Frequency support method of asynchronous interconnection networking system

Abstract

The invention relates to the technical field of networking type energy storage, and discloses a frequency support method of an asynchronous interconnection networking system. Firstly, two independent new energy isolated network systems are connected through high-voltage direct-current power transmission, and each new energy isolated network system comprises a network construction type energy storage device and new energy power generation equipment; an additional frequency controller is introduced into a constant power controller on a constant power side of high-voltage direct current transmission, and the frequency difference of the two sides of the asynchronous interconnection networking system is monitored in real time; the frequency deviation amplitude limiter is used for limiting the frequency difference of the two sides of the asynchronous interconnection networking system and restraining the frequency fluctuation of the support side power grid; and generating a direct-current power additional quantity according to the frequency difference of the two sides of the asynchronous interconnection networking system, and dynamically adjusting the direct-current power reference value of the VSC at the fixed power side to realize sharing of standby frequency resources at the two sides. According to the method, isolated network systems of different network construction type energy storage supporting new energy power generation are interconnected through high-voltage direct current power transmission, additional frequency control is combined to reduce the frequency deviation of the two sides of the interconnection system, and the double-side frequency support capability of the asynchronous interconnection system is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-type energy storage technology, and in particular to a frequency support method for an asynchronous interconnected grid-type system. Background Art

[0002] In the construction of new power systems, energy storage, as a resource with excellent regulation capabilities and flexible use, plays a vital role. Energy storage can provide services in power systems, including frequency regulation, peak shaving and valley shifting, and smoothing fluctuations in the integration of renewable energy sources. Because energy storage devices have an independent energy source, they are ideal vehicles for grid-connected control technologies. Grid-connected energy storage offers all the functions of conventional energy storage and can play a supporting role in power systems similar to conventional synchronous generators.

[0003] Grid-connected energy storage technology uses control strategies to make the converter behave like a voltage source. Crucially, it can fully accommodate renewable energy generation equipment such as wind and photovoltaic power generation, something its grid-following counterpart cannot. Grid-connected converters actively establish reference frequency and voltage, while grid-following converters require external support and references for stable operation. Therefore, a converter control strategy configuration combining grid-following renewable energy generation and grid-connected energy storage can operate within a complete power electronics system without traditional generators, achieving 100% renewable energy island operation.

[0004] The overall control structure diagram of a typical grid-type converter is as follows: Figure 1 First, collect the voltage U at the grid connection point abc and current I abc The collected voltage and current signals are input into the power calculation link; the outputs P and Q of the power calculation link are input together with the given reference value of the system into the outer loop control link of the network to calculate and generate U and θ, which are then fed into the inner loop. The inner loop control part outputs the rotating coordinate system voltage e of the converter dq Finally, the coordinate transformation generates the PWM modulation signal. Compared with the grid-following control, the above control process does not require the use of the PLL link in the overall control link of the grid-forming converter to synchronize with the grid.

[0005] The outer-loop control portion of the aforementioned grid-type converter control structure is key to achieving grid-type control functionality and distinguishing between various types of grid-type control strategies. This outer-loop control primarily simulates the operating mechanism of synchronous motors, using control strategies to impart performance, regulation, and analysis methods to power electronics devices similar to those of synchronous motors. Synchronous generators exhibit active power-frequency (Pf) and reactive power-voltage (QU) droop characteristic curves, as well as rotor inertia and electromagnetic properties. By simulating these distinct synchronous generator operating characteristics, power electronics can implement corresponding grid-type control functionality.

[0006] The power grids at both ends of the asynchronous interconnection are connected to each other through HVDC. Under normal operating conditions, constant active power is transmitted from one end to the other to achieve power exchange.

[0007] Asynchronous interconnected power grids can operate independently at different frequencies, but they cannot share the frequency regulation backup resources of the power grids on both sides in asynchronous operation mode. They can only respond to power fluctuations through the frequency regulation capabilities of their own power grids. In the system where renewable energy power generation is supported by grid-connected power grids, photovoltaic, wind turbine and other power generation equipment are in maximum power point operation mode (MPPT) and do not participate in system frequency regulation. Therefore, only the grid-connected energy storage controlled by VSG responds to load changes in frequency. The power-frequency characteristics of the power system are as follows: Figure 2 shown.

[0008] Depend on Figure 2 It can be seen that when the system has a load change ΔP D0 The frequency change is:

[0009] ΔP D0 =ΔP G -ΔP D =-K G Δf-K D Δf

[0010] =-(K G +K D )Δf

[0011] Where: K G is the power-frequency characteristic coefficient of the grid-type energy storage using the VSG control strategy, K D is the frequency regulation effect coefficient of the load.

[0012] As the above equation shows, when a power imbalance occurs at one end of the AC system in asynchronous operation, maintaining power balance relies entirely on the power-frequency characteristics of the grid-connected energy storage and the load, placing high demands on the frequency regulation capabilities of the energy storage unit. If the system's frequency regulation capacity is insufficient, an accident could lead to excessive deviations in the system's quasi-steady-state frequency, triggering the activation of low-frequency load shedding (high-frequency generator shedding) devices and compromising the safe and stable operation of the system and equipment.

[0013] Existing technology and its shortcomings:

[0014] Existing Solution 1: Zhou Baorong et al., in their paper "Research on the Transition from Synchronous to Asynchronous Operation of the Southern Power Grid," proposed asynchronous interconnection, linking the Yunnan and Southern Power Grids. This approach effectively mitigates the risk of system instability caused by complex faults such as multi-circuit DC pole blocking in the Southern AC / DC synchronous grid, significantly improving the power angle stability of the Southern Power Grid's main grid. However, this measure fails to address the overall frequency stability of the asynchronous interconnected system and only provides frequency control on one side of the interconnected system.

[0015] Existing Solution 2: In the paper "Analysis of Security and Stability Characteristics of the Southwest Power Grid After Asynchronous Interconnection," Chen Gang et al. implemented asynchronous operation between the Central China Power Grid and the Sichuan-Chongqing Power Grid, effectively mitigating the stability risks of 500kV cross-regional long-chain AC interconnection. However, this measure only considers the asynchronous interconnection of grid-type systems and does not incorporate additional frequency control. Summary of the Invention

[0016] To address the above issues, the present invention aims to provide a frequency support method for an asynchronous interconnected grid system. This method interconnects isolated grid systems with different types of energy storage supporting renewable energy generation through high-voltage direct current transmission (HVDC). This method, combined with additional frequency control, reduces the frequency difference between the two sides of the interconnected system, thereby enhancing the frequency support capability of the asynchronous interconnected grid system. The technical solution is as follows:

[0017] A frequency support method for an asynchronous interconnected networking system comprises the following steps:

[0018] (a) connecting two independent isolated new energy grid systems via high-voltage direct current transmission, wherein the isolated new energy grid system includes a grid-forming energy storage device and a new energy power generation device; the grid-forming energy storage device adopts a virtual synchronous generator grid control strategy, and the new energy power generation device adopts a grid-following control strategy;

[0019] (b) Introducing an additional frequency controller into the constant power controller on the constant power side of the HVDC transmission system to monitor the frequency difference Δf on both sides of the asynchronous interconnected network system in real time 12 ;

[0020] (c) Limit the frequency difference Δf on both sides of the asynchronous interconnected network system through the frequency deviation limiter 12 , suppress the frequency fluctuation of the supporting side power grid and realize the sharing of frequency regulation resources on both sides;

[0021] (d) Frequency difference Δf on both sides of the asynchronous interconnection system 12 Generate additional DC power ΔP AFC , and at the same time, the frequency difference Δf on both sides of the system 12Limiting is performed and coordinated with the FM response dead zone to avoid frequent actions and prevent the frequency support side from excessively participating in regulation.

[0022] The beneficial effects of the present invention are:

[0023] The present invention proposes a frequency regulation method and interconnected system mutual support method for DC additional frequency control of quasi-synchronous operation of an asynchronous interconnected networking system, which can automatically adjust the DC transmission power according to the power disturbance of the power grids on both sides of the interconnection to realize the sharing of frequency regulation resources. Compared with the existing solution 1, the present invention takes into account the overall frequency stability of the asynchronous interconnected networking system; compared with the existing solution 2, the present invention considers adding additional frequency control to the asynchronous interconnected networking system to realize the sharing of frequency regulation backup resources on both sides of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the control structure block diagram of the grid-type converter.

[0025] Figure 2 It is the power-frequency static characteristic curve.

[0026] Figure 3 This is the block diagram of the active power-frequency control of the virtual synchronous generator.

[0027] Figure 4 This is the block diagram of the reactive power-voltage control of the virtual synchronous generator.

[0028] Figure 5 Schematic diagram of asynchronous interconnection network system.

[0029] Figure 6 This is the frequency deviation limit control logic diagram.

[0030] Figure 7 Additional frequency control logic diagram.

[0031] Figure 8 The frequency comparison diagram is with and without additional frequency control.

[0032] Figure 9 The comparison chart of DC power with and without additional frequency control.

[0033] Figure 10 The frequency comparison diagram is with and without frequency deviation limiting.

[0034] Figure 11 The figure shows the comparison of DC power with and without frequency deviation limiting. DETAILED DESCRIPTION

[0035] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] The present invention mainly analyzes the control technology of grid-type virtual synchronous generator (VSG).

[0037] The primary frequency modulation and rotor motion process of VSG are simulated by the active power-frequency control branch, such as Figure 3 As shown in the figure, the active power-frequency droop characteristic is characterized; the excitation regulation process of the VSG is simulated by the reactive power-voltage control branch, as shown in the figure. Figure 4 As shown in the figure, it can be used to characterize the reactive power-voltage droop characteristics. Its mathematical expression is:

[0038]

[0039] The VSG control mathematical expression (1) is transformed into:

[0040]

[0041] Since ω0 is the angular frequency reference value and is a constant, equation (2) can be simplified as follows:

[0042]

[0043] As can be seen from the mathematical expression, VSG control not only simulates the primary frequency and voltage regulation characteristics of synchronous generators, but also incorporates their inertia and damping properties. Grid-connected energy storage using VSG control strategies not only provides frequency and voltage references for renewable energy generation but also participates in system frequency regulation, leveraging the primary frequency regulation characteristics of synchronous generators.

[0044] During the quasi-synchronous operation of the asynchronous interconnected network system, quasi-synchronous operation, as a special operating mode, introduces additional frequency control (AFC) to regulate the DC power during the power transmission process, keeping the frequencies of the power grids on both sides at a relatively close level, sharing the frequency regulation resources on both sides, giving full play to the frequency support capabilities of both sides, reducing the frequency variation on the side where the accident occurs, and ensuring the smooth operation of the entire power grid.

[0045] The present invention is Figure 5 The asynchronous interconnected grid system shown in the figure is formed by connecting two grid-supported isolated renewable energy power generation systems with HVDC. The model mainly consists of wind turbine photovoltaic power generation equipment, grid-type energy storage, converter station, and DC transmission line. In the asynchronous interconnected grid system, VSC1 is controlled by constant voltage and VSC2 is controlled by constant power. The DC transmission line transmits power P refTaking VSC1 to VSC2 as the positive direction, the auxiliary frequency control (AFC) is introduced into the VSC2 constant power controller to adjust the power transfer amount according to the frequency difference between the two sides of the interconnected system, thereby realizing the sharing of frequency regulation reserve resources on both sides.

[0046] (1) Firstly, considering the need for mutual support between the two sides of the asynchronous interconnected network system and the need to avoid frequent operation, the DC frequency limiter (Frequency Limit Control, FLC) in the present invention adopts the control method of "dead zone + power regulation" to suppress system frequency fluctuations. The frequency deviation limiting control logic and the additional frequency control logic are as follows: Figure 6 and Figure 7 As shown; where f N The rated frequency is 50Hz; f1 and f2 are the operating frequencies of the power grids at both ends of the interconnected system; Δf max and Δf min The maximum and minimum values of the frequency deviation output are 0.4Hz and -0.4Hz respectively; ±f H is the dead zone size; K Pr , K I are proportional and integral coefficients respectively; ΔP max and ΔP min are power adjustment limit values; P ref is the DC power reference value of the VSC (voltage source converter) on the constant power side.

[0047] (2) Figure 6 As shown in the figure, when a system fault occurs, the frequencies of the power grids at both ends fluctuate. The measured frequencies f1 and f2 are subtracted from the rated frequencies to obtain the frequency deviations Δf1 and Δf2 of the power grids at both ends, which are then limited. Finally, the two frequency deviations are subtracted to obtain the frequency difference of the systems at both ends.

[0048] Assume that a fault occurs in the power grid on the f2 side (constant power side), the frequency deviation Δf2 input to the additional frequency controller on this side is controlled within Δf due to the frequency deviation limiting effect. min to Δf max At this time, the frequency difference Δf between the two sides of the asynchronous interconnected network system is 12 The size of can be expressed as:

[0049]

[0050] When the frequency deviation on the fault side is too large, the frequency deviation limiter is used to limit the frequency difference Δf on both sides. 12 , thereby limiting the frequency change on the support side, ensuring the safe and stable operation of the support side power grid, and reducing the scope of the accident impact.

[0051] (3) Figure 7 As shown, the frequency difference Δf of the two-end system is calculated. 12 Subtract the upper and lower limits of the dead zone respectively and send the calculated results to different PI adjustments. 12 The result of the dead zone upper limit calculation is adjusted (integrated) by "I" to obtain x1, and the upper limit of the amplitude is limited; and Δf 12 The result of the dead zone lower limit calculation is adjusted by "I" to obtain x2, and the amplitude is limited to the lower limit. Then the results of the two PI adjustment outputs are again limited to obtain ΔP1 and ΔP2. Finally, ΔP1 and ΔP2 are added to obtain the DC power addition ΔP AFC .

[0052] When the frequency difference Δf on both sides of the asynchronous interconnected network system 12 When the value is within the dead zone, due to the action of the limiter, the output DC power addition ΔP AFC is 0, thus avoiding frequent actions. 12 >+f H When ΔP2=0 due to the limitation, ΔP AFC Determined by the following formula:

[0053]

[0054] Among them, ΔP2 is the power deviation calculated according to the set lower limit of the dead zone in the accessory frequency control, and ΔP1 is the power deviation calculated according to the set upper limit of the dead zone. ±f H is the dead zone size, K Pr and K I are proportional and integral coefficients respectively; ΔP max The maximum value of the power adjustment limit is set; x max It is the maximum value of the intermediate value limit of PI regulation output.

[0055] From formula (5), we can see that when Δf 12 >+f H When the DC power addition ΔP AFC The DC transmission power will continue to increase until the frequency difference Δf at both ends of the interconnected system 12 <+f H When Δf 12 <+f H After that, dx1 / dt turns from positive to negative, and FLC enters the reverse recovery stage. At this time, x1 will continue to decrease to 0, and the DC additional power will gradually decrease to 0. Similarly, the frequency difference Δf on both sides of the interconnected network system can be obtained 12 <-f HDC additional power ΔP AFC process of change.

[0056] (4) Connect the designed additional frequency controller to the Figure 5 The simulation model shown in the figure was used for simulation verification, and the frequency and power waveforms of each asynchronous interconnected grid system were measured. The photovoltaic and wind turbine new energy generation equipment adopted grid-following control, while the energy storage equipment adopted the VSG grid-forming control strategy.

[0057] Example verification:

[0058] As Figure 5 The calculation and verification are performed using the asynchronous interconnected network system shown as an example.

[0059] Comparison of grid frequency and DC transmission power at both ends of the asynchronous interconnected network system with and without additional frequency control Figure 8 and Figure 9 shown.

[0060] Without additional frequency control, the power grids at both ends of the interconnected system are in an asynchronous operation state, and different power grids can operate at different frequencies. When load fluctuations occur on one side of the power grid, only the power grid on that side participates in frequency regulation, which will not change the operating frequency of the other side of the power grid, and the DC line transmission power remains unchanged. After the introduction of additional frequency control in the interconnected system, the power grids on both sides turn to quasi-synchronous operation, and the frequencies remain relatively close. Figure 9 As shown in Figure 2, when a step load disturbance occurs in system 2, the DC transmission power is changed through the fast power regulation function of HVDC transmission, and part of the load disturbance is transmitted to system 1. The load disturbance is shared by the power grids on both sides of the interconnected system. Figure 8 As shown in Figure 2, introducing AFC for frequency regulation can effectively reduce the frequency variation on the load disturbance side, improve the system's anti-interference ability, and reduce the demand for reserve capacity.

[0061] Comparison of grid frequencies and DC transmission at both ends of the asynchronous interconnected network system with and without frequency deviation limiting Figure 10 and Figure 11 As shown. When the frequency deviation is not limited, a large load increment disturbance will cause the power grids at both ends of the interconnected system to operate at a lower frequency, which is not conducive to the safe and stable operation of power electronic equipment and AC loads. By coordinating the frequency deviation limit and the dead zone, the power of DC transmission during the frequency support response is reduced, and the frequency fluctuation on the support side can be limited to [-0.3Hz, 0.3Hz], limiting the impact range of the load disturbance, as shown. Figure 10 、 Figure 11 At this time, the AC frequency on the side where the load disturbance occurs is low, and low-frequency load shedding and other device actions can be taken to restore the system to normal operating levels.

Claims

1. A frequency support method for an asynchronous interconnected networking system, characterized in that: The following steps are involved: (a) connecting two independent isolated new energy grid systems via high-voltage direct current transmission, wherein the isolated new energy grid system includes a grid-forming energy storage device and a new energy power generation device; the grid-forming energy storage device adopts a virtual synchronous generator grid control strategy, and the new energy power generation device adopts a grid-following control strategy; (b) Introducing an additional frequency controller into the constant power controller on the constant power side of the HVDC transmission system to monitor the frequency difference Δf on both sides of the asynchronous interconnected network system in real time 12 ; (c) Limit the frequency difference Δf on both sides of the asynchronous interconnected network system through the frequency deviation limiter 12 , suppress the frequency fluctuation of the supporting side power grid and realize the sharing of frequency regulation resources on both sides; (d) Frequency difference Δf on both sides of the asynchronous interconnection system 12 Generate additional DC power ΔP AFC , and at the same time, the frequency difference Δf on both sides of the asynchronous interconnection network system 12 Limiting is performed and coordinated with the FM response dead zone to avoid frequent actions and prevent the support side from excessively participating in regulation.

2. The frequency support method of the asynchronous interconnected networking system according to claim 1, characterized in that: In step (b), the frequency difference Δf between the two sides of the asynchronous interconnected network system is monitored. 12 When a fault occurs in the fixed power side grid, the frequency deviation input to the additional frequency controller is controlled at the minimum frequency deviation output value Δf min To the maximum frequency deviation output Δf max At this time, the frequency difference Δf between the two sides of the asynchronous interconnected network system is 12 The size is expressed as: Among them, Δf1 is the frequency fluctuation amplitude of the constant voltage side grid during the fault, and Δf2 is the frequency fluctuation amplitude of the constant power side grid during the fault.

3. The frequency support method of the asynchronous interconnected networking system according to claim 1, characterized in that: In step (d), the system frequency fluctuation is suppressed by a DC frequency limiter, which adopts a "dead zone + power regulation" logic, specifically: When the frequency difference Δf on both sides of the asynchronous interconnected network system 12 When the size is within the dead zone, that is, -f H ≤Δf 12 ≤+f H When the DC power addition ΔP AFC =0; When the frequency difference Δf on both sides of the asynchronous interconnected network system 12 >+f H When the DC power addition ΔP AFC Calculated by the following formula: Among them, ±f H is the dead zone size, K Pr and K I are proportional and integral coefficients respectively; ΔP max The maximum value of the power adjustment limit is set; x max is the maximum value of the intermediate quantity limit of the PI regulation output; ΔP1 is the power deviation calculated based on the set dead zone upper limit; x1 is the intermediate quantity in the calculation process and has no special meaning; Similarly, the frequency difference Δf on both sides of the interconnected network system is obtained 12 <-f H DC additional power ΔP AFC process of change.

4. The frequency support method of the asynchronous interconnected networking system according to claim 1, characterized in that: The virtual synchronous generator control strategy of the grid-type energy storage device satisfies the following mathematical model: Among them, ω0 is the angular frequency reference value, ω is the angular frequency, K P is the adjustment coefficient, J s is the virtual moment of inertia, D P is the damping coefficient, P0 is the given reference value of active power, and P is the measured active power.

5. The frequency support method of the asynchronous interconnected networking system according to claim 1, characterized in that: The new energy power generation equipment includes a photovoltaic power generation unit and a wind turbine generator set, both of which operate in a maximum power point tracking mode and do not participate in system frequency regulation.

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