Earticle

In recent years, with the rapid development of China's nuclear power industry, nuclear generator has also been actively developed. Because the unit capacity of the generator is increasing, so working life and operational reliability of the generator are also being tested. Among them, transient thermal running ability of the generator is one of the key factors affecting working life and operational reliability of the generator. This paper mainly analyzed the negative sequence current and the transient negative sequence component of the generator during asymmetric operation, and elaborates the principle of the generator negative sequence component harmonic. The paper has reference value for in-depth study of the generator negative sequence component.

As the grid voltage of the V/v power supply system for high-speed railway is distorted, the detection method for harmonic and negative sequence current is complex, and there are errors in the detection result. A method is proposed in order to solve these issues, which is capable of carrying out fast and accurately comprehensive compensation for grid current without the PLL circuit. During the detection process, a group of sine and cosine function with arbitrary initial phase angle is needed, the grid fundamental positive sequence voltage is extracted to participate into the detection of fundamental active power component to obtain the command signal. The calculation process is fast and simple and not affected by the distorted grid voltage. The correctness and feasibility of this method is verified by the result of simulation

The negative-sequence eddy current field of the first AP1000 third-generation 1,250 MVA nuclear half-speed (4-pole) turbo-generator is deeply researched in this paper. In order to find out the influence rules of using different rotor slot wedge materials, the eddy current loss densities, which are generated by negative-sequence current, are compared and analyzed. It is well known that aluminium and aluminum-bronze are widespread materials in electric machine industry. Hence, the two materials are adopted respectively as the slot wedge material, the eddy current loss calculations of slot wedges, rotor teeth and damping bars are given, and the loss characteristic analysis results are also obtained in this work. The calculation and analysis results indicate that different materials have different effects on eddy current loss, especially for the loss density of slot wedges, eddy current loss density of the big tooth area is bigger than the small tooth, and damping bars have the biggest loss density on the rotor surface, which all should be attached importance to in actual large capacity generator design.

The three-phase cascaded H-bridge rectifier (CHBR) is widely used in high power and high voltage applications. This paper focuses on the load removed from the DC port of a three-phase CHBR, which can cause single-phase DC-port voltage unbalance issues and three-phases DC-port voltage unbalance issues. This paper proposes a voltage balance control strategy based on negative sequence current injection for three-phase CHBR to deal with the unbalance problem. An individual DC-port voltage control strategy is designed to balance the DC-port voltage in the faulty phase. In addition, a negative sequence current injection control strategy is responsible for balancing the DC-port voltages in the three phase CHBR. The effectiveness and feasibility of the proposed voltage control strategy based on negative sequence current injection is verified by the experimental results.

본 논문은 계통에 전력을 변동하여 계통 임피던스를 추정하는 기법의 문제점을 분석하고 개선된 역상분 전류 주입을 이용한 임피던스 추정 기법을 제안한다. 기존의 계통 임피던스 추정 기법에서는 유, 무효 전력의 크기를 변화시키고, 그에 따른 두동작점에서 측정된 전압의 변동량을 통해 계통 임피던스를 계산한다. 하지만 일반적인 계통 연계형 인버터에는 전원 전압의 위상을 추종하는 PLL이 동작하고 있으며, 이 PLL은 정상 상태에서 얻을 수 있는 전류 주입에 따라 발생하는 d-q축 전압의 변동량을 왜곡시켜 추정 오차를 유발한다. 따라서 본 논문에서는 불평형 전원을 독립적으로 제어하는 듀얼 제어기와 정상분 PLL 기법을 사용하여 역상분 전류에 의한 역상분전압의 변동량만으로 PLL에 의한 PCC 전압의 정상 상태 측정 오차를 제거하여 계통 임피던스를 정확히 추정하는 기법을 제안한다.

A new algorithm is proposed for the estimation of equivalent grid impedance at the point of common coupling of a grid-tie inverter output. The estimated impedance parameter can be used for the improvement of the performance and the stability of the distributed generation system. The estimation error is inevitable in the conventional estimation method because of the axis rotation due to PLL. In the conventional estimation error, the d-q voltage and current are used for the calculation of the impedance with active and reactive current injections. Conversely, in the proposed algorithm, the negative sequence current is injected, and then the negative sequence voltage is measured for the impedance estimation. As the positive and negative sequence current controller is independent and the PLL is based on the positive sequence component only, the estimation of the equivalent impedance can be achieved with high accuracy. Simulation and experimental results are compared to validate the proposed algorithm.

This paper proposes an active islanding detection method for the BESS (Battery Energy Storage System) with 3-phase inverter which is connected to the AC grid. The proposed method adopts the DDSRF (Decoupled Double Synchronous Reference Frame) PLL (Phase Locked-Loop) so that the independent control of positive-sequence and negative-sequence current is successfully carried out using the detected phase angle information. The islanding state can be detected by sensing the variation of negative-sequence voltage at the PCC (Point of Common Connection) due to the injection of 2-3% negative-sequence current from the BESS. The proposed method provides a secure and rapid detection under the variation of negative-sequence voltage due to the sag and swell. The feasibility of proposed method was verified by computer simulations with PSCAD/EMTDC and experimental analyses with 5kW hardware prototype for the benchmark circuit of islanding detection suggested by IEEE 1547 and UL1741. The proposed method would be applicable for the secure detection of islanding state in the grid-tied Microgrid.

본 논문은 계통과 연계된 3상 전압원 인버터를 기반으로 한 BESS의 능동 단독 운전 검출 방법을 제안한다. 계통 전압의 불평형에서도 안정적으로 위상을 추종할 수 있는 DDSRF_PLL(Decoupled Double Synchronous Reference Frame_PLL)방식을 적용 하였으며, 검출된 위상각 정보를 통해 정상분 전류 제어기와 역상분 전류 제어기를 독립적으로 제어할 수 있게 된다. 이를 위해 IEEE 1547과 UL1741에서 제시하는 단독 운전 기준 시험 회로를 구성하여 PSCAD/EMTDC 소프트웨어를 통한 시뮬레이션과 5kw프로토타입 하드웨어 장치를 통해 제안된 단독 운전 검출 방법을 검증하였다.