Medium Voltage Applications Research Articles

Low Switching Frequency Operation Control of Line Voltage Cascade Triple Converter

With the increasing power level of wind power generation system, the traditional topology of power converters can no longer meets the demand of high-power wind power generation systems due to the limitation of device performance. The line voltage cascade type multiple PWM converter (LVC-VSC) is a kind of converter that uses the traditional two-level and six-switch voltage source converter as the basic component unit, and each unit is combined with the line voltage cascade method. This type of converter is suitable for medium-voltage and high-power applications such as wind power generation and metallurgical drives because of its easy modularization, strong scalability and low number of isolated power supplies required. However, for medium-voltage and high-power applications, the switching frequency of power devices in the converter is low, usually limited to a few hundred hertz. The traditional modulation method of line voltage cascade converter has a large number of redundant states, and simply reducing the carrier ratio will cause serious degradation of control performance and system instability. To address this problem, this paper proposes a modulation strategy and a corresponding control method for low switching frequency. The modulation strategy is based on the vector relationship of finite switching states, and the optimal switching sequence is selected according to the modulation system by removing redundant states, thus ensuring the application of different modulation sequences under different modulation depths and ensuring the current quality on the basis of the minimum switching frequency, which effectively solves the control problems at low switching frequency. The experimental results show the correctness and effectiveness of the proposed modulation strategy and control method.

Reliability and Cost-Oriented Analysis, Comparison and Selection of Multi-Level MVdc Converters

DC technology has gained considerable interest in the medium voltage applications due to the benefits over the AC counterpart. However, to utilize the full capacity of this development, the selection of a suitable power electronic converter topology is a key aspect. From the pool of voltage source converters (VSC's), it is unclear which topology is suitable for multi-megawatt applications at medium voltage dc (MVdc) levels. To address this, the paper proposes a selection guideline based on reliability and optimum redundancy levels of VSCs for MVdc applications. This will be combined with other functional factors such as operational efficiency and return-on-investment. Three candidate multi-level topologies namely three-level neutral point clamped converter (3L-NPC), modular multi-level converter (MMC) and cascaded 3L-NPC (which is being used for the first MVdc link in the U.K.) have been evaluated over two-level-VSC from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 10 kV to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 50 kV. Results show that with the increase of MVdc voltage level MMC shows better performance whereas at low MVdc voltage levels 3L-NPC is the prominent topology.

A Multilevel Inverter Topology Using Diode Half-Bridge Circuit with Reduced Power Component

This paper presents a new multilevel converter with a reduced number of power components for medium voltage applications. Both symmetric and asymmetric structures of the presented multilevel converter are proposed. The symmetric topology requires equal dc source values, whereas the asymmetric topology uses minimum switch count. However, both structures suffer from high blocking voltage across the switches. To reduce the blocking voltage on switches, an optimal topology is presented and analyzed for the selection of the minimum number of switches and dc sources, while maintaining a low blocking voltage across the switches. A comparative analysis with recently published topologies was performed. The simulation results, as well as the comparative analysis, validated the robustness and effectiveness of the proposed topology in terms of the reduced power loss, lowered number of components, and cost. Furthermore, in addition to the simulation results, the performance of the proposed topology was verified using experimental results of 9, 17, and 25 levels.

Thermal management of MV GIS filled with pure air

Pure air could be a true environment-friendly substitute for SF6, while keeping the same proven technologies and benefits of a hermetically encapsulated gas insulated switchgear (GIS). Given its environmental impact, SF6 should be replaced. Thanks to vacuum interrupter technology in medium voltage (MV) applications, SF6 is no longer necessarily needed to break load current or short circuit currents. For a modern MV GIS with vacuum technology, the gas mainly provides dielectric insulation and acts as a cooling medium for convective heat transfer. Both, electric insulation and cooling capabilities of a gas, depend on its density. When going from SF6 to pure air, there is a reduction factor of 5 in density. Specific heat and thermal conductivity, however, are higher in pure air while radiation is not impacted at all and external cooling of the switchgear is virtually the same as for the SF6 GIS. It is therefore easily enough possible to achieve similar performances with pure air as with SF6 for both, electric insulation and cooling, by design modification and pressure increase while keeping the same dimensions.

ASSESSING REAL ALTERNATIVES TO MV SF6 GAS-INSULATED SWITCHGEAR

The purpose of this paper is to analyse the different alternatives to SF6 filled metal-enclosed switchgear and controlgear in medium-voltage secondary distribution application, in order to improve the environmental footprint, having in mind that they are designed to keep the high-voltage parts insensible to different environments by enclosing them inside gas-filled sealed compartment(s). With this purpose, it is also very important to analyse and prioritise the criteria and attributes used to select the alternatives. Today, several gases and gas mixtures have been investigated and proposed as possible alternatives. Even, some of them are presented in commercial products as alternatives to SF6-filled ones. However, there&apos;s still short experience on their behaviour for their usual expected long life of more than 30 years, based on the service history of SF6-filled ones. Bearing in mind that these equipment are typically installed in electrical networks and without any maintenance in their inner high-voltage parts.

A simplified SVPWM method for cascaded multilevel inverters

A highly popular alternative in medium voltage and high-power applications is multilevel converters because of their superior performance over conventional two-level converters. The most commonly used control methods in the case of multilevel inverters are sine pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM) methods. Among these two control strategies, SVPWM has superior performance over SPWM in terms of DC bus voltage utilization along with a reduction in total harmonic distortion (THD) of line voltages. The classical SVPWM method has various drawbacks such as computational complexity for identifying the location of reference voltage vector, sector identification, region identification, memory requirement to store lookup tables for switching vectors. The novel simplified SVPWM technique is presented for cascaded H-Bridge multilevel inverter (CHBMLI) in this paper. This simplified SVPWM method has overcome the drawbacks of the classical SVPWM method. This new technique has been implemented into a five-level CHBMLI to evaluate performance and also to compare with the SPWM method. The simulation has been performed in MATLAB software.

Power Quality Phenomena, Standards, and Proposed Metrics for DC Grids

This work addresses the problem of power quality (PQ) metrics (or indexes) suitable for DC grids, encompassing low and medium voltage applications, including electric transports, all-electric ships and aircrafts, electric vehicles, distributed generation and microgrids, modern data centers, etc. The two main pillars on which such PQ indexes are discussed and built are: (i) the physical justification, so the electric phenomena affecting DC grids and components (PV panels, fuel cells, capacitors, batteries, etc.), causing, e.g., stress of materials, aging, distortion, grid instability; and (ii) the existing standardization framework, pointing out desirable coverage and extension, similarity with AC grids standards, but also inconsistencies. For the first point, each phenomenon is discussed with quantitative conclusions on relevant thresholds: in many cases some percentage of distortion (ripple) is acceptable (stress on capacitors and storage, impact on fuel cells, and PV panels), whereas in other cases, much higher levels may be tolerated (interference to protection and monitoring devices). Standards are reviewed for indications not only of low-order harmonics and voltage fluctuations typical of old DC grid schemes, but also for high-frequency noise, including thus supraharmonics and common-mode disturbance, and filling the gap with the electromagnetic compatibility domain. However, phenomena typical of EMC and electrical safety (such as various types of overvoltages and fast transients) are excluded. Suitable PQ indexes are then reviewed, suggesting integrations and modifications, to cover the relevant phenomena and technological progress, and to better follow the normative exigencies: ripple is considered in time and frequency domain, in particular with a band limited implementation; for transients and pulsed loads, more traditional indexes based on area, energy, and half duration are confronted with indexes evaluating the power trajectory and its derivative.

(Invited) Multi-Channel AlGaN/GaN Power Rectifiers: Breakthrough Performance up to 10 kV

High-voltage power rectifiers are widely used in renewable energy processing, electric grids, industrial motor drives, pulse power systems, among other applications. Today’s high-voltage rectifier market is dominated by bipolar Si diodes up to 6.5 kV, which suffer from slow reverse recovery. Wide-bandgap SiC unipolar diodes have been pre-commercialized up to 10 kV, which allows for a much higher switching speed. Recently, we have developed a new generation of high-voltage rectifiers based on the multi-channel AlGaN/GaN platform, which highlight a series of novel device designs incorporating the stacked two-dimensional electron gas (2DEG) channels, p-n junctions, and 3-D fin structures. With these innovations, the performances of our unipolar 1.2-10 kV multi-channel AlGaN/GaN Schottky rectifiers well exceed the Si and SiC 1-D limit, at the same time possessing a lower cost as compared to SiC counterparts. This paper reviews our efforts in the design, fabrication and characterization of these GaN devices. Our results show the tremendous promise of GaN for medium-voltage and high-voltage power electronics applications.

Predictive current control based pseudo six-phase induction motor drive

Asymmetrical six-phase induction machines are widely used in high-power safety-critical applications. The so-called pseudo six-phase winding layout has recently been proposed as a promising contender for high power medium voltage applications in terms of torque density, phase current quality, simple single-layer-based stator winding design, and fault-tolerant capability. This winding layout employs quadruple three-phase stator winding sets connected to provide only six terminals, while being fed from two three-phase inverters. Therefore, the same conventional six-phase-based control structure can still be employed. Among the different control techniques, model predictive current control is considered as one of the cutting-edge technologies for multiphase drive systems, thanks to its simplicity and flexibility in defining new control objectives. This paper investigates how the available 64 voltage vectors of a six-phase inverter are classified among different subspaces when employing a pseudo six-phase stator winding. Moreover, the required modifications to classical predictive current control (PCC) are introduced in order to properly control the machine in a manner similar to conventional asymmetrical six-phase machines while ensuring minimum circulating xy current components. The concept of virtual voltage vectors (VVVs) has also been employed to further enhance the stator current quality. The proposed controller is validated using a 2Hp prototype pseudo six-phase machine under different operating conditions. An experimental comparative study with an asymmetrical six-phase machines is also carried out to verify the claimed equivalence.

Multi-module capacitor voltage measuring technique of modular multilevel converters based on sampling instant classification

The measurement of the sub-module (SM) capacitor voltages serve as the premise and basis for the realization of control goals of the modular multilevel converter (MMC). This paper presents an improved measuring technique based on sampling instant classification (SIC-MT) with a smaller number of sensors, where an improved calculation process with a correction function is applied for estimation of the capacitor voltages to enhance the measurement accuracy. Furthermore, in practical applications, the impact on the voltage estimation resulting from the auxiliary circuit within each SM is also taken into account to further improve the voltage estimation accuracy. In addition, a grouping strategy is introduced to ease the voltage range/bandwidth requirements of the sensors employed for medium voltage applications. The simulation and the experimental results validate the feasibility and effectiveness of the proposed technique. The results show that the proposed method has higher measurement accuracy than the overall measuring technique (O-MT).

State of the Art of Low and Medium Voltage Direct Current (DC) Microgrids

Direct current (DC) microgrids (MG) constitute a research field that has gained great attention over the past few years, challenging the well-established dominance of their alternating current (AC) counterparts in Low Voltage (LV) (up to 1.5 kV) as well as Medium Voltage (MV) applications (up to 50 kV). The main reasons behind this change are: (i) the ascending amalgamation of Renewable Energy Sources (RES) and Battery Energy Storage Systems (BESS), which predominantly supply DC power to the energy mix that meets electrical power demand and (ii) the ascending use of electronic loads and other DC-powered devices by the end-users. In this sense, DC distribution provides a more efficient interface between the majority of Distributed Energy Resources (DER) and part of the total load of a MG. The early adopters of DC MGs include mostly buildings with high RES production, ships, data centers, electric vehicle (EV) charging stations and traction systems. However, the lack of expertise and the insufficient standards’ framework inhibit their wider spread. This review paper presents the state of the art of LV and MV DC MGs in terms of advantages/disadvantages over their AC counterparts, their interface with the AC main grid, topologies, control, applications, ancillary services and standardization issues. Overall, the aim of this review is to highlight the possibilities provided by DC MG architectures as well as the necessity for a solid/inclusive regulatory framework, which is their main weakness.

Active voltage balancing strategy of asymmetric stacked multilevel inverter

Multilevel inverters (MLI) play an important role in AC applications and are undergoing continuous development in topology and control. In higher levels inverters, conventional MLIs have high components count which calls for modification of these topologies to obtain the same number of levels with fewer components to reduce cost and size. Balancing of the capacitors voltages is crucial for the operation of the MLI and it becomes more challenging in higher levels. This paper presents an active voltage balancing strategy for a reduced switch count five-leve topology which is the asymmetric stacked multilevel inverter (ASMLI). The ASMLI uses fewer components than the conventional MLIs when used in their five-levelconfiguration. The proposed active voltage balancing strategy uses simple measurements and logic to assure a balanced capacitors voltages during steady state and transients. The performance was examined and compared based on two modulation techniques with LCL filter and RL load using MATLAB/Simulink. The results show that the active voltage balancing strategy can trace all capacitors voltages to the reference value simultaneously with less than 1% voltage error, fast dynamic response, and an acceptable total harmonic distortion (THD) which allows the proposed setup to be an available option for medium voltage applications.&lt;br /&gt;&lt;div&gt; &lt;/div&gt;

Selective Harmonic Elimination of Five Level Cascaded H-Bridge Inverter Using the Newton-Raphson Technique

Multilevel inverter becomes an alternative for high power and medium voltage applications recently and is more popular in renewable energy. The cascaded H-bridge multilevel inverter is more suitable for the Photovoltaic application because it acts as a separate DC source for each bridge. This paper presents a selective harmonic elimination (SHE) modulation for single-phase cascaded H-bridge (CHB) five level inverter. The optimum angle of the transcendental equations is solved the fundamental and harmonic elements using the Newton-Raphson (NR) process. The proposed selective harmonic elimination (SHE) scheme is tested using MATLAB/Simulink simulation. These simulation results are then verified through experiment using microcontroller. The proposed SHE is efficient in eliminating the 5th order harmonics and producing a higher quality output waveform with a better harmonic profile.

Investigation of the effect of roughness value on the wettability behavior under electric field in XLPE materials used in medium and high voltage applications

Investigation of the effect of roughness value on the wettability behavior under electric field in XLPE materials used in medium and high voltage applications

Study of Toroidal Core Multilimb Transformer (TCMLT) for High-Power DC Application

Multimodule isolated converters incorporating excellent features of galvanic isolation, bidirectional power flow, and simple control are widely adopted in medium-voltage (MV) high-power dc-dc applications. However, the converter size and dc-link capacitor voltage balancing are significant concerns in galvanically isolated cascaded dc-dc converters. This article proposes a new design of an integrated transformer implementing toroidal cores named toroidal core multilimb transformer (TCMLT), employed in a dc-dc modular isolated bidirectional dual-active-bridge (IB-DAB) resonant converter topology. The modular design structure of the proposed transformer can be conveniently constructed and implemented for more number of participating modules conforming the compactness constraint. Moreover, an external coupling winding is implemented in the TCMLT structure to resolve the dc-link voltage balancing issue. This coupling winding method substitutes the expensive complex control circuits, which consequently results in cost reduction and reliability enhancement. Besides, the reluctance model of the proposed structure is analyzed. Based on the reluctance model, a common mode-difference mode (CM-DM) electrical circuit is realized for the proposed scheme. Furthermore, unlike the conventional approach of implementing U, UI, and EI cores for multilimb transformer (MLT) structure with separate limbs, a synthesis method of TCMLT in toroidal form is proposed. The comparative study of the proposed design with the conventional U-core MLT confirms that the proposed TCMLT offers less leakage inductance with significant size reduction. Finally, to validate the efficacy of TCMLT, a three-limb laboratory-scale prototype is employed in a 6-kW cascaded IB-DAB resonant converter topology.

Quasi Z-Source Hybrid Modular Multilevel converter controlled by Reduced Inserted Cells Modulation Technique for Medium Voltage Applications

Quasi Z-Source Hybrid Modular Multilevel converter controlled by Reduced Inserted Cells Modulation Technique for Medium Voltage Applications

Simulation studies on nano-hybrid polymer composites as covered conductor for enhancing the insulation characteristics of low voltage and medium voltage applications

The development of cover conductors involving nano-hybrid polymers possessing high strength & good insulation properties would really help the electrical industry to avoid fatal accidents due to electrical shock and minimize electrical damage occurring due to short circuit in the event of tree/load falling on the bare overhead lines/conductors. The material selection and design are very essential parameters from the point of manufacturing, processing and application. The material characteristics dictate the mechanical as well as electrical performance in the field. For any material to have long life expectancy, compositions/constituents matter a lot with the combination of the basic raw materials. When any material such as overhead conductor is subjected to heavy electrical & mechanical loads, there is a likelihood of sag in the line and short circuit in the event of bare conductors resulting in fire. In such cases, the propagation of fire, fire retardancy, generation of smoke in addition to line protection are required to be considered from the point of life saving and system reliability. In this context, the electrical properties get affected due to varying loads acting on the conductor from time to time and depend on the season. As a part of remedial measures, cover conductor is best suited to meet the requirements. The design of the cover conductor shall be done in such a way that it meets the requirement of the prevailing standards. In the present work simulation studies will be performed using simulation software to study the behavior of electric field of nano based covered conductor and prove that these conductors will give better reliability and superior performance compared to the existing bare conductor. Further, the matrices and filler addition are envisaged are Acrylonitrile Butadiene Styrene (ABS)/Polytetrafluoroethylene (PTFE) of different combinations and nano-silica/nano-clay as fillers for the preparation of nano insulating covered conductor material.

DC-Link Voltage Ripple Control of Regenerative CHB Drives for Capacitance Reduction

The diode-front-end cascaded H-bridge (CHB) inverters have prevailed in the nonregenerative industry drive domain for high-power medium-voltage applications. The regenerative version of the CHB drives is made possible by adding the extra active-front-end rectifier in each power cell, such as a three-phase PWM rectifier. However, due to the instantaneous power unbalance, the dc-link capacitors of the regenerative power cell need to be overdesigned to maintain a stable low ripple dc-link voltage. To reduce the dc-link capacitance, this article proposes a novel closed-loop voltage ripple controller for the regenerative CHB drive without adding extra sensors. In the proposed method, dc-link voltage ripple amplitude and phase angle are accurately detected with a high-performance adaptive filter. Moreover, a latent instability issue is discussed and is avoided in the proposed controller. The performance of the proposed control strategy is validated experimentally on a seven-level regenerative CHB drive.

Topology, Modeling and Control Scheme for a new Seven-Level Inverter With Reduced DC-Link Voltage

This paper presents a novel single-source three-phase multilevel converter with voltage boosting capability for medium-voltage photovoltaic applications. This converter consists of different switched-capacitor networks, formed by 1 capacitor and 3 active switches, as a basic switching cell to generate variable dc-link voltage that boosts the input dc-source voltage, producing 2n+1 (n2) voltage levels at the output. It can reduce the dc-link voltage requirements by 50% in comparison to the traditional neutral point clamped (NPC), flying capacitors, active NPC (ANPC), hybrid and hybrid clamped ANPC and cascaded H-bridge topologies, and 75% to advanced ANPC topologies. It can also reduce the number of required switches and capacitors as well as their voltage stresses compared to these state-of-the-art topologies reported in the literature so far. A finite control set model predictive control algorithm is derived to control the converter, which has the capability to control the active and reactive power without distorting the generated grid current quality. The capacitor voltage balancing is inherent in the proposed topology, and thus, there is no need for any additional voltage balancing circuit, which reduces the control complexity. As an example, a 7-level version of the proposed topology is compared to other existing 7-level inverter topologies.

Commutation behaviour analysis of a 3L-ANPC-VSC phase-leg PEBB using 4.5kV and 3kA press-pack IGBT

Three-level Neutral-Point-Clamped voltage source converter is extensively used in high-power and Medium-Voltage applications. 3L-ANPC-VSC was put forward to conquer the unequal losses distribution and supply redundant switch states for zero-level output by replacing the clamping diode with IGBT in 3L-NPC-VSC. The stray inductance takes an important role in the commutation path of the switching current during the switching event. This paper shows two stacking methods of press-pack IGBT 3L-ANPC unit busbar have been designed. The main differences between the two structures are the positions of clamping IGBT(T5 and T6). The total switching power losses for all commutation paths are analyzed. The stray inductance of all the commutation paths have been extracted by means of Three-Dimensional Finite-Elements-Analysis simulations based on the theoretical analysis,. The optimal structures are obtained by optimizing structure according to simulation results. The calculated values have been compared with simulation results obtained from the performance of double-pulse tests for each commutation path. This paper presents all the analysis details of the 3L-ANPC-VSC commutation behavior using 4.5kV and 3kA press-pack IGBT modules.