Tuan Ngo

Grid-connected photovoltaic converters: Topology and grid interconnection

With the advances in high power converter technologies, photovoltaic (PV) power has been integrated into the utility system in greater pace in recent years. This paper aims to provide a broad overview of the various grid-connected PV system configurations and their power converter technologies for applications in single- and three-phase utility power systems. Grid-connected PV systems presented include module-integrated, string, and centralized configurations. Single- and three-phase power converters discussed include full-bridge (with an additional switch in the DC link and back-to-back IGBT on the AC terminals), and neutral point clamped converters. The synchronization techniques for grid-connected PV applications are discussed in this paper. An improved method to detect the grid frequency and voltage magnitude for single-phase system is introduced. More importantly, harmonic distortions resulting from the operation of grid-connected photovoltaic systems are addressed and analyzed. Methods for reducing harmonic currents using three types of passive filters, L, LC, and LCL, are presented. The LCL-filter design guidelines for a specific PV system are presented in detail. The effectiveness of these filters is demonstrated on 3 kW and 50 kW photovoltaic systems.

Steady-State Analysis and Performance of Low Frequency AC Transmission Lines

The steady-state performance of low frequency AC (LFAC) systems for bulk power transmission is proposed and investigated in this paper. It is demonstrated that the LFAC is superior to the conventional 60 Hz AC system in terms of power transfer capability and voltage stability. An existing power system can increase its power transfer capability by up to more than eleven times if it operates at 5 Hz. In low operating frequency conditions, the transmission overhead line reactance is significantly lowered and thus results in less voltage drop along the line. If the system operating frequency is in the range of 1 to 10 Hz, the voltage profile along a transmission line is comparable to the surge impedance loading at the no-load condition or approaches that of the HVDC system at the full-load condition. More importantly, the V-P and Q-V characteristic curves show that the dependency of voltage on the real and reactive power flow is greatly reduced. In other words, the sensitivity of voltage on reactive power variations is diminished and results in a more stable voltage and a higher stability margin for the LFAC system compared to the conventional 60 Hz system.

Improving performance of single-phase SOGI-FLL under DC-offset voltage condition

The second-order-generalized-integrator frequency-locked loop (SOGI-FLL) is typically employed to synchronize with the grid in the single-phase grid-connected converter. The SOGI creates two orthogonal signals from an input grid voltage, and estimates the magnitude while the frequency locked-loop identifies the grid frequency. If there is a DC component in the grid voltage due to the measurement process, it degrades the accuracy of SOGI-FLL estimates. The SOGI-FLL, therefore, cannot identify accurate information of the grid voltage. This paper introduces an improved control method for the SOGI-FLL when the grid contains a DC component. The value of the DC component voltage is estimated online and totally removed from the frequency control loop. The controller, thus, responses faster and more precisely detects both the magnitude and frequency of the input voltage. Simulation results are presented to verify the theoretical analyses.

Detecting positive-sequence component in active power filter under distorted grid voltage

In distribution power system, an active power filter (APF) is installed at the point of common coupling (PCC) bus to eliminate harmonic currents injected by non-linear loads. The active power filter, therefore, needs to synchronize with the grid by employing a second-order-generalized-integrator frequency-locked loop (SOGI-FLL) to detect the positive-sequence voltage component. If the grid is highly distorted by harmonic currents, it degrades the accuracy of SOGI-FLL estimation and the APF cannot get correct grid voltage characteristics. This paper introduces a new grid synchronization structure based on the SOGI-FLL under distorted voltage condition. The proposed structure behaves as a high-order filter to completely remove harmonic components and inherits advantages from SOGI-FLL. The grid voltage magnitude and frequency, thus, are detected precisely under distorted grid. Simulation results are presented to verify the proposed system performances.

Operation of three-level single-phase half-bridge NPC inverter-based shunt active power filter under non-ideal grid voltage condition with sliding mode controller

In this paper, a new control method is presented for a single-phase three-level neutral point clamped half-bridge shunt active power filter with the aim of eliminating the grid current harmonics. It is shown that the proposed control method accurately detects the positive-sequence voltage component under distorted grid condition. With this feature, the performance of the single-phase active power filter under distorted grid voltage condition is improved considerably. Furthermore, the imbalance in the capacitor voltages which occurs in the half-bridge topology is eliminated by adding a feedback term to the current control loop. The performance of the proposed control method is verified by simulation results obtained from MATLAB/Simulink.

Analysis of distance protection in low frequency AC transmission systems

This paper extends the analysis of low frequency AC (LFAC) transmission systems and investigates short-circuit fault characteristics and distance protection considerations. Due to operation at a low frequency, transmission line reactance is reduced and thus the power system transfer capability is increased significantly. The transmission line impedance reduction, however, creates a drawback in LFAC systems when faults occur. The analysis shows that fault currents are expected to be much higher than in a 60 Hz system. In addition, LFAC has a longer current wavelength and requires the protection system perform quickly to clear faults. This paper first examines distance protection in LFAC systems and determines that LFAC systems have a larger separation between distance protection zone characteristic impedance and load impedance in comparison to 60 Hz systems. Next, several typical fault types in a power systems are analyzed at different frequencies. A fault clearing time case is also calculated and compared between different systems. The analysis shows that the critical clearing time (in seconds) is less for LFAC systems in comparison to conventional 60 Hz systems.

Power flow solution for multi-frequency AC and multi-terminal HVDC power systems

This paper addresses to a power flow solution for AC-DC power systems. The AC system consists of transmission lines operated at 60 Hz and at low frequencies while the DC system has multi-terminal connections. The paper shows power flow in such power systems can be obtained by using the existing unified approach for AC-DC systems and reflecting impedances of lines operating at low frequencies in AC grids to their equivalent impedances at the fundamental power frequency. The equivalent transmission lines in terms of power flow of those lines operated at low frequencies are first mathematically established. The conventional unified approach to solve power flow in AC-DC systems is then discussed with a modification to reduce the size of the Jacobian matrix. The method is then verified by analyzing the power flow solution of a multi-frequency AC and multi-terminal HVDC power system modeled in PSCAD/EMTDC.

Modal-based voltage stability analysis of low frequency AC transmission systems

The low frequency AC (LFAC) transmission, in which a power system is operated at a low frequency, i.e., below 50/60 Hz, is superior to the conventional 60 Hz system in terms of power transfer capability. In addition, due to low operating frequency, the line reactance is reduced and thus voltage drops along the line are decreased. The low frequency transmission thus offers a higher voltage profile for a power system. In other words, an LFAC system can be more voltage stable in comparison to the conventional 60-Hz system. This paper intends to focus on the voltage stability of an LFAC system. The theoretical foundations of a two-bus system are first discussed based on eigenvalue. A modified stability index calculation is also introduced for low frequency transmission to estimate the system stability accurately. The simulation results from a practical system verify that the LFAC transmission has great benefits over the 60 Hz system in terms of power transfer capability and voltage stability.

Power flow solution for multi-frequency AC power systems

This paper proposes to enhance existing power flow solutions for application in multi-frequency AC power systems. The paper shows such power flow solutions can be obtained using existing methods provided that impedances of lines operating at different frequencies be reflected to their equivalent impedances at the fundamental power frequency. The paper first presents a mathematical proof of equivalent parameters in terms of power flow of a transmission line operated at a low frequency and the fundamental frequency. It is then validated by analyzing the power flow solutions of a power system modeled in PSCAD/EMTDC when it is operated at multi-frequency and at conventional 60 Hz conditions. An application of a low frequency AC transmission line is illustrated to demonstrate its superiority in terms of power carrying capacity over a typical fundamental-frequency AC transmission line.

Voltage stability of low frequency AC transmission systems

This paper investigates the voltage stability of a power system with low frequency AC (LFAC) transmission. Under low frequency conditions, transmission line reactance is reduced and thus the voltage drop along the transmission line is decreased. The LFAC system, therefore, is superior to the conventional 60-Hz system in terms of power transfer capability and, more importantly, voltage instability. In other words, the LFAC can drive a power system further away from instability mode in comparison to the conventional 60-Hz system. A power system voltage stability can be quantified and displayed with the eigenvalues of a Jacobian matrix and the self-sensitivity values by using the modal analysis method. In this paper, the theoretical stability of an LFAC system is discussed first and then validated using several transmission system case studies. The simulation and analysis results show that the LFAC system outperforms the conventional 60 Hz system in terms of voltage stability.