# Island DC Microgrid Hierarchical Coordinated Multi-Mode Control Strategy

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

## 2. DC Microgrid Structure

- (1)
- Photovoltaic array (PV): Integrated into the DC microgrid bus via a one-way DC/DC converter. PV usually adopts the maximum power point tracking (MPPT) mode to fully utilize PV energy.
- (2)
- Energy storage system (ESS): The energy storage system utilizes multiple group battery energy storage (BES) to meet generation and load fluctuation and integrates this with the DC microgrid bus via a two-way DC/DC converter to adjust the power balance of the system.
- (3)
- Common DC load: The common DC load can be integrated into the DC microgrid bus. In addition, it is possible to carry out constant voltage control for important loads through a double closed-loop DC/DC control system. In this way, the voltage fluctuation is reduced and power-supply stability is improved.
- (4)
- Direct current/direct current converter (DC/DC): This is utilized to achieve connections between various generation units in the grid and important loads, which facilitates power exchange of various units.

## 3. Multi-Mode of Island DC Microgrid

- (1)
- Mode 1: PV utilizes MPPT control to fully use the photovoltaic energy and maintain a power balance in the system with the help of BES. The power relationship between photovoltaic energy and storage load in the system at this point is indicated by the following formula:$$\sum _{i=1}^{N}{P}_{\mathrm{BES}.i}^{}}+{\displaystyle \sum _{j=1}^{M}{P}_{\mathrm{PV}.j}^{\mathrm{MPPT}}}\text{}=\text{}{P}_{\mathrm{load}}^{\mathrm{sum}}+{P}_{\mathrm{L}},$$
_{BES.i}refers to the developed power of BES i, ${P}_{\mathrm{PV}.j}^{\mathrm{MPPT}}$ indicates the maximum power point tracking power of PV j, ${P}_{\mathrm{load}}^{\mathrm{sum}}$ represents the common DC overall load of the system, and P_{L}indicates the power consumed by line resistance. - (2)
- Mode 2: In case of the power required by the system exceeding BES safe output capacity with excess power, the system power balance is guaranteed by reducing PV output power. At this point, the system power satisfies the following formula:$$\sum _{i=1}^{N}{P}_{\mathrm{BES}.i}^{\mathrm{max}}}+{\displaystyle \sum _{j=1}^{M}{P}_{\mathrm{PV}.j}^{}}={P}_{\mathrm{load}}^{\mathrm{sum}}+{P}_{\mathrm{L}},$$
- (3)
- Mode 3: When the bus voltage plummets due to the power vacancy of the system, it is necessary to carry out load-shedding control to guarantee the power supply quality of important loads. At this time, the system power satisfies the following formula:$$\sum _{i=1}^{N}{P}_{\mathrm{BES}.i}^{\mathrm{max}}}+{\displaystyle \sum _{j=1}^{M}{P}_{\mathrm{PV}.j}^{\mathrm{MPPT}}}={P}_{\mathrm{load}}^{}+{P}_{\mathrm{L}},$$
_{load}indicates the residual load after load shedding.

## 4. Hierarchical, Coordinated Control Strategy Based on Multi-Mode Smooth Switch of an Island DC Microgrid

_{L}. Therefore, it is hard to achieve accurate mode switching in terms of system power fluctuation, as mentioned in Section 3. In order to solve the problem, a hierarchal, coordinated multi-mode control strategy for island DC microgrids is put forward in this paper, as indicated in Figure 3. This strategy fully considers the maximum charge–discharge capacity of BES, as well as the fluctuation of PV and load. In Section 4.1, the influence of line resistance on precise load power dispatch is analyzed on the basis of traditional droop control. To address the influence of line resistance, current-sharing layer control is introduced in Section 4.2, where the difference discrete consistency algorithm is illustrated in detail. The DDCA implements real-time tracking DC/DC converter output voltage and iteratively converges to rapidly average the values. The average value is utilized as the virtual bus voltage, so as to provide a unified reference input for each DC/DC converter, thus achieving current-sharing control. In Section 4.3, virtual bus voltage information is used as a criterion for multi-mode smooth switching of the system. A smooth switch between different modes can be achieved with voltage information only using real-time monitoring of voltage fluctuation. With the help of the strategy mentioned above, voltage stability of the system and precise load power dispatch can be guaranteed. Flow diagrams of island DC microgrid current-sharing based on Figure 3 and voltage stability control strategy are shown in Section 4.2.

#### 4.1. The First Layer: The Equipment Layer

_{pi}and k

_{ii}represent the proportion and integral terms of current PI controller, respectively. ${U}_{i}^{\mathrm{ref}}$ is the no-load voltage of VSC, i.e., the reference voltage of droop control. ${U}_{\mathrm{VSC}.i}$ and I

_{VSC}.

_{i}represent the output voltage and the current of the DC/DC converter, respectively. K

_{i}refers to the droop coefficient of VSC. D represents the pulse width modulation (PWM) duty ratio.

_{VSC}.

_{i}of VSC, i, is calculated as follows:

_{i}

^{1}+ R

_{i}

^{2}is not equal in Equation (4). Considering that both the virtual resistance and line resistance under a closed-loop state are relatively low, the obvious deviation of the two resistances may lead to a considerable power imbalance and an inaccurate load power dispatch. Considering that the units in the system are connected to the DC microgrid bus in the distributed way, the bus piecewise resistance may aggravate the unbalanced dispatch of power in the system.

#### 4.2. The Second Layer: The Current-Sharing Layer

_{i}(k) refers to the output of unit i after the kth iteration, x

_{j}(k) indicates the output of unit j after the kth iteration, a

_{ij}is the edge weight between node i and node j, a

_{ij}= 0 when the nodes i and j are not neighboring nodes, and N

_{i}refers to the set of indexes of the agents that are connected with agent i.

_{i}(k) in Equation (6) is represented as ${U}_{\mathrm{VSC}.i}\left(k\right)$ and x

_{j}(k) is represented as ${U}_{\mathrm{vsc}.j}\left(k\right)$.

_{ii}(k) and W

_{ij}(k) refer to the weight factor of the kth iteration, respectively.

**W**refers to weight matrix of the communication network:

**E**indicates the N-order unit matrix,

**L**indicates the Laplacian matrix, a refers to the constant edge weight, and L refers to the undirected connected graph composed of the output voltage N information nodes of VSCs.

**1**refers to all column vectors with element of 1.

_{1}(L) refers to the maximum eigenvalue of the Laplacian matrix, L, and λ

_{n−}

_{1}(L) indicates the second minimum eigenvalue of L.

**X**(k)

**−X**(k

**−**1)), which is the predicted value in Equation (19), the DCA is updated as:

**L,**of this structure.

**L**are ${\left[0,1.382,1.382,3.618,3.618\right]}^{T}$. The convergence when $a=2/5$, $a=1/5$, and $a=1/10$ is verified in this paper by Matlab simulation.

#### 4.3. The Third Layer: The Multi-Mode Smooth Switch Layer

_{L}, is hard to acquire accurately, it is not appropriate to utilize the real-time power of the system units in Section 3 as the criterion for the multi-mode smooth switch layer, which may lead to serious deviation. When using DC bus voltage as the key index for the system operation, the power status of the DC microgrid system is reflected in real time. The DC bus voltage is of great significance for the multi-mode smooth switch. Meanwhile, the average voltage, ${U}_{\mathrm{avg}}$, introduced in Section 4.2, reflects the true level of bus voltage. Thus, average voltage, ${U}_{\mathrm{avg}}$, is regarded as a virtual bus voltage in this section to provide a unified voltage criterion for mode switching of the system units. At the same time, it provides a unified reference input for all VSCs. A multi-mode smooth switch control strategy, based on virtual bus voltage information, is proposed in this paper, as indicated in Figure 3. Among them, ${U}_{\mathrm{BES}}^{\mathrm{ref}}$ refers to the no-load voltage of BES-VSC, ${U}_{\mathrm{BES}}^{1\mathrm{L}}$ and ${U}_{\mathrm{BES}}^{1\mathrm{H}}$ indicate the scope of the droop voltage regulation of BES-VSC, ${U}_{\mathrm{PV}}^{\mathrm{ref}}$ represents the no-load voltage of droop control of PV-VSC, ${U}_{\mathrm{PV}}^{1\mathrm{L}}$ indicates the minimum voltage of droop control of PV-VSC, and P

_{PV}refers to the output power of PV.

## 5. Example Simulation Analysis

#### 5.1. Situation 1

#### 5.2. Situation 2

#### 5.3. Situation 3

#### 5.4. Situation 4

#### 5.5. Situation 5

#### 5.6. Situation 6

## 6. Conclusions

- (1)
- In the mode switching layer, the control strategy solves the bus voltage deviation problem caused by line resistance and the influence of power loss on a multi-mode switch. Based on virtual bus voltage information, the smooth switch between different operation modes of an island DC microgrid is achieved.
- (2)
- In the current-sharing layer, which avoids centralized control and upper energy management, the control strategy utilizes DDCA to track the output of each VSC unit in real time, eliminating the influence of line resistance and providing reliable virtual bus voltage information for the equipment layer and the mode switching layer.
- (3)
- In the equipment layer, the output current of each unit with current-sharing control achieves accurate load power dispatch. In addition, the control strategies of each unit are regulated in terms of the fluctuation of virtual bus voltage information to achieve system voltage stability.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 3.**The hierarchical, coordinated control strategy of an island DC microgrid based on a multi-mode smooth switch.

**Figure 9.**Communication topology and its Laplacian matrix. (

**a**) A topological structure with five information nodes; (

**b**) Laplacian matrix.

**Figure 10.**Converging speed comparison under different constant weights, a. (

**a**) Consensus dynamic at a = 2/5; (

**b**) Consensus dynamic at a = 1/2; (

**c**) Consensus dynamic at a = 1/10.

Situation | Operation Mode | Operation Time/s |
---|---|---|

1 | 1 | 0–2.5 s |

2 | 1-3-1 | 2.5–4.4 s |

3 | 1-2 | 4.4–6.2 s |

4 | 2-1 | 6.2–6.8 s |

5 | 1 | 6.8–8 s |

6 | 1 | 0–2.5 s |

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**MDPI and ACS Style**

Zhao, Z.; Zhang, J.; He, Y.; Zhang, Y.
Island DC Microgrid Hierarchical Coordinated Multi-Mode Control Strategy. *Energies* **2019**, *12*, 3012.
https://doi.org/10.3390/en12153012

**AMA Style**

Zhao Z, Zhang J, He Y, Zhang Y.
Island DC Microgrid Hierarchical Coordinated Multi-Mode Control Strategy. *Energies*. 2019; 12(15):3012.
https://doi.org/10.3390/en12153012

**Chicago/Turabian Style**

Zhao, Zhongbin, Jing Zhang, Yu He, and Ying Zhang.
2019. "Island DC Microgrid Hierarchical Coordinated Multi-Mode Control Strategy" *Energies* 12, no. 15: 3012.
https://doi.org/10.3390/en12153012