Real-Time Method for Detecting Harmonic and Reactive Currents of Single-Phase Circuits

According to the characteristics of single-phase circuits and demand of using active filter for real-time detecting harmonic and reactive currents, a detecting method based on Fryze＇s power definition is proposed. The results of theoretical analysis and simula- tion show that the proposed method is effective in realtime detecting of instantaneous harmonic and reactive currents in single-phase circuits. When only detecting the total reactive currents, this method does not need a phase-locked loop circuit, and it also can be used in some special applications to provide different compensations on the ground of different requirements of electric network. Compared with the other methods based on the theory of instantaneous reactive power, this method is simple and easy to realize.

Influence of DFIG Wind Turbine Protection and Control during Voltage Dip on Circuit Breaker Operation

This paper analyzes a DFIG （doubly fed induction generator） WT （wind turbine） fault current after a symmetrical network voltage dip. The goal is to identify the factors determining how fast the first zero crossings of the fault current occur. This is an important subject because the ftmdamental property of the CB （circuit breaker） is that it breaks the current when the current is very near zero. The study was conducted using a hardware-in-the-loop test environment constructed using two real time simulators （dSPACE and RTDS） and a commercial protection relay. It is found that the reactive current injection during a voltage dip demanded by the grid codes enhances the operation of the WT protection because the zero crossings of the currents through CB are attained earlier. In addition, the size of the crowbar resistance has a significant influence on the zero crossings.

Coordinated Control of DFIG Converters to Comply with Reactive Current Requirements in Emerging Grid Codes

The doubly-fed induction generator(DFIG)is considered to provide a low-reactance path in the negative-sequence system and naturally comply with requirements on the negative-sequence reactive current in emerging grid codes.This paper shows otherwise and how the control strategy of converters plays a key role in the formation of the active and reactive current components.After investigating the existing control strategies from the perspective of grid code compliance and showing how they fail in addressing emerging requirements on the negative-sequence reactive current,we propose a new coordinated control strategy that complies with reactive current requirements in grid codes in the positive-and negative-sequence systems.The proposed method fully takes advantage of the current and voltage capacities of both the rotor-side converter(RSC)and grid-side converter(GSC),which enables the grid code compliance of the DFIG under unbalanced three-phase voltages due to asymmetrical faults.The mathematical investigations and proposed strategy are validated with detailed simulation models using the Electric Power Research Institute(EPRI)benchmark system.The derived mathematical expressions provide analytical clarifications on the response of the DFIG in the negative-sequence system from the grid perspective.

Small-signal Model for Dual-active-bridge Converter Considering Total Elimination of Reactive Current

Emerging technologies such as electric vehicles,solid-state transformers,and DC transformers are implemented using the closed-loop bi-directional dual-active-bridge(DAB)converter.In this context,it is necessary to have average models that provide an efficient way of tuning the proportional integral(PI)compensator parameters for large-and small-signal applications.We present a novel small-signal model(SSM)for DAB converter with a single closed-loop PI controller and the total elimination of reactive current(IQ=0).The method applies a modulation technique for IQ=0 and introduces a composite function in the controller while reducing the original nonlinear switching model,which allows to decrease the order of the transfer function and analyze the closed-loop operation.The proposed SSM is analyzed using different response time,load,and DC voltage changes.The simulation and experimental results demonstrate the ease of implementation and effectiveness of the proposed model with respect to other SSM techniques.

Analysis of low voltage ride-through capability and optimal control strategy of doubly-fed wind farms under symmetrical fault

Given the“carbon neutralization and carbon peak”policy,enhancing the low voltage ride-through(LVRT)capability of wind farms has become a current demand to ensure the safe and stable operation of power systems in the context of a possible severe threat of large-scale disconnection caused by wind farms.Currently,research on the LVRT of wind farms mainly focuses on suppressing rotor current and providing reactive current support,while the impact of active current output on LVRT performance has not been thoroughly discussed.This paper studies and reveals the relation-ship between the limit of reactive current output and the depth of voltage drop during LVRT for doubly-fed induction generator(DFIG)based wind farms.Specifically,the reactive current output limit of the grid-side converter is inde-pendent of the depth of voltage drop,and its limit is the maximum current allowed by the converter,while the reac-tive current output limit of the DFIG stator is a linear function of the depth of voltage drop.An optimized scheme for allocating reactive current among the STATCOM,DFIG stator,and grid-side converter is proposed.The scheme maximizes the output of active current while satisfying the standard requirements for reactive current output.Com-pared to traditional schemes,the proposed LVRT optimization strategy can output more active power during the LVRT period,effectively suppressing the rate of rotor speed increase,and improving the LVRT performance and fault recov-ery capability of wind farms.Simulation results verify the effectiveness of the proposed scheme.