Fault tolerance control of solid state transformer
View Patent ↗A solid state transformer (SST) is provided, and is concerned with improving the fault tolerance of the SST. A method of operating the SST and an SST device including the SST are also provided. The SST has three parallel phase branches each having multiple cells, and each phase branch is connectable via a switch to a power grid. The method comprises operating the SST in a first control mode, in which each of the three phase branches is individually controlled. Further, when an open-circuit or short-circuit fault occurs in a cell of a particular phase branch, the method further comprises opening the switch connecting the particular phase branch to the power grid, and operating the SST in a second control mode, in which the two phase branches other than the particular phase branch are jointly controlled as a single phase branch.
1 . A method of operating a solid state transformer (SST) having three parallel phase branches, wherein each of the three parallel phase branches comprises multiple cells and is connected via at least one switch to a power grid, and wherein the method comprises:
operating the SST in a first control mode to open the at least one switch connecting a first of the three parallel phase branches to the power grid based on an open-circuit fault or a short-circuit fault occurring in at least one cell of the first of the three parallel phase branches,
wherein each of the three parallel phase branches is individually controlled; and
operating the SST in a second control mode to adapt reference voltages of the cells of each of second and third of the three parallel phase branches based on a discontinuous pulse width modulation (DPWM) voltage,
wherein the second and third parallel phase branches are jointly controlled as a single phase branch.
2 . The method according to claim 1 , wherein each of the three parallel phase branches, operating in the first control mode, is controlled based on at least one of the following:
an individual grid voltage for each of the three phase branches; and
an individual grid current for each of the three phase branches.
3 . The method according to claim 2 , wherein in the second control mode, the cells of the second and third parallel phase branches are operated in a serial connection.
4 . The method according to claim 1 , wherein the second and third parallel phase branches, operating in the second control mode, are controlled based on at least one of the following:
a single grid voltage for the second and third parallel phase branches; and
a single grid current for the second and third parallel phase branches.
5 . The method according to claim 4 , wherein in the second control mode, the cells of the second and third parallel phase branches are operated in a serial connection.
6 . The method according to claim 1 , wherein the second and third parallel phase branches, operating in the second control mode, are controlled based on at least one of the following:
a single grid voltage for the second and third parallel phase branches; and
a single grid current for the second and third parallel phase branches.
7 . The method according to claim 1 , wherein in the second control mode, the cells of the second and third parallel phase branches are operated in a serial connection.
8 . The method according to claim 1 , the method further comprising adding or subtracting, in the second control mode, the DPWM voltage to or from a reference voltage of each of the cells comprising the second and third parallel phase branches,
wherein the DPWM voltage is subtracted from the reference voltage of one cell of the second and third parallel phase branches to operate the one cell in a clamped mode, and
wherein the DPWM voltage is added to each of the reference voltages of the other cells of the second and third of the three parallel phase branches to operate the other cells in a not-clamped mode.
9 . The method according to claim 8 , wherein the cells of the second and third of the three parallel phase branches are alternatingly operated in the clamped mode and the not-clamped mode.
10 . The method according to claim 8 , wherein a DC voltage of a cell operating in the clamped mode is increased.
11 . The method according to claim 10 , the method further comprising:
sorting DC voltages of all the cells in the second and third parallel branches one or more times, in order to determine the cell having a lowest DC voltage; and
operating the cell determined to have the lowest DC voltage in the clamped mode.
12 . A solid state transformer (SST) device comprising:
an SST having three parallel phase branches, wherein each of the three parallel phase branches comprises multiple cells, and wherein each of the three parallel phase branches is connectable, via at least one switch, to a power grid; and
a controller configured to:
operate the SST in a first control mode to open the at least one switch connecting a first of the three parallel phase branches to the power grid based on an open-circuit fault or a short-circuit fault occurring in at least one cell of the first of the three parallel phase branches,
wherein each of the three parallel phase branches is individually controlled in the first control mode; and
operate the SST in a second control mode to adapt reference voltages of the cells of second and third of the three parallel phase branches based on a discontinuous pulse width modulation (DPWM) voltage,
wherein the second and third parallel phase branches are jointly controlled as a single phase branch.
13 . The SST device according to claim 12 , wherein the controller comprises a DPWM unit and a DC voltage control unit, wherein in the second control mode,
the DPWM unit is configured to provide the DPWM voltage, wherein the DPWM voltage is subtracted from the reference voltage of one cell of the second and third parallel phase branches to operate that cell in a clamped mode, and the DPWM voltage is added to the reference voltages of the other cells of the second and third parallel phase branches to operate these other cells in a not-clamped mode; and
the DC voltage control unit is configured to sort DC voltages of all the cells in the second and third parallel phase branches one or more times, determine the cell having a lowest DC voltage, and operate the cell determined to have the lowest DC voltage in the clamped mode.
14 . The SST device according to claim 12 , wherein the controller is further configured to:
control, in the first control mode, each of the three parallel phase branches based on an individual grid voltage for each of the three parallel phase branches and/or an individual grid current for each of the three parallel phase branches; and/or
control, in the second control mode, the second and third parallel phase branches based on a single grid voltage for the second and third parallel phase branches and/or a single grid current for the second and third parallel phase branches.
15 . The SST device according to claim 12 , wherein each of the three parallel phase branches of the SST comprises:
a cell array comprising the multiple cells of each of the three parallel phase branches connected in series; and
the at least one switch connected in series between the cell array and a connection port for connecting each of the three parallel phase branches of the SST to the power grid.
16 . The SST device according to claim 15 , wherein the at least one switch is connected to the cell array via an inductor and/or a resistor.
17 . The SST device according to claim 15 , wherein each cell comprises a switchable power electronics device connected to a capacitor; and
wherein the capacitor is connected to a load in order to supply power to the load.
18 . A non-transitory computer readable program comprising instructions that, upon being executed by a computer or a controller, causes the computer or the controller to perform a method of operating a solid state transformer (SST), wherein the SST comprises three parallel phase branches, wherein each of the three parallel phase branches comprises multiple cells and is connected via at least one switch to a power grid, and wherein the method comprises:
operating the SST in a first control mode to open the at least one switch connecting a first of the three parallel phase branches to the power grid based on an open-circuit fault or a short-circuit fault occurring in at least one cell of the first of the three parallel phase branches,
wherein each of the three parallel phase branches is individually controlled; and
operating the SST in a second control mode to adapt reference voltages of the cells of each of second and third of the three parallel phase branches based on a discontinuous pulse width modulation (DPWM) voltage,
wherein the second and third parallel phase branches are jointly controlled as a single phase branch.
19 . The non-transitory computer readable program according to claim 18 , wherein each of the three parallel phase branches, operating in the first control mode, is controlled based on at least one of the following:
an individual grid voltage for each of the three phase branches; and
an individual grid current for each of the three phase branches.
20 . The non-transitory computer readable program according to claim 18 , wherein the second and third parallel phase branches, operating in the second control mode, are controlled based on at least one of the following:
a single grid voltage for the second and third parallel phase branches; and
a single grid current for the second and third parallel phase branches.