Elektronski sistem za analizu polifaznih opterećenja baziran na FPGA
Committee membersPetković, Predrag
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Electronic devices industry is characterized by a very dynamic growth. Such quick advancement is not recognized in other technological branches. It is exponential and it can be represented by Moore’s law, which describes long-term tendency: the number of active components integrated on circuit doubles every eighteen months, keeping constant production price. This trend stands almost fifty years, and it will stand in the near future. Moore’s law is closely related to electronic device’s properties: processor speed, memory capacity, resolution, etc. Exponential growth had immense impact on world economy and it led to significant changes in lifestyle. Consequently, electronic devices are cheaper and more available on the market. As a result, electronic equipment takes a bigger portion in overall power consumption. Electronic devices are complex circuits, consisting active semiconductor components that require direct current for polarisation. Electric energy is delivered to end users usin...g three-phase alternating current distribution. Three-phase distribution is suitable for electric power transfer; however it cannot be directly applied to electronic circuits. Alternating current must be converted to direct current, using AC/DC power converters. Converter is electric circuit containing transformer, or switch mode power supply drawing power from the mains in pulses hence it is nonlinear. It can be analysed as two-port network with reactive and nonlinear impedance. Characterisation of power converter is performed by means of determining electrical quantities, power and power quality parameters – power factor and total harmonic distortion. In linear circuits, consisting of linear loads, the currents and voltages are sinusoidal and the power factor effect arises only from the difference in phase between the current and voltage. When nonlinear loads are present one should introduce new quantities in the calculations emanated by the harmonics and related power components. Now the power factor can be generalized to a total or true power factor where the apparent power, involved in its calculations, includes all harmonic components. This is of importance in characterization and design of practical power systems which contain non-linear loads such as rectifiers, and especially, switched-mode power supplies. Phase difference between current and voltage, as well as harmonic distortion has negative impact on distribution system. Therefore, industry standards regulate the limits (minimum) of power factor. One of the most paradigmatic examples is personal computer that typically includes switched-mode power supply (SMPS) with output power ranging from 150 W to 500 W. SMPS with passive power correction factor (PFC) can achieve power factor of about 0.7–0.75, SMPS with active PFC – up to 0.99, while SMPS without any PFC has power factor of about 0.5–0.65 in the best. Since the problem of distortion becomes ubiquitous, it can be either observed at the distribution system level, or one has to take local measurement of the properties of this kind of loads. Measurement of power factor and distortion, however, usually requires dedicated equipment. For example, use of a classical ammeter will return incorrect results when attempting to measure the AC current drawn by a non-linear load and then calculate the power factor. A true RMS multimeter must be used to measure the actual RMS currents and voltages and apparent power. To measure the real power or reactive power, a wattmeter designed to properly work with non-sinusoidal currents must be also used. Contemporary methods and algorithms for spectrum analysis are presented in this thesis. The basic definitions of parameters describing nonlinear loads in one-phase and three-phase circuits are introduced. Alternative definitions for reactive power and their calculation methods are elaborated. A new approach to polyphase load analysis is presented: system for nonlinear load characterization which is flexible, scalable, with advanced options. The solution introduced in the thesis brings all benefits of virtual instrumentation, keeping main advantage of classical instrument – determinism in measurement. The hardware component of the system is implemented using field programming gate array (FPGA) in control of data acquisition. The software part is implemented in two stages, executing on real-time operating system and general purpose operating system. Described realization provides possibility for calculating a large number of parameters that characterize nonlinear loads, which is impossible using classical instruments. This is of great importance particularly in calculation of alternate definitions of reactive power. The system is scalable; it can be upgraded in number of calculated parameters, as well as in number of independent measurement channels or functionality. The system is open; it can be modified to be a part of harmonic compensation circuitry or aimed for hardware-in-the-loop simulations. The system is flexible; it is implemented on different platforms for different purposes: as laboratory equipment for real time measurements (PXI controller equipped with PXI-7813R FPGA card and expansion chassis), as compact industrial device for real time operation (installed on programmable automation controller) or simple portable instrument equipped with computer interface. It consists of three subsystems: acquisition subsystem, real time application for parameter calculations and virtual instrument for additional analysis and data manipulation. Acquisition subsystem consists of acquisition modules for A/D conversion, FPGA circuit and interface for computer or programmable automation controller. A/D resolution is 24-bit, with 50 kSa/s sampling rate and dynamic range ±300 V for voltages and ±5 A for currents. Function of FPGA circuit is acquisition control and harmonic analysis. Real time application calculates power and power quality parameters deterministically and save calculated values on local storage. The application is executed on real time operating system. Virtual instrument for additional analysis and data manipulation represents user interface of described system. It runs on general purpose operating system, physically apart from rest of the system. Communication is achieved by TCP/IP. Parameters and values obtained by means of acquisition and calculations are presented numerically and graphically. The usage of the system is also described. Nonlinear single-phase (SMPS, LED, CFL) and three-phase loads are examined in order to present all possibilities of new system.