Types Of Power Quality Disturbances

A power quality problem is any disturbance manifested in voltage, current or frequency deviations which result in failure of equipment (3) A power disturbance may arise from several sources which include felling tree limbs, lightning strikes, feeder and load switching, operation of non-linear devices, and failure of a power system to maintain synchronous frequency. Power quality is a collection of different event types such as interruptions, rms variations, transients, and harmonics. Voltage sags, which can be caused by faults throughout the electrical transmission or distribution system, are the most serious of these power quality events. They can lead to the undesired shutdown of unprotected critical industrial processing equipment resulting in costly production interruptions. Another power quality phenomena is an oscillatory transient caused by capacitor switching. As more capacitor banks are added for power factor correction, transients during capacitor switching can get magnified. These transients may cause high transient voltages that can cause the malfunction of equipment and protective devices. Harmonic distortion is the non-linear distortion of a system characterized by the appearance in the output of harmonics other than the fundamental component when the input wave is sinusoidal. This is the result of connecting nonlinear loads such as adjustable speed drives, arc furnaces, compact fluorescent lights, and rectifiers. This is dependent on system impedance and the stiffiiess of the voltage source. Most problems occur when power factor capacitors cause the system to become resonant at a significant harmonic frequency. A harmonic-producing load can affect other loads if significant voltage distortion is caused. The resulting voltage distortion is a function of both the system impedance and the amount of current injected. The fact that the load current is distorted does not always mean that there will be adverse effects on other power consumers. If distortion on the voltage wave form exists, potential results include over-heating of transformers and motors, premature operation of protective devices including fuses, and metering inaccuracies. IEEE Standard 519, (4) specifies limits for harmonic current distortion that can be injected into the system. These limits are shown in Table 1. Values shown are in percent of "average maximum demand load current" and are applicable to twelve-pulse rectifiers.

Table 2. Harmonic Current Limits for SCDP Fuel Cell Power Plant

SCR=ISC/IL

h<ll

ll<h<17

17<h<23

23<h<35

h>35

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