Submodule fault tolerance in modular multilevel converters
An example device includes at least one processor configured to receive an indication of a fault in submodules in a first arm of a first phase leg of a modular multilevel converter (MMC), cause it to bypass an equal number of healthy submodules in a second arm of the first phase leg, determine a zero-sequence voltage for each phase leg, and determine, for each phase leg, based at least in part on the zero-sequence voltage, a respective modified phase leg reference signal. The respective modified phase leg reference signal for the first phase leg may be determined further based in part on a reduced utilization factor for the first phase leg. The processor is further configured to determine, based on the respective modified phase leg reference signal for each phase leg, gate driver signals for submodules in the MMC and cause it to operate using the signals.
1 . A device comprising:
at least one processor configured to:
receive an indication of a fault in one or more submodules (SMs) that are in a first arm of a first phase leg of a modular multilevel converter (MMC);
cause the MMC to bypass an equal number of healthy SMs in a second arm of the first phase leg;
determine, a zero-sequence voltage for each phase leg;
determine, for each phase leg, based at least in part on the zero-sequence voltage, a respective modified phase leg reference signal, the respective modified phase leg reference signal for the first phase leg being determined further based in part on a reduced utilization factor (RUF) for the first phase leg;
determine, based on the respective modified phase leg reference signal for each phase leg, gate driver signals for SMs in the MMC; and
cause the MMC to operate using the gate driver signals.
2 . The device of claim 1 , wherein the at least one processor is further configured to:
determine, based on the number of healthy SMs in the first arm, a reduced carrier phase shifted pulse-width modulation (CPS-PWM) signal for the first leg; and
determine the gate driver signals for SMs in the first leg based additionally on the reduced CPS-PWM signal.
3 . The device of claim 1 , wherein determining the zero-sequence voltage comprises determining V ZS =0.5V dc (1−2σ)+(1−σ)V min +0V max , wherein:
V zs represents the respective zero-sequence voltage;
V dc represents the DC link voltage of the MMC;
σ represents a generalized distribution coefficient;
V min represents an instantaneous maximum voltage value; and
V max represents an instantaneous minimum voltage value.
4 . The device of claim 3 , wherein σ is determined by
σ
=
2
(
N
-
N
f
m
N
-
0.5
)
+
(
1
-
m
3
)
2
-
m
3
,
wherein:
N represents the number of SMs in each phase arm;
N fm represents the larger of the number of faulty SMs in the first arm or the number of faulty SMs in the second arm; and
m represents an original modulation index.
5 . The device of claim 3 , wherein σ is selected to be a value between 0 and 1.
6 . The device of claim 1 , wherein the at least one processor is further configured to determine the RUF for the first phase leg by determining
R
U
F
=
N
-
N
f
m
N
,
wherein:
N represents the number of SMs in each phase arm; and
N fm represents the larger of the number of faulty SMs in the first arm or the number of faulty SMs in the second arm.
7 . The device of claim 1 , wherein the device comprises a control module of the MMC.
8 . A method comprising:
receiving an indication of a fault in one or more submodules (SMs) that are in a first arm of a first phase leg of a modular multilevel converter (MMC);
causing, by a device comprising at least one processor, the MMC to bypass an equal number of healthy SMs in a second arm of the first phase leg;
determining, by the device, a zero-sequence voltage;
determining, by the device, for each phase leg, and based at least in part on the zero-sequence voltage, a respective modified phase leg reference signal, the respective modified phase leg reference signal for the first phase leg being determined further based in part on a reduced utilization factor (RUF) for the first phase leg;
determining, by the device and based on the respective modified phase leg reference signal for each phase leg, gate driver signals for SMs in the MMC; and
operating the MMC using the gate driver signals.
9 . The method of claim 8 , further comprising:
determining, based on the number of healthy SMs in the first arm, a reduced carrier phase shifted pulse-width modulation (CPS-PWM) signal for the first leg; and
determining the gate driver signals for SMs in the first leg based additionally on the reduced CPS-PWM signal.
10 . The method of claim 8 , wherein determining the zero-sequence voltage comprises determining V ZS =0.5V dc (1−2σ)+(1−σ)V min +0V max , wherein:
V ZS represents the respective zero-sequence voltage;
V dc represents the DC link voltage of the MMC;
σ represents a generalized distribution coefficient;
V min represents an instantaneous maximum voltage value; and
V max represents an instantaneous minimum voltage value.
11 . The method of claim 10 , wherein σ is determined by
σ
=
2
(
N
-
N
f
m
N
-
0.5
)
+
(
1
-
m
3
)
2
-
m
3
,
wherein:
N represents the number of SMs in each phase arm;
N fm represents the larger of the number of faulty SMs in the first arm or the number of faulty SMs in the second arm; and
m represents an original modulation index.
12 . The method of claim 10 , wherein σ is selected to be a value between 0 and 1.
13 . The method of claim 8 , further comprising determining the RUF for the first phase leg by determining
RUF
=
N
-
N
f
m
N
,
wherein:
N represents the number of SMs in each phase arm; and
N fm represents the larger of the number of faulty SMs in the first arm or the number of faulty SMs in the second arm.
14 . The method of claim 8 , wherein the device comprises a control module of the MMC.
15 . A method performed by a modular multilevel converter (MMC), the method comprising:
monitoring operation of the MMC for faults occurring in one or more of a plurality of submodules (SMs) positioned in the MMC;
when a faulty SM is detected in a first phase of the MMC, isolating the faulty SM;
when the faulty SM is detected in the first phase of the MMC, bypassing a non-faulty SM;
a first calculating of a reduced utilization factor (RUF) due to the faulty SM in the first phase;
a second calculating of a respective zero-sequence voltage (ZSV) for each phase; and
an injecting the ZSV into the phase voltages, resulting in the modifying of a reference signal for each phase (U a_ref , U b_ref , U c_ref ), wherein:
the MMC comprises a leg comprising a first arm and a complementary arm, and each arm comprises at least one of the plurality of SMs,
the faulty SM is positioned in the first arm,
the bypassed non-faulty SM is positioned in the complementary arm,
the first calculating, the second calculating, and the injecting are performed in a control module of the MMC, and
the control module operates the MMC using the modified reference signals, resulting in at least one of a suppression of a circulating current, a balanced leg voltage, a balanced arm current, a stable phase current, or a stable DC-bus current in the MMC.