Maziar Mobarrez

Variable interleaving technique for photovoltaic cascaded DC-DC converters

This paper introduces a variable interleaving technique for photovoltaic cascaded DC-DC converters. A series rather than parallel connection of converters allows higher switch utilization and lower rating of components; however, they suffer from nonhomogeneous irradiations condition. Under partial shading conditions, the input power of all PV panel will not be the same and as a result the output voltage of each converter will not be identical. This causes system to operate in an asymmetric condition, in which, the conventional interleaving techniques are not capable of eliminating the DC link output voltage variations. In this work, an interleaving algorithm is reported on cascaded DC/DC converters under asymmetric condition to minimize the DC link output voltage variations. The effectiveness of this algorithm for cascaded DC-DC converters has been validated by simulation and Hardware-In-the-Loop tests.

Comparative study of DC circuit breakers using realtime simulations

One of the main limitations in using high voltage and medium voltage DC grids is the issues related to the current-limiting devices and circuit breakers. These devices should be able to handle large currents in the normal operation condition and block high DC currents in a few microseconds in the case of DC faults. Moreover, they are required to dissipate huge amounts of energy stored in the transmission line inductances. In this paper, four configurations of DC circuit breakers from three general groups are evaluated and the results are compared in terms of the time required by the breakers to interrupt the current, maximum DC breaking current, rated voltage, efficiency and current state of development. The performance of DC circuit breakers is evaluated when a line to ground DC fault happens in a 9 module/2 terminal DC system. Simulation results were obtained using Real Time Digital Simulator (RTDS) and PSCAD.

Implementation of distributed power balancing strategy with a layer of supervision in a low-voltage DC microgrid

Low-voltage DC microgrids are gaining popularity due to the higher efficiency, reliability, lower costs and better compatibility with DC loads compared to their AC counterparts. One of the challenges associated with employing DC microgrids is to design and implement a reliable control algorithm that ensures all the power converters maintain the power balance in normal operating condition as well as emergency condition. In this paper, a novel power balancing strategy is proposed to harvest maximum power from the solar and minimize the effect of solar output variations on the AC power grid. Moreover, a layer of communication is proposed that can be added to the control algorithm to optimize the usage of battery energy storage systems (BESS) and compensate for voltage drop/rise due to the implementation of linear droop. The feasibility and effectiveness of the proposed power balancing strategy is verified by a lab-scale DC microgrid test bed.

Flexible HF distribution transformers for inter-connection between MVAC and LVDC connected to DC microgrids: Main challenges

Solid-state transformers or so called flexible high-frequency (HF) distribution transformers are likely to become a more efficient inter-connection between medium voltage (MV) AC grids and low voltage (LV) DC grids which can eventually be connected to DC microgrids. This paper addresses the main challenges in order to implement this technology. High-Frequency Transformers (HFTs) are one of the key elements of such converters which are the main contributor to realize the voltage adaption, isolation requirements, as well as high-power density. This paper addresses the essential design considerations taking into account the magnetic materials, leakage inductance integration as well as thermal management of such transformers. Moreover, the limitations of the currently available semiconductors are discussed and the applicability and performance of the latest generations of 10 kV SiC MOSFETs are discussed. The highlighted design considerations within both the transformer and HV SiC devices are then demonstrated using experimental results.

Grounding architectures for enabling ground fault ride-through capability in DC microgrids

Distributed generation in the power grid will result in considerable efficiency improvement and increase in reliability and stability of the grid. And DC microgrids have clear benefits such as higher reliability, higher efficiency, better compatibility with DC loads, expandability and etc., over their AC equivalent systems. Although DC microgrids have clear advantages over the AC microgrids, but there is not sufficient information available on their grounding. Realizing the grounding of DC systems would accelerate employing of these systems in the power grid. Grounding is a complex topic involving many design considerations and trade-offs and it is needed to ensure the safety of personnel and equipment as well as detection of ground fault in the system. Grounding of DC power system should be designed to 1) minimize the leakage current during normal operation, 2) maximize the safety of personnel and equipment under fault conditions. This work examines the different grounding methods and system architectures and discusses the design trade-offs in terms of safety, reliability, detection, mitigation, noise, and cost. We examine impedance grounding, isolation, and bi-polar architectures and discuss their benefits with respect to these criteria.

A novel control method for preventing the PV and load fluctuations in a DC microgrid from transferring to the AC power grid

DC microgrids are gaining popularity due to the higher efficiency, reliability, lower costs and better compatibility with DC loads compared to their AC counterparts. Large-scale deployment of distributed renewable energy resources like solar in the microgrids, has to provide utilities and grid operators the capability to safely and reliably mitigate the impact of solar and loads intermittency on the main AC power grid. In this paper, a novel control method is proposed to prevent solar and load variations inside a DC microgrid from transferring to the AC power grid while the microgrid is operating in grid-tied mode. The control method works based on having multiple slack terminals with different voltage controller response times in parallel. The response times can be adjusted such that the battery converters of the microgrid absorb solar and load fluctuations while the grid-tied inverters contribute to their voltage regulations shares smoothly according to their droop parameters. This method improves upon previously discussed methods in literature in that it does not require single slack terminal, DC bus signaling or converter mode changes.

Smart inverter volt-watt control design in high PV penetrated distribution systems

Advanced control techniques such as Volt-Watt and Volt-VAR Control have been developed for high integration of distributed renewable energy, such as Photovoltaic (PV) resources, on an electric distribution system. However, designing these control parameters which yields the best results in the system is complicated and depends on feeder conditions. This paper proposes a method to properly design the Volt-Watt control parameters in smart PV inverters to increase the benefit of their control action. The intention of this control design is to mitigate the voltage violations in a high PV penetrated distribution feeder, while evenly distributing the weight of energy curtailment among all PV systems. Test results are provided from simulation-only scenarios and a Hardware-In-The-Loop (HIL) test platform.

Control and performance analysis methodology for scale-up of MMC submodules for back-to-back HVDC applications

The modular multilevel converter (MMC) is a promising topology for both Medium Voltage (MV) and High Voltage Direct Current (HVDC) applications. The MMC employs a large number of “submodule” (SM) cascaded in series per phase arm to achieve the MV- or HVDC voltage. The SM numbers per phase arm can be as high as 256 or 512 SMs for +/− 300kV DC bus voltage for typical 300–600MW Back-to-Back (B2B) and also point to point HVDC applications. The MMC AC side interface voltage Total Harmonics Distortion (THD) requirements are normally < 3% which can be achieved by 48-pulse stepped AC waveform. This paper addresses the first step towards scale-up control and performance analysis such that the behavior of a large number of SMs can be predicted by a cumulative set of a smaller number of SMs. The SM capacitor voltage control is analyzed for scale-up of MMC. Simulation results of B2B HVDC application based MMC system with minimum values of MMC components (SMs capacitors and arms inductors) based on a parametric search are performed using Real Time Digital Simulator (RTDS) system.

A multi-loop controller for LCL-filtered grid-connected converters integrated with a hybrid harmonic compensation and a novel virtual impedance

The LCL-filtered converter is widely adapted to interface renewable energy sources and energy storage devices to the grid. While the LCL filter is capable of eliminating current harmonics caused by the high-frequency PWM of the converter, low-order current harmonics are reduced selectively by the stationary-frame resonant harmonic compensators (HCs). There are two types of the harmonic compensator, namely, harmonic current compensator (HCC) and harmonic voltage compensator (HVC). HCCs are in general embedded in parallel with conventional grid current controllers by which the active and reactive power are regulated. If autonomous islanding operation is required, the active and reactive power are controlled by a V-f droop method and a filter capacitor voltage controller with which HCCs or HVCs are added in parallel. In this paper, a multi-loop controller is proposed as one possible alternative to the conventional grid current controller to improve the harmonic compensation performance by using both HCCs and HVCs, which is a hybrid harmonic compensation (HHC). In addition, a novel virtual impedance implementation technique which fits to the multi-loop frame is presented to maintain stability of the controller in case of large grid impedance.