Detecting, Locating and Assessing

Voltage Sags

Presented by

Jilani Saiad

1691210043

Road Map of the presentation

Definition of Voltage Sag.

Detecting VS by Wavelet transformation.

Locating VS by making use of Real current component.

Assessing VS by the method of critical distances.

Conclusion.

References. Audience time

What is Voltage Sag?

• Voltage sag is a short time (10 ms to 1 min) event during which a

reduction in RMS voltage magnitude occurs.

• Caused by multitude of reasons viz. switching heavy loads or

starting large motors.

• Sag is different from voltage dip.(VD=100-VS) expressed in %.

• The production process comes to a halt.-The disturbance

may be over within one second, but the restoration process

may take several hours.

• Characterised by Magnitude and Duration.

Detecting Voltage Sag by Wavelet transformation

Wavelet Transformation???

• A wavelet is a waveform of effectively limited duration that has an average value of zero.

• Decomposing the signal into two parts is called WT.

• 2 parts are approximations, the high-scale, low-frequency components of the signal and details are the low-scale, high-frequency components.

Voltage Signal Decomposition

• Daub4 wavelet is used as mother wavelet in computation here.

• First level of the transformed signal clearly shows a peak at the beginning and end of the voltage sag.

Locating Voltage Sag by

Real current component

Why do we need to locate???

• Only after information about a voltage sag source location is available, can power-quality trouble-shooting, diagnosis and mitigation be carried out.

• Any disputes among the major responsibility party can be resolved fairly.

• Phase angle difference between current and voltage is used to determine the voltage sag source.

• In the method, magnitude of currents and phase angles of voltages and currents are measured at the measuring point.

• The current magnitude is then multiplied with the cosine of the power factor angle and the product is then plotted against time.

• The product polarity is used to indicate the direction of voltage sag source either it is from upstream or downstream relative to the measuring point.

• A positive polarity of the product will indicate the voltage sag source is from downstream.

• A negative polarity of the product will indicate that the voltage sag source is from upstream.

Behind the scene

• Consider a fault at X and two monitoring points MA and MB.

• For a fault at X, using Kirchhoff's second law, the voltage at MA will be:

V=E1-IZ1 V and I are the voltage and current measured directly at MA.

• Multiply both sides of the Eq by I* gives

VI∗ = E1I∗ − I2Z1

• Extracting the real part will lead to

VI cos(ɵ − α) = E1I cos(φ1 − α) − I2R

• where θ and α are the phase angles for voltage and current at the

measuring point MA, respectively. The phase difference (θ − α) is the power factor angle at the monitoring point.

Algorithm • Create a voltage sag condition by simulating a short-circuit fault at

a specified location.

• Obtain the magnitude and phase of voltage and current from the measuring meter at pre-fault and during fault times.

• Calculate the values of I cos(ɵ−α) for a few cycles of pre-fault and

during fault durations.

• Plot coordinates of I cos(ɵ−α) against time of a few cycles of pre-fault and during fault durations.

• Check the polarity of I cos(ɵ− α) at the beginning of fault.

• If it is positive, the source of voltage sag is from downstream.

• If it is negative, the source of voltage sag is from upstream.

Sample plot

Voltage sag calculation:

• This method is a simple prediction technique based on the voltage divider model.

• The voltage sag magnitude at the point of common coupling (pcc) during the fault is represented as

where ZF and ZS are the feeder and source impedance, respectively.

The pre-event voltage is assumed 1 pu, thus E = 1.

This results in the following expression for the sag magnitude as

Method of critical distance:

• For every load, there is a critical voltage, VCrit is defined as the voltage below which the equipment will trip.

• The critical distance is the distance from the pcc at which a fault would lead to voltage below VCrit at the pcc.

The critical distance for a voltage VCrit is given by

where α is the impedance angle, i.e., the angle in the complex plane

between source and feeder impedance 𝑧 is the feeder impedance per

unit length.

The critical distances as a function of the critical voltage can also be expressed in simplified form as in the eq below, which is used to estimate the exposed area at every voltage level in the supply to a sensitive load.

Voltage divider model: Impedance angle

CONCLUSION:

• WT method uses discrete wavelet transforms to determine beginning and ending of the voltage sag with sag magnitude. It has the advantage of finding voltage sag magnitude and duration with real time operation and low cost implementation.

• The proposed real current component method satisfy 100% of the voltage sag source location for the three-phase balanced and unbalanced faults for the one-source system. The method has also been verified for the two-source system in which it has given very promising results.

• The method of critical distance is the more accurate one ,This proposed method provides good accuracy in the results in comparison with the method of fault positions for SLGF, LLF and 3PF.

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industrial distribution systems. IEEE Trans Ind Appl 1998;34(4):682–8.

Parsons AC, Grady WM, Powers EJ. A wavelet-based procedure for

automatically determining the beginning and end of transmission system

voltage sags. In: Proc. IEEE-PES Winter Meeting, vol. 2; 1999. p. 1310–5.

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7. M.H.J. Bollen, Voltage sags in three-phase systems, IEEE Power Eng. Rev. 17 (2001)

• M.H.J. Bollen, Understanding Power Quality Problems, IEEE Press, 2000, pp. 1–34.

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IEEE Trans. Ind. Appl. 29 (2) (1993) 397–403.

• IEEE Std. 1159-1995: Recommended Practice for Monitoring Electric Power Quality. ISBN 1-55937-549-3.

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Queries???

Thank You