Oct 17, 2013

Voltage Sag - (Dip)



voltage-Sag Wave-form

Voltage sags are the foremost common power disturbance. At a typical industrial website, it's common to visualize many sags per annum at the entrance way and much additional at instrumentality terminals.

Sag could be a decrease amongst 0.1 and 0.9 PU in rms current or voltage at the power frequency for duration's from 0.5 cycles to one min. The power quality civic has used the word sag for several years to explain a short duration voltage reduction. Though the word has not be there properly outlined, it's been progressively accepted and employed by manufactures, utilities and final users. The IEC explanation for this occurrence is dip. The 2 terms are thought of exchangeable, with sag being the most well-liked word within the U.S. power quality community. Word wont to define the magnitude of voltage sag is usually confusing. A “20 % sag” will check with a sag that leads to a voltage of 0.8 pu or 0.2 pu. The popular word would be one that leaves little question as to the ensuing voltage level: “a sag to 0.8 pu” or “a sag whose magnitude was 20%.” once not such otherwise, a 20% sag are going to be thought of considered event throughout that the rms voltage attenuated by 20 % to 0.8 pu. The base or nominal voltage level should even be such.

Voltage Sag Caused by SLG Fault

 
RMS-waveform-sag                                                          RMS waveform for Voltage sag
voltage-sag-waveform                                                           Voltage Sag waveform

Voltage sags are typically related to faults however will even be triggered by energization of heavy loads or beginning of huge motors. The fig above has shown the typical sketch voltage sag that may be related to a single-line to ground (SLG) fault on other feeder from the similar substation. Making an allowance for 80% sag exists for almost three cycles till the sub-station breaker is in a position to interrupt the faulty current. Usual fault rectifying times vary from three to thirty cycles, looking on the fault current magnitude and the sort of over-current protection.

voltage-sag-waveform-due-to-motor-starting                                              Voltage Sag Waveform due to motor starting

In this figure we have seen the impact of an oversize motor beginning. Bearing in mind induction motor can draw nearly 6 to 10 intervals its full load current throughout start up. If the current magnitude is massive relative to the on the obtainable fault current within the system at that time, the ensuing voltage sag is vital. During this case, the voltage sags right away to 80% and then gradually returns to traditional in regarding three seconds. Note the distinction in time frame amongst this and sags owed to utility network faults.

Until recent efforts, the period of sag events has not been clearly outlined. Typical sag period is outlined in some publications as ranging from a pair of ms (about common fraction of a cycle) to a couple of minutes. Under voltages that last less than simple fraction cycle can't be characterized effectively by a modification within the rms worth of the elementary frequency worth. Therefore, these events are thought of transients. Under voltages that last longer than one min will usually be controlled by voltage regulation instrumentality and might be related to causes aside from faults. Therefore, this are classified as long duration variations.

Sag duration's can be divided here into 3 categories instantaneous, momentary, and temporary which coincide with the 3 classes of interruptions and swells. These periods are meant to correspond to typical utility protecting device operation times additionally as duration divisions suggested by international technical organizations.


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