Voltage-source converter (VSC) technologies are rapidly evolving and increasing the range of applications in a variety of fields within the power industry. Existing two- and three-level VSC technologies are being superseded by the new modular multilevel converter (MMC) technology for HVDC applications. The computational burden caused by detailed modeling of MMC-HVDC systems in electromagnetic transient-type (EMT-type) programs complicates the simulation of transients when such systems are integrated into large networks. This paper develops and compares different types of models for efficient and accurate representation of MMC-HVDC systems. The results show that the use of a specific type of model will depend on the conducted analysis and required accuracy.

This paper presents an average-value model of a line-commutated converter-based HVDC system using dynamic phasors. The model represents the low-frequency dynamics of the converter and its ac and dc systems, and has lower computational requirements than a conventional electromagnetic-transient (EMT) switching model. The developed dynamic-phasor model is verified against an EMT model of the CIGRE HVDC Benchmark. Simulation results confirm the validity and accuracy of the average value model in predicting the low-frequency dynamics of the ac- and dc-side quantities. Merits and applicability limitations of the average model are highlighted.

Virtual flux (VF)-oriented control (VFOC) and VF-based direct power control (DPC) (VF-DPC) have been developed to improve voltage-oriented control and DPC of three-phase active rectifiers. The VF space vector is utilized in transformations between stationary and rotating coordinates in VFOC and in obtaining instantaneous power in VF-DPC. The VF space vector is calculated by integrating the grid voltage space vector. This integration is usually performed using a first-order low-pass (LP) (FOLP) filter, which counteracts the saturation and dc-drift problems associated with pure integrators. However, the dynamics of FOLP filters can be enhanced to a great extent. This paper presents a new, simple, and fast integration algorithm for VF-based control methods. Simulations and experimental tests on a VF-DPC-based system showed that the proposed algorithm leads to rapid recoveries after grid voltage sags occur. Moreover, the performance of VF-DPC under nonideal grids is discussed.

Power transformers figure to be amongst the most costly pieces of equipment used in electrical systems. A major research effort has therefore focused on detecting failures of their insulating systems prior to unexpected machine outage. Although several industrial methods exist for the online and offline monitoring of power transformers, all of them are expensive and complex, and require the use of specific electronic instrumentation. For these reasons, this paper will present online analysis of transformer leakage flux as an efficient alternative procedure for assessing machine integrity and detecting the presence of insulating failures during their earliest stages. A 12-kVA 400-V/400-V power transformer was specifically manufactured for the study. A finite-element model of the machine was designed to obtain the transient distribution of leakage flux lines in the machine's transversal section under normal operating conditions and when shorted turns are intentionally produced. Very cheap and simple sensors, based on air-core coils, were built in order to measure the leakage flux of the transformer, and nondestructive tests were also applied to the machine in order to analyze pre and post failure voltages induced in the coils. Results point to the ability to detect very early stages of failure, as well as locating the position of the shorted turn in the transformer windings.

High-voltage direct-current (HVDC) systems play an important role in modern energy grids, whereas efficient and accurate models are often needed for system-level studies. Due to the inherent switching in HVDC converters, the detailed switch-level models are computationally expensive for the simulation of large-signal transients and hard to linearize for small-signal frequency-domain characterization. In this paper, a dynamic average-value model (AVM) of the first CIGRE HVDC benchmark system is developed in a state-variable-based simulator, such as Matlab/Simulink, and nodal-analysis-based electromagnetic transient program (EMTP), such as PSCAD/EMTDC. The 12-pulse converters in the HVDC system are modeled with a set of nonlinear algebraic functions that are extracted numerically. The results from the average-value models are compared with the results of the detailed simulation to verify the accuracy of the AVMs in predicting the large-signal time-domain transients. The developed dynamic average models are shown to have computational advantages.

Different techniques have been developed to detect rotor asymmetries in asynchronous motors. Although the reliability and the amount of information these techniques provide about a machine's state are indubitable, they still have a serious limitation: failure detection when the motor is driving a variable load torque. In this case, the motor phase currents are modulated by torque oscillations, and the information they contain about the integrity of bars and end rings is altered. This paper reports a new diagnostic method, based on the measurement of the magnetic flux linked by one stator tooth, which allows perfect simple discrimination between the actual presence of rotor asymmetries and the spurious effects caused by the oscillations in the load torque off the driven machine even when it is part of an adjustable speed drive and the motor is operating under a variable supply frequency.