The Impact of Sympathetic Magnetisation Transients on Harmonic Filters in Auto-Transformer Fed Railway Traction Applications

The operation of mixed fleets of modern PWM rectifier and older thyristor locomotives and the tightening of harmonic emission standards globally will lead to more complex harmonic filter installations in railway applications. This paper shows that track switching operations in an auto-transformer fed system can generate sympathetic transformer magnetization currents that have very significant magnitudes, high harmonic content and last for some seconds. A sympathetic transient can occur when un-energized auto-transformers are switched into service and paralleled with energized transformers at the feeder station or in adjacent track sections. The harmonic loads imposed by these currents need to be considered in the design of the harmonic filter elements and protection

A UPFC with reduced DC bus capacitance for LV distribution networks with high PV penetrations

A low voltage (LV) distribution level unified power flow controller (UPFC) is shown capable of regulating the positive sequence voltage with a network while simultaneously correcting phase unbalance voltages that can be produced by high levels of distributed photovoltaic (PV) generation. Instantaneous reactive power theory shows that DC-bus capacitor power will fluctuate at twice mains frequency during any unbalanced operation. Instantaneous power balance can be maintained by allowing the input converter to draw a small negative sequence current. This allows a hundred-fold reduction in the value of the DC bus capacitance allowing long life ceramic or polypropylene capacitors to replace electrolytic capacitors in this application

A Receding Predictive Horizon Approach to the Periodic Optimization of Community Batery Energy Storage Systems

Community scale battery energy storage systems can improve the utilization of network assets and increase the uptake of intermittent renewable energy sources. This paper presents an efficient algorithm for optimizing the cyclic diurnal operation of battery storages in a low voltage distribution network with a high penetration of PV generation. A predictive control solution is presented that uses neural networks to predict the load and PV generation at hourly intervals for twelve hours into the future. The load and generation forecast, and the previous twelve hours of load and generation history, is used to assemble a 24 hour load profile. A diurnal charge profile can be compactly represented by a vector of Fourier coefficients allowing a direct search optimization algorithm to be applied. The optimal profile is updated hourly allowing the state of charge profile to respond to changing future forecasts in load and PV generation

An Analysis of a Voltage Clamped Zero-Voltage Switching Two-Inductor Boost Converter with a Wide Load Range

The Zero-Voltage Switching (ZVS) two-inductor boost converter has been previously developed for the dc-dc conversion stage in a photovoltaic (PV) Module Integrated Converter (MIC) and is able to operate with variable load condition under variable frequency control. However, the converter only offers a narrow output voltage range and this is limited by the MOSFET voltage stress. In this paper, a voltage clamped ZVS two-inductor boost converter is proposed. The converter is able to operate under a wide output voltage range with a lower MOSFET voltage stress while maintaining the resonant transitions. The state analyses of three different operation modes are provided. The design process is also demonstrated in detail and explicit control functions for a 200-W converter are established. Finally, a brief comparison of the advantages and the disadvantages of the two ZVS converters is provided

Correlation Based Method for Phase Identification in a Three Phase LV Distribution Network

Low voltage distribution networks feature a high degree of load unbalance and the addition of rooftop photovoltaic is driving further unbalances in the network. Single phase consumers are distributed across the phases but even if the consumer distribution was well balanced when the network was constructed changes will occur over time. Distribution transformer losses are increased by unbalanced loadings. The estimation of transformer losses is a necessary part of the routine upgrading and replacement of transformers and the identification of the phase connections of households allows a precise estimation of the phase loadings and total transformer loss. This paper presents a new technique and preliminary test results for a method of automatically identifying the phase of each customer by correlating voltage information from the utility's transformer system with voltage information from customer smart meters. The techniques are novel as they are purely based upon a time series of electrical voltage measurements taken at the household and at the distribution transformer. Experimental results using a combination of electrical power and current of the real smart meter datasets demonstrate the performance of our techniques

A Performance Study for Two Current Sensor Free Single-Cell Maximum Power Point Tracking Methods for High Performance Vehicle Solar Arrays

Two Maximum Power Point Tracking (MPPT) control algorithms have been previously developed for the single high performance dual junction or triple junction solar cells for hybrid and electric vehicle applications. These algorithms are respectively based on the Perturb and Observe (P&O) and the Incremental Conductance (IncCond) methods but remove the need for current sensing devices. This paper provides a comparison of the two MPPT control algorithms and a detailed performance evaluation of the two algorithms under both static and dynamic tests. The Incremental Conduction algorithm achieved slightly better results and static tracking accuracy of 99%

An Elegant Solution Using Hybrid Power Filter to Improve the Line Current Spectrum of Multiphase PWM Locomotive Rectifiers with Load Unbalance

Locomotive PWM Rectifiers employed in A.C traction systems represent several megawatts of electrical load. Typically they use multiple rectifiers/converters in parallel in order to secure high power ratings and high frequency operation. The rectifiers are supplied from a single-phase A.C. traction transformer with multiple secondaries of high leakage inductance. The switching instants of the PWM rectifiers are phase shifted and interlaced in order to achieve high ripple current cancellation, assuming that the converter loads are balanced. This would ensure the rectifiers to provide better harmonic performance and also redundancy of operation with multiple units in operation. However, in practice, rectifiers may be closely coupled to a traction inverter supplying an axle or a group of axles and the real power developed will depend upon the traction conditions. Creep and slip can give rise to variations in power and perfect power balance at the rectifiers is an improbable proposition.There is high degree of interest to examine the possibility locomotive systems that degrade gracefully with equipment failure. Failure of one traction inverter would lead to load unbalance for the rectifier modules. Or sometimes, partial failures may result in unbalanced operation which can cause reduced ripple current cancellation and generate switching frequency harmonic current components. This may adversely impact the signalling systems and/or result in over voltage effects due to resonance in the overhead supply system. This paper examines a 4 MW locomotive with three rectifier modules and a device switching frequency of 900 Hz. This results in a 5400 Hz ripple frequency in the mains current. And this paper will consider what may be the most frequently occurring case; that of loss of load in one traction motor due to wheel slip. Current ripple cancellation is partially lost in this case and lower frequency current components can be produced. Traditionally, number of solutions has been employed in the industry like passive filters, active filters etc., for this type of a problem. However, this paper explores an elegant, attractive and economical solution of using hybrid filters in order to achieve a level of acceptable satisfactory harmonic performance and thereby improving the power quality of the rail systems

A Unity Power Factor Boost Rectifier with a Predictive Capacitor Model for High Bandwidth DC Bus Voltage Control

Single phase rectifiers with power factor correction circuits based on the boost converter find broad application in consumer equipment. A fundamental design difficultly is caused by the need to trade off the input line current wave shape and the dynamic response of the output energy storage capacitor voltage regulation loop. The energy balance requirement inherent in single phase systems forces a significant 100Hz voltage ripple on the output capacitor. A high bandwidth voltage regulation loop will feed back enough of the 100Hz component to distort the current reference signals for the inner current control loop distorting the line wave shape. A reference model can be used to construct a ripple free estimate of the capacitor voltage. This paper shows that improved output capacitor voltage regulation can be achieve simultaneously with a high quality input current spectrum

Variable Frequency Control of the Zero-Voltage Switching Two-Inductor Boost Converter

The two-inductor boost converter has been previously presented in a zero-voltage switching (ZVS) form where the transformer leakage inductance and the MOSFET output capacitance can be utilized as part of the resonant elements. In many applications, such as maximum power point tracking (MPPT) in grid interactive photovoltaic systems, the resonant converter is required to operate with variable input output voltage ratios. This paper studies the variable frequency control of the ZVS two-inductor boost converter to secure an adjustable output voltage range while maintaining resonant switching transitions. The design method of the resonant converter is thoroughly investigated and explicit control functions relating the circuit timing factors and the voltage gain for a 200 W converter are established. Three sets of theoretical, simulation and experimental waveforms are provided for specific operating points. A variation of the basic circuit, the resonant converter with a voltage clamp, which is capable of operating with a wider output voltage range but a lower switch voltage stress, is also given at the end of the pape

An Analysis of the ZVS Two-Inductor Boost Converter under Variable Frequency Operation

The two-inductor boost converter has been previously presented in a zero-voltage switching (ZVS) form where the transformer leakage inductance and the MOSFET output capacitance can be utilized as part of the resonant elements. In many applications, such as maximum power point tracking (MPPT) in grid interactive photovoltaic systems, the resonant two-inductor boost converter is required to operate with variable input output voltage ratios. This paper studies the variable frequency operation of the ZVS two-inductor boost converter to secure an adjustable output voltage range while maintaining the resonant switching transitions. The design method of the resonant converter is thoroughly investigated and explicit control functions relating the circuit timing factors and the voltage gain for a 200-W converter are established. The converter has an input voltage of 20 V and is able to produce a variable output voltage from 169 V to 340 V while retaining ZVS with a frequency variation of 1 MHz to 407 kHz. Five sets of theoretical, simulation and experimental waveforms are provided for the selected operating points over the variable load range at the end of the paper and they agree reasonably well. The converter has achieved part load efficiencies above 92% and an efficiency of 89.6% at the maximum power of 200 W