Patent Application
20150124356
This invention relates to a scheme for blocking Geomagnetically Induced Currents (GIC) in a transformer apparatus of an AC power system by means of a passive device connected to the neutral end of a winding of such apparatus, part of a GIC circuit.
In its most common form, GIC is a phenomenon that takes place when high magnetic fields produced by electric particles emanated from the sun, periodically impinging our planet interact with the conductors of transmission and distribution circuits. Such interaction causes, according to the laws of physics, the induction of currents in these circuits. GIC can therefore flow in the network, the paper by Pirjola, R., Geomagnetically induced currents during magnetic storms; IEEE Transactions on Plasma Science, Volume 28, Issue 6, and December 2000 Page(s): 1867-1873, is a good reference on this matter. Furthermore the current flow reaches the power transformers as well as the instrument transformers, shunt reactors and phase shifters connected to the transmission lines, entering through their phase connections and returning from their earthed neutral. The most important effects are related to the saturation of those apparatus' magnetic circuitry. In general it may cause wave distortion and equipment overheating. Possible outcomes of this disturbance are the malfunction of protective systems and/or failure as well as a deterioration of the grid's performance, including voltage collapse; the paper by Bozoki, B. (chair) et al. (Working group K-11, IEEE power system relaying committee); The effects of GIC on protective relaying, IEEE Trans. Power Delivery, vol 11, 725-739, 1996, refers to this issue. A comprehensive document from the EMP Commission of Congress: Report of the Commission to Assess the Threat to The US from Electromagnetic Pulse Attack-Critical National Infrastructures, April 2008, makes a full discussion of these matters. At the end of Chapter 2, under Recommendations, the group concludes that ‘EMP attack on the electrical power system is an extraordinarily serious problem but one that can be reduced below the level of a catastrophic national consequence through focused effort coordinated between industry and government’. The present invention deals with a mitigation technology in order to block the aforementioned GIC without the use of condensers. On the other hand, it is clear today from searching relevant prior art, there are basically only two patent documents to be found as countermeasures for dealing with these perils i.e. U.S. Pat. No. 7,589,943 and U.S. Pat. No. 8,035,935, respectively. In both cases a linear resistor is the basic component to reduce GIC; moreover a protective Varistor MOV (or surge arrester) is devised in parallel with the resistor to get a by-pass of potential power-system ground currents. While such an approach has widely proven to be cost-effective, actually recommended by the aforementioned EMP Commission, some major technological progress taking place in recent time does afford a transcendental additional refinement; indeed state-of-the-art surge arresters, both for AC and, particularly for DC, plus Hall-effect based monitoring/relaying instruments are the pillars of this invention. In the case of surge arresters a new major short-circuit capability has been established and recognized by the IEEE; IEEE Standard C62.11a™-2008 (Amendment to IEEE C62.11a™-2005) eliminates sub-clauses 8.16, 8.17 and 8.18 (Pressure Relief) replacing them with 8.21, whereas Short Circuit Testing is established instead; Short-Circuit Ratings now appear in updated top manufacturers catalogs. On this rooting, surge arresters, based on their continuous negative volt-ampere characteristic are capable of sustaining a transformer solid neutral grounding under all conceivable system conditions; in other words the provisions of IEEE C62.92 Standard, Guide for the Application of Neutral Grounding in Electrical Utility Systems are key; this normative sets the positive-sequence to zero-sequence ratios of X0/X1≦3 and R0/X1≦1 respectively, which now get rigorously met in this invention. In fact, the typical resistance range for their non-linear characteristic expands from Mega ohms to fractions of one ohm depending on voltage/current; therefore under normal operating conditions such a value is very large, affording a neutral GIC current blocking should the need arise, while for ground faults or transients overvoltages it becomes very small and thus a protecting expeditious bolted connection to earth. In addition, novel Hall-effect Current Transducers (CT) provide both a reliable and accurate quasi-DC detection sparing any need for the traditional AC convoluted second harmonic GIC signature indirect detection, given the fact that DC monitoring is not possible through conventional AC current transformers (CT), as it has been the case up to now. In sum, by taking advantage of these developments the role of a main linear resistor, in parallel with a MOV as in the preceding inventions, can be minimized or plainly obviated in dealing with GIC currents. Consequently, for the purpose of this application such linear resistor is eliminated from the device; a major breakthrough considering such a linear resistor represents typically 95% of size, cost and device design challenge.
This invention relates to a scheme for mitigating Ground-Induced Currents (GIC) from a Geomagnetic Disturbance (GMD) in an electric power transformer by means of a non-linear negative volt-ampere characteristic resistor designed to block the circulation of such currents. The preferred embodiment is based on a simple surge arrester connected from the transformer neutral to ground. Therefore this element passive device, a non-linear resistor, is inserted in series to a circuit made primarily of a transmission lines and a transformer winding with neutral earthed return. Current division, affecting all transmission lines and transformers involved, can basically block the GMD surge into the specific transformer winding to be protected according to basic circuit theory. A mitigation philosophy can be established where a GIC current actual blocking by direct application of Ohm's law is decisively accomplished before a sudden major pulse disturbance.
Consequently, it is an object of this invention to provide an external neutral grounding device to cope with the GIC problem, as defined, simply by applying effectively/carefully basic electricity laws to block a circuit current; this result must be accomplished for a quasi permanent operation without any limitation or infringement of equipment standards, insulation guidelines, and particularly on the transformer grounding code. Furthermore, as known from symmetrical component theory, a neutral resistor is three-times more effective than phase ones to limit ground currents, for they get amplified by a factor of three with respect to the corresponding phase ones (the zero sequence resistance sums three times the neutral one); moreover in a system designed basically to be lossless, i.e. having very low resistances, GIC currents become sensitive to a neutral resistor insertion blocking component.
Nevertheless the basically lossless qualifier remains, since in balanced three-phase systems there is no steady-state flow through the neutral. In addition neutral-to-ground circuit elements do not get any major power duty, neither bear high voltage. It is also an advantage of this invention to consist of only two inexpensive standard components of the Distribution Class. Moreover it becomes yet a novel advantageous transformer neutral grounding means; a traditional utility practice for a host of other applications.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.
Regarding
It should be apparent the present invention adds a major functionality to surge arresters becoming a very cost-effective means for counteracting potentially severe GIC currents in a power transformer. Furthermore it yields the following objects and advantages:
Thus while the preferred embodiments of the invention have been illustrated herein, it is to be understood that changes and variations may be made by those skilled in the art without departing from the spirit and scope of appending claims.
