For a given quantity voltage, current, power, impedance, torque, etc. Pdf every textbook on power system analysis discusses the perunit system, varying in depth with problems and solutions. An example would be a cable modem, speakers, or a printer. Treat each load on the systems as a resistance in series with a reactance. The per unit real and reactive power loads at busses 2 and 4 are p2 0. Circuit models of transmission line, synchronous machines, transformers, and load. Power systems analysis, second edition, describes the operation of the interconnected power system under steady state conditions and under dynamic operating conditions during disturbances. The power system between the generation sources and end consumers is divided into di.
The per unit system of calculation is a method whereby system impedances and quantities are normalized across different voltage levels to a common base. These become useful when performing calculations across a power system where multiple voltage. Lecture 15 power system representation and equations. Nov 04, 2017 12 feb 20 for many years, the per unit or per cent method has been used to define the constants of such this article is only available in pdf. May 26, 2019 fundamental to any power system analysis is the knowhow of per unit systems. Normally in such studies the load is of composite nature with both industrial and domestic components. Per unit pu is commonly used for the calculation of voltage, current, impedance, and power in power flow and related calculations. In a threephase system rather than obtaining the per unit system definition values using per phase base quantities, the per unit values can be obtained directly by using threephase base quantities. We can then calculate the current and impedance bases using the chosen power and voltage bases as. Pdf a gui based perunit representation of power system with. Per unit representation part 01 introduction youtube. Just imagine you are in the united states and need to compare measurements with someone elsewhere in the world. In per unit notation, the physical quantity isin per unit notation, the physical quantity is expressed as a fraction of the reference vava ue. Sketch the perphase, perunit equivalent circuit of the power system in figure 102.
Per unit system is the ratio between the actual value in any unit and the base or reference value in the same unit. The normalization typically follows transformer ratings. Fundamental to any power system analysis is the knowhow of per unit systems. Obtain the per unit impedance reactance diagram of the power system shown in fig. Selecting the generator rating as the base quantities in the generator circuit determine the bus quantities in other parts of the system and evaluate. Sep 16, 2017 per unit system is the ratio between the actual value in any unit and the base or reference value in the same unit. In a power system different power equipment with different voltage and power levels are connected together through. A gui based perunit representation of power system with symmetrical components. The perunit system is widely used in the power system industry to express values of voltages, currents, powers, and impedances of various power equipment. Draw the structure of an electrical power system and describe the components of the system with typical values 16 3. To gain a better understanding, it is worth running through the typical steps required to solve a fault calculation problem. In the computation of power system problems it is more convenient to express impedance, current, voltage and power in terms of perunit p.
The second formula for calculating resistive heating is p. Per unit calculation per unit system examples electrical. Electric power systems pennsylvania state university. Although power system analysis is now done by computer, results are often expressed as per unit values on a convenient system wide base. Computer methods in power systems notes pdf cmps notes. The perunit real and reactive power loads at busses 2 and 4 are p2 0.
The per unit system is a method of expressing quantities in an electrical system e. Vin per unit vin kvv base in kv where the base value is a reference value for magnitude. A single line diagram is diagrammatic representation of power system in which the. Using a 8kv, 100mva base in the transmission lines, draw the positive and negative sequence per unit diagrams. Per unit system definition per unit representation of. The generators perunit voltage is the transmission lines perunit impedance is the other base quantities are 10 0 41.
Power supply a power supply is the component of the system unit that converts the wall outlet ac power into the dc power. 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. Written at a foundational level, including numerous worked examples of concepts discussed in the text, it provides an understanding of how to keep power. Sep 18, 20 power system components and per unit system 96 5. While performing calculations, referring quantities from one side of the transformer to the other side serious errors may be committed. Chapter 8 per unit representation sometimes it is more convenient to represent the value of current, voltage, impedance, power in per unitpu value rather t slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. Per unit system allows the system operator to overlook abnormalities in the system easily. Ppt lecture 15 power system representation and equations. Perunit equivalent reactance network of a three phase power system, numerical problems.
The positive sequence fault current for 3 phase to ground fault level is the same as the 3 phase fault level mva, i. The per unit system was developed to make manual analysis of power systems easier. Information for a small power system is shown below. The per unit system also makes the calculation easier as all the values are taken in the same unit. Power system representation and equations a oneline diagram of a simple power system oil or liquid circuit breaker rotating machine twowinding power transformer wye connection, neutral ground perphase, per unit system 3,base 2 l,base base base l,base base l,base 3,base base 3 3. Nov, 2016 in representation of load curve for various system studies such as load flow and stability studies, it is essential to know the variation of real and reactive power with variation of voltage. 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.
Every textbook on power system analysis discusses the per unit system, varying in depth with problems and solutions. Circuit analysis of systems containing transformers of various transformation ratios is greatly simplified. Aug 02, 2018 prebook pen drive and g drive at teacademy. Representation of load curve different ways of load. Per unit system for power systems calculations what is per unit. Per unit representation in a power system different power equipment with different voltage and power levels are connected together through various step up or step down transformers.
However, these variations are most of the time so small that an algebraic, i. Jan 05, 2017 per unit representation part 01 introduction the per unit system power transmission lines are operated at voltage levels where kv is the most convenient unit to express voltage. Per unit system per unit system is when we normalize the voltage and current at each location. Explain the per phase generator model with required diagrams. The per unit value of fault levels on 100 mva base are 4. Synchronous machines, transformers, transmission lines, one line diagram, impedance and reactance diagram, per unit system. Nov 11, 2016 if the actual impedance is z ohms, its per unit system definition value is given by. Besides being taught in higher education, it is quite commonly used by engineers in the power industry. Circuit analysis of systems containing transformers of various transformation ratios is.
In a power system there are always small load changes, switching actions, and other transients occurring so that in a strict mathematical sense most of the variables are varying with the time. This makes the per unit value of either voltage or current to be around 1. Symmetrical components of unbalanced phasors, power in terms of symmetrical components, sequence. However, it is still not an interactive method for teaching and learning the per unit representation. Voltages, currents and impedances expressed in per unit do not change when they are referred from one side of transformer to the. In power system analysis, per unit pu system is used to express a physical variable as a fraction of base or reference value. Discuss in detail about the modeling of transmission lines. The base voltage varies by the voltage ratio of each transformer in the circuit but the base apparent power stays the same through the circuit. 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.
However the presence of various voltage and power levels causes problem in finding out the currents or voltages at different points in the network. Rather determined per unit values serve to assist in the calculation of other system values. It is mainly used for transformers and ac machines. I2r where p is the power, i is the current, and r is the resistance. Source impedance for fault analysis is furnished in per unit. Energy time and see that, with the charge canceling out, units of current multiplied by units of voltage indeed give us units of power. The following calculators compute various base and per unit quantities commonly used in the per unit system of analysis by power system engineers. Per unit representation in power system studies in a large interconnected power system, the various sections of the system are connected through transformers and have different voltage levels.
Develop a perphase, per unit equivalent system for this power system using a base apparent power of 100 mva and a base line voltage at generator g1 of. 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. The determination of per unit values is not the end objective of a study. Every textbook on power system analysis discusses the perunit system, varying in depth with problems and solutions. Perunit system the base apparent power and base voltage are specified at a point in the circuit, and the other values are calculated from them. With neat diagrams, explain the transformer model used for per phase analysis. To make electrical power system calculations easier, impedance, current, voltage and power are all referred to a common base. In representation of load curve for various system studies such as load flow and stability studies, it is essential to know the variation of real and reactive power with variation of voltage. The generator at bus 3 is supplying a real power p3 0. V in per unit v in kvv base in kv where the base value is a reference value for magnitude. Assume that bus 1 is the slack bus and that it has a voltage v1 1. Single line diagram, impedance and reactance diagrams. It ends with a short discussion of perunit systems and power system representation. So much so, it shows up in all exams pertaining to power.
Clearly explain the basic components of a power system. The components of power system are generators, power. Also, machine and transformer impedances fall within fairly narrow ranges for each type and construction of device while the perunit system is employed. Jun 24, 2019 per unit systems is an important concept in power system analysis. Or in the other words, per unit is a scaling system to make calculation process in analyzing an electrical network system easier, b. Per unit systems is an important concept in power system analysis. Per unit data representation yields valuable relative magnitude information. Pdf a gui based perunit representation of power system. Jan 15, 2015 chapter 8 per unit representation sometimes it is more convenient to represent the value of current, voltage, impedance, power in per unitpu value rather t slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. The per unit value of any quantity is defined as the ratio of the actual value of the any quantity to the base value of the same quantity as a decimal.
To use the perunit system, we normally begin by selecting a threephase power base and a linetoline voltage base. The perunit system puts all the values of a power system on a common base so they can be easily compared across the entire system. Assume that no current is flowing in the network, so that all generator and motor voltages are. It is typically used for transformers and ac machines. Per unit analysis can be used to calculate system three phase fault levels and the current distributions. However, it is still not an interactive method for teaching and learning the perunit representation. Power system representation and equations a oneline diagram of. Question bank with solved 2 mark qs power system analysis. If the actual impedance is z ohms, its per unit system definition value is given by. Perunit representation results in a more meaningful and correlated data. Note that you do not have enough information to actually calculate the values of components in the equivalent circuit. This article, part of a series of articles, will explain step by step how to calculate these parameters for any component anywhere in the power system.
The per unit system is widely used in the power system industry to express values of voltages, currents, powers, and impedances of various power equipment. The per unit value of any quantity is defined as the ratio of the actual value of the. Impedance and voltagelevel conversions are avoided. Another approach to solve circuits containing transformers is the perunit. Per unit data representation yields important information about relative magnitudes. Equivalent generator representation for power system analysis. For a power system, practical choice of base values are. Computer methods in power systems notes pdf cmps notes pdf. Per unit analysis per unit system power system analysis. Per unit system, per unit impedance diagram of the power system.
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