INTERACTIVE SIMULATION OF POWER SYSTEMS: ETAP APPLICATIONS AND TECHNIQUES Keith Brown Student Member, IEEE Ooeration Tichnolow. Inc. Iriine, California -- Farrokh Shokooh Senior Member, IEEE Operation Technoiogy, Inc. Irvine, California Abstract For many years electrical engineers have relied on the power (but not necessarily the convenience) of mainframe computers to ana- lyze and design power systems. Recent advanc- es in microcomputers have brought the power of mainframe computing to the desktop, paving the way for straight-forward, easy to use engineering applications which are designed especially for the personal computer. System studies are an integral part of power system engineering and design. A structured computer program that uses technically cor- rect models, employs a user-friendly inter- face, uses a common data base, and traps user errors is a powerful tool which greatly en- hances the engineer's efficiency and produc- tivity. ETAP is an engineering design and analysis program which satisfies these cri- teria. In addition, ETAP performs numerical calculations with tremendous speed, automati- cally applies industry accepted standards, and provides easy to follow output reports. This paper discusses solution techniques used by ETAP for various problems, as well as other features that make ETAP unique. Introduction The Electrical Transient Analyzer Program, commonly known as ETAP, began as a mainframe program and was rewritten for the PC as an interactive power system analysis and design tool. Significant effort has been made to insure that ETAP will provide the user with the highest possible technical accuracy, while maintaining a thoroughly user-friendly format. The common data base exemplifies the inter- active nature of ETAP. Separate data editors for Bus, Branch, and Machine data allow the user to model the system in a common data base. User-edited libraries provide typical data which can be substituted into the data base upon request. When system studies are to be performed, ETAP automatically extracts the necessary parameters from the common data base. An interface with AutoCad is provided to automatically produce a detailed one-line diagram of the network modeled in the data Herminio Abcede, Member, IEEE Flour Daniel Inc. Irvine, California Gary Donner Member IEEE. Texaco Refiding and harketmg Los Ange1es;California base including (optionally) load flow and/or short circuit results. ETAP, while capable of handling 1000 buses, contains a load schedule program which tracks up to 10,000,000 load items, and reports the voltage and short-circuit current at the terminals of each load item. This capability makes ETAP suitable for large industrial facilities, as well as utility systems. ETAP * * * * * * * * * * * * 5.5 includes the following programs : One Line Diagram Load Flow Short Circuit Dynamic Stability Motor Acceleration Motor Starting Cable Derating Cable Pulling Ground Grid Design Induction Machine Parameter Estimation Induction Machine TorqueISlip Curve Load Schedule Common Data Base A complete system analysis includes load flow, short circuit, and transient stability studies. Because of the unique data sharing capability of ETAP's programs, the data for an entire system needs to be entered only once with three user-friendly editors (Bus, Branch, and Machine). By not having to contin- ually enter data, the user is free to concen- trate his attention on analytical tasks, rather than bookkeeping. Errors in data input are thus reduced and more meaningful study results are obtained. ! 1 Authorized licensed use limited to: UNIVERSITY OF WESTERN ONTARIO. Downloaded on March 9, 2009 at 20:20 from IEEE Xplore. Restrictions apply.
Transcript
INTERACTIVE SIMULATION OF POWER SYSTEMS: ETAP APPLICATIONS AND TECHNIQUES
Keith Brown Student Member, IEEE Ooeration Tichnolow. Inc. Iriine, California -- Farrokh Shokooh Senior Member, IEEE Operation Technoiogy, Inc. Irvine, California
Abstract
For many years electrical engineers have relied on the power (but not necessarily the convenience) of mainframe computers to ana- lyze and design power systems. Recent advanc- es in microcomputers have brought the power of mainframe computing to the desktop, paving the way for straight-forward, easy to use engineering applications which are designed especially for the personal computer.
System studies are an integral part of power system engineering and design. A structured computer program that uses technically cor- rect models, employs a user-friendly inter- face, uses a common data base, and traps user errors is a powerful tool which greatly en- hances the engineer's efficiency and produc- tivity. ETAP is an engineering design and analysis program which satisfies these cri- teria. In addition, ETAP performs numerical calculations with tremendous speed, automati- cally applies industry accepted standards, and provides easy to follow output reports. This paper discusses solution techniques used by ETAP for various problems, as well as other features that make ETAP unique.
Introduction
The Electrical Transient Analyzer Program, commonly known as ETAP, began as a mainframe program and was rewritten for the PC as an interactive power system analysis and design tool. Significant effort has been made to insure that ETAP will provide the user with the highest possible technical accuracy, while maintaining a thoroughly user-friendly format.
The common data base exemplifies the inter- active nature of ETAP. Separate data editors for Bus, Branch, and Machine data allow the user to model the system in a common data base. User-edited libraries provide typical data which can be substituted into the data base upon request. When system studies are to be performed, ETAP automatically extracts the necessary parameters from the common data base. An interface with AutoCad is provided to automatically produce a detailed one-line diagram of the network modeled in the data
Herminio Abcede, Member, IEEE Flour Daniel Inc. Irvine, California
Gary Donner Member IEEE. Texaco Refiding and harketmg Los Ange1es;California
base including (optionally) load flow and/or short circuit results.
ETAP, while capable of handling 1000 buses, contains a load schedule program which tracks up to 10,000,000 load items, and reports the voltage and short-circuit current at the terminals of each load item. This capability makes ETAP suitable for large industrial facilities, as well as utility systems. ETAP
* * * * * * * * * * * *
5.5 includes the following programs :
One Line Diagram Load Flow Short Circuit Dynamic Stability Motor Acceleration Motor Starting Cable Derating Cable Pulling Ground Grid Design Induction Machine Parameter Estimation Induction Machine TorqueISlip Curve Load Schedule
Common Data Base
A complete system analysis includes load flow, short circuit, and transient stability studies. Because of the unique data sharing capability of ETAP's programs, the data for an entire system needs to be entered only once with three user-friendly editors (Bus, Branch, and Machine). By not having to contin- ually enter data, the user is free to concen- trate his attention on analytical tasks, rather than bookkeeping. Errors in data input are thus reduced and more meaningful study results are obtained.
! 1
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Often, when conducting system studies, the engineer does not have a complete set of input data, i.e. data is not available for certain machines, transformers, etc. The ETAP data editors are equipped with a large set of typical data for these situations. Based on information already entered (such as voltage rating), ETAP will automatically substitute typical data into the appropriate data fields upon request.
As indicated in Figure 1, ETAP data editors accept raw manufacturer data in standard NEMA form, as well as in per unit values, giving the user added flexibility.
using a new program often requires spending a considerable amount of time flipping through manuals to figure out what the computer is asking for in a certain entry field. On-line tutorials in the ETAP editors all but eliminate this problem. Each entry field has its own tutorial which is printed at the bottom of the screen. Another useful feature is the comprehensive error trapping that is continually occurring within the program while data is being entered. If the editor detects an unreasonable input, especially one which will result in meaningless study results, the user is immediately alerted, and given instructions on how to correct the mistake.
There are actually four levels of error detec- tion in ETAP. As described above, during data entry, ETAP checks each data input for valid- ity. When exiting a data editor, ETAP again checks the data for errors. A third error check is made before any study is run, check- ing the validity of the data extracted from the common data base. Finally, run-time er- rors are trapped during program execution.
ETAP contains an on-line power conversion calculator which can be invoked from the data editors by pressing a function key. Real and reactive power, voltage, current, and power factor can be calculated and substitute& directly into the editors.
A graphic representation of the network mod- eled in tha common data base can be generated wing ET-'. One-Line Diagram. This program
is an interface with AutoCad whereby the user can create a detailed one-line diagram with- out the assistance of a draftsman or CAD operator. The program automatically extracts data from the common data base and generates the one-line diagram, optionally indicating load flow and/or short circuit study results on the drawing.
10,000 YVA XFRW 13
New XFRW 7.500 YVA 13.80-2.40 kV
- 12.0 L'R
XFRY TRB 7.500 YVA 13.80-2.40 kV
14.0 X/R
Figure 2. ETAP One-Line Diagram
Load Flow
Load flow studies are essential in the plan- ning and operation of electrical systems. The results obtained from a load flow study (in conjunction with short-circuit study results) are used to size capacitors, feeders, trans- formers, and current-limiting reactors. Wheth- er designing a new system, or analyzing an existing one, factors such as voltage drop, load capacity, power factor constraints, steady-state stability limits, transformer tap settings, and generator excitation levels must be considered. The load flow study aids in the evaluation of these factors, and pro- vides a convenient way of predicting the effects of system modification and expansion.
The two most commonly used methods for calcu- lating load flow are Gauss-Seidel and Newton- Raphson [1,2]. The Gauss-Seidel method starts with an initial guess of bus voltages. Using the scheduled power injected into the buses as well as the most immediate voltages of the other buses, the voltage at each bus is recal- culated. These corrected voltages are then used to recalculate the voltages at all buses except the swing bus, continuing until all bus voltages have been evaluated, thus com- pleting the first iteration. Iterations con- tinue until all voltage corrections are less than a specified precision. A modification to this method is the Accelerated Gauss-Seidel method, which uses an acceleration factor tn a p e d up the convergence of the solution.
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Like Gauss-Seidel, the Newton-Raphson method is an iterative algorithm for solving a set of nonlinear simultaneous equations. At each iteration, the Jacobian matrix is used as a linear approximation for calculating a new set of bus voltages. This results in a qua- dratic convergence of the solution. However, this method is sensitive to the initial guess of bus voltages, as well as the level of system nonlinearity. To overcome the initial condition dependency, the Newton-Raphson method is preceded by a few iterations of Gauss-Seidel.
Due to the nature of the Newton-Raphson meth- od, extremely large humps in the admittance matrix could delay or even prevent solution convergence. Depending on the system configu- ration and impedances, the Newton-Raphson and Gauss-Seidel methods converge at different speeds. It is therefore advantageous to have both methods available. The load flow analy- sis package in ETAP provides both the Newton- Raphson and the Accelerated Gauss-Seidel methods, providing the flexibility needed to model loop and radial systems efficiently. Since some systems may be solved even faster by raising or lowering the acceleration factor, ETAP allows the user to adjust this factor. Other adjustable solution controls include the precision and the maximum number
of iterations. All system parameters are extracted from the ETAP common data base.
The standard output report produced by ETAP lists all input parameters on separate pages: bus input parameters, linejcable parameters, and transformer and reactor parameters. The load flow results show bus voltages, loads, generation, and power flows, as well as transformer tap setting.
Equipment exposed to undervoltage or over- voltage can be severely damaged and can pre- vent the efficient operation of machinery. Overloaded cables and transformers have con- siderably reduced life expectancies. When conducting a load flow study, these con- ditions are of paramount importance to the engineer. ETAP provides four optional summary reports to bring this information to the engineer's attention:
* Overvoltage Buses * Undervoltage Buses * Overloaded Branches * Branch Losses and Voltage Drop The user simply specifies the voltage or
loading limits, and ETAP lists all violations in the respective summary reports. Figure 3 shows a sample ETAP load flow output report.
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Short-circuit
X '1
X "
X "
Short-circuit studies are performed for an electric power system in order for the engi- neer to determine an adequate protection scheme for the system and to coordinate the operation of protective devices during fault conditions. Almost instantaneously, short- circuit currents produce powerful magnetic forces and intense heat in the power system, which can result in considerable damage to affected equipment if not promptly inter- rupted. These values of short-circuit currents must be determined by the engineer to ensure that the short-circuit ratings of all equipment are adequate to handle the currents available at their locations. In addition, the engineer can use data about these currents in conjunction with time- current characteristics of protective devices to determine the required relay settings and fuse ratings to achieve coordinated operation during faults. A properly coordinated protec- tion system will generally result in fast and selective isolation of faulted equipment with minimum equipment outage.
Calculation of short-circuit currents for industrial power systems tend to be more complex because of the mixture of sources contributing currents to the fault. In a typical modern industrial system the basic sources of fault currents are the utility, the in-plant generation, and synchronous and induction motors. These sources contribute additional exponentially decaying currents which make fault current magnitudes at Various locations time dependent. ANSI, IEEE, and IEC provide recommendations [3,4,5] as to how the decay of the ac and dc components of fault currents from various sources should be considered in fault calculations.
The ETAP Short-circuit program is a user- friendly, fully interactive program for calculating rms values of fault currents corresponding to different time periods after fault occurrence. Specifically, it calculates the 112 cycle, 1.5 to 4 cycle and 30 cycle m s symmetrical fault currents for three- phase, line-to-ground, line-to-line and line-to-line-to-ground faults.
For calculating device duties, ETAp provides two methods for determining X/R ratios. The first method, which is in full compliance with ANSI and IEEE Standards, finds the equivalent resistance and reactance of the entire system, resulting in a single valued X/R ratio for a given fault location. The second method differs from the first in that it combines the individual branch current contributions (each with a separate X/R ratio) into a total value. This method, unique to ET-, assumes the individual branch contributions to be exponentially decaying currents, with time constants corresponding to the equivalent X/R ratios of the individual branches. For each time period, these exponential currents are evaluated and summed to get the total fault duty at a bus. The IEEE method was designed for simple manual calculations. As a result, it yields conservative results to insure safety. With the power of today's computers, ETAP is able to track individual fault contributions and their rates of decay.
X " X'
X'
X'
By doing so, a more accurate representation of the power system is achieved. The user has the option of choosing either method.
ETAP provides two methods for selecting the X/R ratio of individual machines. The first method, Variable X/R Ratio, calculates the X/R ratio using the subtransient reactances for 112 cycle currents, and transient reac- tances for 1.5 to 4 cycle currents. The second method, Fixed X/R Ratio, calculates the X / R ratio using the subtransient reactance for both 112 cycle and 1.5 to 4 cycle fault currents. This method is consistent with the IEEE adjustment tables which consider the X/R ratio a measure of the time constant of the exponentially decaying dc component during a fault at the machine terminals. Note that the fixed X/R ratio does not imply a varying armature resistance, but is included merely for consistency with the way the multiplying factor tables have been formed.
For branches, the impedances of transformers and current-limiting reactors are adjusted if tolerances are specified. ETAP automatically uses the lower impedance values for conserva- tive results. The reactance representation that ETAP uses for the different sources in calculating the 112, 1.5 to 4 and 30 cycle fault currents are summarized in Table 1, where X I 1 represents utility short-circuit reactance and machine subtransient reactance, and represents machine transient reac- tance. This reactance selection is automa- tically performed by ETAP.
Contributions From
utilities
Synchronous Generators
Synchronous Motors
Induct ion Motors
Cycle Cycle Cycle 2 Table 1. Recommended reactance representation for utilities and machines in short-circuit calculations.
Table 2 presents typical reactances that the program will automatically use for induction motors of varying sizes if the user decides to use this feature of ETAP [ 6 ] . As in the other programs, the user can access typical data for use by the short-circuit program by pressing a function key during data entry. Table 2 presents a locked-rotor current (LRC) of 650% for medium voltage and 600% for low voltage motors. Using these LRC's, the corresponding subtransient and transient reactances for the given HP size and speed ratings are reported based on ANSI and/or IEEE recommendations.
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@ 41800 rpm
X'=3.0 X"
Table 2. Typical parameters recommended for induction motors for use in short-circuit calculations (if actual data is unavailable). Except for a LRC of 650 % for MV motors, all parameters are based on ANSIIIEEE standards.
When calculating 112 cycle and 1.5 to 4 cycle fault currents for ac high-voltage circuit breakers, synchronous generators are modeled with the same reactance (subtransient). Note that this results in the same generator rms symmetrical contribution for both 112 cycle and 1.5 to 4 cycle fault currents, i.e., no ac decay. However, the symmetrical interrupt- ing fault current for each bus is calculated as a combination of the utility, generator and motor contributions. For this reason, the program determines from the network config- uration if a generator contribution to a given fault location is local or remote based on the amount of impedance separating the generator from the fault location. Therefore, for a given fault, the percentage of local and remote contributions is determined to calculate the NACD (No AC Decay) ratio. For the remote component, a multiplying factor with no ac decrement is applied while for the local component a factor with both ac and dc decrement is used.
The short-circuit results are printed in a format that can easily be understood by even the novice user. Selection of appropriate breaker duties and bus bracing for each location in the power system is directly obtained from the printout. ETAP applies all appropriate ANSI, IEEE, IEC, and UL standards and procedures for device duty selection. For relay setting and coordination, fault contri- butions from all branches in the network to a given fault location ?ay be calculated and printed. This calculation is facilitated by an efficient, fast, and reliable algorithm using the bus impedance matrix with nodal injection method. Table 3 summarizes the application of the ETAP Short-circuit program output data.
Mv CI
LV Cf
Fuse
SWGR/ ICC
Relab
- Table
112 Cycle Currents
Closing & Latching Capability
Interrupting Capability
Interrupting Capability
Bus Bracing
Instantaneous Settings
1.5-4 30 Cycle Currents Cycle
Currents
zapability
Overcurrent Settings
---
3 . Device duty and usage of fault cur- rent contributions.
Figure 4 contains a sample ETAP short-circuit output report indicating fault contributions one bus level away from the faulted bus.
Three-phase f a u l t a t bus nuker: 1 Main A , 13.80 kV bus ( Prefault Voltage = 100.00 X ) *.***********.****
Manentary Duty: Spmetrical = 34.848 M rms Aspmetrical = 58.077 W rms ( MF = 1.667, X/R = 53.3 frm separate R B X networks )
Asyrmetrical = 55.756 M rms ( MF = 1.60 a t X/R = 25.0 )
CB Capabil i ty = 77.000 M rms
Interrupting Duty: syrmetrical = 33.936 M rms ( NAU) r a t i o = .66 )
Aspmetrical = 46.238 M rms ( MF = 1.362, X I R = 54.7 frm separate R B X networks )
CB Capabil i ty = 40.200 W rm6 ( 5 cycle CB, tota l current ra t ing bnsis )
Figure 4. ETAP sample short-circuit report.
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mansient (Dynamic) Stability
Over the years, the size and complexity of utility power systems have increased tremen- dously with the additions of large power plants and long distance high-voltage ties between utility power pools. Similarly, industrial power systems, particularly oil refineries, have grown in size and complexity with the popularity of in-plant cogeneration systems and large synchronous and induction motor drives. As these power systems grow bigger and become more complex, the need to predict system behavior more accurately, both dynamic and steady-state, has also increased. In this paper! the words transient and dynamic stabilities are used interchangeably since both are commonly used to indicate studies which include the machine swing equations.
For a typical industrial power system consist- ing of induction and synchronous motors, in-plant synchronous generators, and utility interties, a power system engineer using ETAP investigates two types of system instabilities. The first type of transient stability study investigates the system behav- ior during the first cycle of the machine power angle swing. This study is commonly known as the first swing analysis. First swing analyses can cover disturbances such as faults, generator rejection, abrupt dropping of a large load, and loss of utility ties. A simulation time of 1 second is typically adequate for this type of study. The second type of stability analysis is commonly refer- red to as multi-swing analysis and requires more sophisticated simulation models and numerical solution techniques to accurately predict system performance over a relatively longer simulation time.
A good stability analysis program employs technically accurate simulation models for various power system components, namely, generators, generator excitation systems, prime mover speed-governing systems, synchro- nous motors, induction motors, static loads and system equivalents.
The ETAP stability program includes all of the above simulation models. Moreover, it uses efficient numerical techniques to solve the differential equations of the simulation models and the algebraic equations for the network. All relevant system parameters are extracted from the common data base, and the load flow program is automatically run prior to the stability solution. The ETAP stability program 1s fully menu-driven. An input menu screen with an on-line tutorial is provided for each power system component to be includ- ed in the study. Running the program and accessing the output are as simple as press- ing a series of function keys. Although the ETAP stability program is designed for indus- trial power systems, it could also be used For most utility power systems, except for those with HVDC interties.
The simulation models and the required parameters of the ETAP stability program are deecribed in the following paragraphs.
For synchronoum machines, two modelm are implemented. The simpler model does not
include the effects of damper windings, while the more complex model does [ 7 ] . Machine parameters including subtransient, transient and synchronous reactances and time constants are used in the more sophisticated model. As such, this model is suitable for studies that concentrate on damping of machine oscilla- tions. The effect of magnetic saturation is considered for both models. Total inertia constant for the generator and prime mover is also a required input.
For excitation systems, different models are provided from which a user can select the type that best matches the actual exciter to be used [ 8 ] . These include: IEEE Type 1 - Continuously Acting Regulator With Rotating Exciter System; IEEE Type 2 - Rotating Rectifier Exciter With Static Regulator System; and IEEE Type 3 - Static System With Terminal Potential.
The IEEE Type 1 excitation system is represen- tative of the majority of modern systems now in service and presently being supplied. As variations of the Type 1 excitation system, Type 2 and Type 3 have similarities to Type 1. The Type 2 system applies to units with the main input to the damping loop provided from the regulator output rather than the exciter output. Although the regulator trans- fer function of the Type 3 is similar to Type 1, the regulator output is combined with a signal which represents the self-excitation from the generator terminals. This feature makes Type 3 different from Type 1.
Three models are provided €or governor- turbine systems to represent the most common- ly used turbines and governing systems [ 9 ] . Type ST is used for steam turbine, Type GT for gas turbine, while Type GP models a general turbine prime mover application. The mathematical derivations of these models are consistent with the IEEE Committee Report on governors and turbines.
Induction machines are represented dynamical- ly by their equivalent circuit as obtained either from locked rotor test data or from manufacturer's performance curves. ETAP pro- vides two induction machine models. The sin- pler model (Figure 5) uses a single-rotor equivalent circuit with an internal voltage source behind the machine locked-rotor reac- tance and stator resistance. The slip of the machine is a variable and is calculated from the acceleration equation. Changes in rotor flux linkages are considered in the model.
The second model includes the effects of rotor deep bars and double cage machines. Electrical dynamics in the rotor circuit are, therefore, represented more accurately with this model.
Other parameters that are required for both models include the combined inertia constant of motor and load and the mechanical load torque-speed characteristic. In cases where the circuit parameters are not known, the parameters can be estimated from manufactur- er's performance curves, namely, torque, current, and power factor versus speed curves, using the ETAP induction motor parameter estimation program.
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out# 4
out mtor load: 8.98B N czm P= 48)
9.W PaRr Factor llachlne I l24 llachlne I125
P = 2,458 N 9 - 1.serullR
Unag. =1W.93 x out uoltage:
uang. = -8.86 aeg.
The available models for synchronous motors are identical to those for synchronous genera- tors, however, the excitation system and speed-governing system are not represented. Field excitation is considered constant dur- ing the simulation. The required subtransient and transient parameters for the more sophis- ticated model allow the damping of machine oscillations to be investigated more closely as the system frequency changes during a transient.
Other loads are modeled either as constant kVA or constant impedance loads. In the con- stant kVA model, the load volt amperes is held constant only within a specified range. When the voltage goes out of this range, the model automatically reverts to constant imped- ance. With constant impedance representation, the load real and reactive power vary as a function of voltage squared. In the ETAP stability program, motor representation can be switched from dynamic models to constant kVA and vice versa for a particular simula- tion by pressing a single function key. If the detailed response of a given motor is not important for the case being studied, then the constant kVA representation is used for that particular run. In general this results in less computing time.
In the ETAP stability program, the swing bus is automatically represented as a constant voltage of constant frequency behind a fixed impedance corresponding to the fault avail- ability at the bus. This is done by the pro- gram without user intervention. With this representation, the swing bus acts as a sink or source of power as the system oscillates after a disturbance. Generally, the presence of a swing bus in a given simulation results in a better transient stability margin for the system.
Simulation of system disturbances in the ETAP stability program is carried out by inputting a series of switching events in the dynamic simulation menu. This menu is used to specify the network changes such as short circuit, load/generator rejection, circuit switching, and/or impact loading situations. The menus provide for entry of the type of switching along with their time of occurrences. The specific network changes that are allowed in ETAP are branch addition and deletion, and bus revision (swing, generator, load, delete, fault, and accelerate)
ETAP provides a total of six time events for specifying the occurrence times of the net- work changes. The first time event is default- ed at time T=O.O while all the other time events are user-specified. The seventh time event corresponds to the total simulation time. Up to a maximum of 42 network changes can be specified per time event, which is generally adequate for simulation of most commonly occurring system disturbances.
A print/plot menu is provided in the ETAP stability program for use in selecting the variables to be included in the program out- put report. The calculated values of these variables are printed out at an interval corresponding to the specified time step or multiples thereof. A plot of these values as a time function may also be included in the output if the user so specifies. The vari- ables that the user can specify for inclusion in the program output report are:
* Synchronous machine power angles * Generator exciter voltages * Bus voltages and frequencies * Induction motor slips * Branch power flows * Apparent impedances
The various study categories for which the ETAP stability program may be used are:
Generator Parameter Specifications which may involve determining generator reactances, generator-turbine combined inertia, and unit transformer reactance to achieve a certain stability margin for the generator as part of a system under specified disturbances;
Generator Control which may cover spe- cific areas, namely, excitation re- sponse time, excitation voltage ceil- ing, and generator dynamic breaking for investigation relative to system tran- sient performance;
System Control which may deal wfth controlled separation of various is- lands of a utility system and also load shedding to prevent system collapse during an overload condition of an ascended subsystem;
Plant Equipment which may cover various conditions relating to the starting of large motors and their stability during fault disturbances in the system;
Transmission System Design which could address items such as line design, voltage level, requirements for interme- diate switching stations, voltage sup- port, series compensation and shunt compensation relative to transient stability performance of the system;
Transmission Line Protection which may study breaker and relaying speeds, breaker schemes, automatic reclosing and transfer tripping relative to criti- cal fault clearing time.
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Motor Acceleration
In large industrial power systems and power plants, acceleration or re-acceleration of large motor drives is always necessary. It is therefore crucial for the design engineer to predict accurately whether or not a large motor would be able to successfully start and to determine how long it would take to reach the steady state condition. If analysis indicates that operating problems would arise, the engineer can then identify neces- sary changes or adjustments to the system during its design stage. I f problems are not accurately predicted, then practical solu- tions at the later stage of plant testing may be limited and uneconomical. A computer pro- gram that is based on technically accurate simulation models for motor acceleration and re-acceleration analysis is therefore very useful to an engineer to help investigate all the What ift1 scenarios early in the design of the system.
The ETAP stability program has the capability to simulate motor acceleration or re- acceleration dynamically. Motors to be started are specified in the dynamic simula- tion menu as machines to be accelerated in a specific time event. In addition, other net- work changes can be specified at the instant of motor starting. This feature allows the
user to accelerate motors under a variety of circumstances.
~nd. motor Slip (X) vs Time (Secwds)
.oooo
.0500
.3500
.4000
.4500
.5500
.6000
.6500
.7000
.?so0
.woo +
.a500 +
.E500 +
.woo .I +
1.2500 1.3000 1.3500
1.6500 +x
1.7500 +x
1.8500 +x 1.9000 +x 1.9500 *I
r.moo t*
1.8ooo it
Figure 6.
The required data €or this study are de- scribed previously under the Transient (Dynam- ic) Stability Program. The motor to be start- ed is modeled dynamically, preferably using the induction motor double cage model. This is generally an accurate representation for any ac starting motor since the rotor reac- tance and resistance generally change with the machine speed. The torque-speed character- istics of the driven load are specified by indicating its shape and the machine loading at the steady state condition. ETAP uses a curve fitting program to form a 5th order polynomial for the load torque. For no-load starting, the final value of the load torque can be either zero or a small value to re- flect motor friction and windage losses.
ETAP can provide a compplete load flow report at each time event, i.e., at the first time event (T=O.O before starting), second time event, and so on including the end of simula- tion time. As described earlier in this pa- per, plots and print outs of the different motor variables as functions of time can be requested from the program printlplot menu to obtain a complete picture of the motor accel- eration. A sample motor acceleration curve generated by ETAP is shown in Figure 6.
104 (3500 HP ) - 124 (1250 HP ) - X
33.3
125 (700 HP 1 - +
+
-- X
*
* *
Sample Motor Acceleration Output Plot
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Cable m a t i n g
Electrical power systems are only as reliable as the cables that network them together. A complete analysis of a power system should therefore include an analysis of the cable ampacities. This analysis is complicated since the ampacity of a conductor varies with different installation configurations. Ampaci- ty is defined as "the current in amperes a conductor can carry continuously under the conditions of use (conditions of the surround- ing medium in which the cables are installed) without exceeding its temperature rating." Therefore, a cable ampacity study is the calculation of the temperature rise of the conductors in a cable system under steady state conditions. The National Electrical Code (NEC) [lo] now accepts the Neher-McGrath method [11] of determining the ampacities of conductors. NEC gives ampacity tables for a few specific configurations, but since most practical applications have yore complex configurations than those given in the tables, the engineer must turn to a computer program which calculates the temperatures of each conductor in the installation, and verifies that they do not exceed the rated maximum temperature. ETAP Cable Derating Program (ETAP-CD) is based on the Neher-McGrath method, and not onl-y calculates temperature rise in cables, but also contains options for sizing cables, and optimizing ampacities.
When an electrical current flows through a cable, it generates heat. The type of cable and its location in the installation are two of the factors which determine how many compo- n nts of heat generation are present, such as I R losses, sheath losses, dielectric loss- es, and so on. The heat flows from these sources through a series of thermal resistanc- es to the surrounding environment [12]. The cable operating temperature is directly related to the amount of heat generated and the effective thermal resistance through which it flows. The Neher-McGrath method involves the application of a series of thermal equivalents of Ohm's and Kirchoff's laws to a thermal circuit. This circuit includes several parallel paths with heat entering at several points. The steady state Conductor temperature can be determined by the temperature differential created across the thermal resistances as the heat flows to the ambient temperature.
Important characteristics of the installation that determine thermal resistance between the cable and the ultimate surrounding environ- ment include the type of conduit (steel, PVC, etc.), the depth of the underground installa- tion, the thermal resistivity of the earth surrounding the installation, and the conduc- tor insulation thermal resistivity. Factors that determine the external heat sources include the proximity of nearby cables and the temperature of the surrounding environ- ment. The heat generated in each cable de- pends on the cable attributes, such as the type of conductor and the thermal resistivity of the insulation, as well as the dielectric losses.
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ETAP-CD contains many extra features tailored especially for the design engineer. User-friendly data entry menus as well as a cable library database make the cable derat- ing process straight-forward and hassle free. Since the design engineer starts from scratch, simply finding the temperature rise of cables in a given installation is not sufficient. This is why ETAP-CD includes special options for sizing cables and optimiz- ing ampacities.
Finding ampacities which optimize the amount of current that can flow through a given underground installation would usually be done by trial and error, but with the Find Ampacities option, the engineer simply enters the maximum rated temperature for the cables in the duct bank, and ETAP-CD finds ampaci- ties which will bring each conductor to the specified temperature, thus maximizing the current through the entire installation. Alternatively, it may be desirable to run each unique cable size at the same ampacity. ETAP-CD computes these ampacities just as easily. If the loading requirements are known for each cable, and the cable sizes have not been determined, selecting the Size Cables option tells ETAP-CD to choose adequate cable sizes from the library.
User-friendliness is an important aspect that is often overlooked in engineering applica- tions. Special care has been taken to make data entry as simple as possible in ETAP-CD. A separate menu was designed to aid in the entry of repetitive data which also calcu- lates the spacing of the cables for direct substitution into the appropriate entry field. popup menus allow the user to select a run option (see Figure 7).
- i l l
1 1 I 2 2 2 3 3 3 4 4 4
-
- Clrarlt uller)*ulber or Iu=der Mber
I
lun
Figure 7. ETAP-CD Cable Location Menu
The output report can be printed by pressing a single function key. All input parameters (including any data extracted from the li- brary) are printed, followed by a structured output which graphically shows the location of the cables in the installation, making the report easy to use. Figure 8 shows a sample duct bank system, with the output report shown in Figure 9.
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Induction Machine Parmeter -thation
Available circuit parameters of induction machines are usually sufficient for short- circuit analysis only. Many steady state and transient studies require more sophisticated models which include the resistances and reactances representing the stator, rotor, and magnetizing branches. Additional para- meters used in some models include the effects of saturation, hysteresis, eddy currents, deep rotor bars, and double rotor cages.
An equivalent circuit representing the steady state behavior of a polyphase induction motor is shown in Figure 10 where the deep rotor bars and double cage effects are included in the rotor reactance and resistance as a function of slip and full-load values.
Figure 8. Sample 3x5 duct bank installation
COLUINS: 1 2 3 4 5
IOU 1
RW 2
ROU 3
Cable: Mp.:
Tanp.:
Cable: A p . :
7anp.:
Cable: Mp.:
hap.:
_ - - - - _ _ 500
209.8 75.0
500
75.0
500 185.3 75.0
173.8
- - _ _ - - - 500
175.9 75.0
500 129.4 75.0
500 148.4 75.0
-----.- 350
138.5 75.0
350 99.0 75.0
350 115.1 75.0
_ _ _ _ _ _ _ 350
146.9 75.0
350 110.3 75.0
350 124.8 75.0
_- - - - - - 350
176.3 75.0
350 149.3 75.0
350 157.6 75.0
Figure 9. Output report from ETAP-CD using the Find Ampacities (Uniform Temperatures) option.
Cable pulling
In the design of underground distribution 8y8temSI the limiting factors imposed by cable pulling design criteria must be consid- ered. For most underground installations, the number and location of splices, electrical manholes, pull boxes and other pull points in the system are known to be dependent upon the maximum allowable pulling lengths of cables. Therefore, in order to achieve an optimum design that will be economical and will not cause damage or distortion to the cables being pulled into the conduits, the practical application of cable pulling theory and de- sign criteria is necessary. The ETAP Cable Pulling program, ETAP-CP, was developed using widely accepted cable pulling equations and criteria. It is user-friendly, fully interac- tive, and menu driven, and is certainly a handy tool for a cable run designer. The user can model three dimensional configurations by specifying lengths and slopes of pull seg- ments, angles and radii of bends, coeffi- cients of friction, etc. Cable libraries that can be modified and expanded by the user are provided. These include physical and electri- cal data for various cables of different voltage ratings.
The ETAp Cable Pulling program is used to determine tensions and sidewall pressures that a cable is subjected to when pulled into conduits. A point-by-point calculation method ie performed at every conduit bend and pull point. The program is applicable to simple as well as complex conduit configurations where both the forward and reverse pulling tensions as well as sidewall pressures are calculated to determine the best direction of pull. Additionally, it checks for percent fill, jam ratio and other NEC and cable requirements. The program automatically flags any installa- tion problems.
The task of obtaining machine design data can be difficult, if not impossible for some machines. Manufacturers do not always supply complete data, while owners may misplace data that has been supplied. For most small ma- chines, the only available data appears on their nameplates. Additional data available for some larger motors are the steady-state performance curves, which include the torque, current, and power factor versus slip.
The Parameter Estimation Program (ETAP-PE), assumes that the nameplate data along with two points from the motor performance curves are available. These points, which include the locked rotor torquelcurrent and breakdown torque, may be obtained from actual machine tests or from curves supplied by the manufac- turer. Indeed, these points are often the only reliable information available from the performance curves since the rest of the curve might have been provided merely as an indication of the machine's generic behavior.
The algorithm for estimating the equivalent circuit parameters [13] proceeds by first determining the parameters of a simplified equivalent circuit. Next the method utilizes exact machine equations to obtain a more accurate estimation. A sensitivity analysis of the performance characteristics with re- spect to the circuit parameters is then per- formed to obtain a higher degree of accuracy.
TO illustrate the application of ETAP-PE, a three-phase, 1000 HP, 4.16kV, 60Hz induction motor was selected. This particular example required 20 iterations of sensitivity adjustments until the performance characteris- tics calculated from the estimated Circuit parameters were within 1 percent of the given values, as indicated in the computer output shown in Figure 11.
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Rotor reactance cage factor t Xr = (1 + S*Kx)*Xr,s=o I
(a)
x C - U R - R E - N 1 -
Figure 11. ETAP-PE example output report.
Ground Grid Design
When a short-circuit occurs, large currents must be passed to the earth without endanger- ing personnel in the vicinity. Based on ANSIfIEEE Std 80-1986 [14], the Ground Grid program, ETAP-GRD, calculates the required number of parallel ground conductors and ground rods for safety during a ground fault in the system. For a given grid configuration with a specified number of conductors and rods, the program calculates and reports the step and touch potentials and flags unsafe conditions.
ETAP-GRD provides options which allow the engineer to optimize the design and to deter- mine the most economically sound implementa- tion of a given configuration. For a fixed number of rods, the program finds the optimum number of conductors. Based on the user-specified costs of conductors and rods (including installation costs), ETAP-GRD finds the optimum number of conductors and rods for the configuration. Both of these options find solutions which limit the step and touch potentials to acceptable values. The program also checks the ground conductors and rods for their capability to carry the ground fault current.
Conclusion
Engineers depend on continual development of software to keep in step with fast-paced technological advancement. ETAP's designers are professional electrical engineers who are dedicated to researching and implementing new methods as well as developing new applica- tions which will enhance the productivity and capability of ETAP users.
Today's electrical engineer is limited only by the software available to him. ETAP is a technically accurate, user-friendly software package designed with the working engineer in mind. From the common data base input editors, to the clearly structured output reports, ETAP has pushed forward the frontier of desktop power system analysis software.
References
1. Glenn W. Stagg and Ahmed H. El-Abiad, Computer Methods in Power System Analysis, McGraw-Hill, 1968.
2. J. Arrillaga, C.P. Arnold, and B.J. Harker, Computer Modelling of Electrical Power Systems, John Wiley & Sons, 1983.
3. ANSIfIEEE C37.010-1979, IEEE Application Guide for AC High-Voltage Circuit Breakers Rated on a Symmetrical Current Basis.
4. ANSIfIEEE C37.50-1979, Guide for Calculation of Fault Currents for Application of AC High-Voltage Circuit Breakers Rated on a Total Current Basis.
5. ANSIfIEEE C37.13-1981, IEEE Standard for Low-Voltage AC Power circuit Breakers Used in Enclosures.
6. Walter C. Huening, Jr. , "Calculating short-circuit currents with contributions from Induction Motorsa1, IEEE Trans. Ind. App.
7. Edward Wilson Kimbark, Power system Stability: Synchronous Machines, Dover Publications, 1956.
8. IEEE committee report on computer modelling of excitation systems, "Excitation system models for power system stability analysisIng IEEE Trans. Power App. Sys., Vol. PAS-100, No. 2, Feb. 1981.
Vol. IA-18, No. 2, MarfApr 1982.
Authorized licensed use limited to: UNIVERSITY OF WESTERN ONTARIO. Downloaded on March 9, 2009 at 20:20 from IEEE Xplore. Restrictions apply.
9. IEEE committee report for governors and turbines, "Dynamic models for steam and hydro turbines in power system studies," IEEE Trans. Power App. Sys., Vol. PAS-92, pp. 1904-15, Jul/Dec 1973.
10. National Electrical Code (NEC), NFPA 70-1990.
11. J.H. Neher and M.H. McGrath, "The calculation of the temperature rise and load capability of cable systems, AIEE Trans. Power App. Sys. (pt. 111), Vol. 76, pp. 752-72, Oct. 1957.
12. Farrokh Shokooh and Harold M. Knutson, "Ampacity derating for underground cables,'* IEEE IKPS Conference in Baltimore, Maryland, May 1988.
13. Somchai Ansuj, Farrokh Shokooh, and Roland Shinzinger, "Parameter estimation for, induction machines based on sensitivity analysis," IEEE Trans. Ind. Appl., vol. IAS-25, pp. 1035-40, Nov/Dec 1989.
14. ANSI/IEEE Std 80-1986, IEEE Guide for Safety in AC Substation Grounding.
1941
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