Question

Harmonics

What effects could be experienced as a result of harmonics within

1) star transformer systems

2) delta transformer systems

3) a.c motors

4)Describe the term K factor

5) To imrove the power factor a capacitor bank has been installed. What effects would this have on the harmonics

Answer 1

The effects of the harmonic currents are

1. Additional copper losses due to harmonic currents

2. Increased core losses

3. Increased electro magnetic interference with communication circuits.

On the other hand the harmonic voltages of the transformer cause

1. Increased dielectric stress on insulation

2. Electro static interference with communication circuits.

3. Resonance between winding reactance and feeder capacitance.

------------------------------------------------------------------------------------------------------

1.Yy connection without neutral wires:

1. With both primary and secondary connected in star no closed path exists. As the triplen harmonics are always in phase, by virtue of the Y connection they get canceled in the line voltages. Non-triplen harmonics like fundamental, become ? 3 times phase value and appear in the line voltages.
2. Line currents remain sinusoidal except for non-triplen harmonic currents.
3. Flux wave in each transformer will be flat topped and the phase voltages remain peaked.
4. The potential of the neutral is no longer steady. The star point oscillates due to the third harmonic voltages. This is termed as ”oscillating neutral”.

Yy connection with neutral wires:

When a neutral wire is provided the triplen harmonic current can flow and the condition is similar to the single phase case (with a star connected 4 wire source or with the system earth). The neutral wire carries three times the triplen harmonic current of one transformer as these currents are co-phasal. Unloaded secondary neutral will not be operative.

-----------------------------------------------------------------------------------------

2.Dd connection:

1. In three phase banks with mesh connection on both primary side and secondary side a closed path is available for the triplen harmonics to circulate currents. Thus the supply current is nearly sinusoidal.
2. The triplen harmonic currents inside the closed mesh winding correct the flux density wave to be nearly sinusoidal. The secondary voltages will be nearly sinusoidal.
3. Third harmonics currents flow both in the primary and the secondary and hence the magnitudes of these currents, so also the drops due to them will be lower.

------------------------------------------------------------------------------------------------------

3. Generators and motors are adversely affected by harmonics. Typical effects are:

1. Increased heating due to iron and copper losses at the harmonic frequencies
2. Higher audible noise emission as compared with sinusoidal excitation
3. Harmonic currents in the rotor. These rotor harmonic currents will result in increased rotor heating and pulsating or reduced torque.

----------------------------------------------------------------------------------------

4. K-factor for an electrical network primarily relates to non-linear loads and is defined as a number representing the effect of harmonics on heating of transformer.

The “K” factor has been established to define the ability of a transformer to serve varying degrees of nonlinear load current without exceeding the rated temperature rise.

-----------------------------------------------------------------------------------------------

5.Effects of harmonics on capacitors:

1. Resonance imposes considerably higher voltages and currents in capacitors.
2. The capacitor bank acts as a sink for higher harmonic currents, which increases the heating and dielectric stresses.
3. The losses in a capacitor are proportional to the reactive output (kVAR), which, in turn, is proportional to the frequency. These losses are increased, and the overall capacitor life is shortened with increasing harmonics

------------------------------------------------------------------------------------------------------