CN111563691A - Performance evaluation method for AC/DC hybrid power distribution network accessed with new energy - Google Patents

Abstract

The invention discloses a performance evaluation method for an AC/DC hybrid distribution network connected to new energy, comprising the following steps: connecting new energy based on a radial distribution network, a double-ended distribution network and a ring distribution network; establishing an economical A comprehensive evaluation index system based on four dimensions of reliability, reliability, power quality and new energy consumption capacity, and a comprehensive evaluation index calculation model is constructed. The comprehensive evaluation index calculation model includes an economic index calculation model, a reliability index calculation model, an electric energy A quality index calculation model and a new energy absorption capacity index calculation model; based on the uncertainty of new energy, scene clustering is performed, and the comprehensive evaluation index model is solved; based on the solution results, the AC-DC hybrid distribution network is planned. The comprehensive evaluation index system constructed by the invention takes into account the diversity of loads and the volatility of new energy sources, and provides a practical solution for the planning of future AC and DC distribution networks.

Description

A performance evaluation method for AC/DC hybrid distribution network connected to new energy

technical field

The invention relates to the technical field of AC/DC hybrid distribution network, in particular to a performance evaluation method for AC/DC hybrid distribution network connected to new energy.

Background technique

The traditional AC power distribution system faces a series of problems such as high line loss, power quality disturbance, and voltage sag, and it is difficult to meet the increasing power demand of power users. Therefore, it is the development trend of the future distribution network to build an AC-DC hybrid distribution network on the basis of the AC distribution network.

At present, domestic and foreign researches have been done to different degrees on the establishment of distribution network evaluation indicators. The establishment of evaluation indicators is diverse, involving DC distribution network, traditional AC distribution network and AC-DC hybrid distribution network. However, there is little research on the comprehensive evaluation index system of AC and DC distribution networks, and the index system is relatively simple; considering that my country is vast, the supply and demand of distribution networks in different regions are very different, the access conditions of new energy are different, and the load demand is also different in different places. In this context, how to plan the distribution network after considering the actual situation of different regions and considering the access of new energy has become an urgent problem to be solved in the future planning of the distribution network; however, the current evaluation indicators are relatively single, and the comprehensive indicators are not considered in the new energy Due to the fluctuation and uncertainty after connection, there is a problem that practical solutions cannot be proposed for the planning of AC and DC distribution networks when new energy is connected.

SUMMARY OF THE INVENTION

In view of the above problems, the comprehensive evaluation index system proposed by the present invention takes into account the diversity of loads and the volatility of new energy sources, and provides a practical solution for the planning of future AC and DC distribution networks.

The technical scheme adopted in the present invention is: a method for evaluating the performance of an AC/DC hybrid distribution network connected to new energy, comprising the following steps:

S1: Access to new energy based on radial distribution network, double-ended distribution network and ring distribution network respectively;

S2: Establish a comprehensive evaluation index system with four dimensions of economy, reliability, power quality and new energy consumption capacity, and build a comprehensive evaluation index calculation model, where the comprehensive evaluation index calculation model includes an economic index calculation model, a reliability index Calculation model, power quality index calculation model and new energy absorption capacity index calculation model;

S3: Perform scene clustering based on the uncertainty of new energy, and solve the comprehensive evaluation index model;

S4: Plan the AC/DC hybrid power distribution network based on the solution result.

Preferably, constructing an economic index calculation model in the step S2 includes constructing an investment economic calculation model and an operation economic calculation model; the operation economy includes converter loss rate and comprehensive line loss rate;

1) The investment economy is obtained by the following formula:

C _total =C _I +C _O +C _S (1)

其中N_k为某种设备对应台数，P_k为某种设备对应的单价，K为设备的种类数；运维成本为

其中C_e、C_l分别为设备和线路的运 行检修成本，C_O1为第一年运维成本，m为折现率，N为年份，i为年份；报废处置成本

其中C_{S_k}为某个设备用至报废的处置成本，K为设备数目；In the formula, the investment cost

Among them, N _k is the number of units corresponding to a certain equipment, P _k is the unit price corresponding to a certain equipment, and K is the number of types of equipment; the operation and maintenance cost is

Among them, C _e and C _l are the operation and maintenance costs of equipment and lines respectively, C _O1 is the operation and maintenance cost of the first year, m is the discount rate, N is the year, and i is the year;

Among them, C _{S_k} is the disposal cost of a certain equipment until it is scrapped, and K is the number of equipment;

2) The loss rate of the converter is obtained by the following formula:

In the formula, r _con is the loss rate of the converter; ΔP _{i_VSC} is the loss of the ith converter, which can be calculated from the optimal power flow; n is the number of converters;

3) The comprehensive line loss rate is obtained by the following formula:

In the formula, r _avg is the comprehensive line loss rate; ΔP _i is the power loss value of a certain line i, which can be calculated from the optimal power flow; l is the number of lines; β _m is the probability of the scene obtained by clustering; M is the number of scenes.

Preferably, building a reliability index calculation model in the step S2 includes building a system average power outage frequency calculation model, a system average power outage duration calculation model and an average power supply availability rate calculation model;

1) The average power outage frequency of the system is obtained by the following formula:

In the formula, SAIFI is the average power outage frequency of the system, the unit is times/(user·year); λ _i is the outage probability of load point i; N _i is the number of users of load point i;

2) The average power outage duration of the system is obtained by the following formula:

In the formula, SAIDI is the average power outage duration index of the system, the unit is hour/(user·year); U _i is the annual average power outage time of load point _i ; Ni is the number of users of load point i;

3) The average power supply availability is obtained by the following formula:

In the formula, ASAI is the average power supply availability index; D is the number of hours in a year, which is 8760 hours in the calculation; U _i is the annual average power outage time at load point _i ; Ni is the number of users at load point i.

Preferably, constructing the power quality index calculation model in the step S2 includes constructing a comprehensive voltage deviation calculation model and a voltage harmonic distortion rate calculation model;

1) The comprehensive voltage deviation is obtained by the following formula:

In the formula, f is the comprehensive voltage deviation; △U _m is the voltage deviation of a certain scene; M is the number of scenes; M _l,i is the weight of node i in feeder l; β _m is the probability of the scene obtained by clustering;

2) The voltage harmonic distortion rate is obtained by the following formula:

In the formula, THD is the voltage harmonic distortion rate; U ₁ is the fundamental voltage; U _j is the j-th harmonic corresponding to the voltage; J _max is the highest harmonic detected by each feeder in the distribution network; N is The number of power electronic periods; n is the number of nodes; j is the harmonic order.

Preferably, in the step S2, a new energy consumption capacity index calculation model is constructed, specifically:

In the formula, η is the new energy consumption rate; _PL is the active load, and _PNE is the power generation of the new energy.

Preferably, the average distance from each cluster domain sample to the cluster center in the scene clustering of step S3 is obtained by the following formula:

为平均聚类距离；N_c为聚类中心个数；X为S_j类簇中元素；Z_j为其聚类中心；N_c为聚类类数；S_j为某一类族；j为聚类类数的变量。In the formula,

is the average clustering distance; N _c is the number of cluster centers; X is the element in the S _j cluster; Z _j is the cluster center; N _c is the number of clusters; S _j is a certain family; j is the A variable for the number of cluster classes.

Preferably, the reliability calculation in the step S3 adopts a sequential Monte Carlo method.

Preferably, the calculation of the new energy consumption capacity in the step S3 is specifically the calculation of the power flow of the AC/DC hybrid power distribution network.

Preferably, the operation economic calculation in the step S3 is specifically the power flow calculation of the AC/DC hybrid distribution network.

Preferably, the power flow calculation in the step S3 includes the following sub-steps:

S3-1: Build a converter loss model;

S3-2: Solve by alternate iteration method.

The beneficial effects of the above technical solutions:

(1) The present invention performs comprehensive index evaluation on the AC/DC hybrid distribution network connected to new energy.

(2) The invention overcomes the unscientific problems caused by too many indicators, overlapping and overlapping, and constructs a comprehensive indicator system according to the principles of complete coverage, representative classification indicators, and non-repetitive reflection factors.

(3) The comprehensive evaluation index system disclosed in the present invention takes into account the diversity of loads and the volatility of new energy sources, and provides a practical solution for the planning of future AC and DC distribution networks.

(4) The comprehensive index system disclosed in the present invention improves the accuracy of the index calculation result.

Description of drawings

FIG. 1 is a flowchart of a method for evaluating the performance of an AC/DC hybrid distribution network connected to new energy according to the present invention;

Fig. 2 is the schematic diagram that the new energy source of the present invention is connected to the radial distribution network;

FIG. 3 is a schematic diagram of the new energy access to a double-ended distribution network according to the present invention;

FIG. 4 is a schematic diagram of the access of new energy sources to a ring-shaped distribution network according to the present invention;

Fig. 5 is the framework diagram of the comprehensive evaluation index of the present invention;

FIG. 6 shows 8 typical scenarios and their probability maps obtained by using the ISODATA clustering algorithm in the present invention.

Detailed ways

The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The detailed description of the following embodiments and the accompanying drawings are used to illustrate the principle of the present invention by way of example, but not to limit the scope of the present invention, that is, the present invention is not limited to the described preferred embodiments, and the scope of the present invention is defined by the claims. limited.

Many specific details are set forth in the following description to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein, and those skilled in the art can do so without departing from the connotation of the present invention. Similar promotion, therefore, the present invention is not limited by the specific embodiments disclosed below.

Second, reference herein to "one embodiment" or "an embodiment" refers to a particular feature, structure, or characteristic that may be included in at least one implementation of the present invention. The appearances of "in one embodiment" in various places in this specification are not all referring to the same embodiment, nor are they separate or selectively mutually exclusive from other embodiments.

Example 1

As shown in Figure 1, the present invention provides a method for evaluating the performance of an AC/DC hybrid distribution network connected to new energy, including the following steps:

S1: Access to new energy based on radial distribution network, double-ended distribution network and ring distribution network.

The invention first analyzes the power supply structure of the existing AC and DC power distribution network, and selects a typical power supply structure of the AC and DC power distribution network suitable for absorbing new energy. The AC/DC hybrid distribution network is based on the original AC distribution network, and connects the AC bus and the DC bus through the AC/DC converter, so that the distribution network can supply power to the AC load and the DC load at the same time; distributed photovoltaic, Fans and energy storage are scattered and connected on the low-voltage user side, and the DC side voltage is 750V or 375V, which not only realizes local consumption and reduces network loss, but also saves the investment of AC/DC converters on the AC side; however, the DC equipment (circuit breaker) The increase of inverters, converters) leads to increased economic investment, increased failure rate and serious harmonic pollution. Based on this, the present invention proposes three typical AC and DC power supply structures.

(1) Radial distribution network

As shown in Figure 2, the radial distribution network leads radial DC power supply lines for the A-type AC distribution network; according to the construction requirements of the AC and DC distribution network, the AC and DC radial power supply mode is based on the radial AC distribution network, adding 1 AC/DC converter and 2 DC transformers and 3 circuit breakers are installed.

Equipment such as new energy and energy storage are connected to the low-voltage AC or DC bus respectively, and the radial AC and DC distribution network is converted from a single power supply mode to multi-power supply. When the AC power supply or load side fails, the downstream load can be transferred to the new energy source. Corresponding lines, thereby improving reliability. During normal operation, the complementary operation strategy of wind, solar and storage can be adopted, which reduces the impact of new energy on the system state and improves the local consumption capacity. The power supply structure is suitable for supplying power to a residential area or a residence.

(2) Double-ended distribution network

As shown in Figure 3, the double-ended distribution network is a B-type hand-in-hand AC line connected by a DC bus. The hand-in-hand AC distribution network itself has two power sources, which can perform load transfer after a fault occurs. The AC/DC converter, 2 DC transformers, and 6 DC circuit breakers form a B-type power supply mode.

After the new energy is connected to the distribution network, the power supply mode has three power sources, and some new energy access sides are upgraded from meeting the N-1 criterion to meeting the N-2 criterion. It can be transferred to the backup power supply, and can also be transferred to the power supply connected to the new energy source, which improves the transfer capacity of the load side, thereby improving the reliability of the power supply. The B-type power supply mode adopts a voltage margin control strategy. This method is improved on the basis of master-slave control. It does not require communication and the inverters have priority. One inverter adopts constant voltage control, and the other inverters adopt constant power. Control, if the power fluctuates due to a fault, only the main converter will provide the power difference. If it still cannot be satisfied, it will switch to the next-level converter according to the priority until the power demand is met; when the new energy is connected to the grid, the power When it fluctuates, it can quickly suppress the power fluctuation and effectively improve the new energy consumption. However, compared with the A-type, after the new energy is connected, the investment increases and more complex protection devices are required. The power supply structure is suitable for industrial parks and other reliability requirements. higher places.

(3) Ring distribution network

As shown in Figure 4, the ring distribution network is a C-type hand-in-hand AC line connected by a DC ring network, and the AC distribution network supplies power for two or more power sources, generally "closed-loop design, open-loop operation" On this basis, a C-type AC and DC distribution network is formed by adding 2 AC/DC converters, 4 DC transformers and 14 DC circuit breakers, and a ring power supply structure is formed on the DC side.

After connecting to new energy and DC loads, the DC ring network part can operate in a closed loop. If a line on the DC side fails, both upstream and downstream loads can be transferred to further improve the reliability of the distribution network; The control strategy adopted by the C-type power supply mode is the same as that of the B-type, and the DC load area is concentrated, which greatly promotes photovoltaic consumption. However, the increased equipment investment of the ring distribution network and the complexity of the distribution network are much greater than those of the A-type and B-type two types. In the power supply mode, the corresponding protection is more complex, and the reliability and economy are mutually restricted, which requires further analysis. This power supply structure is suitable for areas with high requirements for power supply reliability and concentrated DC loads.

S2: Establish a comprehensive evaluation index system.

At this stage, the most urgent problem to be solved in the distribution network is the consumption of new energy. According to the establishment principle of the evaluation index system, the establishment of the evaluation index system should have a certain purpose, logic and hierarchy; as shown in Figure 5, the present invention In the establishment of the comprehensive evaluation index system, four dimensions of economic index, reliability index, power quality index and new energy consumption capacity are mainly considered; at the same time, three levels of index system are constructed according to the comprehensive index, classification index and sub-class index; Economic indicators include investment economy and operation economy, and operation economy includes converter loss rate and comprehensive line loss rate; reliability indicators include system average power outage frequency, system average outage duration and average power supply availability; power quality indicators Including comprehensive voltage deviation and voltage harmonic distortion rate; new energy consumption capacity includes new energy consumption rate. In order to focus on reflecting the differences of candidate objects, the present invention overcomes the unscientific problems caused by too many indicators, overlapping and overlapping, and the present invention constructs a comprehensive indicator system according to the principles of complete coverage, representative classification indicators and non-repetitive reflection factors.

(1) Economical

Economy is divided into two categories, namely investment economy and operation economy. The number of AC and DC devices in different power supply modes is different, which not only affects the investment cost, but also increases the operating cost due to the loss of the converter; the operating economy includes the loss rate of the converter and the comprehensive line loss rate.

1) Investment economy

Considering the cost of the whole life cycle, the cost model of the AC and DC distribution network is established. The total cost C _total includes: investment, operation and maintenance and maintenance, and residual value. Among them, the investment cost _CI includes the cost of all equipment (AC and DC lines, converters, circuit breakers, DC transformers, tie switches, etc.) and the cost of land occupation. The annual operation and maintenance cost of the equipment is _CO ; The end-of-life disposal cost is C _S .

C _total =C _I +C _O +C _S (1)

其中N_k为某种设备对应台数，P_k为某种设备对应的单价，K为设备的种类数；运维成本为

其中C_e、C_l分别为设备和线路的运 行检修成本，C_O1为第一年运维成本，m为折现率，N为年份，i为年份；报废处置成本

其中C_{S_k}为某个设备用至报废的处置成本，K为设备数目。In the formula, the investment cost

Among them, N _k is the number of units corresponding to a certain equipment, P _k is the unit price corresponding to a certain equipment, and K is the number of types of equipment; the operation and maintenance cost is

Among them, C _e and C _l are the operation and maintenance costs of equipment and lines respectively, C _O1 is the operation and maintenance cost of the first year, m is the discount rate, N is the year, and i is the year;

Where C _{S_k} is the disposal cost of a certain equipment until it is scrapped, and K is the number of equipment.

2) Operational economy

①Converter loss rate

Assuming that there are n converters in the distribution network, the average loss rate of the n converters is calculated as the converter loss rate of the distribution network. The converter loss rate is obtained by the following formula:

In the formula, r _con is the loss rate of the converter; ΔP _{i_VSC} is the loss of the ith converter, which can be calculated from the optimal power flow; n is the number of converters.

②Comprehensive line loss rate

Assuming that the number of trunk lines in the distribution network is l, the average value of the line loss rate of the l lines is calculated as the comprehensive line loss rate; the comprehensive line loss rate is obtained by the following formula:

In the formula, r _avg is the comprehensive line loss rate; ΔP _i is the power loss value of a certain line i, which can be calculated from the optimal power flow; l is the number of lines; β _m is the probability of the scene obtained by clustering; M is the number of scenes.

(2) Reliability

The reliability of AC/DC hybrid distribution network refers to the ability to continuously supply power to DC and AC users. The failure probability of DC equipment itself increases the number of distribution network failures, and multi-terminal power supply can improve reliability through load transfer. Power supply reliability is Comprehensive reflection of the number, type, network structure, load rate and load distribution of equipment.

1) System average interruption frequency index (SAIFI)

The system average outage frequency index (SAIFI) refers to the average number of outages per unit time for each user powered by the system, which is obtained by the following formula:

In the formula, SAIFI is the average power outage frequency of the system, the unit is times/(user·year); λ _i is the outage probability of load point i; N _i is the number of users of load point i.

2) System average interruption duration index (SAIDI)

The System Average Outage Duration Indicator (SAIDI) refers to the average outage duration experienced by each user powered by the system in a year, and is obtained by the following formula:

In the formula, SAIDI is the average power outage duration index of the system, the unit is hour/(user·year); U _i is the annual average power outage time of load point _i ; Ni is the number of users of load point i.

3) Average power supply availability (Average service availability index, ASAI)

The Average Supply Availability Index (ASAI) refers to the ratio of the total number of outage hours experienced by a user to the total number of hours of supply required by the user in a year, and is obtained by the following formula:

In the formula, ASAI is the average power supply availability index; D is the number of hours in a year, which is 8760 hours in the calculation; U _i is the annual average power outage time at load point _i ; Ni is the number of users at load point i.

(3) Power quality

Since the discussion is about the AC/DC hybrid distribution network, the comprehensive indicators of DC and AC should be integrated, so frequency-related indicators are not considered, and the main indicators are voltage-related indicators; in addition, the access of DC loads and new energy will increase power electronic devices. , the power electronic devices will add harmonic sources to the distribution network, so the second-level indicators corresponding to the power quality are the comprehensive voltage deviation and the voltage harmonic distortion rate.

1) Comprehensive voltage deviation

Considering the uncertainty of new energy and load access, it will cause line voltage fluctuations. Different power supply modes have different risks of new energy fluctuations. Voltage deviation refers to the difference between the actual voltage and the nominal voltage of each feeder point. The deviation is obtained by the following formula:

In the formula, U is the voltage deviation; U _t is the actual voltage; U _N is the nominal voltage.

The comprehensive voltage deviation f is calculated by the product of the voltage deviation and the weight and then summed up separately. The weight depends on the position of the feeder node, the position of the new energy access and the output. The weight of the node i in the feeder l is M _{l, and i} is obtained by the following formula :

In the formula, Z _i is the impedance from node i to the beginning of the branch; Z _max is the maximum impedance between the beginning of the branch and each end; S _t,i is the apparent power of the i-th new energy source at time t; S _t is the sum of the apparent power of the branch load; Z _0l is the impedance from the new energy access point to the balance node; Z _il is the impedance from node i to the new energy access point, when Z _il is greater than Z _0l , correct Z _0l -Z _il is zero.

In the formula, f is the comprehensive voltage deviation; △U _m is the voltage deviation of a certain scene; M is the number of scenes; M _l,i is the weight of node i in feeder l; β _m is the probability of the scene obtained by clustering .

2) Voltage harmonic distortion rate

Whether it is an AC distribution network or a DC distribution network, there are harmonics, a sine wave component that exists in a certain AC or DC voltage, and the access of new energy increases the use of converters, that is, the use of power electronic devices. Semiconductor components and devices containing arcs and ferromagnetic nonlinearities are harmonic sources. Harmonic problems have a certain representativeness when reflecting power quality. According to GB/T 14549-93, it can be obtained:

In the formula, THD is the voltage harmonic distortion rate; U ₁ is the fundamental voltage; U _j is the j-th harmonic corresponding to the voltage; J _max is the highest harmonic detected by each feeder in the distribution network; N is The number of power electronic periods; n is the number of nodes; j is the harmonic order.

(4) New energy consumption capacity

In recent years, my country has vigorously developed new energy, but the consumption of new energy also brings some problems. Therefore, determining the consumption rate of new energy can effectively improve the power grid structure and load matching degree, and can further improve the stability of the power grid. The energy consumption rate is the ratio of the power generation of new energy to the load base value, which is obtained by the following formula:

In the formula, η is the new energy consumption rate; _PL is the active load, and _PNE is the power generation of the new energy.

S3: Perform scene clustering based on the uncertainty of new energy, and solve the comprehensive index.

S3-1: Scenario clustering based on the uncertainty of new energy

New energy includes photovoltaics, wind turbines and energy storage. The characteristics of wind and light output are closely related to seasons, weather conditions and geographical conditions. In order to calculate various indicators considering uncertainty, scene clustering is used to convert uncertainty into certainty.

The traditional clustering method has high requirements for the initial value, and the final optimal solution may be only a local optimum, and it is difficult to achieve the global optimum. The present invention adopts the improved ISODATA clustering scene division. On the basis of the class algorithm, the "merge" and "split" operations have been added, which are expressed by the following formulas:

为类j对应最大标准差；C为所期望得到聚类总数；若类标准差大于要求值且聚类类数小于期望类数的一半则采取“分裂”措施。反之，分类效果过于分散则选择“合并”。In the formula, θ _s is the sample value of the sample distance distribution in the clustering domain; N _C is the number of clustering classes;

is the maximum standard deviation corresponding to class j; C is the expected total number of clusters; if the standard deviation of the class is greater than the required value and the number of clusters is less than half of the expected number of clusters, "split" measures are taken. Conversely, if the classification effect is too scattered, select "Merge".

Determine and correct each cluster center, that is, the photovoltaic output and wind turbine output are based on the distance from each initial scene to the cluster center, and introduce trial steps and combine human-computer interaction to improve the clustering accuracy, specifically converting the random output sampling set of wind power and photovoltaic It is a point set of a multi-dimensional coordinate system, and each cluster center corresponds to the typical scene parameters of each class, and the optimal typical scene set is obtained by continuously optimizing the distance between the classes; the average value of each cluster domain sample to the cluster center is The distance is obtained by the following formula:

为平均聚类距离；N_c为聚类中心个数；X为S_j类簇中元素；Z_j为其聚类中心；N_c为聚类类数；S_j为某一类族；j为聚类类数的变量。In the formula,

is the average clustering distance; N _c is the number of cluster centers; X is the element in the S _j cluster; Z _j is the cluster center; N _c is the number of clusters; S _j is a certain family; j is the A variable for the number of cluster classes.

S3-2: Solve the comprehensive index

(1) Reliability calculation

Considering the complexity of AC and DC distribution network operation, the sequential Monte Carlo method is used for reliability calculation. The fault scenario is a single fault, and the faulty equipment is divided into: AC side fault, DC side fault and converter fault. Considering the differences in the complexity of the three power supply modes, the ring-shaped AC and DC distribution network is taken as an example to illustrate the failure consequence analysis method. :

1) When the AC side line fails, if the fault current reaches 1.5 to 2 times, the AC/DC converter will be blocked, the initial circuit breaker of the feeder where the line is located will operate, the fault side will be powered off, the converter will return to work, and the AC load downstream of the fault will occur. It can be transferred to the peer power supply;

2) When the DC side line fails, the fault current rises rapidly. If the fault current reaches 1.5 to 2 times, the converter will be blocked until the traveling wave protection action removes the fault, the converter works normally, and the non-faulty part supplies power normally. Simultaneous dual-terminal power supply mode;

3) When the converter on one side fails, the faulty converter will be quickly blocked and out of operation. The AC power supply on the faulty converter side only supplies power to the adjacent AC load, and the DC load powered by the AC power supply on the converter side is transferred to the opposite side. If the faulty converter is the main controller, it will quickly switch to the next-level converter according to the priority. If the converters on both sides are faulty, the converters will be blocked, and the DC side will be coordinated by the new energy and energy storage. powered by.

If the backup power supply or the new energy power supply cannot support all the loads in the non-faulty section after the fault, the traditional switch is dispatched to cut off part of the load, and the present invention adopts the strategy of cutting off according to the important user level.

(2) Calculation of economy, power quality, and new energy consumption

1) Calculation of operating economy, comprehensive voltage deviation and new energy consumption rate

In order to calculate the evaluation indicators such as operating economy, voltage deviation, new energy consumption rate, network loss, etc., it is necessary to calculate the power flow of the AC-DC hybrid distribution network. For a certain scene obtained by clustering, the power flow calculation is performed. The economics of investment is calculated directly according to the formula.

Firstly, the converter loss model is established, see formula [21-22], and the alternate iteration method is used for the power flow calculation. In this method, the nodes on both sides of the converter and the DC/DC converter are equivalent, the AC side is equivalent to a PQ node, and the DC side is equivalent to a constant voltage node, a constant power node and a droop control node according to different control strategies, and then The power flow calculation is carried out on the AC side and the DC side, and the forward-backward substitution method is used for calculation. First, according to the initial value of the converter station, calculate the power flow on the AC side, then calculate the loss of the converter to obtain the voltage and power of the DC side, and then calculate the power flow value on the DC side.

①The flow on the AC side

According to the control strategy of the converter and its initial value, the DC side is equivalent to the voltage and power of the converter side, and then the voltage and power of each node on the AC side are calculated by the forward push-back substitution.

In the formula, P _i (U, δ) is active power; Q _i (U, δ) is reactive power; U _i is the voltage of node i; U _j is the voltage of node j; G _ij is conductance; δ _i is the phase angle of node i ; δ _j is the phase angle of the j node; B _ij is the susceptance.

The active and reactive power flowing into the converter can be regarded as AC power, so the power mismatch vector equation is:

为有功修正量；

为发电有功；

为换流器输入有功；P_si为换流器输出有功；P_i为负荷有功；U^(j)为节点电压；δ^(j)为节点相角；△Q_i ^(j)为无功修正量；

为发电无功；

为换流器输入无功；Q_si为换流器输出无功；Q_i为负荷无功。In the formula,

is the active correction amount;

to generate power;

is the input active power of the converter; P _si is the active power output of the converter; P _i is the active power of the load; U ^(j) is the node voltage; δ ^(j) is the node phase angle; △Q _i ^(j) is the reactive power correction amount ;

for generating reactive power;

is the input reactive power of the converter; Q _si is the output reactive power of the converter; Q _i is the load reactive power.

The most common control strategies for converters are: constant voltage control, droop control, and constant power control. In the case of multiple converters, there is usually one and only one converter that adopts constant voltage control, and the rest adopts constant power or droop control. Assuming that there are k converters, the first converter adopts constant voltage control.

为换流器初始功率；P_sj为换流器有功；k为换流器个数；j为换流器个数变量。In the formula,

is the initial power of the inverter; P _sj is the active power of the inverter; k is the number of inverters; j is the variable of the number of inverters.

Using the power on the AC side, according to the converter loss model, the converter loss can be calculated and then the power flow on the converter side can be calculated.

In the formula, P _loss is the loss of the converter; a, b, and c are the loss coefficients of the converter; I _c is the current passing through the converter; P _c is the input active power of the converter; Q _c is the input reactive power of the converter power; U _c is the inverter voltage.

② DC side flow

From the known power and voltage input to the DC side by the converter in 1), the DC side voltage and power are solved by the forward-backward substitution method. Compared with the AC side, the DC side does not need to consider the reactive power, and the calculation is simpler.

In the formula, P _dc is the DC side power; U _dc is the DC side voltage.

In addition, both the AC side power flow and the DC side power flow cannot exceed the voltage and current limit, that is,

(U _{k,i_min} ) ² ≤U _k,i ² ≤(U _{k,i_max} ) ² (23)

0≤I _k,i ² ≤(I _{k,i_max} ) ² (24)

In the formula, U _k,i is the voltage of the kth feeder node i; U _{k,i_min} and U _{k,i_max} are the maximum and minimum voltages of the kth feeder node i respectively; I _k,i is the kth feeder The current of the i branch; I _{k, i_max} is the rated current of the ith branch of the feeder k.

Through the above formula, the power balance of each node and the voltage and current on each branch are not exceeded.

③ Alternate iterative process

First, input the original data of the distribution network, number the distribution network, and determine the initial value of the converter station and the control method. Second, calculate the power flow on the AC side until convergence, calculate the injected power of the converter, and determine whether the converter exceeds the limit. Next, calculate the power flow on the DC side until it converges and judge whether the AC and DC are both convergent. If so, output the calculation result, otherwise return to continue the calculation.

2) Calculation of voltage harmonic distortion rate

The voltage harmonic distortion rate is obtained by simulating and collecting the corresponding line voltage of each power electronic device and performing fast Fourier transform.

S4: Plan the AC/DC hybrid distribution network based on the solution results.

Example 2

This example is an example analysis:

(1) Basic parameters of the example

Taking the new high-tech zone distribution network in a certain area as an example, the area is currently powered by a 110kV substation through two AC 10V lines, the existing load is 170,000 kW, and the newly built AC and DC load is 2,500kW, of which the DC load is 1800kW and the AC load is 700Kw. The scenery resources are abundant, and a large amount of new energy will be connected in the future. In order to meet the demand for new energy and DC load access, implement green grid transformation and introduce AC-DC hybrid distribution network. The selected converter and DC/DC converter capacity are both 2MW.

The new energy ratio is selected according to the installed capacity of local photovoltaic fans, and a certain proportion of energy storage is provided. As shown in Table 1.

Table 1 Corresponding wind and solar storage capacity for different new energy consumption rates

(2) Scene clustering and index calculation results

1) Scenario clustering: Take the annual wind, light resource and load data in the region, take 15 minutes as a cycle, and select 500 operating scenarios according to the difference of seasonal temperature and load size, as shown in Figure 6. Then use the ISODATA clustering algorithm to obtain 8 typical scenarios and their probabilities.

2) Index calculation: Based on the source, load information and scene probability of 8 scenarios, when the new energy consumption rate is 30%, 60% and 80%, the performance indicators of three AC-DC hybrid distribution network are calculated. The calculation results are shown in Table 2-Table 10.

Table 2 Reliability calculation results of radial power supply mode

Table 3 Reliability calculation results of double-terminal power supply mode

Table 4 Reliability calculation results of ring power supply mode

Table 5 Calculation results of power quality in radial power supply mode

Table 6 Calculation results of power quality in double-terminal power supply mode

Table 7 Calculation results of power quality in ring power supply mode

Table 8 Economic calculation results of radial power supply mode

Table 9 Economic calculation results of double-terminal power supply mode

Table 10 Economic calculation results of ring power supply mode

(3) Based on the above solution results, plan the AC/DC hybrid distribution network with different demands

According to the above solution results, the AC-DC hybrid distribution network is planned for different power supply requirements.

The invention performs comprehensive index evaluation on the AC-DC hybrid distribution network connected to new energy; the invention overcomes the unscientific problems caused by too many indexes, overlapping and overlapping, according to the complete coverage, representative classification indexes, and different reflection factors. The principle of repetition builds a comprehensive indicator system.

The comprehensive evaluation index system disclosed by the invention takes into account the diversity of loads and the volatility of new energy sources, and provides a practical solution for the planning of future AC and DC distribution networks; the comprehensive index system disclosed in the invention improves the accuracy of the index calculation results sex.

While the present invention has been described with reference to the preferred embodiments, various modifications may be made and equivalents may be substituted for parts thereof without departing from the scope of the invention. In particular, as long as there is no structural conflict, each technical feature mentioned in each embodiment can be combined in any manner. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims (10)

1. A performance evaluation method for an AC/DC hybrid power distribution network accessed with new energy is characterized by comprising the following steps:

s1: new energy is accessed based on the radial distribution network, the double-end distribution network and the annular distribution network respectively;

s2: establishing a comprehensive evaluation index system with four dimensions of economy, reliability, electric energy quality and new energy consumption capacity, and constructing a comprehensive evaluation index calculation model, wherein the comprehensive evaluation index calculation model comprises an economy index calculation model, a reliability index calculation model, an electric energy quality index calculation model and a new energy consumption capacity index calculation model;

s3: carrying out scene clustering based on the uncertainty of new energy, and solving the comprehensive evaluation index model;

s4: and planning the alternating current-direct current hybrid power distribution network based on the solving result.

2. The evaluation method according to claim 1, wherein the constructing of the economic indicator calculation model in the step S2 includes constructing an investment economic calculation model and an operation economic calculation model; the operating economy includes converter loss rate and integrated line loss rate;

1) the investment economy is obtained by the following formula:

C_total＝C_I+C_O+C_S(1)

in the formula, investment cost

Wherein N is_kFor a certain equipment to number, P_kThe price is the unit price corresponding to a certain equipment, and K is the number of the equipment; the operation and maintenance cost is

Wherein C is_e、C_lOperating maintenance costs of the equipment and lines, respectively, C_O1The operation and maintenance cost of the first year, m is the discount rate, N is the year, and i is the year; cost of disposal

Wherein C is_{S_k}For disposal cost of a certain facility to scrap, K isThe number of devices;

2) the converter loss rate is obtained by the following formula:

in the formula, r_conIs converter loss rate △ P_{i_VSC}The loss of the ith converter can be obtained by the optimal power flow calculation; n is the number of converters;

3) the comprehensive line loss rate is obtained by the following formula:

in the formula, r_avg△ P as the comprehensive line loss rate_iIs the power loss value of a certain line i, which can be obtained by the calculation of the optimal power flow, wherein, l is the number of the lines, β_mProbability of the scene obtained for clustering; m is the number of scenes.

3. The assessment method according to claim 1, wherein the constructing of the reliability index calculation model in step S2 includes constructing a system average outage frequency calculation model, a system average outage duration calculation model and an average power supply availability calculation model;

1) the average power failure frequency of the system is obtained by the following formula:

in the formula, SAIFI is the average power failure frequency of the system, and the unit is times/(user year); lambda [ alpha ]_iThe power failure probability of the load point i is obtained; n is a radical of_iThe number of users at the load point i;

2) the average power failure duration of the system is obtained by the following formula:

in the formula, SAIDI is the average power failure duration index of the system, and the unit is hour/(user.year); u shape_iThe average annual power failure time of the load point i; n is a radical of_iThe number of users at the load point i;

3) the average power supply availability is obtained by the following formula:

in the formula, ASAI is an average power supply availability index; d is the hour number of one year, and 8760 hours are taken in calculation; u shape_iThe average annual power failure time of the load point i; n is a radical of_iThe number of users at load point i.

4. The evaluation method according to claim 1, wherein the constructing of the power quality index calculation model in the step S2 includes constructing a comprehensive voltage deviation calculation model and a voltage harmonic distortion calculation model;

1) the integrated voltage deviation is obtained by the following formula:

wherein f is the integrated voltage deviation of △ U_mA certain scene voltage deviation; m is the number of scenes; m_l,iIs the weight of node i in feeder l β_mProbability of the scene obtained for clustering;

2) the voltage harmonic distortion rate is obtained by the following formula:

wherein THD is the voltage harmonic distortion rate; u shape₁Is the fundamental voltage; u shape_jIs the j harmonic corresponding to the voltage; j. the design is a square_maxThe highest harmonic detected by each feeder line in the power distribution network; n is the number of power electronic periods; n is the number of nodes; j isThe number of harmonics.

5. The assessment method according to claim 1, wherein the step S2 is implemented by constructing a new energy consumption capability index calculation model, specifically:

wherein η is the new energy consumption rate, P_LFor active load, P_NEThe power generation capacity of new energy.

6. The method for evaluating of claim 1, wherein the average distance between the cluster domain samples in the scene cluster of step S3 and the cluster center is obtained by the following formula:

in the formula (I), the compound is shown in the specification,

is the average clustering distance; n is a radical of_cThe number of clustering centers; x is S_jElements in the cluster; z_jAs its clustering center; n is a radical of_cIs a clustering class number; s_jIs a certain family; j is a variable of the cluster class number.

7. The method of claim 1, wherein the reliability calculation in step S3 employs a sequential monte carlo method.

8. The evaluation method according to claim 1, wherein the new energy consumption capability calculation in step S3 is specifically a power flow calculation of an ac/dc hybrid power distribution network.

9. The evaluation method according to claim 2, wherein the operation economy calculation in step S3 is specifically a power flow calculation of the ac/dc hybrid power distribution network.

10. The evaluation method according to claim 8 or 9, wherein the power flow calculation in the step S3 includes the sub-steps of:

s3-1: constructing a loss model of the current converter;

s3-2: and solving by adopting an alternative iteration method.

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