Updated Transmission Lines for Renewables: Electric Powerways
- Mar 31, 2021 5:15 pm GMT
The electric energy grid is designed to transmit power from central generating stations, such as coal burning power plants, to areas of consumption, cities. As distributed renewable energy sources, for example rooftop solar, gain in popularity and add energy inputs to the grid from a variety of locations, this traditional design creates a challenge. Transmission lines must be updated so that they are able to transmit power from various energy sources, rather than a single generating station.
Another issue is that traditional transmission lines can only be operated at the capacity of the line that loads to full capacity first. While transmission lines may be rated to carry a large amount of electric energy, it is unlikely that most lines will ever carry their rated load. This creates a problem as traditional transmission lines could never carry the maximum output of both central generating stations and distributed renewables.
As the electric power grid evolves, transmission lines need to be converted to electric powerways to optimize power flow. An electric powerway is a transmission element that offers impedance control, so operators aren’t limited to a single power source or size, a major limitation in today’s grid. Once implemented, electric powerways will allow transmission system operators to direct the flow of power from any point of energy generation to any point of energy consumption. With electric powerways, all lines will be able to operate closer to maximum capacity at the same time. Just as highways have optimized transportation, electric powerways will optimize power flow across the interconnected grid.
A key benefit of powerways is that they will not require a total overhaul of grid infrastructure. Instead, new components can be added to the existing infrastructure to modify them. Let’s take a look at the concept of a powerway, the innovations necessary for their implementation, and the benefits we’ll see.
New Components for Electric Powerways: SSRX & T-Caps
Converting transmission lines to powerways requires the introduction of several new components that will be used to optimize powerway impedance. Currently, the impedance of transmission lines is a fixed value based on wire size and line length. The impedance of powerways can be much more variable within the range specified by the system operator. Two devices should be installed along powerways, depending on line impedance: solenoid series reactors (SSRX) and high-current transmission-class series capacitors (T-Caps).
Solenoid series reactors (SSRX) are new devices used to reduce fault current and stabilize voltage. The impedance of short powerways can be increased during normal system operating conditions by installing SSRX at each line terminal. SSRX will further increase impedance when short circuits occur. This will stabilize grid voltage within eight milliseconds. This special feature of SSRX will enable renewable energy supplies to ride through fault conditions.
High-current transmission-class series capacitors (T-Caps) are used to reduce the impedance of longer transmissions line. Once inserted, T-Caps can be activated to reduce the impedance of long powerways, thereby optimizing power flow. When needed, T-Caps can be bypassed to return the powerway impedance to the initial value. T-Caps will be 10 KV, 2000 amp components, and will be located at several locations along a powerway. As a note, T-Caps are still in the design phase.
Once these devices are installed in existing transmission lines, the conversion to electric powerways will be complete.
Case Study: Metropolitan Area Transmission Lines vs. Electric Powerways
Consider, for example, transmission lines that power a metropolitan area. Transmission lines have been built from throughout the state and other nearby areas. Power transfer across the existing network of transmission lines is limited by the line that carries the most current.
Table 1 shows a model of the maximum power capacity of traditional transmission lines. The rating for each line is significantly higher than the actual load carried by each line. In this model, all seven lines can only ever transmit 63% of the combined rated capacity for all seven transmission lines serving the metropolitan area.
If Upstate Transmission Line 1 is out of service, then Upstate Transmission Line 2 is the limiting transmission line, carrying its maximum rating in amps. Maximum power transfer capability is still 63% of the total rated capacity.
In contrast, when transmission lines are converted to powerways, power transfer capability will be increased to 86% of the total rated capacity for all seven lines. Table 2 shows that the rating of each line remains the same, while the network load increases. The values in Table 2 are based on powerways that include SSRx and T-Caps.
The increase in power transfer capability of 3,200 Amps is equivalent to building two additional lines to serve the metropolitan area. Even when one line is out of service, power transfer capability remains at 86% of capacity.
Benefits of Electric Powerways
Electric powerways will provide major benefits for electric utility companies. Powerways will increase the capacity of current transmission lines by over 20%, allowing more power to be sent over existing lines. This eliminates the need to build new transmission lines and other infrastructure.
Energy from distributed renewable sources could be easily transferred once powerways are commonplace, allowing the energy from renewables to be integrated smoothly into the existing grid.
Electric powerways will also benefit the consumer by eliminating price spikes on the power grid because of the increased power flow capability across the grid. Interconnected powerways will optimize power transfer, because all lines will be able to operate at maximum capacity at the same time. Energy shortfalls, like that experienced by California residents in the summer of 2020, should become a thing of the past. Powerways will be able to move significant amounts of power among all connected parts of the grid.
Electric Powerways Will Efficiently Transmit Distributed Renewable Energy
Electric powerways will help prepare the grid for distributed renewable energy sources by allowing for more efficient transfer of power from location to location. While the current grid is designed to transmit power from central generating stations to major points of consumption, new electric powerways will allow power to be transferred among all locations where power is produced or consumed. Cities with abundant rooftop solar could transmit power within the city or to remote areas as power is needed. Power flow would be optimized everywhere.
If SSRX and T-Caps designs are optimized, prototypes are fabricated, and high power testing is completed in 2022, a demonstration conversion of a transmission line to a powerway can be completed as soon as 2024. Electric powerways will be a game changer for the electric power industry.
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