Supply system to a set of loads connected in parallel to a direct current supply bus

Abstract

The invention relates to a supply system for a plurality of loads connected in parallel to a direct current supply bus. The supply system includes a DC supply bus and a plurality of supply lines connected in parallel to the supply bus and supplying the said loads. The supply system includes uncoupling and damping means that is adapted to decrease the unipolar signals travelling within the supply system while the loads are being supplied. The uncoupling and damping means includes at least one inductance arranged in series in at least one of the supply lines. Protective means (are also provided for protection in the event of a fault.

Claims

1. A supply system for a plurality of loads comprising: a direct current (DC) supply bus comprising a positive DC line and a negative DC line; a plurality of supply lines connected in parallel to the positive and negative DC lines of the DC supply bus and each supply line comprising a DC/AC converter being connected at an AC side to a load of the plurality of loads for supplying the loads; and an uncoupling and damping means disposed in each supply line and configured to absorb AC unipolar signals travelling within a respective supply line of a load of the plurality of loads while the loads are being supplied, the uncoupling and damping means in a common mode comprising a first capacitor and a second capacitor both connected directly at one end to ground and opposite ends thereof to the positive DC line and the negative DC line, respectively, and at least one inductance connected in series to a DC side of the DC/AC converter on each supply line and each inductance comprising a first unipolar inductance coil in the positive DC line and a second unipolar inductance coil in the negative DC line; protective means that intervene in the event of a fault in one section of the supply system and wherein the protective means includes a non-return diode in each supply line in such a manner as to prevent current from travelling towards the supply bus; and wherein each non-return diode is connected in series between the respective at least one inductance and the respective DC/AC converter.

2. A supply system in accordance with claim 1, comprising a resistance connected in parallel to the inductance.

3. A supply system in accordance with claim 1, wherein the protective means includes a non-return diode in each supply line in such a manner as to prevent current from travelling towards the DC supply bus.

4. A supply system in accordance with claim 1, wherein the protective means includes a breaker in each supply line with electronic switching components controlled by a control system based on the value of the current travelling within the said supply system section.

5. A supply system in accordance with claim 4, wherein each breaker is connected in series between the respective at least one inductance and the DC supply bus.

6. A supply system in accordance with claim 1, wherein the protective means includes an electro-mechanical switch in each supply line activated in the event of a fault within the said supply system section.

7. A supply system in accordance with claim 6, wherein each electro-mechanical switch is connected in series between the respective at least one inductance and the DC supply bus.

8. A supply system in accordance with claim 1, wherein each inductance further includes a third unipolar inductance coil.

9. A supply system in accordance with claim 8, wherein the resistance is connected in parallel with one of the first coil, the second coil, the first coil and the second coil, and the third coil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following description, which is provided purely as a non-exclusive example, highlights other aims, features, and advantages of the invention. It relates to the drawings where:

(2) FIG. 1 is a schematic diagram of a supply system for a plurality of loads connected in parallel;

(3) FIGS. 2 and 3 are equivalent unipolar diagrams of a supply system comprising two supply lines in parallel;

(4) FIG. 4 is a schematic diagram of a first implementation of an uncoupling and damping means that is not in accordance with the invention;

(5) FIG. 5 is a schematic diagram of a second implementation of the uncoupling and damping means that is in accordance with the invention;

(6) FIG. 6 is a schematic diagram of a third implementation of the uncoupling and damping means that is in accordance with the invention;

(7) FIGS. 7A and 7B illustrate the operation of the filtering inductance in a differential mode;

(8) FIGS. 7C and 7D illustrate the operation of the filtering inductance in a common mode;

(9) FIG. 8 is a schematic diagram of a detailed supply system in accordance with the invention; and

(10) FIGS. 9 and 10 are schematic diagrams illustrating how the protective means of a supply system operate in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

(11) FIG. 1 shows an example of how to implement a supply system for a plurality of loads.

(12) As can be seen, the supply system includes a DC supply bus (labelled DC BUS) to which a plurality of supply lines L1, . . . Ln are connected.

(13) The DC supply bus in the implementation example is a medium voltage direct current (MVDC) supply bus.

(14) Here, the supplied loads are AC motors M. It should be noted, however, that any other loads connected in parallel to a DC supply bus remain within the scope of the invention.

(15) As can be seen, the motors M are supplied by supply lines via DC/AC converters.

(16) As previously indicated, the loads and in particular the DC/AC converters are likely to create voltage or unipolar currents that could generate high voltage potentially detrimental to supply line, converter, and motor isolation means.

(17) FIG. 2 represents the equivalent unipolar diagram of a supply system which, for the purposes of simplicity, only has two supply lines supplied by one shared DC supply bus.

(18) On this diagram, Z.sub.b1 and Z.sub.b2 represent the respective equivalent impedances of the system (converter, load including AC supply line sections and motor impedance) and Z.sub.a represents the equivalent impedance between the supply bus and earth on the DC side. V.sub.zs1 and V.sub.zs2 represent the unipolar voltages generated by the two power supply line converters, and V.sub.b1 and V.sub.b2 represent the voltage at the Z.sub.b1 and Z.sub.b2 impedance terminals. As can be readily understood, the equivalent circuit is highly dependent on the value of its parameters. For example, if Z.sub.b2 is negligible compared with Z.sub.a and Z.sub.b1, the equivalent circuit becomes that of FIG. 3.

(19) This leads to:
V.sub.b1=V.sub.zs2−V.sub.zs1  (Eq1)

(20) Thus, the voltage at the point of motor isolation can equate to the sum of the V.sub.zs1 and V.sub.zs2 unipolar voltages.

(21) As illustrated in FIG. 4, a first implementation to alleviate this problem is to connect the DC supply bus to earth via a capacitor C. The capacitor C is selected so that its value exceeds Z.sub.b1 and Z.sub.b2 in order to minimize interactions between supply lines, and prevent the unipolar signals from adding themselves together in the supply bus.

(22) As illustrated in FIG. 5, a second implementation is to connect an inductance Lh in series with the converters on each of the supply lines on the DC side. As can be readily understood, this inductance absorbs the majority of the AC unipolar voltage component. However, the potential interactions between supply lines are maintained.

(23) Thus, as shown in FIG. 6, to prevent interactions between supply lines whilst decreasing unipolar signals, the supply system should have uncoupling and damping means in each supply line. This should include several inductances Lh in common mode and one or more capacitors C connecting the DC supply bus to earth (or ground). Such an arrangement would reduce the voltage in the loads generated by unipolar signals. This implementation has been found to be particularly suitable for underwater settings.

(24) FIGS. 7A, 7B, 7C, and 7D show the electric circuit and structure of a filtering inductance for filtering the noise generated by the unipolar signals within a differential mode (FIGS. 7A and 7B) and a common (or zero sequence) mode (FIGS. 7C and 7D).

(25) The inductance is provided by two wound coils 1 and 2 arranged around a core 3.

(26) As illustrated by the arrows F1 and F2, which illustrate the currents I.sub.p+ and I.sub.p− within the inductance coils, the I.sub.p+ and I.sub.p− currents of both coils are travelling in the same direction in the common mode, whereas they are travelling in opposite directions in the differential mode. In the differential mode, the magnetic flux generated by the coils 1 and 2 is in opposite directions and cancels each other out (see arrows F3 and F4). In the common mode, the magnetic flux generated by the coils 1 and 2 is in the same direction and is added together, resulting in high impedance. It is possible, but not essential, to add a damping resistance in parallel with one of the unipolar inductance coils 1 or 2, or in parallel to coil 1 and to coil 2. As shown, there is also the option to add a damping resistance Rh in parallel to a third unipolar inductance coil 4.

(27) FIG. 8 represents a detailed supply system implementation according to the invention.

(28) FIG. 8 shows the shared DC supply bus (labelled DC BUS) and supply lines L1 and L2—limited to two here for clarity purposes. FIG. 8 also shows an uncoupling and damping means (or circuit) in a common mode comprising two capacitors C connected between the supply bus and earth (or ground) and two inductances Lh. One capacitor is connected to a first DC line (e.g., a positive line) of the supply bus and the other capacitor is connected to a second DC line (e.g., a negative line) of the supply bus. Each inductance includes primary coils 1 and 2, a secondary coil 4, and a resistance R connected in parallel with the secondary coil 4. The first primary coil 1 is in a first DC line (e.g., a positive line) and the second primary coil 2 is in a second DC line (e.g., a negative line).

(29) Protective means are incorporated in the supply system to prevent a fault such as a short-circuit occurring in one of the supply line loads from reaching the shared supply bus.

(30) Such protective means includes, for each supply line L1 and L2, a breaker 5 with electronic power components, IGBT (Insulated Gate Bipolar Transistor) in this instance, controlled via a control circuit (not shown), based on the level of current travelling within the supply line as measured by an appropriate sensor (not shown).

(31) As soon as the level of current exceeds a predetermined threshold, the control circuit directs the electronic power components of the breaker 5 to open.

(32) As can be readily understood, such a layered protection is particularly effective in that it can intervene very swiftly to isolate a defective circuit section. It may, however, prove insufficiently reliable in as much as it could develop a fault itself. To enhance protective resilience, the protective means further includes an isolation means comprising an electro-mechanical or electro-magnetic switch 6 for each supply line, such as, for instance, a conveniently motorized breaker, which may require longer to open, but which offers improved isolation inasmuch as the circuit is opened by mechanical means.

(33) FIG. 8 also shows that the protective means are supplemented by a non-return diode 7 preventing the current coming from a DC/AC converter or, more generally, any defective load, from travelling towards the shared supply bus.

(34) The operation of the detailed supply system implementation of FIG. 8 when faced with a fault on a supply line will now be described.

(35) Referring to FIG. 9, when a short-circuit occurs on a supply line between the protective means (e.g., the breaker 5) and the DC/AC converter, in stage I the DC/AC converter is stopped by the opening of its own electronic power components (e.g., its IGBTs).

(36) In stage II, the breaker 5 swiftly opens, in order to isolate the faulty circuit branch and prevent the short-circuited current from travelling towards the DC supply bus, and hence to the other loads. The non-return diodes 7 prevent the current arising from the faulty supply line converter from travelling towards the DC supply bus, and prevent the capacitors and other DC/AC converters from discharging into the faulty supply line.

(37) In stage III, the electro-mechanical switch 6 opens in order to isolate the faulty circuit branch mechanically, and to safely enable the maintenance of operations to proceed on that supply line.