Stability Enhancement of Grid-Connected Wind Power Generation System Using PSS, SFCL and STATCOM

The stability related issues may occur in a power system due to disturbances in generating or loading conditions, especially in the presence of distributed generation (DG) based on renewable energy resources (RERs). This paper proposes a novel strategy for the stability enhancement of a wind power generation system (WPGS) by using a combination of three devices, namely, a power system stabilizer (PSS), resistive superconductor fault current limiter (R-SFCL) and static synchronous compensator (STATCOM). The small signal (SS) stability of the test system is enhanced by selecting the best PSS type from the different types of PSS. An R-SFCL is used for improving the rotor angle and the frequency stability of the test system. Two indices, namely, transient stability index and sum of maximum deviations (SMD) index are introduced for determining the optimal locations of different sized R-SFCLs for increasing the rotor angle stability. The sensitivity index (SI) based on the power change between areas is applied for determining the optimal locations of different sized R-SFCLs for enhancing the frequency stability. Along with rotor angle and frequency stability, LVRT capability improvement of the wind farm using STATCOM is also considered. Finally, the combined effect of R-SFCL and STATCOM on the rotor angle and the frequency stability, for different fault locations, is also investigated for determining the optimal location of an R-SFCL in the presence of STATCOM. The results presented in the paper show that STATCOM affects both the number of feasible locations and the optimal locations that can be selected for different sized R-SFCLs for augmenting the rotor angle and the frequency stability of the system during faults. Moreover, it is pointed out that an optimal combination between the different sizes and the locations of R-SFCLs and STATCOM exists to enhance the overall stability of the test system under fault conditions.


I. INTRODUCTION
A power system is considered stable if it can maintain an equilibrium state when subjected to physical disturbances. Small-signal (SS) stability describes the ability of a power The associate editor coordinating the review of this manuscript and approving it for publication was Akin Tascikaraoglu . system to remain in synchronism mode in case of small load variations or decoupled generators. The transient stability deals with the ability of a system to maintain its stability after occurrence of a severe disturbance, i.e., faults. The stability issues may occur in a power system due to a large change in generation or loading conditions. These issues are aggravated due to large scale integration of renewable energy resources (RERs) especially based on wind power generation. Due to intermittent nature of wind power, the transient, frequency and voltage stability may be affected. Moreover, power generated from wind turbines would change the power flow of the system and have an effect on the primary oscillation modes of synchronous generators (SGs). In extreme cases, the tripping of one wind turbine may cause cascaded tripping of other subsequent wind turbines and system generators and might result into a major black out. Moreover, different types of induction generators (IGs), widely deployed in wind energy conversion systems (WECSs), may also suffer due to their high sensitivity to grid disturbances and faults [1]. Thus, it is important to devise a strategy to operate various system generators within their stability margins especially in presence of a large share of wind generation.
The use of PSS is described for damping of low frequency oscillations (LFOs) of wind power systems in [2], [3] and for enhancing the rotor angle stability of doubly fed induction generator (DFIG) based wind power systems in [4].
For the transient stability enhancement, use of braking resistors [5], [6], Superconducting Fault Current Limiters (SFCLs) [7], [8], [9], Flexible AC Transmission Systems (FACTS) devices [10], [11], [12], and Superconducting Magnetic Energy Storage (SMES) [13], [14], etc., is extensively discussed in literature. Fault Current Limiter (FCL), an active power controlling device, is very effective in improving the transient stability of a power system. Authors in [15] proposed a bridge type FCL with optimal reclosing of the circuit breaker to enhance transient stability. The use of a solid state FCL (SSFCL) for enhancing the dynamic damping of a power network with wind-turbine power generation is described in [16]. The performance of SFCL for improving the transient stability of a power system is analyzed in [17], [18]. A comparison of inductive and resistive types of an SFCL with respect to transient stability enhancement of a power system is described in [19].
In [20], optimal location of SFCL, in order to improve rotor angle stability is determined by using IEEE benchmark four machine two area test system. In that literature, transient stability index and optimal resistive values are determined on the basis of sum of maximum deviations. In [21], optimal location of SFCL to damp out low frequency oscillations is determined by using IEEE benchmark four machine two area test system. In [22], double-integral sliding mode controller (DISMC) based bridge-type flux-coupling non SFCL has been proposed to improve fault ride-through (FRT) capability and rotor angle stability response of a DFIG-based wind power system. In [23], the authors have discussed the relation between the location of SFCL and type of power system stabilizer (PSS) to enhance the transient stability.
FACTS devices including static synchronous compensator (STATCOM) have a number of applications including enhancement of transient stability and Low Voltage Ride Through (LVRT) capability. The effect of fuzzy logic controller based static var compensator and static synchronous compensator on power system stability has been studied in [24]. It is concluded by the authors that STATCOM has a better performance in respect of stability improvement of the system as compared to static Var compensator (SVC). The LVRT capability of a doubly fed induction generator (DFIG) based wind farm is enhanced with STATCOM in dynamic conditions. Metaheuristic optimization algorithms like the Water Cycle Algorithm (WCA), Particle Swarm Optimization (PSO) and a hybrid algorithm of both WCA and PSO are used to enhance the STATCOM performance [25]. The combined interaction of a unified power quality conditioner and superconducting magnetic energy storage is proposed to enhance the LVRT of DFIG and the permanent magnet synchronous generator. The effectiveness of the proposed technique is verified during fault conditions and wind speed variation [26]. Similarly, coordinated design and application of robust supplementary damping controllers for STATCOM and SVC to improve the SS stability of a large power system is described in [27]. A comprehensive review is presented in [28] to enhance LVRT and high voltage ride through for the DFIG system using different techniques including STAT-COM. In [29], coordination of SSSC and PSSs controllers is proposed for improvement of transient stability and LVRT capability of a power system connected with wind and photovoltaic (PV) based generation. In [30], the application of SVC is considered to increase the voltage stability and the LVRT capability of the DFIG based wind farm system. The effect of STATCOM and SVC on dynamic voltage stability during LVRT of wind turbine systems is investigated in [31]. In [32] a nonlinear generalization of the robust coordinated PSS-AVR is proposed to enlarge the stability region and for enhancing the transients response during faults. In [33] the synergetic control (a nonlinear control method) is applied to the decentralized TCSC controller for improving transient stability and voltage regulation, and to schedule TCSC line active power transfer in a wide range of operation conditions. The rotor side crowbar circuit is proposed and analyzed for the impact of future grid impedance with high penetration of distributed generation and improved LVRT ability of the system in [34]. In [35], [36] an additional cascading converter topology has been proposed to mitigate the stator flux oscillation of DFIG and to enhance the LVRT and HVRT capabilities in dynamic conditions.
In [36], the application of PSS, SVC, and STATCOM devices for improving SS and transient stability is discussed and a robust method based on Particle Swarm Optimization (PSO) is proposed for optimizing parameters of their controllers. In [37], a comparison of PSS, SVC and STATCOM controllers regarding damping interarea oscillations has been described. The combined application of PSS and SSSC is described in [38] for enhancing the transient and small signal stability and in [29] for improving the transient stability and LVRT capability of a power system with RERS.
The coordination of PSS and FACTS thyristor controlled series capacitor (TCSC) damping controllers was explained VOLUME 11, 2023 in paper [27] to enhance the SS stability of large-scale power systems. In research [39], the effect of PSS and TCSC on instabilities related to Hopf Bifurcation (HB) has been analyzed. The authors have suggested that a coordinated control of PSS and the TCSC may delay the point of HB as compared to the case when the PSS is acting alone in the system. A new coordinated design between PSSs and a unified power flow controller (UPFC) is proposed in [40] for damping local and interarea modes of oscillations by using genetic algorithm (GA). The combined interaction of a unified power quality conditioner and superconducting magnetic energy storage is proposed to enhance the LVRT of DGIG and the permanent magnet synchronous generator. The effectiveness of the proposed technique is verified during fault conditions and wind speed variation [41]. The literature [42] discusses the application of PSSs and the generalized UPFC-power oscillation damping (GUPFC-POD) in a multimachine power system and proposes a technique based on the novel bat algorithm (NBA) for the parameter design of these devices for enhancing SS stability.
The combined use of SFCL and STATCOM and the combined use of resistive-type SFCL and UPFC has been discussed in [43] and [44] respectively for improving the transient stability of a multi-machine power system. This paper discusses the stability enhancement of an induction generator (IG) based wind power generation system (WPGS). The combined use of PSSs, FCLs and FACTs devices for stability enhancement is increasing worldwide as the traditional power grid is becoming a Smart Grid with the integration of DG based on renewable energy resources (RERs). Damping of generator rotor oscillations is obtained by PSS with the help of excitation controllers on the basis of auxiliary stabilizing signals. The delta Power PSS which is most efficient than other PSSs is selected for the test system.
For improving the rotor angle and the frequency stability, resistive superconductor fault current limiter (R-SFCL) is used. Two indices, namely, transient stability index and sum of maximum deviations (SMD) index are introduced for determining the optimal locations of different sized R-SFCLs for enhancing the rotor angle stability. For increasing the frequency stability, sensitivity index is used and the optimal locations of different sized R-SFCLs is determined on the basis of power change between areas by utilizing sensitivity index (SI). It is important to determine optimal location of R-SFCL considering all stability types; otherwise, R-SFCL will only improve the transient stability of the system in case of a fault at other locations of the system. Along with rotor angle and frequency stability, LVRT capability improvement of a wind farm using STATCOM is also considered. Moreover, combined effect of STATCOM and R-SFCL on rotor angle and frequency stability for different fault locations is also investigated for determining optimal location of R-SFCL to enhance the overall stability of the system. In contrast to the existing literature, this paper considers the improvement in rotor-angle and frequency stability, and LVRT capability of a power system by using three components, namely, PSS, R-SFCL and STATCOM. The results presented in the paper show that STATCOM affects both the number of feasible locations and the optimal locations that can be selected for different sized R-SFCLs of the system for improving the rotor angle and frequency stability during faults. Moreover, it is pointed out that an optimal combination between the different sizes and the locations of R-SFCLs and STATCOM exists in order to enhance the overall stability of the test system under fault conditions. Keeping in view the increasing number of power networks which incorporate PSSs, R-SFCL and FACTS devices jointly, it is important to consider the mutual interaction of these devices for the selection of optimal loaction of R-SFCL for enhancing the system's overall stability. This topic according to the best of the author's knowledge is not so far covered in the existing literature.
The rest of this paper is organized as follows: the test system's description is given in section II. Different methods used to improve rotor angle and frequency stability, and their mathematical expressions are described in section III. Fuzzy Logic Controller (FLC) based STATCOM controller design is also discussed in section III. The results and their analysis is given in section IV. The section V, finally, concludes the paper. Fig. 1 is the single line diagram of the IEEE benchmark four machine two area test system used for the case study. The details of system are given in appendix in table 13 to table 21 [45]. All the test system generators are equipped with power system stabilizers to improve the small signal stability. An R-SFCL is used to improve the rotor angle and frequency stability, and ten different locations are considered for the optimal application of R-SFCL as shown in Fig. 1. An IG based wind power generation system (WPGS) with a total capacity of 9 MVA is integrated into the area 1. A 9 MVA STATCOM is also attached at area 1 to enhance LVRT of the IG based WPGS [46].

III. METHODOLOGY
To analyze the stability of system, various indices have been proposed. Mathematical basis for the stability indices and the methodology to improve the system stability and LVRT has been discussed in this section.

A. SMALL SIGNAL STABILITY IMPROVEMENT USING PSS
Damping in generator rotor oscillations is obtained by PSS with the help of controlling the excitation of generator on the basis of auxiliary stabilizing signal. The common input signals for PSS include shaft speed, terminal frequency and integral of power. On the basis of input signal, PSS has the following types: As name describes, delta ω PSS has turbine speed as stabilizing signal, delta Pa PSS has turbine power as stabilizing signal, multi-band PSS has both turbine speed and turbine power as stabilizing signal, and frequency-based stabilizer has terminal frequency as stabilizing signal.

B. ROTOR ANGLE STABILITY
When a fault occurs in the network, each generator's rotor angle deviates from its nominal value. Stability can be improved by reducing these rotor angle deviations. R-SFCL has been deployed to damp the rotor angle deviations in case of fault. To find out the deviations, an index is calculated based on the sum of mean deviations (SMD) for all machines with reference to the slack bus. System stability is assessed and enhanced based on this value of SMD and Transient Stability Index (TSI).
The above indices are calculated as: Where m = R-SFCL location in the network k = Fault location in the network N = The number of fault locations investigated By introducing R-SFCL, if there is a reduction in the SMD, then it indicates an improvement in the system stability. The Transient Stability Index (TSI) which depends on SMD and resistance of the R-SFCL is calculated as: Note that the positive value of TSI m indicates that the R-SFCL reduces the rotor angle deviations due to which the transient stability of the system increases during a fault. While the negative value of TSI m indicates that stability is degraded and R-SFCL location is non-optimal. So the best location of R-SFCL is when the TSI m is maximum for selected resistive value of R-SFCL. The flow chart in Fig. 2 shows the procedure to select optimal location of R-SFCL on the basis of TSI.

C. FREQUENCY STABILITY
Frequency stability of the system is improved by damping frequency oscillations. This is accomplished by reducingpower change between areas with the help of R-SFCL. Optimal location and optimal resistive value of R-SFCL for frequency stability is determined on the basis of sensitivity index (SI). For optimal location of the R-SFCL, a new index is used [21].
where PCA = Power change between areas P Scheduled is the nominal power flow in steady state, i.e, 413.3 MW and t f is the settling time after clearing the fault and is taken as 20s.
Sensitivity index based on PCA is defined as: The flow chart in Fig. 3 shows the procedure to select the optimal location of R-SFCL.

D. LVRT CAPABILITY IMPROVEMENT USING STATCOM
During the fault, voltage dip occurs at the generator terminals. This voltage dip should be within the specific limits to avoid tripping. By using fuzzy logic controller (FLC) based STAT-COM, the voltage dip is improved during the fault. There are two inputs to the FLC of STATCOM that are error (E) and change in error (CE) of voltage signal. Both inputs and the output have seven linguistic variables of the triangular type. So there are forty nine FLC rules as shown in Table 1. Fig. 4 shows the surface of overall developed FLC. Where linguistic variables are negative small (NS), medium negative (MN),    The STATCOM controller keeps the fundamental voltage component of a voltage-sourced converter (VSC) in phase with the line voltage at PCC during SS conditions. When VSC voltage is not in phase with line voltage, then STATCOM exchanges reactive power corresponding to an error between VSC and line voltage. The magnitude of reactive power exchange depends on transformer leakage reactance (X tr ),the magnitude of VSC voltage and the angle. The VSC voltage angle (θ 2 )which is normally kept close to zero is temporarily phase-shifted according to the reactive power requirement of the system. The reactive power being delivered or absorbed by the STATCOM is given by Eq. 7.
The reactive power exchange between PCC and STATCOM is Q c . V 1 is the line voltage and V 2 is the STATCOM output voltage. The SS performance model can be written as: The size of the compensation devices depending upon the voltage improvement index and the ratio of STATCOM size (Q ST ) to the maximum possible STATCOM size (Q ST max ). The voltage improvement index of the power system is defined as the deviation of the voltage from unity at a bus.
The system voltage improvement index (D v ) is written as: No. of bus represents n, V iref is the nominal voltage at reference ith bus and V i denoted the actual voltage at ith bus. The bus voltage must remain in allowable limit between ± 10 % of the minimal voltage bus voltage. The optimal size of the compensation device is determined from the equation 10 [47].
where P loss , Loss base , m 1 , m 2 and m 3 are active power loss of the network, base system active power loss and coefficients whose values are determined from the above equations.

IV. RESULTS AND DISCUSSION
In

A. SMD FOR THE TEST SYSTEM
When a fault is applied to the network, rotor angle deviation can be observed among all generators. In order to stabilize the network, these rotor angle deviations must be reduced to minimum. First of all, SMD is determined without applying R-SFCL. Fig. 9 compares the value of SMD of each fault location without R-SFCL & without DG, and without R-SFCL & with DG respectively. Total value of SMD comes out to be 1280.3 without R-SFCL and without DG and 1270.5s without R-SFCL and with DG.

B. EFFECT OF DIFFERENT TYPES OF POWER SYSTEM STABILIZERS ON SMD
The effect of using different types of PSS is studied for both cases when DG is connected and when DG is not connected. The SMD is very high without PSS and reduces significantly for multi-band PSS. For △(ω) and △(P a ) PSS, SMD is futher reduced and is minimum for △(P a ) PSS. So, for further simulations, △(P a ) PSS is used. Table 2 shows SMD for ten locations when different types of PSS are used.

C. OPTIMAL LOCATIONS OF DIFFERENT SIZED R-SFCLs USING TRANSIENT STABILITY INDEX
A resistive superconductor fault current limiter (R-SFCL) is employed. Three different values of R-SFCL, i.e., 10 ohm, 20 ohm, and 50 ohm have been considered for the analysis [20] and [21].   Table 3 shows the results of SMD and TSI for the case when R-SFCL of 10 ohm is added at each fault location in two area system along with the DG. A three phase fault is considered that lasts for 100 ms, i.e., the fault is applied at 3s and is cleared at 3.1s. The total simulation time is 20s. For R-SFCL = 10 ohm, the optimal location is L 2 as for this location, SMD is the lowest and the value of TSI is the highest. While for location L 10 , the value of TSI is negative, showing that R-SFCL at this location increases the rotor angle deviations, i.e., it decreases the stability. Table 4 shows the results of SMD and TSI for case when R-SFCL of 20 ohm is added at each fault location in the two area system. For R-SFCL = 20 ohm, the optimal location is L 9 as for this location, SMD is the lowest and value of TSI is the highest. While for location L 10 , the value of TSI is negative, showing that R-SFCL at this location increases the rotor angle deviations, i.e., it decreases the stability. Table 5 shows the results of SMD and TSI for case when R-SFCL of 50 ohm is added at each fault location in the system under study. For R-SFCL = 50 ohm, the optimal location is L 9 as for this location, SMD is the lowest and value of TSI is the highest. While for location L 1 and L 10 , the value of TSI is negative, showing that SFCL at these  locations increases the rotor angle deviations, i.e., it decreases the stability.

D. OPTIMAL LOCATIONS OF DIFFERENT SIZED R-SFCL USING SENSITIVITY INDEX AND PERCENTAGE REDUCTION
In this case optimal location is determined on basis of sensitivity index which is dependent on power change between areas. Tables 6 and 7 show the results of power change between areas, sensitivity index and percentage reduction in power for the following these three cases.
• When no R-SFCL is added and no DG is connected • When R-SFCL of 10 ohm is added at each fault location • When R-SFCL of 20 ohm is added at each fault location

1) OPTIMAL LOCATION OF 10 OHM R-SFCL
The effect of 10 ohm R-SFCL is observed on PCA for each fault location. The R-SFCL is inserted at each fault location and percentage reduction in PCA is observed. Table 6 shows that for locations L 2 , L 3 , L 4 and L 9 , there is significant reduction in PCA and L 9 is the best location. For fault locations L 6 , L 7 and L 8 , percentage reduction in PCA is negative. This shows that these locations are not suitable for inserting 10 ohm R-SFCL as it reduces the stability.

2) OPTIMAL LOCATION OF 20 OHM R-SFCL
The effect of 20 ohm R-SFCL is observed on PCA for each fault location. The R-SFCL is inserted at each fault  location and percentage reduction in PCA is observed. Table 7 shows that for locations L 1 , L 2 , L 4 , L 9 and L 10 , there is significant reduction in PCA and L 10 , there is significant reduction in PCA and L 4 location is the best location. For fault locations L 5 , L 7 , and L 8 , percentage reduction in PCA is negative. This shows that these locations are not suitable for inserting 20 ohm R-SFCL as it reduces the stability. The effect of 10 ohm R-SFCL and 9 MVA STATCOM is observed on SMD for each fault location. STATCOM is connected to system at point of common coupling (PCC) while R-SFCL is inserted at the each fault location and the percentage reduction in SMD is observed. Table 8 shows that, for locations L 1 , L 2 , L 8 and L 9 , there is significant reduction in SMD and L 1 is the best location. For fault location L 10 , the percentage reduction in SMD is negative. This shows that the location is not suitable for inserting 10 ohm R-SFCL as it reduces the stability. The effect of 20 ohm R-SFCL and 9 MVA STATCOM is observed on SMD for each fault location. STATCOM is connected to system at point of common coupling while R-SFCL is inserted at each fault location and the percentage reduction in SMD is observed. Table 9 shows that for locations L 1 , L 6 , L 8 and L 9 , there is significant reduction in SMD and L 9 is the best location. For fault locations L 5 and L 10 , percentage reducation in SMD is negative. This shows that these locations R-SFCL as it reduces the stability. The effect of 10 ohm R-SFCL and 9 MVA STATCOM is observed on PCA for each fault location. STATCOM is connected to system at point of common coupling (PCC) while R-SFCL is inserted at each fault location and percentage reduction in PCA is observed. Table 10 shows that for locations L 3 , L 4 , L 7 , L 8 and L 9 , there is significant reduction in PCA and L 8 is the best location. For fault locations L 1 , L 2 and L 10 , percentage reduction in PCA is negative. This shows that these locations are not suitable for inserting 10 ohm R-SFCL VOLUME 11, 2023  as it reduces the stability. Sections E and G illustrating the interaction of three devices R-SFCLS, (PSS), and STAT-COM's impact on SMD and PCA. The simulated results shown in tables 9, 10, and 11 generated the optimal location for R-SFCL.

2) COMBINED EFFECT OF 20 OHM R-SFCL, (Pa) PSS AND 9 MVA STATCOM ON PCA
The effect of 20 ohm R-SFCL and 9 MVA STATCOM is observed on PCA for each fault location. STATCOM is connected to system at point of common coupling while R-SFCL is inserted at each fault location and percentage reduction in PCA is observed. Table 11 shows that for locations L 3 , L 4 , L 7 , L 8 and L 9 there is significant reduction in PCA and L 3 is the best location. For fault locations L 1 , L 2 , L 5 , L 6 and L 10 , percentage reduction in PCA is negative. This shows that these locations are not suitable for inserting 20 ohm R-SFCL as it reduces the stability.
G. LVRT IMPROVEMENT OF WIND FARM USING STATCOM, PSS AND R-SFCL FACTS devices including STATCOMs are commonly used to improve LVRT capability of WPGS. For analysis of the LVRT capability, a three-phase short circuit fault is simulated at PCC for 100 msec. It is observed that STATCOM with PSS and R-SFCL has a significant impact on the system performance during fault. A reduction in voltage dips is noted in grid/ system voltage as shown in Fig.10. Thus, the combined interaction of these devices has enhanced the LVRT capability of the IG based WPGS. The values of voltage dips and the percentage improvement IG voltage, and the active power generated by the wind farm are shown in Fig.11 and Fig.12 respectively. The STATCOM's reactive power generation/ absorption in the system dynamic conditions is illustrated in Fig.13. This figure shows that the reactive demand of the system reduces due to the mutual interaction of STATCOM, PSS and R-SFCL. At the instant of fault, the rotor speed of the IG-based WPGS increases. Resultantly, the pitch angle of the turbine increases    to limit the rotor speed to safe limits as is illustrated in Fig.14 and Fig. 15 respectively.

V. FINAL ANALYSIS
On the basis of TSI and SI, the locations L 3 -L 9 are feasible for the installation of R-SFCL of 10 in the presence of 30840 VOLUME 11, 2023   Table 8. Similarly, based on TSI and SI, the locations L 3 , L 4 and L 7 , L 8 , and L 9 are suitable for the installation of 20 R-SFCL in the presence of STATCOM as discribed in Table 10. It is clear that the number of feasible locations for 10 R-SFCL is more than 20 R-SFCL. Moreover, based on the percentage reduction in SMD, R-SFCL of 10 located on locations L 1 and L 2 give the best results in the presence of STATCOM as shown in Table 8. However, these two locations are unsuitable for the location of R-SFCL based on SI in presence of STATCOM as described in Table 10. Thus, on the basis of Tables 8 and 10, location L 8 is the best option for the installation of 10 R-SFCL. Similarly, on the basis of percentage reduction in SMD, R-SFCL of 20 located at location L 9 gives the best result in the presence of STATCOM as shown in Table 9. However, based on the value of SI in presence of STATCOM as described in Table 11, R-SFCL of 20 located at locations L 3 produces the best results.

STATCOM as shown in
On the basis of analysis presented in the paper, it can be noticed that STATCOM affects the number of feasible locations that can be selected for the installation of differently sized R-SFCL in a power network for the minimization of rotor angle deviations (i.e., transient stability) and power exchange between areas during faults, (i.e., frequency stability). Moreover, the analysis presented in the paper also helps to determine the best locations for the connection of different sized R-SFCL in a power network to improve the transient and frequency stability of the system. The results are summarised in Table 12.
The distributed generation based on WPGS of 9 MW capacity is integrated into area 2 at location L 4 and of 18 MW  capacity at location L 8 in area 1 without exceeding the stability limit of the system. When the DG penetration level is greater than 27 MW, the system becomes unstable as is described in Fig.16.

VI. CONCLUSION
This paper shows that the rotor angle and frequency stability of a power system can be improved by placing optimally sized R-SFCLs at different location of the network. The optimal locations of different sized R-SFCLs have been determined by using TSI and SMD for increasing the rotor angle stability and by using the sensitivity index (SI) for enhancing the frequency stability. The impact of STATCOM on optimal locations of different sized R-SFCLs has also been discussed in the paper. It is concluded that an optimal combination exists between the different sizes and the locations of R-SFCLs, and STATCOM for augmenting the overall stability of the system during faults.
The proposed strategy may be applied in future to other networks for stability enhancement with a combined application of DFIG based WPGS, nonlinear PSSs, different types of SFCLs and FACTs devices. The comparative analysis of performance of different types of fault current limiters (FCL) for stability improvement in combination with other components is out of the scope of this paper and may be done in the future. See Tables 13-21. VOLUME 11, 2023