Series-Parallel Converter-Based Microgrids

Transmission impedance shows the characteristics of variety in microgrid. Under different transmission impedance types, virtual impedance, angle droop control, and frequency droop control play important roles in maintaining system stability and load sharing among distributed generators (DGs). These approaches have been developed into three totally independent concepts, but a strong correlation exists. This chapter reveals their similarities and differences. Finally, introducing a unified droop control unifies these three independently developed droop control methods into a generalized theoretical framework.

VSGs emulate the kinetic energy of SGs to support the islanded networks to maintain grid frequency. This chapter introduces a virtual synchronous generator (VSG) control based on adaptive virtual inertia to improve dynamic frequency regulation of microgrid. When the system frequency deviates from the nominal steady-state value, the adaptive inertia control can exhibit a large inertia to slow the dynamic process and thus improve frequency nadir. And when the system frequency starts to return, a small inertia is shaped to accelerate system dynamics with a quick transient process. As a result, this flexible inertia property combines the merits of large inertia and small inertia, which contributes to the improvement of dynamic frequency response. The stability of the algorithm is proved by Lyapunov stability theory, and the guidelines on the key control parameters are provided. Finally, both hardware-in-loop (HIL) results demonstrate the effectiveness of the control algorithm.

For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this chapter introduces an improved droop control method. The introduced method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is introduced to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.

Economic dispatch is considered as one of the core problems in microgrid research. Decentralized economical dispatch methods have drawn increasing attention because of its high reliability and no communication. This chapter studies the economical dispatch problems (EDPs) in islanded microgrid and presents a criterion to determine whether the global optimal dispatch (GOD) can be realized via a decentralized manner. Besides, an optimal/suboptimal decentralized economical-sharing scheme is presented when the criterion is met/not met. Finally, both simulation and experiments are performed to validate the methods.

In the modular on-line uninterruptible power supply (UPS) system, the unequal power sharing may exist in the system due to the mismatched line resistance, leading to the circulating current, overloading, and even tripping the system. This chapter introduces a dynamic consensus algorithm (DCA)-based virtual resistance control strategy to share the active power and harmonic power. This approach does not require a central controller, and the communication links are only required between the neighboring UPS modules, which enhances system’s reliability and flexibility. Simulation results and experimental results are provided to verify the effectiveness of the control strategy.

For several reasons, particularly due to the mismatch in the feeder impedance, accurate power sharing in islanded microgrids is a challenging task. This chapter introduces a distributed event-triggered power sharing control strategy. The suggested technique adaptively regulates the virtual impedances at both fundamental positive/negative sequence and harmonic frequencies and, therefore, accurately shares the reactive, unbalanced, and harmonics powers among distributed generation (DG) units. The method requires no information of feeder impedance and involves exchanging information among units at only event-triggered times, which reduces the communication burden without affecting the system’s performance. The stability and inter-event interval are analyzed. Finally, experimental results are presented to validate the effectiveness of the scheme.

In series-type microgrid, the synchronization and power balance of distributed generators become two new issues that need to be addressed urgently. To that end, an f − P∕Q droop control is firstly introduced in this chapter, which is capable to achieve power balance under both resistive-inductive and resistive-capacitive loads autonomously. However, the f − P∕Q droop control for series-type microgrid has the problem of multiple equilibrium points. Then, this chapter introduces a new decentralized power sharing control scheme to address this problem. The uniqueness of equilibrium point and its small signal stability are proved. Besides, the power factor angle droop control is introduced to suit for any types of loads in islanded modes. Finally the feasibility of the methods is verified by simulation.

It is still an important problem to realize the optimal economical dispatch operation of series-type microgrids with capacity constraints in a communication-free manner. To address this problem, a new optimal economical dispatch control scheme is introduced for the islanded series-type microgrids without communications. There are two prominent advantages: (1) global optimal economical operation with capacity constraints is realized and (2) improved load voltage quality is obtained. Then, the stability of the introduced scheme has been analyzed with small-signal analysis method. Finally, the effectiveness of the method is verified by both simulations and experiments.

As unbalance state of charge (SOC) of storage units usually leads to the decrease of lifetime, SOC balancing control is essential. In this chapter, a decentralized SOC balancing method is introduced to balance the SOC of series-type energy storage system (CESS). Since the method does not rely on any communication, it possesses higher reliability. As is well known, SOC is a slowly changing variable compared to other variables such as voltage and current. Thus, the studied system has obvious two-time scale characteristic. In addition, the stability analysis of the system based on the singular perturbation theory is carried out. Simulation and experiments are performed to verify the validity of the SOC balancing scheme.

This chapter presents decentralized control strategies for series-connected inverters, series-connected H-bridge rectifiers, and series-connected H-bridge inverter-based static compensators (STATCOM) in medium/high voltage (MV/HV) power network without any communication, and each module makes decisions based on its own local information. In contrast, the conventional methods are usually centralized control and depend on a real-time communication. Thus, the proposed scheme has advantages of improved reliability and decreased costs. The overall system stability is analyzed, and the stability condition is derived as well. The feasibility of the method is verified by simulations and experimental results.

In this chapter, a novel hybrid voltage/current control scheme with low communication burden is introduced for series-type inverters in a decentralized manner. All the inverter units are controlled by their individual local controllers without a real-time central global communication. Among them, a few inverters serve as the Current-Controlled Inverters (CCIs) to regulate the grid current, and the rest inverters work as the Voltage-Controlled Inverters (VCIs). Under this control architecture, five main control targets are achieved: (1) frequency self-synchronization of each VCI inverter based on local current phase-angle signals; (2) fault-tolerant capability of all inverters without one-to-all-failure; (3) high disturbance-rejection-capability to grid variations; (4) adjustable grid power factor (PF); (5) independent power control for inverters with unbalanced power sources.

In order to ensure the uninterrupted power supply, it is necessary for microgrid to operate in both grid-connected mode and islanded mode. To address this concern, this chapter introduces a unified decentralized control strategy for both grid-connected and islanded operation of the series-type microgrid (CMG). It can realize the smooth mode switching without the need of changing controllers. In terms of the scheme, the system always holds a unique equilibrium point regardless of the grid-connected or islanded operation. Since the CMG is performed in decentralized manner without any communication, it is a reliable and cost-effective solution. Moreover, the self-synchronization of each DG is obtained under both the resistance-inductance (RL) and resistance-capacitance (RC) loads. The small-signal stability is proved and the design of control parameters is given. Finally, the feasibility of the method is verified by simulation and OPAL-RT based real-time simulation results.

Hybrid series–parallel microgrid is becoming a new emerging structure to integrate multiple low-voltage power sources. This chapter briefly compares and analyzes the decentralized power control strategy of parallel microgrid and series microgrid, and try to explore a unified decentralized synchronous control for hybrid series–parallel microgrid. A globally distributed control strategy is presented to implement power sharing control in hybrid series–parallel microgrid under both resistive-inductive (RL) and resistive-capacitive (RC) load, where a sign function is introduced to automatically match load characteristic. Active power and reactive power regulators without frequency drop are developed, and low-bandwidth communication network is employed to support power management and improve system redundancy. Furthermore, small-signal model of hybrid series–parallel microgrid with RL load and RC load is established. Also, small-signal stability and dynamic performance of the proposed distributed control strategy is investigated. Simulation results show that the globally distributed control strategy is able to implement desirable power sharing under different load types with superior control performance. Also, the proposed control strategy is able to improve system redundancy and support plug-and-play operation of microgrid.

This chapter presents a local-distributed control for hybrid series–parallel microgrid in islanded mode. A number of local converters in series type are controlled as a consistent string by distributed cooperative control without a central controller, and power demand among parallel strings is shared in a decentralized manner. Hence only an in-string sparse low-bandwidth communication (LBC) network is sufficient for limited information exchange among local converters. With the control, the frequency synchronization and proper power sharing can be achieved under both the resistive-inductive (RL) and resistive-capacitive (RC) loads. Moreover, the reliability and redundancy of hybrid series–parallel microgrid are improved, and the communication costs are reduced. The overall system stability is analyzed and reasonable design ranges of control parameters are given. Finally, the feasibility and effectiveness of the proposed solution are verified by simulation tests.

Hybrid series–parallel structure provides an effective mean for large-scale energy storage system (ESS) integrating low voltage level energy storage units (ESUs). In ESS, the SOC balancing control plays an essential role. In this chapter, a local-distributed and global-decentralized SOC balancing control strategy is introduced for hybrid series–parallel ESS. Under the proposed scheme, the SOC of ESUs in each local string converge to the same value driven in a distributed manner and the SOC balance among different strings and power sharing of the ESS under both the charging and discharging modes are realized in a decentralized manner. Because there are no communication links between dispersed ESU strings, the scalability of the ESS is improved. Small-signal stability analysis is carried out, which provides a guide for parameter design. Simulation results verify the effectiveness and correctness of the proposed method.

In this chapter, we introduce a leader-distributed follower-decentralized (LDFD) control strategy for series–parallel microgrids (SPMGs) in the islanded mode, under which the economic dispatch (ED) is achieved with voltage quality guaranteed and frequency synchronization. In this control strategy, a distributed control method is presented for the leader of distributed generators (DGs) and an improved droop control method is designed for other DGs (followers) to optimize the generation cost and frequency control of the system. Furthermore, a unified voltage regulator is proposed to make the power factor of each DG in the same string equal. Since only the leader in each string communicates with its neighboring leaders with low data transfer rate, very sparse low-bandwidth communication networks are needed in this scheme, which reduces costs and increases the reliability in the event of a communication failure. In addition, the small-signal stability of the system is analyzed and the design of the related parameters is given. Some cases are designed to evaluate the performance of the strategy, and the simulation results verify the effectiveness of the proposed method.