Nordic Grid Research Articles

Dynamic Virtual Power Plant Design for Fast Frequency Reserves: Coordinating Hydro and Wind

To ensure frequency stability in future low-inertia power grids, fast ancillary services such as fast frequency reserves (FFR) have been proposed. In this work, the coordination of conventional (slow) frequency containment reserves (FCR) with FFR is treated as a decentralized model matching problem. The design results in a dynamic virtual power plant (DVPP) whose aggregated output fulfills the system operator’s (SO’s) requirements in all time scales, while accounting for the capacity and bandwidth limitation of participating devices. This is illustrated in a 5-machine representation of the Nordic synchronous grid. In the Nordic grid, stability issues and bandwidth limitations associated with non-minimum phase zeros of hydropower is a well-known problem. By simulating the disconnection of a 1400 MW importing dc link, it is shown that the proposed DVPP design allows for coordinating fast FFR from wind, with slow FCR from hydro, while respecting dynamic limitations of all participating devices. The SO’s requirements are fulfilled in a realistic low-inertia scenario without the need to install battery storage or to waste wind energy by curtailing the wind turbines.

Superflexible Hydropower for the Nordic Grid: Accelerating the Energy Transition

The efficacy of flexible reservoir-based hydropower for renewable integration is evident throughout the world in places like Norway, Canada, Brazil, India, and China, to name a few. The flexibility of hydropower, both at the reservoir and generation side, could enable higher shares of intermittent renewable sources in regions where hydropower is readily available. In Norway, hydropower is the backbone, enabling extraordinary cheap electricity from new renewables. Compared to Germany, where coal power plants are the main backup producers for weather-dependent renewables, Norwegian hydropower is much more flexible to dispatch when the load changes. Although we are still waiting for the missing link to back up intermittent renewables everywhere, pursuing the flexibility of hydropower can be a transformative solution in places where it applies. This solution is especially true for the Nordic power grid, where Norway alone hosts half of Europe’s entire hydropower storage capacity.

Phase and Amplitude Synchronization in Power-Grid Frequency Fluctuations in the Nordic Grid

Monitoring and modelling the power grid frequency is key to ensuring stability in the electrical power system. Many tools exist to investigate the detailed deterministic dynamics and especially the bulk behaviour of the frequency. However, far less attention has been paid to its stochastic properties, and there is a need for a cohesive framework that couples both short-time scale fluctuations and bulk behaviour. Moreover, commonly assumed uncorrelated stochastic noise is predominantly employed in modelling in energy systems. In this publication, we examine the stochastic properties of the Nordic Grid, focusing on the increments of the frequency recordings. We show that these increments follow non-Gaussian statistics and display spatial and temporal correlations. Furthermore, we report two different physical synchronisation phenomena: a very short timescale phase synchronisation (< 2 s) followed by a slightly larger timescale amplitude synchronisation (2 s–5 s). Overall, these results show that modelling of fluctuations in power systems should not be restricted to Gaussian white noise.

Case Study and Feasibility Analysis of Multi-Objective Life Cycle Energy System Optimization in a Nordic Campus Building

Due to the high energy consumption of buildings, there is a demand for both economically and environmentally effective designs for building energy system retrofits. While multi-objective optimization can be used to solve complicated problems, its use is not yet widespread in the industry. This study first aims to develop an efficient and applicable multi-objective building energy system optimization method, used to dimension energy production and storage retrofit components in a case campus building in Lahti, Finland. Energy consumption data of the building are obtained with a dynamic energy model. The optimization model includes economic and environmental objectives, and the approach is found to function satisfactorily. Second, this study aims to assess the feasibility and issues of multi-objective single-building energy system optimization via the analysis of the case optimization results. The results suggest that economically beneficial local energy production and storage retrofits could not always lead to life cycle CO2-eq emission reductions. The recognized causes are high life cycle emissions from the retrofit components and low Nordic grid energy emissions. The performed sensitivity and feasibility analyses show that correctness and methodological comparability of the used emission factors and future assumptions are crucial for reliable optimization results.

Revealing drivers and risks for power grid frequency stability with explainable AI

SummaryStable operation of an electric power system requires strict operational limits for the grid frequency. Fluctuations and external impacts can cause large frequency deviations and increased control efforts. Although these complex interdependencies can be modeled using machine learning algorithms, the black box character of many models limits insights and applicability. In this article, we introduce an explainable machine learning model that accurately predicts frequency stability indicators for three European synchronous areas. Using Shapley additive explanations, we identify key features and risk factors for frequency stability. We show how load and generation ramps determine frequency gradients, and we identify three classes of generation technologies with converse impacts. Control efforts vary strongly depending on the grid and time of day and are driven by ramps as well as electricity prices. Notably, renewable power generation is central only in the British grid, while forecasting errors play a major role in the Nordic grid.

Spatio-temporal complexity of power-grid frequency fluctuations

Power-grid systems constitute one of the most complex man-made spatially extended structures. These operate with strict operational bounds to ensure synchrony across the grid. This is particularly relevant for power-grid frequency, which operates strictly at 50 Hz (60 Hz). Nevertheless, small fluctuations around the mean frequency are present at very short time scales <2 s and can exhibit highly complex spatio-temporal behaviour. Here we apply superstatistical data analysis techniques to measured frequency fluctuations in the Nordic grid. We study the increment statistics and extract the relevant time scales and superstatistical distribution functions from the data. We show that different synchronous recordings of power-grid frequency have very distinct stochastic fluctuations with different types of superstatistics at different spatial locations, and with transitions from one superstatistics to another when the time lag of the increment statistics is changed.

Variable Speed Hydropower for Provision of Fast Frequency Reserves in the Nordic Grid

The main purpose of this article is to demonstrate the grid support capability of variable speed hydropower (VSHP). The hypothesis is that active power control of VSHP plants can improve both the frequency stability and the transient rotor angle stability by contributing to virtual inertia (VI), fast frequency reserves and power oscillation damping. Fast active power control is made possible by power electronic converters obtaining the energy from the rotating masses of the turbine and generator while the rotational speed is restored by control of the guide vane opening of the hydro turbine. A detailed dynamic simulation model is developed to simulate the VSHP within the Nordic 44-bus grid model. The VSHP controller includes a moving horizon estimator (MHE), model predictive control (MPC) with inertia emulation realised by the frequency converter of the VSHP plant. The results show that VSHP can contribute significantly to reduce the maximum frequency deviation following a sudden loss of generation. In addition, the amplitudes of power oscillations are reduced and the critical fault clearing times are generally improved. The VSHP should, therefore, be regarded as one of the technically viable options for provision of fast balancing system services.

Evaluating component importance and reliability of power transmission networks subject to windstorms: methodology and application to the nordic grid

We present a procedure for the efficient assessment of component importance and network reliability in power transmission grids subject to wind hazards. A stochastic wind load model is learned from windstorm data. Component fragility models are established by combining information on the design wind loads prescribed by the relevant structural codes and the observed failure rate in the network as a whole. Network performance is assessed by a DC power flow model, which accounts for cascading failures and potential islanding due to load redistribution after initial failure events caused by the windstorms. Besides evaluating the network reliability, selected importance measures are defined to rank single components according to their influence on the overall system reliability. We thereby distinguish component importance related to initial failures triggered by the windstorms and component importance related to potential subsequent cascading failures. The procedure is demonstrated by application to the Nordic Grid model of the electrical power transmission network in Denmark, Norway, Sweden, and Finland. We find that the procedure can provide an efficient basis for planning network improvements in terms of (1) strengthening vulnerable line segments against wind loads and (2) increasing line capacities to limit cascading failures.

Impact of deep wind power penetration on variability at load centers

Increasingly, variable renewable energy capacity will be added distant from load centers in much of the world. How such intraregional heterogeneity in variable supply and load will impact the energy system in a deep renewable penetration scenario is studied here. Heterogeneous reliability requirements imposed by such scenarios are not well understood. Some unique geographic settings, such as the Nordic grid with weeks of pumped hydro backup, manage to circumvent this issue without significant curtailment, but most regions have yet to achieve the renewable energy levels at which the issue will arise. Using simulations of wind power expansion in New York State, we illustrate the intraregional effects by quantifying the net load, net load ramping, operating reserve and regulation requirements, and the associated distribution of infrastructure investments and ancillary services. The study finds that only at wind capacities exceeding 100% of the average statewide load does the wind-generated electricity meet significant portions of the distant demands. However, the peak net load in these areas is not reduced, requiring that dispatchable generation capacity be maintained. Moreover, the net load becomes highly variable, with large ramp rates that increase ancillary service requirements (operating reserves and frequency regulation) to ensure reliable system operation. A major finding is that the additional ancillary service requirements become more concentrated in the distant load centers. A second significant finding is that while transmission capacity upgrades greatly increase the wind-generated electricity that reaches load centers, the increased variability in that supply can exacerbate both the magnitude and heterogeneity of ancillary service requirements. These services would presumably be provided by the same local dispatchable resources that would now be operating at lower capacity factors but with higher variability. Such changes in the scale and distribution of intraregional integration measures and infrastructure investments may require new energy planning approaches and market structures to achieve anticipated future renewable energy targets.

A predictive controller based on transient simulations for controlling a power plant

A predictive governor based on an embedded, online transient simulation was commissioned at Tonstad power plant in Norway in December 2014. This governor controls each individual turbine governor by feeding them modified setpoints. Tonstad power plant consists of 4 × 160 MW + 1 × 320 MW high head Francis turbines. With a yearly production of 3888 GWh, it is the largest in Norway. The plant is a typical high head Norwegian plant with very long tunnels and correspondingly active dynamic behaviour. This new governor system continuously simulates the entire plant, and appropriate actions are taken automatically by special algorithms. The simulations are based on the method of characteristics (MOC). The governing system has been in full operational mode since December 19 2014. The testing period also included special acceptance tests to be able to deliver FRR, both on the Nordic grid and on DC cable to Denmark. Although in full operational mode, this system is still a prototype under constant development. It shows a new way of using transient analysis that may become increasingly important in the future with added power from un-regulated sources such as wind, solar and bio.

Analysis on regulation strategies for extending service life of hydropower turbines

Since a few years, there has been a tendency that hydropower turbines experience fatigue to a greater extent, due to increasingly more regulation movements of governor actuators. The aim of this paper is to extend the service life of hydropower turbines, by reasonably decreasing the guide vane (GV) movements with appropriate regulation strategies, e.g. settings of PI (proportional-integral) governor parameters and controller filters. The accumulated distance and number of GV movements are the two main indicators of this study. The core method is to simulate the long-term GV opening of Francis turbines with MATLAB/Simulink, based on a sequence of one-month measurements of the Nordic grid frequency. Basic theoretical formulas are also discussed and compared to the simulation results, showing reasonable correspondence. Firstly, a model of a turbine governor is discussed and verified, based on on-site measurements of a Swedish hydropower plant. Then, the influence of governor parameters is discussed. Effects of different settings of controller filters (e.g. dead zone, floating dead zone and linear filter) are also examined. Moreover, a change in GV movement might affect the quality of the frequency control. This is also monitored via frequency deviation characteristics, determined by elementary simulations of the Nordic power system. The results show how the regulation settings affect the GV movements and frequency quality, supplying suggestions for optimizing the hydropower turbine operation for decreasing the wear and tear.

Role of Statistical and Machine Learning Methods in Special Protection Scheme Logic Design and Failure Assessment

With tremendous growth in variable renewable generation and uncertainties in power system operations, there has been lot of installations of special protection schemes in many parts of the world. A special protection scheme, also termed a remedial action scheme, is the high-end last line of defense mechanism to relieve the system from catastrophic consequences under highly stressed conditions. This article delineates the special protection scheme logic design process and, with the help of a Nordic grid case study, proposes a new paradigm of special protection scheme failure assessment and logic design process, namely the “system-view” framework. In this context, the article also demonstrates the application of decision tree-based methods to derive special protection scheme operational logic and provides a pathway to use efficient statistical sampling techniques for implementing the system-view special protection scheme failure assessment method, thereby advancing the logic redesign process.

Simulations of the Dynamic Load in a Francis Runner based on measurements of Grid Frequency Variations

In the Nordic grid, a trend observed the recent years is the increase in grid frequency variations, which means the frequency is outside the normal range (49.9-50.1 Hz) more often. Variations in the grid frequency leads to changes in the speed of rotation of all the turbines connected to the grid, since the speed of rotation is closely related to the grid frequency for synchronous generators. When the speed of rotation changes, this implies that the net torque acting on the rotating masses are changed, and the material of the turbine runners must withstand these changes in torque. Frequency variations thus leads to torque oscillations in the turbine, which become dynamical loads that the runner must be able to withstand. Several new Francis runners have recently experienced cracks in the runner blades due to fatigue, obviously due to the runner design not taking into account the actual loads on the runner. In this paper, the torque oscillations and dynamic loads due to the variations in grid frequency are simulated in a 1D MATLAB program, and measured grid frequency is used as input to the simulation program. The maximum increase and decrease in the grid frequency over a 440 seconds interval have been investigated, in addition to an extreme event where the frequency decreased far below the normal range within a few seconds. The dynamic loading originating from grid frequency variations is qualitatively found by a constructed variable , and for the simulations presented here the variations in are found to be around 3 % of the mean value, which is a relatively small dynamic load. The important thing to remember is that these dynamic loads come in addition to all other dynamic loads, like rotor-stator interaction and draft tube surges, and should be included in the design process, if not found to be negligible.

Measurements and simulations of turbines on common grid

Speed droop control is of basic importance for the primary governing in the Nordic grid. The speed droop control. a mandatory and build-in regulatory loop on all larger units. is automatically changing the produced power on synchronous units as the grid frequency changes. This part of the governor allows a certain deviance from the nominal 50 Hz grid frequency. If the grid frequency is decreasing this means that the load on the grid is greater than the power delivered into the grid. and the local speed droop regulatory loop on each unit then autonomously increases the production to obtain a new balance between load and production. which will be at a lower frequency than 50 Hz. If the power delivered into the grid is greater than the load. the rotating masses will be accelerated (thus increasing the grid frequency) and the speed droop operation will act to reduce the power produced to obtain a new balance. this time at a higher frequency than 50 Hz. The frequency in the Nordic power grid has in recent years for increasing duration been outside the allowed steady state frequency band of 50 ± 0.1 Hz. In order to study the behaviour of a turbine operating on a common grid, measurements have been done at site. The measurements performed are the generator power, main servo motor position, the rotational speed of the unit and the grid frequency. The purpose of the measurements was to see if it is possible to observe the behaviour of the machine as it is linked together with all the other machines on a synchronous grid. It is interesting to observe the response to deviations in the frequency due to the speed droop operation. In order to better understand the behaviour, a simulation model of two power plants, complete with individual conduit system, turbine and generator, connected to the same grid was used.

Dynamic load on a Francis turbine runner from simulations based on measurements

The frequency of the Nordic power grid has become more volatile during recent years. This gives rise to two effects in synchronous machines. Firstly, as grid frequency changes the rotational speed of synchronous machines must change likewise. Secondly, the Nordic grid uses speed droop operation extensively as the primary governing; hence the power produced is a function of the grid frequency. These two coupled effects will lead to the runner and axle on synchronous machines having to cope with a varying level of torque. Even if the unit is supposedly operating at steady state via a fixed set point for the production, the influence of the varying grid frequency is that the torque in not steady at all. Recent years' new high head Francis runners in Norway have shown a tendency towards experiencing fatigue to a greater extent than what seem to be the case for new runners decades ago. Leading to this paper, measurements have been made of the rotational speed; generator power; main servo motor position and grid frequency at a Francis turbine unit. Based on these measurements simulations that include the hydraulic domain have then been performed. From these simulation results a property is constructed which is intended as a qualitative measure of the material stresses induced in the rotating masses of the unit, and is representative of the dynamical loads on the material of the rotating masses. The work is a part of a longer term goal, namely identifying the stress oscillations in a Francis turbine runner operating at speed of rotation oscillating because of grid frequency variations.

Preprocessing synchronized phasor measurement data for spectral analysis of electromechanical oscillations in the Nordic Grid

Summary Spectral analysis techniques have successfully been applied for near real-time monitoring of power system small-signal electromechanical oscillations using synchronized phasor data from PMUs. The methods used for this purpose commonly assume that random load variations adopt the distribution function of Gaussian noise. Hence, careful attention has to be paid so that the preprocessing of synchronized phasor measurements is capable of providing data with the characteristics expected for these methods to work properly. This can be viewed as a “conditioning” step in the data handling process that has an impact on the results from spectral analysis which consume this data. This article aims to revisit the crucial step of preprocessing of PMU data in a tutorial fashion. The goal is to share the authors' experience when dealing with real PMU data originating from the Nordic Grid. This article offers a systematic and detailed methodology developed by the authors which has been successfully used in studies on estimation of electromechanical modes in the Nordic Grid. Copyright © 2013 John Wiley & Sons, Ltd.

Smart Demand for Frequency Regulation: Experimental Results

As renewable energy sources increase their penetration, the traditional providers of frequency regulation service, i.e., fossil fueled thermal power plants, will be displaced, motivating the search for novel providers such as demand-side resources. This paper presents the results of field experiments using demand as a frequency controlled reserve (DFCR) on appliances with programmable thermostats. The experiments conducted showed the response of a population of thermostatically controlled loads acting as normal reserves (up and down regulation) and disturbance reserves (up regulation only) as defined by the Nordic Grid Codes . In addition, industrial pump loads and relay-controlled loads were tested as DFCR. The tests show that a population of refrigerators was able to deliver frequency reserves approximately equal to their average power consumption. Electric space heaters in the autumn season were able to provide frequency reserves of a magnitude 2.7 times their average power consumption.

Identification of Power System Dominant Inter-Area Oscillation Paths

This paper presents three algorithms for identification of dominant inter-area oscillation paths: a series of interconnected corridors in which the highest content of the inter-area modes propagates through. The algorithms are developed to treat different sets of data: 1) known system model; 2) transient; and 3) ambient measurements from phasor measurement units (PMUs). These algorithms take feasibility into consideration by associating the network variables made available by PMUs, i.e., voltage and current phasors. All algorithms are demonstrated and implemented on a conceptualized Nordic Grid model. The results and comparison among three algorithms are provided. The applications of the algorithms not only facilitate in revealing critical corridors which are mostly stressed but also help in indicating relevant feedback input signals and inputs to mode meters which can be determined from the properties of dominant paths.

Applications of Spectral Analysis Techniques for Estimating the Nordic Grid's Low Frequency Electromechanical Oscillations

For power networks such as the Nordic Grid, that have operation constraints limits imposed by the existence of low-damped electromechanical oscillations, the estimation of electromechanical mode properties is of crucial importance for providing power system control room operators with adequate indicators of the stress of their network. This article addresses the practical application of different spectral analysis techniques that can be used for the estimation of electromechanical mode properties using data emerging from real synchronized phasor measurement units (PMUs) located at both the low-voltage distribution and high-voltage transmission networks of the Nordic grid. Emphasis is made on providing systematic approaches to deal with imperfect data found in practice so that accurate estimates can be computed.

Energy consumption, costs and environmental impacts for urban water cycle services: Case study of Oslo (Norway)

Energy consumption in the operation and maintenance phase of the urban water and wastewater network is directly related to both the quantity and the desired quality of the supplied water/treated wastewater – in other words, to the level of service provided to consumers. The level of service is dependent on not just the quantity and quality of the water but also the state of the infrastructure. Maintaining the infrastructure so as to be able to provide the required high level of service also demands energy. Apart from being a significant operational cost component, energy use also contributes to life-cycle environmental impacts. This paper studies the direct energy consumption in the operation and maintenance phase of the water and wastewater system in Oslo; and presents a break-up among the different components of the network, of quantities, costs and environmental impacts. Owing to the diversity in the periods of time for which comprehensive data for the whole system are available, the study period is restricted to years 2000–2006. The per-capita annual consumption of energy in the operational phase of the system varied between 220 and 260 kWh; and per-capita annual expenses on energy in inflation-adjusted year-2006-Euros ranged between 6.5 and 11 Euros. The energy consumed on the upstream, per unit volume water supplied was around 0.4 kWh on average, while the corresponding value for the downstream was 0.8 kWh per cubic metre wastewater treated. The upstream Greenhouse gas (GHG) emissions ranged between 70 and 80 g per cubic metre of water supplied, about 22% greater on average than the corresponding specific GHG emissions on the downstream. ► Annual per-capita energy consumption in the Operation and Maintenance (O & M) phase of the system varied between 220 and 260 kWh. ► Annual per-capita annual expenses on energy in inflation-adjusted year-2006-Euros ranged between 6.5 and 11 Euros. ► Upstream O & M energy consumption per cubic metre of water supplied was 0.4 kWh on average, on the downstream, it was 0.8 kWh per cubic metre. ► Upstream GHG emissions ranged between 70 and 80 g/m 3 of water supplied, about 22% greater (on average) than the downstream. ► Consumption of biogas at Vestfjorden Avløpsselskap (VEAS) precludes the need for natural gas or electricity from the Nordic grid and in both cases, contributes to a reduction in GHG emissions.