Method for evaluating dc carrying capacity security stability domain considering multiple transient characteristic quantities

Abstract

This invention provides a method for evaluating the safe and stable domain of DC carrying capacity, taking into account multiple transient characteristics. When a fault occurs on the inverter side, the DC transmission power under fault conditions is obtained based on system constant parameters and the ratio of wind to thermal power output. Voltage constraints are obtained based on the sending-end transient overvoltage, rectifier transmission power, synchronous machine voltage, sending-end equivalent reactance, and the proportion of thermal power output. The allowable decrease in DC power is obtained based on the initial mechanical power, sending-end equivalent reactance, maximum equivalent mechanical power, and the steady-state active power of the synchronous machine. Based on a stability domain coordinate system, a voltage stability domain is obtained based on the voltage constraints; a power angle stability domain is obtained based on the proportion of thermal power reactance and the allowable decrease in DC power; and a frequency stability domain is obtained based on the DC transmission power under fault conditions. The intersection of these three domains yields the safe and stable domain for evaluating DC carrying capacity. This invention obtains a safe and stable domain diagram from three perspectives: transient overvoltage, transient power angle, and transient frequency, thus visualizing DC carrying capacity.

Description

Technical Field

[0001] This invention relates to the field of safety analysis technology for new energy power stations, specifically to a method for evaluating the safe and stable domain of DC carrying capacity that takes into account multiple transient characteristics. Background Technology

[0002] To achieve the goal of "carbon peaking and carbon neutrality," my country has significantly increased its installed wind power capacity. In recent years, the construction of large-scale wind power bases, focusing on desert, Gobi, and arid regions, has accelerated markedly, with investment increasing dramatically. By the end of 2022, my country's investment in desert, Gobi, and arid wind power construction reached 154.7 billion yuan, a year-on-year increase of 326.7%. With the continued rapid growth of new energy installed capacity in desert, Gobi, and arid regions, the DC carrying capacity of these new energy bases faces significant challenges. The construction of large-scale AC / DC hybrid transmission systems in these regions places considerable pressure on the DC carrying capacity of the new energy bases. Currently, the DC carrying capacity of new energy bases is mainly limited by safety and stability constraints, such as transient overvoltage, frequency, and power angle. When the grid disturbs the system voltage fluctuations, new energy units are prone to grid disconnection, triggering cascading accidents and disrupting system safety and stability. Therefore, overvoltage, power angle instability, and frequency instability issues in large-scale new energy projects have become serious constraints on ultra-high voltage direct current (UHVDC) transmission, necessitating the search for corresponding quantitative indicators. Summary of the Invention

[0003] This invention addresses the problems existing in the prior art by providing a method for evaluating the DC carrying capacity safety and stability domain of a new energy DC transmission system during the transient period of DC commutation failure from three perspectives: transient overvoltage, transient power angle, and transient frequency.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for evaluating the safe and stable domain of DC carrying capacity that takes into account multiple transient characteristics, comprising the following steps: obtaining transient voltage thresholds;

[0005] Based on the acquisition of system constant parameters and equivalent reactance of the sending end of the new energy DC transmission system;

[0006] During normal operation, the steady-state active power of the synchronous machine of the new energy DC transmission system is obtained;

[0007] The proportion of thermal power reactance in the new energy DC transmission system is obtained based on the equivalent reactance at the sending end.

[0008] After a fault occurs on the inverter side of the new energy DC transmission system: the transient overvoltage at the sending end, the active power transmitted by the rectifier, the rectifier power factor angle, the synchronous machine voltage, the initial mechanical power, and the maximum electromagnetic power are obtained respectively.

[0009] The thermal power output ratio, the wind-thermal power output ratio, and the maximum equivalent mechanical power are obtained based on the initial mechanical power and the maximum electromagnetic power, respectively.

[0010] The DC transmission power under fault conditions is obtained based on the system constant parameters and the wind and fire output ratio.

[0011] Voltage constraints are obtained based on the transient overvoltage at the sending end, the active power transmitted by the rectifier, the rectifier power factor angle, the synchronous machine voltage, the equivalent reactance at the sending end, the proportion of thermal power output, and the transient voltage threshold.

[0012] The allowable DC power reduction is obtained based on the initial mechanical power, the equivalent reactance at the sending end, the maximum equivalent mechanical power, and the steady-state active power of the synchronous machine.

[0013] Construct a stable domain coordinate system, where the horizontal axis of the stable domain coordinate system represents the proportion of thermal power output, and the vertical axis of the stable domain coordinate system represents the proportion of power that can be reduced under DC conditions.

[0014] Based on the stability domain coordinate system, the voltage stability domain is obtained according to the voltage constraint, the power angle stability domain is obtained according to the thermal power reactance ratio and the allowable DC power reduction, and the frequency stability domain is obtained according to the DC transmission power under fault conditions.

[0015] The safe stability region is obtained by finding the intersection of the voltage stability region, the power angle stability region, and the frequency stability region.

[0016] The DC carrying capacity of the new energy DC transmission system is evaluated based on the aforementioned safety and stability domain.

[0017] In some embodiments, the sending-end equivalent reactance includes the wind turbine-side equivalent reactance and the synchronizing machine-side equivalent reactance.

[0018] In some embodiments, the thermal power reactance ratio is the proportion of the equivalent reactance on the synchronous machine side in the total reactance at the sending end, and the total reactance at the sending end is the sum of the equivalent reactance on the wind turbine side and the equivalent reactance on the synchronous machine side.

[0019] In some embodiments, the sending-end transient overvoltage is expressed as a per-unit value.

[0020] In some embodiments, the voltage constraint is:

[0021] ;

[0022] In the formula, This represents the per-unit value of the transient overvoltage at the sending end. The active power delivered to the rectifier. The rectifier power factor angle. This is the voltage of the synchronous machine. For the equivalent reactance on the wind turbine side, For the equivalent reactance on the synchronous machine side, Contributing to the forces of wind and fire This is the transient voltage threshold.

[0023] In some embodiments, the system constant parameters include the equivalent damping coefficient, the equivalent unit regulating power coefficient of the synchronous generator set, the time constant of the synchronous generator governor, the equivalent inertial constant of the synchronous generator set, and the steady-state frequency of the system.

[0024] In some embodiments, the DC transmission power under fault conditions includes the DC transmission power at the initial moment of the fault and the DC transmission power at the moment of the frequency extremum.

[0025] In some embodiments, the DC transmission power at the initial moment of the fault is:

[0026] ;

[0027] In the formula, The DC power supplied at the initial moment of the fault. This is the equivalent damping coefficient. The equivalent unit adjustment power coefficient for synchronous generator sets. This is the first intermediate expression. This is the second intermediate expression. This is the third intermediate expression. The time at which the frequency extremum point is located;

[0028] ;

[0029] In the formula, The time constant of the synchronous generator governor. This is the fourth intermediate expression;

[0030] .

[0031] In some embodiments, the DC transmission power at the moment of the frequency extremum is:

[0032] ;

[0033] In the formula, This represents the DC power transmitted at the moment of the frequency extremum. The equivalent inertial constant of the synchronous generator unit, For the system steady-state frequency, The effort put into the wind and fire is comparable.

[0034] In some embodiments, based on the stability domain coordinate system, MATLAB software is used to obtain the voltage stability domain according to the voltage constraint, the power angle stability domain according to the thermal power reactance ratio and the allowable DC power reduction, and the frequency stability domain according to the DC transmission power under the fault.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] This invention evaluates the DC carrying capacity of a new energy DC transmission system during the transient period of DC commutation failure from three perspectives: transient overvoltage, transient power angle, and transient frequency. By constructing a stable domain coordinate system and obtaining a safe stable domain diagram that simultaneously characterizes transient overvoltage, transient power angle, and transient frequency, the DC carrying capacity is visualized. Attached Figure Description

[0037] Figure 1 This is a flowchart illustrating the DC carrying capacity safety stability domain evaluation method of the present invention, which takes into account multiple transient characteristics.

[0038] Figure 2 This is a schematic diagram of the overall circuit of the system model built based on the new energy DC transmission system in an embodiment of the present invention;

[0039] Figure 3 This is an equivalent circuit diagram of the new energy DC transmission system in an embodiment of the present invention;

[0040] Figure 4 This is the equivalent topology diagram for reactive power distribution in an embodiment of the present invention;

[0041] Figure 5 This is a vector diagram for calculating transient overvoltage at the new energy generator terminal in an embodiment of the present invention;

[0042] Figure 6 This is a schematic diagram of the DC power flow model at the sending end of the new energy DC transmission system in an embodiment of the present invention;

[0043] Figure 7 This is a schematic diagram of the equivalent model of the transient frequency response of the new energy DC transmission system in an embodiment of the present invention;

[0044] Figure 8 This is a voltage stability domain diagram based on the stability domain coordinate system in an embodiment of the present invention;

[0045] Figure 9 This is a power angle stability domain diagram based on the stability domain coordinate system in an embodiment of the present invention;

[0046] Figure 10 This is a frequency stability domain diagram based on the stability domain coordinate system in an embodiment of the present invention;

[0047] Figure 11This is a safe and stable domain diagram based on the stable domain coordinate system in an embodiment of the present invention. Detailed Implementation

[0048] To clearly illustrate the technical features of this solution, the implementation methods of this application will be described in detail below with reference to the accompanying drawings and embodiments. This will allow for a full understanding and implementation of how this application uses technical means to solve technical problems and achieve corresponding technical effects. The embodiments of this application and the various features within them can be combined with each other without conflict, and the resulting technical solutions are all within the protection scope of this application.

[0049] This invention provides a method for evaluating the safe and stable domain of DC carrying capacity that takes into account multiple transient characteristics, including the following steps: obtaining transient voltage thresholds. ;

[0050] The system constant parameters and sending-end equivalent reactance are obtained based on the new energy DC transmission system; the system constant parameters include the equivalent damping coefficient. Equivalent unit regulating power coefficient of synchronous generator set Synchronous generator speed governor time constant Equivalent inertial constant of synchronous generator units and system steady-state frequency The equivalent reactance at the sending end includes the equivalent reactance on the fan side. and equivalent reactance of the synchronous machine side ;

[0051] Acquire the steady-state active power of the synchronous machine of the new energy DC transmission system during normal operation. ;

[0052] The proportion of thermal power reactance in the new energy DC transmission system is obtained based on the equivalent reactance at the sending end. Thermal power reactor ratio Equivalent reactance of the synchronous machine side The proportion of the total reactance at the sending end; the total reactance at the sending end is the equivalent reactance on the wind turbine side. Equivalent reactance of synchronous machine side The sum of ;

[0053] After a fault occurs on the inverter side of a new energy DC transmission system: acquire the transient overvoltage at the sending end. Active power transmitted by the rectifier rectifier power factor angle Synchronous machine voltage Initial mechanical power and maximum electromagnetic power ;

[0054] Based on the transient overvoltage at the sending end Active power transmitted by the rectifier rectifier power factor angle Synchronous machine voltage Equivalent reactance at the sending end, and the proportion of thermal power output Voltage constraints are obtained using transient voltage thresholds;

[0055] A system model was built based on the new energy DC transmission system. (See [link]) Figure 2 Based on the topology of the new energy DC transmission system, it can be Figure 2 The system model in the text is simplified to Figure 3 The equivalent circuit shown; Figure 3 middle, The equivalent voltage of the sending-end AC system. For the equivalent sending-end reactance, For the transient overvoltage at the sending end, This represents the active power deviation value, which is the unbalanced active power injected into the sending-end AC system. This represents the reactive power deviation value, which is the unbalanced reactive power injected into the sending-end AC system. The equivalent voltage of the receiving-end AC system. For the receiving end equivalent reactance, This is a transient overvoltage at the receiving end;

[0056] In normal operating mode Figure 3 Both active power deviation and reactive power deviation are 0. However, when a fault occurs on the inverter side, the DC system may experience commutation failure. Figure 3 , and Rapid changes, and It will also change drastically. This is the DC current of the rectifier. This is the DC voltage of the rectifier. The active power consumed by the rectifier. The reactive power transmitted to the rectifier is affected by the imbalance between reactive and active power injected into the transmitting side. This indicates that the sending-end AC system has excess active power. This indicates that the sending-end AC system has excess reactive power;

[0057] The active power deviation and reactive power deviation can be expressed as follows:

[0058] ;

[0059] In the formula, This is the DC active power supply during normal operation. To transmit active power via AC during normal operation. The reactive power provided to the AC filter To provide reactive power to DC during normal operation. To supply reactive power via AC during normal operation;

[0060] The equivalent sending-end reactance can be expressed as: ;

[0061] The transient overvoltage at the sending end can be expressed as:

[0062] ;

[0063] Transverse component of the sending voltage Longitudinal component of the sending end voltage for:

[0064] ;

[0065] In the formula: Equivalent reactance of the sending-end AC system;

[0066] Transverse component of the sending voltage Longitudinal component of the sending end voltage Substituting the transient overvoltage at the sending end, we get:

[0067] (1)

[0068] Substituting the short-circuit ratio expression into equation (1), we get:

[0069] (2)

[0070] According to the per-unit value conversion formula Equation (2) can be written as:

[0071] (3)

[0072] The longitudinal component of active power usually has a small impact on voltage, so equation (3) can be simplified to:

[0073] (4)

[0074] Since the reactive power generated by the capacitor in the AC bus reactive power compensation should be proportional to the square of the voltage, therefore:

[0075] (5)

[0076] In the formula, The rated reactive power provided for the AC filter, This is the equivalent capacitance of the AC filter;

[0077] According to equation (5) and the per-unit value conversion formula, we can obtain:

[0078] (6)

[0079] Therefore, the reactive power deviation can be expressed as:

[0080] (7)

[0081] Substituting equation (3) into equation (7), and solving the quadratic equation, we obtain the analytical expression for the voltage near 1 pu:

[0082] (8)

[0083] For further quantification of reactive power allocation by synchronous machines, see [link to documentation]. Figure 4 Equivalent topology diagram of reactive power distribution. Figure 4 In the text, W refers to the fan, G refers to the synchro, and j is the imaginary unit;

[0084] The reactive power inrush at the wind turbine terminal is relatively small, and the wind turbine exhibits "open-circuit" characteristics in reactive power distribution during commutation failure. However, the transient overvoltage of the synchronous machine is smaller compared to the fluctuation of the converter bus, and its internal potential remains almost unchanged. The reactive power of the wind turbine line can be calculated by calculating the reactive power of the synchronous machine line. The calculation formula is as follows:

[0085] (9)

[0086] (10)

[0087] In the formula, This is the voltage of the synchronous machine. This refers to the reactive power of the synchronous machine line.

[0088] Then we have:

[0089] (11)

[0090] In the formula, This refers to the reactive power of the wind turbine circuit.

[0091] When calculating the peak transient overvoltage at the new energy generator terminal, the influence of the voltage transverse component can be considered, and a vector diagram can be drawn as follows: Figure 5 As shown: Figure 5 middle, This refers to the actual voltage at the new energy generator terminal. For the angle of attack, This is the transverse component of the voltage;

[0092] according to Figure 5 The expression for the transient overvoltage at the new energy generator terminal can be calculated as follows:

[0093] (12)

[0094] In the formula, This refers to the rated transient overvoltage at the new energy generator terminal.

[0095] Distributing reactive power to the wind turbine branch, DC system control equations Substituting into equation (12), we have:

[0096] (13)

[0097] Equation (13) describes the voltage constraint during DC commutation failure. The above equation is greater than the transient voltage threshold. Subsequently, there is a high probability that new energy sources will disconnect from the grid, resulting in a significant reduction in DC transmission power and a rise in transient voltage threshold. Typically, 1.3 is taken; that is, the voltage constraint is obtained as follows: the transient overvoltage at the sending end adopts the per-unit value expression;

[0098] ;

[0099] In the formula, This represents the per-unit value of the transient overvoltage at the sending end. The active power delivered to the rectifier. The rectifier power factor angle. This is the voltage of the synchronous machine. For the equivalent reactance on the wind turbine side, For the equivalent reactance on the synchronous machine side, Contributing to the forces of wind and fire This is the transient voltage threshold.

[0100] Based on initial mechanical power and maximum electromagnetic power The respective proportions of thermal power output Wind and fire output ratio and maximum equivalent mechanical power Maximum equivalent mechanical power This is the maximum equivalent mechanical power that the synchronous machine can withstand after DC blocking;

[0101] Thermal power output ratio Initial mechanical power With maximum electromagnetic power The ratio, that is: ;

[0102] Wind and fire output ratio The ratio of wind turbine output to thermal power unit output: This shows that thermal power output accounts for a significant proportion. Compared with the output of wind and fire The following relationship is satisfied between them: ;

[0103] After a fault occurs on the inverter side of a new energy DC transmission system, it goes through an initial equilibrium point, a post-blocking equilibrium point, and an unstable equilibrium point, then:

[0104] (14)

[0105] In the formula, For transient work angle, The transient work angle at the initial equilibrium point. The transient work angle at an unstable equilibrium point;

[0106] Integrating equation (14) yields:

[0107] (15)

[0108] Equation (15) is equivalent to:

[0109] (16)

[0110] After rearranging equation (16), we get:

[0111] (17)

[0112] In the formula, is The transient work angle at the equilibrium point after locking;

[0113] Further rearranging equation (17) yields:

[0114] (18)

[0115] Will Substituting into equation (18), we get:

[0116] (19)

[0117] Based on equation (19), the initial mechanical power can be determined. and maximum electromagnetic power To obtain maximum equivalent mechanical power ;

[0118] At the same time, substitute It can solve for different initial mechanical power. The corresponding maximum DC blocking power ;

[0119] Based on initial mechanical power Equivalent reactance at the sending end, maximum equivalent mechanical power and the steady-state active power of the synchronous machine Obtain DC allowable power drop ;Specifically:

[0120] Further consideration should be given to the distribution of unbalanced power in the sending-end units after DC blocking. Establishment Figure 6 The figure shows the DC power flow analysis model of the sending-end system;

[0121] Figure 6 middle, The power angle of the synchronizing machine. For the equivalent work angle of the sending system, For the fan power angle, This refers to the steady-state active power of the synchronous machine, which is the active power generated by the synchronous machine during steady-state operation. This represents the active power generated by the synchronous machine after DC blocking. For DC transmission power, For DC allowable power reduction, This refers to the steady-state active power of the wind turbine, which is the active power generated by the wind turbine during steady-state operation. This represents the active power generated by the fan after DC blocking.

[0122] Based on the DC power flow analysis model of the sending-end system, according to the DC power flow:

[0123] In steady state, equation (20) is satisfied:

[0124] (20)

[0125] Simplifying equation (20) yields:

[0126] (twenty one)

[0127] After DC blocking, the following conditions are met:

[0128] (twenty two)

[0129] Simplifying equation (22) yields:

[0130] (twenty three)

[0131] because:

[0132] (twenty four)

[0133] By combining equations (23) and (24), we can obtain:

[0134] (25)

[0135] Therefore, the power change of the synchronous machine after DC blocking is as follows: The equivalent mechanical power of the synchronous machine after DC blocking is ;

[0136] When the synchronous machine is subjected to the maximum equivalent mechanical load after DC blocking, we can obtain:

[0137] (26)

[0138] In equation (25) Substituting into equation (26), we get:

[0139] (27)

[0140] make ,and Substituting equation (21) into equation (27), we get:

[0141] (28)

[0142] Further rearranging equation (28), we get the allowable DC power reduction. :

[0143] (29)

[0144] According to equation (29), under different DC allowable power reduction The corresponding allowable DC power reduction can be calculated below. If the DC allowable power reduction is exceeded This leads to instability of the power angle at the sending end;

[0145] Based on the system constant parameters and the wind-fire output ratio Obtain the DC transmission power under fault conditions; the DC transmission power under fault conditions includes the DC transmission power at the initial moment of the fault. DC transmission power at the time of the frequency extremum point ;

[0146] Frequency stability refers to the ability of a power system (new energy DC transmission system) to maintain or recover its frequency to within an acceptable range after being subjected to small or large disturbances, without frequency oscillations or collapse. The frequency stability criterion given in my country's "Guidelines for the Safety and Stability of Power Systems" is: the system frequency can quickly recover to near its rated frequency and continue operating, without sustained frequency oscillations or collapse, and without prolonged suspension at an excessively high or low value.

[0147] If, after large-scale grid connection of new energy sources, other inertial support resources besides synchronous generators do not participate in the power system's inertial response, then the system's inertial level depends only on the grid-connected capacity of conventional synchronous generators, without considering the ability of new energy sources (i.e., wind turbines) to provide inertial support for the system. In this case, the system's equivalent inertial constant will gradually decrease, expressed as:

[0148] (30)

[0149] In the formula, This represents the equivalent inertia level of the entire power system. The inertia provided for the synchronous generator set; Rated capacity of grid-connected synchronous generating units; For the rated capacity of grid-connected new energy generating units, This is the sequence number of the synchronizer. This represents the total number of synchronizing machines;

[0150] For new energy DC transmission systems, the output of thermal power units is assumed to remain constant, i.e., the output of thermal power units is maintained at the rated capacity of grid-connected synchronous units. Let the wind power output be ,Right now = The power output ratio of wind power to thermal power units, i.e., the wind-to-thermal power output ratio, is: Then we can obtain:

[0151] (31)

[0152] By combining equations (30) and (31), we can obtain:

[0153] (32)

[0154] Considering only the inertial response of the synchronous generator set and primary frequency regulation measures, the dynamic response of the system frequency can be simplified from the rotor motion equation of the equivalent generator set:

[0155] (33)

[0156] In the formula, The system's inertial center frequency, This refers to the active power adjustment amount for primary frequency regulation of the generating unit. The active power imbalance caused by the disturbance. For time;

[0157] Establish as Figure 7 The system transient frequency response equivalent model is shown. When the grid-connected wind turbines do not participate in system frequency regulation, only the synchronous generators participate in frequency regulation within the system. Figure 7 middle, Let be the equivalent inertial constant of the system. This is the equivalent damping coefficient. The equivalent unit adjustment power coefficient for synchronous generator sets. The time constant of the synchronous generator governor. For complex variables in the Laplace transform, For frequency deviation, This is due to the short-term power deficit caused by the low voltage ride-through and recovery of wind turbine units.

[0158] When the system has a size of Δ P L (s)=Δ P L During a short-term power deficit of / s, the frequency deviation... The expression is:

[0159] (34)

[0160] This can be further expressed as:

[0161] (35)

[0162] In the formula:

[0163] ;

[0164] ;

[0165] By performing an inverse Laplace transform on equation (35), the frequency deviation can be obtained. The time-domain expression is:

[0166] (36)

[0167] In the formula:

[0168] ;

[0169] ;

[0170] .

[0171] (1) Initial frequency change rate

[0172] At the initial moment of the disturbance Time: The system's active power imbalance is at its maximum, determined by Δ P G ( t = t 1)=0, Δ f ( t = t 1)=0, which gives the maximum value of the rate of change of frequency. for:

[0173] (37)

[0174] Combining equation (37) with equation (32), we can obtain:

[0175] (38)

[0176] (2) Frequency extreme value deviation

[0177] Differentiating the frequency deviation shown in equation (36) yields:

[0178] (39)

[0179] in:

[0180] .

[0181] At the time of the frequency extremum point, i.e., time... When, dΔ f ( t ) / d t =0, where:

[0182] (40)

[0183] Substituting equation (40) into equation (36), we can obtain the frequency extreme value deviation. for

[0184] (41)

[0185] (3) Steady-state frequency deviation

[0186] Assuming the system failure lasts for a long time, the steady-state frequency that the system will reach under the current active power deficit can be obtained using the final value theorem to calculate the steady-state frequency deviation. :

[0187] (42)

[0188] According to the relevant provisions of the national standard GB / T 15945-2008 regarding system frequency, in order to meet the condition that the maximum frequency deviation of the system, i.e., the extreme frequency deviation, does not exceed ±0.2Hz, that is:

[0189] (43)

[0190] Equation (43) combined By simplification, we can obtain:

[0191] (44)

[0192] In the formula, This represents the DC power output during normal operation. This refers to the DC power output during a fault.

[0193] In the most severe case, when bipolar blocking occurs... At the initial moment of the fault, the DC transmission power is:

[0194] ;

[0195] In the formula, The DC power supplied at the initial moment of the fault. This is the first intermediate expression. This is the second intermediate expression. This is the third intermediate expression. The time at which the frequency extremum point is located;

[0196] According to the power system stability guideline ENSTO-E, the rate of change of frequency (ROCOF) should not exceed ±2 Hz / s within 0.5 seconds after a disturbance occurs. That is:

[0197] (45)

[0198] Equation (45) combined We can obtain:

[0199] (46)

[0200] If a fault occurs at the receiving end of the system at this time, considering the worst-case scenario, i.e., the active power drops to 0, At 2Hz / s, the DC transmission power at the moment of the frequency extreme point is:

[0201] ;

[0202] In the formula, This represents the DC power transmitted at the moment of the frequency extremum. The equivalent inertial constant of the synchronous generator unit, For the system steady-state frequency, The force exerted by wind and fire is comparable;

[0203] This ensures that frequency instability will not occur at the sending end;

[0204] Construct a stable domain coordinate system, where the horizontal axis represents the proportion of thermal power output and the vertical axis represents the proportion of power that can be reduced under DC conditions.

[0205] Based on the stability domain coordinate system, MATLAB software was used to obtain the voltage stability domain according to voltage constraints, the power angle stability domain according to the proportion of thermal power reactance and the allowable decrease in DC power, and the frequency stability domain according to the DC transmission power under fault conditions; the voltage stability domain is described in [reference missing]. Figure 8 Different proportions of thermal power reactors ( , , The work angle stability region under (see) Figure 9As can be seen, a smaller percentage means handling more unbalanced power, thus reducing the proportion of DC-DC power that can be reduced. The area to the left of the intersection with the dashed line can withstand the power surge from DC blocking, while the area to the right cannot withstand the power surge from DC blocking; see the frequency stability domain... Figure 10 ;

[0206] Frequency stability is evaluated by considering two constraint indicators: frequency extreme deviation and maximum frequency change rate. Specifically, the DC transmission power is taken as min{ , }. Change the wind and fire output ratio The DC transmission power is obtained by calculating the maximum rate of frequency change and the frequency extreme deviation. An approximately linear curve can be obtained. Figure 10 As shown. By Figure 10 It can be seen that the frequency stability region is mainly constrained by the maximum rate of frequency change, that is, the region formed by the curve constrained by the maximum rate of frequency change is taken as the frequency stability region.

[0207] The safe stability region is obtained by finding the intersection of the voltage stability region, power angle stability region, and frequency stability region. This safe stability region is the transient stability safe stability region of the new energy DC transmission system. Specifically, based on the new energy DC transmission system in this embodiment, [the following is taken as an example]. Stability region of the work angle at that time;

[0208] The DC carrying capacity of a new energy DC transmission system can be evaluated from three aspects: transient overvoltage, power angle stability, and frequency stability. In order to comprehensively evaluate the DC carrying capacity of a new energy DC transmission system, it is necessary to find the intersection of the voltage stability domain, power angle stability domain, and frequency stability domain in the stability domain coordinate system. The overlapping part of the voltage stability domain, power angle stability domain, and frequency stability domain is the transient stability safe stability domain of the new energy DC transmission system.

[0209] The DC carrying capacity of the new energy DC transmission system is assessed based on the safety and stability domain.

[0210] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, and is not intended to limit the scope of protection of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for evaluating the safe and stable domain of DC carrying capacity considering multiple transient characteristics, characterized in that: Includes the following steps: Obtain the transient voltage threshold; Based on the acquisition of system constant parameters and equivalent reactance of the sending end of the new energy DC transmission system; During normal operation, the steady-state active power of the synchronous machine of the new energy DC transmission system is obtained; The proportion of thermal power reactance in the new energy DC transmission system is obtained based on the equivalent reactance at the sending end. After a fault occurs on the inverter side of the new energy DC transmission system: the transient overvoltage at the sending end, the active power transmitted by the rectifier, the rectifier power factor angle, the synchronous machine voltage, the initial mechanical power, and the maximum electromagnetic power are obtained respectively. The thermal power output ratio, the wind-thermal power output ratio, and the maximum equivalent mechanical power are obtained based on the initial mechanical power and the maximum electromagnetic power, respectively. The DC transmission power under fault conditions is obtained based on the system constant parameters and the wind and fire output ratio. Voltage constraints are obtained based on the transient overvoltage at the sending end, the active power transmitted by the rectifier, the rectifier power factor angle, the synchronous machine voltage, the equivalent reactance at the sending end, the proportion of thermal power output, and the transient voltage threshold. The allowable DC power reduction is obtained based on the initial mechanical power, the equivalent reactance at the sending end, the maximum equivalent mechanical power, and the steady-state active power of the synchronous machine. Construct a stable domain coordinate system, where the horizontal axis of the stable domain coordinate system represents the proportion of thermal power output, and the vertical axis of the stable domain coordinate system represents the proportion of power that can be reduced under DC conditions. Based on the stability domain coordinate system, the voltage stability domain is obtained according to the voltage constraint, the power angle stability domain is obtained according to the thermal power reactance ratio and the allowable DC power reduction, and the frequency stability domain is obtained according to the DC transmission power under fault conditions. The safe stability region is obtained by finding the intersection of the voltage stability region, the power angle stability region, and the frequency stability region. The DC carrying capacity of the new energy DC transmission system is evaluated based on the aforementioned safety and stability domain.

2. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 1, characterized in that: The equivalent reactance at the sending end includes the equivalent reactance on the wind turbine side and the equivalent reactance on the synchronous machine side.

3. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 2, characterized in that: The thermal power reactor ratio is the proportion of the equivalent reactor on the synchronous machine side in the total reactor at the sending end, and the total reactor at the sending end is the sum of the equivalent reactor on the wind turbine side and the equivalent reactor on the synchronous machine side.

4. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 2, characterized in that: The transient overvoltage at the sending end is expressed using a per-unit value.

5. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 4, characterized in that: The voltage constraint is: ； In the formula, This represents the per-unit value of the transient overvoltage at the sending end. The active power delivered to the rectifier. The rectifier power factor angle. This is the voltage of the synchronous machine. For the equivalent reactance on the wind turbine side, For the equivalent reactance on the synchronous machine side, Contributing to the forces of wind and fire This is the transient voltage threshold.

6. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 1, characterized in that: The system constant parameters include the equivalent damping coefficient, the equivalent unit regulating power coefficient of the synchronous generator set, the time constant of the synchronous generator governor, the equivalent inertial constant of the synchronous generator set, and the steady-state frequency of the system.

7. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 6, characterized in that: The DC transmission power under fault conditions includes the DC transmission power at the initial moment of the fault and the DC transmission power at the moment of the frequency extreme point.

8. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristic quantities according to claim 7, characterized in that: The DC transmission power at the initial moment of the fault is: ； In the formula, The DC power supplied at the initial moment of the fault. This is the equivalent damping coefficient. The equivalent unit adjustment power coefficient for synchronous generator sets. This is the first intermediate expression. This is the second intermediate expression. This is the third intermediate expression. This refers to the time when the frequency extremum point is located; ； In the formula, The time constant of the synchronous generator governor. This is the fourth intermediate expression; 。 9. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristic quantities according to claim 7, characterized in that: The DC transmission power at the time of the frequency extremum point is: ； In the formula, The DC transmission power at the moment of the frequency extremum. The equivalent inertial constant of the synchronous generator unit, For the system steady-state frequency, The effort put into the wind and fire is comparable.

10. The DC carrying capacity safety stability domain evaluation method considering multiple transient characteristics as described in claim 1, characterized in that: Based on the aforementioned stability domain coordinate system, the voltage stability domain is obtained using MATLAB software according to the voltage constraints, the power angle stability domain is obtained according to the thermal power reactance ratio and the allowable DC power reduction, and the frequency stability domain is obtained according to the DC transmission power under fault conditions.

Images