Martin Wolter

Application of adaptive agents in decentralized energy management systems for the purpose of voltage stability in distribution grids

Nowadays, local utilizes face new challenges by operating their grids. This is mainly due to the increasing amount of distributed generation in the low and medium voltage level. If the injected power is not consummated locally but has to be transmitted over a long distance, voltages at busbars close to the source will rise which on the one hand is a potential threat to the equipment and on the other hand may lead to illegal system states and outages. Furthermore, measurement is sparsely spread in distribution grids. Thus, the grid state is not entirely observable and violations may not be detected. So local utilities trying to achieve a highly automated energy supply do not aim at a global governor but the grid has to be controlled locally using small adaptive systems which are able to make decisions autonomously. In this paper a new approach on the exploitation of reactive power reserves of inverter dominated distributed sources is introduced. Although X/R ratio differs from the transmission system reactive power offers a constricted but cost-effective possibility to control nodal voltages. So, first, a method to control single nodal voltages by using reactive power reserves of inverters is presented. After that, voltage stability is improved by intelligent agents which are able to work together.

Multi-agent based distributed power flow calculation

Against the background of the increasing amount of distributed generation and the intention of local utilities to offer system services at the distribution level, a high degree of automation gets more and more necessary.

Congestion management and determination of optimal grid expansion and retreat strategies using a modified boundary load flow method

The conventional boundary load flow method is an algorithm calculating interval bounds and distribution functions of nodal voltages or deduced values of electric power systems at which density functions of all influencing variables are known. In contrast, the modified boundary load flow was developed to estimate state space bounds of distribution grids which are not entirely observable. But the field of application is not limited to determining voltage bounds and equipment overstressing. Instead, the modified boundary load flow can be used to detect congestions and furthermore to uniquely determine the contribution of each load and generator which is highly relevant to transmission system operators. Due to the proposed method is able to trace power flows throughout the grid it additionally enables operators the find optimal grid expansion and retreat strategies for future load scenarios. In this paper the modified boundary load flow and its original intention is presented. After that the adoptions for the congestion detection and the determination of future grid expansion and retreat strategies are shown.

General estimation of the impact of additional DG sources on distribution grids

Nowadays, DG sources have a major effect on the operation of distribution grids. Due to in the future even more power will be fed in decentrally, it is imperative that small producers offer system services like reactive power provision and voltage control in the distribution layer. In this paper the impact of different types of DG sources on voltage bounds and grid losses are analyzed depending on the installed capacity and the location in the grid. It can be seen that DG sources at first positively effect grid losses but after exceeding a certain amount — which is almost reached nowadays — grid losses will virtually always rise. Generally, injected power leads to higher voltages especially in the low voltage layer. It is shown that the most limiting constraint of DG sources is not the transport capacity of the grid but the voltage tolerance band.

Grid reduction approach for state identification of distribution grids

Against the background of the increasing amount of DG sources and the intention of local utilities to offer system services like voltage control and demand management at the distribution level which is not entirely observed, knowledge of the grid state becomes more and more important. In this paper an entirely new approach on grid state identification is introduced. This new method uses already measured nodal and terminal values as input variables and extensively exploits typical properties of distribution grid topology. In doing so, it is able to more accurately estimate the grid state even without installation of new measurement facilities which is a further advantage. To estimate nodal voltages, the grid is reduced to the observed central busbars and a minimum of auxiliary nodes on remaining tie-lines. Thereby the equation system size is reduced as well and the resulting equation system gets solvable. After the reduced grid state is calculated a recursive algorithm is applied to back-reference to the original grid by re-adding a small number of nodes in each recursion step until the original grid is reconstructed again. The obtained results are more accurate than the ones obtained by previous methods. Furthermore, the grid reduction approach does not need a linearized model and thus approximation errors are independent of the deviation from an arbitrary chosen operation point.

About the impact of burdening and relieving partial power flows caused by loop flows in interconnected networks on ITC amount

Expenses caused by energy trade have to be covered by their originators. 3rd party TSOs are entitled for compensation payment if both source and sink of each regarded power flow is situated outside the borders of its member state. Furthermore, power flows contributing to reduce tie-line utilization should be considered to lessen compensation payment. In the past, focus was on transit flows, starting and ending in different countries and using the grid of a third one. However, Loop Flows evoke utilization of 3rd party grids as well and thus have to be considered in fair Inter-TSO-compensation (ITC) schemes. In this paper a model from Wolter and Hühnerbein, which is based on the law of superposition and is by now the only model to identify partial power flows solely based on physical principles, is used to figure out the Loop Flow amount in interconnected networks. It is clearly shown that Loop Flows have a significant effect on tie-line utilization and therefore cannot be neglected in the calculation of compensation payment in the future.

Identification of cross-border power flows in integrated networks based on the principle of superposition

Expenses caused by energy trade have to be covered by their originators. 3rd party TSOs are entitled for compensation payment if both source and sink of each regarded power flow is situated outside the borders of its member state. Furthermore, power flows contributing to reduce tie-line utilization should be considered to lessen compensation payment. Current methods do not match regulatory requirements due to their lack of technical soundness. In this paper a method entirely based on physical rules is developed. It is based on a complex-valued power flow and a subsequent superposition of nodal currents. Thus, partial power flows are physically correctly determined. Furthermore, a unique identification of transits, loop flows, imports, exports and internal flows and an allocation to their originators are possible. A comparison with the overall flows classifies all partial flows as burdens or reliefs to either increase or decrease the compensation payment amount.

Agentenbasiertes Energiemanagement zur Spannungshaltung in Verteilnetzen

Zusammenfassung Heutzutage gehen Verteilnetzbetreiber neue Wege bei der Führung ihrer Netze. Hauptsächlich ist dies der steigenden Anzahl von Kleinerzeugern in ihren Netzen geschuldet. Darüber hinaus ist die Messtopologie unterbesetzt, so dass der Netzzustand nicht vollständig erfasst werden kann und unzulässige Systemzustände unentdeckt bleiben. Netzbetreiber, die intelligente Energiemanagementsysteme einsetzen, können sowohl einen globalen Optimierer als auch dezentrale Strukturen verwenden, um ihre Netzführung zu automatisieren. Letzterer Ansatz ist im Sinne des angestrebten plug´n´play-Verteilnetzes die bessere Variante. Abstract Nowadays, local utilities face new challenges by operating their grids. This is mainly due to the increasing amount of distributed generation. Furthermore, measurement is sparsely spread in distribution grids. Thus, the grid state is not entirely observable and violations may not be detected. So, local utilities trying to achieve a highly automated energy supply do not aim at a global governor but the grid has to be controlled locally using small adaptive systems due to missing infrastructure. The final aim is a plug´n´play distribution system.

Agent based power system management — Concept of congestion management

Congestion management is one of the technical challenges in the power system. Privatization and market liberalization lead to stressed operating conditions in existing and future power systems. Additionally, the increasing complexity of the power system results in an increased need for information on the actual grid condition and intelligent control methods. Unfortunately, measurement data from each node is not in the distribution level. Introduction of decentralized control methods allows overcoming the limitations associated with centralized control, like failure of main controller. In this paper, a concept of congestion management using a multi-agent system is presented. In this approach, control processes are realized by active power adjustments based on Power Transfer Distribution Factor methodology. Depending on localization of congested line, appropriate agent initiates the whole redispatch procedure.

Technical integration of virtual power plants into German system operation

This paper critically evaluates the operational perspective of Virtual Power Plants (VPP) in Germany by analyzing key factors to replace conventional power plants in the future power system. Therefore, its necessity for the secure operation as well as the technical and economic benefits for the German power system are pointed out. The single sections describe in detail how the requirements on technical functions and standardized communication can be reconciled with the increasing challenges on volatile generation. Furthermore, a new sensitivity calculation concept based on accounting areas and under consideration of VPP are presented. Based on that, different operation concepts are assessed with respect to their mathematical algorithms and their practical feasibility under consideration of given circumstances and future developments.