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Chapter 2

Composition and classification of the microgrid

Abstract

Composition and classification of the microgrid, describes the composition, operation, and control modes, integration voltage, and classification of microgrids.

Keywords

distribution network dispatch layer

centralized control layer

local control layer

grid-connected operation

islanded operation

master–slave control

peer-to-peer control

combined control

P/Q control

U/f control

droop control

integration voltage

classification by function demand

capacity

AC/DC type

This chapter introduces the composition, structure, operation, and control modes and integration voltages of the microgrid, as well as classification of microgrids by function demand, capacity, and AC/DC type.

2.1. Composition

A microgrid is composed of distributed generation (DG), loads, energy storage (ES), and control devices. It acts as a single entity with respect to the grid, and connects to the grid via a single point of common coupling (PCC). Figure 2.1 shows the composition and structure of a microgrid.

1. DG: It can be various types of new energy, such as photovoltaic (PV), wind, and fuel cell; or combined heat and power (CHP) or combined cooling, heat, and power (CCHP), which provides heat for users locally, thereby increasing efficiency and flexibility of DG.

2. Loads: It includes common load and critical load.

3. ES: It includes physical, chemical, and electromagnetic forms, for storage of renewable energy, load shifting, and black-start of microgrid.

4. Control devices: They constitute the control system for DGs, ESs, and transfer between grid-connected mode and islanded mode, facilitating real-time monitoring and energy management.

Figure 2.1 Composition and structure of microgrid.

2.2. Structure

Figure 2.2 shows the three-layer microgrid control scheme implemented in the demonstration project based on “multiple microgrid structures and controls.” The top layer is the distribution network dispatch layer, which coordinates and dispatches the microgrid to maintain security and economy of the distribution network, and the microgrid is regulated and controlled by the distribution network. The middle layer is the centralized control layer. It forecasts the DG output and load demand, develops operation plans, and adjusts the plans and controls start and stop of DG, load, and ES in real-time based on information such as current, voltage, and power, thereby ensuring voltage and frequency stability of the microgrid. In grid-connected operation, the microgrid operates for best economic efficiency; in islanded operation, the microgrid regulates the output of DGs and consumption of loads to ensure stable and secure operation. The bottom layer, local control layer, executes coordination of DGs, charging and discharging control of ESs, and load control within the microgrid.

2.2.1. Distribution network dispatch layer

The distribution network dispatch layer coordinates and dispatches the microgrid to keep the distribution network in a safe and economic operation, and the microgrid is regulated and controlled by the distribution network.

The microgrid acts as a single controllable and flexible entity with respect to the macrogrid.

1. It can operate either in parallel with the macrogrid or in islanded mode in the case of macrogrid failure or when necessary.

2. In special situations (e.g., an earthquake, snowstorm or flood), the microgrid can serve as reserve to the distribution network, thereby providing effective support to the macrogrid and speeding up recovery of the macrogrid from failure.

3. In case of power shortage in the macrogrid, the microgrid can shift load with its own energy storage, thereby avoiding widespread tripping of the distribution network and reducing the reserve capacity for macrogrid.

4. Under normal circumstances, the layer participates in macrogrid dispatch for best economy.

2.2.2. Centralized control layer

The centralized control layer is the microgrid control center (MGCC) and the core of the microgrid control system. It centrally manages DGs, ESs, and loads, and monitors and controls the entire microgrid. It optimizes the control strategy in real time based on the operating conditions to ensure smooth transfer between grid connection, islanding, and shutdown. In grid-connected operation, the layer regulates the microgrid for best performance; in islanded operation, the layer adjusts the DG output and load consumption to ensure stable and safe operation of the microgrid.

1. In grid-connected operation, the layer dispatches the microgrid for best economic performance and coordinates various DGs and ESs for load shifting to smooth the load curve.

2. During transfer between grid-connected mode and islanded mode, the layer coordinates the local controller to realize quick transfer.

3. In islanded operation, the layer coordinates various DGs, ESs, and loads to maintain supply to important loads and safe operation of the microgrid;

4. When the microgrid stops operation, the layer initiates black-start to rapidly resume operation.

2.2.3. Local control layer

The local control layer of the microgrid is composed of local protection and local controller. The local controller realizes primary regulation of frequency and voltage of DG, while local protection provides quick fault protection for the microgrid. The two work together for quick self-healing of the microgrid from faults. The DG is controlled by the MGCC and adjusts its active and reactive output according to dispatch orders.

1. The local controller controls the automatic transfer between U/f control and P/Q control.

2. The load controller rejects unimportant loads based on system frequency and voltage to ensure system security.

3. The local control layer communicates with the MGCC through weak ties. The former is responsible for microgrid transient control while the latter for steady-state control and analysis.

2.3. Operation modes

A microgrid may operate either in grid-connected or in islanded mode, and grid-connected operation is further divided into power-matched operation and power-mismatched operation according to power exchange. As shown in Figure 2.3, the microgrid is connected to the distribution network via a PCC, the active and reactive power flowing through the PCC is, respectively, ∆P and ∆Q. When ∆P = 0 and ∆Q = 0, the current through PCC is zero, indicating that the DG output and load reach a balance and no power exchange occurs between the distribution network and microgrid. This is the most economic operation mode of the microgrid, known as power-matched operation. When ∆P ≠ 0 or ∆Q ≠ 0, the current through the PCC is not zero, indicating that power exchange occurs between the distribution network and the microgrid. This mode is known as power-mismatched operation. In this mode, if ∆P < 0, the excessive active power from DGs after meeting load demand is injected to the distribution network; if ∆P > 0, the electricity from DGs is insufficient for meeting load demand, requiring the distribution network to provide the deficit. Likewise, reactive power is excessive if ∆Q < 0 and deficient if ∆Q > 0. All these operations are power-mismatched operation variants.

Figure 2.3 Power exchange between distribution network and microgrid.

2.3.1. Grid-connected operation

In grid-connected mode, the microgrid is connected to and exchanges power with the distribution system of the utility grid via PCC.

Figure 2.4 shows the schematic diagram of transfer between operation modes.

1. When the microgrid stops operation, it can transfer to grid-connected mode directly by grid connection control; when it is connected to the grid, it can be disconnected from the grid by disconnection control.

2. When the microgrid stops operation, it can transfer to islanded mode directly by disconnection control, and when it is in islanded operation, it can be connected to the grid-by-grid connection control.

3. When the microgrid operates in parallel with the grid or in islanded mode, the microgrid can be shut down by shutdown control.

Figure 2.4 Transfer between operation modes.

2.3.2. Islanded operation

Islanded operation means that the microgrid is disconnected from the distribution system of the main grid at the PCC following a grid failure or as scheduled, and that the DGs, ESs, and loads within the microgrid operate independently. In islanded mode, since the electricity produced by the microgrid itself is generally small and insufficient to meet the demand of all loads, it is necessary to prioritize loads based on their importance and ensure uninterrupted supply to important loads.

2.4. Control modes

2.4.1. Microgrid control modes

Three microgrid control strategies are commonly used, respectively, master–slave mode, peer-to-peer mode, and combined mode. For a small microgrid, master–slave mode is most commonly used.

2.4.1.1. Master–slave mode

In master–slave mode, different DGs within a microgrid are controlled with different methods and assigned with different functions, as shown in Figure 2.5. One or more DGs act as the master while the others as the slave. In grid-connected operation, all DGs are under P/Q control. In islanded operation, the master DG switches to U/f control to provide voltage and frequency reference for other DGs. The master DG also traces load fluctuation, and therefore, its power output has to be controllable to some extent, and the DG should be able to respond fast enough to load fluctuation. The slave DGs remain under P/Q control.

Figure 2.5 Architecture of microgrid under master–slave control.

This control mode has some disadvantages. First, with the master DG under U/f control, its voltage output is constant. To increase the power output, the only way is to increase the current output. Instantaneous load fluctuations are usually first balanced by the master DG, and therefore, it has to have a certain adjustable capacity. Second, as the system relies on the master DG to coordinate and control all slave DGs, once the master DG fails, the whole microgrid will collapse. Third, master–slave control requires accurate and timely islanding detection, while islanding detection itself is accompanied by error and time delay. Hence, without a communication channel, transfer between control strategies is likely to fail.

To enable quick transfer between control modes, the following forms of DG could be used as master DGs:

1. DGs of randomness such as PV and wind;

2. ES, microturbine, fuel cell, and other stable and easily controlled DGs; and

3. DG + ES, for example, PV + ES or PV + fuel cell.

The third form is advantageous over the first two in that it can make best use of quick charge and discharge functions of ES and the capability of DGs in maintaining islanded operation of the microgrid for an extended time. In this mode, the ES can provide power support for the microgrid immediately after it switches to islanded operation, thus effectively suppressing significant fluctuation of voltage and frequency resulting from slow dynamic response of DG.

2.4.1.2. Peer-to-peer mode

Peer-to-peer mode is a control strategy based on ideas of “plug-and-play” and “peer-to-peer” used in power electronic technologies. In this mode, all DGs in the microgrid are equal and there is no master or slave DG. All DGs participate in regulation of active power and reactive power in a preset control mode to maintain stability of the system voltage and frequency. Droop control is adopted in the peer-to-peer mode, with the architecture shown in Figure 2.6. In this mode, all DGs under droop control participate in voltage and frequency regulation of the microgrid in islanded operation. When the load changes, the changes will be automatically distributed among the DGs according to the droop factor, that is, all DGs will adjust the frequency and amplitude of their output voltage to establish a new steady state for the microgrid and finally achieve reasonable distribution of output power. The droop control model enables automatic distribution of load variations among DGs, but the voltage and frequency of the system also vary after load variation, and therefore, this control mode is actually a proportional control. The droop control model of DGs can remain unchanged for grid-connected operation and islanded operation, making it easy for smooth transfer between the two modes.

Figure 2.6 Architecture of microgrid under peer-to-peer control.

The droop control model allows for independent control of DGs according to the voltage and frequency at the PCC, thus making it possible for automatic regulation of voltage and frequency, plug-and-play of DG without communication links, and flexible and convenient deployment of the microgrid. Unlike the master–slave mode where power imbalance is compensated by the master DG, power imbalance is dynamically distributed to all DGs in the peer-to-peer mode. This kind of control is simple, reliable, and easy to deploy, but at the sacrifice of voltage and frequency stability; it is currently under laboratory test.

2.4.1.3. Combined mode

Master–slave control and peer-to-peer control have advantages and disadvantages. A microgrid may contain multiple types of DGs, such as DG of randomness (e.g., PV and wind), or stable and easily controlled DG or ES (e.g., microturbine and fuel cell). Control characteristics differ greatly for different types of DG. Apparently, a single control mode cannot meet the operation requirements of a microgrid. In view of the dispersive DGs and loads within a microgrid, different control strategies may be adopted for different types of DGs, that is, master–slave control and peer-to-peer control could be used in conjunction in a microgrid.

2.4.2. Inverter control modes

The DGs integrated to a microgrid may operate either in parallel with the grid or in islanded mode. In the former case, the DGs only need to control their own power output to maintain balance within the microgrid. As the total capacity of a microgrid is much smaller than that of a grid, the rated voltage and frequency are supported and regulated by the grid, and the inverters are usually under P/Q control. In the latter case, the microgrid is isolated from the grid. To maintain the rated voltage and frequency within the microgrid, one or more DGs need to play the role of the grid to provide rated voltage and frequency. These DGs are usually under U/f and droop control.

2.4.2.1. P/Q control

As the interface between the microgrid and macrogrid, the basic function of inverters is to control the active and reactive output. In P/Q control, the inverters can produce active power and reactive power, and the determination of reference power is the prerequisite for power control. For purpose of power control, the DGs with a mediate or small capacity can be integrated to the grid with a constant power, the grid provides rigid support for voltage and frequency, and the DGs do not participate in frequency and voltage regulation and just inject or absorb power. This can avoid direct participation of DG in the regulation of feeder voltage, thus eliminating adverse impacts on the electric power system.

P/Q control is based on the grid voltage oriented P/Q decoupled control strategy, in which the outer loop adopts power control and the inner loop adopts current control. The mathematical model is like this: the three-phase voltage is first rotated to the dq coordinate through Park transformation to get the following inverter voltage equation:

$\begin{array}{l} v_{d} = R i_{d} + L \frac{d i_{d}}{d t} - ω L i_{q} + u_{d} \\ v_{q} = R i_{q} + L \frac{d i_{q}}{d t} + ω L i_{q} + u_{q} \end{array}\}$ $\begin{array}{l} v_{d} = R i_{d} + L \frac{d i_{d}}{d t} - ω L i_{q} + u_{d} \\ v_{q} = R i_{q} + L \frac{d i_{q}}{d t} + ω L i_{q} + u_{q} \end{array}\}$

(2.1)

where u_d and u_q are the voltage at the inverter terminal, and ωLi_q and ωLi_d are cross-coupling terms. They will be eliminated by feedforward compensation in subsequent control.

The PI controller is usually used for outer-loop power control. Its mathematical model is expressed as follows:

$\begin{array}{l} i_{dref} = (P_{ref} - P) (k_{p} + \frac{k_{i}}{s}) \\ i_{qref} = (Q_{ref} - Q) (k_{p} + \frac{k_{i}}{s}) \end{array}\}$ $\begin{array}{l} i_{dref} = (P_{ref} - P) (k_{p} + \frac{k_{i}}{s}) \\ i_{qref} = (Q_{ref} - Q) (k_{p} + \frac{k_{i}}{s}) \end{array}\}$

(2.2)

where P_ref and Q_ref are the reference active power and reference reactive power, respectively, and i_dref and i_qref are the d-axis reference current and q-axis reference current, respectively.

If the grid voltage u is constant, the active output of the inverter is proportional to d-axis current i_d and the reactive output proportional to q-axis current i_q, respectively.

The transfer function between v_d1/v_q1 and i_d/i_q is a first-order lag, which means that the d-axis and q-axis voltages can be controlled by the d-axis and q-axis currents. On this basis, the inner-loop current controller, usually PI controller, can be designed. Its mathematic model is expressed as follows:

$\begin{array}{l} v_{dl} = (i_{dref} - i_{d}) (k_{p} + \frac{k_{i}}{s}) \\ v_{ql} = (i_{qref} - i_{q}) (k_{p} + \frac{k_{i}}{s}) \end{array}\}$ $\begin{array}{l} v_{dl} = (i_{dref} - i_{d}) (k_{p} + \frac{k_{i}}{s}) \\ v_{ql} = (i_{qref} - i_{q}) (k_{p} + \frac{k_{i}}{s}) \end{array}\}$

(2.3)

Then, by adding compensation terms, the effects of grid voltage and d–q cross-coupling can be eliminated and current decoupling control can be achieved. The inverter control wave can be obtained by reverse Park transformation of d-axis and q-axis voltages, and then the three-phase voltage output of the inverter can be derived by sinusoidal pulse width modulation. Figure 2.7 shows the schematic diagram of P/Q control.

Figure 2.7 Schematic diagram of P/Q control.

2.4.2.2. U/f control

In U/f control, the inverters output constant voltage and frequency to ensure continual operation of slave DGs and sensitive loads after the microgrid is isolated from the grid. Given the limited capacity of the microgrid in islanded operation, once power shortfall occurs, it is necessary to shed some less important loads to ensure continuous supply to sensitive loads. As such, this control mode requires the ability to respond to and trace load switching.

In this control mode, the AC-side voltage is regulated according to voltage feedback from the inverter to maintain a constant output, and the dual-loop control scheme with outer-loop voltage control and inner-loop current control is often adopted. Outer-loop voltage control can maintain stable voltage output, and inner-loop current control constitutes the current servomechanism system, and can significantly accelerate the dynamic process to defend against disturbances. This dual-loop control can make the best use of system status information, and has a high dynamic performance and steady-state precision. Furthermore, inner-loop current control increases the bandwidth of the inverter control system, thereby speeding up the dynamic response of the inverter, enhancing the inverter’s adaptability to nonlinear load disturbance, and reducing harmonic distortion of the output voltage.

The U/f control is similar to P/Q control in terms of decoupling and control mechanism. Figure 2.8 shows the schematic diagram of U/f control, where outer-loop voltage control and inner-loop current control are adopted and the reference voltages

U_{ldd}^{*}

$U_{ldd}^{*}$

and

U_{ldq}^{*}

$U_{ldq}^{*}$

and measured voltages U_ldd and U_ldq are specified.

Figure 2.8 Schematic diagram of U/f control.

2.4.2.3. Droop control

Droop control is realized by simulating the droop characteristic of generators in a traditional grid and controlling the output voltage and frequency of the voltage source inverter (VSI) according to variation of the output power. The control strategy is based on inverter parallel-connection technology. As all DGs are integrated to the microgrid via inverters, the microgrid in islanded operation is equivalent to multiple inverters being connected in parallel, and the active and reactive output of individual inverters are, respectively

$\begin{matrix} P_{n} = \frac{U U_{n}}{X_{n}} δ_{n} \\ Q_{n} = \frac{U U_{n} - U^{2}}{X_{n}} \end{matrix}\}$ $\begin{matrix} P_{n} = \frac{U U_{n}}{X_{n}} δ_{n} \\ Q_{n} = \frac{U U_{n} - U^{2}}{X_{n}} \end{matrix}\}$

(2.4)

where U is the integration voltage, U_n the output voltage of the inverter power supply, X_n the output impedance of the inverter power supply, and δ_n the included angle between U_n and U.

According to Eq. (2.4), the delivery of active power mainly depends on δ_n and that of reactive power mainly depends on the output voltage amplitude of the inverter power supply U_n. U_n can be directly controlled, and the phase can be controlled by adjusting the output angular frequency or frequency of the inverter, as expressed in Eq. (2.5):

$f_{n} = \frac{ω_{n}}{2 π} = \frac{d δ_{n}}{d t}$ $f_{n} = \frac{ω_{n}}{2 π} = \frac{d δ_{n}}{d t}$

(2.5)

It can be seen from Eqs (2.4) and (2.5) that the output voltage of the inverter can be regulated by regulating its reactive output, and the output frequency can be regulated by regulating its active output. Figure 2.9 shows the droop control characteristic so derived.

Figure 2.9 Droop control characteristic.

Reverse droop control is to control the active and reactive outputs by measuring grid voltage amplitude and frequency to trace the predefined droop characteristic. This is a total reversion of the control mode where the output voltage is regulated by measuring the output power and therefore called reverse droop control. As the name implies, the reactive output and active output of the inverter are regulated by regulating the output voltage amplitude and output frequency, respectively.

To make the microgrid operational, inverters may adopt P/Q control, droop control or reverse droop control, and with these control modes, the output power of DG can be controlled by simply measuring local data.

2.5. Integration voltage class

Microgrids can be integrated into grids at the following three voltages:

1. 380 V (mains)

2. 10 kV

3. A hybrid of 380 V and 10 kV.

Figure 2.10 shows the integration voltages of a microgrid. Figure 2.10a represents integration to a 380 V (mains) LV distribution network, Figure 2.10b represents integration to a 10 kV distribution network with the voltage being increased to 10 kV from 380 V by a step-up transformer, and Figure 2.10c represents integration to a 380 V (mains) LV distribution network and a 10 kV distribution network.

Figure 2.10 Microgrid integration voltage classes.
(a) 380 V, (b) 10 kV, and (c) hybrid of 380 V and 10 kV.

2.6. Classification

Different types of microgrids should be established according to capacity, location, and types of DRs to suit the local situation. Microgrids can be classified as follows.

2.6.1. By function demand

Microgrids are classified into simple microgrid, multi-DG microgrid, and utility microgrid by function demand.

2.6.1.1. Simple microgrid

A simple microgrid contains only one type of DG, has simple functions and design, and is intended for use of CCHP or continuous supply to critical loads.

2.6.1.2. Multi-DG microgrid

A multi-DG microgrid is composed of multiple simple microgrids or multiple types of complementary, coordinated DGs. Compared with a simple microgrid, the design and operation of such a grid are much more complicated. Some loads need to be identified as sheddable loads in case of emergency to maintain power balance during islanded operation.

2.6.1.3. Utility microgrid

All DGs and microgrids that meet specific technical conditions can be integrated into a utility microgrid. In such a microgrid, loads are prioritized based on users’ requirements on reliability, and high-priority loads will be powered preferentially in an emergency.

Classifying microgrids by function demand clearly defines the ownership of a microgrid during operation: simple microgrids can be operated and managed by customers, utility microgrids can be operated by utilities, and multi-DG microgrids can be operated either by utilities or customers.

2.6.2. By capacity

Microgrids are classified into simple microgrid, corporate microgrid, feeder area microgrid, substation area microgrid, and independent microgrid by capacity, as shown in Table 2.1.

Table 2.1

Classification of Microgrids by Capacity

Type	Capacity (MW)	Grid to be Connected
Simple microgrid	<2	Common grid
Corporate microgrid	2–5
Feeder area microgrid	5–20
Substation area microgrid	>20
Independent microgrid	Depending on loads on an island, a mountainous area or a village	Diesel-fueled grid

2.6.2.1. Simple microgrid

A simple microgrid has a capacity below 2 MW and is intended for independent facilities and institutes with multiple types of loads and of a small area, such as a hospital or school.

2.6.2.2. Corporate microgrid

A corporate microgrid has a capacity of 2–5 MW, and comprises CCHPs of varying sizes and some small household loads, generally no commercial or industrial loads.

2.6.2.3. Feeder area microgrid

A feeder area microgrid has a capacity of 5–20 MW and comprises CCHPs of varying sizes and some large commercial and industrial loads.

2.6.2.4. Substation area microgrid

A substation area microgrid has a capacity above 20 MW and generally comprises common CCHPs and all nearby loads (including household, commercial, and industrial loads).

These four types of microgrids are connected to common grids, and therefore, are collectively called grid-connected microgrid.

2.6.2.5. Independent microgrid

An independent microgrid is mainly intended for remote off-grid areas such as an island, a mountainous area, or a village, and the distribution system of the main grid uses a diesel generator or other small units to meet the power demand of such areas.

2.6.3. By AC/DC type

Microgrids are classified into DC microgrid, AC microgrid, and AC/DC hybrid microgrid.

2.6.3.1. DC microgrid

As shown in Figure 2.11, in a DC microgrid, DG, ES, and DC load are connected to a DC bus via a converter and the DC bus is connected to AC loads via an inverter to power both DC and AC loads.

Figure 2.11 Structure of a DC microgrid.

The advantages of the DC microgrid are as follows:

1. As DG control solely depends on DC voltage, it is easier to realize coordinated operation of the DGs.

2. DG and load fluctuations are compensated by ES on the DC side.

3. Compared with an AC microgrid, a DC microgrid is easier to control, does not involve synchronization among DGs, and thus is easier to suppress circulating current.

The disadvantage of the DC microgrid is that inverters are required for the power supply to AC loads.

2.6.3.2. AC microgrid

An AC microgrid connects to the distribution network via an AC bus, and the AC bus controls the microgrid’s connection to and disconnection from the distribution network through the circuit breaker at the PCC. Figure 2.12 shows the structure of an AC microgrid, in which DG and ES are connected to the AC bus via inverter. The AC microgrid is a dominant type of microgrid, and the major topic of this book in the following chapters.

Figure 2.12 Structure of an AC microgrid.

The advantage of the AC microgrid is that as the microgrid is connected to the grid through an AC bus, no inverter is required for power supply to AC loads. The disadvantage is that control and operation are difficult.

2.6.3.3. AC/DC hybrid microgrid

An AC/DC hybrid microgrid is a microgrid consisting of an AC bus and a DC bus. Figure 2.13 shows the structure of such a microgrid, in which the AC bus and DC bus allow for direct supply to AC loads and DC loads. With special power sources connected to an AC bus, an AC/DC hybrid microgrid is essentially a special AC microgrid on the whole.

Figure 2.13 Structure of an AC/DC hybrid microgrid.

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Table of Contents for Chapter 2: Composition and classification of the microgrid

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