Transient Simulation Study HV Vietnam

8/12/2019 Transient Simulation Study HV Vietnam

1/8

1

TRANSIENT SIMULATION STUDY FOR THE 1500 KM NORTH-

SOUTH 500 kV INTERCONNECTION IN VIETNAM

by

Q. Bui-Van(1)

, B. Khodabakhchian, H. Huynh and B. de-Metz-Noblat(2)

(1)Transnergie - Hydro-Qubec International (HQI) (2)Merlin-Grin - Schneider Electric (MG-SE)P.O Box 10 000, Complexe Desjardins A2 plant, 38050 Grenoble cedex 9 - France

East Tower 10th

floor, Montreal, Quebec, Canada H5B 1H7 Phone: 33-4 76 57 95 98 Fax.: 33-4 76 57 98 60Phone: (514) 289 2211 (ext. 3054) Fax.: (514) 289 3164 E-mail: [email protected]

E-mail: [email protected]

Abstract : To assess the performance required for the500 kV equipment, an exhaustive electromagnetic transientstudy was performed for the design of the existing North-South series-compensated 500 kV interconnection inVietnam. This study covered several aspects such as : thetransient recovery voltages (TRVs) across the 500 kV circuitbreakers (CBs), the temporary and switching overvoltages(TOVs and SOVs) in the 500 kV system, the energy stresseson the metal-oxide varistors (MOVs) protecting series-capacitor banks as well as the analysis of secondary arc

current extinction. All those aspects were thoroughlyinvestigated. These study results have allowed the selection ofthe 500 kV circuit breakers to withstand severe TRVconditions on series compensated lines. A special protectionscheme has also been devised to control temporaryovervoltages in the 500 kV system in case of total loadrejection. Moreover, the MOV design criteria have beenestablished in light of study results. Finally, the efficiency ofthe installed reactor neutral circuits for secondary arc currentextinction has also been verified.

1. INTRODUCTION.

In 1994, the North-South 1487-km 500-kV transmission line

in Vietnam was put into service interconnecting the threeregional sub-transmission systems: The Northern, The Centraland The Southern region. This interconnection has been builtfor the purpose of : unifying the national electrical network,exchanging the generated electric power between the threeregions as well as optimizing the electricity production costfrom various resources during the rainy and dry seasons in thecountry. The 500 kV line has interconnected, from North toSouth, to five substations namely : Hoa Binh, Ha Tinh, DaNang, Pleiku and Phu Lam. From Hoa Binh to Phu Lam, each500 kV line section is series compensated to approximately60% (30% on each end) and shunt compensated to 70% (35%

on each end). Furthermore, in order to improve the systemtransient stability following a spurious single-phase fault onthe 500 kV line, single-phase auto-reclosing (SPAR) systemswere used on the four 500 kV line sections. Fig. 1 presents theexisting North-South 500 kV interconnection.

Fig. 1: The Vietnamese North-South 500 kV Interconnection.

A turn-key contract for the construction of the abovementioned 500 kV substations was awarded to MG-SE. Thispaper presents the results of electromagnetic transient studythat was subcontracted to HQI by MG-SE for the design andspecification of 500 kV equipment supplied for thesesubstations. This study covered several aspects such as : thetransient recovery voltages on 500 kV breakers, thetemporary and switching overvoltages, the energy stresses onMOV as well as the analysis of secondary arc current

extinction. The analysis of electromagnetic transientphenomena in the 500 kV interconnection was performedwith DCG/EPRI EMTP and based on the 1995 systemconfigurations established in [1]. For the interpretation ofstudy results, 1 p.u. of TRV or overvoltage presented in thepaper is defined on the base voltage of 449 kV-peak.

2. TRVs ACROSS LINE CIRCUIT BREAKERS.

It is well known that the addition of series-capacitor banks ontransmission line will substantially increase the TRVs acrossline circuit breakers due to the presence of trapped charges on


2/8

2

series-capacitor banks at the time of line trip [2]. Moreover,high temporary overvoltages on long radial transmission linesin case of total load rejection will result in severe TRVstresses on line breakers during out-of-phase and unloadedline interruptions. Therefore, TRV stresses on the 500 kVcircuit breakers were analyzed for the following interruptionconditions :

2.1 TRVs caused by fault clearings. To determine the worst

TRV stresses on the 500 kV line breakers, the followingparameters have been evaluated :

Both light and full load system configurations. Four types of faults : phase-to-ground (p-g), phase-to-

phase ungrounded (p-p), phase-to-phase-to-ground(p-p-g) and three-phase-to-ground (3p-g).

Five fault locations from 1 to 5 along the line section. Random fault applications and clearings.

Fig. 2: Simulation of TRV on Pleiku-Phu Lam line CBs.

Study results for the Pleiku-Phu Lam line section as presentedin Fig. 2 have shown that TRVs for the light load system areless severe than for the full load system. This confirms theeffect of trapped charges on series-capacitor banks in theincreasing of TRVs as mentioned earlier. For other linesections, only the full load system configurations wereanalyzed. This will give more conservative results. The most

severe TRVs are caused by 3p-g or p-p faults. The worstTRVs are listed in the Table-1 for any combination of faulttypes and locations :

Line section North line end South line end

Hoa Binh-Ha Tinh 2.7 2.4Ha Tinh-Da Nang 3.0 2.6Da Nang-Pleiku 2.9 2.5Pleiku-Phu Lam 3.1 2.5

Table-1: Maximum TRV levels (in p.u. based on 449 kV-peak)

Fig. 3 illustrates a waveform of TRV caused by a 3p-g fault.

Fig. 3: TRV on the Pleiku-South CB during a 3p-g fault at 5.

2.2 TRVs during unloaded line openings under overvoltageconditions. For the Hoa Binh-Phu Lam interconnection, theseparation of the network at Phu Lam will create important

temporary overvoltages of which the duration must be limitedby remote tripping of the Da Nang and Pleiku line breakers.These remotely controlled trips require to have the concernedbreakers capable of withstanding the worst TRVs associatedwith the unloaded line openings under overvoltage conditions.The simulation of unloaded line interruptions at Da Nang andPleiku subsequent to a network separation at Phu Lam wasperformed on both light and full load system configurations.The results obtained show that the worst TRVs will lie

between 2.9 and 3.5 p.u. for the various conditions evaluated.There is no significant difference between the Da Nang andPleiku TRV levels. Fig. 4 shows the waveform of the PleikuTRV during unloaded line opening subsequent to a systemseparation at Phu Lam.

Fig. 4: TRV on Pleiku CB during unloaded line opening

subsequent to the system separation at Phu Lam.

2.3 TRVs associated to out-of-phase interruptions. Twomethods are proposed to energize the 500 kV interconnection.The first one [1] recommends the energizing of Hoa Binh-to-Phu Lam sections followed by a synchronizing at Phu Lam.The second method [3] rather suggests a synchronizing at DaNang preceded by the energizing of Hoa Binh-to-Da Nangsections and Phu Lam-to-Da Nang sections. During a falsesynchronizing operation, there could be a situation whereboth parts of the system have opposite phasing at the time thatthe breaker is closed or tripped. Simulations were performedto determine the TRVs under these conditions according toboth energizing methods previously mentioned. The results

show a maximum TRV of 2.9 p.u. in both cases. Fig. 5illustrates the TRV on the Da Nang CB during out-of-phaseinterruption.

Fig. 5: TRV on the Da Nang CB during out-of-phase interruption .

The high TRVs present on the Vietnamese North-South

500 kV interconnection can be explained by two main factors:the series-capacitor banks with their trapped chargesconsiderably increase the TRVs and the particular systemconfiguration which features a very long single line that willpromote the creation of overvoltages each time a breaker istripped. These high TRV conditions are not unusual eventsand there is no simple and efficient solution to reduce theseTRVs to acceptable level for any system operating condition.Therefore, following this study and according to anagreement between MG-SE and the Vietnamese Authority, itwas decided to install the 500 kV circuit breakers having fourinterrupting chambers to withstand the worst TRV conditions


3/8

3

on the North-South series compensated 500kV inter-connection.

3. TEMPORARY AND SWITCHING

OVERVOLTAGES.

Important transient and temporary overvoltages can occur onlong radial AC transmission lines in case of total loadshedding. The use of single-phase tripping and auto-reclosing

technique to maintain the system stability following spurioussingle phase faults can also produce high transientovervoltages should this scheme not operate properly.Temporary and switching overvoltages on the Hoa Binh-PhuLam 500 kV interconnection were analyzed for the followingevents :

System separation. Faulty operation of SPAR systems. Energizing of a line section with single phase fault at the

open end. Line energizing. Transformer energizing.3.1 System Separation. The particular configuration of HoaBinh - Phu Lam interconnection which features a very longsingle line subjects to network separation by the tripping of asingle line breaker, either accidentally or by the systemprotection. The study has covered 7 possible separationlocations along the interconnection as illustrated in Fig. 6.

Fig. 6: Separation possibilities along the Hoa Binh-Phu Lam line.

a) System separations at the locations 1 and 7. Under thecondition of power transfer from Hoa Binh to Phu Lam, asystem separation at the location 1 leaving the unloaded auto-transformers connected on the lightly loaded long 500 kVHoa Binh-Phu Lam line will generate high magnitude andextended low frequency oscillation superimposed to thefundamental frequency voltages along the line as shown inFig. 7. Similar overvoltage condition would be observed for asystem separation at the location 7 in case of power transferfrom Phu Lam to Hoa Binh. In order to avoid dangerousconditions associated with extended oscillation voltages, it isstrongly suggested to initiate a fast shutdown of the auto-transformers at Phu Lam or Hoa Binh by tripping the 220 kV

as well as the 500 kV breakers following a system separationat the location 1 or 7.

Fig. 7: Overvoltages caused by a system separation at the loc. 1

b) System separations at the locations from 2 to 6. Thetemporary overvoltages followed the system separations atthe locations from 2 to 6 are caused by the Ferranti effect ofthe lightly loaded line sections that are still connected to thesource side of the interconnection. Therefore, in the case ofpower transfer from Hoa Binh to Phu Lam, the highestovervoltage is observed during the system separation at thelocation 2. On contrary, under the condition of power transferfrom Phu Lam to Hoa Binh, the system separation at the

location 6 would generate the worst overvoltage condition.The network separations at the locations from 2 to 6 along theinterconnection were simulated for both light and full loadsystem configurations. The presence or absence of a faultprior to the separation has also been analyzed. Furthermore,the isolation of the Da Nang local grid from the 500 kVsystem as well as the outage of one shunt reactor along theinterconnection have also been considered. Simulation resultsreveal that the temporary overvoltages for the light loadsystem are more severe than for the full load system. Thepresence of a fault prior to the separation increases thetransient overvoltage level immediately followed the networkseparation.The worst overvoltage condition for a power transfer from

Hoa Binh to Phu Lam results from a separation at the location2 on the light load system in which one shunt reactor at Pleikuis out of service and the Da Nang local grid is isolated fromthe 500 kV system. As illustrated in Fig. 8, importanttemporary overvoltages of 1.95 p.u. appear following thesystem separation. These overvoltages will impose verysevere stresses on the 500 kV equipment such as: 468 kVsystem surge arresters, shunt reactors, power transformers andother internally insulated equipment. Therefore, protectivemeasures to quickly limit the duration of those overvoltagesare required.

Fig. 8 : The worst temporary overvoltages at Phu Lam followed asystem separation at the location 2.

c) Remote transfer tripping and overvoltage protectionschemes. As mentioned earlier overvoltages following asystem separation are caused by the Ferranti effect of the linesections that are still connected to the source side. Fastremoval of these line sections will reduce the magnitude as

well as the duration of these overvoltages. The two followingprotection schemes have been suggested :

Initiate the remote transfer tripping of the Da Nang-Pleiku and Pleiku-Phu Lam line sections at both endswhen one of the line breakers at Pleiku or Phu Lamreceives a three phase trip order. This remote transfertripping scheme could also be applied for the Da Nang-Ha Tinh and Ha Tinh-Hoa Binh line sections to cover thecase of power transfer from Phu Lam to Hoa Binh asillustrated in Fig. 9


4/8

4

Implement an overvoltage protection in every 500 kVsubstation to limit the overvoltage maximum duration.

Fig. 9: Remote transfer tripping scheme for the Hoa Binh-PhuLam interconnection.

The worst case of overvoltages following a system separationat the location 2 has been tested to verify the efficiency of theremote transfer tripping scheme up to Da Nang substation.

a) Overvoltages at Phu Lam

b) Overvoltages at Pleiku

c) Overvoltages at Da Nang.

Fig. 10: Effect of remote transfer tripping up to Da Nang onovervoltages following a system separation at the location 2.

It can be seen in Fig. 10 that the overvoltages at Phu Lam,Pleiku and Da Nang following a system separation at thelocation 2 were rapidly reduced once the remote trippingcompleted. Moreover, as shown in Fig. 11, maximum energystress in the 468 kV system surge arrester is 5.5 MJ.Furthermore, it has also been suggested to implementovervoltage protection in every 500 kV substation in order tolimit the overvoltage maximum duration following a systemseparation.

Fig. 11: System separation at the location 2 - Energy stresses in468-kV surge arresters at Phu Lam, Pleiku and Da Nang.

3.2 Faulty operation of SPAR systems. The use of singlephase tripping and reclosing technique to improve the systemstability following a spurious single phase fault mightproduce transient and temporary overvoltages should thisscheme not operate properly. Overvoltages can occur underthe following conditions :

a) Single phase faulty reclosing at one line end. When a linephase is tripped followed a spurious single phase fault, theload current will cease to flow in that phase. The situation offaulty reclosing at one line end is similar to the network

separation previously analyzed; here however, this is a linesingle phase separation as shown in Fig. 12-a.

a) Single phase separation during tripping/reclosing sequences.

b)Overvoltage at Phu Lam open end when reclosing at Pleiku.

c) Overvoltage at Pleiku open end when reclosing at Phu Lam.

Fig. 12: Overvoltages caused by single phase reclosing at one line

end only.

On the upstream sections (source side), the Ferranti effect onthe open phase will generate overvoltages, and the reclosingon this side of the network will make the situation worse asillustrated in Fig. 12-b since the reclosed section susceptanceis added on the open phase. However, on the downstreamsections (load side), the open phase is still connected to theload grid, and recorded overvoltages when reclosing on thisside are usually smaller as seen in Fig. 12-c.


5/8

5

b) Single phase false tripping at one line end. A falseoperation of a single phase trip system can produce anopening of the line phase without having a fault. Thissituation produces the overvoltages similar to the line singlephase separation previously described. The maximumovervoltage reach 1.6 p.u. upon a breaker pole false trip atPhu Lam as illustrated in Fig. 13.

Fig. 13: Overvoltage caused by a single phase false trip atPhu Lam.

c) Remedy measures. Generally, overvoltages caused byfaulty operation of SPAR systems are less severe than thosein system separation. However, these overvoltages will last aslong as the line phase stays open at one end due to a stuckbreaker pole or a failure of SPAR system. Therefore, thefollowing measures have been suggested to limit themagnitude and duration of overvoltages :

To limit the reclosing overvoltage magnitude, it wassuggested to apply the sequential SPAR systems thatallow to reclose the load side line end before the sourceside line end.

To control the maximum overvoltage duration in eachline phase, it was suggested that the overvoltagemeasurement for overvoltage protection in eachsubstation must be performed on single phase basis.

It was also suggested to implement a line backupprotection feature which could detect the condition onephase open at only one line end and initiate a three phasetripping of the line at both end.

3.3 Energizing of a line section with a single phase fault atthe open end. High transient and temporary overvoltages canappear on the healthy phases when energizing a line sectionwith a permanent single phase fault applied to the open end.Overvoltage conditions were analyzed according to bothmethods of 500 kV line energizing: first according to [1], the

energizing from Hoa Binh to Phu Lam followed by thesynchronizing at Phu Lam and second, as proposed by [3], thesynchronizing at Da Nang after energizing the Hoa Binh-to-Da Nang and Phu Lam-to-Da Nang sections. The highesttemporary overvoltage of 1.70 p.u. were recorded in Fig. 14when energizing the Pleiku-Phu Lam section with a singlephase fault applied at Phu Lam open end. However, themaximum overvoltage recorded for the energizing of the HaTinh-Da Nang section with a single phase fault applied at theDa Nang open end only reaches 1.30 p.u.

Fig. 14: Overvoltages at Phu Lam when energizing the Pleiku-PhuLam line with a single phase fault applied at Phu Lam

Therefore, in order to minimize the overvoltage stresses toequipment when energizing the interconnection the followingmeasures were suggested :

Disable the SPAR systems on the four line section whenenergizing the interconnection. This measure allows toprevent reclosing on a permanent fault when energizingthe interconnection.

Apply the method proposed by [3], i.e. the synchronizingat Da Nang after energizing the Hoa Binh-to-Da Nangand Phu Lam-to-Da Nang sections, for energizing theHoa Binh-Phu Lam interconnection.

3.4 Line energizing. The 500 kV line CBs installed on theHoa Binh-Phu Lam interconnection were equipped withclosing resistors. They have the following characteristics:1000 for the Pleiku - Phu Lam section, 600 for the otherthree sections. The specified insertion time of these closingresistors was 10 2 ms. To assess the performances of theseclosing resistors, switching overvoltages caused by lineenergizing were analyzed according to both methods of 500kV line energizing previously described [1], [3].

a) Line energizing from Hoa Binh to Phu Lam as proposedby [1]. The maximum switching overvoltage of 1.55 p.u. wasrecorded when energizing the Pleiku-Phu Lam section asillustrated in Fig. 15.

Fig. 15: Switching overvoltage at Phu Lam when energizing thePleiku-Phu Lam section with 1000 - 10 2 ms closing resistor

On these wave shapes, the beat around 10 Hz are

superimposed to the fundamental frequency voltage.

Fig. 16: System impedance seen at Pleiku once energized fromHoa Binh-to-Phu Lam with 4 to 8 generators at Hoa Binh


6/8

6

This condition is originated by the excitation of the firstsystem impedance pole to approximately 60 Hz during theline closing at Pleiku as seen in Fig. 16. Due to this lowfrequency beat, the short insertion time closing resistors haveonly little effect on the overvoltage level.

b) Line energizing from Hoa Binh-to-Da Nang sections andfrom Phu Lam-to-Da Nang sections according to [3]. Themaximum overvoltage of 1.40 p.u. was recorded when

energizing the Ha Tinh-Da Nang section as shown in Fig. 17.The beat phenomena were also observed on the fundamentalfrequency voltages. However the beat frequencies are higherbecause of shorter lines involved. In this case, the first systemimpedance pole seen at Ha Tinh was around 78-82 Hz asindicated in Fig. 18. Again, due to these low frequency beats,the short insertion time closing resistors have only little effecton the overvoltage level.

Fig. 17: Switching overvoltages at Da Nang when energizing theHa Tinh-Da Nang section with 600 - 10 2 ms closing resistor

Fig. 18: System impedance seen at Ha Tinh once energized fromHoa Binh-to-Da Nang with 4 to 8 generators at Hoa Binh

Although the specified closing resistors and their insertiontime were not optimum to damp the natural systemoscillations at the time of line energizing according to bothproposed methods, they were considered acceptable since theovervoltages involved remain relatively low (1.55 p.u. max.)

3.5 Transformer energizing. Once the Hoa Binh-Phu Lamline is energized with either one of the above mentionedmethods, the 500-225 kV, 450 MVA transformers will beprogressively energized at Ha Tinh, Da Nang and Pleikusubstations by the 500 kV CBs having the samecharacteristics as the 500 kV line breakers. To maximize theinrush currents, the transformer energizing was performed at

the zero voltage crossing. The residual flux of 75% of ratedflux was set for one of the three transformer phases at thetime of transformer energizing. Simulation results reveal thatthe transformer energizing along the Hoa Binh-Phu Laminterconnection will not generate harmful overvoltages.

4. ENERGY STRESSES ON MOVs.

The series-capacitor banks on the existing Hoa Binh-Phu Lam500 kV interconnection are protected by MOVs connected inparallel. This provides the advantage of a reinsertion timepractically instantaneous following a fault thus greatly

improving the system transient stability following a majordisturbance. During a fault, the MOVs limit the maximumovervoltages across the series-capacitor bank to theirprotective level (normally 2-2.5 time the series-capacitor bankrated peak voltage). Therefore, they must be designed toabsorb the energy associated to such a condition. Once thefault is cleared, the MOVs cease to conduct and the bank issomewhat instantaneously reinserted in the circuit.However, it is not economic to size the MOVs to take care of

the worst possible faults. In extreme cases, it is acceptable tohave the MOVs quickly bypassed by the protective controlledgap when their energy absorption capacities are likely to beexceeded. In general, for external faults to the line where theseries capacitors are located, no bypass is allowed except forvery severe faults (multiple faults or excessively longduration faults). For an internal fault where no fast reclosingis provided, MOV bypass is acceptable since the line will betripped anyway and the series capacitor will be lost.For the Hoa Binh-Phu Lam interconnection, the line must bequickly reclosed to maintain the system stability following aspurious single phase internal fault, the fast bypass of theseries capacitors on the faulty phase can be acceptableprovided that these bypasses will help to accelerate the

secondary arc current extinction and ensure a trouble free linereclosing. In order to establish reasonable design criteria, theenergy stresses on MOVs were evaluated for 4-cycle single-phase and three-phase faults for the 1995 full load systemconfiguration as illustrated in Fig. 19.

Fig. 19: Fault locations along the Hoa Binh-Phu Lam

interconnection for the evaluation of energy stresses in MOVs.

Simulation results were summarized in the table-2 indicatingfor each series-capacitor bank, the external and internal faultlocations that produce the worst energy absorption conditionsin the MOVs and the worst associated energy levels.

Internal fault External faultSeries-capacitor bank Pos. E p-g

(MJ)E 3p-g(MJ)

Pos. E p-g(MJ)

E 3p-g(MJ)

Hoa Binh (S) 2 32 26 5 3.8 7.4

Ha Tinh (N) 4 3 6 5 4.4 7.6

Ha Tinh (S) 5 4.8 8 8 0.9 2.6

Da Nang (N) 7 1.1 2.1 8 1.2 2.7Da Nang (S) 8 4.3 3.1 11 1.3 2.2

Pleiku (N) 10 0.9 1.4 11 1.4 2.0

Pleiku (S) 11 2 3.3 9 0.2 0.8

Phu Lam (N) 12 10.8 5.6 13 0.9 0.5Table-2: Worst energy stresses on MOVs for various fault locations

along the Hoa Binh-Phu Lam interconnection.

E p-g : energy in MOVs during phase-to-ground fault.E 3-pg : energy in MOVs during three-phase-to-ground fault


7/8

7

From these simulation results, the following MOV designcriteria have been established for the series-capacitor bankson the existing Hoa Binh-Phu Lam interconnectionconsidering the use of SPAR systems on the four linesections:

Fault location MOV design criteria proposed for the Hoa Binh-Phu Lam interconnection.

External faults

The most constraining of the two following

events : Three phase to ground fault of 4 cycle

duration. The first phase-to-ground fault of 4 cycle

duration (see note 1) followed by 0.5 s deadtime followed by the second phase-to-ground fault cleared in 12 cycles (backup

protection time delay). The bypass isallowed only 4 cycle after the initiation ofthe second phase-to-ground fault.

(1) It has to be added the energy stresses onMOVs due to power oscillation.

Internal faults Bypass on single phase basis through externalcontrol signal is allowed when required.

5. ANALYSIS OF SECONDARY ARC CURRENT

EXTINCTION.

In order to improve the system transient stability following aspurious single phase fault, the SPAR systems were used onthe four 500 kV line sections of the Hoa Binh-Phu Laminterconnection. The reclosing dead times recommended by[1] for the four line sections: Hoa Binh-Ha Tinh, Ha Tinh-DaNang, Da Nang-Pleiku and Pleiku-Phu Lam are respectively0.6 s, 0.9 s, 1.0 s and 1.0 s. Therefore, the prime condition ofa successful single phase reclosing is to have the secondaryarc current extinguished on time and the arc canal sufficientlyde-ionized to withstand the reclosing overvoltages.

On the Vietnamese 500 kV series-compensated lines withshunt reactors connected to the middle between the linebreakers and the series-capacitor banks, the secondary arccurrent during single phase reclosing is composed of thefollowing four components :

A component due to the inter-phase inductive coupling. A component due to the inter-phase capacitive coupling. A component due to the series capacitor discharge in the

fault. A dc component due to the energy trapped in the shunt

reactors at the time when the fault is initiated.

The component resulting from the inter-phase inductive

coupling do not create problems since its magnitude isgenerally less than 10 A.For the four line sections of the Hoa Binh-Phu Laminterconnection, the magnitudes of the capacitive couplingcomponents which are given in Table-3, were calculatedassuming perfectly transposed lines according to [4].

Linesection

Hoa Binh -Ha Tinh

Ha Tinh -Da Nang

Da Nang -Pleiku

Pleiku -Phu Lam

Is (A rms) 53.8 61.0 40.6 77.8Table-3: Capacitive coupling components of secondary arc

currents.

The recovery voltage, also calculated according to [4], is43.1 kV rms.Considering that above 40 Arms, the secondary arc extinctionwithin 1.0 second is unlikely to take place and that othercomponents will prolong the arc extinction time as well, it isnecessary to reduce the capacitive coupling components byadding the reactor neutral circuits. For the four line sectionsof the Hoa Binh-Phu Lam interconnection the installedreactor neutral circuits have the following characteristics :

Neutral reactor Neutral resistorLine section Tap no. X () RDC()

Hoa Binh - Ha Tinh 2 650 28Ha Tinh - Da Nang 0 800 33Da Nang Pleiku 0 800 33Pleiku Phu Lam 1 550 25

Table-4 : Characteristics of the installed shunt reactor neutralcircuits.

To verify the efficiency of these reactor neutral circuits, thesecondary arc currents on the four line sections have beenanalyzed for the 1995 full load system configurations. Thesimulation results reveal that in case of no series-capacitorbypass, the secondary arc currents are of high magnitudes anddominantly the 5-7 Hz oscillating discharge currents as seenin Fig. 20. These currents will extend the arc quenching timeand make it difficult to achieve the reclosing dead timesrecommended by [1]. Therefore, it will be necessary toeliminate the series capacitor discharge currents in order toaccelerate the arc quenching process. The best way to do thisis to short the series capacitors at both ends of the faultyphase by giving single phase closing orders to both bypassbreakers. These closing orders could be initiated by lineprotection for single phase fault and must be operate asquickly as possible within the first time-out periods(Tbypass). At the instant of series-capacitor bypass theoscillating discharge currents disappear and the dc

components show up on the secondary arc currents.

Fig. 21: Shunt reactor dc flow paths after series capacitor bypass.

These dc components will dissipate through the arc resistance,the neutral circuit resistance and the system resistance asshown in Fig. 21. Simulation results, as illustrated in Fig. 20,indicate that under the effect of arc resistance, the shuntreactor dc currents can ultimately cause a maximum delayTdc of 200 ms in the creation of the first current zero crossingand consequently, in the secondary arc maximum extinctiontime.


8/8

8

a) Capacitor discharge and dc components of secondary arc

current on the Hoa Binh-Ha Tinh line section

b) Capacitor discharge and dc components of secondary arccurrent on the Ha Tinh-Da Nang line section

c) Capacitor discharge and dc components of secondary arccurrent on the Da Nang-Pleiku line section

d) Capacitor discharge and dc components of secondary arccurrent on the Pleiku-Phu Lam line section

Fig. 20: Capacitor discharge and dc components of secondary arccurrents on the four 500 kV line sections

Once the major components of secondary arc current areeliminated and damped out, the remaining arc currentcomponents are associated with inductive and residualcapacitive couplings. These residual arc currents have to bequenched before the line reclosing takes place. The test data

gathered in [5] were used to predict the residual arc quenchingtime Tarcin function of the product of the residual arc currentby prospective recovery voltage (Is x Vs) for the four linesections. Furthermore, assuming a time delay Tdielectric of100 ms for the arc path dielectric regeneration, the total deadtime for secondary arc current extinction should be :

Tdead-time = Tbypass+ Tdc + Tarc + Tdielectric (1)

Various dead time components for the four line sections weregiven in the Table-5.

Table-5: Residual arc current and various dead time componentsestimated for the four line sections

Line

section

Is

Arms

Vs

kV rms

Tarc

ms*

Tdc

ms

Tdielectric

ms

Tbypass**ms

HB-HT 17.8 35.4 200 200 100 100HT-DN 28.3 35.4 470 200 100 130DN-PK 18.4 43.1 320 200 100 380PK-PL 25.0 30.0 270 200 100 430(*) estimated residual arc quenching time in function of

Is x Vs [5](**) maximum allowable for series capacitor bypass time to

meet the total dead time recommended by [1].

From these results, the reclosing dead time of 0.6 s, 0.9 s, 1. sand 1. s. respectively recommended by [1] for the four linesections: Hoa Binh-Ha Tinh, Ha Tinh-Da Nang, Da Nang-Pleiku and Pleiku-Phu Lam are achievable with the installedreactor neutral circuits and the application of fast bypassactions to the series capacitors at both end of the faulty phase.However for the Hoa Binh-Ha Tinh line section, the seriescapacitors bypass should be completed within the first 100 msof the time-out.

6. CONCLUSIONS.

Extensive electromagnetic transient study has been performedfor the design of the existing North-South series-compensated500 kV interconnection in Vietnam. From these study resultsthe following main conclusions could be drawn :

The 500 kV circuit breakers are subjected to severe TRVconditions of very long series-compensated single linethat promote the creation of high transient overvoltageseach time a breaker is tripped.

Transient and temporary overvoltages in the North-South500 kV system in case of total load rejection can beefficiently controlled by using the remote transfer

tripping and overvoltage protection schemes. The reclosing dead times recommended by [1] tomaintain the system stability following a spurious singlephase fault are achievable with the installed shuntreactor neutral circuits and the application of fast bypassactions to the series capacitors at both ends of the faultyphase.

REFERENCES.

[1] Nippon Koei Co. Ltd. Tokyo Japan, North-South 500 kVTransmission System Project - Technical Report, September1992.[2] B. Khodabakhchian and al., TRV and the Non-zero

Crossing Phenomenon in Hydro-Quebecs Projected 735 kVSeries-Compensated System, CIGR Paris 1992.[3] Pacific Power International - SECV International, North-South 500 kV Transmission Line Project - System Studiesparts A and B, December 1992.[4] N. Knudsen, Single Phase Switching of TransmissionLines Using Reactors for Extinction of the Secondary Arc,CIGR report no. 310, Session 1962.[5] D. E. Perry et al., tude et valuation du R-enclenchement Monophas dans les Rseau Ultra HauteTension des tats-Unis, rapport 39-08 de la CIGR Session1984.

Documents

Transient Simulation Study HV Vietnam