Design and Construction of Synchronizing Check Relay

Transcription

1 Design and Construction of Synchronizing Check Relay M.J.A.A.I.Jayawardene,, R.W.Jayawickrama, M.D.R.K.Karunarathna,S.A.P.U.Karunaratne, W.S.Lakmal Abstract This document contains an introduction about our final year project which is Design and Construction of Synchronizing Check Relay. The design procedure, current results and further enhancements are also included in this document. Keywords: synchronizing, Synch-check relay, in phase point detection, Pulse Width Modulation, Capture, Compare Synchronizing Concept I. INTRODUCTION When closing a circuit breaker between two energized parts of the power system, it is crucial to match voltages on both sides of the circuit breaker before closing. If this matching or "synchronizing" process is not done correctly, a power system disturbance will result and equipment such as generators can be damaged. Therefore in order to correctly synchronize the generator following conditions or synchronizing variables must be satisfied. 1. The frequency of the voltages produced by the generator must be equal to the ac power network frequency. 2. The value of the voltages produced by the generator must be equal to the ac power network voltage. 3. The phase sequence of the voltages produced by the generator must be the same as that of the ac power network. 4. The voltages produced by the generator must be in phase with the ac power network voltages. If the direction of rotational is always maintained the same and appropriately the connections are changed to get the correct phase sequence first time, it is not needed to check the condition number 3 always. Synchronizing relays allow unattended synchronization of a machine with a system. Today these are digital microprocessor instruments, but in the past electromechanical relay systems were applied. Loss of synchronization can occur because of multiple reasons, but causes severe amounts of damage if not stopped. Protection system must be able to detect and disconnect the generator from the system as soon as possible before any harmful effects take place. [5]That s why it is set up with its own protective relay to trip the system when detected. Synchro-check relay electrically determines if the difference in voltage magnitude, frequency and phase angle falls within allowable limits. The allowable limits will vary with the location on the power system. Typically, the further away from generation and load, the more phase angle difference can be tolerated. Synch-check relays typically do not provide indication of the voltage magnitude, frequency or phase angle. It decides internally whether its conditions for closing are satisfied. It will either allow or prevent closing depending on its settings. [3][4].Following table shows the relevant standard ranges of interconnections of distributed generators to the power system. [1] Aggregate rating of DG units (kva) Freque ncy differen ce Voltage difference (.V, %) (.f, Hz) Phase angle difference (.Φ, ) Table1: IEEE 1547 Standard for Interconnections of distributed power sources to the area electric power system

2 II. METHODOLOGY A. Frequency Controlling In order to set the difference of frequencies of the generator and the supply side according to the relevant tolerable ranges, frequency controlling is needed. Also terminal voltage of a synchronous generator is proportional to the rotating speed of the rotor and to the excitation current in the field winding. Therefore to control the terminal voltage by controlling the excitation current, rotor angular velocity must be kept constant. Hence the frequency controlling should be introduced. signal frequency of 5 KHz is generated. According to the frequency error signal, the duty ratio of the PWM signal output is changed. This output is fed to the IGBT driver circuit in order to control the speed of the prime mover. Figure 1: Frequency controlling Block Diagram Frequency Detector Unit consists of three sections. 1. Voltage Divider. 2. Zero crossing detector circuit. 3. Frequency calculation inside the microcontroller. The Zero Crossing circuit detects the zero crossing instances of the output voltage waveform. When the output voltage goes to zero the circuit will generate a pulse. The waveform goes to zero only once during a cycle. i.e. pulse frequency is same as the terminal voltage frequency. These pulses are fed in to the microcontroller PIC18F452A as an interrupt. Figure 2: Complete Frequency Controlling Circuit Diagram Pulses generated by the zero crossing detector circuit are fed to the microcontroller as an interrupt. Here PIC18F452 microcontroller is programmed to detect the frequency. The system frequency is given as the reference. Inside the microcontroller initially Frequency feedback value and the frequency reference value are converted in to same scale and the frequency error is calculated. From the microcontroller continuous PWM

3 220v B. Voltage controlling IGBT Switching cct Excitation winding PWM Signal generator Step down rectify F/B system Step down rectify Ref The main function of the voltage regulator is to change the generated output terminal voltage of the synchronous generator according to the voltage variation of system in order to match the generated voltage with the system voltage before get synchronized. This voltage regulation is achieved by controlling the dc excitation voltage supplied to the rotor of the generator where pwm signal is used to control DC excitation voltage as the generated output voltage depends on excitation voltage the output pwm signal using an embedded PID controller in microcontroller as it detects the error between a measured process value and a desired set point and outputs a value determines the reaction to the current error, The relevant tuning parameter Kd, Ki.Kp are designed to observed through matlab simulink model using practical data. The voltage controlling algorithm with PID operation is being developed. PID controller microcontroller Figure 3: Voltage controlling Block Diagram As shown in block diagram here, the terminal voltage of generator is fed to the microcontroller as feed back signal while system voltage is fed as reference signal to microcontroller where both signals are step downed & rectified.the duty ratio of the pulse width modulation signal of the microcontroller is varied according to the error signal fed to the embedded PID controller to change the generator output according to the system voltage. The PWM signal issued by microcontroller is fed to MOSFET through a gate driver cct. Here MOSFET IRF730 is selected according to the excitation current ratings of generator while IR2110 gate driver is selected according to the required voltage input signal of the selected MOSFET cct.18f452 microcontroller is used as it provide the required PWM outputs including enhanced programming features. In our design, the mathematical relation between excitation voltage & output voltage is derived practically by measuring the changes in the voltage generated with the variation of excitation voltage supply as the theoretical calculations are less reliable Here, we are designing to obtain Figure 4: Complete Voltage Controlling Circuit Diagram B. In phase point detection Once the voltage and the frequencies of both generator and system are matched, external interrupt is triggered with an input from the Frequency control circuit, which initiates the instantaneous phase difference calculation program in PIC 18F452 microcontroller. Pulses generated in zero crossing circuits are fed to RC1 (CCP2) and RC2 (CCP1) pins of the PIC 18F452 and the timer1 is updated. Rising edges of pulses fed to RC2 pin, are captured from the microcontroller, by triggering the CCP1

4 capture interrupt. With the interrupt timer1 value stored in CCP1 register is passed to a variable rise1 and inside the interrupt service routine (ISR), the flag rise is set to 1. Once the input from RC1 becomes low to high with the flag rise is 1, current updated timer1 value is passed to a variable rise to resetting the timer1. Then the magnitude of (rise1 rise2) is calculated and passed to a variable difference. When the difference = 0, both voltage signals are in phase and considered to be synchronized. Then the next synchronizing point is captured as previous case, and the time duration of two consecutive synchronizing points are calculated and the total time of circuit breaker operation is deducted from that value. The result is stored in a variable time, once the next synchronizing point is detected, after counting a time period of time, output signal from RD7 is fed to the isolator to close the circuit breaker. Start Enable capture interrupt Enable timer1 generator.therefore Protection against frequency deviation is a high priority.also protection against undesirable voltage deviations is also a must to avoid of occuring harmful effects. [5] Over frequency operation: Over frequency results from the excess generation and it can easily be corrected by reducing in the power outputs by means of governors. Under frequency operation: Under frequency occurs due to the excess load. During an overload, generation capability of the generator increases and frequency reduction is occurred. The power system endures only if we maintain a proper load shading scheme. Over voltage protection: Over voltage occurs due to the increase in the speed of the prime mover because of sudden loss in the load on the generator. Normally the over voltage protection is provided by two over voltage relays and it may occur due to the defective voltage regulator and also due to manual control errors. Under voltage protection: When several generators supply the load and due to some reason one generator is suddenly trip, then another generators try to supply the load. Each of these generators will experience a sudden increase in current and thus decreases the terminal voltage. Usually under voltage relay type-27 is also used for the under voltage protection.[2] Start timer1 III. RESULTS Capture interrupt from CCP1 A. Zero crossing circuit results Reset timer1 Pass timer1 value to the variable rise1 Pass timer1 value to the variable rise2 Calculate difference from (rise1 - rise2) magnitude No difference = 0 RC1 pin high These images shown below have been saved during the checking process of the zero crossing circuit, which is a part of the governor control circuit. The purpose of this zero crossing circuit is to generate pulses when there is a zero passes in the input sinusoidal signal. The generated pulses are counted in the microcontroller to calculate the frequency of the output terminal voltage of the generator to compare with the frequency reference to calculate the error between those two values. Yes In phase point Figure 5: In phase point detection algorithm D. Further Developments Initially, we decided to embed following protection function for the synchronizing check relay. But due to the restricted time limitation, they were not tested and implemented. Eventhough it couldn t be achieved; it can be effectively embedded for the design and enhanced its performance. Figure 6: pulse output obtained from zero crossing circuit Even the smallest deviation from the operatingfrequency cause un-repairable damage to the prime mover of the

5 B. Frequency controlling circuit results When the speed of the generator is increased, it is reduced to the synchronous speed (1500rpm) by the controlling circuit by increasing the duty of the output PWM signal which drives the MOSFET switching circuit. Then it controls the voltage of the field winding of the DC motor which is our prime mover. The same reverse function is occurred when the speed of the generator needs to be increased.. Figure 9: PWM observed when the system voltage is increased Figure 7: PWM output when system frequency get Increased Figure 10: PWM observed when the system voltage is decreased IV. CONCLUSION Figure 8 : PWM output when system frequency get decreased C. Voltage Controlling Circuit Results When the system voltage is increased, the generator terminal voltage will also be increased by increasing the duty of the PWM signal which is controlling the excitation voltage of the generator, similarly the program is coded in such a way that a decrease in the system voltage would result in a decrease in the duty Our final year project titled as Design and Construction of Synchronizing Check relay is mainly aimed at providing a low cost automatic relay with basic features, specifically for the purpose of demonstration when the synchronization of distributed generators is explained using the dark lamp method in the machines laboratory. It is supposed to show the students that in place of manual synchronizing, automatic synchronizing can be used quite reliably and effectively. We were able to implement our test circuits collectively in achieving the targeted output quite successfully. However protection functions could not be tested and implemented due to the restricted time limitation. We designed our casing also in a user friendly manner providing indicator lamps and labels wherever we felt necessary. As IGBT switches are used and a well designed thermal control is employed the relay assembly is expected to work stable for a reasonable time span.

6 The design basically involves three major circuits for voltage regulation, frequency regulation and in phase point detection. These three circuits have a unique microcontroller each and they communicate with each other during the synchronizing process. In this model the prime mover was taken as a DC motor in which the speed can be controlled by the excitation of the field winding. In practical situations this prime mover would be a turbine which is rotated using either steam, water or fuel combustion. Therefore in order to control the prime mover these input valves should be controlled. With some changes done to the frequency controlling microcontrollers coding, these occasions can be also covered with our model. However it should be noted again that what we have achieved in time permitting conditions is only a basic model and should not be compared with sync-check relays in the industry having a vast range of added features and functions. If provided with more time and financial means, this model can be developed into a higher level which could be compared with advanced sync-check relays currently in use. ACKNOWLEDGEMENT First of all we would like to offer our sincere gratitude to the project supervisors Dr. J.P. Karunadasa and Eng.W.A.D.S.Wijayapala for the invaluable guidance given for us to come up with a successful project completion. We also wish to thank all the staff members of the Department of Electrical Engineering of the University of Moratuwa for their immense concern on our projects and the given encouragement throughout the period of our project design. We also wish to thank the laboratory technical staff of the department for their assistance in successful completion of the project. REFERENCES [1] G. 1547TM IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems.-Sponsored by thestandards Coordinating Committee 21 on Fuel Cells, Photovoltaics, Dispersed Generation, and Energy Storage. [2] B Ramandeep Kaur Aujla,S.NO , Generator Stator Protection, under/over voltage, under /over frequencyand unbalanced loading. [3] Terrell Croft and Wilford Summers (ed), American Electricans' Handbook, Eleventh Edition, McGraw Hill, New York (1987) ISBN pages 7-45 through [4] Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition,McGraw-Hill, New York, 1978, ISBN X pp. 3-64,3-65. [5] Dr. Ramesh Bansal,School of Information Technology and Electrical Engineering, Axon Bldg, 47/212,University of Queensland, St Lucia, 4072, GENERATOR TECHNOLOGY DESIGN &APPLICATIONS.