CN114696323B - High-proportion new energy power system frequency safety judgment method - Google Patents
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- H—ELECTRICITY
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Abstract
The invention discloses a high-proportion new energy power system frequency safety judging method, which comprises the steps of obtaining system parameters of a target proportion new energy power system; determining the relation between the equivalent inertial time constant of the system after the new energy is accessed into the system and the equivalent inertial time constant of the original system; calculating a system inertia time constant safety margin index of a target high-proportion new energy power system, a system frequency change rate safety margin index at the occurrence time of disturbance and a transient frequency deviation margin index in the transient response process; and judging the system frequency safety of the target high-proportion new energy power system according to the index value. The method can judge the system frequency safety of the high-proportion new energy power system through the innovative design safety judging and calculating process and calculating method, and has the advantages of high reliability, good stability and high accuracy.
Description
Technical Field
The invention belongs to the field of electric automation, and particularly relates to a high-proportion new energy power system frequency safety judging method.
Background
Along with the development of economic technology and the improvement of living standard of people, electric energy becomes an indispensable secondary energy source in the production and living of people, and brings endless convenience to the production and living of people. Therefore, ensuring stable and reliable supply of electric energy becomes one of the most important tasks of the electric power system.
Inertia in an electrical power system, which is commonly represented by an inertia time constant, appears to be a resistance to power disturbances and provides the most rapid, direct response to frequency changes. The synchronous generator in the traditional power grid has good inertia and damping characteristics, and can absorb or release energy through inertia response to maintain stable system frequency. With the increasing prominence of environmental problems, a large-scale new energy system starts grid-connected power generation. In general, when no special grid connection rule exists, the new energy power generation system lacks primary frequency modulation capability, and cannot provide stable and effective inertia support for a power grid. Under the background of large-scale new energy grid connection, the electric power system is forward developed in the direction of low inertia, and the low inertia characteristic not only weakens the power disturbance bearing capacity of the power grid, but also makes the dynamic behavior of the power grid system more complex.
As the permeability of new energy of the electric power system is continuously improved, the inertia of the system is reduced. The capability of the system for resisting frequency change in disturbance is reduced, the frequency stability of the power grid is greatly threatened, and various indexes of the frequency are increasingly close to the safe and stable boundary. Therefore, the system frequency safety of the high-proportion new energy power system is particularly important. However, the current method for determining the system frequency safety of the power system is generally only applicable to the conventional power system, and is not applicable to the high-ratio new energy power system. This poses a great threat to the safety of current high-proportion new energy power systems.
Disclosure of Invention
The invention aims to provide a high-proportion new energy power system frequency safety judging method with high reliability, good stability and high accuracy.
The invention provides a frequency safety judging method of a high-proportion new energy power system, which comprises the following steps:
s1, acquiring system parameters of a target high-proportion new energy power system;
s2, analyzing the target high-proportion new energy power system according to the system parameters acquired in the step S1, and determining the relation between the system equivalent inertia time constant of the new energy access system and the equivalent inertia time constant of the original system;
s3, calculating a system inertia time constant safety margin index of the target high-proportion new energy power system;
s4, calculating a safety margin index of the system frequency change rate of the target high-proportion new energy power system at the disturbance occurrence moment;
s5, calculating a transient frequency deviation margin index of the target high-proportion new energy power system in a transient response process;
s6, judging the system frequency safety of the target high-proportion new energy power system according to the index values calculated in the steps S3 to S5.
The step S2 of determining the relationship between the system equivalent inertial time constant after the new energy is accessed to the system and the equivalent inertial time constant of the original system specifically comprises the following steps:
the following formula is adopted as the relation between the equivalent inertial time constant of the system after the new energy is accessed into the system and the equivalent inertial time constant of the original system:
H' T ＝H T (1-η)
in H' T The system equivalent inertia time constant is obtained after the new energy is accessed into the system; h T Is the equivalent inertial time constant of the original system; η is the permeability after the new energy is accessed into the system.
The step S3 of calculating the system inertia time constant safety margin index of the target high-proportion new energy power system specifically comprises the following steps:
calculating the safety margin index lambda of the inertia time constant of the system by adopting the following calculation formula H ：
Wherein H is the equivalent inertial time constant of the system; h T The equivalent inertial time constant of the system when the new energy permeability is 0; h cr Is the critical value of the equivalent inertial time constant of the system, and
P step for step disturbance values, rocofs max Is the maximum value of the rate of change of frequency.
The step S4 of calculating the safety margin index of the system frequency change rate of the target high-proportion new energy power system at the disturbance occurrence time specifically comprises the following steps:
the safety margin index lambda of the system frequency change rate at the disturbance occurrence moment is calculated by adopting the following formula RoCoF ：
Wherein the RoCoF isThe rate of change of the frequency of the system; rocofs max Is the maximum value of the rate of change of frequency.
The step S5 of calculating the transient frequency deviation margin index of the target high-proportion new energy power system in the transient response process specifically comprises the following steps:
calculating a transient frequency offset margin index lambda in the transient response process by adopting the following formula nadir ：
F in nadir Is the lowest frequency of the system in the transient response process; f (f) cr A safety threshold value for the lowest frequency offset of the system; f (f) N Is the rated frequency value of the system.
The step S6 of determining the system frequency safety of the target high-proportion new energy power system according to the index values calculated in the steps S3 to S5 specifically includes the following steps:
A. the system frequency security criterion delta is calculated using the following equation:
δ＝λ H λ RoCoF λ nadir (-1) n
lambda in H The safety margin index is the system inertia time constant; lambda (lambda) RoCoF Safety margin indexes of the system frequency change rate are obtained; lambda (lambda) nadir Is a transient frequency offset margin index; n is an index parameter, and the value rule is
B. C, judging the system frequency safety of the target high-proportion new energy power system according to the system frequency safety criterion delta obtained in the step A:
if delta is more than 0, judging that the system frequency of the target high-proportion new energy power system is safe;
if delta=0, judging that the system frequency of the target high-proportion new energy power system is critical safety;
if delta is less than 0, judging that the system frequency of the target high-proportion new energy power system is unsafe.
According to the high-proportion new energy power system frequency safety judging method provided by the invention, through the innovative design safety judging calculation process and calculation method, the method not only can judge the system frequency safety of the high-proportion new energy power system, but also has the advantages of high reliability, good stability and high accuracy.
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FIG. 1 is a schematic flow chart of the method of the present invention.
FIG. 2 is a schematic diagram of a second order simplified model of the method of the present invention.
Fig. 3 is a schematic diagram of an IEEE30 node standard test system for an embodiment of the method of the present invention.
Fig. 4 is a schematic diagram of a full time domain simulation of an embodiment of the method of the present invention and a frequency response curve simulation of a simplified ASF model.
Fig. 5 is a schematic diagram of a system frequency response curve of an embodiment of the method according to the present invention in a scenario where the IEEE30 node system is entirely a conventional unit and in a scenario where the new energy permeability of the system is 23.5% based on an ASF model.
Fig. 6 is a schematic diagram of a system frequency response curve of an embodiment of the method according to the present invention in a scenario where the IEEE30 node system is entirely a conventional unit and in a scenario where the new energy permeability of the system is 35.3% based on an ASF model.
Detailed Description
A schematic process flow diagram of the method of the present invention is shown in fig. 1: the invention provides a frequency safety judging method of a high-proportion new energy power system, which comprises the following steps:
s1, acquiring system parameters of a target high-proportion new energy power system;
s2, analyzing the target high-proportion new energy power system according to the system parameters acquired in the step S1, and determining the relation between the system equivalent inertia time constant of the new energy access system and the equivalent inertia time constant of the original system; the method specifically comprises the following steps:
the following formula is adopted as the relation between the equivalent inertial time constant of the system after the new energy is accessed into the system and the equivalent inertial time constant of the original system:
H' T ＝H T (1-η)
in H' T The system equivalent inertia time constant is obtained after the new energy is accessed into the system; h T Is the equivalent inertial time constant of the original system; η is the permeability of the new energy after being accessed into the system;
in specific implementation, the analysis process is as follows:
in a multi-machine system, the frequency response of each machine set is not necessarily completely consistent, so that the rotor motion equation of the system can be defined into an equivalent machine set, and the average dynamic behavior of all the machine sets can be reflected; according to the definition of the inertia time constant, the equivalent system inertia time constant provided by the ith unit for the whole system is as follows:
wherein H is i,sys The unit of the equivalent system inertia time constant provided for the ith unit for the system is s; h i The unit is s for the inertia time constant of the ith unit; s is S i The capacity of the ith unit; n is the number of conventional units of the system; s is S sys Is the total installed capacity of the system, and
the center of inertia frequency of the system can thus be defined and expressed as:
f in COI Is the inertial center frequency of the system; f (f) i The frequency of the ith unit is the frequency of the ith unit;
the basic concept of frequency response model derivation is based on a uniform or average frequency, and it is assumed that synchronous oscillations between generators have been filtered; the basic frequency response model equalizes the dynamic behaviors of all units in the whole large system into a single machine model; for the system mainly comprising the reheat steam turbine, links with smaller time constants in the system are ignored, factors such as amplitude limiting and the like are not considered, and a second-order simplified model shown in fig. 2, namely a system frequency response model (System Frequency Response Model, SFR model), can be obtained, and the model omits inertia links with smaller influence and amplitude limiting links.
In FIG. 2, R is a difference adjustment coefficient, F H The working ratio T of the equivalent high-pressure cylinder of the steam turbine R Is equivalent reheat time constant(s), K of steam turbine m For coefficients relating to system rotational redundancy, P d The method is characterized in that the method is used for carrying out active power disturbance per unit value on a load, H is an inertia time constant(s) of an equivalent rotor of a generator, D is an equivalent damping coefficient, and delta omega is a system frequency deviation per unit value;
the expression of the systematic frequency deviation can be further simplified as:
then, the following steps are obtained:
wherein P is step Is a step disturbance;
then, the expression of Δω is converted into the time domain:
wherein the method comprises the steps of
P d (t)＝P step u(t)，
From the above equation, the equivalent machine rotor angular velocity time response can be obtained, and then the expression f (t) =f of the frequency response can be obtained 0 [1+Δω(t)]Wherein f 0 For the nominal frequency of the system omega 0 The per unit value of the system frequency deviation;
the characteristics of equivalent system response:
the initial frequency change rate value of the frequency response curve of the model depends on P step And H:
when no new energy is available in the system, the equivalent inertial time constant of the ith unit of the system for the whole system can be calculated by the following formula:
when the ratio of new energy access is eta, the system equivalent inertia time constant is required to be calculated and analyzed, and the analysis process is as follows:
assuming that the equivalent inertial time constant of the system is H T The total capacity of the system is S sys If a conventional unit is replaced by a new energy unit in the system, the equivalent inertial time constant H of the equivalent system T Will become smaller. When (when)When the system units are conventional units, the equivalent inertia time constant H of the system T Can be expressed as
Under different scenes, solving and analyzing the equivalent inertial time constant of the system under the condition that the system is connected with new energy:
the system equivalent inertia after the new energy is considered to replace the conventional power supply is accessed in the first scene can be calculated and analyzed through the following formula, and N units are assumed to exist in the whole system, wherein the N conventional units are replaced by the new energy unit, and the equivalent inertia time constant of the system is H '' T ：
η represents the permeability of the new energy after being accessed into the system, and min { H } can be known according to the inertia time constants of different units i }≤H T ≤max{H i E (1, N), if H T ≥max{H j E (N-n+1, N), then we can get by the above equation:
if max { H j }≤H T Then it can be obtained by the formula:
if max { H j }≤H T Then it can be obtained by the formula:
thus, in the above analysis, when considering the new energy permeability as η, H is the ratio of H T ≥max{H j Sum max { H } j }≤H T The two conditions are simultaneously satisfied, and the equivalent inertia time constant of the system is H' T ：
H' T ＝H T (1-η)
Scene II: the conventional power supply of the system is kept unchanged, and the total capacity of the unit connected with the new energy is S RES Equivalent inertial time constant H 'of the system' T Can be expressed as:
scene III: the system is added with a conventional power supply unit and a new energy unit simultaneously, and the total capacity of the new energy unit is S RES The total capacity of the connected conventional power unit is S CON The corresponding inertia time constant is H CON Equivalent inertial time constant H 'of the system' T Can be expressed as:
according to the formula, the inertia time constant of the system in the third situation depends on the inertia time constant of the newly added conventional unit and the inertia time constant of the original system. Considering that the newly added unit is a high-capacity unit under the actual condition, the moment of inertia provided by the newly added unit is larger, and can be considered as H CON ＞H T Thus H' T ≥H T (1-. Eta.) in which H 'is taken at the time of analysis' T ＝H T (1-. Eta.). Through the three scene analysis, the relation between the equivalent inertial time constant of the system after the new energy is accessed into the system and the equivalent inertial time constant of the original system is H' T ＝H T (1-eta), eta represents the permeability after the new energy is accessed into the system.
S3, calculating a system inertia time constant safety margin index of the target high-proportion new energy power system; the method specifically comprises the following steps:
calculating the safety margin index lambda of the inertia time constant of the system by adopting the following calculation formula H To describe the system inertia level:
wherein H is the equivalent inertial time constant of the system; h T The equivalent inertial time constant of the system when the new energy permeability is 0; h cr Is the critical value of the equivalent inertial time constant of the system, and
P step for step disturbance values, rocofs max Is the maximum value of the frequency change rate; based on given constraint rocofs max Conditions according to the H obtained cr Value, by H' T ＝H T (1-eta) can be deduced to +.>
η max The maximum ratio and capacity of the new energy accessible to the system can be obtained by the maximum value of the new energy permeability of the system;
s4, calculating a safety margin index of the system frequency change rate of the target high-proportion new energy power system at the disturbance occurrence moment; the method specifically comprises the following steps:
the safety margin index lambda of the system frequency change rate at the disturbance occurrence moment is calculated by adopting the following formula RoCoF ：
Where rocofis the rate of change of the frequency of the system; rocofs max Is the maximum value of the frequency change rate;
s5, calculating a transient frequency deviation margin index of the target high-proportion new energy power system in a transient response process; the method specifically comprises the following steps:
calculating a transient frequency offset margin index lambda in the transient response process by adopting the following formula nadir ：
F in nadir The lowest frequency of the system in the transient response process can be obtained according to a frequency response curve; f (f) cr The unit is Hz for the safety threshold value of the lowest frequency offset of the system; f (f) N The frequency value is the rated frequency value of the system, and the unit is Hz;
s6, judging the system frequency safety of the target high-proportion new energy power system according to the index values calculated in the steps S3 to S5; the method specifically comprises the following steps:
A. the system frequency security criterion delta is calculated using the following equation:
δ＝λ H λ RoCoF λ nadir (-1) n
lambda in H The safety margin index is the system inertia time constant; lambda (lambda) RoCoF Safety margin indexes of the system frequency change rate are obtained; lambda (lambda) nadir Is a transient frequency offset margin index; n is an index parameter, and the value rule is
B. C, judging the system frequency safety of the target high-proportion new energy power system according to the system frequency safety criterion delta obtained in the step A:
if delta is more than 0, judging that the system frequency of the target high-proportion new energy power system is safe;
if delta=0, judging that the system frequency of the target high-proportion new energy power system is critical safety;
if delta is less than 0, judging that the system frequency of the target high-proportion new energy power system is unsafe.
The method of the invention is further described in connection with one embodiment as follows:
taking an IEEE30 node system as an example (topological structure is shown in fig. 3), the installed capacity of the system is 425MW, the load of the system is 285MW, and the disturbance occurs when the disturbance is 45MW and t=5s. The frequency response of the system is compared under the same disturbance by adopting full time domain simulation and simplified ASF model. As can be seen from fig. 4, the full-time domain simulation and the frequency response curve of the simplified ASF model can be well matched, and the simplified ASF model can more accurately describe the frequency dynamic characteristics of the system.
The 100MW conventional unit at the node 2 is replaced by a new energy unit, the new energy permeability is 23.5%, and the new energy unit is regarded as a constant power source and does not participate in the frequency regulation process of the system. Setting load disturbance at t=5s, wherein the disturbance is 45MW, and taking the safety threshold value f of the lowest frequency offset of the system in the transient process cr =49.25 Hz. When the system is a thermal power generating unit, the equivalent inertia time constant of the system is 6.5s. When the 100MW unit of the system is replaced by a new energy unit, the new energy permeability is 23.5%. According to the three scene analysis, the equivalent inertia time constant value of the system is greater than or equal to 4.97s, so that the condition that the inertia time constant is minimum is considered, and the equivalent inertia time constant of the system is 4.97s.
Based on ASF model, analyzing system frequency response curves of IEEE30 node system under the traditional unit scene and the scene with the new energy permeability of 23.5%, and the result is shown in figure 5.
When the new energy permeability is 23.5%, the rocofs of the system are=0.533 Hz/s, and when the new energy permeability is the thermal power generating unit, the rocofs of the system are=0.406 Hz/s. When the fixed disturbance size is 45MW, the RoCoF is taken max =1 Hz/s, the critical value of the system inertia time constant H can be obtained cr The new energy permeability η= 59.27% at this time is further calculated by calculation after 2.65s, and the new energy access capacity is 252MW. Therefore, according to analysis, when the new energy permeability in the system reaches 59.27%, the frequency change rate RoCoF of the system under disturbance condition max =1 Hz/s. As can be seen from the analysis of fig. 5, when the system energy permeability is 0, the transient minimum frequency of the system is 49.49Hz; when the new energy permeability is 23.5%, the systemIs 49.33Hz.
When the new energy permeability is further increased to 35.3%, the frequency response curve of the system is shown in fig. 6. When the new energy permeability is 35.3%, the system inertia time constant is 4.21s, the rocof0.622 Hz/s of the system, the system transient minimum frequency value is 49.21Hz/s, the first-round low-frequency load shedding is triggered, and the system frequency is unsafe and stable.
The corresponding index values under different scenes are calculated according to the following formulas, and the results are shown in table 1:
TABLE 1 first schematic table of corresponding index values in different scenes
| Permeability η=0% | Permeability η=23.5% | Permeability η=35.3% | |
| λ H | 1.0 | 0.60 | 0.41 |
| RoCoF | 0.406Hz/s | 0.533Hz/s | 0.622Hz/s |
| λ RoCoF | 0.594 | 0.467 | 0.378 |
| λ nadir | 0.32 | 0.107 | -0.053 |
| |
0 | 0 | 0 |
| δ | 0.190 | 0.030 | -0.008 |
When δ >0, the system frequency is safe and stable; when δ <0, the system frequency is unsafe and unstable; delta=0, the system frequency is critically safe and stable.
If RoCoF max =1 Hz/s, while the system equivalent inertial time constant constraintH cr =5.0 s, λ is then in the case of new energy permeability η=23.5% H The delta = -0.001 calculated at this time is unsafe and unstable for the system frequency.
If RoCoF max ＝0.5Hz/s，H cr The system new energy permeability limit was calculated to be 18.62 s at this time, and the new energy access capacity maximum was 79MW. The results obtained from the simulation result analysis are shown in table 2:
TABLE 2 second schematic table of corresponding index values in different scenes
| Permeability η=0% | Permeability η=23.5% | Permeability η=35.3% | |
| λ H | 1.0 | -0.26 | -0.89 |
| RoCoF | 0.406Hz/s | 0.533Hz/s | 0.622Hz/s |
| λ RoCoF | 0.188 | -0.066 | -0.244 |
| λ nadir | 0.32 | 0.107 | -0.053 |
| |
0 | 1 | 0 |
| δ | 0.06 | -0.002 | -0.012 |
As shown by the analysis of the results of the table, as the permeability of the new energy of the system increases, the rate of change of the system frequency RoCoF increases, the equivalent inertial time constant of the system and the corresponding evaluation index lambda thereof H Reduction, system minimum frequency reduction, transient frequency offset margin index lambda nadir The less secure the system transient frequency offset is. Therefore, according to the analysis and calculation, various indexes of the system under different permeability can be obtained, and the safety and stability characteristics of the system can be judged. For different systems, the equivalent inertial time constant of the system, the change rate of the system frequency and the frequency requirement of the lowest point in the transient process are different, so that corresponding indexes also have differences, and the differences of the indexes can be better solved through the defined comprehensive evaluation index delta, so that the safety and the stability of the system frequency are comprehensively and accurately judged. Meanwhile, the maximum value of the new energy permeability of the system under the condition of meeting certain safety and stability constraint conditions can be obtained according to constraint conditions on different indexes, so that the method is favorable for guiding the access capacity selection of a new energy unit, and has very important engineering practice significance in actual power grid operation. In addition, the method of the invention can be used for tying a high-proportion new energy power systemThe system frequency safety is judged, and the system frequency safety judgment method is high in reliability, good in stability and high in accuracy.
Claims (1)
1. A high-proportion new energy power system frequency safety judging method comprises the following steps:
s1, acquiring system parameters of a target proportion new energy power system;
s2, analyzing the target high-proportion new energy power system according to the system parameters acquired in the step S1, and determining the relation between the system equivalent inertial time constant of the new energy access system and the equivalent inertial time constant of the original system;
s3, calculating a system inertia time constant safety margin index of the target proportion new energy power system
；
S4, calculating a system frequency change rate safety margin index of the target proportion new energy power system at the disturbance occurrence moment
；
S5, calculating a transient frequency deviation margin index of the target proportion new energy power system in the transient response process
；
S6, judging the system frequency safety of the target high-proportion new energy power system according to the index values obtained by calculation in the steps S3-S5;
the step S2 of determining the relationship between the system equivalent inertial time constant after the new energy is accessed to the system and the equivalent inertial time constant of the original system specifically comprises the following steps:
the following formula is adopted as the relation between the equivalent inertial time constant of the system after the new energy is accessed into the system and the equivalent inertial time constant of the original system:
s3, calculating a system inertia time constant safety margin index of the target high-proportion new energy power system
The method specifically comprises the following steps:
calculating the safety margin index of the inertia time constant of the system by adopting the following calculation formula
：
s4, calculating a system frequency change rate safety margin index of the target high-proportion new energy power system at the disturbance occurrence moment
The method specifically comprises the following steps:
calculating the safety margin index of the system frequency change rate at the disturbance occurrence moment by adopting the following formula
：
Where rocofis the rate of change of the frequency of the system; />
Is the maximum value of the frequency change rate;
step S5 of calculating transient frequency deviation margin index of target high-proportion new energy power system in transient response process
The method specifically comprises the following steps: />
Calculating a transient frequency offset margin index in a transient response process by adopting the following formula
：
In->
Is the lowest frequency of the system in the transient response process; />
A safety threshold value for the lowest frequency offset of the system; />
Is the rated frequency value of the system;
and step S6, judging the system frequency safety of the target high-proportion new energy power system according to the index values calculated in the steps S3-S5, and specifically comprising the following steps:
A. calculating the frequency safety criterion of the system by adopting the following formula
：/>
In->
The safety margin index is the system inertia time constant; />
Safety margin indexes of the system frequency change rate are obtained; />
Is a transient frequency offset margin index;nis an index parameter, and the value rule is
；
B. And C, according to the system frequency safety criterion obtained in the step A
Judging the system frequency safety of a target high-proportion new energy power system:
if it is
Then determine that the target is high-proportion newThe system frequency of the energy power system is safe;
if it is
Judging the system frequency of the target high-proportion new energy power system as critical safety;
if it is
And judging that the system frequency of the target high-proportion new energy power system is unsafe. />
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| CN112564127A (en) * | 2020-10-30 | 2021-03-26 | 武汉大学 | Photovoltaic grid-connected power system frequency transient stability analysis method |
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| Power System Over-frequency Control Strategy for High Proportion of Renewable Energy;Yuzheng Xie等;《2022 IEEE 6th Information Technology and Mechatronics Engineering Conference（ITOEC）》;380-385 * |
| 风、光高渗透率电网中考虑频率稳定的可再生能源承载力研究;郭小龙等;《可再生能源》;第38卷(第01期);84-90 * |
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