Christian Romeis

Real-Time Adaption of Dead Time for Single-Phase Autoreclosing

An approach for a real-time adaption of the dead time for autoreclosing on high-voltage lines was proposed. The objective was the optimal adaption of the dead time of a currently running single-phase autoreclosing cycle by calculating the secondary arc current based on measurements. The algorithms for optimization were performed for real-time applications. For this purpose, the fault location plus the load currents of the parallel nonfaulty conductors were measured. The measurement and the calculation of the dead time took place during the fault and the dead-time period. In this way, an optimized dead-time setting with regard to the actual fault location and loading conditions were achieved. This approach results in an online determination of the minimum necessary dead time of a successful reclosing under consideration of the fault location and prevailing loading conditions. Analytical calculations and numerical examples as well as high-voltage field tests verified the results.

Innovative strategies for protection security assessment

Protection security assessment of power grids is getting an important task in the course of a decarbonized power generation and a competitive energy business. The analysis of past blackouts studied by the North American Electric Reliability Council (NERC) shows that protection relays are involved in about 75% of all major disturbances. Protection security assessment can be investigated under two different aspects: on the one hand the behavior regarding selectivity, speed and sensitivity and on the other hand the behavior regarding the response on dynamic network phenomena as voltage stability and transient stability. This paper is focused on the protection response on dynamic network phenomena and presents innovative strategies for this investigation aspect. Applicable protection models and the simulation environment based on PSS®NETOMAC will be described. Case studies on real networks show the way of investigation and the achieved improvement of dynamic protection security based on the applied innovative strategies.

Dynamic protection security assessment, a technique for blackout prevention

In most cases of blackouts, protection systems play a significant role. The maloperation of one protection system can lead to a cascade-tripping of many other protection devices, and so a wide-area interruption of the power grid can affect millions of people. Therefore one important task of a transmission system operator (TSO) is to initiate remedial measures to prevent the grid from such interruptions. The TSO has different tools to operate a secure and reliable transmission grid, e.g. state estimator, dynamic security assessment (DSA) tools and so on. The dynamic protection security assessment (DPSA) is another tool to assess the situation in the grid depending on different contingencies. In addition to the transient network models, protection system models are also included for the simulation of stability problems. In contrast to DSA, DPSA assesses the time-dependent behavior of the protection systems and shows their influence on the grid security.

Anwendung der Dualen Planungsmethodik

Im Folgenden werden Vorgehensweise und Ergebnisse der Dualen Planungsmethodik am Beispiel der Netzanbindung von Windkraftanlagen (WKA) im Modelllandkreis, auch unter Einbeziehung vorhandener und zukunftiger Photovoltaikanlagen, dargelegt.

Veranlassung, Problemstellung und Notwendigkeit

Die Netze der elektrischen Energieversorgung erfahren einen stetig wachsenden Zubau an Regenerativen Energieeinspeiseanlagen (REA). Die Forschungs‐ und Entwicklungsanstrengungen auf dem Gebiet der REA und deren netzkompatible Anbindung haben seither ebenfalls zugenommen. Im Zuge dessen hat das Bayerische Staatsministerium fur Umwelt und Verbraucherschutz das Forschungsprojekt „Neue Methoden der elektrischen Netzplanung zur nachhaltigen Anbindung von Windkraftanlagen im Binnenland“ initiiert. Die Ergebnisse dieses Projektes bilden die Grundlagen des vorliegenden Buches. Die dabei erarbeiteten Methoden der Netzplanung wurden auf das Versorgungnetz eines bayerischen Landkreises angewandt, in dem ein massiver Zubau an REA in den letzten Jahren zu verzeichnen war. Die Nutzung der Windkraft und der Photovoltaik haben daran den grosten Anteil. Aus Grunden der Ubersichtlichkeit und Vertraulichkeit wurden alle Netzdaten und topographischen Gegebenheiten in eine aquivalente Modellregion uberfuhrt.

Influence of ion current on current transfomers of AC systems in near vicinity of DC systems

Equipping one support structure with DC and AC systems in parallel can increase the transport capacity of existing transmission corridors. The constant electric field of the DC system induces a constant DC component into the AC systems in near vicinity. All magnetic equipment including an iron core is in danger to saturate. Especially current transformers (CTs) for protection issues can be influenced in the way that the time-to-saturation is reduced. Therefore, maybe the protection relays are affected adversely and the selectivity is harmed. This paper evaluates this major impact by determining the transient dimensioning factor and the time-to-saturation in presence of a constant DC component in the fault current and remanent flux. The influence on the transient dimensioning factor is marginal and can be almost neglected. But for metering cores it could be a problem and should be tested because of the different excitation curves.

Calculation of the Electric Field below Hybrid Overhead Lines

The demand for increased transmission capacities in Germany will be covered in part by high voltage direct current (HVDC) lines. In order to reduce the need for new corridors, hybrid systems with AC and DC circuits together on the same tower are planned. Therefore, the characteristics of such arrangements need to be studied. In the first part of this paper, electromagnetic coupling mechanisms between overhead lines are summarized. Next, the method of image charges as a way to calculate the electric field around overhead lines is presented. The method is then used to analyze the electric field on ground level below three different hybrid line configurations. KeywordsHVDC; hybrid line; electric field I. HYBRID LINES FOR THE GERMAN “ENERGIEWENDE” The remaining nuclear power plants in Germany are scheduled to be shut down in a few years according to the agreements of the Energiewende. As they constitute the largest generating units in southern Germany today, a considerable amount of power plant capacity will have to be substituted. Furthermore, continuing expansion of volatile renewable power generation with large regional differences and often far from load centers will lead to increased load flows over large distances. In order to retain network stability and reliability in spite of these challenges, the transmission capacities of the electrical grid have to be increased. For the first time in Germany, four high voltage direct current (HVDC) lines are among the projected measures [1]. Finding new line corridors is often extremely difficult, therefore the possibility of operating hybrid AC/DC power lines on existing towers is currently investigated by transmission grid operators (TSOs). The TSOs Amprion and TransnetBW plan to put into operation a 340 km hybrid line from Osterath to Philippsburg in 2019, when the Philippsburg nuclear plant will be decommissioned [2]. While the mechanical design of hybrid overhead lines is not very different to that of conventional ones, the close proximity of AC and DC conductors causes electrical coupling effects that have to be studied precisely. Also, it must be ensured that electromagnetic fields around hybrid lines do not exceed the permissible values [3]. In the first part of this paper, coupling mechanisms and their consequences are discussed in a general way. Next, the method of image charges as an approach to calculate the electrostatic field around overhead lines is explained in detail. Finally, this method is used to calculate the electric field below hybrid overhead lines. Three different tower types and the influence of different placement of the DC poles are considered regarding the maximum field values on ground level. For one configuration, the instantaneous field distribution along an AC cycle is discussed. II. COUPLING MECHANISMS BETWEEN OVERHEAD LINE CIRCUITS In general, three different coupling mechanisms can be distinguished: inductive, (quasi-) ohmic and capacitive coupling. A. Inductive Coupling Magnetic fields resulting from time-dependent current flows in one conductor produce longitudinal voltages in adjacent conductors according to Maxwell’s laws. Thus, high transients in one system will induce large overvoltages in systems nearby. In closed circuits, these voltages will also cause currents. On a hybrid line, stationary load flow in the AC conductors results in an alternating current component in the DC system because the DC sources appear as a short circuit to alternating currents. In addition to an increased voltage drop across a DC reactor, these currents can be harmful especially to any equipment with iron cores. Line commutated HVDC converters will turn the induced fundamental frequency current mainly into a second harmonic and a DC component on the AC side. In the worst case, this offset can lead to core saturation and endanger a safe transformer operation [4]. B. Ohmic Coupling High electric field strength on the surface of conductors leads to ionization of surrounding air molecules, the so called corona discharge. While ionized field charges from AC conductors stay close to the wires, field charges originating from DC conductors can move over large distances and thus reach neighboring systems. AC conductors collect the free charges, resulting in a small DC current being injected. The problems from such undesired current components have been discussed in the previous chapter.