Current Transformer Saturation Research Articles

Researching the Blocking of Current Protection of Electrical Power Units with Transformers

The magnetizing current inrush appears in electrical power units equipped with transformers in case of no-load energizing of the power transformers and in a number of other cases. This phenomenon could cause a false triggering of the current protection. To prevent incorrect actions of the current protection during the magnetizing current inrush a protection blocking is carrying out. The blocking principle operation is based on the fact that in a three-phase system in normal mode and in case of symmetrical short circuit the first harmonics contained in the phase currents of electrical installations form a direct sequence and the second ones – the negative sequences. In case of an asymmetric mode, including an asymmetric short circuit, the negative sequence appears, formed by the first harmonics that are part of the phase currents of the specified system. In magnetizing current inrush modes, second harmonics of significant magnitude are present in phase currents, which form the negative sequence. Based on the analysis of the information parameters of the specified sequences currents, identification of the magnetizing current inrush and short-circuit modes is performed with the implementation of the protection blocking if necessary. The study of the current protection blocking functioning was performed using computational experiment by analyzing the calculated changes of blocking parameter compared with the threshold setpoint. The specified researching is done by using the digital model that is implemented in the dynamic modeling environment MatLab-Simulink. As a result of the performed calculations, the principal operability of the proposed current protection blocking was confirmed that provides a fairly reliable identification of the magnetizing current inrush and short-circuits modes, regardless of the degree of saturation of current transformers. It was found that the proposed principle of the current protection blocking has a higher sensitivity in comparison with the classical one based on the estimation of the ratio of the second and first harmonics of the phase currents. In short-circuit modes in an electrical power units the proposed blocking causes a current protection operation delay that can be reduced by digital filters performance improvement.

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.

Prevention of the current transformer saturation by using negative resistance

In the power grid, various conditions may occur that saturate the core of a current transformer. Saturation of the protection core of the current transformers can cause a malfunction in the protection relays. To prevent malfunction of the protection relays caused by the current transformer saturation, software and hardware methods can be used. In the software method changes in the protection relay algorithm are used. In the hardware method special devices are used. A new hardware method is presented here, which uses compensation to prevent the occurrence of current transformer saturation. In the proposed method, compensation is done by controlling an applied voltage to the current transformer secondary terminals using negative resistance. The proposed method, despite its simplicity and cost-effectiveness, has good performance in preventing saturation in various conditions. The high reliability in terms of the risk of secondary open circuit and its application in various saturation conditions, even in the case of severe saturation are the main advantages of the proposed method. Also, the proposed method does not produce distortion in the transformation of high-order harmonics by the current transformer. To verify the effectiveness of the proposed method, prototype design and construction have been performed. The simulation and experimental results of laboratory tests have been presented and analysed.

Formation of Instantaneous Differential and Restraining Cur-rents for Differential Protection of Busbar Assemblies

The methods of forming differential and restraining currents for busbar differential protection are reviewed; their advantages and disadvantages are considered. It is noted that differential protection according to instantaneous values has a shorter proper response time than for current ones, since it does not use digital filters. The response characteristic and principles of setting selection are studied. The effect of sampling on the operation of differential protection according to instantaneous values is analyzed. It was found that without the use of special measures, depending on the sampling step and the frequency of the signal, the response current would fluctuate within the mathematical error caused by sampling. A solution to this problem has been proposed. The method consists in applying piecewise quadratic interpolation and determining the values of inflection points of instantaneous differential and restraining current signals. The efficiency of the proposed method has been evaluated. It was found that its use reduces the error in determining the response current. For a sinusoidal signal, the maximum possible error was 0.02 %. The trajectories of operating point s of differential protection in case of external fault with saturation of current transformers have been analyzed. In this mode, protection for instantaneous values is more susceptible to false positives than for active ones. The method of exponential smoothing of the restraining current was considered and investigated. An exponential smoothing algorithm has been proposed and analyzed. It is concluded that exponential smoothing increases the stability of the differential protection according to instantaneous values to external faults. Exponential smoothing does not exclude the possibility of false positive of differential protection in case of external fault.

A novel complex current ratio-based technique for transmission line protection

With respect to sensitivity, selectivity and speed of operation, the current differential scheme is a better way to protect transmission lines than overcurrent and distance-based schemes. However, the protection scheme can be severely influenced by the Line Charging Capacitive Current (LCCC) with increased voltage level and Current Transformer (CT) saturation under external close-in faults. This paper presents a new UHV/EHV current-based protection scheme using the ratio of phasor summation of the two-end currents to the local end current, instead of summation of the two-end currents, to discriminate the internal faults. The accuracy and effectiveness of the proposed protection technique are tested on the 110 kV Western System Coordinating Council (WSCC) 9-bus system using PSCAD/MATLAB. The simulation results confirm the reliable operation of the proposed scheme during internal/external faults and its independence from fault location, fault resistance, type of fault, and variations in source impedance. Finally, the effectiveness of the proposed scheme is also verified with faults during power swing and in series compensated lines.

Mitigation of current transformer saturation on medium voltage switchgears in APR1400 of Nuclear power plants

Selection of current transformers (CTs) must be done carefully to ensure protection, control, and measuring systems function correctly. Some of the conventional CTs on medium voltage (MV) switchgear in nuclear power plants (NPPs) perform poorly due to accuracy errors and saturation. This paper suggests the use of electronic current transformers (ECTs) as a solution. Simulations were done on a 13.8 kV switchgear using electromagnetic transient program restructured version (EMTP‐RV) software to compare the performance of conventional CTs and ECTs under high fault currents. It was found that ECTs show significant improvement in performance with the secondary current output of 97.37% and eliminate saturation even in systems with high symmetrical and asymmetrical short‐circuit currents. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Quartile Based Differential Protection of Power Transformer

A new technique for power transformer protection, that depends on the calculation of fault detection ratio (FDR) based on quartile of superimposed differential currents is presented. Internal faults are detected by comparing FDR with the predetermined threshold derived through analytical analysis and verified based on simulation results. The performance of the presented scheme is tested by modeling an existing Indian power transmission network using PSCAD/EMTDC software. Various types of abnormal conditions such as magnetizing inrush (including sympathetic and recovery) and over-excitation along with several types of internal (winding, inter-winding and turn-to-turn) faults have been investigated. Its validity has also been checked on the data of magnetizing inrush and simultaneous inrush with the internal fault in the transformer, which was obtained from the real network. The results reveal the effectiveness of the suggested technique in differentiating internal faults from other abnormal conditions. Further, it remains stable during heavy through fault with current transformer saturation. It is equally applicable for different ratings and winding connections of power transformers. Finally, comparative evaluation of the proposed scheme with the conventional and several existing techniques shows its superiority in terms of higher sensitivity during internal faults and improved stability in case of non-internal faults.

A new protection scheme for transmission lines utilizing positive sequence fault components

In this paper, a novel integrated approach of protective relaying methodology has been adopted for detecting faults and its location estimation in long transmission line. First, the detection algorithm has been formulated using positive sequence admittance. Subsequently, fault location algorithm has been developed using positive sequence voltage and current phasors. The proposed algorithms have been implemented on a 50 Hz, 400 kV single-circuit transmission line in MATLAB environment. A wide range of diversified and challenging cases have been simulated to test the reliability of this scheme. The results confirm the preciseness of the algorithm, as it can produce accurate results under stressed conditions, viz. high impedance fault (HIF), current transformer (CT) saturation/ capacitive voltage transformer (CVT) transient, close-in faults, swing condition, source strength variation, etc. Moreover, this scheme has also been successfully tested in series-compensated and double-circuit transmission line, which brings out the superiority of this scheme. The comparative analysis with recent protective relaying techniques institutes the potentiality and robustness of the scheme.

Sampled value‐based bus zone protection scheme with dq ‐components

This study presents a novel algorithm based on dq-components, which has been effectively used for bus zone protection. The analogue current transformer (CT) secondary signals are acquired and converted into sampled values. Then, the fundamentals of an instantaneous current-based differential protection scheme (87B) have been considered to establish the trip logic of the proposed scheme. Its performance has been validated by simulating faults on an existing 400-kV substation with a double-bus-single-breaker configuration. Moreover, a laboratory test bench has been developed to test the authenticity of the proposed scheme for various fault scenarios. The obtained results from the simulation model and laboratory prototype testify the claims of higher sensitivity during internal faults and better stability during external faults with CT saturation. The proposed scheme has been able to provide high-speed busbar protection against a wide range of internal faults. Comparative evaluation with contemporary busbar protection schemes clearly indicates its superiority.

A Novel Fuzzy Theory-Based Differential Protection Scheme for Transmission Lines

Abstract- Ever-increasing consumption of electrical energy has forced extension of power systems to wider regions, which will augment concerns about the stability and reliability of power systems. Therefore, a special attention has been given to protection systems and protection relays to improve these concerns. By appropriate design of protection systems and proper function of protection relays, it is possible to reduce the adverse effects of undesirable faults. A fuzzy logic-based algorithm is presented to improve differential relay performance. The algorithm is employed for protection of short transmission lines against internal faults with/without resistance. By using fuzzy logic and selecting the best stability characteristics, the possibility of malfunction of protection relays will be negligible. In this algorithm, sensitivity, reliability and speed of the relay performance are preserved at suitable levels. The proposed algorithm is also very beneficial for external faults accompanied with the saturation of current transformers (CTs). The purpose of this study is to present an algorithm based on the fuzzy logic, which can decide on the slope of stability characteristics of a differential relay in the best way during various conditions. The performance of the presented algorithm is analyzed in PSCAD/EMTDC program and compared to conventional methods.

A phasor-distance based faulty phase detection and fault classification technique for parallel transmission lines

AbstractThis paper presents a phasor-distance based faulty phase detection and fault classification technique for parallel transmission lines. Detection and classification of faulty phase(s) have been carried out by deriving indices from the change in phasor values of current with a distance of one cycle. The derived indices have zero values during normal operating conditions whereas the index corresponding to the faulty phase exceeds the pre-defined threshold in case of occurrence of a fault. A separate ground detection algorithm has been utilized for the identification of involvement of ground in a faulty situation. The performance of the proposed technique has been evaluated for intra-circuit, inter-circuit and simultaneous faults with wide variations in system and fault conditions. The suggested technique has been evaluated for over 23,000 diversified simulated fault cases as well as 14 recorded real fault events. The performance of the proposed technique remains consistent under Current Transformer (CT) saturation as well as different amount and direction of power flow. Moreover, suitability to different power system network has also been studied. Also, faults having fault current less than pre-fault conditions have been detected accurately. The results obtained suggest that it is able to detect faulty phases as well as classify faults within quarter-cycle from the inception of fault with impeccable accuracy. Besides, as modern digital relays have been already equipped with phasor computation facility, phasor-based technique can be easily incorporated with relative ease. At last, a comparative evaluation suggests its superiority in terms of fault classification accuracy, fault detection time, diversify fault scenarios and computational requirement among other existing techniques.

A Novel Methodology to Sense CT Saturation Using Numerical Protection Relays

The necessity for dependability and safety in power system is rising at an alarming rate as the power system designs are getting more & more complex. In order to ensure maximum reliability, the numerical protection relays must receive accurate measurements. One of the most important input measurements desired by the relay is current. Nevertheless, the current measurements expected from current transformers (CTs) can be inaccurate due to occurrence of a phenomenon called as CT saturation. CT saturation causes distortion in the secondary current, which is not linearly proportional to the primary current, leading to mal-operation of protection devices. Therefore it becomes extremely vital that the numerical protection relay, also known as intelligent electronic device (IED) senses the CT saturation condition and blocks its operation so as to avoid mal-operation of the IED which may lead to supply interruption, further resulting in production losses and undesirable switching of the sensitive equipments affecting system reliability and customer satisfaction. This paper includes a review of the background about CTs & proposes a novel methodology to exclusively sense the CT saturation using the numerical protection relays reliably and cost effectively.

Simulink model of transformer differential protection using phase angle difference based algorithm

<p class="p1">An application of phase-angle-difference based algorithm with percentage differential relays is presented in this paper. In the situation where the transformer differential relay is under magnetizing inrush current, the algorithm will be utilized to block the process. In this study, the technique is modeled and implemented using Simulink integrated with MATLAB. The real circuit model of power transformer and current transformers are considered in the simulation model. The results confirmed the effectiveness of the technique in different operation modes; such as, magnetizing inrush currents, current transformers saturation and internal transformer faults.</p>

Ratios-based universal differential protection algorithm for power transformer

For safe operation of power systems, it is essential that the transformer protection operate reliably if a fault occurs. There are many challenges in developing a fast and accurate differential protection scheme that not only disconnects the transformer during internal fault conditions but also restrains during energizing and external fault conditions. A universal algorithm based on the ratios of primary and secondary voltages and currents, supplemented by current direction and wave-shape criteria, for power transformers with any type of winding connection, star or delta, is proposed. The ratios are the ratio of the absolute difference and absolute sum of primary and secondary line currents and the terminal phase voltages. The primary objective is to avoid incorrect operation of the conventional differential protection approach during energization, energizing on an existing internal fault and external faults under current transformer saturation. Performance studies confirm that the proposed algorithm can provide fast, efficient and reliable protection of a power transformer for all types of winding configurations.

Electromagnetic compatibility and protection of electric energy meters from strong magnetic fields

Power theft through electric energy meters tampering causes significant revenue losses. One of the meter tampering method involves manipulation using strong magnetic fields to distort the process of energy calculation by saturation of current transformers’ cores. This paper presents an approach towards effective shielding of the power meters’ sensitive elements from external magnetic fields. The theory of magnetic materials and the methods to calculate magnetic fields in the presence of these materials are used to develop a refined protective shield.

Study on the transformer protection scheme based on double thresholds and waveform similarity

As the key device of power transmission, the power transformer has higher requirements for the protection sensitivity and reliability. However, the operation time of the differential protection is limit to the data window of the Fourier transform algorithm. In addition, the differential protection cannot recognize the slight transformer turn-to-turn fault. Moreover, it may maloperate when the current transformer (CT) is saturated. To solve above problems, a novel transformer protection scheme based on double thresholds and waveform similarity is proposed. By comparing the current waveform similarity with the double thresholds, the external faults, internal faults and CT saturation scenarios can be effectively identified, while the waveform similarity is obtained by calculating the Hausdorff distance of the current at each side of the transformer. The PSCAD/EMTDC based simulation results show that the proposed protection scheme can operate quickly and the operation time is as low as 5.5ms. In addition, it has high sensitivity and can effectively identify 0.5% or higher turn-to-turn faults, and it will not be affected by CT saturation.

Zero-Sequence Differential Current Protection Scheme for Converter Transformer Based on Waveform Correlation Analysis

Through the analysis of the recovery inrush current generated by the external fault removal of the converter transformer, it is pointed out that the zero-sequence current caused by the recovery inrush may result in the saturation of the neutral current transformer (CT), whose measurement distortion contributes to the mis-operation of zero-sequence differential current protection. In this paper, a new scheme of zero-sequence differential current protection based on waveform correlation is proposed. By analyzing the characteristics of zero-sequence current under internal fault, external fault and external fault removal, the waveform correlation of the zero-sequence current measured at the terminal of the transformer and the zero-sequence current measured at the neutral point of the transformer is used for identification. The polarity of the CT is selected to guarantee the zero-sequence currents at the terminal and neutral point of the transformer exhibit a "ride through" characteristic under external fault, then the waveform similarity is high, and the correlation coefficient is positive. On the other hand, when internal fault occurs, zero-sequence current waveforms on both sides differ from each other largely, and the correlation coefficient is negative. Through a large number of simulations verified by PSCAD/EMTDC, this criterion can accurately identify internal and external faults, exempt from effects of the recovery inrush. Moreover, it presents certain ability for CT anti-saturation.

Novel Fault Distance Estimation Method for Three-Terminal Transmission Line

A new faulty section identification and fault distance estimation technique for three-terminal transmission line (TTL) is proposed in this paper. It is based on the derivation of three indices by solving non-linear equations with iterative method utilizing synchronized measurements from three ends of the line. In the absence of synchronization, an angle synchronization operator is derived for building a common reference among the three terminals. The efficacy of the technique is verified by generating several fault cases on an existing Indian 400 kV TTL using PSCAD/EMTDC software. Reported results demonstrate the capability of the presented technique for all types of fault on three-terminal transposed/untransposed as well as the non-homogeneous transmission line. It also provides accurate fault location under comprehensive variation in type of fault, line length, fault inception angle, complex fault impedance, source impedance, and current transformer (CT) saturation condition. Its accuracy remains almost constant despite errors in line parameters, as well as synchronization and noise in the measured signals. The initial guess has a minimal impact on the accuracy and convergence of the presented technique. The attained results reveal higher accuracy and better robustness of the proposed technique in comparison with those of several existing techniques.

Avoid Current Transformer Saturation Using Adjustable Switched Resistor Demagnetization Method

In some conditions, the flux in the core of current transformers (CT) increases. If this increase in flux leads to saturation of the protective core, it can create a lot of problems in power system protection. One of the hardware solutions to prevent the occurrence of core saturation in CT is the fixed switched resistor demagnetization method. Due to disadvantages such as dependence on CT parameters and inefficiencies in severe core saturation, the use of this method has not been expanded. This paper proposes a new method to improve the fixed switched resistor demagnetization method. In the proposed method, instead of using a fixed resistor in the circuit topology, an adjustable resistor with an auxiliary transformer is used. In the previous method, the control process is based on flux estimation from the secondary terminal voltage. The control process in the proposed method is based on the second derivative of CT secondary current. To verify the effectiveness of the suggested method, first simulation is done using COMSOL and MATLAB softwares and then all necessary electronic and control circuitry are designed and built. Experimental test is carried out on a 20 kV- 500/5 A CT. The simulation and experimental results show that this method prevents the CT from saturation in case of short circuit faults. The main advantages of this method are independent to CT parameters and efficient in severe core saturation.

Analysis of Measures that Exclude the Non-Selective Actions of Differential Collecting Busbar Protection in External Two-Phase Short Circuits with the Saturation of Current Transformers Included in Fault-Free Phases

An analysis is made on the non-selective actuation of the differential protection of busbars with a voltage of 110 kV of an electrical distribution system substation in the transition mode of an external two-phase short circuit. The protection effect is found to be due to the saturation of the transformer cores of class P current included in fault-free phases. Methods are also analyzed to ensure the correct operation of protection in the considered mode, and methods for calculating the protection parameters at which these methods are workable are proposed.