Olav Krause

Bottom-Up Self-Organization of Unpredictable Demand and Supply under Decentralized Power Management

In the DEZENT project we had established a distributed base model for negotiating electric power from widely distributed (renewable) power sources on multiple levels in succession. Negotiation strategies would be intelligently adjusted by the agents, through (distributed) reinforcement learning procedures. The distribution of the negotiated power quantities (under distributed control as well) occurs such that the grid stability is guaranteed, under 0.5 sec. The major objective in this paper was to deal, on the same level of granularity, with short-term power balance fluctuation, in terms of a peak demand and supply management exhibiting highly dynamic, self-organizing, autonomous yet coordinated algorithms under fine-grained distributed control. Our extensive experiments show very clearly that these short-term fluctuations could be leveled down by 70 - 75 %. In this way we have tackled, for the quickly increasing renewable power systems, a crucial problem of its stability, in a novel way that scales very easily due to the completely decentralized control.

An Inductively Active Filtering Method for Power-Quality Improvement of Distribution Networks With Nonlinear Loads

This paper proposes an inductively active filtering (IAF) method to comprehensively improve the power quality (PQ) for not only the distribution network (public grid) but also the power-supply system (nonlinear load) connected to the network. At first, a new main-circuit topology for implementing the IAF method is presented, which consists of an inductively filtered transformer and a fully tuned branch controlled by an inverter. Its operating principle and technical features are introduced by comparing with the traditional active filtering method. Based on this, the equivalent circuit is established and by means of mathematical modeling, the unique filtering mechanism of the IAF method is revealed in theory. Further, the control strategy of the FT branch and the impedance coordination for the inductively filtered transformer are designed based on theoretical analysis. A case study is investigated in detail to illustrate the operating characteristics of the IAF method. Both the theoretical and the case studies show that the IAF method can effectively prevent harmonic components from flowing into the primary (grid) winding of the transformer. Since the harmonic components are suppressed near the harmonic source, it is good for the power-supply system and especially good for the converter transformer. Besides, since the harmonic flow is limited to near the harmonic source, the PQ of the public network can be guaranteed completely.

Distributed Learning Strategies for Collaborative Agents in Adaptive Decentralized Power Systems

For regenerative electric power the traditional top- down and long-term power management is obsolete, due to the wide dispersion and high unpredictability of wind and solar based power facilities. In the R&D DEZENT1 project we developed a multi-level bottom- up solution where autonomous software agents negotiate available energy quantities and needs on behalf of consumers and producer groups. We operate within very short time intervals of assumedly constant demand and supply, in our case 0.5 sec (switching delay for a light bulb). We prove security against a relevant variety of malicious attacks. In this paper the main contribution is to make the negotiation strategies themselves adaptive across periods. We adapted a reinforcement Learning approach for defining and discussing learning strategies for collaborative autonomous agents that are clearly superior to previous (static) procedures. We report briefly on extensive comparative simulation.

Support vector machines for an efficient representation of voltage band constraints

Future Smart Grids emphasize the (at least) partial coordination of a large number of stochastic consumers and producers to balance consumption and generation of electrical energy. However, this coordinated and often time-synchronous activation and deactivation of demand and supply may result in the violation of the grids feasibility constraints, since it may increase statistical simultaneity above the level assumed in the initial design of such networks. The detection and avoidance of such operational infeasibilities is essential for secure grid operations. In order to handle the complexity of the optimization problem at hand, autonomous software agents will play a vital role in future power management systems. The systems complexity is reduced down to several less complex sub-problems, which may then be solved in parallel, based on locally available information. In this paper we present ongoing work on the development of autonomous grid management under stability constraints imposed by the power grid.

Stability enhancement in distribution systems with DFIG-based wind turbine by use of SFCL

A significant amount of renewable energy based power is being integrated at the distribution or sub-transmission level worldwide, known as Distributed Generation (DG). And the Doubly-Fed Induction Generator (DFIG) based wind turbine is one of the utmost important branches of DG development. However, the DFIGs suffer from high sensitivity from system disturbance that they cannot keep connection with utility under severe faults. This paper proposed to apply a superconducting fault current limiter (SFCL) in a distribution network with DFIG to limit the short-circuit currents so as to improve system stability. This presented study is aiming at providing deeper insight into the implication of the two main SFCL deployment strategies, DG-side and feeder deployment. The study is carried out using PSCAD/EMTDC with special focuses on stator currents, voltage sags, active and reactive power of the DFIG-based wind turbine effected by the short-circuit faults and the SFCL.

Establishing Large-Scale Renewable Reserve Capacity through Distributed Multi-Agent Support

A world-wide trend towards renewable and ecologically clean forms of energy has been steadily growing. Private investments are encouraged and heavily subsidized in most of the European countries, through tax deductions, and even more through a very favorable refund program for feeding electric power from renewable sources into the public network. Due to the limited predictability of the output of renewable power capacities it has long become the policy of grid operators and large power distributors to cover the differences between demand and supply with immense reserve and balancing power capacities based on fossil, and thus predictable, energy sources. With growing renewable power feed-in the demand for reserve and balancing power grows over-proportionally. In 2005 the European Union for the Coordination of Transmission of Electricity (UCTE) demanded to impose an obligation on grid operators to reduce integration costs for renewable energy capacities. This could obviously be possible once the renewable capacities sources could serve as reserve capacity. Since these are widely distributed and dispersed, their combined effect may well be used to guarantee a stable supply. The remaining problem behind is that the largely unpredictable character of wind and solar power supply is to be administered financially and in terms of timely transmission. We introduce a novel solution for the distributed negotiation process, which is compatible with electric distribution procedures. This is part of our DEZENT (decentralized management of electric power distribution) project.

A Power Factor-Oriented Railway Power Flow Controller for Power Quality Improvement in Electrical Railway Power System

Focusing on the freight-train dominant electrical railway power system (ERPS) mixed with ac-dc and ac-dc-ac locomotives (its power factorε[0.70,0.84]), this paper proposes a power factor-oriented railway power flow controller (RPFC) for the power quality improvement of ERPS. The comprehensive relationship of the primary power factor, converter capacity, and the two-phase load currents is built in this paper. Besides, as the main contribution of this paper, the optimal compensating strategy that suited the random fluctuated two-phase loads is analyzed and designed based on a real traction substation, for the purposes of satisfying the power quality standard, enhancing RPFCs control flexibility, and decreasing converters capacity. Finally, both the simulation and the experiment are used to validate the proposed conceive.

Influence of renewable energy based microgrid on low frequency oscillation of power systems

Increase proliferation of microgrid (MG) technologies with extended use of renewable energy sources (RES) can affect the system stability performance since dynamic characteristics of this kind of MG are different from conventional generation. Sharing certain portion of or entire generated power from synchronous generator with MG results in decrease of total system inertia. On the other hand, connecting RES based MG near a central load eventually enhance system performance due to improvement in reliability, reduction of power loss and congestion on transmission line. This paper investigates the impact of RES based MG on local and inter-area oscillatory modes of power systems. This study focuses on low oscillatory eigenvalues in the frequency range of 0.1-2 Hz. Eigenvalues analysis is performed to observe damping ratios and stability margins of the system due to MG integration. Furthermore, time domain simulation is then carried out to validate the result from eigenvalues analysis.

Dezentrales autonomes Energiemanagement

Zusammenfassung Bislang wird das Lastverschiebepotenzial an einzelnen Anschlüssen in den Verteilnetzen lediglich für einen Abgleich von Erzeugung und Verbrauch in Situationen besonders hohen oder besonders niedrigen Angebots erneuerbarer Energien in Betracht gezogen (z. B. Demand Side Management). Eine koordinierte Regelung über die Anschlüsse bezogener bzw. eingespeister Leistungen lässt sich aber auch für eine effizientere Auslastung bestehender Netze und zum Einhalten von Betriebsgrenzen heranziehen, da mit einer solchen Regelung direkt Einfluss auf Leitungsströme oder Spannungsniveaus ausgeübt werden kann. Abstract The potential of adjusting the demand of certain appliances with time-flexible duty cycles is currently used for load balancing purposes only (e. g. Demand Side Management). However, a coordinated operation scheme for consumers and producers of electrical energy may also be used for grid stabilization or in general for a more efficient utilization of existing distribution grids, directly influencing line currents and voltage profiles along the network.

The effect of PV on transformer ageing: University of Queensland's experience

Rooftop PV has an advantage that it can reduce the load supplied by nearby distribution transformers. This in theory should make the transformer run cooler, and therefore extend the remaining life of its paper insulation. However, the relative impact of harmonics on transformer life is sometimes neglected. In this article we present the findings of an analysis on the new rooftop PV installation at University of Queensland, in Brisbane. The load profile of three transformers was investigated, with IEC thermal models applied to estimate hot-spot temperature. The relative effect of PV on the weighted hot-spot temperature was investigated, and then a determination made on the likely loss of life. The harmonics injected by the PV was then explored.