Olav Bjarte Fosso

Short-term hydro scheduling in a liberalized power system

The paper addresses the challenges of short-term hydropower scheduling in a liberalized power system. Hydropower systems may have quite complex topologies with many cascaded reservoirs/power plants in the same river system. The reservoirs may have very different storage capacity with significant water travel time that makes the decisions coupled between several time steps. In other words, the decisions in one time interval have strong impact on whats possible to do in later time steps. In systems where some of the reservoirs have significant storage capacity, the use of the hydropower resources short-term will be coupled with the long-term strategic decisions. It is not feasible to have the short-term decisions within the same model as the long-term decisions because modeling of uncertainty is important. The problem is decomposed into a planning hierarchy where the long/mid-term models provide boundary conditions for the short-term models. The major items of this paper are: challenges and possible solutions in hydro system modeling, a flexible coupling principle between short-term and mid-term scheduling, strategies for bidding in a spot market.

Control system tuning and stability analysis of Virtual Synchronous Machines

Virtual Synchronous Machines (VSMs) have been introduced as a control concept for emulating the behavior of traditional synchronous machines with power electronic converters. This paper analyses a VSM control scheme where an outer loop for inertia emulation provides references for two cascaded voltage and current controllers. This scheme offers several advantages compared to alternative VSM implementations. However, the interactions between the cascaded control loops and the complex functional dependency of the system dynamics with respect to the controller parameters prevents the effective application of classical tuning methods to this scheme at low switching frequencies. Therefore, a tuning approach for the VSM guided by the eigenvalue sensitivity matrix of a linearized system model is proposed in the paper. The method is implemented in the form of an iterative optimization procedure enforcing 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 dynamic performance of the system when tuned with the presented algorithm compared to the results achieved by a more conventional tuning method.

Automatic Tuning of Cascaded Controllers for Power Converters Using Eigenvalue Parametric Sensitivities

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.

Small-signal stability study of the Cigré DC grid test system with analysis of participation factors and parameter sensitivity of oscillatory modes

This paper presents a detailed case study of small-signal stability analysis for the Cigré DC-grid test system. The presented investigations are intended for identifying critical modes of interaction between different parts of the electrical system and the controllers in multi-terminal HVDC (MTDC) schemes. Oscillatory and critical modes of the system are investigated by modal analysis, including participation factor analysis and studies of the parameters sensitivity. The potentially unstable modes are investigated by calculating the participation factors of the system, which identify the influencing components. The sensitivity of the eigenvalues to the control parameters is also presented in order to reach an improved control design for MTDC with sufficient damping and enhanced dynamic response. Time-domain simulation results are further presented in order to verify the transient performance of the control system.

Flow based activation of reserves in the Nordic power system

In the Nordic market, manually activated tertiary control based on bids for upward and downward regulation is used for system balancing. Although a system wide merit order list is used, the resulting regulation is suboptimal because of the congestion and the effect of losses, which are not taken into account. This paper proposes an algorithm for the dispatch of regulation resources based on an incremental DC optimal power flow formulation. The results of this model are compared with todays practice for some cases of up- and downward regulation, and a potential for cost reduction is observed. However, the method requires Automatic Generation Control that is not in use in the system, although it is presently evaluated. Also pricing of regulation is an issue, because Location Marginal Prices probably are unacceptable to market participants.

Small-signal modelling and parametric sensitivity of a Virtual Synchronous Machine

The Virtual Synchronous Machine (VSM) concept is emerging as a flexible approach for controlling power electronic converters in grid-connected as well as 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 reference for cascaded voltage and current control in a synchronous reference frame. The control system includes also a virtual impedance and an outer loop frequency droop controller which is functionally equivalent to the governor of a traditional SM. A linearized small-signal model of the VSM including the converter and its grid side filter, the control system and a simple grid equivalent, is developed and verified by time-domain simulations of a nonlinear system model. The small-signal model is used to identify the critical modes of the system and to investigate their parametric sensitivity.

Optimal bidding in sequential physical markets — A literature review and framework discussion

There are expectations of increased trading of electricity in markets with shorter time horizons than the day-ahead market. As the importance of these markets grows, there is a question of whether the potential for profit in subsequent markets such as the intraday market or balancing market should be taken into account already when preparing the bid for the day-ahead market. This paper reviews studies of optimal bidding where subsequent markets are modeled and summarizes if, and under which conditions, the inclusion of shorter-term markets makes a difference. Finally, the assumptions of the modelers are systemized and discussed in order to prepare a framework for future studies.

Electric vehicles charging in a smart microgrid supplied with wind energy

This paper compares the performance of smart charging and dumb charging applied to Electric Vehicles (EVs) by implementing a simulation model. The model was developed based on the trend of current electric vehicle adoption in Norway. The electrical network system considered includes wind turbine to supply electric power to a group of 38 households. The assumed scenario is that the households own 31 EVs, which is equivalent to 60% of current EV adoption in Norway. The network is an almost stand-alone network as it mainly relies on wind power and if needed, power is purchased from the Nordic spot market. The obtained results indicate that the smart charging has several advantages if it is used with the support of energy storage: it prevents the event of peaks in electricity demand, it increases the use of wind power and it reduces the dependency from the spot market.

Implication of Grid Code Requirements on Reactive Power Contribution and Voltage Control Strategies for Wind Power Integration

The exploitation of wind power is often limited due to the weak interconnection to the main grid. Little consideration given to the different boundary conditions, can either limit the exploitation of potential sites, or increase considerably the costs of integration. This paper explains the relations between grid strength and requirements for wind power generators on reactive power contribution and voltage control. The reactive power needed to support the voltage is mapped, in a generic grid equivalent, by varying the short circuit power and the RX ratio at the point of common coupling (PCC) of the wind-farm. The methods presented are relevant for any project involving the exploitation of wind energy in proximity to weak electric grids.

Moving towards the Smart Grid: The Norwegian case

Encompassing the global developments towards more sustainable and environment-friendly energy solutions for the future, Norway has been developing its own Smart Grid strategy. This strategy follows a path defined by the specific characteristics of the Norwegian energy system and the societal context. This article presents the Norwegian Smart Grid case by collecting the experiences and actions taken by industry, academic and research sectors. The role of power electronics technologies in smart grid research and in industry-driven innovation is also addressed in the paper.