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Accelerating electric transmission systems

A look at the current status of transmission system tools and technologies that are ready for large-scale deployment by electric utilities.

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Electricity, Grid

The high-voltage transmission systems that we use today are, in effect, the “backbone” of our power delivery systems. This highly complex infrastructure transmits very large amounts of electric energy between regions, and among sub-regions. Despite the public perceptions of rampant failures, transmission system equipment fails and causes power outages much less frequently than distribution equipment.

But when transmission equipment does fail, many more customers are affected, and outage costs can be much higher, compared with the impact of a distribution equipment-related outage.

What is the current status of technologies, especially those which are truly ready for larger-scale deployment and adoption by electric utilities? Several technologies have undergone significant research and development and are now available for commercial use, but have not been widely deployed.

As with any technology, advances are continuing in most areas. The adoption of these technologies by the electric utility industry is limited, typically by the business cases that include the cost/benefits analysis each capital investment must undergo before implementation.

For instance, the adoption of Dynamic Thermal Circuit Ratings (DTCRs) requires more than just R&D—it also requires new business case development. Dynamic rating and real-time monitoring of transmission lines are becoming important tools to maintain system reliability while optimising power flows.

Dynamic ratings can be considered a low-cost alternative for increased transmission capacity. Dynamic ratings are typically 5% to 15% higher than conventional static ratings. Application of dynamic ratings can benefit system operation in several ways, in particular by increasing power flow through the existing transmission corridors with minimal investments.

Dynamic rating increases the functionality of the smart grid because it involves the monitoring of real-time system data that can be used in various applications:

Real-time monitors yield a continuous flow of data to system operations—line sag, tension or both, wind speed, conductor temperature—traditionally not available to operators.

Monitored data can be processed to spot trends and patterns.

Real-time monitored data may be turned into useful operator predictive intelligence (for instance, critical temperature and percent load reduction needed in real time).

While additional research may be needed to fully integrate DTCRs with future control schemes, a more robust business case for the application of this technology must be developed so that more utilities adopt this technology.

System planners are concerned that you cannot count on the qualifying conditions that offer the ability to benefit from DTRC to be present when DTRC may be needed. Therefore, when doing the business case, you almost always have to plan around, or even plan out the corridors that could benefit from DTCR.

Another big priority is increasing the capabilities of the Fault Current Limiters (FCLs) or Short-Circuit Current Limiters (SCCLs). The SCCLs or FCLs are a family of technologies that can be applied to utility power delivery systems to address the growing problems associated with fault currents.

The present utility power delivery infrastructure is approaching its maximum capacity and yet demand continues to grow, leading in turn to increases in generation. The strain to deliver the increased energy demand results in a higher level of fault currents.

As a result, more SCCLs are needed. However, additional development is needed to reduce their cost and physical size. The SCCLs fall into two categories: superconducting devices and non-superconducting devices.

Superconducting SCCLs are either resistive or inductive. In a resistive device the current passes through the superconductor and when the current increases, the superconductor quenches. In the inductive device, the simplest form is a transformer with a closed-loop superconducting secondary. Non-superconducting devices can be simple inductors or variable resistors.

The power-electronics-based SCCL is designed to work with the present utility system to detect a fault current and act quickly to insert impedance into the circuit to limit the fault current to a level acceptable for normal operation of the existing protection systems.

Many utilities are coping with mandates to realize cost reductions in transmission power flow control technologies. This area of could also be called ‘Power Electronics Based Systems’. They are also known in the industry as Flexible AC Transmission (FACTS) technologies.

A number of these technologies are commercially available today. These all incorporate power electronics and can be applied to the transmission system. These include both the control and operation of the power system and applications that will extend eventually to transformers themselves.

These types of devices can be put to a wide variety of uses: power flow control, loop flow control, load sharing among parallel corridors, voltage regulation, enhancement of transient stability, and mitigation of system oscillations.

Such devices include the thyristor-controlled series capacitor (TCSC); thyristor-controlled phase angle regulator (TCPAR); static condenser (STATCON); and the unified power flow controller (UPFC).

American Electric Power (AEP), the US-based utility, installed the first UPFC at its Inez substation in eastern part of the US State of Kentucky. While these technologies are more than 20 years old and well understood, they are almost always considered too expensive when compared with simply building more assets. Accelerating development of advance power electronics so as to dramatically reduce the cost of FACTS devices should remain a priority.

Numerous utilities are interested in accelerating the deployment of voltage source converters (VSCs). These are self-commuted high-voltage direct current (HVDC) converters. Contrary to “traditional” HVDC converters, the self-commuted HVDC converters do not have to rely on synchronous machines in the ac system for its operation.

The increased controllability improves harmonic performance and provides VAR support. VSCs permit power flow to be reversed without reversing the polarity of the cable, thereby enabling the use of extruded cables (cables insulated with extruded polyethylene-based compounds, such as XPLEs). It makes undergrounding (cables, instead of overhead lines) more attractive. More VSCs are needed in the North American power system.

Advanced analytics and visualization applications are needed to maximize use of phasor measurement unit (PMU) data. PMUs, or synchrophasors, provide real-time information about the power system’s dynamic performance. Specifically, they take measurements of electrical waves (voltage and current) at strategic points in the transmission system 30 times per second.

These measurements are time stamped with signals from Global Positioning System (GPS) satellites, which enable PMU data to be time-synchronized and combined to create a comprehensive view of the broader electrical system. Widespread installation of PMUs, which is occurring now, will enhance the ability to monitor and manage the reliability and security of the grid over large areas.

PMUs can provide system operators with feedback about the state of the power system with much higher accuracy than the conventional SCADA systems which typically take observations every four seconds.

What will it take to accelerate development of intelligent electronic devices (IEDs)? IEDs encompass a wide array of microprocessor-based controllers of power system equipment, such as circuit breakers, transformers, and capacitor banks.    

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