KR101769795B1 - Superconducting magnetic energy storage system in microgrids for eddy current losses reduction and method of controlling the same - Google Patents

Abstract

According to an embodiment of the present invention, a superconduction magnetic energy storage (SMES) system for reducing an eddy current loss in a micro-grid comprises: a superconducting coil for storing power; a power conversion device connected between the superconducting coil and an AC track of the micro-grid; an inner control unit based on model predictive control (MPC) for controlling an operation of the power conversion device; and an outer control unit for providing a command value for controlling a voltage and a frequency of the micro-grid to the inner control unit based on the MPC. The SMES system can reduce an eddy current loss generated by harmonics included in the DC current of the superconductor coil by reducing an output power, a frequency, and a ripple of DC current of the SMES system.

Description

TECHNICAL FIELD [0001] The present invention relates to a superconducting power storage system for reducing eddy current loss in a microgrid, and a control method thereof. [0002] Generally,

The present invention relates to a superconducting power storage system and a control method thereof for reducing eddy current loss in a microgrid.

As the introduction of renewable energy sources such as grid-connected wind power generators and photovoltaic power generators is increasing, researches on micro grid, which is a small power system based on renewable energy sources, are actively being carried out.

The introduction of such a micro grid not only reduces CO2 emissions, but also has many advantages such as reliability of power supply and improvement of power quality. However, as the introduction of renewable energy sources with intermittent output characteristics increases, fluctuations in output due to environmental factors can adversely affect the power system. In order to solve the problem of power quality deterioration due to such output variations, studies have been conducted using a superconducting magnetic energy storage (SMES) system. SMES system has many advantages such as unlimited charge / discharge frequency, fast response speed, high efficiency and high energy density.

In general, SMES systems can use a variety of control techniques such as PI controllers, fuzzy controllers, and adaptive control. However, PWM (Pulse Width Modulation), which is commonly used in the control technology described above, generates harmonics in the DC current in the superconducting coil, and the generated harmonics (especially the second and third harmonics) cause loss of the eddy current . These eddy currents can cause losses of up to 5% for conventional metals and up to 1% for second harmonics, but up to 20% for superconductors.

Accordingly, there is a need for a superconducting power storage system and a control method thereof that can reduce eddy current loss in a superconducting coil.

H.-M. Kim, Y. Lim, and T. Kinoshita, " An Intelligent Multiagent System for Autonomous Microgrid Operation, " Energies, vol. 5, no. 9, pp. 3347-3362, Sep. 2012. I. Ngamroo and T. Karaipoom, "Improving Low-Voltage Ride-Through Performance and Alleviating Power Fluctuations of DFIG Wind Turbine in dc Microgrid by Optimal SMES with Fault Current Limiting Function, IEEE Trans. Appl. Supercond., Vol. 24, no. 5, pp. 1051-8223, Oct. 2014, Art. ID. 5700805. M. G. Molina & E. Merado, " Power Flow Stabilization and Control of Microgrid with Wind Generation by Superconducting Magnetic Energy Storage, " IEEE Trans. Power Electron., Vol. 26, no. 3, pp. 910-922, Mar. 2011. A.-R. Kim, G.-H. Kim, S. Heo, M.-W. Park, I.-K. Yu, and H-M. Kim, " SMES Application for Frequency Control During Islanded Microgrid Operation, " Physica C, vol. 484, pp. 282-286, Jan. 2013. M. H. Ali, W. Bin, and R. A. Dougal, "An Overview of SMES Applications in Power and Energy Systems," IEEE Trans. Sustain. Energy, vol. 1, no. 1, pp. 38-47, Apr. 2010. M. H. Ali, T. Murata, and J. Tamura, "Transient Stability Enhancement Fuzzy Logic Controlled SMES Considering Coordination with Optimal Reclosing of Circuit Breakers," IEEE Trans. Power Syst., Vol. 23, no. 2, pp. 631-639, May 2008. S. C. Wang and J. X. Jin, " Design and Analysis of a Fuzzy Logic Controlled SMES System, " IEEE Trans. Appl. Supercond., Vol. 24, no. 5, Oct. 2014, Art. ID. 5701205. H. M. Hasanien, " A Set-Membership Affine Projection Algorithm-Based Adaptive-Controlled SMES Units for Wind Farms Output Power Smoothing, IEEE Trans. Sustain. Energy, vol. 5, no. 4, pp. 1226-1233, Oct. 2014. V. Sokolovsky, V. Meerovich, M. Spektor, G. A. Levin, and I. Vajda, "Losses in Superconductors under Non-Sinusoidal Currents and Magnetic Fields," IEEE Trans. Appl. Supercond., Vol. 19, no. 3, June 2009. T.-T. Nguyen, H.-J. Yoo, and H.-M. Kim, " Application of Model Predictive Control to BESS for Microgrid Control, " Energies, vol. 8, no. 8, pp. 8798-8813, Aug. 2015. J. Rodriguez, J. Pontt, C. A. Silva, P. Correa, P. Lezana, P. Cortes, and U. Ammann, "Predictive Current Control of a Voltage Source Inverter," IEEE Trans. Ind. Electron., Vol. 54, no. 1, pp. 495-503, Feb. 2007. J. Rodriguez and P. Cortes, Predictive Control of Power Converters and Electrical Drives, 1st ed. Chichester, U. K .: IEEE press-Wiley, Mar. 2012, pp. 147-161.

SUMMARY OF THE INVENTION A problem to be solved by the present invention is to reduce eddy current loss in a microgrid that can reduce ripple of output power, frequency, dc current, and reduce eddy current loss caused by each harmonic included in the dc current of a superconducting coil And to provide a superconducting power storage system.

Another problem to be solved by the present invention is to reduce the ripple of the output power, the frequency, the dc current, and the eddy current loss in the microgrid, which can reduce the eddy current loss caused by each harmonic included in the dc current of the superconducting coil And to provide a method of controlling the superconducting power storage system.

According to an aspect of the present invention, there is provided a superconducting power storage system for reducing eddy current loss in a micro grid,

Superconducting coils for power storage;

A power converter connected between the superconducting coil and the AC line of the microgrid;

A model predictive control (MPC) based internal controller for controlling the operation of the power converter; And

And an external controller for providing an instruction value for controlling the frequency and voltage of the microgrid to the model predictive control based internal controller.

In a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention,

A voltage-type converter (VSC) connected to an AC line of the microgrid; And

And a dc-dc chopper connecting the superconducting coil and the voltage converter.

In addition, in the superconducting power storage system for reducing the eddy current loss in the micro grid according to an embodiment of the present invention, the model predictive control (MPC)

A VSC switching state determiner for determining an optimal VSC switching state among all the number of switching states associated with the plurality of switching elements included in the voltage converter;

A dc-dc chopper switching state that determines an optimal dc-dc chopper switching state among all the number of switching states associated with the plurality of switching elements included in the dc-dc chopper based on the determined optimum VSC switching state A decision unit; And

Dc chopper switching control signal in accordance with the determined optimal dc-dc chopper switching state is provided to the dc-dc chopper by providing a VSC switching control signal according to the determined optimal VSC switching state to the voltage-type converter And a control signal output unit.

Further, in the superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, the VSC switching state determiner may include:

A VSC current predicting unit for predicting output currents of the voltage-type converter in accordance with all the switching states of the plurality of switching elements included in the voltage-type converter; And

And a first cost function minimizing unit that determines a VSC switching state corresponding to a VSC output current having the smallest error from the predicted VSC output currents from the command value provided by the external control unit as the optimal VSC switching state .

In a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention,

Further comprising a DC link capacitor connected to a DC link which is an interconnection point of the dc-dc chopper and the VSC,

The dc-dc chopper switching state determination unit determines,

A DC link capacitor voltage predicting unit for predicting voltages of the DC link capacitor according to the switching states of all the cases of the plurality of switching elements included in the dc-dc chopper based on the determined optimum VSC switching state; And

Dc chopper switching state corresponding to the DC link capacitor voltage having the smallest error from the target voltage command value of the DC link capacitor among the predicted DC link capacitor voltages is determined as the optimal dc- Cost function minimization unit.

Further, in the superconducting power storage system for reducing the eddy current loss in the micro grid according to an embodiment of the present invention, the VSC current predicting unit may include:

에 기반하여 상기 전압형 컨버터의 출력 전류들을 예측하고,

Based on the output currents of the voltage-type converters,

는 시간 k에서의 AC 전압,

는 시간 k에서의 VSC 의 출력 전압, T_s는 샘플링 시간, R 및 L은 각각 VSC의 출력과 AC 선로 사이의 필터의 저항과 인덕턴스,

는 시간 k에서의 VSC의 전류를 나타낼 수 있다.In the above,

Is the AC voltage at time k,

Is the output voltage of the VSC at time k, T _s is the sampling time, R and L are the resistance and inductance of the filter between the output of VSC and the AC line,

Can represent the current of VSC at time k.

In the superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, the DC link capacitor voltage predicting unit may include:

에 기반하여 상기 DC 링크 커패시터의 전압들을 예측하고, 상기에서.

이며, u_C(k+1)은 시간 (k+1)에서 예상되는 상기 DC 링크 커패시터의 전압, i_C(k)과 V_dc(k)는 각각 시간 k에서 측정되는 상기 DC 링크 커패시터의 전류와 전압, i_C(k+1)은 시간 (k+1)에서의 상기 DC 링크 커패시터의 전류, C는 상기 DC 링크 커패시터의 커패시턴스를 나타낼 수 있다.

And estimates the voltages of the DC link capacitors based on the voltages of the DC link capacitors.

And, u _C (k + 1) is the voltage of the DC link capacitor which is estimated at time _{(k + 1), i C} (k) and V _dc (k) is the current of the DC link capacitor is measured at each time k And the voltage, i _C (k + 1), may be the current of the DC link capacitor at time (k + 1), and C may be the capacitance of the DC link capacitor.

Further, in the superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, the first cost function minimizing unit may include:

에 기반하여 상기 예측된 VSC 출력 전류들과 상기 외부 제어부에서 제공되는 지령치와의 오차와 관련된 제1 비용함수들을 계산하고,

Calculates first cost functions related to an error between the predicted VSC output currents and an instruction value provided from the external control unit,

Determining a VSC switching state corresponding to a VSC output current in the case of a minimum of the first cost functions as the optimal VSC switching state,

와

는 각각 상기 외부 제어부에서 제공하는 상기 VSC 의 유효 전류 지령치 및 무효 전류 지령치를 나타내고,

와

는 각각 시간 (k+1)에서 예상되는 상기 VSC의 무효 전류 및 유효 전류를 나타낼 수 있다.In the above,

Wow

Respectively indicate the effective current command value and the reactive current command value of the VSC provided by the external control unit,

Wow

May represent the reactive current and the effective current of the VSC expected at time (k + 1), respectively.

Also, in the superconducting power storage system for reducing the eddy current loss in the micro grid according to an embodiment of the present invention, the second cost function minimizing unit may include:

에 기반하여 상기 예측된 DC 링크 커패시터 전압들과 상기 DC 링크 커패시터의 목표 전압 지령치와의 오차와 관련된 제2 비용함수들을 계산하고,

Calculating second cost functions related to an error between the predicted DC link capacitor voltages and a target voltage command value of the DC link capacitor based on the second cost functions,

Determining a dc-dc chopper switching state corresponding to a DC link capacitor voltage in a case of a minimum of the second cost functions as an optimal dc-dc chopper switching state,

는 상기 DC 링크 커패시터의 목표 전압 지령치를 나타내고,

는 시간 (k+2)에서 상기 DC 링크 커패시터의 예상되는 전압을 나타낼 수 있다.In the above,

Represents a target voltage command value of the DC link capacitor,

May represent the expected voltage of the DC link capacitor at time (k + 2).

According to another aspect of the present invention, there is provided a method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid,

A power conversion device connected between the superconducting coil and an AC line of the micro grid, a model predictive control (MPC) based internal control for controlling the operation of the power conversion device, And an external control unit for providing an instruction value for controlling the frequency and voltage of the microgrid to a predictive control based internal control unit, wherein the power conversion apparatus includes a voltage converter (VSC) connected to an AC line of the microgrid, A method of controlling a superconducting magnetic energy storage (SMES) system for reducing eddy current loss in a microgrid, comprising a dc-dc chopper coupling a coil and the voltage converter,

(a) determining, in the model predictive control (MPC) based internal control unit, an optimum VSC switching state among all the number of switching states associated with the plurality of switching elements included in the voltage-type converter ;

(b) determining an optimum dc-dc chopper switching state among all the number of switching states associated with the plurality of switching elements included in the dc-dc chopper based on the determined optimum VSC switching state; And

(c) providing a VSC switching control signal according to the determined optimal VSC switching state to the voltage-type converter, and supplying a dc-dc chopper switching control signal according to the determined optimal dc-dc chopper switching state to the dc- .

In the method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, the step (a)

(a-1) predicting output currents of the voltage-type converter in accordance with all the number of switching states associated with the plurality of switching elements included in the voltage-type converter; And

(a-2) determining the VSC switching state corresponding to the VSC output current having the smallest error from the predicted VSC output currents from the command value provided from the external control section as the optimal VSC switching state have.

In the control method of a superconducting power storage system for reducing an eddy current loss in a micro grid according to an embodiment of the present invention, the superconducting power storage system may include a dc-dc chopper and a DC link And a DC link capacitor connected to the DC link capacitor,

The step (b)

(b-1) estimating voltages of the DC link capacitor according to the switching states of all the cases of the plurality of switching elements included in the dc-dc chopper based on the determined optimal VSC switching state; And

(b-2) the dc-dc chopper switching state corresponding to the DC link capacitor voltage having the smallest error from the predicted DC link capacitor voltages with respect to the target voltage command value of the DC link capacitor is referred to as the optimal dc-dc chopper switching state As shown in FIG.

In the method of controlling a superconducting power storage system for reducing an eddy current loss in a micro grid according to an embodiment of the present invention, in the step (a-1), output currents of the voltage-

에 기반하여 예측되고,

Lt; / RTI >

는 시간 k에서의 AC 전압,

는 시간 k에서의 VSC 의 출력 전압, T_s는 샘플링 시간, R 및 L은 각각 VSC의 출력과 AC 선로 사이의 필터의 저항과 인덕턴스,

는 시간 k에서의 VSC의 전류를 나타낼 수 있다.In the above,

Is the AC voltage at time k,

Is the output voltage of the VSC at time k, T _s is the sampling time, R and L are the resistance and inductance of the filter between the output of VSC and the AC line,

Can represent the current of VSC at time k.

In the method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, the voltages of the DC link capacitors in (b-1)

에 기반하여 예측되고, 상기에서.

이며, u_C(k+1)은 시간 (k+1)에서 예상되는 상기 DC 링크 커패시터의 전압, i_C(k)과 V_dc(k)는 각각 시간 k에서 측정되는 상기 DC 링크 커패시터의 전류와 전압, i_C(k+1)은 시간 (k+1)에서 예상되는 상기 DC 링크 커패시터의 전류, C는 상기 DC 링크 커패시터의 커패시턴스를 나타낼 수 있다.

And is predicted based on the above.

And, u _C (k + 1) is the voltage of the DC link capacitor which is estimated at time _{(k + 1), i C} (k) and V _dc (k) is the current of the DC link capacitor is measured at each time k And the voltage, i _C (k + 1), may be the current of the DC link capacitor at time (k + 1), and C may be the capacitance of the DC link capacitor.

In the method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, in the step (a-2)

에 기반하여 상기 예측된 VSC 출력 전류들과 상기 외부 제어부에서 제공되는 지령치와의 오차와 관련된 제1 비용함수들을 계산하고,

Calculates first cost functions related to an error between the predicted VSC output currents and an instruction value provided from the external control unit,

Determining a VSC switching state corresponding to a VSC output current in the case of a minimum of the first cost functions as the optimal VSC switching state,

와

는 각각 상기 외부 제어부에서 제공하는 상기 VSC 의 유효 전류 지령치 및 무효 전류 지령치를 나타내고,

와

는 각각 시간 (k+1)에서 예상되는 상기 VSC의 무효 전류 및 유효 전류를 나타낼 수 있다.In the above,

Wow

Respectively indicate the effective current command value and the reactive current command value of the VSC provided by the external control unit,

Wow

May represent the reactive current and the effective current of the VSC expected at time (k + 1), respectively.

In the method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, in the step (b-2)

에 기반하여 상기 예측된 DC 링크 커패시터 전압들과 상기 DC 링크 커패시터의 목표 전압 지령치와의 오차와 관련된 제2 비용함수들을 계산하고,

Calculating second cost functions related to an error between the predicted DC link capacitor voltages and a target voltage command value of the DC link capacitor based on the second cost functions,

Determining a dc-dc chopper switching state corresponding to a DC link capacitor voltage in a case of a minimum of the second cost functions as an optimal dc-dc chopper switching state,

는 상기 DC 링크 커패시터의 목표 전압 지령치를 나타내고,

는 시간 (k+2)에서 상기 DC 링크 커패시터의 예상되는 전압을 나타낼 수 있다.In the above,

Represents a target voltage command value of the DC link capacitor,

May represent the expected voltage of the DC link capacitor at time (k + 2).

According to the superconducting power storage system and the control method thereof for reducing the eddy current loss in the micro grid according to an embodiment of the present invention, the ripple of the output power, frequency, and dc current of the SMES system is reduced to be included in the dc current of the superconducting coil It is possible to reduce the eddy current loss caused by the respective harmonics.

FIG. 1 illustrates a superconducting power storage system for reducing eddy current losses in a microgrid according to an embodiment of the present invention. FIG.
2 is a flowchart illustrating a method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention.
3 is a diagram illustrating a structure of a microgrid to which a superconducting power storage system for reducing eddy current loss is applied in a microgrid according to an embodiment of the present invention.
4 is a graph showing the output power of each component in the microgrid.
5 is a graph showing the frequency of the microgrid.
6 is a graph showing the voltage of the microgrid.
7 is a graph showing the DC current of the superconducting coil.
8 is a graph showing harmonics included in the DC current for a superconducting coil.
9 is a graph showing a decrease in eddy current loss compared to PI.

BRIEF DESCRIPTION OF THE DRAWINGS The objectives, specific advantages and novel features of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG.

Prior to that, terms and words used in the present specification and claims should not be construed in a conventional and dictionary sense, and the inventor may properly define the concept of the term in order to best explain its invention Should be construed in accordance with the principles and the meanings and concepts consistent with the technical idea of the present invention.

It should be noted that, in the present specification, the reference numerals are added to the constituent elements of the drawings, and the same constituent elements are assigned the same number as much as possible even if they are displayed on different drawings.

Also, the terms "first", "second", "one side", "other side", etc. are used to distinguish one element from another, It is not.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description of the present invention, a detailed description of known arts which may unnecessarily obscure the gist of the present invention will be omitted.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

A superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention includes a superconducting coil 101 for storing electric power, an AC line 101 connected between the superconducting coil 101 and an AC line 103 of a micro grid A model predictive control (MPC) based internal control unit 104 for controlling operations of the power conversion apparatuses 100 and 102, and a model predictive control based internal control unit 104 to provide an instruction value for controlling the frequency and voltage of the microgrid.

The power converters 100 and 102 that control power between the micro-grids include the VSC-based PCS 102 and the VSC-based PCS 102 and the superconducting And a dc-dc chopper 100 that coils the coil 101.

The model predictive control (MPC) -based internal control unit 104 calculates an optimum value of the switching states of all the switching states associated with the plurality of switching elements 134a to 134f included in the voltage-type converter 102 A VSC switching state determination unit 109 for determining a VSC switching state of the dc-dc chopper 100 based on the determined optimum VSC switching state, A dc-dc chopper switching state determination unit 113 for determining an optimal dc-dc chopper switching state among the switching states of the number of switching elements, and a dc-dc chopper switching state determination unit 113 for determining a VSC switching control signal S_opt according to the determined optimal VSC switching state, Dc chopper switching control signal G_opt according to the determined optimum dc-dc chopper switching state to the dc-dc chopper 100. The control signal output unit 116 ).

The VSC switching state determination unit 109 determines the VSC switching state of the voltage converter 102 based on the switching states of all the cases related to the plurality of switching elements 134a to 134f included in the voltage converter 102. [ And a VSC switching state corresponding to the VSC output current having the smallest error between the predicted VSC output currents and an instruction value provided from the external control unit 106, And a first cost function minimizing unit 110 for determining the optimal VSC switching state.

)와의 오차가 가장 작은 DC 링크 커패시터(105)의 전압에 대응하는 dc-dc 초퍼 스위칭 상태를 최적의 dc-dc 초퍼 스위칭 상태로 결정하는 제2 비용함수 최소화부(114)를 포함한다.The superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention includes a dc-dc chopper 100 and a DC link 100 connected to a DC link, The dc-dc chopper switching state determination unit 113 further includes a capacitor 105. The dc-dc chopper switching state determination unit 113 determines the switching state of the plurality of switching elements 136a , 136b) for estimating the voltages of the DC link capacitor (105) according to the number of switching states of all the voltages of the DC link capacitor (105) The target voltage command value (DC) of the DC link capacitor 105

Dc chopper switching state corresponding to the voltage of the DC link capacitor 105 having the smallest error between the dc-dc chopper switching state and the second dc-dc chopper switching state.

The external control unit 106 provides an instruction value for controlling the frequency and the voltage of the microgrid to the model predictive control based internal control unit 104. The external control unit 106 receives an active power reference value / And a plurality of components 122, 124, 126, 128, 130, 132 for performing PI (proportional-integral) based control in accordance with the reactive power reference value / voltage reference value.

Model predictive control (MPC) techniques do not use pulse width modulation (PWM), which reduces harmonic ripple of control variables such as output voltage and current.

Therefore, in the superconducting power storage system for reducing the eddy current loss in the micro grid according to an embodiment of the present invention, an MPC technique that can be applied to the power conversion apparatuses 100 and 102 of the SMES system is proposed.

In order to examine the effect of SMES control based on MPC, we compared the performance of SMES based on PI controller and analyzed the harmonics of DC current generated in MPC based SMES and DC current generated in PI controller based SMES Eddy current losses were compared.

MPC  Based superconducting power storage system modelling

A superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention is a VSC-based SMES system, which includes a 2-level VSC 102 and a dc-dc chopper, (100).

The MPC technique is applied to the VSC 102 and the dc-dc chopper 100 in a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention. Further, the current of the VSC 102 is predicted by predictive current control, and the current is controlled according to the voltage of the DC link.

로 표시되고 수학식 1과 같다.The dynamic current of VSC 102 is

And is expressed by Equation (1).

는 AC 시스템 전압, R 및 L은 각각 VSC(102)의 출력과 AC 선로(103) 사이의 필터의 저항과 인덕턴스,

는 VSC(102)의 출력 전압, 그리고 x는 VSC(102)의 출력 전압 개수를 나타낸다.In the above,

R and L denote the resistance and inductance of the filter between the output of the VSC 102 and the AC line 103, respectively,

Is the output voltage of the VSC 102, and x is the number of output voltages of the VSC 102.

A discrete-time model with a sampling time T _s is used to predict the current of the VSC 102 and can be expressed as di / dt according to the Euler approximation principle as shown in equation (4).

는 시간 k에서의 VSC(102)의 전류를 나타내고,

은 시간 (k+1)에서의 예측되는 전류를 나타낸다.In the above,

Represents the current of VSC 102 at time k,

Represents the predicted current at time (k + 1).

Thus, at time k, the VSC current and voltage are used to predict the future VSC current. The current of the VSC 102 using Equations (1) to (4) can be expressed by Equation (5).

는 시간 k에서의 AC 전압,

는 시간 k에서의 VSC(102)의 출력 전압, T_s는 샘플링 시간, R 및 L은 각각 VSC(102)의 출력과 AC 선로 사이의 필터의 저항과 인덕턴스,

는 시간 k에서의 VSC(102)의 전류를 나타낸다.In the above,

Is the AC voltage at time k,

T _s is the sampling time, R and L are the resistance and inductance of the filter between the output of VSC 102 and the AC line, respectively,

Represents the current of VSC 102 at time k.

The DC link voltage is controlled based on the expected VSC current and SMES current in a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention. The SMES current in the discrete time model is shown in Equation (6).

(K) represents the current of the SMES coil 101 at time k, and V _S (k) represents the inductance of the possible dc-dc chopper 101 at time k, where L _S represents the inductance of the SMES coil 101, i _L (100). The output voltage vector of the dc-dc chopper 100 can be expressed as shown in Table 1.

At time (k + 1), the current of the capacitor 105 is expressed by Equation (7).

S1, S2, S3 and G1 denote the switching state (VSC 102, dc-dc chopper 100).

The voltage of the capacitor 105 is expressed by Equation (11).

Where C represents the capacitance of the capacitor 105 of the DC link.

The predicted DC voltage of the capacitor 105 can be obtained based on the dynamic dynamic voltage of the capacitor 105. The Euler approximation is used to replace the capacitor voltage du _C / dt and can be calculated as: < EMI ID = 12.0 >

In the above, u _C (k + 1) is the voltage of the DC link capacitor 105 expected at time (k + 1), i _C (k) and V _dc (k) The current and voltage of the link capacitor 105, i _C (k + 1) represents the current of the DC link capacitor 105 expected at time (k + 1), and C represents the capacitance of the DC link capacitor 105 .

The expected current of the VSC 102 and the expected dc voltage of the DC link capacitor 105 can be obtained based on Equations (5) and (12), respectively, and the first cost function (g ₁ ) second cost function (g ₂₎ is equal to the equation (14) and equation (15).

The variables included in the cost function are as follows.

와

는 각각 상기 외부 제어부에서 제공하는 상기 VSC (102)의 유효 전류 지령치 및 무효 전류 지령치를 나타내고,

와

는 각각 시간 (k+1)에서 예상되는 상기 VSC(102)의 무효 전류 및 유효 전류를 나타낸다.In the above,

Wow

Respectively indicate an effective current command value and a reactive current command value of the VSC 102 provided by the external control unit,

Wow

Represents the reactive current and the effective current of the VSC 102 expected at time (k + 1), respectively.

는 상기 DC 링크 커패시터의 목표 전압 지령치를 나타내고,

는 시간 (k+2)에서 상기 DC 링크 커패시터의 예상되는 전압을 나타낸다.In addition,

Represents a target voltage command value of the DC link capacitor,

Represents the expected voltage of the DC link capacitor at time (k + 2).

The number of cases of the cost function is equal to the number of cases of the switching state. For the three-phase two-level VSC 102, the number of possible switching states (N ₁ = 2 ³ = 8) and the number of cases of dc-dc chopper 100 (N ₂ = 2 ² = 4) have 8 and 4 switching states, respectively. Therefore, although a total number of 32 switching states occurs, the MPC proposed in the superconducting power storage system for reducing the eddy current loss in the micro grid according to an embodiment of the present invention reduces the number of possible switching states to 12 switching states have.

After the VSC optimal switching state for controlling the VSC current is obtained, the optimal switching state of the dc-dc chopper 100 corresponding to the DC link voltage is obtained, and the obtained optimum switching state is controlled by the control signal output section 116 ) To the VSC 102 and the dc-dc chopper 100, respectively.

In a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, a switching signal is directly input to the VSC 102 and the dc-dc chopper 100 to control the SMES system based on MPC PWM is not used.

FIG. 2 is a flowchart illustrating a method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention. Referring to FIG. 2, MPC SMES system control algorithm.

)와 AC 선로(103)의 전압(

)이 측정되고(단계 S200_1), 시간 k에서의 초전도 코일(101)의 전류(i_L(k)), 커패시터(105)의 전류(i_C(k)), 및 커패시터(105)의 전압(V_dc(k))이 측정된다(단계 S200_2).Referring to FIG. 2, in a method of controlling a superconducting magnetic energy storage (SMES) system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, (

And the voltage of the AC line 103

Is measured (step S200_1), the current i _L (k) of the superconducting coil 101 at time k, the current i _C (k) of the capacitor 105, and the voltage of the capacitor 105 V _dc (k)) is measured (step S200_2).

In the model predictive control (MPC) -based internal control unit 104, an optimum value of the number of switching states in all cases related to the plurality of switching elements 134a to 134f included in the voltage-type converter 102 (Steps S202, S204, S206, S208, S210, S212, and S214).

In the model predictive control (MPC) based internal control unit 104, a plurality of switching elements 136a and 136b included in the dc-dc chopper 100 based on the determined optimal VSC switching state, (Step S216, S218, S220, S222, S224, S226, S228, S230, S232) among the switching states of all the related cases.

In the model predictive control (MPC) based internal control unit 104, the VSC switching control signal according to the determined optimal VSC switching state is provided to the voltage-type converter 102, and the determined optimal dc- dc chopper switching control signal corresponding to the dc chopper switching state to the dc-dc chopper 100 (step S234).

In a control method of a superconducting magnetic energy storage (SMES) system for reducing eddy current loss in a micro grid according to an embodiment of the present invention shown in FIG. 2, step S206 is performed based on Equation (5) k + 1), step S208 is a step of estimating a first cost function g1 based on equation (14), and step S210 is a step of storing an optimum value .

In the control method of a superconducting magnetic energy storage (SMES) system for reducing eddy current loss in a micro grid according to an embodiment of the present invention shown in FIG. 2, step S216 is performed in Equations (7) to (K + 1), and step S224 is a step of calculating the current of the capacitor 105 predicted at time (k + 2) based on equations (12) and And calculating the voltage. Also, step S226 is a step of estimating a second cost function g2 based on equation (15), and step S228 is a step of storing an optimum value.

The method of controlling a superconducting magnetic energy storage (SMES) system for reducing eddy current loss in a micro grid according to an embodiment of the present invention shown in FIG. The present invention relates to a method of controlling a superconducting power storage system for reducing eddy current loss in a micro grid, and detailed steps have been described in detail with reference to FIG. 1, and detailed description of each step will be omitted.

In Micro Grid MPC  Based SMES  system

In one embodiment of the present invention, the microgrid comprises a wind power generator (WG), a load, a diesel generator (DG), and a superconducting power storage system (SMES) as shown in FIG. The SMES system performs the frequency and voltage control of the microgrid, which consists of an external control unit using a PI controller. That is, the entire SMES control system consists of MPC-based current controller and PI controller-based external controller.

The microgrid operates in two operating modes: grid-connected mode and islanded mode. In the grid-connected mode, the microgrid is maintained in frequency and voltage by a utility grid, so the SMES is a constant power (point of common coupling) point at the point of common coupling (PCC) that links the microgrid and utility grid flow, and when the microgrid is switched to the stand-alone operation mode, the SMES performs frequency and voltage control of the microgrid.

Simulation result

In order to examine the control performance of the MPC-based SMES system, the PI-based SMES system and control performance are compared in a superconducting power storage system and a control method thereof for reducing eddy current loss in a micro grid according to an embodiment of the present invention. In order to compare the control performance effectively, the gain value of the PI controller was used in the method proposed in the non-patent document [10].

Initially, the microgrid operates in a grid-connected mode operating in conjunction with the grid, and in a 5-second stand-alone mode of operation.

In the grid-connected mode, the algae control at the PCC point is controlled for efficient charging and discharging of the SMES system, the load of the micro grid is increased (50 kW) in 3 seconds, and the power flow at the PCC point is assumed to be 0 kW .

The power of each element of the microgrid in the grid-connected mode is as shown in FIG. 4, and it is confirmed that the power flow at the PCC point is maintained constant due to the charge / discharge of the SMES despite the output fluctuation of the wind power generator .

5 and 6, the frequency and voltage of the microgrid in the independent operation mode are controlled by the SMES system. In addition, the load of 50kW is reduced in 8 seconds. SMES can control the output power amount according to the load reduction, so that the frequency and the voltage of the micro grid are always controlled constantly.

The MPC-based SMES system and the PI controller-based SMES system use the PI controller in the external controller for power and frequency control, so that the dynamic response is similar. However, the MPC-based SMES system performs the current control based on the MPC, so that the ripple of the output voltage is reduced.

Harmonics included in DC current in superconducting coils are analyzed to compare eddy current loss of superconducting coils according to control techniques. In the superconducting power storage system for reducing the eddy current loss in the micro grid according to an embodiment of the present invention, the parameters such as the internal resistance of the SMES are set the same for the objective test of the SMES system according to the control technique.

The eddy current loss of a superconducting coil is proportional to the magnitude of the DC current harmonics and the internal resistance, so the eddy current loss of the superconducting coil can be measured according to the magnitude of the harmonics included in the DC current.

The DC current of the superconducting coil is shown in FIG. 7, and the harmonic magnitude of the DC current flowing in the superconducting coil based on the PI controller and MPC is as shown in FIG. The switching frequency by the PI controller and the switching frequency by the MPC are similar, but the 5th harmonic in the SMES system using the MPC technique is remarkably reduced. 9 shows the eddy current loss when compared with the case using the PI controller.

As a result, by using the MPC technique, the eddy current loss caused by the second harmonic can be reduced by about 5%, and the eddy current loss caused by the third harmonic can be reduced by about 30%. It can be seen that the eddy current loss caused by the higher harmonics is considerably reduced compared with the PI control technique.

conclusion

In a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, an MPC-based SMES system applied simultaneously to a VSC and a dc-dc chopper in a microgrid has been proposed. In order to investigate the effect of the proposed control method in a superconducting power storage system for reducing eddy current loss in a micro grid according to an embodiment of the present invention, the control effect was examined by comparing the performance with the PI-based control technique .

Simulation results show that the dynamic responses of SMES system based on MPC and SMES system based on PI control scheme are similar. However, in the case of the MPC-based SMES system, it was confirmed that the ripple of the output power, frequency, and dc current is reduced as compared with the PI controller.

For this reason, the eddy current loss caused by each harmonic included in the dc current of the superconducting coil can be reduced by using MPC rather than the PI controller. The MPC control technique has the advantage that the eddy current loss can be reduced without changing the SMES specification and structure, and the difficulty of determining the control gain value can be avoided.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. It is clear that the present invention can be modified or improved.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

100: dc-dc chopper 101: superconducting coil
102: Voltage type converter (VSC) 103: AC
104: Model predictive control based internal controller 105: Capacitor
106: PI-based external control unit 108: VSC current predicting unit
110: First cost function minimization part 112: Capacitor voltage predicting part
114: second cost function minimization unit 116: control signal output unit
118, 120, 124: conversion section 122: PLL
126, 128: PI 130, 132:

Claims (16)

Superconducting coils for power storage;
A power converter connected between the superconducting coil and the AC line of the microgrid;
A model predictive control (MPC) based internal controller for controlling the operation of the power converter; And
And an external control unit for providing an instruction value for controlling the frequency and voltage of the microgrid to the model predictive control based internal control unit,
The power conversion apparatus includes:
A voltage-type converter (VSC) connected to an AC line of the microgrid; And
And a dc-dc chopper connecting the superconducting coil and the voltage converter,
Wherein the model predictive control (MPC) based internal control unit comprises:
A VSC switching state determiner for determining an optimal VSC switching state among all the number of switching states associated with the plurality of switching elements included in the voltage converter;
A dc-dc chopper switching state that determines an optimal dc-dc chopper switching state among all the number of switching states associated with the plurality of switching elements included in the dc-dc chopper based on the determined optimum VSC switching state A decision unit; And
Dc chopper switching control signal in accordance with the determined optimal dc-dc chopper switching state is provided to the dc-dc chopper by providing a VSC switching control signal according to the determined optimal VSC switching state to the voltage-type converter A superconducting magnetic energy storage (SMES) system for reducing eddy current losses in a microgrid, including a control signal output. delete delete The method according to claim 1,
The VSC switching state determination unit determines,
A VSC current predicting unit for predicting output currents of the voltage-type converter in accordance with all the switching states of the plurality of switching elements included in the voltage-type converter; And
And a first cost function minimizing unit for determining a VSC switching state corresponding to a VSC output current having the smallest error from the predicted VSC output currents from an instruction value provided from the external control unit, as the optimal VSC switching state. A superconducting power storage system for reducing eddy current loss in a grid. The method of claim 4,
Further comprising a DC link capacitor connected to a DC link which is an interconnection point of the dc-dc chopper and the VSC,
The dc-dc chopper switching state determination unit determines,
A DC link capacitor voltage predicting unit for predicting voltages of the DC link capacitor according to the switching states of all the cases of the plurality of switching elements included in the dc-dc chopper based on the determined optimum VSC switching state; And
Dc chopper switching state corresponding to the DC link capacitor voltage having the smallest error from the target voltage command value of the DC link capacitor among the predicted DC link capacitor voltages is determined as the optimal dc- A superconducting power storage system for reducing eddy current losses in a microgrid comprising a cost function minimization unit. The method of claim 5,
The VSC current predicting unit may include:

Based on the output currents of the voltage-type converters,
In the above,

Is the AC voltage at time k,

Is the output voltage of the VSC at time k, T _s is the sampling time, R and L are the resistance and inductance of the filter between the output of VSC and the AC line,

Is a superconducting power storage system for reducing eddy current losses in microgrid, representing the current of VSC at time k. The method of claim 6,
Wherein the DC link capacitor voltage predicting unit comprises:

And estimates the voltages of the DC link capacitors based on the voltages of the DC link capacitors.

And, u _C (k + 1) is the voltage of the DC link capacitor which is estimated at time _{(k + 1), i C} (k) and V _dc (k) is the current of the DC link capacitor is measured at each time k And the voltage, i _C (k + 1), represents the current of the DC link capacitor at time (k + 1), and C represents the capacitance of the DC link capacitor. The method of claim 7,
Wherein the first cost function minimization unit comprises:

Calculates first cost functions related to an error between the predicted VSC output currents and an instruction value provided from the external control unit,
Determining a VSC switching state corresponding to a VSC output current in the case of a minimum of the first cost functions as the optimal VSC switching state,
In the above,

Wow

Respectively indicate the effective current command value and the reactive current command value of the VSC provided by the external control unit,

Wow

Represents a reactive current and an effective current of the VSC expected at a time (k + 1), respectively, for a superconducting power storage system for reducing eddy current losses in a microgrid. The method of claim 8,
Wherein the second cost function minimization unit comprises:

Calculating second cost functions related to an error between the predicted DC link capacitor voltages and a target voltage command value of the DC link capacitor based on the second cost functions,
Determining a dc-dc chopper switching state corresponding to a DC link capacitor voltage in a case of a minimum of the second cost functions as an optimal dc-dc chopper switching state,
In the above,

Represents a target voltage command value of the DC link capacitor,

Represents the expected voltage of the DC link capacitor at time (k + 2). ≪ Desc / Clms Page number 13 > A power conversion device connected between the superconducting coil and an AC line of the micro grid, a model predictive control (MPC) based internal control for controlling the operation of the power conversion device, And an external control unit for providing an instruction value for controlling the frequency and voltage of the microgrid to a predictive control based internal control unit, wherein the power conversion apparatus includes a voltage converter (VSC) connected to an AC line of the microgrid, A method of controlling a superconducting magnetic energy storage (SMES) system for reducing eddy current loss in a microgrid, comprising a dc-dc chopper coupling a coil and the voltage converter,
(a) determining, in the model predictive control (MPC) based internal control unit, an optimum VSC switching state among all the number of switching states associated with the plurality of switching elements included in the voltage-type converter ;
(b) determining an optimum dc-dc chopper switching state among all the number of switching states associated with the plurality of switching elements included in the dc-dc chopper based on the determined optimum VSC switching state; And
(c) providing a VSC switching control signal according to the determined optimal VSC switching state to the voltage-type converter, and supplying a dc-dc chopper switching control signal according to the determined optimal dc-dc chopper switching state to the dc- To < RTI ID = 0.0 >
The step (a)
(a-1) predicting output currents of the voltage-type converter in accordance with all the number of switching states associated with the plurality of switching elements included in the voltage-type converter; And
(a-2) determining, from the predicted VSC output currents, the VSC switching state corresponding to the VSC output current having the smallest error with the command value provided from the external control section, as the optimal VSC switching state. Control Method of Superconducting Power Storage System for Reducing Eddy Current Loss in Micro Grid. delete The method of claim 10,
Wherein the superconducting power storage system further comprises a DC link capacitor connected to a DC link which is an interconnection point of the dc-dc chopper and the VSC,
The step (b)
(b-1) estimating voltages of the DC link capacitor according to the switching states of all the cases of the plurality of switching elements included in the dc-dc chopper based on the determined optimal VSC switching state; And
(b-2) the dc-dc chopper switching state corresponding to the DC link capacitor voltage having the smallest error from the predicted DC link capacitor voltages with respect to the target voltage command value of the DC link capacitor is referred to as the optimal dc-dc chopper switching state , The method comprising the steps of: < RTI ID = 0.0 > determining < / RTI > The method of claim 12,
In the step (a-1), the output currents of the voltage-

Lt; / RTI >
In the above,

Is the AC voltage at time k,

Is the output voltage of the VSC at time k, T _s is the sampling time, R and L are the resistance and inductance of the filter between the output of VSC and the AC line,

/ RTI > of the superconducting power storage system for the reduction of the eddy current loss in the microgrid, which represents the current of the VSC at time k. 14. The method of claim 13,
The voltages of the DC link capacitor in (b-1)

And is predicted based on the above.

And, u _C (k + 1) is the voltage of the DC link capacitor which is estimated at time _{(k + 1), i C} (k) and V _dc (k) is the current of the DC link capacitor is measured at each time k (K + 1) is the current of the DC link capacitor at time (k + 1), C is the capacitance of the DC link capacitor, i _C (k + / RTI > 15. The method of claim 14,
In the above step (a-2)

Calculates first cost functions related to an error between the predicted VSC output currents and an instruction value provided from the external control unit,
Determining a VSC switching state corresponding to a VSC output current in the case of a minimum of the first cost functions as the optimal VSC switching state,
In the above,

Wow

Respectively indicate the effective current command value and the reactive current command value of the VSC provided by the external control unit,

Wow

(K + 1) represents the reactive current and the effective current of the VSC, respectively, at time (k + 1). 16. The method of claim 15,
In the step (b-2)

Calculating second cost functions related to an error between the predicted DC link capacitor voltages and a target voltage command value of the DC link capacitor based on the second cost functions,
Determining a dc-dc chopper switching state corresponding to a DC link capacitor voltage in a case of a minimum of the second cost functions as an optimal dc-dc chopper switching state,
In the above,

Represents a target voltage command value of the DC link capacitor,

(K + 2) represents the expected voltage of the DC link capacitor at time (k + 2).

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