Publication: Western Electrician
Chicago, IL, United States
Single-Phase Power Transmission in
A hydro-electric power plant has been installed recently at Rotorua, in New Zealand, which possesses some features of interest. The town of Rotorua is situated in the North Island of New Zealand on Lake Rotorua. It decided to install a modern system of sewerage and drainage, and a steam pumping plant was designed for this work. Attention was drawn to the possibility of utilizing available waterpower in the vicinity to generate electricity for the purpose, and as a result of the investigation which followed the present pumping station is operated by electric power instead of steam.
The power house, shown in the accompanying illustration, is situated on Kaituna River, 13 miles from Rotorua. A succession of small cascades occurs in the river there, from which, if the total head were used, many thousands of horsepower could be obtained. As the present needs are small, only two of these falls were utilized, the added bead of the two being 14 feet, and the horsepower developable being 200. This is considerably more than is used by the pumping station, but the town uses part of the surplus for light and power purposes.
A single-phase alternating-current system of generation was adopted, with a machine voltage of 4,000 A drop of 1,000 volts is allowed on the line, and the current is transformed down at the pumping station in the town to 100 volts. This line loss is considerable, but the available power is so much in excess of what is used that it is not of material consideration, at least for the present.
The intake for the flume (see cut) is taken off just above the first of the two falls used. The flume is calculated to carry 10,000 cubic feet of water per minute, with a velocity of two feet per second, the low fall necessitating a large quantity of water. When the flume is full it contains about 700 tons of water. The water is conducted from the flume to the turbines by riveted iron pipes five feet 10 inches in diameter.
Two turbines, made in Scotland, of the single-discharge pattern, with horizontal shafts, are used. The wheels are 40 inches in diameter, of cast iron with steel shafts. They are rated to give 100 brake horsepower at a speed of 117 revolutions per minute. The governors are King's patent pattern; they are fixed close to the wall behind; the turbines. As long as the speed of the turbine is below the normal, the governor will continue opening the gate. In the event of the speed being kept low, owing to the overloading of the alternator, this might lead to serious results by overwinding the gate. To prevent this a somewhat ingenious arrangement is fitted to the governor. It consists of a train of wheels driven by the main operating shaft, and they operate a screw and sliding block, which by the time the gate is full open, comes in contact with a small lever connected with the main lever and belt fork, and holds it in the normal position, keeping the belt on the center loose pulley, and thus preventing any further movement of the gate by depriving it of its actuating movement. These governors act on a six per cent. variation of speed.
Each turbine is belted to a 50-kilowatt alternator of the Mordey inductor type, in which all the coils and wires are stationary, the revolving part, or rotor, consisting merely of a steel casting. These alternators are wound to give the full output at any pressure between 3,200 and 4,200 volts. They are run at constant speed, and the pressure variation is obtained by varying the field excitation. This variation of pressure is necessary in order to compensate for the varying loss in the line, depending upon the load, and to maintain a constant voltage of 3,000 at the town end. The exciters are direct-connected to the shafts of the generators, and are of the two-pole type, with slotted armature.