An improved hybrid unified power flow controller (IHUPFC) is developed in this paper to control power flow and increase utilization rate of the existing transmission lines. The IHUPFC is composed of an improved "Sen" transformer (IST) and a small capacity UPFC. After a detail introduction of the structure and voltage regulation performance of the IST, this paper derives the capacity relationship between the IST and the UPFC. Then this paper introduces the technology of IHUPFC in power flow regulation and transmission capability improvement. Besides, the advantages of the IHUPFC in performance and structure are analyzed. The IST has simpler structure and larger power flow control area than "Sen" transformer and the IHUPFC provides a continuous active and reactive power flow control. Simulation and experiment studies are used to verify the practicability and superiority of the IHUPFC.

If current transformer (CT) is saturated in operation, it will lead to incorrect current measurements and may cause relay maloperation, which can greatly threaten the safety of power grid. This paper analyzed the basic measuring characteristics of the proposed novel current transformer based on virtual air gap. This method can successfully compensate the distorted secondary current and expand the measuring range. Based on the simplified magnetization curve model, its working state is divided into linear working state, partially saturated working state and saturated state. Furthermore, its basic measuring characteristics are deducted theoretically. The theory is verified by FEM simulation results and prototype experiments. The difference between this method and other methods is also discussed. Compared with CT, this method adopts partial-gapped magnetic core and magnetic field sensor (MFS) for current reconstruction. When it works in rated state, this method is like a normal CT. Once the primary current saturated the small section core, it will be like a virtual gap, making the whole core more difficult to saturate and allowing the measurement of leakage flux. It has the advantages of simple and compact structure and can be embedded into the electronic current transformer (ECT) system. In this sense, it can have a good application prospect. The main limitation is the requirement of high permeability material for accuracy performance.

The fault current limiter (FCL) is one of the most useful measures for limiting fault currents in power systems. This paper introduces a new structure for a parallel-type permanent magnet (PM)-biased saturation-based FCL (PPFCL). The PPFCL consists of two magnetic devices connected in parallel and in magnetic counter opposite to each other. The operating principle and the core inductances in different states were analyzed using an electromagnetic equivalent circuit. In addition, a 220 V/10 A experimental prototype was designed and tested in a simple system and compared with Maxwell-3-D simulation and analytical methods. Finally, a 10 kV case-study model was created and compared with a series-type FCL, which verified the effectiveness of the limiter in high-voltage, large-capacity application. Experimental and simulation results have demonstrated that the PPFCL exhibits fast response and low impedance during normal operations, as well as high bias capability and low possibility of demagnetization of the PM during fault conditions.

This paper proposes a novel ground-fault location method in resonant grounded systems based on distributed modulation and compensation adjustment. After the permanent single-phase ground fault arc has been suppressed, the Petersen coil will be adjusted to make the zero-sequence currents at the upstream monitoring points of the fault point exceed a certain threshold and be modulated in situ. At this time, the corresponding modulated signals can be extracted from the neutral-to-earth voltage on the busbar for measuring these zero-sequence currents. Taking the carrier-frequency of every monitoring point as its identification tags and the variation characteristic of the zero-sequence currents along feeders as another detection criterion, the fault point can be determined online accurately without a high-performance real-time monitoring network. In this paper, the carrier-frequencies allocation and signal conditioning scheme are proposed for a good detection accuracy of the zero-sequence currents, then the signal detection principle with the control of Petersen coil is described in detail. Simulations and experiments verified the feasibility of the proposed fault location method.

Soft magnetic composites (SMC) are composed of ferromagnetic particles surrounded by electrical insulation. To establish a homogenization technique for the magnetic field analysis of an electrical machine with a SMC core, we have already proposed a method to evaluate the effective permeability and iron loss of the SMC core using a cell model of one particle taking account of the nonlinear magnetic characteristics and eddy currents. However, the measured eddy current loss cannot be represented by using the proposed method. In this paper, a 3-dimensional eddy current analysis method modeling the anomalous eddy current loss due to the movement of domain walls in the cell model is developed based on the Pry and Bean magnetic domain model. The effect of the nonuniformity of the domain structure on the anomalous eddy current loss is investigated. It is shown that the measured eddy current loss can be almost represented by adding the anomalous eddy current loss to the classical one.

To reduce the requirement of DC-biasing capacity and improve the biasing ability of a permanent magnet (PM), a novel hybrid saturated-core fault-current limiter (HSCFCL) is proposed in this paper. Compared with traditional saturated-core fault-current limiter (SCFCL), the HSCFCL has the advantages of small size, low DC-biasing capacity, a high biasing ability of the PM and excellent limiting performance. Firstly, the principle and the magnetic circuit model of the HSCFCL are introduced. Then, the improvement of DC-biasing capacity with a PM is analyzed. In addition, the influence of the leakage-flux effect on the biasing ability of the PM is presented in detail, and a small-section optimal structure is proposed to improve the biasing ability of the PM. Finally, to validate the principle and performance of the HSCFCL, various electromagnetic simulations, optimization studies and experiments are carried out. The simulation and experimental results demonstrate the effectiveness of the proposed method.