Multi-element current differential protection method based on DG integrated power distribution system

Abstract

A multi-element current differential protection method based on a DG integrated power distribution system comprises the following steps: step 1, specifying a DPZ protection area range in a distribution network with a DG, simplifying the criterion of traditional differential current protection, and determining the inapplicability of the DG in processing differential protection coordination and intermediate DG feed; step 2, providing action criteria of multiple DR elements so as to realize coordination between a transverse protection device of a DPZ protection area and a continuous differential protection interval; step 3, providing an action criterion of a sensitive time delay DR element so as to realize a correct action of a high-impedance fault; and step 4, providing an action criterion of differential current protection of a corresponding branch after fault isolation so as to avoid interference tripping caused by DG feed. A network part as small as possible can be disconnected during a fault period, so that as many as possible loads and DG units are kept running, and the running reliability of the power distribution network is improved.

Description

Technical Field

[0001] The present invention belongs to the field of secondary protection of distribution network equipment, and in particular relates to a multi-element current differential protection method based on a DG integrated distribution system. Background Art

[0002] Considering the inapplicability of traditional overcurrent protection under certain conditions, the prospect of large-scale integration of DG into distribution systems requires the design of new protection schemes. Most of the proposed related protection solutions utilize directional overcurrent protection schemes based on optimization algorithms, adaptive logic or pilot logic. A fundamental limitation of the above solutions is that they do not solve the coordination problem with existing lateral protection means.

[0003] Since the operating conditions of modern / future distribution systems with DG (i.e., bidirectional short-circuit current and / or closed-loop configuration) are similar to those of transmission systems, it can be considered that the line current differential protection method and the distance protection method are effective solutions for DG integrated distribution systems. However, the disadvantage of the distance protection scheme is that it requires voltage transformers and a large number of DRs. The line current differential protection method has become the mainstream solution for solving the DG integrated distribution system. Most of the current differential protection schemes proposed for DG integrated distribution systems do not address the impact of intermediate DG feeding or load on the differential protection performance. The DG feeding effect is particularly important in terms of false tripping, not only during normal system operation, but also during external faults, which may require coordination between consecutive DRs. Some of the differential protection schemes are correctly set to prevent false tripping due to intermediate loads, but the coordination with the corresponding branch protection devices or the impact of intermediate DGs is not checked. Some schemes take into account the IIDG (inverter type DG) T distribution network within the DPZ, and propose a longitudinal differential protection method based on composite differential impedance, which eliminates the dead zone of this method under three-phase metallic short-circuit faults, but the coordination with the lateral protection has not been solved. Some schemes are based on the equivalent fault model of IIDG and branch load, and establish an adaptive differential protection method that coordinates the fault current amplitude and phase angle threshold, but do not take into account the impact of intermediate DG feeding. Some schemes consider the impact of intermediate DG feeding, but are only applicable to low-feed IIDG units and can only solve the DR coordination problem during external faults. Therefore, it is urgent to propose a multi-element current differential protection method for DG integrated distribution systems to ensure the reliability of power grid operation. Summary of the invention

[0004] The object of the present invention is to solve the above-mentioned deficiencies of the prior art, thereby providing a multi-element current differential protection method based on a DG integrated distribution system, which can disconnect as small a network portion as possible during a fault, so that as many loads and DG units as possible can remain in operation, thereby improving the operating reliability of the distribution network.

[0005] The technical solution adopted by the present invention is: a multi-element current differential protection method based on a DG integrated power distribution system, comprising the following steps:

[0006] Step 1: Define the scope of the DPZ protection zone in the distribution network with DG, simplify the criteria of traditional differential current protection, and clarify its inapplicability in handling differential protection coordination and intermediate DG feeding;

[0007] Step 2: Propose the action criteria of multiple DR elements to achieve the coordination between the lateral protection device of the DPZ protection zone and the continuous differential protection zone;

[0008] Step 3: Propose the action criteria of sensitive time-delay DR elements to achieve correct action for high-impedance faults;

[0009] Step 4: Propose the action criteria of the corresponding branch differential current protection after fault isolation to avoid interference tripping caused by DG feeding.

[0010] In step 1, the scope of the DPZ protection zone in the distribution network with DG is specified, the criteria of the traditional differential current protection are simplified, and its inapplicability in handling differential protection coordination and intermediate DG feeding is clarified, specifically:

[0011] In a distribution network with DGs, the DPZ protection zone is set to cover all areas between a pair of adjacent DRs, covering part of the main feeder and the branch line connected to the farthest MV / LV distribution transformer;

[0012] The DPZ protection zone provides both main protection for main feeder faults and backup protection for branch faults. i and DR j A percentage current differential element is enabled in the DPZ to protect ij , the operating current of this percentage current differential element is I op,ij Equal to DPZ ij The current vector sum measured by the CT at both ends is the limiting current I res,ij Equal to DR i and DR j Measure half of the sum of the current magnitudes;

[0013] When each DR trips, the action criterion of its own circuit breaker is as shown in formula (1):

[0014]

[0015] Where: b ij To define the current differential characteristic slope; I i ,I j DR i and DR j The measured current; Imin-op,ij is the minimum pickup differential current threshold; t p,ij is the operation time of internal MPD or external DR; t CTi The time interval requested for CT measurements; t d,ij time delay for tripping the circuit breaker;

[0016] When a fault occurs inside a DPZ, it is necessary to maintain coordination between the DR and the lateral MPD of the DPZ and time-current graded coordination between consecutive DPZs, and each DPZ is set to not detect faults in the low-voltage part of the distribution system; when a fault occurs outside a DPZ, the DR of the non-faulty DPZ may sense differential current due to the DG feeding effect;

[0017] To ensure that the DR of each DPZ must operate reliably under all critical fault and normal conditions and all possible grid operating states, the DR should be set to: 1) prevent tripping under normal system conditions, regardless of load / generation levels and grid configuration; 2) clear every fault occurring within the DPZ, at least within the specified fault resistance range; 3) provide delayed tripping commands to ensure coordination with the lateral MPD in the event of a lateral fault; 4) do not detect faults occurring in the low-voltage network; 5) maintain selectivity between consecutive DPZs to avoid tripping caused by DG feeding effect interference.

[0018] The action criteria of multiple DR elements are proposed to achieve the coordination between the lateral protection device of the DPZ protection zone and the continuous differential protection zone, specifically:

[0019] In order to achieve coordination between the lateral protection device of the protection zone and the continuous differential protection zone, the DR needs a time delay t marg-d,ij The settings are shown in formula (2):

[0020]

[0021] Where: t lat-d,ij For DPZ to occur ij The internal transverse fault resistance reaches the specified value R sp-f The specific time delay set; t max-MPD,ij For DPZ ij The maximum MPD operating time for all common short-circuit fault types of branch circuit inspection; t lat-d,mn For DPZ ij Adjacent DPZ mn The specific time delay set by the transverse fault resistor occurs; t ext-d,ij The specific time delay selected for the occurrence of an external fault; t marg-d,ij The time delay required for DR action;

[0022] However, when a fault occurs in the main feeder section covered by the DPZ, it can be considered that the time delay setting is very high. For this reason, multiple time-delay current differential elements are used in the DR of each DPZ. ij The number of DR elements n is set in , and the calculation formula is shown in formula (3):

[0023]

[0024] Where: Δt d,ij For DPZ ij The time difference between the DR element action time delay and the fastest delay; t fast-d,ij For DPZ ij The fastest delay setting of the DR element; Δt des,ij A constant time period set for the DR element;

[0025] DPZ ij The time delay t of the additional current differential element k enabled k-d,ij The calculation formula is shown in formula (4):

[0026] t k-d,ij =round(t fast-d,ij +kΔt des,ij ),k=0,1,…,n-1 (4)

[0027] Where: t k-d,ij is the time delay of the additional current differential element k enabled; Δt des,ij A constant time period set for the DR element.

[0028] The action criteria of sensitive time-delay DR elements are proposed to achieve correct action of high-impedance faults, specifically:

[0029] For high impedance faults with a fault resistance ≥ 40Ω, an additional time-delay current differential element is enabled in each DR pair. The pick-up differential current threshold I min-op is set to be slightly higher than the differential current during normal system operation, picking up the differential current threshold I min-op,ij The calculation formula is shown in formula (5):

[0030] I min-op,ij >c ij I max-diff,ij (5)

[0031] Where: I max-diff,ij For DR i and DR j The maximum differential current sensed before all possible normal operating system faults; c ij is the safety factor;

[0032] The delay setting of the additional delay current differential element enabled to handle high impedance faults will limit the acceptable upper limit time t imposed by this limit. utl The calculation formula is shown in formula (6):

[0033] t utl ≥t d,ij >I min-res,ij (6)

[0034] Where: I min-res,ij Minimum current limit of the time-delay current differential element enabled to handle high impedance faults.

[0035] The action criteria of the corresponding branch differential current protection after fault isolation are proposed to avoid interference tripping caused by DG feeding, specifically:

[0036] Fault isolation can be divided into two cases: correct tripping of the circuit breaker and failure to start after the circuit breaker fault. The change in the distribution network topology after fault isolation will also affect the action criteria of the corresponding DR element. For the case of topology changes after the circuit breaker action, when the DR pair trips correctly, part of the distribution system area will be disconnected. After tripping, the differential current measured by the remaining DR in the operating system may change greatly. For this reason, a post-fault topology delay current differential element is introduced. This element is pre-set in each DR pair and is only enabled when any other differential element in the system has previously tripped, causing the network topology to change. The post-fault topology protection element uses a single current differential characteristic, and the relevant DR a The delay t d,a , Suppression current I min-res,a and the pickup differential current threshold I min-op,a The calculation is shown in formula (7) to formula (9):

[0037] t d,a ≥t p,a +t CTa (7)

[0038]

[0039] Where: t p,a is the operation time of adjacent DRs; t CTa is the time interval for requesting coordination; I1 and I2 are the currents measured by the CTs at both ends; b a is the slope of the current differential characteristic; c a is the safety factor; I max-diff,a I is the maximum differential current sensed by the CTs at both ends before all possible normal operating system faults; min-res,a The minimum current limit value of the enabled post-fault topology delay current differential element;

[0040] In case of circuit breaker failure, although the corresponding DR has issued a trip command, an additional BFI is required to enhance the protection safety, ensuring that when the CB fails to disconnect after receiving the trip signal issued by the corresponding DR, the same DR will trip another CB to clear the fault. ij Failure occurs, DR i and DR j The CB at the position should be tripped by these relays. Now suppose that DR i Controlled CB i Correctly tripped, but by DR j Controlled CB j If the DR fails to disconnect, j Will make CB m or CB n Tripping; To avoid misoperation caused by noise and other interference inputs, the time delay t for the corresponding circuit breaker to trip again RT The calculation is shown in formula (10):

[0041] t RT =(1~2)T(10)

[0042] Where: T is the period of the sinusoidal current.

[0043] The advantages of the present invention are:

[0044] 1: The present invention applies multiple DR elements to each DPZ to form a time-grading scheme to achieve coordination between the lateral protection device of the protection zone and the continuous differential protection zone. The sensitive delay differential element can protect relatively high resistance faults, while the instantaneous differential element can maximize the protection speed for the most serious faults.

[0045] 2: The complementary use of multiple DR elements can be used to handle circuit breaker fault conditions and post-fault topology changes to avoid nuisance tripping of the zone during external faults caused by DG feeding, thereby maximizing the operational reliability of DPZ protection and distribution system after tripping.

[0046] 3: Based on a real medium-voltage distribution system model with ring topology and distributed generation, the enhanced performance of the designed protection scheme is verified through time-domain fault simulation, which shows the superiority of the proposed method compared with the traditional methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is the DPZ structure in a typical power distribution system;

[0048] Figure 2 Set up procedures for multiple differential elements;

[0049] Figure 3 For DR iCircuit breaker failure tripping logic;

[0050] Figure 4 For power distribution systems;

[0051] Figure 5 For DPZ 1,1’ Differential component characteristics;

[0052] Figure 6 For DPZ 2,2' Internal LG fault and CB2 failure fault binary logic;

[0053] Figure 7 For DPZ 3,3' Latching current differential threshold setting;

[0054] Figure 8 is the result of differential current calculation. DETAILED DESCRIPTION

[0055] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments:

[0056] A multi-element current differential protection method based on a DG integrated power distribution system comprises the following steps:

[0057] Step 1: Specify the range of DPZ (differential protection zone) in the distribution network with DG, simplify the criteria of traditional differential current protection, and clarify its inapplicability in handling differential protection coordination and intermediate DG feeding.

[0058] by Figure 1 The partial structure of the MV (medium voltage) distribution feeder with DG is explained in the following figure. The DPZ (differential protection zone) is set as all areas between a pair of adjacent DRs (differential relays), covering part of the main feeder and the branch line connected to the farthest MV / LV distribution transformer. The DPZ provides both primary protection for main feeder faults and backup protection for branch faults. i and DR j A percentage current differential element is enabled in the DPZ to protect ij , working current I op,ij Equal to DPZ ij The current vector and magnitude measured by the CT at both ends, and the limiting current I res,ij Equal to DR i and DR j The measured current is half the sum of the magnitudes of the currents. If the specified time delay t is maintained d,ij , then the action criterion for each DR to trip its own circuit breaker (CB) is as shown in formula (1):

[0059]

[0060] Where: b ij To define the current differential characteristic slope; I i ,I j DR i and DR j The measured current; I min-op,ij is the minimum pickup differential current threshold; t p,ij is the operation time of internal MPD or external DR; t CTi The time interval requested for CT measurements; t d,ij Time delay for tripping the circuit breaker.

[0061] When a fault occurs inside a DPZ, it is necessary to maintain coordination between the DR and the lateral MPD (main protection device) of the DPZ and time-current graded coordination between consecutive DPZs, and each DPZ is set not to detect faults in the low-voltage part of the distribution system; when a fault occurs outside the DPZ, the DR of the non-faulty DPZ may sense differential currents due to the DG feeding effect. To ensure that the DR of each DPZ must operate reliably under all critical faults and normal conditions and all possible grid operating conditions, the DR should be set to: 1) prevent tripping under normal system conditions, regardless of load / generation levels and grid configuration; 2) clear every fault occurring within the DPZ, at least within the specified fault resistance range; 3) provide a delayed trip command to ensure coordination with the lateral MPD in the event of a lateral fault; 4) do not detect faults occurring in the low-voltage network; 5) maintain selectivity between consecutive DPZs to avoid tripping caused by interference from the DG feeding effect.

[0062] Step 2, propose the action criteria of multiple DR elements to achieve the coordination between the lateral protection device of the protection zone and the continuous differential protection zone.

[0063] In order to achieve coordination between the lateral protection device of the protection zone and the continuous differential protection zone, the DR needs a time delay t marg-d,ij The settings are shown in formula (2):

[0064]

[0065] Where: t lat-d,ij For DPZ to occur ij The internal transverse fault resistance reaches the specified value R sp-f The specific time delay selected; t max-MPD,ij For DPZ ij The maximum MPD operating time for all common short-circuit fault types of branch circuit inspection; t lat-d,mn For DPZ ij Adjacent DPZ mn The specific time delay set by the transverse fault resistor occurs; t ext-d,ijThe specific time delay selected for the occurrence of an external fault; t CTi is the coordination time interval of the request; t marg-d,ij The time delay required for DR action.

[0066] However, when a fault occurs in the main feeder section covered by the DPZ, it can be considered that the time delay setting is very high. To this end, the present invention proposes to use multiple delayed current differential elements in the DR of each DPZ, and the setting procedure is as follows Figure 2 As shown in DPZ ij The number of DR elements n is set in , and the calculation result is shown in formula (3):

[0067]

[0068] Where: Δt d,ij For DPZ ij The time difference between the time delay of the DR element and the fastest delay; Δt des,ij For DPZ ij The constant time period required for the DR element.

[0069] DPZ ij The time delay calculation formula of the enabled current differential element k is shown in formula (4):

[0070] t k-d,ij =round(t fast-d,ij +kΔt des,ij ),k=0,1,…,n-1(4)

[0071] Where: t k-d,ij is the time delay of the additional current differential element k enabled; Δt des,ij A constant time period set for the DR element.

[0072] Step 3: Propose the action criteria of the sensitive time-delay DR element to achieve correct action for high-impedance faults.

[0073] For high impedance faults with fault resistance ≥ 40Ω, an additional time-delay current differential element is enabled in each DR pair. The pick-up differential current threshold I min-op is set to be slightly higher than the differential current during normal system operation, picking up the differential current threshold I min-op The calculation formula is shown in formula (5):

[0074] I min-op,ij >c ij I max-diff,ij (5)

[0075] Where: I max-diff,ij For DR i and DR jThe maximum differential current sensed before all possible normal operating system faults; c ij is the safety factor.

[0076] The delay setting of the sensitive current differential element will limit the acceptable upper limit time t imposed by this limitation. utl The calculation formula is shown in formula (6):

[0077] t utl ≥t d,ij >I min-res,ij (6)

[0078] Where: I min-res,ij Minimum current limit of the time-delay current differential element enabled to handle high impedance faults.

[0079] Step 4: Propose the action criteria of the corresponding branch differential current protection after fault isolation to avoid the interference tripping caused by DG feeding, thereby maximizing the operation reliability of the protection and distribution system after DPZ tripping.

[0080] Fault isolation can be divided into two cases: correct tripping of the circuit breaker and failure to start after the circuit breaker fault. The change in the distribution network topology after fault isolation will also affect the action criteria of the corresponding DR elements. For the case of topology changes after the circuit breaker is operated, when the DR pair trips correctly, part of the distribution system area will be disconnected. After tripping, the differential current measured by the remaining DR in the operating system may change greatly. For this reason, a post-fault topology delay current differential element is introduced. This element is pre-set in each DR pair and is only enabled when any other differential element in the system has previously tripped, causing the network topology to change. The post-fault topology protection element uses a single current differential characteristic, and the relevant DR a The delay t d,a , Suppression current I min-res,a and the pickup differential current threshold I min-op,a The calculation is shown in formula (7) to formula (9):

[0081] t d,a ≥t p,a +t CTa (7)

[0082]

[0083] Where: t p,a is the operation time of adjacent DRs; t CTa is the time interval for requesting coordination; I1 and I2 are the currents measured by the CTs at both ends; b a is the slope of the current differential characteristic; c a is the safety factor; I max-diff,aI is the maximum differential current sensed by the CTs at both ends before all possible normal operating system faults; min-res,a The minimum current limit value of the enabled post-fault topology delay current differential element;

[0084] In case of circuit breaker failure, although the corresponding DR has issued a trip command, an additional BFI (Breaker Failure Initiation) scheme is required to enhance the protection safety. The latter ensures that when the CB fails to open after receiving the trip signal issued by the corresponding DR, the same DR will trip another CB to clear the fault.

[22] If DPZ ij Failure occurs, DR i and DR j The CB at the position should be tripped by these relays. Now suppose that DR i Controlled CB i Correctly tripped, but by DR j Controlled CB j Failed to disconnect, at this time, DR j The designed circuit breaker failure tripping logic is as follows Figure 3 As shown, DR j To CB j When the trip command is issued, the BFI signal is also issued at the same time, and the BFI signal lasts for an appropriate duration t BF Remain valid within, if CB j In t BF If the closed state remains after the expiration, DR j Will make CB m or CB n To avoid misoperation caused by noise or other interference input, the corresponding circuit breaker is tripped again with a time delay t RT The calculation is shown in formula (10):

[0085] t RT =(1~2)T (10)

[0086] Where: T is the period of the sinusoidal current.

[0087] Example

[0088] exist Figure 4 In the real distribution system model shown in FIG, the proposed protection scheme is tested by simulation. The simulation test system consists of two 20 kV hybrid overhead-underground distribution feeders, namely feeder 23 and feeder 24. Assume that each feeder is connected to a 3 MW PV (photovoltaic) unit (maximum steady-state short-circuit current is 1 p.u.) and two 1.5 MW SG (synchronous generators). The minimum and maximum load conditions of the system are set to S total 40% and 100%, S totalThe maximum expected load consumption when feeder 23 and feeder 24 are simultaneously powered is set to 15.93 MVA, and the minimum and maximum DG penetrations are set to S total The slope b is set to 0.8 in all DRs to improve protection safety, and the safety factor c = 1.1, Δt des = 100ms. The feeders of the overhead line section and the distribution transformers distributed in the power grid are protected by fuses installed on their departure and primary sides respectively. Due to the different line characteristics and configurations of the overhead network and the underground network, these line sections are analyzed separately. On this basis, one DPZ is formed to protect the entire overhead line section of each feeder, and the other two DPZs are responsible for protecting the underground section. All zones are in Figure 4 They are represented by different line styles and colors.

[0089] 1) First, for all marginal faults / system conditions, the maximum total clearing time t of the main transverse fuse within each DPZ is calculated by performing all types of faults at multiple transverse points max-c and the minimum total clearing time t min-c , as shown in Table 1. By adding the typical t CTi = 0.3s, combined with equation (1), the minimum time delay t required for the differential element enabled in each DPZ can be determined marg-d,ij and the fastest time delay t fast-d .

[0090]

[0091] Where: t lat-d,ij For DPZ to occur ij The internal transverse fault resistance reaches the specified value R sp-f The specific time delay selected; t ext-d,ij The specific time delay selected for the occurrence of an external fault; t CTi is the coordination time interval of the request; t min-MPD,ij For DPZ ij Minimum purge time among all purge times of the internal MPD.

[0092] The total number n of differential elements that must be enabled in each DPZ to achieve the proposed multi-element coordinated operation is calculated by the following formula. The complete DR setting in each DPZ is shown in Table 2.

[0093]

[0094] Where: Δt d,ij For DPZ ij The time difference between the DR element action time delay and the fastest delay; t fast-d,ij For DPZij The fastest delay setting of the DR element; Δt des,ij A constant time period set for the DR element.

[0095] For DPZ 1,1' and DPZ 5,5' , calculate the time delay setting t for the differential value of the slowest and fastest intermediate differential elements using the following formula k-d,ij , as determined in Table 1.

[0096] t k-d,ij =round(t fast-d,ij +kΔt des ),k=0,1,…,n-1

[0097] Table 1 shows the time data and the number of differential elements;

[0098]

[0099] 2) Next, the differential current value for handling high impedance faults is set based on the detailed simulation results. The I of each differential element in the DPZ is calculated by the following formula: min-op,ij and I min-res,ij , as shown in Table 2, to ensure that under normal operating conditions, a fault detected by the element does not cause uncoordinated or accidental tripping between the corresponding DR and fuse within the DPZ.

[0100] I min-op,ij >c ij I max-diff,ij

[0101] Table 2 shows the DPZ differential component settings;

[0102]

[0103] The DR in a DPZ can also detect faults occurring in adjacent DPZs. To avoid overlap, appropriate checks are performed to ensure that the time delay setting of this element is safely higher than the delay setting of the trip element of the faulty DPZ. The time delay is set to 2s because of the upper time limit imposed by the main transformer damage curve. utl The delay setting is large enough to achieve coordination with other differential elements in the same DPZ. min-op and I min-res The setting allows tripping only for severe faults in the first main feeder section, i.e. before the first transverse section of each DPZ. 3,3' There is no such main feeder section in the DPZ, so the transient element is not enabled in this area. 1,1' The differential element characteristics are as follows Figure 5 shown.

[0104] 3) Circuit breaker fault test: The test simulation scenario of the BF solution is DPZ 2,2' A single-phase ground fault occurs internally, where CB2 cannot trip. Table 3 shows the differential relay DR in this case. 2’ and DR2 determine the tripping sequence 314ms after the fault occurs. The tripping sequence of DR2 is in the form of binary 1. Figure 6 As shown, the BFI command is also issued at the same time. CB2 should be Figure 6 The vertical arrow in the middle indicates a normal trip at time t = 414ms, but as shown by the red line, CB2 remains closed, that is, it remains in state 0. After a delay of 10 cycles, BF After that, the adjacent circuit breaker CB 1’ Finally, the circuit breaker trips due to the fault scenario, and DR2 issues a re-trip command to cause CB2 to trip. RT Exactly 2 cycles after the initial trip signal, but without success, resulting in CB 1’ Tripping.

[0105] Table 3 shows the DPZ differential component settings.

[0106]

[0107] 4) Topology change after fault: The delay t of the topology protection element after fault is calculated by the following formula: d,a , Suppression current I min-res,a and the pickup differential current threshold I min-op,a .

[0108] t d,a ≥t p,a +t CTa (6)

[0109]

[0110] Where: t p,a is the operation time of adjacent DRs; t CTa is the time interval for requesting coordination; I and I2 are the currents measured by the CTs at both ends; b a is the slope of the current differential characteristic; c a is the safety factor; I max-diff,a I is the maximum differential current sensed by the CTs at both ends before all possible normal operating system faults; min-res,a The minimum current limit of the current differential element for the enabled post-fault topology delay.

[0111] Since the network topology changes from ring to radial, it is now more prone to nuisance tripping of the differential element due to external faults, so I bl-pf It is crucial. Figure 7Provides DPZ 3,3' Differential element setting after fault I bl-pf Example, considering the edge failure scenario for this particular case, namely DPZ 3,3' Internal LG fault, a simulated fault is detected in the working area of ​​DPZ3,3' and the corresponding element of the adjacent DPZ2,2' is enabled, which usually leads to an unexpected double tripping due to the very small difference in the delay of the 2 elements. 2,2' The time delay of the main topology components after the fault, Figure 7 The horizontal dotted line is DPZ 3,3' Properly set I bl-pf , so that once the former calculates I op Beyond this threshold, DPZ 3,3' The component will be DPZ 2,2' The element sends a blocking signal. This setting is selected to apply to any fault that may cause the adjacent DPZs to trip simultaneously.

[0112] In addition to the static fault simulation studies required to set up the DR checks, the performance of the designed protection scheme is tested through time domain fault simulations. Figure 8 A single-phase ground fault of 10Ω occurs at the horizontal L 4’ The end point of the horizontal direction is protected by the fuse and the DPZ 4,4' Protection. The fault is cleared by the transverse fuse after about 0.35s. During the fault, DPZ 4,4' The first differential element of the DPZ is also started, but it is delayed by 0.708s and will not trip, coordinating with the transverse fuse. 2,2' In the main line section of the protection, the second differential element in this area detects the fault, trips and clears the fault after 0.314s. If the traditional setting method is used instead of the proposed multi-element method, the fault will be cleared after 0.438s, because only the first differential element will be enabled in practice, and the I measured during the fault period is op The voltage suddenly dropped 0.16s after the fault started. This was because the undervoltage interconnection protection disconnected the DG unit and the DPZ 2,2' The DR operates correctly, which further proves the preventive effect of the proposed setting method on the intermittent output of DG.

[0113] The method of the present invention is particularly suitable for multi-element current differential protection applied to DG integrated distribution system, because the present invention introduces in detail the basic principle of the proposed multi-element current differential protection method. Then, the normal setting procedure of the proposed multi-element current differential protection method is elaborated in detail. The multiple time-delay differential elements in the DPZ ensure the coordination between the lateral protection device of the protection zone and the continuous differential protection zone. The sensitive time-delay differential element can protect relatively high resistance faults, and the instantaneous differential element can maximize the protection speed. Finally, the proposed scheme is applied to the actual medium-voltage distribution system model with ring topology and distributed generation, and the simulation results prove the practicality and superiority of the proposed scheme.

Claims

1. A multi-element current differential protection method based on a DG integrated power distribution system, characterized in that: The following steps are involved: Step 1: Define the scope of the DPZ protection zone in the distribution network with DG, simplify the criteria of traditional differential current protection, and clarify its inapplicability in handling differential protection coordination and intermediate DG feeding; Step 2: Propose the action criteria of multiple DR elements to achieve the coordination between the lateral protection device of the DPZ protection zone and the continuous differential protection zone; Step 3: Propose the action criteria of sensitive time-delay DR elements to achieve correct action for high-impedance faults; Step 4: Propose the action criteria of the corresponding branch differential current protection after fault isolation to avoid interference tripping caused by DG feeding.

2. According to claim 1, a multi-element current differential protection method based on a DG integrated power distribution system is characterized in that: In step 1, the scope of the DPZ protection zone in the distribution network with DG is specified, the criteria of the traditional differential current protection are simplified, and its inapplicability in handling differential protection coordination and intermediate DG feeding is clarified, specifically: In a distribution network with DGs, the DPZ protection zone is set to cover all areas between a pair of adjacent DRs, covering part of the main feeder and the branch line connected to the farthest MV / LV distribution transformer; The DPZ protection zone provides both main protection for main feeder faults and backup protection for branch faults. i and DR j A percentage current differential element is enabled in the DPZ to protect ij , the operating current of this percentage current differential element is I op,ij Equal to DPZ ij The current vector sum measured by the CT at both ends is the limiting current I res,ij Equal to DR i and DR j Measure half of the sum of the current magnitudes; When each DR trips, the action criterion of its own circuit breaker is as shown in formula (1): Where: b ij To define the current differential characteristic slope; I i ,I j DR i and DR j The measured current; I min-op,ij is the minimum pickup differential current threshold; t p,ij is the operation time of internal MPD or external DR; t CTi The time interval requested for CT measurements; t d,ij time delay for tripping the circuit breaker; When a fault occurs inside a DPZ, it is necessary to maintain coordination between the DR and the lateral MPD of the DPZ and time-current graded coordination between consecutive DPZs, and each DPZ is set to not detect faults in the low-voltage part of the distribution system; when a fault occurs outside a DPZ, the DR of the non-faulty DPZ may sense differential current due to the DG feeding effect; To ensure that the DR of each DPZ must operate reliably under all critical fault and normal conditions and all possible grid operating states, the DR should be set to: 1) prevent tripping under normal system conditions, regardless of load / generation levels and grid configuration; 2) clear every fault occurring within the DPZ, at least within the specified fault resistance range; 3) provide delayed tripping commands to ensure coordination with the lateral MPD in the event of a lateral fault; 4) do not detect faults occurring in the low-voltage network; 5) maintain selectivity between consecutive DPZs to avoid tripping caused by DG feeding effect interference.

3. According to the multi-element current differential protection method based on DG integrated power distribution system of claim 1, it is characterized by: In step 2, the action criteria of multiple DR elements are proposed to achieve the coordination between the lateral protection device of the DPZ protection zone and the continuous differential protection zone, specifically: In order to achieve coordination between the lateral protection device of the protection zone and the continuous differential protection zone, the DR needs a time delay t marg-d,ij The settings are shown in formula (2): Where: t lat-d,ij For DPZ to occur ij The internal transverse fault resistance reaches the specified value R sp-f The specific time delay set; t max-MPD,ij For DPZ ij The maximum MPD operating time for all common short-circuit fault types of branch circuit inspection; t lat-d,mn For DPZ ij Adjacent DPZ mn The specific time delay set by the transverse fault resistor occurs; t ext-d,ij The specific time delay selected for the occurrence of an external fault; t marg-d,ij The time delay required for DR action; However, when a fault occurs in the main feeder section covered by the DPZ, it can be considered that the time delay setting is very high. For this reason, multiple time-delay current differential elements are used in the DR of each DPZ. ij The number of DR elements n is set in , and the calculation formula is shown in formula (3): Where: Δt d,ij For DPZ ij The time difference between the DR element action time delay and the fastest delay; t fast-d,ij For DPZ ij The fastest delay setting of the DR element; Δt des,ij A constant time period set for the DR element; DPZ ij The time delay t of the additional current differential element k enabled k-d,ij The calculation formula is shown in formula (4): t k-d,ij =round(t fast-d,ij +kΔt des,ij ),k=0,1,…,n-1 (4) Where: t k-d,ij is the time delay of the additional current differential element k enabled; Δt des,ij A constant time period set for the DR element.

4. According to the multi-element current differential protection method based on DG integrated power distribution system of claim 1, it is characterized by: In step 3, the action criteria of the sensitive time-delay DR element are proposed to achieve the correct action of the high-impedance fault, specifically: For high impedance faults with a fault resistance ≥ 40Ω, an additional time-delay current differential element is enabled in each DR pair. The pick-up differential current threshold I min-op is set to be slightly higher than the differential current during normal system operation, picking up the differential current threshold I min-op,ij The calculation formula is shown in formula (5): I min-op,ij >c ij I max-diff,ij (5) Where: I max-diff,ij For DR i and DR j The maximum differential current sensed before all possible normal operating system faults; c ij is the safety factor; The delay setting of the additional delay current differential element enabled to handle high impedance faults will limit the acceptable upper limit time t imposed by this limit. utl The calculation formula is shown in formula (6): t utl ≥t d,ij >I min-res,ij (6) Where: I min-res,ij Minimum current limit of the time-delay current differential element enabled to handle high impedance faults.

5. According to the multi-element current differential protection method based on DG integrated power distribution system of claim 1, it is characterized by: In step 4, the action criteria of the corresponding branch differential current protection after fault isolation are proposed to avoid interference tripping caused by DG feeding, specifically: Fault isolation can be divided into two cases: correct tripping of the circuit breaker and failure to start after the circuit breaker fault. The change in the distribution network topology after fault isolation will also affect the action criteria of the corresponding DR element. For the case of topology changes after the circuit breaker action, when the DR pair trips correctly, part of the distribution system area will be disconnected. After tripping, the differential current measured by the remaining DR in the operating system may change greatly. For this reason, a post-fault topology delay current differential element is introduced. This element is pre-set in each DR pair and is only enabled when any other differential element in the system has previously tripped, causing the network topology to change. The post-fault topology protection element uses a single current differential characteristic, and the relevant DR a The delay t d,a , Suppression current I min-res,a and the pickup differential current threshold I min-op,a The calculation is shown in formula (7) to formula (9): t d,a ≥t p,a +t CTa (7) Where: t p,a is the operation time of adjacent DRs; t CTa is the time interval for requesting coordination; I1 and I2 are the currents measured by the CTs at both ends; b a is the slope of the current differential characteristic; c a is the safety factor; I max-diff,a I is the maximum differential current sensed by the CTs at both ends before all possible normal operating system faults; min-res,a The minimum current limit value of the enabled post-fault topology delay current differential element; In case of circuit breaker failure, although the corresponding DR has issued a trip command, an additional BFI is required to enhance the protection safety, ensuring that when the CB fails to disconnect after receiving the trip signal issued by the corresponding DR, the same DR will trip another CB to clear the fault. ij Failure occurs, DR i and DR j The CB at the position should be tripped by these relays. Now suppose that DR i Controlled CB i Correctly tripped, but by DR j Controlled CB j If the DR fails to disconnect, j Will make CB m or CB n Tripping; To avoid misoperation caused by noise and other interference inputs, the time delay t for the corresponding circuit breaker to trip again RT The calculation is shown in formula (10): t RT =(1~2)T (10) Where: T is the period of the sinusoidal current.

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