Lake Coleridge Hydro-Electric Scheme

Progress, Volume XI, Issue 8, 1 April 1916, Page 597

Lake Coleridge Hydro-Electric Scheme

By ROBT. WHITSON, Engineer.

Christchurch well supplied with Electrical Power. Three Units now in operation.

The "Water Power Act of 1903, vested the rights of all water power within the Dominion of New Zealand in the Government, and conferred on the Min-

ister of Public Works the necessary powers to construct and operate hydro-electric installations. The comprehensiveness of this Act was not fully brought home to the people of the Dominion until the opening of the initial undertaking at Lake Coleridge in November, 1914, and even now it is only those who come into close contact with the direct effects of the installation who realize that this is only the small beginning of bigger things. In 1904 the Public Works Department investigated all the available sources of power within the Dominion, gauged the rivers and instituted a system of recording the

rainfall and the flow of the most important rivers, Mr. L. M. Hancock of America at this time also reported on the general possibilities of hydro-elec-

trie development. A full report was prepared by the late Engineer-in-Chief Mr. P. S. Hay dealing entirely with the available sources of power, which has formed the basis of most subsequent investigations on the subject. The Government after much discussion decided to carry out the development works themselves in preference to leasing them, and in 1910 the “Aid to Water Power Act” came into force. A special branch cf the Public Works Department was established to carry out the work and placed under the charge of Mr. E. Parry.

We are indebted to the Department of Public Works for the photographs shown herein and we append a brief description of the various units and machinery employed. LAKE COLERIDGE Lake Coleridge is situated about sixty-five miles west of Christchurch at a height of 1,667 feet above sea level. Approximately it is eleven miles long by two broad.

Despite the height of the lake it is said never to freeze, and the total absence of driftwood and refuse round the shores make it an ideal source from which to draw power supplies. The natural outflow of the lake is at the western end, where the waters after functioning with the

Harper and Wilberforce rivers empty into the Rakaia. From this point the Rakaia river runs eastwards nearly parallel to the shore of the lake, and divided therefrom by a ridge or bar of shale and shingle varying from one and a half to two and a half miles wide. As from the western end of the lake to the point opposite the intake of the power tunnel, the distance is some sixteen miles, and the average drop is about

30 feet per mile on the river bed, the head available for power purposes works out at about 480 feet or to be exact it has come out at 485 feet. The lake has four feeder streams and the outflow by the lake stream is estimated at 160 cubic feet per second.

THE TUNNEL The water is conveyed from the lake to the top of the river bank by a concrete lined tunnel 6,970 feet long. This tunnel is semi-eliptic in shape 8 ft. x 8 ft., and has an area of 50.35 square feet on the cross section. The grade of the tunnel floor is one ft. in 1,000, or just 7 ft. fall in the total length of the tunnel. Midway along the line of tunnel a shaft was sunk to a depth of 198 ft, so obtaining two additional working faces during construction. This shaft is lined with concrete for some distance above

lake level, thus serving as an additional surge chamber to assist in taking up the fluctuations of energy, and adjusting the flow of water in the tunnel to varying conditions. The lake entrance to the tunnel is provided with duplicate gates and screening chambers, so designed that the screens are accessible at any time without cutting off the supply and so that the pressure on the gates may be equalized before raising. The tunnel outlet is provided with a surge chamber 38 ft. in diameter and with gates and screens operated on the same principle as at the inlet. The surge chamber plays an important part in conjunction with the governors and reactances of the generating sets in minimising the disturbances due to changes in the load, and enables the output to respond automatically to the requirements of the service.

From the surge chamber the main penstocks—steel pipes 52 inches in diameter and 2,730 ft. long run down to the power house on the river bank giving an effective head of 485 ft. At present only two of the ultimate four pipes are installed. These pipes branch at the lower end into two 36 inch pipes, each of these supplying one of the main turbines. A 15 inch bus pipe connecting the main penstocks at the lower ends is used to drive the exciters, and this connection further equalizes the flow in the main pipes.

TURBINES AND GENERATORS The main turbines, three of which are already installed and at work are of the Francis Type, each 2,150 horse power normal output, at 500 R.P.M. but capable of handling an overload up to 2,700 B.H.P. if necessary. The exciter wheels are of the Pelton type of 225 horse power normal output. An order has been placed for a fourth unit of 2,150 horse power capacity and tenders are at present being called for a fifth unit of 4,300 horse power and for a third pipe line. The generators at present in work are three in number, each 1,500 KW. 3 phase, 50 cycle, 6,600 volt units. The exciters are of 150 K.W. 110 volts direct current generators. In addition to the exciter generators, a battery of 56 cells of 800 ampere hour capacity is installed to insure a supply

of energy for excitation under any emergency which may arise. In the original lay out of the power house six units of equal capacity, i.c. 1,500 KW. were provided for, but we understand that owing to the prospective demands for electrical power the Department have decided to install 3,000 KW. units in the place of the last two 1,500 K.W. ones provided for. This will give an ultimate capacity of 12,000 KW. for the whole installation. The step-up transformers at

Lake Coleridge Power House, at present consist of two banks, each of these 1,500 KW. single phase oil and water-cooled transformers, raising the voltage from 6,600 to 66,000 volts for transmission. TRANSMISSION LINES The transmission lines from the power house to the main sub-station at Addington, Christchurch are some 66 miles long, and in order to obviate as far as possible breakdowns in the transmission system the two lines have been kept distinct and apart from each other. At places the distance between the lines is as much as forty .miles, and in case of accident to either line, the other one is capable of handling the entire output of the power plant. The current is transmitted at a pressure of 66,000 volts, and each transmission line consists of three alumin-

ium conductors, each 7/.135 ins. The conductors are carried on ironbark poles, on pin type insulators spaced trinangularly six feet apart. Over the majority of the transmission routes the difficulties were immense, as in addition to having to haul great weights over partly made, and in some cases altogether unmade roads, none of the bridges along the route could be used, as a weight of five tons was all they were guaranteed to stand up under. This of course meant haulage through watercourse

beds, and the making of temporary roadways, that added immensely to the difficulties of transport. The average span between poles on the transmission lines is about 400 feet, and the maximum span 1,300 feet. At the Addington sub-station the 66,000 volt pressure is reduced by two banks of step-down transformers identical with the step-up transformers at the main power house, but here they are star connected on their secondary side to give 11,000 volts, 3 phase for the primary distribution. A branch of the 66,000 volt transmission line will eventually run to Timaru and other centres, with substations at intervals transforming down to 11,000 volts, at , which voltage the current wdl be distributed throughout the province. Probably no work of the size of the Lake Coleridge installation has ever been put into commission

with greater freedom from interruption or mishap, and those temporary stoppages that have at infrequent times been necessary, have been from causes outside the main power house, and the majority of these incidents have been caused by malicious damage to parts of the transmission line system. On the cost of the installation, as far as it has gone, no great profits were to be expected, but with the installation of the fourth unit (already arranged for) the Department confidently expect that the returns will more than pay working expenses and interest on expenditure, thus leaving the returns from the two final units to be installed, for capital charges, payment of interest paid out of capital during construction, and losses on working during the preliminary stages, after winch the surplus can be applied towards making a reduction in the charges for current.

An interesting development is the possibility of retail reticulation to farmers and dairy companies in proximity to the lines of transmission, and through contracting companies, retail contracts have been already let to individual users. Having regard to adverse comments of critics, when the Government made the announcement that it intended to handle the hydro-electric proposition with its own officers and staff, one cannot but admit that the gloomy anticipations of these “experts” have proved to be without foundation, and that despite the difficulties of the undertaking, including the getting into shape of a practically new engineering section, the Government have, so far, handled the proposition in a masterly manner. The contractors for the power house at the lake site were Messrs Taylor Bros, and Moorhead of Christchurch, and it is worthy of note that the cement used throughout the erection was supplied by Messrs Reese Bros, the Canterbury agents for the Golden Bay Cement, and that every shipment was accepted as being fully up to sample. We are indebted to the Construction Engineer Mr. F. T. M. Kissel, B.Sc., A.M.1.C.E., for his courtesy in supplying data and figures quoted above.