This cache was not created specifically to highlight the Takapu Road Substation but it happens to be right in your face at the chosen cache location. So it makes sense to provide a little bit of information about it.
The cache is on the walkway that leads from the car park at the end of Takapu Road, that was restored in 2022 following the completion of the Transmission Gully Motorway, to the Takapu Ridge walkway which heads into Belmont park and up to Cannons's Head, the Dress Circle, Belmont Trig and eventually Lower Hutt.
The substation is bounded by a decent fence which is there for good reason. You should obviously keep outside it.
At first glance this may just look like a colloquial alternative to "really high voltage" (like extra strong coffee) but "extra high voltage" has an actual meaning in electrical engineering although this is muddied by different standards using different voltage ranges for each term. e.g. for AS/NZS 61000, which one would expect to be used in New Zealand, defines the terms as:
| LV (Low Voltage) |
below 1 kV |
| MV (Medium Voltage) |
1k V to 35 kV |
| HV (High Voltage) |
35 kV to 230 kV |
| EHV (Extra High Voltage) |
Over 230 kV |
where kV is "kilovolts" or thousands of volts. So given that that Takapu takes in "only" 110 kV lines it is not "extra high voltage" according to AS/NZS 61000. But, given that this is a sign for the public and not any electrical engineers that happen to be walking by, the term "high voltage" perhaps didn't feel like it conveyed enough danger. It's not kidding about the danger though. You'd be well advised to keep away from the fence.
A little bit about the substation
A substation is infrastructure equipment on a power grid whose main job is to connect to the high voltage main grid and step down the voltage for distrubtion to surrounding consumers. For this reason it is also called a Grid Exit Point (or GXP). Takapu's job is to step down the 110 kV main lines carried on the nearby pylons and step it down to 33 kV for distrubution to the west coast population from about Johnsonville to Porirua. It has two transformers rated at 90 MW each. It powers further zonal substations at Waitangirua, Porirua, Tawa, Kenepuru, Ngauranga and Johnsonville which each get their own 33 kV line delivered from here. Those zonal substations step down the voltage again from 33 kV to 11 kV for regional distrubtion before finally getting stepped down one last time to 230V (between a single phase and neutral - it's 400V between each of the 3 phases) on roadside or pole transformers. Voltage stepping is trivially easy with alternating current (you just need a transformer) which is why the whole power network is AC. The Cook Straight Cable is a special beast being an underwater cable and is direct current which gets special treatment at the Haywards substation.
Takapu Road is at the junction of 4 110 kV lines which go to the Pauatahanui, Haywards, Wilton and Ngauranga substations. Interestingly the main 220 kV line that goes to Wilton and Haywards bypasses Takapu just to the south of here on its way to Bunnythorpe.
The reason for all the voltage stepping is mainly a matter of efficiency. Power loss in transmission lines is proportional to the square of the current it is carrying so by using very high voltages a given amount of power can be transmitted at lower current and therefore with lower loss. For example, by doubling the voltage (other factors permitting), the current is halved and the power loss in the cable is quartered. That is extremely significant with high power and long cables.
Each successive substation lowers the voltage and supplies the power at higher current so these lower voltage sub-networks need to be reasonably small - typically having a reach of a few km. Another important reason for stepping down the voltage to 33 kV and lower is that it then becomes practical to run cables underground which is desirable once inside urban areas.
The left picture above shows one of the 110 kV line 3-phase inputs. The red, yellow and blue markings are one (yes, there are several) of the standard phase identification colours for phases 1, 2 and 3 respectively. The middle picture is of one of the two gigantic transformers that actually do the voltage step-down. The picture on the right is a nearby pylon that carries the 110 kV into the substation. Note that each phase is well separated on the pylon (one on each side and one in the middle) and that each phase has two conductors to share the load. The conductors do not touch the pylon but are connected to the structure with long insulators. The power jumps past the pylon on the dodgy-looking loops hanging underneath the insulators.
The cache
Okay, enough about power infrastructure. The cache is on the now-reopened (since the motorway finished) track that goes past the substation. It's not quite big enough to be a regular but it is 1 litire in volume. It's as straightforward as the D-rating suggests. You do not have to leave the path and don't climb up to the fence - the cache is not up there.