A Blocking Method for Bus Protection of CT Disconnection and Opening Method of HIF Based on Current Characteristics

Current transformer (CT) disconnection in busbar can lead to erroneous operation of busbar differential protection. The existing methods for handling CT disconnection have not taken into account how to re-enable the protection in the case of high-impedance faults (HIFs). If the two types of faults cannot be distinguished in a timely manner, it can adversely affect the safe operation of the power grid. This paper provides a theoretical analysis of the differences between differential current and restraining current in the case of busbar CT disconnection faults and HIFs. Based on these characteristics, this paper proposes a criterion for CT disconnection fault blocking and a criterion for HIFs opening. Simulation results demonstrate that this method can effectively and accurately identify HIFs even in the presence of CT disconnection faults.


Introduction
With the development of power systems, current circuits play a crucial role in relay protection, automation, and online monitoring [1].However, as the complexity of power equipment increases and the number of interconnected nodes grows, the operation of current circuits becomes more intricate, posing potential risks to their safe operation.Current transformers (CT), as the core components for secondary current transmission, directly affect the accuracy of relay protection devices.The occurrence of secondary current circuit disconnection is a common anomaly that can result in increased open-circuit voltage, triggering false tripping of protective devices, equipment damage, and even risks to personnel safety.Especially during grid faults, changes in the unbalanced differential current can affect the normal operation of differential protection [2].Therefore, studying current transformer disconnection has become an important research direction for addressing these abnormal situations.
Currently, there are relatively well-developed detection and preventive measures for CT secondary circuit disconnection [3][4][5].Reference [6] proposes the admittance method, which is applied to intelligent online detection of CT secondary open and short circuit faults in load management terminals.It diagnoses the operational status of the CT secondary side by detecting the impedance characteristics of the secondary circuit.Reference [7] proposes a criterion for detecting CT disconnection based on voltage transients on both sides.This criterion can effectively reduce the impact of CT disconnection detection on the sensitivity of line differential protection.Reference [8] proposes that for busbar protection with voltage input, a sensitive voltage criterion is used as an auxiliary criterion.For busbar protection without voltage input, the ratio and difference between the zero-sequence current of the branch and the differential current are used as auxiliary criteria.Reference [9] achieves accurate identification of internal faults, data anomalies, or CT disconnection scenarios by constructing redundant data from arbitrary incoming and outgoing line currents and comparing the similarity of waveforms between measured data and redundant data using the Hausdorff distance algorithm.Reference [10] utilizes differential current and the amplitude and phase characteristics of branch sequence currents to distinguish busbar CT disconnection and improve the adaptability of CT disconnection strategies, thereby avoiding the occurrence of busbar incidents caused by CT disconnection lockout errors.
To address the problem of effective identification of CT disconnection and high-impedance faults (HIFs), this paper starts with theoretical analysis and distinguishes them by analyzing the characteristics of differential current and restraining current under these two types of faults.Subsequently, CT disconnection blocking criterion and high-impedance fault opening criterion are proposed based on the fault characteristics.Finally, the effectiveness of the proposed methods is verified through simulations.

Characteristics Analysis of Busbar CT Disconnection and Short Circuit Faults
To achieve reliable identification of CT disconnection and prevent busbar protection from failing to trip in the event of a short circuit fault after a false tripping, this section focuses on the characteristics of busbar differential current and restraining current.A comparative analysis is conducted between the differences in busbar differential current and restraining current under the conditions of busbar CT disconnection and busbar short circuit fault.

Characteristics of Busbar CT Disconnection
The busbar differential current IΣ is the vector sum of the currents of all branches connected to the busbar.Taking phase A as an example, during normal operation, the busbar differential current is denoted as Id.A = I1A+ I2A +…+ InA=0, with subscript numbers representing the branches connected to the busbar, totaling n branches.Assuming that a CT disconnection occurs in branch f with phase A. The illustration of the busbar differential current is shown in figure 1.The current of phase A in branch f with CT disconnection is 0, while the currents of non-CT disconnection branches remain unchanged.Therefore, the busbar differential current of the CT disconnection phase can be calculated as follows: where Id.A is busbar differential current of phase A during normal operation, and I′d.A is busbar differential current of phase A after a CT disconnection.From (1), it can be observed that the busbar differential current of the CT disconnection phase exhibits an increase in numerical value compared to the normal condition, while it does not have an impact on the non-CT disconnection phases.
where Ir.A is the restraining current of phase A during normal operation, and I′r.A is the restraining current of phase A after a CT disconnection.From (2), it can be observed that the restraining current of the CT disconnection phase decreases due to the absence of fault branch current, but it has no effect on the non-CT disconnection phase.
Therefore, due to the non-CT disconnection branch measuring current being unaffected by the CT disconnection, its variation before and after the break is nearly zero.On the other hand, the CT disconnection branch experiences a significant change in current before and after the break.It can be observed that the CT disconnection phase has an increased differential current and decreased restraining current, with a noticeable abrupt change in the current before and after the CT disconnection.This makes it distinguishable from the normal operating state.

Characteristics of Short Circuit Faults
To differentiate between busbar short circuit faults (single-phase grounding faults) and CT disconnections, especially in the case of high impedance faults (HIF) and CT disconnections, a comparative analysis of the differential current and restraining current trends is required.The schematic diagram of the differential current during a short circuit fault on the busbar is shown in figure 2.
According to figure 2, the busbar differential current can be obtained as follows: Similar to CT disconnections, when busbar short circuit faults occur, the differential current of the faulted phase will increase compared to normal conditions, while the differential current of the nonfaulted phases will remain relatively unchanged.
When considering the variation of the restraining current, following formula can be obtained.
From ( 4), it can be observed that the restraining current of the CT disconnection phase increases due to the additional current introduced by the short circuit faults.
Therefore, when a busbar experiences a short circuit fault, the differential current of the faulted phase will increase, similar to the case of CT disconnection.However, the restraining current will still increase, which is contrary to the changing trend of restraining current in the case of CT disconnection.Therefore, the variation in restraining current can be used to further differentiate between CT disconnection and ground fault.

CT Disconnection Blocking Criterion and High Impedance Faults Opening Criterion
Based on the above analysis, this article proposes a criterion for CT disconnection detection based on restraining current, and a criterion for protection tripping based on the asymptotic increase of highimpedance fault current under protection blocking condition.It should be noted that this method is applicable to single-phase single CT disconnection and single-phase high-impedance ground fault situations.

CT Disconnection Blocking Criterion
The CT disconnection blocking criterion is primarily used to quickly identify CT disconnection faults and prevent differential protection from mistakes due to CT disconnection faults in heavily loaded branches.The definition of the φ-phase restraining current is as follows: where Ir.(k) is the φ-phase restraining current at the kth calculation point and Ii.(k) is the root mean square (RMS) value of the φ-phase current collected by the ith CT, and n is the total number of CT.
Based on the previous analysis, when a CT disconnection fault happens, the restraining current of the fault phase will decrease, while in the case of a short circuit fault, the restraining current of the fault phase will increase.Therefore, by utilizing the characteristic of the restraining current mutation, it is possible to determine whether a CT disconnection fault has occurred.This can be expressed by the following criterion: where Ir.set is the threshold value of the restraining current and N is the number of samples within one cycle.Equation ( 6) uses the difference between the restraining current at the kth calculation point and the restraining current one cycle ago as the basis for judgment.If this difference is below the threshold value, which means the restraining current decreases, it indicates the occurrence of a disconnection fault.
Additionally, to select the specific faulted phase, the method of calculating the effective value of the differential current is used for judgment.The specific procedure is as follows: where Id.A, Id.B and Id.C are the effective values of the differential currents for phases A, B, and C respectively and Id.set1 is threshold value.When the differential current of a certain phase satisfies (7), it is determined that the respective phase is the faulted phase, noted as f.
In the case where a CT disconnection has already been determined, the faulted branch can be further determined based on the absolute value of the variation in the effective values of the faulted phase in each CT.
where ΔIi.f is the absolute value of the variation in the effective value of the current for faulted phase collected on the ith CT.Among them, the branch with the maximum value is considered the faulted branch.

High Impedance Faults Opening Criterion
The aforementioned method allows for the detection of CT disconnection faults and preventing false operations of the protection devices.However, if another short-circuit fault occurs during the CT disconnection tripping, the protection device may fail to operate, especially in the case of highimpedance faults.Therefore, it is necessary to study the opening criterion for short-circuit faults in the presence of CT disconnection tripping.Due to the continuously changing transient resistance of highimpedance faults, which generally tends to decrease, the current during high-impedance faults exhibits a progressively increasing characteristic.A specific criterion for detecting the opening of highimpedance faults can be constructed as follows.
where Id.f is the effective value of differential current for faulted phase at the kth calculation point and Id.set2 is the threshold value.By comparing the effective values of the differential current for the faulted phase in three consecutive cycles, one can identify high-impedance faults by determining whether they exhibit the characteristic of progressive increase.

Flowchart of CT Disconnection Blocking and High-Impedance Faults Opening Strategy
The flowchart for CT disconnection blocking protection and high-impedance fault opening protection is shown in figure 3. First, current information is collected through the CT device.Then, the differential current is calculated to determine if a fault has occurred.In the event of a fault, the faulted phase and faulted branch are selected by using ( 7) and (8).Next, the restraining current for the faulted phase is calculated.The CT disconnection blocking criterion is applied to determine if a CT disconnection fault has occurred.If a CT disconnection fault is detected, the bus differential protection is blocked.Otherwise, the original signal is filtered by using WT.The high impedance fault opening criterion is then used to determine if a high-impedance fault has occurred.If confirmed, the bus differential protection is opened.

Simulation and Verification
A simulation model of a 500kV busbar with two-thirds connection is established in PSCAD.The system schematic diagram is shown in figure 4. K1~K3 are different fault points; and T2 is a 500kV transformer; and S1 is the equivalent power source of the 500kV system; and S2 is the equivalent power source of the 220kV system; and G1 is a generator; and DG1 is a photovoltaic power source with an output power ranging from 0 to 5MW; and DG2 is a wind turbine power source with an output power ranging from 0 to 50MW; DG3 is a wind turbine power source with an output power ranging from 0 to 150MW.
The main parameters of the model are as follows: line positive sequence resistance is 0.016079 Ω/km; and line positive sequence inductive impedance is 0.306804 Ω/km; and line positive sequence capacitive reactance is 0.248314 MΩ*km; and line zero sequence resistance is 0.172458 Ω/km; and line zero sequence inductive impedance is 0.978052 Ω/km; and line zero sequence capacitive reactance is 0.402700 MΩ*km; and line length is 200km; and CT is 2000/1; and PT is 500kV/100V.The effectiveness of the proposed criteria can be verified with a specific example.Consider a scenario where a phase A CT disconnection fault occurs in CT1 first, followed by a HIF in phase A of bus 1.The differential current of phase A is illustrated in figure 5.
Figure 5 shows that the system works in normal operation before 400th sampling point because id.A is around zero, and then id.A increases due to the presence of CT disconnection, and at 807th sampling point id.A continues increasing and its waveform becomes distorted which indicates the presence of HIF.The proposed method in this paper will be verified in following steps.First, the effective value of the differential current for each phase of the bus is used to determine if a fault has occurred.The three-phase differential current is shown in figure 6.It can be observed that around the 400th sampling point, the differential current for phase A experiences a sudden increase.This indicates that a fault has occurred at that moment.Meanwhile, based on the maximum differential current observed in phase A, it can be determined that the faulted phase is phase A.
Then, the faulted branch can be selected by examining the changes in the effective values of the currents in each branch with respect to the faulted phase.The absolute values of the changes in effective values of the currents in each branch for the faulted phase are shown in figure 7. The figure indicates that branch 1 has the greatest absolute change in current effective value when fault occurs, thus it is determined to be the faulted branch.Afterwards, the occurrence of CT disconnection faults can be determined using the restraining currents of each phase.As shown in figure 8, it can be observed that the current in phase A exhibits a significant downward trend at the initial moment of the fault, while the currents in phases B and C show no significant changes.Therefore, it can be concluded that a CT disconnection fault has occurred in phase A.
Finally, the HIF is identified by using the HIFs opening criterion.The judgment result is shown in figure 9, which indicates the occurrence of a HIF after the CT disconnection fault.Therefore, the protection should be opened.This method is capable of accurately detecting HIF even in the case of CT disconnection faults.
Table 1 presents the results of CT disconnection and HIF detection in various fault scenarios, demonstrating the effectiveness of the proposed method in identifying both types of faults.

Conclusion
This paper provides a theoretical analysis of the differences between differential current and restraining current in the case of busbar CT disconnection faults and HIFs.The theoretical analysis indicates that both faults can cause an increase in differential current, while CT disconnection faults result in a decrease in restraining current and HIFs lead to an increase in restraining current.Based on these characteristics, this paper proposes a criterion for CT disconnection fault blocking and a criterion for HIFs opening, taking into account the asymptotic increase of differential current in the presence of HIFs.
Simulation results demonstrate that this method can effectively identify HIFs even in the presence of CT disconnection faults, with high accuracy and effectiveness.

Figure 1 .
Figure 1.Schematic diagram of bus differential current of CT disconnection.

Figure 2 .
Figure 2. Schematic diagram of bus differential current of short circuit fault.

OpenFigure 3 .
Figure 3. Flowchart of CT disconnection blocking and high-impedance faults opening strategy.

Figure 4 .
Figure 4. Schematic diagram of fault point configuration in the two-thirds connection simulation model.

Figure 5 .
Figure 5. Waveform of differential current of phase A.

Figure 6 .
Figure 6.Effective value of three-phase differential current.

Figure 7 .
Figure 7. Absolute values of ΔId in each branch for the faulted phase.

Figure 8 .
Figure 8.The restraining currents of the three phases.

Figure 9 .
Figure 9.The results of HIF detection.

Table 1 .
Results of CT disconnection and HIF detection in different fault scenarios.