Three-phase Medium Voltage Research Articles

Hybrid Modular Multilevel Converters for High-AC/Low-DC Medium-Voltage Applications

With ever-increasing power-density requirements, technologies such as energy storage systems and electric-vehicles can benefit greatly from interfacing medium-voltage (MV)-AC grid like 13.8kV or 30kV using high-AC/low-DC voltage converter. Using modular high-AC/low-DC voltage converter can help increase power-density and efficiency, while reducing total conversion steps and providing flexibility. Full-bridge modular multilevel converters (FB-MMC) and solid-state transformers are existing solutions for such operations, but suffer from limitations of high semiconductor requirements, large submodule capacitors and/or many high-frequency transformers. Three new hybrid-MMC (HMMC) topologies are proposed in this paper as alternative solutions for such high-AC/low-DC voltage operations. Each of the three developed HMMCs utilizes a unique combination of low-frequency high-voltage switches and fast-switching low-voltage switch based submodules to generate multilevel-AC voltage. HMMCs are compared extensively to state-of-the-art FB-MMC and are shown to have semiconductor savings of over 27%, 38% lower submodule capacitor size, and 53% lower losses for 13.8-kV-AC/6-kV-DC operation. Due to these benefits like higher efficiency, significantly smaller submodule capacitance requirements, and fewer semiconductors, HMMCs can be an excellent option for high-AC/low-DC applications. Practical considerations like snubber and DC split-capacitor requirement are also elaborated for developing and commercializing HMMCs. Comparison results are verified using a 17.5 kW three-phase MV laboratory prototype.

1 kV, 10-kW SiC-Based Quadruple Active Bridge DCX Stage in a DC to Three-Phase AC Module for Medium-Voltage Grid Integration

Interfacing low voltage dc to medium voltage three-phase ac grid is often based on series-stackable modular converter architectures. To minimize energy storage requirements, it is advantageous to employ a quadruple active bridge (QAB) stage operating as a “dc transformer” in each stackable module. The QAB stage offers three isolated dc link voltages, which then allow for flexible stacking of three single-phase dc-to-ac inverter stages. Each of the module phases processes a pulsating power having a component at twice the line frequency. This presents a challenge in maintaining zero voltage switching (ZVS) on the secondary sides of the QAB during low-power portions of the line cycle. This article is focused on the design of the QAB stage. A detailed analysis of ZVS switching waveforms is presented, including effects of nonlinear device capacitances. It is shown how ZVS can be achieved at all times using a relatively small circulating current provided by the magnetizing inductance of the high-frequency transformer. Analytical expressions are given for the optimal values of the magnetizing inductance and the dead times of the QAB primary and secondary bridges. The approach is verified by experimental results on a 1 kV, 10-kW SiC-based prototype, demonstrating a relatively flat efficiency curve with a peak efficiency of 97.1% at 75% load.

Multilevel Converter With Nearest Level Control for Integrating Solar Photovoltaic System

This article presents a new Scott-ternary solar multilevel converter (ST-MLC)-based solar photovoltaic (PV) system for medium voltage applications. This system employs a single solar PV array as an input source and two voltage source converters (VSCs) to feed a three-phase grid. Solar power is fed from a two-phase to a three-phase medium voltage grid with balanced grid currents. The legs of VSCs are cascaded to form a two-phase system. The multiwinding transformers are connected in a Scott manner to develop a nine-level three-phase power conditioning system. The turn ratios of the transformers have ternary progression for the generation of higher levels. The modeling of the system is presented, and a closed-loop nearest level control strategy is incorporated to have minimum losses. Simulation results show the performance of the solar PV system under steady-state and dynamic irradiance. Varying solar profile is considered on basis of the daily average global solar irradiance from the geographical information system for New Delhi, India. The harmonic performance is analyzed for the converter voltage and grid current. Moreover, a detailed comparative analysis is covered for the ST-MLC to evaluate its superiority over its existing counterparts. Performance validation is done in a real-time test bench to validate the control and operation of the large-scale solar PV system.

Unidirectional medium voltage rectifier utilizing sinusoidal input currents

Due to recent advances in the field of wide bandgap semiconductors, medium voltage converters are starting to regain momentum. This is mainly facilitated as SiC MOSFETs with blocking voltages of up to 10 kV and switching frequencies in the 10th of kilohertz range are starting to become commercially available. Therefore, a unidirectional three-phase medium voltage rectifier based on a diode bridge rectifier and active current injection network is proposed which benefits from a relatively low number of active semiconductor devices compared to already existing medium voltage solutions. Furthermore, sinusoidal mains currents can be achieved due to third harmonic injection techniques.

Grid current unbalance compensation using a fuzzy controlled smart transformer

This paper proposes a novel method that uses a hybrid fuzzy controller (FC) to control a smart transformer (ST) to compensate three-phase medium voltage (MV) currents arising from unbalanced renewable generation in the low voltage (LV) grid. An averaged macro-model of the ST is used to handle current unbalances in the LV secondary, while providing balanced share of active power in the MV primary. The ST benefits from proportional derivative (PD)-like plus Integrator fuzzy controllers (FPD+I) to provide a regulated voltage in LV grid in addition to unity power factor in MV grid and one PD-like plus gain update mechanism to balance currents in MV grid. Moreover, a three-phase power-sharing strategy is proposed to guarantee active power equality in both sides of ST. The proposed ST macro-model is tested by two modified IEEE 5-bus and IEEE 33-bus with an unbalanced LV grid connected through an ST. Three different scenarios for imbalance conditions are considered. The effectiveness of the proposed scheme is verified by measuring the current unbalance factor (CUF) indices. The proposed controller shows almost 1% reduction in the voltage unbalance factor (VUF) and 9% in CUF, while the response time is reduced to 20 ms.

A hybrid redundancy scheme for medium‐voltage three‐phase cascaded H‐bridge electronic power transformer

A hybrid redundancy (HR) scheme is proposed for the medium-voltage three-phase cascaded H-bridge electronic power transformer (CHB-EPT) based on the combination of the global redundant power modules and local redundant power modules. A “HOT-FIRST-LOCAL-FIRST” transition strategy is designed to achieve a short transition time. Furthermore, to reduce the power loss of the redundant power modules, an intermittent charging (IC) strategy is proposed for the hot-reserved redundant power modules. The detailed analysis of the topology shows that the proposed scheme can achieve low cost, high reliability, low power loss, and good transition performance. The performances are verified by simulations and experimental results.

A Gen-3 10-kV SiC MOSFET-Based Medium-Voltage Three-Phase Dual Active Bridge Converter Enabling a Mobile Utility Support Equipment Solid State Transformer

The emergence of medium-voltage silicon carbide (SiC) power semiconductor devices, in ranges of 10&#x2013;15 kV, has led to the development of simple two-level converter systems for medium-voltage applications. A medium-voltage mobile utility support equipment-based three-phase solid state transformer (MUSE-SST) system, based on Gen3 10 kV SiC MOSFETs, is developed to interconnect a three-phase 4160 V/60 Hz grid to a three-phase 480 V/60 Hz grid to provide a shore-to-ship power interface for naval vessels. The MUSE-SST system consists of three power conversion stages, namely, MVac/MVdc stage (MV: active front-end converter), MVdc/LVdc stage (dual active bridge converter), and LVdc/LVac stage (LV: active front-end converter). The galvanic isolation is introduced in the MVdc/LVdc stage using MV/LV high-frequency transformers (HFTs). This article demonstrates the operation of the three-phase Y&#x2013;<inline-formula> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> connected dual active bridge converter used in the MVdc/LVdc stage of the MUSE-SST system. Equations for phase currents, power flow, and zero-voltage switching (ZVS) boundaries are derived for all possible modes for the three-phase Y&#x2013;<inline-formula> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> configuration. A detailed parasitic simulation model is derived by measuring and experimentally verifying the parasitic elements of the HFT. A brief discussion regarding the design considerations required for the hardware development of the medium- and low-voltage sides of the three-phase dual active bridge converter is also provided. Successful tests demonstrating the operation and feasibility of the medium-voltage dual active bridge converter, at medium-voltage levels (7.2 kV dc-link voltage), are shown. The results indicate that these devices can accelerate the growth and deployment of the medium-voltage SiC-based converter for isolated and bidirectional medium- to low-voltage dc systems.

Thermal Performance and Reliability Analysis of a Medium-Voltage Three-Phase Inverter Considering the Influence of High $dv/dt$ on Parasitic Filter Elements

In recent years, the use of silicon carbide (SiC) power semiconductor devices in medium-voltage (MV) applications has been made possible due to the development of high blocking voltage (10–15 kV)-based devices. While the use of these devices brings in a lot of advantages, the semiconductor devices are exposed to high peak stress (of up to 15 kV) and a very high $dv/dt$ (of up to 100 kV/ $\mu \text{s}$ ). The high $dv/dt$ across the devices leads to a high $dv/dt$ across other components connected to the system. This makes the effect of the parasitic capacitance across the components to be of paramount importance since an additional current flows through the components and, consequently, through the switching device. This additional current flows during each switching transition and leads to increased switching losses in the device. This article analyzes the effect of these additional losses on the lifetime of the device. The thermal performance of a three-phase inverter power block is provided, and a mission profile (solar irradiance and temperature)-based analysis is carried out to account for the additional junction temperature rise. The rainflow counting method is implemented to identify the mean and amplitude of each thermal cycle. An empirical device lifetime model is used to calculate the number of cycles to failure. Finally, the Palgrem Miner rule is used to quantify the total damage in the device. Comparisons have been carried out on basis of lifetime for both the cases (with and without the influence of parasitic capacitances). This analysis can be helpful in validating the importance of the design of filter inductors in these MV applications.

High-power medium-voltage three-phase ac–dc buck–boost converter for wind energy conversion systems

This paper proposes three topologies of single-stage three-phase ac–dc buck–boost converters suitable for medium-voltage and high-power wind energy conversion systems (WECS). The proposed converters draw sinusoidal input currents with nearly unity power factor from wind generators over wide range of ac voltages and frequencies. The proposed converters reduce the voltage and current stresses on the self-commutated switching device compared to existing solutions; thus, promising for new generation of relatively cheap WECSs. The proposed converters operate in Discontinuous Condition Mode (DCM), with their ac terminals connected to three-phase generator with open ended windings, or to ac supply using three single-phase transformers with isolated windings at converter side. The validity of the proposed converters and its detailed theoretical analysis and discussions are confirmed using PSCAD/EMTDC simulations, and further corroborated by experimentations, considering different operating conditions.

A Short-Time Transition and Cost Saving Redundancy Scheme for Medium-Voltage Three-Phase Cascaded H-Bridge Electronic Power Transformer

Reliability is one of the major concerns when the electronic power transformer (EPT) is employed in the field, and redundancy is a common approach to improve the reliability. For hot and cold redundancy schemes, the redundancy performance will be degraded due to the small number of power modules (PMs) when the EPT is employed in the medium-voltage power grid. For the three-phase EPT, at least three redundant PMs are needed (one redundant PM per phase), which leads to a high redundancy cost. In this paper, a novel redundancy scheme is proposed. The proposed scheme can accomplish the faulty PM replacement process within tens of microseconds with nearly no transition. And only one PM and a set of switches are needed for the three-phase redundancy, which can effectively save the redundancy cost. The proposed scheme is analyzed and supported by simulations and experimental results.

Four-Arm Three-Phase Modular Multilevel Converter (4A-TPMMC)

In this paper a Four-Arm Three-Phase MMC (4A-TPMMC)-based DC–AC converter is proposed. The proposed converter generates a three-phase output voltage using a lower number of components, which reduces the converter complexity and cost. The concept of the proposed approach is presented in this paper. A comparison between the conventional Six-Arm Three-Phase MMC (6A-TPMMC) and the proposed 4A-TPMMC is conducted. Two simulation models are built, one for the 6A-TPMMC, and the other for the proposed 4A-TPMMC. The simulations are carried out for the same number of submodules per arm and the same AC output voltage magnitude. Moreover, a case study using the proposed 4A-TPMMC, as a three-phase medium voltage motor drive, is considered. Finally, an experimental validation for the proposed 4A-TPMMC is presented to prove the viability of the claimed contributions.

Harmonic Analysis and Controller Design of 15 kV SiC IGBT-Based Medium-Voltage Grid-Connected Three-Phase Three-Level NPC Converter

Cascaded converters are generally used for medium-voltage (MV) grid-connected applications due to the limitation in the voltage rating of available silicon (Si) power devices. These converters find application in active power filters, STATCOM or as the active front end converters for solid state transformers at the distribution voltage levels. The high voltage wide bandgap semiconductor devices have enabled the grid connected operation of noncascaded converters. This results in high power density, less number of switching devices, and high efficiency for three-phase MV grid interface. This also results in control simplicity without the need for complex dc bus balancing algorithms otherwise needed for cascaded converters. However, such noncascaded, grid-connected converters introduce challenges in maintaining power quality at low currents. This paper investigates the harmonic performance and current distortion of the grid-connected, three-level neutral point clamped converter using 15 kV silicon carbide Insulated Gate Bipolar Transistor (IGBTs). A suitable control scheme for stable harmonic compensation is proposed. The challenges and control performance are explained through frequency domain analysis, simulations, and experimental validation on a developed prototype of the three-phase converter up to 4.16 kV, three-phase MV grid-connected operation.

Probabilistic Assessment and Evaluation of Transients in a Medium-Voltage Three-Phase Capacitor Bank Energized by Unsynchronized Vacuum Switchgears

This paper presents a case study of a medium-voltage three-phase capacitor bank (CB) energization, based on an unsynchronized switching scheme using three independent pole vacuum switches simultaneously commanded. The motivations are cost reduction and simplification of the switching scheme. The maximum current and voltage peaks during transients caused by connection of CB with three independent pole switches or a three-phase mechanically coupled switch are compared. A probabilistic assessment for the overcurrents and overvoltages based on Monte Carlo simulation is also performed. The obtained results show that three single-phase unsynchronized switches could be used in a CB, with good transitory response.

Geometrical Shapes Impact on the Performance of ABS-Based Coreless Inductive Sensors for PD Measurement in HV Power Cables

In this paper, a series of investigations on sensitivity and bandwidth for four types of Rogowski coil (RC) sensors was demonstrated. The proposed RC sensor design using acrylonitrile butadiene styrene material carries the advantages, such as lighter weight, robust, and higher temperature resistance. The prototype development first was carried out with the assistance of Solidwork 3D computer aided design software and MakerBot 2X 3D printer. Subsequently, the sensors were modeled and simulated using electromagnetic transient program-alternative transient program software environment based on the lumped parameter identification. A single-end measuring technique was used to locate the PD signal on the three-phase medium voltage underground cross-link polyethylene insulated power cable with a 240-mm2 nominal area copper conductor. In order to verify the results achieved from the simulation, an experiment was carried out. This experiment was repeated concurrently for each design of the RC sensors. In order to achieve the best PD detection, the precise selection for the best sensor is paramount. In this case, the results indicated that certain geometrical shape performed better with regard to the detection of the PD signal.

Power Loss Analysis of Medium Voltage Three-Phase Converters using 15 kV/40 A SiC N-IGBT

Medium-voltage (MV) silicon carbide (SiC) devices such as the 15-kV SiC N-insulated gate bipolar transistor (IGBT) have better thermal withstanding capability compared with silicon (Si)-based devices. These devices also have lower switching and conduction losses at high switching frequencies and high power levels, respectively. The maximum safe operating junction temperature for the 15-kV SiC IGBT is 175 °C. This enables high power density design of the MV converters using this device. Heat sink with forced air cooling is considered for dissipating the heat generated during converter operation. In this paper, the power loss analysis of three-phase MV converters based on 15-kV/40-A SiC N-IGBT is discussed. The converter thermal analysis is carried out based on the experimental loss data and the continuous heat-run test of the device. It is supported by analytical calculations, PLECS thermal simulations, and FEM simulations in COMSOL Multiphysics software. Hardware prototypes of the converters are developed and the experimental results support the analysis. Experimental results are given for both hard-switched grid-connected converter and soft-switched dual active bridge converter. The paper mainly focuses on the semiconductor losses in the converter.

Solid-State Transformer and MV Grid Tie Applications Enabled by 15 kV SiC IGBTs and 10 kV SiC MOSFETs Based Multilevel Converters

Medium-voltage (MV) SiC devices have been developed recently which can be used for three-phase MV grid tie applications. Two such devices, 15 kV SiC insulated-gate bipolar transistor (IGBT) and 10 kV SiC MOSFET, have opened up the possibilities of looking into different converter topologies for the MV distribution grid interface. These can be used in MV drives, active filter applications, or as the active front end converter for solid-state transformers (SSTs). The transformerless intelligent power substation (TIPS) is one such application for these devices. TIPS is proposed as a three-phase SST interconnecting a 13.8 kV distribution grid with a 480 V utility grid. It is an all SiC device-based multistage SST. This paper focuses on the advantages, design considerations, and challenges associated with the operation of converters using these devices keeping TIPS as the topology of reference. The efficiency of the TIPS topology is also calculated using the experimentally measured loss data of the devices and the high-frequency transformer. Experimental results captured on a developed prototype of TIPS along with its measured efficiency are also given.

Two-Wire Distribution System for Power Supply to Three-Phase Rural Facilities

Nowadays, the electric power supply to rural facilities is commonly composed by single-phase medium voltage grids. Eventually, some of these rural facilities require the use of more significant driving loads, making necessary the replacing of the single-phase networks by a new medium voltage three-phase network. Thus, despite the fact it is a new or an existing facility, the construction costs related to a new medium voltage three-phase grid, which may have some miles long, could cripple investment to be made by the farmer as well as increase utility tariffs due to higher costs of the three-phase distribution networks in rural areas. In this context, this paper presents an innovative and low cost power supply to attend three-phase rural facilities through a new two-wire distribution system.

Sample Time Staggered Space Vector Modulation for Medium Voltage Cascade Seventeen-level Static Synchronous Compensator

Abstract An implementation method of applying the sample time staggered space vector modulation technique (STS-SVM) to three-phase medium voltage cascade H-bridge static synchronous compensator (STATCOM) was proposed. Proposing an equivalent topology relation between two-level three-phase bridge and three-phase cascade H-bridge based on combined transformer, according to which, using the three left and three right bridge legs of each stage three-phase H-bridge as two adjacent three-phase full bridge to SVM modulate, thus realized the implementation of using STS-SVM technique to the cascade H-bridge STATCOM. Simulation results show that the method has the advantages of high DC voltage utilization, easy-to-digital implementation, low switching losses, and so on, which is easy to use in medium voltage cascade H-bridge STATCOM. The practical application proved that the medium voltage cascade seventeen-level STATCOM based on STS-SVM, which is of fast dynamic response, could realize comprehensive compensation of harmonics and passive power precisely and efficiently at lower switch frequency in actual power grid.

PWM Algorithms for Multilevel Inverter Supplied Multiphase Variable-Speed Drives

Multiphase variable speed drives are nowadays regarded as serious contenders for various applications, due to the certain advantages that they possess when compared to three-phase drives. On the other hand, the multilevel (predominantly three-level) voltage source inverter (VSI) has become an industrially accepted technology for medium-voltage three-phase drives. The logical next step is to integrate multilevel VSI and multiphase drive technologies, since benefits of each can then be combined. This paper will review recent advances in this area. Two different topologies of the multiphase supply will be considered and the emphasis will be placed on appropriate pulse width modulation (PWM) techniques that can be used in conjunction with the supply structure. The first topology utilises multilevel (three-level) VSI and the machine’s stator multiphase winding is star-connected with an isolated neutral point. In the second topology the winding is left open-ended and each side of the winding is connected to an isolated two-level VSI. This topology can also achieve three-level operation. Carrier-based and space vector based PWM strategies are considered and the obtainable performance is illustrated using experimental results.

Electric Field Under an MV Line Supported by Concrete Poles

This paper presents a study of the electric field under a three-phase medium voltage (MV) power line supported by concrete poles. The analyzed configuration is a pole-top-pin construction. An analytical model has been developed for the three-phase wires assuming three straight cylindrical conductors whose axes have been put parallel to each other and to the ground, and for the concrete pole a cylindrical conductor placed in a vertical position. Simple analytical formulas have been obtained for the three components of the electric field in the vertical plane containing the axis of the conductor representing the central top wire. The validity range of the model is from the ground to one quarter of the height of the concrete pole. The analytical expressions have been checked against the numerical data obtained by a 3-D numerical analysis performed by using the charge simulation method. Finally, the results have been experimentally validated. The analytical model reported here is a new easy tool for the evaluation of the electric field for the analyzed geometry.