Download Three Phase Power Experiment: Measuring Real and Reactive Power - Prof. Donald Zinger and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! Experimant 4 Fall 2000 ELE 340 Page 1 of 5 ELECTRIC POWER SYSTEMS Experiment 4 THREE PHASE POWER Purpose: To investigate the behavior of a three phase power systems under various loading conditions. Background: As in all ac systems, the apparent power delivered in a three phase system is related to both the real and reactive power delivered. Depending on the actual components used, therefore, different amounts of current can flow for the same amount of real power delivered. By adjusting the reactive component of the load the current in being used to feed the load can be reduced. Measurement of power in a three phase system is done by measuring voltages and currents as in any electrical system. These systems become more complicated since the total power is the sum of the power in all three phases. If each of the three phases are balanced the power in each phase is equal and the total average power is give by three times the power in a single phase or P3φ = 3 VLN IL cos θ where P3φ = average real power VLN = rms line to neutral voltage IL = rms line current θ = phase angle between voltage and current Often only the line to line voltage is capable of being measured. But knowing the relationship between the magnitudes of line to neutral and line to line voltages the above relationship can be given as P3φ = 3 VLL IL cos θ where VLL = rms line to line voltage Note the power factor angle in this equation is the angle between line to neutral and line current. This is desirable for calculating the power factor. Wattmeters Experimant 4 Fall 2000 ELE 340 Page 2 of 5 Wattmeters work by finding the average of the product of voltage and current . This is done either electronically in newer meters or with the interaction of magnetic fields as in meters with coil movements. In either case connects must be made for both the current and the voltage. A typical wattmeter connection is shown in fig. 1. It is always a good idea to put an ammeter in series with the wattmeter to make sure the current coil of the wattmeter is not being overloaded. Wattmeter c v L O A D ± ± S O U R C E I V A Ammeter Fig. 1. Typical single phase wattmeter connection. A standard wattmeter measures only single phase power. If a it is desired to measure power in a completely balanced three phase system, a wattmeter could be used to measure the power in one phase and the result multiplied by three. Most systems, however, have slight voltages imbalances between the phases making this method of measurement inaccurate. To measure the power more accurately power could be measured in all three phases and summed together. The result of this measurement is could be mathematically represented by P3φ = 1 T ∫0 T ia(t) van(t) dt + 1 T ∫0 T ib(t) vbn(t) dt + 1 T ∫0 T ic(t) vcn(t) dt this can be simplified to P3φ = 1 T ∫0 T (ia(t) van(t) +ib(t) vbn(t) + ic(t) vcn(t))dt For a system with no neutral connection ia + ib + ic = 0 or ib = - (ia + ic) The three phase power becomes P3φ = 1 T ∫0 T (ia(t) van(t) -(ia(t)+ic(t)) vbn(t) + ic(t) vcn(t))dt or
