Passive fault protection arrangement for a measuring voltage transformer or a power voltage transformer for use in high voltage applications

Technical Field

The invention relates to a passive fault protection arrangement for a measuring voltage transformer or a power voltage transformer for use in high voltage applications, comprising the voltage transformer comprising a composite insulator with a first high voltage terminal and a post insulator comprising a second high voltage terminal configured for connecting a high voltage line. The invention further relates to a method for passive fault protection of a measuring voltage transformer or a power voltage transformer for use in high voltage applications, comprising the voltage transformer comprising a composite insulator with a first high voltage terminal and a post insulator comprising a second high voltage terminal connecting a high voltage line.

Background Art

Voltage transformers, VTs, have been used for about a century to transform high voltage down to a value easy to handle for protection relays and measuring instruments. Voltage transformer are usually designed to present a negligible load in order to guarantee high accuracy in terms of voltage ratio and phase displacement, and therefore to enable accurate reading from secondary side. Derived from voltage transformers, a station service voltage transformer, SSVT, is a line-to-ground connected single-phase transformer that is used for supplying single-phase loads. Station service voltage transformers are typically connected to high voltage, HV, lines through a disconnector or even without such disconnector.

In an unlikely case of an internal fault, for example a phase to ground short circuit, in the VT or SSVT, a HV network such as a grid would be temporarily perturbated by a short circuit current that would only be cleared when upstream circuit breakers, i.e. circuit brokers that energize the HV line, would open. As a result, the HV line would remain de-energized for seconds, until the disconnector is completely open or, in case there is no disconnector, the HV line would remain de-energized for hours or days, until the faulty VT or SSVT is dismantled from HV. During this time the HV line stays unavailable thereby causing troubles to the HV network and consequential economical losses.

Summary of invention

It is therefore an object of the invention to provide a passive fault protection arrangement for a measuring voltage transformer or a power voltage transformer for use in high voltage applications which reduces an outage time in case of an internal fault.

The object of the invention is solved by the features of the independent claims. Preferred implementations are detailed in the dependent claims.

Thus, the object is solved by a passive fault protection arrangement for a measuring voltage transformer or a power voltage transformer for use in high voltage applications, comprising the voltage transformer comprising a composite insulator with a first high voltage terminal, and a post insulator comprising a second high voltage terminal configured for connecting a high voltage line, whereby the voltage transformer comprises a moving contact electrically connected with a first end to the first high voltage terminal and a spring charged operating mechanism configured for moving the moving contact from a connected state, in which a second opposed end of the moving contact is electrically connected to the second high voltage terminal, to an unconnected state, in which the second end is electrically disconnected from the second high voltage terminal, and the post insulator comprises an activator configured for, if an overcurrent occurs in the high voltage line, electrically disconnecting the second end from the second high voltage terminal such that the spring charged operating mechanism moves the moving contact from the connected state into the unconnected state. A key point of the invention is therefore to provide a so-called passive fault protection, FPD, capability respectively device for reducing an outage time up to 0.3 seconds in case of internal fault, while such FPD does not have the functionality of a (classical respectively existing) circuit breaker, but may work in parallel, respectively technically in particular in series, with the circuit breaker. Said speed of the FPD guarantees that the high voltage, HV, line can be energized again after 0.3 seconds, in particular at a first reclosing of the circuit breaker. Such FPD is in particular realized by the proposed moving contact, operating mechanism and activator.

Thus, the FPD should be understood as a passive device that in particular does not interface with an existing protection and control system, for example does not depend on an external input or trip such that the existing protection and control system does not require changes when implementing the FPD. Specifically, the FPD is activated only by a short circuit current i.e. the overcurrent, which is typically thousands time higher than a nominal current absorbed by the voltage transformer, VT, or by a station service voltage transformer, SSVT. Thus, the proposed solution is also suitable for installations in remote and/or inaccessible areas. In sum, the FPD provide a simple and robust functionality, which increases a HV network reliability at affordable cost.

In other words, the FPD is intended to be a simple and robust device, composed by the activator which gets activated in presence of the short circuit current, the in particular fast-moving contact, which physically isolates a faulty voltage transformer in particular within a time between a fault current extinction is done by the existing circuit breaker and reclosing of the same circuit breaker, and the operating mechanism, which provides energy to the moving contact once the activator has detected an anomalous current. Thus, the FPD is completely passive and does therefore not require any engineering work on secondaries, such as the protection and control system.

The FPD can be used as additional protecting device not only in sub-station perimeter, but also in remote and inaccessible areas where the installation of any other type of protecting switchgear would be difficult, for example because other switch- gears would require an interconnection via fibre-optic cables to the existing protection and control system and that would be anti-economical or even impossible. This makes the proposed FPD ideal for rural electrification, in particular as a concept where limited amounts of power are tapped from the HV line directly to low voltage, LV, for household needs, providing to local population a low-cost access to electricity. With such FPD, utilities can increase reliability of their network at affordable cost.

In even other words, the proposed FPD can be installed onboard on each and every VT or SSVT connected to the HV line and being energized be the existing circuit breaker, CB, in particular with one CB with several FPDs. Thereby, the FPD withstands in a normal condition a nominal current absorbed by a transformer on a primary winding, in particular in a range of milli amperes for a VT and a few amperes for a SSVT. The FPD preferably withstands inrush current absorbed by the transformer at first energization, which is indicatively up to 20 times higher than the nominal current. The FPD may concurrently work with the circuit breaker that energizes the HV line.

Further, the FPD is passively activated, which means that the FPD device is in particular not connected to any intelligent electronic device, IED, triggered by a fault current, for example a tripping command received by an overcurrent relay, which fault current can be as low as for example 10% of the rated short circuit current of the network in a point where the transformer is installed. For example, a 245 kV network has typically a rated short circuit current of 50 kA. Therefore, the FPD could be activated with a current of 10% * 50 = 5000 A. As a result, the proposed solution is fast enough to isolate the faulty transformer in less than 300 ms, which may be a defined time for circuit breaker for rapid auto-reclosing. Conventional disconnectors are normally significantly slower in a range of a few seconds, while their operating time is not defined in IEC 62271 -102.

Once the CB has reclosed, the faulty transformer is isolated while other transformers of the network are re-energized again. The FPD can be replaced once activated, along with a replacement of the faulty transformer. In sum, the FPD may not have a breaking capability, but may require external intervention, such as from the upstream circuit breakers, to clear the fault current. Further, the proposed solution may guarantee a phase-to-earth insulation distance, instead of a phase-to-phase distance, as it is not considered constituting a classical disconnector. Lastly, the proposed solution can be adopted in any voltage levels as not being a fuse, while fuse standards typically cover up to 145 kV.

Voltage transformers typically consist of a single-phase design intended for connection between phase and ground on a, for example, 46 kV to 362 kV HV grounded neutral high voltage networks to supply power to panels at low voltage or medium voltage. Such design allows for convenient siting within a substation environment for simple mounting to single phase supports. Thereby, the voltage transformer is typically connected to the HV network and provides power to the panel within the substation, or to remote loads directly supplied for the HV service. Voltage transformers can provide output voltages from 120 V to MV levels, while power levels typically range from 1 kVA to 333 kVA and more. Voltage transformers, in particular service station voltage transformers, SSVTs, can be supplied for power only, or power plus optional metering windings having either single or two tapped windings for voltage measurement.

Spring charged operating mechanism means that a spring force impinges on the operating mechanism for moving the moving contact by the spring force into the unconnected respectively disconnected state. The spring can a housed in a housing or the like. Occuring an overcurrent means in particular that the overcurrent, respectively in particular when a current flowing through the moving contact exceeds a specified respectively predefined current level, releases the electrical connection between the second end and the second high voltage terminal such that the spring force moves the second end away from the second high voltage terminal towards the unconnected state of the moving contact.

According to a preferred implementation the spring charged operating mechanism is configured for swivelling the moving contact in a vertical and/or horizontal plane. Therefore, the moving contact is preferably attached in a swivelling manner to the composite insulator respectively to the first high voltage terminal. Thus, when the spring force has actuated the moving contact, the second end is swivelled away from the second high voltage terminal. Most preferably, the spring charged operating mechanism respectively the moving contact is attached to the composite insulator such that a swivelling axis matches the longitudinal axis of the comp side insulator.

In another preferred implementation the activator comprises a fused link arranged between the second end and the second high voltage terminal. The fused link is preferably provided as a metal rod having a diameter smaller, in particular much smaller, than the moving contact. The diameter of the fused link is preferably dimensioned such that the overcurrent destroys the fused link so that as consequence the moving contact is moved away from the second high voltage terminal by the operating mechanism.

According to a further preferred implementation the activator is provided destructive by the overcurrent and/or provided for single operation. Preferably, the fused link is provided destructive by the overcurrent and/or provided for single operation. In other words, the activator respectively the fused link is preferably provided not self-restoring respectively for operating only once in lifetime. Thus, when the activator interrupts the electrical connection between the high-voltage line and the fault transformer, in particular due to a fault of the voltage transformer, replacement of the faulty voltage transformer and the activated activator are preferably carried out at the same time

In another preferred implementation the moving contact is provided as metal rod. The metal rod extends between the first end and the second end. The metal rod may comprise a diameter of 2, 3, 4, 5, 10 or 15 cm and/or may comprise a length of 0,5, 0,75, 1 , 1 ,25 or 1 ,5 meter. Besides that other diameters and/or lengths are possible.

According to a further preferred implementation the composite insulator and the post insulator are each provided as bushings arranged parallel to each other. The bushing is in particular provided as an epoxy-impregnated fiberglass tube with silicone rubber sheds, for example according to IEC 60137. The first high voltage terminal and the second high voltage terminal is preferably provided on a top of the composite insulator and/or on a top of the post insulator. The composite insulator and the post insulator preferably extend vertically along their main axis. A bottom of the post insulator may be attached to a bottom of the composite insulator, for example by means of a metal sheet or rod extending in horizontal direction.

In another preferred implementation the voltage transformer is provided as a station service voltage transformer, as an inductive station service voltage transformer and/or as a gas insulated or oil insulated high voltage to medium or low voltage power voltage transformer. Preferably, the voltage transformer is provided as a gas insulated or oil insulated high voltage transformer, VT, or as a gas insulated or oil insulated high voltage power VT, which converts power from high voltage to low voltage or from high voltage to medium voltage. Station service voltage transformers, SSVTs, are intended to provide low voltage control power for substations, cell tower installations, and at switching stations by tapping directly from the high voltage line. Sizes may comprise 10 kVA to 333 kVA with primary voltages ranging from 46 kV to 550 kV and secondary voltages being, for example, 120/240 V AC, 240/480 V AC, 277 V AC, 600 V AC and others.

According to a further preferred implementation the passive fault protection arrangement comprises a circuit breaker connected on one end to the second high voltage terminal and configured for connecting on another end to the high voltage line. The circuit breaker may be provided as known from prior art, for example for serving a three-phase passive fault protection arrangement comprising three the passive fault protection arrangements.

The object is further solved by a tree phase protection arrangement comprising three passive fault protection arrangements according to any of the previous claims, whereby each of the three passive fault protection arrangements is connected to one of the three phases.

The object is even further solved by a method for passive fault protection of a measuring voltage transformer or a power voltage transformer for use in high voltage applications, comprising the voltage transformer comprising a composite insulator with a first high voltage terminal, and a post insulator comprising a second high voltage terminal connecting a high voltage line, whereby the voltage transformer comprises a moving contact electrically connected with a first end to the first high voltage terminal and a spring charged operating mechanism configured for moving the moving contact from a connected state, in which a second opposed end of the moving contact is electrically connected to the second high voltage terminal, to an unconnected state, in which the second end is electrically disconnected from the second high voltage terminal, and comprising the step of: if an overcurrent occurs in the high voltage line, electrically disconnecting the second end from the second high voltage terminal by an activator such that the spring charged operating mechanism moves the moving contact from the connected state into the unconnected state.

According to another preferred implementation the method comprises the step of: swivelling the moving contact in a vertical and/or horizontal plane for electrically disconnecting the second end from the second high voltage terminal.

In a further preferred implementation, the activator comprises a fused link arranged between the second end and the second high voltage terminal.

According to another preferred implementation of the method the activator is provided destructive by the overcurrent and/or provided for single operation.

In a further preferred implementation, the moving contact is provided as metal rod.

Further implementations and advantages of the method are directly and unambiguously derived by the person skilled in the art from the arrangement as described before.

Brief description of drawings These and other aspects of the invention will be apparent from and elucidated with reference to the implementations described hereinafter.

In the drawings:

Fig. 1 shows in a schematic view a passive fault protection arrangement according to a preferred implementation,

Fig. 2 shows in a perspective side view a passive fault protection arrangement according to another preferred implementation,

Fig. 3 shows in a perspective side view a portion of the fault protection arrangement according to Figs. 1 and 2,

Fig. 4 shows in a perspective side view a further portion of the fault protection arrangement according Fig. 3,

Fig. 5 shows in a schematic view three passive fault protection arrangements according to Figs. 1 and 2, and

Fig. 6 shows a timeline operating the passive fault protection arrangement according to Figs. 1 and 2.

Description of implementations

Fig. 1 shows in a schematic view a passive fault protection arrangement according to a preferred implementation. The passive fault protection arrangement is intended for a measuring voltage transformer or a power voltage transformer 1 for use in high voltage applications having, for example, 245 kV. The voltage transformer is provided as a station service voltage transformer, SSVT, comprising an aluminium tank attached to ground with a composite insulator 2 installed thereon, which is provided as a vertically extending bushing connected with one end to the aluminium tank and having a first high voltage terminal 3 arranged on an opposite top end of the composite insulator 2.

A vertically extending post insulator 4, which is also provided as bushing, is arranged in parallel and at a dielectric clearance distance, for example 1 m distant, to the composite insulator 2. A lower end of the post insulator 4 is attached to ground. Fig. 2 shows a further implementation of the passive fault protection arrangement in a perspective side view, which equals the implementation shown in Fig. 1 except that the aluminium tank is provided on a post installed on ground and in that the post insulator 4 is not installed on ground. Instead, the lower end of the post insulator 4 is connected via a horizontally extending metal sheet respectively metal rod 5 to a lower end of the composite insulator 2. The post insulator 4 comprises a second high voltage terminal 6, which is connected via a circuit breaker 7, indicated in Fig. 5, to a high voltage line 8.

The voltage transformer 1 further comprises a moving contact 9, which is provided as horizontally extending metal rod. A first end of the moving contact 9 is electrically connected to the first high voltage terminal 3 via a spring charged operating mechanism 10. Figs. 1 to 4 show the moving contact 9 in a connected state electrically connecting the first high voltage terminal 3 and the second high voltage terminal 6 via a second opposed end of the moving contact 9. The spring charged operating mechanism 10 holds the first end in a swivelling manner, which means that the moving contact 9 can be swivelled around an axis defined by the composite insulator 2 in a horizontal plane.

Now in particular referring to Figs. 3 and 4, showing a portion of the fault protection arrangement according to Figs. 1 and 2, with Fig. 4 showing a further portion of Fig. 3, the post insulator 4 comprises an activator 11 , which is provided as fused link 12 electrically connected between the second end of the moving contact 9 and the second high voltage terminal 6. In particular, the fused link 12 is provided as vertically extending metal rod have a much smaller diameter than the moving contact 9. Thus, in case an overcurrent occurs in the high voltage line 8, the fused link 12 becomes destroyed by the overcurrent and thereby interrupts the electrical connection between the second high voltage terminal 6 and the moving contact 9. As a consequence, the spring charged operating mechanism 10 due to the spring force swivels the moving contact from the connected state shown in Figs. 1 to 4 to an unconnected state, in which the second end is electrically disconnected from the second high voltage terminal 6.

Fig. 5 shows in a schematic view three passive fault protection arrangements according to Figs. 1 and 2, namely three voltage transformer 1 , one for each phase, connected via before described moving contact 9 including the spring charged operating mechanism 10 and the activator 11 together and via the circuit breaker 7 to the high voltage line 8.

Finally, Fig. 6 shows a timeline operating the passive fault protection arrangement according to Figs. 1 to 4. In case of an internal fault in a transformer of the voltage transformer 1 , the passive fault protection mechanism in particular realized by the moving contact 9, operating mechanism 10 and the activator 11 , referred to as FPD, fault protection device, is passively activated by the fault current, which can be as low as 10% of a rated short circuit current of a HV network supplying the HV line 8 in a point where the transformer is installed. For example, a 245 kV HV network has typically a rated short circuit current of 50 kA, therefore the FPD shall be activated with a current of (10%)* 50 = 5000 A. Thus, the FPD is fast enough to isolate the faulty transformer in less than 300ms, which is a defined time for the circuit breaker 7 for rapid auto-reclosing. Once the circuit breaker 7 has reclosed, the faulty transformer is isolated while the other transformers are re-energized again.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed implementations. Other variations to be disclosed implementations can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.

Reference signs list

1 voltage transformer

2 composite insulator 3 first high voltage terminal

4 post insulator

5 metal sheet

6 second high voltage terminal

7 circuit breaker 8 high voltage line

9 moving contact

10 operating mechanism

11 activator

12 fused link