Minigrid With Soft Grid Connection

Interconnection of Two Micro Hydro Units Forming a Mini-grid System Using Soft Connection B.Adhikary, Member, IEEE, B.Ghimire, P. Karki Depart ment of Electrical and Electronics Engineering Kathmandu Un iversity Kathmandu, Nepal

Abstract— This paper presents the soft connection between two micro-hydro units forming a Mini-grid system. It describes the operation and the design of the induction generator controller (IGC), used for the power balancing at varying consumers load as required for standalone micro-hydro generators and the synchronizer unit, used for the Mini-grid interconnection. Three phase induction motors are used as single phase generators by using C-2C capacitor configuration for supplying single phase resistive load. This paper proposes a simple soft connection scheme based on classical Zero Crossing Detector. Operation and experimental tests of IGC based Mini-grid system with soft connection is also presented. Through the results it has been shown that the proposed model is feasible for rural electrification schemes. Keywords- Induction generator, Induction generator controller (IGC), Synchronizer unit, Ballast load, Standalone system, Minigrid.

I. INT RODUCTION This decade has seen an unprecedented drive towards renewable energy sources for rural and sustainable electricity systems. The number of micro and mini hydro power system for rural electrification has been growing rapidly and has shown a trend for further growth. More and more research works and investigation are being carried out to make the Minigrid system more stable, cheap and easy to use. Extensive studies have been done on self excited induction generators operating in standalone mode [1]-[2]-[3]-[4]. On ly few are available on parallel operation of induction generators [5]-[6] and limited literatures are available on synchronization of parallel operated induction generators [5]-[7]-[8]-[9]. Different authors have specified various aspects of Mini-grid connection [10]. Past research indicate that soft starters are widely used during starting of induction motors in industrial drives to reduce mechanical stress on mechanical system and electrical stress on supply system [7]. In Mini-grid connection, induction generator requires soft connection scheme to minimize these stresses on the system during grid connection. Basically grid synchronization schemes for interconnection of renewable system are (i) Zero crossing detector based connection scheme, (ii) Phase lock loop (PLL) based connection scheme and (iii) Inverter interface for interconnection. The PLL scheme is more convenient for three phase system and inverter interface is more economical for DC sources. [9]

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Soft connection based on SCR/ TRIA C, Inverters are found on the existing literatures [9]. These soft connection schemes are extensively used for higher rating of induction generator. Such synchronization/soft connection is expensive in the sector of rural electrification and cannot be easily implemented. Direct connection or ‘hard connection’ of induction generators to the grid is possible but it results in high inrush current and switching transients which are undesirable in case of Mini-grid/weak grid [13]. This paper proposes a simple soft connection scheme for interconnection of two micro hydro units forming weak/Minigrid system. This paper also deals with design, operation and experimental test of IGC based Mini-grid system with soft connection. A. Soft Grid Connection When two or more standalone micro-hydro plants are interconnected, proper synchronization is required for generators. While connecting hydro schemes or any power production schemes with hard connection, it involves rapid flow of large transient current. Thus, care has to be taken to control this transient current flow which may put electrical stress on generator as well as grid during interconnection. These transient can cause damage of protection system as well as connected loads. To avoid such consequences, it is worthy and efficient to employ soft grid connection. The method used to synchronize two induction generators to form a Min i-grid is based on ZCD method and is also known as ‘soft connection’. For the synchronism, the generators voltage, frequency and phase must be same. After synchronization, the parallel operation of induction generators is more stable. Soft Grid Connection involves a controlled sequence to connect two or more system together. B. Induction Generator in Micro Hydro Plants Standalone systems often employ single-phase generation and distribution schemes due to low cost, easy maintenance, robustness and simp licity in protection [4]. In addition to that for small power system they are easier to regulate than three phase generators. However three phase induction generators are readily availab le in the market and used for high power generation. An induction generator consumes reactive power to magnetize the air gap and supply active power to load. The reactive power

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can be either taken fro m grid or can be fed through excitation capacitors. The choice left for standalone micro hydro plants is excitation through the capacitors. Such generators are called as self excited induction generator (SEIG). The most widely used configuration for SEIG is C-2C configuration. The schematic diagram of a C-2C SEIG system is shown in Figure 1.

kept constant. In this case, the decrease in the load resistance will result in a drop in the stator frequency to provide higher torque to match the increment in the power demand. Magnetizing current needed for induction motor used as generator can be supplied from grid or fro m the exc itation capacitors connected across the phases of the induction motor. The capacitor of required rating is connected on the leading phase and another capacitor of double rating is connected in the lagging phase otherwise the performance of generator reduces drastically. This connection is used to convert a three phase induction motor to a Self Excited single phase induction generator feeding a single phase load.

Figure 1. C-2C connection of SEIG

When the induction machine is driven at the speed higher than the synchronous speed, the residual magnetic flu x in the rotor will induce a small e.m.f. in the stator winding. The appropriate capacitor bank causes this induced voltage to continue to increase until a steady state is attained due to magnetic saturation of the machine.

Figure 2.

II. DESCRIPTION OF THE SYSTEM The proposed system, under design consists of two generators producing different power outputs. For design purpose, two MG sets are used to produce required power output. These two generators were connected to a point of common coupling to form a Min i-grid. The amount of power required is sent to the Mini-grid and remaining power is dumped in the ballast load. This mechanism was controlled by two independent IGCs. The synchronization was required for the generators to be connected to the Mini-grid, and the synchronization was done by the synchronizer unit. This is performed in by self-excited induction generators which are coupled to DC motors working as prime mover for induction generators. The consumer loads are considered by using resistive filament bulbs. The power output of both generators is fed to the same main load connected through Mini-grid. In the experiment the SEIG 1 is taken as reference and SEIG 2 is synchronized to it. Fig 3 shows the schematic diagram of the proposed Mini-grid system.

No-load magnetization characteristic and capacitive impedance line

Figure 2 shows both the magnetizing curve of an unloaded SEIG and the terminal voltage–magnetizing current characteristic of SEIG using a capacitor bank plotted on the same set of axes. The intersection of the two curves is the operating point at which the capacitor bank exactly supplies the reactive power demanded by the generator. As shown in the above figure, the no-load terminal voltage and operating frequency of the generator may be determined from this point [2]-[3]-[4]. When a SEIG is loaded, both the magnitude and frequency of the induced e.m.f. are affected by: the prime mover speed, the capacitance of the capacitor bank and the load impedance. In this paper all losses in the generator are ignored, the connected load is purely resistive and the rotor speed is

Figure 3. The Schematic diagram for mini grid Connection

fro m generator and the grid voltage. At the instant of perfect phase match the incoming generator is connected to grid.

A. IGC

Figure 4. Block Diagram of IGC

The Induction Generator Controller is basically designed to regulate the power flo w in the main load and ballast load. During the time of s mall load, IGC d iverts the power developed by generator to ballast load but at the time of full load, the ballast load is cut off and total power is supplied to the consumer load or main load. Figure 4 shows the control mechanism of the IGC. Init ially the generated voltage is tapped fro m the line and this voltage is stepped down by transformer and applied to zero crossing detectors. Zero crossing detectors are used for positive half and negative half detection of the input sine wave and produce square waves. These square waves are converted to ramp signal by Ramp Converter. Rectifier (center tapped diode rectifier) rectifies the supply ac voltage to dc voltage. This dc voltage is subtracted by a reference voltage to get the error signal (voltage) using differential amplifier. The subtracted voltage is given to PI Controller. The ramp signal and the error signal fro m PI Controller are co mpared by the comparator to give out the pulse signal to the gate of TRIAC for firing. The width of the pulse determines the firing angle of the TRIAC. Thus power to the ballast load is controlled by IGC.

The generated voltage by the SEIG is stepped down by transformer (TxF 220/12V) and converted to square wave by ZCD. Similarly, the grid voltage is step down by transformer (TxF 220/12V) and converted to square wave by ZCD. The phase of grid voltage and generated voltage is compared by phase matcher. After the phases are matched the relay is in ‘ON’ state, such that the generated voltage is fed to gird line. The phase matcher is designed by using simple XNOR logic. High signals generated are fed to the delay circuit which produces delay of 3 seconds. This time delay is determined by the resistance capacitance (RC) circuit. Thus mo mentary fluctuation of the phase and frequency doesn’t affect the grid transients. As a result once the micro hydro is synchronized this delay provides the sufficient time required by the relay to connect micro hydro to the grid without electrical stresses. The synchronization of the generators causes high stability and the transient effects are minimized. Proper synchronization of the generators limits the inrush current. III.

EXPERIM ENTS AND RESULTS

The follo wing are the experiments for the mini grid and the results obtained from the experiments. A. Experiment of SEIG This experiment is carried out for two sets of motors and generators. The experimental setup connections is shown in the figure 6.

B. Synchronizer Unit Figure 6. The Experimental Setup

Figure 5. Block Diagram of Synchronizer Unit

The synchronizer unit is basically designed to connect generators so as to feed generated power into the grid. The synchronizer unit compares the waveform of generated voltage

The experiment is done for measurement of the load current, load voltage, phase current and phase to phase voltages. A separately excited dc motor rated at 190V field voltages, 3000rp m, 1.1KW, is used as a prime mover for 3 phase squirrel cage induction motor of rating 50Hz, 0.7KW, 400V, 2A and 4 no. of poles. The induction motor is operated as induction generator with star connection. Various authors have described the method of finding the value of excitation capacitance for SEIG [2]-[5]-[6]-[7]-[8]. The value of capacitor for self excitation in this generator is found to be 8.21µF between phase ‘a’ and phase ‘b’ and 18.25µF is connected in between phase ‘b ‘and phase ‘c’. These two capacitors formed C-2C connection for self excited induction generator. The dc motor working as prime mover is run at 1620rp m. Then the corresponding phase voltages, phase currents, and

load current were noted at different load conditions with constant speed of 1620rp m. The recorded voltages and currents are shown in figure 7 and figure 8.

excitation in this generator is found to be 11.12µF between phase ‘a’ and phase ‘b’ and 18.32µF is connected in between phase ‘b’ and phase ‘c’. The graph is plotted between three phase terminal voltages and the load current; and between the phase currents and the load current are as shown in figure 9 and figure 10 respectively.

Figure 7. Load current Vs Phase Voltages of Generator 1

Figure 9. Load current Vs Phase Voltage of Generator 2

Figure 8. Load current Vs Phase currents of Generator 1

In the figure 7, the graph is plotted between 3-phase terminal voltages Vab , Vbc, and Vca; and load current IL using the terminal capacitors in C-2C configuration with star connection of stator winding of induction generator. When the load is less or light, the terminal voltages Vab , Vbc, and Vca are not equal. The voltage Vca is less than other voltages because of parallel connection of 2C in that phase. When the load increases, the terminal voltages Vab , Vbc, and Vca converge to the operating voltage i.e. 220V. Similarly, in fig. 8 graph shows plot between 3-phase terminal currents Ia, Ib , and Ic; and the load current IL . At fewer loads, the terminal currents are not equal and the current Ic is less than others due the 2C in that phase. When the load increases the terminal currents converge to the operating current i.e. 1.5A of load current. The min imu m speed for generation (voltage build up) voltage for the induction machine is 1015rpm. At this speed generated voltage as well as frequency is very low. When the load of 440W is used (60% of 750W), the generator terminal voltage is obtained to be 220V and load current to be 1.4A and frequency is found out to be 49.9Hz and the speed is 1593rp m. The similar experiment is performed for another set of SEIG. A separately excited dc motor rated at 190V field voltages, 1500rp m, 2.75KW, is used as a prime mover for 3 phase squirrel cage induction motor of rating 50Hz, 1.1KW, 400V, 2.5A and 4 no. of poles. The value of capacitor for self

Figure 10. Load current Vs Phase currents of Generator 2

B. Experiment of Mini Grid The two motor generator sets are first operated as standalone generators refer as in figure 1. The first generator is connected to the main load. The power to the ballast load and main load is controlled by IGC. This SEIG with higher rating is chosen as reference. The synchronizing unit is added to the second SEIG. Init ially the frequency of generator is below grid frequency or the phase match between both voltages is not obtained. In this case the switching relay remained open and the power generated is all diverted to the ballast load. When the generator frequency matched the frequency of grid then the trip signal is generated by synchronization unit which closed the synchronization relay. Then the generated power is added to the grid. Once the stand alone generator is connected or synchronized with grid, the operating frequency and voltage is in step and the power sharing followed the droop characteristic.

After weak grid connection if the voltage of IGC is greater than the grid voltage, then IGC co mpletely cutoffs ballast load and send total generated power to grid. If the voltage of IGC is less than the voltage of the grid, then IGC sees the voltage higher than its reference as a result it gives power to ballast load. This reduces the amount of power to be fed to grid. Both cases confirm that the power is fed to the grid as per the load demand. The experiment shows that the voltage of SEIG 1 and SEIG 2 are 232V at 50Hz and 218V at 50Hz respectively before synchronism. After the synchronization the system voltage stabilized at 222V at 50Hz. IV.

A UT HORS Brijesh Adhikary was born in Kathmandu, Nepal on April 15, 1966. He received the B.Sc. degree fro m Tribhuvan University Nepal in 1988, B.E degree fro m Karnataka University India in 1994, M.E degree fro m BITS Pilani India in 1997and currently doing PhD fro m NTNU Trondheim Norway. His research interest includes renewable energy, distributed generation and power generators.

CONCLUSION

This paper has presented Interconnection of Two M icro Hydro Units Forming a Min i-grid System Using Soft Connection. The self-excitation process of a SEIG is described by using the C-2C capacitors configuration. The design and the performance of the IGC for SEIG has been presented and verified with experimental results in the standalone operation. In addition to that the design of the synchronizer unit for two SEIGs along with two IGCs for the interconnection of the generators to form the mini grid has been presented. The performance and the results from the experiments of the IGC and the synchronizer unit for the Mini-grid connection are satisfactory. REFERENCES [1]

[13] Muhammad H. Rashid, “Power Electronics Circuits, Devices and Applications”, Prentice-Hall, India, 2005

R.C. Bansal, “Three-phase self-excited induction generators: an overview”, IEEE Transactions on Energy Conversion, vol. 20, no. 2, pp.292-299, June 2005. [2] T. F. Chan and L. L. Lai, “A Novel Excitation Scheme for a StandAlone Three-Phase Induction Generator Supplying Single-Phase Loads”, Volume 19, “IEEE Transactions on Energy Conversion”, March 2004 [3] T. F. Chan and L. L. Lai, “ Capacitance Requirements of Self-Excited Induction Generators”, Volume 8, “IEEE Transactions on Energy Conversion”, June 1993 [4] S. S Murthy, “A Novel Self-Excited Self-Regulated Single Phase Induction Generator”, Volume 8, “IEEE Transactions on Energy Conversion”, September 1993 [5] A.H. Al-Bahrani N.H. Malik “ Voltage Control of parallel operated self excited induction generators” IEEE Transactions on Energy Conversion, Vol. 8, No. 2, June 1993 [6] C.Chakraborty, S.N. Bhadra, A.K. Chattopadhayay, “ Analysis of parallel operated self–excited induction generators”, IEEE Transaction on Energy Conversion, vol 14, No.2, June 1999 [7] R. Ahshan and M.T. Iqbal and George K.I Mann, “Power resistors based soft-starter for a small grid connected Induction Generator based wind turbine”,Proceedings, 17th IEEE NECEC, November 8, 2006, St. John’s, NL. [8] R. Carlson and H. Voltolini, “ Grid synchronization of brushless doubly fed asynchronous generators in wind power systems”, 6th International conference on Electrical Engineering, ICEENG 2008, Cairo. [9] Power converters and control of renewable energy system tutorial 2, 37th IEEE power Electronics Specialists Conference PECS 2006 [10] Felix A. Farret, M. Godoy Simones, ”Integration of Alternative Sources of Energy”, IEEE Press, 2006 [11] Dr. Stephen .J. Finney,” Power electronic interface for distribution and micro generation”, Department of Electrical Engineering, University of Strathclyde, Glasgow, UK [12] R. Krishann, “Dynamic Modeling of Induction Machines”, PrenticeHall, India, 2004

Bijaya Gh imire was born in Kathmandu in 1985. He received his B.E Degree in Electrical and Electronics Engineering fro m the Kathmandu University in 2008. Since 2008 Mr. Gh imire has been with Department of Electrical and Electronic Engineering Kathmandu University. His current research interest is High Voltage Engineering, Interconnection of Renewable energy sources, Power electronics and Drives.

Paras Karki received his Bachelor of Engineering in Electrical and Electronics Engineering fro m the Kathmandu University in 2008. Since 2008 he has been working in RIDSNepal (NGO) as Research Assistant. He is working in the holistic communities development projects for rural electrification in the very remote and impoverished north western district of Hu mla through RIDS-Nepal. His current research interests are self-excited a.c. generators with reactive and capacitive load, hybrid system of PV and wind generator.