Minghao Wen

Study of Fault Current Characteristics of the DFIG Considering Dynamic Response of the RSC

During nonsevere fault conditions, the crowbar protection is not activated and the rotor windings of a doubly fed induction generator (DFIG) are still excited by the ac/dc/ac converter. In these cases, the dynamic response of a rotor-side converter (RSC) has a large influence on the fault current characteristics of the DFIG. In this paper, an analysis method for the fault current characteristics of the DFIG under nonsevere fault conditions is proposed. First, the dynamic response of the RSC is analyzed on condition that the external power control loop is shut down and the reference signals of the inner rotor current control loop are kept constant. Then, the simplified calculation models of the rotor fault current are established according to different design principles of the inner rotor current controller. Based on it, the fault characteristics of the stator current are studied and the analytical expressions of the stator fault current are obtained. Finally, digital simulation results validate the analytical results. The proposed analysis method is applicable for the study of fault current characteristics of DFIG with different control strategies for low-voltage ride through. The research results are helpful to the construction of adequate relaying protection for the power grid with penetration of DFIGs.

A Novel Compensation Technology of Static Synchronous Compensator Integrated With Distribution Transformer

This paper presents a novel compensation technology of static synchronous compensator integrated with distribution transformer (DT-STATCOM). STATCOM is connected with the transformer taps at the high-voltage (HV) windings. By selecting the appropriate tap voltage level, it can help to make full use of the spare capacity of the distribution transformer, simplify the main circuit topology of STATCOM, and reduce the current level of power devices. The theoretical basis for DT-STATCOM reactive power compensation is explained. For the Dyn-type transformer, the structure of DT-STATCOM with multigroup taps at the transformer HV windings is mainly designed, and its influence on the current distribution in the transformer windings is analyzed. Besides, according to the characteristics of DT-STATCOM, this paper also proposes a reactive command current calculating method based on the principle of power balance, and adopts a hysteresis current control method with a variable hysteresis band. Simulation and experimental results verify the correctness of the proposed DT-STATCOM structure and its control method.

A novel traveling wave based fault location scheme for power distribution grids with distributed generations

A traveling wave fault location method based on multi measuring points for distribution network with distributed generation is proposed in this paper. A novel fault location algorithm based on the time difference between the first aerial-mode and zero-mode wave-fronts of the traveling wave are used to obtain the preliminary fault location without the wide area precision time synchronization. This method acquires the information gathered by measuring points at the distribution network busbar and the terminals of the trunk feeder and the random error is eliminated by using network redundant data. Its verified that the method proposed in this paper can achieve accurate location of the fault section and fault distance for distribution network with distributed generation effectively by using the IEEE 34 node network simulation.

The research of regional intelligent protection based on regional power system information

In order to incarnate area wide protection in engineering application and improve the capability of relay protection, the paper refers to regional intelligent protection system. In compared with wide area protection, regional intelligent protection system pays attention to improving the capability of relay protection by using regional power network information. In regional intelligent protection system, the relay protection devices could obtain more abundant information from regional power network than local area, including fault direction information, breaker information. And the paper also introduces some regional protection means by using the information, which is useful to improve the reliability of relay protection.

Fault Location in Neutral Non-effectively Grounded Distribution Systems Using Phase Current and Line-to-line Voltage

Abstract—A scheme of single-line-to-ground fault location in neutral non-effectively grounded distribution systems is proposed in this article. The characteristics of phase current traveling waves and line-to-line voltage traveling waves are analyzed. The wavelet transform technique is used to extract the information of the arrival times, polarities, and amplitudes of the recorded traveling waves. The fault location scheme can simultaneously realize faulty feeder identification, fault section location, and fault position location even if only two-phase current transformers are installed in the Phases A and C and feeder terminal units only to provide line-to-line voltage information. The effectiveness and practicability of the proposed scheme have been verified by PSCAD/EMTDC simulation results.

A cascaded DSTATCOM integrated with D-Y connection distribution transformer for reactive power compensation

To decrease the initial investment of the compensation system in distribution networks, this paper proposes a cascaded distribution static synchronous compensator (DSTATCOM) integrated with D-y connection distribution transformer for reactive power compensation. The cascaded H-bridge DSTATCOM is connected to a group vertex-taps on the primary windings of the D-y connection distribution transformer, which the connection voltage range is chose from zero to the rated ac line voltage in theory. The proposed structure can achieve a suitable connection voltage selection based on the actual reactive power compensation requirements, thus, the voltage stress, initial investment and size of the compensation system can be greatly reduced. The equivalent model is built and the winding currents distribution is also analyzed. Based on the results of winding current distribution analysis, the compensation capacity constraint is deduced. Finally, both simulation and experimental results demonstrate the effectiveness of the proposed system.

A comprehensive control strategy suitable for reactive power compensation and harmonic elimination

This paper proposes a comprehensive compensation strategy for DSTATCOM which is suitable for both reactive power compensation and harmonic elimination in distribution network. The DSTATCOM is connected with the transformer taps on the high voltage (HV) windings and the transformer adopts the Dyn11 connection group. Firstly, according to the converter model, the decoupled control strategy is presented for the independent control of the active and reactive current components. Then, the detection method of reactive command current and harmonic command current is analyzed. Conventional PI controller and quasi-PR controller are adopted respectively for the control of the reactive current and harmonic current. Simulation test results based on MATLAB/SIMULINK show that the proposed control strategy has a good performances.

Stateflow based carrier phase-shifted SPWM and its application in DT-STATCOM

Combining switching characteristics of power electronic devices and state transitions in carrier phase-shifted SPWM (CPS-SPWM), this paper uses graphic Stateflow toolbox of Matlab/Simulink to design the process of CPS-SPWM. Then it is applied to STATCOM which is integrated with distribution transformer (DT-STATCOM) for PWM signal generation. Simulation results are presented to prove that the method is valid and effective for reactive power compensation. Meanwhile, software-in-loop (SIL) test and automatic code generation are also carried out. Results of experimental test with DSP TMSF28335 have validated the theoretical contributions of the paper. PWM signal comparison of both simulation and experiment result shows that this method has both the feasibility and effectiveness of software and hardware design. With this method, design of CPS-SPWM can be more rapid, efficient and convenient.

The effects of the reverse current caused by the series compensation on the current differential protection.

The series capacitor compensation is one of the key technologies in the EHV and UHV long distance power transmission lines. This paper analyzes the operation characteristics of the main protection combined with the engineering practice when the transmission line overcompensation due to the series compensation system is modified and analyzes the influence of the transition resistance and the system operation mode on the current differential protection. According to the simulation results, it presents countermeasure on improving the sensitivity of differential current protection.