Synchronization of parallel-connected inverter units or frequency converters

• US 20050179419A1
• Filed: 02/15/2005
• Published: 08/18/2005
• Est. Priority Date: 02/16/2004
• Status: Active Grant

• Current Assignee: Vacon Oy (Danfoss A/S)
• Original Assignee: Vacon Oyj (Danfoss A/S)
• Inventors: Ollila, Jaakko
• Granted Patent: US 7,327,588 B2
• Time in Patent Office: Days
• US Class Current: 318/801
• CPC Class Codes: H02M 1/0043 Converters switched with a phase shift, i.e. interleaved non-isolated DC/DC converters H02M3/1586
• CPC Class Codes: H02M 7/493 the static converters being arranged for operation in parallel

Abstract

A method for the synchronization of parallel-connected inverter units (INU1, INU2) or frequency converters and an inverter apparatus or a frequency converter apparatus wherein the inverter units are provided with inverter-specific modulators, in which method the circulating current between the inverter units is measured, and the modulators are synchronized to each other on the basis of the current measurement data.

Claims

1. A method for synchronizing parallel-connected inverter units (INU1, INU2), said inverters comprising inverter-specific modulators, characterized in that, in the method,

the circulating current between the inverter units is measured, and

the modulators are synchronized to each other on the basis of the current measurement data.

2. A method according to claim 1, characterized in that, to measure the circulating current, in the inverter units functioning as mains inverters

the phase currents are measured simultaneously and a sum of the phase currents is formed,

the instantaneous value of switching-frequency circulating current is detected by multiplying the sum of the phase currents by the polarity of a triangular wave or a corresponding modulation signal, and

a circulating current sum is formed by summing the successive instantaneous value results.

3. A method according to claim 1, characterized in that, in the synchronization, in order to prevent the flow of circulating current in the parallel-connected inverter units functioning as mains inverters the triangular waves or corresponding modulation signals of the modulators are synchronized via the following operations, which are performed in all the units:

a circulating current sum is formed,

if the circulating current sum exceeds a preset positive limit value, then the switching frequency is increased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, and at the same time the circulating current sum is reset to zero,

if the circulating current sum is lower than a preset negative limit value, then the switching frequency is decreased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, and at the same time the circulating current sum is reset to zero.

4. A method according to claim 3, wherein the synchronization of the triangular waves or corresponding modulation signals is effected by adjusting their frequency in proportion to the detected switching-frequency circulating current.

5. A method according to claim 1 which is used when the parallel-connected inverter units serve as a motor feeding device and feed the same motor winding via a filter consisting of at least an inductor.

6. A method according to claim 1 which is used in all the parallel-connected inverter units except one.

7. An inverter apparatus comprising parallel-connected inverter units (INU1, INU2), which inverter units are provided with inverter-specific modulators, characterized in that the apparatus:

measures the circulating current between the inverter units, and

synchronizes the modulators to each other on the basis of the current measurement data.

8. An apparatus according to claim 7, characterized in that in the synchronization, in order to prevent the flow of circulating current in the parallel-connected inverter units functioning as mains inverters the triangular waves or corresponding modulation signals of the modulators are synchronized in such manner that in the modulators in all the units

a circulating current sum is formed,

the switching frequency is increased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, if the circulating current sum exceeds a preset positive limit value, and at the same time the circulating current sum is reset to zero,

the switching frequency is decreased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, if the circulating current sum is lower than a preset negative limit value, and at the same time the circulating current sum is reset to zero.

9. An apparatus according to claim 7, which is used when the parallel-connected inverter units serve as a motor feeding device and feed the same motor winding via a filter consisting of at least an inductor.

10. An apparatus according to any one of claim 7, which is used in all the parallel-connected inverter units except one.

11. A method for synchronizing parallel-connected frequency converters (FC1, FC2) re-injecting braking energy into the supply network, said frequency converters having inverter unit-specific modulators both in the mains bridges and in the load bridges, characterized in that, in the method:

the circulating current between the frequency converters is measured, and

the mains bridge modulators are synchronized to each other and the load bridge modulators are synchronized to each other on the basis of the current measurement data.

12. A method according to claim 11, characterized in that the switching frequency (f_K1) of the mains bridges has been set to be mutually the same but different from the switching frequency (f_K2) of the load bridges.

13. A method according to claim 11, characterized in that, to measure the circulating current in the inverter units,

the phase currents are measured simultaneously and a sum of the phase currents is formed,

the instantaneous value of switching-frequency circulating current is detected by multiplying the sum of the phase currents by the polarity of a triangular wave or a corresponding modulation signal, and

a circulating current sum is formed by summing the successive instantaneous value results.

14. A method according to claim 11, characterized in that, to prevent the flow of circulating current in the parallel-connected frequency converters, the triangular waves or corresponding modulation signals of the modulators are synchronized via the following operations, which are performed in all the inverter units:

a circulating current sum is formed,

if the circulating current sum exceeds a preset positive limit value, then the switching frequency is increased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, and at the same time the circulating current sum is reset to zero,

if the circulating current sum is lower than a preset negative limit value, then the switching frequency is decreased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, and at the same time the circulating current sum is reset to zero.

15. A method according to claim 11, characterized in that the synchronization of the triangular waves or corresponding modulation signals is effected by adjusting their frequency in proportion to the detected switching-frequency circulating current.

16. A method according to claim 11 which is used in all except one of the inverter units of the mains bridges and/or load bridges of the parallel-connected frequency converters.

17. An apparatus consisting of frequency converters (FC1, FC2) re-injecting braking energy into the supply network, which frequency converters are provided with inverter unit-specific modulators both in the mains bridges and in the load bridges, characterized in that the apparatus:

measures the circulating current between the inverter units, and

synchronizes the modulators to each other on the basis of the current measurement data.

18. An apparatus according to claim 17, characterized in that the switching frequency (f_K1) of the mains bridges has been set to be mutually the same but different from the switching frequency (f_K2) of the load bridges.

19. An apparatus according to claim 17, characterized in that, to prevent the flow of circulating current between the parallel-connected frequency converters, the triangular waves or corresponding modulation signals of their inverter unit-specific modulators are synchronized in such manner that in all the units:

a circulating current sum is formed,

the switching frequency is increased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, if the circulating current sum exceeds a preset positive limit value, and at the same time the circulating current sum is reset to zero,

the switching frequency is decreased by a fixed step for a certain time, e.g. for the time of one half of a modulation cycle, if the circulating current sum is lower than a preset negative limit value, and at the same time the circulating current sum is reset to zero.

20. An apparatus according to claim 17, which is used in all except one of the inverter units of the mains bridges and/or load bridges of the parallel-connected frequency converters.