VOLTAGE SAG AND IT’S MITIGATION

By –Mayur Dilip DhandeB.E. ElectricalGovernment College of Engineering, Jalgaonmayurdhande11@gmail.com

GUIDED BY :Prof. D. P. YAVALKAR

CONTENTS

Voltage sag Sources of voltage sag General causes of voltage sag Characteristics of voltage sag Effects of voltage sag on commercial installations Measurement and characterization of voltage sag Impact and cost of voltage sag Mitigation of voltage sag Conclusion References

VOLTAGE SAG A decrease in RMS voltage at the power frequency

for durations from 0.5 cycles to 1 minute, reported as the remaining voltage.

TYPES1)Single phase sags2) Phase to phase sags3) Three phase sags

SOURCES OF SAGS AND SHORT INTERRUPTIONS 

Power systems have non-zero impedances, so every increase in current causes a corresponding reduction in voltage.

There are two sources of voltage sags: 1) Large increases in current. 2) Increases in system impedance. Think of the power system as a tree Facility-sourced voltage sags Utility-sourced voltage sags

GENERAL CAUSES OF VOLTAGE SAGS VOLTAGE SAGS DUE TO FAULTS May be symmetrical or unsymmetrical in nature

 VOLTAGE SAGS DUE TO MOTOR STARTING symmetrical in nature

VOLTAGE SAGS DUE TO TRANSFORMER ENERGIZING Unsymmetrical in nature

CHARACTERISTICS OF VOLTAGE SAG MAGNITUDE OF SAG:• Type of fault• Fault impedance• System Configuration• Distance of the fault from the point of consideration DURATION OF SAG:

PHASE-ANGLE JUMP: affects power electronic convertors  POINT-ON-WAVE: phase angle at which the sag occurs

EFFECTS OF VOLTAGE SAG ON COMMERCIAL INSTALLATIONS

CHILLER SYSTEMo causing the trip LIGHTING CIRCUITSo flickering of the lights (HID) ESCALATORSo control contactors and PLC of the escalator may drop off GENERAL CIRCUITSo Tripping of MCB and RCD

MEASUREMENT & CHARACTERISATION OF VOLTAGE SAGS

measured using specialised power quality monitoring instrumentation

IMPACT & COST OF VOLTAGE SAGS Typical Financial Loss for Voltage Sags based

on Industry

Industry Financial loss per event

Semiconductor industry 3800000Financial Trading 6000000 per hour

Paper manufacturing 300000Telecommunications 30000 per minute

Steel works 350000Glass industry 250000

Automobile industry 75000

MITIGATION OF VOLTAGE SAG

DYNAMIC VOLTAGE RESTORER (DVR)   D-STATCOM PWM switched auto-transformer SOLID STATE TRANSFER SWITCH (SSTS)

DYNAMIC VOLTAGE RESTORER (DVR) The DVR is a series connected power electronic device

used to inject voltage of required magnitude and frequency.

Basic structure

DVR OPERATION:

The nominal or rated voltage is compared with the voltage variation and the DVR injects the difference voltage that is required by the load.

Here Vs is the supply voltage, Vinj is the voltage injected by the DVR and VL is the load voltage.

Equivalent Circuit Diagram of DVR 

D-STATCOM

D-STATCOM is a shunt connected device designed to regulate the voltage either by generating or absorbing the reactive power.

Basic Structure

OPERATION OF D-STATCOMA D-STATCOM is capable of compensating either bus voltage or line current. It can operate in two modes based on the parameter which it regulates.

Voltage Mode OperationCurrent Mode Operation Inductive OperationCapacitive Operation

Block Diagram of the Control Circuit of D-STATCOM 

PWM SWITCHED AUTO-TRANSFORMER

The auto transformer is controlled by a PWM operated power electronic switch.

Voltage Sag Mitigation Scheme Using Auto Transformer

CONTROL STRATEGY

When there is voltage sag then an error occurs and based on the error value PWM generator generates pulses to the IGBT switch. Accordingly, the auto-transformer operates and the load voltage is maintained constant.

Single-phase Circuit Diagram during Voltage Sag

CONCLUSIONAmong the different methods to mitigate the voltage sag, the use of FACT

devices is the best methodThe FACT devices like DVR, D-STATCOM are helpful in overcoming the

voltage unbalance problems in power systemD-STATCOM is a shunt connected device and injects current into the systemThese devices are connected to the power network at the point of interest

protect the critical loadsThese devices also have other advantages like harmonic reduction, power

factor correctionThe amount of apparent power infusion required by D-STATCOM is higher

than that of DVR for a given voltage sagDVR acts slowly but is good in reducing the harmonic contentBoth DVR and D-STATCOM require more number of power electronic

switches and storage devices for their operation. To overcome this problem, PWM switched auto-transformer is used for mitigating the voltage sag

Here the number of switches required are less and hence the switching losses are also reduced

 

REFERENCES[1] R.C. Dugan, M.F. McGranaghan, and H.W. Beaty, “Electric Power Systems Quality,” McGraw-

Hill, 1996.

[2] “IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems,” IEEE Std. 493-1997, December 1997.

[3] IEEE Standards Board (1995), “IEEE Std. 1159-1995”, IEEE Recommended Practice for Monitoring Electric Power Quality”. IEEE Inc. NewYork

[4] AmbraSannino. Mitigation of voltage sags and short interruptions through distribution system design. Dept. of Electrical Engineering University of Palermo, pp 1-6,2000.

[5] David Chapman, Power Quality Application Guide - The Cost of Poor Power Quality, Copper Development Association, 2001.

[6].Haque, M. H., "Compensation of distribution system voltage sag by DVR and DSTATCOM,"Power Tech Proceedings, 2001 IEEE Porto , vol.1, no., pp.5 pp. vol.1,, 2001.

[7]Venkatesh, C.; Reddy, V.P.; Siva Sarma, D.V.S.S., "Mitigation of voltage sags/swells using PWM switched autotransformer," Harmonics and Quality ofPower, 2008. ICHQP 2008. 13th International Conference on , vol., no., pp.1,6, Sept. 28 2008-Oct. 1 2008.

[8] K. Chan, A. Kara, and G. Kieboom, “Power quality improvement with solid state transfer switches,” in Proc. 8th ICHQP 1998, Athens, Greece, Oct. 1998, pp. 210-215