The present invention relates to high voltage, direct current (HVDC) switchgear, and particularly to HVDC switchgear comprising circuit breakers and methods for operating such circuit breakers. BACKGROUND\

Circuit breakers are an essential component of electrical power networks. Their main purpose is to safely clear, when instructed by a protection system, any fault that might occur on any part of the electrical power network in order to protect operational personnel, connected equipment or the network itself. Direct current (DC) power transmission is used in a number of different applications. High- voltage DC (HVDC) is particularly useful for power transmission over long distances and/or interconnecting alternating current (AC) networks that operate at different frequencies. A first station may therefore transmit electrical energy to a second station over a DC transmission line, e.g. an overhead line or subsea or buried cable. The first station may generate the DC supply by conversion from a received AC input supply. The second station then typically provides conversion back from DC to AC. Each of the first and second stations may therefore typically comprise a voltage source converter (VSC) for converting from AC to DC or vice versa. The AC supply will typically have three phases, each of which is input to the VSC to contribute to the DC output.

In certain VSC designs, circuit breakers in the AC domain act as the primary protection in the event of a fault being detected in the network or the VSC. The circuit breakers operate by rapidly separating two contacts surrounded by an insulating medium in order to interrupt the current. However, current interruption does not occur immediately: there is a short arcing period in which current continues to flow between the opened contacts. In AC systems, the current naturally crosses zero twice in each cycle. When the current is zero, the magnetic energy stored in the system is also zero, and AC system circuit breakers rely on this property to extinguish the arc which forms between the contacts after they have opened. Certain faults, however, can cause a significant injection of DC into one or more of the AC phases. This means that the current in those phases may have a delayed zero crossing, or may never cross zero at all, making it difficult or impossible to operate the AC circuit breakers effectively.

US patent publication no 2014/0268942 addresses this problem, and describes an interface arrangement for coupling between an AC system and a DC system. A transformer is provided with a third set of windings in order to provide auxiliary power to a plant or converter station in which the interface arrangement is provided. Once a fault is detected, this third set of windings is short-circuited, reducing the voltage between the converter and the transformer and lowering the voltage on the phase arms such that zero crossings occur. The problem with this solution is that it requires an additional shorting device. This will not be desirable in all situations, and in general it is preferable to avoid extra circuit elements if possible. An alternative solution is therefore required.

SUMMARY OF THE INVENTION

The inventors have recognised that, while certain phases of the AC power may have delayed zero crossings or no zero crossings at all in the event of certain faults, at least one of the phases will have a zero crossing. What is more, the act of opening the circuit breaker for this AC phase ensures that zero crossings will occur on the other phases.

According to a first aspect of the invention there is provided a method of operating high-voltage direct-current (HVDC) switchgear in response to a defined type of fault. The HVDC switchgear provides electrical isolation for an interface between a multi- phase AC power transmission system and a DC power transmission system, and comprises a breaker for each phase of the AC power transmission system. The method comprises: following identification of the defined type of fault, identifying which of the multiple AC phases will experience a zero crossing of current and transmitting an open command to the breaker associated with the identified AC phase; and subsequently transmitting one or more further open commands to the breakers associated with the remaining phases after a time delay.

In a second aspect of the invention, there is provided a control system for operating high-voltage direct-current (HVDC) switchgear. The HVDC switchgear provides electrical isolation for an interface between a multi-phase AC power transmission system and a DC power transmission system, and comprises a breaker for each phase of the AC power transmission system. The control system comprises: one or more inputs for receiving signals from a fault detection system; one or more outputs for providing open signals to the breakers; and processing circuitry, responsive to a signal from the fault detection system identifying a defined type of fault, for identifying which of the multiple AC phases will experience a zero crossing of current, transmitting via the one or more outputs an open command to the breaker associated with the identified AC phase, and subsequently transmitting one or more further open commands to the breakers associated with the remaining phases after a time delay. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only, with reference to the accompanying drawings, of which:

Figure 1 illustrates an example of a DC power transmission network;

Figure 2 illustrates an example of a station according to embodiments of the invention; and

Figure 3 illustrates a method according to embodiments of the invention.

DETAILED DESCRIPTION

Figure 1 shows an example of a DC power transmission network 10 according to embodiments of the invention. The network 10 shown in Figure 1 comprises a DC link connected between two AC systems; however, the invention is applicable in general to any AC/DC coupling in power transmission systems. The network 10 comprises a first AC power source 12. For example, the first AC power source may be a power generator such as a wind farm, or an electric grid operating at a first frequency. The AC supply is provided to the upper arm and the lower arm of a station 13 (also known as a converter station). The station 13 acts to convert the AC supply to a DC supply for onward transmission on the DC network. In practice, the AC supply comprises multiple phases (e.g. three), with each phase being provided to the station.

Each arm has an AC breaker system 14a, 14b, which is operative under the control of a protection system (not illustrated) to break the circuit and stop current flow in the event of a fault being detected. Further detail concerning the operation of the protection system and the breaker system will be provided below. The AC supply is provided from the breakers 14a, 14b to transformers 16a, 16b, which serve to transform the voltage of the supply from that provided by the power source 12, to the voltage required by the converter design. The AC supply is then coupled from the transformers 16a, 16b to respective voltage source converters (VSCs) 18a, 18b, which serve to convert the input AC voltage to an output DC voltage. The VSC 18a on the upper arm of the station 13 generates a positive output DC voltage on a DC transmission cable 20a, while the VSC 18b on the lower arm generates a corresponding negative output DC voltage on a DC transmission cable 20b. Again, further detail will be provided below with respect to the operation of the VSCs.

The DC transmission cables 20a, 20b are connected at their distal ends to the upper and lower arms of a second station 21, which comprises corresponding components to the first station 13, arranged in the reverse order, so as to provide an output AC supply to a second AC network 28. That is, the second station 21 comprises VSCs 22a, 22b coupled respectively to the positive and negative DC transmission cables 20a, 20b, for converting the DC supplies to corresponding AC supplies; transformers 24a, 24b, for converting the AC voltage output from the VSCs 22a, 22b from that required for operation of the VSCs, to that required for the second AC network 28; and AC breaker systems 26a, 26b coupled to the transformers 24a, 24b, operating under control of a protection system (not illustrated) to break the circuit and stop current flow in the event of a fault being detected. The protection system controlling the AC breaker systems 26a, 26b will typically be different from the corresponding protection system controlling the AC breaker systems 14a, 14b in the first station 13, but could be the same without departing from the scope of the invention.

The system 10 shown in Figure 1 provides what is known as bipolar DC transmission. That is, each station has a pair of VSCs, each of which is coupled to ground and generates an output DC voltage such that the transmission system provides a pair of conductors (i.e. cables 20a, 20b), each at a high potential with respect to ground, connected in opposite polarity. However, the invention is equally applicable to alternative HVDC transmission schemes, including that known as asymmetrical monopole.

In asymmetrical monopole transmission schemes, each station comprises a single VSC converting an AC supply to a single DC supply or vice versa. One terminal of each converter is coupled to a high-voltage DC transmission cable, while the other is connected to ground. The DC power transmission network between stations may comprise a single cable held at a high potential with respect to ground (known as monopole and earth return), or a pair of cables— one held at a high DC potential, the other at ground (known as monopole and metallic return). The invention is also applicable to other HVDC schemes.

Figure 2 shows a station 100 according to embodiments of the invention. The station 100 generates a single-pole DC output signal, and can be employed in the network 10 shown in Figure 1. For example, the station 100 may be employed in either of the stations 13, 21 to generate either of the DC signals carried by cables 20a or 20b. Alternatively, the station 100 could be operated to provide the single-pole DC output signal for an asymmetrical monopole transmission network.

Figure 2 shows on the lefthand side an AC power supply, and in particular shows the multiple phase components of the AC supply, labelled A, B and C. The invention does not rule out the possibility of different numbers of phases (i.e. two or more), but the convention is for the AC supply to be formed from three separate phases. Each phase is input to a respective breaker 102a, 102b, 102c (collectively 102), each of which receives and is controlled by a control signal from a protection system 104. The operation of the protection system 104 will be described in greater detail below, but it may form part of the station 100 and control only components within the station, or part of the network as a whole.

The breakers 102 act, in the closed position, to conduct current from the AC transmission system into the station 100 and the converter circuitry therein. In the open position, the breakers 102 act to interrupt that load current, and any fault currents that may occur. The breakers operate by rapidly separating two contacts surrounded by a liquid or gaseous insulating medium in order to interrupt the current, and are generally classified according to the insulating medium used. For example, air blast (in which compressed air is used during separation to cool any arc that has formed and remove ionized gas), oil, sulphur hexafluoride and vacuum breakers are known. The invention is not limited to any particular type of circuit breaker, save that it be suitable for use in a HVDC converter station of a power transmission network.

Each of the AC phases A, B and C is then coupled to a transformer 106. The transformer 106 is operative to transform the voltage of the AC supply from that of the input power supply, to a value required for operation of the converter equipment. The transformer comprises a primary set of windings 108, coupled to receive the input AC power supply via the breakers 102 and inductors 105, and a secondary set of windings 110 coupled to the converter equipment for conversion to a DC supply. In the illustrated embodiment, the primary set of windings 108 is in a wye configuration with a grounded neutral point, while the secondary set of windings 110 is in a delta configuration. The neutral point may be connected directly to ground, or via a limitation element such as an impedance or a surge arrestor. A "wye-delta" configuration arranged in this way can provide some advantages, such as preventing third-harmonic currents from flowing in the supply line. However, alternative configurations may be used without departing from the scope of the invention. For example, the primary and/or secondary sets of windings 108, 110 may be arranged in delta or wye configurations, such that the transformer 106 in other embodiments may be arranged in "delta-wye", "wye-wye", or "delta-delta" configurations. Other arrangements using different configurations (i.e. neither delta or wye) may also be possible without departing from the scope of the invention.

Each of the transformed AC phases, now labelled U, V and W, is input to the respective arms of a converter 112. In the illustrated embodiment, the converter 112 is a voltage source converter (VSC), and in general the converter 1 12 may be any non fault-blocking type of VSC, such as a half-bridge modular multi-level converter (HB MMC) as illustrated, or a two-level VSC. In still further embodiments, the converter may employ a diode bridge rectifier.

In the illustrated bipole arrangement, the converter 112 comprises respective upper arms 114u, 114v, 114w (collectively 114), arranged in parallel, for coupling each of the phases U, V and W to a first pole 118 of the DC transmission system, and respective lower arms 116u, 116v, 116w (collectively 116), arranged in parallel, for coupling each of the phases U, V and W to a second pole 120 of the DC transmission system. The second pole 120 is coupled to ground such that the station 100 outputs a single DC signal VDC on pole 118.

Each upper arm 114 comprises a series-connected inductor 113u, 113v, 113w (collectively 113) and one or more cells also arranged in series. Each lower arm 116 similarly comprises a series-connected inductor 115u, 115v, 115w (collectively 115) and one or more cells arranged in series. The one or more cells may be collectively known as a "valve", owing to historical reasons in which mercury-arc valves or thyristor valves formed this part of the circuit. The illustrated embodiment shows a plurality of such cells in each arm, making the illustrated converter 112 an MMC type of VSC. It will be understood by those skilled in the art that multiple such converters may be connected together in series or in parallel within the station 100, without departing from the scope of the invention. Figure 2 comprises a zoomed- in portion showing one of the cells 124 in more detail. Each cell 124 comprises a first switch 121 coupled in parallel with a second switch 122 and a capacitor 123. The switches may be formed by transistors, which allow both turn-on and turn-off, such as insulated-gate bipolar transistors (IGBT). In operation, the first and second switches are operated in a complementary fashion, such that when the first switch 121 is closed the second switch 122 is open, and vice versa. In this way, the capacitor 123 can be switched into and out of the arm in which the cell is located. By careful switching of each cell in the upper and lower arms 1 14, 116 in a known manner, introducing one or multiple capacitors between the phases U,V and W and the poles 118, 120, a DC voltage can be generated from the three input AC phases.

Figure 2 shows a station 100 which is operative to produce a monopole DC output supply. As set out above, however, the invention is equally applicable to stations producing bipole DC output supplies. In the bipole configuration, respective breakers, a transformer and converters are provided to generate an output DC signal on a second cable, the output DC signal having an opposite polarity to that output on pole 118.

Further, it will be apparent to the skilled person that the station 100 may be operated as a rectifier, i.e. synthesizing a DC output supply from an AC input supply, or as an inverter, synthesizing an output AC supply from an input DC supply.

As with any system, faults can occur in the station shown in Figure 2 and it is important that the breakers 102 are able to operate effectively to prevent risk to equipment and operators. It is explained above that the breakers 102 are required to open at a zero crossing of current if the arc between the breaker contacts is to be successfully extinguished.

Figure 2 shows a highlighted area 125 (dashed box), that includes the secondary windings 110 of the transformer 106, the inductors 113, 115 in each arm of the converter 112, and all connections therebetween. In general, the highlighted area 125 includes all components between the secondary windings 110 and the cells 124 of each arm 114, 1 16.

One fault case is a single-phase-to-ground fault, in which one of the AC phases shorts to ground. If such a fault occurs in the highlighted area 125 of the station 100, it can cause a significant injection of DC current into one or more of the phases A, B, C, such that the current flowing through the breakers 102 for those one or more phases will have a delayed zero crossing or no zero crossing at all. This creates a serious problem for the correct functioning of the AC breakers 102, as the breakers require a zero crossing of current to open successfully. Figure 3 illustrates a method for operating the breakers 102 according to embodiments of the invention. The method may be carried out, for example, in the protection system 104.

In step 200, a signal identifying a particular type of fault is received. The station 100 and/or the network itself may comprise a fault detection system (not illustrated) which is operative to identify faults in the network and the station 100. The station 100 in general will comprise a plurality of sensors (not illustrated) which are operative to measure the voltage, current and other electrical parameters at various locations around the station 100 and the network. Thus a large amount of data is gathered on the current operation and performance of the station 100. The fault detection system is coupled to receive this information and identify faults, i.e. values of the sensed parameters which are unusual or otherwise indicative of a fault which is occurring. For example, the fault detection system may compare the sensed values to one or more predefined thresholds, or generate differential values (i.e. the differential between two sensed values) and compare those differential values to one or more predefined thresholds. In this way, the fault detection system is able to detect both the type of fault that has occurred, and the location of that fault.

In step 200, therefore, the protection system 104 receives a signal from the fault detection system identifying a particular type of fault. In one embodiment, that particular type of fault is a single-phase-to-ground fault, which occurs in the highlighted area 125, i.e. between the secondary windings 110 of the transformer 106 and the cells 124 in the arms of the converter 112. For example, such a fault may be detected based on measurements of phase-to-ground voltages; however, other methods are possible. In step 202, the protection system 104 identifies which of the three phases A, B, C will experience a zero crossing of current.

The phase can be identified in a number of different ways. For example, the protection system 104 may comprise memory storing a look-up table which contains information identifying the phase that will experience a zero crossing based on one or more of: the phase in which the fault was detected; and the phase sequence between the phases (i.e. the order of the phases A, B, C). By determining the phase in which the fault occurred (which may be indicated in the signal received from the fault detection system, for example), and with knowledge of the phase sequence, the protection system 104 is able to consult the look-up table and determine which of the phases A, B, C will experience a zero crossing of current.

In an alternative embodiment, the protection system 104 measures the currents flowing in each phase A, B, C, and identifies the phase which experiences a zero crossing of current. For example, the protection system 104 may measure the currents within a time window of the fault being detected, such as half a cycle of the AC signal. If a zero crossing of current is detected within that time window, then the phase in which the zero crossing occurs may be identified.

In a yet further alternative embodiment, the protection system 104 measures the currents flowing in each phase A, B, C and identifies the phase with the lowest current. For example, the protection system 104 may measure the currents within a time window of the fault being detected, such as half a cycle of the AC signal. The phase with the lowest value of current within that time window may be identified. The lowest value of current may be zero (i.e. a zero crossing occurs within the time window) or greater than zero (i.e. if no zero crossing occurs within the time window). In step 204, an open command is transmitted to the breaker 102 associated with the phase identified in step 202. The identified breaker 102 opens in response to the open command and, because it experiences a zero crossing of current shortly after opening, any arc which forms between the contacts of the identified breaker 102 is quickly extinguished.

Thus, one of the multiple circuit breakers 102 is successfully opened at the conclusion of step 204, while the breakers for the remaining phases (experiencing currents without zero crossings, or with delayed zero crossings) are closed. However, because one of the breakers is now open, the currents in the remaining phases will be reduced. The driving voltage in the faulty phase is reduced and therefore the DC components of current in the other phases start to reduce. Thus the breakers for these phases will see zero crossings of current.

In step 206, one or more further open commands are transmitted to the other breakers 102, after a time delay with respect to the first open command transmitted in step 204. In one embodiment, the time delay may be a predefined value, such as the time required for the breaker 102 identified in step 204 to open (e.g. 30-40 ms). In another embodiment, the time delay is based on the capability of the circuit breakers 102 to handle non-zero current cycles, i.e. cycles of the AC supply in which no zero crossing occurs. For example, if a circuit breaker is able to handle two cycles without zero crossings, and it observes a zero crossing after four cycles of the AC supply (based on simulation of the scenario in software), then an open command can be sent to the circuit breaker after a time delay of two cycles of the AC supply.

The present invention thus provides methods and devices for controlling the operation of HVDC switchgear. In the event of a particular type of fault, such as a single- phase-to-ground fault, the phase which experiences a zero crossing of current is identified, and an open command sent to the breaker associated with that phase. By opening one of the multiple AC breakers which has a zero crossing of current, the current flowing in the other phases will also have zero crossings. Open commands can thus be sent to the other AC breakers after a time delay, and the current flowing in all AC phases successfully stopped.

Those skilled in the art will appreciate that various amendments and alterations can be made to the embodiments described above without departing from the scope of the invention as defined in the claims appended hereto.