Fault detection and isolation method for connection converter of alternating-current and direct-current hybrid micro-grid

Abstract

The invention discloses a fault detection and isolation method for a connection converter of an AC / DC hybrid microgrid. According to the method, the three-phase current value of the alternating current side of the converter is calculated by directly measuring the three-phase current value of the alternating current side of the converter and measuring the bridge arm current of the converter. And respectively performing dq conversion on the measured value and the calculated value to obtain a dq-axis current based on the measured value and a dq-axis current based on the calculated value, and respectively calculating a d-axis differential current and a q-axis differential current. A fault occurrence position is judged by using detection logic constructed by d-axis and q-axis differential current, after an internal fault is detected, the converter is isolated, fault return logic is continuously judged, and when a return criterion is detected to be met, a fault isolation command is withdrawn. The method is suitable for fault detection and isolation of the connection converter of the AC-DC hybrid microgrid, and all common fault types of the connection converter can be sensitively reflected.

Description

Technical Field

[0001] The present invention belongs to the technical field of AC-DC hybrid microgrid operation and control, and particularly relates to a method for fault detection and isolation of a connection converter between an AC sub-network and a DC sub-network in an AC-DC hybrid microgrid. Background Art

[0002] With the large-scale access of distributed photovoltaic power sources, new energy storage systems, and electric vehicle charging loads, developing an AC-DC hybrid microgrid has become an important technical route to promote the coordinated operation of power sources, energy storage, and loads in a distribution network. In an AC-DC hybrid microgrid, a connection converter completes rectification and inversion functions and is a key device connecting the AC sub-network and the DC sub-network. After a connection converter fails, it will directly lead to the outage of the AC-DC hybrid microgrid. Therefore, comprehensive, sensitive, and fast fault detection and isolation are key technologies to ensure reliable power supply for the AC-DC hybrid microgrid.

[0003] Currently, the technical solutions commonly used for fault detection and isolation of connection converters in AC-DC hybrid microgrids include two categories: detection technologies based on overcurrent principles and detection technologies based on current differential principles.

[0004] (1) Detection technology based on overcurrent principle

[0005] To avoid severe overcurrent damage to devices such as thyristors caused by connection converter failures, fault detection devices for valve overcurrent are configured in the converter area. The principle of valve overcurrent fault detection is simple and easy to implement, but the entire converter will be blocked after fault isolation. Aiming at the drawback that the traditional valve overcurrent fault detection blocks the entire converter as soon as any bridge arm has overcurrent, this technology has been improved to bridge arm blocking, where only the bridge arm that reaches the overcurrent threshold is blocked, while other bridge arms can operate normally. However, connection converter failures usually cause a large increase in current within a very short time, and it is difficult to set the overcurrent threshold for bridge arm blocking, making it difficult to accurately isolate the faulty bridge arm.

[0006] (2) Detection technology based on current differential principle

[0007] To accurately and sensitively isolate the faulty components of the converter, a fault detection technology based on the current differential principle is also adopted. According to different subdivided regions, this technology is further divided into four types: bridge arm differential detection method, converter differential detection method, differential detection method based on bridge arm phase units, and pole current differential detection method.

[0008] The arm differential detection method is based on the principle that the currents flowing into and out of the same arm are equal. It measures the currents at both ends of the cascaded sub-modules of the same upper arm or the same lower arm respectively, so as to construct a fault detection criterion. The minimum detection unit of the arm differential detection technology is a single upper arm or lower arm. The principle of the arm differential detection technology is simple, only involving the currents at both ends, and it can locate the fault in a half-bridge arm, which is beneficial to accurate fault location and isolation. However, detection units need to be separately configured for each of the 6 arms of the converter, and the number of detection units is relatively large.

[0009] In order to reduce the number of configured detection units, the converter differential detection method divides the 6 arms into an upper arm group and a lower arm group, and configures fault detection units respectively, reducing the number of detection units to 2, which are responsible for detecting the faults of 3 upper arms and 3 lower arms respectively. After the number of detection units is reduced, the detection criterion is the same as that of the arm differential detection method, and the fault detection range is also the same as it, only including the part from the arm to the DC side outlet, and neither can cover the entire connected converter area.

[0010] In order to cover the entire converter area as much as possible for fault detection, a differential detection method based on arm phase units is proposed. This method first divides the entire converter area into 3 "phase units", and a phase unit consists of the upper and lower arms of the same phase and the AC side leads connecting the arms of this phase; then, according to the principle that the currents flowing into and out of the same phase unit should satisfy Kirchhoff's current law, detection units are configured for the 3 phase units of the converter respectively. The ratio restraint criterion adopted by this detection method, although ensuring reliable restraint during external faults, also reduces the detection sensitivity during internal faults.

[0011] For the purpose of sensitively reflecting various faults in the DC part of the connected converter, a pole current differential detection method is also adopted in the converter area. This method calculates the differential current by summing the positive pole current and the negative pole current on the DC side of the converter, and compares it with a fixed threshold to identify the fault location. The criterion is simple, which is beneficial to improving the speed of fault detection. However, the pole current differential method can only detect the grounded faults inside the converter, and cannot detect the ungrounded faults.

[0012] To sum up, the existing fault detection methods for connected converters are designed based on different principles, and any single detection method cannot sensitively and reliably detect all the faults in the entire converter area. Therefore, there is an urgent need to design a detection and isolation method that can cover the complete connected converter area and sensitively detect various types of faults, so as to improve the operation and control level of the AC-DC hybrid microgrid connected converter. Summary of the Invention

[0013] To overcome the above problems existing in the prior art, the present invention discloses a method for fault detection and isolation of a connecting converter in an AC-DC hybrid microgrid. This method transforms the three-phase current on the AC side of the connecting converter into dq-axis current, calculates the differential current using the dq-axis current, and then constructs a fault detection criterion and a return criterion to complete fault detection and isolation.

[0014] The specific technical solution adopted by the present invention is as follows:

[0015] A method for fault detection and isolation of a connecting converter in an AC-DC hybrid microgrid, comprising the following steps

[0016] 1. Directly collect the three-phase current on the AC side of the converter and the converter arm current

[0017] Using the current transformer CT1 installed on the connecting line of the AC side of the converter, obtain the three-phase current i vj_M (k) of the AC side of the converter. The subscript j in the current symbol i vj_M takes a, b, and c, respectively representing the A-phase current i va_M (k), the B-phase current i vb_M (k), and the C-phase current i vc_M (k); the subscript M represents the direct measurement value, which is distinguished from the current calculation value in the subsequent steps; k represents the sampling value number. At the same time, use the current transformer CT2 installed between the upper arm of the converter and the DC positive pole to obtain the upper arm current i pj (k), and use the current transformer CT3 installed between the lower arm of the converter and the DC negative pole to obtain the lower arm current i nj (k). The meaning of the subscript j in the current symbols i pj (k) and i nj (k) is the same as above; the subscript p represents the quantity of the upper arm, and the subscript n represents the quantity of the lower arm. The fault detection of the connecting converter takes CT1, CT2, and CT3 as the boundaries, and the internal faults within the area surrounded by these three current transformers are regarded as internal faults of the converter.

[0018] 2. Indirectly calculate the three-phase current on the AC side of the converter

[0019] Based on the Kirchhoff's current law relationship, calculate the calculated value i pj (k) of the three-phase current on the AC side of the converter according to the directly measured upper arm current i nj (k) and lower arm current i vj_C (k) of the converter as shown in Equation (1).

[0020]

[0021] In the formula, i va_C (k), i vb_C (k), and ivc_C (k) represent the calculated values of the converter AC side phase A, phase B, and phase C currents; i pa (k), i pb (k), and i pc (k) are the upper arm currents of the converter phase A, phase B, and phase C respectively; i na (k), i nb (k), and i nc (k) are the lower arm currents of the converter phase A, phase B, and phase C respectively; the subscript C represents the indirectly calculated value to distinguish it from the directly measured values i vj_M (k) of the three-phase currents on the AC side of the converter.

[0022] 3. Perform dq transformation on the directly collected and indirectly calculated AC side currents of the converter

[0023] For the directly collected measured values i vj_M (k) of the three-phase currents on the AC side of the converter and the indirectly calculated calculated values i vj_C (k) of the three-phase currents on the AC side of the converter, perform dq transformation respectively:

[0024]

[0025]

[0026] In the formula, i d_M (k) and i q_M (k) are the d-axis current and q-axis current obtained after performing dq transformation on i vj_M (k) respectively; i d_C (k) and i q_C (k) are the d-axis current and q-axis current obtained after performing dq transformation on i vj_C (k) respectively; T abc-dq is the dq transformation matrix, specifically:

[0027]

[0028] In the formula, θ represents the angle between the d-axis and the phase A current.

[0029] When the converter is operating normally and there is no fault in the converter area, the calculation results of formula (2) and formula (3) satisfy:

[0030]

[0031] When a fault occurs inside the converter area, formula (5) no longer holds.

[0032] 4. Calculate the d-axis differential current and q-axis differential current

[0033] According to the relationship of formula (5), using the transformed d-axis current id_M (k) and i d_C (k) constructs the d-axis differential current i dif_d (k), using the transformed q-axis current i q_M (k) and i q_C (k) constructs the q-axis differential current i dif_q (k):

[0034]

[0035] When the converter is operating normally, Equation (5) holds, so i dif_d (k) and i dif_q (k) are both close to 0; when a fault occurs inside the converter area, Equation (5) does not hold, so i dif_d (k) and i dif_q (k) are both much greater than 0.

[0036] 5. Issue or withdraw the fault isolation command according to the detection result

[0037] During the normal operation of the connected converter, execute the fault detection process according to Step 5.1; after a internal fault has been detected and the isolation command has been issued, then execute the isolation command withdrawal process according to Step 5.2.

[0038] 5.1 Fault detection process

[0039] Use the d-axis differential current i dif_d (k) and the q-axis differential current i dif_q (k), and judge whether a fault occurs inside the converter according to the fault detection criterion shown in Equation (7).

[0040] i dif_d (k) > i d0_d or i dif_q (k) > i d0_q (7)

[0041] In the formula, i d0_d and i d0_q are the d-axis differential current threshold and the q-axis differential current threshold respectively.

[0042] i d0_d and i d0_q are determined as follows: Determine by avoiding the maximum unbalanced current during normal operation and external faults and considering a certain margin. The specific determination formula is:

[0043]

[0044] In the formula, k0 is the margin coefficient, and the margin coefficients for the d-axis and q-axis take the same value; i un_d and i un_qThe maximum unbalanced currents on the d-axis and q-axis respectively.

[0045] The d-axis differential criterion i dif_d (k) ≥ i d0_d and the q-axis differential criterion i dif_q (k) ≥ i d0_q are taken as an "OR" logical relationship. That is, when any of the d-axis and q-axis differential currents exceeds the threshold, it can be determined that the internal fault criterion is satisfied; when neither the d-axis nor the q-axis differential current exceeds the threshold, it is determined that the internal fault criterion is not satisfied. When continuously satisfying the fault detection criterion formula (7) within the time period t d1 , it is determined that an internal fault has occurred, and a fault isolation command is issued. In specific implementation, the continuous judgment time t d1 is converted into the number of sampling points n1 under the sampling frequency f s , and the calculation method is n1 = t d1 × f s .

[0046] 5.2 Process of withdrawing the isolation command

[0047] After the fault isolation command has been issued, continuously detect whether the fault has been isolated to determine when to withdraw the command. Using the d-axis differential current i dif_d (k) and the q-axis differential current i dif_q (k), judge whether the fault has been reliably isolated according to the return criterion shown in formula (9).

[0048] i dif_d (k) < i d0_d and i dif_d (k) < i d0_q (9)

[0049] The d-axis differential criterion i dif_d (k) ≤ i d0_d and the q-axis differential criterion i dif_q (k) ≤ i d0_q are taken as an "AND" logical relationship. That is, when both the d-axis and q-axis differential currents are less than the threshold, it can be determined that the return criterion is satisfied; when any of the d-axis and q-axis differential currents is greater than the threshold, it is determined that the return criterion is not satisfied.

[0050] When continuously satisfying the return criterion formula (9) within the time period t d2 , it is considered that the fault has been isolated, and the isolation command is withdrawn; otherwise, the fault isolation command is continuously sent until it is confirmed that the fault has been isolated. In specific implementation, the continuous judgment time t d2 is converted into the number of sampling points n2 under the sampling rate f s , and the calculation method is n2 = t d2 × f s .

[0051] The beneficial effects of the present invention include:

[0052] (1) The present invention can identify ground faults and non - ground faults in the converter - connected area, and can identify faults occurring at all positions in both the AC part and the DC part of the entire converter - connected area, reflecting fault types and fault positions more comprehensively than the prior art.

[0053] (2) The present invention uses the dq - axis differential current of DC nature as the judgment basis, designs the fault detection criterion as a direct comparison method between the differential current and the threshold, and uses the OR logic of the dq - axis differential current criterion to detect the fault position, which not only simplifies the criterion but also improves the sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a schematic diagram of the AC - DC hybrid micro - grid structure;

[0055] Figure 2 is a schematic diagram of the topology structure of the converter - connected part and the fault positions;

[0056] Figure 3 is a flow chart of fault detection and isolation;

[0057] Figure 4 are the dq - axis current, dq - axis differential current, and fault isolation signal during internal faults. DETAILED DESCRIPTION OF THE INVENTION

[0058] The following further describes in detail the specific implementation manners of the present invention in conjunction with the drawings and embodiments, but it does not limit the protection scope of the present invention. Any technical solutions obtained by equivalent replacement or equivalent transformation are within the protection scope of the present invention.

[0059] A typical schematic diagram of the AC - DC hybrid micro - grid structure is as Figure 1 shown. Among them, the AC sub - micro - grid and the DC sub - micro - grid are interconnected through a converter - connected part, and the internal topology structure of the converter - connected part is as Figure 2 shown. Figure 2 In, the current transformer CT1 is installed on the AC - side connection line of the converter, CT2 is installed on the upper bridge arm of the converter, and CT3 is installed on the lower bridge arm of the converter. The fault detection of the converter - connected part is bounded by CT1, CT2, and CT3, and internal faults within the area surrounded by these 3 current transformers are regarded as internal faults of the converter. The flow chart of the fault detection and isolation of the AC - DC hybrid micro - grid converter - connected part involved in the present invention is as Figure 3 shown.

[0060] The faults of the converter - connected part are summarized in Table 1, and the specific fault positions are marked in Figure 2 Among them, the internal faults of the converter - connected part include f 11Arm grounding fault, f 12 Arm short - circuit fault, f 13 Fault of valve - side AC connection line; External faults include f2 DC bus fault and f3 AC grid fault.

[0061] Table 1 Faults of the connected converter

[0062]

[0063] Under the above conditions, the maximum unbalanced current during the normal operation and external faults of the connected converter is obtained through digital simulation, approximately i un_d = i un_q = 0.001 kA. Taking the margin coefficient k0 = 20, the d - axis differential current threshold i d0_d and the q - axis differential current threshold i d0_q are respectively set as:

[0064]

[0065] In the embodiment, the sampling frequency f s = 4 kHz, and taking t d1 = 2 ms, t d2 = 1 ms, then it is converted to get n1 = 8, n2 = 4. The following further clarifies the specific implementation steps of the present invention through embodiments of internal and external faults of the connected converter.

[0066] Embodiment 1:

[0067] An internal fault occurs in the connected converter, and the fault point is located at Figure 2 shown as f 11 . The fault type is the upper - arm grounding fault, the fault occurrence time is t = 0.5 s, and the fault duration Δt = 0.1 s.

[0068] 1. Directly collect the three - phase current on the AC side of the converter and the arm current of the converter

[0069] Using the current transformer CT1 installed on the AC connection line of the converter to obtain the three - phase current i vj_M (k) on the AC side of the converter; Using the current transformer CT2 installed on the upper arm of the converter to obtain the upper - arm current i pj (k); Using the current transformer CT3 installed on the lower arm of the converter to obtain the lower - arm current i nj (k).

[0070] Taking the sampling point number at t = 0 s as k = 1, at f sUnder the condition of = 4 kHz, the sampling point number corresponding to the fault occurrence time t = 0.5 s is k = 2000. The sampling point numbers of 1 sampling point before the fault, the sampling point at the fault time, and 8 consecutive sampling points after the fault are k = 1999 to k = 2008 respectively. The current values directly measured by the above method are shown in Table 2.

[0071] Table 2 Measurement data of three-phase AC current and arm current on the AC side of the converter near the fault occurrence time

[0072]

[0073] 2. Indirectly calculate the three-phase AC current on the AC side of the converter

[0074] For the upper arm current i pj (k) and the lower arm current i nj (k) of the converter directly measured, calculate the calculated value i vj_C (k) of the three-phase AC current on the AC side of the converter according to Equation (11).

[0075]

[0076] The calculation results for the sampling point numbers k = 1999 to k = 2008 are shown in Table 3.

[0077] Table 3 Calculated value data of the three-phase AC current on the AC side of the converter

[0078] Sampling point serial number k <![CDATA[i va_C (k) / kA]]> <![CDATA[i vb_C (k) / kA]]> <![CDATA[i vc_C (k) / kA]]> 1999 0.942 -1.683 0.742 2000 0.716 -1.293 0.577 2001 0.758 -1.479 0.702 2002 0.942 -1.006 0.941 2003 0.959 -0.725 0.958 2004 0.936 -0.674 0.937 2005 0.860 -0.645 0.862 2006 0.774 -0.624 0.775 2007 0.681 -0.559 0.682 2008 0.580 -0.542 0.581

[0079] 3. Perform dq transformation on the directly collected and indirectly calculated AC side currents of the converter

[0080] For the measured values i vj_M (k) of the three-phase AC current on the AC side of the converter directly collected and the calculated values i vj_C (k) of the three-phase AC current on the AC side of the converter indirectly calculated, perform dq transformation respectively:

[0081]

[0082]

[0083] The calculation results for the sampling point numbers k = 1999 to k = 2008 are shown in Table 4.

[0084] Table 4 dq-axis current data near the fault occurrence time

[0085] Sampling point serial number k <![CDATA[i d_M (k) / kA]]> <![CDATA[i q_M (k) / kA]]> <![CDATA[i d_C (k) / kA]]> <![CDATA[i q_C (k) / kA]]> 1999 1.468 -0.184 1.469 -0.183 2000 1.468 -0.184 1.469 -0.183 2001 1.474 -0.182 1.247 0.300 2002 1.385 -0.188 0.422 1.586 2003 1.289 -0.191 0.034 1.708 2004 1.327 -0.188 -0.142 1.698 2005 1.358 -0.193 -0.236 1.536 2006 1.401 -0.207 -0.314 1.402 2007 1.437 -0.223 -0.397 1.251 2008 1.489 -0.238 -0.433 1.085

[0086] 4. Calculate the d-axis differential current and the q-axis differential current

[0087] Using the transformed direct-axis current \(i_d(k)\) and \(i_d'(k)\) to calculate the direct-axis differential current \(i_{dd}(k)\), and using the transformed quadrature-axis current \(i_q(k)\) and \(i_q'(k)\) to calculate the quadrature-axis differential current \(i_{qq}(k)\): d_M \(i_d(k)\) and \(i_d'(k)\) d_C to calculate the direct-axis differential current \(i_{dd}(k)\), and using the transformed quadrature-axis current \(i_q(k)\) dif_d and \(i_q'(k)\) q_M to calculate the quadrature-axis differential current \(i_{qq}(k)\): q_C \(i_q(k)\) and \(i_q'(k)\) dif_q (k):

[0088]

[0089] The calculation results for the sampling point numbers \(k = 1999\) to \(k = 2008\) are shown in Table 5.

[0090] Table 5 Data of dq-axis currents and dq-axis differential currents near the fault occurrence time

[0091]

[0092] 5. Issue or withdraw a fault isolation command based on the detection result

[0093] Using the direct-axis differential current \(i_{dd}(k)\) and the quadrature-axis differential current \(i_{qq}(k)\), check whether the fault detection criterion shown in Equation (15) is satisfied. dif_d \(i_{dd}(k)\) and \(i_{qq}(k)\) dif_q (k), check whether the fault detection criterion shown in Equation (15) is satisfied.

[0094] \(i_{dd}(k)>0.02 kA\) or \(i_{qq}(k)>0.02 kA\) (15) dif_d (k)>0.02 kA or \(i_{qq}(k)\) dif_q (k)>0.02 kA (15)

[0095] Table 5 data shows that at the sampling points \(k = 1999\) and \(k = 2000\), that is, before and at the time of fault occurrence, the dq-axis differential currents are both less than the differential current threshold \(i_{th}=i_{ddth}=i_{qqth}=0.02 kA\), and the fault detection criterion of Equation (15) is not yet satisfied. d0_d \(=i_{ddth}\) d0_q \(=i_{qqth}=0.02 kA\), and the fault detection criterion of Equation (15) is not yet satisfied.

[0096] At the sampling points \(k = 2001\) to \(k = 2008\), that is, the consecutive \(n_1 = 8\) sampling points after the fault, the dq-axis differential currents are continuously greater than the direct-axis differential current threshold \(i_{ddth}\) d0_d \(=i_{ddth}\) d0_q \(=i_{qqth}=0.02 kA\), satisfying the fault detection criterion of Equation (15). Therefore, an internal fault is detected at the 8th sampling point after the fault, and then a fault isolation command is sent.

[0097] After the fault isolation command is issued, it will always be output. During the continuous duration of the fault, the differential current is continuously calculated to determine whether the direct-axis differential current \(i_{dd}(k)\) and the quadrature-axis differential current \(i_{qq}(k)\) satisfy the return criterion shown in Equation (16). dif_d \(i_{dd}(k)\) and the quadrature-axis differential current \(i_{qq}(k)\) dif_q (k) satisfy the return criterion shown in Equation (16).

[0098] i dif_d (k) < 0.02kA and i dif_d (k) < 0.02kA (16)

[0099] The waveforms of the dq-axis current and the dq-axis differential current during the fault are as Figure 4 shown. The differential current data from the k = 2014th sampling point to the k = 2018th sampling point are listed in Table 6, corresponding to Figure 4 the data during the period from 0.507 s to 0.509 s in

[0100] Table 6 dq-axis current and dq-axis differential current data during the fault duration

[0101]

[0102] According to Table 6, during the fault duration, there is no situation where four consecutive sampling points satisfy the fault isolation return criterion formula (16). Therefore, the fault isolation command is continuously issued to ensure reliable fault isolation.

[0103] The dq-axis current and the dq-axis differential current data before and after the actual fault isolation moment are listed in Table 7. The data in Table 7 show that four consecutive sampling points from the k = 2402nd sampling point to the k = 2405th sampling point satisfy the return criterion of formula (16), and the fault isolation command is withdrawn. Thus, the fault detection and isolation process ends.

[0104] Table 7 dq-axis current and dq-axis differential current data before and after withdrawing the isolation command

[0105]

[0106] Example 2:

[0107] An internal fault occurs in the connected converter, and the fault point is located at Figure 2 the f shown in 13 . The fault type is a ground fault of phase A of the connecting line on the AC side of the converter, the fault moment is t = 0.5 s, and the fault duration Δt = 0.1 s.

[0108] Steps 1 to 5 in Example 2 are the same as those in Example 1 in terms of process. Only the differences will be elaborated below.

[0109] Step 4, after calculation, the dq-axis current and the dq-axis differential current data are shown in Table 8.

[0110] Table 8 dq-axis current and dq-axis differential current data for the internal fault on the AC side of the connected converter

[0111]

[0112] Step 5. After judgment, it is found that the continuous n1 = 8 sampling points after the fault all satisfy the fault isolation criterion formula (15). Therefore, at the 8th sampling point after the fault, that is, 2 ms after the fault, an internal fault of the connecting converter is detected. During the fault, there are no continuous n2 = 4 sampling points that satisfy the return criterion formula (16). Therefore, the fault isolation signal has been reported and output all the time, and the fault is reliably isolated. At 101.25 ms after the fault, that is, when the sampling point serial number is k = 2405, continuous n2 = 4 sampling points satisfy the return criterion formula (16), and the fault isolation command is withdrawn.

[0113] Embodiment 3:

[0114] An external fault occurs in the connecting converter, and the fault point is located at Figure 2 the position of f2 shown in the figure. The fault type is a DC bus positive pole grounding fault. The fault time is t = 0.5 s, and the fault duration is Δt = 0.1 s.

[0115] Steps 1 to 5 in Embodiment 3 are the same as those in Embodiment 1 in terms of process. Only the differences are described below.

[0116] Step 4. After calculation, the dq-axis current and dq-axis differential current data are shown in Table 9.

[0117] Table 9 dq-axis current and dq-axis differential current data of external faults

[0118]

[0119] Step 5. After judgment, during the fault duration, the dq-axis differential current does not satisfy the fault detection criterion:

[0120] i dif_d (k) > 0.02 kA or i dif_q (k) > 0.02 kA (26)

[0121] The fault detection result is an external fault, and no fault isolation command is issued.

[0122] The method for summarizing the fault detection results of the 3 embodiments is shown in Table 10. The present invention can correctly detect faults and clearly distinguish internal faults and external faults.

[0123] Table 10 Summary of fault detection results of embodiments

[0124]

[0125] As described above, the above is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for detecting and isolating a fault in a connected converter of an AC / DC hybrid microgrid, characterized in that: The following steps are involved: (1) Directly collect the three-phase current on the AC side of the converter and the current on the bridge arm of the converter The three-phase current i on the AC side of the converter is obtained by using the current transformer CT1 installed on the AC side connection line of the converter vj_M (k), where j is a, b and c, corresponding to the A phase current i va_M (k), B phase current i vb_M (k) and C phase current i vc_M (k), k represents the serial number of the kth sampling value, and the M in the subscript represents the measurement value; At the same time, the current transformer CT2 installed between the upper bridge arm of the converter and the DC positive pole is used to obtain the upper bridge arm current i pj (k) The current transformer CT3 installed between the lower bridge arm of the converter and the DC negative pole is used to obtain the lower bridge arm current i nj (k), where j has the same meaning as above; (2) Indirect calculation of the three-phase current on the AC side of the converter The converter upper arm current i is obtained by direct measurement. pj (k) and the lower bridge arm current i nj (k), a set of three-phase current calculation values ​​i on the AC side of the converter is obtained by indirect calculation vj_C (k), subscript C indicates indirect calculation value; (3) Calculate the dq axis current using the AC side current of the converter The three-phase current measurement value i on the AC side of the converter vj_M (k) Perform dq transformation to obtain the d-axis current i d_M (k) and q-axis current i q_M (k), and at the same time, the calculated three-phase current value i on the AC side of the converter vj_C (k) Perform dq transformation to obtain another set of d-axis current i d_C (k) and q-axis current i q_C (k) The connected converter meets the following requirements during normal operation: When a fault occurs inside the connected converter, equation (1) no longer holds; (4) Calculate the d-axis differential current and the q-axis differential current Using the transformed d-axis current i d_M (k) and i d_C (k) Calculate the d-axis differential current i dif_d (k), using the transformed q-axis current i q_M (k) and i q_C (k) Calculate the q-axis differential current i dif_q (k), the calculation method is shown in formula (2); (5) Issue or withdraw fault isolation commands based on the detection results During normal operation of the connected converter, the d-axis differential current i dif_d (k) and q-axis differential current i dif_q (k), judging whether an internal fault of the converter occurs according to the fault detection criterion shown in formula (3); In the formula, i d0_d and i d0_q are the d-axis differential current threshold and the q-axis differential current threshold respectively; After a fault isolation command has been issued, the d-axis differential current i dif_d (k) and q-axis differential current i dif_q (k) According to the return criterion shown in formula (4), determine whether the fault is reliably isolated; 2. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: In step (2), the three-phase current calculation value i of the converter AC side is indirectly calculated. vj_C The specific calculation method of (k) is: In the formula, i va_C (k), i vb_C (k) and i vc_C (k) respectively represent the calculated values ​​of the currents of phase A, phase B and phase C on the AC side of the converter; i pa (k), i pb (k) and i pc (k) are the upper arm currents of phase A, phase B and phase C of the converter respectively; i na (k), i nb (k) and i nc (k) are the lower arm currents of phase A, phase B and phase C of the converter respectively.

3. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: The specific calculation method of the dq transformation in step (3) is: Where, T abc-dq is the dq transformation matrix, specifically: Wherein, θ represents the angle between the d-axis and the A-phase current.

4. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: In step (5), the d-axis differential current threshold i is set according to the maximum unbalanced current that avoids normal operation and external faults, and a certain margin is considered. d0_d and q-axis differential current threshold i d0_q , the specific determination formula is: In the formula, k0 is the margin coefficient, and the margin coefficients of the d-axis and q-axis take the same value; i un_d and i un_q are the maximum unbalanced currents of the d-axis and q-axis respectively.

5. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: In step (5), the fault detection criterion formula (3) is "criterion 1: d-axis differential criterion i dif_d (k)≥i d0_d " and " Criterion 2: q-axis differential criterion i dif_q (k)≥i d0_q " is taken as an "or" logical relationship. When any differential current of the d-axis or q-axis exceeds the threshold, it can be determined that the internal fault criterion is met; when the differential current of both the d-axis and the q-axis does not exceed the threshold, it is determined that the internal fault criterion is not met.

6. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: Step (5) returns to "Criterion 3: d-axis differential criterion i" in criterion formula (4) dif_d (k)<i d0_d " and " criterion 4: q-axis differential criterion i dif_q (k)<i d0_q " is taken as an "AND" logical relationship. When the differential currents of the d-axis and q-axis are both less than the threshold value, it can be determined that the return criterion is met. When any differential current of the d-axis or q-axis is greater than the threshold value, it is determined that the return criterion is not met.

7. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: Step (5) requires the d-axis differential current i dif_d (k) and q-axis differential current i dif_q (k)Continuous d1 Only when the time meets the fault detection criterion formula (3) can it be determined as an internal fault. In specific implementation, the continuous judgment time t d1 Convert to sampling frequency f s The number of sampling points n1 under the condition is calculated as n1 = t d1 ×f s .

8. The method for detecting and isolating a fault of a connected converter of an AC / DC hybrid microgrid according to claim 1, characterized in that: Step (5) requires the d-axis differential current i dif_d (k) and q-axis differential current i dif_q (k)Continuous d2 Only when the time satisfies the return criterion (4), it is determined that the fault has been isolated and the fault isolation command is withdrawn. Otherwise, the isolation command signal continues until the fault is isolated. In specific implementation, the continuous judgment time t d2 Convert to sampling frequency f s The number of sampling points n2 under the condition is calculated as n2 = t d2 ×f s .

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