Abstract: This study presents, the analysis of power outages in transmission lines associated with the Nigerian grid. The study revealed that the Nigeria power transmission network is characterised by prolonged and frequent outages. It was observed that planned outages on the 132 kV recorded the highest value of only 7% while, the remaining 93% were due to either forced outages or emergency/urgent outages. This suggested that the reliability of the network is very low resulting in very low efficiency and disruption in the lives of the citizenry. Recommendations to reduce these outages are proffered in this study to ensure good power quality and security in the network.

INTRODUCTION

The electrical utility is probably the largest and most complex industry in the world. The electrical engineer, who researches in this industry will encounter challenging problems in designing future power systems to deliver increasing amounts of electrical energy in a safe, clean and economical manner (Glover and Sarma, 2002). The transmission network in Nigeria is characterised by several outages leading to disruption in the lives of the citizenry. According to Anil et al. (2007), the level of disruption is a function of the dependency of people on electricity, which can be very high for a developed country and not as much as developing countries. In Nigeria, the available energy generated is not enough to meet the demands of the users leading to constant load shedding and blackouts. Outages can be planned or forced. The National Control Centre (NCC), a unit of the Power Holding Company of Nigeria (PHCN), stipulated in its operational procedure No. 10 (OP 10) ‘(NCC and PHCN, 2006)’ that power stations and transmission stations are required to forward their planned outages schedules for the following year to NCC, latest by end of the month of November. This enables the NCC to plan a master programme of planned outages properly co-ordinated to ensure maintenance of Grid integrity after a thorough study and analysis of the various outages. Forced outages can be associated with aging equipment/defects, lightning, wind, birds/animals, vandalization, accidents and poor job execution by contractors. However, forced outages can be minimised if the system is properly designed and maintained but this will not completely eliminate interruptions. In this study, the outage data gathered on the 132 and 330 kV networks in Nigeria are presented. Thus, the objective of this study therefore, is to examine the power outages in Nigeria and make recommendations to minimise its occurrences.

MATERIALS AND METHODS

The methodology adopted for this study is as follows:

Test system for transmission outages: The power stations in Nigeria are mainly hydro and thermal plants. Power Holding Company of Nigeria (PHCN) generating plants sum up to 6200 MW as at December 2006 out of which 1920 MW is hydro and 4280 MW is thermal-mainly gas fired. Onohaebi (2006) provides a more general analysis of the generating stations in Nigeria in his Ph.D Thesis. The Nigerian Electricity Network comprises 11,000 km transmission lines (330 and 132 kV), 24000 km of sub-transmission line (33 kV), 19000 km of distribution line (11 kV) and 22,500 substations (Sadoh, 2005). It has only one major loop system involving-enin-Ikeja West-Ayede-Oshogbo and Benin. The absence of loops accounts mainly for the weak and unreliable power system in the country. The single line diagram of the existing 28 bus 330 kV Nigerian transmission network used as the test system is shown in Fig. 1 and the bus identification are as shown in Table 1.

Fig. 1:	The Nigerian 330 kV transmission grid used for the case study

Table 1:	Bus identifications

RESULTS AND DISCUSSION

Analysis of power outages in Nigeria transmission networks: The Nigeria transmission network is characterised by frequent outages due to aging of equipment/lines leading to frequent conductor/jumper cuts, frequent earth faults resulting from reduction in overhead clearance and refuse burning, circuit breaker problems, etc. According to the (NEPA) Technical Committee Report (2004), the last transmission line in Nigeria was built in 1987 while none of the on-going ones have been completed. A summary of outages recorded for 2003, 2004 and 2005 is presented in Table 2. Figure 2 showed the diagrammatic representation of different types of outages in the Nigeria transmission network as reflected in Table 2. Figure 3 depicts the percentage contributions of the various forms of outages for 2003, 2004 and 2005 on the 330 and 132 kV networks. Figure 4 and 5 showed the monthly analysis in terms of value and percentage contributions of various types of outages on 330 and 132 kV networks for 2003- 2005.

Table 2:	Summary of outages in Nigeria for 2003, 2004 and 2005 (PHCN, 2005; NCC, 2006)

A total of 3585 outages were recorded in 2005 in the 132 kV network, out of which 65.78% were emergency outages. The outages on 330 kV grid in 2005 was 529 with forced outages constituting 225 representing 42.53% compared to 2004 value 277 (35.1%) and 2003 of 252 (54.47).

Causes and effects of power outages in the Nigeria transmission network: The outages that occurred in transmission network for 2004 and 2005 are grouped into transmission lines constraints, shunt reactor problems, overloading of transformers and vandalisation of the lines (Onohaebi, 2007) (Table 3).

Over-loading of transformers: Many transformers in the system are experiencing over loading above 100%. Table 4 shows some transformers loaded above 100% in 2004 and 2005 in the transmission network. PHCN and NCC (2006) for transformer loadings. Many of the distribution transformers are also characterised by overloads which often lead to very low voltages and these voltages can be as low as 40 V in some areas as contained in Ali (2005). The distribution transformers are not well protected. It is a common practice in the Nigeria power system to see feeder pillars without properly rated fuses but iron bars inserted into their fuse compartments.Political considerations allocate transformers to areas where they are less required to satisfy self ego, thus, preventing areas that are actually in dire need of them.

Fig. 2:	Number of various types of outages on the 330 and 132 kV for 2003- 2005

Fig. 3:	Comparison of various types of outages in the 330 kV network for 2003- 2005

Vandalisation: Vandalisation of transmission lines by unscrupulous individuals for selfish gains is very rampant in the Nigerian power system. Ali (2005) gives a summary of the vandalised transmission lines in Nigeria in <8 months and the effects it had on the consumers as shown in Appendix 1.

Simulation of various aspects of faults on the test system: In order to examine the effects of faults on the network, the test system shown in Fig. 1 was redrawn in the Power World Simulator (PWS) (Power World Co-opration, 1996-2000) environment as shown in Fig. 6.

Table 3:	Summary of faults, causes and effects on the network

Table 4:	Transformer loaded above 100% in 2004 and 2005

Load flow analysis was carried out to determine the bus voltages, as shown in Fig. 7 under normal operating condition. The scenarios considered in this analysis include unbalanced faults involving single phase to ground, line to line and double line to ground faults. These faults were simulated at buses 1-3. Bus 1 was selected because it is the major bus linking the southern and northern parts of the grid and also the location of the National Control Centre (NCC). Bus 2 represents the bus, which links the Eastern, Western and Northern parts of the network. Bus 3 is the highest loaded bus in the entire network and also tied to the highest generating stations located at Egbin and AES.

Singe line to ground fault: The bus voltages after the simulation of singe line to ground fault are shown Fig. 8 for buses 1-3. The results showed low voltage values at bus 4, 9, 16, 23, 24, 26, 27 and 28 when bus 1 was affected. The highest voltage value of 2.52/units was recorded at bus 8, when bus 2 was subjected to the same fault.

Fig. 4:	Monthly outage analysis in values and percentages in the 132 kV network for 2003- 2005

Fig. 5:	Monthly outage analysis in values and percentages in the 330 kV network for 2003- 2005

Line to line fault: The bus voltages after the simulation are shown in Fig. 9 for buses 1, 2 and 3. This scenario showed that most of the bus voltages are within limits except buses 5, 8 and 16. The voltage profiles showed a great resemb-lance to the normal operating condition as shown in Fig. 7.

Double line to ground fault: The bus voltages after the simulation are shown in Fig. 8 for buses 1-3. The voltages were very high in this scenario except for buses 5, 8, 16 and 28, which recorded very low values. There was no bus voltage in the network that was within the acceptable limits.

Fig. 6:	The Nigeria 330 kV transmission network (simulated in the run mode)

Fig. 7:	Voltage profiles under normal conditions

This study revealed the following:

Fig. 8:	Voltage profiles for single line to ground fault at buses 1- 3

Fig. 9:	Voltage profiles resulting from simulation of line to line fault at buses 1-3

Fig. 10:	Voltages profiles due to double line to ground faults

RECOMMENDATIONS

In order to reduce transmission outages in the Nigerian transmission network, the following should be given due consideration:

CONCLUSION

The various causes and effects of power outages in the Nigeria transmission network have been examined in this study. Outages in the network are due to aging of equipment/defects, lightning, vandalisation, poor maintenance, etc. The fault analysis showed that the system needs to be properly protected to ensure safety and security of network.

ACKNOWLEDGEMENT

The authors are highly indebted to Power World Co-operation for the Power World Simulator software, Version 8.0 Glover/Sarma Build 11/02/01, licensed only for Evaluation and University Educational Use. They are also grateful to PHCN for providing relevant data necessary for the analysis of power outages.

Appendix 1:	Summary of vandalized transmission lines between 17-11-02 and 05-07-03 Ali (2005)

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