Fault section detection system for 275-kV XLPE-insulated cableswith optical sensing technique

An on-line fault section detection system, which continuously monitors a 275-kV cross-linked polyethylene-insulated underground transmission line and instantaneously determines the section of fault at a ground fault, was developed. In the case of a ground fault, the system detects the fault current which flows from the power source to the point of ground fault and determines the fault section from the magnitude and phase information of the fault current. Optical magnetic field sensors based on the Faraday effect are used for detection of fault current, and optical fibers are used for signal transmission eliminating effects of electromagnetic induction due to a large ground fault current (5000-50000 A) or switching surges on signal transmission lines. The distance between an optical magnetic field sensor and the fault section detector/indicator can be as long as 10 km without the use of any repeaters     

Study of 1000-kV network protection

Electric power systems in Japan are developing due to economic growth. Especially, recent demand for electrical power in metropolitan areas has increased by more than 250 percent since 1973 when the 50-kV system was first used. Increasing power demands will continue in the future. To satisfy this demand, utilities developed new transmission lines to make networks substantial with stable and efficient operation of the power system. However, this countermeasure causes extremely large fault currents at the substations where large power sources are concentrated. Problems such as large fault currents beyond interrupting the capacity of a circuit breaker or electromagnetic interference have become more severe. To solve these difficulties, Tokyo Electric Power Company is going to develop 1000-kV networks. These increase the capacity of transmitted power and suppress the fault current level by splitting the 500-kV system into several parts. Considering the associated difficulties of 1000-kV networks (such as increasing capacitive current on transmission line), prolonged dc time constants due to large diameter conductors and other electrical characteristics of 1000-kV networks, we examine the applications of a reliable protection system. The results are summarized as follows:

• 1 1) To adopt multiple phase autoreclosing with highspeed grounding switches to suppress secondary arcing on faulted phases.
• 2 2) To adopt air-core type CT for 1000-kV busbar protection as a countermeasure against CT saturation.
• 3 3) To adopt the current comparison scheme to realize sensitive protection for paralleled transformer bank.

A new protection system will be field-tested in 1995.     

Development of a 275-kV liquid cooling-type gas-insulated transformer

Construction of 275-kV and 500-kV substations in densely populated areas is thought to be necessary in the future to maintain a stable supply of electricity. The use of gas-insulated transformers is now considered for substations because such transformers are non-flammable and nonexplosive. Since the completion of the 77-kV gas-insulated transformer in 1979, we have continued research to develop gas-insulated transformers applicable to a higher voltage level up to 500 kV. As the results of the research, 275-kV, 300-MVA a liquid cooling-type gas-insulated transformer has been developed.     

A novel digital directional transformer protection technique based on wavelet packet

This paper presents a novel digital technique for transformer protection. The technique is based on deriving a directional quantity proportional to the fault current signal and the prefault voltage signal. Standard fast wavelet transform (FWT) schemes may not be as effective for data that has chiefly oscillatory features. An effective solution to discrimination involves examining the signal in both the time and frequency domains simultaneously. The wavelet packet transform is an extension of the FWT that allows for finer characterization of signal content for both time and frequency together. A 11/132-kV transformer connected to a 132-kV power system was simulated using Alternative Transient Program/Electromagnetic Transient Program (ATP/EMTP). Results indicate that the proposed technique is stable, reliable, and fast during the discrimination between internal and external faults, magnetizing inrush currents, and internal faults, ratio-mismatch, and saturation of current transformers (CTs).     

Development of a high-voltage,large-capacity gas-cooled,gas-insulated transformer

Gas-insulated transformers are suited for urban underground substations because of their nonflammable and nonexplosive nature and compact installation space, and their use has been growing in urban areas. However, to meet the demand for large-capacity gas-cooled transformers, cooling difficulty caused by the small heat capacity and thermal conductivity of SF₆ gas must be solved. A 275-kV, 300/3-MVA gas-cooled, gas-insulated shell-form transformer has been developed by use of a new cooling structure and is now under a long-term energization test. This paper describes the main features of the transformer as well as the cooling and insulation characteristics obtained by component models of the transformer and from a full-scale model transformer. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 19–26, 1997     

Development of fault detector for 33-kV ungrounded power line

A fault section detection system is being developed for 33-kV ungrounded power line. The fault section is obtained from the relation of two point zero-phase currents on power line. This fault detector is composed of optical current sensors and the processing unit and solar battery system. The optical current sensor consists of a Faraday cell and a porcelain bushing. Zero phase current on the processing unit is obtained through a three-phase composer with auto gain control and residual current eliminator. This fault detector has the ability of 0.15 A zero-phase current detection fault factory test; in field tests, this fault detector detects ground fault.     

Complex Percentage Differential Relay

1.IntroductionBecauseofitsgoodselectivity,highsensitivityandrapidaction,thedifferentialprotectionisusedwidelyasmain-protectionforelectricequipment.BecauseverylargeunbalancedcurrentmaytakeplaceduringexternaIfaultcausingmistrippingbytraditionalbusdifferentialreIay.Andduringinternalfault,ifthereisout-flowingcurrent,thesensitivityofthetraditionalrelaywillbemuchreduced.Althoughthehighimpedancebusdifferentialrelaycanensureitsselectivityduringexternalfault,highvoItagewillbeinducedinCT'ssecondarycir…     

Development of an advanced voltage control technique to improve voltage and reactive power characteristics of a large-scale power cable system

The power supply system for a metropolitan area consists of multiple 275-kV power cable systems to supply power to a large number of consumers. Each system has a large charging capacity (capacitive reactive power). When 275-kV power cable systems experience a voltage drop, their charging capacity decreases. However, the reactive power losses increase in the 275-kV overhead transmission lines, which supply large power to power cable systems, thus causing the performance of the power supply system to deteriorate. In this paper, the following three points are reported:

• 1 When the secondary power system of the trunk substation is a large-scale power cable system, new techniques can greatly improve the voltage and reactive power characteristics by controlling the rise of the sending-end voltage at the secondary side of the trunk substations and maintaining the voltage of power cable systems at a constant level.
• 2 In the use of this control technique, it has been demonstrated that controlling by the direct-detecting method of a voltage drop in the primary power system is superior to one that controls by the indirect-detecting method, which increases load power in the cable system.
• 3 The results of simulations obtained by using a 275-kV cable system model and a performance power system model show that the use of this control technique can produce a good effect despite the allowances made for the opposite effect, i.e., a decrease in the capacity of power capacitors resulting from voltage drop at the tertiary voltage of a 500-kV transformer.

This study is carried out to examine the possibility of lightning control using a photoionized plasma produced by a UV laser without an optical air breakdown. As the fundamental experiment, the characteristics of a laser-triggered spark gap (LTSG) are examined, where a laser beam is not irradiated on the surface of the electrodes. In this experiment, the KrF excimer laser is very effective in increasing the plasma density n_e and the reduction ratio p of 50 percent breakdown voltage against the self-breakdown voltage. The relation between n_e and p is increased by generating a long and high-density plasma parallel to the discharge axis. On the other hand, the abnormal discharge process in triggered lightning using a rocket is examined, and it is indicated that the most important condition to trigger a lightning discharge is the production of a plasma channel of length 200 m and density about 10¹⁹ m^?3. It is suggested that such a plasma channel could be produced by a KrF excimer laser with an energy of only about 3.67 J.     

Prevention of reignition overvoltages by phase-angle-controlled switching of a gas circuit breaker at shunt reactor current interruption

Shunt reactors installed at gas-insulated switchgear substations (GIS-substations) are switched relatively frequently by large-capacity gas circuit breakers (GCBs). Surge voltages appear at such switching operations through various mechanisms. Among them, high-frequency current interruption and subsequent multiple reignition might cause the highest overvoltage and may be harmful to the insulation of the components of the substation. In this paper, shunt reactor current-interrupting tests are conducted in high-power laboratory. The existence of critical arcing time above which reignitions never occur is shown through these tests. From this fact, a phase-angle-controlled-interruption system is proposed. With this system, arcing time is set artificially so that reignitions should be prevented. The reliability of this system is confirmed through the reactor current-interrupting test in the laboratory as well. Afterward the phase-angle-controlled-interruption system is applied to the shunt reactor switching at an actual 275-kV substation. The switching performance of this system is measured over three months. Reignition-free interruption is shown to be attained with this system.     

Relay performance considerations with low-ratio CTs and high-faultcurrents

Low-ratio toroidal current transformers (CTs) are in existence in many locations on power systems. The majority of these CTs are in the auxiliary switchgear at generating stations, but are not limited to this application. The ratio of the CTs used in this application is usually selected to accommodate metering and overload protection. The problem addressed is the absence of consideration in this ratio selection for fault protection. In many cases the fault duty of this switchgear is very high and, because the low-ratio CTs also are of low accuracy classes, the CTs will saturate during a fault. The analysis of some fault cases shows that the protective relaying will not operate, or will not operate properly, due to the saturated CTs. The significance and consequences of the use of these CTs and the accompanying saturation during high-fault currents are discussed. Considerations when testing low-ratio CT applications for saturation are presented as well as a computer program to demonstrate the volt-time concept of CT saturation. Alternate CT connections and corrective alternatives are given along with conclusions to aid in resolving low-ratio/high-fault current situations     

Ground Fault Protection for Bus-Connected Generators in an Interconnected 13.8-kV System

This paper is based on modifications implemented in the protection and grounding systems of a large paper mill and describes selective ground fault protection for a 13.8-kV system with multiple bus-connected generators, synchronous bus ties, and utility interconnections. The ground fault current in the system is reduced from the existing 3400 A to 500 A, and a hybrid grounding system is implemented for each of the generators. As the ground fault currents are reduced to limit the fault damage, the sensitivity and selectivity of the ground fault protection become important. Directional ground fault relays with coordinating pickup settings are applied to achieve this objective. The new platform for directional elements (numerical relays) drives its performance from sequence impedance measurements     

在线CT导纳测试及其应用

讨论了常见电流互感器(CT)的故障及其特征,提出了一种实现在线电流互感器测试的新方法——导纳测试法;介绍了澳大利亚红相电力设备公司505型导纳测试仪的工作原理及实用性。     

Technological progress in high-voltage XLPE power cables in Japan.II

For pt.I see ibid., vol.4, no.5, p.9-16 (1988). Cables designed for particular applications and improved performance are described. The products discussed are: (1) water-impervious cable; (2) composite XLPE/optical fiber cable; (3) 66-kV submarine cable; (4) commercial long-distance 275-kV line; and (5) 500-kV short-distance line. The discussion is confined to the Japanese cable industry     

Mechanical characteristics related to fault currents in bundle conductors

The flow of fault currents in bundled conductors induces an electromagnetic attraction between subconductors and thereby the bus conductor system to an extremely high level of tension. This tension, which is the most important factor in designing the mechanical strength of the system, is determined by a large number of parameters—the magnitude of the fault current, the size and number of the subconductors, spacer interval, the spring of the structure, and so on. Full-scale 63-kA class fault current tests were carried out on eight types of bus conductors used in 275-and 500-kV subconductors, and measurements of fluctuation in tension were used to clarify the relationship between the various parameters and maximum tension at fault time. A method also was devised for calculating maximum tension at fault time on the basis of the various parameters, and it has been applied in the mechanical strength design of the bus conductor system.     

A series multiresolution morphological gradient-based criterion to identify CT saturation

The differential relay possibly mal-operate due to the inconsistent transforming characteristics of current transformers (CTs) when experiencing heavy through fault. According to the investigation of CTs of differential protection, the time interval between the occurrence of differential current and the sudden change of phase current can be utilized to distinguish between the external faults and the internal faults. A new CT saturation-blocking scheme for differential protections, which aims at identifying this time interval by virtue of series multiresolution morphological gradient (SMMG) is proposed. Using the excellent singularity detecting together with the noise-repressing abilities of SMMG, this time interval can be identified in real time with high accuracy. The EMTDC-based simulation results show that the scheme can effectively prevent the differential protection from mal-operating meanwhile guarantee the high-speed response to the internal fault.     

Phasor Estimation in the Presence of DC Offset and CT Saturation

A hybrid algorithm for phasor estimation is proposed that is immune to DC offset and current transformer (CT) saturation problems. The algorithm utilizes partial sum (PS)-based and multistage least-squares (MLS)-based methods before and after CT saturation is detected, respectively. The MLS-based method is initiated when the third difference of the secondary current detects the start point of the first saturation period. The determination of each saturation period is based on the sum of the secondary current from the start point of the first saturation period. A least-squares (LS) technique estimates the DC offset parameters from the single-cycle difference of the secondary current in the unsaturated periods. Removal of DC offset from the secondary current yields the sinusoidal waveform portion. Finally, the LS technique is used once again to estimate the phasor from the sinusoidal waveform portion. The performance of the algorithm was evaluated for a-g faults on a 345-kV 100-km overhead transmission line. The electromagnetic transient program was used to generate fault current signals for different fault angles and remanent fluxes. The performance evaluation shows that the proposed algorithm accurately estimates the phasor of a current signal regardless of DC offset and CT saturation. The paper concludes by describing the hardware implementation of the algorithm on a prototype unit based on a digital signal processor.     

基于两相电流行波的接地选线方法

中性点非有效接地配电系统大多只安装两相电流互感器,其接地选线问题是长期困扰实际运行的技术难题。为了解决只装有两相电流互感器配电系统的单相接地选线问题,在分析了中性点非有效接地配电系统接地所产生的电流行波特征的基础上,提出了基于两相初始电流行波和小波变换的接地选线新算法。该算法仅仅利用接地时所产生的两相行波,不受中性点接地方式和系统正常运行时不平衡电流的影响,解决了现场的实际运行需要。仿真和试验结果验证了所提出的方法的可靠性和有效性。     

A new regenerative snubber circuit for large-capacity three-level GTO inverter systems

Increase in the capacity of GTOs has made remarkable progress in recent years. At present, 4.5-kV, 4.0-kA GTOs are commercially available, and 6.0-kV, 6.0-kA GTOs made from 6-inch silicon wafers are appearing. The 6-inch GTOs will be applied to our three-level GTO inverter system. In order to apply GTOs to voltage-source inverters, snubber circuits are necessary for limiting on the turn-on di/dt and turn-off dv/dt. To realize high efficiency of the system, regenerative snubber circuits are often applied. A conventional circuit applied to three-level GTO inverters had the problem of long paths created for snubber circuits of the inner GTOs. Another circuit using a current transformer for recovering the energy trapped in the snubber circuits of the inner GTOs has been presented. In this paper, a new regenerative snubber circuit is proposed, which is more suitable for three-level GTO inverter systems with many phase-legs. By applying the snubber circuit, all snubber energy generated by each GTO switching can be regenerated to the dc link. In addition, high current turn-off performance of both the inner and the outer GTOs is verified by several successful experimental results using 6-inch 6.0-kV, 6.0-kA GTOs. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 41–48, 1997     

Structural Analysis and Testing of HV Busbar Assemblies with Rigid Conductors under Short-Circuit Conditions

A general method is presented for calculating the dynamic stresses and displacements of busbar structures with rigid conductors under various short-circuit conditions, with or without reclosing on a fault. Based on a finite-element technique and modal-response superposition, this method can be used to study a complex busbar structure in its entirety, taking into account both the three-dimensional aspect of the structural components and the paths followed by the fault currents. An explicit solution for the specific modal responses enhances the efficiency of the procedure. Laboratory tests on a three-dimensional busbar model and, also, extensive field tests on the busbars of a 315-kV substation have been performed kinder various fault conditions. The experimentally determined dynamic responses are compared with the finite-element solutions. Specific conclusions are drawn regarding the structural behavior of Hydro-Québec's 315-kV substations. Practical recommendations of general interest for the fault analysis of busbar structures are included.