Christof Martin Sihler

Damping Torsional Interharmonic Effects of Large Drives

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.

Torsional Mode Damping For Electrically Driven Gas Compression Trains In Extended Variable Speed Operation.

The oil and gas industry has a growing demand for electrically driven trains operated at variable speeds. Variable frequency electrical drives enable increased operational flexibility and energy efficiency. This is of great importance in applications requiring high power, such as gas compression. Load commutated inverters (LCIs) are one of the most widespread technologies for driving large gas compression trains because of excellent reliability records. One drawback of power electronics driven systems is the generation of nonfundamental air-gap torque ripple components due to electrical harmonics. The air-gap torque ripple can interact with the mechanical system at torsional natural frequencies of the drive train. Torsional vibration is an oscillatory angular motion that causes alternating twisting in shaft sections and machinery couplings. A consequence of uncontrolled excited torsional vibration may be a protective trip of the motor, to prevent mechanical damage, such as a failed coupling or a broken shaft. This paper discusses illustrative design details of applying a torsional mode damping control system to LCI driven multi-Megawatt centrifugal gas compressors. The coincidence of electrical drive harmonics and torsional natural frequencies of the mechanical system is sometimes unavoidable due to the large variable speed range of the compressor such as for process requirements. For these types of applications, a power electronic damping system technology can be applied to new units or as a retrofit solution to existing variable speed trains. The so-called integrated torsional mode damping (ITMD) unit is based on a torsional vibration measurement in the mechanical system and an interface to the existing inverter control of the drive system. The dc-link inductor of the LCI is partially used as an integrated energy storage unit and is combined with a smart damping controller, which reacts to a torsional vibration by modulating a small amount of the stored energy and sending it to the motor without impacting the normal operation of the system. As a result, the active power modulation at a torsional natural frequency of the mechanical system has a strong damping effect for torsional vibrations. Intensive simulations and several tests were performed on large LCIs (up to 50 MW) over the last three years. Selected experimental results will be presented and discussed to validate the suggested

A General Approach of Damping Torsional Resonance Modes in Multimegawatt Applications

In large-drive applications, load-commutated inverters (LCIs) are one of the most used technologies mainly because of their excellent reliability records. However, LCIs are known to generate interharmonics. They can interact with the mechanical system at torsional natural frequencies of the rotating train, on both the inverter and rectifier sides in weakly connected power systems, such as offshore oil and gas platforms. These interactions can lead to accelerated shaft fatigue, lifetime reduction, gear damage, and system blackouts. On the other hand, pulsewidth-modulated (PWM) voltage-source inverters (VSIs) are known to produce less torque ripple compared to LCIs. Consequently, VSIs are supposed to be less subject to exciting torsional resonances in mechanical shaft systems. Even with reduced torque ripple, mechanical failures have been consistently reported due to motor air-gap torques supplied by PWM drives. This paper is focused on solving torsional vibration issues. Due to system uncertainties, these issues cannot be excluded in the design phase of LCIs and VSIs for high-power applications. With regard to LCIs, the dc-link inductance is used as an integrated energy-storage unit. Motor and generator interactions are therefore decoupled. Excited eigenmodes on the grid side are damped with the rectifier; the inverter is driving the variable-speed motor. This approach is also successfully applied to damping resonance modes in the motor side. Simulation and selected experimental results on a 30-MW LCI system are provided to validate the proposed design approach. As for VSIs, a more general approach of damping or controlling excited shafts is proposed. This method is successfully applied using the dc-link capacitor as an integrated energy-storage source. This approach is used to optimize the design of new systems; otherwise, it is used to improve the performance of existing systems with minor modifications.

Modeling of Torsional Resonances for Multi-Megawatt Drives Design

Several torsional issues have been reported in drive train applications such as oil and gas or mining, where the drive rated power exceeds the megawatt range. This paper analytically explains why mechanical engineers worry about this phenomenon in such applications. Simple and comprehensive relationships are established, and electrical and mechanical similarities are pointed out. The proposed model can be easily implemented in common simulation software using passive electrical elements. This paper is a key design tool to support torsional analysis of large drive trains. It has been written to avoid communication gaps between mechanical and electrical engineers involved in the design of complex drive trains. Several tests have been performed in an integrated-motor- compressor system (5.5 MW at 10,000 rpm) using the proposed modeling approach. Only intensive simulation results are reported in this paper, in order to validate the suggested modeling approach.

Damping torsional interharmonic effects of large drives

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 for pumps and compressors because of excellent reliability records.

A general approach of damping torsional resonance modes in multi-megawatt applications

In large drive applications load-commutated inverters (LCIs) are one of the most used technologies, mainly because of their excellent reliability records. However, LCIs are known to generate interharmonics. They can interact with the mechanical system at torsional natural torsional frequencies of the rotating train, both on the inverter and on rectifier side in weakly connected power systems, such as offshore oil and gas platforms. These interactions can lead to accelerated shaft fatigue, lifetime reduction, gear damage and system blackouts.

A Modular Subsea Direct-Current Electrical-Power System

Summary Subsea processing has been increasingly accepted by the offshore oil and gas industry as a solution to boost production and reduce cost. Accordingly, the subsea power demand is growing to support various processing loads, including pumps and compressors. Depending on the application, the power rating of a field ranges from tens of kilowatts to tens of megawatts, and the step-out distance ranges from a few kilometers to hundreds of kilometers. Considering the hostile and remote environment, a reliable subsea electrical-power system that is suitable for subsea deployment is clearly desired. This paper presents a modular direct-current electrical-power system that is designed for use in a subsea field with medium or long step-out distance. The proposed system consists of multiple modular converters in the subsea station to achieve the required power-conversion functions. It features high reliability, high flexibility, and reduced installation weight. The system operation and protection are presented, and the performance is verified by a laboratory-scale demonstration.

Electronic Torsional Vibration Elimination for Synchronous Motor Driven Turbomachinery

The oil and gas industry has a growing demand for electrically driven trains operated at variable speeds. Variable frequency electrical drives enable increased operational flexibility and energy efficiency. One drawback of power electronics driven systems is the generation of non-fundamental air-gap torque ripple components due to electrical harmonics. The air-gap torque ripple can interact with the mechanical system at natural torsional frequencies of the drive train. Uncontrolled excited torsional vibration can silently lead to coupling failure due to fatigue. The coincidence of electrical drive harmonics and natural torsional frequencies of the mechanical system is sometimes unavoidable, due to the large variable speed range of the compressor as for process requirements. For those types of applications, a damping system utilizing available power electronics has been developed that can be applied to new units but also as a retrofit solution in existing variable speed trains. Electronic torsional vibration elimination (eTVe) is based on an angular vibration measurement in the mechanical system and an interface to the existing inverter control of the electrical drive. An important milestone of the eTVe development was achieved in 2010, in site testing this new solution to Liquid Natural Gas (LNG) production trains and demonstrating that it can completely eliminate torsional vibrations. With eTVe a residual torsional vibration level was achieved that was lower than the vibration level measured while the LNG train was only gas turbine driven. This torsional performance was achieved with a standard load commutated inverter drive (LCI). LCIs are one of the most widespread electrical drive technology for gas compression trains because of excellent reliability records, and it is the only one referenced solution for electric power larger than 45 MW.© 2011 ASME