commutation failures [6], [7]. One of the key elements of HVDC transmission systems is the AC to DC converter units [1]. Without safe operation of the converters there will be no power transmission. There are a lot of studies concerning the converter protection and control during faulted or abnormal conditions [8]-[11]. [12] introduces the traditional overcurrent protection combined with semiconductor fuse, extremely inverse and definite-time current characteristics equipped with an algorithm based on the variation in the fault current. [13] investigates the performance of overcurrent and differential HVDC converter protections against common malfunctions such as rectifier misfire, backfire and inverter commutation failures. [14] explains that the differential protection of HVDC converters can fail to operate if fault current is not flowing simultaneously into the current transformers (CTs) installed on the delta and star connections of the converter transformer. [15] simulates various situations of earth faults inside of HVDC converters, investigates the current and voltage waveforms and presents information of current and voltage behavior, voltage drop and measurements for safe converter operation. [16] describes the development and feasibility of a microprocessor protection unit designed for dealing with a combination of HVDC converter and system fault conditions. [17] studies the detrimental effects of lightning strikes on single-pole HVDC transmission line, describes the abnormal blocking behavior of the sound converter pole and presents countermeasures for preventing unintended events. There are also some studies for other types of the HVDC converter protection systems such as voltage source converters (VSCs) [18], [19] and modular multi-level converters (MMCs) [19], [20]. A lot of these studies are focused on detecting the converter internal faults and encountering them using solely the HVDC control strategy. Knowing that, this principle not only provides small degree of control over converter faults but is also only functional if the internal HVDC faults are recognized. Therefore the internal converter faults may not be fully detected. On the other hand, if the converter fault acts as a permanent fault (due to high amount of DC component in the fault current), the conventional protection methods are not satisfactorily effective nor fast enough to remove the fault from the system. F. Dehghan. Marvasti, A. Mirzaei, Member, IEEE A Novel Method of Combined DC and Harmonic Overcurrent Protection for Rectifier Converters of Monopolar HVDC Systems H