Recent Trends in Transmission Power Systems

 By Bo Yang, Frank Kreikebaum, and Debrup Das

After the establishment of the transmission power systems (TPSs) a century ago, bulk grids are challenged by aging infrastructure, penetration of intermittent renewable generations, the changing nature of loads, and regulatory reforms. Power companies turn to advanced grid monitoring and control technologies for help, which drives various emerging industry trends orchestrating control, power electronics, communications, and information technologies.

After the establishment of the transmission power systems (TPSs) a century ago, investments in TPSs have been primarily driven by demand growth, where centralized power generation and long-haul transmission lines are both economically attractive and technically efficient. The operation of TPSs relies on supervisory control and data acquisition (SCADA), substation automation and control, and Flexible AC Transmission Systems (FACTS) technologies. These estimate the status of the entire network via measurements at substations and make control decisions relying either on state estimation or local measurements.

As the locations of generation and load pockets don’t usually change, the centralized monitoring and control of TPSs at substations, seems to be technically sound. This approach, however, doesn’t provide much visibility for most transmission infrastructures that are outside of substations.

Today’s TPSs are challenged by aging infrastructure, penetration of intermittent renewable generation, the changing load dynamics, and regulatory reforms. The aftermath of natural disasters repeatedly confirms the necessity of more reliable, flexible and sustainable TPSs. Planning and operation of TPSs become more difficult accounting for the uncertainty of generation and load, and traditional substation-oriented TPS technologies seem to be less sufficient. For example, the intermittent nature of wind makes it difficult to predict and usually requires more fast-ramping spinning reserves to keep frequency stable.

Another example is the ubiquitous integration of solar photovoltaic (PV) generation; a large portion of the existing solar PVs are interconnected at low voltage without utility control. This is the fast growing PV sector, so that impacts will accelerate and become more difficult to predict and control, considering the distributed and stochastic nature. It may induce significant changes to the normal operating conditions in a short period of time. For TPSs with significant renewable power penetrations, real power can vary at any time, by location and in either direction (ramping up or down), which challenges forecast and dispatch, as well as operation and stabilization. Most voltage control and power flow control devices, are sized at high capacities to provide wide area economic and reliability benefits. They may not be flexible enough to address the power and voltage fluctuations happening at different locations due to local cloud movements or variation of wind speeds.

A trend of decentralization on the bulk grid is also closely coupled with the changing nature of demand. With the advances of technology and increasing demands for a cleaner environment, electric cars become more popular. They could show greater impact on the residential demand, for instance, through load distribution, and the peaks and valleys of the demand. The load profile will be more complicated, which may highly depend on the charging behaviors of EV owners. It certainly will cause more concerns for the operation of TPS, which is likely required to be more flexible and robust.

To address these challenges, power companies turn to advanced grid monitoring and control technologies for help. The following industry trends are:

• Wide-area situational awareness - When a TPS is often operating close to the stability limits, the benefits of wide area monitoring and control systems (WAMAC) are more apparent. WAMS aim to provide frequent synchronized measurements to improve situational awareness and coordination of regional control. Development and deployment led by NASPI (North America Sychro Phasor Initiatives) members explore benefits of PMU and WAMS on voltage stability, state estimation, operation and modeling.
• Modular FACTS (M-FACTS) solutions - As it becomes more difficult to add new lines and substations to the TPS as well as accurately predict reinforcement needs, utilities are turning to advanced, modular solutions. These solutions augment the increased transfer capability and reliability of FACTS with the ability to deploy incrementally and redeploy at will. Emerging technologies in this space include the PowerLine Guardian (also known as the distributed series reactor (DSR)), and the continuously variable series reactor (CVSR). Some of these technologies provide additional benefits such as the lack of a single point failure, rapid installation and ability to redeploy on facilities of different voltage and current ratings. Previous installations, funded by the DOE ARPA-E GENI program, show promising results after many years of field experience. For example, PowerLine Guardians have been deployed at TVA since October of 2012.
• Sustainable de-batching of mega-grid - Microgrid and distributed energy resources (DERs) are also seen as promising to cancel out peak load and facilitate renewable energy integration. Campus, military and large commercial sites have demonstrated promising applications around the world and show great potential. In Japan, a few cities have successfully integrated solar PV with residential households, ground transportation and water purification systems, which enables detachable operation from the commercial grid.
• Large-scale renewable integration and resource optimization - In countries where TPSs are under tremendous development and expansion, such as China and India, the concept of Global Energy Internet (GEI) is seen as the future direction. GEI was proposed to expand power systems to an internet of energy resources and loads, where end users have an equal opportunity to plug and play. GEI further advances and promotes the optimization of energy generation and consumption globally through large-scale renewable generation and ultra-high voltage power interconnections across continents.
• Data-driven diagnosis and maintenance - With the increasing availability of advanced sensors, asset owners and operators find it more affordable to have more intelligent electronic devices (IEDs) and sensing devices in their equipment, transmission lines, and substations. When the granularity and coverage of sensors are enhanced, data analysis has become a highly efficient tool to assess the health of critical infrastructure and drive maintenance activities. This may enable the migration from traditional time based maintenance to a more effective condition-based maintenance.
• Multi-disciplinary collaboration and innovation - All of the above technical innovations can have significant impacts on TPSs and may lead to advancements of the power system information and communication technologies. There are various ongoing initiatives among power companies to redesign the architecture of their existing IT systems, to address the challenges introduced by devices such as phasor measurement units and smart meters. These initiatives are facilitated by powerful tools and cross-disciplinary knowledge including artificial intelligence, deep learning, parallel computation and data analytics.

The utility industry is experiencing an exciting evolution of the TPSs, driven by innovations in control, power electronics, communications, and information technologies. The orchestration of IT, communication, data management and analysis will eventually become key enablers for a reliable, sustainable and flexible transmission grid.

For a downloadable copy of  May 2016 eNewsletter which includes this article, please visit the IEEE Smart Grid Resource Center.

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