New energy off-network risk assessment method and device for direct current layered access system

Abstract

The invention discloses a new energy off-network risk assessment method and device for a direct-current layered access system, and belongs to the technical field of power system control, and the method comprises the steps: enabling all alternating-current systems, loads and new energy units in a receiving-end power grid to be equivalent to voltage sources with impedance; and substituting the power signal and the equivalent voltage source parameter into a receiving-end power grid voltage expression of the direct-current layered access system to calculate and obtain a first transient low voltage of a receiving-end commutation bus, thereby calculating a first evaluation index of a new energy off-network risk for judging whether the system generates new energy low-power-on and off-network. According to the method, on the basis of the AC-DC-new energy power coupling relation in the receiving-end power grid, the transient voltage of the receiving-end commutation bus is calculated under the DC locking condition, then the evaluation index of the new energy off-network risk is obtained, and meanwhile, the new energy off-network risk possibly occurring in the system after the DC locking occurs is evaluated on the basis of the causal relation between cascading failures. The strong impact of direct current blocking on an alternating current-direct current-new energy system is revealed.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system control, and more specifically, relates to a method and device for evaluating the new energy disconnection risk of a DC hierarchical access system. Background Art

[0002] UHV DC transmission technology is the key to promoting the large-scale development and long-distance transmission of clean energy. However, since the UHV DC project is still in the initial stage and the continuous construction of the DC hierarchical access system, the AC-DC coupling characteristics have become very complex. AC-DC coupling, AC-AC coupling, and DC-DC coupling may expand the fault range and cause a large impact on the receiving-end AC grid. As a very serious fault in DC transmission, DC blocking will cause power deficits and surpluses, which will have an extremely serious impact on the safe and stable operation of the AC system. Therefore, studying the DC blocking mechanism and its impact is of great significance to the safe and stable operation of the power system.

[0003] With the continuous increase in the new energy penetration rate in the power system, the receiving-end AC system with the original strong DC and weak AC problems and the weak sending-end AC system with a weak grid structure and insufficient reactive power support capacity in the DC hierarchical access system have further reduced the ability to resist large-power impacts. The occurrence of a DC system blocking fault will cause the reactive power imbalance to interact and transfer among the AC-DC-new energy systems, resulting in a large voltage drop at the receiving-end system. In severe cases, it may cause large-scale chain disconnection of new energy units in the vicinity of the DC.

[0004] In summary, DC blocking will bring a huge impact to the system. Local new energy units may enter the high-voltage or low-voltage ride-through stage due to the fault impact, and even may experience disconnection. The occurrence of these situations will further aggravate the degree of voltage instability of the receiving-end grid and expand the impact range, playing a key role in promoting the evolution of power system cascading faults. However, there is a lack of technical solutions in the prior art to analyze the system operation risk after DC blocking, resulting in low operation safety of the power system. Summary of the Invention

[0005] In view of the above deficiencies or improvement requirements of the prior art, the present invention provides a method and device for evaluating the new energy disconnection risk of a DC hierarchical access system, aiming to solve the technical problem that the lack of means for evaluating the system operation risk after DC blocking in the prior art leads to low operation safety of the power system.

[0006] To achieve the above object, according to one aspect of the present invention, a method for evaluating the new energy disconnection risk of a DC hierarchical access system is provided, including:

[0007] S1: After the DC blocking in the DC hierarchical access system, collect the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively; Equivalent all AC systems, loads, and new energy units in the receiving-end power grid to a voltage source with impedance to obtain the equivalent voltage source parameters;

[0008] S2: Substitute the power signals and the equivalent voltage source parameters into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the first transient low voltage of the receiving-end converter bus;

[0009] S3: Use the first transient low voltage to calculate the first evaluation index of the new energy tripping risk;

[0010] S4: If the first evaluation index is lower than the first low voltage threshold, it is determined that the system has new energy low-ride-through tripping; Among them, if the new energy low-ride-through tripping is single-end new energy low-ride-through tripping, obtain the corresponding power signals and equivalent voltage source parameters after tripping, and substitute them into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the second transient low voltage of the receiving-end converter bus, and use the second transient low voltage to calculate the second evaluation index of the new energy tripping risk; If the second evaluation index is lower than the second low voltage threshold, it is determined that the system has cascading tripping.

[0011] Further, collecting the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively in S1 includes: Based on the power balance equation of the DC hierarchical access system on the first layer Collect the corresponding power signals; Among them, P ac1 is the active power absorbed by the combined AC power grid and load corresponding to the converter bus on the first layer, Q ac1 is the reactive power absorbed by the combined AC power grid and load corresponding to the converter bus on the first layer, P dc1 is the active power generated by the DC system corresponding to the converter bus on the first layer, Q dc1 is the reactive power absorbed by the inverter station of the DC system corresponding to the converter bus on the first layer, P new1i is the active power generated by the i-th new energy unit corresponding to the converter bus on the first layer, Q new1i is the reactive power generated by the i-th new energy unit corresponding to the converter bus on the first layer, Q f1 is the reactive power generated by the AC filter corresponding to the converter bus on the first layer, P acac 、Q acac are the active power and reactive power absorbed by the converter bus on the first layer through the AC tie line.

[0012] Further, the power signals of the AC system, the tie line, the DC system, the new energy system, and the reactive power compensation device collected in S1 include: based on the power balance equation of the DC hierarchical access system on the second layer collect the corresponding power signals; where, P ac2 is the active power absorbed by the combined AC power grid and load corresponding to the converter bus on the second layer, Q ac2 is the reactive power absorbed by the combined AC power grid and load corresponding to the converter bus on the second layer, P dc2 is the active power generated by the DC system corresponding to the converter bus on the second layer, Q dc2 is the reactive power absorbed by the inverter station of the DC system corresponding to the converter bus on the second layer, P new2i is the active power generated by the i-th new energy unit corresponding to the converter bus on the second layer, Q new2i is the reactive power generated by the i-th new energy unit corresponding to the converter bus on the second layer, Q f2 is the reactive power generated by the AC filter corresponding to the converter bus on the second layer, is the active power and reactive power transmitted by the converter bus on the second layer through the AC tie line.

[0013] Further, the active power and reactive power of the power loss transmitted on the tie line of the AC system are respectively: where, P loss , Q loss are respectively the active power and reactive power of the power loss transmitted by the AC tie line, U s1 , U s2 are respectively the calculated AC voltages of the converter buses on the first and second layers, δ is the power factor angle of the AC tie line, R l , X l are respectively the equivalent resistance and equivalent reactance of the AC tie line.

[0014] Further, the equivalent of all AC systems, loads, and new energy units in the receiving-end power grid as a voltage source with impedance includes: using to respectively equivalent all AC systems, loads, and new energy units in the receiving-end power grid of the DC hierarchical access system on the first and second layers as a voltage source with impedance; where, U s1 , U s2 are respectively the calculated AC voltages of the converter buses on the first and second layers, U o1 , U o2 are respectively the internal electromotive forces after equivalent of the receiving-end power grids corresponding to the converter buses on the first and second layers, R1 and R2 are respectively the resistance values after equivalent of the receiving-end power grids corresponding to the converter buses on the first and second layers, X1 and X2 are respectively the reactance values after equivalent of the receiving-end power grids corresponding to the converter buses on the first and second layers, P dc1 , Pdc2 are the active powers generated by the DC systems corresponding to the first-layer and second-layer commutation buses respectively, P new1i , P new2i are the active powers generated by the i-th new energy unit corresponding to the first-layer and second-layer commutation buses respectively, Q new1i , Q new2i are the reactive powers generated by the i-th new energy unit corresponding to the first-layer and second-layer commutation buses respectively, Q f1 , Q f2 are the reactive powers generated by the AC filters corresponding to the first-layer and second-layer commutation buses respectively, P acac , Q acac are the active power and reactive power absorbed by the first-layer commutation bus through the AC tie line respectively, are the active power and reactive power generated by the second-layer commutation bus through the AC tie line respectively, are the power factor angles of the DC inverters corresponding to the first-layer and second-layer commutation buses.

[0015] Further, the S3 includes: using respectively represent the first evaluation indexes of the new energy disconnection risks on the first layer and the second layer of the DC hierarchical access system, and the corresponding first low-voltage threshold n(t) is a time-segmented function that changes with time; where U s1 , U s2 are the calculated AC voltages of the first-layer and second-layer commutation buses respectively, U b1 , U b2 are the AC voltage reference values of the first-layer and second-layer commutation buses respectively.

[0016] Further, the second evaluation index for calculating the new energy disconnection risk using the second transient low voltage includes: using respectively represent the second evaluation indexes of the new energy disconnection risks on the first layer and the second layer of the DC hierarchical access system after disconnection, and the corresponding second low-voltage threshold n(t)' is a time-segmented function that changes with time; where U s1 ', U s2 ' are the calculated AC voltages of the first-layer and second-layer commutation buses of the DC hierarchical access system after disconnection respectively, U b1 ', U b2 ' are the AC voltage reference values of the first-layer and second-layer commutation buses of the DC hierarchical access system after disconnection respectively.

[0017] According to another aspect of the present invention, there is provided a new energy disconnection risk assessment device for a DC hierarchical access system, including:

[0018] An acquisition equivalent module is used to acquire the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively after the DC blocking of the DC hierarchical access system; all the AC systems, loads, and new energy units in the receiving-end power grid are equivalent to a voltage source with impedance to obtain the equivalent voltage source parameters;

[0019] A voltage calculation module is used to substitute the power signal and the equivalent voltage source parameters into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the first transient low voltage of the receiving-end converter bus;

[0020] An index calculation module is used to calculate the first evaluation index of the new energy disconnection risk by using the first transient low voltage;

[0021] A risk assessment module is used to determine that the system has new energy low-through disconnection if the first evaluation index is lower than the first low voltage threshold; wherein, if the new energy low-through disconnection is single-end new energy low-through disconnection, the corresponding power signal and equivalent voltage source parameters after disconnection are obtained and substituted into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the second transient low voltage of the receiving-end converter bus, and the second evaluation index of the new energy disconnection risk is calculated by using the second transient low voltage; if the second evaluation index is lower than the second low voltage threshold, it is determined that the system has cascading disconnection.

[0022] According to another aspect of the present invention, a power system is provided, including a memory and a processor, the memory stores a computer program, and when the processor executes the computer program, the steps of the above method are implemented.

[0023] According to another aspect of the present invention, a computer-readable storage medium is provided, on which a computer program is stored, and when the computer program is executed by a processor, the steps of the above method are implemented.

[0024] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following beneficial effects can be achieved:

[0025] (1) The present invention provides a method for evaluating the new energy disconnection risk of a DC hierarchical access system. Based on the power coupling relationship between AC-DC-new energy in the receiving-end power grid, the transient voltage of the receiving-end converter bus is calculated under the condition of DC blocking, and then the evaluation index of the new energy disconnection risk is obtained. At the same time, based on the causal relationship between cascading failures, the possible new energy disconnection risk after DC blocking is evaluated, revealing the strong impact of DC blocking on the AC-DC-new energy system. The innovation of this method lies in the comprehensive analysis and evaluation of the cascading disconnection path after DC blocking in the receiving-end power grid with DC hierarchical access and large-scale new energy access, and the research on the field of receiving-end power grid transient stability, which has been less studied. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the flowchart of the new - energy islanding evaluation method after DC blocking in the DC hierarchical access system provided in Embodiment 1 of the present invention;

[0027] Figure 2 This is the topology diagram of the direct - drive wind farm integrated into the UHV DC hierarchical access system in the PSCAD simulation scenario provided in Embodiment 1 of the present invention;

[0028] Figure 3 This is the diagram of the chain - evolution process of key events including DC commutation failure, DC blocking, and new - energy high / low - voltage ride - through and islanding after an AC fault occurs in the receiving - end power grid with DC hierarchical access and new - energy access in Embodiment 1 of the present invention;

[0029] Figure 4 This is the variation curves of the active power, voltage and current of the DC system, the voltage waveform of the receiving - end commutation bus, the variation curve of the inverter extinction angle γ, the output power variation curve of the direct - drive wind farm, the reactive power compensation amount variation curve of the AC filter, and the dynamic process of fault chain - evolution during the process of commutation failure, new - energy islanding, and DC bipolar blocking occurring step by step in the system when a short - circuit fault occurs in the receiving - end power grid in Embodiment 1 of the present invention. Detailed implementation manners

[0030] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0031] Embodiment 1

[0032] As Figure 1As shown in the figure, this example provides a new energy disconnection risk assessment method for a DC hierarchical access system, including: S1: After the DC blocking in the DC hierarchical access system, collect the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively; Equivalent all AC systems, loads, and new energy units in the receiving-end power grid as a voltage source with impedance to obtain the equivalent voltage source parameters. S2: Substitute the power signal and the equivalent voltage source parameters into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the first transient low voltage of the receiving-end converter bus. S3: Use the first transient low voltage to calculate the first evaluation index of the new energy disconnection risk. S4: If the first evaluation index is lower than the first low voltage threshold, it is determined that the system has a new energy low-ride-through disconnection; among them, if the new energy low-ride-through disconnection is a single-end new energy low-ride-through disconnection, obtain the corresponding power signal and equivalent voltage source parameters after disconnection, and substitute them into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the second transient low voltage of the receiving-end converter bus, and use the second transient low voltage to calculate the second evaluation index of the new energy disconnection risk; if the second evaluation index is lower than the second low voltage threshold, it is determined that the system has a cascading disconnection. It should be noted that in this embodiment, the following descriptions are all exemplified with the first layer being 500 kV and the second layer being 1000 kV.

[0033] Specifically, collect the power signals of the AC, DC, new energy, and reactive power compensation devices according to the power balance equation of the receiving-end converter bus, including the power signals of the AC grid, AC tie lines, and loads, the DC transmission active power and the reactive power absorbed by the DC inverter station, the output power signals of each new energy unit, and the reactive power output signals of the AC filters; According to the idea of electromechanical transient simulation, equivalent all AC systems (including loads) and new energy units in the receiving-end power grid as a voltage source with impedance. First, determine the equivalent impedance of the voltage source according to the voltage fluctuation amplitude of the receiving-end converter bus under small disturbances, and then jointly determine the equivalent electromotive force of the voltage source according to the equivalent impedance and the steady-state voltage value of the receiving-end converter bus; Substitute the power signal and the equivalent voltage source parameters into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the transient low voltage of the receiving-end converter bus, and substitute the transient voltage into the low voltage threshold corresponding to the new energy low-ride-through disconnection to evaluate the new energy disconnection risk. If the evaluation index is less than or equal to the disconnection low voltage threshold, it is regarded that the system has a new energy low-ride-through disconnection; If a single-end new energy low-ride-through disconnection occurs, collect the power signal and the equivalent voltage source parameters after disconnection again and substitute them into the receiving-end power grid voltage expression of the DC hierarchical access system to calculate the transient low voltage of the receiving-end converter bus, and substitute it into the low voltage threshold corresponding to the new energy low-ride-through disconnection for the second time to evaluate the cascading disconnection risk.

[0034] Further, in S1, collecting the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively includes: Based on the power balance equation of the DC hierarchical access system on the first layer Collect the corresponding power signals. Further, in S1, collect the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively, including: based on the power balance equation of the DC hierarchical access system on the second layer Collect the corresponding power signals.

[0035] Specifically, in the power balance relationship of the receiving-end converter bus of the DC hierarchical access system, there are two receiving-end converter buses in a DC hierarchical access system, and there is an AC tie line coupling. The power balance equations of the receiving-end converter buses can be expressed by the following two sets of equations:

[0036]

[0037] Among them, P ac1 、P ac2 are the active powers absorbed by the combined AC power grid and load corresponding to the 500kV and 1000kV converter buses respectively, Q ac1 、Q ac2 are the reactive powers absorbed by the combined AC power grid and load corresponding to the 500kV and 1000kV converter buses respectively, P dc1 、P dc2 are the active powers generated by the DC system corresponding to the 500kV and 1000kV converter buses respectively, Q dc1 、Q dc2 are the reactive powers absorbed by the inverter stations of the DC system corresponding to the 500kV and 1000kV converter buses respectively, P new1i 、P new2i are the active powers generated by the i-th new energy unit corresponding to the 500kV and 1000kV converter buses respectively, Q new1i 、Q new2i are the reactive powers generated by the i-th new energy unit corresponding to the 500kV and 1000kV converter buses respectively, Q f1 、Q f2 are the reactive powers generated by the AC filters corresponding to the 500kV and 1000kV converter buses respectively, P acac 、Q acac are the active power and reactive power absorbed by the 500kV converter bus through the AC tie line respectively, are the active power and reactive power generated by the 1000kV converter bus through the AC tie line respectively.

[0038] The above formula corresponds to the power balance equation of the 500 kV converter bus, and the following formula corresponds to the power balance equation of the 1000 kV converter bus. It consists of five parts: the AC power grid (including loads), the DC system, new energy units, AC filters, and AC tie lines. In terms of power balance, there is AC-DC coupling in the DC hierarchical access system: one part is the coupling of the AC tie line, where there is power exchange between two AC regions and the exchanged power changes with the change of the converter bus voltage; the other part is the series coupling of the DC system inverters. Adjacent inverters correspond to the same DC current, so there is coupling in the corresponding DC power. Especially for DC blocking faults, the blocking of one inverter will inevitably lead to the simultaneous blocking of adjacent inverters.

[0039] Furthermore, the active power and reactive power of the power loss transmitted on the tie line of the AC system are respectively: where, P loss and Q loss are respectively the active power and reactive power of the power loss transmitted on the AC tie line, U s1 and U s2 are respectively the calculated AC voltages of the first-layer and second-layer converter buses, δ is the power factor angle of the AC tie line, R l and X l are respectively the equivalent resistance and equivalent reactance of the AC tie line.

[0040] Specifically, when calculating the reactive power compensation amount of the AC filter, the reactive power generated by the AC filter before the fault and the reactive power absorbed by the DC system inverter are balanced with each other, and the switching of the AC filter is not considered within a short time after the DC blocking occurs. The reactive power compensation amount is proportional to the square of the bus voltage. The reactive power compensation amount of the AC filter can be expressed by the following set of equations: Q f1 and Q f2 are respectively the reactive powers generated by the AC filters corresponding to the 500 kV and 1000 kV converter buses, U s1 and U s2 are respectively the calculated AC voltages of the 500 kV and 1000 kV converter buses, U e1 and U e2 are respectively the rated values of the AC voltages of the 500 kV and 1000 kV converter buses, P dc0 is the rated value of the active power transmitted by the DC system, is the power factor angle before the DC system fault.

[0041] Among them, when calculating the power loss value of the AC tie line transmission, due to the equivalent impedance of the transformer and the line impedance of the AC tie line, there is power loss during the power transmission process. The power loss value of the AC tie line transmission can be expressed by the following set of equations: P loss and Qloss are the active power and reactive power of the transmission power loss of the AC tie line, P acac , Q acac are the active power and reactive power absorbed by the 500 kV converter bus through the AC tie line, are the active power and reactive power sent out by the 1000 kV converter bus through the AC tie line.

[0042] Among them, only considering the voltages at both ends of the AC tie line and the line impedance, the transmission power loss value of the AC tie line can also be expressed by the following set of equations:

[0043]

[0044] P loss , Q loss are the active power and reactive power of the transmission power loss of the AC tie line, U s1 , U s2 are the calculated AC voltages of the 500 kV and 1000 kV converter buses respectively, δ is the power factor angle of the AC tie line, R l , X l are the equivalent resistance and equivalent reactance of the AC tie line respectively. Since the line reactance value in the power system is much larger than the resistance value, it can be simplified as the above formula.

[0045] Specifically, the transient low voltage of the receiving-end converter bus may cause the new energy to trip off the grid during low voltage ride-through. Compared with the DC sending end, the degree to which the reactive power compensation device at the receiving end cannot respond to the reactive power required by the sending-end converter station in a timely manner is relatively light, and there is no serious overvoltage problem in the receiving-end power grid. On the contrary, there is a serious transient low voltage problem when a fault occurs at the receiving end, and the transient overvoltage is not sufficient to cause the new energy to trip off the grid during high voltage ride-through. The characteristic of the new energy connected to the receiving end tripping off the grid in a chain is manifested as continuous low voltage ride-through chain tripping off the grid. The main mechanism is as follows: After the DC is blocked, a large amount of power deficit is directly caused, triggering problems such as frequency and voltage. The voltage of the receiving-end converter bus drops significantly. After the short-circuit fault is cleared, the voltage recovers partially and fluctuates continuously. As the low voltage lasts for too long, it will lead to the chain tripping off the grid of the new energy.

[0046] Furthermore, the voltage expression of the receiving-end converter bus in the DC hierarchical access system. After the receiving-end power grid is equivalent to an impedance voltage source, the voltage expression of the receiving-end converter bus in the DC hierarchical access system can be expressed by the following set of equations:

[0047]

[0048] Among them, U s1 , U s2 are the calculated AC voltages of the 500 kV and 1000 kV converter buses respectively, U o1 , U o2They are the equivalent electromotive forces of the receiving-end power grids corresponding to the 500 kV and 1000 kV commutation buses respectively. R1 and R2 are the equivalent resistance values of the receiving-end power grids corresponding to the 500 kV and 1000 kV commutation buses respectively. X1 and X2 are the equivalent reactance values of the receiving-end power grids corresponding to the 500 kV and 1000 kV commutation buses respectively. P dc1 and P dc2 are the active powers generated by the DC systems corresponding to the 500 kV and 1000 kV commutation buses respectively. P new1i and P new2i are the active powers generated by the i-th new energy unit corresponding to the 500 kV and 1000 kV commutation buses respectively. Q new1i and Q new2i are the reactive powers generated by the i-th new energy unit corresponding to the 500 kV and 1000 kV commutation buses respectively. Q f1 and Q f2 are the reactive powers generated by the AC filters corresponding to the 500 kV and 1000 kV commutation buses respectively. P acac and Q acac are the active power and reactive power absorbed by the 500 kV commutation bus through the AC tie line respectively, are the active power and reactive power generated by the 1000 kV commutation bus through the AC tie line respectively, is the power factor angle of the DC inverter corresponding to the 500 kV and 1000 kV commutation buses.

[0049] Furthermore, S3 includes: using to represent the first evaluation indexes of the new energy disconnection risks on the first layer and the second layer of the DC hierarchical access system respectively, and the corresponding first low voltage threshold n(t) is a piecewise function of time; where U s1 and U s2 are the calculated AC voltages of the commutation buses on the first layer and the second layer respectively, and U b1 and U b2 are the AC voltage reference values of the commutation buses on the first layer and the second layer respectively. Furthermore, using the second transient low voltage to calculate the second evaluation index of the new energy disconnection risk includes: using to represent the second evaluation indexes of the new energy disconnection risks on the first layer and the second layer of the DC hierarchical access system after disconnection respectively, and the corresponding second low voltage threshold n(t)' is a piecewise function of time; where U s1 ' and U s2 ' are the calculated AC voltages of the commutation buses on the first layer and the second layer of the DC hierarchical access system after disconnection respectively, and U b1 ' and U b2 ' are the AC voltage reference values of the commutation buses on the first layer and the second layer of the DC hierarchical access system after disconnection respectively.

[0050] Specifically, after DC blocking, for the low-voltage threshold corresponding to the new energy low-voltage ride-through and disconnection from the grid, the new energy low-voltage ride-through and disconnection from the grid assessment can be expressed by the following set of equations: Where, U s1 and U s2 are the calculated AC voltages of the 500 kV and 1000 kV converter buses respectively, U b1 and U b2 are the reference values of the AC voltages of the 500 kV and 1000 kV converter buses respectively, and n(t) is the low-voltage threshold corresponding to the new energy low-voltage ride-through and disconnection from the grid that changes with the fault occurrence time, which can be specifically expressed by the following set of equations: Where, t is the system fault occurrence time, and the value of this piecewise function is determined by the new energy unit fault voltage ride-through curve. Once the per-unit value of the receiving-end converter bus voltage is less than or equal to the low-voltage threshold corresponding to the new energy low-voltage ride-through and disconnection from the grid, it can be considered that the new energy low-voltage ride-through ability is insufficient to support the system fault level, and new energy low-voltage ride-through and disconnection from the grid occur.

[0051] The power deficit caused by the new energy disconnection from the grid can be coupled and transmitted through the AC tie line to affect the voltage stability of the adjacent AC area, and further may lead to the aggravation of the new energy low-voltage ride-through degree or even a serious disconnection from the grid fault. Therefore, if new energy low-voltage ride-through and disconnection from the grid occur at one end, it is necessary to collect the power signal and equivalent voltage source parameters after disconnection again and substitute them into the receiving-end grid voltage expression of the DC hierarchical access system to calculate the transient low voltage of the receiving-end converter bus, and then substitute it into the low-voltage threshold corresponding to the new energy low-voltage ride-through and disconnection from the grid for the second time to evaluate the risk of cascading disconnection from the grid.

[0052] Embodiment 2

[0053] This embodiment provides a new energy disconnection from the grid risk assessment device for a DC hierarchical access system, including: a collection and equivalent module, a voltage calculation module, an index calculation module, and a risk assessment module.

[0054] The collection and equivalent module is used to collect the power signals of the AC system, tie line, DC system, new energy system, and reactive power compensation device respectively after DC blocking in the DC hierarchical access system; and equivalent all the AC systems, loads, and new energy units in the receiving-end grid into a voltage source with impedance to obtain equivalent voltage source parameters.

[0055] The voltage calculation module is used to substitute the power signal and equivalent voltage source parameters into the receiving-end grid voltage expression of the DC hierarchical access system to calculate the first transient low voltage of the receiving-end converter bus.

[0056] The index calculation module is used to calculate the first assessment index of the new energy disconnection from the grid risk by using the first transient low voltage.

[0057] A risk assessment module is used to determine that the system has experienced new energy low penetration and disconnection from the grid if the first evaluation index is lower than the first low voltage threshold. Among them, if the new energy low penetration and disconnection from the grid is single - end new energy low penetration and disconnection from the grid, the corresponding power signal and equivalent voltage source parameters after disconnection are obtained and substituted into the receiving - end grid voltage expression of the DC hierarchical access system to calculate the second transient low voltage of the receiving - end commutation bus, and the second evaluation index of the new energy disconnection risk is calculated using the second transient low voltage. If the second evaluation index is lower than the second low voltage threshold, it is determined that the system has experienced cascading disconnection from the grid.

[0058] The following is an example for illustration of Embodiment 1 of the present invention. Among them, Figure 2 is the topology diagram of a direct - drive wind farm connected to a UHV DC hierarchical access system in a PSCAD simulation scenario; Figure 3 is the diagram of the chain - evolution process of key events including DC commutation failure, DC blocking, new energy high / low penetration and disconnection from the grid, etc. after an AC fault occurs in the receiving - end grid with DC hierarchical access and new energy access.

[0059] First, according to the power balance equation of the receiving - end commutation bus, power signals of AC, DC, new energy, and reactive power compensation devices are collected, specifically including power signals of the AC grid, AC tie lines, and loads, DC transmitted active power and reactive power absorbed by the DC inverter station, output power signals of each new energy unit, and reactive power output signals of AC filters. The collected signals are as Figure 4 shown. Among them, Figure 4 in (a) is the voltage curve diagram of each bus and the grid - connection point of the wind farm; Figure 4 in (b) is the output power curve diagram of each wind farm; Figure 4 in (c) is the exchanged power curve diagram of the AC system and AC tie lines; Figure 4 in (d) is the electrical quantity change curve diagram of the DC system; Figure 4 in (e) is the commutation - angle curve diagram of the converter station; Figure 4 in (f) is the schematic diagram of the fault chain - evolution process.

[0060] Then, according to the idea of electromechanical transient simulation, all AC systems (including loads) and new energy units in the receiving - end grid are equivalent to a voltage source with impedance. First, the equivalent impedance of the voltage source is determined according to the voltage fluctuation amplitude of the receiving - end commutation bus under small disturbances, and then the equivalent electromotive force of the voltage source is determined jointly according to the equivalent impedance and the steady - state voltage value of the receiving - end commutation bus. It is calculated and determined that the equivalent electromotive force of the 500kV commutation bus corresponding voltage source is 525kV, and the equivalent impedance is 0.05Ω. It is determined that the equivalent electromotive force of the 1000kV commutation bus corresponding voltage source is 1050kV, and the equivalent impedance is 0.1Ω.

[0061] Secondly, substitute the power signal and equivalent voltage source parameters into the receiving-end grid voltage expression of the DC hierarchical access system to calculate the transient low voltage of the receiving-end commutation bus. Substitute the transient voltage into the low voltage threshold corresponding to the new energy low voltage ride-through to evaluate the new energy tripping risk. If the evaluation index is less than or equal to the tripping low voltage threshold, it is regarded that the system has new energy low voltage ride-through tripping; combined with Figure 4 Analysis: After the short-circuit fault of the 500 kV commutation bus occurs at 15.0 s, the voltages of each commutation bus, the grid connection point voltage of the wind farm, and the DC voltage decrease rapidly. Subsequently, consecutive commutation failure faults occur at the 500 kV converter station and the 1000 kV converter station, and the number of consecutive commutation failures reaches 3 times at 15.042 s, resulting in DC blocking. Since the positive and negative DC converter stations of the DC hierarchical access system share the same commutation bus and the transient voltages are the same, bipolar blocking occurs, causing a large power deficit of 12,000 MW. After blocking, the power transmitted by the DC transmission line is directly interrupted to 0. At this time, the reactive power consumption of the converter station drops suddenly to 0. According to the receiving-end grid voltage expression of the DC hierarchical access system, the voltage of the 500 kV commutation bus drops to the lowest 0.15 p.u., and the subsequent voltage continues to fluctuate, with the amplitude gradually decreasing and stabilizing between 0.25 and 0.57 p.u. At the same time, Wind Farm 1 directly connected to the 500 kV commutation bus trips due to low voltage ride-through failure at 15.056 s because the calculated voltage is lower than 0.2 p.u. Wind Farm 2 with a longer electrical distance enters the low voltage ride-through stage, with reduced active power output and increased reactive power output, and has not tripped directly. After the short-circuit fault is cleared at 15.1 s, since the reactive power consumption of the DC converter station in the receiving-end system is 0, and the reactive power support provided by the AC system, AC filters, and Wind Farm 2 is much greater than the reactive power consumption of the receiving-end system, according to the receiving-end grid voltage expression of the DC hierarchical access system, the voltage of the commutation bus rises significantly, reaching a maximum of 0.94 p.u. Subsequently, due to the blocking of the DC system and the large power deficit caused by the low voltage ride-through failure tripping of Wind Farm 1, the calculated voltage drops back and continues to fluctuate, gradually stabilizing between 0.25 and 0.57 p.u. During this process, since the grid connection voltage of Wind Farm 2 is relatively low and the low voltage duration is too long, it trips due to low voltage ride-through failure at 16.18 s, which belongs to the cascading tripping of new energy in the receiving end caused by DC blocking. The entire fault cascading evolution process is as shown in Figure 4 (f). To sum up, substitute the transient voltage into the low voltage threshold corresponding to the new energy low voltage ride-through to evaluate the new energy tripping risk. The low voltage thresholds corresponding to the two new energy low voltage ride-throughs are both less than the tripping low voltage threshold, which is regarded as new energy low voltage ride-through tripping occurring at the 500 kV commutation bus. The simulation results and the evaluation results confirm each other.

[0062] Finally, if new energy low penetration and grid disconnection occur at a single end, the power signal after grid disconnection and the equivalent voltage source parameters are collected again and substituted into the expression of the receiving-end grid voltage of the DC hierarchical access system to calculate the transient low voltage of the receiving-end converter bus. Then, substitute it into the low voltage threshold corresponding to new energy low penetration and grid disconnection for the second time to evaluate the risk of cascading grid disconnection. As Figure 4 shown in (a) of , the low voltage thresholds corresponding to new energy low penetration and grid disconnection calculated by the second substitution are all higher than the grid disconnection low voltage threshold, which is regarded as no new energy low penetration and grid disconnection occurring at the 1000 kV converter bus, and the simulation results and evaluation results confirm each other.

[0063] The new energy grid disconnection assessment method for the DC hierarchical access system after DC blocking provided by the embodiment of the present invention constructs a PSCAD model of a direct-drive wind farm and a UHV DC hierarchical access system, provides a new energy grid disconnection assessment method for the DC hierarchical access system after DC blocking. Based on the power coupling relationship of AC-DC-new energy in the receiving-end grid, the calculation expression of the transient voltage of the receiving-end converter bus is deduced under the condition of DC blocking. At the same time, based on the causal relationship between cascading faults, the risk of new energy grid disconnection that may occur in the system after DC blocking is evaluated, revealing the strong impact of DC blocking on the AC-DC-new energy system. The innovation of this method lies in the comprehensive analysis and evaluation of the cascading grid disconnection path after DC blocking in the receiving-end grid with DC hierarchical access and large-scale new energy access, and the research on the field of transient stability of the receiving-end grid, which has been less studied.

[0064] Embodiment 3

[0065] This embodiment provides a power system, including a memory and a processor. The memory stores a computer program, and when the processor executes the computer program, the steps of the above method are implemented.

[0066] Embodiment 4

[0067] This embodiment provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the steps of the above method are implemented.

[0068] It is easy for those skilled in the art to understand that the above are only preferred embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A new energy disconnection risk assessment method for a DC hierarchical access system, characterized in that Including: S1: After the DC blocking of the DC hierarchical access system, collect the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively; Equivalent all AC systems, loads, and new energy units in the receiving-end power grid as a voltage source with impedance to obtain the parameters of the equivalent voltage source; S2: Substitute the power signal and the parameters of the equivalent voltage source into the voltage expression of the receiving-end power grid of the DC hierarchical access system to calculate the first transient low voltage of the receiving-end converter bus; S3: Calculate the first evaluation index of the new energy disconnection risk using the first transient low voltage; S4: If the first evaluation index is lower than the first low voltage threshold, it is determined that the system has a new energy low-ride-through disconnection; among them, if the new energy low-ride-through disconnection is a single-end new energy low-ride-through disconnection, obtain the corresponding power signal and equivalent voltage source parameters after disconnection, and substitute them into the voltage expression of the receiving-end power grid of the DC hierarchical access system to calculate the second transient low voltage of the receiving-end converter bus, and calculate the second evaluation index of the new energy disconnection risk using the second transient low voltage; If the second evaluation index is lower than the second low voltage threshold, it is determined that the system has a cascading disconnection.

2. The new energy disconnection risk assessment method for the DC hierarchical access system according to claim 1, characterized in that In S1, the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device are collected, including: based on the power balance equation of the DC hierarchical access system on the first layer Collect the corresponding power signals; Among them, P ac1 is the active power absorbed after combining the AC power grid and load corresponding to the first-layer commutation bus, Q ac1 is the reactive power absorbed after combining the AC power grid and load corresponding to the first-layer commutation bus, P dc1 is the active power generated by the DC system corresponding to the first-layer commutation bus, Q dc1 is the reactive power absorbed by the inverter station of the DC system corresponding to the first-layer commutation bus, P new1i is the active power generated by the i-th new energy unit corresponding to the first-layer commutation bus, Q new1i is the reactive power generated by the i-th new energy unit corresponding to the first-layer commutation bus, Q f1 is the reactive power generated by the AC filter corresponding to the first-layer commutation bus, P acac 、Q acac are the active power and reactive power absorbed by the first-layer commutation bus through the AC tie line.

3. The new energy off-grid risk assessment method for the DC hierarchical access system according to claim 1, characterized in that, In S1, the power signals of the AC system, the tie line, the DC system, the new energy system, and the reactive power compensation device are collected, including: based on the power balance equation of the DC hierarchical access system on the second layer Collect the corresponding power signals; Among them, P ac2 is the active power absorbed after the AC power grid and load corresponding to the second-layer commutation bus are combined, Q ac2 is the reactive power absorbed after the AC power grid and load corresponding to the second-layer commutation bus are combined, P dc2 is the active power generated by the DC system corresponding to the second-layer commutation bus, Q dc2 is the reactive power absorbed by the inverter station of the DC system corresponding to the second-layer commutation bus, P new2i is the active power generated by the i-th new energy unit corresponding to the second-layer commutation bus, Q new2i is the reactive power generated by the i-th new energy unit corresponding to the second-layer commutation bus, Q f2 is the reactive power generated by the AC filter corresponding to the second-layer commutation bus, is the active power and reactive power transmitted through the AC tie line by the second-layer commutation bus.

4. The new energy grid - disconnection risk assessment method for the DC hierarchical access system according to claim 2 or 3, characterized in that, The active power and reactive power of the power transmission loss on the tie line of the AC system are respectively: Among them, P loss and Q loss are the active power and reactive power of the transmission power loss of the AC tie line respectively, U s1 and U s2 are the calculated AC voltages of the first-layer and second-layer converter buses respectively, δ is the power factor angle of the AC tie line, R l and X l are the equivalent resistance and equivalent reactance of the AC tie line respectively.

5. The new energy disconnection risk assessment method for the DC hierarchical access system according to claim 1, characterized in that The equivalent of all AC systems, loads, and new energy units in the receiving-end power grid as a voltage source with impedance includes: Utilize Equivalently represent all AC systems, loads, and new energy units in the receiving-end power grid of the DC hierarchical access system on the first layer and the second layer as voltage sources with impedances respectively; Among them, U s1 and U s2 are the calculated AC voltages of the first-layer and second-layer commutation buses respectively. U o1 and U o2 are the electromotive forces after equivalent of the receiving-end power grids corresponding to the first-layer and second-layer commutation buses respectively. R1 and R2 are the resistance values after equivalent of the receiving-end power grids corresponding to the first-layer and second-layer commutation buses respectively. X1 and X2 are the reactance values after equivalent of the receiving-end power grids corresponding to the first-layer and second-layer commutation buses respectively. P dc1 and P dc2 are the active powers generated by the DC systems corresponding to the first-layer and second-layer commutation buses respectively. P new1i and P new2i are the active powers generated by the i-th new energy unit corresponding to the first-layer and second-layer commutation buses respectively. Q new1i and Q new2i are the reactive powers generated by the i-th new energy unit corresponding to the first-layer and second-layer commutation buses respectively. Q f1 and Q f2 are the reactive powers generated by the AC filters corresponding to the first-layer and second-layer commutation buses respectively. P acac and Q acac are the active power and reactive power absorbed by the first-layer commutation bus through the AC tie line respectively. are the active power and reactive power generated by the second-layer commutation bus through the AC tie line respectively. is the power factor angle of the DC inverter corresponding to the first-layer and second-layer commutation buses.

6. The new energy disconnection risk assessment method for the DC hierarchical access system according to claim 1, characterized in that The S3 includes: using respectively represent the first evaluation index of the new energy disconnection risk of the DC hierarchical access system on the first layer and the second layer, and the corresponding first low voltage threshold n(t) is a time-segmented function that changes with time; where U s1 , U s2 are the calculated AC voltages of the converter buses on the first layer and the second layer respectively, and U b1 , U b2 are the AC voltage reference values of the converter buses on the first layer and the second layer respectively.

7. The new energy disconnection risk assessment method for the DC hierarchical access system according to claim 6, wherein The second evaluation index for calculating the new energy disconnection risk using the second transient low voltage includes: using respectively represent the second evaluation indexes of the new energy disconnection risk on the first layer and the second layer of the DC hierarchical access system after disconnection, and the corresponding second low voltage threshold n(t)' is a time-segmented function that changes with time; where U s1 ', U s2 ' are the calculated AC voltages of the converter buses on the first layer and the second layer of the DC hierarchical access system after disconnection, and U b1 ', U b2 ' are the reference values of the AC voltages of the converter buses on the first layer and the second layer of the DC hierarchical access system after disconnection.

8. A new energy disconnection risk assessment device for a DC hierarchical access system, characterized in that Including: A collection module, used to collect the power signals of the AC system, tie lines, DC system, new energy system, and reactive power compensation device respectively after the DC blocking of the DC hierarchical access system; Equivalent all AC systems, loads, and new energy units in the receiving-end power grid as a voltage source with impedance to obtain the parameters of the equivalent voltage source; A voltage calculation module, used to substitute the power signal and the parameters of the equivalent voltage source into the voltage expression of the receiving-end power grid of the DC hierarchical access system to calculate the first transient low voltage of the receiving-end converter bus; An index calculation module, used to calculate the first evaluation index of the new energy disconnection risk using the first transient low voltage; A risk assessment module, used to determine that the system has a new energy low-ride-through disconnection if the first evaluation index is lower than the first low voltage threshold; among them, if the new energy low-ride-through disconnection is a single-end new energy low-ride-through disconnection, obtain the corresponding power signal and equivalent voltage source parameters after disconnection, and substitute them into the voltage expression of the receiving-end power grid of the DC hierarchical access system to calculate the second transient low voltage of the receiving-end converter bus, and calculate the second evaluation index of the new energy disconnection risk using the second transient low voltage; If the second evaluation index is lower than the second low voltage threshold, it is determined that the system has a cascading disconnection.

9. A power system includes a memory and a processor, the memory storing a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the method described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the method described in any one of claims 1 to 7.

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