Reliability Analysis of Circuit Breakerin the Nigerian 330-kV Transmission Network.

Reliability Analysis of Circuit Breakerin the Nigerian 330-kV Transmission Network.

1Ademola Abdulkareem, 2C. O. A. Awosope, 3A. U. Adoghe, 4Okelola. M. O

1,2,3Electrical and Information Engineering Department, School of Engineering, College of Science and Technology, covenant University, Ota, Nigeria

4Ladoke Akintola University, Ogbomosho, Nigeria.

Abstract This paper is concerned with using the fault analysis to establish the requirements for the proper selection of circuit breaker; A Case Study of Power Holding company of Nigeria (PHCN) 330-kV Transmission Grid System. The work is modelled for Fault Analysis and it is written in a flexible MATLAB programs to accommodate addition or reduction in the Transmission Grid System. It aimed at establishing the Circuit Breaker Capacity at any point in the system. The result is then compared with the existing circuit breaker capacity of PHCN 330-kV system. The short-circuit fault is simulated by combining a solution of algebraic equations describing the changes in the network with a numerical solution of the differential equations. Two MATLAB programs were written and simulated; one for Lord Flow study to know the pre-fault bus voltages based on Gauss-Seidel Method; the other for Short Circuit Studies which made use of Thevenins theorem application. The highest Circuit Breaker Capacity established by the result of this study is relatively lower and the investments needed for this are smaller compared with the normal practice with PHCN system. This reveals that PHCN system can be protected with this low capacity circuit breaker with a reduced cost effectiveness and equal sensitivity which is a break-through in terms of Circuit Breaker Capacity in the field of power system protection.

Keywords Fault studies, circuit breaker, 330kV transmission grid, MATLAB program, Gauss Seidel load flow solution

1. INTRODUCTION

   The current trends of erratic power supply and system collapse in Nigeria have made this study a paramount importance to the nations power industry. The purpose of an electrical power system is to generate and supply electrical energy to consumers with reliability and economy. The operation of a power system is affected by disturbances that could be due to natural occurrences such as lightning, wind, trees, animals, and human errors or accidents. These disturbances could lead to abnormal system conditions such as short circuits, overloads, and open circuits. Short circuits, which are also referred to as faults, are of the greatest concern because they could lead to damage to equipment or system elements and other operating problems including voltage drops, decrease in frequency, loss of synchronism, and complete system collapse. There is, therefore, a need for a device or a group of devices that is capable of recognizing a disturbance and acting automatically to alleviate any ill effects

   on the system element or on the operator. Such capability is provided by the protection system. The protection system is designed to disconnect the faulted system element automatically when the short circuit currents are high enough to present a direct danger to the element or to the system as a whole. The objective of the system will be defeated if adequate provision for fault clearance is not made. The installation of switchgear alone is insufficient, discriminative protective gear, designed according to the characteristics and requirements of the power system must be provided to control the switchgear [1]. Security of supply, therefore, can be better by improving plant design, increasing the spare capacity margin and arranging alternative circuits to supply loads. Majority of the faults are unsymmetrical. However, the circuit breaker rated MVA breaking capacity is based on 3phase fault MVA. Since a 3phase fault inflicts greatest damage to the power system, except in a situation where a single line to ground fault is very close to a solidly grounded generators terminal. In this instance the severity of single line to ground fault is greater than that of 3phase balance fault.

2. BRIEF REVIEW OF SHORT-CIRCUIT ANALYSIS

   Fault studies form an important part of power system analysis. In the context of electrical fault-calculation, a power system fault may be defined as any condition or abnormality of the system which involve the electrical failure of the primary equipment, the primary equipment implying equipment such as generators, transformers, busbars, overhead lines and cables and all other items of plant which operate at power system voltage (330kV for this case).

   Faults on power system are divided into three-phase balanced faults and unbalanced faults. The different types of unbalanced faults are single line-to-ground fault (LG), line-to-line fault (LL), and double line-to-ground fault (LLG). The problem consists of determining the voltages and currents during various types of faults. The information gained from fault studies are usedfor proper relay setting and coordination. The three-phase balanced fault (LLL) information is used to select and set phase relays.

   Majority of the faults are unsymmetrical and the current which a breaker must interrupt is usually asymmetrical since it still

   contains some of the decaying dc component [2]. However, the circuit breaker rated MVA breaking capacity is based on three-phase balanced fault MVA. Since a three-phase fault inflicts greatest damage to the power system, except in a situation where LG fault is very close to a solidly grounded generators terminal. In this instance the severity of single line to ground fault is greater than that of three-phase balanced fault.

   The condition of the power system during the fault condition can be explained from the equation for short circuit studies. The equation for the short circuit uses the sequence

   components theory in the method of calculation.

   Vp

   =

   GenIpo

   Bus p

   ypo

   In an attempt to establish short circuit studies, various forms

   Figure 1: A typical fault model at bus P

   of faults were simulated to obtain the current which the

   breaker must interrupt and comparison was made between LLL fault and LG fault either of which is likely to cause

   I po

   Pp jQ

   p

   *

   (3)

   greater damage to a power system. This current is properly called the required symmetrical interrupting capacity or simply the rated symmetrical short-circuit current. [3]

3. PROBLEM FORMULATION (LOAD REPRESENTATION)

   During sub-transient period, power system loads, other than motors are represented by the equivalent circuit as static impedance or admittance to ground.The symmetrical three phase fault current in per unit is given by

   V p

   Where; Pp and Qp = the scheduled bus load.

   VP = the calculated voltage which can only be determined if Qip is given or known

   The injected current Ipo flows from bus P to ground, that is, to bus 0.

   The magnitude and power factor angle of Ipo remain constant.

   y I po .

   = 0

   (1)

   po V p

   Where 0 is the per unit Perfault bus voltage and =

   the p.u reactance to the point of Fault

   (4)

   The base current

   = ×103

   3

   (2)

4. NETWORK PERFORMANCE EQUATION

   Where SB is the base MVA and VB is the line to line base voltage in kV

   The interrupting rating of a circuit breaker was specified in

   The Gauss-Seidel Method of solution used for the load flow

   equation can be applied to describe the performance of a network during a su-transient period, using the bus

   KVA or MVA.

   admittance matrix with ground as reference. The voltage

   From (2), it implies that the interrupting KVA equal 3 times

   equation for bus P is given by:

   the kV of the bus to which the breaker is connected times the current which the breaker must be capable of interrupting

   Pp

   jQp Lp

   p1

   k 1 '' k

   when its contact part. This current is of course, lowers than the momentary current and depends on the speed of the breaker [2].

   V p

   * YLpqV q q1

   V

   p

   1

   YLpqV q ….(5)

   q p1

   Also, for the purpose of short circuit analysis in order to select appropriate circuit breaker to clear a fault instantly before transient condition on a power system, pre-fault condition of the system (i.e, pre-fault voltages and currents) should be

   where;

   YLpq Y pq Lp ; Lp

   Pp jQ

   Y pp

   known and this can be obtained from the load flow solution for the power system. Detail of the initial value of the current for a constant current representation is obtained from model of

   Theterm

   p in equation (5) represents the load current at bus P.

   V

   *

   p

   fig 1.

   for the cons tantload current representation,

   Pp jQp k

   k

   | I po | p p…………………………………(6)

   V p *

   where;

   p

   the power factor angle,

   p

   and k the angle of voltage with respect to the reference.

   When the constant power is used to represent the load, (Pp jQp) Lp will be constant but the bus voltage Vp will change in

   o

   f

   V

   jX

   I ' '

   f

   ……………………………………………………..(9)

   Z

   any iteration [4]. When the load at bus P is represented by a

   static admittance to ground, the impressed current at the bus is TH

   zero and the

   Pp

   jQp Lp

   V

   *

   0

   p

   (7)

   1. COMPARISON OF SLG FAULT AND THREE- PHASE FAULT (LLL) CURRENTS

      This comparison[5]is necessary because of the earlier

      For a sub-transient analysis in short circuit studies, the parameters of equation (5) must be modified to include the effect of the equivalent element required to represent synchronous, induction and loads. The line parameters YLpq must be modified for the new elements and additional line parameter must be calculated for each new network element.

      V. METHOD OF SOLUTION

      The methods and concepts employed to implement this work includes:

      • Developing an algorithm and hence a programme for fault level calculation at the location of fault in a 330-kV transmission Grid system.

      • Determine the fault current for various types of fault simulation.

      • Recommend the appropriate circuit breaker capacity to clear any detected fault.

        Note that it is necessary to do a load flow calculation before one can proceed on fault analysis. This is important so as to know the pre-fault voltages and currents necessary for further calculation. The network representation for the short circuit studies includes among other things, the Grid components parameter i.e the generators system buses, transmission lines and transformers. Modification of the admittance matrix to impedance matrix is done on the load flow calculation [4] to reflect fault analysis.

        These pre-fault conditions can be obtained from the result of

        statement in this project study that single line-to-ground fault is more severe than that of 3-phase fault if the fault is located very close to the terminal of a solidly grounded generator.

        The fault impedance can be assumed to be zero because of the enormous effect of the fault current. In addition, if the impedances Z1, Z2 and Z0 are assumed to be pure reactances (X1, X2 and X0), then for a 3-phase fault.

        Ia

        E ………………………………………………………………….(10).

        1

        jX

        and that of sin gle line to ground fault is given as;

        I

        3E …………………………………………….(11).

        1

        2

        0

        a jX jX jX

        The three practical possibilities are as follow;

        1. Fault at the terminals of neutral solidly grounded generator, (for generator X0<< X1), and it is assumed that X1 = X2 for sub-transient condition which is the case for the short circuit studies. At this instance single line to-ground fault is more severe than a 3-phase fault

        2. If a generator is grounded through a reactance Xn, this does not have any effect on a 3-phase fault current, but a single line-to-ground fault will have a fault current:

           load flow solution by Gauss-Seidel iteration method using YBUS, the flowchart of which is illustrated in Fig.2.

           The pre-fault machine currents are calculated from load flow

           I a jX X

           3E

           2 X

           0 3X n

           1

           by Gauss-Seidel iterative method from:

        3. to this end the relative severity of 3-phase fault

           V

           I Pki

           jQ

           ki ;i 1,2,………., m…………………………………(8)

           and single line-to-ground fault will depend on the value of Xn.

           ki *

           ki

           where;

        4. For a fault on a transmission line (which is the case study) X0>> X1 so that for a fault on a line

        Pki

        Q

        and

        ki

        the scheduled

        or calulated

        machine real and

        sufficiently far away from the generator

        retaercmtiivnealst,er3-mphinasae l fapuoltwceurrsr.ent is more than single line-to-ground fault current.

        V

        ki

        * the last iteration

        voltage.

        m the number of machines in the system.

        The network is then modified to correspond to the desired representation for short circuit studies. Being a linear network of several voltage sources, further calculation can be computed by application of Thevnins theorem [5].

        Figure 2: Flow Chart for Load-Flow Solution: Gauss-Seidel Iteration

        Figure 3: Flow Chart for 3-Phase Symmetrical Fault

        					3
        5	14	Beni	Omotos	120	0.0	0.0	0.228
        		n	ho		04	36
        					3	5
        5	18	Beni	Oshogb	251	0.0	0.0	0.954
        		n	o		08	76
        					9	3
        10	8	Sape	Aladja	63	0.0	0.0	0.239
        		le			02	19
        					3
        11	8	Delta	Aladja	32	0.0	0.0	0.239
        					02	19
        					3
        12	5	Ikeja	Benin	280	0.0	0.0	1.162
        				10	77
        					0	9
        12	9	Ikeja	Aiyede	137	0.0	0.0	0.521
        					04	41
        					9	6
        12	13	Ikeja	Papanlat	30	0.0	0.0	0.057
        			o		01	09
        					1	1
        12	14	Ikeja	Omotos	160	0.0	0.0	0.304
        			ho		05	48
        					7	6
        12	15	Ikeja	Akangb	18	0.0	0.0	0.257
        			a		02	17
        					2	2
        12	17	Ikeja	Egbin	62	0.0	0.0	0.257
        					02	17
        					2	2
        12	18	Ikeja	Oshogb	252	0.0	0.0	0.521
        			o		04	41
        					9	6
        13	9	Papa	Aiyede	60	0.0	0.0	0.114
        		lanto			02	18
        					1	2
        17	16	Egbi	Aja	14	0.0	0.0	0.257
        		n			02	17
        					2	2
        18	9	Osho	Aiyede	115	0.0	0.0	0.437
        		gbo			04	34
        					1	9
        18	27	Osho	Jebba(T	157	0.0	0.0	0.597
        		gbo	S)		05	47
        					6	7
        20	21	Kadu	Kano	230	0.0	0.0	0.874
        		na			08	69
        					2	9
        20	23	Kadu	Jos	197	0.0	0.0	0.748
        		na			07	59
        					0	9
        20	24	Kadu	Shiroro	96	0.0	0.0	0.364
        		na			03	29
        					4	2
        23	22	Jos	Gombe	265	0.0	0.0	1.01
        					09	81
        					5
        24	25	Shiro	Katamp	144	0.0	0.0	0.598

   2. RESULT OF SYSTEM MODELING

      There is a necessity to have the knowledge of pre-fault voltages and currents in order to proceed with the calculation of the fault currents and hence achieving the aims of the research study. Hereunder are one-line diagram of the existing National 330-kV Network (Fig.4) and the systems data(Table) employed in the load flow calculation:

      Figure 4: The 28-Bus System of the Nigerian Transmission 330-kV Grid as a Case Study [6]

      Table 1: Transmission line data on 33kV, 100MVA base (All values are in per unit) [7]

      BU S – NO FR O M	B U S – N O T O	FRO M BUS	TO BUS	LENG TH(km )	R( pu)	X( pu)	ADMIT TANCE (b/2)
      1	2	Alao ji	Afam	25	0.0 09	0.0 07	0.104
      1	4	Alao ji	Onitsha	138	0.0 49	0.0 42	0.524
      3	4	New Have n	Onitsha	96	0.0 03	0.0 29 2	0.365
      4	6	Onits ha	Okpai	80	0.0 09	0.0 07	0.104
      5	4	Beni n	Onitsha	137	0.0 04 9	0.0 41 6	0.521
      5	7	Beni n	Ajaokut a	195	0.0 07	0.0 56	0.745
      5	10	Beni n	Sapele	50	0.0 01 8	0.0 13 9	0.208
      5	11	Beni n	Delta	107	0.0 02	0.0 19	0.239

      		ro	e(Abuja )		05 2	40 1
      24	27	Shiro	Jebba(T	244	0.0	0.0	0.927
      		ro	S)		06	70
      					7	2
      26	28	Beni	Kainji	734	0.0	0.0	1.178
      		n			11	94
      		Kebb			1	2
      		i
      27	19	Jebb	Jebba(T	8	0.0	0.0	0.0322
      		a(GS	S)		00	02
      		)			3	2
      28	27	Kain	Jebba(T	81	0.0	0.0	0.308
      		ji	S)		02	24
      					9	6

      16	IKEJA-WEST	-5.1500	-2.2900
      17	AJAOKUTA	0.0000	0.0000
      18	BENIN	-2.4000	-1.1200
      19	ONITSHA	-1.0200	-0.4400
      20	ALADJA	-1.5600	-0.8500
      21	ALAOJI	-2.1600	-1.0400
      22	NEW-HAVEN	-1.1000	-0.1800
      23	AKANGBA	-3.0750	-1.5400
      24	AJA	0.0000	0.0000
      25	KATAMPE (ABUJA)	0.0000	0.0000
      26	AIYEDE	0.0000	0.0000
      27	PAPALANTO	0.0000	0.0000
      28	OMOTOSHO	0.0000	0.0000

      Table 2; Voltage-Control Bus Data

   3. LOAD FLOW RESULTS

      BU S NO.	BUS NAME	QG	QD	QMI N	QMA X	VSP
      		SLAC K BUS
      1	KAINJI	0.0000	0.000	–	2.790	1.050
      			0	2.790	0	0
      				0
      2	JEBBA	0.0000	0.240	–	3.230	1.000
      			0	3.230	0	0
      				0
      3	SHIROR	0.0000	0.180	–	2.000	1.000
      	O		0	2.000	0	0
      				0
      4	SAPELE	0.0000	0.000	–	4.670	1.000
      			0	4.670	0	0
      				0
      5	DELTA	0.0000	0.370	–	3.430	1.000
      	(IV)		0	3.430	0	0
      				0
      6	AFAM	0.0000	0.000	–	36700	1.000
      	(IV)		0	3670		0
      				0
      7	EGBIN	0.0000	0.000	–	5.820	1.000
      			0	5.820	0	0
      				0

      The bus-bar pre-fault voltage, pre-fault current and pre-fault powers, which flow out of the bus bars, are tabulated in Table 4 hereunder

      Table 3: Load Bus Data

      BUS NO	BUS NAME	ACTIVE	REACTIVE
      		POWER (PG)	POWER (QG)
      8	JEBBA (T.S)	-0.7200	-0.4300
      9	BIRNIN- KEBBI	-0.3900	-0.1800
      10	KADUNA	-1.6100	-0.8200
      11	KANO	-2.0400	-0.8000
      12	JOS	-0.9800	-0.3460
      13	GOMBE	-1.5300	-1.0800
      14	OSOGBO	-1.5600	-0.8800
      15	IBADAN	-1.8000	-0.9300

      Table 4; Output of Load-Flow Results (in p.u )

      BUS NO.	VOLTAGE	POWER ANGLE	POWER FLOW	CURRENT
      1	1.0500	0.0000	2.4787	2.3605
      2	1.0000	-0.4060	7.2392	7.2394
      3	1.0000	-8.1200	3.6954	3.6954
      4	1.0000	12.9979	7.0150	7.0150
      5	1.0000	13.9877	3.6998	3.6998
      6	1.0000	18.2990	4.4075	4.4075
      7	1.0000	2.0316	4.3869	4.3869
      8	1.1219	-3.8503	0.8403	0.7443
      9	1.0081	-0.6090	0.4238	0.4208
      10	1.0173	-12.9984	1.8070	1.7760
      11	1.0050	-21.0013	2.1898	2.1982
      12	1.0601	-21.4268	1.0387	0.9522
      13	1.0599	-27.4552	1.8735	1.7563
      14	1.0220	-0.5700	1.7934	1.7518
      15	1.0042	-2.3010	2.0273	2.0202
      16	0.9899	-0.1259	5.6455	5.6960
      17	1.0417	10.4510	0.0022	0.0020
      18	1.0201	10.6305	1.1065	1.0860
      19	1.0340	11.9994	0.4296	0.4068
      20	0.9980	13.3385	1.8026	1.8015
      21	1.0005	17.3005	2.4028	2.4018
      22	1.0350	10.2956	1.1223	1.0764
      23	0.9875	-0.5500	3.4326	3.4798
      24	1.0002	2.0225	0.0164	0.0168

   4. RESULT OF SHORT CIRCUIT STUDIES

      In the short circuit studies, a base current of 174.9546A and a base voltage of 330kV are computed together with the load flow output result of the pre-fault condition in the input data for the various fault current calculations using the Power System Matlab Programming.[9].

      Simulations were made for different types of short circuit faults i.e, 3-phase fault; single line-to-ground fault, line- to-line fault and double line-to-ground fault. The summary of is shown in Table 5.

      MVAsc 3 phase 3 Vpf 1 I sc .

      where;

      MVAsc 3 phase 3 phase short circuit MVA.

      VALUE)

      Fault Current Result (IN ACTUAL

      Vpf 1 Pr e fault line voltage in kV.

      BASE CURRENT = 1,749.546kA BASE MVA = 100MVA

      BASE VOLTAGE = 330kV

      TABLE 5: TYPE OF FAULT (SUMMARY)

      I sc Short circuit current in kA.

      If voltages and currents are in per unit values on a 3-phase basis, then,

      MVAsc (3-phase) = |V|pre-fault x |I|sc xMVAbase

      BUS NO.	3- PHASE	SLG.	LL.	DLG
      1	21878	11296	19335	19684
      2	11688	4321	9897	10369
      3	12082	6345	9879	9802
      4	15568	8442	12680	12582
      5	9157	4913	7723	7675
      6	2312	1382	2255	2196
      7	24989	13021	21958	22013
      8	2228	1180	2012	2032
      9	10012	5014	9146	8887
      10	5826	2798	4988	5102
      11	1895	894	1687	1698
      12	1768	852	1485	1551
      13	1062	598	986	993
      14	8896	4140	7734	7984
      15	28984	13982	24968	25437
      16	21787	9603	18475	19004
      17	5719	2870	4970	5042
      18	20593	10143	18042	18275
      19	3678	1830	3234	3290
      20	9006	4367	7911	7960
      21	2218	1150	2019	2050
      22	2462	1146	2203	2239
      23	25432	11951	22204	22597
      24	20998	10246	18269	18804

      For example: The Short Circuit Current from the output result of the program for the short circuit studies is 29036A with a pre-fault voltage of 331.4520kV.

      MVAsc 3 phase 3 Vpf 1 I sc .

      3 330kV 28,971kA

      16,559MVA

      From the summarized result in Table 5 above, it can be inferred that 3-phase fault causes greatest magnitude of fault current to the system and hence, should be a point of reference upon which the circuit breaker to clear faults on 330kV transmission line is based.

   5. CONCLUSION AND RECOMMENDATION

      To select the most appropriate size of Circuit Breaker for the Grid System, it has to be borne in mind that rated momentary current and rated symmetrical interrupting currents are required for the computation of circuit breaker ratings. Symmetrical current to be interrupted is computed by using sub-transient reactance for synchronous generators. Momentary current (rms) is then calculated by multiplying the symmetrical momentary current by a factor of 1.6 to account for the presence of D.C. offset current.

      The 3-phase short circuit MVA to be interrupted can be computed as in the following equation [8]

   6. CONCLUSION

      From the above, and as a result of the research study, it can be concluded that the Circuit Breaker capacity for PHCN 330kV Transmission Grid should be 20,000MVA. The research result is a break-through in terms of Circuit Breaker Capacity in the field of Power System Protection.

      The Birnin-Kebbi bus (B8) on which the receiving end voltage is higher than the sending end voltage, it is agreed upon and confirmed that the line is an open-ended one. This is what causes the abnormality. The solution to it is to connect a reactor to the line.

   7. RECOMMENDATIONS

From the Nameplate of a Sf6 Gas Circuit Breaker (manufactured by GEC ALSTHOM T & D) on 330kV Transmission Line at the Area Transmission station in Osogbo:

Line Voltage VL = 362kV, Frequency f = 50Hz,

Short Circuit Current Isc = 40kA Line Current IL = 400A,

Current Interruption Capacity = 3,150A.

From the above data, the capacity of the Circuit Breaker is calculated to be 25,080.09569MVA. This is too high compared with the result of this project (i.e, 20,000MVA capacity) and it will be expensive in term of cost. The higher value of 25,080.09569MVA apart from cost, it will be more insensitive to any fault detected.

It is therefore recommended that the result of this paper should be of value to PHCN in the daily operation of the National Grid.

REFERENCES

1. W. D. Stevenson, Element of Power System Analysis, McGraw-Hill New York, 1982.

2. J. J. Grander, and W. D. Stevenson, PowerSystem Analysis, New York.

   McGraw-Hill, 1994

3. H. Saadat Power System Analysis Published by McGraw-Hill publishing company limited, New

   York 2006.

4. Braess, D. and Grebe, E. A. Numerical Analysis of load-Flow Calculation Methods, .IEEE Trans,

   1981 July, No. 7, Vol. Pas-100, pp. 3642-3647.

5. B. R. Gupta, Power System Analysis and Design, Revised Edition, 2011

6. PHCN National Control Centre Oshogbo, 2010, one line diagram of the 28 bus systems of Nigerian

   Transmission 330-kV Grid

7. PHCN Line Parameters for 330-kV Circuits, 2011.

8. I. J. Nagrath and D. P. Kothari, Power System Engineering, Fifth Print, 1998

9. Brown, H. W., et al, Digital Calculation of Three-Phase Short Circuits by Matrix Method, AIEE Trans, 1960, Vol. 79, pp. 1277