WO2014135220A1

WO2014135220A1 - Protection of a thyristor valve

Info

Publication number: WO2014135220A1
Application number: PCT/EP2013/054771
Authority: WO
Inventors: Carl HEYMAN; Gunnar INGESTRÖM
Original assignee: Abb Technology Ltd
Priority date: 2013-03-08
Filing date: 2013-03-08
Publication date: 2014-09-12

Abstract

The present invention relates to an electrical arrangement comprising: a series capacitor (SC) connected in series with an electrical transmission line; a thyristor controlled reactor comprising a thyristor valve connected over and in parallel with the SC and arranged for controlling the series capacitor by regulating a current through the reactor by means of the thyristor valve; and a closing device comprising an electromagnetic coil based actuator, connected over the thyristor valve and arranged for protection of said thyristor valve by allowing the thyristor valve to be bypassed when the closing device has passed from an open state to a closed state.

Description

PROTECTION OF A THYRISTOR VALVE

TECHNICAL FIELD

The invention relates to an electrical arrangement comprising a series capacitor (SC) connected in series with an electrical transmission line, and a thyristor controlled reactor comprising a thyristor valve connected over and in parallel with the SC and arranged for controlling the series capacitor by regulating a current through the reactor by means of the thyristor valve.

BACKGROUND

Series capacitors (SC) are used for reactive power compensation in

alternating current (AC) transmission systems. In order to control the series capacitor, a thyristor controlled reactor (TCR) can be connected over the series capacitor to form a thyristor controlled series capacitor (TCSC). Such a TCSC is schematically shown in figure l, where a thyristor valve is connected over a series capacitor. A TCSC functions such that a thyristor valve is opened (activated) and conducts current past the SC at different phase positions to create different currents through a reactor which is series connected with the thyristor valve in the bypass loop over the SC, depending on which voltage is desired over the SC. A thyristor switched series capacitor (TSSC) has a similar main circuit setup but the thyristor valve is only blocked or fully conducting.

For protecting the series capacitor in overvoltage situations, a spark gap can be used for allowing fast bypassing of the series capacitor until a bypass switch can be closed. Additionally or alternatively, a metal oxide varistor (MOV) can be used. The publication "Field experience of encapsulated fast protective device for series capacitors", C. Heyman et ah, XII symposium of specialists in electric operational and expansion planning, 20-23 May 2012 in Rio de Janeiro, Brazil, discloses the CapThor™ based fast protective device concept of ABB. CapThor is a switch consisting of an arc plasma injector, a fast closing mechanical contact and an operation and supervision unit. CapThor does not require any electrode adjustments for project specific capacitor voltages or fault currents. It does not suffer from the conventional spark gap dilemma - electrodes having to be close enough for secure operation, but separated enough not to unintentionally spark over - as it does not require a high electrical field between the electrodes to operate. The modules consist of one Arc Plasma Injector (API) and one Fast Contact (FC) respectively and are enclosed in composite insulator housings. The two modules are connected in parallel and are compact when compared with conventional spark gaps.

Figure 2 illustrates a CapThor protected SC. In the figure, C refers to the series capacitor, Z refers to a MOV, D refers to a damping reactor, T refers to the CapThor and B refers to a bypass switch. The FC of the CapThor is based on a Thompson coil magnetic actuator.

SUMMARY

The inventors have now realised that the thyristor valve(s) of a TCSC, TSSC or similar electrical arrangements where an SC is connected in parallel with a thyristor controlled reactor may advantageously be provided with protection for avoiding overheating of the thyristor valve e.g. in case of a fault current, in addition to any general overvoltage protection of the SC. The thyristor valve is usually able to conduct the fault currents during the whole fault sequence. However, in new and future applications for TCSCs, the inventors see a need for improved thermal protection of the thyristor valve in view of higher fault currents. Thus, the present disclosure is related to protection of the thyristor valve of a TCSC, apart from any protection provided for the SC.

According to an aspect of the present invention, there is provided an electrical arrangement comprising: a series capacitor (SC) connected in series with an electrical transmission line; a thyristor controlled reactor comprising a thyristor valve connected over and in parallel with the SC and arranged for controlling the series capacitor by regulating a current through the reactor by means of the thyristor valve; and a closing device comprising an

electromagnetic coil based actuator, connected over the thyristor valve and arranged for protection of said thyristor valve by allowing the thyristor valve to be bypassed when the closing device has passed from an open state to a closed state.

According to another aspect of the present invention, there is provided a method of protecting a thyristor valve of an electrical arrangement, the method comprising: closing a closing device by means of an electromagnetic coil based actuator in response to a current passing through the thyristor valve, thereby bypassing the thyristor valve; holding the closing device closed for a period of time; and opening the closing device.

The method of the present invention may be performed by an embodiment of the TCSC of the present invention.

By means of the fast contact, the thyristor valve can be bypassed in case of e.g. a too high fault current through the valve, generating heat that risk damaging the valve. Thus, the purpose of the fast contact is to protect the thyristor valve, regardless of any protection of the series capacitor that may also exist. When the thyristor valve is activated (i.e. conducts current), the voltage over the thyristor valve is reduced to close to zero. The thyristor valve thus efficiently protects the SC from fault currents and over voltages.

However, if the fault current is large, the thyristor valve may be subjected to heat due to the high current passing through it. By means of the FC, the risk of overheating the thyristor valve is greatly reduced or removed. Thus, in case of a fault current, the thyristor valve protects the SC and the FC can then protect the thyristor valve. Since the fast contact comprises a Thompson coil actuator for closing the contact, the fast contact can obtain a higher closing speed compared with a traditional bypass switch but it does not require any voltage across it to operate which is the case for a spark gap. With the traditional spark gap the thyristor valve must first be blocked in order to create the necessary voltage across the spark gap for it to operate. This does not only creates a difficult synchronization problem but also makes it necessary that the valve must conduct until the next zero crossing and then block, which results in a high voltage across the thyristor valve. But with the Thompson coil based FC it is now made possible to bypass the valve in a very short time without the mentioned problems of a spark gap. By being able to bypass the thyristor valve, the valve can be protected and may be more cheaply produced since it may not need to conduct high fault currents.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which:

Fig l schematically illustrates a TCSC.

Fig 2 schematically illustrates a CapThor protected SC. Fig 3 is a schematic diagram of an embodiment of an electrical arrangement in the form of a TCSC, in accordance with the present invention.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

The TCR comprises a reactor and a thyristor valve controlling the current through said reactor. The thyristor valve can comprise a single thyristor or a plurality of thyristors, arranged to conduct a current in one or both directions when a voltage exceeds the voltage threshold of the thyristor(s) of the thyristor valve. In case of a fault current, the thyristor valve may conduct this as well, but for high fault currents, where the fault current may reach e.g. above 50 kA, the thyristor valve may need to be bypassed rapidly in order to avoid overheating. The thyristor valve may also be allowed to have a lower current rating (fault current duration) than without the fast contact and can thus be made cheaper.

The closing device is connected over the thyristor valve, and not over the series capacitor. The closing device may thus be connected in parallel with the thyristor valve, but in series with the reactor of the thyristor controlled reactor. The closing device has an electromagnetic coil based actuator, e.g. comprising a Thompson coil, allowing it to be closed very fast by magnetic action (why the closing device is herein sometimes called a fast contact, FC). In some embodiments, the closing device is arranged to pass from the open state to the closed state (closing to bypass the thyristor valve) in less than 10 ms (milliseconds) after having been activated e.g. by a fault current, e.g. in between 5 and 10 ms. In some embodiments, the closing device is arranged to be able to remain in the closed state for at least 100 ms, e.g. at least 300 ms or at least 500 ms, after having passed from the open state to the closed state. This allows the closing device to bypass the valve e.g. either throughout a whole transient fault current event or until a regular bypass switch can be closed (closing a bypass loop over the thyristor valve or over the whole TCSC) and/or until line breakers are able to disconnect the transmission line (the line which the series capacitor is connected in series with). In some embodiments, the electrical arrangement further comprises a bypass switch connected over the thyristor valve, arranged for allowing the closing device to return to the open state while still allowing current to bypass the thyristor valve. The bypass switch may be a regular bypass switch which is slower than the closing device. The bypass switch may e.g. be configured to close in less than 50 ms, e.g. less than 30 ms such as in between 20 and 30 ms. The closing device may thus close first, after which the slower bypass switch closes whereby the FC may open again while the thyristor valve is still bypassed. The bypass switch thus bypasses the thyristor valve but also the fast contact. As with the closing device, the bypass switch is arranged to bypass and protect the thyristor valve, regardless of any protection

additionally provided for the SC.

In some embodiments, the electrical arrangement further comprises an arc plasma injector (API) connected in parallel with the closing device. The closing device may thus e.g. be part of a CapThor unit arranged to protect the thyristor valve. The API may allow the thyristor valve to be bypassed and protected even faster than by means of the FC. The API may e.g. be triggered in less than 5 ms of a triggering event (e.g. a fault current), such as in between 1 and 4 ms. In some embodiments, the electrical arrangement is a thyristor controlled series capacitor (TCSC) or a thyristor switched series capacitor (TSSC), preferably a TCSC, but other arrangements comprising parallel connected series capacitor and thyristor controlled reactor are also contemplated.

Figure 3 schematically illustrates an embodiment of an electrical

arrangement in the form of a TCSC 1, of the present invention. A series capacitor 2 is connected in series with a high power AC transmission line. The SC 2 is controlled by a thyristor valve 3 connected in parallel with the SC 2. A reactor L (e.g. a damping reactor) is connected in series with the thyristor valve 3 (forming a thyristor controlled reactor with the thyristor valve), and in parallel with the series capacitor 2. A closing device 4 is connected over the thyristor valve 3 and is arranged to close a bypass loop for bypassing the thyristor valve 3 if needed. The bypass loop is arranged to only bypass the thyristor valve 3, not e.g. the reactor L. Thus, the FC 4 is connected in parallel with the thyristor valve 3, but in series with the damping reactor L. A bypass switch 5 may be arranged in parallel with the closing device 4, as well as with the thyristor valve 3, but in series with the reactor L. As discussed herein, the bypass switch 5 is slower than the closing device 4, but is able to close a bypass loop over the thyristor valve for a longer time period than may be convenient with the closing device 3 which is closed by means of e.g. a Thompson coil. One or several line breakers 6 may be arranged to connect and disconnect, the TCSC 1 from the AC line.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

1. An electrical arrangement (l) comprising: a series capacitor, SC, (2) connected in series with an electrical transmission line; a thyristor controlled reactor comprising a thyristor valve (3) connected over and in parallel with the SC (2) and arranged for controlling the series capacitor by regulating a current through the reactor (L) by means of the thyristor valve; and a closing device (4) comprising an electromagnetic coil based actuator, connected over the thyristor valve (3) and arranged for protection of said thyristor valve by allowing the thyristor valve to be bypassed when the closing device has passed from an open state to a closed state.

2. The electrical arrangement of claim 1, wherein the closing device (4) is arranged to pass from the open state to the closed state in less than 10 ms after having been activated.

3. The electrical arrangement of claim 1 or 2, wherein the closing device (4) is arranged to be able to remain in the closed state for at least 100 ms, after having passed from the open state to the closed state.

4. The electrical arrangement of any preceding claim, wherein the electrical arrangement (1) is a thyristor controlled series capacitor, TCSC, or a thyristor switched series capacitor, TSSC.

5. The electrical arrangement of any preceding claim, wherein the electromagnetic coil based actuator comprises a Thomson coil.

6. The electrical arrangement of any preceding claim, further comprising: a bypass switch (5) connected over the thyristor valve (3), arranged for allowing the FC (4) to return to the open state while still allowing current to bypass the thyristor valve.

7. The TCSC of any preceding claim, further comprising: an arc plasma injector, API, connected in parallel with the closing device (4).

8. A method of protecting a thyristor valve (3) of an electrical arrangement (1), the method comprising: closing a closing device (4) by means of an electromagnetic coil based actuator in response to a current passing through the thyristor valve (3), thereby bypassing the thyristor valve; holding the closing device (4) closed for a period of time; and opening the closing device (4).