61 CHAPTER 3 TRANSIENT STABILITY ENHANCEMENT IN A REAL TIME SYSTEM USING STATCOM 3.1 INTRODUCTION The modeling of the real time system with STATCOM using MiPower simulation software is presented in this chapter. The system is analyzed under severe disturbance to study the transient behavior by simulating three phase to ground fault at various buses. To enhance the transient stability of the system, STATCOM is inserted and tested to show the effect of the same on the transient stability under severe disturbance. The swing curve in degree, real power in MW, reactive power in MVAr and the voltage in p.u. of 11 kv generator bus(11) and the voltage in p.u. for 11 kv bus(14), 33 kv bus(6) and 110 kv grid bus(1) are taken for analysis. The potential application of STATCOM on the improvement of voltage profile of the various buses and reduction in rotor angle oscillation of the generator is evaluated from the implementation results. 3.2 LITERATURE REVIEW STATCOM includes the GTO and diode valves together with their necessary snubber circuits that make a valuable tool in the power system design process. The STATCOM is given this name because in a steady state operating regime, it replicates the operating characteristics of a rotating synchronous compensator (Gyugyi 1998 and Hingorani and Gyugyi 2000).
62 Abido (2005) presented a singular value decomposition (SVD) based approach to assess and measure the controllability of the poorly damped electromechanical modes by the different control channels of STATCOM. It is observed that the electromechanical modes are more controllable via phase modulation channel. It is also concluded that the STATCOM-based damping stabilizers extend the critical clearing time and enhance greatly the power system transient stability. Haque (2004) demonstrated that by the use of energy function, the STATCOM is capable to provide additional damping to the low frequency oscillations. The damping characteristics of STATCOM have also been analyzed and addressed, where different approaches to STATCOM-based damping controller design have been adopted, such as loop-shaping (Rahim et al 2002), pole placement (Lee and Sun 2002), multivariable feedback linearization (Sahoo et al 2002, 2004), H control (Al-Baiyat 2005) and intelligent control (Morris et al 2003). 3.3 PROBLEM STATEMENT Transient stabilization is an important factor in the power system control. The key factor in transient stability prediction is the way in which the transient swings either converges or diverges. It is important to prevent generators from losing synchronism and damping the subsequent oscillations quickly. The problem is formulated as the insertion of STATCOM in real time system that is to be analyzed using MiPower simulation software for enhancing the transient stability. 3.4 GENERAL REPRESENTATION OF STATCOM The basic electronic block of a STATCOM is a VSC that converts a DC voltage at its input terminal into a three-phase set of AC voltages at fundamental frequency with controllable magnitude and phase angle. The
63 VSC can be made up of three-phase, two-level, six pulse converters connected by an appropriate magnetic circuit into a multi-pulse structure to meet the practical harmonic, current and voltage rating requirements; or threephase, three-level, twelve-pulse converters in a multi-pulse structure; or simply PWM controlled three phase, two-level converters. A STATCOM has no inertia and can basically act in a fraction of a second, which is an advantage over a synchronous compensator. Furthermore, STATCOM does not significantly alter the existing system impedance, which gives it an advantage over the SVC. In all STATCOM applications implemented in transmission systems so far, only two of these methods have been used; PWM are still considered uneconomical due to high switching losses of available semiconductor switches with intrinsic turn-off capabilities. A STATCOM can be used for voltage regulation in a power system, having as an ultimate goal the increase in transmittable power, improvements of steady state transmission characteristics and of the overall stability of the system. Under light load conditions, the controller is used to minimize or completely diminish line overvoltage; on the other hand, it can also be used to maintain certain voltage levels under heavy loading conditions. In its simplest form, the STATCOM is made up of a coupling transformer, a VSC, and a DC energy storage device. The energy storage device is a relatively small DC capacitor, and hence, the STATCOM is capable of only reactive power exchange with the transmission system. If a DC storage battery or other DC voltage sources are used to replace the DC capacitor, the controller can exchange real and reactive power with the transmission system, extending its region of operation from two to four quadrants. A functional model of a STATCOM is shown in Figure 3.1. A STATCOM can support system voltage at extremely low voltage conditions as long as the DC capacitor can retain enough energy to supply losses. The STATCOM also has increased transient ratings in both capacitive and inductive regions.
64 Figure 3.1 Functional model of a STATCOM The overload capability is about 20% for several cycles in both regions. It is also worth noticing that the inductive transient current rating is slightly larger due to the fact that the GTOs in the inductive region are naturally commutated and hence, the amount and duration of this temporary overload capability is limited by the maximum current of the free-wheel diode. The capacitive transient rating is determined by the maximum current turn-off capability of the GTO thyristors. 3.4.1 Modeling of STATCOM STATCOM model shown in Figure 3.2, used to improve the transient stability of the real time system (real time system is discussed in section 2.6) considered, has been modeled in MiPower6.0. STATCOM is modeled using FPB available in MiPower. This FPB is executed to verify the correctness of the model. FPB is converted into FPD and this FPD is called as a function for the simulation of transient stability.
K=2.23 N[4] K=0.9416 Max = -0.529 + N[9] + N[2] N[14] N[18] N[5] N[6] K K Vref [1] N[3] - K=0.0584 K 1 st K = 1 K = 1 T = 0.015 K 1 st + N[7] Min = 0.9959 N[13] T=0.025 T = 0.01 Terminal Voltage [12] K 1 st N[10] N[17] STATCOM Q Supply N[16] Figure 3.2 Modeling of STATCOM 65
66 3.5 RESULTS WITH DISCUSSIONS The effect of STATCOM on transient stability of power system is analyzed by creating three phase to ground fault at 11 kv bus(14) and 33 kv bus(6) using MiPower. The transient stability is considered with a fault initiated at 1 s and cleared at 1.1 s. i.e., protection system cleared the fault within 100 ms. Transient stability is executed upto 10s to view the response. By inserting the STATCOM in the 33 kv bus(6) for the fault at 11 kv bus(14) and in the 11 kv bus(14) for the fault at 33 kv bus(6), the responses are presented. Swing curve and voltages are taken at different conditions like normal, at fault and after the insertion of FACTS controllers. During the steady state condition (without any disturbance) and fault condition (with disturbance) for the fault at 11 kv bus(14), the various responses of the system are discussed in sections 2.7.1 and 2.7.2. 3.5.1 Transient Stability of System with STATCOM at 33 kv Bus(6) The STATCOM is placed at 33 kv bus(6) of the system to improve the voltage and to enhance the transient stability. Three phase to ground fault is simulated for a period of 100 ms in 11 kv bus(14). The swing curve and voltage of 11 kv generator bus(11) and voltage of 110 kv grid bus(1) and 33 kv bus(6) are presented below. STATCOM at 33 kv bus(6), for a three phase to ground fault of 100 ms duration at 11 kv bus(14), leads the generator to oscillate from 3.5 to 9 degrees (during disturbance its value ranges from 3 to 9.6 degrees) whereas at the steady state condition, it is 6.3 degrees with respect to grid. This is depicted in Figure 3.3.
67 Figure 3.3 Swing curve of generator at fault condition (with STATCOM at 33 kv bus(6)) Figure 3.4 110 kv grid bus(1) voltage in p.u. at fault condition (with STATCOM at 33 kv bus(6))
68 STATCOM at 33 kv bus(6), for three phase fault at 11 kv bus(14), significantly contributes the reactive power and improves the voltage profile. Since 110 kv grid bus(1) is far away from STATCOM at 33 kv bus(6), the voltage profile improves from 0.96 p.u. to 0.983 p.u. This is depicted in Figure 3.4. Figure 3.5 33 kv bus(6) voltage in p.u. at fault condition (with STATCOM at 33 kv bus(6)) STATCOM at 33 kv bus(6), for three phase fault at 11 kv bus(14), significantly contributes the reactive power and improves the voltage profile. Since the STATCOM is connected in the same bus, the impact on voltage profile improvement is high. Even in the steady state, the STATCOM increases the voltage profile to 0.995 p.u. and limits the voltage dip to 0.955 p.u. during fault. This is depicted in Figure 3.5.
69 Figure 3.6 11 kv generator bus(11) voltage in p.u. at fault condition (with STATCOM at 33 kv bus(6)) STATCOM at 33 kv bus(6), for three phase fault at 11 kv bus(14), significantly contributes the reactive power and improves the voltage profile. It limits the voltage drop from 0.98 p.u. to 0.99 p.u. This is depicted in Figure 3.6. Considering the STATCOM at 33 kv bus(6) for the fault at 11 kv bus(14), it improves voltage stability to great extent by maintaining the voltage profile during steady state and improves the voltage profile significantly during fault. However, the impact of STATCOM on angular stability is good.
70 3.5.2 Transient Stability of System with STATCOM at 11 kv bus(14) The transient stability is evaluated with the help of swing curve for a three phase fault at 33 kv bus(6) for 100 ms duration. It is assumed that protection system cleared the fault within 100 ms. Three phase fault at 33 kv bus(6) is simulated from 1 s to 1.1 s and the response is plotted up to 10 s. The swing curve, real power, reactive power and voltage of 11 kv generator bus(11) and voltage of 33 kv bus(6) and 11 kv bus(14) are presented below. Figure 3.7 Swing curve of generator at fault condition in 33 kv bus(6) A 100 ms three phase to ground fault at 33 kv bus(6) leads the generator to oscillate from -2 to 15 degrees whereas at the steady state condition, it is 6.3 degrees with respect to grid. This is depicted in Figure 3.7. The system is considered stable, for the three phase to ground fault at 33 kv bus(6) for 100 ms duration, since the swing is well within the transient limit of 180 degrees.
71 Figure 3.8 Real power generation by generator in MW at fault condition in 33 kv bus(6) Whenever there is a three phase fault in the network, the real power transfer and consumption will come down. However, mechanical input of the generator remains constant. This will increase the accelerating torque in the generator and in turn leads to unstable condition if not controlled properly. Even after the clearance of fault, oscillation will not damp to zero immediately. Since the droop characteristics of the generator are slow in nature (high time constant), the oscillation takes longer time to damp. Figure 3.8 indicates the oscillation of real power of the generator that varies from 6.5 MW to 63 MW.
72 Figure 3.9 Reactive power generation by generator in MVAr at fault condition in 33 kv bus(6) Whenever there is a three phase fault in the network, the voltage will come down to zero at fault point and drastically reduce the voltage profile in the vicinity of the fault. The reactive power of the generator drastically increases from 14 MVAr to 75 MVAr during the fault without any time delay because the exciter characteristics are fast in nature (very low time constant). This is depicted in Figure 3.9. Also, after the fault clearance, the voltage profile is improved to normal value.
73 Figure 3.10 33 kv bus(6) voltage in p.u. at fault condition in 33 kv bus(6) Since the fault is at this bus, the voltage is zero during fault and recovers back to the normal after clearance of the fault. This is depicted in Figure 3.10. Figure 3.11 11 kv generator bus(11) voltage in p.u. at fault condition in 33 kv bus(6)
74 Though the 11 kv generator bus(11) is far away from the faulty 33 kv bus(6) and two transformer impedances exist between the two buses, the generator connected to the 11 kv generator bus(11) limits the voltage drop to small value (from 1 p.u. to 0.97 p.u.) during fault and recovers back to normal after clearance of the fault. This is depicted in Figure 3.11. Figure 3.12 11 kv bus(14) voltage in p.u. at fault condition in 33 kv bus(6) Since the 11 kv bus(14) is far away from the faulty 33 kv bus(6) and two transformer impedances exist between the two buses, the voltage reduction is severe (from 1 p.u. to 0.95 p.u.) during fault and recovers back to normal after clearance of the fault. This is depicted in Figure 3.12.
75 The effect of STATCOM on transient stability of power system is analyzed by simulating three phase to ground fault at 33 kv bus(6) using MiPower, by inserting the STATCOM in the 11 kv bus(14). The swing curve and voltage of 11 kv generator bus(11) and voltage of 11 kv bus(14) are presented below. Figure 3.13 Swing curve of generator at fault condition in 33 kv bus(6) (with STATCOM at 11 kv bus(14)) STATCOM at 11 kv bus(14), for a three phase to ground fault of 100 ms duration at 33 kv bus(6), leads the generator to oscillate from -1 to 14 degrees (during disturbance its value ranges from -2 to 15 degrees) whereas at the steady state condition it is 6.3 degrees with respect to grid. This is depicted in Figure 3.13.
76 Figure 3.14 11 kv generator bus(11) voltage in p.u. at fault condition in 33 kv bus(6) (with STATCOM at 11 kv bus(14)) STATCOM at 11 kv bus(14), for three phase fault at 33 kv bus(6), significantly contributes the reactive power and improves the voltage profile. It limits the voltage drop from 0.97 p.u. to 0.985 p.u. This is depicted in Figure 3.14. STATCOM at 11 kv bus(14), for three phase fault at 33 kv bus(6), significantly contributes the reactive power and improves the voltage profile. Since the STATCOM is connected in the same bus, the impact on voltage profile improvement is higher and maintained at 1 p.u. This is depicted in Figure 3.15.
77 Figure 3.15 11 kv bus(14) voltage in p.u. at fault condition in 33 kv bus(6) (with STATCOM at 11 kv bus(14)) Considering the STATCOM at 11 kv bus(14), it improves voltage stability to great extent by maintaining the voltage profile during steady state and improves the voltage profile significantly during fault. However, the impact of STATCOM on angular stability is good. The reactive power supplied by STATCOM at 11 kv bus(14) to nearby buses 13 and 15 is depicted in the Figures 3.16 and 3.17 respectively.
78 Figure 3.16 Reactive power Q supplied to bus13 (with STATCOM at 11 kv bus(14)) Figure 3.17 Reactive power Q supplied to bus15 (with STATCOM at 11 kv bus(14))
79 The ability of the STATCOM to supply the reactive power during voltage dip / drop improves the voltage profile of buses, which in turn prevents the large motors connected to those buses to stall. 3.6 CONCLUSION Modeling of real time system with the STATCOM using MiPower simulation software is presented. STATCOM is modeled using MiPower for enhancing the transient stability of the system. From the transient stability analysis, it is observed that STATCOM improves the voltage profile of the system for the following conditions: i) STATCOM at 33 kv bus(6), three phase to ground fault at 11 kv bus(14) ii) STATCOM at 11 kv bus(14), three phase to ground fault at 33 kv bus(6) Also, the impact of STATCOM on damping the rotor angle oscillation of generator is effective in the system considered. Compared with the performance of SVC discussed in chapter 2, the STATCOM provides much better control in damping the rotor angle oscillation of the generator in addition to the improvement of voltage profile. The damping of the rotor angle oscillation of the generator may further be improved by increasing the inertia of the generator by means of adding additional weight or flywheel.