Anuncio

Control of an island Micro-hydropower Plant with Self-excited AVR and combined ballast load frequency regulator Guillermo Castillo, Leonardo Ortega, Marcelo Pozo, Xavier Domínguez Departamento de Automatización y Control Industrial Escuela Politécnica Nacional Ladrón de Guevara, E11-253. Quito-Ecuador {guillermo.castillom, leonardo.ortega, marcelo.pozo, xavier.dominguez}@epn.edu.ec Abstract—This project describes the design and construction of an Automatic Voltage Regulator (AVR) and an Electronic Load Controller (ELC) for the voltage and the frequency regulation in an island Micro-hydropower Plant (MHP). For the frequency control, the speed regulation by ballast load method has been used. To this approach, a combined binary-continuous load regulation was employed. The implemented AVR is totally self-excited by means of an energy transfer system which allows an isolated operation of the MHP. The entire system has been designed considering the current standard regulations of the Ecuadorian Agency of Electricity Control and Regulation (ARCONEL). The frequency and the voltage regulation were properly achieved through the implementation of digital PI controllers tuned based on mathematic models obtained from experimental data of frequency and voltage. The control of the system was validated by both, software simulations and field tests performed. Keywords— AVR, hydropower Plant, I. digital PI controller, ELC, Micro- simplification in turbine design, lower cost, easy operation and maintenance, avoiding water pipe overpressure and faster response to load variations [2]. The designed ELC uses ballast load regulation with a mixed strategy which consists in using both binary and continuous load regulation. In contrast, the voltage regulation has been achieved through an AVR with a self-excited voltage system. The AVR input ac voltage was rectified by a single phase half controlled rectifier to obtain dc current which was then applied to the generator field winding [1]. The proposed control structure is illustrated in Fig. 1. TURBINE GENERATOR USER LOAD PG PB FIELD WINDING ELC AVR SET POINT INTRODUCTION Various methods for frequency and voltage regulation have been studied and reported in the related literature. Frequency regulation by water flow control is presented in [3], where velocity of the turbine-generator group is controlled by regulating the water flow by means of a valve operated by a servomotor. Reference [2] presents a frequency regulation strategy which uses a combined regulation of the velocity of the turbine-generator unit by both, water flow and ballast load control. A popular method consists in using auxiliary ballast or dump loads to regulate the frequency [4]-[7]. In this approach a constant water flow and a constant load connected to the generator are required. In the present project, to regulate the frequency of the MHP, an Electronic Load Controller (ELC) has been implemented due to the benefits compared to the other methods such as 978-1-5090-1629-7/16/$31.00 ©2016 IEEE FREQUENCY MEASUREM. VOLTAGE MEASUREM. ACTUATOR Voltage and frequency regulation are key aspects in Microhydropower Plants (MHP) operation because of a relative small deviation of the control variables could cause permanent damages or lifespan reduction. Therefore, the development of low-cost, practical and robust control systems to regulate MHPs is highly important, especially in countries where the hydric resources are abundant and the conditions to install isolated or grid-connected MHPs for public and private projects are favorable [1]-[2]. PU CONTROLLER SET POINT Fig. 1. CONTROLLER ACTUATOR BALLAST LOAD Diagram of the proposed control system. Regarding the ELC, the ballast or auxiliary load was connected in parallel with the user load, thus the power consumed by the total load (ballast and user) was kept constant and equal to the power generated by the MHP (PG). This relationship is shown in (1), where PU is the power consumed by the user and PB is the power dissipated in the ballast load. PB is controlled depending on the frequency of the generated voltage. PG = PB + PU = cte (1) For the modeling of the turbine-generator unit, the response of the plant to a step input was employed. This method demonstrated to be a convenient and useful approach when the system parameters are unknown as in the case of the present project. II. SYSTEM DESCRIPTION A. Voltage Regulation The regulation of the generator output voltage is achieved by means of the variation of the voltage applied to the field winding. An ac-dc converter with half controlled bridge configuration was used to control the field voltage. Fig. 2 shows the implemented circuit to control the field excitation voltage. This scheme also includes a diode bridge to supply enough field voltage for the machine start-up process using its own excitation system. circuit that allows the load connection only when the instantaneous voltage is zero. Thus, the noise caused by current transients produced when load is not connected at zerocrossing is eliminated. Fig. 3. Diagram of mixed regulation for ballast load III. Fig. 2. Energy transfer circuit for generator field winding. Once the generator output voltage has reached a suitable value to power the control system and assure an appropriate performance of the electronic circuits as well of the power converters, the control panel is turned on. At that instant the main contactor of the system is connected and the voltage and frequency controllers initiate the operation. Simultaneously, the transfer relay is operated to switch the energy transfer from the diode bridge to the ac-dc converter. Since then, the voltage regulation is performed by the AVR. B. Frequency Regulation and load connection system for the ballast load. The proposed frequency regulation system is based in the ballast load regulation method with a combined strategy which uses three loads in discrete steps and one load in continuous regulation. Fig. 3 shows the diagram of this mixed regulation. Firstly, a continuous regulation is employed to connect a percentage of the ballast load RL4 (continuous load) so that the ballast load dissipates the surplus power from the generator which is not required by the load. When the power dissipated over RL4 is increased and a value equal to the nominal power of RL1, RL2, or RL3 (ballast loads controlled by discrete steps regulation) is reached, the power consumed by RL4 is transferred to the discrete loads. Therefore, if the power dissipated in the ballast load keeps increasing, the continuous regulation is used on RL4 until the power is enough to activate another discrete ballast load. The process could be repeated again until all the discrete loads have been connected. For the connection of both, discrete and continuous loads TRIACs were used. For the continuous load, a single phase acac converter was implemented to control the power transferred to the load by means of the trigger angle alpha (α). Whereas for the discrete loads, the TRIACs were used as solid state relays to implement an ON-OFF control with a zero-crossing detector MODELING AND CONTROL A. Plant Modeling To obtain the model of the plant, experimental data for voltage and frequency were obtained for step variations of the control variables. The AVR and the ELC were designed for a nominal power of 1600 watts in the MHP. Two models were obtained, one for the frequency plant and the other for the voltage plant. Frequency Plant Model: This model establishes the relationship between the frequency as output variable and the percentage of load connected to the generator as input variable. The test results are shown in Fig. 4. The variation range of the step input (75% to 100%) was selected considering a frequency response near the operation point of the plant (60 hertz) within the allowed range of variation. As it can be seen, the response of the plant corresponds to the behavior of a first order system. Thus, the transfer function of the system can be expressed by (2). G(s) = K τ s +1 (2) Where, Κ= Gain of the process or steady state gain. τ= Time constant of the process. Mathematically, the gain K is defined by (3) [8]. K= ΔO ΔI = Variation of the output variable Variation of the input variable (3) For the step response showed in Fig. 4, the transfer function of the plant can be obtained. This procedure was repeated to obtain several transfer functions for both positive and negative variations of the input variable. Then, the average of the gain and the time constant from all transfer functions were calculated and the transfer function which represents the model of the plant was obtained (4). G(s) = −0.145 1.697 s + 1 (4) Fig. 6. Response of the voltage plant to an alpha step input. Fig. 4. Response of the frequency plant to a percentage load step input Finally, the reached model was validated by means of comparison of the response of the system from the mathematic model versus the response obtained from experimental tests. This comparison is expounded in Fig. 5. As can be observed, the mathematical model of the plant is highly similar to the experimental behavior of the plant. The maximum error was 1.5 hertz, which represents a 2.5% of the frequency nominal value. Fig. 7 shows the response of the system obtained from both, the mathematic model and the experimental test. For this case, the maximum error was 2 volts, which is equivalent to 0.9% of the MHP’s nominal voltage. Fig. 7. Comparison between the mathematic model and the actual behavior of the voltage plant B. Frequency Digital Controller Fig. 5. Comparison between the mathematic model and the experimental behavior of the frequency plant Voltage Plant Model: To obtain the voltage plant model, variations in the firing angle (alpha) of the half controlled rectifier (input variable) were performed to observe the MHP voltage (output variable). Fig. 6 shows a firing angle variation from 79.2° to 72°, which achieves a voltage variation near to the operation point of the plant (220Hz) and within the allowed range of variation. Equation (5) details the obtained model of the voltage plant. G(s) = 1.143 0.65s + 1 (5) The frequency closed-loop block diagram is exhibited in Fig. 8. The Control Digital Action (CDA) Delay block represents the delay introduced by the instrumentation and the digital controller. The value of Ts corresponds to the time that the controlled rectifier takes to update its trigger signal [9], this is at every new period of the sine voltage signal (16.67 milliseconds). PI Controller E(s) R(s) + CDA Delay Plant U(s) - Fig. 8. Frequency Closed Loop Block Diagram C(s) To achieve a successful regulation of the system and avoid steady state errors, PI controllers were used. To define the proportional and integral gains, the current standard regulations regarding to the Dispatch and Operation Procedures for Generators in Backup Systems from the Ecuadorian Electricity Regulator (ARCONEL) were considered [10] (See Table I). TABLE I Admissible frequency ranges for generators Operation conditions Admissible frequency range Without action of generator’s 57.5 - 62 Hz instantaneous disconnection relays For 10 seconds maximum period 57.5 - 58 Hz; y 61.5 - 62 Hz For 20 seconds maximum period 58 - 59 Hz; y 61 - 61,5 Hz Without time limit 59 y 61 Hz. C (s) = C (s) = 11.33s + 22.35 s U [ k ] = U [ k − 1] + 11.42 * E[ k ] − 11.23 * E[ k − 1] IV. (8) (9) EXPERIMENTAL RESULTS TO LOAD VARIATION TESTS The practical implementation was performed in Quillán Micro-hydropower Plant, which is located in the Province of Tungurahua-Ecuador and generates 1600 watts. The nominal parameters of the MHP are shown in Table II. Several test procedures were performed to validate an appropriate operation of the implemented AVR and ELC. In order to meet with the mentioned regulation, a Maximum Percentage Overshoot (MPO) of 4.17% and a settling time (ts) of 200 milliseconds were chosen. For the design of the PI frequency controller, the symmetrical optimum PI technique was used to find the proportional and integral regulator parameters (Kp=-236.85 and Ki=-82.93). Thus, the controller’s transfer function is presented in (6). −236.85 s − 82.93 transfer function and the difference equation can be expressed as (8) and (9) respectively. (6) s TABLE II Parameters of Quillán MHP Parameter Nominal Value Power 1600 watts Voltage 220 volts Current 9.09 amperes 0.8 Cos θ Fig. 10 shows the external and internal view of the control panel including all the sensors, electronic boards, protections and operation elements. Once the controller transfer function was obtained, the difference equation can be expressed as in (7). U [ k ] = U [ k − 1] − 238.23 * E[ k ] + 235.46 * E[ k − 1] (7) C. Voltage Digital Controller To obtain the digital controller to perform the voltage regulation, the same procedure detailed for the frequency controller was used. For this case, the closed-loop block diagram is shown in Fig. 9. Ts is 8.33 ms. PI Controller E(s) R(s) + CDA Delay Plant U(s) C(s) Fig. 10. Control panel implemented for the tests. - A. Load Variation Tests Fig. 9. Voltage Closed Loop Block Diagram. To define the PI controller parameters, the regulation regarding to the Quality of Electrical Distribution Service [11] from the ARCONEL was considered. The maximum admissible voltage variation in urban areas is ±8% of the nominal voltage (220 volts), which corresponds to an acceptable range from 202.4 volts to 237.6 volts. For this controller, a MPO of 4% and a settling time (ts) smaller than 200 ms where selected. The controller parameters which fulfill the previous conditions were Kp=11.33 and Ki=22.35.Thus, the controller The AVR and the ELC were tested for the nominal power of the MHP (1600 watts). However, to consider a safety band, a ballast load of 2000 watts was selected. From tests performed to obtain the model of the plant, it was determined that a load variation up to 500 watts produced frequency variation within the ranges allowed according to Table I. Therefore, four 500 watts loads were used, one of them was controlled by continuous regulation by means of phase control and the other three with discrete ON-OFF control. During the tests, different types of user loads (See Table III) were connected and disconnected to evaluate the response of the frequency and voltage controllers. TABLE III Data sheet of the employed user loads Load Type Voltage [V] Power [W] Incandescent Lights 210-230 100 Cell phone 100-240 5 Laptop 100-240 120 Blender 120 600 Iron 110 1000 B. Response of the Frequency Controller When the power consumed by the user was higher, the frequency deviation was also higher as in the case of the electric iron connection (Fig 11). Table IV presents the frequency response when connecting and disconnecting different user loads. TABLE IV Response of frequency controller to load variations Max. Relative Response Event Frequency Error time (s) Variation (Hz) (%) Blender connection 0 0.93 1.5 8 lights connection 3.7 1.73 2.9 Electric iron connection 4.5 1.95 3.3 Electric iron disconnection 4.3 1.85 3.1 Full load disconnection 8 2.91 4.9 Fig. 12. Results obtained during the frequency controller test C. Response of the Voltage Controller Fig. 13 presents the voltage response of the MHP when an electric iron was connected. As can be seen when a voltages deviated from the reference value occur, the voltage controller changed the firing angle of the half-controlled bridge to correct the generated voltage in less of 0.4 seconds. The voltage response to different load variations is detailed in Table IV. Fig. 11. Response of the controller to electric iron connection and disconnection Fig. 12 shows the achieved results for the frequency controller during a 45 minutes test. At the beginning, the MHP started up without any user load connected, and the frequency increased up to the nominal frequency value. Then, the MHP was subjected to load connections and disconnections for different power rated loads. When a frequency variation was detected, the controller responded immediately and changed the percentage of ballast load to correct the frequency to the reference value. Although several load variations where performed during the whole test, the frequency kept within the admissible range established by ARCONEL in [3]. Fig. 13. Response of the voltage controller TABLE IV Response of voltage controller to load variations Event Response time (s) Max. Voltage Variation (V) Blender connection Blender disconnection Electric iron connection Electric iron disconnection Full load disconnection 0.3 1 0.4 1.5 1.5 2 2 4 4 6 Relative Error (%) 0.9 0.9 1.81 1.81 2.73 Finally, the results obtained during the whole test time of the voltage controller operation are shown in Fig. 14. The test lasted 30 minutes and proved that voltage remained within the admissible range established by ARCONEL in [5] when different user loads where connected or disconnected, independently of their power. Furthermore, the voltage controller output remained in an almost constant value, which is characteristic of the ballast load frequency regulation method. Fig. 14. Results obtained during the voltage controller test for differents disturbances V. CONCLUSIONS The achieved results for the obtained mathematic models where highly similar to the real behavior of the plants, which demonstrates the effectiveness and reliability of the modeling procedure used. The maximum error obtained in the validation of the voltage plant model was 2 volts, which corresponds to 0.9% of the reference voltage. The maximum error obtained in the validation of the frequency plant model was 1.5 hertz, which corresponds to 2.5% of the reference frequency. The control strategy for the proposed AVR topology allows the MHP to start up without an external power supply, which turns the MHP in an isolated and autonomous generation system. The ELC implemented in this project reduces the amount of loads used for discrete steps and decreases the power managed by the AC-AC converter used for continuous regulation related with classical frequency controllers. The AVR and ELC implemented keep the voltage and frequency within the ranges stablished in the current standard regulations of the ARCONEL for load variations up to 62.5% respect to the nominal generation power. The maximum voltage deviation registered was 2.73% and the maximum frequency deviation was 3.33%, which demonstrated an effective and alternative voltage and frequency control. REFERENCES [1] C. Greaced, R. Engel and T. Quetchenbach, “A guidebook on grid connection and island operation of mini grid power system up to 200 kW,” Lawrence Berkeley National Lab, One Cyclotron Road, Berkeley & Schatz EnergyResearch Center, Harpst St., Arcata, April 2013. [2] Intermediate Technology Development Group, ITDG-Perú., Manual de Mini y Microcentrales Hidráulicas, Lima-Perú: ITDG, 1995, pp. 30-58. [3] L. Giuliani and R. Figueiredo, “Frequency and voltage control of micro hydro power stations based on hydraulic turbine’s linear model applied on induction generators”, IEEE XI Brazilian Power Electronics Conference, in press. [4] B. Singh and S. Murthy, “Analysis and design of electronic load controller for self-excited induction Generators”, IEEE Transactions on Energy Conversion, vol 21, No. 1, march 2006, in press. [5] B. Singh and V. Rajagopal, “Decoupled Solid State Controller for Asynchronous Generator in Pico-hydro Power Generation”, IEEE 5th Int. Conference on Industrial and Information Systems, in press. [6] Y. Sofian and M. Iyas, “Design of Electronic Load Controller for a Self Excited Induction Generator Using Fuzzy Logic Method Based Microcontroller”, IEEE Electrical Engineering and Informatics International Conference, in press. [7] B. Singh, G. Kumar, A. Chandra, K. Al-Haddad, “Voltage and Frequency Controller for an Autonomous Micro Hydro Generating System” IEEE Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, in press. [8] C. Smith y A. Corripio, Control Automático de Procesos, México: Limusa, 1991, pp. 91-103. [9] S. Buso y P. Mattavelli, Digital Control in Power Electronics, Morgan & Claypool, 2006, pp. 15-28,53-65. [10] Consejo Nacional de Electricidad, «Calidad del Servicio Eléctrico de Distribución. Regulación No. CONELEC-004/01,» 2001. [En línea]. Available: http://www.regulacionelectrica.gob.ec/wpcontent/uploads/downloads/2015/12/CONELEC-CalidadDeServicio.pdf [Último acceso: 19 08 2016]. [11] Consejo Nacional de Electricidad, «Procedimientos de Despacho Y Operación. Regulación No. CONELEC – 006/00,» 09 08 2000. [En línea]. Available: http://www.regulacionelectrica.gob.ec/wpcontent/uploads/downloads/2015/10/ProcedimientosDespacho.pdf [Último acceso: 19 08 2016].