Po-Hsu Huang

A Novel Droop-Based Average Voltage Sharing Control Strategy for DC Microgrids

This paper introduced a decentralized voltage control strategy for dc microgrids that is based on the droop method. The proposed distributed secondary voltage control utilizes an average voltage sharing scheme to compensate the voltage deviation caused by the droop control. Through nonexplicit communication, the proposed control strategy can perform precise terminal voltage regulation and enhance the system reliability against system failures. The distributed voltage compensators that resemble a centralized secondary voltage controller are implemented with the bi-proper anti-wind-up design method to solve the integration issues that necessarily lead to the saturation of the controller output efforts. The proposed concept of pilot bus voltage regulation shows the possibility of managing the terminal voltage without centralized structure. Moreover, the network dynamics are illustrated with a focus on cable resonance mode based on the eigenvalue analysis and small-signal modeling; analytical explanations with the development of equivalent circuits give a clear picture regarding how the controller parameters and droop gains affect the system damping performance. The proposed derivative droop control has been demonstrated to damp the oscillation and to improve the system stability during transients. Finally, the effectiveness and feasibility of the proposed control strategy are validated by both simulation and experimental evaluation.

Subsynchronous Resonance Mitigation for Series-Compensated DFIG-Based Wind Farm by Using Two-Degree-of-Freedom Control Strategy

This paper investigates a special class of dynamic power system problem, namely subsynchronous resonance (SSR) resulted from a series-compensated network connecting doubly-fed induction generator (DFIG) based wind farms. A novel two-degree-of-freedom (2DOF) control strategy combined with a damping control loop is designed and analyzed for enhancing the system stability and alleviates the SSR that may arise due to the induction generator effect (IGE). The proposed control strategy is tested at different operating conditions of series compensation levels and low wind speeds to ensure the system stability. The doubly-fed induction generator based wind farms without the proposed control strategy leads to overall system instability during high series compensation and low wind speeds. Hence, the mitigation of the SSR and damping enhancement are critical to the entire power system stability. A reliable way of analyzing the system and designing effective control strategies against SSR based on the eigenvalue analysis and impedance based stability criterion is deployed. Moreover, analytical explanations have been elaborated to verify the procedure of the controller design. Fault ride-though capability has also been investigated with the proposed control strategy that is flexible to be integrated with the FRT schemes so as to assist the wind farm in mitigating the SSR during the fault recovery stage. Finally, time domain simulations are carried out to demonstrate the effectiveness of the proposed control strategy for mitigating the SSR and damping power system oscillations.

Novel Fault Ride-Through Configuration and Transient Management Scheme for Doubly Fed Induction Generator

This paper proposed a novel fault ride-through (FRT) configuration and transient management scheme to enhance the FRT capability of doubly fed induction generator-based wind turbines. The new configuration of the grid-side converter introduces shunt and series compensation for normal operation and voltage dips, respectively. A braking resistor is added to smooth switching transients from shunt to series interfaces and dissipate excessive power from the grid-side converter. To attain a flexible control solution for balanced and unbalanced fault conditions, the proposed transient management scheme employs positive and negative sequence controllers. A small-signal linear model is developed and examined to analyze the system dynamics for the series compensation topology. Based on the mathematical model, the controller is tuned to balance both voltage regulation performance and transient stability margins with consideration of various operating conditions. The combination of shunt and series interfaces demonstrates a low component count, simple protection structure, and improved performance of FRT with effective compensation to the electric grid. A comprehensive simulation verified the capability of the new configuration and transient management scheme.

Novel Fault Ride-Through Scheme and Control Strategy for Doubly Fed Induction Generator-Based Wind Turbine

This paper presents a novel modulated series dynamic braking resistor (MSDBR) control strategy for enhancing the fault ride-through (FRT) of doubly fed induction generator-based wind turbines. The proposed cost-effective protection scheme introduces a voltage booster that offers series voltage compensation capability and provides a means of power evacuation to mitigate the power imbalance during grid faults. To attain flexible and robust control solution for handling both balanced and unbalanced grid faults, the proposed scheme employs a modulated pulse width modulation (PWM) switching technique to control the stator phase voltage individually. The proposed transient management scheme allows the MSDBR to mitigate the impact from different types of grid faults and to fulfill with the recent grid code requirement. Also, reactive current injection capability during faults is also investigated with the proposed voltage reference algorithm. For the controller design, small-signal modeling is utilized with consideration of measurement dynamics for the tuning of controller parameters in order to ensure the system robustness and stability. Finally, the simulation results demonstrate the satisfactory performance of the MSDBR with its preferred allocation for enhancing the FRT performance against both balanced and unbalanced faults.

Comprehensive Parameterization of Solar Cell: Improved Accuracy With Simulation Efficiency

Simulating photovoltaic (PV) power systems becomes important when studying integration issues of the intermittent solar energy into electric grids. The accuracy of the maximum power point (MPP) in PV output model is a concern since it represents the capacity of power generation. This paper proposes a comprehensive approach to identify PV cell parameters and avoid model errors of the MPP based on standard and simplified equivalent circuits. The estimation accuracy has been improved while maintaining the computational simplicity. This paper begins with the definition of the performance index used to quantify the model accuracy at the MPP. Based on the availability of information from the manufacturers datasheets, the accuracy and complexity of different equivalent circuits are discussed and investigated. The proposed approach has been successfully tested for solar cells made of mono and multi crystalline material.

A practical load sharing control strategy for DC microgrids and DC supplied houses

Microgrid (MG) research mainly focuses on AC-based power flow control techniques. Nevertheless, the rise of DC output sources such as photovoltaic (PV) systems, fuel cells, and distributed batteries leads to the immediate need for DC MGs. In this paper, a hierarchical control strategy for a droop-controlled DC MG is proposed, which fits the smart house infrastructure to adopt online renewable generation and load sharing. The improved control strategies combined with the hierarchical approach includes three loops of controllers: the primary control, the secondary control, and the tertiary control. The issues of internal current limiter and anti-windup are also discussed in this study. The simulation results of the proposed approach are presented to verify the feasibility, and the experimental results are carried out to evaluate the droop concept and secondary voltage compensation.

Fault Ride-Through Configuration and Transient Management Scheme for Self-Excited Induction Generator-Based Wind Turbine

The paper proposes a novel fault ride-through (FRT) configuration with a transient management scheme to provide dynamic grid supports for self-excited induction generator (SEIG)-based wind turbines. The new configuration is capable of supporting both shunt and series compensation modes for enhancing FRT capability and transient stability. The proposed control scheme, utilizing positive and negative sequence controllers, provides a transient management solution for both balanced and unbalanced fault conditions. An active damping control loop for the shunt compensation configuration is developed to mitigate system oscillation and improve system performance during and following faults. A comprehensive simulation has verified the FRT capability of the new configuration and transient management scheme, and showed that the proposed topology increases system stability under various short circuit ratios, even in case of a weak grid. The experimental results are also carried out to show the effectiveness of the proposed shunt-series reconfiguration topology.

Non-invasive winding fault detection for induction machines based on stray flux magnetic sensors

Non-intrusive monitoring of health state of induction machines within industrial process and harsh environments poses a technical challenge. In the field, winding failures are a major fault accounting for over 45% of total machine failures. In the literature, many condition monitoring techniques based on different failure mechanisms and fault indicators have been developed where the machine current signature analysis (MCSA) is a very popular and effective method at this stage. However, it is extremely difficult to distinguish different types of failures and hard to obtain local information if a non-intrusive method is adopted. Typically, some sensors need to be installed inside the machines for collecting key information, which leads to disruption to the machine operation and additional costs. This paper presents a new non-invasive monitoring method based on GMRs to measure stray flux leaked from the machines. It is focused on the influence of potential winding failures on the stray magnetic flux in induction machines. Finite element analysis and experimental tests on a 1.5-kW machine are presented to validate the proposed method. With time-frequency spectrogram analysis, it is proven to be effective to detect several winding faults by referencing stray flux information. The novelty lies in the implement of GMR sensing and analysis of machine faults.

Adaptive Roles of Islanded Microgrid Components for Voltage and Frequency Transient Responses Enhancement

This paper introduces a novel framework of coordinated voltage and frequency control strategy for islanded microgrid (MG) operation. The proposed control schemes rely on local measurements as communication-free control approach. Therefore, the distributed controllers of the MG components have been deployed based on their slow, medium, and fast dynamic responses to maintain the voltage and frequency in adherence to IEEE Standards 1547 and 929. The various voltage and frequency control responses associated with reactive power management scheme are efficiently utilized based on well-defined states of operation and transient management scheme. In each state, the roles of each device for voltage and frequency regulations are defined with its regulation capability, and response time based on its local measurements. Consequently, the fast reactive power compensation and rapid frequency regulation are ensured based on the inverter-based devices at challenging operating conditions. As a result, the proposed control strategy improves the voltage and frequency regulation, transient response, and MG stability. A comprehensive simulation study has verified the superior performance of the communication-free approach during steady state and in response to severe disturbances.

Improved Sample Value Adjustment for Synchrophasor Estimation at Off-Nominal Power System Conditions

The phasor measurement unit (PMU) commonly implements enhanced discrete Fourier transformation (EDFT) to compensate leakage and signal discontinuity errors at offnominal frequencies. However, EDFT results in a high total vector error (TVE) at large offnominal frequency and worsens with harmonics in power signals. Although the sample value adjustment (SVA) technique addressed such demerits, it generates an asymmetrical TVE profile. In addition, the frequency estimator was not taken into account in the previous publication on SVA. This paper proposes an improved sample value adjustment (ISVA) as a pre-DFT technique to further enhance the phasor estimation. The ISVA generates a symmetrical TVE profile and performed better than EDFT, SVA, and Taylor weighted least-square method at steady-state conditions. Subsequently, the ISVA is integrated with a recursive least-square frequency estimator, which enabled it to track dynamic signals as well. The algorithm is validated in MATLAB simulation and in a prototyped PMU. Static and dynamic test signals are based on the IEEE C37.118.1a-2014 standard.