The methodology proposed in this work consists of development of block diagram of the protective device, background to the most commonly arising problems in the power system network and a brief discussion about the micro controller used in this work.
2.1 Functional block diagram of proposed protection scheme
The aim of the proposed work is to develop a system that monitors the voltage variation and provides breakpoint based on low and high voltage tripping mechanism that avoids any damage to the load, various industrial and domestic systems consisting of fluctuation in the AC mains. In the tripping mechanism shown in Fig. 1, a quad comparator IC is used with two more comparator ICs as window comparator. When the system delivers error, the input voltage falls out of the window range. This trigger then operates a relay that cut off the lamp load to avoid any damage. So the high quality power can be obtained with the help of this device and it will also improve the power factor and thus power can be fully utilized. The proposed protection circuit protects refrigerator, IM and other electrical appliances from abnormal conditions.
2.2 Over voltage
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as over voltage [5]. The conditions may be hazardous. Depending on its duration, the over voltage event can be transient (a voltage spike as shown in Fig. 2) or permanent, leading to a power surge. Over voltages are less common than under voltage but they also arise due to system faults. Overvoltage can occur due to single line to ground fault, which in turn will raise the voltage of the other phases. It can also happen due to disconnection of heavy industrial loads or switching on the capacitor banks or energization of capacitor bank. This is generally due to ungrounded or floating ground delta systems, where a change in ground reference would give voltage rise to the ungrounded system. It can also be generated by sudden load deduction. Due to the disconnection of load there is a sudden reduction of current, which will gives rise to over voltage, where L is the inductance of the line. The effects of over voltage are more severe and destructive. It may cause the electrical equipment to fail, due to overheating caused by high voltage. Also electronic and other sensitive equipment are prone to malfunction.
2.3 Under voltage
Under voltage is defined as a sudden drop in the root mean square (r.m.s.) voltage and is usually characterized by the remaining (retained) voltage [5]. Under voltage is thus, short duration reduction in r.m.s. voltage, caused mainly by short circuit, starting of large motors and equipment failure. Under voltages are the most common power disturbance whose effect is quite severe especially in industrial and large commercial customers such as the damage of the sensitivity equipments and loss of daily productions and finances. The examples of the sensitive equipments are Programmable Logic Controller (PLC), Adjustable Speed Drive (ASD) and Chiller control. Under voltage at the equipment terminal can be due to a short circuit fault hundreds of kilometers away in the transmission system.
2.4 Over current
In an electric power system, over current or excess current is a situation where the value larger than intended electric current exists through a conductor, leading to excessive generation of heat which leads to the risk of fire or damage to equipment [5,6,7]. Possible causes for over current include short circuit, excessive load, incorrect design, or a ground fault. Fuses, circuit breakers, temperature sensors and current limiters are commonly used protection mechanisms to control the risks of over current. Over current protection also includes the protection against over loads and this is most widely used protection. Over loading of a machine (or equipment) means the machine is taking more current than its rated current. Hence, with overloading there is an associated temperature rise. The permissible temperature rise has a limit based on insulation class and material. Over current protection against overload is generally provided by thermal relays. Over current protection also includes protection against short-circuit. Short circuits may leads to phase faults, earth faults or winding faults. Short circuit currents are generally several times more than full load current. Hence fast fault clearance is always desirable at the time of short-circuits.
2.5 Microcontroller
The microcontroller [8,9,10] is a small, low cost, on-chip computer. It essentially includes:
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An 8 or 16 bit microprocessor as central processing unit (CPU).
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A small amount of RAM.
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Programmable ROM and/or flash memory.
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Parallel and/or serial I/O.
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Timers and signal generators.
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Analog to Digital (A/D) and/or Digital to Analog (D/A) conversion.
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Often used to run dedicated code that controls one or more tasks in the operation of a device or a system.
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Also known as embedded controller because the microcontroller and supporting circuits are often built into, or embedded in, the devices they control.
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Devices that utilize microcontrollers include car engines, consumer electronics (VCRs, microwaves, cameras, pagers, cell phones), computer peripherals (keyboards, printers, and modems), test/measurement equipment (signal generators, multimeters, and oscilloscopes).
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Microcontrollers usually must have low-power requirements (~ 0.05–1 W compared to ~ 10–50 W of general purpose desktop CPUs) since many devices they control are battery operated.
The microcontroller performs the major functions of decision and control. The input supply voltage is fed to the microcontroller via current measuring unit and voltage measuring unit. The microcontroller was used in this circuit in order to reduce the complexity of the design and to ensure an easy interface with a liquid crystal display (LCD).
