1 st Langaroud, s Conference On Electrical Engineering (LCEE2015) Mohammad Azimi Ashpazi University of Tabriz Tabriz, Iran

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1 An Approach to Determine the Optimal Location of Thyristor-controlled Phase Shifting Transformer to Improve Transient Stability in Electric Power System Mohammad Azimi Ashpazi University of Tabriz Tabriz, Iran Abstract This paper presents an approach to determine the optimal location of thyristor-controlled phase shifting transformer (TCPST) for enhancing the transient stability of power system. TCPSTs are a cost-effective means to ensure reliable and efficient power flow control in overloaded transmission lines. This feature leads to reduction of the rotor angle separation of synchronous generators during disturbance occurrence which, results in an improvement of transient stability of network. To select the optimal location of TCPST, the angular separation of the rotors of generators in different positions of TCPST installation was calculated. Implementation of the proposed method on WSCC 9-bus and New England 39- bus test system shows the good efficiency of this method for improving transient stability. The results show that installation of TCPST in proper possession decreases the angular separation of the rotors of generators, which effectively improves the system s transient stability. 1-1 Motivation I. INTRODUCTION (HEADING 1) Due to recent attitude in creation of a deregulated utility industry, the power systems face considerable challenges. Deregulation is aimed at a competitive energy market, in which results the power system to operate near its static and dynamic limits. It is simply possible, by equipment outages or power disturbances the synchronism of power system generators losses and instability occurs. This aspect of instability is influenced by the dynamics of generator s rotor angle. In power system, the necessary condition for satisfying the system operation is that all synchronous machines should remain in synchronism when transient disturbances such as loss of transmission line, increase of load or short-circuit appears. In this paper, an approach is presented to determine the optimal location of TCPST for improving the transient stability of power system. Control of transmitted power in transmission line is possible through the control of at least one of its three parameters: terminal voltages, impedance or phase angle between two ends of transmission lines. Practically, control of the phase angle appears the most suitable [1]. It is performed that using transformers can makes power flows shift in transmission lines during disturbance and improve the transient stability of power system. These transformers generate phase quadrature voltage components to be injected into lines via series connected boosting transformers. 1-2 Literature Review In this regard many efficient studies are assigned to the idea of improving transient stability of power system. The preference of using the TCSC operation for transient stability improvement of a multi-machine power system is explained in [2], by using trajectory sensitivity analysis (TSA). The optimal location of shunt FACTS controllers for transient stability improvement by employing genetic algorithm has been presented in [3]. In [4] a simplified nonlinear method is proposed to enhance the transient stability of multi machine power system using a SSSC. The method that presented in [5] determines the security margin against voltage collapse in power system and improves it using FACTS devices in the optimal continuous power flow framework. Finding the best location of series controllers in order to improve the transient stability margin of a power system has been studied in [6]. In this study, in order to achieve this aim, an approach has been developed based on an index of proximity to instability and trajectory sensitivity analysis. The optimal location of superconducting fault current limiter (SFCL), to improve the transient stability, is presented in single machine infinite bus 1

2 systems in [7]. The proposed algorithm is able to calculate and analyze the stability margin of system. The effects of using different types of TCSC and static compensator (STATCOM) on power system s transient stability by TSA are studied in [8]. In [9] TCPST was applied to improve transient stability of power system by determining control strategies which was obtained from mathematical modeling of TCPST in power flow equations. [10] presents the ways of entering effects of PST into power systems energy functions and shows that PST improves transient stability of power system. In [11] TCPST was used to shift power flow of the transmission lines where an accident occurs into other lines. II. DESCRIPTION OF TCPST IN POWER SYSTEM The TCPSTs built for transmission grids are generally 3- phase shunt transformers with series windings and have a thyristor based controllable tap changer, which makes it possible to bypass a winding or reverse its voltage polarity. The TCPST models in power system as a reactance in series with a phase angle shift, which had shown in Fig. 1. Depends on the type of TCPST, both the voltage magnitude and phase angle can be controlled. The electrical output power of generator in Fig. 1 is given by: Fig.1: Model of transmission line with TCPST Where is the sum of generator transient reactance, TCPST reactance and equivalent reactance of transmission line. The generator dynamics can be represented by the following first order differential equations: Where,,, and are the rotor angle, speed, input mechanical power, output electrical power and inertia of the generator, respectively. In (3), and are considered as constant, so is the main factor that determines the dynamic behavior of the Synchronous machine. The addition of a quadrature voltage to the bus voltage leads to increase or decrease of the voltage phase-angle. From (1) it is clear that the phase shift of makes it possible to control the power flow of a transmission lines. This feature makes it possible to increase the power transfer in remaining lines of transmission corridor, due to mitigate lines that would be over-loaded in an N-1 situation. In such a case, usually the TCPST could be on stand-by and only switched-in as quickly as possible. (1) (2) (3) 2 III. EXPLANATION OF PROPOSED METHOD In an electrical power system, when transient disturbances like three-phase short-circuits occur, the synchronicity of synchronous machines should be satisfied in order to maintain the stability of the network. This feature is influenced by dynamics of generators rotor angle and power-angle relationships. Disturbance causes changes in the operating point of generators. Till the fault is cleared, the angular separation between the rotor positions of each generator is modified. This angular separation depends on the location of the fault, the fault clearing time and the type of fault. When a three-phase short-circuits occurs near the generator buses, the magnitude of rotor angular separation increases and the probability of transient instability increases. Also, if the fault cleared before critical clearing time, the power system is perturbed and the angle oscillates between two extreme values, but a stable point will be found again after a few seconds. On the other hand, if the fault is not cleared till critical clearing time, the kinetic energy gained during the fault wouldn t be completely expended to the system. The power system is not capable of returning to a stable position and the system will experience the loss of synchronicity. To determine the optimal location of the TCPST, the method focuses on the angular separation between the rotors of all synchronous generators in case of fault. Assuming that generator is taken as reference and its rotor angle is expressed as. Then, the angular separation of all other generators (relative to reference generator) will be obtained and is expressed as. Where, is the number of generators of power system. Figure 2 presents the evolution of the angular separation of a sample power system, during a three-phase short circuit. In the presented figure, the angular separation expresses the difference between the angular positions of the rotor of one of the generators related to reference generator. Installing TCPST in the suitable place can reduce the two boundary markers in Fig. 2 and consequently, it improves the transient stability of the system. To determine the optimal location of the TCPST in power system, the index sum of maximum deviation of (SMD) is defined, as mentioned in (4-6). Fig.2: Evolution of the angular separation of the rotors. (4)

3 Which and is the number of faults that are considered in the system and the location of TCPST that will be installed, respectively. is the maximum deviation of of generator related to reference generator. In this algorithm is the number of possible places that TCPST can be installed. The number of depends on the number of transmission lines of power system. The TCPST can be installed in the beginning or ending bus of transmission lines. So, if the number of transmission lines of network is, the will be. Regarding this algorithm, effects of lots of faults are considered, so the index could be proper decision criteria to install the TCPST in optimum place. IV. CASE STUDY To assess the efficiency of the proposed method, it has been applied on WSCC 9-bus and 39 bus New England test systems. In Fig. 3 the schematic diagram of WSCC 9-bustest system is represented. As shown in this figure, there are six transmission lines. So, the number of possible places to install TCPST is 12. In the time based simulation for simplicity, all disturbances were considered as three-phase short circuit, which occurs in middle of transmission lines. It is assumed that the faults will be cleared after 10 cycles, which is too long time. To determine the optimal location of TCPST, the value of index SMD is evaluated. Calculation of this index needs time based simulation, which is done by Dig-silent software. In this algorithm, first, one of the TCPSTs is installed in the appropriate place, by considering a specific tap adjustment. Next, the three-phase short circuit runs and the diagrams of for all synchronous machines related to reference machine obtains for each considered three-phase short circuit. Now, it is possible to calculate the index SMD, as explained in (6). Afterward, the tap position of TCPST changes and the same process done again and the SMD index calculates for each tap adjustments. The process explained above, runs for other TCPSTs one by one. So, the SMD index for each of TCPSTs that could be installed in power system is available. It is clear that, the minimum value of SMD is appropriate. The result of index SMD for WSCC 9-bus test system shows that the minimum value of SMD is obtained when the TCPST is installed in line 5 to 4. In Fig. 4 and Fig. 5 the diagram of all synchronous machines related to reference machine of WSCC 9-bus test system are represented during three-phase short circuit before and after installing TCPST, respectively. In the presented diagram, it is assumed that three-phase short circuit occurred in line 5 to 7. These diagrams show that by installing TCPST in line 5 to 4, the rotor angle separation of all (5) (6) 3 synchronous machines is reduced and consequently, the transient stability of system improves. Fig.3: schematic diagram of WSCC 9-bus test system. Fig.4: diagram of synchronous machines during threephase short circuit in line 5 to 7 before installing TCPST. Fig.5: diagram of synchronous machines during threephase short circuit in line 5 to 7 after installing TCPST. Figure 6 shows the schematic diagram of 39 bus New England test system. In this system only one TCPST is not able to improve the stability. In this case we can split the power system into smaller areas depending on the coherency of the areas when disturbances occurs. The optimal location of

4 TCPSTs in each area can be determined by applying the proposed method to each area by considering three-phase short circuits at different locations. As shown in Fig. 6, we divided system into three areas. Figure 7, 8 and 9 shows the value of SMD for areas 1, 2 and 3, respectively. According to these diagrams its clear that the optimal location of TCPST in area 1 is which is the begining line 26 to 27, which is the beginning of line 16 to 24 in area 2 and which is the beginning of line 6 to 5 in area 3, respectively. In Fig. 10 the diagram of four generators of bus 31, 32, 34 and 36 before and after installing TCPSTs are represented, respectively. It is assumed that three-phase short circuit is in middle of line 17 to 16. From these diagrams it is clear that by installing TCPSTs in optimum places of each area, the rotor angle separation is reduced and consequently, the transient stability of system improves. Fig.8: Value of SMD for each candidate location of area 2. Fig.9: Value of SMD for each candidate location of area 3. Fig.10: Rotor angle of the four generators before and after TCPST installation when a fault occurs at line 17 to 16. Fig.6: schematic diagram of 10 generator New England test system. V. CONCLUSION This paper presents an approach to determine the optimal location of TCPST for improving the transient stability of power system. TCPSTs are able to control the power flow in overloaded transmission lines. Due to this, they are capable of increasing the transient stability of network. This feature leads to reduction of the rotor angle separation of synchronous generators during disturbances. In this paper, via the difference of rotor angular of synchronous machines, SMD index is defined to finding the optimal location of the TCPST. The effects of possible faults in power system are considered in this index. The low value of SMD index indicates the optimum location to install TCPST. Fig.7: Value of SMD for each candidate location of area 1. REFERENCES 4 [1] G. El-Saady, A variable structure static phase shifting transformer for power system stabilization, Electric Power Systems Research 50 (1999) [2] D. Chatterjee and A. Ghosh TCSC control design for transient stability improvement of a multi-machine power system using trajectory sensitivity, Electrical. Power Systems Research, vol. 77, pp , 2007.

5 [3] S. Panda and R. N. Patel, Optimal location of shunt FACTS controllers for transient stability improvement employing genetic algorithm, Electric Power Components and Systems, vol.35, no.2,pp , [4] Jean de Dieu Nguimfack-N dongmo, Godpromesse Kenné, René Kuate-Fochie, André Cheukem Hilaire Bertrand Fotsi, Françoise Lamnabhi-Lagarrigue, A simplified nonlinear controller for transient stability enhancement of multi-machine power systems using SSSC device, Electrical Power and Energy Systems 54 (2014) [5] J. Aghaei, M. Gitizadeh, M. Kaji, Placement and operation strategy of FACTS devices using optimal continuous power flow Scientia Iranica D (2012) 19 (6), [6] A. Zamora-Cárdenas, Claudio R. Fuerte-Esquivel, Multi-parameter trajectory sensitivity approach for location of series-connected controllers to enhance power system transient stability Electric Power Systems Research 80 (2010) [7] B. Sung et al., Study on a series resistive SFCL to improve power transient stability: Modeling, simulation, experimental verification, IEEE Trans. Ind. Electron., vol. 56, no. 7, pp , Jun [8] D. Chatterjee and A. Ghosh, Transient stability assessment of power systems containing series and shunt compensators, IEEE Trans. Power Syst., vol. 22, no. 3,, pp , Aug [9] P. Kumkratug, Improvement of Transient Stability of Power System by Thyristor Controlled Phase Shifter Transformer American Journal of Applied Sciences 7 (11): , [10] U. Gabrijel, R. Mihalic, Transient Stability Assessment of Power Systems with Phase Shifting Transformers EUROCON 2003 Ljubljana, Slovenia. [11] C. N. Huang, Preventive Flow Control for Transient Stability by Phase Shifting Transformers, Transmission and Distribution Conference and Exposition, 2003 IEEE PES, pp vol.2. 5