Control Strategies for Hybrid AC/DC Transmission Power Grids with Renewable Energy Integration
Hybrid AC/DC transmission power grid has been deployed in several transmission networks to reinforce the power grid and to achieve seamless integration of renewable power plants (e.g. PV & Wind). It brought several advantages such as increased flexibility of operation, enhanced power transfer capability and simpler interconnection of renewable energy sources (RES). Therefore, it becomes evident that hybrid AC/DC Transmission grids are the future for transmission networks especially for connecting offshore industrial and generation platforms such as offshore oil rigs and offshore wind parks. Although AC grid characterized by large inertia and its dynamic responses to either voltage or frequency disturbances are well studied, the DC grid lacks system inertia with low tolerance to DC grid disturbances (e.g. DC faults, failure of protection, simultaneous commutation failures in multiple converter stations due to AC fault,…etc). Thus, the interconnected AC/DC transmission grid will have new characteristics due to the complex dynamic and transient interactions of the AC and DC grids resulted from their different response times and control strategies. This presentation outlines advanced control strategies of the voltage source converters (VSCs) based multi-terminal HVDC (MTDC) system.
In the first part of this talk, the structure of the basic dc voltage droop control for MTDC system and its enhanced forms will be discussed. DC voltage regulation is essential for stable operation of MTDC system. For this, dc voltage droop control is the widely used control technique since it is more reliable compared to its counterparts. However, the conventional droop control has drawbacks such as the converters’ power sharing is impacted by the MTDC grid topology and also a significant steady-state dc voltage deviation can occur during contingencies. Therefore, a more advanced forms of dc voltage droop control have been developed including power-sharing index (PSI) communication based dc voltage droop control and communication-free accurate power sharing mechanism to mitigate the impact of grid topology on converters’ power allocation. Moreover, to limit the dc voltage deviation within the allowable range, secondary average dc voltage regulation technique was proposed to regulate the average dc voltage to the nominal value. The transient dc voltage deviation can also go beyond the permissible range during disturbances in the system. Hence, a novel control strategy to reduce the transient dc voltage deviation through swift power rerouting using direct power control (DPC) has been proposed.
Later in the presentation, an overview of the inertia and primary frequency support strategies for MTDC system will be presented. When the penetration level of the MTDC system increases, the inertia of the hybrid ac/dc system will decrease due to the decoupling effect of HVDC system. Therefore, MTDC systems are required to participate in the frequency support of the interconnected ac grids to make the hybrid power system secure. A selective inertia and frequency support approach based on consensus algorithm combined with identification mechanism for the disturbed ac grid will be presented. On the other hand, to mitigate high-bandwidth communication dependency of the consensus algorithm based mechanism, a less communication dependent frequency support technique based on dc voltage vs frequency (Vdc – f) droop control structure was proposed. More recently, primary frequency regulation strategy for MTDC system considering the loading of converters is proposed to effectively utilize the available capacity of converters. The performance of the aforementioned control strategies will be presented and evaluated in response to piratical testing scenarios.
Flexible Power Grid Technologies Toward 100% Renewable Energy Integration
Power System Flexibility represents an important aspect of the transformation of current power grids towards one dominated by renewable energy resources (RES). Therefore, future power systems with higher penetration levels of RES will require harnessing flexibility from all parts and employed technologies in the power grids. In addition, the integration of several evolving technologies and techniques such as optimization, IOT, artificial intelligence (AI), communication, and control will be mandatory to enhance the efﬁciency and security of several services including energy and electrified transportation. To transform into flexible power grid, many physical systems should be tightly intertwined to enable the co-operation/co-optimization via connected networks of digital devices. On the other hand, the integration of RES and electrified mobility will impose several security challenges, which stem from changes in weather conditions and customers’ consumption behaviors. Consequently, improving power system flexibility with accurate RES forecasting component becomes mandatory to satisfy the key flexibility indices (i.e. ramping limits (ρ), power capacity (π), energy capacity (ε) and response time (θ)). In nutshell, the flexible power grid is characterized with their ability to adjust power generation and/or consumption in response to both anticipated and unanticipated variability across all relevant timescales. In this context, this presentation introduces a novel FlexGrid platform for effective and least-cost assessment, planning and operation of flexible AC/DC power grids. The framework of the proposed FlexGrid platform along with the dynamic modelling and economic analysis of the different sources of flexibility which are associated with supply-side, grid-side and demand-side will be presented. For each type, comprehensive modeling of individual components and technologies will be performed to simulate their behavior across different time scales such that real-world challenges are properly addressed. Furthermore, the presentation outlines the utilization of advanced machine/deep learning techniques to train accurate prediction models for different types of technologies that are involved in the generation-side such as solar and wind power generation along with the aggregation/classification of virtual power plants, demand response and load profiling. Moreover, the FlexGrid platform will devise an optimization module to plan for flexible hybrid AC/DC grids with high RES integration and energy storage systems. Furthermore, optimal coordination (e.g. TSO and DSO coordination) of operational flexibility exchange between hybrid AC/DC transmission and distribution networks will be analyzed to pave the way for providing sufficient flexibility at both sides. Finally, efficient ancillary services on the supply-side, grid-side and demand-side, which can be provided/traded to enhance specific features of flexibility will be analyzed.
Optimal Design and Operation of Hybrid Renewable Power Plant with Dispatching Capability
In traditional power grids, variability was mainly caused by frequent changes in load patterns or system contingencies during the entire day and across all seasons of the year. Meanwhile, flexibility of the power grid is ensured through commissioning additional spinning reserve units and/or utilizing the support from the interconnected power networks. Although conventional power plants provide reliable and secure power supply, they have contributed to the largest share of global carbon emissions. As a result, significant transformation is being witnessed across the power sector towards large-scale integration of Renewable Power Plant to reduce these emissions. Nevertheless, the increasing penetration of RES at both transmission and distribution networks introduce several challenges to power system operation. For example, the output power of these resources is highly unpredictable and volatile as it strongly depends on weather conditions. Therefore, high penetrations of RES increase system requirements for ensuring the balance between supply and demand. Hence, this presentation outlines the design, operation and interconnections requirements of Hybrid Renewable Power Plant (HRPP) such as PV/ESS and PV/Wind/ESS at transmission level to enable considerable level of dispatching capability as well as efficient ancillary services. In this context, an optimal design for HRPP based on various types of RES generation and ESS will be presented. In addition, the proposed HRPP will be augmented with Renewable Energy Management system (REMS), which employs a risk-constrained dispatching algorithm based on stochastic optimization to compute the optimal operating points of the HRPP. Moreover, the SCADA of the HRPP will be equipped with intelligent systems and data analytics techniques to identify the failures in response to grid disturbances and verify the performance in connection with the grid operation and environmental impact. The operation of HRPP in grid-connected mode as well as islanded mode of operation will be presented. Consequently, the control strategies for enabling the black start capability of the HRPP for efficient system restoration in case of system blackout will be introduced. The HRPP equipped with REMS tool will enable the grid operator to simulate multiple HRPP dispatching scenarios to assess participation in scheduling RES power generation over various time horizons in connection with the requirements of economic dispatch, unit commitment and the energy market operation. Finally, the HRPPs flexibility and its benefits of adjusting efficient ancillary services in connection with grid codes requirements will be analyzed for a practical test system.
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