Voltage control method based on active coordination and reactive coordination

Abstract

The invention provides a voltage control method based on active coordination and reactive coordination, and the method comprises the steps: carrying out the local prevention and control of the voltage of an AC system, and detecting whether the voltage of a photovoltaic grid-connected node in the AC system is out of limit or not; if the voltage is out of limit, performing voltage control on the alternating current system based on reactive power coordination until the reactive power compensation capacity is used up; if the voltage of the alternating-current system is not recovered to normal, performing voltage control on the alternating-current system based on active coordination, and detecting whether the voltage of other alternating-current systems participating in voltage regulation reaches a critical value or not; and if the voltage reaches the critical value, carrying out on-site voltage prevention control on the alternating current system, and if the voltage of the alternating current system is out of limit at the moment, carrying out voltage control on the alternating current system again until the voltage of the alternating current system returns to normal. Through active coordination and reactive coordination of the alternating current system, reactive voltage regulation resources of the system are utilized to the maximum extent under low communication cost, and rapid voltage regulation is realized.

Description

Technical Field

[0001] The present invention relates to the field of optimized operation and control of a distribution network, and in particular to a voltage control method based on active power coordination and reactive power coordination. Background Art

[0002] Distributed photovoltaic (PV) power generation is highly random and intermittent due to the influence of meteorological conditions. After a high proportion of distributed photovoltaics is connected to the distribution network, the random changes in its output power are likely to cause voltage fluctuations and flickers, voltage over-limit and other problems, affecting the safe operation of the distribution network. Therefore, solving the voltage over-limit problem caused by the high proportion of distributed photovoltaics connected to the distribution network has become the key to improving the safe and efficient use of photovoltaics. In response to the voltage over-limit problem caused by the high proportion of distributed photovoltaics connected to the distribution network, traditional voltage regulation methods mainly include voltage and reactive power control of reactive controllable devices such as capacitor banks and on-load tap-changing transformers. However, the parallel capacitor bank cannot continuously adjust the voltage and the adjustment speed is slow. The on-load tap-changing transformer has a limited number of adjustment times and is not suitable for frequent operation. These traditional discrete voltage regulation methods are difficult to deal with the voltage over-limit problem caused by the high proportion of photovoltaics connected.

[0003] With the rapid development of power electronics technology, intelligent soft switch (Soft Open Point, SOP) as a new type of power electronic device with real-time and precise power control characteristics is gradually applied to distribution networks. The flexible interconnected distribution network based on SOP not only includes important voltage regulating equipment such as photovoltaic grid-connected inverters and SOP, but also can realize the flexible consumption of photovoltaic power between different systems and give play to the advantages of power mutual assistance between systems. In the flexible interconnected distribution network, SOP is composed of multiple back-to-back voltage source converters (VSC). Photovoltaic inverters and converters can not only adjust the voltage through reactive power compensation, but also optimize the distribution of power flow between different AC subsystems to achieve voltage regulation.

[0004] In order to achieve power coordination between different AC subnets in the flexible interconnected distribution network and avoid voltage over-limit, it is necessary to coordinate and control the VSCs at different ports of the SOP. According to the communication configuration conditions, the voltage control methods of the distribution network can be divided into centralized control, local control and distributed control. Among them, centralized control uses the power flow information of each node to achieve global control of voltage; compared with centralized control, local control makes decisions based on local measurement data and has a fast response speed. In local voltage control, photovoltaic local reactive compensation is an effective way to achieve rapid regulation of over-limit voltage, and is usually used as a preventive control measure.

[0005] However, centralized control is highly dependent on the communication architecture and has high computational difficulty. In addition, local control cannot ensure full utilization of voltage regulation resources due to the lack of communication between nodes. Distributed control achieves global regulation through information exchange between local controllers and adjacent controllers. Each controller is relatively autonomous and coordinated with each other, taking into account the advantages of local control and centralized control. However, existing distributed voltage control strategies often only use photovoltaic power for voltage regulation, which is prone to cause abandoned light problems and cannot effectively utilize the reactive resources of photovoltaic inverters in each area. To this end, a voltage control method based on active coordination and reactive coordination is proposed. Summary of the invention

[0006] In view of this, the purpose of the present invention is to provide a voltage control method based on active coordination and reactive coordination. Through the active coordination and reactive coordination of the AC system, the role of the flexible interconnection device in power mutual assistance between different AC systems is effectively brought into play, and the reactive voltage regulation resources of the system can be maximized at a low communication cost, thereby realizing rapid voltage regulation of the AC system.

[0007] In a first aspect, an embodiment of the present invention provides a voltage control method based on active power coordination and reactive power coordination, characterized in that it includes:

[0008] S1: Performing local preventive control on the voltage of the AC system and detecting whether the voltage of the photovoltaic grid-connected node in the AC system exceeds the limit;

[0009] S2: If the voltage of the photovoltaic grid-connected node exceeds the limit, the AC system is subjected to reactive coordinated control by means of a distributed reactive coordinated control method until the reactive compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, and the voltage of the AC system is detected to see whether it returns to normal;

[0010] S3: If the voltage of the AC system has not returned to normal, the AC system is controlled by active power flow distribution coordination control method, and the voltage of other AC systems implementing the active power coordination control is detected to see whether it reaches a critical value;

[0011] S4: If the voltage of the other AC systems implementing the active coordinated control reaches the critical value, the voltage of the AC system is subject to on-site preventive control. If the voltage of the AC system exceeds the limit at this time, the reactive coordinated control of the AC system is performed through the distributed reactive coordinated control method until the voltage of the AC system returns to normal.

[0012] Preferably, in step S1, a photovoltaic inverter is used to perform the voltage local preventive control on the AC system through reactive power compensation. During the voltage local preventive control process, the maximum adjustment amount ΔQ PVmax The expression is as follows:

[0013]

[0014] In the formula, S PV is the photovoltaic capacity; P PV is the photovoltaic active output; Q PV is the photovoltaic reactive power output; is the minimum power factor;

[0015] In the above expression, when When both are positive, ΔQ PVmax is the maximum reactive power regulation during step-down regulation; When both are negative, ΔQ PVmax It is the maximum reactive power regulation amount during boost regulation.

[0016] Preferably, when the voltage of the photovoltaic grid-connected node reaches the upper limit of the critical value, the photovoltaic grid-connected node absorbs reactive power to prevent the voltage of the photovoltaic grid-connected node from exceeding the limit; when the voltage of the photovoltaic grid-connected node reaches the lower limit of the critical value, the photovoltaic grid-connected node outputs the reactive power to prevent the photovoltaic grid-connected node from undervoltage.

[0017] Preferably, in step S2, the reactive coordinated control is performed on the AC system by the distributed reactive coordinated control method until the reactive compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, including:

[0018] S201: the photovoltaic grid-connected node sends a first reactive power compensation request, and after receiving the first reactive power compensation request, the downstream photovoltaic grid-connected node performs reactive power compensation on the voltage of the photovoltaic grid-connected node until the reactive power compensation capacity of the downstream photovoltaic grid-connected node is exhausted;

[0019] S202: the downstream photovoltaic grid-connected node sends a second reactive power compensation request, and the upstream photovoltaic grid-connected node performs reactive power compensation on the voltage of the photovoltaic grid-connected node after receiving the second reactive power compensation request, until the reactive power compensation capacity of the upstream photovoltaic grid-connected node is exhausted;

[0020] S203: The upstream photovoltaic grid-connected node sends a third reactive power compensation request, the SOP receives the third reactive power compensation request, and performs reactive power compensation on the voltage of the photovoltaic grid-connected node through the VSC connected to the off-limit AC subsystem until the reactive power compensation capacity of the VSC is exhausted, and then sends a reactive power exhaustion signal to the photovoltaic grid-connected node.

[0021] Preferably, if it is detected that the voltage of the other AC systems implementing the active coordinated control does not reach the critical value, it is detected whether the voltage of the AC system returns to normal, and if so, the voltage control is terminated; if not, step S3 is repeated.

[0022] Preferably, in the step S3, the active power coordinated control is performed on the AC system by the active power flow distribution coordinated control method, including:

[0023] The constant voltage control VSC sends an active power adjustment request to the constant power control VSC, and the constant power control VSC receives the active power adjustment request and adjusts the active power distribution in the AC system;

[0024] Among them, when the voltage on the AC side of the constant voltage control VSC exceeds the limit, the constant power control VSC allocates the active power increase amount according to the remaining capacity ratio, periodically adjusts the active power distribution in the AC system, and performs voltage control on the AC system; when the voltage on the AC side of the constant power control VSC exceeds the limit, the constant power control VSC periodically adjusts the active power distribution in the AC system by changing its own power parameters to control the voltage of the AC system.

[0025] Preferably, the constant power control VSC can obtain the active power change ΔP after allocating the active power increase amount according to the remaining capacity ratio d VSC , which is expressed as follows:

[0026]

[0027] Where: S d,adj is the adjustable active power of the constant power control VSC at node d; S k,adj is the adjustable active power of the constant power control VSC at node k; ΔP set is the single active power adjustment amount; h is the number of the constant power VSCs participating in the active coordinated control.

[0028] Preferably, when voltage control is performed on the AC system, a maximum voltage regulation operating point is obtained, and the maximum voltage regulation operating point includes a maximum voltage regulation amount, a maximum active power regulation operating point, and a maximum reactive power regulation operating point.

[0029] Preferably, the maximum voltage regulation amount ΔU i,max The expression is as follows:

[0030]

[0031] During the step-down regulation process, the maximum active power regulation operating point The expression is as follows:

[0032]

[0033] In the step-down regulation process, the maximum reactive power regulation operating point The expression is as follows:

[0034]

[0035] During the step-up regulation process, the maximum active power regulation operating point The expression is as follows:

[0036]

[0037] During the step-up regulation process, the maximum reactive power regulation operating point The expression is as follows:

[0038]

[0039] Where: P VSC is the active power of the VSC, Q VSC is the reactive power of the VSC; S VSC is the VSC capacity; k1 is the sensitivity of the node voltage to the change of the node active power; k2 is the sensitivity of the node voltage to the change of the node reactive power. The expressions of k1 and k2 are as follows:

[0040]

[0041] Where: U N is the rated value of the node voltage; B i,vsc is the intersection of the line set of the line from the photovoltaic grid-connected node i to the power inflow node of the distribution network and the line set from the photovoltaic grid-connected node where the VSC is located to the power inflow node of the distribution network; R L and X L are the resistance and reactance of line L respectively.

[0042] Preferably, step S4 also includes that during the AC system voltage regulation process, if a certain constant power control VSC has a problem of exceeding the system voltage limit when increasing the active power, the constant power control VSC exits the active coordinated control, and the other constant power control VSCs continue to be adjusted according to the original margin ratio until the AC system voltage returns to normal.

[0043] The beneficial effects of the present invention are as follows:

[0044] The present invention provides a voltage control method based on active power coordination and reactive power coordination, which effectively brings into play the role of flexible interconnection devices in power mutual assistance between different substations, controls the voltage by adopting a distributed reactive power coordination control method for the AC system, and maximizes the use of the reactive power compensation capability of the system, thereby realizing rapid voltage control with low communication cost; it also optimizes the power distribution between AC subsystems through active power coordination of the AC system, and realizes the regulation of over-limit voltage and cross-regional consumption of photovoltaic power while ensuring that the VSC is load balanced.

[0045] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention are realized and obtained by the structures particularly pointed out in the description, claims and drawings.

[0046] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0048] Figure 1 A schematic flow chart of a voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0049] Figure 2 A three-terminal power supply flexible interconnected distribution network structure diagram of a voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0050] Figure 3 A schematic diagram of photovoltaic output active power of each node in a voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0051] Figure 4 A schematic diagram of the voltage condition of AC1 after distributed reactive power coordinated control in scenario 1 of a voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0052] Figure 5 A schematic diagram of the voltage condition of AC1 after distributed reactive power coordinated control in scenario 2 of a voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0053] Figure 6 The voltage condition of AC1 during the active power regulation process of a voltage control method VSC based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0054] Figure 7 The voltage change of each node in AC3 during the active power regulation process of a voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention;

[0055] Figure 8 A voltage control method based on active power coordination and reactive power coordination provided by an embodiment of the present invention controls the voltage conditions of the system before and after distributed control;

[0056] Fig. 9 The present invention provides a voltage control method based on active power coordination and reactive power coordination in scenario 3, and provides a relationship between various changes in the VSC during active power regulation and the voltage regulation amount. DETAILED DESCRIPTION

[0057] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0058] To facilitate understanding of this embodiment, Figure 1 A voltage control method based on active power coordination and reactive power coordination disclosed in an embodiment of the present invention is introduced in detail.

[0059] A voltage control method based on active power coordination and reactive power coordination disclosed in an embodiment of the present invention comprises the following specific steps:

[0060] S1: Perform local preventive control on the voltage of the AC system and detect whether the voltage of the photovoltaic grid-connected node in the AC system exceeds the limit;

[0061] S2: If the voltage of the PV grid-connected node exceeds the limit, the AC system is controlled by the distributed reactive coordination control method until the reactive compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, and the AC system voltage is detected to see if it returns to normal;

[0062] S3: If the AC system voltage does not return to normal, the AC system is controlled in active power coordinated manner through the active power flow distribution coordinated control method, and the voltage of other AC systems implementing active power coordinated control is detected to see whether it reaches a critical value;

[0063] S4: If the voltage of other AC systems implementing active coordinated control reaches a critical value, the voltage of the AC system is preventively controlled on site. If the voltage of the AC system exceeds the limit at this time, the AC system is reactively coordinated controlled through the distributed reactive coordinated control method until the voltage of the AC system returns to normal.

[0064] Based on the above steps, this embodiment is further described:

[0065] S1: Perform local preventive control on the voltage of the AC system and detect whether the voltage of the photovoltaic grid-connected node in the AC system exceeds the limit.

[0066] Furthermore, in this embodiment, before detecting whether the voltage of the photovoltaic grid-connected node in the AC system exceeds the limit, the network parameters of the flexible interconnected distribution network are obtained and initialized, and the initial values ​​of the voltage regulator devices of each AC subsystem in the flexible interconnected distribution network are determined. The network parameters include topology data, communication architecture data, line parameters, and load parameters.

[0067] Preferably, in step S1, a photovoltaic inverter is used to perform local voltage prevention control on the AC system through reactive power compensation. During the local voltage prevention control process, the maximum adjustment amount ΔQ PVmax The expression is as follows:

[0068]

[0069] In the formula, S PV is the photovoltaic capacity; P PV is the photovoltaic active output; Q PV is the photovoltaic reactive power output; is the minimum power factor;

[0070] In the above expression, when When both are positive, ΔQ PVmax is the maximum reactive power regulation during step-down regulation; When both are negative, ΔQ PVmax It is the maximum reactive power regulation during boost regulation.

[0071] Preferably, when the voltage of the photovoltaic grid-connected node reaches the upper limit of the critical value, the photovoltaic grid-connected node absorbs reactive power to prevent the photovoltaic grid-connected node from exceeding the voltage limit; when the voltage of the photovoltaic grid-connected node reaches the lower limit of the critical value, the photovoltaic grid-connected node outputs reactive power to prevent the photovoltaic grid-connected node from undervoltage.

[0072] S2: If the voltage of the PV grid-connected node exceeds the limit, the AC system is controlled by the distributed reactive power coordinated control method until the reactive power compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, and the AC system voltage is detected to see if it returns to normal.

[0073] In this embodiment, in step S2, if the voltage of the photovoltaic grid-connected node exceeds the limit, it means that the local photovoltaic reactive compensation capacity is exhausted, and it is necessary to control the voltage of the AC system through the distributed reactive coordination control method; after completing the voltage control, if the AC system voltage returns to normal, the voltage control is terminated; if the reactive compensation capacity is exhausted, if the AC system voltage returns to normal, the voltage control is also terminated.

[0074] Preferably, in step S2, reactive coordination control is performed on the AC system by using a distributed reactive coordination control method until the reactive compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, including:

[0075] S201: The photovoltaic grid-connected node sends a first reactive power compensation request, and after receiving the first reactive power compensation request, the downstream photovoltaic grid-connected node performs reactive power compensation on the voltage of the photovoltaic grid-connected node until the reactive power compensation capacity of the downstream photovoltaic grid-connected node is exhausted;

[0076] S202: the downstream photovoltaic grid-connected node sends a second reactive power compensation request, and the upstream photovoltaic grid-connected node performs reactive power compensation on the voltage of the photovoltaic grid-connected node after receiving the second reactive power compensation request, until the reactive power compensation capacity of the upstream photovoltaic grid-connected node is exhausted;

[0077] S203: The upstream photovoltaic grid-connected node sends a third reactive power compensation request, and the SOP receives the third reactive power compensation request, and performs reactive power compensation on the voltage of the photovoltaic grid-connected node through the VSC connected to the over-limit AC subsystem until the reactive power compensation capacity of the VSC is exhausted, and sends a reactive power exhaustion signal to the photovoltaic grid-connected node.

[0078] Among them, the upstream photovoltaic grid-connected node is a photovoltaic grid-connected node located upstream of the voltage over-limit node based on the power flow distribution characteristics; the downstream photovoltaic grid-connected node is a photovoltaic grid-connected node located downstream of the voltage over-limit node based on the power flow distribution characteristics; SOP is an intelligent soft switch (Soft Open Point, SOP), VSC is a voltage source converter (VSC) that can form SOP, which can interconnect multiple AC subsystems and participate in the reactive power compensation and voltage regulation process of the interconnected AC subsystems; the over-limit AC subsystem is an AC subsystem with voltage over-limit.

[0079] In this embodiment, when the VSC connected to the over-limit AC subsystem is used to perform reactive power compensation on the voltage of the photovoltaic grid-connected node, the voltage reduction amount ΔU of the photovoltaic grid-connected node is 2,VSC The expression is as follows:

[0080]

[0081] Where: U1 is the end node voltage of line L1; U2 is the end node voltage of line L2; R1 is the resistance of line L1; R2 is the resistance of line L2; X1 is the reactance of line L1; X2 is the reactance of line L2; ΔP VSC1 is the change in the active power transmitted by the VSC; ΔQ VSC1 is the change in reactive power absorbed by the VSC.

[0082] S3: If the voltage of the AC system does not return to normal, the AC system is controlled in an active coordinated manner through the active power flow distribution coordinated control method, and the voltage of other AC systems implementing active coordinated control is detected to see whether it reaches a critical value.

[0083] Preferably, if it is detected that the voltage of other AC systems implementing active coordinated control does not reach the critical value, it is detected whether the voltage of the AC system returns to normal, and if so, the voltage control is terminated; if not, step S3 is repeated.

[0084] Preferably, in step S3, active power coordinated control is performed on the AC system by using an active power flow distribution coordinated control method, including:

[0085] The constant voltage control VSC sends an active power regulation request to the constant power control VSC, and the constant power control VSC receives the active power regulation request and adjusts the active power distribution in the AC system;

[0086] Among them, when the voltage on the AC side of the constant voltage control VSC exceeds the limit, the constant power control VSC distributes the active power increase according to the remaining capacity ratio, periodically adjusts the active power distribution in the AC system, and controls the voltage of the AC system; when the voltage on the AC side of the constant power control VSC exceeds the limit, the constant power control VSC periodically adjusts the active power distribution in the AC system by changing its own power parameters to control the voltage of the AC system.

[0087] In this embodiment, the VSC includes a constant voltage control VSC and a constant power control VSC, wherein the constant voltage control VSC is a master controller and the constant power control VSC is a slave controller; the constant voltage control VSC is connected to each constant power control VSC in communication, wherein the active power adjustment amount of the constant voltage control VSC is the same as the active power adjustment amount of the constant power control VSC, but in the opposite direction.

[0088] Furthermore, in this embodiment, the remaining capacity ratio is the remaining reactive power compensation capacity ratio. The method of periodically adjusting the active power distribution in the AC system is: setting the active power adjustment amount ΔP set , based on ΔP setThe active power distribution in the AC system is adjusted in a single round. After the adjustment in this round is completed, the VSC receives an active power adjustment request to perform the next round of adjustment until the VSC no longer receives an active power adjustment request. The periodic adjustment of the active power distribution in the AC system has the following functions: preventing the voltage of the AC subsystem that has not exceeded the limit from exceeding the limit due to the large change in active power transmission between VSCs.

[0089] Furthermore, the constant power control VSC can obtain the active power change by allocating the active power increase amount according to the remaining capacity ratio. Its expression is as follows:

[0090]

[0091] Where: S d,adj is the adjustable active power of the constant power control VSC at node d; S k,adj is the adjustable active power of the constant power control VSC at node k; ΔP set is the single active power adjustment amount; h is the number of constant power VSCs participating in active coordinated control.

[0092] Figure 2 The following is a schematic diagram of the structure of a three-terminal power supply flexible interconnected distribution network. Figure 2 As shown, VSC1 is a VSC based on constant voltage control, VSC2 and VSC3 are VSCs based on constant power control, and the VSC inversion direction is the positive direction; L1~L18 are system lines; AC1, AC2, and AC3 are all AC subsystems; T1~T3 are transformers; Node 2, Node 5, Node 8, Node 10, Node 16, and Node 17 are photovoltaic grid-connected nodes; Node 6, Node 13, and Node 20 are VSC grid-connected nodes, that is, the equivalent transmission nodes on the AC side of each VSC in the SOP; Node 0, Node 7, and Node 14 are balancing nodes in each AC subsystem; and the remaining nodes are load nodes.

[0093] For example, in combination Figure 2 When VSC1 sends active power regulation requests to VSC2 and VSC3, the active power of VSC2 increases by ΔP VSC2 According to the power balance relationship of SOP, the change in active power of VSC1 is its opposite number ΔP VSC1 =-ΔP VSC2 At this time, the voltage changes of node 2 in AC1 and node 10 in AC2 are ΔU2 and ΔU 10 They are:

[0094]

[0095] Where: U8, U9, U 10and R7, R8, and R9 are the end node voltage and line resistance of lines L7, L8, and L9 respectively.

[0096] Preferably, when voltage control is performed on the AC system, a maximum voltage regulation operating point is obtained, and the maximum voltage regulation operating point includes a maximum voltage regulation amount, a maximum active power regulation operating point, and a maximum reactive power regulation operating point.

[0097] In this embodiment, due to the capacity constraint of the VSC, the voltage control effect of the VSC on the AC system first increases and then decreases. Therefore, the maximum voltage regulation operating point is calculated to evaluate the voltage regulation capability of the VSC on the AC system.

[0098] Furthermore, the maximum voltage regulation amount ΔU i,max The expression is as follows:

[0099]

[0100] During the step-down regulation process, the maximum active power regulation operating point The expression is as follows:

[0101]

[0102] During the step-down regulation process, the maximum reactive power regulation operating point The expression is as follows:

[0103]

[0104] During the boost regulation process, the maximum active power regulation operating point The expression is as follows:

[0105]

[0106] During the step-up regulation process, the maximum reactive power regulation operating point The expression is as follows:

[0107]

[0108] Where: P VSC is the active power of VSC, Q VSC is the reactive power of VSC; S VSC is the VSC capacity; k1 is the sensitivity of the node voltage to the change of the node active power; k2 is the sensitivity of the node voltage to the change of the node reactive power. The expressions of k1 and k2 are as follows:

[0109]

[0110] Where: U N is the rated value of the node voltage; Bi,vsc is the intersection of the line set from the photovoltaic grid-connected node i to the power inflow node of the distribution network and the line set from the photovoltaic grid-connected node where the VSC is located to the power inflow node of the distribution network; R L and X L are the resistance and reactance of line L respectively.

[0111] S4: If the voltage of other AC systems implementing active coordinated control reaches a critical value, the voltage of the AC system is preventively controlled on site. If the voltage of the AC system exceeds the limit at this time, the AC system is reactively coordinated controlled through the distributed reactive coordinated control method until the voltage of the AC system returns to normal.

[0112] In this embodiment, when the AC system voltage returns to normal, the voltage control is terminated.

[0113] Preferably, step S4 also includes that during the AC system voltage regulation process, if a certain constant power control VSC has a problem of exceeding the system voltage limit on its side when increasing the active power, the constant power control VSC exits the active coordinated control, and other constant power control VSCs continue to adjust according to the original margin ratio until the AC system voltage returns to normal.

[0114] The following content takes the flexible interconnected distribution network as an example, combined with Figure 2 A specific analysis is performed on the situations of different photovoltaic output active powers of each node in the AC system to gradually verify the effectiveness of the voltage regulation strategy described in this embodiment.

[0115] Among them, the reference voltage and reference capacity of the system are set to 10kV and 10MVA respectively; the capacity of each AC side VSC in the SOP is 1MVA, and the transmission powers of VSC1, VSC2 and VSC3 in the initial state are 300kW, -50kW and -250kW respectively; the minimum power factor φmin of photovoltaic in the AC system is 0.95, the allowable voltage variation range is set to 0.95pu~1.05pu, and the upper and lower limits of the critical voltage are 1.045pu and 0.955pu respectively; the photovoltaic installation capacities corresponding to nodes 2, 5, 8, 10, 16 and 17 are 2.5MVA, 2MVA, 1.5MVA, 1.2MVA, 1.6MVA and 2MVA respectively, and nodes 6, 13 and 20 in the AC system are equivalent transmission nodes on the AC side of each VSC in the SOP, and are connected without load.

[0116] In the following analysis scenarios, Figure 2 The active power output of photovoltaic power at each node in the AC system shown is as follows: Figure 3 shown.

[0117] (1) Scenario 1: The system’s PV active output increases by 15%;

[0118] Figure 4 When the photovoltaic active output suddenly increases by 15%, the voltage of AC1 after distributed reactive power coordinated control is shown. Before control, the highest voltage of the AC system is 1.05475pu, and AC1 voltage exceeds the limit. When the photovoltaic voltage controller detects that the voltage of the photovoltaic grid-connected node in the AC system reaches the critical value, the voltage of the photovoltaic grid-connected node in the AC system is prevented and controlled through reactive power compensation. It is determined that the AC system has voltage exceeding the limit, and the voltage of the AC system is controlled through distributed reactive power coordinated control.

[0119] Depend on Figure 4 It can be seen that the voltage of the AC system can be effectively reduced by controlling the voltage of the AC system through the distributed reactive power coordinated control method. When the voltage exceeds the limit to a small extent, the VSC restores the voltage to the normal operating range by adjusting the reactive power compensation amount calculated by the voltage regulation amount.

[0120] (2) Scenario 2: The system’s PV active output increases by 25%;

[0121] Figure 5 When the photovoltaic active power output suddenly increases by 25%, the highest voltage of the AC system is 1.06152pu without control, and AC1 voltage exceeds the limit. When the photovoltaic voltage controller detects that the voltage of the photovoltaic grid-connected node in the AC system reaches the critical value, the voltage of the photovoltaic grid-connected node in the AC system is prevented and controlled through reactive power compensation, and the voltage of the AC system is determined to be over-limited. The AC system is controlled through the distributed reactive power coordination control method, and the node 2 located at the most upstream transmits the reactive power request to VSC1 until the reactive power compensation capacity of the VSC is exhausted, and a reactive power exhaustion signal is sent to the photovoltaic grid-connected node.

[0122] through Figure 4 and Figure 5 By comparison, it can be seen that when the voltage exceeds the limit to a large extent, the reactive power compensation of VSC can only reduce the voltage to a certain extent, but cannot adjust the voltage of all nodes to the normal range. At this time, the AC system still has voltage exceeding the limit. Therefore, when the photovoltaic grid-connected node receives the reactive power exhaustion signal sent by VSC, the active power flow distribution coordinated control method is used to control the voltage of the AC system.

[0123] Set the single active power adjustment value ΔP set =50kW, the corresponding voltage regulation is about 0.015pu. Based on this, it can be known that VSC2 and VSC3 adjust 22.8261kW and 27.1739kW respectively in each adjustment process. The voltage of AC1 during VSC active power regulation is as follows Figure 6 As shown, after the fourth adjustment, the voltage returns to normal within the normal range.

[0124] Figure 7 In order to control the voltage of the AC system through the active power flow distribution coordinated control method, that is, the change of the voltage of each node in AC3 during the active power regulation process, in which node 17 performs local preventive control every time the voltage reaches the critical upper limit, and maintains the node voltage at the critical value, while the voltage of other nodes also decreases due to the change of reactive power in the AC system. This active power regulation process not only reduces the possibility of AC3 voltage exceeding the limit, but also realizes the cross-regional consumption of photovoltaic power in different subsystems.

[0125] Table 1 shows the power regulation amount of each AC subsystem in the process of active power regulation of VSC. As shown in Table 1, based on the power transmission characteristics of VSC, VSC2 and VSC3 quantitatively adjust the power according to the load rate. After four adjustments, the active power of VSC2 and VSC3 is increased by 91.3044kW and 108.6956kW respectively. Through the above active power regulation process, node 17 compensates a total of 100.7248kVar of reactive power after local preventive control, so as to reduce the voltage of the AC system and improve the active power regulation and control ability of VSC. Therefore, through the active power regulation of VSC to coordinate the power between different AC systems, the voltage over-limit problem caused by the sudden change of AC1 photovoltaic output power can be solved, and the cross-regional consumption of distributed photovoltaic can be realized.

[0126] Table 1 Power regulation of each AC subsystem

[0127]

[0128] Figure 8 It is the voltage status of the system before and after distributed control. Before distributed control indicates that the voltage of the AC system is controlled by the distributed reactive power coordinated control method and the active power flow distributed coordinated control method; after distributed control indicates that the voltage of the AC system is controlled by the distributed reactive power coordinated control method and the active power flow distributed coordinated control method. Figure 8 As shown, the constant power side AC system, while participating in voltage regulation, ensures that the AC system voltage is within the normal range through its own reactive power coordination. Therefore, the voltage control strategy proposed in this application can effectively utilize the reactive power regulation resources of each AC distribution network, and solve the voltage limit problem after a high proportion of photovoltaic access through active power coordination between different AC subsystems.

[0129] Furthermore, in order to verify the effectiveness of the voltage control strategy proposed in this application, a decentralized local control strategy, a distributed control strategy that only uses photovoltaic inverters to regulate voltage, and the distributed control strategy proposed in this application are compared and analyzed. Table 2 is a comparison of the voltage regulation effects of the above control strategies.

[0130] Table 2 Comparison of voltage regulation effects of different control strategies

[0131]

[0132]

[0133] As can be seen from Table 2, the decentralized local control only uses local photovoltaics for voltage regulation, and the reactive compensation used only accounts for 12.03% of the adjustable reactive power of photovoltaics in the AC subsystem. A large amount of photovoltaic reduction will cause serious light abandonment problems. Although the distributed control strategy that only uses photovoltaic inverters for voltage regulation can fully utilize the reactive compensation capacity in the subsystem, the reactive compensation amount will also decrease as the photovoltaic active power is reduced. After the voltage regulation, the reactive compensation amount in the subsystem is reduced by 13.76% compared with that before the voltage regulation.

[0134] Compared with the distributed control strategy that only uses photovoltaic inverter voltage regulation, the distributed control strategy proposed in this application uses SOP to accurately control power, optimizes the power flow distribution between subsystems, and realizes cross-regional photovoltaic consumption through active power transmission between VSCs. The reactive power compensation amount is increased by 32.71%, which effectively improves the flexible system's utilization of overall photovoltaic reactive resources. At the same time, in the active power regulation and voltage regulation process under this situation, the VSC is operating at full capacity, and the reduction of active power increases the available reactive capacity of the VSC, which increases the reactive compensation amount of the VSC by 39.49%, thereby avoiding the use of active power reduction voltage regulation method and greatly improving the photovoltaic utilization rate.

[0135] (3) Scenario 3: The system’s photovoltaic active output increases by 30%;

[0136] The photovoltaic active output of each node in the AC system is set to increase by 30% to analyze the maximum voltage regulation operating point of the VSC during the coordination of active power. Through experimental simulation, in the absence of control, the system maximum voltage reaches 1.06488pu, and AC1 voltage exceeds the limit.

[0137] Fig. 9 The relationship between each change and the voltage regulation amount during the active power regulation of the VSC when the active power output of each node in the AC system increases by 30%. Fig. 9 The regulation conditions of each set regulation point in the figure, where point a is the distributed control point; point b is the maximum voltage regulation working point; point c is the test point where the active regulation amount is greater than the maximum voltage regulation working point.

[0138] As shown in Table 3, during the active power regulation at point a, the working state of the VSC switches from inverter to rectifier, and the available reactive capacity increases first and then decreases. The regulation amount of the VSC in the inverter state is greater than that in the rectifier state. Therefore, the available reactive capacity is increased compared with that before the active power regulation, and the VSC performs 34.747 kVar reactive power compensation. Since the active power regulation at points b and c is large in the rectifier state, the available reactive capacity at points b and c is greatly reduced due to the capacity limitation of the VSC, and the larger the active power regulation amount, the smaller the available reactive capacity. Among them, compared with point c, the active power regulation amount at point b is smaller, the reactive power compensation amount change suppression is also small, and the voltage regulation amount is 0.0006 pu higher than that at point c.

[0139] Therefore, in the rectification state, due to the limitation of VSC capacity during the active power coordination process, the regulation of active power will cause a reduction in the amount of reactive compensation. If the active power regulation exceeds the maximum voltage regulation operating point, the voltage regulation effect will be suppressed.

[0140] Table 3 Comparison of voltage regulation effects at different active power regulation points of VSC

[0141]

[0142] Finally, it should be noted that the above-described embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The protection scope of the present invention is not limited thereto. Although the present invention is described in detail with reference to the above-described embodiments, ordinary technicians in the field should understand that any technician familiar with the technical field can still modify the technical solutions recorded in the above-described embodiments within the technical scope disclosed by the present invention, or can easily think of changes, or make equivalent replacements for some of the technical features therein; and these modifications, changes or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A voltage control method based on active power coordination and reactive power coordination, characterized in that: include: S1: Performing local preventive control on the voltage of the AC system and detecting whether the voltage of the photovoltaic grid-connected node in the AC system exceeds the limit; S2: If the voltage of the photovoltaic grid-connected node exceeds the limit, the AC system is subjected to reactive coordinated control by means of a distributed reactive coordinated control method until the reactive compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, and the voltage of the AC system is detected to see whether it returns to normal; S3: If the voltage of the AC system has not returned to normal, the AC system is controlled by active power flow distribution coordination control method, and the voltage of other AC systems implementing the active power coordination control is detected to see whether it reaches a critical value; S4: If the voltage of the other AC systems implementing the active coordinated control reaches the critical value, the voltage of the AC system is subject to on-site preventive control. If the voltage of the AC system exceeds the limit at this time, the reactive coordinated control of the AC system is performed through the distributed reactive coordinated control method until the voltage of the AC system returns to normal.

2. A voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: In step S1, a photovoltaic inverter is used to perform the voltage local preventive control on the AC system through reactive power compensation. During the voltage local preventive control process, the maximum adjustment amount ΔQ PVmax The expression is as follows: In the formula, S PV is the photovoltaic capacity; P PV is the photovoltaic active output; Q PV is the photovoltaic reactive power output; is the minimum power factor; In the above expression, when When both are positive, ΔQ PVmax is the maximum reactive power regulation during step-down regulation; When both are negative, ΔQ PVmax It is the maximum reactive power regulation amount during boost regulation.

3. A voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: When the voltage of the photovoltaic grid-connected node reaches the upper limit of the critical value, the photovoltaic grid-connected node absorbs reactive power to prevent the photovoltaic grid-connected node from exceeding the voltage limit; when the voltage of the photovoltaic grid-connected node reaches the lower limit of the critical value, the photovoltaic grid-connected node outputs the reactive power to prevent the photovoltaic grid-connected node from undervoltage.

4. The voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: In the step S2, the reactive coordination control is performed on the AC system by the distributed reactive coordination control method until the reactive compensation capacity of the VSC connected to the over-limit AC subsystem is exhausted, including: S201: the photovoltaic grid-connected node sends a first reactive power compensation request, and after receiving the first reactive power compensation request, the downstream photovoltaic grid-connected node performs reactive power compensation on the voltage of the photovoltaic grid-connected node until the reactive power compensation capacity of the downstream photovoltaic grid-connected node is exhausted; S202: the downstream photovoltaic grid-connected node sends a second reactive power compensation request, and the upstream photovoltaic grid-connected node performs reactive power compensation on the voltage of the photovoltaic grid-connected node after receiving the second reactive power compensation request, until the reactive power compensation capacity of the upstream photovoltaic grid-connected node is exhausted; S203: The upstream photovoltaic grid-connected node sends a third reactive power compensation request, the SOP receives the third reactive power compensation request, and performs reactive power compensation on the voltage of the photovoltaic grid-connected node through the VSC connected to the off-limit AC subsystem until the reactive power compensation capacity of the VSC is exhausted, and then sends a reactive power exhaustion signal to the photovoltaic grid-connected node.

5. The voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: If it is detected that the voltage of the other AC systems implementing the active coordinated control does not reach the critical value, it is detected whether the voltage of the AC system returns to normal. If so, the voltage control is terminated; if not, step S3 is repeated.

6. The voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: In the step S3, the active power coordinated control is performed on the AC system by using the active power flow distribution coordinated control method, including: The constant voltage control VSC sends an active power adjustment request to the constant power control VSC, and the constant power control VSC receives the active power adjustment request and adjusts the active power distribution in the AC system; Among them, when the voltage on the AC side of the constant voltage control VSC exceeds the limit, the constant power control VSC allocates the active power increase amount according to the remaining capacity ratio, periodically adjusts the active power distribution in the AC system, and performs voltage control on the AC system; when the voltage on the AC side of the constant power control VSC exceeds the limit, the constant power control VSC periodically adjusts the active power distribution in the AC system by changing its own power parameters to control the voltage of the AC system.

7. A voltage control method based on active power coordination and reactive power coordination according to claim 6, characterized in that: The constant power control VSC distributes the active power increase amount according to the remaining capacity ratio to obtain the active power change amount. Its expression is as follows: Where: S d,adj is the adjustable active power of the constant power control VSC at node d; S k,adj is the adjustable active power of the constant power control VSC at node k; ΔP set is the single active power adjustment amount; h is the number of the constant power VSCs participating in the active coordinated control.

8. The voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: When voltage control is performed on the AC system, a maximum voltage regulation operating point is obtained, and the maximum voltage regulation operating point includes a maximum voltage regulation amount, a maximum active power regulation operating point, and a maximum reactive power regulation operating point.

9. A voltage control method based on active power coordination and reactive power coordination according to claim 8, characterized in that: Maximum voltage regulation ΔU i,max The expression is as follows: During the step-down regulation process, the maximum active power regulation operating point The expression is as follows: In the step-down regulation process, the maximum reactive power regulation operating point The expression is as follows: During the step-up regulation process, the maximum active power regulation operating point The expression is as follows: During the step-up regulation process, the maximum reactive power regulation operating point The expression is as follows: Where: P VSC is the active power of the VSC, Q VSC is the reactive power of the VSC; S VSC is the VSC capacity; k1 is the sensitivity of the node voltage to the change of the node active power; k2 is the sensitivity of the node voltage to the change of the node reactive power. The expressions of k1 and k2 are as follows: Where: U N is the rated value of the node voltage; Bi,vsc is the intersection of the line set from the photovoltaic grid-connected node i to the power inflow node of the distribution network and the line set from the photovoltaic grid-connected node where the VSC is located to the power inflow node of the distribution network; R L and X L are the resistance and reactance of line L respectively.

10. The voltage control method based on active power coordination and reactive power coordination according to claim 1, characterized in that: The step S4 also includes that during the AC system voltage regulation process, if a certain constant power control VSC has a problem of exceeding the system voltage limit when increasing the active power, the constant power control VSC exits the active power coordination control, and the other constant power control VSCs continue to be adjusted according to the original margin ratio until the AC system voltage returns to normal.

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