Medium Voltage Grid Research Articles

Distributed Wind Energy Conversion System Interface to the Grid Using Cascaded Multi-Level Converter

ABSTRACTIn this paper, a distributed wind energy conversion system based on the cascaded H-bridge multi-level converter topology has been proposed to be used for the medium-voltage grid interface application. This includes load compensation and injection of real power requirement of the load. The load compensation includes reactive power compensation, load harmonic mitigation, and correction for the unbalancing problem. An algorithm has been proposed for generating references for the voltage source converter in current control mode in order to inject real power together with the reactive and harmonic components of the load. Modular structure of the wind turbine-alternator sets and cascaded multi-level inverter using H-bridges suits the distributed wind energy conversion system interface to the medium-voltage grid. The factors like equal distribution of switching stress and power losses, reduced power ratings of the inverter, redundancy, and high quality of inverter output voltage make the proposed arrangement efficient for high-power applications. The cascaded topology with phase-shifted multi-carrier pulse width modulation (PWM) has the features of increasing the effective switching frequency and reducing the ripple magnitude. The proposed scheme has been verified through the simulation study using power system transient analysis tool (PSCAD/EMTDC).

Optimal reactive power flow in a HV grid connected to several distribution grids with predictive Volt Var Control

This paper proposes an optimal power flow to supervise several distribution networks connected to a transmission grid. MV (Medium Voltage)grids can support the voltages of the HV (High Voltage) network by controlling adequately the reactive power exchange at the HV/MV interface. The interaction between the distribution grids equipped with predictive Volt-Var Controllers (VVC) and the optimal coordination is investigated. From a case study involving a rural network, several recommendations are given to tune and combine these controllers.

Reverse-blocking modular multilevel converter for battery energy storage systems

Energy storage systems with multilevel converters play an important role in modern electric power systems with large-scale renewable energy integration. This paper proposes a reverse-blocking modular multilevel converter for a battery energy storage system (RB-MMC-BESS). Besides integrating distributed low-voltage batteries to medium or high voltage grids, with the inherited advantages of traditional MMCs, the RB-MMC-BESS also provides improved DC fault handling capability. This paper analyzes such a new converter configuration and its operating principles. Control algorithms are developed for AC side power control and the balancing of battery state of charge. The blocking mechanism to manage a DC pole-to-pole fault analyzed in depth. Comprehensive simulation results validate both the feasibility of the RB-MMC-BESS topology and the effectiveness of the control and fault handling strategies.

Toward the Universal DC Distribution System

Due to an increasing number of power generation units and load devices operating with direct current (DC) at distribution level, there is a potential benefit of leading efforts toward building a DC distribution system. However, the implementation of DC distribution systems faces important challenges, including the market inertia of AC systems and standardization. Many of the benefits that are attributed to DC can only be realized if a complete DC system is developed, and not if only a few components are replaced. This paper presents the concept of a universal DC distribution system, as envisioned by the authors. The universal DC distribution system could be implemented in various use cases, but could also completely replace AC distribution grids. The paper covers the possibilities of having DC nanogrids inside buildings, DC microgrids in neighborhoods, and the connection to AC and DC medium voltage grids. Furthermore, considerations regarding flexibility, electricity market design, control, and protection are presented.

Multiple-Site Amplitude and Phase Measurements of Harmonics for Analysis of Harmonic Propagation on Bornholm Island

The motivation for harmonic propagation measurements in distribution grids is given. An island site with substantial renewable generation and smart grid elements is chosen as a site for a measurement campaign, and the description of the island distribution network and demographics is given. The measurement infrastructure including a set of GPS synchronized digitizers, transducers, and data collection and communication infrastructure is described. A selection of the results is analyzed and presented for power and harmonic amplitude is given. The phase between the measurement sites is also presented at the fundamental and harmonic frequencies. Interpretation of the results against simplified models of the island medium voltage grid is given. The role of the intersite harmonic phase measurements in interpreting localized harmonic power levels is discussed.

Influence of increasing numbers of RE-inverters on the power quality in the distribution grids: A PQ case study of a representative wind turbine and photovoltaic system

This paper presents the selected power quality (PQ) indicia of a wind generator and a photovoltaic installation considered to be the representative of medium voltage and low voltage distribution grids. The analysis of measured values suggests that the decrease in PQ is a case of specific combination of distributed generation, grid parameters and load behaviour. Modern generators have a limited impact on PQ. On the other hand, fluctuations in power generation are regarded as an emerging PQ indicator. The growing number of distributed renewable installations causes stochastic, variable, and hardly predictable power flows in the distribution grid. The nature of fluctuations in wind and solar generation is different. In both cases, new indexes for the quantification of fluctuations are needed and are yet not standardised. Proper assessment of these fluctuations enables definition of useful fluctuation limits and rules for optimal storage system integration.

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.

Thermal Stress Analysis of Medium-Voltage Converters for Smart Transformers

A smart transformer (ST) can take over an important managing role in the future electrical distribution grid system and can provide many advanced grid services compared to the traditional transformer. However, the risk is that the advanced functionality is balanced out by a lower reliability. To address this concern, this work conducts thermal stress analysis for a modular multilevel converter (MMC), which is a promising solution for the medium voltage stage of the ST. The focus is put on the mission profiles of the transformer and the impact on the thermal stress of power semiconductor devices. Normal operation at different power levels and medium voltage grid faults in a feeder fed by a traditional transformer are considered as well as the electrical and the thermal stress of the disconnection and the reconnection procedures. For the validation, the thermal stress of one MMC cell is reproduced on a laboratory setup with high-speed temperature measurement. It is concluded that the investigated conditions, including inrush currents and power variations, do not cause a significant lifetime reduction.

Three‐phase battery storage system with transformerless cascaded multilevel inverter for distribution grid applications

A distributed generator (DG) based on renewable energy is a promising technology for the future of the electrical sector. DGs may benefit utility companies and customers in a variety of perspectives. However, DGs suffer from intermittent behaviour. Storage systems appear as an attractive solution to support the continuous operation of DGs. The technology within the storage also plays an important role, since DGs and storage are connected in medium-voltage grids. The use of batteries and the DC/AC converter in its conventional structure presents drawbacks in such grids. In this context, this study presents a three-phase transformerless battery storage system (BSS) based on a cascaded H-bridge inverter applied to a medium-voltage grid. The BSS is composed of eight equal series connected H-bridge converters, without bulk transformers, for connection to a distribution grid. Each converter contains 75, 12V/600Ah lead-acid batteries. The converters are controlled through pulse-width modulation at 600Hz. The BSS is able to keep working even with a failure of one of its converters. Reactive energy compensation not compensated by an existent passive filter is also performed. A case study with simulated and experimental results obtained through a hardware-in-the-loop system is presented showing the efficacy of the proposed BSS.

Reverse Power Flow Control in a ST-Fed Distribution Grid

The massive integration of distributed generation in the grid poses new challenges to the system operators, like the reverse power flow from the low voltage (LV) to medium voltage (MV) grid. In the case of high DG power production and low load absorption, the voltage rises in the line reaching the upper voltage limit. At this regard the smart transformer (ST) offers a new possibility to limit the reverse power flow in the MV grids. The ST can adapt the voltage waveform modifying the frequency in order to interact with the local distributed generation, that are normally equipped with droop characteristic. However, when a fast change in the frequency is applied to avoid reverse power flow to MV grid, stability problems, so far not investigated, arise. In this paper, the stability analysis has been performed analytically and validated by means of control-hardware-in-loop in a real time digital simulator and with experimental results in laboratory.

Optimum Number of Cascaded Cells for High-Power Medium-Voltage AC–DC Converters

For power electronic systems to interface medium-voltage grids, e.g., in future electric ships, usually cascaded cells converters need to be employed, whereby either few cells featuring power semiconductors with high blocking voltage capability or a larger number of cells using low-voltage (LV) semiconductors can be used. As shown in this paper, physics-inspired empirical models of the dependence of Insulated-Gate Bipolar Transistor (IGBT) power modules' loss-relevant characteristics on the blocking voltage enable an analytic optimization of the efficiency of a cascaded H-bridges (ac-dc) converter, which is complemented by a full efficiency versus power density ηρ-Pareto optimization. For a 10-kV grid, 1200V or 1700V are identified as optimum blocking voltages, resulting in a suitable trade-off between efficiency and power density. Significant efficiency and power density gains can be realized by replacing silicon IGBTs by LV silicon carbide (SiC) devices in multi-cell systems, whereas single-cell designs based on high-voltage SiC devices suffer from the high dv/dt and di/dt values required to limit switching losses. Reliability is analyzed considering redundancy, showing that the reliability of designs based on lower blocking voltages can be comparable with that of designs using higher blocking voltages, and hence fewer cells, if similar effort concerning additionally installed power capability is considered.

Overload Control in Smart Transformer-Fed Grid

Renewable energy resources and new loads—such as electric vehicles—challenge grid management. Among several scenarios, the smart transformer represents a solution for simultaneously managing low- and medium-voltage grids, providing ancillary services to the distribution grid. However, unlike conventional transformers, the smart transformer has a very limited overload capability, because the junction temperature—which must always be below its maximum limit—is characterized by a short time constant. In this work, an overload control for smart transformer by means of voltage and frequency variations has been proposed and verified by means of simulations and experiments.

Suppression of Time-Varying Interharmonics Produced by Medium-Frequency Induction Melting Furnaces by a HAPF System

This paper describes the design and field implementation of a hybrid active power filter (HAPF) system to suppress time-varying interharmonics injected into the grid by medium-frequency coreless induction melting furnaces (IMFs). In a sample steel melt shop, variable-frequency load-resonant inverters of works coils are supplied from the medium-voltage grid via 12- and 24-pulse thyristor rectifiers. The cross-modulation phenomenon in ac–dc–ac link of the medium-frequency coreless IMF installations produces interharmonics and characteristic and uncharacteristic harmonics in the grid-side line current waveforms. Furthermore, frequencies of dominant interharmonics are migrating in time inside a frequency window as the operating frequency of the load-resonant inverter varies in wide range during a melting cycle. The HAPF system developed for this application consists of nine HAPF units operating in parallel in current control mode and is connected to the grid via a coupling transformer. Current control is achieved by extracting high-order fixed frequency characteristic harmonic components from the reference current signal and defining a fixed hysteresis band for each HAPF unit. The input LC filter of the HAPF system is optimized in the design stage by taking into account the frequency range of prominent interharmonics. Theoretical findings are verified in a sample steel melt shop by extensive field measurements. Field test results have shown that the developed HAPF system suppresses successfully the dominant time-varying interharmonics and harmonics in the frequency range from 250 to 650 Hz.

Flicker Caused by Operation of Industrial Technology

There are more and more electrical devices operating in industrial plants which impact negatively on the distribution network. The EN 50160 standard defines the voltage characteristics of electricity supplied by the public distribution system and it is a problem for a distribution network operator when the voltage quality in the distribution network does not comply with the requirements of the EN 50160 standard because of complaints about voltage quality. Such a complaint is justified and the distribution network operator has to pay a penalty and has to remedy the situation. This paper describes a problem of flicker in the medium-voltage distribution grid when the flicker is produced by one customer operating a forging press. The remedies from the side of the distribution network operator and the customer with the aim of reducing the flicker level in the grid are described.

Superordinate Control Structure for Hybrid Compensation Systems

Medium voltage grids are subject to a change in load situation due to the installation of decentralized generation plants. Predominantly load symmetry, active and reactive power as well as the required capacity of earth fault compensation is affected. Inverter-based plants as well as consumers using switching power supplies or phase control cause distortions of frequency. Simultaneously electrical energy storage is gaining importance within rural distribution networks to compensate fluctuating generation plants. A hybrid compensation system includes a transformer, a multi-phase inverter, diverse storage systems and a communication-capable control unit, which may be connected to the control center of the distribution system operator (DSO). It is object to current research activities to design and evaluate the functions of the superordinate control unit, which has to parametrize individual components, coordinate the energy flows between them and to embed several measuring instruments. Within the present work, basic considerations for design of system control are outlined.

A Review of Power Electronics for Grid Connection of Utility-Scale Battery Energy Storage Systems

The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium-voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.

Inverter-based hybrid compensation systems contributing to grid stabilization in medium voltage distribution networks with decentralized, renewable generation

Medium voltage grids are subject to a change in load situation due to the installation of decentralized generation plants. Predominantly load symmetry, active and reactive power as well as the required capacity of earth fault compensation are affected. Inverter-based plants as well as consumers using switching power supplies or phase control cause distortions of frequency. Simultaneously, electrical energy storage is gaining importance within rural distribution networks to compensate fluctuating generation plants. A hybrid compensation system includes a transformer, a multi-phase inverter, diverse storage systems and a communication-capable control unit, which may be connected to the control center of the distribution system operator (DSO). It is object to current research activities to design and evaluate the functions of the superordinate control unit, which has to parametrize individual components, coordinate the energy flows between them and to embed several measuring instruments. Within the present work, basic considerations for design of system control are outlined.

Minimum Voltage Tracking Balance Control Based on Switched Resistor for Modular Cascaded Converter in MVDC Distribution Grid

This paper provides a characterizationr analysis for the voltage balance of modular cascaded converter in a medium-voltage direct-current distribution grid. Specifically, voltage stability and traditional auxiliary resistor solution are analyzed from the theoretical level. On this basis, a minimum voltage tracking balance (MVTB) control based on a switched resistor is proposed. The voltage balance during startup and normal locked processes can be adjusted effectively by MVTB control. Moreover, the resistor does not need to absorb power continuously; instead, it is just switched into operation for a while when a difference occurs, and the power loss can be decreased after the system recovers to a steady state. Finally, experimental results verify the effectiveness of the MVTB control.

From old to new: Assessing cybersecurity risks for an evolving smart grid

Future smart grids will consist of legacy systems and new ICT components, which are used to support increased monitoring and control capabilities in the low- and medium-voltage grids. In this article, we present a cybersecurity risk assessment method, which involves two interrelated streams of analyses that can be used to determine the risks associated with an architectural concept of a smart grid that includes both legacy systems and novel ICT concepts. To ensure the validity of the recommendations that stem from the risk assessment with respect to national regulatory and deployment norms, the analysis is based on a consolidated national smart grid reference architecture. We have applied the method in a national smart grid security project that includes a number of key smart grid stakeholders, resulting in security recommendations that are based on a sound understanding of cybersecurity risks.

An On-line Monitoring System for Over-voltages Based on a Two-stage Voltage Divider and Field Measurement Results in Medium-voltage Grids

An on-line monitoring system based on a two-stage voltage divider is presented for monitoring the switching and lightning over-voltages in 10–35-kV medium-voltage grids. A low-damped capacitive divider (voltage ratio of 1000:1) is designed as an over-voltage monitoring sensor for primary voltage division, and an R-C damped divider is designed for the secondary voltage division (voltage ratio of 10:1 for the 10-kV grid and 20:1 for the 35-kV grid). The divider's designing methods and response characteristics are analyzed. The result of the measurement indicates that the voltage divider has good frequency response, which can meet the requirements for monitoring switching and lightning over-voltages. Moreover, with the data double-buffering and high-speed sampling techniques, this device can collect longer data (64 Mbytes) and shorten the interval (less than 1 sec) at a high sampling rate (up to 20 MHz). Finally, the device has been installed in a 110-kV substation in parallel with a 10-kV bus for two years. Many over-voltage waveforms (including switching, induced lightning, and back-flashover lightning over-voltages) are captured on site. The actual measured results indicate that the device can monitor fast transient over-voltages precisely.