Ines Romero

Bio: Ines Romero is an academic researcher. The author has contributed to research in topics: AC power & Dynamic load testing. The author has an hindex of 1, co-authored 1 publications receiving 27 citations.

Dynamic Power System Load -Estimation of Parameters from Operational Data

Abstract: The significance of load modeling for voltage stability studies has been emphasized by several disturbances, which have taken place in the past years. They have shown that the loads in combination with other dynamics are among the main contributors of prolonged low voltage conditions, voltage instability and collapse in the power system. As a result of these disturbances new investigations have come up to better understand the nature of the load. However, power system loads keep being very difficult to model; the load generally aggregates a large number of individual components of different nature, different load dynamics are excited depending on the time frame of actuation and the type of disturbance affecting the system, and the load is highly dependent on external factors such as weather conditions. This thesis investigates the load-voltage characteristic during two different time scales, long-term over several minutes, and short-term covering ms to several seconds, for different sized disturbances, and its impact on the calculation of transfer limits and security margins in voltage stability studies. The accurate determination of transfer limits will be an increasingly important task to maintain the operational security and economic dispatch of the power system. The location of the stability limits and the determination of transfer limits depend on the load-voltage characteristic since load relief due to the load-voltage dependency results in larger transfer limits. Moreover, the importance of using dynamic load models instead of static ones in stability studies is highlighted in this thesis. Due to the large amount of electrical heating loads in Sweden and its effect on voltage stability, a dynamic load model with exponential recovery, previously proposed by Hill and Karlsson, [Karlsson and Hill, 1994], has been the starting point for the investigations. Field measurements from continuous normal operation at the 20 kV-level from a substation in Sweden have provided a large amount of data covering all seasons during the time period July 2001-June 2002, and have resulted in extensive, unique and interesting recordings of active and reactive load characteristic and its dependency with small voltage variations. The data have revealed the variation of the load parameters and their dependency with weather and season of the year. The work has also contributed to a better approach for the normalization of traditional reactive load models. Furthermore the load-voltage characteristic during large disturbances has been investigated based on field measurements of phase-to-phase faults in a non-effectively earthed 50 kV system in Sweden. Three-phase currents and voltages have been used to estimate the active and reactive power. The recordings exhibited voltage dips up to 30% in the positive sequence voltage. The severity of the disturbances accentuates the nonlinear behavior of the load; the active and reactive power rapidly increase after fault clearing to levels even above the pre-disturbance value due to the re-acceleration of motors. The full recovery of the voltage is delayed due to the re-connection of tripped load. Moreover, it is shown that traditional load models do not accurately reflect the load behavior during these disturbances, for voltage dips around 12 % or larger due to the nonlinearities. An alternative load model, which represents the nonlinearities, has been tested. The superior behavior is demonstrated with the field measurements. Finally, some guidelines for industry to better account for the load in future stability studies have been included as a corollary of this thesis. (Less)

Microgrids of commercial buildings: strategies to manage mode transfer from grid connected to islanded mode

Abstract: Microgrid systems located within commercial premises are becoming increasingly popular and their dynamic behavior is still uncharted territory in modern power networks. Improved understanding in design and operation is required for the electricity utility and building services design sectors. This paper evaluates the design requirements for a commercial building microgrid system to facilitate seamless mode transition considering an actual commercial building microgrid system. A dynamic simulation model of the proposed microgrid system is established (utilizing DIgSILENT Power Factory) to aid the development of planning and operational philosophy for the practical system. An economic operational criterion is developed for the microgrid to incorporate selective mode transition in different time intervals and demand scenarios. In addition, a multi-droop control strategy has been developed to mitigate voltage and frequency variations during mode transition. Different system conditions considering variability in load and generation are analyzed to examine the responses of associated microgrid network parameters (i.e., voltage and frequency) with the proposed mode transition strategy during planned and unplanned islanding conditions. It has been demonstrated that despite having a rigorous mode transition strategy, control of certain loads such as direct online (DOL) and variable-speed-drive (VSD) driven motor loads is vital for ensuring seamless mode-transition, in particular for unplanned islanding conditions.

對稱座標法 = Symmetrical components

Abstract: Transformation matrices and the decoupling that occurs in balanced three-phase systems. Physical significance of zero sequence.

On Stability of Sustainable Power Systems: Network Fault Response of Transmission Systems with Very High Penetration of Distributed Generation

Abstract: Power systems are undergoing a historic structural and technological transformation. The increase of distributed generation (DG), recently mostly wind power park modules (WPPMs) and photovoltaic power park modules (PVPPMs), is already changing the way power systems are structured and operated. Distribution systems are turning from ‘passive’ into ‘active’ parts of the system (ADS). This structural and technological transformation influences the power system’s network fault response and stability. This thesis investigates the network fault response of transmission and distribution systems with very high penetration of distributed generation. The impact of DG on transient stability, large disturbance voltage stability, and frequency stability is analysed. The analysis is limited to symmetrical, three-phase network faults only but extended to adequately represent all voltage levels, including DG connected at low voltage (LV). Requirements for the response of DG to network faults are defined in grid connection requirement (GCR). The massive insertion of DG into distribution systems (DSs) leads to new challenges like the regular occurrence of reverse power flow (RPF) from the distribution to transmission level etc. Further, DG is more likely to be exposed to fault-induced delayed voltage recovery (FIDVR) events than transmission and sub-transmission connected generating facilities. The overall objective of this thesis is, therefore, to critically review the necessity and the specification of current and proposed grid connection requirements with regard to the network fault response of transmission systems with very high penetration of distributed generation and to propose changes to the specification where needed. The scientific contributions of this thesis are (1) a proposed comprehensive methodology of aggregation of DGs and dynamic equivalencing to derive highly accurate dynamic equivalent models of ADSs that considers the composition of ADS with DG classes in terms of their technology type and grid code performance (performance legacy); (2) a case study demonstrating that nowadays undervoltage protection schemes for small- and medium-scale DG connected in LV distribution networks may become a risk for power system stability; (3) identification of minimum requirements and improvement of existing grid connection requirements for the network fault response of DG to maintain power system stability.

Integrated modeling and control of a load-connected SOFC-GT autonomous power system

Abstract: In this paper we have developed low complexity, control-relevant models of all the components of the SOFC-GT hybrid system which is connected to a load through a bus bar. A control structure is designed by analyzing the complete system and simulation results are presented.