The fundamental question to answer is: “What is the optimal type, architecture and control strategy of STATCOM technologies to increase the hosting capacity of renewable energy sources in distribution networks and to provide flexibility from the distribution network to the transmission network?” This question is addressed from both theoretical and practical perspective. Indeed, the analyses are based on the literature and at the same time on a specific case study of the benefits of integrating a STATCOM into SIL existing Medium Voltage (MV) network. A summary of the activities carried out to date to answer this question is provided below.
A literature review of a multitude of STATCOM structures was carried out. After evaluating and comparing the different structures, the Modular Multilevel Cascaded Converter (MMCC) was selected. These structures can operate over a wide voltage range due to their modularity, they are robust to network disturbances and are capable of operating even during a network imbalance or fault. Specifically, in this type of converter, the Single Star Bridge-Cells (SSBC) topologies, also known as Cascaded H-Bridge (CHB) in star connection, Single Delta Bridge-Cells (SDBC), also known as CHB in delta connection, and Double Star Chopper-Cells (DSCC), also known as Modular Multilevel Converter (MMC), offer these advantages without being the most complex and expensive topologies of this category.
An important criterion for the use of a STATCOM in the electricity network is its ability to operate in an unbalanced network or in the event of a network fault (such as a short-circuit). When a voltage imbalance or short-circuit occurs, the power per phase in the STATCOM converter is unbalanced. The difficulty with these 3 topologies is the absence of a common DC bus, which prevents an automatic exchange of energy between phases. As a result, an additional control strategy must be used to manage this power imbalance per phase. After an in-depth analysis of the control strategies, it emerged that the SDBC topology could not meet all the project criteria. On the other hand, detailed simulations carried out on the SSBC topology with the additional Zero Sequence Voltage Control (ZSVC) control strategy demonstrate that it operates permanently under all types of voltage imbalance, even during short circuits. Finally, detailed simulations and experimental tests of the prototype of the DSCC topology under real conditions with energy control proposed in this project have demonstrated that this topology can continue to operate in all possible network states on a permanent basis except for the two phases line-to-earth short-circuit. In this case, the DSCC can only continue to operate for few seconds before excessive internal unbalancing occurs. Given that the duration of a short circuit is generally no more than 5s, this topology with this control could still meet all the criteria. Other types of control could improve this operating state. This is one of the project's development points.
An analysis of STATCOM technologies available on the market today has been carried out. It covers 11 different manufacturers and describes the products available on the market. In addition, the products are compared by topology and by manufacturer to provide a more detailed analysis of the market. In particular, it shows that modular converters (SDBC and SSBC) are becoming increasingly popular, especially for high-power applications. Finally, a list of STATCOMs that could potentially be implemented in the SIL network studied is proposed.
From a power grid point of view, part of the work also involved studying the improvements brought about by STATCOMs on the operation of SIL's MV power grid by massively integrating new Renewable Energy Sources (RES). To this end, a model of a part of the MV network with loads and PV generation was created using data supplied by SIL and MeteoSwiss. On the basis of annual simulations, the effects of different STATCOM locations and control modes under steady-state conditions were compared. The results show that the STATCOM power required for optimal operation can be relatively high, up to 5 Mvar. In this case, STATCOMs should be located close to the HV/MV feeder substation, or at well-interconnected nodes. If the power available to the STATCOM is limited (due to space or financial constraints), the best location is a well-interconnected node, in the "middle" of a MV feeder. The efficiency of exporting reactive power from the distribution system to the transmission system was analysed for two relatively simple cases. The results show that the complex interconnections between the two distribution and transmission systems in Western Switzerland limit the potential of such a concept for a MV STATCOM. Better results would be achieved by placing the STATCOM in High-Voltage (HV) or Extra-High-Holtage (EHV).
Thanks to an in-depth analysis of the MV network supplied by SIL, it was possible to create this network on a reduced scale in the HEIG-VD Intelligent Network reconfigurable laboratory (ReIne), using a load and line aggregation principle explained in this report. This reduced-scale network was used to test the DSCC prototype (developed by HEIG-VD) operating in STATCOM mode under real conditions in different scenarios. Three scenarios were carried out:
- underloaded network
- highly loaded network
- network with high production based on the forecasted PV installation according to the 2050+ swiss energy perspectives
As the ReIne laboratory can measure currents, voltages and power at any location, it has been possible to assess the benefits of STATCOM on the network depending on the scenario. The results show that, whatever the scenario, STATCOM is capable of reducing the voltage deviation from the nominal voltage. It should be noted that STATCOM is most profitable when the network is highly loaded. On the other hand, it is least profitable under conditions of high production.
The final part of the project involved sharing and discussing the main results of the COSTAM project with Swiss DSOs and Swissgrid. The first step was to carry out a survey of reactive power management in distribution networks. The results of this survey were used to prepare the organisation and content of a workshop. The second stage was to hold a workshop to share the main findings of the COSTAM project and the results of the survey, and also to discuss the various issues relating mainly to reactive power in the distribution network. Seven DSOs and Swissgrid took part in this workshop, which gave them a better understanding of the current technologies available for reactive power compensation and the points of view of each party on the subject.
