Micro hydro

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Micro hydro in northwest Vietnam

Micro Hydro is a term used for hydroelectric power installations that typically produce up to 100 kW of power. They are often used in water rich areas as a Remote Area Power Supply (RAPS). There are many of these installations around the world, including several delivering around 50 kW in the Solomon Islands, supplying energy for small communities.

Micro hydro is frequently accomplished with a pelton wheel for high head, low flow water supply. The installation is often just a small dammed pool, at the top of a waterfall, with several hundred feet of pipe leading to a small generator housing.

In low-head installations, maintenance and mechanism costs often become important. A low-head system moves larger amounts of water, and is more likely to encounter surface debris. For this reason a Banki turbine, a pressurized self-cleaning crossflow waterwheel, is often preferred for low-head microhydropower systems. Though less efficient, its simpler structure is less expensive than other low-head turbines of the same capacity. Since the water flows in, then out of it, it cleans itself and is less prone to jam with debris.

Micro hydro systems complement photovoltaic solar energy systems because in many areas, water flow, and thus available hydro power, is highest in the winter when solar energy is at a minimum.

[edit] Frequency stability

The frequency of the alternating current generated needs to match the local standard utility frequency. Typically, the controller valves the water supply to generate a constant frequency for motors and clocks. The normal controller is a small programmable logic controller with a custom program that uses a deadband to minimize valve motion so the valve wears out as slowly as possible, while conserving water.

A grid-linked system slaves its generator to the grid by measuring current, to assure that the power is always output, so the grid never drives the turbine. The usual scheme is to measure voltage across a shunt resistor on one of the phases. The external utility's grid controller provides precision frequency controls.

An independent system usually governs its long-term frequency from an external time standard. The hydropower's AC time may vary by several seconds per hour, but over many days, it doesn't vary at all. Traditionally a caretaker would compare a simple AC clock driven by the hydropower system to a shortwave clock broadcast and adjust the mechanical governor on the hydropower system until the AC clock read the same as the broadcast for a few minutes. Over time, the result would be good. With a modern PLC-based system, the caretaker can just set the PLC's clock periodically from a radio clock, say once per week. Some more-professional systems automatically set the controller's clock from a radio clock.