Decentralized Power Flow Control Strategy Using Transition Operations of DC-Bus Voltage for Detection of Uncertain DC Microgrid Operations

Abstract

To enhance the reliability and flexibility of DC microgrids (DCMGs), this paper presents a decentralized power flow control strategy (PFCS) by using the transition operation modes. The transition operation modes are introduced as an effective communication method among power units, eliminating the use of additional digital communication links (DCLs) for the purpose of ensuring the power balance as well as the voltage regulation even under uncertain conditions. During the transition operation modes, the power unit which transmits the information shifts the DC-link voltage level, and the power unit which receives the information continuously monitors the DC-link voltage with predetermined time. When uncertain conditions occur in a particular power unit, this power unit triggers the transition operation modes to send this information to all power units in the DCMG system. The proposed PFCS can maintain the DC-link voltage at the nominal value for steady-state conditions both in the grid-connected mode and islanded mode. Moreover, the proposed PFCS significantly enhances the overall reliability of the decentralized DCMG system by effectively addressing several uncertainties stemmed from electricity price fluctuations, grid availability, battery state-of-charge (SOC) levels, and wind power variations. The scalability of the DCMG system is also demonstrated by incorporating an electric vehicle (EV) unit as an additional energy storage system (ESS). The EV unit seamlessly cooperates with the existing battery unit, functioning as additional ESS to regulate the DC-link voltage when the battery SOC level is low. Simulation and experimentation results under various conditions demonstrate the effectiveness of the proposed PFCS.

1. Introduction

The global energy infrastructure is undergoing a transformation, driven by the need for sustainable and resilient power systems. As traditional centralized power generation models face challenges of efficiency, reliability, and environmental impact, the demand for decentralized energy solutions attracts worldwide attention from researchers. In this context, microgrids have emerged as a promising concept that offers a new paradigm for efficient and flexible power distribution and management. A microgrid, defined as a localized and self-contained power system, provides a reliable and resilient electricity supply. In general, the microgrid integrates various distributed energy resources (DERs), renewable energy, energy storage systems (ESSs), and controllable loads [1]. As the world’s energy situation keeps changing, microgrids are becoming increasingly important in driving the shift toward a sustainable and reliable energy future.

In the microgrid system, the DC microgrid (DCMG) has gained significant attention as a promising solution to realize efficient and sustainable power distribution systems. The DCMG system offers several advantages over the AC microgrid system because the issue regarding the synchronization of distributed generators, inrush currents caused by transformers, reactive power flow, harmonic currents, and three-phase unbalance does not exist in the DCMG system [2,3,4]. Furthermore, DCMG offers easy integration for DC energy sources such as photovoltaics (PV), fuel cells, and ESSs like batteries or supercapacitors [5,6].

In general, the DCMG system operation can be categorized into two modes: the grid-connected mode and islanded mode, depending on the availability of the main grid [7]. In the grid-connected mode, the utility grid primarily coordinates the exchange power in the DCMG to achieve power balance as well as voltage regulation. On the other hand, in the islanded mode, the power balance of the DCMG is maintained through a coordinated operation of specific power units, such as distributed generators (DGs) and ESSs. In the grid-connected mode, optimizing the cost of power consumption in a DCMG is a significant concern. Various methods such as time-of-use pricing (TOU), real-time pricing (RTP), and stepwise power tariff (SPT) are used worldwide to calculate electricity bills [8,9,10,11]. To optimize the power consumption cost of a DCMG system, previous studies have considered the electricity price change of the utility grid [12,13,14].

In terms of communication links, the DCMG control methods can be categorized as centralized control, distributed control, and decentralized control. The centralized control uses a central controller to determine the proper operation modes of each power unit by communicating with all power units through digital communication links (DCLs) [15]. In contrast, the distributed control also utilizes DCLs to collect data from adjacent power units without using the central controller. In this approach, each power unit independently determines its operation mode by considering data collected from its surroundings [16]. On the other hand, the decentralized control operates without the use of DCLs. In this scheme, the DCMG system autonomously maintains power balance by relying solely on local measurements of each unit [17,18]. The absence of DCLs in the decentralized control eliminates communication delays and simplifies system installation, thereby enhancing the system’s scalability and flexibility [19]. The elimination of DCLs also has other potential benefits such as lower implementation costs and less dependency on communication systems, which can increase overall reliability and robustness [19,20,21]. Furthermore, the DCMG’s vulnerability to cybersecurity threats may also be decreased, and the effort for maintenance and troubleshooting is reduced, which can potentially lead to faster problem resolution [21,22].

In a decentralized control scheme, the operation modes of each power unit in a DCMG system are determined using the DC-link voltage as a key indicator [23]. When there is surplus power in the DCMG, the DC-link voltage increases. On the contrary, a deficit of power in the DCMG leads to a decrease in the DC-link voltage. To enable power sharing among power sources in the DCMG system, voltage-based droop control methods have been introduced in the study [24]. These methods present the implementation of a virtual resistor to achieve the power balance of each converter in the DCMG system. Voltage/current (V/I) or voltage/power (V/P) characteristics are commonly used in the droop control method for the decentralized DCMG to achieve not only voltage regulation but also power balance [25].

Despite the effective power-sharing characteristic of the droop control, it still has the drawback of DC-link voltage deviation [26]. This deviation in DC-link voltage impacts the delivered power to the load. A significant deviation affects power accuracy, while even a slight deviation may influence power-sharing precision [27]. To overcome this issue, a voltage restoration scheme for a decentralized DCMG is presented in [28], in which the DC-link voltage is maintained at a constant value by injecting or absorbing power. For optimal power-sharing, another study successfully implements the DC-link voltage controller in the centralized and distributed control schemes [29,30]. In these schemes, the DC-link voltage controller should be implemented by at least one power unit in every operation mode of the DCMG.

Aside from the DC voltage deviation, another concern for a reliable decentralized DCMG operation is to carefully manage the battery state-of-charge (SOC) level to avoid faster battery degradation [31]. It is imperative to ensure that the battery operates primarily at high SOC levels for the extension of its lifespan. In the study [32], the low SOC level is only used for certain emergency operations to increase the reliability of DCMG operations. In addition to the battery SOC management, the reliability of the DCMG can be further improved by separately considering the electrical loads as non-critical and critical loads with different priorities [33]. Numerous studies in the field of microgrids have emphasized the importance of maintaining critical loads as well as improving the resilience and survivability of critical loads [34]. Motivated by these concerns, the battery SOC levels will be categorized according to the respective significance in this paper. During periods of low SOC levels, only critical loads are supplied based on the given priority. In this case, non-critical loads are subject to load shedding to achieve high resilience and reliability for the microgrid operation.

To enhance the reliability and optimize the operation of a microgrid system, this paper presents a power flow control scheme (PFCS) for a decentralized DCMG system. The transition operation modes are utilized in the proposed scheme to transmit the information on the DCMG system to other power units without the use of additional DCLs. This is achieved by continuously monitoring the DC-link voltage value in all power units. The DCMG system considered in this study consists of a utility grid unit, battery unit, wind turbine unit, and load units. In the grid-connected mode, the DC-link voltage control method is employed by the utility grid unit. On the other hand, in the islanded mode, the battery and wind turbine units alternate the regulation of the DC-link voltage to the nominal value. In the proposed PFCS, various factors such as the generation power of the wind turbine unit, battery SOC level, grid availability, load demand, and electricity price conditions are considered to develop an overall control strategy for the decentralized DCMG system. In addition, the scalability issue of the DCMG system is also studied by considering an electric vehicle (EV) unit as an additional ESS in the DCMG system. The contribution of the proposed method is to enhance the reliability and optimize the operation of the DCMG system without using the DCLs for significantly reduced system price. The PFCS is proposed to achieve the optimum operating condition and maintain the DC link voltage at steady-state conditions. The DCMG system inevitably has uncertain conditions such as grid electricity price change, grid fault, battery SOC reaching emergency level, or variation of wind power generation. To deal with such uncertain conditions effectively without DCLs, the transition mode operations are used in the proposed scheme for the purpose of detecting the uncertainties in the DCMG operation and changing the operations of power units accordingly.

The simulation is carried out to verify the effectiveness of the proposed decentralized control scheme by using the Powersim (PSIM) software (9.1, Powersim, Rockville, MD, USA). The simulation results confirm the effectiveness of the proposed approach in achieving optimal power-sharing, even in the presence of diverse conditions such as fluctuations in electricity prices, grid availability, uncertainties in battery SOC in the islanded mode, wind power variation, and EV unit involvement. The experimental results are also presented to support the simulation results by using a prototype DCMG test setup. The main contributions of this paper are summarized as follows:

(i)

The proposed PFCS for a decentralized DCMG is capable of maintaining the DC-link voltage at the nominal value for steady-state conditions both in the grid-connected mode and islanded mode. In addition, the proposed PFCS autonomously changes the DCMG operation to guarantee the power balance even in the presence of uncertain DCMG conditions such as electricity price changes, grid availability, battery SOC level, wind power variation, and EV unit involvement.

(ii)

In the proposed PFCS for a decentralized DCMG, the transition operation modes are introduced as an effective way to communicate between power units without the use of additional DCLs for the purpose of ensuring the power balance as well as the voltage regulation even under uncertain conditions. During the transition operation modes, the power unit which transmits the information shifts the DC-link voltage level. Meanwhile, the power unit which receives the information continuously monitors the DC-link voltage with predetermined time. When an uncertain condition occurs in a particular power unit, it uses the transition operation modes to send this information to all power units in the DCMG system. After this uncertain condition is acknowledged by all power units, the DCMG operation is switched back from the transition operation mode to the optimum steady-state operation.

(iii)

The proposed PFCS improves the reliability of the DCMG system by carefully managing the battery SOC level. Furthermore, the proposed PFCS allows the easy integration of an EV unit within the DCMG system as additional ESS to support the battery unit that functions as the primary ESS. When the battery SOC approaches a low level, the EV unit undertakes the DC-link voltage regulation to ensure the power balance.

This paper is organized as follows: Section 2 presents the system configuration of a decentralized DCMG system. Section 3 describes the proposed control strategy of a decentralized DCMG. Section 4 addresses the proposed PFCS of a decentralized DCMG. Section 5 and Section 6 present the simulation and experimental results, respectively. Finally, Section 7 concludes the paper.

2. System Configuration of a Decentralized DCMG

2.1. Configuration of DCMG

The configuration of a decentralized DCMG system presented in this paper is illustrated in Figure 1, comprising four primary power units: a utility grid unit, a wind turbine unit, a battery unit, and a load unit. An EV unit can also be considered as an additional ESS.

The utility grid unit connects the main grid to the DC-link through a transformer and a bidirectional AC/DC converter. The wind turbine unit uses a permanent magnet synchronous generator (PMSG) to convert mechanical power into electrical power, which is then fed into the DC-link through a unidirectional AC/DC converter. Both the battery unit and the EV unit utilize bidirectional interleaved DC/DC converters to exchange powers with the DC-link. The load unit which consists of critical load (R_C) and non-critical load (R_NC) employs electronic switches for load shedding and load reconnection.

In the DCMG system, several power units such as the utility grid, battery, and EV have bidirectional power flow, enabling them to either supply power into the DC-link, or absorb power from the DC-link. In contrast, the wind turbine unit only supplies power into the DC-link, and the load unit absorbs power from the DC-link. As shown in Figure 1, P_G, P_B, P_W, and P_L denote the power flow of the utility grid unit, the battery unit, the wind turbine unit, and the load unit, respectively. The power injection and current flow into the DC-link are represented as negative (−) values, while power absorption and current flow from the DC-link are represented as positive (+) values. The power of the EV unit (P_EV) functions similarly to the battery unit, serving as an additional ESS. The operational details and control methods of converters are determined by the proposed PFCS of the DCMG, which will be discussed in subsequent sections. The EV unit is dealt with separately from the proposed PFCS and functions as an additional plug-and-play ESS.

2.2. Management of Battery SOC

The management strategy of the battery SOC level used in the proposed decentralized PFCS of the DCMG system is shown in Figure 2. Battery operation is determined according to SOC levels and connected loads as specified in

Table 1. SOC levels for battery unit.

SOC	Values	Purposes
SOCB,Min	5%	Level to stop discharging in islanded mode
SOCB,Emg-L	20%	Level to activate RC load reconnection in islanded mode
SOCB,Emg-H	25%	Level to activate RNC load shedding in islanded mode
		Level to stop discharging in grid-connected mode with high electricity price condition
SOCB,LR	50%	Level to activate RNC load reconnection in islanded mode
SOCB,Max	90%	Level to stop charging in both islanded and grid-connected modes

.

First, to prevent the critical situation, SOC_B,Min is defined as 5%, which is the level to stop discharging in the islanded mode. At a battery SOC level lower than SOC_B,Min, the battery unit is always in charging mode. The second SOC level is SOC_B,Emg-L, at which the load reconnection process is activated for the critical load R_C. The third SOC level defined as SOC_B,Emg-H is used for two purposes. This level is used to stop discharging the battery in grid-connected mode with high electricity price condition. It is also used to activate load shedding for the non-critical load R_NC in the islanded mode. The fourth SOC level is defined as SOC_B,LR which is used to activate load reconnection for non-critical load R_NC if the battery unit has activated load shedding previously for R_NC. The last SOC level defined as SOC_B,Max is used to stop charging in both islanded and grid-connected modes.

3. Proposed Control Strategy of Decentralized DCMG

3.1. Operation Modes of Utility Grid Unit

Figure 3 shows the operation modes of the utility grid unit which are composed of one steady-state mode and two transition modes. The first mode, denoted by Rd_G1, in Figure 3 represents the steady-state mode in which the DC-link voltage V_dc is always maintained at the nominal DC-link voltage V_Nom. In Rd_G1 operation, the utility grid unit works in the region between $-P_G^{max}$ and $P_G^{max}$ to supply power to the DC-link by rectifier (REC) mode, or to absorb power from the DC-link by inverter (INV) mode.

The second mode Rd_G2, is used during the transition period when the electricity price condition changes from normal to high. When this condition happens, the utility grid unit regulates V_dc to V_L1 by adopting the Rd_G2 mode in order that the battery unit identifies the electricity price condition change from normal to high by detecting the DC-link voltage variation. Then, the battery unit changes the operation in order to work properly under the high electricity price condition. After some predetermined time, the operation of the utility grid unit switches back to Rd_G1 to maintain V_dc to V_Nom.

Another transition mode Rd_G3 is used when the electricity price condition changes from high to normal, or when the grid is reconnected after a fault. Under these conditions, the utility grid unit temporarily changes from V_dc to V_H1 by the Rd_G3 mode for the purpose of transmitting this information to other power units. After some predetermined time, the operation of the utility grid unit is returned to steady-state mode Rd_G1.

3.2. Operation Modes of Battery Unit

The operation modes of the battery unit consist of five steady-state modes and four transition modes as illustrated in Figure 4 and Figure 5. Figure 4 shows the steady-state operation modes of the battery unit which are separated into the operations in the grid-connected mode and islanded mode.

In steady-state grid-connected mode operation, the battery unit can work in the battery maximum current discharging mode (BDM) with Rd_B1, the battery maximum current charging mode (BCM) with Rd_B2, or idle mode with Rd_B3. In steady-state islanded mode operation, the battery unit can operate in BCM with Rd_B2, idle mode with Rd_B3, battery secondary voltage control discharging mode (BVCM_Dis) with Rd_B4, or battery secondary voltage control charging mode (BVCM_Cha) with Rd_B5. The Rd_B1 mode is utilized in the grid-connected mode when the utility grid has high electricity price conditions. The Rd_B2 mode is deployed either in the grid-connected mode with normal electricity price conditions, or in the islanded mode with the wind turbine unit regulating the DC-link voltage to V_Nom. The Rd_B3 mode indicates the idle mode which is used when the battery SOC level reaches SOC_B,Max in the charging condition, or when the battery SOC level falls to SOC_B,Emg-H in the high electricity price condition. The Rd_B4 and Rd_B5 modes, which are only used in the islanded mode, implement BVCM_Dis and BVCM_Cha. To regulate the DC-link voltage at V_Nom, the operation of BVCM_Dis and BVCM_Cha is achieved by the primary droop control and secondary voltage control as shown in Figure 5. The Rd_B4 mode is utilized when the generated power of the wind turbine unit is less than the demanded load in the islanded mode. In contrast, the Rd_B5 mode is deployed to guarantee the DC-link voltage to V_Nom when the power of the wind turbine unit is in the region [P_L, $\left(P_L+P_B^{max}\right)$].

Figure 5 shows the secondary and primary controls to implement Rd_B4 and Rd_B5. The block diagram for the primary control and secondary control is shown in Figure 5a in which the primary control generates reference power (P_ref) to achieve power balance, and the secondary control regulates the DC-link voltage to V_Nom. In secondary control, the DC-link voltage is compared to the V_Nom and the error is fed to the proportional-integral (PI) controller. The PI controller output is added with the initial no-load voltage ($V_{N{L}_0}$) to produce a no-load voltage (V_NL) in the primary control. Finally, the V_NL is compared to the nominal voltage (V_Nom) to generate reference power (P_ref) by using the droop constant (R_d). Figure 5b shows the V-P droop curves for P_ref and V_NL.

Figure 6 shows the transition operation modes of the battery unit used only in the islanded mode. The first mode Rd_B6 denotes the battery voltage control emergency mode (BVCM_Emg1) which is used to decrease the DC-link voltage to V_L2 when the battery SOC level drops to SOC_B,Emg-H in discharging. The Rd_B6 mode is used to allow another ESS to respond to low DC-link voltage and to regulate DC-link voltage back to V_Nom. The second transition mode Rd_B7 implements the battery voltage control emergency discharging mode (BVCM_Emg2,Dis) and battery voltage control emergency charging mode (BVCM_Emg2,Cha). These modes are used to maintain the DC-link voltage at V_L2 after Rd_B6 activation for R_NC load shedding in the islanded mode.

The last transition mode Rd_B8 implements the battery voltage control load reconnection discharging mode (BVCM_LR,Dis) and battery voltage control load reconnection charging mode (BVCM_LR,Cha). These modes are deployed to increase the DC-link voltage at V_H1 for load reconnection in the islanded mode. In the Rd_B8 mode, the critical load R_C is reconnected as the battery SOC level reaches SOC_B,Emg-L. When the battery SOC level further increases to SOC_B,LR, the non-critical load R_NC is also reconnected.

3.3. Operation Modes of Wind Turbine Unit

Figure 7 shows the operation of the wind turbine unit which has two operation modes in steady-state and one operation mode in transient condition. The first mode, Rd_W1, denotes the wind voltage droop control mode (WVDCM) in the islanded mode during transition condition. In this mode, a primary droop control is utilized to reduce the wind power generation. The second mode, Rd_W2 is a steady-state operation and happens when the wind turbine unit regulates the DC-link voltage in the islanded mode using wind voltage control mode (WVCM). When the absolute of the wind turbine power(|P_W|) is higher than the sum of P_L and $P_B^{max}$, the WVCM mode uses a PI controller for DC-link voltage regulation. The third mode, Rd_W3, is the maximum power point tracking (MPPT) mode that is utilized to supply the maximum power to the DC-link either in the grid-connected mode or islanded mode.

3.4. Load Unit

The load unit consists of two categories of load, which are critical load (R_C) and non-critical load (R_NC). The total load (R_T) represents the case that both R_C and R_NC are connected. The load unit also detects the DC-link voltage for load shedding and load reconnection purposes. The details of the DC-link voltage detection are discussed in the next subsection.

3.5. Voltage Detection

In the proposed decentralized PFCS for a DCMG system, the power balance and voltage regulation are effectively achieved by detecting the DC-link voltage values without DCLs. All the power units can properly change the operation modes according to the DC-link voltage values after some predetermined time.

Figure 8 shows the transition operation of DC-link voltage and information transmission in the case of V_dc = V_H1. Figure 8a shows the process of R_C reconnection in the islanded mode. As soon as the battery SOC level reaches SOC_B,Emg-L, the battery unit starts to operate with Rd_B8 mode within 5 s to increase V_dc to V_H1. The load unit continuously monitors the V_dc level. Finally, if the V_dc value reaches V_H1, the load unit reconnects R_C after =3 s.

Figure 8b shows the process of R_NC reconnection in the islanded mode. As the battery SOC level reaches SOC_B,LR, the battery unit starts the Rd_B8 mode within 10 s to increase the DC-link voltage to V_H1. The load unit continuously monitors whether the DC-link voltage V_dc is increased to V_H1. If the V_dc reaches to V_H1, the load unit reconnects R_NC after =8 s, resulting in the total load of R_T as shown in Figure 8b.

Figure 8c shows the process of the utility grid unit reconnection from fault in high electricity price condition. In this condition, the utility grid unit operates with Rd_G3 to increase the DC-link voltage to V_H1. After 3 s, R_C is reconnected if R_C has been disconnected previously by load shedding. After 8 s, R_NC is reconnected if R_NC has been disconnected previously by load shedding. After 13 s, by observing the DC-link voltage value, the battery and the wind turbine units recognize that the utility grid is reconnected from the fault. Then, the wind turbine unit is returned to the MPPT mode to supply the maximum power to the DC-link. Depending on the SOC level, the battery unit works in BDM, BCM, or idle mode. After the transmission of the utility grid recovery information to other power units has been accomplished during 15 s, the utility grid unit returns to steady-state Rd_G1 mode to regulate the DC-link voltage at V_Nom.

Figure 8d shows the process of the utility grid unit reconnection from fault in normal electricity price condition. Similar to Figure 8c, the utility grid unit operates with Rd_G3 to increase the DC-link voltage to V_H1. If the load shedding has been activated previously, similar load reconnection is achieved after 3 s and 8 s, respectively. After 13 s, the wind turbine unit identifies the utility grid recovery, and starts the MPPT mode. It is worth mentioning that the wind turbine unit identifies only the utility grid recovery without having the exact information of the utility grid price condition. Only the battery unit recognizes the utility grid recovery as well as the normal electricity price condition after 18 s, if the Rd_G3 operation by the utility grid unit lasts until 20 s. Then, the battery unit operates either in BCM or idle mode depending on the SOC level. Similarly, after the utility grid recovery information has been sent to other power units within 20 s, the utility grid unit changes the transition operation Rd_G3 to steady-state operation Rd_G1 to regulate the DC-link voltage at V_Nom.

Figure 8e shows the process of the utility grid unit electricity price change from high to normal. The utility grid unit operates with Rd_G3 to increase the DC-link voltage to V_H1. After 18 s, the battery unit detects this voltage level to identify the electricity price change. Then, the battery unit changes the operation either in BCM or idle mode. Finally, after 20 s, the utility grid unit changes the operation to Rd_G1 to regulate the DC-link voltage at V_Nom in the steady-state condition.

Figure 9 shows the transition operation of DC-link voltage and information transmission in case of different values of V_dc. Figure 9a shows the process of the utility grid unit electricity price change from normal to high by using V_dc = V_L1. In this condition, the utility grid unit first operates in Rd_G2 and decreases the DC-link voltage to V_L1. After 3 s, the battery unit acknowledges the electricity price condition change from normal to high by monitoring the V_dc level, and changes operation mode to BDM, idle, or BCM depending on the battery SOC level. After 5 s, the utility grid unit stops operating the Rd_G2 and returns to steady-state operation mode Rd_G1.

Figure 9b,c show how the battery unit detects the utility grid unit fault with two different high-voltage and low-voltage conditions, respectively. If a fault occurs in the utility grid unit during INV mode operation, the DC-link voltage is rapidly increased. Figure 9b shows the detection process of the battery unit for this case. On the contrary, Figure 9c shows the detection process of the battery unit for grid fault with low voltage when the utility grid unit works in REC mode before the fault. If those DC-link voltage conditions last for 10 ms, the battery unit confirms the grid fault.

Figure 9d shows the process of R_NC load shedding in the islanded mode. As the battery unit SOC level reaches SOC_B,Emg-H in the islanded mode, the battery unit changes operation to Rd_B6 for BVCM_Emg1, and reduces the DC-link voltage to the second level of low DC-link voltage (V_L2). As soon as the DC-link voltage decreases to V_L2, the battery unit changes the operation to Rd_B7 after 0.5 s, and the load unit activates load shedding for R_NC after 3 s.

Figure 9e,f show the process of load shedding for R_NC and R_C, respectively. Figure 9e shows the process of R_NC shedding after 0.1 s when the DC-link voltage drops to the third level of low DC-link voltage (V_L3). Figure 9f shows the process of R_C shedding after 0.3 s when the DC-link voltage falls to V_L3.

Figure 10 shows the transition operation of DC-link voltage and information transmission in the islanded mode the in case of V_dc > V_H2 − V_D. Figure 10a shows the process of the battery unit operation change from BVCM_Dis to BVCM_Cha in case of |P_W| > P_L in the islanded mode. When |P_W| is higher than P_L, the wind turbine unit operates in WVDCM and V_dc increases to the value of V_H2 − V_D. By detecting this increased voltage, the battery unit operation is changed from Rd_B4 to Rd_B5 after 1 s. Then, the wind turbine unit operates back in the MPPT mode as the battery unit regulates DC-link voltage in BVCM_Cha.

Figure 10b shows the process of the wind turbine unit regulating the DC-link voltage to V_Nom. When |P_W| is higher than P_L even with battery charging operation, the wind turbine unit operates in WVDCM in Rd_W1 and V_dc increases to the value of V_H2 − V_D. Then, after 3 s, the battery unit changes the operation from BVCM_Cha in Rd_B5 to BCM in Rd_B2. After 5 s, the wind turbine unit changes operation from WVDCM in Rd_W1 to WVCM in Rd_W2.

4. Power Flow Control Strategy of Decentralized DCMG

In this section, the proposed PFCS is explained to realize an effective decentralized DCMG control and cooperation method with the EV unit. The PFCS determines the operating modes of all power units within the DCMG, by taking into account the factors such as wind power generation, load demand, battery SOC level, and availability of the utility grid unit. In addition, the grid-connected operation of the DCMG system also considers the electricity price condition to determine the suitable operation of the power units. The proposed PFCS is categorized into three main operation modes: grid-connected mode operation, islanded mode operation, and transition mode operation.

4.1. PFCS of Decentralized DCMG in Grid-Connected Mode

Table 2. Steady-state DCMG states and operation modes in grid-connected mode (1 = True, 0 = False).

DCMG States	Conditions				Operation Modes
	High Electricity Price	Battery SOC
		CB	DB or EB	FB
1	0	0	0	1	OP1
2	0	0	1	0	OP2
3	0	1	0	0	OP2
4	0	0	0	0	OP2
5	1	0	0	1	OP3
6	1	0	1	0	OP3
7	1	1	0	0	OP1

shows the DCMG states and operation modes in the steady-state grid-connected mode. In this study, the DCMG states in the grid-connected mode are divided according to the electricity price conditions and battery SOC level as defined in Figure 2. As shown in Table 2, grid-connected operation modes at steady-state are determined according to the DCMG states. For example, DCMG state 1 represents the condition that the utility grid unit has the normal electricity price condition and the battery unit SOC level is in the F_B region. In DCMG state 1, the DCMG works with the operation mode OP₁.

Table 3. Steady-state operation modes for individual power units of decentralized DCMG in grid-connected mode according to operation modes in Table 2.

Operation Modes	Operations of DCMG Units
	Utility Grid Unit	Battery Unit	Wind Turbine Unit	Load Unit
OP1	$R{d}_{G1}$	$R{d}_{B3}$	$R{d}_{W3}$	$R_T$
OP2	$R{d}_{G1}$	$R{d}_{B2}$	$R{d}_{W3}$	$R_T$
OP3	$R{d}_{G1}$	$R{d}_{B1}$	$R{d}_{W3}$	$R_T$

shows the steady-state operation modes for individual power units in the grid-connected mode of the decentralized DCMG system.

Operation mode OP₁: In this mode, the utility grid unit operates with Rd_G1 mode, the battery unit is in idle mode with Rd_B3, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. This operation mode is used in DCMG states 1 and 7. In either case, the battery enters idle mode to prevent further charging or discharging.

Operation mode OP₂: In this mode, the utility grid unit operates with Rd_G1 mode, the battery unit is in BCM mode with Rd_B2, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. This operation mode is used in DCMG state 2 through 4. In this case, the battery works in BCM mode to charge the battery to SOC_B,Max.

Operation mode OP₃: In this mode, the utility grid unit operates with Rd_G1 mode, the battery unit is in BDM mode with Rd_B1, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. This operation mode is used in DCMG states 5 and 6. In this case, the battery works in BDM mode to supply power to DC-link.

4.2. PFCS of Decentralized DCMG in Islanded Mode

The proposed PFCS for the decentralized DCMG in the islanded mode are divided according to the load operation, the battery SOC level, and the wind turbine unit power condition.

Table 4. Steady-state DCMG states and operation modes in islanded mode with condition |P_W| < P_L (1 = True, 0 = False).

DCMG States	Conditions					Operation Modes
	Load	Battery SOC
		BB	CB	DB	EB or FB
8	$R_T$	0	0	0	1	OP4
9	$R_T$	0	0	1	0	OP4
10	$R_C$	0	0	1	0	OP5
11	$R_C$	0	1	0	0	OP5
12	$R_C$	1	0	0	0	OP5

,

Table 5. Steady-state DCMG states and operation modes in islanded mode with condition P_L + P_B > |P_W|> P_L (1 = True, 0 = False).

DCMG States	Conditions						Operation Modes
	Load	Battery SOC
		AB	BB	CB	DB	EB
13	$R_T$	0	0	0	0	1	OP6
14	$R_T$	0	0	0	1	0	OP6
15	$R_C$	0	0	0	1	0	OP7
16	$R_C$	0	0	1	0	0	OP7
17	$R_C$	0	1	0	0	0	OP7
18	No load	0	1	0	0	0	OP8
19	No load	1	0	0	0	0	OP8

and

Table 6. Steady-state DCMG states and operation modes in islanded mode with condition |P_W|> P_L + P_B (1 = True, 0 = False).

DCMG States	Conditions							Operation Modes
	Load	Battery SOC
		AB	BB	CB	DB	EB	FB
20	$R_T$	0	0	0	0	0	1	OP9
21	$R_T$	0	0	0	0	1	0	OP10
22	$R_T$	0	0	0	1	0	0	OP10
23	$R_C$	0	0	0	1	0	0	OP11
24	$R_C$	0	0	1	0	0	0	OP11
25	$R_C$	0	1	0	0	0	0	OP11
26	No load	0	1	0	0	0	0	OP12
27	No load	1	0	0	0	0	0	OP12

show the DCMG states and operation modes in the steady-state islanded mode, mainly according to the wind turbine power conditions. Table 4 denotes DCMG states and operations in the islanded mode for condition |P_W| < P_L. For example, the DCMG state 8 represents the condition that the load unit operates with total load R_T and the battery SOC level is in E_B or F_B region with |P_W| < P_L. In DCMG state 8, the DCMG works with the operation mode OP₄.

Table 5 denotes DCMG states and operations in the islanded mode for condition P_L + P_B > |P_W| > P_L. In this Table, the DCMG state 13 represents the condition that the load unit operates with total load R_T and the battery SOC level is in the E_B region with P_L + P_B > |P_W| > P_L. In DCMG state 13, the DCMG works with the operation mode OP₆.

Table 6 denotes DCMG states and operations in the islanded mode for condition |P_W| > P_L + P_B. In this Table, DCMG state 20 represents the condition that the load unit operates with total load R_T and the battery SOC level is in the F_B region. In DCMG state 20, the DCMG works with the operation mode OP₉.

Table 7. Steady-state operation modes for individual power units of decentralized DCMG in islanded mode according to operating modes in Table 4, Table 5 and Table 6.

Operation Modes	Operations of DCMG Units
	Utility Grid Unit	Battery Unit	Wind Turbine Unit	Load Unit
OP4	-	$R{d}_{B4}$	$R{d}_{W3}$	$R_T$
OP5	-	$R{d}_{B4}$	$R{d}_{W3}$	$R_C$
OP6	-	$R{d}_{B5}$	$R{d}_{W3}$	$R_T$
OP7	-	$R{d}_{B5}$	$R{d}_{W3}$	$R_C$
OP8	-	$R{d}_{B5}$	$R{d}_{W3}$	No load
OP9	-	$R{d}_{B3}$	$R{d}_{W2}$	$R_T$
OP10	-	$R{d}_{B2}$	$R{d}_{W2}$	$R_T$
OP11	-	$R{d}_{B2}$	$R{d}_{W2}$	$R_C$
OP12	-	$R{d}_{B2}$	$R{d}_{W2}$	No load

shows the steady-state operation modes for individual power units in the islanded mode of the decentralized DCMG system.

Operation mode OP₄: In this mode, the battery unit is in BVCM_Dis with Rd_B4, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. This operation mode is used in DCMG states 8 and 9. In either case, the battery SOC level is higher than SOC_B,Emg-H. The battery unit regulates DC-link voltage to V_Nom and the wind turbine unit operates in MPPT mode as |P_W| < P_L.

Operation mode OP₅: This mode is similar to operation mode OP₄ except for the load unit that only operates with R_C. This mode is used in DCMG states 10 through 12. In this case, the battery unit regulates DC-link voltage to V_Nom and the wind turbine unit operates in MPPT mode. This operation mode is used after the R_NC load shedding because the battery SOC level is lower than SOC_B,Emg-H or it does not fulfill R_NC reconnection condition.

Operation mode OP₆: In this mode, the battery unit is in BVCM_Cha with Rd_B5, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. This mode is used in DCMG states 13 and 14. In either case, the battery SOC level is higher than SOC_B,Emg-H but lower than SOC_B,Max.

Operation mode OP₇: This mode is similar to OP₆ except for the load unit that only operates with R_C. This mode is used in DCMG states 15 through 17. In this case, the battery unit regulates DC-link voltage to V_Nom and the wind turbine unit operates in MPPT mode with condition P_L + P_B > |P_W| > P_L. This mode is used after the R_NC load shedding because the battery SOC level is lower than SOC_B,Emg-H or it does not fulfill the R_NC reconnection condition.

Operation mode OP₈: This mode is similar to OP₆ except that the load is not connected. This mode is used in DCMG states 18 and 19. In this case, the battery unit regulates DC-link voltage to V_Nom and the wind turbine unit operates in MPPT mode as P_L + P_B > |P_W| > P_L with P_L = 0. This mode is used after the R_NC and R_C load shedding caused by insufficient power as the battery SOC level reaches SOC_B,Min, or if the battery SOC level still does not reach SOC_B,Emg-L for R_C reconnection.

Operation mode OP₉: In this mode, the battery unit is in idle mode with Rd_B3, the wind turbine unit operates in the WVCM mode with Rd_W2, and all loads are connected. This mode is only used in DCMG state 20. In this case, the battery SOC level is higher than SOC_B,Max. The wind turbine unit regulates DC-link voltage to V_Nom in charging mode as |P_W| > P_L + P_B with P_B = 0.

Operation mode OP₁₀: In this mode, the battery unit is in BCM with Rd_B5, the wind turbine unit operates in the WVCM mode with Rd_W2, and all loads are connected. This mode is used in DCMG states 21 and 22. In either case, the battery SOC level is higher than SOC_B,Emg-H and lower than SOC_B,Max.

Operation mode OP₁₁: This mode is similar to OP₁₀ except for the load unit that only operates with R_C. This mode is used in DCMG states 23 through 25, in which the battery unit is in maximum power charging and the wind turbine unit operates in WVCM mode as |P_W| > P_L + P_B. This mode is used after the R_NC load shedding because the battery SOC level is lower than SOC_B,Emg-H, or it does not fulfill the R_NC reconnection condition.

Operation mode OP₁₂: This mode is similar to OP₁₀ except that the load is not connected. This mode is used in DCMG states 26 and 27, in which the battery unit is in maximum power charging and the wind turbine unit operates in WVCM mode as |P_W| > P_L + P_B with P_L = 0. This mode is used after the R_NC and R_C load shedding caused by insufficient power as the battery SOC level reaches SOC_B,Min, and the battery SOC level still does not reach SOC_B,Emg-L for R_C reconnection.

4.3. Transition Mode Operations of Decentralized DCMG

Table 8. Transition conditions and operation modes of decentralized DCMG.

Conditions								Operation Modes
Utility Grid Unit	Electricity Price	Battery SOC	Mode of Battery Unit	Mode of Wind Turbine Unit	PW	Load Unit	Vdc
Normal	Normal → High	AB − FB	RdB1, RdB2, or RdB3	RdW3	$\left[-P_W^{max},0\right]$	RT	VL1	T1
Normal	High → Normal	AB − FB	RdB1, RdB2, or RdB3	RdW3	$\left[-P_W^{max},0\right]$	RT	VH1	T2
Normal	High	AB, BB → CB	RdB2	RdW3	$\left[-P_W^{max},0\right]$	RT	VNom	T3
Fault → Normal	Normal	AB − FB	RdB2, RdB3, RdB4, or RdB5	RdW2 → RdW3	$\left[-P_W^{max},0\right]$	RT, RC, No load	VH1	T4
Fault → Normal	High	AB − FB	RdB2, RdB3, RdB4, or RdB5	RdW2 → RdW3	$\left[-P_W^{max},0\right]$	RT, RC, No load	VH1	T4
Fault	-	CB	RdB6	RdW3	|PW| < PL	RT	VNom → VL2	T5
Fault	-	BB − CB	RdB7	RdW2, RdW3	$\left[-P_W^{max},0\right]$	RT → RC	VL2	T6
Fault	-	AB	RdB5	RdW3	|PW|< PL	RC → No load	< VL3	T7
Fault	-	CB	RdB8	RdW2, RdW3	|PW| > PL	No load → RC	VH1	T8
Fault	-	EB − FB	RdB8	RdW2, RdW3	|PW| > PL	RC → RT	VH1	T9
Fault	-	AB − FB	RdB4 → RdB5	RdW1	|PW| > PL	RT, RC, No load	>VH2 − VD	T10
Fault	-	AB − FB	RdB5 → RdB2	RdW1	|PW| > PL + PB	RT, RC, No load	>VH2 − VD	T11
Fault	-	FB	RdB3	RdW1	|PW| > PL	RT, RC, No load	>VH2 − VD	T12
Fault	-	BB − FB	RdB2, RdB3, RdB5 → RdB4	RdW2 → RdW3	|PW| < PL + PB	RT or RC	<VL2	T13

shows transition conditions and operation modes for the proposed PFCS which are used for transmitting information to other power units in this study. Thirteen transition states of T₁ through T₁₃ are defined as follows:

Conditions for mode T₁: This transition condition happens when the utility grid unit electricity price is changed from normal to high. When this condition occurs, the utility grid unit reduces the DC-link voltage V_L1 as shown in Figure 9a. Then, the battery unit acknowledges this information in the predetermined time.

Conditions for mode T₂: This transition condition happens when the utility grid unit electricity price is changed from high to normal. When this condition occurs, the utility grid unit increases the DC-link voltage to V_H1 as shown in Figure 8e. Then, the battery unit acknowledges this information by detecting the voltage level.

Conditions for mode T₃: This transition condition happens when the utility grid unit electricity price is high and the battery SOC level is lower than SOC_B,Emg-L.

Conditions for mode T₄: This transition condition happens when the utility grid unit is reconnected after a fault. By observing the DC-link voltage value, the battery unit recognizes that the utility grid is reconnected with normal or high electricity prices as in Figure 8c,d. The wind turbine unit also recognizes the reconnection of the utility grid unit, and returns to the MPPT mode. The load unit reconnection is also initiated.

Conditions for mode T₅: This transition condition happens when the battery SOC level reaches SOC_B,Emg-H in the islanded mode. The battery unit reduces the DC-link voltage to V_L2 in discharging mode using BVCM_Emg1 for 0.5 s as shown in Figure 9d. This is to enable another ESS to regulate DC-link voltage to V_Nom.

Conditions for mode T₆: This transition condition happens after T₅ when the battery SOC level reaches SOC_B,Emg-H in the islanded mode. The battery unit maintains the DC-link voltage at V_L2 with Rd_B7 during 5 s as shown in Figure 9d. The load unit then recognizes the DC-link voltage value and activates R_NC shedding.

Conditions for mode T₇: This transition condition happens when the battery SOC level reaches SOC_B,Min in the islanded mode. The battery unit operation is switched to BVCM_Cha and the load unit activates R_C shedding as shown in Figure 9f.

Conditions for mode T₈: This transition condition happens when the battery SOC level reaches SOC_B,Emg-L in the islanded mode after previous decrease below SOC_B,Min. The battery unit increases the DC-link voltage to V_H1 as in Figure 8a. The load unit continuously monitors the DC-link voltage level and reconnects R_C.

Conditions for mode T₉: This transition condition happens when the battery SOC level reaches SOC_B,LR in the islanded mode after previous decrease below SOC_B,Emg-H. The battery unit increases the DC-link voltage to V_H1 as in Figure 8b. The load unit continuously monitors the DC-link voltage level and reconnects R_NC.

Conditions for mode T₁₀: This transition condition happens when the wind turbine unit power is higher than the load requirement, and the battery unit is in BVCM_Dis. The wind turbine unit increases the DC-link voltage and operates in WVDCM as in Figure 10a. The battery unit changes operation from BVCM_Dis to BVCM_Cha after 1 s. The wind turbine unit operates back to the MPPT mode as the battery unit regulates DC-link voltage in BVCM_Cha.

Conditions for mode T₁₁: This transition condition happens when the wind turbine unit power is higher than the load requirement, and the battery unit charges power. The wind turbine unit increases the DC-link voltage and operates in WVDCM as in Figure 10b. The battery unit changes operation from BVCM_Cha to BCM after 3 s. The wind turbine unit operates in WVCM and regulates DC-link voltage after 5 s.

Conditions for mode T₁₂: This transition condition happens when the wind turbine unit power is higher than the load requirement, and the battery unit is in idle mode after reaching SOC_B,Max. The wind turbine unit increases the DC-link voltage and operates in WVDCM. After 5 s, the wind turbine unit operates in WVCM and regulates DC-link voltage.

Conditions for mode T₁₃: This transition condition happens when the wind turbine unit power is decreased. The wind turbine unit changes operation to the MPPT mode as soon as the DC-link voltage falls below V_L2. When the DC-link voltage reduces to V_L2, the battery unit also changes operation from BVCM_Cha, BCM, or idle mode, to BVCM_Dis, and then regulates the DC-link voltage.

Table 9. Transition operation modes for individual power units of decentralized DCMG according to operating conditions in Table 8.

Operation Modes	Operation Modes				Next Operation Mode
	Utility Grid Unit	Battery Unit	Wind Turbine Unit	Load Unit
T1	$R{d}_{G2}$	RdB2, or RdB3	RdW3	RT	OP1, OP3, or T3
T2	$R{d}_{G3}$	RdB1, RdB2, or RdB3	RdW3	RT	OP1 or OP2
T3	$R{d}_{G1}$	RdB2	RdW3	RT	OP1
T4	$R{d}_{G3}$	RdB2, RdB3, RdB4, or RdB5	RdW2, RdW3	RT	OP1, OP2, OP3, or T3
T5	-	RdB6	RdW3	RT	T6
T6	-	RdB7	RdW2, RdW3	RT	OP5, OP7, or OP11
T7	-	RdB4	RdW1	RC	OP8, or OP12
T8	-	RdB8	RdW2, RdW3	No load	OP5, OP7, or OP11
T9	-	RdB8	RdW2, RdW3	RC	OP4, OP6, or OP10
T10	-	RdB4	RdW1	RT, RC, No load	OP6, OP7, or OP8
T11	-	RdB5	RdW1	RT, RC, No load	OP10, OP11, or OP12
T12	-	RdB3	RdW1	RT	OP9
T13	-	RdB2, RdB3, or RdB5	RdW2	RT, RC, No load	OP4, OP5, or T10

shows the transition operation modes for individual power units of the decentralized DCMG system.

Operation mode T₁: In this mode, the utility grid unit operates with Rd_G2 mode to reduce the DC-link voltage to V_L1 as in Figure 9a, the battery unit operates either in BCM with Rd_B2 or idle mode with Rd_B3, and the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. The battery unit then changes operation to BDM with Rd_B1, BCM with Rd_B2, or idle mode with Rd_B3 depending on the battery SOC level. If the battery unit operates in BDM, then the next mode is OP₃. If the battery unit changes operation to idle mode, then the next mode is OP₁. If the battery SOC level is in A_B or B_B region, then the battery unit operates in BCM to charge until SOC_B,Emg-H and the next mode is T₃.

Operation mode T₂: In this mode, the utility grid unit operates with Rd_G3 mode to increase the DC-link voltage to V_H1 as in Figure 8e, the battery unit operates in BDM with Rd_B1, BCM with Rd_B2, or idle mode with Rd_B3, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. The battery unit then changes operation to either BCM with Rd_B2, or idle mode with Rd_B3 depending on the battery SOC level. If the battery unit operates in BCM, then the next mode is OP₂. If the battery unit changes operation to idle mode, then the next mode is OP₁.

Operation mode T₃: In this mode, the utility grid unit operates with Rd_G1 and regulates the DC-link voltage to V_Nom, the battery unit operates in BCM with Rd_B2, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. This mode follows T₁ when the battery SOC level is lower than SOC_B,Emg-L.

Operation mode T₄: In this mode, the utility grid unit operates with Rd_G3 to increase the DC-link voltage to V_H1, the battery unit is in BCM with Rd_B2, idle mode with Rd_B3, BVCM_Dis with Rd_B4, or BVCM_Cha with Rd_B5, the wind turbine unit operates either in the MPPT mode with Rd_W3, or WVCM mode with Rd_W2, and the load unit operates with R_T, R_C, or no load connection. As shown in Figure 8c,d, if the load shedding has been previously activated, the load reconnection process is initiated. The wind turbine unit recognizes the DC-link voltage and returns to the MPPT mode from WVCM. The battery unit recognizes electricity price, and changes operation to BDM with Rd_B1, BCM with Rd_B2, or idle mode with Rd_B3 depending on the battery SOC level. If the battery unit operates in BDM, then the next mode is OP₃. If the battery unit changes operation to idle mode, then the next mode is OP₁. If the battery unit operates in BCM with the normal electricity price condition, then the next mode is OP₂. If the battery unit operates in BCM with high electricity price, then the next mode is T₃.

Operation mode T₅: In this mode, the battery unit operates in the BVCM_Emg1 with Rd_B6 and regulates DC-link voltage to V_L2, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. The next mode is T₆ to maintain the DC-link voltage with the battery unit operating in Rd_B7 as in Figure 9d.

Operation mode T₆: In this mode, the battery unit operates with Rd_B7 to maintain the DC-link voltage to V_L2, the wind turbine unit operates in the MPPT mode with Rd_W3, and all loads are connected. The load unit continuously monitors the DC-link voltage level and starts R_NC shedding as in Figure 9d. The next mode is OP₁₁ if the previous steady-state operation of the battery unit is in BCM with Rd_B2. If the previous steady-state operation of the battery unit is in BVCM_Cha with Rd_B5, then the next mode is OP₇ to change from BVCM_Emg2,Cha to BVCM_Cha, or OP₅ to change from BVCM_Emg2,Dis to BVCM_Dis.

Operation mode T₇: In this mode, the battery unit is in BVCM_Cha with Rd_B5 as the battery SOC level falls below SOC_B,Min, the wind turbine unit operates in the MPPT mode with Rd_W3, and only the critical load is connected. When the wind power is not sufficient for the load, the voltage is reduced, and R_C shedding is triggered as in Figure 9f.

Operation mode T₈: In this mode, the battery unit regulates the DC-link voltage to V_H1 either in BVCM_LR,Cha, or BVCM_LR,Dis with Rd_B8 as the battery SOC level reaches SOC_B,Emg-L, the wind turbine unit operates either in the MPPT mode with Rd_W3 or in the WVCM with Rd_W2, and the load is not connected. The load unit monitors the DC-link voltage to reconnect R_C as in Figure 8a. The next mode is OP₁₁ if the previous steady-state operation of the battery unit is in BCM with Rd_B2. If the previous steady-state operation of the battery unit is in BVCM_Cha with Rd_B5, the next mode is OP₇ to change from BVCM_LR,Cha to BVCM_Cha, or OP₅ to change from BVCM_LR,Dis to BVCM_Dis.

Operation mode T₉: In this mode, the battery unit regulates the DC-link voltage to V_H1 either in BVCM_LR,Cha, or in BVCM_LR,Dis with Rd_B8 as the battery SOC level reaches SOC_B,LR, the wind turbine unit operates either in the MPPT mode with Rd_W3 or in the WVCM with Rd_W2, and only the R_C load is connected. The load unit monitors the DC-link voltage to reconnect R_NC as in Figure 8b. The next mode is OP₁₀ if the previous operation of the battery unit is in BCM with Rd_B2. If the previous operation of the battery unit is in BVCM_Cha with Rd_B5, the next mode is OP₆ to change from BVCM_LR,Cha to BVCM_Cha, or OP₄ to change from BVCM_LR,Dis to BVCM_Dis.

Operation mode T₁₀: In this mode, the battery unit is in BVCM_Dis with Rd_B4, the wind turbine unit operates in WVDCM with Rd_W1, and the load unit operates with R_T, R_C, or no load connection. The DC-link voltage is increased by the wind turbine unit. The battery unit operation is changed from BVCM_Dis with Rd_B4 to BVCM_Cha with Rd_B5 and the wind turbine unit operates back in the MPPT mode as in Figure 10a. If the total load is connected, the next mode is OP₆. If only the R_C load is connected, the next mode is OP₇. Finally, if there is no load connection before T₁₀, the next mode is OP₈.

Operation mode T₁₁: In this mode, the battery unit is in BVCM_Cha with Rd_B5, the wind turbine unit operates in the WVDCM with Rd_W1, and the load unit operates with R_T, R_C, or no load connection. The DC-link voltage is increased by the wind turbine unit, the battery unit operation is changed from BVCM_Cha with Rd_B5 to BCM with Rd_B2, and the wind turbine unit regulates the DC-link voltage to V_Nom in WVCM with Rd_W2 as in Figure 10b. If the total load is connected, the next mode is OP₁₀. If only R_C load is connected, the next mode is OP₁₁. Lastly, if there is no load connection, the next mode is OP₁₂.

Operation mode T₁₂: In this mode, the battery unit is in idle mode with Rd_B3, the wind turbine unit operates in WVDCM with Rd_W1, and all the loads are connected. The DC-link voltage is increased by the wind turbine unit, and the wind turbine unit regulates the DC-link voltage to V_Nom in WVCM with Rd_W2 as in Figure 10b with the battery unit still working in idle. The next mode is OP₉.

Operation mode T₁₃: In this mode, the battery unit is in BCM with Rd_B2, BVCM_Cha with Rd_B5, or idle mode with Rd_B3, the wind turbine unit operates in WVCM with Rd_W2, and the load unit operates with R_T, R_C, or no load connection. The DC-link voltage is decreased as the wind turbine unit power is insufficient to regulate the DC-link voltage to V_Nom. As soon as the DC-link voltage falls below V_L2, the wind turbine unit operates in the MPPT mode with Rd_W3 and the battery unit regulates the DC-link voltage to V_Nom with BVCM_Dis with Rd_B4. If all the loads are connected, the next mode is OP₄. If only the R_C load is connected, the next mode is OP₅. If the load is not connected, the next mode is T₁₀, in which the DC-link voltage is increased, and the battery unit changes operation to BVCM_Cha with Rd_B5.

4.4. Co-Operation with EV Unit

The integration of an EV unit as additional ESS is also studied to improve the scalability of the proposed decentralized DCMG system. Figure 11 shows the operation modes for the EV unit. The operation modes of the EV unit only work at steady-state and consist of three grid-connected modes and four islanded modes. For the EV unit, operation modes are simplified without considering load shedding or load reconnection as in the battery unit because the EV can be disconnected from the DCMG at any time.

The first mode, Rd_EV1, is the EV maximum current discharging mode (EDM) that is used when the utility grid is in the high electricity price condition. The second mode, Rd_EV2, is the EV maximum current charging mode (ECM) that is used under the normal electricity price condition and the condition that the wind power unit regulates the DC-link voltage. The third mode, Rd_EV3, is the idle mode to stop charging when the EV SOC reaches SOC_EV,Max, or to stop discharging when the EV SOC reaches SOC_EV,Min, respectively. The fourth mode, Rd_EV4, is the EV voltage control mode by discharging (EVCM_Dis) that is used in the islanded mode when the battery unit cannot regulate the DC-link voltage because of the low battery SOC level. The fifth mode, Rd_EV5, is the EV voltage control by charging (EVCM_Cha) that is used to absorb excessive power from the DC-link. The EVCM_Dis and EVCM_Cha use the PI control to maintain the DC-link voltage at V_Nom.

Table 10. SOC levels for EV unit.

SOC	Values	Purposes
SOCEV,Min	30%	Level to stop discharging
SOCEV,Max	90%	Level to stop charging

shows the EV SOC levels used in this study. Because the EV unit may connect and disconnect at any time, the transition modes for load shedding and load reconnection are not considered for the EV unit. The minimum and maximum SOC levels for the EV unit are used solely to prevent overdischarge and overcharge.

The EV unit also has the transition times to be able to cooperate with the battery unit. Figure 12 shows the transition operation of DC-link voltage and information transmission to cooperate with the EV unit. Figure 12a shows the battery and EV unit detection times for the utility grid unit fault when the previous utility grid unit operation is in the REC mode. Even while the EV unit acknowledges the utility grid fault at the same instant as the battery unit, it operates in idle mode with Rd_EV3. The EV unit continuously monitors the DC-link voltage and regulates the DC-link voltage after 100 ms when the DC-link voltage is higher than V_H2 − V_D. Figure 12b shows the process of utility grid fault identification by the EV unit when the previous utility grid unit operation is in INV mode. The entire process is similar to Figure 12a except that the EV unit takes the regulation of the DC-link voltage with Rd_EV4 with the condition V_dc < V_L2. Figure 12c shows the EV detection process of insufficient power in the DCMG system. After 100 ms when the DC-link voltage is lower than or equal to V_L2, the EV unit regulates the DC-link voltage at V_Nom with Rd_EV4.

Figure 13 shows the transition operation of DC-link voltage and information transmission for the EV unit cooperation in the islanded mode in the case of V_dc > V_H2 − V_D. Figure 13a shows the process of the EV unit operation change from idle to EVCM_Dis and the battery unit operation change from BVCM_Cha to BCM caused by |P_W| > P_L + P_B in the islanded mode. This process occurs after the battery unit changes operations from BVCM_Dis to BVCM_Cha in Figure 10a. When |P_W| is higher than P_L and P_B, the wind turbine unit operates in WVDCM and V_dc increases to the value of V_H2 − V_D. By detecting this increased voltage, the EV unit operation is changed from Rd_EV4 to Rd_EV5 after 3 s. At the same time, the battery unit operation is changed from Rd_B5 to Rd_B2. Then, the wind turbine unit operates back in the MPPT mode as the EV unit regulates DC-link voltage in EVCM_Cha.

Figure 13b shows the process of the wind turbine unit regulating the DC-link voltage to V_Nom. When |P_W| is higher than P_L even with the battery unit and the EV unit in charging operation, the wind turbine unit operates in WVDCM in Rd_W1 and V_dc increases to the value of V_H2 − V_D. Then, after 5 s, the EV unit changes the operation from EVCM_Cha in Rd_EV5 to ECM in Rd_EV2. After 5 s, the wind turbine unit changes operation from WVDCM in Rd_W1 to WVCM in Rd_W2.

5. Simulation Results

In this section, the simulations for a decentralized DCMG are conducted by using the PSIM software to assess the feasibility and reliability of the proposed control scheme. To validate the effectiveness under various uncertain DCMG operations, the simulation tests consider several scenarios such as electricity price changes, transition from the grid-connected mode to islanded mode, emergency SOC in the islanded mode, load reconnection in the islanded mode, voltage regulation by the wind turbine in the islanded mode, and the involvement of EV units. Detailed system parameters can be found in

Table 11. System parameters of a decentralized DCMG system.

Power Units	Parameters	Symbol	Value
Utility grid unit	Grid voltage	$V_G^{rms}$	220 V
	Grid frequency	$f_G$	60 Hz
	Transformer $\mathrm{Y}/\mathsf{\Delta }$	T	380/220 V
	Inverter-side inductance of LCL filter	$L_1$	1.7 mH
	Parasitic resistance in $L_1$	$R_1$	0.5 Ω
	Grid-side inductance of LCL filter	$L_2$	1.7 mH
	Parasitic resistance in $L_2$	$R_2$	0.5 Ω
	Filter capacitance of LCL filter	$C_f$	4.5 μF
Wind turbine unit	PMSG stator resistance	$R_S$	0.64 Ω
	PMSG dq-axis inductance	$L_{dq}$	0.82 mH
	PMSG number of poles	$p$	6
	PMSG inertia	$J$	0.111 kgm2
	PMSG flux linkage	$\psi$	0.18 Wb
	Converter filter inductance	$L_W$	7 mH
	Maximum power of the wind turbine unit	$P_W^{max}$	−1500W
Battery unit	Rated capacity	C	25 Ah
	Maximum power	$P_B^{max}$	450W
	Maximum voltage	$V_B^{max}$	180 V
	Initial no-load voltage	$V_{N{L}_0}$	400 V
	Droop constant	Rd	0.0111
	Converter filter inductance L	$L_B$	7 mH
Load unit	Power of non-critical load	$P_{NC}$	200 W
	Power of critical load	$P_C$	200 W
DC-link	Nominal DC-link voltage	${\mathrm{V}}_{Nom}$	400 V
	First level of low DC-link voltage	$V_{L1}$	385 V
	Second level of low DC-link voltage	$V_{L2}$	370 V
	Third level of low DC-link voltage	$V_{L3}$	360 V
	First level of high DC-link voltage	$V_{H1}$	415 V
	Second level of high DC-link voltage	$V_{H2}$	430 V
	Third level of high DC-link voltage	$V_{H3}$	440 V
	Droop voltage constant	$V_D$	5 V
	Capacitance	$C_{dc}$	4 mF

. Throughout the simulation results, P_G, P_W, P_B, and P_L stand for the power of the utility grid unit, wind turbine unit, battery unit, and load unit, respectively. It is worth mentioning that the power flow direction follows the definition in Section 2.

5.1. Case of Electricity Price Changes

Figure 14 shows the simulation result for the grid-connected case with the transition of the electricity price from normal to high as well as the wind power increase at t = 5 s and decrease at t = 7 s. Initially, the DCMG system starts at t = 1 s with OP₂ in the grid-connected mode. The DC-link voltage V_dc is regulated to V_Nom by the utility grid unit. The battery unit is in BCM mode, and the wind turbine unit is in the MPPT mode. When the electricity price is changed to high at t = 3 s, the utility grid unit regulates V_dc to V_L1 for 5 s until t = 8 s in mode T₁. In 3 s after V_dc is reduced, the battery unit recognizes the high electricity price condition and changes operation to BDM. As explained in Figure 9a, the transition process is completed at t = 8 s and the DCMG system operates in OP₃ at steady-state with V_dc returning to V_Nom. In mode OP₃, the battery is in maximum power discharging to minimize the electricity cost by maximizing the power injection into the utility grid if SOC_B is higher than SOC_B,Emg-H.

5.2. Case of Grid-Connected Mode to Islanded Mode

Figure 15 shows the simulation result for the transition from the grid-connected mode to islanded mode. Initially, the DCMG system is in mode OP₁ at t = 3 s. In this mode, the wind turbine unit operates in the MPPT mode, the battery unit is in idle mode with Rd_B3 as the battery SOC level is higher than SOC_B,Max, the utility grid unit regulates V_dc to V_Nom with Rd_G1, and all the loads are connected. As the utility grid unit fault occurs at t = 4 s, V_dc decreases. The battery unit monitors V_dc and changes operation to BVCM_Dis with Rd_B4 when V_dc drops below V_L2 after 10 ms as shown in Figure 9c. As a result, the operation mode is changed to OP₄, in which the battery unit regulates V_dc to V_Nom in the islanded mode, the wind turbine unit operates in the MPPT mode, and all the loads are connected.

5.3. Case of Emergency SOC in Islanded Mode

To verify the reliability of the proposed battery SOC management feature, the simulation result in case of emergency SOC level in the islanded mode is presented in Figure 16. The DCMG system initially operates in OP₄ at t = 1 s. The wind turbine unit operates in the MPPT mode, the battery unit regulates V_dc to V_Nom in BVCM_Dis with Rd_B4 in the islanded mode, and all the loads are connected. The battery SOC level slowly decreases because of discharging operation. As soon as the battery SOC level reaches SOC_B,Emg-H in the islanded mode, the DCMG operation is changed to mode T₅, in which the battery unit works as BVCM_Emg1 to reduce V_dc to V_L2 during 0.5 s. After that, the DCMG operation is changed again to T₆, in which the battery unit maintains V_dc at V_L2 with BVCM_Emg2,Dis. The load unit continues to monitor V_dc, and triggers R_NC shedding after 3 s when V_dc is reduced as shown in Figure 9d. Then, after 5 s when V_dc is reduced, the DCMG works as steady-state mode OP₅, in which the battery unit regulates V_dc at V_Nom using BVCM_Dis with Rd_B4, the wind turbine unit operates in the MPPT mode, and the load unit only operates with R_C.

5.4. Case of Load Reconnection in Islanded Mode

Figure 17 presents the simulation result for the proposed PFCS in case of load reconnection in the islanded mode. First, the DCMG system operates with OP₇ at t = 1 s. In this mode, the wind turbine unit operates in the MPPT mode, the battery unit regulates V_dc with BVCM_Cha with Rd_B5, and the load unit only operates with R_C. The battery SOC level increases when operated with BVCM_Cha. As soon as the battery SOC level reaches SOC_B,LR around t = 4 s, the battery unit changes operation to T₉. In this mode, the battery unit operates in BVCM_LR,Dis to increase V_dc immediately to V_H1 and then changes operation to BVCM_LR,Cha to maintain V_dc at V_H1. The load unit monitors V_dc, and reconnects the R_C load after 8 s (around t = 12 s). After load reconnection, the battery unit operation is changed to BVCM_LR,Dis to maintain V_dc to V_H1. After the transition for load reconnection is finished, the DCMG operates in steady-state mode OP₄ around t = 14 s, in which the wind turbine unit operates in the MPPT mode, the battery unit is in BVCM_Dis with Rd_B4, and all the loads are connected.

5.5. Case of Voltage Regulation by Wind Turbine Unit in Islanded Mode

Figure 18 shows the simulation result for the proposed PFCS in case of voltage regulation by the wind turbine unit in the islanded mode. Initially, the DCMG system operates with OP₆ at t = 1 s, in which the wind turbine unit operates in the MPPT mode, the battery unit regulates V_dc with BVCM_Cha with Rd_B5, and all the loads are connected in the load unit. As soon as the battery SOC level reaches SOC_B,Max around t = 4 s, the battery unit changes operation to idle mode with Rd_B3. Because this produces a V_dc rise, the DCMG operation is changed to mode T₁₂, in which the wind turbine unit operates in WVDCM with Rd_W1. The wind turbine unit monitors V_dc and changes the operation mode after 5 s (around t = 9 s) to WVCM with Rd_W2, which results in the DCMG operation change to OP₉. In this operation, the wind turbine unit regulates V_dc, the battery unit is in idle mode, and all the loads are connected.

5.6. Case of EV Unit Involvement

To demonstrate the scalability of the proposed PFCS for the decentralized DCMG system, Figure 19 shows the simulation result in the case of the EV unit connection when the battery SOC level reaches SOC_B,Emg-H in the islanded mode. Initially, the wind turbine unit is in the MPPT mode, the battery unit regulates V_dc in BVCM_Dis with Rd_B4, and the EV unit is in idle mode with Rd_EV3. As the battery SOC level reaches SOC_B,Emg-H around t = 2.5 s, the battery unit changes operation to BVCM_Emg1 with Rd_B6 and decreases V_dc to V_L2. The EV unit monitors V_dc, and changes operation to EVCM_Dis with Rd_EV4 after 100 ms to regulate V_dc back to V_Nom. The battery unit remains idle when the EV undertakes the regulation of V_dc to V_Nom.

6. Experimental Results

The decentralized DCMG experimental setup, depicted in Figure 20, is used to demonstrate the feasibility of the proposed PFCS with the system parameters in Table 11. The experiment was performed with a laboratory-scale prototype DCMG system which has a similar power rating to the studies in [12,13,14]. The experimental DCMG system comprises four power electronic converters, which interface the DC-link with the utility grid unit, battery unit, wind turbine unit, and EV unit. The utility grid unit is connected to a three-phase main grid via a Y-Δ transformer and bidirectional AC/DC converter equipped with an LCL filter. The wind turbine unit combines a PMSG with an induction machine to transmit power to the DC-link through a unidirectional AC/DC converter. The battery and EV units utilize bidirectional interleaved DC/DC converters to connect a bidirectional DC power source, serving as a battery. The magnetic contactors are used in the load unit. Power converters are implemented using a digital signal processor (DSP) TMS320F28335. The experimental tests are performed considering six scenarios: electricity price change, transition from the grid-connected to islanded mode, emergency SOC operation in the islanded mode, load reconnection in the islanded mode, voltage regulation by the wind turbine unit, and involvement of the EV unit.

6.1. Case of Electricity Price Changes

Figure 21 shows the experimental result of the proposed PFCS for a decentralized DCMG system when the grid electricity price changes from normal to high. The DCMG system initially works in OP₂, in which the wind turbine unit operates in the MPPT mode, the battery unit is in BCM mode with Rd_B2, and the utility grid unit regulates V_dc to V_Nom with Rd_G1. When the electricity price is changed to high, the operation is changed to T₁. In this mode, the utility grid unit operates with Rd_B2 and regulates V_dc to V_L1. The battery unit monitors V_dc, and changes operation to BDM with Rd_B1 in 3 s after the instant that the V_dc reduces to V_L1. After 5 s from the instant the V_dc reduces to V_L1, the DCMG operation is returned to OP₃, in which the utility grid unit regulates V_dc back to V_Nom, the battery unit is in BDM mode with Rd_B1, and the wind turbine unit operates in the MPPT mode.

Figure 22 shows the experimental result of the proposed PFCS when the grid electricity price changes from high to normal. The DCMG system initially runs in OP₃, in which the wind turbine unit operates in the MPPT mode, the battery is in BDM mode with Rd_B1, and the grid regulates V_dc to V_Nom with Rd_G1. When the electricity price is changed to normal, the operation is changed to T₂. In this mode, the utility grid unit operates with Rd_B3 and regulates V_dc to V_H1. The battery unit monitors V_dc, and changes operation to BCM with Rd_B2 in 18 s after that V_dc increased to V_H1. In the 20 s after V_dc increased to V_H1, the DCMG operation is returned to OP₂, in which the utility grid unit regulates V_dc back to V_Nom, the battery unit is in BCM mode with Rd_B2, and the wind turbine unit operates in the MPPT mode.

6.2. Case of Transition from Grid-Connected Mode to Islanded Mode

Figure 23 shows the experimental result of the proposed PFCS under the transition from the grid-connected mode to islanded mode. Initially, the DCMG system is in OP₂, in which the wind turbine unit operates in the MPPT mode, the battery unit is in BCM mode with Rd_B2, the utility grid unit regulates V_dc to V_Nom with Rd_G1, and all the loads are connected. When the utility grid is disconnected due to a fault, the V_dc starts to decrease. The battery unit continuously monitors V_dc and changes operation to BVCM_Dis with Rd_B4 in the 10 ms after V_dc falls to V_L2. As a result, the DCMG operation is changed to OP₄, in which the battery regulates V_dc to V_Nom in the islanded mode, the wind operates in the MPPT mode, and all the loads are connected.

6.3. Case of Emergency SOC in Islanded Mode

To verify the battery SOC management feature of the proposed PFCS, Figure 24 presents the experimental result when the battery SOC reaches SOC_B,Emg-H in the islanded mode. The DCMG system initially operates in OP₄, in which the wind turbine unit operates in the MPPT mode, the battery unit regulates V_dc to V_Nom using BVCM_Dis with Rd_B4, and all the loads are connected. As soon as the battery SOC level reaches SOC_B,Emg-H in the islanded mode, the DCMG operation is changed to T₅ as the battery unit changes operation to BVCM_Emg1 to reduce V_dc to V_L2 during 0.5 s. After that, the DCMG operation is changed again to T₆, in which the battery unit maintains V_dc at V_L2. The load unit monitors V_dc and triggers R_NC shedding in 3 s after that V_dc is reduced. In 5 s after that V_dc is reduced to V_L2, the DCMG operation is changed to OP₅, in which the battery unit regulates V_dc at V_Nom using BVCM_Dis with Rd_B4, the wind turbine unit operates in the MPPT mode, and only R_C is connected in the load unit.

6.4. Case of Load Reconnection in Islanded Mode

Figure 25 shows the experimental result of the proposed PFCS when the battery SOC level reaches load reconnection level, SOC_B,LR in the islanded mode. First, the DCMG system operates in OP₇, in which the wind turbine unit operates in the MPPT mode, the battery unit regulates V_dc with BVCM_Cha with Rd_B5, and the load unit only operates with R_C. The battery SOC level increases with BVCM_Cha operation. As soon as the battery SOC level reaches SOC_B,LR, the battery unit changes operation to BVCM_LR,Cha with Rd_B8 and the DCMG starts to operate in T₉ in which the battery unit increases V_dc to V_H1. The load unit monitors V_dc and then reconnects R_C load after 8 s. After load reconnection, the DCMG operates in steady-state OP₄, in which the battery operates in BVCM_Dis with Rd_B4, the wind turbine unit operates in the MPPT mode, and all loads are connected.

6.5. Case of Voltage Regulation by Wind Turbine Unit in Islanded Mode

Figure 26 shows the experimental result of the proposed PFCS when the wind turbine unit regulates V_dc in the islanded mode. Initially, the DCMG system operates in OP₆, in which the wind turbine unit operates in the MPPT mode, the battery unit regulates V_dc in BVCM_Cha with Rd_B5, and all loads are connected in the load unit. As soon as the battery SOC level reaches SOC_B,Max, the battery unit operation is changed to idle mode with Rd_B3. Because it results in the wind turbine power being higher than the load requirements, V_dc increases, and the wind turbine unit operates in WVDCM with Rd_W1. Then, the DCMG operation is changed to T₁₂. As shown in Figure 10b, 5 s after the instant of V_dc increase, the wind turbine unit changes the operation to WVCM with Rd_W2, which changes the DCMG operation to OP₉. In this mode, the wind turbine unit regulates V_dc, the battery unit is in idle mode, and all loads are connected.

6.6. Case of EV Unit Involvement

To demonstrate the scalability of the proposed PFCS of the decentralized DCMG system, Figure 27 shows the experimental result in case of the EV unit connection specifically when the battery SOC level reaches SOC_B,Emg-H in the islanded mode. Initially, the wind turbine unit is in the MPPT mode, the battery unit regulates V_dc in BVCM_Dis with Rd_B4, and the EV unit is in idle mode with Rd_EV3. When the battery SOC level reaches SOC_B,Emg-H, the battery unit operation is changed to BVCM_Emg1 with Rd_B6 and V_dc is decreased to V_L2. By monitoring V_dc level, the EV unit changes operation to EVCM_Dis with Rd_EV4 100 ms after the instant that V_dc is decreased to V_L2, as shown in Figure 12c. The V_dc is then gradually increased to V_Nom as the EV unit injects the maximum power. It is confirmed from this result that the proposed PFCS exhibits the scalable feature because the EV unit can operate as additional ESS to support the battery when the battery SOC level approaches a low level.

7. Conclusions

With the aim of enhancing the reliability and optimizing the DCMG operation, this paper has presented a PFCS for a decentralized DCMG system. The proposed PFCS utilizes the transition mode operations to transmit the information on the DCMG system to other power units without using additional DCLs. During the transition operation modes, the transmitting power unit adjusts the DC-link voltage level, while the receiving power unit continuously monitors the DC-link voltage with the predetermined intervals. If uncertain conditions arise in a specific power unit, this power unit activates the transition operation mode to propagate this information to all power units in the DCMG system. After this uncertain condition is acknowledged by all power units, the DCMG operation is switched back from the transition operation mode to the optimal steady-state operation. The proposed PFCS greatly improves the flexibility of the decentralized DCMG system even under various factors such as electricity price change, grid availability, battery SOC level, and wind power variation. The DC-link voltage can be effectively maintained at the nominal value at steady-state conditions by the proposed PFCS regardless of the grid-connected or islanded mode. The DCMG system reliability is also improved by incorporating the battery SOC management strategy. Furthermore, the scalability issue of the DCMG system is also studied by considering an EV unit as an additional ESS in the DCMG system. Simulation and experimental results under various conditions have been presented to verify the performance of the proposed PFCS. The results clearly confirm the reliability and flexibility of the proposed PFCS.