WO2012100831A1

WO2012100831A1 - Circuit breaker apparatus

Info

Publication number: WO2012100831A1
Application number: PCT/EP2011/051161
Authority: WO
Inventors: Colin Donald Murray Oates; Eswar Kumar Chukaluri; William R. CROOKES
Original assignee: Alstom Technology Ltd
Priority date: 2011-01-27
Filing date: 2011-01-27
Publication date: 2012-08-02

Abstract

A circuit breaker apparatus (10) is for use in high voltage direct current (HVDC) power transmission and reactive power compensation. The circuit breaker apparatus (10) comprises first and second DC terminals (12, 14) for connection in use to first and second DC networks (22, 24); the circuit breaker apparatus (10) further comprising first, second and third circuit branches (16, 18, 20) connected in parallel between the first and second DC terminals (12, 14), the first circuit branch (16) including a circuit breaker (26), the second circuit branch (18) including at least one switching element (28) and at least one capacitor bank (30) connected in series, the or each switching element (28) being controllable in use to switch the second circuit branch (18) into and out of circuit, and the third circuit branch (20) including a surge arrester (36), wherein the capacitor bank (30) of the second circuit branch (18) is pre-charged to a predetermined voltage so that the second circuit branch (18) has a lower impedance than the first circuit branch (16).

Description

CIRCUIT BREAKER APPARATUS

Technical Field and Prior Art

This invention relates to a circuit breaker apparatus for use in high voltage direct current (HVDC) power transmission and reactive power compensation.

In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometre of the lines and/or cables. Conversion from AC to DC thus becomes cost- effective when power needs to be transmitted over a long distance.

The conversion of AC to DC power is also utilized in power transmission networks where it is necessary to interconnect AC networks operating at different frequencies.

In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion and one such converter is a voltage source converter. A voltage source converter typically includes one or more circuit breakers, which are employed to switch specific components of the voltage source converter out of circuit in response to various circumstances.

In one such circumstance, during operation of HVDC power transmission networks, voltage source converters may be vulnerable to DC side faults that present a short circuit with low impedance across the DC power transmission lines or cables. Such faults can occur due to damage or breakdown of insulation, movement of conductors or other accidental bridging between conductors by a foreign object.

The presence of low impedance across the DC power transmission lines or cables is detrimental to a voltage source converter because it can cause current flowing in the voltage source converter to increase to a fault current level many times above its original value. In circumstances where the voltage source converter was only designed to tolerate levels of current below the level of the fault current, such a high fault current damages components of the voltage source converter.

A circuit breaker may be used on the DC side of the voltage source converter to isolate the fault to allow the fault in the DC network to be repaired,

Circuit breakers may also be used in voltage source converters to minimise the detrimental effects associated with the generation of "earth- currents" .

In a common configuration of an HVDC network, AC power is converted to DC power using a rectifier station and transmitted over long distance DC power transmission cables before being converted back to AC power using an inverter station. These rectifier and inverter stations typically rely on the use of monopole or bi-pole voltage source converters to carry out the necessary power conversion.

A mono-pole voltage source converter includes DC terminals, which are respectively connected to a DC voltage and earth ground. During normal operation of the mono-pole voltage source converters at the rectifier and inverter stations, current flows between the earthed terminals of the two mono-pole converters. This "earth-current" can lead to electrochemical corrosion in buried metal infrastructure such as pipelines or affect the water chemistry in the vicinity of undersea earth electrodes.

A bi-pole voltage source converter includes DC terminals, which are respectively connected to DC voltages having different polarities. During normal operation, negligible "earth-current" flows between the earthed terminals of the rectifier and inverter stations. However, the development of a fault relating to either pole necessitates monopolar operation of the bi-pole voltage source converter, which in turn leads to an increase in "earth-current".

The flow of "earth-current" can be minimised by diverting the current to a metallic return interconnecting the earthed terminals of the rectifier and inverter stations. The diversion of this current is performed using a metallic return transfer breaker, which incorporates a conventional mechanical circuit breaker, to commutate the flow of "earth-current" from the respective earthed terminal to the metallic return.

However, during the operation of a circuit breaker in either of the circumstances mentioned above, the non-zero direct current flowing in the voltage source converter results in the formation of a sustained power arc between the opened breaker contacts, which reduces the efficiency of the circuit breaker. It is therefore necessary to use specialised DC circuit breaking equipment to increase the operational efficiency of the associated circuit breaker .

US 4,216,513 A discloses the use of an SF₆ alternating current circuit breaker connected in parallel to a resonant inductor-capacitor circuit. During the opening of the circuit breaker, the resonant circuit reacts against the negative impedance of the generated power arc to generate an oscillatory current alongside the direct current flowing through the circuit breaker. The oscillatory current continues to build up until it is equal to the direct current, at which point the net current flowing through the circuit breaker is zero, which leads to the quenching of the power arc and thereby allows the safe opening of the circuit breaker. The build-up of the oscillatory current can however be tenuous during its initial stages and this inherent uncertainty in achieving a zero net current leads to an unreliable circuit breaker.

DE 102005040432 Al discloses an apparatus including a circuit breaker connected in parallel with a series connection of a switch and a capacitor and with a surge arrester. To prevent the formation of a sustained arc in the circuit breaker, the switch is turned on during the opening of the circuit breaker to commutate the current in the apparatus to flow through the series connection of the switch and the capacitor instead of the circuit breaker. There are however difficulties in ensuring successful commutation of the current as a result of the hyperbolic and stochastic behaviour of the arc resistance.

In addition, the existing circuit breakers mentioned above are designed to cope with voltages of approximately lOOkV, which are much lower than DC transmission line voltages of 500kV to 800kV. To meet the power requirements of HVDC power transmission, these circuit breakers may be modified to include larger capacitive and inductive elements and surge arresters, but this would lead to increases in the overall size and weight and associated costs of the power plant.

Disclosure of the invention

According to an aspect of the invention, there is provided a circuit breaker apparatus for use in high voltage direct current (HVDC) power transmission and reactive power compensation, the circuit breaker apparatus comprising first and second DC terminals for connection in use to first and second DC networks; the circuit breaker apparatus further comprising first, second and third circuit branches connected in parallel between the first and second DC terminals, the first circuit branch including a circuit breaker, the second circuit branch including at least one switching element and at least one capacitor connected in series, the or each switching element being controllable in use to switch the second circuit branch into and out of circuit, and the third circuit branch including a surge arrester, wherein the or each capacitor of the second circuit branch is pre-charged to a predetermined voltage so that the second circuit branch has a lower impedance than the first circuit branch .

The provision of the pre-charged capacitor bank ensures that turning-on of the switching element in use results in the successful commutation of the current flowing between the first and second DC terminals from the first circuit branch to the second circuit branch. The subsequent reduction in current flow through the first circuit branch leads to the extinguishing of a power arc formed between the contacts of the circuit breaker. This not only minimises the risk of damage to the circuit breaker arising from a sustained power arc between its contacts, but also ensures a rapid response to instructions to carry out current interruption and thereby improves the efficiency of the circuit breaker apparatus .

Preferably, in use, the voltage across the pre-charged capacitor of the second circuit branch is opposite in polarity to the voltage across the first circuit branch before the second circuit branch is switched into circuit.

The circuit breaker apparatus may be configured in this manner so that the second circuit branch has lower impedance than the first circuit branch . In embodiments of the invention, the second circuit branch may further include at least one inductor connected in series with the or each switching element and the or each capacitor.

The inclusion of the inductor bank simplifies the design of the circuit breaker apparatus while helping to ensure adequate control over the rate of change in current in the second circuit branch. In particular, including the inductor bank means that the control of the current characteristics in the second circuit branch is not dependent only on the parasitic inductance of the circuit.

In other embodiments, the or each switching element may include a semiconductor device. In such embodiments, the semiconductor device may be an insulated gate bipolar transistor, a gate turn-off thyristor, a field effect transistor, an insulated gate commutated thyristor, an integrated gate commutated thyristor, or any other self commutated semiconductor switch .

Such semiconductor devices have a fast response time which allows the second circuit branch to be rapidly switched into circuit in response to the opening of the circuit breaker and thereby improves the efficiency of the circuit breaker apparatus.

In further embodiments, the surge arrester may have variable impedance and may be configured so that the third circuit branch has lower impedance than the first and second circuit branches when the voltage across the capacitor is equal to or larger than the knee voltage of the surge arrester. Configuring the surge arrester in this manner results in the commutation of the current flowing between the first and second DC terminals to the third circuit branch when the aforementioned conditions are met. This allows the surge arrester to minimise the current flowing through the circuit breaker apparatus and thereby complete the current interruption process.

Preferably the surge arrester is controllable in use to provide a voltage to oppose the flow of current in the third circuit branch.

This allows the surge arrester to reduce the current flowing between the first and second DC terminals to zero and thereby complete the current interruption process.

The surge arrester preferably includes at least one metal oxide varistor.

The non-linear electrical characteristics of metal oxide varistors are not only suitable for use in the surge arrester outlined above, but also allow the surge arrester to provide the requisite voltage to minimise the current flowing through the circuit breaker apparatus . Brief description of the drawing

Preferred embodiment of the invention will now be described, by way of non-limiting example, with reference to the accompanying drawing in which:

Figure 1 shows a circuit breaker apparatus according to an embodiment of the invention. Detailed description of the preferred embodiment

Figure 1 shows a circuit breaker apparatus 10 comprising first and second DC terminals 12,14 and first, second and third circuit branches 16,18,20 connected in parallel between the first and second DC terminals 12,14.

The first DC terminal 12 is connected to a first portion 22 of a DC network and the second DC terminal 14 is connected to a second portion 24 of the DC network. In normal operation the circuit breaker apparatus 10 allows, e.g. a DC power source connected to the first terminal 12 to supply a load connected to the second terminal 14. In one example of a fault condition the second terminal 14 becomes connected to ground and a fault current flows. Other fault conditions may also occur.

The first circuit branch 16 includes a circuit breaker 26, which is in the form of an SF₆ alternating current circuit breaker.

The second circuit branch 18 includes a power electronic switch (PES) , such as a thyristor 28, connected in series with a capacitor 30 and an inductor 32. The thyristor 28 is controllable to selectively allow current to flow in the second circuit branch 18. In particular, the thyristor 28 is normally maintained off (by not turning it on) so that the second circuit branch 18 is out of circuit.

In use the capacitor 30 of the second circuit branch 18 is pre-charged to a negative voltage so that the voltage across the capacitor 30 is opposite in polarity to the voltage across the first and second DC terminals 12,14. A charging unit 34, such as a DC power supply is connected in parallel with the capacitor 30 for the purpose of pre-charging the capacitor 30. The charging unit 34 can be decoupled from the capacitor 30 so that it does not charge the capacitor 30 during the operation of the circuit breaker apparatus 10 to interrupt current flow.

It is envisaged that, in other embodiments, the thyristor 28 may be replaced by a different PES in the form of a different semiconductor device. For example, an insulated gate bipolar transistor, a field effect transistor, a gate-turn-off thyristor, a gate- commutated thyristor, an insulated gate-commutated thyristor, an integrated gate-commutated thyristor or another self commutated semiconductor switch may be used .

It is also envisaged that in further embodiments, the capacitor 30 and inductor 32 may be replaced by a plurality of capacitors and inductors respectively so as to provide different values of capacitance and inductance and thereby modify the electrical characteristics of the circuit breaker apparatus 10 to suit the associated power application.

The third circuit branch 20 includes a surge arrester 36 having a metal oxide varistor.

During normal operation, i.e. non-faulted operation, current flows from the first DC terminal 12 to the second DC terminal 14 predominantly via the circuit breaker 26. This is because the second circuit branch 18 is switched out of circuit due to the thyristor 28 being maintained off and the surge arrester 36 has a very high resistance due to the electrical characteristics of the metal oxide varistor.

When a fault condition occurs (e.g. when the second portion 24 of the DC network becomes connected to ground) , it is necessary to interrupt the fault current now flowing between the first and second DC terminals 12,14.

A current interruption process is triggered by the receipt of instructions to open the circuit breaker 26. This results in the separation of the circuit breaker contacts, which leads to an increase in voltage across the circuit breaker 26 and thereby results in the formation of a power arc between the separated contacts. Consequently current continues to flow in the first circuit branch 16 via the circuit breaker 26.

The thyristor 28 of the second circuit branch 18 is then turned on to switch the second circuit branch 18 into circuit. When this happens, the current flowing between the first and second DC terminals 12,14 begins to commutate from the first circuit branch 16 to the second circuit branch 18. This is because the second circuit branch 18 presents a lower impedance than the first circuit branch 16 as a result of the capacitor 30 being pre-charged to a negative voltage.

The commutation of current from the first circuit branch 16 to the second circuit branch 18 leads to a reduction in current flowing in the first circuit branch 16. When the current in the first circuit branch 16 is reduced to a sufficiently low value, the power arc formed across the contacts is no longer sustainable. The subsequent extinguishing of the power arc enables the circuit breaker 26 to be safely opened without risk of damage to the contacts of the circuit breaker 26.

The rate of increase in current in the second circuit branch 18 upon the turn-on of the thyristor 28 is dependent on the total inductance value of the inductor 32 and the parasitic inductance in the circuit breaker apparatus 10. The total inductance value is preferably configured so that the rate of increase in current in the second circuit branch 18 is within the operational range of the thyristor 28 so as to avoid the risk of damage to the thyristor 28.

The flow of current in the second circuit branch 18 causes the negatively charged capacitor 30 to be charged towards a positive voltage. During the charging of the capacitor 30, the voltage across the capacitor 30 increases toward the voltage across the circuit breaker 26 while the voltage of the thyristor 28 reduces towards its on-state voltage, which is negligible compared to the capacitor voltage. When the voltage across the capacitor 30 turns positive, the capacitor 30 provides a voltage that opposes the current flowing through the second circuit branch 18 and thereby causes a reduction in the rate of increase in current in the second circuit branch 18. This continues until the voltage across the capacitor 30 is greater than the voltage across the first and second DC terminals 12,14, which in turns leads a reduction in current flowing between the first and second DC terminals 12,14.

The voltage across the capacitor 30 is imposed across the third circuit branch 20, which means that any change in voltage across the capacitor 30 leads to a change in voltage across the third circuit branch 20. Consequently, when the voltage across the capacitor 30 increases to a point where it is equal to the knee voltage of the surge arrester 36 (which preferably is configured to be higher than the DC Power Source voltage and, in particular, approximately 1.5 times higher) , the current flowing in the second circuit branch 18 begins to commutate from the second circuit branch 18 to the third circuit branch 20. This is because when the voltage across the third circuit branch 20 is equal to or exceeds the knee voltage of the surge arrester 36, the impedance of the surge arrester 36 drops to a level that is lower than the impedance in each of the first and second circuit branches 16,18. While there is current flowing in the surge arrestor 36 then the voltage across it is equal to the voltage across the circuit breaker apparatus 10.

In this manner the surge arrestor 36 limits the extent to which the capacitor 30 charges, and so helps to minimise the risk of the insulation of the various components in the circuit breaker apparatus 10 becoming compromised.

The extent to which the capacitor 30 is pre-charged preferably ensures that the impedance of the second circuit branch 18 is lower than the impedance of the first circuit branch 16 at least until the voltage across the capacitor 30 is zero. This allows the arc formed in the circuit breaker 26 to be extinguished before the voltage between the first and second DC terminals 12,14 becomes positive.

When the voltage across the capacitor 30, and hence the voltage across the surge arrester 36, is greater than the DC power source voltage it opposes the flow of current flowing in the third circuit branch 20. As a result, the current flowing between the first and second DC terminals 12,14 reduces to zero, i.e. the current flowing between the first and second terminals 12,14 is interrupted, as required.

The circuit breaker apparatus can be used in a variety of applications.

The circuit breaker apparatus can be used in a metallic return transfer breaker to ensure successful commutation of current from earthed terminals of two mono-pole converters to the metallic return interconnecting the mono-pole converters so as to minimise the detrimental effects of "earth-current" on the surrounding environment.

The circuit breaker apparatus can also be used as part of a multi-terminal DC network.

In the event of a DC power transmission line fault, the circuit breaker apparatus can be used to disconnect the affected electrical network from the multi-terminal DC network so as to allow continued power supply to other networks connected to the multi- terminal DC network. The circuit breaker apparatus can also be employed in a multi-terminal DC network to switch one or more electrical networks into or out of circuit with the multi-terminal DC network so as to control power flow within the multi-terminal DC network .

Claims

1. A circuit breaker apparatus (10) for use in high voltage direct current (HVDC) power transmission and reactive power compensation, the circuit breaker apparatus (10) comprising first and second DC terminals (12, 14) for connection in use to first and second DC networks (22, 24); the circuit breaker apparatus (10) further comprising first, second and third circuit branches (16, 18, 20) connected in parallel between the first and second DC terminals (12, 14), the first circuit branch (16) including a circuit breaker (26), the second circuit branch (18) including at least one switching element (28) and at least one capacitor (30) connected in series, the or each switching element (28) being controllable in use to switch the second circuit branch (18) into and out of circuit, and the third circuit branch (20) including a surge arrester (36), wherein the or each capacitor (30) of the second circuit branch (18) is pre-charged to a predetermined voltage so that the second circuit branch (18) has a lower impedance than the first circuit branch (16).

2. A circuit breaker apparatus (10) according to Claim 1 wherein, in use, the voltage across the pre-charged capacitor of the second circuit branch (18) is opposite in polarity to the voltage across the first circuit branch (16) before the second circuit branch (18) is switched into circuit.

3. A circuit breaker apparatus according to Claim 1 or Claim 2 wherein the second circuit branch

(18) further includes at least one inductor (32) connected in series with the or each switching element

(28) and the or each capacitor.

4. A circuit breaker apparatus (10) according to any preceding claim wherein the or each switching element (28) includes a semiconductor device.

5. A circuit breaker apparatus (10) according to Claim 4 wherein the semiconductor device is an insulated gate bipolar transistor, a gate turn- off thyristor, a field effect transistor, an insulated gate commutated thyristor, an integrated gate commutated thyristor, or any other self-commutated semiconductor switch.

6. A circuit breaker apparatus (10) according to any preceding claim wherein the surge arrester (36) has variable impedance and is configured so that the third circuit branch (20) has lower impedance than the first (16) and second (18) circuit branches when the voltage across the capacitor is equal or larger than the knee voltage of the surge arrester (36) .

7. A circuit breaker apparatus (10) according to any preceding claim wherein the surge arrester (36) is controllable in use to provide a voltage to oppose the flow of current in the third circuit branch (20) .

8. A circuit breaker apparatus (10) according to any preceding claim wherein the surge arrester (36) includes at least one metal oxide varistor .