Reliability Analysis of Distribution Transformer A Case Study of Gwiwa Business Unit , Sokoto

In this paper, the reliability of 11/0.415KV town feeder transformers comprising four sub-feeders (Town, GRA, Water Works and industrial) located at Gwiwa Business Unit, Sokoto were studied. The cost implications of failed transformers were also studied, using the data collected from Central Bank of Nigeria (CBN, 2009). It was found that the total amount lost was N37,997,732.40, N16,522,245.88, N20,934,463.70 and N25,559,391.64 for Town, GRA, Water Works and Industrial respectively, over the four years. It is revealed that the approximate time any transformer under Town, GRA, Water Works and industrial feeder may be expected to function before fault or failure (Mean Time Between Failures, MTBF) is 9 months, 39 months, 20 months and 14 months respectively. It was also found that reliability of Town feeder is 52%, GRA79%, Water works 62% and Industrial feeder 50%. It was deduced that town and GRA are under semi-reliable conditions because they are subjected to load beyond their identified plate capacity.


INTRODUCTION
Over the years, the Nigerian Electricity Power Authority (NEPA) has been performing extremely below expectation.The supply situation is never near satisfaction, as Nigerians are not sure of good 12 hours uninterrupted power supply.This situation persists due to a combination of cluster of problems ravaging the most viable sector against meaningful economic growth.The problems are multi-faceted and spanning all the divisions of the power industry (Generation, Transmission and Distribution) (Abdullahi, 2009).
This study enumerates the huge loss of amount of investment, poor operations, monitoring and maintenance procedures of distribution transformers.
The distribution transformer is a critical item of equipment in power systems and its adequate functioning is essential for reliable supply of power.It is therefore necessary to monitor the operating condition and performance of distribution transformer in order to be able to avoid or reduce disruption due to unexpected failure.It also helps to save running cost by optimizing maintenance schedules (Bartley, 2001).
Several monitoring systems for power transformers have been developed, but little is known of distribution transformers.Generally, the trend of Transformer Monitoring Systems (TMS) is from data acquisition to data interpretation in giving clear information to the operator, and controlling of the distribution network (Vashishtha, 2000).
The reliability of operation of distribution networks can be increased by using automatic monitoring systems for transformers.Remote monitoring can provide selective sharing of data among multiple sites in the most efficient and cost effective manner.An information centre of utility acquires information on power plants and on HV/MV substation from Supervisory Control and Data Acquisition (SCADA) system, and information on lower voltage energy consumption of end users from automatic metering recorders.However, online data on the condition of distribution transformer substations are currently not available for remote diagnosis (Vekera, 2003).Utilities find the implementation of communication flow between numerous monitoring systems too expensive and thus they have been ignored entirely.However, the development of the infrastructure of wireless communication such as the mobile phone networks gives rise to cost effective possibilities of monitoring the distribution transformers substations.Remotely, this offers several advantages over the traditional methods (Abdullahi, 2009).
The main aim of carrying out this study is to determine the reliability of transformers under the town feeder of Gwiwa Business Unit, Sokoto with following specific objectives:

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To study the cost implication of the failed transformers under the feeder.

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To investigate the Mean-Time-Between Failures for each of the four sub-feeders.

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To determine which among the four the sub-feeders is more reliable.

MATERIALS AND METHOD OF ANALYSIS
Sokoto 11 KV town feeder transformers located at Gwiwa business unit are classified into four sub-feeders, namely Town, G.R.A, Water works and Industrial.
The transformers under town sub-feeders are summarized in Table 2.1.The most useful index for reliability is how often the transformer fails.This is specified in two ways, depending on whether the transformer is repairable or non-repairable.
• Mean time between failures (MTBF) for repairable transformers.

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Mean time to failure (MTTF) for non-repairable transformers.
Below are the PHCN approved costs of distribution transformers that are commonly found in Nigerian network, PHCN sokoto (2009).N16,522,245.88, N20,934,463.70 and N25,559,391 respectively.The total loss of the failed transformer will be N101, 013,833.60.This is more than two times the cost of purchase, importation and installation of the transformer monitoring device at each and every substation (N40, 499,300.00).

CONCLUSION/RECOMMENDATIONS
It shall be recalled that the above reliability tests were conducted only on 11 KV Town feeder; whilst Sokoto has 20 number distribution feeders .One can therefore imagine the quantum of losses if same test has been extended to the remaining feeders.
This definitely calls for much more economically viable approach to transformer fault detection and clearance to avoid the loss of this very important equipment in power distribution prematurely.
Therefore, it has been found that the average time any transformer under town, GRA, Water Works and Industrial, may be expected to function before fault or failure (MTBF) is 9 months, 39 months, 20 months and 14 months respectively.The table 3.1above shows a typical test conducted to determine MTTF for 65number of transformers on 11KV town sub-feeder.The mean number of transformers energized each month is multiplied by the monthly load to obtain the total operational time, in hours per month.At the end of year 2009,it was found that the number 31, 15, 16 and 23 transformers have failed over a period of one year for the four sub-feeders and approximately 196, 430, 235 and 238 operational hours have accumulated for Town, GRA, W/works and Industrial sub-feeders respectively.
It was found that the approximate time any transformer under Town, GRA, Water Works and industrial feeder may be expected to function before fault or failure (Mean Time Between Failures, MTBF) are 9 months, 39 months, 20 months and 14 months, and their reliabilities are found to be 0.52, 0.79, 0.62 and 0.50 respectively.

Table 2
Transformer with circuit failures due to over load or excess loads (over loaded transformers) are better upgraded than replaced.Upgrading entails not only replacing and upgrading the bad or weak transformer, but upgrading associated incoming and outgoing cables and improving the earth resistance of the mass of earth.Table2.3contains the complete cost of replacing and upgrading transformers.Below are detailed costs of failed transformers in the town feeder, See table (2.4). is under semi-reliable condition.The number of failures under that feeder is 8 under the category of 200KVA.The estimated cost of failures of all the four sub-feeders namely: Town, GRA, Water Works and Industrial feeders in monetary term is N75,481,49.00,if table 2.2 is used to compute the loss which includes up rating of the failed substation , total cost of failures for the Town, GRA, Water Works and Industrial feeders are: N37,997,732.40, Source: PHCN Sokoto (2009).Summary/Discussion of ResultFigure2.1 reveals that 500KVA transformers under town sub-feeder have the highest number of failure and therefore the total amount lost is significantly high.The table(2.5)andfigure (2.1) above revealed that the industrial sub-feeder

Table 3
below shows the typical test conducted to determine the reliability and Mean Time Between Failures (MTBF) for transformers under Town Feeder.The mean number of transformers (Survivals) energized each month is multiplied by the monthly load to obtain the total operational time.The figure 3.1 below shows monthly cummulative failure of the town sub-feeder transformers.