Management of inland capture fisheries and challenges in fish production in Nigeria

The paper critically examines the trend in fish production in Nigeria. The problem of excessive mismanagement and lack of attention by relevant agencies are still common place in inland water bodies. The paper discusses these mismanagement practices which are non compliance with the existing rules and regulations on good fishing methods, uncontrollable, unorthodox and obnoxious fishing practices, destruction of the natural breeding grounds and the collapse of the fishery due to massive over fishing. The challenges posed by the fishing methods as well as the effect of different gears and mechanization of fishing crafts on fish production are discussed. The paper recommends ways to increase domestic fish production in inland water bodies, which include a well planned strategy of restocking the existing reservoirs after careful scientific study, enforcement of the existing laws and regulation based on community participation. Training of stakeholders on the code of practice for responsible fisheries (CPRF), extension of subsidies to fisher folks, the traditional practices, which encourage the adherence to close season and other fish conservation and utilization strategies, are also advocate

Sensorless multi-loop control of phase-controlled series-parallel resonant converter

This paper proposes a multi-loop controller for the phase-controlled series-parallel resonant converter. Output voltage is solely measured for control and inner loop is used to enhance closed loop stability and dynamic performance compared to single-loop control. No additional sensors are used for inner loop variables. These are estimated using a Kalman filter, based on a linearized converter model. The advantage of this sensorless scheme is not only reducing the number of sensors but more significantly providing an alternative to sensing high frequency resonant tank variables which require high microcontroller resolution in real time. First, the converter non-linear large signal behavior is linearized using a state feedback based scheme. Consequently, the converter preserves its large signal characteristics while modeled as a linear system. Comparison is made between the most suitable state variables for feedback, according to a stability study. Finally, simulation and experimental results are demonstrated to validate the improved system performance in contrast with single-loop control

Modified half-bridge modular multilevel converter for HVDC systems with DC fault ride-through capability

One of the main challenges of voltage source converter based HVDC systems is DC faults. In this paper, two different modified half-bridge modular multilevel converter topologies are proposed. The proposed converters offer a fault tolerant against the most severe pole-to-pole DC faults. The converter comprises three switches or two switches and 4 diodes in each cell, which can result in less cost and losses compared to the full-bridge modular multilevel converter. Converter structure and controls are presented including the converter modulation and capacitors balancing. MATLAB/SIMULINK simulations are carried out to verify converter operation in normal and faulty conditions

Analysis and hardware testing of cell capacitor discharge currents during DC faults in half-bridge modular multilevel converters

This paper focuses on the behaviour of the cell capacitor discharge currents during DC faults in half-bridge modular multilevel converters. Active switches, not designed for fault conditions, are tripped to minimize discharge currents effect on the semiconductor switches. Two levels of device protection are commonly in place; driver level protection monitoring collector-emitter voltage and overcurrent protection with feedback measurement and control. However, unavoidable tripping delay times, arising from factors such as sensor lags, controller sampling delays and hardware propagation delays, impact transient current shape and hence affect the selection of semiconductor device ratings as well as arm inductance. Analytical expressions are obtained for current slew rate, peak transient current and resultant I2t for the cell capacitor discharge current taking into account such delays. The study is backed by experimental testing on discharge of a 900V MMC capacitor

State feedback linearized model for phase-controlled series-parallel resonant converters

This paper proposes a linearized large-signal state space model for phase-controlled series-parallel resonant converter. The model combines multiple-frequency and average state-space modeling techniques to generate a universal model with DC state variables that are easier to control compared to the fast resonant tank dynamics. In order to perform linearization, the proposed model utilizes a state feedback scheme from output filter inductor current. The model also serves as a tool for large signal prediction/estimation of converter state variables. The model accuracy was verified by comparing with a detailed switching model of the converter built in MATLAB simulation environment

Bronchoscopic Extraction Of Aspirated Foreign Bodies In Children In Harare Central Hospital, Harare, Zimbabwe

A CAJM article on Aspiration of foreign bodies in Zimbabwean sick children.Aspiration of foreign bodies results in the death of more than 500 children per year in the United States. Foreign body aspiration may be manifested by acute respiratory distress as well as insidious chronic pulmonary problems leading to diagnostic delay in some patients.2,3 This study describes the experience with 58 cases of foreign body aspiration in infants and children managed at Harare Central Hospital, Cardio-thoracic Unit in Harare, Zimbabwe

Comprehensive steady state analysis of bidirectional dual active bridge DC/DC converter using triple phase shift control

Several papers have been published recently on TPS control of dual active bridge (DAB) converter, however, no complete study of the converter operation behaviour exists, that takes into account all switching modes in both charging and discharging (bidirectional) power transfer. In this paper, six switching modes and their complements with opposite power transfer direction are defined with their operational constraints. Exact expressions for power transferred are derived with no fundamental frequency assumptions and range of power transfer for each mode is also defined to characterize mode limitations. Detailed constraints for zero voltage switching (ZVS) are also obtained. A new definition for converter reactive power consumption is introduced. This is based on calculation of inductor apparent power which avoids fundamental frequency approximations as well as the vague negative (back flowing) power definitions in recent papers. All known DAB phase shift modulation techniques including conventional, dual and extended phase shift, represent special cases from triple phase shift, therefore the presented analysis provides a generalised theory for all phase shift based modulation techniques

On the use of length scale parameters to assess the static strength of notched 3D-printed PLA

This paper aims to investigate the accuracy of the Theory of Critical Distances (TCD) in estimating static strength of notched additively manufactured PLA as both notch sharpness and infill angle vary. The TCD takes as its starting point the assumption that the extent of damage under static loading can be assessed successfully by using two different material parameters, i.e. (i) a critical distance whose length is closely related to the material microstructural features and an inherent (i.e., a defect free) material strength. Plain and notched specimens of 3D-printed PLA were manufactured horizontally by making the deposition angle vary in the range 0°-90°. Using the TCD in the form of the Point Method, failures were predicted by directly post-processing the linear-elastic stress fields estimated through the well-known analytical solutions due to Glinka and Newport. Independently of the notch sharpness, the estimates being obtained were found to be highly accurate, falling within an error interval of about 20%. This result fully supports the idea that the TCD can successfully be used in situations of practical interest to design against static loading notched components of additively manufactured PLA by directly post-processing the results from simple linear-elastic Finite Element (FE) models