Prediction of viltage sag in the transmission system of vietnam,a case study

E] KHOA HQC • CdNG NGHf:

Prediction of Vtoltage Sag in Tlie Transmission

SYSTEM OF ViETNAM. A CASE STUDY

Bach Quoc Khanh, Phung The Anh, Nguyen Hong Phuc

Abstract— In thli paptr* i novtl tffort (or pradtctlon of voltag* sag In the entire transmission

lystMi of Vittnam It praMrttid. As the Vietnamese electricity Industry moves toward the electricity

market, prediction will help utlHtles have eariy assessment of power quality In transmission system.

The proposed prediction approach uses a fault position method In which the fault distribution in the

tranimifslon synem Is cfMted based on an actual fault occurrence In the entire 220kV and SOOkV

transmission system throughout Vietnam that took placa In 2008. Tha research also makes use of the

SARFICURVE with ITIC and SEMI curve, which takes Into account of the actual fault clearing time of

protective devlcesused In transmission system In Vietnam. By using SARFICURVE, a better assessment of

voltage sag parformance Is obtained In the transmission system with regard to load's voltage tolerance.

IndtK Ttirms-transmlssion system, power quality, voltage sag frequency, stochastic prediction,

fault distribution, fault clearing time, ITIC, SEMI curve.

1. Introduction

Among power quality phenom￾ena, voltage sag (dip) Is defined by

IEEE 1159 (1995) as a decrease in

RMS voltage to between 0.1 to 0.9 of

nominal voltage at power frequency

for duration of 0.5 cycle to 1 minute.

Interests in voltage sag has been get￾ting much greater recently in Vietnam

due to its impact on the performance

of sensitive electronic equipment

like variable speed drives, computer￾controlled production lines that are

widely used, especially in industry.

Although voltage sags are more com￾mon in distribution system, many

causes leading to voltage sag are de￾rived from transmission systems. An

assessment of voltage sag in trans￾mission systems is important for utili￾ties and customers in Vietnam now.

Voltage sag assessment normally

comes prior looking for the solution

of voltage sag mitigation. Voltage sag

assessment is usually related with the

basic process known as a "compat￾ibility assessment" [1] which includes

three steps; (i). Obtain the voltage sag

performance of the system of interest,

(ii). Obtain equipment voltage toler￾ance and (ill). Compare equipment

voltage tolerance with the voltage sag

performance and estimate expected

28 I Dien Ji D&i song

impact of voltage sag on the equip￾ment. The permissible voltage toler￾ance for electric equipment, normally

defined by the manufacturers and the

well-known PQ curves for susceptibil￾ity of computer equipment displays

are CBEMA, ITIC or SEMI [1] whereas

power quality assessment of power

supply system is utilities duty. This

paper is the first effort to assess the

voltage sag performance in the trans￾mission system of Vietnam by using

the method of stochastic prediction of

voltage sags [1], [2], [3] using SARFI￾CURVE-X that Is derived from SARFIX

with regard to fault clearing time of

protective devices currently used in

the transmission system in Vietnam.

II. Indices for voltage sag

assessment

Voltage sag assessment often

relies on voltage sag characteristics:

magnitude and duration. There are

many indices proposed for voltage

sag quantification. [1], [4] In this pa￾per the authors use one of the fre￾quently used indices, SARFIX. It is de￾fined as follows

SARFI^ = -

lA^. .

/ rms voltage threshold;

possible values - 10-90% nominal

voltage

NX(i) ; Number of customers

experiencing voltage sag with

magnitudes below X% due to

measurement event L

N: number of customers served

from the section of the system to be

assessed

Despite being widely used,

SARFIX only considers the magni￾tude of voltage sag. Unfortunately,

the magnitude value maybe much

greater than the actual number of

tripped electrical appliances, espe￾cially when the duration of sags is

small enough (less than a half sec￾ond), such as for transmission sys￾tem in Vietnam where the total fault

clearing time of protection system is

typically less than 5 to 7 cycles of the

mains frequency. To take the voltage

sag duration into account, SARFIX is

developed into SARFICURVE-X [5], [6]

which is defined below

SARFIr,

1^.( 0