A transformer neutral point voltage analysis method considering inverter-type distributed light storage capacity

Abstract

The present application relates to a kind of considering the analysis method of transformer neutral point voltage of inverter type distributed light storage capacity, first, obtain the transformer neutral point voltage formula expressed in line parameters and transformer rated phase voltage;Second, obtain the neutral point voltage only with the short-circuit current of distributed power output and load impedance is related;Then, according to the ratio of distributed photovoltaic capacity and load, the short-circuit current of distributed power output is divided into two cases, namely constant output maximum short-circuit current and the short-circuit current that changes with load;Respectively, the neutral point voltage formula is obtained, after setting gap breakdown voltage, the load proportion critical value that can cause neutral point overvoltage can be obtained;Finally, function about inverter type distributed photovoltaic capacity, energy storage proportion and load proportion is established, and the load proportion critical value that can cause neutral point overvoltage under certain energy storage proportion is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power distribution network fault feature analysis, and particularly relates to a transformer neutral point voltage analysis method considering inverter type distributed light storage capacity. BACKGROUND

[0002] For a traditional passive 110kV power distribution system with transformer gap grounding, when an asymmetric ground fault occurs in the 110kV transmission line, the line protection on the grid side operates correctly, the offset voltage of the neutral point of the main transformer disappears, and the insulation of the neutral point is less threatened. However, when an inverter type distributed power source is connected to the low voltage side of the transformer, the continuous fault current provided by the distributed power source may further raise the offset voltage of the neutral point of the main transformer, threatening the insulation of the neutral point. If the capacity of the distributed power source is greater than the load power, the gap of the neutral point of the transformer may be broken down. At this time, the gap protection operates, causing the main transformer to exit the system operation and stop power supply to the load side. Since the control strategies of inverter type distributed power sources are different, the power characteristics of the output of the distributed power sources after they are disconnected from the power grid are also different. Therefore, it is necessary to study the influence of the connection of inverter type distributed power sources on the neutral point voltage and gap protection of the power distribution network.

[0003] At present, there are few studies on the influencing factors of the neutral point voltage of the transformer. Some literature theoretically analyzes the influence of different types of distributed power sources on the neutral point voltage when a single-phase ground fault or a broken line fault occurs in the 110kV line with distributed power sources connected to the low voltage side of the 110kV transformer, and proposes suggestions for preventing overvoltage of the neutral point. However, this research does not consider the influence of the further rise of the neutral point voltage on the safe operation of the transformer after the operation of the grid side protection. Some literature also analyzes the influencing factors of the neutral point voltage after the occurrence of a fault in the 110kV line and the operation of the line protection, and verifies that the neutral point overvoltage event occurs when the load power is less than the capacity of the distributed power source after the distributed power source is disconnected from the power grid. However, this research only analyzes the connection of distributed photovoltaic power sources without island operation capability to the power distribution network, and does not consider the influence of different control strategies of different types of distributed power sources, such as master-slave control of light storage simultaneously connected, on the neutral point voltage and gap protection. SUMMARY

[0004] The application aims to overcome the shortcomings of the prior art and provide a transformer neutral point voltage analysis method considering the capacity of an inverter-type distributed light storage. First, the transformer neutral point voltage formula expressed by the line parameters and the rated phase voltage of the transformer is obtained according to the composite sequence network diagram when a single-phase ground fault occurs on the 110kV line. Second, when the grid-side protection acts, there is no zero sequence loop on the high-voltage side of the transformer, and there is no zero sequence current at the fault point, so the neutral point voltage is irrelevant to the high-voltage side, and it can be summarized that the neutral point voltage is only related to the short-circuit current output by the distributed power supply and the load impedance. Third, since the short-circuit current output by the inverter-type distributed power supply has an upper limit, and the distributed power supply does not necessarily output the maximum short-circuit current at the time of fault, the short-circuit current output by the distributed power supply can be divided into two cases according to the ratio of the distributed photovoltaic capacity to the load, i.e. constant maximum short-circuit current output and short-circuit current changing with load. In these two cases, the neutral point voltage formula is calculated respectively, and after setting the gap breakdown voltage, the critical value of the load ratio leading to the neutral point overvoltage can be obtained. Finally, considering that a large number of photovoltaic accesses need to be configured with corresponding energy storage units, and when the capacity of the energy storage itself is limited, the energy storage cannot completely absorb the power output from the photovoltaic when operating off-grid, which may also lead to neutral point overvoltage. Based on this, a function about the capacity of the inverter-type distributed photovoltaic, the proportion of the energy storage and the proportion of the load is established, and the critical value of the load ratio leading to the neutral point overvoltage under a certain energy storage proportion is solved.

[0005] The application solves the technical problem by the following technical scheme:

[0006] A transformer neutral point voltage analysis method considering the capacity of an inverter-type distributed light storage, characterized in that the steps of the method are:

[0007] S1, for the neutral point grounded through gap transformer, when the power transmission line connected thereto fails, the neutral point of the transformer generates a voltage, the voltage size is affected by the ratio of the zero sequence impedance to the positive sequence impedance in the line parameter, and the relationship between the neutral point voltage at the time of fault and the rated phase voltage of the transformer is calculated by using the feature;

[0008] S2, when the grid-side protection acts, there is no zero sequence loop on the high-voltage side of the transformer, and there is no zero sequence current at the fault point, but the distributed power supply will still output positive and negative sequence currents to the fault point, and the relationship between the neutral point voltage and the short-circuit current output by the distributed power supply and the load impedance is calculated based on the system composite sequence network diagram;

[0009] S3, based on the fact that the inverter-type distributed power supply has a maximum short-circuit current limit at the time of fault, i.e. m=1.2-2.0 times the rated current, and the ratio of the distributed power supply capacity to the load active power, i.e. the load ratio, affects the change of the short-circuit current, the transformer neutral point voltage after the protection acts is divided into two cases:

[0010] A load smaller than m2 The neutral point voltage is closely related to the load ratio when the distributed power capacity is more than m

[0011] A method for determining the maximum short-circuit current of a distributed power supply when the load is greater than m 2 The neutral point voltage is closely related to the load ratio when the distributed power capacity is more than m

[0012] S4, a function is established between the distributed photovoltaic access capacity, the energy storage capacity ratio and the load ratio, and the relationship curve between the energy storage capacity ratio and the load ratio is obtained, and the critical value of the load ratio that leads to the neutral point overvoltage of the transformer under a certain energy storage ratio is solved.

[0013] Moreover, the S1 is specifically:

[0014] When a single-phase metallic ground fault occurs on a 110kV line, the positive sequence short-circuit current expression at the fault point is:

[0015]

[0016] Wherein: Z m(1) and Z m(2) are the positive sequence and negative sequence equivalent impedances of the low voltage side of the main transformer, and have Z m(1) = Z m(2) .

[0017] Z s is the equivalent impedance of the total impedance of the composite sequence network; Z 1(1) = Z 1(2) , that is, the positive sequence and negative sequence equivalent impedances of the grid-connected transmission line are equal.

[0018] For different types of transmission lines, the zero sequence impedance of the line is taken as n times the positive sequence impedance, that is, Z 1(0) = nZ 1(1) , and in general cases, n < 3, and since the sum of the line impedance and the load impedance on the 10kV side is much greater than the equivalent impedance of the high voltage side of the main transformer, that is, Z m(1) ≥ Z 1(1) , then the positive sequence short-circuit current at the fault point is rewritten as:

[0019]

[0020] Since there is no path in the zero sequence network of the low voltage side of the main transformer, the zero sequence voltage at the fault point is expressed as:

[0021]

[0022] According to the above formula, the neutral point voltage is the system phase voltage under the working condition, and if n is determined, the neutral point voltage remains unchanged, and the neutral point voltage is always lower than the system phase voltage, which will not cause gap breakdown.

[0023] Further, the S2 is specifically:

[0024] When the network side protection acts, there is no zero sequence network access, and the fault point has no zero sequence current. At this time, the zero sequence voltage of the fault point is equal to the neutral point voltage, which is expressed as:

[0025]

[0026] Wherein: is the sum of the short-circuit currents of each distributed power output;

[0027] The size of the main transformer neutral point voltage is determined by the output current of the low-voltage side distributed power when off-grid and the local load, and is irrelevant to the high-voltage side of the main transformer.

[0028] Further, the S3 is specifically:

[0029] Based on the fact that the active power output by the light storage cannot change instantaneously before and after the line protection acts, the change of the voltage at the grid connection point of the light storage depends on the active power demand of the load and the imbalance degree of the active power output by the light storage. According to the change of the active power output by the light storage, the voltage at the grid connection point of the light storage when off-grid for a short time is obtained as:

[0030]

[0031] Wherein: and are the voltage at the grid connection point of the distributed power and the rated voltage when the distributed power is in grid-connected operation, respectively;

[0032] P g and P L are the active power components of the total access capacity of the light storage and the load, respectively;

[0033] P pv and P es represent the active power output by the distributed photovoltaic and energy storage, respectively;

[0034] When the active power demand of the load is greater than the power provided by the light storage, the voltage decreases and the output current increases; when the active power demand of the load is less than the power provided by the light storage, the voltage rises and the output current decreases;

[0035] Define the variable load ratio as k=P g / P L , which represents the ratio of the power of the distributed light storage to the power of the load when off-grid, and the rated phase voltage of the main transformer high-voltage side is 1.0 p.u., and the per-unit value of the transformer neutral point voltage is obtained as:

[0036]

[0037] The transformer neutral point voltage in a short time after the line protection is tripped is a function of the ratio k of the total capacity of the optical storage and the active power of the load demand, and the ratio m of the maximum current output by the distributed power supply to the rated current. After setting the gap breakdown voltage, the critical value of the load ratio that causes the neutral point overvoltage can be obtained according to the above formula.

[0038] Moreover, the S4 is specifically:

[0039] When the distributed optical storage is disconnected from the large power grid, the distributed photovoltaic adopts PQ control, and the energy storage should be switched to V / f control, which needs to provide reference voltage and frequency for the entire micro-grid system to ensure the stable operation of the micro-grid. The energy storage plays the role of power balance, which can be mainly divided into two states: charging and discharging states.

[0040] For the discharging state, due to the effect of V / f control, the energy storage can effectively adjust its own output power to ensure the balance between the total output power of the distributed power supply and the load, and there is no neutral point overvoltage.

[0041] For the charging state, it is expressed as follows:

[0042] P pv >P L

[0043] After the distributed photovoltaic supplies the load, there is surplus power that needs to be charged to the energy storage. If the capacity of the energy storage can effectively absorb the surplus power of the photovoltaic after meeting the load supply, there will be no power transmission to the upper line. However, if the energy storage cannot completely absorb the surplus power, the neutral point voltage may be further lifted, which may threaten the insulation of the neutral point. It is described by the formula as follows:

[0044] ΔP g =P pv -(P L +P es )>0

[0045] Among them, the load power is expressed as:

[0046]

[0047] According to relevant regulations, when the photovoltaic is connected, a certain proportion of energy storage needs to be configured, and the ratio of the two is defined as x. The above formula is expressed as:

[0048]

[0049] The fundamental reason for the transformer neutral point overvoltage is that the output power of the distributed power supply is greater than the load when the distributed power supply operates off-grid. According to the load ratio critical value obtained in S3, assuming that the neutral point gap breakdown voltage is 1.04 p.u., a target function about the distributed photovoltaic capacity, energy storage ratio, and load ratio can be obtained:

[0050]

[0051] Let F≥0, the energy storage ratio and the load ratio value range when the neutral point overvoltage is obtained, no matter how large the photovoltaic capacity is, the critical value of x leading to the neutral point overvoltage can be expressed as:

[0052]

[0053] The size of the energy storage capacity ratio is closely related to the load ratio, and the critical value of the energy storage capacity ratio leading to the neutral point overvoltage is calculated.

[0054] The advantages and beneficial effects of the present application are:

[0055] 1, the present application is based on whether the distributed power output maximum short-circuit current, the ratio of distributed power capacity and load is divided into two parts, theoretically prove that the neutral point voltage and the output short-circuit current of distributed power, capacity and load size are related;

[0056] 2, the present application defines the ratio of distributed power capacity and load as load ratio, gives the critical value of load ratio leading to neutral point overvoltage when only distributed light storage without island operation capability is accessed in distribution network and PQ control is used;

[0057] 3, the present application defines the ratio of distributed energy storage capacity and photovoltaic capacity as energy storage ratio, gives the critical value of load ratio leading to neutral point overvoltage under any energy storage ratio when distributed light storage with island operation capability is accessed in distribution network and master-slave control is used. BRIEF DESCRIPTION OF DRAWINGS

[0058] Figure 1 is the wiring diagram of the distribution network of the present application containing inverter type distributed light storage access;

[0059] Figure 2 is the single-phase ground fault composite sequence network diagram of the present application;

[0060] Figure 3 is the system composite sequence network diagram after protection action of the present application;

[0061] Figure 4 is the relationship diagram of neutral point voltage and load ratio and energy storage ratio of the present application;

[0062] Figure 5 is the curve diagram of energy storage ratio x of the present application;

[0063] Figure 6 is the influence diagram of transformer neutral point voltage and gap protection when 10MW photovoltaic is accessed in the present application;

[0064] Figure 7is the simulation diagram of the influence of different capacity photovoltaic access on the neutral point voltage and gap protection of the transformer of the application;

[0065] Figure 8 is the influence diagram of the neutral point voltage and gap protection when the photovoltaic 5MW and energy storage 1MW of the application. DETAILED DESCRIPTION

[0066] The application will be further described in detail through specific examples below, and the following examples are only descriptive and not limiting, and cannot limit the protection scope of the application.

[0067] A transformer neutral point voltage analysis method considering inverter type distributed light storage capacity, comprising:

[0068] As Figure 1 shown is a structure diagram of a 110kV substation with inverter type distributed power supply access on the low voltage side, containing 220kV and 110kV two substations. Among them, the 110kV substation is connected to the grid through a transmission line, the grounding mode of the high voltage side neutral point of the main transformer is that the grounding gap and the disconnector are connected in parallel, and the low voltage side exists distributed photovoltaic and energy storage access. The current protection is configured between the grid-connected transmission lines Bus2 and Bus3, wherein protection 1 is configured on the Bus2 side and protection 2 is configured on the Bus3 side.

[0069] If a fault occurs as shown in Figure 1 , the composite sequence network after the fault is as shown in Figure 2 . Since the inverter type distributed power supply in the model in this paper is a positive sequence control strategy, the distributed photovoltaic can be equivalent to a constant current source that only outputs positive sequence current. In the figure, is the equivalent main power supply of the 110kV neutral point grounded through the gap; Z1, Z2 are the line impedance between the grid-connected transmission lines Bus2 and Bus3, and the equivalent impedance of the 10kV distribution network; Z T1 and Z T2 are the equivalent impedances of the main transformers of the 220kV and 110kV substations respectively; Z L is the equivalent impedance of the load; subscripts (1), (2), (0) represent positive sequence, negative sequence and zero sequence components respectively.

[0070] According to Figure 2 , the positive sequence current expression at the fault point can be obtained as:

[0071]

[0072] Z m(1) = Z T1(1) + Z T2(1) + Z L(1) + Z 1(1) (2)

[0073] Zm(2) = Z T1(2) + Z T2(2) + Z L(2) + Z 1(2)

[0074]

[0075] wherein, Z m(1) and Z m(2) are the positive and negative sequence equivalent impedances of the low voltage side of the main transformer, and have Z m(1) = Z m(2) ; Z s is the equivalent impedance of the total impedance of the composite sequence network; Z 1(1) = Z 1(2) , i.e. the positive and negative sequence equivalent impedances of the grid-connected transmission line are equal; for different types of transmission lines, the zero sequence impedance of the line is taken as n times the positive sequence impedance, i.e. Z 1(0) = nZ 1(1) (generally n < 3). Since the sum of the line impedance and the load impedance at the 10kV side is much greater than the equivalent impedance of the high voltage side of the main transformer, i.e. Z m(1) ≥ Z 1(1) . Then formula (1) can be rewritten as:

[0076]

[0077] From the above formula, it can be seen that the fault current provided by the system side after the fault occurs is significantly increased, and its amplitude is inversely proportional to the size of the high voltage side transmission line impedance and is irrelevant to the low voltage side of the main transformer. When only considering the effect of the distributed power supply on the fault point current, the positive sequence fault current at the fault point is:

[0078]

[0079] wherein, and are the positive sequence fault currents of the distributed energy storage and photovoltaic output, respectively. Due to the large equivalent impedance of the 10kV side after reduction, and due to the characteristics of the photovoltaic grid-connected control strategy and the current limitation of the power electronic device, the fault current provided by the distributed photovoltaic power supply is small and can be ignored compared to the fault current provided by the system side. Then the short-circuit current at the fault point is:

[0080]

[0081] According to the above analysis, the fault current at the fault point is mainly provided by the system side and is not affected by the access of the distributed photovoltaic power supply. At this time, the fault current flowing through the protection 1 at the B side of the transmission line is significantly increased, and the protection 1 can reliably act.

[0082] Since the zero sequence network of the low voltage side of the transformer is open, the zero sequence voltage of the neutral point is equal to the zero sequence voltage of the fault point. The zero sequence voltage is:

[0083]

[0084] The offset voltage of the neutral point of the transformer can be expressed as:

[0085]

[0086] The through voltage of the neutral point is generally slightly higher than the phase voltage of the system under the working condition. When a single-phase grounding fault occurs, the voltage offset of the neutral point is caused by the unbalanced voltage drop of the three-phase winding of the transformer, and is not affected by the low voltage side. At this time, the fault characteristics are approximately the same as when the transmission line is faulty with the low voltage side open. Therefore, according to the above formula, the voltage of the neutral point is n / (n+2) of the phase voltage of the system under the working condition. If n is determined, the voltage of the neutral point remains unchanged, and the voltage of the neutral point is always lower than the phase voltage of the system, which will not cause gap breakdown.

[0087] When the starting element of the protection 1 on the transmission line B side detects the fault current, the protection action disconnects the distribution network containing the distributed power supply from the system. At this time, the distributed power supply and the local load are disconnected from the network for a short time, and all the energy in the network is provided by the distributed power supply. According to Figure 1 , after the protection 1 acts and the distributed power supply is disconnected, the composite sequence network diagram of the system is shown in Figure 3 .

[0088] As can be seen from Figure 3 , when the line protection 1 is tripped, there is no fault current at the fault point due to the open zero sequence network. At this time, the zero sequence voltage of the fault point and the voltage of the neutral point are expressed as:

[0089]

[0090] wherein is the sum of the short-circuit currents output by each distributed power supply. As can be seen from the above formula, the voltage of the neutral point of the transformer is determined by the output current of the distributed power supply on the low voltage side when it is disconnected from the network and the local load, and is independent of the high voltage side of the transformer. When the output power of the inverter is large and the load is small, that is, when the short-circuit current provided by the inverter and the load impedance are large, the voltage of the neutral point will be further raised. If the voltage of the neutral point is raised to be greater than the gap breakdown voltage, the gap will be broken down.

[0091] Based on the fact that the active power output by the optical storage cannot change instantaneously before and after the line protection acts, the change in the voltage at the grid connection point of the optical storage depends on the active power demand of the load and the imbalance between the active power output by the optical storage. According to the change in the active power output by the optical storage, the voltage at the grid connection point of the optical storage when it is disconnected from the network for a short time is:

[0092]

[0093] where, and are the voltage at the point of common coupling (PCC) and the rated voltage when the distributed generator is in operation, respectively; P g and P L are the total active power output of the photovoltaic and energy storage and the active component of the load, respectively; P pv and P es represent the active power output of the distributed photovoltaic and energy storage, respectively. Generally, the load and the output power of the distributed generator need to be balanced. However, due to the uncertainty of system operation, the load can be greater or less than the power of the distributed generator. It can be seen that when the active demand of the load is greater than the power provided by the photovoltaic and energy storage, the voltage decreases and the output current increases; when the active demand of the load is less than the power provided by the photovoltaic and energy storage, the voltage rises and the output current decreases. Then the output current of the photovoltaic and energy storage at this time is:

[0094]

[0095] Considering that the output current of the inverter-type distributed generator has an upper limit, which is generally about 1.2-2 times the rated current of the inverter-type distributed generator. Considering the worst case, assume that the photovoltaic and energy storage inverters reach their maximum short-circuit current at the same time during a fault, and the maximum short-circuit current output is m times the rated current, i.e.,

[0096]

[0097] where, is the maximum value of the sum of the output currents of the photovoltaic and energy storage; and are the rated currents of the distributed photovoltaic and energy storage when they are in operation. According to the above formulas, when the output current of the photovoltaic and energy storage reaches the maximum value, the relationship between the load power and the active power output of the photovoltaic and energy storage is:

[0098] P L = m 2 P g (13)

[0099] Therefore, when the load power is small, the following relationship exists:

[0100]

[0101] At this time, the neutral point voltage changes with the change of the output current of the photovoltaic and energy storage, which can be represented as:

[0102]

[0103] where, is the load current, The voltage of the high-voltage side of the main transformer in the 110 kV substation. When the load power is large, that is,

[0104] P L ≥m 2 P g (16)

[0105] Further represented as:

[0106]

[0107] At this time, the sum of the short-circuit currents output by the optical storage is maximum and is a certain value, and the neutral point voltage can be represented as:

[0108]

[0109] The above analysis shows that the voltage of the neutral point of the main transformer will further rise after the protection is tripped, and the degree of voltage deviation is related to the capacity of the distributed power source and the ratio of the load power, and the coefficient m of the maximum output current of the optical storage at the time of fault.

[0110] Define variable k = P g / P L , which represents the ratio of the capacity of the distributed power source to the load power when operating off-grid. Take the rated phase voltage of the high-voltage side of the main transformer as the reference value, that is, 1.0 p.u., and the per-unit value of the neutral point voltage of the transformer can be obtained as:

[0111]

[0112] As can be seen from the above formula, the neutral point voltage of the transformer in the short time after the fault occurs and the grid-side protection is tripped is a function of the ratio k of the active power output by the optical storage to the active power required by the load, and the ratio m of the maximum output current to the rated current of the distributed power source. According to the above formula, the relationship between the neutral point voltage and k at different m values can be obtained.

[0113] As shown in Figure 4 , the neutral point breakdown voltage is set to 1.04 p.u.. As can be seen from the above figure, when k≤1 / m 2 , as m increases, the photovoltaic and energy storage inverters will output the maximum short-circuit current when k is small, but the neutral point voltage will always not exceed the breakdown voltage; when k>1 / m 2 , since the neutral point voltage is only related to k, therefore, regardless of the value of m, when the breakdown voltage is 1.04 p.u., the value of k is 1.0816. When k≥1.0816, the main transformer of the 110 kV substation has a risk of neutral point breakdown. The above results show that when multiple distributed power sources are connected, whether the neutral point overvoltage exists or not is irrelevant to the ratio m of the maximum output current to the rated current of the inverter.

[0114] Since the inverter-type distributed power supply is divided into island operation and non-island operation, when the distributed power supply has island operation, it can balance the power output inside the microgrid after being disconnected from the grid. Taking the distributed photovoltaic storage disconnected from the grid as an example, the distributed photovoltaic generally continues to use PQ control, and the storage should switch to V / f control, which needs to provide reference voltage and frequency for the whole microgrid system to ensure the stable operation of the microgrid. The storage plays the role of power balance, which can be mainly divided into two states: charging and discharging state. For the discharging state, due to the effect of V / f control, the storage can effectively adjust its own output power to ensure the balance of the total output power of the distributed power supply and the load, and it can be considered that there is no neutral point overvoltage; for the charging state, it can be expressed as formula 20:

[0115] P pv >P L (20)

[0116] The above formula shows that the distributed photovoltaic appears surplus power after supplying the load, and needs to charge the storage. If the capacity of the storage can effectively absorb the surplus power of the photovoltaic after meeting the load supply, there will be no power transmission to the upper line. However, if the storage cannot completely absorb the surplus power, the neutral point voltage may be further lifted, which may threaten the insulation of the neutral point. The above situation can be described by formula as follows:

[0117] ΔP g =P pv -(P L +P es )>0 (21)

[0118] Among them, the load power can be expressed as:

[0119]

[0120] According to the relevant provisions, when the photovoltaic is connected, a certain proportion of storage needs to be configured, and the ratio of the two is defined as x, then formula 22 can be expressed as:

[0121]

[0122] Substituting formula 23 into formula 21 can obtain

[0123]

[0124] The above inequality shows that the storage cannot completely absorb the surplus power, and there is a risk of neutral point overvoltage. However, the neutral point voltage may be greater than the rated phase voltage, and there may also be a case where it is not greater than the breakdown voltage. Only when k>1.0816, the neutral point will produce overvoltage, that is:

[0125]

[0126] At this point, the load power is approximately 0.9245 times the sum of the distributed power sources (1 / 1.0816).

[0127] P L <0.9245P g (26)

[0128] According to Formula 26, the critical value of residual power that causes neutral point overvoltage when the distributed power source outputs power continuously and stably is:

[0129] ΔP g =P g -0.9245P g =0.0755P g (27)

[0130] Combining formulas 24 and 27, if the neutral point voltage is greater than the breakdown voltage, then:

[0131]

[0132] ΔP g ≥0.0755(1+x)P pv (29)

[0133] Finally, based on formulas 28 and 29, a formula for P can be established. pv Functions of x and k:

[0134]

[0135] According to Formula 30, when F < 0, the neutral point voltage may exceed the rated phase voltage, but the possibility of overvoltage is relatively small. When F ≥ 0, there is a higher probability of neutral point overvoltage and gap breakdown. Based on all the above analyses, an objective function with multiple constraints can be obtained:

[0136]

[0137] Let F≥0. Observing the above equation, we can see that the relationship between x and k is not affected by the magnitude of the distributed photovoltaic output power. Regardless of the photovoltaic output power, the critical value of x that causes neutral point overvoltage can be expressed as:

[0138]

[0139] From the above formula, as Figure 5 As shown, a graph depicting the relationship between x and k can be drawn. From Figure 5It can be seen that, due to x>0, the invalid value region needs to be removed. As can be seen from the above figure, no matter how large the photovoltaic access capacity is, or how the load changes, as long as the proportion of energy storage configured with it is greater than x corresponding to each k in the green indicated area of the figure, the neutral point overvoltage will not occur; if the proportion of energy storage is less than or equal to x corresponding to each k in the red indicated area of the figure, the neutral point overvoltage is most likely to occur.

[0140] Simulation verification:

[0141] A system model containing distributed photovoltaic and energy storage grid-connected as shown in Figure 1 is established in PSCAD, both of which are simultaneously connected using independent inverters. The neutral point of the high-voltage side of the 110kV substation transformer is grounded through a gap. In the simulation experiment, a single-phase ground fault at the end of the transmission line is set at 2s, and the protection 1 of the 110kV transmission line is tripped at 2.1s. In order to avoid the large transient voltage after the line protection is tripped, the switch at the ground gap is closed again after a delay of 0.5s. The gap uses the "Spark Gap" element in the PSCAD software, which can set the gap breakdown voltage. When the neutral point voltage is greater than the set breakdown voltage, it will switch between conduction and non-conduction. The gap protection is mainly divided into gap zero sequence overcurrent protection and overvoltage protection. Among them, the gap zero sequence overcurrent protection can be used as the main protection, and the setting value is set to 100A, and the data is sampled from the ground gap; the overvoltage protection is a backup protection, which acts when the gap zero sequence current does not reach the setting value, and its setting value can be set to 150V according to experience, and the data is sampled from the 110kV side bus.

[0142] As can be seen from Figure 6 , when k=0.6, the neutral point voltage is still much smaller than the breakdown voltage, and the gap zero sequence current is also 0A. When k=1, the neutral point voltage is about 63.55kV, which is greater than the rated phase voltage, although it is less than the breakdown voltage, but it still threatens the transformer insulation, also indicates that if the output power of photovoltaic is further increased, even if the load power does not decrease, the neutral point voltage is still likely to be greater than the gap breakdown voltage, resulting in the gap being broken down; when k=1.2 and 1.5, the neutral point voltage is much greater than the breakdown voltage, resulting in the gap being broken down, and the maximum value of the effective value of the gap zero sequence current is also greater than 100A, at this time, the gap zero sequence overcurrent protection should act.

[0143] Then, the output power of the photovoltaic inverter is changed to 1MW-20MW to observe the influence of different capacities of distributed power on the neutral point voltage and gap protection of the distribution network. As can be seen from Figure 7It can be seen that when the photovoltaic access capacity is small and the load is not large, the neutral point overvoltage is small. When the photovoltaic access capacity is large, with the continuous increase of the load ratio, the neutral point overvoltage is more and more serious. When the photovoltaic access capacity is 1 MW and 2 MW, due to the small photovoltaic capacity, the short-circuit current output by the photovoltaic is small, resulting in that the gap zero sequence current is less than 100 A, and the zero sequence voltage protection needs to be operated at this time; when the photovoltaic capacity is greater than 5 MW, the effective value of the gap zero sequence current is greater than 100 A, and the gap zero sequence overcurrent protection is operated at this time.

[0144] As shown in Figure 8 , the photovoltaic output power is 5 MW, the energy storage output power is 1 MW, and the influence of off-grid operation photovoltaic PQ control and energy storage V / f control on the neutral point voltage and gap protection is shown. It can be seen that when k = 1, 1.2 and 1.5, due to the capacity of the energy storage is still sufficient to coordinate the power, therefore, the neutral point does not appear overvoltage, and the gap is not broken down. However, when the load is reduced to 3 MW, that is, k = 2, since the photovoltaic is always in PQ control, after meeting the load demand, the excess 2 MW power is transmitted to the energy storage for charging. However, the capacity of the energy storage itself is 1 MW, which is not enough to bear the excess 2 MW power. At this time, the neutral point appears overvoltage, resulting in that the gap is broken down, and the effective value of the gap zero sequence current is about 105 A. This corresponds to the theoretical analysis that the 20% energy storage ratio can only guarantee that k < 1.65, and the neutral point overvoltage event does not occur.

[0145] Finally, in order to observe the change of the neutral point voltage in detail when the energy storage ratio is different, the energy storage ratio is set to 10% and 25% respectively, and the photovoltaic capacity is set to 10 MW and 20 MW respectively.

[0146] Table 1 Neutral point voltage Bus zero sequence voltage secondary value and gap zero sequence current

[0147]

[0148] (energy storage ratio 10%)

[0149]

[0150]

[0151] Table 2 Neutral point voltage Bus zero sequence voltage secondary value and gap zero sequence current

[0152]

[0153] (energy storage ratio 25%)

[0154]

[0155] As shown in the charts 1 and 2, the k corresponding to the occurrence of the gap breakdown at each energy storage ratio conforms to the aforementioned theoretical analysis, which indicates the accuracy of the theoretical analysis and simulation. According to the simulation results, the following conclusion can be obtained: for the case of the photovoltaic PQ and the energy storage V / f control in off-grid operation, when the energy storage ratio is small, if the load change is also small, the neutral point overvoltage will not occur; if the load reduction degree is large, the neutral point is more likely to have overvoltage. When the energy storage ratio is large, even if the load reduction degree is large, the neutral point overvoltage will not occur, unless the photovoltaic power supply load has surplus power and the surplus power is greater than a certain range. At this time, the energy storage cannot effectively balance the power, so that the excess power of the photovoltaic output is transmitted to the upper line, resulting in the lifting of the neutral point voltage.

[0156] In summary, for the case of the inverter-type distributed power supply access without island operation capability and adopting the PQ control strategy, as the access capacity of the distributed power supply continues to increase, the risk of the neutral point overvoltage gradually increases. When the access capacity is large, the neutral point overvoltage is more likely to occur, resulting in the breakdown of the gap. For this case, the gap protection should be added under most of the access capacity. For the case of the inverter-type distributed power supply access with island operation capability and adopting the master-slave control strategy, when the energy storage ratio is small, it is necessary to add the gap protection; when the energy storage ratio is large, the gap protection can not be added, but the economic benefits, land area and safe operation and the like need to be considered.

[0157] The technical scheme provided by the application analyzes the influence of the inverter-type distributed photovoltaic storage access to the distribution network on the transformer neutral point voltage and the gap protection under different control strategies and capacities. The simulation obtains the transformer neutral point voltage, the gap zero sequence current and the secondary value of the main transformer high-voltage side bus zero sequence voltage, and the results show that when the distributed power supply is accessed without island operation capability and adopting the PQ control, the larger the capacity is, the more obvious the neutral point overvoltage is, and it is necessary to add the gap protection; when the distributed photovoltaic storage is accessed with island operation capability and adopting the master-slave control, the larger the energy storage capacity is, the smaller the neutral point overvoltage degree is, so whether to add the gap protection can be determined according to the energy storage access capacity and the control strategy and the like factors.

[0158] The neutral point voltage deviation problem when fault occurs and after line protection action is analyzed theoretically. The influence of the ratio of the distributed generation without islanding capability and the load to the neutral point voltage is discussed when the photovoltaic storage is simultaneously connected to the distribution network. The influence of the different control strategies and the different capacity of the inverter type distributed photovoltaic storage on the transformer neutral point voltage and the gap protection action is studied. Finally, the suggestion of setting the gap protection for the 110 kV transformer is given by comprehensively considering the two working conditions of the photovoltaic access and the simultaneous access of the photovoltaic storage. The simulation results show that the distributed generation without islanding capability and using PQ control will lead to the significant increase of the transformer neutral point voltage; the distributed generation with islanding capability and using master-slave control needs to consider the proportion of the energy storage to determine whether the neutral point is overvoltage.

[0159] Although the embodiments of the present application and the drawings are disclosed for the purpose of illustration, it is understood that various substitutions, changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the present application and the appended claims, therefore, the scope of the present application is not limited to the disclosed content of the embodiments and the drawings.

Claims

1. A method for analyzing the neutral point voltage of a transformer considering inverter-type distributed photovoltaic-storage capacity, characterized in that: The steps of the method are as follows: S1. For a neutral point grounded transformer, when a fault occurs in the transmission line it is connected to, a voltage is generated at the neutral point of the transformer. The magnitude of the voltage is affected by the ratio of zero-sequence impedance to positive-sequence impedance in the line parameters. Using this characteristic, the relationship between the neutral point voltage and the rated phase voltage of the transformer during the fault can be calculated. S2. When the grid-side protection operates, there is no zero-sequence circuit on the high-voltage side of the transformer and no zero-sequence current at the fault point. However, the distributed power source will still output positive-sequence and negative-sequence currents to the fault point. The relationship between the neutral point voltage and the output short-circuit current and load impedance of the distributed power source is calculated based on the system composite sequence network diagram. S3. Based on the maximum short-circuit current limit of inverter-type distributed power sources during faults, which is m = 1.2 to 2.0 times the rated current, and considering that the ratio of distributed power source capacity to load active power (i.e., the load ratio) affects the short-circuit current change, the transformer neutral point voltage after protection operation is divided into two cases: One type is for loads less than m 2 When the capacity of distributed power sources is doubled, the neutral point voltage is closely related to the load ratio; One type is a load greater than m 2 When the capacity of the distributed power source is doubled, the short-circuit current of the distributed power source is the largest, and the neutral point voltage is related to the maximum short-circuit current coefficient and the load ratio. S4. Establish a function relating distributed photovoltaic access capacity, energy storage capacity ratio, and load ratio. After obtaining the relationship curve between energy storage capacity ratio and load ratio, solve for the critical load ratio value that causes overvoltage at the neutral point of the transformer under a certain energy storage ratio.

2. The transformer neutral point voltage analysis method considering inverter-type distributed photovoltaic-storage capacity according to claim 1, characterized in that: Specifically, S1 is: When a single-phase metallic ground fault occurs on a 110kV line, the expression for the positive sequence short-circuit current at the fault point is as follows: Among them: Z m(1) and Z m(2) These are the positive and negative sequence equivalent impedances of the low-voltage side of the main transformer, respectively, and Z... m(1) =Z m(2) ; Z s Z is the equivalent impedance of the total impedance of the composite sequence network; 1(1) =Z 1(2) That is, the positive sequence and negative sequence equivalent impedances of grid-connected transmission lines are equal; For different types of transmission lines, the zero-sequence impedance is taken as n times the positive-sequence impedance, i.e., Z. 1(0) =nZ 1(1) Generally, n < 3. Since the sum of the line impedance and load impedance on the 10kV side is much greater than the equivalent impedance of the transmission line on the high-voltage side of the main transformer, i.e., Z... m(1) ≥Z 1(1) The positive sequence short-circuit current at the fault point is then rewritten as: Since there is no path in the zero-sequence network on the low-voltage side of the main transformer, the zero-sequence voltage at the fault point is expressed as: According to the above formula, the neutral point voltage is the system phase voltage under operating conditions. If the value of n is determined, the neutral point voltage remains unchanged. The neutral point voltage is always lower than the system phase voltage, and will not cause gap breakdown.

3. The transformer neutral point voltage analysis method considering inverter-type distributed photovoltaic storage capacity according to claim 1, characterized in that: Specifically, S2 is: When the grid-side protection trips, since there is no path in the zero-sequence network, there is no zero-sequence current at the fault point. At this time, the zero-sequence voltage at the fault point is equal to the neutral point voltage, which is expressed as: in: This is the sum of the short-circuit currents output by each distributed power source; The magnitude of the neutral point voltage of the main transformer is determined by the output current of the distributed power source on the low-voltage side when it is off-grid and the local load, and is independent of the high-voltage side of the main transformer.

4. The transformer neutral point voltage analysis method considering inverter-type distributed photovoltaic storage capacity according to claim 1, characterized in that: Specifically, S3 is: Since the active power output of the photovoltaic energy storage system cannot change instantaneously before and after the line protection operation, the change in the grid-connected voltage of the photovoltaic energy storage system depends on the active power demand of the load and the degree of imbalance in the active power output of the photovoltaic energy storage system. Based on the change in the active power output of the photovoltaic energy storage system, the grid-connected voltage of the photovoltaic energy storage system during short-term off-grid operation is obtained as follows: in: and These are the voltage at the grid connection point of the distributed power source and the rated voltage during grid-connected operation, respectively. P g and P L These represent the total capacity of the photovoltaic and energy storage system connected to the grid, and the active power component of the load, respectively. P pv and P es These represent the active power output of distributed photovoltaic and energy storage, respectively. When the active power demand of the load is greater than the power provided by the photovoltaic and energy storage, the voltage drops and the output current increases; when the active power demand of the load is less than the power provided by the photovoltaic and energy storage, the voltage rises and the output current decreases. Define the variable load ratio as k = P g / P L This represents the ratio of distributed photovoltaic storage power to load power during off-grid operation, taken as the rated phase voltage on the high-voltage side of the main transformer. Using the base value, i.e., 1.0 pu, the per-unit value of the transformer neutral point voltage is obtained as follows: The neutral point voltage of a transformer in the short time after the line protection trips is a function of the ratio k of the total photovoltaic and energy storage capacity to the active power demand of the load, and the ratio m of the maximum output current of the distributed power source to the rated current. After setting the gap breakdown voltage, the critical load ratio value that makes the neutral point overvoltage can be obtained according to the above formula.

5. The transformer neutral point voltage analysis method considering inverter-type distributed photovoltaic-storage capacity according to claim 1, characterized in that: Specifically, S4 is: When distributed photovoltaic and energy storage are disconnected from the main grid, distributed photovoltaic adopts PQ control, while energy storage should switch to V / f control. It needs to provide reference voltage and frequency for the entire microgrid system to ensure the stable operation of the microgrid. Energy storage plays the role of power balancing and can be mainly divided into two states: charging and discharging. During discharge, due to the V / f control, the energy storage can effectively regulate its own output power to ensure the balance between the total output power of the distributed power source and the load, and there will be no neutral point overvoltage. The charging status is represented as follows: P pv >P L Distributed photovoltaic (PV) systems, after supplying the load, have surplus power that needs to be charged by energy storage. If the energy storage capacity can effectively absorb the surplus power after the PV system has met the load demand, there will be no power transmission to the upstream line. However, if the energy storage cannot fully absorb the surplus power, it may further increase the neutral point voltage, threatening the neutral point insulation. This can be described by the following formula: ΔP g =P pv -(P L +P es )>0 The load power is expressed as: According to relevant regulations, a certain proportion of energy storage must be configured when photovoltaic power is connected to the grid. Let the ratio of the two be x, then the above formula can be expressed as: The root cause of transformer neutral point overvoltage is that the output power of distributed generation exceeds the load when it is off-grid. Based on the load ratio critical value obtained in S3, assuming the neutral point gap breakdown voltage is 1.04 pu, an objective function can be obtained regarding the distributed photovoltaic capacity, energy storage ratio, and load ratio: s.t.P pv >0,k≥1,0<x≤1 By setting F≥0, the range of energy storage ratio and load ratio during neutral point overvoltage can be calculated. Regardless of the photovoltaic capacity, the critical value of x that leads to neutral point overvoltage can be expressed as: The proportion of energy storage capacity is closely related to the load ratio. The critical value of the proportion of energy storage capacity that leads to neutral point overvoltage is calculated.

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