Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks

In the last decade, distribution systems are experiencing a drastic transformation with the advent of new technologies. In fact, distribution networks are no longer passive systems, considering the current integration rates of new agents such as distributed generation, electrical vehicles and energy storage, which are greatly influencing the way these systems are operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system through power electronics converters, is unlocking the possibility for new distribution topologies based on AC/DC networks. This paper analyzes the evolution of AC distribution systems, the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409

Operation and control strategy of coupled microgrid clusters

A standalone remote area microgrid may frequently experience overloading due to lack of sufficient power generation or excessive renewable-based generation that can cause unacceptable voltage and frequency deviation. This can lead the microgrid to operate with less resiliency and reliability. Such problems are conventionally alleviated by load-shedding or renewable curtailment. Alternatively, autonomously operating microgrids in a geographical area can be provisionally connected to each other to facilitate power exchange for addressing the problems of overloading or overgeneration. The power exchange link among the microgrids can be of different types such as a three-phase ac, a single-phase ac, or a dc-link. Power electronic converters are required to interconnect such power exchange networks to the three-phase ac microgrids and control the power-sharing amongst them. Such arrangement is also essential to interconnect microgrid clusters to each other with proper isolation while maintaining autonomy if they are operating in different standards. In this thesis, the topologies, and structures of various forms of power exchange links are investigated and appropriate operation and control frameworks are established under which power exchange can take place properly. A decentralised control mechanism is employed to facilitate power-sharing without any data communication. The dynamic performance of the control mechanism for all the topologies is illustrated through simulation studies in PSIM® while the stability and robustness of the operation are evaluated using numerical studies in MATLAB®

Assessment of HVDC technologies for an offshore MTDC grid

This thesis examines various HVDC converter technologies that could be used in offshore Multi-Terminal DC (MTDC) grids. MTDC grids rely on AC/DC converters to interface with AC systems and also for control services. Two AC/DC topologies were compared, the half-bridge Modular Multi-level Converter (MMC) and the Alternate Arm Converter (AAC). As new DC system voltages emerge the DC/DC converter could be an enabling technology for interconnection and future MTDC networks. As yet there is no consensus on DC/DC converter topology and a critical comparison of several potential designs was conducted. An MMC based DC/DC converter had distinct advantages compared with other designs. Several average value converter models of the converters were developed to allow efficient simulation of MTDC networks, while maintaining a high level of accuracy of the converter characteristics. These models were verified with full switching models for steady state and fault conditions. Two o shore MTDC networks were studied; a four-terminal network, and a MTDC network. The four-terminal network used a normally open point to connect two existing point-to-point links, allowing reconfiguration in the event of a DC fault. The MTDC network uses a DC/DC converter to interconnect a bipole HVDC link with the previously studied four-terminal network. Several simulation studies show how new converters can improve the operation of a MTDC and provide additional capabilities such as DC fault blocking.Open Acces

Multi-terminal HVDC grids with inertia mimicry capability

The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.Peer ReviewedPostprint (author's final draft

Modified dual active bridge DC/DC converter with improved efficiency and interoperability in hybrid LCC/VSC HVDC transmission grids

DC transmission grids are the promising electrical networks in the near future especially with the high penetration of large scale renewables. This paper proposes a modified version of the dual active bridge (DAB) DC/DC converter with AC link capacitors generating reactive power to compensate for non-active power consumption; hence mitigating current stresses and losses to improve efficiency. The proposed topology also enables the connectivity of current source line-commutated HVDC and voltage source HVDC technologies particularly during power reversal; a feature which conventional DAB is incapable of doing. Analysis and detailed design of the proposed converter are addressed and a comparative performance analysis is carried out with conventional DAB. Converter principle of operation is explained and Matlab/Simulink simulations are carried out to verify converter operation particularly under adverse conditions such as rated power reversal and DC fault conditions. A low scale prototype substantiates the theoretical analysis and simulation results

Stability of microgrids and weak grids with high penetration of variable renewable energy

Autonomous microgrids and weak grids with high penetrations of variable renewable energy (VRE) generation tend to share several common characteristics: i) low synchronous inertia, ii) sensitivity to active power imbalances, and iii) low system strength (as defined by the nodal short circuit ratio). As a result of these characteristics, there is a greater risk of system instability relative to larger grids, especially as the share of VRE is increased. This thesis focuses on the development of techniques and strategies to assess and improve the stability of microgrids and weak grids. In the first part of this thesis, the small-signal stability of inertia-less converter dominated microgrids is analysed, wherein a load flow based method for small-signal model initialisation is proposed and used to examine the effects of topology and network parameters on the stability of the microgrid. The use of a back-to-back dc link to interconnect neighbouring microgrids and provide dynamic frequency support is then proposed to improve frequency stability by helping to alleviate active power imbalances. In the third part of this thesis, a new technique to determine the optimal sizing of smoothing batteries in microgrids is proposed. The technique is based on the temporal variability of the solar irradiance at the specific site location in order to maximise PV penetration without causing grid instability. A technical framework for integrating solar PV plants into weak grids is then proposed, addressing the weaknesses in conventional Grid Codes that fail to consider the unique characteristics of weak grids. Finally, a new technique is proposed for estimating system load relief factors that are used in aggregate single frequency stability models

Power Management and Protection in MT-HVDC Systems with the Integration of High-Voltage Charging Stations

Due to the significant increase of the long-distance electricity demand, effective use of Distributed Generations (DGs) in power system, and the challenges in the expansion of new transmission lines to improve the reliability of power system reliability, utilizing Multi-Terminal HVDC (MT-HVDC) technology is an applicable, reliable, and cost-effective solution in hybrid AC/DC grids. MT-HVDC systems have flexibility in terms of independent active and reactive power flow (reversible control) and voltage control. Interconnecting two AC grids with different frequencies and transmitting electricity for the long-distance with low power-losses, which leads to less operation and maintenance costs, can be done through the MT-HVDC systems. The integration of large-scale remote DGs, e.g., wind farms, solar power plants, etc., and high-voltage charging stations for Electric Vehicles (EVs) into the power grid have different issues, such as economic, technical, and environmental challenges of transmission and network expansion/operation of both AC and DC grids. In details, damping oscillation, voltage support at different buses, operation of grid-connected inverters to the off-shore and on-shore AC systems, integrating of existing converter stations in MT-HVDC systems without major changes in control system, evaluation of communication infrastructure and also reactive power and filtering units’ requirements in MT-HVDC systems are the technical challenges in this technology. Therefore, a reliable MT-HVDC system can be a possible mean of resolving all the above-mentioned issues. MT-HVDC systems need a control system that can bring stability to the power system during a certain period of the operation/planning time while providing effective and robust electricity. This thesis presents an improved droop-based control strategy for the active and reactive power-sharing on the large-scale MT-HVDC systems integrating different types of AC grids considering the operation of the hybrid AC/DC grids under normal/contingency conditions. The main objective of the proposed strategy is to select the best parameters of the local terminal controllers at the site of each converter station (as the primary controller) and a central master controller (supervisory controller) to control the Power Flow (PF) and balance the instantaneous power in MT-HVDC systems. In this work, (1) various control strategies of MT-HVDC systems are investigated to propose (2) an improved droop-based power-sharing strategy of MT-HVDC systems while the loads (e.g., high-voltage charging stations) in power systems have significant changes, to improve the frequency response and accuracy of the PF control, (3) a new topology of a fast proactive Hybrid DC Circuit Breaker (HDCCB) to isolate the DC faults in MT-HVDC grids in case of fault current interruption. The results from this research work would include supporting energy adequacy, increasing renewable energy penetration, and minimizing losses when maintaining system integrity and reliability. The proposed strategies are evaluated on different systems, and various case scenarios are applied to demonstrate their feasibility and robustness. The validation processes are performed using MATLAB software for programming, and PSCAD/EMTDC and MATLAB/Simulink for simulation