Jens C. Boemer

Coordinated Active Power-Dependent Voltage Regulation in Distribution Grids With PV Systems

High penetrations of photovoltaic (PV) systems in distribution grids have brought about new challenges such as reverse power flow and voltage rise. One of the proposed remedies for voltage rise is reactive power contribution by PV systems. Recent German Grid Codes (GGC) introduce an active power dependent (APD) standard characteristic curve, Q(P), for inverter-coupled distributed generators. This study utilizes the voltage sensitivity matrix and quasi-static analysis in order to locally and systematically develop a coordinated Q(P) characteristic for each PV system along a feeder. The main aim of this paper is to evaluate the technical performance of different aspects of proposed Q(P) characteristics. In fact, the proposed method is a systematic approach to set parameters in the GGC Q(P) characteristic. In the proposed APD method the reactive power is determined based on the local feed-in active power of each PV system. However, the local voltage is also indirectly taken into account. Therefore, this method regulates the voltage in order to keep it under the upper steady-state voltage limit. Moreover, several variants of the proposed method are considered and implemented in a simple grid and a complex utility grid. The results demonstrate the voltage-regulation advantages of the proposed method in contrast to the GGC standard characteristic.

Dynamic models for transient stability analysis of transmission and distribution systems with distributed generation: An overview

Distributed Generation is increasing in nowadays power systems. Small scale systems such as photovoltaic, biomass or small cogeneration plants are connected to the distribution level, while large wind farms will be connected to the transmission level. Both trends lead to a replacement of large synchronous generators as the dominating generation technology. Up to now, transient stability of transmission systems has been analysed to a satisfactory degree of accuracy with a simplified representation of the distribution systems. In future, distributed generation will more and more influence the behaviour of the system. Stiff, inverter-based local generation technologies may improve the system stability; however, increasing electrical distances between large synchronous generators in operation will impede the system stability. These (and other) diverging effects have to be studied in detail. This overview paper summarises the latest findings and reveals future research questions. It is concluded that the accuracy and validity of the currently applied dynamic models for transient stability analysis of power systems with high penetration of DG should be further investigated.

Fault ride-through requirements for onshore wind power plants in Europe: The needs of the power system

Wind power plants show different behavior than conventional (synchronous) generators. As the traditional power systems mainly consisted of centralized generation by synchronous machines feeding passive loads, it was well-understood how the system reacted in normal operation as well as during disturbances. As wind power plants are foreseen to increase in size and the amount of installed wind power will grow, the relative contribution of equipment not exhibiting this common behavior increases. At the same time power electronics offer opportunities for additional features to stabilize the power system. Transmission system operators impose requirements on the (dynamic) capabilities of connected new generation resources (including wind power plants) which are specified in grid codes. In this paper, the importance of such requirements is explained by looking at the needs of the power system and by showing simulation results for a test network. The paper facilitates a detailed understanding of the underlying phenomena related to grid code requirements with a focus on low-voltage ride-through and voltage support by reactive current boosting.

Compliance with technical codes turns into precondition for support and system services bonus for wind power plants in Germany

When amending the Renewable Energy Sources Act (EEG) in June 2008 the German government acknowledged the importance of enhanced technical requirements. From now on, for wind power plants, full grid code-compliance will become a necessary precondition for privileged network access, receipt of feed-in tariff, and extra payments (system services bonus). Thus, the amended EEG ensures further development of renewable energy sources in line with targets without compromising security of supply. The technical basis for the technical requirements in the EEG has been determined by the recently revised technical code for dispersed generators in German distribution networks (Medium Voltage Directive 2008) and the Transmission Code 2007 respectively. However, the latter needed careful review and some clarifying specifications were proposed. The new technical requirements and their implications for renewable energy sources generators, with special regard to wind power plants, are presented in this paper.

Contribution of negative-sequence controlled distributed generation to power system stability under unbalanced faults: A discussion paper

The transformation of the power system in terms of efficiency and sustainability will further lead to increasing converter-coupled generation and demand. This changes the systems characteristics and influences its stability. Despite these fundamental changes, the secure operation of the power system must be maintained at all times. DG and any converter-coupled generation of relevant size are requested to stay connected to keep power equilibrium and to support the voltage during faults. The balanced fast voltage control currently applied for all types of faults could be improved by an unbalanced injection of short-circuit current among the three phases depending on the fault. A technical solution is an active control of the negative-sequence during unbalanced faults. However, introducing a general requirement, e.g. in grid codes, for this new feature must be well justified. This paper is intended to start a structured discussion of unbalanced fast voltage control by converter-coupled generation for unbalanced faults from a system perspective.

Small disturbance angle stability indication in the electrical networks with variable speed wind turbines

The fast growing application of sustainable energy sources imposes major structural changes on the current electric power systems. One of these structural changes is to make use of large variable speed wind turbines within the conventional electrical power networks. The installation of these wind turbines has, indeed, indispensable impacts on the dynamic behavior of the existing electric power systems. Thus, it is important to gain a rather generalized overview on how these wind turbines, which mostly use Doubly Fed Induction Generators (DFIGs), affect the system stability. This paper performs an analytical analysis for the indication of small disturbance rotor angle stability in the power systems equipped with variable speed wind turbines using DFIGs. Also, in order to consider the stochastic characteristic of the sustainable energy sources, the paper applies an iterative-stochastic method to analyze the small disturbance angle stability. The suggested iterative-stochastic methodology is numerically verified, within this paper, by getting applied to an electric power test system.