CN106877340B - Classified low-voltage load shedding method and system based on load characteristics - Google Patents
Classified low-voltage load shedding method and system based on load characteristics Download PDFInfo
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Abstract
The invention relates to a classified low-pressure load shedding method and a system thereof based on load characteristics, which comprises the following steps: 1) analyzing the load dynamic response characteristic to obtain a Q-V relation characteristic curve of the load reactive power changing along with the voltage; 2) setting an initial low-voltage load reduction scheme according to the existing method by taking the dynamic motor load as a basic load; 3) determining the replacement coefficients of the loads under different voltage levels according to the Q-V relation characteristic curve; 4) according to the replacement coefficient, a mode of configuring classified low-pressure load reduction measures to cut off various loads is provided; 5) and (4) checking and determining a final classification low-pressure load shedding scheme through simulation calculation. The technical scheme of the invention is to make up the deficiency of the existing low-voltage load reduction measures and reduce the risk of large-area power failure of the power grid.
Description
Technical Field
The invention relates to the technical field of power system stability control, in particular to a classified low-voltage load shedding method and system based on load characteristics.
Background
The following common points exist in large load centers in China, such as Beijing, Shanghai, Guangdong and the like: (1) due to the restriction of energy, environmental protection and land resources, the number of power plants in a central load area is reduced, and the power transmission proportion outside the area is increased. (2) The proportion of the air conditioning load in the load center is larger and larger, and the quantity of the air conditioning load is increased and decreased violently along with weather change, so that the air conditioning load is difficult to predict. (3) The number of parallel devices (parallel capacitance compensation devices, filters, etc.) for capacitive discontinuous regulation in the power grid is large. (4) With the widespread use of power electronics, the sensitivity of many loads to voltage is reduced, similar to constant power properties, which is not conducive to voltage recovery. Due to the characteristics of the load center, on one hand, the power grid of the load center is lack of necessary dynamic reactive power support, and on the other hand, the demand for dynamic reactive power is greatly increased; therefore, once disturbed, the system is prone to reactive starvation, causing voltage stability problems.
Low voltage load shedding is a basic and effective control measure to solve the voltage stability problem. Since the load dynamic response characteristic is a key factor in the voltage stability of the power system, the voltage dynamic change process excited by different types of loads has large difference and may determine whether voltage collapse occurs, so that, unlike the low-frequency load shedding scheme, the influence of the load characteristic needs to be considered when the low-voltage load shedding scheme is prepared. However, when each grid actually makes a low-voltage load shedding scheme, a method and a mode similar to a low-frequency load shedding configuration are still adopted, and setting parameters only include load shedding voltage starting values, places, load shedding amounts, action time and the like, and do not consider the influence of load characteristics, and the reasons are as follows: (1) the load cell itself has uncertainty and time variability; (2) the difference of the dynamic characteristics of various load elements enables the comprehensive load characteristics of the power receiving side to have uncertainty and time-varying property; (3) the low-voltage load reduction scheme required by the power grid side is difficult to implement to the user side at the tail end of the power distribution network; (4) the low-voltage load shedding scheme needs to verify the validity of the scheme and the parameters through dynamic simulation, and although the importance and the necessity of classifying the low-voltage load shedding scheme are generally recognized in voltage stability research, due to the time delay of simulation program development, most of the current research is still limited to the influence of certain comprehensive load model characteristics.
Disclosure of Invention
In order to solve the defects in the prior art, the invention aims to provide a classified low-voltage load shedding method based on load characteristics and a system thereof, and provides a configuration method and a configuration step of classified low-voltage load shedding measures based on load characteristics on the basis of research contents required by the traditional low-voltage load shedding method and the problems of research on the influence of different load characteristics on a low-voltage load shedding scheme, the replacement and coordination of various load shedding load quantities and the like. The invention aims to make up the deficiency of the existing low-voltage load reduction measures and reduce the risk of large-area power failure of the power grid.
The purpose of the invention is realized by adopting the following technical scheme:
the invention provides a classified low-pressure load shedding method based on load characteristics, which is improved in that the load shedding method comprises the following steps:
1) analyzing the load dynamic response characteristic to obtain a Q-V relation characteristic curve of the load reactive power changing along with the voltage;
2) setting an initial low-pressure load reduction scheme by taking the dynamic motor load as a basic load;
3) determining the replacement coefficient lambda of the load under different voltage levels according to the Q-V relation characteristic curve;
4) according to the replacement coefficient, a mode of configuring classified low-pressure load reduction measures to cut off various loads is provided;
5) and (4) checking and determining a final classification low-pressure load shedding scheme through simulation calculation.
Further, in the step 1), according to a mathematical expression relational expression or through simulation calculation, simulation calculation curves of active power changing with frequency and reactive power changing with voltage of various load elements, namely Q-V relation characteristic curves, are respectively obtained;
no matter static load or dynamic load, the active power of the load is approximately linear with the change of frequency, and after the same number of different characteristic loads are cut off at low frequency, the frequency, voltage and active power of the system after reaching a steady state have no great difference.
Further, in the step 2), a low-voltage load shedding configuration scheme is formulated by adopting a low-voltage load shedding measure configuration principle and method aiming at an actual power system, and the loads cut off by the low-voltage load shedding configuration scheme are all dynamic motor loads;
important indicators for low pressure load shedding arrangements include: starting voltage, delay time, number of cutting wheels and cutting amount, and the configuration flow in specific implementation is as follows:
i identify regions where there is a voltage stability problem:
the region is obtained by analyzing the simulation results of the transient state and medium and long-term voltage stability of the whole network;
ii determining the total amount of shed load in the deployment area:
comprehensively determining the amount of the load which can be cut and the total amount of the cut load of each round according to the load level, the load constitution and the importance of different loads of the configuration area;
and iii, determining a voltage recovery target value after the low-voltage load shedding action:
the voltage recovery target value is an important index for evaluating the rationality of the low-voltage load shedding scheme, and after the voltage recovery target value is given, the determination of a large number of parameters in the low-voltage load shedding measures is performed around the voltage recovery target value;
iv, determining a low-pressure load shedding preliminary configuration scheme:
compared with the common low-voltage load shedding configuration algorithm, the common low-voltage load shedding configuration algorithm comprises a PV curve method and a QV curve method, and a preliminary low-voltage load shedding configuration scheme can be determined by adopting a simulation calculation result according to the existing experience;
v, checking the adaptability of the preliminary configuration scheme to form a final scheme:
and (3) analyzing the adaptability of the preliminary configuration scheme by adopting a simulation program, and simultaneously considering the requirements of transient voltage stability and medium-long term voltage stability on low-voltage load shedding measures.
The current low-pressure load shedding configuration area is basically determined according to simulation calculation results, the set quantity value is also set according to actual conditions and simulation results, and a unified regulation, a method and a set value are not actually available.
Further, in the step 3), the cut load amount of the dynamic motor is taken as a basic value, and the ratio of the cut load amount required by other types of loads to the cut load amount of the dynamic motor load is defined as a replacement coefficient lambda of the loads;
setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0The reactive power and the load power factor of the constant impedance load are respectively Qz0And cos θz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the dynamic load P is cut off during the low-voltage load shedding turn with the voltage level Vm0When the effect is relatively good, the active power P of the constant impedance load needing to be cut offz0The following were used:
wherein: thetam0Load power factor angle, which is the motor load; thetaz0Is a constant impedance load power factor angle.
Further, in the step 4), configuring a classified low-pressure load shedding measure to cut off various loads includes: determining the total load amount of other types of loads needing to be cut by using a replacement coefficient when the load of the dynamic motor is at the reactive maximum value, and distributing the load amount of each round of cutting according to the proportion requirement of each round of cutting the load; secondly, in a Q-V relation characteristic curve, respectively measuring and calculating the replacement coefficient between various loads and dynamic motor loads under the voltage level corresponding to each low-voltage load shedding turn, and then replacing a part of the dynamic load amount cut in each turn in the step 2) with other types of loads according to the proportion of various loads in an actual power grid or the requirement on the load cutting characteristic of a low-voltage load shedding device.
Further, in the step 5), whether the ratio of the loads of each round and all the loads of each type of load to be cut exceeds the load amount which can be cut by the actual system is analyzed through simulation calculation, and if the ratio exceeds the load amount, the load cutting amount of each type of load needs to be redistributed; if the total load shedding amount of all rounds exceeds the limit value of the load shedding amount, other stability control measures matched with the low-voltage load shedding measures need to be researched or suggestions are made for the operation and construction work of the power grid.
The present invention also provides a classified low pressure load shedding system based on load characteristics, the improvement wherein the system comprises:
a Q-V curve determination module: the load reactive power characteristic analysis module is used for analyzing the load dynamic response characteristic to obtain a Q-V relation characteristic curve of the load reactive power changing along with the voltage;
a formulating module: the method is used for establishing an initial low-pressure load reduction scheme according to the existing method by taking the dynamic motor load as a basic load;
a permutation coefficient determination module: determining the replacement coefficients of the loads under different voltage levels according to the Q-V relation characteristic curve;
load shedding module: the method is used for providing a mode of configuring and classifying low-pressure load reduction measures to cut off various loads according to the replacement coefficients;
a classification low-pressure load shedding scheme determination module: and the method is used for checking and determining a final classification low-pressure load shedding scheme through simulation calculation.
Further, the Q-V curve determination module is further configured to: according to the mathematical expression relational expression or through simulation calculation, respectively obtaining simulation calculation curves of the active power changing along with the frequency and the reactive power changing along with the voltage of various load elements, namely Q-V relational characteristic curves;
the active power of the static load or the dynamic load is approximately linear with the frequency change; after the same number of different characteristic loads are cut off at low frequency, the system frequency, voltage and active power after reaching a steady state have no big difference; the load characteristics have an influence on the amount of load shedding and the stabilization process and results of the low-pressure load shedding.
Further, the permutation coefficient determination module is further configured to: taking the load shedding amount of the dynamic motor as a basic value, and defining the ratio of the load shedding amount required by other types of loads to the load shedding amount of the dynamic motor as a replacement coefficient lambda of the loads;
setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0The reactive power and load power factors of the constant impedance load are Pz0, Qz0 and cos thetaz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the dynamic load P is cut off during the low-voltage load shedding turn with the voltage level Vm0Constant resistance to excision when effectiveAnti-load active power Pz0The following were used:
wherein: thetam0Load power factor angle, which is the motor load; thetaz0Is a constant impedance load power factor angle.
Further, the load shedding module is further configured to: the mode for configuring classified low-pressure load reduction measures to cut off various loads comprises the following steps: determining the total load amount of other types of loads needing to be cut by using a replacement coefficient when the load of the dynamic motor is at the reactive maximum value, and distributing the load amount of each round of cutting according to the proportion requirement of each round of cutting the load; secondly, in a Q-V relation characteristic curve, respectively measuring and calculating the replacement coefficient between various loads and dynamic motor loads under the voltage level corresponding to each low-voltage load shedding turn, and then replacing a part of the dynamic load amount cut in each turn in the step 2) with other types of loads according to the proportion of various loads in an actual power grid or the requirement on the load cutting characteristic of a low-voltage load shedding device.
Further, the classification low-pressure load shedding scheme determining module is further configured to: analyzing whether the ratio of the loads cut in each round and all the loads cut in total exceeds the load amount which can be cut by the actual system or not through simulation calculation, and if so, redistributing the load cut amount of each load; if the total load shedding amount of all rounds exceeds the limit value of the load shedding amount, other stable control measures matched with the low-voltage load shedding measures need to be researched, or suggestions are provided for the operation and construction work of the power grid; and according to the calculation and analysis results, a final classification low-pressure load reduction scheme is provided.
Compared with the closest prior art, the technical scheme provided by the invention has the following excellent effects:
the low-voltage load shedding is a basic and effective control measure for solving the problem of voltage stability, and because the dynamic response characteristic of the load is a key factor in the voltage stability of the power system, the difference of the dynamic change process of the voltage excited by different types of loads is large, and the application effect of the low-voltage load shedding scheme is seriously influenced. The invention provides a classified low-voltage load shedding method based on load characteristics on the basis of the existing low-voltage load shedding configuration method of a power system. On the basis of the research content required by the traditional low-voltage load shedding method, the problems that the influence of different load characteristics on a low-voltage load shedding scheme and the replacement and coordination of various load shedding quantities and the like need to be researched are provided, and on the basis, a configuration method and steps of classification low-voltage load shedding measures based on the load characteristics are provided. The invention aims to make up the deficiency of the existing low-voltage load reduction measures and reduce the risk of large-area power failure of the power grid.
For the purposes of the foregoing and related ends, the one or more embodiments include the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and are indicative of but a few of the various ways in which the principles of the various embodiments may be employed. Other benefits and novel features will become apparent from the following detailed description when considered in conjunction with the drawings and the disclosed embodiments are intended to include all such aspects and their equivalents.
Drawings
FIG. 1 is a flow chart of an additional control method based on instability characteristic quantity provided by the present invention;
FIG. 2 is an active (reactive) power-frequency (voltage) characteristic curve of various load models provided by the present invention;
FIG. 3 is a comparison curve of the low frequency switching static or dynamic load stability condition of the equivalent two-machine system provided by the invention;
FIG. 4 is a comparison curve of the low-voltage-switching static or dynamic load stability condition of the equivalent two-machine system provided by the invention;
FIG. 5 is a instability fault stability curve for the power grid in Jiangxi province provided by the invention;
FIG. 6 is a dynamic load stability curve of low-voltage switching of power grid instability fault in Jiangxi province provided by the invention;
FIG. 7 is a stabilized fault stability curve of the stabilized district of power grid in Beijing according to the present invention
Fig. 8 is a low-voltage-switching dynamic load stability curve of the instability fault of the ballast area provided by the invention.
Detailed Description
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
The following description and the drawings sufficiently illustrate specific embodiments of the invention to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. The examples merely typify possible variations. Individual components and functions are optional unless explicitly required, and the sequence of operations may vary. Portions and features of some embodiments may be included in or substituted for those of others. The scope of embodiments of the invention encompasses the full ambit of the claims, as well as all available equivalents of the claims. Embodiments of the invention may be referred to herein, individually or collectively, by the term "invention" merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed.
Example one
The classified low-pressure load shedding method based on load characteristics and the application flow chart thereof provided by the invention are shown in figure 1, and comprise the following steps:
(1) analyzing the dynamic response characteristics of various loads to obtain Q-V relation curves of various load reactive power changes along with voltage;
(2) an initial low-voltage load reduction scheme is established according to the existing method by taking the dynamic motor load as a basic load;
(3) determining the replacement coefficient lambda load of various loads under different voltage levels according to the Q-V curve;
(4) calculating the load quantity of other types needing to be cut off according to the replacement coefficient;
(5) and analyzing the reasonability of setting various loads in each turn and in total removal through simulation calculation, and providing a final classification low-voltage load reduction scheme or a proposal for the operation, construction and other works of the power grid.
6) From the viewpoint of system stability, a requirement is made for the setting of a constant value of low-voltage protection of a load, disordered actions of the low-voltage protection of the load are orderly managed, and the load of the part cut by the low-voltage protection actions becomes a part of the load cut by the low-voltage load shedding device.
Specifically, the method comprises the following steps: in the step 1), according to the mathematical expression relational expression or through simulation calculation, respectively obtaining the Q-V relational curve of reactive power of each type of load along with the voltage change in the system voltage change process.
No matter static load or dynamic load, the active power of the load is approximately linear with the change of frequency, and after the same number of different characteristic loads are cut off at low frequency, the frequency, voltage and active power of the system after reaching a steady state have no great difference.
For the reactive power changing with the voltage and different characteristic loads, the changing relation shows a very large difference, and especially the characteristic that the reactive power demand of the dynamic load is increased inversely with the reduction of the voltage is very unfavorable for the voltage stability of the system. Compared with the influence of the frequency-active power characteristic on the low-frequency load shedding measure, the voltage-reactive characteristic in the load characteristic has great influence on the setting and the stable recovery process of the low-voltage load shedding measure.
Specifically, the method comprises the following steps: in step 2), aiming at an actual power system, a low-voltage load shedding configuration scheme is made by adopting a traditional low-voltage load shedding measure configuration principle and method, but different from the traditional low-voltage load shedding scheme without considering load characteristics, the cut loads are all dynamic motor loads.
Aiming at an actual power system, a low-voltage load shedding measure configuration principle and a low-voltage load shedding measure configuration method are adopted to formulate a low-voltage load shedding configuration scheme, and loads cut off by the low-voltage load shedding configuration scheme are all dynamic motor loads;
important indicators for low pressure load shedding arrangements include: starting voltage, delay time, number of cutting wheels and cutting amount, and the configuration flow in specific implementation is as follows:
i identify regions where there is a voltage stability problem:
the region is obtained by analyzing the simulation results of the transient state and medium and long-term voltage stability of the whole network;
ii determining the total amount of shed load in the deployment area:
comprehensively determining the amount of the load which can be cut and the total amount of the cut load of each round according to the load level, the load constitution and the importance of different loads of the configuration area;
and iii, determining a voltage recovery target value after the low-voltage load shedding action:
the voltage recovery target value is an important index for evaluating the rationality of the low-voltage load shedding scheme, and after the voltage recovery target value is given, the determination of a large number of parameters in the low-voltage load shedding measures is performed around the voltage recovery target value;
iv, determining a low-pressure load shedding preliminary configuration scheme:
compared with the common low-voltage load shedding configuration algorithm, the common low-voltage load shedding configuration algorithm comprises a PV curve method and a QV curve method, and a preliminary low-voltage load shedding configuration scheme can be determined by adopting a simulation calculation result according to the existing experience;
v, checking the adaptability of the preliminary configuration scheme to form a final scheme:
and (3) analyzing the adaptability of the preliminary configuration scheme by adopting a simulation program, and simultaneously considering the requirements of transient voltage stability and medium-long term voltage stability on low-voltage load shedding measures.
The current low-pressure load shedding configuration area is basically determined according to simulation calculation results, the set quantity value is also set according to actual conditions and simulation results, and a unified regulation, a method and a set value are not actually available.
Specifically, the method comprises the following steps: in step 3), a concept of classifying the load shedding amount substitution coefficient is proposed, which is defined as follows: the ratio of the load shedding amount required by other types of loads to the load shedding amount of the dynamic motor is defined as a displacement coefficient lambda load of the loads by taking the load shedding amount of the dynamic motor as a base value.
The method of using the load replacement factor will be described by taking the dynamic motor load and the constant impedance load as examples. Setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0Reactive power and load power of constant impedance loadFactor is respectively Qz0And cos θz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the dynamic load P is cut off during the low-voltage load shedding turn with the voltage level Vm0When the effect is relatively good, the active power P of the constant impedance load needing to be cut offz0The following were used:
wherein: thetam0Load power factor angle, which is the motor load; thetaz0Is a constant impedance load power factor angle.
Specifically, the method comprises the following steps: in step 4), two replacement methods for configuring low-pressure load shedding and cutting off various loads are provided. Firstly, determining the total load amount of other types of loads needing to be cut by using a replacement coefficient when the load of the dynamic motor is at the reactive maximum value, and then distributing the load amount of each round of cutting according to the proportion requirement of each round of cutting the load; secondly, in a Q-V curve, the replacement coefficients of various loads and dynamic motor loads under the voltage levels corresponding to the low-voltage load shedding rounds are respectively measured and calculated, and then according to the proportion of various loads in an actual power grid or the requirement on the load shedding characteristics of a low-voltage load shedding device, a part of the dynamic load shedding amount of each round in the step 2) is replaced by other types of loads.
Specifically, the method comprises the following steps: in the step 5), the rationality of setting of various loads in each turn and in total removal is analyzed through simulation calculation, and a final classification low-pressure load reduction scheme is provided. If the load shedding amount of a certain round or the total load shedding amount of a certain type is too large, the load shedding amount of various types of loads needs to be redistributed; if the total load shedding amount of a certain round or the total load shedding amount is too large, other stable control measures matched with low-voltage load shedding measures need to be researched according to the actually-allowed load shedding condition of the power grid, or suggestions are made on the work of power grid operation, construction and the like.
Specifically, the method comprises the following steps: in the step 6), the low-voltage protection setting of the load equipment is accurately controlled by investigation and fine operation control management work carried out on the load side, and the fixed value of the low-voltage protection is set from the angle of influence on system stability, so that the condition of disordered tripping of the low-voltage protection of the motor can be avoided, the disordered action of the low-voltage protection of the load can be orderly managed according to the system stability requirement, and the dynamic load of the low-voltage protection action is cut off when needed, so that the load of the part cut off by the low-voltage protection action becomes a part of the load cut off by the classified low-voltage load reducing device.
Example two
The classified low-pressure load shedding method based on load characteristics and the application flow chart thereof provided by the invention are shown in figure 1, and comprise the following steps:
(1) analyzing the dynamic response characteristics of various loads to obtain Q-V relation curves of various load reactive power changes along with voltage;
(2) an initial low-voltage load reduction scheme is established according to the existing method by taking the dynamic motor load as a basic load;
(3) determining the replacement coefficient lambda load of various loads under different voltage levels according to the Q-V curve;
(4) calculating the load quantity of other types needing to be cut off according to the replacement coefficient;
(5) and analyzing the reasonability of setting various loads in each turn and in total removal through simulation calculation, and providing a final classification low-voltage load reduction scheme or a proposal for the operation, construction and other works of the power grid.
Specifically, the step (1) includes:
taking a two-machine equivalent system as an example, by setting an excitation system, a speed regulation system and parameters of the generator, simulation calculation curves of active power variation with frequency and reactive power variation with voltage of various load elements are obtained respectively, as shown in fig. 2. In fig. 3, under the power shortage after the fault, the system has low-frequency instability, and after 10% of different characteristic loads are cut off at low frequency, the system voltage, frequency and load power recovery level have no difference basically. In fig. 4, the required load shedding amounts for restoring the system voltage when the low voltage is shed at different characteristic loads after the occurrence of the low voltage instability fault in the same system are shown in table 1. The results of the calculations in the figures and tables were analyzed as follows:
1) the active power of the static load or the dynamic load is approximately linear with the change of the frequency, and the change degree is different, for example, when the system frequency is reduced from 50Hz to 47Hz, the active power of the static constant impedance load is reduced by about 7 percent, and the load of the dynamic industrial large motor is reduced by about 14 percent. For example, for a constant impedance load with static characteristics and an industrial large motor load with dynamic characteristics, when the node voltage is reduced from 1.0pu to 0.85pu, the reactive power of the two loads is in a reduction trend, and the reduction rates are respectively about 28% and 21%, and the difference is not large; however, if the voltage continues to decrease, the static load reactive power continues to decrease, but the dynamic load shows a trend that the reactive power demand increases inversely with the voltage decrease, for example, when the node voltage decreases from 0.85pu to 0.49pu, the reactive power required by the node voltage increases by 121% compared with the initial reactive power, and this inversely increasing reactive power demand with the voltage decrease is very detrimental to the voltage stability of the system.
2) As can be seen from fig. 3(a), (b) and (c), after the same number of different characteristic loads are cut off at low frequency, although the stability recovery process is different, the system frequency, voltage and active power after reaching the steady state are not greatly different. The low-frequency load shedding result is insensitive to the load characteristic, so that only the load shedding amount problem can be considered in the low-frequency load shedding scheme, and the influence of the load characteristic is not required to be considered.
3) As can be seen from fig. 4(a), (b) and (c) and table 1, the load characteristics have a great influence on the amount of load shedding for low-pressure load shedding and the stabilization process and results. For example, if the low voltage cuts off the dynamic motor load, the system voltage can recover after 29% of the total load is cut off; if the constant power load is firstly cut off, after all the constant power loads (30%) are cut off, the system voltage can be recovered after 14% of the dynamic motor load is cut off; by adopting the existing uniform load cutting method, the total load cutting amount is more than 50 percent, and is more than 20 percent more than that of the scheme of cutting the motor only.
It can be seen that compared with the influence of the frequency-active power characteristic on the low-frequency load shedding measure, the voltage-reactive characteristic in the load characteristic has a great influence on the setting and the stability recovery process of the low-voltage load shedding measure, and therefore, it is necessary to consider the classification of the low-voltage load shedding measure.
The Q-V relationship curve of various load reactive power changes along with the voltage is shown in figure 2 (b).
TABLE 1 Low Voltage cut different characteristic load demand statistics under the same fault
The step (2) comprises the following steps:
still taking the two-machine equivalent system as an example, after a low-voltage instability fault occurs, 29% of the dynamic motor load needs to be cut off at low voltage, so that the system can be recovered to be stable. The initial low pressure de-rating scheme thus established requires a total of 29% dynamic motor load shedding.
The step (3) comprises the following steps:
for implementing the classified low-voltage load reduction measures, the problem of equivalent replacement of different types of loads needs to be solved, and therefore, the concept of the classified load shedding amount replacement coefficient is provided and defined as follows: the ratio of the load shedding amount required by other types of loads to the load shedding amount of the dynamic motor is defined as a displacement coefficient lambda load of the loads by taking the load shedding amount of the dynamic motor as a base value.
The method of using the load replacement factor will be described by taking the dynamic motor load and the constant impedance load as examples. Setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0Absence of constant impedance loadThe power factor and the load power factor are respectively Qz0And cos θz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the step (4) comprises the following steps:
at low voltage load shedding turns with a voltage level V, and the dynamic load P is cut offm0When the effect is relatively good, the active power P of the constant impedance load needing to be cut offz0The following were used:
also taking the two-machine equivalent system as an example, the constant impedance load λ can be approximated by the Q-V characteristic curve in FIG. 2 and equation (3)Constant impedanceConstant power load λ ═ 8Constant power2. Assuming that the initial power factors are the same, it can be seen from the simulation calculation results in table 1 that if 15% of the motor loads are replaced with constant power loads, the system can recover to be stable after 30% of the constant power loads and 14% of the motor loads are removed, and the removal amount of the constant power loads is consistent with the result calculated by using the replacement coefficients; if 4% of the motor load is replaced by the constant impedance load, the system recovers stability after 30% of the constant impedance load and 25% of the motor load are cut off, and the cut-off amount of the constant impedance load is not much different from the result calculated by using the replacement coefficient. Therefore, the various load replacement amounts estimated by using the replacement coefficients basically meet the requirements of classified low-voltage load shedding after the power system fails in the example problems.
The step (5) comprises:
in an actual power grid, the operating power factors of various loads are not always the same, and the recovery of the system voltage is also related to the situations of excitation of a generator, configuration and operation of reactive compensation equipment and the like, so that the replaced classified low-voltage load reduction scheme is not always capable of obtaining the same effect as the original scheme, and the following are calculation examples of two actual power grids.
Example three: jiangxi power grid
The power grid in the west of the Yangtze river is in a grid-shaped power grid structure, the power flow distribution is uniform, and the power of the Jiangxi river receives 5575MW which accounts for 27.6% of the load in the research mode. When the power grid in the west and the river is in failure oscillation and is disconnected, the isolated power grid voltage can be recovered to a stable state, but the low voltage can cause low-voltage protection action; the continued rise in all plant frequencies leads to grid frequency collapse (fig. 5(a) and (b)). By adopting the classified low-voltage load shedding method, the voltage of the Jiangxi power grid can be recovered after 15% of the motor load (accounting for about 7.5% of the total load) is cut off at low voltage, and then the system is stabilized after the low-frequency load shedding part load (fig. 6(a) and (b)). If the low-voltage load reduction measure set by the existing method is adopted, the load shedding proportion needs to reach 12 percent, and the system voltage can be recovered. If the substitution coefficient estimation is used, the required load shedding ratio should be around 18%, that is, the actually required low pressure reduction amount is smaller than the amount estimated using the substitution coefficient.
Example four: beijing electric network
The Beijing 500kV main network is a double-circuit ring network structure, and an internal 220kV power grid is divided into a plurality of power supply areas which are respectively connected with 2-3 500kV transformer substations. Because of the characteristics of large power receiving proportion and important load of the Beijing power grid, the isolated network operation of each regional power grid is generally not allowed after the power grid is severely disturbed, and the power supply structure of the regional power grid of the hand-in-hand power and other strict security measures also reduce the possibility that the Beijing power grid becomes the isolated network to be extremely low. Fig. 7(a) and (b) are stability curves of a stabilized substation after a serious accident of 500kV line in and out full shutdown, and it can be seen from the diagrams that after the accident, only the power plant in the stabilized area is out of step and the continuous low voltage phenomenon of each node occurs in the area. After the low voltage cut off 50% of the motor load (about 30% of the total load proportion), the ballast zone voltage recovers above 0.6p.u. (fig. 8(a) and (b)). If the low-voltage load reduction measure set by the existing method is adopted, the node voltage recovery value of the stable area is still below 0.5p.u. when the load shedding proportion reaches 50%. Because there is a generator being cut out of step during a fault, it is clear that the amount of low voltage reduction actually required is greater than that estimated using the substitution coefficient.
EXAMPLE five
The invention also provides a classified low-pressure load shedding system based on load characteristics, which comprises:
a Q-V curve determination module: the load reactive power characteristic analysis module is used for analyzing the load dynamic response characteristic to obtain a Q-V relation characteristic curve of the load reactive power changing along with the voltage;
a formulating module: the method is used for establishing an initial low-pressure load reduction scheme according to the existing method by taking the dynamic motor load as a basic load;
a permutation coefficient determination module: determining the replacement coefficients of the loads under different voltage levels according to the Q-V relation characteristic curve;
load shedding module: the method is used for providing a mode of configuring and classifying low-pressure load reduction measures to cut off various loads according to the replacement coefficients;
a classification low-pressure load shedding scheme determination module: and the method is used for checking and determining a final classification low-pressure load shedding scheme through simulation calculation.
The Q-V curve determination module is further configured to: according to the mathematical expression relational expression or through simulation calculation, respectively obtaining simulation calculation curves of the active power changing along with the frequency and the reactive power changing along with the voltage of various load elements, namely Q-V relational characteristic curves;
the active power of the static load or the dynamic load is approximately linear with the frequency change; after the same number of different characteristic loads are cut off at low frequency, the system frequency, voltage and active power after reaching a steady state have no big difference; the load characteristics have an influence on the amount of load shedding and the stabilization process and results of the low-pressure load shedding.
The replacement coefficient determination module is further configured to: taking the load shedding amount of the dynamic motor as a basic value, and defining the ratio of the load shedding amount required by other types of loads to the load shedding amount of the dynamic motor as a replacement coefficient lambda of the loads;
setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0The reactive power and the load power factor of the constant impedance load are respectively Qz0And cos θz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the dynamic load P is cut off during the low-voltage load shedding turn with the voltage level Vm0When the effect is relatively good, the active power P of the constant impedance load needing to be cut offz0The following were used:
wherein: thetam0Load power factor angle, which is the motor load; thetaz0Is a constant impedance load power factor angle.
The load shedding module is further configured to: the mode for configuring classified low-pressure load reduction measures to cut off various loads comprises the following steps: determining the total load amount of other types of loads needing to be cut by using a replacement coefficient when the load of the dynamic motor is at the reactive maximum value, and distributing the load amount of each round of cutting according to the proportion requirement of each round of cutting the load; secondly, in a Q-V relation characteristic curve, respectively measuring and calculating the replacement coefficient between various loads and dynamic motor loads under the voltage level corresponding to each low-voltage load shedding turn, and then replacing a part of the dynamic load amount cut in each turn in the step 2) with other types of loads according to the proportion of various loads in an actual power grid or the requirement on the load cutting characteristic of a low-voltage load shedding device.
The classified low-pressure load shedding scheme determining module is further configured to: analyzing whether the ratio of the loads cut in each round and all the loads cut in total exceeds the load amount which can be cut by the actual system or not through simulation calculation, and if so, redistributing the load cut amount of each load; if the total load shedding amount of all rounds exceeds the limit value of the load shedding amount, other stable control measures matched with the low-voltage load shedding measures need to be researched, or suggestions are provided for the operation and construction work of the power grid; and according to the calculation and analysis results, a final classification low-pressure load reduction scheme is provided.
Therefore, the replacement coefficient is only a method for estimating various types of load configurations, when the classified low-voltage load reduction scheme is actually configured, the load shedding scheme obtained by the replacement coefficient needs to be corrected through simulation calculation, and if the load shedding amount of a certain round or the total load shedding amount of a certain type is too large, the load shedding amounts of various types need to be redistributed; if the total amount of the load shedding of a certain round or the total load shedding is overlarge, the schemes of power grid operation, power grid transformation and the like for reducing the low-voltage load shedding load are researched according to the actually allowed load shedding condition of the power grid. Finally, the reasonability of setting various loads in each turn and in total removal is analyzed through simulation calculation, and a reasonable classification low-voltage load reduction scheme is provided or suggestions are provided for the operation, construction and other works of the power grid.
Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.
Claims (8)
1. A classified low-pressure load shedding method based on load characteristics, characterized by comprising the steps of:
1) analyzing the load dynamic response characteristic to obtain a Q-V relation characteristic curve of the load reactive power changing along with the voltage;
2) setting an initial low-pressure load reduction scheme by taking the dynamic motor load as a basic load;
3) determining the replacement coefficient lambda of the load under different voltage levels according to the Q-V relation characteristic curve;
4) according to the replacement coefficient, a mode of configuring classified low-pressure load reduction measures to cut off various loads is provided;
5) checking and determining a final classification low-voltage load shedding scheme through simulation calculation;
in the step 3), the load shedding amount of the dynamic motor is taken as a basic value, and the ratio of the load shedding amount required by other types of loads to the load shedding amount of the dynamic motor is defined as a replacement coefficient lambda of the loads;
setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0The reactive power and the load power factor of the constant impedance load are respectively Qz0And cos θz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the dynamic load P is cut off during the low-voltage load shedding turn with the voltage level Vm0When the effect is relatively good, the active power P of the constant impedance load needing to be cut offz0The following were used:
wherein: thetam0Load power factor angle, which is the motor load; thetaz0Is a constant impedance load power factor angle.
2. The classified low-voltage load shedding method according to claim 1, wherein in the step 1), a simulation calculation curve of reactive power of each type of load elements along with voltage change, namely a Q-V relation characteristic curve, is obtained according to a mathematical expression relation or through simulation calculation.
3. The classified low-voltage load shedding method according to claim 1, wherein in the step 2), a low-voltage load shedding configuration scheme is formulated by adopting a low-voltage load shedding measure configuration principle and method aiming at an actual power system, and loads cut by the low-voltage load shedding configuration scheme are dynamic motor loads;
important indicators for low pressure load shedding arrangements include: the starting voltage, the delay time, the number of cutting rounds and the cutting amount are implemented by the following configuration flows:
determining a region with a voltage stability problem;
ii determining the total amount of shed load in the configuration area;
iii, determining a voltage recovery target value after the low-voltage load shedding action;
iv, determining a low-pressure load shedding preliminary configuration scheme;
and v, checking the adaptability of the preliminary configuration scheme to form a final scheme.
4. The classified low pressure load shedding method according to claim 1, wherein the step 4) of configuring the classified low pressure load shedding measure to shed various types of load comprises: determining the total load amount of other types of loads needing to be cut by using a replacement coefficient when the load of the dynamic motor is at the reactive maximum value, and distributing the load amount of each round of cutting according to the proportion requirement of each round of cutting the load; secondly, in a Q-V relation characteristic curve, respectively measuring and calculating the replacement coefficient between various loads and dynamic motor loads under the voltage level corresponding to each low-voltage load shedding turn, and then replacing a part of the dynamic load amount cut in each turn in the step 2) with other types of loads according to the proportion of various loads in an actual power grid or the requirement on the load cutting characteristic of a low-voltage load shedding device.
5. The classified low-voltage load shedding method according to claim 1, wherein in the step 5), whether the duty ratios of various types of loads cut in each turn and in total exceed the load amount available for cutting by an actual system is analyzed through simulation calculation, and if the duty ratios exceed the load amount, the load cutting amount of various types of loads needs to be redistributed; if the total load shedding amount of all rounds exceeds the limit value of the load shedding amount, other stability control measures matched with the low-voltage load shedding measures need to be researched or suggestions are made for the operation and construction work of the power grid.
6. A classified low pressure load shedding system based on load characteristics, the system comprising:
a Q-V curve determination module: the load reactive power characteristic analysis module is used for analyzing the load dynamic response characteristic to obtain a Q-V relation characteristic curve of the load reactive power changing along with the voltage;
a formulating module: the method is used for establishing an initial low-pressure load reduction scheme according to the existing method by taking the dynamic motor load as a basic load;
a permutation coefficient determination module: determining the replacement coefficients of the loads under different voltage levels according to the Q-V relation characteristic curve;
load shedding module: the method is used for providing a mode of configuring and classifying low-pressure load reduction measures to cut off various loads according to the replacement coefficients;
a classification low-pressure load shedding scheme determination module: the system is used for checking and determining a final classification low-voltage load shedding scheme through simulation calculation;
the Q-V curve determination module is further configured to: respectively obtaining a simulation calculation curve of reactive power of various load elements along with voltage change, namely a Q-V relation characteristic curve, according to a mathematical expression relation or through simulation calculation;
the replacement coefficient determination module is further configured to: taking the load shedding amount of the dynamic motor as a basic value, and defining the ratio of the load shedding amount required by other types of loads to the load shedding amount of the dynamic motor as a replacement coefficient lambda of the loads;
setting the reactive power and the load power factor of the dynamic motor load of the load needing low-voltage removal before disturbance to be respectively Qm0And cos θm0Constant impedance load reactive power and load powerFactor is respectively Qz0And cos θz0(ii) a After disturbance occurs, when the system voltage is reduced to V, the fact that the reactive power required to be absorbed by the dynamic motor load is Q is measured by using a Q-V curvemConstant impedance load requires the reactive power absorbed to be Qz(ii) a Defining constant impedance load displacement factor lambdazThe following were used:
the dynamic load P is cut off during the low-voltage load shedding turn with the voltage level Vm0When the effect is relatively good, the active power P of the constant impedance load needing to be cut offz0The following were used:
wherein: thetam0Load power factor angle, which is the motor load; thetaz0Is a constant impedance load power factor angle.
7. The classified low pressure offloading system of claim 6 wherein the load shedding module is further configured to: the mode for configuring classified low-pressure load reduction measures to cut off various loads comprises the following steps: determining the total load amount of other types of loads needing to be cut by using a replacement coefficient when the load of the dynamic motor is at the reactive maximum value, and distributing the load amount of each round of cutting according to the proportion requirement of each round of cutting the load; secondly, in a Q-V relation characteristic curve, the replacement coefficients of various loads and dynamic motor loads under the voltage levels corresponding to various low-voltage load shedding rounds are measured and calculated respectively, and then according to the proportion of various loads in an actual power grid or the requirement of the load shedding characteristics of a low-voltage load shedding device, a part of the dynamic load shedding amount of each round is replaced by other types of loads.
8. The classified low pressure load shedding system of claim 6, wherein the classified low pressure load shedding scheme determination module is further configured to: analyzing whether the ratio of the loads cut in each round and all the loads cut in total exceeds the load amount which can be cut by the actual system or not through simulation calculation, and if so, redistributing the load cut amount of each load; if the total load shedding amount of all rounds exceeds the limit value of the load shedding amount, other stable control measures matched with the low-voltage load shedding measures need to be researched, or suggestions are provided for the operation and construction work of the power grid; and according to the calculation and analysis results, a final classification low-pressure load reduction scheme is provided.
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