AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT ELECTRIC POWER DISTRIBUTION SYSTEM INCORPORATING CURRENT MULTIPLYING DC-DC CONVERTERS The following statement is the full description of this invention, including the best method of performing it known to me: Electric Power Distribution System incorporating current multiplying DC-DC converters 5 The Power Distribution System covered in this invention can be used wherever there is a need to apply constant electrical DC current to the load, rather than constant voltage. It specifically targets applications where distribution of low voltage / high DC currents is employed over long distances into loads at remote or difficult to access areas, or loads that reside in aggressive or corrosive environments which are not suitable for housing standard electric and electronic equipment and power supplies. 0 Typical examples of such applications include Impressed Current Cathodic Protection of steel structures in sea water and marine environments, DC current supply in anodising, electroplating, electrowinning and electrorefining industries, mining and lighting applications, etc. 5 Usually the loads are anodes, or groups of anodes, typically all working in parallel from the same power supply system. Anode voltages are normally up to 10 Vdc or less, while currents range from tens to hundreds of Amperes per anode. Typically power distribution systems in the above mentioned applications, as shown in Figure 1, use .0 Transformer Rectifiers 1 (with or without thyristor controlled output) as low voltage (ie. < 50V) DC power supplies. Low voltage Distribution Cable 2 network usually employ large cable cross sections and may include a set of adjusting power resistors for balancing the currents to each individual Load 3 (anode). These systems suffer from poor energy efficiencies, primarily due to huge power losses in distribution cables .5 and associated adjusting resistors, with consequent high energy consumption throughout the life of the system. In addition, such systems tend to have a poor and variable power factor, introducing harmonic distortion back into the AC mains supply and therefore creating additional power losses in the AC feeding network. 0 It is the object of this invention to reduce power losses in distribution cables, and to reduce cross section size and weight of distribution cables, between the AC powered converter and the remote load or loads. The invention is a Power Distribution System having two power processing stages. The first stage incorporates a Power Supply unit on the AC side of the Power Distribution System, and a long high voltage 5 distribution cable. The second stage incorporates a Current Multiplying DC/DC converter, located close to the load, and a short low voltage load cable. The function of the first stage is to generate a combined power and control signal for the remote load and to distribute it with low power losses employing high voltage / low current approach. The function of the second 40 stage is to convert the combined power and control signal to a low voltage / high current output signal suitable for powering the load. The combined power and control signal is a constant DC current, controllable and adjustable at the Power Supply unit. Thus, the full control and adjustment of the load current is retained in the first stage of the 45 Power Distribution System, where Power Supply serves as a master unit, while Current Multiplying DC/DC converter is a slave unit delivering the load current. This invention is based on the use of high switching frequency (ie. much higher than 50 Hz) switch-mode power conversion, both in the Power Supply unit (AC to DC) and the Current Multiplier converter (DC to DC), 50 to achieve the reduction of distribution power losses and reduction in size and weight of distribution cables. The architecture of the Power Distribution System is shown in its minimal configuration in Figure 2. The system consists of a separate AC/DC Power Supply unit 4 and a DC/DC Converter (Current Multiplier) 5. Dedicated Distribution cable pair 6 connects the Power Supply unit 4 to the Current Multiplier 5, while 55 dedicated Load cable pair 8 connects the Current Multiplier 5 to the Load 7. This configuration is used as an individual channel to power and control a single load. In the case where there are multiple independent loads, the configuration from Figure 2 can be replicated in parallel as many times as there are loads, creating a multiple channel Power Distribution System.
Power Supply unit 4 is a self-contained AC/DC converter working in a constant current output mode, feeding the regulated DC current ISUPP to the input of the Current Multiplier 5. This invention does not require the use of specially constructed Power Supply units - they can be commercially available off-the-shelf products capable of constant current mode operation, or custom built units constructed using standard solutions for 5 achieving constant current output mode. They should contain, however, some form of active power factor correction to aid the increased power efficiency approach achieved by the invention. Current Multiplier 5 transforms the Power Supply current IsUPP into the load current IMULT with the pre-defined ratio - a current multiplication factor Kc >> 1 (typically 10 to 50). The output current of the Power Supply 0 IsUPP is therefore much smaller than the Current Multiplier output current IMULT. This invention is based on the use of high frequency switch-mode DC power conversion in the Current Multiplier 5 to achieve the proper current multiplication factor Kc. The reduction of Power Supply current ISUPP significantly decreases power losses in the Distribution cable 6. 5 Therefore, Distribution cable 6 can be of reduced cross section compared to cables in a traditional approach of distributing load currents from the transformer rectifier to the load, as shown in Figure 1. If the same voltage drop along the Distribution cable 6 is allowed as in the traditional approach, then the cross section of the Distribution cable 6 in this invention can be reduced by a factor which equals Kc. Power loss in the Distribution cable 6 is also reduced by the same factor Kc. .0 Because of the smaller overall diameter of Distribution cable 6, it can run as an un-interrupted lead from Power Supply unit 4 to Current Multiplier 5, thus eliminating the need for junction boxes along the way. Because of the reduced power losses in the Distribution cable 6, and of the applied principle of constant current drive, Power Supply unit 4 can be located hundreds or thousands of metres away from the Current .5 Multiplier 5. This allows for Power Supply units placement in the convenient and accessible location, away from adverse environmental influences. Current Multiplier units, which can have more rugged construction, can be located close to loads, at different distances from the cental Power Supply units position. The length of the Load cable 8 should be minimised by installing the Current Multiplier 5 close to the Load 7. 0 This decreases power losses in the Load cable 8, through which flows the high load current IMULT. Thus, load cables can also have somewhat reduced cross section compared to cables in a traditional approach, while still keeping associated power losses and voltage drops at low levels. When the Power Supply current IsUPP is kept constant (Power Supply unit working in constant current mode), 5 then the Current Multiplier output current IMULT will also be constant. Consequently, as the output current IMULT is regulated and constant, the output voltage VMULT is defined only by the load impedance and VMULT value changes as the load impedance changes. As a further consequence to scaling up of the load current IMULT by a factor Kc, the Power Supply output -to voltage VSUPP is also scaled up compared to Current Multiplier output voltage VMULT by a voltage multiplication factor Kv, similar in value to Kc. This factor is usually 5% to 20% higher than Kc, as it includes effects of the power losses within the Current Multiplier 5 and of any voltage drop developed along the Distribution cable 6. 45 Dynamic range of the output current IMULT is typically 1:10 in the practical applications. As stated above, the output voltage VMULT of the Current Multiplier is determined only by the load impedance, so dynamic range of output voltage VMULT will also be about 1:10. This will in turn create a dynamic range of about 1:10 for the Power Supply output voltage VSUPP. For values of Kc between 10 and 50, and for maximum output voltage VMULT value of 10 Vdc, the maximum Power Supply output voltage VSUPP levels would range from about 100 50 up to 600 Vdc, which includes effects of voltage drop developed along the Distribution cable. The preferred and the most superior configuration of the proposed system is the one in which each Power Supply unit 4 feeds just one Current Multiplier 5 via dedicated Distribution cable 6, and each Current Multiplier 5 feeds a single Load 7, also via dedicated Load cable 8, thus creating a one-to-one channel 55 system, as shown in Figure 2. Optionally, as shown in Figures 3, 4 and 5, each Power Supply unit 4 can feed one or more Current Multipliers 2, each via dedicated Distribution cables 3 (including both positive and negative leads). Each Current Multiplier 5 can feed one or more Loads 4, again via dedicated Load cables 5 (including both 60 positive and negative leads).
If Current Multiplier is used to feed several loads in parallel, as shown in Figure 3 forming one-to-many load configuration, distribution of individual load currents will be dictated by the individual load impedances, and only the sum of these currents will be constant. Common output voltage of the Current Multiplier will be the result of the equivalent load impedance and corresponding current distribution between loads. 5 If Power Supply unit is used to feed several Current Multipliers in parallel, as shown in Figure 4 forming one to-many distribution configuration, distribution of individual load currents is more complex, and only the sum of these currents will be constant. Common output voltage of the Power Supply unit will be the result of a complex relationship between load impedances, current multiplication factors and distribution cables 0 impedances. If Power Supply unit is used to feed several Current Multipliers in parallel, and each Current Multiplier is used to feed several loads in parallel, as shown in Figure 5 forming many-to-many distribution configuration, distribution of individual load currents is even more complex and less controlled. 5 The complete Power Distribution System can then consist from one to several parallel branches of one-to one, one-to-many and / or many-to-many configuration channels, as shown in Figures 3, 4 and 5 and described above. The advantages of using Current Multipliers in all of the above cases are preserved, ie. huge reduction in size of distribution cables and distribution power losses. However, the flexibility and ease .0 of system design, and the precise control of current distribution into individual loads are somewhat compromised for all versions employing other than the one-to-one configuration channels. One additional benefit of this invention is found in case of multiple dependent loads, as is the case with multiple anodes all sharing common negative connection in cathodic protection or electroplating applications. .5 As each Current Multiplier output has its own negative return path, the electrical continuity network between loads' negative connections is far less critical when several Current Multipliers are used to power the combination of loads. The continuity network then undertakes more of a potential equalisation role, rather than a current distribution role, as return currents are distributed locally rather then globally through a common connection. 0 Current Multiplier converter, as described in this invention, is of novel construction shown in a block diagram in Figure 6. Current Multiplier has only four terminal connections, two for input and two for output. There are no separate control inputs or front panel controls or interfaces. 5 Electrical signal at the input terminals of the Current Multiplier provides two functions. The first function is to serve as a source of power both for the Current Multiplier and the load, as most of the Current Multiplier input power is transmitted to the load. The second function is to serve as the control signal defining Current Multiplier's output current. -t0 Switching power block 9, which contains voltage step-down transformers and power switches, converts the input current into the output current. Step-down transformer in the Switching power block 9 is where electrical isolation of the output versus the input takes place. Switching power block 9 can be constructed by employing: - any of the standard or proprietary step-down switching converter topologies including, but not limited to, 45 half-bridge and full bridge, push-pull and forward, and - any of the standard or proprietary switching types including, but not limited to, hard-switching (square wave) or soft-switching (resonant, quasi resonant, zero-voltage switching, zero-current switching). 50 Input filter 10 and Output filter 11 remove the switching conversion artefacts and maintain low ripple in the input and output currents. Optionally, these filters can contain circuitry for high frequency EMC filtering providing interference immunity, and circuitry for surge protection from externally induced over-voltages at input and output terminals. 55 The separate DC/DC converter 12, attached to the input terminal connections of the Current Multiplier, generates regulated supply voltage to bias the electronics in Control circuit 13 and Switching power block 9. As Current Multiplier faces a wide input range of up to 1:10 (voltage Vsupp) during operation, the DC/DC converter 12 must have at least a 1:10 or higher operational input voltage range, in order to properly bias the internal Current Multiplier circuitry at all times. 60 Control circuit 13 defines and regulates the current conversion ratio (current multiplication factor Kc) by controlling the switching duty cycle in the Switching power block 9. Switching duty cycle is defined as a ratio of on-time duration, when power switches allow flow of power towards the converter's output, and a cycle period duration, which is the sum of 'on' and 'off' times. Current Multiplier mode of operation, of converting 5 input current into proportional output current, can be achieved by employing one of several regulation control techniques in the Control circuit 13. Some of the regulation control techniques are, but not limited to, the following: - fixed switching duty cycle in an open loop, without feedback or loop regulation of the Current Multiplier output current or voltage, 0 - switching duty cycle forced to a maximum value in a saturated closed loop, using the feedback from Current Multiplier output but keeping the output current or voltage outside of regulation, - actively adjusted switching duty cycle in a closed loop regulation, by comparing the Current Multiplier output current to the input current. 5 The preferred and the most superior regulation control technique employed in Current Multiplier converter is a closed loop regulation which actively adjusts switching duty cycle by comparing the output current to the input current. Modulation of switching duty cycle employed in Control circuit 13 can have any form of standard or .0 proprietary control including, but not limited to, pulse-width modulation, current-mode control (peak or average), fixed or variable frequency modulation. Methods for establishing the desired value of the current multiplication factor Kc are, but not limited to, in open loop versions: .5 - by selecting the appropriate turns ratio for the power transformer in the Switching power block 9, and - by fixing the switching duty cycle to a particular value in the Control circuit 13, or, in closed loop versions: - by selecting the appropriate turns ratio for the power transformer in the Switching power block 9, and - by actively adjusting the switching duty cycle in the Control circuit 13 cycle against the reference value 0 and thus keeping the ratio of output and input currents constant. The preferred and the most superior method employed in Current Multiplier converter for establishing the desired value of the current multiplication factor Kc is an active adjustment of switching duty cycle against the reference value in a closed loop and thus keeping the ratio of output and input currents constant. 5 BRIEF DESCRIPTION OF DRAWINGS -t0 Figure 1 shows a schematic illustration of a traditional approach to distribution of power to remote low voltage / high current DC loads. Figure 2 shows a schematic illustration of a Power Distribution System incorporating current multiplying converter with one-to-one channel configuration. 45 Figure 3 shows a schematic illustration of a Power Distribution System incorporating current multiplying converters with one-to-many load configuration. Figure 4 shows a schematic illustration of a Power Distribution System incorporating current multiplying 50 converters with one-to-many distribution configuration. Figure 5 shows a schematic illustration of a Power Distribution System incorporating current multiplying converters with many-to-many distribution configuration. 55 Figure 6 shows a schematic illustration of the Current Multiplier converter block diagram.