EFFECTS OF HARMONICS ON THE PERFOR1VlANCE OF DISTRIBUTION SYSTEMS

Abstract

The recent fast development and increasing use of power semiconductors switching devices for frequency or magnitude conversion and control have a profound impact on modern electrical power technology. The main effect of this development is the frequent use of nonsinusoidal voltage sources supplying nonlinear loads at higher power levels. The main problems of switch controlled systems are the generation of harmonics and the low power factor generally associated with them. However, when dealing with such systems, the apparent power can be decomposed into three orthogonal components namely, active power, reactive power and distortion power. The compensation of the reactive power alone does not necessarily yield unity power factor.

In the first pm1 of the thesis, an attempt is made to clarify the different definitions of powers associated with systems characterized by nonsinusoidal sources and/or nonlinear loads in the time and frequency domains. The relationship between these definitions in both domains is established. An expression for the required static element(s) for reactive power compensation is developed, keeping in mind the minimization of the apparent power. This analysis is carried out for five different examples. which include theoretical and experimental cases. These cases are: Case l: a sinusoidal voltage applied to a linear load. Case 2: a sinusoidal voltage applied to a nonlinear load. Case 3: a nonsinusoidal voltage applied to a linear load. Case 4: a nonsinusoidal voltage applied to a nonlinear load. CaseS: practical measurements of terminal voltage and armatute current of a microalternator loaded with thyristor bridge. Numerical calculations using spreadsheet analysis are carried out. For each example, the power components, current components, test of their orthogonality and the equivalent model circuit are calculated. The thesis will present the results obtained.

This analysis is extended. in the second part of the seminar, and applied to the Greater Cairo Metro line distribution system. The

ii

(GCM) project is a practical distribution system of a·large power rating. It is a good example of the nonlinear load type that generates harmonics. The aim of the second part of the thesis is to simulate and analyze this system using V-HARM software program.

The analysis will identify the resultant harmonics, and their levels at different loading conditions. Furthermore, the thesis presents the design of harmonic tuned filters based on minimizing the total apparent power provided by the network. The results of this part will demonstrate the effect of these filters on the harmonic levels in the distribution network, the power requirements and the power factor of the system.