Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3)

Energy is recognized as the prime factor for economic and social development of populations and for their welfare. However, as of 2016, more than 15% of the world’s 7.5 billion people had no access to commercial energy sources and electricity. Increasing energy and electricity consumption is the driver for investment in infrastructures and therefore also in T&D systems, which are projected to have a larger percentage development compared to other industrial sectors. The first part of this contribution to INMR by industry expert Alessandro Clerici, senior advisor to CESI and Chair of the WEC Working Group on Integration of Renewables, reviews present reserves of fossil fuels that still dominate the market and the potential of different types of renewables; the electricity sector is also analyzed in detail. Then, trends over the past 15 years are shown, underlying the increasing importance of non-OECD countries and that of electricity and of renewable energy sources (RES). In the second part, reports on present global investments with special reference to electricity and to the T&D share. Main factors affecting future development are reviewed together with projections that take account of the increasing importance of environmental concerns and of the widespread introduction of ICT technologies. The last part of the paper emphasizes the key role of transmission that is usually not adequately considered in many smart grid approaches, which are concerned instead mainly with distribution. The key role of overhead transmission lines (OHTL) is another area of focus, with various opportunities for upgrading existing lines or for construction of new lines. Finally, mandatory application of bulk transmission systems in Africa is stressed so as to best exploit enormous available energy resources, to eradicate regional poverty and to provide the 600 million inhabitants of the sub-Saharan region the electricity they still lack.

Key Role of Transmission in Expanded View of Smart Grids

Deregulation and the opening of markets have pushed for unbundling of: Production (P); Transmission (T); Distribution (D); and Sale (S). With this came a proliferation of entities, split responsibilities, different/conflicting interests. At the same time, environmental issues, increased penetration of RES and requests for demand response, ever increasing difficulties to obtain approvals for new transmission lines, development of technologies in both the electric power sector and in ICT are all pushing for better and indispensable integration of the operation of P – T – D and customers (prosumers).

Many things can be called ‘smart’ but current discussion of ‘smart grids’ tends only to involve distribution systems. More advanced hardware (power system infrastructure) and advanced ICTs (a huge volume of data is now involved in communications) are key ingredients for smart grids. ICT is an asset but without adequate infrastructure does not solve the problem, e.g. ICT cannot control flow of electrons if there are no adequate overhead lines (OHTL’s) and substations. Yet there is no optimum utilization of power system infrastructures without ICT.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) Zibo ad 14

Proposed definition of smart grid:

A smart grid (or better a smart electrical system) is an evolved system from any type of production to consumers that manages the electricity production, transmission, distribution and demand through measuring, communicating, elaborating and controlling all the on line quantities of interest with transparent info accessible to all the involved stakeholders; this to optimize the valorization of assets and the reliable and economic operation allowing adequate global savings with smart sharing of costs and benefits among all the involved.

Considering the consequences of possible bottlenecks in transmission, quick application of smart grid concepts to transmission should be implemented as soon as possible and should include, for OHTLs:

• ‘smart upgrading/uprating’ of existing line corridors;

• new types of ‘eco’ towers/lines;

• dynamic loading; and

• above all, interconnection between countries and transmission networks of bulk power from remote/more economic sources.

It is worth noticing that FACTS devices help by increasing transmission capacity in power systems, by avoiding loop power flows, by improving transient and dynamic stability, etc. But these do not increase the inherent transmission capacity of a line with its specific conductors.

Upgrading Existing Right of Way (ROW) Corridors

Many alternatives exist, including:

1. Use of new high temperature low sag (HTLS) conductors that allow a 1.5 to 2-fold increase in power transfer capability;

2. Voltage upgrading of an AC line, maximizing use of present towers/conductors by means of application of horizontal V type insulators;

3. Transformation of an existing AC line to DC, making maximum use of conductors and towers with up to a 3 to 5 time increase in power transfer capacity;

4. Substitution of an existing AC line with one that is compact at higher voltage or with a DC one in the same right-of-way (ROW)

Some examples are reported below:

Use of HTLS Conductors

Different types of HTLS conductors are now available on the market but, due to their initial high costs, have been used mainly to achieve reduced numbers of spans. Their great advantage is no need for modification of towers, foundations and insulators (the last to be verified for high temperature of conductors) with appropriate choice of conductor diameter and weight. Adequate fittings must of course be provided. Decrease in their cost will allow application to long lines on the basis of comparing conductor and re-stringing cost against the value of increased power transfer capability. In Italy, such an analysis has been completed for a 40 km long 400 kV line that saw substitution of the original conductors with HTLS conductor. The same has now also been done on 145 kV and 245 kV lines.

Upgrading AC Line

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) transmission tower

The figure on the left shows a study performed on a 245 kV double-circuit line equipped with single ø 31 mm ACSR conductor per phase. The lighter background is the original tower, on which and with the same foundations 6 arms with horizontal V pivoting insulator assemblies were substituted. A 420 kV circuit with 2 conductors per phase was thereby obtained. Both mechanical stresses to the tower and foundations and electrical effects (RIV, corona, electric and magnetic fields, etc.) were investigated and resolved.

Power transfer increase was 1.7 times.

Role Of Insulator Assemblies Is Essential.

Transformation of AC Line to DC

Studies have been performed since the 1990s to as much as possible utilize the components of an existing line (conductors, towers, foundations) to increase transmissible power through an HVDC mode. Clearly, AC insulators must be replaced by special DC insulators. Type and numbers of DC insulators will depend on contamination conditions and affect maximum DC voltage level applicable in each specific case. The type of existing AC line tower that is the best suited for such transformation in order to obtain significant increase in power throughput is the double circuit type, with one circuit in vertical configuration on both sides of the tower. According to the length/type of the AC insulator strings as well as clearances to tower and ground, the easiest transformation is the one that sees simple substitution of AC insulators with the maximum possible length of DC insulators (and therefore maximum DC voltage). The 3 phases on the left serve as one pole of the new DC line and the 3 phases on the right as the second, although this does not allow substantial power transfer improvement. Rather, maximum upgrading is given by eliminating the arms of existing AC towers and substituting them with two new arms in suitable position to match mechanical and electrical stresses. From various studies performed, power upgrade can be in the range of 3 to 5 times. Possible elimination of the steel arms and adoption of horizontal V hinged insulator assemblies has been found to be the most interesting option. The figure below summarizes results of studies on the same 245 kV Italian double circuit line discussed above, where previous AC transfer capacity of 300 MW was increased to 550 MW by the simplest transformation and use of composite insulators. A power increase to 1560 MW is achieved with modification of tower arms. These reported values refer to operation at the same current density of 1A/mm2.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) Possible transformations of Italian 245 kV AC line to HVDC
Possible transformations of Italian 245 kV AC line to HVDC.

Substitution of Existing AC Line with Compact Line at Higher Voltage or with DC Line in Same ROW

Figure 1 shows substitution in the same ROW of a 245 kV Italian double circuit line equipped with single ø 31 mm ACSR conductor per phase (original tower in background) with a 400 kV compact line using horizontal V insulator assemblies and lower spans. By adopting the same conductors as the original line, the new 400 kV line equipped with twin ø 31 mm ACSR conductors per phase has increased transfer capacity by 1.7 times. By adopting new larger conductors per phase, it is possible to achieve larger power transfer capacity with the new line.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) tower
Fig. 1
electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) transmission
Fig. 2

Figure 2 shows replacing an AC double circuit OHTL with a bipolar DC one having appropriate voltage/conductors to realize the desired power upgrading. This transformation does not pose limits to the power increase by adopting a DC OHTL with poles in a vertical position.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) CSL compressed

There are no limits to possible solutions to improve power transfer capacity of existing transmission lines or transmission line corridors. Different possible solutions with different power increase/costs can be considered and key issues are:

• actual possibility to take the specific AC line out of service and for how much time during the ‘transformation period’;

• specific problems relevant to existing anchor towers, angle towers, etc.;

• length of line involved, even if entering/exiting substations one could consider a new transformed line as a series of 2 to 3 original AC lines;

• cost/space of DC converter stations in case of transformation of AC into DC;

• actual ‘power increase’ acceptable by networks at both ends of the line;

• local standards (including possible hot line maintenance, RIV, AN, EMF limits, etc.) and conditions posed by technical problems (e.g. local contamination), by access and logistics and by local costs including cost of losses.

The key issue is development of technologies/equipment/organization of works in order to minimize unavailability of power transmission to make a power increase feasible.

New Types of Towers & AC lines

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) Towers AC lines 1

For AC lines, main effort has been placed in developing:

new towers, allowing increased transfer capacity for long transmission (high SIL= surge impedance load) with the minimum ROW width;

eco towers/compact lines with more acceptable impact on the environment;

With respect to the first category the table on the left is valid to provide the main varieties of towers built around the world. The figures (from top to bottom) close to each tower compare, in %, the corresponding values of a conventional self supported Y tower with conductors in flat configuration. The relevant quantities are:GMD geometric mean distance between phases

• ROW width


• SIL/ROW (power density in ROW) 

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) towers
electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) Examples of realization of eco towers
Examples of realization of eco towers.

New types of compact lines with pivoted tower bases and horizontal V insulators have been developed and applied for many hundreds of kilometers of lines in Italy, Switzerland and Egypt (photo at left for OHTL arriving at Cairo South substation) in the range from 145 to 400 kV, as an more economical alternative to tubular poles while still presenting light bi-dimensional towers and small foundations. Among the tubular poles types ,the one as here below on the left is of interest.

There are no limits to imagination for improvements but not only CAPEX costs but also O&M costs must be carefully analyzed. For all the above towers/lines, insulator assemblies and type of insulators play a fundamental role.

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Dynamic Loading of OHTLs

On line monitoring of conductor current/ temperature/ sags and ambient conditions will allow:

• larger transfer capacity in the great majority of hours;

• alleviation of N-1 conditions (to be re-considered).

Adequate sensors and communication facilities are involved.

Interconnections & Transmission of Bulk Power from Remote but Economic Sources

Benefits of interconnections are: political, technical, economical and environmental. Development of interconnection capacity between countries (or areas) allows for greater electrical system reliability and flexibility in the generation mix and operations, thereby improving security of energy supply. Availability of cross-border/cross areas transmission capacity could help in selecting power from cheaper units (including environmental costs) located in other areas, countries or regions and also to ‘capture’ the best of renewables. Both AC and DC technologies are available for systems above 1000 kV for OHTLs (1200 kV AC in India and 1100 kV DC in China) and up to 600 kV for sea/underground cables. Subsea cables have been developed for depths exceeding 3000 m. DC transmission up 3500 km is under construction in China.

FACTS technologies have had great improvements in AC systems. There are no limits for technologies but the key issue is, as mentioned, strong opposition and very long times to implement OHTLs and even cables. From conceptual engineering of a new OHTL of some dozens of km to its commissioning, average time in EU is now more than 10 years and, disregarding those cases of infinite time (impossibility to arrive at the final completion).

Interconnectors are also a mean to foster higher penetration of RES with consequent reduction of GHG emissions. Sub-Sahara Africa has huge potential of both fossil and renewable resources, largely unexploited which, connected to the development of powerful long transmission systems, could create strong inter regional exchanges of electrical energy. This could bring substantial contributions to the eradication of poverty and access to electricity to 600 million people in the region.

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Construction of hydro power plants, or of power plants in general, and of OHTLs, is a labor-intensive activity providing local jobs. Considering the hydro potential with economic development of large plants with high load factors and discovery of enormous gas reserves (e.g. in Mozambique) that could provide cheap conventional electricity in some areas, possible plants linked to long transmission systems could lead to a sub-Sahara region energy renaissance.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) OFIL banner 300X125 July1

From studies on development of long DC transmission systems in Africa, the figures below show the cost relevant to transmission for delivered kWh and the total cost of delivered kWh as a function of production cost. This with 2 main extreme hypotheses relevant to annual rates (AR) and cost of losses (CL). The 6% value of AR corresponds to a discount rate close to 3.5 to 4% over 25 years, as indicated by IMF for special projects with grants/soft financing, while the10.5% relates to private financing.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) Screen Shot 2017 12 18 at 09
Transmission cost of delivered kWh on AC side of receiving end. Optimum voltage level for different powers (P) and length.
electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) Screen Shot 2017 12 18 at 09
Cost of energy delivered on AC side of receiving end as function of generation cost.

Considering the huge hydro potential (RDC with Inga, Ethiopia, Cameroons, Angola) and the large new gas discoveries, appropriate utilization of these resources to feed different regions in the sub-Sahara seems mandatory. With 2 c$/kWh production cost (cheap large hydro) electricity can be delivered at 3÷3.5 c$/kWh for 1000 MW at 1000 km or 4000 MW at 3000 km. With 4 c$/kWh production cost (GTCCPP with gas at 3-4 $/MBTU), electricity can be delivered at 5÷6 c$/kWh for 1000 MW at 1000 km or 4000 MW at 3000 km. The potential for transmission development is huge.

electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) BG Big Box banner 300x250

The role of insulators in DC lines is more critical than for AC lines and has fundamental impact on line optimization.

electrical power electrical power Trends in Electrical Power & Impact on T&D Systems (Part 3 of 3) large plants and    virtual plants    fed through large HVDC interconnections
Africa: Actual & feasible cheap/large plants and “virtual plants” fed through large HVDC interconnections.


1. No shortage of fossil fuels; socio-political problems for their location and environmental concerns. Fossil fuels have still a share of 86% in consumption of primary energy and 66% in electricity production with respect to 10% and 23% for RES. Reduced investment in drilling activity for low oil price could have remarkable effects on future energy consumption.

2. With strong development of RES, mainly in the electricity sector, and increasing consumption of oil in transport, share of fossil fuels will decrease faster in the electricity sector with respect to primary energy consumption. By 2035, projected share of fossil fuels in electricity production is around 45%, close to the share of RES that will have a contribution close to 2/3 from wind and solar.

3. Due to rationalization and energy efficiency policies, rate of decoupling between GDP growth and energy consumption growth is increasing. Percent growth of prime energy demand will slow. Intensity of CO2 emissions in tCO2 per $1000 of GDP has a general downward trend (now lead by China), reaching a projected value by 2040 in the range 0.75-1.5 for all the main countries.

4. Over the next 15 years, yearly growth of population is projected at 0.85% versus 1.2% for energy consumption and 2% for electricity, that is becoming the sector with the highest growth.

5. Growth in the energy sector and in GDP is now and will be mainly in non-OECD countries that have overtaken those of the OECD. The difference is going to increase. Real GDP annual % change is expected to be 1.7 % in OECD and 5% in non-OECD countries.

6. The largest market for energy and electricity is Asia Pacific, with a share around 50% and with China at around 25%.

7. Wind and solar PV have had and are having a large development due both to initial generous incentives and to continuous reduction of CAPEX per kW installed. Their intermittent production of energy is posing challenges to electrical power systems in terms of massive integration. This will need flexibility and storage devices.

8. Regulations and technology are key drivers for development all energy sectors.

9. Environmental concerns are having an increasing impact on regulation, government policies and public behavior in regard to strong opposition to new energy infrastructure. Socio-economic implications of climate change include increase in energy prices, stranded assets, stranded energy resources.

10. Over the past 20 years, the electricity sector, apart from having the highest development and the largest investments per year, has experienced major structural modifications in all its subsectors, with massive integration of ICT technologies and evolution of smart grids. New business models and new ‘players’ will arrive. Much growth is expected for electric vehicles and heat pumps.

11. T&D has a large share of investment in the electricity sectors, close to that of RES, and is undergoing continued development, with expected market size increase of more than 50% over the coming 10 years. The sub-sector with the largest increase is that relevant to protections & relays, control systems (share of around 50% of that sector) and smart meters/smart grids.”

12. Investments in transmission with respect to those in distribution vary strongly according to the regions; world average is around 1 to 2 but in EU countries over recent years has been close to 1 to 4 or 5.

13. Projected shares of T&D in 2025 envisage Asia (excluding India) at around 50%, Western Europe 12.5%, North America 11.5%, Sub-continental India 6%, followed in order of share by Africa, South America and ME with close to 5% each.

14. Advanced infrastructure and advanced ICT are key ingredients for smart grids. ICT is an asset but cannot control flow of electrons if there are no adequate OHTLs. But, vice versa, there is no optimum utilization of power system infrastructure without ICT. But special attention must be paid to the possible impact of cyber security on power system reliability and security of supply.

15. The smart grid concept should include and give special attention to transmission and not be confined mainly to distribution. Smart upgrading/uprating of existing line corridors, new types of eco towers/lines and, above all, interconnections with their outstanding technological developments should be better considered in the family of smart grids.

16. Africa should be a target of special concern for exploitation of its enormous energy resources with introduction of a great number of bulk transmission systems involving thousands of km of OHTLs, mainly DC. This to eradicate poverty and provide social development.

17. In all the above-mentioned applications of transmission lines, insulator assemblies and composite insulators are playing an important role.