A Hybrid Discontinuous Voltage Controller for DSTATCOM Applications

This paper proposes a hybrid discontinuous control methodology for a voltage source converter (VSC) based distribution static compensator (DSTATCOM) connected to a 3-phase distribution system. It is assumed that the DSTATCOM controls the point of common coupling (PCC) voltage. An LC output filter is connected at the output of the VSC. With the help of both filter inductor current and filter capacitor voltage control, the voltage across the filter capacitor is controlled. Based on the voltage error, the control is switched between voltage and current control mode. In this scheme, an extra diode state is used that makes the VSC output current discontinuous. This diode state reduces the switching losses because half of the total switching occurs at nearly zero converter current. The effects of different source conditions on voltage at PCC are studied. Voltage angle at PCC is controlled using a PI controller. Simulation studies are performed using PSCAD to validate the efficacy of the proposed controller

Implementing natural PWM digitally

For dynamic closed loop control of a multilevel converter with a low switching frequency, natural sampled PWM is the best form of modulation. However previous natural sampled PWM implementations have generally been analog. A digital implementation has advantages, particularly for a multilevel converter. Re-sampled uniform PWM, a modulation technique which approaches the performance of natural PWM, is implemented both in software and hardware. Results demonstrating the improvement over uniformPWM techniques are presented

An isolated MOSFET gate driver

Traditional methods of isolated MOSFET/IGBT gate drive are presented, and their pros and cons assessed. The best options are chosen to meet our objective— a small, high speed, low cost, low power isolated gate drive module. Two small ferrite bead transformers are used for isolation, one transmits power at 2.5MHz, the other sends narrow set reset pulses. On the secondary these pulses drive a transistor totem pole to ensure high current drive, and the value is held by CMOS buffers with positive feedback. An alternative design for driving logic level devices uses only an HC buffer on the secondary. Double sided SMDconstruction (primary one side, secondary on the other) yields an upright module 40x18x5mm. Propagation delaywas 20ns, and rise/fall time 15ns with a 1nF load. The design places no limits on frequency of operation or duty cycle. Power supply requirementswere 5V@20mA for operation below 100kHz, dominated by magnetising current

Relaxation studies to identify aging with oil filled transformers: Experiments

Requirement of reliable condition indicators is needed in the energy sector with web based information structure and with increased number of aged power apparatus operating in digital switching and loading environment. This project for oil-filled power transformers is taken up by Cooperative Research Centre for Integrated Engineering Asset Management (CIEAM) to develop knowledge models for prediction of remaining life of operating transformers. The aims at Queensland University of Technology (QUT) are to come up with partial discharge (PD) and relaxation condition indicators. In this paper, part of the promising results on relaxation measurement is presented. Two 11 kV/220 V ‘SWER’ (Single Wire Earth Return) and one 70 kV/220V HV test transformers were tested. They were tested with Step Polarization and Low Frequency Domain Spectroscopy techniques. The collected time and frequency domain polarization responses provide the aged and new status of transformers. The dynamic variation of the relaxation responses with heating reveals the aging status information of the transformers. With heating, NEW transformer’s leakage current magnitude and changes with heating time are significantl

Tracking variable periodic signals with fixed sampling rate: feedforward control

Recent research has shown that the feedback repetitive control is very efficient in tracking periodic signals. However, the existing repetitive control algorithms require an integer number of samples in each period. In some industry applications where the signal period varies but other requirements on the data acquisition system force a fixed sample rate, the number of samples per period may be a non-integer. To address this problem, adaptive feedback repetitive controllers have been proposed. Although the adaptive feedback repetitive control provides good steady state performance, the transient response is not fast. This paper presents a new adaptive feedforward control with fixed sampling rate to track periodic signals with variations in periods. The experimental result on a servomotor demonstrates that the proposed feedforward tracking controller has fast transient response

Power Quality Enhancement using Custom Power Devices

Power Quality Enhancement Using Custom Power Devices considers the structure, control and performance of series compensating DVR, the shunt DSTATCOM and the shunt with series UPQC for power quality improvement in electricity distribution.\ud Also addressed are other power electronic devices for improving power quality in Solid State Transfer Switches and Fault Current Limiters. Applications for these technologies as they relate to compensating busses supplied by a weak line and for distributed generation connections in rural networks, are included. In depth treatment of inverters to achieve voltage support, voltage balancing, harmonic suppression and transient suppression in realistic network environments are also covered. New material on the potential for shunt and series compensation which emphasizes the importance of control design has been introduced.\ud Power Quality Enhancement Using Custom Power Devices is appropriate for distribution engineers, graduate engineers and designers working in the area of power electronic applications for power systems. Sections of the book on power quality issues and generation connection make for a timely reference for undergraduates studying distribution engineering. \ud Written for:\ud Distribution engineers, graduate engineers and designers working in the area of power electronic applications for power systems, student

Mathematical analysis of planar busbar with distributed capacitors for voltage source inverters

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Load model: induction motors in large disturbance

The aim of this work is to propose a simple\ud methodology that facilitates characterization of the power system load, particularly induction motors, taking into account the different possible responses of induction motors to large disturbance. Since the nonlinearities associated with induction motors imply different responses if an induction motor reaccelerates, trips or stalls in the post-disturbance period, the induction motor load model can be improved identifying the percentages of reaccelerated, stalled and tripped induction motors.\ud The aim is to build these models from system measurement

Load modelling : induction motors

Modelling the power systems load is a challenge since the load level and composition varies with time. An accurate load model is important because there is a substantial component of load dynamics in the frequency range relevant to system stability. The composition of loads need to be charaterised because the time constants of composite loads affect the damping contributions of the loads to power system oscillations, and their effects vary with the time of the day, depending on the mix of motors loads.\ud \ud This chapter has two main objectives: \ud \ud 1) describe the load modelling in small signal using on-line measurements; and \ud 2) present a new approach to develop models that reflect the load response to large disturbances. \ud \ud Small signal load characterisation based on on-line measurements allows predicting the composition of load with improved accuracy compared with post-mortem or classical load models. Rather than a generic dynamic model for small signal modelling of the load, an explicit induction motor is used so the performance for larger disturbances can be more reliably inferred. The relation between power and frequency/voltage can be explicitly formulated and the contribution of induction motors extracted. One of the main features of this work is the induction motor component can be associated to nominal powers or equivalent motor

A new technique for optimal allocation and sizing of capacitors and setting of LTC

An iterative based strategy is proposed for finding the optimal rating and location of fixed and switched capacitors in distribution networks. The substation Load Tap Changer tap is also set during this procedure. A Modified Discrete Particle Swarm Optimization is employed in the proposed strategy. The objective function is composed of the distribution line loss cost and the capacitors investment cost. The line loss is calculated using estimation of the load duration curve to multiple levels. The constraints are the bus voltage and the feeder current which should be maintained within their standard range.\ud \ud For validation of the proposed method, two case studies are tested. The first case study is the semi-urban 37-bus distribution system which is connected at bus 2 of the Roy Billinton Test System which is located in the secondary side of a 33/11 kV distribution substation. The second case is a 33 kV distribution network based on the modification of the 18-bus IEEE distribution system. The results are compared with prior publications to illustrate the accuracy of the proposed strategy.\u