Steady-State Analysis of Electric Springs With a Novel δ Control

A novel control strategy is proposed for the recent proposed electric springs (ESs), which are connected in series with noncritical loads to form smart loads for enhancing stability of smart grid with renewable energy sources. δ control is the key concept in this paper, which is realized by controlling the phase angle of the predefined reference in a proportional resonant controller. Four critical operating functions of the ESs are analyzed with different critical loads such as resistive, inductive and capacitive types, where vector diagrams and geometric relationships are explored for δ calculation with which the ac mains voltage is regulated to the predefined value and the phase angle between the ES voltage and current is determined. With the proposed δ control, the operating modes of the ES can also be determined automatically as the input voltage varies. Operating limitations and constraints of the ESs and guidelines on how to distribute the ESs in the distributed systems are provided with δ control. Both simulation and experiment are carried out to verify the effectiveness of the proposed control strategy and theoretical analysis for the ESs.