## 2 The Overall Structure of the Control System  
Operating at constant speed, this type of system requires a minimum water flow rate as low as 30% of the nominal flow rate [26], which means such a system cannot be operated during a rainy season. This disadvantage is solved via operation in a variable speed mode. The best mode of operation for \(AST\)s is variable speed [27]. The control system works on the rotational speed of the \(AST\), increasing or decreasing its speed depending on the available flow rate. The other option is for the turbine to operate at a fixed speed and to vary the flow rate through an automated airlock. Variable speed operation of an \(AST\) offers several benefits [27]: (i) it is quieter; (ii) it is \"backlash free\"; (iii) it reduces mechanical stress and therefore extends the lifetime of the gearbox; (iv) it reduces energy consumption with dry weather flows when the \(AST\) must continue operating; and (v) it reduces load losses and therefore improves the overall efficiency of the system. A micro-hydropower plant mainly includes an Archimedes screw turbine (\(AST\)), a gearbox, a squirrel-cage asynchronous generator (\(SCAG\)), and a back-to-back dual \(PWM\) converter. Figure 2 shows the control structure of the system. This structure includes two parts: the generator-side control and the grid-side control system. The mission of the gearbox in this system is to convert the slow, high-torque rotation of the \(AST\) into a much faster rotation of the \(SCAG\). The gearbox has been identified as a sensitive part of the system, as it gives rise to a multitude of failures [28]. \(AC\) electrical generators can be classified into synchronous and asynchronous generators. In turn, asynchronous electrical generators can be classified into squirrel-cage  
Figure 2: A micro-hydropower grid-connected control system. machines and wound rotor machines. Asynchronous wound rotor generators are used with wind power plants. On the other hand, squirrel-cage asynchronous generators are used with micro-hydropower plants. Squirrel-cage asynchronous generators are often used with micro-hydropower plants given their low cost, low maintenance, high operational reliability, and high resistance to external environmental factors. A back-to-back dual \(PWM\) converter is comprised of a rectifier, an inverter, and a capacitor between both (see Figure 2). The rectifier and the inverter are made up of \(IGBT\) and these are controlled via space vector pulse width modulation (\(VPWM\)). Back-to-back dual \(PWM\) converters separate the generator power transmission process (generator-side control system) and the grid connection process (grid-side control system). The two are connected via a high voltage \(DC\) bus to ensure a constant frequency in the grid connection process while the generator operation mode is variable speed. These two control systems operate independently, but are closely linked. A generator-side control system is comprised of an \(AST\), a gearbox, a squirrel-cage asynchronous generator, an \(AC/DC\) converter, a \(PLC\), and a speed sensor. With a generator-side control system, the \(AST\) is coaxially connected to the gearbox input to provide torque, whereas the gearbox output is coaxially connected to the \(SCAG\) to provide torque. The \(SCAG\) produces three-phase \(AC\) power. This three-phase \(AC\) power is converted to \(DC\) power via the \(VPWM\) converter. Control in a generator-side control system comes from the speed of the \(AST\). This control system adjusts the \(SCAG\) speed in real time to track the maximum power output point of the \(AST\) and transfers the generated power to the \(DC\) bus with maximum efficiency. A grid-side control system is comprised of a \(DC\)-link bus, a \(DC/AC\) converter, an \(LCL\) filter, and the power grid. With a grid-side control system, the \(DC\) bus voltage is stabilised, and the \(DC\) power is converted into three-phase \(AC\) power and then integrated into the grid. Control in a grid-side control system is exercised by the \(DC\) bus voltage.