A Study on Energy Management and Cooperative Control Considering LVRT in a Hybrid Microgrid

Abstract

Recently, the establishment of technical standards for grid connection has gained interest in academia and industry. These standards have focused on the reactive power control function of the grid-connected inverter and maintenance of grid operation, and include detailed information about the grid support function. However, remote control communication and control devices for grid support functions, and other distributed sources, such as wind power and energy storage systems, other than inverters have not been addressed. In this paper, the control of the interlinking converter (ILC) in a hybrid microgrid considering low voltage ride-through (LVRT) among grid support functions is investigated. The proposed method consists of an energy management system considering LVRT and a cooperative control scheme. In the energy management system, an algorithm capable of mode selection was constructed by applying the LVRT curve. Then, considering the LVRT situation, the allowable reactive power range of the ILC was mathematically analyzed through the cooperative control of the energy storage device and the ILC. The proposed method enables us to perform active and reactive power control of the ILC in a hybrid distribution network, considering the power factor under various conditions. This functionality, such as supplying reactive power, significantly contributes to the enhanced grid resilience with distributed power sources, including renewable energy. The proposed strategies were verified through experiments after configuring an experimental set of distributed power sources.

1. Introduction

As global environmental issues have emerged, there has been a shift toward expanding the supply of renewable energy sources (RESs) to reduce greenhouse gas emissions in order to increase energy efficiency and carbon neutrality [1,2]. The electricity industry has set a goal to provide 100% of the electricity used by companies with RESs. In the power system field, this goal has led to the spread of distributed power sources (DPSs) including RESs and energy storage systems (ESSs), and the establishment of technical standards for grid connection [3,4]. In particular, in a distribution system linked with such DPSs, grid support functions of inverters for active and reactive power control, system operation maintenance, and emergency functions are required [5]. In order to establish detailed requirements for these grid support functions, the Republic of Korea adopted UL 1741 and IEEE 1547 for the KSGA-025-15-1 group standards for system support function requirements and test methods for smart inverters for photovoltaic power generation [6,7,8]. However, remote control communication and control facilities for the grid support function have not been addressed, and studies on grid supports considering wind power and ESS in addition to the grid support function of the inverter are insufficient [9]. In addition, as interest in hybrid microgrids (MGs) in the form of small scale power grids has recently increased, it is necessary to prepare a regional power supply system. Additionally, studies on interlinking converters (ILC), which play a role in bidirectional power exchange in connection with both systems, have been conducted [10,11,12]. In this case, an energy management system (EMS) is needed to manage DPSs according to the power situation in the MG by linking multiple power sources [13,14,15].

Although studies have been conducted on ILCs, most of them have focused on their stable operation in consideration of the power situation within the MG [16,17]. However, since the hybrid microgrid is connected to the AC utility grid, it is necessary to expand the scope of the study into an EMS strategy that considers the grid support function, which is increasing in importance with the spread of RES. In addition, owing to the generation of abnormal voltage on the AC grid, the consideration of an autonomous operation is required during the grid separation operation defined in the LVRT standard. In addition, after an AC grid recovers, reconnection to the grid should also be considered as one of the grid support functions along with EMS and cooperative control.

In this paper, we define an EMS scheme considering low voltage ride-through (LVRT) among the grid support functions under emergency conditions. For this scheme, an operation mode selection algorithm that meets the LVRT standard base in the energy management method is presented. In addition, cooperative control of the power electronics device in the MG according to each operation mode is defined. Through this scheme, we mathematically analyzed the allowable range of reactive power of the ILC considering the power factor in case of an instantaneous low voltage accident during cooperative control linked to DGSs. Moreover, an integrated study was conducted including an operation plan for autonomous operation after a separation operation by abnormal voltage on the AC grid and a reconnection function after the restoration of the AC voltage. To verify the feasibility of energy management and cooperative control considering the Korea Electric Power Corporation (KEPCO) LVRT, a hybrid MG set was built and experimented on a laboratory scale using the ILC, ESS, photovoltaic (PV), and wind turbine (WT) converters.

The rest of this paper is structured as follows. Section 2 discusses the hybrid MG and the KEPCO standards for the LVRT. Section 3 deals with the proposed EMS considering the LVRT condition. Section 4 presents the proposed power electronics-based cooperative control. Subsequently, Section 5 introduces the configuration of the experimental setup. Finally, conclusions are given in Section 6.

2. Hybrid AC/DC Microgrid and LVRT Requirements for Grid-Connection

This section describes the typical hybrid MG configurations. In addition, the grid support function is covered, and, in particular, the LVRT requirements applied in this study are explained.

2.1. Configuration of the Hybrid Microgrid

A general configuration of a hybrid MG with distributed generation (DG) and loads connected to both the AC and DC buses is shown in Figure 1 [18]. A converter between both systems, called an ILC, controls the voltage and current. When the AC system is normally operated, it generally operates in the grid connection mode, and the ILC may perform DC bus voltage control through active current control and regulate the unit power factor by reactive current control. On the other hand, in the autonomous mode, as the AC utility grid is disabled, the ILC performs AC voltage control for the AC loads [19,20].

2.2. Distribution System Connection Technical Standards by KEPCO

This section introduces the related standards and configuration diagrams for connecting distributed sources (DSs) to the power grid, as well as the grid support functions. It also covers the LVRT. Currently, studies on the LVRT standard are conducted using the AC grid; however, studies that proposed the LVRT standard were actively conducted in the field of wind power systems using a DC MG [21,22,23].

Figure 2 shows the requirements of the LVRT according to the KEPCO standards. A general description of the LVRT curve is as follows. In region 1, the system voltage is greater than 0.9 pu (per unit, the ratio of actual to nominal voltage) and less than 1.1 pu. This region is called the normal voltage state regardless of time, and belongs to the normal voltage and operation region, where normal output is maintained without separation from the grid. In region 2, when the grid voltage is greater than 0.7 pu and less than 0.9 pu, the operation duration is 1.5 s from the time of abnormal voltage detection, and the grid connection and operation continue. The corresponding voltage is maintained, and within the separation time of 2 s, it corresponds to region 3 and is connected to the grid. However at this point, the operation is stopped. After the separation time elapses, the voltage corresponds to region 4 and must be separated from the grid. If the grid voltage is more than 0.5 pu and less than 0.7 pu, the grid connection and operation continue for an operation duration of 0.16 s (region 2). After a separation time of 2 s, the voltage is separated from the grid (region 4), and the operation is stopped between the end of the operation duration and separation time (region 3). If the grid voltage is less than 0.5 pu, the grid connection and operation continue only for the relatively short operation duration of 0.15 s (region 2), and the operation is stopped within the separation time of 0.5 s (region 3). After the separation time has elapsed, the grid support converter must be separated from the AC grid (region 4) [24,25].

3. Proposed Energy Management Considering the LVRT

In this section, an EMS considering LVRT is proposed. The proposed strategy aims to perform energy management in conjunction with DG in MGs and uses ILCs to perform LVRT functions based on the KEPCO Technical Standards. To implement the proposed method, a laboratory scale of the MG is constructed as shown in Figure 3. In order to construct a hybrid microgrid, ILC, converters for PV and WT, and ESS converters for surplus energy storage were built. In addition, various loads were additionally configured, and MGCC was established for a central control-based energy management system. The detailed description of each converter is covered in Section 4.1.

The proposed algorithm for the EMS, considering LVRT, is shown in Figure 4. The algorithm is divided into grid-connected mode and islanded operation mode. The operation mode is determined according to the range and duration of the LVRT voltage, and the operation of power electronic devices in the premises is specifically defined.

In this scheme, the instantaneous power amount and information of the MG are first received. At this time, the collected information is as follows: P_ILC, the active power of the ILC; P_PV, the generated power of the photovoltaic (PV) converter; P_WT, the active power of the wind-turbine power converter; and P_ESS, the instantaneous output power of the ESS converter. In addition, state of charge (SOC), which is the remaining capacity of the ESS battery, and V_g, which is the pu value of the AC system voltage, are received through the ILC. In addition, in order to perform grid-reconnection after operating in DC/AC autonomous operation, the Sync_{_Clr} variable indicates the state in which the phase synchronization of the grid voltage and AC bus voltage completion signal is received. Afterwards, mode, which is determined in the previous algorithm, T_LVRT duration of LVRT, and T_recon measuring return delay time after synchronization are defined. If the previous operation mode is 3 or less, it operates in grid-connected mode, and, from 4, it is divided into self-operating mode and performs the operation. When the initial operation is performed in a state where the AC system is normal, the operation is performed from the grid-connected mode in which the operation mode is 1.

In compliance with the grid code described in the previous Section 2.2, a mode selection algorithm is provided, detailed in Section 3.1.

3.1. Mode Selection Algorithm

The mode selection algorithm and operation mode of the MG, considering the LVRT support function of the proposed energy management, are described in this section. The operation mode of the MG was assigned to the curved region, as shown in Figure 2, by using the same voltage range, duration, and trip time of the LVRT operation standard defined in the existing distributed distribution grid connection KEPCO standard. In order to implement the grid support function according to these standards, a mode selection algorithm is defined to determine the operation mode from modes 1 to 4. The selection algorithm is defined by dividing the normal voltage and low voltage ranges as 1 to 3, as shown in Figure 5.

This LVRT-based mode selection algorithm outputs the operation Mode according to the AC system voltage and the duration after the occurrence of the abnormal voltage in the graph shown in Figure 2. In the first step of the algorithm, the voltage of the AC system uses V_g. The MG central controller (MGCC) receives information through communication lines regarding the value calculated from the ILC. When low voltage occurs and continues in the MG, it starts from operation mode 1 and sequentially operates in operation modes 2, 3, and 4 as T_LVRT increases. Then, the operation of the distributed generators in the MG is defined in detail according to the derived operation mode.

3.2. Grid-Connected Mode (Modes 1 to 3)

Power flow diagrams for each operating mode are shown in Figure 6. Power flow diagrams, in particular, are depicted for modes 1 to 3 in the grid-connected operation.

3.2.1. Mode 1 (Power Balance)

Mode 1, the power balance operation mode, refers to normal operation when the AC grid voltage is 0.9 pu. In this mode, an operation is performed to balance power supply and demand between the AC bus and DC bus. The power flow diagram for each operation mode is shown in Figure 6. In mode 1, shown in Figure 6a, the AC grid and hybrid MG operate in a grid-connection mode through a static transfer switch (STS). In addition, the ILC controls the active current while ensuring the DC bus voltage is 380 V. The reactive current component is maintained as 0 so that the ILC can perform unit power factor control. At this time, the ESS converter performs charge/discharge power control. The power reference value $P_{ESS}^{*}$ of the ESS is determined by Equation (1).

$$P_{ESS}^{*}=P_{DCL}-(P_{WT}+P_{PV})$$

(1)

where P_DCL is the amount of power of the DC load, P_WT is the amount of power generated by the converter for wind power generation, and P_PV is the amount of power generated by the converter for the PV system. When the power reference value of the ESS has a positive value, it becomes a discharge power reference, and when it has a negative value, it becomes a charging power reference. In the case of an AC load, power is supplied from the AC system. When performing these operations, if the condition of Equation (2) is satisfied, the power supply–demand balance condition is satisfied.

$$P_{ESS\_rated}^{}\ge \left|P_{WT}+P_{PV}-P_{DCL}\right|$$

(2)

where P_{ESS_rated} is the rated power of the ESS converter. In the case where the amount of power generated is greater than the amount of load power, the condition for exceeding the rated power amount of the ESS converter is shown in Equation (3).

$$P_{WT}+P_{PV}-P_{DCL}>P_{ESS\_rated}^{}$$

(3)

The ESS converter charges as much as P_{ESS_rated}, and the remaining power is regenerated to the AC system side as the ILC performs DC voltage control. The active power regenerated by the ILC at this time is as shown in Equation (4).

$$P_{ILC}=P_{WT}+P_{PV}-P_{DCL}+P_{ESS\_rated}^{}$$

(4)

where P_ILC is the active power of the ILC. Under the condition of Equation (3), which is the active power of the ILC generated under the condition of Equation (4), the P_ILC becomes positive, and the power is regenerated from the DC bus to the AC bus side. Conversely, when the amount of power of the DC load is greater than the amount of generated power, the condition for exceeding the rated power of the ESS converter is shown in Equation (5).

$$P_{DCL}-P_{WT}-P_{PV}>-P_{ESS\_rated}^{}$$

(5)

In this case, the active power P_ILC of the ILC has a negative value, and power is supplied from the AC bus to the DC bus side through the DC bus voltage control of the ILC. In the power balance operation of operation mode 1, the power reference P_REF of the ESS converter through P_DCL is calculated as in Equation (1) to operate the effective power P_ILC of the ILC close to zero.

3.2.2. Mode 2 (Reactive Power Supply)

The reactive power supply operation is performed in mode 2, which is determined by the mode selection algorithm according to the duration and separation time when the AC grid voltage is less than 0.9 pu. Even though the AC voltage is less than 0.9 pu, the STS of the AC system is continuously connected, and the grid is supported through the supply of reactive power, which is different from operation mode 1, which is used to perform power factor control of 0.9 or higher. Cooperative control with the ESS converter is required. The power flow diagram in operation mode 2 is shown in Figure 6b. The ILC performs DC bus voltage control as in operation mode 1. At this time, the reactive power is supplied according to the magnitude of the AC grid voltage. As such, the reactive current reference value versus the rated current of the ILC, according to the AC voltage, is shown in Figure 7.

$$I_{d\_ILC}^{*}=\left\{\begin{array}{l}{I}_{ILC\_rated},\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}0\le V_g<0.5\\ (2.25-2.5V_g)I_{ILC\_rated},\hspace{1em}\hspace{1em}0.5\le V_g<0.9\\ 0,\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}0.9\le V_g\end{array}\right.$$

(6)

Equation (6) is the reference of the reactive current of the ILC. Here, I_{ILC_rated} is based on the rated current of the ILC. After determining the reactive current reference according to the grid voltage magnitude according to Equation (6), the active power range in which the power factor of ILC is 0.9 or more was considered. In addition, if the power reference for cooperative control with the ESS converter is derived according to the power condition in the MG, it is as shown in Equation (7).

$$P_{ESS}^{*}=(3.097)V_q^e{I}_{d\_Conv}^{e*}-(P_{WT}+P_{PV}-P_{DCL})$$

(7)

When controlling the reactive power of the ILC according to the magnitude of the grid voltage, active power occurs through cooperative control of the ESS converter through Equation (7), and the condition of a power factor of 0.9 or more can be satisfied. The cooperative control method with the ESS converter considering such power factor control is covered in detail in Section 4.

3.2.3. Mode 3 (DC Autonomous)

Operation mode 3, the DC autonomous mode, is configured to operate in the corresponding curve of LVRT region 3. This is between the minimum mandatory operation duration after abnormal voltage occurrence according to the LVRT standard and the time the ILC should be separated from the grid. Therefore, the ILC is not separated from the AC grid but is in an idle state. This is so that the system can return to normal operation when the voltage of the AC grid is restored. The power flow diagram in operation mode 3 is shown in Figure 6c. The ILC in operation mode 3 meets the requirements of the LVRT, but in order for it to perform DC autonomous operation that can continuously supply power to DSs and loads connected in the DC bus, the ESS converter performs DC bus voltage control instead of charge/discharge control.

If the low voltage situation persists, the ESS converter continuously performs DC voltage control even in operation mode 4, according to the LVRT-based mode selection algorithm. In this DC autonomous operation, the power sharing that the ESS converter must handle must satisfy Equation (8).

$$P_{ESS\_rated}\ge \left|(P_{WT}+P_{PV})-P_{DCL}\right|$$

(8)

This means that the absolute value obtained by subtracting the amount of power of the DC load from the amount of power generated by the WT converter in the DC bus and the amount of power generated by the PV converter must exist below the rated capacity of the ESS converter. The voltage on the bus cannot be kept constant.

3.3. Stand-Alone Mode

In the grid-connected mode, if the low voltage situation persists in the AC grid, operation modes 1 to 3 are performed by the LVRT-based mode-selection algorithm, and, after the separation time, the DC/AC autonomous mode, which is operation mode 4, is determined.

Mode 4 (DC/AC Autonomous)

In mode 4, the MG is operated in a state separated from the AC grid, and the synchronization and reconnection processes are performed when the grid is restored.

Figure 6d shows the power flow during the DC/AC autonomous mode in operation mode 4. The ILC is electrically separated from the AC grid and supplies active power as much as the AC load power amount P_ACL to the AC bus side by controlling the AC bus voltage. The ESS converter performs DC bus voltage control. The amount of power required by the ESS converter at this time is given in Equation (9), considering all the generated power, and load power in the hybrid MG.

$$\begin{array}{l}{P}_{ESS}=P_{ILC}+P_{DCL}-(P_{WT}+P_{PV})\\ \hspace{1em}\hspace{1em}=P_{ACL}+P_{DCL}-(P_{WT}+P_{PV})\end{array}$$

(9)

In the DC autonomous operation of mode 4, the rated current of the ESS can be defined as Equation (10) after considering the power condition in the MG.

$$P_{ESS\_rated}\ge \left|(P_{WT}+P_{PV})-P_{DCL}-P_{ILC}\right|$$

(10)

Subsequently, when the grid voltage is restored during the AC bus voltage control of the ILC converter and the DC voltage control of the ESS converter, the phase of the AC bus voltage and the phase of the AC grid voltage must be synchronized. Figure 8 was applied as an algorithm for phase synchronization.

In the phase synchronization algorithm of Figure 8, it is assumed that Sync_{_Clr}, the phase synchronization completion signal, is set to 0. Then, in order to adjust the phase ${\theta }_{inv}$ of the AC bus voltage to match the phase ${\theta }_{grid}$ of the AC system voltage, the phase difference ${\theta }_{comp}$ is first calculated. Afterwards, if the phase ${\theta }_{inv}$ of the AC bus voltage is regenerated, it obtained as shown in Equation (11).

$$\left.\begin{array}{l}{\theta }_{comp}[k]={\theta }_{grid}[k]-{\theta }_{inv}[k-1]\\ {\theta }_{inv}[k]={\theta }_{inv}[k-1]+({\omega }_{inv}-K\cdot {\theta }_{comp}[k])\cdot T_s\end{array}\right\}$$

(11)

While performing phase synchronizations through Equation (11), if the phase difference ${\theta }_{comp}$ is less than a specific value, synchronization is complete, and the phase synchronization completion signal Sync_{_Clr} is set to 1. If the phase synchronization completion signal, Sync_{_Clr}, outputs 1, T_{Sync_Clr}, the time after phase synchronization completion, is increased, and if the refeeding delay time T_recon is exceeded, the reconnection and operation mode is set to 1, and the system returns to the grid-connected mode.

The LVRT requirements of the grid-connected inverter were applied to the ILC through the operation mode selection algorithm that considers LVRT and applies the KEPCO standard. After ILC was separated from the AC grid, the contents of reconnection through autonomous operation and phase synchronization were described in detail. The proposed cooperative control method of the power converter used to perform the energy management of the hybrid MG considering LVRT is described in the next section.

4. Proposed Cooperative Control Method Considering LVRT

This section describes the configuration of the MG for the proposed cooperative control method, mathematically analyzes the allowable reactive power range of the ILC, and proposes a cooperative control with the ESS considering the power factor control.

4.1. Configuraion of Microgrid for the Proposed Cooperative Control

The control block diagram of the entire system for performing the proposed cooperative control is shown in Figure 9. A communication line was constructed between the central controller and individual local controller to share necessary information and reference values.

The ILC used a 4-leg inverter and was configured to perform voltage control and power control according to the operation mode. An interleaved buck-boost DC/DC converter was applied to the circuit of the ESS and PV generation converter. The ESS converter controls the battery-side input current of the buck-boost DC/DC converter according to the operation mode determined from the energy management system. As a control method of the PV converter, the input current is controlled. The input current reference value of the PV converter was derived through the maximum power point tracking (MPPT) algorithm based on perturbation and observation. The circuit diagram and control block diagram of the WT converter applied a 3-phase 3-level neutral point clamped AC/DC converter. By performing q-axis current control according to the variable reverse torque reference, the WT converter was controlled through power regeneration to the DC side.

4.2. Analysis of the Allowable Reactive Power in Cooperative Control of the ILC with ESS Converter

When the ILC performs current control, the magnitudes of the active power P_ILC, reactive power Q_ILC, and apparent power S_ILC of the interlinking converter are as in Equation (12) (assume that the three-phase voltages are in balance). Here, $I_{q\_conv}^{e\ast }$ represents a reactive current component and $I_{q\_conv}^{e\ast }$ represents an active current component [26,27].

$$\left.\begin{array}{l}{P}_{ILC}=\frac{3}{2}{V}_{q\_Conv}^e{I}_{q\_Conv}^e\\ Q_{ILC}=\frac{3}{2}{V}_{q\_Conv}^e{I}_{d\_Conv}^e\\ S_{ILC}=\sqrt{{(P_{ILC})}^2+{(Q_{ILC})}^2}\end{array}\right\}$$

(12)

In operation mode 2, the reactive power supply operation takes place, the voltage of the grid drops, and, based on Equation (12), the magnitude of active and reactive power may fluctuate. Therefore, in addition to obtaining the reactive power using Equation (6), which is dependent on the magnitude of the grid voltage, it is necessary to analyze the power according to the power status of the MG and the magnitude of the active power by the ESS converter. Accordingly, the allowable reactive power of the ILC to apply a cooperative control method was analyzed.

(1)

Case 1: When the ESS does not exist (grid voltage 1.0 pu)

When the ESS converter does not exist, the allowable reactive power Q_{ILC_avail_1} within the rated apparent power S_{ILC_rated} of the ILC is as given in Equation (13).

$$\sqrt{{(P_{ILC\_1})}^2+{(Q_{ILC\_avail\_1})}^2}\le S_{ILC\_rated}$$

(13)

where P_{ILC_1} is the active power of the case 1 interlinking converter and can be obtained using Equation (14).

$$P_{ILC\_1}=P_{WT}+P_{PV}-P_{DCL}$$

(14)

As the ILC controls the DC bus voltage, the active power of the ILC is determined by the power of the WT converter, PV converter, and DC load, and can be regarded as surplus power on the DC bus side. Substituting the ILC active power equation, Equation (14), into Equation (13), the range of allowable reactive power Q_{ILC_avail_1} within the rated apparent power of the ILC by the generation and load power in the hybrid MG in the absence of an ESS converter can be expressed in Equation (15). Substituting the ILC active power equation, Equation (14), into Equation (13), the range of allowable reactive power Q_{ILC_avail_1} within the rated apparent power of the ILC by the MG internal power generation and load power can be obtained in Equation (15).

$$\begin{array}{l}-K_1\le Q_{ILC\_avail\_1}\le K_1\\ K_1=\sqrt{{(S_{ILC\_rated})}^2-{(P_{WT}+P_{PV}-P_{DCL})}^2}\end{array}$$

(15)

Figure 10a shows the range of the allowable reactive power values of the ILC in case 1.

When the ESS converter of case 1 does not exist, the range of allowable reactive power Q_{ILC_avail_1} decreases as the active power P_{ILC_1} of the ILC increases.

(2)

Case 2: When an ESS converter exists and performs cooperative control of operation mode 1 within the rated power of the ESS converter (grid voltage 1.0 pu)

In operation mode 1, the condition for generating surplus/shortage power within the rated power of the ESS converter can be defined as Equation (16).

$$P_{ESS\_rated}\ge \left|P_{DCL}-(P_{WT}+P_{PV})\right|$$

(16)

In Equation (16), as the absolute value $\left|P_{DCL}-(P_{WT}+P_{PV)}\right|$ of the insufficient power in the DC bus is smaller than the rated power P_{ESS_rated} of the ESS converter, the ESS converter can charge or discharge all of the surplus or insufficient power in the DC bus. In case 2, when the ESS converter performs cooperative control in operation mode 1, the active power P_{ILC_2} of the ILC becomes 0 W according to Equation (17).

$$\begin{array}{l}{P}_{ILC\_2}=P_{WT}+P_{PV}-P_{DCL}+P_{ESS}^{*}\\ \hspace{1em}\hspace{1em} =P_{WT}+P_{PV}-P_{DCL}+\{P_{DCL}-(P_{WT}+P_{PV})\}=0\end{array}$$

(17)

The ranges of allowable reactive power Q_{ILC_avail_2} and allowable reactive power $I_{d\_Conv\_avail\_2}^{e*}$ within the rated apparent power in case 2 are defined using Equation (12) and Equation (17) as follows, respectively.

$$-S_{ILC\_rated}\le \frac{3}{2}{V}_{q\_Conv\_rated}^e{I}_{d\_Conv\_avail\_2}^e=Q_{ILC\_avail\_2}\le S_{ILC\_rated}$$

(18)

$$-\frac{2}{3V_{q\_Conv\_rated}^e}{S}_{ILC\_rated}\le I_{d\_Conv\_avail\_2}^e\le \frac{2}{3V_{q\_Conv\_rated}^e}{S}_{ILC\_rated}$$

(19)

where $V_{q\_Conv\_rated}^e$ is the q-axis rated voltage and refers to the phase voltage peak value at 1.0 pu of the AC system voltage. The range of allowable reactive power Q_{ILC_avail_2} of the ILC in case 2 is shown in Figure 10b. As the active power P_{ILC_2} of the interlinking converter becomes 0 W, owing to the cooperative control of operation mode 1 of the ESS converter, the allowable reactive power Q_{ILC_avail_2} is equal to the rated apparent power S_{ILC_rated}.

(3)

Case 3: Cooperative control is performed in operation mode 1 when surplus power exceeding the rated power of the ESS converter is generated

When performing cooperative control in operation mode 1, the condition under which surplus power exceeds the rated power of the ESS converter is generated, as is shown in Equation (20).

$$P_{DCL}-(P_{WT}+P_{PV})<-P_{ESS\_rated}$$

(20)

The left term of Equation (20) is the surplus power, and the right term is the rated power of the negative ESS converter, indicating the rated power during charging. Therefore, if the ESS converter has a negative value and has surplus power that is lower than the rated power when charging, the ESS converter cannot charge all the surplus power.

In case 3, when the ESS converter performs cooperative control, the active power P_{ILC_3} of the interlinking converter can be obtained using Equation (21).

$$\begin{array}{l}{P}_{ILC\_3}=P_{WT}+P_{PV}-P_{DCL}+P_{ESS}^{*}\\ \hspace{1em}\hspace{1em} =P_{WT}+P_{PV}-P_{DCL}+(-P_{ESS\_rated})>0\end{array}$$

(21)

where the active power P_{ILC_3} of the interlinking converter has a positive value, and the active power is supplied to the AC grid side. If the range of allowable reactive power Q_{ILC_avail_3} within the rated apparent power S_{ILC_rated} in case 3 is defined using Equations (12) and (21), then Equation (22) is obtained.

$$\begin{array}{l}-K_2\le Q_{ILC\_avail\_3}\le K_2\\ K_2=\sqrt{{(S_{ILC\_rated})}^2-{(P_{WT}+P_{PV}-P_{DCL}-P_{ESS\_rated})}^2}\end{array}$$

(22)

Figure 10c shows the range of allowable reactive power Q_{ILC_avail_3} of the ILC in case 3. The active power of the ILC, considering the power situation in the MG, is reduced by the rated power amount of the ESS converter by the charging operation of the ESS converter, thereby generating active power corresponding to P_{ILC_3}. Accordingly, the range of allowable reactive power Q_{ILC_avail_3} is as shown in Equation (22).

(4)

Case 4: When the ESS converter performs cooperative control of operation mode 1 considering the magnitude of the AC grid voltage (AC grid voltage 0$\le \alpha \le$1 pu)

The range of allowable reactive power Q_{ILC_avail_4} when considering the change in the magnitude of the AC system voltage as 0 ≤ $\alpha$ ≤ 1 pu is derived. It is assumed that the generation and load power conditions in the MG in case 2, discussed above, are the same. The allowable reactive power Q_{ILC_avail_2} in case 2 can be obtained using Equation (18). If the allowable reactive power Q_{ILC_avail_2} is expressed as the allowable reactive current $I_{d\_Conv\_avail\_2}^e$ and the q-axis rated voltage $V_{q\_Conv\_rated}^e$ at 1.0 pu, Equation (23) can be obtained.

$$Q_{ILC\_avail\_2}=\frac{3}{2}(1.0)V_{q\_Conv\_rated}^e{I}_{d\_Conv\_avail\_2}^e$$

(23)

where when the allowable power Q_{ILC_avail_4} is as expressed, and in the case where the AC grid voltage of case 4 is α pu instead of the q-axis rated voltage $V_{q\_Conv\_rated}^e$ at 1.0 pu, Equation (24) is obtained.

$$Q_{ILC\_avail\_4}=\frac{3}{2}(\alpha )V_{q\_Conv\_rated}^e{I}_{d\_Conv\_avail\_2}^e=(\alpha )Q_{ILC\_avail\_2}$$

(24)

Through this, the range of allowable reactive power Q_{ILC_avail_4} considering the magnitude of the AC grid voltage is

$$-\alpha S_{ILC\_rated}\le \frac{3}{2}(\alpha )V_{q\_Conv\_rated}^e{I}_{d\_Conv\_avail\_2}^e=Q_{ILC\_avail\_4}\le \alpha S_{ILC\_rated}$$

(25)

Figure 10d shows the range of allowable reactive power Q_{ILC_avail_4} of the ILC when the cooperative control in case 4 is performed considering the magnitude of the AC system voltage. All of the active power P_{ILC_4} of the ILC operates at 0 W owing to the cooperative control of operation mode 1 of the ESS converter. It is shown that the allowable reactive power Q_{ILC_avail_4} is reduced in proportion to the magnitude of the grid voltage α pu.

When performing cooperative control in operation mode 1, the magnitude of allowable reactive power is increased by performing cooperative control through the ESS converter compared to the range of allowable reactive power of the interlinking converter in the condition without the ESS converter. Even in the case where the reactive power supply operation of operation mode 2 is performed, the following process is required to maintain a power factor of 0.9 or more when applying the cooperative control of the ESS converter as in mode 1.

4.3. Cooperative Control with the ESS Converter Considering the Power Factor of ILC in Reactive Power Supply Operation

First, the d-axis current command value $I_{d\_Conv}^{e*}$ of the reactive current component for reactive power control in operation mode 2 is as defined in Equation (6), according to the magnitude of the AC grid voltage. The reactive power reference $Q_{ILC}^{*}$ when reactive power control is performed in operation mode 2 can be calculated using Equations (6), (12) and (26).

$$Q_{ILC}^{*}=\frac{3}{2}{V}_q^e{I}_{d\_Conv}^{e*}=\left\{\begin{array}{l}\frac{3}{2}{V}_q^e(\sqrt{2}{I}_{ILC\_rated}),\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}0\le V_g<0.5\\ \frac{3}{2}{V}_q^e\{\sqrt{2}(2.25-2.5V_g)I_{ILC\_rated}\},\hspace{1em} 0.5\le V_g<0.9\\ 0,\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} 0.9\le V_g\end{array}\right.$$

(26)

If the power factor PF is 0.9 or higher, Equation (27) is obtained.

$$PF=\frac{P_{ILC}}{S_{ILC}}=\frac{P_{ILC}}{\sqrt{(P_{ILC})+{(Q_{ILC}^{*})}^2}}=\frac{P_{ILC}}{\sqrt{(P_{ILC})+{(\frac{3}{2}{V}_q^e{I}_{d\_Conv}^{e*})}^2}}\ge 0.9$$

(27)

Through Equation (27), the range of active power P_ILC that satisfies the condition for a power factor of 0.9 or more when operating with the reactive power reference $Q_{ILC}^{*}$ can be defined as follows.

$$\left.\begin{array}{l}{P}_{ILC}\ge (3.097)V_q^e{I}_{d\_Conv}^{e*}\\ P_{ILC}\le -(3.097)V_q^e{I}_{d\_Conv}^{e*}\end{array}\right\}$$

(28)

If the positive active power and negative power of the ILC for a power factor of 0.9 or higher are defined as P_{ILC_Pos} and P_{ILC_Neg}, respectively, Equation (29) is obtained.

$$\left.\begin{array}{l}{P}_{ILC\_Pos}=(3.097)V_q^e{I}_{d\_Conv}^{e*}\\ P_{ILC\_Neg}=-(3.097)V_q^e{I}_{d\_Conv}^{e*}\end{array}\right\}$$

(29)

As operation mode 2 performs DC bus voltage control, the active power of the ILC is determined as shown in Equation (30) by the power reference of the WT converter, PV converter, DC load, and ESS converter.

$$P_{ILC}=P_{WT}+P_{PV}-P_{DCL}+P_{ESS}^{*}$$

(30)

The above equations are summarized as follows.

$$\left.\begin{array}{l}{P}_{ESS}^{*}\ge (3.097)V_q^e{I}_{d\_Conv}^{e*}-(P_{WT}+P_{PV}-P_{DCL})\\ P_{ESS}^{*}\le -(3.097)V_q^e{I}_{d\_Conv}^{e*}-(P_{WT}+P_{PV}-P_{DCL})\end{array}\right\}$$

(31)

Therefore, even in operation mode 2, according to the conditions described in Equation (31), the ILC can satisfy the condition that the power factor should be at least 0.9 by performing reactive power control considering the magnitude of the AC grid voltage and cooperative control with the ESS converter. In order to control the inactive power during LVRT operation, in this study, the minimum positive value was defined as the condition needed to satisfy the condition that the power factor should be at least 0.9 when operating with the reactive power command value, as shown in Equation (32).

$$P_{ESS}^{*}=(3.097)V_q^e{I}_{d\_Conv}^{e*}-(P_{WT}+P_{PV}-P_{DCL})$$

(32)

Figure 11 illustrates an operating state at the time of the minimum requirement that satisfies the power factor of 0.9 when supplying reactive power, and shows the operating point in operating mode 2 of this paper. Therefore, through Equation (32), cooperative control that can satisfy the condition that the power factor should be 0.9 based on the minimum discharge power of the ESS converter during reactive power operation, according to the magnitude of the grid voltage, is possible.

5. Experiment Results

To verify the validity of the proposed energy management and cooperative control, a hybrid AC/DC MG was constructed, as shown in Figure 12.

In the AC grid, the low voltage situation was simulated using Slidac, and the separation operation with the AC grid was performed using the STS.

Table 1. Components and specifications of the power devices.

Component		Value
ILC	Rated capacity	10 [kVA]
	AC side line-to-line voltage	220 [${\mathrm{V}}_{\mathrm{rms}}$]
	Switching frequency	10 [kHz]
	DC-link output voltage	380 [${\mathrm{V}}_{\mathrm{dc}}$]
PV converter	Rated output power	5 [kW]
WT converter		5 [kW]
ESS converter		5 [kW]
AC Load		7 [kW]
DC Load		5 [kW]

shows the parameters of the entire system for the proposed energy management and cooperative control configuration. The power change state of each power converter for each operation mode is shown in

Table 2. Power change conditions of all components for each mode.

	Modes	I					II			III	IV			Unit
Comp		${\mathbf{T}}_0$$-{\mathbf{T}}_1$	${\mathbf{T}}_1$$-{\mathbf{T}}_2$	${\mathbf{T}}_2-{\mathbf{T}}_3$	${\mathbf{T}}_3-{\mathbf{T}}_4$	${\mathbf{T}}_4-{\mathbf{T}}_5$	${\mathbf{T}}_6-{\mathbf{T}}_7$	${\mathbf{T}}_7-{\mathbf{T}}_8$	${\mathbf{T}}_8-{\mathbf{T}}_9$	${\mathbf{T}}_9-{\mathbf{T}}_{10}$	${\mathbf{T}}_{11}-{\mathbf{T}}_{12}$	${\mathbf{T}}_{12}-{\mathbf{T}}_{15}$	${\mathbf{T}}_{15}-{\mathbf{T}}_{16}$
ILC	P	−1.4	−0.4	−0.3	−0.3	−0.2	−0.2	4.2	4.2	4.2	2	4	0.3	kW
	Q	-	-	-	-	-	0.1	2	2	2	-	-	-	kVAR
ESS Conv.		Idle	Idle	0.3	1.6	−1.6	−1.6	2.6	2.6	2.6	0.4	2.4	−1.6	kW
PV Conv.		Idle	1	1	1	1	1	1	1	1	1	1	1	kW
WT Conv.		Idle	Idle	Idle	Idle	3.2	3.2	3.2	3.2	3.2	3.2	3.2	3.2	kW
DC Load		1.3	1.3	1.3	2.6	2.6	2.6	2.6	2.6	2.6	2.6	2.6	2.6	kW
AC Load		-	-	-	-	-	-	-	-	-	2	4	4	kW

.

5.1. Grid-Connected Mode (Modes 1–3)

The operation mode of the proposed energy management and cooperative control, considering the KEPCO LVRT requirements, consists of modes 1–4. The operation mode is determined according to the low voltage occurrence of the AC grid voltage and the power condition of the MG, and each power electronics-based device is constructed to operate through CAN communication with the MG central controller (MGCC).

5.1.1. Mode 1 (Power Balance)

Mode 1 is an MG operation under normal conditions where the AC grid voltage is 1.0 pu. At this time, the goal is to drive the output active power of the AC grid close to 0 W by using the system itself to balance power supply and demand. The experimental waveforms of the ILC and DC load in the linkage experiment performed under operation mode 1 when the proposed scheme is applied are shown in Figure 13a. Each waveform represents the DC bus voltage and DC load current waveform in operation mode 1, and represents the line-to-line voltage on the grid side and the a-phase output current of the ILC. The operation and amount of power of the power converters for each time section are shown in the experimental waveforms.

Figure 13b shows the output power waveforms of the ESS converter, WT converter, and PV converter during the linkage experiment performed under operation mode 1. In the T0–T1 section of operation mode 1, the ILC performs DC bus voltage control after the initial charging of the DC bus while being connected to the AC grid. Then, a DC load of 1.3 kW is applied. In order for the ILC to supply power to the DC load, the ILC generates an output of −1.4 kW, and the a-phase input current increases.

In the time from T1 to T2, the operation of the PV converter starts and 1.0 kW power generation control is performed through MPPT control. As the amount of the generated power increases, the output power of the ILC is reduced to −0.4 kW. In the time period from T2 to T3, the ESS converter is started and the charge/discharge power control is performed by the power reference value transmitted from the MGCC by the proposed energy management and cooperative control method considering the LVRT. At time T3, DC load is additionally applied, and as the DC load power consumption increases to 2.6 kW, the discharge power amount of the ESS converter also increases. Therefore, it can be confirmed that the magnitude of the output power of the ILC is maintained at −0.3 kW without additional increases. In the section from T4 to T5, the converter for WT generation is started, and then the power generation situation of the MG is such that the power generation amount increases. As the amount of wind power generated by the WT converter increases to 3.2 kW, the ESS converter changes from a discharge control of 1.6 kW to a charge operation of −1.8 kW. In addition, it can be confirmed that the power supply–demand balance operation is achieved through the a-phase current waveforms of the ILC.

5.1.2. Mode 2 (Reactive Power Supply)

In operation mode 2, reactive power is supplied according to the voltage level of the AC grid through a reactive power supply operation. In addition, in order for an operation that considers the power factor to occur, the active power of the ILC must be supplied to the AC grid through the additional discharge operation of the ESS converter.

As mentioned above, Table 2 shows the power status of each power converter for each mode. In mode 2, active and reactive power are separately marked in the power for each section of the ILC. When the AC grid voltage drops from T6, operation mode 2, reactive power supply operation is performed by the LVRT-based mode selection algorithm. Figure 14a is a linkage experiment waveform that simulates the LVRT situation according to the voltage drop of the AC grid during operation mode 2 of the reactive power supply, and Figure 14b illustrates the output power of the ESS, PV, and WT converters during operation mode 2 of the proposed method.

Prior to T6, the operation mode is the mode 1 section, and the magnitude of the AC voltage is 1.0 pu. After T6, the voltage drop of the AC grid was induced using Slidac. In the section T6 to T7, the AC voltage is higher than 0.9 pu, so the ESS converter operates in the −1.6 kW charging mode. Accordingly, the output active power of the ILC is −0.2 kW, and the reactive power is 0 kVAR. The amount of power generated by the WT converter and PV converter is 3.2 kW and 1 kW, respectively, and the DC load is 2.6 kW. According to the voltage drop of the AC voltage after T7, the output power of the ESS converter is switched from the −1.6 kW charging mode, which is a balance mode, to the 2, 2.6 kW discharge control for active power generation of the ILC. Therefore, the reactive power supply of mode 2 is performed after T7. In this section, the ILC supplies reactive power to the AC utility according to the AC voltage level, and can perform active power control of the ILC considering the power factor according to the cooperative additional discharge operation of the ESS.

Figure 15 shows the waveforms of the AC voltage, ILC output current, and active/reactive power in mode 2. In this mode of the proposed method, the output current of the ILC was generated through the control of the active power through the additional discharge of the ESS converter and the reactive power, according to the level of the grid voltage.

Accordingly, from the T8 section onward, the ILC controls the active power of 4247 W by cooperative control with the ESS converter and the reactive power of 2031 Var. The period T7 to T8, which is less than 0.9 pu, is 1.5 s, and mode 2 operation in the low voltage range is performed. At this time, the power factor based on the ILC power is 0.9021, which satisfies the power factor of 0.9 or higher required by the AC grid. At the same time, LVRT operation can be performed with the ability to supply reactive power according to the drop in AC grid voltage.

When the AC grid voltage is 0.8 pu, the theoretical values of active and reactive power are 4132 W and 2000 Var, respectively, as obtained through Equation (32), and the power factor is 0.9. A comparison of these results with the experimental results reveals that the active power has an error of −2.78%, the reactive power has an error of −1.55%, and the power factor has an error of −0.23%, owing to the measured value error and various parameter variations.

5.1.3. Mode 3 (DC Autonomous)

After the LVRT duration, the connection with the AC grid is maintained, but the operation of the ILC is stopped, and the MG system is operated in the DC autonomous operation in mode 3 in which the ESS converter performs DC bus voltage control. The DC bus voltage, AC grid-side voltage, and output current of the ILC in operation mode 3 are shown in Figure 16a. The reactive power supply operation period of operation mode 2 occurs prior to T9. In this section, the ESS converter performs a discharging operation. However, in operation mode 3 between T9 and T10, the ESS converter operates as a DC autonomous operation and controls the DC bus voltage; at this time, the ILC stops PWM control.

The output power experimental waveforms of the ESS, PV, and WT converters in operation mode 3 are shown in Figure 16b. At time T9, the conversion to the DC voltage control operation changes the output power of the ESS converter from a 2.6 kW discharge operation to a −1.6 kW charge operation, and, thus, DC autonomous operation can be performed. At this time, the power generation of the PV and the WT converters is 1.0 kW and 3.2 kW, respectively, and the DC load power is 2.6 kW. Therefore, it can be seen that the ESS converter controls the DC bus voltage and supplies 1.6 kW, which is the amount of insufficient power, into the DC bus.

5.2. Stand-Alone Mode

From time T11, DC/AC autonomous operation in mode 4 performs AC bus voltage control, and the ESS converter performs DC bus voltage control.

Mode 4 (DC/AC Autonomous)

The output current of the ILC and the output powers of ESS, WT, and PV are shown in Figure 17a. This shows that current is supplied to the AC load of 2 kW connected to the AC bus through the a-phase output current of the ILC. At the time of T12, the power of the three-phase load changes from 2 kW to 4 kW, and, accordingly, the ESS output discharge power increases to 2.4 kW. The AC grid voltage was maintained at 0.8 pu until the time T13 through the Slidac, and it increased to 1.0 pu until the time T14 to simulate the recovery situation of the AC voltage. Figure 17b shows the voltage and phase angle waveforms of the ILC and AC grid in mode 4. After phase synchronization, for performing reconnection with the AC grid, is performed, a reconnection operation with the AC grid is performed at T15 to return to the power balance operation mode of mode 1. Through the balance operation, the ESS converter performs a discharge operation of 2.4 kW and then converts to a charge operation of −1.6 kW. At this time, after switching the ILC to DC bus voltage control, it is shown that the magnitude of the a-phase output current of the ILC is reduced so that the output power becomes −0.2 kW. Through these experiments, the power supply of AC loads through ILC in DC/AC autonomous operation in mode 4, phase synchronization according to the restoration of the AC grid, and reconnection experiments to mode 1 were verified.

6. Conclusions

Hybrid AC/DC microgrids require energy management according to various power generation situations and load conditions within the microgrid, as distributed power sources are connected. In this study, the grid support function of the ILC connected to the distribution on both sides was considered. To this end, the allowable reactive power range and power factor of the ILC were analyzed when cooperative control between power converters and LVRT situations occurred. In particular, the reference power value of the ESS was defined to maintain the power factor of the ILC at 0.9 or higher using cooperative control through additional discharge with the ESS converter during reactive power operation of the ILC. In order to implement these methods, the paper is composed of (1) EMS according to the operation mode selection considering LVRT, and (2) cooperative control of the converters in the microgrid according to the selected mode. In particular, this study stipulates the operation of other power converters in detail when an LVRT situation occurs from the viewpoint of a hybrid microgrid, not the LVRT application of a single grid-connected inverter. The findings of this paper are anticipated to provide valuable insights into LVRT scenarios in a hybrid microgrid, contributing to the expansion of grid resilience with renewable energy sources.

A laboratory-scale hybrid microgrid experiment set was established to conduct an experimental verification of the effectiveness of the EMS and cooperative control, considering the KEPCO LVRT.