Renewable Energy Grid Integration Week 2021, Germany, 27 Sep - 01 Oct 2021 (RE Grid Integration Week 2021)
Germany, 27 September - 01 October 2021

Solar Integration Workshop 2021

Citizens' Energy Cooperatives: Key Drivers of the Energy Transition in Baden-Wuerttemberg
Submission-ID 011
Andreas Ensinger 1, Daria Kern 1, Karlheinz Bozem 2, Anna Nagl 1, David K. Harrison 3, Bruce M. Wood 3
1 Aalen University Beethovenstraße 1 73430 Aalen, Germany
2 bozem | consulting associates | munich Kunstmannstraße 5 80997 München, Germany
3 School of Computing, Engineering and Built Environment Glasgow Caledonian University Glasgow G4 0BA, United Kingdom

Keywords: Renewables, Citizens’ energy cooperatives, Digitization, Servitization, Innovation

Purpose: Within the context of the innovative research project “Citizen Energy Transition”, subsidized by the Ministry of Science, Research and the Arts Baden-Wuerttemberg (funding code Kap. 1403 title group 75), digital business models for the economic, platform-based marketing of PV power will be developed and implemented by 2022. The fundamental problem addressed in this research project is to ensure the continued operation of renewable energy systems, which are no longer eligible for funding under the Renewable Energy Sources Act (EEG). The withdrawal of renewable energy intallations from the EEG funding since 1 January 2021 and the resulting possible shutdown are likely to lead to a breakdown of renewable energy generation capacities. This would be problematic from the perspective of environmental protection, but also with reference to the consistent implementation of the energy transition. Therefore, it is all the more important to develop sustainable business models for community energy cooperatives, operators of renewable energy systems, electricity consumers, network operators, direct marketers, billing service providers and small and medium-sized municipal utilities.

Design/Methodology/Approach: In order to implement user-experienced business models, a hypothesis-based survey of all citizens' energy cooperatives (full survey) in Baden-Wuerttemberg was carried out in this research project in spring 2021. The aim of this empirical survey was to record the current situation and options for action of the citizens' energy cooperatives with regard to the discontinued EEG remuneration for PV systems. Within the context, empirical findings from the customer survey are presented and conclusions are derived.

Findings: By evaluating this survey, key conclusions could be drawn for the development of new, innovative platform-based marketing approaches for the energy generated from renewable energy systems. Among other things, it was found that the current business model of the citizens' energy cooperatives is mainly based on the pure generation of renewable electricity, which is marketed within the framework of the EEG.

Originality/Value: The research team developed a hypothesis-based questionnaire to empirically identify the needs of citizens’ energy cooperatives and thus be able to create platform-based and service-oriented business models. With this in mind, the new innovative business models have the aim of enabling the citizens' energy cooperatives and the energy companies to continue to exist in a mutually meaningful cooperation.

System non-synchronous penetration (SNSP) metric and potential alternatives in low-carbon grids
Submission-ID 012
Julian Eggleston 1, Andrew Halley 2, Pierluigi Mancarella 3
1 DIgSILENT Pacific, Australia
2 TasNetworks, Australia
3 University of Melbourne, Australia

With the increasing penetration of variable renewable energy sources based on inverter-connected technologies, the system non-synchronous penetration (SNSP) metric has been used in several power systems to describe, in simple terms, the extremities of operation and reduced reliance on traditional synchronous generators. It has provided an effective operational tool to monitor power system security, as well as provide an indicator of the long-term progression towards higher penetrations of inverter based resources (IBR). The metric can be easily calculated in both operational and planning timeframes, providing a useful tool in a variety of settings. However, it is potentially misleading to compare SNSP values derived from different power systems which may use a diverse range of solutions to manage system security issues including inertia and system strength. On this premise, this paper discusses the SNSP metric and its applications in the Australian National Electricity Market (NEM). Scenarios are considered where the metric can be used effectively and where caution is required. An alternative form of SNSP that may be more representative of the operational state of renewables-rich and technology diverse power systems is also proposed.

International electrical interconnection to unlock solar photovoltaic potential and accelerate progress towards carbon neutrality in Japan and South Korea
Submission-ID 013
Romain Zissler 1, Tatsuya Wakeyama 2, Jeffrey S. Cross 1
1 Tokyo Institute of Technology, Japan
2 Kyushu University, Japan

In October 2020, both Japan and South Korea separately pledged to reach carbon neutrality by 2050. These announcements marked decisive turning points in the two countries’ long-term energy policies and will lead to major transformations of energy systems, including electricity, heating & cooling, and transportation. Low carbon electricity and further electrification, for examples of some industrial activities and transportations, are likely to be key pillars of decarbonization, meaning there is a need to further expand renewable energy and/or nuclear technologies. In Europe, the leading continent towards carbon neutrality, international electrical interconnections play a key role in cost-efficiently integrating low carbon technologies. Our study aims at demonstrating why and how international electrical interconnection could also accelerate progress towards carbon neutrality in Japan and South Korea by unlocking the potential of low carbon technologies in these two countries.

From a methodology perspective, the present analysis is based on a computer simulation of Japan and South Korea power systems using the commercial electricity market simulation software called “PROMOD,” provided by the company Hitachi ABB. This software incorporates an algorithm that notably enables to simulate dispatch, short-term marginal cost and electricity prices by area and transmission flows between areas. Into more details, PROMOD performs a security constrained unit commitment and an hourly chronological dispatch algorithm that minimizes costs while simultaneously satisfying a number of operating constraints, among which: electricity consumption, generating unit characteristics, transmission grid capacity, fuel and environmental considerations, and ancillary service requirements.

Regarding data input and validation process, sources of data referred to notably include: Japan and South Korea’s power companies and power exchanges, as well as Japan’s Organization for Cross-regional Coordination of Transmission Operators, and South Korea’s Electric Power Statistics Information System.

Our study first economically demonstrates that an electrical interconnection between Japan and South Korea should send a strategic investment signal in favor of new low carbon low marginal cost electricity. It then discusses solar photovoltaic, wind, and nuclear power as the three main new low carbon low marginal cost electricity options. It is found that under current conditions and in the framework of the interconnection considered, the potential of solar photovoltaic should be unlocked, especially in the area of Kyushu, Japan. The realization of such a project should thus contribute to accelerate progress towards similar carbon neutrality objectives in the region.

Overcurrent Suppression Control for Grid Forming Inverter
Submission-ID 025
Takumi Narita 1, Satoshi Sugimori 1, Tatsuhito Nakajima 1, Yasuaki Mitsugi 2, Hiroshi Hashiguchi 2
1 Tokyo City University, Japan
2 Toshiba Mitsubishi-Electric Industrial Systems Corporation, Japan

A large number of distributed energy resources using renewable energy (DER) and battery storage systems (BESS) have been widely introduced. The authors have researched the grid forming (GFM) inverter for providing virtual inertia to the grid-connected inverter for those DER and BESS. However, the GFM inverter has a problem that an overcurrent is often observed in simulation studies, in the case that a power grid disturbance such as a load change or a grid fault occurs in the power grid. As a countermeasure for the overcurrent issue, the paper proposes an original overcurrent suppression control method for the GFM inverter. The paper also describes the results of Simulink simulations for validating the proposed methods.

The authors have researched a drop-type GFM inverter composed of a frequency controller and a voltage controller. The frequency controller is based on the generator swing equation. The phase angle of the GFM inverter output voltage is obtained by integrating the angular frequency, which is the output signal of the swing equation. The voltage controller is based on the droop relationship between the reactive power and the inverter output voltage amplitude. No current controller is used in the frequency and voltage controllers.

In the overcurrent suppression control method originally proposed by the authors, an AC current controller is implemented after the droop-type GFM inverter controller. When the inverter current level is lower than an overcurrent threshold, the current controller does not affect the GFM output voltage reference. The GFM inverter operates as if there is no current controller after the GFM controller. In contrast, when the inverter current level exceeds the threshold, and a new current reference lower than the threshold level is created in the current controller using the GFM output voltage reference. A new GFM voltage reference is calculated instead by a feedback loop by the new current reference and detected current, and a feedforward offset by the old GFM output voltage reference or the grid voltage detected at the point of common coupling.

For validating the proposed overcurrent suppression control method, MATLAB/Simulink simulation studies were carried out. A 10MVA GFM inverter is connected to a 115kV transmission line via a boost transformer in the simulation model. A three-phase-to-ground fault was made occurred at the midpoint of the transmission line. Without the proposed suppression method, an overcurrent of more than 2pu was observed in the GFM inverter output current. On the other hand, the GFM inverter output current was reduced effectively less than 1.3pu when the threshold was set at 1.3pu, and the proposed method was applied. It was confirmed that good suppression results were obtained thanks to the proposed overcurrent suppression control method.

Cross-Current Power Control for Coordination of Multiple Grid Forming Inverters
Submission-ID 026
Satoshi Sugimori 1, Takumi Narita 1, Tatsuhito Nakajima 1, Yasuaki Mitsugi 2, Hiroshi Hashiguchi 2
1 Tokyo City Univercity, Japan
2 Toshiba Mitsubishi Electric Industrial Systems Corporation, Japan

A large number of distributed energy resources using renewable energy (DER) and battery storage systems (BESS) have been introduced. The authors have researched the grid forming (GFM) inverter for providing virtual inertia to the grid-connected inverter for those DER and BESS. However, the GFM inverter has a problem that a cross-current power in the GFM inverters is often observed in simulation studies, in the case that multiple GFM inverters are operated in the same solar farm or a local area. Power sharing between the inverters is made unbalanced by the cross-current power. As a countermeasure for the cross-current power issue, the paper proposes an original cross-current power control method to coordinate the multiple GFM inverters. The paper also describes the results of Simulink simulations for validating the proposed methods.

The authors have researched a drop-type GFM inverter composed of a frequency controller and a voltage controller. The frequency controller is based on the generator swing equation. The phase angle of the GFM inverter output voltage is obtained by integrating the angular frequency, which is the output signal of the swing equation. The voltage controller is based on the droop relationship between the reactive power and the inverter output voltage amplitude. No current controller is used in the frequency and voltage controllers.

In the cross-current power control method originally proposed by the authors, the active and reactive power references of the droop-type GFM controller are adjusted by subtracting the cross-current power. The cross-current power circulating between the multiple GFM inverters is defined as a difference in the output power of the GFM inverter concerned and the averaged output power of the multiple GFM inverters. When there is a difference in the rated capacity of the GFM inverters, weight coefficients corresponding to the ratio of the rated capacity can be taken into consideration for calculating the cross-current power.

MATLAB/Simulink simulations were carried out to validate the proposed cross-current power control method. In the simulation model, three GFM inverters were connected in parallel. The inverter was connected to a 630V distribution feeder each, and these feeders are joined at the same 630V bus. The 630V bus is connected to a 115kV transmission line via a boost transformer. The simulation condition was a load change in the power grid. The simulation results showed that the three GFM inverters supply and absorb power according to the load change and provide virtual inertia to the power grid as expected. However, the output power waveforms of the three GFM inverters are not matched with each other because of a difference in the impedance of the three feeders. On the other hand, it was confirmed that power sharing between the multiple GFM inverters is successfully balanced, thanks to the proposed cross-current power control method.

A Multi-Objective Operational Strategy for a Utility-Scale PV Plus Energy Storage System
Submission-ID 030
Fahad Alsaeed, Mesut Baran
North Carolina State University, United States

Recently, battery energy storage (BES) has emerged as an economically viable technology to be adopted in large-scale photovoltaic (PV) and wind farms to facilitate their integration into the system and increase their economic value. This paper focuses on the determining a proper BES for such a system that will enable the system to respond to the power price variations and thus maximize the BES benefits. Additionally, this paper proposes a detailed dispatching scheme that can handle various operation constraints in order to maximize the BES benefits. This paper also takes into account the factors affecting the degradation of BES during its operation and shows that this is a critical factor in determining economic viability of the BES. A case study for a 300 MW solar power plant is given to illustrate the proposed method and assess the economic viability of the storage for this case. The results show the importance of adopting a detailed BES model to improve the accuracy of the estimated economic benefits.

Modelling PV Self-consumption at Portfolio Level
Submission-ID 034
Steffen Karalus 1, Benedikt Köpfer 1, Philipp Guthke 2, Sven Killinger 1, Elke Lorenz 1
1 Fraunhofer Institute for Solar Energy Systems ISE, Germany
2 TransnetBW GmbH, Germany
Self-consumption of the energy generated by photovoltaics (PV) plays an increasingly important role in the power grid. “Prosumer” systems consume part of the produced energy immediately to meet the local demand reducing the feed-in into as well as the demand from the grid. As PV generation and consumption profiles are diverse and heterogeneously distributed, the adaptation of feed-in and grid load estimates is a complex task.

In order to analyse the effects of PV self-consumption in the power grid, we introduce a stochastic bottom-up model of PV power generation and local consumption in the control area of the German transmission system operator TransnetBW. We set up a realistic portfolio of more than 100,000 PV/prosumer systems to generate representative time series of PV generation, self-consumption and feed-in. The model allows to investigate their time-dependent behaviour in detail for the full portfolio whereas measurements are presently only available as aggregated feed-in time series over a non-representative subset of systems.

We analyse the variation of self-consumption with PV generation and consumption at portfolio level and its seasonal, weekly and diurnal cycles. The results are expected to be helpful to identify suitable parameters for data-driven models of PV self-consumption and feed-in.

Hybrid-Approach for Unit Commitment Simulation combining a Fundamental Model with a Neural Network
Submission-ID 041
Henrik Sommer, Martin Winkelkotte, Fabian Bauer, Peter Bretschneider
Technische Universität Ilmenau Department of Electrical Engineering and Information Technology Energy usage optimization Group, Germany

Due to the low storage capacities of the electrical energy supply system (ESS), feed-in and feed-out must be balanced at all times. In Germany, a power plant park with various technologies is available to cover the load. In a liberalized ESS, the decision which power plants are used to cover the load is not made by a single authority but in a decentralized way. Which power plants are used is determined in control rooms, on the electricity market, by the regulatory framework and, last but not least, by the fed-in from wind and solar depended renewables. The resulting unit commitment (UC) directly relates to the CO2 emissions, electricity costs as well as the amount and type of primary energies consumed. Consequently, the UC is a vital component to understand and optimize the interdependencies in an ESS.

Many publications focus on finding the optimal UC regarding cost efficiency or CO2 emissions. However, in our work we aim to model the sub-optimal real-world UC. The difficulty to do so is that inefficiencies in the UC decision are complex and intransparent and therefore cannot be formulated as a closed mathematical description. Examples for such inefficiencies are market dominance by single market participants, lack of information or forecast errors. To include these inefficiencies we expanded an existing parametric fundamental model following the merit-order-approach that finds the most cost-efficient UC with a neural network that includes the inefficiencies of the real ESS. Thereby, the output of the fundamental model functions as input for the neural network. The neural network is trained with historical data of a real-world scenario for the German market region. To increase model accuracy a hyperparameter tuning and an input variable selection is conducted for the neural network.

Finally, the hybrid model is compared to the results of standalone fundamental and neural network models. The results show that the hybrid model outperforms the standalone models significantly.

Evaluation of advanced reserve sizing methods based on RE variability and uncertainty
Submission-ID 043
Artelia, France

The large-scale integration of variable renewable energy (RE) into the electricity mix imposes to deal with the uncertainty and variability associated with such generation. As a result, the power system reserve management, which aims to ensure real-time balancing between production and load, has to evolve in order to guarantee the reliable operation of the electrical grid.

This paper focuses on the evaluation of advanced reserve sizing methods identified in the literature, based on real operation data, from RE plants under operation. More precisely, the paper details a field-data oriented analysis, where fine time-resolution (few minutes time step) data is analyzed (1) to characterize RE variability and uncertainty, and (2) to evaluate the benefits from taking into account such variability and uncertainty for advanced reserve sizing.

The paper details the following methodology :

  1. Analysis of the variability and predictability of PV and Wind assets based on field operation data, including a review of the associated metrics used for the variability and predictability;
  2. Analysis of power system reserve classification for different areas (Europe, US, other areas); Analysis of the state of the art regarding the evolution of reserve sizing methods in the case of large scale integration of RE. Limits from existing methods which have been set for electricity mix mainly based on conventional generation, will be highlighted. The identified advanced reserve sizing methods consider both the variability and the limited predictability of the RE;
  3. Application of the most relevant reserve sizing methods and evaluation of the benefits from such methods through simplified simulation of the power system operation, based on use cases with field operation data.

The results will include two different use cases, which correspond to two configurations regarding RE aggregation; for both territories, high RE penetration is expected and the results are derived from operation data from exiting RE plants in these territories:

  1. a first use case is related to an interconnected power system in Africa (Western Africa Power Pool, WAPP), where the wind and solar power units are distributed over a large territory, and thus benefit from fluctuation compensation and variability reduction (smoothing effect);
  2. a second use case is related to an island power system with few RE plants (island of Santiago , Cape Verde) and low fluctuation compensation

What was the fundamental cause for long-continued high market price in Japan in 2021? – negative impact to Japan’s renewable energy policy –
Submission-ID 062
Yoh Yasuda 1, Hajime Matsukubo 2, Hiroshi Takahashi 3, Kenichi Oshima 4
1 Kyoto University, Japan
2 Citizens’ Nuclear Information Center, Japan
3 Tsuru University, Japan
4 Ryukoku University, Japan

The Japan’s electricity market operated by JEPX (Japan Electric Power eXchange) faced extremely high prices in the spot market for three weeks from the end of December 2020 to mid-January 2021. The highest spot price was 251 JPY/kWh (approx. 2300 USD/MWh), and the daily average price exceeded more than 60 JPY/kWh (approx. 550 USD/MWh) more than 15 days.

Many literatures, including by the Japan government and mass-media, argued what caused the event. Some argued it was caused by "unstable PV (photovoltaic)", and others said it was due to colder weather, otherwise it was because of lack of LNG (Liquid Natural Gas) import due to ship congestion in the Panama Canal, and so on. The event in Japan was sometimes compared to that which occurred in Texas in February 2021.

However, these arguments can be denied by data analysis with market information and other published data. It was neither short-term “spikes" as a normal market response, nor was it caused by extreme weather conditions like that in Texas. Generated power by PVs was actually more than that in the same period in past years. It was clarified that PVs also contributed to curbing daytime spot prices during the event period. Though it was certainly colder than the previous year, but not so much compared with the past several years.

While the lack of LNG seems one of the major causes of the event, this was not a direct and essential cause that triggered the long-continued high price. The information on the lack of LNG had not been opened to the public, but hidden as insider information among limited incumbents that dominate the electricity market.

Nuclear power can be pointed out as another major cause of this event. After the nuclear disaster in Fukushima in 2011, restarting a nuclear reactor is now extremely uncertain, even for nuclear generators, from both technical and legal viewpoints. Though a short-term adequacy was estimated enough in this season, the lack of LNG suddenly came about in mid-December 2020, when a nuclear reactor failed to restart due to a regulatory reason.

Thus, in the oligopolistic Japanese electricity market, new-comers in the retail market have suffered enormously, but incumbents not so much because they own both retail and generation businesses. Safety nets to avoid inadequate market behaviours have been installed in Japan. How to improve imbalance price rules was under discussion and the upper limit of imbalance price during such extreme events has not been enforced. It can be strongly estimated that these immature market designs cause the unusual market response that has rarely been seen in the world.

The negative impact by this event hit small retailers, especially those who have bought renewable energy using FIT (Feed-in Tariff) through the market. In the end, the event resulted in weakening Japanese renewable policy.

PV as an ancillary service provider – Laboratory and field experiences from different IEA PVPS countries
Submission-ID 063
Markus Kraiczy 1, Denis Mende 1, Roland Bründlinger 2, Stefan Siegl 1, Jonathan Schütt 1, Gunter Arnold 1, Sebastian Wende von Berg 1, Martin Braun 1, Gerd Heilscher 3, Shuo Chen 3, Niraj Lal 4, Iain MacGill 5, Andreas Knobloch 6, Daniel Premm 6, Ricardo Guerrero Lemus 7, Christof Bucher 8, Marine Cauz 9, Lionel Perret 9, Arnulf Jäger-Waldau 10, Giovanna Adinolfi 11, Giorgio Graditi 11
1 Fraunhofer Institute for Energy Economics and Energy System Technology, Germany
2 AIT Austrian Institute of Technology GmbH, Austria
3 University of Applied Sciences Ulm, Germany
4 Australian National University, Australia
5 University of New South Wales, Australia
6 SMA Solar Technology AG, Germany
7 Universidad de la Laguna, Spain
8 Bern University of Applied Sciences, Switzerland
9 PLANAIR, Switzerland
10 European Commission, Joint Research Centre, Italy
11 Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy

The paper at hand provides selected highlights from the upcoming IEA PVPS Task 14 Report “PV as an ancillary service provider – Laboratory and field experiences from different IEA PVPS Countries”.

The PV penetration in many countries is continuously growing and PV is becoming a major energy source in the future electricity grid worldwide. Therefore, PV systems and PV hybrids need to take over more and more system responsibility by providing ancillary services. Ancillary services are services provided by generators and other grid users, which are necessary to maintain the power quality and reliability in power system operation, such as maintaining grid voltage and grid frequency within the specified limits. The specifications, types, needs, and procurement procedures of these services can vary in different power systems and are changing with the energy transition in many countries. The report highlights the status and the potential of PV and PV hybrids as an ancillary service provider. The focus is set on mainly good practice examples. Nevertheless, improvement and further development potential and needs for the application of PV as an ancillary service provider are addressed and discussed in this report.

Estimating the socio-economic drivers of residential photovoltaic systems adoption in a Swedish city
Submission-ID 064
Umar Hanif Ramadhani, Bezawit Tsegai, Caroline Apelryd, Sofia Ekbring, Joakim Munkhammar, Joakim Widén
Built Environment Energy Systems Group, Department of Civil and Industrial Engineering, Uppsala University, Sweden

Motivated by the need for more sustainable energy, power generation is increasingly shifted to small scale distributed generation, which enables residential customers to produce photovoltaic electricity and participate in the energy generation mix. Hence, insight into the expected spatial spread of new residential photovoltaic systems is of great importance to predict and mitigate the potential impacts on the electricity distribution system. Previous studies have shown that socio-economic factors are among the main drivers when installing residential photovoltaic systems. This paper identifies the socio-economic drivers of residential photovoltaic systems adoption in the Swedish city of Uppsala. The spatial adoption of photovoltaic systems is then predicted at the district level using regression analysis, and the results are compared to the spatial potential of photovoltaic electricity generation in Uppsala from a purely technical perspective. Property ownership type, average income, the share of households with cars, age group, and unemployment rate are found to be significant factors. The fraction of installed residential photovoltaic systems is predicted and a comparison to the technical-based spatial potential of photovoltaic electricity generation is made. The models can form the basis of spatial adoption forecast methods for photovoltaic systems for grid impact analysis to allow more extensive integration of residential photovoltaic systems in electricity distribution systems.

Synchronous Energy Storage System with Inertia Capabilities for Angle, Voltage and Frequency Stabilization in Power Grids
Submission-ID 068
Andreas Knobloch 1, Christian Hardt 1, Andreas Falk 1, Thorsten Buelo 1, Simon Scheurich 1, Chokri Khalfet 1, Richard Hesse 1, Tobias Becker 1, Rahul Bhatia 2
1 SMA Solar Technology, Germany
2 SMA Solar India, India

In today’s electric power systems loads and renewable generators rely on the voltage and frequency that are formed by power plants with synchronous generators and their grid stabilizing mechanisms. Due to a tremendous increase of cost-efficient renewable energy sources and the consequential phaseout of fossil fuel generators there is a need for substitution and even a supplement of the grid stabilizing mechanisms in electric power grids [1,2]. In future energy systems voltage and frequency will be formed by synchronous inverter-based power plants with advantageous capabilities compared to synchronous machines [3]. First requirements specifications and testing approaches on grid forming are being set up, preparing regular operation of grid forming converters for stabilization of future power grids [4].

This paper will introduce a grid forming energy storage system solution for voltage, angle and frequency strength improvement in future distribution and transmission networks. Configurable control modes for inertia and damping provision will be presented. The performance capabilities for inertial response, instantaneous fault current, power oscillation response as well as power reserve provision will be shown based on simulation results and practical laboratory experiments. Application examples will be given as well.

  1. The German TSOs: Grid Development Plan 2035, April 2021
  2. NationalGrid ESO: Operability Strategy Report, December 2020
  3. Knobloch, A. et al.: Grid stabilizing control systems for battery storage in inverter-dominated island and public electricity grids, Energy Transition in Power Supply - System Stability and System Security; 13th ETG/GMA-Symposium, Berlin, 18.-19.09.2019
  4. GC0137 Workgroup Consultation and “Draft Grid Code – Grid Forming Converter Specification”, National Grid ESO, 2021

Fault operation of grid forming converters with focus on system stability
Submission-ID 077
Nils Wiese 1, Daniel Duckwitz 1, 2, Maria Nuschke 2, Yonggang Zhang 1, Martin Braun 1, 2
1 Universität Kassel, Germany
2 Fraunhofer IEE, Germany
The increasing integration of renewable energies and battery storages into the electrical grid poses many challenges. Those power-electronically coupled units do not have any mechanical inertia, so that new control methods, like grid-forming controls, are necessary to ensure system stability [1]. Grid-forming converters have to co-operate with different shares of synchronous machines and conventional converters. Our paper studies these interactions during short circuit faults through power system simulations. Overcurrent capability increases hardware costs, so appropriate control methods are used in order to protect the converter. We use a virtual synchronous machine control scheme and present two types of current limitation methods which keep grid-forming capability during short circuits. In the study, converter penetration of the test system is varied from 0% to 100%. Due to the different overcurrent capabilities of synchronous machines and converters the voltage support during fault differs. The effect of the typical electrical length of feeder lines of the generation technologies is evaluated. We show that an adopted current limitation method from [2] and another method based on PWM blocking are suitable for overcurrent protection while keeping system stability. The first method feeds in a sinusoidal fault current with controlled amplitude. The second method injects a fault current with high harmonic content. Both methods perform well regarding angular stability during and after the fault. This is in contrast to synchronous machines and leads to increased critical fault clearing times. Despite the different characteristics of grid-forming converters and synchronous machines, parallel operation is possible and system stability is maintained for all considered scenarios. Fault currents of the grid-forming converters are limited to rated values (1 pu) under all considered scenarios. We conclude that suitable current limitation schemes in grid-forming devices prevent excessive current rating and avoid related additional cost. References [1] Duckwitz, Daniel, Power System Inertia: Derivation of Requirements and Comparison of Inertia Emulation Methods for Converter-based Power Plants, Universität Kassel, 2019 [2] Duckwitz, Daniel & Welck, Friedrich & Glöckler, Christoph, “Operational Behavior of the Virtual Synchronous Machine,” in 12. ETG/GMA-Fachtagung Netzregelung und Systemführung, Berlin, 2017

Determination of the Voltage and Frequency Dependent Behaviour of Low Voltage Grids – Test Procedure for a Modified Mobile Generator
Submission-ID 105
Sebastian Seifried 1, Tobias Lechner 1, Daniel Stenzel 4, Steffen Herrmann 2, Georg Kerber 5, Kathrin Schaarschmidt 3, Michael Finkel 1, Rolf Witzmann 4
1 University of Applied Sciences Augsburg, Germany
2 AVS Aggregatebau GmbH, Germany
3 LEW Verteilnetz GmbH, Germany
4 Technical University of Munich, Germany
5 University of Applied Sciences Munich, Germany

In the LINDA-project, concluded in 2019, a concept for an emergency power supply to provide electrical energy to critical infrastructures during long lasting, large scale blackouts using local islanded grids was developed. During such a blackout a decentralized energy resource, called leading power station, sets up an island grid. The control concept of the leading power station, using the LINDA control approach, allows renewable energy sources (following RES) to feed in to the island grid to reduce the consumption of the main energy resource or to supply a larger grid area that the leading power station could actually supply. To identify proper parameters for the control algorithm of the leading power station, the static and dynamic frequency and voltage dependent behaviour of loads and other RES in the grid is essential. Furthermore, transmission system operators are interested in the aggregated grid behaviour too. This behaviour gives knowledge about the dynamic stimulation in the transmission grid and helps during the grid restoration process.

The database regarding the frequency and voltage dependent load behaviour is not up to date any more, since it was determined around 1993. Also, those investigations were conducted during “normal” operation. To get an actual database, the aggregated grid behaviour in low voltage grids is planned to be determined during maintenance work using a mobile generator.

The grid frequency during the operation of a conventional mobile generator is above 51.5 Hz to suppress the infeed of RES, since RES can destabilize the grid due to their volatility. The infeed of the RES depends on the grid frequency and voltage and is specified in different standards, which were developed during the last years. Therefore, several generation systems with different frequency and voltage dependent behaviours regarding their start-up date are installed in the grid. Furthermore, the number of inverter based loads, e.g. LED lightning in low voltage grids has increased, too.

The mobile generator, used for grid maintenance, is supplemented with a load bank and upgraded with an algorithm to stimulate frequency and voltage to record the aggregated grid behaviour. The voltage and frequency stimulation is planned in the range of 47,5 Hz ≤ f ≤ 51,5 Hz and f > 51,5 Hz to identify the voltage and frequency dependent behaviour with and without RES. Mobile generators are normally driven by diesel engines, which are not able to absorb a reversal load flow. The load bank is required to absorb the excess power generated by RES, which is not consumed by the loads in the grid in order to keep the frequency at 50 Hz as long as possible. In the full paper the measuring procedure to stimulate the frequency and voltage and to record the aggregated grid behaviour of loads and generation systems, as well as the control algorithm for the load bank of the measurement system is introduced and described.

Real-Time Simulation Modeling for PV-Battery hybrid System
Submission-ID 108
Mohd Zamri Che Wanik, Antonio Sanfilippo
Qatar Environment and Energy Research Institute, Qatar

Simulation of power system operation is executed for both technical reasons, and some are for non-technical reasons. The power system software is utilised as the process is quite complex and time-consuming. This paper describes modelling for real-time simulation of a photovoltaic (PV) and battery-based system and presents the simulation results. The hybrid system under study consists of a PV system as an example of an unreliable resource, a battery representing energy storage and loads. The modelling approach and the study analysis performed are described. The model and operation scenarios have successfully been simulated in real-time using the OPAL-RT digital simulator. The operation of this hybrid system in performing load levelling and voltage regulation in the local power system is demonstrated. The dynamic response of the hybrid system is presented when subjected to sudden changes in solar irradiance. Battery energy storage is demonstrated to be very effective in complementing and compensating for the fluctuation of generated power from the PV. The system is also shown very effective in regulating the voltage. The presented works are part of the ongoing process of developing a digital twin of the actual power system, which can be used for various purposes such as for fast prototyping of microgrid control solutions and intelligent management of power networks, to name a few.

Evaluating the optimal portfolio of VRE capacity to be integrated into the power system: A case study of Zambia
Submission-ID 115
Lars Falsen Habostad 1, Thomas Haugstenrød 2, Hossein Farahmand 1
1 Norwegian University of Science and Technology, Norway
2 Multiconsult Norge As, Norway

The Zambian power system is today dominated by hydropower. In recent years, low rainfalls and increasing electricity demand have led to massive power shortages. Integrating variable renewable energy (VRE) has been proposed as a measure for diversifying the generation portfolio and increase energy security. This paper evaluates the optimal solar PV and wind power capacity portfolio to be integrated into the Zambian power system within 2030. System cost is used as the metric for evaluating the economic impact of integrating different portfolios of solar PV and wind power capacity. The optimal portfolio is found as the one resulting in the lowest system cost, and is identified as a portfolio consisting of 1470 MW solar PV capacity and 630 MW wind power capacity. Brief load flow analyses, with the optimal portfolio of VRE capacity integrated into the system, suggests that the current grid is suited for integrating the optimal portfolio of new VRE capacity, given the assumed distribution of new power plants. From a technical perspective, the flexibility of the dispatchable hydropower plants in the system is therefore considered the most critical factor when integrating renewable energy into the system. However, increasing the share of wind power capacity in the portfolio decreases the magnitude of average daily ramping required from hydropower plants. The findings presented in this paper can serve as a basis for government strategies in Zambia, and in similar countries in Southern Africa, for assessing integration of renewable energy sources in power systems.

Application of dynamic line rating for solar power integration
Submission-ID 117
Rena Kuwahata
Ampacimon, Belgium

A dynamic line rating system was installed on a sub-transmission line which is expected to host increasing amounts of solar power. Data was collected for 10 months with line-mounted sensors as well as tower-mounted weather stations to compared the measurements of different instrumentation and evaluate the results in terms of accuracy, usefulness in providing capacity information, and suitability to integrate more solar power without reinforcing the line. The results of this project is reported in this paper, in the form of data comparison results and description of how conclusions were reached by the grid company.

A Methodology to Improve the Predictability of Solar Energy Generation with Confirmatory Evidence from Germany
Submission-ID 120
Kevin Forbes
UCD Energy Institute, University College Dublin, Ireland
Using data from Germany's 50Hertz electric power system, this paper begins by observing that the current intraday solar energy forecasts do not fully reflect the day-ahead weather forecasts' information. Solar energy generation also has the property of being highly autoregressive. Based on those properties, an Autoregressive Moving Average with Exogenous Inputs (ARMAX) model is formulated. The model was estimated using 15-minute data from Jan 1, 2014, through Dec 31, 2017. The model is evaluated using out-of-sample data from Jan 1, 2018, to Aug 30, 2020. The 15 minute-ahead out-of-sample predictions have a weighted-mean-absolute-percentage-error (WMAPE) of about 3.84%, substantially less than the WMAPE associated with the intraday solar energy forecasts reported by the system operator over the same period.

Supervisory Optimal Control for Photovoltaics Connected to an Electric Power Grid
Submission-ID 121
Joseph Young 1, David Wilson 2, Wayne Weaver 3, Rush Robinett III 3
1 OptimoJoe, United States
2 Sandia National Laboratories, United States
3 Michigan Technological University, United States

The following research presents an optimal control framework called Oxtimal that facilitates the efficient use and control of photovoltaic (PV) solar arrays. This framework consists of reduced order models (ROM) of photovoltaics and DC connection components connected to an electric power grid (EPG), a discretization of the resulting state equations using an orthogonal spline collocation method (OSCM), and an optimization driver to solve the resulting formulation. Once formulated, the framework is validated using realistic solar profiles and loads from actual residential applications.

Efficiency in Renewables-BESS Hybrid Systems O&M
Submission-ID 122
Gian Schelling
Hitachi ABB Powergrids, Switzerland

Hitachi ABB Power Grids
Efficiency in Renewables-BESS Hybrid Systems O&M

Executive Summary
During the last years, focus in co-location projects combining Battery Energy Storage (BESS) with Renewables has been on successfully planning, permitting, engineering, procuring and connecting such systems in an emerging application market place.
Procurement of components has classically been split between PV- or Wind equipment on the one hand side, BESS on the other hand side and EBoP (incl. substation) as a third plant component, due to historic market developments where substations are the most proven assets and partly operated and even procured by grid operators, PV and Wind equipment is advanced in the meantime and procured by the Renewables plant EPCs / GenCos, and BESS systems incl. their monitoring + controls are the youngest component in Renewables-BESS Hybrid Systems and classically also procured by the plant EPC (developers or GenCos for major components often free-issued). With BESS-PV hybrid power systems accounting for the majority of the applications and relying on similar power conversion technologies, first synergies have been developed by first PV inverter manufacturers developing and offering both PV and BESS solutions and starting combining such particularly through DC-coupled power inverter systems offering synergies in engineering, equipment procurement, space, installation and Operations + Maintenance (O&M) phases.
In the utility-scale market, in the meantime, >3’000 MW of BESS co-located with PV and >1000 MW of BESS co-located with wind have been installed globally by the end of 2020 (IHS, Energy Storage Market Tracker Q2 2020, 02/2021). The market is slowly moving into a more mature status where experience with O&M in Renewables-BESS Hybrid Systems has been increasingly accumulated. Life-times are classically designed for 5-15 years depending the use-cases, locations and environmental conditions. First painfull lessons have been learned in the O&M phase of Renewables-BESS Hybrid Systems. As further expansion of this market is forecasted to be exponential, with 24’000 MW of BESS co-located with PV and 4’000 MW of BESS co-located with Wind expected to be installed additionally between 2021 and 2025 (IHS, Energy Storage Market Tracker Q2 2020, 02/2021), this presentation will explore key challenges and potential solutions using the example of Hitachi ABB Powergrid’s e-mesh™ solutions and project references in the following top 4 challenges in Renewables-BESS Hybrid Systems O&M:
1. Dispersed monitoring + controls of PV, Wind, BESS and substation equipment resulting in inefficient operations + maintenance in BESS systems.
2. Actual vs. expected cycle life of BESS systems
3. Optimal real-time operation of BESS systems in light of an ever-increasing available revenue stack on the markets
4. Cyber-security despite digitalization

Short-Circuit Analysis of Grid-Connected PV Power Plants Considering Inverter Limits and Grid Support
Submission-ID 129
Jie Song, Marc Cheah-Mane, Eduardo Prieto-Araujo, Oriol Gomis-Bellmunt
Polytechnical University of Catalonia – UPC, Spain
Grid-connected PV systems account for the majority of the total installed capacity compared to the stand-alone systems. Voltage Source Converters (VSCs) have been widely accepted as the most common devices integrating the PV modules into the AC transmission grid. The VSC operation in different control modes introduces a non-linear characteristic to the grid. Also, converters protect themselves from being overloaded by saturating the current. Such current-saturation represents a different VSC operation that should be considered in the steady-state analysis of PV power plants. This is especially critical for the short-circuit analysis where VSCs are more likely to operate in current-saturated states in response to a reduced voltage level. Short-circuit equilibrium points in a PV power plant may exist for any combination of VSCs current-saturation
states. In other words, whether a converter is unsaturated or saturated during a specific short-circuit scenario is uncertain. Also, the studied system might have multiple equilibrium points depend on the converters that reach the current limitations.

The IEC 60909 standard established that converter-based generating units can be modeled as a current source for steady-state short-circuit calculation. However, it has not been clearly defined how to obtain the current angle considering the converter operation in a specific control mode. This paper presents a novel methodology for short-circuit analysis of PV power plants. The studied system is modeled with an element-based formulation in steady-state where the converters’ operation in various potential current-saturation states is included. Two different approaches are proposed to identify the short-circuit equilibrium points with different options regarding the existence of multiple equilibrium points and results in different computing efficiencies:
Comprehensive approach: this approach solves all systems of equations corresponding to different converters’ currentsaturation states and identifies all possible short-circuit equilibrium points. This approach is suggested for PV power plants with a small number of inverters in order to ensure a short computing time.
Direct approach: this approach defines an iterative process where the VSCs saturation states are updated based on the solution from the previous iteration in order to identify a short-circuit equilibrium point that satisfies VSCs operation limits. This approach has the advantage in computing efficiency compared to the comprehensive approach as it goes through only selected combinations of converters saturation states and therefore is suggested for systems with a large number of inverters.

Numerical case studies will be presented in the full paper including the short-circuit analysis of PV power plants with different numbers of inverters. Short-circuit equilibrium points can be obtained where several converters are unsaturated or saturated utilizing the developed methodologies. The obtained results show that significant error might exist in short-circuit calculation if converters’ current-saturation is not considered. Also, multiple equilibrium points might exist for different combinations of VSCs current-saturation states. In addition, the grid-support operation of PV inverters shows a significant impact on the system equilibrium points during the fault.