A High-Performance SPWM Controller for Three-Phase UPS Systems Operating Under Highly Nonlinear Loads

The increased use of rectifiers in critical loads employed by the information technologies, and medical and military equipment mandate the design of uninterruptible power supplies (UPS) with highquality outputs. The highly nonlinear currents drawn especially by high-power single-phase rectifier loads greatly distort the uninterruptible power supplies (UPS) outputs. The distorted uninterruptible power supplies (UPS) voltages cause generation of low dc voltage at the output of the rectifier loads, which causes high current flow, increased power losses, and possibly the malfunction of the critical load or the uninterruptible power supplies (UPS). As a result, different harmonics mitigating techniques have gained a lot of attention. The main objective of this project is to develop simulation of a high-performance Pulse Width Modulation (PWM) technique based AC-DC converter system operating under highly nonlinear loads using MATLAB/SIMULINK. Here, controlled Insulated-gate bipolar transistor (IGBT) based AC-DC converter is used to supply the load instead of Diode or Thyristor Bridge. The pulse width modulation method is quite effective in controlling the root mean square (RMS) magnitude of the AC-DC converter output voltages and shape of input current. This enables automatic harmonic compensation by Rectifier itself. Therefore, the device is controlling Power Flow as well as does Power Conditioning. In this project the performance of proposed system will be analysed for different pulse width modulation techniques like sinusoidal pulse width modulation (SPWM), using MATLAB/SIMULINK. Keywords— Harmonic Compensation, sinusoidal pulse width modulation (SPWM) technique based ACDC Converter, different Non-linear loads, current harmonics mitigation techniques Introduction In recent years power quality has become an important and growing problem due to the proliferation of nonlinear loads such as power electronic converters in typical power distribution systems. Particularly, voltage harmonics and power distribution equipment problems are the result of current harmonics produced by nonlinear loads. Eminent issues always arise in three-phase four-wire systems. It is well-known that zero line may be overheated or causes a fire as a result of excessive harmonic current going through the zero line three times or times that of three. Thus a perfect compensator is necessary to avoid the negative consequences of harmonics. Though several control techniques and strategies have been developed they still have contradictions with the performance of filters. The distortion is resulted mainly by the voltage drop across the inductive element of the LC filter due to the non-sinusoidal current at the output of the inverter. In a uninterruptible power supplies (UPS) system, the inverter is responsible for synthesizing sinusoidal voltages from a dc source through the pulse width modulation (PWM) of the dc voltage. The inductive element here is needed to remove the switching frequency harmonics from the current waveform that are generated by the pulse width modulation (PWM) operation of the inverter. The inductance value can be reduced if the switching frequency is increased .But, in practice; it has an upper limit at high power inverters due to the efficiency concerns and the switching device limitations. So, for the selected switching frequency and the power level, an optimum filter with a NOVATEUR PUBLICATIONS INTERNATIONAL JOURNAL OF INNOVATIONS IN ENGINEERING RESEARCH AND TECHNOLOGY [IJIERT] ISSN: 2394-3696 VOLUME 2, ISSUE 5, MAY-2015 2 | P a g e smallest inductance can be designed, but the distortion cannot be completely avoided, and the regulations and the customer specifications may not be satisfied. The solution to this problem is to design a proper inverter controller such that it generates a control signal with multiple functionalities. This signal must carry information to produce sinusoidal voltages with small steady-state RMS error, to provide fast transient response and corrective actions to reduce distortion even under highly nonlinear loads. Therefore, a major research has been conducted to design such controllers for the high-performance uninterruptible power supplies uninterruptible power supplies (UPS) Systems. The high performance controllers in general employed multi-loop state feedback control strategies to achieve the regulation specifications Moreover, the dead-beat control method and the predictive and repetitive control methods have been widely investigated and proposed among researchers. In addition, the iterative and adaptive learning control methods, the Hinfinity control method the feedback linearization method, and recently the multi sampled control approach to improve the control performance have been studied and evaluated. High-quality output voltages with substantially low total harmonic distortion (THD) and fast dynamic response have been demonstrated with these methods. However, the disadvantages such as implementation complexity and the problems caused by highly unbalanced loading may limit some of the benefits of these methods. POWER QUALITY PROBLEMS & ISSUES A recent survey of Power Quality experts indicates that 50% of all Power Quality problems are related to grounding, ground bonds, and neutral to ground voltages, ground loops, ground current or other ground associated issues. Electrically operated or connected equipment is affected by Power Quality and determining the exact problems requires sophisticated electronic test equipment. The following symptoms are indicators of Power Quality problems: • Piece of equipment misoperates at the same time of day. • Circuit breakers trip without being overloaded. • Equipment fails during a thunderstorm. • Automated systems stop for no apparent reason. • Electronic systems fail or fail to operate on a frequent basis. • Electronic systems work in one location but not in another location. The commonly used terms those describe the parameters of electrical power that describe or measure power quality are Voltage sags, Voltage variations, Interruptions Swells, Brownouts, Blackouts, Voltage imbalance, Distortion, , Harmonic resonance, Inter harmonics, Notching, Noise, Impulse, Spikes (Voltage), Ground noise, Common mode noise, Critical load, Crest factor, Electromagnetic compatibility, Dropout, Fault, Flicker, Ground, Raw power, Clean ground, Ground loops, Voltage fluctuations, Transient, Dirty power, Momentary interruption, Over voltage, Under voltage, Nonlinear load, THD, Triples, Voltage dip, Voltage regulation, Blink, Oscillatory transient etc. The issue of electric power quality is gaining importance because of several reasons: • The society is becoming increasingly dependent on the electrical supply. A small power outage has a great economical impact on the industrial consumers. A longer interruption harms practically all operations of a modern society. • New equipments are more sensitive to power quality variations. • The advent of new power electronic equipment, such as variable speed drives and switched mode power supplies, has brought new disturbances into the supply system. Maintaining the Integrity of the Specifications The template is used to format your paper and style the text. All margins, column widths, line spaces, and text fonts are prescribed; please do not alter them. You may note peculiarities. For example, the head margin in this template measures proportionately more than is customary. This measurement and others are deliberate, using specifications that anticipate your paper as one part of the entire proceedings, and not as an independent document. Please do not revise any of the current designations. NOVATEUR PUBLICATIONS INTERNATIONAL JOURNAL OF INNOVATIONS IN ENGINEERING RESEARCH AND TECHNOLOGY [IJIERT] ISSN: 2394-3696 VOLUME 2, ISSUE 5, MAY-2015 3 | P a g e POWER QUALITY STANDARDS Power quality is a worldwide issue, and keeping related standards current is a never-ending task. It typically takes years to push changes through the process. Most of the ongoing work by the IEEE in harmonic standards development has shifted to modifying Standard 519-1992. IEEE 519 IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, established limits on harmonic currents and voltages at the point of common coupling (PCC), or point of metering . The limits of IEEE 519 are intended to: 1) Assure that the electric utility can deliver relatively clean power to all of its customers; 2) Assure that the electric utility can protect its electrical equipment from overheating, loss of life from excessive harmonic currents, and excessive voltage stress due to excessive harmonic voltage. Each point from IEEE 519 lists the limits for harmonic distortion at the point of common coupling (PCC) or metering point with the utility. The voltage distortion limits are 3% for individual harmonics and 5% THD. All of the harmonic limits in IEEE 519 are based on a customer load mix and location on the power system. The limits are not applied to particular equipment, although, with a high amount of nonlinear loads, it is likely that some harmonic suppression may be necessary. IEEE 519 Standards for Current Harmonics • General Distribution Systems [120V69 kV] Below current distortion limits are for odd harmonics. Even harmonics are limited to 25% of the odd harmonic limits. For all power generation equipment, distortion limits are those with ISC/IL<20. ISC is the maximum short circuit current at the point of coupling “PCC”.IL is the maximum fundamental frequency 15-or 30minutes load current at PCC. TDD is the Total Demand Distortion (=THD normalized by IL) General Sub-transmission Systems [69 kV-161 kV]. The current harmonic distortion limits apply to limits of harmonics that loads should draw from the utility at the PCC. Note that the harmonic limits differ based on the Isc /IL rating, where ISC is the maximum short circuit current at