Coordinated control strategy for improving frequency stability of power systems with high proportion of renewable energy

Abstract

The present invention discloses a coordinated control for improving the frequency stability of a high-proportion renewable energy power system. The zero points of the secondary derivative of a local measured frequency curve are connected in sequence to obtain a piecewise linear curve, which approximately replaces the system inertia center frequency curve. If the absolute value of the difference between the frequency at which the measured secondary derivative is zero and the rated frequency of the system exceeds a set frequency event start threshold, it is determined that a frequency event has occurred. If the difference is less than the threshold, the measured local frequency is returned. After the frequency event occurs, the rate of change of the system inertia center frequency is estimated. The system power shortage is estimated by combining the system equivalent inertia evaluated after the last frequency disturbance and the local measured frequency of this event. According to the ratio of the renewable energy installed capacity to the total installed capacity of the system, the renewable energy unit is arranged to bear the power shortage or surplus of the system, so as to provide frequency support for the AC system. The additional power control strategy provided by the present invention has the advantages of estimating the system inertia center frequency without communication, simple calculation, and rapid response.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system control, and specifically relates to a collaborative control strategy for improving the frequency stability of a high-proportion renewable energy power system. Background Art

[0002] In recent years, renewable energy power generation has been vigorously developed with the advantages of being clean and renewable, and the proportion of renewable energy power systems has been increasing. However, there is a lack of coupling between the power electronic converter type power supply and the system frequency, making it difficult to provide power support for the system after it is disturbed. As traditional synchronous power supplies are gradually replaced by converter type power supplies, the inertia level of the power system is decreasing, and the frequency changes rapidly during the dynamic process of the system and is prone to exceed the specified frequency fluctuation range. Therefore, a high proportion of renewable energy power systems have the characteristics of low inertia, and how to ensure the frequency stability of the system becomes a challenge.

[0003] Many scholars have studied converter control to improve system frequency stability, which can be roughly divided into two categories: droop control and virtual synchronization control. Both control strategies have certain limitations:

[0004] 1. Droop control can only provide strong power support when the system frequency deviation is large;

[0005] 2. Virtual synchronous control inherits the electromechanical transient characteristics of synchronous generators. How to match control parameters such as virtual inertia and virtual damping to suppress the electromechanical oscillation of the system becomes a difficulty;

[0006] 3. The access of multiple virtual synchronous machines makes the dynamic characteristics of the power grid more complex, which is not conducive to the analysis and design of virtual synchronous control;

[0007] 4. The designs of these two types of frequency controllers are based on the changes in transient frequency and cannot utilize the fast power modulation of the converter, resulting in rapid frequency changes and large frequency offsets in the early stages of disturbances, and the problem of transient frequency stability cannot be well resolved. Summary of the invention

[0008] The present invention overcomes the shortcomings of the prior art and provides a collaborative control strategy for improving the frequency stability of a high-proportion renewable energy power system. The strategy can estimate the unbalanced power of the system, thereby quickly adjusting the converter power and providing frequency support for the AC system. The strategy has a clear principle, simple control logic, and does not require communication.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] The coordinated control strategy for improving the frequency stability of a high-proportion renewable energy power system includes the following steps:

[0011] S1, based on the local measurement frequency signal, that is, the frequency f of the common coupling point PCC PCC , f PCC The zero points of the second derivative of the curve are connected in sequence to obtain a piecewise linear curve to approximately replace the system center of inertia (COI) frequency curve, thereby obtaining a piecewise linear approximation of the center frequency of inertia of the entire system;

[0012] S2, if the f obtained in step S1 PCC If the absolute value of the difference with the rated frequency of the system exceeds the set frequency event start threshold, it is determined that a frequency event has occurred and step S3 is executed. If it is less than the threshold, the process returns to step S1.

[0013] S3. Calculate f PCC Second-order difference sequence, estimating the system COI frequency change rate;

[0014] S4. Estimate the system equivalent inertia of the last frequency event based on the system unbalanced power, system synchronous inertia and generator output change obtained from the post-analysis of the last frequency event. Combine the system equivalent inertia of the last frequency event and the system COI frequency change rate of this event to estimate the system power shortage of this event.

[0015] S5. Arrange new energy units to bear the power shortage or surplus of the system according to the ratio of new energy installed capacity to the total installed capacity of the system, and provide frequency support for the AC system.

[0016] Specifically, in step S2, the f measured in step S1 is PCC Perform the following calculations:

[0017] |f PCC -f n |>Δf set

[0018] Among them, f n is the rated frequency of the system; Δf set Start threshold for frequency events.

[0019] Specifically, in step S3, the system COI frequency change rate can be estimated as follows:

[0020]

[0021] Among them, f COI is the estimated value of system COI frequency; f PCC1 and f PCC2 are the values ​​of the first and second second-order derivatives of the PCC measured frequency after the frequency event when their signs change, and t1 and t2 are the corresponding moments.

[0022] Specifically, in step S4, the system unbalanced power ΔP′ is calculated as follows:

[0023]

[0024] Among them, H sys is the equivalent inertia of the system at the last frequency event.

[0025] Furthermore, the estimation formula of the equivalent inertia of the system in the last frequency event is as follows:

[0026]

[0027] Where ΔP is the unbalanced power of the system in the last frequency event, and its value is obtained by the post-analysis of the last frequency event; ΔP G is the total output change of the generator; H G is the synchronous inertia of the system.

[0028] Furthermore, the system synchronous inertia H G The total output change of the generator is ΔP G It can be calculated as follows:

[0029]

[0030] Among them, H i is the inertia constant of the i-th generator; ΔP Gi is the output change of the i-th generator.

[0031] The output of the i-th generator changes as follows:

[0032]

[0033] Among them, Δ represents the change; P Gi The output of the i-th generator; f i is the frequency of the i-th generator in the last frequency event.

[0034] Specifically, in step S5, the output command value of the new energy is:

[0035]

[0036] Among them, P inv and are the command value and reference value of the inverter station power outer loop control, K r It is the ratio of new energy installed capacity to system installed capacity.

[0037] Compared with the prior art, the present invention has the following beneficial effects.

[0038] The present invention improves the collaborative control strategy of frequency stability of high-proportion renewable energy power systems, and quickly estimates the unbalanced power of the AC system in frequency events only through the frequency data of the PCC point of the renewable energy access inverter station, which is beneficial to quickly estimate the unbalanced power and change the converter output in the early stage of frequency accidents.

[0039] Furthermore, the synchronous inertia and system power change are used to indirectly estimate the inertia level of the system, which avoids the more complicated load and new energy inertia assessment and simplifies the calculation.

[0040] Furthermore, based on the local measurement frequency (f PCC ) estimates the system COI frequency without the need for communication equipment, avoiding the reliability and communication delay issues that exist in the centralized method of measuring the system inertial center frequency, and is conducive to fast calculation.

[0041] In summary, the control strategy of the present invention has the advantages of estimating the COI frequency of the system without communication, simple calculation, and rapid response. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention will be further described below in conjunction with the accompanying drawings.

[0043] Figure 1 is a flow chart of the control strategy of the present invention;

[0044] Figure 2 This is a flow chart of local frequency inflection point detection;

[0045] Figure 3 Schematic diagram of the new energy power system being tested;

[0046] Figure 4 for Figure 3 Simulation diagram of the dynamic process of the system when the power absorbed by node 6 suddenly increases by 0.4 pu, where (a) is simulation result one, (b) is simulation result two, and (c) is simulation result three.

[0047] Figure 5 for Figure 3 Simulation diagram of the dynamic process of the system when the power consumption of node 5 suddenly drops by 0.6 pu, where (a) is simulation result one, (b) is simulation result two, and (c) is simulation result three. DETAILED DESCRIPTION

[0048] The present invention is further described below in conjunction with specific embodiments.

[0049] The present invention proposes a system COI frequency estimation method based on local measurement, and provides a system equivalent inertia estimation method, which combines the two to quickly calculate the unbalanced power in the system dynamic process, and then uses the rapid regulation capability of new energy to provide power support for the AC system.

[0050] See also Figure 1 , a coordinated control strategy for improving the frequency stability of a high-proportion renewable energy power system includes the following steps:

[0051] S1. Estimate system COI frequency;

[0052] Since the system COI frequency curve passes through the second derivative zero point of the local measurement frequency curve, the second derivative zero points of the local measurement frequency curve are connected in sequence, and the obtained piecewise linear curve can approximately replace the system COI frequency curve; for the local frequency inflection point detection method, please refer to Figure 2 .

[0053] S2: Determine whether the absolute value of the difference between the PCC point frequency estimated in step S1 and the system rated frequency exceeds the frequency event start threshold. If |f PCC -f n |>Δf set , then determine the frequency event and execute step S3; if it is less than the threshold, return to step S1.

[0054] S3, estimate the system COI frequency change rate;

[0055] First calculate f PCC Second-order difference sequence, the system COI frequency change rate is:

[0056]

[0057] Among them, f PCC1 and f PCC2 are the values ​​of the first and second second-order derivatives of the PCC measured frequency after the frequency event when their signs change, and t1 and t2 are the corresponding moments.

[0058] S4. Estimate system power shortage;

[0059] The inertia constant of the i-th generator is:

[0060]

[0061] Among them, J i is the moment of inertia of the i-th generator; S i is the rated capacity of the i-th generator.

[0062] The output change of the i-th generator is:

[0063]

[0064] Among them, Δ represents the change; P Gi The output of the i-th generator; f iis the frequency of the i-th generator.

[0065] System synchronous inertia H G The total output change of the generator is ΔP G for:

[0066]

[0067] The system COI frequency is:

[0068]

[0069] When the system contains only synchronous inertia, combining equations (3), (4) and (5) we get:

[0070]

[0071] Similarly, when taking into account the inertia provided by the asynchronous motor and the virtual inertia provided by the new energy source in the system, we have:

[0072]

[0073] Among them, H sys is the equivalent inertia of the system; ΔP is the unbalanced power of the system. Since ΔP takes into account the power changes of the non-synchronous components in the system, ΔP>ΔP G .

[0074] Combining equations (6) and (7), we can get the calculation formula for the equivalent inertia of the system:

[0075]

[0076] Among them, △P, △P G and H G Available through post hoc analysis.

[0077] It is worth noting that formula (8) approximately assumes that the ratio of the system equivalent inertia to the synchronous inertia is equal to the ratio of the system power change to the generator power change when the system is disturbed and returns to steady state; in the early stage after the system is disturbed, the unbalanced power of the entire system is difficult to obtain quickly; since the subsequent control does not require the accurate calculation of the unbalanced power, the system inertia does not need to be very accurate, so it is reasonable to use formula (8) to estimate the equivalent inertia.

[0078] It can be seen from formula (7) that the system equivalent inertia and unbalanced power cannot be obtained at the same time. The system equivalent inertia evaluated after the last frequency disturbance is used to approximately replace the current system equivalent inertia to calculate the system unbalanced power △P′ of this event:

[0079]

[0080] Among them, Hsys is the system equivalent inertia of the last frequency event, f COI is the inertia center frequency of this frequency event.

[0081] S5. Calculate the output command value of the new energy unit:

[0082] Specifically:

[0083]

[0084] Among them, P inv and are the command value and reference value of the inverter station power outer loop control, K r It is the ratio of new energy installed capacity to system installed capacity.

[0085] See also Figure 3 , Figure 3 The schematic diagram of the tested new energy power system has a total of 6 nodes; the AC power grid includes three equivalent generators SG1, SG2 and SG3. Each generator is represented by a classic second-order model and is equipped with a corresponding speed regulation system. The load adopts a constant impedance model and is connected to the generator through an overhead line represented by an RX model; the photovoltaic power generation PV is centrally boosted and connected to node 3 through a DC-AC interconnected converter. In order to smooth the fluctuation of new energy output, an energy storage system with fast power modulation capability is installed on the DC side, P dc The power provided by the new energy source; the inverter control mode is changed to test the ability of the present invention to improve the system frequency stability under the conditions of sudden increase and decrease of system load.

[0086] See also Figure 4 , Figure 4 for Figure 3 The dynamic process of the system when the power absorbed by node 6 of the test system suddenly increases by 0.4pu; when the load suddenly increases, since the power output of the generator is less than the power absorbed by the system, the generator rotor slows down to release rotational kinetic energy to provide power support for the AC system, and the system frequency drops immediately.

[0087] Figure 4 (a) is the frequency response of the system when the new energy does not participate in the inertia response and frequency modulation process; Figure 4 (a) It can be seen that in the dynamic process of the system, the generator frequency curve fluctuates around the system COI frequency curve, and the piecewise linear curve obtained by connecting the zero points of the second-order derivative of the generator frequency curve is the COI frequency estimation curve; it can be seen that the calculation result obtained by the COI frequency estimation method proposed in the present invention almost coincides with the actual system COI curve, which well verifies the accuracy of the proposed frequency estimation method.

[0088] Figure 4 (b) and Figure 4(c) is the dynamic process of the system under droop control and the control proposed by the present invention, Figure 4 The droop control curve and the proposed control curve marked in (b) represent the frequency changes of SG1, SG2 and SG3 under droop control and the proposed control, respectively. Figure 4 (b) It can be seen that due to the support of new energy to the AC system, the system frequency deviation is significantly less than when there is no additional control; in addition, droop control can only provide strong power support when the system frequency deviates greatly from the rated value, and the corresponding lowest frequency point is still not ideal (the lowest frequency point is about 0.9932pu); Figure 4 (c) It can be seen that the control strategy proposed in the present invention can quickly estimate and compensate for the power shortage of the system at the initial stage of frequency drop, and thus has a better inhibitory effect on the frequency drop of the system (the lowest frequency point is about 0.9944pu); therefore, the control strategy proposed in the present invention can improve the frequency dynamic process of the system after disturbance and enhance the first swing stability of the system.

[0089] Figure 5 for Figure 3 The dynamic process of the system when the power absorption of node 5 in the test system suddenly drops by 0.6pu.

[0090] Figure 5 (a) is the system dynamic process when the new energy source does not respond to the change of AC system frequency. It can be seen that after the load is reduced, the generator generates a power surplus, which accelerates the rotor and increases the system frequency. The highest frequency point is about 1.0064pu. In addition, the piecewise linear estimation curve is very close to the actual system COI frequency curve, which once again verifies the accuracy of the proposed COI frequency estimation method.

[0091] Figure 5 (b) and Figure 5 (c) are the frequency response process of the system and the output change of the converter when the load suddenly drops. Figure 5 The droop control curve and the proposed control curve marked in (b) represent the frequency changes of SG1, SG2 and SG3 under droop control and the proposed control, respectively. Figure 5 (b) It can be seen that under the droop control and the control proposed by the present invention, the highest point of the system frequency is significantly reduced, which are 1.0056 and 1.0041 pu respectively; Figure 5 (c) It can be seen that the control proposed in the present invention has a faster response speed than the traditional droop control, and can provide frequency support for the AC system in the early stage of a frequency accident; the simulation results once again verify that the control strategy proposed in the present invention can effectively suppress the frequency deviation after the system is disturbed.

[0092] In summary, the collaborative control strategy for improving the frequency stability of a high-proportion renewable energy power system provided by the present invention, on the one hand, proposes a system COI frequency estimation method based on local measurement according to the unique relationship between the frequency curve of a certain node in the system and the system COI frequency curve; on the other hand, according to the synchronous inertia and power change information of the system after the frequency event, the equivalent inertia level of the system can be evaluated; combining the two can quickly calculate the unbalanced power of the system during the dynamic process of the system, and then use the rapid adjustment capability of renewable energy to provide power support for the AC system; the strategy does not require communication equipment and is simple to implement. Compared with traditional droop control, the proposed strategy can respond quickly at the initial stage of frequency change and has a better inhibitory effect on the frequency drop or surge of the system.

[0093] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Those familiar with the technology may modify or improve the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or changes made by those with ordinary knowledge in the technical field without departing from the spirit and technical ideas disclosed by the present invention should still be covered by the claims of the present invention.

Claims

1. A collaborative control strategy for improving the frequency stability of a high-proportion renewable energy power system, characterized in that: The following steps are involved: S1, based on the local measurement frequency signal, that is, the frequency f of the common coupling point PCC PCC , f PCC The zero points of the second derivative of the curve are connected in sequence to obtain a piecewise linear curve to approximately replace the system center of inertia (COI) frequency curve, thereby obtaining a piecewise linear approximation of the center frequency of inertia of the entire system; S2, if the f obtained in step S1 PCC If the absolute value of the difference with the rated frequency of the system exceeds the set frequency event start threshold, it is determined that a frequency event has occurred and step S3 is executed. If it is less than the threshold, the process returns to step S1. S3. Calculate f PCC Second-order difference sequence, estimating the system COI frequency change rate; S4. Estimate the system equivalent inertia of the last frequency event based on the system unbalanced power, system synchronous inertia and generator output change obtained from the post-analysis of the last frequency event. Combine the system equivalent inertia of the last frequency event and the system COI frequency change rate of this event to estimate the system power shortage of this event. S5. Arrange new energy units to bear the power shortage or surplus of the system according to the ratio of new energy installed capacity to the total installed capacity of the system, and provide frequency support for the AC system.

2. The coordinated control strategy for improving frequency stability of a high-proportion renewable energy power system according to claim 1 is characterized in that: In step S3, the system COI frequency change rate can be estimated as follows: Among them, f COI is the estimated value of system COI frequency; f PCC1 and f PCC2 are the values ​​of the first and second second-order derivatives of the PCC measured frequency after the frequency event when their signs change, and t1 and t2 are the corresponding moments.

3. The coordinated control strategy for improving frequency stability of a high-proportion renewable energy power system according to claim 1 is characterized in that: In step S4, the calculation formula of the system unbalanced power △P′ of this event is as follows: Among them, H sys is the system equivalent inertia of the last frequency event.

4. The coordinated control strategy for improving frequency stability of a high-proportion renewable energy power system according to claim 3 is characterized in that: The system inertia level at the time of the last frequency event is estimated as follows: Among them, ΔP is the unbalanced power of the system in the last event; ΔP G is the total output change of the generator; H G is the synchronous inertia of the system.

5. The coordinated control strategy for improving frequency stability of a high-proportion renewable energy power system according to claim 1 is characterized in that: In step S5, the output command value of the new energy is: Among them, P inv and are the command value and reference value of the inverter station power outer loop control, K r It is the ratio of new energy installed capacity to system installed capacity.

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