This paper investigates a static synchronous compensator (STATCOM) based on a nine-leg converter topology consisting of three conventional three-phase voltage source inverters (VSIs). By providing a high-quality multilevel output voltage, the nine-leg converter offers an interesting alternative to multilevel converters, which are often complicated and expensive. The three-phase grid-connected system enables the generation of positive sequence components for reactive power compensation. In addition to the positive sequence, the nine-leg topology also allows the generation of the zero-sequence component (ZSC) for voltage unbalance compensation. The system incorporates a special five-leg transformer for homopolar magnetic flux. The performance of the system and the proposed algorithms, including compensation and DC-link voltage balancing, have been validated experimentally on a laboratory prototype.

This paper investigates the utilization of the zero-sequence component (ZSC) to increase the voltage and enhance the operation area of five-phase electric drives with permanent magnet synchronous machines (PMSM). Although the ZSC allows reaching higher voltage, its application in multiphase drives is constrained by the usage of x-y planes activated by harmonic components. Since some odd harmonics allow torque enhancement, these planes are typically involved in voltage and current control as part of field-oriented control strategies (FOC) to maximize the torque while minimizing the losses (e.g., Joule losses in the Maximum Torque per Ampere strategy). In five-phase systems, the third harmonic is used for torque enhancement, which affects the maximum value and shape of the resulting voltage waveform across the torque-speed map. Consequently, the space available for ZSC injection is limited and depends on the actual operation point. This paper presents an analysis of how ZSC can be leveraged within these constraints to improve the torque-speed operating map of a five-phase PMSM drive, especially in the field-weakening region. Experimental results obtained on a laboratory five-phase PMSM setup validate the theoretical findings and demonstrate the practical benefits of the proposed approach.

Maximum torque per current (MTPA) and other optimal curves are well-known for three-phase machines. Since multiphase machines dispose by more degrees of freedom than their three-phase counterparts, the solution is much more complicated and a variety of regimes can be found. In this paper, optimal current setpoints for five-phase synchronous machines are analyzed. Besides the first harmonic, the optimization includes also the third harmonic, used for both torque enhancement and waveform shaping. It is shown that the MTPA curves, field weakening (FW) and maximum current (MC) regions have an inner structure with multiple regimes. This variety comes from the phase voltage and phase current limits that yield optimizing waveforms containing multiple peaks (peak shaving). A taxonomy of these regimes is provided, and their mutual relations are demonstrated on a case study machine. The optimal current setpoints in their waveforms are analyzed in selected operating points.

This paper deals with an experimental injection of harmonics to a stator winding of a nine-phase machine. Different combinations of the harmonic components are applied with the optimal magnitudes calculated by a linear programming to maximize exploitation of a magnetic circuit. The theoretical analysis provides magnetic flux waveforms including distribution in the air gap. Thanks to the prototype of the nine-phase induction machine with in-built test coils wound on the stator tooth and the magnetic pole, the magnetic flux waveforms calculated in the theoretical analysis are verified by those obtained by the experimental measurements.

A carrier-based PWM technique proposed for open-end winding drives, which are fed from dual three-phase inverters, is discussed in the paper. The proposed method is based on minimization of infinity norm of a searched vector containing instantaneous modulation values, entering PWM blocks of both inverters. Due to the optimization criterion, the proposed technique is able to generate optimized modulation signals ensuring high utilization of the de-link voltage and achieve comparable performance to classical SVPWM in the normal operation mode. However, besides the normal mode, the method discussed in the paper is designed for attractive fault tolerant operation mode which enables to significantly increase robustness and reliability of an electric drive.

Recently, arc suppression coils are often completed by active systems connected to the secondary winding such as ground fault neutralizers, active earthing and fault localization systems, or systems for current injection. These systems often operate with quantities related to the primary winding. However, direct measurement on the high-voltage side is often not possible. This paper deals with a frequency-independent mathematical modeling of arc suppression coils (Petersen coils) which would enable to simply recalculate quantities from the secondary to the primary side of the coil. Unlike traditional transformers, mathematical modeling is more complicated due to significant difference between high-voltage primary and low-voltage secondary side, high leakage flux, variable coil inductance and variable magnetic circuit topology caused by tunable air gap.

This paper presents a new pulse width modulator dedicated basically for under determined systems. The solution of the under determined problem is performed by minimum 1-inflnity norm solution. One of known under determined system is a four-leg three-phase converter because four reference signals must be created from three control commands. The infinity norm enables to find proper solution to generate corresponding reference signals for the PWM modulator. The designed modulator is implemented with a control system of the four-leg three-phase converter working as an universal supply unit but the proposed modulator is able to operate in arbitrary application with the four-leg converter, generally for arbitrary under-deteremined systems. It is shown in the paper that the known carrier based PWM algorithm represents a subset of modulators utilizing the minimum 1-infinity norm. The theoretical assumptions and the design of the modulator were verified by simulations as well as by experiments on a laboratory prototype of 17 kVA.

This paper proposes a novel PWM for four-leg voltage-source converter operating as an earth fault current compensator in power grids with unearthed or high-impedance earthed neutral. For proper earth fault compensation, the converter has to draw zero-sequence currents from the three-phase grid. Its control is based on three independent proportional-resonant controllers. The main task of a new modulator is to generate from three outputs coming from the control block four modulation signals entering the PWM. In order to minimize a dc-link voltage of the converter, the least norm method is used to eliminate high difference of amplitudes of the modulation signals.

This study proposes a 4-phase voltage-source inverter replacing conventional 3-phase inverters, traditionally used in renewable energy resources, which is able to compensate a fault current, when a single-phase earth fault of the power grid occurs. The investigated method is able to suppress the earth fault current and recover current symmetry in 3-phase insulated or high-impedance grounded power grids. Proposed compensation is made by the 4-phase voltage-source inverter with earthed 4thphase, which can operate, under normal operation condition, equally as a traditionally used 3-phase inverter and supply the ac power grid from the renewable resource. The single-phase earth fault in the power grid is analyzed in the paper. Both simulations and experiments were performed on a mathematical model and on a designed laboratory prototype, respectively, in order to verify the proposed compensation method.