Abstract
The issue of frequency stability of microgrids under islanded operation mode and mode transfer has attracted particular attention recently. In this paper, a cooperative frequency control method, which consists of a microgrid central control (MGCC) and microgrid local control (MGLC), is proposed to achieve a seamless transfer from grid-connected to islanded mode, and hence increase the frequency stability of islanded microgrids during both primary and secondary frequency control. A power deficiency prediction and distribution method is proposed in MGCC to effectively distribute and utilize the power and loads, and accomplish the cooperative control of all microgrid units. With regards to MGLC, a Hopfield fuzzy neural network control (HFNNC) is applied to make the corresponding frequency control of DFIG-SMES more adaptive. Meanwhile a state of capacity (SOC) control is utilized in battery energy storage (BES) to extend battery life. Simulation results indicate that the proposed frequency control approach can maintain the frequency stability of islanded microgrids even in emergency conditions.
Highlights
The requirements to alleviate the energy crisis and the concerns over the potentially adverse effects on the environment have led to the rapid expansion of microgrid technology in recent years [1,2]
According to the differences on response time, the frequency control in islanded operation mode can be divided into two parts, the ESs and rotor kinetic energy (RKE) act in the primary frequency control, and the others act in the secondary frequency control
ESs during primary frequency control; Step 3: microgrid central control (MGCC) predicts the power deficiency ΔP according to Equation (6), and distributes the power deficiency to DGs and reserved power control (RPC) of DFIG as well as loads in secondary frequency control; Step 4: The active power references calculated by MGCC is distributed to the DGs and controllable loads, the power deficiency is balanced by the above control strategies, the power outputs of the ESs could be restored to secure the maximum emergency reserved power
Summary
The requirements to alleviate the energy crisis and the concerns over the potentially adverse effects on the environment have led to the rapid expansion of microgrid technology in recent years [1,2]. In [20], a cooperative control strategy of microsources and an energy storage system during islanded operation is presented and evaluated by simulation and experiment This scheme utilizes stored energy to make up the power deficiency in primary frequency control. This paper addresses a two-layer cooperative control strategy, which consists of a microgrid central control (MGCC) and microgrid local control (MGLC), to achieve a seamless transfer from grid-connected to islanded mode, and increase the frequency stability of islanded microgrids. In. MGCC, a power deficiency prediction and distribution method based on rate of change of frequency (ROCOF) and equivalent inertia is proposed to achieve cooperative control and effectively utilize the power and load. The remainder of the paper is organized as follows: Section 2 gives a brief introduction on the frequency control of islanded microgrids with a DFIG-SMES module and BES; Section 3 introduces the proposed cooperative frequency control strategy for autonomous microgrid; the proposed method is illustrated and investigated with a simulation system in Section 4; the conclusions are duly drawn
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