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Friday, April 26, 2013

Power Quality Issues I: Voltage Sag and Swell


      Power Quality is one of the trend topics in energy industry. The term "Power Quality" simply focuses on the measurable features of the supplied power in a power system. Supplied power for some load is basically said to have quality (actually this is an hypothetical term), if the supplied voltage magnitude is at rated value and the frequency of it is constant at system synchronous frequency with no other frequency components. In addition to this simple definition, the term power quality can be enlarged to general term to represent a power system that does not contain any unintended, harmful components of voltage, current or power and any unintended phenomena at supplied voltage.
     All loads in a power system are designed to operate at its design specifications which are determined considering the rated values of an ideal mains supply. Thus, any unintended failures, voltage phenomena or harmful current components nearby are harms most of the loads. Actually, some of them are very vulnerable to unintended behaviours of mains. A supply with less quality (do not forget this is an hypothetical term) causes trouble to industrial loads and consequently this means the loss of labor, time and money for industrial plants. The power quality problems may be seen in supply voltage, frequency or nearby currents.In this article, I will mention one of the supply voltage problems that the industrial plants mostly face namely "voltage sag and swells".
     These kind of distortions  are usually due to overloading of huge loads, short-circuit failures or lightning discharges. Voltage sag is defined as the case when the magnitude of the mains voltage is reduced below 90% of rated voltage for a short while. Similarly, voltage swell is defined as the case when the mains voltage is increased above 110% of rated voltage for a short while. Actually, the interconnected network can not prevent these situations; however, it can only be protected by the protection devices after the failure is detected. In medium voltage level, the typical time that should be passed for a protection device to act on a failure is 100 msec. Thus the voltage sags and swells may last typically from 100 msec to a couple of seconds. During this time the voltage magnitude is reduced/increased significantly and the vulnerable loads should be protected during this time interval. Figure 1 illustrates a real voltage sag which is measured by a Class A power quality measurement device in field. As can be seen in figure 1, the three phase voltage sag is so severe that the supply voltage is dropped down to 90 V from 210 V.

Figure 1: Three Phase Simultaneous Voltage Sag

Similarly figure 2 illustrates a real voltage sag and swell happenning at the phase voltage components at 20kV Medium Voltage (MV) level.

Figure 2: Voltage Swell and Sag in a Three Phase Voltage

Actually, the voltage sag or swell lasts for a very short time in degrees of some hundreds of miliseconds; however, they may cause harmful results for industry. The harmful results of voltage sag/swell may be stoppage of pruduction, damages on motor drives, damages on raw or processed materials etc. For example, figure 3 represents the supply voltage and load current of an industrial printing house at the instant of the voltage sag occus.


As can be seen from figure 3, after voltage sag the load current is significantly reduced, which means a production stoppage. For these kind of instants ,actually, their motor drives loses their states and the paper inside the rolling machines are cut due oscillations in motor torque. This situation causes printing office to lose 2 hours in production and they should be throw away both the printed and raw paper which is inside the machine at the instant that voltage sag/swell occurs.
   
     The solution for this issue
     Power electronic sytems can solve this problem, since they detect the failure and act in about some tens of microseconds. Some of the systems that can solve this problem is Dynamic voltage restorer (DVR) and dynamic UPS.

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