MAKING READY

Evening Post, Volume CVII, Issue 84, 8 April 1924, Page 7

MAKING READY

MANGAHAO POWER NEARER

TRANSFORMER STATION AT KHANDALLAH

IN. AT 110,000, OUT AT 11,000 VOLTS.

Although, for a variety of reasons, the building of the transformer station for Mangahao power at Khandallah is somewhat behind tune-table, necessitating the temporary, storage of some of the heavier equipment on tne wharves for luck oi ready housing, the construction work proper is now well on towards completion and forms a new skyline feature to tho north, while further away again can be seen if the air is clear, the first two or three of the hundred steel lattice towers now being erected to carry the cables in from Paekakariki.' The lattice tower has several advantages^ver the wooden; fewer are required in a given distance, and it is a much easier task to handle the comparatively light steel sections over such difficult country as has to be crossed between Paekakariki and the station than.to transport heavy ironbark poles. The cost is greater, however, and consequently wooden towers win out over country t where transport problems are sasier. Tho line of wooden towers is now practically complete. The two features of the transformer station are the switchboard, repair shop, and'office building, and the great slab of concrete adjoining on the northern1 side on which : will be placed the transformers, each weighing from 28 to 30 tons when their cases are filled with oil. Tho slab is rougldy four feet in thickness, and is, for technical reasons, not reinforced, but is laid down in squares ao that the fracture lines, should Wellington some day experience '. another shake, may follow the joint lines and bring about a minimum >ot damage. In the •centre of the slab is the oil house, partly above and partly below the concrete level in which will be stored the several thousand1 gallons of oil necessary for the cooling of the six giant' transformers •which will ultimately be installed. STEPPING DOWN. The cable line-from over the hilis will carry power at the tremendous voltage of 110,000, a current pressure so great that the expert alone realises the significance of the four noughts, direct to the transformers, while leads will1 be' taken • away from the secondary windings ol the transformers at. 11,000 volts to the switchboards of the distributing gallery in the building. la. the case of break-ing-down alternating current—in which the current runs through the cable circuit first one way and1 then the other, fifty changes to the second—no whirling machinery is required, the breaking, down, or stepping down, being a result of the induction of current in a secondary winding by the, passing of dorrent through the primasy coils. The induction coil, often sold for medicinal purposes, but too often applied for the shocking of maiden aunts, is the direct opposite of the breaking-down transformer, for it steps the voltage upwards, while the transformer steps it down. The principle is the same, the different results being brought about by different' winding.'- Apart from abstruse technical considerations \ the degree of stepping down depends 'upon the relation of the number of turns in the secondary (11.000 volts) winding to the number in the primary (110,000 volts) coiling.' DIRECT ANDINDIRECT. ■ The transforming: of direct current— "which runs through the. electrical . cir- j cuit smoothly without change of direction—is another matter altogether, requiring the installation 61 complicated rotary plant, as does also the changingl of the number of alternations per second in alternating" current necessary for the linking up of the present city lighting power station,' which supplies current with an alternation of 80 cycles per second, with the new supply at 50 cycles, while the change-over is in progress. Plant of this type is now installed at the Jervois quay station, but this is purely a city matter with which the Government engineers at Khandallah are not directly concerned. ; THE DISTRIBUTION GALLERY. The actual work of\the station, then, will bs carried out in the steel-jacketed and oil-bathed transformers outside the buildings; inside the most important work will be the distribution control. Ono full floor or gallery^ will be given over to this work, and *as soon as the building construction work is completed a start Will be made with the installation of the table switchboards and controls, the power as stepped down outside being led through earthenware pipes set in the concrete slab to a cellar immediately bolow the switchboards gallery and carried tip to the controls by a couple of hundred conductors through the concrete floor. Each section of the Wellington city and suburban district will have its table and controls in this gallery, a maze oi instruments which the expert reads as a book in clear type. Kunning parallel to the gallery but on a slightly lower level is the workshop, equipped with an overhead gantry and a pit for the handling and examination of tho 30-ton transformers when these arerun in from the slab on a system of rail tracks. ' A fair idea of the immense size of the transformers is gathered when one stands in the doorway through which they will run to tho workshop; it is 24 feet high: big plant for a big job. Instrument rooms, office accommodation, and storage space are also provided in the building, which is being carried out by the Fletcher Construction Company, the installation of the plant and all technical matters being carried out by' or under the supervision of the Public Works Department. ~ . CITY SUB-STATIONS. The breaking-down of Mangahao power from 110,000 to 11,000 volts at Khandallah is merely one step on the way to consumers' premises in the -city, and from this Government transformer station underground cables will carry the power—as far as the city is concerned— to the Jervois quay sub-station, where it will be stepped down again to 230 volts or sent out again to the Newtown or other smaller sub-stations to be distributed at usable voltages in various sections of . the city. Each transformer station may be likened to the hub of a cart wheel, of\ which the distributing cables are the spokes and the ring of consumers the rim. .