Hybrid island detection method and system for V2G grid-connected system

Abstract

The invention discloses a hybrid island detection method and system for a V2G grid-connected system, and the method comprises the steps: obtaining the voltage and frequency information of a grid-connected side of the V2G system in real time, and building an equivalent model and an island state model of the V2G on-load grid-connected system; a passive island detection system is constructed through monitored voltage and frequency change information, and passive monitoring and protection are carried out on island crisis; an improved active frequency deviation method is used for constructing an active island protection system, the system is started when a passive island detection system considers that disturbance level fluctuation occurs, and the detected speed of the island condition is accelerated by injecting distorted waveforms into a power grid; according to the active and passive hybrid island detection result, corresponding system state switching and protection behaviors are carried out, the influence of island detection on the quality of energy injected into a power grid and the misjudgment and missed judgment of the island condition are reduced, meanwhile, the island detection speed is increased, and stable and safe operation of the V2G grid-connected system is ensured.

Description

Technical Field

[0001] The present invention relates to the field of grid-connected island detection for V2G charging piles, and particularly to a hybrid island detection method and system for a V2G grid-connected system. Background Art

[0002] With the rapid development of electric vehicle technology and the widespread use of renewable energy, the power system is facing new challenges in achieving supply-demand balance. The V2G technology has emerged accordingly, which allows for two-way power flow between electric vehicles and the power grid, providing great regulation potential for the power grid. In a V2G system, electric vehicles can not only provide peak shaving services by discharging during peak power demand periods but also charge during low power demand periods to achieve valley filling, thereby optimizing power grid load management and energy optimization.

[0003] However, the application of V2G technology also brings new challenges. Especially when a power grid fault occurs, it is necessary to ensure that electric vehicles are safely disconnected from the power grid to avoid the islanding effect (which can cause damage to power grid workers and equipment. The islanding effect may lead to electric shock of power grid workers without their knowledge, the decline in power quality may damage the equipment of customers and utilities, and the reverse reclosing during the power grid restoration process may damage the protection equipment. Therefore, implementing an effective island detection and protection mechanism is crucial for the safe operation of the V2G system.

[0004] To solve this problem, researchers have proposed various island detection technologies, which can be divided into two categories: passive island detection and active island detection. This patent aims to propose a hybrid island detection method for a V2G system to improve the safety and reliability of the system and provide technical support for the development of V2G technology and the modernization of the power grid. Summary of the Invention

[0005] The present invention proposes a hybrid island detection method combining active and passive methods, which reduces the impact of island detection on the power quality injected into the power grid and the misjudgment and missed judgment of island situations, and at the same time accelerates the speed of detecting islands, ensuring the stable and safe operation of the V2G grid-connected system.

[0006] To achieve the above object, the present invention adopts the following technical solutions, including the following steps:

[0007] S1. Real-time obtain information such as voltage and frequency on the grid-connected side of the V2G system, and establish an equivalent model and an island state model of the V2G grid-connected system with load.

[0008] S2. Construct a passive island detection system based on the monitored voltage and frequency change information, and divide the interference level (0.3Hz / s) and the island level (1Hz / s) crisis according to the frequency change amount, and passively monitor and protect the island crisis.

[0009] S3. Construct an active islanding protection system using the improved active frequency shift method. This system is activated when the passive islanding detection system detects fluctuations at the interference level, and accelerates the detection speed of the islanding situation by injecting distorted waveforms into the power grid.

[0010] S4. According to the results of the hybrid active and passive islanding detection, perform corresponding system state switching and protection actions.

[0011] Furthermore, in S1, obtaining the V2G grid-connected side information and the grid-connected equivalent model information includes:

[0012] The instantaneous voltage V at the point of common coupling (PCC) between the output port of the V2G charging pile and the power grid PCC , the instantaneous frequency f at the point of common coupling (PCC) between the output port of the V2G charging pile and the power grid PCC , the voltage change amount ΔV per unit time at the point of common coupling (PCC) between the V2G charging pile and the power grid PCC , the frequency change amount Δf PCC , the changed frequency f PCC ’ . The load is equivalent to an RLC circuit, and the actual output power P of the V2G charging pile V2G +jQ V2G , the power P consumed by the local load L +jQ L , the power P provided by the grid side G +jQ G .

[0013] Furthermore, constructing the grid-connected state model and the islanding state model includes:

[0014] 1) Grid-connected state:

[0015] The active power transfer formula in the V2G grid-connected state with load is as follows:

[0016]

[0017] The reactive power transfer formula in the V2G grid-connected state with load is as follows:

[0018]

[0019] 2) Islanding state:

[0020] The formula for the voltage change at the point of common coupling caused by the active power mismatch in the V2G islanding state is as follows:

[0021]

[0022] The formula for the frequency change at the point of common coupling (PCC) caused by reactive power mismatch in the V2G islanded state is as follows:

[0023]

[0024] Furthermore, in the above S2, a passive islanding detection strategy is proposed, including:

[0025] Monitor the frequency of the point of common coupling (PCC). If the frequency change within 1 s satisfies the formula:

[0026] 0.1 Hz < Δf PCC < 0.3 Hz;

[0027] Or the voltage change of the point of common coupling (PCC) satisfies the formula:

[0028] 0.9U N < V PCC < 0.95U N Or 1.05U N < V PCC < 1.1U N ;

[0029] Then it is considered that an interference-level crisis has occurred. This may be an interference phenomenon, and the possible causes include grid transient fluctuations, sampling errors and fluctuations, and local load switching, etc. It is not a real islanding crisis situation, but there is also a possibility that islanding has occurred.

[0030] Monitor the frequency of the point of common coupling (PCC). If the frequency change within 1 s satisfies the formula:

[0031] 0.3 Hz < Δf PCC < 0.7 Hz;

[0032] Or the voltage change of the point of common coupling (PCC) satisfies the formula:

[0033] V PCC < 0.9U N Or V PCC > 1.1U N ;

[0034] Then the possibility of interference is excluded, and islanding must have occurred.

[0035] Furthermore, in the above S3, an active islanding detection strategy is proposed, including:

[0036] Using the improved positive feedback active frequency offset method, a certain truncation and frequency change are generated in the current transmitted by the V2G charging pile to the power grid. In the grid-connected state, due to the existence of the power grid, the charging pile will not detect a change in the frequency at the connection point; if in the island state, the V2G charging pile will detect a slightly deviated frequency signal and superimpose a frequency change on this basis to output current. After the cumulative frequency change over multiple cycles, the passive island detection will finally detect the island-level crisis.

[0037] The formula for the real-time truncation coefficient for active frequency offset is as follows:

[0038] cf k =α·(f k-1 -f k-2 )cf k-1 +k·(f k-1 -f k )

[0039] In the formula, cf k is the truncation coefficient of the current cycle, cf k-1 is the truncation coefficient of the previous cycle, f k is the sampling frequency of this cycle, f k-1 is the sampling frequency of the previous cycle, f k-2 is the sampling frequency of the cycle before the previous cycle, α is the frequency gain coefficient, and k is the positive feedback coefficient.

[0040] Furthermore, in S4, a main-passive hybrid island detection strategy and island protection behavior are proposed, including:

[0041] When the passive island system does not detect an interference-level island, the active island system is turned off to ensure the power quality at the power grid connection point to the greatest extent, but no other island protection behaviors are performed; after the passive island detects an interference-level island, the active island system is turned on and continues to operate until the interference-level island is no longer detected. At this time, if it is an interference situation, the active island system will automatically turn off after the interference ends; if it is a real island situation, the active island system can accelerate the discovery speed of the island situation, accelerate the amplitude and speed of the frequency change at the point of common coupling (PCC), until it enables the passive island detection to detect the island-level protection. At this time, the output of the V2G charging pile should be immediately cut off, and relevant personnel should be dispatched for maintenance.

[0042] The beneficial effects of the present invention:

[0043] In the present invention, the interference to the power quality of the output current is reduced under normal power grid conditions, and the THD is about half of that of continuous active island detection. In the event of an island crisis, the occurrence of the island situation can be judged more quickly.

[0044] In the present invention, a hybrid islanding detection method combining active and passive methods reduces the impact of islanding detection on the power quality injected into the power grid and the misjudgment and missed judgment of islanding situations. At the same time, it accelerates the speed of detecting islanding and ensures the stable and safe operation of the V2G grid-connected system. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 FIG. is a schematic flow chart of a hybrid islanding detection method and system for a V2G grid-connected system provided by an embodiment of the present invention;

[0046] Figure 2 FIG. is a schematic diagram of an equivalent model for a V2G grid-connected system provided by an embodiment of the present invention.

[0047] Figure 3 FIG. is a flow chart of the active frequency drift method provided by an embodiment of the present invention;

[0048] Figure 4 FIG. is a schematic diagram of the reference current waveform of the active frequency drift method provided by an embodiment of the present invention;

[0049] Figure 5 FIG. is a schematic diagram of the output current of the hybrid islanding detection provided by an embodiment of the present invention;

[0050] Figure 6 FIG. is a schematic diagram of the output current of pure active islanding detection provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0052] In the description of the present application, it should be understood that the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present application, unless otherwise specified, the meaning of "a plurality" is two or more.

[0053] Embodiment 1

[0054] Please refer to Figure 1 , an embodiment of the present invention provides a hybrid islanding detection method for a V2G grid-connected system, including:

[0055] S1. Obtain information such as voltage and frequency on the grid connection side of the V2G system in real time, and establish an equivalent model of the V2G grid-connected system with load and an island state model;

[0056] S2. Construct a passive island detection system through the monitored voltage and frequency change information, and divide the interference level (0.3Hz / s) and island level (1Hz / s) crises based on the frequency change amount to passively monitor and protect the island crisis;

[0057] S3. Use an improved active frequency shift method to construct an active island protection system. This system is turned on when the passive island detection system detects fluctuations at the interference level, and accelerates the detection speed of the island situation by injecting distorted waveforms into the power grid;

[0058] S4. Perform corresponding system state switching and protection actions according to the results of the hybrid active and passive island detection.

[0059] Please refer to Figure 2 , based on the above S1, obtain the V2G grid connection side information and grid connection equivalent model information, including:

[0060] The instantaneous voltage V at the point of common coupling PCC between the output port of the V2G charging pile and the power grid PCC , the instantaneous frequency f at the point of common coupling PCC between the output port of the V2G charging pile and the power grid PCC , the voltage change amount ΔV per unit time at the point of common coupling PCC between the V2G charging pile and the power grid PCC , the frequency change amount Δf PCC and the changed frequency f PCC ’ .

[0061] Equivalent the load to an RLC circuit, the actual output power P V2G +jQ V2G of the V2G charging pile, the power P L +jQ L consumed by the local load, and the power P G +jQ G provided by the power grid side;

[0062] At this time: construct the grid-connected state model and the island state model, including:

[0063] Grid-connected state, including:

[0064] The active power transfer formula in the V2G grid-connected state with load is as follows:

[0065]

[0066] The reactive power transfer formula in the V2G grid-connected state with load is as follows:

[0067]

[0068] Island state, including:

[0069] The formula for the voltage change at the point of common coupling (PCC) caused by the active power mismatch in the V2G island state is as follows:

[0070]

[0071] The formula for the frequency change at the point of common coupling (PCC) caused by the reactive power mismatch in the V2G island state is as follows:

[0072]

[0073] Based on the above embodiments, this embodiment proposes a passive islanding detection strategy, including:

[0074] Monitor the frequency of the point of common coupling (PCC). If the frequency change within 1 s satisfies the formula:

[0075] 0.1 Hz < Δf PCC < 0.3 Hz;

[0076] Or the voltage change of the point of common coupling (PCC) satisfies the formula:

[0077] 0.9U N < V PCC < 0.95U N Or 1.05U N < V PCC < 1.1U N ;

[0078] It is considered that an interference-level crisis has occurred. This may be an interference phenomenon, and the possible reasons include the transient fluctuations of the power grid, sampling errors and fluctuations, and the switching of local loads, etc. It is not a real islanding crisis situation, but there is also a possibility that islanding has occurred.

[0079] Monitor the frequency of the point of common coupling (PCC). If the frequency change within 1 s satisfies the formula:

[0080] 0.3 Hz < Δf PCC < 0.7 Hz;

[0081] Or the voltage change of the point of common coupling (PCC) satisfies the formula:

[0082] V PCC < 0.9U N Or V PCC > 1.1U N ;

[0083] Then, to rule out the possibility of interference, an islanding must have occurred.

[0084] Refer to Figure 3 As shown, this embodiment proposes an active islanding detection strategy, including:

[0085] Using the improved positive feedback active frequency shift method, the current transmitted by the V2G charging pile to the power grid is made to have a certain truncation and frequency change. In the grid-connected state, due to the presence of the power grid, the charging pile will not detect a change in the frequency at the connection point; if in the islanding state, the V2G charging pile will detect a slightly deviated frequency signal and superimpose a frequency change on this basis to output the current. After the cumulative frequency change over multiple cycles, the passive islanding detection will ultimately detect an islanding-level crisis.

[0086] Please refer to Figure 4 , in the reference current waveform of the improved active frequency shift method proposed in this embodiment, the real-time truncation coefficient formula is as follows:

[0087] cf k = α·((f k-1 - f k-2 )cf k-1 + k·(f k-1 - f k );

[0088] In the formula, cf k is the truncation coefficient of the current cycle, cf k-1 is the truncation coefficient of the previous cycle, f k is the sampling frequency of this cycle, f k-1 is the sampling frequency of the previous cycle, f k-2 is the sampling frequency of the cycle before the previous cycle, α is the frequency gain coefficient, and k is the positive feedback coefficient.

[0089] To verify the reliability and advancement of the embodiment, a simulation model is built in matlab-simulink, adapted to the hybrid detection system in the simulation, and compared with the traditional active islanding detection.

[0090] The hybrid strategy is as follows: when the passive islanding system does not detect an interference-level islanding, the active islanding system is turned off to ensure the power quality at the power grid connection point to the greatest extent, but no other islanding protection actions are taken; after the passive islanding detects an interference-level islanding, the active islanding system is turned on and continues to operate until no interference-level islanding is detected. At this time, if it is an interference situation, the active islanding system automatically turns off after the interference ends; if it is a real islanding situation, the active islanding system can accelerate the discovery speed of the islanding situation, accelerate the amplitude and speed of the frequency change at the point of common coupling (PCC), until it enables the passive islanding detection to detect an islanding-level protection. At this time, the output of the V2G charging pile should be immediately cut off, and relevant personnel should be dispatched for maintenance.

[0091] Please refer to Figure 5 When the output current of the hybrid island detection provided by the embodiment of the present invention is in the range of 0s - 0.04s, the power grid is normal, the island detection system does not actively inject interference, the THD of the grid-connected current is below 2%, and the power quality is relatively high; at 0.04s, an island crisis of grid disconnection occurs. At this time, passive island detection detects interference-level fluctuations, activates the active island detection strategy, controls the output current, introduces cf, the waveform of the output current changes more severely in the range of 0.04s - 0.08s, and at 0.08s, passive island detection believes that an island has been generated and determines that the output should be cut off.

[0092] Please refer to Figure 6 When the output current of the pure active island detection provided by the embodiment of the present invention is in the range of 0s - 0.04s, the power grid is normal, and the active island detection still injects interference, the THD of the grid-connected current is about 4%; at 0.04s, an island crisis of grid disconnection occurs. The active island detection strategy is adopted to control the output current, introduce cf, the waveform of the output current changes more severely in the range of 0.04s - 0.2s, and at 0.2s, it is considered that an island has been generated and determines that the output should be cut off.

[0093] Reduce the interference to the power quality of the output current under normal power grid conditions. The THD is about half of that of continuous active island detection. When an island crisis occurs, the island situation can be judged more quickly.

[0094] The hybrid island detection method combining active and passive of the present invention reduces the impact of island detection on the quality of the energy injected into the power grid and the situations of misjudgment and missed judgment of the island situation, and at the same time accelerates the speed of detecting the island, ensuring the stable and safe operation of the V2G grid-connected system.

[0095] Embodiment 2

[0096] Based on the above embodiment, this embodiment provides a hybrid island detection system for a V2G grid-connected system, including:

[0097] Data acquisition module: Real-time acquire information such as the voltage and frequency on the grid-connected side of the V2G system, and establish an equivalent model of the V2G grid-connected system with load and an island state model;

[0098] Data monitoring module: Construct a passive island detection system through the monitored voltage and frequency change information, and rely on the frequency change amount to divide the interference level (0.3Hz / s) and the island level (1Hz / s) crisis, and passively monitor and protect against the island crisis;

[0099] Data processing module: An active islanding protection system is constructed using an improved active frequency offset method. This system is activated when the passive islanding detection system detects fluctuations at the interference level, and it accelerates the detection speed of the islanding situation by injecting distorted waveforms into the power grid.

[0100] System protection module: Based on the results of the hybrid active and passive islanding detection, corresponding system state switching and protection actions are carried out.

[0101] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and supplements can be made, and these improvements and supplements are also regarded as the protection scope of the present invention.

Claims

1. A hybrid island detection method for a V2G grid-connected system, characterized in that: The steps include: S1. Obtain the voltage and frequency information of the grid-connected side of the V2G system in real time, and establish an equivalent model and an island state model of the V2G grid-connected system with load; S2. A passive islanding detection system is constructed by monitoring the voltage and frequency change information. The interference level (0.3 Hz / s) and islanding level (1 Hz / s) crises are divided based on the frequency change, and passive monitoring and protection are performed on the islanding crises. S3. Use the improved active frequency shift method to build an active island protection system. The system is turned on when the passive island detection system believes that the interference level fluctuates. The system accelerates the speed at which the island situation is detected by injecting distorted waveforms into the power grid. S4. Perform system state switching and protection actions based on the results of active-passive hybrid islanding detection.

2. A hybrid island detection method for a V2G grid-connected system according to claim 1, characterized in that: The information obtained from the V2G grid-connected side and the grid-connected equivalent model information include: The instantaneous voltage V between the output port of the V2G charging pile and the public connection point PCC of the power grid PCC , the instantaneous frequency f of the common connection point PCC between the V2G charging pile output port and the power grid PCC , the voltage change per unit time ΔV between the V2G charging pile and the public connection point PCC of the power grid PCC , frequency change Δf PCC , the frequency after the change is f PCC ’ The load is equivalent to an RLC circuit, and the actual output power P of the V2G charging pile is V2G +jQ V2G , the power consumed by the local load P L +jQ L , the power P provided by the grid side G +jQ G .

3. A hybrid island detection method for a V2G grid-connected system according to claim 2, characterized in that: The grid-connected state model includes: The active power transfer formula of V2G in the grid-connected state with load is as follows: The reactive power transfer formula of V2G in the grid-connected state with load is as follows:

4. A hybrid island detection method for a V2G grid-connected system according to claim 2, characterized in that: The island state model includes: The formula for the voltage change at the common connection point caused by active power mismatch in the V2G island state is as follows: The formula for the frequency change at the common connection point caused by reactive power mismatch in the V2G island state is as follows:

5. A hybrid island detection method for a V2G grid-connected system according to claim 1, characterized in that: Passive island detection strategies include: Monitor the frequency of the common connection point PCC. If the frequency change within 1s satisfies the formula: 0.1Hz<Δf PCC <0.3Hz Or the voltage change at the common connection point PCC satisfies the formula: 0.9U N <V PCC <0.95U N or 1.05U N <V PCC <1.1U N It is considered that an interference-level crisis has occurred. This may be an interference phenomenon. Possible causes include transient fluctuations in the power grid, sampling errors and fluctuations, and switching of local loads. It is not a true island crisis, but there is a possibility that an island has occurred. Monitor the frequency of the common connection point PCC. If the frequency change within 1s satisfies the formula: 0.3Hz<Δf PCC <0.7Hz Or the voltage change at the common connection point PCC satisfies the formula: V PCC <0.9U N or V PCC >1.1U N Then, eliminating the possibility of interference, an island must have been created.

6. A hybrid island detection method for a V2G grid-connected system according to claim 1, characterized in that: Active island detection strategy, including: An improved positive feedback active frequency offset method is used to cause a certain degree of truncation and frequency change in the current transmitted by the V2G charging pile to the power grid. In the grid-connected state, due to the existence of the power grid, the frequency detected by the charging pile at the connection point will not change. If it is in an island state, the V2G charging pile will detect a slightly deviated frequency signal, and on this basis, superimpose a frequency change to output current. After multiple cycles of frequency changes are accumulated, the passive island detection will eventually detect an island-level crisis.

7. A hybrid island detection method for a V2G grid-connected system according to claim 1, characterized in that: The real-time cutoff coefficient formula for active frequency offset is as follows: cf k =α·(f k-1 -f k-2 )cf k-1 +k·(f k-1 -f k ); In the formula, cf k is the cutoff coefficient of the current period, cf k-1 is the cutoff coefficient of the previous period, f k is the sampling frequency of the cycle, f k-1 is the sampling frequency of the previous cycle, f k-2 is the sampling frequency of the previous cycle, α is the frequency gain coefficient, and k is the positive feedback coefficient.

8. A hybrid island detection method for a V2G grid-connected system according to claim 1, characterized in that: Active and passive hybrid island detection strategy and island protection behavior, including: When the passive islanding system does not detect an interference-level island, the active islanding system is turned off to ensure the power quality at the grid connection to the greatest extent, but no other islanding protection actions are performed; after the passive islanding system detects an interference-level island, the active islanding system is turned on and continues to operate until no interference-level island is detected.

9. A hybrid island detection method for a V2G grid-connected system according to claim 8, characterized in that: If it is an interference situation, the active island system will automatically shut down after the interference ends; if it is a real island situation, the active island system can accelerate the speed of discovery of the island situation and accelerate the frequency change amplitude and speed of the common connection point PCC until the passive island detects the island-level protection. At this time, the output of the V2G charging pile should be cut off immediately and relevant personnel should be sent for maintenance.

10. A hybrid island detection system for a V2G grid-connected system, characterized in that: The system is used to perform the hybrid island detection method according to any one of claims 1 to 9, comprising: Data acquisition module: real-time acquisition of voltage, frequency and other information on the grid-connected side of the V2G system, and establishment of an equivalent model and an island state model for the V2G grid-connected system with load; Data monitoring module: A passive islanding detection system is built through the monitored voltage and frequency change information. The interference level (0.3Hz / s) and islanding level (1Hz / s) crises are divided based on the frequency change, and passive monitoring and protection against islanding crises are performed. Data processing module: Use the improved active frequency offset method to build an active island protection system. The system is turned on when the passive island detection system believes that there is a fluctuation in the interference level. It accelerates the speed at which the island situation is detected by injecting distorted waveforms into the power grid. System protection module: performs corresponding system state switching and protection actions according to the results of active and passive hybrid islanding detection.

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