KR102425037B1 - Method for controlling of dc micro grid system - Google Patents

Abstract

The present invention relates to a control method of a DC microgrid system, comprising the steps of: when a rectifier of a long-distance DC microgrid system is driven, the controller sets a thermal resistance model; when the thermal resistance model is set, the control unit receives line information and calculates line resistance; receiving, by the control unit, an output current (I) and a line temperature of the rectifier, and calculating a real-time voltage drop value (V _drop ) based thereon; And when the voltage drop value (V _drop ) of the DC line is calculated in real time, the control unit reflects the voltage drop value (V _drop ) of the DC line calculated in real time in real time, for the converter of the DC microgrid system and performing droop control.

Description

Control method of DC microgrid system

The present invention relates to a control method of a DC microgrid system, and more particularly, to accurately estimate the voltage drop value occurring at the line end of a DC microgrid network having a long-distance line in real time to perform droop control. It relates to a control method of a DC microgrid system that can be performed.

In general, a low voltage direct current (DC) power distribution system has a relatively large current capacity.

At this time, the voltage drop to be considered in the distribution is caused by the resistance (ie, impedance) of the line, and as the transmission current increases, the degree of the voltage drop increases.

When the voltage drop becomes severe, the power conversion means (eg, inverter, converter, etc.) connected to the output side of the line is dropped due to the low voltage protection operation, resulting in a power outage at the consumer or a sudden decrease in power quality and device efficiency. Occurs.

On the other hand, in the case of a DC microgrid system with a long-distance line, a voltage difference occurs between the line input side and the end side due to a voltage drop due to the line impedance, and it is difficult to perform droop control by this voltage difference.

Therefore, a method for accurately estimating the current voltage drop value and a method for performing droop control in a DC microgrid system having a long-distance line are required.

Background art of the present invention is disclosed in Korean Patent Publication No. 10-2010-0093078 (published on August 24, 2010, power control).

According to one aspect of the present invention, the present invention was created to solve the above problems, and accurately estimates the voltage drop value occurring at the line end of a direct current (DC) microgrid network having a long-distance line in real time to droop. An object of the present invention is to provide a control method of a DC microgrid system that can perform (droop) control.

A control method of a DC microgrid system according to an aspect of the present invention comprises the steps of: when a rectifier of a long-distance DC microgrid system is driven, the controller sets a thermal resistance model; when the thermal resistance model is set, the control unit receives line information and calculates line resistance; receiving, by the control unit, an output current (I) and a line temperature of the rectifier, and calculating a real-time voltage drop value (V _drop ) based thereon; And when the voltage drop value (V _drop ) of the DC line is calculated in real time, the control unit reflects the voltage drop value (V _drop ) of the DC line calculated in real time in real time, for the converter of the DC microgrid system and performing droop control.

In the present invention, the line resistance is calculated using Equation (1).

(Equation 1)

)는 고유저항을 의미한다.where A is the cross-sectional area of the conductor [mm ² ], l is the length of the line, and a proportional constant (

) means specific resistance.

In the present invention, the line resistance is calculated as a line resistance value according to the temperature of the electric wire using Equation (2).

(Equation 2)

는 저항의 온도계수를 의미한다.Here, t ₀ is the reference temperature, R is the wire resistance value at the reference temperature, t is the elevated temperature,

is the temperature coefficient of resistance.

In the present invention, the real-time voltage drop value (V _drop ) is characterized in that it is calculated using Equation 3 below.

(Equation 3)

Here, I is the output current of the DC line, and R is the line resistance.

In the present invention, the thermal resistance model is calculated using a value calculated by attaching a temperature sensor to the surface of the wire, wherein the thermal resistance is between the conductor temperature (T _c ) and the temperature of the sheath (T _s ) The thermal resistance (R _cs ), the thermal resistance (R si ) between the temperature of the sheath (T _s ) and the temperature of the sheath (T _i ), and the thermal resistance (R _si ) between the temperature of the sheath (T _i ) and the temperature of the temperature sensor (T _m ) It is characterized in that it includes a thermal resistance (R _im ).

In the present invention, a thermal capacitor model is connected in parallel to the thermal resistance model, and the thermal capacitor model is a thermal capacitor C _cs between the conductor temperature (T _c ) and the temperature of the sheath (T _s ), the temperature of the sheath A thermal capacitor (C _si ) between (T _s ) and the temperature of the sheath (T _i ), and a thermal capacitor (C _im ) between the temperature of the wire sheath (Ti) and the temperature of the temperature sensor (T _m ) characterized.

According to one aspect of the present invention, the present invention enables droop control to be performed by accurately estimating a voltage drop value occurring at the end of a line of a direct current (DC) microgrid network having a long-distance line in real time.

1 is an exemplary diagram showing a schematic configuration of a control device of a DC microgrid system according to an embodiment of the present invention.
2 is an exemplary diagram illustrating a graph for explaining a droop control characteristic according to an embodiment of the present invention.
3 is a flowchart for explaining a control method of a DC microgrid system according to an embodiment of the present invention.
4 is an exemplary diagram showing the structure of a cable for explaining a method for calculating line resistance in which real-time temperature is reflected according to an embodiment of the present invention;
5 is an exemplary view showing a model of thermal resistance between a conductor and a temperature sensor set according to an embodiment of the present invention.

Hereinafter, an embodiment of a control method of a DC microgrid system according to the present invention will be described with reference to the accompanying drawings.

In this process, the thickness of the lines or the size of the components shown in the drawings may be exaggerated for clarity and convenience of explanation. In addition, the terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to intentions or customs of users and operators. Therefore, definitions of these terms should be made based on the content throughout this specification.

1 is an exemplary diagram showing a schematic configuration of a control device of a DC microgrid system according to an embodiment of the present invention.

As shown in FIG. 1 , the control apparatus of the DC microgrid system according to the present embodiment includes a voltage drop value calculating unit 110 , a control unit 120 , and a gate pulse generating unit 130 .

The voltage drop value calculating unit 110 is a voltage drop value (V _drop ) using the current (I) of the DC line, the line resistance (R), and the temperature (T _m ) measured using a temperature sensor (not shown) to calculate

When a voltage drop occurs in the DC line, the controller 120 reflects the voltage drop value (V _drop ) of the DC line calculated in real time to the droop control in real time.

Here, the controller 120 may be a proportional integral (PI) controller.

Hereinafter, the characteristics of the droop control will be described with reference to the graph of FIG. 2 .

2 is an exemplary diagram illustrating a graph for explaining a droop control characteristic according to an embodiment of the present invention.

For example, as shown in (a) of Figure 2, while detecting the voltage of the DC line, the current is discharged when it falls below 750V, and, conversely, when it exceeds 750V, the current is charged. As shown in (b) of FIG. 2 , when a voltage drop occurs, the voltage drop value (V _drop ) of the DC line calculated in real time is reflected in real time as shown in the droop control characteristic graph. At this time, the rectifier (not shown) with the voltage control right does not change in the existing control characteristic curve, and the voltage drop value (V _drop ) is calculated by reflecting the line distance between the rectifier (not shown) and the connected device, and this value is real-time continues to change with On the other hand, as shown in (c) of Figure 2, when the voltage drop value (V _drop ) becomes a negative value, that is, the line end voltage by the power generation amount of the power generation source (PV, wind power generator, etc.) connected to the line end When this rises, the battery (not shown) receives current from the line and is charged.

The gate pulse generator 130 generates a gate pulse (eg, a gate pulse for controlling an internal switching element of the converter) under the control of the controller 120 . For example, by increasing or decreasing the frequency of the gate pulse, the switching frequency of the switching element of the converter is increased or decreased.

For example, the gate pulse generator 130 generates and outputs a gate pulse that controls an internal switching element such as a general 2-level rectifier or 3-level NPC rectifier that receives an AC voltage and outputs a DC output, or is connected to a DC line. A gate pulse that controls the internal switch element of the converter that charges the battery can be generated and output.

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

3, when the rectifier is driven (S101), the controller 120 sets the thermal resistance model (S102).

For example, as shown in FIG. 4, if a temperature sensor is attached to the surface of the electric wire, a thermal resistance model as shown in FIG. 5 can be obtained.

At this time, each of the thermal resistances (R _cs , R _si , R _im ) is the thermal resistance (R _cs ) between the conductor temperature (T _c ) and the temperature of the sheath (T _s ), the temperature of the sheath (T _s ) and the temperature of the sheath. The thermal resistance (R _si ) between the temperature (T _i ), the thermal resistance (R _im ) between the temperature (T _i ) of the covering and the temperature (T _m ) of the temperature sensor is set. These values may be obtained from the cable manufacturer or may be obtained by experimentation.

In addition, a thermal capacitor model may be set with reference to FIG. 5 .

That is, the thermal capacitor model is modeled by a thermal capacitor by the conduction delay of heat generated from the components, and is due to each size or material, and the thermal capacitor model is connected in parallel to the thermal resistance model and can be modeled. have.

The thermal capacitor model is, as shown in FIG. 5, a thermal capacitor C _cs ) between the conductor temperature (T _c ) and the temperature of the sheath (T _s ), the temperature of the sheath (T _s ) and the temperature of the sheath (T _i ) A thermal capacitor (C _si ) between the thermal capacitor (C _im ) between the temperature of the sheath (Ti) and the temperature (T _m ) of the temperature sensor is set. At this time, like the thermal resistance, the thermal capacitor may be obtained from a cable manufacturer or obtained through an experiment.

Meanwhile, when the thermal resistance model is set as described above, the control unit 120 receives line information (S103) and calculates the line resistance (S104).

For example, the line resistance can be calculated using Equation 1 below.

)는 고유저항을 의미하며, 도전율을 C[%]라고 하면 상기 비례정수(

)는 다음과 같다.where A is the cross-sectional area of the conductor [mm ² ], l is the length of the line, and a proportional constant (

) means specific resistance, and if the conductivity is C [%], the proportional constant (

) is as follows.

(C:연동선의 경우 100, 경동선의 경우 97)

(C: 100 for interlocking copper wire, 97 for hard copper wire)

At this time, the line resistance (R) is determined by the cross-sectional area of the conductor and the line distance, but since the value does not change, it may be set once initially.

That is, when the cross-sectional area (A) of Equation 1, the line length (l) from the rectifier (not shown), the type of conductor, etc. are input at the beginning of the driving of the DC system-connected converter, the resistance value of the entire line is set, Alternatively, line resistance information may be collected and inputted from the cable manufacturer.

Meanwhile, in order to increase the accuracy in the calculation of the line resistance value, the temperature of the wire (cable) must be reflected.

Usually, the line resistance is based on 20℃, but in general, the resistance of the conductor increases as the temperature rises. Therefore, the resistance value according to the temperature of the wire can be calculated using Equation 2 below.

는 저항의 온도계수로서 기준온도에 따라 그 값이 조금씩 달라지는 것인데 일반적으로 20℃의 값을 기준으로 한다. 예컨대

의 값은 copper 0.003930, aluminum 0.004100, iron 0.005671, steel 0.003 등이다.Here, t ₀ is the reference temperature, R is the wire resistance value at the reference temperature, t is the elevated temperature,

is the temperature coefficient of resistance, and its value varies slightly depending on the reference temperature, and is generally based on a value of 20°C. for example

The values of copper are 0.003930, aluminum 0.004100, iron 0.005671, steel 0.003, etc.

Accordingly, the controller 120 receives the output current I and the line temperature of the rectifier (S105), and calculates a real-time voltage drop value (V _drop ) based thereon (S106).

For example, as described above, the real-time voltage drop value (V _drop ) is calculated using the conductor temperature value (T _c ) obtained by the line resistance (R) and the thermal resistance model, and the current sensor (CT) of the DC line (not shown) time), the output current (I) of the DC line (eg, the DC line that is converted to DC and output) is received and calculated as in Equation 3 below.

Here, the line resistance (R) changes depending on the temperature of the wire, but the current value has a dominant influence, and as the load increases, the voltage drop value (V _drop ) increases.

As described above, when the voltage drop value (V _drop ) of the DC line is calculated in real time, the controller 120 controls the droop as much as the voltage drop value (V _drop ) of the DC line calculated in real time when a voltage drop occurs in the DC line. is reflected in real time (S107).

As described above, this embodiment relates to droop control of DC microgrid, and unlike AC microgrid, droop control was not easy in conventional DC microgrid due to a relatively large amount of current and voltage drop due to line resistance.

In other words, when the droop control technique is used, two or more power converters control the DC voltage of the DC grid, and when each component controls the voltage, a difference in the voltage controlled by the line impedance occurs. A circulating current inevitably occurs. Therefore, a control method for suppressing this is required. In the technique of this embodiment, the voltage drop due to the line impedance is calculated in real time so that it can be reflected in the DC microgrid system.

In other words, the present embodiment can accurately estimate the voltage drop value occurring at the end of the line of the DC microgrid network in real time, and implement functions such as adding a control function and measuring the line conductor temperature by a thermal resistance model. A great effect can be obtained, and it also has the effect of stably implementing droop control in a DC microgrid system where the amount of load and power generation change.

As described above, the present invention has been described with reference to the embodiments shown in the drawings, but these are merely exemplary, and various modifications and equivalent other embodiments are possible therefrom by those skilled in the art to which the art pertains. will understand the point. Therefore, the technical protection scope of the present invention should be defined by the following claims.

110: voltage drop value calculation unit
120: control unit
130: gate pulse generator

Claims (1)

When the rectifier of the long-distance DC microgrid system is driven, the control unit sets the thermal resistance model;
when the thermal resistance model is set, the control unit receives line information and calculates line resistance;
receiving, by the control unit, an output current (I) and a line temperature of the rectifier, and calculating a real-time voltage drop value (V _drop ) based thereon; and
When the voltage drop value (V _drop ) of the DC line is calculated in real time, the controller reflects the voltage drop value (V _drop ) of the DC line calculated in real time in real time, and droops for the converter of the DC microgrid system including; performing control;
Further comprising; generating a gate pulse for controlling the internal switching element of the converter by the gate pulse generator under the control of the control unit;
The control unit, a proportional integral (PI) controller is used,
The control unit increases or decreases the frequency of the gate pulse by increasing or decreasing the switching frequency of the switching element of the converter,
The real-time voltage drop value (V _drop ) is,
A control method of a DC microgrid system, characterized in that calculated using Equation 3 below.
(Equation 3)

Here, I is the output current of the DC line, and R is the line resistance.

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