Harmonic effects

Effects of harmonics in the power system can be categorised into two main categories;

network level and system component level.

Table 2. Type of harmonics and their sources [5].

Figure 2. Harmonic voltage propagation due to non-linear loads [5].

Figure 2. depicts a small network comprising of linear and non-linear loads connected through the system impendence to a sinusoidal voltage source. Here, non-linear loads draw a harmonic current from the network. This current while flowing through the system impedance will cause a voltage drop (harmonic voltage drops to be precise) across the network. Hence, after subtracting this harmonic voltage drop from the source voltage it will be noticed that resultant voltage at the point of common coupling (PCC) has become distorted due to the harmonics. Thus, harmonics caused at the local level (non-linear load) have the widespread effect (network level) in power system, if no curative step is taken at the point of origin. For instance, voltage harmonics at the connection point of motor cause the harmonic fluxes to be produced inside the motor windings. These har-monic fluxes effect the rotation frequency in a way that motor starts rotating with a fre-quency other than the synchronous one. Hence, the additional fluxes, caused by voltage harmonics, produce high frequency currents inside the motor which further results in additional losses, heating, decreased efficiency and vibration in rotor. [5]

Harmonics have negative impacts on various system components. Speaking of trans-former, additional heat is the most vital effect caused by the harmonic content available in the network. This extra heat deteriorates the transformer’s rating to operate within the defined temperature limit. Moreover, in the presence of harmonics, a transformer’s in-ductance may cause resonance with the system’s capacitance. [7] Here the concept of resonance has been explained in detail at the end of this subchapter. Moving further, Elmoudi et al. [8] suggested that current harmonics contribute to increase in eddy current and stray losses of the transformer. Distortion in current may cause the protection relays to trip in unfaulty condition and failing in tripping during a fault [7]. In the case of rotating machines, harmonics result in increasing the copper and iron losses along with pulsating torque, which is produced due to the interaction of harmonic and fundamental component of the magnetic field [9].

As mentioned earlier, in term of phasor rotation with respect to fundamental frequency, harmonics can be categorised as positive, negative, and zero sequence component.

Each of these harmonic sequences affect the power system differently. Due to same phaser rotation with the fundamental component, positive sequence harmonics add up with the fundamental component and result in a composite waveform. Peak to peak value of this composite waveform might be higher than the peak to peak value of fundamental waveform alone. Thus, this higher peak to peak value or increased magnitude in general may contribute in causing the overloading of conductors, transformers and power lines.

[5]

On the contrary, due to the opposite rotation with the fundamental component, negative sequence harmonics give birth to pulsating mechanical torque inside an induction motor.

Being the worst ones, zero sequence harmonics (also known as triplens when system is balanced) circulate between the phases and neutral (or ground) wire of a three-phase system. Amount of zero sequence harmonic current in neutral wire is three times higher than its value in phase wire, since all phase currents coincide in the neutral. Conse-quently, the main effects due to the zero sequence harmonics are overloading of neutral conductor and telephone interference. Type of transformer winding connection plays a big role in preventing the propagation of zero sequence harmonics. For instance, in

“grounded wye-delta” type of transformer zero sequence harmonics enter from the wye side and due to their nature of zero displacement, they sum up in the neutral conductor.

However, on the other side of the transformer where windings arrangement is in delta form, these harmonics can enter and flow inside the delta due to its ampere-turn balance property. But these harmonics remain trapped there and do not appear in the delta side-line current of the transformer. [5]

Capacitor banks used for the purpose of power factor correction and voltage support may sometimes contribute to causing harmonic resonance. [10] Capacitor may appear also from cables, which have high capacitance. Basically, every circuit comprised of ca-pacitance and inductance may have one or more natural frequency of resonance and when this frequency matches with the frequency of harmonic current produced by a non-linear load then amplification of that harmonic current often occurs. Such a phenomenon of harmonic current amplification due to the system resonance is known as harmonic resonance. [11] This amplified harmonic current flowing in the network may result in breaker tripping, blown fuses, audible noises in the capacitor, and disrupting the opera-tion of neighboring equipment [10].

Harmonic resonance can be further divided into two main types as series and parallel resonance. Parallel resonance occurs when the system inductance and power factor correction (PFC) capacitor are in parallel to the harmonic producing source. On the con-trary, when system inductance and PFC’s capacitance are in series with respect to har-monic sources, then series resonance occurs. [6] As Eghtedarpour et al. [10] discussed, in the case of parallel resonance, the net impedance at resonant frequency, seen from the harmonic source side become very large. Further, multiplying this impedance with the harmonic current (even small) flowing through the network will cause significant dis-tortion in voltage. High disdis-tortion in voltage will be at the remote point in case of series resonance and harmonic source (e.g., non-linear load) side in case of parallel resonance [6]. Traditional solutions to prevent harmonic resonance is to use reactor in series with

the PFC capacitor, which further act as a tuned filter circuit [12]. However, the main problem with such combination of the filter circuit is that it gets de-tuned due to ever changing dynamics of the system network; change in system impedance caused by net-work switching, change in load, etc [12].