EFFECT OF DISTRIBUTION SYSTEM UNBALANCE ON MAGNETIC FIELD LEVELS Hassan A.N. Hejase, Ahmed M. Abd-Rabou and Adel A. Hussein Department of Electrical Engineering, United Arab Emirates University P.O. Box 17555, Al-Ain, United Arab Emirates Email: hhejase@uaeu.ac.ae Abstract: This work aims at evaluating the effect of load unbalance on the resultant magnetic flux density from three-phase distribution lines. The field is expressed in terms of the symmetrical components of the unbalanced three-phase currents. We simulate the effect of current unbalance factor (magnitude and phase) on the magnetic field levels as the degree of magnitude and argument unbalance is increased. The effect of the three-phase line configuration on of the resulting magnetic flux density was also addressed. An experimental lab setup was used to measure the magnetic field for flat and bundled line arrangements due to current magnitude unbalance factors between 10% and 50%. The setup load elements were designed to produce the required current unbalance factors. Computed and measured field results are in very good agreement. An increase in zero-sequence unbalance factor (four-wire line) produces higher field levels than a similar increase in negative-sequence unbalance factor. The resultant magnetic flux density increases linearly with the magnitude of the zero- sequence unbalance factor. As much as 240 % increment over the balanced line case is observed for a 50 % zero-sequence current unbalance (m 0 ) factor. Measurements show that the zero-sequence current phase unbalance remains small (within 5 degrees) and unaffected by the variations in the magnitude component for all cases under study. I. INTRODUCTION The major cause of power distribution system unbalance is unbalanced single-phase loads. Residential loads in particular are dependent on daily activities as well as on seasonal weather. Other large single-phase industrial loads employing large motors draw large unbalanced currents. Previous and continuing research work focuses on addressing power quality problems as a result of power system unbalances [1-4]. In fact, harmonics are produced by nonlinear loads such as converters, motors, transformers, fluorescent lamps, computer peripherals, copiers, etc., which are commonly found in residential, business and industrial loads. These have an important effect on the system power factor and line losses as well as the magnetic field strength which is greatly influenced by current unbalances. The minimum field occurs when the line currents are balanced. Olsen et al. [4] have found that magnetic fields are very sensitive to small deviations from positive sequence currents due to load unbalance. This may cause magnetic field levels off the right-of- way to be higher than those predicted in a balanced line. The objective of this work is mainly to study the effect of current unbalance on resultant magnetic field levels. We first derived the analytical expressions for the resultant magnetic flux density in terms of the unbalanced three-phase currents using the symmetrical components approach. Then we wrote a computer program to simulate the influence of the zero-sequence and negative-sequence current unbalance factors on the resultant magnetic flux density for flat (A-B-C, B-A-C and C-B-A) and bundled three-phase distribution-line configurations. Finally, we build a small-scale lab setup to measure the magnetic field strength under current unbalance conditions for the aforementioned line configurations. For voltages below 11 kV, most underground distribution lines are in bundled 3-wire or 4-wire cables. Only, 11-kV overhead lines may be found in a flat configuration. However, multiple underground circuits are placed in a flat arrangement close to each other. We followed Chen's definition [1] for the complex unbalance factor in terms of the symmetrical current components (I o , I 1 , I 2 ) with the positive-sequence current being the base current (m 0 =I 0 /I 1 = m 0 ∠β 0 ; m 2 =I 2 /I 1 = m 2 ∠β 2 ). Both the magnitude and argument of the unbalance factors were considered in our computer simulation. We mainly focused on two unbalance cases, namely, (Case 1) m 0 ≠0 and m 2 =0, and (Case 2) m 2 ≠0 and m 0 =0 to isolate the effect of the zero- sequence and negative-sequence currents under different unbalanced conditions. Case 1 applies when a neutral wire is connected for a four-wire distribution line, while Case 2 applies to a three-wire distribution line with no neutral wire connected (hence the zero- sequence component can not be supported). The case when both the zero- and negative- sequence current unbalance factors are present was done but not discussed here. In each case, we compute the resultant magnetic field directly under (or above for buried lines) the distribution line as a function of the unbalance argument (β 0 or β 2 ) for different values of the unbalance magnitude (m 0 or m 2 ). This approach was previously followed by Chen [1] to study the effect of load unbalance on line losses. In both computer-simulation cases, we considered a base