Engineer's Notes: networks and substations
The open ring is the workhorse
Most urban cable networks in the GCC, Australia, New Zealand and Asia are open rings. Each substation has an RMU with two ring switches, and one switch on the ring is kept open as the NOP. A cable fault costs customers only the switching time, not the repair time.
Automation buys minutes
Manual switching takes about an hour once a crew is on site. Remote control from the control room cuts it to minutes, and automatic fault location, isolation and restoration (FLISR) to about a minute.
Plan the room early
The substation location, size and access are fixed by the utility's rules and the transformer delivery route. Agree them at concept stage: moving a substation late in design is one of the most expensive changes on a project.
Heat and oil drive the details
Transformer losses set the louver sizes or the air conditioning load. Oil volume sets the bund and fire separation. Dry-type or ester-filled transformers change both.
Frequently asked questions
What is a normally open point (NOP)?
An open switch in a ring network that splits the loop into two radial feeders. After a cable fault, the faulted section is isolated and the NOP is closed so that every substation is fed from one end or the other.
Why are urban HV networks built as open rings and not closed rings?
An open ring gives a second supply route using simple ring main units with load-break switches and simple overcurrent protection at the source. A closed ring avoids any interruption but needs circuit breakers and unit protection at every substation, which costs more.
What is SAIDI?
System Average Interruption Duration Index: the total customer minutes of interruption in a year divided by the number of customers. Network topology and how fast supply is restored after a fault have the biggest effect on it.
How big should a substation room be?
It depends on the RMU, the number and size of transformers, whether the LV board is in the same room, and the utility's clearances. Add equipment widths, the gaps between them, wall clearances and the operating aisle, then check door size, height, ventilation and oil containment against the utility's standard drawings.
How much ventilation does a transformer room need?
Enough to remove the transformer losses. For natural ventilation a common estimate is an inlet free area of about 0.18 x P / sqrt(H) square metres, where P is the losses in kW and H the height between inlet and outlet, with the outlet about 10 % larger. Hot climates often need forced ventilation.
Topics: radial network · open ring · normally open point · closed ring · unit protection · dual radial · FLISR · SAIDI · customer minutes lost · RMU · fuse-switch · HV metering · kiosk substation · substation room layout · clearances · transformer delivery · ventilation · oil bund · fire separation
Related labs
