Without going into the highly complex electrical engineering behind this, there are two basic ways to connect hydro to the grid: using a fixed-speed induction generator or via a grid-tied inverter.
Fixed-speed induction generator
This is the most common method for grid connecting hydropower systems and is generally used for all turbine types except Archimedean Screws, which can use this method but can also benefit from using a grid-tied inverter because this allows variable-speed operation.
A fixed-speed induction generator is essentially identical to an induction motor, only it spins at above synchronous speed because it is being pushed around by the turbine, rather than pushing a load around.
An induction generator is grid-excited, which means that the magnetic field that must be created by the generator’s stator windings is energised by the grid. This has the advantage that by default the electricity generated must be perfectly grid-synchronised because the grid is providing the excitation. However, this does mean that if there is a power cut the excitation ceases and the generator stops working, so during a power cut the hydro system will shut down.
An inverter is a power-electronic device that can convert DC electricity into AC. In hydropower applications two inverters are actually used in this format:
This essentially makes the generator independent of the grid. The inverter connected to the grid is ‘tied’ to the grid, which effectively means it is excited by the grid in a similar way to a fixed-speed induction generator (albeit using power electronics and a control system). The inverter connected to the generator is controlled by the system controller which allows it to operate over a range of voltages and frequencies, and by doing this the generator will spin over a range of speeds – hence is ‘variable speed’. Although this means that the electricity generated by the generator will be at variable-voltage and frequency which is completely incompatible with the grid, this doesn’t matter because the rectifier in-between rectifies the ‘dirty AC’ from the generator to smooth DC, and then the grid-tied inverter produces perfect grid-synchronised AC electricity.
Sequence of events to connect hydro to the grid
The following sequence applies broadly to fixed and variable-speed hydropower systems:
- Water inlet to hydro system slowly opens, turbine starts to rotate.
- As flow rate through the turbine increases the generator begins to excite and produce AC electricity, but at a lower voltage and frequency than the grid can accept.
- The control system monitors the voltage and frequency, and adjusts to flow rate through the turbine to make it the same as the grid.
- Once within specification, the electricity waveforms of the generator and grid must by synchronised so that they are ‘in phase’; this is done by either adjusting the flow rate by a tiny amount and waiting, for by diverting a small amount of generated energy to a dump load.
- Once synchronised, a ‘contactor’ is closed (automatically) that directly connects the generator (or grid-tied inverter) to the grid – it is now ‘grid connected’.
From this point forward the hydro system is controlled by the flow available in the river and will generate as much power as possible from the available flow.
Obviously this is quite a simplistic explanation, but the process is clear. From a technical point of view a variable-speed system operates in a slightly different way, but the principle is the same.
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