Patent Description:
The electricity distribution network (i.e. electrical grid) is a system to transmit and distribute electrical power from electrical suppliers (e.g. power stations) to electrical consumers (e.g. large industry or a home). The transmission network of the electrical grid operates at a high voltage, and is used for some transmission and sub-transmission routes. Distribution networks operate at medium voltage and lower voltages and constitute the network supplying the domestic and some commercial consumers. Known state of the art are <CIT> and <CIT>.

The network topology is mainly radial, originating from large power stations/producers. Feeders are part of the electricity distribution network and can typically supply about <NUM> customers. With the advent of renewable technologies such as rooftop solar arrays, customers can now supply power to the electricity distribution grid. Under certain circumstances power flow could flow out from a feeder - this could ultimately cause cable insulation failure or damage transformers and power lines. Thus, the possibility of feeder faults increases with a more distributed electricity supply network.

To protect feeders from faults it is known to have multiple isolating switches (reclosers) along the feeder length, and open points at the end of the feeders, such as a Normal Open Point (NOP). NOPs provide an interconnection with adjacent feeders, and operate like a standard switch that can electrically connect the adjacent feeders to each other.

An alternative to a NOP is a Soft Open Point (SOP). A SOP is a power electronic device usually using back-to-back voltage source converters (VSC) installed in place of a NOP. Unlike mechanically operated switches, such as a NOP, there are no physical opening and closing of contacts in an SOP. Instead the required real power (P) and reactive power (Q) flowing through a SOP is achieved through controlled switching of power electronic switches. Thus, the SOP controls the flow of power between the two adjacent feeders, and is always on. Hence the term, 'Soft' Open Point.

SOPs are typically made from arrangements of VSCs in varying ratings and quantity. There are various topologies of SOPs, however, the main flavours include back-to-back, multi-terminal, and unified power flow controllers (UPFC).

A number of studies have been carried out to investigate the effectiveness of SOPs in reducing power losses, balancing feeder loads, and network reinforcements, improving voltage profile, and increased distributed generation connections. However, the majority of these studies focus on the utilisation of SOPs during normal operating conditions of the network. For example, the paper "<NPL>et al) discloses the monitoring of a fault index at an SOP during normal network operating conditions in order to provide for instant detection of an arising fault.

A SOP operating as it would under normal operating conditions behaves like a current source during a fault on the network. The magnitude of current injected from the SOP is limited by the physical current limit of its power electronic switches. However, this current could interfere with existing protection co-ordination (e.g. feeder automation), since protection in distributed networks is largely current based. Therefore, the SOP is typically disconnected from the rest of the electricity distribution network during AC faults.

If a fault occurs on a feeder that is not connected to an adjacent feeder via a NOP or SOP, then hundreds of domestic and/or commercial customers could be left without power while the network operators send someone to investigate and fix the fault.

Feeder automation (FA) is a way of automatically restoring a feeder to an operating state after a temporary fault and redirecting power during a permanent fault. FA schemes improve the restoration time of the network during faults. FA schemes also ensure that the network follows predetermined steps to automatically isolate the faulted section. Power can then be re-routed to the unaffected feeder sections (reconnecting those consumers) through a healthy feeder by closing the NOP, or restarting a SOP.

Historically, FA schemes include switching devices, such as auto-reclosers, along the feeder. In the event of a fault these auto-reclosers detect the fault current flowing through them and open (disconnecting the fault from the rest of the electricity distribution network). Auto-reclosers then blindly attempt to close after a pre-defined period of time. If the fault is temporary then when the auto-recloser closes, the VSC/SOP/NOP resumes normal operation. If the fault persists, the auto-recloser detects the fault current again and re-opens. The auto-recloser then attempts to close again after a further pre-defined period of time. Each reclosing attempt is called a shot, and the fault is considered permanent after a set number of shots.

Disadvantages of this historical feeder automation methodology include the following:.

According to one aspect, the invention provides a soft open point, SOP, for an electricity distribution network comprising a first and second VSCs, and a DC link connecting the two VSCs. Each VSC can be configured to apply voltage corrections to a respective feeder of an electricity distribution network while the feeders are operating normally. The first VSC is configured to, when a first feeder connected to the first VSC is faulty, apply a diagnostic voltage to the first feeder and make current and voltage measurements at the connection of the first VSC to the first feeder while the diagnostic voltage is being applied. In this way, the SOP, which is normally used for applying voltage corrections to the feeder, is repurposed to diagnose a fault on the feeder.

The first VSC is configured to generate the diagnostic voltage from a DC voltage applied to the DC link, and the second VSC is configured to generate the DC voltage from a voltage picked up from a second feeder to which the second VSC is operatively coupled.

In some embodiments, the SOP includes processing means configured to estimate from the measurements and a per-unit-length impedance or resistance of the first feeder the distance along the first feeder from the point of application of the diagnostic voltage to a fault along the first feeder. The processing means may also or alternatively be configured to determine from the measurements the type of a fault on the first feeder. The processing means may be arranged to, in response to determining that the fault has cleared, send a command for a feeder isolation device to close. The processing means may be arranged to, in response to determining that the fault persists a predetermined time after being detected, send a command for a feeder isolation device to open. In some embodiments, the processing means comprises one or more processors executing instructions retrieved from associated memory. In some embodiments, the controller is external to the SOP.

According to another aspect, the invention provides a method of fault assessment for a feeder in an electricity distribution network. According to the method, supply of a voltage correction to a feeder from a voltage source converter, VSC, of a soft open point, SOP, is ceased. A diagnostic voltage is generated by the VSC and applied to the feeder. At least one voltage measurement is made at the connection of the VSC to the feeder. At least one current measurement is made at the connection of the VSC to the feeder. At least one characteristic of a fault on the feeder is determined from the measurements. In this way, the SOP, which is normally used for applying voltage corrections to the feeder, is repurposed to diagnose a fault on the feeder.

In some embodiments, determining at least one characteristic comprises calculating from the voltage and current measurements the distance along the first feeder from the point of application of the diagnostic voltage to the fault along the feeder. Alternatively or additionally, determining at least one characteristic comprises determining from the voltage and current measurements the type of the fault.

In all embodiments and aspects, it is determined from the voltage and current measurements that the fault has cleared and a command for a feeder isolation device to close is sent.

In some embodiments, it is determined from the voltage and current measurements that the fault has not cleared within a predetermined period of being detected and a command for a feeder isolation device to open is sent.

According to another aspect, the invention provides a voltage source converter, VSC, for connection to a first feeder in an electricity distribution network. The VSC is configured to, when a feeder connected to the VSC is faulty, apply a diagnostic voltage to the feeder and make current and voltage measurements at the connection of the VSC to the feeder while the diagnostic voltage is being applied. In this way, a VSC can be repurposed for diagnostic use.

According to a yet further aspect, the invention provides a voltage source converter, VSC, for connection to a first feeder in an electricity distribution network. The VSC comprising a voltage source configured to produce an output voltage, current and voltage sensors, and a controller configured to adjust electrical parameters of the output voltage. The controller is configured to, when a feeder connected to the VSC is faulty, direct the voltage source to apply a diagnostic voltage to the feeder and make current and voltage measurements at the connection of the VSC to the feeder via the sensors while the diagnostic voltage is being applied. In this way, a VSC can be repurposed for diagnostic use.

The diagnostic voltage may be fixed, in the sense that it is magnitude is nominally constant during the process of using the diagnostic voltage to assess a feeder fault.

By way of example only, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which:.

<FIG> shows an example of a Soft Open Point (SOP) <NUM> which could be connected to feeders. Specifically, SOP <NUM> is a back-to-back SOP. The SOP <NUM> comprises two Voltage Source Controllers (VSCs) <NUM>, <NUM>. Each VSC <NUM>, <NUM> can convert between DC and AC. The two VSCs <NUM>, <NUM> are connected via a common DC link <NUM>. VSC <NUM> is connected to the end of a feeder <NUM>, and VSC <NUM> is connected to the end of a second feeder <NUM>. The internal converter impedances of each VSC <NUM>, <NUM> are shown as Zc<NUM> and Zc<NUM> respectively.

Other types of SOPs can also be connected to feeders, such as a multi-terminal SOP, or a unified power flow controllers (UPFC) SOP, but others are possible. Although it is shown in <FIG> that SOP <NUM> comprises VSCs <NUM>, <NUM>.

<FIG> schematically illustrates the architecture of VSC <NUM>. It will be understood that VSC <NUM> is a two-level VSC. It is common (but not essential) for VSCs in a SOP to be of the same type, thus, VSC <NUM> is typically also a two-level VSC. Other VSCs can also be used in SOPs, such as multi-level converters. Multi-level converters are common for higher voltage level systems. It can be assumed that, for the purposes of this illustrative and exemplary description, that VSC <NUM> has the same architecture as VSC <NUM>.

As shown in <FIG>, VSC <NUM> has six arms of insulated-gate bipolar transistors (IGBTs), indicated <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, in a bridge configuration. Two-level VSC <NUM> could equally have six arms of high-power metal-oxide-semiconductor field-effect transistors (MOSFETs), or the arms might use another type of electronic switching device. The IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> in the arms of VSC <NUM> are commutated to control the flow of current, this is achieved through pulse width modulation (PWM). Smooth voltages are generated at the VSC <NUM> terminal using low pass filters <NUM>. Voltage measurements are taken by voltage sensors <NUM>, and current measurements are taken by current sensors <NUM>. The capacitors <NUM> and <NUM> limit the DC current ripple in the DC link <NUM>, and can provide for decoupled real power transfer. The VSC <NUM> can be connected to the feeder <NUM> through an isolating transformer (not shown). The VSC <NUM> can be similarly connected to the second feeder <NUM> through an isolating transformer (not shown).

Ignoring the power losses, the magnitude of real power flowing between the two VSCs <NUM>, <NUM> is equal during steady state operation. This is illustrated by the power balance Eqn. (<NUM>): <MAT> where P<NUM> is the real power flowing through VSC <NUM> and P<NUM> is the real power flowing through VSC <NUM>.

As may also be seen from <FIG>, the VSC <NUM> has a controller <NUM>. The controller <NUM> receives voltage and current measurements from the point of connection between the feeder <NUM> and the VSC <NUM>, to control the drive signals <NUM> of the IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. (To avoid cluttering the diagram, the connections delivering the drive signals <NUM> to the IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> are not shown in <FIG>. ) A number of control schemes will be known to the skilled person, such as the use of linear proportional-integral (PI) controllers to control the sinusoidal network quantities. The controller <NUM> is capable of controlling parameters such as real power, reactive power, frequency, AC terminal voltage, and DC link voltage. The controller <NUM> controls these parameters using the drive signals <NUM>.

<FIG> shows the feeder <NUM> in an electricity distribution network connected to the rest of the electricity distribution network at point <NUM>, and to the SOP <NUM>. The feeder <NUM> shows feeder isolation devices (e.g. reclosers) <NUM>, <NUM>, line impedances (Zg<NUM>, Zg<NUM>, Zg<NUM>), loads L<NUM> and L<NUM>, and a fault <NUM>. Alternatively, the feeder <NUM> can have any number of feeder isolation devices and loads. The line impedances (Zg<NUM>, Zg<NUM>, Zg<NUM>) are quantised representations of the impedance (or resistance) over a unit length of the feeder <NUM>.

VSC <NUM> can be configured by the controller <NUM> to operate in at least one of three operational modes; power control mode, diagnostic mode, and/or restoration mode. However, the VSC <NUM> can only operate in one mode at a time.

The controller <NUM> operates the VSC <NUM> in the power control mode during an unfaulted, grid-connected condition. In the power control mode, the VSC <NUM> controls the flow of power between the feeder <NUM> and the DC link <NUM>. Thus, the SOP <NUM> controls the flow of power between the two feeders. To control the flow of power, the controller <NUM> produces voltage reference signals using the inverse Park's transformation. The voltage reference signals can be used by the controller <NUM> to generate control signals <NUM> to fire the IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> to generate the three voltage phases (Va, Vb, Vc) of the VSC <NUM> at point <NUM>. Control of the power flow reduces the possibility that one feeder has a cable insulation failure caused by too much power in a feeder's cable.

The controller <NUM> operates the VSC <NUM> in the diagnostic mode during a fault condition. In a fault condition, the fault <NUM> is disconnected from the rest of the electricity distribution network by the feeder isolation device <NUM>. In the diagnostic mode, the controller <NUM> can determine the presence of a fault <NUM> on the feeder <NUM>, the type of fault <NUM>, and/or the distance along the feeder <NUM> to a fault <NUM>, from the connection of the VSC <NUM> to feeder <NUM>.

The feeder <NUM> voltage under normal operating conditions is roughly the voltage of the electricity distribution network as a whole, and similar to the voltage the VSC <NUM> generates in the power control mode. In diagnostic mode, the controller <NUM> of the VSC <NUM> is configured to apply a diagnostic voltage to the feeder <NUM>. The diagnostic voltage is an attenuated version of the feeder <NUM> voltage under normal operating conditions. For example, the diagnostic voltage could be <NUM>% of the feeder <NUM> voltage under normal operating conditions. With the VSC <NUM> applying a diagnostic voltage it is possible to make current and voltage measurements on the feeder <NUM> using voltage and current sensors <NUM>, <NUM> at the connection point <NUM> of the VSC <NUM> to the feeder <NUM>.

The diagnostic voltage is used so as to not interfere with existing protection co-ordination, since protection in electricity distribution networks is largely current based. A diagnostic voltage also ensures no physical damage (e.g. wear) to connected components on the feeder <NUM> (e.g. power electronic switches, devices, etc.).

Phasors and phasor diagrams are a method of describing an AC voltage or a current in terms of its amplitude and its relative phase. For three-phase systems, the three phases a, b, c, can be plotted relative to each other. A three-phase system with three unbalanced phasors can be resolved into three symmetrical components:.

For the three-phase electricity distribution network in a normal operation, there are only positive sequence components, with no negative and zero sequence components, neglecting the imbalance introduced by the load. During a fault <NUM>, the voltage and current phasors of the feeder <NUM> will be unbalanced and can thus be represented by the three symmetrical components. Negative sequence components are present for unbalanced faults. The zero sequence components exist in the network only if a ground path is available for the flow of currents.

<FIG> illustrates the feeder <NUM> having a fault <NUM>. The VSC <NUM>, of the SOP <NUM>, is operating in the diagnostic mode. Specifically, the VSC <NUM> connected to the feeder <NUM> behaves as a voltage source. Therefore, the sequence voltage components (i.e. positive, negative and zero sequence) are quantified to confirm the continued presence of a fault <NUM>. The voltage and current are measured by the controller <NUM>, at points <NUM> and <NUM> respectively. The controller <NUM> uses these voltage and current measurements to calculate the following Fault-Index (FI): <MAT> where x represents phase a, b, or c. <MAT> is the root mean square (RMS) value of positive sequence voltage, and <MAT> is the RMS value of the negative sequence voltage and <MAT> is the zero sequence voltage. <MAT> is the nominal RMS voltage of the VSC <NUM> during diagnostic mode.

It is necessary to clearly distinguish between voltage imbalance and a fault. The FI is a ratio that is equal to <NUM> when there is only the positive component (i.e. no voltage imbalance). In reality there will always be some voltage imbalance depending on the loads and the topology of the electricity distribution network so any threshold to distinguish between voltage imbalance and a fault will be tuned to the deployment environment. However, a value that is considered generally acceptable by the present inventors is FIthreshold = <NUM>, and so any value of FI < <NUM> may be used to indicate the presence of a fault in the network. This normalized, dimensionless FI is applicable for any network by using the measured sequence quantities and corresponding nominal voltage of the network under consideration. In the diagnostic mode, the controller <NUM> continuously calculates the fault index to determine whether the fault remains present.

The measured phase voltage and the line current at the grid connection point <NUM> depends on the type of fault. Each type of fault is uniquely characterised by three conditions of respective phase voltage and line currents. The table below shows the conditions for three types of faults; a) a line-to-ground fault (e.g. 'La - G' when line 'a' is connected to ground); b) a line-to-line fault (e.g. 'La - Lb' when line 'a' and line 'b' are connected); c) a three-phase fault (e.g. 'La - Lb - Lc' when line 'a', line 'b', and line 'c' are connected). Similar equations can be written for faults involving other phases. The apparent positive sequence impedance can be calculated by using the fault-loop applicable for the respective fault type. The fault-loop is a well-known fault analysis method. (An example of fault-loop in literature is in "<NPL>. ) The corresponding equations to calculate the Vg and Ig values are shown in Table <NUM> below.

In Table <NUM>, the zero sequence line impedance is represented by <MAT> and positive sequence line impedance is represented by <MAT>. For line-to-ground faults, the phase to neutral voltage is used and the current includes zero sequence components ( <MAT>). For phase faults, phase to phase voltages and currents are both used. <FIG> shows the voltage and current phasors for the three types of faults above in a network with loads not neglected (i.e. a realistic application). The use of threshold values can be used so that Table <NUM> applies to a realistic application. For example, in <FIG>, a small enough voltage or current condition could be approximated to zero using a threshold value. Using this technique the type of fault can be calculated using Table <NUM> above.

In the diagnostic mode, the controller <NUM> estimates the location of a fault by calculating the apparent impedance of the network with single end measurement (e.g. from sensors <NUM>, <NUM>). Algorithms based on impedance calculation using single end measurement are advantageous since they are simple to implement, do not require any communication or remote data and can deliver reasonably accurate results. <FIG> shows an equivalent single line diagram of the feeder <NUM>. Vg and Ig are calculated from the measured phase voltage and the line current at the grid connection point <NUM> (measured by sensors <NUM>, <NUM>). From Vg and Ig it is possible to calculate the apparent positive sequence impedance Zapp of the line between the grid connection point <NUM> and the fault <NUM>. Using Kirchhoff's law, the impedance seen from grid connection point <NUM> can be mathematically expressed as shown in Eqn. <NUM>: <MAT>.

<NUM>, the distance to the fault can be calculated by the controller <NUM>. d is the per unit distance of the fault from grid connection point <NUM> defined with the total feeder length (D) as a base. For example, if the fault is a quarter of the way along the total feeder length, <MAT>. The fault current at the location of the fault <NUM> is represented by If. The fault resistance is represented by R, and the total feeder impedance is represented by Ztotal.

Other ways of determining fault location are possible and are known to the skilled person in the art, such as the travelling wave method, or a method using digital fault recorders for example.

The VSC <NUM> operates in the restoration mode when there is a fault <NUM>, and the fault <NUM> has been isolated from the rest of the electricity distribution network (e.g. by feeder isolation device <NUM>) and from the VSC <NUM> (e.g. by feeder isolation device <NUM>). The restoration mode is used to resume power supply to the un-faulted out-of-service loads, thus restoring power to some electrical consumers (e.g. large industry or homes). <FIG> shows this un-faulted out-of-service load L<NUM> connected to the SOP <NUM>. The controller <NUM> is then used to generate the converter terminal voltage (Va, Vb, Vc) through inverse Park's transformation, similar to the power control mode.

The power control mode cannot be used since the voltage of the VSC <NUM> is no longer dictated by the electricity distribution network. Therefore, the controller <NUM> implements a strategy to generate and control the terminal voltage and the grid frequency to be within the standard operating range for an un-faulted feeder. The VSC <NUM> connected to the un-faulted feeder <NUM> continues to operate in the power control mode; controlling the flow of power between the second feeder <NUM> and the DC link <NUM>. This is to maintain the DC link voltage as the VSC <NUM> draws power from the DC link voltage in order to supply the un-faulted out-of-service loads.

<FIG> shows a method of operation for the VSC <NUM> which is capable of operating in a diagnostic mode. When there is no fault on the feeder <NUM> the VSC <NUM> (also by extension the SOP <NUM>) operates in the power control mode <NUM>. When a fault <NUM> occurs, the controller <NUM> which is continuously processing the Fault Index (FI), indicates that there is a fault <NUM> on the feeder <NUM>. The VSC <NUM> then shuts down until the fault has been isolated from the rest of the network. Using the FI, the controller <NUM> can detect when the fault has been isolated and then changes its operational state to the diagnostic mode <NUM>, and a timer is started. Alternatively, the isolation device can command the VSC <NUM> to change its operational state to the diagnostic mode <NUM>. If the fault <NUM> resolves itself (i.e. FI is maintained above the FIthreshold continuously for a set number of AC cycles [e.g. <NUM>], called the confirmation time ΔTr) within a pre-defined duration ΔTp (from the start of the diagnostic mode) then the VSC <NUM> changes operational state back to power control mode <NUM>. ΔTp defines the duration for a fault to be considered permanent, and it will be tuned to the deployment environment. A ΔTp on the magnitude of seconds (e.g. <NUM> seconds) can be long enough for any temporary faults to resolve themselves. If the fault <NUM> is permanent, then the VSC <NUM> changes operational state to restoration mode <NUM>.

<FIG> shows how the method of <FIG> can be implemented in the cases of both a temporary fault <NUM> and permanent fault <NUM>. The feeder isolation device <NUM> of <FIG> is represented in <FIG>, the feeder isolation device <NUM> of <FIG> is also represented. The graphs <NUM>, <NUM> corresponds to the SOP <NUM> mode of operation.

It can be seen in <FIG> that the power control mode <NUM> halts as soon as a fault <NUM> is detected. After time Δt the diagnostic mode <NUM> starts, feeder isolation device <NUM> opens <NUM>, disconnecting the faulty feeder <NUM> from the rest of the electricity distribution network. If the fault <NUM> is of a temporary type <NUM>, a command <NUM> is sent (from controller <NUM>) for the feeder isolation device <NUM> to close <NUM>, and the power control mode <NUM> resumes.

If the fault <NUM> is of a permanent type <NUM>, then after time ΔTp from the start of the diagnostic mode <NUM>, a command <NUM> is sent (from controller <NUM>) for the feeder isolation device <NUM> to open <NUM>. Thus, isolating the fault <NUM> so the SOP <NUM> can operate in restoration mode <NUM> to reconnect customers that were disconnected due to the fault <NUM>. The isolation of the fault <NUM> can also allow for further manual investigation and repair, while ensuring the maximum number of customers are connected to the electricity distribution network.

An advantage of a diagnostic mode being used by VSCs and SOPs is that in the event of a fault on a feeder, an improved feeder automation methodology can be implemented.

The main advantages in comparison to the restoration using auto-reclosers include the following:.

It is described above that the VSC <NUM> is connected at the end of a feeder, however, in an alternate embodiment, the VSC <NUM> could be connected at any point along the feeder. This embodiment would require extra feeder isolation devices while the VSC <NUM> is in a diagnostic mode, to ensure a single path to a fault.

The VSC <NUM> draws power from the DC link <NUM> in order to perform its operations in the power control, diagnostic and restoration modes. In an alternative embodiment, the VSC <NUM> does not have to be part of a SOP <NUM>, as long as the VSC <NUM> is connected to suitable DC supply of some sort.

As described above, the controller <NUM> not only synthesises the drive signals <NUM> that control the IGBTs <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> to operate the VSC <NUM> in the power control, diagnostic and restoration modes, but also carries out the necessary analytic processing of the voltage and current measurements obtained by sensors <NUM> and <NUM> in order to determine the continuing presence, type and location of a fault. In other embodiments, some or all of this analytic processing is done outside the SOP <NUM> in another computer or processor, to which one or more of the voltage and current measurements, or one or more values synthesised therefrom, have been sent. Such a variant is shown, at a high level, in <FIG>.

In the VSC shown in <FIG>, features carried over from VSC <NUM> of <FIG> retain the same reference numerals. In <FIG>, the functionality of the controller <NUM> of <FIG> has been divided between internal controller <NUM> and external controller <NUM>, which communicate via connection <NUM>. Connection <NUM> could, for example, be a cable directly connecting the controllers <NUM> and <NUM>. Alternatively or connection <NUM> could be a logical connection, fulfilled physically via a telecommunications network and/or the internet. Internal controller <NUM> receives current and voltage measurements from the sensors <NUM> and <NUM> and processes them to make any feedback adjustments to the drive signals <NUM> that may be required for the SOP to perform correctly in whichever of the power control, diagnostic and restoration modes the SOP is currently operating. The current and voltage measurements from the sensors <NUM> and <NUM> collected by the internal controller <NUM> are also communicated to the external controller <NUM> via the connection <NUM>. The external controller <NUM> carries out the aforementioned diagnostic-mode analytic processing to determine the continuing presence, location and type of feeder fault.

Claim 1:
A soft open point (<NUM>), SOP, for an AC electricity distribution network comprising a first and second voltage source converters (<NUM>, <NUM>), VSCs, and a DC link (<NUM>) connecting the two VSCs, wherein:
in a power control mode, each VSC (<NUM>, <NUM>) is configured to apply voltage corrections to a respective AC feeder (<NUM>, <NUM>) of an AC electricity distribution network while the AC feeders are operating normally; characterised in that:
the first VSC (<NUM>, <NUM>) is configured to, when a first AC feeder (<NUM>, <NUM>) connected to the first VSC is faulty, operate in a diagnostic mode to apply a diagnostic AC voltage to the first AC feeder (<NUM>, <NUM>) and make current and voltage measurements at the connection of the first VSC to the first AC feeder while the diagnostic AC voltage is being applied, wherein the diagnostic mode is used to continuously determine whether the fault remains present; and
in the diagnostic mode, if the fault resolves itself within a pre-defined duration, then the first VSC (<NUM>, <NUM>) changes operational state to the power control mode.