Assignment 4 Electric Power Transmission Distribution

Michael Kurniawan 2018123456 University of Cincinnati Distribution & Transmission Assignments Protection in Electric Power Transmission 1. Differential Relays Differential Relay is one part of a protection system and takes its place as the main protection system. Based on the name, it can be imagined that this relay works based on a difference (different). In simple terms, differential relays work by comparing two or more input currents (in this case not only the magnitude, but also the angle). The current being compared is the current entering into and leaving the area protected by the relay. If the difference is zero, it is assumed that there is no internal fault. Likewise, if the difference is not zero then there is an internal fault. The illustration in Figure 1 can simply describe a differential relay in securing a power transformer. Figure 1 Protection system using a differential relay The use of differential relays is not only limited to power transformers, but can be applied in almost all power system protections such as generators, motors, busbars, transmission lines, capacitors, reactors or a combination of them. However, this paper is more concerned with differential relays in power transformer protection. The function of the differential relay on a transformer is as the main safety against short circuit faults both between transformer coils and between coils with main tanks. In other words, the geometric mean is defined as the nth root of the product of n numbers. It should be noted that the geometric mean is different from the arithmetic mean. Because, in arithmetic mean, we add up the data values and then divide it by the total number of values. But in geometric mean, we multiply the given data values and then take the root with the radical index for the total number of data values. For example, if we have two data, take the square root, or if we have three data, then take the square root, or if we have four data values, then take the 4th root, and so on. Because it is the main protection system, this relay works instantly. In normal operation a diferential relay will only see disturbances within its working area and has no effect when there is a disturbance from outside. This is because the current vector when there is an external disturbance does not change the phase difference, which is 180 degrees, which means that in the calculation the phasors will negate each other as in Figure 2. However, when there is an internal disturbance (within the working area), the current phasors entering and leaving the protection zone will be the same value or at least almost the same. So that in the calculation the two phasors will add to each other which causes the difference will not be zero. Figure 2 External and internal faults Under normal conditions and under external fault conditions, the value of Ir (also commonly called Ioperation or Iops) is zero. However, in reality this condition is rare and may even be very difficult to achieve. This can be caused by losses in the protected area or the difference between the two CTs that are the reference for reading the current. In power transformer protection, this current imbalance can be caused by several things, namely: A. Effect of saturation of CT and ACT Saturation of a CT or ACT can cause the current reading by the relay to be chaotic. B. Effect of ACT tap, ACT or Auxilary Current Transformer is a device needed to equalize the secondary side current of two CTs if they have unequal ratio values. Generally, the ACT tap has been determined by the factory and cannot be changed. So there is a possibility that the ACT tap value has a price that does not match the loading reality. C. Effect of tap changer The presence of OLTC (On Load Tap Changer) on a power transformer can create opportunities for changing transformation comparisons in operating conditions. While the tap position of the ACT will always remain fixed which causes the appearance of circulating current in the differential relay. D. The influence of the power transformer entry process to the network When a power transformer is inserted into the network, it will cause an initial excitation current (inrush current) whose value is very large and flows on one side where the transformer is entered. Current imbalances at points no. 1, 2 and 3 can be overcome by using a differential bias relay that has characteristics (slope) as shown in Figure 3. However, current imbalances at point no. 4 can generally be overcome by harmonic restraint or harmonic blocking methods. Figure 3 slope characteristics Io is Ioperation while IR is Irestrain. The determination of these two currents both Io operation and Irestrain is not always the same for each relay. For differential relays of the ALSTOM GEC family of MBCH types, for example, the value of Ioperation is defined as the difference between the input current on the primary side of the transformer and on the secondary side of the transformer. While Irestrain is defined as half of the sum of the two input currents. Mathematically it can be described as follows: Where: i1 is the input current from the primary side of the transformer. i2 is the input current from the secondary side of the transformer. A differential bias relay will only work if the relationship between the two currents (IR vs Io) is in the TRIP (operating) region. Meanwhile, if the relationship between the two currents is in the BLOCK (restrain) region, the relay will not work even though the difference between i1 and i2 is quite large. 2. Temperature Relay Temperature Relay serves to protect the transformer from excessive temperature. If the transformer temperature exceeds the specified limit, the temperature relay will work. The amount of temperature increase is proportional to the loading factor and the outside air temperature of the transformer. Temperature relays can be divided into two types, namely winding temperature relays (windings) and oil temperature relays (transformer oil) which work at two stages: working on the alarm and ordering a trip to the PMT.