There are a number of relatively high-head hydroelectric plants in British Columbia and the Pacific coast states. In December 1952, the Wahleach Plant was placed in service by the British Columbia Electric Company, Ltd. To serve it, water is diverted through a 2-mile tunnel from Wahleach Lake to a penstock partly underground and partly on the surface of the steeply sloping Four Brothers' Mountain to a powerhouse on the left bank of the Fraser River approximately 78 miles from the river mouth. The single generating unit representing the full development operates under supervisory control from the company's central office in Vancouver, a distance of 72 miles. A transmission line, the first of its kind on the North American continent, is associated with this development, but designed for later extension to the company's Bridge River Plant, approximately 110 miles to the north. This line is designed for 345-kv operation with two parallel conductors per phase. It is presently operating at 230-kv with only one of the two conductors, but full insulation has been provided for the higher voltage and hardware is in place to permit the second conductor to be added with a minimum of change when it ts desired' to operate at 345 kv.
Commencement of Construction.—After preliminary studies had indicated the practicability of the project, detailed field studies were commenced in May 1949. A Vancouver firm of consulting engineers was employed on this work and in later phases of the project. The field studies for the location of the damsite and reservoir were completed by the end of 1949; investigation of the various sites for the components of the development was resumed in 1950 as soon as weather conditions permitted. The powerhouse was located a 1/2 mile from the site for the initial plan, and the tunnel line established from the intake to the powerhouse.
Powerhouse and Tailrace.— The substructure of the powerhouse is of reinforced concrete on rock foundations. The lowest portion of the structure is at elevation 41 feet, the center line of the turbine at 70 feet, and the generator supports at 90 feet.
The superstructure is structural steel with precast aerocrete roof and asbestos cement siding. The frame supports two 130-ton travelling electric overhead cranes. These cranes were transferred from another plant and one was returned after erection of the unit was completed. On the upstream side of the building is a steel and concrete wing housing the control equipment and low-voltage switch-gear. The main building is 87 feet long by 61 feet wide and the wing is 70 feet long by 18 feet wide.
Turbine.— The turbine is of the 6-nozzle vertical-shaft impulse type, rated to deliver 82,000 horsepower at an effective head of 1,880 feet and at 360 rpm. Efficiencies of 88 per cent at full load and 90 per cent at 3/4 load have been guaranteed, but tests have not yet been made to determine actual values. The six nozzles are supplied from a spiral casing made up of five steel castings, the first having an inlet diameter of five feet 10 inches. Diameter of each nozzle is 85/32 inches. Jet deflectors are provided to reduce the amount of water striking the buckets if load is reduced rapidly. The runner, bolted to the bottom of the 25-inch diameter shaft, has 22 buckets each 23 inches wide. Outside diameter of the runner is 1253/4 inches and pitch diameter 1051/2, inches.
The turbine is controlled by a governor which transmits the proper movement to the needle valves by a conventional servomotor mechanism through a rock-shaft linkage. The governor is actuated by a hydraulic unit mounted on the top of the generator shaft. Although this type of unit is now commonly used with steam turbines, it is comparatively new in its application in hydroelectric plants.
Generator and Transformer.— The generator is rated 75,000 kva, 0.8 lagging power factor, 13,800 volts, 3 phases, 60 cycles, 360 rpm, 60-degree-centigrade rise, and has class B insulation. It is of conventional design, weight of both turbine and generator being supported by one thrust bearing near the upper end of the shaft. The main exciter, pilot exciter, and governor control unit mounted above the thrust bearing are direct-connected to the generator shaft. Due to the relatively high speed and the small number of poles for a water wheel driven generator of this capacity, the stator has an outside diameter of only 20 feet 8 inches. To assist in maintaining rigidity, the stator is encased in a concrete enclosure 23 feet square and 16 feet high between two floors of the building. The overall height from the bottom of the turbine to the top of the generator is slightly more than 49 feet.
The stator windings are wye-connected, with the neutral completely insulated from ground. Ground detection is provided by a 25-kva 13,800/220-volt transformer, having its primary connected between the neutral and ground, and with a suitable alarm device on its secondary. The stator winding is in two parallel circuits per phase, with six line and six neutral leads brought out of the machine, permitting the use of double primary current transformers with primaries in opposition for split-phase protection. Additional current transformers on line and neutral leads are utilized for differential protection. Indication and record of temperatures within the windings are obtained from 10-ohm resistors imbedded between coils in the stator slots.
The rotor is made up of a fluted forged-steel shaft, supporting a laminated steel rim to which the field poles are in turn attached. A noncontinuous amortisseur winding is provided in the pole faces. The flutes being integral with the shaft contribute materially to its rigidity, making it possible to maintain the critical frequency well above the overspeed of the turbine. Maximum overspeed is 667 rpm. A segmental braking surface on the underside of the spider provides for bringing the machine to rest. The brake mechanisms can also be used as jacks to lift the rotor off the thrust bearing for maintenance or adjustment.
The main exciter is rated 240 kw, 250 volts, and the pilot exciter 7 kw, 250 volts. The main transformer is rated 75,000 kva, 3 phases, 60 cycles, 13.8-kv delta to 230-kv wye, with the high-voltage neutral solidly grounded. The high-voltage winding has full capacity taps at 250, 240, and 220 kv, with one of reduced capacity at 210 kv. Two forced-oil water-cooled heat exchangers external to the transformer provide the necessary cooling, each cooler having sufficient capacity to keep the transformer in operation at 75-per-cent load. The transformer, complete with oil, weighs 157 tons.
Protective Relaying.—-The generator is provided with split-phase, differential, overvoltage, overtemperature, overspeed, and current-balance protection. The neutral has an overcurrent relay in the secondary of the grounding transformer. Operation of any of these devices opens the generator circuit breaker and field circuit breaker and shuts down the unit. The main and 60-kv transformers have phase-differential and ground differential protection with overcurrent relays for stand-by.
A gas-pressure relay fitted into the cover of the main transformer operates to disconnect the transformer from the circuit if an arc or explosion should increase the oil pressure rapidly. Each of the transformers in the 13.8/60-kv bank is also equipped with this type of protection.
- For protection of the transmission line,
carrier-current type relaying is provided with a 3-zone impedance relay
on each phase, and a di
rectional
overcurrent relay for ground faults. Out-of-step blocking is provided to
prevent disconnection of the line under out-of-phase conditions,
unless the second zone of the relay comes into operation.
Transmission-line relays are presently arranged to trip the generator
circuit breaker. Transformer protective relays are arranged to trip the
generator circuit breaker and close the motor-operated grounding blade
on
the 230-kv disconnecting switch to open the circuit breaker at the
other end of the line.
Automatic and Supervisory Control. — The station is designed for operation without attendants. Supervisory control devices operated from the central control office in Vancouver, 72 miles away, function to place the automatic starting relays in operation, adjust voltage and load, and stop the machine.
The supervisory signals are sent from the central control office over a leased telephone pair to the receiving substation near Vancouver, thence by carrier current over the 230-kv transmission line to the generating station. Continuous kilowatt output is recorded over a separate telemetering channel in the central control office by a telemetering unit, which also registers the kilowatt-hour output. The unit can also be started, stopped, and controlled manually in the powerhouse, by the simple expedient of moving a control switch from the "Supervisory" to "Local Automatic" or "Manual" control position*.
Transmission Line.— Although presently operating at 230 kv, the transmission line is designed and fully insulated for operation at 345 kv, using an additional conductor per phase. Provision has been made and hardware provided for the installation of this second-phase conductor; it will be strung prior to conversion to 345 kv, which is anticipated in the course of the next 2 or 3 years.
Apart from the extra-high voltage aspect, however, there are a number of features of design and construction which are worthy of mention. The first concern the use of aerial photography for acquisition of the right-of-way as well as spacing and location of the line structures.
Initial reconnaissance of the route was made by helicopter, and aerial photography was used to make final selection; then photographs of this route were obtained to a scale of 200 feet to the inch. On these photographs were superimposed all property lines, road boundaries, the boundaries of the proposed right-of-way and legal descriptions of the property traversed. Final land survey for registration purposes followed at a later date. The right-of-way acquired is 450 feet in width, to provide for two additional similar circuits at some future date.
Concurrent with the
selection
of a suitable route was the design
and
fabrication of the towers. The conductor selected was 795,000
circular-mil 26/7 steel-reinforced aluminum cable, using a twin bundle
per phase at 18-inch centers; phase spacing was 35 feet. The conductor
was
suspended from 21 unit insulator strings, with specially designed
grading
rings attached at the lower ends. The maximum design tension
in the
conductor was one-half its ultimate strength. The maximum design
loading
was 1/2 inch of radial ice, plus 4 pounds per square foot wind
pressure
at zero degrees Fahrenheit. As the lightning incidence in this
area is
very low, ground wires were installed for only 1/2 mile at the
line terminals.
After considerable study as to the type of tower to be employed on this line, the portal type was finally selected. Fig. 26 shows a tangent tower. This type of tower offers a number of distinct advantages for this application. By the use of two masts, instead of the quadruped construction normally used, the weight of redundant steel is considerably reduced, particularly in the tower head. At extra-high voltages, this reduction becomes increasingly important. Another advantage is that the two masts offer very little obstruction to the use of agricultural equipment around the tower. This was a factor, as 59 of the 64 miles of line pass through highly cultivated farmland. The third advantage lay in the ease of erection for this type of tower.
The specification called for a standard mast to be designed to meet the requirements for tangent, angle, and dead-end towers. On angle towers, the transverse load was to be taken by internal guys, and similarly, on deadend towers the conductor tension was to be taken by guys. Thus, the mast designed for the tangent tower could be used for all towers, and only separate crossarms need be detailed. This effected a considerable saving in detailing cost and simplified erection.
The maximum line span was 1,222 feet and the minimum 514 feet. The average span was 995 feet.
Every suspension tower was to be capable of withstanding a longitudinal load due to both conductors of one phase being broken.
In order to reduce the dynamic load on the tower masts when a conductor breaks, it was decided that the crossarm should be designed to fail at 60 per cent of the actual broken-wire load, that is, a safety factor of one applied to the broken-wire load as described above. Upon failure the crossarm would swing into the line, thus reducing both the dynamic and the static load on the tower.
A total of 340 towers were constructed on this line over a period of 8 months. The great majority of towers were erected by completely assembling the masts on the ground and then erecting them by means of a mobile crane. A 2-masted tower took approximately one day to assemble on the ground and 2 hours to erect.
Twenty-three months after commencement the line was completed; it was energized at 230 kv on November 30, 1952.
М.А. Беляева и др. «Сборник технических текстов на англ. языке»