By Peter Kohnstam
As the power grid continues to become more complex with the introduction of new generation, both on-grid and decentralized, and the proliferation of sensors and data collection, transmission grid planning becomes extremely challenging. New solutions, including hardware and software, are needed to ensure the reliable transmission of power throughout the network while still conducting accurate outage planning.
The transmission system in the U.S. has grown from small clusters of customers served by local generating stations to a nationwide system described by the National Academy of Engineering as “the greatest achievement of the 20th Century.” As demand for electricity grew, larger power stations were built, typically close to good sources of fuel like coal. Transmission lines were built to transport the power to the growing load centers. The growth was unprecedented, with electricity demand increasing at up to 12 percent a year resulting in construction of lines and generation to keep pace until the pressure eased. The pressure was somewhat relieved in the latter part of the last century when the transmission system that we know today was largely complete.
We are now faced with a new set of challenges for our electricity system: thermal generating stations are coming to the end of their operational lives; economic and environmental pressures are causing other sources of generation to close; and large scale additions of renewable energy sources are being made. Traditional generation has always been the main voltage supporting mechanism of the interconnected system for many years. Renewable energy sources bring a number of technical challenges to the operation of the transmission system. Renewable energy is intermittent in nature, does not typically provide similar levels of reactive power compensation or short circuit current contribution, and is generally not located anywhere near the original generating stations or load centers. In addition, we see an increasing amount of residential renewable generation and distributed generation being implemented that directly impacts the flow pattern seen by the electricity system.
A traditional solution to overcome these new problems would have been to reconfigure and expand the transmission system. The necessity of installing new transmission lines, and the public’s lack of understanding or appetite for new infrastructure makes acceptance problematic. There are a few exceptions to this. The CREZ Initiative in Texas supported the construction of over 3,000 miles of new transmission lines that connect large amounts of wind power located in remote locations to highly populated areas. Other areas of the country however are not necessarily as fortunate – while California is increasing its level of renewable generation (50 percent by 2030) it is also closing traditional thermal generation plants. This significantly changes the power flow pattern across the state and creates challenges for the system planners and operators to maintain the mandated system conditions.
Transmission planners are tasked with ensuring that the transmission system is reliable, efficient and cost-effective. Hence, they now face the challenge of enhancing the existing transmission system to support the new generation mix and locations so that it meets the needs of the country for the foreseeable future. It is a significant challenge. However, there are a number or modern Transmission technologies available that will help achieve the goal.
One key technology is synchronous condensers. They have existed for decades, but with a modern twist to old equipment, they are proving invaluable in stabilizing the system. Synchronous condensers provide short-circuit power contribution, reactive power compensation, short-term overload capability, and provide inertia to the transmission system. They can be connected to existing transmission substations or, alternatively, retired generators can be retrofitted to provide the required functionality.
Another key technology solution is Flexible AC Transmission Systems (FACTS). The IEEE defines FACTS systems as “a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability.” These systems provide dynamic voltage regulation, increased power transfer over long AC lines, damping of active power oscillations, and power flow control in meshed systems, thereby significantly improving the stability and performance of existing and future transmission systems.
FACTS devices include static VAR compensators (SVC), a quick and reliable means of controlling the voltage in a transmission system. With average response times ranging from 30 to 40 ms, SVCs are much faster than conventional mechanically switched reactors and capacitors (100 to 150 ms) and can also be used to actively damp power oscillations. When system voltage decreases, an SVC generates capacitive reactive power. When system voltage increases, it absorbs inductive reactive power. The reactive power is changed by selectively switching and or controlling three-phase capacitors and reactors connected to the secondary side of a step down transformer. A capacitor bank (TSC) is switched on and off by thyristor valves and the reactors (TCR) can be either switched or controlled variably by thyristor valves.
Another FACTS device that can stabilize the transmission system is a static synchronous compensator (STATCOM) that provides similar functionality to an SVC however its dynamic performance is better. In addition STATCOMs are based on voltage source converter technology which enables it to provide the required compensation across almost all of its operating range independently of the voltage of the system it is connected to.
High voltage direct current (HVDC) systems provide a mechanism by which highly reliable and controllable power can be injected into a load center from a generation source, irrespective of its type and location. HVDC systems have been traditionally associated with bulk power transfer however advances in control technology now allow HVDC to provide a range of additional services that can support the transmission system including frequency and voltage support, spinning reserve sharing and power system oscillation damping. The choice of HVDC technology is dependent on the application and both technologies offer benefits to the system, but for this challenge, there are two HVDC technologies that can be used; line commutated converter (LCC) and voltage source converter (VSC).
While we cannot predict the significant changes the transmission system will be exposed to in the foreseeable future, we do know that changes will come. The transmission system will need to adapt and accommodate new requirements. Application of the various available technologies discussed will help transmission planners overcome the challenges that are coming and ensure that the transmission system is there to support the electrical needs of the country through the 21st Century, as it has so ably done in the 20th Century.
Peter Kohnstam began his career in the electric industry in the UK and has been involved in transmission operation and construction for the majority of his career. He has been involved in projects ranging from the rollout of SCADA systems through substation refurbishments to large area network improvement projects. Peter joined Siemens in 2012 as an HVDC Business Development Manager and is now focused on promoting and supporting HVDC projects in the US.