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Transformers Improvement and Environment Conservation by Using Synthetic Esters in Egypt

: Distribution transformer (DT) is a crucial component in power systems as it exchanges energies between different voltage levels or between utility grid and DC microgrids. Nevertheless, the operation of an oil-immersed DT is limited by the thermal and electrical capabilities of the internal insulating liquid. This paper aims to raise the efﬁciency of distribution transformers and preserve the environment by using a biodegradable insulating liquid instead of the conventional mineral insulating oil (MIO). This work examines the Egyptian case, where a real distribution network located in middle Egypt is selected as a pilot project. Study and analysis of the status que of the insulation system inside DTs are done with the aid of fault-tree analysis. The deﬁciency of the insulation system is conﬁrmed by conducting an electronic survey of 100 expert participants. The most appropriate solution among three different alternatives is conﬁrmed using the weighting and ranking method. The best choice suitable for the selected area is the substitution of MIO by synthetic ester (SE). The technical and environmental advantages achieved by the presented solution are discussed. The feasibility studies have proven that the solution is positively acceptable in all aspects. An execution plan is established for the application of proposed solution on the selected Egyptian distribution network.


Introduction
For more than a hundred years, mineral insulating oils (MIOs) have been used inside distribution transformers (DTs) to guarantee electrical insulation, suppress arcing in case of a fault and keep thermal stability. In order for the oil to fulfill these purposes, its appearance must remain clear, free from sediment or any suspended matters. New MIOs have to comply the international electrotechnical commission standard IEC 60296 especially in the requirement of lower viscosity, higher flash point and good resistance to oxidation [1,2]. Since its discovery by Eliu Thomson, MIOs were widely used due to availability, good dielectric properties and reasonable price. Nevertheless, MIOs are non-renewable fossil liquid with low biodegradable nature. Additionally, the periodic tests constantly affirmed the degradation of MIO properties by time even after following the maintenance guidance of IEC 60422 [3].
In general, distribution transformers are responsible for transferring energy between the different voltage levels in the utility network. Recently, this task has extended to include energy exchanges between the utility grid and the DC microgrids. However, the operation of oil-immersed transformers is limited by the thermal and electrical capabilities of the A real district located in the middle of Egypt is highlighted as a practical application of the proposed solution. The process of checking and maintaining the insulation system inside DTs is analyzed. Furthermore, the existing state of the system is evaluated by conducting an electronic survey. Fault-tree analysis is performed to catch the root-causes that deteriorate the efficiency of insulation system as a preliminary step to make the wright decision. The authors aim in this research to raise the efficiency of transformers and preserve the environment in Egypt by selecting the most appropriate substitute to the existing insulation and cooling system in Egyptian distribution transformers. This coincides with the vision of the Egyptian Electricity Holding Company (EEHC) and the objectives of its subsidiaries.
The rest of the paper has been organized in five following sections. The existing situation of the insulation system inside the Egyptian DTs is examined in Section 2 focusing on the case-study region. Section 3 clarifies the workable alternatives to improve the insulation efficiency inside DTs. Differentiation between the alternatives using the weighting and ranking method is described in Section 4 to determine the most appropriate solution for the selected district. The execution plan as applied to the selected district is discussed in Section 5 focusing on the feasibility studies, execution stages and execution period. Eventually, Section 6 summarizes the main conclusions.

Examination of the Egyptian Case
The Egyptian distribution network extends for more than half a million kilometers to serve more than 36 million customers through about 197,000 distribution transformers of about 86.3 GVA total capacity. Middle Egypt Electricity Distribution Company (MEEDC) is one of nine distribution subsidiaries of the EEHC. As shown in Figure 1, MEEDC covers a part of Upper Egypt, known for its high summer temperatures [31]. MEEDC supplies electricity to 3.9 million customers through 26,100 transformers of about seven GVA installed capacity. All of the transformers owned to MEEDC are oil-immersed transformers filled by the conventional MIO.
Dir-Mawas, one of 56 districts belongs to MEEDC, was selected as the research area because a large number of its transformers are located near the very hot western desert. The total installed capacity in Dir-Mawas is 75.55 MVA distributed over 295 transformers. According to 2020 inventory, seven of Dir-Mawas transformers exceed the economic loading limits. The economic loading percentage of DTs is considered to range between 40 up to 80%. Loading below 40% is not desirable due to the low rate of investment return and the high effect of iron losses. Loading above 80% is not desirable also due to the danger of overheating in addition to the high effect of copper losses. During the last five years, MEEDC has added 20 new transformers to Dir-Mawas distribution network. Nevertheless, Energies 2021, 14,1992 4 of 15 three transformers, rated 50, 100 and 160 kVA, were burned in Dir-Mawas during only the last summer.
Dir-Mawas, one of 56 districts belongs to MEEDC, was selected as the research area because a large number of its transformers are located near the very hot western desert. The total installed capacity in Dir-Mawas is 75.55 MVA distributed over 295 transformers. According to 2020 inventory, seven of Dir-Mawas transformers exceed the economic loading limits. The economic loading percentage of DTs is considered to range between 40 up to 80%. Loading below 40% is not desirable due to the low rate of investment return and the high effect of iron losses. Loading above 80% is not desirable also due to the danger of overheating in addition to the high effect of copper losses. During the last five years, MEEDC has added 20 new transformers to Dir-Mawas distribution network. Nevertheless, three transformers, rated 50, 100 and 160 kVA, were burned in Dir-Mawas during only the last summer. To identify the existing state of DT cooling, a flow-chart of the periodic oil-inspection process in Egypt is drawn and given in Figure 2. The chart indicates all the process elements: process inputs, activities, outputs, owner, customer, components and feedback. To identify the existing state of DT cooling, a flow-chart of the periodic oil-inspection process in Egypt is drawn and given in Figure 2. The chart indicates all the process elements: process inputs, activities, outputs, owner, customer, components and feedback. Studying the status-que clarifies that there is a problem, the symptoms of which are illustrated in Figure 3. These symptoms can be summarized in the following list:


Increased number of burned transformers, especially in summer.


The operating temperature of some transformers is higher than its rated values.  Increased number of faulted transformers that leads to service interruption.  Low level of oil inside some transformers.  The degradation in oil properties by time, especially the breakdown voltage.  Difficulty of drawing oil samples for testing. Studying the status-que clarifies that there is a problem, the symptoms of which are illustrated in Figure 3. These symptoms can be summarized in the following list:

•
Increased number of burned transformers, especially in summer.

•
The operating temperature of some transformers is higher than its rated values. • Increased number of faulted transformers that leads to service interruption.  Studying the status-que clarifies that there is a problem, the symptoms of which are illustrated in Figure 3. These symptoms can be summarized in the following list:


Increased number of burned transformers, especially in summer.


The operating temperature of some transformers is higher than its rated values.  Increased number of faulted transformers that leads to service interruption.  Low level of oil inside some transformers.  The degradation in oil properties by time, especially the breakdown voltage.  Difficulty of drawing oil samples for testing.  To confirm, an electronic survey is conducted to assess the cooling and insulation system of distribution transformers. A number of 100 experts from various parties and companies affiliated with the electricity sector in Egypt were responded to the survey. The results of the questionnaire confirm the existence of a deficiency in the existing cooling and insulation system of the Egyptian DT as shown in Figure 4. To confirm, an electronic survey is conducted to assess the cooling and insulation system of distribution transformers. A number of 100 experts from various parties and companies affiliated with the electricity sector in Egypt were responded to the survey. The results of the questionnaire confirm the existence of a deficiency in the existing cooling and insulation system of the Egyptian DT as shown in Figure 4.   [32]. From this fault-tree, a list of the possible causes can be created. The possible causes are then filtered using the root-cause analysis technique to identify the root-causes of the problem [33][34][35]. The resultant rootcauses are: As is evident, most of the root-causes are related to MIO. Even root-causes that are not directly related to MIO, MIO causes them indirectly. Although moist is normally released during heat exchange with insulation paper, this moisture weakens the dielectric strength of MIO. As temperature rises, MIO as a hydrocarbon mixture reacts with oxygen to form black sediments. These sediments stick to the surface of transformer coils, creating heat isolators and reducing the surface of cooling area. Therefore, the next section of this paper discusses different alternatives to eliminate those causes.   [32]. From this fault-tree, a list of the possible causes can be created. The possible causes are then filtered using the root-cause analysis technique to identify the root-causes of the problem [33][34][35] As is evident, most of the root-causes are related to MIO. Even root-causes that are not directly related to MIO, MIO causes them indirectly. Although moist is normally released during heat exchange with insulation paper, this moisture weakens the dielectric strength of MIO. As temperature rises, MIO as a hydrocarbon mixture reacts with oxygen to form black sediments. These sediments stick to the surface of transformer coils, creating heat isolators and reducing the surface of cooling area. Therefore, the next section of this paper discusses different alternatives to eliminate those causes.

Solution Alternatives
Focus in this section will be concentrated on three alternatives that contribute to solving the deficiency of the insulation and cooling system inside oil-immersed distribution transformers. These alternatives are ranging between the substitution of MIO by ester-based liquids and replacing the oil-immersed transformers by dry-type ones.

MIO Substitution by Ester
As compared to MIO, the use Ester fluids achieves the following benefits:

Suppress Transformers Burn
For any fire to occur, three elements must be combined: fuel, air, and temperature. No burn occurs if any of these elements are vanished. Thus, the presence of mineral oil, which is a petroleum derivative, facilitates transformers fire as the temperature rises. Ester fluids have a flash point exceeds 300 °C. This moves an ester-immersed transformer to the highest fire safety class according to the IEC 61039 standard [23]. Many tests have been conducted to confirm the superiority of fire diffusion characteristics in Ester over MIO [36].

Environment Conservation
According to environmental laws, used mineral oils are classified as hazardous wastes due to its low biodegradability. Therefore, it is forbidden to trade these oils without prior license. On the other hand, ester fluids are degradable materials according to the Organization for Economic Co-operation and Development (OECD). Following the guideline for chemicals testing OECD 301B, Figure 6 compares the biodegradation rate of different insulating fluids [37,38]. Figure 6 confirms that ester fluids, whether synthetic or natural, are superior to other insulating liquids in terms of preserving the environment.

Renewable Resources
MIO are generally produced by distilling the fossil crude oil that are gradually depleted. Therefore, MIO prices are subject to increase year by year. The need for renewable resources of insulating liquids that have a sustainable characteristic becomes

Solution Alternatives
Focus in this section will be concentrated on three alternatives that contribute to solving the deficiency of the insulation and cooling system inside oil-immersed distribution transformers. These alternatives are ranging between the substitution of MIO by esterbased liquids and replacing the oil-immersed transformers by dry-type ones.

MIO Substitution by Ester
As compared to MIO, the use Ester fluids achieves the following benefits:

Suppress Transformers Burn
For any fire to occur, three elements must be combined: fuel, air, and temperature. No burn occurs if any of these elements are vanished. Thus, the presence of mineral oil, which is a petroleum derivative, facilitates transformers fire as the temperature rises. Ester fluids have a flash point exceeds 300 • C. This moves an ester-immersed transformer to the highest fire safety class according to the IEC 61039 standard [23]. Many tests have been conducted to confirm the superiority of fire diffusion characteristics in Ester over MIO [36].

Environment Conservation
According to environmental laws, used mineral oils are classified as hazardous wastes due to its low biodegradability. Therefore, it is forbidden to trade these oils without prior license. On the other hand, ester fluids are degradable materials according to the Organization for Economic Co-operation and Development (OECD). Following the guideline for chemicals testing OECD 301B, Figure 6 compares the biodegradation rate of different insulating fluids [37,38]. Figure 6 confirms that ester fluids, whether synthetic or natural, are superior to other insulating liquids in terms of preserving the environment. urgent. Despite the fact that the Arab-Gulf countries are among the world's largest oil exporters, they have tended to replace MIO by ester in their transformers [39].

Low Maintenance
Due to its stability at higher temperatures and humidity levels, ester fluids are ideal solution for low maintenance applications. The use of ester achieves the following [26,27]:


Keeps the breakdown voltage high even at high levels of moisture content. Regarding this point, Figure 7 verifies the superiority of ester fluids over other insulating liquids.  Reduces the requirements of refining, which saves time, effort and money.

Low Maintenance
Due to its stability at higher temperatures and humidity levels, ester fluids are ideal solution for low maintenance applications. The use of ester achieves the following [26,27]: • Keeps the breakdown voltage high even at high levels of moisture content. Regarding this point, Figure 7 verifies the superiority of ester fluids over other insulating liquids.

•
Reduces the requirements of refining, which saves time, effort and money. urgent. Despite the fact that the Arab-Gulf countries are among the world's largest oil exporters, they have tended to replace MIO by ester in their transformers [39].

Low Maintenance
Due to its stability at higher temperatures and humidity levels, ester fluids are ideal solution for low maintenance applications. The use of ester achieves the following [26,27]:


Keeps the breakdown voltage high even at high levels of moisture content. Regarding this point, Figure 7 verifies the superiority of ester fluids over other insulating liquids.


Reduces the requirements of refining, which saves time, effort and money.  Obviously, K-class cooled transformers can be designed for smaller dimensions than those of O-class cooled ones. For example, Figure 8 represents an ester-immersed DT of 2300 kVA rated power and 22/0.4 kV transformation ratio. This transformer is designed for KNAN cooling system. The length × width × height dimensions of this transformer are 233 cm × 77 cm × 167 cm respectively. The dimensions of a transformer of the same rate but MIO-immersed (ONAN cooling system) are 240 cm × 187 cm × 238 cm. i.e., the width of ester-immersed DTs can be reduced by up to 58.8% of the width of MIO-immersed ones. The height can also be reduced by up to 29.8%.

Long lifetime
Reference [40] confirmed that NE fluids maintain its good condition that matches the requirements of new fluids after 7 years of continuous operation. Moreover, IEC 60076-14 [41] indicates that the use of ester insulating fluids extends the life of insulation paper inside DTs at different temperatures, as shown in Figure 9. This feature gives DTs the ability to withstand overloads up to 20% more than rated capacity.

Long Lifetime
Reference [40] confirmed that NE fluids maintain its good condition that matches the requirements of new fluids after 7 years of continuous operation. Moreover, IEC 60076-14 [41] indicates that the use of ester insulating fluids extends the life of insulation paper inside DTs at different temperatures, as shown in Figure 9. This feature gives DTs the ability to withstand overloads up to 20% more than rated capacity. Obviously, K-class cooled transformers can be designed for smaller dimensions than those of O-class cooled ones. For example, Figure 8 represents an ester-immersed DT of 2300 kVA rated power and 22/0.4 kV transformation ratio. This transformer is designed for KNAN cooling system. The length × width × height dimensions of this transformer are 233 cm × 77 cm × 167 cm respectively. The dimensions of a transformer of the same rate but MIO-immersed (ONAN cooling system) are 240 cm × 187 cm × 238 cm. i.e., the width of ester-immersed DTs can be reduced by up to 58.8% of the width of MIO-immersed ones. The height can also be reduced by up to 29.8%.

Long lifetime
Reference [40] confirmed that NE fluids maintain its good condition that matches the requirements of new fluids after 7 years of continuous operation. Moreover, IEC 60076-14 [41] indicates that the use of ester insulating fluids extends the life of insulation paper inside DTs at different temperatures, as shown in Figure 9. This feature gives DTs the ability to withstand overloads up to 20% more than rated capacity.

Miscibility
Miscibility is a property that determine the ability of an insulating liquid to be mixed with other equivalent liquids by any quantity without any changes affecting their efficiency or validity for insulation inside the electrical equipements. Both NE and SE are miscible with MIO in all proportions but not miscible with silicone fluids at all. Small amount of silicone cause foaming when mixing with ester fluids or MIOs.

Effect of Tank Design
Oil-immersed DTs are manufactured in either sealed tanks or free breathing tanks. Free breathing design is predominant in Egypt due to ease of maintenance and other economic considerations. Free breathing design allows the expansion and contraction of insulating oil through the conservator. The top level of oil inside the conservator is in direct contact with air. Although a desiccating device containing silica-gel crystals is used to keep the air inside conservator as dry as possible, this does not prevent the insulating liquid from being oxidized. As concluded from Table 1, this design is not appropriate for natural ester due to the high susceptibility of natural ester to oxidation [42]. Therefore, natural Energies 2021, 14, 1992 9 of 15 ester requires special fittings in non-sealed design to prevent oxidation and minimize air exposure, which increases the manufacturing cost. Despite the continuous efforts to improve the oxidation stability of natural ester by antioxidant additives [43], synthetic esters are best suited to retrofill both sealed and free breathing transformers.

Dry Cooling Technologies
Some features distinguish the dry-type transformers over the oil-immersed ones. Low maintenance requirements, the ability to be loaded at higher levels compared to MIOimmersed DTs and safely tap-changer switching are examples of these features. However, dry transformers in Egypt have remained limited to specific private-applications such as hospitals, factories, and hypermarkets. Dry-type transformers have not been popularly used due to the following reasons: • Dry transformers are uneconomical for small power rates.

•
The total area required to install dry transformer is relatively large due to the requirement of some extra accessories such as the enclosure and fans. • Losses are relatively higher in dry transformers, especially at low loading conditions. • The cost of the dry transformer is relatively higher.

•
The windings in dry transformers are non-repairable.

•
The noise level is relatively higher in dry transformers. • Dry transformers are sensitive to polluted environment.

The Appropriate Solution
Throughout this section, three distinct alternatives are compared to distinguish the most appropriate one for the Egyptian case. As shown in Figure 10a, the three alternatives are identified as: The weighting and ranking method is chosen to differentiate between the alternatives. This method is based on arranging the alternatives according to given criteria. As indicated in Figure 10b, four criteria are selected to differentiate between the three alternatives. The four criteria are the cost, the quality, the applicability and the execution time. The quality criterion has assigned the highest priority, and then the criteria of cost, applicability and execution time are arranges respectively. The quality criterion includes not only the technical assessment of each alternative but also the environmental impacts such as fire suppression, biodegradability and renewability.  Table 2 indicates the estimated weights and ranks of each criterion per each alternative as for the Egyptian case. Figure 11 represents the result of the weighting and ranking method indicating that alternative (A) is the high ranked alternative. Therefore, the most appropriate alternative for Dir-Mawas is the gradual substitution of MIO by SE.    Table 2 indicates the estimated weights and ranks of each criterion per each alternative as for the Egyptian case. Figure 11 represents the result of the weighting and ranking method indicating that alternative (A) is the high ranked alternative. Therefore, the most appropriate alternative for Dir-Mawas is the gradual substitution of MIO by SE.  ranking method indicating that alternative (A) is the high ranked alternative. Therefore, the most appropriate alternative for Dir-Mawas is the gradual substitution of MIO by SE.

Execution Plan
Ester price was expected to be higher than that of MIO, which affects the transformer cost [44]. However, the cost-to-benefit analysis may lead to change that belief as will be explained hereafter. As given in Figure 12, the execution plan can proceed in two phases: the first phase include the retrofill of existing highly loaded distribution transformers of less than five years lifetime. The second phase is the replacement of highly loaded old transformers by new SE-immersed ones of the same power ratings. As applied to Dir-Mawas, eleven DTs are included in the first phase and three DTs are included in the second phase.

Execution Plan
Ester price was expected to be higher than that of MIO, which affects the transformer cost [44]. However, the cost-to-benefit analysis may lead to change that belief as will be explained hereafter. As given in Figure 12, the execution plan can proceed in two phases: the first phase include the retrofill of existing highly loaded distribution transformers of less than five years lifetime. The second phase is the replacement of highly loaded old transformers by new SE-immersed ones of the same power ratings. As applied to Dir-Mawas, eleven DTs are included in the first phase and three DTs are included in the second phase.

Feasibility Studies
Technical and operational conditions of Dir-Mawas transformers show that eleven DTs shall be included in the first phase while three DTs shall be included in the second one. The traditional solution for DTs of the first phase is to perform transformers replacement chain while the traditional solution for DTs of the second phase is to replace these MIO-immersed DTs by other MIO-immersed ones of higher rates. The cost difference between the proposed solution and the existing solution was estimated for around 228,000 Egyptian pound (EGP) as detailed in Table 3. Cost calculations throughout this research are based on the price list given in Appendix A.

Feasibility Studies
Technical and operational conditions of Dir-Mawas transformers show that eleven DTs shall be included in the first phase while three DTs shall be included in the second one. The traditional solution for DTs of the first phase is to perform transformers replacement chain while the traditional solution for DTs of the second phase is to replace these MIO-immersed DTs by other MIO-immersed ones of higher rates. The cost difference between the proposed solution and the existing solution was estimated for around 228,000 Egyptian pound (EGP) as detailed in Table 3. Cost calculations throughout this research are based on the price list given in Appendix A. An economic feasibility study was performed on a MIO-immersed DT of 300 kVA with 85% loading percentage as a sample. As shown in Figure 13, the traditional solution is the replacement of this transformer by another MIO-immersed one of 500 kVA. This provides an additional capacity in the new transformer of 170 kVA for a cost rate of 382 EGP per each available kVA. The application of the proposed solution provides only substitution of the MIO by SE inside the existing transformers. This substitution increases the transformer efficiency and its ability to withstand up to 120% of its rated capacity of continuous operation without harmful effects. This provides an additional capacity of 105 kVA for a cost rate of only 75 EGP per each available kVA. This confirms that the proposed solution can achieve economic savings of up to 80%.

Execution Stages
The plan to apply the solution on Dir-Mawas is divided into five stages:  Persuading stage: during which a series of meetings and seminars are held with stakeholders to highlight the importance and feasibility of the proposed solution.  Planning stage: during which feasibility studies are confirmed, technical specifications are modified and funding sources are identified.  Procedures stage: started with the official approval for bidding and followed by the technical and financial analysis of proposals, notification to proceed, and contracts signing.  Actual execution stage: includes training, material supplying, retrofilling, installing the new SE-immersed DTs, and testing.  The final stage is the follow-up during which periodic assessments of SE-immersed DTs are performed.

Execution Period
A project schedule for the execution plan including the project's phases and activities was established. With the aid of Microsoft Project software, the Gantt chart was plotted to relate activities with each other to study the possibility of reducing the overall implementation time. As illustrated in Figure 14, Gant chart determines the total execution time where the accumulated time of the project as applied to Dir-Mawas district is estimated as 213 working days. Although the retrofilling process does not take long, the longer time is for administrative and organizational procedures. The project lasts for approximately 290 calendar days considering vacations, waiting periods for decisions and

Execution Stages
The plan to apply the solution on Dir-Mawas is divided into five stages:

•
Persuading stage: during which a series of meetings and seminars are held with stakeholders to highlight the importance and feasibility of the proposed solution. • Planning stage: during which feasibility studies are confirmed, technical specifications are modified and funding sources are identified.

•
Procedures stage: started with the official approval for bidding and followed by the technical and financial analysis of proposals, notification to proceed, and contracts signing. • Actual execution stage: includes training, material supplying, retrofilling, installing the new SE-immersed DTs, and testing.

•
The final stage is the follow-up during which periodic assessments of SE-immersed DTs are performed.

Execution Period
A project schedule for the execution plan including the project's phases and activities was established. With the aid of Microsoft Project software, the Gantt chart was plotted to relate activities with each other to study the possibility of reducing the overall implementation time. As illustrated in Figure 14, Gant chart determines the total execution time where the accumulated time of the project as applied to Dir-Mawas district is estimated as 213 working days. Although the retrofilling process does not take long, the longer time is for administrative and organizational procedures. The project lasts for approximately 290 calendar days considering vacations, waiting periods for decisions and waiting periods for performance monitoring.

Conclusions
This paper highlights the deficiency problem in the existing MIO-immersed DTs. Different tools are used to define the problem, examine its symptoms, and analyse its rootcauses. Dir-Mawas (in the middle of Egypt) was selected as a research area. The researcher suggested three different alternatives to solve the detected problem. The first alternative was substituting MIO by synthetic ester (SE) liquids. The second one was substituting MIO by natural ester (NE) liquids. The third was replacing oil-immersed transformers by dry ones. The weighting and ranking method was used to differentiate between the three alternatives and select the most appropriate one. Four differentiation criteria were used: the cost, the quality, the applicability and the execution time. The most suitable solution for Dir-Mawas area was replacing MIO by SE liquids.
The presented solution achieved many advantages, including preventing the occurrence of any transformers fire, preserving the environment and maximizing the reliance on renewable materials. Furthermore, the replacement of MIO by SE increased the transformer life-time, reduced the need for maintenance and offered the possibility to design transformers with smaller sizes. The researchers had drawn up an execution plan to implement the proposed solution on the DTs of the selected area. The execution plan was divided into five stages: persuasive, planning, procedures, execution and follow-up. A Gantt chart was developed to monitor the timeline of each stage and estimate the overall project time. The overall project time as applied to Dir-Mawas was 213 working days including all administrative, organizational and retrofilling procedures. The feasibility

Conclusions
This paper highlights the deficiency problem in the existing MIO-immersed DTs. Different tools are used to define the problem, examine its symptoms, and analyse its root-causes. Dir-Mawas (in the middle of Egypt) was selected as a research area. The researcher suggested three different alternatives to solve the detected problem. The first alternative was substituting MIO by synthetic ester (SE) liquids. The second one was substituting MIO by natural ester (NE) liquids. The third was replacing oil-immersed transformers by dry ones. The weighting and ranking method was used to differentiate between the three alternatives and select the most appropriate one. Four differentiation criteria were used: the cost, the quality, the applicability and the execution time. The most suitable solution for Dir-Mawas area was replacing MIO by SE liquids.
The presented solution achieved many advantages, including preventing the occurrence of any transformers fire, preserving the environment and maximizing the reliance on renewable materials. Furthermore, the replacement of MIO by SE increased the transformer life-time, reduced the need for maintenance and offered the possibility to design transformers with smaller sizes. The researchers had drawn up an execution plan to implement the proposed solution on the DTs of the selected area. The execution plan was divided Energies 2021, 14,1992 13 of 15 into five stages: persuasive, planning, procedures, execution and follow-up. A Gantt chart was developed to monitor the timeline of each stage and estimate the overall project time. The overall project time as applied to Dir-Mawas was 213 working days including all administrative, organizational and retrofilling procedures. The feasibility studies had proven that the use of SE is positively acceptable in all aspects. Economically, using SE instead of MIO can reduce the cost per available kVA in DTs by up to 80%.  Acknowledgments: The first author is grateful to the leadership development center LDC of the Egyptian electricity sector and his collogues in the LDP29 program for their valuable help during the preparation of this project. He also acknowledges the co-workers in MEEDC for providing the pricing data.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A
List of the reference prices used in the evaluation and cost estimation throughout this research are given in this Appendix. Reference prices for 11/0.4 kV transformers of power ratings and different cooling types are given in Table A1. Table A2 represents a separate price list for different cooling liquids.