Multi-converter parallel connection system and off-grid startup method

1 . A multi-converter parallel connection system, comprising N converters, wherein

alternating-current sides of the N converters are connected to an alternating-current bus, wherein N is an integer greater than or equal to 2;

M converters among the N converters are configured, when the N converters are disconnected from a power grid or the power grid fails, to switch from a current-source operation mode to a voltage-source operation mode to form an alternating-current bus voltage, wherein M is an integer less than or equal to N; and

remaining N-M converters among the N converters are configured, after the alternating-current bus voltage is formed, to switch from the current-source operation mode to the voltage-source operation mode.

2 . The system according to claim 1 , further comprising a monitoring unit, wherein

the monitoring unit is connected to the alternating-current bus;

the monitoring unit is powered by the alternating-current bus after the alternating-current bus voltage is formed; and

the monitoring unit is configured to control the remaining N-M converters to switch from the current-source operation mode to the voltage-source operation mode.

3 . The system according to claim 1 , wherein

the M converters, operating in the voltage-source operation mode, are configured to change an amplitude and/or a frequency of the alternating-current bus voltage, wherein the N-M converters, when detecting that the alternating-current bus fails, stop operating in the current-source operation mode.

4 . The system according to claim 1 , wherein

at least one converter of the M converters is configured, after the alternating-current bus voltage is formed, to transmit a predetermined voltage signal to the alternating-current bus; and

the remaining N-M converters are configured, when detecting the predetermined voltage signal, to switch from the current-source operation mode to the voltage-source operation mode.

5 . The system according to claim 4 , wherein

the at least one converter of the M converters is configured to transmit a predetermined voltage signal, with an amplitude varying according to a predetermined rule and/or a frequency varying according to a predetermined rule, to the alternating-current bus, wherein the frequency of the predetermined voltage signal is less than or equal to a fundamental frequency of the multi-converter parallel connection system.

6 . The system according to claim 4 , wherein

the at least one converter of the M converters is configured to transmit a predetermined voltage signal, with an amplitude varying according to a predetermined rule and/or a frequency varying according to a predetermined rule, to the alternating-current bus, wherein the frequency of the predetermined voltage signal is greater than a fundamental frequency of the multi-converter parallel connection system and less than a cut-off frequency of a filter at alternating-current sides of the converters.

7 . A voltage regulation method for a multi-converter parallel connection system, comprising:

switching M converters, among N converters comprised in the multi-converter parallel connection system, from a current-source operation mode to a voltage-source operation mode to form an alternating-current bus voltage when the N converters are disconnected from a power grid or the power grid fails, wherein alternating-current sides of the N converters are connected to an alternating-current bus, N is an integer greater than or equal to 2, and M is an integer less than or equal to N; and

switching remaining N-M converters among the N converters from the current-source operation mode to the voltage-source operation mode after the alternating-current bus voltage is formed.

8 . The method according to claim 7 , comprising:

controlling, by a monitoring unit comprised in the multi-converter parallel connection system and connected to the alternating-current bus, the remaining N-M converters to switch from the current-source operation mode to the voltage-source operation mode after the alternating-current bus voltage is formed.

9 . The method according to claim 7 , further comprising:

changing, by the M converters operating in the voltage-source operation mode, an amplitude and/or a frequency of the alternating-current bus voltage, wherein the N-M converters stop operating in the current-source operation mode when detecting that the alternating-current bus fails.

10 . The method according to claim 7 , wherein the switching remaining N-M converters among the N converters from the current-source operation mode to the voltage-source operation mode after the alternating-current bus voltage is formed comprises:

transmitting, by at least one converter of the M converters, a predetermined voltage signal to the alternating-current bus after the alternating-current bus voltage is formed; and

switching the remaining N-M converters from the current-source operation mode to the voltage-source operation mode when detecting the predetermined voltage signal.

11 . The method according to claim 10 , wherein the transmitting, by at least one converter of the M converters, a predetermined voltage signal to the alternating-current bus comprises:

transmitting, by the at least one converter of the M converters, a predetermined voltage signal with an amplitude varying according to a predetermined rule and/or a frequency varying according to a predetermined rule to the alternating-current bus, wherein the frequency of the predetermined voltage signal is less than or equal to a fundamental frequency of the multi-converter parallel connection system.

12 . The method according to claim 10 , wherein the transmitting, by at least one converter of the M converters, a predetermined voltage signal to the alternating-current bus comprises:

transmitting, by the at least one converter of the M converters, a predetermined voltage signal with an amplitude varying according to a predetermined rule and/or a frequency varying according to a predetermined rule to the alternating-current bus, wherein the frequency of the predetermined voltage signal is greater than a fundamental frequency of the multi-converter parallel connection system and less than a cut-off frequency of a filter at alternating-current sides of the converters.

13 . The system according to claim 2 , wherein

the M converters, operating in the voltage-source operation mode, are configured to change an amplitude and/or a frequency of the alternating-current bus voltage, wherein the N-M converters, when detecting that the alternating-current bus fails, stop operating in the current-source operation mode.

14 . The method according to claim 8 , further comprising:

changing, by the M converters operating in the voltage-source operation mode, an amplitude and/or a frequency of the alternating-current bus voltage, wherein the N-M converters stop operating in the current-source operation mode when detecting that the alternating-current bus fails.