Evidence shall show that knowledge has been acquired of safe working practices and providing solutions for problems in complex polyphase power circuits. All knowledge and skills detailed in this unit should be contextualised to current industry practices and technologies. KS01-EG149A Polyphase power circuit analysis Evidence shall show an understanding of polyphase power circuit analysis to an extent indicated by the following aspects: T1 Polyphase supply system encompassing: advantage of three phase system compared to single phase systems double subscript notation phase sequence 120 degree operator given circuit component parameters, solve practically based problems using: equivalent circuits of transformers, lines and loads. component values using rectangular and polar notation. current divider and potential divider rules using complex impedances. The “per unit” values of voltage, current, VA and impedance to a common VA base. T2 Types of three phase system connections encompassing: supply to balanced star, 3 and 4 wire loads supply to delta connected loads effects of phase reversal representation of currents and voltages as complex phasors for 3 phase and 3 phase and neutral quantities. calculation the values of and draw labeled phasor diagrams, not to scale, to represent complex values of current and voltage for balanced and unbalanced loads for star and delta systems. calculation of values of P, Q and S for balanced and unbalanced systems. draw and label single phase diagrams to represent 1 phase of a complex 3 phase system. represent unbalanced voltages or currents as symmetrical components. Phase to phase currents Phase to neutral/earth currents. T3 Balanced three phase loads encompassing: calculations of balanced loads connected in star calculations of balanced loads connected in delta calculation of steady state values of fault current for various configurations. evaluation of the symmetrical component impedances for the various distribution system components. Transformers (earthed neutral case). Generators (high impedance earth) calculation of fault currents using the per unit approach. calculation using the “worst case” values based on transformer impedance only (ie., a short circuit fault) estimation of peak values using accepted multipliers. effects of the d.c. component on the instantaneous magnitudes of fault currents in transformers and generators. T4 Unbalanced three phase loads encompassing: Star – 4 wire systems Delta systems Star – 3 wire systems Star 4 wire with neutral impedance T5 Power in three-phase circuits encompassing: summation of phase powers and power in balanced loads measurement of power in balanced loads – 2 Wattmeter methods T6 Reactive three phase power encompassing: power triangle calculation measurement of VAR power factor correction T7 Fault currents encompassing: symmetrical components positive, negative and zero sequence impedance fault current breaking and let-through energy capacities of circuit breakers, fuses importance of fault/arc impedance calculation of fault currents - phase-to-earth faults calculation of fault currents - phase-to-phase faults analysis of asymmetrical faults currents. T8 Harmonics in three phase systems encompassing: presence of triple in harmonics in 3 phase systems effects of 3 phase harmonics for different star and delta connections. methods for reducing harmonics in three phase systems. |