Control method for hybrid energy storage to participate in system frequency modulation

Abstract

The invention is suitable for the technical field of new energy, and provides a control method for hybrid energy storage to participate in system frequency modulation, which comprises the following steps: step 1, a pumped storage system model is established, a pumped storage unit is a hydroelectric coupling system, and an adjusting system is mainly composed of a speed regulator, an electro-hydraulic servo system, a pump turbine, a water diversion system and a motor part; the flywheel energy storage system is mainly composed of a flywheel, a power generation system and a converter, electric energy is released and absorbed through conversion between kinetic energy stored in a rotating body and the electric energy, and the flywheel energy storage system is connected with the power generation system for a large-amplitude and high-frequency power grid AGC frequency modulation signal instruction; the novel frequency modulation control strategy adopted by the invention can well predict the output size change of the pumped storage system, and further adjust the output size of the flywheel energy storage system.

Description

Technical Field

[0001] The present invention relates to the field of new energy technology, and in particular to a control method for hybrid energy storage participating in system frequency modulation. Background Art

[0002] Frequency is one of the three elements of power system operation. Ensuring its stability is one of the important goals for achieving safe operation of the national power system. Automatic generation control (AGC) is one of the main technical means for the national power system to achieve stable operation, and it can issue frequency regulation indicators of units in real time.

[0003] Flywheel energy storage is a physical energy storage system that meets the dual carbon goals while having the advantages of fast response speed, two-way output, and high power generation and reception efficiency. It is a very flexible and controllable clean energy. However, flywheel energy storage as a single system frequency regulation device has certain limitations, such as the capacity of the flywheel energy storage system, the terrain problems of the flywheel energy storage system construction, and the later maintenance problems of the flywheel energy storage system. In order to deal with the above problems to a certain extent, the advantages of each energy storage system can be fully utilized by adding a pumped storage system to build a hybrid energy storage system.

[0004] For hybrid energy storage systems, the most important thing is to study their control methods. How to reasonably dispatch the resources of various energy storages is an important research direction of current research. Appropriate control methods can make greater use of system resources, reduce the accident rate, and reduce equipment losses. This invention studies a new control method for hybrid energy storage frequency modulation based on model predictive control, which can achieve advanced regulation to a certain extent, so that the system has higher sensitivity and better stability.

[0005] With the rapid development of new energy power generation systems, the large-scale and extensive access of new energy, mainly photovoltaic and wind power, is an inevitable trend in the transformation and development of power grid energy. Since new energy power generation systems are random and volatile, and there are problems such as unbalanced layout of new energy installed capacity and reverse distribution of power generation and load, the problem of new energy system consumption is prominent, and the phenomenon of wind and solar power abandonment is obvious. Therefore, the present invention studies a control method for hybrid energy storage participating in system frequency modulation based on model predictive control, which is of great significance for guiding the efficient consumption of new energy. Summary of the invention

[0006] The purpose of the present invention is to solve at least one of the technical problems existing in the prior art and to provide a control method for hybrid energy storage participating in system frequency regulation, which can solve the problems of the background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: a control method for hybrid energy storage participating in system frequency modulation, comprising the following steps:

[0008] Step 1: Pumped storage system model. The pumped storage unit is a hydroelectric coupling system. Its regulation system is mainly composed of a speed governor, an electro-hydraulic follow-up system, a water pump turbine, a water diversion system and a motor.

[0009] Step 2: Flywheel energy storage system. The flywheel energy storage system is mainly composed of a flywheel, a power generation system and a converter. It releases and absorbs electrical energy by converting the kinetic energy stored in the rotating body into electrical energy. The energy stored in the flywheel energy storage system is related to the rotational inertia and speed of the flywheel rotor. The hybrid energy storage system control strategy. The pumped storage system is a commonly used frequency-regulated energy storage device that is widely used and stable. The addition of flywheel energy storage can make up for its shortcomings in response speed and output limit, and achieve complementarity in energy and power. However, the overall system still needs a reasonable control strategy to ensure that the pumped storage and flywheel can work in the healthiest state, and on this basis, optimize the overall frequency regulation performance of the system as much as possible.

[0010] This paper adopts the most commonly used droop control method for the pumped storage unit to make it track the AGC frequency modulation index of the power grid to output, and adopts the model predictive control method (MPC) for the output of the flywheel energy storage system to predict the output increment of the pumped storage system in real time, so as to control the output power of the flywheel energy storage system.

[0011] Step 3: Simulation analysis. Based on the above model and control strategy, a simulation platform for pumped storage units and flywheel energy storage units is built in the Matlab / Simulink software platform. The parameters of the pumped storage units and flywheel energy storage units in the system are shown in Table 1. The rated power and rated capacity of the flywheel energy storage system are 3MW / 0.15MWh. Figure 6 The fluctuation signal shown is simulated as the grid AGC frequency modulation signal input in the system, verifying the frequency modulation capability of the overall system under steady-state conditions, predicting the output variation curve of the pumped storage unit under a certain frequency modulation signal, and comparing it with the measured signal. Finally, the advantages and effectiveness of the system proposed in this paper in grid frequency modulation are verified.

[0012] Preferably, the third step: real-time output prediction of the pumped-storage unit is to simulate and study the feasibility and effect of the prediction model of the pumped-storage system in this article, input a certain disturbance signal into the overall system, and detect the guide vane opening of the pumped-storage unit and its output power change curve.

[0013] Preferably, the third step: comparison of frequency regulation effects of different energy storage schemes. In order to compare the advantages of this scheme in frequency regulation, this paper proposes three simulation schemes to compare their frequency regulation effects. The three schemes are as follows: only the pumped storage system participates in frequency regulation; the hybrid energy storage system participates in frequency regulation only in droop control mode; the hybrid energy storage system participates in frequency regulation under model predictive control.

[0014] Preferably, the third step: step disturbance, at t=2s, for a system in a stable operating state, a step load disturbance signal with an input amplitude of 4.7MW is input, and the frequency change curves of the overall system under three different schemes are statistically analyzed, such as Figure 8 As shown in the figure, the system frequency recovery speed is slow when only pumped storage participates in frequency regulation. Compared with the single energy storage system, the hybrid energy storage system has better resistance to step disturbances. The collaborative control strategy proposed in this paper reduces the time for the system frequency to recover stability by about 30%, and can raise the system frequency to a stable level faster.

[0015] Preferably, the fourth step: continuous disturbance. The simulation in the above section verifies the effect of the system under the anti-step disturbance signal. The access of the hybrid energy storage system can make the overall system have a better frequency regulation effect, and the addition of the collaborative control strategy can make the flywheel energy storage system operate in a safer and healthier state. However, the frequency fluctuations in reality are usually caused by continuous and irregular rapid fluctuations of small loads. Therefore, in order to ensure the feasibility of the scheme proposed in this paper, this paper inputs the grid AGC frequency modulation signal received by a power plant on a typical day in MATLAB to simulate a random continuous disturbance signal, and compares the system frequency changes under the three schemes to verify the effectiveness of this scheme.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] (1) The control method of the hybrid energy storage participating in the system frequency regulation, for the large-amplitude, high-frequency power grid AGC frequency regulation signal instructions, the new frequency regulation control strategy adopted by the present invention can well predict the output size change of the pumped storage system, and then adjust the output size of the flywheel energy storage system.

[0018] (2) The control method for the hybrid energy storage participating in the system frequency regulation aims to solve the problem that the capacity of the flywheel energy storage is difficult to consider. The present invention introduces a pumped storage system with controllable capacity and stable operation to compensate for the lack of frequency regulation margin.

[0019] (3) The control method for the hybrid energy storage participating in the system frequency regulation is aimed at the problems of slow frequency regulation response speed, long time for unit switching operation state, and frequency regulation vacancy period existing in the pumped storage unit. The present invention introduces a flywheel energy storage system with high power generation density and fast regulation speed to give full play to the advantages of each energy storage system in frequency regulation.

[0020] (4) The control method of the hybrid energy storage participating in the system frequency regulation introduces a SOC control scheme for the flywheel energy storage system, which limits its output to prevent overcharging or over-discharging, and can extend its service life to a certain extent and reduce operation and maintenance costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention is further described below in conjunction with the accompanying drawings and embodiments:

[0022] Figure 1 This is a structural diagram of the pumped storage system of the present invention;

[0023] Figure 2 This is a model block diagram of the pumped storage system of the present invention;

[0024] Figure 3 This is a charge and discharge curve diagram of the flywheel energy storage of the present invention;

[0025] Figure 4 This is a control block diagram of the hybrid energy storage system of the present invention;

[0026] Figure 5 This is the AGC frequency modulation curve diagram of the power grid of the present invention;

[0027] Figure 6 This is a prediction curve diagram of the guide vane opening of the pumped storage unit of the present invention;

[0028] Figure 7 The output power prediction curve diagram of the pumped storage unit of the present invention;

[0029] Figure 8 It is the frequency variation curve diagram of the system of the present invention;

[0030] Fig. 9 This is the output power diagram of the flywheel energy storage system of the present invention;

[0031] Fig.10 This is a frequency difference variation curve of the system of the present invention. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] In order to further understand the content of the present invention, the present invention is described in detail in conjunction with the accompanying drawings.

[0034] See also Figure 1-10The present invention provides a technical solution: a control method for hybrid energy storage participating in system frequency modulation.

[0035] Step 1: Pumped Storage System Model

[0036] The pumped storage unit is a hydroelectric coupling system, and its regulation system is mainly composed of a speed governor, an electro-hydraulic follow-up system, a water pump and turbine, a water diversion system and a motor. Figure 1 shown.

[0037] The pumped storage system has a variety of operating conditions, such as power generation, static, pumping, etc. When it is working and pumping, it can be simplified as a motor system with constant power. Therefore, this paper only considers its dynamic state when it is a generator. This paper considers the situation where the power system works near the stable operating point and undergoes small changes. Therefore, the pump turbine can be simplified as a linear state, and its transfer function is

[10] :

[0038]

[0039] In the formula, m t is the mechanical torque, q is the water flow rate, They are the transmission coefficients of torque to speed, guide vane opening and water head; They are the transfer coefficients of system flow to speed, guide vane opening and water head respectively.

[0040] When the power system is stable, the typical value of the turbine transfer coefficient can be used, that is, e y =1, e h =1.5, e qy =1, e qh = 0.5, after linearizing the turbine output and constructing the prime mover transfer function, the classic turbine rigid water hammer model can be obtained:

[0041]

[0042] Where y is the ideal guide vane opening, P m is the mechanical power, T w is the water flow inertia time constant.

[0043] At present, the electro-hydraulic servo system usually uses a digital electro-hydraulic speed control model to describe the operation of the speed regulator, and its transfer function is:

[0044]

[0045] In the formula, R T 、T R is the transient parameter of the system when water hammer occurs in the turbine; R d It is the speed regulator differential parameter.

[0046] The overall model block diagram of the pumped storage unit is as follows: Figure 2 As shown:

[0047] 2. Flywheel energy storage system

[0048] The flywheel energy storage system is mainly composed of a flywheel, a power generation system and a converter. It releases and absorbs electrical energy by converting the kinetic energy stored in the rotating body into electrical energy. The energy stored in the flywheel energy storage system is related to the rotational inertia and speed of the flywheel rotor, and its expression is as follows:

[0049]

[0050] Where E is the kinetic energy stored in the flywheel rotor; J is the moment of inertia of the flywheel rotor; ω is the operating speed of the rotor.

[0051] In simulation, the first-order inertia link is usually used to study the frequency modulation characteristics of the flywheel energy storage system, and its transfer function is:

[0052]

[0053] Where, T F is the time inertia constant of the flywheel energy storage system, which usually depends on the characteristics of the flywheel energy storage system itself.

[0054] The frequency regulation capability of the flywheel energy storage system is not only related to its own performance, but also affected by the real-time residual charge (State of Charge, SOC). The charge state of the flywheel energy storage system can be expressed as:

[0055]

[0056] In the formula, SOC t is the real-time charge state of the flywheel energy storage at time t; SOC0 represents the initial charge of the flywheel energy storage, is the output curve of the flywheel energy storage system, Reserve total energy for the flywheel energy storage system.

[0057] In order to ensure that the residual charge of the flywheel energy storage is in the optimal output range, this paper built a flywheel energy storage system model in Matlab / Simulink software and simulated the optimal charge range of the flywheel energy storage system. The simulation results are as follows: Figure 3 As shown:

[0058] According to the simulation results, when the SOC of the flywheel energy storage system exceeds 0.7, the system's ability to absorb electrical energy declines significantly. When the SOC is lower than 0.3, the system's discharge capacity also weakens rapidly. Therefore, when the SOC of the flywheel energy storage system is in the range of 0.3-0.7, the system has the best charging and discharging capacity.

[0059] According to this, the charge of the flywheel energy storage system is divided into three intervals: SOC h is the maximum energy storage limit point of flywheel energy storage, SOC l It is the minimum charge point of the flywheel energy storage.

[0060] In the optimal frequency modulation range, the Sigmod function is introduced for the flywheel energy storage system to smooth and limit the output of the flywheel energy storage system. The flywheel charging and discharging power constraint is:

[0061]

[0062] In the formula, SOC max is the maximum charge of the flywheel energy storage, SOC min is the minimum charge of flywheel energy storage, P m is the rated power of the flywheel, and SOC represents the real-time state of charge of the flywheel energy storage.

[0063] The logistic function is introduced to constrain the maximum output of the flywheel energy storage system at time t. The maximum discharge power at time t is as follows:

[0064]

[0065] Where P out (t) is the maximum discharge force of the flywheel energy storage system at the moment, P m is the rated power of the flywheel energy storage system, K, P0, P, r, and b are constants and are the characteristic parameters of the Logistic regression function.

[0066] 2 Hybrid Energy Storage System Control Strategy

[0067] The pumped storage system is a commonly used frequency regulation energy storage device that is widely used and stable in use. The addition of flywheel energy storage can make up for its shortcomings in response speed and output limit, and achieve complementarity in energy and power. However, the overall system still needs a reasonable control strategy to ensure that the pumped storage and flywheel can work in the healthiest state, and on this premise, optimize the overall frequency regulation performance of the system as much as possible.

[0068] This paper adopts the most commonly used droop control method for pumped storage units to track the AGC frequency modulation index of the power grid to output, and adopts the model predictive control method (MPC) for the output of the flywheel energy storage system to predict the output increment of the pumped storage system in real time, so as to control the output power of the flywheel energy storage system. The specific control strategy is as follows Figure 4 As shown:

[0069] To predict the incremental output of the pumped storage unit model, it is necessary to explore the relationship between the unit output and the guide vane opening, water hammer intensity and water level change. When performing a frequency modulation, the delay of the water level and the inertia of water hammer have limited impact on the unit output, so the change of the guide vane opening is mainly considered in the analysis.

[0070] The output power model of the pumped storage unit under the condition of ideal guide vane opening can be derived from equations (2) and (3):

[0071]

[0072] In the formula, ΔN p is the output power of the pumped storage unit.

[0073] After discretizing the variable relationship, an output power prediction model for pumped storage systems under MPC is obtained.

[0074]

[0075] Based on the above control strategy, flywheel energy storage can predict the output changes of the pumped storage system when the power system is modulating the frequency, and adjust its own output in advance according to the difference between its output and the grid frequency regulation demand, so as to ensure that the hybrid energy storage system has sufficient frequency regulation capability during frequency regulation and speed up the response speed of the frequency regulation system.

[0076] 3. Simulation analysis

[0077] Based on the above model and control strategy, a simulation platform for pumped storage units and flywheel energy storage units is built in the Matlab / Simulink software platform. The parameters of the pumped storage units and flywheel energy storage units in the system are shown in Table 1. The rated power and rated capacity of the flywheel energy storage system are 3MW / 0.15MWh. Figure 5 The fluctuation signal shown is simulated as the grid AGC frequency modulation signal input in the system, verifying the frequency modulation capability of the overall system under steady-state conditions, predicting the output variation curve of the pumped storage unit under a certain frequency modulation signal, and comparing it with the measured signal. Finally, the advantages and effectiveness of the system proposed in this paper in grid frequency modulation are verified.

[0078] Table 1 Hybrid energy storage system parameters

[0079]

[0080]

[0081] 3.1 Real-time output prediction of pumped storage units

[0082] In order to simulate the feasibility and effect of the prediction model of the pumped storage system in this paper, a certain disturbance signal is input into the overall system to detect the guide vane opening and output power change curve of the pumped storage unit. Figure 6 , Figure 7 They are the guide vane opening and output power change curves of the pumped storage unit respectively. It can be seen from the figure that the output change curve predicted by the MPC module cannot be completely fitted with the actual change curve, but the predicted curve has the same trend as the actual curve. By introducing the root mean square error, it can be seen that the root mean square error between the predicted output and the actual output is 0.991. This result verifies that the prediction model can meet the feasibility requirements of the scheme proposed in this paper.

[0083] 3.2 Comparison of frequency regulation effects of different energy storage solutions

[0084] In order to compare the advantages of this scheme in frequency modulation, this paper proposes three simulation schemes and compares their frequency modulation effects. The three schemes are as follows:

[0085] Only pumped storage systems participate in frequency regulation;

[0086] The hybrid energy storage system only participates in frequency regulation in droop control mode;

[0087] The hybrid energy storage system participates in frequency regulation under model predictive control.

[0088] 3.2.1 Step disturbance

[0089] At t = 2s, the system is in a stable operating state. A step load disturbance signal with an input amplitude of 4.7MW is input. The frequency change curves of the overall system under three different schemes are statistically analyzed, as shown in Figure 2. Figure 7 As shown in the figure, the system frequency recovery speed is slow when only pumped storage participates in frequency regulation. Compared with the single energy storage system, the hybrid energy storage system has better resistance to step disturbances. The collaborative control strategy proposed in this paper reduces the time for the system frequency to recover stability by about 30%, and can raise the system frequency to a stable level faster.

[0090] Figure 8 It is a detection curve of the output power of the flywheel energy storage system, comparing the output states of the flywheel energy storage system in Scheme 2 and Scheme 3. Compared with Scheme 2, the flywheel energy storage system in Scheme 3 can adjust its output in advance when receiving the frequency regulation signal of the system, and make up for the frequency regulation output gap of the pumped storage system when the output gradient is increased with a faster output increment.

[0091] 3.2.2 Continuous disturbance

[0092] The simulation in the above chapter verifies the effect of the system under the anti-step disturbance signal. The access of the hybrid energy storage system can make the overall system have a better frequency regulation effect, and the addition of the collaborative control strategy can make the flywheel energy storage system operate in a safer and healthier state. However, the frequency fluctuations in reality are usually caused by continuous and irregular rapid fluctuations of small loads. Therefore, in order to ensure the feasibility of the scheme proposed in this paper, this paper inputs the grid AGC frequency modulation signal received by a power plant on a typical day in MATLAB to simulate a random continuous disturbance signal, and compares the system frequency changes under the three schemes to verify the effectiveness of this scheme.

[0093] Fig. 9 The frequency change diagram of the overall system after the continuous disturbance signal is input into the system is compared with the frequency change diagrams of only the pumped storage system, the hybrid energy storage system using only droop control, and the coordinated control based on model predictive control.

[0094] Fig.10 The output power curve of the flywheel energy storage system under Scheme 2 and Scheme 3.

[0095] Through data analysis and calculation of the simulation structure, the hybrid energy storage system has better resistance than the single energy storage system when facing continuous and frequent irregular frequency fluctuations. The peak value of frequency change is reduced from 0.072Hz to below 0.05Hz, and the hybrid energy storage system under the collaborative control strategy reduces the peak value of frequency change from 0.05Hz to 0.040Hz, verifying the practicality of this solution.

[0096] Compared with the traditional droop control scheme, the collaborative control scheme can effectively smooth the output fluctuation of the flywheel energy storage system, improve the unit life of the flywheel energy storage system, and reduce the economic losses caused by equipment loss.

[0097] The embodiments of the present invention are described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in the technical field without departing from the purpose of the present invention.

Claims

1. A control method for hybrid energy storage participating in system frequency modulation, characterized in that: The following steps are involved: Step 1: Pumped storage system model. The pumped storage unit is a hydroelectric coupling system. Its regulation system is mainly composed of a speed governor, an electro-hydraulic follow-up system, a water pump turbine, a water diversion system and a motor. Step 2: Flywheel energy storage system. The flywheel energy storage system is mainly composed of a flywheel, a power generation system and a converter. It releases and absorbs electrical energy through the conversion between kinetic energy stored in the rotating body and electrical energy. The energy stored in the flywheel energy storage system is related to the rotational inertia and speed of the flywheel rotor. The hybrid energy storage system control strategy. The pumped storage system is a commonly used frequency-regulating energy storage device that is widely used and stable in use. The addition of flywheel energy storage can make up for its shortcomings in response speed and output limit, and achieve complementarity in energy and power. However, the overall system still needs a reasonable control strategy to ensure that the pumped storage and flywheel can work in the healthiest state, and on this basis, optimize the overall frequency regulation performance of the system as much as possible; This paper adopts the most commonly used droop control method for pumped storage units to make them track the AGC frequency modulation index of the power grid to output power, and adopts the model predictive control method (MPC) for the output of the flywheel energy storage system to predict the output increment of the pumped storage system in real time, so as to control the output power of the flywheel energy storage system; The third step: simulation analysis. Based on the above model and control strategy, a simulation platform for pumped storage units and flywheel energy storage units is built in the Matlab / Simulink software platform. The parameters of the pumped storage and flywheel energy storage units in the system are shown in Table 1. The rated power and rated capacity of the flywheel energy storage system are 3MW / 0.15MWh. The fluctuation signal shown in Figure 6 is intercepted and simulated as the grid AGC frequency modulation signal input in the system to verify the frequency modulation capability of the overall system under steady-state conditions. The output change curve of the pumped storage unit under a certain frequency modulation signal is predicted and compared with the measured signal. Finally, the advantages and effectiveness of the system proposed in this paper in grid frequency modulation are verified.

2. A control method for hybrid energy storage participating in system frequency modulation according to claim 1, characterized in that: The third step: real-time output prediction of the pumped storage unit is to simulate the feasibility and effect of the pumped storage system prediction model in this article, input a certain disturbance signal into the overall system, and detect the guide vane opening of the pumped storage unit and its output power change curve.

3. A control method for hybrid energy storage participating in system frequency modulation according to claim 1, characterized in that: The third step: comparison of frequency regulation effects of different energy storage schemes. In order to compare the advantages of this scheme in frequency regulation, this paper proposes three simulation schemes and compares their frequency regulation effects. The three schemes are as follows: only the pumped storage system participates in frequency regulation; the hybrid energy storage system participates in frequency regulation only in droop control mode; the hybrid energy storage system participates in frequency regulation under model predictive control.

4. A control method for hybrid energy storage participating in system frequency modulation according to claim 3, characterized in that: The third step: step disturbance. At t=2s, the system is in a stable operating state, and a step load disturbance signal with an input amplitude of 4.7MW is input. The frequency change curve of the overall system under three different schemes is statistically analyzed, as shown in Figure 8. The system frequency recovery speed is slow when only pumped storage participates in frequency regulation. Compared with the single energy storage system, the hybrid energy storage system has better resistance to step disturbances. The collaborative control strategy proposed in this paper reduces the time for the system frequency to recover stability by about 30%, and can raise the system frequency to a stable level more quickly.

5. The control method for hybrid energy storage participating in system frequency modulation according to claim 4 is characterized by: The fourth step: continuous disturbance. The simulation in the above chapter verifies the effect of the system under the anti-step disturbance signal. The access of the hybrid energy storage system can make the overall system have a better frequency regulation effect, and the addition of the collaborative control strategy can make the flywheel energy storage system operate in a safer and healthier state. However, the frequency fluctuations in reality are usually caused by continuous and irregular rapid fluctuations of small loads. Therefore, in order to ensure the feasibility of the scheme proposed in this paper, this paper inputs the grid AGC frequency modulation signal received by a power plant on a typical day in MATLAB to simulate a random continuous disturbance signal, and compares the system frequency changes under the three schemes to verify the effectiveness of this scheme.

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