DC island detection method based on power difference judgment

Abstract

The present invention proposes a DC island detection method based on power difference judgment. The switching control power threshold Ps, switching control voltage Us, and switching control time T are set according to the output of the photovoltaic unit; based on the obtained setting value, the power difference between each single PV unit and the local load is detected, and whether to start the switching control process and the locking control is determined according to the detected power difference; and whether the system is in island operation is determined according to the change in the voltage amplitude measured at the PCC. In the present invention, the power difference threshold is used to realize switching control only when the power matching degree is high, thereby reducing the impact on the power quality of the system; and the single unit is independently adjusted, and the switching control is also independently operated when it is running, and the multi-machine control strategies do not interfere with each other, and the island detection accuracy is high; relying on the same judgment logic as the traditional passive over / undervoltage method to determine whether the system is in island operation, the judgment method is accurate and reliable, and has high practicality.

Description

Technical Field

[0001] The present invention relates to the technical field of power system analysis, and in particular to a DC island detection method based on power difference judgment. Background Art

[0002] The integration of a large number of photovoltaic generators into the grid has brought about a series of safety issues. Among them, unplanned islanding of photovoltaic generators can pose a significant threat to the safety of operating equipment and maintenance personnel. Therefore, reliably identifying unplanned islanding of photovoltaic generators is particularly important.

[0003] Research on AC system islanding detection is relatively well established. Detection methods are primarily categorized as local and communication-based. Local islanding detection can be further divided into passive and active methods. Passive islanding detection primarily relies on changes in electrical quantities before and after islanding to identify the system state. Common passive methods include over / undervoltage, over / underfrequency, and harmonic impedance. Active islanding detection primarily relies on injecting a disturbance signal into the system and determining the system state based on the difference in electrical quantities caused by the disturbance signal before and after islanding. Common active methods include active frequency shift, power perturbation, and impedance measurement. Communication-based islanding detection identifies the system by monitoring the closed state of key equipment or switches.

[0004] There is relatively little research on DC islanding detection. The current approach is to draw on AC islanding detection methods. However, compared to AC systems, DC systems have fewer observable electrical quantities, including those for frequency and reactive power. Therefore, most passive AC islanding detection methods are difficult to apply directly to DC systems. Communication methods rely on communication means, which limits their application scenarios and makes them difficult to apply on a large scale. Therefore, active islanding detection solutions are the main approach used in DC systems.

[0005] Active islanding detection requires injecting a disturbance signal into the system, which will affect the system power quality. At the same time, in a multi-machine system, different disturbance injection methods may interfere with each other, which is not conducive to accurately detecting islanding operation. Considering that in order to maximize efficiency, photovoltaic units are always operated at the maximum power point, combined with the problems existing in active islanding detection, it is hoped that there will be an islanding detection method that uses power judgment to overcome or at least alleviate the above-mentioned defects of the existing technology. Summary of the Invention

[0006] The purpose of the present invention is to propose a DC islanding detection method based on power difference judgment. The detection method takes advantage of the fact that photovoltaic units always operate at the maximum power point under normal circumstances. When the power matching degree is high, switching control is proposed to achieve islanding detection. The method is simple to implement and has accurate identification.

[0007] A DC islanding detection method based on power difference judgment includes the following steps:

[0008] Step 1: According to the output of the photovoltaic unit, the switching control power threshold P is independently adjusted for each unit. s , switching control voltage U s and switching control time T;

[0009] Step 2: Set the power threshold P s Set to a single PV unit, detect the power difference between each single PV unit and the local load, and calculate the power difference based on the power difference and the power threshold P s Comparison is made to determine whether to initiate switching control and locking control;

[0010] Step 3: Determine whether the system is in islanding operation based on the change in the voltage amplitude at the PCC.

[0011] Preferably, when the photovoltaic unit is under MPPT control, the voltage at the PCC is adjusted so that the voltage is at the critical value after the islanding occurs, and the grid voltage rating U N 85%-110% is the critical range, and the power threshold P is obtained by setting s .

[0012] Preferably, the power threshold P s It is 0.17 times the maximum output power of the photovoltaic unit.

[0013] Preferably, when the local load and the PV unit output are fully matched, the switching control voltage U s After that, the voltage measurement reaches the critical situation of detecting islanding, and the setting value U S .

[0014] Preferably, the voltage U S For U m -U1, where U m is the maximum power point voltage of the photovoltaic cell, and U1 is the critical change voltage corresponding to the photovoltaic operating point voltage.

[0015] Preferably, when setting T, the harmonic content is ensured to be about 4% within the switching control time T, the detection time is delayed to t0, t0 is without disturbance injection, and disturbance injection is maintained at t1, T=t0+t1.

[0016] Preferably, when the power difference is detected to be less than the power threshold P s When the switching control is locked for time t0, the power difference is always less than the threshold value P s Afterwards, the control of the optical generator set is switched from MPPT control to constant voltage control, the disturbance injection time is t1, and the voltage is measured to determine whether islanding occurs.

[0017] Preferably, if the power difference is not always less than the power threshold P within the time t0 s , or if the voltage is not detected to be over the limit within t1, the constant voltage control is locked for t2 time, and the PV unit switches to MPPT control, where t2 = T.

[0018] Preferably, if the power difference is not less than the power threshold P s , the PV unit is still under MPPT control, and the voltage is measured to determine whether it exceeds the limit and whether islanding occurs.

[0019] Preferably, the occurrence of islanding is detected by using a traditional passive over / under voltage method.

[0020] The method of the present invention can achieve the following beneficial effects:

[0021] (1) Combined with the MPPT control of the photovoltaic unit, the power difference threshold is used to realize switching control only when the power matching degree is high, and the disturbance signal is injected. In other cases, the switching control is not started, reducing the impact on the power quality of the system;

[0022] (2) All parameters required for setting are local quantities, making setting simple and easy;

[0023] (3) The switching control is independently adjusted for each unit, and the switching control is also operated independently during operation, which will not cause the multi-machine control strategies to interfere with each other and reduce the accuracy of island detection;

[0024] (4) Multiple control logics are used to judge the power difference detected, greatly reducing the impact on power quality during the injection period;

[0025] (5) The same judgment logic as the traditional passive over-voltage and under-voltage method is used to determine whether the system is in island operation. The judgment method is accurate, reliable and highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Connecting DC load structure diagram to multi-machine parallel photovoltaic power generation system;

[0027] Figure 2 This is a simplified schematic diagram of the DC system structure;

[0028] Figure 3 This is a schematic diagram of photovoltaic unit output;

[0029] Figure 4 is a flow chart of the method of the present invention;

[0030] Figure 5 This is a structural diagram of a six-terminal flexible DC power system used in an example of the present invention;

[0031] Figure 6 In the example of the present invention, P S Threshold verification plot;

[0032] Figure 7 In the example of the present invention, U S Threshold verification plot;

[0033] Figure 8 This is a T threshold verification diagram in the example of the present invention;

[0034] Figure 9 This is a comparison diagram of island waveforms of a single photovoltaic unit in an example of the present invention;

[0035] Figure 10 This is the island waveform diagram of a single unit with underpower load in the example of the present invention;

[0036] Figure 11 This is the island waveform diagram of a single unit with overpower load in the example of the present invention;

[0037] Figure 12 This is the island waveform diagram of multiple units in the example of the present invention. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the implementation of the present invention clearer, the technical solutions in the embodiments of the present invention will be described in more detail below in conjunction with the drawings in the embodiments of the present invention. In the drawings, the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The described embodiments are part of the embodiments of the present invention, not all of the embodiments. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain the present invention, and should not be understood as limiting the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. The embodiments of the present invention are described in detail below in conjunction with the drawings.

[0039] The structure of connecting multi-machine parallel photovoltaic power generation system to DC load is as follows Figure 1 As shown in the figure, the photovoltaic power source is connected to the local DC load through a DC boost converter to form a DC power system, and is connected to the grid through a DC-AC conversion MMC. When a fault or a large disturbance occurs in the system, the circuit breaker K MMC The photovoltaic power generation system is disconnected from the grid and forms a DC island with the DC load.

[0040] For the convenience of explanation, Figure 1 The structure diagram shown is simplified to Figure 2The DC bus is simplified to an infinite grid. There is no reactive power flow in the DC system. The relevant load is simulated by a resistive load. Assume that the active power provided by the photovoltaic system to the load is P, the active power provided by the infinite grid to the load is △P, and the active power required by the load is P load .

[0041] According to the law of conservation of energy, the power at the PCC should satisfy:

[0042] P load =P+ΔP (1)

[0043] Combine Figure 2 In normal operation, the active power provided by the photovoltaic system to the load (the output power of the photovoltaic system) is P, and the active power required by the load (the power consumed) is P load , the active power provided by the infinite grid to the load is △P. At this time, the photovoltaic system is under MPPT control, and the voltage at the PCC (common connection) is determined by the grid. When the grid is operating normally, no abnormal situation will occur. Generally, DCT adopts a constant current control strategy. Therefore, in the two steady states before and after the islanding is formed, the output current of the DCT remains unchanged, that is:

[0044] I0=I (2)

[0045] Where I is the output current of the photovoltaic system when it reaches a steady state after islanding is formed, and I0 is the steady-state value corresponding to I before islanding is formed.

[0046] Analysis of the system circuit before islanding occurs:

[0047]

[0048] Where U0 is the voltage at the PCC before islanding occurs, U is the voltage at the PCC after islanding occurs, and R is the equivalent impedance of the local load.

[0049] After an island occurs:

[0050] U=IR (4)

[0051] Combining the above two equations, we can get:

[0052]

[0053] It can be seen from the above formula that when ΔP is not 0, the voltage at the PCC will change significantly before and after the islanding occurs. However, when the PV operates in a fully matched state, ΔP is approximately 0. According to formula (5), the voltage will not change significantly before and after the islanding occurs, and the over / undervoltage method cannot correctly identify it.

[0054] In order to ensure maximum efficiency during normal operation, each PV unit is under MPPT control and operates at the maximum power point. When the PV output differs greatly from the local load, that is, △P is much greater than 0, the occurrence of islanding can be judged based on the passive method. At this time, the control method of the PV unit does not change; when the difference between the PV output and the local load is small, that is, △P is approximately 0, the passive method enters the detection blind spot. At this time, the control method of the PV unit is still MPPT control and operates at the maximum power point.

[0055] Combine Figure 3 ,Analysis of PV output characteristics: When the system is operating normally, the PV unit is at the maximum power point (Q point). At this time, when the most serious island operation occurs, there are:

[0056] P load =P=P m (6)

[0057] Among them, P m is the maximum output power of the PV unit. In this case, the active power △P provided by the grid to the load is approximately 0.

[0058] Combining equations (3) and (6), the equivalent impedance R of the local load remains unchanged before and after the islanding, and it can be deduced that:

[0059]

[0060] like Figure 3 , artificially shift the PV operating point to point A or B. Since the local load is equal to the maximum PV output power, a power difference will be generated after the artificial shift. At this time, the circuit relationship of the system is:

[0061]

[0062] During islanded operation, the power difference P1 cannot be absorbed by the larger system, causing voltage fluctuations at the PCC. When the voltage fluctuation exceeds the over / undervoltage detection threshold, an islanding condition is detected. However, due to the unimodal output of the PV unit, operating point shifts can be achieved simply by changing the PV unit's operating voltage.

[0063] To achieve output-based disturbance injection, it is necessary to consider the accuracy of island detection and the impact on the original system. Therefore, three parameters need to be adjusted, namely: the judgment standard of the switching control strategy, that is, the switching control power threshold P s , switch control voltage value U s And the switching control time T.

[0064] (1) Switching control power threshold P s Setting

[0065] P sThe key to setting is to ensure the island identification effect while minimizing the disturbance injection time. The method used in the present invention uses the over / under voltage method as the judgment method for island identification. Therefore, for P s The setting should be based on the critical value of the voltage at the PCC being in the island detection blind zone when the PV unit is under MPPT control. The present invention uses (85%-110%) U N (U N is the rated value of the grid voltage and assuming that the grid voltage is the rated value during normal operation) is the critical range. According to formula (7), we have

[0066]

[0067] According to formula (9), the smaller one can be used to obtain the setting value P s .

[0068]

[0069] (2) Switching control voltage value U s Setting

[0070] U s The key to setting U is to accurately identify the occurrence of islanding after switching to PV control, while minimizing voltage deviation and reducing the impact on power quality. s The setting idea is (assuming that there is no transition process in PV constant voltage control): when the local load and the PV unit output are fully matched, island operation occurs and the U is switched to switching control. s Finally, the over / under voltage method is able to detect the critical situation of islanding.

[0071] Since the switching control power threshold P s It has been determined that according to the setting principle, the system should still satisfy equation (8) after islanding. Based on this, the output power of the PV after the switching strategy can be determined as:

[0072]

[0073] Combined with the known Figure 3 The output characteristic curves of each PV unit shown in Figure 1 can be used to obtain the corresponding set voltage U according to P1 obtained from formula (11). m -U1 is U s . Among them U m is the maximum power point voltage of the photovoltaic unit, P m is the maximum power of the photovoltaic unit, P1 is the critical power difference of the calculated photovoltaic output, and U1 is the critical change voltage corresponding to the photovoltaic operating point voltage.

[0074] (3) Setting of switching control time T

[0075] Different from the setting of the switching control power threshold P s and the switching control voltage U s The setting of the switching time T affects the power quality of the system. Therefore, the setting of T should be based on the principle of minimizing the interference to the power quality of the system. Currently, there are no requirements for DC harmonics in relevant standards, but in AC systems, the limit standard for users injecting harmonics into the grid is 5%. After considering a certain margin, the setting idea of T is as follows:

[0076] When the power difference detection element detects that it is less than the threshold P s First, lock the switching control for t0 time. If the power difference is always less than the threshold within t0 time, then open the switching control. The PV control changes from MPPT control to switching control, and the disturbance injection time is t1. There is

[0077] T = t0 + t1 (12)

[0078] The system state is judged by the over / under voltage method throughout the whole period. Through pre-experiment on a single PV unit, during the entire injection period T, there is no disturbance injection in t0 and disturbance injection is maintained in t1, ensuring that the harmonic content is about 4% in each injection period T. Generally, t0 < t1, and in order to ensure the minimum impact on the system, the sum of the two should be less than 0.1 s.

[0079] The reason for dividing T into t0 and t1 is that the volatility of PV output and local load makes the detected power difference always in a fluctuating state. Without setting the detection time, the control of PV will be in a constantly changing state, and repeated switching has a great impact on the power quality of the system. Therefore, it is necessary to set a certain detection time t0 and injection time t1. In order to minimize the interference to the system to the greatest extent, after the island operation is not detected within the injection period T, the whole set of switching strategies is briefly locked for t2 time. For island detection, it meets the requirements if the island state is detected within 2 s. The disturbance injection period T is generally much less than 2 s. Therefore, set t2 = T to greatly reduce the injection time.

[0080] Figure 4 It is the operation flowchart of the DC island detection method based on power difference judgment.

[0081] Monitor the power difference △P of a single PV unit, and compare the power difference △P with the set threshold P s If the power difference △P is less than the power threshold P s , continue to monitor for t0 time; if the power difference △P is not less than the power threshold P s, the system is still under MPPT control. At this time, the common passive over / under voltage method is used to determine whether the voltage at the PCC exceeds the limit. If it exceeds the limit, it means that islanding has occurred. If the voltage does not exceed the limit, continue monitoring. During the time t0, if the power difference △P is constantly less than the power threshold P s , then the control of the photovoltaic unit becomes switching control, and the photovoltaic operating point voltage changes from U m Change to U s , continue to monitor time t1, if the voltage exceeds the limit, islanding occurs; if the power difference △P is not always less than the power threshold P within time t0 s , or if no voltage exceeding the limit is detected within t1, the PV unit switches to MPPT control, switches the control to lock for t2 time, and then continues to monitor the power difference △P of the PV unit.

[0082] The following uses a more specific example to illustrate the technical effect of the DC island detection method based on power difference judgment of the present invention. In order to show the detection effect of the invention in complex scenarios, a system is constructed as follows: Figure 5 The simulation environment shown in FIG. 1 is a diagram illustrating examples (1) to (3) of the present invention, wherein examples (1) to (3) introduce the threshold setting method using simulation examples, and examples (4) to (6) are simulation results using the invention.

[0083] (1) Adjust the local load so that the PCC voltage in island operation is at the upper critical value of over / under voltage. At this time, the power difference between the PV output and the local load is the power threshold P. s The normal PV output of the simulation system is 2MW, and the DC bus voltage is 20KV. In the simulation model, by adjusting the local load, the voltage after islanding is 22KV. The system has PV3 islanding operation at 1s. The corresponding voltage waveforms and power waveforms are as follows Figure 6 As shown. Figure 6 It can be seen from the figure that when the voltage of PV3 after islanding is set to the detection blind zone limit, the power difference is about 0.3MW, which is close to the threshold value of 0.34MW calculated by formula (10).

[0084] (2) After the PV3 load and output are fully matched, the island operation state is maintained. When PV3 is switched to switching control, the PV unit voltage is adjusted so that the voltage at the PCC is at the over / under voltage detection critical value, and the switching control voltage U is obtained. s At this time, the PV3 output can be calculated from formula (11) to be 1.58MW. The critical voltage simulation diagram is realized in the constructed model, as shown in the figure below: Figure 7 As shown. It can be seen that the corresponding PV3 output is 1.6MW, and the corresponding PV3 working voltage is 0.4, which is the set voltage U s .

[0085] (3) Through simulation experiments, the total harmonic content in the T period is obtained. Based on the simulation experience and harmonic standards, the injection period is finally determined to be T = 0.01 + 0.05 (s). The corresponding harmonic content and total harmonic component are shown in the figure below. Figure 8 As shown. Figure 8 It can be seen that according to the above injection time, the total harmonic content within the disturbance injection period T is 4.08%, which is less than the standard 5% and close to the 4% standard of harmonic content considering a certain threshold in the method, thus meeting the setting requirements.

[0086] (4) Figure 9 The figure shows the case of single-machine islanding when the local load and output of PV3 are fully matched. (a) and (b) are the voltage conditions at the PCC of PV3 unit without and with the DC islanding detection method proposed in the present invention, respectively. The islanding occurs at 1s. Figure 9 The middle dotted line is the detection limit of the over / under voltage method (85%-110% U N As standard), from Figure 9 As can be seen from (a), without using the method of the present invention, it is impossible to judge the operating status of the unit by relying solely on the over / under voltage method. However, after using the method proposed by the present invention, Figure 9 (b) The voltage quickly crosses the limit after the islanding occurs, which can accurately identify the occurrence of islanding.

[0087] (5) When island operation occurs in general conditions (when the local load is significantly different from the PV unit output), the method proposed in this invention performs as follows: Figure 10-11 As shown. Figure 10 When the local load is greater than the output, Figure 11 The local load is less than the output. As can be seen from the figure, no matter whether the load is less than or greater than the unit output, the power difference cannot be less than the power threshold P within the time t0. s , the method proposed in the present invention will not start, and only relies on the over / under voltage method to judge the system operation status, and will not inject disturbance signals into the system.

[0088] (6) Figure 12 The figure shows the operation diagrams of the units where PV3 and PV4 units experienced islanding operation. It can be seen from the figure that PV3 and PV4 units experienced islanding operation. The switching control was put into operation, causing the voltage to shift rapidly and the island to be accurately identified. However, the voltage at the PCC point of the units that were operating normally did not exceed the limit and no disturbance occurred.

[0089] Therefore, by using the DC island detection method based on power difference judgment of the present invention in combination with the condition that the photovoltaic unit is in MPPT operation under normal circumstances, it is clear that the switching control is only started when the power matching difference is large, and the control signal is selectively locked during the switching period, thereby greatly reducing the impact of the disturbance signal introduced by the switching control on the system power quality; at the same time, the present invention uses local quantities as the conditions for setting and identifying island operation, and is not related to the electrical quantities of the remote units, and does not have the defect of offsetting multiple control methods; finally, the present invention relies on the same judgment criteria as the traditional overvoltage and undervoltage method to identify the system operating status, and is simple to operate and has strong universality.

[0090] It should be noted that the above-mentioned embodiments are only preferred embodiments of the present invention and cannot be understood as limiting the scope of protection of the present invention. Any minor changes and modifications made to the present invention without departing from the concept of the present invention are within the scope of protection of the present invention.

Claims

1. A DC island detection method based on power difference judgment, It is characterized in that The method comprises the following steps: Step 1: According to the output of the photovoltaic unit, the switching control power threshold P is independently set for each unit. s , Switching control voltage U s and switching control time T; When the PV unit is under MPPT control, the voltage at the PCC is adjusted so that the voltage is at the critical value after the islanding occurs. N 85%-110% is the critical range, and the power threshold P is obtained by setting s , the formula is P m is the maximum output power of the PV unit; When the local load and the PV unit output are fully matched, the control voltage U is switched to the island operation. s After that, the voltage measurement reaches the critical situation where the island can be detected, and the setting is U S The voltage U S For U m -U 1 , where U m is the maximum power point voltage of the photovoltaic cell, U 1 is the critical change voltage corresponding to the photovoltaic operating point voltage; When setting T, the harmonic content is guaranteed to be 4% within the switching control time T, and the delay t 0 Detection time, t 0 No disturbance injection, keep disturbance injection t 1 , T = t 0 +t 1 ; Step 2: Set the switching control power threshold P s Set to a single PV unit, detect the power difference between each single PV unit and the local load, and calculate the power difference based on the power difference and the power threshold P s Compare to decide whether to start switching control and locking control; monitor the power difference △P of a single photovoltaic unit, compare the power difference △P with the set threshold P s The power is compared, if the power difference △P is less than the power threshold P s , then continue to monitor t 0 time; if the power difference △P is not less than the power threshold P s , the system is still under MPPT control. At this time, the commonly used passive over / under voltage method is used to determine whether the voltage at the PCC exceeds the limit. If it exceeds the limit, it means that an island has occurred. If the voltage does not exceed the limit, continue to monitor; at t 0 If the power difference △P is constantly less than the power threshold P s , then the control of the photovoltaic unit becomes switching control, and the photovoltaic working point voltage changes from U m Change to U s , continue to monitor t 1 If the voltage exceeds the limit at t 0 The power difference △P within the time is not always less than the power threshold P s , or at t 1 If the voltage is not detected to be beyond the limit within the time, the photovoltaic unit will switch to MPPT control and the switching control will be locked. 2 time, and then continue to monitor the power difference △P of the PV unit.

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