Transcript File
Submitted By Ravi Raj Rajeev Kumar Vipul Batra FUNCTIONS PERFORMED BY THE RELAY MADE 1.OVER VOLTAGE PROTECTION 2.OVERCURRENT PROTECTION Power System Protection Modern Power Systems use different types of relays for the protection of the total power system during faults, the modern relay which is used for the protection of power systems are the microcontroller relays. The power equipments which are used in a Power station are very costly and hence to protect these equipments from the faults generated within them i.e. the ground faults we need an earth fault relay. An earth fault relay detects the ground fault or earth fault where the phase is shorted to the earth or ground; it monitors the amount of earth current and accordingly follows the characteristics which are defined to it and trips the circuit. In selecting an earth fault relay the grounding of the system is considered. For e.g. if the grounding is solid grounding then a fast acting relay i.e. the speed of the relay has to be very fast in detecting and tripping of the circuit, if the grounding is high impedance grounding then the sensitivity of the relay has to be increased to detect the earth fault in the system. In this project an overvoltage and overcurrent relay which is made using a microcontroller .The microcontroller is programmed in such a way that it will perform its action whenever there is abnormal conditions such as overvoltage or overcurrent situations. A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal (with complete electrical isolation between control and controlled circuits), or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays were used extensively in telephone exchanges and early computers to perform logical operations.". A type of relay that can handle the high power required to directly drive an electric motor is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protective relays . A simple electromagnetic relay consists of a coil of wire surrounding a soft iron core, an iron yoke which provides a low reluctance path for magnetic flux, a movable iron armature, and one or more sets of contacts (there are two in the relay pictured). The armature is hinged to the yoke and mechanically linked to one or more sets of moving contacts. It is held in place by a spring so that when the relay is deenergized there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. When an electric current is passed through the coil it generates a magnetic field that attracts the armature, and the consequent movement of the movable contact(s) either makes or breaks (depending upon construction) a connection with a fixed contact. If the set of contacts was closed when the relay was deenergized, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. A microcontroller -based system can be used for detecting faults in the Power system. The real time data monitoring of various electrical parameters in the Power system helps us in detecting electrical faults. In this system the abnormal conditions are detected by the microcontroller and necessary initiation of the trip signal to the circuit breaker is given. For this process to happen, real time monitoring of the data is required. Since the microcontroller understands only binary language we need to convert our analog signal to digital by using ADC. After getting this data based upon the programming in the memory (ROM/RAM) the microcontroller takes the decision of the tripping of electrical system i.e. it detects faults based on the conditions of the program written. A microcontroller is a multipurpose, programmable, clock-driven register based electronic device that reads binary instructions from a storage device called memory, accepts binary data as input and processes data according to those instructions, and provides results as output. A typical programming machine is used for performing a specific function/ task. It can be represented with three components 895C1 microcontroller , memory, and input/output. These three components work together or interact with each other to perform a given task, thus they comprise a system. The physical components are called hardware. A set of instructions written for the microcontroller to perform a task is called a program and a group of programs is called software. STEP DOWN TRANSFORMER A/D CONVERTER MICROCONTROLLE R 895C1 OPERATOR Previous figure represents a block diagram of proposed microcontroller-based over current relay. The limited dc voltage output of the measuring unit is proportional to the operating current, i.e., V=K1*I…. (1) The voltage is fed to a microcomputer, whose function is to perform the following jobs sequentially through an appropriate programming 1) analog-to-digital conversion; 2) fault detection; 3) function generation; 4) automatic variable time delay achievement; 5) pulse generation. The microcomputer first converts the dc analog input voltage into a digital equivalent in terms of the hexadecimal system, i.e., (X)h= V….(2) This digital value is tested. If there is a fault, this means that tested value exceeds a stored digital pick-up value, then it will be processed through a software function generator, which consequently determines the type of the resulting overcurrent relay, i.e., (Y)h= K2/g(x)....(3) The operation time of the relay is determined by a specified time delay subroutine depending upon the value of (Y)h, i.e.,t=K3(Y)h …. (4) To make the microcontroller work as an over voltage/under voltage relay, another set of three ADC’s are required, which have addresses different from that of current Converting ADC’s. Here the voltage is stepped down by a P.T and the output of the P.T is directly fed to the bridge rectifier such that it converts the sinusoidal signal into positive half cycles, this positive cycle signal is fed to the ADC’s. The circuit is designed in such a way that the rated voltage of the line is represented by +4v positive half cycles. The output +4v positive half cycles of the three phases are fed to an ADC0801 IC’s with addresses 1000H, 0800H, 0400H. To implement the microcontroller as an over voltage and under voltage relay the program above is modified. Circuit for detecting Over voltage/Under voltage Faults Microcontroller-based relays offer many advantages and benefits over electromechanical relays they are: I. Reduced installation costs 2. Reduced maintenance cost 3. Application flexibility 4. Improved monitoring and control functions The use of microcontroller-based relays has become very common. Many utilities are taking advantage of the new features and innovations offered in these relays. New developments in microcontroller based relays offer added benefits by further reducing costs and by improving the relay functions and features.