The development of a Silicon Carbide (SIC)-powered electronic device.
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3.3. Types of FCLs
Work on the modification of FCLs’ performance has attracted a lot of researchers. This helps in finding new enhanced types that match with the applications of higher ratings. Generally, FCLs can be classified as:
3.3.1. Fault Current Limiting Reactor
It is the simplest one that consists of a limiting coil
[35]. This coil has large inductive reactance and low ohmic resistance
[29]. Such FCLs can be divided into two types: air core and iron core, where the limiting impedance depends on the magnitude of the fault current. Unlike the iron core, the air core does not suffer from saturation, so its impedance is independent
[29].
3.3.2. Pyrotechin Fault Current Limiter (Is-Limiter)
The Is-Limiter, which was created in 1995, consists of an ultrafast-acting switch and a contactor connected in parallel to a high interrupting fuse
[29]. At a fault condition, a high external trigger provides the main path for the fault current to transfer to the fuse. This fuse can limit the fault current to 0.5 ms and then interrupt it in the next zero-voltage moment
[39]. Finally, the Is-limiter is disconnected after operation by the service. This operation needs an electronic measurement device as an additional circuit to determine if the passing current needs to be interrupted or not. This limiter can let the existing devices run as they are not replaced but it is not non-resettable
[29].
3.3.3. Superconducting Fault Current Limiters (SCFCLs)
Superconducting materials are those that can carry electrons from one atom to another without any resistance
[39]. These materials have two states, the normal and superconducting states, where the transition from one state to another mainly depends on the current, magnetic field, and temperature. Below the critical current
[40], the material has no resistance (superconducting state), while, when the current increases until it reaches its critical value, the quench operation occurs, and the material transfers to its normal state. In this state, there is a great resistance besides high temperature, so a cooling system must be created. According to the temperature generated, superconductors can be categorized as low-temperature superconductors (LTSC) and high-temperature superconductors (HTSC)
[41][42]. For LTSC, the transition temperature is below 25 Ko, and the cooling system, which is liquid helium, reduces the temperature to 4.2 Ko. The new version of superconductors, HTSC, can be cooled by liquid nitrogen and can operate at 77 ko
[29].
The nature of the superconducting materials has been harnessed to be implemented in FCLs where the first SCFCL is 12–100 FCL. It can operate at 12 kV and 100 A. This has been fabricated by Nexans superconductors (NSC) GmbH and installed in Bamber Bridge, UK as a busbar coupler. The second one has been developed to work in 12 kV and 800 A systems. It is the first HTS device to work in a thermal power plant. Both SCFCLs have been used since the last quarter of 2009
[43]. According to the structure and operation, SCFCLs can be divided into inductive and non-inductive-type SCFCLs. Non-inductive-type SCFCLs consist of two superconducting coils, which can be formulated as a current-limiting coil and a trigger coil. The two coils are connected antiparallelly and magnetically coupled
[17]. There are many configurations, such as coaxial coil and bifilar winding, which are superior as they have a high impedance ratio. On the other hand, the inductive type consists of primary and secondary coaxial coils with a magnetic core
[44]. The primary coil is made of copper, while the secondary one is made of HTS.
Inductive Shield SCFCL
It is about the secondary side, which is the superconducting coil (SC coil) and the bypass winding lapped around the iron core, and the primary side as the power line
[29]. During the normal operation, where the material appears in its superconducting state, the secondary coil shields the iron core from the primary coil. This means that the magnetic flux generated by the primary is not able to penetrate the iron core, so the impedance transferring to the primary is very low. On the other side, the superconducting coil losses its superconductivity at the fault condition so the magnetic field can pass through the iron core, forming a high impedance at the primary side. In this state, bypass winding can provide a path for the flowing current. This in turn reduces the energy produced in the SC coil and produces a counter-induction, reducing the current in the primary coil. Generally, the idea of this device depends on the full diamagnetism of the superconducting materials, which was first discovered by Meibner and Oshsenfeld
[29].
Saturated Iron Core-Type SCFCL
It is composed of two iron cores (each for a half cycle), AC windings, superconducting DC winding wrapped around each core, DC power, and a control circuit
[29]. During normal operation, the DC source can feed the superconducting winding which in turn produces DC magnetic field. This field saturates the two iron cores; therefore, there is no impedance, whereas when a disturbance occurs, the control circuit plays its role as it can disconnect the DC magnetizing coil just after the fault happens. This fault current produces a large inductive electromagnetic force in the two AC coils, which in turn limits the current. This type has superiority, as the superconducting material does not have to transfer to its normal state.
Transformer-Type SCFCL
The primary side of the transformer is connected in series with the load, while the secondary one is connected in series with superconductors
[39]. The transformer is connected to a vacuum interrupter as a switch, where L1 and L2 are the self-inductance of the primary and secondary, respectively, and M is the mutual inductance between the two coils. During normal operation, the material can exist in its superconductivity so there is no impedance. However, at the fault condition, quench operation occurs, and the current is limited in the secondary side, so it is limited dependably in the primary one. This type is implemented to enhance both the system stability and reliability, as in
[45]. The transformer type has some advantages as it provides isolation between the power line, and the current-limiting part beside it has a flexible design
[46].
Resistive-Type SCFCL
Such a type is used to enhance the transient stability of the system as it suppresses the fault current in a quick and efficient manner
[47]. It consists of two resistances which are known, according to their function, as stabilizing and superconducting resistances beside a series-connected coil
[17]. During normal operation, the resistance keeps its superconductivity, so it does not pose losses where the coil has a small value as well that forms small AC losses. However, the resistance is quenched and limits the fault current in the faulty condition. This type has a high length of superconductor which makes it uneconomical.
Hybrid SCFCL
This type can be used to improve the dynamic performance of the system
[48] besides solving the difference of the critical currents among the units, which can be observed in the resistive and inductive types. This type
[48], is composed of the primary winding and several secondary windings which are connected in series with superconductor resistance. This resistance is obtained through normal operation, so its value is zero while it is transformed to its normal state to limit the fault current during the fault condition.
Flux Lock-Type SCFCL
Flux lock SCFCL,
[17], consists of two parts: a current-limiting part and a current-interrupting part. The first part has two parallel connecting coils with one connected with HTSC, whilst the current-interrupting part is composed of an over-current relay, which takes its signal from one coil, in addition to a circuit breaker. During normal operation, zero voltage is deduced across the coils whilst the current is limited during the fault condition throughout the voltage deduced across the two coils. Flux lock SCFCL has less power burden compared between the other SCFCLs.
Magnetic Shield-Type SCFCL
This type consists of a primary coil (copper coil) and a secondary coil (HTSC tube), which are wound around a magnetic iron core
[49]. At the steady state condition, the flux does not penetrate the iron magnetic core, as the HTSC tube is sited between the copper coil and the magnetic core, so there are no losses at this state, while in the fault condition, the HTS tube is quenched, transforming into its normal operation, so its resistance increases, and consequently, the resistance of the primary coil increases the limiting of the fault current.
3.3.4. Non-Superconducting Fault Current Limiters (Solid)
Unlike SCFCLs, FCLs can be implemented with passive nonlinear elements as inductors and switches. Such FCLs are known as non-superconducting fault current limiters (non-SCFCLs). The revolution of semiconducting switches helps in developing non-SCFCLSs with minimal cost as compared to superconducting ones. Within the distribution current level, non-SCFCLs are exposed to high thermal stress, so they should be cooled using either forced air or liquid cooling. Additionally, the voltage, during switching, rises sharply in a short time; therefore, properly tamed snubbers are used beside the bypass and clear switches to take FCLs out from service for maintenance or fixing
[50]. There are different types of solid-state FCLs, such as:
Series Switch-Type FCL
It is composed of a bidirectionally controlled semi-conductor switch (Sss) and bypass network
[50]. This network consists of a normal bypass (Sbp), fault current resistance (Zf), overvoltage protection (Zno), and a snubber circuit. The normal bypass is about a low resistance path for decreasing both switching losses and waveform distortion, while (Zf) is used to limit the fault current. Zno is important to provide an alternative current path to limit the voltage across switching as well as absorb some of the energy stored in the inductance, while the snubber circuit function is to keep with allowable limits
[50].
Series Dynamic Braking-Type FCL (SDBRFCL)
It is used in fault rides through the capability enhancement of the wind system
[51]. During normal operation, an insulated gate bipolar transistor (IGBT) is tuned on (Vpcc is the voltage of the point of common coupling, and Vref is the minimum voltage on which is the IGBT), and the resistor is bypassed. On the other side, at fault conditions, IGBT is turned off, and the resistor is connected in a series system
[44].
Bridge-Type FCL (BFCL)
This type is composed of two parts: the bridge part and the shunt branch part
[17]. During normal operation, the IGBT is turned on, and the current has two paths. In a positive half-cycle, the current flows through the LDC, RDC, and D4. However, the path of D2, LDC, RDC, and D3 is considered its path during a negative half-cycle. Additionally, an LDC of a small value acts like a short circuit; therefore, there is a negligible voltage drop, whereas, through the fault condition, the IGBT is turned off, so the bridge is out, and the current is limited by the shunt branch (Rsh + Lsh). Ds, which is the freewheeling diode, can discharge the current through the LDC so it protects IGBT from high voltages.
Modified Bridge-Type FCL (MBFCL)
In MBFCL, the shunt branch in the BFCL is modified as the shunt-inductance LDC is omitted
[52]. The LDC discharges when the shunt inductance is disconnected during normal operation. On the other side, the resistance of the shunt branch can limit the fault current when the IGBT is turned off. This type is used in fixed- and variable-speed wind farms to enhance their low-voltage ride
[53].
DC Link FCL (DLFCL)
This type is implemented in fault rides through the capability enhancement of an inverter-based DG system
[54]. It contains a diode bridge type and a limiting branch of inductance Ld and a small-resistance Rd
[17]. During normal operation, the limiting branch is negligible while it can limit fault current and suppress high voltages during fault operation.
Transformer-Coupled BFCL
A transformer-coupled BFCL is used for low voltage rides through the capability enhancement of a doubly fed induction generator (DFIG), as in
[55]. During the steady state condition, all the thyristors are turned off, making the reactors bypassed, but they are inserted into the system during the fault condition. This is achieved by turning on the thyristors via a control circuit. Rb is a bypass resistor which absorbs most of the harmonics generated during operation, and it can reduce voltage spikes.
Resonant-Type FCL
Instead of using heterogeneous circuits either for the normal state or fault state, resonant FCL, has switches, which in turn reconfigure the circuit to suit both states, normal and fault
[50]. During the normal state, the series resonant tank is tuned to the line frequency, forming zero resistance, whilst this resistance turns to a large value to limit the fault current in the fault condition as the resonance is lost. Despite its simplicity, the resonant-type FCL may cause harmonics
[50].
3.3.5. Electromechanical Dynamic FCL
Its nomenclature, dynamic, refers to the fact that its impedance varies with the current magnitude
[29]. The electromechanical dynamic FCL adjusts automatically and instantaneously its own resistance depending on the current magnitude. The more the current increases, the more the limiting action exists. That is why it creates a low impedance at normal operation and, therefore, low voltage regulation. Such a type can thermally afford high currents for a definite time. On the other side, it is complex and reconciles high-power capacitive systems
[29].
3.3.6. Hybrid FCL
This type is a combination of mechanical switches, solid-state FCLs, superconducting materials, and other technologies to limit the fault currents
[56]. It consists of inductance (L) and capacitance (C), which are approximately zero at the nominal power frequency in addition to ZnO that protects both the switch SW2 and TVS (triggered vacuum switch) from high voltage. In normal operation, the TVS and SW2 are off, whilst in fault one, a signal is sent to the TVS, and the contactor turns on the bypass capacitor C1. This makes the reactor L limit the fault current, and both C2 and SW1 form a series compensation
[56].