Harmonic differential protection method and system for distribution network containing IIDG based on energy injection

Abstract

A harmonic differential protection method and system for a distribution network containing IIDG based on energy injection, wherein intermittent injection of high-order harmonic energy is added to the IIDG current inner loop controller, the characteristic signal length of each distributed power source injected each time is 40ms, and the injection signal amount is 10% of the IIDG inner loop control current reference value, and the FFT signal acquisition method is used to perform active injection protection of the distribution network based on characteristic signals and / or active protection based on multiple IIDG characteristic signals. The present invention makes protection more flexible through the active participation of IIDG in protection. With the constraints that the power quality of the DG grid-connected point is qualified and the IIDG output power does not exceed the limit, the IIDG current inner loop is selected to inject characteristic high-order harmonic energy, and the activation time and effective action period of active energy regulation are determined, thereby constructing an active action mechanism for the IIDG grid-connected inverter.

Description

Technical Field

[0001] The present invention relates to a technology in the field of smart grid control, specifically to a harmonic differential protection method and system for an IIDG distribution network based on energy injection. Background Art

[0002] With the large-scale access of inverter interfaced distributed generation (IIDG) in the distribution network, the network power structure and fault electrical quantity distribution have also changed, and the fault characteristics have become more complex, which has increased the difficulty of distribution network protection. The IIDG in the distribution network has the ability to actively adjust the output by adjusting the control link. Its active participation in regulation can quickly, accurately and orderly remove the fault, thereby restoring the stable operation of the distribution network. In existing research, there has been the use of active signal injection into the power grid to realize fault identification, but most of them have been applied to DC distribution networks.

[0003] The decentralized access of IIDGs in the AC distribution network is equivalent to adding distributed measurement points along the feeder, which can be fully utilized for fault detection and section location. With the continuous multi-point decentralized access of a large number of IIDGs, the active participation of multiple IIDGs in the equivalent section during the fault period is very beneficial to the fault location and protection action in the distribution network. It is necessary to propose an active protection scheme for the AC distribution network that combines the activeness and rapid controllability of IIDG access.

[0004] However, with the rise of distributed power generation technology, the penetration rate of IIDG in AC distribution networks continues to increase, the post-fault characteristics are becoming more and more complex, and the probability of false protection failure is gradually increasing. Current protection schemes rarely consider the high controllability of IIDG grid-connected inverters. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the present invention proposes a harmonic differential protection method and system for a distribution network containing IIDG based on energy injection, which makes the protection more flexible through the active participation of IIDG in protection. With the constraints that the power quality of the DG grid connection point is qualified and the IIDG output power does not exceed the limit, the IIDG current inner loop is selected to inject characteristic high-order harmonic energy, and the activation time and effective action period of active energy regulation are determined, so as to construct an active action mechanism of the IIDG grid-connected inverter.

[0006] The present invention is achieved through the following technical solutions:

[0007] The present invention relates to a harmonic differential protection method for a distribution network containing an IIDG based on energy injection. Intermittent injection of high-order harmonic energy is added to an IIDG current inner loop controller. The length of a characteristic signal injected by each distributed power source each time is 40ms, and the amount of the injected signal is 10% of the IIDG inner loop control current reference value. An FFT signal acquisition method is adopted to perform active injection protection of the distribution network based on characteristic signals and / or active protection based on multiple IIDG characteristic signals.

[0008] Technical Effects

[0009] The present invention solves the problem that the IIDG in the prior art always plays a passive role in the distribution network protection, and makes the protection more flexible by actively injecting signals into the power grid and actively participating in the protection of the IIDG.

[0010] Compared with the prior art, the present invention can accurately identify faults within the zone and faults outside the zone, and the protection can operate reliably under different fault types and fault lines. Give full play to the controllability of the inverter, inject characteristic signals into the current inner loop control module, and this control method can be implemented in the current inner loop without the need to add additional equipment. Characteristic signals of different frequencies are selected to be injected to distinguish different distributed power sources, so it is also suitable for distribution networks with multiple distributed power sources. In the field of AC distribution networks with decentralized access to inverter-type distributed power sources, fault identification is achieved by actively injecting signals into the power grid. Based on this, a harmonic differential protection scheme for distribution networks containing IIDG based on energy injection is proposed. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 The inverter control block diagram with active characteristic signal injection;

[0012] Figure 2 Schematic diagram of characteristic signal injection fitting;

[0013] Figure 3 A simplified distribution network diagram for a single IIDG access;

[0014] In the figure: Grid is the AC grid; CB is the AC circuit breaker; LD is the AC load; IIDG is the inverter distributed power source;

[0015] Figure 4 is the CD fault current flow diagram;

[0016] Figure 5 This is a schematic diagram of the process of implementing the injection protection of the present invention;

[0017] Figure 6 It is the AB fault current flow diagram;

[0018] Figure 7 It is the AE fault current flow diagram;

[0019] Figure 8 This is the structure diagram of the distribution network containing dual IIDGs;

[0020] Fig. 9 It is a schematic diagram of the simulation model constructed in PSCAD / EMTDC;

[0021] In the figure: (a) a single IIDG intervention distribution network simulation model; (b) multiple IIDG access distribution network simulation model;

[0022] Fig.10 This is a schematic diagram of the FFT result of the grid voltage after the characteristic signal is injected;

[0023] In the figure: (a) BusA; (b) IIDG access point;

[0024] Fig.11 This is a schematic diagram of the FFT result of the grid current after the characteristic signal is injected;

[0025] In the figure: (a) BusA; (b) IIDG access point;

[0026] Fig.12 This is a schematic diagram of the simulation of a single-phase ground fault in an AC line;

[0027] In the figure: (a) current flowing through protection 4; (b) current flowing through protection 5;

[0028] Fig.13 It is a schematic diagram of AC line phase-to-phase short circuit fault simulation;

[0029] In the figure: (a) current flowing through protection 4; (b) current flowing through protection 5;

[0030] Fig.14 Simulates three-phase ground fault in AC line

[0031] Fig.15 This is a schematic diagram of the fault current FFT calculation results of AC line fault protection 4 and protection 5;

[0032] In the figure: (a) single-phase grounding protection 4 current, (b) single-phase grounding protection 5 current, (c) phase-to-phase fault protection 4 current, (d) phase-to-phase fault protection 5 current, (e) three-phase grounding protection 4 current, (f) three-phase grounding protection 5 current;

[0033] Fig.16 This is a schematic diagram of the FFT calculation results and protection action of a single-phase ground fault on an AC line;

[0034] In the figure: (a) FFT calculation result; (b) protection criterion. DETAILED DESCRIPTION

[0035] like Figure 1 As shown, this embodiment relates to a harmonic differential protection system for a distribution network containing an IIDG based on energy injection, comprising: a power outer loop control module, a current inner loop control module, an inverter drive signal generation module with a phase-locked loop (PLL), a PWM inverter, an LC oscillator and a busbar connected in sequence, wherein: the power outer loop control module collects the real-time output power of the IIDG and compares it with the rated power P ref and Q ref After making the difference, the current reference value I required by the current inner loop is obtained through the PI control loop. d_ref and I q_ref The current inner loop control module performs a dq-abc rotation transformation on the current reference value to obtain the required three-phase current reference values ​​Iaref, Ibref, and Icref. The inverter drive signal generation module couples the three-phase current reference value with the injected characteristic signal and applies it to the PWM inverter.

[0036] The power outer loop control module obtains the real-time output power of the IIDG and obtains the current reference value required by the current inner loop through the PI control loop after making a difference between the power and the rated power.

[0037] The current inner loop control module obtains the current signal output by the power outer loop control module and calculates the inverter drive signal.

[0038] The inverter drive signal generation module includes: a phase-locked loop, a drive signal acquisition unit connected thereto, and a trigger signal generation unit, wherein: the drive signal acquisition unit obtains the inverter drive signal m obtained by the current inner loop control module d and m q , the inverter switch is driven on and off by the inverter drive signal; the trigger signal generation unit modulates the signal into a sinusoidal voltage and current signal according to the PWM algorithm, and outputs the voltage and current signal result to the IIDG outlet through the filtering link.

[0039] The injected characteristic signal is in: f1, T represents the angular frequency, frequency and period of the fundamental wave, h is the harmonic order of the injected characteristic high-order harmonic energy, β h Indicates the initial phase angle of the hth harmonic component, I sprms is the effective value of the injected characteristic signal.

[0040] In order to ensure the stability of the system and the power quality of the system, the injected characteristic signal adopts an intermittent injection mode. Considering the time margin required for communication delay and data processing, each rising edge is selected as the time to start injecting the characteristic signal. The length of the characteristic signal injected by each distributed power source is 40ms, which is equivalent to two fundamental wave cycles. The signal is set to be injected once a minute during normal operation. If there is no abnormality, the injection will be canceled after 40ms. If an abnormality is detected 40ms after the signal is injected, the injection will continue until subsequent operations such as protection action are detected. If a system abnormality is detected during the period when the signal is not injected, the signal injection will be started immediately until the abnormality is resolved.

[0041] The amplitude of the injected characteristic signal is 10% of the IIDG inner loop control current reference value.

[0042] The present embodiment relates to a harmonic differential protection method for a distribution network containing IIDGs based on energy injection of the above system, including: active injection protection of the distribution network based on characteristic signals and / or active protection based on characteristic signals of multiple IIDGs.

[0043] like Figure 3 As shown, the specific application environment of the protection system of this embodiment is a simplified distribution network to which a single IIDG is connected, including: an AC grid Grid, AC circuit breakers CB0-CB6, AC loads LD1, LD2 and an inverter-type distributed power source IIDG, wherein: the second to fifth AC circuit breakers CB1-CB4 of the AC grid Grid connected to the AC grid are connected in series in sequence, the sixth and seventh AC circuit breakers CB5 and CB6 of the AC grid Grid connected to the AC grid are connected in series, the first and second AC loads LD1 and LD2 are connected to the fifth and seventh AC circuit breakers CB4 and CB6 respectively, and the inverter-type distributed power source IIDG is connected in parallel with the fourth AC circuit breaker CB3.

[0044] The active injection protection of the distribution network based on characteristic signals refers to: the power outer loop control module collects the real-time output power of the inverter distributed power source IIDG set at the grid-connected relay and compares it with the rated power P ref and Q ref Make a difference, such as Figure 4 As shown:

[0045] When the fault occurs at the downstream line CD, that is, the fourth AC circuit breaker CB3, according to the flow direction of the fault current, the current flowing through the AC circuit breaker CB3 is provided by the upper system and IIDG, and the current flowing through the AC circuit breaker CB4 is 0. At this time, the characteristic sub-signal current harmonic content HRI flowing through each AC circuit breaker CB is detected h , specifically: Where: I n_1Represents the effective value of the fundamental current flowing through the nth protection action unit, I n_h It represents the effective value of the hth harmonic current flowing through the nth protection action unit. The subscript n represents the nth protection action unit, and h represents the effective value of the hth harmonic current.

[0046] For AB line, when the fault occurs outside the line, the characteristic sub-signal current harmonic content rate HRI of the second and third AC circuit breakers CB1 and CB2 is detected. h Since the current provided by DG cannot flow through AC circuit breakers CB1 and CB2, the characteristic harmonic energy injected into DG cannot be detected at AC circuit breakers CB1 and CB2, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, ΔHRI of line AB is hAB =|HRI hB1 -HRI hB2 |;

[0047] For CD line, when the fault occurs inside the line, the characteristic sub-signal current harmonic content rate HRI of the fourth and fifth AC circuit breakers CB3 and CB4 is detected. h Since the current provided by DG cannot flow through AC circuit breaker CB4, the characteristic harmonic energy injected into DG cannot be detected at AC circuit breaker CB4, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the CD line is hCD =|HRI hB3 -HRI hB4 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is: Where: ΔHRI hset It is the setting value of the characteristic sub-signal current harmonic content difference.

[0048] like Figure 5 As shown in FIG. 1 , a protection action unit based on the inverter control system according to the present embodiment is provided. Figure 1 / Figure 2 / Figure 3 / Figure 4The protection action unit specifically includes: an acquisition module, an FFT calculation module, a difference comparison module and a start module, wherein: the acquisition module arranged in the current relay acquires the AC current of each line, the FFT calculation module performs FFT analysis and calculation on the current passing through the fifth and sixth AC circuit breakers 4 and 5, the difference comparison module sets the protection criterion by determining the difference in the harmonic content rate of the characteristic sub-signal current at both ends of the line, and the start module completes the tripping of the relay and the interaction of the tripping information.

[0049] like Figure 6 As shown, the protection criterion for setting the content rate difference is obtained in the following way: when a fault occurs in the upstream line AB, the characteristic sub-signal current harmonic content rate HRI of the AC circuit breaker CB1 and the AC circuit breaker CB2 is detected. h Since the current provided by DG cannot flow through AC circuit breaker CB1, the characteristic harmonic energy injected into DG cannot be detected at AC circuit breaker CB1, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, ΔHRI of line AB is hAB =|HRI hB1 -HRI hB2 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is: Where: ΔHRI hset It is the setting value of the characteristic sub-signal current harmonic content difference.

[0050] like Figure 7 As shown, when a fault occurs in the adjacent line AE, the characteristic sub-signal current harmonic content rate HRI of the sixth and seventh AC circuit breakers CB5 and CB6 is detected. h Since the current provided by DG cannot flow through AC circuit breaker CB6, the characteristic harmonic energy injected into DG cannot be detected at AC circuit breaker CB6, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the AE line is hAE =|HRI hB5 -HRI hB6 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is: Where: ΔHRI hset It is the setting value of the characteristic sub-signal current harmonic content difference.

[0051] The active protection based on multiple IIDG characteristic signals is aimed at Figure 8The power distribution network connected to multiple distributed power sources shown includes: a first branch consisting of buses BusA and BusB and a second branch consisting of a bus BusC.

[0052] When a fault occurs in the line between BusA and BusB ine1 When the fault current reaches point f1, the upper power grid, IIDG1 and IIDG2 will provide fault current to point f1 according to the power flow direction. For line AB, the characteristic sub-signal current harmonic content rate HRI of AC circuit breaker CB1.1 and AC circuit breaker CB1.1 is detected. h , where HRI h1 HRI is the current harmonic content rate of the characteristic subharmonic injected by IIDG1 h2 The current harmonic content of the characteristic subharmonic injected by IIDG2. The fault current on the left side of the fault point is provided by the system and IIDG2, while the fault current on the right side of the fault point is provided by IIDG1. The current provided by IIDG2 cannot flow through the AC circuit breaker CB1.2, so the characteristic harmonic energy injected into IIDG2 cannot be detected at the AC circuit breaker CB1.2. Similarly, the current provided by IIDG1 cannot flow through the AC circuit breaker CB1.1, so the characteristic harmonic energy injected into IIDG1 cannot be detected at the AC circuit breaker CB1.1, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the AE line is h1AE =|HRI h1B1.1 -HRI h1B1.2 |,ΔHRI h2AE =|HRI h2B1.1 -HRI h2B1.2 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is:

[0053]

[0054] like Fig. 9 As shown, it is a hybrid structure of single IIDG access and multiple IIDG access, including: a first branch composed of buses BusA and BusB and a second branch composed of bus BusC. The characteristic signal injection protection strategy under this hybrid structure includes: protection implementation of single IIDG characteristic signal injection and protection implementation of multiple IIDG characteristic signal injection, specifically:

[0055] ① Single IIDG characteristic signal injection protection implementation: Fig. 9 (a) shows the characteristic signal injected by the IIDG1 current control loop, where the 22nd harmonic is selected and the amplitude of the harmonic energy is I sprms =0.1I refrms, the grid voltage level is 10.5kV, the IIDG limit is 1.5 times, and the IIDG rated power is 3MW. When the distribution network is operating normally, observe the harmonic voltage and current conditions of the grid connection point after the harmonic energy is injected, and perform FFT calculation on the voltage and current signals. The results are as follows Fig.10 , Fig.11 As shown in Figure 1, the harmonic energy intensity injected meets the harmonic content standard of the grid. Different types of faults occurring on the AC line are simulated, and the results are shown in Figure 1. Figure 12-14 The FFT analysis and calculation of the current flowing through the AC circuit breakers CB4 and CB5 under the above fault conditions is performed. The FFT analysis results of the current flowing through the AC circuit breaker CB under the fault are shown as follows: Fig.15 shown.

[0056] Similarly, different types of fault conditions of AB line and CD line are simulated, FFT calculation is performed on the fault current, and the obtained ΔHRI is compared. hf and the set value ΔHRI hset , the results are shown in Table 1.

[0057] Table 1 Single IIDG characteristic signal injection protection action

[0058]

[0059] ②Protection realization of multiple IIDG characteristic signal injection: Fig. 9 The three rated capacities shown in (b) are: S IIDG1 =3MW; S IIDG2 =2MW and S IIDG3 =1MW is injected into the characteristic signal from the power supply current control loop, and the amplitude of the harmonic energy is I sprms =0.1I refrms , where IIDG1 injects 11th harmonic, IIDG2 injects 17th harmonic, and IIDG3 injects 23rd harmonic. When a single-phase grounding fault occurs on the AB line at 4s, FFT analysis and calculation are performed on the current flowing through the AC circuit breakers CB1.1 and CB1.2. The FFT analysis results and protection action of the current flowing through the protection under the fault are shown in Fig.16 As shown, it can be seen that ΔHRI hf The protection action conditions are met quickly and the protection action is successful.

[0060] Comparing the ΔHRI of the above fault conditions hf ΔHRI of IIIDG with larger capacity under the same fault condition hf The value is larger, and the protection criterion is more sensitive.

[0061] Compared with the prior art, the present invention adds an additional control strategy of intermittently injecting high-order harmonic energy into the IIDG current inner loop controller, and proposes a harmonic differential protection scheme for the distribution network containing IIDG based on energy injection. The characteristic signal length injected by each distributed power source each time is 40ms, and the injection signal amount is 10% of the IIDG inner loop control current reference value. When the IIDG capacity increases, the proportion of the provided fault current increases, that is, the protection criterion is more sensitive; when the IIDG penetration rate increases, that is, when the distribution network is connected to multiple IIDGs, the fault can be judged more accurately based on the location of each IIDG, and the identification of multiple protection criteria is more accurate; when multiple IIDG criteria are combined, when the IIDG capacity is the same, the protection criterion of the IIDG closer to the fault point is more sensitive.

[0062] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A harmonic differential protection method for a distribution network containing IIDG based on energy injection, characterized in that: Add intermittent injection of high-order harmonic energy in the IIDG current inner loop controller, the characteristic signal length of each injection of each inverter-type distributed power source is 40ms, the injection signal amount is 10% of the IIDG inner loop control current reference value, and the FFT signal acquisition method is used to perform active protection based on a single IIDG characteristic signal and / or active protection based on multiple IIDG characteristic signals; The active protection based on the single IIDG characteristic signal is for a distribution network connected to a single IIDG, the distribution network comprising: an AC power grid, an AC circuit breaker, an AC load and an IIDG, wherein: the second to fifth AC circuit breakers of the AC power grid connected to the AC power grid are sequentially connected in series, the sixth and seventh AC circuit breakers of the AC power grid connected to the AC power grid are connected in series, the first and second AC loads are respectively connected to the fifth and seventh AC circuit breakers, and one end of the inverter-type distributed power source is arranged between the third and fourth AC circuit breakers; The active protection based on the single IIDG characteristic signal refers to: the power outer loop control module collects the real-time output power of the inverter-type distributed power source IIDG set at the grid-connected relay and compares it with the rated power P ref and Q ref The difference includes: When the fault occurs at the fourth AC circuit breaker, according to the direction of the fault current, the current flowing through the fourth AC circuit breaker is provided by the upper system and IIDG, while the current flowing through the fifth AC circuit breaker is 0. At this time, the characteristic sub-signal current harmonic content rate HRI flowing through each AC circuit breaker CB is detected. h , specifically: Where: I n_1 Represents the effective value of the fundamental current flowing through the nth protection action unit, I n_h represents the effective value of the hth harmonic current flowing through the nth protection action unit, the subscript n represents the nth protection action unit, and h represents the effective value of the hth harmonic current; The protection action unit includes: an acquisition module, an FFT calculation module, a difference comparison module and a start module, wherein: the acquisition module arranged in the current relay acquires the AC current of each line, the FFT calculation module performs FFT analysis and calculation on the current passing through the fifth and sixth AC circuit breakers, the difference comparison module sets the protection criterion by determining the difference in the harmonic content rate of the characteristic sub-signal current at both ends of the line, and the start module completes the tripping of the current relay and the interaction of the tripping information; For the upstream line AB, that is, the line between the AC grid and the third AC circuit breaker, when the fault occurs outside the line, the characteristic sub-signal current harmonic content rate HRI of the second and third AC circuit breakers and h Since the current provided by the IIDG cannot flow through the second AC circuit breaker, the characteristic harmonic energy injected into the IIDG cannot be detected at the second AC circuit breaker, so ΔHRI h is the harmonic content difference of the characteristic sub-signal current flowing through the circuit breakers at both ends of the line. At this time, the harmonic content difference of the characteristic sub-signal current of the upstream line AB is ΔHRI hAB =|HRI hB1 -HRI hB2 |; For the downstream line CD, that is, the line between the third AC circuit breaker and the fifth AC circuit breaker, when the fault occurs inside the line, the characteristic sub-signal current harmonic content rate HRI of the fourth and fifth AC circuit breakers and h Since the current provided by the IIDG cannot flow through the fifth AC circuit breaker, the characteristic harmonic energy injected into the IIDG cannot be detected at the fifth AC circuit breaker, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the line is hCD =|HRI hB3 -HRI hB4 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set. The action criterion of the protection scheme is: the difference in the harmonic content rate of the characteristic sub-signal current of the downstream line CD Where: ΔHRI hset It is the setting value of the characteristic sub-signal current harmonic content difference; The active protection based on multiple IIDG characteristic signals is aimed at a distribution network connected to multiple inverter-type distributed power sources, the distribution network comprising: a first branch consisting of buses BusA and BusB and a second branch consisting of bus BusC, wherein: AC circuit breakers CB1.1 and CB1.2 are sequentially arranged in series between buses BusA and BusB, and AC circuit breakers CB1.3 and CB1.4 are respectively connected to the output end of AC circuit breaker CB1.2; The active protection based on multiple IIDG characteristic signals specifically includes: When a fault occurs in the line between BusA and BusB ine1 When the fault current reaches point f1, the upper power grid, IIDG1 and IIDG2 will provide fault current to point f1 according to the power flow direction. For the upstream line AB, the characteristic sub-signal current harmonic content rate HRI of the AC circuit breaker CB1.1 and the AC circuit breaker CB1.2 is detected. h , where HRI h1 HRI is the current harmonic content rate of the characteristic subharmonic injected by IIDG1 h2 is the current harmonic content rate of the characteristic subharmonic injected by IIDG2. The fault current on the left side of the fault point is provided by the system and IIDG2, while the fault current on the right side of the fault point is provided by IIDG1. The current provided by IIDG2 cannot flow through the AC circuit breaker CB1.2, so the characteristic harmonic energy injected into IIDG2 cannot be detected at the AC circuit breaker CB1.

2. Similarly, the current provided by IIDG1 cannot flow through the AC circuit breaker CB1.1, so the characteristic harmonic energy injected into IIDG1 cannot be detected at the AC circuit breaker CB1.1, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the upstream line AE is h1AE =|HRI h1B1.1 -HRI h1B1.2 |,ΔHRI h2AE =|HRI h2B1.1 -HRI h2B1.2 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is: The upstream route AE is the route between BusA and point f1.

2. A harmonic differential protection system for distribution network containing IIDG based on energy injection for implementing the method described in claim 1, characterized in that: include: The power outer loop control module, the current inner loop control module, the inverter drive signal generation module with a phase-locked loop, the PWM inverter, the LC oscillator and the bus are connected in sequence, wherein: the power outer loop control module collects the real-time output power of the IIDG and compares it with the rated power P ref and Q ref After making the difference, the current reference value I required by the current inner loop is obtained through the PI control loop. d_ref and I q_ref The current inner loop control module performs dq-abc rotation transformation on the current reference value to obtain the required three-phase current reference values ​​Iaref, Ibref, and Icref. The inverter drive signal generation module couples the three-phase current reference value with the injected characteristic signal and acts on the PWM inverter. The power outer loop control module obtains the real-time output power of the IIDG and obtains the current reference value required by the current inner loop through the PI control loop after subtracting the power from the rated power. The current inner loop control module obtains the current signal output by the power outer loop control module and calculates the inverter drive signal; The injected characteristic signal is in: ω1, f1, T represents the angular frequency, frequency and period of the fundamental wave, h is the harmonic order of the injected characteristic high-order harmonic energy, β h represents the initial phase angle of the hth harmonic component, I sprms is the effective value of the injected characteristic signal.

3. The energy injection-based harmonic differential protection system for distribution network containing IIDG according to claim 2 is characterized in that: The injected characteristic signal adopts an intermittent injection mode, specifically: each rising edge is selected as the time to start injecting the characteristic signal, and the length of the characteristic signal injected by each inverter-type distributed power supply is 40ms, which is equivalent to two fundamental wave cycles; the signal is set to be injected once a minute during normal operation. When there is no abnormality, the injection is canceled after 40ms. When an abnormality is detected 40ms after the signal is injected, the injection continues until the subsequent operation of the protection action is detected. When a system abnormality is detected during the period when the signal is not injected, the signal injection is started immediately until the abnormality is resolved.

4. The energy injection-based harmonic differential protection system for distribution network containing IIDG according to claim 2 is characterized in that: The inverter drive signal generation module includes: a phase-locked loop, a drive signal acquisition unit connected thereto, and a trigger signal generation unit, wherein: the drive signal acquisition unit obtains the inverter drive signal m obtained by the current inner loop control module d and m q , the inverter switch is driven on and off by the inverter drive signal; the trigger signal generation unit modulates the signal into a sinusoidal voltage and current signal according to the PWM algorithm, and outputs the voltage and current signal result to the IIDG outlet through the filtering link.

5. The energy injection-based harmonic differential protection system for distribution network containing IIDG according to claim 2 is characterized in that: The protection criteria for content rate difference setting, for the distribution network connected to a single IIDG, specifically include: When a fault occurs in the upstream line AB, that is, the line between the second AC circuit breaker and the third AC circuit breaker, the characteristic sub-signal current harmonic content rate HRI of the second and third AC circuit breakers CB1 and CB2 is detected. h Since the current provided by the IIDG cannot flow through the second AC circuit breaker CB1, the characteristic harmonic energy injected into the IIDG cannot be detected at the second AC circuit breaker CB1, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the upstream line AB is hAB =|HRI hB1 -HRI hB2 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is: Where: ΔHRI hset It is the setting value of the characteristic sub-signal current harmonic content difference; When a fault occurs in the adjacent line AE, that is, the line between the AC grid and the seventh AC circuit breaker, the characteristic sub-signal current harmonic content rate HRI of the sixth and seventh AC circuit breakers CB5 and CB6 is detected. h Since the current provided by the IIDG cannot flow through the seventh AC circuit breaker CB6, the characteristic harmonic energy injected into the IIDG cannot be detected at the seventh AC circuit breaker CB6, so ΔHRI h is the difference in harmonic content of the characteristic subsignal current flowing through the circuit breakers at both ends of the line. At this time, the ΔHRI of the adjacent line AE is hAE =|HRI hB5 -HRI hB6 |; By determining the difference in the harmonic content rate of the characteristic harmonic energy current at both ends of the line, the protection criterion is set, and the action criterion of the protection scheme is: Where: ΔHRI hset It is the setting value of the characteristic sub-signal current harmonic content difference.

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