IMPEDANCE COMPENSATION

• US 20160028325A1
• Filed: 07/17/2015
• Published: 01/28/2016
• Est. Priority Date: 07/23/2014
• Status: Active Grant

• Current Assignee: Delta Electronics Thailand Public Company Limited (Delta Electronics Incorporated)
• Original Assignee: DET International Holding Limited (Delta Electronics Incorporated)
• Inventors: Redmann, Michael
• Granted Patent: US 10,141,869 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G05F 1/44 semiconductor devices only
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 3/12 for adjusting voltage in ac networks by changing a characteristic of the network load
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02M 7/53871 with automatic control of output voltage or current
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers

Abstract

The invention relates to a method for operating a power converter (1), preferably a solar inverter, for feeding a load current (I_load) into a grid (3). The method includes the steps of determining the load current (I_load), monitoring an output voltage (U_out) of the power converter (1), and controlling the load current (I_load), to avoid that the output voltage (U_out) exceeds a voltage limit (U_out,lim), wherein the voltage limit (U_out,lim) is adjusted in dependency of the load current (I_load). Furthermore the invention relates to a corresponding power converter (1) for connecting to a grid (3), preferably it relates to a solar power converter. This power converter (1) includes a load current sensor (142), an output voltage sensor (143) and a load current controller (141).

Claims

1. Method for operating a power converter, preferably a solar inverter, for feeding a load current into a grid, including the steps of

a) determining the load current,

b) monitoring an output voltage of the power converter,

c) controlling the load current, to avoid that the output voltage exceeds a voltage limit,

d) adjusting the voltage limit, in dependency of the load current.

2. Method according to claim 1, wherein adjusting the voltage limit, includes the steps of

a) determining a voltage correction value,

b) adding the voltage correction value to a grid voltage limit.

3. Method according to claim 2, wherein the voltage correction value is determined by estimating a voltage drop due to a line impedance of a line between the power converter and the grid.

4. Method according to claim 3 wherein estimating the voltage drop includes the steps of

a) injecting test currents into the line and

b) measuring the output voltage of the power converter for each test current,

c) identifying the line impedance of the line between the power converter and the grid.

5. Method according to claim 4 wherein the voltage drop is estimated as linear dependent on the load current.

6. Method according to claim 4, in that at least one test current is zero.

7. Method according to claim 6, where the steps of injecting three consecutive test currents at equally spaced time intervals,

a) the first test current being zero,

b) the second test current being between 60% to 95% of the load current before the test, preferably between 75% and 85% of the load current before the test,

c) the third test current being zero,

d) the time intervals between the test currents being chosen to be in the range of 10 ms to 2500 ms, preferably between 100 ms and 500 ms.

8. Method according to claim 2, wherein the voltage correction value is scaled such that the output voltage does not exceed a maximum output voltage of the power converter.

9. Method according to claim 1, wherein controlling the load current includes the step of defining a limiting characteristics of the load current in dependency of the output voltage, the limiting characteristics

a) being equal to the nominal load current, below a first output voltage level,

b) having a negative gradient between a first output voltage level and a second output voltage level,

c) covering a load current value range between zero and the nominal load current,

d) being zero above the second output voltage level,

e) wherein the first output voltage level being smaller than the second output voltage level,

f) the second output voltage level being equal or smaller than the maximum output voltage.

10. Method according to claim 9, in that the limiting characteristics between the first output voltage level and the second output voltage level is chosen to be linear.

11. Method according to claim 10, wherein controlling the load current includes the step of adjusting the load current stepwise to meet the limiting characteristics by generating intermediate load current reference values.

12. Method according to claim 11, wherein the intermediate load current reference values I_load,ref are determined by the formula:

13. Method according to claim 9, wherein the first voltage level and the second voltage level are adapted in dependency of the voltage limit.

14. A power converter for connecting to a grid, preferably a solar power converter, including a load current sensor, an output voltage sensor and a load current controller for performing the step of adjusting the voltage limit in dependency of the load current according to claim 1.