Two-level voltage-source converters and half-bridge modular multilevel converters are among the most popular types of HVDC converters. One of their serious drawbacks is their vulnerable nature to ...dc-side faults, since the freewheeling diodes act as a rectifier bridge and feed the dc faults. The severity of dc-side faults can be limited by connecting double thyristor switches across the semiconductor devices. By turning them on, the ac current contribution into the dc side is eliminated and the dc-link current will freely decay to zero. The main disadvantages of this method are: high dv/dt stresses across thyristors during normal conditions, and the absence of bypassing for the freewheeling diodes during dc faults since they are sharing the fault current with thyristors. This paper proposes combining and connecting the double thyristor switches across the ac output terminals of the HVDC converter. The proposed protection scheme provides advantages, such as lower dv/dt stresses and lower voltage rating of thyristor switches in addition to providing full segregation between the converter semiconductor devices and ac grid during dc-side faults. A simulation case study has been carried out to demonstrate the effectiveness of the proposed scheme.
The nine-phase six-terminal induction machine has recently been proposed as a promising contender to the conventional six-phase asymmetrical winding machine in terms of torque density, phase current ...quality, stator winding simplicity, and fault-tolerant capability. However, the relatively lower dc-link voltage utilization of a single neutral arrangement in multiphase machines with multiple three-phase windings represents, in general, a technical challenge when compared to windings with isolated star points. Therefore, this paper proposes a new pseudo six-phase winding layout suitable for medium-voltage high-power induction machines, which employs quadruple three-phase stator winding sets, while providing the same terminal behavior of a nine-phase six-terminal winding. Additionally, like the traditional six-phase winding, two possible neutral arrangements can be configured. The proposed winding configuration provides the same dc-link voltage utilization as in conventional dual three-phase winding machines with isolated neutrals. The effect of the circulating zero-sequence current component experienced with a single neutral arrangement can also be avoided. A 1.5 Hp prototype induction machine is used to experimentally validate the proposed six-phase winding layout under both healthy and fault conditions.
Power generation through wind is expected to play a major role in the world's future energy portfolio. Nevertheless, wind power integration remains a challenging research area due to the special ...characteristics of wind power generation. Specifically, offshore wind has received significant attention worldwide due to the vast generation potential available. The electrical infrastructure of offshore wind farms is thus of significant importance. The multi-terminal HVDC solution represents a preferable solution and has shown promise in solving wind farm interconnection problems. Droop control techniques have been proposed as a means to regulate the DC voltage and facilitate the automatic coordination between different converters without the need for fast communication between units. Different methodologies have been developed to select the droop gains to satisfy the system performance specifications. In this work, a control design methodology is proposed for power sharing among the multi-terminal HVDC feeders providing that the power transmission efficiency is optimized. A simulation study on a 400-kV/1000-MW four-terminal HVDC transmission topology is conducted to ensure the validity of the proposed methodology.
The nine-phase six-terminal induction machine (IM) has been recently proposed as a promising contender to the conventional asymmetrical six-phase type in terms of torque density, stator winding ...simplicity, and fault-tolerant capability. The stator is composed of nine phases, which are connected in a fashion to only provide six stator terminals. Therefore, this connection combines the high performance of a nine-phase winding with the terminal behavior of a six-phase machine. This paper introduces the machine mathematical model based on the vector space decomposition (VSD) modeling approach. The required current and voltage sequence transformation matrices are derived such that the machine is mathematically regarded as an equivalent six-phase IM with only three decoupled subspaces. This way, the same VSD-based controller structures conventionally applied to six-phase-based systems can be preserved. A 1.5-hp prototype IM is used to experimentally validate the machine model under both healthy and open-phase conditions.
In the recent literature, a rewinding approach was proposed to construct a rewound five-phase machine with either star-connected or combined star-pentagon winding using existing off-the-shelf ...three-phase stator frames. The power per phase drops by a factor of 3/5 in the five-phase machine compared to the three-phase machine. This indicates that either the rated current or voltage must be rescaled. The two options are compared in terms of drive cost and performance. The constant current - reduced voltage option is analytically investigated to determine whether reducing the operating voltage affects the operating speed/output power or not. The second contribution of this work is to determine the optimal slot/pole combinations of the existing off-the-shelf three-phase stators that ensure balanced five-phase star-connected and/or combined star-pentagon windings. This has been done by comparing the harmonic mapping of the rewound machines for different slots/poles combination with the conventional five-phase machines using standard vector-space-decomposition. In addition, the leakage inductance mismatch for different slots/poles combinations of the rewound five-phase machines has been compared to the standard conventional five-phase machines. The last contribution of this paper is to compare the performance of the rewound combined star-pentagon winding with an optimally designed conventional symmetrical one (60-slots/4-poles). This has been done using 2D Ansys Maxwell transient simulation. Finally, experimental results are used to support the theoretical and analytical conclusions.
This paper investigates the performance of synchronous reluctance motors when the stator is equipped with a combined star-delta winding layout. The conventional star winding is used as a benchmark in ...this study to compare different possible single-layer winding layouts. Among these different winding layouts, those which maximize the fundamental magneto-motive force component are selected. A simple mathematical formula is then derived to calculate the equivalent winding factor for different shares between star and delta subwindings. It has been proved that for the same copper volume and line current magnitude, the star-delta connection can offer an enhancement in the torque density of approximately 5.2% over the conventional star case under rated conditions. However, this gain is affected by the employed number of poles and stator slots. On the other hand, the effect of the winding layout on either power factor or core loss can be merely neglected over a wide range of speeds and currents. Nevertheless, the machine efficiency under a combined star-delta connection is relatively improved under light loading as a result to the machine torque/current ratio enhancement. The theoretical findings are experimentally validated using two identical 5.5 kW prototype machines, having star and combined star-delta winding connections.
One of the main merits offered by multiphase machines is their high fault-tolerant capability. Literature has demonstrated that the performance of multiphase induction machines under fault conditions ...is affected by the employed stator winding connection. In open-loop controlled five-phase machines, the star connection is favorable under healthy conditions while the pentagon connection is favorable and yields a lower derating factor under the open phase condition. In this paper, a new combined star/pentagon single layer winding layout that combines the advantages of both star and pentagon connections is proposed for a five-phase induction machine. Although the proposed winding is intrinsically an asymmetrical ten-phase machine, the proposed connection allows for only five-phase terminals. Moreover, the proposed winding not only yields better flux distribution compared to a conventional single layer winding, but also provides a complete cancellation of the third-order harmonic flux component caused by the induced third sequence currents due to the saturation effect and/or under unbalanced operation. Hence, the machine losses are decreased, which improves the overall machine efficiency. For the healthy case, the machine is similar to a conventional star connected five-phase machine. However, with one phase open, the proposed connection results in a lower derating factor compared to conventional connections for both open-loop and optimal current control techniques. A 1-kW prototype machine is used for experimental verification.
This paper proposes a new winding layout for high-power medium-voltage nine-phase induction machines (IMs) based on a single-layer concentrated winding layout having a unity winding factor. The ...machine is fundamentally an asymmetrical nine-phase IM, where phases are connected in such a way as to provide six terminals that are fed from two three-phase inverters. Compared to a conventional asymmetrical six-phase IM with the same stator and copper volumes, it provides improved torque density, a higher torque/current ratio, and a simpler winding layout. Finite-element simulation is used to compare the proposed winding layout with a conventional split-phase six-phase IM to assess the claimed merits. A 1.5-hp prototype IM is also used for experimental verification. The experimental results are given under both healthy and fault conditions, where the faulty converter is completely disabled. The achievable derating factors under this case are then given and compared with those of conventional six-phase IMs.
This paper presents an algorithm that enables single-sensor operation of vector-controlled three-phase permanent-magnet-synchronous-motor voltage source inverter (VSI)-based drive systems. The ...closed-loop operation is performed through measuring only the VSI dc-link current. The measured VSI dc-link current is fed to a space-vector-based lookup table, which reconstructs the motor line currents. Three Luenberger-style observers are used to filter out the reconstructed motor line currents and estimate the rotor position and speed. The effect of observer bandwidth on the accuracy of the motor's estimated parameters is also investigated. The drive system's physical dynamic stiffness is used as a key metric to compare the drive system performance in single-sensor mode and fully sensored mode, in order to find the lowest steady-state speed limit for the single-sensor drive system operation.
•A new DC-hub is proposed for interconnection of VSC-based HVDC ports based on thyristor device.•Zero-reactive power control across the common AC link is proposed to achieve reduced RMS ...current.•Comprehensive mathematical analysis is illustrated for circuit design parameters for the DC-Hub under study.•Transformation from Sinusoidal to Quasi-Trapezoidal to accommodate the utilization of thyristor based DC-Hub.•Small scale hardware setup is built to test the proposed DC-Hub under different healthy and unhealthy scenarios of power flow.
The promising features of HVDC technology have led to the possibility of numerous renewable resources integration and enormous DC grids interconnection. In spite of the obstacles, these interconnections encounter such as the necessity to block DC faults, achieving isolation between different schemes, the ability to maintain power flow throughout different power flow profiles, and the interfacing with various infrastructures, the DC-Hub arises to overcome these interconnection obstacles being the excellent approach to enhance the DC grid capabilities. This paper proposes a new monolithic modular thyristor-based multilevel converter, which serves as the fundamental building block of the DC-Hub, offering advantages such as lower switch count, bidirectional power flow, and DC fault blocking capability. Moreover, a control algorithm, for zero reactive power circulation in the DC-Hub, is introduced. The proposed algorithm successfully mitigates the circulation of reactive power throughout the entire range of power flow. A comprehensive mathematical analysis, optimum design of converter parameters, and the proposed control technique, which suppress the circulating reactive power at full range of power flow, are illustrated. Finally, simulation modelling and hardware test rig are established to validate the claims of the DC-Hub at different normal and faulty scenarios.
