CHAPTER 4 INCREASE IN CRITICAL CLEARING TIME USING SVC AND STATCOM IN TNEB

80 CHAPTER 4 INCREASE IN CRITICAL CLEARING TIME USING SVC AND STATCOM IN TNEB 4.1 INTRODUCTION The preliminary study conducted to evaluate the potential due to the application of SVC and STATCOM on the TNEB 400 kv transmission network, generators and their respective step-up transformers directly connected to 400 kv substations in Tamil Nadu and nearby 400 kv substations (Trivendrum, Chittoor etc.,) is presented in this chapter. The study compares the results and effectiveness of SVC and STATCOM for enhancing the transient stability of the system through the critical clearing time using MiPower simulation software. The fault clearing time is increased up to the critical clearing time to test the robustness of the system and the effectiveness of the SVC and STATCOM. 4.2 LITERATURE REVIEW An occurrence of three phase fault on the bus and clearance of the fault within a short period of time (up to 100 ms) does not represent a large risk to the power system in terms of transient stability. For this reason, it is necessary to ensure that generators operating in the electric power system have a critical clearing time higher than 100 ms. The electric power system instability can be interpreted using various methods depending on the system configuration and operational

81 status. Traditionally, the question of stability has been connected to maintain synchronous operations. The production of electricity is secured primarily using synchronous generators. For this reason, it is important to secure their synchronism and therefore the question of stability has mainly hinged on the transient stability of synchronous machinery and on the relationship between the active power and the rotor angle of the generator. Electric power system instability can also appear even if synchronous operation of the generators is not interrupted (Eleschova et al 2010). On 1st July 1957, TNEB came into being and has remained as the energy provider and distributor all these years. During the period, the Government has extended the electrical network to all villages and towns throughout the state. After fifty three years of journey, on 1st November 2010, TNEB restructured itself into TNEB Limited, Tamil Nadu Generation and Distribution Corporation (TANGEDCO) Limited and Tamil Nadu Transmission Corporation (TANTRANSCO) Limited. Ministry of Power, Government of India, has planned to supply Power for all by 2012. To achieve this, TANGEDCO Limited is making progress in generation and distribution sector. TNEB limited has completed the electrification of all villages and towns. To satisfy the energy needs of the state, TANGEDCO Limited has installed generating stations of capacity 10, 214 MW which includes state, central share and independent power producers. Besides, the state has installations in renewable energy sources like windmill up to 5400 MW. Due to the astronomical increase in the energy demand in future, the state has proposed new generation projects for the next five years. TANTRANSCO Limited has a total of 1309 substations with HT(High Tension) and EHT(Extra High Tension) lines to a length of 1.69 lakh km. The voltage levels in use in TANTRANSCO Limited are 400 kv,

82 230 kv, 110 kv and 66 kv. In order to evacuate bulk power from one region to another region, there is more scope for enhancing the transmission capability to 765 kv level and setting up of 800 kv High Voltage DC system. The Government of India has approved nondiscriminatory open access for the transmission system. The Government of Tamil Nadu has also permitted third party sale of power produced by independent power producer (IPPs), captive power plants (CPPs) and other private power producers through short term intra-state open access to HT consumers within Tamil Nadu. TANGEDCO has a consumer base of about 212.76 lakh consumers. 100% rural electrification has been achieved. Per Capita consumption of Tamil Nadu is 1080 Units. To achieve reliable and quality power supply, and to minimize the loss of energy, Ministry of Power, Government of India, has launched the restructured APDRP (Accelerated Power Development and Reform Program) scheme under the eleventh five year plan (www.cea.nic.in\reports\planning\ power_scenario.pdf.). 4.3 PROBLEM STATEMENT Load growth, equipment outage, unplanned maintenance, aging of substation equipment and increasing stress on grid leads poor voltage profile, network overloading, increase in losses and thermal damage of the equipment. A better method to improve the utilization ratio of the existing electric network is expected. As the awareness of environmental protection is increasing, it becomes critical that the existing transmission and distribution resources can be fully utilized. As the system network grows in complexity and load consumption continues to increase, interface loading conditions are expected to worsen, resulting in more acute transient problem and may lead to disasters and blackouts, which have not only huge commercial loss, but also needs high restoring time. Here, Tamil Nadu 400 kv transmission network

83 and nearby interconnected 400 kv substations (Trivendrum, Chittoor etc.) are considered for the study. This has prompted the need to consider broad utilization of FACTS technology, including SVC and STATCOM. SVC and STATCOM can be inserted independently in the TNEB system to achieve control function including enhancement of the transient stability using MiPower simulation software. 4.4 GENERAL REPRESENTATION OF SVC AND STATCOM The representation of both SVC and STATCOM are discussed in sections 2.5 and 3.4 respectively. The SVC and STATCOM model used here is shown in Figures 4.1 and 3.2 respectively. 4.5 TNEB 400 kv TRANSMISSION NETWORK Tamil Nadu 400 kv transmission network and nearby interconnected substations (Trivendrum, Chittoor etc.) are considered for the purpose of identifying the critical clearing time at all 400 kv buses to ensure rapid fault clearance and thereby maintaining the transient stability. The system model includes the representation of thirty four 400 kv buses, eleven generators, eleven transformers etc. Transmission network (only 400 kv) of TNEB is modeled using MiPower. Transmission line parameters of various conductors (Moose and Zeebra) are used as per manufacturer s standard. Line length and type of conductor are considered as per information available in the Power Grid and Ministry of Power website. The generators directly connected to TNEB 400 kv transmission network are modeled along with the generator step-up transformers. The generator, AVR, TG and transformer data are given in the Tables 4.1 to 4.4 respectively. The block diagrams of type1 AVR and TG are depicted in Figures 4.2 and 4.3 respectively.

Vref[1] Max=0.529 T=0.2 T=0.003 Max=1 N[5] N[6] N[12] N[2] N[13] N[4] N[8] N[9] K=2.23 Min=-1 K = 1.04 Min=-0.278 SVC Q Supply N[3] K = 1 N[7] T=0.003 N[10] Figure 4.1 Modeling of SVC 84

Table 4.1 Generator data in TNEB 400 kv transmission network Generator Name Rated MVA AVR Type TG No. Model type Rated Voltage(kV) P Schedule Q min Q max Neyveli-1 247 1 1 2 11 163 0 130.03 4 Neyveli-Ext 295 1 1 2 11 225 0 156.60 4 Kudamkulam 1111.11 1 1 2 21 850 0 484.32 4 Tuticorin-ST4 555.56 1 1 2 21 450 0 242.17 4 Tuticorin 555.56 1 1 2 21 450 0 242.17 4 Ind-Bharath 666.67 1 1 2 21 600 0 290.60 4 Coastal Ene 666.67 1 1 2 21 510 0 290.60 4 Mettur 555.56 1 1 2 21 450 0 242.17 4 Chennai JV 733.33 1 1 2 21 637.5 0 319.64 4 North Chennai 666.67 1 1 2 21 510 0 290.60 4 North Chennai ABAN 666.67 1 1 2 21 510 0 290.60 4 Droop Constant 85

- - - Figure 4.2 Block diagram of type1 AVR 86

87 Table 4.2 Type1 AVR data in TNEB 400 kv transmission network Variable Description Data T rec Input rectifier time constant in s 0.05 K a Amplifier gain 200 T a Amplifier time constant in s 0.1 K e Exciter gain -0.05 T e Exciter time constant in s 0.5 K f Regulator stabilizing circuit gain 0.05 T f Regulator stabilizing circuit time constant 0.5 V se1 Saturation function at 0.75 times maximum field voltage 0.06 V se2 Saturation function at maximum field voltage 0.3 V rmax Maximum amplifier voltage 1 V rmin Minimum amplifier voltage -1 E fdmax Maximum field voltage 4.3 E fdmin Minimum field voltage 0

- - Figure 4.3 Block diagram of TG 88

89 Table 4.3 TG data in TNEB 400 kv transmission network Variable Description Data Droop 0.04 P max Maximum power limit 1.1 P min Minimum power limit 0 C max Rate of valve opening 0.1 C min Rate of valve closing - 1 K 1 K 2 Power extraction at HP turbine 0.276 K 3 K 4 Power extraction at IP turbine 0.324 K 5 K 6 Power extraction at LP turbine 0.4 T 1 Phase compensation 1 0.1 T 2 Phase compensation 2 0.03 T 3 Servo time Constant 0.4 T hp HP section Time constant in s 0.26 T rh Reheat section Time constant in s 10 T lp LP section Time constant in s 999 T ip IP section Time constant (including reheater) in s 0.5

90 Table 4.4 Transformer data in TNEB 400 kv transmission network Sl. No. Bus Name MVA Rating Primary Voltage (kv) Secondary Voltage (kv) Zp.u. Taps(Min and Max) OCTC (Taps) 1 Neyveli-1 250 400 11 0.14 1 and 9 OCTC(5) 2 Neyveli-Ext 300 400 11 0.14 1 and 9 OCTC(5) 3 Kudamkulam 1200 400 21 0.14 1 and 9 OCTC(5) 4 Tuticorin- ST4 560 400 21 0.14 1 and 9 OCTC(5) 5 Tuticorin 560 400 21 0.14 1 and 9 OCTC(5) 6 Ind-Bharath 670 400 21 0.14 1 and 9 OCTC(5) 7 Coastal Ene 670 400 21 0.14 1 and 9 OCTC(5) 8 Mettur 560 400 21 0.14 1 and 9 OCTC(5) 9 Chennai JV 740 400 21 0.14 1 and 9 OCTC(5) 10 11 North Chennai North Chennai ABAN 670 400 21 0.14 1 and 9 OCTC(5) 670 400 21 0.14 1 and 9 OCTC(5) Single line diagram of the TNEB 400 kv transmission network system is shown in Figure A 1.2.

91 4.6 RESULTS WITH DISCUSSIONS Critical clearing time is the principal criterion for the assessment of transient stability. Fault should be cleared well within the critical clearing time to maintain the transient stability of the power system, even while critical clearing time is not the sufficient criterion to evaluate the transient stability when considering various scenarios of severe fault occurrence in the power system. The oscillation of generators is basically measured with respect to infinite bus / grid which are represented as slack bus. The oscillation level of the generator depends on the disturbance severity and its time duration. If the generator oscillation goes beyond 180 degrees, the generator loses its synchronism and will not be able to regain the steady state. That is, the generator will not be synchronous with rest of the system. Hence, 180 degrees is standardized as the transient stability limit. Since a severe fault (three phase fault) is assumed, the ability of the generator to remain in synchronism depends on the fault clearing time. The time to clear the fault is slowly increased up to the critical clearing time with the help of SVC and STATCOM. The effect of SVC and STATCOM on the transient stability of the power system is analyzed by creating three phase to ground fault at various buses using MiPower through the critical clearing time. By placing the SVC and STATCOM independently at Sholinganallur, Neyveli, Karaikudi and Salem 400 kv substations, the fault clearing time is increased up to the critical clearing time. Critical clearing time for the various buses with SVC at the 400 kv substations, are tabulated in Table 4.5 and compared with critical clearing time without SVC.

92 Table 4.5 Critical clearing time of various 400 kv buses with and without SVC Bus No. Bus Name Bus Description Without SVC Critical clearing time in ms SVC at Neyveli SVC at Sholinganallur SVC at Karaikudi SVC at Salem 402 ALAM_4 Alamanthi 120 148 145 149 149 404 SVC_4 SVC 200 208 209 208 208 405 MLKTYR Malekuttaiyur 200 225 220 225 225 406 SHLGNR Sholinganallur 260 268 268 268 268 407 PONDI_4 Pondicherry 550 565 560 585 565 410 TRCHY_4 Trichy 250 290 295 290 290 411 KARKDI Karaikudi 180 190 190 188 188 412 MDRAI_4 Madurai 140 149 149 148 148 415 KYTHR_4 Kaythar 180 185 200 188 185 416 TM_WND TM_Wind 170 172 180 172 170 417 TRNVL_4 Tirunelveli 120 125 128 145 125 419 TRNDRM Trivandrum 190 700 800 800 800 420 TRVLM_4 Tiruvelam 170 250 260 250 260 421 SNGRPT Singarpet 220 220 225 228 228 422 PGLR_4 Pugalur 180 188 187 188 188 423 SALEM_4 Salem 170 172 175 170 170 424 UDMLPT Udumalpet 180 188 189 188 188 426 ARSR_4 Arasur 290 290 310 310 310 427 KRMDI_4 Karmadai 480 488 488 488 488 428 MVTPHA Muvattupuzha 310 315 320 310 310 429 N_TRCHR N_Trichur 310 310 310 325 310 431 HOSUR_4 Hosur 410 410 430 430 410 Total 5280 6056 6238 6253 6056 Critical clearing time for the various buses with STATCOM at the 400 kv substations, are tabulated in Table 4.6 and compared with critical clearing time without STATCOM.

93 Table 4.6 Critical clearing time of various 400 kv buses with and without STATCOM Bus No. Bus Name Bus Description Without STATCOM Critical clearing time in ms STATCOM STATCOM at at Sholinganallur Neyveli STATCOM at Karaikudi STATCOM at Salem 402 ALAM_4 Alamanthi 120 149 149 150 152 404 SVC_4 SVC 200 215 208 210 209 405 MLKTYR Malekuttaiyur 200 225 225 230 228 406 SHLGNR Sholinganallur 260 275 273 270 272 407 PONDI_4 Pondicherry 550 565 565 580 572 410 TRCHY_4 Trichy 250 290 300 295 300 411 KARKDI Karaikudi 180 190 195 185 189 412 MDRAI_4 Madurai 140 145 146 149 145 415 KYTHR_4 Kaythar 180 185 188 210 185 416 TM_WND TM_Wind 170 175 175 180 175 417 TRNVL_4 Tirunelveli 120 120 130 150 125 419 TRNDRM Trivandrum 190 750 900 850 850 420 TRVLM_4 Tiruvelam 170 240 250 260 250 421 SNGRPT Singarpet 220 225 230 230 230 422 PGLR_4 Pugalur 180 190 190 195 190 423 SALEM_4 Salem 170 175 170 172 170 424 UDMLPT Udumalpet 180 185 190 185 185 426 ARSR_4 Arasur 290 300 290 290 310 427 KRMDI_4 Karmadai 480 490 490 495 490 428 MVTPHA Muvattupuzha 310 315 315 310 310 429 N_TRCHR N_Trichur 310 310 310 310 310 431 HOSUR_4 Hosur 410 420 425 425 430 Total 5280 6134 6314 6331 6134 Impact of FACTS device on the transient stability enhancement is analyzed with the help of critical clearing time. Critical clearing time at Alamanthi 400 kv substation without FACTS is 120 ms. Considering the relay delay time (40 ms) and circuit breaker opening time (30 ms) in the Alamanthi substation, the relay and circuit breaker fails to isolate the fault during N-1 contingency ( i.e during struck breaker operation - after struck breaker, all other breaker should open to isolate the fault) within the critical clearing time. However, with FACTS installation at various buses, the critical

94 clearing time is increased from 120 ms to around 150 ms. This increased cushion of 30 ms (1.5 cycle) will maintain the stability for three phase fault at Alamanthi even for N-1 contingency. The swing curves of generators in Neyveli-1, Neyveli-Ext, Kudamkulam, Tuticorin-ST4 for a three phase fault at Alamanthi 400 kv substation with and without SVC are presented here. Figure 4.4 Swing curves of generators without SVC The swing curves of generators in Neyveli-1, Neyveli-Ext, Kudamkulam, Tuticorin-ST4 for 120 ms duration of a three phase fault at Alamanthi 400 kv substation without SVC are shown in Figure 4.4. If the

95 fault clearing time exceeds 120 ms, the oscillations of generators exceed 180 degrees. Figure 4.5 Swing curves of generators with SVC at Sholinganallur SVC at Sholinganallur, increases the critical clearing time from 120 ms to 148 ms for a three phase fault at Alamanthi 400 kv substation. The swing curves of generators in Neyveli-1, Neyveli-Ext, Kudamkulam, Tuticorin-ST4 for 148 ms duration of a three phase fault at Alamanthi 400 kv substation with SVC at Sholinganallur are shown in Figure 4.5. If the fault clearing time exceeds 148 ms, the oscillations of generators exceed 180 degrees.

96 Figure 4.6 Swing curves of generators with SVC at Neyveli SVC at Neyveli, increases the critical clearing time from 120 ms to 145 ms for a three phase fault at Alamanthi 400 kv substation. The swing curves of generators in Neyveli-1, Neyveli-Ext, Kudamkulam, Tuticorin-ST4 for 145 ms duration of a three phase fault at Alamanthi 400 kv substation with SVC at Neyveli are shown in Figure 4.6. If the fault clearing time exceeds 145 ms, the oscillations of generators exceed 180 degrees.

97 Figure 4.7 Swing curves of generators with SVC at Karaikudi SVC at Karaikudi, increases the critical clearing time from 120 ms to 149 ms for a three phase fault at Alamanthi 400 kv substation. The swing curves of generators in Neyveli-1, Neyveli-Ext, Kudamkulam, Tuticorin-ST4 for 149 ms duration of a three phase fault at Alamanthi 400 kv substation with SVC at Karaikudi are shown in Figure 4.7. If the fault clearing time exceeds 149 ms, the oscillations of generators exceed 180 degrees.

98 Figure 4.8 Swing curves of generators with SVC at Salem SVC at Salem, increases the critical clearing time from 120 ms to 149 ms for a three phase fault at Alamanthi 400 kv substation. The swing curves of generators in Neyveli-1, Neyveli-Ext, Kudamkulam, Tuticorin-ST4 for 149 ms duration of a three phase fault at Alamanthi 400 kv substation with SVC at Salem are shown in Figure 4.8. If the fault clearing time exceeds 149 ms, the oscillations of generators exceed 180 degrees. Without FACTS, the critical clearing time at Trichy 400 kv substation is 250 ms. With FACTS, this is improved to around 300 ms. This

99 will be certainly helpful to maintain the stability, when the backup protection isolates the fault due to primary relay failure. Similarly at all other buses, with FACTS devices the critical clearing time has increased or maintained for three phase to ground faults. The total critical clearing time of the system has significantly improved with the help of FACTS devices. 4.7 CONCLUSION The TNEB 400 kv transmission network is modeled using MiPower. The critical clearing time is found at various 400 kv buses of TNEB system and is tabulated. The proposed SVC and STATCOM model is placed independently at Sholinganallur, Neyveli, Karaikudi and Salem 400 kv substations to identify the critical clearing time of various 400 kv buses to ensure the rapid fault clearance and thereby maintaining the transient stability. The critical clearing time increases appreciably after placing the SVC and STATCOM independently in the TNEB 400 kv transmission network. This is shown in Tables 4.5 and 4.6. It is evidenced that the transient stability of the system has improved. The result obtained in the TNEB system is valid only for the present scenario. The study should be repeated as and when the considerable capacity of new generation (> 500 MW) is added.