High performance conductors improve grid capacity, efficiency
Access to affordable and reliable electricity is paramount to building stable and healthy economies. Without it we would be unable to pump water to grow crops, build products to compete in a global economy, or provide basic services to a growing population. But delivering power is no easy task and building new generation is expensive and complex – especially as world leaders have begun to recognise the impact fossil fuel based generation has on climate change. It is therefore imperative that we leverage high performance conductors to build an efficient, reliable and robust grid.
Historically, as demand for electricity grew, many utilities invested in more efficient sources of generation to offset rising fuel costs. Though costs fluctuated, the challenges associated with building new generation remained substantial. Many utilities supported, often subsidised, the deployment of more efficient demand side appliances to push out the need to build new generation. Investment in efficiency made sense. This held true for transformers as well. While more efficient transformers were offered at a price premium, their reduced life cycle costs were easy to recognise, to the point that efficient transformers became the norm.
Fig. 1: Conventional ACSR and modern
ACCC conductors
The efficiency of transmission and distribution lines which were often developed 40 years ago or more, however, have degraded over time. This is not solely due to the fact that many of these lines are operating at capacities previously not considered, it is also due in part to the fact that lower upfront capital cost objectives took precedence over increased efficiency investment, as line losses were generally socialised and passed through to the consumer. The highly loaded lines operating at higher temperatures not only subjects them to excessive conductor sag and greater susceptibility to sag trip outages, it also subjects them to exponentially higher I2R line losses.
Historical perspective of conductors
In the nineteenth century, copper was used for overhead conductors due to its excellent conductivity. Due to the first world-war effort, and the associated demand for copper, copper was replaced with much lighter, but less conductive aluminum. Over time, various aluminum alloys were introduced which offered improved strength (with some reduction in conductivity), and steel core strands were added to increase the conductors’ overall strength to accommodate greater spans with reduced sag. During the second world-war, aluminum supply was directed toward aircraft manufacturing and some transmission lines built during that period reverted back to copper conductors (some of which manufactured with steel cores).
Fig. 2: ACCC power plant upgrade.
While the pre-1970s electric grid in the US became heavily loaded as the country’s demand for electricity grew and new corridors became more difficult to secure, the existing electrical grid needed increased capacity, often on existing rights-of-way (ROW). At that point, new conductors were introduced to address the challenges. Trapezoidal strand (TW) aluminum conductors packed more aluminum into a diameter to minimise wind load on structures and offered added ampacity. Conductors capable of operating at higher temperatures were introduced. SSAC, now known as ACSS (Aluminum Conductor Steel Supported) was deployed to increase line capacity; although it’s relatively high thermal sag characteristics limited its deployment to some degree.
Fig. 3: Clean power plan lines.
The ACSS conductor’s high operating temperatures also reflected increased line losses which resulted to some degree in increased fuel consumption (or depletion of hydro resource), and additional generation requirements to offset line losses.
Today there are a vast number of conductor types and sizes available, but the basic formula has not changed much in nearly a hundred years. All aluminum-alloy conductors are still widely utilised (such as AAC, AAAC and ACAR) in certain applications, and ACSR (Aluminum Conductor Steel Reinforced) is generally the basic conductor of choice. In addition to ACSS, other high-temperature, low-sag (HTLS) conductor types were developed to increase capacity with reduced sag.
While high temperature/low sag capabilities can be extremely important on key spans where clearance requirements are challenging, they can also be very important during N-1 or N-2 conditions when adjacent lines are out of service and the remaining line(s) are relied upon to handle additional current. However, high temperature operation has historically been reserved for special circumstances, as high temperature operation also reflects high (I2R) losses and their associated costs.
Line loss mitigation
To this day, line loss mitigation is not widely considered a first tier priority. Increased capacity is by far the more prominent objective. Fortunately, modern high performance conductors offer improvements in both categories.
Consider the following example:
In 2010, demand for electricity in southern Texas reached an all-time high during a peak winter season. Power provided to this region was supplied by two 345 kV lines 193 km long. In 2013 American Electric Power (AEP) began upgrading the existing lines. Using a live-line technique previously used in South Africa, they replaced over 2300 km of double-bundled aluminium conductor steel-reinforced cable (ACSR) with double-bundled aluminum conductor composite core (ACCC) lines. ACSR is conventional steel reinforced conductor. ACCC replaced the conventional steel wires with a single composite core strand. The composite core’s lighter weight allows it to incorporate 28% more aluminum using trapezoidal shaped strands without a weight or diameter penalty.
Fig. 4: ACCC survived a tornado.
The use of the ACCC conductor allowed AEP to double the capacity of both of the 345 kV circuits. The utility also achieved a 30% reduction in line losses due to the higher aluminum content and decreased electrical resistance. The reduction in line losses not only saved over US$16-million per year in wholesale electricity costs, it also freed up 38 MW of generation capacity that was otherwise wasted supporting the line losses.
The added aluminum content not only helps to increase the conductor’s ampacity, because its current carrying capability that is about twice that of a conventional conductor, it also significantly reduces line losses compared to any other conductor on the market today. Line loss reductions can range from 25 to 40% depending on load level. The ability of the ACCC lines to carry twice the current of a conventional conductor without exhibiting excessive conductor sag, coupled with its improved efficiency characteristics, obviously offers huge advantages.
The composite core is highly resistant to environmental ageing and will not corrode and, more importantly, as those in the aerospace, automotive and other industries appreciate, it also resists degradation from cyclic load fatigue. In the case of powerlines, everything is cyclic including tension, temperature, vibration and stress.
Considering these efficiency-driven improvements, without even considering the increased power sales, this project’s payback time could be measured in months. This was especially true because the utility was able to use existing structures.
Another benefit, not previously considered, was the fact that the 30% reduction in line losses also translated into a CO2 emission reduction of over 160 000 t per year. This was the equivalent of removing about 34 000 cars from the road. The emission reduction performance on this project was based on all combined sources of generation in the state of Texas, including wind, solar, geothermal, natural gas and coal.
Fig. 5 : Conductor comparison showing ampacity capabilities.
While design engineers of transmission systems and grid operators are generally not directed to consider the impact line losses might have on global warming or climate change, the reality is that improved grid efficiency can play an important role in helping to reach certain emission reduction objectives and political mandates. Though team members on the generation side have many things to consider, including capacity factors and costs per MWh of delivered power, there is very little, if any, added cost on the transmission side to substantially improve grid efficiency and reduce emissions, as the first tier benefits of transmission investment are typically captured in the negotiated return on investment.
A typical example
ETESA, the grid operator/transmission company in Panama, is currently working to build “the most efficient grid in the world”. The reason is that they get to “pocket” any system loss reduction saving that drops below 4%. This added cash flow is helping ETESA to secure international bonds that offer very attractive payback to investors which are becoming even more attractive to the rapidly growing green community. Panama has truly cracked the code on soliciting investment in their grid and we should all pay attention. Meanwhile, other countries and utilities are establishing efficiency standards for conductor technologies. Between all of these new cards being played, we can expect to see improvements across the board.
Contact Dave Bryant, CTC Global, dbryant@ctcglobal.com
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