1380_Transient Stability Analysis

JGC-DESCON ENGINEERING (PVT.) LIMITED Document No.: 1380-DP-00X

Rev.: 0

Guide For Transient Stability Analysis

Guide for Transient Stability Analysis

Date: 23-Jan-08

Guide for Transient Stability Analysis

Rev: 0

TABLE OF CONTENTS

1 SCOPE...........................................................................................................................1 2 GENERAL .....................................................................................................................2 3 PURPOSE .....................................................................................................................2 4 TIMING OF STUDY.......................................................................................................3 5 SYSTEM MODELLING & DATA REQUIREMENTS.....................................................3 6 STUDY RESULTS (INTERPRETATION & RECOMMENDATIONS)...........................5 6.1 Results.....................................................................................................................................................................5 6.2 Recommendations..................................................................................................................................................5

7 RECOMMENDED CODES & STANDARDS.................................................................5

Rev.

Date

Description

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23-Jan-08

Issued for Information

1

Scope

Prepared By Checked By Approved By Initial Signature Initial Signature Initial Signature THA

ZAZ

ZAZ

Guide for Transient Stability Analysis

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This Design Guide provides guidelines for Transient Stability Analysis. 2

General A stable system can be characterized as the one which when subjected to one or more bounded (i.e. less than infinite magnitude) disturbance, the resulting system response is also bounded. The disturbances can be in the following form:  Faults (Short circuits etc.)  Switching operations (Loss or shedding of on site generation or utility supply etc.)  Putting loads on line and off line (Large Motor starting/stopping, impact loading of motors, abrupt decrease of electrical load on generators etc.) A stable power system has the capability to withstand sudden changes in generation, load, or system characteristics without a prolonged loss of synchronism. System stability limits in terms of max real power that can be delivered to a simplified two machine system containing a generator with terminal voltage denoted by EG, a load with terminal voltage V and the reactance(s) of the machines and transmission line X, can be formulated as follows: Pmax = (EG x V) / X Any change in the parameters of the aforementioned equation or an attempt to transmit more power than Pmax will cause the machines to pull out of step and loose synchronism. Effectively, the following three characteristics of power system have major impact on system stability.  Internal voltages of generators  Internal voltages of loads mainly motors  Reactance(s) of the machines and transmission lines. The effects of asynchronous operation produce transient mechanical torque and currents which in turn causes mechanical and thermal damage to the system. The out of step operation also causes large oscillatory flows of real and reactive power over the circuits connecting the machines. Depending on the level of these oscillations, the electromechanical equilibrium in the system is affected and the instability could occur. Furthermore, these power surges can be falsely interpreted by the system’s protection as system line fault, causing unnecessary tripping of breakers and process interruptions. Synchronous machines play a decisive role in the power system stability because during and after disturbances their power angles (also referred as rotor angles) will oscillate to cause power flow oscillations in the system. As a result of system disturbances, electrical system may experience the following problems:

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 Over/under frequency  Over/under voltage  In plant oscillations  Unnecessary tripping of breakers and process interruptions. Purpose The purpose of performing a transient stability study is the following:

Guide for Transient Stability Analysis

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 To investigate the machine power angles and speed deviations, system electrical frequency, real and reactive power flows of the machines, power flows of lines and transformers, as well as the voltage levels of the buses in the system.  To investigate that the system is stable following a severe disturbance, all synchronous machines reach their steady-state operating condition without prolonged loss of synchronism or going out of step with other machines. 4

Timing of Study The purpose of study and data used in the study depends on the timing of the study.  Early Design Stage A preliminary study at early design stage is performed for initial assessment and selection of equipment, typical/In-house data is used at this stage.  Final Stage (Or studies of existing Facility) As built studies using as built data, such as actual data from vendor, impedance data available from test results etc are performed to analyze the system stability and capability for future expansion or modifications.

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System Modelling & Data Requirements Basic system configuration used for load flow or short circuit studies is used; however, certain additional data is required for transient stability study. Summary of required data is shown, it may be noted that some of the information is not essential; omitting it merely limits the accuracy of the results. The more essential items are marked by an asterisk (*).  System Single Line Diagram*  System data  Impedance (R+ jX) of all significant transmission lines, cables, reactors, and other series components*  For all significant transformers and autotransformers o kVA rating* o Impedance* o Voltage ratio* o Winding connection* o Available taps and tap in use* o For regulators and load tap-changing transformers: regulation range, tap o step size, type of tap changer control*  Short-circuit capacity (steady-state basis) of utility supply, if any*  kvar of all significant capacitor banks*  Description of normal and alternate switching arrangements*  Load data Real and reactive electrical loads on all significant load buses in the system*  Rotating machine data

Guide for Transient Stability Analysis

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For major synchronous machines (or groups of identical machines on a common bus) o Mechanical and/or electrical power ratings (kVA, hp, kW, etc.)* o Inertia constant H or inertia Wk2 of rotating machine and connected load or prime mover* o Speed* o Real and reactive loading, if base-loaded generator* o Speed torque curve or other description of load torque, if motor* o Direct-axis subtransient,* transient,* and synchronous reactances* o Quadrature-axis subtransient, transient,* and synchronous reactances o Direct-axis and quadrature-axis subtransient and transient* time constants o Saturation information o Potier reactance o Damping data o Excitation system type, time constants, and limits o Governor and steam system or other prime mover type, time constants, and limits

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For minor synchronous machines (or groups of machines) o Mechanical and/or electrical power ratings* o Inertia* o Speed* o Direct-axis synchronous reactance*

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For major induction machines or groups of machines o Mechanical and/or electrical power ratings* o Inertia* o Speed* o Positive-sequence equivalent circuit data (e.g.,R1,X1,XM)* o Load speed-torque curve* o Negative-sequence equivalent circuit data (e.g.,R2,X2)* o Description of reduced-voltage or other starting arrangements, if used*

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For small induction machines: detailed dynamic representation not needed, represent as a static load

 Disturbance data 

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General description of disturbance to be studied, including (as applicable) initial switching status; fault type, location, and duration; switching operations and timing; manufacturer, type, and setting of protective relays; and clearing time of associated breakers* Limits on acceptable voltage, current, or power swings*

To run a transient stability study in ETAP, the data required for load flow calculation can be used. In addition to that, machine dynamic model data, load model data, and any control units, such as exciter and governor data is also required. Refer to

Guide for Transient Stability Analysis

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ETAP help file on Transient Stability Analysis under “Transient Stability Required Data” for details of system data needed to run ETAP module. 6

Study Results (Interpretation & Recommendations) For transient stability study, following are the main results & typical recommendations:

6.1

Results  Check the voltage level, angle, and frequency at various buses during the specified operational conditions.  Check the power angles also referred as rotor angles of the synchronous machines specified operational conditions. The transient stability limit for any synchronous machine is that its power angle is less than 180 degrees.

6.2

Recommendations          

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Operating restrictions. Changing power flow patterns. Decreasing the duration of disturbance. Use of induction instead of synchronous motors. Selecting synchronous machines with a higher moment of inertia. Selecting synchronous machines with low transient reactance. Connecting the machines by low impedance circuits. Proper setting of regulator and exciter voltage. Proper selection of fast system protection devices to limit instability. Use of power system stabilizers

Recommended Codes & Standards For further details refer to the following Codes:  IEEE Std. 399-1997 Blue Book (Chapter 8 Stability Studies)  IEEE Std. 1110-2002 IEEE Guide for Synchronous Generator Modelling Practices and Applications in Power System Stability Analysis