Current Differential Protection Research Articles

A novel current amplitude differential protection for active distribution network considering the source-effect of IM-type unmeasurable load branches

The current amplitude differential protection is one of the effective means to overcome the deficiencies of the conventional over-current protection in the active distribution network. However, the existing criterions are rarely immune to the unmeasurable load branches especially the IM-type unmeasurable load branches. Therefore, this paper derives the positive-sequence equivalent model of the IM during fault occurrence. Then, the general variation law of the current amplitude difference between both terminals is revealed. For the passive unmeasurable load branch, the current amplitude difference is greater than its original value under internal faults while smaller under external faults. But for the IM-type unmeasurable load branch, such difference can exceed its original value temporarily under external low-resistance faults. To cover the maximum possible current amplitude difference during external fault occurrence, the adaptive operation characteristic curve is established. Accordingly, a novel protection scheme based on the pre-fault and post-fault current amplitude differences is proposed. The simulation results show that the proposed protection can avoid the risk of mal-operation in various external fault scenarios and identify internal faults sensitively.

An adaptive fault-component-based current differential protection scheme for distribution networks with inverter-based distributed generators

The high penetration of inverter-based distributed generators (IBDGs) T-connected into distribution networks has changed their structure from single-source networks to multi-source multi-branch networks; therefore, conventional distribution network protection schemes are difficult to apply. To this end, a mathematical model of IBDG fault component current considering the control strategy of IBDG is established, and then, a method for estimating the IBDG fault component current using only electrical information at both terminals of the protected line is proposed. On this basis, an adaptive fault-component-based current differential protection scheme suitable for distribution networks with IBDGs is proposed. The proposed scheme only needs to exchange the electrical information at both terminals of the protected line; that is, it can adapt to multiple IBDGs that are T-connected into the distribution networks, reducing the requirements of multi-terminal protection for communication channels and thus lowering costs. Finally, the effectiveness of the proposed protection scheme is demonstrated in the PSCAD/EMTDC software. The results show that the proposed protection scheme is not affected by the location, capacity of the IBDGs, or the fault type and has a high sensitivity.

An improved Bergeron differential protection for half-wavelength AC transmission line

Half-wavelength AC transmission line has the characteristics of long transmission distance and high voltage level, and its fault characteristics are significantly different from conventional transmission line. In order to reduce the interference of distributed capacitive current on half-wavelength AC transmission line on the calculation of current differential protection, this paper proposes a new current differential protection scheme based on Bergeron model. In order to solve the problem of small differential current located at the midpoint when a short circuit fault occurs, a solution using different methods to calculate setting value in different areas is proposed. The protection can move quickly near the terminal and delay to act in the middle area. After simulation and verification on the PSCAD experimental platform, it is found that when there is a fault at both terminals of the line, the protection can quickly operate in about 10 ms; when fault occurs in the middle area, the protection can delay its operation. The experimental results show that the various actions and performance of the protection device can meet the requirements of safe operation of half-wavelength transmission line.

The study on the Mal-operation of the transformer current differential protection during external faults cause by CT saturation

This paper analyses the unusual mal-operation of the differential protection for transformer current differential protection during the external fault. The mal-operation reason is researched in detail, based on the mathematical analysis, the simulation, and the field recorded fault data analysis. It is disclosed that the CT saturation leads to the maloperation. Based on this, this paper analyses the improvement for the CT saturation identification method, and discusses the CT selection principles. This paper is aimed at attracting the attention on the problem of the CT saturation and proposing the reference of the analysis method the countermeasure for this problem.

Determining alpha plane characteristics of line current differential protection in two-terminal high voltage transmission with different current transformer ratio

Determining alpha plane characteristics of line current differential protection in two-terminal high voltage transmission with different current transformer ratio

A reliable differential protection algorithm for delta hexagonal phase-shifting transformers

Phase-shifting transformers (PST) are exhibited to continuous variations in phase angle. This continuously-varied phase shift can cause miss operation for the current-based differential protection relay by generating false trip signal. This paper discusses the experimental validation of a proposed current differential protection technique. The currents of the exciting winding are calculated based on the relationship between the magnetic and electric characteristics of the transformer. Then, these currents are provided for calculating the restraint and operating currents of the protective relay. The first stage is to design and fabricate the delta hexagonal (Delta/Hex) phase-shifting transformer and, then, constructing a complete laboratory system. PST terminal currents and tapping value are sensed and, then, they are provided to the relay for calculating the excitation current. To check the relay security, different types of external faults are considered. Similarly, extra internal faults, both with and without impedance, are represented to test the relay dependability and sensitivity. Also, the protective relay is checked to discriminate between fault currents that occur during the transformer energization and healthy inrush currents.

Study on effects of DSE-Based instrumentation channel error correction on protective relays

This paper presents the effects of dynamic state estimation (DSE)-based instrumentation channel error correction on protective relays. It discusses the possible mis-operations of relays caused by current transformer (CT) saturation for three typical protection schemes: distance protection, current differential protection and setting-less protection. For each protection scheme, it compares the relay responses with the estimated measurement samples and with the legacy measurement samples, respectively. Extensive cases have demonstrated that when large errors introduced by CT saturation, the legacy measurement samples can lead to slower operations for distance relay in case of internal faults, and unnecessary trip decisions during external faults for current differential relay and setting-less relay. In contrast, with the estimated measurement samples, the relays can correctly identify internal faults and avoid mis-operation in case of external faults.

Extra‐sensitive impedance differential protection immune to CT saturation

For extra-high voltage transmission lines configured with a communication channel, the current differential protection is facing a dilemma between sensitivity and security. On the one hand, the resistive coverage will not be sufficiently high if security is guaranteed. Although the sequence-current-based differential elements are more sensitive, it may cause a longer time delay and deteriorate sensitivity under the open-phase operating condition. On the other hand, if high sensitivity is guaranteed at the cost of security, the main protection element will be blocked in the current transformer (CT) saturation scenario. To solve this problem, a novel type of impedance differential protection (IDP) is proposed. Under the open-phase condition awaiting an automatic reclosing, the proposed IDP can assist in accelerating the speed of fault-clearance for healthy phases, which is faster and more sensitive than using the zero-sequence current differential protection. For an external fault with heavy CT saturation, the proposed IDP can take over the fault identification capability before the recovery of CT, in which duration the current differential protection is blocked. Simulation results indicate that the proposed IDP coordinate perfectly with the current differential protection in the above two scenarios. The performance of the proposed IDP is also assessed using field data.

Ultra-fast current differential protection with high-sensitivity for HVDC transmission lines

Protection of transmission lines is the most important defense against faults in multi-terminal HVDC system. Up until now, no single protection can balance the operation speed and sensitivity needed for all fault types. To solve this problem, a novel current differential protection for HVDC transmission line is proposed in this paper. The intrinsic properties of a HVDC transmission line are analyzed first. Then, essence of ideal current differential protection is revealed and fundamental relations between line models and performance degradation of existing differential protections are discussed. Based on this analysis, a novel current differential protection scheme, including fault detection and faulty pole selection, is developed, and Bergeron model is used to describe the electrical wave propagation process along line, resulting in only simple computations are needed in each calculation cycle. Finally, a simulation model of Wudongde ± 800 kV HVDC project in China is established in PSCAD/EMTDC for verification. By discussing diversified faults and transient processes, simulation results prove that regardless of fault type, distance and transition impedance, the proposed differential protection can respond correctly. Main contribution of this paper is reducing the operation time of differential protection from hundreds of milliseconds to several milliseconds.

Novel Disturbance Blocking Criterion for Reliable Current Differential Protection of LCC-HVDC Lines

The existing current differential protection has been designated as the backup protection for LCC-HVDC lines. However, its reliability is very limited under the dc line high-impedance faults. In this letter, the challenges of the existing current differential protection are firstly revealed to offer a basic background. These challenges are shown to be caused by the existing disturbance blocking criterion, which imposes the unnecessary long time-delay under the dc line high-impedance faults. Moreover, a novel disturbance blocking criterion is proposed to identify the concerned disturbances, which is realized by a combination detection of the ac faults and commutation failures in LCC inverters. Compared with the existing disturbance blocking criterion, the proposed criterion has no long time-delay even under the dc line high-impedance faults. This salient feature makes the concomitantly proposed current differential protection operate much more quickly under the dc line high-impedance faults, and thus much more reliable than the existing one. Finally, the simulation results based on an actual bipolar LCC-HVDC link using the PSCAD/EMTDC program validate the proposed criterion and protection.

A Novel Method to Distinguish Internal and External Faults During Power Swing

The immunity to external faults is very important for any power swing unblocking scheme to improve its reliability. However, most of the available schemes to differentiate faults from power swing have poor reliability as they are not immune to external faults and switching. Phase comparison and current differential protection schemes can be suitable for this purpose, although their accuracy depend on the removal of sub-synchronous frequency and decaying dc component which is very prominent in current signals especially after close-in faults. In this context, the present article proposes a new method to distinguish internal and external faults during power swing on the transmission line by determining the orientation of Lissajous figure of two terminal current signals with the objective to improve the reliability without removing decaying dc component. An index is developed mathematically based on the change of orientation of Lissajous figure due to a fault to distinguish internal and external faults at swing condition and to detect the faulty phase accurately at various conditions. Performance of the method is also evaluated for close-in internal and external faults, switching events, noisy conditions and in presence of decaying dc component and is compared to the available methods to prove its merit.

110 kV substation relay protection

In this paper, the main electric wiring mode of 110kV substation is selected, the structure of substation is determined, and then the main wiring diagram is drawn. According to the design and load of the primary electrical connection, select the maximum and minimum operating modes to calculate the short circuit separately. Then, according to the short-circuit current parameters, the relay protection of transmission lines, transformers, busbars, etc. is set, and the configured protections include current quick-break protection, gas protection, and longitudinal differential protection. Finally, a comprehensive evaluation of the selected protection devices is carried out. Adding relay protection device in substation can send out fault signal and cut off fault line in time to reduce the occurrence of substation fault, so as to ensure the reliable power supply of users and enterprises.

Transformer Differential Protection

Overcurrent and earth fault protective equipment employing time grading and directional detection cannot provide correct discrimination on all power networks and in many cases clearing times for some faults would not be acceptable. Differential protection is an alternative overcurrent protective scheme, which is used to protect individual sections of networks or pieces of equipment, such as transformers, generators, e.t.c. Thus, where protection co-ordination is difficult using time delayed over current and earth fault protection, or where fast fault clearance is critical, then differential protection may be used. Kirchhoff’s first law, which states that the sum of the currents flowing to a node must be equal to the sum of the currents flowing out from it is the basic principle of the differential protection scheme. It detects the difference between the current entering a section and that leaving it. Under normal operating conditions, the current leaving the protected unit would be equal to that entering it at every instant. If the current flowing into the protected unit is the same as the current leaving, then the fault is not in the protected unit and the protective equipment or relay should not operate. If there is a difference in either the phase or magnitude between input and output, then the fault is in the protected unit and the protection should operate. This paper investigates how power transformers can be protected using the current-differential protection schemes.

Transformer Differential Protection

Overcurrent and earth fault protective equipment employing time grading and directional detection cannot provide correct discrimination on all power networks and in many cases clearing times for some faults would not be acceptable. Differential protection is an alternative overcurrent protective scheme, which is used to protect individual sections of networks or pieces of equipment, such as transformers, generators, e.t.c. Thus, where protection co-ordination is difficult using time delayed over current and earth fault protection, or where fast fault clearance is critical, then differential protection may be used. Kirchhoff’s first law, which states that the sum of the currents flowing to a node must be equal to the sum of the currents flowing out from it is the basic principle of the differential protection scheme. It detects the difference between the current entering a section and that leaving it. Under normal operating conditions, the current leaving the protected unit would be equal to that entering it at every instant. If the current flowing into the protected unit is the same as the current leaving, then the fault is not in the protected unit and the protective equipment or relay should not operate. If there is a difference in either the phase or magnitude between input and output, then the fault is in the protected unit and the protection should operate. This paper investigates how power transformers can be protected using the current-differential protection schemes.

Enhanced Current Differential Protection for HVDC Grid Based on Bergeron Model: A Parameter Error Tolerable Solution

Benefiting to the immunization of distributed capacitance, Bergeron model based current differential protection has become a promising backup protection in HVDC grid. However, its performance strongly relies on the precise transmission line parameter, whereas the error of parameter is inevitable due to the frequency-dependent nature. To solve this problem, an enhanced current differential protection with parameter error tolerability is proposed in this paper. The influence of transmission line parameter error on the compensated current is investigated, where the parameter error is embodied by the characteristic impedance and velocity of traveling wave. It is discovered that the major influence comes from the error of traveling wave velocity. The main idea behind the proposed protection is to mitigate the influence from the perspective of velocity. Wavelet transform modulus maxima (WTMM) is adopted to locate and calibrate the arrival time of traveling wave. The differential current under external fault is reduced effectively, which could have been higher than that of the internal fault when the parameter error exists, and thus avoiding the protection misoperation. Finally, the feasibility and robustness of the proposed protection are validated in a four-terminal VSC-HVDC grid.

Study and Solution of Current Differential Protection Disability for High Series Compensation Transmission Line

With the increasing day by day integration of large-size generators and tighter among the power systems, in internal fault, the fault current of Series Compensated (SC) lines could also be upturned. Therefore, the differential protection of SC transmission lines may stop working to operate because of less sensitivity. In this research article the causes for the current overturning in SC lines and its effects differential protection. The working characteristics of differential protection are analyzed by focusing on multiple-factors, for example, different series compensation degrees, types of fault, system operation mode, the difference of power angle, points of the fault, and transition resistance. An enhanced standard based on the current amplitudes and phases on both sides of the series compensated lines is proposed. Furthermore, to make stronger the operation characteristics of differential protection, the triple-fold line is chosen to get better sensitivity for internal faults in the article. The results are efficient to improve the protection sensitivity of SC lines during the internal fault. The PSCAD/EMTDC simulation and the power system dynamic physics simulation demonstrate that protection sensitivity is increased in the improved scheme. Keywords : Differential Protection, Series compensation, EHV/ UHV, Current reverse, sensitivity analysis. DOI: 10.7176/CTI/10-06 Publication date: August 31 st 2020

Current differential protection of double-circuit transmission lines in modal domain

In this paper, we present a new approach to protection of double-circuit transmission lines using modal transformation. The current differential principle is applied in the modal domain, and the relay characteristics are plotted in the alpha plane for numerical differential protection applications. Synchronized voltages and currents from both the ends are transformed to modal values. The transmission lines are modeled as equivalent $$\pi $$ model in modal domain, and line charging current is compensated. Electromagnetic transient program EMTP-ATP is used to simulate the double-circuit transmission systems, and the algorithm is implemented in MATLAB. The extensive case studies demonstrate the utility and the superiority of the proposed current differential scheme.

An Advanced Fault Control of Transformerless Modular Multilevel Converters in AC/DC Hybrid Distribution Networks Under the Single-Phase Grounding Fault

This paper presents an advanced fault control for transformerless modular multilevel converters (MMCs) in AC/DC hybrid distribution networks (HDNs) under the single-phase grounding (SPG) fault. The characteristics of voltages and transmission paths of zero sequence currents in AC networks with different voltage levels and neutral grounding types under SPG faults are analyzed. According to the analysis, the SPG fault results in two issues: 1) the voltage imbalances in nonfaulted networks, which have negative influence on the operation of nonfaulted networks; 2) the overcurrent of MMC-based terminals results in the shutdown of MMCs instead of fault ride-through. To solve these two issues, two types of zero sequence fault control are proposed, which are suitable for different fault ride-through abilities of transformerless MMCs in AC/DC HDNs. The proposed fault control is implemented by an advanced zero sequence current controller (ZSCC). Furthermore, to reduce the effect of fault control on the sensitivity of current differential protection based on the zero sequence fault current, an improved set of current reference value is addressed. Finally, the theoretical analysis and proposed fault control are verified through simulations performed in MATLAB/Simulink.

High-speed directional pilot protection for MVDC distribution systems

Selective dc protection with high speed is the key technology for the dc distribution system. The single-ended protections applied in high voltage dc (HVDC) grid are not suitable for the dc distribution system, due to the absence of the boundary elements. Differently, the pilot protection shows a good application prospect in dc distribution systems, because it can protect a clear bounded zone. In this paper, the characteristic of backward traveling waves on the fault cable and healthy cable is analyzed in detail. Then a high-speed directional pilot protection is proposed for dc distribution systems, which identifies the fault direction mainly according to the high-frequency backward traveling wave. Compared with the traditional pilot protections, such as the current differential protection, directional protections based on current change value (Δi) or change rate (di/dt), the proposed protection is immune to the cable distributed capacitor current. In addition, the proposed method does not depend on the boundary of the line terminals, thus being more suitable for the medium voltage dc (MVDC) distribution system. Simulation cases based on the PSCAD/EMTDC are carried out to verify the feasibility and advantages of the proposed scheme.

Ultra‐high‐speed non‐unit non‐differential protection scheme for buses of MMC‐HVDC grids

Half-bridge modular multilevel converter-based high-voltage direct-current (MMC-HVDC) grids are technically and economically viable solutions to transmit bulk electrical energy generated from geographically dispersed and remote renewable resources. However, due to the half-bridge converter's inability to control fault currents, these grids need specialised protection schemes to increase reliability and equipment safety. This study proposes a non-unit non-differential protection scheme founded on a comparative orientation measure of fault components for buses of half-bridge MMC-HVDC grids. Considering the selected protection strategy, the protection zone of this scheme can also cover a part of the converter station's ac side; in which severe short-circuit faults should be quickly detected and isolated as well since they may cause the entire grid outage. This scheme can be used as the main protection or a backup of the conventional differential current protection, and subsequently, increase the protection system dependability by increasing the functional diversity. The test results disclose that the proposed scheme has superior sensitivity to identify single-pole and double-pole faults with resistances up to 1500 and 2000 Ω, respectively, at the protected dc bus with the maximum delay of 1 ms, while it is robust against all ac and dc faults outside the designated protection zone.