4.1 Introduction

Port microgrid is an organic combination of the distributed generator (DG), energy storage, and load, with two modes of operation: grid-connected and islanded, and is one of the most important ways to effectively use renewable energy [1, 2]. Microgrids are positioned in medium and low-voltage distribution networks and support plug-and-play and seamless switching by efficiently coordinating the operation of micropower, energy storage, and load to provide self-control, protection, and management [3, 4]. The microgrid provides a platform for liaison and interaction between the grid, DG, energy storage, and load due to its flexible mode of operation and high dependability of power supply. With the ongoing implementation and promotion of new energy policies and the increasing penetration of DG, particularly renewable energy generation [5,6,7], it is difficult for a single microgrid to fully consume and effectively utilize a large quantity of dispersed connected DG [8].

As an extension of a single microgrid's structure and function, multiple microgrids (MMGs) can increase the DG consumption and control capacity by clustering the operation of numerous sub-microgrids [9], as well as the microgrid's reliability and economics. Because of this, the mode of microgrid building has evolved from a single small-scale demonstration to the clustering of numerous microgrids in locations with abundant distributed energy supplies [10, 11].

MMGs are combinations of sub-microgrids aggregated to increase the interconnected interactive coordination of sub-microgrids. The EU Framework Plan for Science and Technological Development FP6 pioneered the control architecture of MMGs, which consists of a distribution network management layer, a centralized control layer, and a microgrid control layer [9]. Literature [10] suggested an AC contact line-based interconnection strategy for MMGs to accomplish reactive power synergy during normal system operation and frequency control during islanded operation while enabling the sub-microgrid black start. Direct connection of several microgrids across stations, voltage levels, and control systems to form complementary interconnections is challenging to do using the AC interconnection approach [11].

MMGs coordinate many microgrids by complementing one another to achieve energy balance, mutual aid, and other operational objectives under various operating scenarios [12, 13]. Current research on MMGs focuses on control, optimization, and management in order to improve the reliability and economics of operation through the design and development of practical control and operation systems. Literature [14] reduces the control hierarchy of MMGs into two layers and employs the cooperative operation of several microgrids to enhance reliability by creating a standby capacity mechanism. Based on the peculiarities of the operation mode of MMGs, the literature [15] suggests an intertemporal strategy and real-time scheduling strategy to achieve the flexible and economical operation of the system. To meet the operational aims and needs of MMGs, the previous research presented coordinated operation solutions from various perspectives. Nevertheless, the sub-microgrids are connected to the grid in an AC synchronous manner and coordinate with one another in a uniform manner, which severely restricts the operational flexibility and economic potential of MMG. Actual operation of AC interconnection MMGs reveals that the sub-microgrids have a significant impact on one another and that the control flexibility is limited. In some cases, the parallel operation of multiple microgrids is unable to perform the originally specified regulation and control functions, and the islanding mode makes it difficult to operate in a stable manner due to microgrid disturbances or abnormalities.

The use of multi-terminal DC to construct flexible interconnection of sub-microgrids is a novel approach to structurally altering MMGs, and there are few pertinent research results and papers. ABB's Hrithik Majumder first proposed connecting various numbers and types of microgrids using voltage-sourced converters [16], but he did not elaborate on the synergy and interaction of MMGs. MD. Jahangir Hossain et al. recommended the use of multi-terminal flexible DC technology to regulate MMG islanding operating currents in order to increase system operating stability [17], but they did not take the flexibility of operating modes into account. The literature [18, 19] proposed using droop-controlled flexible converters to connect different areas of the distribution network to achieve reasonable power distribution and stable system operation; the literature [20] combined flexible DC transmission technology with distribution networks to establish a multi-terminated flexible DC distribution network across station areas; and the related studies provided technical ideas for the application of flexible DC in distribution networks. Currently, the cost of voltage source converters based on mature topology architecture is gradually decreasing, and it is increasingly used in soft switches, active filters, and low-voltage DC distribution networks. The investment cost and technical economy meet the requirements, and the technical economy is still improving [20]. MMGs use voltage source converters to establish interconnection structures, which not only improves the access and consumption capacity of new energy sources, but also reduces the investment cost and improves the technical economy.

To improve the flexibility and coordination of MMGs operation, this chapter first analyzes the typical connection mode of MMGs and then proposes a hybrid unit of common coupling (HUCC) for the microgrid to establish the flexible interconnection structure of MMGs; based on the combination of the switch, bidirectional converter, and power conditioning unit in HUCC, the connection mode of MMGs flexible interconnection system is proposed with a flexible interconnection string. The connection mode and control mode of flexible interconnection system are proposed based on the combination of the switch, bi-directional converter, and power regulation unit in HUCC; three operation modes of grid connection, islanding, and emergency and their switching methods are analyzed; the multi-layer control system of flexible connected MMGs (FCMMGs) is established; and the control methods of central layer, an interface layer, and a micro network layer are presented. The simulation model is built, and the simulation results indicate that FCMMGs and their control scheme have good operating performance and are an important approach to access and consume widely distributed power sources.

4.2 Typical Structure and Characteristics of MMGs

Microgrids offer the benefits of independence and autonomy, coordination and optimization with high efficiency and dependability, but their scale and operational capacity are determined by DG penetration, control methods, and the capacity of the primary power source [3, 4]. In regions with a significant number of DGs, several independently dispersed sub-microgrids serve as the foundation for resolving this technical issue, and the electrical connection between them is enhanced using a rational connection mechanism and organizational structure. MMGs generally use AC interconnection [7, 12] with the following three characteristics: (1) the presence of two or more adjacent sub-micro-networks; (2) the existence of energy and information interactions between sub-micro-networks; and (3) the ability of each sub-micro-network to operate independently or collaboratively to accomplish common goals.

Figure 4.1 depicts the usual configuration of a MMGs system based on AC connection, which consists of four independent sub-microgrids from MG1 to MG4; each sub-microgrid has complete control and protection devices [21] and can access different types of DGs, energy storage, and local loads, etc. MMGs provide the foundation for energy mutualization, frequency and voltage support, and cooperative operation between sub-microgrids.

Fig. 4.1
figure 1

A typical MMG structure and organization

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As depicted in Fig. 4.1 According to the “source-feed” connection of access, the fundamental structure of MMGs can be categorized into one of three types.

  1. (1)

    Same-source and same-feeder (SSSF) type: several microgrids are connected to the same feeder and interchange power with the same superior power source, such as MG1 and MG2;

  2. (2)

    Same-source and different-feeder (SSDF) type: many microgrids are connected to the grid by separate feeders, but are connected to the same power source, and the electrical connection between multiple microgrids is near, as in MG1 and MG3;

  3. (3)

    Different-source and different-feeder (DSDF) type: multiple microgrids are connected to the grid via different feeders and connected to different superior power sources, and the superior power sources are typically in the unlisted state between them, and the electrical relationship between sub-microgrids is weak, as in MG1 and MG4.

Currently, MMGs generally establish interconnection relationships through communication methods. The three types of structures, SSSF, SSDF, and DSDF, form the basis of interconnecting MMGs, and the three basic structures can be switched through a point of common coupling (PCC) according to operational needs. As shown in Fig. 4.2, PCC, as a key node connecting MMGs, can adjust the topology of MMGs by switching states, controlling the grid connection and disconnection between microgrids and between MMGs and higher-level power grids. At the same time, PCC can provide measurement information on electrical quantities and operational status, supporting the coordinated control and operation of a group of microgrids. As the number of interconnected microgrids continues to increase, the combination of basic structures and switching of PCC states enables MMGs to adapt to complex operating environments.

Fig. 4.2
figure 2

Structure and function of microgrid PCC

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MMGs feature significant electromagnetic coupling between AC interconnected sub-microgrids, and there are technical issues with synchronous grid connection, fault isolation, flexible operation, etc. MMGs achieve flexible connections by decoupling active and reactive components via voltage source converters and configuring a set amount of energy storage to form HUCCs with two types of grid connection interfaces, AC and DC. Compared with AC interconnection, the HUCC of MMGs can, on the one hand, smooth out disturbances and quickly isolate faults, reduce the impact of disturbances, and improve the operational stability of the system and individual microgrid; on the other hand, through the decoupling control of active and reactive components, it increases the control dimension of the system, has the capability of asynchronous interconnection, meets the demand of multiple operation modes, and reduces the impact of disturbances on the system and individual microgrid.

4.3 Flexible Interconnection of MMGs

4.3.1 HUCC Structure and Operation Mode

The flexibility and coordinated operation of MMGs are based on their interface technology. This chapter proposes a hybrid point of common coupling (PCC) unit that integrates both AC and DC connection methods. On the one hand, microgrids are connected to the higher-level power grid through the AC interface, fully utilizing the support function of the power grid and improving the frequency and voltage stability of MMGs. On the other hand, flexible interconnection between microgrids is achieved through the DC interface, fully leveraging the compatibility and control capabilities of the DC unit, and improving the scalability and transient stability of multi-microgrid systems.

HUCC is an organizational and control unit for interconnecting MMGs. As shown in Fig. 4.3, it consists of AC and DC interfaces, power regulation units, control and protection systems, etc. To ensure the reliability and flexibility of HUCC operation, a three-bus structure is adopted for its primary wiring, where the common connection bus serves as a bridge between the microgrid and AC/DC interfaces, responsible for connecting the AC bus, DC bus, and power regulation unit. The AC and DC buses provide external interfaces, with the AC interface typically used for connection to the public grid and the DC interface used for interconnecting microgrids. To flexibly combine the SSSF, SSDF, and DSDF structures and minimize the isolated fault area, corresponding types of circuit breakers are configured for the AC and DC interfaces, respectively.

Fig. 4.3
figure 3

Configuration fundamentals of the HUCC

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As indicated in Fig. 4.3, the DC bus is connected to the common connection bus via the converter, and the voltage source type is preferred for the converter in order to increase the control dimension. HUCC can configure modular multi-level converter (MMC) to increase the efficiency and power quality of multi-microgrid operation due to MMC’s small size, low loss, and high output waveform quality. The power regulation unit primarily employs an energy storage system (ESS) to optimize the regulation of sub-microgrid grid characteristics, such as power fluctuations, etc. To increase the economics, the energy storage system can be constructed individually or by utilizing existing energy storage equipment in the microgrid.

As a unified interface for the flexible interconnection of MMGs, the HUCC relies on a coordinated combination of switching states, MMC control modes, and quick ESS response to perform its duties. Based on the actual operation of MMGs, the operation mode of the HUCC consists of two layers: connection mode and control mode, and the switching of the HUCC connection mode is based on the combination of switching states, which can be categorized into three types: AC mode, DC mode, and hybrid mode, with the following operating characteristics.

  1. (1)

    AC mode: AC mode is used to connect sub-microgrids to the upper-level distribution networks, and the steady voltage and frequency of distribution networks is utilized to support MMGs and increase system operating stability.

  2. (2)

    DC mode: Based on MMC to realize flexible interconnection between mutual mass sub-microgrids, MMC control mode and characteristics can effectively solve the technical problems of AC connection, such as power balance and electromagnetic coupling. This mode enables access to sub-microgrids and improves DG's consumption capacity.

  3. (3)

    Hybrid mode: Hybrid mode is a combination of AC and DC modes with the benefits of 2 independent modes; also, AC and DC connections in hybrid mode can be backed up by each other, making hybrid mode the most prevalent operating mode of FCMMGs.

Matching the connection mode, HUCC employs the flexible control of MMC and ESS to enhance the operational stability and dependability of the system. Due to the limited capacity of individual MMCs in MMGs, the active power-DC voltage (P-U) droop control is used for the DC interface MMCs of controllable sub-microgrids to jointly participate in the voltage stability control of the interconnection system [24, 25]; for uncontrollable sub-microgrids (microgrids that cannot operate independently and require voltage and frequency support from the grid) then constant AC voltage/frequency (v/f) control is used [26]. The ESS functions as a regulator for power balancing and fluctuation reduction and collaborates with the MMC to meet the multi-flexible microgrid's control objectives. The HUCC control mode is determined by the operating needs of MMGs and is subdivided as follows: P–U droop mode and v/f mode.

  1. (1)

    P–U droop mode: MMC adopts P–U droop control, which can provide voltage support for DC-connected networks and accomplish flexible power dispatching, and ESS as an auxiliary smoothing fluctuation employs P–U mode.

  2. (2)

    v/f mode: MMC utilizes fixed AC voltage and frequency control, ESS functions primarily for peak regulation and valley filling, and sub-microgrid HUCC without an adjustable power source often utilizes this mode.

As the core of flexible interconnection in MMGs, HUCC not only has structural compatibility but also has the ability to coordinate control and optimize operation. Compared with the PCC interface, HUCC uses the DC interface to isolate the electromagnetic coupling between different microgrids, supports asynchronous interconnection of multiple microgrids, and can provide power to passive systems. By decoupling the power components in the multi-microgrid system through MMC, HUCC achieves separate control of active and reactive power, improving the controllability and flexibility of the system operation.

4.3.2 Flexible Interconnection Solution for HUCC-Based MMGs

The AC interface is utilized to connect the microgrid to the higher-level grid, while the DC interface is used for the flexible connectivity of many sub-microgrids. The flexible interconnection of MMGs is based on the classic multi-microgrid structure, and the configuration and interconnection regulations of MMGs are established by the three types of fundamental units, SSSF, SSDF, and DSDF. The flexible interconnection rules of MMGs are outlined in the next section.

  1. (1)

    Several sub-microgrids under the basic structure of SSSF have the same “source-feeder” characteristics, are interconnected by DC interfaces, and can be connected to the higher grid via AC interfaces, allowing for grid-connected or islanded operation;

  2. (2)

    The connection rules of sub-microgrid under SSDF basic structure are the same as (1);

  3. (3)

    The sub-microgrid under DSDF basic structure uses a DC interface to form a flexible interconnection on the one hand, and on the other hand, the AC interfaces of its sub-microgrid are all connected to the superior grid due to the access to different power sources.

Figure 4.4 depicts the FCMMGs developed in accordance with the above flexible interconnection rules, based on the usual construction of MMGs depicted in Fig. 4.1. MMGs have a more adaptable mode of operation due to their connectivity layout. During normal operation, FCMMGs can operate in two modes: grid-connected and islanded. In the event of an emergency, HUCC can be utilized to rapidly reconfigure the system in order to restore power to important locations and ensure power quality.

Fig. 4.4
figure 4

Structure of HUCC-based FCMMGs

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4.3.3 Operation Modes and Mode Switching of FCMMGs

  1. (1)

    Operation Modes of FCMMGs

FCMMGs offer versatile operation modes, the most common of which being grid-connected mode, islanding mode, and emergency mode. The HUCC connection and control modes corresponding to each mode are distinct. Take the MMGs depicted in Fig. 4.4 as an example, where MG1, MG2, and MG4 are controllable microgrids and MG3 is a non-controllable microgrid; Table 4.1 illustrates the associated HUCC states and control techniques for various operation modes.

Table 4.1 Operation modes of FCMMGs
Full size table

Note that in Table 4.1, 1 indicates the switch is closed, whereas 0 indicates the switch is open. To maximize DG utilization, FCMMGs typically operate in grid-connected mode, as depicted in Fig. 4.4 and Table 4.1, where the HUCC of MG1 and MG4 is in hybrid connection mode and the HUCC of MG2 and MG3 is in DC connection mode. MG1 and MG4 of DSDF exchange electricity with the superior grid via the AC interface, while relying on the superior grid to provide voltage and frequency support

When there is a defect or anomaly in the higher-level grid, HUCC swiftly modifies the connection mode and MMGs enter islanding operation. During islanding operation, the HUCC of all sub-microgrids adopts DC connection mode and disconnects the AC connection with the superior AC grid, the sub-microgrids operate in cooperation with one another to improve operational stability. The MMGs grid structure and control mode are essentially identical to grid-connected operation, as shown in Table 4.1.

MMGs have emergency operating modes such as black start and fault recovery, in addition to grid-connected and islanding operation modes. The structure of MMGs in emergency mode is highly variable, and each sub-microgrid can operate independently in the islanding state as depicted in Table 4.1, or in local interconnection states such as MG1 and MG2 interconnection, depending on operational requirements. Consequently, the control mode in emergency mode is also highly variable. Each sub-microgrid system is in a decentralized islanding state when the system fails and needs to be “black-started” by the microgrid. After all sub-microgrid DC connections have been completed, some HUCCs convert to hybrid connection and return to grid-connected mode, which can offer voltage and frequency support for the higher-level grid, as the power supply is progressively restored.

  1. (2)

    Operation Mode Switching of FCMMGs

FCMMGs have three typical operation modes that can be changed based on operational objectives and restrictions when various types of system events occur. FCMMGs will switch between grid-connected mode, islanding mode, and emergency mode, as illustrated in Table 4.2. When there is a defect in the superior grid or low power quality, FCMMGs will disconnect from the superior grid and switch from grid-connected operation to islanding operation to ensure the safe and stable functioning of MMGs and good power quality. During the islanding operation, the power output of each sub-microgrid DG and energy storage system will be regulated to maintain the power balance of MMGs. Similarly, when a defect or low power quality in MMGs negatively impacts the higher-level grid, MMGs will convert to islanding operations to protect the higher-level system. When the aforementioned faults and anomalies have been rectified, FCMMGs will reconnect to the higher-level power grid and resume grid-connected operation.

Table 4.2 Operation modes switch conditions of FCMMGs
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Contingency MMGs extreme fault removal, switch to islanding, then switch to grid connection MMGs severe fault removal—switch to islanding, then switch to grid connection.

In addition, when there is a significant fault in MMGs and the system cannot maintain grid-connected or islanded operation, MMGs will be delisted and each sub-microgrid will enter emergency operation mode. After the fault has been cleared, each isolated microgrid will be black-started, gradually restoring flexible interconnection operation and eventually returning to grid-connected or islanded operation mode.

Due to the strong adaptability of the control methods used in MMC and ESS in HUCC, and the use of peer-to-peer control in controllable microgrids, the control methods of HUCC and microgrids can autonomously adapt during the mode switching of FCMMGs. During the switching process, only the suppression of impacts needs to be considered to improve the stability of FCMMGs mode switching. The switch from grid-connected mode to islanded mode includes two types: planned switching and unplanned switching. Planned switching adjusts the control of HUCC and microgrids to exchange power with the higher-level power grid through the AC interface until the power exchange drops to zero, and then disconnects the connection when the voltage crosses zero, smoothly switching modes. Unplanned switching usually refers to abnormalities or faults in the higher-level distribution network or MMGs, where the AC interface circuit breaker isolates the fault. At this time, the MMGs system responds quickly and ensures power balance within the system by adjusting the microgrids and HUCC, improving the stability of MMGs switching from grid-connected to islanded mode. The switch from islanded mode to grid-connected mode is a planned switch. MMGs control the ESS of HUCC, adjust the voltage amplitude, phase angle, and frequency of the AC interface bus, and close the AC circuit breaker when the grid connection conditions are met, smoothly switching to grid-connected mode.

4.4 HUCC-Based Control Strategies for FCMMGs

4.4.1 FCMMGs Control Architecture

The control approach of MMGs is crucial to their flexible connectivity and flexible functioning. Taking into account various operation scenarios, FCMMGs employ a three-layer design for coordination and control, as depicted in Fig. 4.5. The interface layer control uses the center layer command to determine the switching state, MMC control mode, and ESS control method, where MMC is used to meet the operation target under specific conditions.

Fig. 4.5
figure 5

Control architecture of HUCC-based MMGs

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Figure 4.5 depicts the control system architecture and control information flow of FCMMGs. The central layer controller generates control targets and constraints based on operational scenarios and transmits control information to the interface layer and the microgrid layer; the interface layer consists of the switch controller, the MMC controller, and the ESS controller; the switch controller controls the HUCC connection mode and switches accordingly; the MMC controller primarily completes the cooperative operation of MMGs; and the ESS controller suppresses power outages.

After the flexible interconnection of MMGs, power exchange between microgrids is carried out in the form of DC, but the overall operation of MMGs needs to cooperate with the scheduling strategy of the higher-level AC distribution network. The distribution network scheduling system collects real-time information and predictive information of micro power sources and loads in FCMMGs through the central layer, mainly achieving short-term power balance and long-term energy management of MMGs. The former is mainly used to control the voltage and frequency within the system and the smooth switching of MMGs operating modes, while the latter mainly performs economic dispatch of microgrid groups to achieve optimized operation. The distribution scheduling system formulates scheduling strategies and issues them to the central layer of MMGs. The central layer combines operating constraints and goals to achieve power balance, stability, and economic operation.

4.4.2 Control Technique for the Central Layer of FCMMGs

  1. (1)

    Grid-Connected Operation Mode

The goal of FCMMGs is to fully utilize and maximize the use of renewable energy under reliable and stable conditions. When FCMMGs are grid-connected, they use the voltage and frequency support provided by the higher-level distribution network, and optimize the distribution of sub-microgrid currents through HUCC to improve the utilization rate of DG. Therefore, the operating goals of FCMMGs can be summarized as:

  1. 1.

    Maximizing the consumption of renewable energy generation;

  2. 2.

    Flexibly controlling the active and reactive power between multiple microgrids, and optimizing the control of power distribution.

To achieve these operating goals, DG in each sub-microgrid operates at maximum power, the MMC in HUCC allocates excess power between sub-microgrids, and the ESS smoothes power fluctuations. The remaining power or power shortage of FCMMGs is consumed or supplemented by the higher-level distribution network, usually through local consumption of FCMMGs. When regulating the grid-connected operation of FCMMGs, the system should meet operating constraints to ensure stability, safety, and quality, and the grid-connected operation constraints of FCMMGs are shown in Eq. (4.1):

$$\left\{ {\begin{array}{*{20}c} {\left| {P_{T}^{i} } \right| \le P_{T\,\max }^{i} } \\ {\left| {P_{C}^{j} } \right| \le P_{C\,\max }^{j} } \\ {\left| {P_{E}^{j} } \right| \le P_{E\,\max }^{j} } \\ {P_{T}^{j} + P_{C}^{j} - P_{L}^{j} + P_{E}^{j} + P_{G}^{j} = 0} \\ {\left| {\frac{{f_{MG}^{j} - f_{{{\text{MG}}}}^{j\;\;\;*} }}{{f_{{{\text{MG}}}}^{j\;\;\;*} }}} \right| \le \varepsilon_{1} } \\ {\left| {\frac{{U_{{{\text{MG}}}}^{j} - U_{{{\text{MG}}}}^{j\;\;\;*} }}{{U_{{{\text{MG}}}}^{j\;\;\;*} }}} \right| \le \varepsilon_{2} } \\ {\left| {\frac{{U_{{{\text{DC}}}}^{j} - U_{{{\text{DC}}}}^{j\;\;\;*} }}{{U_{{{\text{DC}}}}^{j\;\;\;*} }}} \right| \le \varepsilon_{3} } \\ {T_{{{\text{THDMG}}}}^{j} \le \delta } \\ \end{array} } \right.$$
(4.1)

where the superscript \(i \in \left[ {1,N} \right]\), \(j \in \left[ {1,M} \right]\) denotes the ith AC feeder transformer and the jth microgrid, respectively; N denotes the total number of AC feeder transformers; M denotes the total number of microgrids; \(P_{T}^{i}\) and \(P_{T\max }^{i}\) denotes the power and maximum power of AC feeder transformers; \(P_{T}^{j}\) is the grid-connected transformer of the jth microgrid, the positive direction of power flow to the microgrid; \(P_{C}^{j}\) and \(P_{C\max }^{j}\) denotes the power and maximum power of MMC, the are the total harmonics limits of the sub-microgrid. The variables \(P_{E}^{j}\) and \(P_{E\max }^{j}\) denotes the the power and maximum power of the ESS, with the positive direction indicating the energy storage discharge direction; \(P_{G}^{j}\) denotes the total power of DG in the microgrid. \(P_{L}^{j}\) is the total load of the microgrid; \(U_{{{\text{DC}}}}^{j}\) and \(U_{{{\text{DC}}}}^{j\;\;\;*}\) denote the DC voltage and rated voltage of HUCC. \(f_{{{\text{MG}}}}^{j}\) and \(f_{{{\text{MG}}}}^{j\;\;\;*}\) denote the frequency and rated frequency of the microgrid, respectively; \(U_{{{\text{MG}}}}^{j}\) and \(U_{{{\text{MG}}}}^{j\;\;\;*}\) denote the AC voltage and rated voltage of HUCC, respectively; \(T_{{{\text{THDMG}}}}^{j}\) denotes the total distortion of the sub-microgrid j; \(\varepsilon_{1}\), \(\varepsilon_{2}\), \(\varepsilon_{3}\) and \(\delta\) represent the limits of frequency deviation, AC voltage deviation, DC voltage deviation, and total harmonic distortion of the sub-microgrid, respectively.

  1. (2)

    Islanded Operation Mode

When FCMMGs operate in island mode, the higher-level distribution network no longer provides voltage and frequency support for MMGs. Therefore, the operating goals of FCMMGs are:

  1. 1.

    Stable operation of each sub-microgrid, including voltage stability and frequency stability;

  2. 2.

    Smoothing the impact of renewable energy output fluctuations;

  3. 3.

    Minimizing the power outage load in MMGs.

To achieve these operating goals, each sub-microgrid first meets the power demand of local loads and uses energy storage to balance the output fluctuations of renewable energy. Secondly, the central layer coordinates the MMC of the interface layer to dispatch the power between sub-microgrids and achieve complementary balance. The constraints of multi-microgrid system island operation are shown in Eq. (4.2), where the power of energy storage and MMC should not exceed the limit, and the DC voltage deviation and the power quality of each sub-microgrid should be controlled within the specified range.

$$\begin{array}{*{20}c} {\left\{ {\begin{array}{*{20}c} {\left| {P_{C}^{j} } \right| \le P_{C\max }^{j} } \\ {\left| {P_{E}^{j} } \right| \le P_{E\max }^{j} } \\ {\begin{array}{*{20}c} {P_{C}^{j} - P_{L}^{j} + P_{E}^{j} + P_{G}^{j} = 0} \\ {\left| {\frac{{U_{{{\text{DC}}}}^{j} - U_{{{\text{DC}}}}^{j\;\;\;*} }}{{U_{{{\text{DC}}}}^{j\;\;\;*} }}} \right| \le \varepsilon_{3} } \\ {T_{{{\text{THDMG}}}}^{j} \le \delta } \\ \end{array} } \\ \end{array} } \right.} \\ \end{array}$$
(4.2)
  1. (3)

    Contingency Operation Mode

In addition to normal operation mode, to improve the reliability and stability of the system power supply, FCMMGs should also have the ability to operate in emergency mode. The emergency operation mode of FCMMGs mainly refers to the process of black starting the system or gradually interconnecting sub-microgrids to restore power supply in the event of a serious fault. The overall goal of FCMMGs’ emergency operation is to improve the transient stability of the multi-microgrid system, which can be divided into two aspects:

  1. 1.

    Increase the system inertia of the sub-microgrid that has been restored power supply;

  2. 2.

    Reduce transient impacts during the power restoration process.

To achieve these operating goals, the constraints of emergency operation should be satisfied as shown in Eq. (4.3):

$$\begin{array}{*{20}c} {\left\{ {\begin{array}{*{20}c} {\left| {P_{C}^{j} } \right| \le P_{C\max }^{j} } \\ {\left| {P_{E}^{j} } \right| \le P_{E\max }^{j} } \\ {U_{{{\text{DC}}}}{\prime} < \sigma } \\ {T_{{U_{{{\text{DC}}}} }}^{j} \le \Delta t} \\ {\left| {\frac{{f_{{{\text{NMG}}}}^{j} - f_{{{\text{NMG}}}}^{j\;\;\;\;\;*} }}{{f_{{{\text{NMG}}}}^{j\;\;\;\;\;\;*} }}} \right| \le \varepsilon_{1} } \\ {\left| {\frac{{U_{{{\text{NMG}}}}^{j} - U_{{{\text{NMG}}}}^{j\;\;\;\;\;\;*} }}{{U_{{{\text{NMG}}}}^{j\;\;\;\;\;\;*} }}} \right| \le \varepsilon_{2} } \\ \end{array} } \right.} \\ \end{array}$$
(4.3)

where the subscript NMG denotes the microgrid during emergency operation; \(T_{{U_{{{\text{DC}}}} }}^{j}\) is the DC voltage recovery time; \(\Delta t\) denotes the permitted recovery time; \(U_{{{\text{DC}}}}{\prime}\) denotes the DC voltage variation rate; \(\sigma\) denotes the DC voltage variation rate limit; and represents the DC voltage variation rate.

4.4.3 Control Strategies for the Interface Layer and Microgrid Layer of FCMMGs

FCMMGs’ grid-connected, islanded, and emergency operation modes are controlled through the interface layer and microgrid layer of HUCC and sub-microgrids based on the goals and constraints formulated by the central layer. As shown in Fig. 4.6, there are interface layer and microgrid layer control strategies for controllable and uncontrollable sub-microgrids. The interface layer receives instructions from the central layer and selects the HUCC connection status and operation mode according to the strategy shown in Table 4.1. The DC interface MMC of the controllable sub-microgrid adopts P–U droop control, while the DC interface MMC of the uncontrollable sub-microgrid adopts v/f control. The ESS in the sub-microgrid HUCC adopts a unified PQ control strategy.

As a key component in the control of FCMMGs, the microgrid layer is the regulatory basis for the interdependent energy production, storage, and mutual assistance of the system. The controllable sub-microgrid adopts a peer-to-peer control strategy, where controllable micro-sources have equal status in control and participate in voltage/frequency regulation. Controllable micro-sources use P–f and Q–U droop control methods, and intermittent micro-sources use PQ control as shown in Fig. 4.6a. The droop control used in the peer-to-peer control mode can automatically participate in power distribution and is easy to implement plug-and-play and mode switching for microgrids. The uncontrollable sub-microgrid is supported by the MMC for voltage and frequency, and intermittent micro-sources in the sub-microgrid use PQ control as shown in Fig. 4.6b.

Fig. 4.6
figure 6

FCMMG control scheme for the interface layer and microgrid layer

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Compared with grid-connected and islanding modes, emergency mode control is relatively complex and involves structural changes and control strategy adjustments. During emergency operation, the controllable sub-microgrid operates independently, and the MMC in HUCC does not work, while the ESS participates in power adjustment as needed. During black start-up, the controllable micro-sources first start to restore the islanded operation of the controllable sub-microgrid, then start the MMC to raise its DC voltage to the set value, establish connection relationships among multiple sub-microgrids, and increase the output power to the set value, restoring MMGs to islanded operation. Finally, based on the status of the superior distribution network, the system can restore grid-connected operation or provide black start-up power for the upper-level network.

4.5 Simulations

4.5.1 System Architecture and Settings for MMGs

The FCMMGs simulation system shown in Fig. 4.7 is based on the multi-microgrid demonstration project in Rizhao Port. To fully verify the multi-scenario operation capability of MMGs, the simulation system includes four sub-microgrids, where MG1, MG2, and MG4 are controllable sub-microgrids, and MG3 is an uncontrollable sub-microgrid. The HUCC, AC interface switch, DC interface switch, ESS, and MMC corresponding to MG1 to MG4 are HUCC1 to HUCC4, ACB1 to ACB4, DCB1 to DCB4, ESS1 to ESS4, and MMC1 to MMC4, respectively.

Fig. 4.7
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FCMMG simulation model

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MMGs form a flexible interconnection structure through HUCC. The AC voltage at the grid connection point for MG1 and MG3 is 10 kV, while for MG2 and MG4, it is 10.5 kV. The frequency of the sub-microgrid is 50 Hz, and the MMC in HUCC uses 21 levels with a DC voltage of ± 2.5 kV and a capacity of 1 MW. The DG type, power, and load parameters for each sub-microgrid are shown in Table 4.3.

Table 4.3 Configuration parameters of the simulation model
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MG1, MG2, and MG4 have the ability to operate in both grid-connected and islanded modes, using peer-to-peer control. Controllable micro-sources operate under droop control, while intermittent micro-sources (wind and solar) operate under PQ control. MG3 does not have the ability to operate independently, and its photovoltaic system uses PQ control. The MMCs in HUCC1, HUCC2, and HUCC4 operate under P-U droop control, while HUCC3 uses v/f control. The ESS in HUCC uses PQ control to mainly suppress power fluctuations.

Based on PSCAD/EMTDC, a simulation model of FCMMGs is established for steady-state, transient, and emergency operation analysis. According to the IEEE 519-1992 and IEC 61000-2-2 standards, the operating boundary constraint parameters of MMGs are 2%, 10%, 10%, 5%, and 0.5 s for \(\varepsilon_{1}\), \(\varepsilon_{2}\), \(\varepsilon_{3}\) and \(\delta\) respectively.

4.5.2 Simulation Results

  1. (1)

    Steady-State Operation Simulation of FCMMGs System

As shown in Fig. 4.7, during the parallel operation of FCMMGs, MG1 and MG4 are connected to the upstream distribution network through AC interfaces, while MG2 and MG3 are flexibly interconnected through DC interfaces with MG1 and MG4. The control methods of DG, energy storage and MMC in each sub-microgrid are shown in Table 4.1. The energy storage systems in MG1, MG2 and MG4 can flexibly operate to absorb (compensate for) the surplus (shortage) power of the FCMMGs system and maintain the power balance of the system. Distributed new energy generation (wind and solar) in MG2 and MG4 operates at maximum power, and in addition to meeting their own load demand, can also transmit surplus power to the connected sub-microgrid and upstream distribution network through HUCC. To improve the operational efficiency, each sub-microgrid balances reactive power locally, so the reactive component of MMC can be set to 0, and the active component is matched according to the power difference.

As shown in Fig. 4.8, the simulation results of the parallel operation of FCMMGs show that the system operates stably at the rated working state from 0.6 to 1 s, and the DC voltage is stable and the output power is constant, with the power exchange between FCMMGs and the upstream distribution network (AC connection) being basically 0. During the stable operation, the maximum deviation of the DC voltage is 3.1%; MG1 and MG4 are supported by the upstream distribution network for voltage and frequency, with small deviations during operation; The frequency and voltage deviations of MG2 and MG3 are relatively large, with maximum values of 0.2% and 0.3%, respectively.

Fig. 4.8
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Simulation results of grid-connected operation

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During the islanded operation of FCMMGs, MG1 and MG4 are disconnected from the AC connection and flexibly interconnected through DC interfaces. As shown in Fig. 4.9, the simulation results of islanded operation show that MG1 to MG4 operate stably, with stable DC voltage and effective support formed between sub-microgrids, and flexible power mutual assistance. Due to the lack of support from the upstream distribution network, the maximum deviation of the DC voltage is 3.9%, and the maximum deviation of frequency and voltage between sub-microgrids becomes 0.5% and 0.7%, respectively.

Fig. 4.9
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Simulation results during islanded operation

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  1. (2)

    Transient Operation Simulation of FCMMGs System

To verify the transient operation performance of the flexible interconnection structure and control method of MMGs, the simulation system shown in Fig. 4.7 is used to simulate two scenarios to verify the transient operation capability of the system during power adjustment, and to compare and analyze the simulation results with those of MMGs system using AC interconnection.

Scenario 1: Parallel operation mode, MG1 to MG4 operate in parallel

During the parallel operation of FCMMGs, the system structure is complete, and MG1 to MG4 are all in parallel operation. At 0.7 s, the new energy output of MG2 decreases by 0.2 MW, and its DC output power is adjusted from 0.5 to 0.3 MW, resulting in a power shortage in the system. Due to the P–U droop control of the MMC in FCMMGs, the DC voltage of the system will experience a brief drop process, and at the same time, the MMC of each sub-microgrid will automatically adjust the DC output power until the power on the DC interconnection line reaches a new equilibrium point. As shown in Fig. 4.10, the voltage and power regulation and response curves of FCMMGs during the system adjustment process show that during the system adjustment process, the DC voltage quickly reaches a new stable working point after a brief decrease and remains stable, and the DC output power of each sub-microgrid is also quickly adjusted to a new stable working point according to their respective droop control curve parameters, while the voltage and frequency of each sub-microgrid remain basically stable. The entire transient adjustment process of FCMMGs is relatively short, with a stable time of DC voltage and DC output power of 0.12 s and an power variation overshoot of 25%.

Fig. 4.10
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Voltage and power curves of FCMMGs during MG2 power adjusting

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To verify the transient operational performance of FCMMGs and for the sake of generality, a simulation case of interconnected AC MMGs was designed for comparison. The AC MMGs have the same parameters as the FCMMGs, but their interconnected structure and controllable elements are different. The simulation was conducted for 0.7 s under the scenario where the output power of MG4 renewable energy source increased by 0.2 MW. Interconnected AC MMGs refer to a cluster of microgrids that are interconnected with each other to form a community system, as described in literature [10]. Based on the design methods of interconnected AC microgrids in literatures [8] and [11], the DCBs 1-4 of the system shown in Fig. 4.7 were disconnected and the ACBs 1-4, IB1, and IB2 were closed to form an interconnected AC microgrid.

Figure 4.11 shows the power adjustment response curves of the FCMMGs and the interconnected AC MMGs. In the adjustment process of the FCMMGs, due to the excess DC power in the system, the DC voltage gradually increased and rapidly reached a new steady-state operating point, which remained stable. The DC output power of the sub-microgrid also quickly reached a new steady-state operating point through autonomous adjustment, as shown in Fig. 4.11a. The entire transient adjustment process of the FCMMGs was relatively short, with a stabilization time of 0.09 s for the DC voltage and DC output power, and an overshoot of 23% in power change. As shown in Fig. 4.11b, the transient adjustment process of the interconnected AC microgrid was relatively long, with a stabilization time of about 0.66 s, an overshoot of 30% in power change, and oscillations during the adjustment process. Due to the increase in DG output power, the voltage at the internal node of MG4 increased, and the active power of the load increased. After stabilization, the power of the AC interconnection line was 0.497 MW. The comparison shows that the flexible interconnection method has better transient characteristics for power adjustment in scenarios with fluctuating renewable energy generation under grid-connected mode.

Fig. 4.11
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Power curves of FCMMGs and AC-interconnected MMGs during MG4 power adjusting

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Based on the above simulation and analysis, the change in renewable energy output of the sub-microgrid leads to a change in DC output power. The DC voltage and DC output power of each sub-microgrid will be autonomously adjusted according to their respective MMC droop control curve parameters. Compared with the interconnected AC MMGs, the FCMMGs have faster adjustment speed, shorter transient process, and higher stability.

Scenario 2: Islanding mode, MG4 out of operation

Considering the change in operating mode to islanding and the structural adjustment of the MMGs (MG4 out of operation), the transient operational performance of the flexible interconnection and interconnected AC MMGs was compared through simulation. In the islanding operation of the FCMMGs, when MG2 increased its power output by 0.1 MW at 0.7 s, the DC voltage, HUCC interface power, and AC voltage and frequency change curves of the sub-microgrid are shown in Fig. 4.12. It can be seen that the system operates stably during power adjustment, with values of \(\varepsilon_{1}\), \(\varepsilon_{2}\) and \(\varepsilon_{3}\) are 0.07%, 1.5%, and 5.8% respectively, and the transient adjustment time is about 0.1 s.

Fig. 4.12
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Power adjusting curves during gird-connected operation of FCMMGs

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Figure 4.13 shows the voltage, frequency, and interconnection line power of the interconnected AC MMGs during power adjustment in islanding mode with MG4 out of operation. During the power adjustment process, the voltage and frequency of the interconnected AC MMGs continue to fluctuate, and when stabilized, the frequency deviation is 0.12% and the voltage deviation is 5.5%, both of which are larger than those of the FCMMGs.

Fig. 4.13
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Power adjusting Curves during gird-connected operation of AC-connected MMGs

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Compared with the interconnected AC MMGs, the flexible interconnection MMGs can balance the system power and suppress disturbances through the control of MMC, energy storage, and sub-microgrid during transient operation. The operation control of FCMGs is more flexible and stable in grid-connected mode, islanding mode, and network structure changes compared to the control of AC interconnection MMGs.

  1. (3)

    Emergency Operation Simulation of FCMMGs System

To verify the emergency operation ability of FCMMGs, a severe fault was simulated by disconnecting the HUCC interconnection and DC switches of MG1 to MG4. MG1, MG2, and MG4 were placed in isolated island operation mode, while MG3 was out of service. After the fault was cleared, MG1, MG2, and MG4 resumed DC connection, and MG3 was reconnected to the grid and restored power supply after 0.7 s, as shown in Fig. 4.14.

Fig. 4.14
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Simulation results during emergency operation

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As shown in Fig. 4.14, when MG3 is not connected to the grid, FCMMGs operate at a new equilibrium point with stable DC voltage and output power after MG1, MG2, and MG4 are connected in DC. When MG3 is reconnected to the grid after 0.7 s, the DC voltage and power of the system will undergo transient adjustment due to the P-U droop control of the MMCs in MG1, MG2, and MG4. As shown in Fig. 4.14b, after MG3 is connected to the multi-microgrid, the DC voltage of the system drops slightly, and the DC output power of each microgrid increases, and MG3 gradually restores power supply.

During the emergency operation of FCMMGs, the transient process is stable, and after autonomous adjustment, it can quickly stabilize to a new equilibrium point. The indicators of FCMMGs’ emergency operation are shown in Table 4.4, and the frequency deviation, AC voltage deviation, DC voltage adjustment time, and total voltage harmonic are all better than the requirements.

Table 4.4 Operation indexes during emergency operation
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4.6 Conclusion

This chapter systematically studied the basic structure, unified interface, flexible interconnection scheme, and control system and methods of multi-microgrids. A hybrid public connection unit structure with integrated AC and DC interfaces was designed, and a flexible interconnection multi-microgrid system scheme was proposed, which expanded the diversity and flexibility of multi-microgrid interconnection. A multi-layer control system for flexible interconnection multi-microgrids was constructed, and control methods for the central layer, interface layer, and microgrid layer were proposed. The control methods fully considered the operating characteristics of each sub-microgrid and distributed power source, achieved the coordinated control of the hybrid public connection unit and sub-microgrid, and completed the operational objectives and constraints formulated by the central layer.

Based on the hybrid public connection unit, the flexible interconnection multi-microgrid integrates AC and DC connection modes. On the one hand, it fully utilizes the frequency and voltage support function of the AC interface to improve the operational stability of the multi-microgrid system. On the other hand, by connecting each sub-microgrid through the DC interface, it reduces the coupling effect between sub-microgrids, improves the scalability and stability of the multi-microgrid system, and fully utilizes the compatibility and control ability of the DC unit to expand the control dimension, thereby improving the flexibility and coordination of multi-microgrid operation.

The simulation results of the flexible interconnection multi-microgrid in port show that the proposed flexible interconnection scheme and control methods meet the operational requirements in different scenarios and have good operational performance. The hybrid public connection unit is suitable for connecting multi-microgrid groups containing high-penetration distributed power sources and requiring high operational flexibility, and the flexible interconnection multi-microgrid formed by it is a feasible way to scale up the consumption of distributed new energy. With the continuous development of the application technology of voltage source converters in distribution networks, the application of flexible interconnection multi-microgrids not only improves the stability, flexibility, and reliability of the system but also optimizes the technical and economic performance of the system. It is a supplement and extension to the application research of existing multi-terminal DC technology in distribution networks.