The last full review of load models used for power system studies occurred in the 1980s. Since then, new types of loads have been introduced and system load mix has changed considerably. The examples of newly introduced loads include drive-controlled motors, low energy consumption light sources and other modern power electronic loads. Their numbers have been steadily increasing in recent years, a trend which is expected to escalate. Accordingly, the majority of load models used in traditional power system studies are becoming outdated, as they are unable to accurately represent power demand characteristics of existing and future loads. Therefore, in order to accurately predict both active and non-active power demand characteristics of aggregated modern power system loads in different load sectors (e.g. residential, commercial or industrial), existing load models should be updated and new models developed. This thesis aims to fill this gap by developing individual, generic and aggregated steady state models of the most common loads in use today, as well as of those expected to show significant growth in the future. The component-based approach is adopted for load modelling, where individual load models are obtained in detailed simulations of physical devices. Whenever possible, the developed individual load models are validated by measurements. These detailed individual load models are then simplified and expressed as equivalent circuit and analytical models, which allowed the establishment of generic load models that can be easily aggregated. It should be noted that since all non-active power characteristics are correctly represented, the developed aggregated load models allow for a full harmonic analysis, which is not the case with the standard steady state load models. Therefore, the proposed load models form an extensive library of comprehensive load models that are suitable for use in multiple areas of power system research. Based on the results of research related to typical domestic/residential sector load mix, the newly developed load models are aggregated and then applied to a typical UK/Scotland distribution network. Considerable differences are seen between network characteristics of newly proposed and previously developed models. The voltage distortion of a typical distribution system bus is investigated, and it is shown that distortion of the system voltage is likely to increase significantly in the future. The results of the presented research also suggest that neglecting the harmonic characteristics from the set of general load attributes may introduce errors in standard load flow studies.

When wind power and other intermittent renewable energy (IRE) sources begin to supply a significant part of the load, concerns are often raised about the inherent intermittency and unpredictability of these sources. In order to study the impact from higher IRE penetration levels on the power system, integration studies are regularly performed. The model package presented and evaluated in Papers I–IV provides a comprehensive methodology for simulating realistic time series of wind generation and forecasts for such studies. The most important conclusion from these papers is that models based on coarse meteorological datasets give very accurate results, especially in combination with statistical post-processing. Advantages with our approach include a physical coupling to the weather and wind farm characteristics, over 30 year long, 5-minute resolution time series, freely and globally available input data and computational times in the order of minutes. In this thesis, I make the argument that our approach is generally preferable to using purely statistical models or linear scaling of historical measurements. In the variability studies in Papers V–VII, several IRE sources were considered. An important conclusion is that these sources and the load have very different variability characteristics in different frequency bands. Depending on the magnitudes and correlations of these fluctuation, different time scales will become more or less challenging to balance. With a suitable mix of renewables, there will be little or no increase in the needs for balancing on the seasonal and diurnal timescales, even for a fully renewable Nordic power system. Fluctuations with periods between a few days and a few months are dominant for wind power and net load fluctuations of this type will increase strongly for high penetrations of IRE, no matter how the sources are combined. According to our studies, higher capacity factors, more offshore wind power and overproduction/curtailment would be beneficial for the power system.

This thesis describes the generic modeling of a Doubly-Fed Induction Generator (DFIG) based wind turbine. The model can also represent a wind plant with a group of similar wind turbines lumped together. The model is represented as a controlled current source which injects the current needed by the grid to supply the demanded real and reactive power. The DFIG theory is explained in detail as is the rationale for representing it by a regulated current source. The complete model is then developed in the time-domain and phasor domain by the interconnection of various sub-systems, the functions of which have been described in detail. The performance of the model is then tested for steady-state and dynamic operation. The model developed can be used for bulk power system studies and transient stability analysis of the transmission system. This thesis uses as its basis a report written for NREL [1]. / text

abstract: Constraint relaxation by definition means that certain security, operational, or financial constraints are allowed to be violated in the energy market model for a predetermined penalty price. System operators utilize this mechanism in an effort to impose a price-cap on shadow prices throughout the market. In addition, constraint relaxations can serve as corrective approximations that help in reducing the occurrence of infeasible or extreme solutions in the day-ahead markets. This work aims to capture the impact constraint relaxations have on system operational security. Moreover, this analysis also provides a better understanding of the correlation between DC market models and AC real-time systems and analyzes how relaxations in market models propagate to real-time systems. This information can be used not only to assess the criticality of constraint relaxations, but also as a basis for determining penalty prices more accurately. Constraint relaxations practice was replicated in this work using a test case and a real-life large-scale system, while capturing both energy market aspects and AC real-time system performance. System performance investigation included static and dynamic security analysis for base-case and post-contingency operating conditions. PJM peak hour loads were dynamically modeled in order to capture delayed voltage recovery and sustained depressed voltage profiles as a result of reactive power deficiency caused by constraint relaxations. Moreover, impacts of constraint relaxations on operational system security were investigated when risk based penalty prices are used. Transmission lines in the PJM system were categorized according to their risk index and each category was as-signed a different penalty price accordingly in order to avoid real-time overloads on high risk lines. This work also extends the investigation of constraint relaxations to post-contingency relaxations, where emergency limits are allowed to be relaxed in energy market models. Various scenarios were investigated to capture and compare between the impacts of base-case and post-contingency relaxations on real-time system performance, including the presence of both relaxations simultaneously. The effect of penalty prices on the number and magnitude of relaxations was investigated as well. / Dissertation/Thesis / Doctoral Dissertation Engineering 2016