. Introduction

Economic growth and greater energy utilization are essential in raising people’s living standard, as well, rural electrification using renewable and sustainable energy has a significant role in it. World Bank shows much interest in universal electricity access and clean cooking fuels through increasing the renewable energy sources [1]. Hence, the Universe focuses on the development of economically viable renewable energy for rural and remote areas [2], [3], [4], [5], [6], [7]. Among renewable energy sources, hydroelectric energy is a major energy source [8], in which micro hydropower plant (MHP) is the best suitable for rural electrification as it does not require dam and reservoir as of convention hydropower plant [9], [10], [11]. In general, MHP operates in stand-alone and it is built with either self-excited synchronous generator or asynchronous generator for continuously varying load that creates voltage unbalance and frequency fluctuations. These perturbations develop mechanical vibrations and thermal effect, that consequence to generator lifespan deterioration [12]. Therefore, control system on turbine side or generator side must be employed to regulate output voltage and frequency during load variation. As can be seen in Fig. 1, the generator output voltage and frequency can be controlled from turbine side known as mechanical control, or it can be controlled from generator side known as electrical control, or it can be controlled from both sides known as electro-mechanical control. In turbine side control system, generator voltage and frequency is controlled by regulating the speed of turbine, which is governed by controlling the water flows. These water flows are controlled through regulating the inlet valve or opening and closing the valves in multi-pipe system [13]. These hydro turbine speed governing system is carried out using electrical controllers (analog /digital) for improving the system performance. Also, various control techniques adopted to govern the turbine speed such as, gate limit control, speed level control, speed droop control, etc., [13], [14], [15], [16], [17], [18]. However, due to the less dynamic response and high expenses, the mechanical and electromechanical regulators are less preferred in MHP [19], [20], [21], [22].

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Fig. 1. Generator Output Controller.

Similarly, the generator side control system regulates the voltage and frequency by employing dummy resistors during load variation which modifies the amount of power sent to dissipation circuit. The electronic controller with resistive load is also known as electronic load controller (ELC) or dump load controller (DLC). ELC advances over mechanical control system in dynamic response, reduced cost, less complexity, and maintenance [23]. In this regard, a comprehensive review of generator output control in the perspective of controller, control techniques and power electronic configuration is discussed in this paper.

In 1980, Woodward, et.al, introduced power relay based ELC scheme in hydropower plant to regulate the voltage and frequency of generator for providing good isolation between control circuitry and the power lines [24]. Soon after, in 1984, S. Kormilo, et.al replaces power relays with phase control power electronic switches for providing smooth regulation in generation system [25]. These power electronic regulatory switches are connected in between the generator terminals and the dump load to control the generator output voltage and frequency as in [13], [21], [26]. Later on, in 1990, Bonert R, et.al reforms the power electronics circuit configuration to minimize the line distortion in standalone induction generator based ELC system [27]. Eventually, ELC schemes developed with different switches and configuration like TRIAC based regulators in [28], [29], [30], [31], [32], [33], [34], [35], [36], SCR based controlled rectifier in [21], [27], [37], [38], contactors with resistive load [39], and contractors with discreet ballast [40]. Meanwhile, in 1998, Insulated Gate Bipolar Transistor (IGBT) is introduced in ELC for chopping the uncontrolled rectified voltage [41]. Later, IGBT chopper is employed in ELC with various configurations of rectification circuit such as, three phase uncontrolled rectifiers with IGBT chopper [37], [42], [43], [44], [45], [46], [47], [48], [49], single phase uncontrolled rectifier with IGBT chopper [50], [51], 6-pulse diode rectifier with IGBT chopper [52], [53], [54], [55], 24-pulse diode rectifier with IGBT chopper [56]. Also in recent, bidirectional IGBT switches are used for chopping [57], [58], [89] and matrix converter operation [84]. Likewise, in Micro hydro generation system, IGBT switches are employed in different converter configurations such as 2-leg IGBT based voltage source converter [61], [62], 3-leg IGBT based voltage source converter [63], 4-leg IGBT based voltage source converter [64], [65], and 6-leg IGBT based voltage source converter [66].

Initially, Self-Excited Induction Generator (SEIG) is employed in MHG as it is reliable, robust and cost-effective. To regulate the output voltage and frequency of the SCIG based system, numerous controllers and control techniques are used such as analog controller [24], microprocessor based ELC with the current control algorithm [28], microcontroller based fast feedforward control [34], [37], [68], and digital signal processor based ELC [48], [69]. In addition, various control techniques are adopted to enhance the efficiency of SEIG based ELC system like, Proportional controller based ELC [70], [71], PI controller based ELC [36], [41], [43], [44], [58], [59], [72], [73] PID control [74] and harmonics elimination control using p-q control theory [75]. Later, Synchronous Generators (SG) are employed in MHP due to their high efficiency and regulation in energy production, such generator can be implemented using various controller and control strategies like, AIM 65 microcomputer [25], Proportional controller [40], [54], [55], PI controller [45], [76], Fuzzy logic control based battery charging system [77], Micro-controller based ELC [78], PLC based load controller [39], etc. Similarly, asynchronous generators with external excitation (also known as separately excited induction generator) employed in MHP adopting various control techniques such as PI based decoupled voltage and frequency controller [64], [65], [79], [80], [81], [82], PI controller based active and reactive power control [63], [66], deadbeat-current-controller-based active power filter [83], Zig-zag transformer using PI controller [61], [62], [84] integrated electronic load controller using PLL technique [85], [86], [87], automatic generation control technique [88], transformer based ELC [89], IGBT converter based ELC [90], genetic algorithm based dumped load and multivalve control [35], DSP based reactive power control [91], quasi-oppositional grey wolf optimization algorithm based control [60], [92], and other optimization control [93].

Although existing ELC is utilized for regulating the output voltage and frequency, it creates power wastage. Typical ELC required cooling facility for cooling dump load, especially, during the summer period and tropical regions, which increases system complexity and cost (installation cost and running cost) [85]. Some of these controllers are used for utility purpose [58], [59] whereas, the efficiency and lifetime of the machine is concerned, still an issue. As the capacity of the dump load resembles plant capacity, it requires more cooling system, which affects the generator’s life span. As per the survey of CIGRE [54], a major source of breakdown in hydroelectric generator is due to electrical, mechanical, and thermal stresses. Understanding these stresses are important to increase efficiency and lifespan of the generator. The lifetime of a generator depends upon environmental condition and operating stresses (electrical, mechanical and thermal). Variation of dump load with respect to the main load creates electrical and mechanical stresses, as well as, thermal stress in generator are created due to unbalance loading, continuous loading, bearing fault, etc. In existing ELC with typical dump load, afore discussed stresses are accountable. Considering this problem in existing ELC, 20 percent of dump load is reduced in the proposed strategy by increasing the reference frequency. Performance of generator and domestic appliances under reduced dump load have been experimentally investigated in this paper. Moreover, lifespan of the generator under existing and proposed strategy is estimated with their thermal effects. In addition, protection of system from load disturbances and sensor fault is embedded in the controller. Overall the paper includes; design and operation of electronic load controller with experimental arrangement and testing in Section 2, investigation of generator and domestic loads with 20 percent voltage and frequency variation in Section 3, lifetime estimation of hydroelectric generator with existing and proposed strategy is deduced in Section 4, sensor fault detection and isolation technique is implementation in Section 5, Section 6 discusses the realization of proposed strategy with result and discussions in Section 7 and concluded in Section 8.

2. Design and operation of electronic load controller

In this section, the design of an electronic load controller with hardware setup and functioning procedures is discussed.

2.2. Experimental arrangement

Experimental arrangement (Fig. 2a) consists of a 3 kVA synchronous generator (S.G) driven by 3.5 kW prime mover (DC motor drive), rating of both machines is given in Table 4. As well, the enforcement of dump load controller mainly consists of sensing unit, control unit and switching unit (Fig. 2b). Frequency of output voltage is sensed and compared with reference value to control firing angle of switches.

• i.

  Sensing Unit: The sensing circuit consists of potential transformer (VDE 0570) for measuring and providing voltage signals to zero-crossing detector circuit (ZCD). ZCD is circuited using LM741 IC for developing square wave proportional to the voltage signals. ZCD’s output is fed to phase locked loopbased frequency multiplier circuit (CD4046 & 74LS294) for higher resolution during F to V conversation. The circuit locks the rising and falling edge of input square wave signal with the falling edge of output signal for increasing frequency of output (approximately 10 times). As the phase locked loop is driven by positive signal transitions, output of ZCD block is locked with falling edge. Finally, the multiplied frequency signal is converted into a voltage signal using LM2907 IC and fed to comparator where the processed signals are compared with the reference value for getting error signal.

• ii.

  Control Circuit: error signal is amplified by PI controller and reduces the steady-state error (kp = 1.13 & Ki = 0.512), the controller gain values are obtained by trial and error tuning method. Initially, the proportional gain is tuned until the constant rate output by setting the integral term to zero. Once the obtained response is fast enough, the integral gain is tuned to reduce the steady-state error. PI controller’s output is compared with high frequency carrier wave for providing pulses to switches through driver circuits. The high frequency is generated using SG3524 IC. The non-inverting amplifier based driver circuit with opto-isolator is used to provide isolation between the signals and power circuits.

• iii.

  Switching Unit: Three-single phase rectifier (GBPC3506) with IGBT chopper(STGW40N120KD) and ACNW3190-1335 IC based opto-isolated gate driver circuit for engaging resistive load depend up on controller signal. In addition, output quantities of generator are measured and analyzed using three phase power quality analyzers (PQA – FLUKE 435) is used and input quantities of the generator (torque and speed) are measured using spring dial arrangement and tachogenerator respectively. Moreover, the surface temperature of generator under various operating conditions are measured and analyzed using a thermal analyzer (FLUKE Ti-32) and smart view software respectively.