Synchronization of parallel-connected inverter units or frequency converters

• US 7,327,588 B2
• Filed: 02/15/2005
• Issued: 02/05/2008
• 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
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US11/057,130
• Publication Number:
  US 20050179419A1
• Time in Patent Office: 1,085 Days
• Field of Search: 363/65, 363/71, 363/72, 363/157, 363/159, 307/82
• US Class Current: 363/71
• 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, said inverters including inverter-specific modulators, or parallel-connected frequency converters re-injecting braking energy into a supply network, said frequency converters having inverter unit-specific modulators both in the mains bridges and in the load bridges, the method comprising;

measuring the circulating current between the inverter units; and

synchronizing the modulators to each other on the basis of the current measurement data;

wherein measuring the circulating current, in the inverter units functioning as mains inverters or in the frequency converters, includes;

measuring the base current simultaneously and forming a sum of the phase currents;

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

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

2. A method according to claim 1, wherein, 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.

3. A method according to claim 2, 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.

4. 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.

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

10. A method according to claim 1, wherein 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.

6. A inverter apparatus, comprising:

parallel-connected inverter units, which inverter units are provided with inverter-specific modulators or an apparatus consisting of frequency converters re-injecting braking energy into a supply network, which frequency converters are provided with converter unit-specific modulators both in the mains bridges and in the load bridges;

wherein 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;

wherein the apparatus measuring the circulating current, in the inverter units functioning as mains inverters or in the frequency converters, includes measuring the phase currents simultaneously and forming a sum of the phase currents, detecting the instantaneous value of the switching-frequency circulating current by multiplying the sum of the phase currents by the polarity of a triangular wave or corresponding modulation signal, and forming a circulating current sum by summing the successive instantaneous value results.

7. An apparatus according to claim 6, wherein 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.

8. An apparatus according to claim 6, 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.

9. An apparatus according to any one of claim 6, which is used in all the parallel-connected inverter units except one.

11. An apparatus according to claim 6, wherein 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.

12. An apparatus according to claim 6, wherein, 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.