Enhanced primary frequency control from EVs: a fleet management strategy to mitigate effects of response discreteness

Electric vehicle (EV) chargers can be controlled to support the grid frequency by implementing a standard-compliant fast primary frequency control (PFC). This study addresses potential effects on power systems due to control discreteness in aggregated EVs when providing frequency regulation. Possible consequences of a discrete response, as reserve provision error and induced grid frequency oscillations, are first identified by a theoretical analysis both for large power systems and for microgrids. Thus, an EV fleet management solution relying on shifting the droop characteristic for the individual EVs is proposed. The PFC is implemented in a microgrid with a power-hardware-in-the-loop approach to complement the investigation with experimental validation. Both the analytical and the experimental results demonstrate how the controller performance is influenced by the response granularity, and that related oscillations can be prevented either by reducing the response granularity or by applying appropriate shifts on the droop characteristics for individual EVs. This is an open access article published by the IET under the Creative Commons Attribution -NonCommercial License (http://creativecommons.org/licenses/by-nc/3.0/) Document type: Articl

Transient stability analysis in Multi-terminal VSC-HVDC grids

A novel approach to transient stability analysis in multi-terminal high voltage direct current (MTDC) grids is presented in this paper. A symmetrical three-phase fault in an ac grid connected to a rectifier terminal of the MTDC grid causes the power injected into the dc grid to decrease, which in turn leads to a lower dc voltage in the MTDC grid. If dc voltage drops below a critical voltage limit before the ac fault is cleared, then the dc grid becomes unstable and its operation is disrupted. An analytical approach is proposed in this paper to calculate the critical clearing time of a fault in an ac grid behind a rectifier terminal beyond which dc voltage collapse occurs. A five-terminal MTDC grid modeled in EMTDC/PSCAD is used to validate the results obtained with the analytical method

Convex Optimization-Based Control Design for Parallel Grid-Connected Inverters

This paper presents a novel frequency-domain approach toward the control design for parallel grid-connected voltage source inverters (VSIs) with LCL output filters. The proposed method allows the controllers of multiple VSIs to be designed in a single step, and inherently attenuates the resonances introduced by the output filters and coupling effects while guaranteeing stability. Performance specifications such as desired closed-loop bandwidth, decoupling or robustness toward multi-model uncertainty can be specified through frequency-domain constraints. Furthermore, controllers can be designed in a plug-and-play fashion. The designed controllers are equivalent in structure to multi-variable PI controllers with filters. As the control design is based on the frequency response of the system, the algorithm is independent of the model order, which allows the use of large and high-order models. The performance of the method is demonstrated on a relevant example of a low-voltage distribution grid with five VSIs, and the results are validated both in numerical simulation using MATLAB/Simulink as well as in power-hardware-in-the-loop experiments

Essays on fiscal policy in developing countries and microstates

Descripció del recurs: 5 nov. 2013Els efectes i el paper de la Política Fiscal en els països en vies de desenvolupament i els microestats contínua sent una branca relativament poc explorada en la literatura especialitzada. Aquesta tesi és un intent de contribuir en tres àrees importants de la literatura sobre Política Fiscal: els efectes a curt termini de la Política Fiscal i el comportament dins del cicle econòmic de la Política Fiscal en els països en vies de desenvolupament i, finalment, la relació entre la Política Fiscal i la Balança per Compte Corrent en els microestats. La primera part de la tesi tracta d'avaluar els efectes dels xocs de despesa pública sobre l'economia dels països en vies de desenvolupament. Aquesta tesi utilitza una nova metodologia basada en el Model SVAR (Structural Vector Autoregression) on la identificació es realitzada a través de restriccions en el signe. L'esquema d'identificació assumeix que els xocs de despesa pública són els únics que augmenten la despesa pública, la producció, el dèficit i els ingressos fiscals dins el període d'impacte. La tècnica mencionada anteriorment es aplicada utilitzant dades de 9 països diferents. Els resultats mostren que un augment de la despesa pública conduirà a un creixement de la producció i del consum a curt termini, una immediata deterioració de les exportacions netes i a una apreciació, que pot ser no significativa, de la taxa de canvi. A més a més, els multiplicadors fiscals indiquen ser més grans que u per a tots els països menys un, durant el període d'impacte. Aquest resultats indica que un estímul fiscal pot tenir efectes d'expansió sobre la producció i el consum. De totes maneres, aquests efectes semblen ser de curt termini. En la segona part de la tesi es considera el tema de la Política Fiscal pro-cíclica en els països en vies de desenvolupament. En la literatura existeixen dues possibles explicacions per aquest fenomen. Una d'elles senyala que la Política Fiscal pro-cíclica és conseqüència de la falta d'integració fiscal amb la resta del món que pateixen els països en vies de desenvolupament mentre l'altre responsabilitza la capacitat institucional d'aquests països. En aquesta tesi s'analitza, tenint en compte els diferents estats del cicle econòmic, la importància de l'obertura financera i la qualitat de les institucions per determinar l'habilitat dels països de dur a terme Política Fiscal contra-cíclica. Aquest anàlisi es duu a terme mitjançant un model de regressió multiplicativa de planell amb termes interactius i dades de 109 països. L'anàlisi mostra que durant períodes de bonança la qualitat de les institucions juga un paper determinant en el cicle de la Política Fiscal, mentre que durant períodes de recessió son tant la integració fiscal com la qualitat de les institucions els factors claus en determinar la capacitat dels països d'emprar polítiques contra-cícliques. La tercera i última part de la tesi, escrita conjuntament amb Charles Amo-Yartey i Therese Turner Jones, examina la relació empírica entre Política Fiscal i la Balança per Compte Corrent en microestats (aquells amb una població inferior a 2 milions d'habitants entre 1970 i 2009). Donat el fet que els microestats estan caracteritzats per condicions especials com ara la reduïda dimensió del mercat domèstic i de recursos de base, gran grau d'obertura i un sector públic normalment gran, els resultats de la Política Fiscal poden ser diferents dels d'altres tipus d'economies. En aquesta parts, s'utilitza un Model PVAR (Panel Vector Autoregression) per tal d'estimar l'efecte de la Política Fiscal en la Balança per Compte Corrent dels microestats. Resumint, els resultats indiquen que el dèbil efecte dels preus fa l'ajust fiscal força més difícil en els microestats.Fiscal policy in the context of developing countries remains a relatively under explored area in the literature. This thesis is an attempt to address three important areas in the literature on fiscal policy in developing countries, namely, the short-run effects of fiscal policy, the cyclical behavior of fiscal policy, and the link between fiscal policy and the current account. The first part of the thesis assesses the effects of government spending shocks on the economy of developing countries. I use a recent Structural Vector Autoregression (SVAR) technique where identification is achieved via sign restrictions. The identification scheme applies the restrictions that government spending shocks are the only shocks that raise government spending, output, deficit and tax revenue in the impact period. I gather data on 9 countries and employ the above outlined technique. The results show that an increase in government spending would lead to a short-lived expansion of output and consumption, an immediate deterioration of net exports, and an appreciation or no effect on exchange rates. Moreover, the calculated output multipliers give values that are greater than one for all but one country in the impact period. The results suggest a fiscal stimulus could have expansionary effects on output and consumption, however these effects would be short-lived. In the second part of the thesis, I consider the issue of procyclicality of fiscal policy in developing countries. In the literature, there exist two competing plausible explanations. One espouses the view that procyclical fiscal policy is a result of lack of financial integration with the world economy while the other view attributes it to weak institutions within the country. I analyze, by taking into consideration the different states of the economy, the role of financial openness and quality of institutions on the ability of countries to conduct counter-cyclical fiscal policy. I develop a multiplicative panel regression model with interactive terms and use data from 109 countries. The analysis shows during good times the quality of institutions has a dominant role to play in the cyclicality of fiscal policy, and during bad times both financial integration and institutions are important in the ability of countries to run counter-cyclical fiscal policy. The third and last part of the thesis, coauthored with Charles Amo-Yartey and Therese Turner Jones, examines the empirical link between fiscal policy and the current account focusing on microstates. Microstates are defined as countries with a population of less than 2 million between 1970 and 2009. Due to microstates being characterized by special features such as small size of domestic markets, small domestic resource base, high degree of openness and large size of the public sector, among others, findings from other countries may not be applicable to such states. In this part, panel regression and Panel Vector Autoregression (PVAR) are employed to estimate the impact of fiscal policy on the current account in microstates. Overall, the results suggest that the weak relative price effect makes fiscal adjustment much more difficult in microstates

Spectral Decomposition of Regulatory Thresholds for Climate-Driven Fluctuations in Hydro- and Wind Power Availability

Abstract Climate-driven fluctuations in the runoff and potential energy of surface water are generally large in comparison to the capacity of hydropower regulation, particularly when hydropower is used to balance the electricity production from covarying renewable energy sources such as wind power. To define the bounds of reservoir storage capacity, we introduce a dedicated reservoir volume that aggregates the storage capacity of several reservoirs to handle runoff from specific watersheds. We show how the storage bounds can be related to a spectrum of the climate-driven modes of variability in water availability and to the covariation between water and wind availability. A regional case study of the entire hydropower system in Sweden indicates that the longest regulation period possible to consider spans from a few days of individual subwatersheds up to several years, with an average limit of a couple of months. Watershed damping of the runoff substantially increases the longest considered regulation period and capacity. The high covariance found between the potential energy of the surface water and wind energy significantly reduces the longest considered regulation period when hydropower is used to balance the fluctuating wind power

Essays on Fiscal Policy in Developing Countries and Microstates

Els efectes i el paper de la Política Fiscal en els països en vies de desenvolupament i els microestats contínua sent una branca relativament poc explorada en la literatura especialitzada. Aquesta tesi és un intent de contribuir en tres àrees importants de la literatura sobre Política Fiscal: els efectes a curt termini de la Política Fiscal i el comportament dins del cicle econòmic de la Política Fiscal en els països en vies de desenvolupament i, finalment, la relació entre la Política Fiscal i la Balança per Compte Corrent en els microestats. La primera part de la tesi tracta d'avaluar els efectes dels xocs de despesa pública sobre l'economia dels països en vies de desenvolupament. Aquesta tesi utilitza una nova metodologia basada en el Model SVAR (Structural Vector Autoregression) on la identificació es realitzada a través de restriccions en el signe. L'esquema d'identificació assumeix que els xocs de despesa pública són els únics que augmenten la despesa pública, la producció, el dèficit i els ingressos fiscals dins el període d'impacte. La tècnica mencionada anteriorment es aplicada utilitzant dades de 9 països diferents. Els resultats mostren que un augment de la despesa pública conduirà a un creixement de la producció i del consum a curt termini, una immediata deterioració de les exportacions netes i a una apreciació, que pot ser no significativa, de la taxa de canvi. A més a més, els multiplicadors fiscals indiquen ser més grans que u per a tots els països menys un, durant el període d'impacte. Aquest resultats indica que un estímul fiscal pot tenir efectes d'expansió sobre la producció i el consum. De totes maneres, aquests efectes semblen ser de curt termini. En la segona part de la tesi es considera el tema de la Política Fiscal pro-cíclica en els països en vies de desenvolupament. En la literatura existeixen dues possibles explicacions per aquest fenomen. Una d'elles senyala que la Política Fiscal pro-cíclica és conseqüència de la falta d'integració fiscal amb la resta del món que pateixen els països en vies de desenvolupament mentre l'altre responsabilitza la capacitat institucional d'aquests països. En aquesta tesi s'analitza, tenint en compte els diferents estats del cicle econòmic, la importància de l'obertura financera i la qualitat de les institucions per determinar l'habilitat dels països de dur a terme Política Fiscal contra-cíclica. Aquest anàlisi es duu a terme mitjançant un model de regressió multiplicativa de planell amb termes interactius i dades de 109 països. L'anàlisi mostra que durant períodes de bonança la qualitat de les institucions juga un paper determinant en el cicle de la Política Fiscal, mentre que durant períodes de recessió son tant la integració fiscal com la qualitat de les institucions els factors claus en determinar la capacitat dels països d'emprar polítiques contra-cícliques. La tercera i última part de la tesi, escrita conjuntament amb Charles Amo-Yartey i Therese Turner Jones, examina la relació empírica entre Política Fiscal i la Balança per Compte Corrent en microestats (aquells amb una població inferior a 2 milions d'habitants entre 1970 i 2009). Donat el fet que els microestats estan caracteritzats per condicions especials com ara la reduïda dimensió del mercat domèstic i de recursos de base, gran grau d'obertura i un sector públic normalment gran, els resultats de la Política Fiscal poden ser diferents dels d'altres tipus d'economies. En aquesta parts, s'utilitza un Model PVAR (Panel Vector Autoregression) per tal d'estimar l'efecte de la Política Fiscal en la Balança per Compte Corrent dels microestats. Resumint, els resultats indiquen que el dèbil efecte dels preus fa l'ajust fiscal força més difícil en els microestats.Fiscal policy in the context of developing countries remains a relatively under explored area in the literature. This thesis is an attempt to address three important areas in the literature on fiscal policy in developing countries, namely, the short-run effects of fiscal policy, the cyclical behavior of fiscal policy, and the link between fiscal policy and the current account. The first part of the thesis assesses the effects of government spending shocks on the economy of developing countries. I use a recent Structural Vector Autoregression (SVAR) technique where identification is achieved via sign restrictions. The identification scheme applies the restrictions that government spending shocks are the only shocks that raise government spending, output, deficit and tax revenue in the impact period. I gather data on 9 countries and employ the above outlined technique. The results show that an increase in government spending would lead to a short-lived expansion of output and consumption, an immediate deterioration of net exports, and an appreciation or no effect on exchange rates. Moreover, the calculated output multipliers give values that are greater than one for all but one country in the impact period. The results suggest a fiscal stimulus could have expansionary effects on output and consumption, however these effects would be short-lived. In the second part of the thesis, I consider the issue of procyclicality of fiscal policy in developing countries. In the literature, there exist two competing plausible explanations. One espouses the view that procyclical fiscal policy is a result of lack of financial integration with the world economy while the other view attributes it to weak institutions within the country. I analyze, by taking into consideration the different states of the economy, the role of financial openness and quality of institutions on the ability of countries to conduct counter-cyclical fiscal policy. I develop a multiplicative panel regression model with interactive terms and use data from 109 countries. The analysis shows during good times the quality of institutions has a dominant role to play in the cyclicality of fiscal policy, and during bad times both financial integration and institutions are important in the ability of countries to run counter-cyclical fiscal policy. The third and last part of the thesis, coauthored with Charles Amo-Yartey and Therese Turner Jones, examines the empirical link between fiscal policy and the current account focusing on microstates. Microstates are defined as countries with a population of less than 2 million between 1970 and 2009. Due to microstates being characterized by special features such as small size of domestic markets, small domestic resource base, high degree of openness and large size of the public sector, among others, findings from other countries may not be applicable to such states. In this part, panel regression and Panel Vector Autoregression (PVAR) are employed to estimate the impact of fiscal policy on the current account in microstates. Overall, the results suggest that the weak relative price effect makes fiscal adjustment much more difficult in microstates

Wind Turbine Model Validation with Measurements

The objective of this work is to validate wind turbine models available in commercial simulation tools with measurements. Results are shown for two turbines located in two different wind farms; wind turbine 1 is a fixed speed turbine with induction generator, and wind turbine 2 is a variable speed turbine with converter-interfaced synchronous generator. Simulated active and reactive power transient responses to voltage dips have been compared to measured responses, as suggested by IEA Wind Annex 21. For the fixed speed turbine quite good agreement between measurement and simulation is obtained. Shaft parameters are seen to have significant influence on the simulated active power response. For the variable speed turbine the active and reactive power responses are to a high degree determined by the power electronics interface and corresponding controllers, and particularly the control strategy applied during voltage dips. Wind turbine manufacturers are generally very restrictive on giving out this type of information, and thus typical configurations and parameters have been used in this work. The agreement between measurement and simulation can to some degree be improved by changing the inverter controller parameters by trialand error, but detailed knowledge on the control of the converter would be required in order to achieve a very good agreement. Copyright © 2012 Published by Elsevier Ltd.publishedVersio

Using Decentralized Control Techniques for Interaction Analysis in Hybrid AC/DC Grids

One of the ancillary services that can be provided by Multi-terminal Direct Current (MTDC) grid to connected ac grids is power oscillation damping (POD). However, using PODs at multiple terminals of an MTDC grid results in multi-loop, multi-variable control system. Such control systems inherently have control loop interactions challenge, which can result in reduced performance of one or more controllers. This entails that PODs installed at multiple converter terminals to damp oscillations in respective ac grids could be affected due to unfavorable interactions among the controllers. Thus, compromising the stability of the connected ac grids. This paper presents analyses of interaction between multiple POD controllers installed on MTDC. For a three-terminal study system, insights on interactions between POD controllers at two different converter terminals of an MTDC are obtained using relative gain array and performance relative gain array measures.Using Decentralized Control Techniques for Interaction Analysis in Hybrid AC/DC GridsacceptedVersio

Stability Analysis of High Voltage Hybrid AC/DC Power Systems

Interest in large-scale integration of power from renewable energy sources (RES) has grown in the last decade as a result of energy policies adopted by governments in an effort to reduce CO2 and greenhouse gas emissions. Both large scale, and distributed solar and wind energy have proliferated the power system and will continue to do so in the future. Thus, large and complex transmission systems are needed for robust, flexible and secure operation of the future power system. Multi-terminal HVDC (MTDC) grids are expected to play an important role in an efficient socio-economic operation of the electric power system by acting as a means for integration of RES, exchange of balancing power, crossborder power market trading, grid reinforcement, etc. As the introduction of MTDC grids will eventually result in a hybrid ac/dc power system, it is necessary to carry out a global analysis that considers the entire hybrid ac/dc power system, which includes both dc and all synchronous areas of the power systems. The main objective of this PhD is to study the stability of hybrid ac/dc power systems, with a particular focus on the ac grids. The work investigates how the dynamic characteristics of ac grids will be affected by the introduction of the MTDC grids and/or by control methods implemented in MTDC converter controllers. Modal analysis, in particular eigenvalues, mode shapes, and participation factors, was used to identify and analyze interactions (dynamic coupling) between different subsystems in a hybrid ac/dc power system. Mode shapes were used to identify electromechanical interactions between generators located in different synchronous areas (asynchronous grids). The inter-grid electromechanical interactions are generally weak but are influenced by dc grid control strategy, controller tuning and damped frequency of electromechanical modes. The source of the interactions is dynamic coupling between ac and dc grids. When several terminals share the duty of dc voltage regulation, as in the case of dc voltage droop control operation mode, the dynamics of the ac grids behind those terminals are coupled to a common dc grid dynamics. This leads to indirect coupling of dynamics of different ac grids through dc grid dynamics. A qualitative analysis of state matrix of a single 2-level converter with and without connection to a detailed ac grid model was used to supplement the findings of the quantitative modal analysis. It was shown that there is a two-way dynamic coupling between ac and dc grids when a converter is operated in constant dc voltage or dc voltage droop control modes, i.e. ac grid dynamics is coupled with dc grid dynamics and dc grid dynamics is coupled with ac grid dynamics. However, there is only a one-way coupling between an ac grid and a dc grid if the converter is operated in constant power control mode. In such cases, the ac grid dynamics is coupled with the dc grid dynamics, but the dc grid dynamics is not coupled with the ac grid dynamics. Decentralized control techniques were used to study interactions between power oscillation damping (POD) controllers on multiple terminals of an MTDC that interconnects several asynchronous ac grids. Interaction between the selected control loops was assessed using dynamic relative gain array and performance relative gain array techniques in the frequency domain. In addition, modal and time domain analyses carried out for the study case supported the findings from the frequency domain analysis. For the study case analyzed, it was found that due to control loop interactions the performance of one of the controllers was augmented, while the performance of the other controller deteriorated. The analyses clearly showed that control loop interaction should be considered while tuning PODs on converters even if they are connected to different grids. Finally, a coordinated control strategy for terminal converters of a dc grid was proposed to address the issue of frequency disturbance in other ac grids when one grid receives frequency support from an offshore wind farm. It was shown that by coordinating converter controllers at the terminals of an offshore wind farm and one ac grid, it is possible to maximize frequency support contribution of the offshore wind farm and avoid disturbance in other ac grids connected to the MTDC. However, the proposed method works when only one ac grid is receiving frequency support and the remaining ac grids are connected MTDC terminals, which are operating in dc droop or constant power control mode. If more than one ac grids are to receive frequency support through MTDC grid, then negative interactions occur when the proposed controller is used. Therefore, in such cases, distributed dc voltage and frequency droop control is the best control option. However, it should be noted that with distributed dc voltage and frequency droop control method, the frequency support comes not only from the wind farm but also from other ac grids behind an MTDC terminal operating in dc voltage droop control mode