Optimal Reclosing Time Of

7/30/2019 Optimal Reclosing Time Of

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OPTIMAL RECLOSING TIME OFTRANSMISSION LINES AND ITS APPLICATIONIN REAL POWER SYSTEMP. Li*, B.H. Zhang*, Z.G. llao*, Y.F. Rao*, Y. T. Wang*, Z.Q. Bo t , A. Klimekt, Q. Zhao#, W. He'

*Schoolof Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China.E-mail: roclarry(~igmail.com

tAREVA T&D Automation, UK.*Ningxia Electric Power Corporation,Yinchuan 750001 ,China

Keywords: Optimal reclosing time (ORT), system stability,transient energy function.AbstractTh e study of various stability-control methods for atransmission system indicates that the reclosing time of atransmission line has a distinct influence on system stability.And there is an optimal reclosing time (ORT) for alltransmission line faults. Utilizing the transient energyfunction (TEF) with classical models considered, the ORTsetting computation method is proposed. This paper showsthe validity and practicability of ORT. Based on analyzing thecharacteristic and influencing factors of the ORT, the settingcriteria of reclosing time for transmission line is presented. Itwill conduct simulations with a complex real system inNorthwest of China. Simulation studies show that thistechnique can significantly increase the security margin anddamp the oscillation of the power system. It is indicated thatthe variation of optimal reclosing time of a certaintransmission line is insensitive to these influencing factors:the fault location, fault clearing time and pre-fault power flowof this line. Finally, the setting strategy is drawn to optimizethe reclosing time of the key transmission lines in NingXianetwork under current technological conditions.1 IntroductionHolding steady run is one of the primary missions of powersystem. Modem power system are facing continuouslyrestricting stability margins, as social and economic obstaclesset limits to the realization of new generation andtransmission structure. Hence, it is very important to focusattention on all measures able to improve stability-relatedaspects. Nowadays, to enhance the continuousness of powersupply and the stability of power system, the reclosure deviceis extensively used for high voltage transmission lines inpower networks.

As taking the advantages, also some technical problem for theauto-reclosure device should be solved. Presently, it isdifficult to distinguish an instantaneous fault from apermanent one in practice. Most reclosures take place whenthe nature of a fault is unknown. The contemporary auto-reclosure techniques adopt the scheme of fixed time reclosureand re-trip if a permanent fault. The impacts on systemstability and power apparatuses due to reclosing on apermanent fault could be worse than that caused by the fault.For some power networks, the transmission capacity mayhave to be limited so that the system stability requirement issatisfied if reclosing on a permanent fault at a fixed reclosingtime.Th e study of various stability-control methods for atransmission system indicates that the reclosing time of atransmission line has a distinct influence on system stability.There is an optimal reclosing time (ORT) for allinstantaneous or permanent transmission line faults [1.-3].Th e power transfer capacity may be increased significantly ifan optimal reclosing time is adopted [4-1d0]. This paper isorganized as following: Firstly, based on the OrdinaryDifferential Equation theory and Transient Energy Function(TEF) method, the qualitative analysis and quantitativecomputation method for the OR T is presented. Secondly, toshow the validity of the ORT. extensive studies andsimulations are carried out. After analyzing the characteristican d influencing factors of the ORT, the optimal reclosingcriterion for transmission lines is introduced. It will conductsimulations with a real complex system in Northwest Grid ofChina to verify the practicability of the ORT. Simulationstudies show that this technique can significantly increase thesecurity margin of the power system. More, the simulationresults indicate that the variation of optimal reclosing time isinsensitive to the changes of the fault location, fault clearingtime and pre-fault power flow, and so on. Finally, the settingscheme is drawn to optimize the reclosing time of the networkunder current technological condition.

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2 Optimal Reclosing Time2.1 Qualitative analysis of the ORTIn multi-machine power system, given that 1=0' is theinstance at which the last system disturbance occurred justly,and the stable equilibrium point (SEP) is the origin of thestate space. Neglecting the damp, the generator adaptesclassical models, and the movement equations of rotors arefollowing:

W'i 12 ,_ _I

system stability is proposed, and the instance correspondingto the minimum TEF after the last network's operation isselected as the optimal reclosing time (ORT).Given classical model adopted, the multi-machine systemTEF based on the center of inertia (COI) frame is [18,19]

f"Ih Mi (t), 2 Z Pi (O "i) [. Cs0 o y)2 = i-I Y Y+

+ Di, fcos 9~d(O,9Oj)Where 0'15 - C0,

(2-2)

(2-1)Where Mi Inertia constant of generator i;

5i,o Power angle and angular velocity of generator i;PTi Mechanical power input of generator i;Pei, Electromagnetic power of generator i;The system dynamic behavior after the last disturbance ischaracterized by equations (2-1). The stability of system is thestability of the solution of system equations (2- 1), which is anautonomous system. According the qualitative theory of theOrdinary Differential Equation, the solution of (2-1) dependson the initial value: if the initial value is inside the stableregion of SEP, the system will be stable and converge to theSEP finally, or else the system will lose stability. The initialvalue of (2-1) is the power angle and angular velocity of eachgenerator at the instance of the last disturbance justaccomplished. Zero initial value is an ideal situation, namely,the last system operation is completed at the SEP, and thesystem will reach the steady state at once, but it is impossiblein general. In practice, the optimal instance that the lastoperation accomplished is instance at which the system statevariables (5i, iv, i =I. - N) are best closed to the SEP[2].For instantaneous fault, the last disturbance of system issuccessful reclosure. It must ensure that the reclosing time islonger than the least reclosing time which is determined bythe circuit breaker (CB). If the fault is a permanent one, thelast disturbance is the CB re-tripping following anunsuccessful reclosure. In any case, the fault must be clearedas quickly as possible, which is decided by the protection(about 0.1 s). Selecting optimal last operation time isequivalent to selecting optimal reclosing time. As far assystem stability is concerned, reclosing on a permanent faultma y not always result in negative effects. Utilizing suitablereclosing time, the accelerative energy yielded by the secondfault striking may counteract the decelerative energy carriedby the rotor back-swing of generators, and the powerimbalance of the generators is decreased significantly.2.2 Quantitative computation of OR TIt is well known that, Transient Energy Function (TEF)method is a direct method for studying the power systemtransient stability. The TEF value indicates the oscillationsmagnitude of an autonomous system: the larger the TEF is,the severer the system condition is, and vice versa. While theTEF is larger than the critical value, the system will beunstable [18, 19]. Based on this, using the TEF as an index of

MT XM ,

I n 1 1MTH MT ,i=

Ol SEP of the generator i with the CO I frame;Sf5j, wco Angle and angular velocity of the system COI;Gii,4 Real and imaginary components of the U-th

elements of the network's admittance matrix;[i,i Angle and angular velocity of the generator

with the COI frame;Given V~h is TEF just after the last disturbance. The optimalreclosing criterion can be calculated by [10~- 17]

1fn 2 f_-,hmin (-YXM (toi+,o 2P(:, 9ii+);2, I i=1

S[C!. (cosOf~ii- cos 64,)s)]+~ D' Jcos09 d(0,++90)1YjY - -iI 9Where the superscript "+" denotes the value of system statevariable just at the last disturbance accomplished; and thesuperscript "P" denotes the variable depending on the gridstructure after the last disturbance, such as the SEP and theadmittance matrix. In general, it is difficult to obtain theanalytical solution of equation (2-3), so the TEF curve iscalculated as the numerical integral method of the transientstability calculation, and the local minimum point is soughtout as the ORT.3 Setting computation of the optimize reclosingtimeThe OR T is interrelated with the system condition. Toachieve the optimal effect in deed, it is necessary to calculateand set the OR T online in real time. But this is unpractical inreal power system. In this section, based on the analysis of theinfluencing factors and natures of the ORT, the offline settingscheme of reclosing time of transmission line is suggested.3.1 Influencing factors analysisIn the above formula (2-2) and (2-3), the TEF value is mainlyrelated with the following factors:

a) reclosing instance;b) inertia of power system;c) system configuration;d) structural changes caused by the reclosure;

ill

pi = pT i 1Ei 12GiiDy =dEi IEj I Gy



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e) fault clearing time;f) fault location;g) pre-fault operation conditions. etc.Clearly, factor a) is the objective that will be optimized. Thechange of b) -g) will influence the TEF value, and may

induce to the variation of the ORT. Normally, it is difficult toreset the reclosing time online to follow these changes. Allthese influence factors ma y reduce the practicability of theORT. After numerous analysis and simulation, someconclusions and strategies are obtained: First, the inertia ofthe system mainly depends on the generator installed capacity.which ma y not be changed heavily in short term, especiallyfor large-scale real system. Second, the grid structure will notbe regulated massively and frequently, and the smalladjustment has little influence on the ORT. Third, thestructural changes caused by the reclosure have beenconsidered in the computation procedure. There is a smalldifference in fault clearing time of relay. And the OR T isinsensitive to the small difference of the fault clearing time.Simulations indicate that the ORT varies slightly with systemoperation conditions and fault location for a given systemconfiguration. In a word, of all influencing factors, the greatchange of inertia and configuration of the power system isimportant for the ORT, and others have little impact on theORT. All above analysis give a support to apply the OR T inreal power system.3.2 Nature analysis of the ORTThe ORT calculated from the TEF is the minimum point ofthe TEF curve, and exactly speaking, it is a local minimumpoint of TEF curve, which is formed by the TEF valuecorresponding to different reclosing time. The TEF iscontinuously depended on the system state variable (angleand angular velocity of generator in classical model) whichare solutions of ordinary differential equations. So the TEFcurve is smoothness, which ensures that the curve is flat andthe TEF is also smaller in the neighborhood of a localminimum point. Fo r the operation parameter variation orsetting error, the proposed ORT will yield optimal orsuboptimal results, no t the worst. This nature is important forthe application of the OR T in practice. After disturbance, theangle and angular velocity of generators is swing in someperiods. The minimum point of the TEF will appearperiodically. It is also useful for catching the ORT.3.3 Setting calculation of the ORTMore often, the auto-reclosure device adopts simple timingunit in field. The reclosing time cannot be reset online tofollow the changes of system practical situation. More,considering the requirement and reliability of the system, it isnot suitable to vary the reclosing time frequently [8].Therefore, as setting the reclosing time offline, it is ought totake various cases into account. An d it is reasonable andrealistic to propose a relatively optimum reclosing time. Sinceno practical method is available to avoid reclosing on apermanent fault presently, and also as far as system stabilityis concemned, reclosing on a permanent fault may not always

result in negative effects. From the point of view of systemstability, the optimizing reclosing time should be calculatedunder the system maximum power-flow condition with thepermanent fault, which the transient stable limit is restricted[5,8,17]. As such case, the reclosure operation has greatinfluence on the system stability. Fo r the faults on otheroperation mode of the power network, even if it isn't the best,only the magnitude of system oscillations can be affected.However, it will not threaten the system stability. Consideringthe natures of the ORT, this approach will achieve optimal orsuboptimal results. However, it should be noted that thereclosing instance need to be re-calculated, and updated todevices in time as system configuration changes greatly.4 Application of ORT in real power systemTh e Northwest Grid in China, including about 1100 buses,150 generators and 650 AC-lines, has been investigated bysimulation. The simulations have been conducted mainly inkey transmission sections in NingXia province network ofNorthwest Grid (as simply shown in Fig. 1). In the simulationstudies, the ORT of the single-phase reclosure under a single-phase to earth permanent fault are calculated. Thecontemporary reclosing time is 0.7s in NingXia network. Thefault location is at the first 2% of relevant line. 'The faulthappens at 0.0s, and fault clearing time is 0.lIs. Because of theleast reclosing time (about 0.5s), normally, the TEF curvesstart at 0.5s and end at 2.0s. The TEF of instance t is thetransient energy as the last operation achieved by reclosing atthis instance.

/330kV I......YinRci-YinChuan YinChuan-YinNan Run-out Section

Section SectionFig. 1Key transmission sections in NingXia network4.1 Calculation of the ORTFig.3 shows the single-phase fault and single-phase reclosureTEF curve of the 220-ky line BuQiao-JinFeng of the YinBei-YinChuan transmission section (as shown in Fig.2). Th e faultlocation is at the 2% of the line to BuQiao. It is clearly thatthe TEF reaches its first local minimum point atapproximately 0.95s. Also the curve is smoothness and flat inthe neighborhood of 0.95s.

DaWK ToeBUQuao 'aChang~hg U

yitiBet 0 N

Yinflhuan

(D LuHua linI'eng YueCiYinChuan 0'2 kVS~imn

Fig.2 YinBei-YinChuan transmission section in NingXiaNetwork

112



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2

1,5Tin W~Fig.3 TEE Curve of fault on the line BuQiao-Jin~eng

125

o120+

,,100,

0 1 2 Time(s) '3Eig.4 Angle swing curves between YinRe and LueYang PlantEig.4 shows the angle-oscillation curves between thegenerating units of the YinRe and LueYang Plant, betweenwhich the magnitude of angle-oscillation is relatively larger,under different reclosing time. Eig.5 shows the maximal angledifference oscillation curves under different reclosing time(0.70s and 0.95s). It can be seen that reclosing at the instancein the interval about 0.95s-l .00s the system oscillation can bereduced obviously. It is consistent with the ORTcorresponding to the local minimum point of the TEEcurve.The ORT of transmission lines in YinBei-YinChuanSection of NingXia network in heavy-load running mode in2007 and 2008 winter is presented in Table 1.

110

0 110

1052 4 U 0 1

0 2 4~Toime(s) 6 8 1Fig.5 Max angle difference swing curves

TransmissionLine OptimalReclosing TimeSection Line 2007 W 2008 WYinBei- DaWK-YinChuan 0.95s /YinChuan ChangShg-LuHua -0.93s 0.83sSection BuQiao-JinFeng 0 .95s 0.85s

______YueYH-TaoLe 0.92s 0.82sTable 1 The ORT of transmission line in Yinbei-Yinchuantransmission section of NingXia network

; /.. .. . . .

Time(s)Eig.7 TEE Curve of different fault clearance timeSecondly, the impact of the different fault clearing time onthe ORT of a certain line is verified. Nowadays, the relay cantrip the fault line quickly (less than 0.1is for high voltagetransmission lines). But the time stray of the protection action,it should verify the influence of different fault clearing timeon ORT. Eig.7 shows the single-phase fault and single-phasereclosure TEE curve of the line YinChuan-DaBa withdifferent fault clearing time (0.08s, 0.09s, 0.10s, 0.1 Is and

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Notation: "/" indicates that the line DaWK-YinChuan doesno t exist in 2008 winter running mode.4.2 Influencing factors SimulationIn order to verify the practicability of the ORT, some maininfluence factors are simulated in the NingXia network onheavy-load run mode in 2007 winter.Firstly, different fault location of the transmission line caninfluence on the TEF curve. Fig.6 shows the single-phasefault and single-phase reclosure TEF curve of the lineYinChuan-DaBa faulted at different location (distance to thehead-terminal of the line 2% , 25%, 50%, 75% and 98%,respectively). It can be seen from the fig.6 is that: The TEF islarger as the head or end-terminal fault, and the stability isrelatively weak, which accords with the actual situation of thesystem running. Moreover, the minimum point of TEE,namely the ORT, varies slightly (about in 0.90s-'0.95s), andthe TEE curve is very flat near the ORT. It can be concludedthat the fault location has little impact on the OR T of a certainline.

8 2%1

98%

IT 'rmS) d

Fig.6 TEE Curve of different fault location



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0. 12s, respectively). What can be seen from the fig.7 is that:The longer is the fault clearing time, the higher is TEF curve,and the weaker is the system stability, which is the actualsituation. More, the minimum point of TEF, namely the ORT,vary slightly (about 0.95s), and the TEE curve is very flatnear the ORT. It can be concluded that the fault clearing timeof the protection has little impact on the ORT of a certaintransmission line.Thirdly, the impact of the different pre-fault power-flow levelon the OR T of a certain line is verified. Fig.8 shows thesingle-phase fault and single-phase reclosure TEF curve ofthe line YinChuan-DaBa in different pre-fault power-flow(0.8, 0.9, 1.0, 1.1 and 1.2 times of basic power flow,respectively). What can be concluded from the fig.8 is that:The severer is the pre-fault power flow, the higher is TEFcurve, and the weaker is the stability, which also is the actualsituation. More, the minimum point of TEF, namely the ORT,vary slightly (about 0.90s'A .O0s), and the TEE curve is veryflat near the ORT. It is obvious that the OR T varies slightlywith pre-fault operation condition for a certain systemconfiguration.

-PL-O.SPL10i'

PL-17 Pt=1.2

2-I,

11

Time(s) 1.52Fig.8 TEF Curve of different pre-fault power flow5 Setting strategy of ORT for Ningxia Network5.1 Practicability analysisAs the change of the system inertia and structure, the OR Twill vary. In order to achieve good result, and alsoconsidering the requirement and reliability of the systemrunning, the OR T should be recalculated as systemconfiguration changes greatly. The device should be reset intime. The unit of update period can be "year". Moreover, itcan be seen from the most TEE curve of the lines in NingXianetwork that the period of TEF curves are almost longer than1 2s, and the distance between continuous minimum andmaximum TEE point is almost longer than 0.5s. Anotherexciting result is that the TEF curve is very flat near the localminimum point, and ascends slowly. Therefore, there is alarge time interval near the ORT, in which the TEE is smaller.It benefits improving the practicability and increase the usedeadline of the ORT.It is worth noting that the ORT, based on the TEE method, iscalculated for a transmission line respectively. Namely, the

OR T of a certain line is independent to the reclosing time ofothers lines, and there is nothing concerned mutualcoordination. Considering the workload and the systemreliability, the OR T calculation and setting can be just carriedou t on some key transmission lines and sections. It is useful toimprove the flexible of the application of the ORT.5.2 Setting strategy of the ORT for NingXia NetworkUsing the TEE theory, the OR T has been calculated for thelines of main transmission sections in the NingXia network.The angle-swing curve reclosed on different time has verifiedthe validity of the ORT. Utilizing the ORT can damp theswing of the system, and improve the capability to suppresstransmission line fault.The ORT is a local optimal reclosing time, obtained from acertain operation condition and fault. For NingXia powersystem, the OR T is calculated on some main running modes(e.g. heavy load mode in 2007 winter, 2008 winter and so on).Comprehensive considered the natures, influencing factorsand the setting method of the ORT, the reclosing time ofrelevant lines can be set offline, and will yield optimal orsuboptimal results. Within the same voltage lines of a section,the computation results indicate that the OR T is nearly thesame. Therefore, the ORT setting can be carried outaccording to sections with the same voltage level. Table 2shows the OR T setting scheme of the main transmissionsections in NingXia network on the voltage level. The OR Tneed to be recalculated for system configuration changesgreatly to maintain its effectiveness.

Transmission lines220kV Sections330kV Sections750kV Lines

IORT0 80s'-0.90s1.00s- .1Osl.05s'-l.15s I

Table 2 The OR T setting strategy of the main transmissionsections in NingXia network on voltage6 ConclusionTh e reclosing time of a transmission line has distinct impacton power system stability. Based on the transient energyfunction (TEE), the OR T can be calculated accurately, andalso has actual physical meaning. Theoretical analysis andnumerical application with a complex real power system hasclearly underlined the success, feasibility and practicability ofthis technique. Considering the realistic situation, the settingscheme is drawn to optimize the reclosing time of powersystem under current technological condition. Simulationstudies with NingXia network show that this technique cansignificantly increase the security margin and damp theoscillation of the power system. It can be concluded that theOR T is insensitive to the influence of the fault location, faultclearing time and pre-fault power flow, etc. The OR T can beset only on the lines of key transmission sections of a powernetwork. All these works are helpful to improve flexible ofthe application of the OR T to realistic power networks.

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Optimal Reclosing Time Of