Two-stage hierarchical control method for harmonic control in medium and low voltage distribution networks based on photovoltaics

Abstract

The present invention relates to the technical field of harmonic control in distribution networks, and discloses a two-stage hierarchical control method for harmonic control in medium and low voltage distribution networks based on photovoltaics. First, a medium and low voltage hierarchical control architecture is constructed, first controlled by the medium voltage control level, then controlled by the low voltage control level, and interactive coordination control is performed between the medium voltage layer and the low voltage layer; then, the objective function is established for the medium voltage layer and the low voltage layer respectively with the lowest harmonic control cost of the medium and low voltage distribution network, and constraints are set and solved to construct a hierarchical optimization model for harmonic control in the medium and low voltage distribution network; finally, based on the medium and low voltage hierarchical control architecture and the hierarchical optimization model for harmonic control in the medium and low voltage distribution network, a two-stage hierarchical coordination control process for the medium and low voltage distribution network is obtained. The present invention uses the residual capacity after photovoltaic active power is connected to the grid to generate compensation current, which greatly improves the utilization rate of the equipment, takes the lowest harmonic control cost of the medium and low voltage distribution network as the optimization goal, and has more advantages in economy.

Description

Technical Field

[0001] The present invention relates to the technical field of harmonic control in distribution networks, and in particular to a two-stage hierarchical control method for harmonic control in medium and low voltage distribution networks based on photovoltaics. Background Art

[0002] In recent years, the development of distribution network has provided technical support for large-scale distributed power generation grid connection. It is an intelligent grid with certain automatic control capabilities. However, there are a large number of harmonic sources inside the distribution network, mainly including inverter-type distributed power sources, small rectifiers and other nonlinear power electronic devices. They are widely distributed and dense, and the current and voltage are not linearly related, which will cause the current waveform to be distorted, generate harmonics, and may cause resonance, posing a threat to the safe operation of load equipment. As the problem of harmonic pollution in distribution network becomes more and more serious, it has attracted widespread attention from scholars at home and abroad. Therefore, it is particularly important to effectively solve the problem of harmonic source pollution in distribution network. In the actual operation process of photovoltaic, it will be affected by factors such as light conditions and temperature. Changes in light cause the output power of photovoltaic cells to fluctuate continuously, rather than always maintaining the maximum power generation. Therefore, there will always be a certain amount of residual capacity after photovoltaic is actively connected to the grid through the inverter. The residual capacity can be used to inject reverse harmonic compensation current into the distribution network to realize the function of harmonic control, thereby improving equipment utilization and reducing harmonic control costs.

[0003] At present, the methods for harmonic control mainly include active control and passive control. Passive control is to use special harmonic control equipment to control harmonics. Commonly used harmonic control equipment mainly includes passive power filters and active power filters. Passive power filters are usually installed near the harmonic source and the node with the most serious harmonic pollution. They use capacitors and inductors to form a resonant circuit to block the propagation of harmonics of the corresponding frequency. Passive power filters have a simple structure, large capacity, and low investment cost, but can only compensate for harmonics of a specific frequency. Active power filters are generally connected in parallel to the incoming line of nonlinear loads. By real-time monitoring of the voltage and current in the power grid, power electronic devices are used to generate compensation currents with equal amplitude and opposite polarity to the harmonic current of the power grid, and injected into the power grid, thereby compensating or offsetting the harmonic current and actively eliminating power harmonics. However, their prices are usually expensive and are not suitable for scenarios where multiple nodes have excessive harmonics. Active control is achieved by improving the topological structure or control strategy of power electronic equipment and changing the harmonic current output by itself. Microgrid harmonics can be controlled from the root. Active governance generally establishes an optimization model with the goal of minimizing the total harmonic voltage distortion rate of the network, and uses an intelligent optimization algorithm to solve the compensation current emitted by the governance equipment for harmonic governance, thereby achieving harmonic governance in the distribution network. However, this method usually does not take the cost of harmonic governance into consideration.

[0004] The use of photovoltaic inverters for harmonic compensation is an emerging harmonic control method. The purpose of harmonic control is achieved by generating a harmonic compensation current with a phase opposite to the original harmonic current in the distribution network through the residual capacity after the photovoltaic active power is connected to the grid. However, the existing methods only consider the harmonic control problem of a single-layer distribution network, such as only a medium-voltage distribution network or only a low-voltage distribution network. In real life, the distribution network system is usually composed of a medium-voltage distribution network and a low-voltage distribution network. In addition, the current harmonic control strategy only technically solves the problem of excessive harmonics at nodes in the distribution network to ensure that the power quality meets the standard. It is often accompanied by a high cost of control economically, which to a certain extent limits its widespread application.

[0005] In summary, the prior art has the following disadvantages:

[0006] 1) Existing harmonic control methods mainly focus on the harmonic control of single-level distribution networks, and there is a gap in the research on multi-level distribution networks. However, in real life, the distribution network system is usually composed of a medium-voltage distribution network and a low-voltage distribution network.

[0007] 2) Existing harmonic control methods do not take into account the harmonic control capabilities of photovoltaics.

[0008] 3) Existing harmonic control methods usually establish optimization models with the goal of minimizing the total harmonic voltage distortion rate of the network to ensure that the harmonics of all nodes in the distribution network do not exceed the standard. This only considers technical issues but not costs, and the control costs are usually expensive. Summary of the invention

[0009] In view of the above problems, the purpose of the present invention is to provide a two-stage hierarchical control method for harmonic control of medium and low voltage distribution networks based on photovoltaics, to achieve harmonic control of medium and low voltage distribution networks, to use the residual capacity of photovoltaic active power after grid connection to generate harmonic compensation current for harmonic control, to reduce the harmonic control cost of medium and low voltage distribution networks and to ensure that all node harmonics do not exceed the standard, and to solve the technical and economic problems of harmonic control of medium and low voltage distribution networks under various constraints. The technical solution is as follows:

[0010] The two-stage hierarchical control method for harmonic control of medium and low voltage distribution networks based on photovoltaics includes the following steps:

[0011] Step 1: Build a medium- and low-voltage hierarchical control architecture, use the remaining capacity of photovoltaic active power after grid connection to generate compensation current, first control it through the medium-voltage control level, then control it through the low-voltage control level, and perform interactive and coordinated control between the medium- and low-voltage control levels;

[0012] Step 2: Establish the objective function with the lowest cost of harmonic control in medium and low voltage distribution networks, set constraints and solve them to build a hierarchical optimization model for harmonic control in medium and low voltage distribution networks;

[0013] Step 3: Based on the medium and low voltage hierarchical control architecture and the medium and low voltage distribution network harmonic control hierarchical optimization model, the two-stage hierarchical coordination control process of the medium and low voltage distribution network is obtained.

[0014] Furthermore, step 1 specifically includes:

[0015] Step 1.1: Medium voltage control level control:

[0016] First, the medium-voltage layer centralized control master station obtains the status information of the medium-voltage line and each medium-voltage node, and uses the large-capacity photovoltaic power station inverter group directly connected to the medium-voltage control level as a harmonic control device;

[0017] Secondly, the medium voltage control level takes the lowest harmonic control cost and the harmonic voltage distortion rate of all nodes not exceeding the maximum harmonic voltage distortion rate allowed under the voltage level as the optimization target, takes the amplitude and phase of each harmonic compensation current emitted by the photovoltaic power station as the control variables, and establishes and solves the optimization model based on the optimal power flow;

[0018] Then, the control command is transmitted to the photovoltaic power station, and the harmonic voltage distortion rate of the medium-voltage distribution network node is controlled below the standard by issuing the corresponding harmonic compensation current;

[0019] Finally, the harmonic compensation current emitted by the photovoltaic power station remains unchanged, and the central control master station at the medium voltage layer sends harmonic control instructions to the intelligent terminal in the low voltage area;

[0020] Step 1.2: Low voltage control level control:

[0021] After receiving the instructions from the central control master station at the medium voltage layer, the intelligent terminals in each low voltage area obtain the status information of each low voltage area node and the capacity of household photovoltaics; the low voltage control level performs the optimal power flow calculation with the goal of minimizing the harmonic control cost and not exceeding the harmonic voltage distortion rate of each low voltage node, and obtains the amplitude and phase of each harmonic compensation current that should be emitted by the household photovoltaics connected to each node of the low voltage control level; the intelligent terminals in the distribution area realize flexible regulation of household photovoltaics in this area by communicating with the household photovoltaic inverter and issuing instructions;

[0022] Step 1.3: Interactive coordination control between the medium voltage control level and the low voltage control level:

[0023] When managing the low-voltage control level, ensure that the harmonic voltage distortion rate of all nodes in the medium-voltage control level does not exceed the standard; the basic principle followed by the regulation frequency of the medium-voltage control level and the low-voltage control level is: harmonic regulation is triggered when harmonics exceed the standard, and if there is no problem, the current mode of operation is maintained.

[0024] Furthermore, step 2 specifically includes:

[0025] (1) Establish the objective function:

[0026] (1);

[0027] In the formula, Indicates the number of days in a year when the harmonic source generates harmonics; Indicates the 24 moments of the day; Indicated in Costs incurred by harmonic control at all times;

[0028] in:

[0029] (2);

[0030] In the formula, Indicated in The cost of harmonic control in medium voltage distribution network at any time; Indicated in The cost of harmonic control in low voltage distribution network at any time;

[0031] Split formula (2) into two parts and use the objective function as shown below:

[0032] 1) Medium voltage control level:

[0033] The objective function of the medium voltage control level is expressed as follows:

[0034] (3);

[0035] In the formula, Indicates the limit value of harmonic voltage distortion rate in medium voltage distribution network; Indicated in The harmonic voltage distortion rate of the nodes in the medium voltage distribution network at the moment;

[0036] in:

[0037] (4);

[0038] In the formula, Indicated in The effective value of the fundamental voltage of the node in the medium voltage distribution network at the moment; Indicated in The nodes in the medium voltage distribution network at the moment Subharmonic voltage effective value; Indicates the maximum order of harmonics;

[0039] in:

[0040] (5);

[0041] In the formula, Indicates the time period of photovoltaic power generation; It represents the unit price of harmonic control fee paid by the distribution network operator to the medium voltage photovoltaic power station; Indicates the time period when photovoltaic power generation is not in progress; Indicates the unit price of photovoltaic power purchased from the grid; Indicated in The distorted power used for harmonic control generated by the inverter group of the medium-voltage photovoltaic power station at any time;

[0042] Ignore the harmonic voltages and only keep the fundamental voltage. The expression is as follows:

[0043] (6);

[0044] In the formula, Indicated in The medium voltage photovoltaic power station inverter group sends Subharmonic compensation current; Indicated in The effective value of the fundamental voltage of the inverter group of the medium-voltage photovoltaic power station at the moment;

[0045] 2) Low voltage control level:

[0046] The objective function of the low-voltage control level is expressed as follows:

[0047] (7);

[0048] In the formula, Indicates the limit value of harmonic voltage distortion rate in low voltage distribution network; Indicated in Harmonic voltage distortion rate of nodes in low-voltage distribution network at every moment;

[0049] in:

[0050] (8);

[0051] In the formula, Indicated in The effective value of the fundamental voltage of the node in the low-voltage distribution network at the moment; Indicated in The nodes in the low voltage distribution network at the moment Subharmonic voltage effective value;

[0052] in:

[0053] (9);

[0054] In the formula, It represents the unit price of harmonic control fee paid by distribution network operators to low-voltage household photovoltaics; Indicated in Distorted power generated by low-voltage household photovoltaics at all times;

[0055] (10);

[0056] In the formula, Indicated in Low voltage household photovoltaic power generation Subharmonic compensation current; Indicated in The effective value of the fundamental voltage of low-voltage household photovoltaic at all times;

[0057] (2) Establish constraints:

[0058] The constraints of the hierarchical optimization model for harmonic management of medium and low voltage distribution networks include fundamental power flow constraints, harmonic power flow constraints, node voltage upper and lower limit constraints, photovoltaic regulation capacity constraints, and medium voltage distribution network node constraints; among them, the first four constraints are included in both the medium voltage control level and the low voltage control level, and the last constraint only exists in the low voltage control level; as follows:

[0059] 1) Fundamental power flow constraints:

[0060] The state variables of the power system include voltage amplitude, voltage phase angle, bus active power and bus reactive power; the power flow calculation is to solve a set of nonlinear algebraic equations, in which the nonlinear algebraic equation refers to the system power balance equation, which is expressed as follows:

[0061] (11);

[0062] (12);

[0063] In the formula, Representation and Node All directly connected nodes ; and Respectively Time Node Active power and reactive power; and They represent the conductance and susceptance in the system node admittance matrix at the fundamental frequency respectively; express Time Node and nodes The fundamental voltage phase angle difference between and Respectively Time Node With Node The fundamental RMS voltage;

[0064] in, and The expression is as follows:

[0065] (13);

[0066] (14);

[0067] In the formula, and Respectively Photovoltaic at the node Active and reactive power generated; and Respectively Time Node The active power and reactive power consumed by the load;

[0068] 2) Harmonic power flow constraints:

[0069] Harmonic power flow calculation is used to calculate the distribution of each harmonic voltage or current in the network when the harmonic source model or harmonic injection amount is known. The expression is as follows:

[0070] (15);

[0071] In the formula, express The number of nodes injected at each time Subharmonic current column vector; express Distribution network at all times Subharmonic admittance matrix; express Each node at a time Subharmonic voltage column vector;

[0072] in:

[0073] (16);

[0074] (17);

[0075] (18);

[0076] In the formula, express Time Node of Subharmonic current effective value; express Time Node of Subharmonic voltage effective value; and Respectively time The self-admittance and mutual-admittance sub-matrices of the nodal admittance matrix at subharmonic frequencies; is the total number of nodes; is the transpose symbol;

[0077] 3) Node voltage upper and lower limit constraints:

[0078] The node voltage satisfies the following expression:

[0079] (19);

[0080] In the formula, and They represent the upper and lower limits of the voltage allowed at the distribution network nodes respectively; express Time Node The actual voltage is as follows:

[0081] (20);

[0082] 4) Photovoltaic regulation capacity constraints:

[0083] (twenty one);

[0084] In the formula, express The distorted power generated by photovoltaic at every moment; Indicates the rated power of the photovoltaic inverter; and Respectively Active power and reactive power output by the photovoltaic inverter at all times; express The remaining capacity of the photovoltaic inverter at any moment;

[0085] 5) Medium voltage distribution network node constraints:

[0086] When managing the low-voltage control level, it is necessary to ensure that the harmonic voltage distortion rate of all nodes in the medium-voltage control level does not exceed the standard. The expression is as follows:

[0087] (twenty two);

[0088] (3) Solution algorithm:

[0089] A balanced optimizer is adopted to solve the proposed optimization problem.

[0090] Furthermore, step 3 specifically includes:

[0091] Step 3.1: Determine the time scale for centralized control of the medium voltage distribution network ;

[0092] Step 3.2: The medium-voltage layer centralized control master station obtains the load parameters of each node of the medium-voltage distribution network, and adjusts the capacity and output power parameters of the photovoltaic power station in the medium-voltage distribution network;

[0093] Step 3.3: Based on the line topology and impedance information, the amplitude and phase of each harmonic compensation current emitted by photovoltaic power generation are used as control variables, and the harmonic control cost of the medium voltage control level and the harmonic voltage distortion rate of each node are used as the objective function to build a centralized control optimization model for the medium voltage distribution network, and use the balance optimizer to solve formulas (3) to (6) and (11) to (21);

[0094] Step 3.4: The medium-voltage centralized control master station sends the solved control variables to the medium-voltage distribution network photovoltaic; the medium-voltage distribution network photovoltaic uses the remaining capacity to send the corresponding harmonic compensation current according to the control variables, completing the control time scale The harmonic control of the medium-voltage distribution network is carried out; then the control master station sends harmonic control instructions to each intelligent terminal in the low-voltage control layer, the harmonic compensation current emitted by the photovoltaic power generation at the medium-voltage control layer remains unchanged, and the low-voltage area control is started;

[0095] Step 3.5: Determine the control time scale of the distributed control of the low-voltage distribution network ;

[0096] Step 3.6: The intelligent terminal in the substation obtains the line parameters of the low-voltage distribution network and the data of each node; the low-voltage distribution network distributed control optimization model is constructed with the harmonic control cost of the low-voltage control level and the harmonic voltage distortion rate of each node as the objective function, and the balanced optimizer is used to solve formulas (7) to (10) and (11) to (22);

[0097] Step 3.7: The power quality monitor collects data of the nodes in the medium-voltage distribution network and sends it to the intelligent terminal in the substation area; if the harmonic voltage distortion rate of the node in the medium-voltage distribution network exceeds the standard, return to step 3.6; if not, proceed to the next step;

[0098] Step 3.8: The intelligent terminal in the substation area sends the solved control variables to the low-voltage distribution network photovoltaic; the low-voltage distribution network photovoltaic sends the corresponding harmonic compensation current according to the control variables to complete the regulation time scale + The task of harmonic control in medium and low voltage distribution networks is completed.

[0099] Compared with the prior art, the present invention has the following beneficial effects:

[0100] 1) The present invention adopts a two-stage hierarchical coordinated control strategy, which can be used for harmonic control in medium and low voltage distribution networks, filling the gap in harmonic control technology in multi-level distribution networks.

[0101] 2) The present invention uses photovoltaics as a harmonic control device and utilizes its residual capacity after active grid connection to generate compensation current for control. This can not only reduce harmonic pollution in the power grid, but also greatly improve the utilization rate of the equipment, which is in line with the development trend of green energy.

[0102] 3) The present invention establishes an optimization model for medium and low voltage distribution networks, with the goal of reducing the harmonic control costs of medium and low voltage distribution networks and ensuring that the harmonics of all nodes do not exceed the standard. Under multiple constraints, the present invention simultaneously solves the technical and economic problems of harmonic control in medium and low voltage distribution networks. BRIEF DESCRIPTION OF THE DRAWINGS

[0103] Figure 1 This is a two-stage hierarchical coordination control flow chart of the present invention. DETAILED DESCRIPTION

[0104] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0105] Unlike ordinary single-layer distribution networks, medium and low voltage distribution networks consist of two layers: medium voltage distribution network and low voltage distribution network. Harmonics on the medium voltage side can be transmitted to the low voltage side through transformers and lines, affecting the operation of downstream equipment. Under certain conditions, the coupling of harmonics between different levels may lead to harmonic amplification, increasing the difficulty of governance. Therefore, in order to achieve harmonic governance, this paper proposes a two-stage hierarchical coordinated control strategy for medium and low voltage distribution networks, which is described in detail as follows:

[0106] 1. Medium and low voltage hierarchical control architecture

[0107] Considering that the medium-voltage side regulation has a wider impact range and a greater impact on the low-voltage distribution network during the optimization and regulation of the medium- and low-voltage distribution network, the number of regulation of the medium-voltage distribution network should be reduced as much as possible, and priority should be given to the control of the harmonic exceeding the standard in the medium-voltage distribution network. Therefore, when studying the medium- and low-voltage coordinated control strategy of the active distribution network, the present invention adopts a two-stage hierarchical coordinated control architecture of "medium voltage first, low voltage later, medium- and low-voltage coordination", and the control architecture is analyzed as follows:

[0108] (1) Medium voltage control level:

[0109] First, the medium-voltage layer centralized control master station obtains the status information of the medium-voltage line and each medium-voltage node, including the load at the PCC, the capacity of the photovoltaic power station, the output power, etc. The large-capacity photovoltaic power station inverter group directly connected to the medium-voltage control level has a strong harmonic control capability and is the main control method considered by the medium-voltage control level. Secondly, the medium-voltage control level takes the lowest harmonic control cost and the harmonic voltage distortion rate of all nodes not exceeding the maximum harmonic voltage distortion rate allowed under the voltage level as the optimization target, and takes the amplitude and phase of each harmonic compensation current emitted by the photovoltaic power station as the control variable, and establishes and solves the optimization model based on the optimal power flow. Finally, the control command is transmitted to the photovoltaic power station, and the harmonic voltage distortion rate of the medium-voltage distribution network node is controlled below the standard by issuing the corresponding harmonic compensation current. Subsequently, the output of the photovoltaic power station remains unchanged, and the medium-voltage layer centralized control master station sends harmonic control instructions to the low-voltage intelligent terminal.

[0110] (2) Low voltage control level:

[0111] After receiving the instructions from the control master station, each low-voltage area intelligent terminal obtains the status information of each area node and the capacity of household photovoltaics. The low-voltage control level also performs optimal power flow calculation with the goal of minimizing harmonic control costs and not exceeding the harmonic voltage distortion rate of each low-voltage node, and obtains the amplitude and phase of each harmonic compensation current that should be emitted by each node of the low-voltage control level connected to household photovoltaics. It should be noted that the distribution station area intelligent terminal can realize flexible adjustment of household photovoltaics in this area through communication and instruction issuance with household photovoltaic inverters, that is, to achieve the adjustment of distorted power from 0 to full power. The relevant edge computing functions are implemented in the intelligent terminal to reduce the computing pressure of the centralized control layer.

[0112] (3) Interaction and coordination mechanism between the medium voltage control level and the low voltage control level:

[0113] An important feature of the new distribution network is the access of large-scale distributed power sources, that is, both the medium-voltage control level and the low-voltage control level contain adjustable resources. The medium- and low-voltage coordinated governance control system should be an inter-layer interactive coordination and optimization control system. When conducting harmonic governance of medium-voltage or low-voltage distribution networks, due to some coupling effects and interactive influence mechanisms, the problem of excessive harmonics on the other side may be affected and even aggravated. Therefore, when conducting governance at the low-voltage control level, it is necessary to ensure that the harmonic voltage distortion rate of all nodes at the medium-voltage control level does not exceed the standard. In addition, regardless of the medium-voltage control level or the low-voltage control level, the basic principle that should be followed in the frequency of regulation is: regulation is triggered by excessive harmonics, and if there is no problem, it will continue to operate in the current manner.

[0114] 2. Hierarchical optimization model for harmonic control in medium and low voltage distribution networks

[0115] In order to solve the technical and economic problems of harmonic control in medium and low voltage distribution networks, a hierarchical optimization model for harmonic control in medium and low voltage distribution networks was constructed.

[0116] (1) Establish the objective function:

[0117] (1);

[0118] In the formula, Represents the number of days in a year when the harmonic source generates harmonics. During the day, the harmonic emission level of the harmonic source is the same every day. Represents the 24 moments of the day. Indicated in The cost of harmonic control at any time is At the moment when the harmonic source does not generate harmonics, is 0. Among them:

[0119] (2);

[0120] In the formula, represent The cost of harmonic control in medium voltage distribution network at all times. represent The cost of harmonic control in low-voltage distribution networks at all times.

[0121] Since the present invention adopts a two-stage hierarchical coordinated control strategy, formula (2) can be divided into two parts, and the objective function shown below is adopted:

[0122] 1) Medium voltage control level:

[0123] (3);

[0124] In the formula, It is the limit value of harmonic voltage distortion rate in medium voltage distribution network; for The harmonic voltage distortion rate of a node in the medium voltage distribution network at the moment is defined as follows:

[0125] (4);

[0126] In the formula, for The effective value of the fundamental voltage at a node in the medium voltage distribution network at the moment; for The node of a medium voltage distribution network at a certain time Subharmonic voltage effective value; Indicates the maximum order of harmonics;

[0127] in:

[0128] (5);

[0129] In the formula, Indicates the time period of photovoltaic power generation. It represents the unit price ($ / kVA) paid by the distribution network operator to the medium voltage PV power station for harmonic control. Indicates the time period when photovoltaic power generation is not taking place. It indicates the unit price ($ / kVA) of electricity purchased by photovoltaic power from the grid. Indicated in The distorted power used for harmonic control generated by the inverter group of the medium-voltage photovoltaic power station at any time.

[0130] Considering that the amplitudes of each harmonic voltage and harmonic current are small, and the product of the harmonic voltage and harmonic current is much smaller than the product of the fundamental voltage and harmonic current, the harmonic voltages can be ignored in the calculation, and only the fundamental voltage is retained, that is:

[0131] (6);

[0132] In the formula, for The medium voltage photovoltaic power station inverter group sends Subharmonic compensation current. for The effective value of the fundamental voltage of the inverter group of the medium-voltage photovoltaic power station at this moment.

[0133] 2) Low voltage control level:

[0134] Similar to the medium voltage control level, the objective function of the low voltage control level is as follows:

[0135] (7);

[0136] In the formula, It is the limit value of harmonic voltage distortion rate in low voltage distribution network; for The harmonic voltage distortion rate of a node in the low-voltage distribution network at a certain moment is defined as follows:

[0137] (8);

[0138] In the formula, for The effective value of the fundamental voltage at a node in the low-voltage distribution network at the moment; for The current of a node in the low voltage distribution network at a certain moment Subharmonic voltage effective value;

[0139] (9);

[0140] In the formula, Indicates the time period of photovoltaic power generation. It represents the unit price ($ / kVA) paid by the distribution network operator to low-voltage household photovoltaics for harmonic control. Indicates the time period when photovoltaic power generation is not taking place. It indicates the unit price ($ / kVA) of electricity purchased by photovoltaic power from the grid. Indicated in The distorted power generated by low-voltage household photovoltaics at all times is as follows:

[0141] (10);

[0142] In the formula, for Low voltage household photovoltaic power generation Subharmonic compensation current. for The effective value of the fundamental voltage of low-voltage household photovoltaic at all times.

[0143] (2) Establish constraints:

[0144] The constraints of the medium and low voltage distribution network harmonic management hierarchical optimization model mainly include fundamental power flow constraints, harmonic power flow constraints, node voltage upper and lower limit constraints, photovoltaic regulation capacity constraints and medium voltage distribution network node constraints. Among them, the first four constraints are included in both the medium voltage control level and the low voltage control level, and the last constraint only exists in the low voltage control level. As follows:

[0145] 1) Fundamental power flow constraints:

[0146] The state variables of the power system mainly include voltage amplitude, voltage phase angle, bus active power and bus reactive power. If the two basic parameters of each node are known, the other two parameters can be obtained through the simultaneous equations. The power flow calculation is essentially to solve a set of nonlinear algebraic equations, in which the nonlinear algebraic equation refers to the system power balance equation, as follows:

[0147] (11);

[0148] (12);

[0149] In the formula, , They are Time Node Active power and reactive power; and are the conductance and susceptance in the system node admittance matrix at the fundamental frequency respectively; for Time Node and nodes The fundamental voltage phase angle difference between and They are Time Node With Node The fundamental RMS voltage;

[0150] in , The expression is as follows:

[0151] (13);

[0152] (14);

[0153] In the formula, , They are Photovoltaic at the node Active and reactive power generated; , They are Time Node The active power and reactive power consumed by the load.

[0154] 2) Harmonic power flow constraints:

[0155] Different from fundamental power flow, the basic function of harmonic power flow calculation is to calculate the distribution of each harmonic voltage or current in the network when the harmonic source model or harmonic injection amount is known, as follows:

[0156] (15);

[0157] in, for The number of nodes injected at each time Subharmonic current column vector. The value of the node without harmonic injection is 0. for Distribution network at all times Subharmonic admittance matrix. for Each node at a time The subharmonic voltage column vector is as follows:

[0158] (16);

[0159] (17);

[0160] (18);

[0161] In the formula, for Time Node of Subharmonic current effective value, for Time Node of Subharmonic voltage effective value, and They are time The self-admittance and mutual-admittance sub-matrices of the nodal admittance matrix at subharmonic frequencies; is the total number of nodes; is the transpose symbol.

[0162] 3) Node voltage upper and lower limit constraints:

[0163] In order to ensure the voltage quality of the system and ensure the safe, stable and efficient operation of the distribution network, the node voltage should satisfy the following expression:

[0164] (19);

[0165] In the formula, and are the upper and lower limits of the voltage allowed at the distribution network nodes, for Time Node The actual voltage is as follows:

[0166] (20);

[0167] 4) Photovoltaic regulation capacity constraints:

[0168] In order to complete the harmonic control task and ensure the safe operation of the photovoltaic inverter, the following capacity constraints need to be met:

[0169] (twenty one);

[0170] in, for The distorted power generated by photovoltaic at every moment, Represents the rated power of the PV inverter. and Respectively represent The active power and reactive power output by the photovoltaic inverter at all times. represent The remaining capacity of the PV inverter at this moment.

[0171] 5) Medium voltage distribution network node constraints:

[0172] When conducting harmonic control on the low-voltage distribution network, the coupling effect and interactive influence mechanism between the medium- and low-voltage distribution networks will affect the harmonic exceeding standard problem on the other side, and may even aggravate it. Therefore, when conducting control on the low-voltage control level, it is necessary to ensure that the harmonic voltage distortion rate of all nodes in the medium-voltage control level does not exceed the standard. As follows:

[0173] (twenty two);

[0174] (3) Solution algorithm:

[0175] The equilibrium optimizer is used to solve the proposed optimization problem. The equilibrium optimizer is a physics-based metaheuristic algorithm that is inspired by the mass balance equation in physics. The mass balance equation describes the physical process of mass entering, leaving, and being generated in a control volume. The equilibrium optimizer has been shown to have superior performance compared to traditional algorithms such as genetic algorithms, particle swarm optimization, and gray wolf optimizer. In the equilibrium optimizer algorithm, the concentration of each particle is updated in a random manner until an equilibrium state is reached.

[0176] 3. Two-stage hierarchical coordination control strategy process for medium and low voltage distribution networks

[0177] Based on the medium and low voltage hierarchical control architecture and the medium and low voltage distribution network optimization model, the two-stage hierarchical coordination control process of the medium and low voltage distribution network can be obtained, such as Figure 1 The specific steps are as follows:

[0178] (1) Determine the time scale for centralized control of medium voltage distribution networks Considering the communication and computing capabilities of the medium-voltage distribution network, the control time scale is usually about 10 minutes.

[0179] (2) The medium-voltage layer centrally controls the master station to obtain the load parameters of each node in the medium-voltage distribution network, as well as the capacity, output power and other parameters of the control resources (photovoltaic power stations in the medium-voltage distribution network).

[0180] (3) According to the line topology and impedance information, the amplitude and phase of each harmonic compensation current emitted by photovoltaic power generation are taken as control variables, and the harmonic control cost of the medium voltage control level and the harmonic voltage distortion rate of each node are taken as objective functions to construct a centralized control optimization model for the medium voltage distribution network. The balanced optimizer is used to solve Formulas (3) to (6) and (11) to (21) corresponding to the optimization model.

[0181] (4) The medium-voltage centralized control master station sends the solved control variables to the medium-voltage distribution network photovoltaic. The medium-voltage distribution network photovoltaic uses the remaining capacity to emit the corresponding harmonic compensation current based on the control variables to complete the control time scale. The control master station then sends harmonic control instructions to each intelligent terminal in the low-voltage control layer, and the photovoltaic output of the medium-voltage control layer remains unchanged, and the low-voltage area control is started.

[0182] (5) Determine the control time scale of distributed control of low-voltage distribution networks Due to the weak centralization of low-voltage distribution network control, its regulation time scale is relatively small, usually around 1 minute.

[0183] (6) The intelligent terminal in the substation obtains the line parameters of the low-voltage distribution network and the data of each node, including distributed photovoltaic, load and other parameters. The low-voltage distribution network distributed control optimization model is constructed with the harmonic control cost of the low-voltage control level and the harmonic voltage distortion rate of each node as the objective function, and the balanced optimizer is used to solve the formulas (7) to (10) and (11) to (22) corresponding to the optimization model.

[0184] (7) PQM (Power Quality Monitor) collects data from medium-voltage distribution network nodes and sends it to the substation intelligent terminal. If the harmonic voltage distortion rate of any node in the medium-voltage distribution network exceeds the standard, return to step (6); if not, proceed to the next step.

[0185] (8) The intelligent terminal in the substation sends the obtained control variables to the low-voltage distribution network photovoltaic. The low-voltage distribution network photovoltaic sends the corresponding harmonic compensation current according to the control variables to complete the regulation time scale. + The task of harmonic control in medium and low voltage distribution networks is completed.

[0186] The method proposed in the present invention adopts a two-stage hierarchical coordinated control strategy, which can be used for harmonic control in medium and low voltage distribution networks, filling the gap in harmonic control technology in multi-level distribution networks. The residual capacity after photovoltaic active power is connected to the grid is used to generate compensation current, which greatly improves the equipment utilization rate. In addition, an optimization model for medium and low voltage distribution networks is established in the strategy, with the lowest cost of harmonic control in medium and low voltage distribution networks as the optimization goal. Compared with other control methods, it has more advantages in economy.

Claims

1. A two-stage hierarchical control method for harmonic control of medium and low voltage distribution networks based on photovoltaics, characterized in that: The following steps are involved: Step 1: Build a medium- and low-voltage hierarchical control architecture, use the remaining capacity of photovoltaic active power after grid connection to generate compensation current, first control it through the medium-voltage control level, then control it through the low-voltage control level, and perform interactive and coordinated control between the medium- and low-voltage control levels; Step 2: Establish the objective function with the lowest cost of harmonic control in medium and low voltage distribution networks, set and solve the constraints, and build a hierarchical optimization model for harmonic control in medium and low voltage distribution networks; Step 3: Based on the medium and low voltage hierarchical control architecture and the medium and low voltage distribution network harmonic governance hierarchical optimization model, a two-stage hierarchical coordination control process for medium and low voltage distribution network harmonic governance is obtained; Step 1 specifically includes: Step 1.1: Medium voltage control level control: First, the central control master station at the medium voltage control level obtains the status information of the medium voltage line and each medium voltage node, and uses the large-capacity photovoltaic power station inverter group directly connected to the medium voltage control level as the harmonic control device; Secondly, the medium voltage control level takes the lowest harmonic control cost and the harmonic voltage distortion rate of all nodes not exceeding the maximum harmonic voltage distortion rate allowed under the voltage level as the optimization target, takes the amplitude and phase of each harmonic compensation current emitted by the photovoltaic power station as the control variables, and establishes and solves the optimization model based on the optimal power flow; Then, the control command is transmitted to the photovoltaic power station, and the harmonic voltage distortion rate of the medium-voltage distribution network node is controlled below the standard by issuing the corresponding harmonic compensation current; Finally, the harmonic compensation current emitted by the photovoltaic power station remains unchanged, and the central control master station at the medium voltage layer sends harmonic control instructions to the intelligent terminal in the low voltage area; Step 1.2: Low voltage control level control: After receiving the instructions from the central control master station at the medium voltage layer, the intelligent terminal in each low-voltage area obtains the status information of each low-voltage area node and the capacity of household photovoltaics; the low-voltage control level performs optimal power flow calculation with the goal of minimizing harmonic control costs and ensuring that the harmonic voltage distortion rate of each low-voltage node does not exceed the standard, and obtains the amplitude and phase of each harmonic compensation current that should be emitted by household photovoltaics connected to each node at the low-voltage control level; the intelligent terminal in the distribution area realizes flexible regulation of household photovoltaics in this area by communicating with household photovoltaic inverters and issuing instructions; Step 1.3: Interactive coordination control between the medium voltage control level and the low voltage control level: When managing the low-voltage control level, ensure that the harmonic voltage distortion rate of all nodes in the medium-voltage control level does not exceed the standard; the basic principle followed by the regulation frequency of the medium-voltage control level and the low-voltage control level is: harmonic regulation is triggered when harmonics exceed the standard, and if there is no problem, the current mode of operation is maintained.

2. According to claim 1, the two-stage hierarchical control method for harmonic control of medium and low voltage distribution networks based on photovoltaics is characterized in that: Step 2 specifically includes: (1) Establish the objective function: （1）； In the formula, Indicates the number of days in a year when the harmonic source generates harmonics; Indicates the 24 moments of the day; Indicated in Costs incurred by harmonic control at all times; in: （2）； In the formula, Indicated in The cost of harmonic control in medium voltage distribution network at any time; Indicated in The cost of harmonic control in low voltage distribution network at any time; Split formula (2) into two parts and use the objective function as shown below: 1) Medium voltage control level: The objective function of the medium voltage control level is expressed as follows: （3）； In the formula, Indicates the limit value of harmonic voltage distortion rate in medium voltage distribution network; Indicated in The harmonic voltage distortion rate of the nodes in the medium voltage distribution network at the moment; in: （4）； In the formula, Indicated in The effective value of the fundamental voltage of the node in the medium voltage distribution network at the moment; Indicated in The nodes in the medium voltage distribution network at the moment Subharmonic voltage effective value; Indicates the maximum order of harmonics; in: （5）； In the formula, Indicates the time period of photovoltaic power generation; It represents the unit price of harmonic control fee paid by the distribution network operator to the medium voltage photovoltaic power station; Indicates the time period when photovoltaic power generation is not in progress; Indicates the unit price of photovoltaic power purchased from the grid; Indicated in The distorted power used for harmonic control generated by the inverter group of the medium-voltage photovoltaic power station at any time; Ignore the harmonic voltages and only keep the fundamental voltage. The expression is as follows: （6）； In the formula, Indicated in The medium voltage photovoltaic power station inverter group sends Subharmonic compensation current; Indicated in The effective value of the fundamental voltage of the inverter group of the medium-voltage photovoltaic power station at the moment; 2) Low voltage control level: The objective function of the low-voltage control level is expressed as follows: （7）； In the formula, Indicates the limit value of harmonic voltage distortion rate in low voltage distribution network; Indicated in Harmonic voltage distortion rate of nodes in low-voltage distribution network at every moment; in: （8）； In the formula, Indicated in The effective value of the fundamental voltage of the node in the low-voltage distribution network at the moment; Indicated in The nodes in the low voltage distribution network at the moment Subharmonic voltage effective value; in: （9）； In the formula, It represents the unit price of harmonic control fee paid by distribution network operators to low-voltage household photovoltaics; Indicated in Distorted power generated by low-voltage household photovoltaics at all times; （10）； In the formula, Indicated in Low voltage household photovoltaic power generation Subharmonic compensation current; Indicated in The effective value of the fundamental voltage of low-voltage household photovoltaic at all times; (2) Establish constraints: The constraints of the hierarchical optimization model for harmonic management of medium and low voltage distribution networks include fundamental power flow constraints, harmonic power flow constraints, node voltage upper and lower limit constraints, photovoltaic regulation capacity constraints, and medium voltage distribution network node constraints; among them, the first four constraints are included in both the medium voltage control level and the low voltage control level, and the last constraint only exists in the low voltage control level; as follows: 1) Fundamental power flow constraints: The state variables of the power system include voltage amplitude, voltage phase angle, bus active power and bus reactive power; the power flow calculation is to solve a set of nonlinear algebraic equations, in which the nonlinear algebraic equation refers to the system power balance equation, which is expressed as follows: （11）； （12）； In the formula, Representation and Node All directly connected nodes ; and Respectively Time Node Active power and reactive power; and They represent the conductance and susceptance in the system node admittance matrix at the fundamental frequency respectively; express Time Node and nodes The fundamental voltage phase angle difference between them; and Respectively Time Node With Node The fundamental RMS voltage; in, and The expression is as follows: （13）； （14）； In the formula, and Respectively Photovoltaic at the node Active and reactive power generated; and Respectively Time Node The active power and reactive power consumed by the load; 2) Harmonic power flow constraints: Harmonic power flow calculation is used to calculate the distribution of each harmonic voltage or current in the network when the harmonic source model or harmonic injection amount is known. The expression is as follows: （15）； In the formula, express The number of nodes injected at each time Subharmonic current column vector; express Distribution network at all times Subharmonic admittance matrix; express Each node at a time Subharmonic voltage column vector; in: （16）； （17）； （18）； In the formula, express Time Node of Subharmonic current effective value; express Time Node of Subharmonic voltage effective value; and Respectively time The self-admittance and mutual-admittance sub-matrices of the nodal admittance matrix at subharmonic frequencies; is the total number of nodes; is the transpose symbol; 3) Node voltage upper and lower limit constraints: The node voltage satisfies the following expression: （19）； In the formula, and They represent the upper and lower limits of the voltage allowed at the distribution network nodes respectively; express Time Node The actual voltage is as follows: （20）； 4) Photovoltaic regulation capacity constraints: （21）； In the formula, express The distorted power generated by photovoltaic at every moment; Indicates the rated power of the photovoltaic inverter; and Respectively Active power and reactive power output by the photovoltaic inverter at all times; express The remaining capacity of the photovoltaic inverter at any moment; 5) Medium voltage distribution network node constraints: When managing the low-voltage control level, it is necessary to ensure that the harmonic voltage distortion rate of all nodes in the medium-voltage control level does not exceed the standard. The expression is as follows: （22）； (3) Solution algorithm: A balanced optimizer is adopted to solve the proposed optimization problem.

3. The two-stage hierarchical control method for harmonic control of medium and low voltage distribution networks based on photovoltaics according to claim 1 is characterized in that: Step 3 specifically includes: Step 3.1: Determine the time scale for centralized control of the medium voltage distribution network ; Step 3.2: The medium-voltage layer centralized control master station obtains the load parameters of each node of the medium-voltage distribution network, and adjusts the capacity and output power parameters of the photovoltaic power station in the medium-voltage distribution network; Step 3.3: Based on the line topology and impedance information, the amplitude and phase of each harmonic compensation current emitted by photovoltaic power generation are used as control variables, and the harmonic control cost of the medium voltage control level and the harmonic voltage distortion rate of each node are used as the objective function to build a centralized control optimization model for the medium voltage distribution network, and use the balance optimizer to solve formulas (3) to (6) and (11) to (21); Step 3.4: The medium-voltage centralized control master station sends the solved control variables to the medium-voltage distribution network photovoltaic; the medium-voltage distribution network photovoltaic uses the remaining capacity to send the corresponding harmonic compensation current according to the control variables, completing the control time scale The harmonic control of the medium-voltage distribution network is carried out; then the control master station sends harmonic control instructions to each intelligent terminal in the low-voltage control layer, the harmonic compensation current emitted by the photovoltaic power generation at the medium-voltage control layer remains unchanged, and the low-voltage area control is started; Step 3.5: Determine the control time scale of the distributed control of the low-voltage distribution network ; Step 3.6: The intelligent terminal in the substation obtains the line parameters of the low-voltage distribution network and the data of each node; the low-voltage distribution network distributed control optimization model is constructed with the harmonic control cost of the low-voltage control level and the harmonic voltage distortion rate of each node as the objective function, and the balanced optimizer is used to solve formulas (7) to (10) and (11) to (22); Step 3.7: The power quality monitor collects data of the nodes in the medium-voltage distribution network and sends it to the intelligent terminal in the substation area; if the harmonic voltage distortion rate of the node in the medium-voltage distribution network exceeds the standard, return to step 3.6; if not, proceed to the next step; Step 3.8: The intelligent terminal in the substation area sends the solved control variables to the low-voltage distribution network photovoltaic; the low-voltage distribution network photovoltaic sends the corresponding harmonic compensation current according to the control variables to complete the regulation time scale + The task of harmonic control in medium and low voltage distribution networks is completed.

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