Low Head Hydro Near Moffat

Located near Moffat in Dumfries and Galloway the original installation used three Canadian LH1000 Stream Engine turbines over vertical 150mm draft tubes. Each of the three Stream Engine turbines was in a separate compartment within the fore-bay tank. The offtake and screening was upstream of a small weir and supplied the first compartment in the fore-bay tank through two 305mm underground pipes. When the water supply was greater than the capacity of turbine No, 1, the water overflowed into the second compartment and turbine No. 2, and then into the third compartment and turbine No. 3. Whilst this arrangement was intended to accommodate a considerable variation in available flow it also caused substantial aeration of the water as it flowed over the partitions between the compartments, the Stream Engine turbines were each connected to SMA Windy Boy inverters through SMA overvoltage units.

Since the system was installed approximately 6 years ago the client has experienced several high voltage events resulting in the failure of two inverters and all three overvoltage units. In the last six months two of the Stream Engine turbines have failed with burnt out alternator windings.

Westflight were asked to survey the existing system and recommend a long term and viable solution to maintain the hydro-electric output at this location. We considered that the three compartment concept was seriously flawed in that it introduced considerable aeration to the water as well as a turbulent flow due to a high approach velocity and vertical flows within each compartment. We also considered that the use of three inverters, each with its own over-voltage protection and a separate air heater dump load was un-necessarily complicated. Our solution was to use two PowerSpout LH Pro Low Head turbines over 250mm draft tubes connected to a single Enasolar 2kW inverter which has a max input voltage of 600VDC. With the PowerSpout turbines wired for a maximum output voltage of 550VDC under “no-load” conditions (Voc) there would be no possibility of an over-voltage causing damage to either the turbines or the inverter in the future.

The PowerSpout LH Pro turbine has a unique brush assembly which rotates around the turbine inlet every five minutes and completely removes leaves and similar debris which may have become lodged and reducing the inlet flow. This makes this unit especially suited to areas where there may be leaf and pine needle fall in Autumn without the need for additional screening.

Removing the existing compartment partitions in the fore-bay tank would reduce the approach velocity to each turbine and eliminate aeration. The water level in the tank would then be controlled by cutting two spillways in the tank sides at an appropriate height for turbine operation. The new spillways needed to be sufficiently large to accommodate the maximum flows expected when the river was in spate condition.

 The internal partitions and the new spillways were gas cut (Oxy-acetylene) before a new tank floor was fitted and sealed. The new 250mm draft tubes were then installed and secured at their base using clamps and guy wires. With the new draft tubes plugged the inlet gate was opened and the water flow and spillway height assessed.

The plugs were then removed and the turbines inserted into the draft tube adaptors. With the intake gate fully open the Turbine “no-load” (Voc) was recorded at 440 VDC. The inverter was then switched on and the generation output following commissioning was stable at 1.2kW.

The client was exceedingly pleased.