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Electrical power plays an important role in the over-all development of a country and is considered as the back-bone of the industrial world today. The exponential growth in the demand of electricity for different purposes such as industrial, agricultural, commercial and domestic activities has forced power system engineers to develop additional power stations and expand the grid systems that interconnect different generating stations located at different places.
The power system of today is a complex network comprising of several sub-networks such as generation, transmission and distribution. Planning the operation of such systems under existing conditions, determining its best operating conditions as well as its improvements and future expansion requires load flow i.e. planning studies and short circuit studies i.e fault analysis and stability studies.
The generation network consists of 3-phase synchronous generators which are designed and constructed to generate 3-phase balanced voltages. Generated voltages are typically above 35kV for generators used in large electrical power stations. The generation sub-network is symmetric in nature. Transmission sub-networks consist of 3-phase lines which are arranged symmetrically on poles or transposed at regular intervals thereby balancing it. Sub- transmission voltages are typically 66-138kV and transmission voltages are typically above 138kV. Distribution networks include loads such as heating coils, lights, induction motors, synchronous motors etc which can be categorized under static, composite and dynamic loads such. Distribution voltages are typically 10-60kV.
Understanding the components and developing a mathematical model for power system network is essential for power system studies such as load flow studies, short circuit studies or transient stability studies. The model should reflect correctly the terminal behavior of each component of the network for the purpose for which the model has been developed. This chapter deals with the main components like synchronous generator, transformer, transmission lines and loads along with their mathematical models.
Fig1.1- Power System NetworkC:\Users\fathima\Pictures\power system.jpg
1.2 Synchronous Generator
Synchronous machines play a predominant role in the power system and are mainly used as alternating current generators. It works on the principle of electromechanical energy conversion. They operate in series or parallel thus forming a large power system that supplies power to loads or consumers. Generators supply power to various sectors ranging from domestic activities, agriculture, commerce and industry. Synchronous machines are built in large units and their ratings vary from tens to hundreds of megawatts based on the application.
The source driving the prime movers of synchronous generators vary based on the speed required. For high speed machines, the prime movers are generally steam turbines which employ fossil or nuclear energy resource and for low speed machines, hydro turbines which run on water power is employed.
Synchronous machines are broadly classified as rotating armature type and rotating field type
1.2.1Rotating armature type
The armature winding is on the rotor and the field system is on the stator. Current generated is brought out to the load via slip-rings. Transmitting large currents via the brushes is difficult in this type thus limiting the maximum power output. Hence it is used only in small units and its main application is as the main exciter in large alternators with brushless excitation systems.
1.2.2Rotating field type
In this type, the armature winding is on the stator and the field system is on the rotor. Field current is supplied from the exciter via two slip-rings, while the armature current is directly supplied to the load. This type of machine can deliver very high power.
According to the shape of the field, synchronous machines are classified as cylindrical rotor machines and salient pole machines.
The cylindrical-rotor construction is used in generators that operate at high speeds such as steam-turbine generators (usually two-pole machines) and the salient-pole construction is used in low-speed alternating current generators such as hydro-turbine generators, and also in synchronous motors.
1.3Working principle of cylindrical rotor synchronous generator
A balanced voltage is generated in the armature winding of a synchronous generator when the rotor windings are excited with field current and is driven at a constant speed. When a balanced load is connected to the armature winding, a balanced current at the same frequency as the emf will flow. The frequency of generated emf is related to the rotor speed and the armature mmf rotates synchronously with the rotor field. An increase in rotor speed causes a rise in the frequency of emf and current, while the power factor depends on the nature of the load.
The effect of the armature mmf on the resultant field distribution is called armature reaction. The three phase and per-phase equivalent circuit for steady-state performance analysis is shown below. The effects of armature reaction and armature winding leakage produce an equivalent internal voltage drop across the synchronous reactance Xs and the field excitation is accounted for by the open-circuit armature voltage Ef. The impedance Zs = (R + jXs) represents the synchronous impedance of the synchronous generator, where R is the armature resistance.
Fig1.2- 3 phase equivalent circuit Fig1.3- 1 phase equivalent circuit
The circuit equation of the synchronous generator is: = V + I
The voltage phasor diagram of a cylindrical-rotor synchronous generator supplying a lagging-power-factor load is given below. The terminal voltage is less than the open-circuit voltage Ef due to the synchronous impedance drop. For generator operation, the Ef phasor leads the terminal voltage phasor V by the load angle Î´.
Fig1.4-Phasor diagram of cylindrical rotor synchronous generator supplying a lagging power factor load Î´ - load angle; Ï† - power factor angle.
Fig1.5a) floating condition ; (b) unity power factor operation; (c) lagging power factor operation; (d) leading power factor operation.
Transformers are static machines that comprise an electromagnetic circuit. It consists of a primary winding and secondary winding. Electrical energy is transferred from one winding to another through magnetic field. A transformer works on the principle of mutual inductance between two conducting coils. Ideally it Fig1.6-TransformerC:\Users\fathima\Pictures\s11.jpg
is assumed that all the flux linked in the primary winding also links the secondary winding. Transformers raise or lower the voltage with a corresponding decrease or increase in current based on the number of turns in the primary and secondary.
The equivalent circuit of a single phase two winding transformer referred to the primary is give below. Shunt branches represent magnetizing current and core loss; series resistance represent winding resistance and series reactance represent leakage reactance with respect to the primary.
Fig1.7-Equivalent circuit of transformer referred to primary
Transmission lines are designed to guide electrical energy from one point to another. All transmission lines have two ends. The end of a two-wire transmission line connected to a source can be called input end, transmitter end, sending end or generator end. The other end of the line is called the output end, receiving end or load end.C:\Users\fathima\Pictures\s22.jpg
1.5.1Short transmission lines
Transmission lines of length less than 100km are termed short transmission lines. The relation between sending end and receiving end voltages and currents are expressed as =
Fig1.8- Short transmission line
1.5.2Medium transmission linesC:\Users\fathima\Pictures\s13.jpg
Transmission lines of length 100-250 km are termed medium transmission lines. The matrix in ABCD form is give below
Fig1.9-Medium Transmission line
1.5.3Long transmission lines
For lines over 250km, the parameters of the line are not lumped but distributed uniformly throughout its length.
Fig-1.10 Long transmission line
The loads can be classified mainly as
1. Continuous load and intermediate load.
2. Static and Dynamic load
3. Load may be inductive or capacitive.
Resistive load: The voltage and current peaks for a resistive load coincide and are therefore in phase and the power factor is in unity. Eg: heaters and incandescent lights
Inductive load: In an inductive load the current waveform is lagging behind the voltage waveform. Hence the voltage peaks and current peaks are not in phase. The amount of phase delay is given by the cosine of the angle between the vectors representing voltage and current. Eg: transformers and motors
Capacitive Load: The capacitive load has a current waveform which is leading the voltage waveform. Thus the voltage peaks and current peaks are not in phase. The cosine of the angle between the vectors representing voltage and current denotes the amount of phase delay. Eg: Capacitors, wiring, cable etc.
PER UNIT CALCULATIONS
2.1Single Line Diagrams
Single line diagrams are used to represent the various components of the power system mentioned above and their interconnections. A balanced three- phase system is generally analyzed on per phase basis by considering one of the three phase lines and neutral. In a single line diagram representation, the neutral is omitted and components such as generators, circuit breakers, transformers, capacitors, bus bars and transmission lines of the system are represented by standard symbols and single line respectively. The ratings and impedances of the components are marked besides the symbols. These diagrams are largely used in power flow studies.C:\Users\fathima\Pictures\s1.jpg
Fig2.1- Symbols used in single line diagram
Fig2.2- Example of a single line diagramC:\Users\fathima\Pictures\s2.jpg
2.2Per Unit Representation
The various components or sections of the power system may operate at different voltage and power levels and have their ratings in kV, kVA, kA , â„¦ etc. Hence expressing the power, current and impedance ratings of components with reference to a base value will be convenient for power system analysis. Thus for analysis purpose, a base value is chosen for voltage, power, current and impedance. The voltage, power, current and impedance ratings of the components are usually specified in per unit on taking the name plate rating as the base value.
Per unit value of any quantity is defined as the ratio of the actual value of the quantity to the arbitrarily chosen base value of the quantity.
Per unit value = Actual value/Base value
% Per unit value = Actual value/Base value x 100
2.3Advantages of per-unit computations
Name plate ratings of most devi0ces and machines specify impedance in percent or per unit basis
Ohmic values differ widely for machines of different ratings but their corresponding per unit impedances lie within a narrow range and hence it easy for analysis.
The per unit impedance of transformers is the same on either side. Whether a transformer is star connected or delta connected, it will not affect the per unit impedance value.
2.4 Per Unit Calculation for a Single Phase System
kVAb = Base kVA
kVb = Base voltage in kV
Ib = Base current in Amp
Zb = Base impedance in â„¦
Base current, Ib=
Base impedance, Zb =
= = =
Per Unit Impedance =
The per unit value of a 3-phase kVA on the 3-phase kVA base is identical to the per unit value of the kVA per phase on the kVA per phase base.
The base impedance and base current of 3-phase system can be computed directly from 3-phase value of base kVA and line value of base kV.
kVb= Line to line base kV
kVAb = 3-phase base kVA
Ib= Line value of base current
Zb= = =
2.5 Changing the base of per-unit quantities
When a system is formed by interconnecting various devices, it will be convenient for analysis if all the impedances are converted to a common base. The p.u. impedance expressed in one base value (old base) is converted to another base (new base) as follows:
The above equation can be used to convert the p.u impedance expressed on one base value to another base.
2.6 Impedance Diagram
The impedance diagram of a power system is the equivalent circuit of the power system in which the various components of the power system are represented by their simplified equivalent circuits. This equivalent circuit of power system is used to analyze the performance of a system under load conditions and is applied in load flow studies.
While forming an impedance diagram, the following approximations are made:
Under balanced conditions, no current flows through the neutral. Hence the current limiting impedances connected between the generator neutral and ground are neglected.
Shunt branches in the equivalent circuit of the transformer can be neglected as the magnetizing current of a transformer is very low when compared to load current.
If the inductive reactance of a component is very high when compared to resistance, then the resistance can be neglected.
When the impedance diagram is formed, all the impedances should be expressed in per unit calculated on a common base value.
2.7 Admittance Matrix
This power system network consists of two generating stations, three transmission lines, one load and a static capacitor connected to load bus 3.
Fig2.3- 3 bus system
The node voltage equations for this network are as follows:
Thus, in the matrix formC:\Users\fathima\Pictures\s6.jpg
This is written as
For a network having n nodes, the matrix is of the form
Where = bus current vector =
= Bus Voltage vector =
2.8 Impedance Matrix
Impedance matrix is formed through these equations,
LOAD FLOW STUDIES
3.1 What is Load Flow?
Load flow or power flow analysis is the determination of current, voltage, active power, and reactive voltamperes at various points in a power system network operating under normal steady-state or static conditions. Load flow studies are made to plan the best operation and control of the existing system as well as to plan the future expansion to keep pace with the load growth.
Such studies help in understanding the effects of interconnections with other power systems,of new generating stations, new transmission lines , new loads etc. before they are installed. This prior information plays a major role in minimizing system losses and provides a check on the system stability.
It also ensures the following
Ensure that generation supplies the demand plus losses.
Bus voltage magnitudes remain close to rated values
Generation operates within specified real and reactive power limits
Transmission lines and transformers are not overloaded.
Load flow studies are of great importance to system planners who continuosly evaluate the performance of existing systems as well as plan a power system that will come into existence 10 or 20 years later. A power company must know far in advance the problems associated with the location of the plant and the best arrangement of lines to transmit the power to load centres and this requires an indepth understanding of load flow studies. It is also necessary for economic scheduling.
3.2 Load Flow Formulation
The main information obtained from a load-flow study is the magnitude and phase angle of the voltage at each bus and the real and reactive power flowing in each line. The load flow solution also gives the initial conditions of the system when the transient behavior of the system is to be studied.
A load flow study of a power system generally requires the following steps:
Representation of the system by single line diagram
Determining the impedance diagram using the information in single line diagram.
Formulation of network equations.
Solution of network equations.
3.3 Buses and its various types
In electrical power distribution, a bus bar is a thick strip of copper or aluminum that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus. Bus bars are used to carry very large currents, or to distribute current to multiple devices within switchgear or equipment.
Buses are considered as meeting points of various components. The generator will feed energy to buses and loads will draw energy from buses. In the network of a power system the buses becomes nodes and thus a voltage can be specified for each bus. Each bus in a power system is associated with four quantities such as real power, reactive power, magnitude of voltage and phase angle of voltage. In a load flow problem two quantities are specified for each bus and the remaining two quantities are obtained by solving the load flow equations.
3.3.1 Types of buses
Three types of buses or nodes are identified in a power system network for load flow studies. In each bus two variables are known and two are to be determined. The buses of a power system can be classified into following three types based on the quantities being specified for the buses.
Swing bus / Reference bus / Slack bus: Voltage magnitude |Vi| and phase angle Î´i are specified for this bus. Usually one of the generator buses is selected as the slack bus and this bus is the first to respond to a changing load condition. It is assumed that the slack bus generates the real and reactive power required for transmission like
Generator bus or voltage control bus of PV bus: Here |Vi| and Pi are specified. The load flow equation can be solved to find the reactive power and phase of bus voltage. Often the upper and lower limits of Q are also specified.
Load Bus or PQ Bus: Here the active power Pi and reactive volt-amperes Qi are specified. The load equations can be solved to find the magnitude and phase of bus voltage.
For buses with neither generator nor load Pi = Qi = 0 and for bused with both generator and load, load is generally treated as negative generation.
Quantities to be obtained
3.4 Static Load Flow Equations
Load flow formulation can be done in both rectangular coordinates and polar coordinates. The Gauss Siedel technique is computed using the rectangular coordinates and Newton Raphson technique can be done using polar as well as rectangular co-ordinates. In this chapter, the computational procedure for Newton-Raphson method is done using rectangular coordinates.
Based on the Y bus matrix, the total current entering the ith bus of an n-bus system is given as
Ip = Yp1V1+ Yp2V2+â€¦â€¦.+ YppVp+â€¦â€¦â€¦+ YpnVn =
Static Load Flow Equations in Polar Coordinates:
If; , Ii =
The complex power injected into the pth bus is and âˆ
Separation of real and imaginary parts gives
Static load flow equations using rectangular coordinates:
The above equations are called static load flow equations. It gives n real and reactive power flow equations thus representing 2n power flow equations. In order to find a solution it is necessary to specify two variables at each bus. The solution of the 2n variables out of the 4n variables is done by numerical methods because the above equations are non linear and finding an exact solution is not possible. Hence iterative techniques are employed where in which an initial value is assumed for each of the unknown independent variable. These numerical values are substituted in the original equation to obtain a new set of corrected values of unknown variables. The second set is used to find the third set and so on. Ultimately these approximations converge upon a solution that is within the required limits.
3.5 Methods of load flow solution
Several methods are available for load flow solution, but the two main methods that are used are Gauss Siedel and Newton Raphson method. Based on requirements such as fast convergence, size of system, memory requirement, simplicity and ease of programming either of the two methods are chosen.
The two methods are discussed in detail below:
3.5.1 Method 1 - Gauss Siedel Method
This method is an iterative algorithm for solving a set of non-linear load flow equations.The equation for Ip has been mentioned in the previous section. Expanding the summation, we get the following.
The right hand side of the equation is obtained from (
Thus, . The values of node voltages V1,V2,V3â€¦â€¦.Vn is obtained from this equation by assuming an initial value for voltages such as . This is substituted in the equation and by taking p=1 we obtain the revised value of bus-1 voltage .is replaced for initial value and the revised bus-2 voltage is calculated. The iterative process is continued and till the bus voltage converges within the prescribed accuracy.
For a load bus, the iterative form the equation is
= kth iteration value of bus voltage Vi
= (k+1)th iteration value of bus voltage Vi
(k+1)th iteration values are used for all buses less than p and kth iteration values are used for all buses greater than or equal to p.
For a generator bus the above equation is rearranged to obtain the phase of the voltage and reactive power.
The reactive power of bus-p during (k+1)th iteration is given by
Stepwise procedure for load flow solution using Gauss Siedel method
Step 1: The Y matrix is formed using the admittances. An initial voltage value of 1+j0 is assumed for all buses except the slack bus for which the voltage is specified and remains unmodified in any iteration.
Step2: A suitable value is assumed for the convergence criterion . is a specified change in bus voltage that is used to compare the actual change in bus voltage between kth and (k+1)th iteration.
Step3: Set iteration count k=0 and initial value for voltages are denoted as
Step4: Set bus count p=1
Step5: Check for slack bus. If it is a slack bus go to step 12, else next step
Step6: Check for generator bus. If it is a generator bus go to next step. If it is a load bus go to step 9.
Step7: If bus-p is a generator bus with |Vp| being the specified magnitude of voltage, temporarily set |and phase of as the kth iteration value. Calculate the reactive power of the generator bus using the following equation.
If the calculated reactive power is within the specified limits for reactive power, then the bus is considered as generator bus and for this iteration. Go to step 8 after this.
If the calculated reactive power violates the specified limits, then consider this bus as load bus. The following changes are then made
If, then or then
Since the bus is treated as load bus, take actual value of for (k+1)th iteration
Step8: For generator bus the magnitude of voltage does not change and remains as the specified value for all iterations. The phase of the bus voltage is calculated from
The (k+1)th iteration of generator bus voltage is given by =|. After calculating for generator bus, go to step 11.
Step9: For the load bus the (k+1)th iteration value of load bus-p voltage,is calculated using the equation
Step10: An acceleration factor can be used for faster convergence. If it is used then the (k+1)th iteration value of bus-p voltage is modified as . Then set
Step11: Calculate the change in the bus p-voltage, using the relation Î”
Step12: Increment the bus count by 1 and go to step 5. Repeat steps 5 to 11 until all the bus voltages have been calculated. Continue this process till the bus count is n.
Step13: Find out the largest of the absolute value i.e. . Let the largest value be |. If this is less than the convergence criterion then move to next step, else increment the iteration count and go to step 4.
Step14: Calculate the line flows and slack bus power using the bus voltages.
3.5.2 Method 2: Newton Raphson
This is an iterative method which approximates the set of non-linear simultaneous load flow equations to a set of linear simultaneous load flow equations using Taylor's series expansion and the terms are limited to first.
= Real and imaginary part of respectively
= Real and imaginary part of respectively
= Conductance and susceptance of the admittance respectively.
On substituting we get,
On separating the real and imaginary parts,
The above 2 equations are called load flow equations of Newton-Raphson
The unknown variables to be calculated for load bus and generator bus remains the same. In order to calculate of load bus and and phase of for generator bus we have to first calculate the real and imaginary parts of bus voltages, i.e. and respectively.
For an n-bus system, bus-1 is considered as slack bus and the real and imaginary part of remaining (n-1) bus voltages have to be solved. Thus 2(n-1) variables need to be solved through a set of 2(n-1) equations.
22.214.171.124 Formations of 2(n-1) equations for a system with n buses
Case (i): When all the (n-1) buses are load buses
Here bus 1 is slack bus and the remaining buses are load buses
are the specified real powers and are the specified reactive powers of (n-1) load buses. The matrix thus formed to obtain the unknown variables and â€¦. is given below :C:\Users\fathima\Pictures\s4.jpg
Case (ii): When the system has both load and generator buses.
Even in this method, bus 1 is considered as slack bus, buses 2 to m are load buses and bus (m+1) to bus n are generator buses. P2, P3 â€¦â€¦ Pn are the specified real power of (n-1) buses. Q2,Q3â€¦â€¦..Qm are the specified reactive powers of load buses.Let |Vm+1|â€¦.|Vn| be the specified magnitude of voltages of generator buses. The unknowns are same as above and the matrix equation for this form is:
C:\Users\fathima\Documents\4th year\APS\2010-10 (Oct)\2010-10 (Oct)\scan0002.jpg
126.96.36.199 Procedure for load flow solution by Newton-Raphson Method
Step1: The Y matrix is formed using the admittances. An initial voltage value of 1+j0 is assumed for all buses except the slack bus for which the voltage is specified and remains unmodified in any iteration.
Step2: A suitable value is assumed for the convergence criterion . is a specified change in the residue that is used to compare the actual residues at the end of each iteration.
Step3: Set iteration count k=0 and initial value for voltages are denoted as except for the slack bus. The real part of the voltage are denoted as and the reactive part as for p=1,2,3..n
Step4: Set bus count p=1
Step5: Check for slack bus. If it is a slack bus go to step 13, else next step
Step6: Calculate the real and reactive power of bus-p using the following equation
Step7: Calculate the change in real power. Change in real power, where = Specified real power for bus-p.
Step8: Check for generator bus. If it is a generator bus, go to next step otherwise go to step 12.
Step9: Check for reactive power limit violation of generator buses. For this compare the calculated reactive power with specified limits. If the limit is violated go to step 11, else go to next step.
Step10: This bus is a generator bus if the calculated reactive power is within the specified limits. The voltage residue is calculated using the following equation.
- . Then go to step 13.
Step11: If the reactive power limit is violated then treat this bus as a load bus. Now the specified reactive power for this bus will correspond to the limit violated.
i.e, if then
Step12: Calculate the change in reactive power for load bus (or for the generator bus treated as load bus)
Change in reactive power,
Step13: Repeat steps 5 to 12 until all residues are calculated. For this increment the bus count by 1 and go to step5 until the bus count is n.
Step14: Determine the largest of the absolute value of the residue (i.e among Let this largest change be.
Step15: Compare and . If then go to step-20. If go to next step.
Step16: Determine the elements of jacobian matrix by partially differentiating the load flow equations and evaluating the equations using the kth iteration values.
Step17: Calculate the increments in real and reactive part of voltages, and by solving the matrix B=JC. The elements of matrix B are calculated in the previous step. The elements of C matrix are and which are voltage increments to be solved.
Step18: Calculate the new bus voltages as given below:
Step19: Advance the iteration count, i.e k=k+1 and go to step 4
Step20: Calculate the line flows.
3.6Comparison of Gauss Siedel and Newton Raphson Method of Load Flow Study
1. The variables in GS method are expressed in rectangular coordinates while in NR it can be expressed both in polar and rectangular coordinates. Rectangular coordinates method requires more memory.
2. Since the number of iterations in GS method is less than NR method, computation time per iteration is less in GS method.
3. NR method converges faster than GS method because GS method has linear convergence and NR method has quadratic convergence
4. In GS method, number of iterations increases with number of buses but in NR method the number of iterations remains constant and does not depend on the size of the system.
5. Convergence in GS method is affected by the choice of slack bus and the presence of series capacitors but NR method is less sensitive to these factors.
6. GS method requires large number of iterations in contrast to GS method for the same level of accuracy.
Advantages of G-S Method
Calculations are simple and so the programming task is lesser.
The memory requirement is less
Useful for small size system
Disadvantages of G-S Method
Requires large number of iterations to reach convergence.
Not suitable for large systems
Convergence time increases with size of the system
Advantages of N-R Method
The N-R method is faster, more reliable and the results are accurate.
Requires less number of iterations for convergence
The number of iterations are independent of the size of system(number of buses)
Suitable for large size systems
Disadvantages of N-R method
The programming logic is more complex than GS method.
The memory requirement is more.
Number of calculations per iterations is higher than GS method.
A 5 bus system is solved using Gauss Siedel method and the same network is solved using the simulation software-Power world simulator and is discussed in detail in chapter 5.
APpLICATIONS OF LOAD FLOW STUDIES
4.1 Applications of Load Flow Studies
The applications of Load Flow Studies can be broadly categorized into two:
Study of an existing network
Load flow is used to evaluate the performance of an existing system under various load conditions such as peak load and light load under normal and outage conditions. It gives an understanding of how the network behaves with all elements in service as well as under emergency conditions as mentioned in the planning criteria.
Under peak load conditions, the system load which is a summation of individual loads at various substations will be a maximum. The reactive power requirement will be high. More loads also draw more current resulting in voltage drop at the buses.
Under light load conditions, voltage in the substations can exceed due to cable charging. This is known as the Ferranti effect.
Problems like unacceptable voltage conditions and equipment overloading are identified and remedial measures such as reactive compensation, addition of new feeders etc are adopted.
4.2 Planning Criteria
Certain basic principles of planning criteria are:
Bus voltages should be within its specified limits under normal and outage conditions.
Equipments (lines, transformers etc) should operate within its normal thermal ratings, normal line capacity and normal equipment voltage limits when the system is operating with all elements in service.
Equipments should operate within emergency thermal ratings, line loading limits and emergency voltage limits immediately after a disturbance involving the loss of an element for a short period of time. The system should be capable of such performance at all times including operations during minimum and maximum forecasted load conditions.
The two types of operation requirements in power system planning are given below.
4.4.1Normal operation requirements
When power system equipment is operated under good conditions, various operating standards are ensured. For example, line transmission power, generator output, voltage level and so on is within the rated range
4.4.2 Contingency operation requirements
When an equipment fault or load disturbance occurs, the electricity supply reliability requirements should be satisfied.
N-1 reliability criterion means that the sudden loss of a single element mainly a line, a transformer etc. even during summer peak should not result in the loss of power supply to consumers. It is predominantly used especially for the planning and design of the 132kV network including power lines, transformers etc. The transmission network should be planned in such a way that there should not be any bottle neck to evacuate the entire power from any generating plant under single contingency.
N-2 reliability criterion means that the sudden loss of two lines even during summer peak, should not result in system collapse nor lead to a widespread loss of power supply to consumers. This is mainly used for 400kV network in U.A.E as it is the backbone of the power system. The logic diagram for transmission expansion study is shown in Figure below.
4.5 Voltage Control
The various components or equipments connected to a power system are designed to work satisfactorily at rated voltages. Therefore the voltages at various buses in a power system should remain constant or vary within the specified limits. If the voltage variations are more than a specified limit then the performance of the equipment will be poor and the life of the equipment will reduce. Hence voltage control is very important in a power system.
It can be proved that the voltage variations in a bus or node are directly related to reactive power. If the reactive power injected to a bus is less than the reactive power drawn from it then the voltage of the bus decreases and vice versa.
Therefore the reactive power Q should be adjusted at the load buses in order to maintain their voltages constant. Thus sufficient level of reactive support has to be provided to ensure that satisfactory voltage control of the power system is maintained under conditions following a loss of critical network element or generator particularly under high summer loading conditions. This is possible by generation of reactive power by means of shunt capacitors or synchronous motors. For capacitive loads or very light loads the reactive power can be generated by shunt inductor.
Reactive power used for voltage control is provided in a number of ways:
Synchronous compensators and Static Var Compensators
Tap changing, regulating and boosting transformers
The first two sources are fast acting and can be used to provide continuous control of voltage. They are able to provide rapid reactive support following loss of a critical network element or generator. Capacitor banks provide discrete blocks of reactive power and are appropriate to compensate slowly changing reactive demands associated with daily load variations.
Performance evaluation of an existing system with all the devices in service under normal, peak load and light load conditions.
Identification of potential problems in terms of unacceptable voltage conditions, overloading of facilities, decreasing reliability, or any failure of the transmission system to meet performance criteria.
How a system re-routes power under contingency conditions.
Future expansion of the system and technical and commercial feasibilities of alternative plans under both normal and emergency conditions.
By changing the location, size, and number of transmission lines, the planner can achieve to design an economical system that meets the operating and design criteria.
4.1 Planning Objective
To cope up with the huge growth in power system network, the power system infrastructure has also increased tremendously with bulk of transmission system at 400kV, 132kV etc. Planning is the determination an optimal network configuration to meet demand growth and generation plans for the planning period. Thus planning is a very crucial stage in the development of a power system network. All parts of the power system must be planned, and designed so that all the future requirements for power generation, transmission, distribution and consumption can be suitably met at the lowest possible cost. All this needs to be done by maintaining adequate reliability and taking into account various constraints in line with the best international standards and practices.
4.2 Major Factors Affecting the System Planning Process
The system planning process is strongly influenced by a wide variety of external and internal factors such as load growth, cost, availability of sites corridors, etc as shown in the figure.C:\Users\fathima\Documents\4th year\APS\scan0039.jpg
Fig4.1 - Factors affecting system planning process
4.3 Planning Stages
The various planning stages are:
Load forecasting: The first and one of the most important activities in the Power System Planning process is the electricity demand/load forecast. A reasonably accurate load forecast is a fundamental requirement for successful planning and operation of any power system. For generation planning, it is adequate to have the total demand and energy forecast on whole of the system basis, but for transmission and distribution planning, the geographical location of the load is of prime importance, where the facility is required to be created.
Evaluation of existing system capabilities: Once the forecast of a system is available, the existing system is comprehensively analyzed and the weak points are identified. This involves
Determination of the capabilities of the existing components in the system.
Modeling of the static and dynamic system for the network using computer software.
Evaluation of the system performance against adopted standard values.
Defining the range of operating conditions for which system performance must meet those standards.
Identification of System Deficiencies: Deficiencies in the system are identified on the basis of existing system capabilities, future requirements etc.
Formulation of schemes to meet system requirements: New proposals are formulated for overcoming the identified deficiencies.
Evaluation of Techno- Economic schemes: The various options to meet the existing and future inadequacies of the power system are compared in order to determine which of them best satisfies the requirement. Some of the features are technical sufficiency, technical feasibility, cost effectiveness etc.
Selection of Most Optimal Option: On the basis of economic analysis carried out, the most cost effective proposal is selected for implementation if it satisfies the evaluation criteria.
Plan Release and Approval: The complete plan is issues as a draft to the concerned department for their comments and feedback. After getting the feedback the plan is sent to the management for approval.
Implementation of the most preferred scheme
Project Monitoring and Reviews
Diagram below shows a functional block diagram of a typical transmission-system-planning process. This process may be repeated, with diminishing detail, for each year of a long-range (15-20 year) planning horizon. The key objective is to minimize the long-range capital and operating costs involved in providing an adequate level of system reliability, with due consideration of environmental and other relevant issues.
System performance without violating planning criteria
Expand present system
Total cost acceptable?
15-20 year expansion plan complete?
Investigation of different alternatives
Fig 4.2 - System Planning Process
4.6 Applications of Load Flow Studies
Load Flow and Short Circuit Study
Consider all generation schedules
Study system performance at high and low load conditions.
Study contingency analysis and identify overloads and low voltages
Investigate different alternatives to reinforce and expand transmission system
Have the studies provided satisfactory results?
Conduct stability study for major transmission expansion
Make necessary modifications to the network
Fig4.3 - Expansion of a power system network
POWER WORLD SIMULATOR
5.1 Introduction to Power World Simulator
Power World Simulator 15 Beta is the software that has been used in this project to conduct load flow studies. It is an extremely visual, high voltage interactive power system simulation, analysis and visualization package that is very much useful for serious engineering analysis. Due to its interactive and graphical features it can also be used to explain power system operations to non-technical audience as well.
This software is a powerful solution tool which is capable of efficiently performing power flow analysis on systems containing up to 100,000 buses. Unlike other commercially available power flow tools, Simulator enables the user to visualize the system through full-color animated one line diagrams. System models can be either modified or built from scratch using Simulator's full-featured graphical case editor.
Transmission lines can be switched in or out of service and new transmission or generation can be added as and when required. Thus, it increases the user's understanding of system characteristics, problems, constraints and techniques to remedy them thereby proving to be an effective tool for load flow studies.
The Power world simulator also contains tools necessary to perform fault analysis, integrated economic dispatch, area transaction economic analysis, power transfer distribution factor computation, short circuit analysis, and contingency analysis.
5.2 Case Study
The load flow problem solved in chapter 3 is solved using power world simulator. This case is constructed using 5 buses, AC transmission lines, generators and loads. The steps carried out are as follows:
When a component is drawn on the SLD, a dialog box appears and the relevant data should be filled in the space provided.
Bus bar: Insert the bus number, name of the bus, nominal voltage, bus voltage in p.u, angle and a tick mark on the slack bus check box if it is so.
Transmission LineÂ : Input the Impedance (R + jX). In constructing a line, a pie chart will be observed. Do not remove this, as this is the line loading chart.
GeneratorÂ : Input the Nominal MW and MVAR values, as well as the Maximum & Minimum MW and MVAR values.
Â Load: Input the load details in MW and MVAR consumed at each bus.
Once the necessary parameters have been input, the simulation can be begun. To obtain an accurate power flow solution, it is necessary to ensure that the circuit diagram and data inputs are free from error. Once there are no errors run modeÂ button is pressed. The power flow can be seen by pressing the play button.
The result obtained is as follows:C:\Users\fathima\Pictures\LF5.jpg
The report generated from the simulation is as follows:
Bus Flows C:\Users\fathima\Pictures\LF6.jpgC:\Users\fathima\Pictures\LF&.jpg
Fig 5.6 - 7 bus system