KR102549305B1 - Microgrid system and method of controlling thereof - Google Patents

Abstract

Provided is a microgrid system capable of controlling power quality to be constantly maintained without a power failure in a small-scale power system. The microgrid system comprises: a distributed power supply configured to generate electricity using renewable energy as an energy source; a plurality of energy storage devices connected to the distributed power supply so as to form a microgrid, and configured to store power supplied from the distributed power supply or to output pre-stored power to an outside; and a central controller configured to control the distributed power source and the plurality of energy storage devices, wherein the central controller may set one of the energy storage devices as a master device and the other energy storage devices as slave devices, among the plurality of energy storage devices, and wherein the master device may output a constant voltage and a constant frequency in a normal state, and may change an output frequency in the normal state to another value within a predetermined allowable range when an overload is applied.

Description

Microgrid system and its control method {Microgrid system and method of controlling its}

The present invention relates to a microgrid system and a control method thereof, and more specifically, to a microgrid system that can be controlled to maintain constant power quality without power failure in a small-scale power system using an energy storage device as a base power source, and It is related to the control method.

In remote areas, such as islands or mountain tops, where it is difficult to connect to a large-scale power grid, a small-scale power system consisting of a load and a generating device such as a diesel generator is constructed. However, in recent years, in order to reduce emissions of materials that accelerate global warming, such as carbon dioxide, electricity is supplied using renewable energy instead of diesel generators.

When the renewable energy is used, since the generation amount of the renewable energy does not match the consumption pattern of the load, an energy storage device is used to charge when the generation amount of the renewable energy is greater than the load, and the load is more than the generation amount of the renewable energy. Discharge in case

Even if the energy storage device is used, a diesel generator is continuously used to supply electricity when the amount of renewable energy generation is insufficient or the energy storage device is out of order. That is, the energy storage device operates as a base power supply that maintains the frequency and voltage of the power system.

In a conventional small-scale power system using an energy storage device, a plurality of energy storage devices may be installed in preparation for failure of the energy storage device. However, even if a plurality of energy storage devices are installed, only one set of energy storage devices operates and the rest of the energy storage devices stand by in a stopped state. In the case of installing only one set of energy storage devices, a plurality of power converters are used in preparation for failure of the power converter, but even in this case, only one power converter operates and the remaining power converters stand by in a stopped state.

In a small-scale power system in which the ESS operates as base power, when a failure occurs in an ESS, a minimum of several ms to hundreds of seconds until another energy storage device, power converter, or diesel generator on standby operates to supply electricity at a constant frequency. It takes ms and leads to power failure.

Even if multiple energy storage devices or multiple power converters are installed, the reason why one set or one unit is operated instead of operating simultaneously is that a failure occurs in one power converter in a situation where a small number of power converters share power in parallel. This is because the frequency of the power system cannot be maintained constant.

In a small-scale power system, problems that may occur when an energy storage device operates with base power using a conventional control method are as follows.

First, since an excessively large power conversion device is installed from the beginning by reflecting the expected increase in power consumption after installing an energy storage device operating on base power, the initial investment cost is high and the economic feasibility is low.

Second, when a failure occurs in the energy storage device, the reliability of power supply is low because an instantaneous power outage occurs.

Third, when a battery needs to be additionally installed due to a decrease in battery life after long-term operation, maintenance costs are excessive because the entire battery must be replaced.

One technical problem to be solved by the present invention is to provide a microgrid system and its control method, which can be controlled so that power quality is constantly maintained without blackout in a small-scale power system using an energy storage device as a base power source. .

The technical problem to be solved by the present invention is not limited to the above.

In order to solve the above technical problem, the present invention provides a microgrid system.

According to one embodiment, the microgrid system includes a distributed power source that generates power using renewable energy as an energy source; A plurality of energy storage devices that are connected to the distributed power source to form a microgrid, store power supplied from the distributed power source, or output previously stored power to the outside; And a central controller for controlling the distributed power source and a plurality of energy storage devices, wherein the central controller sets one energy storage device as a master device among the plurality of energy storage devices, and the other energy storage devices It is set as a slave device, and the master device outputs a constant voltage and a constant frequency in a normal state, and when an overload is applied, the output frequency in the normal state can be changed to another value within a preset allowable range.

According to one embodiment, the energy storage device may include a battery capable of charging and discharging; a power conversion device that converts power supplied from the distributed power source to charge the battery or converts power stored in the battery and discharges it to the outside; and a device controller controlling the power conversion device to charge or discharge the battery, wherein when the energy storage device is a slave device, the device controller monitors an output frequency of the master device, You can check whether the master device is operating normally.

According to one embodiment, when it is confirmed through monitoring the output frequency of the master device that the output frequency of the master device is changed to another value, the slave device performs frequency-active power droop control. , can share the active power applied to the master device.

According to an embodiment, the central controller commands a new active power reference value to the slave device when active power applied to the master device is shared through frequency-active power droop control for the slave device, It is possible to return the output frequency change value of the master device to the output frequency value in a normal state.

According to one embodiment, the central controller gives priority to the slave device, and when the output frequency of the master device is monitored as out of the predetermined allowable range, the slave device determines that the master device is a microcontroller. Recognized as being separated from the grid, a slave device having a higher priority among the slave devices may be converted into a master device by itself.

According to an embodiment, when the slave device does not recognize the situation in which the master device is separated from the microgrid, the central controller converts a slave device having a higher priority among the slave devices into a master device, and the remaining slave devices Priorities can be newly assigned to devices.

Meanwhile, the present invention provides a microgrid system control method.

According to an embodiment, the microgrid system control method includes a distributed power source that generates power using renewable energy as an energy source and a microgrid connected to the distributed power source to form a microgrid, and stores power supplied from the distributed power source Alternatively, as a method of controlling a microgrid system including a plurality of energy storage devices that output previously stored power to the outside, any one energy storage device among the plurality of energy storage devices is set as a master device, Setting the remaining energy storage devices as slave devices; In a normal state, a constant voltage and a constant frequency are output from the master device, and when an overload is applied, the step of changing the output frequency in the normal state to another value within a preset allowable range; And monitoring the output frequency of the master device through the slave device to check whether the master device is normally operating, but when it is confirmed that the output frequency of the master device is changed to another value, the slave device Through frequency-active power droop control for devices, active power applied to the master device may be shared.

According to an embodiment of the present invention, a distributed power source for generating power using renewable energy as an energy source; A plurality of energy storage devices that are connected to the distributed power source to form a microgrid, store power supplied from the distributed power source, or output previously stored power to the outside; And a central controller for controlling the distributed power source and a plurality of energy storage devices, wherein the central controller sets one energy storage device as a master device among the plurality of energy storage devices, and the other energy storage devices It is set as a slave device, and the master device outputs a constant voltage and a constant frequency in a normal state, and when an overload is applied, the output frequency in the normal state can be changed to another value within a preset allowable range.

Accordingly, in a small-scale power system using an energy storage device as a base power source, a microgrid system and a control method thereof capable of controlling power quality to be constantly maintained without blackout can be provided.

That is, according to an embodiment of the present invention, since a plurality of energy storage devices are used and failures occurring in the operating energy storage device are quickly detected and responded to, it is possible to prevent blackouts in a small-scale power system by using the energy storage device as a base power source. Through this, it is possible to supply electricity with excellent power quality.

In addition, according to an embodiment of the present invention, since a plurality of energy storage devices can be operated simultaneously, a small capacity renewable energy source and energy storage device are installed in accordance with power demand in the initial stage of installing the energy storage device, and then, Additional renewable energy sources and energy storage can be installed as the load increases.

That is, according to an embodiment of the present invention, it is possible to secure economic feasibility by reducing the initial investment cost.

1 is a configuration diagram for explaining a microgrid system according to an embodiment of the present invention.
2 is a block diagram showing an energy storage device of a microgrid system according to an embodiment of the present invention.
3 is a flowchart illustrating a master-slave control process of a central controller for a plurality of energy storage devices in a microgrid system according to an embodiment of the present invention.
4 is a control block diagram for a master mode of a master device in a microgrid system according to an embodiment of the present invention.
5 is a control block diagram for a slave mode of a slave device in a microgrid system according to an embodiment of the present invention.
6 is a reference diagram for explaining the relationship between frequency setting values.
7 is a diagram for explaining operation mode conversion of a slave device when a failure occurs in a master device in a microgrid system according to an embodiment of the present invention.
8 is a configuration diagram of a simulation model for verifying a microgrid system according to an embodiment of the present invention.
9 is a simulation model circuit diagram of FIG. 8 .
10 is a diagram showing an initial startup of a simulation model.
11 is a simulation result when the nominal frequency is maintained even when an overload is applied to the master device.
12 is a simulation result when the frequency is moved when an overload is applied to the master device.
13 is a simulation result of Comparative Example 1.
14 is a simulation result of Example 1.
15 is a simulation result of Comparative Example 2 Example 2.
16 is a simulation result of Comparative Example 3.
17 is a simulation result of Example 3.
18 is a simulation result of Example 4.
19 is a flowchart illustrating a microgrid system control method according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the technical spirit of the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosed content will be thorough and complete, and the spirit of the present invention will be sufficiently conveyed to those skilled in the art.

In this specification, when an element is referred to as being on another element, it means that it may be directly formed on the other element or a third element may be interposed therebetween. Also, in the drawings, shapes and sizes are exaggerated for effective description of technical content.

In addition, although terms such as first, second, and third are used to describe various elements in various embodiments of the present specification, these elements should not be limited by these terms. These terms are only used to distinguish one component from another. Therefore, what is referred to as a first element in one embodiment may be referred to as a second element in another embodiment. Each embodiment described and illustrated herein also includes its complementary embodiments. In addition, in this specification, 'and/or' is used to mean including at least one of the elements listed before and after.

In the specification, expressions in the singular number include plural expressions unless the context clearly dictates otherwise. In addition, the terms "comprise" or "having" are intended to designate that the features, numbers, steps, components, or combinations thereof described in the specification exist, but one or more other features, numbers, steps, or components. It should not be construed as excluding the possibility of the presence or addition of elements or combinations thereof. In addition, in this specification, "connection" is used to mean both indirectly and directly connecting a plurality of components.

In addition, in the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description will be omitted.

1 is a configuration diagram for explaining a microgrid system according to an embodiment of the present invention, and FIG. 2 is a block diagram showing an energy storage device of a microgrid system according to an embodiment of the present invention.

As shown in FIG. 1, the microgrid system 100 according to an embodiment of the present invention is for supplying power to consumers in remote areas where it is difficult to connect to a large-scale power grid, such as an island or a mountain top. It is a small power grid system.

To this end, the microgrid system 100 according to an embodiment of the present invention includes a distributed power source 110, an energy storage system (ESS) 120 and a central controller 130 can do. At this time, each of the distributed power source 110 and the energy storage device 120 is electrically connected to the consumer side through a distribution line, and a circuit breaker and a transformer may be installed in the distribution line.

Also, according to an embodiment of the present invention, the distributed power source 110 and the energy storage device 120 may be communicatively connected to the central controller 130 through a low-speed communication network 131.

The distributed power source 110 is a power generation facility that generates power using renewable energy as an energy source. Here, the renewable energy may be, for example, wind power and sunlight. Accordingly, the distributed power source 110 may include a wind power generator and a solar power generator.

According to an embodiment of the present invention, the operation of the distributed power source 110 may be controlled by the central controller 130 connected through a low-speed communication network 131 in communication.

In one embodiment of the present invention, a wind power generator and a solar power generator have been exemplified as the distributed power source 110, but this is only an example, and only a wind power generator can be provided as the distributed power source 110, and only a solar power generator is provided. It could be. In addition, a geothermal generator may be further provided as the distributed power source 110 in addition to a wind power generator or a solar power generator.

At this time, each generator may be provided with one or two or more, which may be determined according to the possibility of obtaining renewable energy at the site where the microgrid system 100 is installed, the capacity of each generator, and the size of the load they will be responsible for. there is.

The distributed power source 110 has a characteristic in which output varies depending on weather, season, and the like. To compensate for this, the microgrid system 100 according to an embodiment of the present invention may further include an emergency power generation facility 140.

The emergency power generation facility 140 may be a power generation facility that generates power using fossil fuel as an energy source. For example, the emergency power generation facility 140 may be equipped with a diesel generator. The diesel generator uses diesel as an energy source, and together with the distributed power source 110 and the energy storage device 120 may be responsible for supplying power to a plurality of loads, for example, a customer side.

This emergency power generation facility 140 may be communicatively connected to the central controller 130 through a low-speed communication network 131. The emergency power generation facility 140 controls the central controller 130 when the amount of power generated by the distributed power source 110 that generates power using renewable energy as an energy source is insufficient or when a serious problem occurs in the energy storage device 120. can be driven according to

In one embodiment of the present invention, a diesel generator is exemplified as the emergency power generation facility 140 using fossil fuel as an energy source, but is not limited thereto. For example, a power generation facility using coal as a fuel is an emergency power generation facility ( 140), of course.

The energy storage device 120 is connected to the distributed power source 110 to form a microgrid. The energy storage device 120 is a device that stores power supplied from the distributed power source 110 or outputs previously stored power to the outside.

That is, the energy storage device 120 receives and stores surplus power from the distributed power source 110 when the amount of power generated by the distributed power source 110 is greater than the load, and stores the surplus power when the load is greater than the amount of power generated by the distributed power source 110. The current power is output to a load, for example, a consumer side. Here, the customer may mean a power consumer who consumes power, such as a general household or a factory.

Referring to FIG. 2 , the energy storage device 120 may include a battery 121, a power conversion device 122, and a device controller 123.

The battery 121 may be provided as a secondary battery capable of charging and discharging. In addition, the power converter 122 converts the power supplied from the distributed power source 110 to charge the battery 121 when the amount of power generated by the distributed power source 110 is greater than the load, or the load is distributed to the distributed power source 110. ), the electric power charged in the battery 121 may be converted and discharged to the outside.

The device controller 123 may control the power converter 122 to charge or discharge the battery 121 . That is, the device controller 123 may output a switching signal to the power conversion device 122 so that the battery 121 is charged or discharged.

Meanwhile, the microgrid system 100 according to an embodiment of the present invention may include a plurality of energy storage devices 120.

In one embodiment of the present invention, three energy storage devices 120 composed of a first energy storage device 120a, a second energy storage device 120b, and a third energy storage device 120c have been exemplified, but this is an example. Only two energy storage devices 120 may be provided, or four or more may be provided. However, for master-slave control to be described later, it may be preferable that at least three energy storage devices 120 are provided.

According to an embodiment of the present invention, any one of the first energy storage device 120a, the second energy storage device 120b, and the third energy storage device 120c is a master device by the central controller 130. unit) can be set.

For example, the first energy storage device 120a may be set as a master device by the central controller 130 . Accordingly, the second energy storage device 120b and the third energy storage device 120c may be set as slave units.

According to an embodiment of the present invention, the first energy storage device 120a set as a master device may output a constant voltage and constant frequency (CVCF) in a steady state.

In this state, when an overload is applied to the first energy storage device 120a set as the master device, the magnitude of the overload borne by the first energy storage device 120a set as the master device in the transient state and the duration of the transient state In order to reduce, the output frequency in a normal state may be changed to another value within a predetermined allowable range.

On the other hand, in the conventional master-slave control, when an error occurs in the master device, the central controller separately issues an operation command to the slave device. In this case, communication delay occurs due to the physical communication distance between the central controller and the slave device. And this communication delay causes a power outage.

According to an embodiment of the present invention, the device controller 123 of each of the second energy storage device 120b and the third energy storage device 120c set as a slave device is configured as a first energy storage device 120a set as a master device. The output frequency of can be constantly monitored.

Through this, each device controller 123 of the second energy storage device 120b and the third energy storage device 120c set as a slave device checks whether the first energy storage device 120a set as a master device is normally operating. can

In the process of monitoring the output frequency of the first energy storage device 120a set as a master device, the second energy storage device 120b and the third energy storage device 120c set as slave devices ( When it is confirmed that the output frequency of 120a) is changed to another value, the active power applied to the master device may be shared through frequency-active power droop control.

As such, according to an embodiment of the present invention, the slave device can check whether the master device is normally operating while constantly monitoring the output frequency, and when an abnormality occurs in the frequency, the droop can be controlled immediately. Accordingly, it is possible to prevent communication delays and resulting power outages that have conventionally occurred between the central controller and the slave devices.

Meanwhile, according to an embodiment of the present invention, the second energy storage device 120b and the third energy storage device 120c set as slave devices may be given priority by the central controller 130. Here, the priority means the priority to be switched to the master device, which will be described in more detail below.

Referring back to FIG. 1 , the central controller 130 may be communicatively connected to the distributed power source 110 provided by the wind power generator and the solar power generator through the low-speed communication network 131 . In addition, the central controller 130 may be communicatively connected to the first energy storage device 120a, the second energy storage device 120b, and the third energy storage device 120c through the low-speed communication network 131. In addition, the central controller 130 may also communicate with the emergency power generation facility 140 provided as a diesel generator through the low-speed communication network 131.

Through this, according to an embodiment of the present invention, the central controller 130 can individually control at least one distributed power source 110, a plurality of energy storage devices 120, and at least one emergency power generation facility 140. there is.

According to an embodiment of the present invention, the central controller 130 sets any one energy storage device 120 as a master device operated in master mode among a plurality of energy storage devices 120, and stores the remaining energy. Device 120 can be configured as a slave device operating in slave mode.

The central controller 130 may set the energy storage device 120 to be operated as a master device by integrating the state of charge of the batteries 121 of the plurality of energy storage devices 120 and various state data. For example, the central controller 130 may set the energy storage device 120 having the highest charging rate of the battery 121 among the plurality of energy storage devices 120 as the master device.

In this way, the energy storage device 120 set as the master device by the central controller 130 outputs a constant voltage and a constant frequency in a normal state, and then, when an overload is applied, the output frequency in the normal state is set in advance. It can be changed to another value within the permissible range.

At this time, as described above, through monitoring of the output frequency of the master device, the slave device that has confirmed that the output frequency of the master device has changed to a different value is transmitted to the master device through frequency-active power droop control. It shares the applied active power, and thus, the central controller 130 may return the output frequency change value of the master device to the output frequency value in a normal state.

In addition, according to an embodiment of the present invention, as the central controller 130 sets any one of the plurality of energy storage devices 120 as the master device, priority is given to the energy storage device 120 set as the slave device. can be granted.

Accordingly, at least two or more slave devices may recognize that the master device is separated from the microgrid when it is monitored that the output frequency of the master device is out of a preset allowable range.

Accordingly, according to an embodiment of the present invention, a slave device having a higher priority among slave devices may be converted into a master device by itself.

According to another embodiment, a slave device with the highest priority may monitor the output frequency of the master device, and if the output frequency of the master device is monitored to be out of a preset tolerance range, it may switch itself to the master device. there is.

On the other hand, when the slave device does not recognize the situation in which the master device is separated from the microgrid, the central controller 130 may convert a slave device having a high priority among the slave devices into a master device.

Also, according to an embodiment of the present invention, the central controller 130 may newly assign priorities to the remaining slave devices.

Hereinafter, with reference to FIG. 3, a master-slave control process of a central controller for a plurality of energy storage devices will be described in more detail.

3 is a flowchart illustrating a master-slave control process of a central controller for a plurality of energy storage devices in a microgrid system according to an embodiment of the present invention.

Referring to FIG. 3, the central controller may start checking the microgrid system (S1).

That is, a monitoring start signal for the microgrid from the central controller can be transmitted to a plurality of energy storage devices (ESS Units).

Accordingly, each of the plurality of energy storage devices may respond to the state (S2).

The central controller may output a command to set any one energy storage device as a master unit based on the state information of the plurality of energy storage devices that have been responded (S3).

Accordingly, the energy storage device may be set to the master mode (S4-1). In addition, the remaining energy storage devices (slave units) may be set to a slave mode, and priorities may be assigned to each energy storage device (S4-2).

Meanwhile, the central controller may output a command for starting the master mode (S5-1). Accordingly, control of the master device set to the master mode may be initiated (S5-2).

Also, the central controller may output a command to initiate the slave mode (S5-3). Accordingly, control of the slave devices set to the slave mode may be initiated (S5-4).

Here, the master device set to the master mode can be controlled by constant voltage constant frequency (CVCF) (S6-1). For example, when an overload is applied to the master device, the master device may be controlled to change the frequency to another value within a predetermined allowable range.

At this time, if an error occurs in the constant voltage constant frequency (CVCF) control for the master device set to the master mode (S6-2), for example, if the current is out of the allowable range, the master device set to the master mode It can be separated from the microgrid (S6-3).

Meanwhile, when control of the slave devices set to the slave mode starts (S5-4), the slave devices set to the slave mode can be operated by power control (S7-1).

At this time, if it is confirmed that the master device is separated from the microgrid (S7-2), the slave device with a high priority automatically switches the operation mode from slave mode to master mode, and accordingly, it is controlled by constant voltage constant frequency (CVCF). It can be (S8).

Accordingly, the central controller 130 may output a command for newly assigning priorities to the remaining slave devices (S9-1).

Accordingly, priorities of the remaining slave devices are set, and operation can be performed by power control (S9-2).

Meanwhile, FIG. 4 is a control block diagram for a master mode of a master device in a microgrid system according to an embodiment of the present invention, and FIG. 5 is a control block diagram of a slave device in a microgrid system according to an embodiment of the present invention. This is the control block diagram for the mode.

First, referring to FIG. 4 , the master mode may output a voltage of a constant frequency. At this time, in the master mode, the size of the output voltage may be controlled with a reference value set by the central controller 130 or the power converter itself (122 in FIG. 2), or the size of the output voltage may be kept constant.

Unlike the conventional master-slave control method in which an overload is applied to the master device due to a sudden change in output or load of another device, according to an embodiment of the present invention, the master device reduces the magnitude and duration of the overload. The frequency can be shifted from the nominal frequency to another frequency.

을 계산할 수 있다.In a normal state, the device controller (123 in FIG. 2 ) of the master device may increase the phase angle θ _t of the output voltage by a constant size and keep the frequency constant. However, if the absolute value of the master device active power P _master is higher than the nominal power, the phase angle compensation value through Equation 1 below

can be calculated.

[Calculation 1]

Here, s is the sign of the active power of the master device. When active power is supplied from the energy storage device to the microgrid, the sign is (-), and when active power is supplied from the microgrid to the energy storage device, the sign is (+ )am. Also, K _θ is a coefficient for calculating the phase angle compensation value. In order to limit the frequency variation range (allowable range), the maximum and minimum values of the phase angle compensation value calculated as in Equation 1 above may be limited.

The phase angle of the voltage output from the master device can be calculated through Equation 2 below.

[Calculation 2]

Here, θ _t−1 is the phase angle in the previous period, θ _s is the amount of phase angle change during one sampling period of the device controller, and the amount of phase angle change can be calculated through Equation 3 below.

[Calculation 3]

Here, f _sampling is a sampling frequency in master mode.

If the phase angle compensation value is 0, the voltage phase angle may increase by θ _s at every sampling period to become a nominal frequency.

When the sign of the phase angle compensation value is (+), the phase angle increases faster and the frequency becomes higher than the nominal frequency. Conversely, when the sign of the phase angle compensation value is (-), the phase angle increases more slowly and the frequency becomes lower than the nominal frequency.

When the master device shifts the frequency to a value different from the nominal frequency, in slave mode, frequency-active power droop control is activated so that the slave devices share a portion of the master device's active power that exceeds the nominal power. Then, when the central controller reflects the overload of the master device and commands a new active power reference value to the slave device, the frequency output from the master device, that is, the frequency of the microgrid, can be restored to the nominal value.

Next, referring to FIG. 5, in the slave mode, active power and reactive power are controlled according to the command of the central controller, and frequency-active power droop and voltage-reactive power droop operate according to the frequency and voltage size of the microgrid. It can assist the active power and reactive power controller.

In addition, in the slave mode, the state of the master device can be checked through frequency. If the frequency of the microgrid is out of the permissible range, the slave device with higher priority may determine that the master device is separated from the microgrid and switch itself to the master device to maintain the frequency constant.

When active power is supplied from the slave device to the microgrid, that is, when active power is supplied to the microgrid through battery discharge, the sign of the active power is (+). The central controller may command an active power reference value to each slave device in consideration of the amount of power generated by the distributed power source, the power demand of the load, and the state of charge of the batteries of the energy storage devices.

If the central controller commands the active power reference value, in the slave mode, the active power can be controlled by adding the active power reference value sent from the central controller and the droop compensation value according to the frequency of the microgrid. In the slave mode, the active power of the slave device may be controlled according to the reference value P ^* calculated through Equation 4 below.

[Calculation 4]

Here, P ^* _central is the active power reference value sent from the central controller, K _active is the frequency-active power droop coefficient, and f _m is the measured frequency of the microgrid.

If the frequency is higher than the set range, i.e. at a frequency higher than (f _nom +Δf _min ), the slave device operates in a direction that reduces the active power sent to the microgrid or increases the active power taken from the microgrid. will do Conversely, at frequencies lower than (f _nom -Δf _min ), the active power sent to the microgrid increases or the active power taken from the microgrid decreases. When the active power reference value is determined, the q-axis current reference value I ^* _q is calculated.

Under normal conditions, the frequency of the microgrid is kept constant at the nominal frequency because the master device keeps the frequency constant. However, the frequency may vary in three cases.

First, when the active power of a load or distributed power source changes rapidly, a frequency measured in a slave device may differ from a nominal frequency for a short period of time. In this case, the frequency of the microgrid can be restored to the nominal frequency before the device controller of the slave device reacts. Second, as described in the master mode, when the master device has to bear transient power exceeding the nominal power due to rapid fluctuations in the active power or load of the distributed power source, the master mode operates to lower the frequency within the allowable range. or can be raised. Third, when the master device is separated from the microgrid by accident, the frequency fluctuates.

Another function of the slave mode is to check whether the master device is operating normally by measuring the frequency of the microgrid. As described above, if the master device is connected to the microgrid and operated, the frequency may be maintained within a certain range from the nominal frequency. However, if the master device is separated from the microgrid, the frequency may fluctuate depending on circumstances.

If the active power burdened by the master device is not large and the droop control of the slave device sufficiently compensates, the frequency of the microgrid remains within the allowable range, and the slave device may not determine that the master device is separated from the microgrid. In this case, the central controller may command a slave device having a high priority to switch to a master device through communication.

If the active power borne by the master device is large or if the droop control of the slave device does not sufficiently compensate, the frequency of the microgrid may be outside the allowable range. In this case, the slave device may determine that the master device is disconnected from the power grid, and the slave device having a higher priority may decide to switch to the master device itself.

On the other hand, referring to FIG. 6, if the master device is connected to the microgrid, the frequency measured in the slave device is higher than (f _nom -Δf _{master_low} ) and lower than (f _nom +Δf _{master_high} ), which is displayed in gray. can keep

The frequency-active power droop control of the slave device may operate at a frequency lower than (f _nom -Δf _min ) or higher than (f _nom +Δf _max ). Even if the frequency-active power droop control of the slave device operates, if the frequency is lower than (f _nom -Δf _low ) or higher than (f _nom +Δf _high ), it is determined that the master device is separated from the microgrid, and priority is given to it. A high slave device can be turned into a master device to supply a constant frequency.

Δf _low may be set to be 0.1 Hz or more lower than Δf _{master_low} in consideration of the accuracy of a clock device used in a device controller of each energy storage device. Similarly, Δf _high can be set higher than Δf _{master_high} by 0.1 Hz _or more.

According to an embodiment of the present invention, when a failure occurs in the master device, a slave device having a higher priority among slave devices is converted to a constant voltage and constant frequency and operates with base power.

7 is a diagram for explaining operation mode conversion of a slave device when a failure occurs in a master device in a microgrid system according to an embodiment of the present invention. Referring to FIG. 7, a failure occurs in a master device. Previously, the frequency deviation remains within an acceptable range. At this time, even if a failure does not occur in the master device, the frequency may appear to fluctuate depending on the measurement location. However, in the event of a failure in the master device, the frequency continues to go lower or higher.

The slave device starts droop control when the frequency deviation is out of the allowable range. When the frequency is lowered, the slave device increases the effective power discharge amount or decreases the charge amount.

However, if the frequency is out of the allowable range even with droop control, the slave device determines that the master device has been separated from the microgrid due to a failure, and is converted into a master device operating in master mode. Then, the central controller can operate the standby energy storage device as a slave device or operate another power source.

Meanwhile, FIG. 8 is a configuration diagram of a simulation model for verifying a microgrid system according to an embodiment of the present invention, and FIG. 9 is a circuit diagram of the simulation model of FIG. 8 .

Referring to FIGS. 8 and 9 , in order to shorten the simulation period, a diesel generator, photovoltaic power generation, and wind power generation were not included in the simulation model. Devices that control output in connection with the power system are the same as current sources, so one set of three-phase constant current source lcs was used in the simulation model.

Three sets of 100 kW energy storage devices were used, and they were marked as ESS1, ESS2, and ESS3 in order. ESS1 is designated as the master device and starts operating, and is disconnected from the power grid after 2.8 seconds. ESS2 and ESS3 are designated as slave devices, and ESS2 has a higher priority.

The load is simplified into a pure resistive load and a rectifying load. Two sets of single-phase loads and one set of three-phase loads were used as rectifying loads, and a total of five sets of resistive loads were used. Loads are connected or disconnected from the power grid at preset times.

The nominal voltage of the energy storage device is 380V, and the nominal voltage of the distribution network is 6.4 kV. The leakage impedance of the transformer is 6.5%, and the wiring line has a dominant resistance component.

The distributed power source is composed of energy storage devices ESS1, ESS2, and ESS3, and the constant current source is indicated by CS. The energy storage device controller was implemented as a DLL file, and the sampling frequency was 10 kHz and the PWM frequency was 5 kHz. The central controller of the microgrid was not separately implemented, and the active power reference value was designated as a pattern for each device. At this time, the reactive power was set to 0.

10 shows the initial startup of the simulation model. Referring to FIG. 10, active power supplied to the microgrid from three sets of energy storage devices is P _ESS1 , P _ESS2 , P _ESS3 , active power supplied from the constant current source is Pcs, and power converted to DC from the rectifier The active power of the load consuming P _rect1 , P _rect2 , P _rect3 , and the active power of the resistive load consuming AC power are respectively P _Rd1 , P _Rd2 , P _Rd3 , P _Rd4 , and P _Rd5 .

The master device ESS1 starts operating and supplies the voltage V _{ab_ESS1} to the microgrid. Even if a constant frequency is supplied from the master device, the frequency varies slightly at the moment the load or generation amount fluctuates in each device.

The current (l _{a_ESS1} , l _{a_ESS2} , l _a - _{_ESS3} ) supplied to the microgrid from each ESS varies depending on the load of the microgrid and the operation of each device. The current of the slave device is determined according to the standards of active power and reactive power, but the current l _{a_ESS1} of the master device is determined according to the demand and supply of active and reactive power in the microgrid. When the output of is changed, it fluctuates rapidly. The current (l _DL1 , l _DL2 ) flowing through the 2 sets of distribution impedance of the microgrid is determined according to the state of the load of the energy storage device.

In order to verify the microgrid system according to an embodiment of the present invention, a simulation was performed for the frequency movement of the master device.

11 is a simulation result when the nominal frequency is maintained even when an overload is applied to the master device, and FIG. 12 is a simulation result when the frequency is shifted when an overload is applied to the master device.

The low frequency trip level is set to 57 Hz, the lower limit of the frequency at which the slave device starts droop control (Δf _min ) is 0.1 Hz, and the lower limit of the frequency of the master device (Δf _{master_low} ) is 59.1 Hz. When maintaining the nominal frequency, the effective power of the master unit is approximately 145 kW before the resistive load Rd5 disconnects at 3.2 seconds. When shifting the frequency, the effective power of the master device before the resistive load Rd5 is disconnected is approximately 128.5 kW.

That is, it can be confirmed that the overload of the master device is improved by the frequency shift. If the lower frequency limit is further lowered or the droop coefficient of the slave device is increased, the active power borne by the slave device increases, thereby further reducing overload of the master device. When the load is reduced or the active power is increased by commanding a new power reference value to the slave device from the central controller, the overload condition of the master device is resolved and the frequency is restored to the nominal frequency.

After the master device is separated from the microgrid, the simulation compares the case where the slave device is converted to the master device and the case where it is not.

At the moment when the master device (ESS1) is separated from the grid, the simulation was performed under the conditions of the active power of the slave device (ESS2) with high priority, the droop coefficient setting of the slave devices (ESS2, ESS3), and whether or not to switch to the master device of ESS2. , simulation conditions and results are shown in Table 1 below.

ESS2 droop coefficient control Case transient

60kW Discharge
lowness Maintain droop control Case1 stability switch mode Case2 stability
height Maintain droop control Case3 Ripple generation switch mode Case4 Ripple generation

10kW discharge
lowness Maintain droop control Case5 blackout switch mode Case6 stability
height Maintain droop control Case7 Ripple generation switch mode Case8 stability

60kW charging
lowness Maintain droop control Case9 blackout switch mode Case10 stability
height Maintain droop control Case11 blackout switch mode Case12 stability

After ESS1, the master device, was separated from the microgrid, when ESS2, a slave device with a high priority, maintained droop control, transient ripple or power failure occurred depending on the size of the droop coefficient.

However, when ESS1, the master device, was separated from the microgrid and ESS2, a slave device with high priority, was switched to the master device, most of them operated stably, and transient ripples occurred in some cases where the droop coefficient of the slave device was high. . That is, when ESS1, the master device, was separated from the microgrid and ESS2, a slave device with a high priority, was switched to the master device, the frequency could be stably maintained if a low droop coefficient was used.

Comparison example 1

Comparative Example 1 simulated the case where ESS1 is disconnected from the power grid and ESS2 maintains droop control in a situation where the droop coefficient is low and ESS2 discharges 60 kW.

Referring to FIG. 13 , in Comparative Example 1, when ESS1 is disconnected from the power grid at 2.8 seconds, the frequency is lowered, and droop control of ESS2 and ESS3 is operated to increase the output of active power.

The frequency stabilizes at about 59.2 Hz before 3 seconds and at about 58.2 Hz after load Rd4 is connected to the power grid at 3 seconds. In order to restore the frequency to the nominal frequency, the active power reference value sent from the central controller to ESS2 and ESS3 must be increased.

Example 1

Example 1 simulated the case where ESS1 is disconnected from the power grid and ESS2 is switched to a master device in a situation where the droop coefficient is low and ESS2 discharges 60 kW.

Referring to FIG. 14 , in Example 1, like Comparative Example 1, when ESS1 is disconnected from the power grid at 2.8 seconds, the frequency is lowered and the droop control of ESS2 and ESS3 is operated to increase the output of active power. However, at 3 seconds, when the load Rd4 is connected to the power grid and the frequency drops below 59 Hz, ESS2 switches to the master device and keeps the frequency at the nominal frequency.

However, since ESS2 is overloaded after being converted into a master device, the microgrid frequency is moved to 59.1 Hz to mitigate the overload condition.

Comparative Example 2

Comparative Example 2 simulated the case where ESS1 is disconnected from the power grid and ESS2 continues to maintain droop control in a situation where the droop coefficient is high and ESS2 discharges 60 kW.

Example 2

Example 2 simulated the case where ESS1 is disconnected from the power grid and ESS2 is switched to a master device in a situation where the droop coefficient is high and ESS2 discharges 60 kW.

Referring to FIG. 15 , in Comparative Example 2 and Example 2, when ESS1 is disconnected from the power grid at 2.8 seconds, the frequency is lowered, and the droop control of ESS2 and ESS3 increases the output of active power. In this case, in the case of Comparative Example 2 and Example 2, since the droop coefficient is high, the frequency drop width is lower than that of Comparative Example 1, but ripple is generated in a transient state. Even if Rd4 is connected at 3 seconds, since the frequency remains above 59.4 Hz, ESS2 does not determine that the master device is separated from the microgrid and maintains the slave mode. In this case, the central controller must command ESS2 to switch to master mode.

Comparison Example 3

Comparative Example 3 simulated the case where ESS1 is disconnected from the power grid and ESS2 maintains droop control in a situation where the droop coefficient is low and ESS2 discharges 10 kW.

Referring to FIG. 16 , in Comparative Example 3, when ESS1 is disconnected from the power grid at 2.8 seconds, the frequency is lowered and droop control of ESS2 and ESS3 is operated. However, since ESS2 and ESS3 with low droop coefficients cannot respond quickly, ESS2 and ESS3 stop due to voltage or frequency abnormalities.

That is, the microgrid becomes a power outage state. The fact that the frequency seems to have recovered to 60 Hz is due to the operation of the ESS2 and ESS3 controller PLLs after the trip stop.

Example 3

Example 3 simulated the case where ESS1 is disconnected from the power grid and ESS2 is switched to a master device in a situation where the droop coefficient is low and ESS2 discharges 10 kW.

Referring to FIG. 17, in Example 3, when ESS1 is disconnected from the power grid at 2.8 seconds, the frequency is lowered and droop control of ESS2 and ESS3 is activated. However, the frequency continues to decrease, and ESS2, which determines that ESS1 is disconnected from the power grid, switches to the master device. When ESS2 is converted into a master device, the microgrid is quickly stabilized because it controls the frequency constantly.

If Rd4 is connected to the power grid after 3 seconds, the output of ESS2 momentarily becomes higher than the nominal power, so the frequency is shifted from ESS2.

Example 4

Example 4 simulated the case where ESS1 is disconnected from the power grid and ESS2 is switched to a master device in a situation where the droop coefficient is low and ESS2 charges 60 kW.

Referring to FIG. 18, in Example 4, when ESS1 is disconnected from the power grid at 2.8 seconds, the frequency is lowered and the droop control of ESS2 and ESS3 is operated, reducing the amount of charge in ESS2 and increasing the amount of power generated in ESS3.

However, the frequency continues to drop, eventually out of acceptable range and ESS2 switches to the master device. When ESS2 is converted into a master device, ESS2 automatically switches from charging to discharging to constantly control the frequency. Since ESS2 was charging, the drop in frequency was larger than in other cases.

Hereinafter, a microgrid system control method according to an embodiment of the present invention will be described with reference to FIG. 19 . Here, reference numerals of each component refer to FIGS. 1 and 2 .

19 is a flowchart illustrating a microgrid system control method according to an embodiment of the present invention.

Referring to FIG. 19 , the microgrid system control method according to an embodiment of the present invention may include steps S110 to S130.

Step S110

In step S110, one energy storage device 120 among the plurality of energy storage devices 120 may be set as a master device, and the remaining energy storage devices 120 may be set as slave devices.

At this time, in step S110, a master device and a slave device may be set based on the state of charge of the plurality of energy storage devices 120 and various data. For example, in step S110, the energy storage device 120 having the highest charging rate of the battery 121 may be set as the master device.

Meanwhile, in step S110, priority may be given to the slave device. When the master device is separated from the microgrid, the slave device with the highest priority can be switched to the master device.

S120 step

In the step S120, a constant voltage and a constant frequency are output from the master device, and when an overload is applied, the output frequency in a normal state may be changed to another value within a predetermined allowable range. Through this, it is possible to reduce the size of the overload and the duration of the transient state borne by the master device.

Step S130

In step S130, the output frequency of the master device may be monitored through the slave device. Through this, in step S130, it is possible to check whether the master device is normally operating.

In step S130, when it is confirmed that the output frequency of the master device is changed to another value, active power applied to the master device may be shared through frequency-active power droop control for the slave device.

In addition, in step S130, when the active power applied to the master device is shared through the frequency-active power droop control for the slave device, the output frequency change value of the master device can be returned to the output frequency value in a normal state.

Here, when the slave devices monitor that the output frequency of the master device is out of the preset tolerance range, the slave device may recognize that the master device is separated from the microgrid. Accordingly, a slave device having a high priority may be converted into a master device by itself.

As another example, the slave device with the highest priority may monitor the output frequency of the master device, and if the output frequency of the master device is monitored as out of the preset allowable range, the master device is recognized as separated from the microgrid , can be turned into a master device by itself.

Meanwhile, in step S130, when the slave device does not recognize the situation in which the master device is separated from the microgrid, a slave device having a high priority among the slave devices may be converted into a master device.

Also, in step S130, priorities may be newly assigned to the remaining slave devices.

In the above, the present invention has been described in detail using preferred embodiments, but the scope of the present invention is not limited to specific embodiments, and should be interpreted according to the appended claims. In addition, those skilled in the art should understand that many modifications and variations are possible without departing from the scope of the present invention.

100; microgrid system
110; distributed power
120; energy storage device
120a; 1st energy storage device
120b; 2nd energy storage device
120c; 3rd energy storage device
121; battery
122; power converter
123; device controller
130; central controller
131; low speed network
140; emergency power plant

Claims (7)

Distributed power generating electricity using renewable energy as an energy source;
A plurality of energy storage devices that are connected to the distributed power source to form a microgrid, store power supplied from the distributed power source, or output previously stored power to the outside; and
A central controller for controlling the distributed power source and a plurality of energy storage devices; including,
The central controller sets one energy storage device among the plurality of energy storage devices as a master device and sets the remaining energy storage devices as slave devices,
The master device outputs a constant voltage and a constant frequency in a normal state, and when an overload is applied, the output frequency in the normal state is changed to another value within a preset allowable range,
The energy storage device,
A battery that can be charged and discharged;
a power conversion device that converts power supplied from the distributed power source to charge the battery or converts power stored in the battery and discharges it to the outside; and
A device controller controlling the power conversion device so that the battery is charged or discharged;
When the energy storage device is a slave device, the device controller monitors the output frequency of the master device to determine whether the master device is normally operating,
Through monitoring of the output frequency of the master device, if it is confirmed that the output frequency of the master device has changed to another value,
The slave device shares active power applied to the master device through frequency-active power droop control;
The central controller changes the output frequency of the master device by commanding a new active power reference value to the slave device when the active power applied to the master device is shared through the frequency-active power droop control for the slave devices. The microgrid system, which returns the value to the output frequency value in steady state.
delete delete delete According to claim 1,
The central controller gives priority to the slave device,
When the output frequency of the master device is monitored as being out of the preset allowable range, the slave device recognizes that the master device is separated from the microgrid, and the slave device having a higher priority among the slave devices is itself the master device. Transitioning to a microgrid system.
According to claim 5,
When the slave device does not recognize the situation in which the master device is separated from the microgrid,
Wherein the central controller converts a slave device having a high priority among the slave devices into a master device and newly assigns priority to the remaining slave devices.
A distributed power source that generates power using renewable energy as an energy source and a microgrid connected to the distributed power source to form a microgrid, storing power supplied from the distributed power source, or outputting the previously stored power to the outside. As a method of controlling a microgrid system including an energy storage device,
setting one of the energy storage devices as a master device and setting the remaining energy storage devices as slave devices among the plurality of energy storage devices;
In a normal state, a constant voltage and a constant frequency are output from the master device, and when an overload is applied, the step of changing the output frequency in the normal state to another value within a preset allowable range; and
Monitoring the output frequency of the master device through the slave device to check whether the master device is normally operating; Including,
When it is confirmed that the output frequency of the master device is changed to another value, the active power applied to the master device is shared through frequency-active power droop control for the slave device,
The energy storage device,
A battery that can be charged and discharged;
a power conversion device that converts power supplied from the distributed power source to charge the battery or converts power stored in the battery and discharges it to the outside; and
A device controller for controlling the power conversion device so that the battery is charged or discharged;
When the energy storage device is a slave device, the device controller monitors the output frequency of the master device to determine whether the master device is normally operating,
Through monitoring of the output frequency of the master device, if it is confirmed that the output frequency of the master device is changed to another value,
The slave device shares active power applied to the master device through frequency-active power droop control;
When the active power applied to the master device is shared through the frequency-active power droop control for the slave device, the central controller controlling the distributed power source and the plurality of energy storage devices sets a new active power reference value to the slave device. A microgrid system control method for instructing the device to return the output frequency change value of the master device to the output frequency value in a normal state.

Images