How does electricity get from the power station to a building?
Generators produce power at around 11 to 24 kV. A generator step-up transformer raises it to transmission voltage, such as 400 kV in the UAE or 330 to 500 kV in eastern Australia, because higher voltage means lower current and lower losses over long distances. At grid substations it is stepped down to 132 kV or similar, then to MV (11 kV in most places) at primary or zone substations. MV cables run through the streets to distribution substations, where a transformer drops it to 400/230 V for the building's main LV board.
PractitionerThe voltage at each level is regulated. On-load tap changers on the primary substation transformers hold the 11 kV busbar voltage, while distribution transformers only have off-circuit taps. That is why LV voltage complaints often need a tap change at the distribution transformer and a check of the MV voltage profile along the feeder.
Radial, open ring or closed ring: which MV network is best?
It depends on how much an outage costs compared with the cost of the network:
- Radial: cheapest. It is used for rural overhead lines and small sites, where a fault leaves customers off until the repair.
- Open ring: the standard for urban underground networks. It needs only RMUs with load break switches. A fault causes a short outage on one feeder, then switching restores supply.
- Closed ring: no outage for a single cable fault, but it needs circuit breakers and unit protection at every substation. It is used for critical loads, airports and some CBD networks.
- Automated ring (FLISR): an open ring with remote-controlled RMUs, FPIs and SCADA. Fault location, isolation and service restoration happens automatically, often in under a minute.
ExpertUtilities measure reliability with SAIDI (average outage minutes per customer per year) and SAIFI (average interruptions per customer per year). Automating the open rings mainly cuts SAIDI, because most of the outage time in a manual open ring is the time to find the fault and send a crew to switch.
What does neutral earthing do?
On an MV system, the transformer star point (or an earthing transformer) can be connected to earth directly, through a resistor or reactor, or not at all. This sets how much current flows in an earth fault, the most common fault on cable networks. High fault current is easy to detect but causes more damage and higher touch voltages. Very low fault current is safer for fire and damage, but the healthy phases rise to line voltage to earth, so cables and equipment must be rated for that, and the protection has to be more sensitive.
PractitionerThe earth fault current also sets the screen rating of MV cables (see MV-502) and the earth grid design of substations (MV-508). Always get the utility's maximum earth fault current and clearance time before designing either.
Regional notes
GCC
Transmission is mainly 400 kV and 132 kV (220 kV in parts of Qatar and Oman), with 11 kV distribution in urban open rings fed from 132/11 kV or 33/11 kV substations. The GCC interconnection ties the national grids together at 400 kV. Saudi Arabia runs at 60 Hz with 380 kV transmission and 13.8 kV and 33 kV distribution.
Australia
The National Electricity Market runs 500, 330, 275 and 220 kV transmission in the east, with 132 and 66 kV subtransmission. Distribution is 11 kV and 22 kV, plus SWER lines in rural areas. REFCLs are required at specified zone substations in high bushfire risk areas of Victoria.
New Zealand
Transpower runs the 220 kV and 110 kV national grid, including the HVDC link between the islands. Lines companies run 33 kV subtransmission and 11 kV or 22 kV distribution.
Asia
India uses 765 and 400 kV transmission with 11, 22 and 33 kV distribution. Malaysia (TNB) uses 500, 275 and 132 kV with 11 to 33 kV distribution. Singapore's SP Group runs 400, 230 and 66 kV with 22 kV and 6.6 kV distribution.