Circuit analysis of systems containing transformers of various transformation ratios is greatly simplified. The base power may be the rating of a single piece of. We discuss circuit breaker and fuse selection in section 7. Balanced threephase power systems are assumed throughout this chapter. Defining base quantities, per unit voltage per unite current and per unit impedance, selecting the appropriate mva and kv base, calculating the base current and impedance, changing bases, per unit problem solving worksheets, per unit calculation examples and problems. Selected problems, psa and design, chapter 3 per unit system tutorial 1 1 for the system shown in figure 1, draw an impedance diagram in per. Per unit system summary modern power systems comprise several transmission lines interconnected in subsystems of different voltage levels through various stepup or stepdown transformers. Consider a pair of voltage sources sitting side by side with. On the second day, jim builds upon the per unit system and takes you to the next level with symmetrical components and unbalanced power system calculations. Impedance and voltagelevel conversions are avoided. Basekvaperphase kvaperphase phasebasekva phasekva ie.
Using a 8kv, 100mva base in the transmission lines, draw the positive and negative sequence per unit diagrams. Per unit identities university of minnesota duluth. Given the system single line diagram, construct and simplify the per unit impedance diagram. The determination of per unit values is not the end objective of a study. Ppt transformers, per unit calculations powerpoint.
Construction site demand requirement a 200 b 200 c 300 the cost of transporting 1 ton of concrete from each plant to each site is shown in the gure 8 in emalangeni per ton. Pdf per unit system practice problem solved for easy. L05 power system representation and introduction to per unit system ee3230 spring 2014 duration. Just imagine you are in the united states and need to compare measurements with someone elsewhere in the world. Per unit and symmetrical components electrical power.
Singlephase per unit system university of texas at. Perunit representation results in a more meaningful and correlated data. Another approach to solve circuits containing transformers is the perunit system. With neat diagrams, explain the transformer model used for per phase analysis. Calculations are simplified because quantities expressed as per unit do not change when they are referred from one side of a transformer to the other. When values are expressed in pu, the comparison of electrical quantities with their normal values is straightforward. All p and q quantities are three phase voltage magnitudes are represented as a fractional part of their standard or base value all phase angles are represented in the same units as normally used advantages 1. It would be very difficult to continually have to refer impedances to the different sides of the transformers this problem is avoided by a normalization of all variables. Per unit notation in per unit notation, the physical quantity isin per unit notation, the physical quantity is expressed as a fraction of the reference vava ue. Calculations are simplified because quantities expressed as perunit do not change when they are referred from one side of. How to model bus admittance matrix ybus of a given power. The per unit system also makes the calculation easier as all the values are taken in the same unit. Measurement conversion word problems liquid volume 1. Per unit analysis for single and three phase systems udemy.
The main idea of a per unit system is to absorb large differences in absolute values into base relationships. Thus, representations of elements in the system with per unit values become more uniform. The transformer can be included as a simple impedance. The generators perunit voltage is the transmission lines perunit impedance is the other base quantities are 10 0 41. Per unit system practice problem solved for easy understanding. Explain the per phase generator model with required diagrams. Treat each load on the systems as a resistance in series with a reactance.
Threephase per unit system the same advantages apply to a threephase system if the following rules are obeyed. So much so, it shows up in all exams pertaining to power. Jun 24, 2019 per unit systems is an important concept in power system analysis. In many respects the method is similar to the use of percentage values. In power system analysis, per unit pu system is used to express a physical variable as a fraction of base or reference value. Pick a voltage base for each different voltage level, vb. Mar 26, 2018 solving per unit system numerical and impedance diagram in power ystem analysis. If she serves 250 ml per student, how many liters of juice will she need to buy. The per unit system allows engineers to analyze a single phase network where. Source impedance for fault analysis is furnished in per unit. Also, machine and transformer impedances fall within fairly narrow ranges for each type and construction of device while the perunit system is employed.
The per unit system is used in power flow, short circuit evaluation, motor starting studies etc. The following calculators compute various base and per unit quantities commonly used in the per unit system of analysis by power system engineers. Inventory management example problems with solutions. Per unit calculation per unit system examples electrical. Mar 14, 2017 l05 power system representation and introduction to per unit system ee3230 spring 2014 duration. The perunit value of any quantity is defined as the ratio of actual value in any unit to the base or reference value in the same unit.
Fault current calculations let choose the perunit base values for this power system to be 100 mva and 115 kv at the highvoltage side and. To gain a better understanding, it is worth running through the typical steps required to solve a fault calculation problem. A common threephase voltampere base is used throughout the system, where sbase,3. This makes the per unit value of either voltage or current to be around 1. Besides being taught in higher education, it is quite commonly used by engineers in the power industry.
View notes per unit calculation from electrical keee4269 at university of malaya. Generally base values of power and voltage are chosen. In the power systems analysis field of electrical engineering, a perunit system is the expression of system quantities as fractions of a defined base unit quantity. Clearly explain the basic components of a power system. Per unit data representation yields valuable relative magnitude information. Unit rate word problem worksheet 1 integers this problem worksheet features word problems where you will calculate the unit rate for everyday situations like points per game and miles per hour. Information for a small power system is shown below.
In the power systems analysis field of electrical engineering, a per unit system is the expression of system quantities as fractions of a defined base unit quantity. Measurement conversion word problems lengthdistance. The perunit system was developed to make manual analysis of power systems easier. Question bank with solved 2 mark qs power system analysis. Discuss in detail about the modeling of transmission lines. Per unit pu is commonly used for the calculation of voltage, current, impedance, and power in power flow and related calculations. Voltage bases are related by transformer turns ratios.
However, this makes it difficult to figure out the various voltage and power levels at different points in the network. Circuits are simplified voltages have same range in per unit in all parts of the system from ehv. The annual demand is approximately 1,200 batteries. Per unit calculations a key problem in analyzing power systems is the large number of transformers. Fault calculation methods university of new south wales. An auto parts supplier sells hardybrand batteries to car dealers and auto mechanics. Per unit system per unit system is when we normalize the voltage and current at each location. This initial solution can also be presented together with the costs per unit as shown in the table 8.
Learn how to more easily use positive, negative and zero sequence components and draw zero sequence diagrams. Per unit analysis understanding the per unit system and its uses. As the per unit values of parameters of a machi f i d i ll f llhine of a given design normally falls. The plants are able to supply the following numbers of tons per week. For example, a motor with a 100 horsepower hp nameplate rating which delivers 90 hp to a dynamic load is said to be 90% loaded or 0. Per unit calculation per unit system practice problem. Advantages and disadvantages of per unit system in power system analysis, per unit pu system is used to express a physical variable as a fraction of base or reference value. The subtransient reactance of the two generators to the system base is 2 given new new given new given v. The per unit system is a method of expressing quantities in an electrical system e.
In order to do this in a satisfactory way the state of the system, i. Any quantity is converted into per unit quantity by dividing the numeral value by the chosen base value of the same dimension. Assume that no current is flowing in the network, so that all generator and motor voltages are. This allows the removal of the complexity of transformer ratios in the fault calculations.
The per unit system overview page 1 of 6 overview the per unit system is based on the formula shown in equation 1. Although powersystem analysis is now done by computer, results are often expressed as perunit values on a convenient systemwide base. Plant supply capacity 1 300 2 300 3 100 the requirements of the sites, in number of tons per week, are. Stepper motors usually have 200 or 400 full steps per one rotation of its shaft. Only whole numbers are included in the problems and answers. Introduction to per unit calculations pages 1 11 text. Inventory management example problems with solutions 1. Per unit system is the ratio between the actual value in any unit and the base or reference value in the same unit. Provides a more meaningful comparison ofprovides a more meaningful comparison of parameters of machines with different ratings. Per unit analysis can be used to calculate system three phase fault levels and the current distributions. Per unit system explaination with solved example free download as powerpoint presentation. Note that you do not have enough information to actually calculate the values of components in the equivalent circuit.
Advantaggyes of per unit system normally we are dealing with numericsnormally we are dealing with numerics near unity rather than over a wide range. The per unit system of calculation is a method whereby system impedances and quantities are normalized across different voltage levels to a common base. Rather determined per unit values serve to assist in the calculation of other system values. Or in the other words, per unit is a scaling system to make calculation process in analyzing an electrical network system easier, b. The normalization typically follows transformer ratings. For example, a transient voltage reaching a maximum of 1. The subtransient reactance of the two generators to the system base is 2 given new new given new given v s zz vs. Here are the main reasons for using the perunit system.
Vin per unit vin kvv base in kv where the base value is a reference value for magnitude. Per unit systems is an important concept in power system analysis. All values here are given in per unit, we will enlist conversion formulas for per unit system at the end it is essential to convert all given and. The object of the power system analysis and the analysis tools are shown in table 2. By definition, the per unit value of a quantity is the ratio of the original quantity to its base value which results in a dimensionless per unit or pu value.
The system consists of 4 numbered in circles buses, 0 bus is a ground or reference bus, a generator with an emf of 1. That means this this one, this is that single line diagram that that is the schematic diagram of all these thing even and this is your per unit diagram right. Sketch the perphase, perunit equivalent circuit of the power system in figure 102. Steps per unit value in further text as spu defines how many steps will stepper motor have to make in order to move the axis for distance of one unit. The per unit value of a 3phase kva on the 3phase kva base is identical to the per unit value of kva per phase on the kva per phase base. Per unit system for the analysis of electrical machines or electrical machine system, different values are required, thus, per unit system provides the value for voltage, current, power, impendence, and admittance. Power system analysis for solving problems with expanding. It ends with a short discussion of per unit systems and power system representation. Original circuit 8 per unit example, contd same circuit, with values expressed in per unit. When they say 5 meters, you pull out your calculator and convert to feet and inch. This class is loaded with many in class examples and problems for a hands on. How to model bus admittance matrix ybus of a given. Nissin electric has achieved successful results in power system analysis in the time domains of surge.