CN113839386B - Control method for restraining power grid frequency fluctuation under island medium-voltage micro-grid - Google Patents
Control method for restraining power grid frequency fluctuation under island medium-voltage micro-grid Download PDFInfo
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- H02J2300/22—The renewable source being solar energy
- H02J2300/24—The renewable source being solar energy of photovoltaic origin
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- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
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- H02J2300/28—The renewable source being wind energy
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- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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- H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
- H02J2300/40—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation wherein a plurality of decentralised, dispersed or local energy generation technologies are operated simultaneously
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Abstract
The invention discloses a situation awareness and guide method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid; the situation sensing and beneficial guiding method is combined with the island medium-voltage micro-grid for the first time, so that the island medium-voltage micro-grid can continuously stabilize the frequency when the output power of the distributed power supply fluctuates; modeling the island medium-voltage micro-grid; situation awareness based on historical data; three parts of situation profit guiding method for restraining frequency fluctuation; according to the method provided by the invention, when the island medium-voltage micro-grid is operated, the information acquisition unit firstly extracts the wind speed of the fan and the frequency information of the power grid as historical data, then carries out situation awareness, carries out advanced prediction on the frequency situation of the power grid system, then obtains the predicted values of active power and reactive power according to the coupling relation of the island medium-voltage micro-grid, carries out situation decision system and situation execution system control to realize a situation beneficial guide function, can effectively inhibit frequency fluctuation, reduces impact on the power grid system, and enables the frequency of the system to keep stable when the output power of the distributed power supply fluctuates.
Description
Technical Field
The invention relates to the technical field of monitoring, analysis and control of micro-grid systems, in particular to a control method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid.
Background
On islands with larger areas, the power consumption requirement is large, the distance is far, the islands cannot be connected with a large power grid, and the distance between power generation facilities and electric equipment on the islands is far, so that the medium-voltage island micro-power grid is established for reducing the electric energy loss and improving the power transmission efficiency. On islands, the light energy and wind energy are sufficient, the duty ratio of photovoltaic power generation and wind power generation is continuously improved, and the use of diesel engines and energy storage equipment can be reduced so as to reduce the cost. However, with the continuous improvement of photovoltaic power generation and wind power generation, the system power unbalance caused by the intermittence and fluctuation of the light energy and the wind energy causes frequency fluctuation, and reduces the electric energy quality. Therefore, how to stabilize the frequency of the isolated medium-voltage micro-grid has been studied and paid attention to.
Photovoltaic and wind generators are typically connected to the grid through an inverter. The inverter simulates the primary frequency modulation balance power of a traditional generator by using a droop control method, thereby causing the frequency shift of a power grid. Several researchers introduced virtual impedance techniques to achieve reasonable distribution of their output power. The method for simulating the secondary frequency modulation of the power system adjusts the output power of the distributed power supply, and reduces the frequency offset is paid attention to by a plurality of researchers.
The power control method adopted in the frequency modulation process is different due to different power grid line parameters of different voltage classes. In low-voltage power networks, the line resistance is far greater than the line inductance, the power network is resistive, the frequency is proportional to reactive power, and the reactive power needs to be controlled to reduce frequency fluctuation. In a high-voltage power grid, the line inductance is far greater than the line resistance, the power grid is inductive, the frequency is proportional to the active power, and the active power needs to be controlled to reduce the frequency fluctuation. The medium voltage power network is different from the low voltage power network and the high voltage power network, and the line resistance and the line reactance value are approximately equal, so that strong coupling is presented. In the medium-voltage micro-grid, only active power or reactive power is regulated, and a good frequency modulation effect cannot be achieved.
Disclosure of Invention
In order to solve the technical problems, the invention provides a control method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid. According to the method, through research on coupling characteristics of the medium-voltage island micro-grid, the principle of frequency modulation by using distributed energy reserve power is analyzed, and situation awareness and situation profit are combined. According to the method, an optimal frequency modulation scheme is formulated according to situation awareness information, the running state and constraint conditions, and the island micro-grid is ensured to run in a stable state.
In order to achieve the above purpose, the invention adopts the following technical scheme: the synchronous inverter grid-connected operation control system comprises the following components: the system comprises a synchronous inverter control module, an inverter output end positive-negative sequence current/voltage decomposition unit, a grid side power supply end positive-negative sequence voltage decomposition unit and a system parameter situation prediction method during voltage unbalance: the method comprises the steps of an inverter output end current acquisition module, a current situation identification and evaluation method, a current situation prediction model and a system state potential guide execution method when voltage is unbalanced: the method comprises a current imbalance situation presenting unit, a situation profit judging condition setting and a condition threshold value control negative sequence controller active decision making method.
A control method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid is characterized by comprising the following steps of: the system comprises a data extraction and storage module, a situation awareness module and a situation benefit guiding module, wherein the data extraction and storage module comprises a wind speed data acquisition unit, a power grid frequency data acquisition unit and a data storage unit, the situation awareness module comprises a data preprocessing unit, a data classification unit, a prediction model training unit, a medium-voltage island micro-power grid frequency and active power reactive power relation type and a data prediction unit, and the situation benefit guiding module comprises a decision unit, a scheme storage unit, a scheme decomposition unit and an execution distribution unit.
A control method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid comprises the following steps:
step one, situation awareness is carried out on frequency historical data of the island medium-voltage micro-grid, the frequency historical data is divided into training data and test data, the training data is used for model training, a frequency trend prediction model is established, the test data is used for model prediction, a grid frequency prediction value is obtained,
and step two, simplifying the coupling relation between the frequency voltage of the medium-voltage micro-grid and the active power and the reactive power to obtain a simplified relation. The predicted value of the power grid frequency is used as input quantity, the active power and the reactive power required by the power grid system are obtained by calculating according to the coupling relation of the medium-voltage micro-grid frequency and the active power and the droop relation of the frequency and the active power,
step three, the obtained active power and reactive power are manufactured into input quantities, and the effective plan is made and reasonably distributed by combining the running condition of the power grid and the state potential profits of the frequency modulation capability,
and step four, respectively establishing an active power control system and a reactive power control system of the two-stage photovoltaic power generation system in the island medium-voltage micro-grid, taking the required active power and reactive power as tracking objects of the controller, synthesizing the output signals of the controller into PWM modulation signals, regulating the active power and reactive power output of the two-stage photovoltaic power generation system, and stabilizing the frequency.
In the first step, wind speed data and frequency data at the same moment are extracted by a wind speed acquisition module and a power grid frequency acquisition module to serve as historical state information. The wind speed value and the frequency value at the moment T are collected by a wind speed collection module and a power grid frequency collection module to be used as a sequence S T ＝[t T ,f T ,V T ]And synthesizes the sequences into a sequence group K= [ S ] 1 ,S 2 ,…,S m …]m is E N, the normalized treatment is carried out on the decomposed components by adopting the min-max standardization, and the components are transformed into [0,1 ]]Normalization is performed according to the following formula:
wherein x is norm Is the value after normalization; x is x min Is the minimum in the dataset; x is x max Is the maximum in the dataset.
Inputting the normalized sequence group K into an LSTM network for prediction, and dividing the preprocessed data K into training data K 1 And test data K 2 Training data K 1 For training LSTM, after obtaining corresponding model, inputting predictive data K 2 The prediction is performed such that,and carrying out inverse normalization on the predicted result to obtain a final predicted value.
In the second step, the inverter power supply is controlled by sagging to obtain command values of output voltage and frequency, and the active power and reactive power of the system are reasonably distributed by adjusting the output voltage and frequency. Likewise, the voltage and frequency can be adjusted by controlling the active and reactive power of the system. The inverter of the distributed power supply adopts droop control, so that a calculation formula of a power reference value can be obtained:
in the medium-voltage independent microgrid, the values of the resistor R and the reactance X are approximately equal, and r≡x=a can be set to have strong coupling.
In a medium voltage independent microgrid, the power transfer relationship of DG and PCC is as follows:
wherein S, P, Q is respectively the complex power, active power and reactive power output by DG, U 1 U is the DG side voltage value 2 The PCC side voltage value is referred to as delta, and the power angle is referred to as delta.
The power angle delta is very small, sin delta is approximately equal to delta, cos delta is approximately equal to 1, and the expression of the phase angle delta can be obtained:
and calculating the power grid frequency predicted value as an input quantity according to the coupling relation between the medium-voltage micro-grid frequency and the active power and reactive power to obtain the active power and reactive power value required by the power grid system.
In the control method for suppressing the power grid frequency fluctuation under the island medium-voltage micro-grid, in the third step, situation profit is divided into situation decision and situation execution. The situation decision is mainly based on the state of the current system operation main body and the prediction direction of the future situation. After the power reference value is obtained, the power reference value is distributed to each distributed power supply according to the reserve power condition of the distributed power supply in a certain proportion.
Wherein alpha is i The i-th part of the micro-net is assigned a coefficient,
to achieve reasonable power distribution of each part, the distribution coefficient should be compared with the rated capacity thereof, i.e
α 1 :…:α n ＝P 1.N :…:P n.N β 1 :…:β n ＝Q 1.N :…:Q n.N
Wherein P is n.N 、Q n.N Active and reactive power is reserved for the nth DG.
Task allocation requires the following conditions:
wherein x is i For regulating the task quantity g of the ith distributed power supply of the system i (x i ) For the effect of the ith distributed power supply of the system after task adjustment, a is the total task amount required to be distributed to the system.
When the tasks are distributed, the tasks are distributed according to the capacity of each part of the system and a certain task starting time is spaced to achieve the optimal adjustment effect, as shown in the formula:
where t is the switching interval time, K is the system sensitivity, G is the switching part occupation, and G is the total system.
In the fourth step, the two-stage photovoltaic power generation system refers to a control method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid, wherein after voltage amplitude change is carried out on a photovoltaic array through a boost circuit, direct current is changed into alternating current through an inverter, and grid connection is achieved. In a boost circuit, a cycle can be analyzed in two stages according to the on and off states of a controllable device. The mathematical model of the on state of the switching device is:
wherein I is PV U is the output current of the photovoltaic array PV The output voltage of the photovoltaic array is L is Boost chopper circuit inductance, C is Boost chopper circuit capacitance, u L(on) 、i C(on) Is the capacitance voltage and inductance current in the on state of the switching device.
The mathematical model of the off state of the switching device is
Wherein u is L(off) 、u C(off) For the capacitive voltage and inductive current in the off state of the switching device). The average value of the inductance voltage and the capacitance current in one switching period is as follows:
when the circuit reaches a steady state, the average value of the inductance voltage and the capacitance current is 0 in one switching period, and the mathematical expression is as follows:
u in PV Is the output voltage of the photovoltaic array, U dc Is the voltage after passing through the Boost chopper and D is the duty cycle.
By controlling the duty cycle, photovoltaic operating point control is achieved, thereby regulating its output active power. In a two-stage photovoltaic power generation system, reactive power is output by using the residual capacity of an inverter, and the maximum reactive power output capacity is:
wherein Q is PV.max For the maximum output reactive power of the inverter at the moment, S PV For the rated capacity of the inverter, P PV The active power output by the inverter at this moment.
After the active power predicted value is obtained, the reactive power Q is calculated, and the reactive power Q is input into an inverter for reactive power regulation. Under the combined action of active power and reactive power adjustment, the medium-voltage island micro-grid frequency is stable.
The invention has the beneficial effects that:
the grid frequency and wind speed data are first stored for use as historical data. The historical data is divided into training data and prediction data, and the training data forms a frequency prediction model through a training long-term and short-term memory network. The method comprises the steps of inputting predicted data into a frequency, obtaining a frequency predicted value by a prediction model, calculating the relation between active power, reactive power and frequency of a medium-voltage island micro-grid to obtain the active power and the reactive power required by the medium-voltage island micro-grid in the future, inputting the predicted information into a memory system in a situation decision system as predicted information, storing a large number of historical decision schemes in the memory system, searching the predicted information in the memory system, finding out and outputting the corresponding historical decision schemes, if not, transmitting the corresponding historical decision schemes to the decision system, designing a new decision scheme, and transmitting the new decision scheme to the memory system for storage and output. According to the current situation of the current system, the scheme is decomposed into a plurality of targets and transmitted to the target distribution system. The target distribution system can match the target with the capacity of each part of the system, and the matched target is transmitted to each part of the system for execution, so that the scheme is completed, and the suppression of the frequency fluctuation of the medium-voltage island micro-grid is realized. The whole control process realizes the closed-loop control of the frequency of the medium-voltage island micro-grid, can normally operate under the condition of fluctuation of power variation frequency, can balance power, inhibits the fluctuation of the frequency of the system, avoids the influence of the system on the system, and improves the autonomous capacity of the independent micro-grid.
Drawings
Figure 1 is a frame diagram of the overall structure of the system of the invention,
figure 2 is a graph of power transfer from a distributed power source to a common node of a medium voltage island micro-grid in accordance with the method of the present invention,
figure 3 is an internal operation diagram of the situation decision system of the method of the invention,
FIG. 4 is an internal operation diagram of the method scheme execution system of the invention.
Detailed Description
The invention is further described below with reference to the drawings and examples.
As shown in fig. 1, the system overall structure of the invention comprises a data extraction and storage module, a situation awareness module and a situation benefit guiding module, wherein the data extraction and storage module comprises a wind speed data acquisition unit, a power grid frequency data acquisition unit and a data storage unit, the situation awareness module comprises a data preprocessing unit, a data classification unit, a prediction model training unit, a medium-voltage island micro-power grid frequency and active power reactive power relation type and a data prediction unit, and the situation benefit guiding module comprises a decision unit, a scheme storage unit, a scheme decomposition unit and an execution distribution unit. Firstly, collecting wind speed data and frequency data of a micro-grid through a sensor to form historical data, preprocessing the historical data, and classifying the historical data into training data and prediction data. The training data is used for training a frequency prediction model, and the frequency prediction value is obtained by the frequency prediction model through the prediction data. Inputting the prediction information into a memory system in the situation decision system, storing a large number of historical decision schemes in the memory system, searching the prediction information in the memory system, finding out the corresponding historical decision scheme and outputting, if not, transmitting the prediction information to the decision system, designing a new decision scheme, and transmitting the new decision scheme to the memory system for storage and output. According to the current situation of the current system, the scheme is decomposed into a plurality of targets and transmitted to the target distribution system. The target distribution system can match the target with the capacity of each part of the system, and the matched target is transmitted to each part of the system for execution, so that the scheme is completed, and the suppression of the frequency fluctuation of the medium-voltage island micro-grid is realized.
Droop control simulates one-time adjustment of a traditional power system, is realized by controlling an inverter, and is essentially realized by converting output voltage and current of a micro power supply into active power and reactive power by using a power detection device, and then performing approximate decoupling treatment on the active power and the reactive power by using reference power, rated voltage and frequency. The inverter power supply can obtain instruction values of output voltage and frequency through droop control, and the active power and reactive power of the system are reasonably distributed through adjusting the output voltage and the frequency. Likewise, the voltage and frequency can be adjusted by controlling the active and reactive power of the system. Fig. 2 shows the power transfer relationship of DG and PCC. From the graph, it can be derived that DG output complex power expression is
The active power P and reactive power Q delivered to the PCC by DG are:
in the medium voltage line, since the values of the resistance R and the reactance X are approximately equal, r≡x=a can be assumed. If the power angle delta is small, sin delta is approximately equal to delta, cos delta is approximately equal to 1, and then the expression of the voltage phase angle delta and the voltage drop delta U can be obtained:
and is also provided withIt can thus be seen that in an island medium voltage micro grid, the frequency is affected by both active and no power. The frequency of the isolated island medium-voltage micro-grid is stabilized, and active power and reactive power need to be adjusted simultaneously.
Fig. 3 is an internal operation diagram of a situation decision system of the method, and the situation decision is mainly to make corresponding decisions according to the state of the current system operation main body and the prediction direction of the future situation. The situation decision comprises two major parts, namely a decision system and a memory system. The decision system comprises a decision principle and a calculation principle. The decision principle is a system decision criterion manually specified during system construction. The calculation principle is that the situation direction faced by a future system operation main body is calculated by manually specifying a predicted value when the system is built. The decision system calculates the future situation direction of the system according to the predicted value under the rule of the decision principle through the calculation principle.
First, history prediction data and corresponding history schemes are stored in a memory system:
D n ＝{[f TP1 ,M plan1 ],[f TP2 ,M plan2 ],…,[f TPn ,M plann ]}n∈N
wherein D is n To memorize the set of all historical prediction data and corresponding historical schemes in the system,the data and corresponding historical schedule sets are predicted for the nth history.
The new prediction data is input into the memory system for searching, the corresponding history scheme is found out and output, if not, the prediction data is input into the decision system to make a new decision scheme:
M prog ＝V(f TP )+E
wherein M is prog For decision-making systems to formulate corresponding schemes, V (F TP ) E is a variable related to the current system state, which is a value obtained according to the calculation principle.
The newly formulated decision scheme and the corresponding prediction information are combined into a combination input memory system for storing and outputting the scheme:
D n+1 ＝D n +[f TPn+1 ,M plann+1 ]
wherein D is n+1 D for updated historical prediction data and corresponding historical scheme group n To update the pre-update historical prediction data and the corresponding set of historical schemes,is new data.
FIG. 4 is an internal operation diagram of the method scheme execution system of the invention. When the predicted information is input into the situation decision-making system, searching whether the same situation exists in the memory system, if so, directly inputting the corresponding decision into the lower-level execution system, otherwise, inputting the predicted information into the decision-making system, calculating a new decision, and transmitting the new decision to the scheme execution system. At the same time, the memory system will memorize the new predicted value and decision scheme.
The scheme execution refers to reasonably decomposing and distributing the scheme formed by situation decision according to the current form of the system and the capability of the system, and is effective in scheme execution. After the decision-making system receives the upper new scheme, the scheme is decomposed into a plurality of targets according to the current situation of the current system, and the targets are transmitted to the target distribution system. The target distribution system matches the target with the capabilities of the parts of the system and transmits the matched target to the parts of the system for execution to complete the scheme.
After the scheme is input into a situation execution system, the scheme is divided into a plurality of plans according to the running condition and execution capacity of the system and is reasonably distributed:
M prog ＝Plan 1 +Plan 1 +…+Plan n n∈N
wherein Plan is n The n-th plan is after the scheme is according to the system running condition and execution capacity.
After the power reference is obtained, the power reference is distributed to each distributed power supply according to the reserve power condition of the distributed power supply in a certain proportion.
Wherein alpha is i The i-th part of the micro-net is assigned a coefficient,to achieve reasonable power distribution of each part, the distribution coefficient is proportional to the rated capacity, i.e
α 1 :…:α n ＝P 1.N :…:P n.N
Wherein P is i,N Is the rated power of the ith DG.
Task allocation requires the following conditions:
wherein x is i For regulating the task quantity g of the ith distributed power supply of the system i (x i ) For the effect of the ith distributed power supply of the system after task adjustment, a is the total task amount required to be distributed to the system.
When the tasks are distributed, the tasks are distributed according to the capacity of each part of the system and a certain task starting time is spaced to achieve the optimal adjustment effect, as shown in the formula:
where t is the switching interval time, K is the system sensitivity, G is the switching part occupation, and G is the total system.
To further analyze the effectiveness of the methods used herein, a determination is made regarding the state when the active power imbalance causes frequency fluctuations, establishing a correlation indicator.
Index 1: frequency fluctuation coefficient M
A frequency fluctuation coefficient is provided which reflects the magnitude of the frequency fluctuation over a period of time. The frequency fluctuation coefficient is a numerical value obtained by extracting the difference between the corresponding frequency and the standard frequency and accumulating the data obtained by squaring the difference in a fixed time and every small time interval, so as to obtain the frequency fluctuation amplitude value reflected by people in a period of time. The frequency fluctuation coefficient is calculated as follows:
wherein M is a frequency fluctuation coefficient, f n For rated frequency of the power grid, Y is the number of segments of interception time, f 1 ,f 2 ,f 3 ...f n Is the corresponding frequency of the y-period time.
Index 2: frequency out of limit times N
The voltage frequency of China is 50Hz, various distributed energy sources and micro-grids are designed by taking 50Hz as a reference, and the normal running range of the frequency in a power system of China is 50+/-0.2 Hz. The frequency out-of-limit number N refers to the number of times that the average fluctuation amplitude of the frequency per minute exceeds the normal operation range in the operation of the power grid.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.
Claims (4)
1. A control method for restraining power grid frequency fluctuation under an island medium-voltage micro-grid comprises the following steps:
step one, situation awareness is carried out on frequency historical data of the island medium-voltage micro-grid, the frequency historical data is divided into training data and test data, the training data is used for model training, a frequency trend prediction model is established, the test data is used for model prediction, a grid frequency prediction value is obtained,
step two, simplifying the coupling relation between the frequency voltage of the medium-voltage micro-grid and the active power and the reactive power to obtain a simplified relation, taking a predicted value of the frequency of the grid as an input quantity, calculating according to the simplified coupling relation between the frequency of the medium-voltage micro-grid and the active power and the reactive power and the sagging relation between the frequency and the active power to obtain the active power and the reactive power value required by the grid system,
step three, the obtained active power and reactive power are manufactured into input quantities, an effective plan is made and reasonable distribution is carried out by combining the running condition of a power grid and the state benefit of frequency modulation capability, the state benefit is divided into state decision and state execution, and the state decision is mainly to make corresponding decisions according to the state of a current system running main body and the prediction direction of future states; after the power reference value is obtained, distributing the power reference value to each distributed power supply according to a certain proportion according to the reserve power condition of the distributed power supply;
wherein alpha is i The i-th part of the micro-net is assigned a coefficient,
to achieve reasonable power distribution of each part, the distribution coefficient should be compared with the rated capacity thereof, i.e
α 1 :…:α n ＝P 1.N :…:P n.N β 1 :…:β n ＝Q 1.N :…:Q n.N
Wherein P is n.N 、Q n.N Reserve active and reactive power for nth DG;
task allocation requires the following conditions:
wherein x is i For regulating the task quantity g of the ith distributed power supply of the system i (x i ) The method comprises the steps that the effect generated by an ith distributed power supply of the system after task adjustment is performed is that a is the total task quantity required to be distributed to the system;
when the tasks are distributed, the tasks are distributed according to the capacity of each part of the system and a certain task starting time is spaced to achieve the optimal adjustment effect, as shown in the formula:
wherein t is the switching interval time, K is the system sensitivity, G is the switching part occupation, and G is the total system;
and step four, respectively establishing an active power control system and a reactive power control system of the two-stage photovoltaic power generation system in the island medium-voltage micro-grid, taking the required active power and reactive power as tracking objects of the controller, synthesizing the output signals of the controller into PWM modulation signals, regulating the active power and reactive power output of the two-stage photovoltaic power generation system, and stabilizing the frequency.
2. The control method for suppressing grid frequency fluctuation under an island medium-voltage micro-grid according to claim 1, wherein in the first step, the wind speed data and the frequency data at the same moment are extracted as historical state information by a wind speed acquisition module and a grid frequency acquisition module; the wind speed value and the frequency value at the moment T are collected by a wind speed collection module and a power grid frequency collection module to be used as a sequence S T ＝[t T ,f T ,V T ]And synthesizes the sequences into a sequence group K= [ S ] 1 ,S 2 ,…,S m …]m is E N, the normalized treatment is carried out on the decomposed components by adopting the min-max standardization, and the components are transformed into [0,1 ]]Normalization is performed according to the following formula:
wherein x is norm Is the value after normalization; x is x min Is the minimum in the dataset; x is x max Is the maximum in the dataset;
inputting the normalized sequence group K into an LSTM network for carrying outPredicting, dividing the preprocessed data K into training data K 1 And test data K 2 Training data K 1 For training LSTM, after obtaining corresponding model, inputting predictive data K 2 And predicting, and performing inverse normalization on the prediction result to obtain a final prediction value.
3. The control method for suppressing the fluctuation of the grid frequency under the island medium voltage micro-grid according to claim 2, wherein in the second step, the command values of the output voltage and the frequency can be obtained by controlling the inverter power supply through sagging, the active power and the reactive power of the system are reasonably distributed by adjusting the output voltage and the frequency, the voltage and the frequency can be adjusted by controlling the active power and the reactive power of the system, and the inverter of the distributed power supply adopts sagging control, so that the calculation formula of the power prediction value can be obtained:
in a medium voltage independent microgrid, the power transfer relationship of DG and PCC is as follows:
wherein S, P, Q is respectively the complex power, active power and reactive power output by DG, U 1 U is the DG side voltage value 2 For the PCC side voltage value, delta is the power angle,
sin δ≡δ, cos δ≡1, the expression of phase angle δ can be obtained:
and calculating the power grid frequency predicted value as an input quantity according to the coupling relation between the medium-voltage micro-grid frequency and the active power and reactive power to obtain the active power and reactive power value required by the power grid system.
4. The control method for suppressing the fluctuation of the grid frequency under the island medium-voltage micro-grid according to claim 2, wherein in the fourth step, the two-stage photovoltaic power generation system is that the photovoltaic array changes the voltage amplitude through a boost circuit and then changes the direct current into alternating current through an inverter to realize grid connection; in a boost circuit, a period can be divided into two stages for analysis according to the on and off states of a controllable device; the mathematical model of the on state of the switching device is:
wherein I is PV U is the output current of the photovoltaic array PV The output voltage of the photovoltaic array is L is Boost chopper circuit inductance, C is Boost chopper circuit capacitance, u L(on) 、i C(on) For the capacitive voltage and the inductive current in the on state of the switching device,
the mathematical model of the off state of the switching device is
Wherein u is L(off) 、u C(off) For the capacitance voltage and the inductance current of the switching device in the off state, the average value of the inductance voltage and the capacitance current in one switching period is:
when the circuit reaches a steady state, the average value of the inductance voltage and the capacitance current is 0 in one switching period, and the mathematical expression is as follows:
wherein U is PV Is the output voltage of the photovoltaic array, U dc Is the voltage after passing through the Boost chopper, D is the duty cycle;
the control of the photovoltaic operation point is realized by controlling the duty ratio, so that the output active power of the photovoltaic operation point is regulated; in a two-stage photovoltaic power generation system, reactive power is output by using the residual capacity of an inverter, and the maximum reactive power output capacity is:
wherein Q is PV.max For the maximum output reactive power of the inverter at the moment, S PV For the rated capacity of the inverter, P PV Active power output by the inverter at this moment;
after the active power predicted value is obtained, calculating reactive power Q, and inputting the reactive power Q into an inverter for reactive power regulation; under the combined action of active power and reactive power adjustment, the medium-voltage island micro-grid frequency is stable.
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