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Journal of Electrical Engineering & Technology Vol. 7, No. 3, pp. 297~303, 2012 http://dx.doi.org/10.5370/JEET.2012.7.3.297
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. Pre-strikes 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
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
† Corresponding Author: Power & Industrial Systems R&D Center, Hyosung Corporation, Korea. ([email protected])
* School of Information and Communication Engineering, Sungkyunkwan Univerity, Korea. ([email protected])
Received: April 29, 2011; Accepted: February 3, 2012
Analysis of Transient Overvoltages within a 345kV Korean Thermal Plant
298
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.
2.2 Pre-strike
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].
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1
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scopeVw
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1
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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.
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.
+ 570
R
1
+
0 .0463nF C9
+1.2uH
L2
P2P1
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.
+R1 ?i
0.0001
+
Icramp1
1.2us/63kA
+R2 ?v
400BUS1
a
a
Fig. 3. Lightning surge model
3. Simulation and Results
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.
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
299
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. The other components are modeled by similar procedure [4, 6-13].
(a) Open status (b) Close status
Fig. 5. The equivalent model of the circuit breaker 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.
386E
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Fig. 6. The model of control circuit
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.
(a) The layout of real grounding system
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71
P2
P1
DEV
675
P2
P1
DEV
681
P2
P1
DEV
684
P2
P1
DEV
687
P2P1
DEV68 9
P2P1
DEV6 90
P2P1
DEV6 93
P2P1
DEV6 94
P2P1 P2P1
DEV6 97
P2
P1
DEV
698
P2
P1
DEV
700
P2
P1
DEV
702
P2
P1
DEV
705
P2
P1
DEV
706
P2
P1
DEV
708
P2
P1
DEV
709
P2
P1
DEV
710
P2
P1
DEV
711
P2
P1
DEV
712
P2
P1
DEV
713
P2
P1
DEV
714
P2
P1
DEV
715
P2
P1
DEV
717
P2P1
DEV7 18
P2P1
DEV7 19
P2P1
DEV72 0
P2P1
DEV7 23
P2P1
DEV72 4
P2P1
DEV7 27
P2
P1
DEV7
29 P2
P1
DEV7
30 P2
P1
DEV7
31 P2
P1
DEV7
32 P2
P1
DEV7
33 P2
P1
DEV7
38
P2P1
DEV7 39
P2P1
DEV74 0
P2P1
DEV7 41
P2P1
DEV74 2
P2P1
DEV7 43
P2P1
DEV7 44
P2P1
DEV7 45
P2P1
DEV74 6
P2P1
DEV7 47
P2P1
DEV7 48
P2P1
DEV7 49
P2P1
DEV75 0
P2P1
DEV7 51
P2P1
DEV7 52
P2P1
DEV7 53
P2P1
DEV7 54
P2P1
DEV75 5
P2P1
DEV7 56
P2
P1
DEV
757
P2
P1
DEV
758
P2
P1
DEV
759
P2
P1
DEV
760
P2
P1
DEV
761
P2
P1
DEV
762
P2
P1
DEV
763
P2
P1
DEV
764
P2
P1
DEV
765
P2
P1
DEV
766
P2
P1
DEV
767
P2
P1
DEV
768
P2
P1
DEV
769
P2
P1
DEV
770
P2
P1
DEV
771
P2
P1
DEV
772
P2
P1
DEV
773
P2
P1
DEV
774
P2
P1
DEV
775
P2
P1
DEV
776
P2
P1
DEV
777
P2
P1
DEV
778
P2
P1
DEV
779
P2
P1
DEV
780
P2
P1DE
V781 P2
P1DE
V782 P2
P1DE
V783 P2
P1DE
V784 P2
P1DE
V785 P2
P1DE
V786 P2
P1DE
V787P2
P1DE
V788
P2
P1
DEV
789
P2
P1
DEV
790
P2
P1
DEV
791
P2
P1
DEV
792
P2
P1
DEV
793
P2
P1
DEV
794
P2
P1
DEV
795
P2
P1
DEV
796
P2
P1
DEV
797
P2
P1
DEV
798
P2
P1
DEV
799
P2
P1
DEV
800
P2
P1
DEV
801
P2
P1
DEV
802
P2
P1
DEV
803
P2
P1
DEV
804
P2
P1
DEV
805 P2
P1
DEV
806 P2
P1
DEV
807 P2
P1
DEV
808 P2
P1
DEV
809 P2
P1
DEV
810 P2
P1
DEV
811P2
P1
DEV
812
P2
P1DE
V813 P2
P1DE
V814 P2
P1DE
V815 P2
P1DE
V816 P2
P1DE
V817 P2
P1DE
V818 P2
P1DE
V819P2
P1DE
V820
P2
P1
DEV
821
P2
P1
DEV
822
P2
P1
DEV
823
P2
P1
DEV
824
P2
P1
DEV
825
P2
P1
DEV
826
P2
P1
DEV
827
P2
P1
DEV
828
P2
P1
DEV
829
P2P1
DEV8 30
P2P1
DEV83 1
P2P1
DEV8 32
P2P1
DEV83 3
P2P1
DEV8 34
P2P1
DEV8 35
P2P1
DEV83 6
P2P1
DEV8 37
P2P1
DEV8 38
P2P1
DEV83 9
P2P1
DEV8 40
P2P1
DEV84 1
P2P1
DEV8 42
P2P1
DEV8 43
P2P1
DEV8 44
P2P1
DEV84 5
P2P1
DEV8 46
P2P1
DEV8 47
P2P1
DEV84 8
P2P1
DEV8 49
P2P1
DEV85 0
P2P1
DEV8 51
P2P1
DEV8 52
P2P1
DEV8 53
P2P1
DEV85 4
P2P1
DEV8 55
P2P1DEV8 56
P2P1DEV85 7
P2P1DEV8 58
P2P1DEV85 9
P2P1DEV8 60
P2P1DEV8 61
P2P1DEV8 62
P2P1DEV86 3
P2P1DEV8 64
P2P1
DEV8 65
P2P1
DEV86 6
P2P1
DEV8 67
P2P1
DEV86 8
P2P1
DEV8 69
P2P1
DEV8 70
P2P1
DEV8 71
P2P1
DEV87 2
P2P1
DEV8 73
P2P1
DEV8 74
P2P1
DEV87 5
P2P1
DEV8 76
P2P1
DEV87 7
P2P1
DEV8 78
P2P1
DEV8 79
P2P1
DEV8 80
P2P1
DEV88 1
P2P1
DEV8 82
P2P1
DEV8 83
P2P1
DEV88 4
P2P1
DEV8 85
P2P1
DEV88 6
P2P1
DEV8 87
P2P1
DEV8 88
P2P1
DEV8 89
P2P1
DEV89 0
P2P1
DEV8 91
P2P1
DEV8 92
P2P1
DEV8 93
P2P1
DEV8 94
P2P1
DEV8 95
P2P1DEV89 6
P2P1
DEV89 7
P2P1
DEV89 8
P2P1
DEV89 9
P2P1
DEV90 0
P2P1DEV90 1
P2P1
DEV90 2
P2P1
DEV90 3
P2
P1
DEV
904
P2
P1
DEV
905
P2
P1
DEV
906
P2
P1DE
V907
P2
P1
DEV
908
P2
P1
DEV
909
P2
P1
DEV
910
P2
P1DE
V911
P2
P1
DEV
912
P2
P1
DEV
913
P2
P1
DEV9
14
P2
P1DE
V915
P2
P1
DEV
918
P2
P1
DEV
919
P2
P1
DEV
920
P2P1
DEV9 22
P2P1
DEV92 3
P2P1
DEV9 24
P2P1
DEV92 5
P2P1
DEV9 26
P2P1
DEV9 27
P2P1
DEV9 28
P2P1
DEV92 9
P2P1
DEV9 30
P2P1
DEV93 1
P2
P1
DEV
932
P2
P1
DEV
933
P2
P1
DEV
934
P2
P1
DEV
935
P2
P1
DEV
936
P2
P1
DEV
937
P2
P1
DEV
938
P2
P1
DEV
939
P2
P1DE
V940P2
P1DE
V941 P2P1
DEV94 2
P2P1
DEV9 43P2P1
DEV94 4
P2P1
DEV94 5
P2P1
DEV94 6
P2P1
DEV9 47
P2
P1
DEV
948
P2
P1
DEV
949
P2P1
DEV95 0
P2P1
DEV9 51
P2P1
DEV9 52
P2P1
DEV9 53
P2P1
DEV9 54
P2
P1
DEV
955
P2P1
DEV95 6
P2P1
DEV9 57
P2P1
DEV6 5
P2P1
DEV66
P2P1
DEV18 1
P2P1
DEV18 2
P2P1
DEV28 4
P2P1
DEV9 58
P2P1
DEV9 59
P2P1
DEV96 0
P2P1
DEV9 61
P2P1
DEV9 62
P2P1
DEV96 3
P2P1
DEV96 4
P2P1
DEV9 65
P2P1
DEV96 6
P2P1
DEV9 67
P2P1
DEV96 8
P2P1
DEV9 69
P2P1
DEV9 70
P2P1
DEV9 71
P2P1
DEV97 2
P2P1
DEV97 3
P2P1
DEV64
+
1
R9
+
1
R23
+
1
R24
+
1
R2 5
+
1
R2 6
+
1
R2 7
+
1
R28
+
1
R30
+
1
R49
v (t)
p 1
scopeGnd 56
v(t)
p 2
scopeGnd 34
v (t)
p3
scopeGn d1 2
P2P1
DEV1 7
P2P1
DEV18
P2P1
DEV4 0
P2P1
DEV44
P2P1
DEV4 5
P2P1
DEV4 6
P2P1
DEV47
P2P1
DEV48
P2P1
DEV4 9
P2P1
DEV50
P2P1
DEV5 1
P2P1
DEV5 7
P2P1
DEV58
P2P1
DEV6 3
P2P1
DEV68
P2P1
DEV6 9
P2P1
DEV8 1
P2P1
DEV8 2
P2P1
DEV85
P2P1
DEV8 6
P2P1
DEV91
P2P1
DEV92
P2P1
DEV9 7
P2P1
DEV99
P2P1
DEV1 01
P2P1
DEV1 02
P2P1
DEV10 3
P2P1
DEV10 4
+
1
R2
P2
P1
DEV1
05
P2
P1
DEV
106
P2
P1
DEV1
07
+
1
R1
+
1
R3
(b) the model of the entire grounding system
Fig. 7. Model of the grounding system
disconnector
DS_CRpLp
7071DS_C
PI+
PI1 PI
+PI
2 CP+
55.4
3
TLM
1
CP+
6.16
TLM
2
PQ
Load
1
54
5MW
29M
VAR
PQ
Load
2
29
2MW
46M
VAR
PQ
Load
3
34
2MW
46M
VAR
PQ
Load
4
54
9MW
32M
VAR
PQ
Load
5
54
5MW
41M
VAR
CP+
55.4
3
TLM
3
CP+
6.16
TLM
4
CP+
77.4
3
TLM
5
CP+
8.60
TLM
6
CP+
77.4
3
TLM
7
CP+
8.60
TLM
8D
S_C R
pLp
7622
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Lp75
22
DS
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Lp73
22
DS
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Lp72
22
DS
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Lp7A
22
DS
_C Rp
Lp7B
22
DS
_C Rp
Lp71
21
DS
_C Rp
Lp72
21
DS
_C Rp
Lp73
21
DS
_C Rp
Lp74
21
DS
_C Rp
Lp76
21
BUS2BUS1GIS Bus Bar
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Rp
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3
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4
21
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12
MAI
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20
.9/3
45
pinst./pu
Gen4
pinst./pu
Gen3
pinst./pu
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pinst./puSSUBTR4_2ND
pinst./pu
SSUBTR6_2ND
+
0.05
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2 3
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126.9/
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pinst./pu
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DY_
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v(t)
i(t)
p3
v(t)
i(t)
p4
v(t)
i(t)
p6
v(t)
i(t)
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scopeTR1N_VscopeTR1N_C
scopeTR2N_VscopeTR2N_C
scopeTR4N_VscopeTR4N_C
scopeTR6N_VscopeTR6N_C
v(t)
i(t)
p7
scopeCTR1N_VscopeCTR1N_C
v(t)
i(t)
p8
scopeCTR2N_VscopeCTR2N_C
v(t)
i(t)
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scopeCTR3N_VscopeCTR3N_C
v(t)
i(t)
p10
scopeCTR4N_VscopeCTR4N_C
v(t)
i(t)
p11
scopeCTR5N_VscopeCTR5N_C
v(t)
i(t)
p12
scopeCTR6N_VscopeCTR6N_C
v(t)
i(t)
p1
scopeTR5N_VscopeTR5N_C
v(t)
i(t)
p2
scopeTR3N_VscopeTR3N_C
+
0.05nF C9
+
0.05nF C1
SM22
kV62
0MVA
SM1SM
22kV
620M
VA
SM2
LF Load7
184MW2MVAR
LF Load6
154MW2MVAR
LF Load8
164MW2MVAR
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
DEV5
full_gnd
IN_SGNIN_VAC
GND Out_Sgn
Control & DC Part
CTRL_Block
CTRL1
IN_SGNIN_VAC
GND Out_Sgn
Control & DC Part
CTRL_Block
CTRL2
IN_SGNIN_VAC
GND Out_Sgn
Control & DC Part
CTRL_Block
CTRL3
IN_SGNIN_VAC
GND Out_Sgn
Control & DC Part
CTRL_Block
CTRL4
IN_SGNIN_VAC
GND Out_Sgn
Control & DC Part
CTRL_Block
CTRL5
IN_SGNIN_VAC
GND Out_Sgn
Control & DC Part
CTRL_Block
CTRL6
c
b
a
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 pre-
strike 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
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.
PI
+
PI1 PI
+
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9
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W46
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11
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549M
W32
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GISbusbar
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V0 mag rad
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PrestrikeCswVsw
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BUS2BUS1GIS Bus Bar
GISbusbar
DEV7
BUS2BUS1GIS Bus Bar
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a
b
c
a
b
c
Fig. 9. System model for the switching transients
Table 1. Simulation conditions for very fast transients
Time step 3ns Maximum simulation time 30us
System condition 76BUS have been de-energized Simulation condition 75-76 DS close when simulation starts
Table 2. Simulation conditions for switching transients
Time step 125ns Maximum simulation time 1.3s
System condition 76BUS have been de-energized Simulation condition 75-76 DS close when simulation starts
Table 3. Simulation conditions for switching transients
Time step 3ns Maximum simulation time 100us
Simulation condition Lightning strike transmission lines at 20us
3.3 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
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.
(a) #4 Generator (b) #6 Generator
Fig. 12. Voltage waveforms for some generators in the frequency domain
Fig. 10. Voltage waveform at 76BUS
(a) #4 Generator (b) #6 Generator
Fig. 11. Voltage waveforms for some generators in the time domain
Sang-Min Yeo and Chul-Hwan Kim
301
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
Voltage at each generator (unit : p.u.) Measured Point #1 Gen. #2 Gen. #3 Gen. #4 Gen. #6 Gen.
Max. voltage 1.184 1.311 1.377 1.336 1.111Max. voltage
deviation 0.432 0.651 0.729 0.787 0.270
Table 5. Measured voltage at each bus
Voltage at each bus (unit: p.u.) Measured Point 70BUS 71BUS 75BUS 76BUS
Max. voltage 1.27937 1.11887 1.22162 1.89149 Max. voltage
deviation 0.55189 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
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.
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.
Fig. 15. Ground voltage at low-voltage control circuits
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
(a) Instantaneous voltages
(b) Frequency Spectrum
Fig. 13. 3-phase voltage at the GIS bus bar
Analysis of Transient Overvoltages within a 345kV Korean Thermal Plant
302
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
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.
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
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Sang-Min Yeo and Chul-Hwan Kim
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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.