EEP Basics of Subtransmission Systems (web)

A short general technical explainer (excerpt from the book Electric Power Distribution Equipment and Systems) on subtransmission systems from Electrical Engineering Portal, used to ground the wiki’s existing “no fixed voltage cutoff” claim in the general international literature rather than only the Swedish-specific regionnät regulation.

Note: the article includes Figures 1.15 (radial) and 1.16 (looped) subtransmission systems.

Key findings

• Definition: subtransmission systems are the circuits that supply distribution substations, normally themselves fed from bulk transmission lines at subtransmission substations; distribution substations are only occasionally fed directly from high-voltage transmission such as 230 kV, which makes for expensive high-side substation equipment
• Voltage levels: common subtransmission voltages are 34.5, 69, 115, and 138 kV; higher voltages carry more power with less loss over greater distances. The article also mentions 23 kV and 230 kV circuits when describing lower- and higher-voltage subtransmission practice (as utility examples, not as the common-voltage list)
• No fixed cutoff: the article says there is “much crossover in functionality and voltage” — for some utilities one system serves as both subtransmission and transmission, and neighbouring utilities differ (a 23 kV subtransmission system feeding 4 kV distribution substations at one, a 138 kV system feeding a 34.5 kV distribution system at another).
• Topology: radial is the simplest and cheapest arrangement but the least reliable (one fault can interrupt several distribution substations); redundant options include dual circuits and looped or meshed circuits. Lower-voltage subtransmission (69, 34.5, 23 kV) tends to be designed and operated like distribution lines (radial or simple loop, wood-pole construction along roads, reclosers and regulators, often no shield wire, time-overcurrent protection); higher-voltage (115, 138, 230 kV) like bulk transmission (loop or mesh, tower configurations on private right-of-way, shield wire(s) for lightning protection, directional or pilot-wire relaying from two ends); generators may or may not interface at subtransmission level, which can affect protection practices
• Construction: most subtransmission is overhead, often along roads; some higher-voltage lines use private right-of-way; some new lines go underground as solid-insulation cable costs fall. Design (and evolution) depends on how the circuit developed, present and future load, distribution circuit voltages, where bulk transmission is available, rights-of-way and economics

Relevance to existing wiki topics

• Electric Grid Structure — international grounding for the page’s existing “no fixed voltage threshold” framing (line: “the TSO/DSO boundary sitting at a specific substation type, not a fixed voltage threshold”) and for topology detail on subtransmission (the mapping of Swedish regionnät onto subtransmission, and any voltage ranges attached to it, are the wiki’s own, not the article’s)