Abstract
Numerous research studies on high capacity DC-DC converters have been put forward in recent years, targeting multi-terminal medium-voltage direct current (MVDC) and high-voltage direct current (HVDC) systems, in which renewable power plants can be integrated at both medium-voltage (MV) and high-voltage (HV) DC and AC terminals; hence, leading to complex hybrid AC-DC systems. Multi-port converters (MPCs) offer the means to promote and accelerate renewable energy and smart grids applications due to their increased control flexibilities. In this paper, a family of MPCs is proposed in order to act as a hybrid hub at critical nodes of complex multi-terminal MVDC and HVDC grids. The proposed MPCs provide several controllable DC voltages from constant or variable DC or AC voltage sources. The theoretical analysis and operation scenarios of the proposed MPC are discussed and validated with the aid of MATLAB-SIMULINK simulations, and further corroborated using experimental results from scale down prototype. Theoretical analysis and discussions, quantitative simulations, and experimental results show that the MPCs offer high degree of control flexibilities during normal operation, including the capacity to reroute active or DC power flow between any arbitrary AC and DC terminals, and through a particular sub-converter with sufficient precision. Critical discussions of the experimental results conclude that the DC fault responses of the MPCs vary with the topology of the converter adopted in the sub-converters. It has been established that a DC fault at high-voltage DC terminal exposes sub-converters 1 and 2 to extremely high currents; therefore, converters with DC fault current control capability are required to decouple the healthy sub-converters from the faulted one and their respective fault dynamics. On the other hand, a DC fault at the low-voltage DC terminal exposes the healthy upper sub-converter to excessive voltage stresses; therefore, sub-converters with bipolar cells, which possess the capacity for controlled operation with variable and reduced DC voltage over wide range are required. In both fault causes, continued operation without interruption to power flow during DC fault is not possible due to excessive over-current or over-voltage during fault period; however, it is possible to minimize the interruption. The above findings and contributions of this work have been further elaborated in the conclusions.
Highlights
The advantages of using DC grids and power electronic components in the renewable energy revolution are substantially identified [1,2,3,4,5,6,7,8,9]
This paper has investigated the enhanced power control offered by multi-port converters (MPCs), namely the two-port and tri-port system as practically sound representatives of MPC with multiple DC and AC terminals, which can be adopted for large-scale integration of renewable power generations into future high-voltage direct current (HVDC) and UHVDC grids at different DC and AC voltage levels
Theoretical principles that underpin the operation of the MPC under investigation have been described, and substantiated quantitatively, by means of simulations and experimentations, which show that the MPC can control DC and real power flow between multiple DC and AC terminals, and facilitate DC voltage matching and tapping in complex HVDC grids
Summary
The advantages of using DC grids and power electronic components in the renewable energy revolution are substantially identified [1,2,3,4,5,6,7,8,9]. In [37], a partially isolated modular DC transformer named auto-transformer was proposed, with the partial ability to block DC faults, and its main features are that the semiconductor switches of the low voltage side and isolation transformer that links upper and lower sub-converters do not experience the full load currents when the rated power is being exchanged between the high and low voltage sides. Many variants of this auto DC transformer, including front to front and multi pole models, to connect different HVDC transmission topologies, are discussed in [37]. The theoretical basis that underpins the working principles of the presented MPC is explored, and further supported quantitatively by aid simulations, and corroborated experimentally, considering various operating conditions
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