2.1 State THREE advantages of a three-phase distribution system over a single-phase distribution system - NSC Electrical Technology Electronics - Question 2 - 2017 - Paper 1

A diagram depicting a three-phase voltage waveform should include three sine waves, each representing the phases R (Red), Y (Yellow), and B (Blue). The waves should be evenly spaced, with a phase difference of 120° between each:

• The first wave (R) peaks at $+V_m$.
• The second wave (Y) peaks at $+V_m$ but lags by 120°.
• The third wave (B) peaks at $+V_m$ but lags by 240°.

The x-axis should represent time, while the y-axis should represent voltage.

One disadvantage of the two-wattmeter method is that it can yield inaccurate readings if the power factor is very low (close to 0), as the method may not effectively account for the reactive power component, leading to poor measurement of total power within the system.

To calculate the line voltage ($V_L$), we use the formula:

V_L = rac{S}{rac{ ext{√3}}{1} imes I_L}

Substituting the given values:

V_L = rac{25 imes 10^3}{rac{ ext{√3}}{1} imes 38} = 379.84 ext{ V}

To find the phase voltage ($V_{PH}$), we can use the formula:

V_{PH} = rac{V_L}{ ext{√3}}

Thus:

V_{PH} = rac{379.84}{ ext{√3}} = 219.31 ext{ V}

The impedance per phase ($Z_{PH}$) can be calculated using the formula:

Z_{PH} = rac{V_{PH}}{I_{PH}}

With $I_{PH}$ being equal to the line current $I_L$ in a star connection:

Z_{PH} = rac{219.31}{38} = 5.77 ext{ Ω}

Eskom could benefit from consumers improving their power factor by reducing wasted energy in their transmission systems. A higher power factor leads to more efficient use of energy, which in turn decreases the demand on generation capacity. This allows Eskom to have more power available for distribution, thereby enhancing the reliability of the power supply and potentially lowering electricity costs for consumers.