Grid-forming control (GFC) has seen numerous technological advances in their control types, applications, and the multitude of services they provide. Some examples of the services they provide include black start, inertial frequency response, and islanded operation capabilities with the possibility of re-synchronization without the need of additional support from other devices such as storage. State of the art literature proposes a variety of GFCs which can provide single or multiple of these services. However, study of these different GFCs for weakly-connected offshore wind power plants (WPPs) based on time-domain simulation and focusing on the large signal disturbance is not well covered. This paper reviews some of the most researched grid-forming control methods applicable to offshore WPPs and provides a comparative investigation and discussion of their stability properties and applicability, especially when connected to a weak-grid. The paper also provides a discussion on the prerequisites and challenges surrounding the comparative study of different GFCs.
Recently, the concept of dynamic virtual power plants (DVPP) has been proposed to collectively provide desired dynamic ancillary services such as fast frequency and voltage control by a heterogeneous ensemble of distributed energy resources (DER). This paper presents an experimental validation of a recent DVPP control design approach on a multi-converter power hardware-in-the-loop (PHIL) test bed system. More specifically, we consider a DVPP composed of a wind generation system, a photovoltaic (PV) system, and a STATCOM with small storage capacity to collectively provide grid-following fast frequency regulation in the presence of grid-frequency and load variations. The performance of the aggregated DVPP response is evaluated with respect to its ability to match a desired dynamic behavior while taking practical limitations of the individual DVPP units into account.
Grid Forming (GFM) Inverters and their capabilities are critical to enable growing penetration of Distributed Energy Resources (DER) into the electrical grid. The electrical inertia brought by GFM inverters to the network can replace or supplement the inertia of rotating machines that is a key element for power system stability. The German research project “Netzregelung 2.0” (“Grid Control 2.0”) investigated the operation of power systems with GFM inverters and developed inverter controls and possible grid code compliance testing approaches. First outcomes regarding the testing approaches for inertia provision are presented. A major portion of grid-forming control approaches can be described following the well-known equations of synchronous machines and thus can also be characterized in their frequency behaviour with reference to the first order swing equation of a rotating system, mainly characterized by the acceleration time constant (Ta) and damping (D). The accurate determination of these values is important for system operators and future ancillary service markets. Measurements were performed based on existing and upcoming standards and guidelines with a special focus on parameter determination and measurement uncertainty.
With the shutdown of conventional power plants as part of the decarbonisation of the energy system, new capacities for system services are needed. Therefore, according to the EU's Electricity Balancing Guideline from 2017, Demand Response is to be integrated non-discriminatorily into the joint European mechanism for procurement of system services, which is currently being developed. In Germany, the procurement of system services was last adjusted in 2020, so that flexibility options available at short notice can participate recently. This offers a new marketing option for industrial flexibility options. Expected revenues in this balancing energy market are higher than in the continuous intraday market. Nevertheless, the introduction of the balancing energy market has not fulfilled its objective of increasing competition and the costs for balancing energy are immense. However, complex analyses are necessary for optimised marketing in the balancing energy market, compared to the intraday market, where the attractiveness of flexibility marketing is also increasing. In this paper, it is shown, that the prequalification mechanism for participation in the german energy balancing markets is currently too high, a hurdle for industrial demand response. Within the framework of European procurement, it must be possible to intensively integrate modern virtual power plants.
Conventionally, wind turbine (WT) manufacturers are performing grid compliance tests at the WT level primarily in the field to verify the WT capabilities and performances against certain grid code requirements and validate simulation models accuracy and performance. Such compliance tests are becoming more challenging and numerous due to the growing number of relevant grid code requirements and the rapid growth of WT's size and rating, which in turn require more powerful test equipment and larger sites to test such turbines. Upcoming standards such as IEC 61400-21-4 and FGW AK KEZ and other industry initiatives aim to address these challenges through the replacement of site-specific tests with tests performed on WT components and subsystems in a controlled test bench environment. This paper presents the experience gathered so far on various test benches including nacelle, electrical generation, converter, and real time digital simulator test benches. The results of these different test benches brought important lessons regarding the transferability of the results through the evaluation and comparison with one another and benchmarking against site specific tests to assess their accuracy in representing the performance, capability, and functionality at WT level. Finally, a long-term grid compliance strategy, revolved around on moving part of the testing campaign to subsystem and component testing and modelling, is presented with a proposal of new test bench and acceptance strategies for the industry.
Integrating renewable energy is one of the current and future subjects for the energy transition to CO2-neutral energy generation. New requirements, including grid and converter perspectives, must be considered as renewable power generation is carried out with power electronic converters. Inverter-based resources (IBRs) must meet grid connection regulations, forecasting options, time scheduling, storage options, and voltage quality. In this article, some essential requirements from the manufacturing perspective are described, and their testing procedure is displayed with the validation of simulation models. This paper further highlights the SICAM Power Plant Controller (PPC) for photovoltaic plants, enabling the plant operator to create generation forecasts for the next 2-3 days of power and energy. To be grid-code compliant, the PPC includes several functions to meet requirements, and its start-up sequence is also described. The system operator gains a grid-compliant model validated against site measurements. The procedures have been successfully applied to several IBR projects. Previews of new concepts to reach the future demands of integrating IBRs into the public grid are presented. Possible impacts on the power generation and components are assessed, and necessary countermeasures are applied with the influence and feasible solution for energy trading with IBRs. Future aspects of supra-harmonics and their impacts on public grids are described and presented as an outlook. This paper highlights the challenges of integrating IBRs and lessons learned based on project executions.
The existing impedance-based stability criterion is effective for analyzing local control interactions; however, it is difficult to scale the existing criterion to analyze wide-area control interactions among numerous IBRs through a complex power system network. The scaled version of the existing criterion requires the impedance response of each IBR in the system as well as of the network looking from all the IBRs. It is quite challenging to obtain all these impedance responses because of the computational effort and the requirement of separately scanning the impedance of the network from the IBRs. We propose a reversed criterion for the impedance-based stability analysis to address these problems. In contrast to the existing criterion, the reversed criterion analyzes the stability of a power system when an IBR is disconnected from the system. The reversed criterion estimates the impact of an IBR on the frequency and damping of power system oscillation modes using the impedance scans of only the IBR and the grid at its terminal. It can be sequentially applied at different IBRs to evaluate their impact on the power system stability. In addition to scalability, the reversed criterion gives flexibility to focus only on a few selected IBRs, depending on their rating, the magnitude of oscillations observed at their terminals, and the vendor support available for implementing stabilizing control system updates. The reversed criterion is demonstrated on a 14-bus power system with 100% IBRs.
In this paper, we report on two analyses that support the need to reform processes for transmission and generator interconnection planning in the US. The first develops methodologies to quantify the multiple benefits of transmission and applies these methodologies in two sample transmission projects. Using 40 years of weather data, a methodology is developed to examine resilience to extreme weather events. Extensive sensitivity analysis is conducted across renewable energy and fossil-fuelled generator levels, gas prices, and load growth rates, to examine risk mitigation. Loss-of-load expectation analysis is used to examine resource adequacy benefits. Adjusted production cost, emissions, and enablement of high-quality renewable resources are also quantified. These methodologies are applied to two types of example transmission projects in ERCOT (Electric Reliability Council of Texas) to demonstrate a value-stacking approach. The second investigates cost savings for new generator interconnections through a proactive planned approach. A proactive planned approach can study larger numbers of projects or even look forward to anticipate projects due to public policies and may be able to right-size transmission facilities for the longer term rather than sequentially building many small facilities. In this analysis, we compare generator interconnection costs across three different levels of proactiveness in MISO (Midcontinent ISO) and SPP (Southwest Power Pool) to understand the importance of a proactive, planned approach for generator interconnection.
The main aim of the paper is to highlight one of the major problems faced during the 'one-shot' harmonic assessment based on site measurements. Grid code compliance is mandatory for any renewable plant to successfully interconnect with the transmission grid. Harmonic compliance is one such aspect that is a pre-requisite from utilities. Pre-commissioning stage compliance is showcased through simulation results. However, on interconnection, based on the various utilities' general practices, harmonic compliance verification is carried out at site. 'One-Shot' assessment method is followed for verification by few utilities. The major assumption for the assessment is that the grid condition is to remain constant over the period of measurement. The paper highlights the difficulty faced from the major assumption of the 'one-shot' assessment and how it is overcome in this scenario. The demonstration is carried out based on the 5th harmonic measurements as it was one of the major orders of non-compliance based on measurements. The paper also highlights the importance of quality of data and the need to capture appropriate events in the system over the course of the 'one-shot' assessment.
Hydrogen plays a key role in the deep decarbonization of energy systems. In this study, the demand for hydrogen and its derivatives in five major German energy system studies and a total of nine scenarios is surveyed by a meta-study approach. Special focus was set on the transition year 2030, which shows higher uncertainty in rank and size of sectoral demands than later transition years. In 2030 as well as 2045 the industry sector is generally seen as the most important sector with demand ranging from 12 to 66 TWh in 2030. For other sectors, the rank and demand differ strongly in 2030 and show no correlation to the target electrification rate of the scenarios. This applies to the overall demand as well, ranging from 14 to 123 TWh in 2030. In general, uncertainty and heterogeneity are higher for demands in 2030 than in 2045. Additionally, a structured overview of methodological differences between studies and results is presented and result reporting guidelines are provided to facilitate future scenario comparisons. A data annex to the study is provided.
Over the past ten years, the paradigm has shifted from conventional power generation to renewable generation. Large integration of these renewable energy sources (RES) into the power system poses challenges to system operators, leading them to put stringent requirements for their grid connection. Displacement of synchronous generators by RES reduces system inertia and consequently decreases the system damping capability of electromechanical oscillations. Poorly damped interarea oscillations reduce the transmission line's capacities and may damage power system components. Hence, future grid codes will require wind and solar power plants to provide damping to the system. Several papers have proposed adding an auxiliary damping controller to the wind turbine control algorithm to damp the low-frequency oscillations (LFOs) by modulating active or reactive power. However, these studies have not mentioned if small power plants can damp LFOs in a multimachine system. Therefore, this paper investigates the influence of reactive power capacity on the damping of LFOs and its effects on optimal controller parameters using a simplified SVC model connected at the midpoint of the tie line of a two-area test system. A local feedback signal is selected as the input signal to the SVC damping controller. Controller parameters are optimized using the particle swarm optimization algorithm. Time-domain simulations performed in PowerFactory software demonstrate the damping behavior of the controller at different SVC ratings. The results show a minimum reactive power capacity is required for effective damping of power system oscillations.
With increasing renewable energy penetration globally, Energy Storage Systems (ESS) act as a vital component for transforming the current energy sector. In the form of Grid connected ESS, Lithium-Ion Battery (LIB) technology is presently the most popular form of ESS, especially because of its fast response capability, efficiency, and reducing market prices, but is not always preferred for long-term storage, due to its relatively shorter lifetime. A Redox Flow Battery (RFB) on the other hand has a higher lifetime and better long-term storage capability, but has a higher upfront cost and reduced round trip efficiency. A Hybrid ESS (HESS) consisting of LIB and RFB offers the advantages of both the technologies, thus making the individual ESS more economical and flexible to use while also improving its cycle lifetime. Such a grid-connected HESS is planned and installed for a student residence at Bruchsal accommodating 150 students and equipped with 220kWp photovoltaics (PV) and 10.5kWp wind-power. In order to control this conglomerate, an Energy Management System (EMS) is deployed which not only controls but also optimizes its operations in real-time. The EMS aims at achieving multiple objectives which include reducing the ESS aging, operating the system at reduced losses, and the most important, improving building self-sufficiency. This paper focuses on the charging strategy of the HESS which is optimized in two folds. First the HESS is operated with a fixed priority-based strategy where the operation efficiency of the High Energy ESS, i.e. RFB is improved. Secondly, based on generation and consumption forecasts of the setup the EMS optimizes the charging of the High Power ESS, i.e. LIB. With the forecasts available, the EMS strategically schedules delayed charging of the individual ESS, which avoids longer relaxation periods at higher SOC and thus the aging caused due to it. Additionally, the optimization algorithm iteratively searches and operates with the possible optimal operation point of the ESS, where conversion losses are minimal under the given circumstances. Results of real-life operation of the setup based on these operation strategies are provided in this work.
The electrical grid frequency must be continuously maintained close to its nominal value of 50Hz to ensure a safe operation of the grid and its components. Transmission Systems Operators are responsible for monitoring this stability in real-time and anticipating future challenges such as the growing share of renewable generation. Prospective studies regarding frequency quality require access to generation and demand data sampled at a very small time step (~ second). However, available data for long-term studies are very often 1-hour time series. The methodology described in this article explains how to turn 1-hour time series into realistic 10-second time series for load and photovoltaic and wind generation. The general idea consists in building a 10-second trend time series with the same hourly energy as the original data and then adding noise with zero mean on each hour. This noise is computed with an autoregressive model fitted for each type of generation or load, thanks to high-resolution historical measurements on a limited set of assets. The model calibration requires analyzing relevant variables (installed capacity, load factor, etc.) that impact its results. The assessment of the quality of this method is addressed through different technical indicators.
The transmission grid in Denmark undergoes tremendous reinforcement and development due to massive integration of renewables and PtX, stronger market coupling with foreign grids and connection of the Energy Islands. The political goal is that new transmission lines shall, to the most technically feasible extent, be established as underground cables (UGC) and part of overhead lines (OHL) substituted with UGC. The Danish experience has shown that especially 400 kV UGC may cause resonance conditions, which coincide with harmonic orders of the harmonic distortion and increase the harmonic voltage distortion. Establishment of harmonic filters with the right properties, ratings and locations in the transmission grid is among mitigation solutions of excessive harmonic distortion. In the meshed transmission grid, a new harmonic filter may, on the one hand, dampen a specific harmonic order in the given substation, but, on the other hand, become inefficient or even increase harmonic distortion in other substations. Therefore, the harmonic filter design for the meshed transmission grid is a challenging task. This presentation will explain the algorithm developed by Energinet, Transmission System Operator of Denmark, for automated design of harmonic filters in the meshed transmission grids by simulations. The algorithm will be demonstrated using a validated simulation model of the Eastern Danish 400 kV transmission grid for harmonic assessment as a case study.
The Eye2Sky network is a measurement network in north-western Germany consisting of multiple all-sky imagers (ASI), meteorological and solar irradiance measurements. The network provides high temporal and spatial resolution data for meteorological and especially solar energy related applications. With increasing photovoltaic (PV) capacity in electrical grids fluctuations in solar irradiance due to changing cloud cover may have adverse effects on the grid stability. Within Eye2Sky, new technologies and methodologies facing the demand for more accurate solar irradiance forecasts are being developed. The ASIs used in Eye2Sky record 180° field of view hemispherical sky images from fish-eye lensed cameras. Accompanied with local measurements of solar irradiance components (global, direct and diffuse) a very short-term forecast of the solar resource is possible. These nowcasts provide minutely updated information up to 20 minutes ahead with 1-minute temporal and 50 m x 50 m spatial resolution. This approach shows more precise forecasting results for the next minutes ahead compared to traditional and less detailed methods based on satellite or numerical weather prediction models. In the network, multiple ASIs are used to enlarge the spatial coverage and the forecast horizon requested by many applications. Moreover, the forecast error can be reduced with a network of cameras. In this article, the Eye2Sky network, its research results and applications are introduced.
The variation of output power, due to wind speed variation, turbulence and wind gust, as well as periodic disturbances such as tower shadow and wind shear cause flicker emission in the grid during continuous operation of grid connected wind turbines. Researches were mainly focused on the flicker contribution of stall-controlled wind turbines (WT) only. This paper aims to analyze both flicker contribution as well as damping capabilities of modern wind turbines using a simulation model of a MW-level wind turbine (WT) with full converter. The effect of the wind turbine control as well as grid side converter control on flicker is studied using flicker measurement method of recently completed IEC 61400-21-4 [1] standard and its test cases are applied for comparing the flicker damping capability of the modelled grid-following (GFL) and grid-forming(GFM) converter controller at the point of common coupling (PCC).
As a vital part of the ongoing energy transition, an increasing number of residential as well as small and medium-sized commercial PV systems are and will be installed in the distribution grids. In terms of PV integration, the physical integration of grid-interconnected PV systems often stands in the focus. However, system integration on the information level must be handled as well regarding the scalability of decentralised energy systems. It is not feasible to manually integrate millions of decentralised PV systems into the utility grid management systems. Hence, a model-driven automation approach should be considered to standardise the integration process of distributed PV systems. This paper investigates this issue at the data model and communication level. Two highlights are: 1) the utilisation of an IEC 61850 compliant comprehensive data model for advanced use cases, such as PV curtailment and application of PV forecast; 2) an IEC 61850 model-based automation framework as the data model interface for an experimental distribution grid control centre. The proposed concept has been validated at different testing levels and could assist DSOs in handling the controllability and scalability of distributed PV systems, realise the high capacity utilisation scenarios of future distribution grids and reduce grid reinforcement investments.
Tremendous, fast green transition in Denmark initiates large-scale grid-integration of renewable energy sources, electrification of energy consumption, establishment of PtX and Energy Islands set goals for the transmission grid development, such as establishment of new connections, and grid reconstruction such as extensive substitution of overhead lines (OHL) with underground cables (UGC). The share of UGC in the Danish transmission grid is increasing. Presence of UGC has resulted in that resonances of the harmonic impedance characteristics of the transmission grid are brought within the harmonic order range coinciding with the harmonic distortion and causing systemwide increase of the harmonic voltage distortion in the 400 kV meshed transmission grid. Transformation of the 400 kV transmission grid arises needs of predicting the harmonic voltage distortion using simulation models to secure an adequate power quality and support investment decisions and harmonic mitigation for the grid stage which is not yet established and differs from the present grid. This paper presents a method of direct simulation of the harmonic voltage distortion, which is developed and applied by Energinet, Transmission System Operator of Denmark, using Eastern Danish 400 kV transmission grid as a case study. The main advantage of direct simulation is possibility to predict whether, where in the transmission grid, and for which harmonic orders, the not yet commissioned connections may cause violation of the IEC planning levels and which mitigation is necessary for bringing the harmonic distortion down below the planning levels with a given margin. Further, this paper briefly presents directions for the harmonic assessment in Denmark as part of an industrial PhD project by Energinet and Aalborg University. This joint work shall result in a guideline for prediction of the harmonic distortion in a meshed grid such as where and how an analytical approach can replace observational studies with many numerical simulations.
This paper analyses the impedance passivity of selective harmonic resonant control for voltage-source converter. It is found that the phase-lead compensation of harmonic resonant controller may fail to eliminate the negative damping above certain harmonic frequency, depending on the size of time delay. Further, a trade-off between the internal stability margin of harmonic control and the impedance-passivity shaping is identified. To overcome these drawbacks, a feedforward control scheme is proposed in this work, which guarantees both impedance passivity and sufficient internal stability margin for the selective harmonic resonant control. Electromagnetic transient simulations verify the theoretical analyses and the effectiveness of the control method.
The article considers the possibility of gaining access to the grid for new renewable energy sources (mainly photovoltaic farms) by sharing the grid infrastructure with existing wind farms. Due to the negative correlation of the generation capabilities of wind farms and photovoltaic farms, it is possible to use lines, transformers and other devices by both technologies, without the need to adapt this infrastructure to the power that results from the algebraic summation of nominal powers. As shown by the calcu-lations, the control of the power of a photovoltaic farm allows the use of the connection capacity reserved for the wind farm with very slight curtailment of the annual energy production.
