Medium Voltage Distribution Grid Research Articles

Power Quality Enhancement in Residential Smart Grids Through Power Factor Correction Stages

The proliferation of nonlinear loads and the increasing penetration of distributed energy resources in medium-voltage (MV) and low-voltage (LV) distribution grids make it more difficult to maintain the power quality levels in residential electrical grids, especially in the case of weak grids. Most household appliances contain a conventional power factor corrector (PFC) rectifier, which maximizes the load power factor ( $\text{PF}$ ) but does not contribute to the regulation of the voltage total harmonic distortion ( $\text{THD}_V$ ) in residential electrical grids. This paper proposes a modification for PFC controllers by adapting the operation mode depending on the measured $\text{THD}_V$ . As a result, the PFCs operate either in a low current total harmonic distortion ( $\text{THD}_I$ ) mode or in the conventional resistor emulator mode and contribute to the regulation of the $\text{THD}_V$ and the $\text{PF}$ at the distribution feeders. To prove the concept, the modification is applied to a current sensorless nonlinear controller applied to a single-phase boost rectifier. Experimental results show its performance in a PFC front-end stage operating in the continuous conduction mode connected to the grid with different $\text{THD}_V$ .

An Improved DC Solid State Transformer Based on Switched Capacitor and Multiple-Phase-Shift Shoot-Through Modulation for Integration of LVDC Energy Storage System and MVDC Distribution Grid

This paper proposes an improved dc transformer (NDCT hereinafter) based on switched capacitor with reduced switches for the integration of low-voltage dc energy storage systems and medium-voltage dc power distribution grid. The topology, phase-shift (PS) shoot-through and multiple PS modulations, and voltage, current, and power characterizations, as well as commutation characterization and control strategy, are proposed and analyzed comprehensively. Compared with the traditional schemes, the proposed NDCT not only can enhance reliability and flexibility but can increase voltage adjustment and power transfer capabilities as well. Moreover, the current ripple performance and soft-switching performance are improved, and the number and capacity of switches are reduced, thereby increasing the efficiency and cost of the system. Finally, an NDCT prototype is built. Experimental results verify the correctness and effectively of the proposed solution.

The use of power restoration systems for automation of medium voltage distribution grid

The use of power restoration systems for automation of medium voltage distribution grid

Novel strategy of updating Volt/VAr control set-points for eliminating interactions between different voltage regulating devices and DG units

ABSTRACTIn modern medium voltage distribution grids, voltage and reactive power (Volt/VAr) control can effectively be achieved using the time delayed responses of on-load tap changer, line voltage regulators, capacitor banks and distributed generation (DG) units. The timely coordination among different Volt/VAr control (VVC) devices can be assured through online control processes and updating the control parameters (i.e. set-points) of VVC devices. This is essential for efficient operation of the distribution grids while achieving steady-state voltage recovery and other control objectives such as loss minimisation; as the control interactions among VVC devices and Volt/VAr support DG units caused by poor coordination can lead to operational conflicts. In this article, a novel control approach is proposed for updating VVC set-points for eliminating operational interactions between different voltage regulating devices; thereby, minimising power losses in a network. The proposed strategy has been tested on a practical distribution system derived from the state of New South Wales, Australia, and the simulation results are reported. The results have demonstrated that the interactions among VVC devices and Volt/VAr support DG units can be avoided while minimising power losses, with the implementation of the proposed strategy.

Benefits of High Voltage SiC Applications in Medium Voltage Power Distribution Grids

This paper evaluates potential benefits of high voltage (HV) SiC devices in medium voltage (MV) distribution grids. The MV microgrid, that HV SiC devices can benefit most, is selected as the “killer application” and focused in this paper. The design and simulation are carried out to compare Si-and SiC-based grid interface converters for the quantitative benefit assessment both at converter level and system level. The SiC-based converter has significant benefits in weight and size, and shows enhanced performance and functionality on power quality, system stability and low voltage ride through (LVRT) as well.

A High Step-Up Ratio Soft-Switching DC–DC Converter for Interconnection of MVDC and HVDC Grids

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency dc–dc converters with high voltage step-up ratio to interconnect medium-voltage (MV) dc distribution grids and high-voltage (HV) dc transmission grids. This paper presents an isolated bidirectional soft-switching dc–dc converter combining two-level converters in parallel on the MV side and a modular multilevel converter (MMC) on the HV side. A dedicated control method of the proposed converter is presented. By the proposed method, a certain reactive current is injected into the MV side by the MMC to ensure soft-switching on the MV side. The proposed converter presents low power-semiconductor total device rating and low semiconductor losses over a wide power range at variable input and output voltages. Simulation of a 50 kV/400 kV, 400 MW converter is conducted to evaluate semiconductor losses and verify the validity of this work.

On the Distribution of Lightning Current among Interconnected Grounding Systems in Medium Voltage Grids

This paper presents the results of a first investigation on the effects of lightning stroke on medium voltage installations’ grounding systems, interconnected with the metal shields of the Medium Voltage (MV) distribution grid cables or with bare buried copper ropes. The study enables us to evaluate the distribution of the lightning current among interconnected ground electrodes in order to estimate if the interconnection, usually created to reduce ground potential rise during a single-line-to-ground fault, can give place to dangerous situations far from the installation hit by the lightning stroke. Four different case studies of direct lightning stroke are presented and discussed: (1) two secondary substations interconnected by the cables’ shields; (2) two secondary substations interconnected by a bare buried conductor; (3) a high voltage/medium voltage station connected with a secondary substation by the medium voltage cables’ shields; (4) a high voltage/medium voltage station connected with a secondary substation by a bare buried conductor. The results of the simulations show that a higher peak-lowering action on the lighting-stroke current occurs due to the use of bare conductors as interconnection elements in comparison to the cables’ shields.

A Nearest Level PWM Method for the MMC in DC Distribution Grids

For modular multilevel converters (MMCs) applied to medium-voltage dc distribution grids, using the traditional nearest level modulation (NLM) as in HVdc systems can lead to severe current distortion due to significantly reduced module numbers. This paper proposes a hybrid modulation method combining NLM and pulse width modulation (PWM) where only one module per arm operates under PWM mode. The proposed nearest level PWM (NL-PWM) method not only significantly reduces the current distortion, but also avoids the complicated voltage balancing control in each module. The harmonic characteristics of NL-PWM are derived using double Fourier transform, which provides a theoretical basis for selecting the module number and switching frequency for medium-voltage applications in accordance with grid harmonic requirements. Finally, the harmonic characteristics and feasibility of the proposed modulation method are validated by simulation and experimental studies on an MMC with six modules per arm. The simulated and experimental results reveal that NL-PWM has better voltage and current harmonic characteristics over NLM and CPS-PWM, thereby suiting the application of MMC with few models.

Reliable and efficient operation of closed-ring distribution grids supported by distribution automation

Abstract Closed-ring operation of distribution grids has several advantages over the more commonly used open-ring distribution grid topology. Power flows will naturally balance out between the feeders of a (typically) ring shaped medium voltage distribution grid. This leads to reduced peak loading of components, and therefore reduced grid losses. As such, investments in grid reinforcement can be postponed or avoided, while operational costs can be reduced. The negative side-effect of closed-ring operation, reduced reliability of supply, can be (largely) mitigated by implementing distribution automation. This paper discusses and analyses the potential grid loss reduction of closed-ring operation, the effects on reliability and possible mitigations and finally field-test results to validate the theoretical analysis.

Smart fault handling in medium-voltage distribution grids

This study presents first results from a pilot project on using fault indicators and self-healing in medium-voltage distribution grids. Directional earth-fault indicators were chosen and deployed in real environments to test their functionalities and quantify their benefits for efficient grid operations. Self-healing concepts using these indicators will be tested in the described project. Different communication schemes will be explored in order to find the most cost-effective solutions. Theoretical potentials of the expected benefits of the fault indicators located at different positions in the grid were modelled and will be compared to the results from field tests.

Predictive management of low-voltage grids

This study presents the functional model that provides net-load forecasts for each low-voltage (LV) node (including PV generation and self-consumption), developed for the UPGRID (real proven solutions to enable active demand and distributed generation flexible integration, through a fully controllable low-voltage and medium-voltage distribution grid) framework project. Several tests scenarios were simulated and the results regarding forecast accuracy and computational performance are given. Results demonstrate the applicability of the distribution in memory solution in a practical operational scenario, offering a highly scalable forecasting system for LV networks. Based on forecasts and available real-time information, an architecture for preventive control of LV grids is built upon chronological analysis capabilities of DPlan. An illustration on how such capabilities are used in the context of the foreseen UPGRID preventive control framework is provided.

Impact of wind power plant operation on MV distribution grids

The study summarises the results of short-term voltage quality (VQ) measurements in medium-voltage distribution grids with connected wind power plants. The measurements were made in grids on supply territory of the utility E.ON Czech Republic in 2013–2016 period. VQ parameters were evaluated according to the standard EN 50160. Power flows of individual wind power plants were also evaluated and mutually were compared to find the pattern of their performance in various year seasons. Finally, an impact of wind power plant operation on VQ parameters, mainly on voltage variations and flicker, was analysed.

Distributed voltage control coordination between renewable generation plants in MV distribution grids

This study focuses on distributed voltage control coordination between renewable generation plants in medium-voltage distribution grids (DGs). A distributed offline coordination concept has been defined in a previous publication, leading to satisfactory voltage regulation in the DG. However, here, it is shown that reactive power provision increases the power losses in the grid to a significant extent. A real-time coordination concept is developed which utilises communication systems with online signal exchange between the assets. Performed case studies reveal that this coordination scheme reduces the power losses within the DG to a measurable extent.

Analysis of voltage quality parameters in MV distribution grid

This study describes the impact of operation of big customers on voltage quality (VQ) parameters in medium-voltage (MV) distribution grids. The survey is based on the evaluation of 45 VQ measurements which were carried out in 15 different delivery points of other customers. These 45 measurements were made in years 2014, 2015 and 2016 (15 measurements a year) and were evaluated according to the standard EN 50160. The impact on voltage variations and other VQ parameters was analysed.

PMU-based power system analysis of a medium-voltage distribution grid

The installation of phasor measurement units (PMUs) by Enduris, one of the distribution network operators in the Netherlands to monitor their 50 kV ring distribution grid has resulted in increased observability of the power flows and voltages. However, with the increased observability, there is a need to analyse the distribution grid based on the data generated by the PMU monitoring system. This study provides steady-state and dynamic analysis of the 50 kV Enduris ring distribution grid in terms of voltage and power flow behaviour based on the real-time data acquired from the PMUs. First, the Enduris 50 kV distribution grid under study is modelled in PowerFactory software. This model is subsequently used for analysis of the distribution grid behaviour for two contingencies. The contingencies are actual grid events observed by the PMUs, and using this data, the contingencies are simulated and analysed. The analysis results of each contingency are compared with the data from the PMU monitoring system.

Advanced Droop Control in Islanded Microgrids Using Dynamic Phasor Models

A dynamic phasor model for low- and medium-voltage distribution grids is presented. This model accurately represents the transient dynamics of this type of grid, and its low order makes it well suited for control design. A novel fixed-structure, robust control design method based on convex optimization is then used to design a decentralized low-order controller that shows a significantly improved transient performance compared to classical droop controllers. The performance of the designed controller is then validated in a simulation example for voltage and frequency control of an islanded medium-voltage distribution grid.

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.

Innovative planning method for the construction of electrical distribution network master plans

This paper proposes a method to optimize the feeders’ routes for medium-voltage distribution grids. The study aims to provide a static model for the construction of master plans and can also be applied to expansion planning. The optimization of the network is performed by a three-stage methodology. The first stage minimizes the total length of the conductors with an adapted simulated annealing algorithm. The secured feeder architecture is directly modelled in the structure of the solution. The second stage performs the balancing of the loads between the feeders in order to respect the different technical constraints. The last stage is the allocation of remote-controlled switches according to the objectives of supply quality. The methodology is applied to a real distribution network of 136 nodes. The complexity of the example is significant because there are no preselected possible routes. Instead of that, all the connections are considered, taking into account the geographical topology of urban areas.

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.

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.