https://eprints.ncl.ac.uk/file_store/production/222574/E2E14D0B-B28F-4DAA-80C1-EF8847AA07D4.pdf

Investigating a conventional and retrofit power plant on-board a Roll-on/Roll-off cargo ship from a sustainability perspective – A life cycle assessment case study

In many rural areas, due to unavailability of three-phase AC power, three-phase Variable Frequency Drives (VFDs) are often required to operate from single-phase AC source. Powering up three-phase VFDs from single-phase AC source requires addressing many issues - higher rms value of input diode current, higher ripple voltage across the DC bus capacitor, higher peak input current, higher input current distortion, lower power factor, and poorer system efficiency. All of this leads to severe de-rating of VFDs for single-phase applications. A new low-cost, single-phase active circuit based on injecting current into the mid-point of the DC bus employing only one bidirectional switch is proposed here. The bidirectional switch carries current for less than a quarter-cycle and is rated accordingly. The presence of a three phase diode bridge rectifier lends itself better to distribute the higher input rms current so that the diodes need not carry current higher than their nominal rating. The proposed method shows that VFDs need not be de-rated to achieve rated output power. Test results from a 15hp, 230V system are given to prove the concept.

An improved single-phase active front end rectifier system for use with three-phase variable frequency drives

In many rural areas, due to unavailability of three-phase ac power, three-phase variable-frequency drives (VFDs) are often required to operate from a single-phase ac source. Powering up three-phase VFDs from single-phase ac source requires addressing many issues: higher RMS value of input diode current, higher ripple voltage across the dc bus capacitor, higher peak input current, higher input current distortion, lower power factor, and poorer system efficiency. All of these lead to severe derating of VFDs for single-phase applications. A new low-cost single-phase active circuit based on injecting current into the midpoint of the dc bus employing only one bidirectional switch is proposed here. The bidirectional switch carries current for less than a quarter cycle and is rated accordingly. The presence of a three-phase diode bridge rectifier lends itself to better distribute the higher input root-mean-square current so that the diodes need not carry current higher than their nominal rating. The proposed method shows that VFDs need not be derated to achieve rated output power. Test results from a 15-hp 230-V system are given to prove the concept.

An Improved Single-Phase Active-Front-End Rectifier System for Use With Three-Phase Variable-Frequency Drives

In this master’s thesis, a direct current (DC) hybrid energy system for a liquified natural gas (LNG) ferry is developed and studied. The system is constructed in Matlab and Simulink and consists of two generator sets with gas engines and synchronous generators, six-pulse diode rectifiers, battery storage, bi-directional DC/DC converter and a load. A control structure for the gas engine, excitation system of the generators and battery converter is developed. The gas engine is modelled with the GAST model and a speed governor, while the excitation system is modelled with the AC1A excitation model and droop control with respect to the DC link voltage. Two control methods are developed for the control of the converter, droop and peak shaving. The performance of the controllers and system is tested for generator sets with droop control, generator sets and battery with droop control and peak shaving with generator sets and battery. The specific energy consumption (SEC) of the gas engines is then calculated for fixed and variable speed during stationary operation with respect to the engine speed and engine load. The SEC values are compared for the cases and with respect to variable and fixed speed. Variable speed resulted in lower SEC for all cases presented, and the highest difference was for the case with the peak shaving method. For the droop control configuration between the generator sets and the battery, the SoC limits of 80-65 % was violated with a δB at equal 0.05. Then, the options for the system were to increase δB at and reduce the contribution from the battery or install a larger battery that is able to contribute the load power without violating the SoC limits. With the peak shaving method, stored power was available in the battery since the SoC was in the range of 80-76 %. With the discharge and charging power (±1800 kW) delivered from the battery, a smaller battery with a capacity of 720 kWh could obtain the same operation. An opportunity to reduce the SEC of the operation is to develop a control system for connecting and disconnecting generators sets, which makes the generator sets operate at a more optimal loading rate.

Modeling and operation of hybrid ferry with gas engine, synchronous machine and battery

Variable frequency drives (VFDs) draw discontinuous current from the power system resulting in current distortion and consequently voltage distortion. Limiting current distortion to levels recommended in IEEE 519-1992 improves system efficiency, increases reliability, and limits voltage distortion that can adversely influence sensitive loads. This paper revisits a non regenerative active front end system that has been studied in the past but offers significant improvements to make it acceptable to the drives industry. A three-phase current injecting inductor, required in the topology, is constructed in a five limb fashion and is shown to be helpful in limiting non characteristic harmonics. Since the current injected into the split dc bus capacitor is a 360Hz ripple, it is proposed that the main dc bus capacitor be split into film capacitor for ripple current and optimally reduced electrolytic capacitor for the main energy storage requirements. Experimental results are given based on the suggested improvement and low distortion levels that were not achieved in the past have been shown to be attainable. Comparison between a standard three-limb inductor and a five-limb inductor is made to highlight the performance improvements achievable. Adoption of the topology for common dc bus applications is also proposed.

Mahesh Swamy

Papers

An improved active front end non-regenerative rectifier system employing a five-limb inductor