A method and device for evaluating the new energy carrying capacity of a multi-DC fed power grid

Abstract

The present invention belongs to the field of new energy carrying capacity evaluation, and specifically relates to a method and device for evaluating the new energy carrying capacity of a multi-DC-fed power grid. The evaluation method includes the following steps: S1. Considering the control mode and operation characteristics of new energy access to a multi-DC-fed power grid, as well as the mutual coupling effect between DC and new energy, and comprehensively considering the voltage and frequency safety and stability, an evaluation index system for new energy access to a multi-DC-fed power grid is established; S2. With the goal of maximizing the new energy access capacity and taking into account the grid security and stability constraints, an optimization model for evaluating the new energy carrying capacity of a multi-DC-fed power grid is established; S3. The optimization model for evaluating the new energy carrying capacity of a multi-DC-fed power grid is linearized and solved to obtain the maximum new energy carrying capacity of the system. The present invention effectively evaluates the new energy carrying capacity of a multi-DC-fed power grid and provides a basis for determining the optimal new energy access scheme.

Description

Technical Field

[0001] The present invention belongs to the field of new energy carrying capacity assessment, and particularly relates to a method and device for assessing the new energy carrying capacity of a multi-DC fed power grid. Background Art

[0002] In order to address global climate change, countries around the world are actively promoting the low-carbon, clean, and sustainable transformation of the energy system. China has also put forward the grand goal of "achieving carbon peak before 2030 and carbon neutrality before 2060", and will build a new power system with new energy as the main body as the next major development direction. In the new power system, a large number of wind turbines and photovoltaics are connected to the grid, and new energy has become the main energy form on the power source side; as the core hub for absorbing large-scale new energy, the power grid will also give full play to the advantages of DC transmission in terms of transmission distance, transmission capacity, transmission flexibility, transmission cost, etc., forming a future scenario in which a large amount of new energy is connected to an AC power grid fed by multiple DCs.

[0003] As the scale of new energy connected to the multi-DC fed power grid continues to expand, the power grid will exhibit significant characteristics such as reduced system moment of inertia and weakened frequency regulation performance in terms of safety and stability. At the same time, the output of new energy units is affected by the external environment, seriously threatening the voltage quality of the power grid. After the power grid is subjected to power disturbances or faults, the power system is extremely prone to transient frequency over-limits; in addition, the mutual coupling characteristics of the multi-fed DC and the dynamic process of new energy make the power system exhibit a more complex transient process, and it is extremely easy to cause a chain of faults of multi-circuit DC commutation failure and new energy disconnection from the grid after a fault. Therefore, in order to ensure the safe and stable operation of the power grid, studying the capacity of new energy units that can be connected to the multi-DC fed power grid, that is, the new energy carrying capacity of the multi-DC fed power grid, has increasingly become an important research topic for building a new power system.

[0004] In existing research, the new energy carrying capacity of the power grid is defined as the maximum new energy unit capacity that can be connected to the power grid under various constraint conditions and operation modes, and is also known as the penetration limit power, admissible capacity, maximum penetration rate, grid connection limit, etc. Based on static security constraints, documents such as "Calculation of Distributed Generation Admissible Capacity and Optimal Connection Location Considering Static Security Constraints" proposed a two-stage optimization solution method for evaluating the admissible capacity and optimal connection location of distributed generation. Documents such as "Analysis of Distributed Generation Admissible Capacity Considering Voltage Quality and Short-Circuit Capacity Constraints" analyzed the impact of different types of distributed generation and different connection capacities on the distribution network through actual distribution network examples, and gave the maximum connectable capacity boundaries under different output characteristic combinations of distributed wind turbines and photovoltaics. Another document, "Determining maximum allowable PV penetration level in transmission networks: case analysis - Northern Cyprus Power System", proposed an evaluation model for the maximum wind power connection capacity considering the static voltage stability of the system. The above-mentioned documents only considered the voltage security stability of the power grid when evaluating the new energy carrying capacity of the power grid, and did not consider the impact of new energy connection on the frequency stability of the power grid.

[0005] In response to this situation, some documents considered the frequency stability of the system when evaluating the new energy carrying capacity. Documents such as "A Method for Evaluating the Grid Connection Limit of New Energy Considering the Frequency Security of the Power System" proposed a method for evaluating the new energy carrying capacity considering frequency stability. By establishing a frequency security constraint model, the change of the maximum frequency deviation of the power grid with inertia and equivalent regulation constant was used as a rigid constraint to determine the carrying capacity. Documents such as "Calculation of Wind Power Penetration Power Limit Considering Frequency Constraint and Wind Turbine Frequency Modulation" considered the transient frequency stability of the system after new energy connection, and each had its own characteristics in terms of index design, model establishment, and solution method. The above-mentioned documents studied the evaluation of new energy carrying capacity considering the frequency stability of the power grid, but did not consider the voltage and frequency stability of the system simultaneously in the evaluation, nor did they consider the interaction between multi-infeed DC and new energy in the study of frequency stability.

[0006] In summary, in the assessment of the new energy carrying capacity of the power grid, the deficiencies of existing research are mainly reflected in two aspects: First, the assessment of the new energy carrying capacity is not comprehensive, and the voltage and frequency safety and stability of the power grid are not comprehensively considered during the assessment. However, as the proportion of new energy in the new power system continues to increase, new energy will have a greater impact on the voltage and frequency safety and stability of the power grid. Therefore, when assessing the new energy carrying capacity of the power grid, voltage and frequency safety and stability constraints need to be considered simultaneously. Second, existing research has not studied the system characteristics of a multi-DC-fed power grid after a large amount of new energy is connected, and the mutual coupling effect between multi-fed DC and new energy is not considered in the assessment of the new energy carrying capacity of the power grid. Therefore, the assessment results cannot be fully applied to the assessment of the new energy carrying capacity of a multi-DC-fed power grid, and further research on the assessment of the new energy carrying capacity of a multi-DC-fed power grid needs to be carried out. Summary of the Invention

[0007] To overcome the above defects, the present invention aims at the control mode and operation characteristics of new energy connected to a multi-DC-fed power grid, as well as the mutual coupling effect between DC and new energy, and comprehensively considers voltage and frequency safety and stability, and provides a method and device for assessing the new energy carrying capacity of a multi-DC-fed power grid, effectively assessing the new energy carrying capacity of a multi-DC-fed power grid, and providing a basis for determining the optimal new energy access scheme.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for assessing the new energy carrying capacity of a multi-DC-fed power grid, comprising:

[0010] S1. A evaluation index system for new energy connected to a multi-DC-fed power grid is established by aiming at the control mode and operation characteristics of new energy connected to a multi-DC-fed power grid, as well as the mutual coupling effect between DC and new energy, and comprehensively considering voltage and frequency safety and stability;

[0011] S2. Based on the evaluation index system established in step S1, with the maximum new energy access capacity as the goal and taking into account the power grid safety and stability constraints, an optimization model for assessing the new energy carrying capacity of a multi-DC-fed power grid is established;

[0012] S3. For different power grid safety and stability constraints, the optimization model for assessing the new energy carrying capacity of a multi-DC-fed power grid is linearized and solved to obtain the maximum new energy carrying capacity of the system.

[0013] Preferably, the evaluation index system in step S1 includes line current carrying capacity, node voltage amplitude, node voltage sensitivity factor, and transient frequency stability index;

[0014] The line current carrying capacity refers to the current carrying capacity of each line in the power grid, denoted as I i , i ∈ Ωl , where I i is the current of line i, Ω l is the set composed of all branches in the power grid; the node voltage amplitude is the voltage amplitude U of each node (bus) in the power grid i , i ∈ Ω N , Ω N is the set composed of all nodes

[0015] Preferably, the node voltage sensitivity factor formula is:

[0016]

[0017] ΔU and ΔQ are the node voltage change and the node reactive power change respectively;

[0018] According to the power flow equation in the form of branch power of the system:

[0019]

[0020] Write the node power imbalance equation:

[0021]

[0022] In the formula, P ij and Q ij are the active and reactive powers of line ij respectively; ΔP i and ΔQ i are the active change and the reactive power change of node i respectively, P i and Q i are the active and reactive powers injected by node i respectively; δ ij is the phase angle difference of line ij; g ij and b ij are the conductance and susceptance of line ij respectively; B i is the equivalent shunt susceptance at node i;

[0023] Furthermore, its incremental form can be deduced:

[0024]

[0025] Transform the above formula to get:

[0026]

[0027] In the formula, J is the system Jacobian matrix, J UQ is the node voltage reactive power sensitivity matrix, J UP is the node voltage active power sensitivity matrix, J δQis the node phase angle reactive power sensitivity matrix, ΔP is the column vector of node active power disturbances, Δδ is the column vector of node phase angle fluctuations, ΔU is the column vector of node voltage fluctuations, and ΔQ is the column vector of node reactive power disturbances;

[0028] Therefore, considering the change of node voltage caused by the reactive power disturbance injected into the node, the node voltage sensitivity factor matrix VSF can be expressed as:

[0029] VSF = ΔU(ΔQ) -1 = J UQ

[0030] When DC power is fed into the power grid and new energy is connected, under different control modes of DC and new energy, the matrix J needs to be corrected. For the most common constant current control on the rectifier side and constant extinction angle control on the inverter side of high-voltage DC, the expressions of the output active power and reactive power are respectively:

[0031]

[0032]

[0033] In the formula, P dc,i , Q dc,i are the active power and reactive power output by DC i respectively; I dc,i is the DC side current of DC i; m i is the number of 6-pulse converters of DC i; X r,i is the commutation reactance per phase of the inverter of DC i; U i is the AC voltage of the PCC node of DC i; γ i is the turn-off angle of the inverter of DC i; T i is the transformation ratio of the converter transformer of DC i;

[0034] Thus, the sensitivities of DC power to the voltage amplitude and phase angle of the commutation bus are shown in the following formula:

[0035]

[0036] In the formula, δ i is the voltage phase angle of node i;

[0037] New energy units often adopt maximum power point tracking control and set the power factor to 1. Under this control mode, the sensitivities of the output power of new energy units to the voltage amplitude and phase angle of the connected node are zero, that is:

[0038]

[0039] In the formula: P DG,i is the active power of the grid-connected node i of the new energy unit; Q DG,iThe reactive power of the new energy unit connected to grid node i; U is the amplitude of the node voltage; δ is the phase angle of the node voltage;

[0040] When there are DC and new energy in the power grid, calculate the partial derivatives of the active and reactive power outputs of the DC and new energy with respect to the node voltage amplitude and phase angle, and correct the corresponding elements in matrix J. Then calculate the node voltage sensitivity factor and use it as the voltage stability index for the multi-DC fed power grid.

[0041] Preferably, the transient frequency stability index includes the maximum frequency deviation, the maximum rate of change of frequency, and the steady-state frequency deviation, and its formula is:

[0042]

[0043] In the formula, is the maximum frequency deviation; is the maximum rate of change of frequency; is the steady-state frequency deviation, t os is the moment when the frequency deviation drops the most, H eq is the equivalent inertia time constant of the system, T Geq is the equivalent time constant of the speed regulation system of thermal power units, k Geq is the equivalent unit regulation power of the system, D is the active power frequency response coefficient of the load, is the active power disturbance of the system, α, β, λ, ω are the correlation coefficients of the time-domain expression; s is the complex frequency in the Laplace transform; Δf * (s) is the frequency deviation in the Laplace transform.

[0044] Preferably, the new energy carrying capacity evaluation and optimization model for the multi-DC fed power grid is:

[0045]

[0046] In the formula, C DG,i is the access capacity of new energy unit i, Ω DG is the set of nodes where new energy units are connected.

[0047] Preferably, the power grid security and stability constraints include system power flow constraints, thermal power unit output constraints, system reserve constraints, DC system constraints, line current-carrying capacity constraints, node voltage amplitude constraints, node voltage fluctuation constraints, and frequency security and stability constraints.

[0048] Preferably, the node voltage fluctuation constraint is: ΔU i ≤ΔU i,max

[0049] Among them, ΔU i =∑VSF ij ·ΔQj ,

[0050] where ΔU i is the voltage fluctuation of node i, ΔQ j is the reactive power disturbance of node i, VSF ij is the sensitivity of the reactive power injection change of node voltage i to node j, ΔU i,max is the upper limit of the voltage fluctuation of node i.

[0051] Preferably, the solution method of the new - energy carrying capacity evaluation model for the multi - HVDC - fed power grid is as follows:

[0052] 1) Select the operation scenario with the largest system load and determine the new - energy accessible nodes in the system;

[0053] 2) Solve the new - energy carrying capacity evaluation model for the multi - HVDC - fed power grid without voltage - fluctuation constraints to obtain the output powers of each thermal power unit, HVDC, and new - energy unit in the system, the access capacity of new energy, and the network - node voltage and phase angle;

[0054] 3) Calculate the node - voltage sensitivity factor based on the optimization result of step 2) to determine whether the voltage - fluctuation constraint is satisfied. If the verification result meets the requirements, the optimization result of step 2) is the maximum new - energy carrying capacity of the system. If the verification result does not meet the requirements, reduce the step size under this maximum new - energy access capacity and return to step 2) to solve again until the system voltage - fluctuation constraint is satisfied.

[0055] Preferably, the step - size reduction in step 3) is ΔC DG , and add the constraint:

[0056]

[0057] where C DG,i is the new - energy access capacity of node i; C' DG,i is the new - energy access capacity of node i obtained in step 2).

[0058] A new - energy carrying capacity evaluation device for a multi - HVDC - fed power grid includes:

[0059] A system - establishment module, which, aiming at the control mode and operation characteristics of new - energy access to a multi - HVDC - fed power grid, as well as the mutual coupling effect between HVDC and new energy, and comprehensively considering voltage and frequency security and stability, establishes an evaluation index system for new - energy access to a multi - HVDC - fed power grid;

[0060] A modeling module, which, with the goal of maximizing the new - energy access capacity and taking into account the grid security and stability constraints, establishes an optimization model for the new - energy carrying capacity evaluation of a multi - HVDC - fed power grid;

[0061] A calculation module linearizes and solves the new energy carrying capacity evaluation and optimization model for a multi - HVDC - fed power grid to obtain the maximum new energy carrying capacity of the system.

[0062] The positive and beneficial effects of the present invention:

[0063] 1. Aiming at the control mode and operation characteristics of new energy access to a multi - HVDC - fed power grid, as well as the mutual coupling effect between HVDC and new energy, the present invention proposes a new energy carrying capacity evaluation index system for the power grid that simultaneously considers voltage and frequency security and stability, comprehensively evaluates the voltage and frequency security and stability of the system, and the established evaluation index system is more comprehensive.

[0064] 2. The present invention establishes a new energy carrying capacity evaluation and optimization model for a multi - HVDC - fed power grid. The optimization goal of the energy carrying capacity evaluation model is to maximize the new energy access capacity, and a linearization transformation and solution method for the optimization model is proposed according to the model characteristics. Under the condition of meeting the system safety and stability constraint conditions, the new energy carrying capacity evaluation of the multi - HVDC - fed power grid is realized, and the maximum new energy carrying capacity of the system is obtained, providing a basis for determining the optimal new energy access scheme. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 is the frequency response model of the system of the present invention;

[0066] Figure 2 is the block diagram of the solution method of the new energy carrying capacity evaluation model for a multi - HVDC - fed power grid of the present invention;

[0067] Figure 3 is the topology diagram of the multi - HVDC - fed network of the present invention;

[0068] Figure 4 is the curve diagram of the system frequency change of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0069] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0070] A method for evaluating the new energy carrying capacity of a multi - HVDC - fed power grid includes:

[0071] S1. Considering the safety and stability of system voltage and frequency comprehensively, an evaluation index system for a multi - HVDC - fed power grid with new energy access is established, including: line current - carrying capacity, node voltage amplitude, node voltage sensitivity factor, transient frequency stability index, and the transient frequency stability index includes maximum frequency deviation, maximum frequency change rate, and steady - state frequency deviation; meanwhile, the impacts of HVDC and new energy on system safety and stability are considered, and the node voltage sensitivity factor is corrected according to the control mode and operation characteristics of the multi - HVDC - fed power grid with new energy access.

[0072] S2. Based on the established evaluation index system, with the goal of maximizing the new - energy access capacity and considering the grid safety and stability constraints, an evaluation and optimization model for the new - energy carrying capacity of a multi - HVDC - fed power grid is established.

[0073] S3. For different grid safety and stability constraints, the new - energy carrying capacity evaluation model is linearly transformed and solved to obtain the maximum new - energy carrying capacity of the system. The specific steps are as follows:

[0074] First, determine the typical operation mode of the system and the new - energy accessible nodes: select the operation scenario with the maximum system load, and considering the limitations of new - energy resource quantity and geographical location of each node, as well as the load concentration and power - flow evacuation ability of each node, determine the new - energy accessible nodes in the system.

[0075] Second, solve the new - energy carrying capacity evaluation model of the multi - HVDC - fed power grid without voltage - fluctuation constraints to obtain the output powers of each thermal power unit, HVDC, and new - energy unit in the system, the new - energy access capacity, and the network - node voltage and phase angle.

[0076] Finally, calculate the node voltage sensitivity factor based on the optimization results and check the voltage fluctuation to determine whether the voltage - fluctuation constraints are met. If satisfied, the optimization result is the maximum new - energy carrying capacity of the system; if not, reduce the step size at this maximum new - energy access capacity and solve again until the system voltage - fluctuation constraints are met.

[0077] In step S1, the evaluation index system for a multi - HVDC - fed power grid with new energy access covering voltage and frequency safety and stability:

[0078] (1) Line current - carrying capacity

[0079] The line current - carrying capacity refers to the current - carrying capacity of each line in the power grid, denoted as \(I_i\), \(i\in\Omega\) i , where \(I_i\) l is the current of line \(i\), and \(\Omega\) i is the set composed of all branches in the power grid. l

[0080] (2) Node voltage amplitude

[0081] The node voltage amplitude is the voltage amplitude U of node i (bus) in the power grid i , i ∈ Ω N , Ω N is the set composed of all nodes.

[0082] (3) Node voltage sensitivity factor

[0083] After the new energy is connected to the power grid, it not only changes the power flow distribution of the power grid, but also the volatility of its output will seriously affect the voltage stability of the power grid. In the transmission grid, due to the fact that the line reactance is much larger than the line resistance, the voltage of the system is mainly affected by the node reactive power. Usually, the node voltage sensitivity factor (voltage stability factor, VSF) is used to describe the degree of voltage fluctuation of the system, and its calculation formula is:

[0084]

[0085] In the formula, ΔU and ΔQ are the node voltage change and the node reactive power change respectively;

[0086] VSF represents the sensitivity of the node voltage to the change of the injected reactive power at the node. VSF>0 indicates that the system is statically voltage stable; the larger the VSF, the more sensitive the node voltage is to the reactive power disturbance, and the worse its voltage stability; while the smaller the VSF, the less sensitive the node voltage is to the reactive power disturbance, and the better its voltage stability;

[0087] According to the power flow equation in the form of branch power of the system:

[0088]

[0089] Write the node power imbalance equation:

[0090]

[0091] In the formula, P ij and Q ij are the active and reactive powers of line ij respectively; ΔP i and ΔQ i are the active change and reactive power change of node i respectively, P i and Q i are the active and reactive powers injected by node i respectively; δ ij is the phase angle difference of line ij; g ij and b ij are the conductance and susceptance of line ij respectively; B i is the equivalent shunt susceptance at node i, ΔP i is the active power imbalance of node i; ΔQ i is the reactive power imbalance of node i;

[0092] Furthermore, its incremental form can be derived as follows:

[0093]

[0094] By transforming the above equation, we get:

[0095]

[0096] In the formula, J is the system Jacobian matrix, J UQ is the reactive power sensitivity matrix of the node voltage, J UP is the active power sensitivity matrix of the node voltage, J δQ is the reactive power sensitivity matrix of the node phase angle, ΔP is the column vector of node active power disturbances, Δδ is the column vector of node phase angle fluctuations, ΔU is the column vector of node voltage fluctuations, and ΔQ is the column vector of node reactive power disturbances;

[0097] Therefore, considering the change of the node voltage due to the node injection reactive power disturbance, the node voltage sensitivity factor matrix VSF can be expressed as:

[0098] VSF = ΔU(ΔQ) -1 = J UQ (8)

[0099] The reactive power sensitivity matrix J UQ of the node voltage is used to describe the node voltage stability of the system;

[0100] When DC power is fed into the power grid and new energy is connected, under different control modes of DC and new energy, the matrix J needs to be corrected. For the most common rectifier side constant current control and inverter side constant extinction angle (C / E) control modes of high-voltage DC, the expressions of the output active and reactive powers are as follows:

[0101]

[0102] In the formula, P dc,i , Q dc,i are the active and reactive powers output by DC i respectively; I dc,i is the DC side current of DC i; m i is the number of 6-pulse converters of DC i; X r,i is the commutation reactance per phase of the DC i inverter; U i is the AC voltage amplitude of node i; γ i is the turn-off angle of the DC i inverter; T i is the transformation ratio of the DC i converter transformer.

[0103] From this, the sensitivities of the DC power to the voltage amplitude and phase angle of the commutation bus are shown in the following formula:

[0104]

[0105] In the formula, δ i is the voltage phase angle of node i;

[0106] New energy units often adopt maximum power point tracking control and set the power factor to 1. Under this control mode, the sensitivities of the output power of new energy units to the voltage amplitude and phase angle of the connected node are zero, that is:

[0107]

[0108] In the formula: P DG,i is the active power of the grid-connected node i of the new energy unit; Q DG,i is the reactive power of the grid-connected node i of the new energy unit; U is the voltage amplitude of the node; δ is the voltage phase angle of the node;

[0109] When there are direct currents and new energies in the power grid, it is necessary to calculate the partial derivatives of the active and reactive powers output by the direct currents and new energies with respect to the voltage amplitude and phase angle of the node according to formulas (11) and (12), and correct the corresponding elements in matrix J, and then substitute them into formula (8) to calculate the node voltage sensitivity factor, and use it as the voltage stability index of the multi-direct current fed power grid.

[0110] (4) Frequency stability index

[0111] As the penetration rate of new energy in the power grid gradually increases, the equivalent moment of inertia and frequency regulation ability of the system continuously decrease, resulting in the deterioration of the frequency stability of the system. At the same time, due to the fluctuations in the output of new energy units, the frequency stability of the system is also affected. In order to describe the influence of new energy on the frequency stability of the multi-direct current fed system, a system frequency response model is established, and three indicators, namely the maximum frequency deviation, the maximum frequency change rate, and the steady-state frequency deviation, are selected as the frequency stability indicators.

[0112] Usually, new energy adopts maximum power point tracking control and does not participate in the frequency regulation of the power grid. After the new energy is connected, the equivalent inertia time constant and equivalent unit regulation power of the system are shown in formulas (13)-(14).

[0113]

[0114] In the formula, H eq , k Geq are the equivalent inertia time constant and unit regulation power of the system respectively; C G,i , C dc,i , C DG,i are the capacities of thermal power unit i, DC i, and new energy unit i respectively; Ω G , Ω dc , Ω DGThey are the access node sets of thermal power units, DC, and new energy units respectively; H G,i , k G,i They are the inertia time constant and the unit regulation power of thermal power unit i respectively.

[0115] The frequency response model of the system is as Figure 1 shown, where D is the active power frequency response coefficient of the load, and T Geq is the equivalent time constant of the speed control system of the thermal power unit. When the power disturbance of the system is ΔP0, the expression of the system frequency in the complex frequency domain is:

[0116]

[0117] Taking the inverse Laplace transform of the above formula, we can get:

[0118]

[0119] In the formula: T Geq is the equivalent time constant of the speed control system of the thermal power unit, k Geq is the equivalent unit regulation power of the system, and α, β, λ, ω are the correlation coefficients of the time-domain expression. Furthermore, the maximum frequency deviation, the maximum frequency change rate, and the steady-state frequency deviation of the system can be deduced as shown in Eqs. (17)-(19):

[0120]

[0121] In the formula, is the maximum frequency deviation; is the maximum frequency change rate; is the steady-state frequency deviation; t os is the moment when the frequency deviation drops the most, is the active power disturbance amount of the system; s is the complex frequency in the Laplace transform; Δf * (s) is the frequency deviation in the Laplace transform.

[0122] In step S2, based on the above evaluation index system, an evaluation and optimization model for the new energy carrying capacity of a multi-DC fed power grid is established. The optimization variable of this model is the access capacity of new energy in the multi-DC fed power grid.

[0123] 1. Objective function

[0124] The optimization objective of the energy carrying capacity evaluation model is to maximize the access capacity of new energy, that is, the new energy carrying capacity of the power grid.

[0125]

[0126] In the formula, C DG,i is the access capacity of new energy unit i, and Ω DGis the set of nodes for new energy units to connect to.

[0127] 2. Constraints

[0128] (1) System power flow constraint

[0129] Linearize the power flow equation in the form of branch power for model optimization and solution. The linearized power flow equation is as shown in the formula:

[0130]

[0131] In the formula: U i is the voltage amplitude of node i, U j is the voltage amplitude of node j, P ij and Q ij are the active and reactive powers of line ij respectively, g ij and b ij are the conductance and susceptance of line ij respectively, δ ij is the phase angle difference of line ij.

[0132] (2) Thermal power unit output constraint

[0133] The output of each thermal power unit in the power grid needs to satisfy:

[0134]

[0135] In the formula, P G,i,min and P G,i,max are the minimum and maximum active powers of thermal power unit i respectively; Q G,i,min and Q G,i,max are the minimum and maximum reactive powers of thermal power unit i respectively, P G,i is the active power of thermal power unit i, Q G,i is the reactive power of thermal power unit i, Ω G is the set of nodes where thermal power units connect;

[0136] (3) System reserve constraint

[0137] Due to the fluctuations in new energy and load power in the system, a certain amount of reserve power needs to be reserved in the system to meet the power balance requirement:

[0138]

[0139] In the formula, and are the upper and lower reserve capacities of thermal power unit i respectively; ε L , ε DG are the volatility rates of load and new energy unit output respectively; P L is the total load power of the system; P DG,iActive power output of new energy unit i.

[0140] (4) DC system constraints

[0141] The active power transmitted by the DC system should satisfy:

[0142] P dc,i,min ≤P dc,i ≤P dc,i,max , i ∈ Ω dc (24)

[0143] Wherein, P dc,i is the active power of DC i, P dc,i,min and P dc,i,max are the lower and upper limits of the active power of DC i respectively, and Ω dc is the set of DC access nodes;

[0144] The DC voltage should satisfy:

[0145] U dc,i,min ≤U dc,i ≤U dc,i,max , i ∈ Ω dc (25)

[0146] Wherein, U dc,i is the DC voltage of DC i, U dc,i,min and U dc,i,max are the lower and upper limits of the DC voltage of DC i respectively, and Ω dc is the set of DC access nodes.

[0147] The reactive power compensation constraint of the DC system is:

[0148]

[0149] Wherein, Q c,i is the reactive power compensation equipment capacity of DC i; B c,i is the susceptance of the reactive power compensation equipment of DC i, and Ω dc is the set of DC access nodes.

[0150] (5) Line current-carrying capacity constraint

[0151] The current-carrying capacity of each line in the power grid should be less than the maximum current-carrying capacity of the line, that is:

[0152] |I i | ≤ I i,max , i ∈ Ω l (27)

[0153] Wherein, I i is the current of line i, and I i,max is the maximum current-carrying capacity of line i, and Ω lThe set composed of all branches in the power grid;

[0154] (6) Node voltage magnitude constraint

[0155] According to the relevant regulations of GB / T 12325-2008 "Power Quality - Supply Voltage Deviation", the magnitudes of the voltages at each node in the power grid should be within the allowable range, that is:

[0156] U i,min ≤U i ≤U i,max , i ∈ Ω N (28)

[0157] In the formula, U i is the voltage magnitude of node i, and Ω N is the set composed of all nodes. U i,min and U i,max are respectively the lower limit value and the upper limit value of the allowable voltage range of node i.

[0158] (7) Node voltage fluctuation constraint

[0159] According to the relevant regulations of GB / T 12326—2008 "Power Quality - Voltage Fluctuation and Flicker", when the voltage fluctuation in the power grid is large, it will seriously affect the voltage quality and endanger the normal operation of each component in the system. The voltage fluctuation at each node in the power grid should be within the allowable range, that is:

[0160] ΔU i ≤ΔU i,max (29)

[0161] In the formula, ΔU i,max is the upper limit value of the voltage fluctuation of node i, and ΔU i is the voltage fluctuation amount of node i;

[0162] According to the definition of the node voltage sensitivity factor, the voltage fluctuation of each node in the system is:

[0163] ΔU i =∑VSF ij ·ΔQ j (30)

[0164] In the formula, ΔQ j is the reactive power disturbance amount of node i, and VSFi j is the sensitivity of the voltage of node i to the change in the reactive power injected by node j.

[0165] (8) Frequency security and stability constraint

[0166] According to the relevant regulations of GB / T 15945—2008 "Power Quality - Frequency Deviation of Power System", when there are power disturbances in new energy and loads in the power grid, the maximum frequency deviation, maximum frequency change rate, and steady-state frequency deviation of the system should be within the constraint range, that is:

[0167]

[0168] In the formula, Δf * is the maximum frequency deviation, is the maximum frequency change rate, is the steady-state frequency deviation, [Δf * max , and are the maximum allowable frequency deviation, maximum frequency change rate, and maximum steady-state frequency deviation allowed by the system respectively.

[0169] In step S3, the solution of the new energy carrying capacity evaluation model for a multi-HVDC fed power grid:

[0170] The new energy carrying capacity evaluation model for a multi-HVDC fed power grid established above is a non-linear programming model. Among them, the DC power constraint equations are equations (24)-(26), and the voltage fluctuation constraint equations (4)-(8), equations (29)-(30) are non-linear constraints, which are difficult to directly solve by analytical methods. Therefore, the present invention simplifies the above model, linearly processes the DC power constraint equations (24)-(26) by the piecewise linear method, and proposes a solution method for the new energy carrying capacity evaluation model for a multi-HVDC fed power grid according to the specific characteristics of the voltage fluctuation constraint equations (4)-(8) and equations (29)-(30). As Figure 2 shown, its main process is as follows:

[0171] 1) Determine the typical operation mode of the system and the new energy accessible nodes: To obtain the maximum new energy carrying capacity of the system, select the operation scenario with the maximum system load. At the same time, without considering the change of the system topology, considering the limitations of the new energy resource quantity and geographical location of each node, as well as the load concentration degree and power flow evacuation ability of each node, determine the new energy accessible nodes in the system;

[0172] 2) Solve the new energy carrying capacity evaluation model for a multi-HVDC fed power grid without voltage fluctuation constraints to obtain the output of each thermal power unit, DC and new energy unit in the system, the access capacity of new energy, as well as the network node voltage and phase angle;

[0173] ​3) Calculate the node voltage sensitivity factors in formulas (4)-(12) based on the optimization results in step 2), and verify the voltage fluctuation formula (30). If the verification result meets the requirements, the optimization result in step 2) is the maximum new energy carrying capacity of the system; if the verification result does not meet the requirements, reduce the step size ΔC at this maximum new energy access capacity DG , that is, add constraint (32), and return to step 2) to solve again until the system voltage fluctuation constraint is met.

[0174]

[0175] In the formula: C DG,i is the new energy access capacity of node i; C' DG,i is the new energy access capacity of node i obtained in step 2); ΔC DG is the step size;

[0176] A new energy carrying capacity evaluation device for a multi-HVDC fed power grid, comprising:

[0177] A system establishment module, aiming at the control mode and operation characteristics of new energy access to a multi-HVDC fed power grid, as well as the mutual coupling effect between HVDC and new energy, comprehensively considering voltage and frequency safety and stability, and establishing an evaluation index system for new energy access to a multi-HVDC fed power grid;

[0178] A modeling module, aiming at the maximum new energy access capacity, taking into account the grid security and stability constraints, and establishing an optimization model for evaluating the new energy carrying capacity of a multi-HVDC fed power grid;

[0179] A calculation module, linearizes and solves the optimization model for evaluating the new energy carrying capacity of a multi-HVDC fed power grid, and obtains the maximum new energy carrying capacity of the system.

[0180] To verify the effectiveness of the proposed method for evaluating the new energy carrying capacity of a multi-HVDC fed power grid, the present invention is based on the IEEE 10-machine 39-bus system, feeds two DC lines to form a multi-HVDC fed network, and node 31 is the balancing node, and its topology is as Figure 3As shown in the figure, the system base capacity is 100MW. Two HVDCs are respectively fed into Node 4 and Node 16, with a rated power of 800MW. The upper and lower limits of the DC voltage are 1.5pu and 0.9pu respectively, the commutation reactance is 0.005pu, the extinction angle is 18.22°, the compensating susceptance is 3pu, the turn ratio of the converter transformer is 1pu, and the control mode of constant current at the rectifier side and constant extinction angle at the inverter side is adopted. The equivalent inertia time constant of the thermal power unit is 4.8s, the equivalent time constant of the speed control system is 0.5s, the equivalent unit regulation power of the thermal power unit is 20pu, the load regulation coefficient is 1.5pu, the maximum allowable frequency deviation of the system is 0.5Hz, the maximum frequency change rate is 0.5Hz / s, and the maximum steady-state frequency deviation is 0.2Hz. The upper and lower limits of the node voltage are 1.1pu and 0.9pu respectively, and the upper limit of the voltage fluctuation is 2.5%. The load disturbance amount is 5%, and the new energy power disturbance amount is 10%. After screening, the new energy can be connected to Nodes 1, 3, 4, 5, 9, 14, 16, 17, and 26. The simulation environment of the present invention is Matlab2018a, Yalmip is called for solving, and the solver used is Gurob9.1.2.

[0181] For the improved IEEE-39 node example network, an evaluation model for the new energy carrying capacity is established, and the Figure 2 method shown in the figure is used to solve the model. The maximum new energy carrying capacity of the system obtained by solving is 3248.15MW. The new energy power connected to each node is shown in Table 1, the output of each thermal power unit in the system is shown in Table 2, and the power of the HVDCs in the system is shown in Table 3.

[0182] Table 1 New energy capacity connected to the system

[0183]

[0184] Table 2 Output of thermal power units

[0185]

[0186] Table 3 HVDC power

[0187]

[0188] As can be seen from the above table, the new energy connection capacity of Nodes 4, 5, and 14 is relatively small, while that of Nodes 1, 9, 16, and 26 is relatively large. The reason is that Nodes 4, 5, and 14 are relatively close to the HVDC connection nodes, and the HVDC output power is large. To avoid violating the line current carrying capacity and voltage constraints due to large transmission power, Nodes 1, 9, 16, and 26 are far from the HVDC connection nodes and generator nodes. At the same time, the load of the nearby nodes is large, so that the power is balanced nearby, reducing the power transmission.

[0189] When the access capacity of new energy in the system increases, the output of thermal power units will decrease accordingly, thereby reducing the operating cost of thermal power units and improving the economic efficiency of system operation. However, the system disturbances brought by new energy also increase accordingly, which in turn affects the voltage and frequency stability of the system. Under this new energy access capacity and access scheme, the node voltage fluctuations of the system are shown in Table 4, all within the allowable range of system voltage fluctuations. At the same time, the voltages of all nodes are within the operating range, meeting the requirements of grid voltage safety and stability.

[0190] Table 4 Node Voltage Fluctuations

[0191]

[0192]

[0193] Under this access scheme, under the disturbances of load and new energy power, the maximum frequency deviation of the system is 0.26 Hz, the maximum frequency change rate is 0.44 Hz / s, and the steady-state frequency deviation is 0.2 Hz, all within the allowable range of system frequency fluctuations, meeting the requirements of grid frequency safety and stability. The frequency change curve is as Figure 4 shown.

[0194] In summary, the new energy carrying capacity evaluation method for multi-DC fed power grids proposed by the present invention comprehensively considers the voltage and frequency safety and stability of multi-DC fed power grids, as well as the operating characteristics of DC and new energy, and realizes the effective evaluation of the system's new energy carrying capacity.

[0195] It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for evaluating the new energy carrying capacity of a multi - DC - fed power grid, characterized in that, Including: S1. Aiming at the control mode and operation characteristics of a multi - DC - fed power grid with new energy access, as well as the mutual coupling effect between DC and new energy, comprehensively considering the voltage and frequency safety and stability, an evaluation index system for a multi - DC - fed power grid with new energy access is established; S2. Based on the evaluation index system established in step S1, with the goal of maximizing the new energy access capacity and taking into account the power grid safety and stability constraints, an evaluation and optimization model for the new energy carrying capacity of a multi - DC - fed power grid is established; S3. For different power grid safety and stability constraints, linearize and solve the evaluation and optimization model for the new energy carrying capacity of a multi - DC - fed power grid to obtain the maximum new energy carrying capacity of the system; The evaluation index system described in step S1 includes line current - carrying capacity, node voltage amplitude, node voltage sensitivity factor, and transient frequency stability index; The line carrying capacity refers to the carrying capacity of each line in the power grid, denoted as I i , i ∈ Ω l , where I i is the current of line i, and Ω l is the set composed of all branches in the power grid; The node voltage magnitude is the voltage magnitude U of each node in the power grid i , i ∈ Ω N , Ω N is the set composed of all nodes; The power grid safety and stability constraints include system power flow constraints, thermal power unit output constraints, system reserve constraints, DC system constraints, line current - carrying capacity constraints, node voltage amplitude constraints, node voltage fluctuation constraints, and frequency safety and stability constraints; The solution method for the evaluation model of the new energy carrying capacity of a multi - DC - fed power grid is as follows: 1) Select the operation scenario with the maximum system load and determine the new energy - accessible nodes in the system; 2) Solve the evaluation model of the new energy carrying capacity of a multi - DC - fed power grid excluding the voltage fluctuation constraint to obtain the output powers of each thermal power unit, DC, and new energy unit in the system, the new energy access capacity, and the network node voltage and phase angle; 3) Calculate the node voltage sensitivity factor based on the optimization result in step 2) to determine whether the voltage fluctuation constraint is satisfied. If the verification result meets the requirements, the optimization result in step 2) is the maximum new energy carrying capacity of the system. If the verification result does not meet the requirements, reduce the step size under this maximum new energy access capacity and return to step 2) to solve again until the system voltage fluctuation constraint is satisfied; The step size reduction described in step 3) is ΔC DG , add the constraint: Where C DG,i is the new energy access capacity of node i; C' DG,i is the new energy access capacity of node i obtained in step 2). The transient frequency stability index includes maximum frequency deviation, maximum frequency change rate, and steady - state frequency deviation, and its formula is: Wherein, is the maximum frequency deviation; is the maximum rate of change of frequency; is the steady-state frequency deviation, t os is the moment when the frequency deviation drops the most, H eq is the equivalent inertia time constant of the system, T Geq is the equivalent time constant of the speed control system of thermal power units, k Geq is the equivalent unit regulation power of the system, D is the active power frequency response coefficient of the load, is the disturbance of the active power of the system, α, β, λ, ω are the correlation coefficients of the time-domain expression; s is the complex frequency in the Laplace transform; Δf * (s) is the frequency deviation in the Laplace transform.

2. The new energy carrying capacity evaluation method for a multi - DC - fed power grid according to claim 1, wherein, The matrix VSF of the node voltage sensitivity factor can be expressed as: VSF = ΔU(ΔQ) -1 = J UQ Among them, where J is the system Jacobian matrix, J UQ is the nodal voltage reactive power sensitivity matrix, J UP is the nodal voltage active power sensitivity matrix, J δQ is the nodal phase angle reactive power sensitivity matrix, ΔP is the nodal active power perturbation column vector, Δδ is the nodal phase angle fluctuation column vector, ΔU is the nodal voltage fluctuation column vector, and ΔQ is the nodal reactive power perturbation column vector; The sensitivities of DC power to the voltage amplitude and phase angle of the converter bus are shown as follows: where P dc,i and Q dc,i are the active and reactive power output by DC i respectively; I dc,i is the DC side current of DC i; m i is the number of 6-pulse converters of DC i; X r,i is the commutation reactance per phase of the inverter of DC i; U i is the AC voltage at the PCC node of DC i; γ i is the turn-off angle of the inverter of DC i; T i is the turns ratio of the commutation transformer of DC i; δ i is the voltage phase angle of node i. New energy units often adopt maximum power point tracking control and set the power factor to 1. The sensitivities of the output power of new energy units to the voltage amplitude and phase angle of the access node are zero, that is: Where: P DG,i is the active power of the grid-connected node i of the new energy unit; Q DG,i is the reactive power of the grid-connected node i of the new energy unit; U is the node voltage amplitude; δ is the node voltage phase angle; When there are DC and new energy in the power grid, calculate the partial derivatives of the active and reactive powers output by DC and new energy with respect to the voltage amplitude and phase angle of the node, and correct the corresponding elements in the matrix J, and then calculate the node voltage sensitivity factor.

3. The new energy carrying capacity evaluation method for a multi - DC feeding power grid according to claim 1, characterized in that The evaluation and optimization model for the new energy carrying capacity of a multi - DC - fed power grid is: Where C DG,i is the access capacity of the new energy unit i, and Ω DG is the set of nodes where the new energy units are connected.

4. The new energy carrying capacity evaluation method for a multi - DC feeding power grid according to claim 1, wherein The node voltage fluctuation constraint is: ΔU i ≤ΔU i,max where, ΔU i = ∑VSF ij ·ΔQ j , where, ΔU i is the voltage fluctuation of node i, ΔQ j is the reactive power disturbance of node i, VSF ij is the sensitivity of the voltage of node i to the change in the reactive power injected by node j, ΔU i,max is the upper limit of the voltage fluctuation of node i.

5. An evaluation device for the new energy carrying capacity of a multi - DC - fed power grid, which is used to implement the evaluation method described in any one of claims 1 - 4, includes: A system establishment module, aiming at the control mode and operation characteristics of a multi - DC - fed power grid with new energy access, as well as the mutual coupling effect between DC and new energy, comprehensively considering the voltage and frequency safety and stability, establishes an evaluation index system for a multi - DC - fed power grid with new energy access; A modeling module, aiming at maximizing the new energy access capacity and considering the grid security and stability constraints, establishes an evaluation and optimization model for the new energy carrying capacity of a multi - HVDC - fed power grid; A calculation module linearizes and solves the evaluation and optimization model for the new energy carrying capacity of a multi - HVDC - fed power grid to obtain the maximum new energy carrying capacity of the system.

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