Reactive power and active power comprehensive control method based on photovoltaic grid-connected inverter

Abstract

The invention relates to a reactive power and active power comprehensive control method based on a photovoltaic grid-connected inverter. The method comprises the following steps: calculating the influence of a photovoltaic grid-connected inverter on the voltage of a power distribution network; calculating a voltage out-of-limit problem caused when a plurality of photovoltaic power supplies are connected to the power distribution network according to the influence of one photovoltaic grid-connected power supply on the voltage of the power distribution network; aiming at the voltage out-of-limit problem, on the premise of not changing the line structure of the power distribution network, not adding a reactive power compensation device and not using an on-load tap changer, a reactive power and active power comprehensive control strategy is constructed by taking the low-voltage power distribution network as a management node; the inverter is used for absorbing reactive power, unnecessary active loss is avoided, and the voltage of the power distribution network is reduced by cutting off part of active power. According to the invention, active power adjustment is added on the basis of reactive power adjustment. And when the reactive power cannot adjust the voltage to a specified range, the active power is increased to enable the voltage to reach the standard.

Description

Technical Field

[0001] The invention belongs to the technical field of power grid control, and in particular to a reactive and active power comprehensive control method based on a photovoltaic grid-connected inverter. Background Art

[0002] With the increasing depletion of global renewable energy and the intensification of climate change, fossil energy has become an inevitable energy source. The trend of vigorously developing energy to replace fossil energy is driving the transformation and development of society. China is carrying out a new round of power system reform and energy Internet. In the traditional distribution network, there are many uncertain factors affecting the voltage control effect due to the fluctuation distribution of load. Since the characteristics of photovoltaic power generation and its output power change with the changes in the environment, it is easy to cause power quality problems after it is connected to the distribution network.

[0003] With the development of technology, the drawbacks of traditional centralized power systems, long-distance power transmission and large-system interconnection have become increasingly prominent. The combination of distributed generation and traditional power sources has been recognized as the main way to save investment, reduce energy consumption, and improve grid reliability and flexibility. It has significant economic and social benefits.

[0004] However, the expansion of the scale of distributed photovoltaic power generation connected to the grid will change the structure of the distribution network, which is bound to affect the distribution of the original network flow, thereby deteriorating the power quality of the distribution network. The main factors affecting the power grid by distributed photovoltaics are: voltage over-limit, fluctuation, flicker and harmonic emission. The change in solar radiation intensity caused by cloud movement leads to changes in the output power of the photovoltaic system, which is the main cause of voltage fluctuations and flicker and is an uncontrollable factor. At present, the harmonic distortion of the inverter is within 3%, which meets the grid connection requirements. Therefore, voltage over-limit is regarded as a constraint condition for power quality management. In recent years, more and more researchers have proposed the use of photovoltaic inverters to solve the problem of voltage over-limit at the point of interconnection (POI) of large-scale photovoltaic grid-connected systems (LPVGS). At present, voltage reactive power control based on photovoltaic grid-connected inverters has been studied by many scholars. A variety of control schemes have been proposed, such as constant reactive power control, constant power factor control, and control based on PV active power output. (P) control method and Q(U) control method based on POI voltage amplitude. Constant reactive power control is to output a certain amount of reactive power on the basis of delivering active power, but when the system does not need reactive power, reactive power will also be emitted, increasing network losses. Constant power factor control is to generate reactive power based on the power factor, but when the photovoltaic output is very small, in order to maintain a constant power factor, additional reactive power needs to be emitted, which will also increase network losses. In the (p) control method, the power factor is set by the PV reactive output, which overcomes the disadvantages of constant power factor control. When the PV active power output is large and it happens to be the load peak, the POI voltage does not exceed the limit, and the excess reactive power output on the line will increase the grid loss. The Q(U) control based on the POI voltage amplitude takes into account the POI voltage factor, but does not take into account the PV active power output and load. Compared with constant power factor control and Compared with the (P) control method, its total reactive power and network loss are the smallest, but its voltage regulation ability is weaker. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and propose a reactive and active power comprehensive control method based on a photovoltaic grid-connected inverter, which uses active power and reactive power comprehensive control to increase active power regulation on the basis of reactive power regulation; when the reactive power cannot adjust the voltage to the specified range, the voltage is brought up to standard by increasing the active power.

[0006] The present invention solves the technical problem by adopting the following technical solutions:

[0007] A reactive and active power integrated control method based on a photovoltaic grid-connected inverter comprises the following steps:

[0008] Step 1: Calculate the impact of a photovoltaic grid-connected unit on the distribution network voltage;

[0009] Step 2: Calculate the voltage over-limit problem caused by connecting multiple photovoltaic power sources to the distribution network according to step 1;

[0010] Step 3: For the voltage over-limit problem in step 2, without changing the line structure of the distribution network, adding reactive power compensation devices, or using on-load tap changers, a comprehensive reactive and active power control strategy is constructed with the low-voltage distribution network as the management node;

[0011] Step 4: Use the inverter to absorb reactive power, avoid unnecessary active power loss, and reduce the distribution network voltage by cutting off part of the active power.

[0012] Moreover, the specific implementation method of step 1 is:

[0013] (1) Before photovoltaic power generation is connected to the grid: Ignoring line losses, the voltage drop between node i and node i-1 is:

[0014]

[0015] Among them, U i is the voltage at node i, U i-1 is the voltage of node i-1, N is the node of the distribution network, R i +jX iis the line impedance between node i and node i-1, P i +jQ i is the power consumed by the load node i; the voltage drop is the active and reactive power consumed by the user. Therefore, U i is less than 0, the feeder voltage drop is positive, the farther node i is from the power source, the lower its voltage. The specific voltage value of node i is as follows:

[0016]

[0017] Among them, U 0 is the voltage of the load line head, U i0 is the voltage of node i before PV connection;

[0018] (2) After PV access: The PV power access point is p, and the PV power capacity is P PV +jQ PV ;

[0019] a. When the load node i is before p: 0 < i < p, the voltage drop between node i and node i-1 is:

[0020]

[0021] The voltage of the load node i is:

[0022]

[0023] When the PV power source is connected, the voltage increases;

[0024] If the power factor of the PV grid-connected inverter is 1, the voltage drop between node i and node i-1 is:

[0025]

[0026] When ΔU i = U i - U i-1 < 0, thus reducing the voltage along the line; when ΔU i = U i - U i-1 > 0, so the voltage along the line increases;

[0027] b. When the load node i is after p: p < i < N, the voltage drop between node i and node i-1 is:

[0028]

[0029] The voltage of the load node i is:

[0030]

[0031] PV grid connection increases the voltage of node i, but from equation (6), when i ≥ p, ΔU i =U i -U i-1 <0, therefore, the voltage of the node after p gradually decreases along the line. When PV power is connected to the grid, the node voltage increases compared with the node voltage before PV power is connected in equations (4) and (7). When the PV output power is large, the line voltage presents two possible trends: the line voltage increases first and then decreases, and the line voltage decreases first, then increases, and finally decreases. When the capacity of the grid-connected photovoltaic is large, the voltage near the POI may exceed the upper limit. When the capacity is low, the voltage at the terminal of the power line is less than the lower limit.

[0032] Moreover, the specific implementation method of step 2 is: there are N nodes on the feeder, and the load capacity of node i is P i +jQ i , the photovoltaic grid-connected capacity of node i is P vi +jQ vi ; The voltage at the head of the line is U 0 , the voltage at node i is U i , the line impedance between node i and node i-1 is R i +jX i ;

[0033] Before the PV power is connected to the grid, the voltage drop ΔU between node i and node i-1 is i for:

[0034]

[0035] The voltage U at node i i0 for:

[0036]

[0037] After the PV power source is connected, the voltage drop between node i and node i-1 is ΔU i for:

[0038]

[0039] The voltage Ui at node i is:

[0040]

[0041] Based on equation (11), when PV power is connected to the grid, the node voltage at any point of the line will increase. According to equation (10), large-capacity PV connected to the grid can increase the probability of the line voltage exceeding the upper limit.

[0042] Moreover, the specific implementation method of step 3 is:

[0043] (1) The power factor of the photovoltaic grid-connected inverter is 1 at the beginning;

[0044] (2) Assume the voltage range is U min i max , U max =1.07U n , U min =0.93U n , U n is the nominal voltage;

[0045] (3) According to the power factor range of the inverter, the reactive power range of the inverter is Q min v max Q min =-P pv *0.95, Q max =P pv *0.95;

[0046] (4) Check the voltage of the POI. If the voltage exceeds the specified range, add reactive power. Adjust the power factor It starts from 1 and gradually decreases with an empirical step size of 0.01 until the voltage is within the specified range or the reactive power Q v Exceeding the prescribed range;

[0047] (5) When Q v Out of range and voltage is still greater than 1.07U n When the active power is increased, it is adjusted.

[0048] Moreover, the specific implementation method of step 4 is: the inverter adopts a feedforward decoupling control strategy based on Park transformation to enable the grid-connected inverter to achieve decoupling control of active and reactive power axes.

[0049] The advantages and positive effects of the present invention are:

[0050] ​​​​The present invention calculates the impact of one photovoltaic grid-connected power plant on the voltage of the distribution network; calculates the voltage over-limit problem caused by multiple photovoltaic power sources connected to the distribution network based on the impact of one photovoltaic grid-connected power plant on the voltage of the distribution network; for the voltage over-limit problem, without changing the line structure of the distribution network, without adding reactive power compensation devices, and without using on-load tap-changing switches, a low-voltage distribution network is used as a management node to construct a comprehensive control strategy for reactive and active power; an inverter is used to absorb reactive power to avoid unnecessary active power loss, and the voltage of the distribution network is reduced by cutting off part of the active power. The present invention proposes to add active power regulation on the basis of reactive power regulation. When the reactive power cannot adjust the voltage to the specified range, the voltage is brought up to standard by increasing the active power. The simulation results show that under a given voltage regulation target, the above control method will be beneficial to the stable operation of the distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 It is a load distribution model of a single distributed photovoltaic power source connected to a low-voltage distribution line of the present invention.

[0052] Figure 2 It is a load distribution model of multiple distributed photovoltaic power sources connected to a low-voltage distribution line in the present invention.

[0053] Figure 3 It is a flow chart of the active and reactive comprehensive control strategy of the present invention.

[0054] Figure 4 It is a control diagram under the reactive and active power control strategy of the inverter of the present invention.

[0055] Figure 5 Schematic diagram of voltage change of a photovoltaic access point according to an embodiment of the present invention. DETAILED DESCRIPTION

[0056] The present invention is further described in detail below with reference to the accompanying drawings.

[0057] A reactive and active power integrated control method based on a photovoltaic grid-connected inverter, characterized in that it comprises the following steps:

[0058] Step 1: Calculate the impact of a photovoltaic grid-connected device on the distribution network voltage.

[0059] When a single distributed photovoltaic power source is connected to a distribution user, it may cause serious voltage over-limit on the distribution lines around the POI. The load distribution model of a single distributed photovoltaic power source connected to a low-voltage distribution line is as follows: Figure 1 shown.

[0060] (1) Before PV power generation is connected to the grid: Ignoring line losses, the voltage drop between node i and node i-1 can be described as:

[0061]

[0062] The user consumes active power and reactive power (mainly inductive loads). Therefore, U i is less than 0, and the feeder voltage drop is positive. The farther node i is from the power source, the lower its voltage. The specific voltage value of node i is as follows:

[0063]

[0064] where U 0 is the voltage at the load terminal, and U i0 is the voltage at node i before PV connection.

[0065] (2) After PV connection: Assume that the PV power source connection point is p, and the PV power source capacity is P PV +jQ PV .

[0066] a. When load node i is before p: 0 < i < p, the voltage drop between node i and node i - 1 is:

[0067]

[0068] The voltage of load node i is:

[0069]

[0070] Therefore, when the PV power source is connected, the voltage increases.

[0071] Assume that the power factor of the PV grid-connected inverter is 1. Then the voltage drop between node i and node i - 1 is:

[0072]

[0073] When , ΔU i = U i - U i-1 < 0, thus reducing the voltage along the line; when , ΔU i = U i - U i-1 > 0, so the voltage along the line increases.

[0074] b. When load node i is after p: p < i < N, the voltage drop between node i and node i - 1 is:

[0075]

[0076] The voltage of load node i is:

[0077]

[0078] PV grid connection increases the voltage of node i, but from equation (6), it can be seen that when i ≥ p, ΔU i =U i -U i-1 <0, therefore, the voltage of the node after p gradually decreases along the line. In summary, when PV power is connected to the grid, the node voltage increases compared to the node voltage before the PV power is connected according to equations (4) and (7). When the PV output power is large, the line voltage presents two possible trends: the line voltage increases first and then decreases, and the line voltage decreases first, then increases, and finally decreases. When the capacity of the grid-connected photovoltaic is large, the voltage near the POI may exceed the upper limit. When the capacity is low, the voltage at the terminal of the power line may be less than the lower limit.

[0079] Step 2: Based on step 1, calculate the voltage over-limit problem caused by connecting multiple photovoltaic power sources to the distribution network.

[0080] The load distribution model of multiple distributed photovoltaic power sources connected to low-voltage distribution lines is as follows: Figure 2 Assume that there are N nodes on the feeder and the load capacity of node i is P i +jQ i , the photovoltaic grid-connected capacity of node i is P vi +jQ vi The voltage at the head of the line is U 0 , the voltage at node i is U i , the line impedance between node i and node i-1 is R i +jX i .

[0081] Before the PV power is connected to the grid, the voltage drop ΔU between node i and node i-1 is i for:

[0082]

[0083] The voltage U at node i i0 for:

[0084]

[0085] After the PV power source is connected, the voltage drop between node i and node i-1 is ΔU i for:

[0086]

[0087] The voltage Ui at node i is:

[0088]

[0089] Based on equation (11), when PV power is connected to the grid, the node voltage at any point of the line will increase. According to equation (10), large-capacity PV connected to the grid can increase the probability of the line voltage exceeding the upper limit.

[0090] Step 3: In response to the voltage over-limit problem in step 2, without changing the line structure of the distribution network, adding reactive power compensation devices, or using on-load tap-changing switches, a comprehensive reactive and active power control strategy is constructed with the low-voltage distribution network as the management node.

[0091] From the above analysis, it can be seen that the access of photovoltaic power sources has changed the original network power flow distribution and affected the voltage quality of the network. If the above voltage over-limit problem cannot be solved well, it will directly limit the scale of photovoltaic grid-connected power generation systems. Therefore, voltage management of distribution networks has become one of the biggest challenges for the widespread development of photovoltaic grid-connected power generation systems in China. In response to the voltage over-limit problem, a reactive and active power comprehensive control strategy based on photovoltaic grid-connected inverters is proposed without changing the line structure of the distribution network, adding reactive compensation devices, or using on-load tap-changing switches.

[0092] The strategy uses the low-voltage distribution network as the management node, and the deviation of the three-phase power supply voltage of 20kV and below is ±7% of the nominal voltage. It can be seen from (4) and (11) that when the voltage is outside the standard range, the POI voltage can be adjusted by using the photovoltaic grid-connected inverter to transmit or absorb reactive power (inductive). According to the standard, the power factor of the inverter is in the range of (-0.95, 0.95). When the reactive power with a power factor in this range cannot adjust the voltage to the specified range, the active power is adjusted to make the voltage meet the standard.

[0093] like Figure 3 As shown, the control strategy process includes the following steps:

[0094] (1) The power factor of the photovoltaic grid-connected inverter is 1 at the beginning.

[0095] (2) Assume the voltage range is U min i max (U max =1.07U n , U min =0.93U n , U n is the nominal voltage).

[0096] (3) According to the power factor range of the inverter, assuming that the reactive power range of the inverter is Q min v max (Q min =-P pv *0.95, Q​​​​max =P pv *0.95).

[0097] (4) Check the voltage of the POI. If the voltage exceeds the specified range, add reactive power. Adjust the power factor Start from 1 and gradually decrease (the empirical step size is 0.01) until the voltage is within the specified range or the reactive power Q v Out of the specified range.

[0098] (5) When Q v Out of range and voltage is still greater than 1.07U n When the active power is increased, it is adjusted.

[0099] Step 4: Use the inverter to absorb reactive power, avoid unnecessary active power loss, and reduce the distribution network voltage by cutting off part of the active power.

[0100] The inverter adopts a feedforward decoupling control strategy based on Park transformation to enable the grid-connected inverter to achieve decoupling control of the active and reactive power axes. is the reference value of DC voltage under maximum power point tracking (MPPT), Q ref and P ref They are the reference values ​​for reactive power and active power regulation respectively. Figure 4 The control diagram of the inverter under reactive and active power control strategies is shown. The specific steps are as follows:

[0101] (1) At the beginning, the power factor is 1, and control paths 2 and 4 are the inputs of the current inner loop control.

[0102] (2) When the voltage exceeds the specified range, reactive power is regulated first, and control paths 2 and 3 are selected as the current inner loop control inputs.

[0103] (3) When the reactive power exceeds the specified range but the voltage is still greater than the maximum value, active power regulation is added and paths 1 and 3 are selected as the inner loop control inputs.

[0104] According to the above-mentioned reactive and active power comprehensive control method based on photovoltaic grid-connected inverter, calculation is performed by taking the IEEE-33 node standard distribution network as an example to verify the effect of the present invention.

[0105] The normal voltage of the transmission line is 380V, and the maximum voltage of the distribution network is 1.05Un. The capacity of the transmission line is 400kVA. A single PV power source is connected to the distribution network at node 6. The voltage fluctuations of nodes 5, 17 and 25 during the day are as follows: Figure 5As shown in the figure, the initial PV power is 2kW. As the PV active power increases, the voltages at nodes 5 and 25 exceed 7%, resulting in voltage exceeding the limit.

[0106] To ensure that the voltage of each node meets the operating requirements, the reactive power control strategy is used to solve the voltage over-limit problem. The voltages before and after the control strategy at nodes 0, 5, 25, 6, 8, 21, and 30 are shown in Table 1. When PV generation is connected to node 5, the voltages at nodes 5, 6, and 25 exceed the limit. After adding the control strategy, the node voltages are within the normal range.

[0107] Table 1 Voltage before and after control strategy

[0108] Examples Node 0 Node 5 Node 25 Node 6 Node 8 Node 21 Node 30 Before control strategy 220V 243V 239V 238V 235V 212V 218V Before control strategy 220V 232V 228V 229V 223V 211V 215V

[0109] The present invention analyzes the voltage over-limit problem caused by the access of single / multiple photovoltaic power sources to the distribution network. Taking the 380V low-voltage distribution network as the management node, the reactive and active power control strategies are proposed in combination with the standard. First, the inverter is used to absorb reactive power first to avoid unnecessary active power loss. If the voltage is still over-limit, the distribution network voltage is further reduced by cutting off part of the active power. The results show that under a given voltage regulation target, the above control method will be conducive to the stable operation of the distribution network.

[0110] It should be emphasized that the embodiments described in the present invention are illustrative rather than restrictive. Therefore, the present invention includes but is not limited to the embodiments described in the specific implementation manner. Any other implementation manners derived by those skilled in the art based on the technical solution of the present invention also fall within the scope of protection of the present invention.

Claims

1. A reactive and active power integrated control method based on a photovoltaic grid-connected inverter, characterized in that: It includes the following steps: Step 1: Calculate the impact of a single photovoltaic grid connection on the distribution network voltage; Step 2: Calculate the voltage over-limit problem caused by multiple photovoltaic power sources connecting to the distribution network according to Step 1; Step 3: For the voltage over-limit problem in Step 2, without changing the distribution network line structure, without adding reactive power compensation devices, and without using on-load tap changers, taking the low-voltage distribution network as the management node, construct a comprehensive control strategy for reactive and active power; Step 4: Use the inverter to absorb reactive power to avoid unnecessary active power loss, and reduce the distribution network voltage by cutting off part of the active power.

2. The method for comprehensive control of reactive and active power based on photovoltaic grid-connected inverter according to claim 1, characterized in that: The specific implementation method of Step 1 is as follows: (1) Before photovoltaic power generation is connected to the grid: Ignoring line loss, the voltage drop between node i and node i-1 is: Among them, U i is the voltage at node i, U i-1 is the voltage of node i-1, N is the node of the distribution network, R i +jX i is the line impedance between node i and node i-1, P i +jQ i is the power consumed by load node i; the voltage drop is the active power and reactive power consumed by the user, so U i Less than 0, the feeder voltage drop is positive. The farther the node i is from the power source, the lower its voltage. The specific voltage value of node i is as follows: Among them, U0 is the voltage of the load line end, U i0 is the voltage at node i before PV is connected; (2) After PV access: PV power access point is p, PV power capacity is P PV +jQ PV ; a. Before load node i at p: 0 < i < p, the voltage drop between node i and node i-1 is: The voltage of load node i is: When a PV power source is connected, the voltage increases; If the power factor of the photovoltaic grid-connected inverter is 1, the voltage drop between node i and node i-1 is: when When ΔU i =U i -U i-1 <0, thus reducing the voltage along the line; when When ΔU i =U i -U i-1 >0, so the voltage along the line increases; b. After load node i at p: p < i < N, the voltage drop between node i and node i-1 is: The voltage of load node i is: PV grid connection increases the voltage of node i, but from equation (6), when i ≥ p, ΔU i =U i -U i-1 <0, therefore, the voltage of the node after p gradually decreases along the line. When PV power is connected to the grid, the node voltage increases compared with the node voltage before PV power is connected in equations (4) and (7). When the PV output power is large, the line voltage presents two possible trends: the line voltage increases first and then decreases, and the line voltage decreases first, then increases, and finally decreases. When the capacity of the grid-connected photovoltaic is large, the voltage near the POI may exceed the upper limit. When the capacity is low, the voltage at the terminal of the power line is less than the lower limit.

3. The method for comprehensive control of reactive and active power based on photovoltaic grid-connected inverter according to claim 1, characterized in that: The specific implementation method of step 2 is: there are N nodes on the feeder, and the load capacity of node i is P i +jQ i , the photovoltaic grid-connected capacity of node i is P vi +jQ vi ; The voltage at the head of the line is U0, and the voltage at node i is U i , the line impedance between node i and node i-1 is R i +jX i ; Before the PV power is connected to the grid, the voltage drop ΔU between node i and node i-1 is i for: The voltage U at node i i0 for: After the PV power source is connected, the voltage drop between node i and node i-1 is ΔU i for: The voltage Ui of node i is: Based on Equation (11), when PV power is connected to the grid, the node voltage at any point on the line will increase. According to Equation (10), connecting a large-capacity PV to the grid can increase the probability of the line voltage exceeding the upper limit.

4. The method for comprehensive control of reactive and active power based on photovoltaic grid-connected inverter according to claim 1, characterized in that: The specific implementation method of Step 3 is as follows: (1) The power factor of the photovoltaic grid-connected inverter is 1 at the beginning; (2) Assume the voltage range is U min i max , U max =1.07U n , U min =0.93U n , U n is the nominal voltage;​​ (3) According to the power factor range of the inverter, the reactive power range of the inverter is Q min v max Q min =-P pv *0.95, Q max =P pv *0.95;​​ (4) Check the voltage of the POI. If the voltage exceeds the specified range, add reactive power. Adjust the power factor It starts from 1 and gradually decreases with an empirical step size of 0.01 until the voltage is within the specified range or the reactive power Q v Exceeding the prescribed range; (5) When Q v Out of range and voltage is still greater than 1.07U n When the active power is increased, it is adjusted.

5. The method for comprehensive control of reactive and active power based on photovoltaic grid-connected inverter according to claim 1, characterized in that: The specific implementation method of Step 4 is: The inverter adopts a feed-forward decoupling control strategy based on Park transformation to enable the grid-connected inverter to achieve decoupling control of the active and reactive power axes.

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