Systems and methods for preventing islanding of grid-connected electrical power systems

• US 6,429,546 B1
• Filed: 11/19/1999
• Issued: 08/06/2002
• Est. Priority Date: 11/20/1998
• Status: Expired due to Fees

• Current Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Original Assignee: Georgia Tech Research Corporation (University System of Georgia)
• Inventors: Rohatgi, Ajeet, Begovic, Miroslav M., Ropp, Michael Eugene
• Primary Examiner(s): Huynh, Kim
• Assistant Examiner(s): DEBERADINIS, ROBERT L
• Application Number:
  US09/443,619
• Time in Patent Office: 991 Days
• Field of Search: 307/43, 307/31, 307/32, 307/52, 307/38, 307/51, 307/87, 361/186, 361/187, 363/34, 363/42, 363/47, 363/97, 363/441
• US Class Current: 307/31
• CPC Class Codes: H02J 2300/24 of photovoltaic origin
• CPC Class Codes: H02J 2300/28 The renewable source being wind energy wind motors F03D
• CPC Class Codes: H02J 3/01 Arrangements for reducing harmonics or ripples
• CPC Class Codes: H02J 3/381 Dispersed generators
• CPC Class Codes: H02J 3/388 Islanding, i.e. disconnection of local power supply from the network
• CPC Class Codes: Y02E 10/56 Power conversion systems, e.g. maximum power point trackers
• CPC Class Codes: Y02E 10/76 Power conversion electric or electronic aspects
• CPC Class Codes: Y02E 40/40 Arrangements for reducing harmonics

Abstract

A preferred embodiment of the electrical power system of the present invention includes a power conditioning unit which is configured to receive the DC electrical output signal to deliver an AC output signal to a grid-connected load. Preferably, the power conditioning unit includes a controller which is configured to monitor the AC output signal so that the power conditioning unit may cease delivering the AC output signal when a characteristic of the AC output signal satisfies an established criterion.

Claims

1. A method for reducing size of a non-detection zone in a grid-connected power system having an electrical power generator with an interface to the grid, the method comprising the steps of:

measuring frequency deviation between an output signal of the grid and an output signal of the grid-connected power system;

applying the measured frequency deviation to the output signal of the grid-connected power system; and

when the measured frequency of the output signal of the grid-connected power system satisfies an established cut-off level, discontinuing the output signal of the grid-connected power system.

2. The method of claim 1, wherein the step of applying the measured frequency deviation includes the step of applying the measured frequency deviation to an output signal of the grid-connected power system such that the application thereof accelerates a shift in frequency of the output signal delivered from the grid-connected power system.

3. The method of claim 1, wherein the step of applying the measured frequency deviation includes the step of using the measured frequency deviation as an argument of a function F.

4. The method of claim 3, wherein the function F is a linear function.

5. A power conditioning unit for use in an electrical power system, the electrical power system being adapted to supply AC electrical power to a grid-connected load, the electrical power system being configured to convert energy into a DC electrical output signal, said power conditioning unit comprising:

an inverter configured to receive the DC electrical output signal of the electrical power system, to convert the DC electrical output signal of the electrical power system into an AC output signal, and to deliver said AC output signal to the grid-connected load;

a controller configured to measure the frequency deviation between an output signal of the grid and an output signal of the grid-connected power system, apply the measured frequency deviation to the output signal of the grid-connected power system, and discontinue the output signal of the grid-connected power system when the measured frequency of the output signal of the grid-connected power system satisfies an established cut-off level; and

a feedback loop interconnecting said AC output signal and said controller such that said feedback loop provides said controller with information regarding said AC output signal;

said power conditioning unit being further configured to cease delivering said AC output signal therefrom when a characteristic of said AC output signal satisfies an established criterion.

6. The power conditioning unit of claim 5, further comprising an over-frequency relay and under-frequency relay, said over-frequency relay being adapted to prevent delivery of said AC output signal from said power conditioning unit in response to an established over-frequency threshold of said AC output signal, said under-frequency relay being adapted to prevent delivery of said AC output signal from said power conditioning unit in response to an established under-frequency threshold of said AC output signal;

and

wherein said controller is configured to accelerate a shift in frequency of said output signal via a modification signal from said controller such that one of said relays prevents delivery of said AC output signal from said power conditioning unit.

7. The grid-connected power system of claim 5, wherein said controller comprises means for measuring a frequency deviation of said AC output signal and means for generating a modification signal corresponding to said frequency deviation.

8. A computer readable medium having a computer program for reducing size of a non-detection zone in a grid network, the grid network having a grid-connected load and a grid-connected power system configured for supplying power thereto, the grid-connected power system having a DC electric generator and a power conditioning unit, said computer readable medium comprising:

a first code segment configured to measure frequency deviation between an output signal of the grid and an output signal of the grid-connected power system;

a second code segment configured to apply the measured frequency deviation to the output signal of the grid-connected power system; and

a third code segment configured to discontinue output signal of the grid-connected power system at the power conditioning unit when the measured frequency of the output signal of the grid-connected power system satisfies an established cut-off level.

9. The computer readable medium of claim 8, wherein the second code segment comprises a code segment configured to utilize the measured frequency deviation as an argument of a function F.

10. The computer readable medium of claim 9, wherein the function F is a linear function.