Abstract
A novel hybrid control strategy for energy storage (ES) is applied in accordance to the recommendations by the IEEE Std. 1676. The strategy mainly consists of the model predictive controller (MPC), two-degree of freedom algorithm and modified droop control. In inverter control layer, MPC and two-degree of freedom algorithm are adopted to construct a generalized controller to enable ES operate in microgrid grid-connected and islanded operation modes with a single control structure, motivating the smooth transition between microgrid operating modes. And MPC and two-degree of freedom algorithm are used in the control layer's current inner loop and voltage outer loop, respectively. Comparing with the traditional double loop control from the perspective of structure, with the novel strategy the control layer's current inner loop is without pulse width modulation (PWM) and only a single proportional-integral (PI) controller is applied in its voltage outer loop, so that ES's robust performance can be enhanced. And because of the high frequency inverter's equivalent transfer function with the smaller time constant, MPC's transfer function approximated as being unitary. Therefore, when the two-degree of freedom algorithm construct the inverse model for the controlled object in control layer's voltage outer loop, from the application layer perspective, the control layer can be recognized as unity gain. Thus, the control layer's harmful disturbances associated with the operation mode transitions can be removed. In the application layer, through adding decoupling inductance and introducing microgrid's voltage and angular frequency feedforward compensations to carry out PQ and modified droop controls, the grid-connected and island control targets of ES application layer can be realized. In particular, when microgrid operation mode is switched to island, with the help of modified droop control, microgrid's voltage and frequency can be quickly restored to standard values. Through the theoretical derivation and simulation results, it can be proved that under microgrid's plan or non-plan operation mode switching, the proposed method can not only can not only implement the microgrid smooth switching, but also strengthen its transient performance.
Highlights
Microgrid can reduce the negative impacts from intermittent distributed generations (DGs) on power distribution network, and maximize DGs’ utilization efficiency [1]–[3]
Owing to the value function fi constructed by inverter output current and its reference quantity, and if the inverter operation frequency is set at higher value, from voltage outer loop perspective, the equivalent transfer function of model predictive controller (MPC) can be approximately equal to the equation, k/(Ts + 1) ≈ 1 (T is the switching period and k is gain coefficient), referring to [36,37,38]
This paper proposes a novel control method for energy storage (ES) to realize the smooth switching of microgrid operation mode
Summary
Microgrid can reduce the negative impacts from intermittent distributed generations (DGs) on power distribution network, and maximize DGs’ utilization efficiency [1]–[3]. To solve the first problem, the pre-synchronization technology [10], the adaptive phase-locked loop (PLL) [11], [12] and the modified droop control [13] are adopted They are through amending the input voltage, frequency and other reference signals of ES control system to achieve microgrid seamless switching. Under microgrid’s operation mode switching process, owing to the transfer function unitization characteristic of the control layer, the harmful perturbation influences on ES control system can be eliminated, promoting the microgrid seamless transition. Through adding decoupling inductance and introducing microgrid voltage and angular frequency feedforward compensation to carry out PQ and modified droop controls, the smooth transition of microgrid’s voltage and frequency can be realized when microgrid’s operation modes switching is performed. The functions of the hardware layer can refer to the IEEE Std. 1676
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