What Is Power Transmission?

Power transmission is the bulk transfer of electrical energy from generating stations to the substations that feed distribution networks and industrial customers. It operates at high voltages, typically ranging from 115 kV to 765 kV for alternating current (AC) systems and up to 1,100 kV for some direct current (HVDC) links, because higher voltage reduces the current required to deliver a given amount of power and thereby reduces resistive losses in the conductors. The transmission system forms the backbone of the electric grid, linking geographically dispersed generators to load centers that may be hundreds or thousands of kilometers away.

The physics and engineering of power transmission draw on electromagnetic field theory, circuit analysis, and control systems. It is closely related to generation planning, distribution engineering, and electricity market design, all of which constrain or are constrained by the capacity and configuration of the transmission network.

Transmission Network Architecture

A transmission network consists of overhead lines, underground cables, transformers, circuit breakers, and switchgear arranged to form a meshed or radial topology. Overhead lines carry the large majority of bulk power transmission because of their lower cost per kilometer and the relative ease of inspection and repair; conductors are typically aluminum cable steel reinforced (ACSR), suspended from steel lattice towers or concrete poles. Underground transmission cables are used in urban areas and in submarine crossings where overhead construction is impractical. The network is interconnected across control area boundaries through tie lines, allowing regions to exchange power and share reserve capacity. Power system interconnection at continental scale, as with the Eastern and Western Interconnections in North America or the synchronous areas of the European network, requires careful management of the frequency and angle relationships between all connected generators.

Load Flow and Power Factor

Load flow analysis, also called power flow analysis, is the fundamental computational tool of transmission planning and operations. It solves a system of nonlinear algebraic equations to determine the voltages, currents, and power flows throughout the network for a given combination of generation dispatch and load demand. The results identify whether any lines or transformers are operating above their thermal ratings and whether any buses are experiencing voltage levels outside acceptable limits. Power factor correction is an essential practice in transmission because reactive power, which results from inductive and capacitive elements in the network, increases line current without delivering useful work, adding to losses and limiting active power transfer capacity. Shunt capacitor banks, series capacitors, and power electronic devices such as static VAR compensators supply or absorb reactive power to keep the power factor close to unity and maintain voltage profiles within acceptable bounds.

Power Markets and Grid Operations

In deregulated electricity systems, the transmission network is the physical infrastructure over which competitive power markets operate. Generators and load-serving entities schedule energy deliveries through day-ahead and real-time markets administered by independent system operators (ISOs) and regional transmission organizations (RTOs). The IEEE Power and Energy Society overview of power markets and grid operations covers the interaction between physical transmission constraints and economic dispatch. Electric current control devices, including phase-shifting transformers and thyristor-controlled series capacitors (TCSCs), allow operators to redirect power flows on congested corridors and relieve transmission bottlenecks. As renewable generation with variable output grows, transmission planning increasingly accounts for uncertainty in generation patterns, requiring probabilistic studies in addition to traditional deterministic contingency analysis.

Applications

Power transmission engineering has applications across the electricity supply chain and adjacent industries, including:

• Long-distance bulk power transfer connecting hydroelectric and wind resources to distant load centers
• HVDC links for asynchronous interconnections and submarine cable projects
• Transmission expansion planning to accommodate new generation and load growth
• Congestion management and capacity allocation in electricity markets
• Integration of offshore wind farms through submarine transmission cables