4 Equipment Requirements (DS, ES)
4.3 Bus-transfer switching by Disconnectors .1 Introduction .1 Introduction
4.3.2 Background of technical requirements .1 General requirements .1 General requirements
4.3.2.3 Bus-transfer calculation for current UHV projects
The rated bus-transfer current for both air-insulated and gas-insulated disconnectors is 80 % of rated normal current and normally not expected to exceed 1600 A [4]. The bus-transfer current depends on the impedance ratio of long and short circuit of the loop and on the rated current during the switching sequence. Calculations were performed to verify the ratio of long and short circuit of the loop. For the calculation the actual schemes from the 1100 kV systems were used [3]. It should be noted that in a substation built on a 2 CB layout or 1-1/2 CB layout with double busbar, the bus-transfer can be accomplished using a CB to transfer the current (See figure 4.3.2 a-c). Thus, in normal operation conditions, the DS will not be stressed with transfer switching operations. Only in the case of a failure in a CB, bus-transfer switching capability for the DS will be required to disconnect the faulty CB for maintenance. The calculation for these cases was performed only for comparison. Figure 4.3.2 shows the configurations used. The parameters used for the calculation are shown in table 4.3.3.
Table 4.3.3 Electrical parameter used for calculation Reactance
[µΩ/m]
Inductance [µH/m]
Velocity [m/µs]
Impedance [Ω]
1100 kV GIS busbar (China) 94 0.3 275-300 90
1100 kV AIS busbar (China) 377 1.2 300 360
1100 kV GIS busbar (Japan) 104 0.33 273 90
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a) China 110 0kV GIS, 2CB layout
Usually Transfer current will
(b) China 1100 kV Hybrid-IS, 1-1/2CB layout
Long loop
(c) China 1100 kV Hybrid-IS, 1-1/2CB layout
Line Line Line Line
(d) Japan 1100 kV GIS, double busbar layout, busbar coupler farthest away from DS
Line Line Line Line
(e) Japan 1100 kV GIS, double busbar layout, busbar coupler near-by the DS Figures 4.3.2 Bus-transfer current switching, calculated examples according to [3]
Table 4.3.4 shows the calculation results. The ratios, calculated by using actual design, vary between 70 % and 94 %. Considering only the double busbar layout, the ratios vary between 85 % and 94 %. In case of a double busbar configuration two cases were investigated. In case 1, the bus coupler is far away from the switching DS (figure 4.3.2 d) and in case 2 the bus coupler is nearby the switching DS (figure 4.3.2 e).
The typical application for bus-transfer switching is the double busbar layout scheme.
Therefore this configuration was investigated more in detail. Two different bus-transfer operations are possible.
Case 1: operation at outgoing/incoming bay (figure 4.3.3) Case 2: operation at the other bay (figure 4.3.4)
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Table 4.3.4 Bus-transfer calculation results
Figure Long loop Short loop Ratio
Length [%]
[m]
Impedance [Ω]
Length [m]
Impedance [Ω]
AIS GIS Total AIS GIS Total
China 1100 kV Full-GIS*1 4.3.3 a 0 84 0.00792 0 36 0.00339 70.0 China 1100 kV Hybrid-IS*2 4.3.3 b 112 78.4 0.04961 30 39.2 0.015 76.8 China 1100 kV Hybrid-IS*3 4.3.3 c 256 36 0.0999 112 36 0.04562 68.7
Japan 1100 kV GIS*4 Bus coupler bay away
4.3.3 d 0 175.9 0.01824 0 10.7 0.00111 94.3
Japan 1100 kV GIS*4 Bus coupler bay nearby
4.3.3 e 0 63.6 0.00659 0 10.7 0.00111 85.6
India 800 kV AIS*5 n/a n/a n/a n/a n/a n/a
*1 China 1100 kV GIS, 2CB scheme [3]
*2 China 1100 kV Hybrid-IS, 1-1/2CB scheme [3]
*3 China 1100 kV Hybrid-IS, 1-1/2CB scheme [3]
*4 Japanese 1100 kV GIS, double busbar scheme [3]
*5 Indian 800 kV AIS, 1-1/2CB scheme [3]
1) At initial time power flow route is Line 2 -> Bus1 -> Bank1.
2) Then by the Bus-transfer over at Line 2 bay operation, DS1 is closed. At this moment, there are two power flows in parallel Line 2 (DS1) -> Bus1 -> Bank1. and,
Line 2 (DS2) -> Bus2 -> Tie 1 -> Bus 2 ->Bank1.
3) Finally by opening operation of DS1, DS1 have to break Bus-transfer current.
Line1 Line2 Line3 Line4
Tie1 Tie2
Section
Bank1 Bank1
Bus 1
Bus 2
Line1 Line2 Line3 Line4
Tie1 Tie2
Section
Bank1 Bank1
Bus 1
Bus 2 1) close (DS2)
2) open (DS1)
Figure 4.3.3 Bus Bus-transfer current switching, case 1
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1) At initial time power flow route is Line 2 -> Bus1 -> Tie 1 ->Bank1.
2) By the bus-transfer over operation at Line 1 bay, both of DS1 and DS2 of Line 1 are closed.
At this moment, there are two power flow in parallel
Line 2 -> Bus1 -> DS1 and DS2 of Line 1 -> Bus2 ->Bank1. and, Line 2 -> Bus1 -> Tie 1 -> Bus2 ->Bank1.
3) Finally by opening operation of DS1, DS1 have to break Bus-transfer current.
Line1 Line2 Line3 Line4
Figure 4.3.4 Bus Bus-transfer current switching, case 2
The model configuration is shown in figure 4.3.5. The arrangement and the length of the busbar are based on the information given in [3].
Figure 4.3.5 Model configuration [3]
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Table 4.3.5 and table 4.3.6 show the calculated loop length and current distribution ratio for the bus-transfer current for both cases. There are some special cases which can be omitted by considering actual operation condition. These cases are marked as special cases in the tables.
The highest ratio of current distribution is 98 %. By considering these aspects, the most conservative value for transfer current may be equal to the current at the moment of bus-transfer operation. This current may reach the rated current of the bus coupler or the line feeder.
Table 4.3.5 Bus-transfer calculation results – double busbar scheme case 1 Bay of Power flow Loop length (m)
Ratio Special cases From To (Change Over
assumed)
Long loop
(via Coupler) Short loop
Line 1 Line 2 48.6 1.6 96.8%
*1 Negligible if nearer bus coupler bay is closed
*2 Unusual power flow
Table 4.3.6 Bus-transfer calculation results – double busbar scheme case 2
Bay of Power flow
Change over Loop length (m)
Ratio Special cases
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Bay of Power flow
Change over Loop length (m)
Ratio Special cases
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4.3.3 Recommendations for specifications
Figure 4.3.6 shows the bus transfer voltage for GIS applications, by using typical values for resistance and reactance. It can be seen that the bus-transfer voltage is linearly increasing with length of current carrying loop. As IEC 62271-102 specifies 1600 A as loop current, these values are given in comparison to 2000 A, 4000 A and 8000 A loop current. The vertical lines indicate what current carrying loop appears between bays with typical distances (20 m x
2nd bay 3rd bay 4th bay
GIS 8000 A
Figure 4.3.6 Bus-transfer voltage as a function of length of GIS current carrying loop (IEC value for 800 kV) Similar calculations have been conducted for AIS or Hybrid-IS where the busbars will be AIS lines. AIS lines have increased inductance values compared to GIS. Due to higher inductance values, the bus transfer voltage is higher for Hybrid-IS substations, see figure 4.3.7.
0
Voltage at bus transfer [V] IEC62271-102
AIS 1600 A AIS 2000 A AIS 4000 A
2nd bay 3rd bay 4th bay
AIS 8000 A
Figure 4.3.7 Bus-transfer voltage as a function of length of AIS / Hybrid-IS current carrying loop (IEC value for 800 kV)
Thus for UHV, the bus-transfer current has to be defined in dependence of the actual current rating. Since the maximum rated current is increased up to 8000 A in the UHV systems, the bus-transfer current should be chosen according to the rated current for the application.
Bus-transfer currents have to be defined in dependence of the actual current ratings, the type of substation and the maximum loop length. As can be learnt from figure 4.3.6 and 4.3.7, for
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GIS a bus-transfer voltage of 60 V belongs to 1600 A and of 300 V to 8000 A, while for AIS and Hybrid-IS a voltage of 400 V belongs to 1600 A.
As WG A3.22 has not yet enough information on bus-transfer current and the loop length, any conclusion is too premature. Further investigations are necessary to give recommendations.