CONTROL SYSTEM AND METHOD FOR A UNIVERSAL POWER CONDITIONING SYSTEM

• US 20080205096A1
• Filed: 02/22/2008
• Published: 08/28/2008
• Est. Priority Date: 02/22/2007
• Status: Active Grant

• Current Assignee: Virginia Tech Intellectual Properties, Inc.
• Original Assignee: Virginia Tech Intellectual Properties, Inc.
• Inventors: Lai, Jih-Sheng, Chen, Chien-Liang, Park, Sung Yeul
• Granted Patent: US 8,824,174 B2
• Time in Patent Office: Days
• US Class Current: 363/40
• CPC Class Codes: H02J 3/38 Arrangements for parallely feeding a single network by two or more generators, converters or transformers
• CPC Class Codes: H02M 7/53871 with automatic control of output voltage or current
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

A new current loop control system method is proposed for a single-phase grid-tie power conditioning system that can be used under a standalone or a grid-tie mode. This type of inverter utilizes an inductor-capacitor-inductor (LCL) filter as the interface in between inverter and the utility grid. The first set of inductor-capacitor (LC) can be used in the standalone mode, and the complete LCL can be used for the grid-tie mode. A new admittance compensation technique is proposed for the controller design to avoid low stability margin while maintaining sufficient gain at the fundamental frequency. The proposed current loop controller system and admittance compensation technique have been simulated and tested. Simulation results indicate that without the admittance path compensation, the current loop controller output duty cycle is largely offset by an undesired admittance path. At the initial simulation cycle, the power flow may be erratically fed back to the inverter causing catastrophic failure. With admittance path compensation, the output power shows a steady-state offset that matches the design value. Experimental results show that the inverter is capable of both a standalone and a grid-tie connection mode using the LCL filter configuration.

Claims

1. A power conditioning system comprising:

a power inverter;

a filter interface between the power inverter and a power utility grid, the filter interface comprising a solid-state relay for interconnecting the power conditioning system and a utility power grid; and

a current loop controller for admittance compensation.

2. The power conditioning system of claim 1, wherein the current loop controller is a proportional resonant controller.

3. The power conditioning system of claim 1, wherein the current loop controller is a second-order lead-lag current controller.

4. The power conditioning system of claim 1, wherein the filter interface further comprises an inverter-side filter capacitor.

5. The power conditioning system of claim 1, further comprising one or more circuit elements for feeding back signals from the filter interface to the current loop controller.

6. The power conditioning system of claim 1, wherein the filter is an inductor-capacitor-inductor filter.

7. The power conditioning system of claim 6, wherein the inductor-capacitor-inductor filter has an inverter-side inductor and a grid-side inductor and an output voltage from the inverter-side inductor provides a feedback voltage and a feedback current to the current loop controller.

8. The power conditioning system of claim 6, further comprising a plurality of circuit elements for feeding back an inverter output current and a utility power grid voltage to the current loop controller.

9. The power conditioning system of claim 6, wherein the inductor-capacitor-inductor filter allows the inverter to operate in at least one of a grid-tie connection mode and a standalone operation mode.

10. The power conditioning system of claim 1, wherein the power conditioning system is a fuel cell power conditioning system.

11. A system for eliminating a steady-state error caused by an undesired admittance path in a current loop controller for a grid-tie inverter, the system comprising:

an admittance compensation means for avoiding low stability margin while maintaining sufficient gain at a fundamental frequency; and

a means for operating in at least one of a grid-tie connection mode or a standalone operation mode.

12. The system of claim 11, wherein the grid-tie inverter is a fuel cell grid-tie inverter.

13. A method for admittance compensation in a power conditioning system having a grid-tie inverter, the method comprising the steps of:

receiving an inverter output power command at a digital signal processor;

multiplying the inverter output power command by a scaling factor and a synchronization signal produced by a digital phase lock loop;

feeding back a utility power grid voltage and a feedback sensed current from the grid-tie inverter to the current loop controller.

compensating at a current loop controller for an error between an output current command and the feedback sensed current;

outputting from the current loop controller a duty cycle control signal;

feeding the duty cycle control signal to a pulse width modulation block;

outputting from the pulse width modulation block a gate signal;

feeding the gate signal and applying a dc-link voltage to the grid-tie inverter;

outputting a power circuit output from the grid-tie inverter to an inverter-side filter inductor; and

interconnecting the power circuit output to a utility power grid through a solid-state relay.