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

IndtK Ttirms-transmlssion system, power quality, voltage sag frequency, stochastic prediction, fault distribution, fault clearing time, ITIC, SEMI curve.. This paper is the first effort

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 t h l i 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

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

(ii) Obtain equipment voltage

toler-ance and (ill) Compare equipment

voltage tolerance with the voltage sag

performance and estimate expected

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^ =

-l A ^

/ 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

^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

be-ing its magnitude and duration on the graph of the

equip-ment compatibility curve, SARFICURVE-X corresponding

to voltage sags falling out of the equipment voltage

toler-ant 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

cal-culating SARFIX-CURVE using ITIC and SEMI curve

(SARFI-ITIC-X and SARFISEMI-X) as case studies

3iTB 20ins OSa DumUon ol Dtoturbanc* In C y d M (c) and Sscond* (*)

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

re-sulted from short-circuits, hereby called faults, and it is

possible to use fault modelling and short-circuit

calcula-tion tdols to simulate and predict voltage sags In the

pow-er 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 8}

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)

KHOA HQC • CdNG N G H I Q

4 Cumulating system voltage sags that cause equip-ment to trip and obtaining SARFICURVE-X

To obtain SARFIX-CURVE, the voltage sag duration

that depends on the fault clearing time of protective

sys-tem should be considered.This work takes the typical trip-ping 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 eharacteristics for the transmission system of Vietnam

- Fault position: The fault can occur anywhere in the transmission system including SOOkV and 220kV networks step-down transformers, faults in llOkV networks and distribution networks should not considerably impact

on voltage sags in transmission system because of large the power generating sources should be included in the faults at the 220kV step-up transformers Therefore, this 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 portion of network that cause voltage sags to Idad nddes 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.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 <t Dai songl

E] KHOA HQC - CONG NGH$

generation Therefore, transformer fault positions will be

11 for SOOkV substations and 63 for 220kV substations

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 SOOkV OHL, each line segment should be

50km In this case study, fault positions are selected at 76

locations for SOOkV OHL and 169 locations for 220kV OHL

Therefore, there are 319 fault positions in total

Fig 2.The Transmission System of Vietnam in 2008

- 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

reliabil-ity and stabilreliabil-ity, 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 ratesof 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 subsldiaryagencles(PowerTransmisslon Companies, PTC) for 2008 is shown in the Table 1 below

TABLE 1 Regional fault rate performance }^^

Regional Power Transmission C o m p a n y

PTCI (North) PTC2 ( N o r t h Center) PTC3 (South Center) PTC4 (South)

NPT

Line fault rate

(per l(in.yesf)

SOOkV 0.00093 0.00S62 0.00173 0.0077 0.00407

220kV 0X)2S04 O.OOS36 0X)1279 0.00808 0.01478

Substation! fauftrate ' (per year) 0.0397 0.0408

(Mitei

0.0229 0.0306

It is noticeable that the ^ u l t 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 1 lOkV 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 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

30 I Dien «f Doi song

KHOA HQC • CONG N G H t \3

Select Ihe load node

Short-circuit

cnlculBtion and

determine voltage sag

magnitude at selected

load node by PSS/E

Fault I modeling, dctennme fault rale of the fsull under cBlculolion

Calculate the frequency

of voltage sag at

the selected load node

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 N

• SPG

• PP-C

• P-P

• 3P-G •

z-z '-'

r 14

12

10

8

0

.-; Sag magnitude

Fig 4 Voltage sag frequency spectrum (per year) by fault

types at load node 220kV Mai Dong substation

ll

O-D 1 Q 1-Q3Q2-Q3a3-0404-0605-0Bi

s _

•Q7D7.0BQS-Oa M S

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

per-forms 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

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

ll

<01 <02 <0.3 <0d <05 <06 <07 <08 <09

Fig 6 Cumulative Voltage Sag Frequency (per year)

at 220kV Mai Dong Substation, Hanoi, Vietnam

Dien A D&i songl 31

E] KHOA HQC - CdNG NGHf:

For other load nodes, the calculation issimilarlyperibrmed

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

12

A ^

J

a s ? s s s

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 SARFimC-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 pu

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 fnam 0.5 cycle to 10 cycles (Rg 9)

(^ a>

Fig 7 Transmission system average voltage sag

frequency by magnitude eharacteristics

SARFlx

Fig 8 SARFIX and SARFICURVE-X of the transmission

system of Vietnam

SARFimC-X calculation: S/IRF/X-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

the tele-communication links of power line carrier or

fibre-optical ground wire integrated in power carrying lines or

(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 SOOkV to 60ms for 220kV circuit breakers Besides the

i '•)

!!!!!!!!]

1

i

a^

CBEMA

mc

SBM F 4 7

] ^\:W-Ji.i,.t -'l -; ] ^\:W-Ji.i,.t -'l -; 'ill l'f\ i

0001 < aoii ' ; at! : to I lOO : looo dda*

TkTM 1 , 3<H u ^ ' ™ a s 02 10 SaconA

Figure 9.The difference between m c curve and SEMI cun/e

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)

32 Dien Jk D&i song

KHOA HQC - CdNG N G H E : Q

- Shallow sags (0.7-0.9 p.u) feature a rather high

fre-quency while the frefre-quency of deep sags is very small

Fur-thermore, 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 much lower than total faults in

transmis-sion system (about 110 times per year) That's because the

goegraphieal 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

im-pacts on voltage sag for loads in other regions

Fig 9 Voltage sag frequency of selective load nodes

(220kV substations) throughout of Vietnam

- 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 volt-age sag performance for a large transmission system as a case study of Vietnam In this work, fault distribution mod-eling is proposed basing on actual fault performance for different regions in Vietnam Using SARFIITIC-X gives a bet-ter 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 mar-ket 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 References

[1 ] M.H.J Bollen, Understanding power quality prob-lems - voltage sags and interruptions, IEEE Press, 2000 [2] M.R.Qader, M.HJ.Bollen, and R.N.Allan, "Stochas-tic Prediction of Voltage Sags in a Large Transmission

Sys-tem" IEEE Trans Industry Applications, vol.35, no.1,

pp.152-162,Jan./Feb 1999, [3] Bach Quoc Khanh, Dong Jun Won, Seung II Moon,

"Fault Distribution Modeling Using Stochastic Bivariate Models For Prediction of Voltage Sag in Distribution

Sys-tems", IEEE Trans Power Delivery, pp 347-354, Vol.23, No 1,

January 2008

[4] D L Brooks, R C Dugan, MarekWaclawiak, Ashok Sundaram, "Indices for Assessing Utility Distribution

Sys-tem RMS Variation Performance", IEEE Trans Power

Deliv-ery, vol.13, no.l, pp.254-259, Jan 1998

[5] Juan A Martinez, Jacinto Martin-Arnedo, "Voltage Sag Studies in Distribution Networks - Part II: Voltage Sag

As-sessment, Part III -Voltage Sag Index Calculation", IEEE Trans

PowerDe/jve/y, pp 1679-1697,VoL21, No.3,July 2006 [6] Bach Quoc Khanh, Prediction of Voltage Sags

in Distribution Systems With Regard to Tripping Time of

Protective Devices, Proceedings, EEE.CR.ASPES2009, Tech

Section 2.1., Hua Hin,Thailand, September 28-29,2009 [7] J.V.MIIanovic, M.TAung and C.P.Gupta,"The Influ-ence of Fault Distribution on Stochastic Prediction of

Volt-age Sags'; IEEE Trans Power Delivery, vol.20, no 1,

pp.278-285, Jan 2005

[8] R Saninta, S Premrudeepreechacharn "Assess-ment and prediction of voltage sag in transmission sys-ternational Conference Harmonics and Quality of Power, ICHQP, Sept.28-Qet.l 2008, Wollongong, NSW, Australia [9] E Inan, B Alboyaci, C Leth Bak,"A Case Study Of

Turkish Transmission System ForVoltage Dips", The Journal

on Power and Energy Engineering, Vol 1, No 2, April 2010

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

2006

Dien Jk D&i song\ 33