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Washford Mill Hydroelectrical, a designer and installer of private hydroelectric plants, has developed a dependable, efficient and cost-effective solution based on inverter drives from Parker SSD.

When refurbishing the hydroelectric installation at Vivod Hall, a stately home in North Wales, engineers from Washford Mill 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 bidirectional inverter modules are connected back-to-back via their DC buses.

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 also has other benefits – as it is electrically bidirectional, 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 and eliminating mechanical shock problems.

Another 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 with 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.

This is the basis for the new hydroelectric installation at Vivod Hall.

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 need to source special alternators.

The engineers at Washford Mill Hydroelectrical chose Parker SSD AC890 drives for this project because of their versatility and also because of their support for encoder feedback, which makes it easier to optimise the operation of the installation to achieve maximum energy efficiency.

Parker SSD AC890 drives of the types used at Vivod Hall are available with ratings up to 1,200kW and are specifically designed for use in common DC bus applications.

They feature compact constructions and offer versatility with support for V/F, sensorless vector, closed-loop flux vector and servo-position control.

Parker Hannifin – Electromechanical and Drives

Parker Hannifin is the world’s leading diversified manufacturer of motion and control technologies and systems, providing precision-engineered solutions for a wide variety of mobile, industrial and aerospace markets. With a comprehensive portfolio of electromechanical actuators, servo motors, servo drives, operator displays, communications devices and variable speed AC and DC drives, the Electromechanical and Drives Division are uniquely placed to meet all of your motion control and drives requirements.

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