CN115514008A - New energy system online inertia configuration method based on average system frequency model - Google Patents
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Abstract
The invention discloses an on-line inertia configuration method of a new energy system based on an average system frequency model. In a given operation period, firstly, a speed regulating system model of the commissioning generator is accessed into a power grid according to a power grid operation mode, and a power grid average system frequency model is obtained by combining an inertia response model of a new energy system and an equivalent inertia and damping loop of the power grid. And calculating the lowest frequency point of the power grid under a given frequency safety check event based on a periodically updated frequency dynamic analysis model, and then calculating critical inertia and inertia deficit according to the margin between the lowest frequency point and a safety boundary, wherein the inertia deficit is distributed to the new energy station by taking the capacity as a reference, so that the effective support of the power grid frequency is realized, and the safety of the power grid is guaranteed.
Description
Technical Field
The invention relates to the technical field of virtual inertial support of a power grid, in particular to an on-line inertial configuration method of a new energy system based on an average system frequency model.
Background
The new energy permeability of renewable energy systems in modern power grids is constantly increasing. New energy power generation based on large-scale power electronics brings opportunities and challenges to the power grid. The frequency in a traditional power grid is mainly regulated by a speed regulation system and a rotor of a synchronous generator, and the new energy generally runs in a constant power output mode and hardly provides frequency support. Therefore, the new energy permeability is increased, so that the occupation ratio of the grid synchronous generator is reduced, and the grid frequency regulation capacity and the anti-interference capacity are reduced.
In order to improve the frequency stability and the safety of the power grid, a control mode of a new energy system needs to be changed, so that the new energy system can provide virtual inertial support for the power grid. In addition to how inertial support is achieved, it is important how the virtual inertias of multiple new energy systems provide the tasks to determine and distribute. The key of the virtual inertia distribution is inertia demand analysis based on frequency safety analysis, and inertia configuration needs to have certain timeliness due to the importance of frequency safety, so that online frequency safety analysis and inertia demand analysis are necessary. Although frequency dynamics has a number of characteristics, such as frequency nadirs, frequency change rates, new steady-state frequencies, etc., the most interesting characteristic for frequency safety assessment is frequency nadir. When the lowest point of frequency exceeds some threshold, low frequency load shedding will be activated and the synchronous generator will be tripped by the low frequency relay. Therefore, the online inertial configuration method of the new energy system is designed by taking the lowest frequency point of the power grid under large disturbance as a frequency safety evaluation object.
The prior art measures have the defects
1) Measure 1: a frequency lowest point prediction model is provided based on an average system frequency model for online evaluation of the grid frequency safety, and the following references can be referred to:
[ reference 1] Y.ZHang, Q.Guo, Y.ZHou and H.Sun, "Online frequency security assessment based on an analytical model conditioning limiting module," CSEE J.Power Energy Syst., early access.
The method provides an online frequency minimum point estimation method considering the power amplitude limiting of the generator and online frequency safety evaluation based on an Average System Frequency (ASF) model, but the virtual inertia characteristic of new energy is not considered in the model, and the configuration of a virtual inertia control task is not designed.
2) And 2, measure 2: by predicting the frequency nadir and calculating the critical inertia through time domain simulation, reference may be made to the following references:
[ reference 2] H.Gu, R.Yan, T.K.saha, "Minimum syndrome attenuation of renewable power systems," IEEE trans.Power Syst., vol.33, no.2, pp.1533-1543, mar.2018.
[ reference 3]A.S.Ahmadyar,S.Riaz,G.
A.Chapman,and D.J.Hill,“A framework for assessing renewable integration limits with respect to frequency performance,”IEEE Trans.Power Syst.,vol.33,no.4,pp.4444-4453,July 2018.
The method establishes a large power grid time domain simulation model, predicts the lowest oscillation point of the power grid under disturbance through time domain simulation, and then calculates the critical inertia. However, an accurate simulation model of the synchronous generator in an actual system is difficult to obtain, and along with the enlargement of the scale of the power grid, the establishment and the solution of the simulation model become difficult, and the rapidity and the accuracy of the critical inertia calculation are difficult to guarantee.
3) And (4) measure 3: obtaining the lowest point of frequency and calculating the inertia demand based on the differential algebraic equation of the power grid, and referring to the following references:
[ reference 4 ]]B.K.Poolla,D.Groβ,and F.
“Placement and implementation of grid-forming and grid-following virtual inertia and fast frequency response,”IEEE Trans.Power Syst.,vol.34,no.4,pp.3035–3046,Jul.2019.
According to the method, a detailed differential algebraic equation set of the large power grid is established, the lowest oscillation point of the power grid under disturbance is calculated by iteratively solving the equation set, and then the critical inertia is determined. The method has the defects similar to a method based on time domain simulation, namely, along with the enlargement of the scale of a power grid, the number of differential equation sets is increased extremely rapidly, the rapid solution is very difficult, and a large amount of computing resources are occupied.
4) And 4, measure 4: calculating the frequency nadir and calculating the inertial demand based on the system frequency response model, reference may be made to the following references:
[ reference 5]H.
A.Atarodi,S.Amini,A.R.Messina,B.Francois and H.Bevrani,“Optimal energy storage system-based virtual inertia placement:a frequency stability point of view,”IEEE Trans.Power Syst.,vol.35,no.6,pp.4824-4835,Nov.2020.
According to the method, a frequency domain frequency response (SFR) model of a large power grid is established, the frequency domain model is solved to obtain the oscillation lowest point of the power grid under disturbance, and then critical inertia is determined and an inertia control task is distributed. Due to the self-constraint of the SFR model, the accuracy of the SFR model is relatively deficient, so that the precision of an inertial distribution result is greatly limited. In addition, the SFR model represents all the generator speed regulating system models in the power grid by using a unified model, and when the structure or the operation mode of the power grid is changed, the integral model of the speed regulating system needs to be identified or established again, so that the rapid analysis capability of the method is restricted to a great extent.
Disclosure of Invention
Aiming at the problems, the invention aims to provide an online inertial configuration method of a new energy system based on an average system frequency model, which can quickly update an ASF (automatic system function) model of a power grid according to a power grid structure or an operation mode, does not need to perform integral modeling on the power grid again, and is very suitable for online frequency safety assessment and inertial configuration scenes; accurate real-time critical inertia can be obtained by periodically updating the ASF model and performing frequency safety assessment, so that a virtual inertia support task is periodically distributed to the new energy system, and the frequency safety of a power grid is ensured. The technical scheme is as follows:
an online inertia configuration method of a new energy system based on an average system frequency model is characterized by comprising the following steps:
step 1: establishing a synchronous generator speed regulation system model in an off-line manner:
the pre-storage module is used for taking a transfer function from the frequency signal to the power signal as a model of the speed regulating system of the synchronous generator and identifying the model as a power grid average system frequency model in an off-line manner;
step 2: establishing an inertial response model of the new energy station:
introducing a response to frequency differentiation in the new energy power control, simulating the inertial response of the synchronous generator, and calculating the additional active power generated by the virtual inertial control; when the inertial response model of the new energy station is connected into a power grid, the new energy permeability is considered, and the inertial response power of the new energy is obtained;
and step 3: defining a frequency security check event: defining the accident seriously threatening the frequency security as a frequency security check event;
and 4, step 4: establishing a new energy access power grid average system frequency model on line:
connecting the identified synchronous generator speed regulating system model and the new energy inertial response model into a power grid to obtain a new energy access power grid average system frequency model; periodically updating a power grid average system frequency model according to a power grid operation mode, and substituting power shortage caused by a preset frequency safety check event into a solution to obtain the lowest point frequency based on the real-time updated power grid average system frequency model;
and 5: the online inertial configuration of the new energy system comprises the following steps:
obtaining the critical inertia of the power grid according to the frequency safety evaluation result and the frequency safety margin, and solving according to the power shortage caused by the preset frequency safety check event and the time of the lowest point of the frequency to obtain a critical inertia time constant; obtaining a virtual inertia demand of the power grid by comparing the current total equivalent inertia with the critical inertia; and distributing the virtual inertia demand to the new energy station according to the new energy capacity.
Furthermore, the model of the speed regulating system is provided by a manufacturer of the synchronous generator or is obtained by identifying the model through a frequency signal disturbance injection mode.
Further, the step 2 specifically includes:
step 2.1: determining the additional active power reference generated by the virtual inertia control as:
ΔP f ＝-2H RES sΔf (1)
where s is the differential operator in the frequency domain, Δ P f For additional power reference provided for new energy sources due to virtual inertia control, H RES Providing a virtual inertial time constant for the new energy system, when the new energy does not provide a virtual inertial response, H RES =0; delta f is the grid frequency offset;
step 2.2: the power generation system of a conventional unit in a power grid is defined as follows:
wherein K is the power generation coefficient of a conventional unit, alpha i Converting the speed regulating system model from the capacity reference of a single generator to the total capacity reference of the system for a per unit value conversion coefficient; n is a radical of G The number of generators participating in frequency response in the power grid; defining the new energy permeability as (1-K), when the grid does not contain new energy, K =1;
step 2.3: considering the permeability of the new energy, the inertial response power of the new energy is as follows:
ΔP RES ＝(1-K)ΔP f ＝-(1-K)2H RES sΔf (3)
wherein, Δ P RES Power is responded to for inertia from the new energy source.
Further, the step 4 specifically includes:
step 4.1: approximating an input frequency signal of the speed regulating system by a quadratic function; decoupling the speed regulating system and the disturbed system inertia link to obtain an average system frequency open-loop model; simulating the overall response of the governor system to system power deficit with a constant slope, i.e.
In the formula, P d Is a power deficit; t is t nadir The time when the maximum frequency deviation occurs, namely the time of the lowest point of the frequency, is an unknown constant; delta P m The total increment of the mechanical power of the speed regulating system; t is a time variable after the system is disturbed;
and 4.2: the equivalent inertial damping link equation of the average system frequency model obtained according to the universal frequency dynamic analysis open-loop model is as follows:
in the formula, H sys Representing the equivalent inertia constant, Δ P, of all synchronous generators e Is the electromagnetic power variation;
will be the formula (4) and delta P e ＝P d Substituting formula (5) to obtain:
the time domain expression of the frequency offset integrated by equation (6) to obtain the parabolic approximation is:
step 4.3: based on a frequency offset time domain expression of the parabolic approximation, the total primary frequency response of the system is obtained as follows:
in the formula (I), the compound is shown in the specification,
intermediate variables, with no actual meaning; g sys (s) is the equivalent primary frequency response transfer function of the system, in particular
G sys (s)＝G K (s)+G 1-K (s) (9)
In the formula, G K (s) is the total equivalent transfer function of the generator governor, G 1-K (s) is the total equivalent transfer function of the new energy station; g i (s) is the low-order universal governor transfer function of generator i, G j (s) is a universal frequency response model transfer function of the new energy station j; alpha is alpha j Converting the coefficient for the per unit value of the new energy station; n is the total number of power supplies in the system;
step 4.4: substituting the equations (10) and (11) into the equation (8) and performing inverse Laplace transform to obtain the total primary frequency response P of the system PFR,sys (ii) a The total primary frequency response P of the system when the frequency reaches the lowest point PFR,sys Equal to power deficit P d Thus, there are:
in the formula, P PFR,sys (t nadir ) The total primary frequency response of the system as the time of the lowest point of the frequency; p PFR,n (t nadir ) As time of lowest point of frequencyThe nth power supply of the system has primary frequency response; alpha (alpha) ("alpha") n Converting coefficient for per unit value of power supply; t (T) nadir ) Intermediate variables, with no actual meaning;
step 4.5: the time t at which the maximum frequency deviation occurs is determined from equation (12) nadir Substituting the formula (7) to obtain an analytic formula of a frequency offset time domain expression delta f (t); and (3) obtaining an extreme value of the delta f (t), wherein the lowest point frequency is as follows:
in the formula (f) nadir Is the lowest point frequency, f 0 Is a rated frequency;
will be out of power P d The frequency nadir is obtained by substituting formula (13) for prediction.
Further, the step 5 specifically includes:
step 5.1: calculating critical inertia of power grid
According to formula (12) and formula (13)
Wherein H c Is the critical time constant of inertia, Δ f max Is the maximum frequency deviation defined by the frequency safety margin;
p in formula (14) d Is the power shortage caused by the preset frequency safety check event, and the power shortage is summed with the solved t nadir Solving by substituting formula (14) to obtain H c (ii) a Using the concept of calculating inertia, i.e.
E c ＝S sys H c (15)
Wherein E is c Is a critical calculated inertia, S sys Is the total capacity of all power sources in the grid;
step 5.2: the virtual inertia demand is distributed to the new energy station according to the new energy capacity, namely
Wherein H j Is the virtual inertia command of the jth new energy station, S RE,j And P RE,j Is the capacity and output power of the jth new energy station, E sys The total calculated inertia of the power grid before virtual inertia configuration.
Further, the frequency safety boundaries for frequency safety evaluation are specified by grid codes, or determined by grid protection action thresholds, or given by grid operators.
The invention has the beneficial effects that:
1) The online inertial configuration method of the new energy system is realized based on an ASF model with both flexibility and accuracy; based on the synchronous generator speed regulation system model and the new energy inertial response model established off-line, the ASF model of the power grid can be rapidly updated according to the structure or the operation mode of the power grid, the power grid does not need to be integrally modeled again, and the method is very suitable for online frequency safety assessment and inertial configuration scenes; accurate real-time critical inertia can be obtained by periodically updating the ASF model and performing frequency safety assessment, so that a virtual inertia support task is periodically distributed to the new energy system, and the frequency safety of a power grid is ensured.
2) The method is based on the ASF model updated periodically, calculates the lowest frequency point of the power grid under the given frequency safety check event, then calculates the critical inertia and the inertia shortage according to the margin between the lowest frequency point and the safety boundary, and the inertia shortage is distributed to the new energy station by taking the capacity as the reference, so as to realize the effective support of the power grid frequency.
3) Compared with the measure 1, the inertial response link of the new energy system is considered in the ASF model, on-line frequency safety assessment is performed functionally, and new energy inertial support task configuration facing frequency safety improvement is realized;
4) Compared with measures 2 and 3, the method provided by the invention has the advantages that the order of the used model is low, a large amount of calculation is not needed, and the method is not limited by the difficulty in obtaining the generator model in practical application;
5) Compared with the measure 4, the method provided by the invention carries out frequency lowest point calculation based on the ASF model, compared with the SFR model used in the measure 4, the ASF model independently establishes the model of each generator speed regulating system, the accuracy is higher, the model can be quickly updated when the structure or the operation mode of the power grid is changed, and the method is very suitable for online frequency safety evaluation and inertial support task configuration.
Drawings
FIG. 1 is a generic frequency dynamics analysis open loop model.
Fig. 2 is a schematic diagram of an online periodic virtual inertia configuration strategy.
Fig. 3 is a case study system.
Fig. 4 is a simulation result of online and offline new energy virtual inertia configuration.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
The invention provides an online configuration method of virtual inertia of a new energy system in an electric power system. Firstly, a speed regulating system model of each synchronous generator in a power grid is obtained through off-line identification, and then a frequency analysis model and an inertia configuration strategy are periodically updated. In a given operation period, firstly, a speed regulation system model of the commissioning generator is accessed into a power grid according to a power grid operation mode, and an Average System Frequency (ASF) model of the power grid is obtained by combining an inertia response model of a new energy system and an equivalent inertia and damping loop of the power grid. And calculating the lowest frequency point of the power grid under a given frequency safety check event based on a periodically updated frequency dynamic analysis model, and then calculating critical inertia and inertia shortage according to the margin between the lowest frequency point and a safety boundary, wherein the inertia shortage is distributed to the new energy station by taking the capacity as a reference, so that the effective support of the power grid frequency is realized, and the safety of the power grid is guaranteed.
1. Offline speed regulation system model establishment
Before establishing the power grid frequency analysis model, a model of the governor system (i.e. a transfer function from the frequency signal to the power signal) is identified offline as a pre-stored module of the power grid ASF model. The model of the speed regulating system can be provided by a synchronous generator manufacturer or identified by a frequency signal disturbance injection mode, and once the model of the speed regulating system is obtained, repeated modeling of the speed regulating system in the operation process can be avoided.
2. Establishing an inertial response model of a new energy station
The response to frequency differentiation is introduced into the new energy power control, and the inertia response of the synchronous generator is simulated. The additional active power generated by the virtual inertia control is referenced as
ΔP f ＝-2H RES sΔf (1)
Where s is the differential operator in the frequency domain, Δ P f Is an additional power reference provided for new energy sources due to virtual inertia control, H RES Is the virtual inertial time constant provided by the new energy system (when the new energy does not provide a virtual inertial response, H RES = 0), Δ f is the grid frequency offset.
The permeability of the new energy in the power grid influences the overall frequency response of the new energy, so when the inertia response model of the new energy station is connected into the power grid, the permeability of the new energy needs to be considered. The power generation system of a conventional unit in a power grid is defined as
Wherein, K is the power generation coefficient of the conventional unit, alpha i For the conversion coefficient of the per unit value of the generator, the speed regulating system model is converted from the capacity reference of a single generator into the total capacity reference of the system, N G The number of generators participating in the frequency response within the grid.
Thus, the new energy penetration may be defined as (1-K), when the grid does not contain new energy, K =1. Considering the permeability of the new energy, the inertial response power of the new energy is
ΔP RES ＝(1-K)ΔP f ＝-(1-K)2H RES sΔf, (3)
Wherein, Δ P RES Is the inertial response power from the new energy source.
3. Defining frequency security check events
In addition to the frequency dynamic model, the setting of security check events is also an important step in the frequency security assessment. A security check event should be an accident that may seriously threaten the frequency security. Therefore, typical N-1 or N-2 accidents can be selected and widely applied to safety verification of power systems. Optional emergencies may be 3% to 10% load steps, large capacity feed-in HVDC system lockouts, loss of power generation due to synchronous generator outages, new energy grid disconnection, etc. In practical application, the frequency safety check event can be designed by referring to historical operating data and meteorological conditions.
4. Establishing ASF model for new energy access power grid
Connecting the identified synchronous generator speed regulation system model and the new energy inertial response model into the power grid to obtain a new energy penetration power grid ASF model, as shown in FIG. 1, in which H sys Representing the equivalent inertia constant, D, of all synchronous generators sys Representing the equivalent damping coefficient of all synchronous generators.
The input frequency signal of the governor system is approximated by a quadratic function. And decoupling the speed regulating system and the disturbed system inertia link to obtain an ASF open loop model. After the power grid is disturbed, the mechanical power increment generated by the generator in response to the power disturbance is approximately linearly increased along with the time, and reaches the maximum when the frequency drops to the lowest point. Thus, the overall response of the throttle system to system power deficit can be modeled with a constant slope, i.e.
In the formula, P d Is a power shortage, t nadir Is the time at which the maximum frequency deviation occurs, i.e. the time of the lowest point of the frequency, Δ P m Is the total increment of mechanical power of the speed regulating system.
The equation of the equivalent inertial damping link of the ASF model obtained from the diagram 1 is
In the formula,. DELTA.P e Is the amount of electromagnetic power variation.
Will be the formula (4) and delta P e ＝P d Is substituted by formula (5) to obtain
The time domain expression of the frequency offset of the parabolic approximation obtained by integrating the formula (6) is
The time t in equation (7) is an unknown variable, and t nadir Is an unknown constant, so the time of the lowest point is needed to obtain the lowest point of the frequency.
The total primary frequency response of the system is obtained based on a frequency offset time domain expression of parabolic approximation
Wherein
G sys (s) is the equivalent primary frequency response transfer function of the system, G K (s) is the total equivalent transfer function of the generator governor, G 1-K (s) is the total equivalent transfer function of the new energy station, specifically
G sys (s)＝G K (s)+G 1-K (s) (9)
In the formula, G i (s) is the low-order universal governor transfer function of generator i, G j And(s) is a universal frequency response model transfer function of the new energy station j.
Substituting the formulas (10) and (11) into the formula (8) and performing inverse Laplace transform to obtain P PFR,sys . And because the frequency reaches the lowest point P PFR,sys Is equal to P d Thus there are
The minimum point time t can be obtained from the equation (12) nadir Then, the frequency analysis formula Δ f (t) is obtained by substituting the formula (7).
Obtaining an extreme value for Δ f (t), i.e. the lowest point frequency is
In the formula (f) nadir Is the lowest point of frequency, f 0 Is the nominal frequency.
Will be out of power P d The lowest frequency point can be predicted by substituting formula (13).
5. Online inertial configuration method for new energy system
The frequency safety margin for the frequency safety evaluation may be specified by the grid code, may be determined by the grid protection action threshold, or may be given by the grid operator. And obtaining the critical inertia of the power grid according to the frequency safety evaluation result and the frequency safety margin.
According to the formulae (12) and (13), the compounds
Wherein H c Is the critical time constant of inertia, Δ f max Is the maximum frequency deviation defined by the safety margin.
P in formula (14) d Is the power shortage caused by the preset frequency safety check event, and the power shortage is summed with the solved t nadir Substitution equation (14) can be solved to obtain H c . To avoid confusion of different base values, the invention uses the concept of calculating inertia, i.e. calculating the inertia
E c ＝S sys H c (15)
Wherein E is c Is a critical calculated inertia, S sys Is the total capacity of all power sources (i.e. conventional units and new energy) in the grid.
The virtual inertia demand of the grid is obtained by comparing the current total equivalent inertia with the critical inertia. The virtual inertial demand is then distributed to the new energy stations according to the new energy capacity, i.e. the size of the new energy capacity
Wherein H j Is the virtual inertia command of the jth new energy station, S RE,j And P RE,j Is the capacity and output power of the jth new energy station, E sys The total calculated inertia of the power grid before virtual inertia configuration.
In the steps, the step 1-3 is a pre-finished off-line link, and the step 4-5 is an on-line link. The specific online steps are as follows: periodically updating the ASF model of the power grid according to the operation mode of the power grid, namely the unit combination, the output power level of the synchronous generator, the frequency control parameter of the new energy and the like, wherein the pre-obtained model of the speed regulating system of the synchronous generator is used. And based on a real-time updated power grid ASF model, substituting the power shortage caused by the frequency safety check event set by budget into a solution to obtain a lowest frequency point, calculating critical inertia if the lowest frequency point exceeds a predefined safety boundary, and then acquiring a virtual inertia command and distributing the virtual inertia command to the new energy station. According to the requirement of a system operator, the above procedure is executed every 5 to 20 minutes, so as to realize the online inertia configuration of the new energy system facing the frequency safety improvement, and a schematic diagram of the online inertia configuration is shown in fig. 2.
The invention verifies the on-line inertia configuration method of the new energy system in an IEEE10 machine 39 bus system on PSCAD/EMTDC. The case study system is shown in FIG. 3, and comprises a turbine G1 with an IEEE G3 type governor system and turbines G2-G10 with an IEEE G1 type governor system. The rated capacity of the synchronous generator is 1000MVA, and the total load of the power grid is 6192.8MW. The parameters of the synchronous generator are shown in table 1.
TABLE 1 example System parameters
The verification scheme comprises the following steps:
in the research of the embodiment, the safety boundary is designed to be +/-1 Hz (namely 49Hz to 51 Hz) around the rated value of 50Hz, the designed frequency safety check event is a load step of 7.5 percent, the synchronous generator speed regulating system model is obtained by a frequency disturbance signal identification method, and the online virtual inertia configuration program runs once every 20 s. The verification scheme was designed as follows:
the original capacity of the power grid is 5650MW, the original permeability of the new energy is 32.3% (G10, G8, G7 and G3 in the system shown in FIG. 4 are replaced by the wind power plant), the original inertia time constant is 4.49s, and the original calculated inertia is 25369MW · s. When t =110s, the operation state of the power grid changes, at this time, G2 is disconnected from the power grid, and the power grid subtracts 572.8MW load corresponding to the output power of G2. A stable 9-machine 39-node system is obtained, the total capacity is 5150MW, the inertia time constant is 4.35s, and the permeability of new energy is 35.6%. t =140s, the grid is disturbed by a 7.5% load step.
Before the operation state of the power grid changes, the maximum frequency deviation of the power grid under the frequency safety check event is calculated to be 1.0654Hz based on the ASF model, the maximum frequency deviation exceeds a safety boundary, and virtual inertia configuration needs to be carried out on new energy. The critical inertia to raise the frequency nadir to 49Hz is 5.47s, and the critical calculated inertia is 30906MW · s. Therefore, the inertial demand is calculated to be 5537MW · s, which is apportioned to the new energy system by capacity, i.e.
H G10 ＝2.3s,H G8 ＝2.47s,H G7 ＝2.56s,H G3 ＝2.75s (17)
And (4) obtaining a new energy inertial configuration result in the formula (17), namely, a non-online virtual inertial configuration scheme without considering the change of the power grid operation state.
Since the online virtual inertia configuration program is operated once every 20s, and the operating state of the power grid changes at t =110s, the ASF model of the power grid is updated at t =120s, and the virtual inertia is redistributed. After the operation of a power grid changes, the maximum frequency deviation under a frequency safety check event calculated based on an ASF model is 1.1381H, the maximum frequency deviation exceeds a safety boundary, the calculated inertia requirement is 11124MW & s, and a virtual inertia instruction of a wind power plant is designed to be 11124MW & s
H G10 ＝4.64s,H G8 ＝5.01s,H G7 ＝5.19s,H G3 ＝5.56s (18)
The result obtained by the formula (18) is the online virtual inertia configuration scheme considering the change of the operation state of the power grid.
The system response results without the new energy virtual inertia support, the off-line virtual inertia configuration, and the on-line virtual inertia reconfiguration were tested in PSCAD/EMTDC as shown in fig. 4. As shown in fig. 4, the lowest point of frequency at 7.5% load step is 48.7902Hz when there is no virtual inertial support for the new energy source. When the new energy supports the power grid by using the non-online virtual inertia configuration scheme, namely the virtual inertia is not reconfigured online, the lowest frequency is 48.9076Hz. When the virtual inertia of the new energy is reconfigured on line by the method provided by the invention, the lowest point of the frequency is raised to 48.9925Hz, the requirement of a safety boundary is met, and the relative error (the maximum frequency deviation error compared with the frequency safety boundary 49 Hz) is less than 1%. Simulation results show that if the ASF model is updated on line in real time without considering the operation state of the power grid, the calculated critical inertia and new energy inertia configuration results may not meet the frequency safety requirements. Conversely, if an online virtual inertia configuration method is used, frequency security can be maintained.
Claims (6)
1. An online inertia configuration method of a new energy system based on an average system frequency model is characterized by comprising the following steps:
step 1: establishing a synchronous generator speed regulation system model in an off-line manner:
a pre-storage module which takes a transfer function from the frequency signal to the power signal as a model of a synchronous generator speed regulating system and identifies the model as a power grid average system frequency model in an off-line manner;
step 2: establishing an inertial response model of the new energy station:
introducing a response to frequency differentiation in the new energy power control, simulating the inertial response of the synchronous generator, and calculating the additional active power generated by the virtual inertial control; when the inertial response model of the new energy station is connected into the power grid, the new energy permeability is considered, and the inertial response power of the new energy is obtained;
and step 3: defining a frequency security check event: defining the accident seriously threatening the frequency security as a frequency security verification event;
and 4, step 4: establishing a new energy access power grid average system frequency model on line:
connecting the identified synchronous generator speed regulation system model and the new energy inertial response model into a power grid to obtain a new energy access power grid average system frequency model; periodically updating a power grid average system frequency model according to a power grid operation mode, and substituting power shortage caused by a preset frequency safety check event into a solution to obtain the lowest point frequency based on the real-time updated power grid average system frequency model;
and 5: the online inertial configuration of the new energy system comprises the following steps:
obtaining the critical inertia of the power grid according to the frequency safety evaluation result and the frequency safety margin, and solving according to the power shortage caused by the preset frequency safety check event and the time of the lowest point of the frequency to obtain a critical inertia time constant; obtaining a virtual inertia demand of the power grid by comparing the current total equivalent inertia with the critical inertia; and distributing the virtual inertia demand to the new energy station according to the new energy capacity.
2. The new energy system online inertia configuration method based on the average system frequency model according to claim 1, wherein the model of the governing system is provided by a manufacturer of the synchronous generator or identified by means of frequency signal disturbance injection.
3. The online inertia configuration method for the new energy system based on the average system frequency model according to claim 1, wherein the step 2 specifically comprises:
step 2.1: determining the additional active power reference generated by the virtual inertia control as:
ΔP f ＝-2H RES sΔf (1)
where s is the differential operator in the frequency domain, Δ P f For additional power reference provided for new energy sources due to virtual inertia control, H RES Providing a virtual inertial time constant for the new energy system, when the new energy does not provide a virtual inertial response, H RES =0; delta f is the grid frequency offset;
step 2.2: the power generation system of a conventional unit in a power grid is defined as follows:
wherein K is the power generation coefficient of a conventional unit, alpha i Converting the speed regulating system model from a single generator capacity reference to a system total capacity reference for per unit value conversion coefficients of the generators; n is a radical of hydrogen G The number of generators participating in frequency response in the power grid; the new energy permeability is defined as (1-K),when the power grid does not contain new energy, K =1;
step 2.3: considering the permeability of the new energy, the inertial response power of the new energy is as follows:
ΔP RES ＝(1-K)ΔP f ＝-(1-K)2H RES sΔf (3)
wherein, Δ P RES Power is responded to for inertia from the new energy source.
4. The new energy system online inertial configuration method based on average system frequency model according to claim 3, wherein the step 4 specifically includes:
step 4.1: approximating an input frequency signal of the speed regulating system by a quadratic function; decoupling the speed regulating system and the disturbed system inertia link to obtain an average system frequency open-loop model; simulating the overall response of the governor system to system power deficit with a constant slope, i.e.
In the formula, P d Is a power deficit; t is t nadir The time when the maximum frequency deviation occurs, namely the time of the lowest point of the frequency, is an unknown constant; delta P m The total increment of the mechanical power of the speed regulating system; t is a time variable after the system is disturbed;
step 4.2: the equivalent inertial damping link equation of the average system frequency model obtained according to the universal frequency dynamic analysis open-loop model is as follows:
in the formula, H sys Representing the equivalent inertia constant, Δ P, of all synchronous generators e Is the electromagnetic power variation;
will be the formula (4) and delta P e ＝P d Substituting formula (5) to obtain:
the time domain expression of the frequency offset integrated into the parabolic approximation of equation (6) is:
step 4.3: based on a frequency offset time domain expression of the parabolic approximation, the total primary frequency response of the system is obtained as follows:
in the formula (I), the compound is shown in the specification,
intermediate variables, with no actual meaning; g sys (s) is the equivalent primary frequency response transfer function of the system, in particular
G sys (s)＝G K (s)+G 1-K (s) (9)
In the formula, G K (s) is the total equivalent transfer function of the generator governor, G 1-K (s) is the total equivalent transfer function of the new energy station; g i (s) is the low-order universal governor transfer function of generator i, G j (s) is a universal frequency response model transfer function of the new energy station j; alpha is alpha j Is the conversion coefficient of the per unit value of the new energy station,n is the total number of power supplies in the system;
step 4.4: substituting the equations (10) and (11) into the equation (8) and performing inverse laplace transform to obtain the total primary frequency response P of the system PFR,sys (ii) a The total primary frequency response P of the system when the frequency reaches the lowest point PFR,sys Equal to power deficit P d Thus, there are:
in the formula, P PFR,sys (t nadir ) The total primary frequency response of the system as the time of the lowest point of the frequency; p is PFR,n (t nadir ) The system nth power supply primary frequency response is the time of the lowest frequency point; alpha is alpha n Converting the coefficient for the per unit value of the power supply; t (T) nadir ) Intermediate variables, with no actual meaning;
step 4.5: the time t at which the maximum frequency deviation occurs is determined from (12) nadir Substituting the formula (7) to obtain an analytic formula of a frequency offset time domain expression delta f (t); and (3) obtaining an extreme value of the delta f (t), wherein the lowest point frequency is as follows:
in the formula (f) nadir Is the lowest point frequency, f 0 Is a rated frequency;
will be out of power P d The lowest frequency point is predicted by substituting formula (13).
5. The online inertia configuration method for the new energy system based on the average system frequency model according to claim 4, wherein the step 5 specifically comprises:
step 5.1: calculating critical inertia of power grid
According to formula (12) and formula (13)
Wherein H c Is the critical time constant of inertia, Δ f max Is the maximum frequency deviation defined by the frequency safety margin;
p in formula (14) d Is the power shortage caused by the preset frequency safety check event, and the power shortage is summed with the solved t nadir Solving by substituting formula (14) to obtain H c (ii) a Using the concept of calculating inertia, i.e.
E c ＝S sys H c (15)
Wherein E is c Is a critical calculated inertia, S sys Is the total capacity of all power sources in the grid;
step 5.2: the virtual inertia demand is distributed to the new energy stations according to the new energy capacity, i.e.
Wherein H j Is the virtual inertia command of the jth new energy station, S RE,j And P RE,j Is the capacity and output power of the jth new energy station, E sys The total calculated inertia of the power grid before virtual inertia configuration.
6. The new energy system online inertial configuration method based on the average system frequency model according to claim 5, characterized in that a frequency safety boundary for frequency safety evaluation is explicitly specified by grid specifications, or is determined by grid protection action thresholds, or is given by grid operators.
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