High Voltage Direct Current Applications Research Articles

Practical Analytical Model and Comprehensive Comparison of Power Loss Performance for Various MMCs Based on IGCT in HVDC Application

This paper gives a comprehensive analysis and characterization of power loss of a high-power integrated gate-commutated thyristor (IGCT) in a modular multilevel converter (MMC) for a high voltage dc application. The practical scheme, power loss models, calculation methods, characterization of MMC based on IGCT are analyzed in detail. Especially, a comprehensive comparison of both half-bridge MMC (HB-MMC) and full-bridge MMC (FB-MMC) based on IGCT, press-pack insulated gate bipolar transistor (IGBT), module-type IGBT, press-pack injection enhanced gate transistor (IEGT), and module-type IEGT is analyzed in this paper. According to the study in this paper, the snubber power loss of MMC based on IGCT is far less than the conduction and switching power losses. Both the conduction and switching power losses of IGCT are always lower than that of press-pack and module-type IGBTs and IEGTs in MMC. The power loss of FB-MMC is higher than that of HB-MMC with the same transmission power because more switches are employed in FB-MMC. If taking diodes into account, the HB-MMC and FB-MMC based on IGCT can decrease power loss about 1.9%–49.3% and 8.3%–45.1% under different operation states, respectively. The study in this paper will provide a valuable reference and promote the application of IGCT in MMC.

Generating High Voltage DC with Cockcroft-Walton Voltage Multiplier for Testing Locally Assemble Electric Field Sensor

This paper describes the construction of a high voltage DC generator using single-phase transformer and Cockcroft-Walton voltage multiplier. The aim is to simulate high voltage DC for simulating electric field produced before a lightning strike near the ground. Lower AC voltage is step up to high AC voltage using a neon transformer. The AC voltage is then converted into high DC voltage using variable stages of Cockcroft-Walton voltage multiplier. The generator HVDC is connected to the adjustable parallel plate to ground. Variable stages of Cockcroft-Walton voltage multiplier produce high voltage magnitude proportional to each stage of it. The electric field generated from high voltage DC can also be varied depending on the distance between DC output plate and ground. The application of high voltage DC in generating electric field useful in testing locally assembled Electric Field Sensor.

Series LC DC circuit breaker

The article proposes a mechanical DC circuit breaker (CB) based on a series LC circuit. It requires two switches (a fast disconnector and an AC circuit breaker), an inductor and a capacitor, and therefore the cost is expected to be low. A series LC circuit is analysed and it is concluded that fault current will always have natural zero-current crossings which enable use of simple AC CBs. The current commutation into a capacitor is investigated since this is important for successful operation. A number of analytical conditions are derived for the voltage stress across disconnector contacts which enable arc-less contact opening. Experimental results on a 900 V laboratory prototype LC DC CB illustrate successful DC fault clearing, with commutation of 130 A and peak DC current of 190 A. A detailed PSCAD model for 320 kV LC DC CB is developed and DC fault clearing is evaluated in order to understand the possible benefit for high-voltage direct current applications. Further comparisons with the commercialised hybrid DC CB and mechanical DC CB on 320 kV system illustrate some benefits in terms of performance and simplicity. The mechanical LC DC CB operates very fast because of early capacitor insertion, and this results in low peak current and energy dissipation.

Operation, analysis and experiments of DC transformers based on modular multilevel converters for HVDC applications

This paper studies the operation, analysis and experiments of multilevel high frequency link transformers (MHFLT) based on modular multilevel converters (MMC) for high voltage DC applications. The multilevel dual active phase shift is proposed to operate the MHFLT using a high switching frequency, which brings about many advantages and makes the operation quite different from that of the traditional DC transformer (DCT) based on a dual active bridge and the fundamental frequency MMC widely used in flexible HVDC transmission. Specifically, MHFLT is suitable for high voltage levels, which is due to its good switching characterization, ability to cut itself off from an external fault, and it can also achieve redundancy operations when a sub-module fault occurs. In this paper, the operation, modulation method, multilevel high frequency link voltage, current, and power characterization, high frequency commutation, and switching characterization are analyzed comprehensively; the pulse rotation and distributed delay control, and fault handling strategies of MHFLT are then proposed. Finally, a prototype is built, and the experimental results verify the correctness and effectiveness of the proposed solution.

Improved hybrid modular multilevel converters with dc short‐circuit fault ride‐through capacity

Modular multilevel converter (MMC) has become a promising converter alternative for transmission voltage-source converter high-voltage direct current applications. The main issue and challenge of MMC is how to handle the dc short-circuit fault. Based on the half-bridge MMC, this study proposed an improved hybrid MMC topology which has the dc faults ride-through capability and requires less switching devices contrast with the present hybrid topology. The improved hybrid topology consists of m half-bridge submodules and n modified half-bridge sub-modules (e-RBSM) which consists of an insulated gate bipolar transistor (IGBT), two reverse blocking IGBTs (RB-IGBTs), two diodes and a resistance. The required number of e-RBSM n is deduced according to the dc short-circuit fault ride-through requirement. Moreover, the analysis of the fault current blocking mechanism and electrical stress of the power switches is made in detail. Furthermore, the improved topology allows all RB-IGBTs not to conduct simultaneously by employing the parallel circuit including a diode and a resistance, therefore, it reduces the requirement of trigger pules operation. Simulation results on PSCAD/EMTDC verify the efficiency of the proposed topology and strategy.

UUDAT based on the modular multilevel converter and diode bridges for high‐voltage DC application

Conventional unidirectional DC/DC converters for high-voltage direct current application employ DC/AC/DC technology and suffer from high converter cost and power loss. To solve these issues, a unidirectional step-up DC–DC autotransformer (UUDAT) is studied. UUDAT is composed of the series connection of diode bridges and voltage source converters. Topologies of UUDAT is detailed. The harmonic and uncontrollability issues are discussed. Control and the possible application scenario for UUDAT are proposed. The ways of dimensioning the voltage and power rating of the components in UUDAT are studied. Extensive simulations on the power system level verified the technical feasibility of UUDATs.

Multiple modular DC transformer for flexible HVDC application

This study proposes a novel multiple modular dc transformer (MMDCT) scheme for flexible high-voltage dc (HVDC) application. The MMDCT is composed of n consistent modular dual-active-bridge converters (MDABs), and every MDAB can work independently. The MDABs are series connected and also connects to the HVDC bus. The multiple distributed design decreases the manufacturing difficulty in high-frequency transformer and its installation. Besides, modular discrete design decreases commissioning difficulty of system in practice and also increases reliability and fault-treatment ability. Moreover, the MMDCT still has good soft-switching performance and low operation current for switches. The MMDCT is a practical scheme for HVDC application, and it will be welcomed by engineers.

Investigating Developments in Modular Multilevel Converters [From the Editor

The modular multilevel converter (MMC) has come a long way since first proposed for high-voltage dc (HVDC) applications nearly 15 years ago. Ongoing improvements and advances in MMC technology have extended the reach of the MMC to many more applications in the last 10–15 years, which include flexible ac transmission systems and high-power motor drives. To better understand the MMC, our theme for this issue is the modular multilevel converter.

Modular Embedded Multilevel Converter for MV/HVDC Applications

Many of the renewable energy resources, like offshore wind, utility PV farms, are located far away from the load centers, which requires dc power transmission system to efficiently deliver power. Voltage-source converter solution is required in many applications due to the stability and compensation requirement. This paper presents a new voltage source converter topology for medium to high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three-level topology structure and consists of series-connected insulated-gate bipolar transistor (IGBT) bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor or press-pack high voltage integrated gate-commutated thyristor or IGBT stacks. The basic operation principle, control solutions, and the methods to commutate the thyristors are explained in detail. Due to the three-level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume, and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. There are other topology variations of MEMC with a similar operational principle but different characteristics.

Space charge and conductivity measurement of XLPE nanocomposites for HVDC insulation–permittivity as a nanofiller selection parameter

Cross-linked polyethylene (XLPE) insulation is successfully used for high-voltage AC transmission. However, it is still under development for high-voltage DC application due to space charge accumulation, which distorts the internal electrical field distribution and leads to its aging/failure. Therefore, the space charge should be measured and carefully analysed. On the other side, conductivity measurement helps to forecast the degradation probability of the insulation. Higher conductivity represents the severe degradation. Nanofiller addition, such as SiO2, TiO2, MgO and so on (<5 wt%), particularly surface-modified nanofiller due to its better dispersion significantly suppresses the space charge accumulation and conductivity. Nevertheless, the choice of suitable nanofiller has still remained a challenge. With this context, space charge and conductivity of XLPE-silica and XLPE-magnesium oxide (MgO) surface-modified nanocomposites are measured. This study proposes a parameter for nanofiller selection that will deliver optimal properties for the intended application. Results show that nanocomposites with higher nanofiller permittivity (i.e. MgO) have less space charge accumulation and low conductivity and are justified with the help of a band gap theory model.

Current Rating Considerations in Designing HVDC Cable Installations

Ampacity calculations for high voltage direct current (HVDC) cables are more difficult than those for the ac cables because of the absence of suitable analytical formulas. The current carrying capability of such cables is not only limited by the operating temperature, but also by the electrical stress within the insulation. The IEC60287 standard only considers the thermal limitations of the conductor current and, therefore, cannot be used for some HVDC applications because it fails to include the analytical derivations of the stress-limited rating equations. However, applications of the HVDC cables are becoming more common and so there is a need to extend the IEC standard methodology so that ampacity calculations can be performed for such installations. This paper considers various aspects of the HVDC cable design and how they influence cable rating. It focuses on the temperature difference across the electrical insulation and the entire cable as a means of representing the maximum dielectric stress as the current limiting factor when calculating ratings for HVDC cables.

Stepped Two-Level Operation of Nonisolated Modular DC/DC Converter Applied in High-Voltage DC Grid

Modular dc/dc converter (MDCC) based on cascaded submodules (SMs) has been an attractive converter topology for the interconnection of high-voltage dc (HVDC) grids for its low cost and high efficiency. The MDCC with sine-wave modulation has been developed and poses satisfactory performance; however, the high-frequency sorting of SM capacitor voltages, which aims to achieve their balance, results in quite large computation burden in HVDC application. To resolve the problem, the stepped two-level operation of the MDCC is proposed in this paper. Compared with the traditional sine-wave modulation, the stepped two-level modulation offers the following merits: 1) smaller SM capacitance requirement; 2) smaller ac circulating current; 3) the switching frequency is equal to the fundamental frequency, which avoids the extra switching actions existing in the sine-wave modulation; and 4) the sorting algorithm of the SM capacitor voltages needs only to be executed twice in a period, which poses a significant reduction of the computation burden, especially for ultrahigh voltage power conversion requiring hundreds of SMs. The simulation and experimental results verify the theoretical analysis.

High voltage DC partial discharge and flashover characteristics with surface charging on solid insulators in air

Electrical insulation for high voltage DC (HVDC) applications exhibits many features that differ from those of conventional AC insulation. It is therefore necessary to concentrate research activities on aspects that will facilitate a better understanding of its performance, e.g., DC-partial discharge activity, charging characteristics on solid insulator surfaces, and electric field distributions. All of these are complex phenomena and are not yet well understood [1]–[7].

Environmentally friendly polypropylene/thermoplastic elastomer composites with modified graphene oxide for HVDC application

In this work, polypropylene (PP) composites filled with functionalized graphene oxide (f-GO) and thermoplastic elastomer (SEBS) were prepared by melt blending and were studied. The introduction of SEBS improves the flexibility of PP/f-GO and its mechanical modulus and crystallinity decreases. Space charge in the blends was suppressed and their short-term electrical breakdown strengths increased. This is attributable to PP/f-GO interfacial traps which reduces the amount and energy of free charges. These results provide important information for the design and development of advanced environmentally friendly polymer composites for high voltage direct current applications.

High improvement in trap level density and direct current breakdown strength of block polypropylene by doping with a β-nucleating agent

Polypropylene is one kind of eco-friendly insulating material, which has attracted more attention for use in high voltage direct current (HVDC) insulation due to the long-distance transmission, low loss, and recyclability. In this work, the morphology and thermal and electrical properties of the block polypropylene with various β-nucleating agent (β-NA) contents were investigated. The relative fraction of the β-crystal can reach 64.7% after adding 0.05 wt. % β-NA. The β-NA also greatly reduced the melting point and improved the crystallization temperature. The electrical property results showed that the alternating and direct current breakdown strength and conduction current were obviously improved. In addition, space charge accumulation was significantly suppressed by introducing the β-NA. This work provides an attractive strategy of design and fabrication of polypropylene for HVDC application.

Linearized DC-MMC Models for Control Design Accounting for Multifrequency Power Transfer Mechanisms

The dc-modular multilevel converter (DC-MMC) is one of a new class of single-stage modular multilevel dc-dc converters that has recently emerged for high-voltage dc applications. This paper presents the first small-signal state-space model for the DC-MMC that is able to account for the multifrequency power transfer mechanisms within the converter. Derived from a dynamic phasor model representation of the DC-MMC, the developed model is linear time-invariant (LTI), allowing for the application of conventional LTI tools for both analysis and design. The small-signal dynamics are validated by simulation results from a full switched model demonstrating its accuracy. A simplified model derived from the full LTI system is presented that readers can utilize to develop dynamic controls for the DC-MMC. As a case study, this benchmark model is leveraged to propose a dynamic controller that regulates dc power transfer between networks and balances the capacitor voltages. Control block diagrams are also provided that enable systematic control design of the DC-MMC via standard linear methods. Case study simulations verify the efficacy of the developed controls for dc network applications. The presented small-signal modeling and control design methodology can be readily applied to any MMC-based topology.

A Three-Level Modular DC/DC Converter Applied in High Voltage DC Grid

Modular dc/dc converter (MDCC) based on cascaded submodules (SMs) has been an attractive converter topology for the interconnection of high voltage dc (HVDC) grids for its low cost and high efficiency; however, the quite large filter inductor implemented at lower voltage side significantly increases the system volume. In this paper, a buck three-level type MDCC (Buck-TL-MDCC) is derived from the classic buck three-level converter (Buck-TLC) by replacing the switches and blocking capacitor with cascaded SMs for HVDC application. Just like the major merit of the Buck-TLC, the filter inductance of the Buck-TL-MDCC can be reduced, and the high voltage blocking capacitor is eliminated by the cascaded SMs. Instead of the sine-wave modulation employed by prior arts of MDCCs, the stepped two-level modulation is adopted; it offers the following merits: 1) smaller SM capacitance requirement; 2) smaller ac circulating current; 3) the switching frequency is equal to the fundamental frequency, which avoids extra switching actions; 4) the computation burden is dramatically reduced. Finally, the simulation and experimental results verify the effectiveness of the proposed Buck-TL-MDCC.

Investigation of plasma-enhanced-fluorination treatment on epoxy resins for high-voltage DC applications

Charge build-up under high voltage DC is a significant concern in the transmission system as its presence may distort the local electric field. By chemically treat polymeric insulation via plasma-enhanced-fluorination (PEF) treatment, the charge transport characteristics of the material can be modified. In doing so, excellent surface properties similar to those of fluoropolymers can be attained without compromising the bulk properties of the original polymeric insulation. The change in chemical components at the surface of polymeric insulation should lead to a corresponding change in dielectric properties at the surface and consequently may suppress the occurrences of charge build up. In this paper, the performance of plasma-treated epoxy resins was investigated in term of its dielectric performances, and valid comparison is made between PEF and direct-fluorination treatment from author's previous work. The tests include surface current measurement and surface flashover performances using a pair of finger electrodes. The effect of moisture absorption on the treated surface was also discussed and proved to be the limiting factor in the improvement of dielectric properties of fluorination treatment. This research proved that the introduction of the fluorinated surface layer on epoxy resins does play a major role in improving the surface dielectric properties for the use as insulation spacer in high voltage DC GIS systems.

A multi-loop control technique for the stable operation of modular multilevel converters in HVDC transmission systems

A multi-loop control strategy based on a six-order dynamic model of the modular multilevel converter (MMC) is presented in this paper for the high-voltage direct current (HVDC) applications. For the initial analysis of the operation of MMC, a capability curve based on active and reactive power of the MMC is achieved through a part of the six order dynamic equations. According to the MMC’s control aims, the first loop known as the outer loop is designed based on passivity control theory to force the MMC state variables to follow their reference values. As the second loop with the use of sliding mode control, the central loop should provide appropriate performance for the MMC under variations of the MMC’s parameters. Another main part of the proposed controller is defined for the third inner loop to accomplish the accurate generation of reference values. Also, for a deeper analysis of the MMC’s dc link voltage stability, two phase diagrams of the dc-link voltage are assessed. Matlab/Simulink environment is used to thoroughly validate the ability of the proposed control technique for control of the MMC in HVDC application under both load and MMC’s parameters changes.

Improved Two-Level Voltage Source Converter for High-Voltage Direct Current Transmission Systems

This paper presents an improved two-level voltage source converter for dc transmission systems with relatively low rated power and dc operating voltage. Unlike conventional two-level converter, the presented converter employs two distributed cell capacitors per three phase; thus, do not contribute any current when converter is blocked during dc short-circuit fault as in modular multilevel converter case. The use of three-phase cells is proven to be beneficial because the arm currents do not contain second-order harmonic currents, and cell capacitors tend to be small as they only experience high-order harmonic current associated with the switching frequency. For the same rated dc-link voltage and switching devices, the rated power of the improved two-level converter will be twice than that of the conventional two-level converter. Averaged, switching function, and electromagnetic transient simulation models of the improved two-level converter are discussed and validated against detailed switch model. The viability of the improved two-level converter for high-voltage dc applications is examined, considering dc and ac short-circuit faults. Besides, reduced complexity of the control and power circuit of the improved two-level converter, it has been found that its transient responses to ac and dc faults are similar to that of the modular multilevel converter.