SE1300779A1 - En Hybridcell för en spänningsstyrd omvandlare - Google Patents

Abstract

Uppfinningen hänför sig till en spänningsstyrd omvandlare innefattande celler och en cell i en sådan spänningsstyrd omvandlare. Cellen innefattar en första och en andra anslutningsterminal (TE1, TE2), vardera erbjudande en koppling för cellen till ett fasben i omvandlaren, ett första energilagringselement (Cl) tillhandahållande en spänning, en första sträng med seriekopplade switchenheter (SI, S2), där den första strängen är kopplad parallellt med. energilagringselementet, där switchelementen är. anordnade att selektivt koppla energilagringselementet (Cl) mellan de första och andra anslutningsterminalerna (TE1, TE2), och en andra sträng kopplad parallellt med den första strängen, där den andra strängen innefattar en ytterligare switchenhet (FS) tillsammans med ett spänningsavgivande element (VPE).Fig. 5

Description

15 20 25 30 phase legs PL1, PL2 and PL3 in order to enable connection to a three-phase AC transmission system. It should however be realized that as an alternative there may be for instance two phase legs or even one phase leg. Each phase leg PL1, PL2, PL3 has a first and second end point. In a converter of the type depicted in fig. 1 the first end points of all the phase legs PL1, PL2 and PL3 are connected to a first DC terminal DC+ while the second end points are connected to a second DC terminal DC-.

Each phase leg PL1, PL2, PL3 of this first type of voltage source converter 10 further includes a lower and upper phase leg half and at the junction where the halves of a leg meet, there is provided an AC terminal.

In the exemplifying voltage source converter 10 there is here a first phase leg PL1 having an upper half and a lower half, a second phase leg PL2 having an upper half and a lower half and a third phase leg PL3 having an upper half and a lower half. At the junction between the upper and lower halves of the first phase leg PL1 there is provided a first AC terminal ACl, at the junction between the upper and lower halves of the second phase leg PL2 there is provided a second AC terminal AC2 and at the junction between the upper and lower halves of the third phase leg PL3 there is provided a third AC terminal AC3. Each AC terminal ACl, AC2, AC3 is here connected to the corresponding phase leg via a respective inductor LACl, LAC2, LAC3. Here each half furthermore includes one current limiting inductor Lul, Lu2, Lu3, Lll, Ll2, and Ll3 connected to the corresponding DC terminal DC+ and DC-. 10 15 20 25 30 It should be realized that the phase inductors can be split in half, where one half is provided in one phase leg half and the other in the other phase leg half of a phase leg. It is also possible to completely omit the phase inductors and/or the current limiting inductors described above.

The phase legs all comprise cells that are used for forming the AC voltages. The cells are typically connected in series or in cascade in the phase legs.

In the present example there are three cells in each phase leg half. Thus the upper half of the first phase leg PL1 includes three cells Clul, C2ul, C3u1, while the lower half of the first phase leg PL1 includes three cells C111, C211, and C311. In a similar fashion the upper half of the second phase leg PL2 includes three cells C1u2, C2u2, C3u2, while the lower half of the second phase leg PL2 includes three cells C112, C212, and C312. Finally the upper half of the third phase leg PL3 includes three cells Clu3, C2u3, C3u3, while the lower half of the third phase leg PL3 includes three cells C113, C213, and C313. The numbers are here only chosen for exemplifying a possible layout of a voltage source converter. It is often favorable to have many more cells in each phase leg, especially in HVDC applications. It can also be seen that the cells of a phase leg are with advantage provided symmetrically around the AC terminal.

There is furthermore a control unit 12 which is set to control the cells. Control of each cell in a phase leg half is normally done through providing the cell with 10 15 20 25 30 control signals that control the contribution of that cell to an AC waveform provided by converter 10. The cells are also controlled for handling fault currents.

Here the common control unit 12 controls the cells for converting AC power to DC power or vice versa. The cells further provide a voltage based on energy stored in energy storage elements.

In the exemplifying converter 10 the cells in the upper half of a phase leg, such as the cells Clul, C2u1 and C3ul of the upper half of the first phase leg PL1, are typically controlled so that they provide a DC component corresponding to a positive DC voltage of the first DC terminal DC+ and an AC component corresponding to the full AC voltage of a corresponding AC terminal AC1, AC2 or AC3, while the cells of the corresponding lower half of the phase leg, such as the cells C111, C211 and C311 of the first phase leg PL1, typically provide a DC component corresponding to a negative DC voltage of the second DC terminal DC- and an AC component corresponding to the full AC voltage of the corresponding AC terminal AC1, AC2 or AC3. The instantaneous AC voltage values provided by the cells on opposite sides of an AC terminal of a phase leg here typically have opposite polarities.

The converter 10 may here be operated in two directions. If three-phase AC voltages are applied on the AC terminals AC1, AC2 and AC3 a DC voltage is generated, while if a DC voltage is applied between the DC terminals DC+ and DC-, a three-phase AC voltage is generated on terminals AC1, AC2 and AC3. The control 10 15 20 25 30 furthermore typically involves generating control signals by the control unit 12 in known fashion based on PWM modulation.

Figure 2 shows another simplified schematic of the converter from fig. 1, which is also termed a M2LC converter. The main advantages of this converter are modularity of using series connection of cells and lower cost and loss. However, having large storage energy within the distributed cells in each arm of the converter is the drawback of this converter in DC side fault which will result in a high fault current through the antiparallel diodes and also cell capacitor overcharge.

Figure 2 presents M2LC with series connection of half- bridge cells (2 x HBC). Fig. 3 shows a blocking mode equivalent of the M2LC converter with half-bridge cells. As shown, an asymmetrical commutation through diode or capacitor in each arm depends on current direction. Therefore, half-bridge cells in one current direction can block a DC fault by inducing opposite voltage, while in other direction current follows through diodes. Therefore, an asymmetrical DC fault blocking capability only in one current direction (Ib current direction) is formed by half-bridge cells.

As a result: Before blocking of all IGBTs (this happens around 100 us) 0 dc fault current is driven by the inserted CTL caps voltage (roughly same for all phases). 10 15 20 25 30 0 Fault current rate of rise in each arm is not related to the phase angles of ac voltages. 0 All inserted CTL caps will be discharged.

After blocking of all IGBTs (after 100 ps) 0 DC fault current is driven by the rectified ac voltages. 0 Currents in both ac and dc sides rise rapidly.

Using 4-quadrant cell is one way of blocking DC fault by inserting reverse voltage polarity in MMC arm during the fault. However, this will add extra component and loss to the total system. Besides, it requires more voltage rating for 4-quadrant cells due to converter bus fault. Figure 4 shows various 4-quadrant Cell structures.

To solve this problem, 4-quadrant cells such as mixed cells or clamped-double cells are the good candidate to limit the fault current by inserting of reverse voltage polarity in converter arm; however, this will add extra cost and loss to the total converter station and may result in capacitor over charge in some cases such as converter bus fault.

As discussed earlier, DC fault current through valves of MMC converter is driven by rectified AC sources after blocking all switches. To be able to limit the DC fault current as well as breaking the current, the proposed invention proposes a VSC converter with fault blocking and limitation capability through the exploitation of a new cell structure as shown in Figure 10 15 20 25 30 5, which shows the proposed cell with auxiliary switch capacitor path in parallel with upper position.

As can be seen in fig. 5, the cell comprises a first energy storage element Cl, in the form of a capacitor which provides a voltage, a first string of series connected switching units S1, S2, where the first string is connected in parallel with the energy storage element and controllable by the control unit to selectively connect the energy storage element C1 between the first and second connection terminals TEl, TE2, where the first connection terminal is provided in the middle of the first string and the second terminal at a first end of the energy storage element, in this case a positive end. The first switching unit S1 is connected between the positive end of the energy storage element Cl and a midpoint of the first string.

Consequently the second switching unit S2 is connected between the midpoint and a negative end of the energy storage element. There is also a second string connected in parallel with the first switching unit of the first string, where the second string comprises a further switching unit FS together with a voltage providing element VPE, which in this example is a further capacitor. The voltage of the voltage providing element has a polarity that is opposite to the voltage of the first energy storage element.

The first switching unit may be able to conduct and block currents in both directions through the first string cell. Also the further switching unit is able to conduct and block currents in both directions through the second string. The second switching unit is however 10 15 20 25 30 a switching unit that is only able to block currents in one direction. The second switching unit may be realized as a transistor, for instance an Insulated Gate Bipolar Transistor (IGBT) with anti-parallel diode. Also the further switching unit may be a switching unit that is only able to block currents in one direction, such an IGBT with anti-parallel diode.

The type of switching unit that is able to conduct and block currents in both directions is typically named a four quadrant switching unit. It can be realized in a number of different ways. One realization is through a reverse blocking Insulated Gate Bipolar Transistors (RB-IGBT), i.e. through two anti-parallel transistors with the ability to block reverse voltages each in series with a diode. It is also possible with two series connected transistors, for instance IGBTs, with anti-parallel diodes, where these pairs have the opposite orientation in relation to each other. Another type may be through a diode bridge made up of two bridge legs connected in parallel in the second string, where a first leg comprises two diodes with current conducting directions towards each other and the other leg with two diodes with current conducting directions away from each other. A transistor is then connected between the midpoints of the two legs.

This cell consists: 1. An Alternate half-bridge cell configuration for DC fault blocking by having 4-quadrant switch FS in upper position in normal operation. As a result, this cell performs as a half-bridge cell in normal operation. 10 15 20 25 30 2. An auxiliary switched capacitor VPE in parallel with upper position of half-bridge cell to limit or block the DC fault in fault operation. As a result, this cell performs as a full-bridge in fault CaSe .

In normal operation the further switch FS is not active, it is turned off or blocked by the control unit, which is shown in fig. 6, where operation of the proposed cell for generation of a voltage O or -U is shown, where U is the voltage across the first energy storage element Cl. However, the further switch FS is operated in fault current handling mode. In this mode the first and second switches S1 and S2 are turned off or blocked. The further switch FS is operated. However, it may be operated differently based on fault current direction. If the fault current has a direction that is opposite to the current conduction direction of the anti-parallel diode of the second switch S2, i.e. if the fault current is blocked by the disabled second switch S2, then the further switch FS is on or conducting, while it may be turned off in case the fault current has the same direction as the current conduction direction of the anti-parallel diode of the second switch S2, i.e. if the fault current passes through the disabled second switching unit S2. This situation is schematically shown in fig. 7, which shows fault operation and generating reverse voltage polarity if needed +U or -U. 3. In the proposed cell structure, as the auxiliary capacitor VPE does not come in the current 10 15 20 25 30 10 flow path in normal conditions, its voltage will not deviate significantly over a large period of time.

Moreover, as MMC provides enough redundancies in switching combinations for a given output voltage level, the auxiliary capacitors may be charged to its reference value if required, once in several cycles, without disturbing the converter outputs. 4. The voltage providing element is not necessarily a capacitor. The auxiliary capacitor may be even replaced by a resistor, or an inductor, or a varistor or even energy storage such as a battery for that matter. So this cell may be configured as a distributed hybrid circuit breaker using the varistors in place of the auxiliary capacitor.

It is also possible to use the proposed cell with auxiliary switch capacitor path in parallel with the lower position, as shown in fig. 8. The first switching unit S1 is thus connected between the negative end of the energy storage element and the midpoint of the first string and the second switch S2 is connected between the positive end of the energy storage element and the midpoint of the first string. Fig. 9 shows (b) Normal operation and generation O or +U with this alternative placing, while fig. 10 shows fault operation and generating reverse voltage polarity if needed +U or -U.

Advantages of this kind of converter structure are multiple: 10 15 20 25 30 ll 0 Fault tolerant cell structure for any kind of cascaded converter 0 Lower number of components compared 0 On the basis of required voltage rating, lower number of devices in conduction path as the mixed half and full-bridge cells and clamped double cell due to having only one position 4-quadrant cell 0 Modular and simple cell design structure compared 0 Mixed connection of proposed cell and half-bridge (50%-50%) allows DC fault blocking. 0 As opposed to the full-bridge and other cell structures shown before in Figs. 4 and 5 where at- least two devices per cell conduct in normal operating mode, the number of devices conducting in the proposed cell configuration is only, which is the same as in the half-bridge configuration.

Therefore, the conduction loss using this cell remains limited.

As shown in fig. 11, the equivalent MMC circuit in Fault operation using the proposed cell, the converter is similar to full-bridge cell converter. This can limit as well as block the fault current. Also mixed series connection of proposed cell with half-bridge cells is possible and only 50% should be enough to block the fault completely.

The above described invention thus provides the following: 1. A cell for DC fault current limitation and blocking. 2. Retaining of the modularity of the structure in MMC. 10 12 3. Optionally a mixed connection of proposed new cell structure and conventional half-bridge cells (50%-50%) for allowing DC fault blocking. 4. Minimize number of components. 5. Minimize conduction loss. 6. The voltage providing element auxiliary capacitor may be even replaced by a resistor, or an inductor, or a varistor or even an energy storage for that matter.

So this cell may be configured as a distributed hybrid circuit breaker using the varistors in place of the auxiliary capacitor.

Claims (13)

10 15 20 25 30 13 CLAIMS

1. A cell (Clul) for connection in series with other cells in a phase leg (PL1, PL2, PL3) of a voltage source converter (10) and comprising: - a first and a second connection terminal (TE1, TE2) each providing a connection for the cell to the phase leg, - a first energy storage element (Cl) providing a voltage, - a first string of series connected switching units (S1, S2), said first string being connected in parallel with the energy storage element, the switching units being configured to selectively (Cl) connect the energy storage element between the first and second connection terminals (TE1, TE2), - a second string connected in parallel with a first switching unit of the first string, said second string comprising a further switching unit (FS) together with a voltage providing element (VPE).

2. The cell according to claim 1, wherein the voltage providing element is an element in the group of capacitor, resistor, battery, inductor and varistor.

3. The cell according to claim 1 or 2, wherein the voltage of the voltage providing element has a polarity that is opposite to the voltage of the first energy storage element.

4. The cell according to any previous claim, wherein the first switching unit (S1) is able to 10 15 20 25 30 14 conduct and block currents in both directions through the first string.

5. The cell according to claim 4, wherein the further switching unit (FS) is able to conduct and block currents in both directions through the second string.

6. A cell according to any previous claim, wherein the first switching unit (S1) is connected between a positive end of the energy storage element and a midpoint of the first string.

7. The cell according to any of claims 1 - 5, wherein the first switching unit (S1) is connected between a negative end of the energy storage element and a midpoint of the first string.

8. The cell according to any previous claim, wherein the further switching unit is configured to be non-conducting in normal operation and selectively conducting in a fault current handling mode based on the fault current direction.

9. A voltage source converter (10) including at least one phase leg (PL1, PL2, PL3), where each phase leg includes a number of cells (Clul, C2u2, C3u3, C111, C211, C311, Clu2, C113, C2u2, C3u2, C112, C212, C312, Clu3, C2u3, C3u3, C2l3,C3l3) and at least one cell in each phase leg is a cell according to any of claims 1 - 8 with a second string. 10 15 20 15

10. The voltage source converter (10) according to claim 9, further comprising a control unit (12) configured to provide control signals to cells in each branch in order to control the operation of the voltage source converter.

11. A voltage source converter (10) according to claim 9 or 10, wherein at least one cell in each phase leg is a half-bridge cell.

12. The voltage source converter (10) according to claim 11, wherein there is mixture of cells having second strings and half-bridge cells.

13. The voltage source converter according to claim 12, wherein half of the cells are cells with second strings and half of the cells are half-bridge cells.