Power distribution network photovoltaic permeability improving method considering voltage control

Abstract

The invention discloses a power distribution network photovoltaic permeability improvement method considering voltage control. The method comprises the following steps: S1, constructing a photovoltaic bearing capacity objective function; s2, constructing a gear-voltage control model based on an on-load voltage regulating transformer (OLTC); s3, constructing a reactive power-voltage control model based on the SVG; s4, constructing an active power-voltage control model of the photovoltaic inverter; s5, constructing an energy storage voltage-active power control model; s6, calculating voltage deviation before and after distributed photovoltaic access, and setting a voltage deviation limit value of the distribution network; and S7, according to the voltage deviation in the step S6, solving the maximum access capacity of each node under different voltage control models, calculating bearing capacity values under different voltage control models according to the step 1, comparing the values, and determining the voltage control model with the maximum bearing capacity value. According to the strategy provided by the invention, the photovoltaic bearing capacity can be improved, and the distributed photovoltaic development requirements of the distribution network can be better met.

Description

Technical Field

[0001] The present invention relates to a distribution network voltage control method, and belongs to the field of distribution network operation and analysis, and specifically to a distribution network photovoltaic penetration rate improvement method considering voltage control. Background Art

[0002] Reverse current and intermittent generation of renewable energy will cause line voltage to rise, which will seriously limit the grid-connected capacity of renewable energy. With the continuous implementation of relevant policies, the proportion of distributed photovoltaic installed capacity in the distribution network is increasing. In addition to traditional voltage control equipment, including on-load tap-changing transformers, SVG, etc., the voltage-active power control technology of photovoltaic inverters and energy storage voltage-active power control technology can control the voltage of the distribution network, but different voltage control technologies have different effects on photovoltaic carrying capacity.

[0003] The size of the photovoltaic carrying capacity is closely related to the degree of voltage over-limit of the distribution network. Different voltage control technologies have different control effects on voltage. How to evaluate the impact of different voltage control technologies on photovoltaic carrying capacity is an urgent problem to be solved. However, existing evaluation methods rarely consider the impact of voltage control technology on carrying capacity, nor do they analyze different voltage control strategies and evaluate different voltage control technologies from the perspective of maximum photovoltaic carrying capacity. 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 distribution network voltage control method, a distribution network photovoltaic penetration rate improvement method taking voltage control into account, and the provided strategy can improve the photovoltaic carrying capacity and better adapt to the development needs of distributed photovoltaic distribution networks.

[0005] A method for improving photovoltaic penetration rate in a distribution network considering voltage control comprises the following steps:

[0006] Step S1: constructing the photovoltaic carrying capacity objective function;

[0007] Step S2: constructing a gear-voltage control model and constraint conditions based on an on-load tap-changing transformer OLTC;

[0008] Step S3: constructing a reactive power-voltage control model and constraint conditions based on SVG;

[0009] Step S4: constructing a photovoltaic inverter active power-voltage control model and constraint conditions;

[0010] Step S5: constructing an energy storage voltage-active power control model and constraint conditions;

[0011] Step S6: Calculate the voltage deviation before and after the distributed photovoltaic access, and set the voltage deviation limit of the distribution network;

[0012] Step S7: According to the voltage deviation in step S6, the maximum access capacity of each node under the four different voltage control models in steps S2-S5 is calculated, and the carrying capacity values ​​under the four different voltage control models are calculated according to step 1, and the values ​​are compared to determine the voltage control model with the largest carrying capacity value.

[0013] Furthermore, step S1 is specifically as follows:

[0014] Assume that the peak load capacity of all feeders in the distribution network area is S DN,max , the total peak capacity of the connected distributed photovoltaic is S PV,max , then the photovoltaic carrying capacity can be calculated by formula (1):

[0015]

[0016] In the formula, HC is the value of photovoltaic carrying capacity;

[0017] Taking the maximum value of photovoltaic carrying capacity as the goal, the photovoltaic carrying capacity objective function is constructed as follows:

[0018] maxF=HC{VC1,VC2,...,VC i ,...,VC n} (2)

[0019] Where, VC i is the i-th control technology.

[0020] Step S2 is specifically as follows:

[0021] The voltage control strategy of OLTC is target-dead-time control. When the difference ΔV between the measured voltage and the set voltage exceeds the threshold voltage V BW When the voltage difference is maintained for a time ΔT i Exceeds the set threshold time ΔT set , OLTC will start to adjust the gear. The time delay setting is to ensure that the voltage deviation is not instantaneous, but a continuous process;

[0022] The voltage control model is as follows:

[0023]

[0024] Where, ΔN tap V is the tap position that needs to be adjusted by OLTC; N is the rated voltage of the distribution network; V a is the average voltage at the measuring point; V step is the OLTC single-step voltage regulation value;

[0025] The constraints of OLTC voltage control technology are as follows:

[0026]

[0027] In the formula, is the number of gears of OLTC at time t; They are the minimum and maximum values ​​of the OLTC gear respectively; is the adjustable number of OLTC at node i; The maximum number of times that OLTC can be adjusted; is the number of gears of OLTC at time t+Δt.

[0028] Furthermore, step S3 is specifically as follows:

[0029] The relationship between reactive power and voltage of SVG equipment is expressed as:

[0030] ΔU SVG =K SVG ΔQ SVG (5)

[0031] In the formula, ΔU SVG K is the voltage change of the SVG output node; SVG is the equivalent droop coefficient; ΔQ SVG is the reactive output change of SVG;

[0032] The constraints of SVG voltage control technology are as follows:

[0033]

[0034] In the formula, is the reactive power output by SVG; They are the minimum and maximum values ​​of the reactive power output by SVG respectively.

[0035] Furthermore, step S4 is specifically as follows:

[0036] If the voltage U meas Lower than the upper limit of the allowable voltage U up When the photovoltaic power generation is connected to the grid according to the maximum power point tracking MPPT power; if the voltage of the grid-connected node exceeds the upper limit of the allowable voltage, the photovoltaic power generation is no longer connected to the grid according to the MPPT power, but the photovoltaic grid-connected active power is cut according to the predetermined voltage-active power curve; the active power reduction amount P can be calculated according to the following formula R ,

[0037]

[0038] Where: β is the proportional coefficient, which can be set according to actual conditions;

[0039] The constraint condition of the active power-voltage control model of the photovoltaic inverter is the active power output of the photovoltaic inverter, that is:

[0040] P PV,min ≤P PV ≤P PV,max (8).

[0041] Furthermore, step S5 is specifically as follows:

[0042] The relationship between the adjustable active power of the energy storage device and the energy storage SOC is:

[0043]

[0044] Where: S(t) is the energy storage SOC at time t; S(t-Δt) is the energy storage SOC at time t-Δt; ΔS(t) is the energy storage SOC change at time t; P ESS (t) is the active power of energy storage at time t; C ESS is the capacity of the energy storage device; Δt is the time interval;

[0045] Energy storage needs to meet active power and SOC constraints, namely:

[0046]

[0047] In the formula, S i is the SOC of energy storage; S i and are the lower and upper limits of energy storage SOC respectively; P i,ESS is the active power output of the energy storage; They are the minimum and maximum values ​​of the active power output by the energy storage respectively.

[0048] Furthermore, step S6 is specifically as follows:

[0049] Now assume that the voltage of node i before connecting to distributed photovoltaic is U i0 , the voltage after connecting to distributed photovoltaic is U i1 , then the voltage deviation ΔU i for

[0050]

[0051] Where P and Q are the active and reactive power generated by the photovoltaic power source respectively; P L and Q L are the local active and reactive loads of the grid-connected point respectively; R and X are the line resistance and reactance.

[0052] Set the voltage deviation range to ΔU i∈[-0.10,0.07], when there is no voltage control strategy, if the voltage deviation of distributed photovoltaics exceeds the range after connection, it cannot be connected to the grid. When different voltage control strategies are adopted, if the voltage deviation of distributed photovoltaics after connection is within the set range, photovoltaics are allowed to be connected to the grid.

[0053] Furthermore, step S7 is specifically as follows:

[0054] According to formula (11) in step 6, formula (9) in step S5, formula (7) in step S4, formula (5) in step S3 and formula (3) in step S2, the maximum access capacity of each node under the four voltage control models can be calculated, and then the access capacity of each node is accumulated;

[0055] According to formula (1), the carrying capacity of photovoltaic access when using four voltage control technologies can be calculated as HC OLCT , HC SVG , HC PV and HC ESS , and then compare the calculated carrying capacity values. The VVC technology with the largest carrying capacity value can be used as a method to improve the photovoltaic carrying capacity of the distribution network.

[0056] The present invention starts from the principles of different voltage control technologies, analyzes the impact of different voltage control technologies on carrying capacity, and seeks the most suitable voltage control technology with the goal of maximizing carrying capacity. Compared with existing methods, the strategy provided by the present invention can improve the carrying capacity of photovoltaics and better adapt to the development needs of distributed photovoltaic distribution networks. Compared with existing technologies, the voltage regulation resources considered are more comprehensive, the voltage control strategies involved are richer, and are more in line with the actual situation of distribution network stations. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a flow chart of the method of the present invention;

[0058] Figure 2 Flowchart of the voltage control strategy for OLTC. DETAILED DESCRIPTION

[0059] In order to make the implementation and advantages of the technical method of the present invention clear, the implementation process is described in detail. The implementation process is as follows: Figure 1 The present invention provides a method for improving photovoltaic penetration rate of a distribution network considering voltage control, and its implementation steps are as follows:

[0060] Step S1: PV carrying capacity objective function

[0061] In order to quantify the impact of different voltage control technologies on carrying capacity, it is necessary to define the ratio of PV peak capacity to feeder load peak capacity. Now assume that the peak load capacity of all feeders in the distribution network area is SDN,max , the total peak capacity of the connected distributed photovoltaic is S PV,max , then the photovoltaic carrying capacity can be calculated by formula (1).

[0062]

[0063] Where HC is the value of photovoltaic carrying capacity.

[0064] In order to select the most appropriate voltage control (VC) technology, it is necessary to maximize the PV carrying capacity, so the objective function is:

[0065] maxF=HC{VC1,VC2,...,VC i ,...,VC n} (2)

[0066] Where, VC i is the i-th control technology.

[0067] Step S2: Gear-voltage control model based on on-load tap-changing transformer (OLTC)

[0068] The voltage control strategy of OLTC proposed in the present invention is target-dead zone-delay control, such as Figure 2 As shown, when the difference ΔV between the measured voltage and the set voltage exceeds the threshold voltage V BW When the voltage difference is maintained for a time ΔT i Exceeds the set threshold time ΔT set , OLTC will start to adjust the gear. The time delay setting is to ensure that the voltage deviation is not instantaneous, but a continuous process. The voltage control model is as follows:

[0069]

[0070] Where, ΔN tap V is the tap position that needs to be adjusted by OLTC; N is the rated voltage of the distribution network; V a is the average voltage at the measuring point; V step is the OLTC single-step voltage regulation value.

[0071] The constraints of OLTC voltage control technology are as follows:

[0072]

[0073] In the formula, is the number of gears of OLTC at time t; They are the minimum and maximum values ​​of the OLTC gear respectively; is the adjustable number of OLTC at node i; The maximum number of times that OLTC can be adjusted; is the number of gears of OLTC at time t+Δt.

[0074] Step S3: Reactive power-voltage control model based on SVG

[0075] The relationship between reactive power and voltage of SVG equipment is expressed as:

[0076] ΔU SVG =K SVG ΔQ SVG (5)

[0077] In the formula, ΔU SVG K is the voltage change of the SVG output node; SVG is the equivalent droop coefficient; ΔQ SVG is the change in reactive output of SVG

[0078] The constraints of SVG voltage control technology are as follows:

[0079]

[0080] In the formula, is the reactive power output by SVG; They are the minimum and maximum values ​​of the reactive power output by SVG respectively.

[0081] Step S4: PV inverter active power-voltage control model

[0082] If the voltage U meas Lower than the upper limit of the allowable voltage U up When the photovoltaic power generation is connected to the grid according to the maximum power point tracking (MPPT) power; if the voltage of the grid-connected node exceeds the upper limit of the allowable voltage, the photovoltaic power generation is no longer connected to the grid according to the MPPT power, but the photovoltaic grid-connected active power is cut according to the predetermined voltage-active power curve; the active power reduction amount P can be calculated according to the following formula R .

[0083]

[0084] Where: β is the proportional coefficient, which can be set according to actual conditions;

[0085] The constraint condition of the active power-voltage control model of the photovoltaic inverter is the active power output of the photovoltaic inverter, that is:

[0086] P PV,min ≤P PV ≤P PV,max (8)

[0087] Step S5: Energy storage voltage-active power control model

[0088] The relationship between the adjustable active power of the energy storage device and the energy storage SOC is:

[0089]

[0090] Where: S(t) is the energy storage SOC at time t; S(t-Δt) is the energy storage SOC at time t-Δt; ΔS(t) is the energy storage SOC change at time t; P ESS (t) is the active power of energy storage at time t; C ESS is the capacity of the energy storage device; Δt is the time interval.

[0091] Energy storage needs to meet active power and SOC constraints, namely:

[0092]

[0093] In the formula, S i is the SOC of energy storage; S i and are the lower and upper limits of energy storage SOC respectively; P i,ESS is the active power output of the energy storage; They are the minimum and maximum values ​​of the active power output by the energy storage respectively.

[0094] Step S6: Now assume that the voltage of node i before connecting to distributed photovoltaic is U i0 , the voltage after connecting to distributed photovoltaic is U i1 , then the voltage deviation ΔU i for

[0095]

[0096] Where P and Q are the active and reactive power generated by the photovoltaic power source respectively; P L and Q L are the local active and reactive loads of the grid-connected point respectively; R and X are the line resistance and reactance.

[0097] Considering the national standard of distribution network voltage deviation, the voltage deviation range is set to ΔU i ∈[-0.10,0.07], when there is no voltage control strategy, if the voltage deviation of distributed photovoltaics exceeds the range after access, it cannot be connected to the grid. When different voltage control strategies are adopted (OLTC gear, SVG reactive power, photovoltaic inverter and energy storage active power control), if the voltage deviation of distributed photovoltaics after access is within the set range, photovoltaics are allowed to be connected to the grid.

[0098] Step S7: According to formula (11) in step S6, formula (9) in step S5, formula (7) in step S4, formula (5) in step S3 and formula (3) in step S2, the maximum access capacity of each node under the four voltage control technologies can be calculated, and then the access capacity of each node is accumulated. According to formula (1), the carrying capacity of photovoltaic access when using the four voltage control technologies can be calculated as HC OLCT , HC SVG , HC PV and HC ESS Then, the calculated carrying capacity values ​​are compared, and the VVC technology with the largest carrying capacity value can be used as a method to improve the photovoltaic carrying capacity of the distribution network.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for improving photovoltaic penetration rate in a distribution network considering voltage control, characterized in that: The following steps are involved: Step S1: constructing the photovoltaic carrying capacity objective function; Step S2: constructing a gear-voltage control model and constraint conditions based on an on-load tap-changing transformer OLTC; Step S3: constructing a reactive power-voltage control model and constraint conditions based on SVG; Step S4: constructing a photovoltaic inverter active power-voltage control model and constraint conditions; Step S5: constructing an energy storage voltage-active power control model and constraint conditions; Step S6: Calculate the voltage deviation before and after the distributed photovoltaic access, and set the voltage deviation limit of the distribution network; Step S7: According to the voltage deviation in step S6, the maximum access capacity of each node under the four different voltage control models in steps S2-S5 is calculated, and the carrying capacity values ​​under the four different voltage control models are calculated according to step 1, and the values ​​are compared to determine the voltage control model with the largest carrying capacity value.

2. The method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 1, characterized in that: Step S1 is specifically as follows: Assume that the peak load capacity of all feeders in the distribution network area is S DN,max , the total peak capacity of the connected distributed photovoltaic is S PV,max , then the photovoltaic carrying capacity is calculated by formula (1): In the formula, HC is the value of photovoltaic carrying capacity; Taking the maximum photovoltaic carrying capacity as the goal, the photovoltaic carrying capacity objective function is constructed as follows: maxF=HC{VC1,VC2,...,VC i ,...,YOU n } (2) Where, VC i is the i-th control technology.

3. The method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 1, characterized in that: Step S2 is specifically as follows: The voltage control strategy of OLTC is target-dead-time control. When the difference ΔV between the measured voltage and the set voltage exceeds the threshold voltage V BW When the voltage difference is maintained for a time ΔT i Exceeds the set threshold time ΔT set , OLTC will start to adjust the gear. The time delay setting is to ensure that the voltage deviation is not instantaneous, but a continuous process; The voltage control model is as follows: Where, ΔN tap V is the tap position that needs to be adjusted by OLTC; N is the rated voltage of the distribution network; V a is the average voltage at the measuring point; V step is the OLTC single-step voltage regulation value; The constraints of OLTC voltage control technology are as follows: In the formula, is the number of gears of OLTC at time t; and They are the minimum and maximum values ​​of the OLTC gear respectively; is the adjustable number of OLTC at node i; The maximum number of times that OLTC can be adjusted; is the number of gears of OLTC at time t+Δt.

4. The method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 1, characterized in that: Step S3 is specifically as follows: The relationship between reactive power and voltage of SVG equipment is expressed as: ΔU SVG =K SVG ·ΔQ SVG (5) In the formula, ΔU SVG K is the voltage change of the SVG output node; SVG is the equivalent droop coefficient; ΔQ SVG is the reactive output change of SVG; The constraints of SVG voltage control technology are as follows: In the formula, is the reactive power output by SVG; and They are the minimum and maximum values ​​of the reactive power output by SVG respectively.

5. The method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 1, characterized in that: Step S4 is specifically as follows: If the voltage U meas Lower than the upper limit of the allowable voltage U up When the photovoltaic power generation is connected to the grid according to the maximum power point tracking MPPT power; if the voltage of the grid-connected node exceeds the upper limit of the allowable voltage, the photovoltaic power generation is no longer connected to the grid according to the MPPT power, but the photovoltaic grid-connected active power is cut off according to the predetermined voltage-active power curve; the active power reduction amount P is calculated according to the following formula R , Where: β is the proportional coefficient, which can be set according to actual conditions; The constraint condition of the active power-voltage control model of the photovoltaic inverter is the active power output of the photovoltaic inverter, that is: P PV,min ≤P PV ≤P PV,max (8)。 6. The method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 1, characterized in that: Step S5 is specifically as follows: The relationship between the adjustable active power of the energy storage device and the energy storage SOC is: Where: S(t) is the energy storage SOC at time t; S(t-Δt) is the energy storage SOC at time t-Δt; ΔS(t) is the energy storage SOC change at time t; P ESS (t) is the active power of energy storage at time t; C ESS is the capacity of the energy storage device; Δt is the time interval; Energy storage needs to meet active power and SOC constraints, namely: In the formula, S i is the SOC of energy storage; S i and are the lower and upper limits of energy storage SOC respectively; P i,ESS is the active power output of the energy storage; and They are the minimum and maximum values ​​of the active power output by the energy storage respectively.

7. The method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 1, characterized in that: Step S6 is specifically as follows: Assume that the voltage of node i before connecting to distributed photovoltaic is U i0 , the voltage after connecting to distributed photovoltaic is U i1 , then the voltage deviation ΔU i for Where P and Q are the active and reactive power generated by the photovoltaic power source respectively; P L and Q L are the local active and reactive loads of the grid-connected point respectively; R and X are the line resistance and reactance. Set the voltage deviation range to ΔU i ∈[-0.10,0.07], when there is no voltage control strategy, if the voltage deviation of distributed photovoltaics exceeds the range after connection, it cannot be connected to the grid. When different voltage control strategies are adopted, if the voltage deviation of distributed photovoltaics after connection is within the set range, photovoltaics are allowed to be connected to the grid.

8. A method for improving photovoltaic penetration rate of distribution network considering voltage control according to claim 7, characterized in that: Step S7 is specifically as follows: According to formula (11) in step S6, formula (9) in step S5, formula (7) in step S4, formula (5) in step S3 and formula (3) in step S2, the maximum access capacity of each node under the four voltage control models is calculated, and then the access capacity of each node is accumulated; According to formula (1), the carrying capacity of photovoltaic access when four voltage control technologies are used is HC OLCT , HC SVG , HC PV and HC ESS , and then compare the calculated carrying capacity values. The VVC technology with the largest carrying capacity value is used as a method to improve the photovoltaic carrying capacity of the distribution network.

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