A photovoltaic reactive power evaluation method and control strategy for a “photovoltaic + thermal power” combined transmission system

Abstract

The present invention discloses a photovoltaic reactive power evaluation method and control strategy for a "photovoltaic + thermal power" combined transmission system, and belongs to the field of electrical engineering technology. The method comprises the following steps: (1) obtaining a photovoltaic station terminal voltage mathematical model through the "photovoltaic + thermal power" combined transmission system node equation, and clarifying the functional relationship between the photovoltaic station terminal voltage and reactive power; (2) clarifying the terminal voltage amplitude requirement for stable operation of the photovoltaic station (maintained at 1.0pu), and under a given system flow condition, quantitatively solving the reactive power required by the photovoltaic station through the functional relationship between the photovoltaic station terminal voltage and reactive power. It can provide a quantitative evaluation method for the reactive power demand of the photovoltaic station for stable operation in the actual operation scenario, and improve the efficiency and accuracy of the reactive compensation capacity setting of the photovoltaic station.

Description

Technical Field

[0001] The present invention belongs to the field of electrical engineering technology and relates to a photovoltaic reactive power evaluation method and a control strategy for a "photovoltaic + thermal power" combined transmission system. Background Art

[0002] In recent years, large-scale new energy units and nearby conventional power sources have been centrally connected to the main grid of the receiving power grid to supply power to the load center, and the adjacent DC feed-in has formed a large-scale new energy unit combined with thermal power unit transmission system. Compared with the traditional power system, the stability characteristics of the large-scale new energy unit combined with thermal power transmission system have changed significantly.

[0003] In terms of voltage stability, conventional new energy units are different from synchronous units and cannot provide reactive power support to the system. Therefore, conventional new energy units need to be equipped with a certain proportion of reactive power compensation devices for stable operation to prevent reactive power shortages at the grid connection points of new energy units, which may cause voltage drops. At present, most new energy stations use reactive power compensation devices such as STATCOM and phase shifters to improve the problem of wind power voltage drops. According to the "GB_19964-2012 Technical Regulations for Photovoltaic Power Station Access to Power System" and "GBT_19963-2011 Technical Regulations for Wind Farm Access to Power System", the capacity of reactive power compensation devices in new energy stations can be configured at 20% of the total capacity of the stations, but the commissioning of reactive power compensation devices such as STATCOM will increase operating costs. At the same time, there is currently no quantitative evaluation method for reactive power demand in new energy stations. The configuration method of reactive power compensation device capacity at 20% of the total capacity of new energy units may cause the risk of reactive power shortage or reactive power surplus of new energy units. Summary of the invention

[0004] The purpose of the present invention is to solve the deficiencies of the above-mentioned prior art, thereby providing a photovoltaic reactive power assessment method for a "photovoltaic + thermal power" combined transmission system, which can better solve the problem of reactive power demand assessment of photovoltaic stations, improve the voltage stability of photovoltaic stations, and reduce the cost problem caused by the capacity configuration error of reactive compensation devices.

[0005] The present invention also aims to provide a control strategy for the "photovoltaic + thermal power" combined transmission system of the above-mentioned "photovoltaic + thermal power" combined transmission system photovoltaic reactive power evaluation method.

[0006] A photovoltaic reactive power evaluation method for a "photovoltaic + thermal power" combined transmission system includes the following steps:

[0007] (1) Through the node equation of the "photovoltaic + thermal power" joint transmission system, the mathematical model of the photovoltaic station terminal voltage is obtained, and the functional relationship between the photovoltaic station terminal voltage and reactive power is clarified;

[0008] (2) The terminal voltage amplitude requirement for stable operation of the PV station is clarified (maintained at 1.0 pu). Under given system power flow conditions, the reactive power required by the PV station is quantitatively solved through the functional relationship between the terminal voltage and reactive power of the PV station.

[0009] Step (1) specifically includes the following steps:

[0010] Construct an equivalent model of the "photovoltaic + thermal power" joint transmission system and write the system node equations.

[0011] (1)

[0012] The mathematical model of the voltage at the photovoltaic station terminal is derived from the node equation (1).

[0013] The mathematical model of the photovoltaic station terminal voltage is derived from the node equation (1), which is specifically:

[0014] The voltage at the photovoltaic station can be derived from formula (1): The expression of is,

[0015] (2)

[0016] Among them, U L represents the voltage of the convergent bus L. From equation (1), we can get U L The specific function expression is:

[0017] (3)

[0018] Combining equations (2) and (3), we can solve the voltage at the photovoltaic station: Mathematical expressions,

[0019] (4)

[0020] The parameters K1 and K2 satisfy:

[0021] (5)

[0022] in is the transformer impedance of the synchronous unit;

[0023] In the “PV + thermal power” combined transmission system, the PV station adopts a directional phase-locked control method based on the d-axis voltage of the grid-side converter. d =U W , q-axis component u q =0; PV output current d-axis component i d =I W cosf i , q-axis component i q =IW sinf i , f i is the power factor angle of the photovoltaic station;

[0024] will u d 、u q 、i d 、i q Substituting into formula (4) we can get:

[0025] (6)

[0026] By simplifying equation (6) with trigonometric functions, we can get the voltage amplitude expression of the photovoltaic station terminal, that is, the mathematical model of the photovoltaic station terminal voltage is:

[0027] (7)

[0028] Where P W , Q W They are respectively the active power and reactive power output by the photovoltaic station, and the parameter P W , Q W 、A satisfies:

[0029] (8).

[0030] Step (2) specifically includes the following steps:

[0031] When the active output of the synchronous unit and photovoltaic power in the "photovoltaic + thermal power" joint transmission system is given and the system flow is determined, the photovoltaic active power and synchronous machine power angle values ​​can be determined. At this time, a corresponding functional relationship can be established between the photovoltaic terminal voltage amplitude and reactive power.

[0032] (9)

[0033] When the system is operating under normal conditions, the voltage at the photovoltaic station terminal should be 1.0pu, which can be used to quantitatively calculate the reactive power Q of the photovoltaic station. W ,

[0034] (10).

[0035] The reactive power control strategy of the photovoltaic station is modified so that the photovoltaic station can output reactive power of corresponding capacity according to the reactive power demand solved by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system, so as to maintain the stable operation of the terminal voltage.

[0036] The modification of the reactive power control strategy of the photovoltaic station enables the photovoltaic station to output reactive power of corresponding capacity according to the reactive power demand solved by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system, so as to maintain the stable operation of the terminal voltage, specifically:

[0037] The photovoltaic grid-side converter adopts a voltage-current dual closed-loop control strategy, in which the d-axis voltage outer loop controls the photovoltaic active power P W , to maintain the DC capacitor voltage stable, the q-axis voltage outer loop control target is to make the photovoltaic output reactive power Q W Follow reactive power reference value Q Wref According to the reactive power demand Q obtained by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system described in the above technical solution, the reactive power reference value Q in the photovoltaic grid-side converter is set. Wref Equal to Q, ensuring that the reactive power output of photovoltaic meets the evaluation requirements.

[0038] Compared with the existing photovoltaic control strategy, this control strategy combines the evaluation results of photovoltaic reactive power demand to achieve fixed reactive power control of photovoltaic units, which is convenient for configuring the capacity of photovoltaic units and the capacity of other reactive compensation devices in the station. At the same time, the output of a certain amount of reactive power by photovoltaics helps to solve the problem of low terminal voltage when its output accounts for a large proportion.

[0039] The present invention constructs a mathematical model of the voltage amplitude at the photovoltaic station terminal in the "photovoltaic + thermal power" combined transmission system, and clarifies the functional relationship between the photovoltaic station terminal voltage and reactive power.

[0040] Based on the functional relationship between the photovoltaic station terminal voltage and reactive power, the present invention can quantitatively evaluate the reactive power required for the stable operation of the photovoltaic station under given system flow conditions, thereby preventing the occurrence of reactive power shortage and reactive power surplus.

[0041] The present invention modifies the reactive power control strategy of the photovoltaic station so that the photovoltaic station can output reactive power of corresponding capacity according to reactive power evaluation requirements, thereby supporting the terminal voltage and reducing the cost problem caused by the commissioning of reactive power compensation devices such as STATCOM.

[0042] The present invention establishes a mathematical model of the voltage at the photovoltaic station end in the "photovoltaic + thermal power" joint transmission system, clarifies the functional relationship between the voltage at the photovoltaic station end and reactive power, and thereby quantitatively evaluates the reactive power demand of the photovoltaic station. At the same time, according to the calculation results of the reactive power demand of the photovoltaic station, the reactive power control strategy of the photovoltaic station is modified so that the photovoltaic station can output the required capacity of reactive power to support the voltage at the grid connection point. The present invention can better solve the problem of reactive power demand assessment of photovoltaic stations, improve the voltage stability of photovoltaic stations, and reduce the cost problem caused by the capacity configuration error of reactive power compensation devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 "Photovoltaic + thermal power" joint transmission system structure diagram;

[0044] Figure 2 “PV + thermal power” joint transmission system equivalent model;

[0045] Figure 3 The control block diagram of the grid-side converter after the PV reactive power control strategy is modified;

[0046] Figure 4 Output characteristic simulation curve when the photovoltaic station adopts conventional reactive power control;

[0047] Figure 5 Modify the output characteristic simulation curve after reactive power control of the photovoltaic station. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples.

[0049] The photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system has the following specific implementation steps:

[0050] Step 1: Based on the system node equation, a mathematical model of the voltage amplitude at the photovoltaic station end is established in the "photovoltaic + thermal power" joint transmission system;

[0051] This paper takes the "photovoltaic + thermal power" combined transmission system as the analysis object, and its system structure is as follows Figure 1 As shown, make a system equivalent model according to the system structure, see Figure 2 ,|U W | is the voltage amplitude of the photovoltaic terminal, |U0| is the infinite bus voltage amplitude, |E g | is the potential inside the synchronous machine, and the photovoltaic passes through the equivalent impedance X l Connect to the busbar L, and the synchronous machine passes through the equivalent impedance X lc Connect to the collection bus L, the line impedance between the collection bus and the infinite bus is X l1 .i d 、i q are the photovoltaic d-axis current and q-axis current, δ w , δ g , δ0 are the phase angles of photovoltaic, synchronous machine and infinite bus voltage respectively. The specific parameters in the system diagram and system equivalent model diagram as well as the parameter variable definitions and corresponding values ​​involved in the subsequent formula derivation are given in Tables 1 and 2.

[0052] Table 1 Parameters of “PV + thermal power” combined transmission system

[0053]

[0054] Table 2 System node equations and equivalent model parameters of the "photovoltaic + thermal power" combined transmission system

[0055]

[0056] The system node equations of the equivalent model of the "photovoltaic + thermal power" combined transmission system are listed as shown in formula (1).

[0057] (1)

[0058] The voltage at the photovoltaic station can be derived from formula (1): The expression is shown in formula (2), where UL represents the voltage of the convergent bus L. From formula (1), we can get The specific function expression is shown in formula (3).

[0059] (2)

[0060] (3)

[0061] Combining equations (2) and (3), we can solve the voltage at the photovoltaic station: The mathematical expression is shown in formula (4).

[0062] (4)

[0063] The parameters K1, K2, X i , X lc satisfy:

[0064] (5)

[0065] In this system, the photovoltaic station adopts a directional phase-locked control method based on the d-axis voltage of the grid-side converter. d =U W , q-axis component u q =0; PV output current d-axis component i d =I W cosf i , q-axis component i q =I W sinf i , f i is the power factor angle of the photovoltaic station. d 、u q 、i d 、i q Substituting into formula (4) we can get:

[0066] (6)

[0067] By simplifying equation (6) through trigonometric functions, the voltage amplitude expression at the photovoltaic station end can be obtained as follows.

[0068] (7)

[0069] Where P W , Q W They are respectively the active power and reactive power output by the photovoltaic station, and the parameter P W , Q W 、A satisfies:

[0070] (8)

[0071] From the above derivation, it can be seen that the mathematical model of the terminal voltage of the photovoltaic station is composed of equations (7) and (8). The terminal voltage amplitude is affected by the active power and reactive power output of the photovoltaic station and the synchronous machine power angle δg.

[0072] Step 2: Clarify the terminal voltage amplitude requirement for stable operation of the photovoltaic station (maintained at 1.0pu), and quantitatively solve the reactive power required by the photovoltaic station through the functional relationship between the terminal voltage and reactive power of the photovoltaic station under given system power flow conditions;

[0073] The active power, reactive power output by the photovoltaic station and the power angle of the synchronous unit will affect the voltage amplitude of the photovoltaic terminal. When the active output of the synchronous unit and the photovoltaic station in the "photovoltaic + thermal power" joint transmission system is given and the system flow is determined, the photovoltaic active power and the synchronous machine power angle can be determined. At this time, a corresponding functional relationship can be established between the voltage amplitude of the photovoltaic terminal and the reactive power.

[0074] (9)

[0075] When the system operates under normal conditions, the voltage at the photovoltaic station should be 1.0pu. This condition can be used to quantitatively calculate the reactive power required by photovoltaic power generation. The calculation formula is as follows:

[0076] (10)

[0077] Step 3: Based on the control strategy of the "photovoltaic + thermal power" joint transmission system photovoltaic reactive power evaluation method, modify the reactive control strategy of the photovoltaic station according to the quantitative evaluation results of reactive power, so that the photovoltaic station can output reactive power of corresponding capacity to support the terminal voltage.

[0078] The photovoltaic grid-side converter adopts a voltage-current dual closed-loop control strategy, in which the d-axis voltage outer loop controls the photovoltaic active power P W, to maintain the DC capacitor voltage stable, the q-axis voltage outer loop control target is to make the photovoltaic output reactive power Q W Follow reactive power reference value Q Wref According to the reactive power demand Q obtained by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system, the reactive power reference value Q in the photovoltaic grid-side converter is set Wref Equal to Q, ensuring that the reactive power output of photovoltaic meets the evaluation requirements.

[0079] According to the reactive power value calculated by formula (10), the reactive power control strategy of the photovoltaic station is modified. Compared with the conventional photovoltaic station that does not output reactive power, the present invention sets the q-axis current reference value to be not 0 in the photovoltaic grid-side converter control link, and its value is determined by the reactive power calculated by formula (10). The modified photovoltaic station grid-side converter control block diagram is as follows: Figure 3 As shown in the figure, the definition of the parameter variables in the figure is shown in Table 3. At this time, the photovoltaic station can emit reactive power of corresponding capacity according to the quantitative calculation result of reactive power to maintain the terminal voltage running at 1.0pu.

[0080] Table 3 Parameter table of photovoltaic grid-side converter control block diagram

[0081]

[0082] Figure 3 The control block diagram of the photovoltaic grid-side converter is described. Its control strategy adopts dq decoupled voltage and current double closed-loop control.

[0083] The outer loop is voltage control, which is divided into d-axis voltage control and q-axis voltage control. The d-axis voltage outer loop control goal is to maintain the DC voltage U of the photovoltaic station by controlling the active power output of the unit. dc Stable, the specific operation is the photovoltaic station DC voltage U dc The DC voltage reference value U of the photovoltaic station dcref Make the difference, through the PI controller (proportional coefficient is K p1 , the integral gain is K i1 ) to obtain the d-axis component reference value i of the grid-side converter output current gdref When the active output P of the unit is less than the active reference value Pref, there is a difference in the active power on both sides of the DC capacitor, resulting in a DC voltage U dc Increase, photovoltaic station DC voltage U dc The increase causes the d-axis component reference value i of the grid-side converter output current to increase gdref Increase, so that the unit active output increases, gradually equal to the active reference value Pref, and vice versa; the q-axis voltage outer loop control target is to make the photovoltaic station reactive power Q g Follow the PV station reactive power reference value Q grefThe specific operation is the reactive power Q of the photovoltaic station g and the reactive power reference value Q of the photovoltaic station gref Make the difference, through the PI controller (proportional coefficient is K p5 , the integral gain is K i5 ) to obtain the q-axis component reference value i of the grid-side converter output current gqref When the reactive power Q of the unit is less than the reactive power reference value Q of the photovoltaic station gref At this time, the reactive power reference value of the photovoltaic station is Q gref The reactive power Q of the photovoltaic station g The difference causes the q-axis component reference value i of the grid-side converter output current after PI control. gqref Increase, so that the reactive output of the unit increases, gradually reaching the reactive reference value Q gref Equal and vice versa.

[0084] The inner loop is current control, which is divided into d-axis current control and q-axis current control. The d-axis current inner loop control goal is to make the grid-side converter output current after filtering the d-axis component i gdm The d-axis component reference value i of the output current of the grid-side converter follows gdref The specific operation is that the grid-side converter outputs the filtered d-axis component i gdm The d-axis component reference value i of the grid-side converter output current gdref Make the difference, through the PI controller (proportional coefficient is K p2 , the integral gain is K i2 ) and ω g L g i gqref Make a difference and add the d-axis component u after filtering the output voltage of the grid-side converter gdm , get the d-axis component reference value u of the grid-side converter output voltage gcdref ;

[0085] The q-axis current inner loop control goal is to make the q-axis component i of the grid-side converter output current after filtering gqm The q-axis component reference value i of the output current of the grid-side converter follows gqref The specific operation is that the grid-side converter outputs the q-axis component i after filtering gqm The q-axis component reference value i of the grid-side converter output current gqref Make the difference, through the PI controller (proportional coefficient is K p3 , the integral gain is K i3 ) and ω g L g i gdref Sum, plus the q-axis component u of the grid-side converter output voltage after filtering gqm , get the q-axis component reference value u of the grid-side converter output voltage gcqref.

[0086] Example

[0087] For the above-mentioned "photovoltaic + thermal power" joint transmission system, the synchronous machine capacity is set to 2756MVA, the output active power is 1736MW, the photovoltaic station capacity is 1711MVA, and the output active power is 1540MW. According to the power flow calculation results, when the system is running stably, the synchronous machine power angle δ g =33.12°. Set the system parameters K1, K2, X i , PV output active power P W , synchronous machine power angle δ g Substituting into equation (10), we can get that under this condition, the reactive power required to output when the photovoltaic terminal voltage is running at 1.0pu is Q CAL =288.2MVar.

[0088] Based on the above theoretical calculation results, time domain simulation verification is carried out in PSCAD / EMTDC simulation software to determine the accuracy of quantitative evaluation of unpowered power.

[0089] When the photovoltaic station adopts conventional reactive power control, the simulation curves of the photovoltaic station output power and terminal voltage under normal operating conditions are as follows: Figure 4 As shown, Figure 4 (a) is the simulation curve of the voltage at the photovoltaic station port when conventional reactive power control is adopted. Figure 4 (b) is the PV active power simulation curve when conventional reactive power control is used. Figure 4 (c) is the PV reactive power simulation curve when conventional reactive power control is used; according to the quantitative reactive power evaluation results, Figure 3 Modify the reactive power control strategy of the photovoltaic station so that the photovoltaic station can output 288.2MVar reactive power on the basis of meeting the active output demand. At this time, the simulation curves of the photovoltaic station output power and terminal voltage are as follows: Figure 5 As shown, Figure 5 (a) is the simulation curve of the voltage at the PV station port when the modified PV station reactive power control strategy is adopted. Figure 5 (b) is the PV active power simulation curve when the modified PV station reactive power control strategy is adopted. Figure 5 (c) is the photovoltaic reactive power simulation curve when the modified photovoltaic station reactive power control strategy is adopted;

[0090] contrast Figure 4 , Figure 5It can be seen that under this working condition, when the photovoltaic station adopts conventional reactive power control, the photovoltaic terminal voltage is lower than 1.0pu due to insufficient reactive power compensation; the reactive power control strategy of the photovoltaic station is modified to make the reactive power output of the photovoltaic equal to the reactive power obtained by quantitative evaluation, so that the photovoltaic terminal voltage can operate at 1.0pu. This verifies the accuracy of the reactive power quantitative evaluation method of the photovoltaic station proposed in the present invention.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A photovoltaic reactive power evaluation method for a "photovoltaic + thermal power" combined transmission system, characterized by: The following steps are involved: (1) Through the node equation of the "photovoltaic + thermal power" joint transmission system, the mathematical model of the photovoltaic station terminal voltage is obtained, and the functional relationship between the photovoltaic station terminal voltage and reactive power is clarified; (2) Clarify the terminal voltage amplitude requirements for stable operation of the photovoltaic station, and quantitatively solve the reactive power required by the photovoltaic station through the functional relationship between the terminal voltage and reactive power of the photovoltaic station under given system power flow conditions; Step (1) specifically includes the following steps: Construct an equivalent model of the "photovoltaic + thermal power" joint transmission system and write the system node equations. in: Represents the current injected by the system equivalent voltage source; Indicates the system collection busbar injection current; Indicates the synchronous machine node injection current; Indicates the current injected into the photovoltaic station node; Indicates the system equivalent voltage source terminal voltage; Indicates the system bus terminal voltage; Indicates the potential voltage inside the synchronous machine; Indicates the voltage at the photovoltaic station; Y 00 represents the system equivalent voltage source node self-admittance; Y L0 , Y 0L represents the mutual admittance between the voltage source and the collection bus; Y G0 , Y 0G represents the mutual admittance between the voltage source and the synchronous machine; Y W0 , Y 0W represents the mutual admittance between the voltage source and the photovoltaic station; Y LL Y represents the self-admittance of the busbar node; GL , Y LG Indicates the mutual admittance between the busbar and the synchronous machine; Y WL , Y LW represents the mutual admittance between the busbar and the photovoltaic station; Y GG represents the synchronous machine node self-admittance; Y WG , Y GW represents the mutual admittance between the synchronous machine and the photovoltaic station; Y WW Represents the self-admittance of the photovoltaic station node; The mathematical model of the voltage at the photovoltaic station terminal is derived from the node equation (1); The mathematical model of the photovoltaic station terminal voltage is derived from the node equation (1), which is specifically: The voltage at the photovoltaic station can be derived from formula (1): The expression of is, in, represents the voltage of the convergent bus L, and from formula (1) we can get The specific function expression is: Combining equations (2) and (3), we can solve the voltage at the photovoltaic station: Mathematical expressions, The parameters K1 and K2 satisfy: Where X T is the transformer impedance of the synchronous unit, X l1 Indicates the impedance of Line 1; X l2 Indicates the impedance of Line 2; X lc Indicates the total impedance from the potential inside the synchronous machine to the collection bus; X d ” indicates the internal impedance of the synchronous machine; In the "photovoltaic + thermal power" joint transmission system, the photovoltaic station adopts a directional phase-locked control method based on the d-axis voltage of the grid-side converter. d =U W , q-axis component u q =0; PV output current d-axis component i d =I W cosφ i , q-axis component i q =I W sinφ i ,φ i is the power factor angle of the photovoltaic station; will u d 、u q 、i d 、i q Substituting into formula (4) we can get: By simplifying equation (6) with trigonometric functions, we can get the voltage amplitude expression of the photovoltaic station terminal, that is, the mathematical model of the photovoltaic station terminal voltage is: Where P W , Q W They are respectively the active power and reactive power output by the photovoltaic station, and the parameter P W , Q W 、A satisfies:

2. A photovoltaic reactive power evaluation method for a "photovoltaic + thermal power" combined transmission system as claimed in claim 1, characterized in that: Step (2) specifically includes the following steps: When the active output of the synchronous unit and photovoltaic power in the "photovoltaic + thermal power" joint transmission system is given and the system flow is determined, the photovoltaic active power and synchronous machine power angle values ​​can be determined. At this time, a corresponding functional relationship can be established between the photovoltaic terminal voltage amplitude and reactive power. |U W |=f(Q W ) (9) When the system is operating under normal conditions, the voltage at the photovoltaic station terminal should be 1.0pu, which can be used to quantitatively calculate the reactive power Q of the photovoltaic station. W , 3. A control strategy for a "photovoltaic + thermal power" combined transmission system based on the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" combined transmission system described in any one of rights 1-2, characterized in that: Modify the reactive power control strategy of the photovoltaic station so that the photovoltaic station can output reactive power of corresponding capacity according to the reactive power demand solved by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system described in any one of rights 1-4, so as to maintain the stable operation of the terminal voltage.

4. The control strategy of the "photovoltaic + thermal power" combined transmission system based on the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" combined transmission system according to any one of claims 1 to 2 as claimed in claim 3, characterized in that: The modification of the reactive power control strategy of the photovoltaic station enables the photovoltaic station to output reactive power of corresponding capacity according to the reactive power demand solved by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system described in any one of rights 1-4, so as to maintain the stable operation of the terminal voltage, specifically: The photovoltaic grid-side converter adopts a voltage-current dual closed-loop control strategy, in which the d-axis voltage outer loop controls the photovoltaic active power P W , to maintain the DC capacitor voltage stable, the q-axis voltage outer loop control target is to make the photovoltaic output reactive power Q W Follow reactive power reference value Q Wref According to the reactive power demand Q obtained by the photovoltaic reactive power evaluation method of the "photovoltaic + thermal power" joint transmission system described in any one of the rights 1-2, the reactive power reference value Q in the photovoltaic grid-side converter is set. Wref Equal to Q, ensuring that the reactive power output of photovoltaic meets the evaluation requirements.

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