The ever increasing progress of high-voltage high-power fully controlled semiconductor technology continues to have a significant impact on the development of advanced power electronic apparatus used to support optimized operations and efficient management of electrical grids, which, in many cases, are fully or partially deregulated networks. Developments advance both the HVDC power transmission and the flexible ac transmission system technologies. In this paper, an overview of the recent advances in the area of voltage-source converter (VSC) HVdc technology is provided. Selected key multilevel converter topologies are presented. Control and modeling methods are discussed. A list of VSC-based HVdc installations worldwide is included. It is confirmed that the continuous development of power electronics presents cost-effective opportunities for the utilities to exploit, and HVdc remains a key technology. In particular, VSC-HVdc can address not only conventional network issues such as bulk power transmission, asynchronous network interconnections, back-to-back ac system linking, and voltage/stability support to mention a few, but also niche markets such as the integration of large-scale renewable energy sources with the grid and most recently large onshore/offshore wind farms.

VSC-Based HVDC Power Transmission Systems: An Overview

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

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DC Microgrids—Part II: A Review of Power Architectures, Applications, and Standardization Issues

The continuously increasing demand for electric power and the economic access to remote renewable energy sources such as off-shore wind power or solar thermal generation in deserts have revived the interest in high-voltage direct current (HVDC) multiterminal systems (networks). A lot of work was done in this area, especially in the 1980s, but only two three-terminal systems were realized. Since then, HVDC technology has advanced considerably and, despite numerous technical challenges, the realization of large-scale HVDC networks is now seriously discussed and considered. For the acceptance and reliability of these networks, the availability of HVDC circuit breakers (CBs) will be critical, making them one of the key enabling technologies. Numerous ideas for HVDC breaker schemes have been published and patented, but no acceptable solution has been found to interrupt HVDC short-circuit currents. This paper aims to summarize the literature, especially that of the last two decades, on technology areas that are relevant to HVDC breakers. By comparing the mainly 20+ years old, state-of-the art HVDC CBs to the new HVDC technology, existing discrepancies become evident. Areas where additional research and development are needed are identified and proposed.

HVDC Circuit Breakers: A Review Identifying Future Research Needs

The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.

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Short-Circuit and Ground Fault Analyses and Location in VSC-Based DC Network Cables

A VSC-MTDC (multi-terminal dc) system consists of voltage-source converters (VSCs) connected to a dc network at their dc terminals. The MTDC is most vulnerable to a dc fault which paralyses all the VSCs until the dc fault is cleared. As dc circuit breakers are expensive, this paper proposes a solution based on extinguishing the dc fault current by opening all the ac-circuit breakers (ac-CBs) which the VSCs are already equipped with on the ac-sides. However, it is necessary to identify which dc line is the faulted line (in case it is a permanent fault) so that it can be isolated by fast dc switches (which are much more economical than the dc circuit breakers), prior to restoring the MTDC system by re-closing all the ac-CBs. This paper presents the handshaking method, which locates and isolates the faulted dc line and restores the MTDC without telecommunication.

Locating and Isolating DC Faults in Multi-Terminal DC Systems

Voltage-source converters (VSCs), by themselves, are defenseless against DC faults. Their anti-parallel diodes conduct as rectifier bridges to feed the fault. Their IGBTs (insulated gate bipolar transistors) are helplessly by-passed, unable to extinguish the fault current. This paper shows how a VSC-multi-terminal high voltage direct current (MTDC) transmission system can survive DC faults by a using protection scheme based on: (1) fast, reliable fault detection; (2) blocking of IGBT-circuit breakers (IGBT-CBs) and blocking of the IGBTs of VSCs; (3) locating of the faulted DC line; (4) isolation of the faulted DC line by economical fast DC switches; and (5) de-blocking of IGBT-CBs and IGBTs of VSCs to resume normal service.

Protection of VSC-multi-terminal HVDC against DC faults

ldquoHarmonic transfer through convertersrdquo has caused waveform distortion in multiterminal dc systems based on a voltage-source converter (VSC-MTDC). The harmonic transfer can be by the positive and negative sequences or by the zero sequence. A comprehensive solution consists of decoupling the ac side from the dc side of the VSC. Such decoupling has other benefits in VSC-MTDC because disturbances on the ac side of any VSC do not propagate to other VSCs (except the VSC assigned to function as the dc Voltage Regulator). Decoupling also allows a VSC to be planned and designed independently of the other VSCs of the MTDC.

Elimination of “Harmonic Transfer Through Converters” in VSC-Based Multiterminal DC Systems by AC/DC Decoupling

The 3-phase bridge voltage-source converter (VSC) which has been and continues to be the work-horse of variable-speed AC-motor drives is making penetration into the electric utility environment. In the electric utility environment, there is a greater emphasis on: (i) grounding and (ii) the need of the VSC to survive all conceivable faults. Fault analysis decomposes the unbalanced 3-phase voltages and currents into positive-, negative- and zero-sequence components. The impact of the zero-sequence on the VSC has not been well understood and a common practice is to side-step this issue by excluding the zero-sequence from the VSC at the utility/VSC interface by connecting the transformers in the Y-/spl Delta/ configuration. This paper shows that the Y-/spl Delta/ configuration can lead to destructively high over-voltage or permanent dc voltage unbalance. These problems cease to exist in transformer configurations in which the zero-sequence currents are admitted to the VSC. In order to use these transformer configurations, it is necessary to manage the zero sequence in the VSC. This paper presents the mathematical model of the zero sequence and its equivalent circuit as it affects the voltage source converter.

Managing zero sequence in voltage source converter

Integral harmonics come from the switching pattern of the modulation method Nonintegral harmonics, on the other hand, are initiated by random noise that becomes amplified and sustained by the AC network resonating with the DC network A comprehensive theory of their origins is presented Its correctness is justified by digital simulations It is then shown that an efficient method of predicting their potential occurrences is by eigenvalue analysis The objective of THD compliance can be met by: (i) avoidance by design of network parameters; and (ii) suppression by active filtering Some directions of parameter design are given A simulation result is presented showing that an active filtering method is promising

Lianxiang Tang

Papers

Locating and Isolating DC Faults in Multi-Terminal DC Systems

Protection of VSC-multi-terminal HVDC against DC faults

Elimination of “Harmonic Transfer Through Converters” in VSC-Based Multiterminal DC Systems by AC/DC Decoupling

Managing zero sequence in voltage source converter

Converter nonintegral harmonics from AC network resonating with DC network