“INTEGRATED ELECTRICAL AND LONG-TERM ANALYSIS” FOR THE MEDIUM AND LOW VOLTAGE GRIDS OF KWH NETZ

The increasing demand for electricity and the penetration of renewable energy resources into the electrical grid are triggering the electric power system to undergo changes and complex modifications at high, medium and low voltage levels. There are large and complex projects like Smart Grid Gotland that are developing solutions to deal with these challenges. However, there are several scenarios that could be more practical to be first studied at the simulation level. Therefore, the purpose of this study is to propose and develop a co-simulation platform that allows performing real-time monitoring and control tests and simulations for MV/LV grids. This platform combines both electrical and information and communication technology (ICT) infrastructure simulations. It consists of a real-time power system simulator (Opal-RT) connected to a communication network emulator (OPNET) through an automatic meter reading interface that is based on commercial off-the-shelf (COTS) and low cost devices like Raspberry Pi and Arduino boards. The platform allows the analysis of realistic MV/LV grids and communication networks and will facilitate the simulation and comparison of low voltage control strategies for real-time test scenarios that could be more complicated to test in the reality due to cost and complexity aspects (i.e. high power losses on the low voltage grid).

Investigations of power quality can be performed for every stages of power systems, i.e. low, medium and high voltage levels. Power quality disturbances at low voltage levels are mainly originated from nonlinear loads and switching processes. For the high voltage levels, disturbances originated by the load variabilities in the grid and the power plant itself can be counted as other important factors. In this study, firstly, some important recent studies related to power quality in smart grid has been reviewed. Then, a simulation software developed for power quality analysis that can be integrated to the smart grid infrastructure has been introduced.

With a large penetration of low carbon technologies (LCTs) at medium voltage and low voltage levels, electricity distribution networks are undergoing rapid changes. Much research has been carried out to analyse the impact of employing LCTs in distribution networks based on either real or synthetic network samples. Results of such studies are usually case specific and of limited applicability to other networks. Topological properties of a distribution networks describe how different network components are located and connected, which are critical for the investigation of network performance. However, the number of network modelling and simulation platforms are limited in the open literature which can provide random realistic representations of electricity distribution networks. Thus, it is difficult to arrive to generalized and robust conclusions on impact studies of LCTs. As the initial step to bridge this gap, this paper studies the topological properties of real-world electricity distribution networks at the medium voltage level by employing the techniques from complex networks analysis and graph theory. The networks have been modelled as graphs with nodes representing electrical components of the network and links standing for the connections between the nodes through distribution lines. The key topological properties that characterize different types (urban and sub-urban) of distribution networks have been identified and quantified. A novel approach to obtain depth-dependent topological properties has also been developed. Results show that the node degree and edge length related graph properties are a key to characterize different types of electricity distribution networks and depth dependent network properties are able to better characterize the topological properties of urban and sub-urban networks.

The future of safe and reliable electrical power supply will not be based on fossil resources. Thus, sustainable electric power generation as well as their transmission and distribution gain more and more attention. Especially in Germany, this leads to a steadily increasing level of renewable energies. Due to a significant part of the electrical power generation shifts into the medium and low voltage levels. Their increasing importance to the electrical power supply requires a new way of energy distribution management. This paper outlines a solution for an energy distribution management system for medium and low voltage levels, focusing on bidirectional, vertical power flow between medium and high voltage levels. Control concepts for storages and distributed energy resources are outlined. Based on numerical case studies the characteristics of the developed energy management system were analyzed. It is shown, that a desired power flow between grid levels can be realized, without creating critical grid situations.

The growing demand for electrified heating, electrified transportation, and power-intensive data centres challenge distribution networks. If electrification projects are carried out without considering electrical distribution infrastructure, there could be unexpected blackouts and financial losses. Datasets containing real-world distribution network information are required to address this. However, the existing dataset at NERC that covers the whole of Great Britain (GB) does not provide information about demand and capacity, which is insufficient for evaluating the connection feasibility. Although each distribution network operator (DNO) has detailed network information for their supply area, the information is scattered in separate files and different formats even within the same DNO, which limits usability. On the other hand, studying the coupling between energy systems and societal attributes such as household heating is important in promoting social welfare, which calls for more comprehensive datasets that integrate the social data and the energy network data. However, social datasets are usually provided on a regional basis, and the link to energy networks is not straightforward, which explains the lack of the comprehensive datasets. To fill these gaps, this paper introduces two datasets. The first is the main dataset for the GB distribution networks, collecting information on firm capacity, peak demands, locations, and parent transmission nodes (grid supply points, namely GSPs) for all primary substations (PSs). PSs are a crucial part of UK distribution networks and are at the lowest voltage level (11 kV) with publicly available data. Substation firm capacity and peak demand facilitate an understanding of the remaining room in the existing network. The parent GSP information helps link the released datasets to transmission networks. These datasets are collected, standardised, and merged from various files with different formats published by the six DNOs in GB, using a Python script and manual validation. The second dataset extends the main network dataset, linking each PS to the number of households that use different types of central heating recorded in census data (Census in year 2021 for England and Wales, and Census 2011 for Scotland as the up-to-date Census 2022 data is not fully released). The derivation of the second dataset is based on the locations of PSs collected in the main dataset with appropriate assumptions. The derivation process may be replicated to integrate other social datasets.The datasets have the following reuse potentials: 1) Given the PS demand, capacity, and locations in our datasets, users can estimate the connection feasibility and evaluate the optimal deployment locations for different energy technologies, including electric vehicles, heat pumps, and the growing data centres, under different scenarios and at a national scale. These evaluations are beneficial not only for academic research, but also for industrial planning and policy making. 2) Our extended dataset links household information to distribution networks. The integrated information facilitates cross-disciplinary research and analysis across social science, energy policy, and power systems. 3) The network demand and capacity information provided by the datasets can also help with realistic parameter settings to improve the accuracy of case studies in broader power system research.

Utility-scale and small scale wind and solar power installations along with electric vehicle charging stations, and other active sources of energy are increasing at the medium and lower voltage levels in the distribution grid. This situation requires a better understanding of the impact of high penetration of weather-dependent renewable energy sources on the operating conditions of the distribution network at both medium and low voltage levels. Despite the need, a multi-voltage level distribution network model, based on real network data and weather-dependent renewable generation data, has not been presented for distribution grid studies. This paper presents a comprehensive multi-voltage level active distribution network model based on real network data along with load and generation time-series for about a year. The network topology is modelled based on geographical data for various rural, semi-urban, and urban locations. The distribution network is embodied with a large share of renewable generation sources, with generation time-series simulated from meteorological data. The network is also flexible to incorporate other assets such as electric vehicle charging stations, storage, etc. The presented active distribution network model can be used to study, optimize, and control the effects of weather-dependent generation and other network assets in the distribution grid.

Short term load forecasting and the effect of temperature at the low voltage level

In a large-scale transmission network, such as the European power system, the regional characteristics of electrical demand may have an impact on local voltage stability. The dynamic load behaviour depends on the sectoral composition of demand, which is subject to regional and seasonal variation. An aggregate load model is presented, which accounts for the sectoral characteristics of 210 European regions for all hours a year from the perspective of the high, medium, and low voltage levels. The proposed load model is used to better understand the influence of regional characteristics on voltage recovery during a short-circuit event, as well as the static point of voltage collapse. Several countries are considered in terms of their sectoral composition, to represent intermediate regions or such dominated by the industry and households. Moreover, the dynamic load model is benchmarked against two static load models, assuming a constant power (PQ) load or a constant impedance (Z) load. The conducted study demonstrates that regional differences in the composition of demand highly impact local voltage stability.

The safety of workers and equipment in the power grid requires the shutdown of power plants in case of maintenance or malfunction. The shutdown relies on protective devices that must be properly coordinated to isolate only the part of the grid affected by the malfunction. Using the DIgSILENT PowerFactory software, an analysis was conducted on a model of an accurate real grid integrating a photovoltaic power plant with a capacity of 390 kW. Protection coordination testing was carried out for three-phase, two-phase, and single-phase short circuit currents at arbitrarily selected locations at medium and low voltage levels. Protection in the time domain is coordinated on lines and busbars to determine the speed and selectivity of protective devices. The analysis results indicate that adequately adjusting the three-phase short circuit at the main transformer output 110/10 kV with an impedance of 0 Ω and an allowed protection operation time of up to two seconds can be correctly addressed within 36.7 ms.

New approaches for operating distribution networks are required to handle highly volatile voltage profiles resulting from distributed generation capacities with intermittent feed-in characteristics connected at the medium and low voltage level. This paper discusses the use of a power-electronics-based inline voltage regulator in a low voltage network. Simulations for one year are carried out to investigate the influence of the regulator on a network's voltage profile and to derive a mission profile for the device. Based on this mission profile an analysis of the stresses on the power semiconductors is carried out to illustrate the process of extracting the stresses relevant for a specific component from the mission profile of the device.

Solid state transformers (SSTs) are at the forefront of emerging technologies, enhancing distribution and transmission networks. These transformers facilitate the development of highly controllable and robust smart grids, seamlessly integrating renewable energy sources and accommodating connections for both alternating current and direct current components/lines at medium or low voltage levels. This research paper introduces an innovative SST incorporating the inductive power transfer (IPT) technology, currently under development within SSTAR, a Horizon Europe project. This novel device operates at a frequency of 50 kHz and provides with an output power of 50 kW. The IPT system is submerged in an ester-based, biodegradable dielectric fluid, providing effective cooling and insulation characteristics. The primary focus of this research is to highlight the guidelines for the optimal design of the forced liquid cooling system of the SST module. The analysis employs a multi-faceted simulation strategy to calculate the cooling aspects of the transformer, utilizing the commercial software ANSYS. This involves 3D coupled Electromagnetic (EMAG) - Computational Fluid Dynamics (CFD) simulations, as well as stand-alone CFD and thermal FEA simulations, ensuring robust cross-verification of the obtained results. This novel SST exhibits improved thermal performance, achieving a hot-spot temperature of 58 °C, the lowest value found in similar research studies. The findings underscore the efficiency and robustness of the proposed design, marking a significant step towards the realization of active and environmentally-friendly electrical networks.

An approach for simultaneous distribution, sub-transmission, and transmission networks expansion planning

The large-scale integration of rooftop PVs stalls due to the voltage limit violations they provoke, the uncontrolled reactive power flow in the superordinate grids and the information and communications technology (ICT) related challenges that arise in solving the voltage limit violation problem. This paper attempts to solve these issues using the LINK-based holistic architecture, which takes into account the behaviour of the entire power system, including customer plants. It focuses on the analysis of the behaviour of distribution grids with the highest PV share, leading to the determination of the structure of the Volt/var control chain. The voltage limit violations in low voltage grid and the ICT challenge are solved by using concentrated reactive devices at the end of low voltage feeders. Q-Autarkic customer plants relieve grids from the load-related reactive power. The optimal arrangement of the compensation devices is determined by a series of simulations. They are conducted in a common model of medium and low voltage grids. Results show that the best performance is achieved by placing compensation devices at the secondary side of the supplying transformer. The Volt/var control chain consists of two Volt/var secondary controls; one at medium voltage level (which also controls the TSO-DSO reactive power exchange), the other at the customer plant level.

DC distribution system at low and medium voltage level is undergoing rapid development, driven by the growth of power electronic technology and integration of distributed generation. Virtual synchronous generator (VSG), which can operate in grid-connected mode and islanded mode, provides a convenient interconnecting interface between DC distribution system and AC grid. However, the multiple mode operation and transition of VSG in grid-connected and islanded process is not thoroughly studied, including start-up and reconnection. On the other hand, most research into oscillation mechanism in the past does not consider the outer frequency droop loop. To address these issues, this paper proposed to use VSG in DC distribution system. A phase and frequency regulator is designed to achieve smooth start-up and a method is proposed to realize reconnection of VSG with AC grid in the current research. Stability analysis considering the frequency droop loop is also conducted, based on which proper damping is selected to mitigate the oscillation in transition and grid-connected mode. The proposed strategy is verified by electro-magnetic type simulation on PSCAD/EMTDC.

Connecting solar power plants at low and medium voltage level is the most efficient way to integrate this type of renewable energy source power plant into the power system. With this approach, it is achieved that the generation is close to the consumption/demand and therefore, active power flows on the overhead lines and cables are lower which consequently leads to a reduction of losses both in transmission and distribution system. This estimation will be carried out by using real historical measurements of the consumption/demand and active power losses values for every hour in the one year time period. Those values present inputs for the transmission system simulation model as well as the reference values for comparison of the results after the integration. Distributed generation will be modelled as load reduction on substations 110/X kV/kV taking into account hourly profiles of both solar power plant generation and consumption/demand on the relevant substations.