Active Load Balancing in a Three-Phase Network by Reactive Power Compensation

Abstract

1.1 Brief overview of the causes, effects and methods to reduce voltage unbalances in three-phase networks During normal operating condition, a first cause of voltage unbalance in three-phase networks comes from the asymmetrical structure of network elements (electrical lines, transformers etc.). Best known example is the asymmetrical structure of an overhead line, as a result of asymmetrical spatial positioning of the conductors. Also the total length of the conductors on the phases of a network may be different. This asymmetry of the network element is reflected in the asymmetry of the phase equivalent impedances (self and mutual, longitudinal and transversal). The impedance asymmetry causes then different voltage drop on the phases and therefore the voltage unbalance in the network nodes. As an example of correction method for this asymmetry is the well-known method of transposition of conductors for an overhead electrical line, which allows reducing the voltage unbalance under the admissible level, of course, with the condition of a balanced load transfer on the phases. But the main reason of the voltage unbalance is the loads supply, many of which are unbalanced, single-phase connected between two phases or between one phase and neutral. Many unbalanced loads, having small power values (a few tens of watts up to 5-10 kW), are connected to low voltage networks. But the most important unbalance is produced by high power single-phase industrial loads, with the order MW power unit, that are connected to high or medium voltage electrical networks, such as welding equipment, induction furnaces, electric rail traction etc. Current and voltage unbalances caused by these loads are most often accompanied by other forms of disturbance: harmonics, voltage sags, voltage fluctuations etc. (Czarnecki, 1995). Current unbalance, which can be associated with negative and zero sequence components flow, lead to increased longitudinal losses of active power and energy in electrical networks, and therefore lower efficiency. Voltage unbalance causes first negative effects on the rotating electrical machines. It is associated with increased heating additional losses in the windings, whose size depends on amount of negative sequence voltage component. It also produces parasitic couples, which is manifested by harmful vibrations. Both effects can reduce the useful life of electrical machines and therefore significant material damage. Transformers, capacitor banks, some protection systems (e.g. distance protection), threephase converters (three-phase rectifiers, AC-DC converters) etc. are also affected by a threephase unbalanced system supply voltages.