Use of Induction Generators in Small Hydro Power Generation System Feeding Isolated Load in Remote Mountainous Regions of Himalayas

Abstract

Providing reliable and clean power from conventional grid in remote mountainous regions is always a challenging task due to tough geographical and climatic conditions. Renewable energy sources-based power plants such as small hydro power plants play a significant role in meeting the power requirements in these remote locations in mountainous regions. Synchronous generators are the most commonly used generators in small hydro power plants. However, with the advancement in controller technology for voltage and frequency control, induction generators are nowadays preferred in renewable energy conversion systems. Self-excited induction generators (SEIG) in small hydro power plants feeding isolated domestic loads are more suitable due to their inherent advantages as compared to conventional synchronous generators. This chapter deals with the usefulness of electronic load controller used in voltage and frequency control of self-excited induction generator used in small hydro power plant feeding isolated load in remote mountainous regions of Himalayas.

1. Introduction

To meet electricity demand of people living in mountainous terrain is always a challenging task. Due to adverse geographical conditions, the cost of laying and maintaining electrical power distribution network increases. Under such conditions, using renewable energy sources is the best option to meet their energy requirement. It will also cause less adverse impact on environment as compared to impact caused by using conventional energy sources. Advancement in technology and growing concern on environmental issues worldwide have motivated and compelled the world to maximize the use of renewable sources of energy such as wind, small hydro, solar, geothermal, tidal, biomass, ocean thermal etc. [1]. These renewable energy sources are best choice to compliments the conventional sources of energy and also to meet the growing energy demand of the people living in remote areas where use of conventional grid for power distribution is difficult. An estimated 20–25% of the global final energy consumption comes from various renewable energy sources and is increasing day by day.

Hydroelectric power is the most appropriate, cost effective and environment friendly renewable energy source suitable for providing electrical power in remote mountainous regions which actually have the huge hydro power potential for electricity generation. Especially small hydro power plants play a significant role in meeting the electricity requirement of people living in remote mountainous regions in isolated mode. In comparison to large hydro power plants, micro/pico hydro power plants are environment friendly; require less investment and easy operating with minimal impact on environment. An estimated electric power potential of more than 200,000 MW from small hydro exists worldwide, out of which around 50,000 MW has been tapped so far. India also has an estimated potential of more than 5000 MW capacity of small hydro identified through more than 2000 sites in various states of India mostly located in Himalayan region. In hydro power plants, the erratic rainfall affects the water discharge in catchment areas which further affects the generated kWh output. Most of the small hydro power plants are located in mountainous terrains. However, due to the effect of global warming on overall environment around the globe and erratic rainfall due to disturbance in environment, the output of these small hydro-electric power plants (SHEPP) is not consistent. Similar varying water input conditions exist almost in the entire mountainous region as the water flow rate depends upon the average rainfall and quantity of snow on the mountain peaks in concerned catchment area which feed the small hydro power plants. Annual power generation trend based on the data collected from a cluster of small hydro power plants in Himalayan region shows the variation in power generation against the rated capacity due to varying inputs as shown in Figure 1 [2].

Figure 1.

Annual kWh generations in 1 MW Manjhal HEPP.

Another crucial factor in deciding the appropriate technology for isolated type of pico-hydro power plants is the type of loading on the power plant. Generally the load requirement in remote locations varies as the load fluctuates as per the requirement. Daily load requirement varies during 24 hours in remote areas as most of the electrical load comprises of lighting and heating. Electrical loading profile in the rural remote areas can be best generated using the data from any 11 kV feeder feeding these regions. Figures 2 and 3 show the electrical loading pattern as the per logged data collected from various 11 kV feeders feeding the rural villages in District Chamba, Himachal Pradesh situated in the Himalayan mountainous region during winter and summer months of 2013 and 2015 [2, 3].

Figure 2.

Electrical loading pattern in remote rural areas in hilly regions.

Figure 3.

Electrical loading profile of mountainous regions.

Domestic electrical load in mountainous regions generally comprises of lighting and heating load. Apart from this there is few small industrial load too operate mostly in day times. Electrical loading pattern as shown in Figures 2 and 3, indicate that during the evening time between 6:30 PM to 10 PM, electrical load is maximum with peak loading at around 8 to 9 PM. The load is minimal during mid night to 6 AM and which is about 20% of the peak load. This loading pattern is of much importance when the rural isolated load is fed from a small hydro power plant without conventional grid and operating in isolated mode. Under these conditions, the use of appropriate power conversion technologies such as use of asynchronous generators (induction generators) [1] is useful due to their adaptability to load variation and to keep voltage and frequency within permissible limits.

Recent advancement in technology and widespread use of advanced computer based controllers in renewable energy conversion systems for power generation, have increased the use of induction generators which offer increased efficiency, better control and better co-ordination with grid based power supply systems or in isolated modes [1]. For isolated load, pico-hydro power plants involving self-exciting induction generators are cost effective and reliable and are best suited under varying input and fluctuating load conditions in mountainous remote locations. In these isolated pico-hydro power plants during off-load periods, the generated power can be used for other applications such as flour mills, water pumping [3], and heating of domestic homes etc. As the effect of water discharge variation is less on pico-hydro power plants, these small hydro power plants use uncontrolled hydro-turbines due to continuous availability of required water discharge. These small hydro power plants are installed in the remotest part of the mountainous region feeding isolated load for small populated areas having small electrical load requirement. Isolated Pico hydro power plants are useful in meeting the electricity requirements of these remote locations where supplying electrical power using conventional grid is not feasible due to tough climatic and geographical conditions.

This chapter presents the suitability of self-excited induction generators feeding isolated load in remote locations. After evaluating the performance characteristics of self-excited induction generators using MATLAB/Simulink based models, the simulated results are validated experimentally. The setup comprises of 2.2 kW, 4 pole, 415 V, 3-phase, squirrel cage induction motor operated as 3- phase self-excited induction generator (SEIG) driven by a 3.7 kW rated DC shunt motor which is acting as prime-mover. For self-excitation of SEIG, a 3-phase capacitor bank with different required ratings of capacitors in star and delta mode has been used. Experimental electrical load comprises of a 3-phase lamp load and a single/three-phase inductive load. The performance of SEIG is evaluated at no load and at different loads (balanced & un-balanced load) emulating the actual electrical load conditions in remote mountainous regions.

2. Induction generators

Induction generators are most suitable for renewable energy conversion systems involving wind & small hydro. The difference between induction motor and induction generator lies in their rotor speed. The rotor speed is more than the synchronous speed in induction generator and in induction motor the rotor speed is slightly less than the synchronous speed of the induction machine. In induction generators, reactive power is consumed rather than supplying from it. Induction generator supplies only the real power (kW) to the system to which it is connected. The kVAr required by the induction generator and loads on the system must be supplied from separate source such as capacitor banks. The induction generators are classified [4, 5, 6] as Self Excited Induction Generator (SEIG), Doubly Fed Induction Generator (DFIG) and Permanent Magnet Induction Generator (PMIG).

2.1 Self-excited induction generator

Self-excited induction generator is suitable for micro or pico hydro power plants feeding an isolated load. In stand-alone mode of operation of SEIG, the reactive power must be fed externally using capacitors as shown in Figure 4 to establish the required magnetic field. Capacitors banks are connected in parallel with the induction generator terminals. These capacitor banks are connected in either star or delta modes. When the speed of induction machine rotor exceeds the synchronous speed, the residual magnetic field in the rotor starts generating a voltage across the SEIG terminals and which is further supplemented by the capacitor current to strengthen or reinforce the magnetic field and system builds up voltage with an increasing excitation.

Figure 4.

Three phase SEIG with (a) delta connected capacitor bank, (b) star connected capacitor bank.

As the speed increases, the capacitor impedance further decreases and excitation increases. This further results in increase in generated voltage of the induction generator till the voltage is limited by saturation of magnetic circuit in the induction machine. For voltage build up, there must be some residual magnetism present in the rotor of induction generator. The required value of capacitance depends upon the actual value of kVAr required and there are various techniques to calculate the exact required value of capacitance in SEIG system [7, 8, 9, 10]. The minimum capacitance required is inversely proportional to square of the speed of rotation and to the peak value of saturated magnetizing reactance. The value of capacitance is also affected by load impedance and power factor. The maximum power output from the isolated SEIG depends upon its terminal capacitance and the speed of the generator. There must be a threshold speed called cut-off speed below which no excitation is possible at any capacitor value.

2.2 Control mechanism in SEIG

The main aim of the control strategies in self-excited induction generator is to regulate and maintain the required voltage and frequency of the generated output. This control is achieved using various types of controllers involving power electronics based devices. These controllers are called electronic load controllers. Electronic load controller in pico-hydro power plant using SEIG in isolated mode maintains the output load constant as seen by the SEIG under different load conditions. The SEIG supplies output to two loads connected in parallel. The main load is consumer load and the other load is called dump load or auxiliary load. In this dump load, power dissipated is generally wasted. However, this can be used for other useful applications such as pumping of water etc. [3]. The chopper in this ELC circuit regulates the real & reactive power so that load seen by the generator is always constant. The conventional controller has discontinuous nature of control having discrete steps in connecting and disconnecting dump load. This results in power quality problems. The advanced electronic load controllers are designed to meet the demand of various types of consumer loads which may be of linear, non-linear, balanced or unbalanced type and also maintain the power quality in the system [11, 12, 13, 14, 15, 16, 17, 18, 19].

Conventional diode rectifier based ELC circuit consists of an uncontrolled 3-phase rectifier using diodes, a filtering capacitor, an IGBT or MOSFET based chopper circuit and resistive dump load as shown in Figure 5. The uncontrolled rectifier converts generated AC voltage at terminals of SEIG into DC voltage and filtering capacitor removes ac ripples from rectified output. The chopper circuit consists of an insulated gate bipolar transistor (IGBT) or MOSFET; which is used as electronic switch operated by a close loop controller based driver circuit. The controller circuit is used to switch ON or switch OFF the IGBT to connect the auxiliary dump load in such a way that total load seen by the SEIG terminal remains constant. However, the conventional ELC suffers from power quality problems. The improved or modern electronic load controllers have the capability of harmonics elimination, load balancing and maintaining desired voltage and frequency at various operating conditions. These improved electronic load controllers consist of voltage source converters (VSC) for voltage regulation, chopper circuit and auxiliary dump load. There are various topologies of these electronic load controllers based on the type of systems such as 3-phase-3 wire or 3-phase-4 wire systems to which SEIG is connected and also on the number of switches used in the VSC. One of the improved electronic load controllers which are the combination of conventional ELC and VSC based ELC called de-coupled electronic load controller is shown in Figure 6. In this topology, ELC maintain the desired voltage & frequency like conventional controller, while the VSC serves the purpose of voltage regulation, eliminating harmonics and load balancing [16, 19].

Figure 5.

Conventional ELC for isolated 3-phase SEIG in pico-hydro power generation system.

Figure 6.

De-coupled electronic load controller for 3-phase SEIG system feeding isolated load.

3. Simulated performance evaluation of SEIG using MATLAB/Simulink

Before experimental validation of SEIG, performance characteristics of the SEIG under steady state condition were simulated using MATLAB-Simulink based model as shown in Figure 7 of a 3-phase induction machine of 2.2 kW, 415 Volts, 4-Poles rating which is actually used in real time practical application [20, 21, 22, 23].

Figure 7.

MATLAB/Simulink based model of 3 HP, 3-phase, 415 V, 50 Hz SEIG in isolated pico-hydro power generation system.

After selecting the required capacitance value in SEIG system under consideration for simulation, the capacitors are connected in delta mode in the model and various simulation tests are conducted under steady state conditions of rated voltage and speed. The different simulation conditions are created and the effect on capacitance rating is observed with variation in speed, variation in power factor, change in inductive load and various other loading conditions. Simulation results show the voltage build-up at a required capacitance and various of capacitance with speed. Results show that capacitance value increases with the decrease in power factor and also affected by the variation in connected load. For rated voltage, the value of excitation capacitance is slightly less than the value required at certain value of connected load. The terminal voltage of SEIG is affected by the change in load. As the load on SEIG increases, its terminal voltage decreases slightly. The MATLAB/Simulink based model and some of the results obtained from simulation such as developed terminal voltage and its variation at change in load, variation of capacitance at different load and power factors are shown in Figures 8–11. In the simulated model, the consumer load, controller circuit and measurement sections are shown as sub-systems [23].

Figure 8.

Generated 3-phase output voltage of SEIG.

Figure 9.

Variation in terminal voltage with change in load.

Figure 10.

Variation in terminal SEIG speed with change in load.

Figure 11.

Capacitance v/s load current curves.

4. Experimental evaluation of performance of 3-φ SEIG in pico-hydro power generation system

Simulated results are validated to prove the efficacy and suitability of self-excited induction generator feeding isolated load in pico-hydro power generation system using a hardware setup. Figure 12 shows the complete experimental set-up in the lab to perform the experiments. The main components of hardware set-up are [23]:

• 3HP, 3-phase, 415 V, 4 pole, 50 Hz squirrel cage induction motor operated as 3-phase SEIG

• 5HP, DC shunt motor acting as prime-mover and coupled with induction machine

• Capacitor banks (3-phase/1-phase)

• Control Panels for induction machine and prime mover.

• Data logging, metering equipment and power meter for measurement of voltage, current, power, frequency, power factor and total harmonics distortion (THD)

• 3-phase resistive load panel containing lamp loads on each phase.

• Inductive load of 5A, 415 V rating.

• Electronic load controller circuit.

• Resistive dump load of 2.2 kW rating

Figure 12.

Hardware setup: (a) induction machine coupled with prime-mover, (b) terminals and metering panel, (c) prime-mover motor control panel, (d) excitation capacitor bank panel, (e) load control panel (f) complete experimental set-up.

The complete experimentation for operating the 3-phase induction motor as induction generator involves running the induction machine first as induction motor for few hours to initiate the residual magnetic field. Once it is sure that residual magnetism exists in the induction machine, it is the coupled with prime mover. To emulate the pico-hydro power plant which has constant power prime-mover characteristics, the selected machine as prime-mover should supply the constant power to the induction machine. In this set up, a DC shunt motor of 5 HP rating is selected to drive 3HP (2.2 kW) induction machine and its input power is monitored under all operating conditions. Now power to prime mover machine is switched on to drive the coupled induction machine to its rated speed. Prime-mover speed is further increased to take the 3-phase induction motor speed at above the synchronous speed (1500 RPM for 4-pole machine). Induction machine is driven at 1545 RPM for certain period without connecting the capacitor bank across SEIG terminals. The measurement devices are connected in the system at appropriate places to record the SEIG parameters at various excitation capacitance values and at different speeds. Initially the self-excited induction generator is run at no load. Then the first set of excitation capacitor bank (in delta or star mode) is switched ON to generate the no-load voltage across SEIG terminals. Then the value of the connecting capacitor banks is increased in steps until the required voltage is build up in the SEIG. And when the required voltage is build-up, load is connected across the SEIG and its effect on the SEIG speed, voltage, power factor and excitation capacitance is recorded under different loading and operating conditions.

4.1 Voltage build-up process in SEIG

At speed slightly higher than the synchronous speed of induction machine, a set of excitation capacitance (6 μF/phase) is switched ON and then the generated voltage at the terminals of SEIG is observed. In this set-up, full voltage development across the SEIG terminals is recorded when 12 μF/phase excitation capacitance in delta mode is connected for 2.2 kW rated induction generator at no load. Voltage generated at no load is slightly higher than the rated voltage of SEIG, but when load is switched, the voltage falls and is brought to its rated value by varying the excitation capacitance value connected across SEIG terminals. The required value of the capacitance as shown in Table 1 can also be calculated by observing the terminal voltage and capacitor line current readings recorded during the experimentation. In this experimental process, the connected capacitance value in the capacitor bank connected in delta mode matched with the calculated value of excitation capacitance experimentally.

3-φ, star connected resistive lamp load (W) across SEIG				Capacitance/phase in delta mode (μF)	Cap. bank line voltage (V)	Cap. bank line current (A)	Cap. bank phase current (A)	Calculated value of capacitance/phase (μF)	Frequency of generated voltage	SEIG speed in RPM
Rph	Yph	Bph	Total
0	0	0	0	12	455	3.10	1.79	12.09	49.8	1525
100	100	100	300	12	430	2.89	1.67	11.9	49.8	1518
200	200	200	600	12	415	2.76	1.59	12.1	49.6	1510

Table 1.

Experimental values of excitation capacitance per phase in delta mode.

4.2 SEIG under loaded conditions

Domestic consumer electrical load in remote mountainous regions generally comprises of lighting and heating load using incandescent and compact fluorescent lamps. Apart from this; for a very short period electronic gadgets and house-hold single phase appliances are also used. To emulate these loading conditions, a three phase lamp load with multiple loading steps for each phase has been used to connect the lamp load on each phase in the experimental set-up. With greater emphasis on the maximum use of compact fluorescent lamps (CFLs) from energy conservation point of view, CFLs have also been used along with conventional incandescent lamps to evaluate the performance of SEIG. Table 2 shows the experimental recorded parameters [23].

3-φ, star connected resistive lamp load (W) across SEIG				C/ph (μF) delta mode	VLL (V)	IL(C) cap. line current (A)	Iph (load) (A)	N (RPM)	PF	F (Hz)	Prime mover (DC shunt motor) parameters
Rph	Yph	Bph	Total								VA (V)	VF (V)	IA (A)	IF (A)
0	0	0	0	6	0	0	0	1550	1	—	210	186	0.9	0.73
0	0	0	0	12	470	3.21	0	1552	1	50.8	209	185	2.5	0.70
0	0	0	0	12	415	2.66	0	1460	0.5	48.3	211	190	2.5	0.71
100	100	100	300	12	440	3.0	0.49	1539	0.90	50.3	208	184	4.3	0.69
200	200	200	600	12	425	2.82	0.95	1535	0.95	50.1	208	182	5.4	0.69
300	300	300	900	12	411	2.65	1.31	1530	0.94	49.8	207	183	6.9	0.68
340	0	320	660	12	445	3.0	1.35	1564	0.92	51.3	214	189	6.0	0.69
400	400	400	1200	12	383	2.50	1.52	1525	0.95	49.5	208	182	7.5	0.67
400	400	400	1200	12	417	2.65	1.60	1574	0.84	51.5	217	187	8.0	0.69
500	500	500	1500	12	355	2.30	1.77	1559	0.85	51	215	188	7.7	0.65
500	500	500	1500	18	415	2.7	1.9	1448	0.85	46.8	203	194	11	0.67

Table 2.

Experimental values of SEIG & prime-mover parameters under different operating conditions.

Experimental results show that a minimum value of excitation capacitance is required to build up the voltage at SEIG terminals on no load. For 2.2 kW, 4-pole SEIG, the terminal voltage built-up at 12 μF/phase in delta connected capacitor bank and it will be 36 μF/phase if connected in star connected mode [23]. With the increase in load, the SEIG terminal voltage slightly decreases along with the slight reduction in SEIG speed. This decrease in terminal voltage of SEIG due to increased load can however be balanced by increasing prime mover speed. Further increase in electrical load leads to increased requirement of excitation capacitance at rated speed. Since most of the domestic consumer electrical load is of single-phase type, 4-wire electrical distribution system is used for electrifying the villages which results in the un-balancing of load in each phase. To emulate this, unequal loading of SEIG has been done in the experimental study. The requirement of the excitation capacitance increases when inductive load is added to compensate for the lagging reactive power caused by the inductive load. Table 3 shows the SEIG parameters recorded under resistive and inductive load.

3-φ, star connected resistive lamp load (W) across SEIG				Parameters of 3-φ, star connected inductive load across SEIG			C/ph (μF) in delta mode	VLL (V) SEIG terminal voltage	IL (A) SEIG line current	IL(C) (A) cap. circuit line current	Iph (load) (A) load phase current	N (RPM)	F (Hz)
Rph	Yph	Bph	Total	VLL (V)	IL (A)	Q (kVAr)
100	100	100	300	0	0	0	12	443	3.11	3.0	0.49	1540	50.2
100	100	100	300	452	1.20	0.95	18	452	3.39	3.02	0.49	1522	49.8
200	200	200	600	438	1.15	0.89	18	436	3.35	3.1	0.98	1519	49.7
400	400	400	1200	395	1.03	0.70	18	390	3.28	2.55	1.52	1512	48.8
400	400	400	1200	427	1.11	0.83	18	427	3.4	2.90	1.50	1542	50.2

Table 3.

Experimental values of SEIG parameters under different resistive & inductive loads.

5. Conclusions

The simulated performance of 3-phase SEIG system feeding isolated load have been investigated in this chapter and results are further validated using an experimental setup. The observed results validate the usefulness and suitability of self-excited induction generators for feeding the isolated load in remote mountainous regions as it adapts well to meet the varying loading requirements and keeping the voltage and frequency of the system under permissible limits. During less load period, the wasted energy across dump load can further be utilized for other useful applications such as pumping of water to meet the potable and irrigation requirement in the hilly terrains. The vast use of these self-excited induction generator based systems can be useful to harness the maximum small hydro potential without affecting the environment much and also to meet the electricity requirement of remote Himalayan mountainous regions where distributing electricity is itself a challenging task due to harsh climatic and tough terrain conditions.

Acknowledgments

Our sincere thanks to the management of Himachal Pradesh State Electricity Board Limited to collect the daily electrical load data from their 33 kV/11 kV substation feeders feeding the remote locations presented in this paper.