Title of Invention

HIGH IMPEDANCE HIGH VOLTAGE WINDING OF POTENTIAL TRANSFORMER FOR AC TRANSMISSION SYSTEMS

Abstract

High impedance high voltage winding (01) of potential transformer (A) for AC transmission systems, the winding being made of special winding material/alloy having optimal conductivity, mechanical strength and cross-section for the application, characterized in that the increased impedance of the winding is imparted by distributed resistance (07) and distributed inductive (08) reactance due to the characteristics and length of the winding material.

Full Text

Field of the invention:- The invention relates to potential transformers in general, and to a high impedance high voltage winding of potential transformer for AC transmission systems in particular. Background and prior art:- A potential transformer (PT) is primarily used to measure system voltage and provides proportional signals for metering and protection. A PT is in general connected at receiving side (incomer), load side (feeder) or substation bus bar. Inductive voltage/ potential transformers are used to measure voltage in electric circuits. A potential transformer is a conventional step down transformer having primary and secondary windings. The primary winding is connected directly to the power circuit either between two phases or between one phase and ground depending on system configuration. Their main role is to condition (step down) the voltage to be measured by a measuring instrument/device. The transformer acts as an electrical isolation between instrumentation and the power system. The proportional secondary voltage is expected to be in phase with primary voltage for accuracy of three-phase measurement and the protection. To maintain precise voltage regulation, instrumentation load on PTs' is kept to bare minimum. The PTs are designed with high input impedance to avoid system loading. During evaluation and type testing of these devices, at higher voltages i.e., 110% and 125% of rated voltages specified by IEC, temperature of winding increases close to levels specified signifying a scope for improvement in material and optimization. Further, because of the low resistance of windings, the over-voltages which are encountered during switching/ ferro-resonance (due to inductance of PT and capacitance of long cables/transmissions lines) persist for longer durations for low clamping coefficient of the network, damaging equipment and insulation. In conventional potential transformers, high conductivity material like copper is used for preparation of primary and secondary windings. Because of low resistance of these windings, dc current flow in primary and secondary windings is abnormally high for higher system voltages. The ac magnetizing currents also have higher magnitude for reduced overall impedance. These flaws are conventionally controlled by use of current limiters. More clearly, a fuse or current limiter (CL) is often connected in series with primary winding of transmission PTs, for safety and ease of disconnecting the PT from the circuit (Refer Figure 1). Conventionally, for limiting ferro-resonance over voltage and their adverse effect, a limiting resistor is connected in neutral of the primary winding. Sometimes a limiting resistor connected across secondary winding of the PT helps in ferro-resonance damping (Refer Figure 2). Such resistors may cause thermal damage of the potential transformer in case of sustained unbalance in the supply network. To overcome these problems, protection circuit has been employed for potential transformers (refer patent No. WO 2006/ 126904 A1). The disadvantage of such systems is that the reliability is limited by the performance of limiting resistance and protection circuit. In order to overcome above drawbacks, a high impedance high voltage winding has been proposed in the present invention for the potential transformers, which are being used for AC transmission applications. Objects of the invention: An object of the present invention is to reduce failure of PTs in transmission systems due to DC discharge by suitably modifying the resistive impedance of the PT. Another object of the invention is to improve reliability by eliminating protection circuits and/or by de-rating the limiting resistor connected in neutral of the PT for ferro- resonance damping. A further object of the invention is to eliminate or to de-rate the current limiter (CL) connected in series with primary and secondary windings. Yet another object is to increase resistive impedance of the winding which enhances thermal capability of the PT. A still further object of the PT is to optimize the resistive impedance of primary winding by improving discharge capability of PT and ferro-resonance damping by keeping phase angle error and ratio errors within specified levels. Brief description of the accompanying drawings: The invention is described with the help of Figures 1 to 5, where: Figure 1: Shows the schematic of single phase potential transformer in prior art. Figure 2: Shows the schematic of three phase potential transformer in prior art. Figure 3: Shows the equivalent circuit of substation bay during DC discharge of potential transformer. Figure 4: Shows schematic of the single phase potential transformer in accordance with the invention. Figure 5: DC discharge capability of PT in accordance with the invention for various resistances of primary winding. The invention will now be described in details in an exemplary embodiment as depicted in the accompanying drawings. However, there can be several other embodiments of the present invention, all of which are deemed covered by this description. Description of the invention: A potential transformer (PT) is a step down transformer having primary (01) and secondary (02) windings. A limiting resistor (03) is connected in neutral of the primary winding (01) to limit ferro-resonance over voltages and their adverse effects. At times a limiting resistor (04) is connected across the secondary winding (02) to help ferro- resonance damping. In high voltage substation, if potential transformer is connected on line side terminated by high capacitance components like long transmission lines [05] which may be solid insulation cables, gas insulated transmission lines or overhead lines, it is essential for PT to have a high DC discharge capability. The capacitance of long lines is in the order of few hundred pF to few tens of μF. During opening operation circuit breaker (CB), on either side of PT, charge left on the line may appear as trapped voltage [06]. In general this DC voltage discharges through PT as its input impedance is less than the support insulators and other components of the network. The aim of the present invention is to increase the impedance of the primary winding. This is achieved primarily by increasing the resistance of the winding. It improves the DC discharge capability and ferro-resonance damping of the potential transformer. The resistance and inductance of high voltage winding depend on rated voltage of potential transformer, number of turns used, gauge/ cross section of copper wire and material characteristics. In general, the resistance of primary winding is about few kΩ and design is made based on requirements of phase angle and ratio errors. For these parameters of PT, the direct current (DC) discharge capability is limited. In other words, due to limited thermal rating of PT, it is prone to fail during continuous O-C-O operation of breakers (CB) present in bus and in line side of substation bay. Figure 3 shows the equivalent circuit of substation bay and its connecting line during DC discharge. For conventional PTs, OFF time required between two O-C-O operations is about few hours' for their reliable operation. This may not be acceptable for transmission applications. The OFF time can be reduced by keeping CL, however, it may lead to thermal damage of CL itself. In the present invention, the winding self resistance is increased by selecting low conductivity material for its design in comparison to high conductivity copper. Reduction in area of high conductivity copper conductor for primary winding is not recommended for practical limitations in transmissions class. The invention proposes using a special winding material/alloy which has optimal conductivity, mechanical strength and the cross-section for the application. Precisely, the copper and alloying element composition of the winding material is modified based on resistance requirement. The resistivity of winding material is increased from 2.25 μΩ-cm (about 50% more than the High conductivity copper) to about 7.0 μΩ-cm (approximately 300% more than the high conductivity copper) by modifying its copper composition. Other elements, which primarily control the resistivity of the winding material is Beryllium and Nickel. In the proposed design increased impedance of the PT winding is imparted by distributed resistance [07] and distributed inductive [08] reactance due to the characteristics and length of winding material. These parameters are selected by satisfying the following conditions: 1. DC current flown through primary winding shall be as low as possible. 2. Discharge time shall be as low as possible. 3. OFF-Time shall be as low as possible. 4. Ferro-resonance damping shall be as fast as possible. 5. Phase angle error and ratio error shall be within specified limits. 6. Thermal rating should be as high as possible. DC current through high voltage winding is reduced by increasing resistance and inductance of winding. This helps in significant reduction of current and decrease in discharge time. The significant decrease in discharge time leads to a corresponding reduction in failure of such PTs in transmission systems, making such systems more reliable and reducing expenditure and downtime. Because of significant reduction in discharge time and current amplitude, OFF time between consecutive Open-Close-Open operations of circuit breakers(CB) can also be minimized. The ratio of resistance and inductive reactance at normal service conditions is maintained in such a way, that the phase angle error and ratio error are within acceptable levels. The uniformly distributed resistance of this special winding helps in effective damping of all kinds of over voltages, identical to placement of a fixed high voltage neutral resistance [03]. It provides uniform distributed temperature rise at full rating of the transformer/during transients and dc discharges. The distributed resistance improves effective heat dissipation, resulting in enhancement of thermal rating of transformer. The present innovation also addresses PT damage against sudden drop in burden (in low resistance networks). The invention is aimed to optimize the PT design, eliminate some of PT accessories enhancing overall reliability. The conductivity of the material is selected based on requirements of following parameters: 1. Rated voltage of the potential transformer. 2. DC discharge time and OFF time of potential transformer. 3. Metering VA rating of PT. 4. Thermal VA rating of PT. 5. Over voltage level during service or during transients. 6. Input/output impedance of the primary and secondary windings. Figure 4 shows the schematic of proposed potential transformer. A sample calculation is made for 145 kV system to understand the effect of resistance of primary winding on DC discharge capability of potential transformer (refer Figure 5). From the figure, it is seen that with the increase of resistance of primary winding from 7 kΩ to 15 kΩ, current level decreases and potential transformer discharges the same at faster rate. We Claim:- 1. High impedance high voltage winding (01) of potential transformer (A) for AC transmission systems, the winding being made of special winding material/alloy having optimal conductivity, mechanical strength and cross-section for the application, characterized in that the increased impedance of the winding is imparted by distributed resistance (07) and distributed inductive (08) reactance due to the characteristics and length of the winding material. 2. High impedance high voltage winding (01) as claimed in claim 1, wherein the distributed resistance(07) improves effective heat dissipation, enhancing thermal rating of the potential transformer (A). 3.High impedance high voltage winding (01) as claimed in claiml, wherein said distributed resistance and inductance of said high voltage winding reduces the discharge time significantly which results in corresponding reduction in failure of such PTs (A). 4. High impedance high voltage winding (01) as claimed in claim3, wherein the reduced discharge time of the winding allows the OFF time between Open-Close-Open operations of the circuit breakers (CB) to be minimized. 5. High impedance high voltage winding (01) as claimed in claiml, wherein low conductivity copper is selected, along with beryllium and nickel as the material for the said winding. 6. High impedance high voltage winding (01) as claimed in claiml, wherein the resistivity of the winding material is increased from approximately 2.25 μ Ω -cm to approximately 7.0 μ Ω-cm, i.e. by more than 300% of the high conductivity copper. High impedance high voltage winding (01) of potential transformer (A) for AC transmission systems, the winding being made of special winding material/alloy having optimal conductivity, mechanical strength and cross-section for the application, characterized in that the increased impedance of the winding is imparted by distributed resistance (07) and distributed inductive (08) reactance due to the characteristics and length of the winding material.

Documents:

1682-KOL-2008-(09-06-2014)-ABSTRACT.pdf

1682-KOL-2008-(09-06-2014)-CLAIMS.pdf

1682-KOL-2008-(09-06-2014)-CORRESPONDENCE.pdf

1682-KOL-2008-(09-06-2014)-DESCRIPTION (COMPLETE).pdf

1682-KOL-2008-(09-06-2014)-DRAWINGS.pdf

1682-KOL-2008-(09-06-2014)-FORM-1.pdf

1682-KOL-2008-(09-06-2014)-FORM-2.pdf

1682-KOL-2008-(09-06-2014)-FORM-5.pdf

1682-kol-2008-abstract.pdf

1682-kol-2008-claims.pdf

1682-kol-2008-correspondence.pdf

1682-kol-2008-description (complete).pdf

1682-kol-2008-drawings.pdf

1682-kol-2008-form 1.pdf

1682-kol-2008-form 18.pdf

1682-kol-2008-form 2.pdf

1682-kol-2008-form 3.pdf

1682-kol-2008-gpa.pdf

1682-kol-2008-specification.pdf

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