Power supplies harmonic distortion

A distribution network 33 kV, three-phase 50 Hz feeding an industrial belt with a number of medium-sized factories some with non-linear loads and some with static drives and some with both. It was observed that while the lines were apparently running reasonably loaded, the active power supplied was much below the capacity of the network. Accordingly, a harmonic study of the network was conducted and it was found that despite localized p.f. control by most factories, the p.f. of the network itself was well below the optimum level and the voltage was also distorted by more than was permissible. To improve this network to an acceptable level, we have considered the following load conditions, as were revealed through the analysis. 

The terms displacement and true power factor, are widely mentioned in power factor studies. Displacement power factor is the cosine of the angle between the fundamental voltage and current waveforms. The fundamental waveforms are by definition pure sinusoids. But, if the waveform distortion is due to harmonics (which is very often the case), the power factor angles are different than what would be for the fundamental waves alone. The presence of harmonics introduces additional phase shift between the voltage and the current. True power factor is calculated as the ratio between the total active power used in a circuit (including harmonics) and the total apparent power (including harmonics) supplied from the source ... 

In recent times there has been a rapid development in the design of high-power transistors, to such an extent that they are feasible alternatives to thyristors for many applications. The main advantage of transistors is that they can be switched on and off at any point in the conducting half-cycle that can appear across their emitter and collector terminals. They must be protected against the reversal of voltage when the second half-cycle appears across the terminals. It is therefore possible to synthesise the waveforms in such a manner as to reduce the harmonic distortion at the supply terminals to a low level. 

The rectilier(s) must do this job with high reliability, high efficiency, and minimum disruption to the external power supply. The last-named objective requires a high power factor, to avoid excessive transmission losses or burden to the supplier s kV A capacity, and an acceptably low feedback of harmonic currents into the power supply. The problem of low power factor and methods for its correction (in this case, installation of capacitor banks, dealt with in Section 8.3.2.3) are familiar subjects even to many nonelectrical engineers. The consideration of harmonic distortion of the waveform, which can cause distortions in the power grid voltages and interference with other customers devices, is less so. 

The generation of harmonics also affects the power factor by distorting the waveform. This distortion results from the combination of the higher-frequency harmonics with the fundamental frequency. A six-pulse rectifier, as pointed out in Section 8.3.1.3A, produces the 5th, 7th, 11th, 13th,. .. harmonics. These have frequencies of 250, 350, 550,650,... Hz when connected to a 50-Hz supply. The output signal is the sum of the... 

The second component is caused by the different harmonic quantities present in the system when the supply voltage is non-linear or the load is nonlinear or both. This adds to the fundamental current, /,- and raises it to Since the active power component remains the same, it reduces the p.f of the system and raises the line losses. The factor /f/Zh is termed the distortion factor. In other words, it defines the purity of the sinusoidal wave shape. 

All such loads generate harmonics and cause variations in the fundamental power frequency of the supply sy.stem which leads to distortion in the sinusoidal waveform of the voltage. This distortion may affect the quality of the supply system (voltage) beyond desirable limits, A non-sinusoidal and distorted supply system may adversely affect the different loads connected on the system, besides leading to outage of the sy.stem itself. 

The load uses current (7i) in a nonlinear fashion. The harmonic portion of the current Ii, is— by definition—everything that is not the fundamental frequency component. The APLC becomes the source of Ih for the load circuit, leaving only the fundamental frequency portion (If) to be supplied by the power system. Because the harmonic portion of the load current is supplied locally by the APLC, the distortion power factor of the load is near unity. 

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