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(11) | EP 3 349 233 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | DC POWER SWITCHING UNIT |
| (57) A DC power switching unit comprises a first terminal and a second terminal (24, 25),
a sub-unit comprising a current limiter (L, L1, L2) and a diode (D1, D2) and a semiconductor
device (Q1, Q2). The sub-unit is connected to a first side (31) of the semiconductor
device. The power switching unit further comprises a snubber circuit (30) connected
to a second side (32) of the semiconductor device. The power switching unit comprises
two symmetrical sub-units, each sub-unit being connected on one side to one of the
first and second power switching unit terminals and on the other side to the semiconductor
device (Q1, Q2).
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[0001] This invention relates to a DC power switching unit, in particular for an offshore platform or vessel.
[0002] On offshore platforms or vessels, drilling rigs, aircraft, HVDC systems, wind power grids, or similar DC systems, there may be equipment which is deemed to be critical. For offshore operation in particular, regulatory requirements specify the necessary availability of power in the event of a fault. Consequentially, it has been normal practice to separate the equipment on the platform, vessel, or rig, into sections and provide separate power to each section, with redundancy, so that if a fault occurs in one section, it does not transfer to the other and not all operational capability is lost. This separation has been achieved by operating with bus ties between the sections normally open and only in limited circumstances closing those bus ties to enable one side to receive power from the other. However, such bus ties are only available for low voltage DC systems. For high voltage systems, mechanical breakers must be used.
[0003] In accordance with a first aspect of the present invention, a DC power switching unit comprises a first terminal and a second terminal; a sub-unit comprising a current limiter and a diode, wherein the sub-unit is connected to a first side of a semiconductor device, the power switching unit further comprising a snubber circuit connected to a second side of the semiconductor device; wherein the power switching unit comprises two symmetrical sub-units, each sub-unit being connected on one side to one of the first and second power switching unit terminals and on the other side to the semiconductor device.
[0004] The sub-units and semiconductor devices are electrically coupled between the first terminal and the second terminal to control current flow between the first terminal and the second terminal.
[0006] Preferably, the snubber circuit is connected between the outputs of the semiconductor device and a negative pole.
[0007] Preferably, the snubber circuit further comprises a break circuit, comprising break resistor and semiconductor device in series, the break circuit being connected in parallel across the snubber circuit.
[0008] This provides energy dissipation of reverse power in the DC power distribution system in which the power switching unit is being used.
[0013] Preferably, the snubber circuit comprises a resistor and capacitor in series, or a capacitor and diode in series, the capacitor having a resistor in parallel.
[0015] In accordance with a second aspect of the present invention, a DC power distribution system comprises first and second DC power distribution bus sections and a DC power switching unit according to the first aspect, wherein a first terminal of the assembly is electrically coupled to the first bus section of the power distribution bus and the second terminal is electrically coupled to the second bus section of the power distribution bus.
[0016] Preferably, the voltage at one side of the power switching unit is greater than or equal to 1KV.
[0017] Preferably, the voltage at one side of the power switching unit is within the range 1KV to 15KV.
[0018] An example of a DC power switching unit according to the present invention will now be described with reference to the accompany drawings in which:
Figure 1 illustrates an example of circuitry for a low voltage diesel electric propulsion system;
Figure 2 illustrates a first example of a power switching unit according to the invention;
Figure 3 is an electrical equivalent circuit for the power switching assembly of Fig.2;
Figure 4 illustrates a second example of a power switching unit according to the invention;
Figure 5 illustrates current and voltage curves against time, for a first embodiment of the example of Fig.4;
Figure 6 illustrates current and voltage curves against time, for a second embodiment of the example of Fig.4;
Figure 7 illustrates a third example of a power switching unit according to the invention;
Figure 8 illustrates a forth example of a power switching unit according to the invention;
Figure 9 is a flow diagram of a method of operation of a power switching unit according to the invention.
[0019] DC power distribution systems on offshore vessels, or platforms, or remote drilling rigs, or HVDC connections, typically comprise a power source such as a prime mover, a generator, or an energy store, together with DC bus sections which are joined by a bus tie switch. In order to meet regulatory requirements for safe operation, the bus tie switch must be able to disconnect the DC bus sections from one another to prevent a fault on one side of the system from propagating to the other side and potentially losing all power to critical systems, such as thrusters or essential parts of the drilling equipment.
[0020] Typically, a bus tie switch or breaker function is provided for AC distribution (high and low voltage) solutions with mechanical breakers or low voltage DC distribution systems, typically up to 1000V DC, such as that shown in Fig.1. Currently, there are limited options available for high voltage static DC switches for use in marine and offshore systems. High voltage equivalents, typically for operation at 15kV are not available. Where two independently operating power systems are required to meet classification standards, there is a need for very fast disconnection of DC bus tie connections to prevent faults from propagating from one side to another. Existing low voltage bus tie switches are not able to operate at voltages above 1000V and certainly not for voltage in the region of 10kV to 15kV, or higher.
[0021] The example of Fig. 1 is a diesel electric propulsion system based on low voltage DC distribution and comprises a plurality of diesel engines 1, each connected to a generator G1, G2, G3, G4 within respective generator protections systems P1, P2, P3, P4. The generator protection systems include a generator cubicle K1, K2, K3, K4 incorporating generator control 2. Each generator is coupled to DC main switchboard S1, S2 via line 3 which includes a diode 4 and isolation switch 5. Generators G1 and G2 are coupled to switchboard S1. Generators G3 and G4 are coupled to switchboard S2. From each of the switchboard S1, S2, switches 6 and fuses 7 are provided in lines 8 to inverters 9 between the DC main switchboard and motors 10, or to a shaft generator with motor function, which is coupled to AC auxiliary switchboard A1, A2 via filter 11 and transformer 12. In addition, the DC main switchboard S1, S2 supplies a battery 19 through a DC to DC converter 20. The AC auxiliary switchboard is coupled via bypass 13 and isolation switches 14. The DC main switchboard is connected via bus tie switch 15 comprising an isolation switch 16 and transistor diode arrangement 17 at each side of a di/dt reactor 18. The example of Fig.1 may be adapted for high voltage DC distribution by replacing the bus tie switch 15 with a power switching assembly 22 as described hereinafter.
[0022] Fig.2 illustrates a first example of a power switching unit for a power switching assembly according to the present invention, in particular one that is suitable for use in high voltage DC systems. The bus tie switch 15 of Fig.1 may be replaced by a power switching unit, or circuit breaker, as shown in any of Figs 2, 4 or 5 and described in more detail hereinafter. The power switching units of the present invention reduce cost and complexity.
[0023] In the example of Fig.2, a DC bus tie switch unit comprises a pair of semiconductor devices Q1, Q2, such as diodes or transistors, or a combination thereof, one device of which is coupled to a side 1 positive terminal 24 by a first current limiter L1 and the other to a side 2 positive terminal 25, by a second current limiter L2. Each semiconductor device Q1, Q2 may comprise a plurality of series connected transistors to build up to a required voltage level. To enable voltage sharing across transistors during switching, a parallel balancing circuit may be provided for the transistors. A diode D1, D2 connects on one side to the same input/output terminal 31 of the semiconductor device Q1, Q2 as the current limiter L1, L2 connects to and on the other side connects to the negative pole 22, 23. The diode D1, D2 may comprise a plurality of series connected diodes to build up to a required voltage level. An energy absorbing circuit 30, or snubber, may provide damping, or overvoltage protection. Typically, the snubber comprises a combination of capacitor, diode and resistor, for example a resistor and capacitor in series, or a capacitor and diode in series, the capacitor having a resistor in parallel, but other combinations may be used. The snubber 30 connects between the positive and negative poles within the switch unit 21 between the input/output terminal 32 of each semiconductor device Q1, Q2 on the opposite side of the semiconductor device to the input/output terminal 31 of the semiconductor device which is connected to the current limiter L1, L2.
[0024] Fig.3 illustrates a typical electrical equivalent for a failure on one of the sides of the bus tie switch 21. In the example shown, the failure is on side 2, but the bus tie switch unit works in both directions, for whichever side the failure occurs on. A large capacitor or voltage source 26, 27 is represented on side 1 and side 2. Between this and the positive and negative pole terminals 22, 23, 24, 25 of the switch unit are stray inductances 28, 29. The semiconductor devices Q1, Q2 conduct current in the switch unit 21 when the device is in the ON state. If a short circuit occurs on one side of the unit, the inductance L1, L2 limits the short circuit rate of change of current, di/dt. When the breaker, the transistor that blocks the current, opens, the semiconductor device Q1, Q2 turns off and the current through inductor L1, L2 is diverted through the snubber circuit 30 instead.
[0025] In a second example, shown in Fig.4, the bus tie switch unit is provided with the same arrangement of semiconductor device, current limiter and diode on each side as in the examples of Figs.2 and 3. The DC power switching unit 21 comprises a pair of semiconductor devices Q1, Q2, such as diodes or transistors, or a combination thereof, one device of which is coupled to a side 1 positive terminal 24 by a first current limiter L1 and the other to a side 2 positive terminal 25, by a second current limiter L2. A diode D1, D2 connects on one side to the same input/output terminal 31 of the semiconductor device Q1, Q2 as the current limiter L1, L2 connects to and on the other side connects to the negative pole 22, 23. However, in this example, the central snubber circuit 30 is modified by the addition of a break chopper circuit, comprising a resistor R1 in series with a semiconductor device Q3, such as a transistor, provided in parallel with the capacitor or snubber circuit 30, as shown in Figure 4. The resistor R1 bums up power from the DC circuit, The provision of energy breaking devices for burning up reverse power that increases the DC voltage is a common requirement for offshore DC grids and can be met by modifying the circuitry of the power switching unit in this way. Optionally, a resistor R2 may be provided in series with the capacitor or snubber to the positive side.
[0026] Fig.5 illustrates the change of voltage VR1 33 across resistor R1 in the break circuit with time, in response to a short circuit for an inductance L1 of 100µH and change of current IL1 34 through inductance L1. In normal operation there is a current IL1 of about 1KA flowing through L1 and a voltage VR1 of about 5KV across resistor R1. Upon occurrence of a short circuit at time 1 ms, the voltage VR1 drops instantaneously to about 3KV, then climbs steeply and oscillates about 6KV for about 0.2ms before gradually returning to a steady 5KV at time 1.9ms. The current IL1 drops more gradually over 0.2ms to - 0.2KA before climbing back to zero, with some oscillation and eventually settling.
[0027] Fig.6 illustrates the change of voltage 35 across resistor R1 in the break circuit with time, in response to a short circuit for an inductance L1 of 10mH. In normal operation, as with the Fig.5 example, there is a current IL1 36 of about 1KA flowing through L1 and a voltage VR1 35 of about 5KV across resistor R1. However, with L1 equal to 10mH, upon occurrence of a short circuit at time 1 ms, the voltage VR1 drops instantaneously to about 3KV, then climbs steeply and oscillates very quickly about 6KV for a far longer period, as much as 10ms before the oscillations only slowly drop towards 5KV. Even after 20ms, the voltage 35 has not returned to its steady state. The drop in current IL1 takes about 10 ms and even after another 10ms, the current 36 has not settled at zero, but continues to oscillate. Although the switch-off procedure in Fig. 6 takes much longer, because the larger inductance of 10mH instead of 100uH contains much more energy at the same current level, the energy is safely dissipated, without causing higher overvoltage than for small inductances, small energy values. Dissipation of the higher energy in the chopper takes more time at the same power level. The decay oscillation at the end of the chopper operation is not harmful. The damping is defined by the resistance of the network and the residual energy, which is higher for the large inductance.
[0028] Fig.7 illustrates another alternative. As with the previous examples, the DC power switching unit 21 comprises a pair of semiconductor devices Q1, Q2, such as diodes or transistors, or a combination thereof, one device of which is coupled to a side 1 positive terminal 24 by a first current limiter L1 and the other to a side 2 positive terminal 25, by a second current limiter L2. A diode D1, D2 connects on one side to the same input/output terminal 31 of the semiconductor device Q1, Q2 as the current limiter L1, L2 connects to and on the other side connects to the negative pole 22, 23. In this example, the damper circuit is not common to the two semiconductor devices, but each semiconductor device Q1, Q2 is provided with its own damper 30a, 30b across the semiconductor device.
[0029] An further alternative arrangement, shown in Fig.8, shares a common current limiter L between the two semiconductor devices Q1, Q2 of the power switching unit 21. The current limiter L is connected to one side of each device Q1, Q2. For each semiconductor device, a diode circuit 40a, 40b is connected to the same side of the semiconductor device as the current limiter. Snubbers 30 are connected to the other side of the semiconductor devices.
[0030] Fig.9 is a flow diagram showing an example of a method of operation of a DC power switching assembly according to the invention. A load may be connected 50 to the circuit and the bus tie switches of the power switching unit are set to be closed 51, for normal operation. A system controller (not shown) monitors 52 system voltage and current during operation. Upon detection 53 of a short circuit on one side of the power switching unit 21, the controller causes the breakers to open 54 and opens 55 the transistor diode circuitry to block current, or voltage. After the cause of the short circuit has been removed 56 and cleared, the system may close 57 the breaker again, closing the transistor diode circuit to allow current or voltage again and normal operation is resumed 58, with the system controller continuing to monitor 52 current and voltage to determine if another short circuit occurs.
[0031] The present invention provides a number of embodiments of a static DC circuit breaker including an over voltage protective device. This static DC circuit breaker may be used for any DC voltage level based on generally available components and in particular may be used at high voltage levels, i.e. above 1000V, in the range 1KV to 15KV, or even above 15KV.
1. A DC power switching unit comprising a first terminal and a second terminal; a sub-unit
comprising a current limiter and a diode, wherein the sub-unit is connected to a first
side of a semiconductor device, the power switching unit further comprising a snubber
circuit connected to a second side of the semiconductor device; wherein the power
switching unit comprises two symmetrical sub-units, each sub-unit being connected
on one side to one of the first and second power switching unit terminals and on the
other side to the semiconductor device.
2. A unit according to claim 1, wherein the snubber circuit is connected in parallel
across the semiconductor device.
3. A unit according to claim 1, wherein the snubber circuit is connected between the
outputs of the semiconductor device and a negative pole.
4. A unit according to any preceding claim, wherein the snubber circuit further comprises
a break circuit, comprising break resistor and semiconductor device in series, the
break circuit being connected in parallel across the snubber circuit.
5. A unit according to any preceding claim, wherein each of the sub-units further comprise
a resistor in series with the snubber.
6. A unit according to any preceding claim, wherein the semiconductor device comprises
one of a diode, or a transistor.
7. A unit according to any preceding claim, wherein the semiconductor device comprises
an insulated gate bipolar transistor.
8. A unit according to any preceding claim, wherein the snubber circuit comprises at
least one of a capacitor, diode or resistor.
9. A unit according to any preceding claim, wherein the snubber circuit comprises a resistor
and capacitor in series; or a capacitor and diode in series, the capacitor having
a resistor in parallel.
10. A unit according to any preceding claim, wherein the pair of sub-units share a common
current limiter.
11. A DC power distribution system comprising first and second DC power distribution bus
sections and a DC power switching unit according to any preceding claim, wherein a
first terminal of the assembly is electrically coupled to the first bus section of
the power distribution bus and the second terminal is electrically coupled to the
second bus section of the power distribution bus.
12. A system according to claim 11, wherein the voltage at one side of the power switching
assembly is greater than or equal to 1KV.
13. A system according to claim 12, wherein the voltage at one side of the power switching
assembly is within the range 1KV to 15KV.