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Abstract: Use of power electronic equipment has increased and introduced new dynamical phenomena in power systems. For example, new electric rail vehicles (locomotives) equipped with modern power electronic traction chains have caused situations of low-frequency power oscillations and instability in single-phase railway power supply systems. This paper presents the development and implementation of an instantaneous value model and simplified fundamental frequency (RMS) models of such an advanced electric rail vehicle in order to investigate their representation of low-frequency dynamics. The dynamical behaviour is studied by use of both time-domain simulations and linear analysis (eigenvalues) and the degree of simplifications regarding controller dynamics and power system dynamics are presented and discussed. An enhanced RMS model is tested in order to account for the impact of fast current dynamics on the low-frequency behaviour. The results show that this enhanced model is corresponding more closely to the instantaneous value model than what can be obtained by the traditional RMS simplifications and indicate that current dynamics should be included in stability studies involving power electronic inverters. [Copyright &y& Elsevier]

Presents information on a study examined an overview of the deregulated electricity supply system in Norway with emphasis on the generation scheduling approach. Description of the system; Alternative algorithm for medium term scheduling.

The concept of Virtual Synchronous Machines (VSMs) is emerging as a flexible approach for controlling power electronic converters in grid-connected as well as in stand-alone or microgrid applications. Several VSM implementations have been proposed, with the emulation of inertia and damping of a traditional Synchronous Machine (SM) as their common feature. This paper investigates a VSM implementation based on a Voltage Source Converter (VSC), where a virtual swing equation provides the phase orientation of cascaded voltage and current controllers in a synchronous reference frame. The control system also includes a virtual impedance and an outer loop frequency droop controller which is functionally equivalent to the governor of a traditional SM. The inherent capability of the investigated VSM implementation to operate in both grid-connected and islanded mode is demonstrated by numerical simulations. Then, a linearized small-signal model of the VSM operated in islanded mode while feeding a local load is developed and verified by comparing its dynamic response to the time-domain simulation of a nonlinear system model. Finally, this small-signal model is applied to identify the dominant modes of the system and to investigate their parametric sensitivity. [ABSTRACT FROM AUTHOR]

The ongoing evolution of the power system towards a “SmartGrid” implies a dominant role of power electronic converters, but poses strict requirements on their control strategies to preserve stability and controllability. In this perspective, the definition of decentralized control schemes for power converters that can provide grid support and allow for seamless transition between grid-connected or islanded operation is critical. Since these features can already be provided by synchronous generators, the concept of Virtual Synchronous Machines (VSMs) can be a suitable approach for controlling power electronics converters. This paper starts with a discussion of the general features offered by the VSM concept in the context of SmartGrids. A specific VSM implementation is then presented in detail together with its mathematical model. The intended emulation of the synchronous machine characteristics is illustrated by numerical simulations. Finally, stability is assessed by analysing the eigenvalues of a small-signal model and their parametric sensitivities. [ABSTRACT FROM AUTHOR]

Control structures containing cascaded loops are used in several applications for the stand-alone and parallel operation of three-phase power electronic converters. Potential interactions between these cascaded loops and the complex functional dependence between the controller parameters and the system dynamics prevent the effective application of classical tuning methods in the case of converters operating with a low switching frequency. A tuning approach guided by the eigenvalue parametric sensitivities calculated from a linearized model of the converter and its control system is proposed in this paper. The method is implemented in the form of an iterative procedure enforcing the stability of the system and ensuring that the system eigenvalues are moved away from critical locations. Numerical simulations in the time domain are presented to verify the improvement in the dynamic performance of the system when tuned with the presented algorithm compared with a conventional rule-based tuning method. [ABSTRACT FROM PUBLISHER]

Power systems with high penetration of variable renewable generation are vulnerable to periods with low generation. An alternative to retain high dispatchable generation capacity is electric energy storage that enables utilization of surplus power, where the electric energy storage contributes to the security of supply. Such systems can be considered as energy-constrained, and the operation of the electric energy storage must balance the minimization of the current operating costs against the risk of not being able to meet the future demand. Safe and efficient operation requires stochastic methods with sufficient foresight. Operation dependent storage degradation is a complicating factor. This paper proposes a linear approximation of battery state-of-charge degradation and implements it in a stochastic dual dynamic programming based energy-management model in combination with cycling degradation. The long-term implications of degradation modeling in the daily operation are studied for a small Norwegian microgrid with variable renewable power generation and limited dispatchable generation capacity as well as battery and hydrogen storage to balance supply and demand. Our results show that the proposed strategy can prolong the expected battery lifetime by more than four years compared to the naive stochastic strategy but may cause increased degradation for other system resources. • Multi-stage stochastic energy management model including battery degradation. • Linear battery state-of-charge degradation formulation for optimization. • Analysis of microgrid operation over a full year to valuate degradation modeling. [ABSTRACT FROM AUTHOR]

• Stochastic dynamic programming based multi-period optimal power flow model for power system analysis under uncertain power generation. • Electricity price formation in networks dominated by variable renewable generation where energy storage systems secure the supply. • Stage-wise storage end value description with cubic splines. • Iterative solution method yielding infinite horizon optimal strategy. This paper studies the electricity price formation in a competitive market when introducing generation from variable renewable energy technologies with zero marginal cost and electric energy storage systems. A power system is analyzed with a stochastic optimization model combining multi-period optimal power flow with stochastic dynamic programming. The results illustrate how variable renewable energy, in this case solar photo-voltaic generation, displaces some of the expensive thermal generation and reduces the price. Electric energy storage will reduce the price variations caused by the variable renewable generation and demand as the time with price cap and zero price is reduced. In systems with only variable renewable generation and energy storage, the price will be set by the probability of scarcity similar to the price formation in hydro power dominated systems. The price will indicated the future cost of scarcity as a stochastic expectation value. This paper assumes that the demand is inflexible. However, the resulting electricity prices will remunerate provision of flexibility, which in turn will contribute to securing the supply and reducing the price volatility. [ABSTRACT FROM AUTHOR]