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This article develops a new closed-loop analytical steady-state model of low-capacitance half-bridge Modular Multilevel Converters (MMCs) operated with the average arm energy control and Sub-Module Capacitor Voltage (SMCV) ripple above the conventional value of 10%. In this control approach the level of average SMCVs is regulated directly. As has been already shown in the literature, if the average SMCVs are not controlled, the circular interactions of the inherent low-frequency SMCV ripple and the modulation signal which affect the dc arm voltages will also lead to the variations in the average SMCVs. In order to compensate for these variations a dc component has to be introduced in the modulation signal (in addition to the 2nd harmonic which suppresses the 2nd order circulating currents). This is included in the proposed analytical modelling where the three key non-linear equations are established and solved iteratively using Newton Raphsons method to calculate the steady state modulation signal parameters. The effects of the introduced dc component in the modulation signal on the MMC electrical quantities are demonstrated and analyzed. The developed model is also compared with the existing analytical steady-state MMC models in the literature. It is further employed to optimize the parameters of the grid connected MMC. The optimization is based on the improved utilization of the available modulation signal window and is performed with the aim to minimize the voltage stresses on the power devices. The analysis is supported by simulation and experimental results.

This paper develops a new closed-form analytical solution to the harmonic spectrum of the Pulse Width Modulated (PWM) output voltage of the single-phase inverter connected to the dc-bus with considerable voltage ripple. The solution is based on a double Fourier series expansion in two variables. As the single phase inverter forms the basic building block of most converters, the developed expressions can be applied to various topologies. This study first identifies the interactions of a single-frequency sinusoidal modulation signal and the carrier signal with the dc-bus voltage harmonics. The dc-bus voltage harmonics are constrained in this work to multiples of the inverter output voltage fundamental frequency. The analysis is then extended to include a multi-frequency modulation signal. Starting from the general solution, new expressions for the output voltage harmonics of the Modular Multilevel Converter (MMC) are developed. The low-capacitance MMC is chosen in this analysis due to inherent low-frequency voltage oscillations in the converter internal capacitors. The MMC analytical harmonic spectrum can incorporate the effects of the circulating current control and third harmonic injection PWM, by including the second and third order harmonic components in the modulation signal, respectively. The MMC analytical harmonic spectrum is benchmarked against the experimental results.

The design of power electronic converters for the integration of wind generated power into the\ud grid is more and more important due to a new class of Superconducting Generators (SG) with power\ud ratings of up to 20 MW. High efficiency of power converters for high power applications is mandatory in\ud order to reduce the overall cost of the system. This paper proposes a design method to minimise the cost of\ud the system by finding the optimal number of power devices and capacitors for different high power\ud converter topologies. The investigation focuses on determining the optimal number of voltage levels for a\ud Back-To-Back (BTB) Neutral Point Clamped (NPC) converter. The design method is demonstrated by\ud estimating the cost of different BTB NPC power converter topologies for the integration of a 10 MW SG\ud to the grid.

This paper studies the peak armature and peak field winding currents for three different topologies of 10 MW partial High Temperature Superconducting Generators (HTSGs) under Short-Circuit Conditions (SCC) by simulation. The investigated partial HTSGs employ copper armature windings and superconducting field windings with different armature and rotor topologies, i.e. iron cored armature and rotor, air cored armature and rotor, and iron cored armature and air cored rotor. For each HTSG topology, the investigation includes: (i) the field winding current control strategies, (ii) the influence of operating field current, and (iii) the ratings of circuit breakers for limiting the peak armature and peak field winding currents. The results can provide guidelines for determining the peak armature and peak field currents of HTSGs and also the possibility of limiting them by employing circuit breakers under SCC.

This paper studies the peak armature and peak field winding currents for three different topologies of 10 MW partial high temperature superconducting generators (HTSGs) under short-circuit conditions (SCC) by simulation. The investigated partial HTSGs employ copper armature windings and superconducting field windings with different armature and rotor topologies, i.e., iron-cored armature and rotor, air-cored armature and rotor, and iron-cored armature and air-cored rotor. For each HTSG topology, the investigation includes: 1) the field winding current control strategies; 2) the influence of operating field current; and 3) the ratings of circuit breakers for limiting the peak armature and peak field winding currents. The results can provide guidelines for determining the peak armature and peak field currents of HTSGs and also the possibility of limiting them by employing circuit breakers under SCC.\ud

This paper proposes an simplified instantaneous\ud output power control to suppress the second harmonic DC bus\ud voltage due to asymmetry in a three-phase PWM converter\ud system without any sequential component decomposers. Normally,\ud in the instantaneous output power control, the positive and\ud negative output voltages and currents are required and they are\ud usually decomposed by notch filters or dual second-order\ud generalized integrals that are difficult to tune and complicated.\ud While in the proposed method, the positive-sequence and\ud negative-sequence currents are regulated by PI plus resonant\ud (PI-R) controllers in dq-frame. Therefore, the sequential current\ud decomposers can be avoided. Meanwhile, the positive and\ud negative sequence output voltages, which are essential for\ud calculating the positive-sequence and negative-sequence current\ud references, are simply obtained from the outputs of the PI and\ud resonant controllers, respectively. The proposed method is robust\ud to any asymmetries and its effectiveness is verified on a prototype\ud asymmetric three-phase permanent magnet synchronous\ud generator (PMSG) system with inherent and externally added\ud asymmetries.