Mechanical damage of rotating shafts has been reported for several years from various high-power applications. This paper shows that the variable frequency drive incorporated in a rotating shaft is one of the main root causes of mechanical-shaft failures. Simple analytical relationships show that the frequencies of the motor air-gap torque have a more significant impact on the mechanical-shaft failure than their magnitudes. Effects of mechanical damping are analytically derived and analyzed. Motor air-gap torque is successfully reconstructed using only the motor's voltage and current, thus avoiding torque sensors, which are subject to failure and errors. Simple relationships between frequencies of current harmonics and frequencies of motor pulsating torques are proposed. For pulsewidth-modulated inverters (two and multilevel), possible drive operating points that might excite the shaft's eigenmodes are predicted. Simulation results of four interleaved three-level neutral-point-clamped converters are analyzed for validation purposes. Experimental tests up to 35 MW are performed on a compressor test bed. The presented results confirm the accuracy of the proposed approach, which is particularly valuable for multimegawatt drive applications.

Current source thyristor converters are the most widespread technology for large drives, and today still a suitable choice for supplying high-power variable speed drives because of excellent reliability records. The integer and noninteger harmonics generated by line-commutated converters cause pulsating torque harmonics on the motor and on the grid side of the converter. Intersections of harmonic excitation frequencies with torsional natural frequencies of mechanical drive trains (motor-driven load or generator train) cannot always be avoided in the operating-speed range of the motor. Continuously generated harmonic torque excitations could have a critical impact on the torsional behavior of the entire train. It is therefore mandatory to perform torsional analyses during the design stage of large drives, as specified in API 617 [1] to avoid torsional interaction issues. In multiunit plants, more sophisticated analyses have to be performed, considering the fully coupled electrical and mechanical system. The risk for operational problems increases with increasing percentage of converter loads (converter power relative to the short circuit level of the grid). Some oil and gas production sites, e.g., offshore platforms, are based on island or island-like power systems. To mitigate the risk of torsional issues with increasing percentage of converter loads in weak power systems, new devices for damping torsional resonance modes have been developed and successfully tested. The effect of torsional mode damping will be explained by coupled electromechanical simulations and by measurement results from applying an integrated torsional mode damper (TMD) to a 30 MW gas compression train. The TMD does not require changes to the mechanical or electrical system design and can be designed as a retrofit control system extension for variable speed drives from different manufacturers.

This paper presents new, simple, and linear mathematical relationships to predict pulsating torque frequencies in industrial multi-megawatt pulse width-modulated (PWM) drives. The proposed relationships provide direct definition of possible drive operating points where the torsional resonance modes of the mechanical shaft assembly might be excited. The calculation accuracy of natural frequencies and the induction motor slip have been taken into account in the definition of a critical excitation band. Variable switching frequency PWM strategies have also been analyzed. Pulsating torque frequencies of complex load-commutated-inverter configurations have been proposed as complementary results. Several tests have been performed on an integrated-motor-compressor system operating at high-speed (3.3 kV, 6 MW at 10 000 r/min). Intensive simulation results of a higher power PWM drive for compressor applications are shown to validate the suggested approach. This paper is a key tool for electrical engineers involved in the design of multi-megawatt drive systems such as in liquefied natural gas compression and transportation, mining, or cements.

This paper proposes an approach to control a photovoltaic (PV) cascaded H-bridge multilevel inverter with failed cells and changing meteorological conditions for large-scale grid-connected applications. The controller development is based on an analysis of the interaction between the inverter common-mode and differential-mode quantities, which is done in the time domain, supported by a space vector representation analysis. The proposed approach is able to produce balanced three-phase line-to-line voltages and currents even if there is a failed cell or if the fluctuating meteorological conditions cause uneven power distribution among the PV panels. This is achieved through the modification of the PWM reference phase voltage angles combined to the injection of a dynamic homopolar component. Such approach avoids tripping the system due to the protective functions #25 and #87 for per grid applicable codes and standards. Numerical simulations and laboratory experiments performed on a 7-level converter with different abnormal conditions to confirm the effectiveness of the suggested control strategy.

This paper proposes a generalization of the model-free predictive current controller (MFPCC) for multilevel voltage source inverters (VSIs). MFPCC is an alternative to mitigate the parameter sensitivity faced by model-based PCC (MPCC), but it requires a constant update of the stored current variations (CVs) associated with each VSI state to provide a prediction which results in a satisfactory closed-loop response. When a multilevel VSI is considered the number of states increases, which results in a decrease of the CV update rate, since only one state can be updated at each sample. To solve this issue, the extended adjacent state scheme is used to reduce the possible number of solution candidates and then a controlled CV is used to compensate the updated CV. Simulation and experimental results obtained with a sampling rate of 5 kHz show that the proposed MFPCC exhibits performance similar to the one of MPCC for nominal parameters. However, a better current response is obtained in the case of the load parameter mismatch, especially at steady state.

This paper provides a methodology to estimate power versus distance envelops of high voltage alternating current (HVAC) transmission systems for subsea applications. Alternating current (AC) technology is mature and is proven for land-based applications and relatively short offshore tiebacks. However, for long tiebacks, due to increased conductor capacitance, large reactive power needs to be supplied, leading to higher cable current rating, losses, and expensive umbilical. Subsea ac cables are limited in their capability to transmit power beyond a certain distance, depending on cable characteristics, installation conditions, and system operating mode. Power transfer boundary charts for ac technology based on analytical methods are presented. Analytical calculations and computer simulations of HVAC power cables were performed on more than 30 cables with rated voltage between 35 and 150 kV and cable cross sections between 95 and 400 mm ² . Effects of reactive compensation on system tieback distance have been analyzed, as well as low-frequency transmission and different modes of system operation. Cable models employing multiple pi sections and distributed parameters were used. Well-known power flow and simulation tools were used for validation. AC transmission boundaries were estimated based on the voltage and current limits for various cables under various operating modes.