Bernd M. Buchholz

Dynamic simulation of renewable energy sources and requirements on fault ride through behavior

Currently, power generation from renewable energy sources is of global significance and will continue to grow during the coming years. The grid integration of intermitting renewable energy sources is not only an issue of distribution networks, but effects the transmission grids as well. The largest amount of new installations are connected to the transmission grid, especially into 110-kV. Further, large wind farms with some 100 MW have already been connected to the 400-kV-system. The renewable energy sources are connected to the power network via power electronic converters, and this type of connection schema can change the short circuit conditions in the network. Today, usually a tripping command is generated within the first period, but in the future contributions to the short circuit current by renewable sources are necessary to fulfil the selective clearing of faults. To investigate the new requirements some novel simulation models are necessary, and described in the paper, for: converter connected DC sources (fuel cell, PV plants), doubly fed induction generator, DFIG, converter connected synchronous generator. Using these models the simulation allows for investigating the behavior for various control design. Requested is the delivery of rated current up to voltage dips of 100% for the whole duration of the fault. The results of some simulations and discussion of them have led to new rules for grid access. These rules are implemented into the grid code

Vision 2020 — Security of the network operation today and in the future. German experiences

Nowadays the supply of electrical energy from dispersed generation units has an increasing significance. Hence, security related power system operation strategies have to be adapted to face the new situation. In this paper the security of power system operation from the industry and research point of view will be discussed. After presenting an overview about the current situation, the problems will be pointed out. The promising ongoing research topics which have the ability to help the network operators to manage the turn towards a sustainable power system that will allow for exploiting as much regenerative energy as possible by reaching the high standards of security will be described.

Stability analysis for large power system interconnections in Europe

A worldwide trend in the development of power systems is to build interconnections with the goal to achieve economical benefits. The interconnections are mostly realized by synchronous links where such solutions are technically feasible and economically justified (AC solution). In cases where the synchronous interconnection is technically not feasible, an HVDC coupling station or HVDC log distance transmission (DC solution) can be used. Finally, the DC solution can also be applied in combination with a synchronous interconnection in order to support the operation of the interconnected systems and thus makes the synchronous AC link more reliable (hybrid solution). The evaluation of the best solution for large system interconnections requires detailed system models for stability analysis. In the paper, examples of very large power system simulations are shown. Simulation results are given for DC and AC interconnections. There are practically no limits for the size of the systems, which can be handled by modern computer simulation tools, e.g. the described advanced simulation program. Finally, the challenges of further system interconnections in Europe are considered on the base of the existing experience in stability analysis.

Integration of renewable and dispersed resources: lessons learnt from German projects

In particular, the installation of renewable decentralized energy sources in the distribution system will increase the importance of this subsystem in future power system operation. The secure operation of a power system depends on the provision of system services such as frequency stability, power balance, voltage stability or system management. The current responsibilities for these system services will be discussed in this paper taking into account German experiences using decentralized energy sources. Furthermore, some guide lines on planning and operation, which take into account the impact of this new situation as well as recent experiences in Germany, was discussed. The following topics were elaborated and described: decentralized energy management, reserve power provision, coordination with a transmission system operator (TSO); voltage stability; fault ride through and supply restoration; power balance in island operation. The description takes into account the modeling issues, the operation of the wind parks, the planning and operation of the dispersed generation and will lead to a proposal of a benchmark for the medium voltage network. Finally, a general conclusion and recommendations for future development were given

Communication Requirements and Solutions for Secure Power System Operation

The further increase of the contribution of D&RES in the peak power balance up to 60 % in accordance with the goals of the European Communities for the year 2010 requires innovative approaches to keep the security of the power supply on the actual high level. The communication will be the key for the further suitable operation of the power system. New communication facilities will be necessary to provide the reliable data for e.g. decentralized energy management, integrated planning tasks and to ensure the provision of system services by D&RES. The communication network with different physical communication channels has been investigated in scope of some pilot projects. Further more, the application of communication standards is investigated and in the result the use of the data models and the services of the communication standard IEC 61850 (for substation communication) is recommended. Also the standard IEC 61968 has been discussed. Finally, it becomes clear that the implementation of more communication in the distribution level is necessary for the secure power system planning and operation.

Coordinated operation of HVDC and FACTS

Over the course of three decades of commercial applications, the HVDC technique has been established as a conventional technology in the area of back-to-back and two-terminal long-distance and submarine cable systems up to 2000 MW and higher. As thyristor-controlled shunt compensation schemes have been introduced into high-voltage transmission for more than two decades, FACTS (flexible AC transmission systems) devices are established as technical and economic means for improving the overall system performance of AC systems. Today, only limited HVDC and FACTS systems are operated in a coordinated manner. However, with increasing complexity of system conditions, e.g. in very weak power systems with enhanced stability requirements, parallel operation of HVDC and FACTS devices is gaining impetus. Innovative technologies can further optimize the HVDC performance under weak and very weak system conditions.

A comprehensive simulation program for subsynchronous resonance analysis

Subsynchronous resonance (SSR) is a condition that can exist in a power system, especially for long distance transmission systems with series compensated line. It can cause shaft fatigue and possible damage or failure of the generator involved. The analysis of the SSR phenomena can be performed usually by using three individual analytical methods-frequency scanning, eigenvalue analysis in the frequency domain and transient torque analysis in the time domain. In this paper the comprehensive simulation program NETOMAC with a complete solution for analyzing SSR is presented. Based on a common data set of the system concerned, both frequency and transient torque analysis can be performed, taking interactions among the spring-mass system of the turbine-generator units and the electrical network into consideration. Two case studies are demonstrated in the paper. The results achieved validate the functionality of the program for SSR analysis.

Network security management tool for distribution systems

In this paper a network security management tool, which is integrated in a power system simulator, has been introduced. The network security management (NSM) is used by network operators to avoid insecure network states due to high penetration of DGs. Especially in situations of low load and strong wind the operation parameters e.g. current or voltage can exceed the allowable values. The NSM tool introduced in this paper is performing new NSM algorithms within the power flow calculations. On the one hand, it can be used as an online application to support the network operator in his decisions using measurements and short time forecasts. On the other hand, by using the tool for long time simulations it can be used to optimize the NSM algorithms, to optimally plan network reinforcement and to predict the intensity of the NSM system on the profitability of the DG operators.

Prevention of large grid disturbances through advanced power automation technology

The analysis of the large power outages in 2003 indicate that innovate protection- and transmission functions are well suitable to significantly avoid or reduce such severe system disturbances. To do so, the aspects (i) safe transmission of information, (ii) decision support for network control, (iii) protection concepts, (iv) prevention of voltage collapse, (v) controlled homogeneous net load play an important role. The technical solutions to realize these standards are explained.

Vision and Strategy for the Electricity Networks of the Future

The vision for the electric power system of the future was developed by a European group of experts in the framework of the Technology Platform “Smart Grids”. The driving forces, the definitions and the core elements of the Smart Grid concept are presented separately for both the transmission and distribution systems. The growing importance of renewable energy sources for electricity generation is considered and the related challenges are specified. The transformation of todays’ electricity networks into Smart Grids is the best way to meet these challenges.