Analysis of Contingencies Leading to Transients and Transient Stability Assessment in Power

System NetworkSupriya BimalKumar Rakshit1,Prakash Makhijani2 and Falguni Bhavsar3

1PG Student, Department of Electrical Engineering,

Parul Institute of Engineering and Technology, Limda2Director,

OHMENCON Pvt. Ltd, Makarpura3Professor,

Department of Electrical Engineering,Parul Institute of Engineering and Technology, Limda

1sup2624rak@gmail.com 2prakash@ohmencon.com

3falu_29@ymail.com

Abstract - Recently, the security and stability of power system are the concerned issues, especially the transient stability. In this paper large power system network is modeled in detail in order to assess the transient stability. Transient stability is important because the severity of fault can be determined which otherwise would lead to severe loss or damage to the equipment. Further, the computation of critical clearing time (CCT) in different scenarios is proposed and simulation is also done to analyze the different characteristic. The result of CCTs in different scenarios can evaluate the impact of transient stability. Keywords - Critical clearing time, ETAP, transient stability.

1. I. INTRODUCTIONElectrical power systems are highly complex and dynamic in nature: circuit breakers are closing and opening, faults are being cleared, generation is varying in response to load demand, and the power systems are subjected to atmospheric disturbances, that is, lightning [1]. Assuming a given steady state, the system must settle to a new acceptable steady state in a short duration. Power-system stability is a term applied to alternating current electric power systems, denoting a condition in which the various synchronous machines of the system remain in synchronism, or "in step," with each other. Conversely, instability denotes a condition involving loss of synchronism, or falling "out of step." Transient stability is the ability of the power system to maintain synchronism when subjected to a severe transient disturbance. Power system stability problems are classified into three basic types i.e. steady state, dynamic and transient [2].

• Steady state stability – A Power system is said to be steady state stable for a specific steady state operating

condition following a disturbance. Such disturbances are generally small in nature.• Transient stability – Larger disturbances may change the operating state significantly, but still into an acceptable steady state. Such a state is called transient state.• Dynamic stability – It is generally associated with excitation system response and supplementary control signals involving excitation system.

2. II. CLASSIFICATION OF TRANSIENTS Transient are studied in two categories, based upon on their origin: [3]

• Of atmospheric origin, that is lightning• Of switching origin, that is, all switching operations, load rejection and faults.

Another classification can be done based upon mode of generation of transients:• Electromagnetic transients - Generated predominantly by the interaction between the electrical fields of capacitance and magnetic fields of inductances in the power systems. The electromagnetic phenomena may appear as traveling waves on transmission lines, cables, bus sections, and oscillations between inductance and capacitance• Electromechanical transients - Interaction between the electrical energy stored in the system and the mechanical energy stored in the inertia of the rotating machines, that is, generators and motors.

III. ORIGIN OF TRANSIENTSTransients are disturbances that occur for a very short duration and the electrical circuit is quickly restored to original operation. For transients to occur there must be

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some of the more common causes of transients [4]. • Atmospheric phenomena• Switching loads on or off• Interruption of fault currents• Switching of power lines• Switching of capacitor banks

3. IV. ANALYSIS OF CONTIGENCIES LEADING TO TRANSIENTS

Transient stability of the generator is dependent on the following: [1] • How heavily generator is overloaded • Generator output during fault. • Fault clearing time • Post fault transmission system reactance• Generator reactance • Generator inertia• Infinite bus voltage magnitude• Generator internal voltage magnitude.Single line diagram of power system scheme is shown below. Transient stability is important since system is interconnected and if the fault persists for the longer period of time it can lead to system instability and voltage collapse of the system. Depending on critical clearing angle the time required should be estimated because of dc component present.

Fig.1 One Line DiagramDifferent contingencies which lead to transient conditions are:Case 1: Fault on bus at different locations.Case 2: Fault in transmission line.Case 3: Fault at lumped load.

Buses 33Branches 32Generators 23Loads 31Load-MW 4053.052Load-Mvar 2986.25Generation-MW 849Generation-Mvar 526.165Loss-MW 9.829Loss-Mvar 480.485

Table.1 General Information

Lump1 100000 kVA 68109 42210 4202Lump2 30000 kVA 25731 15946 1581Lump3 400000 kVA 341531 211661 1753Lump4 50000 kVA 42502 26340 2624Lump5 400000 kVA 338711 209913 1746Lump6 150000 kVA 102219 63349 6305Lump7 75000 kVA 50977 31592 3148Lump8 400000 kVA 339180 210204 1747Lump9 150000 kVA 101491 62898 6283Lump10 30000 kVA 25714 15936 1581Lump11 400000 kVA 343272 212740 1758Lump12 25000 kVA 21447 13291 1318Lump13 400000 kVA 339390 210334 1748Lump14 150000 kVA 102414 63470 6311Lump15 400000 kVA 340509 211028 1751Lump16 150000 kVA 102739 63671 6321Lump17 50000 kVA 33862 20985 2095Lump18 400000 kVA 338966 210072 1747Lump19 100000 kVA 68465 42430 4213Lump20 100000 kVA 67603 41896 4187Lump21 400000 kVA 339200 210217 1748Lump22 100000 kVA 67649 41925 4188Lump23 150000 kVA 101543 62930 6284Lump24 50000 kVA 42500 26339 2624Lump25 50000 kVA 42500 26339 2624Lump26 50000 kVA 42500 26339 2624Lump27 50000 kVA 42500 26339 2624Lump28 50000 kVA 42500 26339 2624Lump29 50000 kVA 42500 26339 2624Lump30 50000 kVA 42500 26339 2624Lump31 50000 kVA 42500 26339 2624

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Table.2 Load Data

ID MW Mvar Amp % Gen.Gen1 398.997 307.958 26454 88.7Gen2 411.94 323.606 27495 91.5Gen3 392.592 287.379 25536 87.2Gen4 361.029 264.196 23481 80.2Gen5 404.541 319.455 27055 89.9Gen6 400.785 313.368 26703 89.1Gen7 417.02 322.627 27674 92.7Gen8 417.148 321.496 27643 92.7Gen9 45 27.889 2776 90Gen1

0 45 27.889 2766 90

Gen11 45 27.889 2778 90

Gen12 45 27.889 2776 90

Gen13 45 27.889 2779 90

Gen14 90 55.777 5571 90

Gen15 45 27.889 2766 90

Gen16 84 52.059 5162 84

Gen17 45 27.889 2773 90

Gen18 45 27.889 2769 90

Gen19 90 55.777 5569 90

Gen20 45 27.889 2769 90

Gen21 45 27.889 2787 90

Gen22 45 27.889 2786 90

Gen23 90 55.777 5570 90

Table.3 Generation Data

4. V. TRANSIENT STABILITY ASSESSMENTCase 1: Fault on bus at different locationa. Fault at bus no. 24 – The bus 24 is located at the lower level towards the load side so the fault at this particular bus will have critical clearing time of 1sec and beyond this limit the system will undergo transient condition. As it is located far away from the grid the critical clearing time is more and therefore severity of fault at this location is less.

Fig.1 Generator Electrical Power

Fig.2 Generator Exciter Voltage

Fig.3 Generator Absolute Power Angle

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Fig.4 Generator Reactive PowerThe entire above graph presented in section a indicates that as the fault is located on the bottom end at it does not result in the total failure of the system. Also it indicates that as the fault does not affect the generator located on the grid side but it would definitely affect the generators located at Industrial Power Plant (IPP).

b. Fault at bus no. 16 – Bus 16 is located on the grid side. Fault at this location may lead generator to go out of step. Therefore in such case it is necessary to have less critical clearing time. Critical clearing time for such case is found to be 0.8secs. If the fault continues more than determined time it can lead to the severe damage.

Fig.5 Generator Absolute Power Angle

Fig.6 Generator Reactive Power

Fig.7 Generator Electrical Power

Fig.8Generator Exciter VoltageThe entire above graph presented in section b indicates that as the fault is located on the grid side it result in the total failure of the system starting from top to bottom i.e. from grid towards distribution end. Also it indicates that as the

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fault moves away from the location the effect gets minimized due to presence of reactor in the line, so the generator 1 is having less effect then generator 8 and also due to lower inertia of the generator at lower end Industrial Power Plant (IPP) it also gets affected.

Case 2: Fault in transmission lineFault in transmission line1 causes the transient condition in the power system network. If the fault persists for longer period of time then circuit breaker CB1 is opened. The critical clearing time for such case is found to be 0.6secs.

Fig.9 Generator Absolute Power Angle

Fig.10 Generator Electrical Power

Fig.11 Generator Reactive Power

Fig.12 Generator Exciter VoltageThe entire above graphs presented in section indicates that as the fault occurs on transmission line result in the total failure of the system starting from top to bottom i.e. from grid towards distribution end. Also it indicates that as the fault moves away from the location the effect gets minimized due to presence of reactor in the line, so the generator 1 is having less effect then generator 8 and also due to lower inertia of the generator at lower end i.e. Industrial Power Plant (IPP) it also gets affected.

Case 3: Fault at lumped load.Fault at lumped load also result in transients. Lumped load consist of static as well as motor load. Whenever a large amount of load is removed from the system it would result in the instability of the system.

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Fig.13 Generator Absolute Power Angle

Fig.14Generator Reactive Power

Fig.15Generator Electrical Power

Fig.16 Generator Exciter VoltageThe entire above graphs presented in section indicates that as the fault occurs at load it does not affect the generators located in grid side but it would definitely affect the generators which are having less inertia and located at IPP i.e. industrial power plant

VI. CONCLUSIONTransient stability is the ability of the power system to maintain synchronism after subjected to severe disturbance. The synchronism is assessed with relative rotor angle violations among the different machines. Accurate analysis of the transient stability requires the detailed modeling of generating units and other equipment. Transient stability is critical even small disturbance can make the generator to go out of step. This analysis allows to assess that the system is stable, unstable and also allows to determine the critical clearing time of power system with three-phase faults.

REFERENCES1. Power system stability and control, P. Kundur, McGraw-Hill International Editions, 1994.2. Modern Power system analysis, I. J. Nagrath and D. P. Kothari, Tata McGraw-Hill, 2003.3. Transients in electrical systems analysis, recognition and mitigation, J. C. Das, McGraw-Hill Companies, 2010.4. Power quality, C. Sankaran, CRC Press LLC, 2002.

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