Earth Return Currents Research Articles

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.

Potential Compensation Method for Restraining the DC Bias of Transformers During HVDC Monopolar Operation

The earth return currents (ERCs) of high-voltage direct current transmission can cause severe half-cycle saturation of transformers. This study presents a novel and highly beneficial potential compensation method (PCM) to mitigate the detrimental effects of ERCs. The simulation model of the PCM was first derived and verified through a laboratory test. Analysis has suggested an association of network configuration and mutual resistance to the compensatory factor. The proposed method has been applied in two representative substations with different types of transformers in various operating modes. Results show that as opposed to the neutral blocking device (NBD), the PCM can adjust the dc voltage equilibrium of ac systems rather than intervene in the neutral path of transformers. Moreover, the PCM can eliminate the direct magnetomotive force of autotransformers with proper selection of the injected current. An application shows that the PCM can outperform NBDs and line-series capacitors in restraining the dc bias of autotransformers.

Steady State Assessment of Shunt Compensated EHV Insulated Cables by Means of Multiconductor Cell Analysis (MCA)

The author has already presented some papers which allow studying cable systems by means of the multiconductor cell analysis (MCA). This method considers the cable system in its real asymmetry without simplified and approximated hypotheses. The multiconductor matrix procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of screen bonding, possible multiple circuits (single and double circuit or more), allows predicting the steady-state regime of any cable system. In the previous papers, these matrix algorithms have been presented with reference to a short extra-high voltage (EHV) double-circuit cross-bonded (CB) underground cable (UGC) system. Since the cable link was short, the shunt reactive compensation was not necessary and consequently not considered. In this paper the procedure is generalized in order to take into account three single-phase (or also one three-phase) reactors installed at the cable ends or also at intermediate locations.

경부선 전철화 구간에서의 공동접지에 의한 귀선전류 분석

전기철도는 전차선을 통해 차량을 구동할 수 있는 전력을 공급하며, 공급된 전류는 선로 및 접지망을 통해 변전소로 귀환한다. 국내의 경우, 이러한 귀선전류에 영향을 주는 접지망 구성은 대부분 단독 접지방식을 사용하였으나, 경부고속선의 도입과 함께 공동 접지방식의 개념이 전기철도에 적용되었다. 본 논문에서는 고속철도의 기존선 운행을 위한 전철화 구간에서 측정, 분석한 귀선전류를 기반으로 전철화 구간에 있어서의 접지방식과 귀선전류의 관계를 분석하였다. 이러한 분석은 경부고속선 궤도회로에서 측정된 귀선전류 분석값과 경부고속선과 기존선 전철화 구간의 접지방식 비교에 따른 귀선전류의 예측값을 활용하였다. Electrical Railroads provide electric power, which can operate vehicles, via feeder wires. And the supplied current returns to the transformer substation through lines and ground net. In domestic cases, the configuration of ground net affecting such a return current mostly uses an exclusive earth method. However, along with the introduction to Gyeongbu HSL(High Speed Line), the concept of Common Earth was applied to Electrical Railroads. In this paper, based on the return currents analyzed to be measured in the electrified sections for the operation of High Speed Rails in existing lines, we analyze the relations between earth methods and return currents in electrified sections. For analysis, we utilize the measured values of return currents measured in track circuits in Gyeongbu HSL, and predictive values of those compared to the earth methods between Gyeongbu HSL and the existing electrified sections.

Multiconductor analysis of underground power transmission systems: EHV AC cables

It is widely ascertained that the multiconductor analysis is a powerful tool which can solve any structurally complex circuit (e.g., high-speed railway supply system, gas insulated lines, etc.). Moreover, this method can be also used as an in-depth analysis of the electric networks after the power-flow studies when they introduce simplifying hypotheses especially in the presence of asymmetry. In this paper, the multiconductor cell analysis has been applied to AC underground cable lines (UGC). This multiconductor procedure based on the use of admittance matrices, which account for the line cells (with earth return currents), different types of sheath bonding, possible multiple circuits, allows predicting the steady-state (and faulty) regime of any cable system. The method calculates the proportion and behavior of the phase currents carried by each parallel conductor, the circulating current in the sheath of each cable and the stray current in the earth. Moreover, some comparisons have been made with traditional programs showing the great accuracy of multiconductor cell model.

Sheath Circulating Current Analysis of a Crossbonded Power Cable Systems

The sheath in underground power cables serves as a layer to prevent moisture ingress into the insulation layer and provide a path for earth return current. Nowadays, owing to the maturity of manufacturing technologies, there are normally no problems for the quality of the sheath itself. However, after the cable is laid in the cable tunnel and is operating as part of the transmission network, due to network construction and some unexpected factors, some problems may be caused to the sheath. One of them is the high sheath circulating current. In a power cable system, the uniform configuration of the cables between sections is sometimes difficult to achieve because of the geometrical limitation. This will cause the increase of sheath circulating current, which results in the increase of sheath loss and the decrease of permissible current. This paper will study the various characteristics and effects of sheath circulating current, and then will prove why the sheath current rises on the underground power cable system. A newly designed device known as the Power Cable Current Analyser, as well as ATP simulation and calculation equation are used for this analysis.

Accurate Calculation of Magnetic-Field Intensity Due to Overhead Power Lines With or Without Mitigation Loops With or Without Capacitor Compensation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> An accurate method for the evaluation of 50-Hz magnetic fields produced by overhead power lines is presented. The method, which is based on the matrix formalism of multiconductor transmission lines (MTL), allows for a correct evaluation of all the currents flowing in the MTL structure, including the currents in the subconductors of each phase bundle, the currents in the ground wires, the currents in the mitigation loop (if present), and also the earth return currents. Furthermore, the analysis also incorporates the nonuniform character of the structure arising from conductor sagging between towers. Frequency domain magnetic field calculations are performed by vectorial sum of the magnetic effects originated by all the existing currents. For exemplification purposes the case of an actual 400-kV–1.4-GVA single-circuit three-phase line is examined. Results presented include a discussion on the effects of mitigation loops with or without capacitor compensation. </para>

Distribution Line Carrier: Analysis Procedure and Applications to DG

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper deals with the analysis of carrier signal transmission in medium-voltage (MV) distribution networks. A multiconductor matrix procedure based on the bus admittance matrix, which accounts for the earth return currents, enables predicting the high-frequency behavior of the distribution networks and, hence, the effectiveness of transmitted signals. To this aim, suitable models have been implemented based on the Carson's theory for overhead lines with ground return and Wedepohl's theory for cable lines. A comparison with the modal analysis demonstrates the advantages of the multiconductor matrix algorithm with respect to the modal analysis. In order to validate the theoretical results, two measurement campaigns have been carried out on real MV networks. A good agreement between measured and computed values has been verified. Since distribution networks are generally unlike each other, the procedure becomes a valuable tool, which makes it easy to assess the feasibility of distribution line carrier (DLC) applications in any system. In particular, a protection system to prevent the islanding of dispersed generators is proposed. In addition, in a foreseeable hypothesis of allowed islanding operation of portions of distribution networks (microgrids), DLCs are proposed as communication control of the interface device at the point of common coupling. </para>

Failure of the London Underground's 22 kV power supply system – an exercise in logic and transport management

The power supply to the north-east sector of London Underground Ltd&apos;s Central Line, and therefore the train service, was out of service for the majority of the time between the 24th and the 29th November 1993. During this period the power system faults were investigated, identified and corrected. This incident was the most extensive power fault on London Underground&apos;s power supply system in its recent history. The key cause was identified as an earth fault on a neutral cable of an alternator, which was located several kilometres from the part of the network that became inoperative. London Underground&apos;s power 22 kV generation and distribution system is described, and the feed arrangements historically made to minimise the possibility of such an extensive outage are explained. The events leading to the loss of supply, subsequent identification and correction of the underlying main fault are described, and a model of the 150 Hz harmonic earth-return current, which was the immediate cause of tripping owing to a series resonant circuit condition, is included. The effect of the neutral cable fault was to increase the 150 Hz current to the point where it caused simultaneous multiple feeder breaker trips from earth-fault relays for the feeders supplying the north-east sector of the Central Line, resulting in loss of power to that sector. A comparison is made between the model and practically measured results.

Mathematical verification of Dubanton's simplified calculation of overhead transmission line parameters and its physical interpretation

For the calculation of overhead transmission line impedances with earth return Dubanton has given explicit formulae based on the assumption, that the earth return current is concentrated in a plane being in a fictitious complex depth. In this paper the authors give the mathematical basis started from Carson's formulae and the physical interpretation of this heuristic method.