Direct-current bearing capacity safety and stability domain assessment method considering various transient characteristic quantities

Abstract

The invention provides a direct-current bearing capacity safety and stability domain evaluation method considering various transient characteristic quantities, which comprises the following steps of: when an inversion side has a fault, obtaining direct-current transmission power under the fault according to a system constant parameter and a wind-fire output ratio; according to the transmitting end transient overvoltage, the rectifier transmission electric quantity, the synchronous machine voltage, the transmitting end equivalent reactance and the thermal power output proportion, voltage constraint is obtained; according to the initial mechanical power, the equivalent reactance of the sending end, the maximum equivalent mechanical power and the steady-state active power of the synchronous machine, obtaining direct-current allowable reduction power; based on the stability domain coordinate system, a voltage stability domain is obtained according to voltage constraint, a power angle stability domain is obtained according to the thermal power reactance ratio and the direct current allowable drop power, a frequency stability domain is obtained according to the direct current transmission power under the fault, and the intersection of the three is solved to obtain a safety stability domain for evaluating the direct current bearing capacity. According to the method, the safety and stability domain graph is obtained from the three angles of transient overvoltage, transient power angle and transient frequency, and visualization of the direct-current bearing capacity is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy power station safety analysis, and particularly relates to a method for evaluating the safety and stability region of the DC carrying capacity considering multiple transient characteristic quantities Background Art

[0002] In order to achieve the goals of "carbon peak and carbon neutrality", the installed capacity of wind power in China has increased significantly. In recent years, the construction of large-scale wind power bases with deserts, gobi, and desert areas as the focus has accelerated significantly, and the investment has accelerated significantly. By the end of 2022, the investment in wind power construction in deserts, gobi, and deserts in China reached 154.7 billion yuan, a year-on-year increase of 326.7%. With the continuous high-speed growth of new energy installed capacity in deserts, gobi, and deserts, the DC carrying capacity of new energy bases is severely tested. Due to the continuous high-speed growth of new energy installed capacity in deserts, gobi, and deserts, building a large-scale new energy AC-DC hybrid external transmission system in the context of deserts, gobi, and deserts severely tests the DC carrying capacity of 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, power angle, etc. When the grid is disturbed and the system voltage fluctuates, new energy units are prone to tripping, triggering chain accidents, and destroying the system safety and stability. Therefore, problems such as overvoltage, power angle instability, and frequency instability in large-scale new energy have severely restricted UHV DC transmission, and it is urgent to find corresponding quantitative indicators. Summary of the Invention

[0003] Aiming at the problems existing in the prior art, the present invention provides a method for evaluating the safety and stability region of the DC carrying capacity of a new energy DC external transmission system during the transient period of DC commutation failure by simultaneously considering transient overvoltage, transient power angle, and transient frequency from three perspectives, which is a method for evaluating the safety and stability region of the DC carrying capacity considering multiple transient characteristic quantities.

[0004] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for evaluating the safety and stability region of the DC carrying capacity considering multiple transient characteristic quantities, comprising the following steps: obtaining a transient voltage threshold;

[0005] Based on the new energy DC external transmission system, obtaining system constant parameters and the equivalent reactance at the sending end;

[0006] Obtaining the steady-state active power of the synchronous machine of the new energy DC external transmission system during normal operation;

[0007] Obtaining the proportion of thermal power reactance of the new energy DC external transmission system according to the equivalent reactance at the sending end;

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

[0009] Obtain the thermal power output ratio, the wind-fire power output ratio, and the maximum equivalent mechanical power according to the initial mechanical power and the maximum electromagnetic power respectively;

[0010] Obtain the DC transmission power under faults according to the system constant parameters and the wind-fire power output ratio;

[0011] Obtain the voltage constraint according to the sending-end transient overvoltage, the active power transmitted by the rectifier, the power factor angle of the rectifier, the synchronous machine voltage, the sending-end equivalent reactance, the thermal power output ratio, and the transient voltage threshold;

[0012] Obtain the DC allowable power reduction according to the initial mechanical power, the sending-end equivalent reactance, the maximum equivalent mechanical power, and the steady-state active power of the synchronous machine;

[0013] Construct a stability region coordinate system, where the abscissa of the stability region coordinate system is the thermal power output ratio, and the ordinate of the stability region coordinate system is the ratio of the power that can be reduced under DC;

[0014] Based on the stability region coordinate system, obtain the voltage stability region according to the voltage constraint, obtain the power angle stability region according to the thermal power reactance ratio and the DC allowable power reduction, and obtain the frequency stability region according to the DC transmission power under faults;

[0015] Obtain the secure and stable region by taking the intersection of the voltage stability region, the power angle stability region, and the frequency stability region;

[0016] Evaluate the DC carrying capacity of the new energy DC external transmission system according to the secure and stable region.

[0017] In some embodiments, the sending-end equivalent reactance includes the equivalent reactance on the fan side and the equivalent reactance on the synchronous machine side.

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

[0019] In some embodiments, the sending-end transient overvoltage adopts a per-unit value expression.

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

[0021]

[0022] In the formula, U Lr.pu is the per-unit value of the sending-end transient overvoltage, P dr is the active power transmitted by the rectifier, is the power factor angle of the rectifier, U Gis the synchronous machine voltage, X 1 is the equivalent reactance on the fan side, X 2 is the equivalent reactance on the synchronous machine side, K w is the ratio of thermal power to wind power output, U * is the transient voltage threshold.

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

[0024] In some of these embodiments, the DC transmission power under a fault 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.

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

[0026]

[0027] In the formula, P drN1 is the DC transmission power at the initial moment of the fault, D is the equivalent damping coefficient, K m is the equivalent unit regulation power coefficient of the synchronous generator set, is the first intermediate expression, α is the second intermediate expression, ω n is the third intermediate expression, t nadir is the moment of the frequency extreme point;

[0028]

[0029] In the formula, T G is the governor time constant of the synchronous generator, ω r is the fourth intermediate expression;

[0030]

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

[0032]

[0033] In the formula, P drN2 is the DC transmission power at the moment of the frequency extreme point, H s0 is the equivalent inertia constant of the synchronous unit, f 0 is the system steady-state frequency, K w is the ratio of thermal power to wind power output.

[0034] In some of these embodiments, based on the stable region coordinate system, the voltage stability region is obtained using Matlab software according to the voltage constraint, the power angle stability region is obtained according to the thermal power reactance ratio and the allowable DC power drop, and the frequency stability region is obtained according to the DC transmission power under the fault.

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

[0036] The present invention starts from three perspectives of transient overvoltage, transient power angle, and transient frequency, evaluates the DC carrying capacity of the new energy DC external transmission system during the transient period of DC commutation failure, and realizes the visualization of the DC carrying capacity by constructing a stable region coordinate system and obtaining a safety and stability region diagram that simultaneously characterizes transient overvoltage, transient power angle, and transient frequency. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 is a schematic flow chart of the safety and stability region evaluation method for the DC carrying capacity of the present invention considering multiple transient characteristic quantities;

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

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

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

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

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

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

[0044] Figure 8 is the voltage stability region diagram based on the stable region coordinate system in the embodiment of the present invention;

[0045] Figure 9 is the power angle stability region diagram based on the stable region coordinate system in the embodiment of the present invention;

[0046] Figure 10 is the frequency stability region diagram based on the stable region coordinate system in the embodiment of the present invention;

[0047] Figure 11This is the security and stability region diagram based on the stable region coordinate system in the embodiments of the present invention. Detailed implementation manners

[0048] To clearly illustrate the technical features of this solution, the following will combine the accompanying drawings and embodiments to detail the implementation manners of this application, so as to fully understand how this application uses technical means to solve technical problems and the implementation process of achieving corresponding technical effects and implement accordingly. Each feature in the embodiments of this application and the embodiments can be combined with each other on the premise of not conflicting, and the formed technical solutions are all within the protection scope of this application.

[0049] The embodiments of the present invention provide a method for evaluating the security and stability region of the DC carrying capacity considering multiple transient characteristic quantities, including the following steps: Obtain the transient voltage threshold U * ;

[0050] Based on the new energy DC transmission system, obtain the system constant parameters and the equivalent reactance at the sending end; the system constant parameters include the equivalent damping coefficient D, the equivalent unit regulation power coefficient K m of the synchronous generator set, the governor time constant T G of the synchronous generator, the equivalent inertia constant H s0 of the synchronous unit, and the system steady-state frequency f 0 ; the equivalent reactance at the sending end includes the equivalent reactance X 1 on the fan side and the equivalent reactance X 2 on the synchronous machine side;

[0051] When operating normally, obtain the steady-state active power P G of the synchronous machine in the new energy DC transmission system;

[0052] Obtain the proportion K X of the thermal power reactance in the new energy DC transmission system according to the equivalent reactance at the sending end; the proportion K X of the thermal power reactance is the proportion of the equivalent reactance X 2 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 X 1 on the fan side and the equivalent reactance X 2 on the synchronous machine side, that is

[0053] After a fault occurs on the inverter side of the new energy DC transmission system: respectively obtain the transient overvoltage U Lr at the sending end, the active power P dr transmitted by the rectifier, the power factor angle of the rectifier, the synchronous machine voltage U G , the initial mechanical power P m0 and the maximum electromagnetic power P e ;

[0054] According to the transient overvoltage ULr , the active power P transmitted by the rectifier dr , the power factor angle of the rectifier , the synchronous machine voltage U G , the equivalent reactance at the sending end, the proportion K of thermal power output GW and the transient voltage threshold to obtain the voltage constraint;

[0055] Based on the new energy DC transmission system, a system model is built. See Figure 2 . According to the topology of the new energy DC transmission system, the Figure 2 system model in it can be simplified to the Figure 3 equivalent circuit shown; Figure 3 In, E r is the equivalent voltage of the sending-end AC system, X eqr is the equivalent sending-end reactance, U Lr is the sending-end transient overvoltage, ΔP r is the active power deviation value, that is, the unbalanced active power injected into the sending-end AC system, ΔQ r is the reactive power deviation value, that is, the unbalanced reactive power injected into the sending-end AC system, E i is the equivalent voltage of the receiving-end AC system, X eqi is the equivalent reactance at the receiving end, U Li is the receiving-end transient overvoltage;

[0056] In the normal operation mode, Figure 3 the active power deviation value and the reactive power deviation value are both 0. However, when a fault occurs on the inverter side, the DC system may experience commutation failure. According to Figure 3 , I dr and U dr change sharply, and P dr and Q dr also change sharply. I dr is the DC current of the rectifier, U dr is the DC voltage of the rectifier, P dr is the active power consumed by the rectifier, Q dr is the reactive power transmitted by the rectifier. Due to the imbalance of reactive power and active power injected into the sending side, ΔP r > 0 indicates that the sending-end AC system has an excess of active power, and ΔQ r > 0 indicates that the sending-end AC system has an excess of reactive power;

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

[0058]

[0059] In the formula, P drN is the DC transmitted active power during normal operation, P acNThe active power transmitted by AC under normal operation, Q cr The reactive power provided for the AC filter, Q drN The reactive power transmitted by DC under normal operation, Q acN The reactive power transmitted by AC under normal operation;

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

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

[0062]

[0063] The transverse component of the sending-end voltage ΔU Lr and the longitudinal component of the sending-end voltage δΔU Lr are:

[0064]

[0065] In the formula: X eq is the equivalent reactance of the sending-end AC system;

[0066] Substitute the transverse component of the sending-end voltage ΔU Lr and the longitudinal component of the sending-end voltage δΔU Lr into the sending-end transient overvoltage, and we can get:

[0067]

[0068] Substitute the short-circuit ratio expression into Equation (1), and we can get:

[0069]

[0070] According to the per-unit value conversion formula U Lr.pu = U Lr / U LrN , Equation (2) can be written as:

[0071]

[0072] Usually, the longitudinal component of the active power has little influence on the voltage, and Equation (3) can be simplified to:

[0073]

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

[0075]

[0076] In the formula, Q CrN is the rated reactive power provided for the AC filter, C ris the equivalent capacitance of the AC filter;

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

[0078]

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

[0080]

[0081] Substitute Equation (3) into Equation (7), and by solving the quadratic equation, the analytical voltage expression near 1 pu can be obtained:

[0082]

[0083] To further quantify the reactive power distribution of the synchronous machine, refer to Figure 4 the equivalent topology diagram of reactive power distribution. Figure 4 In it, W refers to the wind turbine, G refers to the synchronous machine, and j is the imaginary unit;

[0084] The reactive power influx at the wind turbine terminal is small. The wind turbine has an "open circuit" characteristic in the reactive power distribution during commutation failure. However, the transient overvoltage of the synchronous machine fluctuates less compared to the commutation bus, and its internal potential remains almost unchanged. By calculating the reactive power of the synchronous machine line, the reactive power of the wind turbine line can be calculated. The calculation formula is:

[0085]

[0086] In the formula, U G is the voltage of the synchronous machine, and ΔQ G is the reactive power of the synchronous machine line;

[0087] Then there is:

[0088]

[0089] In the formula, ΔQ W is the reactive power of the wind turbine line;

[0090] When calculating the peak value of the transient overvoltage at the new energy machine terminal, considering the influence of the voltage transverse component, a phasor diagram can be drawn as Figure 5 shown: Figure 5 In it, U PCC is the actual voltage at the new energy machine terminal, δ is the power angle, and ΔU is the voltage transverse component;

[0091] According to Figure 5 , the expression of the transient overvoltage at the new energy machine terminal can be calculated as follows:

[0092]

[0093] In the formula, ΔPPCC_N is the rated transient overvoltage at the new energy machine terminal;

[0094] Distribute reactive power in the fan branch, and the DC system control equation Substitute it into Equation (12), then we have:

[0095]

[0096] Equation (13) describes the voltage constraint during commutation failure of the DC. If the above formula is greater than the transient voltage threshold U * then there is a high probability of new energy grid disconnection, resulting in a significant reduction in DC transmission power. The transient voltage threshold U * is usually taken as 1.3; that is, the voltage constraint is obtained as: The transient overvoltage at the sending end is expressed in per-unit value;

[0097]

[0098] In the formula, U Lr.pu is the per-unit value of the transient overvoltage at the sending end, P dr is the active power transmitted by the rectifier, is the power factor angle of the rectifier, U G is the synchronous machine voltage, X 1 is the equivalent reactance on the fan side, X 2 is the equivalent reactance on the synchronous machine side, K w is the ratio of thermal power and wind power output, U * is the transient voltage threshold;

[0099] According to the initial mechanical power P m0 and the maximum electromagnetic power P e respectively obtain the proportion of thermal power output K GW and the ratio of thermal power and wind power output K W and the maximum equivalent mechanical power P m , the maximum equivalent mechanical power P m is the maximum equivalent mechanical power that the synchronous machine can withstand after DC blocking;

[0100] The proportion of thermal power output K GW is the ratio of the initial mechanical power P m0 to the maximum electromagnetic power P e , that is:

[0101] The ratio of thermal power and wind power output K W is the ratio of the output of the wind turbine to the output of the thermal power unit: It can be seen that the proportion of thermal power output K GW and the ratio of thermal power and wind power output K W satisfy the relationship:

[0102] After a fault occurs on the inverter side of the new energy DC external power transmission system, passing through the initial equilibrium point, the post-blocking equilibrium point, and the unstable equilibrium point, we have:

[0103]

[0104] In the formula, δ is the transient power angle, δ 0 is the transient power angle of the initial equilibrium point, δ c2 is the transient power angle of the unstable equilibrium point;

[0105] After integrating Equation (14), we get:

[0106] P m (δ c2 -δ 0 )+P e (cosδ c2 -cosδ 0 )=0 (15)

[0107] Equation (15) is equivalent to:

[0108]

[0109] After arranging Equation (16), we get:

[0110]

[0111] In the formula, δ c is the transient power angle of the post-blocking equilibrium point;

[0112] Further arranging Equation (17), we get:

[0113]

[0114] Substitute into Equation (18) to get:

[0115]

[0116] Based on Equation (19), the maximum equivalent mechanical power P m0 and the maximum electromagnetic power P e can be used to obtain the maximum equivalent mechanical power P m ;

[0117] At the same time, substituting P m =P m0 +dP, the maximum DC blocking power dP corresponding to different initial mechanical powers P m0 can be solved;

[0118] According to the initial mechanical power P m , the equivalent reactance at the sending end, and the maximum equivalent mechanical power P mAnd the steady-state active power P of the synchronous machine G Obtain the allowable DC power reduction ΔP D ; Specifically:

[0119] Further consider the distribution problem of the unbalanced power in the sending-end units after DC blocking. Establish Figure 6 The DC power flow analysis model of the sending-end system shown;

[0120] Figure 6 In, δ 1 Is the power angle of the synchronous machine, δ D Is the equivalent power angle of the sending-end system, δ 2 Is the power angle of the wind turbine, P G Is the steady-state active power of the synchronous machine, that is, the active power generated by the synchronous machine during steady-state operation, P' G Is the active power generated by the synchronous machine after DC blocking, P D Is the DC transmission power, ΔP D Is the allowable DC power reduction, P W Is the steady-state active power of the wind turbine, that is, the active power generated by the wind turbine during steady-state operation, P' W Is the active power generated by the wind turbine after DC blocking;

[0121] Based on the DC power flow analysis model of the sending-end system, according to the DC power flow:

[0122] At steady state, it satisfies Equation (20):

[0123]

[0124] Simplify Equation (20) to get:

[0125] δ 1 -δ 2 =P G X 2 -P W X 1 (21)

[0126] After DC blocking, it satisfies:

[0127]

[0128] Simplify Equation (22) to get:

[0129] δ 1 -δ 2 =P' G X 2 -P' W X 1 (23)

[0130] Since:

[0131] P'G +P' W =P D -ΔP D (24)

[0132] Combining equations (23) and (24) gives:

[0133]

[0134] Therefore, the power change of the synchronous machine after DC blocking is P m0 -P' G , and the equivalent mechanical power of the synchronous machine after DC blocking is 2P m0 -P' G ;

[0135] When the synchronous machine bears the maximum equivalent mechanical force after DC blocking, we get:

[0136] 2P m0 -P' G =P m (26)

[0137] Substitute P' in equation (25) G into equation (26) to get:

[0138]

[0139] Let P D =P m0 and P W =0, substitute equation (21) into equation (27) to get:

[0140] (X 1 +X 2 )(2P m0 -P m )=(δ 1 -δ 2 )+X 1 (P D -ΔP D ) (28)

[0141] Further arranging equation (28) gives the DC allowable power drop ΔP D :

[0142]

[0143] According to equation (29), at different DC allowable power drops P m , the corresponding DC allowable power drop ΔP D can be calculated. If it exceeds the DC allowable power drop ΔP D , it will cause the sending-end power angle to become unstable;

[0144] According to the system constant parameters and the thermal-wind output ratio K W Obtain the DC transmission power under faults; the DC transmission power under faults includes the DC transmission power P at the initial moment of the fault drN1 and the DC transmission power P at the moment when the frequency extreme point is located drN2 ;

[0145] Frequency stability means that after the power system (new energy DC external transmission system) is subjected to small or large disturbances, the system frequency can maintain or recover within the allowable range, and the ability not to have frequency oscillations or collapses. The frequency stability criterion given in the "Power System Safety and Stability Guide" of our country is: the system frequency can quickly recover to near the rated frequency and continue to operate, without continuous frequency oscillations or frequency collapses, nor staying at a certain too high or too low value for a long time.

[0146] If, after large-scale new energy grid connection, other inertial support resources do not participate in the power system inertia response except for synchronous units, the inertia level of the system at this time only depends on the grid-connected capacity of conventional synchronous units, without considering the ability of new energy (i.e., wind turbines) to provide inertia support for the system. Then the equivalent inertia constant of the system will gradually decrease, and the expression is:

[0147]

[0148] In the formula, H s is the equivalent inertia level of the entire power system; H G is the inertia provided by synchronous generator sets; S G is the rated capacity of grid-connected synchronous units; S RES is the rated capacity of grid-connected new energy units, i is the serial number of synchronous machines, and N is the total number of synchronous machines;

[0149] For the new energy DC external transmission system, considering that the thermal power unit output remains unchanged, that is, the thermal power unit output remains at the rated capacity S of the grid-connected synchronous unit G , assume the wind power output is S w , that is, S RES = S w . The ratio of wind power to thermal power unit output, that is, the thermal-wind output ratio, is K w , then it can be obtained that:

[0150] S w = K w S G (31)

[0151] By combining formula (30) and formula (31), it can be obtained that:

[0152]

[0153] In the case of only considering the inertial response of synchronous generator sets and primary frequency regulation measures, the dynamic response of the system frequency can be obtained by simplifying the rotor motion equation of the equivalent generator set:

[0154]

[0155] where f(t) is the frequency of the system's center of inertia, ΔP G is the active power adjustment amount of the unit's primary frequency regulation, ΔP L is the active power imbalance caused by the disturbance, and t is the time;

[0156] Establish an equivalent model of the system's transient frequency response as Figure 7 shown. When the grid-connected wind turbines do not participate in the system frequency regulation, only the synchronous generator sets in the system participate in the frequency regulation; Figure 7 where H is the equivalent inertia constant of the system, D is the equivalent damping coefficient, K m is the equivalent unit regulation power coefficient of the synchronous generator set, T G is the time constant of the synchronous generator governor, s is the complex variable in the Laplace transform, Δf is the frequency deviation, and ΔP L (s) is the short-term power deficit caused by the low-voltage ride-through and recovery of the wind turbines.

[0157] When a short-term power deficit of size ΔP L (s) = ΔP L / s occurs in the system. The expression for the frequency deviation Δf is:

[0158]

[0159] It can be further expressed as:

[0160]

[0161] where:

[0162]

[0163] Taking the inverse Laplace transform of Equation (35), the time-domain expression for the frequency deviation Δf can be obtained as:

[0164]

[0165] where:

[0166]

[0167] (1) Initial frequency change rate

[0168] At the initial moment t 1 of the disturbance: the active power imbalance corresponding to the system is the largest, and from ΔP G (t = t1 ) = 0, Δf(t = t 1 ) = 0, the maximum rate of change of frequency ROCOF| max is:

[0169]

[0170] Combining Equation (37) with Equation (32) gives:

[0171]

[0172] (2) Frequency extreme value deviation

[0173] Taking the derivative of the frequency deviation shown in Equation (36) gives:

[0174]

[0175] Where:

[0176]

[0177] At the moment when the frequency extreme value point occurs, that is, at time t nadir , dΔf(t) / dt = 0, where:

[0178]

[0179] Substituting Equation (40) into Equation (36), the frequency extreme value deviation Δf nadir is

[0180]

[0181] (3) Steady-state frequency deviation

[0182] Assume that the system fault lasts for a long time. To study the steady-state frequency that the system will reach under the current active power deficit, the steady-state frequency deviation Δf can be obtained through the final value theorem ∞ :

[0183]

[0184] According to the relevant regulations of the national standard GB / T 15945-2008 on system frequency, to meet the condition that the maximum system frequency deviation, that is, the frequency extreme value deviation, does not exceed ±0.2 Hz, that is:

[0185]

[0186] Combining Equation (43) with ΔP L = P drN - P d , through simplification, we get:

[0187]

[0188] Wherein, P drN is the DC transmission power during normal operation, and P d is the DC transmission power during a fault;

[0189] In the most severe case, i.e., when bipolar blocking occurs, P d = 0, and the DC transmission power at the initial moment of the fault is:

[0190]

[0191] Wherein, P drN1 is the DC transmission power at the initial moment of the fault, is the first intermediate expression, α is the second intermediate expression, ω n is the third intermediate expression, and t nadir is the moment when the frequency extreme point is located;

[0192] According to the power system stability guidance criterion ENSTO-E, it can be known that: within 0.5 s after the disturbance occurs, the rate of change of frequency ROCOF does not exceed ±2 Hz / s. That is:

[0193]

[0194] Combining Equation (45) with ΔP L = P drN - P d we can obtain:

[0195]

[0196] At this time, if a fault occurs at the receiving end of the system, considering the worst case, that is, the active power drops to 0, when ROCOF| max = 2 Hz / s, the DC transmission power at the moment when the frequency extreme point is located is:

[0197]

[0198] Wherein, P drN2 is the DC transmission power at the moment when the frequency extreme point is located, H s0 is the equivalent inertia constant of the synchronous generator set, f 0 is the steady-state frequency of the system, and K w is the ratio of wind and fire power output;

[0199] It can ensure that frequency instability does not occur at the sending end;

[0200] Construct a stability region coordinate system. The abscissa of the stability region coordinate system is the proportion of thermal power output, and the ordinate of the stability region coordinate system is the proportion of the power that can be decreased under DC;

[0201] Based on the stable domain coordinate system, the voltage stability domain is obtained by using Matlab software according to the voltage constraint, the power angle stability domain is obtained according to the proportion of thermal power reactance and the allowable power reduction of the DC, and the frequency stability domain is obtained according to the DC transmission power under faults; see the voltage stability domain in Figure 8 ; The power angle stability domains under different proportions of thermal power reactance (K X = 0.5, K X = 0.3, K X = 0.1) are shown in Figure 9 . It can be seen that a smaller proportion means bearing more unbalanced power, so the proportion of the allowable power reduction of the DC decreases. To the left of the intersection point with the dotted line, the power impact caused by DC blocking can be tolerated, while to the right of the intersection point, the power impact of DC blocking cannot be tolerated; see the frequency stability domain in Figure 10 ;

[0202] Two constraint indicators, namely the frequency extreme deviation and the maximum frequency change rate, are considered to evaluate the frequency stability. That is, the DC transmission power is taken as min{P drN1 , P drN2}. By changing the ratio K w of the wind and thermal power output, the DC transmission power P drN is calculated under the maximum frequency change rate and the frequency extreme deviation. An approximately linear curve can be obtained as shown in Figure 10 . As can be seen from Figure 10 , the frequency stability domain is mainly constrained by the maximum frequency change rate, that is, the domain formed by the curve constrained by the maximum frequency change rate is taken as the frequency stability domain;

[0203] The safe and stable domain is obtained by taking the intersection of the voltage stability domain, the power angle stability domain and the frequency stability domain. The safe and stable domain is the transient stability safe and stable domain of the new energy DC external transmission system. Among them, based on the new energy DC external transmission system in this embodiment, the power angle stability domain when K X = 0.1 is taken;

[0204] The DC carrying capacity of the new energy DC external transmission system can be evaluated from three aspects: transient overvoltage, power angle stability and frequency stability. To comprehensively evaluate the DC carrying capacity of the new energy DC external transmission system, it is necessary to take the intersection of the voltage stability domain, the power angle stability domain and the frequency stability domain in the stable domain coordinate system. The overlapping part of the voltage stability domain, the power angle stability domain and the frequency stability domain is the transient stability safe and stable domain of the new energy DC external transmission system;

[0205] Evaluate the DC carrying capacity of the new energy DC external transmission system according to the safe and stable domain.

[0206] Finally, it should be noted that the above content is only used to illustrate the technical solution of the present invention, rather than limiting the protection scope of the present invention. Any simple modification or equivalent replacement made by those of ordinary skill in the art to the technical solution of the present invention shall not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for evaluating the safety and stability domain of DC carrying capacity taking into account multiple transient characteristic quantities, characterized by: The following steps are involved: Get the transient voltage threshold; Based on the new energy DC transmission system, obtain the system constant parameters and the equivalent reactance at the sending end; During normal operation, the synchronous machine steady-state active power of the new energy DC transmission system is obtained; Obtaining the thermal power reactance ratio of the new energy direct current transmission system according to the sending-end equivalent reactance; After a fault occurs on the inverter side of the new energy DC transmission system: obtaining respectively the transient overvoltage at the transmission end, the active power transmitted by the rectifier, the power factor angle of the rectifier, the voltage of the synchronous machine, the initial mechanical power and the maximum electromagnetic power; According to the initial mechanical power and the maximum electromagnetic power, the thermal power output ratio, the wind-fire power output ratio and the maximum equivalent mechanical power are respectively obtained; Obtaining the DC transmission power under fault conditions according to the system constant parameters and the wind-fire output ratio; Obtaining a voltage constraint according to the transient overvoltage at the sending end, the active power transmitted by the rectifier, the power factor angle of the rectifier, the synchronous machine voltage, the equivalent reactance at the sending end, the thermal power output ratio and the transient voltage threshold; Obtaining the allowable DC power drop according to the initial mechanical power, the sending-end equivalent reactance, the maximum equivalent mechanical power and the synchronous machine steady-state active power; Constructing a stable domain coordinate system, wherein the abscissa of the stable domain coordinate system is the thermal power output ratio, and the ordinate of the stable domain coordinate system is the power reduction ratio under direct current; Based on the stability domain coordinate system, a voltage stability domain is obtained according to the voltage constraint, a power angle stability domain is obtained according to the proportion of thermal power reactance and the allowable DC power reduction, and a frequency stability domain is obtained according to the DC transmission power under the fault; Obtaining a safe and stable domain by finding the intersection of the voltage stable domain, the power angle stable domain and the frequency stable domain; The DC carrying capacity of the new energy DC transmission system is evaluated according to the safety and stability domain.

2. The DC carrying capacity safety and stability domain assessment method taking into account multiple transient characteristic quantities according to claim 1 is characterized by: The sending-end equivalent reactance includes the wind turbine side equivalent reactance and the synchronous machine side equivalent reactance.

3. The DC carrying capacity safety and stability domain assessment method taking into account multiple transient characteristic quantities according to claim 2 is characterized by: The thermal power reactance ratio is the ratio of the equivalent reactance on the synchronous machine side to the total reactance on the sending end, and the total reactance on the sending end is the sum of the equivalent reactance on the wind turbine side and the equivalent reactance on the synchronous machine side.

4. The method for evaluating the DC carrying capacity safety and stability domain taking into account multiple transient characteristic quantities according to claim 2 is characterized in that: The sending-end transient overvoltage is expressed in per-unit value.

5. The method for evaluating the DC carrying capacity safety and stability domain taking into account multiple transient characteristic quantities according to claim 4 is characterized in that: The voltage constraint is: Where U Lr.pu is the per unit value of the transient overvoltage at the sending end, P dr is the active power delivered by the rectifier, is the rectifier power factor angle, U G is the synchronous machine voltage, X1 is the equivalent reactance on the wind turbine side, X2 is the equivalent reactance on the synchronous machine side, K w is the wind-fire output ratio, U * is the transient voltage threshold.

6. The method for evaluating the DC carrying capacity safety and stability domain taking into account multiple transient characteristic quantities according to claim 1 is characterized in that: The system constant parameters include equivalent damping coefficient, equivalent unit regulation power coefficient of synchronous generator set, synchronous generator governor time constant, synchronous unit equivalent inertia constant and system steady-state frequency.

7. The method for evaluating the safety and stability domain of DC carrying capacity taking into account multiple transient characteristic quantities according to claim 6 is characterized in that: The DC transmission power under fault condition includes the DC transmission power at the initial moment of the fault and the DC transmission power at the moment when the frequency extreme value point is located.

8. The method for evaluating the safety and stability domain of DC carrying capacity taking into account multiple transient characteristic quantities according to claim 7 is characterized in that: The DC transmission power at the initial moment of the fault is: Where P drN1 is the DC transmission power at the initial moment of the fault, D is the equivalent damping coefficient, K m Adjust the power factor for the equivalent unit of the synchronous generator set, is the first intermediate expression, α is the second intermediate expression, ω n is the third intermediate expression, t nadir is the moment when the frequency extreme point is located; Where, T G is the synchronous generator speed regulator time constant, ω r is the fourth intermediate expression; 9. The method for evaluating the DC carrying capacity safety and stability domain taking into account multiple transient characteristic quantities according to claim 7 is characterized in that: The DC transmission power at the moment when the frequency extreme point is located is: Where P drN2 is the DC transmission power at the moment of the frequency extreme point, H s0 is the equivalent inertia constant of the synchronous unit, f0 is the system steady-state frequency, K w It is the ratio of wind and fire output.

10. The method for evaluating the safety and stability domain of DC carrying capacity taking into account multiple transient characteristic quantities according to claim 1, characterized in that: 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 proportion of thermal power reactance and the allowable DC power reduction, and the frequency stability domain according to the DC transmission power under the fault.

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