New M.E. Thesis Submitted from EE Student


An ideal power system is one which has constant voltage and frequency at every point, no interference of one load with another, unity power factor and sinusoidal voltage and current waves (no harmonics).However, none of these conditions are fulfilled in practice. Understanding power systems distortion and containing it to acceptable proportions has been a concern of power engineers from early days of alternating current. The recent growing concern for this problem results from the increasing numbers and power ratings of the non-linear power electronic devices used in the control of power apparatus and systems. The deviations from perfect sinusoids are generally expressed in terms of harmonic components. The quality of power & voltage waveforms is now-a-days an issue of the utmost important for electric energy consumers and also for manufactures of electric and electronic equipment. The voltage waveform is expected to be a pure sinusoidal at a given frequency and amplitude. Power systems are designed to operate at frequencies of 50 Hz. However, certain types of loads produce currents and voltage with frequencies that are integer multiples of the 50 Hz fundamental frequency. These higher frequencies are a form of electrical pollution known as power system harmonics. The ever increasing demand of industry and commerce for stability, adjustability and accuracy of control in electrical equipment led to the development of relatively low cost power diodes, IGBTs, SCRs and other power semi-conductors, now used widely in rectifier circuits for uninterrupted power supply systems, static converters and A.C. & D.C. motor control, these modern devices replace the mercury arc rectifiers of earlier years and create new and challenging conditions for the power engineer of today. Similarly the application equipments are constructed by combinations of non-linear devices. Although solid state Devices such as thyristors, have brought significant improvements in control designs and efficiency, they have the disadvantages of producing harmonic currents. Harmonics can cause:
1. Capacitor bank failure due to dielectric break-down or reactive power over-load.
2. Amplification of harmonics level due to series and parallel resonance causing expensive causalities.
3. Heating and saturation of the equipment.
4. Ageing of apparatus insulation.
5. Plant mal-operation due to interference in controls, particularly in solid state and microprocessor control systems.
6. Errors in different types of measurements.
7. Inductive interference with communication circuits and degradation of telephone communication caused by induced harmonic noise.
8. Mechanical oscillations in induction and synchronous machines.
All the above mentioned effects are leading to the tangible & non-tangible losses to the operations. For example:
A. Unplanned outages due to application cards failures
B. Current unbalancing in system & heating of neutral conductor
C. Tripping of circuit breakers frequent fuse blowing, capacitor failures, overloading of Transformer neutrals
D. Telephone interference
E. Mal-operation of PLC and computer failures-“frozen” screens, insulation failures
F. Overheating of motors and transformers etc.
The survey on harmonics was conducted to firstly exterminate the extent of harmonics and secondly to apprise whether any of the locations that had significant level of harmonics could lead to an electrical problem in the near future. These days industry is very much aware about the problem of harmonics.For harmonics studies, Fourier series and Fourier analysis are fundamental concepts. In non-sinusoidal situations, the conventional electric quantities are used in sinusoidal environment need to be redefined. However, power definitions as well as harmonic phase sequences under unbalanced three-phase systems are still under investigations. Several harmonic indices have been defined for the evaluation of harmonic effects on power system response to harmonic requires accurate models for power system elements and harmonic-generating loads. A simple technique for harmonic analysis is the current injection method which is performed in the frequency domain. Other analysis methods include time domain and frequency/time domain techniques.In order to study the impact of harmonics due to non linear loads we selected telecommunication switching centre electrical site in our experimental set up. Observation was recorded at this electrical site using harmonic analyzer, multi-meters etc. It was Revealed from the observations carried out during our experimental study that there are large variations in the readings with or without capacitor banks, loading patterns, location of loads & type of power connection. By adapting a well-engineered approach and using the harmonics filter solutions harmonics distortion in electrical network can be kept within the stipulated standards. The use of harmonic filters in this context shall provide economic benefits to the user through reduction in overall costs and enhanced reliability of the network.

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