Improvements of Reliability of Micro Hydro Power Plants in Sri Lanka

Transcription

1 Improvements of Reliability of Micro Hydro Power Plants in Sri Lanka S S B Udugampala, V Vijayarajah, N T L W Vithanawasam, W M S C Weerasinghe, Supervised by: Eng J Karunanayake, Dr. K T M U Hemapala Department of Electrical Engineering University of Moratuwa- Sri Lanka Abstract: This research paper includes about improvements of reliability of micro hydro plants. The proposed solutions include techniques to eliminate existing harmonics in the system. One method is by means of filtering techniques. Other one is changing the resonance of the excitation capacitance with the inductance of the generator to a value far away from the present significant harmonics by adding capacitors and inductors. Another improvement proposed is a new Induction Generator Controller (IGC) circuit based on microcontroller. Key words: Micro hydro generation, Induction generator controller, harmonics, resonance, filters. I. Introduction Micro hydro power plants are basically small-scale hydroelectric installations which are particularly used as isolated power supply scheme for village electrification where utility power is well out of reach. In most cases induction generators are used for generation. Basically an induction motor is used as the generator. Since low maintenance and establishment cost are accepted through micro hydro plants, generally there are no governor controllers used [1]. Generation-demand balance is achieved by activating dummy load through a circuit called Induction Generator Controller (IGC). Capacitors are used to supply the excitation to the induction generators and there are some situations where these capacitors tend to burn or explode themselves and sometimes it initiates huge fires causing severe damages to the installation system. This should be considered as one of the major problems since it happens repeatedly. IGC is a power electronic device. There are several occasions where this device is also burnt. Also it is difficult to repair these devices because those should be taken back to the IGC manufacturer to figure out the problems. So during this project it is supposed to state some methods to improve the reliability of these micro hydro plants by giving some solutions to overcome these problems. This research paper contains introduction about micro hydro plants, main causes to the existing problems, the experiment done at plant, our observations, the possible solutions to overcome these problems, proposed calculation method of excitation capacitance, developed software to find the accurate calculation of excitation capacitance and some further suggestions to enhance the efficiency of the micro hydro plants. II. Methodology This project is involved with a micro hydro plant near Rath Ganga in the Rathnapura district. Power capacity of the plant is 55 kw. Per phase excitation capacitance of 385uF has been. There are three ballast loads and the switching signals from IGC to these ballast loads are given through triacs. Block diagram of the micro-hydro plant is shown by figure 1. G Excitation capacitors C Induction Generator Controller circuit Ballast load Figure 1: Block diagram of the micro hydro plant a) Experiments in the Power Plant In the process of identifying the main causes for the problems, some measurements had been taken via a power analyzer at a micro hydro plant. The power analyzer is connected to the generator output terminal, capacitor input terminal and village load output terminal respectively. In each case the load was varied and the data in the power analyzer observed. With the results it is observed that 3 rd, 5th and 7 th order of current harmonics are present in the system. The current waveform obtained IGC L O A D 1

2 from the power analyzer is shown in figure 2. It shows the distorted current waveforms. Figure 2: Current waveforms observed in power analyzer Also it could be observed that each time when the village loads decreases and ballast load is switched on through triacs; and the current harmonic contents had gone up. That means when the village load decreased, Total Harmonic Distortion (THD) increased. It can be seen that the triac switching causes harmonics. The Total Harmonic Distortion of currents observed is given in table1. So the idea of a low pass filter has been moved towards a detuned filter which would solve the above mentioned problems. Detuned filter can be developed by using shunt capacitance and inductance which suppressed only the selected harmonics. Here it is designed to filter out 3 rd, 5 th and 7 th order harmonics. The circuit of the detuned filter for eliminating fifth harmonic is shown in figure 4. Table 1: Total Harmonic Distortion (THD) in currents in three phases THD(%) I1 THD(%) I2 THD(%) I b) Design of Filter By the site measurements it could be verified that there are harmonics present in the system. It can lead to capacitor failures and other adverse effects. The idea of eliminating harmonics and providing secured protection to the complete system has been considered. Ad a low pass filter design was suggested to eliminate harmonic currents. Although it is theoretically successful, it couldn t be implemented because of some practical issues. Capacitor values in filter design are very high; so the cost is too high and the filter would be massive in size if parallel connection is made with several capacitors. The figure 3 shows the simulation of the low pass filter for fifth harmonic. Figure 4: Circuit diagram of detuned filter (only 5 th harmonic filter is shown). The capacitor and inductor values were calculated by the following equations. = = 1 Here the ω f is the fundamental frequency, ω is the harmonic frequency that is to be eliminated and Q is the reactive power compensation of the capacitor. V is the system voltage. c) Excitation capacitance Generally in the micro hydro power plants required excitation capacitance were determined by just a plug and test methodology. Capacitances are connected and continuously varied until the required voltage and the frequency are obtained. That procedure consists with many drawbacks and it is somewhat hazardous to carry on depending on the environment. Over estimation of 2

3 excitation capacitance would results in decreasing the generator efficiency as it may consume unnecessary reactive power which in turn decrease the useful active power output. Therefore it was found a necessity for calculating the exact values of required excitation capacitance while enhancing the reliability of the micro hydro plants. The equations for calculating the capacitor value are given below. = 3. Apparent power =. = = 3 So the reduced reactive power would be Q=V ω (C C ) To give that Reactive power required inductance would be; L= In order to calculate that internal impedance of the generator should be calculated and it can be done using no load test data and locked rotor test data of the generator. According to the calculations carried out, internal impedance of the generator of the second plant being visited was found to be as 0.35Ώ. So, by using the earlier equations harmonic resonance for the second plant for the actual excitation capacitance was; = 1 = 1 ( ) = = 2 The required excitation capacitance for the second plant being visited was calculated and it was found to be µf while the actual capacitance used in the plant was 385 µf. so the plant is not run at its maximum efficiency and it can be improved by effective selection of excitation capacitance according to the theoretical results obtained. d) Harmonic Resonance Mainly the power system impedance is inductive and increases with frequency, consequently the higher frequency components of current give a correspondingly greater distortion in the voltage waveform. There is an intermediate range of frequencies where the capacitive and inductive effects can combine to give very high impedance [2]. A small harmonic current within this frequency range can give a very high and undesirable harmonic voltage. This is the condition which is called resonance. This high voltage can cause over heating of the dielectric of capacitor with a risk of explosion. The size of the capacitor bank was calculated in terms of the reactive power (Qc) and the fault level at the point of capacitor connection (FL) [3]. Then resonance would occur at harmonic order; = = n= Xc Xs = =4.86 Therefore the affected order of harmonic would be 5. If the excitation capacitance is µf, then the harmonic resonance would be; = 1 = 1 ( ) =15.4 = = =6.63 Therefore the affected order of harmonic would be 7. By looking at these orders of resonance values, it can be seen that these values are near the order of the harmonic values present in the system. Therefore these orders should be shifted to higher values which are not present in the system. By introducing capacitors it can b achieved. To shift resonance order to 12, the inductor value to be added can be calculated from the equation derived earlier. = ( ) =(12) 0.35 =50.4 Ω There is risk of harmonic resonance if this number is close to a harmonic order present in the system. To avoid parallel resonance at the harmonic order n what is commonly known as detuning is applied. This is achieved by connecting an inductance in series with the capacitor bank. Q =V ω (C C ) 1 Q = π π 50.4 = VAr 3

4 f) 2.4 New Induction Generator Controller Circuit A study of the existing Induction Generator Control circuit was started and found out the problems with it. Like this the inductor value to be added for the existing capacitor value (385uF) is calculated. L = mh. e) Software implementation In order to ease the calculation process, two softwares were developed. One is to calculate required excitation capacitance for the plant and the other is to calculate the harmonic resonance of the plant. Through the first software, the requirement of excitation capacitance calculation is automated by just entering the nameplate parameters of the generator including rated voltage and currents, rated power factor, rated power. Graphical user interface of the excitation capacitance calculator is shown in figure 5. By analyzing that circuit it was able to find what circuit components are burning frequently. Those are the resistors used to send the switching signals to the switching devices through them. But the suggestion proposed here is create isolation between the control circuit and the switching device. This could be easily achieved by using optocoupler gate driving chips. These optocouplers provide required isolation and protect the control circuit from the surges. The block diagram of the IGC circuit is shown in figure 6. Figure 6: Block diagram of the IGC circuit The existing control circuit is not suitable for the systems with more than 10 kw capacities. The proposed design doesn t have that kind of problem. This new design is a microcontroller based control circuit. Normally active power is controlling by detecting the system frequency. Figure 5: GUI of the excitation capacitance calculator Instead of carrying out manual calculations, accurate and efficient result for the harmonic resonance can be obtained through the second software by easily providing the required parameters such as no load test and lock rotor test data. As discussed in earlier, by adding some inductor value, the harmonic order of the system can be moved to a certain value which is far away from the harmonics present in the system. Since the 12 th harmonic doesn t exist in the system, the software functionality was extended to find out the required inductance value so that the harmonic resonance in the system is shifted towards the 12 th order. a. Transformer g) 2.4.1Components Description of the IGC circuit 230/7.5 V transformer is used for this application. Also it gives current of 200 ma, enough to drive the circuit. b. Rectification Unit Supply of 5V DC was needed to the microcontroller as well as to the sine to square wave conversion unit. So the output of the transformer, 7.5 V AC is rectified using a diode bridge. Then that is regulated through a 7805 voltage regulator and after regulating, the ripples are removed through smoothing capacitors. So the output voltage 5V DC is now achieved. This is the supply of the micro-controller and the sine to square wave conversion unit. 4

5 c. Sine to square wave conversion Frequency is calculated by the micro-controller. The sine wave cannot be applied to the micro-controller to calculate the frequency. Therefore certainly sine wave should be converted into a square wave which has only a positive half and zero. To do this a 4-pin optocoupler HCPL 817 is used. Normally optocouplers are used for isolation purposes. But here it is used for converting sine wave to square wave. There is a light emitting diode and a photo-transistor inside the optocoupler. When the voltage applied across the diode is large enough, then that triggers the transistor. At the on state the potential at the collector will be applied to the emitter. So if 5V DC signal is applied to the collector, when the transistor is triggered on, the emitter voltage will be 5V. When the input voltage across the diode is sinusoidal, the output from the emitter will be a square wave which varies between 0 and 5V. d. Microcontroller 18F452 micro-controller has been used in this circuit. Converted square wave from the optocoupler is applied to the capture pin of the microcontroller. Then the microcontroller calculates the frequency by means of calculating the time between two rising edges of the square wave. After calculating the frequency, the switching signals are generated at the output pins of the microcontroller and these switching signals can be fed to any switching devices. By this the frequency is maintained between 49.5 Hz and 50.5 Hz. If the frequency is greater than 50.5 Hz (that means the village load is reduced), the ballast should be switched on part by part while checking the frequency. If the frequency lowers than 49.5 Hz (that means village load has increased), the ballast should be switched off part by part, while checking the frequency. So the control circuit functions according to the system frequency changes. Subsequently active power of the system is now controlled by checking the system frequency. The switching signals, which are given by the microcontroller, can not apply to the switching device directly, because the current output is not sufficient to drive the switching devices. Thus the following arrangement can be used to give the switching signals to the switching devices. Here D400 transistor is used and it operates in the ON state when the microcontroller gives the switching signal. Then switching device starts to conduct. Otherwise the switch is there in the OFF state when switching signal is absent. Depending up on the capacity of the plant number of switching devices can be used will be varied. h) Implementation of IGC The designed circuit of IGC was implemented and tested in the laboratory. Figure 7 shows the picture of the implemented IGC. And the IGC with the relay ciruit is shown in figure 8. Figure 7: Implemented IGC Figure 8: Implemented IGC with relay circuit i) Testing IGC has been tested in the laboratory in the following manner. The input must be 7.5V (rms), variable frequency sinusoidal wave. This signal has taken from the signal generator. But the current given from the signal generator is not enough to drive the microcontroller, so separate 5V dc was supplied to the circuit. To represent the ballast loads, incandescent lamps have been used. In the switching, normally open configuration of the relay has used and common leg has given to one node of the lamp, and other leg has connected to the line. The neutral was connected to the next node of the lamp. Testing arrangement is shown in Figure 9 in a block diagram. 5

6 Signal generator IGC 5 V DC Figure 9: testing arrangement of IGC in a block diagram III. Results and Conclusion After the several tests that had been done, current harmonics had been identified as the major cause for capacitor bank failures. By the analysis, it can be seen that the THD of this system exceeds the standards of limit of harmonic current in the system. So it is necessary to minimize the harmonic content in the system. Introducing a detuned filter to eliminate the selected harmonics would be a healthy solution. Since this solution incurs high cost we could not implement that also the consumers don t care about the quality of the power. Therefore it is concerned that, only to protect the capacitor banks from the harmonic currents, not the full system. If the system is producing current harmonics around resonance frequency that would cause a very high undesirable voltage and that could affect capacitors badly. The suggestion for overcome this problem by adding inductors in series with the excitation capacitor bank to pull the resonance frequency to a value that is not present in the system. So it can make sure the capacitor banks are safe. Also a new design of Induction Generator Controller is implemented (IGC) based on microcontroller. It is a very simple circuit and easy to modify and maintain. It can most effectively replace the existing IGC. Also it is cheaper than the existing one. This project has given us an opportunity to explore the most of the areas of electrical engineering as well as other engineering disciplines such as software engineering. And this project gave us the chance to deal with the industry sector and to get some new the experience. Although there were some problems like acquiring the required information from the plants for the project development and funding problems for R1 R2 R3 R4 L1 L2 L3 L4 L implementing the proposed solution, the help from N supervisors and plant owners were very helpful to us. As future improvements the IGC circuit can be modified with further protection features like overvoltage, over-current protection. The micro hydro power plant is one of the most suitable power generation methods for country like Sri Lanka which is a renewable energy. If these plants are developed with more innovative and efficient adoptions they can be utilized to fulfill majority of electricity demand requirement as green and cost effective power sources. IV. References [1] Nigel Smith, Motors as geneators for micro hydro power, 4th ed. ITDG publishing, Page [2] Integral Energy Power Quality Centre: Technical Note No.3, Harmonic Distortion in the Electric Supply System, March Page-2-3 [3] Sarath perera Harmonic Resonance and Magnification, Transactions of the Integral Energy power Quality Centre, December Page-6-7 [4] Suriadi, Analysis of harmonics current minimization on power Distribution system using voltage phase shifting concept, Thesis submitted in fulfillment of the requirements for the degree of Master of Science, June [5] J. B. Ekanayake, Induction generators for small hydro schemes. 6