Active Distribution Network Voltage Governance Method Based on Cooperative Control of Energy Storage and Photovoltaic Inverters

Abstract

An active distribution network voltage governance method based on coordinated control of energy storage and photovoltaic inverters, comprising the following steps: Step 1: Establish a voltage-active / reactive double-layer control model with coordinated control of OLTC, CB, SVC, SVG, energy storage and photovoltaic inverters; Step 2: Construct an objective function of the double-layer control model with the objectives of minimizing the node voltage deviation and balancing the three-phase voltage; Step 3: Construct the constraint conditions of the objective function. Fully consider the time response characteristics of each regulating device and the charge and discharge characteristics of the energy storage system, ensure the minimization of the photovoltaic voltage deviation while also ensuring the three-phase voltage balance of the nodes, the stability of the line voltage, reduce the reverse power flow, suppress the voltage fluctuation, and guarantee the customer power quality.

Description

Technical Field

[0001] The present invention relates to the fields of distributed photovoltaic grid connection and distribution network voltage governance, and particularly to an active distribution network voltage governance method based on coordinated control of energy storage and photovoltaic inverters. Background Art

[0002] After high proportion of photovoltaic grid connection, the distribution network faces many risks. Therefore, studying the distribution network voltage control problem under high proportion of photovoltaic access and reducing the adverse effects brought by photovoltaic access have become one of the key problems to be solved urgently.

[0003] Currently, for the overvoltage problem of photovoltaic access to the distribution network, it is mainly solved by means of power regulation. Photovoltaic inverters have the ability to regulate active power and reactive power, and meet the requirements of frequent regulation and fast response speed, so they are widely used in the distribution network overvoltage problem. The dual characteristics of power supply and load of the energy storage system bring a new solution to the overvoltage problem of the photovoltaic distribution network. However, most of the existing studies focus on solving the voltage over-limit problem and rarely pay attention to the three-phase balance of node voltages, and the three-phase voltage imbalance problem is also an important factor affecting voltage quality. Summary of the Invention

[0004] The purpose of the present invention is to solve the above-mentioned deficiencies of the prior art, and thus provide an active distribution network voltage governance method based on coordinated control of energy storage and photovoltaic inverters.

[0005] An active distribution network voltage governance method based on coordinated control of energy storage and photovoltaic inverters includes the following steps:

[0006] Step 1: Establish a voltage-active / reactive power two-layer control model coordinated by OLTC, CB, SVC, SVG, energy storage and photovoltaic inverters;

[0007] Step 2: Construct the objective function of the two-layer control model with the minimum node voltage deviation and three-phase voltage balance as the control objectives;

[0008] Step 3: Construct the constraint conditions of the objective function.

[0009] The voltage-active / reactive power two-layer control model includes an upper-layer voltage-reactive power control model and a lower-layer voltage-active / reactive power control model;

[0010] The upper-layer voltage-reactive power control model is: when the grid voltage exceeds the limit, the tap of the OLTC is adjusted and the CB is switched on and off to adjust the low-voltage side bus voltage, and then the voltage distribution of other nodes on the feeder is changed to reduce the voltage deviation of the whole feeder; if the voltage is still not within the qualified range after the adjustment of the OLTC and CB, voltage-reactive power control is carried out by the reactive power regulation of SVC and SVG in turn according to the requirements;

[0011] The lower - layer voltage - active / reactive power control model is as follows: OLTC, CB, SVC, and SVG operate according to the daily plan. When the voltage exceeds the limit due to the deviation of photovoltaic output and load prediction, the voltage - limit control of the over - limit point is carried out first by reactive power and then by active power through the fast dynamic response capabilities of the photovoltaic inverter and energy storage system.

[0012] In the upper - layer voltage - reactive power control model, the voltage - reactive power control of the reactive power regulation devices OLTC, CB, SVC, and SVG is as follows:

[0013] (1) Voltage - reactive power control of OLTC

[0014] By analyzing the data of the distribution network monitoring to determine whether the voltage is within the allowable range, and using the relay to issue an action command to change the position of the tap to change the tap ratio, the voltage - reactive power control is carried out.

[0015] (2) Voltage - reactive power control of CB

[0016] When the grid load is large, the CB is put into operation for reactive power compensation to achieve the purpose of voltage boost and smooth the voltage fluctuation in the circuit.

[0017] (3) Voltage - reactive power control of SVC

[0018] When the voltage goes below the lower limit, the SVC compensates for capacitive reactive power; when the voltage goes above the upper limit, the SVC compensates for inductive reactive power.

[0019] (4) Voltage - reactive power control of SVG

[0020] By changing the phase angle and amplitude of the AC voltage of the internal bridge circuit of the SVG to output the required reactive current to achieve reactive power compensation; when the fundamental component of the output voltage of the bridge circuit is higher than the grid system component, the SVG sends reactive power to the grid system; when the fundamental component of the output voltage of the bridge circuit is equal to the grid system voltage, the SVG operates in a zero - reactive - power state; when the fundamental component of the output voltage of the bridge circuit is lower than the grid system voltage, the SVG absorbs reactive power from the grid system.

[0021] When the voltage exceeds the limit due to the deviation of photovoltaic output and load prediction, the voltage - limit control of the over - limit point is carried out first by reactive power and then by active power through the fast dynamic response capabilities of the photovoltaic inverter and energy storage system. Specifically:

[0022] When the voltage goes below the lower limit, the photovoltaic inverter outputs capacitive reactive power to compensate for the reactive power of the grid and achieve the purpose of voltage boost; when the voltage goes above the upper limit, at this time, the grid has an excess of reactive power, and the photovoltaic inverter is used to output inductive reactive power to achieve the purpose of suppressing voltage.

[0023] When the adjustable reactive power capacity of the PV inverter is exhausted and the problem of voltage exceeding the upper limit still exists, the active power of the PV will be reduced. The reduced part is stored in the energy storage system to reduce the active power of the power grid and control the voltage from exceeding the upper limit. If the problem of voltage falling below the lower limit is still not solved, the energy storage is controlled to discharge to increase the active power of the power grid and achieve the purpose of voltage elevation.

[0024] The apparent power of the PV inverter and the active power it outputs jointly affect the adjustable reactive power capacity of the PV inverter. The adjustable reactive power capacity of the PV inverter has a fast adjustment speed and can achieve continuous adjustment. The relationship between the adjustable reactive power capacity of the PV inverter and the inverter capacity is:

[0025]

[0026] In the formula: Q pv is the reactive power output by the inverter; S pv is the rated capacity of the inverter; P pv is the active power of the PV power generation.

[0027] Taking the minimum node voltage deviation and three-phase voltage balance as the control objectives, the objective function of the double-layer control model is constructed as follows:

[0028] Objective 1: Minimize the node voltage deviation

[0029]

[0030] In the formula: U i,t is the node voltage value of node i at time t; U i,N is the rated voltage value of node i.

[0031] Objective 2: Three-phase voltage balance

[0032]

[0033] In the formula: Φ is the set composed of three phases {a, b, c}; is the maximum three-phase voltage amplitude of node i at time t; avg i,t represents the average value of the squares of the three-phase voltage amplitudes of node i at time t;

[0034] Taking into account the two factors of voltage deviation and three-phase voltage balance, the comprehensive objective function of the double-layer control model can be expressed as:

[0035] F = λ 1 f 1 + λ 2 f 2

[0036] In the formula: λ 1 、λ 2 are weight coefficients.

[0037] The objective function constraint is:

[0038] OLTC constraint:

[0039]

[0040] In the formula: is the regulation step of the OLTC; is the regulation transformation ratio of the OLTC; is the minimum regulation transformation ratio of the OLTC, is the number of steps of the OLTC; and are the maximum and minimum values of the OLTC step respectively;

[0041] CB constraint:

[0042]

[0043] In the formula: is the total reactive power of the CB; is the reactive power of a single group of CB; is the number of operating groups of the CB; is the maximum number of groups of the CB; Δ CB is the maximum number of groups change constraint of the CB within the optimization time;

[0044] SVC constraint:

[0045] The reactive power of the static var compensator should be less than its allowable maximum value and greater than its allowable minimum value,

[0046] Q svc,min ≤Q svc,i,t ≤Q svc,max

[0047] In the formula: Q svc,min 、Q svc,max are the upper and lower limits of the SVC reactive power, Q svc,i,t is the reactive power compensation of the SVC at node i at time t;

[0048] SVG constraint:

[0049] The reactive power of the static var generator should be less than its allowable maximum value and greater than its allowable minimum value,

[0050] Q svg,min ≤Q svg,i,t ≤Q svg,max

[0051] In the formula: Q svg,min 、Q svg,max are the upper and lower limits of the SVG reactive power, Qsvg,i,t The actual reactive power value of the SVG at node i at time t;

[0052] Node voltage constraint:

[0053] (1 - δ low )U i,N ≤U i,t ≤(1 + δ up )U i,N

[0054] Where: δ low 、δ up are the node voltage deviation limit values;

[0055] Node voltage three-phase unbalance constraint:

[0056] To ensure the relative balance of the three-phase node voltages, the voltage unbalance degree should not exceed its allowable value. It is set that the square unbalance degree of the node voltage amplitude should not exceed 6%,

[0057]

[0058] Energy storage siting and sizing constraint:

[0059]

[0060] Where: is the maximum installed capacity of the energy storage for phase φ of node i; W min 、W max are the maximum and minimum energy multiples of the energy storage respectively; f i is a 0-1 variable, 1 indicates that a three-phase energy storage is installed at node i; is the rated power of the energy storage for phase φ of node i;

[0061] Energy storage charge and discharge constraint:

[0062]

[0063] Where: is the state of charge of the energy storage for phase φ of node i, are its upper and lower limits; are the charge / discharge states of the energy storage for phase φ of node i respectively, and are 0-1 variables; P dis,i,t 、P ch,i,t are the charge / discharge powers of the energy storage for phase φ of node i respectively; η is the charge and discharge efficiency of the energy storage for phase φ; E is the rated capacity of the energy storage device for phase φ.

[0064] Photovoltaic inverter constraint:

[0065] The active power adjustment range of the photovoltaic inverter:

[0066] Ppv = [0, P pv,max

[0067] Where: P pv,max is the maximum value of the active power output of the photovoltaic system;

[0068] Reactive power regulation range of the photovoltaic inverter:

[0069] Q pv = [-Q pv,max , Q pv,max

[0070]

[0071] Where Q pv,max is the maximum reactive power limit of the photovoltaic output; θ is the power factor angle of the inverter.

[0072] This invention analyzes the impact of the supply-demand relationship between distributed photovoltaic power output and system load on the voltage violation of the distribution network. Aiming at the fact that the current voltage governance does not fully consider the differences in response speed, regulation capacity, and regulation range among OLTC, CB, SVC, SVG, energy storage, and photovoltaic inverters, and lacks an optimization scheme that can coordinate and control different reactive power devices and energy storage systems. Therefore, this invention establishes a voltage-active / reactive double-layer control model considering the coordinated control of OLTC, CB, SVC, SVG, energy storage, and photovoltaic inverters. Through a double-layer adjustment mode of reactive power first and then active power, while regulating the voltage violation, it ensures the three-phase voltage balance of the nodes, explores a new idea for the voltage control problem in the distribution network, and contributes a new method to improve the consumption capacity of photovoltaic power sources.

[0073] This invention establishes a voltage-active / reactive double-layer control model considering the coordinated control of on-load tap changer (OLTC), capacitor bank (CB), static var compensator (SVC), static var generator (SVG), energy storage, and photovoltaic inverter, making full use of the regulation characteristics of different regulating devices, aiming to solve the voltage violation problem while ensuring the three-phase balance of the node voltage.

[0074] Compared with the prior art, this invention has the following beneficial effects:

[0075] ​​The present invention establishes a voltage-active power / reactive power two-layer control model coordinated by OLTC, CB, SVC, SVG, energy storage, and photovoltaic inverters. By fully considering the time response characteristics of each regulating device, the charge and discharge characteristics, and the fast response characteristics of the energy storage system, it ensures the minimization of photovoltaic voltage deviation while guaranteeing the three-phase voltage balance of nodes, the stability of line voltage, reducing power reverse transmission, suppressing voltage fluctuations, and ensuring the power quality of customers. Description of the Drawings

[0076] Figure 1 is the equivalent circuit for photovoltaic grid connection;

[0077] Figure 2 is the framework diagram of the two-layer active power / reactive power optimization control scheme;

[0078] Figure 3 is the capacity curve diagram of the photovoltaic inverter. Detailed Implementation Manner

[0079] To make the implementation and advantages of the technical solution of the present invention clear, the present invention will be described in detail below with reference to the accompanying drawings.

[0080] Figure 1 is the equivalent circuit diagram after distributed photovoltaic grid connection. Analyzing Figure 1 it can be seen that the voltage value V at the photovoltaic grid connection point Bus2 2 is:

[0081] V 2 = V 1 - I 1，2 (R + jX) (1)

[0082] In the formula: V 1 is the voltage at Bus1; I 1，2 is the line current flowing from Bus1 to Bus2; R + jX is the line impedance.

[0083] Then the voltage difference ΔV between the lines can be expressed by formula (2).

[0084] ΔV = V 1 - V 2 = I 1，2 (R + jX) (2)

[0085] Also, according to S = UI, it can be obtained:

[0086]

[0087] That is, the current I 1，2 expression (4)

[0088]

[0089] Wherein, is the conjugate current of I 1，2 ; P s and Q s are the active and reactive powers flowing from Bus1 to Bus2.

[0090] Therefore, the voltage difference ΔV between lines can be expressed by Equation (5).

[0091]

[0092] Wherein, is the conjugate voltage of V 2 .

[0093] Ignoring the imaginary part of ΔV, we can obtain:

[0094]

[0095] Assume that the active power of the load is P l , the reactive power is Q l , the active power incorporated by the distributed PV is P pv , the reactive power is Q pv , then

[0096]

[0097] According to Equation (7), it can be seen that when the active and reactive powers of the PV and the load are different, different voltage differences will be caused.

[0098] Based on the above analysis, the active distribution network voltage governance method based on the coordinated control of energy storage and PV inverter of the present invention includes the following steps:

[0099] Step 1: Establish a voltage-active / reactive double-layer control model jointly controlled by OLTC, CB, SVC, SVG, energy storage, PV inverter, etc. The block diagram of the double-layer control model is shown in Figure 2 .

[0100] (1) Upper-layer voltage-reactive power control model

[0101] The upper-layer optimization model mainly formulates the device parameters of the reactive power regulation devices OLTC, CB, SVC, SVG through the PV output prediction data and load prediction data, and performs voltage over-limit control. The specific control model is as follows: when the grid voltage exceeds the limit, the tap of the OLTC is switched and the CB is switched on and off to adjust the low-voltage side bus voltage, thereby changing the voltage distribution of other nodes on the feeder and reducing the voltage deviation of the entire feeder. If the voltage is still not within the qualified range after the regulation of the OLTC and CB, the reactive power regulation of the SVC and SVG is carried out in sequence according to the requirements for voltage-reactive power control.

[0102] (2) Lower - layer Voltage - Active / Reactive Power Control Model

[0103] Subject to the uncertainties of PV output and load, the prediction accuracy is low in actual operation. However, due to the long response times of OLTC, CB, SVC, and SVG and their ability to only make large - range adjustments and not accurately regulate voltage, in the lower - layer voltage control model, PV inverters and energy storage systems are proposed for voltage control. The specific control model is as follows: OLTC, CB, SVC, and SVG operate according to the day - ahead plan. When voltage violations occur due to PV output and load prediction deviations, the rapid dynamic response capabilities of PV inverters and energy storage systems are used to perform voltage violation control, first reactive power and then active power, at the violation points.

[0104] The upper - layer voltage control model - the voltage - reactive power control of reactive power regulation devices OLTC, CB, SVC, and SVG is as follows:

[0105] (1) Voltage - Reactive Power Control of OLTC

[0106] Analyze whether the voltage is within the allowable range through the distribution network monitoring data, and use the relay to issue an action command to change the position of the tap to change the tap ratio for voltage - reactive power control.

[0107] (2) Voltage - Reactive Power Control of CB

[0108] Since CB is a constant - impedance element, it cannot perform directional compensation for a specific line. Therefore, voltage - reactive power control is achieved through the power switching of CB. When the grid load is large, CB is switched in to perform reactive power compensation to achieve the purpose of voltage elevation and smoothing the voltage fluctuations in the circuit.

[0109] (3) SVC Voltage - Reactive Power Control

[0110] When the voltage drops below the lower limit, SVC compensates for capacitive reactive power; when the voltage exceeds the upper limit, SVC compensates for inductive reactive power.

[0111] (4) SVG Voltage - Reactive Power Control

[0112] By changing the phase angle and amplitude of the AC voltage of the internal bridge circuit of SVG, the required reactive current is output to achieve reactive power compensation. When the fundamental component of the output voltage of the bridge circuit is higher than the grid system component, SVG sends reactive power to the grid system; when it is equal to the grid system voltage, SVG operates in a zero - reactive - power state; when it is lower than the grid system voltage, SVG absorbs reactive power from the grid system.

[0113] When the voltage exceeds the limit due to the deviation between the photovoltaic output and the load prediction, the voltage limit control of reactive power first and then active power is carried out on the limit point through the fast dynamic response capabilities of the photovoltaic inverter and the energy storage system. Specifically:

[0114] When the voltage drops below the lower limit, the photovoltaic inverter outputs capacitive reactive power to compensate the reactive power of the power grid and achieve the purpose of voltage boosting; when the voltage exceeds the upper limit, at this time, the reactive power of the power grid is excessive, and the photovoltaic inverter is used to output inductive reactive power to achieve the purpose of suppressing the voltage.

[0115] When the adjustable reactive power capacity of the photovoltaic inverter is exhausted, if the problem of voltage exceeding the upper limit still exists, the active power of the photovoltaic will be reduced, and the reduced part will be stored in the energy storage system to reduce the active power of the power grid and control the voltage from exceeding the upper limit; if the problem of voltage dropping below the lower limit is still not solved, the energy storage will be controlled to discharge to increase the active power of the power grid and achieve the purpose of voltage boosting.

[0116] Figure 3 This is the regulation principle of the photovoltaic inverter. The maximum active power output of the photovoltaic inverter is P pv,max , which is point A. At this time, the maximum reactive power output capacity of the inverter is ±Q pv,max1 . It can be seen that by appropriately increasing the rated capacity of the inverter, the reactive power regulation range of the inverter can be increased. When the real-time active power output of the photovoltaic inverter is reduced to βP pv,max (β ∈ (0, 1)), the operating point of the inverter drops from point A to point B, and the maximum reactive power output capacity of the inverter is ±Q pv,max2 .

[0117] The apparent power of the photovoltaic inverter and its output active power jointly affect the adjustable reactive power capacity of the photovoltaic inverter. The adjustable reactive power capacity of the photovoltaic inverter has a fast adjustment speed and can achieve continuous adjustment. The relationship between the adjustable reactive power capacity of the photovoltaic inverter and the inverter capacity is:

[0118]

[0119] In the formula: Q pv is the reactive power output by the inverter; S pv is the rated capacity of the inverter; P pv is the active power of photovoltaic power generation.

[0120] It can be seen from formula (7) that when the photovoltaic output is greater than the load demand, the voltage will exceed the upper limit. On the contrary, the voltage will drop below the lower limit.

[0121] When the voltage drops below the lower limit, that is, U i,t < U min , the photovoltaic inverter outputs active power or capacitive reactive power.

[0122] When the voltage is within the allowable range, i.e., U min <U i,t <U max the PV inverter operates in the maximum power operation mode, i.e., P pv =P pv,max and the power factor is 1 at this time.

[0123] When the voltage exceeds the upper limit, i.e., U i,t >U max the PV inverter reduces the active power or outputs inductive reactive power.

[0124] Energy storage has the characteristics of both power source and load, and is a common means to solve the voltage over-limit problem caused by high-proportion distributed PV access. By installing energy storage phase by phase, the present invention not only solves the voltage over-limit problem, but also ensures the three-phase balance of the node voltage.

[0125] When the voltage over-limit occurs in the low-voltage distribution network, it is preferred to use the remaining capacity of the PV inverter for voltage regulation. If the remaining capacity is insufficient and the voltage over-limit problem still exists, an energy storage system is introduced on the user side. Specifically: when the available remaining capacity of the PV inverter is exhausted and the voltage exceeds the upper limit, the energy storage system is used to absorb the excess PV output, and this process is to charge the energy storage; if the voltage lower limit problem is still not solved, the energy storage will play its own power source characteristics and release active power for voltage control, and this process is to discharge the energy storage.

[0126] Step 2: Taking the minimum node voltage deviation and three-phase voltage balance as the control objectives, construct the objective function of the double-layer control model;

[0127] Use OLTC, CB, SVC, SVG, energy storage, and PV inverter to adjust reactive power and active power, so as to achieve the control objectives of minimum node voltage deviation and three-phase voltage balance.

[0128] (1) Objective 1: Minimize the voltage deviation

[0129]

[0130] where: U i,t is the node voltage value of node i at time t; U i,N is the rated voltage value of node i.

[0131] (2) Objective 2: Three-phase voltage balance

[0132]

[0133] where: Φ is the set composed of three phases {a, b, c}; is the maximum three-phase voltage amplitude of node i at time t; avg i,tDenote the average value of the square of the three-phase voltage amplitude of node \(i\) at time \(t\).

[0134] The present invention comprehensively considers the factors of voltage deviation and three-phase voltage balance. The comprehensive objective function of the model can be expressed as:

[0135] \(F = \lambda_1f_1+\lambda_2f_2\) (11) 1 \(f_1\) 1 +\(\lambda_2\) 2 \(f_2\) 2 (11)

[0136] In the formula: \(\lambda_1\), \(\lambda_2\) are weight coefficients and can be selected according to actual requirements. 1 \(\lambda_1\), 2 \(\lambda_2\)

[0137] Step 3: Constraint conditions of the objective function

[0138] The constraint conditions of the objective function mainly include the equality constraints and inequality constraints of the upper-layer voltage-reactive power control model, and the equality constraints and inequality constraints of the lower-layer voltage-active / reactive power control model:

[0139] (1) The upper-layer voltage-reactive power control has the following equality constraints and inequality constraints:

[0140] OLTC constraint:

[0141]

[0142] In the formula: \(\Delta k\) is the adjustment step; \(k\) is the transformation ratio; \(k_{min}\) is the minimum transformation ratio, \(n\) is the number of steps; \(k_{max}\) and \(k_{min}\) are the maximum and minimum values of the step respectively.

[0143] CB constraint:

[0144]

[0145] In the formula: \(Q_{c}\) is the reactive power of a single group; \(m\) is the number of operating groups; \(m_{max}\) is the maximum number of groups; \(\Delta m\) CB is the maximum number of group changes of the capacitor bank within the optimization time.

[0146] SVC constraint:

[0147] The reactive power of the static var compensator should be less than its allowable maximum value and greater than its allowable minimum value.

[0148] \(Q_{SVC}^{min}\) svc,min \(\leq Q_{SVC}\) svc,i,t \(\leq Q_{SVC}^{max}\)svc,max (14)

[0149] Where: Q svc,min and Q svc,max are the upper and lower limits of the SVC reactive power, and Q svc,i,t is the reactive power compensation of the SVC at node i at time t.

[0150] SVG constraint:

[0151] The reactive power of the static var generator should be less than its allowable maximum value and greater than its allowable minimum value.

[0152] Q svg,min ≤Q svg,i,t ≤Q svg,max (15)

[0153] Where: Q svg,min and Q svg,max are the upper and lower limits of the SVG reactive power, and Q svg,i,t is the actual value of the SVG reactive power at node i at time t.

[0154] Node voltage constraint:

[0155] (1 - δ low )U i,N ≤U i,t ≤(1 + δ up )U i,N (16)

[0156] Where: δ low and δ up are the node voltage deviation limit values.

[0157] Node voltage three-phase unbalance constraint:

[0158] To ensure the relative balance of the three-phase node voltages, the voltage unbalance should not exceed its allowable value. In the present invention, it is set that the square unbalance of the node voltage amplitude should not exceed 6%.

[0159]

[0160] (2) The lower-layer voltage-active / reactive power control has the following equality constraints and inequality constraints:

[0161] Energy storage site selection and capacity determination constraint:

[0162]

[0163] Where: is the maximum installed capacity of the energy storage for phase φ of node i; W min and W max are the maximum and minimum energy multiples of the energy storage respectively; fi is a 0-1 variable, where 1 indicates that a three-phase energy storage is installed at node i; is the rated power of the φ-phase energy storage at node i.

[0164] Energy storage charge and discharge constraints:

[0165]

[0166] In the formula: is the state of charge of the φ-phase energy storage at node i, are its upper and lower limits; are the charge / discharge states of the φ-phase energy storage at node i respectively, which are 0-1 variables; P dis,i,t , P ch,i,t are the charge / discharge powers of the φ-phase energy storage at node i respectively; η is the charge / discharge efficiency of the φ-phase energy storage; E is the rated capacity of the φ-phase energy storage device.

[0167] Photovoltaic inverter constraints:

[0168] Active power regulation range of the photovoltaic inverter:

[0169] P pv = [0, P pv,max (20)

[0170] In the formula: P pv,max is the maximum value of the active power output of the photovoltaic.

[0171] Reactive power regulation range of the photovoltaic inverter:

[0172] Q pv = [-Q pv,max , Q pv,max (21)

[0173]

[0174] In the formula, Q pv,max is the maximum reactive power output limit of the photovoltaic; θ is the power factor angle of the inverter.

[0175] The node voltage constraints and the node voltage three-phase unbalance constraints are consistent with the upper-layer model and will not be elaborated here.

Claims

1. A method for voltage management of an active distribution network based on coordinated control of energy storage and photovoltaic inverters, characterized in that: The following steps are involved: Step 1: Establish a voltage-active / reactive double-layer control model coordinated by OLTC, CB, SVC, SVG, energy storage and PV inverter; Step 2: Taking the minimum node voltage deviation and voltage three-phase balance as the control objectives, construct the objective function of the two-layer control model; Step 3: Construct the constraints of the objective function; The voltage-active / reactive double-layer control model includes an upper-layer voltage-reactive control model and a lower-layer voltage-active / reactive control model; The upper voltage-reactive power control model is as follows: when the grid voltage exceeds the limit, the low-voltage bus voltage is adjusted by adjusting the OLTC tap and switching the CB, thereby changing the voltage distribution of other nodes on the feeder and reducing the voltage deviation of the entire feeder; if the voltage is still not within the qualified range after adjustment by the OLTC and CB, the voltage-reactive power control is then performed through the reactive power adjustment of the SVC and SVG in turn according to the demand; The lower-layer voltage-active / reactive control model is as follows: OLTC, CB, SVC, and SVG are operated according to the day-ahead plan. When the voltage exceeds the limit due to the deviation of photovoltaic output and load forecast, the photovoltaic inverter and energy storage system are used to perform reactive power control first and active power control later at the voltage exceeding the limit point through the fast dynamic response capability of the photovoltaic inverter and energy storage system. Specifically: When the voltage is below the line, the photovoltaic inverter outputs capacitive reactive power to compensate the grid for reactive power, thereby achieving the purpose of voltage raising; when the voltage is above the line, the grid has excess reactive power, and the photovoltaic inverter is used to output inductive reactive power to achieve the purpose of voltage stabilization; When the adjustable reactive capacity of the photovoltaic inverter is exhausted, if the problem of voltage exceeding the upper limit still exists, the photovoltaic active power will be reduced, and the reduced part will be stored in the energy storage system, reducing the active power of the grid and controlling the voltage to exceed the upper limit; if the problem of voltage exceeding the lower limit is still not solved, the energy storage discharge will be controlled to increase the active power of the grid and achieve the purpose of voltage raising; The apparent power of the photovoltaic inverter and its output active power jointly affect the adjustable reactive capacity of the photovoltaic inverter. The adjustable reactive capacity of the photovoltaic inverter is fast to adjust and can be adjusted continuously. The relationship between the adjustable reactive capacity of the photovoltaic inverter and the inverter capacity is: Where: Q pv Output reactive power for the inverter; S pv is the rated capacity of the inverter; P pv is the active power of photovoltaic power generation.

2. The method for voltage management of active distribution network based on coordinated control of energy storage and photovoltaic inverter according to claim 1, characterized in that: The voltage-reactive power control of reactive power regulation equipment OLTC, CB, SVC, and SVG in the upper voltage-reactive power control model is: (1) OLTC voltage-reactive power control By analyzing the distribution network monitoring data to see if the voltage is within the allowable voltage range, and using the relay to issue an action command to change the position of the tap to change the tap ratio, voltage-reactive power control is performed; (2) CB voltage-reactive power control When the grid load is large, reactive power compensation is performed by inputting CB to achieve the purpose of voltage raising and smoothing voltage fluctuations in the circuit; (3) Voltage-reactive power control of SVC When the voltage is lower than the line, SVC compensates for capacitive reactive power, and when the voltage is higher than the line, SVC compensates for inductive reactive power; (4) SVG voltage-reactive power control Reactive power compensation is achieved by changing the phase angle and amplitude of the AC voltage in the bridge circuit inside the SVG to output reactive current that meets the requirements. When the fundamental component of the bridge circuit output voltage is higher than the grid system component, the SVG sends reactive power to the grid system. When the fundamental component of the bridge circuit output voltage is equal to the grid system voltage, the SVG operates in a zero reactive power state. When the fundamental component of the bridge circuit output voltage is lower than the grid system voltage, the SVG absorbs reactive power from the grid system.

3. The method for voltage management of active distribution network based on coordinated control of energy storage and photovoltaic inverter according to claim 1, characterized in that: Taking the minimum node voltage deviation and the voltage three-phase balance as the control objectives, the objective function of the two-layer control model is constructed, which is as follows: Goal 1: Minimize node voltage deviation Where: U i,t is the node voltage value of node i at time t; U i,N is the rated voltage value of node i; Goal 2: Voltage three-phase balance Where: Φ is the set consisting of three phases {a, b, c}; is the maximum three-phase voltage amplitude of node i at time t; avg i,t It represents the average value of the square of the three-phase voltage amplitude at node i at time t; Taking into account the factors of voltage deviation and voltage three-phase balance, the comprehensive objective function of the two-layer control model is expressed as: F=λ1f1+λ2f2 Where: λ1, λ2 are weight coefficients.

4. The method for voltage management of active distribution network based on coordinated control of energy storage and photovoltaic inverter according to claim 1 or 3, characterized in that: The objective function constraints are: OLTC constraints: Where: is the adjustment step size of OLTC; is the regulation ratio of OLTC; is the minimum regulation ratio of OLTC, is the number of gears of OLTC; and They are the maximum and minimum values ​​of the OLTC gear respectively; CB Constraints: Where: is the total reactive power of CB; is the reactive power of a single group of CBs; is the number of CB groups in operation; is the maximum number of CB groups; Δ CB Change the constraint for the maximum number of groups of CB within the optimization time; SVC constraints: The reactive power of the static VAR compensation device should be less than its maximum allowable value and greater than its minimum allowable value. Q svc,min ≤Q svc,i,t ≤Q svc,max Where: Q svc,min , Q svc,max is the upper and lower limits of SVC reactive power, Q svc,i,t is the reactive compensation amount of the SVC at node i at time t; SVG constraints: The reactive power of the static VAR generator should be less than its maximum allowable value and greater than its minimum allowable value. Q svg,min ≤Q svg,i,t ≤Q svg,max Where: Q svg,min , Q svg,max is the upper and lower limits of SVG reactive power, Q svg,i,t is the actual value of reactive power of SVG at node i at time t; Node voltage constraints: (1-d low )U i,N ≤U i,t ≤(1+δ up )U i,N Where: low ,δ up is the node voltage deviation limit; Constraints on the three-phase unbalance of node voltage: To ensure the relative balance of the three-phase voltage at the node, the voltage imbalance should not exceed its allowable value, and the square imbalance of the node voltage amplitude should not exceed 6%. Energy storage site selection and capacity constraints: Where: is the maximum installed capacity of energy storage at phase i; W min , W max are the maximum and minimum energy rates of energy storage, respectively; f i It is a 0-1 variable, 1 means that the i node is equipped with three-phase energy storage; is the rated power of phase energy storage of node i; Energy storage charging and discharging constraints: Where: is the energy storage charge state of phase φ at node i, Its upper and lower limits; are the energy storage charge / discharge state of the φ phase of the i-node, which is a 0-1 variable; P dis,i,t , P ch,i,t are the energy storage charging / discharging power of phase φ at node i respectively; η is the charge and discharge efficiency of the energy storage of the φ phase; E is the rated capacity of the energy storage device of the φ phase; PV inverter constraints: Active power adjustment range of photovoltaic inverter: P pv =[0,P pv,max ] Where: P pv,max is the maximum value of photovoltaic active output; Reactive power adjustment range of photovoltaic inverter: Q pv =[-Q pv,max ,Q pv,max ] In the formula, Q pv,max is the maximum reactive power limit of photovoltaic output; θ is the power factor angle of the inverter.

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