RENEWABLE ENERGY Unlocking the benefits of micro hydro power
If implemented correctly, micro hydro power plants can give communities affordable access to renewable energy, with minimal environmental impact. However, regulating their voltage and frequency output well enough for safe and reliable connection to the grid creates challenges. This article describes a project that uses PID control as a solution to this issue.
Under the right circumstances, micro hydro power (MHP) can make a sustainable and valuable contribution to a region’s electrical power requirements. It can provide energy reliably and efficiently with reasonable consistency. Small-scale operation is viable, with flow rates as low as two gallons a minute, or a drop of just two feet, being sufficient for useful operation. It can supply energy continuously, peaking in winter, when demand is at its highest. MHP operates as a ‘run of the river’ system, meaning the river water passes directly through the generator, without need for a reservoir and associated environmental impact.
MHP is typically versatile, durable, and low cost to build and maintain, allowing successful implementation by small, remote communities in developing countries. However, MHP can also have disadvantages and factors to be considered before commissioning a project. Although environmental impact is minimal, some stream water will be diverted through the generator, so consequences to local ecology or civil infrastructure must be checked and mitigated. Further issues arise if the plant is intended for connection to the grid. Grid connection requires compliance to strict standards for safe and effective operation, and a need for well-regulated voltage and frequency. Fulfilling this can be challenging, as rainfall levels worldwide have been erratic for several years, and are predicted to remain so; this means that water supply to a microhydro plant will vary over time.
Accordingly, much research has been done to find answers to some or all of these issues. One interesting project is described in a paper titled ‘A Control Scheme for Variable-Speed Micro-Hydropower Plants’, published in the MDPI Sustainability Journal, November 21, 2018, by Youping Fan, Dai Zhang, and Jingjiao Li of the School of Electrical Engineering, Wuhan University, Wuhan 430072, China.
Micro hydro power project design
The project described was intended to select a Pelton Wheel moving-water turbine and synchronous generator combination suitable for supplying electricity to a grid at a fixed voltage. The generator, driven by the turbine, operates at variable speed, and hence with varying voltage and frequency. Accordingly, the challenge was to design and build a control system to manage the turbine/generator performance under varying upstream water flow and electrical load conditions. In the project, the turbine mechanical output power is determined by the upstream water flow and a spear valve (an integrated valve and nozzle assembly that gives continuous water flow control), whilst the generator electrical output power is determined by the turbine output power and the electrical load.
The spear valve is used to control the generator output power at different water and load conditions. A power electronics system was designed to regulate the variable outputs and increase MHP efficacy. An algorithm was also proposed to further improve system efficiency, by optimizing the rotation rate of the permanent magnet synchronous generator.
PID control was used to control the system. The aim was to obtain two different results from controlling the turbine. The first was to apply different loads to the electrical generator, simulating different levels of electricity usage, while keeping the system at a constant frequency. The second was to apply different upstream flows, simulating different water level conditions, while keeping the system at the maximum efficiency possible (varying frequency).
The control system was designed using LabVIEW (Laboratory Virtual Instrument Engineering Workbench). A water tank with 32 m head within the University of Leicester was used to produce water flow to drive the turbine. The generator could extract electrical power while the turbine was rotating. A resistance load connected to the generator received the electrical power.
The turbine characteristics were managed by a valve, in turn driven by a stepper motor. By controlling the valve position, the turbine characteristics could be adjusted for maximum efficiency and required power output under different water conditions. The LabVIEW application was run on a PC, which was connected to a National Instruments NI-USB 6221 card; this acquired data from the system’s analogue and digital sensors, and generated signals to control the stepper motor.
In the variable-speed hydropower project, the upstream water flow is hard to predict exactly, and the generator is difficult to model accurately. Therefore, a proportional, integration and differentiation (PID) control can be used to increase the control performance. It could also ensure stability, which was crucial to the system design. Feedback control is necessary when the system includes:
- Uncertain signals arising from unknown disturbances
- Uncertain models
- An unstable plant
Tuning the PID algorithm involves optimizing the P, I, and D gain terms. Autotuning can be used to calculate these and apply them to the controller automatically. The proportional parameter P is used to increase the control response speed; the integral gain is used to adjust the oscillations. Increasing integral gain can be used to reduce the steady-state error, but can increase the overshoot. The derivative parameter D can be used to control sudden changes, but can increase the measurement noise. The derivative time is normally set to zero, since the D gain often decreases controller performance.
The Ziegler-Nichols algorithm can be used to calculate optimal PID control parameters, but it is difficult to automate. Accordingly, the project used a relay autotuning technique developed by Åström and Hägglund. It is based on the Ziegler-Nichols frequency design formula, yet it can determine ultimate PID gains automatically. In fact, the system could be switched to operate either in autotuning or in normal PID control mode.
Outcome of the project
By implementing this set of hardware, software and PID algorithms, the project succeeded in demonstrating control and maximizing system performance under different upstream water flow and electrical load conditions. Its achievements can be listed more specifically:
- 1. A stepper motor system has been designed to control the turbine valve
- 2. A LabVIEW program has been built to control the stepper motor
- 3. An autotuning PI arithmetic-based controller has been implemented to obtain optimal PI gains automatically
From a user perspective, the output electrical supply under the different upstream water flow and electrical load conditions can be controlled in various ways:
- Maintaining the frequency by automatic regulation of the turbine valve.
- Maintaining the voltage by automatic regulation of the turbine valve.
- Searching for the maximum efficiency point by automatic regulation of the turbine valve and manually entering the output current into the program
The project members have proposed further research that could build on the foundation established by the study:
- 1. An online adaptive arithmetic-based controller could be applied to the control system for variable PI gains, and then the control system could determine the time needed to obtain a new PI gain while the water flow condition fluctuates.
- 2. A search for the maximum energy integrated into the power system.
A valuable contribution to sustainability?
Research projects like this, which facilitate the viability and efficiency of micro hydro power plants could contribute to a more sustainable future. At a casual glance, hydro power of any type may appear to be renewable and green – but it is not that simple. Large scale hydro plants create emissions due to construction and site traffic. Also, if they use reservoirs, the vegetation submerged releases methane into the atmosphere as it rots. The loss of land and environment also affects local flora, fauna, and human population.
However, micro hydro plants, if designed correctly and located appropriately, can mitigate these problems while offering their renewable benefits. Their small scale minimizes disruption to the local landscape, while their ‘run of the river’ mode eliminates the need for a reservoir.