FAULT ANALYSIS FOR CIRCUIT BREAKERS RATINGS DETERMINATION ON NIGERIAN 330kv TRANSMISSION GRID

Transcription

1 The International Journal Of Engineering And Science (Ije) Volume 2 Iue 3 Page In: Ibn: FAULT ANALYSIS FOR CIRCUIT BREAKERS RATINGS DETERMINATION ON NIGERIAN 330kv TRANSMISSION GRID 1, Gafari A. Adepoju, 2, Oluola A. Komolafe, 3, Muhammed A. Tijani 4, Akeem O. Biiriyu 1, Department of Electronic & Electrical Engineering, Ladoke Akintola Univerity of Technology, Ogbomoo, Nigeria. 2, Department of Electronic & Electrical Engineering, Obafemi Awolowo Univerity, Ile-Ife, Nigeria. 3, Department of Electrical/Electronic Engineering, Federal Polytechnic, Ede, Nigeria. 4, Departmet of Electrical/Electronic Engineering, Oun State Polytechnic, Iree, Nigeria Abtract Fault Studie form an important part of power ytem analyi for table and economical operation of a Power Sytem. Fault on a power ytem are divided into ymmetrical and unymmetrical fault. In thi paper, threephae ymmetrical fault wa imulated on the Nigerian 330kV National Grid uing Nigerian 24 bu power ytem from Power Holding Company of Nigeria. Two different MATLAB baed program were written; one program wa for Load Flow Studie to determine the pre-fault condition baed on Newton-Raphon method, while the other wa for three-phae hort-circuit tudie. It wa oberved that the fault current were motly exceively high. The information gained from the fault tudie were ued for the determination of circuit breaker rating on the power ytem. Keyword: Circuit breaker, Load Flow, Power Sytem, Short-Circuit Current, Three-Phae Fault Date Of Submiion:26,Jan, 2013 Date Of Publication:15 March I. Introduction The Nigerian Power Sytem ha recently been expanded; therefore, probability of fault require new device etting, co ordination and calculation in order to withtand a fault. A fault i defined a any failure which interfere with the normal current flow [1]. A fault will caue current of high value (hort-circuit current) to flow through the network to the faulted point. Short-circuit current generate heat proportional to the quare of the current magnitude; thi large amount of heat may damage the inulation of power ytem device uch a bu bar, cable, circuit breaker and witche [2].The purpoe of an electrical power ytem i to generate and upply electrical energy to cutomer with reliability and economy. The greatet threat to thi purpoe of a power ytem i the hort circuit. When the ytem i o large like the Nigerian ytem conidered in thi paper, the chance of a fault occurring and the diturbance it will caue are both o enormou that without equipment to remove fault, the ytem will collape [3]. The evaluation of fault current and determination of circuit breaker rating on a power ytem i therefore ignificant becaue the reliable and ecure operation of the power ytem depend on thee. Fault analyi can be broadly grouped into ymmetrical and unymmetrical fault. A fault involving all the three phae on the power ytem i known a ymmetrical fault or three-phae fault while the one involving one or two phae i known a unymmetrical fault. Single Line-to-ground, Line-to-line and Double line-toground fault are unymmetrical fault [3]. The caue of fault were found to include lightning, inulation aging, heavy wind, tree falling acro line, vehicle colliding with pole, bird, kite, etc. [3], [4]. The effect of fault on power ytem are: (i) Due to overheating and mechanical force developed by fault, electrical equipment uch a bu-bar, generator and tranformer may be damaged. (ii) The voltage profile of the ytem may be reduced to unacceptable limit a a reult of fault. A frequency drop may lead to intability. [5].Majority of fault occurring on power ytem are unymmetrical fault, however, the circuit breaker rated MVA breaking capacity i baed on three-phae ymmetrical fault. The reaon i that a three-phae fault produce the greatet fault current and caue the greatet damage to a power ytem. The only exception to thi i a ingle line-to-ground fault occurring very cloe to a olidly The IJES Page 116

2 Fault Analyi For Circuit Breaker rounded generator terminal [4]. Short circuit tudie involve finding the voltage and current ditribution throughout the power ytem during fault condition o that the protective device may be et to detect and iolate the faulty portion of the power ytem o a to minimize the harmful effect of uch contingencie [6], [7], [8].The preent dilapidated tate of the power ytem infratructure of the Nigeria Grid i attributed to poor maintenance [9]. Any country with poor level of power availability like Nigeria hould firt think about improvement of generation, tranmiion and ditribution before thinking of indutrialization [10]. Power ytem fault analyi i one of the baic problem in power ytem engineering. The reult of power ytem fault analyi are ued to determine the type and ize of the protective ytem to be intalled on the ytem o that continuity of upply i enured even when there i a fault on the power ytem. The current trend of erratic power upply and ytem collape in Nigeria ha made thi tudy important to the nation newly expanded power indutry. Figure1 i a ingle line diagram of the Nigeria 24-Bu, 330kV Tranmiion Grid. Kano B Kebbi Kaduna Gombe Kainji G/S Jo Jebba G/S Shiroro G/S Jebba Ajaokuta Abuja Ayede Oogbo Benin Sapele G/S Ikeja Wet Delta G/S Akangba Onitha Aladja Egbin G/S Aja N - Haven Alaoje Afam G/S Figure 1: Nigeria 24 Bu 330kV National Grid [11] II. Reearch Method In hort circuit tudie, it i neceary to have the knowledge of pre-fault voltage and current. Thee pre-fault condition are obtained from the reult of load flow tudie by the Newton-Raphon iteration method. The goal of a power flow tudy i to obtain complete voltage angle and magnitude information for each bu in a power ytem for pecified load and generator real power and voltage condition. Once thi information i known, real and reactive power flow on each branch a well a generator reactive power output can be analytically determined. [12]. There are everal different method of olving the reulting nonlinear ytem of equation. The mot popular i known a the Newton-Raphon Method. The Newton-Raphon method i preferred to other method (Gau-Seidel and Fat-Decoupled Method) in thi reearch work becaue of it advantage which include maller time to perform one iteration of the computation, the number of iteration i more or le independent of the ize of the power ytem and vary between 4 to 7 iteration. Alo, the convergence characteritic of the Newton-Raphon method are not affected by the election of lack bu [13], [14].Thi method begin with initial guee of all unknown variable (voltage magnitude and angle at Load Bue and voltage angle at Generator Bue). Next, a Taylor Serie i written, with the higher order term ignored, for each of the power balance equation included in the ytem of equation. The reult i a linear ytem of equation that can be expreed a: (1) where ΔP and ΔQ are called the mimatch equation: (3) and J i a matrix of partial derivative known a a Jacobian: (2) The IJES Page 117

3 Data and form Ybu Fault Analyi For Circuit Breaker (4) The linearized ytem of equation i olved to determine the next gue (m + 1) of voltage magnitude and angle baed on: (5) (6) The proce continue until a topping condition i met. A common topping condition i to terminate if the norm of the mimatch equation are below a pecified tolerance. A rough outline of olution of the power flow problem uing Newton-Raphon method i depicted in Figure 2:A omewhat implified, although approximate, hort circuit tudy i made by neglecting the pre-fault current. Thi mean that all the bu voltage are 1 p.u immediately before the fault. For a ymmetrical fault, the negative and zero equence are abent. The poitive equence network preent and modified for fault analyi i hown in Figure 3. The equation relating the equence quantitie are; V 0-bu = - I 0-bu (7) V 1-bu = E bu - I 1-bu (8) V 2-bu = - I 2-bu (9) START Read Load Flow Aume bu voltage V (0) i i = 1...n, i Set iteration count r=0 Set bu count i=0 Determine the type of bu (Slack, PV or PQ bu) Compute P(r)I, Vri for PV bue and P(r)I, Q for PQ bue. Solve the mimatch equation for each bu uing equation (1), (2) & (3). Advance iteration count r = r+1 Determine max. change in power ΔP r, ΔQ r and Δ V r 2 Determine J -1 ; and determine the next gue from equation (5) & (6). Are all max Δ within tolerance? No Aemble Jacobian, uing equation (4) YES Compute and print voltage, angle, etc STOP Figure 2 : Newton-Raphon Load Flow Analyi Flow Chart. 1 E General Poitive Sequence Network n k I 1 k V 1 k Reference Figure 3: Poitive Sequence Network Modified for Fault Analyi [15]. Since pre-fault current are neglected, vector E contain 1 in all the entrie. The current are all zero until the network i terminated externally. At a time only one bu (i.e. the faulted bu k) i terminated. Thu, only have non-zero entry. Very frequently, are aumed to be identical to reduce computer memory requirement. [3], [4], [15].For a ymmetrical fault, the negative and zero equence are abent, i.e., V 0-bu, V 2-bu, I 0-bu and I 2-bu are zero. The IJES Page 118

4 But all current except at the faulted bu, i.e., (11) If Z f i the fault impedance (12) From equation (11) and (12), (13) The voltage at i th bu i Where; V 1 k = Poitive equence bu voltage for bu k. I 1 k = Poitive equence bu current for bu k. Z 1 kk = Poitive equence bu impedance for bu k. E = Induced e.m.f. under load condition. are zero. Therefore, Fault Analyi For Circuit Breaker The hort-circuit fault current I 1 kdetermined from equation (13) were converted to per unit value and the ka (Kilo-Amp) value were calculated from the following relation: Bae Current = bae MVA/ 3 X Bae Voltage Bae MVA = 100MVA Bae Voltage = 330kV Bae Current = 100X10 6 / 3 X 330 X 10 3 Bae Current = A Actual Value of current = Per Unit Value X Bae Value. Figure 4 how the implified computer flowchart for calculating fault current, voltage for the three-phae fault conidered and electing appropriate circuit breaker. (i) (ii) Two of the circuit breaker rating which require computation of hort circuit current are: Rated momentary current and Rated ymmetrical interrupting current. Symmetrical hort circuit current i obtained by uing ub tranient reactance for ynchronou machine. Momentary current i (rm) then calculated by multiplying the ymmetrical momentary current by a factor of 1.6 to account for the preence of dc offet current (Nagrath and Kothari, 1994). The current that a circuit breaker can interrupt i inverely proportional to the operating voltage over a certain range of time. If voltage and current are in per unit value on a three-phae bai, then; (15) Obviouly, rated MVA interrupting capacity of a circuit breaker i to be more than (or equal to) to the hort circuit MVA required to be interrupted. For the election of a circuit breaker for a particular location, the maximum poible hort circuit MVA to be interrupted mut be found with repect to the type and location of fault and generating capacity connected to the ytem. A three-phae fault though rare i generally the one which give the highet hort circuit MVA and a circuit breaker mut be capable of interrupting it. An exception i a line to ground fault due to a ynchronou generator. III. Reult and Analyi The load flow analyi (pre-fault analyi) wa carried out uing the Newton-Raphon load flow method. Thi analyi determine the voltage magnitude and angle in degree at each bu in the power ytem. The reult of the load flow i hown in Table 1. It can be oberved that the voltage magnitude and the angle compared with the nominal value are imilar. Where there are difference, they are within the tolerance range of ± 10% except for Kano and Gombe. Thee two bue low voltage profile can be improved by incorporating voltage control device on the line. After the load flow analyi, a three phae fault wa imulated; voltage and current on the bue were calculated. Table 2 how the voltage magnitude and their angle in degree when a three phae fault occur on bue 5, 9, 15, and 20 (a example). Table 3 how the fault current magnitude and the angle in degree for fault on bue 5, 9, 15, and 20 repectively.when a hort-circuit occur, the voltage at faulted point i reduced to zero [8]. One of the effect of fault on power ytem i that it lower the voltage (10) (14) The IJES Page 119

5 Fault Analyi For Circuit Breaker START Read data Formulate and tore Z 1 -bu, Z 2 -bu and Z 0 -bu Set E=1 0 o and Zf = 0. Specify k, i.e., Faulted bu Find I 1 k uing equation (13) Calculate Ia, Ib, and Ic. PRINT Ia, Ib, and Ic. Set bu count i = 1. Find V 1 k uing equation (14) Find Va, Vb and Vc. PRINT V 1 i, V 2 i, V 0 i, Va, Vb and Vc. Find SCMVA uing equation (15) Select appropriate Ciecuit Breaker Advance bu count by 1 No I i n STOP Ye Figure 4: Flow Chart for Fault Calculation and Circuit Breaker Selection magnitude. Comparing the voltage magnitude in Table 1 with the voltage magnitude in Table 2, it i oberved that the voltage magnitude fall below the acceptable level of ± 10%. The voltage magnitude of the faulted bue are lowered to zero. One of the aumption afely made in hort-circuit calculation i that all the prefault current are zero [7]. From Table 3, it can be oberved that current magnitude of the bue when fault occur are exceively high compared to the pre-fault current aumed to be zero. Current of abnormally high magnitude flow through the network to the point of fault. A een from Table 3, current magnitude on bue 5, 9, 15, and 20 are the highet when the fault were imulated on thee bue a compared to current magnitude on other bue. Table 1: Power Flow Solution by Newton-Raphon Method Nigerian 24 Bu, 330kV Sytem Bu Name Bu No. Voltage (degree) EGBIN DELTA AJA AKANGBA IKEJA-WEST AJAOKUTA ALADJA BENNIN AYEDE OSHOGBO AFAM ALAOJI NEW-HAVEN ONITSHA BIRNIN-KEBBI GOMBE JEBBA JEBBAG JOS KADUNA KAINJI KANO SHIRORO SAPELE Figure 5 how the total fault current magnitude on each bu when fault occur on the repective bue. The value of the fault current magnitude in Kilo-Ampere (ka) are plotted againt each bu in the graph The IJES Page 120

6 Bu Name Fault Analyi For Circuit Breaker Table 2: Voltage and for Fault on Bue 5, 9, 15, and 20. Bu No. Bu 5 Voltage Angle degree Bu 9 Voltage Magnitu de Angle degre e Bu 15 Voltage Angle degree Bu 20 Voltage Magnitud e degree EGBIN DELTA AJA AKANGBA IKEJAWEST AJAOKUTA ALADJA BENIN AYEDE OSOGBO AFAM ALAOJI NEWHAVEN ONITSHA BIRNINKEBBI GOMBE JEBBA JEBBAG JOS KADUNA KAINJI KANO SHIRORO SAPELE From Bu Table 3: Line Current and for Fault on Bue 5, 9, 17, and 20. To Bu 5 Bu 9 Bu 15 Bu 20 Bu Current degree Current degree Current degree Current degree F F The IJES Page 121

7 Current (ka) Fault Analyi For Circuit Breaker F F F Bue Figure 5: Fault Current in ka The three-phae Short Circuit MVA which determine the rating of the Circuit Breaker to be intalled were calculated uing equation (15). Table 4 how the Short Circuit MVA rating for the fault current on each bu and the correponding rating of the Circuit Breaker to be intalled. The range of the circuit breaker determined for the Nigerian Power Sytem i within 100MVA and 650MVA. Table 4: Short Circuit MVA and Circuit Breaker Rating Bu No Current (ka) SCMVA (MVA) Circuit Breaker Rating (MVA) The IJES Page 122

8 Fault Analyi For Circuit Breaker IV. Concluion Fault analyi on power ytem involve knowing the ytem performance at teady tate and calculating the value of the current flowing in the ytem when fault occur. Load flow analyi wa carried out on the Nigeria power ytem to determine the teady tate value, the reult were found to be atifactory. Fault analyi wa ubequently carried out to determine the voltage and current when fault occur and the reult how that exceively high current flow in the power ytem when there i fault. The reult of the fault analyi were ued to determine the circuit breaker rating for the power ytem. A could be oberved from the reult of thi reearch work and for the reaon that the ytem data were ourced from Power Holding Corporation of Nigeria (PHCN), the regular calculation of the current which flow in the power ytem when a three-phae fault ymmetrical fault occur and the election of appropriate circuit breaker are required for the proper operation of the power ytem becaue of the continuou expanion of the National Grid in Nigeria. Reference [1] A. Muhammad, Simulation of Different Type of Fault on Northern Iraq Power Sytem, IGEC VI 028, [2] A. Ibe, and N.Uzonwa, Power Sytem Simulation for Short Circuit Current in the Selection of Switchgear, Nigerian Journal of Indutrial and Sytem Studie, Vol. 4, No. 3, pp 9 15, [3] M. Tijani and D. Olatunji, An Evaluation of Three-Phae Fault Current on the Nigerian 330kV Tranmiion Grid. Journal of Reearch in Technology and Engineering Management, Vol. 4, No. 2, pp , [4] Okelola, et al., Fault Analyi: An Application of Venin Method to 330kV Tranmiion Grid Sytem in Nigeria, LAUTECH Journal of Engineering and Technology, Vol. 3, No. 1, pp 30 43, [5] O. Okemiri, Baic Protection Scheme on Power Sytem, The Nigerian Tribune, pp 24, Tueday 12 th February 2008, [6] M. El-Hawary, Introduction to Electrical Power Sytem, Wiley, IEEE Pre, [7] I.J. Nagrath, and D.P. Kothari, Power Sytem Engineering, Tata McGraw-Hill Publihing Company Limited, New Delhi, [8] V.K. Mehta and R. Mehta, Principle of Power Sytem, S. Chand & Company Ltd, New Delhi, India, [9] E. Igweonu and R. Johua, A Maintenance Paradigm for the Power Infratructure in the 21 ST Century Nigeria, Continental Journal of Engineering Science, Vol. 6, No. 1, Pp 1 7, [10] Arobieke, et al., Electrical Power Outage in Nigeria: Hitory, Caue and Poible Solution, Journal of Energy Technologie and Policy, Vol. 2, No. 6, Pp 18 24, [11] Adepoju, et al., Power Flow Analyi of the Nigerian Tranmiion Sytem Incorporating FACTS Controller, International Journal of Applied Science and Technology, Vol. 1, No. 5, pp , [12] S. Ravi Kumar, and S. Siva Nagaraju, Lo Minimiation by Incorporation of UPFC in Load Flow Analyi, International Journal of Electrical and Power Engineering, Vol. 1, No. 3, pp , [13] Adepoju, et al., Load Flow: An Application of Gau-Seidel Iterative Method to 330kV Electrical Power Sytem in Nigeria, LAUTECH Journal of Engineering and Technology, Vol. 2. No. 1. pp 11 20, [14] C.L. Wadhwa, Electrical Power Sytem, John Wiley and Son, New Delhi, India, [15] B.R. Gupta, Power Sytem Analyi and Deign, S. Chand and Company Ltd. India, The IJES Page 123