D. Cvoric

A FACTS Device: Distributed Power-Flow Controller (DPFC)

This paper presents a new component within the flexible ac-transmission system (FACTS) family, called distributed power-flow controller (DPFC). The DPFC is derived from the unified power-flow controller (UPFC). The DPFC can be considered as a UPFC with an eliminated common dc link. The active power exchange between the shunt and series converters, which is through the common dc link in the UPFC, is now through the transmission lines at the third-harmonic frequency. The DPFC employs the distributed FACTS (D-FACTS) concept, which is to use multiple small-size single-phase converters instead of the one large-size three-phase series converter in the UPFC. The large number of series converters provides redundancy, thereby increasing the system reliability. As the D-FACTS converters are single-phase and floating with respect to the ground, there is no high-voltage isolation required between the phases. Accordingly, the cost of the DPFC system is lower than the UPFC. The DPFC has the same control capability as the UPFC, which comprises the adjustment of the line impedance, the transmission angle, and the bus voltage. The principle and analysis of the DPFC are presented in this paper and the corresponding experimental results that are carried out on a scaled prototype are also shown.

New Three-Phase Inductive FCL With Common Core and Trifilar Windings

Fault current limiters (FCLs) are expected to play an important role in the protection of future power grids. Inductive FCLs are particularly interesting due to their inherent reaction to a fault, but are not commercialized because of a too large amount of magnetic material and an induced overvoltage across dc windings. This paper introduces a new three-phase FCL with a common core and trifilar windings. With the new FCL topology, the phase windings are placed on a single core, resulting in significant reduction of the amount of required magnetic material. Furthermore, the phase windings are wound simultaneously, so that the flux coupling between the phases is increased considerably. It cancels out the magnetic field and reduces the FCL normal impedance significantly. The dc windings are used only to compensate for possible asymmetry in the system currents. Consequently, the number of dc turns and the induced overvoltage are considerably decreased. Testing of a scaled-down FCL lab prototype made it possible to verify the FE simulation results and demonstrated the principle of operation of the new topology. Experimental and simulation results matched very well.

Improved Configuration of the Inductive Core-Saturation Fault Current Limiter with the Magnetic Decoupling

Due to the increasing levels of the short-circuit currents, the fault current limiters (FCLs) are expected to play an important role in the protection of the future power grids. The inductive FCLs, that have a dc winding that drives the core into saturation, are particularly interesting due to their inherent reaction on the fault. To avoid an over-voltage on the dc winding during a fault, FCL with decoupled ac and dc magnetic circuits has been proposed [5]. However, this FCL configuration reduces the value of the normal current, since its ac leg operates around saturation knee, i.e. the FCL impedance increases during the nominal operation. The size of the core and the peak value of the limited fault current have to be increased if the FCL impedance in the normal operation is to be diminished. This paper presents an improved version of the inductive FCL with magnetic decoupling of ac and dc circuits. The new FCL design reduces the device weight and size, while decreasing the FCL impedance to a low value during the normal operation. The increase of the peak value of the limited fault current is avoided. Comparison results are obtained through simulations in software SaberDesigner.

New saturable-core Fault Current Limiter topology with reduced core size

Installation of new generating units and increased penetration of distributed generators (DGs) in the power systems increase the total power transmitted in the power grid. In the case of a fault, the amount of power captured by a short circuit is enlarged, leading to higher peaks of the fault currents and, consequentially, to higher stresses to the power system components. Installations of Fault Current Limiters (FCLs) are expected to prevent expensive upgrade and replacement of the components exposed to the over-stresses. Inductive FCL are particularly interesting due to their inherent reaction to a fault. However, some challenges are to be solved: excessive weight and induced over-voltage across the dc winding. The goal of this paper is to introduce a new configuration of the inductive FCL, where the amount of the required material has been reduced considerably and the induced over-voltage across the dc winding is decreased. Employment of one core per-phase instead of two reduces the amount of magnetic material. The core has three legs, where the middle leg is used as a shunt path for the ac flux. It enables a gap insertion in the ac magnetic circuit without influencing the dc magnetic circuit, i.e. amount of dc winding material. This means that a smaller core (less magnetic material) can be used for a same power level. The simulation results obtained from FCL models created in SaberDesigner are presented. They have been confirmed through the testing of the lab-scale prototype.

Comparison of the four configurations of the inductive Fault Current Limiter

Fault current limiters (FCLs) are expected to play an important role in the protection of the future power networks, since the increase of loads and expansion of the power networks lead to much higher short-circuit power. This paper presents the comparison of four different configurations of inductive FCL, with respect to the FCL weight (magnetic core and winding material) and losses during both the nominal and the fault state of operation. Two main challenges in the inductive FCL design are reduction of the material weight and reduction of the induced dc winding over-voltage during the fault period. So far, solutions (core configurations) proposed in the literature are: decoupling of the dc and the ac magnetic circuits to avoid high voltages across the dc winding during a fault and the so-called open- core configuration. The presented results reveal the merits and drawbacks of each of the configurations and compare them to the conventional inductive FCL design characteristics. The results are obtained through the simulations in SaberDesinger and by experiments.

CB stress reduction and comparison of energy dissipation for two types of FCLs

Fault current limiters (FCLs) are expected to play an important role in the protection of the future power networks, since the expanding of the power networks leads to much higher short-circuit power. This paper presents an analysis of the influence of different types of FCLs on the short-circuit energy dissipation and signal waveforms of the power line upon the fault occurrence. Dissipated energy upon the fault inception is determined for the resistive FCL considering two types: solid-state FCL and hybrid FCL. The reduction in the amount of energy imposed on a CB by employing a resistive or an inductive FCL is analyzed. The results of the analysis of the power system behavior upon the fault occurrence, from the aspect of energy dissipation in different components, are presented. Also, the dependency of the line current waveform on the moment of the fault inception and the FCL reaction time is shown.