Zuoshuai Wang

The study of sampled value differential protection

The sampled value differential protection is described in this paper. The selections of S, R of the protection are investigated with respect to different restraint modes. The basic selective rule is to ensure the restraint effectiveness of the sampled value differential protection equal or superior to the corresponding phasor differential protection during external faults. The particular fuzzy operating zone of the proposed protection is also discussed and its expressions are presented. The operating characteristic phasor analyzing method is proposed to analyze the operating characteristic of sampled value differential protection, which uses the phasor relationships of input currents of the protected system to represent the operating zone, fuzzy operating zone and restraint zone.

Pseudo-parallel genetic algorithm for reactive power optimization

In this paper, static voltage stability is considered in reactive optimization via the minimum singular value of the Jacobian matrix of converged power flow, and a pseudo-parallel genetic algorithm is introduced to find the global optimal results and avoid premature of conventional simple genetic algorithm. Two simple test systems are employed to verify the effectiveness of the proposed model and algorithm. Simulations results show that both operational and economical performances of test power systems are improved after optimization, and either the optimal results or the convergent characteristics of the proposed algorithm are superior to those of the simple genetic algorithm.

Application of SMES in the Microgrid Based on Fuzzy Control

The concerns about the reliability of SMES have always been the obstacle to the practical application of SMES. In this paper, a fuzzy logic-based operating current regulation strategy, aiming at improving the reliability of SMES, has been proposed. In this control strategy, the current of SMES will be maintained in the best range during normal operation. When the temperature of magnet sharply rises after deep power exchange, the current of SMES will be down regulated to reduce ac loss and obtain more thermal stability margin. An application of SMES in microgrid is taken as an example to verify the control strategy. Simulation results based on MATLAB/Simulink demonstrate the feasibility and correctness of the control strategy.

Status Evaluation Method for SMES Used in Power Grid

Superconducting magnetic energy storage (SMES) is expected to be utilized in the power grid for dynamic power compensation. However, a SMES status such as magnet current (including parameters of power conditioning system) and temperature of superconducting (SC) magnet will restrict the output capability of SMES. This paper proposes a new method to evaluate SMES status, which will guarantee the dynamic thermal stability of SC magnet applied in power grid. First, it is generally analyzed that the output capability (Pmax) of a SMES is restricted by the initial (standby) current (I0) in the SC magnet for a specific application of SMES. To prove that Pmax also depends on the temperature (T0) of the SC magnet, the thermal characteristics of the SC magnet is simulated with a 5 MJ SMES model. Based on these restricted conditions and the simulation results, a SMES status evaluation method (SSEM) is constructed and implemented. Then, with combination of test data gotten from open-loop experiments of a real 150-kJ/100-kW conduction-cooled SMES, the presented SSEM is realized by simulation in MATLAB/Simulink, the simulation results preliminarily verify the feasibility and effectiveness of SSEM. Furthermore, SSEM is applied to dynamic experiments, whereas the 150-kJ/100-kW SMES is set for damping power oscillation after the short-circuit fault in a physical simulated power grid. The proposed SSEM presents a potential way to optimize SMES operation in the power system.

Design of Superconductivity Windings of a 35-kV/3.5-MVA Single-Phase HTS-Controllable Reactor

In order to compensate flexibly reactive power, it is necessary to develop a controllable reactor in long-distance and ultrahigh-voltage system. High-temperature superconductivity-controllable reactor (HTS-CR) is a potential equipment in power system. A 35-kV/3.5-MVA single-phase HTS-CR is being developed. The HTS windings of the HTS-CR are made of double pancakes that are wound with HTS tapes connected in parallel. The relationship between the number of double pancakes and the number of HTS tapes, which has great effect on the current of each HTS tape, is studied. Furthermore, the solution that reduces the ac loss of the HTS winding is also investigated, which discloses the optimized design parameters and the calculation results of the HTS windings. The study here is believed to provide some references for developing HTS-CR with larger capacity in the future.

A novel PWM scheme to eliminate common-mode voltages in cascaded multi-level inverters

In this paper, mechanism of common-mode voltage of voltage source inverter is discussed in details, and it is pointed out that among the pulse width modulation (PWM) strategies for cascaded multi-level inverters (CMLI), those who based on inphase modulation of power-cell can cause common-mode voltage. Also, a novel SPWM for reducing common-mode voltage of cascaded multi-level inverter is presented, the proposed SPWM strategy using non-inphase modulation signals and third harmonic injection can totally eliminate common-mode voltage without extra hardware, and maintain high utilizing ratio of DC-Link voltage simultaneously. Theoretical analysis, simulation results are presented to verify the proposed strategy.

Simulation Analysis and Experimental Tests of a Small-Scale Flux-Coupling Type Superconducting Fault Current Limiter

Superconducting fault current limiter (SFCL) is able to efficiently solve the short-circuit problems occurred in the modern power system. In this paper, the simulation analysis and experimental tests of a small-scale flux-coupling type SFCL (FC-SFCL) are carried out. In regard to the developed 400 V/20 A small-scale prototype of the FC-SFCL, it is composed of two coaxial HTS windings carrying current in reverse directions, two metal oxide varistors, and a controlled switch. The parameter design of the FC-SFCL prototype is presented. Based on the simulations, the design schemes effectiveness can be confirmed from the higher current-limiting ratio, and the SFCL has lower average ac losses under normal condition. The experimental results are in accord with the simulation ones, and the current-limiting characteristics of the FC-SFCL can be well verified.

Performance Analysis and Prototype Design of a D-Core-Type Single-Phase HTS Controllable Reactor

Controllable reactor is widely applied as a reactive power compensation device to improve the stability of power grid. This paper describes the principle and equivalent magnetic circuit of a new D-core-type HTS controllable reactor in detail. A D-shaped core was assembled with one straight limb, on which an alternating-current working winding was wound, with two parallel rectangular magnetic yokes and four excitation cores between the parallel yokes with direct-current HTS control windings wound around and connected in series with each other. In general, the magnetic flux of the windings for different purposes is orthogonal at the joint and parallel in the parallel yokes. Compared with traditional saturable controllable reactor, the proposed layout can weaken electromagnetic induction between working winding and control windings significantly. Meanwhile, the HTS windings were used to reduce the volume and supply a more effective excitation. In our studies, a 380-V/7.6-kVA single-phase controllable reactor was designed. The reactance, harmonic, and inductive voltage features were analyzed to demonstrate the performance of this type of reactor.

Conceptual Design and Performance Evaluation of a 35-kV/500-A Flux-Coupling-Type SFCL for Protection of a DFIG-Based Wind Farm

In this paper, a flux-coupling-type superconducting fault current limiter (SFCL) is suggested to protect a 15-MW class doubly fed induction generator (DFIG)-based wind farm. Detailed conceptual design and performance evaluation of the SFCL are conducted. From the technical requirements and operational parameters of the DFIG wind farm, a 35-kV/500-A flux-coupling-type SFCL is designed, taking into consideration: 1) the winding type, core selection, and inductance values of the coupling transformer (CT); and 2) the structural style, tape length, and quenching resistance of the superconducting coil (SC). By use of different simulation tools, the electrical and electromagnetic characteristics of the SFCL are evaluated. The results show that the maximum magnetic field of the CT is 2.4 T, and the ac loss of the SC is 0.51 W. Not only the electromagnetic properties and loss of the SFCL are acceptable, but also using the SFCL in the wind farm enables to suppress the fault current, compensate the voltage sag, and mitigate the wind power fluctuation. Thus, the robustness of the DFIG-based wind farm against the fault is enhanced, and the effectiveness of the proposed SFCL design is verified.

Study on the Current Limiting Performance of a Novel SFCL in DC Systems

With the development of modern power systems, the occurrence of fault current and system sensitivity to fault current have increased. Along with extensive damage to network hardware, considerable consumer losses can result from fault current events. However, with the extensive application of voltage source converter based power systems, large dc fault current becomes an important issue due to the bottleneck of the capacity of the dc breaker. To provide effective dc power system fault protection, a novel dc superconducting current fault limiter (dc SFCL) was designed. Here, the working principles and the mathematical model of the designed SFCL are presented. The simulation test based on SIMULINK and the demonstrator experiment were performed to confirm the correctness of the theoretical analysis. The test results show that the dc SFCL exhibits a rapid response time and good current limiting performance in dc systems.