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VOLTAGE SAG AND IT’S MITIGATION
By –Mayur Dilip DhandeB.E. ElectricalGovernment College of Engineering, [email protected]
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