In-zone Faults Research Articles

Modern numerical relays in protection of power transformer

This paper presents various protection schemes and their respective modern numerical protection relays to protect the power transformer. Proper protection is needed for the economical and safe operation of electrical power systems. The power transformer protective relay should block the tripping during an external fault or magnetizing inrush and speedily operate the tripping during internal faults. The objective of this paper is to implement protection during in-zone and outzone faults and to operate the relay with proper selective discrimination. The basic approach is to disconnect the power transformer feeder against in-zone faults and prevent disconnection of the power transformer feeder due to other operating conditions. The protection relay was able to discriminate between 2nd harmonic inrush, in-zone fault, and out-zone fault conditions. The protection scheme consists of high-speed protection relays and circuit breakers, which are required for the protection of substation equipment against abnormal conditions. A protective relay senses the fault and determines the location of the fault. Then, the protective relay sends the tripping command to the circuit breaker. Therefore, proper care should be taken in designing and selecting an appropriate relay that is reliable, efficient, and fast in operation. The PT and CT continuously measure the voltage and current of the substation and are connected to the relays to enable them to detect abnormal conditions. This paper describes numerical protection for power transformers, especially when the rating of the system is 50 MVA, 110/33 Kv, and Dyn11 at the substation. The protection relay employed in the transformer feeder is 7SR24 type (Siemens make).

An enhanced sensitive power differential protection for series compensated transmission lines in renewable generation intensive power systems

Series compensation is widely used to improve the electrical characteristics of long transmission lines and increase the power transfer capability. However, the insertion of a series capacitor introduces various phenomena that affect the performance of traditional distance relays. Moreover, the fault characteristics of renewable generation systems coupled with generation outputs that vary with weather patterns produce additional complications to the protection of transmission lines. This paper proposes the application of an enhanced sensitive power differential protection (SPDP) algorithm to the protection of series compensated transmission lines. The algorithm incorporates a criterion that improves the security of the protection by inhibiting the SPDP algorithm from operating for faults external to the protected line. Performance of the enhanced SPDP algorithm has been evaluated on a test system built in Digsilent Powerfactory software. Various in-zone and out-of-zone fault cases were simulated at different compensation levels and fault resistances. The results showed that the enhanced SPDP protection technique can detect in-zone faults along practically the entire transmission line (99.99 %) for compensation levels up to 90 %. The method operated reliably for faults with resistances up to 330 Ω.

Discrimination between in-zone and out-of-zone faults during power swing condition using synchrophasor data

A new algorithm to discriminate between in-zone fault and out-of-zone fault conditions under power swing condition for wind/synchronous generators (WG/SG) integrated power transmission network is presented in this paper. The proposed algorithm is based on the calculation of a transmission line positive sequence power loss index, derived by calculating the absolute difference between the calculated and measured values of positive sequence real power loss. The authenticity of the proposed technique is verified by performing Hardware-in-Loop (HIL) simulation in the Real-Time Digital Simulator (RTDS) framework along with physical Phasor Measurement Units (PMUs), Phasor Data Concentrator (PDC), Global Positioning System (GPS) clock, and voltage and current amplifiers. Various events such as power swing, in-zone (including close-in and high resistance) faults during power swing and out-of-zone faults during power swing are simulated by modeling WG/SG integrated IEEE-9 bus system in RSCAD (Real-Time Simulator Computer-Aided Design) framework. The single pole tripping, impact of noise and CT saturation on the proposed algorithm is also analyzed. The proposed algorithm offers accurate discernment between in-zone and out-of-zone faults during power swing conditions. The results obtained reveal increased sensitivity and improved distinguishing capability of the proposed algorithm in contrast to those of many other approaches proposed in the literature.

A Novel Pilot Protection Scheme for AC Line Connected to LCC-HVDC Inverter Based on Composite-Mode Model

In this paper, a new pilot protection scheme for AC line connected to LCC-HVDC inverter based on composite-mode model is proposed. First, taking commutation failure into consideration, 1-mode and 2-mode equivalent instantaneous models of inverter in different operating conditions are built. Second, according to the equivalent instantaneous inductances of inverter in different combinations of operating conditions, the equivalent models of inverter are summarized into 4 operation states. Then, the 1-mode and 2-mode equivalent models are combined to establish the composite-mode model of AC/DC hybrid system. And then, using multi-time section information, the instantaneous inductances of backside composite-mode model are calculated. Thus, protection criterion is constructed on the difference between composite-mode models of in-zone faults and that of out-of-zone faults for fault locating. Finally, RT-LAB simulation tests show that, the proposed method offers high sensitivity and reliability, unaffected by commutation failure.

Superimposed components-based distance protection of lines emanating from DFIG-based wind farms

Type-III wind turbines with doubly fed induction generators (DFIGs) are generally equipped with crowbar circuits in practice to realize the low voltage ride-through (LVRT) during the fault period. Under this situation, DFIGs exhibit significantly different fault behavior by contrast with synchronous generators (SGs). This difference poses inevitable challenges to the existing distance protections that are normally designed based on SGs’ fault signature. In this paper, some easily overlooked issues regarding the superimposed components-based distance protection (SCBDP) when applied in lines emanating from DFIG-based wind farms (WFs) are revealed. It is shown that the unified phase angle property in the additional fault network may be jeopardized in the presence of DFIG-based WFs, thereby resulting in the high probability of failure under the in-zone fault conditions or undesirable operation under the out-of-zone fault conditions for the conventional SCBDP. To address these problems, based on the “phase angle congruency” idea, this paper proposes an improved SCBDP to mitigate the adverse impact of the crowbar-activated DFIGs. By adaptively regulating the phase angle of operating voltage, the improved SCBDP can work properly independent from the fault types and the sizes of crowbar resistance. Also, compared with conventional SCBDP, the improved SCBDP shows higher sensitivity and reliability while inheriting its strong tolerance capability against fault resistance. Extensive simulation results under different fault conditions show the expected performance of the improved SCBDP.

Pilot Directional Protection Scheme for LCC-HVDC Transmission Lines Based on the Voltage difference Between Positive and Negative Poles

Currently available DC line protection methods for AC/DC hybrid systems present poor immunity to fault resistance for in-zone faults and mal-operation problems for out-of-zone faults. This paper proposes a new pilot directional protection scheme based on the voltage difference between positive and negative poles (VDPN). The proposed method is not affected by the fault location, the fault resistance, or lightning interference. By analyzing the conducting states of the converter in different time intervals, the differential expressions of the injected current from AC system to DC system are derived. Calculation models of VDPN in different conducting states are built. Then, according to the consistency between the calculated value and actual value of voltage difference, the protection criterion for DC line fault is constructed. Finally, simulation tests in RT-LAB verify that the proposed scheme can correctly identify in-zone and out-of-zone faults in different fault cases. Besides, the proposed scheme has low demand on data synchronicity, thus it is simple and reliable with a good operation speed.

Impact of DC bias on differential protection of converter transformers

Converter transformers in high-voltage direct current (HVDC) systems are often subjected to DC bias induced by earth return currents (ERCs) from HVDC systems or by geomagnetically induced currents (GICs). DC bias may adversely affect the performance of converter transformers, but its impact on the protection of converter transformers remains uncertain. This paper thoroughly investigates the response of converter transformer differential protection under the effect of DC bias. The network was simulated in PSCAD/EMTDC, and the modeled components included the converter transformers, the current transformers (CTs), and the DC bias signatures. The fault currents generated by the simulator were downloaded into a numerical relay and its operating behavior was observed. The results indicated the pre-existing harmonics, induced by DC bias, do not cause harmonic blocking to incorrectly inhibit the relay for in-zone faults; and any spurious trip, caused by DC bias, can be prevented by appropriate harmonic blocking and the use of a higher slope 2 in the differential current versus bias current characteristic. It was demonstrated the low-frequency characteristic of GIC did not pose any new threats to differential protection. This indicates the low-frequency characteristic of GIC is negligible when investigating the impact of GIC.

A New Differential Protection Scheme for Busbar Using d–q–0 Transformation

This paper presents a new differential protection scheme for busbar using d–q–0 transformation. The proposed method is based on derivation of operating and restraining currents by utilizing direct axis (d) component of current transformer (CT) secondary current signals of all bays. The dual slope relay characteristic (derived from the operating and restraining current) of the proposed scheme operates in conjunction with a CT saturation detection algorithm and hence, provides better stability during external faults with CT saturation. The performance of the proposed scheme has been evaluated by modeling a 400 kV sub-station with two different busbar configurations, namely Breaker-and-Half configuration (BHC) and single breaker double-bus configuration (SBDBC) in PSCAD/EMTDC software package. The proposed algorithm has been tested for various fault cases with diversified system and fault parameters. The proposed scheme is highly sensitive against in-zone faults even with a considerable value of fault resistance as the average fault detection time remains within one-and-half cycle. At the end, a comparative evaluation of the proposed scheme with the conventional bus differential protection scheme (87B) clearly indicates its superiority in terms of stability against close-in out-of-zone faults with CT saturation and sensitivity for high impedance internal faults.

An adaptive algorithm to prevent distance relay overreach during CCVT transient

In this paper, the impact of coupling capacitor voltage transformer (CCVT) subsidence transients on the performance of distance relay is investigated. The reach accuracy of distance relay is not maintained during CCVT subsidence transient and for this the responsible factors are system source-impedance-ratio (SIR), CCVT secondary burden, and fault inception angle. Relay generally overreaches during CCVT subsidence transient and in order to mitigate this problem an integrated adaptive logic is proposed. The adaptive zone-1 algorithm uses two criteria: (1) online SIR calculation using prefault and fault positive sequence voltage and current measured at relay location and (2) the maximum value of the transient monitor function obtained from the three-phase voltages to discriminate the in-zone fault from out-of-section fault. For accurate zone discrimination and to avoid relay overreach issue during end zone fault, the importance of adaptive threshold is crucial and can be set based on the online computed SIR value. The simulation study is performed using EMTDC/PSCAD. As the CCVT available in master library (ML) is not able to develop sufficient subsidence transient, a 500kV, 60Hz CCVT is modelled. Distance relay performance is analysed for different critical cases using the developed CCVT model and also the response of proposed method is observed. Results for different critical cases prove that the proposed method is able to maintain proper selectivity between in-zone and end-zone faults and also using the method the time-delay operation can be avoided during subsidence transient in the secondary side of CCVT

Regional protection scheme for distribution network based on logical information

In view of the incorrect operation of local protection caused by multiple distributed generations (DGs) and branches in distribution system, a regional protection scheme for distribution network based on logical information is proposed here. Moreover, a regional centralised protection system in parallel with and independent from the existing protection system is constructed. First, the problems in local current protection of lines with multiple DGs and branches are analysed. On this basis, protection regionalisation is conducted according to the configuration of circuit breakers. Also then, by combining the start-up information of existing over-current protection set-I, set-II, and set-III, the operation criterion of regional protection is formed. Simulation tests on the IEEE 33-node distribution system verify that, the proposed scheme could accurately locate in-zone fault and block out-of-zone fault, accelerate protection operation, and shorten the outage time. Besides, the proposed criterion is fault tolerant due to redundancy in the multi-source information used. With small amount of information transmission, the requirement on information synchronisation is low.

Enhancing Power Transformer Differential Protection to Improve Security and Dependability

Current differential principle is a well-known principle used for protection of transformers, motors, generators, buses, and any other type of power equipment with input and output current measurements. Further the principle is used in developing percent differential protection which can be programmed to the desired sensitivity for detecting in-zone faults, and security during external faults. This protection dependability is usually achieved by modeling a differential-retraining characteristic with two regions, operating and non-operating, and tracking the real differential restraint ratio during faults. Some external faults with high DC offset and high X/R system time constant would easily saturate the installed current transformers (CTs), which in return would cause high differential/restraint ratio above the preset characteristic into the operating region. In such cases the differential protection would operate and cause unwanted transformer trip. This paper focuses on a number of enhancements applied to the differential principle of the main differential protection, and defines guidance on how to setup the protection for better sensitivity and security. The paper is supported by fault cases, showing the improved security and dependability during internal/external faults with and without CT saturation.

Protection of Double-circuit Transmission Line Using Superimposed Current

This article presents a differential protection scheme for a double-circuit transmission line using a superimposed component of current. The proposed scheme is based on the ratio of directional current difference setting value to directional current bias setting value of the respective phase at both ends. These two quantities are derived based on the difference and the summation of the superimposed currents of the respective phase. The proposed scheme provides proper discrimination between in-zone faults and out-of-zone faults during complete loss of generation at one of the buses. Further, it is highly sensitive toward high-resistance faults and provides instantaneous operation to the entire line. Moreover, the relay provides reliability and selectivity for discriminating cross-country faults and internal faults and remains stable against external faults. The proposed scheme has been tested under different fault conditions with varying fault resistance between 0–200 Ω. A series of tests using test signals generated by PSCAD/EMTDC (Manitoba HVDC Research Centre Inc., Winnipeg, Manitoba, Canada) on realistic data of a part of the Indian 400-kV power transmission system network have proved the accuracy and effectiveness of the proposed scheme.

Protection of Transmission Line Paralleled along the Same Right-of-way

This article presents a novel scheme for the protection of parallel transmission line paralleled along the same right-of-way using wavelet packet transform (WPT). The proposed scheme is based on the difference between the absolute of the additions of wavelet coefficients computed using WPT at the third level with the help of db4 wavelet function. This concept has been applied to the corresponding phases of both the lines in parallel. The WPT is used to capture important features of the decomposed signal that are sensitive to infeed current and mutual coupling and provides stability against in-zone faults. The wavelet packets are highly sensitive toward high-resistance faults and a computationally more flexible way to conduct signal discrimination than wavelet transforms-based methods. The proposed method completely avoids voltage signal used in conventional relays and also ignores the requirement of communication channel. The relay provides reliability and selectivity for discriminating cross-country faults and internal faults and remains stable against external faults. The proposed scheme has been tested under different fault and system conditions with varying fault resistance between 0–200 Ω. A series of tests using test signals generated by Power System Computer Aided Design/Electro-magnetic Transients in DC Systems (PSCAD/EMTDC) for a 400-kV parallel transmission line have proven the accuracy and effectiveness of the proposed scheme.