Arrangement and method for connecting and/or disconnecting a plurality of dc sub-networks

Abstract

An arrangement for connecting and disconnecting DC networks includes a main bus for connecting a plurality of DC lines with each other. First and second DC lines are connected between first and second DC networks and the main bus via respective main switches. The DC networks contain a DC operating equipment, such as a converter, an energy storage device, a DC chopper, a DC cable, an energy source and/or a load. A transfer bus that is electrically connected to the main bus has a transfer bus disconnector for disconnecting the main bus from the transfer bus, a transfer switch, and a current limiting device with a resistor and a parallel resistor bypass switch. The first DC line is connected to the transfer bus via a first transfer disconnector and the second DC line is connected to the transfer bus via a second transfer disconnector.

Claims

1. An arrangement for selectively connecting and disconnecting a plurality of DC networks to one another, the arrangement comprising: a main bus for connecting a plurality of DC lines with each other; a first DC line for connection to a first DC network having DC operating equipment, and a first main switch connecting said first DC line to said main bus; a second DC line for connection to a second DC network having DC operating equipment, and a second main switch connecting said second DC line to said main bus; a transfer bus electrically connected to said main bus, said transfer bus including: a transfer bus disconnector for disconnecting said main bus from said transfer bus; a transfer switch; a current limiting device having a resistor and a parallel resistor bypass switch for bypassing said resistor; a first transfer disconnector connected between said first DC line and said transfer bus; and a second transfer disconnector connected between said second DC line and said transfer bus.

2. The arrangement according to claim 1, wherein said current limiting device further comprises an inductance.

3. The arrangement according to claim 2, wherein said current limiting device further comprises a parallel inductance bypass switch for bypassing said inductance.

4. The arrangement according to claim 1, further comprising a discharge switch connected between said transfer bus and ground potential.

5. The arrangement according to claim 1, wherein said transfer bus further comprises at least one of a current sensing device or a voltage sensing device.

6. The arrangement according to claim 1, wherein said transfer switch is a mechanical switch.

7. The arrangement according to claim 1, wherein said resistor is an actively cooled resistor.

8. The arrangement according to claim 7, wherein said resistor is a resistor cooled by a cooling fluid.

9. The arrangement according to claim 1, wherein said transfer bus further comprises a converter for actively damping voltage fluctuations.

10. A method for connecting a first DC sub-network to a second DC sub-network, the method comprising: providing an arrangement according to claim 1 with the first main switch closed; closing the second transfer disconnector and the transfer bus disconnector; closing the transfer switch; closing the resistor bypass switch; closing the second main switch; opening the transfer switch; opening the second transfer disconnector; opening the resistor bypass switch; and opening the transfer bus disconnector.

11. The method according to claim 10, wherein at least one of the first DC sub-network or the second DC sub-network comprises actively controllable operating equipment and a first DC line current is controlled to a level below 100 A or to a level below 50 A or to zero.

12. The method according to claim 11, wherein the actively controllable operating equipment is a converter.

13. A method for disconnecting a second DC sub-network from a first DC sub-network, the method comprising: providing an arrangement according to claim 1 with the first main switch and the second main switch closed; closing the second transfer disconnector; closing the transfer bus disconnector; closing the transfer switch; closing the resistor bypass switch; commutating the current to the transfer bus by opening the second main switch; opening the resistor bypass switch; opening the transfer switch; and opening the transfer bus disconnector and the second transfer disconnector.

14. The method according to claim 10, wherein at least one of the first DC sub-network or the second DC sub-network comprises actively controllable operating equipment and a first DC line current is controlled to a level below 100 A or to a level below 50 A or to zero.

15. The method according to claim 11, wherein the actively controllable operating equipment is a converter.

16. A method for discharging a DC line of a DC sub-network, the method comprising: providing an arrangement according to claim 1 with the first main switch closed; closing the first transfer disconnector; closing the discharge switch; closing the transfer switch; closing the resistor bypass switch; and opening the transfer bus disconnector and the first transfer disconnector.

17. The method according to claim 10, wherein at least one of the first DC sub-network or the second DC sub-network comprises actively controllable operating equipment and a first DC line current is controlled to a level below 100 A or to a level below 50 A or to zero.

18. The method according to claim 11, wherein the actively controllable operating equipment is a converter.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic view of a first embodiment of the arrangement according to the invention; and

(2) FIGS. 2-4 are schematic views of different variants of the arrangement according to the invention.

(3) Identical or functionally similar elements and components are identified with the same reference numerals throughout the figures.

DETAILED DESCRIPTION OF THE INVENTION

(4) Referring now to the figures of the drawing in detail and first, in particular, to FIG. 1 thereof, there is shown an arrangement 1 for connecting or disconnecting a plurality of DC networks to each other. In the example of FIG. 1 the plurality of DC networks comprises a first DC network 4, a second DC network 5 and a third DC network 15. Any number of additional DC networks can also be connected in this way as it is indicated by the dotted line 7 in FIG. 1.

(5) Each of the first DC network 4 and the second DC network 5 comprise several DC lines and DC operating equipment, such as a converter, an energy storage device, a DC chopper, a DC cable, a DC overhead line, an energy source and/or a load. These are indicated in FIG. 1 by a cloud symbol. Since the invention is basically independent of the operating equipment present in the networks, it is not described here in more detail. The third DC network 15 comprises an AC/DC converter 6 which on its AC side is connected to an AC network (i.e., a non-illustrated AC grid). It is worthwhile mentioning that the figure shows the installation for one of the DC poles. Similar arrangements are suitably provided for each of the DC poles (positive, negative and/or neutral).

(6) The arrangement 1 comprises a main bus 2 for connecting a plurality of DC lines, namely a first DC line 8, a second DC line 9 and a third DC line 16, with each other. The first DC line 8 connects the main bus 2 with the first DC network 4 via a first main switch MD.sub.1. The second DC line 9 connects the main bus 2 with the second DC network 5 via a second main switch MD.sub.n. The third DC line 16 connects the main bus 2 with the converter 6 via a third main switch MD.sub.n+1. The first DC line 8 further comprises grounding equipment 10 and a current measurement facility 13. The other DC lines 9, 16 are equipped accordingly. The main bus 2 also comprises a voltage sensing device 12.

(7) The arrangement 1 further comprises a transfer bus 3 that is electrically connected to the main bus 2. The transfer bus 3 comprises a transfer bus disconnector DS.sub.T for electrically disconnecting the main bus 2 from the transfer bus 3, a transfer switch T.sub.S which is a fast mechanical switch and a current limiting device having a resistor R.sub.T and a parallel resistor bypass switch R.sub.BS to bypass said resistor R.sub.T. The transfer bus 3 further comprises a discharge switch DS.sub.D connected between the transfer bus 3 and ground potential, an inductance L.sub.T, a current sensing device 14 and a voltage sensing device 11. The first DC line 8 is connected to the transfer bus 3 via a first transfer disconnector DS.sub.TL1, the second DC line 9 is connected to the transfer bus 3 via a second transfer disconnector DS.sub.TLn and the third DC line 16 is connected to the transfer bus 3 via a further transfer bus disconnector DS.sub.TLn+1.

(8) FIG. 2 shows an arrangement 100 for connecting or disconnecting a plurality of DC networks to each other (three DC networks, namely a first DC network 4, a second DC network 5 and are third DC network 15 are explicitly shown in FIG. 2).

(9) Functionally identical and similar elements in the arrangements 1 and 100 are identified with the same reference numerals. Same applies to the arrangements 101 and 102

(10) Generally, the arrangements 1 and 100 are functionally similar. To avoid repetitions, only the differences between the embodiments will be described in detail. Same applies also to the embodiments shown in the following FIGS. 3 and 4.

(11) In contrast with the arrangement 1 of FIG. 1, the transfer bus 3 of the arrangement 100 does not have a discharge switch. The discharging is instead achieved via other discharging equipment that is provided, for example, within the DC networks.

(12) FIG. 3 shows an arrangement 101 for connecting or disconnecting a plurality of DC networks to each other (three DC networks, namely a first DC network 4, a second DC network 5 and are third DC network 15 are explicitly shown in the figure).

(13) In contrast with the arrangement 1 of FIG. 1, the transfer bus 3 of the arrangement 101 does not have an additional inductance. This allows an even more compact design of the arrangement 101.

(14) FIG. 4 shows an arrangement 102 for connecting or disconnecting a plurality of DC networks to each other (three DC networks, namely a first DC network 4, a second DC network 5 and are third DC network 15 are explicitly shown in FIG. 2).

(15) In contrast with the arrangement 1 of FIG. 1, the transfer bus 3 of the arrangement 102 comprises an inductance L.sub.T and a inductance bypass switch LBS arranged in parallel to the inductance L.sub.T. Bypassing the inductance L.sub.T can in certain situations reduce the stress on the main bus disconnectors.

(16) With reference to FIG. 1, different examples of a method for connecting/disconnecting two DC sub-networks will be described below.

(17) In a basic state of the arrangement 1, the transfer bus disconnector DS.sub.T is open (non-conducting). Also, all main switches (in this embodiment the main switches are given the first, second and third main disconnector) DS.sub.TL1DS.sub.Tn+1 are open, so that all DC sub-networks are galvanically disconnected from the main bus 2. In its basic state, the arrangement 1 can be serviced independently of the status of the sub-networks 4, 5, 15. In particular, it can be serviced while the operating equipment in any of the sub-networks 4, 5, 15 is active/in operation.

Example 1: Connect Discharged Converter to Energized HVDC Grid with Peak Current Suppression

(18) The converter 6 is initially grounded via the corresponding grounding device on its DC side. The first DC sub-network 4 is connected with the main bus 2 via the first main disconnector MD.sub.1. In a first step, the grounding device of the converter 6 is opened, the transfer bus disconnector DS.sub.T and the third transfer disconnector DS.sub.TLn+1 is closed (switched on). In a second step, the transfer switched T.sub.S is closed, while the resistor bypass switch R.sub.BS is still open. The resistor R.sub.T bears the voltage difference between the main bus 2 and the DC side of the converter 6 and limits the current. If the converter is not yet or only partly charged (in case the converter is a modular multilevel converter with switching modules each comprising a separate energy storage, then the energy storages are being charged), then the resistor R.sub.T limits the charging currents in the transfer bus 3 and the converter 6. Subsequently, the resistor bypass switch R.sub.BS is closed to establish direct connection with the converter 6. The inductance L.sub.T limits transient compensating currents which may occur in the arrangement. Such compensating currents particularly occur in case of sudden changes at the switching equipment during switching operations. This might particularly occur in the case of sudden changes in the power flow or during fault events. Meanwhile, the converter 6 is further charged. The time duration between the closing of the transfer switch T.sub.S and the closing of the resistor bypass switch R.sub.BS basically defines the amount of energy provided for charging of the converter 6. By closing of the third main disconnector MD.sub.n+1 the converter 6 is connected to the main bus 2, thus establishing a parallel connection between the first sub-network 4 and the converter 6. Subsequently, the transfer switch T.sub.S, the bypass switch R.sub.BS and the transfer bus disconnector DS.sub.T as well as the third transfer disconnector DS.sub.TLn+1 are re-opened. The arrangement returns to its basic state and is ready for the next connecting or disconnecting action.

Example 2: Connect Energized Converter to Energized HVDC Grid with Peak Current Suppression

(19) The initial configuration is similar to the one in example 1. However, in this example 2, the converter 6 is energized and in operation but at a different DC voltage level than the first sub-network 4. The first DC sub-network 4 is connected to the main bus via the first main disconnector MD.sub.1. Whether the second sub-network 5 is connected to the main bus 2 or not is not relevant. In a first step, the transfer bus disconnector DS.sub.T and the third transfer disconnector DS.sub.TLn+1 are both closed (switched on). The next step requires the voltage difference between the main bus 2 and the converter 6 to be as small as possible. The voltage at the main bus 2 is measured by the voltage sensing device 12 and the voltage at the transfer bus 3 is measured by the voltage sensing device 11 in order to level both voltages with the help of appropriate control of converter 6. In a second step, the transfer switched T.sub.S is closed, while the resistor bypass switch R.sub.BS is still open. The resistor R.sub.T then bears the remaining voltage difference between the main bus 2 and the DC side of the converter 6 and limits the corresponding current. The transfer bus current is measured by the current sensing device 14. The measured currents and voltages are transferred to the corresponding converter control to control the DC side converter voltage, particularly to adjust said DC side voltage to equal the voltage at the DC node (i.e., the main bus 2) with the goal of a zero current in the transfer bus 3.

(20) Subsequently, the resistor bypass switch R.sub.BS is closed to establish direct connection with the converter 6. The inductance L.sub.T limits transient compensating currents which may occur in the arrangement. Such compensating currents particularly occur in case of sudden changes at the switching equipment during switching operations. This might particularly occur in the case of sudden changes in the power flow or during fault events. By the closing of the third main disconnector MD.sub.n+1 the converter 6 is connected to the main bus 2 establishing a parallel connection between the first sub-network 4 and the converter 6. Subsequently, the transfer switch T.sub.S, the bypass switch R.sub.BS and the transfer bus disconnector DS.sub.T as well as the third transfer disconnector DS.sub.TLn+1 are opened again. The arrangement returns to its basic state and is ready for the next connecting or disconnecting action.

Example 3: Connect Discharged HVDC Line (Cable) to Energized HVDC Grid with Peak Current Suppression

(21) In the initial state the first main disconnector MD.sub.1 is closed. The second DC line 9 is not grounded. In a first step the second transfer disconnector DS.sub.TLn and the transfer bus disconnector DS.sub.T are closed. In a second step the transfer switch T.sub.S is closed. In a third step the resistor bypass switch R.sub.BS is closed. In a fourth step the second main disconnector MD.sub.n is closed. Subsequently, the transfer switch T.sub.S, the bypass switch R.sub.BS and the transfer bus disconnector DS.sub.T as well as the third transfer disconnector DS.sub.TLn+1 are opened again. The arrangement returns to its basic state and is ready for the next connecting or disconnecting action.

Example 4: Connect Energized HVDC Line (Cable) to Energized HVDC Grid with Peak Current Suppression

(22) In the initial state the first main disconnector MD.sub.1 is closed. The second DC line 9 is energized but at a different voltage level than the main bus voltage. In a first step the second transfer disconnector DS.sub.TLn and the transfer bus disconnector DS.sub.T are closed. For the next step it is advantageous to have the voltage difference between the main bus 2 and the DC line 9 as small as possible. The voltage at the main bus 2 is measured by the voltage sensing device 12 and the voltage at the transfer bus 3 is measured by the voltage sensing device 11 to level both voltages with the help of appropriate control of converter 6. In a second step the transfer switch T.sub.S is closed. The remaining voltage difference between the two DC networks 4 and 5 is now present at the resistor R.sub.T. The transfer bus current is measured by the current sensing device 14. The measured currents and voltages are transferred to the corresponding converter control to control the DC main bus voltage, particularly to adjust said DC line voltage to equal the voltage at the DC node (i.e., the main bus 2) to achieve zero current in the transfer bus 3. After the voltages of the networks are sufficiently synchronized, i.e., the transfer bus current is zero or close to zero, in a third step the resistor bypass switch R.sub.BS is closed. In a fourth step the second main disconnector MD.sub.n is closed. Subsequently, the transfer switch T.sub.S, the bypass switch R.sub.BS and the transfer bus disconnector DS.sub.T as well as the second transfer disconnector DS.sub.TLn are opened again. The arrangement returns to its basic state and is ready for the next connecting or disconnecting action.

Example 5: Disconnect Converter from Energized HVDC Grid

(23) In an initial state the first and third main disconnector MD.sub.1, MD.sub.n+1 are both closed. The converter 6 and the DC grid 4 are both energized and in operation. First, the transfer bus disconnector DS.sub.T, the transfer switch T.sub.S, the resistor bypass switch R.sub.BS and the third transfer disconnector DS.sub.TLn+1 are closed to establish a parallel connection between the converter 6 and the DC grid 4. Then, by opening the third main disconnector MD.sub.n+1 the current is commutated to the transfer bus 3. The converter current is controlled to a value close to zero (ideally equal to zero). Then, the current is commutated to the resistor R.sub.T by opening the resistor bypass switch R.sub.BS to further limit the current. Immediately after opening the bypass switch, the transfer switch T.sub.S is opened. The converter 6 is thus disconnected from the first sub-network 4. The arrangement returns to its basic state by opening the transfer bus disconnector DS.sub.T and the third transfer disconnector DS.sub.TLn+1.

Example 6: Disconnect Charged HVDC Line from Energized DC Grid

(24) In an initial state the first and second main disconnector MD.sub.1, MD.sub.n are both closed. The DC grids (sub-networks) 4 and 5 are both energized and in operation. First, the transfer bus disconnector DS.sub.T, the transfer switch T.sub.S, the resistor bypass switch R.sub.BS and the second transfer disconnector DS.sub.TLn are closed to establish a parallel connection between the first and second DC grid 4 and 5. Then, by opening the second main disconnector MD.sub.n the current is commutated to the transfer bus 3. The current in the arrangement 1 is controlled to a value close to zero (ideally equal to zero). Then, the current is commutated to the resistor R.sub.T by opening the resistor bypass switch R.sub.BS to further limit the current. Immediately after opening the bypass switch, the transfer switch T.sub.S is opened. The second DC sub-network 5 is now cut off the first sub-network 4. The arrangement returns to its basic state by opening the transfer bus disconnector DS.sub.T and the second transfer disconnector DS.sub.TLn.

Example 7: Discharge and Ground HVDC Line (HVDC Cable) with Peak Current Suppression

(25) The arrangement 1 can further be applied for discharging a partially charged second DC line 9. Initially, the main disconnector MD.sub.n is open. Also, the transfer bus disconnector DS.sub.T and the resistor bypass switch R.sub.BS are open. To prepare the discharging, the discharge switch DS.sub.D is closed. The second DC line 9 is connected to the arrangement 1 by closing the second transfer disconnector DS.sub.TLn. In a following step the transfer switch T.sub.S is closed to start the discharging of the second DC line 9. As soon as the discharge current decreases below a predefined threshold, the resistor bypass switch R.sub.BS is closed. The inductance L.sub.T limits the current change rate. Afterwards, the DC line 9 can be grounded by its own grounding device or devices. Finally, the second transfer disconnector DS.sub.TLn and the transfer bus disconnector DS.sub.T are reopened and the arrangement 1 returns to its basic state.

(26) Various acronyms appear in the above description, including: DC=direct current, d.c.; AC=alternating current, a.c.; PV=photovoltaic; HV=high voltage; HVDC=high voltage, direct current. The terms grid and network are used interchangeably.