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Publication# White paper on DC technologies for Switzerland’s electricity transmission and distribution

Alexandre François Christe, Drazen Dujic, Stefan Milovanovic, Mario Paolone, Farhad Rachidi-Haeri, Reza Razzaghi, Milan Utvic, Zhaoyang Wang

*Work Package 3 of the SCCER-FURIES, *2019

Rapport ou document de travail

Rapport ou document de travail

Résumé

Existing AC power systems, established more than a century ago, are increasingly challenged by DC technologies, enabled by significant advancements in the power electronics and related scientific fields. Three major application areas for DC transmission are established: transferring bulk power over long distances, interconnecting grids and connecting offshore wind. Medium and low voltage DC application become more appealing based on the improved controllability, more effective integration of renewable energy sources, higher power density and better compatibility with underground cables. Such technologies will be attractive for the Swiss energy transition as they might provide more effective solutions for the densification of power systems, the integration of converter-based renewable energy sources and pumped-storage plants. In order to achieve this, several research challenges, however, need to be overcome and standardization must further advance. Several academic partners from Switzerland contribute to these research problems in WP3: “Multi-Terminal AC-DC Grids and Power Electronics” within the SCCER FURIES. In this paper six major topics are presented: -“General overview of DC options” where present and future applications of DC technologies as wells as MVDC grids development issues are discussed. -“MMC-based MVDC converters” where a selection of modular multilevel converter as a platform in order to provide flexibility in addressing a multitude of applications and conversion needs is shown. Several topological adaptations are proposed, leading to novel converter topologies. -“AC/DC resonance analysis” where analysis allowing to find the resonance location and to analyzeres-onance nodes contribution to critical mode. These frequency analysis methods permit to foresee net-work frequency behavior that is becoming an important issue due to the growing number of power electronics converters in the network. -“Overview of HVDC breaker technologies” where basic requirements for fast and reliable HVDC circuit breakers as well as the differences to HVAC technology are introduced. -“HVDC circuit breakers: testing methods and challenges” where the limits of HVDC circuit breakers are explored. In this section, a flexible, modular high current source is presented. The source is intended to act as a hardware-in-the-loop test benchfor future HVDC circuit breakers, by driving highly dynamic and arbitrary current waveforms through dynamic loads (e.g. DC arc). -“Fault location principles” where the significant influence of fault location on the network security of supply and quality is drawn. A newly-developed technique which is based on the electromagnetic time reversal (EMTR) theory that can be applied to radial/meshed AC/DC power transmission or distribution networks is presented and compared to other Travelling Wave –based methods. The outputs and outlooks are drawn in order to conclude the paper.

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Jean-Yves Le Boudec, Mario Paolone, Styliani Sarri, Lorenzo Zanni

In the operation of power systems, the knowledge of the system state is required by several fundamental functions, such as security assessment, voltage control and stability analysis. By making reference to the static state of the system represented by the voltage phasors at all the network buses, it is possible to infer the system operating conditions. Until the late 1970s, conventional load flow calculations provided the system state by directly using the raw measurements of voltage magnitudes and power injections. The loss of one measurement made the calculation impossible and the presence of measurement errors affected dramatically the computed state.To overcome these limitations, load flowtheory has been combined with statistical estimation constituting the so-called state estimation (SE). The latter consists in the solution of an optimization problem that processes the measurements together with the network model to determine the optimal estimate of the system state. The outputs of load flow and SE are composed of the same quantities, typically the voltage magnitude and phase at all the network buses, but SE uses all the types of measurements (e.g., voltage and current magnitudes, nodal power injections and flows, synchrophasors) and evaluates their consistency using the network model. The measurement redundancy is key to tolerate measurement losses, identify measurement and network parameter errors, and filter out the measurement noise. The foregoing properties of SE allow the system operator to obtain an accurate and reliable estimate of the system state that consequently improves the performance of the functions relying on it. Traditionally, SE has been performed at a relatively low refresh rate of a few minutes, dictated by the time requirements of the related functions together with the low measurement acquisition rate of remote terminal units (RTUs). Nowadays, the emerging availability of phasor measurement units (PMUs) allows to acquire accurate and time-aligned phasors, called synchrophasors, with typical streaming rates in the order of some tens of measurements per second. This technology is experiencing a fast evolution, which is triggered by an increasing number of power system applications that can benefit from the use of synchrophasors. SE processes can exploit the availability of synchrophasor measurements to achieve better accuracy performance and higher refresh rate (sub-second). PMUs already compose the backbone of wide area monitoring systems in the context of transmission networks to which several real-time functionalities are connected, such as inter-area oscillations, relaying, fault location and real-time SE. However, PMUs might represent fundamental monitoring tools even in the context of distribution networks for applications such as: SE [5, 6], loss of main [7], fault event monitoring, synchronous islanded operation [9] and power quality monitoring. The recent literature has discussed the use of PMUs for SE in distribution networks both from the methodological point of view and also via dedicated real-scale experimental setups. Since the pioneering works of Schweppe on power system SE in 1970, most of the research on the subject has investigated static SE methods based on weighted least squares (WLS). Static SE computes the system state performing a “best fit” of the measurements belonging only to the current time-step. Another category of state estimators are the recursive methods, such as the Kalman filter (KF). In addition to the use of the measurements and their statistical properties, they also predict the system state by modelling its time evolution. In general, recursive estimators are characterized by higher complexity and the prediction introduces an additional source of uncertainty that, if not properly quantified, might worsen the accuracy of the estimated state. Besides, their ability to filter out measurement noise could not be exploited due to the low SE refresh rate: even in quasi-steady state conditions, the measurement noise was smaller than the state variations between two consecutive time-steps. However, the effectiveness of power system SE based on KF has been recently reconsidered thanks to the possibility to largely increase the SE refresh rate by using synchrophasor measurements. The chapter starts by providing the measurement and process model of WLS and KF SE algorithms and continues with the analytical formulation of the two families of state estimators, including their linear and non-linear versions as a function of the type of available measurements. Finally, two case studies targeting IEEE transmission and distribution reference networks are given.

Mario Paolone, Paola Pongiglione, Fabrizio Sossan

This paper presents a method to determine the optimal location, energy capacity, and power rating of distributed battery energy storage systems at multiple voltage levels to accomplish grid control and reserve provision. We model operational scenarios at a one-hour resolution, where deviations of stochastic loads and renewable generation (modeled through scenarios) from a day-ahead unit commitment and violations of grid constraints are compensated by either dispatchable power plants (conventional reserves) or injections from battery energy storage systems. By plugging-in costs of conventional reserves and capital costs of converter power ratings and energy storage capacity, the model is able to derive requirements for storage deployment that achieve the technical-economical optimum of the problem. The method leverages an efficient linearized formulation of the grid constraints of both the HV (High Voltage) and MV (Medium Voltage) grids while still retaining fundamental modeling aspects of the power system (such as transmission losses, effect of reactive power, OLTC at the MV/HV interface, unideal efficiency of battery energy storage systems) and models of conventional generator. A proof-of-concept by simulations is provided with the IEEE 9-bus system coupled with the CIGRE’ benchmark system for MV grids, realistic costs of power reserves, active power rating and energy capacity of batteries, and load and renewable generation profile from real measurements.

2020This paper focuses on the problem of the probabilistic optimal day-ahead scheduling of energy resources in Active Distribution Networks (ADNs). These resources include both dispersed energy storage systems (DESSs) and volatile renewable embedded generators. Technical constraints related to both energy resources and electrical network are modeled and taken into account in the proposed optimization problem. The paper first proposes a convex formulation of a specific optimal power flow (OPF) used to compute the resources schedule. Its objective function aims at achieving the minimum of the following quantities: network and DESSs losses, energy cost imported from the external grid, and deviations from the day-ahead scheduled power flow with the same external grid. In addition, the ability of using the substation transformer tap-changer is incorporated into the problem with a suitable cost function. The initial OPF formulation is then enhanced thanks to the use of the Mixed Integer Second Order Cone Programming approach in order to formulate a stochastic AC-OPF. The uncertainties of the problem are due to the forecast errors of the PV generation, load consumption and energy prices. The applicability and the effectiveness of the proposed scheduling approach are tested by using a modified version of the IEEE 34 buses test feeder.