Abstract— In this paper, a novel effort for prediction of

voltage sag in the entire transmission system of Vietnam is presented. As the Vietnamese electricity industry moves toward the electricity market, prediction will help utilities have early assessment of power quality in transmission system. The proposed prediction approach uses a fault position method in which the fault distribution in the transmission system is created based on an actual fault occurrence in the entire 220kV and 500kV transmission system throughout Vietnam that took place in 2008. The research also makes use of the SARFICURVE with ITIC and SEMI curve, which takes into account of the actual fault clearing time of protective devices used in transmission system in Vietnam. By using SARFICURVE, a better assessment of voltage sag performance is obtained in the transmission system with regard to load’s voltage tolerance.

Index Terms--transmission system, power quality, voltage sag frequency, stochastic prediction, fault distribution, fault clearing time, ITIC, SEMI curve.

I. INTRODUCTION mong 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 1 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, computer-controlled 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 (iii). Compare equipment voltage tolerance with the voltage sag performance

Bach Quoc Khanh is a faculty member with Electric Power Systems

Department, Electrical Engineering Faculty, Hanoi University of Science and Technology, 1 Dai Co Viet Rd., Hanoi, Vietnam (e-mail: bq_khanh-htd@mail.hut.edu.vn).

Nguyen Hong Phuc is a master student with Electric Power System Department, Electricity Faculty, Hanoi University of Science and Technology, 1 Dai Co Viet Rd., Hanoi, Vietnam (e-mail: cllx2003@gmail.com).

and estimate expected 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 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 paper the authors use one of the frequently used indices, SARFIX. It is defined as follows

N

NSARFI i

iX

X

∑=

)(

(1)

where X : 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 i. 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 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

N

NSARFI

m

iiX

XCURVE

∑=

− = 1

')(

(2)

where '

)(iXN : Number of customers tripped when experiencing

voltage sag with magnitudes below X% due to measurement

Prediction of Voltage Sag in The Transmission System of Vietnam, A Case Study

Bach Quoc Khanh, Nguyen Hong Phuc

A

978-1-61284-788-7/11/$26.00 ©2011 IEEE

event i. 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.

Fig 1. 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 tools 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 1. Modeling the fault distribution of the transmission system

of Vietnam – event modeling (Sub section B) 2. Calculating the short-circuit current and voltage sags at all

influenced load nodes – event indices (Sub section C) 3. Quantifying voltage sag frequency at load nodes (site

indices) and cumulating system sags with different characteristics and obtaining SARFIX (system indices)

4. 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 Modeling and Assumptions - Fault distribution modeling: Fault distribution modeling considers the occurrence of all faults in the whole transmission system of Vietnam that cover 500kV 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 characteristics for the transmission system of Vietnam. - Fault position: The fault can occur anywhere in the transmission system including 500kV and 220kV networks. Since load nodes of the transmission system are 220kV step-down transformers, faults in 110kV 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 for other nearby short-circuit faults in a portion of network that cause voltage sags to load nodes with the similar characteristics (similar magnitudes). Voltage sag magnitude normally divides in 9 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. That are overhead line OHL faults (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 generation. Therefore, transformer fault positions will be 11 for 500kV substations and 63 for 220kV substations respectively. For OHL faults, fault positions are selected depending on the length of each branch. According to the above said principle of fault position selection, we divide the line branches into some segments and each segment is represented by one fault position, normally at one of two ends of the line segment. For 220kV OHL, the line segment length shoud be from 10km to 40km depending on the line branch length. For 500kV OHL, each line segment should be 50km. In this case study, fault positions are selected at 76 locations for 500kV OHL and 169 locations for 220kV OHL. Therefore, there are 319 fault positions in total.

SARFIX-CURVE qualified

SARFIX-CURVE disqualified

Fig 2. The Transmission System of Vietnam in 2008

Vietnam National Power Transmission Corporation Total 500kV OHL length: 3441km Total 220kV OHL length: 76541km Number of 500kV substation: 11 Number of 220kV substation: 51 Total 500kV transformer capacity: 8756MVA Total 500kV transformer capacity: 14761MVA

220/110/35kV Mai Dong substation, 2x250MVA, Hanoi

- Fault type: This calculation considers all types of short circuit with well known contributory percentages of different fault type are assumed as follows

Single phase to ground (SP-G): 65% Two phase to ground (PP-G): 10% Two phase together (P-P): 20% Three phase to ground (3P-G): 5% For the transmission system that requires high reliability

and stability, short-circuits are prone to permanent fault. Therefore, in this work, transitory faults are not considered.

- Fault rate: The occurrence of short circuits depends on many factors [3] and the rates of occurrence of different faults (fault position, fault type) are normally not the same. However, because, in reality, recorded fault data does not consider detailed fault distribution, this work assumes that fault distribution for each fault type follows uniform model within each regions in Vietnam. For example, phase-to-ground faults remain unchanged anywhere in the section of transmission system within a region. The transmission system is Vietnam is divided in four regions. The data of fault performance recorded by NPT and its subsidiary agencies (Power Transmission Companies, PTC) for 2008 is shown in the Table 1 below.

TABLE 1. REGIONAL FAULT RATE PERFORMANCE Regional Power

Transmission Company

Line fault rate (per km.year)

Substation fault rate (per year) 500kV 220kV

PTC1 (North) 0.00093 0.02504 0.0397

PTC2 (North Center) 0.00562 0.00536 0.0408

PTC3 (South Center) 0.00173 0.01279 0.0161

PTC4 (South) 0.0077 0.00808 0.0229

NPT 0.00407 0.01478 0.0306

It is noticeable that the fault rates stated in Table 1 are for all four fault types as mentioned above. Therefore, for each fault type, the fault rate should multiply by contributory percentage of different fault types. For the fault that represents OHL faults within a line segment, fault rate should be calculated based on the length of the line segment.

- Selection of load nodes for voltage sag calculation: In the transmission system, load nodes are 220kV substations feeding to downstream 110kV and medium voltage networks. The topology of transmission network is complicated and many branches also have switching devices at both ends. When a fault occurs on a certain branch (a line or a transformer), the two switching devices at both ends of that branch will trip and isolate it from the network. Therefore, many load nodes normally experience voltage sags. Only the loads on or nearby the fault position (for transformer fault) suffers an interruption. So, voltage sags at all 63 load nodes had to be considered in this work.

- System loading condition when faults occur: It is also notable that for short-circuit calculation in the transmission system where limited power sources are connects to, the short- circuit current and voltage sags depend heavily on the pre-fault loading condition when the fault occurs. The heavier the

True

False

True

False

True

False

True

Select the load node (among 63 nodes) for sag calculation

START

Select the fault position (among 319 positions)

Select the fault type (SP-G, PP-G, P-P, 3P-G)

Short-circuit calculation and

determine voltage sag magnitude at selected load node by PSS/E

Fault distribution modeling, determine fault rate of the fault

under calculation

Calculate the frequency of voltage sag at

the selected load node

Are all fault type

selected ?

Are all fault position

selected ?

Are all load nodes

selected ?

Sag frequency spectrum by

the fault under calculation

(event index)

Sag frequency spectrum at

selected load node by all

faults

Sag frequency spectrum at all load nodes by

all faults (site index) SARFIX

calculation

Check ITIC curve ?

SARFIX-CURVE

STOP

Fig 3. Block diagram of the problem of prediction of voltage sag in

the transmission system of Vietnam.

(system index)

load on the system is, the higher short-circuit current will be generated and the deeper voltage sags will be at load nodes. Therefore, the most interested prefault loading condition is obviously that of full loaded and this work performs the short-circuit calculation in the maximum loading condition.

C. Short circuit calculation and voltage sag determination for the transmission system of Vietnam

Short circuit calculation and voltage sag determination for the whole transmission system of Vietnam is carried out by program PSS/E (Power System Simulation for Engineering). The block diagram of the calculation is depicted in Fig. 3.

- SARFIX calculation: With fault distribution modeling for the transmission system proposed in Part B, this work performs short-circuit calculation using the program PSS/E for a certain individual fault (fault position, fault type) and then voltage sag magnitude at a selected load node is calculated. After assigning fault rate to this fault, the frequency of sag at the selected load node resulted by this fault will be obtained. By repeating this calculation for all other faults (fault position and fault type), and gather them together, we obtains the frequency spectrum of voltage sag with different magnitude characteristics at the selected load nodes caused by all faults in the transmission system. Fig. 4, Fig. 5 and Fig. 6 show an example of voltage sag performance for an individual load node (220kV Mai Dong substation in Hanoi, Fig. 3). Fig. 4 shows voltage sag frequency spectrum by sag magnitude NEW

Fig 4. Voltage sag frequency spectrum (per year) by fault types at load node 220kV Mai Dong substation

Fig 5. Voltage sag frequency spectrum (per year) for all fault events at 220kV Mai Dong Substation, Hanoi, Vietnam

(per unit) intervals for different fault types. Fig. 5 is voltage sag frequency spectrum for all fault types. Fig. 6 is the cumulative voltage sag frequency.

Fig 6. Cumulative Voltage Sag Frequency (per year) at 220kV Mai Dong Substation, Hanoi, Vietnam

For other load nodes, the calculation is similarly performed and then we obtain voltage sag frequency spectrum of all other load nodes. Finally, the average frequency spectrum per load node is calculated and plotted on the Fig. 7 and SARFIX of the whole transmission system of Vietnam is calculated as the formula (1). The voltage sag performance of transmission system – SARFIX is shown in Fig. 8.

Fig 7. Transmission system average voltage sag frequency by magnitude characteristics

Fig 8. SARFIX and SARFICURVE-X of the transmission system of Vietnam

Sag

Mag

nitu

de

(p.u

)

Sag

Mag

nitu

de

(p.u

) Sa

g M

agni

tude

(p

.u)

SARFIX

SARFIITIC-X

Sag

Mag

nitu

de

(p.u

)

SARFIITIC-0.7

SARFISEMI-X

SARFISEMI-0.5

Sag magnitude (p.u)

SP-G PP-G

P-P 3P-G

- SARFIITIC-X calculation: SARFIX-CURVE can be achieved by taking fault clearing time of protective system into account. For the transmission system of Vietnam, the primary functions currently used for transformer protection is biased differential protection using differential relays of SIEMENS (SIPROTEC 7UT613) or ALSTOM (MiCOM P340). For OHL line protection, the primary functions currently in use are also the differential protection as above said using the tele-communication links of power line carrier or fibre-optical ground wire integrated in power carrying lines or the distance protection using differential relays of SIEMENS (SIPROTEC 7SA6) or ALSTOM (EPAC 3000, MiCOM P440). All those protective relay system is of instantaneous tripping type that is typically less than 100ms. The switching devices are almost SIEMENS, SCHNEIDER or ABB products manufactured in Europe with typical breaking time of 40ms for 500kV to 60ms for 220kV circuit breakers. Besides the above mentioned operating times of protective relays and circuit breakers, additional time delays are also included for auxiliary relay trips and operating time of tele-protection with total additional operating time not exceeding two more cycles (20-24ms). Therefore, the total fault clearing time is 160ms to 180ms that defines the voltage sag duration. If posing this duration on the ITIC curve, it’s obviously that only sags lower than 0.7 p.u. will be out of load voltage tolerance and qualified for SARFIITIC-X. The upper 0.7 p.u. sags with duration defined by the above said fault clearing time definitely fall inside the voltage tolerance envelope and thus, they are not qualified as SARFIITIC-X. Therefore, SARFIITIC-X is a part of SARFIX with X lower than 0.7 p.u. as also shown on the SARFIX chart (Fig. 8). For X from 0.7 p.u to 0.9 p.u, the value of SARFIITIC-X remains unchanged and equal to SARFIITIC-0.7.

If we use SEMI curve for assessment of sag duration, it is noticeable that there is a small difference between ITIC curve and SEMI curve for X from 0.5 cycle to 10 cycles (Fig. 9). Figure 9. The difference between ITIC curve and SEMI curve.

Within this range, ITIC ridethrough voltage is 0.7 p.u whereas this voltage level for SEMI F47 is just 0.5 p.u. Therefore, with the total fault clearing time (160ms to 180ms) for the transmission system in Vietnam, only voltages sag with X lower than 0.5 p.u are qualified for SARFICURVE-X using the SEMI curve (SARFISEMI-X). With X greater than 0.5 p.u, voltage sags fall inside SEMI’s ridethorugh area and not qualified for SARFISEMI-X. So, for X from 0.5 p.u to 0.9 p.u, the value of SARFISEMI-X remains unchanged and equal to SARFIITIC-0.5. SARFISEMI-X is also shown on Fig 8.

D. Result Analysis From the results, there’re some following remarks: - The SARFIX and SARFICURVE-X values obtained from this calculation are useful for utilities as a benchmark for reducing the frequency of fault for solving the problem of voltage sag. This result also helps customers know the voltage sag performance and choose suitable location for less voltage sag frequency. - The frequency of voltage sag as the result of an individual fault type is proportional to fault rate of that fault type for shallow sags (Fig. 4). - Shallow sags (0.7-0.9 p.u) feature a rather high frequency while the frequency of deep sags is very small. Furthermore, the frequency of voltage sag with X lower than 0.9 for either the 220kV Mai Dong substation (about 33 times, Fig. 5) and the system average load node (about 22 times, Fig. 7) is also khanh

Fig 9. Voltage sag frequency of selective load nodes (220kV substations) throughout of Vietnam

Sag magnitude (p.u)

Sag magnitude (p.u)

Sag magnitude (p.u)

much lower than total faults in transmission system (about 110 times per year). That’s because the goegraphical shape of Vietnam is rather long (about 1700 km) and thin in the middle (the narrowest is just 60 km) and the short-circuit faults occur on one region has almost no impacts on voltage sag for loads in other regions. - Cumulative frequency of voltage sag for the node 220kV Mai Dong substation in Hanoi (33 times for X < 0.9 p.u) is higher than the SARFI0.9 (22 times) because there’re more load nodes (220kV substations) located surrounding Hanoi and vicinity. In the center and in the south of Vietnam, the density of 220kV substation is lower than in the north and faults has less impact on voltage sag of load nodes. Fig. 10 shows sag frequency charts for selective load nodes in the north (upper), in the center (in the middle) and in the south (lower) of Vietnam that indicates the above said difference. - Also because of high frequency of 0.7-0.9 p.u. voltage sag, SARFIX-CURVE is very much lower than SARFIX despite voltage sags with the magnitude up to 0.7 p.u are qualified enough for SARFIX-CURVE. Therefore, voltage sags due to faults in the transmission system of Vietnam have less influence on loads than faults in distribution system when the frequency of deep sag is normally very high [6]. It is a remarkable finding in power quality assessment in the power system of Vietnam.

IV. CONCLUSIONS This paper presented the first effort of predicting voltage

sag performance for a large transmission system as a case study of Vietnam. In this work, fault distribution modeling is proposed basing on actual fault performance for different regions in Vietnam. Using SARFIITIC-X gives a better assessment of voltage sag influence on loads operation. The results of this work will be a useful reference for utilities in power system quality assessment toward electricity market operation. This research still needs to develop as faults in the generation part has yet to take into consideration. Besides, if a better fault data is achieved (by monitoring), a more detailed fault distribution can be made and finally a better voltage sag performance can be obtained.

V. ACKNOWLEDGEMENT It is acknowlegded that this work received helps from

Phung The Anh (Msc), Nguyen Anh Tu (Msc) with Power Engineering Consulting Joint-Stock Company No. 1, Hanoi, Vietnam for data collection and power system simulation as well as technical consultancy from Professor David A. Cartes, Senior Member, IEEE, and Dr. Bhuvaneswari Ramachandran with the Center for Advanced Power Systems, Institute for Energy Systems, Economics and Sustainability, Florida State University, USA.

VI. REFERENCES [1] M.H.J. Bollen, Understanding power quality problems - voltage sags and

interruptions, IEEE Press, 2000. [2] M.R.Qader, M.H.J.Bollen, and R.N.Allan, “Stochastic Prediction of

Voltage Sags in a Large Transmission System”, IEEE Trans. Industry Applications, vol.35, no.1, pp.152-162, Jan./Feb. 1999,

[3] Bach Quoc Khanh, Dong Jun Won, Seung Il Moon, “Fault Distribution Modeling Using Stochastic Bivariate Models For Prediction of Voltage Sag in Distribution Systems”, IEEE Trans. Power Delivery, pp. 347-354, Vol.23, No.1, January 2008.

[4] D. L. Brooks, R. C. Dugan, Marek Waclawiak, Ashok Sundaram, “Indices for Assessing Utility Distribution System RMS Variation Performance”, IEEE Trans. Power Delivery, vol.13, no.1, pp.254-259, Jan. 1998.

[5] Juan A. Martinez, Jacinto Martin-Arnedo, “Voltage Sag Studies in Distribution Networks - Part II: Voltage Sag Assessment, Part III - Voltage Sag Index Calculation”, IEEE Trans. Power Delivery, pp. 1679-1697, Vol. 21, No. 3, July 2006.

[6] Bach Quoc Khanh, Prediction of Voltage Sags in Distribution Systems With Regard to Tripping Time of Protective Devices, Proceeding, EEE.CR.ASPES2009, Tech. Section 2.1., Hua Hin, Thailand, September 28-29, 2009.

[7] J.V.Milanovic, M.T.Aung and C.P.Gupta, “The Influence of Fault Distribution on Stochastic Prediction of Voltage Sags”, IEEE Trans. Power Delivery, vol.20, no.1, pp.278-285, Jan. 2005.

[8] P. Saninta, S. Premrudeepreechacharn “Assessment and prediction of voltage sag in transmission system in northern area of Thailand”, Proceeding, 13th International Conference Harmonics and Quality of Power, ICHQP, Sept.28-Oct.1 2008, Wollongong, NSW, Australia.

[9] E Inan, B. Alboyaci, C. Leth Bak, “A Case Study Of Turkish Transmission System For Voltage Dips”, The Journal on Power and Energy Engineering, Vol 1, No. 2, April 2010

[10] National Institute of Vietnam, Master Plan VI, 2006.

VII. BIOGRAPHIES Bach Quoc Khanh received B.S., M.S.

and Ph.D. degrees in power network and systems from Hanoi University of Science and Technology, Hanoi, Vietnam in 1994, 1997 and 2001 respectively. He has been a faulty member of Electric Power System dept., Electricity Faculty, Hanoi University of Science and Technology since 2002. He is currently a visiting scholar in the Center for Advanced Power System, IESES, Florida State University. His research interests include power system analysis, DSM, power system quality, distributed generation.

Nguyen Hong Phuc received BS in Electrical Engineering Faculty,

University of Thai Nguyen in 2006, Vietnam. He is currently a master student in the Electric Power Systems Department, Electricity Faculty, Hanoi University of Science and Technology, Hanoi, Vietnam.