Understanding Voltage Sag Calculation And Solutions

Demonstrating deep understanding of voltage sag calculations and describe 5 short problems also.

How to effectively calculate the dip duration of voltage sag?

How can we effectively calculate the dip duration of voltage sag?

The measurement of the sag duration is quite difficult than what it seems to the entire world.  Right from recording the duration of the sag which is quite obvious but to complete it with the automatic way for better monitoring of power quality to obtain the sag duration which is not so straightforward. The threshold will be quite different for each of the monitor but all the typical values are nearly 90% of the normal voltage (Deepa, & Rajapandian, 2010) . The power quality monitor easily calculates the value of rms value for each cycle. The post fault sag can stand for several seconds but not much in comparison to the original sag. Hence, the sag duration is defined to be no longer equal to the time required for clearing fault. The principal form of protection in the distribution system is just pass the current protection technique which requires a certain amount time grading that increases the clearing time of the fault, even an exception is formed in the entire system in which the current limiting fuses are often used for better functioning.

The magnitude of the voltage sag determination is obtained from the rms voltage and hence the rms voltage is calculated over a one cycle window that is sliding.

The voltage is due to the finite measurement of the window that is often used for the calculation of the rms value. The value of rms is calculated during the sag as the process is not completely constant and so does the voltage does not immediately recover after the fault is committed. There are various methods to obtain the magnitude of sag right from the rms voltage ("Effects of Transformer Connection on Voltage Sag Characterization", 2016). The power quality of the monitors take up the lowest value that are obtained during the event and normally the sags have a constant rms value that is the most deep part of the sag by using the lowest value that is a near acceptable approximation.

There is a short circuit in the power system just not in the case of the voltage sag but also changes the phase angle of the voltage. The sudden change in the phase angle is often termed as the phase angle jump.  If the feeder and the source impedance have an equal ratio of X/R then there will no phase angle jump in the voltage at the PCC. This is also quite similar in the case for fault transfer in the transmission system as normally these are not responsible for the faults that are present in the distribution system (Heffernan, Watson, & Watson, 2014). For several unsymmetrical faults these analysis become quite complicated for voltage sag. The consequence of the unsymmetrical fault is the single phase load experience that has a phase angle jump that is even equal to the X/R   ratio of the source and feeder impedance.

To make proper use of the duration of the magnitude plot it is very important for all the customers that they must have a proper knowledge of the equivalent system or the immunity process of the voltage dips that are quite certain and has the maximum duration of the equipment that can standby. If we plot these for different dip magnitudes then the voltage tolerance curve can be easily created highlighting the immunity of the customers in respect to voltage dips (Jowder, 2009). The simple representation of the community is quite rectangle according to the magnitude duration window that grow right from the lowest right corner. The events of the dip that are inside the rectangle cause a great damage for the equipment and the ones that are outside the rectangle is harmless. Further extensive information on creation and interpretation is highlighted in the voltage tolerance curve.

How to accurately calculate the dip magnitude of voltage sag?

To have a clear idea of the entire picture of the voltage dip situation is quite different from the graphical presentation of all the calculated dips are produced. The mat lab analysis program helps to scatter plots and other voltage dip density chart and voltage dip coordination chart (Martinez, & Martin-Arnedo, 2006). The voltage dip coordination chart is quite helpful in estimating the severity of the dip situation of many customers simply by introducing the voltage tolerance curve. With the use of the cumulative chart and the voltage dip density chart we can generate a graphical presentation of the power quality as per the requirement.

Voltage Sag is a term that is often used for reduction in short duration in respect to rms voltage that can be easily caused by overloading of the electric motors or a short circuit. Voltage sag usually occurs when the rms voltage decreases from 10 to 90 percent of the normal voltage for more than one half cycles which is equivalent to one minute (Md. Shah Majid, 2008). There are several references that define the duration of the sag from the period of point half cycle to a few seconds and for a much longer duration of the low voltage section that is known as sustained sag.

There are several causes of voltage swag but generally it is caused by unknown increase in the load like motor starting, electric heaters while turned on, short circuit or else they are caused with increase in the source of impedance which is actually caused by loose connection. The voltage swells are actually caused by a sudden reduction in the circuit load and that too with a very poor or damaged voltage regulator (Shamsul Aizam Zulkifli., 2005). Voltage swag can also be caused because of natural loose connection or damage.

There are many factors that affect voltage sag or swells. All the important factors that are responsible for the above process are stated below:

• Power Distributor tolerances are not suitable for the sensitive voltage equipment.
• Unbalanced load all the three layered phase system
• Long distance from the transformer that is distributed with interposed loading.
• Rural remote location from the point of power source (T, & M, 2015)
• Grid system that is unreliable
• Unsuitable equipments for supply in the local area.
• Switch of the Heavy Loads

There are several methods of dealing with the sags and swells and all the important factors that support the above method are mention below:

• Constant Voltage transformer that is often known as ferro-resonant
• Transformer that comes with a tap changer
• Connects huge loads to a point of coupling
• Equipment should be chosen with dip resiience
• Saturable  reactor
• Huge electronic equipments that comes with a soft starters (Tanaka, & Sakashita, 2010)
• Power supply with switch mode
• Voltage stabilizer with servo controlled

In this modern business environment all the equipments that are used needs to be resilient to the characteristics that has several defects on the supply and in this case will go out of the shelf equipment. The correction cost is much less if the actions are taken right at the design stage of the equipment that also requires ample knowledge of the probability and nature of the defects, this technique is termed as the most cost effective approach ("Voltage Boosting and Restitution of Voltage Sag by making use of DVR", 2016). There is even some equipment makers that can easily recognize the problems but in todays competitive market that demands the manufacturer to work according to the requirement of the customer.

References

Deepa, S., & Rajapandian, S. (2010). Voltage Sag Mitigation Using Dynamic Voltage Restorer System. International Journal Of Computer And Electrical Engineering, 821-825. https://dx.doi.org/10.7763/ijcee.2010.v2.234

Effects of Transformer Connection on Voltage Sag Characterization. (2016). International Journal Of Science And Research (IJSR), 5(2), 2071-2075. https://dx.doi.org/10.21275/v5i2.nov161681

Heffernan, W., Watson, N., & Watson, J. (2014). Heat-pump performance: voltage dip/sag, under-voltage and over-voltage. The Journal Of Engineering. https://dx.doi.org/10.1049/joe.2014.0180

Jowder, F. (2009). Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation. IET Generation, Transmission & Distribution, 3(6), 547-560. https://dx.doi.org/10.1049/iet-gtd.2008.0531

Martinez, J., & Martin-Arnedo, J. (2006). Voltage Sag Studies in Distribution Networks—Part III: Voltage Sag Index Calculation. IEEE Transactions On Power Delivery, 21(3), 1689-1697. https://dx.doi.org/10.1109/tpwrd.2006.874111

Md. Shah Majid,. (2008). Simulation of voltage sag mitigation using D-STATCOM (1st ed.). Skudai, Johor: Pusat Pengurusan Penyelidikan, Universiti Teknologi Malaysia.

Shamsul Aizam Zulkifli.,. (2005). Simulation of linear feedback control of D-STATCOM for voltage sag mitigation (1st ed.). Serdang: Universiti Putra Malaysia.

T, G., & M, D. (2015). Voltage Sag/ Voltage Swell Compensation and Harmonic Distortion using Dynamic Voltage Restorer. IJIREEICE, 3(12), 97-102. https://dx.doi.org/10.17148/ijireeice.2015.31221

Tanaka, K., & Sakashita, T. (2010). Estimation of system voltage sag profile using recorded sag data. Electrical Engineering In Japan, 173(4), 9-19. https://dx.doi.org/10.1002/eej.21061

Voltage Boosting and Restitution of Voltage Sag by making use of DVR. (2016). International Journal Of Science And Research (IJSR), 5(4), 867-870. https://dx.doi.org/10.21275/v5i4.nov162682