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Power System Stabilization by Fault Current Limiter and Thyristor Controlled Braking Resistor Masaki Yagami Junji Tamura Member, IEEE Hokkaido Institute of Technology 7-15 Maeda, Teine-ku, Sapporo-shi, 006-8585, Japan [email protected] Senior Member, IEEE Kitami Institute of Technology 165 koen-cho, Kitami-shi, 090-8507, Japan [email protected] Abstract -- This paper presents a power system stability enhancement scheme using combination of fault current limiter and thyristor controlled braking resistor. The fault current limiter operates for limiting of fault currents, enhancement of the power system stability and suppression of turbine shaft torsional oscillations. After that, the thyristor controlled braking resistor operates with the objective of fast control of generator disturbances. The effectiveness of both devices has been demonstrated by considering 3LG (three-lines-to-ground) fault in a two-machine infinite bus system. Also, temperature rise effect of both devices has been demonstrated. Simulation results indicate a significant power system stability enhancement and damping turbine shaft torsional oscillations as well as with allowable temperature rise. system enhancement scheme using combination of FCL and TCBR for the purpose of a significant power system stability enhancement and damping turbine shaft torsional oscillations. If both devices-FCL and TCBR operate at the same bus, the stabilization control scheme can be carried out continuously and with flexibility. Namely, FCL operates from the fault occurrence instance to the fault clearing, and then TCBR operates dynamically until the generator disturbance becomes small. Through the simulation analysis, the effectiveness of combination of FCL and TCBR on power system stability enhancement is demonstrated. Moreover, this paper presents the results of analyses about temperature rise effect of resistance materials of both devices. FCL and TCBR dissipate accelerating energy of the generator in the form of heat. Therefore, temperature of the resistor material rises above ambient temperature. In particular, TCBR may become high-temperature state because it operates for a long duration. Higher temperature may cause the resistor material to melt away and due to higher resistance value, the system response may be somewhat affected. In order to confirm the temperature rise effect, both devices with various resistance values are tested in the simulation. These analyses are performed using EMTP. Index Terms—Power system stability, Fault current limiter, Thyristor controlled braking resistor, Temperature rise effect I. INTRODUCTION The use of fault current limiter (FCL) is being evaluated as one element necessary to limit the fault currents and enhance the power system stability [1]-[3]. FCL is a device that limits the fault currents by generating an impedance when a fault occurs. In addition, the limiting impedance generated to limit fault currents proves helpful in increasing generator output degraded by a fault, thus providing stabilization of power system. However, as FCL installed in series with transmission line can be just operated during the period from fault occurrence to fault clearing, FCL cannot control the generator disturbances after the clearing of fault. The braking resistor is also known as a very effective device for power system stability control. Besides, with the recent development of power electronics technology, replacing the circuit breaker with the semiconductor device is becoming feasible. Several thyristor-based control techniques have been proposed in the literature for the switching of braking resistor [4]-[6]. Since the thyristor controlled braking resistor (TCBR) can control an accelerating power in generator with flexibility, the power system stability may be enhanced more than that of the use of braking resistor controlled by mechanical device. However, as TCBR is installed in parallel with the transmission line, TCBR cannot be operated before the clearing of faults. From these viewpoints, we have proposed the power 978-1-4244-2893-9/09/$25.00 ©2009 IEEE II. MODEL SYSTEM A. Power system The two-machine infinite bus system model used for the simulation is shown in Fig. 1. It consists of two generators, an infinite bus, transformers, FCL, TCBR and double-circuit transmission lines. The line constants in the diagram are given in terms of R + jX (jB/2) per circuit. Turbine shaft model has six masses namely high-pressure (HP) turbine, an intermediate-pressure (IP) turbine, two low-pressure turbines (LPA, LPB), the generator (GEN) and exciter (EXC) as shown in diagram. Table I and Table II show the generator parameters [7] and turbine shaft parameters [8] respectively. The models of AVR and governor are shown in Fig. 2. FCL is installed on Y-side of the transformer, Tr.1. In general, FCL should be installed wherever fault current is extremely strong. However, FCL is installed only at the generator terminal because this study is focused mainly on 548 Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on April 23,2010 at 16:33:37 UTC from IEEE Xplore. Restrictions apply. (P/Q=1.0/0.35) (P/Q=1.9/0.1) RFCL 0.017+j0.144 (j0.0373) j0.0625 0.0238+j0.2016 (j0.0523) j0.0586 (P/V=1.2/1.02) G1 G2 FCL Tr.1 Tr.2 Δω PI 0.064+j0.322 (j0.0765) 3LG 0.078+j0.340 (j0.0895) RTCBR TCBR 0.020+j0.170 (j0.044) 0.034+j0.184 (j0.0395) (P/Q= 1.25/0.5) *Unit is pu value on system base (100 MVA) Fig. 1. (pu) (pu) (pu) (pu) (pu) (pu) (pu) 0.003 0.102 1.651 1.59 0.232 0.38 0.171 G2 ra Xl Xd Xq X’d X’q X’’d (pu) (pu) (pu) (pu) (pu) (pu) (pu) 0.004 0.078 1.22 1.16 0.174 0.25 0.134 200 (pu) (pu) (sec) (sec) (sec) (sec) (sec) 0.171 0.13 5.9 0.535 0.033 0.078 9.0 Rating (MVA) 130 (pu) (pu) (sec) (sec) (sec) (sec) (sec) 0.134 0.13 8.97 1.5 0.033 0.141 6.0 X’’q X0 T’d0 T’q0 T’’d0 T’’q0 H X’’q X0 T’d0 T’q0 T’’d0 T’’q0 H TABLE II TURBINE SHAFT PARAMETERS G1 HP Turbine LP Turbine LPA Turbine LPB Turbine Generator Exciter G2 HP Turbine LP Turbine LPA Turbine LPB Turbine Generator Exciter Inertia constant (s) 0.288890 0.483849 2.670287 2.749726 2.700840 0.106406 Inertia constant (s) 0.192593 0.322566 1.780191 1.833151 1.800560 0.070937 Spring constant (pu T/rad) 19.303 34.929 52.038 70.858 2.82 IP LPA LPB GEN EXC High-Pressure Turbine Intermediate-Pressure Turbine Low-Pressure Turbine Generator Exciter Power system model Vso Rating (MVA) HP HP: IP: LPA, LPB: GEN: EXC: Infinite bus TABLE I GENERATOR PARAMETERS G1 (P/Q= 0.9/0.3) j0.0576 Tr.3 1.04 ∠ 0 .0 ra Xl Xd Xq X’d X’q X’’d Turbine Shaft System Rout (α ) - Vs + Efdo 25 1+0.2S 4.0 + + Efd -4.0 (a) AVR ωm0 ωm - + Po 20 1+2.0S (b) Governor Fig. 2. 1.05 + + P 0.0 AVR and governor models the stabilization of the generator. Commutation-type FCL which consists of a shunt resistor with RFCL value and a variable resistor is used. TCBR is installed in parallel at the same bus. Resistor with RTCBR value is connected to the grid through the thyristor switching circuit. Thyristor switch is controlled by firingangle α which is calculated based on the generator speed deviation Δω . In the simulation study, it has been considered that the three-lines-to-ground (3LG) fault occurs near the generator G1 at 0.1 (sec). The circuit breakers on the faulted line are opened at 0.2 (sec) and reclosed at 1.0 (sec). It is assumed that the circuit breakers clear the line when the current through it crosses the zero level. B. Spring constant (pu T/rad) 19.303 34.929 52.038 70.858 2.82 FCL Commutation-type FCL which consists of the shunt resistor with RFCL value and the variable resistor is used. The value of the variable resistor is 0 (pu) during a normal condition. When the fault currents exceed a critical value which is set to 2.5 (pu), the resistance value of the variable resistor increases linearly to 25 (pu) within 1 (ms) (such a characteristic can be achieved by a superconducting coil [9]). 549 Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on April 23,2010 at 16:33:37 UTC from IEEE Xplore. Restrictions apply. Therefore, majority of the current flows into the shunt resistor, and then limited. To avoid an excess load, we have assumed that the resistance value of the variable resistor returns to 0 (pu) at the same time when the circuit breakers are opened at 0.2 (sec). TCBR As shown in Fig. 1, the generator speed deviation Δω and the desired resistance value Rout are selected as the signals of input and output respectively. Following a fault in the power system, Δω of the generator is measured, and then Rout is determined by PI controller device which is tuned by trial and error method. Firing-angle α for the thyristor switch is calculated from the output of the PI controller (i.e., Rout ). The desired power consumption determined by Rout and the real power consumption determined by RTCBR are equal and hence α can be calculated from this relationship. When Δω becomes less than 0.001 (pu), TCBR is removed from the grid. temperature coefficient (0.0012°C-1 [13]). TABLE III RESISTOR MATERIAL PARAMETERS OF BOTH DEVICES specific heat mass heat exchange coefficient surface area C. D. Resistor Material In this work, we have used an austenitic stainless steel [10] as resistor materials of both devices. For simplicity, the same values of material parameters (i.e., mass, surface area, heat exchange coefficient and specific heat) are used for both devices. The material parameters are shown in Table III. When a current, I, flows into the resistor (i.e., the device is in ‘ON’ condition), temperature of the resistor can be calculated from the following equation [12]: C M K S 520 (J/(kg·°C)) [11] 300 (kg) 16.5 (W/(m2·°C)) [11] 8 (m2) III. SIMULATION RESULTS A. Power system stability Fig. 3 shows the load angle responses of each generator in cases of “no devices”, “only FCL with RFCL=1.1 (pu)”, “only TCBR with RTCBR=0.6 (pu)” and “both devices-FCL and TCBR with RFCL=1.1 (pu) and RTCBR=2.5 (pu)”. Because of the absorption of real power by FCL and TCBR, the load angle oscillations are restrained in the case of “with devices”. In particular, the load angle oscillations in the case of “both devices” are restrained effectively for the overall duration of simulation. In “both devices”, FCL dissipates majority of the accelerating energy of the generator, and then TCBR dissipates the accelerating energy of the remainder. As a result, the use of both devices makes the system stabile quickly more than the use of “one-unit”. 100 2 RI tON CM + KS (1) where R is resistance of the resistor, tON is ‘ON’ duration time of the device and TON0 is initial temperature of the resistor. On the other hand, when the device is not in the circuit (i.e., the device is in ‘OFF’ condition), it gets cooled down by dissipating heat into the air. Therefore, temperature of the resistor decreases exponentially. The temperature decrease can be calculated from the following Newton’s law of cooling: TOFF = Ta + ( TOFF 0 − Ta ) e − nt OFF 60 N o d e v ic e s FC L TCBR FCL & TCBR 40 20 0 1 2 3 4 5 6 7 T im e (s ) (a) G1 (2) where TOFF is temperature of the resistor in ‘OFF’ condition, Ta is ambient temperature (20 °C), TOFF0 is initial temperature of the resistor, n is positive constant (0.01 for this work) and tOFF is ‘OFF’ duration time of the device. When temperature of the resistor changes, resistance value of the resistor also changes according to the following equation [13]: R = R0 { 1 + γ ( TON / OFF − Ta ) } Load angle (deg) 80 100 N o d e v ic e s FCL TCBR FCL & TCBR 80 Load angle (deg) TON = TON 0 + (3) 60 40 20 0 1 2 3 4 5 6 T im e ( s ) where R is resistance after the temperature change, R0 is initial resistance in the ambient temperature, γ is (b) G2 Fig. 3. Load angles of each generator 550 Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on April 23,2010 at 16:33:37 UTC from IEEE Xplore. Restrictions apply. 7 Fig. 4 shows the firing-angle responses of the thyristor switch for phase ‘a’ in the cases of “only TCBR” and “both devices”. The firing-angle α varies from 0 to 180 (deg) according to the value of Δω . When Δω becomes less than 0.001 (pu), α is compulsorily set to 180 (deg) (i.e., TCBR is removed from the grid). As can be seen in Fig. 4(a), α in the case of “only TCBR” varies until about 2.7 (sec). On the other hand, α in the case of “both devices” has a constant value of 180 (deg) from the early time. This is because majority of the accelerating energy are dissipated by FCL before TCBR operates. This indicates that the temperature rise of TCBR can be limited as shown in section III-C. Fig. 5 shows the stability index Wc for the resistance values of each device (i.e., RFCL and RTCBR) varied in the range from 0.6 to 2.6 (pu) with a step of 0.1 (pu). In “both devices”, RFCL is set to the constant value of 1.1 (pu). From the results in the cases of “one-unit”, the most effective value of FCL (RFCL) and TCBR (RTCBR) are 1.1 (pu) and 0.6 (pu) respectively. Also, it can be seen that the resistance which can dissipate large energy is effective. In the case of “both devices”, although all results indicate less than the most effective value of “one-unit”, in particular, large value is effective (minimum point is 2.5 pu). This is because TCBR does not need to dissipate the accelerating energy of the generator so much. 3 .0 FCL 2 .5 120 Wc (s) Firing-angle (deg) 180 60 TCBR 2 .0 FC L & TC BR 1 .5 0 0 1 2 3 4 5 6 7 1 .0 T im e (s ) 0 .5 1 .0 (a) TCBR 1 .5 2 .0 2 .5 R e s is ta n c e (p u ) Fig. 5. Stability index Wc for resistance of each device 180 Firing-angle (deg) B. 120 60 0 0 1 2 3 4 5 6 7 T im e (s ) (b) FCL and TCBR Fig. 4. Firing-angle of TCBR (phase ‘a’) To evaluate numerically the power system stability, we have used a stability index Wc given by Wc (sec) = T 0 d Wtotal dt / dt system base power (4) where Wtotal is a summation of kinetic energy of each generator and T is a simulation time selected to 7 (sec). By using Wtotal, we can evaluate the stability of overall power system, taking each generator power rating into account. Wc is the integrated variation of the energy exchanged between the generator rotor and the system; thus, the lower the value of the parameter, the smaller the transient swing, and the better the overall system stability. Turbine shaft torsional oscillations The rotor of generator has a very complex mechanical structure consisting of several predominant masses (such as rotors of turbine sections, generator rotor, couplings, and exciter rotor) connected by shafts of finite stiffness. Therefore, when the generator is perturbed, torsional oscillations occur between different sections of the turbinegenerator rotor. The certain electrical system disturbance can significantly reduce the life expectancy of turbine shafts [14]. Therefore, sufficient damping is needed to reduce the turbine shaft torsional oscillations. To analysis the torsional oscillations of the turbine shaft, we have used the structure of a typical lumped-mass of the generator driven by a tandem turbine as shown in Fig. 1. It consists of six torsional masses. Fig. 6 shows the turbine shaft torque responses of generator G1. In the case of “both devices”, the torques of turbine shafts between each mass are effectively restrained. Fig. 7 shows the integrated variation of the turbine shaft torque in the range of 0 to 7 (sec). The lower value improves the life expectancy of turbine shaft. As seen, the use of FCL is effective in comparison with the use of TCBR on the damping of shaft torque oscillations. In the case of “both devices”, all results indicate less than the most effective value of “one-unit” for all shafts. 551 Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on April 23,2010 at 16:33:37 UTC from IEEE Xplore. Restrictions apply. H P -IP 2 .5 IP -L P A L P A -L P B L P B -G E N 1 .8 Integrated value Turbine shaft torqeue (pu) 2 .0 1 .5 1 .0 0 .5 1 .2 TCBR FCL 0 .6 0 .0 FCL & TCBR 0 .0 -0 .5 0 1 2 0 .5 3 1 .0 IP -L P A 2 .0 2 .5 (a) HP-IP (a) No devices H P -IP 2 .5 1 .5 R e s is ta n c e (p u ) T im e (s ) L P A -L P B L P B -G E N 1 .8 Integrated value Turbine shaft torque (pu) 2 .0 1 .5 1 .0 0 .5 1 .2 TCBR FC L 0 .6 0 .0 FCL & TCBR -0 .5 0 .0 0 1 2 3 0 .5 1 .0 T im e (s ) Fig. 6. 1 .5 2 .0 2 .5 R e s is ta n c e (p u ) (b) FCL and TCBR Turbine shaft torque of G1 (b) IP-LPA 1 .8 Integrated value 1 .2 FC L 0 .6 FC L & TCBR 0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 R e s is ta n c e ( p u ) (c) LPA- LPB 1 .8 TCBR Integrated value Temperature Fig. 8 shows the responses of temperature and resistance of the resistor in the case of “both devices”. RFCL and RTCBR are set to 1.1 and 2.5 (pu) at 20 (°C) respectively. As mentioned earlier, FCL operates from the fault occurrence instance (0.1 sec) to the fault clearing (0.2 sec), and then TCBR operates dynamically until the generator disturbance becomes small. In the case of “both devices”, as shown in Fig. 4, TCBR only operates from about 0.3 to 0.7 (sec). As seen, temperature and resistance of FCL rise up to about 65 (°C) and 1.16 (pu) respectively. Those of TCBR rise up to about 70 (°C) and 2.65 (pu) respectively. As TCBR operates for a long time in comparison with FCL, temperature of TCBR rises more than that of FCL. However, the temperature rise of TCBR is not so large, and the resistance rise of TCBR is also small. Fig. 9 shows the maximum temperature for each resistance value. In “both devices”, RFCL is set to the constant value of 1.1 (pu). As can be seen, the maximum temperature of RFCL is in inverse proportion to the resistance value. On the other hand, RTCBR is not always in inverse proportion to the resistance value due to the dynamic operation of TCBR. Japanese Industrial Standards (JIS) [15] indicates that an upper temperature limit of the austenitic stainless steel is 300 (°C). As seen, the maximum temperatures of all cases are below the upper temperature limit. However, majority of the TCBR C. 1 .2 FC L 0 .6 FCL & TCBR 0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 R e s is ta n c e (p u ) (d) LPB- GEN Fig. 7. Integrated values of the turbine shaft torque responses 552 Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on April 23,2010 at 16:33:37 UTC from IEEE Xplore. Restrictions apply. 80 1.30 Temperature (deg C) A phase B phase C phase tem perature 40 1.20 1.15 Resistance (pu) 1.25 60 maximum temperatures in the case of “only TCBR” exceed 200 (°C). Therefore, if a large disturbance such as an unsuccessful reclosing occurs, the temperature of RTCBR may exceed the limit value of 300 (°C). On the other hand, the maximum temperature in the case of “both devices” can be decreased to about 70 (°C). IV. CONCLUSION 20 re sistance 1.10 0 0.0 0.2 0.4 0.6 0.8 1.0 T im e (s) (a) FCL 2.9 80 Temperature (deg C) te m p e ra tu re A p h a se B p h a se C p ha se 2.7 40 Resistance (pu) 2.8 60 In order to enhance the power system stability and damp the turbine shaft torsional oscillations, the use of combination of the fault current limiter and the thyristor controlled braking resistor is proposed in this paper. Furthermore, the temperature rise effect of both devices is included in the simulation. The simulation results indicate a significant power system stability enhancement and damping turbine shaft torsional oscillations as well as with allowable temperature rise. REFERENCES [1] 2.6 20 re sista n ce 2.5 0 0.0 0 .2 0 .4 0 .6 0.8 1 .0 T im e (s) Fig. 8. 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Maximum temperature for each resistance 553 Authorized licensed use limited to: UNIVERSIDADE DO PORTO. Downloaded on April 23,2010 at 16:33:37 UTC from IEEE Xplore. Restrictions apply.