KR20210034262A - Voltage control device of renewable energy power system based on p-q characteristic - Google Patents

Abstract

The present invention relates to a voltage control device of a renewable energy power system based on P-Q characteristics, and more specifically, to the voltage control device of the renewable energy power system based on the P-Q characteristics, wherein linear P-Q characteristic value for each active power section calculated by analyzing a power system in advance is installed in DC power output unit of a renewable energy generator, so that reactive power for active power is calculated, and the power system is analyzed to estimate a polynomial composed of nonlinear Q variation with respect to P, and the reactive power is calculated from the estimated polynomial to determine the calculated reactive power as a reactive power reference value, and a programmable logic controller (PLC) controller converts the DC power into AC power using the determined reactive power reference value to output stable AC power.

Description

Voltage control device for renewable energy power system based on P-Q characteristics {VOLTAGE CONTROL DEVICE OF RENEWABLE ENERGY POWER SYSTEM BASED ON P-Q CHARACTERISTIC}

The present invention relates to a voltage control device for a renewable energy power system based on P (active power)-Q (reactive power) characteristics, and in particular, linear PQ characteristic data for each active power section by pre-power system analysis is a direct current power supply of a renewable energy generator. It is installed at the output to calculate reactive power for active power, or by estimating a nonlinear polynomial consisting of Q fluctuations for P by prior power system analysis, and calculating reactive power for active power from this estimated polynomial. PQ characteristics that determine the calculated reactive power as the reactive power reference value, and the PLC (Programmable Logic Controller) controller converts the DC power to the AC power using the determined reactive power reference value so that stable AC power can be output. It relates to a voltage control device of the based renewable energy power system.

Recently, many projects related to new and renewable energy are being actively promoted in Korea, and the "Renewable Energy Portfolio Standard" (Renewable Energy Portfolio Standard: In addition to the RPS) system, the 2017 "Renewable Energy 3020 Implementation Plan" announced by the Ministry of Trade, Industry and Energy attracted attention from many private businesses and investors. Compulsory suppliers of the RPS system must be notified in advance to introduce new and renewable energy generation facilities or purchase a Renewable Energy Certificate (REC) from another renewable energy generator to fill the mandatory quota.

In addition, in the Renewable Energy 3020 Implementation Plan, when local governments select a site and supply it to a private business, the business unit develops and develops the site. Due to these systems and plans promoted by the government, the number of businesses entering into new and renewable energy is increasing, and the demand for it is also increasing. However, as this demand increases, it does not always have a positive effect on the distribution system. Most of the renewable energy generators use natural energy to generate power, so they are characterized by intermittent power generation due to weather or environment. Accordingly, the output is also connected to the distribution system in a non-uniform state, and the system voltage may become unstable.

In order to expand renewable energy, it is important to minimize voltage fluctuations in the distribution system due to the connection of renewable energy. Existing methods include a method of exchanging reactive power using a direct current generator connected in parallel to an AC system, and a method of controlling an extended renewable energy generator with a constant power factor.

However, the method of exchanging reactive power is difficult to apply when a DC generator such as a renewable energy generator is expanded, and constant power factor control has a limit in voltage control capability when the total amount of renewable energy to be expanded is very large.

Therefore, when a plurality of renewable energy generators are input to the distribution system, there is a need to minimize voltage fluctuations at the connection points of each renewable energy generator and to prevent a voltage control conflict between the plurality of renewable energy generators.

Domestic Patent Registration No. 1132107 (Registration Date: March 23, 2012)

Therefore, the present invention has been made in consideration of the above points, and when a plurality of renewable energy generators are input to the distribution system, voltage fluctuations at the connection points of each renewable energy generator are minimized, and the voltage control conflict between the plurality of renewable energy generators. An object of the present invention is to provide an apparatus for controlling a voltage of a renewable energy power system based on PQ characteristics, which makes it possible to prevent this from occurring.

In order to achieve the above object, the apparatus for controlling the voltage of the renewable energy power system based on the PQ characteristic according to the embodiment of the present invention divides the active power by section by pre-power system analysis, ) PQ characteristic-based reactive power calculation unit configured to linearize the curve and calculate the reactive power for the active power by calculating the linear PQ characteristic for each active power section by applying the Q (reactive power)-V characteristic to the linearized PV curve. ; And a PQ configured to determine the reactive power calculated by the reactive power calculation unit based on the PQ characteristics as a reactive power reference value, and convert DC power to AC power by PLC (Programmable Logic Controller) control using the determined reactive power reference value. It characterized in that it comprises a; characteristic-based converter.

In the apparatus for controlling the voltage of the renewable energy power system based on the P-Q characteristic according to the embodiment, the reactive power calculation unit based on the P-Q characteristic may calculate the reactive power by the following [Equation 7].

[Equation 7]

는 산출된 무효전력을 나타내고,

는 구간별 P-Q 드룹(droop)의 역수를 나타내며,

는 사전에 결정되지 않은 유효전력값을 나타내며,

는 사전에 결정된 유효전력값을 나타내며,

는 사전에 결정된 무효전력값을 나타냄][here,

Represents the calculated reactive power,

Represents the reciprocal of the PQ droop for each section,

Represents an active power value that has not been determined in advance,

Represents a predetermined active power value,

Represents a predetermined reactive power value]

In order to achieve the above object, a voltage control device for a renewable energy power system based on a PQ characteristic according to another embodiment of the present invention applies a Kalman filter to the data acquired by the pre-power system analysis to achieve the amount of reactive power fluctuation for nonlinear active power. A Kalman filter-based reactive power calculator configured to estimate a polynomial consisting of and calculate reactive power from the estimated polynomial; And Kalman configured to convert the DC power to AC power by PLC (Programmable Logic Controller) control by determining the reactive power calculated by the Kalman filter-based reactive power calculation unit as a reactive power reference value, and using the determined reactive power reference value. It characterized in that it comprises a; filter-based converter.

)이 구해지며,In the voltage control apparatus of the renewable energy power system based on the PQ characteristic according to the other embodiment, the estimated polynomial is obtained by substituting active power as an input value and reactive power as an output value to the Kalman filter, and the estimation If the polynomial is modified, the amount of change in reactive power (

) Is obtained,

[Equation 11]

는 전압의 변화량을 나타내고,

및

는 유효전력의 변화량을 나타내며,

,

,

및

는 칼만필터의 상태 변수로, 추정 과정에서 칼만필터의 입력 및 출력 값에 대해 최소의 RMSE(root-mean-squre-error)를 갖는 곡선이 표현되도록 매회 업데이트 되는 값을 나타냄][here,

Represents the amount of change in voltage,

And

Represents the amount of change in active power,

,

,

And

Is the state variable of the Kalman filter, and represents a value that is updated each time so that a curve with the minimum root-mean-squre-error (RMSE) is expressed for the input and output values of the Kalman filter in the estimation process.]

Reactive power (Q) can be obtained by the following [Equation 12].

[Equation 12]

는 칼만필터의 i번째 상태 변수를 나타내고,

는 y의 i제곱을 나타냄][here,

Denotes the ith state variable of the Kalman filter,

Represents y to the power of i]

)이 구해지며,In the voltage control apparatus for a renewable energy power system based on a PQ characteristic according to the other embodiment, the estimated polynomial is obtained by substituting active power as an input value and a voltage as an output value to the Kalman filter to obtain a first equation, The Kalman filter is obtained by substituting reactive power as an input value and a voltage as an output value, filtering to obtain the second equation, and then calculating the filtered equations 1 and 2, and modifying the estimated polynomial is as follows: As in [Equation 11], the amount of change in reactive power (

) Is obtained,

[Equation 11]

는 전압의 변화량을 나타내고,

및

는 유효전력의 변화량을 나타내며,

,

,

및

는 칼만필터의 상태 변수로, 추정 과정에서 칼만필터의 입력 및 출력 값에 대해 최소의 RMSE(root-mean-squre-error)를 갖는 곡선이 표현되도록 매회 업데이트 되는 값을 나타냄][here,

Represents the amount of change in voltage,

And

Represents the amount of change in active power,

,

,

And

Is the state variable of the Kalman filter, and represents a value that is updated each time so that a curve with the minimum root-mean-squre-error (RMSE) is expressed for the input and output values of the Kalman filter in the estimation process.]

Reactive power (Q) can be obtained by the following [Equation 12].

[Equation 12]

는 칼만필터의 i번째 상태 변수를 나타내고,

는 y의 i제곱을 나타냄][here,

Denotes the ith state variable of the Kalman filter,

Represents y to the power of i]

According to the voltage control device of the renewable energy power system based on the PQ characteristic according to the embodiments of the present invention, it is installed at the DC power output of the renewable energy generator and calculates the linear PQ characteristic for each active power section according to the pre-power system analysis information. Reactive power is calculated for active power, or the power system is analyzed to estimate a polynomial consisting of the Q fluctuation for P of nonlinearity, and reactive power is calculated from the estimated polynomial, and the calculated reactive power is used as a reference value for reactive power. And the PLC (Programmable Logic Controller) controller converts DC power to AC power using this determined reactive power reference value to output stable AC power, so that a plurality of renewable energy generators are put into the distribution system. In this case, there is an excellent effect of minimizing voltage fluctuations at the connection points of each renewable energy generator and preventing collision of voltage control between a plurality of renewable energy generators.

1 is a control block diagram of an apparatus for controlling a voltage of a renewable energy power system based on a PQ characteristic according to a first embodiment of the present invention.
2 is a control block diagram of an apparatus for controlling a voltage of a renewable energy power system based on a PQ characteristic according to a second embodiment of the present invention.

In describing the embodiments of the present invention, when it is determined that a detailed description of known technologies related to the present invention may unnecessarily obscure the subject matter of the present invention, a detailed description thereof will be omitted. In addition, terms to be described later are terms defined in consideration of functions in the present invention, which may vary according to the intention or custom of users or operators. Therefore, the definition should be made based on the contents throughout the present specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be construed as limiting. Unless explicitly used otherwise, expressions in the singular form include the meaning of the plural form. In the present description, expressions such as "comprising" or "feature" are intended to refer to certain features, numbers, steps, actions, elements, some or combination thereof, and one or more It should not be construed as excluding the presence or possibility of other features, numbers, steps, actions, elements, any part or combination thereof.

In each of the systems shown in the drawings, the elements in some cases each have the same reference number or a different reference number, suggesting that the elements represented may be different or similar. However, elements may have different implementations and operate with some or all of the systems shown or described herein. The various elements shown in the figures may be the same or different. Which is referred to as the first element and which is referred to as the second element is arbitrary.

In the present specification, "transmitting", "transmitting" or "providing" data or signals from one component to another component means that one component directly transmits data or signals to another component, as well as It includes transmitting data or signals to other components through at least one other component.

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

[First Example]

1 is a control block diagram of an apparatus for controlling a voltage of a renewable energy power system based on a P-Q characteristic according to a first embodiment of the present invention.

The voltage control device of the renewable energy power system based on the PQ characteristic according to the first embodiment of the present invention is installed between the DC power output terminal and the AC system of each of a plurality of renewable energy generators, and as shown in FIG. 1, based on the PQ characteristic. And a reactive power calculator 100 and a converter 200 based on PQ characteristics.

The PQ characteristic-based reactive power calculation unit 100 receives DC power from the output of the DC generator installed in the power system, analyzes the power system, divides the active power by section, and divides the active power by section. ) The curve is linearized, and the linear PQ characteristic for each active power section is calculated by applying the Q (reactive power)-V characteristic to the linearized PV curve to calculate the reactive power for the active power.

The process of calculating reactive power will be described in detail.

)는 다음의 [수학식 1]과 같다.The relationship between the voltage of the power system and the active power (

) Is as the following [Equation 1].

[Equation 1]

는 모선

에 투입된 유효전력을 나타내고,

및

는 각각 모선

의 전압 및 위상을 나타내며,

및

는 각각 모선

의 전압 및 위상을 나타내며,

및

는 각각 모선

에서 바라본 등가 어드미턴스의 크기 및 위상을 나타내며,

및

는 각각 모선

와

사이의 어드미턴스의 크기 및 위상을 나타냄][here,

The mothership

Represents the active power input to

And

Each mothership

Represents the voltage and phase of

And

Each mothership

Represents the voltage and phase of

And

Each mothership

It represents the magnitude and phase of the equivalent admittance viewed from

And

Each mothership

Wow

Indicate the magnitude and phase of admittance between]

)는 다음의 [수학식 2]와 같다.The relationship between the voltage of the power system and the reactive power (

) Is as the following [Equation 2].

[Equation 2]

는 모선

에 투입된 무효전력을 나타냄][here,

The mothership

Represents the reactive power inputted to]

P-V and Q-V relationship fluctuations due to voltage and phase fluctuations are respectively derived as shown in [Equation 3] below.

[Equation 3]

및

로 근사화 할 수 있으므로, [수학식 3]은 다음의 근사식인 [수학식 4]로 나타낼 수 있다.The main component of the power system impedance is inductance, and the difference in phase between each bus is very small,

And

Since it can be approximated by [Equation 3], it can be expressed as [Equation 4], which is the following approximation.

[Equation 4]

That is, according to the fluctuation of the grid voltage, the P-V droop characteristic does not change, and only the Q-V droop characteristic changes. Likewise, only the P-V droop characteristic is affected by the change of the phase operation point, and the Q-V droop characteristic is not.

As can be seen from the following [Equation 5], the phase is affected by the input of the active power, but the voltage is affected small. In addition, since the voltage is kept constant, only the phase of the connection point is changed by the input of active power of renewable energy. Previously, since it was said that only the P-V droop characteristic is affected by the phase change, the Q-V droop characteristic has a constant linear characteristic regardless of the active power input of renewable energy. On the other hand, the P-V droop characteristic changes according to the input of active power, and the P-V droop characteristic has a nonlinear curve depending on the amount of active power applied.

[Equation 5]

In an embodiment of the present invention, the input of active power is divided by section, the PV curve of each section is linearized, and the linear PQ characteristic for each section is calculated by applying the QV characteristic to it, and the amount of reactive power input relative to the amount of active power input is calculated. . Each of the P-V and Q-V characteristics prevents fluctuations in the connection point voltage before and after the input of renewable energy, so applying the P-Q characteristic keeps the connection point voltage constant before and after the input of renewable energy. The active power section is defined as the following [Equation 6].

[Equation 6]

[Here, I, m, k are integers]

For example, when the maximum amount of renewable energy input is 1MW, if each section is 0.1MW, k=10, m=10.

At this time, the reactive power is calculated as shown in [Equation 7] below.

[Equation 7]

는 산출된 무효전력을 나타내고,

는 구간별 P-Q 드룹(droop)의 역수를 나타내며,

는 사전에 결정되지 않은 유효전력값을 나타내며,

는 사전에 결정된 유효전력값을 나타내며,

는 사전에 결정된 무효전력값을 나타냄][here,

Represents the calculated reactive power,

Represents the reciprocal of the PQ droop for each section,

Represents an active power value that has not been determined in advance,

Represents a predetermined active power value,

Represents a predetermined reactive power value]

The PQ characteristic-based converter 200 determines the reactive power calculated by the PQ characteristic-based reactive power calculation unit 100 as a reactive power reference value, and is reproduced by PLC (Programmable Logic Controller) control using the determined reactive power reference value. It plays a role of stably converting DC power from an energy generator to AC power. The description of PLC control is obvious to those skilled in the art, so it will be omitted here.

According to the voltage control device of the renewable energy power system based on the PQ characteristic according to the first embodiment of the present invention configured as above, it is installed at the DC power output of the renewable energy generator to analyze the power system and determine the linear PQ characteristics for each active power section. By calculating the reactive power for the active power, the calculated reactive power is determined as the reactive power reference value, and the PQ characteristic-based converter converts the DC power to the AC power using the determined reactive power reference value. By being configured to output power, when a plurality of renewable energy generators are put into the distribution system, voltage fluctuations at the connection points of each renewable energy generator are minimized, and voltage control conflicts between the plurality of renewable energy generators are prevented. can do.

[Second Example]

2 is a control block diagram of an apparatus for controlling a voltage of a renewable energy power system based on a P-Q characteristic according to a second embodiment of the present invention.

The voltage control device of the renewable energy power system based on the PQ characteristic according to the second embodiment of the present invention is installed between the DC power output terminal and the AC system of each of a plurality of renewable energy generators, and as shown in FIG. 2, the Kalman filter-based It includes a reactive power calculator 100' and a Kalman filter-based converter 200'.

The Kalman filter-based reactive power calculation unit 100 ′ receives DC power from the output unit of the DC generator installed in the power system, analyzes the power system, and uses a Kalman filter to obtain a polynomial consisting of variations in reactive power for nonlinear active power. It is responsible for estimating and calculating reactive power from this estimated polynomial.

The process of calculating the reactive power will be described in detail.

In the second embodiment, unlike the first embodiment, the curve is not linearized, and the curve itself is estimated and used as a nonlinear polynomial. The polynomial can be estimated by various methods, but the Kalman filter was used in the second embodiment.

First, the system equation can be expressed as the following [Equation 8].

[Equation 8]

는 (n+1)번째 상태변수를 나타내고,

는 시스템방정식을 나타내며,

는 n번째 상태변수를 나타내며,

는 프로세스노이즈에 대한 공분산값을 나타내며,

는 프로세스노이즈를 나타내며,

는 초기 상태변수를 나타내며,

은 출력값을 나타내며,

는 기저함수를 나타내며,

은 추정방정식을 나타내며,

은 측정오차를 나타냄][here,

Represents the (n+1)th state variable,

Represents the system equation,

Represents the nth state variable,

Represents the covariance value for the process noise,

Represents the process noise,

Represents the initial state variable,

Represents the output value,

Represents the basis function,

Represents the estimation equation,

Indicates measurement error]

The measurement-based update of the Kalman filter is as shown in [Equation 9] below.

[Equation 9]

은 칼만게인값을 나타내고,

은 공분산값을 나타내며,

는

의 변환값을 나타내며,

는 튜닝민감도를 나타내며,

는 상태추정값을 나타내며,

는 측정기반 상태 변수 추정 값를 나타내며,

은 유효전력 변화량을 나타냄][here,

Represents the Kalman gain value,

Represents the covariance value,

Is

Represents the converted value of,

Indicates tuning sensitivity,

Represents the state estimate,

Represents the estimated value of the measurement-based state variable,

Represents the amount of change in active power]

The model-based update of the Kalman filter is as follows [Equation 10].

[Equation 10]

는 모델기반 상태 변수 추정 값을 나타내고,

는 모델기반 공분산 추정값 나타내며,

는

의공분산을 나타내며,

는 프로세스 노이즈의 공분산 전치행렬 나타냄][here,

Denotes the model-based state variable estimated value,

Represents the model-based covariance estimate,

Is

Represents the covariance,

Represents the covariance transposition matrix of the process noise]

로 정의하고, 칼만필터의 출력을

로 정의하면, 상태변수 x는 전압 변동에 대한 유효전력과 무효전력 변동량으로 구성된 다항식의 계수를 나타내게 된다. 추정 다항식을 변형하면 다음의 [수학식 11]과 같다.The basis function

And the output of the Kalman filter

If defined as, the state variable x represents the coefficient of a polynomial consisting of the amount of variation in active power and reactive power for voltage variation. If the estimated polynomial is modified, it is as follows [Equation 11].

[Equation 11]

는 전압의 변화량을 나타내고,

및

는 유효전력의 변화량을 나타내며,

,

,

및

는 칼만필터의 상태 변수로, 추정 과정에서 칼만필터의 입력 및 출력 값에 대해 최소의 RMSE(root-mean-squre-error)를 갖는 곡선이 표현되도록 매회 업데이트 되는 값을 나타냄] [here,

Represents the amount of change in voltage,

And

Represents the amount of change in active power,

,

,

And

Is the state variable of the Kalman filter, and represents a value that is updated each time so that a curve with the minimum root-mean-squre-error (RMSE) is expressed for the input and output values of the Kalman filter in the estimation process.]

Therefore, the reactive power that makes the voltage constant can be calculated by the method of [Equation 12] below.

[Equation 12]

는 칼만필터의 i번째 상태 변수를 나타내고,

는 y의 i제곱을 나타냄][here,

Denotes the ith state variable of the Kalman filter,

Represents y to the power of i]

The Kalman filter-based converter 200 ′ determines the reactive power calculated by the Kalman filter-based reactive power calculation unit 100 ′ as a reactive power reference value, and uses the determined reactive power reference value to control PLC (Programmable Logic Controller). As a result, it plays a role of stably converting DC power from a renewable energy generator to AC power. The description of PLC control is obvious to those skilled in the art, so it will be omitted here.

According to the voltage control device of the renewable energy power system based on the PQ characteristic according to the second embodiment of the present invention configured as described above, it is installed at the DC power output of the renewable energy generator to analyze the power system and calculate the Q variation with respect to the nonlinear P. By estimating the configured polynomial and calculating reactive power from the estimated polynomial, the calculated reactive power is determined as the reactive power reference value, and the Kalman filter-based converter uses the determined reactive power reference value to convert DC power into AC power. As it is configured to output stable AC power by converting, when multiple renewable energy generators are input to the distribution system, voltage fluctuations at the connection points of each renewable energy generator are minimized, and voltage control conflicts between multiple renewable energy generators. You can prevent this from happening.

In the drawings and specification, an optimal embodiment has been disclosed, and specific terms are used, but these are used only for the purpose of describing the embodiments of the present invention, and are used to limit the meaning or to limit the scope of the present invention described in the claims. Was not done. Therefore, those of ordinary skill in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

100: PQ characteristic-based reactive power calculation unit
100': Kalman filter-based reactive power calculation unit
200: PQ characteristic based converter
200': Kalman filter-based converter

Claims (5)

According to the result of the pre-power system analysis, the active power is divided by section, the P (active power)-V (voltage) curve of each section is linearized, and the Q (reactive power)-V characteristic is applied to the linearized PV curve to achieve active power. A PQ characteristic-based reactive power calculator configured to calculate a reactive power with respect to the active power by calculating a linear PQ characteristic for each section; And
Reactive power calculated by the PQ characteristic-based reactive power calculation unit is determined as a reactive power reference value by receiving DC power from the output unit of the DC generator installed in the power system, and PLC (Programmable Logic) by using the determined reactive power reference value. Controller) PQ characteristic-based converter configured to convert DC power to AC power by control; including, PQ characteristic-based voltage control device of the renewable energy power system.
The method of claim 1,
The PQ characteristic-based reactive power calculation unit calculates the reactive power according to the following [Equation 7], the voltage control device of the PQ characteristic-based renewable energy power system.

[Equation 7]

[here,

Represents the calculated reactive power,

Represents the reciprocal of the PQ droop for each section,

Represents an active power value that has not been determined in advance,

Represents a predetermined active power value,

Represents a predetermined reactive power value]
The power system is analyzed by receiving DC power from the output of the DC generator installed in the power system, and a polynomial consisting of the amount of reactive power fluctuations for the nonlinear active power is estimated using a Kalman filter, and the reactive power is calculated from the estimated polynomial. Kalman filter-based reactive power calculation unit configured to; And
A Kalman filter configured to determine the reactive power calculated by the Kalman filter-based reactive power calculation unit as a reactive power reference value, and convert DC power to AC power by PLC (Programmable Logic Controller) control using the determined reactive power reference value. A voltage control device of a renewable energy power system based on PQ characteristics, including; a base converter.
The method of claim 3,
The estimated polynomial is obtained by substituting active power as an input value and reactive power as an output value for the Kalman filter,
If the estimated polynomial is modified, the amount of change in reactive power (

) Is obtained,

[Equation 11]

[here,

Represents the amount of change in voltage,

And

Represents the amount of change in active power,

,

,

And

Is the state variable of the Kalman filter, and represents a value that is updated each time so that a curve with the minimum root-mean-squre-error (RMSE) is expressed for the input and output values of the Kalman filter in the estimation process.]

Reactive power (Q) is obtained by the following [Equation 12], the voltage control device of the PQ characteristic-based renewable energy power system.

[Equation 12]

[here,

Denotes the ith state variable of the Kalman filter,

Represents y to the power of i]
The method of claim 3,
The estimated polynomial is filtered by substituting active power as an input value for the Kalman filter and voltage as an output value to obtain the first equation, and substituting reactive power as an input value for the Kalman filter and voltage as an output value. After obtaining the second equation, it is obtained by calculating the filtered equations 1 and 2,
If the estimated polynomial is modified, the amount of change in reactive power (

) Is obtained,

[Equation 11]

[here,

Represents the amount of change in voltage,

And

Represents the amount of change in active power,

,

,

And

Is the state variable of the Kalman filter, and represents a value that is updated each time so that a curve with the minimum root-mean-squre-error (RMSE) is expressed for the input and output values of the Kalman filter in the estimation process.]

Reactive power (Q) is obtained by the following [Equation 12], the voltage control device of the PQ characteristic-based renewable energy power system.

[Equation 12]

[here,

Denotes the ith state variable of the Kalman filter,

Represents y to the power of i]

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