--Under development--

Mactaquac HydroElectric Plant

on the

Saint John River

NB Power , New Brunswick, Canada

In the province of New Brunswick, on the Saint John River and its tributaries, there are a number of  dams and hydroelectric power plants of which the Mactaquac is the largest.  Most of the dam is a rock- filled structure, with a clay core to prevent percolation of water through it.  The height is 55 metres and the operating head of water for power generation is about 34 metres.  xx insert air photo of dam xx   The powerhouse portion is concrete, with two sets of spillways and six penstocks to feed the six generator units.  The total electrical output capacity is about 650 Megawatts.

General

Plant tour

  These web pages will take you on an illustrated tour of the site, with details of the electrical and mechanical aspects of the installation. Clik on any of the images to to see the full sized image.

Approaches

Highway Road sign	Dam & Sluice Gates	Approaching the Mactaquac site. About 20 kilometers upstream from Fredericton the sign on the highway points to the site of the power plant.  The dam is located only about half a kilometer away from the main highway.
Dam & Power Plant	Entrance to Mactaquac site	One drives across the dam and approaches the entrance to the hydroelectric station marked by the sign (right)

Schematic of the hydroelectric plant

Hydro Plant Schematic

All hydroelectric plants have some sort of storage head pond from where water is taken through large pipes (penstocks ) through a turbine (which extracts power ) to the down stream end of the river. The schematic diagram shown here has been taken from a brochure published by the NB Power and it clearly shows the various elements of the Power plant.  Our tour will take the user along the path of power flow, as the water descends toward the turbines and exits to the tailrace, and then we go up alongside the shaft to the generator and the electrical output portion.

The Penstocks (pipes leading water to turbines from the headpond)

Penstock in foreground	Inside view at bottom of penstock	Another inside view of penstock

Six penstocks of 8.8 m (29 feet) inside diameter bring water from the headpond behind the dam down a slope of  54 m length to each of the turbines.  They have to be of solid construction to withstand the water pressure, especially at the bottom, and to provide the necessary force to make the flow direction change at the elbows.  Such a large volume of water moving at 6 m/s (20 feet/second) carries a lot of energy and momentum with it.

The inside views were obtained in the autumn of 1998 during a time of modifications, when a slice was removed from a penstock for the installation of an expansion joint.   Notice the thickness of  the concrete structure behind the worker.  The saw marks are also visible.

Scroll case and wicket gates

Shape of a scroll case around a turbine

The way in which water is fed to the turbine blades is ingenious.  Turbine efficiency is greatly increased if the entering water has a strong rotational motion or whirl.  To provide such whirl, the water enters a tapered spiral chamber known as a scroll case, which curves right around the whole unit.  Beginning at the full 8.8 m diameter, it narrows down to a slot too small for a person to crawl through.

A similar wireframe drawing at a Hydro-Quebec web site also shows the shape of this spiral. Drawing of spiral

A drawing to show a scroll case structure can also be found in the web pages of Hakan Nilsson.(Scroll case drawing at Hakan Nilsson's pages)

All around the inside of this hollow doughnut, there are stay vanes that draw the flow into the central opening, below which the turbine blades are situated.  As water is taken off at all points along the inner circumference, the diameter of the chamber tapers down correspondingly, and in this way the water speed is maintained right to the end of the scroll case.  This geometry also ensures that the flow is directed equally at all of the turbine blades to provide smoother running.

When repairs are being done, the steel-lined case is a huge, dark and dirty place, with no colour contrasts at all, almost impossible to photograph by the light of bare bulbs and the flames of  workers'  torches. The floor is a curved slope.  Water coming along at that level has to slide up over a shoulder, through the vanes and then past a set of blades known as wicket gates, which are set on a circle just inside the circle of fixed stay vanes.  The photos below show this shoulder portion of the structure. In the region shown in the left photo, the water travels away from the viewer, uphill past the vane.  In the photo on the right, water would come toward the viewer and then through the adjustable wicket gates at the left and into the runners.   Of course the whole pipe will be full of fast-moving water, and the flow at the ceiling must slip down and under a similar shoulder and into the turbine area.

Bottom of a vertical guide vane	Wicket gate at left with guide vane at right

Kaplan turbine

This type of turbine, invented by Viktor Kaplan in 1913, is commonly found in power plants operating with a medium head of water.

A Kaplan  turbine runner	Runner blade at Mactaquac

The axis is vertical and the runner is mounted in a circular chamber somewhat below the level of the scroll case and wickets.  Water entering past the top of the vanes finds its way to the portion of the blades near the axis while water from the bottom of the vanes is swept down the outer wall and encounters the tips of the blades.  Notice that the blades are curved in the usual fashion of a ship propellor, the angle being smaller at the tip than at the blade root, because the peripheral velocity is larger at the blade tip.  Thus the water can flow through with a fairly uniform velocity from the centre to the outside.

The collection of pictures posted by Martin Roth includes a helpful drawing showing the water velocities, with a large whirl in the flow just above the blades changing to a more vertical flow as the water passes through the blades. Martin Roth's Waterflow images The great loss of angular momentum in the water corresponds to the torque applied to the runner and electrical generator.  Torque times rotation rate = power output.

Note that the entire flow from a 8.8 m diameter penstock is coming past these blades.   The turbine runner diameter is 6.64 m, and the blade tips travel a few millimetres from the wall of the chamber.  The rotation rate is kept very steady at 112.5 rpm so that the system produces a reliable 60 Hertz power output.  Pitting is noticeable in these harsh conditions, but the maintenance people are using a clever process for adding steel to the walls to build the thickness back up again and reduce the gap between the runner and the casing.  It is like dentistry on a large scale.

Draft tube

Macatquac turbine installation drawings: NBPower

The flow exits the turbine and power house through a large outlet called the draft tube.  Although the structure is simple in appearance, the shape must be designed with great care in order to maximize the turbine output.  It has been known for some millenia that water emerging from an orifice can be drawn out at a greater volume flow rate if the nozzle is flared like a horn.  The tapered tube causes the water to decelerate before entering the open river.  This results in an enormous suction which creates a large pressure difference between the turbine inlet and outlet.  From the energy point of view, the enlargement of the pipe as the water reaches the river again, causes the final flow speed to be quite low, and this is consistent with the goal of  extracting energy from the water. The upper drawing is a top view of the scroll case while the lower drawing is a top view of a draft tube.  The circular part is the pipe directly below the runner.   From there it turns more than ninety degrees and changes into a more rectangular shape.  The side view in the schematic above shows this elbow.   Note that the draft tube splits into three sections at the outlet from the turbine.

Wicket gate control arms

     Having dealt with the flow of water from one end to the other, we can now examine the transfer of power up the shaft to the generator.  Above the turbine level is an accessible space where one can see the control mechanism for adjusting the angle of all the wicket gates

generator from below and above

electrical output lines

speed sensor and control rooms

hydraulic control mechanisms

dam and alkali-aggregate reaction problems-- slicing the dam!!!

Useful links