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High-voltage dc (HVdc) transmission has been largely used to interconnect asynchronous systems and integrate renewable energy resources into electrical grids. However, the high short-circuit currents and low-tolerance of power electronics equipment impose new challenges for these systems’ protection. To address these challenges, a new distance protection algorithm for HVdc grids with modular multilevel converters (MMCs) is proposed in this article. The proposed algorithm identifies the faulty line/cable using the resonance frequency of a dc capacitor installed in each terminal. The technique was tested in a four-terminal HVdc grid with five cables and ten circuit breakers. The algorithm was highly selective for faults in the lines and provided fast identification, in less than 1 ms, without communication amongst terminals. The algorithm was tested in hardware under high-noise conditions and provided reliable results. [ABSTRACT FROM AUTHOR]

Since dc distribution minimizes the number of power conversion stages, it lowers the overall cost, power losses, and weight of a power system. Critical systems use IT grounding because it is tolerant to the first-fault. Hence, this is an attractive option for hybrid electric aircraft (HEA), which combines gas engines with electric motors driven by power electronic converters. This letter proposes an accurate implementation for the procedure of first-fault detection with IT grounding. The ac component injection along with the sliding discrete Fourier transform (SDFT) is used to estimate the fault impedance. The procedure is very accurate due to the heavy filtering of the implicit moving average filter. Further computation savings are obtained by using the double look-up tables, and the Goertzel algorithm for the SDFT. Results are validated by simulations and experiments. [ABSTRACT FROM AUTHOR]

LCL-filter-based grid-tie inverters require damping for the current-loop stability. There are only software modifications in active damping, whereas resistors are added in passive damping. Although passive damping incurs in additional losses, it is widely used because of its simplicity. This paper considers the active damping in medium-voltage parallel inverters for wind turbines. Due to cost reasons, only minimal software changes are allowed and no extra sensors can be used. The procedure must be robust against line-inductance variations in weak grids. Double-update mode is needed so the resonance frequency is under the Nyquist limit. The bandwidth reduction when using active damping is also required to be known beforehand. Moreover, the design procedure should be simple without requiring numerous trial-and-error iterations. In spite of the abundant literature, the options are limited under these circumstances. Filter-based solutions are appropriate and a new procedure for tuning the notch filter is proposed. However, this procedure requires that the resistance of the inductors is known and a novel filter-based solution is proposed that uses lag filters. The lag filters displace the phase angle at the resonance frequency so that the Nyquist stability criterion is fulfilled. Simulations and experiments with a 100-kVA prototype validate the analysis. [ABSTRACT FROM AUTHOR]

LCL filters are commonly used to connect voltage-sourced converters (VSCs) to the grid. This type of filter is cheaper than a single inductor for the same current total harmonic distortion (THD), but it generates resonance problems if no active or passive damping method is applied. Active damping methods are becoming popular in the literature because they improve efficiency, but they are sometimes difficult to implement and additional measurements are required. This paper proposes an active damping method for VSCs connected to weak grids, which is based on making the open-loop phase zero at the resonance frequency. It will be shown that this strategy provides adequate damping of oscillations and that it can be achieved in two different ways: at the design stage (if the design constraints make it possible) or with an all-pass filter in series with the current controller. Two methods to design the all-pass filter are proposed. Also, the proposed active damping technique is compared with three alternatives already proposed in the literature. All the control algorithms are verified by simulation and in a 15-kW prototype of a three-phase VSC connected to a configurable weak grid via an LCL filter. [ABSTRACT FROM AUTHOR]

This paper presents a fast selective harmonic current mitigation strategy for inverters with active power filter capabilities based on synchronous reference frames and two degrees-of-freedom internal model controllers. The advantage of this control strategy over the conventional proportional integral (PI) control solution is a significant increase in the speed of harmonic detection and mitigation. Furthermore, this control strategy reduces the computational burden when applied in a digital controller. These characteristics make this strategy desirable for applications, where fast/harmonic detection and mitigation are needed. Mathematical analysis and simulations are presented to corroborate the performance of the proposed controller strategy. Finally, the results of this proposal are verified in a 1 kW three-phase multifunctional inverter with harmonic compensation capabilities up to the 17th harmonic. [ABSTRACT FROM PUBLISHER]

Small wind energy conversion systems (WECSs) are becoming an attractive option for distributed energy generation. WECSs use permanent-magnet synchronous generators (PMSGs) directly coupled to the wind turbine and connected to the grid through a single-phase grid-tie converter. The loading produced on the dc link is characterized by large ripple currents at twice the grid frequency. These ripple currents are reflected through the dc bus into the PMSG, causing increased heating and ripple torque. In this paper, the PMSG inverter is used to control the dc-link voltage. In order to avoid reflecting the ripple currents into the PMSG, the feedback dc-link voltage is passed through a filter. The Butterworth filters, notch filters, antiresonant filter (ARF) and moving average filter (MAF) are considered. For a fair comparison, formulas are provided to tune the filter parameters so that dc-link voltage control will achieve the selected bandwidth. The different filtering options produce different levels of torque ripple reduction. The notch filter, ARF, and MAF obtain the best results and there is a tradeoff between the filter implementation complexity, bandwidth, overshoot, and the torque ripple reduction. Simulations and experiments using a 2.5-kW PMSG turbine generator validate the proposals. [ABSTRACT FROM AUTHOR]

Unprecedented expansion of native dc powered equipment (LEDs, computers, and consumer electronics) has increased commercial and residential dc electricity usage over the past decade. Thus, it is foreseeable that hybrid ac/dc buildings featuring both ac and dc infrastructures will coexist. A hybrid ac/dc building will involve an efficient centralized rectifier that supplies all the dc loads, while legacy ac loads will remain connected to the existing ac infrastructure. This paper explores the opportunity of harmonic mitigation at distribution level in small hybrid ac/dc building by using a centralized power factor corrector (PFC) with large bandwidth. The current reference generator for the harmonic mitigation function (HMF) is explained along with power considerations. The PFC uses a proportional resonant controller, instead of a PI controller, without requiring additional sensors in the rectifier. A computationally inexpensive implementation of the phase-locked loop is also proposed along with considerations on parameter selection. The proposals provide all the steps for the straightforward control design of the PFC+HMF with fast calculations. The HMF requires only software modifications in the PFC and one sensor to measure the nonlinear load. Simulation and experiments validate the proposed procedures. [ABSTRACT FROM AUTHOR]

Newly developed VSC-HVDC systems are reaching power levels of up to 1000 MW. At this power level, the nonminimum phase behavior of the VSC-HVDC systems’ dc plant becomes a threat to the stability of the direct voltage for fast dc control-loop dynamics. This paper presents a novel compensation scheme, called RHP-zero shifting+damping, designed to deal with the nonminimum phase dynamics of the dc plant by adding additional compensation loops to the current controller of the VSC-HVDC system. The compensation scheme can work along with linear controllers and allows the closed-loop bandwidth of the direct voltage controller to be increased without affecting the direct voltage stability of high-power VSC-HVDC systems. As a result, the direct voltage variations are significantly reduced during power changes in the ac or dc network. The performance of the compensation scheme is evaluated through simulations and corroborated in a 1-kW experimental test bed. [ABSTRACT FROM PUBLISHER]

Grid-connected converters employ LCL-filters, instead of simple inductors, because they allow lower inductances while reducing cost and size. Active damping, without dissipative elements, is preferred to passive damping for solving the associated stability problems. However, large variations in the grid inductance may compromise system stability, and this problem is more severe for parallel converters. This situation, typical of rural areas with solar and wind resources, calls for robust LCL-filter design. This paper proposes a design procedure with remarkable results under severe grid inductance variation. The procedure considers active damping using lead-lag network and capacitor current feedback. Passive damping is also discussed. The design flow, with little iteration and no complex algorithms, selects the proper ratios between the switching and resonance frequency, the grid and converter inductance, and the filter capacitance and total inductance. An estimation for the grid current total harmonic distortion (THD) is also proposed. Simulation and experiments validate the proposals. [ABSTRACT FROM PUBLISHER]

Three-phase active rectifiers guarantee sinusoidal input currents and unity power factor at the price of a high switching frequency ripple. To adopt an LCL-filter, instead of an L-filter, allows using reduced values for the inductances and so preserving dynamics. However, stability problems can arise in the current control loop if the present resonance is not properly damped. Passive damping simply adds resistors in series with the LCL-filter capacitors. This simplicity is at the expense of increased losses and encumbrances. Active damping modifies the control algorithm to attain stability without using dissipative elements but, sometimes, needing additional sensors. This solution has been addressed in many publications. The lead-lag network method is one of the first reported procedures and continues being in use. However, neither there is a direct tuning procedure (without trial and error) nor its rationale has been explained. Thus, in this paper a straightforward procedure is developed to tune the lead-lag network with the help of software tools. The rationale of this procedure, based on the capacitor current feedback, is elucidated. Stability is studied by means of the root locus analysis in z-plane. Selecting the lead-lag network for the maximum damping in the closed-loop poles uses a simple optimization algorithm. The robustness against the grid inductance variation is also analyzed. Simulations and experiments confirm the validity of the proposed design flow. [ABSTRACT FROM PUBLISHER]