Analysis of Transient Overvoltages within a 345kV Korean Thermal
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Journal of Electrical Engineering & Technology Vol. 7, No. 3, pp. 297~303, 2012 297 http://dx.doi.org/10.5370/JEET.2012.7.3.297 Analysis of Transient Overvoltages within a 345kV Korean Thermal Plant Sang-Min Yeo† and Chul-Hwan Kim* Abstract – This paper presents the simulation results for the analysis of a lightning surge, switching transients and very fast transients within a thermal plant. The modeling of gas insulated substations (GIS) makes use of electrical equivalent circuits that are composed of lumped elements and distributed parameter lines. The system model also includes some generators, transformers, and low voltage circuits such as 24V DC rectifiers and control circuits. This paper shows the simulation results, via EMTP (Electro-Magnetic Transients Program), for three overvoltage types, such as transient overvoltages, switching transients, very fast transients and a lightning surge. Keywords: Surge, Transient Overvoltages, Switching, Lightning, Gas Insulated Switchgear, EMTP 1. Introduction Power systems consist of various facilities such as power stations, substations, transmission lines and various loads, etc. Transient overvoltages may occur within a power system when a system status change occurs due to an event such as a fault, lightning or even normal operation. Generally, modeling of switching devices, such as circuit breakers and disconnector switches, are implemented by ideal switches for analysis within a switching transient simulation. However, the contacts of a disconnector switch move slowly during opening and closing operations, which causes numerous pre-strikes between the contacts. Prestrikes occur when the dielectric strength between the contacts of the disconnector switch are exceeded by overvoltages. Therefore, the effects of pre-strikes due to disconnector switching continue for a relatively long time compared to the time in a circuit breaker [1-5]. Specifically, in gas insulated switchgear (GIS), a large number of restrikes across the switching contacts will occur when disconnector switches or circuit breaker operation occurs. Each restrike generates very fast transient overvoltages (VFTO), which enter the substation and may propagate inside depending on the substation layout. VFTOs typically have a very short rise time, in the range of 4 to 100 ns, and are followed by high frequency oscillations, in the range of 1 to 50 MHz. Therefore, the analysis of switching transients and very fast transients in GIS is extremely important [6-11]. Also, when lightning hits a nearby tower or the shield wire of an incoming line, this can cause a backflashover, which is when the resultant lightning surge enters the substation and may propagate inside depending on the substation layout. Hence, as in the † Corresponding Author: Power & Industrial Systems R&D Center, Hyosung Corporation, Korea. (harc@hyosung.com) * School of Information and Communication Engineering, Sungkyunkwan Univerity, Korea. (hmwkim@hanmail.net) Received: April 29, 2011; Accepted: February 3, 2012 case of switching transients, the transients caused by a lightning surge also need to be analyzed. In 2005, when a disconnector closed within a 345kV Korean thermal plant, the circuit breakers for the exciter protection of two generators tripped except the operation of any protective relay. The tripping of the circuit breakers caused inoperation to occur for two generators, which then caused a productivity decrease for the entire plant. Thus, analysis is required in order to discover the trip cause for the circuit breakers due to disconnector switching. In this paper, the authors analyze the VFTOs due to disconnector switching, including the switching transient and lightning overvoltages. This paper shows the simulation results of transient overvoltages within a GIS. Also, both a 345kV thermal plant and a pre-strike model of a disconnector switch are modeled by EMTP and then it is further simulated to analyze the transient overvoltages via various transients. 2. Background Theory The components in a GIS have already been previously modeled by the authors via several transient simulations [12, 13]. The previous papers include more details about modeling as follows, except for in the case of lightning surges. Therefore, these factors are described briefly within this paper. 2.1 Very fast transients VFTs may occur within a GIS at any time and will cause an instantaneous voltage change, one example being the change caused by disconnector switching. These transient over-voltages are generally below the basic insulation level (BIL) of the GIS and are connected to equipment with Analysis of Transient Overvoltages within a 345kV Korean Thermal Plant 2.2 Pre-strike P2 1.2uH 0.0463nF C9 Fig. 2. Equivalent model of the grounding system per unit length 2.4 Lightning surge Generally, lightning strikes that occur directly on a substation are ignored because it is safely assumed that the substation is shielded appropriately. On the contrary, lightning strikes to the tower or shield wire can cause backflashover due to line insulation flashover between the tower and the phase conductor, and can enter the substation and propagate inside according to the substation layout [9]. In this paper, the standard 1.2x50us impulse waveform is used for the lightning surge model, as shown in Fig. 3. BUS1 R1 ?i + 0.0001 R2 + 400 ?v a + The pre-strike phenomena of the switches are considered in order to simulate the switching transients. In this paper, the authors refer to references 4 and 5 for modeling the pre-strike, which consists of a calculation for the withstand voltage (Vw), a comparison of the withstand voltage, the switch voltage, a controlled switch, and a probe for measuring the switch terminal voltage. Fig. 1 shows the model of a pre-strike in EMTP [14]. L2 + P1 + lower voltage classes. However, they frequently contribute to the life reduction of the insulation within the system [9]. The propagation of VFT through the GIS can be analyzed by representing GIS sections as low-loss distributed parameter transmission lines. The main frequencies depend on the length of the GIS sections affected by the disconnector operation and are in the range of 1 to 50MHz. Also, an internally generated VFT will propagate throughout the GIS and reach the bushing where it will cause both a transient enclosure voltage and a traveling wave that will propagate along the overhead transmission line. R1 + 570 298 1 2 1 2 lim2 1 ?s #KVr# ABS Vw scope 1 Compare 2 ?s Csw Icramp1 a Csw Fm6 1 2 C1 1 0 PROD c_gfun c + - 1 1.2us/63kA Fm7 Gain2 Fig. 3. Lightning surge model DIV Vsw f(t) c Fm5 +Inf sum2 + Vsw v(t) C4 sg2 Fm4 PROD 3. Simulation and Results #toper# t Fig. 1. Model of a pre-strike by EMTP It is confirmed via the simulation results that the verification of the pre-strike model is properly implemented. 3.1 System Model This paper models a 345kV thermal plant via EMTP. The modeled system, as shown in Fig. 4, is comprised of six generators and six transformers, and connects to other systems via six transmission lines. 2.3 Grounding system The operational safety and proper functioning of electric systems are influenced by their earth terminations. Local inequalities of reference potential for a grounding system are sources of malfunction and may cause the destruction of components that are electrically connected to the grounding system. Therefore, ground system modeling is necessary in order to analyze the effects of reference potential disturbances during various operation abnormalities. The presented methodology extends the validity range of well-known grounding system models towards higher frequencies, in the range of a few MHz. The approach is based on the three basic concepts that have been developed [15, 17]. This paper uses a grounding system model via the circuit approach method, as mentioned in [18] and shown in Fig. 2. Fig. 4. System Model 3.1.1 Very fast transients and lightning surge section In order to successfully simulate VFT overvoltage, accurate models are needed for various equipment devices in the range of zero hertz to a few megahertz. However, Sang-Min Yeo and Chul-Hwan Kim component modeling within all frequency ranges is very difficult and impracticable. Thus, for reasons of practicality, the modeling of frequency-dependent power system components is implemented within a specific frequency range [9]. The equivalent GIS model consists of various pieces of equipment that function differently for varying frequency ranges. Therefore, equivalent typical equipment models for very fast transients are used [6-8, 10-13, 17]. For the simulation of the lightning surges, the system model is derived from plant layout drawings and should include the lightning surge characteristics. For this reason, the modeling method is similar to the method used for the case of VFT. Though the plant modeling for VFT is more complicated rather than typical modeling for a lightning surge, when the lightning surge is simulated the authors use the modeled system for the VFT in order to reduce effort and time due to repeated modeling. Fig. 5 shows the equivalent model of the circuit breaker from the equivalent model of GIS components. 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P2 DEV6 47 DEV6 54 DEV6 93 Gn d1 2 s cope DEV72 4 DEV74 0 DEV6 9 P1 P2 DEV9 61 DEV5 92 P1 P2 DEV64 4 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 DEV6 71 DEV6 65 DEV6 62 DEV6 68 DEV70 2 DEV69 8 P1 P2 P1 P2 DEV70 0 P1 P2 DEV70 5 P1 P2 P1 P2 DEV7 32 DEV7 31 DEV7 30 DEV7 29 P1 P2 P1 P2 P1 P2 DEV75 8 DEV75 7 DEV76 6 DEV76 5 P1 P2 P1 P2 DEV774 DEV7 81 P1 P2 DEV773 DEV8 38 P1 P2 P1 P2 DEV6 37 DEV68 9 P1 P2 P1 P2 P1 P2 P1 P2 DEV9 54 P1 P2 DEV94 8 DEV93 8 P1 P2 P1 P2 P1 P2 DEV93 9 P1 P2 P1 P2 t m l _1 _0 P1 P2 P1 P2 DEV65 5 DEV6 35 DEV6 26 P1 P2 P1 P2 P1 P2 DEV6 31 DEV6 25 DEV6 33 DEV6 27 P1 P2 DEV6 34 P1 P2 P1 P2 DEV6 32 DEV64 3 P1 P2 DEV95 6 P1 P2 P1 P2 DEV68 P1 P2 DEV6 39 P1 P2 P1 P2 DEV8 30 P1 P2 DEV9 51 DEV9 53 P1 P2 P1 P2 DEV7 39 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 DEV7 49 DEV96 0 P1 P2 DEV9 52 p3 DEV7 48 P1 P2 DEV9 59 P1 P2 P1 P2 P1 P2 v (t) P1 P2 P1 P2 P1 P2 DEV5 75 P1 P2 DEV6 40 P1 P2 DEV5 7 DEV9 40 P1 P2 P1 P2 DEV7 23 P1 P2 DEV9 58 P1 P2 DEV93 1 P1 P2 P1 P2 DEV9 47 DEV6 29 DEV6 90 DEV6 3 P1 P2 DEV28 4 DEV93 4 P1 P2 P1 P2 P1 P2 DEV62 2 DEV92 P1 P2 DEV9 30 R3 P1 P2 DEV9 27 P1 P2 DEV92 9 DEV93 5 DEV9 26 DEV9 43 P1 P2 P1 P2 DEV62 4 DEV58 P1 P2 DEV9 67 DEV9 28 P1 P2 DEV92 5 P1 P2 DEV94 6 P1 P2 DEV76 4 DEV1 01 1 DEV94 4 DEV94 5 P1 P2 DEV77 2 DEV10 3 DEV9 24 DEV779 DEV1 05 P1 P2 DEV94 2 DEV10 4 P1 P2 DEV94 9 DEV92 3 tm l_2 _3 DEV9 41 P1 P2 DEV93 2 P1 P2 P1 P2 P1 P2 DEV93 3 P1 P2 DEV6 23 R9 R1 + + 1 DEV9 22 P1 P2 1 DEV95 0 + + DEV106 P1 P2 DEV1 07 (a) The layout of real grounding system 1 R24 R23 (b) the model of the entire grounding system Fig. 7. Model of the grounding system a LF Load8 164MW LF Load6 2MVAR 154MW LF Load7 2MVAR 184MW 2MVAR 545MW 41MVAR CP TLM8 + 0.05nF TLM7 77.43 + C3 CP 8.60 + 8.60 CP TLM5 + 0.05nF C4 77.43 7576_2 DEV3 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GISbusbar GISbusbar GISbusbar 76BUS_E inst./pu p 7672 Open Block Lp Rp SWBlock_Open GISbusbar 7600 Close Block Lp Rp 7621 7671 Close Block Lp Rp SAMHAE2 GIS Bus Bar BUS1 BUS2 GISbusbar GIS Bus Bar BUS1 BUS2 Gen6 Lp Rp DS_C 7622 Lp Rp DS_C GISbusbar 7572 SWBlock_Open GIS Bus Bar BUS1 BUS2 Open Block Lp Rp 7522 Lp Rp DS_C 7500 Close Block Lp Rp DSOpen DS_O Lp Rp 7571 GISbusbar GIS Bus Bar BUS1 BUS2 7521 Close Block Lp Rp SAMHAE1 20us| 1E15| 0 Gen5 GISbusbar + 7475_2 GIS Bus Bar BUS1 BUS2 7421 Lp Rp DSClose DS_C Lp Rp DEV8 74GIS GIS Bus Bar BUS1 BUS2 CBClose DSClose 7400 CB_C Lp Rp 7401 DS_C Lp Rp 7471 DS_C Gen4 GIS Bus Bar BUS1 BUS2 + DEV9 DS_C Lp Rp + Close Block Lp Rp GISbusbar 7372 7322 Close Block Lp Rp 7300 Close Block Lp Rp 7321 GISbusbar Lp Rp 7371 Close Block Lp Rp 7200 Close Block Lp Rp DS_C 7271 Gen3 7221 Close Block Lp Rp DS_C GIS Bus Bar BUS1 BUS2 Lp Rp GISbusbar Gen2 GIS Bus Bar BUS1 BUS2 ?vi GISbusbar CP + C5 0.05nF TLM3 55.43 Lp Rp 7272 GIS Bus Bar BUS1 BUS2 SAMCHEONPO1 Close Block Lp Rp GISbusbar DS_C GIS Bus Bar BUS1 BUS2 7222 Lp Rp DS_C DSClose SAMCHEONPO2 CBClose DSClose GISbusbar DS_C Lp Rp CB_C Lp Rp DS_C Lp Rp Close Block Lp Rp GIS Bus Bar BUS1 BUS2 7152 71GIS 7100 7101 7171 7121 GISbusbar DS_C Lp Rp DSClose GISbusbar CBClose GIS Bus Bar BUS1 BUS2 CB_C Lp Rp DSClose DS_C DS_C Lp Rp Lp Rp Gen1 Close Block Lp Rp GIS Bus Bar BUS1 BUS2 7A52 7AGIS 7A00 7A01 7A71 7A22 DS_C SW1 DEV1 7B7A_1 7A71_1 7172_1 7273_1 7374_1 7475_1 7576_1 DEV4 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 Lp Rp DS_C GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GISbusbar GISbusbar GISbusbar GISbusbar GISbusbar GISbusbar 7071DS_C GISbusbar GISbusbar GISbusbar DY_7 0.48/0.15 1 v(t) p6 SM inst./pu p SM1 3 6.9/6.9/21 22kV 620MVA inst./pu p YY1 YY2 6.9/0.48 2 1 1 YgYgD_np1 1 6.9/0.48 2 1 AUXTR6_2ND SUBTR_2ND inst./pu p Out_Sgn CTRL_Block scope scope CTR4N_V CTR4N_C CTRL5 Control & DC Part IN_VAC IN_SGN GND Out_Sgn CTRL_Block scope scope CTR5N_V CTR5N_C inst./pu CTRL6 p Control & DC Part IN_VAC IN_SGN GND v(t) p12 Control & DC Part SSUBTR6_2ND i(t) CTR3N_C scope scope TR6N_V TR6N_C inst./pu p 2 2 CTRL_Block scope scope CTR3N_V SSUBTR4_2ND inst./pu CTRL4 p IN_VAC IN_SGN GND v(t) p11 Out_Sgn i(t) CTR2N_C CTRL3 Control & DC Part IN_VAC IN_SGN GND v(t) p10 CTRL_Block scope scope CTR2N_V inst./pu p v(t) p9 Out_Sgn i(t) CTR1N_C v(t) p8 CTRL_Block scope scope CTR1N_V CTRL2 Control & DC Part IN_VAC IN_SGN GND i(t) i(t) Out_Sgn CTRL_Block scope scope CTR6N_V CTR6N_C DEV5 tml_1_0 tml_1_1 tml_1_2 tml_1_3 tml_2_0 tml_2_1 tml_2_2 tml_2_3 tml_3_0 tml_3_1 tml_3_2 tml_3_3 tml_4_0 tml_4_1 tml_4_2 tml_4_3 tml_5_0 tml_5_1 tml_5_2 tml_5_3 tml_6_0 tml_6_1 tml_6_2 tml_6_3 In addition, the entire grounding system is modeled by the layout of real grounding system and the equivalent model in Figs. 2 and 7 shows the model of the entire grounding system used in this paper. Fig. 8 shows the layout of the 345kV thermal plant, also shown in Fig. 4, when modeled by EMTP. Out_Sgn i(t) SSUBTR3_2ND CTRL1 Control & DC Part IN_VAC IN_SGN GND v(t) p7 Fig. 6. The model of control circuit 2 i(t) DEV10 SM SM2 3 6.9/6.9/2122kV 620MVA 2 AUXTR5 inst./pu p Gen6 scope scope TR5N_V TR5N_C i(t) 2 DY_6 20.9/345 1 v(t) p1 MAINTR5 20.9/345 1 C9 DY_5 0.48/0.15 1 DY_4 0.48/0.15 1 2 + 2 v(t) p5 SM GEN4 3 6.9/6.9/21 22kV 660MVA YY3 6.9/0.48 2 1 YY4 6.9/0.48 2 1 DY_3 0.48/0.15 1 2 YY5 6.9/0.48 2 1 i(t) MAINTR4 21/3451 v(t) p2 1 6.9/6.9/21 AUXTR4 2 660MVA GEN3 322kV 2 AUXTR3 1 1 inst./pu p SUBTR4_2ND 2 0.05nF scope scope TR4N_V TR4N_C inst./pu p AUXTR4_2ND DY_1 0.48/0.15 1 YY6 6.9/0.48 2 1 Gen4 scope scope TR3N_V TR3N_C i(t) MAINTR3 21/3451 C1 Gen3 inst./pu p SM i(t) 2 + 2 v(t) p3 2 2 GEN2 3 6.9/6.9/21 22kV 660MVA AUXTR2 0.05nF scope scope TR2N_V TR2N_C SM i(t) MAINTR1 21/3451 SM 125 660MVA C8 c GEN1 322kV - 1 Compare 2 ?s Detect_125 2 + MAINTR2 21/3451 2 v(t) p4 + Trip_R ?vi + 386E + scope scope TR1N_V TR1N_C DY_2 0.48/0.15 1 v (t) p13 CP TLM6 Load5 P Q 549MW 32MVAR 6.16 + CP TLM4 Load4 P Q 545MW 29MVAR CP + C6 0.05nF TLM1 55.43 PI PI2 + GISbusbar CBClose DSClose Lp Rp 7BGIS 7B22 GIS Bus Bar BUS1 BUS2 DS_C Lp Rp 7B00 CB_C Lp Rp 7B01 DS_C Lp Rp 7B71 GISbusbar GISbusbar DSClose SINGOSUNG1 GISbusbar GIS Bus Bar BUS1 BUS2 GISbusbar DS_C Lp Rp GISbusbar GISbusbar 7B52 GISbusbar SINGOSUNG2 GISbusbar GIS Bus Bar BUS1 BUS2 7374_2 GIS Bus Bar BUS1 BUS2 2 IN_SGN 7273_2 GIS Bus Bar BUS1 BUS2 0.192 In_Sgn scope inst./pu inst./pu p p 7172_2 GIS Bus Bar BUS1 BUS2 Out_Sgn GND BUS1 7576DS_C7576DS_R inst./pu p 7A71_2 GIS Bus Bar BUS1 BUS2 6.9/6.9/21 + 7576DS_L 7B7A_2 GIS Bus Bar BUS1 BUS2 AUXTR1 v (t) p2 C5 !v 15600uF disconnector + DEV2 1 2 sum1 v(t) 1 + GIS Bus Bar BUS1 BUS2 386E_VLL scope v (t) p14 p1 D7 + a DC_24VPU scope + c TRIP_R ?v Tr0_1 Gain2 0.039 DC_24V scope TRIP_E VM + 386E_V IN_VAC DC_125VPU scope + C6 !v 7.794e-3 DC_125V scope Trip_E 2uF v (t) p6 ?vi + D4 D5 D6 + C7 !v 386E_SW Gain1 D1 D2 D3 In order to simulate the effect of various transients on control circuit, the model of control circuit is shown in Fig. 6. Based on the feature of real control circuit, the model of control circuit is simplified. 2uF PI PI1 + + Fig. 5. The equivalent model of the circuit breaker a b c C8 0.05nF (b) Close status Close Block Lp Rp (a) Open status C7 0.05nF 6.16 + CP TLM2 Load1 P Q 342MW 46MVAR Load3 P Q 292MW 46MVAR Load2 P Q b full_gnd Fig. 8. 345kV thermal plant for very fast transients and lightning surges Analysis of Transient Overvoltages within a 345kV Korean Thermal Plant 300 3.1.2 A switching transients section In order to simulate switching transients due to a prestrike on the disconnector switch, the system model uses the pre-strike model mentioned previously. Fig. 9 shows the GIS part of the system model applied to the pre-strike model. The system model is same except the GIS part with the pre-strike model. 3.2 Simulation conditions 554MW 8MVAR Load13 P Q Load1 P Q CP CP TLM14 545MW TLM16 41MVAR 549MW 32MVAR GISBUS DEV7 inst./pu p + DEV6 CP Prestrike Vsw Csw a Prestrike Vsw Csw b z s eq1 DEV19 DEV8 GIS Bus Bar BUS1 BUS2 GIS Bus Bar BUS1 BUS2 GISbus bar GISbus bar + DEV2 + + CP 8.60 + DEV20 inst./pu p 77.43 TLM15 + TLM11 55.43 TLM9 55.43 BUS_Vrad V0 mag rad 77.43 TLM13 8.60 6.16 scope BUS_Vmag scope CP 6.16 CP + PI PI PI1 PI3 + + Fig. 10. Voltage waveform at 76BUS + CP TLM12 Load12 P Q 545MW 29MVAR Load9 P Q CP + 342MW 46MVAR TLM10 292MW 46MVAR Load11 P Q Load10 P Q In order to analyze very fast transients, switching transients, which occur due to disconnector switching within the system model, and lightning surges, the simulation conditions are fixed, as shown in Table 1, 2, and 3, respectively. b c SW2 Prestrike Vsw Csw inst./pu p a c SW3 + DEV3 DEV1 c + c c SW1 GIS Bus Bar BUS1 BUS2 (a) #4 Generator (b) #6 Generator GISbus bar Fig. 9. System model for the switching transients Fig. 11. Voltage waveforms for some generators in the time domain Table 1. Simulation conditions for very fast transients Time step Maximum simulation time System condition Simulation condition 3ns 30us 76BUS have been de-energized 75-76 DS close when simulation starts Table 2. Simulation conditions for switching transients Time step Maximum simulation time System condition Simulation condition 125ns 1.3s 76BUS have been de-energized 75-76 DS close when simulation starts Table 3. Simulation conditions for switching transients Time step Maximum simulation time Simulation condition 3ns 100us Lightning strike transmission lines at 20us 3.3 Simulation results the voltages for the others. As shown in Figs. 10 and 11, overvoltages from the disconnector switching propagate through the GIS and are reflected and refracted at every transition point. (2) Analysis of transients in the frequency domain Fig. 12 shows the frequency spectrum of the measured voltages at each generator terminal. Due to the main frequencies of the VFT overvoltages being in the range of 100kHz to 50MHz, the simulation results shown in Fig. 12 are the same as in the theoretical studies. Additionally, since the propagation route of the traveling wave is different from the GIS layout and length, the main frequency is different from the frequency at the measuring point. Therefore, both an accurate GIS layout and length are very important for obtaining exact simulation results when a VFT is simulated with disconnector switching within a GIS. Tables 4 and 5 show the summarized the simulation results. 3.3.1 Very fast transients (1) Analysis of the transients in the time domain Fig. 10 shows the voltage waveform, which has a 1.9pu maximum value and a high speed oscillation, of the energized 76BUS after closing the disconnect switches (DS). Fig. 11 shows voltage waveforms, which are different at each measuring point, for each generator. The voltage at the #4 generator terminals, close to the 75-76 DSs that caused the overvoltage, is more affected by the VFTOs than the voltage at the terminals of the other generators. Therefore, the voltage for the #4 generator is higher than (a) #4 Generator (b) #6 Generator Fig. 12. Voltage waveforms for some generators in the frequency domain Sang-Min Yeo and Chul-Hwan Kim As shown in the simulation results below, it is clear that the overvoltages generated by disconnector switching propagate through the GIS. The propagated overvoltages affect transformers, generators and buses, especially for a bus that is energized. As mentioned before, VFTOs caused by disconnector switching have very high frequencies and propagate through the GIS and reach all pieces of equipment. Hence, these high frequency overvoltages may cause improper operation of low-voltage control circuits. Table 4. Measured voltage at each generator Measured Point Max. voltage Max. voltage deviation Voltage at each generator (unit : p.u.) #1 Gen. #2 Gen. #3 Gen. #4 Gen. #6 Gen. 1.184 1.311 1.377 1.336 1.111 0.432 0.651 0.729 0.787 301 after the disconnector switching occurred. Also shown is the slight difference in the overvoltage magnitude at the GIS bus bar, of approximately 7kV, from the magnitude of the normal voltage during disconnector switching. The dominant frequency of the transient overvoltages is 60Hz, which is the power frequency, but the disconnector switching within the GIS creates high frequency components rather than the dominant one, even if they have a relatively small magnitude. Moreover, voltages and currents at the neutral point of the main transformer have a large magnitude due to the phase unbalance. Fig. 14 shows the voltage waveforms at the neutral points of the main transformers. 0.270 Table 5. Measured voltage at each bus Measured Point Max. voltage Max. voltage deviation 70BUS 1.27937 0.55189 Voltage at each bus (unit: p.u.) 71BUS 75BUS 76BUS 1.11887 1.22162 1.89149 0.29512 0.49931 1.67597 3.3.2 A switching transients Fig. 13 shows the 3-phase instantaneous voltage waveforms and frequency spectrum for the GIS bus bar Fig. 14. Voltage waveforms at the neutral point of the main transformer Even if the magnitudes are different at each measuring point, the average of the neutral voltage magnitudes is approximately 15kV. As a result of the neutral voltages, the grounding system is affected and the ground voltage is temporarily changed to a non-zero voltage value. Fig. 15 shows the waveforms of the ground voltages for the low-voltage control circuits. The duration is short, about 0.5 seconds, but the voltages have large magnitudes and high frequencies and, because of this, the control circuits would be damaged if they did not have appropriate protection devices, such as a surge protection device. (a) Instantaneous voltages Fig. 15. Ground voltage at low-voltage control circuits (b) Frequency Spectrum Fig. 13. 3-phase voltage at the GIS bus bar 3.3.3 A lightning surge Fig. 16 shows the phase voltages at the terminals of each generator when lightning strikes the BB #2 transmission line. The simulation results for the other transmission lines are slightly different from the results shown in Fig. 16. As shown in Fig. 16, when the lightning surges 302 Analysis of Transient Overvoltages within a 345kV Korean Thermal Plant propagate through the GIS, the generator transformers also experience overvoltages, approximately 140kVpeak, due to lightning surges. Figs. 17-18 show the voltage waveform at the neutral point of the main transformer and ground voltage of the low-voltage control circuits, respectively. Fig. 16. The voltage waveforms at each generator when the lightning strikes BB #2 T/L Fig. 17. Voltage waveforms at the neutral point of the main transformer 5. Conclusions This paper demonstrates an equivalent model for various pieces of equipment implemented via EMTP in order to accurately simulate VFTOs by disconnector switching within GIS. Also, calculated voltages are shown in both the time and frequency domains at various measuring points. It can be concluded from the calculated results that the VFT overvoltages via disconnector switching within GIS propagate and reach all pieces of equipment. Also, the high frequencies of the VFT overvoltage can have an impact on the low-voltage control circuits. Due to the pre-strike phenomenon that causes different reactions for each phase, the phase voltage becomes unbalanced during disconnector switching and then the generated zero sequence voltage has a large magnitude. These zero sequence voltages are revealed as neutral voltages at the neutral point of the transformer and they will affect the grounding system. The simulation results show that the switching transient overvoltages occurring via disconnector switching in GIS are smaller than ones from other origins. Also, when the lightning surge enters the various equipment pieces, such as the GIS, generators, transformers and various devices electrically connected to the GIS in thermal plant, they experience significant overvoltages similar to those stated for the previous cases, i.e. VFTs and switching transients. However, though the magnitudes of the overvoltages are small, it is clear that overvoltages have high frequencies and can cause high voltages in the grounding system and will, therefore, damage low-voltage electronic devices. References [1] Fig. 18. Ground voltage of the low-voltage control circuits The voltages of the neutral point of the main transformer and at the ground point of the low-voltage control circuits should be close to zero. However, each voltage has a large magnitude during a lightning surge strike, as in the case of switching transients. The magnitude at the neutral point of the transformers and at the ground point of the control circuits is approximately 250Vpeak, -580Vpeak, respectively. These magnitudes are smaller than the magnitudes of the switching transients, but the lightning surges and the magnitudes have significantly high frequencies. Therefore, several control circuits may be damaged by the overvoltages. [2] [3] [4] [5] Salih Carsimamovic, Zijad Bajramovic, Miroslav Ljevak, Meludin Veledar, “Very Fast Electromagnetic Transients in Air Insulated Substations and Gas Insulated Substations due to Disconnector Switching”, International Symposium on Electromagnetic Compatibility, 2005. EMC 2005, vol. 2, pp. 382-387, 8-12 Aug., 2005. Z. Haznadar, S. Carsimamovic, R. Mahmutcehajic, “More Accurate Modeling of Gas Insulated Substation Components in Digital Simulations of Very Fast Electromagnetic Transients”, IEEE Trans. on Power Delivery, vol. 7, no. 1, pp. 434-441, Jan., 1992. EPRI, Electromagnetic Transients Program(EMTP) Workbook II, EPRI Final Report, EL-4651, vol. 2, June. 1986. V. Vinod Kumar, Joy Thomas M., M.S. Maidu, “Influence of Switching Conditions on the VFTO Magnitudes in a GIS”, IEEE Trans. on Power Delivery, vol. 16, no. 4, pp. 539-544, Oct., 2001 Lu Tiechen, Zhang Bo, “Calculation of Very Fast Transient Overvoltages in GIS”, 2005 IEEE/PES Sang-Min Yeo and Chul-Hwan Kim [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Transmission and Distribution Conference & Exhibition: Asia and Pacific Dalian, China, pp. 1-5, 2005. Xuzhu Dong, Sebastian Rosado, Yilu Liu, NienChung Wang, E-Leny Line, Tzong-Yih Guo, “Study of Abnormal Electrical Phenomena Effects on GSU Transformers”, IEEE Trans. on Power Delivery, vol. 18, no. 3, pp. 835-842, July, 2003. IEEE Working Group 15.08.09, Tutorial on Modeling and Analysis of System Transients using Digital Programs, IEEE PES Special Publication, 1998. D.A. Woodford, L.M. Wedepohl, “Transmission Line Energization with Breaker Pre-Strike”, 1997 Conference on Communications, Power and Computing WESCANEX '97 Proceedings, pp. 105-108, May 22-23, 1997. D.A. Woodford, L.M. Wedepohl, “Impact of Circuit Breaker Pre-Strike on Transmission Line Energization Transients”, IPST '97, Seattle, pp. 250-253, June 22-26, 1997. A. Ametani, T. Goto, S. Yoshizaki, H. Omura, H. Motoyama, “Switching Surge Characteristics in GasInsulated Substation”, Universities Power Engineering Conference, 2006. UPEC '06. Proceedings of the 41st International, vol. 3, pp. 941-945, Sept. 2006. A. Ametani, K. Ohtsuki, N. Nagaoka, “Switching Surge Characteristics in Gas-Insulated Substations in Particular Reference to Low-Voltage Control Circuits”, UPEC 2004, Bristol, UK, Sept., 2004. S.M. Yeo, H.C. Seo, C.H. Kim, Y.S. Lyu, B.S. Cho, “EMTP Analysis of Very Fast Transients due to Disconnector Switching in a 345kV Korean Thermal Plant”, International Conference of Electrical Engineering, Okinawa, Japan, paper no. P-093, 6-10 July, 2008. S.M. Yeo, H.C. Seo, C.H. Kim, Y.S. Lyu, B.S. Cho, “Investigation of Switching Transients due to Disconnector Switching”, International Conference of Electrical Engineering, Okinawa, Japan, paper no. O-112, 6-10 July, 2008. DCG-EMTP(Development coordination group of EMTP) Version EMTP-RV, Electromagnetic Transients Program. [Online]. Avaliable : http://www.emtp.com. F.E. Menter, L. Grcev, “EMTP-Based Model for Grounding System Analysis”, IEEE Trans. on Power Delivery, vol. 9, no. 4, pp. 1838-1849, Oct. 1994. L. Grcev, “Modelling of Grounding Systems for Better Protection of Communication Installations 303 against Effects from Electric Power System and Lighting”, IEE Conference INTELEC 2001, pp. 461468, 14-18 Oct. 2001. [17] G. Celli, F. Pilo, “A Distributed Parameter model for Grounding Systems in the PSCAD/EMTDC environment”, IEEE Power Engineering Society General Meeting, 2003, vol. 3, pp. 1650-1655, 13-17 July 2003. [18] A. Rakotomalala, Ph. Auriol, A. Rousseau, “Lighting Distribution through Earthing Systems”, IEEE International Symposium on Electromagnetic Compatibility, pp. 419-423, 22-26 Aug., 1994. [19] I. Uglesic, S. Hutter, V. Milardic, I. Ivankovic, B. Filipovic-Grcic, “Electromagnetic Disturbances of the Secondary Circuits in Gas Insulated Substation due to Disconnector Switching”, International Conference on Power Systems Transients, IPST 2003 in New Orleans, USA, 2003. Sang-Min Yeo was born in Korea, 1976. He received his B.S., M.S., and Ph.D. degrees in Electrical Engineering from Sungkyunkwan University in 1999, 2001, and 2009, respectively. Since October 2009, he joined Power & Industrial Systems R&D Center, Hyosung Corporation, Korea. His current research interests include power system transients, modeling and simulation for FACTS/HVDC using EMTP, power system protection and computer application using EMTP, and signal processing. Chul-Hwan Kim was born in Korea, 1961. He received his B.S., M.S., and Ph.D degrees in Electrical Engineering from Sungkyunkwan University, Korea, 1982, 1984, and 1990 respectively. In 1990 he joined Cheju National University, Cheju, Korea, as a full-time Lecturer. He has been a visiting academic at the University of BATH, UK, in 1996, 1998, and 1999. Since March 1992, he has been a professor in the School of Information and Communication Engineering, Sungkyunkwan University, Korea. His research interests include power system protection, artificial intelligence application for protection and control, the modelling/ protection of underground cable and EMTP software.
