Amplification and Analogue to Digital Converters - Case Study Example

PROTECTION RELAY Table of Contents Introduction 3 2. System Details ….4 3. Amplification and Analogue to Digital Converters 5 3.1 amplification computation……………………………………………………………………………………………………..6 3.2 ADCs specification 6 4. Noise to Signal converter….........................................................................................................7 5. Main circuit components..............................................................................................................7 5.1 current/voltage transformer..................................................................................................7 5.2 protection relay.....................................................................................................................7 5.3 circuit breakers/isolators ….................................................................................................7 5.4 batteries …...........................................................................................................................7 5.5 channels of communication …............................................................................................7 6. System evaluation and analysis …..............................................................................................8 7. Determination of performance …...............................................................................................8 8. Thermocouple type k specification ….........................................................................................9 8.1 thermocouple calculations ….............................................................................................10 9. System operation ….................................................................................................................11 10. Winding resistance and sensitivity …....................................................................................12 11. Simulation program …...........................................................................................................12 12. Conclusion ….........................................................................................................................13 13. Bibliography ….....................................................................................................................14 14. Conclusion …........................................................................................................................14 15. Appendix …..........................................................................................................................15 16. Appendix …..........................................................................................................................16 PROTECTION RELAY PROJECT GOAL AND DESIGN A protection relay is an input (current, resistance, temperature or temperature) receiving device, comparing it to set points and giving out outputs which can be visual like indicator lights. Electrical equipment is subjected to many types of disturbances like system faults or routine operations including line de-energization, opening disconnection , and inductive or capacitive loads switching resulting in electrical transients. It prevents the circuit from getting damaged in case of an over current (Theraja 324).This it does by controlling the CNB making the circuit trip instantly without damage to the circuit by the current. Adjusting the set point will make it trip after exceeding the set limit. The design composes of a unit which is the physical magnitude, a measurement range which is the value the counter displays, transducer range which can detect minimal changes in the quantity and resolution which is related to the precision of the measurement. Protective relay performance depends on the signals produced by instrument transformers, and these signals depend on the overall transient response of the instrument transformers. The Capacitive Voltage Transformer (CVT) is a Voltage Transformer (VT) with a capacitive divider which converts input to a CVT. The high voltage capacitors also serve as carrier current coupling. Transient performance is mostly affected by energy stores in the capacitive and inductive elements of the device. Transient errors produced by an instrument transformer cause a major impact on the dependability and security of protective relays and cause desperation, delayed operations, or failure to operate (Theraja 324). System details Amplifiers Amplifiers are current transformers which are used to detect the current within the grid. The detected current is further converted to signal called differential voltage signal through the help of a load resistor. The smallest load resistor should be used in the secondary part of the current transformer. This is necessary so as to reduce any reflected load detected on the primary part of this current transformer. The voltage signal found on the secondary part is often low that is in the form of millivolts though this value can consequently be increased when subjected to common-mode voltage. Due to these reasons, the amplification device should be able to give a high current gain and should allow high common-mode voltages. The amplification gain can be calculated from the following formula; Gain= Vout /Vin = 5/3.474mV = 1439.26 V. ADCs The ADCs support high current input range. This functionality principle of ADCs assists them in increasing the dynamic input signal range thereby helping to improve the signal-noise ratio. In this system, a high resolution ADC is required so as to minimize quantization errors. The chosen ADC should be able to improve the performance of noise overall and to achieve this, faster DACs should be used. The calculation for the number of bits for the ADCs can be computed as follows; Number of Bits From the above equation, the project would require at least a 10 bit. Reference The noise converter function should be good that is should resemble both its input voltage and reference voltage. This reference voltage which is being used by the ADCs should be accurate as well as stable over a given temperature range. Preferred ADCs should have on-chip generator reference voltage. The output of this reference voltage can be centered on the center value and this depends on the system requirement. Processors The calculations of values such as active power, harmonics and impedance require an FFT calculation and this calculation depends on the recorded data. In this protection relay system, a DSP engine is necessary. These DSP engines should be floating-point types since they have a better advantage of performance compared to fixed-point counterparts. Main components Current and voltage transformers These transformers step down all the high currents and voltages present within the electrical system. The reason for this step down is to manage the high voltages/currents so as to limit them to convenient levels which are easily manageable by the protection relays. Protection relays These devices sense any fault within the electrical system. Apart from the faults, these devices also check for any form of disconnection and they initiate a trip within the system. In cases of power discontinuation, protection relays assist the whole system from the effects of increased voltage when the power returns. Circuit breakers/Isolators These devices open or close the whole system. They work on the basis of the relays and the commands of the autorecloser. When the system is overloaded, the circuit breakers or the Isolators close the system. When the operating voltage is above the threshold voltage requirement, the circuit breakers open the system. Batteries They provide power in the situations where there is a blackout or power discontinuation within the system. Blackout affects the operation of the system. The batteries therefore provide a backup system for the whole system. Channels of communication These paths allow the analysis of both voltage and current present at the remote line terminals. The voltage and current present in the system can be analyzed along the line terminals. The line terminals carry more of the voltage and current since they act as points of voltage or current summation. System evaluation and analysis The reference voltage which is also the input and output and is used by the ADCs was set at zero and one. The 5v reading of the reference voltage was equivalent to the value 1 and was recorded in the ADCs while the -5v reading was equivalent to value 0. The Isolators or circuit breakers performed the switching operation on the system while the protective relays performed the control of the system function. The system control acts as the center of operations for the protection relay system. The system takes in voltage as input and gives current as the output. Determination of the performance The input to the protection relay system was in the form of temperature, current, voltage and resistance. The resultant output of the current input was obtained from the alphanumeric indicator. The indicator prints on the screen the current output of the system. Alarm sound was used to determine the voltage. Excess and low voltage gave rise to alarm sound which alerted the operator for consequent corrections. The system was able to go off and on due to high and low resistances respectively. High resistance gave rise to power switch off for the system while the low resistance gave rise to the power switch on. The temperature of the system was determined from the temperature gauge indicator which was fitted with thermocouple thermometer. The high resistance of the electrical wires within the system was able to produce some heating effect on the wires. The heating effect raised the temperature of the system and the increase was recorded on the thermocouple thermometer. Thermocouple type K is common and uses nickel-chromium and nickel-aluminum alloys to generate voltage. Thermocouple type K specification: Omega designed the following specifications from their data sheet. Thermocouple type Overall range °C Standard limits of Error Special Limits of Error Sensitivity K -200 to 1250 Above 0ºC 2.2 °C Below 0ºC 1.1 °C approximately 41 µV/°C From this data-sheet we observe that type K has a wide range of temperature. In my design I am not going to use this range because I am going to use the thermocouple to measure the temperature of the room which has a range of 0°C to 50°C. Calculate accuracy for ± 1 LSB for Vs= 3.6V: Step Size = = 1LSB error % = X 100% = - 0.098 % Performance analysis, output is ± 0.1 % accurate under normal circumstances. Errors appearing at the same time the accuracy is about ± 1% for both ADC and DAC. The error is going to affect the output voltage of the thermocouple affecting the exact reading of the room temperature, when there is no error the range of temperature is 50°C and the range of the corresponding output voltage is 2.023mV. However, the range of temperature will change because of the error (±2. 2°C) =52. 2°C and the range of the equivalent output voltage will be 2.114mV. The percentage of the error can be determined by the following equation: Calculate accuracy for ± 4 LSB for Vs= 3.6V: 4LSB error % = X 100% = - 0.39 % System operation The protection relay operates on the basis of induced magnetic force. Magnetic force was induced at all points within the armature. The winding distance of the armature was designed to exceed forces due to the mechanical parts of the system. The mechanical forces tend to restrain the movement of the armature. The electromagnetic force can be obtained through the following equation; F=2PI (N) 2/A (R0 +X/A) 2 Where A is the pool area of the piece X is the distance between the core and the armature R0 is the reluctance of the Iron part present in the magnetic circuit. N is the ampere turns The pull created by the electromagnetic effect due to the fixed dimensional constraints is directly proportional to the energizing ampere turn given as (N). N value at which the operation relay performs its function well is called the ampere-turn sensitivity. If sensitivity is expressed in terms of power (P) then P=I2R and sensitivity does not vary with the coil dimension. However, power sensitivity is dependent on the volume and conductor space. Power sensitivity is inversely proportional to the coil volume and the space of the conductor which is occupied (Theraja 324). Winding resistance and sensitivity The specification of the performance requirement is necessary when the protection relay system operates with no series components from a power source. The performance requirement is specified in terms of voltage. The winding resistance and the absolute temperature are related according to the following equation; R0/R1= (234.5 +T0) / (234.5 +T1) And T0 and T1 are the temperature of the windings while the 234.5 is the zero point of resistance on the Celsius scale. The power requirement (I2/R) is directly proportional to the absolute temperature. Simulation program Several programs such as Multisim, VisSim viewer and SABER can be used for the simulation and implementation of the protection relay project. The data for the simulation was obtained through successive commissioning test and through manual processes. Through the simulation process it was evident that the faults in the system were present on the primary side of the protective relay. Conclusions The performance of the system was according to the expectations. All the main components of the system were able to perform their functions. The best feature of the system is the time operation feature. The system was able to chatter due to the rippling effects caused by the rippling current. At the same time the system was able to bounce up and down due to the current impulse. The worst feature of the system was the heating effect due to the winding resistance. The system can be improved by using wires with low internal resistance. The high resistance of some wires generated a lot of heat and this heat affected the performance of the system. The protection relay control system can also be installed with a fan. The fan will help to reduce the heat produced as a result of internal resistance of the wires. The fan will also help to reduce other heat sources due to high voltage and current. The high current and voltage also produces eddy currents in the system. The problem of the eddy current in the secondary and primary part of the system can be controlled through proper armature insulation. Work cited Theraja, A. K. Theraja and B.L. A Textbook of electrical technology. New Delhi: S. Chand, 2008 pp 324-329 Appendix Fig. 1: Block diagram of a relay. Fig 2 basic connection diagram of protection relay Read More