Firuz Zare

Improvement of Stability and Load Sharing in an Autonomous Microgrid Using Supplementary Droop Control Loop

This paper investigates the problem of appropriate load sharing in an autonomous microgrid. High gain angle droop control ensures proper load sharing, especially under weak system conditions. However, it has a negative impact on overall stability. Frequency-domain modeling, eigenvalue analysis, and time-domain simulations are used to demonstrate this conflict. A supplementary loop is proposed around a conventional droop control of each DG converter to stabilize the system while using high angle droop gains. Control loops are based on local power measurement and modulation of the d-axis voltage reference of each converter. Coordinated design of supplementary control loops for each DG is formulated as a parameter optimization problem and solved using an evolutionary technique. The supplementary droop control loop is shown to stabilize the system for a range of operating conditions while ensuring satisfactory load sharing.

Power Management and Power Flow Control With Back-to-Back Converters in a Utility Connected Microgrid

This paper proposes a method for power flow control between utility and microgrid through back-to-back converters, which facilitates desired real and reactive power flow between utility and microgrid. In the proposed control strategy, the system can run in two different modes depending on the power requirement in the microgrid. In mode-1, specified amount of real and reactive power are shared between the utility and the microgrid through the back-to-back converters. Mode-2 is invoked when the power that can be supplied by the distributed generators (DGs) in the microgrid reaches its maximum limit. In such a case, the rest of the power demand of the microgrid has to be supplied by the utility. An arrangement between DGs in the microgrid is proposed to achieve load sharing in both grid connected and islanded modes. The back-to-back converters also provide total frequency isolation between the utility and the microgrid. It is shown that the voltage or frequency fluctuation in the utility side has no impact on voltage or power in microgrid side. Proper relay-breaker operation coordination is proposed during fault along with the blocking of the back-to-back converters for seamless resynchronization. Both impedance and motor type loads are considered to verify the system stability. The impact of dc side voltage fluctuation of the DGs and DG tripping on power sharing is also investigated. The efficacy of the proposed control arrangement has been validated through simulation for various operating conditions. The model of the microgrid power system is simulated in PSCAD.

A Hybrid Cascade Converter Topology With Series-Connected Symmetrical and Asymmetrical Diode-Clamped H-Bridge Cells

A novel H-bridge multilevel pulsewidth modulation converter topology based on a series connection of a high-voltage diode-clamped inverter and a low-voltage conventional inverter is proposed in this paper. A dc link voltage arrangement for the new hybrid and asymmetric solution is presented to have a maximum number of output voltage levels by preserving the adjacent switching vectors between voltage levels. Hence, a 15-level hybrid converter can be attained with a minimum number of power components. A comparative study has been carried out to present high performance of the proposed configuration to approach a very low total harmonic distortion of voltage and current, which leads to the possible elimination of the output filter. Regarding the proposed configuration, a new cascade inverter is verified by cascading an asymmetrical diode-clamped inverter, in which 19 levels can be synthesized in output voltage with the same number of components. To balance the dc link capacitor voltages for the maximum output voltage resolution as well as synthesize asymmetrical dc link combination, a new multi-output boost converter is utilized at the dc link voltage of a seven-level H-bridge diode-clamped inverter. Simulation and hardware results based on different modulations are presented to confirm the validity of the proposed approach to achieve a high-quality output voltage.

Droop Control of Converter-Interfaced Microsources in Rural Distributed Generation

This paper proposes new droop control methods for load sharing in a rural area with distributed generation. Highly resistive lines, typical of rural low voltage networks, always create a big challenge for conventional droop control. To overcome the conflict between higher feedback gain for better power sharing and system stability in angle droop, two control methods have been proposed. The first method considers no communication among the distributed generators (DGs) and regulates the converter output voltage and angle ensuring proper sharing of load in a system having strong coupling between real and reactive power due to high line resistance. The second method, based on a smattering of communication, modifies the reference output voltage angle of the DGs depending on the active and reactive power flow in the lines connected to point of common coupling (PCC). It is shown that with the second proposed control method, an economical and minimum communication system can achieve significant improvement in load sharing. The difference in error margin between proposed control schemes and a more costly high bandwidth communication system is small and the later may not be justified considering the increase in cost. The proposed control shows stable operation of the system for a range of operating conditions while ensuring satisfactory load sharing.

Voltage-sharing converter to supply single-phase asymmetrical four-level diode-clamped inverter with high power factor loads

The output voltage quality of some of the single-phase multilevel inverters can be improved when their dc-link voltages are regulated asymmetrically. Symmetrical and asymmetrical multilevel diode-clamped inverters have the problem of dc-link capacitor voltage balancing, especially when power factor of the load is close to unity. In this paper, a new single-inductor multi-output dc/dc converter is proposed that can control the dc-link voltages of a single-phase diode-clamped inverter asymmetrically to achieve voltage quality enhancement. The circuit of the presented converter is explained and the main equations are developed. A control strategy is proposed and explained in details. To validate the versatility of the proposed combination of the suggested dc-dc converter and the asymmetrical four-level diode-clamped inverter (ADCI), simulations and experiments have been directed. It is concluded that the proposed combination of introduced multioutput dc-dc converter and single-phase ADCI is a good candidate for power conversion in residential photovoltaic (PV) utilization.

Angle droop versus frequency droop in a voltage source converter based autonomous microgrid

This paper compares the performance of angle and frequency droops in an autonomous microgrid that only contains voltage source converter (VSC) interfaced distributed generators (DGs). As a VSC can instantaneously change output voltage waveform, power sharing in a microgrid is possible by controlling the output voltage angle of the DGs through droop. The angle droop is able to provide proper load sharing among the DGs without a significant steady state frequency drop in the system. It is shown that the frequency variation with the frequency droop controller is significantly higher than that with the angle droop controller. The angle droop controller is derived from DC load flow. Both the angle and frequency droop controllers are designed through eigenvalue analysis. The performance of these two controllers is then performed through PSCAD simulations.

A hysteresis current control for single-phase multilevel voltage source inverters: PLD implementation

In most high-performance applications of voltage source pulse-width modulation inverters, current control is an essential part of the overall control system. In this paper, a hysteresis current control technique for a single-phase five-level inverter with flying-capacitor topology is proposed. Logic controls and a programmable logic device are suitable for handling a large number of switches and implementation of state transitions. This method also considers how to improve unbalanced voltages of capacitors using voltage vectors in order to minimize switching losses. The simulation and experimental results describe and verify the current control technique for the inverter.

Sensitivity analysis of voltage imbalance in distribution networks with rooftop PVs

A comprehensive voltage imbalance sensitivity analysis and stochastic evaluation based on the rating and location of single-phase grid-connected rooftop photovoltaic cells (PVs) in a residential low voltage distribution network are presented. The voltage imbalance at different locations along a feeder is investigated. In addition, the sensitivity analysis is performed for voltage imbalance in one feeder when PVs are installed in other feeders of the network. A stochastic evaluation based on Monte Carlo method is carried out to investigate the risk index of the non-standard voltage imbalance in the network in the presence of PVs. The network voltage imbalance characteristic based on different criteria of PV rating and location and network conditions is generalized. Improvement methods are proposed for voltage imbalance reduction and their efficacy is verified by comparing their risk index using Monte Carlo simulations.

Control of parallel converters for load sharing with seamless transfer between grid connected and islanded modes

This paper describes control methods for proper load sharing between parallel converters connected to microgrid supplied by distributed generators. The model of the microgrid power system is simulated in PSCAD. It is assumed that the microgrid supplies a load both in grid connected and islanded modes. Both passive loads and inertial loads are considered. A control strategy is proposed to improve the system performance through seamless transfer between islanded and grid connected modes. The controller is capable of handling constant impedance, as well as motor loads. The smooth transition between the grid connected and off grid mode is achieved by changing the control mode from voltage control in islanded mode to state feedback control in grid connected mode. Its efficacy has been validated through simulation for various operating conditions.

Leakage current and common mode voltage issues in modern AC drive systems

Due to rapid developments of IGBT technology, switching time and frequency are dramatically increased. At higher carrier frequencies, IGBTs induce more capacitive coupled current into a rotor and a stator frame and lead to faster bearing damage. Common mode voltage enables motor to create shaft voltage through electrostatic couplings between the rotor and the stator windings and between the rotor and the frame, and it can caused bearing currents when the shaft voltage exceeds a breakdown voltage level of the bearing grease. Also, high frequency leakage current occurs through stray capacitors between stator winding and the motor frame due to a high rate of the common mode dv/dt at motor terminals which can produce induced shaft voltage. Conducted and radiated Electromagnetic Interference (EMI) emissions are major problems in recent motor drives that produce undesirable effects on electronic devices. In modern power electronic systems, increasing power density and decreasing cost and size of system are market requirements. Switching losses, Harmonics and EMI are the key factors which should be considered at the beginning stage of a design to optimise a drive system. In most of power electronic designs, EMI issues have not been taken into account as one of the main factors; and mitigation techniques for EMI are considered at the last stage of design. This paper presents main EMI issues in a modern AC drive system.