AU2011203070A1 - SWER replacement - Google Patents

Abstract

Abstract This invention lies in the field of electrical power transmission. In particular a system for HVDC power transmission, the system comprising an AC power source. A primary isolation transformer and a rectifier forming an AC/DC transformer. One or more SWER transmission lines suitable to be operated at a nominal AC voltage extending from the AC/DC transformer to one or more DC/AC transformers. One or more inverters and secondary isolation transformers forming DC/AC transformers. And one or more local AC distribution systems each connected to an AC output of a DC/AC transformer. The SWER transmission lines are operated at a DC voltage of 1.41 times the nominal AC voltage. (N U-., (9 C-) o 0> - _ II (0 I o I o I -'0

Description

Title SWER Replacement Technical Field 5 This invention lies in the field of electrical power transmission. Background of the Invention 10 Power transmission systems transport power from power sources to power sinks. Power sources include nuclear, gas, or coal power plants as well as wind turbines or solar cells. Power sinks are households, factories, office buildings, farms, processing plants, and the like. Because the power sources and power sinks are distributed over a wider area, most power transmission systems are not a straight line but have a network 15 structure. To operate such a network most economically, the losses through the lines need to be minimised. It can be shown that the losses through the lines depend on the voltage used to transmit the power such that a higher voltage means smaller losses. On the 20 downside, infrastructure such as lines and poles is more expensive for higher voltages. The cost function is a combination of power losses through the line and costs for building the network and this cost function is to be minimised. As a result, high-throughput long-distance lines are operated at extra high voltages, e.g. 25 500kV, whereas low-throughput shorter lines are operated on lower voltages, e.g. 12.7kV. For most consumers of power, for example households, high voltages are impractical and unsafe. Therefore, the power is transformed to a lower voltage of typically I 10V or 230V before being delivered to households. The electrical energy supplier is required to deliver electrical power with a specified level of quality. The 30 quality specification includes voltage variation limits at the sink. At various connection points of the transmission network, the power is transformed from one voltage to another. The cost for building and operating the entire network relies significantly on the costs for transforming voltages at the various connection 35 points. This cost was the main reason why most countries have chosen alternating current (AC) over direct current (DC) for power transmission over their networks 2 because AC can be transformed to different voltages using only induction coil transformers whereas DC transformation involves more complicated methods. The current standard is a 3-phase AC system with different voltage levels depending on the requirements of each part of the grid. In case of a balanced grid, the return current is 5 zero and as a result, no neutral wire is required. In areas of low population density long distances between power sources and power sinks have to be crossed. Most power sinks are relatively small loads such as individual households, farms, or small processing plants. In such a scenario there is 10 less need for a transmission system with high throughput and small losses. The critical parameter contributing to the overall cost of the system is the cost per kilometre, which is minimised by using Single Wire Earth Return (SWER) systems. Instead of three wires required for a three phase system, SWER systems comprise only a single wire and transmit single phase AC power of a relatively low voltage. As a result, the cost 15 for wires and poles is small compared to other systems. In contrast to a balanced three phase system, the return current in a SWER system is not zero and flows back through the earth (or ground). Summary of the Invention 20 The present invention is a system for HVDC power transmission, the system comprising an AC power source, a primary isolation transformer and a rectifier forming an AC/DC transformer, 25 one or more SWER transmission lines suitable to be operated at a nominal AC voltage extending from the AC/DC transformer to one or more DC/AC transformers, one or more inverters and secondary isolation transformers forming DC/AC transformers, and one or more local AC distribution systems each connected to an AC output of a 30 DC/AC transformer, wherein the SWER transmission lines are operated at a DC voltage of 1.41 times the nominal AC voltage. A number of advantages flow from use of the invention, including: 35 (i) increased line transmission capacity for the same conductor size due the increased operating voltage.

3 (ii) mixed AC and DC circuit segments for flexible installation on existing networks. (iii) operation under variable line voltage controlled or transient overload conditions. 5 (iv) regulated single phase or three phase AC supply to the sinks (v) improved fault discrimination due to the removal of the transmission line reactive elements. (vi) integration of energy storage (capacitors and batteries) to improve the transmission line utilisation. 10 (vii) integration of local renewable energy sources and other back up generation to improve the line utilisation. (viii) improved line efficiency by increasing the line capacity without increasing the line losses and removing the line reactive power requirements (additional reactive current reduces the active power capacity). 15 The nominal AC (RMS) voltage of the SWER transmission line may be 12.7kV or 19.1 kV. The AC power source may be a 22kV, 33kV, 66kV or 132kV AC power source. Additional DC power from sources such as wind, sun, water, geothermal heat, and 20 biogas may be connected to the SWER transmission line. The DC transmission voltage may be varied without varying the AC voltage supplied to the sinks or loads. The DC transmission voltage may be strategically reduced or varied. The local AC distribution system may be a 3-phase AC system. 25 Brief Description of the Drawings Examples of the invention will now be described with reference to the accompanying drawings, in which: 30 Fig. I shows a system for high voltage direct current power transmission. Fig. 2 shows another embodiment of the DC transmission system. Fig. 3 shows the system of Fig. I but with a part of the single wire being operated under AC. 35 Fig. 4 shows the structure of a three phase inverter. Fig. 5 shows one of the inverter modules in more detail.

4 Best Modes of the Invention Fig. I shows a system for high voltage direct current (HVDC) power transmission 100 comprising a 22kV three-phase AC network 101, a first recloser 102, a three wire 5 power line 103, a first isolation transformer 104 and rectifier 105, a single wire power line 106 with an earth return circuit 107, a HVDC inverter 108, a second isolation transformer 109, a second recloser 110, a three phase distribution system Il l1, and several buildings 112. 10 The three phase network 101 supplies power to transformer 104 via recloser 102 and three wire power line 103. The recloser opens if there is any malfunction detected on the load side. Because most events of malfunctioning are temporary, after opening due to detected malfunction the recloser will make several attempts to re-energise the line after certain time periods. The transformer 104 transforms the voltage supplied by 15 lines 103 and is connected to rectifier 105, which outputs DC power with a voltage of 17.9kV. The DC power is transmitted over the single wire 106 which spans most of the distance between the power source 101 and the buildings 112. The single wire 106 terminates at HVDC inverter 108, which outputs AC power from the DC supply and delivers it to isolation transformer 109. The isolation transformers 104 and 109 isolate 20 the DC part of the system from the AC part at the supply and the load side. The current flows from the first transformer 104 through the rectifier 105, the single wire 106 and the HVDC inverter 108 to the second transformer 109. The current then flows back through the earth return circuit 107. The transformer 109 supplies the local three phase distribution system Ill via the second recloser 110. The three phase distribution 25 system delivers the power to individual buildings 112. While most of the distance is covered by the single wire DC connection 106, the local three phase distribution system only connects loads which are relatively close to another such as individual buildings of a cattle station. 30 Fig. 2 shows another embodiment of the DC transmission system 200. Fig. 2 shows the system of Fig. I but with a second single wire DC branch 206 connected to the single wire DC connection 106. Also shown are an additional earth return circuit 207, a HVDC inverter 208, a transformer 209, a recloser 210, a three phase supply system 211, and buildings 212. As described above, the 22kV network 101 supplies 35 transformer 104 through recloser 102 and three phase line 103. The rectifier 105 generates DC power from AC supply. The single wire DC line 106 now has a second 5 branch 206 supplying the second HVDC inverter which is in turn connected to transformer 209. The return current flows back through the earth return circuit 207. Thereby, the second local distribution system 211 can be connected via recloser 210 and a second group of buildings or loads 212 can be supplied with power. The concept 5 of additional branches connected to the single wire 106 can be extended to construct a tree structure of single wire lines supplying many groups of buildings. Fig. 3 shows a system 300 similar to Fig. I but with a part of the single wire being operated under AC. Fig. 3 shows the same component as before but now, the single 10 wire 206 is connected directly to transformer 104 without a rectifier. An additional transformer 204 is shown as well as a rectifier 105. The transformer 104 supplies the single wire AC connection 206 with AC power and thereby also the additional transformer 204. The rectifier 105 connected to transformer 204 generates DC power which is fed into a HVDC inverter 108 and transformer 109 to generate three phase 15 AC power. As above, the three phase AC power is delivered to the buildings 112 and other loads by the three phase supply system 111 and recloser 110. The distance between the 22kV supply network and the buildings is covered partly by the single wire AC line and partly by single wire DC line. The additional transformer 204 is optional and the single wire 206 may be directly connected to rectifier 105. This 20 embodiment shows the flexibility of the proposed system as AC lines and DC lines can be combined to cover the entire distance. Fig. 4 shows the structure of the three phase inverter 108, which was used in Figs. 1-3. The inverter 108 comprises a primary side 401, an iron core 402, and a secondary side 25 403. A connection port 404 is also shown. The primary side 401 is built of N inverter modules three of which 410, 420, and 430 are shown. Inverter module 410 comprises three single phase inverters 411, 412, and 413 and three coils 414, 415, and 416. Respectively, inverter module 420 comprises single phase inverters 421, 422, and 423 and coils 424, 425, and 425 and inverter module 430 comprises single phase inverters 30 431, 432, and 433 and coils 434, 435, and 436. The secondary side 403 of transformer 108 comprises three coils 444, 445, and 446 connected between three outputs 451, 452, and 453 respectively and the ground as shown. All coils are wound around the transformer core 402. 35 The operation of inverter module 410 will be outlined in the following and the same can be applied to the other inverter modules 420 and 430. Each inverter module 6 comprises three single phase inverters connected in series, one for each phase and each single phase inverter generates AC power at its output with a fixed frequency and phase controlled by a controller (not shown). The controller controls the single phase inverters such that there is a phase shift of 120 degrees between the output signals of 5 the three inverters 414, 415, and 416. The controller also ensures that the single phase inverters 411, 421, and 431 have no phase shift between each other. The same applies for single phase inverters 412, 422, and 432 and also for single phase inverters 413, 423, and 433. 10 Each of the single phase inverters 411- 413, 421-423, and 431-433 has a maximum input voltage of 1kV resulting in a maximum input voltage of 3kV of an inverter module. In order to connect to an input voltage of 19.] kV, N=7 inverter modules are needed to be connected in series. 15 The coils, which are connected to the single phase inverters, are wound around the iron core 402. The coils with equal phase share one leg of the iron core 402 together with one of the coils of the secondary side 403. In this example, primary coils 414, 424, and 434 are wound on the same leg together with coil 444. Coils 415, 425, and 435 are on the same leg of the core with coil 445, and coils 416, 426, 436, and 446 are wound on a 20 third leg. As a result of this arrangement, the coils with equal phase, for example 414, 424, and 434 generate magnetic fields with equal phases and the superposition of these magnetic fields induce an alternating current in the secondary coil 444. The shifted magnetic fields of the other coils on the primary side induce alternating currents in coils 445 and 446, while the phase of the currents in the three coils 444, 445, and 446 25 on the secondary side are shifted by 120 degrees. The result is a three phase AC output at connection points 451, 452, and 453. Fig. 5 shows one of the inverter modules 410 of Fig. 4 in more detail. The single phase inverter 411 is shown with its internal structure while the other two single phase 30 inverters 412 and 413 have the same structure but this is not shown. Six insulated-gate bipolar transistors (IGBTs) 513, 514, and 516-519 are shown each of which being connected in parallel to a reverse diode. The single phase inverter 411 comprises a boost regulator 5 10 and a H-bridge 511. The boost converter 510 comprises an inductor 512, two of the IGBTs 513 and 514 and a capacitor 515. The H-bridge 511 35 comprises the remaining four IGBTs 516-517 connected in a H-shape fashion. The functionality of the boost regulator 510 will be described in the following. When IGBT 7 513 is off and IGBT 514 is on, inductor 512 is magnetised by current flowing through inductance 512 and IGBT 514. Then, IGBT 514 is turned off and IGBT 513 is turned on and the magnetised inductor 512 maintains a current flow through IGBT 513. At this time, the voltage over capacitor 515 depends on the load and inductor 512 and can 5 be above the input voltage. By adjusting the timing scheme for turning IGBTs 513 and 514 on and off, the voltage over capacitor 515 can be controlled. Capacitor 515 filters out ripples generated by magnetic charging and discharging of inductor 512. The advantage of using a boost regulator is that even when the voltage of the DC input fluctuates, the same constant voltage is supplied to the H-bridge, which is important for 10 generating AC power with a constant peak voltage. The four IGBTs of the H-bridge operate in pairs of two. The aim is to generate an alternating current through coil 414. For generating a first half cycle IGBTs 516 and 519 are turned on and current flows through IGBT 516, coil 414 and IGBT 519. For 50Hz AC one complete cycle takes 20ms. Therefore, to generate a second half cycle, after 1Oms IGBTs 516 and 517 are 15 turned off and IGBTs 517 and 518 are turned on and a current flows through IGBT 517, coil 414 and IGBT 518. Due to the connection of the IGBTs and the coil, this time the current through the coils flows in the opposite direction as during the first half cycle generating an opposite magnetic field. The result is a alternating current with a constant peak to peak voltage through coil 414. In this example, the timing applied to 20 IGBTs 513, 514, and 516-519 by the controller results in a frequency of the AC signal of 50Hz and a fundamental voltage over coil 414 of 690V. The resulting magnetic field induces an alternating current into secondary coil 444 which is connected to the first output 451 of the three phase AC output. The other two phases are generated by the other two single phase inverters 412 and 413 connected to coils 415 and 416, 25 inducing AC into coils 445 and 446 and supplying the power to outputs 452 and 453 respectively. This way a three phase AC output is generated. It will be appreciated by persons skilled in the art that numerous variations 30 and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (8)

1. A system for HVDC power transmission, the system comprising 5 an AC power source, a primary isolation transformer and a rectifier forming an AC/DC transformer, one or more SWER transmission lines suitable to be operated at a nominal AC voltage extending from the AC/DC transformer to one or more DC/AC transformers, one or more inverters and secondary isolation transformers forming DC/AC 10 transformers, and one or more local AC distribution systems each connected to an AC output of a DC/AC transformer, wherein the SWER transmission lines are operated at a DC voltage of 1.41 times the nominal AC voltage. 15

2. A system according to claim 1, wherein the nominal AC (RMS) voltage of the SWER transmission line is 12.7kV or 19.1kV.

3. A system according to claim 1, wherein the AC power source is a 22kV, 33kV, 20 66kV or 132kV AC power source.

4. A system according to claim 1, wherein additional DC power from sources are connected to the SWER transmission line. 25

5. A system according to claim 4, wherein the additional DC power is sourced from wind, sun, water, geothermal heat, or biogas.

6. A system according to claim 1, wherein the DC transmission voltage is varied without varying the AC voltage supplied to the sinks or loads. 30

7. A system according to claim 1, wherein the DC transmission voltage is strategically reduced or varied.

8. A system according to claim 1, wherein the local AC distribution system is a 3 35 phase AC system.