Low-voltage DC power distribution network protection method based on active injection control

Abstract

The invention discloses a low-voltage direct-current power distribution network protection method based on active injection control, and the method comprises the following steps: judging that a low-voltage direct-current power distribution network breaks down when the outlet voltage udc and current idc of the low-voltage side of a direct-current transformer meet a fault starting criterion; when it is judged that the low-voltage direct-current power distribution network breaks down, the direct-current transformer is switched to be in an active injection control mode, the low-voltage side of the direct-current transformer outputs alternating current with the frequency being finj, and the alternating current serves as a characteristic frequency detection signal injected into the low-voltage direct-current power distribution network; all the DC-DC converters except the direct current transformer in the power distribution network lock network side interface switches of the DC-DC converters; and each feeder terminal protection device of the distribution network uploads the information of the characteristic frequency current detected by each line to the protection host, and the protection host discriminates the fault line in a centralized manner. The method can guarantee the safety of the DC transformer during the fault period of the distribution network, reliably identifies the fault line, is simple and reliable in algorithm, and is high in anti-transition resistance capability.

Description

Technical Field

[0001] The invention relates to the technical field of relay protection of a direct current distribution system, and in particular to a low-voltage direct current distribution network protection method based on active injection control. Background Art

[0002] With the increasing number of DC loads such as large data centers and electric vehicle charging stations, distributed power sources such as photovoltaics and wind power, as well as energy storage devices such as lithium batteries and all-vanadium liquid flow, are being integrated into the power grid in large capacities, and the source-load-storage DC characteristics of the power grid are becoming more and more obvious. In this context, the advantages of DC distribution networks over AC distribution networks are reflected: superior power quality, flexible control, fewer commutation links, and the ability to more efficiently accommodate distributed power sources and DC loads.

[0003] When an inter-pole short circuit fault occurs in a DC distribution network, the fault process evolves quickly and the fault current peak is high, threatening the safety of the power electronic devices inside the converter. Therefore, it is necessary to quickly and reliably identify and remove the fault. The radial low-voltage DC distribution network uses a DC transformer (DCT) as the main power supply. After a fault, the DCT will quickly lock to ensure the safety of its own devices, causing the fault current to decay rapidly and eventually to zero. The fault current duration is short, which is not conducive to relay protection to identify the fault. To this end, the present invention designs a low-voltage DC distribution network protection method based on active injection control. By improving the DCT post-fault control strategy, a characteristic frequency detection signal is injected into the low-voltage DC distribution network, and the information of the detection signal detected by each feeder of the distribution network is integrated to centrally identify the fault line, so as to quickly remove the fault line and ensure the safe operation of the low-voltage DC distribution network. Summary of the invention

[0004] Aiming at the problems that fault current of a radial low-voltage DC distribution network has a short duration and the existing protection method has a complex setting, the present invention provides a low-voltage DC distribution network protection method based on active injection control.

[0005] To achieve the above purpose, the technical solution provided by the present invention is:

[0006] A low voltage DC distribution network protection method based on active injection control comprises the following steps:

[0007] When the DC transformer low voltage side output voltage u dc With current i dc When the fault start criterion is met, it is determined that a fault has occurred in the low voltage DC distribution network;

[0008] When it is determined that a fault has occurred in the low-voltage DC distribution network, the DC transformer switches to active injection control mode, and the output frequency of the low-voltage side of the DC transformer is f inj The AC is used as a characteristic frequency detection signal injected into the low voltage DC distribution network;

[0009] When it is determined that a fault occurs in the low-voltage DC distribution network, all DC-DC converters except the DC transformer in the distribution network lock their grid-side interface switches;

[0010] The terminal protection devices of each feeder in the distribution network upload the information of the characteristic frequency current detected by each line to the protection host, which then centrally identifies the faulty line.

[0011] In order to optimize the above technical solutions, the specific limitations adopted also include:

[0012] The low voltage DC distribution network is of radial structure, and the fault type is an inter-pole short circuit fault.

[0013] The fault start criterion is as follows:

[0014] (|u dc |<U set )&(|i dc |>I set )

[0015] Where U set is the voltage setting threshold, I set is the current setting threshold.

[0016] The U set 0.5 times the rated voltage of the low voltage side of the DC transformer, I set It is twice the rated current value of the low voltage side of the DC transformer.

[0017] When it is determined that a fault occurs in the low-voltage DC distribution network, the DC transformer switches to the active injection control mode, and the output frequency of the low-voltage side of the DC transformer is f inj The AC is used as the characteristic frequency detection signal injected into the low-voltage DC distribution network, where the fundamental frequency f of the AC output on the low-voltage side of the DC transformer is inj is 200Hz, the fundamental wave effective value I inj 5% of the rated current value.

[0018] Furthermore, the DC-DC converters other than the DC transformer include photovoltaic, energy storage, and DC load interface converters.

[0019] The feeder terminal protection device of the distribution network uploads the information of the characteristic frequency current detected by each line to the protection host, and the protection host centrally determines the fault line. When the fundamental effective value of the characteristic frequency current flowing through the line is I m When the following overcurrent criteria are met, the feeder terminal protection device uploads information "1", otherwise the upload information is "0":

[0020] I m ≥K set IN

[0021] In the formula, I N is the rated current value of the low voltage side of the DC transformer, K set Take 0.035.

[0022] Furthermore, the algorithm for centrally identifying faulty lines by the protection host includes numbering line switches, generating a fault identification matrix, and identifying faulty lines.

[0023] The process of centralized identification of faulty lines by the protection host is as follows:

[0024] Number the distribution line switches and name them K i (i=1,2,……,n), the numbering order is arbitrary; the definition of the fault discrimination matrix D is as follows:

[0025]

[0026] Among them, the matrix off-diagonal elements d i，j According to the distribution line switch K i With K j The adjacency relationship assignment of K i With K j There is a line between them and K i Point to K j The direction is the positive direction of power, then d i，j =1, otherwise d i，j =0; diagonal element d i，i The logic quantity uploaded by each feeder terminal protection device;

[0027] The method for determining the fault line is: when i satisfies "d i，j =1, and all d i，j =0(j≠i)" or "d i，j =1, and all d i，j =1, the corresponding j all satisfies d j，j =0", the switch K i The corresponding line is the faulty line.

[0028] In a disclosed solution of the present invention, the low voltage DC distribution network protection method based on active injection control comprises the following specific steps:

[0029] (1) Real-time acquisition of the output voltage and current of the low-voltage side of the DC transformer to determine whether the fault start criterion is met. When several consecutive sampling points meet the fault start criterion, proceed to the next step, otherwise return;

[0030] (2) After the fault is initiated, the DC transformer is started to actively inject the characteristic frequency current signal into the grid, and the grid-side interface switches of the photovoltaic, energy storage, and DC load interface converters other than the DC transformer are locked to block the current from being fed into the fault point;

[0031] (3) After a delay of a period of △t, the control enters a steady state and the centralized protection is started;

[0032] (4) The measurement and protection device at each feeder terminal collects the characteristic frequency current flowing through each feeder in real time and calculates its effective value to determine whether it meets the overcurrent criterion; when the effective value calculated by the sliding window meets the overcurrent criterion for several consecutive times, the information "1" is uploaded to the protection host; when the calculated effective value does not meet the overcurrent criterion for several consecutive times, the information "0" is uploaded to the protection host;

[0033] (5) The protection host generates a fault discrimination matrix and identifies the faulty line based on the network structure and the fault information uploaded by the terminal protection device;

[0034] (6) The protection host issues a tripping command and reports the protection action status.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention provides a low-voltage DC distribution network protection method based on active injection control. During a fault, a DC transformer (DCT) is actively injected to control the DC distribution network to inject characteristic frequency current. Each feeder protection terminal detects the characteristic frequency current information and uploads it to a protection center decision unit to centrally identify the fault line. The protection method can solve the problem that DCT locking after a low-voltage DC distribution network fault causes the protection to be unable to provide stable fault characteristics, and has the advantages of simple and reliable algorithm and strong resistance to transition resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A schematic diagram of the wiring structure of a radial low-voltage DC distribution network involved in a specific embodiment of the present invention;

[0038] Figure 2 The topological structure of the DCT capable of active injection control designed for the present invention;

[0039] Figure 3 This is a block diagram of the DCT active injection control strategy adopted by the present invention;

[0040] Figure 4 It is the effective value and setting value of the current of each line with characteristic frequency (200Hz) when f1 fault occurs;

[0041] Figure 5 The present invention provides a flow chart of a low voltage DC distribution network protection method based on active injection control. DETAILED DESCRIPTION

[0042] The above contents of the present invention are further described in detail below in the form of embodiments, but this should not be understood as the scope of the above subject matter of the present invention being limited to the following embodiments, and all technologies realized based on the above contents of the present invention belong to the scope of the present invention.

[0043] The present invention provides a low voltage DC distribution network protection method based on active injection control, comprising the following steps:

[0044] When the DC transformer low voltage side output voltage u dc With current i dc When the fault start criterion is met, it is determined that a fault has occurred in the low voltage DC distribution network;

[0045] When it is determined that a fault has occurred in the low-voltage DC distribution network, the DC transformer switches to active injection control mode, and the output frequency of the low-voltage side of the DC transformer is f inj The AC is used as a characteristic frequency detection signal injected into the low voltage DC distribution network;

[0046] When it is determined that a fault occurs in the low-voltage DC distribution network, all DC-DC converters except the DC transformer in the distribution network lock their grid-side interface switches;

[0047] The terminal protection devices of each feeder in the distribution network upload the information of the characteristic frequency current detected by each line to the protection host, which then centrally identifies the faulty line.

[0048] The technical solution of the present invention is further described in detail below in conjunction with specific implementation methods:

[0049] Low voltage DC distribution network is usually a radial structure. Figure 1 The figure is a schematic diagram of the wiring structure of a radial low-voltage DC distribution network. Figure 2 The circuit structure shown in the figure has a current limiting reactor L configured at the outlet. d=100μH; ESC is an energy storage interface converter, using a bidirectional Buck-Boost circuit; the photovoltaic interface converter (PVC) uses a unidirectional Boost circuit; the circuit structure of the DC load interface converters DC-DC1 and DC-DC2 is related to the load type, but no matter which circuit structure is used, the fault current can be blocked by locking the grid-side switch. Approximately ignoring the ground capacitance branch, the inductance per unit length of the distribution line is 0.05mH / km, and the resistance per unit length is 0.05Ω / km. The lengths of lines 1 to 5 are 0.4km, 0.4km, 0.6km, 0.2km, and 0.2km respectively. During normal operation, DCT adopts constant voltage control to maintain the low-voltage side bus voltage at 0.75kV; PVC adopts constant power control to send out 0.2MW; ESC adopts constant power control to absorb 0.1MW; DC-DC1 and DC-DC2 both adopt constant voltage control to control the load side voltage to 400V, and the corresponding DC load absorbs 0.4MW and 0.25MW respectively. Each line terminal protection device is installed at line switches K1~K6.

[0050] Taking the inter-pole short circuit fault at f1 as an example, the specific implementation operation of the low-voltage DC distribution network protection method based on active injection control disclosed in the present invention is as follows:

[0051] S01, using the DCT outlet undervoltage element and overcurrent element to form a fault start judgment criterion. Real-time detection of the DCT low-voltage side outlet voltage u dc 、DCT low voltage side output current i dc Once the criterion shown in formula (1) is met, it is determined that an inter-pole short circuit fault has occurred in the low-voltage DC distribution network.

[0052] (|u dc |<U set )&(|i dc |>I set (I set ) (1)

[0053] Where U set is the voltage setting threshold, take 0.5U N (U N is the rated voltage value of the low voltage side of DCT), I set is the current setting threshold, take (I N is the rated current value of the DCT low voltage side).

[0054] S02, when it is determined that a fault occurs in the low voltage DC distribution network, the DCT switches to an active injection control mode. The DCT topology designed by the present invention that can perform active injection control is as follows: Figure 2As shown, during the active injection control period, Q5 and Q8 are controlled to be continuously turned on, and Q6, Q7, Q9, and Q 10 , Q 11 Continuously shut down, Q1, Q2, Q3, and Q4 are turned on and off according to the inverter bridge phase shift control. The DCT active injection control strategy block diagram is as follows Figure 3 As shown in the figure, dc is the DCT low voltage side output current, I ref is the injection current command value, d1 is the phase shift of the inverter bridge circuit on the high-voltage side of the high-frequency transformer (HFT), and f is the fundamental frequency of the injection current signal. dc After filtering, the effective value (RMS) of the characteristic frequency current is calculated, and then compared with the command value, and the current loop PI regulator generates the shift phase d1. The fundamental frequency of the injected current signal is designed to be 200Hz, and the fundamental effective value of the injected current signal is set to 5% of the rated current value of the DCT low-voltage side.

[0055] S03, when it is determined that a fault occurs in the low voltage DC distribution network, Figure 1 PVC, ESC, DC-DC1 and DC-DC2 in the distribution network all lock their grid-side interface switches to block photovoltaic, energy storage devices and DC load grid-side capacitors from feeding current to the fault point. At this time, the fault current is only provided by DCT.

[0056] S04, each feeder terminal protection device of the distribution network uploads the information of the 200Hz current detected by each line to the protection host, and the protection host centrally determines the faulty line.

[0057] 1) Characteristic frequency current information judgment:

[0058] Figure 1 The overcurrent elements in the protection devices of the middle lines 1 to 5 judge the characteristic frequency current information. When the effective value I of the 200Hz current component flowing through the line is measured m When the overcurrent criterion shown in formula (2) is met, the feeder terminal protection device uploads information "1", otherwise the upload information is "0".

[0059] I m ≥K set I N (2)

[0060] In the formula, I N is the rated current value of the DCT low voltage side, K set Take 0.035.

[0061] When an inter-pole short circuit fault occurs at f1, the effective value of the 200Hz current component flowing through each line and the overcurrent threshold value are measured as follows: Figure 4It can be judged that the information uploaded by each line terminal protection device to the protection host is shown in Table 1.

[0062] Table 1: Upload information of each line protection device

[0063]

[0064] 2) Fault line identification:

[0065] The algorithm for centrally identifying faulty lines by the protection host includes three steps: numbering line switches, generating a fault identification matrix, and identifying faulty lines.

[0066] First, the distribution line switches are numbered and named K i (i=1,2,……,n), the numbering order is arbitrary. In this embodiment, the numbering and naming of each line switch are as follows Figure 1 shown.

[0067] Then, based on this, the fault discrimination matrix D is generated. The off-diagonal elements of the matrix d i，j According to the distribution line switch K i With K j The adjacency relationship assignment of K i With K j There is a line between them and K i Point to K j The direction is the positive direction of power, then d i，j =1, otherwise d i，j =0; diagonal element d i，i It is the logic quantity uploaded by each feeder terminal protection device. Figure 1 The fault discrimination matrix of the distribution network is shown in formula (3).

[0068]

[0069] Finally, the elements of matrix D are judged. When i satisfies "d i，j =1, and all d i，j =0(j≠i)" or "d i，j =1, and all d i，j =1, the corresponding j all satisfies d j，j =0", the switch K i The corresponding line is the fault line. In formula (3), d i，i =0 (i=2,3,5,6), so the lines corresponding to switches K2, K3, K5, and K6 are not faulty lines; "d 1，1 =1,d 2，2 =0,d 3，3 =0,d 4，4 ≠0", so the line corresponding to switch K1 is not a fault line; "d4，4 =1,d 5，5 =0,d 6，6 =0", so the line corresponding to switch K4 is the fault line. Finally, the protection host determines that line 3 is the fault line, and switch K4 trips to remove the fault.

[0070] The flowchart of a low voltage DC distribution network protection method based on active injection control proposed by the present invention is as follows: Figure 5 The specific description is as follows:

[0071] 1) Real-time acquisition of the DCT low-voltage side outlet voltage and current to determine whether they meet the fault start criterion shown in formula (1). When three consecutive sampling points meet the fault start criterion, proceed to the next step, otherwise return;

[0072] 2) After the fault starts, the DCT active injection control is started to inject the characteristic frequency current signal into the grid, and the grid-side interface switches of the photovoltaic, energy storage, and DC load interface converters other than the DCT are locked to block the current from being fed into the fault point;

[0073] 3) After a delay of △t, the control enters a steady state and the centralized protection is started. △t can be set to 40ms;

[0074] 4) Each feeder terminal measurement and protection device collects the characteristic frequency current flowing through each feeder in real time and calculates its effective value to determine whether it meets the overcurrent criterion shown in formula (2). When the effective value calculated by the sliding window meets the overcurrent criterion for three consecutive times, the information "1" is uploaded to the protection host. When the calculated effective value does not meet the overcurrent criterion for 10ms continuously, the information "0" is uploaded to the protection host;

[0075] 5) The protection host generates a fault discrimination matrix and identifies the fault line according to the network structure and the fault information uploaded by the terminal protection device;

[0076] 6) The protection host issues a tripping command and reports the protection action.

[0077] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any technician familiar with the profession, without departing from the scope of the technical solution of the present invention, according to the technical essence of the present invention, any simple modification, equivalent replacement and improvement made to the above embodiment still falls within the protection scope of the technical solution of the present invention.

Claims

1. A low voltage DC distribution network protection method based on active injection control, characterized in that: The following steps are involved: When the DC transformer low voltage side output voltage u dc With current i dc When the fault start criterion is met, it is determined that a fault has occurred in the low voltage DC distribution network; When it is determined that a fault has occurred in the low-voltage DC distribution network, the DC transformer switches to active injection control mode, and the output frequency of the low-voltage side of the DC transformer is j inj The AC is used as a characteristic frequency detection signal injected into the low voltage DC distribution network; When it is determined that a fault occurs in the low-voltage DC distribution network, all DC-DC converters except the DC transformer in the distribution network lock their grid-side interface switches; The terminal protection devices of each feeder in the distribution network upload the information of the characteristic frequency current detected by each line to the protection host, which then centrally identifies the faulty line.

2. The low voltage DC distribution network protection method based on active injection control according to claim 1 is characterized in that: The low voltage DC distribution network is of radial structure, and the fault type is an inter-pole short circuit fault.

3. The low voltage DC distribution network protection method based on active injection control according to claim 1 is characterized in that: The fault start criterion is as follows: (|in dc |<U set )&(|i dc |>I set ) Where U set is the voltage setting threshold, I set is the current setting threshold.

4. The low voltage DC distribution network protection method based on active injection control according to claim 3 is characterized in that: The U set 0.5 times the rated voltage of the low voltage side of the DC transformer, I set It is twice the rated current value of the low voltage side of the DC transformer.

5. The low voltage DC distribution network protection method based on active injection control according to claim 1 is characterized in that: When it is determined that a fault occurs in the low-voltage DC distribution network, the DC transformer switches to the active injection control mode, and the output frequency of the low-voltage side of the DC transformer is f inj The AC is used as the characteristic frequency detection signal injected into the low-voltage DC distribution network, where the fundamental frequency f of the AC output on the low-voltage side of the DC transformer is inj is 200Hz, the fundamental effective value I inj 5% of the rated current value.

6. The low voltage DC distribution network protection method based on active injection control according to claim 1, characterized in that: The DC-DC converters other than the DC transformer include photovoltaic, energy storage, and DC load interface converters.

7. The low voltage DC distribution network protection method based on active injection control according to claim 1 is characterized in that: The feeder terminal protection device of the distribution network uploads the information of the characteristic frequency current detected by each line to the protection host, and the protection host centrally determines the fault line. When the fundamental effective value of the characteristic frequency current flowing through the line is I m When the following overcurrent criteria are met, the feeder terminal protection device uploads information "1", otherwise the upload information is "0": I m ≥K set I N In the formula, I N is the rated current value of the low voltage side of the DC transformer, K set Take 0.

035.

8. The low voltage DC distribution network protection method based on active injection control according to claim 1 is characterized in that: The algorithm for centrally distinguishing faulty lines by the protection host includes numbering line switches, generating a fault distinguishing matrix, and distinguishing faulty lines.

9. The low voltage DC distribution network protection method based on active injection control according to claim 1, characterized in that: The process of centralized identification of faulty lines by the protection host is as follows: Number the distribution line switches and name them K i (i=1,2,……,n), the numbering order is arbitrary; the definition of the fault discrimination matrix D is as follows: Among them, the matrix off-diagonal elements d i，j According to the distribution line switch K i With K j The adjacency relationship assignment of K i With K j There is a line between them and K i Point to K j The direction is the positive direction of power, then d i，j =1, otherwise d i，j =0; diagonal element d i，i The logic quantity uploaded by each feeder terminal protection device; The method for identifying the faulty line is: when i satisfies "d i，i =1, and all d i，j =0(j≠i)" or "d i，j =1, and all d i，j =1, all satisfy d j，j =0,, when the switch K i The corresponding line is the faulty line.

10. The low voltage DC distribution network protection method based on active injection control according to claim 1, characterized in that: The specific steps are as follows: (1) Real-time acquisition of the output voltage and current of the low-voltage side of the DC transformer to determine whether the fault start criterion is met. When several consecutive sampling points meet the fault start criterion, proceed to the next step, otherwise return; (2) After the fault is initiated, the DC transformer is started to actively inject the characteristic frequency current signal into the grid, and the grid-side interface switches of the photovoltaic, energy storage, and DC load interface converters other than the DC transformer are locked to block the current from being fed into the fault point; (3) After a delay of Δt, the control enters a steady state and the centralized protection is started; (4) Each feeder terminal measurement and protection device collects the characteristic frequency current flowing through each feeder in real time and calculates its effective value to determine whether it meets the overcurrent criterion; when the effective value calculated by the sliding window meets the overcurrent criterion for several consecutive times, the information "1" is uploaded to the protection host; when the calculated effective value does not meet the overcurrent criterion for several consecutive times, the information "0" is uploaded to the protection host; (5) The protection host generates a fault discrimination matrix and identifies the faulty line based on the network structure and the fault information uploaded by the terminal protection device; (6) The protection host issues a tripping command and reports the protection action status.

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