Hybrid modular multilevel converter having fault blocking capability, and control method thereof

Abstract

A hybrid modular multilevel converter having fault blocking capability, and a control method thereof are disclosed. The hybrid modular multilevel converter has an ABC three-phase structure, each phase includes an identical upper arm and lower arm, and the upper arm and the lower arm are each composed of N submodules, including M unipolar full bridge submodules and N-M half bridge submodules, which are connected in series; two identical arm inductors are connected in series between a lower end of the upper arm and an upper end of the lower arm in a same phase; and upper arms and lower arms of three phases are connected in a staggered manner through six thyristor branches, the upper ends of the upper arms of the three phases are short-circuited and are connected to a DC side via a first isolating switch.

Claims

1. A hybrid modular multilevel converter having a fault blocking capability, wherein: the hybrid modular multilevel converter has an ABC three-phase structure, each phase comprises an upper arm and a lower arm, the upper arm and the lower arm are identical and each composed of N submodules, comprising M unipolar full bridge submodules and N-M half bridge submodules, which are connected in series, and a value of M is determined by a calculation of short-circuit parameters; two identical arm inductors are connected in series between a lower end of the upper arm and an upper end of the lower arm in a same phase; a first thyristor branch is connected between the lower end of the upper arm of a phase A and the upper end of the lower arm of a phase B, and a cathode of the first thyristor branch is connected to the lower arm of the phase B; a second thyristor branch is connected between the upper end of the lower arm of the phase B and the lower end of the upper arm of a phase C, and a cathode of the second thyristor branch is connected to the upper arm of the phase C; a third thyristor branch is connected between the lower end of the upper arm of the phase C and the upper end of the lower arm of the phase A, and a cathode of the third thyristor branch is connected to the lower arm of the phase A; a fourth thyristor branch is connected between the upper end of the lower arm of the phase A and the lower end of the upper arm of the phase C, and a cathode of the fourth thyristor branch is connected to the upper arm of the phase B; a fifth thyristor branch is connected between the lower end of the upper arm of the phase B and the upper end of the lower arm of the phase C, and a cathode of the fifth thyristor branch is connected to the lower arm of the phase C; a sixth thyristor branch is connected between the upper end of the lower arm of the phase C and the lower end of the upper arm of the phase A, and a cathode of the sixth thyristor branch is connected to the upper arm of the phase A; and upper ends of upper arms of three phases are short-circuited and are connected to a DC side via a first isolating switch, and lower ends of lower arms of the three phases are short-circuited and are connected to the DC side via a second isolating switch.

2. The hybrid modular multilevel converter having the fault blocking capability-according to claim 1, wherein the unipolar full bridge submodule comprises a first IGBT a second IGBT, a third IGBT, a first diode, and a first electrolytic capacitor, wherein an emitter of the first IGBT is connected to a collector of the second IGBT, and a connection point is used as a positive terminal of the unipolar full bridge submodule; a collector of the first IGBT is connected to a cathode of the first diode and an anode of the first electrolytic capacitor respectively, an anode of the first diode is connected to a collector of the third IGBT, and the connection point is used as a negative terminal of the unipolar full bridge submodule; and an emitter of the third IGBT, a cathode of the first electrolytic capacitor and an emitter of the second IGBT are connected to each other.

3. The hybrid modular multilevel converter having the fault blocking capability according to claim 2, wherein the first IGBT, the second IGBT and the third IGBT are connected to anti-paralleled diodes.

4. The hybrid modular multilevel converter having the fault blocking capability-according to claim 2, wherein the half bridge submodule comprises a fourth IGBT, a fifth IGBT and a second electrolytic capacitor, wherein an anode and a cathode of the second electrolytic capacitor are respectively connected to a collector of the fourth IGBT and an emitter of the fifth IGBT, an emitter of the fourth IGBT is connected to a collector of the fifth IGBT, the connection point is used as the positive terminal of the half bridge submodule, and the emitter of the fifth IGBT is used as the negative terminal of the half bridge submodule.

5. The hybrid modular multilevel converter having the fault blocking capability according to claim 4, wherein the fourth IGBT and the fifth IGBT are connected to anti-paralleled diodes.

6. The hybrid modular multilevel converter having the fault blocking capability according to claim 1, wherein a method of determining the value of M by the calculation of the short-circuit parameters comprises: first, calculating an inductance L.sub.d of a transmission line and a resistance R.sub.d of the transmission line according to a maximum length L.sub.m of the transmission line; then, according to a given DC short-circuit fault blocking time T.sub.int, a fault protection action current I.sub.act, L.sub.d and R.sub.4, calculating a negative voltage V.sub.neg that needs to be injected during a fault blocking process; and finally, calculating a number M of required UFB-SM according to V.sub.neg and a rated capacitor voltage U.sub.cN of the UFB-SM.

7. The hybrid modular multilevel converter having the fault blocking capability according to claim 1, wherein the first thyristor branch, the second thyristor branch, the third thyristor branch, the fourth thyristor branch, the fifth thyristor branch and the sixth thyristor branch are formed by connecting N.sub.TB thyristors in series, and a calculation method of N.sub.TB comprises: first, determining a voltage U.sub.T that each thyristor can withstand according to a model selection of the each thyristor; and then, according to a rated operation voltage V.sub.dcn on the DC side, calculating a number N.sub.TB of series-connected thyristors of each thyristor branch through N.sub.TB=V.sub.dcn/U.sub.T.

8. A control method of the hybrid modular multilevel converter having the fault blocking capability according to claim 4, wherein the following control is performed during a normal operation: during normal operation, the fourth IGBT and the fifth IGBT in the half bridge submodule operate complementarily, the first IGBT and the second IGBT in the unipolar full bridge submodule operate complementarily, the third IGBT remains on, a thyristor branch remains off, and the first isolating switch and the second isolating switch remain closed.

9. The control method of the hybrid modular multilevel converter having the fault blocking capability according to claim 4, wherein a DC side short-circuit fault clearing control method is as follows: sampling a DC side current i.sub.dc, when i.sub.dc is greater than 1.5 times of a rated value, judging that a short-circuit fault occurs on the DC side, at this time, entering a short-circuit fault clearing phase, turning off the first IGBT, the second IGBT and the third IGBT in the unipolar full bridge submodule, turning off both of the fourth IGBT and the fifth IGBT in the half bridge submodule, and turning on thyristors in all thyristor branches.

10. The control method of the hybrid modular multilevel converter having the fault blocking capability according to claim 4, wherein a short-circuit fault clearing control method on an AC side is as follows: sampling a DC side current idc, when i.sub.dc drops to zero, judging that a short-circuit fault is cleared, and sending a disconnection command to the first isolating switch and the second isolating switch on the DC side; after the first isolating switch and the second isolating switch are disconnected, entering an AC side short-circuit fault clearing phase; and turning on the third IGBT in the unipolar full bridge submodule, and removing a driving signal of a thyristor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a circuit diagram of an MMC of the present invention;

(2) FIG. 2 is a control block diagram of an MMC of the present invention;

REFERENCE SIGNS

(3) 2.1 represents the state of a novel MMC during normal operation, 2.2 represents the state of the novel MMC in a DC side short-circuit fault clearing phase, and 2.3 represents the state of the novel MMC in an AC side short-circuit fault clearing phase.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) The technical solutions and the beneficial effects of the present invention will be described in detail below in conjunction with the drawings.

(5) As shown in FIG. 1, the present invention provides a hybrid modular multilevel converter based on interleaved thyristor branches and having fault blocking capability. Each phase of the multilevel converter includes two identical upper and lower arms 1.1 and 1.2, each arm is composed of N submodules (submodule, SM), including M unipolar full bridge submodules (UFB-SM) 1.3 and (N-M) half bridge submodules (HB-SM) 1.4, and the value of M is determined by the calculation of short-circuit parameters; the upper arm and the lower arm are connected in each phase through two identical arm inductors 1.5 and 1.6; the arms of different phases are connected in a staggered manner through 6 thyristor branches (thyristor branch, TB) 1.7, each thyristor branch is formed by connecting NTB thyristors in series, wherein TB.sub.1 is connected to P.sub.1 and P.sub.5, and the cathode thereof points to P.sub.5, TB.sub.2 is connected to P.sub.5 and P.sub.3, and the cathode thereof points to P.sub.3, TB.sub.3 is connected to P.sub.3 and P.sub.4, and the cathode thereof points to P.sub.4, TB.sub.4 is connected to P.sub.4 and P.sub.2, and the cathode thereof points to P.sub.2, TB.sub.5 is connected to P.sub.2 and P.sub.6, and the cathode thereof points to P.sub.6, TB.sub.6 is connected to P.sub.6 and P.sub.1, and the cathode thereof points to P.sub.1, and the upper (lower) ends of the upper (lower) arms are connected to the DC side through isolating switches 1.8 and 1.9.

(6) The number M of UFB-SM is determined by the calculation of short-circuit parameters: firstly, calculating the inductance L.sub.d of a transmission line and the resistance R.sub.d of the transmission line according to the maximum length L.sub.m of the transmission line; then, according to a given DC short-circuit fault blocking time T.sub.int, fault protection action current I.sub.act, L.sub.d and R.sub.d, calculating a negative voltage V.sub.neg that needs to be injected during the fault blocking process; and

(7) $v_{neg}=R_d\times I_{act}\times \frac{e^{-\frac{R_d}{L_d}{T}_{int}}}{1-e^{-\frac{R_d}{L_d}{T}_{int}}}$
finally, calculating the number M of required UFB-SM according to V.sub.neg and the rated capacitor voltage U.sub.cN of the UFB-SM:
M=V.sub.neg/(2U.sub.cN)
The number N.sub.TB of series-connected thyristors of the thyristor branch is determined by the model selection of the thyristor and the DC voltage: firstly, determining the voltage U.sub.T that each thyristor can withstand according to the model selection of the thyristor; and then, according to the rated operation voltage V.sub.dcn on the DC side, calculating the number N.sub.TB of series-connected thyristors of each thyristor branch through N.sub.TB=V.sub.dcn/U.sub.T.

(8) As shown in FIG. 2, the control method of the hybrid modular multilevel converter based on interleaved thyristor branches and having fault blocking capability includes control during normal operation, DC side short-circuit fault clearing control and AC side short-circuit fault clearing control, and the steps are as follows:

(9) 1) during normal operation, T.sub.h1 and T.sub.h2 of the HB-SM operate complementarily, T.sub.u1 and T.sub.u2 of the HB-SM operate complementarily, T.sub.u3 remains on, the thyristor branch remains off, and the isolating switches S.sub.1 and S.sub.2 remain closed.

(10) 2) The DC side current i.sub.dc is sampled and is used as the criterion of detecting a DC side short-circuit fault. When the DC side short-circuit fault is detected, a short-circuit fault clearing phase is performed, all of T.sub.u1, T.sub.u2 and T.sub.u3 in the UFB-SM are turned off, both of T.sub.h1 and T.sub.h1 in the UFB-SM are turned off, and the thyristors in all thyristor branches are turned on.
3) When DC side current drops to zero, it is used as the criterion of clearing the short-circuit fault. When it is detected the DC side short-circuit fault has been cleared, a disconnection command is sent to the isolating switches S.sub.1 and S.sub.2 on the DC side; after the isolating switches are disconnected, the AC side short-circuit fault clearing phase is performed; and T.sub.u3 in the UFB-SM is turned on, and a driving signal of the thyristor is removed.

(11) The above control strategy ensures that the hybrid modular multilevel converter based on interleaved thyristor branches and having fault blocking capability can effectively extinguish the short-circuit current and clear the short-circuit fault.

(12) The above embodiments are only used for illustrating the technical ideas of the present invention, but cannot be used for limiting the protection scope of the present invention. Any changes, made on the basis of the technical solutions according to the technical ideas proposed by the present invention, fall into the protection scope of the present invention.