Bus-tie components able to interrupt very high short-circuit current in time period which is a fraction of current period, are known in the art.

The goal of fault current limiting components of this kind is to allow downsizing of both left and right bus-bar system, in comparison with the total short circuit current supplying a feeder as represented in figure 1.

The downsizing is reached by dimensioning each side of the switchboard for only the short circuit power of its relevant in-coming feeder. This downsizing is possible only when a very fast interrupting device is located in the bus-tie position. In case of a short circuit condition of the out-going feeder, e. g. in the right hand side of the switchboard in figure 1, the very fast interrupting device should be able to avoid any contribution to short circuit peak current coming from the other half of the switchboard not affected by fault conditions, i. e. the left hand side of the switchboard in figure 1.

This downsizing is extremely interesting in economic terms for new electrical installation, where all the new equipment can be purchased for a fraction of the total short circuit power supplying the complete installation plant, with corresponding relevant savings. Cost savings can also be achieved in case of installation up-grade, when an existing plant has to adapted to an increase demand, thus avoiding to up-grade and change all existing apparatus.

The above-described solution can be easily implemented by simply using a fuse in the bus-tie compartment. However, the fuse technology has certain limitations and it is not always possible to have fuses with a satisfactory nominal current.

Therefore, in the current practice, the use of a fuse in the bus-tie compartment is only valid for very small installations.

The solution for bigger installation foresees the use of a fuse, which is supplied

only by the short circuit current, while the nominal current is normally let through a low resistance parallel element.

This parallel element is an explosive-cartridge, which is detonated at the moment the short circuit condition is detected. A control device takes care of detecting this short circuit, by both measuring the current amplitude and its rate of rise.

The present state of the art, although satisfactory for the basic needs, has some disadvantages.

At the end of the short circuit intervention, the whole system, consisting of both the explosive cartridge and the fuse, needs to be replaced before allowing the distribution system to recover the complete functionality. This requires a maintenance operation, which takes time, and requires non-conventional spare parts, as are this explosive cartridge. During this time to maintenance, the system is operating on both side but not with the full short circuit power available, i. e. no loads are disconnected but some operating condition could not be possible due to voltage drop and startup limitations.

Also, all the working functionality is based on an explosive device, which in some countries can create difficulties in transportation and/or property permissions, or in others, simply be banned.

A further disadvantage is given by the fact that the explosive-cartridge requires an electronic control that is separated from the switchboard control system. This means in practical terms to have inside the switchboard at least two different electronic control devices with all difficulties related to this.

The goal of the present invention is to further improve the already valid present solution, by overcoming all the above-described disadvantages or limitation, keeping from the other side all of the advantages.

In particular, a further goal of the present invention is to have a system which allows automatic recovery of the complete system functionality immediately after the short circuit intervention.

Another goal of the present invention is to have a system which does not need the use of any explosive.

Still a further goal of the present invention is to have a system which integrates the specific fault current limiting control requirement inside the main switchboard control system.

These and other goals are achieved by the system according to the present invention, which relates to a novel type of fault current limiting device. The fault current limiting device of the invention is based on the use of combined fast switch and an electrical fuse in parallel; after a fault is detected the fast switch opens in a very short time and transfer the current to the fuse, which is able to blow out thereby interrupting the short-circuit current. Furthermore, an automatic system takes care of replacing the blown-out fuse set with a brand new one.

The fault current limiting system of the invention has the further following main advantages: - It increases the perceived value of the solutions by the customer by providing full system functionality restoration in a short time.

- Increase the potential market.

- It is cheaper than the present solutions.

The invention will be now described with more details. With reference to Figure 3, the system may include of some or all, but not only, the following devices: - Switchboard panel; - Fast switch disconnector ; - A number of sets of fuses; - Fuse revolver switch; - Circuit breaker; - Current sensor; - Voltage sensors; - Voltage transformer;

Switchboard control unit; Withdraw-able track.

The switchboard panel is the enclosure of the other entire components.

The fast switch device is a disconnector having the ability to curry the nominal current for unlimited time, not having any make and break capacity.

Its major characteristics is the capacity to open in a very short time if compared with the normal electrical apparatus, let us say, in a time in the order of one millisecond.

Another important characteristic is the feature of develops a high arc voltage during the opening phase. This arc voltage is used to fast commute (in a time in the order of hundred of micro-second) the current in the parallel low resistance path where a limiting fuse is placed.

In order to be so fast during the open operation a fast and powerful drive is used.

This drive can be based on Thomson coil (also called repulsion drive or electro- dynamic drive) or voice coil drive.

In alternative to the fast switch disconnector an explosive based cartridge can also be used.

The system requires the use of limiting fuses. For each phase several fuses can be used in parallel.

The fuse revolver switch is a disconnector, which function is to automatic exchange of the blown fuses at the end of the fault current limiter system opening operation on fault, avoiding in this way a long out of service of the Fault Current Limiting system in order to exchange the blown fuses with a new set.

The number of fuse sets available in the system can vary upon the expected rate of fault of the specific switchgear, or on the will to limit as much as possible the maintenance operation during time. This can vary from a minimum of 2 sets to a whatever maximum. 3 sets could be an optimum number.

The circuit breaker is used in order to;

- manage open operation made in absence of fault; - manage open operation in case of limited fault current which does not require fault current limiter operation; - act as final switch-off device at the end of the fault current limiter operation; - act as a back-up in case of malfunction of the Fault Current Limiting system.

The current sensors are generic current sensor, with good response time characteristics, to make able the electronic control to detect starting fault conditions.

Voltage sensors, which can be either voltage transformers or resistive or capacitive voltage divider or any other type, in order to allow information on the voltage presence on both side of the bus-bar. These sensors are required only in case of application of the Fault Current Limiting system as bus tie current limiter and for automatic restoration of the Fault Current Limiting operation after a Fault Current Limiting system intervention.

A Voltage transformer can be used as one of the possible supply of the system.

The second one can come from the substation auxiliary supply, which can have an UPS system.

The switchboard control unit is an electronic device, placed either on the switchboard panel or on the substation control room. In the first case the control is seen as distributed, in the second as centralised.

In the following all reference will describe the unit as a distributed system, but the centralised option is also possible. Task of this electronic device are various, some of them are: - Control of all component of the system; - Fast switch protection function; - Classic protection function acted through circuit breaker;

- Measurement of electrical quantities related to the switchboard.

Some of the described components can be mounted on a track, which can be extracted from the switchboard panel when maintenance operation is required.

Advantages of this solution is: - Easy access to all components mounted on the system; - Fast maintenance of the panel; - The truck itself allow to easily integrate two disconnector, which permit once the truck has been extracted, to operate on the fault current limiter system without difficulties or risk for the operator, without the need to shut down the electrical power to the complete switchboard (or one side of it).

In alternative to the truck a standard safety disconnector can be integrated on the opposite site of the Circuit Breaker. The Circuit Breaker in this case will be himself withdraw-able, so to reach, once the safety disconnector is open and the circuit breaker withdrawn.

In normal operating conditions, the fault current limiter truck is inserted, the Circuit Breaker, the fast switch are closed, or in case of alternative solution the safety disconnector is closed, and the Circuit Breaker inserted.

In case of fault the electronic device, through the current sensor signals, analyze the estimated amplitude of the fault, by using both the amplitude and the rate of rise of the signal. In case the foreseen fault overcome a certain threshold, than the command to open is given to the fast switch disconnector. Otherwise the signal to open is given, according to the discrimination definition, to the circuit breaker.

In case of out-going feeder protection this threshold is the max peak bearable by the switchboard, while in case of bus-tie, this is the portion of the max peak bearable by the switchboard that the opposite side of the switchboard can contribute to the fault. Furthermore, in case the switchboard is supplied by only one side because an in-coming feeder is open, then the fault current limiter can be disabled (by a blocking signal from the controller), if the remaining in-

coming feeder can not supply a short circuit power which can be dangerous for the other side of the switchboard.

Once open, the Fast switch disconnector generates a relatively high arc voltage across the contact that forces the current to commutate to the parallel path inside the limiting fuses and the fuse melts. After that the circuit breaker is open in its standard operating time, about lOOms,, and this complete the fault current limiter open operation.

With circuit breaker open, the fuse revolver switch commutates from the present, now blown-out set of three fuses to the next one.

Now the system is ready to automatically restore the full system functionality. In case the switchboard control unit finds the system health, then the system is automatically re-close.

In case the Fault current limiter is used as a bus tie, than system health means the presence of voltage on both side of the bus bar. In case the Fault Current Limiting system is used as out-going feeder protection, than this means the impedance on the load is higher than a certain threshold.

Once the fuse revolver switch is arrived to the last position, which means no more fuse sets are available in addition to the one in service a signal is issued (either a message, for example a SMS, is sent to the control system operator or an alarm is set on) in order to call for preventive maintenance, in order to avoid not having"good"fuse to use in case or further faults.

An optional protection in case of fast switch disconnector fault is foreseen, this protection can work on several principles, either on arc light, or on current detection or on pressure rise in the cubicle, so to inform the control system of the presence of the fault causing all of the in-coming feeder to open.