SOURCES OF UNBALANCE
Unbalance may generate due to the unequal drops in individual phase or unbalanced in three phase loading. Even it may happen due the source, load, improper grounding, etc. The main causes of voltage due to unbalance in power systems are• Unbalanced single-phase loading in a three-phase system: most of the domestic loads and industrial lighting loads are single phase. However, these loads are fed from three phase supply. If the load divisions among different phases are not coordinated, the phase parameters may differ from each other causing unbalanced demand from the supply.
• Overhead transmission lines that are not transposed,
• Blown out fuses in one phase of a three-phase capacitor bank, and
• Severe voltage unbalance (e.g., >5%), which can result from single phasing conditions.
EFFECT OF UNBALANCE
Unbalance in power system is related to the power system stability problem. Unbalance may cause excessive drawl of reactive power, mal-operation of equipment, mal-operation of measuring instruments and shortening of life span of different appliances. Reactive power compensation required for individual phase will differ from each other. Performance of FACTs controller degrades during voltage unbalance.During current unbalance negative sequence component appears. It increases net current in some phase and decreases net current in other phase. This results in unequal loss in phases and unequal heating. Three phase motors draw unbalanced current from unbalanced supply system. In such situation, unequal heating and oscillation in torque hamper motors’ performance. In ungrounded system, unbalance causes neutral shifting. This hampers accurate operation of relays and circuit breakers along with other related problems.