Use of modular, advanced current flow controllers to increase and flexibilize the performance of high-voltage grids – technology, grid design, operation, economics and processes

Abstract:
The green energy transition in Germany and Europe requires a paradigm shift for the electrical system. Not only will the total energy to be generated, transmitted and distributed increase by a factor of about 2, but the power to be controlled will increase disproportionately. The future central energy sources, solar and wind will have a significantly shorter annual service times than classic thermal power plants. Furthermore, power production is no longer local, but increasingly remote to users. The electricity application side is also altering significantly. The electrification of processes, which have so far mainly been operated with chemical energy sources, is creating new extraction focal points. Finally, the effect of the new phenomenon of the million-fold ‘prosumers’ must be considered. As a result, electrical grids become energy hubs that have to cope with significantly increased power with short-term changes in the direction of power flow. All of this requires not only an expansion of the existing network infrastructure, but also the use of flexibility options. The static design of the network for the worst conceivable case with full (n-1) redundancy is neither economically feasible nor can it be financed. Therefore, power electronics has become an essential key component in expanding the capabilities of electrical energy systems. This paper focuses on high voltage distribution grids. For these, there are three major chal-lenges in which FACTS (Flexible AC Transmission Systems) – better called here FACDS (Flexible AC Distribution Systems) – in the form of modular static synchronous series compensators (MSSSCs) can make an important contribution to load flow control and performance increase in meshed grids: 1. the temporary provision of additional line capacities for new, high-performance, but also intermittent and volatile renewable power sources. (The term ‘temporary’ refers to both the hourly range and a multi-year period. Load flows change not only over the course of the day or week, but also when new large renewable generation plants are commissioned) 2. the grid connection of new large electricity customers with strongly fluctuating withdrawal capacity and load ramp-up that is difficult to predict 3. enabling grid shutdowns on a sufficient scale to enable the efficient construction and connection of new lines The aforementioned development options should not be considered in isolation. Interactions are possible, e.g., with large electricity storage systems or current-regulated voltage controllers of EHV/HH transformers. A purely static grid expansion not only leads to an enormously high investment requirement, which cannot be financed by grid operators from the cash flow, it is also time-consuming to implement and determines grid structures in the long term. Thus, it does not represent a satisfactory answer to the uncertainties regarding the development of line requirements. Last but not least, feed-in or withdrawal options that are not available in the medium term have a negative impact on Germany as a business location and thus indirectly also on the grid operator. Since the situation in the grid not only changes during the course of the day and seasonally, but also changing requirements occur in the course of the transformation process, the relocatability of MSSSCs is also an important feature. This provides a response to delays or cancellations of large-scale projects on the generation and demand side. The term ‘dynamic oper-ation’ must therefore be understood comprehensively. It is obvious that projects that can only be implemented in the medium to long term and can hardly be changed must have the character of ‘no regret’ measures. A portfolio of technical solutions that can be used locally at short notice and also for different applications is a great strategic advantage. This means that decisions with medium and long-term effects can also be corrected at least partially, if necessary. The paper describes the technical structure and functionality of a new type of MSSSC called SmartValve. It is discussed where this technology can support and complement conventional, statically oriented grid development planning. From this, key points for the network planner's decision-making process can be derived. Furthermore, business-related issues are dealt with. These include interactions with protective devices and questions about possible disruptions of the MSSSC. These considerations are rounded off by already gained operational and operational experience. All of this is condensed into a complete process flow diagram with decision points. Technically, the process begins with a load flow screening and an analysis of the dimensioning of the MSSSCs to be used. Subsequently, the network area under consid-eration is examined over the course of the year (8,760 h) and the effect of the MSSSCs is confirmed. The modelling of an MSSSC is discussed and the analyses carried out are presented. The results are evaluated economically with cost-benefit analyses, especially to compare the use of MSSSC with conven-tional static grid expansion strategies. This includes the reduction of bottlenecks, shutdowns, grid losses and stability management. Furthermore, added value resulting from additional functions and options is evaluated. These include the significantly higher flexibility of an MSSSC solution during the implementation process, the change in the location de-pending on the evolution of network requirements, the increase in the performance of the MSSSC and the ability to multi-task. Finally, it is shown how the results obtained can be integrated into the planning process of a grid operator.