Design of fault current limiter using core with low initial permeability

To meet the need of increase in electrical energy consumption and generation, setting parallel transmission lines up and existing of complex system as a result of addition of distributed generation sources to grid concludes increase in fault current levels. The increase of fault currents in this way causes the problems of the need to increase of withstand values of power electronics components, the danger occurred on power system hardware caused by higher mechanical forces and excess heating of transformer and the other components. In the case of clearing these increased faults, circuit breakers and similar protection devices are not providing the demanded limitation. Several types of solutions are proposed for this. These solutions are increasing capacity of circuit breakers, bus splitting and sequential breaker opening etc. The solution of increasing capacity of circuit breakers is not feasible because it requires so many of breakers to be changed. Bus splitting causes the loads not to be feeded by power sources in normal case of operation and requires developing additional control methods. Sequential breaker opening increases the total time to clear the fault current. Because of these problems, the fault current limiter usage motivation has been formed. Fault current limiters are expected to be activated so fast so that they can limit first peak of fault current. To make this happen, fault current limiters (FCL) using superconducting material naturally and hysterically passes into normal resistive mode fastly. Some FCL types which uses some switching elements uses magnetic field that is generated by parallel coil connected to them. This type of FCLs can use superconducting module which is missioned to switch the fault current to parallel coil line. Thanks to this method, superconducting material's recovery period is so small that makes it available for other fault current. Also, superconducting element cooling capacity has been decreased by this method. The other method which uses flux-locking method also can have superconducting element or not. In this physical phenomenon, core that is used in design is "locked" by magnetic fluxes with opposite directions to each other and thus no magnetic flux can flow in the core material. As a result of that, the coil that is wounded to core "see" no reactance and becomes invisible to grid. At fault current mode of operation, one of these opposite magnetic fluxes beat the other and no longer the equality of magnetic fluxes happen. Because this change activates so fast, triggering into high reactance mode becomes so quickly that first peak of fault current is limited. FCLs are also expected to consume minimum energy and to drop minimum voltage across its terminal during the normal operation so that they become invisible in the grid. To ensure that, for example saturated core fault current limiters needs to be in saturated condition continuously. In this way, they are invisible with little energy consumption in saturated region for normal mode of operation. However, continuously saturating the cores causes continuous energy consumption from a DC supply or need to have superconducting cables. These requirements cause operating and/or installation costs to increase. This thesis work aims to eliminate the need to saturate cores. In this context primarily, the answer to the question that if design could be invisible or not without saturating the core or by operating the core in unsaturated region is researched. In the case of successing this, the need to saturate the core will be unnecessary and thus there will be no need to use permanent magnets and/or to continuously excite DC coils. From the literature search; some materials that have low magnetic permeability for low magnetic field intensity values which also named initial permeability region are identified. Initial magnetic permeability values of these materials would be accepted to have constant values for low magnetic induction. Firstly, low carbon SAE 1020 material is used and it is tried to be understood if this material could be operated in initial magnetic permeability region for normal mode of operation. For this purpose, shape, dimensions and related parameters of design are determined via calculation methods supporting the necessity of voltage drop to be in constraints and core operation curve to be in initial magnetic permeability region in normal mode of operation. This obtained design is simulated in MAXWELL program which uses finite element method. When obtained results are evaluated, simulations are observed to support the argument we had in beginning. Then, different core materials that are projected or not to be used in initial magnetic permeability region for normal mode of operation have been simulated. When resulted voltage drop and fault current limiting values are considered, low carbon SAE 1020 material is decided to have the best simulation results that satisfies the argument. At first simulations, design dimensions are observed to be useless from the point of practical usage. To overcome this, it was tried to reduce design on size by making definite changes on design and its parameters. In order to reduce the size of design, addition of some air gaps into core is evaluated. Adding some air gap reduced the size of core dimensions however this procedure worsened the fault current limiting performance. Although resulting this performance decrease in the FCL by introducing air gap, this core design shows better performance than air core coil. Same simulation studies are carried out using other some certain core materials without change on design parameters and as a result of air gap in core material dominating the core material, performance outputs on simulations are not significantly different than one another. In addition, the design with air gap is compared to saturated core FCL about fault current limiting device performance criterias and it has been observed that this design performance is worse than saturated type. Also, simulations are carried out to inspect the effect of power consumption in daily hours on same design. This change on power consumption in daily hours is inspected to cause the voltage drop on FCL. As a result of this inspection, least voltage is observed to drop in night period and most voltage is observed to drop in peak period.