Electrical Plant Lab2

AIM:

To measure and explore the effects of varies connections including both star and delta by analysing the waveforms generated by the CRO to calculate the current, voltage and power throughout the three phase AC networks.  

Methodology:

To gather and collect all the relevant information needed for lab 2 we had to measure and calculate the varies load elements of voltage using the voltage probes of the Yokogawa, current using the V/A/W around the relevant conductor and all real powers by clamping again the V/A/W meter around the relevant current carrying conductor and the probes around the related voltage.

Task one was analysing a 4 wire (with neutral) connection with load elements of around 3.0KW and a rated voltage of 240V. By adjusting the variac to 100V we were able to calculate using the Yokogawa wattmeter accurate RMS measurements of the line-neutral voltages, line to line voltages, phase currents, neutral current and the real power for each phase of the circuit. As we progressed through the task 1 we had to record the waveforms onto the CRO by using the differential voltage probe and the Tek current probe. Because we were working with three phase circuits we had to calculate the RMS measurements three times for each phase. Once we had calculated the balanced circuit we had to then repeat the same process but this time for an unbalanced circuit. Reducing the R element down from 3.0KW to 1.0KW at a voltage of 240V.

Task two part B of this second lab consisted of a similar star connected circuit but this one was a 3-wire (no neutral) connection.  By continuing to use the same circuit from part a we only had to remove the neutral connection from the load bank making too distinct separate nodes between the load star point connection and the supply neutral connection. By adjusting the variac to 100V we can again calculate the various RMS measurements as well as using the CRO to plot the various RMS measurements. This task also asked us to calculate using two Yokogawa wattmeters the power readings between the blue phase and the red phase as well as reducing the load element down to 1.0KW to create an unbalanced circuit and repeating all calculations in RMS.

The final task involved making up a delta connected circuit applying the same steps as the previous tasks calculating the RMS values for the various loads.

Results:

4-Wire (With Neutral) connection:

By adjusting the variac to 100V we calculated the following results with the Yokogawa wattmeter:

Notes:

If the loads are not balanced the neutral return will not be close to zero.

3KW for all three loads (phases) neutral return is 20mA.

[pic 1][pic 2][pic 3]

[pic 4]

Phase diagram of the line and phase voltages of the three phase generator.

To create an unbalanced load circuit we had to adjust the load element down to 1.0KW with a voltage of 240V.

ANALYSIS:

We can see that for the neutral current for the balanced circuit we are getting around 0.2A, compared to the unbalanced circuit where we are getting around 3.5A. This is because the neutral current for a balanced circuit is zero as the phase currents are 120 degrees apart as seen above in the phasor diagram. In addition the magnitude of the current in each phase will be equal with each other, whereas for an unbalanced circuit the neutral current will not be zero as a result not cancelling out by the other two currents at the neutral point.