Vol. 5, No. 2 | July - December 2022 SJET | P-ISSN: 2616-7069 | E-ISSN: 2617-3115 | Vol. 5 No. 2 July – December 2022 1 Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses Rehan Ali1, Abdul Saboor Gul1, Ramez Akbar Talani2 Abstract: Load Flow and Short Circuit Analyses have been performed on standard IEEE 9-Bus System. This paper is going to describe in detail how these analyses were performed and what results were achieved. In addition to the performance of these two analyses on the standard test system, they were also performed by removing a voltage- controlled generator from the test system. It is revealed that by removal of a generator, the magnitude of short- circuit fault current in a system would reduce as compared to the case where all generators were present in the system. Keywords: IEEE 9- Bus system, Load flow analysis, Short circuit (SC) analysis 1. Introduction Before installing a power system in the real world, many tests and analyses are digitally performed on computer simulations in order to determine the exact requirements and necessary precautions for the installation of the actual system. As we know that within power systems, electric power flows from the point of generation to the required destination via different paths of network. This flow comprises of reactive and active power known as load flow. It is very important to investigate this load flow so as to plan and determine the steady operation of system. Such investigations employs systematic analytical approaches so as to examine the different voltages on bus, phase angle, as well as reactive and active power that is flowing within various branches of load such as generators and transformers when steady state is attained. The important knowledge gained from such analysis gives the absolute values as well as phase angles of load bus voltage reactive 1 Department of Electrical Engineering, NED University of Engineering and Technology, Karachi, Pakistan 2 Faculty of Electrical Engineering, QUEST, Pakistan Corresponding Author: rahimoon2223@gmail.com powers and voltage phase angles at generator buses, real and reactive power flows on transmission lines together with power at the reference bus. In addition to analytical techniques, numerical methods are also employed to solve load flow equations, as they become nonlinear and requires solution by method of iteration. However, such numerical solutions mostly provide only approximate solution. For more than thirty years different numerical investigations have been carried out to analyze power flow problems. Out of them widely used methods are Fast decoupled methods the Gauss-Seidel and Newton- Raphson and [1-4]. Moreover, due to sudden and mammoth developments in industry the society requires electric systems which keep on enhancing in size and complexity. In such systems the power flow equations range to several thousand with such magnitude of equations it is not feasible for any numerical technique to provide solution which converges. Such mailto:rahimoon2223@gmail.com Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 2 difficulty compels electrical engineers to resort to more suitable and reliable techniques. Hence the issue is that industry is looking for technique that is more feasible for the analysis of power systems Analytical calculations are appropriate when estimation of characteristics of smaller magnitude of circuits is being carried out, however for the more precise and accurate analysis it is imperative to employ specialized programs. Digital computers are being used in calculation s of power flow equations since 1950s. With the rapid advancement in computing power all sort of load flow studies can now be conveniently carried out. Load Flow Analysis is one of such analyses. It is important for planning, economic scheduling, operation, and the distribution of power between different sources. It is also required for many other analyses such as transient stability and contingency studies. This analysis is also required to determine the effects of new loads and new generating stations on the power system, so that the system can be extended. This analysis involves finding voltages, currents, real and reactive power flows at different points in a power system under normal/steady-state conditions. [2, 3] Short-Circuit analysis is mainly required for the protection of the power system and its equipment Sizing. It is performed on different buses under faulty conditions which can cause a short-circuit in the system. It involves finding the huge short- circuit current that flows in the system in the event of a fault. This analysis is also a major part of the system’s protection coordination study, which involves the sizing and placement of protective equipment (fuses, circuit breakers, etc.) in the system, in order to protect the system. [4-6]. 1.1. Bus Classification Any node where more than one component like electric generators and transmission lines are connected is known as bus. In electrical engineering the bus is concerned with four quantities which include potential difference and its phase angle, active and reactive power. [11,12]. Out of them 02 quantities are already known whereas remaining two has to be determined via solution of equation [13]. 1.2. Slack Bus The slack bus is a sort of reference bus used to satisfy the condition of power balance. It is mostly concerned with generation unit which is adjusted to employ the power balance condition [12]. 1.3. Generator (PV) Bus Generator bus is essentially concerned with the voltage control. It is connected with generating unit where power generated from the bus is controlled with the help of prime mover. Whereas the control over voltage so produced is made by varying the generator excitation. Mostly the limits are set for reactive power and it depends on the specifications of machine used. 1.4. Load (PQ) Bus The load bus is not due to generator but we can obtain this from old data, measurements or from forecast. In this case real power provided to the system is taken as positive, whereas the electric power utilized within system is taken as negative. In this bus P and Q are known variable while |V| and δ are unknown variables. [8, 12]. 1.5. Power Flow Analysis Methods From this analysis we mean investigation of various variables of electrical system such as voltage, current, active power etc at various points in a system, once the system has attained steady state.This is carried out by solving simultaneous equations. These solutions form the platform for solving equations for performance of power system [4] 1.6. Gauss-Seidel Method This method is developed based on the Gauss method. It is an iterative method used for solving set of nonlinear algebraic equations [14]. The method makes use of an initial guess for value of voltage, to obtain a Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 3 calculated value of a particular variable. The initial guess value is replaced by a calculated value. The process is then repeated until the iteration solution converges. The convergence is quite sensitive to the starting values assumed. But this method suffers from poor convergence characteristics [15]. 1.7. Fast Decoupled Method This method is considered as speedy and efficient for obtaining the solutions of problems pertaining to power flow. This method is basically an extension of Newton- Raphson method. This method was first presented in 1974 by Stott and Alsac in 1974 [16-18]. Since this method is based on Newton-Raphson method so it provides much simplification in carrying out calculations, also reliable results can be obtained, rapid convergence and this method soon became mostly employed method in power flow problems. In this method polar coordinates and some approximations are used so fast algorithms for power solutions are obtained. However, the use of certain approximations also results in non-convergence in some cases. For instance, in case of large resistance-to-reactance ratios as well as for the case where low voltages appears at some buses. In such cases the solutions do not converge nicely due to approximations, so one has to resort some assumptions so as to simplify Jacobian Matrix. To overcome these difficulties arising due to non convergence of solutions various investigations have been carried out. For instance one of them is co n- convergence of systems with high R/X ratios, and others with low voltage buses. 1.8. Newton-Raphson Method Sir Isaac Newton and Raphson formulated this method so it is named after them. This method employs an iterative method. By using this method we can approximate non linear simultaneous equation into linear ones with the use Taylor series. However, Taylor series is expanded to the first order approximations only. This numerical technique is widely used in the calculation of power flow due to its powerful convergence characteristics as compared with similar techniques. Fig. 1. IEEE 9-Bus System Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 4 2. Methodology For this study, ETAP software has been used. The Load Flow and Short Circuit analyses are first performed on the standard IEEE 9-Bus system, and their results are recorded. The Load Flow has been evaluated by the software using Newton- Raphson iterative method. After this, one generator has been removed from this test system and the above mentioned two analyses are performed again. The results of these analyses on this new system are recorded and then compared with the results obtained from the standard system. 3. Test System Details The test system for this study was the IEEE 9-Bus system. The SLD of this system is shown in figure-1. The system comprises of 3 generators, connected to 3 generator buses. These are buses 1, 2 and 3 respectively. After this, we have 3 step-up transformers, one after each generator bus, which step up the voltage to the transmission level. These transformers are connected to buses 4, 7 and 9 respectively. These buses are further connected to 3 load buses, namely buses 5, 6 and 8. The system has 1 swing generator operating at a voltage of 1.04p.u. and 2 voltage-controlled generators operating at a voltage of 1.025p.u. The system consists of 3 load buses interconnected through 6 transmission lines. The transmission lines are modeled with vertical configuration of conductors and a conductor spacing of 10ft, with a single ground wire. Gymnastic conductor is used. The loads are modeled with a requirement of 230kv and varying real and reactive powers. Also, circuit breakers are connected on each end of the transmission lines and generator buses for protection from overloading or faults. The Load Flow and Short Circuit analyses are performed on this standard system first. After this, generator 3 is removed from the system, and then the above 2 analyses are repeated for this modified system and the results are compared. 4. Simulation and Discussion In this section, we will describe the simulations performed in this study and discuss their results. 4.1. Load Flow Analysis on standard IEEE 9-Bus System Fig. 2. Load Flow Report of IEEE 9-Bus System Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 5 As we have studied theoretically, the real power flows when there is a difference between the magnitudes of voltages between 2 buses, and reactive power flows when there is a difference between the voltage phase angles of 2 buses. This is proven here in the simulation as well, as we can see figure-2. 4.2. Short Circuit Analysis on standard IEEE 9-Bus System Whenever a short-circuit fault occurs, it has 2 components: AC component and DC component. The AC component has further 3 stages; sub transient, transient and steady state. At the instance of fault occurrence, the sub transient current exists. It has the highest magnitude and the shortest time duration (0.5 cycles). 4.2.1. Sub-Transient Sub transient current rapidly decreases to the transient stage. It is relatively longer (from 1.5 cycles to 4 cycles approx.). This transient current further reduces to the steady-state value, which occurs after about 30 cycles, and its magnitude is sustained throughout the rest of the fault. As shown in figure 3. NOTE: The steady-state current mentioned in figure is still the fault current, as its magnitude is still many times greater than the nominal current. Fig. 3. Short Circuit Fault Sub-Transient Component In the above-mentioned results, we can see the sub transient magnitudes of symmetrical and unsymmetrical fault currents (in kA) which flow through the 3 generator buses in the event of a fault. The current is highest during this stage as the generator’s impedance is lowest here. 4.2.2. Transient Fig. 4. Short Circuit Fault Transient Component Comparing the results of the sub transient and transient fault currents, we see that both of these are identical. This is due to the fact that ETAP calculates both these currents using the sub transient impedance values, but in reality, the impedances of both these currents are different, and therefore, there magnitudes are different as well. As shown in Figure 4 This space is intentionally left blank to adjust the figers Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 6 4.2.3. Steady-State Fig. 5. Short Circuit Fault Steady State Component Comparing the steady-state current values with the sub transient and transient current values, we can see that its magnitude is lower than the other two fault currents. This is because the impedance of the generator during the steady-state stage becomes equal to the synchronous reactance. The fault current will not decrease any further than this value. Load Flow Analysis on standard IEEE 9-Bus System after removing one PV Generator. As shown in figure 5. The voltage-controlled generator maintains the voltage throughout the system. Removing it causes a voltage drop at all the buses (except the generator buses). The load supplied by this generator is now being fed by the remaining 2 generators. Therefore, the power flow at the remaining 2 generator buses increases. 4.3. Short Circuit Analysis on standard IEEE 9-Bus System after removing one PV Generator The short-circuit analysis is repeated after removing one of the PV generators. We see that the magnitude of the overall fault current reduces, when compared to the original test system’s fault current. This is due to the obvious fact that now there is one less generator to feed the fault. Shown in figure 6. Fig. 6. Short Circuit Analysis after Removing One PV Generator 4.3.1. Sub-Transient when compared to the original test system’s fault current. This is due to the obvious fact that now there is one less generator to feed the fault. As shown in figure 7. Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 7 Fig. 7. Short Circuit Fault Sub-Transient Component after Removing One PV Generator 4.3.2. Transient Short circuit analysis of transient component on a 9-Bus system after removing one generator is shown in figure 8. Fig. 8. Short Circuit Fault Transient Component after Removing One PV Generator 4.3.3. Steady-State Short circuit analysis of the steady state component on a 9-Bus system after removal of a single generator is shown in figure 9. Fig. 9. Short Circuit Fault Steady-State Component after Removing One PV Generator 4.4. Comparison of the system before and after removing the PV Bus 4.4.1. Load Flow Comparison TABLE I. LOAD FLOW COMPARISON BEFORE AND AFTER REMOVING PV BUS Before removing PV Bus After removing PV Bus *Voltage magnitude at Bus 1 104% 104% *Voltage magnitude at Bus 2 102.5% 102.5% *Voltage magnitude at Bus 3 102.5% - Voltage magnitude at Bus 4 100.534% 99.480% Voltage magnitude at Bus 5 100.523% 99.468% Voltage magnitude at Bus 6 100.527% 99.467% Voltage magnitude at Bus 7 100.530% 99.475% Voltage magnitude at Bus 8 100.524% 99.464% Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 8 Voltage magnitude at Bus 9 100.531% 99.465% Here we can see that after removing the PV generator from bus 3, the voltages at all the buses (except the generator buses) drop, as has been discussed above. Shown in table-I 4.4.2. Short Circuit Comparison: NOTE: The steady-state fault currents in the table II below are 3-phase fault currents. The results will remain same for other types of faults. Shown in table-II TABLE II. SHORT CIRCUIT COMPARISON BEFORE AND AFTER REMOVING PV BUS Before removing PV Bus After removing PV Bus *Steady-State Current at Bus 1 44.518kA 37.872kA *Steady-State Current at Bus 2 39.933kA 34.227kA *Steady-State Current at Bus 3 45.153kA - Here we can see that the fault current reduces after removing a PV generator from the system, as there is one less generator to feed the fault here, as has been discussed above as well. 5. Conclusion In this paper a numerical study has been carried out for load flow and short circuit analysis on standard IEEE 9-bus system. From the above study we conclude that, by removal of a voltage-controlled generator from our system, the magnitude and phase angles of voltages will reduce for all the buses other than the remaining generator buses. Since this was a particularly small test system, the voltage drop was minimum and did not affect the system greatly. But, had it been a large practical system, then such a removal of a voltage-controlled generator would have damaging effects on the transmission system and can also damage the connected loads due to under-voltage. Moreover, it is also found that by removal of a generator, the magnitude of short- circuit fault current in a system would reduce as compared to the case where all generators were present in the system. REFERENCES [1] Mageshvaran, R., Raglend, I.J., Yuvaraj, V., Rizwankhan, P.G., Vijayakumar, T. and Sudheera (2008) Implementation of Non- Traditional Optimization Techniques (PSO, CPSO, HDE) for the Optimal Load Flow Solution. TENCON2008- 2008 IEEE Region 10 Conference, 19-21 November 2008. [2] Elgerd, O.L. (2012) Electric Energy Systems Theory: An Introduction. 2nd Edition, Mc- Graw-Hill. [3] Kothari, I.J. and Nagrath, D.P. (2007) Modern Power System Analysis. 3rd Edition, New York. [4] Keyhani, A., Abur, A. and Hao, S. (1989) Evaluation of Power Flow Techniques for Personal Computers. IEEE Transactions on Power Systems, 4, 817-826. [5] Hale, H.W. and Goodrich, R.W. (1959) Digital Computation or Power Flow—Some New Aspects. Power Apparatus and Systems, Part III. Transactions of the American Institute of Electrical Engineers, 78, 919-923. [6] Sato, N. and Tinney, W.F. (1963) Techniques for Exploiting the Sparsity or the Network Admittance Matrix. IEEE Transactions on Power Apparatus and Systems, 82, 944-950. [7] Aroop, B., Satyajit, B. and Sanjib, H. (2014) Power Flow Analysis on IEEE 57 bus System Using Mathlab. International Journal of Engineering Research & Technology (IJERT), 3. [8] Milano, F. (2009) Continuous Newton’s Method for Power Flow Analysis. IEEE Transactions on Power Systems, 24,50-57. [9] Grainger, J.J. and Stevenson, W.D. (1994) Power System Analysis. McGraw-Hill, New York. [10] Tinney, W.F. and Hart, C.E. (1967) Power Flow Solution by Newton’s Method. IEEE Transactions on Power Apparatus and Systems, PAS-86, 1449-1460. [11] Bhakti, N. and Rajani, N. (2014) Steady State Analysis of IEEE-6 Bus System Using PSAT Power Tool Box. International Journal of Engineering Science and Innovation Technology (IJESIT), 3. [12] Hadi, S. (2010) Power System Analysis. 3rd Edition, PSA Publishing, North York. [13] Kabisama, H.W. Electrical Power Engineering. McGraw-Hill, New York. [14] Gilbert, G.M., Bouchard, D.E. and Chikhani, A.Y. (1998) A Comparison of Load Flow Analysis Using Dist Flow, Gauss-Seidel, and Optimal Load Flow Algorithms. Proceedings of the IEEE Canadian Conference on Electrical and Computer Engineering, Waterloo, Ontario, 24-28 May 1998, 850-853. [15] Glover, J.D. and Sarma, M.S. (2002) Power System Analysis and Design. 3rd Edition, Brooks/Cole, Pacific Grove. [16] Stott, B. and Alsac, O. (1974) Fast Decoupled Load Flow. IEEE Transactions on Power Apparatus and Systems, PAS-93, 859-869. Modeling of IEEE 9-Bus System with Load Flow and Short Circuit Analyses (pp. 01 - 09) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 5 No. 2 July – December 2022 9 [17] Stott, B. (1974) Review of Load-Flow Calculation Methods. Proceedings of the IEEE, 62, 916-929. Adejumobi, I.A., et al. (2014) Numerical Methods in Load Flow Analysis: An Application to Nigeria Grid System. International Journal of Electrical and Electronics Engineering.