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 l y s t M i 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 Introduction Among power quality phenomena, 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 minute Interests in voltage sag has been getting much greater recently in Vietnam due to its impact on the performance of sensitive electronic equipment like variable speed drives, computercontrolled production lines that are widely used, especially in industry Although voltage sags are more common in distribution system, many causes leading to voltage sag are derived from transmission systems An assessment of voltage sag in transmission systems is important for utilities 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 "compatibility assessment" [1] which includes three steps; (i) Obtain the voltage sag performance of the system of interest, (ii) Obtain equipment voltage tolerance 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 equipment The permissible voltage tolerance for electric equipment, normally defined by the manufacturers and the well-known PQ curves for susceptibility 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 transmission system of Vietnam by using the method of stochastic prediction of voltage sags [1], [2], [3] using SARFICURVE-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 assessment voltage sag 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 paper the authors use one of the frequently used indices, SARFIX It is defined as follows / 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 magnitude of voltage sag Unfortunately, the magnitude value maybe much greater than the actual number of tripped electrical appliances, especially when the duration of sags is small enough (less than a half second), such as for transmission system in Vietnam where the total fault clearing time of protection system is typically less than to 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 1^.(0 lA^ SARFI^ = - SARFIr, KHOA HQC • CdNG N G H I Q ^ ^x^,) '• Number of customers tripped when expe:ing voltage sag with magnitudes below X% due to surement event / If we plot voltage sag as a point with co-ordinates being its magnitude and duration on the graph of the equipment compatibility curve, SARFICURVE-X corresponding to voltage sags falling out of the equipment voltage tolerant area (Fig 1) will be obtained So far well known curves are CBEMA, ITIC and SEMI [1] Obviously, SARFICURVE-X can provide a better understanding of the influence of voltage sag on the operation of electric equipment in electric networks This paper presents the method of calculating SARFIX-CURVE using ITIC and SEMI curve (SARFIITIC-X and SARFISEMI-X) as case studies 3iTB 20ins OSa DumUon ol Dtoturbanc* In CydM (c) and Sscond* (*) Fig ITI curve for susceptibility of computer equipment III Prediction of Voltage Sag in The Transmission System of Vietnam A Problem definition The problem with stochastic prediction of voltage sag is that it can only obtain the voltage sag performance of a specific electric system by using data of causal events leading to sags In fact, more than 90% sag events are resulted from short-circuits, hereby called faults, and it is possible to use fault modelling and short-circuit calculation tdols to simulate and predict voltage sags In the power system This work uses the method of "fault position" [1] for voltage sag prediction in the transmission systems with following significant steps Modeling the fault distribution of the transmission system of Vietnam - event modeling (Sub section 8} Calculating the short-circuit current and voltage sags at all influenced load nodes - event indices (Sub section C) Quantifying voltage sag frequency at load nodes (site indices) and cumulating system sags with different characteristics and obtaining SARFIX (system indices) Cumulating system voltage sags that cause equipment to trip and obtaining SARFICURVE-X To obtain SARFIX-CURVE, the voltage sag duration that depends on the fault clearing time of protective system should be considered.This work takes the typical tripping time of protective devices (instantaneous protective relay) and high voltage circuit breakers currently used in the transmission system in Vietnam into its calculation B Fault Distribution fi^odeling and Assumptions - Fault distribution modeling: Fault distribution modeling considers the occurrence of all faults in the whole transmission system of Vietnam that cover SOOkV and 220kV networks The scope of the transmission system of Vietnam starts from the points of energy receiving from generating centers or interconnection points with the transmission system of South China to load nodes that are step-down 220kV substations An individual fault (short-circuit) is characterized by a pair of parameters: fault position, fault type and its occurrence is assigned a fault rate All faults with their assigned rate of occurrence build up a fault distribution model Following are analyses of each fault eharacteristics for the transmission system of Vietnam - Fault position: The fault can occur anywhere in the transmission system including SOOkV and 220kV networks Since load nodes of the transmission system are 220kV step-down transformers, faults in llOkV networks and distribution networks should not considerably impact on voltage sags in transmission system because of large impedance of 220kV step-down transformers Faults at the power generating sources should be included in the faults at the 220kV step-up transformers Therefore, this work only considers faults that occur in the transmission system According to [1], [3], [7], basing on the concept of "area of vulnerability" fault positions should be generally chosen in the manner that a fault position should be the representative ftar other nearby short-circuit faults in a portion of network that cause voltage sags to Idad nddes with the similar characteristics (similar magnitudes) Voltage sag magnitude normally divides in ranges: 0-0.1, 0.1-0.2, , 0.8-0.9 p.u Similar manitudes mean the magnitudes that fall inside a same range of magnitude above said Faults in the transmission system are divided into two groups.Thatare overhead line OHLfaults (or faults on branches) and transformer faults (faults on substations) In the transmission system of Vietnam given in VI Master Plan [10] for the year 2008, 63 substations 220kV will be seen as load nodes for voltage sag assessment The transmission system (Fig 2) includes the 500kV network (11 nodes as 500kV substation and 17 branches of OHL with total length of 3246km) and the 220kV network (63 nodes as 220kV substations and 103 branches of 220kV OHL with total length of 6414km) In Figure 2, the number of 220kV substation is 51 that are under the management of National Power Transmission Corporation (NPT) Other twelve 220kV substations are under the management of power Dien