Decentralized control of a line interactive Uninterruptible Power Supply (UPS)

The decentralized control of a line interactive UPS through p-ω and q-V droop control requires measurement only of variables local to the UPS and does not require a communication link. The droop control strategy has been known to result in unstable systems for large gain constants. This paper begins with a behavioral approach to system representation and presents the transfer function of the controlled system. By determining the location of the poles and zeros of the open loop transfer function, comments are made about the stability of the controlled system for various gains. The value of control gain that causes instability can be determined analytically. Simulations in Scilab are used to show the change in the poles of the controlled system with changes in control gain. Experimental results have been presented to show the operation of the line interactive UPS in transient state.