Comprehensive review of commutation failure in HVDC transmission systems
Graphical abstract
Introduction
High voltage direct current (HVDC) has been widely used in the field of long distance transmission and asynchronous power grid interconnection because of its long transmission distance, large transmission capacity and low transmission loss [1], [2]. To further reduce transmission loss and increase transmission capacity, the number of ultra HVDC (UHVDC) transmission projects with voltage levels of 800kV and above has increased rapidly recently [3], [4]. Commutation failure (CF) is one of the most common issues in HVDC transmission systems. CFs will directly cause a sudden increase in DC current and a sharp decrease in DC voltage. It is precisely because of the larger transmission capacity of UHVDC, the risk of DC pole blocking is increased due to the CFs of its converter station [5]. DC block fault will cause power surplus at the sending end and power shortage at the receiving end, thus causing the interlocking unit to be disconnected, load shaking and other failures [6], [7]. As the demand for transmission capacity increases, more and more receiving ends of HVDC transmission systems are fed into the load center. There is a certain interaction between the converter stations of each HVDC system, thus forming a multi-infeed direct current (MIDC) system. In MIDC, there are many electrical couplings between different inverter stations, and the CFs of the system are more diverse. A single CF may cause subsequent CFs in time and concurrent CFs at adjacent commutation stations in space [8], [9]. Even cascade failures occur in large-scale hybrid AC/DC power grids [10]. From 2004 to 2019, a total of 1353 CFs occurred in the HVDC transmission systems under the jurisdiction of State Grid Corporation of China (SGCC), with an average of 9.1 times CFs per HVDC transmission system per year. Some HVDC projects even exceed 15 times per year, which seriously threatens the safety of the power system [11].
The specific data of commutation failures for each HVDC are shown in Fig. 1 [12].
Commutation failure is an inherent feature of semi-controlled DC commutation valves (such as thyristors) consisting of thyristors. The semi-controlled valve can only control its conduction and cannot control its breaking. When switching from one valve to another, the inductance of the commutation loop prevents the valve current from abruptly changing, which takes a period of time to complete the current conversion between the two valves, a process called commutation. In order to ensure successful commutation, the reverse voltage duration after valve shutdown should be sufficient for the valve to restore its blocking capacity. A commutation failure occurs when the reverse voltage duration is insufficient. Gate-turn-off thyristor (GTO) and insulated gate bipolar transistor (IGBT) are fully controlled devices. The converter valve composed of GTO and IGBT can achieve complete control of on and off, so there is no problem of commutation failure. Therefore, this paper mainly analyzes systems with thyristor.
So far, the methods to prevent CFs can be divided into four categories: (I) optimize protection and control characteristics; (II) install auxiliary equipment; (III) coordinate DC power; and (IV) modify converter structure. However, due to the complex causes of CFs in HVDC transmission systems, especially in multi-infeed systems, the existing methods cannot prevent all CFs under diverse and complex disturbances.
There are few literature reviews on CFs. The analysis of the main literature reviews in recent three years is shown in Table 1. They have the following drawbacks:
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It has limitations and does not provide a good overview of the latest advances in HVDC commutation failure studies.
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There is no in-depth discussion on the challenges in the study of CFs and the direction of future research.
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Most of them are about CFs of traditional HVDC, but lack of study on multi-infeed structure or hierarchical infeed structure.
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Most of them are Chinese literature and lack generality.
Therefore, it is necessary to sort out the related research on CFs of HVDC transmission in recent years and point out the future challenges and research directions.
The main contributions of this review are three-fold:
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We provide a general overview of the mechanism, classification, analysis methods, and discrimination methods of CFs in HVDC Systems.
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We give a comprehensive review of the existing commutation failure suppression methods and point out the advantages and limitations of different methods, which can provide reference for the selection of practical engineering schemes.
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We discuss the challenges faced by HVDC technology in practice and the future direction, so as to provide profound guidance for narrowing the gap between research and practice.
Therefore, to better guide the research on CFs in HVDC, Section 2 of this paper introduces the main concept and classification of HVDC. Section 3 analyzes the mechanism, types and hazards of CFs. Section 4 analyses the influencing factors and coupling characteristics of HVDC CF, and introduces the judgment and evaluation index of HVDC CF. Section 5 summarizes the existing HVDC CF prevention methods, and analyses the characteristics of the four kinds of methods. Section 6 points out the challenges and research directions of HVDC CFs in the future. The main structure of this paper is shown in Fig. 2 for clarity.
Section snippets
Concepts and classification of HVDC transmission systems
HVDC transmission systems use DC mode to transform and transmit power with high voltage and large capacity. HVDC is usually composed of a rectifier that converts AC to DC voltage, a high-voltage DC transmission line, and an inverter that converts DC to AC. Therefore, from a structural point of view, HVDC is a high-capacity AC-DC-AC power electronic conversion circuit. HVDC transmission is the earliest and continuously developing technology applied by power electronics technology in the field of
Mechanism of CFs
During the commutation process of the converter, after the current in the thyristor to be turned off drops to zero, the thyristor still needs to withstand a period of reverse voltage before it can completely resume blocking [21]. This period of time is called the deionization time, which is expressed by . If the deionization time is not enough, the thyristor will resume conduction, resulting in CFs [22]. In engineering, it is generally considered that the minimum extinction angle required for
Analysis of influencing factors of CFs
According to inequality (1), if the maximum commutation supply area is too small or the commutation demand area is too large, CFs of the corresponding valve block will occur [33]. The factors influencing CFs are analyzed from the perspectives of the maximum commutation supply area and the commutation demand area.
Prevention of CFs
The methods to prevent CFs can be divided into five categories: (I) optimize protection and control characteristics; (II) install auxiliary equipment; (III) coordinate DC power; and (IV) modify converter structure. All the above methods can be used in traditional HVDC systems. HVDC Systems has high voltage level, large transmission capacity, and special structure such as multi-infeed or hierarchical infeed structures. In order to adapt to the characteristics of HVDC, the above five methods need
Challenges and directions
At present, in the research of commutation failure, the main research objects, fault types and suppression methods are shown in Fig. 21. At present, the research of red area is relatively mature, while the research of blue area is relatively less and more difficult.
In the future, we will face the following challenges and can be driven by the challenges for further research (Fig. 22).
1) With the increase of DC drop points in adjacent areas, more complex multi-infeed HVDC and hierarchical infeed
Conclusion
Based on the basic concepts of HVDC, it is pointed out the classification of HVDC and analyzes the mechanism, types, and hazards of CFs. Based on the commutation voltage-time area theory, it is pointed out that commutation voltage amplitude, firing angle, harmonic component, DC current, and DC control characteristics all affect CFs of HVDC. Four kinds of CF prevention methods, such as optimizing protection and control characteristics, installing auxiliary equipment, coordinating DC power, and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work is supported by the State Grid Corporation of China under Science and Technology Project (5419-201924207A-0-0-00).
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