A DC island detection method without blind spots based on adaptive current disturbance

Abstract

The present invention discloses a blind-zone-free DC island detection method based on adaptive current disturbance. By adaptively adjusting the resonant frequency points of distributed power generation units at different operating powers, the problem of actual oscillation frequency deviation caused by system power changes in traditional island detection methods based on fixed disturbance frequencies is solved, and blind-zone-free DC island detection under all working conditions is achieved. The present invention introduces the ratio of grid-connected point current to rated current as an adaptive factor in the resonant frequency selection link, constructs an autonomous frequency selection link that changes with system power, and realizes adaptive adjustment of current disturbance frequency at different system power levels. Compared with the traditional detection method based on fixed disturbance frequency, the DC island detection method can autonomously select the resonant frequency at each system power level, effectively eliminate the detection blind spot, and does not need to design a detection threshold, which has important application value for the safety protection of DC microgrids.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic control, and in particular relates to a blind-zone-free DC island detection method based on adaptive current disturbance. Background Art

[0002] With the application of a high proportion of distributed energy and a high proportion of power electronic devices in modern power systems, microgrids have become an effective solution due to their high autonomy. In the past few decades, AC microgrids have been widely studied in academia and industry. However, now that most distributed energy and loads connected to the power system are essentially DC type, DC microgrids have more advantages in operating efficiency and construction cost than AC microgrids, and there is no need to worry about reactive power and synchronization issues. Therefore, DC microgrids have attracted much attention as an important part of the future power system.

[0003] The DC system is divided into two operating modes: grid-connected and islanded. The islanded mode refers to the situation where the main grid is disconnected from the microgrid due to line failure, power outage and maintenance, etc. At this time, only distributed power supplies are supplied to the load in the system, and the bus voltage is out of control, which will seriously threaten the safety of power equipment and operation and maintenance personnel and cause safety accidents. Therefore, the DC islanding detection method without blind spots has important scientific significance and engineering value.

[0004] At present, many experts and scholars at home and abroad have conducted research on DC island detection technology. Existing methods are mainly divided into two types: passive and active. Passive island detection can be achieved by continuously monitoring the changes in grid connection point voltage / current before and after the island event, but it is only applicable to the working condition where the source and load power do not match, and the detection blind area is large. In order to reduce the detection blind area, an active island detection method based on a fixed disturbance frequency is proposed. This method selects the most sensitive disturbance frequency under rated conditions as the resonant frequency, so that the grid connection point voltage oscillates at a specific frequency in the island mode to characterize the island event, which can effectively reduce the blind area of ​​island detection. However, when the system power changes, the selected fixed disturbance frequency is not the most sensitive frequency under the power, causing the deviation between the actual oscillation frequency and the theoretical oscillation frequency to increase with the change of system power, which will further expand the detection blind area. Therefore, the blind area problem of island detection under the condition of system power change needs to be further solved. Summary of the invention

[0005] In view of the above problems existing in the prior art, the present invention proposes a method for detecting DC islands without blind spots based on adaptive current disturbance, which includes the following steps:

[0006] 1) Under rated conditions, the island detection condition with complete source-load power matching is selected as the steady-state operating point, and the grid-connected point voltage v under island mode is established. dg To the disturbance current idis Small signal model S i (s);

[0007] 2) In the frequency domain, S i (s) is expressed as the sum of the real part and the imaginary part, that is, S i (jω)=X(ω)+jY(ω); S i The frequency corresponding to the phase crossing of 0° is the theoretical resonant frequency, and at this frequency S i (jω) has the largest amplitude, so the theoretical resonant frequency is the most sensitive disturbance frequency; at this time, the imaginary part Y(ω) is 0 at the theoretical resonant frequency, so the frequency can be obtained as:

[0008]

[0009] Where P0 is the steady-state voltage, C dg is the equivalent bus capacitance;

[0010] 3) According to the theoretical resonant frequency expression, the change in power will cause the theoretical resonant frequency point to shift; therefore, the grid-connected current i is introduced in the resonant frequency selection link dg The ratio of the steady-state current I0 under rated working conditions is used as the adaptive factor. At this time, the adaptive theoretical resonant frequency expression is:

[0011]

[0012] 4) According to the adaptive theoretical resonant frequency expression, the converter connects the power generation unit to the grid point current i dg The sampling value is sent to the frequency selection link G r (s), the resonant frequency at this power level can be adaptively selected. At this time, the resonant frequency will not be offset due to system power changes, thereby eliminating the detection blind area;

[0013] 5) The converter samples the grid-connected voltage v of the power generation unit dg With voltage reference value v ref The difference is sent to the adaptive resonant controller to obtain the disturbance current i at this frequency. dis , and superimposed on the current loop;

[0014] i dis =(v dg -v ref )·G r (s)

[0015] 6) Detect the oscillation frequency component in the bus voltage. If the bus voltage oscillates three times continuously and the oscillation frequency is equal to the theoretical resonant frequency, it is determined that an islanding event has occurred; otherwise, it is determined that the system is still in the grid-connected mode.

[0016] Preferably, in step 1), the small signal model S i (s) is:

[0017]

[0018] Among them, R L is the load resistance, C dg is the equivalent bus capacitance, K pp is the proportional coefficient of the power controller of the power generating unit, K pi is the integral coefficient of the power controller of the power generation unit, V0 is the steady-state voltage, and s is the Laplace operator.

[0019] Preferably, in step 4),

[0020]

[0021] Among them, K r is the positive feedback gain, ω r is the resonance bandwidth.

[0022] Based on the above scheme, the beneficial effects of the present invention are:

[0023] 1. The present invention introduces an adaptive factor in the frequency selection link, and can adaptively select the theoretical resonant frequency that meets the working conditions at each power level, thereby realizing rapid island detection and having good compatibility.

[0024] 2. Compared with the traditional DC island detection method based on fixed disturbance frequency, the present invention solves the problem that the actual resonant frequency shifts with the change of system power by adaptively adjusting the theoretical resonant frequency, eliminates the detection blind spot, and does not need to design a detection threshold, making the method design simpler.

[0025] 3. The blind-zone-free DC island detection method based on adaptive current disturbance proposed in the present invention is suitable for a multi-machine system containing multiple distributed power sources. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is an equivalent architecture diagram of a single-bus DC microgrid in an embodiment of the present invention.

[0027] Figure 2 4 is a diagram of an adaptive current disturbance control structure based on a Buck converter in an embodiment of the present invention.

[0028] Figure 3 Graph showing the change of the resonant frequency of the grid connection point voltage to the disturbance current with the system power in an embodiment of the present invention.

[0029] Figure 4 This is a performance diagram of island detection of the method proposed in an embodiment of the present invention under rated power operation.

[0030] Figure 5 The figure is a comparison chart of island detection performance between the method proposed in the embodiment of the present invention and the traditional method under 60% rated power operation. DETAILED DESCRIPTION

[0031] In order to describe the method proposed by the present invention more specifically and clearly, the technical implementation scheme of the present invention is described in detail below with reference to the accompanying drawings and actual cases.

[0032] Figure 1 The equivalent architecture diagram of a single-bus DC microgrid is shown in Figure 2. The control goal of the public grid and its interface converter is to maintain system power balance and DC bus voltage stability, so it can be equivalent to a DC source v dc The distributed generation unit and DC converter are responsible for supplying constant power to the terminal load, so they can be equivalent to a constant power source p dg The load is equivalent to a pure resistor R L , because it has the largest island detection blind area. C dg It is expressed as the equivalent bus capacitance. When the circuit breaker is disconnected, the system enters the island mode and the bus voltage is out of control.

[0033] Figure 2 The structure diagram of the adaptive current disturbance control based on Buck converter is mainly composed of power-current double loop. The converter samples the actual output power p of the power generation unit. dg , the output power reference value p of the power generation unit ref The actual output power p dg The difference is used as the input of the power control link. At the same time, the converter samples the grid-connected voltage v of the power generation unit. dg With voltage reference value v ref The difference is sent to the adaptive resonant controller to obtain the disturbance current, and the disturbance current is superimposed on the power control output to obtain the reference value i of the current inner loop control. ref , through the current inner loop to achieve the power generation unit grid point current i dg The error-free tracking can control the output characteristics of the DC converter port.

[0034] Furthermore, the blind-zone-free DC island detection method based on adaptive current disturbance mainly includes the following steps.

[0035] 1) Under rated conditions, the island detection condition with complete source-load power matching is selected as the steady-state operating point, and the grid-connected point voltage v under island mode is established. dg To the disturbance current i dis Small signal model S i (s);

[0036]

[0037] Among them, RL is the load resistance, C dg is the equivalent bus capacitance, K pp is the proportional coefficient of the power controller of the power generating unit, K pi is the integral coefficient of the power controller of the power generation unit, V0 is the steady-state voltage, and s is the Laplace operator.

[0038] 2) In the frequency domain, S i (s) is expressed as the sum of the real part and the imaginary part, that is, S i (jω)=X(ω)+jY(ω); S i The frequency corresponding to the phase crossing of 0° is the theoretical resonant frequency, and at this frequency S i (jω) has the largest amplitude, so the theoretical resonant frequency is the most sensitive disturbance frequency; at this time, the imaginary part Y(ω) is 0 at the theoretical resonant frequency, so the frequency can be obtained as:

[0039]

[0040] Where P0 is the steady-state voltage, C dg is the equivalent bus capacitance;

[0041] 3) According to the theoretical resonant frequency expression, the change in power will cause the theoretical resonant frequency point to shift; therefore, the grid-connected current i is introduced in the resonant frequency selection link dg The ratio of the steady-state current I0 under rated working conditions is used as the adaptive factor. At this time, the adaptive theoretical resonant frequency expression is:

[0042]

[0043] 4) According to the adaptive theoretical resonant frequency expression, the converter connects the power generation unit to the grid point current i dg The sampling value is sent to the frequency selection link G r (s), the resonant frequency at this power level can be adaptively selected. At this time, the resonant frequency will not be offset due to system power changes, thereby eliminating the detection blind area;

[0044]

[0045] Among them, K r is the positive feedback gain, ω r is the resonance bandwidth.

[0046] 5) The converter samples the grid-connected voltage v of the power generation unit dg With voltage reference value v ref The difference is sent to the adaptive resonant controller to obtain the disturbance current i at this frequency. dis , and superimposed on the current loop;

[0047] i dis =(v dg -v ref )·G r (s)

[0048] 6) Detect the oscillation frequency component in the bus voltage. If the bus voltage oscillates three times continuously and the oscillation frequency is equal to the theoretical resonant frequency, it is determined that an islanding event has occurred; otherwise, it is determined that the system is still in the grid-connected mode.

[0049] In order to demonstrate the effect of the DC island detection without blind zone based on adaptive current disturbance proposed in this invention, Figure 1 The verification is carried out under the DC microgrid shown.

[0050] Figure 4 The DC island detection effect of adaptive current disturbance at rated power is demonstrated. In this example, the resonant frequency at rated power is selected as 60Hz. Figure 4 As shown in (a), the PCC voltage is stable in the grid-connected mode. At 1s, an islanding event occurs, and the grid-connected point voltage oscillates due to the loss of support. When the voltage is detected to oscillate three times continuously and the oscillation frequency is 60Hz, it indicates that an islanding event has occurred, as shown in Figure 4 (b) shows that the detection time is 0.37s. After detecting the islanding event, the power generation unit stops supplying power to the load. This result shows that the adaptive current disturbance islanding detection method of the present invention can effectively detect DC islanding events.

[0051] Figure 5 The performance comparison of the island detection between the adaptive current perturbation method proposed in this invention and the traditional fixed perturbation frequency method is shown. The traditional method uses the theoretical resonant frequency at rated power as the fixed current perturbation frequency, and in this example, 60Hz is selected. However, when the system is running at 60% of the rated power, the actual system oscillation frequency is 58Hz. Figure 5 As shown in the figure, the oscillation frequency offset generates a detection blind area, resulting in the failure of island detection. The adaptive current perturbation method proposed in the present invention introduces the grid connection point current in the frequency selection link, and the theoretical resonant frequency of the adaptive adjustment system when operating at 60% of the rated power is 46Hz. Figure 5 As shown, the actual oscillation frequency of the system based on the method of the present invention is also 46 Hz, which effectively eliminates the frequency offset phenomenon. The results show that the adaptive current disturbance island detection method of the present invention can achieve DC island detection without blind spots.

[0052] The technical content and technical features of the present invention have been disclosed above. However, those skilled in the art may still make various substitutions and modifications based on the teachings and disclosures of the present invention without departing from the spirit of the present invention. Therefore, the protection scope of the present invention should not be limited to the contents disclosed in the embodiments, but should include various substitutions and modifications without departing from the present invention, such as using different converter structures, different current inner loop controls, etc., and are covered by the claims of this patent application.

Claims

1. A method for detecting DC islands without blind spots based on adaptive current disturbance, characterized in that: The following steps are involved: Under rated conditions, the island detection condition with complete source-load power matching is selected as the steady-state operating point, and the grid-connected point voltage v in island mode is established. dg To the disturbance current i dis Small signal model S i (s); the small signal model S i (s) is: Among them, R L is the load resistance, C dg is the equivalent bus capacitance, K pp is the proportional coefficient of the power controller of the power generating unit, K pi is the integral coefficient of the power controller of the power generation unit, V0 is the steady-state voltage, and s is the Laplace operator; In the frequency domain, S i (s) is expressed as the sum of the real part and the imaginary part, that is, S i (jω)=X(ω)+jY(ω); S i The frequency corresponding to the phase crossing of 0° is the theoretical resonant frequency, and at this frequency S i (jω) has the largest amplitude, so the theoretical resonant frequency is the most sensitive disturbance frequency; at this time, the imaginary part Y(ω) is 0 at the theoretical resonant frequency, so the frequency can be obtained as: Where P0 is the steady-state voltage, C dg is the equivalent bus capacitance; According to the theoretical resonant frequency expression, the change of power will cause the theoretical resonant frequency point to shift; therefore, the grid-connected current i is introduced in the resonant frequency selection link. dg The ratio of the steady-state current I0 under rated working conditions is used as the adaptive factor. At this time, the adaptive theoretical resonant frequency expression is: According to the adaptive theoretical resonant frequency expression, the converter connects the power generation unit to the grid current i dg The sampling value is sent to the frequency selection link G r (s), the resonant frequency at this power level can be adaptively selected. At this time, the resonant frequency will not be offset due to system power changes, thereby eliminating the detection blind spot; the frequency selection link G r (s) is: Among them, K r is the positive feedback gain, ω r is the resonance bandwidth; The converter samples the grid-connected voltage v of the generating unit dg With voltage reference value v ref The difference is sent to the adaptive resonant controller to obtain the disturbance current i at this frequency. dis , and superimposed on the current loop; i dis =(v dg -v ref )·G r (s) The oscillation frequency component in the bus voltage is detected. If the bus voltage oscillates three times continuously and the oscillation frequency is equal to the theoretical resonant frequency, it is determined that an islanding event occurs; otherwise, it is determined that the system is still in the grid-connected mode.

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