Application of Thyristors in HVDC

QUESTION

Defining/describing thyristors operation

Introducing the topics

Thyristor Valves

Future Development of thyristors

SOURCES

ANSWER

Application of Thyristors in HVDC

Abstract

High Voltage DC Transmission systems (HVDC) utilize high power converters for the purpose of converting power and the control of the quality of power. High power thyristors are a key component in HVDC converters and are being manufactured for even higher power ratings today. This paper is a discussion of the thyristor technology and its improvement, as well as it uses in HVDC systems. The features and properties of thyristors are discussed in reference to their application in HVDC systems. Thyristors connected in series build a thyristor valve, which is used as an HVDC converter. The thyristor valve design and application are also discussed in this paper. Future technological improvements in thyristor valve technology and application are discussed, including a summary of the key design features and parameters of thyristor valve applications.

Introduction

A thyristor is a solid-state semiconductor device that is used as a switch. Thyristors are a type of switch that work in two states; conducting and a non-conducting state. They are not ideal switches because they have some limitations. They are a class of semiconductor devices that are made up of four layers of alternating N-type and P-type substrate. Compared to transistors, thyristors have better high-power handling capability and lower on-state losses. Thyristors have low switching speed and high switching losses. The world is facing a continuous increase in the demand for high quality and high efficiency of power transmission. HVDC systems continue to gain importance and more application in the world’s power transmission systems (Bahrman & Johnson, 2007). These systems use high power electronic converters for efficient power conversion and quality control of power.

The quality and performance of converters in power transmission depend on their main component – high power thyristors. Thyristors have been used in HVDC for more than 50 years, with the technology being developed over the years to achieve a high power rating. The oldest thyristors had a highest blocking voltage of about 1600V and could support a DC of up to 1000A (Bahrman & Johnson, 2007). To achieve higher current ratings, many thyristors would be connected in parallel. Modern thyristors have a blocking voltage of up to 8000V and a DC of 4500A, and there is no need to connect thyristors in parallel to increase current rating. This development is necessary because of the increased demand for efficient and quality high power transmission. There is a need to maximize space utilization for power transmission lines, which necessitates higher transmission voltage and reduced transmission losses.

A majority of HVDC transmission systems across the world utilize 500 kV rated DC voltage (Bahrman, 2006). In recent years, however, many large HVDC systems have been built covering longer transmission distances. These schemes aim to use ultra-high DC voltage ranging 800 kV and power ratings of over 5000MW. For purposes of covering such high DC current and voltage schemes, improved thyristors are being developed to provide improved converter design. The application of thyristors in HVDC systems and the developments in thyristor technology are discussed below.

Application of Thyristors in HVDC Systems

Thyristors, or silicon-controlled rectifiers (SCRs), were first developed in the 1960s. Over the years, thyristors have been improved to increase their power rating and improve their reliability. As earlier stated, thyristors are good but they are not ideal switches. For instance, with an off-state voltage, an off-state current usually flows both in the reverse and the forward direction. In addition, the on-state voltage during conduction makes thyristors a non-ideal switch. The total voltage loss of a thyristor in an HVDC system is two to three volts (Bahrman, 2006). With the increasing demand for higher transmission current over long distances, the need for higher current capabilities arises. A blocking voltage of about 8kV per thyristor is the optimum for total operational losses, which resulted in the development of a six inch thyristor with a blocking voltage of 8kV. This developed thyristor can be used for HVDC transmission with currents up to 4500A. HVDC systems transit huge amounts of power over long distances (Bahrman & Johnson, 2007). These systems can be underground or underwater cables.

HVDC systems are less expensive and have lesser energy losses, compared to AC systems. They use a converter station at either end of the system, where a thyristor or solid-state valve is used for AC and DC current conversion. The thyristor valve at the beginning of the system converts AC to HVDC, which then travels to the intended location through a cable. At the end of the cable, a valve converts the HVDC back to AC.

Thyristor Valves

HVDC systems can either use mercury arc valves or thyristor valves for power conversion. The major problem with the use of the mercury arc valves is the occurrence of backfire or failure to block in the reverse direction, which results in the destruction of the operating capacity of the valves (Edison Tech Center, n.d.). Thyristor valves eliminate this problem, which explains why they have replaced mercury arc valves in HVDC systems. However, thyristor valve ratings cannot be exceeded even for short durations. With technological development and innovation, the power ratings of thyristor valves have been increased significantly (Bahrman, 2006). This has resulted in the enhancement of performance and current-carrying capability. The development of better thyristor valves over the years has also resulted in a decrease in the components of a valve necessary to transmit a certain amount of power. The reliability of thyristors has largely increased, with improved thyristor valve structure, better valve setup, easy assembly and accessibility, and easier maintainability.

Even though thyristors have been improved, resulting in a higher blocking capability, a series connection is still needed to create a valve that has the required high voltage capacity. The number of thyristors that are to be connected depends on the application of the valve (Padiyar, 1990). To fi the thyristor valve to the HVDC application, and to improve its design, modular design is used to create a thyristor valve that has a cost-optimized design. The modules are units that are self-supporting, with an aluminum profile frame that supports all the components. The frame also works as a corona shield in HVDC. The arrangement of the valves and all other equipment in power transmission forms an electric circuit diagram. Thyristor valves result in uniform voltage grading and are easy to test.

Future Development of Thyristors

For efficient HVDC systems, the best and most reliable thyristors are required. However, economic factors have to be taken into consideration. Thyristors that have high continuous current, surge current, and blocking voltage capacity have to be developed. Presently, the maximum diameter of the silicon wafer used in an HVDC thyristor is 6-inches, with the thickness for a blocking capacity of 8000V being about 1.5mm. Improved manufacturing and technologies are solutions to improved thyristor requirements (Bahrman, 2006). Diffusion processes that focus on ensuring high purity, for instance, have resulted in the production of higher power thyristors with high charge carrier lifetimes and homogenous charge carrier distribution on the wafer. Through electron irradiation, the optimization of the trade-off between on-state voltage, as well as the improved reverse recovery charge and turn-off time are achieved.

To achieve a reasonable clamping force and a high surge current capability for a thyristor, a highly efficient thermal coupling of thermal capacity to the silicon wafer is used (Padiyar, 1990). With increasing innovation and investment in technology and research, thyristor technology and its application have a bright future. Better thyristors will be developed and the applications of thyristor valves will increase. This will result in better HVDC systems and improved efficiency in long-distance power transmission.

Conclusion

The world is experiencing an increased demand for efficient long-distance power transmission and distribution systems. HVDC systems are becoming the order of the day, with power demand increase and the increased need to cut down power losses during transmission. High power thyristors play a major role in modern HVDC systems. Thyristor technology has greatly developed over the last decade, resulting in the production of a higher rating and better performing thyristors and thyristor valves. The modern design of thyristor valves utilizes the capability of thyristors to the maximum, meeting the requirements of long-distance transmission systems. Increased power ratings and ultra-high transmission voltage require larger diameter ad higher rating thyristors, which are being developed through advanced technology applications.

References

Bahrman, M. P. (2006, October). Overview of HVDC transmission. In 2006 IEEE PES Power Systems Conference and Exposition (pp. 18-23). IEEE.

Bahrman, M. P., & Johnson, B. K. (2007). The ABCs of HVDC transmission technologies. IEEE Power and Energy Magazine, 5(2), 32-44.

Edison Tech Center. (n.d.). HVDC Power. Retrieved October 9, 2019, from https://edisontechcenter.org/HVDC.html.

Padiyar, K. R. (1990). HVDC power transmission systems: technology and system interactions. New Age International.

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