Performance of rotational speed controlled small-scale head-dependent hydroelectric power plant

Abstract

Small scale or micro scale hydroelectric power production is characterized by high investment costs compared to operation costs. In addition, the repayment time is often long, more than ten years (if not subsidized heavily) which requires that a small-scale hydropower plants can be profitable only if both the investment costs and the running costs are low. The performance and efficiency of a plant is very important because having long repayment time means that the efficiency improvement of one percentage unit often means the ten percentage unit improvement in profit. The most economical way to control the flow is sometimes a two point control where generators are switched on and off depending on high water level used as control variable. The water level cannot be kept constant using this method, but the traditionally used mechanical blade angle adjustment system is too expensive in very small (approximately <100 kW) units. The new method of controlling the upper water level (head) is the use of rotational speed control of turbine, implemented by a four quadrant frequency converter. The efficiency of a 4-quadrant frequency converter is approximately 97 %, which reduces the income compared to the situation where the generators are directly connected to electricity network. However, when the head is kept constant using a frequency converter, it is not required to switch off the generator circuit breakers which have a finite life time and the head height can be kept optimal in terms of performance. In this paper, a system structure, a pilot plant installation and example calculations on the feasibility of a frequency converter driven fixed blade hydro power turbine is given.