Mohammad Kazem Bakhshizadeh

Couplings in Phase Domain Impedance Modeling of Grid-Connected Converters

The output impedance of a power converter plays an important role in the stability assessment of the converter. The impedance can be expressed in different frames such as the stationary frame (phase domain) or in the synchronous frame (dq domain). To treat the three-phase system like a single-phase system, the system can be divided into positive and negative sequences in the phase domain. This paper demonstrates that there exist couplings between the positive and negative sequences, even in a balanced system due to the PLL, which is important for synchronization. Further it will be shown that even though these couplings are very small in magnitude, they are important in the stability of the converter.

Optimal Selective Harmonic Mitigation Technique on Variable DC Link Cascaded H-Bridge Converter to Meet Power Quality Standards

In this paper, power quality standards such as IEC 61000-3-6, IEC 61000-2-12, EN 50160, and CIGRE WG 36-05 are fulfilled for single- and three-phase medium-voltage applications using selective harmonic mitigation-pulsewidth modulation in a cascaded H-bridge (CHB) converter. Furthermore, the ER G5/4 power quality standard, which has the strictest grid codes at the medium-voltage level, will also be met in the whole range of the modulation indices. In order to achieve this goal, a symmetrical CHB with variable dc links are employed, while the converter has a low number of switching transitions. In other words, all of the dc link voltages have the same voltage, which is dependent on the modulation index. Finally, in order to validate the effectiveness and the applicability of the proposed method, it is verified by simulations and experiments for a seven-level CHB converter.

Selective Harmonic Elimination in Asymmetric Cascaded Multilevel Inverters Using a New Low-frequency Strategy for Photovoltaic Applications

Abstract Asymmetric multilevel inverters with a main and auxiliary power circuit can yield maximum number of voltage levels against the number of semiconductor devices. Most of the output power will be produced by the main inverter, which works at low frequency and the auxiliary inverters only improve the voltage quality. Advantages of the asymmetric structure are reduced number of semiconductors and gate drive circuits and better voltage quality and efficiency, which make the asymmetric inverter to be a perfect candidate for photovoltaic applications. This article presents a new switching strategy in asymmetric multilevel inverters, which exploits extra degrees of freedom to eliminate more low-order harmonics. Simulation results show this method can introduce two new switching angles without affecting the power losses of the inverter.

Indirect thermal control for improved reliability of Modular Multilevel Converter by utilizing circulating current

Modular Multilevel Converter (MMC) has recently become a popular multilevel topology for high-power applications, where the reliability performance is a crucial design consideration. In this paper the impacts of the circulating current in the MMC to the loss and thermal loading of power semiconductor devices are comprehensively investigated. Also a novel control strategy by utilizing the circulating current is proposed to enhance the reliability performance of MMC in order to limit the amplitude of thermal cycles. It is concluded that the circulating current may change the losses and thermal loading of the power devices, and by proper controlling the amount of circulating current, the temperature fluctuation or thermal cycling in the power device can be relieved, and thereby contributing to improved reliability performance according to many lifetime models/testing results for power devices.

Harmonic modelling, propagation and mitigation for large wind power plants connected via long HVAC cables: Review and outlook of current research

This paper presents a state-of-the-art review on grid connection of large offshore wind power plants (OWPPs) using extra-long high voltage AC (HVAC) cables. The paper describes research by DONG Energy Wind Power in close collaboration with Aalborg University addressing related challenges through an industrial PhD project. The overall goal is to gain a better understanding of extra-long HVAC cable connected OWPPs, in order to ensure reliability and availability of OWPPs. This will reduce the cost of energy, as the risk of costly delays and modifications after the project has been commissioned can be reduced and operational availability and the reliability can be improved. Challenges hereto include lower resonance frequencies due to the long cables, frequency dependent modelling of long submarine cables, passive filtering, and PLL dynamics in the control loops (from a harmonic stability point of view).

Small-signal model of a decoupled double synchronous reference frame current controller

The converter behavior in harmonic studies or harmonic stability analysis can be evaluated by its output impedance. In order to overcome the nonlinearities introduced by the PLL, linearized models around the operating point are typically used. However, in the case of an imbalance in the system the dq signals are not dc anymore, and therefore, linearization cannot be done. In this paper a Decoupled Double Synchronous Frame PLL is used to eliminate the oscillations in the dq frame signals. The small signal model of this PLL including an unbalanced current controller is presented in this paper.

Using variable DC sources in order to improve the voltage quality of a multilevel STATCOM with low frequency modulation

In this paper, a Selective Harmonic Elimination Pulse Width Modulation (SHE-PWM) method, which utilizes adjustable dc links, is presented for a Static Synchronous Compensator (STATCOM) based on Cascaded H-Bridge (CHB) multilevel converter. The proposed switching algorithm is able to fix the capacitors voltages at optimum and unequal values, while injecting the desired reactive power, in order to eliminate more low order harmonics. An indirect current control method is combined with a loss estimation block to control the dc link voltages that might have different losses. Moreover, by using unequal dc link voltages more levels at the output is achievable compared to the symmetrical multilevel converters that could be used in enhancing the power quality, while keeping the switching frequency near the fundamental (the maximum switching frequency can be kept below 150 Hz). The validity of the proposed method is verified by simulations in Matlab/Simulink environment.

Improvement of device current ratings in Modular Multilevel Converter by utilizing circulating current

Modular Multilevel Converter (MMC) has recently become a popular multilevel topology for high-power applications. Conventionally, the circulating current between the legs of an MMC is an unwanted current that must be minimized in order to reduce the converter loading. In this paper the impacts of the circulating current in the MMC to the current ratings of power semiconductor devices are comprehensively studied. It is shown that by injecting some circulating current instead of completely eliminating it, the device peak current, average current and rms current can be improved. The conduction and switching losses in the power converters are related to the device average and RMS currents. It is concluded that some amount of circulating current can minimize the device current ratings and can also lead to a more equalized usage of the power devices in a sub-module.

Improving the Impedance Based Stability Criterion by Using the Vector Fitting Method

In this paper, some issues related to the impedance-based stability criterion, such as RHP poles in the minor-loop gain, how to specify source and load impedances, and node-dependent stability margins are discussed and the vector fitting method is used to improve the method. This results in an eigenvalue analysis of the system instead of using the Nyquist diagram, and leads to obtain more information about the system. Furthermore, a participation factor analysis can be used to identify the problematic subsystem. The proposed method is examined in the assessment of a stable and an unstable system, and simulation results are used for verification.

A Numerical Matrix-Based Method for Stability and Power Quality Studies Based on Harmonic Transfer Functions

Some couplings exist between the positive- and negative-sequence impedances of a voltage-sourced power converter especially in the low frequency range due to the nonlinearities and low-bandwidth control loops such as the phase-locked loop. In this paper, a new numerical method based on the harmonic transfer function for analysis of the linear time periodic systems is presented, which is able to handle these couplings. In a balanced three-phase system, there is only one coupling term, but in an unbalanced (asymmetrical) system, there are more couplings, and therefore, in order to study the interactions between these couplings a matrix-based method should be used. No information about the structure of the converter is needed and elements are modeled as black boxes with known terminal characteristics. The proposed method is applicable for both power quality (harmonic and interharmonic emissions) and stability studies, which is verified by simulations in this paper.