Mohammad Abusara

Line-Interactive UPS for Microgrids

Line-interactive uninterruptible power supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy, and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery. Power can be also exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. In this paper, a line-interactive UPS and its control system are presented and discussed. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The drooping coefficients are chosen to limit the energy imported by the UPS when reconnecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60-kW line-interactive UPS systems are provided to validate the design.

Design and Control of a Grid-Connected Interleaved Inverter

This paper is concerned with the design and control of a three-phase voltage source grid-connected interleaved inverter. This topology enables the use of low-current devices capable of switching at high frequency, which together with the ripple cancelation feature reduces the size of the output filter and the inverter considerably compared to an equivalent classical two-level voltage source inverter with an LCL output filter using high-current devices with considerably lower switching frequency. Due to its higher switching frequency and low-filter component values, the interleaved inverter also has a much higher bandwidth than the classical inverter, which improves grid voltage harmonics disturbance rejection and increases the speed of response of the inverter and its capability to ride through grid disturbance (e.g., voltage sags and swells). The paper discusses the selection of the number of channels and the filter component values of the interleaved inverter. The design of the digital control system is then discussed in detail. Simulation and practical results are presented to validate the design and demonstrate its capabilities.

Control of Transient Power During Unintentional Islanding of Microgrids

In inverter-based microgrids, the paralleled inverters need to work in grid-connected mode and stand-alone mode and to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using the droop control. Inverters can have different power set points during grid-connected mode, but in stand-alone mode, they all need their power set points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set points to inverters with low power set points, which can raise the dc-link voltage of the inverters causing them to shut down. This paper investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the dc-link voltage and adjusts the power set point in proportion to the rise in the voltage. A small-signal model of an islanded microgrid is developed and used to design the controller. Simulation and experimental results are presented to validate the design.

A Review of Hybrid Solar PV and Wind Energy System

Due to the fact that solar and wind power is intermittent and unpredictable in nature, higher penetration of their types in existing power system could cause and create high technical challenges especially to weak grids or stand-alone systems without proper and enough storage capacity. By integrating the two renewable resources into an optimum combination, the impact of the variable nature of solar and wind resources can be partially resolved and the overall system becomes more reliable and economical to run. This paper provides a review of challenges and opportunities/solutions of hybrid solar PV and wind energy integration systems. Voltage and frequency fluctuation, and harmonics are major power quality issues for both grid-connected and stand-alone systems with bigger impact in case of weak grid. This can be resolved to a large extent by having proper design, advanced fast response control facilities, and good optimization of the hybrid systems. The paper gives a review of the main research work reported in the literature with regard to optimal sizing design, power electronics topologies and control. The paper presents a review of the state of the art of both grid-connected and stand-alone hybrid solar and wind systems.

Control of line interactive UPS systems in a Microgrid

This paper is concerned with the control of a line interactive Uninterruptable Power Supply (UPS) system in a Microgrid. In grid connected mode, power can be imported from the grid to charge the battery or it can be injected into the grid to boost the power supplied by the Microgrid. In island mode, the UPS supplies local critical load in parallel with other sources. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid connected and parallel islanding modes of operation. When the power is imported from the grid, an outer voltage controller that regulates the DC link voltage sets the power demand for an inner power flow controller. The outer voltage loop is designed with the aid of small signal analysis which has been used to model the dynamics of the inner power flow controller. Due to the limited DC link capacitance, the drooping coefficients are chosen carefully to limit transient circulating active power from the grid to the UPS module or from one UPS module to another. The traditional way of measuring active and reactive power which is based on using a low pass filter is replaced by a real time integration method which is shown to improve the response of the controller. A virtual impedance is utilized in grid-connected mode to improve output current THD. Experimental results of two 60kW line interactive UPS modules are provided to validate the design.

Digital control of a three-phase grid connected inverter

This paper presents the design and practical implementation of a digital current controller for a three-phase PWM voltage source inverter connected to the grid via an LCL filter to produce a high quality current injected into the grid. A two-feedback loops control system is proposed, with an outer grid current loop and an inner filter capacitor current loop. The controller also incorporates a grid voltage feedforward loop to compensate for the effect of the utility voltage disturbance. The paper also discusses the effect of the sampling and computational time delay both on system stability and current quality. This time delay is shown to reduce the stability of the inner loop. However, it is also shown that it is better, from the point of view of output current THD quality, to increase the inner loop time delay to a full one sampling period, so that the mains frequency envelope (nearly ripple free) current component is sampled at the peaks and troughs of the PWM, away from the switching instances of the transistors and associated switching noise. To ensure system stability, a capacitor current observer is proposed to compensate for the computational time delay. The design of the capacitor current observer and the controller are discussed. Simulation and practical results are presented to validate the design.

Control of a Point Absorber Using Reinforcement Learning

This work presents the application of reinforcement learning for the optimal resistive control of a point absorber. The model-free Q-learning algorithm is selected in order to maximise energy absorption in each sea state. Step changes are made to the controller damping, observing the associated penalty, for excessive motions, or reward, i.e. gain in associated power. Due to the general periodicity of gravity waves, the absorbed power is averaged over a time horizon lasting several wave periods. The performance of the algorithm is assessed through the numerical simulation of a point absorber subject to motions in heave in both regular and irregular waves. The algorithm is found to converge towards the optimal controller damping in each sea state. Additionally, the model-free approach ensures the algorithm can adapt to changes to the device hydrodynamics over time and is unbiased by modelling errors.

Design of a robust digital current controller for a grid connected interleaved inverter

This paper is concerned with the design and practical implementation of a robust digital current controller for a three-phase voltage source grid-connected interleaved inverter. Each phase consists of 5 half-bridge channels connected in parallel. Due to the current ripple cancellation of the interleaving topology, only small output filter capacitors are required which provide high impedance to grid voltage harmonics and hence better current quality compared to traditional 2-level LCL topology. The current in each channel is controlled via a single feedback loop. A feedforward loop of the grid voltage is incorporated to compensate for the grid disturbance. To control the high resonance frequency of the output filter, high sampling and switching frequencies are required. Alternatively, resistors in series with the filter capacitors are used to provide damping. This method becomes practically possible due to the low magnitude of the current of the capacitors and consequently low power dissipation in the damping resistors. The paper also studies in detail the effects of computational time delay and grid impedance variation on system stability. A phase lag compensator incorporated in the feedback loop is designed to increase system immunity to grid impedance variations. Simulation and practical results are presented to validate the design.

Improved Reactive Power Sharing for Parallel-operated Inverters in Islanded Microgrids

The unequal impedances of the interconnecting cables between paralleled inverters in the island mode of microgrids cause inaccurate reactive power sharing when the traditional droop control is used. Many studies in the literature adopt low speed communications between the inverters and the central control unit to overcome this problem. However, the losses of this communication link can be very detrimental to the performance of the controller. This paper proposes an improved reactive power-sharing control method. It employs infrequent measurements of the voltage at the point of common coupling (PCC) to estimate the output impedance between the inverters and the PCC and then readjust the voltage droop controller gains accordingly. The controller then reverts to being a traditional droop controller using the newly calculated gains. This increases the immunity of the controller against any losses in the communication links between the central control unit and the inverters. The capability of the proposed control method has been demonstrated by simulation and experimental results using a laboratory scale microgrid.

Control of Grid-Connected Inverters Using Adaptive Repetitive and Proportional Resonant Schemes

Repetitive and proportional-resonant controllers can effectively reject grid harmonics in grid-connected inverters because of their high gains at the fundamental frequency and the corresponding harmonics. However, the performances of these controllers can seriously deteriorate if the grid frequency deviates from its nominal value. Non-ideal proportional-resonant controllers provide better immunity to variations in grid frequency by widening resonant peaks at the expense of reducing the gains of the peaks, which reduces the effectiveness of the controller. This paper proposes a repetitive control scheme for grid-connected inverters that can track changes in grid frequencies and keep resonant peaks lined up with grid frequency harmonics. The proposed controller is implemented using a digital signal processor. Simulation and practical results are presented to demonstrate the controller capabilities. Results show that the performance of the proposed controller is superior to that of a proportional-resonant controller.