G. KOEPPL Koeppl Power Experts Switzerland

Transcription

1 PS3: Substation Design: New Solutions and Experiences Bus-Node Substation A Big Improvement in Short-Circuit and Switching Properties at Reduced Substation Costs G. KOEPPL Koeppl Power Experts Switzerland SUMMARY The Bus-Node -concept (BN-concept) for air-insulated substations (AIS) is characterized by: Separated phase layout and HV-cables connecting the feeder bays to the overhead lines and to the transformers. It enables an extremely compact layout for AIS (about 1/3 of land compared to conventional AIS) resulting in a total cost reduction of 30% for a 245kV Double BN substation. The report at hand analyses the conditions regarding short-circuit currents, transient recovery voltage (TRV) and the disconnector bus-transfer switching in such BN-substations. The analysis is based on theoretical considerations and simulations. It shows that 3- and 2-phase fault currents are inexistent in BN-substations. Only single-phase faults may occur, however, at clearly smaller probability of occurrence. The cable capacitance at each feeder bay causes a time delay t d of 4 s in the TRV, compared to 2 s given in the IEC Standard. In sum the BN-concept is a substantial improvement in substation design. Keywords Air-insulated substations, Short-circuit current, Fault interruption, Transient recovery voltage, Bus transfer switching. Georg Koeppl, gskoeppl@bluewin.ch 1

2 1. INTRODUCTION A novel HV-substation concept, the so-called Bus-Node -concept (BN-concept) for air-insulated substations (AIS) was presented recently [1]. The characteristic features of this concept are: Separated phase layout, i.e. no crossings of conductors in the air-insulated area HV-cables connecting the feeder bays to the overhead lines and to the transformers Conventional HV-apparatus and components can be used. The BN-concept enables an extremely compact layout for AIS, i.e. about 1/3 of land area occupied compared to conventional AIS with long busbars. Considering also the substantial savings in conductor material, supporting structures, foundations and earthing grid, a total cost reduction of 30% has been calculated for a 245kV Double Bus-Node substation. Figure 1 shows one phase of a 245kV AIS, 8 feeder bays with double bus-node, Figure 2 a perspective view of the total double bus-node substation [1]. It goes without saying that bus-couplers (not shown in the figure) and even longitudinal bus-sectionalizers are feasible. Figure 1: One Phase of a Double Bus-Node AIS, 245kV, 8 Feeders 2

3 Figure 2: Double Bus-Node AIS, 245kV, 8 Feeders Three- and two-phase short-circuits are no longer possible in such a substation due to phase separation; single phase faults are possible but with a significantly reduced probability. These features and their consequences are dealt with in more detail in the paper at hand. 1. SHORT CIRCUIT CONDITIONS IN BUS-NODE SUBSTATIONS COMPARED TO CONVENTIONAL SUBSTATIONS 1.1 Two Distinct Zones Regarding Fault Consequences There are two areas in the layout of a substation, which determine the fault consequences: (1) The busbar area and the connections between busbar and circuit-breakers (CB) (2) The area between CBs and overhead lines or transformers, respectively Zone (1) is the utmost critical one: a fault there is a busbar fault; it necessarily causes interruption of all feeders connected to this busbar as well as substantial down-time of the substation; if 3- or 2-phase, it mostly leads to heavy damages. A fault in zone (2) is a feeder fault; the busbar is unaffected and only one feeder must be disconnected. Again 3- and 2-phase faults mostly cause substantial damage. The area beyond zone (2) contains the overhead lines and transformers. It is practically identical for the layout of a BN-AIS and a conventional AIS. 3

4 1.2 Short-Circuit Currents in Those Zones Zone (1): Obviously 2- and 3-phase short-circuits in zone (1) of a BN-substation cannot happen due to the separated phase layout. Only single phase faults are possible there but with a significantly reduced probability due to shorter conductor lengths. The length of the phase conductors possibly affected by a busbar fault is determined by the number of busbars, the busbar length and the distance from busbar to CB. In the 8-feeder double BN-AIS described in [1] (see also Figure 1 and 2) the sum of these conductor lengths is about 100m per phase. For the corresponding conventional double busbar AIS compared in [1] this number is 546m per phase. This ratio of 100/546 = is roughly valid also for AIS with other numbers of feeders. It seems to be realistic to assess the probability of a fault occurrence to be proportional to the conductor length (same insulation qualities and overvoltage stresses provided). That means in a Double BN-AIS the probability of a single-phase fault at the bus-bar is less than 20% of a conventional AIS. Here the BN-AIS exhibits a substantial technical advantage. Additionally in favour of the BN-AIS is the fact, that in such a substation there are no conductor crossings, another possible source of short-circuits. These conductor crossings in a conventional AIS are predominantly in the critical zone (1). Zone (2): Again 2- and 3-phase faults are inexistent in zone (2) of a BN-AIS, in contrast to a conventional AIS. The conductor length per phase is given by the number of feeders and the distance from CBs to the overhead lines and to the transformers respectively. These distances are approximately the same for BN-AIS (air-insulated feeder plus HV-cable) and for conventional AIS (air-insulated feeder plus air-insulated connection to overhead line). In this less critical zone (2) the advantage of the BN-AIS is absence of 2- and 3-phase faults and the reduced damage in case of a single phase fault, which might easily develop to a multi-phase fault in a conventional AIS. Short-circuit currents on the overhead lines or in the transformers are obviously not influenced by the substation design. 2. TRANSIENT RECOVERY VOLTAGES at CBs in BUS-NODE-AIS The short-circuit currents in all cases considered in paragraph 2 must be cleared by the CBs next to the fault. In the Bus-Node AIS, the corresponding transient recovery voltage (TRV) of the CB is very positively influenced by the cable sections connected to each feeder bay. This is valid for all fault cases. 2.1 Terminal Faults Since 2- and 3-phase faults are not possible in a BN-substation, only single phase faults must be considered. According to [2], Table 28 the corresponding peak value uc,sp of the TRV is 1.3 times smaller than for the first-phase-to-clear of a 3-phase fault. This is already a substantial alleviation in switching duty. The only conceivable 3-phase fault in a BN-AIS would be closing on a bolted fault which hopefully should be an extremely rare event in a HV-substation however. In this case the damage consequences are zero (no electric fault arc) and the fault is in the less critical zone (2). This extreme case was simulated for the first-phase-to-clear of a CB 245kV, 60Hz, with a rated short-circuit-breaking current of 50kA in a BN-AIS. The corresponding simulation scheme is given in Figure 3. 4

5 Current injector Feeder Bus Feeder Cable Overhead line Net equivalent 10m Z=260 Z=31.5 Z 1 =390 Z 0 =780 X 1 X 0 22m 37m 15km 6x CB B0 B1 Z=260 10m Figure 3: Simulation Scheme for a Bolted 3-Phase Fault 50kA in a Double Bus-Node AIS, 245kV, 8 Feeders The TRV corresponds to the voltage response to a current injector into the ungrounded CB terminal with a current steepness of π 60 A/s. Decisive is the correct modelling of the electrical components in the close electrical vicinity of the CB; this area determines the initial course of the TRV. The more distant components are responsible for the later course of the TRV; they have been chosen to represent the corresponding reference line of the specified TRV of the IEC Standards [2]. Since the considered 3-phase fault is bolted, i.e. rigidly grounded, the connected components on the grounded side may be disregarded (not shown in the scheme). The ungrounded terminal of the CB is connected to an air insulated feeder connection (CB to busbar) of 10m with a typical surge impedance of 260. The very short busbar (10m) is connected to 6 parallel air insulated feeders of 22m (busbar to cable termination) and further on to 6 parallel HV cables (Z = 35.1, l = 37m) of the BN-AIS. Then come 6 parallel overhead lines of 15km length. The components connected to these overhead lines provide a TRV shape which corresponds to the later course of the reference line of the specified TRV [2]. Figure 4 shows the voltage across the first pole-to-clear a short-circuit-breaking current of 50kA, 60Hz for the conditions described above and calculated by means of [3]. Figure 4a gives the total shape compared to the reference line according to [2]; rate of rise of recovery voltage (RRRV) equals to 2kV/ s and the peak value corresponds well to 364kV of the Standard. More interesting is the initial course of the TRV shown in Figure 4b. Reference line of specified TRV Reference line of specified TRV IEC Delay line of specified TRV, td = 2 s td 4 s Figure 4a: TRV First-Phase-to-Clear, Bolted Fault Figure 4b: TRV First-Phase-to-Clear, Initial Course The TRV in red is shaped by the number of (short) cables connected to the feeder bays and by the very compact substation layout with short conductor lengths between bus and cable sealing ends. There is 5

6 practically no initial transient recovery voltage (ITRV) and the delay time td is in the range of 4 s compared to 2 s given in the Standards. Simplified, this delay time is t d = Z C where Z is the resulting surge impedance of the overhead lines and C is the resulting capacitance of the HV-cables between bus-node and overhead lines. The voltage response to a linearly rising current injector di/dt = const. into a parallel Z and C is given by which depicts very well the initial course of the simulated TRV. The resulting surge impedance of overhead lines lies in a rather narrow range (370 to 450 per overhead line). Hence basically the cable lengths determine t d. In our practical case the average cable length is 37m; with C = F/km this yields F, resulting in t d = 3.9 to 4.7 s. With longer cable connections t d would rise. Since this combination of cable capacitance and surge impedance of OH line is valid for each feeder in a BN- AIS, this delay time is valid for any number of feeders and consequently for any magnitude of 100% short-circuit breaking current. It goes without saying that also interruption of single phase faults in a BN-AIS is characterized by this delay time and by the absence of any ITRV. Such an initial shape of the TRV which is inherent to a BN-AIS facilitates substantially the thermal mode of current interruption in a CB. 2.2 Short-Line Faults Even short-line faults near a BN-AIS are easier to be cleared due to the cable capacitance between feeder and overhead line, which is also shown by simulation. The model of Figure 3 is supplemented by a 3-phase cable section and a 3-phase overhead line showing a fault-to-ground in one phase (the overhead lines in the other 2 phases are on-going). The current to be interrupted is 37.5kA according to test duty L75 of [2], based on 50kA, 60Hz. The voltage responses on both CB terminals to ground and across the CB are given in Figure 5a and Figure 5b. TRV across CB TRV source side Reference line of specified TRV TRV across CB TRV source side td 4 s TRV line side Reference line of specified TRV TRV line side Figure 5a: TRV Short-Line Fault L75 Figure 5b: TRV Short-Line Fault L75, Initial Course The line side voltage of the CB terminal (in green) starts from the voltage u 0 (62.3kV) and goes in an oscillatory shape to zero. The shape is not saw-tooth like as in the classical short-line-fault but shows rounded peaks due to the cable capacitance at the beginning of the line. The corresponding time delay t d of 4 s is clearly visible in the initial course of Figure 5b. Also the source side voltage of the CB terminal (in red) exhibits the same time delay due to the effect described in paragraph 2.1. Of course 6

7 this is valid too for the difference of these voltages i.e. the voltage across the CB terminals (in blue). Again no ITRV is discernible on the voltages at both CB terminals. That means that a BN-AIS is synonymous with a tremendously easier CB duty also at short-line faults. 2.3 T60 to T10 Faults, OP1 OP2 Faults (IEC ) The favourable influence of the cable capacitances in a BN-AIS is effective in all fault- and fault switching cases. In [2] Table 26 the TRV requirements for faults with breaking currents less than the 100% rated breaking current (cases T60 to T10) are given. The TRV peaks in all those cases are generally higher than for the 100% case and rise with decreasing fault current. They are all referred to the first-pole-to-clear of a 3-phase fault and use a first-pole to-clear factor of 1.3. Due to the inexistence of 3- and 2-phase faults in a BN-AIS a phase factor of only 1.0 applies to all of such faults providing a substantial safety margin when clearing such faults. Cases T60 to T10 of [2] are characterized by RRRVs 2kV/ s, growing with decreasing current. The reason for this is the assumption that only a transformer or a combination of transformer and 1 or 2 overhead lines might feed the fault. The surge impedance of the transformer being about 5 to 10 times higher than that of an overhead line then is responsible for the higher RRRV. What was deduced in paragraph 2.1 and 2.2 regarding the time delay t d of about 4 s in a BN-AIS is also valid for any other fault case as long as the feeders are overhead lines. If the only feeder is a transformer however (as implied for case T10), then the corresponding surge impedance of a transformer determines the time delay t d = C Z. This is taken into consideration in [2] by higher time delays for T60 to T10 basing on 2 s for T100. If the base t d in a BN-AIS is 4 s however then the corresponding delay times for fault cases T60 to T10 are higher than given in [2] which results in an additional alleviation regarding breaking duty (the absence of ITRV is given here too). Out-of-phase breaking duties are covered by the test duties OP1 and OP2 of [2]. It should be mentioned that out-of-phase faults may be excluded if no generators are connected to the bus or if the generators are synchronized via generator CBs. This condition is fulfilled in many substations. Should out-of-phase switching occur in a BN-AIS, again the cable capacitances connected to the feeders guarantee a slow initial rise of the TRV across the CB. The corresponding time delays at both CB terminals may be easily deduced from the actual feeders affected by the out-of-phase conditions. 3. DISCONNECTORS AT BUS-TRANSFER CURRENT SWITCHING In [4] the duties of disconnectors at bus-transfer current switching are dealt with, i.e. opening and closing of disconnectors under load when this load is not interrupted, but transferred from one bus to a parallel bus. For AIS relatively long loop lengths are assumed in the Standard, e.g. typically 400m for rated voltages 245kV. From this the rated bus-transfer voltages at typical bus impedances are deduced resulting in 175Vrms for 245kV and a rated current of 2000A. It is obvious that the rated bustransfer voltages rise proportionally with the rated currents of the disconnectors. Since in a BN-AIS the disconnectors are immediately connected to the extremely short bus the corresponding loop length is drastically diminished to less than 15m for the 245kV substation described in [1] and shown in Figure 1 and 2. This allows to reduce the rated bus-transfer voltage from 175 Vrms to less than 10Vrms for a rated current of 2000A. The consequences of such a small bus-transfer voltage are: - no problems with bus-transfer current switching 7

8 - practically no wear and tear at the disconnector contacts due to arcing in a BN-AIS - no necessity for bus-transfer current switching tests. 4. CONCLUSIONS The technical properties of a novel and very economic HV-substation concept, the so-called Bus- Node -concept [1] for AIS have been analysed. The most striking quality of such a BN-AIS is the nonexistence of 2- and 3-phase faults (with heavy damages) due to phase separation. Only single phase faults are possible; the most critical zone however where a fault leads to a busbar fault, is in a BN-AIS about 5 times smaller than in a conventional AIS. A correspondingly reduced fault probability for single phase faults may be expected. A further substantial advantage is the easier TRV shape for all faults to be cleared by the CBs in a BN- AIS. The only possible single phase faults show TRV peaks by a factor 1.3 times smaller. The initial shape of the TRV is characterized by the cable capacitance between feeder bay and overhead line or transformer respectively which increases the delay time t d for the TRV rise from 2 s according to [2] to 4 s and more. These cable capacitances and the extremely compact substation layout are responsible for the very smooth rise of the TRV, i.e. there is no ITRV. This applies to all sort of faults, even to short-line faults. Also the delay times given in [2] for breaking currents smaller than the 100% breaking current (Test duties T60 to T10) may be doubled for a BN-AIS. These effects facilitate substantially the thermal mode of current interruption. Bus-transfer current switching is the duty of disconnectors in a multi-bus substation. Here too the BN- AIS offers a great improvement: the usual loop lengths of up to 400m in conventional AIS are reduced to about 15m in a BN-AIS resulting in rated bus-transfer voltages of less than 10V. This means practically no wear and tear at bus-transfer current switching. All these alleviations contribute to a significant increase in substation reliability and availability of a BN-AIS compared to a conventional AIS. BIBLIOGRAPHY [1] G. Koeppl, Th. Aschwanden, P. Zinniker, CIGRE Session 2016, Paris, Paper B3-208 [2] IEC : High-voltage switchgear and controlgear - Part 100: Alternating-current circuit-breakers [3] Alternative Transients Program (ATP), Rule Book, Canadian / American User Group [4] IEC Ed 2: High-voltage switchgear and controlgear - Part 102: Alternatingcurrent disconnectors and earthing switches 8