Fainan Hassan

High-Frequency Operation of a DC/AC/DC System for HVDC Applications

Voltage ratings for HVdc point-to-point connections are not standardized and tend to depend on the latest available cable technology. DC/DC conversion at HV is required for interconnection of such HVdc schemes as well as to interface dc wind farms. Modular multilevel voltage source converters (VSCs), such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), have been shown to incur significantly lower switching losses than previous two- or three-level VSCs. This paper presents a dc/ac/dc system using a transformer coupling two modular multilevel VSCs. In such a system, the capacitors occupy a large fraction of the volume of the cells but a significant reduction in volume can be achieved by raising the ac frequency. Using high frequency can also bring benefits to other passive components such as the transformer but also results in higher switching losses due to the higher number of waveform steps per second. This leads to a tradeoff between volume and losses which has been explored in this study and verified by simulation results with a transistor level model of 30-MW case study. The outcome of the study shows that a frequency of 350 Hz provides a significant improvement in volume but also a penalty in losses compared to 50 Hz.

The Alternate Arm Converter: A New Hybrid Multilevel Converter With DC-Fault Blocking Capability

This paper explains the working principles, supported by simulation results, of a new converter topology intended for HVDC applications, called the alternate arm converter (AAC). It is a hybrid between the modular multilevel converter, because of the presence of H-bridge cells, and the two-level converter, in the form of director switches in each arm. This converter is able to generate a multilevel ac voltage and since its stacks of cells consist of H-bridge cells instead of half-bridge cells, they are able to generate higher ac voltage than the dc terminal voltage. This allows the AAC to operate at an optimal point, called the “sweet spot,” where the ac and dc energy flows equal. The director switches in the AAC are responsible for alternating the conduction period of each arm, leading to a significant reduction in the number of cells in the stacks. Furthermore, the AAC can keep control of the current in the phase reactor even in case of a dc-side fault and support the ac grid, through a STATCOM mode. Simulation results and loss calculations are presented in this paper in order to support the claimed features of the AAC.

Advanced Vector Control for Voltage Source Converters Connected to Weak Grids

This paper addresses an advanced vector current control for a voltage source converter (VSC) connected to a weak grid. The proposed control methodology permits high-performance regulation of the active power and the voltage for the feasible VSC range of operation. First, the steady state characteristics for a power converter connected to a very weak system with a short circuit ratio (SCR) of 1 are discussed in order to identify the system limits. Then, the conventional vector control (inner loop) and the conventional power/voltage control (outer loop) stability and frequency responses are analyzed. From the analysis of the classic structure, an enhanced outer loop based on a decoupled and gain-scheduling controller is presented and its stability is analyzed. The proposed control is validated by means of dynamic simulations and it is compared with classic vector current control. Simulation results illustrate that the proposed control system could provide a promising approach to tackle the challenging problem of VSC in connection with weak AC grids.

Design and simulation of a modular multi-level converter for MVDC application

A modular converter structure for medium voltage DC (MVDC) applications is presented in this paper. The new converter structure is designed in such a way that, the modularity is available not only for voltage but also for current ratings, using the available devices. The new converter will be applicable for a wide range of medium voltage applications and using its modularity it also can be scaled up for high voltage applications. The proposed-converter control is described and the overall operation is explained through simulation. A simple active-reactive power control is utilized for the converter control. It has been highlighted, through simulation, that the proposed converter has a high degree of design flexibility introduced by its high number of redundant switching states. These redundancies are implemented for controlling the capacitor voltages of the converter modules. More importantly, they provide the converter with a DC and AC sides fault-handling capability; a function that is essential for an MV converter. It is explicitly shown that the converter can provide ancillary services to the grid during DC faults.

Direct model predictive control for medium voltage modular multi-level STATCOM with and without energy storage

Static reactive power compensators or STATCOM devices are well known as equipment for controlling the power flow in transmission grids. However, the evolution of distribution grids is calling for similar function adapted to the distribution level. Besides handling the major differences between distribution and transmission systems, with the smart grid vision of the future distribution grids; flexibility, accessibility and open modular equipment are main features of future distribution STATCOMs. This paper proposes the direct model predictive control (D-MPC) as a tool for achieving all the above keywords. D-MPC is simple, easily upgradable and can be easily adapted to facilitate the integration of different active devices at the distribution level. In this paper, D-MPC is implemented for the internal and external control of distribution STATCOMs having modular multilevel designs. It is shown that it is possible to control both active (using energy storage) or reactive power flow at the grid with the same simple control approach in addition to balancing the DC voltage of all modules.

Energy diverting converter topologies for HVDC transmission systems

Grid codes imposed by utilities regulate the operation of Voltage Source Converter - High Voltage Direct Current (VSC-HVDC) interconnected offshore wind farms. Fault ride-through (FRT) specifications require the adoption of specific measures to avoid over-voltages of the HVDC link during faults in order to protect the HVDC equipment. Implementing Energy Diverting Converters (EDC), for instance Dynamic Braking Resistor (DBR) circuits, at the DC link is an established method to comply with the grid codes, where the excess energy of the wind farm is diverted into the parallel circuit during the fault. In this paper an evaluation of three different state-of-the-art DBR circuits is performed in order to establish the advantages and disadvantages of each circuit. The evaluation has shown that although the three solutions meet the FRT requirements, the modular topologies generate reduced slope current and voltage step changes during their operation, while being larger in size and requiring a higher number of semiconductors as compared to the traditional DC chopper circuit employing hard switched series connected semiconductor arrangements.

AC Recharging Infrastructure for EVs and future smart grids — A review

AC Recharging Infrastructures for EVs will be a crucial feature in many smart grid scenarios. EV charging will need to be dynamic and correlated to renewable generation with the effect of reducing the overall net demand variability in order to allow for mass roll-out of EVs. This paper highlights how the functionalities of AC recharging systems may need to evolve to ensure that the optimal operation of the power system is achieved. In the light of future smart grid evolution scenarios, critical parameters of AC infrastructures and their possible evolution are discussed while other potentially useful functions are also put forward. It was found through the analysis of existing infrastructures and connection standards that there are some inherent capabilities that can be readily implemented and may enhance the performance of the AC grids and increase their EVs hosting capacities. Additionally, other capabilities will evolve in the near future serving for better integration of EVs along with renewables.

Choice of AC operating voltage in HV DC/AC/DC system

Demand for DC/DC conversion in HV applications is expected to rise because of the increasing number of HVDC links using different DC voltage levels. This paper presents a DC/AC/DC system consisting of two VSCs connected through an inductor. The two VSCs are Alternate Arm Converters (AAC). Since the two AACs share the same AC voltage level, they cannot be operated at their respective “sweet-spots” at the same time. This results in an energy drift in the valves which is tackled by additional balancing currents. However, the choice of the AC voltage level remains critical as it determines the required amount of balancing current, the number of devices and the cell topology, influencing greatly the total efficiency and volume of the obtained DC/DC converter. A study on a scale-down converter highlights the trade-offs affecting the AC voltage choice.

A flexible modular multi-level converter for DC microgrids with EV charging stations

This paper proposes a modular multilevel converter (MMCon) as a flexible interface for DC microgrids. The operation of this converter is examined for a microgrid integrating highly intermittent loads and storage. Electric vehicle (EV) fast charging stations are the chosen example of intermittent loads in the microgrid. The MMCon provides flexibility for a future increase of the microgrids capacity thanks to its high scalability allowing for the accommodation of new loads and energy sources through retrofitting additional parallel modules. A control strategy for the converter is proposed and the operation during demand side control of the microgrid is validated through computer simulation.

Series Interline DC/DC Current Flow Controller for Meshed HVDC Grids

This paper proposes a DC/DC current flow controller (CFC) for meshed high voltage direct current (HVDC) grids. The CFC has the advantage of a simplified structure that allows us to use the minimum number of switches when unidirectional current flows through the DC lines are expected. An extended CFC topology that is able to operate with all possible current flows is also presented. Then, the operating principle of the CFC is discussed and its structure is compared with a dual H-bridge analyzing advantages and disadvantages. This paper also details the applied modulation strategy and the control methodology. Finally, a five-terminal meshed HVDC grid is used to validate the CFC by means of simulations.