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The transmission system is the high-voltage network that transports electricity across large geographic areas and connects major generators, regional substations, and neighbouring electricity systems.
Beyond simply moving electricity, the transmission system plays a central role in:
Transmission infrastructure forms the backbone of the bulk power system and must be carefully operated and planned to ensure secure grid operation.
Bulk Transmission System
The bulk transmission system is the network of high-voltage transmission lines, substations, and related equipment that transports electricity between major generation resources and regional load centres.
⚡ Electricity does not follow contractual paths—it follows the physics of the network.
Power flows through transmission networks according to electrical laws, meaning flows may distribute across multiple paths rather than along a single line.
This physical behavior creates operational challenges such as transmission congestion and requires sophisticated system coordination.
Transmission networks operate as meshed systems, meaning multiple transmission lines interconnect different regions.
This structure allows electricity to flow through multiple paths and provides redundancy in case of equipment failures.
Ontario’s transmission network includes several major voltage levels:
Higher voltages enable electricity to travel longer distances with lower losses.
Electricity flows through transmission networks according to the physical properties of the grid.
Key constraints affecting power flows include:
When a transmission line approaches its operational limit, the system operator may adjust generation dispatch to maintain safe operating conditions.
Related topic:
Dispatch
Transmission systems must operate according to strict reliability standards.
One of the most important concepts is N-1 reliability.
N-1 Reliability
The ability of the electricity system to continue operating reliably after the failure of any single major component, such as a transmission line or generator.
System operators plan and operate the grid so that the system can withstand such contingencies without widespread disruption.
These reliability standards are coordinated across North America by organizations such as the North American Electric Reliability Corporation (NERC).
Ontario’s transmission system connects with neighbouring electricity systems through interties.
Major interconnections exist with:
These interties allow electricity to be:
Cross-border electricity flows are coordinated through system operator agreements and market mechanisms.
Transmission infrastructure must be planned years in advance to support future electricity needs.
Transmission planning considers factors such as:
In Ontario, transmission planning involves coordination between:
Related topic:
Long-Term Energy Planning
Consider a situation where electricity demand rises sharply in southern Ontario.
This coordination ensures that electricity reaches consumers while maintaining safe grid operation.
➡ Next Topic:
Distribution System
This topic explains how local distribution networks deliver electricity from transmission substations to individual customers.
Last Updated: 2026-03-26