• Energy Research

AbstractDirect voltage fluctuations due to the presence of relatively large DC reactors (as an essential part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in the AC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids. The use of the DC Power System Stabilizer (DC‐PSS) can damp and eliminate voltage oscillations caused by the presence of the DC reactors. However, DC‐PSS cannot address the issues of inertia and unwanted frequency fluctuations. A method to improve inertia is proposed here that can operate well with the droop controller, and DC‐PSS does not interfere with power‐sharing and does not interact with any of these elements. Since the presence of a droop controller in HVAC/HVDC grids associates with power and direct voltage, the method proposed here can improve direct voltage fluctuations by eliminating severe power peaks. Moreover, this method does not change the voltage level of the entire system, so there is no need to change the set‐points of controllers. In addition, all parameters of the controllers are tuned by an intelligent algorithm, and the Participation factor (PF) scheme is used to find the proper placement of the proposed controller.

The current route of achieving the ultimate plan for flawless operation and control of the multi-terminal DC (MTDC) grids can be significantly accelerated by learning from the vast and valuable experiences gained from the operation of the AC power grids for more than a century. This paper introduces concept of flexible DC transmission system (FDCTS), inspired by the successful operation of flexible AC transmission systems (FACTS), to provide voltage regulation, power control and load flow control within MTDC grids. Considering the current advancements in the field of power electronics, this paper recognizes DC-DC converters as the first element of the FDTCS for providing voltage and power control in MTDC grids. By use of DC-DC converters, this paper developes two elements of the FDCTS, namely the cascaded power flow controller (CPFC) and hybrid power flow controller (HPFC). In this paper, to demonstrate the eligibility of the CPFC and HPFC to play the role of an FDCTS, they are included in the DC power flow formulation for DC voltage regulation and power flow control purposes. Peer Reviewed

Future Flexible HVDC Transmission Systems will consist of several active and passive infrastructures including DC Power Flow Controller (DC-PFC) device. The addition of the DC-PFC to the future Multi-Terminal HVDC (MT-HVDC) grids arises some key concerns such as stability, system interoperability, and possible adverse interactions. Hence, a suitable model is necessary to conduct deep frequency domain analysis. In this context, this paper proposes a linearized model for small-signal stability analysis of flexible MT-HVDC grids in state space framework that can be straightforwardly utilized in the process of control design. In this paper, a spick-and-span, systematic and step-by-step process to derive the small signal model of all flexible MT-HVDC grid components is presented such that each sub-system is modeled individually and then all are integrated together. The derived model is cross-verified by time domain simulations of a nonlinear system model in a MATLAB/SIMULINK platform for CIGRE DCS3 MT-HVDC test grid. Peer Reviewed

Abstract This research proposes an approach to select an optimal place and control variable setting for recently proposed Power Flow Controller (PFC)s including series, cascaded, and interline PFCs in Multi-Terminal High-Voltage DC (MT-HVDC) grids based on sensitivity analysis technique to enhance static security. To do so, appropriate static power injection models of the PFCs are obtained. Accordingly, sensitivity relationships between the defined power performance index and each PFCs control variable are obtained. It is for the optimal PFCs placement purpose in order to utilize maximum capacities of the transmission grid. Optimal settings for the PFC as well as the controllable voltage source converters are computed in turn by applying sequential quadratic programming solver to the developed security-based DC optimal power-flow problem which includes several corrective constraints. The study scope is single contingencies affecting HVDC lines with the objective of eliminating the consequent overloaded HVDC lines and thereby enhancing the MT-HVDC grid security by controlling the power flows within the MT-HVDC grid. Static simulations are performed considering two generic four-terminal and eighth-terminal MT-HVDC test grids in order to demonstrate the proposed methods effectiveness and authenticate its robustness. The obtained results indicate that the MT-HVDC grid can remain secure under single HVDC line contingencies in most cases by implementing the proposed method.

Global warming is already having significant and costly effects on our communities, our health, and our climate. Nowadays, In developing countries (G20) around the world, decisions are being considered and a shifting of our dependence from fossil fuels to renewable energy sources (RES). Among RES solar energy, specially photovoltaic system (PV) play a key role in this challenge, thus, this paper provides a comparative study on the performances of three transformer-less inverters for photovoltaic applications (H-Bridge, H5, HERIC and NPC inverters) in terms of power losses, considering the use of different kinds of switches. Silicon (Si) and Silicon Carbide (SiC) diodes have been combined with two topologies of IGBTs in the performed tests. Simulation results have been obtained for each of the systems in different operating conditions by using PLECS software. Power losses and European Efficiency (EE) have been calculated for all the topologies, taking into account the real characteristics of the switches by using the thermal models provided by the software library. Peer Reviewed

Abstract Insufficient control flexibility in multi-terminal HVDC (MT-HVDC) grids is an important motivation to install suitable power electronic-based DC power flow controller (PFC)s to ensure grid controllability, security, and reliability. This article proposes a new static power injection model (PIM) for those (interline) DC-PFCs to enable DC power flow (PF) studies and ease integration of the PFCs in the power system analysis softwares within the well-accepted Newton–Raphson (NR) solver-based framework. HVDC lines shunt conductances are also taken in to account in this newly developed paradigm. For this purpose, the IDC-PFC, as well as other MT-HVDC grid physical/control state variables are modified in cooperation to attain predefined control objective(s). Furthermore, a novel general routine (solution procedure) is proposed to handle several system physical/control limitations during the NR-based DC PF problem solution. Static/dynamic simulations are executed on an eight-bus test MT-HVDC grid to show/confirm the accuracy, effective performance, and excellent convergence property of the proposed IDC-PFC model, NR-based DC PF solver, and technical constraints handling routine. In this situation, it is proved that the original structure and symmetry of the admittance matrix can still be kept, and a few modifications are needed to be done in the Jacobin matrix.

This paper proposes an intelligent control scheme for DC voltage regulationin a DC micro-grid integrating photovoltaic (PV) generation, energy storage and electric loads. The maximum power generation of the PV panel is followed using the incremental conductance (IC) maximum power point tracking (MPPT) algorithm while a high-performance local linear controller (LLC)is developed for the DC voltage control in the micro-grid.The LLC, as a data-driven control strategy, controls the bidirectional converter located between the battery and the DC micro-grid to regulate the micro-grid's voltage at the specified reference value.In order to validate the controller different simulations including variations in the load and in the solar irradiance are performed. Finally a comparison with PI based controller demonstrates the advantages of the proposed control technique for regulating theDC micro-grid's voltage. Peer Reviewed