Fen Tang

Autonomous Active Power Control for Islanded AC Microgrids With Photovoltaic Generation and Energy Storage System

In an islanded ac microgrid with distributed energy storage system (ESS), photovoltaic (PV) generation, and loads, a coordinated active power regulation is required to ensure efficient utilization of renewable energy, while keeping the ESS from overcharge and overdischarge conditions. In this study, an autonomous active power control strategy is proposed for ac-islanded microgrids in order to achieve power management in a decentralized manner. The proposed control algorithm is based on frequency bus-signaling of ESS and uses only local measurements for power distribution among microgrid elements. Moreover, this study also presents a hierarchical control structure for ac microgrids that is able to integrate the ESS, PV systems, and loads. Hereby, basic power management function is realized locally in primary level, while strict frequency regulation can be achieved by using additional secondary controller. Finally, real-time simulation results under various state of charge (SoC) and irradiance conditions are presented in order to prove the validity of the proposed approach.

A Control Architecture to Coordinate Renewable Energy Sources and Energy Storage Systems in Islanded Microgrids

Coordinated operation of microgrids requires that energy management system takes into account both the available power in renewable energy sources (RES) and storage capacity of energy storage systems (ESS). In this paper, a coordinated architecture of islanded ac microgrids with smooth switching droop control (SSDC) is derived. Based on the proposed SSDC approach, flexible power control of each ESS/RES unit can be obtained with seamless modes changes. Furthermore, decentralized power management can be achieved by executing frequency bus-signaling. The power management principle based on different operational modes is explained in detail and small-signal analysis is carried out for SSDC. Real-time hardware-in-the-loop results of an islanded microgrid are provided under several scenarios to validate the proposed coordinated control strategy.

Distributed Voltage Unbalance Compensation in Islanded Microgrids by Using a Dynamic Consensus Algorithm

In islanded microgrids (MGs), distributed generators (DGs) can be employed as distributed compensators for improving the power quality in the consumer side. Two-level hierarchical control can be used for voltage unbalance compensation. Primary level, consisting of droop control and virtual impedance, can be applied to help the positive sequence active and reactive power sharing. Secondary level is used to assist voltage unbalance compensation. However, if distribution line differences are considered, the negative sequence current cannot be well shared among DGs. In order to overcome this problem, this paper proposes a distributed negative sequence current sharing method by using a dynamic consensus algorithm. In clear contrast with the previously proposed methods, this approach does not require a dedicated central controller, and the communication links are only required between neighboring DGs. The method is based on the modeling and analysis of the unbalanced system. Experimental results from an islanded MG system consisting of three 2.2-kVA inverters are shown to demonstrate the effectiveness of the method.

Leakage current analysis of a single-phase transformer-less PV inverter connected to the grid

Due to the large surface of the PV generator, its stray capacity with respect to the ground reaches values that can be quite high. When no transformer is used in a grid-connected PV system, common-mode current, which caused by the common mode voltage, can flow through the stray capacitance between the PV array and the ground. It is quite harmful to the body safety and PV system. In order to avoid leakage current, different inverter topologies that generate no varying common-mode voltages, such as bipolar pulse-width modulation (PWM) full-bridge topology, NPC topology have been proposed. From the safety and energy saving viewpoint, it is necessary to develop a higher efficiency topology. In this paper, the generation mechanism of common mode current is discussed. Then different methods used to eliminate the leakage current are compared. Finally, the full-bridge which generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full-bridge is given. The proposed topology has been verified in the simulation based on MATLAB with satisfactory results.

Tertiary Control of Voltage Unbalance Compensation for Optimal Power Quality in Islanded Microgrids

In multibus islanded microgrids, the power quality requirements for different areas and buses can be different. This paper proposes a hierarchical control to realize optimal unbalance compensation for satisfying the power quality requirements in different areas. Primary and secondary controllers are applied to realize unbalance compensation for critical bus, and at the same time, to make distributed generators (DGs) equally share the compensation efforts. Tertiary control, which inherently is an optimization method, is implemented to adjust the compensating effort of each DG considering the voltage unbalance limits in local buses and DG terminals. This method realizes multipower-quality-level control in a multibus islanded system by optimally utilizing DGs as distributed compensators and saves the investment for additional compensation equipment. Hardware-in-the-loop results demonstrate the effectiveness of the method.

Coordinated Control Based on Bus-Signaling and Virtual Inertia for Islanded DC Microgrids

A low-voltage islanded dc microgrid contains a number of renewable energy sources, local loads, and energy storage systems (ESS). To avoid the over-charging and over-discharging situations of ESS, a coordinated control strategy should be used in islanded dc microgrids. In this paper, a novel bus-signaling method (BSM) is proposed to achieve autonomous coordinated performance of system according to different state of charge conditions. Additionally, a secondary coordinated control is introduced to restore the voltage deviation produced by primary control level without decaying coordinated performance. The proposed control algorithm and controller implementation based on BSM are also presented. Finally, real-time simulation results show the feasibility of the proposed approach by presenting the operation of an islanded dc microgrid in different testing scenarios.

Parallel interleaved grid-connected converters in MW-level wind power generation

With the individual capacity of wind power generation system increasing, the demand for high power grid-connected converters is growing, which is usually limited by the available semiconductor technology. One solution is multiple lower power units connected in parallel. In this paper, direct parallel and interleaved parallel are analyzed and compared. Then it focuses on two parallel interleaved converters. The generating mechanism of high frequency circulating current and low frequency oscillation circulating current are both studied. Also, an interleaved SVPWM and the corresponding control strategy of circulating current are introduced. Finally, the described strategy has been implemented in a 1.5MW grid-connected converter for wind power generation, providing a satisfactory performance.

Design algorithm of grid-side LCL-filter for three-phase voltage source PWM rectifier

A design algorithm for grid-side LCL-filter of three-phase voltage source PWM rectifier is presented, which allows to use reduced values of inductance, improve system dynamic performance and reduce cost compared to traditional L-filter. These advantages are even more attractive in medium and high power applications. In this paper, the design criterion and calculation procedures are introduced in detail. A design example is reported, and the obtained LCL-filter has been realized and tested by simulation and experiments. Experimental results show that the obtained LCL-filter can provide sufficient attenuation of current harmonics and meanwhile ensure a high grid-side power factor. The goodness of this design method is demonstrated.

Distributed Active Synchronization Strategy for Microgrid Seamless Reconnection to the Grid Under Unbalance and Harmonic Distortion

Microgrids can operate in both grid-connected and islanded modes. In order to seamlessly transfer from islanded to grid-connected modes, it is necessary to synchronize microgrid voltage and frequency, and phase to the main grid. However, since the microgrid is often based on power electronic converters, the synchronization process is quite different compared with the quasi-synchronism control in conventional power systems. First, in order to address this concern, the microgrid synchronization criteria are derived. Based on these criteria, a novel distributed active synchronization strategy is proposed, which takes into account not only the fundamental component, but also positive and negative sequences of the harmonic components. This way, a seamless reconnection to the main grid can be performed. The proposed method is implemented in the secondary control level of a hierarchical control structure. Real-time hardware-in-the-loop results show the feasibility of the proposed technique.

Coordinated power control strategy based on primary-frequency-signaling for islanded microgrids

In a flexible microgrid, the power regulation of each electronic-converter-based unit should be not only determined by the load demand, but also controlled according to the power and energy available in each unit. This paper proposes a coordinated control strategy in which each unit can operate in different operation modes taking into account the resource limitation. Firstly, a Primary-Frequency-Signaling (PFS) is introduced to realize coordinated control between units in a distributed way. Then the whole control structure of system is descripted in detail, which includes innerloop control with virtual impedance and primary control based on droop method for modes changes. The four modes changes for operation of ESS and RES are illustrated. Finally, simulation results will be presented to demonstrate and validate the proposed control strategy.