Grid failure

This past week has seen snow falling where I live.  However well Western Power, the electricity distribution company for our area, has done its job of cutting back overhanging trees from the power lines, snow always brings problems.  In this past week we have had repeated power outages and I thought it worth writing a Diary entry on how a grid connected Powerspout pelton behaves in such circumstances.  

When the grid goes down, the turbine will continue to receive water and the energy of that water has to have somewhere to go,- where it goes depends on whether it is an older or newer type Powerspout and on what type of inverter is handling the interface between the turbine and the grid.

... the original "grid enabled" Powerspout, the GE 400, which is the one I have, manages the situation by diverting power which cannot pass to the grid whilst the grid is down by sending it to a small heater load which is splash cooled by the spray within the wet side of the turbine casing: 

 Diversion is controlled by an electronic control board, housed in the dry side of the casing, and this regulates the power fed to the heater element so that system voltage is kept at 380 volts DC.

The necessity of keeping voltage at this level is dictated by the inverter I have which is an early one marketed in 2011. It is unable to accept voltages higher than 400v DC.  The electronic control board locks system voltage at 380 v, just below the inverter's limit and thus keeps the inverter from being damaged by over-voltage.

... later, inverters came on the market capable of seeing an in-coming voltage considerably in excess of 400 v: - the 2 kW Enasolar inverter, for example, is quite happy with 600 v.  This development in inverter technology opened the door to managing grid outage situations in a completely different way, a way which was much simpler and did away with the need for a control board.  

Instead of diverting electrical power to a load, electrical power ceases to be created at all by keeping the system in open circuit: with no load connected what happens is that system voltage rises as the pelton runner goes to its run-away speed but, crucially, no current flows.  And so long as the system voltage rises to no more than 600 v the inverter remains safe.  What becomes important with this way of managing grid outages is that a stator must be selected which is wound in such a way that even at the highest run-away speed possible for the site (which is determined by the net head), the open circuit voltage will never exceed the inverter's limit.

You may well ask "but what happens to all the hydraulic energy in the system if none of it now finds an outlet by being turned into electrical energy?" The answer is that a lot of it never gets as far as being translated into shaft rotational energy. At the run-away speed of the pelton much of the water passes through the pelton runner, which is moving just about as fast as the water jet, without ever hitting the pelton cups.  The water ends up hitting the casing opposite the nozzle and its energy is dissipated as heat and sound. Some extra energy is also lost as heat in the bearings and shaft seal - a greater amount at the higher speed at which the shaft is revolving at run-away speed than is lost at normal operating rpm.

To illustrate some of this, here is a picture of a pelton at run-away speed showing how the water from both nozzles fails to be deflected in the normal way onto the front glazing because the velocities of both runner and jets are little different:

This pelton was coupled to an Enasolar 2 kw inverter which, at the time, was not connected to the grid; the run-away speed of the turbine which resulted was measured at 1440 rpm:

and because the stator of the SmartDrive had been carefully selected to be one which delivered just 0.266v /rpm in open circuit (it was a 100-14S-1P S stator), the open circuit voltage at runaway was measured at 383 v: well below the 600 v limit of the inverter.

So there have been two ways of managing a grid outage with a Powerspout pelton and both are good.  This past week with its numerous grid outages has reminded me just how "bomb proof" my system is: when the grid goes down, the turbine continues happily feeding power to its dump load and after the grid comes back on, seamlessly the turbine re-connects itself to the grid.  It's all clever stuff and it gives me great pleasure to see it in operation.