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

DOI : 10.17577/IJERTV3IS030541

Text Only Version

#### 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

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]

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

= Ã—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 0.228 n ho 4 36 3 5 5 18 Beni Oshogb 251 0 0.0 0.954 n o 8 76 9 3 10 8 Sape Aladja 63 0 0.0 0.239 le 2 19 3 11 8 Delta Aladja 32 0 0.0 0.239 2 19 3 12 5 Ikeja Benin 280 0 0.0 1.162 10 77 0 9 12 9 Ikeja Aiyede 137 0 0.0 0.521 4 41 9 6 12 13 Ikeja Papanlat 30 0 0.0 0.057 o 1 09 1 1 12 14 Ikeja Omotos 160 0 0.0 0.304 ho 5 48 7 6 12 15 Ikeja Akangb 18 0 0.0 0.257 a 2 17 2 2 12 17 Ikeja Egbin 62 0 0.0 0.257 2 17 2 2 12 18 Ikeja Oshogb 252 0 0.0 0.521 o 4 41 9 6 13 9 Papa Aiyede 60 0 0.0 0.114 lanto 2 18 1 2 17 16 Egbi Aja 14 0 0.0 0.257 n 2 17 2 2 18 9 Osho Aiyede 115 0 0.0 0.437 gbo 4 34 1 9 18 27 Osho Jebba(T 157 0 0.0 0.597 gbo S) 5 47 6 7 20 21 Kadu Kano 230 0 0.0 0.874 na 8 69 2 9 20 23 Kadu Jos 197 0 0.0 0.748 na 7 59 0 9 20 24 Kadu Shiroro 96 0 0.0 0.364 na 3 29 4 2 23 22 Jos Gombe 265 0 0.0 1.01 9 81 5 24 25 Shiro Katamp 144 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 -5.15 -2.29 17 AJAOKUTA 0 0 18 BENIN -2.4 -1.12 19 ONITSHA -1.02 -0.44 20 ALADJA -1.56 -0.85 21 ALAOJI -2.16 -1.04 22 NEW-HAVEN -1.1 -0.18 23 AKANGBA -3.075 -1.54 24 AJA 0 0 25 KATAMPE (ABUJA) 0 0 26 AIYEDE 0 0 27 PAPALANTO 0 0 28 OMOTOSHO 0 0

Table 2; Voltage-Control Bus Data

 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

 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

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