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Small scale hydro supplies grid

Small scale hydro supplies grid
Variable speed drives from Parker SSD are at the heart of a refurbished micro hydroelectric generator deep in the Welsh countryside at historic Vivod Hall.

The regulatory framework is now in place to allow small-scale producers of electricity to export and sell their surplus power via the national grid, but transferring that power to the grid is no trivial task. Washford Mill Hydroelectrical Co - a specialist in the design and installation of private hydroelectric plant - has, however, developed a dependable, efficient and cost effective solution based on inverter drives from Parker SSD.

The biggest challenge in transferring energy from a small hydroelectric installation to the national grid is synchronisation: the power being fed to the grid must precisely match the phase and frequency of the grid itself. Achieving accurate synchronisation by conventional methods, such as controlling the speed of the alternator is, however, by no means straightforward. One of the problems is that the flow of water in a small hydroelectric installation fluctuates, which means that the speed of the turbine it is driving and hence the speed of the alternator also fluctuate. Maintaining a steady output frequency from the alternator is, therefore, virtually impossible.
 
A second challenge is to have the turbine working at its peak efficiency all the time even though the supply of water flow and pressure varies considerably requiring the turbine speed to vary with it. A third challenge is to provide smooth shock-free starting for the turbine and alternator. Simply allowing the full force of the water to reach stationary turbine blades would impose large mechanical shock loads, leading to premature - if not immediate - failure.

To address these issues when refurbishing the hydroelectric installation at Vivod Hall, a stately home in North Wales, engineers from Washford Mill Hydroelectrical decided to use Parker SSD AC890 modular inverter drives. The drives are not, however, used in the usual way to control the speed of a motor. Instead, two bi-directional inverter modules are connected back-to-back via their DC buses. In essence, one of the inverters is connected to the hydroelectric installation's generator, the other to the national grid. The generators used are induction generators.

When the induction generator is generating power, it appears to the inverter it feeds just as if it is a motor that is continuously braking. As it would in any regenerative braking system, the inverter rectifies the AC power it receives and passes it to the DC bus. The second inverter takes power from the bus and reconverts to AC, but this time with phase and frequency locked to the grid. With this arrangement, the speed of the induction generator may be varied, but the supply from the hydroelectric installation is always synchronised with the grid. 

The peak efficiency of the system may be set for different water conditions by measuring the induction generator slip between the frequency of the supply and the induction generator speed, by automatically raising and lowering the frequency by small increments the system can set its running frequency to find the most advantageous operating frequency. This arrangement has other benefits, too: as it is electrically bi-directional, it is possible to draw power from the grid to drive the induction generator as a motor. This means that it can be run up to speed when at system start up, bringing the turbine up to  speed prior to turning on the water, thereby eliminating mechanical shock problems.

Another important benefit is that the fault contribution from an induction generator inverter drive system is naturally constrained to only twice its full load current (FLC) for 150ms compared to an alternator at 20 times FLC for up to 500ms. This will often allow a larger output generator to be synchronised to the grid without placing the grid in undue additional stress at times of fault tripping. This may be particularly important in remote rural areas.

Historic interest
This is the basis for the new hydroelectric installation at Vivod Hall, but the plant has an interesting history. The original system was installed in 1922 to supply the house with electricity for lighting. At that time, it comprised two turbines driving dynamo's producing a 110V DC supply. This system operated until the 1950s, when the house was connected to the national grid, after which time the turbines gradually fell into disuse. In the 1980s, however, an attempt was made to revive it, and one of the turbines was fitted with an alternator. This delivered a mediocre performance, however, and there was no provision for exporting energy. With the growing emphasis on renewable energy sources and more attractive tariffs for energy export, interest was again shown in the installation, and Washford Mill Hydroelectrical was approached to implement the most recent upgrade.

The system still has two turbines, one of which is used purely to supply the 8kW needed to drive a heat pump and to provide hot water and lighting for the house. The second and larger of the two, which is rated at 22kW, is configured with the Parker SSD drives and has the capability to export all of its power. In this case, it proved possible to use standard totally enclosed six-pole motors as induction generators operating at frequencies from 40 to 70Hz and coupled directly to the turbine avoiding belt drives, thereby eliminating the expense and inconvenience of sourcing special alternators.

The engineers at Washford Mill Hydroelectrical chose Parker SSD AC890 drives for this project because of their versatility and their support for encoder feedback, which makes it easier to optimise the operation of the installation to achieve maximum energy efficiency. In addition, the company had previous experience with Parker SSD products, and had always found them to be reliable and easy to use, as well as offering excellent value for money.
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