Investigation of FACTS devices to improve power quality in distribution networks

Flexible AC transmission system (FACTS) technologies are power electronic solutions that improve power transmission through enhanced power transfer volume and stability, and resolve quality and reliability issues in distribution networks carrying sensitive equipment and non-linear loads. The use of FACTS in distribution systems is still in its infancy. Voltages and power ratings in distribution networks are at a level where realistic FACTS devices can be deployed. Efficient power converters and therefore loss minimisation are crucial prerequisites for deployment of FACTS devices. This thesis investigates high power semiconductor device losses in detail. Analytical closed form equations are developed for conduction loss in power devices as a function of device ratings and operating conditions. These formulae have been shown to predict losses very accurately, in line with manufacturer data. The developed formulae enable circuit designers to quickly estimate circuit losses and determine the sensitivity of those losses to device voltage and current ratings, and thus select the optimal semiconductor device for a specific application. It is shown that in the case of majority carrier devices (such as power MOSFETs), the conduction power loss (at rated current) increases linearly in relation to the varying rated current (at constant blocking voltage), but is a square root of the variable blocking voltage when rated current is fixed. For minority carrier devices (such as a pin diode or IGBT), a similar relationship is observed for varying current, however where the blocking voltage is altered, power losses are derived as a square root with an offset (from the origin). Finally, this thesis conducts a power loss-oriented evaluation of cascade type multilevel converters suited to reactive power compensation in 11kV and 33kV systems. The cascade cell converter is constructed from a series arrangement of cell modules. Two prospective structures of cascade type converters were compared as a case study: the traditional type which uses equal-sized cells in its chain, and a second with a ternary relationship between its dc-link voltages. Modelling (at 81 and 27 levels) was carried out under steady state conditions, with simplified models based on the switching function and using standard circuit simulators. A detailed survey of non punch through (NPT) and punch through (PT) IGBTs was completed for the purpose of designing the two cascaded converters. Results show that conduction losses are dominant in both types of converters in NPT and PT IGBTs for 11kV and 33kV systems. The equal-sized converter is only likely to be useful in one case (27-levels in the 33kV system). The ternary-sequence converter produces lower losses in all other cases, and this is especially noticeable for the 81-level converter operating in an 11kV network

Trofazni kaskadni pretvarači s izravnom sinkronom širinsko-impulsnom modulacijom

A novel method of direct synchronous pulsewidth modulation (PWM) is applied for control of modular multilevel converters consisting from three standard triphase inverter modules along with an 0.33 p.u. output transformer. The proposed method provides synchronisation of the voltage waveforms for both each module and the composed voltage at the output of the converter. Multilevel output voltage of the converter has quarter-wave symmetry during the whole range including the zone of overmodulation. Both continuous and discontinuous versions of synchronous PWM, based on a vector approach for determination of the pulse patterns, have been analysed and compared using simulations of the systems with low switching frequencies, which normally are used in high power systems.Primijenjena je nova metoda izravne sinkrone šironsko-impulsne modulacije za upravljanje višerazinskim pretvaračima koji se sastoje od tri standardna trofazna izmjenjivačka modula te 0,33 p.u. izlaznog transformatora. Predložena metoda osigurava sinkronzaciju valnih oblika izlaznog napona za svaki izmjenjivački modul i za cjeli pretvarač. Izlazni napon višerazinskog pretvarača ima četvrtvalnu simetričnost u čitavom području vrijednosti, uključivo i područje nadmodulacije. Analizirane su i simulacijski uspoređene kontinuirane i diskontinuirane inačice sinkrone širinsko-impulsne modulacije, zasnovane na vektorskom načelu određivanja impulsnog niza. Simulirani su sustavi s niskom sklopnom frekvencijom, kakvi su standardno sustavi velike snage

Differential Power Processing Converter Design for a Photovoltaic-Powered Charging Bag

Department of Electrical EngineeringTraditional photovoltaic (PV) systems are stationary PV systems mounted in one location and, generally, receive consistent and even illumination across the PV panels. However, solar photovoltaic (PV) power is also getting widely used in lower-power emerging applications, like wearables or internet of things (IoT) devices. One fundamental challenge of using PV power in wearable applications is that individual PV cells may be pointing in different angles, receiving different light intensities. Under these uneven illumination, resulting system efficiency depends on the configurations of the PV cells and converters. Through this thesis, the system efficiencies of five configurations are compared with nine realistic test cases. The five configurations are: PV in series with central converter, PV in parallel with central converter, PV with cascaded converters, PV in series with differential power processing (DPP) converters, and PV in parallel with DPP converters. The nine test cases are composed of an ideal case (all PV cells at 1,000 W/m2) and eight realistic illumination cases based on the weather (sunny or cloudy) and realistic usage scenarios. Based on these cases the system efficiency is calculated for each configuration considering a range of converter efficiencies (70% to 100%). Results show that the parallel DPP configuration shows the highest system efficiency in all cases. Parallel DPP converters can achieve individual PV control and maximizing output power by processing small fraction of the PV power. There are two types of parallel DPP architectures which are with and without a front-end converter. Two parallel DPP architectures are analyzed and compared for a target 5-W wearable application. Between the two architectures, the DPP system without a front-end converter shows consistently high performance and operates properly over a wider range of lighting conditions. Therefore, the proper operation, such as maximum power point tracking (MPPT) of PV cells, using parallel DPP converters without the front-end converter is validated through simulation and hardware experiments. The PV-powered wearable prototype is able to charge a portable battery under low-light and partial shading conditions.ope

Stability Analysis and Compensator design for Cascaded Converters

The present work deals with the modeling and the stability of the cascade converters. When a converter tightly regulates its output, it behaves as a constant power load (CPL). The problem of CPLs is that they show negative incremental resistance that causes instability in the system. Hence the CPL has been modeled and linearizaed to study its effect on the source converter. On this basis, it was found that because of the negative resistance exhibited by the CPL, the control to output transfer function possessed a RHP pole and thus makes system unstable. The effect of the introduction of small resistance in series with inductor of source converter on the stability was analyzed. Also the compensation on the basis of the passive components i.e. snubbers were designed so as to reduce the power dissipation due to the resistance. Type III compensation was designed by taking the input impedance of the load converter as the load for the source converter using K factor method. Further, based on input impedance, minor loop gain concept was used for stabilizing the system

VSC topology comparison for STATCOM application under unbalanced conditions

This paper compares the performance of four different power electronic converter topologies, which have been proposed for STATCOM applications. Two of the topologies are Modular Multilevel Cascaded Converters (MMCC), whilst the remaining circuits utilize magnetic elements and an open-winding transformer configuration to combine individual power modules. It is assumed that the STATCOM has to work under unbalanced conditions, so that it delivers both positive and negative sequence currents. Simulation studies for the four topologies have been carried out using the simulation tool Saber

DC-Voltage-Ratio Control Strategy for Multilevel Cascaded Converters Fed With a Single DC Source

Recently, a multilevel cascaded converter fed with a single DC source has been presented. An analysis of the steady-state working limits of this type of converter is presented in this paper. Limits of the maximum output voltage and the minimum and maximum loading conditions for stable operation of the converter are addressed. In this paper, a way to achieve any DC voltage ratio (inside the stable operation area of the converter) between the H-bridges of the single-DC-source cascaded H-bridge converter is presented. The proposed DC-voltage-ratio control is based on a time-domain modulation strategy that avoids the use of inappropriate states to achieve the DC-voltage-ratio control. The proposed technique is a feedforward-modulation technique which takes into account the actual DC voltage of each H-bridge of the converter, leading to output waveforms with low distortion. In this way, the dc voltage of the floating H-bridge can be controlled while the output voltage has low distortion independently of the desired DC voltage ratio. Experimental results from a two-cell cascaded converter are presented in order to validate the proposed DC-voltage-ratio control strategy and the introduced concepts.Ministerio de Ciencia y Tecnología TEC2006-03863Junta de Andalucía EXC/2005/TIC-117